Note: Descriptions are shown in the official language in which they were submitted.
CA 02899929 2015-08-07
1
SILICA-STABILIZED ULTRAFINE ANATASE TITANIA, VANADIA CATALYSTS, AND
METHODS OF PRODUCTION THEREOF
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED
RESEARCH OR DEVELOPMENT
=
[0002] Not applicable.
BACKGROUND
[0003] The selective catalytic reduction (SCR) of nitrogen oxides produced
during combustion processes using reductants such as NH3 has been a
commercially successful technology for over 30 years. It was originally
introduced for the control of NOx emissions in exhaust gases from stationary
power plants and other industrial facilities. Recently, interest in the
technology
has expanded as the result of its utility for treatment of emissions from
mobile
power sources, such as marine vessels, cars, trucks, and machinery. This
increased interest is driven largely by regulations that govern emissions from
mobile sources. For example,. the US EPA regulations that will be effective
in 2010 for mobile diesel engines set such low emissions levels for NOx that
efficient exhaust after-treatment is essential, and SCR is a leading
technological choice.
[0004] In stationary applications, the requirements on the catalyst are
not very high. For example, stationary engines typically run at close to
steady-state, constant temperature conditions, and with relatively low gas
space velocity. Further, the volumetric requirements for the catalyst are
not too demanding. In on-road mobile applications, however, the catalysts
requirements are much more severe. In this case, engines are not run at
steady-state or at constant . temperature, but instead cycle over wide
variations in load (and hence temperature). In one possible system
configuration, the SCR catalyst is located downstream of a diesel
particulate filter (DPF), and regeneration of the soot-loaded DPF can cause
a high temperature pulse of hot gas to pass through the downstream SCR
catalyst. Furthermore, the mobile applications typically involve much higher
gas space velocities and the volumetric requirements on the catalyst are
severe. For example, in early application of SCR applications to heavy-duty
diesel engines, the catalyst volume was several times greater than the
CA 02899929 2015-08-07
2
engine displacement! For these reasons, it is imperative that improved
catalysts be developed that have higher thermal stability, and improved
volumetric activity, so that cost-effective technological solutions can be
found to meet increasingly strict regulations.
[0005] The
technology that has been utilized for many years in
stationary applications involves catalysts based on metal oxides, and
especially those based on TiO2 as the catalyst support, and the active
catalytic functionality is based on vanadia, V205. Thus, mixtures of TiO2
(80-95%), W03 (3-10%) and optionally with the balance comprising Si02
(such as DT-52Tm and DT-58Tm) have been in use as the catalyst support,
and the active vanadia component is typically present at 0.1 to 3 wt%. In
these catalysts the titania is initially present with relatively high surface
area in the anatase form. The use and limitations of vanadia-based
catalysts for mobile urea-SCR systems are reviewed in "Studies in Surface
Science and Catalysis", Granger, P. and Parvulescu, V. I., ed., Vol. 171,
Chapter 9. There are two considerations that rely on greater stability of the
vanadia-based catalyst. First, the catalysts may be used in mobile
applications in a configuration where a diesel particulate filter (DPF) is
positioned upstream of the vanadia-SCR catalyst. In this configuration, the
vanadia catalyst may be exposed to extremes in temperature associated
with the exothermic regeneration of the DPF. A second consideration is that
it is desirable for a vanadia-based catalyst to maintain its catalytic
activity
at high temperatures (e.g., > 550 C) so as to better compete with base-
metal exchanged zeolite catalysts, which show a high degree of stability
and activity at high temperatures. DT-58Tm contains 10 wt% Si02 9 wt%
W03 and 81% Ti02, and has a surface area of about 90 m2/gm. It is well
known, however, that catalysts based on titania and vanadia are not
particularly thermally stable. There are several reasons for this lack of
thermal stability. First, the titania by itself tends to sinter at elevated
temperature, with an associated loss of surface area. Second, the titania
also undergoes a transformation to the rutile crystalline form at high
temperatures, and this form is generally thought to be a less active
support than the anatase form. Third, unsupported vanadia has a melting
point of about 675 C, and so, even when it is supported on titania, at
elevated temperatures it tends to be somewhat mobile and can eventually
CA 02899929 2015-08-07
3
aggregate to form low surface area.(and less active) vanadia crystals..
[0006] For these reasons, it is imperative to improve the thermal
stability of the final catalyst, and at the same time, maintain or increase
the catalytic activity for selective catalytic reduction of nitrogen oxides
(SCR-DeN0x) from lean-burn mobile engines. Achieving both goals
simultaneously is a significant challenge, since often one can be improved
at the detriment of the other. For example, the incorporation silica and/or
rare-earths into the titania is reported to increase stability, but further
gains in both stability and activity are needed.
[0007] Amorphous silica-stabilized ultrafine anatase titania have
previously been used in catalytic applications. It is known that amorphous
silica improves the anatase phase stability and surface area retention of
ultrafine anatase titania, and hence amorphous silica is an additive in
commercial products like DT-58Tm and DT-S10Tm, and these materials can
be used commercially in selective catalytic control catalysis of diesel
emissions, particularly for DeN0x applications.
[0008] An early patent describes the use of "silicic acid" to stabilize
anatase titania for DeN0x (U.S. 4,725,572). However, a careful reading of
this patent shows that the silica source is in fact a colloidal, particulate
silica. A more recent U.S. patent (U.S. 6,956,006 B1) also describes the
use of colloidal silica to effect an anatase titania with enhanced thermal and
hydrothermal stability. A recent published U.S. patent application (U.S.
2007/0129241 Al) discusses vanadia/titania-based DeN0x catalysts with
improved stability. The silica source used therein is also a colloidal silica.
However, these colloidal silica-based titania catalysts, as noted, lack
stability and acceptable activity after extremes of high temperature. Titania
catalysts which minimize these shortcomings would be of great use and
advantage.
[0009] While the DT-58Tm support material mentioned above is a
state-of-the-art support material for diesel emission catalysts, an improved
titanium support would, in general, be (1) more thermally stable, thus
enabling its placement in closer proximity to the engine, and (2) more
catalytically active, thus enabling use of a smaller canister (say 10L vs.
12L)
for containing the catalyst, thus optimizing (reducing) the, size of the
emission control system.
CA 02899929 2015-08-07
4
[0010] It is to the production of such improved silica supported-titania
substrates, and catalysts made therefrom, that the present invention is
directed.
SUMMARY OF THE DISCLOSURE
[0011] The present disclosure describes compositions and processes
for the production of stable ultrafine anatase titania for use, for example,
as a support material for vanadia catalysts preferably for use in a catalytic
emission control system. The stabilization. involves treatment of the titania
with a soluble, low molecular weight form and/or small nanoparticle form
(< 5 nm) of silica such as, in a preferred embodiment, a
tetra(alkyl)ammonium silicate, such as tetramethylammonium silicate, or
silicic acid, which serves to efficiently maintain the anatase phase and
prevent sintering (crystal growth) under severe thermal and hydrothermal
conditions, even in the presence of vanadia. The novel silica-stabilized
titanias combined with vanadia have equal or improved catalytic activity for
selective catalytic reduction of NOx compared to currently-available silica-
titania based vanadia catalysts.
[0012] In one of its aspects, the invention is a catalyst support
material which comprises anatase titania particles comprising ?.. 85 wt% dry
weight of TiO2 and 5. 10 wt% dry weight of Si02, wherein the Si02 is
substantially in a low molecular weight and/or small nanoparticle form. The
catalyst support material may further comprise, for example, 3% to 10%
W03 and may have a BET surface area of at least 80 m2/gm. The catalyst
support material may comprise ?..85 /0 dry weight of Ti02, 3% - 9% of Si02,
and 3% - 9% dry weight of 'W03 ,for example. The Si02 may be present at
a fractional monolayer value of less than 1.0 before the catalyst support
material is sintered. The small nanoparticle form of the Si02 may comprise a
diameter of <5nm. The low molecular weight form of the Si02 may comprise
a MW of < 100,000. The Si02 may comprise silicon atoms which are
substantially (e.g., >50%) in the Q3, Q2, Q1 and Q coordination
environments. The Si02,may comprise patches which are substantially 55
nm deep after redistribution as seen by scanning electron microscopy or by
transmission electron microscopy. The TiO2 used may optionally not be
prepared in the presence of urea.
[0013] In another aspect, the invention may be a vanadia catalyst
CA 02899929 2015-08-07
comprising a silica-stabilized titania catalyst support material as described
herein which comprises V205 disposed thereon. The vanadia catalyst may
comprise, for example, 0.5% to 5% dry weight of V205 (or more preferably
1.0 to 3%). The V205 may be present at a fractional monolayer value of
less than 1.0 before sintering. The vanadia catalyst may be sintered at a
650 C for example. In another aspect, the invention may be a diesel
emission catalytic device comprising the vanadia catalyst as described
herein. In another aspect the invention may be a diesel emission control
system which comprises the diesel emission catalytic device described
above and a diesel particulate filter, and wherein the diesel emission
catalytic device is positioned upstream of or downstream of the diesel
particulate filter.
[0014] In another one of its aspects, the invention is a method of
catalyzing the conversion of nitrogen oxides to N2 gas, comprising exposing
engine emissions comprising NOx to the vanadia catalyst as described
herein with an added reductant to produce N2 and H20. The reductant may
be for example NH3 and/or urea. In the method the vanadia catalyst may
comprise 0.5%-5% (or more preferably 1.0% to 3%) dry weight of V205, for
example. The engine emissions may be passed through a diesel particulate
filter before or after being exposed to the vanadia catalyst.
[0015] In another of its aspects, the invention is a method of
producing a catalyst support material, comprising providing a slurry
comprising T102, combining the TiO2 slurry with (1) a silica precursor
solution comprising Si02 substantially in a low molecular weight form
and/or Si02 comprising small nanoparticles and with (2) W03 to form a
Ti02-W03-Si02 mixture, wherein the silica precursor solution is combined
with the TiO2 slurry before, after, or while the W03 is combined with the
TiO2 slurry, and then washing and sintering the T102-W03.Si02 mixture to
form a silica-stabilized titania support material. In the method the silica-
stabilized titania support material may comprise, for example, 86%-94%
dry weight of Ti02, 3%-9% dry weight of a Si02, and 3%-7% dry weight of
W03, and the titania support material may primarily comprise a surface
area of at least 80 m2/gm before sintering. The TiO2 of the slurry may
comprise, for example, preformed titanium hydroxide, titanium oxy-
hydroxide or titanium dioxide particles. Optionally, the TiO2 of the slurry is
CA 02899929 2015-08-07
6
not produced in the presence of urea. The small nanoparticle form of the
S102 of the silica precursor solution may substantially comprise a diameter
of < 5 nm. The low molecular weight form of the Si02 of the silica precursor
solution may substantially comprise a MW of < 100,000. The Si02 of the
silica precursor solution may comprise silicon atoms which are substantially
(e.g. >50%) in the Q3, Q2, Q1 and Q coordination environments. The silica
precursor solution may comprise a tetra(alkyl)ammonium silicate solution or
silicic acid. The Si02 may substantially comprise patches which are 55 nm in
depth after redistribution as seen by scanning electron microscopy or by
transmission electron microscopy. The method may further comprise
combining the Ti02-W03-S102 mixture with V205 to form a vanadia catalyst.
The vanadia catalyst thus formed may comprise, for example, 0.5% to 3%
to 5% dry weight of V205. The y205 thereof may be present at a fractional
monolayer value of less than 1.0 before sintering. The vanadia catalyst may
be sintered at ?..650 C for example.
[0016] In
another aspect, the invention contemplates a method of
producing a silica-stabilized titania catalyst support material by providing
,a
TiO2 slurry comprising TiO2 particles, providing a particulate silica source,
combining the TiO2 slurry with the particulate silica source to form a Ti02-
Si02 mixture, and adjusting the Ti02-Si02 mixture to a pH <8.5 and a
temperature <80 C wherein the particulate silica source is dissolved and
reprecipitated on the TiO2 particles to form the silica-stabilized titania
catalyst support material. The method may further comprise the step of
combining the silica-stabilized titania catalyst support material with W03 to-
form a silica-stabilized titania tungsten catalyst support material. The
method may further comprise washing and sintering the silica-stabilized
titania tungsten catalyst support material. The silica-stabilized titania
tungsten catalyst support material may comprise, for example, 86%-94%
dry weight of Ti02, 3%-9% dry weight of a Si02, and 3%-7% dry weight of
W03, and the titania support material may primarily comprise a surface
area of at least 80 m2/gm before sintering. The TiO2 particles of the TiO2
slurry may comprise, for example, preformed titanium hydroxide, titanium
oxy-hydroxide or titanium dioxide particles. The TiO2 particles of the TiO2
slurry optionally are not produced in the presence of urea. The Si02 of the
Ti02-Si02 mixture, after dissolving, may comprise silicon atoms which are
CA 02899929 2015-08-07
7
substantially (e.g., > 50%) in the Q3, Q2, Q1 and Q coordination
environments. The Si02 on the TiO2 particles of the method may
substantially comprise patches which are 55 nm in depth after redistribution
of the S102 as seen by scanning electron microscopy or by transmission
electron microscopy. The method may further comprise combining the Ti02-
W03-Si02 mixture with V20,5 to form a vanadia catalyst. In the method, the
vanadia catalyst may comprise, for example, 0.5%- 3% dry weight of V205.
The V205 of the vanadia catalyst may be present at a fractional monolayer
value of less than 1.0 before sintering, and the vanadia catalyst may be
sintered at 2650 C.
[0017] Other aspects of the invention will become evident upon
consideration of the description below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Figure 1 is a graph showing the effect of calcination
temperature on surface area of vanadia catalyst.
[0019] Figure 2 is a graph showing the effect of calcination
temperature on the percentage of anatase phase of titania in vanadia
catalysts.
[0020] Figure 3 is a graph showing the effect of calcination
temperature on DeNox activity of 1% vanadia catalysts.
[0021] Figure 4 is a graph showing the= effect of calcination
temperature on DeN0x conversion of 3% vanadia catalysts.
[0022] Figure 5 is a graph showing the effect of temperature on DeN0x
activity of various vanadia catalysts.
[0023] Figure 6 is a graph showing the effect of temperature on
surface area of a catalyst support of the present invention versus a
conventional catalyst support.
[0024] Figure 7 is a transmission electron micrograph (TEM) of a silica-
tungsta-titania catalyst of Example 6 showing two-dimensional patches of
silica <2 nm in depth on the titania surface.
[0025] Figure 8 is a scanning electron micrograph of a vanadia-titania
catalyst of Example 10 showing ¨20 nm colloidal silica particles disposed
thereon.
[0026] Figure 9 is a transmission electron micrograph (TEM) of a
catalyst showing ¨20 nm colloidal silica particles on the outer surface of the
CA 02899929 2015-08-07
8
vanadia-titania catalyst of Example 10.
[0027] Figure 10 is
a transmission electron micrograph (TEM) of the
catalytic particles of Example 11, showing anatase crystals with a patchy,
two-dimensional silica layer on the outer crystal surface. In this image, no
silica particles are observable.
[0028] Figure 11 is
a transmission electron micrograph (TEM) of the
catalytic particles of Example 11, showing anatase crystals with a patchy,
two-dimensional silica layer on the outer crystal surface. In this image, one
remnant silica particle that is <5 nm in diameter can be seen.
[0029] Figure 12 is
a transmission electron micrograph (TEM) of the
catalytic particles of Example 12, showing small, two-dimensional patches of
silica on the anatase crystallites.
[0030] Figure 13 is
a transmission electron micrograph of silica patches
present on the surface of the catalytic particles of anatase titania of
Example 13.
[0031] Figure 14 is
a transmission electron micrograph of silica patches
present on the surface of the catalytic particles of anatase titania of
Example 13.
[0032] Figure 15 is
a transmission electron micrograph (TEM) of the
catalyst support before vanadia addition and sintering. The image shows
the lattice fringes associated with anatase titania; the silica is present as
1-
3 nm patches on the titania surface (Example 14).
[0033] Figure 16 is
a transmission electron micrograph (TEM) of the
vanadia catalyst that shows large (>20 nm) three dimensional silica nodules
(see arrows) that are not well dispersed on the titania surface (e.g.,
Example 15).
[0034] Figure
17 is another transmission electron micrograph (TEM) of
the vanadia catalyst that shows large (>20 nm) three dimensional silica
nodules (see arrows) that are not well dispersed on the titania surface (e.g.,
Example 15).
[0035] Figure 18 is a graph showing the effect of various calcination
(activation) temperatures on the deN0x catalytic activities of forms of
titania-supported vanadium catalysts.
DETAILED DESCRIPTION OF THE INVENTION
[0036] A primary goal of the present invention is production of a
CA 02899929 2015-08-07
9
stable, high surface area titania support material in anatase crystal form,
primarily to be used as a support for vanadia (V205) in diesel emission
control catalyst applications. The stabilization involves treatment of the
titania with silica in a low molecular weight form and/or small nanoparticle
form, such as a soluble precursor tetra(alkyl)ammonium silicate (i.e.,
tetramethylammonium silicate) or tetraethylorthosilicate (TEOS). Other
examples of low molecular weight silica precursors which may be used in
the present invention include, but are not limited to aqueous solutions of
silicon halides (i.e., anhydrous SiX4, where X= F, Cl, Br, or I), silicon
alkoxides (i.e., Si(OR)4, where R=methyl, ethyl, isopropyl, propyl, butyl,
iso-butyl, sec-butyl, tert-butyl, pentyls, hexyls, octyls, nonyls, decyls,
undecyls, and dodecyls, for example), other silicon-organic compounds such
as hexamethyldisilazane, fluoro-silicic acid salts such as ammonium
hexafluorosilicate [(NF14)2SIF6], quaternary ammonium silicate solutions
(e.g., (NR4)n, (Si02), where R=H, or alkyls such as listed above, and when
n=0.1-2, for example), aqueous sodium and potassium silicate solutions
(Na2SiO3, K2SiO3, and MS103 wherein M is Na or K in varying amounts in
ratio to Si), silicic acid (Si(OH)4) generated by ion exchange of any of the
cationic forms of silica listed herein using an acidic ion-exchange resin
(e.g., ion-exchange of the alkali-silicate solutions or quaternary
ammonium silicate solutions).
[0037] The term "low molecular weight form" of silica refers to a silica
species having a molecular weight (MW) of less than about 100,000. The
term "small nanoparticle form" refers to silica particles having diameters <
nm.
[0038] The emphasis on thermal/hydrothermal stability improvement
for vanadia-based catalysts is relatively new since this segment of the
mobile emission control market is just developing. It .was only after
extensive characterization by the inventor of traditional catalysts that it
was recognized that optimization of the vanadia-based catalysts required
that (1) overall silica level needed to be minimized, and that (2) the
soluble, low molecular weight and/or small nanoparticle form, forms of
silica were most effective towards providing the requisite stability and
activity.
[0039] The catalytic support materials of the present invention have
CA 02899929 2015-08-07
exceptional retention of the anatase phase of titania and surface area after
severe thermal and/or hydrothermal treatments, even in the presence of
vanadia. The compositions and methods of manufacture of the invention
use low molecular weight and/or small nanoparticle forms of silica to obtain
an exceptional ultrafine anatase titania phase and surface area stability,
while the finished vanadia catalyst demonstrates equal or improved
catalytic activity for vanadia-based selective catalytic reduction of NOx
after accelerated aging. Such compositions= and methods have been
previously unknown in the art.
[0040] Two key aspects of the present invention that differentiate it
over the prior art involve the nature of the amorphous silica and the
manner in which it is incorporated into the titania.
[0041] With regard to the nature of the amorphous silica, it is first
necessary to make the distinction between particulate forms of amorphous
silica, and solution- or gas-phase forms that consist of very low molecular
weight amorphous silicate monomers or clusters that are not considered to
be in particulate form or comprise very small nanoparticles. The suitable
forms of silica for the present invention are described herein and referred
to as low molecular weight and/or small (< 5 nm) nanoparticle silica. Two
references that describe the types of amorphous silica are "Ullmann's
Encyclopedia of Industrial Chemistry, Fifth ed., Vol A23, pp. 583-660,
(1993) and "The Chemistry of Silica", R. K. her, 1979. For example, one
form of particulate amorphous silica is colloidal silica or silica sot. This
type
of silica consists of suspensions of dense, discrete amorphous silica
particles that have diameters in the size range between about 5 nm and
100 nm. In this size range, the particles typically scatter visible light and
thus form turbid to opaque suspensions. These particles can also typically
be analyzed by visible light scattering methods using commonly available
commercial instruments. As will be seen from the examples below, without
further modification, colloidal silica in particulate form (>5 nm) is not a
suitable form of silica for the present invention. One reason that this form
of silica is not desirable (without subsequent modification) according to the
present invention is that most of the mass of the silica in the particle is in
the interior, and is not available at the surface to interact with the
substrate titania. Thus, according to Iler (op cit., p. 8), an amorphous
CA 02899929 2015-08-07
11
silica particle with a diameter of 5 nm has 1500 silicate atoms, and 37% of
these silicate atoms are on the particle surface, whereas a particle of 1 nm
has almost all the silicate atoms on the surface. Thus, for the purposes of
the present invention, it is desirable to use silica sources which
substantially comprise particles which have diameters <5 nm and/or which
have low molecular weights for example wherein the MW < 100,000, and
hence are available for interaction with the titania. An exception, as will be
described below, involves subsequent modification of particulate silica
using conditions of pH and temperature wherein the particle silica has
been dissolved and re-precipitated onto the titania surface.
[0042] Where used herein the term "substantially" is intended to
mean that more than 50% of the process or material in question has the
particular characteristic or condition to which is being referred.
[0043] For example, as noted above, in the present mention the
catalyst support material, in a preferred embodiment comprises silica
which is substantially in a low molecular weight form and/or a small
nanoparticle form. By this is meant that over 50% of the silica is either in
the low molecular weight form (MW < 100,000) or in a small nanoparticle
from (diameter < 5 nm), or is a combination of both. In a more preferred
version the silica comprises .> 60% low molecular weight forms and/or
small nanoparticle forms. In a still more preferred version the silica
comprises > 70% low molecular weight and/or small nanoparticle forms.
In a still more preferred version the silica comprises > 80%, and yet still
more preferred, > 90%, low molecular weight forms and/or small
nanoparticle forms of silica.
[0044] Furthermore, the low molecular weight and small nanoparticle
forms of the present invention preferably have geometric surface areas of
> 450 m2/g.
[0045] Particulate forms of silica (i.e., wherein diameter is > 5 nm)
include silica gel, precipitated silica and fumed silica. While the primary
particles of dense, amorphous silica in these particulate forms can be very
small (e.g., 2.5 nm), the primary particles are irreversibly agglomerated
together to form much larger secondary particles that can range in size
from hundreds of nanometers to many microns in diameter. These
secondary particles obviously do not have a large portion of the silicate
CA 02899929 2015-08-07
12
atoms near the surface and available for interaction with the titania. Of
course, these secondary particles are easily analyzed using the visible light
scattering methods, and when maintained in suspension, the particles are
quite opaque. Particulate silica in any of these forms, without subsequent
modification, is also not suitable for the present invention.
[0046] One class of silica precursor that is suitable for the present
invention is the highly alkaline solutions, referred to as the water soluble
silicates. These are described in Iler (op cit., Chapter 2). These solutions
are typically transparent since the silica particles, if present, are
generally
too small to scatter visible light. However, depending on the silica
concentration and alkalinity, small particles of silica can form in these
solutions. Iler (op cit., p.133) estimates that for a Si02: Na20 molar ratio
of
3.1, the average number of Si02 units per particle in dilute solutions is
about 900, which is less than the 1500 silicate units in the 5 nm particle
described above. Such a silicate precursor, even though it may contain
some nanoparticles above about 5 nm, is suitable for the present invention
since most of the mass of the silica is in the form of smaller, low molecular
weight species. The alkali silicates are not the most preferable form for the
present invention, however, because residual alkali ions such as Na are
extremely effective catalyst poisons for vanadia-based SCR catalysts.
[0047] Recently, the nature of amorphous silica nanoparticles in
alkaline solutions has been examined in more detail by Fedeyko, et. al.,
("Langmuir", 2005, 21, 5197-5206). These authors used a variety of
techniques, including small-angle x-ray scattering (SAXS) and small-angle
neutron scattering (SANS). These methods are able to detect the presence
of nanoparticles down to about 2 to 3 nm in size. The authors showed that
in dilute solution, when [OH]/[Si02] is less than about 1, the silica forms
small nanoparticles, while for [OH]/[Si02] greater than 1, the silica is
present as monomers and oligomers that are too small to be detected in
the scattering experiments. It is the latter type of amorphous silica species,
mostly too small to be easily detected by visible light and x-ray scattering
methods, that are referred to as low molecular weight and/or small
nanoparticle amorphous silica in the present invention, and these are the
preferred forms of silica for the present invention.
[0048] One useful meant of characterizing the silica monomers and
CA 02899929 2015-08-07
13
oligomers in solution is 23Si nuclear magnetic resonance (see, for example,
Chapter 3 in the book "High Resolution Solid-State NMR of Silicates and
Zeolites" by G. Engelhardt and D. Michel, 1987). The method can provide
information on the usual tetrahedral coordination environment around Si,
and in particular, whether or not the Si contains one or more Si next
nearest neighbors (linked by bridging oxygens). Notation that is commonly
used to describe this coordination is as follows: Q refers to a central Si
with no next nearest Si neighbors, i.e., Si(OH)4; Q1 refers to a central Si
with one next nearest Si neighbors, i.e., Si(OH)3(0Sp1; Q2 refers to a
central Si with two next nearest Si neighbors, i.e., Si(OH)2(0502; Q3 refers
to a central Si with three next nearest Si neighbors, i.e., Si(OH)1(0S03; and
Q4 refers to a central Si with four next nearest Si neighbors, i.e., Si(OSi)4.
[0049]
Without being bound by theory, it is believed that in order to
be used directly (without subsequent treatment to change the form of
silica) it is desired to use silicate solutions that consist predominantly of
Q
to Q3 oligomers. On the other hand solutions of silicate oligomers that
consist almost entirely of Q4 species are not desired for the present
invention. Conceptually, it is reasoned that for the latter types of silicate
oligomers, much of the silica is completely surrounded by other silicate
species and hence is unavailable for reaction with the titania surface, where
it is most needed to stabilize the anatase.
[0050] One
form of silica that is suitable for use in the present
invention is the commercially available alkaline
solution
tetramethylammonium silicate. Insight can be gained into the nature of this
solution based on earlier research. Engelhardt and Michel (op cit. p. 92)
describe the 295i nuclear magnetic resonance study of a 1M solution of Si02
(approximately 6 wt%) with TMA/Si = 1.0, which is roughly equivalent to a
TMAOH concentration of 9 wt%. In this solution, the silica is primarily in
the form of a cubic octamer ,that contains 8 silicon atoms, and these have
=
Q3 coordination. This small species represents about 90% of the mass of
the silica. The actual TMA-silicate solution used in the examples of the
present invention has a somewhat higher concentration of silica (9 wt%)
and lower concentration of TMAOH (7 wt%) and so the distribution of
silicate species is somewhat different than the above literature report as
shown in Table 6, below.
CA 02899929 2015-08-07
14
[0051] Another form of silica that is suitable for the present invention
is "silicic acid". This type of silica is described in her (op cit., Chapter
3). A
more detailed characterization of silicic acid is accomplished using the 29S1
nuclear magnetic resonance characterization, as described in G. Engelhardt
and D. Michel (op cit. p. 100). This form of silica can be made by
acidification of alkali silicate solutions, for example by ion-exchange using
acidic ion exchange resins.
[0052] Fractional Monolayer Concept.
[0053] It is of interest to show that the compositions and methods of
the present invention are different from prior art examples, and one means
of doing so involves the notion of fractional monolayer coverage of the
substrate surface with an added oxide. In the definitions below, the
subscript x denotes the added oxide of interest, e.g., silica.
[0054] Cõ = Surface area-basis amount of added oxide for perfect
monolayer coverage, g/m2;
SA = Surface area of mixed oxide;
Mx = Mass basis amount of added oxide for perfect monolayer
coverage; g/g mixed oxide;
= Actual loading of added .oxide on mixed oxide, g/g;
FM, = Fractional monolayer of added oxide on aged mixed oxide;
TFM =Total fractional monolayer on aged mixed oxide.
Mx = C, * SA (Eqn 1).
FM, = Lx/Mx (Eqn. 2)
TFM = Sum(FM) (Eqn. 3)
[0055] First, it was necessary to establish a best estimate for the
monolayer coverage of perfectly well-dispersed added oxides on the
substrate titania or similar oxides, Cx. For vanadia, the literature value for
monolayer coverage of the supported oxide is 7-8 V atoms/nm2, which
corresponds to 1,100 micrograms V205 /nn2. (see I. E. Wachs, et al., 2003).
For tungsta, the literature value of 4.5 W atoms/nm2 was used (I. E.
Wachs, 2006), which corresponds to 1700 microgram W03/m2. For silica,
the literature value of 600 microgram SiO2/m2 was obtained (her, p. 36, op
cit.). Thus, as an example, a mixed oxide consisting of 10 wt% Si02 (0.10
g/g), 9 wt% W03 (0.09 g/g) and 2 wt% V20 (0.02 g/g) with the balance
TiO2, has a measured N2 BET surface area of 250 m2/g. The TFM for this
CA 02899929 2015-08-07
material is TFM = (1/250)* ((0.10/600E-6) + (0.09/1700E-6) +
(0.02/1100E-6)) = 0.95. This number indicates that were the added Si02,
W03 and V205 oxides perfectly well dispersed on the titania surface, the
surface coverage of the final, mixed oxide would be 0.95 monolayers thick
with the added oxides. With respect to silica alone, the fractional monolayer
coverage would be 0.67, or two-thirds of the surface would be covered with
an ideally dispersed silica coating. The compositions of the present
invention, when freshly prepared (i.e., after the addition of the added
oxides but before aging or sintering) typically have surface areas greater
than about 100 m2/g and a total amount of added oxides of 15 wt% or
less, and so the fractional monolayer coverage is about 1.0 or less, and the
fractional monolayer coverage specifically for silica is about 0.80 or less.
[0056] Methods of silica incorporation for the present invention.
[0057] The surface coating of titania using alkali silicates or silicic
acid
such as described above has been practiced industrially for many years in
the paints and coatings industry. See, for example, the review chapters 52
and 53 in the "Colloidal Silica, Fundamentals and Applications", Surfactant
Science Series Vol. 131, E. Bergna, W. 0. Roberts, eds. (2006). As
described in Ch. 52 of Bergna and Roberts, one approach to coating the
titania surface with silica involves exposing the substrate titania particles
to
silica under alkaline conditions with a concentration of silica that is below
the solubility limit for amorphous silica. As described in Ch. 53 of Bergna
and Roberts, another method involves exposing the substrate titania
surface to monosilicic acid at low pH at a silica concentration that is again
very low and below the solubility limit. While the methods for silica
incorporation in the above references represent suitable means of
incorporating silica according to the present invention, there are several
important differences. One difference is that for the prior art, the phase of
titania that is used as the substrate is rutile (because of its higher light
scattering power than anatase), and there is no suggestion of the ability of
silica addition via those methods for the prevention of anatase phase
conversion to rutile. A second important difference is that the substrate
titania particles of the paints and coatings prior art, regardless of whether
they are anatase or rutile, are relatively low surface area substrates, with
N2 BET surface areas of the substrate surface typically being less than
CA 02899929 2015-08-07
16
about 15 m2/g. Third, a key difference is in the surface coverage of the
added oxide such as silica. With the above definition of fractional
monolayer coverage, the compositions of the present invention, if they
were prepared on a low surface area support (15 m2/g) would have total
fractional monolayer of about 5 or greater, and the fractional monolayer
coverage specifically for silica would be about ?.3. Thus, in the prior art,
the silica coating is present over the entire titania particle, and with a
thickness that exceeds the thickness of a monolayer. Indeed, the silica
coating is present so as to completely inactivate the photo-catalytic
activity of the titania surface. Finally, in a preferred embodiment of the
present invention, the titania is coated with silica under conditions where
the added silica is well above the solubility limit of a few hundred ppm. As
will be seen below, in the present invention the silica, when it is deposited
initially, does not completely cover the titania surface, so that the
desirable catalytic functionality of the titania surface for the SCR reaction
is still available. Hence, the goal of the present invention of maximizing
the catalytic activity of the surface can then be met, while preserving the
stability of the support.
[0058] There is one more reason why particulate silicas are not the
preferred forms of silica for the present invention, including forms of
particulate silica with internal porosity and hence high pore volume. It is
well know from the literature (e.g., Wachs et al., J. Cat. 161, 211-221
- (1996)) that silica, by itself, is not a good support for vanadia SCR
catalysts, while titania and tungsta-doped titania are good supports.
Hence, for the present invention, it is desirable to minimize the amount of
silica surface area that is. available to adversely interact with vanadia,
while maximizing the amount of TiO2/W03 surface area, so as to make the
most active catalyst. Thus, only enough silica is used to stabilize the
titania, and it is used in a form (molecularly dispersed on the titania
surface) that has minimal adverse impact on the vanadia catalyst.
[0059] Finally, another approach to make the materials of the present
invention, wherein the particulate forms of silica described above can be
used, is now described. It is well known that particulate amorphous silica
is soluble to an extent that depends on solution pH and temperature, see
for example her, (op cit., p. 42). Above about pH 9 and for temperatures
CA 02899929 2015-08-07
17
higher than ambient temperature, amorphous silica will appreciably
dissolve. This dissolved silica can then be precipitated again, for example
onto a titania surface, by subsequently lowering the temperature and/or
pH to a region that has lower silica solubility. In this manner, particulate
silicas that are finely mixed with anatase titania can be dissolved and
redistributed in a well-dispersed manner onto the titania surface via the
hydrothermal treatment. However, such a post-treatment is not a
preferred method of making the compositions of the present invention,
since this step adds processing time and cost during the mixed oxide
manufacture. It is most preferred to use a suitable silica precursor and
treat the titania directly.
[0060] EXAMPLES
[0061] Titania Starting Material
[0062] In one embodiment of the present invention, a sulfated titania
slurry (see Table 1) was used. Such a sulfated titania slurry can be
obtained as an intermediate product in a production process for making
titanium dioxide using the sulfuric acid process, for example as produced
by the MIC production facility in Thann, France. Such a slurry comprises
about 27% of a high surface area, hydrous anatase titania, Ti02. The TiO2
has primary crystallite particles having sizes of less than 5 nanometers,
and corresponding N2 BET surface areas in excess of 250 m2/g. The slurry
has a viscosity of 0.5-3 poises, a density of 1275 kg/m3, and a low pH of
around 1.5 - 2.0, which results from the fact that the slurry contains about
6.6 wt% S03. However, the present invention is not restricted to use of
this slurry. Any composition comprising hydrous anatase titania could be
used herein. Indeed, it is not necessary to use a sulfated titania slurry as
the starting material. A dried low-sulfate anatase titania precursor could
be used instead.
=
CA 02899929 2015-08-07
18
[0063]
Type Method Unit Specification
Residue on
Calcination TiO2 43
Drying then calcination on 1000 C % (weight) 27 1
Iron Ti02.15 X-Ray Fluorescence Mg/kg 580
SO3 G1.3 S Analyzer/Dryness 105 C Wo (weight) 6.6 1
P205 T102.16 X-Ray Flou % (weight) 50.4
Na T102.47 Atomic Absorption Wo (weight) 50.05
Ti02.5 X-Ray Fluorescence % (weight) 50.01
Pb Ti02.13 X-Ray Fluorescence % (weight) 50.01
Peak Height G1.2 Diffraction X Degree >1
Rutile Ti02.48 Diffraction X None detected
Specific Area G1.1 B.E.T. mzig a250
*Statistical values
Table 1. Composition of Sulfated Titania Slurry
[0064] Preferably however, the titania slurry used herein is produced
with titania which has not been produced in the presence of urea.
[0065] In an embodiment of the invention, the TiO2 component of the
catalyst support material used herein substantially comprises a surface
area <400 m2/g and a pore volume <0.40 crrNg.
[0066] Experimental Methods: The structure and stability of titania- based
catalysts and the changes that occur during exposure to elevated temperatures
were investigated by various means. The methods employed consist of x-ray
diffraction analysis (XRD), transmission electron microscopy (TEM), SEM
(scanning electron microscopy), high resolution solid-state nuclear magnetic
resonance spectroscopy (NMR), nitrogen porosimetry (N2 BET/B3H) and catalytic
evaluation of activity for the reaction of NO with NH3 (DeN0x)
[0067] XRD: Samples were evaluated for crystal phase composition and
crystallite size in the following manner. The samples were prepared for XRD by
pressing into spherical XRD PW1812/00 holders and then analyzed using a
Panalytical X'Pert ProTM diffractometer equipped with a sealed Cu x-ray tube
and
an X-Celerator position sens.itive detector. Instrument conditions were set at
45kV, 40mA, 0.008 20/step and 50 second dwell time. Phase identifications
are performed through search-match of the experimental patterns with both
the ICCD and ICSD databases. The Rietveld method was applied for
Quantitative Phase Analysis by X-Ray diffraction. The crystallite size was
measured on the single peaks from Scherrer formula as employed in the
Panalytical High Score software. The Scherrer's formula depends on the fact
CA 02899929 2015-08-07
19
that crystallite size is inversely related to the full width at half maximum
(FWHM) of an individual peak - the more narrow the peaks, the larger the
crystallite size. The instrument broadening value used for calculation was
from LaB6 standard (NIST profile standard material). In addition, the full
profile method such as Rietveld Analysis, found in X'Pert High-Score PlusTm
software in addition used for calculated grain size as well.
[0068] TEM: The samples were prepared for TEM analysis by dipping
wholey carbon coated Cu TEM grids directly into the provided powder. The
grids were then viewed in the TEM at magnifications ranging from 50 to
400,000X. Analysis was performed using a JEOL 2000FX II TEM operated at
200kV. During the imaging process particular attention was given to
characterizing phase size and distribution. Images were collected with a
Gatan MultiScanTm CCD camera and are in jpeg format.
[0069] SEM: The samples were prepared for SEM analysis by
dispersing the provided powder onto Al SEM stubs covered in colloidal
graphitic carbon. SEM analysis was conducted using a JEOL 7401 at 2kV
without conductive coating.
[0070] 29Si NMR Spectroscopy Characterization of Samples. 2951 Magic
angle spinning nuclear magnetic resonance spectroscopy (29SIMASNMR) is a
useful way of characterizing the coordination of silica in silica-containing
solid samples (see, for example, Engelhardt and Michel, 1987 (op cit.)) as
described above. A problem with 2951 MASNMR spectroscopy is, however,
that the 29S1 nucleus is present at low natural abundance (4.7%), and so the
method is not very sensitive. A common method for increasing the
sensitivity is the cross-polarization approach (see for example "The Colloid
Chemistry of Silica", H. Bergna, ed., ACS Series 234, p. 270 (1994)). In this
technique, spin polarization from a more abundant spin that has a large
nuclear magnetic moment (in this case, 1H) is transferred via double
resonance to a less abundant spin (29Si). This method has the effect of
dramatically increasing the sensitivity for the 29Si NMR signal when the Si
has (OH) connected to it. It is well known that in the silicates, silicon
adopts
tetrahedral coordination and is surrounded by four oxygen nearest-
neighbors, and then either H or Si next-nearest-neighbors. An isolated
silicate tetrahedron that sits on the titania surface would be expected to
have at least one H next-nearest-neighbor, Si-OH, and this proton should
CA 02899929 2015-08-07
increase the sensitivity of the method for the silicon nucleus. 29Si NMR
spectroscopy can also be performed on liquid samples that contain soluble,
low molecular weight silicates, as also described in Engelhardt and Michel
(op cit.).
[0071] Nz Porosimetry: Samples were evaluated for nitrogen
porosimetry using Micronneretics TriStarr" units. The samples were
outgassed overnight at 150 C under flowing nitrogen. They were then
cooled to room temperature for the adsorption measurement.
Adsorption/desorption curves were measured at liquid nitrogen
temperature. Surface area was determined using the BET method, and
pore volumes were measured using the BJH method on the adsorption
branch.
[0072] The vanadia was added by impregnation of either an alkaline
solution (e.g., monoethanolamine) or from acidic (e.g., oxalic acid)
solution. The impregnated materials were then aged at high temperature in
hydrothermal environment (750 C for 16 hr in 10% H20) (or at 600 C-
900 C for 6 hr in an air atmosphere) in order to cause accelerated aging.
It is desirable to have 100% anatase with high surface area (associated
with very small crystallites), and no crystalline tungsta after the age
treatments.
[0073] Examples 1-3:
Benchmarking of Commercial Materials.
[0074] In the following three examples, we sought to benchmark the
performance of several commercial prior art materials that are used in 'SCR
applications, DT-52T" (Example 1), DT-58T" (Example 2) and DT-SlOT"
(Example 3). Properties for these three materials are listed in Table 2. It
can be seen that DT-52T" contains added tungsta (but not silica), DT-S10T"
contains added silica (but not tungsta) and DT-58T" contains both added
tungsta and silica.
[0075]
Material
Property Unit ' DT-52 DT-58 DT-S10
W03 wt% 10.0 9.0 0.0
Si02 wt% 0.0 10.0 10.0
TiO2 wt% Balance (90) Balance (811 Balance (90)
Surface Area m2/g 90 110 110
Crystal Phase Anatase Anatase Anatase
Table 2. Target Properties of Commercial Materials
CA 02899929 2015-08-07
21
[0076] For each of Examples 1-3, the base materials were used as
received, and vanadia was loaded onto them in the following way. A
solution of monoethanolamine (MEA) in deionized water was prepared that
was 0.4 M (24.4 g/L MEA). To this solution was added 10.9 g/L V205, (0.06
M). In order to prepare a catalyst with a final vanadia loading of 1 wt%,
approximately 13.7 g of the above solution was mixed with 15.8 g of the
titania support (loss on ignition = 5 wt%), and the mixture was heated in a
rotary evaporator under vacuum at 75 C until dry. The resulting product as
then calcined in a static muffle furnace at a temperature of 600 C, 700 C or
800 C. Similarly, catalysts were prepared with a final vanadia loading of 3
wt% by using 41.2 g of the MEA/vanadia solution and 15.8 g titania.
[0077] The. materials prepared above were then evaluated for N2
porosity, phase composition and crystal size by XRD and for DeN0x activity
(results are given in Table 3). For the DeN0x activity, a 0.1 g sample of
each vanadia-loaded catalyst sample was pelletized and meshed to -20/+40
mesh, and was loaded into a reactor to determine the conversion of NO in
the presence of NH3. A flowing stream that contained 5% 02, 1000 ppm NH3,
1000 ppm NO, and 3% H20 was passed over the catalyst at a space velocity
of 650 1/g.cat-hr.
-
CA 02899929 2015-08-07
22
.....X if
i...1.i, L
Z 1":¨I "WSN'ASDSMV.ech:LIVPIA
0 . .-
0 A
IIca CI g la ii It Ca CD ca ca a a 0 ca CA Cb CA Ul
P T.:
. a)
4-,
tu
g :: g :: 11 : g :: g :5 :3 :2 :1 :1 :5 :5 :5 :5
al ni
El
L.
La a)
E
ul A Cs CA CA CA il il CA CD CD CD Cs cp CA CD CS CA CA ii
E
ta i 0) o
W U
. 1...
a o
I: :: g :: ; :: :::::::: :: :I :: :4 g 2 1;
il
co
'B.
741 E
g 01 il WA il le MD MA CD Ws MP t: OM 4 rv CD F el
X
x le gt tli :e t: !!
to c
4 o
in
[
.1 ca p. 4, 0, ,0 ca ca ca ca 40 r. ca CD GI CD CI co
J!I 1! fi ti Si 1i g li i! !! Si Si li ig Eg 11 IR Si _
c..
ICI
tn.
E
0
U
ida II !! !i 1 !! !! 5 !! !! 5 !I !! !! !! !! !! !! 5 !! 0
c
yam )0 ft.
le 0
Et ri
al lo
a. J SiN
1`, gCI" 2232fa;g µ
ri228 Ma 67"
Z i I
t
to
A A IN=
U fa
.c
U
=
1- In
c a)
. ci .- .- .- rl el el r. e= r- ra el 4,2 .e. 1.= ir- 47 f7 CA =13
1 r.
All
Sc al i'm
Ca
CO v
o SAVOYMISIS 00 0000
12 it 1:31.' g g g ti ti ti ti 6 g 6 61 tIggi!gg
r 1
03 m
75.
r..
o
11 ................. oe 01 Al Al 04 0.1 f7 1.7 (.2 el ra co
0
1...1 0. u A 1 _
CA 02899929 2015-08-07
23
[0079] Visual
inspection of Table 3 confirms trends reported in the
literature, and that is that higher vanadia loadings and higher temperatures
are associated with loss of surface area, conversion of anatase phase to
rutile,
crystallization of tungsta, and increase in crystal size (sintering). The
individual materials, however, respond differently from one another. As
examples, data for the 3 wt% vanadia samples are plotted in Figures 1 (BET
surface area) and Figure 2 (% anatase phase), which are indicators of the
thermal stability of the catalysts.
[0080] It can
be clearly seen from these Tables 1 and 2 and Figures 1
and 2 that DT-S10" (with silica) has the highest thermal stability, followed
by
DT-58" (with silica and tungsta), followed by DT-52' (with tungsta only). If
thermal stability were the only requirement for a good vanadia catalyst
support, then DT-S10' would be the clear choice.
[0081] Shown
in the next two figures (Figs. 3 and 4) are the DeN0x
conversions at 325 C for the materials loaded with 1% and 3% vanadia,
respectively. It can be clearly seen that the sample with silica only (DT-
510")
has the lowest conversion for most aging temperatures; it is only the sample
with 3% vanadia, aged at 800 C, that has activity equal to that of DT 58TM.
[0082] Thus,
based on the performance of these commercial samples,
there is an obvious need for develop a catalyst with both improved stability
and activity.
[0083]
Examples 4-5. The following examples reveal the performance of
two additional commercial prior art materials (MIC DT-60" and Tayca
Corporation ITAC 115GSTM) in relation to DT-58". Samples of these materials
were analyzed for composition using x-ray fluorescence analysis, with results
shown in Table 4.
CA 02899929 2015-08-07
24
[0084]
Oxide, wt% DT58 MIC DT60 Tayca
TiO2 80.5 84 84.2
W03 9.1 5.3 5.2
Si02 9.8 10.3 10.2
SiO3 0.4 0.2 0.2
Table 4. Compositions of Materials
[0085] The results show that the DT-60T" and TaycaT" samples contain
nominally about 10 wt% S102 and about 5 wt% W03. All three types of
materials were loaded with 0.9 wt% V205 using deposition from MEA solution
as in Examples 1-3. The products were then aged at 800 C for 6 hrs in air in a
static muffle furnace, and the products were analyzed using the N2 BET
method. For the DeN0x activity, (micro-reactor), a 0.1 g sample of each
vanadia-loaded catalyst sample was pelletized and meshed to -20/+40 mesh,
and was loaded into a reactor to determine the conversion of NO in the
presence of NH3. A flowing stream that contained 5% 02, 1000 ppm NH3, 1000
PPm NO, and 3% H20 was passed over the catalyst at a space velocity of 650
1/g .cat-hr.
[0086] The surface area stability of the samples is compared below in
Table 5.
[0087]
Sample Surface Area (m2/g)
DT58rm 45.9
DT60 TM 53.3
Tayca TM 51.5
Table 5. Surface Areas of Commercial Samples
[0088] The data show that the samples with lower tungsta level have
slightly greater stability than does DT-581". However, the DeN0x activities of
the catalysts, as shown in Figure 5, shows that the activity for the DT-60T"
and TaycaT" samples is lower than that for the DT-58T" sample. Thus, as for
Examples 1-3, the Examples 4 and 5 demonstrate the need for greater
stability and activity for the novel materials of the present invention.
CA 02899929 2015-08-07
[0089] Example 6: Silica Surface Stabilization
[0090] As noted elsewhere herein, the present invention is directed to
providing a very stable ultrafine titania using a minimal amount of silica
additive. As noted above, particulate silicas (e.g., colloidal, fumed, and
precipitated) are not ideal sources of silica for use in titania-supported
vanadium catalysts, because much of the silica is not available to interact
with the titania surface. A goal of the present invention was to find another
form of silica that could be used to more effectively stabilize the surface of
the titania, but that would have a minimal adverse impact on the catalytic
activity of vanadia supported on the surface of the titania.
[0091] The use of silica in low molecular weight and/or small
nanoparticle form was considered, rather than in the particulate form which is
present in the conventional amorphous silicas described above. The lowest
molecular weight form of silica in aqueous solution is silicic acid, Si(OH)4.
However, this chemical entity has a very low solubility in water, and
therefore
is restricted to concentrations of a few hundred ppm. (Discussions of the
aqueous chemistry of silica in water, are found for example, in Iler, (op
cit.)
and Brinker, C. J and Scherer, G. W, 1990, Chapter 3).
[0092] In view of the low solubility of Si(OH)4, we turned to
experimentation with tetra(alkyl)ammonium silicate solutions (including, but
not limited to, tetramethylammonium, TMA). These reagents contain silica in
low molecular weight forms (see Engelhardt and Michel, op cit.).
Furthermore, the silica in these solutions is present at fairly high
concentrations (e.g., 9 wt% Si02). Thus, we considered whether or not the
molecular entities in these solutions may be small enough to react selectively
with the titania surface, while not providing separate area for vanadia to
bond which would diminish its catalytic activity.
[0093] "Si NMR spectroscopy of a liquid sample of soluble silicate.
[0094] In order to determine the nature of the silicate species in
commercially available TMA silicate solution, the commercial source of TMA
silicate used in these examples (Alfa TMA-silicate, 9% Si02) was evaluated
using "Si NMR spectroscopy on a 400 MHz instrument by Spectral Data
Services, Inc. Shown in Table 6 are the results.
CA 02899929 2015-08-07
26
[0095]
Description Q2 Q3 Q4
Alfa TMA Silicate (Liquid) 4 17 39 23 16
Example 6 (Solid) 0 4 16 50 30
Example 14 (Solid) 11 5 34 44 6
Table 6. Q Forms of TMA Silicate Solutions and Titania Solids
[0096] It can be seen that the TMA silicate solution contains mostly silica
species with connectivity of Q3 or lower. However, there is some silica with
Q4
connectivity, so the solution does not contain all of the silica in the ideal
form
(connectivity of 1Q3 or lower). However, as will be shown herein, we have made
the novel discovery that the soluble silica sources such as
tetra(alkyl)ammonium silicates can be used to make exceptionally stable
(anatase phase, high surface area) vanadia-based catalysts that exhibit
excellent catalytic activity for selective catalytic reduction of NOx
reactions.
[0097] Stability Improvement
via Novel Silica Treatment.
[0098] The Examples 1-5 above reveal the stability and catalytic
performance of commercial materials loaded with vanadia and subjected to
accelerated aging conditions. Example 6 demonstrates the improvement in
thermal stability that can be attained using the novel silica treatment method
of the present invention. A slurry of production-made sulfated titania
hydrogel
was diluted to a TiO2 content of 21.6 wt%. 112.5 g of this slurry was added
to a round-bottom flask that was equipped with an overhead stirrer. This
slurry was heated to a temperature of 60 C via a temperature controlled
heating mantle, and was maintained at that temperature throughout the
preparation. To this slurry was added 33.3 g of tetramethylammonium silicate
(TMA-Si02, Alfa-Aesar, 9% Si02, TMA/S102 = 0.5). This mixture was allowed
to react for 20 min. The pH was then adjusted up to 6.0 via the addition of
concentrated NH4OH (29%).. 3.07 g of ammonium paratungstate (APT) was
then added, and the final pH was adjusted to 6.5 with addition of more
concentrated NH4OH. This mixture was allowed to react an additional 30 min,
and was then filtered, rinsed with DI water, and dried. The final nominal
CA 02899929 2015-08-07
27
composition of this product on an oxide basis was 81 wt% Ti02, 10 wt% S102
and 9 wt% W03. It was then divided into portions which were calcined over
the temperature range of 600 C to 900 C for 6 hr in air using a static muffle
furnace. A sample of production DT-58'm of the same composition was also
aged under identical conditions. Both samples were evaluated for retention of
surface area using the BET method, with data shown in Figure 6.
[0099] The data in Figure 6 clearly show that while the compositions of
the two products are nominally the same, the sample prepared using the
novel low molecular weight and/or small nanoparticle form of silica of the
present invention is much more= thermally stable (retains surface area to a
greater extent) than the prior art material (DT-58Tm).
[0100] 29Si MASNMR Spectroscopy Characterization of Solid Samples. The
following analysis of the TMA-Si02 material of the invention and the
conventional material (DT-58'm) shows that the silica is present in a very
different morphology for the materials of the present invention. The two
samples, in their "fresh" state (before addition of vanadia but after
calcination
at 500 C) were analyzed in detail by Spectral Data Services, Inc., using 29Si
MASNMR spectroscopy on a 270 MHz instrument. An attempt was made to run
the DT-58'm sample with cross-polarization, but no signal was observed after
1 hr, and under these conditions, a large signal would have been seen if there
were any (OH) groups near the Si nucleus as would be expected for well
dispersed silica on the surface of the titania. Hence, this sample was run for
4
hrs using just the MASNMR method. A weak signal was observed at -111 ppm
relative to tetramethylsilane. This signal is consistent with Si = in a Q4
environment, or Si(OSO4. Hence, both observations (the lack of a cross-
polarization signal and the presence of the Q4 signal) in the NMR experiment
are consistent with particulate silica where most of the Si is in the interior
of
the silica particle, and there are not many Si(OH) groups on the surface of
the
silica.
[0101] The sample of novel catalyst support was run under nominally the
same NMR conditions, and this sample also contained 10 wt% S102, however,
the source of the silica was TMA-Si02, which contains low molecular weight
and/or small nanoparticle forms of silica In this case, a strong signal was
CA 02899929 2015-08-07
28
observed in the 1H-29Si cross-polarization experiment, which demonstrates
that there are hydroxyl groups attached to the Si. This supports the idea that
the silica that is well dispersed on the titania surface in the present
invention.
Furthermore, the spectrum was deconvoluted into four peaks, with the
following positions and relative intensities -110 ppm, 30%; -100 ppm, 50%; -
90 ppm, 16%; and -82 ppm (4%), and these peaks are assigned the
coordinations Q4, Q3, Q2 and
respectively, as shown in Table 6. It can be
seen, then that about 70% of the silica is in a coordination environment (Q3,
Q2 and Q1) such that hydroxyl groups are the next-nearest-neighbors, and this
further supports the notion that the silica is well dispersed on the titania
surface. Thus, we conclude that the use Of the low molecular weight and/or
small nanoparticle silica precursor, such as TMA-Si02 or other compositions
described herein gives rise to silica that is well dispersed on the titania
surface. In particular, a preferred coordination environment of the silicon
atoms of the titania support of the present invention is substantially (at
least
50%) Q3, Q2, Q1 and Q coordinations, as determined by 29Si CP-MASNMR.
One key manifestation of this difference in the nature of the silica is that
the
well-dispersed silica is much more effective, on a mass basis, towards
stabilizing the titania. Therefore, less silica is necessary to stabilize the
titania
when the silica is well-dispersed on the titania surface.
[0102] In
order to further evaluate the nature of the silica coating on the
novel sample, it was subjected to TEM analysis, as shown in Figure 7 which
reveals
that the silica is present as isolated patches on the anatase titania surface.
The
patches exhibit two-dimensional character in that the length is typically less
than 5
nm while the depth (distance away from the titania surface) is typically less
than 2
nm.
[0103]
Example 7: Stability and Activity Advantage of 90:4:6 Ti02:Si0,:W01,.
The following example of the invention demonstrates the stability and activity
benefit for materials produced according to the methods of the present
invention. A
slurry of production-made sulfated titania hydrogel was diluted to a TiO2
content of
21.6 wt%. 208.3 g of this slurry was added to a round-bottom flask that was
equipped with an overhead stirrer. This slurry was heated to a temperature of
60 C
via a temperature controlled heating mantle, and was maintained at that
CA 02899929 2015-08-07
29
temperature throughout the preparation. (In an embodiment of the invention the
titania slurry and silica component used herein are mixed at a temperature <80
C
and at a pH <8.5. Alternatively, the titania slurry and silica component used
herein
may be mixed at a temperature <70 C and at a pH <7Ø) 3.4 g of ammonium
paratungstate (APT, 88% W03) was then added and allowed to react for 30 min.
To
this mixture was added 22.2 g of the tetramethylammonium silicate of Example 6
and the mixture was allowed to react for 10 min. The pH was then adjusted up
to
6.5 via the addition of concentrated NH4OH (29%). This mixture was allowed to
react an additional 20 min, and was then filtered, rinsed with DI water, and
dried at
105 C and then calcined at 500 C for 6 hr. The final nominal composition of
this
product on an oxide basis was 90 wt% Ti02, 4 wt% Si02 and 6 wt% W03. To this
powder was deposited vanadia from. an MEA solution, as in Examples 1-3, above,
so
that the final loading was 2 wt% V205 on a total oxide basis. A portion of the
dried
powder was then heated to 750 C and held at that temperature for 16 hr in an
air
atmosphere that contained 10 wt% H20.
[0104]
Example 8: Stability and Activity Advantage of 90:5:5
Ti07:S107:W03. This example further demonstrates the stability and activity
benefit for materials according to the present invention. A
slurry of
production-made sulfated titania hydrogel was diluted to a TiO2 content of
21.6
wt%. 208.3 g of this slurry was added to a round-bottom flask that was
equipped with an overhead stirrer. This slurry was heated to a temperature of
60 C via a temperature controlled heating mantle, and was maintained at that
temperature throughout the preparation. 2.8 g of ammonium paratungstate
(APT, 88% W03) was then added and allowed to react for 30 min. To this
mixture was added 27.8 g of tetramethylammonium silicate (TMA-Si02, 9 /o
Si021) and the mixture was allowed to react for 10 min. The pH was then
adjusted up to 6.5 via the addition of concentrated NH4OH (29%). This mixture
was allowed to react an additional 20 min, and was then filtered, rinsed with
DI water, and dried at 105 C and then calcined at 500 C for 6 hr. The final
nominal composition of this product on an oxide basis was 90 wt% Ti02, 5 wt%
S102 and 5 wt% W03. To this powder was deposited vanadia from MEA solution
as in Examples 1-3, above, so that the final loading was 2 wt% V205 on a total
oxide basis. A portion of the dried powder was then heated to 750 C and held
CA 02899929 2015-08-07
at that temperature for 16 hr in an air atmosphere that contained 10 wt% H20.
[0105] In order to form a basis set for comparison, 4 different samples of
DT-58" were loaded with 2 wt% vanadia as above, and hydrothermally aged
under the sample conditions. Results from these four samples were then
averaged.
[0106] The materials from Examples 7 and 8, along with the DT-58'm
reference materials, were analyzed by XRD, N2 Porosimetry and DeN0x
activity, with results shown in Table 7, below. In order to evaluate the
materials for DeN0x applications, a 0.1 g sample of each vanadia-loaded and
aged catalyst sample was pelletized and meshed to 20/+40 mesh, and was
loaded into a reactor to determine the conversion of NO in the presence of
NH3. A flowing stream that contained 5% 02, 500 PPm NH3, 500 ppm NO, and
10% H20 was passed over the catalyst at a space velocity of 650 l/g.cat-hr.
For each of the Example 7 and 8 materials, two DeN0x runs were performed.
A total of 10 runs were obtained on the four DT-58' reference materials.
Results are reported in two ways. First, the conversion of NO is reported. A
second method involves calculation of the "rate" of reaction. As persons of
ordinary skill are aware, the SCR reaction is generally thought to be first
order
with respect to NO and zero order with respect to NH3, and under these
conditions, the reaction rate is, proportional to -In(1-x), where x is the
fractional conversion (%conversion/100). Reaction rate is a better method of
comparing samples at high conversions. Basic statistics were computed from
the data, and analysis of variance demonstrated that the materials of the
present invention gave significantly different ("P" value for the null
hypothesis
<0.05) and higher activity than the reference samples.
CA 02899929 2015-08-07
31
[0107]
DT-58*
Example 7 Example 8
Anatase 95.4 100.0 100.0
XRD-Phase %Rutile 2.3 0.0 0.0
%W03 2.3 0.0 0.0
Anatase 391 233 333
XRD-Crystal Size (A) Rutile 89 0 0
WO3_ 498 0 0
BET Surface Area (m2/g) 34.9 , 47.5 30.6
N2 PSD BJH Pore Volume (cm3/g) 0.25 0.29 0.24
250 C 19.4 26.7 31.5
NO Conversion, A) 350 C 64.0 72.6 78.8
450 C , 73.1 80.2 82.4
250 C 0.22 0.31 0.38
NO Rate, Wo 350 C 1.04 1.30 1.57
450 C 1.34 1.62 1.75
*Average of 4 samples
Table 7. Characterization of Samples
by XRD N2 Porosimetry and DeN0x Activity
[0108] Table 7 clearly shows that the samples made according to the
present invention (Examples 7 and 8) retain a greater portion of the anatase
phase, resist crystallization of tungsta, and resist crystal growth (i.e.,
demonstrate less sintering) than the base materials. Furthermore, the
materials of the present invention retain higher surface area and pore volume
than do the reference materials. Finally, the materials made according to the
present invention exhibit higher catalytic activity for the SCR reaction.
[0109] In the following two examples (9 and 10), the dramatic difference
in stability and activity between catalysts made with a particulate
(colloidal)
silica versus the inventive materials is demonstrated.
[0110] Example 9. A novel material of the present invention was prepared
in the following way: A slurry of production-made sulfated titania hydrogel
(comprising 27% Ti02, 7% sulfate, and H20) was diluted with water to give a
21.7 wt% TiO2 dispersion. 207.7, g of this dispersion was heated with stirring
over 20 minutes to 60 C, and 2.3 g ammonium paratungstate (APT-88% W03)
was then added at low pH. The APT was allowed to react for 20 min. 44.4 g of
the soluble, low molecular weight form of silica tetramethylammonium (TMA)
silicate (Alfa Aesar- 9 wt% Si02) was then added and allowed to react for
CA 02899929 2015-08-07
32
another 20 min. The pH was then adjusted to about 6.5 by the addition of
concentrated NH4OH (this step could be performed prior to addition of W03)=
The slurry was then filtered, washed free of ammonium sulfate and then dried
and calcined at 500 C for 6 hr in air. The nominal composition of this base
material was 8 wt% Si02, 4 wt% W03 and 88% TiO2 (1-102: S102: W03 =
88:8:4).
[0111] Example 10. A comparison sample was made using conventional
particulate colloidal silica in the following way: A slurry of production-made
sulfated titania hydrogel (27 A) Ti02) was diluted with water to give a 21.6
wt% TiO2 dispersion. 203.7 g of this dispersion was heated with stirring to
60 C, and 2.3 g ammonium paratungstate (APT-88% W03) was then added.
The APT was allowed to react for 20 min. 13.3 g of the particulate, colloidal
silica AS-30 (W. R. Grace- 30 wt% S102) was then added and allowed to react
for another 20 min. As will be recognized by a person of ordinary skill in the
art, this form of colloidal silica is stabilized with NH4+ ion rather than Na
+ ion,
since the latter is a catalyst poison for the SCR reaction. The pH of the
mixture
was then adjusted to 6.5 by the addition of concentrated NH4OH. The slurry
was then filtered, washed and dried and calcined at 500 C for 6 hr in air. The
nominal composition of this base material is 8 wt% Si02, 4 wt% W03 and the
88% Ti02. Thus, the materials of both Example 9 and Example 10 have
nominally the same overall composition (88:8:4-Ti02:Si02:W03) on an oxide
basis.
[0112] To these two base materials comprising titania, silica, and
tungsten was added vanadia to a target of 2 wt% V205. The vanadia was added
by impregnation of an alkaline MEA solution. The impregnated materials were
then aged at high temperature in hydrothermal environment (750 C for 16 hr
in 10% H20) in order to cause accelerated aging. The aged samples were
evaluated by x-ray diffraction analysis, and the observed diffraction patterns
were analyzed by Rietveld analysis. In order to evaluate the Example 9 and
Example 10 materials for DeN0x applications, a 0.1 g sample of each vanadia-
loaded and aged catalyst sample was pelletized and meshed to -20/+40 mesh,
and was loaded into a reactor to determine the conversion of NO in the
presence of NH3. A flowing stream that contained 5% 02, 500 ppm NH3, 500
CA 02899929 2015-08-07
33
ppm NO, and 10% H20 was passed over the catalyst at a space velocity of 650
lig.cat-hr. NO conversion and rate data are reported as described above.
[0113]
DT-58*
Example 9 Example 10
Anatase 95.4 100.0 91.2
XRD-Phase /oRutile 2.3 0.0 6.1
0/0W03 2.3 0.0 2.7
Anatase 391 200 1863
XRD-Crystal Size (A) Rutile 89 0 NM
W03 498 0 268
BET Surface Area (m2/g) 34.9 54.2 8.1
N2 PSD B.IH Pore Volume (cm3/g) 0.25 0.26 0.04
250 C 19.4 20.1 8.4
NO Conversion, % 350 C 64.0 64.6 34.0
450 C 73.1 70.8 33.2
250 C 0.22 0.22 0.09
NO Rate, % 350 C 1.04 1.04 0.42
450 C 1.34 1.23 0.40
*Average of 4 samples
Table 8. Characterization of Samples
=
[0114] The results
in Table 8 again demonstrate the dramatic benefit in
anatase phase stability (and resistance to sintering) offered by the materials
of the present invention, the benefit in retention of surface area associated
with the inventive materials, and the activity advantage associated with the
present invention (Example 9) relative to the sample made with colloidal
silica
(Example 10).
[0115] The vanadia
loaded and aged catalyst from Example 10 was
evaluated using SEM (Fig. 8) and TEM (Fig. 9) microscopy. The images clearly
show the presence of particulate colloidal silica particles having diameters
of
about 20 nm coating the underlying -400-200 nm diameter vanadia-anatase
titania particles.
[0116] Example 11.
This example demonstrates another embodiment of
the present invention and it involves dissolution of particulate silica
followed
by re-precipitation of a surface coating of silica onto the titania via
hydrothermal treatment at elevated pH. A slurry of production-made sulfated
titania hydrogel was diluted to a TiO2 content of 21.6 wt%. 833.3 g of this
CA 02899929 2015-08-07
34
slurry was added to a round-bottom flask that was equipped with an overhead
stirrer. This slurry was heated to a temperature of 60 C via a temperature
controlled heating mantle, and was maintained at that temperature
throughout the preparation. 13.6 g of ammonium paratungstate (APT, 88%
W03) was then added and allowed to react for 20 min. The pH was then
adjusted up to 6.0 via the addition of concentrated NH4OH (29%). To this
mixture was added 80 g of a dispersion of fumed silica (Cabot M-S, 10% S102
in DI water) and the mixture was allowed to react for 20 min. The pH was
then adjusted up to 9.0 via the addition of concentrated NH4OH (29%), and
this slurry was heated at reflux for 6 hr. It was then slowly cooled to
precipitate the soluble silica, filtered, rinsed with DI water, and dried at
105 C
and then calcined at 500 C for 6 hr. The final nominal composition of this
product on an oxide basis was 90 wt% Ti02, 4 wt% S102 and 6 wt% W03
(90:4:6). Under these conditions, the fractional monolayer coverage of the
silica on the titania is well below 1Ø To this powder was deposited vanadia
from MEA solution as in Examples 1-3, above, so that the final loading was 2
wt% V205 on a total oxide basis. A portion of the dried powder was then
heated to 750 C and held at that temperature for 16 hr in an air atmosphere
that contained 10 wt% H20.
[0117] The
catalyst material from Example 11 was analyzed by XRD, N2
Porosimetry and DeN0x activity and TEM. In order to evaluate the material for
DeN0x applications, a 0.1 g sample of each vanadia-loaded and aged catalyst
sample was pelletized and meshed to -20/+40 mesh, and was loaded into a
reactor to determine the conversion of NO in the presence of NH3. A flowing
stream that contained 5% 02, 500 ppm NH3, 500 ppm NO, and 10% H20 was
passed over the catalyst at a space velocity of 650 1/g.cat-hr. Results are
shown in Table 9.
CA 02899929 2015-08-07
[0118]
DT-58* Example 11
% Anatase , 95.4 100.0
XRD-Phase %Rutile 2.3 0.0
%W03 2.3 0.0
Anatase 391 309
XRD-Crystal Size (A) Rutile 89 0
WO 3 498 0
BET Surface Area (m2ig) _ 34.9 30.3
N2 PSD , BM Pore Volume (cm /g) 0.25 0.21
250 C 19.4 30.6
NO Conversion, % 350 C 64.0 77.6
450 C 73.1 83.5
250 C 0.22 0.36
NO Rate, Of 350 C 1.04 1.50
450 C 1.34 1.80
*Average of 4 samples
Table 9. Characterization of Samples
[0119] TEM analyses, as shown in Figures 10 and 11, indicate that, while
there are a few remnant spherical silica particles that were not completely
dissolved and re-precipitated, these are typically less than 5 nm in size. For
the most part, the fumed silica has largely been dissolved and re-precipitated
onto the anatase surface as a rough coating where it is most effective for
modifying the surface properties of the underlying titania.
[0120] These results reveal the dramatic benefit in anatase phase
stability (and resistance to sintering) offered by the material present
invention, the benefit in retention of surface area associated with the
inventive materials, and the activity advantage associated with the present
invention when the silica, initially in particulate form, is solubilized and
redistributed in nano-particulate form to provide a uniform coating on the
titania surface.
[0121] Example 12. This example is another demonstration of the
beneficial effect of redistribution of the silica via hydrothermal treatment,
only in this case the starting source is colloidal silica. A slurry of
production-
made sulfated titania hydrogel was diluted to a TiO2 content of 21.6 wt%.
208.3 g of this slurry was added to a round-bottom flask that was equipped
with an overhead stirrer. This slurry was heated to a temperature of 60 C via
CA 02899929 2015-08-07
36
a temperature controlled heating mantle, and was maintained at that
temperature throughout the preparation. To this mixture was added 6.7 g of
a dispersion of colloidal silica AS-30 (W. R. Grace- 30 wt% S102) and the
mixture was allowed to react for 30 min. 3.4 g of ammonium paratungstate
(APT, 88% W03) was then added and allowed to react for 10 min. The pH
was then adjusted up to 6.5 via the addition of concentrated NH4OH (29%).
The pH was then adjusted up to 9.0 via the addition of concentrated NH4OH
(29%), and this slurry was heated at reflux for 6 hr. It was then filtered,
rinsed with DI water, and dried at 105 C and then calcined at 500 C for 6 hr.
The final nominal composition of this product on an oxide basis was 90 wt%
Ti02, 4 wt% Si02 and 6 wt% W03 (90:4:6). Under these conditions the
fractional monolayer coverage of the silica on the titania is well under 1Ø
To this powder was deposited vanadia from MEA solution as in Examples 1-3,
above, so that the final loading was 2 % V205 on a total oxide basis. A
portion
of the dried powder was then heated to 750 C and held at that temperature
for 16 hr in an air atmosphere that contained 10 wt% H20.
[0122] The material from Example 12 was analyzed by XRD, N2
Porosimetry and DeN0x activity and TEM. In order to evaluate the Example 12
material for DeN0x applications, a 0.1 g sample of each vanadia-loaded and
aged catalyst sample was pelletized and meshed to 20/+40 mesh, and was
loaded into a reactor to determine the conversion of NO in the presence of
NH3. A flowing stream that contained 5% 02, 500 ppm NH3, 500 ppm NO, and
100/0 H20 was passed over the catalyst at a Space velocity of 650 1/g.cat-hr.
[0123] Results shown in Table 10 below compared to the prep with
colloidal silica (but no hydrothermal treatment, Example 10). The XRD and N2
BET analyses reveal that the Example 12 material has improved anatase phase
stability and resistance to sintering, while the catalytic results show that
the
Example 12 material has improved catalytic activity as well associated with
the
hydrothermal redistribution of silica.
CA 02899929 2015-08-07
37
[0124]
Example 12 Example 10
% Anatase 91.6 91.2
XRD-Phase %Rutile 6.6 6.1
%W03 1.8 2.7
Anatase 562 1863
XRD-Crystal Size (A) Rutile 40 NM
W03 _ 694 268
BET Surface Area (m2/g) 23.3 8.1
N2 PSD &IN Pore Volume (cm3/g) , 0.15 0.04
250 C 31.0 8.4
NO Conversion, % 350 C 76.4 34.0
450 C _ 79.9 33.2
250 C 0.37 0.09
NO Rate, % 350 C 1.40 0.42
=
450 C 1.60 0.40
*Average of 4 samples
Table 10. Characterization of Samples
[0125] A TEM image of the Example 12 material is shown below in Figure
12. The analysis indicates that, while there are a few remnant spherical
silica
particles that were not completely dissolved and re-precipitated (roughly 10
nm in size or less), for the most part, the colloidal silica has substantially
been
dissolved and re-precipitated onto the anatase surface as a rough, patchy
coating where it is more effective for modifying the surface properties of the
titania.
[0126] Example 13: Silicic Acid. This example provides another
embodiment of the present invention, wherein the low molecular weight silica
is in the form of silicic acid generated via ion-exchange of sodium silicate.
First, a dilute solution (3 wt% Si02) of sodium silicate was prepared by
adding
569 g of DI water to 71 g of Philadelphia Quartz "N" sodium silicate, 28.7
wt% Si02. A 650.7 g portion (as received basis) of strong acid ion-exchange
resin (Dowex 650C H-form) was weighed out. Separately, a slurry of
production-made sulfated titania hydrogel was diluted to a TiO2 content of
21.6 wt%. 1666.7 g of this slurry was added to a round-bottom flask that was
equipped with an overhead stirrer. This slurry was heated to a temperature of
60 C via a temperature controlled heating mantle, and was maintained at that
CA 02899929 2015-08-07
38
temperature throughout the preparation. The ion-exchange resin was then
added to the diluted sodium silicate solution with good mixing, and the pH
was monitored. Once the pH indicated that the ion-exchange reaction had
gone to completion (pH < 3.0), the resin was filtered off, and 533 g of the
silicic acid was added to the titania slurry. This mixture was allowed to
react
for 20 min. 27.3 g of ammonium paratungstate (APT, 88% W03) was then
added and allowed to react for 20 min. The pH was then adjusted up to 6.5 via
the addition of concentrated NH4OH (29%). The mixture was then filtered,
rinsed with DI water, and dried at 105 C and then calcined at 500 C for 6 hr.
The final nominal composition of this product on an oxide basis was 90 wt%
Ti02, 4 wt% S102 and 6 wt% W03 (90:4:6). To this powder was deposited
vanadia from MEA solution as in Examples 1-3, above, so that the final
loading was 2 wt% V205 on a total oxide basis. A portion of the dried powder
was then heated to 750 C and held at that temperature for 16 hr in an air
atmosphere that contained 10 'wt% H20. In order to evaluate the Example 13
materials for DeN0x applications, a 0.1 g sample of each vanadia-loaded and
aged catalyst sample was pelletized and meshed to -20/+40 mesh, and was
loaded into a reactor to determine the conversion of NO in the presence of
NH3. A flowing stream that contained 5% 02, 500 PPm NH3, 500 ppm NO, and
100/0 H20 was passed over the catalyst at a space velocity of 650 1/g.cat-hr.
The aged samples were then evaluated by XRD, N2 PSD, DeN0x conversion
and compared against DT-58' as shown in Table 11.
[0127]
DT-58* Example 13
% Anatase 95.4 100.0
XRD-Phase %Rutile 2.3 0.0
%W03 2.3 0.0
Anatase 391 286
XRD-Crystal Size (A) Rutile 89 0
WO3 498 0
BET Surface Area (m2/g) 34.9 40.8
N2 PSD _ BM Pore Volume (cm3/g) 0.25 0.27
250 C 19.4 29.6
NO Conversion, % 350 C 64.0 76.7
450 C 73.1 83.0
*Average of 4 samples
Table 11. Characterization of Samples
CA 02899929 2015-08-07
39
[0128] It can clearly be seen that the material prepared according to the
present invention has higher anatase phase stability, better retention of
surface area (sintering resistance) and higher DeN0x activity compared to DT-
58T"'. TEM analysis of the Example 13 material was conducted, and the
results, highlighted in Figures 13 and 14, reveal that the silica is present
as
two dimensional patches well distributed on the titania surface. There are a
few rare three dimensional particles identifiable as silica present in some of
the images, but these are, for the most part, less than about 5 nm in size.
[0129] Example 14. This example provides another embodiment of the
present invention, wherein the low molecular weight silica is in the form of
silicic acid generated via ion-exchange of sodium silicate. First, a dilute
solution (3 wt% S102) of sodium silicate was prepared by adding 59.7 g of DI
water to 7.0 g of Philadelphia Quartz "N" sodium silicate, 28.7 wt% Si02. A
13.5 g portion (as received basis) of strong acid ion-exchange resin (Dowex
650C H-form) was weighed out and added to a flow-through column.
Separately, a slurry of production-made sulfated titania hydrogel was diluted
to a TiO2 content of 21.6 wt%. 208.3 g of this slurry was added to a round-
bottom flask that was equipped with an overhead stirrer. This slurry was
heated to a temperature of 60 C via a temperature controlled heating mantle,
and was maintained at that temperature throughout the preparation. 66.7 g of
the diluted sodium silicate solution was then passed through the column to
remove the sodium. This Mixture was allowed to react for 20 min. 3.4 g of
ammonium paratungstate (APT, 88% W03) was then added and allowed to
react for 20 min. The pH was then adjusted up to 6.5 via the addition of
concentrated NH4OH (29%). The mixture was then filtered, rinsed with DI
water, and dried at 105 C and then calcined at 500 C for 6 hr. The final
nominal composition of this product on an oxide basis was 90 wt% Ti02, 4
wt% 5102 and 6 wt% W03 (90:4:6). This composition, before addition of
vanadia, had a N2 BET surface area of 221 m2/g, and so the silica is present
at a fractional monolayer coverage of 0.30, well under 1 monolayer. This
sample was evaluated using TEM. An identical sample was prepared,
except that the tungsta was added before the silica, and this sample was
CA 02899929 2015-08-07
analyzed by 29Si-CP-MASNMR spectroscopy. The NMR results shown in
Table 6 demonstrate that most of the silica present in the sample has
coordination of Q3 or less, as would be expected for silica distributed in
two-dimensional patches on the titania surface. The TEM image shown in
Figure 15 reveals that the silica is present as 1-3 nm patchy coating on the
anatase titania crystallite surface, and no distinct, 3-dimensional particles
of silica can be seen larger than 5 nm in diameter.
[0130] To this powder was deposited vanadia from MEA solution as in
Examples 1-3, above, so that the final loading was 2 wt% V205 on a total oxide
basis. A portion of the dried powder was then heated to 750 C and held at
that temperature for 16 hr in an air atmosphere that contained 10 wt% H20.
In order to evaluate the Example 14 materials for DeN0x applications, a 0.1 g
sample of each vanadia-loaded and aged catalyst sample was pelletized and
meshed to -20/+40 mesh, and was loaded into a reactor to determine the
conversion of NO in the presence of NH3. A flowing stream that contained 5%
02, 500 ppm NH3, 500 ppm NO, and 10% H20 was passed over the catalyst at
a space velocity of 650 1/g.cat-hr. The aged samples were then evaluated by
XRD, N2 PSD, and DeN0x conversion and compared against DT-58 as shown in
Table 12.
[0131]
, DT-58TM Example 14
XRD-Phase % Anatase 95.4 100.0
% Rutile 2.3 0.0
% W03 2.3 01.0
XRD-Crystal Size Anatase 391 336
(A) Rutile 89 0
W03 498
N2 PSD BET Surface Area m2/9 34.9 31.9
El3F1 Pore Volume cm3/g) 0.25 0.25
NO Conversion, 250 C 0.22 0.41
Rate 350 C 1.13 1.67
450 C 1.43 1.99
Table 12. Characterization of Samples
[0132] It can clearly be 'seen that the material prepared according to the
present invention has higher anatase phase stability, better retention of
surface area (sintering resistance) and higher DeN0x activity compared to DT-
CA 02899929 2015-08-07
41
58TM.
[0133]
Example 15. This example is directed to showing that various
materials of the prior art are different from those of the present invention.
In
particular, reference is made to U.S. Patent 4,221,768 column 3, 4 (line 3),
Ex. 1, and US 2007/0129241 (paragraph 0026). In this example, a
particulate colloidal silica is incorporated into the titania during the
precipitation of the titania. First, 1169 g of water were added to a 4 L glass
beaker, and this was placed in an ice bath to cool down. Then, 330 g of
TiOCl2 solution (25.9% Ti02) was slowly added with stirring to the cooled
water, such that the temperature of the solution did not rise above 30 C, in
order to make a 5.7% TiO2 solution. 544.6 g of this solution was then placed
in a 1L glass beaker, and was stirred vigorously. To this mixture was slowly
added 4.33 g of Ludox AS-30 colloidal silica (W. R. Grace- 30 wt% Si02). To
this suspension was then added concentrated NH4OH (29%) until the pH
reached 7. The precipitated slurry was aged for 2 hrs. It was then filtered,
rinsed with DI water and then dried at 105 C. The nominal composition, on
an oxide basis, of this powder was 4 wt% Si02, and 96 wt% T102. 27 gm of
dried powder (84.5 % solids) was then slurried in 100 g of DI water, heated
to 60 C, and 1.7 g of APT was then added and allowed to react for 20 min. The
pH was then adjusted to 7.0, and the final mixture was filtered and dried at
105 C and then calcined at 500 C for 6 hr. The final nominal composition of
this product on an oxide basis was 90 wt% Ti02, 4 wt% Si02 and 6 wt% W03.
To this powder was deposited vanadia from MEA solution as in Examples 1-3,
so the final loading of vanadia was 2 wt% V205 on a total oxide basis. A
portion
of the dried powder was then heated to 750 C and held at that temperature
for 16 hr in an air atmosphere that contained 10 wt% H20. In order to evaluate
the Example 15 materials for DeN0x applications, a 0.1 g sample of each
vanadia-loaded and aged catalyst sample was pelletized and meshed to -
20/+40 mesh, and was loaded into a reactor to determine the conversion of
NO in the presence of NH3. Allowing stream that contained 5 /0 021 500 PPm
NH3, 500 ppm NO, and 10 /0 H20 was passed over the catalyst at a space
velocity of 650 1/g.cat-hr. The aged samples were then evaluated by XRD, N2
PSD, DeN0x conversion and TEM, and compared against DT-58Tm as shown in
CA 02899929 2015-08-07
42
Table 13 and Figure 16.
[0134]
DT-58* Example 15
% Anatase 95.4 90.0
XRD-Phase %Rutile 2.3 9.0
%W03 2.3 1.0
Anatase 391 935
XRD-Crystal Size (A) Rutile 89 1567
WO 3 498 190
BET Surface Area (m2/g) 34.9 10.2
N2 PSD BM Pore Volume (cm3/9) 0.25 0.07
250 C 19.4 13.2
NO Conversion, % 350 C 64.0 46.3
450 C 73.1 , 57.9
*Average of 4 samples
Table 13. Characterization of Samples
[0135] Results clearly show that the comparison material of Example 15
(not formed from a low molecular weight and/or small nanoparticle silica)
clearly has lower stability and activity than even the reference DT-58r"
samples. Furthermore, TEM analysis (Fig. 16 reveals that the silica is present
as large three-dimensional nodules (e.g., >20 nm and up to 50 nm or larger
in size).
[0136] Example 16. This embodiment is similar to prior art embodiments
where the silica is incorporated in a soluble form in the precipitation (see
for
example 4,221,768 col. 3, line 36), except in this example the TMA silicate of
the present invention is used, as in Examples 7, 8 and 9. In this example, a
material is prepared wherein silica is again incorporated during the
precipitation of the titania. However, in this case, TMA silicate is used as
the
silica source, and titanyl sulfate solution is used as the titania source.
First,
990 g of titanyl sulfate solution (10.1% h02, ¨29% H2SO4) was added to a 1L
beaker. In a separate beaker, 26.5 g of TMA silicate (9 wt% S102, Alfa Aesar)
was diluted 350 ml with DI water. In a third vessel with a spout for
continuous
removal of precipitated slurry, a 150 g heal of water was added, and this
vessel was stirred. The titanyl sulfate solution was pumped into vessel 3 at a
rate of 20 ml/min, and the TMA silicate solution was also pumped into vessel 3
CA 02899929 2015-08-07
43
at a rate of 10 ml/min. Further, concentrated NH4OH (29%) was also pumped
into vessel 3 to maintain a pH for precipitation of the oxides at 6Ø The
overflow from vessel 3 was captured in another beaker. It will be known to
persons of ordinary skill in the art that vessel 3 is a continuous-flow,
stirred
tank reactor. Once the precipitation of the oxides was complete, the
precipitate
was then filtered, rinsed with DI water and then dried at 105 C. The nominal
composition, on an oxide basis, of this powder was 2.5 wt% Si02, 97.5 wt%
T102.
[0137] 51.2 gm of the dried powder (73% solids) was then slurried in 122
g of DI water, heated to 60 C, and 1.8 g of APT was then added and allowed to
react for 20 min. The pH was then adjusted to 6.5 and allowed to react for 20
min. The final mixture was filtered and dried at 105 C and then calcined at
500 C for 6 hr. The final nominal 'composition of this product on an oxide
basis
was 93.5 wt% T102, 2.5 wt% Si02 and 4 wt% W03 (93.5:2.5:4). To this
powder was deposited vanadia from MEA solution as in Examples 1-3, so that
the final loading of vanadia was 2 wt% V205 on a total oxide basis. A portion
of
the dried powder was then heated to 750 C and held at that temperature for
16 hr in an air atmosphere that contained 10 wt% H2O. In order to evaluate
the Example 16 materials for DeN0x applications, a 0.1 g sample of each
vanadia-loaded and aged catalyst sample was pelletized and meshed to -
20/+40 mesh, and was loaded into a reactor to determine the conversion of
NO in the presence of NH3. A flowing stream that contained 5%'O, 500 ppm
NH3, 500 ppm NO, and 10% H20 was passed over the catalyst at a space
velocity of 650 1/g.cat-hr. The aged samples were then evaluated by XRD, N2
PSD, DeN0x conversion and compared against DT-58Tm as shown in Table 14.
[0138] The results clearly show that the material produced under
conditions wherein a low molecular weight and/or small nanoparticle form
silica is incorporated during the precipitation of the titania has lower
anatase
phase stability, lower sintering resistance and lower DeN0x activity that the
base case materials. Shown in Fig. 17 is a transmission electron micrograph
(TEM) of the vanadia catalyst that shows large (>20 nm) three dimensional
silica nodules that are not well dispersed on the titania surface.
CA 02899929 2015-08-07
44
[0139]
DT-58* Example 16
% Anatase 95.4 82.3
XRD-Phase %Rutile 2.3 14.8
%W03 2.3 2.9
Anatase 391 1456
XRD-Crystal Size (A) Rutile 89 _ 1994
W03 498 353
BET Surface Area (m2/g) 34.9 12.0
N2 PSD BJH Pore Volume (cm3/q) 0.25 0.05
250 C 19.4 12.2
NO Conversion, % 350 C 64.0 50.0
450 C 73.1 _ 55.8
*Average of 4 samples
Table 14. Characterization of Samples
[0140]
Example 17. This embodiment is similar to Example 16, only the
final composition is 90:4:6 wt% Ti02, Si02, W03 prior to vanadia addition. In
this example, a material is prepared wherein silica is again incorporated
during the precipitation of the titania. First, 891 g of titanyl sulfate
solution
(10.1% T102, ¨29% H2SO4) was added to a 1L beaker. In a separate beaker,
44.4 g of TMA silicate (9 wt% Si02, Alfa Aesar) was diluted with 400 ml with
DI
water. In a third vessel with a spout for continuous removal of precipitated
slurry, a 150 g heal of water was added, and this vessel was stirred. The
titanyl sulfate solution was pumped into vessel 3 at a rate of 20 ml/min, and
the TMA silicate solution was also pumped into vessel 3 at a rate of 10
ml/min.
Further, concentrated NH4OH (29%) was also pumped into vessel 3 to maintain
a pH for precipitation of the oxides at 6Ø The overflow from vessel 3 was
- captured in another beaker. It will be known to persons of ordinary skill in
the
art that vessel 3 is a continuous-flow, stirred tank reactor. Once the
precipitation of the oxides was complete, the precipitate was then filtered,
rinsed with DI water and then dried at 105 C. The nominal composition, on an
oxide basis, of this powder was 4.3 wt% Si02, 96.7 wt% Ti02-
[0141] All
of the dried powder was then slurried in ¨150 g of DI water,
heated to 60 C, and 6.8 g of APT was then added and allowed to react for 20
min. The pH was then adjusted to 6.5 and allowed to react for 20 min. The
final mixture was filtered and dried at 105 C and then calcined at 500 C for 6
CA 02899929 2015-08-07
hr. The final nominal composition of this product on an oxide basis was 90
wt% Ti02, 4 wt% Si02 and 6 wt% W03. To this powder was deposited vanadia
from MEA solution as in Examples 1-3, so that the final loading of vanadia was
2 wt% V205 on a total oxide basis. A portion of the dried powder was then
heated to 750 C and held at that temperature for 16 hr in an air atmosphere
that contained 10 wt% H20. In order to evaluate the Example 17 material for
DeN0x applications, a 0.1 g sample of each vanadia-loaded and aged catalyst
sample was pelletized and meshed to -20/+40 mesh, and was loaded into a
reactor to determine the conversion of NO in the presence of NH3. A flowing
stream that contained 5% 02, 500 ppm NH3, 500 ppm NO, and 10% H20 was
passed over the catalyst at a space velocity of 650 1/g.cat-hr. The aged
samples were then evaluated by XRD, N2 PSD, DeN0x conversion and
compared against DT-581m as shown in Table 15.
[0142]
DT-58* Example 17
% Anatase 95.4 94.5
XRD-Phase % Rutile 2.3 3.9
%W03 2.3 1.6
Anatase 391 748
XRD-Crystal Size (A) Rutile 89 795
W03 498 180
N2 PSD BET Surface Area (m2/g) 34.9 18.2
BJH Pore Volume (cm2/0 0.25 0.09
NO Conversion, % 250 C 19.4 16.2
350 C 64.0 58.4
450 C 73.1 67.0
*Average of 4 samples
Table 15. Characterization of Samples
[0143] The results clearly show that the material made wherein a soluble
silica is incorporated during the precipitation of the titania has lower
anatase
phase stability, lower sintering resistance and lower DeN0x activity that the
base case materials.
[0144] Example 18. This embodiment demonstrates the effect of calcination
temperature on the deN0x catalytic activity of the materials of the present
invention. The DT-581m reference catalysts loaded with 2 wt% V205 described in
CA 02899929 2015-08-07
46
Example 8 were used as the benchmark. A 90:4:6 Ti02:Si02:W03 composition of
the present invention, as prepared in Example 13 (batch mode) and Example 14
(continuous mode) were loaded with 2 wt% V205 as described in those Examples.
A 88:8:4 T102:Si02:W03 composition of the present invention, as prepared in
Example 9, was also loaded with 2 wt% V205 as described therein. These
materials
were then exposed to elevated temperatures (calcined) ranging from 500 C to
850 C, and the rate of deN0x catalytic activity was measured as in Example 17.
The data of the results were fitted by regression to polynomial functions, and
the
fitted curves are shown in Fig. 18. Fig. 18 demonstrates that in order to
obtain the
maximum activity for the vanadia catalyst materials of the present invention,
in
particular activity that is greater than the reference DT-58 activity, the
vanadia
catalyst materials must first be exposed to elevated temperatures, i.e.,
temperatures in excess of 650 C.
[0145] UTILITY
[0146] The
present invention is directed, in one embodiment, to
compositions comprising anatase titania, wherein the titania is stabilized by
a
silica provided in a low molecular weight form and/or small nanoparticle form.
Further, the invention is directed to the use of these silica-titania
compositions
as catalyst supports, in particular in combination with added tungsta and
vanadia, for vanadia-based selective catalytic reduction of DeN0x from lean-
burn (diesel) engines. The invention further is directed to methods of
producing these silica-stabilized titania or titania-tungsta supports, and the
vanadia-based catalysts which comprise the silica-stabilized titania or
titania-
tungsta supports, and the methods of production of the vanadia catalysts, and
to catalytic devices comprising these vanadia catalysts.
[0147] The
actual specific composition of the silica-titania or silica-titania-
tungsta catalyst support depends on the requirements of the specific catalytic
application. In one preferred composition, the invention comprises a silica-
stabilized titania catalyst support which comprises particles which comprise
?_90% dry weight of TiO2 and 5.10 wt% Si02. In
another preferred
composition, the invention comprises a silica stabilized titania-tungsta
catalyst
support with .85 /0 dry weight titania, 3%-10% dry weight of Si02, and 3%-
10% dry weight of W03. Alternatively, in one embodiment where the
CA 02899929 2015-08-07
47
application requires particularly good thermal stability, the catalyst support
comprises 85%
dry weight of Ti02, 5.0%-9.0% dry weight of Si02, and
3.0%-7.0% dry weight of W03. More particularly, this stable catalyst support
comprises 87%-89% dry weight of TiO3, 7%-9% dry weight of Si02, and 3%-
5% dry weight of W03. In one 'preferred embodiment the catalyst support
comprises about 88% (+0.5%) dry weight Ti02, about 8% (+0.5%) dry
weight 5'02, and about 4% (+0.5%) dry weight W03. In one embodiment, the
weight % of W03 is less than the weight of % of Si02. In one embodiment, the
catalytic support has a fresh surface area of at least 80 m2/gm, and more
preferably at least 100 m2/gm.
[0148] In
another embodiment where the application requires particularly
good catalytic activity, the catalyst support comprises a 85% dry weight of
Ti02, 3.0%-8.0% dry weight of 5102, and 4.0%-9.0% dry weight of W03. More
particularly, this active catalyst support comprises ?:87% dry weight of TiO3,
3%-6% dry weight of Si02, and 4%-8% dry weight of W03. In one preferred
embodiment the catalyst support comprises about 90% (+0.5%) dry weight
Ti02, about 4% (+0.5%) dry weight Si02, and about 6% (+0.5%) dry weight
W03. In one embodiment, the weight % of W03 is greater than the weight of
% of S102. In one embodiment, the catalytic support has a fresh surface area
of at least 80 m2/gm, and more preferably at least 100 m2/gm.
[0149] In an
embodiment of the invention, the TiO2 component of the
catalyst support material used herein substantially comprises a surface area
<400 m2/g and a pore volume <0.40 cm3/g.
[0150] In an
embodiment of the invention the titania slurry and silica
component used herein are mixed at a temperature < 80 C and at a p1-1 <
8.5. Alternatively, the titania slurry and silica component used herein may be
mixed at a temperature <70 C and at a pH < 7Ø
[0151] In
another embodiment, the invention is a vanadia catalyst
comprising the novel silica-stabilized titania or titania-tungsta catalyst
support
described herein upon which a quantity of vanadium oxide (V205) is disposed.
In the vanadia catalyst, V205 preferably comprises 0.5% to 3% to 5% of the
dry weight thereof. The invention, further is directed to diesel engine
emission
catalytic devices which contain the vanadia catalysts described herein. The
CA 02899929 2015-08-07
=
48
vanadia catalyst materials of the invention may be further treated by
calcination (sintering) at a temperature > 650 C to increase their deN0x
catalytic activity.
[0152] Furthermore, these novel catalytic devices may be used upstream
or downstream of a diesel particulate filter (DPF) in a diesel emission
control
system. In an upstream system the catalytic device is between the engine and
the DPF, and in a downstream system the DPF is between the engine and the
catalytic device.
[0153] Where and used herein the term "silica titania support" is
intended to have the same meaning as "silica-stabilized titania support", and
where the term silica titania tungsta support" is intended to have the same
meaning as "silica stabilized titania-tungsta support".
[0154] Preferably most of the silica particles in the stabilized titania
support particles have diameters < 5 nm, and more preferably < 4 nm and
more preferably < 3 nm, and still more preferably < 2 nm, and/or comprise
low molecular weights (e.g., MW < 100,000, whether or not the particles do,
or do not, have V205 deposited thereon.
[0155] Where the silica titania support particles contain V205, the V205
preferably comprise from 0.5%-3.0% of the dry weight of the support
material.
[0156] Distribution of the W03 and Si02 species on the surface of the
titania support also plays a role in the optimization of DeN0x activity of the
vanadia catalysts. Thus, when the catalysts are freshly prepared, that is,
when the added silica and tungsta oxides are first deposited and before high
temperature treatment, the fractional monolayer coverage should be about
1.0 or less.
[0157] As noted above, the stabilization of titania support material with
silica involves treatment of the titania with silica in a low molecular weight
form and/or small nanoparticle form, such as tetra(alkyl)ammonium silicate
(e.g., tetramethylammonium silicate) or tetraethylorthosilicate (TEOS). Other
examples of low molecular weight and/or small nanoparticle silica precursors
which may be used in the present invention include, but are not limited to
aqueous solutions of silicon halides (Le., anhydrous SiX4, where X= F, Cl, Br,
CA 02899929 2015-08-07
49
or I), silicon alkoxides (i.e., Si(OR)4, where R=methyl, ethyl, isopropyl,
propyl,
butyl, iso-butyl, see-butyl, tert-butyl, pentyls, hexyls, octyls, nonyls,
decyls,
undecyls, and dodecyls, for example), other silicon-organic compounds such as
hexamethyldisilazane, fluoro-silicic acid salts such as ammonium
hexafluorosilicate [(NH4)2SiF6], quaternary ammonium silicate solutions (e.g.,
(NR4)n, (Si02), where R=H, or alkyls such as listed above, and when n=0.1-2,
for example), aqueous sodium and potassium silicate solutions (Na2SiO3,
K25iO3, and MS103 wherein M is Na or K in varying amounts in ratio to Si),
silicic acid (Si(OH)4) generated by ion exchange of any of the cationic forms
of
silica listed herein using an acidic ion-exchange resin (e.g., ion-exchange of
the alkali-silicate solutions or quaternary ammonium silicate solutions). In
preferred embodiments, the titania used herein has not been prepared in the
presence of urea.
[0158] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein.
Moreover, the scope of the present application is not intended to be limited
to
the particular embodiments of the process, items of manufacture,
compositions of matter, means, methods and steps described in the
specification. As one of ordinary skill in the art will readily appreciate
from
the disclosure of the present invention, processes, items of manufacture,
compositions of matter, means, methods, or steps, presently existing or later
to be developed that perform substantially the same function or achieve
substantially the same result as the corresponding embodiments described
herein may be utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such processes,
items of manufacture, compositions of matter, means, methods, or steps.
=
CA 02899929 2015-08-07
[0160] Cited References
[0161] 1. Granger, P. and Parvulescu, V.I. eds. "Studies in Surface
Science and Catalysis. Vol. 171, Ch. 9 (2007).
[0162] 2. Ullmann. "Encyclopedia of Industrial Chemistry." Fifth ed., Vol.
A23, pp. 583-660, (1993).
[0163] 3. Iler, R.K. "The Chemistry of Silica." (1979).
[0164] 4. Fedeyko et at. "Langmuir," Vol. 21, 5179-5206, (2005).
[0165] 5. Engelhardt G. and D. Michel. "High Resolution Solid-State NMR
of Silicates and Zeolites." John Wiley and Sons, NY (1987).
[0166] 6. Wachs, I., et at. "Catalysis Today," 78, p. 17 (2003).
[0167] 7. Wachs, I., et at. "Catalysis Today," 116, p. 162-168 (2008).
[0168] 8. Bergna, H.E., and W.O. Roberts, eds. "Colloidal Silica,
Fundamentals and Applications." Surfactant Science Series, Vol. 131, CRC
Press, Taylor and Francis (2006).
[0169] 9. Wachs, et al. "J. Catalysis," 161, pp. 211-221 (1996).
[0170] 10. Bergna, H. ed. "The Colloid Chemistry of Silica," ACS Series
234 (1994).
[0171] 11. Brinker, C.J. and G.W. Scherer. "Sol-Gel Science," Chapter 3
(1990).
