Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CARBIDE AND OXYCARBIDE BASED COMPOSITIONS AND NANORODS
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to compositions of carbide-based and oxycarbide-based
nanorods, carbon nanotubes including carbide and/or oxycarbide compounds,
rigid
porous structures including these compositions, and methods of making and
using the
same. More specifically, the invention relates to rigid three dimensional
structures
comprising carbon nanotubes bearing carbides and oxycarbides, carbide and/or
oxycarbide-based nanorods having high surface areas and porosities, low bulk
densities, substantially no micropores and increased crush strengths. The
invention
also relates to using the compositions of carbide-based nanorods, oxycarbide-
based
nanorods, carbon nanotubes comprising carbide and oxycarbide compounds and the
rigid porous structures including these compositions as catalysts and catalyst
supports,
useful for many types of heterogenous catalytic reactions frequently
encountered in
petrochemical and refining processes.
Description of the Related Art
Heterogeneous catalytic reactions are widely used in chemical processes in the
petroleum, petrochemical and chemical industries. Such reactions are commonly
performed with the reactants) and products) in the fluid phase and the
catalyst in the
solid phase. In heterogeneous catalytic reactions, the reaction occurs at the
interface
between phases, i.e., the interface between the fluid phase of the reactants)
and
products) and the solid phase of the supported catalyst. Hence, the properties
of the
surface of a heterogeneous supported catalyst are significant factors in the
effective
use of that catalyst. Specifically, the surface area of the active catalyst,
as supported,
and the accessibility of that surface area to reactant chemiabsorption and
product
desorption are important. These factors affect the activity of the catalyst,
i.e., the rate
of conversion of reactants to products. The chemical purity of the catalyst
and the
catalyst support have an important effect on the selectivity of the catalyst,
i.e., the
degree to which the catalyst produces one product from among several products,
and
the life of the catalyst.
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Generally catalytic activity is proportional to catalyst surface area.
Therefore,
a high specific area is desirable. However, that surface area must be
accessible to
reactants and products as well as to heat flow. The chemiabsorption of a
reactant by a
catalyst surface is preceded by the diffusion of that reactant through the
internal
structure of the catalyst.
Since the active catalyst compounds are often supported on the internal
structure of a support, the accessibility of the internal structure of a
support material
to reactant(s), products) and heat flow is important. Porosity and pore size
distribution of the support structure are measures of that accessibility.
Activated
carbons and charcoals used as catalyst supports have surface areas of about
1000
square meters per gram and porosities of less than one milliliter per gram.
However,
much of this surface area and porosity, as much as 50%, and often more, is
associated
with micropores, i.e., pores with pore diameters of 2 manometers or less.
These pores
can be inaccessible because of diffusion limitations. They are easily plugged
and
thereby deactivated. Thus, high porosity material where the pores are mainly
in the
mesopore (>2 manometers) or macropore (>50 manometers) ranges are most
desirable.
It is also important that self supported catalysts and supported catalysts not
fracture or attrit during use because such fragments may become entrained in
the
reaction stream and must then be separated from the reaction mixture. The cost
of
replacing attritted catalyst, the cost of separating it from the reaction
mixture and the
risk of contaminating the product are all burdens upon the process. In other
processes, e.g. where the solid supported catalyst is filtered from the
process stream
and recycled to the reaction zone, the fines may plug the filters and disrupt
the
process. It is also important that a catalyst, at the very least, minimize its
contribution
to the chemical contamination of reactants) and product(s). In the case of a
catalyst
support, this is even more important since the support is a potential source
of
contamination both to the catalyst it supports and to the chemical process.
Further,
some catalysts are particularly sensitive to contamination that can either
promote
unwanted competing reactions, i.e., affect its selectivity, or render the
catalyst
ineffective, i.e., "poison" it. Charcoal and commercial graphites or carbons
made
from petroleum residues usually contain trace amounts of sulfur or nitrogen as
well as
metals common to biological systems and may be undesirable for that reason.
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Since the 1970s carbon nanofibers or nanotubes have been identified as
materials of interest for such applications. Carbon nanotubes exist in a
variety of
forms and have been prepared through the catalytic decomposition of various
carbon-
containing gases at metal surfaces. Nanofibers such as fibrils, bucky tubes
and
nanotubes are distinguishable from continuous carbon fibers commercially
available
as reinforcement materials. In contrast to nanofibers, which have, desirably
large, but
unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios
(L/D) of
at least 104 and often 106 or more. The diameter of continuous fibers is also
far larger
than that of nanofibers, being always >1.0~ and typically 5 to 7~.
U.S. Patent No. 5,576,466 to Ledoux et al. discloses a process for isomerizing
straight chain hydrocarbons having at least seven carbon atoms using catalysts
which
include molybdenum compounds whose active surface consists of molybdenum
carbide which is partially oxidized to form one or more oxycarbides. Ledoux et
al.
disclose several ways of obtaining an oxycarbide phase on molybdenum carbide.
However, their methods require the formation of molybdenum carbides by
reacting
gaseous compounds of molybdenum metal with charcoal at temperatures between
900°C and 1400°C. These are energy intensive processes.
Moreover, the resulting
molybdenum carbides have many similar drawbacks as other catalysts prepared
with
charcoal. For example, much of the surface area and porosity of the catalysts
is
associated with micropores and as such these catalysts are easily plugged and
thereby
deactivated.
While activated charcoals and other materials have been used as catalysts and
catalyst supports, none have heretofore had all of the requisite qualities of
high
surface area porosity, pore size distribution, resistance to attrition and
purity for the
conduct of a variety of selected petrochemical and refining processes. For
example,
as stated above, although these materials have high surface area, much of the
surface
area is in the form of inaccessible micropores (i.e., diameter < 2 nm).
It would therefore be desirable to provide a family of catalysts and catalyst
supports that have high accessible surface area, high porosity, resistance to
attrition,
are substantially free of micropores, are highly active and selective and show
no
significant deactivation after many hours of operation.
Nanofiber mats, assemblages and aggregates have been previously produced
to take advantage of the increased surface area per gram achieved using
extremely
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thin diameter fibers. These structures are typically composed of a plurality
of
intertwined or intermeshed nanotubes.
Objects of the Invention
It is an object of the present invention to provide a composition including a
multiplicity of oxycarbide nanorods having predominately diameters between 2.0
nm
and 100 nm.
It is a further object of the present invention to provide another composition
including a multiplicity of carbide nanorods comprising oxycarbides.
It is a further object of the present invention to provide another composition
including a multiplicity of carbon nanotubes which have predominantly
diameters
between 2.0 nm and 100 nm, which nanotubes comprise carbides and optionally
also
oxycarbides.
It is a further object of the present invention to provide another composition
including a multiplicity of carbon nanotubes having a carbide portion and
optionally
an oxycarbide portion.
It is a further object of the present invention to provide rigid porous
structures
which comprise compositions including a multiplicity of oxycarbide nanorods or
a
multiplicity of carbide nanorods with or without oxycarbides.
It is a further object of the present invention to provide compositions of
matter
which comprise three-dimensional rigid porous structures including oxycarbide
nanorods, carbide nanorods, carbide nanorods comprising oxycarbides, or carbon
nanotubes comprising a carbide portion and optionally an oxycarbide portion.
It is a further object of the present invention to provide methods for the
preparation of and using the rigid porous structures described above.
It is still a further object of the invention to provide improved catalysts,
catalyst supports and other compositions of industrial value based on
composition
including a multiplicity of carbide nanorods, oxycarbide nanorods and/or
carbon
nanotubes comprising carbides and oxycarbides.
It is still a further object of the invention to provide improved catalysts,
catalyst supports and other compositions of industrial value based on three-
dimensional rigid carbide and/or oxycarbide porous structures of the
invention.
It is an object of the invention to provide improved catalytic systems,
improved catalyst supports and supported catalysts for heterogenous catalytic
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reactions for use in chemical processes in the petroleum, petrochemical and
chemical
industries.
It is a further object of the invention to provide improved methods for
preparing catalytic systems and supported catalysts.
It is another object of the invention to improve the economics and reliability
of
making and using catalytic systems and supported catalysts.
It is still a further object of the invention to provide improved,
substantially
pure, rigid carbide catalyst support of high porosity, activity, selectivity,
purity and
resistance to attrition.
The foregoing and other objects and advantages of the invention will be set
forth in or will be apparent from the following description and drawings.
SUMMARY OF THE INVENTION
The present invention which addresses the needs of the prior art provides a
composition including nanorods which contain oxycarbides. Another composition
provided by the present invention includes carbide-based nanorods which also
contain
oxycarbides. Another composition provided by the invention relates to carbon
nanotubes which bear both carbides and oxycarbides. In one composition the
carbides retain the structure of the original aggregates of carbon nanotubes.
However,
a composition is also provided which includes carbide-based nanorods where the
morphology of the aggregates of carbon nanotubes is not retained. The
invention also
provides a composition of carbides supported on carbon nanotubes where only a
portion of the carbon nanotubes have been converted to carbide-based nonorods
and/or carbides.
The present invention also provides rigid porous structures including
oxycarbide nanorods and/or carbide-based nonorods and/or carbon nanotubes
bearing
carbides and oxycarbides. Depending on the morphology of the carbon nanotubes
used as a source of carbon, the rigid porous structures can have a uniform or
nonuniform pore distribution. Extrudates of oxycarbide nanorods and/or carbide-
based nanorods and/or carbon nonotubes bearing oxycarbides and/or carbides are
also
provided. The extrudates of the present invention are glued together to form a
rigid
porous structure.
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The invention also provides for the compositions and rigid porous structures
of the invention to be used either as catalysts and/or catalyst supports in
fluid phase
catalytic chemical reactions.
The present invention also provides methods of making oxycarbide-based
nonorods, carbide-based nanorods bearing oxycarbides and carbon-nanotubes
bearing
carbides and oxycarbides. Methods of making rigid porous structures are also
provided. Rigid porous structures of carbide-nonorods an be formed by treating
rigid
porous structures of carbon nanotubes with a Q-based compound. Depending upon
temperature ranges the conversion of the carbon nanotubes to carbide-based
nanorods
can be complete or partial. The rigid porous structure of carbide nanorods
and/or
carbon nanotubes can be further treated with an oxidizing agent to form
oxycarbide
nanorods and/or oxycarbides. The rigid porous structures of the invention can
also be
prepared from loose or aggregates of carbide-based nonorods and/or oxycarbide-
based nanorods by initially forming a suspension in a medium, separating the
suspension from the medium, and pyrolyzing the suspension to form rigid porous
structures. The present invention also provides a process for making supported
catalysts for selected fluid phase catalytic reactions.
Other improvements which the present invention provides over the prior art
will be identified as a result of the following description which sets forth
the preferred
embodiments of the present invention. The description is not in any way
intended to
limit the scope of the present invention, but rather only to provide a working
example
of the present preferred embodiments. The scope of the present invention will
be
pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 A is an XRD graph of sample 12 as set forth in Table 1. A reference
XRD pattern of hexagonal Mo2C is shown immediately below.
Figures 1 B and 1 C are SEM micrographs of sample 12 as set forth in Table 1.
Figure 2A is an XRD graph of sample 12 as set forth in Table 1. A reference
XRD pattern of hexagonal Mo2C is also shown immediately below.
Figure 2B is an HRTEM micrograph of sample 12 as set forth in Table 1.
Figure 3A is an XRD graph of sample 10 as set forth in Table 1. Reference
XRD patterns of hexagonal Mo2C, cubic Mo2C and graphite are shown immediately
below.
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Figure 3B is an HRTEM micrograph of sample 10 as shown in Table C.
Figure 4 is a thermogravimetric analysis of sample 12 as set forth in Table 1.
Figure SA is an SEM micrograph of SiC extrudates.
Figure SB is an SEM micrograph illustrating micropores among the aggregates
of the extrudates shown in Figure SA.
Figure SC is an SEM micrograph illustrating micropores in the networks of the
intertwined SiC nanorods present in the extrudates shown in Figure SA.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
The terms "nanotube", "nanofiber" and "fibril" are used interchangeably.
Each refers to an elongated hollow structure having a diameter less than 1
micron.
The term "nanotube" includes ''nanofiber" or "fibril" (which refers to an
elongated
solid, (e.g. angular fibers having edges) structures having a cross section of
less than 1
micron. The term "nanotube" also includes "bucky tubes" and graphitic
nanofibers
the graphene planes of which are oriented in herring bone pattern.
"Graphenic" carbon is a form of carbon whose carbon atoms are each linked
to three other carbon atoms in an essentially planar layer forming hexagonal
fused
rings. The layers are platelets only a few rings in diameter or they may be
ribbons,
many rings long but only a few rings wide.
"Graphenic analogue" refers to a structure which is incorporated in a
graphenic surface.
"Graphitic" carbon consists of layers which are essentially parallel to one
another and no more than 3.6 angstroms apart.
The term "nanorod" refers to a rod-like structure having a surface and a
substantially solid core with a diameter less than or equal to 100 nm and at
least 1.0
nm. The structure has an aspect ratio between 10 and 500 and a length up to
SOp.
The diameter of a nanorod is substantially uniform along the entire length of
the
nanorod. A nanorod is solid without being neither hollow with one or two open
ends,
nor hollow with two sealed ends.
The term "carbides"refers to well known compounds of composition QC or Q2
C. Generally, Q is selected from the group consisting of transition metals
(groups 3b,
4b, Sb,6b,7b,8 of periods 4, 5,6 of the Periodic Table) rare earths
(lanthanides) and
actinides. More preferably, Q is selected from the group consisting of B, Ti,
Nb, Zr,
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Hf, Si, Al, Mo, V and W. The term also includes crystalline structures
characterized
by x-ray diffraction (XRD) as QC or QZC by themselves and/or in combinations
with
Q or C, for instance remaining after the sythesis step is substantially
complete.
Carbides can be detected and characterized by x-ray diffraction (XRD). When,
as is
contemplated within the scope of this invention, the carbides are prepared by
carburization of metal oxides or by oxidation of elemental carbon, a certain
amount of
"non- stoichiometric" carbide may appear, but the diffraction pattern of the
true
carbides would still be present. Metal rich non- stoichiometric carbides, such
as might
be formed from a synthesis wherein the metal is carburized, are simply missing
a few
of the carbons that the metal matrix can accommodate. Carbon rich non-
stoichiometric carbides comprise domains of stoichiometric carbides embedded
in the
original carbon structure. Once the carbide crystallites are large enough they
can be
detected by XRD.
Carbides also refers to interstitial carbides as more specifically defined in
"Structural Inorganic Chemistry" by A.F. Wells, 4th Edition, Clarendon Press,
Oxford
1975 and in "The Chemistry of Transition Metal Carbides and Nitrides", edited
by
S.T. Oyama, a Blackie Academic & Professional publication, both of which are
incorporated herein by reference as if set forth in full.
The term "carbides-based nanorod" refers to a Q-based nanorod
predominantly having a diameter greater than 2.0 nm but less than 50 nm,
wherein Q
is an element capable of forming a carbide, Q being selected from the group
consisting of B, Ti, Ta, Nb, Zr, Hf, Si, Al, Mo, V, W, and having an aspect
ratio from
5 to 500. When the carbide nanorod has been made by conversion of the carbon
of
the nanotube to carbide compounds then the conversion has been substantially
complete.
The term "oxycarbides-based nanorod" refers to an M- based nanorod having
a substantially uniform diameter greater than 1.0 nm but less than or equal to
100 nm,
wherein M is any metal capable of forming a oxycarbide such as Ti, Ta, Nb, Zr,
Hf,
Mo, V, W, B, Si and Al. It has an aspect ratio of 5 to 500.
Oxycarbides, unlike carbides, are inherently non-stoichiometric. The
oxycarbides of the present invention have the formula:
MnCx_Y~Y
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where M selected from the group consisting of transition metals (groups 3b,
4b, Sb,
6b, 7b, 8 of periods 4, 5, 6 of the Periodic Table) rare earths (lanthanides)
and
actinides, and more preferably Ti, Ta, Hf, Nb, Zr, Mo, V, W, Si, Al, B; n and
x are
selected to satisfy a known stoichiometry of a carbide of Q, where Q is the
same as
M; y is less than x and the ratio [y/(x-y)] is at least 0.02 and less than 0.9
and more
preferably between 0.05 and 0.50. The term "oxycarbides" also includes but is
not
limited to products formed by oxidative treatments of carbides present in
connection
with carbon nanotubes as a source of carbon or in connection with carbide
nanorods
as a source of carbides. Oxycarbides can also include products formed by
carburization of metal oxides. Oxycarbides also comprise mixtures of unreacted
carbides and oxides, chemisorbed and physisorbed oxygen. M is selected from
the
group consisting of Mo, W, V, Nb, Ta, Ti, Zr, Hf, B, Si and Al. More
specifically,
oxycarbides have a total amount of oxygen sufficient to provide at least 25%
of at
least one monolayer of absorbed oxygen as determined by temperature programmed
desorption (TPD) based on the carbide content of the carbide source.
Oxycarbides
also refer to compounds of the same name as defined in "The Chemistry of
Transition
Metal Carbides and Nitrides", edited by S.T. Oyama, a Blackie Academic &
Professional publication incorporated herein by referenced as if set forth in
full.
Examples of oxycarbides include polycrystalline compounds, wherein M is a
metal
preferably in two valent states. M can be bonded to another metal atom or only
to an
oxygen or only to a carbon atom. However, M is not bonded to both an oxygen
and
carbon atoms.
The term "aggregate" refers to a dense, microscopic particulate structure.
More specifically, the term "assemblage" refers to structures having
relatively or
substantially uniform physical properties along at least one dimensional axis
and
desirably having relatively or substantially uniform physical properties in
one or more
planes within the assemblage, i.e. they have isotropic physical properties in
that plane.
The assemblage may comprise uniformly dispersed individual interconnected
nanotubes or a mass of connected aggregates of nanotubes. In other
embodiments,
the entire assemblage is relatively or substantially isotropic with respect to
one or
more of its physical properties. The physical properties which can be easily
measured
and by which uniformity or isotropy are determined include resistivity and
optical
density.
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The term "pore" traditionally refers to an opening or depression in the
surface
of a catalyst or catalyst support. Catalysts and catalyst supports comprising
carbon
nanotubes lack such traditional pores. Rather, in these materials, the spaces
between
individual nanotubes behave as pores and the equivalent pore size of nanotube
aggregates can be measured by conventional methods (porosimetry) of measuring
pore size and pore size distribution. By varying the density and structure of
aggregates, one can vary the equivalent pore size and pore size distribution.
The term "micropore" refers to a pore which has a diameter of less than 2
micrometers.
The term "mesopore" refers to pores having a cross section greater than 2
nanometers.
The term "nonuniform pore structure" refers to a pore structure occurring
when individual discrete nanotubes are distributed in a substantially
nonuniform
manner with substantially nonuniform spacings between nanotubes.
The term "uniform pore structure" refers to a pore structure occurnng when
individual discrete nanotubes or nanofibers form the structure. In these
cases, the
distribution of individual nanotubes in the particles are substantially
uniform with
substantially regular spacings between the nanotubes. These spacings
(analogous to
pores in conventional supports) vary according to the densities of the
structures.
The term "bimodal pore structure" refers to a pore structure occurring when
aggregate particles of nanotubes and/or nanorods are bonded together. The
resulting
structure has a two-tiered architecture comprising a macrostructure of
nanotube
aggregates having macropores among the bundles of nanotube aggregates and a
microstructure of intertwined nanotubes having a pore structure within each
individual bundle of aggregate particles.
The term "surface area" refers to the total surface area of a substance
measurable by the BET technique.
The term "accessible surface area" refers to that surface area not attributed
to
micropores (i.e., pores having diameters or cross-sections less than 2 nm).
The term "isotropic" means that all measurements of a physical property
within a plane or volume of the structure, independent of the direction of the
measurement, are of a constant value. It is understood that measurements of
such
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non-solid compositions must be taken on a representative sample of the
structure so
that the average value of the void spaces is taken into account.
The term "internal structure" refers to the internal structure of an
assemblage
including the relative orientation of the fibers, the diversity of and overall
average of
nanotube orientations, the proximity of the nanotubes to one another, the void
space
or pores created by the interstices and spaces between the fibers and size,
shape,
number and orientation of the flow channels or paths formed by the connection
of the
void spaces and/or pores. According to another embodiment, the structure may
also
include characteristics relating to the size, spacing and orientation of
aggregate
particles that form the assemblage. The term "relative orientation" refers to
the
orientation of an individual nanotube or aggregate with respect to the others
(i.e.,
aligned versus non-aligned). The "diversity of and "overall average" of
nanotube or
aggregate orientations refers to the range of nanotube orientations within the
structure
(alignment and orientation with respect to the external surface of the
structure).
The term "physical property" means an inherent, measurable property of the
porous structure, e.g., surface area, resistivity, fluid flow characteristics,
density,
porosity, etc.
The term "relatively" means that ninety-five percent of the values of the
physical property when measured along an axis of, or within a plane of or
within a
volume of the structure, as the case may be, will be within plus or minus 20
percent of
a mean value.
The term "substantially" means that ninety-five percent of the values of the
physical property when measured along an axis of, or within a plane of or
within a
volume of the structure, as the case may be, will be within plus or minus ten
percent
of a mean value.
The terms "substantially isotropic" or "relatively isotropic" correspond to
the
ranges of variability in the values of physical properties set forth above.
The term "predominantly" has the same meaning as the term "substantially".
Carbon Nanotubes
The term nanotubes refers to various carbon tubes or fibers having very small
diameters including fibrils, whiskers, buckytubes, etc. Such structures
provide
significant surface area when incorporated into a structure because of their
size and
shape. Moreover, such nanotubes can be made with high purity and uniformity.
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Preferably, the nanotube used in the present invention have a diameter less
than 1 micron, preferably less than about 0.5 micron, and even more preferably
less
than 0.1 micron and most preferably less than 0.05 micron.
Carbon nanotubes can be made having diameters in the range of 3.5 to 70
nanometers.
The nanotubes, buckytubes, fibrils and whiskers that are referred to in this
application are distinguishable from continuous carbon fibers commercially
available
as reinforcement materials. In contrast to nanofibers, which have desirably
large, but
unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios
(L/D) of
at least 104 and often 106 or more. The diameter of continuous fibers is also
far larger
than that of fibrils, being always >1.0 ~m and typically 5 to 7 Vim.
Continuous carbon fibers are made by the pyrolysis of organic precursor
fibers, usually rayon, polyacrylonitrile (PAN) and pitch. Thus, they may
include
heteroatoms within their structure. The graphitic nature of "as made"
continuous
carbon fibers varies, but they may be subjected to a subsequent graphitization
step.
Differences in degree of graphitization, orientation and crystallinity of
graphite
planes, if they are present, the potential presence of heteroatoms and even
the absolute
difference in substrate diameter make experience with continuous fibers poor
predictors of nanofiber chemistry.
Carbon nanotubes are vermicular carbon deposits having diameters less than
1.0 ~, preferably less than 0.5 ~, even more preferably less than 0.2 ~ and
most
preferably less than 0.05 p. They exist in a variety of forms and have been
prepared
through the catalytic decomposition of various carbon-containing gases at
metal
surfaces. Such vermicular carbon deposits have been observed almost since the
advent of electron microscopy. A good early survey and reference is found in
Baker
and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14,
1978, p. 83 and Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993),
each of
which are hereby incorporated by reference. (see also, Obelin, A. and Endo,
M., J. of
Crystal Growth, Vol. 32 (1976), pp. 335-349, hereby incorporated by
reference).
United States Patent No. 4,663,230 to Terment, hereby incorporated by
reference, describes carbon nanotubes or fibrils that are free of a continuous
thermal
carbon overcoat and have multiple ordered graphitic outer layers that are
substantially
parallel to the fibril axis. As such they may be characterized as having their
c-axes,
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the axes which are perpendicular to the tangents of the curved layers of
graphite,
substantially perpendicular to their cylindrical axes. They generally have
diameters
no greater than 0.1 ~ and length to diameter ratios of at least 5. Desirably
they are
substantially free of a continuous thermal carbon overcoat, i.e.,
pyrolytically
deposited carbon resulting from thermal cracking of the gas feed used to
prepare
them. The Tennent invention provided access to smaller diameter fibrils,
typically 35
to 700 !x(0.0035 to 0.0700 and to an ordered, "as grown" graphitic surface.
Fibrillar
carbons of less perfect structure, but also without a pyrolytic carbon outer
layer have
also been grown.
United States Patent No. 5,171,560 to Tennent et al., hereby incorporated by
reference, describes carbon nanotubes free of thermal overcoat and having
graphitic
layers substantially parallel to the fibril axes such that the projection of
said layers on
said fibril axes extends for a distance of at least two fibril diameters.
Typically, such
fibrils are substantially cylindrical, graphitic nanotubes of substantially
constant
diameter and comprise cylindrical graphitic sheets whose c-axes are
substantially
perpendicular to their cylindrical axis. They are substantially free of
pyrolytically
deposited carbon, have a diameter less than 0.1 ~ and a length to diameter
ratio of
greater than 5. These fibrils are of primary interest in the invention.
When the projection of the graphitic layers on the nanotube axis extends for a
distance of less than two nanotube diameters, the carbon planes of the
graphitic
nanotube, in cross section, take on a herring bone appearance. These are
termed
fishbone fibrils. Geus, U.S. Patent No. 4,855,091, hereby incorporated by
reference,
provides a procedure for preparation of fishbone fibrils substantially free of
a
pyrolytic overcoat. These carbon nanotubes are also useful in the practice of
the
invention.
According to one embodiment of the invention, oxidized nanofibers are used
to form rigid porous assemblages. McCarthy et al., U.S. Patent Application
Serial
No. 351,967 filed May 15, 1989, hereby incorporated by reference, describes
processes for oxidizing the surface of carbon nanotubes or fibrils that
include
contacting the nanotubes with an oxidizing agent that includes sulfuric acid
(HZS04)
and potassium chlorate (KC103) under reaction conditions (e.g., time,
temperature,
and pressure) sufficient to oxidize the surface of the fibril. The nanotubes
oxidized
according to the processes of McCarthy, et al. are non-uniformly oxidized,
that is, the
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carbon atoms are substituted with a mixture of carboxyl, aldehyde, ketone,
phenolic
and other carbonyl groups.
Nanotubes have also been oxidized nonuniformly by treatment with nitric
acid. International Application PCT/LJS94/10168 discloses the formation of
oxidized
fibrils containing a mixture of functional groups. Hoogenvaad, M.S., et al.
(''Metal
Catalysts supported on a Novel Carbon Support", Presented at Sixth
International
Conference on Scientific Basis for the Preparation of Heterogeneous Catalysts.
Brussels, Belgium, September 1994) also found it beneficial in the preparation
of
nanotube-supported precious metals to first oxidize the nanotube surface with
nitric
acid. Such pretreatment with acid is a standard step in the preparation of
carbon-
supported noble metal catalysts, where, given the usual sources of such
carbon, it
serves as much to clean the surface of undesirable materials as to
functionalize it.
In published work, McCarthy and Bening (Polymer Preprints ACS Div. of
Polymer Chem. 30 ( 1 )420( 1990)) prepared derivatives of oxidized nanotubes
or
fibrils in order to demonstrate that the surface comprised a variety of
oxidized groups.
The compounds they prepared, phenylhydrazones, haloaromaticesters, thallous
salts,
etc., were selected because of their analytical utility, being, for example,
brightly
colored, or exhibiting some other strong and easily identified and
differentiated
signal. These compounds were not isolated and are of no practical
significance.
The nanotubes may be oxidized using hydrogen peroxide, chlorate, nitric acid
and other suitable reagents.
The nanotubes within the structure may be further functionalized as set forth
in U.S. Patent Application No. 08/352,400, filed December 8, 1995, by Hoch and
Moy et al., entitled "Functionalized Fibrils", hereby incorporated by
reference.
Carbon nanotubes of a morphology similar to the catalytically grown fibrils or
nanotubes described above have been grown in a high temperature carbon arc
(Iijima,
Nature 354 56 1991, hereby incorporated by reference). It is now generally
accepted
(Weaver, Science 265 1994, hereby incorporated by reference) that these arc-
grown
nanofibers have the same morphology as the earlier catalytically grown fibrils
of
Tennent. Arc grown carbon nanofibers are also useful in the invention.
Nanotube Aggregates and Assemblages
The "unbonded" precursor nanotubes may be in the form of discrete
nanotubes, aggregates of nanotubes or both.
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When carbon nanotubes are used, the aggregates, when present, are generally
of the bird's nest, combed yarn or open net morphologies. The more
''entangled" the
aggregates are, the more processing will be required to achieve a suitable
composition
if a high porosity is desired. This means that the selection of combed yarn or
open net
aggregates is most preferable for the majority of applications. However,
bird's nest
aggregates will generally suffice.
As with all nanoparticles, nanotubes aggregate in several stages or degrees.
Catalytically grown nanotubes produced according to U.S.S.N. 08/856,657 filed
on
May 15, 1997 are formed in aggregates substantially all of which will pass
through a
700 micron sieve. About 50% by weight of the aggregates pass through a 300
micron
sieve. The size of as-made aggregates can, of course, be reduced by various
means,
but such disaggregation becomes increasingly difficult as the aggregates get
smaller.
Nanotubes may also be prepared as aggregates having various morphologies
(as determined by scanning electron microscopy) in which they are randomly
entangled with each other to form entangled balls of nanotubes resembling bird
nests
("BN"); or as aggregates consisting of bundles of straight to slightly bent or
kinked
carbon nanotubes having substantially the same relative orientation, and
having the
appearance of combed yarn ("CY") e.g., the longitudinal axis of each nanotube
(despite individual bends or kinks) extends in the same direction as that of
the
surrounding nanotubes in the bundles; or, as, aggregates consisting of
straight to
slightly bent or kinked nanotubes which are loosely entangled with each other
to form
an "open net" ("ON") structure. In open net structures the extent of nanotube
entanglement is greater than observed in the combed yarn aggregates (in which
the
individual nanotubes have substantially the same relative orientation) but
less than
that of bird nest. CY and ON aggregates are more readily dispersed than BN
making
them useful in composite fabrication where uniform properties throughout the
structure are desired.
The morphology of the aggregate is controlled by the choice of catalyst
support. Spherical supports grow nanotubes in all directions leading to the
formation
of bird nest aggregates. Combed yarn and open nest aggregates are prepared
using
supports having one or more readily cleavable planar surfaces, e.g., an iron
or iron-
containing metal catalyst particle deposited on a support material having one
or more
readily cleavable surfaces and a surface area of at least 1 square meters per
gram.
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Moy et al., U.S. Application Serial No. 08/469,430 entitled "Improved Methods
and
Catalysts for the Manufacture of Carbon Fibrils", filed June 6, 1995, hereby
incorporated by reference, describes nanotubes prepared as aggregates having
various
morphologies (as determined by scanning electron microscopy).
Further details regarding the formation of carbon nanotube or nanofiber
aggregates may be found in the disclosure of U.S. Patent No. 5,165,909 to
Tennent;
U.S. Patent No. 5,456,897 to Moy et al.; Snyder et al., U.S. Patent
Application Serial
No. 149,573, filed January 28, 1988, and PCT Application No. US89/00322, filed
January 28, 1989 ("Carbon Fibrils") WO 89/07163, and Moy et al., U.S. Patent
Application Serial No. 413,837 filed September 28, 1989 and PCT Application
No.
US90/05498, filed September 27, 1990 ("Fibril Aggregates and Method of Making
Same") WO 91/05089, and U.S. Application No. 08/479,864 to Mandeville et al.,
filed June 7, 199 and U.S. Application No. 08/329,774 by Bening et al., filed
October 27, 1984 and U.S. Application No. 08/284,917, filed August 2, 1994 and
U.S.
Application No. 07/320,564, filed October 11, 1994 by Moy et al., all of which
are
assigned to the same assignee as the invention here and are hereby
incorporated by
reference.
Nanotube mats or assemblages have been prepared by dispersing nanofibers in
aqueous or organic media and then filtering the nanofibers to form a mat or
assemblage. The mats have also been prepared by forming a gel or paste of
nanotubes
in a fluid, e.g. an organic solvent such as propane and then heating the gel
or paste to
a temperature above the critical temperature of the medium, removing the
supercritical fluid and finally removing the resultant porous mat or plug from
the
vessel in which the process has been carried out. See, U.S. Patent Application
Number 08/428,496 entitled "Three-Dimensional Macroscopic Assemblages of
Randomly Oriented Carbon Fibrils and Composites Containing Same" by Tennent et
al., hereby incorporated by reference.
Extrudates of Carbon Nanotubes
In a preferred embodiment the carbon rigid porous structures comprise
extrudates of carbon nanotubes. Aggregates of carbon nanotubes treated with a
gluing
agent or binder are extruded by conventional extrusion methods into extrudates
which
are pyrolyzed or carbonized to form rigid carbon structures having bimodal
pore
structure. The bundles of carbon nanotubes are substantially intact except
that they
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have been splayed (e.g. by sonication) or partially unravelled to provide a
bimodal
pore structure. The space between bundles ranges from points of contact to
about 1
micron. Within bundles, spaces between carbon nanotubes range from 10 nm to 30
nm. The resulting rigid bimodal porous structure is substantially free of
micropores,
has surface areas ranging from about 250 m2/g to about 400 m2/g and a crush
strength
of about 20 psi for extrudates of 1 /8 inch in diameter. Carbon nanotube
extrudates
have densities ranging from about 0.5 g/cm3 to about 0.7 g/cm3, which can be
controlled by the density of the extrusion paste. The extrudates have liquid
absorption
volumes from about 0.7 cm3/g.
Gluing or binding agents are used to form the paste of carbon nanotubes
required for extrusion processes. Useful gluing or binding agents include
without
limitations cellulose, carbohydrates, polyethylene, polystyrene, nylon,
polyurethane,
polyester, polyamides, poly(dimethylsiloxane), phenolic resins and the like.
The extrudates obtained as described above can be further treated with mild
oxidizing agents such as hydrogen peroxide without affecting the integrity of
the rigid
porous carbon structures. Subsequently, the rigid porous structures can be
impregnated with catalytic particles by ion exchange, generally a preferred
method for
deposition of small size particles. Alternatively, the rigid porous carbon
structure can
also be impregnated with catalysts by incipient wetness, or physical or
chemical
adsorption.
Nanorods
The term nanorods refers to rod-like structures having a substantially solid
core, a surface and a diameter greater than 1.0 nm but less than 100 nm. The
structure
has an aspect ratio between 5 and 500 and a length between 2nm and SOp, and
preferably between 100nm and 20~. The disclosed nanorods are substantially
solid,
being neither hollow with one or two open ends, nor hollow with two sealed
ends.
Carbide Nanorods
Carbide-based nanorods can be prepared by using carbon nanotubes as a
source of carbon. For example, D. Moy and C.M. Niu have prepared carbide
nanorods or nanofibrils as disclosed in U.S. Application No. 08/414,369
incorporated
herein by reference as if set forth in full. They reacted Q-based gas with
carbon
nanofibrils or nanotubes to form, in situ, solid Q-based carbide nanofibrils
or
nanorods at temperatures substantially less than 1700°C and preferably
in the range of
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about 1000°C to about 1400°C, and more preferably at
approximately 1200°C. Q-
based gases were volatile compounds capable of forming carbides. Generally, Q
is
selected from the group consisting of transition metals (groups 3b, 4b, Sb,
6b, 7b, 8 of
periods 4, 5, 6) rare earths (lanthanides) and actinides. Preferably, Q was
selected
from the group consisting of B, Ti, Ta, Nb, Zr, Hf, Si, Al, Mo, V and W.
We call this conversion pseudotopotactic because, even though the dimensions
and crystalline orientations of the starting material and product differ, the
cylindrical
geometry of the starting nanotube is retained in the final nanorod and the
nanorods
remain separate and predominately unfused to each other. The diameters of the
resulting nanorods were about double that of the starting carbon nanofibrils
or
nanotubes ( 1 nm-1 OOnm).
Carbide nanorods have also been prepared by reacting carbon nanotubes with
volatile metal or non-metal oxide species at temperatures between 500°C
and 200°C
wherein the carbon nanotube is believed to act as a template, spatially
confining the
reaction to the nanotube in accordance with methods described in PCT/US
96/09675
by C.M. Lieber, incorporated herein by reference. Carbide nanorods formed by
methods wherein the carbon nanotube serves as a template are also useful in
the
present invention.
Because of the ease with which they can penetrate fibril aggregates and rigid
porous structures, volatile Q compounds are usually preferred. Volatile Q
precursors
are compounds having a vapor pressure of at least 20 torr at reaction
temperature.
Reaction with the volatile Q compound may or may not take place through a non-
volatile intermediate.
Other methods of preparing carbide nanorods include reductive carburization
in which the carbon nanotubes are reacted with Q-based volatile metal oxides
followed by passing a flow of gaseous CH4/H2 mixture at temperatures between
250°C and 700°C. In addition to Q-based metal oxides, volatile Q-
based compounds
useful in preparation of Q-based carbide nanorods include carbonyls and
chlorides
such as, for example, Mo(CO)6, Mo(V) chloride or W(VI)O chloride.
In a preferred method of making useful carbide nanorods for the present
invention, vapors of a volatile Q-based compound are passed over a bed of
extrudates
of carbon nanotubes in a quartz tube at temperatures from about 700°C
to about
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1000°C. By controlling the concentration of the Q-based compound, the
crystallization of the carbides is limited to the space of the nanotube.
In all the methods of providing carbide-based nanorods discussed above, the
extent of conversion of the carbon in carbon nanotubes to carbide nanorods can
be
controlled by adjusting the concentration of the Q-based compound, the
temperature
at which the reaction occurs and the duration of the exposure of carbon
nanotubes to
the volatile Q-based compound. The extent of conversion of the carbon from the
carbon nanotubes is between 40% and 100% and preferably around 95%. The
resulting carbide nanorods have an excellent purity level in the carbide
content, vastly
increased surface area and improved mechanical strength. The surface area of
the
carbide nanorods is from 1 to 400 and preferably 10 to 300m2/g.
Applications for compositions based on carbide nanorods include catalysts and
catalyst support. For example, compositions including carbide nanorods based
on
molybdenum carbide, tungsten carbide, vanadium carbide, tantalum carbide and
niobium carbide are useful as catalysts in fluid phase catalytic chemical
reactions
selected from the group consisting of hydrogenation, hydrodesulfurisation,
hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation,
hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization,
alkyation,
dealkyation and transalkylation.
Similarly, silicon carbide and aluminum carbide-based nanorods are especially
useful as catalyst supports for conventional catalysts such as platinum and
palladium,
as well as for other Q-based carbides such as molybdenum carbide, tungsten
carbide,
vanadium carbide and the like.
Oxycarbide Nanorods
Oxycarbide-based nanorods can be prepared from carbide nanorods. The
carbide nanorods are subjected to oxidative treatments known in the art. For
example,
oxidative treatments are disclosed in U.S. Patent No. 5,576,466 to Ledoux, et
al.; M.
Ledoux, et al. European Pat. Appln. No. 0396 475 A1, 1989; C. Pham-Huu, et
al., Ind.
Eng. Chem. Res. 34, 1107-1113, 1995; E. Iglesia, et al., Journal of Catalysis,
131,
523-544, 1991, incorporated herein by reference as if set forth in full. The
foregoing
oxidative treatments are applicable to the formation of oxycarbide nanorods as
well as
to the formation of nanotubes and/or nanorods comprising an oxycarbide portion
wherein the conversion of the carbide source is incomplete.
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Oxycarbide compounds present in an oxycarbide nanorod, and also present
when the conversion of the carbide source is incomplete, include oxycarbides
having
a total amount of oxygen sufficient to provide at least 25% of at least one
monolayer
of absorbed oxygen as determined by temperature programmed desorption (TPD)
based on the carbide content of the carbide source. For example, by subjecting
carbide nanorods to a current of oxidizing gas at temperatures of between
30°C to
500°C oxycarbide nanorods are produced. Useful oxidizing gases include
but are not
limited to air, oxygen, carbon dioxide, N20, water vapor and mixtures thereof.
These
gases may be pure or diluted with nitrogen and/or argon.
Compositions comprising oxycarbide nanorods are useful as catalysts in many
petrochemical and refining processes including hydrogenation,
hydrodesulfurisation,
hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation,
hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization,
alkylation,
dealkylation and transalkylation.
Supported Carbides and Oxycarbides
According to another embodiment of the present invention, by adjusting the
process parameters, for example, the temperature, the concentration of, and
the length
of exposure to the Q-based volatile compound, it is possible to limit the rate
of
conversion of the carbon in the carbon nanotube. Thus, it is possible to
provide
carbon nanotubes having a carbide portion where the location of the carbide
portion
can be engineered as desired. For example, the carbide portion of the carbon
nanotube can be located entirely on the surface of the carbon nanotube such
that only
parts of the surface comprise nanocarbide compounds. It is possible to have
the entire
surface of the carbon nanotube coated with carbides while the core of the
carbon
nanotube remains substantially carbon. Moreover, it is possible to control the
surface
coverage of carbon nanotubes with carbide compounds from 1 % to 99% of the
entire
surface area. An embodiment wherein the carbon nanotube comprises carbide
covering less than 50% of the surface of the carbon nanotube is preferred. Of
course,
at low percentages large areas of the carbon nanotube surface remain
uncovered.
Nevertheless, as long as the carbide portion of the carbon nanotube is
retained at the
surface, the morphology of the carbon nanotube remains substantially the same.
Similarly, by carefully controlling the process parameters it is possible to
turn
the carbide portion of the nanotube into a carbide nanorod thereby obtaining a
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nanotube-nanorod hybrid structure. The carbide portion can be located anywhere
on
the carbon nanotube. Partial conversion of carbon to carbide compounds
preferably
varies from about 20% to about 85% by weight. When the content of carbide
compounds in the carbon nanotube exceeds 85% by weight, then the carbon
nanotubes have been substantially converted to carbide nanorods. Once in
possession
of the teachings herein, one of ordinary skill in the art can determine as a
routine
matter and without the need for further invention or undue experimentation how
to
control the rate of conversion of carbon nanotubes to carbide nanorods in
order to
convert the carbon in the carbon nanotubes incompletely.
The embodiment of the invention where the carbon nanotubes contain a
carbide portion also encompasses providing the carbide portion of the carbon
nanotube in any manner now known or later developed. For example, in another
method of providing carbide compounds on carbon nanotubes or aggregates
thereof,
the Q-based metal or metal compound, preferably Mo, W or V is placed on the
carbon
nanotubes or aggregates directly and then pyrolyzed, leaving behind carbon
nanotubes
coated with carbide compounds.
In yet another method of providing carbide compounds on carbon nanotubes,
solutions of Q-based salts, such as, for example, salts of Mo, W or V are
dispersed
over the carbon nanotubes or aggregates thereof and then pyrolyzed, again
forming
carbide compounds primarily on the surface of the carbon nanotubes.
An embodiment wherein the core of the carbon nanotube remains carbon and
the location of the metallic carbides is limited is quite desirable as a
catalytic system.
The core of the carbon nanotube acts as a catalyst support or carrier for the
metallic
carbide catalyst.
In yet another embodiment of the invention, it is possible to transform the
core
of the carbon nanotubes into one metal carbide preferably silicon carbide or
aluminum
carbide at temperatures between 1100°C and 1400°C. Thereafter,
by bringing the
silicon carbide nanorod in contact with the volatile compound of another
metal, for
example, MoO, a mixed carbide nanorod is provided which has a silicon carbide
(preferably ~i SiC), core and another Q-based carbide portion. When Mo0 is
used for
example, the SiC nanorod can have a MoC portion that could be an outer layer
or a
MoC-based nanorod. Thus, the resulting nanorod is a mixed carbide-based
nanorod
wherein part of the nanorod is SiC-based and another portion is MoC-based.
There is
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likewise an advantageous presence of molybdenum silicide. The mixed carbide
nanotube or nanorods as discussed above are particularly suitable as catalyst
carriers
or directly as catalysts in high temperature chemical reactions, particularly
in the
petrochemical field.
In yet another embodiment of the improvement discussed above. it is possible
to subject the nanotube having a carbide portion to oxidative treatments such
that the
carbide portion of the nanotube further comprises an oxycarbide portion. The
oxycarbide portion comprises oxycarbide compounds located any place on, in and
within the carbon nanotube or carbide nanorod.
The oxycarbide compounds can be placed on the nanotube in any way now
known or later developed. Similarly, the nanotube having a carbide portion can
be
exposed to air or subjected to carburization or any other means of converting
the
carbide portion of the nanotube partially or completely into an oxycarbide
nanorod
portion. Thus, it is possible to provide a carbon nanotube which is partly
still a
carbon nanotube, partly a carbide nanorod and partly a oxycarbide nanorod also
referred to as a carbon-carbide-oxycarbide nanotube-nanorod hybrid.
Carbide and Oxycarbide Rigid Porous Structures
The invention also relates to rigid porous structures made from carbide
nanorods, oxycarbide nanorods, and supported carbide and oxycarbide carbon
nanotubes and methods for producing the same. The resulting structures may be
used
in catalysis, chromatography, filtration systems, electrodes, batteries and
the like.
The rigid porous structures according to the invention have high accessible
surface area. That is, the structures have a high surface area which are
substantially
free of micropores (i.e., pores having a diameter or cross-section less than 2
nm). The
invention relates to increasing the mechanical integrity and/or rigidity of
porous
structures comprising intertwined carbon nanotubes and/or carbide and/or
oxycarbide
nanorods. The structures made according to the invention have higher crush
strengths
than the conventional carbon nanotube or nanorod structures. The present
invention
provides a method of improving the rigidity of the carbon structures by
causing the
nanotubes and/or nanorods to form bonds or become glued with other nanotubes
and/or nanorods at the nanotube and/or nanorod intersections. The bonding can
be
induced by chemical modification of the surface of the nanotubes to promote
bonding,
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by adding ''gluing" agents and/or by pyrolyzing the nanotubes to cause fusion
or
bonding at the interconnect points.
The nanotubes or nanorods can be in the form of discrete nanotubes and/or
nanorods or aggregate particles of nanotubes and nanorods. The former results
in a
structure having fairly uniform properties. The latter results in a structure
having two-
tiered architecture comprising an overall macrostructure comprising aggregate
particles of nanotubes and/or nanorods bonded together and a microstructure of
intertwined nanotubes and/or nanorods within the individual aggregate
particles.
According to one embodiment, individual discrete nanotubes and/or nanorods
form the structure. In these cases, the distribution of individual nanotube
and/or
nanorod strands in the particles are substantially uniform with substantially
regular
spacing between strands. These spacings (analogous to pores in conventional
supports) vary according to the densities of the structures and range roughly
from 1 S
nm in the densest to an average 50-60 nm in the lightest particles (e.g.,
solid mass
formed from open net aggregates). Absent are cavities or spaces that would
correspond to micropores (<2 nm) in conventional carbon supports.
According to another embodiment, the distribution of individual nanotubes
and/or nanorods is substantially nonuniform with a substantially nonuniform
pore
structure. Nevertheless, there are no cavities or spaces corresponding to
micropores
which are frequently present in other catalysts and catalyst supports.
These rigid porous materials are superior to currently available high surface
area materials for use in fixed-bed reactors, for example. The ruggedness of
the
structures, the porosity (both pore volume and pore structure), and the purity
of the
carbide nanorods and/or oxycarbide nanorods are significantly improved.
Combining
these properties with relatively high surface areas provides a unique material
with
useful characteristics.
One embodiment of the invention relates to a rigid porous structure
comprising carbide nanorods having an accessible surface area greater than
about
l Omz/gm and preferably greater than 50 m2/gm, being substantially free of
micropores
and having a crush strength greater than about I lb. The structure preferably
has a
density greater than 0.5 g/cm3 and a porosity greater than 0.8 cm3/g.
Preferably, the
structure comprises intertwined, interconnected carbide nanorods and is
substantially
free of micropores.
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According to one embodiment, the rigid porous structure includes carbide
nanorods comprising oxycarbide compounds, has an accessible surface area
greater
than about 10 mz/gm, and preferrably greater than 50 m2/gm, is substantially
free of
micropores, has a crush strength greater than about 1 lb and a density greater
than 0.5
g/cm3 and a porosity greater than 0.8 cm3/g.
According to another embodiment the rigid porous structure includes
oxycarbide nanorods having an accessible surface area greater than about 10
mz/gm,
and preferably greater than 50 m2/gm, being substantially free of micropores,
having a
crush strength greater than about 1 lb, a density greater than O.Sg/cm3 and a
porosity
greater than 0.8 cm3/g.
According to yet another embodiment, the rigid porous structure includes
carbon nanotubes comprising a carbide portion. The location of the carbide
portion
can be on the surface of the carbon nanotube or any place on, in or within the
carbon
nanotube or the carbide portion can be converted into a carbide nanorod
forming a
carbon nanotube-carbide nanorod hybrid. Nevertheless, the catalytic
effectiveness of
these rigid porous structures is not affected by the carbide portion on the
resulting
composites. This rigid porous structures has an accessible surface area
greater than
about 10 m2/gm and preferably than 50 mz/gm, is substantially free of
micropores, has
a crush strength greater than about 1 lb, a density greater than O.Sg/cm3 and
a porosity
greater than 0.8 cm3/g.
In another related embodiment the rigid porous structure includes carbon
nanotubes having a carbide portion and also an oxycarbide portion. The
location of
the oxycarbide portion can be on the surface of the carbide portion or any
place on, in
or within the carbide portion.
Under certain conditions of oxidative treatment it is possible to convert a
portion of the carbide nanorod part of the carbon-carbide nanotube-nanorod
hybrid
into an oxycarbide. The rigid porous structure incorporating carbon-carbide-
oxycarbide nanotube-nanorod hybrids has an accessible surface area greater
than
about l Om2/gm, is substantially free of micropores, has a crush strength
greater than
about 1 lb, a density greater than O.Sg/cm3 and a porosity greater than 0.8
cm3/g.
According to one embodiment, the rigid porous structures described above
comprise nanotubes and/or nanorods which are uniformly and evenly distributed
throughout said rigid structures. That is, each structure is a rigid and
uniform
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assemblage of nanotubes and/or nanorods. The structures comprise substantially
uniform pathways and spacings between said nanotubes and/or nanorods. The
pathways or spacings are uniform in that each has substantially the same cross-
section
and are substantially evenly spaced. Preferably, the average distance between
nanotubes and/or nanorods is less than about 0.03 microns and greater than
about
0.005 microns. The average distance may vary depending on the density of the
structure.
According to another embodiment, the rigid porous structures described above
comprise nanotubes and/or nanorods which are nonuniformly and unevenly
distributed throughout said rigid structures. The rigid structures comprise
substantially nonuniform pathways and spacings between said nanorods. The
pathways and spacings have nonuniform cross-section and are substantially
unevenly
spaced. The average distance between nanotubes and/or nanorods varies between
0.0005 microns to 0.03 microns. The average distances between nanotubes and/or
nanorods may vary depending on the density of the structure.
According to another embodiment, the rigid porous structure comprises
nanotubes and/or nanorods in the form of nanotube and/or nanorod aggregate
particles
interconnected to form said rigid structures. These rigid structures comprise
larger
aggregate spacings between the interconnected aggregate particles and smaller
nanotube andlor nanorod spacings between the individual nanotubes and/or
nanorods
within the aggregate particles. Preferably, the average largest distance
between said
individual aggregates is less than about 0.1 microns and greater than about
0.001
microns. The aggregate particles may include, for example, particles of
randomly
entangled balls of nanotubes and/or nanorods resembling bird nests and/or
bundles of
nanotubes and/or nanorods whose central axes are generally aligned parallel to
each
other.
Another aspect of the invention relates to the ability to provide rigid porous
particulates or pellets of a specified size dimension. For example, porous
particulates
or pellets of a size suitable for use in a fluidized packed bed. The method
involves
preparing a plurality of nanotubes and/or nanorods aggregates, fusing or
gluing the
aggregates or nanotubes and/or nanorods at their intersections to form a large
rigid
bulk solid mass and sizing the solid mass down into pieces of rigid porous
high
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surface area particulates having a size suitable for the desired use, for
example, to a
particle size suitable for forming a packed bed.
General Methods of Making Rigid Porous Structures
The above-described rigid porous structures are formed by causing the
nanotubes and/or nanorods to form bonds or become glued with other nanofibers
at
the fiber intersections. The bonding can be induced by chemical modification
of the
surface of the nanofibers to promote bonding, by adding "gluing" agents and/or
by
pyrolyzing the nanofibers to cause fusion or bonding at the interconnect
points. U.S.
Patent Application No. 08/857,383 filed May 15, 1997, incorporated herein by
reference, describes processes for forming rigid porous structures from carbon
nanofibers or nanotubes. These processes are equally applicable to forming
rigid
porous structures including discrete unstructured nanotubes or nanotube
aggregates
comprising carbides and in another embodiment also oxycarbides, wherein the
carbon
nanotube morphology has been substantially preserved. These methods are also
applicable to forming rigid porous structures comprising carbide or oxycarbide
nanorods, unstructured or as aggregates. Additionally, these methods are also
applicable to forming rigid porous structures comprising hybrids of carbon-
carbide
nanotube-nanorods and/or carbon-carbide-oxycarbide nanotube-nanorods.
In several other embodiments rigid porous structures comprising carbide
nanorods are prepared by contacting a rigid porous carbon structure made of
carbon
nanotubes with volatile Q-based compounds under conditions sufficient to
convert all
of the carbon or only part of the carbon of the carbon nanotubes to carbide-
based
compounds.
The rigid, high porosity structures can be formed from regular nanotubes or
nanotube aggregates, either with or without surface modified nanofibers (i.e.,
surface
oxidized nanotubes). Surface oxidized nanotubes can be crosslinked according
to
methods described in U.S. Patent Application No. 08/856,657 filed on May 1 ~,
1997
and U.S. Patent Application No. 08/857,383 also filed on May 15, 1997, both
incorporated herein by reference, and then carbonized to from a rigid porous
carbon
structure having a uniform pore structure, substantially free of micropores.
Preferred Methods of Making
Carbide Based Rigid Porous Structures
There are many methods of preparing rigid porous structures comprising
carbide nanorods. In one embodiment the rigid porous carbon structures
prepared as
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described above are contacted with Q-based compounds under conditions of
temperature and pressure sufficient to convert the carbon nanotubes of the
rigid
porous carbon structure to carbide nanorods. The location of the carbide
portion of
the carbon nanotubes of the rigid porous carbide structure can be on the
surface of the
carbon nanotube or any place on, in or within the carbon nanotube, or when the
conversion is complete then the entire carbon nanotube is transformed into a
substantially solid carbon nanorod. Once in the possession of the teachings
herein,
one of ordinary skill in the art can determine as a routine matter and without
the need
for further invention or undue experimentation how to control the rate of
conversion
of carbon nanotubes present in the rigid porous carbon structure to a rigid
porous
carbide-based structure comprising carbon nanotubes having a carbide portion
which
can vary in location on the carbon nanotube and in an amount from about 20% to
about 85%, preferably in excess of 85% by weight.
The carbide-based rigid porous structures of the present invention have high
accessible surface areas between 10 m2/gm and 100 m2/gm and are substantially
free
of micropores. These structures have increased mechanical integrity and
resistance to
attrition by comparison to individual carbide-based nanorods. Carbide-based
rigid
porous structures have a density greater than 0.5 g/cm3 and a porosity greater
than 0.8
cm3/g. The structure has at least two dimensions of at least 10 microns and
not
greater than 2 cm. Depending on the pore structure of the starting rigid
porous carbon
structure, the porous structure of the carbide-based rigid porous structure
can be
uniform, nonuniform or bimodal.
When the rigid porous structure is uniform the average distance between said
carbide-based nanorods is less than 0.03 microns and greater than 0.005
microns. In
another embodiment the rigid porous structure comprises carbide-based nanorods
in
the form of interconnected aggregate particles wherein the distance between
individual aggregates ranges from point of contact to 1 p. When the carbide-
based
nanorod rigid porous structures was formed from rigid porous carbon structures
comprising nanotube aggregates, the structure has aggregate spacings between
interconnected aggregate particles and carbide nanorod spacings between
nanorods
within the aggregate particles. As a result the rigid porous structure has a
bimodal
pore distribution.
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One embodiment of the invention relates to rigid porous structures comprising
extrudates of aggregate particles of carbide nanorods, wherein the carbide
nanorods
are glued together with binding agents such as cellulose, carbohydrates,
polyethylene,
polystyrene, nylon, polyurethane, polyester, polyamides,
poly(dimethylsiloxane) and
phenolic resins. Without being bound by theory, it is believed that the
conversion of a
rigid porous carbon structure to a carbide-based rigid porous structure
whether
completely or partially is accomplished in pseudo-topotactic manner as
previously
discussed herein above.
Preferred Methods of Making
Oxycarbide Based Rigid Porous Structures
There are many methods of preparing rigid porous structures comprising
oxycarbide nanorods and/or nanotubes comprising a carbide portion and further
an
oxycarbide portion. In one embodiment the carbide based rigid porous
structures are
subjected to oxidative treatments as disclosed in the art and in U.S. Patent
No.
5,576,466 to Ledoux et al. issued November 13, 1996.
In another embodiment rigid porous structure comprising carbon nanotubes
having an oxycarbide portion and/or a carbide portion are prepared by
subjecting to
oxidative treatments disclosed in the art rigid pores carbon structures which
have been
partially converted to carbide nanorods.
In another embodiment discrete carbide nanorods are subjected to oxidative
treatments and then assembled into rigid porous structures according to
methods
similar to those disclosed in U.S. Patent Application No. 08/857,383 filed May
15,
1997 incorporated herein by referenced.
In yet another embodiment discrete carbon nanotubes or aggregate of carbon
nanotubes which have been partially converted to carbide nanorods are further
subjected to oxidative treatments and then assembled into rigid porous
structures
according to methods disclosed in U.S. Patent Application No. 08/857,383 filed
May
15, 1997.
Catalytic Compositions
The carbide and/or oxycarbide nanorods and nanotubes having carbide and/or
oxycarbide portions of the invention, have superior specific surface areas as
compared
to carbide and oxycarbide catalysts previously taught in the art. As a result,
they are
especially useful in the preparation of self supported catalysts and as
catalyst supports
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in the preparation of supported catalysts. The self supported catalysts of the
invention
include catalytic compositions comprising nanotubes and/or nanorods and rigid
porous structures comprising the same. Self supported catalysts of the
invention
constitute the active catalyst compound and can be used without any additional
physical support to catalyze numerous heterogenous reactions as more
specifically
described herein. The supported catalyst of the invention comprises a support
including a nanofiber and/or nanorod rigid porous structure and a
catalytically
effective amount of a catalyst supported thereon.
The uniquely high macroporosity of carbon nanotube structures, the result of
their macroscopic morphology, greatly facilitates the diffusion of reactants
and
products and the flow of heat into and out of the self supported catalysts.
This unique
porosity results from a random entanglement or intertwining of nanotubes
and/or
nanorods that generates an unusually high internal void volume comprising
mainly
macropores in a dynamic, rather than static state. Sustained reparability from
fluid
phase and lower losses of catalyst as fines also improves process performance
and
economics. Other advantages of the nanotube and/or nanorod structures as self
supported catalysts include high purity, improved catalyst loading capacity
and
chemical resistance to acids and bases. As self supported catalysts, carbon
nanotube
and/or nanorod aggregates provide superior chemical and physical properties in
porosity, surface are, separability and purity.
Self supported catalysts made of nanotubes and/or nanorods have a high
internal void volume that ameliorates the plugging problem encountered in
various
processes. Moreover, the preponderance of large pores obviates the problems
often
encountered in diffusion or mass transfer limited reactions. The high
porosities
ensure significantly increased catalyst life.
One embodiment of the invention relates to self supported catalyst which is a
catalytic-composition comprising carbide-based nanorods having a diameter
between
at least 1.0 nm and less than 100 nm, and preferably between 3.5 nm and 20 nm.
The
carbide-based nanorods have been prepared from carbon nanotubes which have
been
substantially converted to carbide nanorods. In the catalytic composition of
this
embodiment the carbide nanorods retain substantially the structure of the
original
carbon nanotubes. Thus, the carbide nanotubes can have uniform, nonuniform or
bimodal porous structure. These catalytic compositions can be used as
catalysts to
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catalyze reactions such as hydrogenation, hydrodesulfurisation,
hydrodenitrogenation,
hydrodemetallisation, hydrodeoxygenation, hydrodearomatization,
dehydrogenation,
hydrogenolysis, isomerization, alkylation, dealkylation and transalkylation.
Catalytic Compositions Supported on Aggregates
of Carbide and Oxycarbide Nanorods
Depending upon the application the rigid porous structures of the invention
can be used as both self supported catalysts and as catalyst supports. As is
true of
catalysts comprising regular nanotubes and/or nanorods, catalysts and catalyst
supports comprising the rigid porous structures of the invention have unique
properties. They are exceptionally mesoporous and macroporous. They are also
pure
and resistant to attrition, compression and shear and consequently can be
separated
from a fluid phase reaction medium over a long service life. The increased
rigidity of
the rigid porous structures of the present invention enables catalysts and
catalyst
supports comprising the structures to be used in fixed bed catalytic
reactions. A
packing containing the sized rigid structures can be formed and a fluid or gas
passed
through the packing without significantly altering the shape and porosity of
the
packing since the rigid structures are hard and resist compression.
Rigid structures formed from nanorod aggregates, preferably silicon carbide
and aluminum carbide-based nanorods are particularly preferred structures for
use as
catalyst supports.
The combination of properties offered by nanorod structures is unique. No
known catalyst supports combine such high porosity, high accessible surface
area and
attrition resistance. The combination of properties offered by the nanorod
structures
is advantageous in any catalyst system amenable to the use of a carbide
catalyst
support. The multiple nanorods that make up a nanorod structure provide a
large
number of junction points at which catalyst particles can bond to multiple
nanorods in
the nanorod structures. This provides a catalyst support that more tenaciously
holds
the supported catalyst. Further, nanorod structures permit high catalyst
loadings per
unit weight of nanorod. However, catalyst loadings are generally greater than
0.01
weight percent and preferably greater than 0.1, but generally less than 5%
weight
based on the total weight of the supported catalyst. Usually catalyst loadings
greater
than 5% by weight are not useful, however catalyst loadings greater than 5% by
weight of active catalyst based on the total weight of the supported catalyst
are easily
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within the contemplation of the invention, i.e., loadings in excess of 100
weight
percent based on the weight of the support of the invention, owing to the
porosity of
nanorod structures and other factors discussed herein. Desirable hydrogenation
catalysts are the platinum group (ruthenium, osmium, rhodium, iridium,
palladium
and platinum or a mixture thereof) and, preferably, palladium and platinum or
a
mixture thereof. Group VII metals including especially iron, nickel and cobalt
are also
attractive hydrogenation catalysts.
Oxidation (including partial oxidation) catalysts may also be supported on
carbide and oxycarbide nanotubes and nanotube structures. Desirable metallic
oxidation catalysts include, not only members of the platinum group enumerated
above, but also, silver and the group VIII metals. Oxidation catalysts also
include
metal salts known to the art including salts of vanadium, tellurium,
manganese,
chromium, copper, molybdenum and mixtures thereof as more specifically
described
in "Heterogeneous Catalytic Reactions Involving Molecular Oxygen," by
Golodets,
G.L& Ross, J.R.H, Studies in Surface Science, 15, Elsevier Press, NYC 1983.
Active catalysts include other carbide compounds such as carbides of Ti, Ta,
Hf, Nb, Zr, Mo, V and W. These supported carbides are particularly useful for
hydrogenation, hydrodesulfurisation, hydrodenitrogenation,
hydrodemetallisation,
hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis,
isomerization, alkylation, dealkylation and transalkylation.
Because of their high purity, carbide nanorod aggregates exhibit high
resistance to attack by acids and bases. This characteristic is advantageous
since one
path to regenerating catalysts is regeneration with an acid or a base.
Regeneration
processes can be used which employ strong acids or strong bases. This chemical
resistance also allows the carbide supports of the invention to be used in
very
corrosive environments.
The supported catalysts are made by supporting a catalytically effective
amount of catalyst on the rigid nanorod structure. The term "on the nanotube
and/or
nanorod structure" embraces, without limitation, on, in and within the
structure and
on the nanotubes and/or nanorods thereof. The aforesaid terms may be used
interchangeably. The catalyst can be incorporated onto the nanotube and/or
nanorod
or aggregates before the rigid structure is formed, while the rigid structure
is forming
(i.e., add to the dispersing medium) or after the rigid structure is formed.
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Methods of preparing heterogeneous supported catalysts of the invention
include adsorption, incipient wetness impregnation and precipitation.
Supported
catalysts may be prepared by either incorporating the catalyst onto the
aggregate
support or by forming it in situ and the catalyst may be either active before
it is placed
in the aggregate or activated in situ.
The catalyst, such as a coordination complex of a catalytic transition metal,
such as palladium, rhodium or platinum, and a ligand, such as a phosphine, can
be
adsorbed by slurrying the nanorods in a solution of the catalyst or catalyst
precursor
for an appropriate time for the desired loading.
These and other methods may be used in forming the catalyst supports. A
more detailed description of suitable methods for making catalyst supports
using
nanotube structures is set forth in U.S. Application Serial No. 08/857,383 by
Moy et
al. entitled ''rigid Porous Carbon Structures, Methods of Making, Methods of
Using
and Products Containing Same" filed May 15, 1997, hereby incorporated by
reference.
Preferred Catalytic Compositions and Their Uses
One embodiment of the invention relates to a catalyst comprising a
composition including a multiplicity of oxycarbide-based nanorods. Each
nanorod
has substantially uniform diameters between 3.5 nm and 20 nm. As previously
described, the oxycarbide-based nanorods have a substantially solid core, form
a
substantially polycrystalline solid and the individual nanorods are
predominantly
unfused.
Another embodiment relates to a catalyst comprising a rigid porous structure
including oxycarbide-based nanorods as described above. Each catalytic
composition
can be used as a catalyst in a fluid phase reaction selected from the group
consisting
of hydrogenation, hydrodesulfurisation, hydrodenitrogenation,
hydrodemetallisation,
hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis,
isomerization, alkylation, dealkylation and transalkylation.
Another embodiment of the invention relates to a catalyst comprising a
composition including a multiplicity of Q-based nanorods, wherein Q is
selected from
the group consisting of B, Si, Al, Ti, Ta, Nb, Zr, Hf, Mo, V and W. The
resulting
carbide nanorods can be distributed nonuniformly, uniformly or can be in the
form of
interconnected aggregate particles.
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In a related embodiment, the catalyst comprises a rigid porous structure based
on the Q-based nanorods described above which have been formed into extrudates
and connected by gluing agents or in any other manner sufficient to form the
rigid
porous structure. Each catalytic composition discussed immediately above can
be
used as catalysts in a fluid phase reaction selected from the group consisting
of
hydrogenation, hydrodesulfurisation, hydrodenitrogenation,
hydrodemetallisation,
hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis,
isomerization, alkylation, dealkylation and transalkylation.
Another embodiment relates to a catalyst comprising a composition including
a multiplicity of carbide-based nanorods which further comprise oxycarbide
compounds any place on, in or within the nanorod, preferably on the surface.
In a related embodiment the catalyst comprises a rigid porous structure
including the carbide-based nanorods comprising oxycarbides which have been
formed into extrudates connected into the rigid porous structure by gluing
agents or in
any other manner sufficient to form the rigid porous structure. Each catalytic
compositions discussed immediately above can be used as a catalyst in a fluid
phase
reaction selected from the group consisting of hydrogenation,
hydrodesulfurisation,
hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation,
hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization,
alkyation,
dealkyation and transalkyation.
Another embodiment relates to a catalyst comprising a composition including
a multiplicity of carbon nanotubes having substantially uniform diameters. In
this
embodiment the carbon nanotubes comprise carbide compounds anywhere on, in or
within the nanotubes, but preferably on the surface of the nanotubes. In yet
another
related embodiment the carbon nanotubes additionally comprise oxycarbide
compounds on, in or within the nanotubes, but preferably on the surface as
more
specifically described in section "Supported Carbides and Oxycarbides" of the
of the
specification. In these embodiments the nanotube morphology is substantially
retained.
In a related embodiment the catalyst comprises a rigid porous structure
including carbon nanotubes comprising carbide compounds and, in another
embodiment, also oxycarbide compounds as described above. Each rigid porous
structure is useful as a catalyst in a fluid phase reaction to catalyze a
reaction selected
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from the group consisting of hydrogenation, hydrodesulfurisation,
hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation,
hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization,
alkyation,
dealkyation and transalkyation.
In another embodiment the catalytic composition includes a multiplicity of
carbon nanotubes having a carbide portion which has been converted to a
carbide
nanorod forming a nanotube-nanorod hybrid structure. In another related
embodiment, the catalytic composition includes a multiplicity of carbon
nanotubes
having a carbide nanorod portion and in addition also an oxycarbide portion
which
has been converted to an oxycarbide nanorod. In yet other related embodiments
the
foregoing carbon nanotubes can be included in rigid porous structures, wherein
the
carbon nanotubes are formed into extrudates and/or are otherwise connected to
form
rigid porous structures. The catalytic compositions are useful as catalysts in
a fluid
phase reaction selected from the group consisting of hydrogenation,
hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation,
hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis,
isomerization, alkyation, dealkyation and transalkytion.
EXAMPLES
The invention is further described in the following examples. The examples
are illustrative of some of the products and methods of making the same
falling within
the scope of the present invention. They are, of course, not to be considered
in any
way restrictive of the scope of the invention. Numerous changes and
modification
can be made with respect to the invention. The materials used in the examples
hereinbelow are readily commercially available.
In all of the experiments which follow the source of carbon was provided by
aggregates of carbon nanotubes as manufactured by Hyperion Catalysis
International
of Cambridge, Mass. The aggregates of carbon nanotubes were of the cotton
candy
("CC") also known as combed yarn ("CY") type as described in the section
entitled
"Nanotube Aggregates and Assemblages" herein above.
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EXAMPLE 1
Preparation of Moybdenum Carbide Precursors by Impregnation of Carbon Nanotube
Aggregates with Molybdenum Acetyl Acetonate
Five grams of powder samples of CC aggregates having porosity of 6.~ cc/gm
were impregnated by the incipient wetness with 35 cc of an ethanol solution
containing the correct amount of Mo02(C; H~ 02)Z or molybdenum acetyl
acetonate
(herein referred to as Moacac) necessary for the desired C:Mo atom ratio
loading.
The resulting mixture was dried at 110°C at full vacuum for 18 hours
during which
the Mo precursor decomposed to a mixture of molybdenum suboxides, generally
designated as Mo03_X, wherein x is 0 orl . The sample was set aside for
conversion to
carbide catalysts by careful calcination under an inert atmosphere as
described in
Examples 5, 6 or 7 hereinbelow.
EXAMPLE 2
Preparation of Moylbdenum Carbide Precursors by Impregnation
of Carbon Nanotube Aggregates with Ammonium Moybdate
A similar procedure as used in Example 1 above was followed, except that the
impregnating solutions were aqueous solutions containing the correct amount of
ammonium heptamolybdate tetrahydrate or (NH4) 6Mo~ 024.4Hz0 herein referred to
as ammonium molybdate necessary for the desired C:Mo atom ratio loading. The
resulting mixtures were dried at 225°C in full vacuum for 18 hours
during which the
heptamolybdate compound was decomposed to Mo03. The sample was set aside for
conversion to carbide catalysts by careful calcination under an inert
atmosphere as
more particularly described in Examples 5, 6 and 7 herein.
EXAMPLES 3
Preparation of Molybdenum Carbide Extrudate Precursors by Impregnation with
Molybdenum Acetyl Acetonate or Ammonium Molybdate
CC or CY type aggregates were oxidized with nitric acid as described in U.S.
Application Serial No. 08/352, 400 filed December 8, 1994 entitled
"Functionalized
Nanotubes" to form oxidized CC aggregates having an acid titer of about 0.6
mg/g).
Five grams of the oxidized CC type aggregates of carbon nanotubes were
well-mixed with either an ethanol solution of Moacac or an aqueous solution of
ammonium heptamolybdate tetrahydrate, each solution containing the correct
amount
of Mo compound necessary for the desired C:Mo loading. The mixing was
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accomplished by kneading in a Braybender kneader until the paste had a
homogeneous consistency. The excess solvent was removed from the kneaded
sample by evaporation until a solids content of from about 8 to about 10% by
weight
was obtained. The material was then extruded by using an pneumatic gun
extruder.
The extrudates were about I /8" diameter and several centimeters in length.
The
extrudates were then dried at 200°C in air for 18 hours during which
some shrinkage
occurred. The dried extrudates were then broken into pieces of about 1/16 inch
by I/4
inch which were set aside for conversion to carbide catalysts by careful
calcination as
described in examples 5, 6 and 7 herein.
EXAMPLE 4
Preparation of Molybdenum Carbide Precursor by Mixing Carbon Nanotube
Aggregates with Ammonium Molybdate or Molybdenum Oxide
As grown CC or CY aggregates were oxidized with nitric acid as described in
example 3 to form oxidized CC aggregates having an acid titer of about 0.6
mg/g.
Five grams of oxidized CC type aggregates of carbon nanotubes were
physically admixed with the correct amount of either ammonium heptamolybdate
tetrahydrate or Mo03 necessary for the desired C:Mo atom ratio by kneading the
sample in a mortar and pestle. A small amount of wetting agent such as water
or
ethylene glycol, was added periodically to keep the oxidized carbon nanotube
powder
dusting under control and to facilitate the contact between the molybdenum
precursor
particles and the carbon nanotube aggregates. After the mix was kneaded to a
homogeneous thick paste, the excess solvent was removed by gentle warming
while
continuing to knead the sample. The mixture was then dried at 200°C for
14 hours in
air and set aside for conversion to carbide be careful calcination as
described in
examples 5, 6 and 7 herein.
EXAMPLE 5
Calcination of Molybdenum Carbide Precursors at 600°C or
625°C
Weighed samples of molybdenum carbide precursors were loaded into
porcelain boats which were then placed horizontally in a I inch quartz tube.
The tube
and boat assembly were placed in a high temperature furnace equipped with a
programmable temperature controller and a movable thermocouple. The
thermocouple was located directly in contact with the end of the boat. The
sample
was heated under a slow flow, i.e., at several standard cc's/min of argon at a
heating
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rate of 5°C/min to 200°C and thereafter at 1°C/min to the
final temperature of 600°C
or 625°C. The sample was held at this temperature for 18 hours. Since
pure Mo2C
reacts violently with atmospheric oxygen, after cooling in argon to ambient
temperature, the samples were passivated by passing 3% OZ/Ar over them for 1
hour.
EXAMPLE 6
Calcination of Molybdenum Carbide Carbon Precursors at 800°C
The same procedure as described in Example 5 above was followed up to
600°C. The samples were then held at 600°C for one hour.
Thereafter, heating was
resumed at the same rate of 1 °C/min to 800°C and held at that
temperature for another
3 hours. After cooling in argon, the samples were passivated using 3% 02/Ar.
EXAMPLE 7
Calcination of Molybdenum Carbide Carbide Precursors at 1000°C
The same procedure as described in Example 6 above was followed up to
800°C, at which temperature the samples were held for 1 hours.
Thereafter, heating
of the samples was resumed at the rate of 1 °C/min to 1000°C,
where the temperature
was maintained for 0.5 hours. After cooling in argon, the samples were
passivated
using 3% 02/Ar.
Results of Eamples 1-7
Unsupported carbide nanorods and carbide nanoparticles supported on carbon
nanotubes were prepared according to Examples 1 to 7 above. Table 1 below
summarizes the experimental conditons and XRD results for selected
experiments.
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TABLE 1. SUMMARY OF RESULTS FOR MOLYBDENUM CARBIDE
PREPARATIONS
2 Mo03 + 7 C --~ Mo2C + 6 CO
C:Mo Weight loss
SAMPLE METHOD T° C initial (theory PHASES,
XRD
1 Moacac (s)b 600 4 (94)e27 (44) C, MoO~, Mo~C
(hex)
2 Mo03 (s)a 800 35 (38)e23 (35) C, MoaC (cub)
3 Mo03 (s)a 800 40 (34)ena C, MozC (hex)
4 Moacac (s)b 800 10 (85)e31 (32) C, Mo~C (hex)
5 Moacac (s)b 800 20 (57)e27 (22) MozC (hex/cub),
Mo
6 Mo03 (S)b 1000 10 (85)e41 (32) Mo~C (hes).
Mo
7 Mo03 (s)b 1000 20 (57)e27 (22) Mo~C (hex/cub),
Mo
8 Mo03 (s)b 1000 10 (85)e38 (32) C, MoaC (hex)
9 Mo03 (s)b 625 30 (43)e20 (17) C, Mo,C (hex/cub)
10 Mo03 (s)b 625 20 (57) e 27 C, MoZC (hex),
(22)
MoOz
11 Mo03 (s)b 1000 50 (28)e12 (11) C, MozC (hex/cub)
12 Mo03 (s) 800 3.5 55 (55) MoZC (hex/cub)
(100)e
a Impregnated powder of aggregates of carbon nanotubes
b Impregnated extrudates of aggregates of carbon nanotubes
Powder of aggregates of carbon nanotubes physically mixed with Mo precursor
a Calculated MoZC loading in final calcined product assuming full conversion
of Mo precursor
to Mo~C
The chemical reaction followed by all experiments summarized in Table 1
above is set forth at the top. In the method column, there is a listing of
molybdenum
precursors which were converted to Mo2C by reaction with carbon nanotubes.
Moacac refers to molybdenyl acetylacetonate and Mo03 refers to molybdenum
trioxide. "(s)" refers to the solid phase of the molybdenum precursor.
Superscripts a,
b and c refer to methods of dispersing the reactants as described in examples
?. 3 and
4, respectively. T°C refers to the final calcination temperature of the
reaction
temperature cycle. "C:Mo initial " refers to the atomic ratio of C:Mo in the
original
reaction mixture before conversion to a carbide compound. For example, the
stoichiometric atom ratio to produce pure carbide with no excess C or Mo,
i.e., pure
Mo2C is 3.5. The number following in parentheses is the calculated loading of
the
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MozC contained in the resulting materials. "Weight loss (theory" refers to the
theoretical weight loss according to the equation at the top of Table 1.
"Phases, XRD"
shows the compounds found in the X-ray diffraction (XRD) analyses. Mo~C exists
in
two distinct crystallographic phases, hexagonal and cubic.
Table 2 below summarizes the XRD results for the samples of Table 1.
T ABLE 2. SUMMARY OF XRD RESULTS
MozC Mo2C Mo02
Sample (hex) (cubic)
MoC > 100 nm
1 1520 nm Minor Component
2 5~8 nm
3 5~8 nm
1015 nm
15---20 nm ~ 15 nm
20 nm
7 3 63 8 nm
g 810 nm 810 nm
9 18 nm Minor Component
2025 nm 5~8 nm
11 35nm
12 26nm
Table 2 summarizes the XRD results for the experiments summarized in Table
1, identifies the compounds made, the phases present, and the calculated
average
10 particle size for the different phases.
The average particle size is a volume-biased average size, such that the value
of one large particle counts more heavily than several medium particles and
much
more than the volume of many small particles. This is a conventional procedure
which is well known to those familiar with XRD methods.
Discussion of Results of Examples 1-7
A. Unsupported Mo~C Nanoparticles and Nanorods
Samples 1 and 12 provided the clearest evidence of the formation of free-
standing Mo2C nanorods and nanopartieles. These were obtained by reacting
stoichiometric or near stoichiometrix mixtures of Mo03 and carbon nanotubes,
either
as powder or as extrudates. Product identification and morphologies were
obtained
by SEM, HRTEM and XRD. In Example 1, with about 15% excess of C, the major
product was identified by XRD as the hexagonal phase of Mo~C. MoO~ and
graphitic
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carbon were seen as minor components. SEM showed the presence of both nanorods
(~10-15 nm diameter) and nanoparticles (~20 nm).
Samples 11 and 12 resulted by reacting carbon nanotubes either with a
stoichiometric mixture of well-dispersed Mo03 powder, or with impregnated
ammonium molybdate.
More evidence of the formation of Mo2C nanorods and nanoparticles was
obtained in Sample 12, which resulted by reacting of a stoichiometric mixture
of
Mo03 and powder of carbon nanotubes. XRD, SEM and HRTEM analyses showed
formation of both Mo2C nanorods and nanoparticles. The SEM analyses showed a
network of nanorods with nanoparticles distributed within the network as shown
in
Figure 1. Accurate dimensions of carbide nanorods have been obtained by HRTEM
as shown in Figure 2, which shows carbide nanorods having diameters similar to
those of carbon nanotubes, namely, about 7 nm. The carbide nanoparticles
particles
ranged from about 7 to about 25 nm in diameter.
Sample 12, which was a stoichiometric mixture, was studied in more detail in
order to learn the course of the reaction. The reaction was tracked by
thermogravimetric analysis (TGA) as shown in Figure 4. Figure 4 shows that the
stoichiometric reaction has occurred in two distinct steps, namely, reduction
of Mo03
by carbon to Mo02 at from about 450 to about 550°C, followed by further
reduction
to MoZC at from about 675°C to about 725°C. SEM and XRD analyses
taken after
calcination at 600°C showed a complete redistribution of oxide
precursor from the
very large, supra-~ particles of Mo03 initially present to about 20-50 nm
particles of
Mo03_X, well-dispersed amongst individual fibrils. This redistribution
probably
occurred through vaporization. Further calcination to 800° C converted
the Mo03_X
(wherein x is o or 0 ) mixture to MoZC nanorods and nanoparticles, with
further
reduction in particle size from about 7 to about 25 nm. Even though
redistribution of
Mo03 probably takes place through vaporization, both chemical transformations
(Mo03 -~Mo02 and MoOZ~Mo2C by reduction by carbon) are believed to occur
through solid-solid phase reactions.
B. Mo2C nanoparticles supported on carbon nanotubes
XRD, SEM and HRTEM analyses of products from Sample 10 provided
evidence for the successful preparation of nanoparticles of Mo2C supported on
individual carbon nanotubes. These products were formed by impregnation of
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ammonium molybdate from aqueous solution onto CC aggregates of carbon
nanotubes and carefully calcined as shown in Table 1. XRD's of both products
showed the cubic form of Mo~C to be the major component along with graphitic
carbon. Hexagonal Mo2C was seen as a minor component. No molybdenum oxide
was detected. The cubic Mo2C particles ranged from about 2 to about 5 nm in
diameter, while the hexagonal particles ranged from about 10 to about 25 nm.
The
cubic particles were mainly deposited on individual carbon nanotubes, while
the
hexagonal particles were distributed between carbon nanotubes. These can be
seen in
Figures 3 and 4, which are copies of HRTEM micrographs taken from Sample 10.
In
these pictures, the particle size can be estimated by direct comaprison with
the fibril
diameters, which range from 7-10 nm.
EXAMPLE 8
_Preparation of Tungsten Carbide Precursors by Impregnation with Ammonium
Tungstate
The same procedure as used in Example 2 above was followed, except that the
impregnating solution was an aqueous solution containing the correct amount of
ammonium paratungstate hydrate or (NH4),oWizOa1.5H20, 72.% W (herein referred
to as ammonium tungstate) necessary for the desired C:W atom ratio loading
(C:W
mole ratios of 3.5:1, 10:1 and 20:1.) The resulting mixture was dried at
225°C in full
vacuum for 18 hours during which the paratungstate compound was decomposed to
W03. The sample was set aside for conversion to carbide catalysts by careful
calcination under an inert atmosphere as more particularly described in
Example 10
herein.
EXAMPLE 9
Preparation of Tungsten Carbide Precursors by Impregnation with
Phosphotungstic
Acid PTA
The same procedure as used in Example 8 above was followed, except that the
impregnating solution was an aqueous solution containing the correct amount of
phosphotungstic acid, H3 P04.12 W03.xH20, 76.6% W, herein referred to as PTA,
necessary for the desired C:W atom ratio loading (C:W mole ratios of 3.5:1,
10:1 and
20:1.) The resulting mixture was dried at 225°C in full vacuum for 18
hours during
which the PTA was decomposed to W03. The sample was set aside for conversion
to
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carbide catalysts by careful calcination under an inert atmosphere as more
particularly
described in Example 10 herein.
EXAMPLE 10
Calcination of Tungsten Carbide Precursors at 1000°C
The same procedure as described in Example 7 above was followed to vonvert
precursors of tungsten carbides to tungsten carbides. After cooling in argon,
the
samples were passivated using 3% 02/Ar. Table 3 below summarizes the
experimental conditions and XRD results for selected experiments.
TABLE 3. SUMMARY OF RESULTS FOR TUNGSTEN CARBIDE
PREPARATIONS
W03(s) + 4C -~ WC + 3 CO
2W03(s) + 7C ~ WZC + 6 CO
SAMPLE COMPONENTS T° C ~I ~L PHASES, XRD
1 PTA and CCa 1000 3.5:1 WC and WzC
2 PTA and CC 1000 10:1 WC and WzC
3 PTA and CC 1000 20:1 WC and WZC
4 A. Tung and CCb 1000 3.5:1 WC, WIC and possibly
W
5 A. Tung and CC 1000 10:1 WC and WZC
6 A. Tung and CC 1000 20:1 WC and WZC
a Impregnated powder of CC aggregates of carbon nanotubes by incipient wetness
with
phosphotungstic acid
b Impregnated powder of CC aggregates of carbon nanotubes by incipient wetness
with
ammonium
paratungstate hydrate
The chemical reactions occurring in these experiments are summarized in
Table 3 above. In the components column, there is a listing of molybdenum
precursors which were converted to WZC/WC by reacting with carbon nanotubes.
PTA refers to phosphotungstic acid and A. Tung refers to ammonium
paratungstate
hydrate. "(s)" refers to solid phase of the tungsten precursor. C:W refers to
the ratio
of C atoms to W atoms in the original mix. The stoichiometric atom ratio to
produce
pure WC with no excess of C or W is 4Ø To produce pure W2C, the atom ratio
C:W
is 3.5. The XRD column lists the compounds observed in the X-ray diffraction
(XRD) analyses.
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EXAMPLES 11-13
Preparation of a Catalyst Support of
Extrudates of Silicon Carbide Nanorods
SiC nanorods were prepared from Hyperion aggregates of carbon nanotubes in
accordance with Example 1 of U.S. Application Serial No. 08/414,369 filed
March
31, 1995 (Attorney Docket No. KM 6473390) by reacting the carbon nanotubes
with
Si0 vapor at high temperature. The resulting SiC nanorods have a uniform
diameter
of l5nm on the average and a highly crystallized ~iSiC structure.
Poly(dimethylsiloxane) as provided by Aldrich Chemicals, Milwaukee, WL,
was used as a binder for the preparation of extrudates of SiC nanorods. 0.16g
of SiC
nanorods and 0.16g of poly(dimethylsiloxane) were mixed to form a uniform
thick
paste. Subsequently, the paste was pushed through a syringe to produce
extrudates of
a green color which were heated under flowing argon atmosphere as follows: at
200°C for 2 hours (Example 11); at 400°C for 4 hours (Example
12); and at 700°C for
4 hours (Example 13). A rigid porous structure of SiC nanorods has formed.
The extrudates obtained in Example 11-13 had a density of 0.97 g/cc and a
bimodal pore structure. The macropores were 1-5 Vim, as shown in Fig. SB among
aggregates and the mesopores were 10-50 nm, as shown in Fig. SC in the
networks of
intertwined SiC nanorods. The diameter of the extrudates was around 1.2 mm as
shown in Fig. SA. The specific surface area of the extrudates of SiC nanorods
was 97
m2/g.
Because of high surface area, unique pore structure and high temperature
stability, the SiC extrudates are attractive for various applications,
including support
for catalysts such as platinum, palladium and the like and carbides of Mo, W,
V, Nb
or Ta. The surface properties of SiC nanorods when used as a catalyst support
are
very close to that of carbon. Therefore conventional carbon supports can be
replaced
with SiC extrudates and thus extend many properties of carbon supported
catalysts to
high temperature regions, as required in particular for oxidative conditions.
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EXAMPLE 14 and 15
Preparation by Reductive Carburization
of Extrudates of Carbon Nanotubes
Including Molybdenum Carbides
Two samples of 5 grams of extrudates of carbon nanotubes bearing a volatile
molybdenum compound prepared according to Example 14 above are charged into
alumina boats. Each boat is placed into a tube furnace and is heated under
flowing
argon for two hours at 250°C and 450°C, respectively. The gas is
changed from
argon to a mixture of CH4/H2 (20% CH4) and the furnace is slowly
(1°C/min) heated
up to 650°C where the temperature is maintained for 1 hour. Molybdenum
carbides
supported on the surface of extrudates of carbon nanotubes are obtained.
EXAMPLE 16
Preparation by Reactive Chemical Transport
of Extrudate of Molybdenum Carbide Nanorods
1 gram of extrudates of carbon nanotubes, 8 grams of molybdenum powder
and SOmg of bromine contained in a glass capsule are placed into a quartz tube
which
is evacuated at 10'3 torr and then sealed. After the bromine capsule is
broken, the tube
is placed into a tube furnace and heated at 1000°C for about one week.
The
extrudates of carbon nanotubes have been substantially converted to molybdenum
carbide nanorods.
EXAMPLE 17
Preparation by Carburization of Molybdenum Carbides
Supported on the Surface of Extrudates of Carbon Nanotubes
A sample of extrudates of carbon nanotubes is placed in a vertical reactor
such
that a bed is formed. The extrudates are heated under flowing H2 gas at
150°C for 2
hours. Thereafter, the extrudates are cooled to 50°C. HZ gas passed
through a
saturator containing Mo(CO)6 at 50°C is flown over the cooled
extrudates of carbon
nanotubes. As a result, Mo(CO)6 becomes adsorbed on the surface of extrudates
of
carbon nanotubes. Following the adsorption of Mo(CO)6 step, the temperature of
the
sample is raised to 150°C in an atmosphere of pure H2. The temperature
is
maintained at 150°C for 1 hour. The temperature of the sample is then
increased at
650°C and maintained at this temperature for 2 hours under flowing H2
gas. A sample
of extrudates of carbon nanotubes bearing molybdenum on their surfaces is
obtained.
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This sample is then kept at 650°C for 1 hour and the gas is switched
from H2 to a
CH4/HZ mixture (20% CH4). The molybdenum adsorbed on the surfaces of carbon
nanotubes is converted to molybdenum carbides. By varying the duration of
adsorption of the Mo(CO)6 over the cooled carbon nanotube extrudates, the
amount of
molybdenum carbide formed on the surface of the extrudate can be controlled.
The terms and expressions which have been employed are used as terms of
description and not of limitations, and there is no intention in the use of
such terms or
expressions of excluding any equivalents of the features shown and described
as
portions thereof, it being recognized that various modifications are possible
within the
scope of the invention.
Thus, while there had been described what are presently believed to be the
preferred embodiments of the present invention, those skilled in the art will
appreciate
that other and further modifications can be made without departing from the
true
scope of the invention, and it is intended to include all such modifications
and
changes as come within the scope of the claims as appended herein.
