Note: Descriptions are shown in the official language in which they were submitted.
1 Field of the Invention
The invention concerns the environmentally important removal
of undesirable nitrogen oxides (hereinafter "N0 ") from gases discharged to
the atmosphere. Examples of such exhaust gases are those from high temperature
combustion of fuel by internal combustion engines, fired furnaces and the
like wherein nitrogen and oxygen react at the high prevailing temperature to
"fix" nitrogen as one or more of the several stable oxides. Exhaust gases
containing objectionable N0 are also generated in such chemical operatlons
as manufacture of nitric acid, nitration of organic chemicals and other
operations of similar nature. Of the several oxides which can be present,
the oxides which form nitrous or nitric acid by combination with water are of
primary importance from the standpoint of air quality. Thus, N0 and N02 re-
moval is a principal aim of techniques for removing N0x.
The oxides of nitrogen generally are suspected of contribution
to photochemical smog and, in general, it is considered desirable that N0
discharges to be held to a minimum, particularly in areas susceptible to
photochemical smog, such as the Los Angeles basin.
Of the several classes of techniques for control of NO
emissions, this invention is particularly concerned with reduction of N0x
to environmentally acceptable nitrogen and water.
Background of the Invention
In recent years, increasingly intensive effort has been
devoted to systems for improvement of air and water quality. For
the most part, those efforts are concerned with control of emissions
-2-
, ~k
:~.,c~
1 from facilities created by man, possibly because these are regarded
as the only sources of contamination subject to effective control
at acceptable cost. One of the sources of air contamination
which has been given attention is emission of NO generated by
fixation of atmospheric nitrogen in high temperature zones of
combustion processes such as in internal combustion engines and
fired furnaces. As noted above, two of the oxides of nitrogen
combine with water to form corrosive acids; all the oxides of
nitrogen are suspected of contribution to photochemical smog.
Several methods of reducing NO emissions have been
investigated and reported. It has been proposed that these air
contaminants be reacted with or absorbed by liquid or solid
agents, thus posing a disposal problem with respect to the spent
reagent. It has been shown that NO can be reacted with various
reducing agents including carbon monoxide, hydrogen and ammonia
to leave innocuous agents for discharge to the atmosphere. For
example, ammonia reacts with NO under proper conditions to yield
nitrogen and water which can be freely discharged without adverse
effect.
It has been demonstrated that the transition metals are
catalysts for conversion of NO by reaction with reducing agents
such as carbon monoxide and ammonia. Patent application 340,809
distributed by National Technical Information Service of the U.S.
Department of Commerce describes the use of ion exchanged zeolite,
specifically mordenite, as catalyst for reduction of NO by carbon
monoxide. Thomas and Pence reported reduction of NO with ammonia
catalyzed by a zeolite in a paper entitled "Reduction of NO with
Ammonia over Zeolite Catalysts" presented at the Air Pollution Control
Association Meeting at Denver, June 10, 1974. The specific
catalyst employed by Thomas et al. was hydrogen mordenite.
1 Those zeolite catalysts have been used in the form of
extrudate pelle~s in beds which produce substantial pressure drop.
In fixed installations such as fired furnaces, the pressure drop
through the bed constitutes a reduction in draft applied by the
stack. In mobile equipment, automobiles and the like, the bed
pressure drop applies a back pressure to engine exhaust which tends
to reduce efficiency of the internal combustion engine. A like
effect of pelleted platinum catalyst for oxidation of carbon
monoxide and unburned hydrocarbons in automotive exhaust was obviated
by applying the metal to "monolithic honeycomb" structures of
refractory support material. Metals on monolithic honeycomb have
also been proposed for reduction of N0 . Similar application of
zeolites to the surface of honeycomb refractory raises problems
of abrasion of the zeolites, which are relatively fragile as
compared to platinum by gases and entrained fly ash or other solids.
British Patent 1,441,448 contemplates the use of a grid
work of honeycombs superficially coated with zeolite crystals as
a catalyst, especially for cracking of petroleum hydrocarbons.
The patent does not disclose catalytic composites in which zeolite
is embedded or disseminated throughout a porous matrix that permits
access to the active zeolitic crystals while minimizing losses
of the crystals due to attrition during use or handling.
The synthesis of zeolites from calcined clays, especially
kaolin clay, is known. For example, it is well-known that metakaolin
(kaolin clay calcined at a temperature of about 1200 to 1500F.)
will react with sodium hydroxide solution to produce sodium zeolite
A. It is also known that when kaolin is calcined under more
severe conditions, for example 1700 to 2000F., it will react
with sodium hydroxide solution, small amounts of metakaolin
preferably being present, to synthesize faujasite-type zeolites
a~
1 useful in hydrocarbon conversion processes. Reference is made to
the following commonly assigned patents of Haden et al.:
U.S. 3,335,098, I].S. 3,338,672, U.S. 3,367,886, U.S. 3,367,887,
U.S. 3,391,994, U.S. 3,433,587, U.S. 3,503,900, U.S. 3,406,594,
U.S. 3,647,718, U.S. 3,657,154 and U.S. 3,663,165. In the processes
of these patents synthetic faujasite-type zeolite is crystallized
either as a pulverulent mass or as composite fluid or pelleted
particles.
In accordance with the teachings of U.S. 3,119,660 to
Howell et al., preformed metakaolin or preformed mixtures of metakaolin
and zeolite A were reacted with caustic to form lO0 percent zeolite
A. By adding sources of soluble silica to the reaction mixture,
zeolite X or zeolite Y was formed as a constituent of pellets or the
like.
U.S. 4,007,134 to Liepa et al. deals with the use of
extruded zeolitic honeycombs in the carbonation of soft drinks.
The honeycombs contain over 40% zeolite and are prepared by extruding
preformed zeolitic molecular sieve crystals, preferably with a known
binder such as clay, and calcining the resulting extrudate to harden
the structures. It is well-known that zeolitic molecular sieve
crystals lack thermal stability when-they are calcined at high
temperatures. While high temperatures favor hardening of clay
binders~ the presence of the zeolite in the unfired honeycomb
structures of U.S. 4,007,137 precludes the use of high temperatures
e.g., 1700F. or above. Temperatures of this order are needed to
secure the high strength required for many uses of catalysts and
catalyst supports. This process limitation is obviated by
practice of the process of the instant invention.
Summary of the Invention
The invention provides an improved manner of applying the
P~3
1 known technique of reducing N0x by reaction with ammonia, carbon
monoxide, or other reducing gas in the presence of zeolite catalyst.
Accordingly, it is a primary object of the invention to provide
zeolite catalyst for that reaction in a physical form which provides
the low pressure drop and freedom from clogging by fly ash ~lich is
found with monolithic honeycomb structure without exposing the
relatively fragile zeolite to erosive effects. It is an important
aspect of the invention that the zeolite is unimpaired as to catalytic
activity and accessibility to reactants as would be the case if the
nolith were prepared by extrusion of a dispersion of zeolite in clay
and firing to form the ceramic honeycomb. In such cases, diffusivity
is limited by the fixed matrix material surrounding the zeolite and
the zeolite is subject to thermal degradation in the firing step.
The present invention relates to a process for reducing
oxide~ of nitrogen contained in an exhaust gas by mixing the gas with
a gaseous reducing agent and contacting the resulting gaseous mixture
with a synthetic molecular sieve catalyst at an elevated temperature
above about 200C. The improvement of the present invention comprises
contacting the gas at a space velocity o 3,000 to 60,000 volumes of
gas per volume of catalyst per hour at a temperature below about
700C. wlth a catalyst in the form of a monolithic body having
channels extending therethrough. The monollthic body consists
essentially of fine crystals of a synthetic zeolitic molecular sieve
formed in situ by calcining a monolithic body of kaolin, treating the
calcined body of kaolin with a caustic solution to develop a synthetic
zeolitic molecular sieve therein and etch the surfaces thereof, the
crystals of molecular sieve being disseminated throughout the body in
an essentially a rphous alumina-silica porous residue of anhydrous
calcined clay having etched diffusion paths to and from the embedded
zeolitic molecular sieve.
~, ,
1 The zeolite catalysts are made available for treating
exhaust gases by novel refractory structures of monolithic honeycomb
or other channeled form in which the zeolite is embedded in the
refractory. The particular structures provided by the invention are
etched by steps in the manufacturing process such that gases readily
diffuse through the refractory to be converted by catalysis at the
embedded zeolite and reaction products diffuse to the surface at rates
which facilitate use at the very high space velocities needed for use -~
in conversion of exhaust gases. These structures of refractory bodies
of high diffusivity containing embedded zeolites exhibit low pressure
drop because they provide parallel channels for free flow of gases in
contact with the thin walls which defined the channels.
An object of the invention is to provide zeolite-binder
composites in the form of a multichanneled monolithic mass, especially
honeycombs, which possess chemical and structural properties
3o
-6a-
required for use as catalysts in reduction of NO contained in
exhaust gases.
One aspect of the invention comprises use of a rigid
catalyst structure in the form of a monolithic body having a plurality
of channels extending therethrough, for example a honeycomb, the
body comprising crystals of a synthetic zeolitic aluminosilicate
molecular sieve formed in situ from a monolithic multichanneled
precursor body of calcined amorphous clay, the crystals of molecular
sieve being disseminated throughout the body in an essentially
amorphous alumina-silica porous residue of anhydrous calcined clay.
Another aspect of the invention comprises reduction of NO
by use of a catalyst formed by the in situ synthesis of a zeolitic
aluminosilicate molecular sieve within a preformed calcined kaolin
clay honeycomb by reacting the preformed honeycomb with an aqueous
solution of a base to effect the synthesis. During the synthesis,
substantial silica and/or alumina is leached from the preformed
honeycomb by the basic solution. This results in adequate
dlffusion during synthesis and then imparts desirable porosity
in the amorphous alumina-silica component of the finished catalyst
composite. In a preferred embodiment the preformed honeycomb is
obtained by calcining a green multichanneled monolith such as
a honeycomb consisting essentially of kaolin clay and free from
fluxes, at a temperature in the range of about 1700 to 2000F.
This calcination step imparts excellent mechanical strength to the
calcined preformed honeycomb so that even when thin-walled honey-
combs are produced, they do not disintegrate during zeolite synthesis.
Furthermore, the high temperature calcination insures excellent
strength and hydrothermal stability in the zeolitized honeycomb
structure and is required to synthesize certain zeolites such as
fau;asite or ZSM-5.
-- 7 --
1 D ~
Known methods for producing ceramic honeycombs may be used
in preparing the given forms to be processed in manufacture of catalyst
for use in practicing the invention. However, it is preferable to
select a forming technique which does not necessitate the incorporation
of a nonfugitive forming aid such as talc or mineral substance (other
than kaolin clay or metakaolin) which might be a deleterious impurity.
Generally fluxes such as alkali or alkaline earth compounds are
excluded from the mixture employed to produce the green honeycomb
since fluxes may react with the clay constituent during the firing step
and thereby prevent the desired subsequent reaction between the
fired clay in the honeycomb body and the alkali in the reaction medium.
Thus, the green honeycomb should preferably contain only fugitive,
soluble and/or pyrolitically decomposable forming aids. Honeycomb
forming techniques which use thermoplastic organic vehicles alone or
in combination with solvents are useful.
One suitable technique for producing a green honeycomb
is described in British Patent 1,371,082 to Langley et al. In
accordance with this pa~ent, a formulation consisting essentially
of precursor inorganic material such as clay and a thermoplastic
vehicle such as a wax is prepared. The formulation is solid at
room temperature and fluid at elevated temperature. The mixture
while warm is forced by a squeegee through a stencil defining a print
(layer) of the honeycomb body in two dimensions. The print is
solidified as a layer after it passes through the stencil. Successive
overlap of prints build up a three dimensional honeycomb which is
fired to eliminate the thermoplastic vehicle. In effect this is a
discontinuous extrusion and is very useful in producing thin-walled
honeycombs, for example honeycombs having from 200 to 300 channels
per square unit. Wall thickness may vary over a substantial range.
1 Honeycombs of any height, for example 1/4 inch up to a foot or more, can
be produced.
Corrugation techniques such as described in RE. 27,747 to
Johnson may also be used. In one corrugation process a ceramic powder is
made into a tape with an organic binder. The tape is formed at room temper-
ature into a honeycomb structure which is heated to burn off the binder and
sinter the ceramic. In practice of the present invention, sintering i9
avoided, as discussed below. Fugitive organic carriers that are removed
during final heating are also useful in some corrugation techniques.
High pressure extrusion techniques, such as described in
British Patent 1,142,800, can also be employed. Other suitable techniques
are described in Re. 28,195 of U. S. 3,755,204 to Sergeys and U. S. 3,837,783
to Bagley.
Irrespective of the honeycomb formation equipment that is
employed, the self-supporting green honeycomb consisting of
hydrated kaolin clay and temporary binder is calcined, preferably
at 1700 to 2000F., for a time sufficient to dehydrate the clay. The
heat treatment should be controlled to avoid slumping, cracking and
fusion. ~ormally heat treatment is conducted by gradually increasing
the temperature of the monolith to volatilize and burn out temporary
binder or vehicle until the desired elevated temperature is reached.
At this time the temperature can be maintained at that level.
During heat treatment the hydrated kaolin clay undergoes
the characteristic kaolin endotherm associated with dehydration when
it is heated to a temperature of about 1350F. Subsequently the
resulting metakaolin undergoes the characteristic exothermic reaction
when it is heated to a temperature of about 1800F.
It is essential to calcine the preformed honeycomb
_9_
8~
1 structure at a temperature of 1700QF. or above in order to convert
it to a state or condition such that it is useful in the synthesis
of a catalytlcally suitable zeolite such as faujasite, mordenite
or ZSM-5. Lower temperatures such as 1350F. will generally limit
zeolite synthesis to the formation of zeolites such as type A (U.S.
2,882,243) unless an additional source of silica, for example
sodium silicate, is employed as a reactant. Furthermore, temperatures
of 1700F. or above lead to the formation of honeycombs that will
be considerably stronger than those obtainable at lower temperatures,
e.g., 1350F. On the other hand, temperatures appreciably above
2000F. result in recrystallization of silica and/or alumina phases
which generally are deleterious to the reactivity of the components
in the precursor honeycomb with basic solutions. Thus, the calcined
honeycomb is preferably amorphous or essentially so when tested by
a conventional X-ray diffraction technique (see the Haden et al.
patents supra~. -
Bases known to be useful in zeolite synthesis, including
alkall metal hydroxides, ammonium bases, as well as mixtures thereof,
may be employed ln practice of the lnvention. The base or mixture
of base~ are dissolved ln water and the solution added ln amount
at least sufflcient to cover the honeycomb to be zeolitized. Depending
on the zeolite to be synthesized, soluble ~ources of silica and/or
alumina may be added to the aqueous reaction medium. Generally
aqueous solutions of 2 to 15% weight concentration are used. The
amount of solution, controlled to provide the desired ratios of
alkali (and/or ammonium oxide) to A1203.2SiO2 in the calcined
preformed honeycomb, will obviou61y vary with the zeolite that is
to be synthesized within the honeycomb and with the concentration
of the solution of the base.
Generally the reactions are carried out at atmospheric
--10--
1 or elevated pressure at elevated temperature for a time sufficient
to achieve crystal formation within the honeycomb structure. In the
case of faujasite synthesis, a low temperature (aging step) may
precede the high temperature crystallization. The zeolite is
crystallized in hydrated form.
Preferably the zeolitized honeycombs will contain about
2 to 90%, most preferably 10-65%, crystalline zeolite as determined
by X-ray diffraction. It will be noted that complete conversion of
the honeycomb to zeolite is avoided since the structures may lack
mechanical strength and diffusity lmparted by the porous nonzeolitic
alumina-silica matrix constituent.
After synthesis the structure may be subjected to ion
exchange treatment in known manner to replace cations present as a
result of synthesis with more desirable cations. For example,
exchangeable sodium may be reduced to 1% or below by ion exchange
with ammonium salts, ammonium and rare earth salts or alkaline earth
metal salts. The exchange is facilitated by the fact that the
channels in the structures provide access of the fluid exchange
medium and the nonzeolite constituent permits diffusion of the
solution to the zeolite crystals. Similarly, the monolithic bodies are
readily separated from liquids by filtration. Problems experienced
in exchanging and filtering powdered zeolites are avoided.
The ion exchange of the crystal-bearing structure may be
conducted in a fashion to take advantage of the known properties of
transition metal or hydrogen form of zeolites for N0 reduction as
described in the prior art reviewed above.
The following examples are illustrative of practice of
the invention but are no~ considered to be limiting to the specific
features utilized therein except as set forth in the appended claims.
In the examples a honeycomb preform was prepared by
1 pretreating 949 parts high purity kaolin clay having a SiO2/A1203 molar
ratio of 2/1 with 9.6 parts silane sold under the trademark "silane Y-9187"
to render the clay particles hydrophobic. The treated clay was dried,
added to a mixture of 314 parts stearic acid, 34 parts gilsonite, 4.1 parts
of a modified lecithin wetting agent and 2.6 parts ethyl cellulose, and heated
to 158 to 176F. The clay mixture was agitated for 1/2 hour and then extruded
through a stencil by the technique described in British Patent 1,371,082 to
Langley et al. at a temperature of 140F. After formation the honeycomb
was solidified in water and the organics were burned out in air by increas-
ing the temperature 0.20F. per minute to 527F. and then holding at this
temperature 2.5 hours. The honeycomb was then calcined for two hours
at 1800F. The honeycomb contained 225 channels per square inch. Wall thick-
ness was 0.02 inch. Length and diameter were 2 inches and 2.5 inches,
respectively.
In one example a monolith containing zeolite ZSM-5 was prepared.
Thirty-nine and three-tenths (39.3) grams of the calcined honeycomb preform,
7.0g. of NaOH, 45 g. of tetra-n-propyl-ammonium bromide, and 210 ml. of
water were charged to a Pyrex-lined autoclave. The mixture was heated with-
out agitation at 275F. for six days. The honeycomb was then removed,
washed with water and dried. A portion of the honeycomb was ground up for
analysis and X-ray diffraction analysis. The peaks at 8.1, 9, 16,
23, 24 and 29.4 2~ corresponding to interplanar spacings in A. of 11.15,
10.01, 5.56, 3.82, 3.64 and 2.98, re~.pectively, indicated that the
product contained approximately 29% ZSM-5. The amount of zeolite was esti-
mated by comparison of the peak intensities with those of ZSM-5 prepared
according to Example 23 of U. S. 3,702,886 and assumed to be 95% ZSM-5.
By substituting other quaternary ammonium bases (known in the art), other
known members of the ZSM-5 type zeolites, such as ZSM-ll and 12, may be
produced.
In another examplei a monolithic horeycomb containing
-12-
1 synthetic crystalline faujasite was produced. Forty-one grams of the
calcined honeycomb, 21 g. of NaOH and 141 ml. of water were added to a
500 ml. resin kettle. The mixture was aged without agitation at 100F.
for six hours and then heated at 180F. for sixteen hours. The resulting
product was washed with water and dried. X-ray diffraction analysis of
the 6.1, 16 and 27 20 peaks showed that the product contained 29%
synethetic crystalline faujasite with a Si02/A1203 ratio of 3.3 as
determined by X-ray, using the Freeman and Stamires curve to correlate
a- and Si2/A123
In still another example, 41 g. of the calcined honeycomb, 21
g. of the NaOH and 141 ml. of water were added to a 500 ml. resin
kettle. The mixture was aged at 100F. for six hours without agitation
and then heated at 180F. for 5-1/2 hours. At this point 100 ml. of
water was added to the reaction vessel to dilute the mixture and retard
crystallization. The mixture was heated at 180F. for an additional 17-
1/2 hours. The honeycomb was removed, washed and dried. X-ray diffrac-
tion analysis indicated the product contained 18% synthetic crystalline
fau~asite having a Si02/A1203 ratio of 3.9 as determined by X-ray
analysis.
A representative zeolitized product useable in practicing
the invention was analyzed by X-ray and found to contain 8% synthetic
crystalline faujasite. By wet chemical analysis the structure
contained 36% Si02. Since the calcined honeycomb preform contained
about 54% si02, a substantial percentage of the silica originally
present in the kaolin was therefore leached from the honeycomb
during the crystallization. The honeycomb was found to contain 0.12
cc./g. in pores of less than 100 A. diameter as measured by the
ni~rogen isotherm method.
In another example 42 g. of the calcined honeycomb
was added along with 8 g. NaOH and 215 ml. of H20 to an autoclave
C~
1 and the mixture heated at 257F. for three days and then at 284F. for
four additional days. The product was water washed and dried. X-ray
diffraction analysis of the 10.0, 13.6, 19.8, 22.3, 26.1 and 27.9
20 peaks corresponding to interplanar spacings in A. of 13.4, 6.49, 4.50,
3.~8, 3.42 and 3.15, respectively, indicated the amount of mordenite to
be approxlmately 30%, as compared to a mordenite reference sold under the
trademark "Zeolon 900" assumed to be 100% H-mordenite.
In applying the catalyst of this invention to reduction of
NOX ln exhaust gas, it is usually preferable to use the acid form of the
zeollte when the reducing agent i8 ammonia. When carbon monoxide is the
reducing agent, transition metal forms of zeolite are preferred. Choice
of zeolite will be influenced by nature of the exhaust gas treated. For
highly acid environment, the more acid resistent zeolites are consldered
to be more stable, for example mordenite and the variations of zeolite
ZSM-5 which show high silica/alumina ratios upwards of about 30.
Space velocity will range upwards of about 3000 volumes
of gas treated per volume of catalyst per hour. At space velocities
of 60,000, efficiency may be severely impaired. Temperatures of
treatment can vary over a broad range and, will generally be selected
to suit operating parameters of the installation where used by designing
the facility for good thermal efficiency and adapting the NOX converter to
a location in the direction of gas flow which is convenient to the
engineering of the plant. The temperatures of treatment will be below those
which tend to impair the crystalline structure of the zeolite and below those
which can lead to thermal fixation of nitrogen downstream of the NO converter.
For most in~tallations, temperatures up to about 800C. can be effective,
but gases containing substantial amounts of water may require lower
temperatures due to the greater sensitivity of zeolites to elevated
-14-
.t~
1 temperature in the presence of steam. The high silica zeolites will
be found stable, even at 10% water vapor, up to about 700qC. In any
event, the temperature should be high enough to promote the desired
reaction, say above about 200C.
Temperatures below the maximum stated above based on zeolite
stability will generally be preferred. ~or most compositions of
exhaust gas plus a reducing agent treatment temperatures will be
below 500C. When using ammonia as the reducing agent, temperature
must be controlled below 500C., preferably below about 400C.
In practicing the invention, a suitable reducing gas, ammonia
carbon monoxide, hydrogen or the like will be added to the exhaust
gas stream upstream of the reactor in an amount such that the added
gas together with any reducing agent presen~ (e.g., carbon monoxide~
will be about equal to the stoichiometric amount required for a
desired reduction of N0x.
An especially preferred embodiment is a honeycomb containing
synthetic mordenite as the æeolite, useful as a catalyst for the
reduction of nitrogen oxides with ammonia such as described in U.S.
3,895,094 to S.L. Carter et al. In accordance with the teachings
of this patent, nitrogen oxides in exhaust gases from a variety of
industrial processes (for example, fossil fuel power plants or nitric
acid plants) are selectively reduced with ammonia, yielding nitrogen
and water. An acid resistant aluminosilicate molecular sieve
composition having substantially uniform intercrystalline pores with
effective diameters of at least 6A. (i.e., mordenite), approximately
a stoichiometric molar quantity of ammonia, and temperatures between
200 to 300C. were employed to treat gas co~positions containing trace to
10% oxygen, trace to 2% nitric oxide, and trace to 2% nitrogen dioxide.
The patent teaches processing at up to 60,000 WHSV (ft. of gas at
standard conditions per ft.3 of catalyst per hour). However, preferred
-15-
1 space velocities are between 3000 to 30,000. In fact, in the
examples, at 15,000 a 96% NO reduction (260 ppm after processing
vs. 6520 ppm before) is achieved while at 40,000 a loss in efficiency
was found (no quantitative data given). The use of a mordenite
S containing honeycomb catalyst offers the ability to go to higher
space velocities without the use of a longer reaction vessel. Mass
transfer limitations can be overcome by use of a honeycomb having
more cells per square inch. Pressure drop is therefore not significantly
affected and greater flexibility in reactor design is permitted.
In addition, problems often encountered in power plant
NO reduction systems involve plugging of fixed catalyst beds with
particulate matter (i.e., fly ash, carbon). A honeycomb is less
affected by such particulate matter.
Other advantages and features of the process using
zeolitized honeycombs according to the invention will be apparent
to those skilled in the art.
-16-
