| PATENT ASSIGNEE'S COUNTRY | USA |
| UPDATE | 08.00 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 15.08.00 |
| PATENT TITLE |
Attrition resistant zeolite catalysts for production of methylamines in fluidized bed reactors |
| PATENT ABSTRACT |
This invention provides an attrition resistant catalyst composition and method for producing such composition. The catalyst is comprised of an acidic zeolite, rho or chabazite, and a particulate binder, kaolin, bentonite, alpha-alumina, or titania, which can be optionally modified by treatment with a compound containing Si, Al, P or B. This invention further provides a process for producing methylamines, preferably dimethylamine, comprising reacting methanol and/or dimethyl ether and ammonia in the presence of a catalytic amount of an attrition resistant catalyst of the invention. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 26.05.95 |
| PATENT REFERENCES CITED | This data is not available for free |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. An attrition resistant catalyst composition comprising one or more acidic zeolites selected from the group consisting of rho and chabazite; said zeolite being uniformly admixed to a ratio of from about 25 to about 75 weight % with one or more particulate binders selected from the group consisting of kaolin, bentonite, alpha-alumina, and titania. 2. The catalyst composition of claim 1 wherein the acidic zeolite is rho. 3. The catalyst composition of claim 2 wherein the rho is NH.sub.4 -rho. 4. The catalyst composition of claim 1 wherein the particulate binder is alpha-alumina and/or titania. 5. The catalyst composition of claim 1 wherein said catalyst composition is modified by treatment with one or more compounds containing elements selected from Si, Al, P and B, said treatment comprising depositing at least 0.05% by weight of the compound onto the catalyst composition. 6. The catalyst composition of claim 5 wherein said catalyst composition has been modified by treatment with tetraethylorthosilicate. 7. The catalyst composition of claim 5 wherein the acidic zeolite is rho and the binder is alpha-alumina present at a ratio of about 50 weight %; and further wherein said catalyst composition is modified by treatment with tetraethylorthosilicate. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
This invention relates to attrition resistant zeolite catalysts which are particularly useful for the production of methylamines in fluidized bed reactors. BACKGROUND OF THE INVENTION Zeolite catalysts, and especially zeolite rho catalysts and their use in fixed bed reactors for conversion of methanol and ammonia to dimethylamine are well known in the art. (U.S. Pat. No. 3,904,738, U.S. Pat. No. 4,683,334, U.S. Pat. No. 4,752,596, U.S. Pat. No. 4,814,503, and U.S. Pat. No. 4,806,689.) The present invention provides an improvement in these catalysts whereby they are blended with one or more microparticulate binders during formation, which renders the catalyst particles attrition resistant and therefore suitable for use in fluidized bed reactor processes. A particularly useful aspect of the invention is the use of these attrition resistant catalysts in fluidized bed reactors for the efficient and cost effective commercial production of methylamine compounds. Other examples of improved related catalysts are known in the art. Gladrow et al., (U.S. Pat. No. 3,609,103) disclose use of faujasite and a deagglomerated clay such as Georgia kaolin matrix with a silica-alumina cogel to form a cracking catalyst. The use of the clay phase increases the cracking activity, and thus is added as an active component for the cracking chemistry. Elliott (U.S. Pat. No. 3,867,308) discloses a process for preparing hydrocarbon cracking catalysts using a silica sol by first adding mineral acid to adjust pH, and then adding clay and zeolitic components followed by spray drying. These zeolites are typically X or Y zeolites. Increased attrition resistance and activity of the catalyst, compared to the pure H.sup.+ form of the zeolite is disclosed. The process and additive are chosen to increase the activity of the catalyst by adding active components to the formulation. Gladrow (U.S. Pat. No. 4,147,613, U.S. Pat. No. 4,151,119 and U.S. Pat. No. 4,182,693) disclose a hydrocarbon conversion process utilizing catalyst comprising major amounts of silica and minor amounts of zirconia and alumina, bulk alumina and aluminosilicate zeolites. (3-16 wt percent zeolite, 50-85 wt percent inorganic oxide gel, mostly consisting of silica and a minor amount of zirconia and alumina, and 15 to 40 wt percent of a porous absorbent, for instance bulk alumina.) The absorbant is in place to absorb heavy metals present in the petroleum crudes, which can deactivate the zeolite. Increased activity/selectivities for these catalysts compared to a more conventional Y zeolite containing kaolin and a silica-alumina hydrogel is claimed. Lim et al. (U.S. Pat. No. 4,206,085) report an improved abrasion resistant zeolite, prepared from a faujasite type zeolite, hydrated alumina and ammonium polysilicate or silica sol and clay to form microspheres. The use of ball clay is present because the clay has pre-cracking activity which is important in the hydrocarbon chemistry. Lim et al. (U.S. Pat. No. 4,325,845) describe a method for producing zeolite cracking catalysts using sodium silicate, derived from silica gel, in combination with clay to form catalysts of good attrition resistance. The authors eliminate the alumina from the formulation (pseudoboehmite), claiming it is a source of coking, or deactivation of the catalyst and sodium silicate is substituted for the alumina hydrate. The silicate is added to the ball clay and zeolite to form the final catalyst in order to enhance catalytic activity. Scherzer (U.S. Pat. No. 4,987,110) claims an attrition resistant cracking catalyst using a molecular sieve (zeolite) having cracking activity, a clay such as kaolin, a silica sol and aluminum chlorohydroxide. In contrast to the present catalysts, the clay disclosed by Scherzer would have significant activity in the methylamines chemistry. Velten et al. (WO 89/01362) claim various zeolites (ZSM-5, ultra stable Y) formulates with binders prepared from amorphous silica, alumina and zirconia, particularly those of colloidal dimensions. Binder formulations include colloidal silica, colloidal alumina, colloidal silica and acid dispersed alumina which may be noncolloidal or colloidal, colloidal silica and colloidal zirconia, or mixtures of these ingredients. Applicants have found that colloidal silicas, aluminas and silica/alumina combinations do not give a satisfactorily attrition resistant rho zeolite at 50 weight percent binder or greater. SUMMARY OF THE INVENTION The present invention provides an attrition resistant catalyst composition comprising one or more acidic zeolites selected from rho or chabazite; said zeolite being uniformly admixed to a final weight % of about 25 to 75 with one or more particulate binders selected from kaolin, bentonite, alpha-alumina, and titania; wherein said catalyst composition is optionally modified by treatment with one or more compounds containing elements selected from Si, Al, P and B, said treatment comprising depositing at least 0.05% by weight of the compound onto the surface of the catalyst particles. The present invention further provides a process for producing a methylamine compound, preferably dimethylamine, comprising reacting methanol and/or dimethylether and ammonia, in amounts sufficient to provide a carbon/nitrogen ratio from about 0.2 to about 2.5, and at a temperature from about 220.degree. C. to about 450.degree. C., in the presence of a catalytic amount of an attrition resistant catalyst composition comprising one or more acidic zeolites selected from rho and/or chabazite; said zeolite being uniformly admixed to a weight % of about 25 to 75 with one or more particulate binders selected from kaolin, bentonite, alpha-alumina, and titania; wherein said catalyst composition is optionally modified by treatment with one or more compounds containing elements selected from Si, Al, P and B, said treatment comprising depositing at least 0.05% by weight of the compound onto the surface of the catalyst particles. Preferably, the above process is used to produce dimethylamine in a fluidized bed reactor. The present invention further provides a process for the production of an attrition resistant catalyst composition comprising one or more acidic zeolites selected from rho and/or chabazite, said zeolite being uniformly admixed with one or more particulate binders selected from kaolin, bentonite, alpha-alumina, and titania; wherein said catalyst composition is optionally modified by treatment with one or more compounds containing elements selected from Si, Al, P and B, said treatment comprising depositing at least 0.05% by weight of the compound onto the catalyst composition, said process comprising the steps of: (a) blending one or more acidic zeolites selected from rho and/or chabazite with one or more particulate binders selected from kaolin, bentonite, alpha-alumina, and titania, at a ratio of from about 25 to about 75 weight %; (b) adding the blend to water to yield a slurry of about 20 to about 55 wt percent solids; (c) spray drying the slurry to form microspherical particles; (d) calcining the particles at about 500.degree. C. to about 750.degree. C.; and optionally; (e) screening the calcined particles to produce a catalyst composition having the desired median particle diameter (d.sub.50). DETAILED DESCRIPTION The advantages of fluid bed catalytic processes over fixed bed processes are well recognized in the art. The advantages in fluid bed processes include improvement of temperature control because of better heat transfer and more efficient solids handling. Particularly in the case of zeolite catalysts for methylamines synthesis, it is recognized that precise temperature control is important to maintain the activity of the catalyst and eliminate the formation of hot spots which are known to occur in fixed bed reactors. Additionally, if the catalyst loses activity with time, it can easily be removed and replaced in a fluid bed reactor. A fixed bed reactor, however, requires the reactor system to be shut down for catalyst removal. The activity, stability and durability of a catalyst in a fluidized bed catalytic process depend on the inherent attrition resistance of the catalyst particle. Most zeolites, as prepared, do not have the correct particle size range for such a reactor. Hence, they must be formed in the correct particle size range. Attrition by abrasion and/or fracture of the particles is a frequent problem in fluidized reactors, which necessitates the addition of a binder to the catalyst particles. Excessive particle attrition in these reactors is caused, for example, by particle-to-particle contact, abrasion with bed walls and bed internals, as well as distributor jet impingement and abrasion in circulation conduits leading to and from the reactor bed. High particle attrition contributes to product contamination, catalyst loss, plugging of down stream equipment, high filtration costs, and unstable fluidization behavior such as channeling, slugging or increased entrainment of reactants. The deleterious effects of fluidized bed operations can be exacerbated by high temperature conditions. Zeolites by themselves cannot be formed in the correct particle size range with sufficient mechanical strength to be attrition resistant. In addition to mechanical strength, particle shape can also have an impact on attrition. Spheroidal particles with smooth surfaces will have lower attrition losses than particles with irregular shapes and rough edges. By spheroidal we mean to include spherical and nearly spherical particles, so long as there are no irregular or sharp edges that would likely cause attrition during handling or fluidization. For a fluid bed methylamines process, a catalyst of high attrition resistance as well as sufficient activity/selectivity is necessary. The use of binders to impart attrition resistance however, introduces additional entities which may have their own reactivities resulting in undesirable competing side reactions. For these reasons, prior literature is not directly applicable in any particular catalytic process. Most previous disclosures in this art concern FCC (fluid cracking catalysts). For these systems, however, the binders are chosen for their catalytic activity towards hydrocarbons. Since fluid cracking is also an acid catalyzed reaction, these FCC catalysts will have undesirable activity on the methylamines reactants. This reactivity is deleterious to the overall selectivity of the catalyst since the molecular sieving characteristic is not a feature of these binders. Thus, in developing the attrition resistant catalysts of the invention for methylamine production in fluidized bed systems, applicants were faced with many obstacles and constraints. Primarily, the goal was to select the appropriate types and amounts of binders to blend with the appropriate zeolites whereby sufficient catalytic activity and attrition resistance of the catalyst particles was attained for use in commercial fluid bed reactors. Constraints included: 1) minimizing reactivity of the binder phase; 2) controlling the selectivity of the zeolite/binder in producing methylamine compounds in the dimethyl form; 3) producing attrition resistant fluidizable material without excessive heating in order to preserve the integrity of the zeolite. The attrition resistant catalysts of the invention are either comprised of acidic zeolites rho or chabazite. These and other zeolites can be described as aluminosilicates characterized by a three-dimensional framework structure occupied by ions and water molecules. Rho zeolite and chabazite contain a common structural characteristic: pores or channels within the zeolite framework, the largest of which are bounded by 8-membered rings of tetrahedral atoms. This structural characteristic is associated with catalytic selectivity for production of dimethylamine from methanol and ammonia; the catalyst possesses a geometric or shape selectivity which permits the release of dimethylamine and monomethylamine from the zeolite pores, but not trimethylamine. Zeolite rho is a small-pore synthetic zeolite which can be described by the formula (Na,Cs).sub.12 Al.sub.12 Si.sub.36 O.sub.96.44H.sub.2 O. The structure and synthesis of this synthetic zeolite are described by Robson et al., "Synthesis and Crystal Structure of Zeolite Rho--A New Zeolite Related to Linde Type A", Advances in Chemistry Series 121 (American Chemical Society 1973), and Robson, U.S. Pat. No. 3,904,738, incorporated by reference herein. The cation species Na.sup.+ and Cs.sup.+ present in rho zeolites can be exchanged with H.sup.+ or ammonium ions to prepare an acid or ammoniated form (NH.sub.4 -rho) which is then converted to the acid form by calcination at elevated temperatures (ion exchange of ammonium for Na.sup.+ and Cs.sup.+ ions may be incomplete in any given experiment, typically leaving 0.5-1.0 Cs per unit cell; the product of this ion exchange is referred to as NH.sub.4 -rho; similarly, deammoniation of NH.sub.4 -rho may not result in complete conversion of all NH.sub.4 sites to H.sup.+ and/or other acid sites). Chabazite, a mineral zeolite, has a structure consisting of identical, near-spherical "chabazite cages", each composed of two 6-rings at top and bottom, six 8-rings in rhombohedral positions, and six pairs of adjacent 4-rings. Each cage is interconnected to six adjacent units by near-planar, chair-shaped 8-rings. Chabazites can be characterized by the formula: M.sub.a.sup.n Al.sub.12 Si.sub.24 O.sub.72.40H.sub.2 O In this formula, the product of a and n is 12, and M generally refers to Ca, Mg, Na and K. As with rho zeolite, the cations can be exchanged for H.sup.+ or by conversion to an ammoniated form which can then be converted to the acid form by calcination at elevated temperatures, generally ranging from 400 to 600.degree. C. Zeolites rho and chabazite are known to be useful as catalysts for methylamines synthesis in fixed bed reactors. See U.S. Pat. Nos. 3,904,738, 4,683,334, 4,752,596, 4,814,503 and 4,806,689. The present invention encompasses such known methods of methylamines synthesis in fixed bed reactors, as well as methylamines synthesis in fluidized bed reactors, wherein the catalyst is attrition resistant per the method of this invention, discussed below. Thus, a process of the present invention comprises reacting methanol and/or dimethylether (DME) and ammonia, in amounts sufficient to provide a carbon/nitrogen (C/N) ratio from about 0.2 to about 2.5, in the presence of a catalytic amount of attrition resistant catalyst composition, at a temperature from about 220.degree. C. to about 450.degree. C. Reaction pressures can be varied from 1-1000 psi (7-7000 kPa) with a methanol/DME space time of 0.01 to 80 hours. The resulting conversion of methanol and/or DME to methylamines is generally in excess of 85% (on a mole basis). The process variables to be monitored in practicing the process of the present invention include C/N ratio, temperature, pressure, and methanol/DME space time. The latter variable is calculated as the mass of catalyst divided by the mass flow rate of methanol and DME introduced to a process reactor (mass catalyst/mass methanol+DME fed per hour.) Generally, if process temperatures are too low, low conversion of reactants to dimethylamine and monomethylamine will result. Increases in process temperatures will ordinarily increase catalytic activity, however, if temperatures are excessively high, equilibrium conversions and catalyst deactivation can occur. Preferably, reaction temperatures are maintained between 270.degree. C. and 350.degree. C. more preferably 290.degree. C. to 330.degree. C. with lower temperatures within the ranges essentially preferred in order to minimize catalyst deactivation. At relatively low pressures, products must be refrigerated to condense them for further purification adding cost to the overall process. However, excessively high pressures require costly thick-walled reaction vessels. Preferably, pressures are maintained at 10-500 psi (70-3000 kPa). Short methanol/DME space times result in low conversions and tend to favor the production of monomethylamine. Long methanol space times may result either in inefficient use of catalyst or production of an equilibrium distribution of the products at very high methanol/DME conversions. Generally, methanol/DME space times of 0.01-80 hours are satisfactory, with methanol/DME space times of 0.10-1.5 hours being preferred (corresponding to methanol/DME space velocities of 0.013-100 g methanol+DME/g of catalyst/hour, preferably 0.67-10 g of methanol+DME/g of catalyst/hour). The molar reactant ratio of methanol and/or dimethylether to ammonia, herein expressed as the C/N ratio (g atoms C/g atoms N), is critical to the process of the present invention. As the C/N ratio is decreased, production of monomethylamine is increased. As the C/N ratio is increased, production of trimethylamine increases. Catalyst deactivation is also greater at high C/N ratios. Accordingly, for best results, C/N ratios should be maintained between 0.2 and 2.5, preferably from 0.5 to 2.2 and most preferably 1 to 2.0 in conducting the process of the present invention. The efficiency of the process of the invention is measured by overall conversion of methanol and/or DME to methylamines, and by selectivity of dimethylamine production. For example, if methanol is used as the sole reactant, overall conversion is determined by comparison of the amount (in moles) of methanol in the product mixture, which is considered to be unconverted, to the amount in the reactant feed. Thus, overall conversion in percent is given by: ##EQU1## Selectivity of methanol to monomethylamine (MMA) in percent, is given by: ##EQU2## Similarly, selectivity of methanol to trimethylamine (TMA), in percent, is given by: ##EQU3## Finally, selectivity to dimethylamine (DMA) is calculated by analysis of product composition. Thus, selectivity to DMA, in percent, is provided by the following expression: ##EQU4## For efficient operation, the catalyst must be selective at high conversion (87-98%) and a C/N ratio of 0.2-2.5, preferably 0.5-2.2, and most preferably 1-2.0. Comparison of selectivities for different samples should be made at similar conversions since selectivity varies with conversion. At low conversions, MMA production is favored, at very high conversions, the reaction will approach an equilibrium distribution and thus result in increased TMA production. Because of its high activity and shape selectivity for monomethylamine and dimethylamine, rho zeolite is preferred over chabazite. The binders of the invention which are admixed with the zeolites may be comprised of one or more of the following metal oxides, most of which are neutral or mildly acidic for use in methylamine synthesis and which have sufficient mechanical properties to confer attrition resistance in microspherical catalysts compositions: alpha-alumina, titania, bentonite and kaolin. Submicron alpha alumina is most preferred because of its hardness and catalytic inertness. Bentonite is preferred because of its exceptional binding efficiency. In order to form the catalyst in microspheres, a spray drying process is employed, the first step of which is the formation of an aqueous slurry containing the binder and the zeolite catalyst. In some cases, the pH of this slurry can be important (pH can be adjusted by the addition of an appropriate acid, such as nitric acid). For instance, a range in pH of the composition from <2 to >9 will not significantly change the attrition characteristics of the composition for the bentonite or titania binders. However, for the alpha alumina system, a pH .ltoreq. about 2 (about 1.8) is preferred. In addition, for the alpha alumina systems, it is desirable to hold the slurry, with high speed stirring, for about 1-2 hours prior to use. The standing particle size of the binders range from 0.2 to 3 micrometers. Alpha alumina is available from various suppliers in the form of powders with a median particle diameter (d.sub.50) between about 0.2 and 3 micrometers. In the case of Alcoa's A16 SC alpha alumina (Alcoa Industrial Chemicals, Bauxite, AR) a high yield of submicron particles can be obtained by slurrying the powders in water and decanting the fine fraction of particles. Bentonite is an aluminosilicate clay consisting of submicron agglomerates of colloidal particles. It can be obtained from various suppliers, one of which is Southern Clay Products, Gonzales, Tex. as Gellwhite H--NF. TiO.sub.2 can be obtained as a submicron powder from Degussa. Much of the TiO.sub.2 used in this study is a fumed titania, Degussa's P25 (Degussa, Pigments Division, Ridgefield Park, N.J.). The ultimate particle size of the binder has an influence on the attrition resistance of the zeolite composites. For instance, <0.5 micron alpha alumina binders (with rho zeolite) imparts a lower attrition rate (by about 50%) than 0.5 micrometer alumina. In addition, the crystallite size of the rho zeolite should be micron sized or lower for proper dispersion. Use of a high speed mixer is preferred for proper dispersion of the aqueous slurry used for spray drying. A preferred catalyst composition is formed using rho zeolite as the catalyst component. In a typical preparation, it was found that the hydrogen form of rho zeolite (calcined) or the ammonium form (uncalcined) could be blended with the appropriate binders by slurrying both components, zeolite and binder with water (water-based solution) to make a 20-50 wt % solids. The slurry is then spray dried to form the microspherical particles. Spray drying conditions are chosen to produce a particle ranging from 20 to 150 microns. Some experimental parameters, such as slurry concentration, atomization pressure and feed rate can affect the particle size distribution and particle microstructure. These parameters will also vary with the spray dryer configuration and nozzle type used to prepare the material. Applicants used a 4.5 ft i.d. spray dryer fitted with a two fluid nozzle in a counter-current, fountain configuration. Typical conditions include a feed rate of 160 ml/min., inlet temperature of 376.degree. C., and outlet temperatures of 160-170.degree. C. The spray dried powders are then calcined in air by heating at about 600.degree. C., and maintained at that temperature for 8 hours. The calcined powder is screened to produce a catalyst in the correct particle size distribution and to minimize particles less than 20 microns in diameter. Typically, a distribution of particles ranging from 20 to about 150 microns in diameter is produced. A median particle diameter (d.sub.50) of 50 to 70 microns is usually obtained. The median particle diameter (d.sub.50) is calculated based on median cumulative volume, assuming all particles are spherical. The median cumulative volume is determined from a gaussian distribution based on particle volume. Additionally, to further enhance selectivity to methylamines, the catalysts of the invention can be modified by treatment with one or more compounds selected from the group consisting of silicon, aluminum, phosphorous, and boron, by depositing at least 0.05 weight percent of the element. Such deposition can be performed at various steps in the catalyst preparation. For a detailed description of such modification methods, see U.S. Pat. Nos. 4,683,334 and 4,752,596. Attrition measurements are performed using an attrition mill which simulates particle attrition near the gas spargers of a fluidized bed. A catalyst charge is loaded into a column fitted with a single 0.016" perforation. Air flows through the perforation, fluidizes the catalyst bed, and causes attrition. For most measurements, the constant air flow through the mill is calibrated to yield a linear velocity of 760 ft/s through the orifice; this compares to a typical velocity of 150 ft/s in a commercial fuel spargers. The attrition mill measurement accelerates attrition by a factor of roughly thirty. A 24 hour attrition measurement is a reliable indicator of attrition in a commercial reactor. Attrited fines (i.e., those particles lower than 20 micrometers in diameter) are collected in an overhead flask which is fitted with a porous thimble. Flask weight, recorded as a function of time, is used to calculate attrition. The determination of attrition is calculated as an attrition ratio, AR: catalyst attrition divided by the attrition rate of a fluid cracking catalyst standard (FCC). The FCC standard is supplied by Davison Chemical, Baltimore Md. (SMR-5-5209-0293). This catalyst, which contains zeolite Y, is typical of the highly attrition resistant catalysts used in FCC Catalytic Crackers for petroleum refining. As used herein, for a catalyst to be considered attrition resistant, the attrition ratio (AR) should be less than or equal to about 3. In all cases, in addition to the attrition resistance determined by weighing the fines collected in the flask, the contents of the bed are analyzed by SEM (scanning electron microscopy) as well as for particle size distribution (Coulter Counter or Microtrack techniques) to check that any fines that are produced are properly elutritated (disengaged) from the attrition mills. A catalyst is considered to be attrition resistant only if the weight of fine particles carried over to the flask is acceptably low, and if the contents of the mill do not show any appreciable quantities of fine particles (particles less than 20 microns in diameter). |
| PATENT EXAMPLES | This data is not available for free |
| PATENT PHOTOCOPY | Available on request |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |