AROMATIZATION REACTIONS WITH ZEOLITES

Abstract: A process for producing aromatic hydrocarbons comprises contacting one or more non-aromatic compounds, such as propane, propylene, or methanol, with a catalyst containing a zeolite, such as ZSM-5, which has been modified with phosphorus oxide by impregnating the zeolite with a source of phosphate ions, such as an aqueous solution of an ammonium phosphate, followed by calcination.

Description: The present invention relates to aromatization reactions with zeolites, such as ZSM-5. Zeolites, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO₄ and AlO₄ in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration. Although zeolites may contain silica and alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. More particularly, GeO₂ is an art recognized substitute for SiO₂ and B₂O₃, Cr₂O₃, Fe₂O₃, and Ga₂O₃ are art recognized replacements for Al₂O₃. Accordingly, the term zeolite as used herein shall connote not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and/or aluminum. On the other hand, the term aluminosilicate zeolite as used herein shall define zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof, as opposed to materials which contain substantial amounts of suitable replacement atoms for such silicon and/or aluminum. U.S. Patent No. 4,350,835 describes a process for converting ethane to liquid aromatics by contacting the ethane with a zeolite catalyst such as ZSM-5 having incorporated therein a minor amount of gallium. U.S. Patent No. 4,175,057 describes a process for producing aromatics by contacting a C₃-C₈ hydrocarbon with a gallium catalyst supported on an aluminosilicate in which the ratio of silica to alumina is between 20:1 and 70:1. U.S. Patent No. 4,157,356 describes a process for producing aromatic hydrocarbons by contacting a C₃-C₈ hydrocarbon with a gallium catalyst on a silica support which has a surface area greater than 500 m²/g and a pore volume less than 0.8 ml/g. U.S. Patent Nos. 3,972,832 and 3,911,041 describe the treatment of various zeolites with phosphorus compounds which are capable of reacting or exchanging with hydrogen in zeolites followed by calcination. This particular phosphorus treatment is, however, taught to result in the substantial reduction of aromatization activity of the zeolites. According to the present invention, there is provided a process for producing aromatic hydrocarbons, said process comprising contacting a feedstock comprising ethylene, propane, propylene, methanol or dimethylether with a catalyst comprising a crystalline zeolite material having a constraint index within the range of 1 to 12 and an oxide of phosphorus which has been incorporated in said catalyst by a method comprising the steps of: (i) impregnating said zeolite with a source of phosphate ions; and (ii) calcining the impregnated zeolite of step (i) to convert said impregnated phosphate ions to said oxide of phosphorus. An important characteristic of the crystal structure of the particular class of zeolites suitable for use in accordance with the present invention is that it provides a selective constrained access to and egress from the intracrystalline free space by virtue of having an effective pore size intermediate between the small pore Linde A and the large pore Linde X, i.e. the pore windows of the structure are of about a size such as would be provided by 10-membered rings of silicon atoms interconnected by oxygen atoms. However, rather than attempt to judge from crystal structure whether or not a zeolite possesses the necessary constrained access to molecules of larger cross-section than normal paraffins, a simple determination of the "Constraint Index" of the zeolite may be made as described in, for example, U.S. Patent No. 4,016,218. Zeolites suitable for the present invention are those having a Constraint Index of about 1 to 12. Constraint Index (CI) values for some typical materials are: Preferably, the zeolite employed also has a silica to alumina molar ratio of at least 12. Preferred zeolites for use herein include ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, and ZSM-48, with ZSM-5 being particularly preferred. Of these materials, ZSM-5 is described in U.S. Patents No. 3,702,886 and Re. 29,948. ZSM-11 is described in U.S. Patent No. 3,709,979. ZSM-12 is described in U. S. Patent No. 3,832,449. ZSM-23 is described in U.S. Patent No. 4,076,842. ZSM-35 is described in U.S. Patent No. 4,016,245 . ZSM-38 is described in U.S. Patent No. 4,046,859. ZSM-48 is described in U.S. Patent No. 4,375,573. It is to be understood that in relying on the foregoing patent documents to describe the specified zeolites it is intended that identification of the zeolites be resolved on the basis of their respective X-ray diffraction patterns. The reference to the identified patent documents should therefore not be construed as limiting the disclosed crystalline zeolites to those having the specific compositions, e.g., silica-alumina mole ratios, discussed therein, since it is now known that such aluminosilicate zeolites may be substantially aluminum-free and yet may be useful or even preferred in some applications. It is the crystal structure, as identified by the X-ray diffraction "fingerprint", which establishes the identity of the specific crystalline zeolite material. The specific zeolites described, when prepared in the presence of organic cations, are substantially catalytically inactive, possibly because the intra-crystalline free space is occupied by organic cations from the forming solution. Thus, before use as catalysts, they are activated by heating in an inert atmosphere at 540°C for one hour, for example, followed by base exchange with ammonium salts and then by calcination at 540°C in air. Similar activation by ammonium exchange and air calcination is effected if the zeolite is synthesized in the alkali metal form. In addition the original alkali metal of the zeolite may be partially replaced by ion exchange with other suitable metal cations of Groups I through VIII of the Periodic Table, including, by way of example, nickel, copper, zinc, palladium, calcium or rare earth metals. Natural zeolites may sometimes be converted to zeolite structures of the class herein identified by various activation procedures and other treatments such as base exchange, steaming, alumina extraction and calcination, alone or in combinations. Natural minerals which may be so treated include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite, and clinoptilolite. In practicing the present conversion process, it may be useful to incorporate the above-described crystalline zeolites with a matrix comprising another material resistant to the temperature and other conditions employed in such processes. Such matrix materials include synthetic or naturally occurring substances as well as inorganic materials such as clay, silica and/or metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be composited with the zeolite include those of the montmorillonite and kaolin families, which families include the sub-bentonites and the kaolins commonly known as Dixie, McNamee-Georgia and Florida clays or others in which the main mineral constitutent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. In addition to the foregoing materials, the zeolites employed herein may be composited with a porous matrix material, such as alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, and silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may be in the form of a cogel. The relative proportions of zeolite component and inorganic oxide gel matrix, on an anhydrous basis, may vary widely with the zeolite content ranging from between 1 to 99 percent by weight and more usually in the range of 5 to 80 percent by weight of the dry composite. An additional component of the conversion catalysts used in the present process is a minor proportion, from 0.05% to 50%, more preferably 0.25% to 25%, and most preferably 0.7% to 15%, of a phosphorus oxide by weight of the catalyst composite. The phosphorus oxide is incorporated in the catalyst by contacting the catalyst with a solution of an appropriate phosphate compound, followed by drying and calcining to convert phosphorus in the catalyst to its oxide form. Particularly preferred phosphates are ammonium phosphates, including ammonium hydrogen phosphate, (NH₄)₂HPO₄, and ammonium dihydrogen phosphate, NH₄H₂PO₄. Calcination is generally conducted in the presence of oxygen at a temperature of at least l50°C, for 3-5 hours, although the heating time may be extended to 24 hours or longer. Preferably, calcination is conducted at a temperature of at least 500°C since this is found to give higher selectively to BTX (benzene, toluene and xylenes) and lower formation of C₉+ aromatics but with a slightly decreased conversion rate. However, if the calcination temperature is too high, in excess of 700°C for paraffin conversion, both selectivity and conversion rate are decreased. For the conversion of olefins to BTX, the upper limit for the calcination temperature appears to extend to at least 800°C. It will be understood that phosphate ions, being negatively charged, are incapable of exchanging or reacting with hydrogen in zeolites in the manner of the phosphorus compounds, such as trimethylphosphite, described in the US Patent Nos. 3,972,832 and 3,911,041. It is further to be noted that the zeolites, when contacted with phosphate ions need not be in the hydrogen form. More particularly, when the zeolites are contacted with phosphate ions, the cation exchange sites of the zeolite may be occupied with cations other than hydrogen, which cations can later be calcined or exchanged to produce catalytically active hydrogen sites. Ammonium cations are preferred cations for occupying cation exchange sites of the zeolite when the zeolite is contacted with phosphate ions. Such ammonium cations are converted to hydrogen when the zeolite is calcined to convert phosphate ions to phosphorus oxide. Additional catalyst modifying procedures which may also optionally be employed to modify catalyst activity or selectivity include precoking and presteaming (e.g., before oxide incorporation), or combinations thereof. The aromatization conditions suitable for use in accordance with the present process include a temperature of from 200°C to 700°C, a pressure of from 10 to 6080 kPa (0.1 to 60 atmospheres), a weight hourly space velocity (WHSV) of from 0.1 to 400 and a hydrogen/hydrocarbon mole ratio of from 0 to 20. In one embodiment, the feedstock to be aromatized comprises ethylene, propylene and/or propane. When these three hydrocarbons are present in admixture, the feedstock to be aromatized may be a refinery off-gas and comprise from 1 to 30 weight percent of ethylene, from 1 to 30 weight percent of propylene and from 1 to 30 weight percent of propane. Alternatively, the feedstock comprises methanol and/or dimethylether. The phosphorus oxide modified zeolites in accordance with the present invention have been observed to have at least one of the following characteristics in aromatization reactions in comparison with corresponding non-phosphorus oxide modified zeolite: (1) they produce less aromatics having 9 or more, especially 10 or more carbon atoms (i.e. C₉⁺ and C + / 10 aromatics); (2) they produce more benzene; and/or (3) they have more activity in terms of overall feedstock conversion. At the same time, these phosphorus oxide modified zeolites have been observed to produce BTX (i.e. a combination of benzene, toluene, xylene and/or ethylbenzene) in an amount of at least 95 percent by weight or even greater than 100 percent by weight of the amount produced by the corresponding non-phosphorus modified zeolites. The invention will now be more particularly described with reference to the following Examples. EXAMPLE 1 Modification of ZSM-5 containing 35% alumina binder was made by treating 200g zeolite in a solution of 80 g diammonium hydrogen phosphate in 300ml water at about 80°C for 2 hours. The phosphorus impregnated zeolite was then filtered, dried and calcined at 500°C for 2 hours. The resulting zeolite contained 3.54% by weight phosphorus present in an oxide form. Conversion of hydrocarbons was carried out by placing 5.0g catalyst in a Quartz reactor and passing through the reactor 40cm³/min of the desired hydrocarbon at the desired temperature. The results are listed below in Tables 1-3 together with the results for comparison from the corresponding non-phosphorus oxide modified catalyst (i.e., HZSM-5). PZSM-5 gave higher benzene and BTX yields for all reactions and higher activity for propane conversions. It is noted that the simulated refinery off gas of Table 3 included 22. 6% ethane, 15.1% ethylene, 6.7% propane and 16% propylene. EXAMPLE 2 In this Example the conversion of methanol over PZSM-5 was compared with the conversion over HZSM-5. The PZSM-5 gave higher yields of BTX, higher yields of C₅-C₉ hydrocarbons and decreased yields of C + / 10 aromatics. Two catalysts were used for comparison, an unmodified HZSM-5 and a PZSM-5. The procedures for preparing the PZSM-5 were as follows:50g ZSM-5 in ammonium form was treated with a solution of 10g diammonium hydrogen phosphate in 80ml water at about 80°C for 4 hours. After drying at 60-80°C for 16 hours and calcination at 500°C, analyses showed that the zeolite contained 2.7% by weight P. Methanol conversion was run at 400 and 450°C over 5.0g catalyst at WHSV = 1.3. Results are shown below in Table 4. Methanol conversion was complete in all cases. The results show that for a given temperature, PZSM-5 gave higher BTX yields than HZSM-5. PZSM-5 also gave higher yields of C₅ to C₉ range hydrocarbons, a gasoline range, than HZSM-5. In this range, the higher yield of toluene would also result in higher octane gasoline. The C₉⁺ aromatics were noticeably lower with PZSM-5, especially those of C + / 10 aromatics, in which the yield of durene was also lower than HZSM-5. The zeolites in this Example contained 35% alumina binder. EXAMPLE 3 Three phosphorus modified ZSM-5 catalysts were prepared in essentially the same manner as in Example 1, except that different calcination temperatures were used to convert impregnated phosphorus compound to phosphorus oxide. More particularly, the first catalyst was calcined at 500°C, the second catalyst was calcined at 700°C, and the third catalyst was calcined at 800°C. These catalyst were then used to convert propane under equivalent conditions at temperatures of 500°C, 550°C and 600°C. Results of these propane conversions are summarized in Table 5. (a) Calcine for 3 h. at specified temperature. EXAMPLE 4 The three phosphorus modified catalysts of Example 3 were used to convert a hydrocarbon in the same manner as Example 3, except that propylene was substituted for the propane reactant. Results of these propylene conversions are summarized in Table 6. (a) Calcine for 3 h. at specified temperature. EXAMPLE 5 The three phosphorus modified catalysts of Example 3 were used to convert hydrocarbons in the same manner as in Example 3, except that a simulated refinery off gas (SROG) was substituted for the propane reactant. This simulated refinery off gas included 22.6% ethane, 15.1% ethylene, 6.7% propane and 16% propylene. Results of these SROG conversions are summarized in Table 7. (a) Calcine for 3 h. at specified temperature.

Claims: A process for producing aromatic hydrocarbons comprising contacting a feedstock comprising ethylene, propane, propylene, methanol or dimethylether with a catalyst comprising a crystalline zeolite having a constraint index within the approximate range of 1 to 12 and an oxide of phosphorus which has been incorporated into said catalyst by a method comprising the steps of: (i) impregnating said zeolite with a source of phosphate ions; and (ii) calcining the impregnated zeolite of step (i) under conditions sufficient to convert said impregnated phosphate ions to said oxide of phosphorus. A process according to claim 1, wherein said source of phosphate ions is an aqueous solution of an ammonium phosphate. A process according to claim 2, wherein essentially all of the cationic exchange sites of said zeolite are occupied by ammonium ions prior to and during the impregnation of said zeolite with said ammonium phosphate. A process according to any preceding claim, wherein said zeolite is an aluminosilicate zeolite having a silica/alumina mole ratio of at least 12. A process according to any preceding claim wherein said zeolite is selected from ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 or ZSM-48. A process according to any preceding claim wherein said aromatization conditions include a temperature of from 200°C to 700°C, a pressure of from 10 to 6080 kPa (0.1 to 60 atmospheres), a weight hourly space velocity (WHSV) of from 0.1 to 400 and a hydrogen/hydrocarbon mole ratio of from 0 to 20. A process according to any preceding claim, wherein said feedstock comprises ethylene, propane and/or propylene. A process according to any one of claims 1-6, wherein said feedstock comprises methanol. A process according to any one of claims 1-6, wherein said feedstock comprises dimethylether. A process according to any preceding claim wherein the calcination temperature of said step (ii) is at least 150°C. A process according to any preceding claim wherein the calcination temperature of said step (ii) is at least 500°C.

Inventors: CHU, CHIN-CHIUN
Applicants: MOBIL OIL CORPORATION
Application year: 1986
Main classification: B01J29
IPC codes: B01J29, C07C1, C07C2