WO1994022791A1

WO1994022791A1 - Catalyst and process for shape selective hydrocarbon conversion

Info

Publication number: WO1994022791A1
Application number: PCT/US1994/002525
Authority: WO
Inventors: Rudolph Michael Lago; David Owen Marler; Sharon Brawner Mccullen
Original assignee: Mobil Oil Corporation
Priority date: 1993-04-05
Filing date: 1994-03-08
Publication date: 1994-10-13
Also published as: AU6399794A; US5349114A; US5610112A

Abstract

A process for shape selective hydrocarbon conversion, particularly toluene disproportionation, involves contacting a hydrocarbon feedstream under conversion conditions with a modified catalytic molecular sieve which has been modified by being preselectivated with a first silicon source, then steamed.

Description

CATALYST AND PROCESS FOR SHAPE SELECTIVE HYDROCARBON CONVERSION

The present invention is directed to a catalyst and process for shape selective hydrocarbon conversion.

The term shape-selective catalysis describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g. by N.Y. Chen, .E. Garwood and F.G. Dwyer, "Shape Selective Catalysis in

Industrial Applications, 3jS, Marcel Dekker, Inc. (1989) . Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, al ylation or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. A final type of selectivity results from configurational diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in the selective disproportionation of toluene to p- xylene. The production of para-xylene is typically performed by methylation or disproportionation of toluene over a catalyst under conversion conditions. Examples are the reaction of toluene with methanol as described by Chen et al., J. Amer. Chem. Sec. 1979. 101, 6783, and toluene disproportionation, as described by Pines in "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press, N.Y., 1981, p. 72. Such methods typically result in the production of a mixture including para-xylene, ortho- xylene, and meta-xylene. Depending upon the para- selectivity of the catalyst and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amount of feedstock actually converted to xylene, is also affected by the catalyst and the reaction conditions.

The equilibrium reaction for the conversion of toluene to xylene and benzene proceeds as follows:

2 Moles Toluene «■ 184.27g I

75.55g 100 .72g

46.09g 62.63g

15.03g 33.02g 13.70g para- meta- ortho- xylene xylene xylene

(24%) (54%) (22%) p-Xylene Yield = 100 x 15.03 = 8.2% 184.27

Yield = Selectivity x Conversion = 15.03 x 0.59 =8.2%

108.72 p-Xylene Purity = 100 x 15.03 = 24%

62.63 One method for increasing para-selectivity of zeolite catalysts is to modify the catalyst by treatment with a "selectivating agent". For example, U.S. Patents 4,950,835, 4,927,979, 4,465,886, 4,477,583, 4,379,761, 4,145,315, 4,127,616,

4,100,215, 4,090,981, 4,060,568 and 3,698,157 disclose specific methods for contacting a catalyst with a modifying compound containing silicon.

U.S. Patent No. 4,548,914 describes another modification method involving impregnating catalysts with difficultly reducible oxides such as those of magnesium, calcium and/or phosphorus followed by treatment with water vapor to improve para- selectivity. Steaming has been used in the preparation of zeolite catalysts to modify the activity of the catalyst or improve its stability. For example, U.S. Patent No. 4,559,314 describes steaming a zeolite/ binder composite at 200"-500° C for at least an hour to enhance activity by raising the alpha. U.S.

Patent No. 4,522,929 describes presteaming a fresh zeolite catalyst so that the alpha activity first rises then falls to the level of the fresh unsteamed catalyst, producing a stable catalyst which may be used in xylene isomerization. U.S. Patent No.

4,443,554 describes steaming inactive zeolites (Na ZSM-5) to increase alpha activity. U.S. Patent No. 4,487,843 describes contacting a zeolite with steam prior to loading with a Group IIIB metal. There has been no suggestion, however, to steam treat silicon pre-selectivated catalysts to enhance shape-selectivity. It has now been found that pre- selectivation treatment followed by steam treatment of a molecular sieve catalyst provides unexpectedly better results in shape selective hydrocarbon conversions than pre-selectivation alone or steam treatment alone. Furthermore, steaming alone has been found to be detrimental in the context of the present invention. Accordingly, the invention resides in one aspect in a catalyst comprising a molecular sieve having an initial Constraint Index of 1-12 which has been modified by being pre-selectivated by contacting with a first silicon source, calcined, and treated with steam.

In a further aspect, the invention resides in a process for shape selective hydrocarbon conversion which comprises contacting a reaction stream comprising a hydrocarbon to be converted with a catalytic molecular sieve which has been modified by being pre-selectivated with a first silicon- containing compound and subsequently steamed. The present invention is useful in shape selective hydrocarbon conversion reactions, for example, in converting various aromatics such as toluene to commercially useful para-substituted benzenes, such as para-xylene.

The catalytic molecular sieves useful herein have an initial Constraint Index from 1 to 12 and include intermediate pore zeolites. Zeolites which conform to the specified values of Constraint Index for intermediate pore zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-50, ZSM- 57 and Zeolite Beta which are described, for example, in U.S. Patent Nos. 3,702,886 and Re. No. 29,949, 3,709,979, 3,832,449, 4,046,859, 4,556,447, 4,076,842, 4,016,245, 4,229,424, 4,397,827, 4,046,859, 3,308,069 and Re. 28,341 and EP 127,399. MCM-22, described in U.S. Patent No. 4,973,784, is also useful herein. The catalyst employed in the present invention preferably has an alpha value greater than 100, for example 150 - 2000, and a silica-alumina ratio less than 1000 preferably 20 - 500. The Alpha Value test is described in U.S. Patent 3,354,078 and in The Journal of Catalysis. Vol. 4, pp. 522-529 (August 1965): Vol. 6, p. 278 (1966); and Vol. 61, p. 395 (1980) .

In the catalyst of the invention, the molecular sieve may be combined with a support or binder material such as, for example, a porous inorganic oxide support or a clay binder. While the preferred binder is silica, other non-limiting examples of such binder materials include alumina, zirconia, magnesia, thoria, titania, boria and combinations thereof, generally in the form of dried inorganic oxide gels or gelatinous precipitates. Suitable clay materials include, by way of example, bentonite and kieselguhr. The relative proportion of suitable crystalline molecular sieve to the total composition of catalyst and binder or support may be 30 to 90 percent by weight and is preferably 50-80 percent by weight of the composition. The composition may be in the form of an extrudate, beads or fluidizable microspheres. The catalyst may be further modified in order to reduce the amount of undesirable by-products, particularly ethylbenzene in the disproportionation of toluene, by the incorporation of a hydrogenation/ dehydrogenation component. The preferred hydrogenation-dehydrogenation component is platinum, but other metals such as palladium, nickel, copper, cobalt, molybdenum, rhodium, ruthenium, silver, gold, mercury, osmium, iron, zinc, cadmium, and mixtures thereof may be utilized. The metal may be added by cation exchange, in amounts of 0.01 - 2%, typically about 0.5% by weight of the catalyst. The metal must be able to enter the pores of the catalyst in order to survive the subsequent calcination step.

To produce the catalyst of the invention, the initial molecular sieve, incorporated with a binder or in unbound form, is treated with a first silicon source. This first silicon treatment will be called pre-selectivation. In pre-selectivation, the silicon compound is deposited on the external surface of the catalyst by any suitable method. For example, the silicon may be dissolved in a solvent, mixed with the catalyst, and then dried. The silicon compound employed may be in the form of a solution, a liquid or a gas under the conditions of contact with a zeolite. Examples of methods of depositing silicon on the surface of the zeolite are found in U.S. Patents 4,090,981, 4,127,616, 4,465,886 and 4,477,583 to Rodewald. The deposited silicon compound extensively covers the surface and resides substantially exclusively on the external surface of the molecular sieve.

The first silicon-containing compounds which are silicon sources for the pre-selectivation include alkoxy silanes, silanes and organoamine silane polymers. Also suitable are silicones which are the high efficiency, p-xylene selectivating agents discussed below which are also used as second silicon sources for trim selectivation. Preferred silicon containing compounds for pre-selectivation include Si(OR)₄ wherein R = CH₃, C₂H₅ or C₃H₇; or a silicone polymer (SiO(R')₂)_n wherein R'= alkyl of C^_o, aryl of ^c ₆-_ιo/ ^or hydroxide and n is greater than 10 and less than 1000; or an organoamine silane polymer where the organoamine is -N(CH₃)₃, -N(C₂H₅)₃ or -N(C₃H₇)₃. The molecular sieve may be contacted with silicon containing compound at a molecular sieve/silicon compound weight ratio of 100/1 to 1/100. Following deposition of the first silicon- containing compound, the catalyst is calcined. For example, the catalyst may be calcined in an oxygen- containing atmosphere, preferably air, at a rate of 0.2" to 5°C./minute to a temperature greater 300^βC. but below a temperature at which the crystallinity of the zeolite is adversely affected. Generally, such temperature will be below 600*C. Preferably the temperature of calcination is within the approximate range of 350* to 550*C. The product is maintained at the calcination temperature usually for 1 to 24 hours.

After pre-selectivation and calcining, the catalyst is subjected to steam treatment at a temperature of 200^βC to 540^βC, preferably 280"C to 400°C with 5% to 100% steam at a pressure of 108 to

445 kPa (0.1 to 50 psig), preferably from 50% to 100% steam, for about two to about twelve hours, preferably from about three to about six hours.

The pre-selectivated molecular sieve catalyst, with or without binder, shows improved selectivity after steaming. If the catalyst is not pre- selectivated before steaming or if the catalyst is pre-selectivated without steaming, as shown in the examples below, the same improvement in selectivity does not occur. Indeed, steaming alone can be detrimental.

After silicon pre-selectivation and steaming, the catalyst may be "trim selectivated" by contact with a mixture of toluene and a second silicon source, which is a high-efficiency selectivating agent, at reaction conditions for converting toluene to xylene. Reaction conditions for this trim selectivation step generally include a temperature of 350" - 540°C and a pressure of 100 to 34600 kPa (atmospheric - 5000 psig) . The feed is provided to the system at a rate of 0.1 - 20 WHSV and hydrogen is preferably fed to the system at a hydrogen to hydrocarbon molar ratio of 0.1-20. The trim selectivation will last for at least one hour, or preferably 50-300 hours, most preferably less than 170 hrs.

The second silicon-containing compound used for the trim-selectivation is preferably a silicone compound which obeys the general formula:

where R. is hydrogen, fluorine, hydroxy, alkyl, aralkyl, alkaryl or fluoro-alkyl. The hydrocarbon substituents generally contain from 1 to 10 carbon atoms and preferably are methyl or ethyl groups. R₂ is selected from the same group as R_{l f} and n is an integer of at least 2 and generally in the range of 3 to 1000. The molecular weight of the silicone compound employed is generally between 80 and 20,000 and preferably within the range of 150 to 10,000.

Representative silicone compounds include dimethyl- silicone, diethylsilicone, phenylmethylsilicone, methylhydrogensilicone, ethylhydrogensilicone, phenylhydrogensilicone, methylethylsi1icone, phenylethylsilicone, diphenylsilicone, methyltri- fluoropropylsilicone, ethyltrifluoropropysilicone, polydimethylsilicone, tetrachlorophenylmethyl silicone, tetrachlorophenylethyl silicone, tetra- chlorophenylhydrogen silicone, tetrachlorophenyl- phenyl silicone, methylvinylsilicone and ethylvinyl- silicone. The silicone compound need not be linear but may be cyclic as for example hexamethylcyclotri- siloxane, octamethylcyclotetrasiloxane, hexaphenylcyclotrisiloxane and octaphenyl- cyclotetrasiloxane. Mixtures of these compounds may also be used as well as silicones with other functional groups. Other silicon-containing compounds, such as silanes, may also be utilized. Preferably, the kinetic diameter of the high efficiency, p-xylene selectivating agent is larger than the zeolite pore diameter, in order to avoid reducing the internal activity of the catalyst.

The modified catalysts of the present invention are suitable for use in a variety of shape selective hydrocarbon conversion processes including cracking hydrocarbons with reaction conditions including a temperature of 300^βC to 700^βC, a pressure of 0.1 atmosphere (bar) to 30 atmospheres and a weight hourly space velocity of 0.1 hr"¹ to 20 hr"¹; dehydrogenating hydrocarbon compounds with reaction conditions including a temperature of 300^βC to 700^βC, a pressure of 0.1 to 10 atmospheres, and a weight hourly space velocity of 0.1 to 20; converting paraffins to aromatics with reaction conditions including a temperature of 300°C to 700^βC, a pressure of 0.1 to 60 atmospheres, a weight hourly space velocity of 0.5 to 400 and a hydrogen/hydrocarbon mole ratio of 0 to 20; converting olefins to aromatics, e.g. benzene, toluene and xylene, with reaction conditions including a temperature of 100"C to 700^βC, a pressure of 0.1 to 60 atmospheres, a weight hourly space velocity of 0.5 to 400 and a hydrogen/hydrocarbon mole ratio of 0 to 20; converting alcohols, e.g. methanol, or ethers, e.g. dimethylether , or mixtures thereof to hydrocarbons including olefins and/or aromatics with reaction conditions including a temperature of 275°C to 600^βC, a pressure of 0.5 to 50 atmospheres and a liquid hourly space velocity of 0.5 to 100; isomerizing xylene feedstock components with reaction conditions including a temperature of from 230^βC to 540^βC, a pressure of 100 to 7000 kPa (0 to 1000 psig) , a weight hourly space velocity of 0.1 to 200 and a hydrogen/hydrocarbon mole ratio of 0 to 100; disproportionating toluene with reaction conditions including a temperature of 200*C to 760^βC, a pressure of atmospheric to 60 atmospheres and a weight hourly space velocity of 0.08 to 20; alkylating aromatic hydrocarbons, e.g. benzene and alkylbenzenes in the presence of an alkylating agent, e.g. olefins, formaldehyde, alkyl halides and alcohols, with reaction conditions including a temperature of 250° to 500*C, a pressure of atmospheric to 200 atmospheres, a weight hourly space velocity of 2 to 2000 and an aromatic hydrocarbon/alkylating agent mole ratio of 1/1 to 20/1; and transalkylkating aromatic hydrocarbons in the presence of polyalkylaromatic hydrocarbons with reaction conditions including a temperature of 340"C to 500"C, a pressure of atmospheric to 200 atmospheres, a weight hourly space velocity of 10 to 1000 and an aromatic hydrocarbon/ polyalkylaromatic hydrocarbon mole ratio of 1/1 to 16/1. In general, therefore, catalytic conversion conditions over the catalyst of the invention include a temperature of 100°C to 760^βC, a pressure of 0.1 atmosphere (bar) to 200 atmospheres (bar) , a weight hourly space velocity of 0.08 hr"^α to 2000 hr"¹ and a hydrogen/organic, e.g. hydrocarbon compound of 0 to 100.

Toluene Disproportionation

Toluene disproportionation will be used as a representative shape selective conversion. A catalyst treated in the manner described herein has a desirable decreased ortho-xylene sorption rate parameter and yields a para-selective product in toluene disproportionation. Diffusion rate constants in toluene disproportionation have been discussed by D.H. Olson and W.O. Haag, "Structure-Selectivity Relationship in Xylene Isomerization and Selective Toluene Disproportionation", Catalytic Materials Relationship Between Structure and Reactivity, ACS Symposium Ser. No. 248 (1984) .

In toluene disproportionation, toluene diffuses into the zeolite with a diffusivity D_τ. The toluene undergoes disproportionation to p-, m-, and o-xylene and benzene at a total rate constant k₀:

D_τ = r²

The degree of para-selectivity depends on the activity and the diffusion characteristics of the catalyst. The primary product will be rich in the para isomer if initially produced m- and o- xylene diffuse out of the zeolite crystal at a lower rate (D-^_o/r²) than that of their conversion to p-xylene ( ._) and the p-xylene diffusion (D_p/r²) out of the catalyst D_m = diffusion of m-xylene

D₀ = diffusion of o-xylene

D_p = diffusion of p-xylene r = length of diffusion path (crystal radius) k_x = rate of interconversion via isomerization of xylene isomers yielding secondary xylene product m-xylene and o-xylene.

It is desirable to reduce the k_x thereby reducing the isomerization of p-xylene to o- and m- xylene in a secondary reaction by adjusting D,_f0/r² downward so that

^kι > — _-

Thus a para-rich primary product will result. It is therefore apparent that if the o-xylene diffusion

(D_o/r²) can be adjusted downward, the p-xylene product will increase.

Selective toluene disproportionation may be carried out under conditions which include a temperature between 200^βC and 600*C, preferably 350^βC to 540"C, a pressure of 100 to 34600 kPa (atmospheric to 5000 psig), preferably 800 to 7000 kPa (100 to 1000 psig) , a weight hourly space velocity (WHSV) of 0.1 to 20, preferably 2 to 10, and a hydrogen/ hydrocarbon mole ratio from 0 to 20, preferably 2 to 6.

The present invention provides a process for obtaining p-xylene at toluene conversion rates of at least 15%, preferably at least 20-25%, with a para- xylene purity of greater than 85%, preferably at least 90%. As used herein, the term "para-xylene purity" means the percentage of para-xylene in all of the xylene products para-xylene, ortho-xylene, and meta-xylene. Those skilled in the art will appreciate that the proximity of the boiling points of these xylene products necessitates expensive separation processes whereas para-xylene may be more readily separated from other components in the product stream such as benzene, toluene, and para- ethyltoluene.

The following non-limiting examples illustrate the invention.

In the examples, the o-xylene sorption rate parameter Dp/r was measured at 120"C and 3.9 torr.

D„ = diffusivity of o-xylene r = crystal size

D_o/r² = the diffusion rate constant (K) which is the rate that o-xylene diffuses out of the crystal

COMPARATIVE EXAMPLES 1-5 EXAMPLE 1 HZSM-5/Al₂0₃ was steamed at 342^βC for 3 hours with 100% steam then tested for toluene disproportionation. One atmosphere toluene was reacted with the steamed ZSM-5/Al₂0₃ at 482°C and the toluene conversion changed by varying the toluene WHSV. The p-xylene selectivity of the unsteamed catalyst was 37.5% at 4% toluene conversion with a TDP (toluene disproportionation) rate constant (K) of 163. After steaming the p-xylene selectivity decreased to 30.6% at 4% toluene conversion and a TDP rate constant of 341. EXAMPLES 2-3 Additional samples of HZSM-5/Al₂0₃ were steamed and tested for toluene disproportionation and TDP rate constant. The results are shown in Table 1. Table 1

Unsteamed Steamed K p-sel^* K p-sel Example 2 190 35.0 363 30.0

Example 3 176 36.9 384 30.0 *p-sel = para selectivity

The results of Examples 1-3 show that steaming a non-pre-selectivated catalyst decreases para- selectivity.

EXAMPLE 4 One gram of water soluble n-propylamine silane polymer was diluted with 1 gram deionized (DI) H₂0.

One gram of as-synthesized ZSM-5 was mixed with 1 gram of the silane polymer/H₂0 solution at room temperature for 2 hours. The sample was dried at 130^βC then calcined in nitrogen followed by air at

538^βC.

The o-xylene sorption was measured at 120^βC before and after treatment. The D/r² had decreased from 8.5 x 10^-6 s^_1 to 6.5 x 10^-7 s"¹.

EXAMPLE 5

A silica modified HZSM-5 catalyst prepared as described in Example 4 was tested for toluene disproportionation. One atmosphere toluene was reacted with the catalyst at 482"C. The conversion was changed by varying the WHSV. 42.9% p-xylene selectivity was obtained at 4% toluene conversion with a rate constant of 105. EXAMPLE 6 The silica modified HZSM-5 prepared as described in Example 4 was steamed at 342°C and 100% H₂0(g) for three hours. After steaming the sample was tested for toluene disproportionation as in Example 5. 73.2% p-selectivity was obtained at 4% toluene conversion. The steam treatment increased the p- xylene selectivity from 42.9% to 73.2%. The rate constant for toluene disproportionation increased after steaming from 105 to 150. The o-xylene sorption rate parameter, D/r², had decreased from 6.5 x lO^'V¹ to 1.85 x Kr's^-1.

COMPARATIVE EXAMPLE 7 One gram of water soluble n-propylamine silane polymer was diluted with 5 grams deionized water.

One gram of HZSM-5/Al₂0₃ was mixed with 1 gram of the silane polymer/H₂0 solution at room temperature for 2 hours. The sample was dried at 130°C then calcined in nitrogen followed by air at 538^βC. The o-xylene sorption parameter was measured before and after silane treatment. The D/r² decreased from 4.8 X 10"⁶ s"¹ before treatment to 6.7 X 10"⁷ s"¹ after treatment.

EXAMPLE 8 The silica modified HZSM-5/Al₂0₃ prepared in Example 7 was steamed at 342°C and 100% H₂0(g) for three hours. The sample was tested for toluene disproportionation before and after steaming. Steam treatment increased the p-xylene selectivity from 40.5% to 67.9% at 4% toluene conversion. The rate constant for toluene disproportionation (k_D) increased after steaming from 91 to 195. COMPARATIVE EXAMPLE 9 Ten grams of water soluble n-propylamine silane polymer was diluted with 10 grams deionized (DI) H₂0. Five grams of ZSM-5/Si0₂ was mixed with five grams of the silane solution at room temperature for 2 hours. The sample was dried at 130°C then calcined in nitrogen followed by air at 538°C.

The o-xylene sorption rate was measured before and after silane treatment. The D₀/r² decreased from 1.5 X 10"⁴ s^_1 before treatment to 9.0 X 10^-7 s^_1 after treatment.

EXAMPLE 10 The modified ZSM-5/Si0₂ prepared in Example 9 was steamed at 342"C and 100% H₂0 (g) for three hours.

The o-xylene sorption rate was measured. The D₀/r² decreased from 9.0 X 10^"7 s^"1 before treatment to 5.6 X 10^"7 s^"1 after treatment.

EXAMPLE 11 A silica modified ZSM-5/Si0₂ catalyst prepared as described in U.S. Patent No. 4,090,981 was steamed at 342°C for 3 hours with 100% steam. The catalyst for toluene disproportionation was tested before and after steaming. At 446^βC, 8 WHSV, 2 H₂/HC and 500 psi (3550 kPa) , the steamed silica modified ZSM- 5/Si0₂ showed 93.4% p-xylene selectivity at 18.9% toluene conversion while the unsteamed catalyst showed 79.2% p-xylene selectivity at 18.3% toluene conversion. EXAMPLE 12 A silica modified ZSM-5/Al₂0₃ catalyst prepared as described in U.S. Patent No. 4,090,981 was steamed at 342°C for 3 hours with 100% steam. After steaming, the sample was tested for toluene disproportionation as described in Example 5. After steaming, the p-xylene selectivity increased from 60.6% to 86.7% at 4% toluene conversion. The rate constant for toluene disproportionation increased from 149 to 238.

EXAMPLE 13 NaZSM-5/Si0₂ was treated with a propylamine silane polymer/H₂0 mixture, 7:1 wt ratio, at room temperature for overnight. The sample was filtered and dried at 130"C and then calcined at 538^βC in nitrogen followed by air.

The calcined sample was exchanged 2-3 times with 1 M NH₃N0₃ at room temperature for 1-2 hours. The o- xylene diffusivity was 3 x 10"⁷ s"¹. NH₃-TPAD was performed to determine the number of acid sites which was 0.43 meq/g.

The exchanged Si0₂-ZSM-5/Si0₂ material was calcined in air at 538^βC and then steamed at 315^βC for 3 hours in 100% steam. Catalytic evaluation of the selectivated-steamed catalyst was conducted in an automated testing unit with on line sampling. The sample was heated to 446^βC in 40 cc/min H₂ at a heating rate of 3.5^βC/min. Pure toluene was then introduced at 446^βC, 4, 8, 16 and 32 WHSV, 2 H₂/HC and 3550 kPa (500 psi) to measure the catalytic performance. A solution of 1 wt.% phenylmethyl-dimethyl silicone copolymer (Dow 550) in toluene was then passed over the catalyst at 466^βC, 4 WHSV, 2 H₂/HC and 500 psi for 4 hours. To deter ine the activity/selectivity performance of the selectivated catalysts, reactor temperature was varied to change toluene conversion. Toluene conversion/p-xylene selectivities are shown below in Table 2.

Table 2

Temp, *C 446 466

Toluene Conv. , % 15.5 27.1 p-xylene sel. , % 96.9 91.4

EXAMPLE 14 a. Five grams of propylamine silane polymer were diluted with 5 grams DI H₂0. 10 grams of Na- ZSM-5/Si0₂ were treated with 10 grams of the propylamine silane polymer/H₂0 solution by impregnation for overnight and then dried at 130^βC. The sample was then calcined in 300 cc/min N₂ using a heating rate of 2°C/min to 538°C then held at 538^βC for 2 hours followed by 300 cc/min air heated at 2^βC/min from 300^βC to 538"C then held for 2 hours. The o-xylene diffusivity was measured at 120°C. The selectivation procedure reduced the D/r² from 1.5 x 10^"4 s^"1 to 1 x 10~⁵ s"¹. To reduce the diffusivity further a second selectivation was performed. b. 7 grams of propylamine silane polymer were diluted with 3 grams DI H₂0. 10 grams of Si0₂//Na-

ZSM-5/Si0₂ described in a. above were treated with 10 grams of the propylamine silane polymer/H₂0 solution by impregnation for overnight and then dried at 130^βC. The sample was N₂/air calcined as described above.

The calcined sample was exchanged 2-3 times with 1 M NH₄N0₃ at room temperature for 1-2 hours to reduce the sodium content to less than 500 ppm. The sample was calcined in air at 538°C for 2 hours then steamed at 343^βC in 100% steam. Representative catalyst selectivities are shown below after 6 hours trim selectivation with 1% methylphenyl-dimethylsilicone in toluene which was passed over the catalyst at 466^βC, 4 WHSV, 2 H₂/HC and 3550 kPa (500 psi) for 6 hours. Toluene conversion/p-xylene selectivities are shown in Table 3.

Table 3 T Teemmpp,, **CC 4 46666 446

WHSV 4 8

Pressure, kPa 3550 3550

H₂/HC 2 2

Toluene Conv. , % 30.9 17.1 pp--xxyylleennee sseell..,, %% 9 900..22 93.5

Claims

CLAIMS :

1. A catalyst comprising a molecular sieve having an initial Constraint Index of about 1-12 which has been modified by being pre-selectivated by contacting with a first silicon source, calcined, and treated with steam.

2. The catalyst of claim 1 wherein the first silicon source is selected from silicones, silanes, alkoxysilanes and organoamine silane polymers.

3. The catalyst of claim 1 wherein the silicon- containing compound is selected from:

Si(OR)₄, wherein R is selected from the group consisting of CH₃, C₂H₅ and C₃H₇; (SiO(R')₂)_n, wherein R' is selected from the group consisting of alkyl of C^_o, aryl of C₆.₁₀, hydroxide and hydrogen and n is greater than 10 and less than 1000; and an organoamine silane polymer, wherein the organoamine is selected from the group consisting of -N(CH₃)₃, -N(C₂H₅)₃ and -N(C₃H₇)₃.

4. The catalyst of claim 1 wherein the steaming comprises treating the molecular sieve with steam under conditions comprising between 5 and 100% water vapor, a temperature between 200°C and 540^βC and a pressure of 108 to 445 kPa (0.1 to 50 psig) , for a time of two to 12 hours.

5. The catalyst of claim 1 wherein a pre- selectivated, calcined, steam treated molecular sieve is subsequently contacted with a mixture comprising toluene and a second silicon source at reaction conditions for converting toluene to xylene for at least one hour.

6. The catalyst of claim 5 wherein the second silicon source is a silicone.

7. The catalyst of claim 6 wherein the silicone comprises a mixture of phenylmethylsilicone and di ethylsilicone.

8. A process for shape selective hydrocarbon conversion which comprises contacting a reaction stream comprising a hydrocarbon to be converted under conversion conditions, with a catalytic molecular sieve which has been modified by being pre-selectivated with a first silicon-containing compound and subsequently steamed.

9. The process of claim 8 wherein the shape selective hydrocarbon conversion is toluene disproportionation.

10. The process of claim 9 wherein the conversion conditions comprise a temperature of 350"C to 540^βC, a pressure of 800 to 7000 kPa, a weight hourly space velocity (WHSV) fo 2 to 10, and a hydrogen/hydrocarbon mole ratio of 2 to 6.