Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By alkyl or aryl transfer between molecules – e.g.,...
Reexamination Certificate
2002-08-26
2004-08-17
Yildirim, Bekir L. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By alkyl or aryl transfer between molecules, e.g.,...
C585S470000, C208S046000
Reexamination Certificate
active
06777583
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a selectivated binder-free catalytic molecular sieve.
The term “shape-selective catalysis” describes catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., by N. Y. Chen, W. E. Garwood, and F. G. Dwyer,
Shape Selective Catalysis in Industrial Applications,
36, 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, alkylation 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. Another type of selectivity results from configurational constraints on diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in the 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 selective toluene disproportionation to para-xylene.
The production of para-xylene is typically performed by methylation of toluene or by toluene disproportionation over a catalyst under conversion conditions. Examples include the reaction of toluene with methanol as described by Chen et al.,
J. Amer. Chem. Soc.
101, 6783 (1979), and toluene disproportionation, as described by Pines in
The Chemistry of Catalytic Hydrocarbon Conversions,
Academic Press, NY, 72 (1981). Such methods typically result in the production of a mixture including para-xylene, ortho-xylene, and meta-xylene. Depending upon the degree of selectivity of the catalyst for para-xylene (para-selectivity) and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amount of xylene produced as a proportion of the feedstock, is also affected by the catalyst and the reaction conditions.
Various methods are known in the art for increasing the para-selectivity of zeolite catalysts. One such method is to modify the catalyst by treatment with a “selectivating agent”. For example, U.S. Pat. Nos. 5,173,461; 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 selectivating agent containing silicon (“silicon compound”).
U.S. Pat. No. 4,548,914 describes another modification method involving impregnating catalysts with oxides that are difficult to reduce, such as those of magnesium, calcium, and/or phosphorus, followed by treatment with water vapor to improve para-selectivity.
European Patent No. 296,582 describes the modification of aluminosilicate catalysts by impregnating such catalysts with phosphorus-containing compounds and further modifying these catalysts by incorporating metals such as manganese, cobalt, silicon and Group IIA elements. The patent also describes the modification of zeolites with silicon compounds.
Traditionally, ex situ pre-selectivation of zeolites has involved single applications of the selectivating agent. It may be noted, however, that the suggestion of multiple treatments was made in U.S. Pat. No. 4,283,306 to Herkes. The Herkes patent discloses the promotion of crystalline silica catalyst by application of an amorphous silica such as ethylorthosilicate. The Herkes patent contrasts the performance of catalyst treated once with an ethylorthosilicate solution followed by calcination against the performance of catalyst treated twice with ethylorthosilicate and calcined after each treatment. The Herkes disclosure, however, shows that the twice-treated catalyst is less active and less selective than the once-treated catalyst as measured by methylation of toluene by methanol. Thus, Herkes indicates that multiple ex situ selectivation confers no benefit and in fact reduces a catalyst's efficacy in shape-selective reactions.
In U.S. Ser. No. 08/269,051, the first multiple ex situ selectivation sequence of catalytic molecular sieves to enhance selectivity in hydrocarbon conversion reactions was described. These catalysts proved particularly useful in toluene disproportionation as demonstrated in U.S. Pat. Nos. 5,365,004 and 5,367,099 which issued on the 15th and 22nd of November 1994, respectively. The disclosures of U.S. Pat. Nos. 5,365,004 and 5,367,099 are herein incorporated by reference.
However, because the para-isomers of alkyl-substituted aromatic hydrocarbons (e.g., para-xylene) are utilized to produce a variety of commercial products, there is still a continuing need in the art to increase the efficiency of production.
Accordingly, it is an object of the present invention to improve the efficiency of producing alkyl-substituted aromatic hydrocarbons utilizing ex situ selectivated catalytic molecular sieves.
SUMMARY OF THE INVENTION
There is provided a method for preparing a catalyst, said method comprising the steps of:
(a) contacting a substantially binder-free catalytic molecular sieve under liquid phase conditions with an organosilicon selectivating agent under conditions sufficient to impregnate said molecular sieve with said organosilicon selectivating agent;
(b) calcining the impregnated molecular sieve of step (a) under conditions sufficient to decompose said organosilicon selectivating agent and leave a siliceous residue of said agent on said molecular sieve; and
(c) repeating steps (a) and (b) at least once.
There is also provided a method for preparing a catalyst, said method comprising the steps of:
(a) mulling and then extruding a mixture comprising water, ZSM-5, sodium ions and no intentionally added binder material under conditions sufficient to form an extrudate having an intermediate green strength sufficient to resist attrition during ion exchange step (b) set forth hereinafter;
(b) contacting the uncalcined extrudate of step (a) with an aqueous solution comprising ammonium cations under conditions sufficient to exchange cations in said ZSM-5 with ammonium cations;
(c) calcining the ammonium exchanged extrudate of step (b) under conditions sufficient to generate the hydrogen form of said ZSM-5 and increase the crush strength of said extrudate;
(d) contacting the substantially binder-free ZSM-5 extrudate of step (c) under liquid phase conditions with an organosilicon selectivating agent under conditions sufficient to impregnate said extrudate with said organosilicon selectivating agent;
(e) calcining the impregnated molecular sieve of step (d) under conditions sufficient to decompose said organosilicon selectivating agent and leave a siliceous residue of said agent on said molecular sieve; and
(f) repeating steps (d) and (e) at least once.
There is also provided a process for hydrocarbon conversions, such as toluene disproportionation reactions, using this catalyst.
Shape selective hydrocarbon conversions over the present modified catalytic molecular sieve may be conducted by contacting a reaction stream comprising an alkyl-substituted aromatic hydrocarbon, under conversion conditions, with the present modified catalytic molecular sieve. The modified catalytic molecular sieve is a substantially binder-free catalytic molecular sieve which had been exposed, preferably, to at least two ex situ selectivation sequences. Each ex situ selectivation sequence includes impregnating the substantially binder-free catalytic molecular sieve with a selectivating agent, followed by calcination after each impregnation. Selectivating agents useful in the present invention include a large variety of silicon-containing compounds, preferably silico
Beck Jeffrey S.
Cheng Jane C.
McCullen Sharon B.
Olson David H.
Stern David L.
ExxonMobil Oil Corporation
Yildirim Bekir L.
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