Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By alkyl or aryl transfer between molecules – e.g.,...
Reexamination Certificate
1998-11-04
2002-11-26
Silverman, Stanley S. (Department: 1754)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By alkyl or aryl transfer between molecules, e.g.,...
C585S481000, C502S060000, C502S064000, C502S063000, C502S071000, C502S077000, C502S085000
Reexamination Certificate
active
06486373
ABSTRACT:
This invention relates to a shape selective zeolite catalyst and its use in the conversion of aromatic compounds, particularly the selective production of para-dialkylaromatic compounds.
The term “shape-selective describes unexpected 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 isomerization, disproportionation, alkylation and transalkylation of aromatics 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 alkyl-substituted benzene dispro-portionation to para-dialkyl-substituted benzene.
A representative para-dialkyl-substituted benzene is para-xylene, which is a valuable chemical feedstock for the production of polyesters. 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, N.Y., 1981, p. 72. Such methods typically result in the production of a mixture of the three xylene isomers, i.e., 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.
Another well known method for producing para-xylene is by isomerization of a C
8
aromatic feedstock containing a high proportion of other xylene isomers, particularly meta-xylene. Commercially available C
8
aromatic feedstocks normally contain significant amounts of ethylbenzene, which is difficult to separate by physical methods, and hence an important object of most xylene isomerization processes is to convert ethylbenzene to more readily removable species without undue loss of xylenes.
Various methods are known in the art for increasing the para-selectivity of zeolite catalysts. These typically involve modifying the diffusion characteristics of the zeolite so that the rate at which the unwanted reaction products can diffuse into and out of the zeolite pores is reduced as compared to the diffusion rate of the desired para-product. For example, U.S. Pat. No. 4,117,026 describes a selectivation process in which the ortho-xylene sorption rate of the zeolite is increased by depositing a layer of coke on the surface of the zeolite.
It is also known to increase the para-selectivity of a zeolite by depositing on the zeolite an oxide of a metal, such as an alkaline earth metal (U.S. Pat. No. 4,288,647), a Group IIIB metal, for example gallium, indium and/or thallium (U.S. Pat. No. 4,276,437), a Group IVA metal, for example titanium and/or zirconium (U.S. Pat. No. 4,302,620) and a Group IVB metal, for example tin and/or germanium (U.S. Pat. No. 4,278,827).
An alternative selectivation process described in, 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, is to contact the zeolite with a selectivating agent containing a silicon compound. Such known methods include both ex-situ and in-situ silicon selectivation. In ex-situ selectivation the zeolite is pre-treated with the silicon-containing selectivating agent outside the reactor used for desired shape selective aromatic conversion process. In in-situ selectivation the zeolite is loaded in the aromatic conversion reactor and, during a start-up phase of the reaction, is contacted with a mixture of the silicon-containing selectivating agent and an organic carrier, such as toluene. A combination of both ex-situ and in-situ silicon selectivation can be used. In either event, the selectivation procedure results in the deposition of a silica coating on the surface of the zeolite which modifies the diffusion characteristics of the zeolite.
Traditionally, ex-situ pre-selectivation of zeolites has involved a single application of the selectivating agent. However, U.S. Pat. No. 5,476,823 discloses a process for modifying the shape selectivity of a zeolite by exposing the zeolite to at least two ex-situ selectivation sequences, each of which includes the steps of contacting the zeolite with a silicon-containing selectivating agent in an organic carrier and subsequently calcining the zeolite.
Unexpectedly, it has now been found that certain large crystal forms of ZSM-5, which have a high aluminum content and an unusual aluminum distribution, with the SiO
2
/Al
2
O
3
ratios in bulk being essentially the same as those on the surface, are more responsive to selectivation than conventional ZSM-5 crystals. Such large crystal forms of ZSM-5 are described in International Application No. PCT/US96/09878 which describes a synthetic porous crystalline material having the structure of ZSM-5 and a composition involving the molar relationship X
2
O
3
:(n)YO
2
, wherein X is a trivalent element, such as aluminum, boron, iron and/or gallium, preferably aluminum; Y is a tetravalent element such as silicon and/or germanium, preferably silicon; and n is greater than about 12, and wherein the crystals have a major dimension of at least about 0.5 micron and a surface YO
2
/X
2
O
3
ratio which is no more than 20% grater than the bulk YO
2
/X
2
O
3
ratio of the crystal.
It is to be appreciated that, although ZSM-5 is normally synthesized as an aluminosilicate, the framework aluminum can be partially or completely replaced by other trivalent elements, such as boron, iron and/or gallium, and the framework silicon can be partially or completely replaced by other tetravalent elements such as germanium.
Accordingly, the invention resides in one aspect in shape-selective catalyst comprising a synthetic porous crystalline material having the structure of ZSM-5 and a composition involving the molar relationship:
X
2
O
3
:(
n
)YO
2
,
wherein X is a trivalent element, such as aluminum, boron, iron and/or gallium, preferably aluminum; Y is a tetravalent element such as silicon and/or germanium, preferably silicon; and n is greater than about 12, and wherein the crystals have a major dimension of at least about 0.5 micron and a surface YO
2
/X
2
O
3
ratio which is no more than 20% less than the bulk YO
2
/X
2
O
3
ratio of the crystal; the catalyst having a diffusion-modifying surface coating of a refractory material.
Preferably, the crystals have a major dimension of at least about 1 micron.
Preferably, the surface YO
2
/X
2
O
3
ratio is no more than 10% less than the bulk YO
2
/X
2
O
3
ratio of the crystal.
Preferably, n is less than about 100 and more preferably is about 25 to about 40.
Preferably, the surface coating is selected from the group consisting of coke, a metal oxide, a non-metal oxide and a non-oxide ceramic and most preferably comprises silica.
In a further aspect, the invention resides in a se
Abichandani Jeevan S.
Lissy Daria N.
Rubin Mae K.
Sharma Sanjay B.
Shihabi David S.
Hantman Ronald D.
Mobil Oil Corporation
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