Selective para-xylene production by toluene methylation

Details Selective para-xylene production by toluene methylation Selective para-xylene production by toluene methylation

2002-05-08

2003-01-07

Dang, Thuan (Department: 1764)

Chemistry of hydrocarbon compounds

Aromatic compound synthesis

By condensation of entire molecules or entire hydrocarbyl...

C585S464000

Reexamination Certificate

active

06504072

ABSTRACT:

There is provided a process for the selective production of para-xylene by catalytic methylation of toluene in the presence of a solid catalyst. There is also provided a method for preparing a catalyst which is particularly suited for this reaction.
Of the xylene isomers, para-xylene is of particular value since it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers. Equilibrium mixtures of xylene isomers either alone or in further admixture with ethylbenzene generally contain only about 24 wt. % para-xylene and separation of p-xylene from such mixtures has typically required superfractionation and multistage refrigeration steps. Such processes have involved high operation costs and resulted in only limited yields. There is therefore a continuing need to provide processes which are highly selective for the production of p-xylene.
One known method for producing xylenes involves the alkylation of toluene with methanol over a solid acid catalyst. Thus the alkylation of toluene with methanol over cation-exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280(1970). These workers reported selective production of para-xylene over the approximate temperature range of 200 to 275° C., with the maximum yield of para-xylene in the mixture of xylenes, i.e. about 50% of the xylene product mixture, being observed at 225° C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in production of para and ortho-xylenes.
U.S. Pat. Nos. 3,965,209 to Butter et al. and 4,067,920 to Kaeding teach processes for producing para-xylene in low conversion and high selectivity by reaction of toluene with methanol over a zeolite having a Constraint Index of 1-12, such as ZSM-5. In Butter et al the zeolite is steamed at a temperature of 250-1000° C. for 0.5-100 hours to reduce the acid activity of the zeolite, as measured by its alpha activity, to less than 500 and preferably from in excess of zero to less than 20.
U.S. Pat. No. 4,001,346 to Chu relates to a process for the selective production of para-xylene by methylation of toluene in the presence of a catalyst comprising a crystalline aluminosilicate zeolite which has undergone prior treatment to deposit a coating of between about 15 and about 75 wt. % of coke thereon.
U.S. Pat. No. 4,097,543 to Haag et al. relates to a process for the selective production of para-xylene (up to about 77%) by disproportionation of toluene in the presence of a crystalline aluminosilicate catalyst which has undergone precoking to deposit a coating of at least about 2 wt. % coke thereon.
U.S. Pat. No. 4,380,685 to Chu relates to a process for para-selective aromatics alkylation, including the methylation of toluene, over a zeolite, such as ZSM-5, which has a constraint index of 1-12 and which has been combined with phosphorus and a metal selected from iron and cobalt. Chu indicates that the catalyst can optionally be modified (without specifying the effect of the modification) by steaming at a temperature of 250-1000° C., preferably 400-700° C. for 0.5-100 hours, preferably 1-24 hours.
U.S. Pat. No. 4,554,394 to Forbus and Kaeding teach the use of phosphorus-treated zeolite catalysts for enhancing para-selectivity in aromatics conversion processes. U.S. Patent No. 4,623,633 to Young relates to the use of thermal shock calcination of aluminosilicates to produce up to 66% para-xylene selectivity.
The use of phosphorus modified ZSM-5 fluid bed catalysts as additive catalysts to improve the olefin yield in fluid catalytic cracking (FCC) is described in U.S. Pat. No. 5,389,232 to Adewuyi et al. and in U.S. Pat. No. 5,472,594 to Tsang et al.
According to the invention, it has now been found that certain porous crystalline materials having specific and closely defined diffusion characteristics, such as can be obtained by unusually severe steaming of ZSM-5 containing an oxide modifier, exhibit improved selectivity for the alkylation of toluene with methanol such that the xylene product contains at least about 90% of the para-isomer at per-pass toluene conversions of at least about 15%.
In one aspect, the invention resides in a process for the selective production of para-xylene which comprises reacting toluene with methanol under alkylation conditions in the presence of a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-15 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
Preferably, the porous crystalline material has a Diffusion Parameter of about 0.5-10 sec
−1
.
Preferably, the catalyst contains at least one oxide modifier and more preferably at least one oxide modifier selected from oxides of elements of Groups IIA, IIIA, IIIB, IVA, VA, VB and VIA of the Periodic Table. Most preferably the oxide modifier is selected from oxides of boron, magnesium, calcium, lanthanum and most preferably phosphorus.
Preferably, the catalyst contains about 0.05 to about 20, more preferably about 0.1 to about 10 and most preferably about 0.1 to about 5, wt % of the oxide modifier based on elemental modifier.
Preferably, the catalyst has an alpha value less than 50 and preferably less than 10.
In a further aspect, the invention resides in a method for producing a catalyst for use in the selective production of para-xylene by reacting toluene with methanol, said method comprising the steps of:
(a) starting with a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane in excess of 15 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa); and
(b) contacting the material of step (a) with steam at a temperature of at least about 950° C. to reduce the Diffusion Parameter thereof for 2,2-dimethylbutane to about 0.1-15 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa), the micropore volume of the steamed material being at least 50% of the unsteamed material.
Preferably, said porous crystalline material used in step (a) comprises an aluminosilicate zeolite having a silica to alumina molar ratio of at least 250.
The present invention provides a process for alkylating to methanol to selectively produce p-xylene in high yield and with a high per-pass conversion of toluene. The process employs a catalyst which comprises a porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1-15 sec
−1
and preferably 0.5-10 sec
−1
, when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
As used herein, the Diffusion Parameter of a particular porous crystalline material is defined as D/r
2
×10
6
, wherein D is the diffusion coefficient (cm
2
/sec) and r is the crystal radius (cm). The required diffusion parameters can be derived from sorption measurements provided the assumption is made that the plane sheet model describes the diffusion process. Thus for a given sorbate loading Q, the value Q/Q
13
, where Q
13
is the equilibrium sorbate loading, is mathematically related to (Dt/r
2
)
½
where t is the time (sec) required to reach the sorbate loading Q. Graphical solutions for the plane sheet model are given by J. Crank in “The Mathematics of Diffusion”, Oxford University Press, Ely House, London, 1967.
The porous crystalline material employed in the process of the invention is preferably a medium-pore size aluminosilicate zeolite. Medium pore zeolites are generally defined as those having a pore size of about 5 to about 7 Angstroms, such that the zeolite freely sorbs molecules such as n-hexane, 3-methylpentane, benzene and p-xylene. Another common definition for medium pore zeolites involves the Constraint Index test which is described in U.S. Pat. No. 4,016,218, which is incorporated herein by reference. In this case, medium pore zeolites have a Constraint Index of about 1-12, as measured on the zeolite alone

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