Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – From nonhydrocarbon feed
Reexamination Certificate
2001-10-30
2004-01-20
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
From nonhydrocarbon feed
C585S640000
Reexamination Certificate
active
06680418
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing light olefins rich in ethylene from methanol and dimethyl ether.
BACKGROUND TO THE INVENTION
A remarkable growth in the production of synthetic fibers, plastics and rubber has taken place in recent decades. This growth, to a very large extent, has been supported and encouraged by an expanding supply of inexpensive petrochemical raw materials such as ethylene, propylene, and other, four and five carbon olefins. Side by side with this growth, there has been an increasing demand for alkylate, made by reacting olefins with isobutane, for use as a high octane gasoline component.
Burgeoning demand for olefins, particularly ethylene, propylene and butenes, has of course led to periods of shortage, which has led to substantial price increases in the feedstocks to the commercialized technologies. These feedstocks are largely C
2
to C
4
olefins co-produced with natural gas and/or paraffinic straight run naphtha. These feedstocks can be substantially more expensive than methane, making it desirable to provide efficient means for converting methane to olefins.
Conversion of methane to methanol followed by conversion of methanol to light olefins is among the most economic routes to make light olefins from methane. In this respect, it is known that methanol or methyl ether can be catalytically converted to olefin-containing hydrocarbon mixtures by contact under certain conditions with particular types of crystalline zeolite materials. U.S. Pat. Nos. 4,025,575 and 4,038,889 for example, both disclose processes whereby methanol and/or methyl ether can be converted to an olefin-containing product over a Constraint Index 1-12 zeolite catalyst, particularly ZSM-5. ZSM-5, in fact, converts methanol and/or methyl ether to hydrocarbons containing a relatively high concentration of light olefins with prolonged catalyst lifetime before catalyst regeneration becomes necessary.
It has also been reported that other types of zeolite catalysts can be used to convert methanol and/or methyl ether to olefin-containing hydrocarbons products containing even higher proportions of light olefins than obtained with ZSM-5. For example, U.S. Pat. No. 4,079,095 discloses that zeolites of the erionite-offretite-chabazite type, and especially ZSM-34, can usefully be employed to promote conversion of methanol and/or methyl ether to products comprising a major amount of ethylene and propylene. However, while erionite-offretite-chabazite type catalysts are highly selective to light olefins production, such smaller pore zeolites tend to age rapidly in comparison to ZSM-5 when used for methanol/methyl ether conversion.
U.S. Pat. Nos. 4,677,242 and 4,752,651 disclose the conversion of methanol to C
2
-C
4
olefins over various silicoaluminophosphates and “non-zeolitic molecular sieves” (such as metal aluminophosphates) and teach that the addition of diluents, such as aromatic materials, having a kinetic diameter greater than the pore size of the molecular sieve increases the ethylene to propylene ratio in the product.
U.S. Pat. No. 4,499,314 discloses that the addition of various promoters, including aromatic compounds, such as toluene, accelerate the conversion of methanol to hydrocarbons over zeolites, such as ZSM-5, which have a pore size sufficient to permit sorption and diffusion of the promoter. In particular, the '314 patent teaches that the increased conversion resulting from the addition of the promoter allows the use of lower severity conditions, particularly lower temperatures, which increase the yield of lower olefins (column 4, lines 17-22). However, the Examples of the '349 patent employ ZSM-5 with a large crystal size, namely in excess of 1 micron (see column 13, lines 60-65). Surprisingly, it has now been found that small crystal ZSM-5 (as defined by having a Diffusion Parameter for 2,2-dimethylbutane of at least 500 sec
−1
) can, in the presence of tetramethylbenzene and under closely controlled conditions, selectively convert methanol to C
2
to C
4
olefins.
U.S. Pat. No. 4,520,219 discloses a process for producing pseudocumene or durene by reacting xylene with methanol over a catalyst comprising at least one crystalline silicate selected from the group consisting of a crystalline borosilicate, a fluorine-containing crystalline borosilicate, a fluorine-containing crystalline aluminosilicate and a fluorine-containing crystalline boroaluminosilicate. According to Example 8 of the '219 patent, using fluorine-containing aluminosilicate ZSM-5 to methylate xylene at 300° C. and 9 kg/cm
2
in a two-reactor system gives a product in which the C
9
and C
10
components consist mainly of pseudocumene and durene, respectively, and an unspecified amount of a gaseous component is produced consisting mainly of ethylene and propylene. However, since the object of the process described in the '219 patent is to enhance the yield of polymethylated benzenes by maximizing methylation of the xylene and minimizing decomposition of the methylating agent, no incentive exists to recycle the polymethylated benzene product in an attempt to increase the yield of olefins.
SUMMARY OF THE INVENTION
The present invention resides in a process for converting methanol and/or dimethyl ether to a product containing C
2
to C
4
olefins which comprises the step of contacting a reaction mixture which contains methanol and/or dimethyl ether and at least 10 wt % of a polymethylbenzene component selected from trimethylbenzenes, tetramethylbenzenes and mixtures thereof with a catalyst comprising a porous crystalline material, the contacting step being conducted under conversion conditions including a temperature of about 250° C. to about 500° C. and a methanol and/or dimethyl ether partial pressure of about 5 to about 250 psia (35 to 1725 kPa), and the porous crystalline material having a Diffusion Parameter for 2,2-dimethylbutane of at least 500 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
Preferably, the polymethylbenzene component comprises at least 10 wt %, and more preferably at least 30 wt %, of at least one tetramethylbenzene.
Preferably, the process includes the further steps of introducing into the reaction mixture an aromatic compound which is selected from the group consisting of benzene, toluene, xylene, a trimethylbenzene and mixtures thereof and which is capable of alkylation by the methanol and/or dimethyl ether under the conversion conditions, and recycling to the contacting step a trimethylbenzene- and/or tetramethylbenzene-containing fraction of said product.
Preferably, the molar ratio of methanol and/or dimethyl ether to the aromatic compound is from about 0.1:1 to about 25:1, and more preferably from about 1:1 to about 10:1.
Preferably, the conversion conditions include a temperature of about 250° C. to about 400° C.
Preferably, the conversion conditions are such that the methanol conversion rate is less than 90% and more preferably less than 80%.
Preferably, the porous crystalline material has a pore size between 5 and 7 Angstrom.
Preferably, the porous crystalline material is an aluminosilicate zeolite which is substantially free of fluorine and most preferably is ZSM-5.
Preferably, the catalyst has an alpha value less than 250 and more preferably less than 150.
Preferably, the porous crystalline material has a Diffusion Parameter for 2,2-dimethylbutane of about 500 to about 2000 sec
−1
when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a catalytic process for selectively converting methanol and/or dimethyl ether to C
2
-C
4
olefins in a reaction mixture which contains at least 10 wt % of a polymethylbenzene component selected from trimethylbenzenes, tetramethylbenzenes and mixtures thereof. The catalyst employed in the process of the invention contains a porous crystalline material which has a Diffusion Parameter for 2,2-dimethylbutan
ExxonMobil Chemical Patents Inc.
Griffin Walter D.
Nguyen Tam M.
LandOfFree
Process for producing light olefins does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for producing light olefins, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing light olefins will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3195270