Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – From nonhydrocarbon feed
Reexamination Certificate
2001-03-23
2003-03-18
Dunn, Tom (Department: 1725)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
From nonhydrocarbon feed
C585S639000
Reexamination Certificate
active
06534692
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for converting methanol to light olefins with increased selectivity to ethylene and propylene. The process comprises contacting the methanol with a catalyst comprising a metallo aluminophosphate molecular sieve having an empirical formula of (EL
x
Al
y
P
z
)O
2
where EL includes silicon and characterized in that “x” has a value from about 0.02 to about 0.08. A preferred catalyst is one where the molecular sieve has predominantly a plate crystal morphology such that the average smallest crystal dimension is at least 0.1 micron and has an aspect ratio of less than or equal to 5.
BACKGROUND OF THE INVENTION
The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of methanol to hydrocarbons and especially light olefins (by light olefins is meant C
2
to C
4
olefins). The interest in the methanol to olefin (MTO) process is based on the fact that methanol can be obtained from coal or natural gas by the production of synthesis gas which is then processed to produce methanol.
Processes for converting methanol to light olefins are well known in the art. Initially aluminosilicates or zeolites were used as the catalysts necessary to carry out the conversion. For example, see U.S. Pat. No. 4,238,631 B1; U.S. Pat. No. 4,328,384 B1, U.S. Pat. No. 4,423,274 B1. These patents further disclose the deposition of coke onto the zeolites in order to increase selectivity to light olefins and minimize the formation of C
5+
byproducts. The effect of the coke is to reduce the pore diameter of the zeolite.
The prior art also discloses that silico aluminophosphates (SAPOs) can be used to catalyze the methanol to olefin process. Thus, U.S. Pat. No. 4,499,327 B1 discloses that many of the SAPO family of molecular sieves can be used to convert methanol to olefins. The '327 patent also discloses that preferred SAPOs are those that have pores large enough to adsorb xenon (kinetic diameter of 4.0 Å) but small enough to exclude isobutane (kinetic diameter of 5.0 Å). A particularly preferred SAPO is SAPO-34.
U.S. Pat. No. 4,752,651 B1 discloses the use of nonzeolitic molecular sieves (NZMS) including ELAPOs and MeAPO molecular sieves to catalyze the methanol to olefin reaction.
The effect of the particle size of the molecular sieve on activity has also been documented in U.S. Pat. No. 5,126,308 B 1. In the '308 patent it is disclosed that molecular sieves in which 50% of the molecular sieve particles have a particle size less than 1.0 &mgr;m and no more than 10% of the particles have a particle size greater than 2.0 &mgr;m have increased activity and/or durability. The '308 patent also discloses that restricting the silicon content to about 0.005 to about 0.05 mole fraction also improves catalyst life by reducing coke formation.
In contrast to this art, applicants have found that a methanol to olefin process using molecular sieves having the empirical formula (EL
x
Al
y
P
z
)O
2
(hereinafter ELAPO) where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof and “x”, “y” and “z” are the mole fractions of EL, Al and P respectively and specifically where “x” has a value from about 0.02 to about 0.08 provides an increased selectivity to ethylene and propylene with a reduction in undesirable C
4s
and C
5+
. A preferred catalyst is one which additionally has a predominantly plate crystal morphology wherein the average smallest crystal dimension is at least 0.1 micron and has an aspect ratio of less than or equal to 5
SUMMARY OF THE INVENTION
As stated, this invention relates to a process for converting methanol to light olefins using a catalyst comprising an ELAPO molecular sieve. Accordingly, one embodiment of the invention is a process for converting methanol to light olefins comprising contacting the methanol with a catalyst at conversion conditions to provide the olefins, the catalyst comprising a crystalline metallo aluminophosphate molecular sieve having a chemical composition on an anhydrous basis expressed by an empirical formula of:
(EL
x
Al
y
P
z
)O
2
where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, “x” is the mole fraction of EL and has a value of about 0.02 to about 0.08, “y” is the mole fraction of Al and has a value of at least 0.01, “z” is the mole fraction of P and has a value of at least 0.01 and x+y+z=1.
In another embodiment, the molecular sieve is characterized in that it has a crystal morphology wherein the average smallest crystal is at least 0.1 micron and has an aspect ratio no greater than 5.
These and other objects and embodiments of the invention will become more apparent after the detailed description of the invention.
DETAILED DESCRIPTION OF THE INTENTION
An essential feature of the process of the instant invention is an ELAPO molecular sieve. ELAPOs are molecular sieves which have a three-dimensional microporous framework structure of AlO
2
, PO
2
and ELO
2
tetrahedral units. Generally the ELAPOs have the empirical formula:
(EL
x
Al
y
P
z
)O
2
where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, “x” is the mole fraction of EL and has a value of at least 0.005, “y” is the mole fraction of Al and has a value of at least 0.01, “z” is the mole fraction of P and has a value of at least 0.01 and x+y+z=1. When EL is a mixture of metals, “x” represents the total amount of the metal mixture present. Preferred metals (EL) are silicon, magnesium and cobalt with silicon being especially preferred. As will be shown in the examples, when “x” has a value of about 0.02 to about 0.08 a greater selectivity to ethylene and propylene with a diminished selectivity to C
4+
components is observed.
The preparation of various ELAPOs are well known in the art and may be found in U.S. Pat. No. 4,554,143 B1 (FeAPO); U.S. Pat. No. 4,440,871 B1 (SAPO); U.S. Pat. No. 4,853,197 B1 (MAPO, MnAPO, ZnAPO, COAPO); U.S. Pat. No. 4,793,984 B1 (CAPO), U.S. Pat. No. 4,752,651 B1 and U.S. Pat. No. 4,310,440 B1, all of which are incorporated by reference. Generally, the ELAPO molecular sieves are synthesized by hydrothermal crystallization from a reaction mixture containing reactive sources of EL, aluminum, phosphorus and a templating agent. Reactive sources of EL are the metal salts such as the chloride and nitrate salts. When EL is silicon a preferred source is fumed, colloidal or precipitated silica. Preferred reactive sources of aluminum and phosphorus are pseudo-boehmite alumina and phosphoric acid. Preferred templating agents are amines and quaternary ammonium compounds. An especially preferred templating agent is tetraethylammonium hydroxide (TEAOH).
The reaction mixture is placed in a sealed pressure vessel, optionally lined with an inert plastic material such as polytetrafluoroethylene and heated preferably under autogenous pressure at a temperature between about 50° C. and 250° C. and preferably between about 100° C. and 200° C. for a time sufficient to produce crystals of the ELAPO molecular sieve. Typically the time varies from about 1 hour to about 120 hours and preferably from about 24 hours to about 48 hours. The desired product is recovered by any convenient method such as centrifugation or filtration.
The ELAPO molecular sieves of this invention have predominantly a plate crystal morphology. By predominantly is meant greater than 50% of the crystals. Preferably at least 70% of the crystals have a plate morphology and most preferably at least 90% of the crystals have a plate morphology. Especially good selectivity (C
2
=
versus C
3
=
) is obtained when at least 95% of the crystals have a plate morphology. By plate morphology is meant that the crystals have the appeara
Barger Paul T.
Vora Bipin V.
Ildebrando Christina
Molinaro Frank S.
Tolomei John G.
UOP LLC
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