Molecular sieve abstract
A method for fluorinating molecular sieve catalysts to increase
selectivity to ethylene during conversion of oxygenates to olefins,
fluorinated catalysts produced by such method, and methods of using
the fluorinated molecular sieve catalysts to increase selectivity
to ethylene during conversion of oxygenates to olefins.
Molecular sieve claims
We claim:
1. A method for increasing selectivity of a silicoaluminophosphate
molecular sieve catalyst to ethylene during conversion of oxygenates
to olefins, said method comprising:
(a) providing a silicoaluminophosphate molecular sieve catalyst
having a framework; and
(b) contacting said silicoaluminophosphate molecular sieve framework
with a fluorinating agent under conditions effective to fluorinate
said framework but insufficient to dealuminate said framework, forming
a fluorinated silicoaluminophosphate molecular sieve catalyst comprising
an amount of fluorine sufficient to increase selectivity of said
catalyst to ethylene.
2. The method of claim 1 wherein said fluorinating agent is selected
from the group consisting of hydrogen fluoride, ammonium hexafluorosilicate,
ammonium hexafluorogermanate, ammonium hexafluorotitanate, ammonium
hexafluorophosphate, ammonium hexafluorozirconate, and ammonium
hydrogen fluoride.
3. The method of claim 1 wherein said framework comprises pores
consisting essentially of a diameter in the range of from about
5 to about 10 Angstroms.
4. The method of claim 2 wherein said framework comprises pores
consisting essentially of a diameter in the range of from about
5 to about 10 Angstroms.
5. The method of claim 1 wherein said framework comprises pores
consisting essentially of a diameter less than about 5 Angstroms.
6. The method of claim 2 wherein said framework comprises pores
consisting essentially of a diameter less than about 5 Angstroms.
7. A silicoaluminophosphate molecular sieve catalyst for increasing
selectivity of said catalyst to ethylene during conversion of oxygenates
to olefins comprising:
(a) a silicoaluminophosphate molecular sieve catalyst having a
framework; and
(b) fluorine incorporated onto said framework in an amount sufficient
to increase selectivity of said silicoaluminophosphate molecular
sieve catalyst to ethylene.
8. The molecular sieve catalyst of claim 7 wherein said fluorine
comprises a fluorine-containing agent selected from the group consisting
of hydrogen fluoride, ammonium hexafluorosilicate, ammonium hexafluorogermanate,
ammonium hexafluorotitanate, ammonium hexafluorophosphate, ammonium
hexafluorozirconate, and ammonium hydrogen fluoride.
9. The molecular sieve catalyst of claim 7 wherein said framework
comprises pores consisting essentially of a diameter in the range
of from about 5 to about 10 Angstroms.
Molecular sieve description
FIELD OF THE INVENTION
The present invention is directed to a method of fluorinating molecular
sieve catalysts, to fluorinated catalysts produced by such method,
and to a method of using fluorinated catalysts to increase selectivity
to ethylene during conversion of oxygenates to olefins.
BACKGROUND OF THE INVENTION
Light olefins (defined herein as "ethylene, propylene, and
butylene") serve as feeds for the production of numerous chemicals.
Light olefins traditionally are produced by petroleum cracking.
Because of the limited supply and/or the high cost of petroleum
sources, the cost of producing olefins from petroleum sources has
increased steadily.
Alternative feedstocks for the production of light olefins are
oxygenates, such as alcohols, particularly methanol, dimethyl ether,
and ethanol. Alcohols may be produced by fermentation, or from synthesis
gas derived from natural gas, petroleum liquids, carbonaceous materials,
including coal, recycled plastics, municipal wastes, or any organic
material. Because of the wide variety of sources, alcohol, alcohol
derivatives, and other oxygenates have promise as an economical,
non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins
are molecular sieve catalysts. Because ethylene and propylene are
the most sought after products of such a reaction, research has
focused on which catalysts are most selective to light olefins.
Methods also are needed for increasing the selectivity of molecular
sieve catalysts to a particular light olefin, such as ethylene.
SUMMARY OF THE INVENTION
The present invention provides a method for increasing selectivity
of a molecular sieve catalyst to ethylene during conversion of oxygenates
to olefins. The method comprises: providing a molecular sieve catalyst
comprising separately synthesized microporous framework comprising
a material selected from the group consisting of silica, alumina,
phosphate, and combinations thereof; and, contacting the framework
with a fluorinating agent under conditions effective to fluorinate
the framework but insufficient to dealuminate the framework. The
result is a fluorinated molecular sieve catalyst comprising an amount
of fluorine sufficient to increase selectivity of the molecular
sieve catalyst to ethylene during conversion of oxygenates to olefins.
DETAILED DESCRIPTION OF THE INVENTION
In the conversion of oxygenates to light olefins, it is desirable
to maximize the production of light olefins and to minimize the
production of undesired by-products, such as methane, ethane, propane,
carbon dioxide, hydrogen gas, and C.sub.4.sup.+ materials, including
aromatics. It also may be desirable at times to maximize the ethylene
or the propylene fraction of the light olefin product. The present
invention maximizes the ethylene fraction of the light olefin product
by fluorinating the molecular sieve catalyst used to promote the
conversion.
Molecular sieve catalysts generally comprise a crystalline, three
dimensional, stable framework enclosing cavities of molecular dimensions.
The cavities form a well-defined microporous system of channels
and cages. The cavities or "pores" in a given type of
molecular sieve have well-defined dimensions which will only allow
molecules up to a certain size to enter the pores.
The present invention is directed towards increasing the selectivity
of substantially any molecular sieve catalyst to ethylene, regardless
of pore size. However, preferred catalysts for use in the invention
are "small" and "medium" pore molecular sieve
catalysts. "Small pore" molecular sieve catalysts are
defined as catalysts with pores having a diameter of less than about
5.0 Angstroms. "Medium pore" molecular sieve catalysts
are defined as catalysts with pores having a diameter in the range
of from about 5 to about 10 Angstroms.
One group of suitable molecular sieve catalysts is the zeolite
group. Several types of zeolites exist, each of which exhibit different
properties and different utilities. Structural types of zeolites
that are suitable for use in the present invention with varying
levels of effectiveness include, but are not necessarily limited
to AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR,
EDI, ERI, GOO, KFI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, and
THO and substituted examples of these structural types, as described
in W. M. Meier and D. H. Olsen, "Atlas of Zeolite Structural
Types," Butterworth-Heineman, Third Edition, 1992 incorporated
herein by reference. Structural types of medium pore molecular sieve
catalysts useful in the present invention include, but are not necessarily
limited to, MFI, MEL, MTW, EUO, MTT, HEU, FER, AFO, AEL, TON, and
substituted examples of these structural types, as described in
the "Atlas of Zeolite Types," previously incorporated
herein by reference.
Preferred zeolite catalysts for use in the present invention include,
but are not necessarily limited to, ZSM-5 ZSM-34 erionite, and
chabazite.
Silicoaluminophosphates ("SAPO's") are another group
of molecular sieve catalysts that are useful in the invention. SAPO's
have a three-dimensional microporous crystal framework of PO.sub.2.sup.+,
AlO.sub.2.sup.-, and SiO.sub.2 tetrahedral units. Suitable SAPO's
for use in the invention include, but are not necessarily limited
to SAPO-44 SAPO-34 SAPO-17 and SAPO-18. A preferred SAPO for
treatment according to the present invention is SAPO-34 which may
be synthesized according to U.S. Pat. No. 4440871 incorporated
herein by reference, and "Zeolites", Vol. 17 pp. 512-522
(1996), incorporated herein by reference.
SAPO's with added substituents also may be useful in the present
invention. These substituted SAPO's form a class of molecular sieves
known as "MeAPSO's." Substituents may include, but are
not necessarily limited to nickel, cobalt, strontium, barium, and
calcium.
In order to fluorinate molecular sieve catalysts according to the
present invention, a suitable fluorinating agent is to be dissolved
in a suitable solvent, and the solution is to be mixed with the
previously synthesized microporous framework for a selected molecular
sieve catalyst and simply allowed to stand for a period of time
at ambient conditions. Substantially any agent comprising fluorine
atoms capable of modifying the catalyst may be used as a fluorinating
agent. Preferred fluorinating agents include, but are not necessarily
limited to, hydrogen fluoride, ammonium hexafluorosilicate, ammonium
hexafluorogermanate, ammonium hexafluorotitanate, ammonium hexafluorophosphate,
ammonium hexafluorozirconate, and ammonium hydrogen fluoride. When
one of the foregoing preferred materials is used as the fluorinating
agent, the mixture comprising the fluorinating agent and the catalyst
is to be allowed to stand at ambient conditions for at least about
one hour.
Thereafter, the mixture is to be dried for an amount of time sufficient
to produce a dry powder. In a preferred embodiment, the mixture
is dried at a temperature of about 110.degree. C. for about two
hours. The dried powder is to be calcined preferably for about 16
hours at a temperature in the range of from about 300.degree. C.
to about 800.degree. C., preferably in the range of from about 350.degree.
C. to about 650.degree. C., most preferably in the range of from
about 500.degree. C. to about 650.degree. C. The resulting powder
may be pressed into pellets and then crushed and sieved to a mesh
size preferably in the range of from about 14 to about 20. In a
preferred embodiment, the pellets are formed by application of about
138 Mpa (20000 psi) of pressure.
Without limiting the present invention to a particular mechanism
of action, it is believed that the fluorine or fluorine-containing
groups in the fluorinating agent replace at least some of the hydroxyl
groups found in the catalysts. As a result, the electronegativity
of the molecular sieve catalyst is changed. The result is an increased
selectivity of the catalyst to ethylene. The presence of silicon,
germanium, titanium, zirconium, and/or phosphorous in the fluorinating
agents is believed to provide further "fine-tuning" of
the selectivity to ethylene.
The process for converting oxygenates to olefins employs an organic
starting material (feedstock) preferably comprising "oxygenates."
As used herein, the term "oxygenates" is defined to include,
but is not necessarily limited to aliphatic alcohols, ethers, carbonyl
compounds (aldehydes, ketones, carboxylic acids, carbonates, and
the like), and also compounds containing hetero-atoms, such as,
halides, mercaptans, sulfides, amines, and mixtures thereof. The
aliphatic moiety preferably is in the range of from about 1 to about
10 carbon atoms and more preferably is in the range of from about
1 to about 4 carbon atoms. Representative oxygenates include, but
are not necessarily limited to, lower straight chain or branched
aliphatic alcohols, their unsaturated counterparts, and their nitrogen,
halogen and sulfur analogues. Examples of suitable compounds include,
but are not necessarily limited to: methanol; ethanol; n-propanol;
isopropanol; C.sub.4 -C.sub.10 alcohols; methyl ethyl ether; dimethyl
ether; diethyl ether; di-isopropyl ether; methyl mercaptan; methyl
sulfide; methyl amine; ethyl mercaptan; di-ethyl sulfide; di-ethyl
amine; ethyl chloride; formaldehyde; di-methyl carbonate; di-methyl
ketone; n-alkyl amines, n-alkyl halides, n-alkyl sulfides having
n-alkyl groups of comprising the range of from about 3 to about
10 carbon atoms; and mixtures thereof. As used herein, the term
"oxygenate" designates only the organic material used
as the feed. The total charge of feed to the reaction zone may contain
additional compounds such as diluents.
Preferably, the oxygenate feedstock is to be contacted in the vapor
phase in a reaction zone with the defined molecular sieve catalyst
at effective process conditions so as to produce the desired olefins,
i.e., an effective temperature, pressure, WHSV (Weight Hourly Space
Velocity) and, optionally, an effective amount of diluent, correlated
to produce olefins. Alternately, the process may be carried out
in a liquid or a mixed vapor/liquid phase. When the process is carried
out in the liquid phase or a mixed vapor/liquid phase, different
conversions and selectivities of feedstock-to-product may result
depending upon the catalyst and reaction conditions.
The temperature employed in the conversion process may vary over
a wide range depending, at least in part, on the selected catalyst.
Although not limited to a particular temperature, best results will
be obtained if the process is conducted at temperatures in the range
of from about 200.degree. C. to about 700.degree. C., preferably
in the range of from about 250.degree. C. to about 600.degree. C.,
and most preferably in the range of from about 300.degree. C. to
about 500.degree. C. Lower temperatures generally result in lower
rates of reaction, and the formation of the desired light olefin
products may become markedly slow. However, at higher temperatures,
the process may not form an optimum amount of light olefin products,
and the coking rate may become too high.
Light olefin products will form--although not necessarily in optimum
amounts--at a wide range of pressures, including but not limited
to autogeneous pressures and pressures in the range of from about
0.1 kPa to about 100 MPa. A preferred pressure is in the range of
from about 6.9 kPa to about 34 MPa, most preferably in the range
of from about 48 kPa to about 0.34 MPa. The foregoing pressures
are exclusive of diluent, if any is present, and refer to the partial
pressure of the feedstock as it relates to oxygenate compounds and/or
mixtures thereof. Pressures outside of the stated ranges may be
used and are not excluded from the scope of the invention. Lower
and upper extremes of pressure may adversely affect selectivity,
conversion, coking rate, and/or reaction rate; however, light olefins
such as ethylene still may form.
The process is to be continued for a period of time sufficient
to produce the desired olefin products. The reaction cycle time
may vary from tenths of seconds to a number of hours. The reaction
cycle time is largely determined by the reaction temperature, the
pressure, the catalyst selected, the weight hourly space velocity,
the phase (liquid or vapor), and the selected process design characteristics.
A wide range of weight hourly space velocities (WHSV), defined
as weight feed per hour per weight of catalyst, for the feedstock
will function in the present invention. The WHSV generally is to
be in the range of from about 0.01 hr.sup.-1 to about 5000 hr.sup.-1
preferably in the range of from about 0.1 hr.sup.-1 to about 2000
hr.sup.-1 and most preferably in the range of from about 1 hr.sup.-1
to about 1000 hr.sup.-1. The catalyst may contain other materials
which act as inerts, fillers, or binders; therefore, the WHSV is
calculated on the weight basis of oxygenate and catalyst.
One or more diluents may be fed to the reaction zone with the oxygenates,
such that the total feed mixture comprises diluent in a range of
from about 1 mol % and about 99 mol %. Diluents which may be employed
in the process include, but are not necessarily limited to, helium,
argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, water,
paraffins, other hydrocarbons (such as methane), aromatic compounds,
and mixtures thereof. Preferred diluents are water and nitrogen.
A preferred embodiment of a reactor system for the present invention
is a circulating fluid bed reactor with continuous regeneration,
similar to a modern fluid catalytic cracker. Moving beds also may
be used. Fixed beds may be used, but are not ideal for the process
because oxygenate to olefin conversion is a highly exothermic process
which requires several stages with intercoolers or other cooling
devices. The reaction also results in a high pressure drop due to
the production of low pressure, low density gas.
The invention will be better understood with reference to the following
examples which are intended to illustrate, but not to limit the
present invention. |