Molecular sieve abstract
A titanium-containing molecular sieve which has been used as an
oxidation catalyst is regenerated to provide a level of performance
comparable to that of freshly prepared catalyst by heating in the
presence of a gas stream comprised of less than 5 volume percent
molecular oxygen. The regeneration temperature may be in the range
of from 150.degree. C. to 700.degree. C. The same batch of catalyst
thus may be used over an extended period of time in a continuous
epoxidation process by periodic practice of the aforedescribed reactivation
method.
Molecular sieve claims
We claim:
1. A method for regenerating a spent titanium-containing molecular
sieve which has framework titanium atoms and has been employed in
an olefin epoxidation process wherein propylene is converted to
propylene oxide using hydrogen peroxide comprising heating the spent
titanium-containing molecular sieve at a temperature of from 150.degree.
C. to 700.degree. C. in the presence of a gas stream which is free
of molecular oxygen for a time effective to enhance the activity
of the spent titanium-containing molecular sieve.
2. The method of claim 1 wherein the spent titanium-containing
molecular sieve has an MFI, MEL, or zeolite beta topology.
3. The method of claim 1 wherein the regeneration is performed
in a static manner.
4. The method of claim 1 wherein the temperature is from 250.degree.
C. to 600.degree. C.
5. The method of claim 1 wherein the regeneration time is from
1 to 150 hours.
6. The method of claim 1 wherein the gas stream is passed over
the spent titanium-containing molecular sieve during the regeneration.
7. The method of claim 1 wherein the gas stream is comprised of
one or more inert gases selected from the group consisting of nitrogen,
helium, carbon dioxide, argon, water vapor, and mixtures thereof.
8. The method of claim 1 wherein the spent titanium-containing
molecular sieve has the composition xTiO.sub.2 :(1-x)SiO.sub.2 where
x is from 0.01 to 0.125.
9. A method for regenerating a spent titanium-containing molecular
sieve having an MFI, MEL, or zeolite beta topology and framework
titanium atoms and has been used in an olefin epoxidation reaction
wherein propylene is converted to propylene oxide using hydrogen
peroxide comprising heating the spent titanium-containing molecular
sieve at a temperature of from 250.degree. C. to 600.degree. C.
in the presence of a flowing gas stream which is free of molecular
oxygen for a period of from 1 to 150 hours.
10. The method of claim 9 wherein the flowing gas stream consists
essentially of nitrogen.
11. The method of claim 9 wherein the spent titanium-containing
molecular sieve has the composition xTiO.sub.2 :(1-x)SiO.sub.2 where
x is from 0.01 to 0.125.
12. The method of claim 9 wherein the spent titanium-containing
molecular sieve has been deployed in the form of a fixed bed within
a reactor vessel during said olefin epoxidation reaction.
13. The method of claim 9 wherein regeneration of the spent titanium-containing
molecular sieve is performed within said reactor vessel.
Molecular sieve description
FIELD OF THE INVENTION
This invention relates to a method of restoring the activity and
selectivity of a titanium-containing molecular sieve which has been
used to catalyze an oxidation reaction such as the epoxidation of
an olefin with hydrogen peroxide or other active oxygen species.
Regeneration is accomplished by heating the spent catalyst in the
presence of a gas stream comprised of less than 5 volume percent
molecular oxygen. Contrary to the expectation of the prior art,
reactivation is effectively achieved even under anaerobic conditions;
the gas stream, for example, may consist essentially of an inert
gas such as nitrogen.
BACKGROUND OF THE INVENTION
In recent years, various titanium-containing molecular sieves have
been developed which usefully catalyze organic transformations such
as the conversion of olefins to epoxides. For example U.S. Pat.
No.4833260 discloses the use of TS-1 titanium silicalite in epoxidation
wherein hydrogen peroxide serves as a source of oxygen. Heterogeneous
catalysts such as titanium silicalite are of great industrial interest,
not only because of their high activity and selectivity, but also
because such catalysts remain insoluble in liquid phase reaction
mixtures and thus can be easily recovered and reused. It would be
highly desirable to use titanium-containing molecular sieves in
continuous processes. Unfortunately, such materials, for reasons
which are not fully understood, tend to slowly deteriorate in performance
when used for a long period of time. Due to the relatively high
cost of synthesizing this type of catalyst, regeneration of the
spent catalyst would be greatly preferred over replacement.
It has previously been proposed to regenerate used titanosilicate
epoxidation catalysts by recalcining the catalysts at elevated temperatures
in the presence of an oxygen-containing gas. For example, G. Perego
et al. Proc. 7th Intern. Zeolite Confer., 1986 Tokyo, p. 827 discloses
that a temperature of 550.degree. C. using air as the oxygen-containing
gas is sufficient for this purpose. Subsequently, other investigators
found that such regeneration could also be accomplished by baking
the spent catalyst in a gas atmosphere at temperatures of from 400.degree.
C. to 500.degree. C. (Japanese Laid-Open Patent Application No.
3-114536), provided at least 5 volume % oxygen is present.
SUMMARY OF THE INVENTION
We have now discovered that a spent titanium-containing molecular
sieve may be reactivated by heating at a temperature of from 150.degree.
C. to 700.degree. C. (more preferably, 250.degree. C. to 600.degree.
C.) in the presence of a gas stream which contains less than 5 volume
percent molecular oxygen. Preferably, the gas stream is passed over
the spent catalyst during reactivation. The restoration in catalyst
performance was unexpected and surprising in view of the express
teaching of the prior art that the presence of at least 5 volume
% molecular oxygen is necessary to regenerate catalysts of this
type.
DETAILED DESCRIPTION OF THE INVENTION
The titanium-containing molecular sieves which may be regenerated
using the process of this invention comprise the class of zeolitic
substances wherein titanium atoms are substituted for a portion
of the silicon atoms in the lattice framework of a molecular sieve.
Such crystalline substances are well-known in the art.
Particularly preferred titanium-containing molecular sieves include
the molecular sieves commonly referred to as "TS-1" (having
an MFI topology analogous to that of the ZSM-5 aluminosilicate zeolites),
"TS-2" (having an MEL topology analogous to that of the
ZSM-11 aluminosilicate zeolites), "TS-3" (as described
in Belgian Pat. No. 1001038), "TS-48" (having a ZSM-48
structure), and "TS-12" (having an MTW-type structure).
Also suitable for use are the titanium-containing molecular sieves
having framework structures isomorphous to zeolite beta as well
as those materials designated "CIT-1", "SSZ-33",
"ETS-4", "ETS-10", and "Ti-MCM-41".
The titanium-containing molecular sieves preferably contain no non-oxygen
elements other than titanium and silica in the lattice framework,
although minor amounts of boron, iron, aluminum, and the like may
be present. Titanium-containing molecular sieves usable in the present
regeneration process are sometimes variously referred to by workers
in the field as "titanium silicalites", "titanosilicates",
"titanium silicates", "silicon titanates" and
the like. The molecular sieve may be admixed with a binder or other
matrix material and may be in any physical form such as powder,
pellets, granules, blocks, or the like. Supported titanium-containing
molecular sieves such as titanium silicalite supported on titania,
silica, or the like may also be regenerated in accordance with the
invention.
Titanium-containing molecular sieves suitable for use in the process
of this invention will generally have a composition corresponding
to the following empirical formula xTiO.sub.2 :(1-x)SiO.sub.2 where
x is between 0.0001 and 0.500. More preferably, the value of x is
from 0.01 to 0.125. The molar ratio of Si:Ti in the lattice framework
of the titanium-containing molecular sieve is advantageously from
9.5:1 to 99:1 (most preferably, from 9.5:1 to 60:1). Large pore
(mesoporous) as well as small pore (microporous) molecular sieves
are suitable for use. Relatively titanium-rich molecular sieves
may also be successfully regenerated. It has been found that spent
titanium-containing molecular sieves typically are contaminated
with organic substances, possibly polymeric or oligomeric in character,
which are not present in fresh catalyst. The regeneration process
herein described is capable of reducing the levels of such contaminants,
as indicated by a decrease in the % C present by elemental analysis.
Prior to regeneration, the titanium-containing molecular sieve
will have been used to catalyze some desired synthetic process.
The present method is particularly useful for restoring the activity
and selectivity of a catalyst employed in olefin epoxidation. Such
epoxidation processes are well-known (see, for example, U.S. Pat.
Nos. 4833260 5354875 5262550 5214168 5374747 5384418
and 5412122) and may be performed using a variety of olefins as
well as different types of oxidizing agents. For example, the catalyst
to be regenerated may have been recovered from a process wherein
propylene is converted to propylene oxide using hydrogen peroxide.
Without wishing to be bound by theory, it is believed that the by-products
which tend to accumulate on the catalyst during epoxidation (e.g.,
epoxide oligomers, olefin oligomers) are capable of being converted
by degradation to more volatile and/or more soluble substances at
relatively low temperatures. Apparently, for reasons which are not
well understood, such degradation does not require the presence
of oxygen.
The spent titanium-containing molecular sieve is preferably separated
in solid form from any liquid components of the reaction mixture
in which it may be present prior to regeneration. For example, where
the molecular sieve has been deployed in the form of a slurry, it
may be readily collected by filtration, centrifugation, decantation,
or other such mechanical means and then transferred into a vessel
which is suitable for carrying out the regeneration. Alternatively,
where the molecular sieve has been used as a fixed bed, the liquid
components may be simply drained or pumped away from the spent catalyst
and regeneration conducted in the same vessel as the catalytic process.
It is not, however, necessary to completely dry the recovered catalyst
prior to regeneration since any minor amounts of solvent, reactants,
and the like adsorbed on the catalyst can be readily removed and
disposed of during such regeneration. An important advantage of
the present method is that reactivation of catalyst may be performed
in vessels of the type conventionally used for olefin epoxidation.
Prior art regeneration processes utilizing oxygen and calcination
temperatures in excess of 400.degree. C. may need to be carried
out in specialized equipment fabricated using relatively high cost
materials of construction in order to avoid metallurgical complications.
The spent titanium-containing molecular sieve is heated in the
presence of a gas stream at a temperature of from 150.degree. C.
to 700.degree. C. The temperature range of from 250.degree. C. to
600.degree. C. is especially suitable. No significant loss in the
crystallinity of the molecular sieve is observed. The temperature
may be kept constant during regeneration or may be periodically
or continuously increased or decreased as may be desired. It has
unexpectedly been found that the presence of molecular oxygen in
the gas stream is not needed in order to accomplish satisfactory
reactivation of the molecular sieve. That is, the gas stream may
be comprised predominantly of one or more inert gases such as nitrogen,
helium, carbon dioxide, argon, water vapor or the like and mixtures
thereof. The gas stream thus contains less than 5 volume % oxygen.
The process of the invention is capable of accomplishing its intended
purpose (i.e., catalyst reactivation) even when the gas stream contains
1 volume % O.sub.2 or less. Anaerobic (oxygen-free) regeneration
conditions may also be employed, if desired.
The process may be conducted such that the gas stream is continually
passed over the titanium-containing molecular sieve in order to
sweep away any volatile products evolved from the catalyst. Alternatively,
the regeneration may be performed in a discontinuous or static manner.
That is, a volume of gas may be introduced into the regeneration
vessel containing the spent catalyst, the vessel sealed and heated
for some period of time before discharging the gas and replacing
it with a fresh volume of gas. The catalyst is heated for such time
as may be necessary to restore the desired level of activity and
selectivity. Typical heating times are from 1 to 150 hours. The
optimum time will vary somewhat depending upon the extent to which
the catalyst has been deactivated, the type of reaction in which
the catalyst has been used, the regeneration temperature, the flow
rate of gas through the catalyst, as well as other factors, but
may be readily ascertained by routine experimentation. A useful
method of monitoring the extent of regeneration is to measure the
% C present in the catalyst by elemental analysis. A spent catalyst
will typically contain 1 weight % carbon or more, with a regenerated
catalyst generally having less than 1 weight % carbon. Broadly speaking,
it will usually be desirable to heat the spent catalyst under conditions
effective to reduce the residual carbon level by at least 50% (more
preferably, at least 85%) relative to the residual carbon level
in the unregenerated catalyst. Activities and selectivities comparable
to that of freshly prepared titanium-containing molecular sieves
may be attained even with only relatively modest decreases in carbon
levels however. Although the regeneration may be performed at any
pressure, atmospheric or subatmospheric pressures are preferred.
Following heat treatment, the regenerated molecular sieve may be
further treated if so desired to further modify its catalytic properties.
For example, the catalyst may be treated with a basic substance
or a silylating agent to neutralize acidic sites which may be present.
Washing with a suitable solvent such as water and/or an organic
solvent such as an alcohol or the like may also be performed. The
regenerated catalyst may be admixed with freshly prepared catalyst
prior to reuse, if so desired.
EXAMPLES
Two portions of spent TS-1 titanium silicalite catalyst which had
been used for olefin epoxidation were heated for 28 hours at 385.degree.
C. in the presence of, in one run (for comparative purposes), a
flowing air stream, and in a second run (to illustrate the process
of the invention), a flowing nitrogen stream. In both runs, removal
of carbon from the catalyst was essentially quantitative as determined
by elemental analysis. |