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 at less
than 400.degree. C. in the presence of molecular oxygen. 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 been used as a catalyst in an olefin epoxidation
reaction comprising heating the spent titanium-containing molecular
sieve at a temperature of less than 400.degree. C. but greater than
150.degree. C. in the presence of molecular oxygen for a time effective
to restore the activity and selectivity of the spent titanium-containing
molecular sieve to levels comparable to that of a freshly prepared
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 a gas comprising molecular oxygen
is passed over the spent titanium-containing molecular sieve during
the regeneration.
4. The method of claim 1 wherein the regeneration is performed
in a static manner.
5. The method of claim 1 wherein the spent titanium-containing
molecular sieve has been used as a catalyst in a propylene epoxidation
reaction.
6. The method of claim 1 wherein the temperature is from 165.degree.
C. to 360.degree. C.
7. The method of claim 1 wherein the regeneration time is from
0.5 to 48 hours.
8. A method for regenerating a spent titanium-containing molecular
sieve having an MFI, MEL, or zeolite beta topology which has been
used as a catalyst in an olefin epoxidation reaction comprising
heating the spent titanium-containing molecular sieve at a temperature
of from 165.degree. C. to 360.degree. C. in the presence of molecular
oxygen for a period of from 0.5 to 48 hours to restore the activity
and selectivity of the spent titanium-containing molecular sieve
to levels comparable to that of a freshly prepared titanium-containing
molecular sieve.
9. The method of claim 8 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.
10. The method of claim 9 wherein regeneration of the spent titanium-containing
molecular sieve is performed within said reactor vessel.
11. The method of claim 8 comprising an additional subsequent step
of treating the spent titanium-containing molecular sieve with a
silylating agent.
12. The method of claim 8 wherein a gas comprising molecular oxygen
is passed over the spent titanium-containing molecular sieve.
13. The method of claim 8 wherein the spent titanium-containing
molecular sieve is in the form of a fixed bed during said heating.
14. The method of claim 8 wherein the spent titanium-containing
molecular sieve is a TS-1 molecular sieve.
15. The method of claim 8 wherein the molecular oxygen is combined
with another gas.
16. The method of claim 8 wherein air is used as a source of molecular
oxygen.
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 at a
moderately elevated temperature in the presence of air or other
oxygen-containing gas.
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.
For example, G. Perego et al. Proc. 7th Intern. Zeolite Confer.,
1986 Tokyo, p. 827 discloses that a temperature of 550.degree.
C. is sufficient for this purpose. Subsequently, other investigators
found that such regeneration could also be accomplished by baking
the spent catalyst at temperatures of from 400.degree. C. to 500.degree.
C. (Japanese Laid-Open Patent Application No. 3-114536). These investigators
also concluded that temperatures lower than 400.degree. C. would
not be adequate to restore the activity of the catalyst.
SUMMARY OF THE INVENTION
We have now discovered that a spent titanium-containing molecular
sieve may be reactivated by heating at a temperature of less than
400.degree. C. but greater than 150.degree. C. in the presence of
molecular oxygen. The restoration in catalyst performance was unexpected
and surprising in view of the express teaching of the prior art
that such temperatures would be insufficient 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. 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 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.
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
when heated at temperatures greater than 150.degree. C.
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.
The regeneration method of this invention may also, however, be
satisfactorily applied to deactivated titanium-containing molecular
sieves utilized in other reactions such as, for example, hydroxylation
of aromatic compounds, ammoximation of ketones, oxidation of saturated
hydrocarbons to alcohols and ketones, and the like, including other
oxidation processes.
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 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 molecular oxygen at a temperature greater than 150.degree.
C., but less than 400.degree. C. The temperature range of from 165.degree.
C. to 360.degree. C. is especially suitable. Due to the relatively
low temperature at which the present process is operated, 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. The molecular
oxygen may be combined with other gases such as nitrogen and the
like; the use of air is especially advantageous due to the low costs
associated with this source of oxygen. The process may be conducted
such that a gas comprising molecular oxygen is passed over the titanium-containing
molecular sieve in order to sweep away any volatile organic products
evolved from the catalyst. Alternatively, the regeneration may be
performed in a static manner. 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 0.5 to 48 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 75%) 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.
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.
The regenerated catalyst may be admixed with freshly prepared catalyst
prior to reuse, if so desired.
|