Molecular sieve abstract
This invention is directed to a method of making an olefin product
from an oxygenate feedstock and a method of protecting catalytic
activity of a silicoaluminophosphate molecular sieve. The methods
comprise providing a silicoaluminophosphate molecular sieve having
catalytic sites within the molecular sieve; shielding the catalytic
sites to protect from loss of catalytic activity; and contacting
the protected sieve in its activated state with an oxygenate feedstock
under conditions effective to produce an olefin product before undesirable
loss of catalytic activity. Undesirable loss in catalytic activity
occurs when activated molecular sieve contacting the oxygenate feedstock
has a methanol uptake index of at least 0.15.
Molecular sieve claims
What is claimed is:
1. A method of storing a silicoaluminophosphate catalyst, comprising
storing silicoaluminophosphate molecular sieve in the form of a
wet filter cake, the stared molecular sieve having a template therein;
blending the template-containing molecular sieve with inert or catalytically
active materials to form a silicoaluminophosphate catalyst; calcining
the catalyst to remove the template; storing the calcined catalyst
in an anhydrous blanket; and contacting the calcined catalyst with
an oxygenate feedstock in a reaction zone of a reactor apparatus
to produce an olefin product.
2. The method of claim 1 wherein the anhydrous blanket is a gas
blanket.
3. The method of claim 2 wherein the gas blanket comprises a gas
selected from the group consisting of nitrogen, helium, CO, CO.sub.2
H.sub.2 argon, O.sub.2 C.sub.1 -C.sub.4 alkanes, cycloalkanes
and mixtures thereof.
4. The method of claim 3 wherein the gas is nitrogen.
5. The method of claim 1 wherein the anhydrous blanket is a liquid
blanket.
6. The method of claim 5 wherein the liquid blanket comprises
a liquid selected from the group consisting of alkanes, cyclo-alkanes,
C.sub.6 -C.sub.30 aromatics, alcohols and mixtures thereof.
7. The method of claim 1 wherein the reactor apparatus has a reactor
loading in excess of 50 kg.
8. The method of claim 1 wherein the reactor apparatus has a reactor
loading in excess of 500 kg.
9. The method of claim 1 wherein the reactor apparatus has a reactor
loading in excess of 5000 kg.
10. The method of claim 2 wherein the gas blanket comprises air.
11. The method of claim 2 wherein the gas blanket is provided
under vacuum conditions.
12. The method of claim 2 wherein the gas blanket is provided
under atmospheric pressure conditions.
13. The method of claim 2 wherein the gas blanket is provided
under greater than atmospheric pressure conditions.
14. The method of claim 2 wherein the gas blanket comprises less
than about 1.2 volume percent water.
15. The method of claim 14 wherein the gas blanket comprises less
than about 0.2 volume percent water.
16. The method of claim 15 wherein the gas blanket comprises less
than about 0.02 volume percent water.
17. The method of claim 5 wherein the liquid blanket comprises
less than about 200 ppm water.
18. The method of claim 17 wherein the liquid blanket comprises
less than about 100 ppm water.
19. The method of claim 18 wherein the liquid blanket comprises
less than about 50 ppm water.
Molecular sieve description
FIELD OF THE INVENTION
This invention relates to a method of protecting the catalytic
activity of a SAPO molecular sieve, and to a method of making an
olefin product by contacting the activated catalyst with an oxygenate
feedstock. In particular, this invention relates to a method of
protecting the catalytic activity of a SAPO molecular sieve by shielding
the catalytic sites of the molecular sieve.
BACKGROUND OF THE INVENTION
Silicoaluminophosphates (SAPOs) have been used as adsorbents and
catalysts. As catalysts, SAPOs have been used in processes such
as fluid catalytic cracking, hydrocracking, isomerization, oligomerization,
the conversion of alcohols or ethers, and the alkylation of aromatics.
In particular, the use of SAPOs in converting alcohols or ethers
to olefin products, particularly ethylene and propylene, is becoming
of greater interest for large scale, commercial production facilities.
As is known in the development of new large scale, commercial production
facilities in the commodity chemical business, many problems arise
in the scale up from laboratory and pilot plant operations. This
is particularly a concern in catalytic reaction systems where large
scale operation will be several orders of magnitude larger than
typical pilot scale facilities. For example, conventional laboratory
scale processes of making olefin products from oxygenate feed are
conducted with catalyst loads of about 5 grams. Conventional large
pilot plant operations may utilize as much as 50 kg of catalyst,
making on the order of 20 kg/hr ethylene and propylene product,
but this is nevertheless minuscule in comparison to what a large
scale, commercial production facility would produce, if one were
in existence today. It would be desirable to operate large scale,
commercial production facilities, which may require a catalyst loading
of anywhere from 1000 kg to 700000 kg, producing anywhere from
600 to 400000 kg/hr of ethylene and propylene product, if a reliable
method of providing such a large quantity of catalyst could be used.
Operating large scale, commercial production facilities clearly
presents great challenges in the development of the catalyst production-to-use
chain. By production-to-use chain is meant the entire area of activities
beginning with the production of molecular sieve, including such
activities as receipt of starting materials, on through the crystallization
process. Also included in the production-to-use chain are intermediate
activities which include formulation of the sieve with binders and
other materials, activation of the manufactured sieve and finished
catalyst; storage, transport, loading, unloading of molecular sieve
and finished catalyst; as well as other practices associated with
the handling and preparation of the sieve and finished catalyst
for its ultimate use. The production-to-use chain ends at the point
when the molecular sieve is introduced into the reaction system.
For purposes of this invention, the end of the production-to-use
chain does not necessarily mean the instant when the molecular sieve
is introduced into the reaction system, since large scale systems
are very large and instantaneous measurements are not practically
feasible. In large scale systems, the production-to-use chain may
be considered as completed some time within 12 hours of loading
an activated catalyst into the reaction system.
Since information to date relating to production of olefin products
by catalytic conversion of oxygenate feedstock has been limited
to laboratory and small pilot plant activities, little if any attention
has been paid to the problems associated with the intermediate activities
in the production-to-use chain. For example, little attention has
been focused on the impact of storage, transport, etc. on catalyst
activity, since small scale activity is rather easily manageable.
While today only relatively small quantities of catalyst are stored
and transported, large quantities of materials will need to be handled
for commercial operations. This may require storage of large quantities
of sieve and catalyst materials for considerable periods of time,
at multiple locations, and under rather rigorous industrial conditions.
As the management of sieve and catalyst in the catalyst production-to-use
chain expands in volume and complexity, there is the likelihood
that millions of dollars will be tied up in catalyst inventory,
and the value of the sieve and catalyst will be lost if quality
is not maintained at every step. Loss of quality will necessarily
translate to loss of product quality as well as loss of product
quantity, and these product losses could far outweigh the cost of
the sieve and catalyst.
Although there has been some work published relating to the intermediate
activities in the catalyst production-to-use chain, few of the problems
associated therewith have been addressed. For example, U.S. Pat.
No. 4681864 to Edwards et al. discuss the use of SAPO-37 molecular
sieve as a commercial cracking catalyst. It is disclosed that activated
SAPO-37 molecular sieve has poor stability, and that stability can
be improved by using a particular activation process. In this process,
organic template is removed from the core structure of the sieve
just prior to contacting with feed to be cracked. The process calls
for subjecting the sieve to a temperature of 400-800.degree. C.
within the catalytic cracking unit.
U.S. Pat. No. 5185310 to Degnan et al. discloses another method
of activating silicoaluminophosphate molecular sieve compositions.
The method calls for contacting a crystalline silicoaluminophosphate
with gel alumina and water, and thereafter heating the mixture to
at least 425.degree. C. The heating process is first carried out
in the presence of an oxygen depleted gas, and then in the presence
of an oxidizing gas. The object of the heating process is to enhance
the acid activity of the catalyst. The acid activity is enhanced
as a result of the intimate contact between the alumina and the
sieve.
Briend et al., J. Phys. Chem. 1995 99 8270-8276 teach that SAPO-34
loses its crystallinity when the template has been removed from
the sieve and the de-templated, activated sieve has been exposed
to air. Data are presented, however, which suggest that over at
least the short term, this crystallinity loss is reversible. Even
over a period of perhaps two years, the data suggest that crystallinity
loss is reversible when certain templates are used.
EP-A2-0 203 005 also discusses the use of SAPO-37 molecular sieve
in a zeolite catalyst composite as a commercial cracking catalyst.
According to the document, if the organic template is retained in
the SAPO-37 molecular sieve until a catalyst composite containing
zeolite and the SAPO-37 molecular sieve is activated during use,
and if thereafter the catalyst is maintained under conditions wherein
exposure to moisture is minimized, the crystalline structure of
the SAPO-37 zeolite composite remains stable.
As seen from the disclosure herein, we have now found that an activated
SAPO molecular sieve will exhibit a loss of catalytic activity when
exposed to a moisture-containing environment, and that this loss
occurs between the time the catalyst is activated and even after
as little as one day of storage. More importantly, we have now found
that the loss of catalytic activity is not reversible after a certain
period of time. It is desirable, therefore, to obtain an activated
SAPO molecular sieve and incorporate that molecular sieve into a
catalytic process before loss of catalytic activity becomes too
great.
SUMMARY OF THE INVENTION
In order to overcome at least one of the many problems inherent
in the prior art, the invention provides a method of protecting
catalytic activity of a silicoaluminophosphate molecular sieve which
is to be used in converting an oxygenate feedstock to an olefin
product, particularly an olefin product comprising ethylene, propylene,
or both. Protection against loss of catalytic activity is provided
by covering catalytic sites of the molecular sieve with a shield
prior to contacting with the oxygenate feedstock. Catalytic contact,
i.e., contact of feedstock with molecular sieve under catalytic
conversion conditions, must be made before a parameter defined herein
as the methanol uptake index drops too low. In addition, the weight
percent methanol conversion of the catalyst, determined at standard
parameters, should not be allowed to drop below a minimum percentage.
Drops in methanol uptake index or methanol conversion which are
too low will likely result in a catalyst that is of little or no
practical use in a large scale process.
In this invention, the shield can be provided in several ways.
The shield can be the template material which is actually used to
make the molecular sieve. As is known in the art, the template forms
the porous structure within the molecular sieve. Conventionally,
the template is removed by calcining, essentially burning it from
the molecular sieve. Leaving the template within the intracrystalline
structure for the proper time will, however, protect the catalytic
sites that are within the porous structure of the molecular sieve.
Even if the template is removed the molecular sieve can still be
protected by providing other types of shields to cover the catalytic
sites. For example, carbonaceous material can be used as a shield.
One way of providing the carbonaceous material is to partially calcine
or burn the template, leaving enough carbon material within the
pores of the molecular sieve to provide the shield.
An anhydrous environment can also act as a shield, even when the
template or carbonaceous material has been removed. An anhydrous
environment is one that is depleted in water content. It can be
either a gas or a liquid environment.
In a particular embodiment of protecting catalytic activity of
a silicoaluminophosphate molecular sieve, the invention comprises
providing a silicoaluminophosphate molecular sieve having catalytic
sites protected against loss of catalytic activity by covering with
a shield, and introducing the molecular sieve into an oxygenate
reaction system, wherein the molecular sieve has a methanol uptake
index of at least 0.15 at time of contact with oxygenate under conditions
effective to convert the oxygenate to olefin product. To provide
protection for storage and transportation the shield should be provided
within the molecular sieve for at least 12 hours prior to contact
with oxygenate. Longer storage and transportation conditions may
require that the shield be provided within the shield for longer
periods of time, e.g., 24 hours, 1 month, or perhaps many months.
The protected molecular sieve is of great benefit in large scale
commercial processes of making olefin product from oxygenate feedstock,
particularly making olefins containing ethylene or propylene from
feedstock comprising methanol or dimethyl ether. In a particular
embodiment of making an olefin product from an oxygenate feedstock,
the invention is to a method which comprises providing a silicoaluminophosphate
molecular sieve having catalytic sites within the molecular sieve;
providing a shield to protect the catalytic sites from contact with
water molecules; removing the shield; and, after removing the shield,
contacting the sieve with an oxygenate feedstock under conditions
effective to produce an olefin product, wherein the activated sieve
contacting the oxygenate feedstock has a methanol uptake index of
at least 0.15 preferably 0.4 more preferably at least 0.6 and
most preferably at least 0.8.
In another embodiment, there is provided a method of making an
olefin product from an oxygenate feedstock, comprising removing
a template from a silicoaluminophosphate molecular sieve and contacting
the molecular sieve with the oxygenate feedstock under conditions
effective to convert the feedstock to an olefin product before the
methanol uptake index drops below 0.15 preferably 0.4 more preferably
0.6 most preferably 0.8.
It is desirable that the activated molecular sieve that is contacted
with oxygenate feedstock have a methanol conversion of at least
10 wt. % at a standard time on stream (TOS) of 5 minutes and a weight
hourly space velocity (WHSV) of 25 hr.sup.-1. Preferably the molecular
sieve should have a methanol conversion of at least 15 wt. % at
a standard time on stream of 5 minutes and a WHSV of 25 hr.sup.-1
more preferably a methanol conversion of at least 20 wt. % at a
standard time on stream of 5 minutes and a WHSV of 25 hr.sup.-1.
The shield can be removed ex situ (outside the reactor per se)
or in situ. In a preferred embodiment, the shield is the template
and the template is removed outside of the reactor unit per se in
order to minimize product contamination, particularly nitrogen contamination
due to nitrogen components within a nitrogen-containing template
that may be used as the shield.
In another preferred embodiment, once the shield has been removed,
the molecular sieve can be maintained at a temperature of at least
150.degree. C., with no shield, with little if any catalyst activity
loss due to exposure of catalytic sites with moisture. In this embodiment,
the molecular sieve is preferably maintained at a temperature of
150 to 800.degree. C., more preferably at a temperature of 175-600.degree.
C., and most preferably at a temperature of 200-500.degree. C. in
order to maintain catalyst activity.
Preferably, the template is a nitrogen-containing hydrocarbon.
Preferably, the nitrogen-containing hydrocarbon is selected from
the group consisting of a tetraethyl ammonium hydroxide salt, cyclopentylamine,
aminomethyl cyclohexane, piperidine, triethylamine, cyclohexylamine,
tri-ethyl hydroxyethylamine, morpholine, dipropylamine, pyridine,
isopropylamine and mixtures thereof. The silicoaluminophosphate
molecular sieve is preferably selected from the group consisting
of SAPO-5 SAPO-8 SAPO-11 SAPO-16 SAPO-17 SAPO-18 SAPO-20
SAPO-31 SAPO-34 SAPO-35 SAPO-36 SAPO-37 SAPO-40 SAPO-41 SAPO-42
SAPO-44 SAPO-47 SAPO-56 metal containing forms thereof, and mixtures
thereof.
The oxygenate feedstock is preferably selected from the group consisting
of methanol; ethanol; n-propanol; isopropanol; C.sub.4 -C.sub.20
alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl
ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic
acid; and mixtures thereof More preferably, the oxygenate feedstock
is methanol or dimethyl ether.
The silicoaluminophosphate molecular sieve can provided with a
binder material, and the template can be removed by heating at a
temperature between 200.degree. C. and 800.degree. C. In order to
convert the oxygenate to olefin product, the process is preferably
performed at a temperature between 200.degree. C. and 700.degree.
C.
The present invention will be better understood by reference to
the Detailed Description of the Invention when taken together with
the attached drawings and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows methanol conversion using a morpholine based molecular
sieve, which has been aged under various environmental conditions;
FIG. 2 shows methanol uptake of a morpholine based molecular sieve;
and
FIG. 3 shows methanol conversion using a SAPO-34 molecular sieve
which has been stored under various conditions.
DETAILED DESCRIPTION OF THE INVENTION
SAPO catalysts, in particular, are susceptible to structural changes
as a result of continued exposure to even low levels of moisture.
Such authorities as Paulitz et al., Microporous Materials, 2 223-228
(1994), however, have shown through X-ray diffration (XRD), nuclear
magnetic resonance (NMR), infrared (IR) and nitrogen (N.sub.2) adsorption
analyses that the structural change is largely reversible. Nevertheless,
we have found that although adsorption analyses indicate that structural
change is largely reversible, this data cannot be relied upon as
an indicator of loss of catalytic activity. In particular, we have
found that SAPO molecular sieves lose catalytic activity when the
catalytic sites are exposed to an open air environment for as little
as a few hours after activation, and that loss of catalytic activity
is irreversible after a certain point.
The possibility of irreversible loss of catalytic activity presents
a problem in the commercial production-to-use chain where storage
and transport of molecular sieve and catalyst can be over a relatively
long period of time. For example, the as manufactured molecular
sieve can be stored anywhere from 12 hours to many months, perhaps
as long as one year, before its final use as an activated catalyst.
SAPO molecular sieve, as well as the SAPO molecular sieve blended
with other catalyst material, can be protected from negative effects
of moisture by properly shielding catalytic sites within the molecular
sieve. Proper shielding can be accomplished in a variety of ways.
The catalytic sites can be shielded by maintaining a template within
the molecular sieve, by covering the sites with a carbonaceous material
or by maintaining the sieve, even without a template, in an anhydrous
environment. Removing template or carbonaceous material from the
active sites of the molecular sieve results in an activated molecular
sieve, meaning that the molecular sieve has its catalytic sites
open and ready to contact feedstock. The anhydrous environment serves
as a shield for the activated molecular sieve in the sense that
it shields active catalyst sites from contact with open air conditions,
particularly moisture in the air.
In order to use the SAPO molecular sieve as a catalyst, the shield
must be removed such that the catalytic sites of the molecular sieve
can be open to contact feedstock. Once the shield is removed, however,
the catalytic sites are open to contact with moisture or other components
that may be present in the localized environment and cause a loss
of catalytic activity. Extended exposure of catalytic sites to such
conditions generally results in irreversible loss of catalytic activity
to the exposed site. At a certain point, the molecular sieve is
not of practical use in a large scale catalytic process. As defined
herein, a large scale catalytic process is one having a reactor
loading in excess of 50 kg, particularly one having a reactor system
loading in excess of 500 kg, especially one having a reactor loading
in excess of 5000 kg.
Extended exposure, or the point at which loss of catalytic activity
becomes undesirable, is defined according to this invention by a
methanol uptake index. According to this invention, the methanol
uptake index is defined as the ratio between the maximum methanol
adsorption capacity (wt %) of an activated SAPO molecular sieve
(i.e., the initial methanol adsorption capacity) and the methanol
adsorption capacity (wt %) of the activated SAPO molecular sieve
at the time of catalytic contact with a feedstock (i.e., the methanol
adsorption capacity at feed contact). At the time of catalytic contact
with a feedstock marks the end of the production-to-use chain.
At the time of catalytic contact means the point in time when the
activated SAPO molecular sieve is contacted with feed under conditions
effective to convert the feed to product, the product containing
measurable portions of ethylene and propylene. This does not imply,
however, that the methanol adsorption capacity at feed contact must
be calculated at the exact instant that feed contacts activated
molecular sieve. This is because it may not be possible to run such
a precise calculation, particularly in evaluating large scale reaction
systems. Therefore, the methanol adsorption capacity at feed contact
must be evaluated as soon as practical before contact with feed.
For molecular sieve activated in situ, the time between activation
and actual contact with feed is short enough such that the initial
methanol adsorption capacity is essentially equivalent to the methanol
adsorption capacity at feed contact, resulting in a methanol uptake
index of approximately 1. For ex situ activation, the methanol adsorption
capacity at feed contact should be evaluated as close as practical
to actual contact with feed under catalytic conversion conditions.
Under some circumstances, especially when dealing with large scale
systems, as close as practical may extend up to as much as 12 hours
between activation and actual contact with feed under catalytic
conversion conditions.
According to this invention, it is preferred that the methanol
uptake index be at least 0.15 preferably at least 0.4 more preferably
at least 0.6 and most preferably at least 0.8. Although some catalytic
activity can occur at a methanol uptake index below 0.15 the molecular
sieve at that state is not of practical value as a commercial scale
catalyst. Irreversible loss of catalytic activity will likely occur
below this point to the extent that the catalyst is no longer of
benefit in a large scale catalytic process.
The catalytic activity of the molecular sieve for use as a catalyst
for converting oxygenate to olefin product is considered sufficiently
preserved or protected when the molecular sieve has the desired
methanol uptake index at time of contact with oxygenate under conditions
effective to convert the oxygenate to olefin product. Since the
methanol uptake index will drop over time if the molecular sieve
is improperly handled, contact with oxygenate under reaction conditions
should occur before the methanol uptake index drops below 0.15.
To calculate methanol uptake index, methanol adsorption capacity
must be measured. Techniques for measuring methanol adsorption capacity
are known to those of ordinary skill in the art. In a preferred
technique, about 5 mg of sample is introduced into a thermogravimetric
analyzer (TGA). The sample is subjected to a heat treatment process,
which includes: (1) heating from room temperature to 450.degree.
C., with a heat up rate of 20.degree. C./min. in air; (2) holding
at 450.degree. C. for 40 min. in air; and cooling to 30.degree.
C. in air. After the sample has reached 30.degree. C., the air flow
is switched to a methanol containing nitrogen flow with a methanol
partial pressure of 0.09 atm. The sample is contacted with this
nitrogen/methanol mixture for 180 minutes. The methanol adsorption
capacity is the weight percent weight increase after the 180 minutes
contact with the methanol vapor.
To obtain a SAPO molecular sieve having the appropriate methanol
uptake index, the shield can be removed in situ. That is, the shield,
whether template, carbonaceous material or anhydrous liquid or gas,
can be removed inside the reactor or the regenerator during operation.
However, in a preferred embodiment, the template or carbonaceous
material is removed ex situ. This means that it is preferred to
activate the catalytic sites of the molecular sieve outside of the
reactor. This is because there is less likelihood that the shield
material will contaminate the reaction products. This is particularly
beneficial when the desired product of the methanol reaction process
is to be very low in any nitrogen or sulfur-containing contaminants.
For example, in cases where the shield is a molecular sieve template
containing a nitrogen component and the desired product of the reaction
process is ethylene, it may be desirable to remove the template
ex situ since the presence of very small amounts of nitrogen in
the ethylene might adversely impact the subsequent conversion of
the ethylene product to polyethylene. In less sensitive reaction
systems, however, catalyst containing a template material can be
added as makeup and activated in situ. Even in more sensitive reaction
processes, makeup addition can be directly to the reactor, i.e.,
in situ addition, since makeup addition can be controlled to add
catalyst at relatively low quantities over a period of time, thereby
minimizing possible product contamination. Preferably, the makeup
addition is directly to a return line which sends regenerated catalyst
from the regenerator back to the reactor, or addition is to the
regenerator itself. Addition of catalyst outside of the reactor
itself is considered ex situ addition, which includes addition at
the return line or the regenerator.
When ex situ activation of the molecular sieve is carried out,
it is important to not let the methanol uptake index drop below
the desirable value of 0.15 preferably 0.4 more preferably 0.6
and most preferably 0.8 before using the molecular sieve in a reaction
process. As mentioned above, this is because irreversible loss of
catalytic activity, which is represented by a significant drop in
methanol uptake index as well as methanol conversion, will reach
a level that is undesirable for practical operation.
It has also been found that once the activated catalyst is loaded
into a heated system, whether reactor, regenerator or any other
part of the operating system, or any type of storage environment,
loss of catalyst activity is greatly reduced, even when a shield
is not present. At a temperature of at least about 150.degree. C.,
catalyst activity has been found to be stabilized. This means that
at a temperature of 150.degree. C. or above, moisture has very little
impact on active catalyst sites. It is preferred to maintain active
molecular sieve at a temperature of 150 to 800.degree. C., more
preferably at a temperature of 175-600.degree. C., and most preferably
at a temperature of 200-500.degree. C. in order to maintain catalyst
activity.
Undesirable drops in methanol uptake index can be controlled by
proper handling during storage or transport. In environments maintained
below 150.degree. C., this means that as long as there is sufficient
shielding of the catalytic sites before use in a catalytic process,
catalytic activity will be acceptable. Sufficient shielding can
be maintained by storing or transporting the molecular sieve or
catalyst containing the molecular sieve with its template or with
an acceptable carbonaceous material which shields the catalytic
sites from contact with moisture. Even when activated, sufficient
shielding can be maintained by storing or transporting the activated
molecular sieve or catalyst containing the molecular sieve in an
anhydrous environment.
It is also preferable that the activated SAPO molecular sieve have
a methanol conversion of at least 10 wt. %, preferably at least
15 wt. %, most preferably at least 20 wt. %, at standard methanol
conversion conditions. For purposes of this invention, standard
methanol conversion conditions means that methanol conversion is
determined at a time on stream (TOS) of 5 minutes and a WHSV of
24 hr.sup.-1. As defined herein, methanol conversion is the weight
percent of methanol converted to product, with any dimethyl ether
present in the product not being included as part of the converted
product. The method for calculating methanol conversion is carried
out using a standard 1/2" diameter SS, fixed bed, continuous
reactor. A sample of the molecular sieve or a catalyst containing
the molecular sieve is added to the reactor and 100% methanol is
added as feed. The reaction is carried out at 450.degree. C., a
reactor pressure of 25 psig (i.e., a methanol partial pressure of
25 psig), and a weight hourly space velocity (WHSV) of 25 hr.sup.-1.
The reaction products are preferably analyzed with an on-line gas
chromatograph (GC). After 5 minutes time on stream (i.e., after
5 minutes of contacting methanol with molecular sieve under reaction
conditions), methanol conversion is calculated as: 100-(wt. % methanol+wt.
% DME) left in the product.
In testing for the methanol conversion, WHSV is defined as the
weight of the feed fed to the 1/2" reactor over time (per hour)
divided by the weight of the silicoaluminophosphate molecular sieve
component of the catalyst in the reactor. The silicoaluminophosphate
molecular sieve component of the catalyst is intended to mean only
the silicoaluminophosphate molecular sieve portion that is contained
within the catalyst. This excludes catalyst components such as non-silicoaluminophosphate
molecular sieves, binders, diluents, inerts, rare earth components,
etc.
The silicoaluminophosphate molecular sieves of this invention comprise
a three-dimensional microporous crystal framework structure of [SiO.sub.2
], [AlO.sub.2 ] and [PO.sub.2 ] tetrahedral units. The way Si is
incorporated into the structure can be determined by .sup.29 Si
MAS NMR. See Blackwell and Patton, J. Phys. Chem., 92 3965 (1988).
The desired SAPO molecular sieves will exhibit one or more peaks
in the .sup.29 Si MAS NMR, with a chemical shift [(Si) in the range
of -88 to -94 ppm and with a combined peak area in that range of
at least 20% of the total peak area of all peaks with a chemical
shift [(Si) in the range of -88 ppm to -115 ppm, where the [(Si)
chemical shifts refer to external tetramethylsilane (TMS).
Silicoaluminophosphate molecular sieves are generally classified
as being microporous materials having 8 10 or 12 membered ring
structures. These ring structures can have an average pore size
ranging from about 3.5-15 angstroms. Preferred are the small pore
SAPO molecular sieves having an average pore size ranging from about
3.5 to 5 angstroms, more preferably from 4.0 to 5.0 angstroms. These
preferred pore sizes are typical of molecular sieves having 8 membered
rings.
In general, silicoaluminophosphate molecular sieves comprise a
molecular framework of corner-sharing [SiO.sub.2 ], [AlO.sub.2 ],
and [PO.sub.2 ] tetrahedral units. This type of framework is effective
in converting various oxygenates into olefin products.
The [P.sub.2 ] tetrahedral units within the framework structure
of the molecular sieve of this invention can be provided by a variety
of compositions. Examples of these phosphorus-containing compositions
include phosphoric acid. organic phosphates such as triethyl phosphate,
and aluminophosphates. The phosphorous-containing compositions are
mixed with reactive silicon and aluminum-containing compositions
under the appropriate conditions to form the molecular sieve.
The [AlO.sub.2 ] tetrahedral units within the framework structure
can be provided by a variety, of compositions. Examples of these
aluminum-containing compositions include aluminum alkoxides such
as aluminum isopropoxide, aluminum phosphates, aluminum hydroxide,
sodium aluminate, and pseudoboehmite. The aluminum-containing compositions
are mixed with reactive silicon and phosphorus-containing compositions
under the appropriate conditions to form the molecular sieve.
The [SiO.sub.2 ] tetrahedral units within the framework structure
can be provided by a variety of compositions. Examples of these
silicon-containing compositions include silica sols and silicium
alkoxides such as tetra ethyl orthosilicate. The silicon-containing
compositions are mixed with reactive aluminum and phosphorus-containing
compositions under the appropriate conditions to form the molecular
sieve.
Substituted SAPOs can also be used in this invention. These compounds
are generally known as MeAPSOs or metal-containing silicoaluminophosphates.
The metal can be alkali metal ions (Group IA), alkaline earth metal
ions (Group IIA), rare earth ions (Group IIIB, including the lanthanoid
elements: lanthanum, cerium, praseodymium, neodymium, samarium,
europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,
ytterbium and lutetium; and scandium or yttrium) and the additional
transition cations of Groups IVB, VB, VIB, VIIB, VIIIB, and IB.
Preferably, the Me represents atoms such as Zn, Mg, Mn, Co, Ni,
Ga, Fe, Ti, Zr, Ge, Sn, and Cr. These atoms can be inserted into
the tetrahedral framework through a [MeO.sub.2 ] tetrahedral unit.
The [MeO.sub.2 ] tetrahedral unit carries a net electric charge
depending on the valence state of the metal substituent. When the
metal component has a valence state of +2 +3 +4 +5 or +6 the
net electric charge is between -2 and +3. Incorporation of the metal
component is typically accomplished adding the metal component during
synthesis of the molecular sieve. However, post-synthesis ion exchange
can also be used.
Suitable silicoaluminophosphate molecular sieves include SAPO-5
SAPO-8 SAPO-11 SAPO-16 SAPO-17 SAPO-18 SAPO-20 SAPO-31 SAPO-34
SAPO-35 SAPO-36 SAPO-37 SAPO-40 SAPO-41 SAPO42 SAPO44 SAPO-47
SAPO-56 the metal containing forms thereof, and mixtures thereof.
Preferred are SAPO-18 SAPO-34 SAPO-35 SAP044 and SAPO-47 particularly
SAPO-18 and SAPO-34 including the metal containing forms thereof,
and mixtures thereof. As used herein, the term mixture is synonymous
with combination and is considered a composition of matter having
two or more components in varying proportions, regardless of their
physical state.
The silicoaluminophosphate molecular sieves are synthesized by
hydrothermal crystallization methods generally known in the art.
See, for example, U.S. Pat. Nos. 4440871; 4861743; 5096684;
and 5126308 the methods of making of which are fully incorporated
herein by reference. A reaction mixture is formed by mixing together
reactive silicon, aluminum and phosphorus components, along with
at least one template. Generally the mixture is sealed and heated,
preferably under autogenous pressure, to a temperature of at least
100.degree. C., preferably from 100-250.degree. C., until a crystalline
product is formed. Formation of the crystalline product can take
anywhere from around 2 hours to as much as 2 weeks. In some cases,
stirring or seeding with crystalline material will facilitate the
formation of the product.
Typically, the molecular sieve product will be formed in solution.
It can be recovered by standard means, however, such as by centrifugation
or filtration. The product can also be washed, recovered by the
same means and dried.
As a result of the crystallization process, the recovered sieve
contains within its pores at least a portion of the template used
in making the initial reaction mixture. The crystalline structure
essentially wraps around the template, and the template must be
removed to obtain catalytic activity. Once the template is removed,
the crystalline structure that remains has what is typically called
an intracrystalline pore system.
The SAPO molecular sieve can contain one or more templates. Templates
are structure directing agents, and typically contain nitrogen,
phosphorus, oxygen, carbon, hydrogen or a combination thereof, and
can also contain at least one alkyl or aryl group, with 1 to 8 carbons
being present in the alkyl or aryl group. Mixtures of two or more
templates can produce mixtures of different sieves or predominantly
one sieve where one template is more strongly directing than another.
Representative templates include tetraethyl ammonium salts, cyclopentylamine,
aminomethyl cyclohexane, piperidine, triethylamine, cyclohexylamine,
tri-ethyl hydroxyethylamine, morpholine, dipropylamine (DPA), pyridine,
isopropylamine and combinations thereof. Preferred templates are
triethylamine, cyclohexylamine, piperidine, pyridine, isopropylamine,
tetraethyl ammonium salts, and mixtures thereof. The tetraethylammonium
salts include tetraethyl ammonium hydroxide (TEAOH), tetraethyl
ammonium phosphate, tetraethyl ammonium fluoride, tetraethyl ammonium
bromide, tetraethyl ammonium chloride, tetraethyl ammonium acetate.
Preferred tetraethyl ammonium salts are tetraethyl ammonium hydroxide
and tetraethyl ammonium phosphate.
In this invention, the templates can be used to shield the catalytic
sites of the SAPO molecular sieve from contact with water molecules.
Since the templates will be present within the microporous structure
of the sieve, water molecules will not be able to enter the pores
of the sieve, preventing any contact with the catalyst sites. This
means that a molecular sieve containing a template can even be stored
in wet filter cake form, without noticeable loss of catalytic activity
once activated. When stored in wet filter cake form, the molecular
sieve will typically be dried without removing the template. Then,
the molecular sieve can be calcined to remove the template.
Carbonaceous material can also be used to shield the catalytic
sites of the SAPO molecular sieve. In this embodiment, carbonaceous
material can be within the microcrystalline pore structure or it
can be deposited to cover the pore entrance. The carbonaceous material
can be placed within the crystalline pore structure by partially
burning the template material so that carbon residue remains within
the molecular sieve. Carbonaceous material can also be directly
applied to the exterior of the molecular sieve to block the pore
openings.
As is known in the art, molecular sieve or catalyst containing
the molecular sieve, must be activated prior to use in a catalytic
process. Activation is performed in such a manner that template
is removed from the molecular sieve, leaving active catalytic sites
with the microporous channels of the molecular sieve open for contact
with feed. The activation process is typically accomplished by calcining,
or essentially heating the template at a temperature of from 200
to 800.degree. C. in the presence of an oxygen-containing gas. In
some cases, it may be desirable to heat in an environment having
a low oxygen concentration. This type of process can be used for
partial or complete removal of the template from the intracrystalline
pore system. In other cases, particularly with smaller templates,
complete or partial removal from the sieve can be accomplished by
conventional desorption processes such as those used in making standard
zeolites.
Once the molecular sieve or catalyst containing the molecular sieve
has been activated, an anhydrous environment can be provided as
a shield against water molecules contacting catalyst sites within
the molecular sieve. Such an environment can be provided by covering
the sieve with a gas or liquid blanket under anhydrous conditions.
As provided herein, the anhydrous gas or liquid blanket will have
a limited amount of water. The anhydrous gas blanket can be provided
under vacuum conditions or under atmospheric or greater pressure
conditions, and will have less than about 1.2 volume percent water,
preferably less than about 0.2 volume percent water, more preferably
less than about 0.02 volume percent water. The anhydrous liquid
blanket will have less than about 200 ppm water preferably less
than about 100 ppm water, more preferably less than about 50 ppm
water. The anhydrous environment can be applied during storage,
transport or loading of the catalyst.
The anhydrous gas blanket is one which is a gas under standard
temperature and pressure conditions and does not react to any significant
degree with the molecular sieve structure. The gas is preferably
selected from the group consisting of nitrogen, helium, CO, CO.sub.2
H.sub.2 argon, O.sub.2 light alkanes (especially C.sub.1 -C.sub.4
alkanes, particularly methane and ethane), cyclo-alkanes and mixtures
thereof, e.g. air. The gas blanket can be maintained at any pressure,
including under vacuum or at pressures above standard, even if the
gas becomes liquid at pressures above standard, as long as the conditions
remain anhydrous.
The anhydrous liquid blanket is a liquid under standard temperature
and pressure conditions, and does not react to any significant degree
with the molecular sieve structure. The liquid is preferably selected
from the group consisting of alkanes, cyclo-alkanes, C.sub.6 -C.sub.30
aromatics, alcohols, particularly C.sub.4.sup.+ branched alcohols.
In this invention, the molecular sieve is made ready for use in
a catalytic process by removing the shielding conditions. If the
gas or liquid blanket is applied under anhydrous conditions to shield
an already activated molecular sieve, the blanket or liquid need
merely be removed by any standard technique. This can be as simple
as merely opening the blanketed system to the atmosphere or by using
any standard filtration or separation technique.
If a carbonaceous material is used as the shield, it can also be
removed by exposing the sieve to sufficient temperature conditions
to decompose the carbonaceous material. Preferably, the carbonaceous
material is removed by calcining at a temperature of about 200-800.degree.
C.
It is preferred that the molecular sieve not be exposed to hydrous
conditions once the shield is removed. Otherwise, there may be irreversible
catalytic loss. However, the molecular sieve can be stored, transported
or loaded into a reactor system, in its unshielded form, in a hydrous
environment as long as the methanol uptake index does not fall below
0.15 preferably 0.4 more preferably 0.6 most preferably 0.8.
The silicoaluminophosphate molecular sieves may be admixed (blended)
with other materials. When blended, the resulting composition is
typically referred to as a silicoaluminophosphate (SAPO) catalyst,
with the catalyst comprising the SAPO molecular sieve.
Materials which can be blended with the molecular sieve can be
various inert or catalytically active materials, or various binder
materials. These materials include compositions such as kaolin and
other clays, various forms of rare earth metals, other non-zeolite
catalyst components, zeolite catalyst components, alumina or alumina
sol, titania, zirconia, quartz, silica or silica or silica sol,
and mixtures thereof These components are also effective in reducing
overall catalyst cost, acting as a thermal sink to assist in heat
shielding the catalyst during regeneration, densifying the catalyst
and increasing catalyst strength. When blended with non-silicoaluminophosphate
molecular sieve materials, the amount of molecular sieve which is
contained in the final catalyst product ranges from 10 to 90 weight
percent of the total catalyst, preferably 30 to 70 weight percent
of the total catalyst.
In one embodiment of this invention, a feed containing an oxygenate
is contacted in a reaction zone of a reactor apparatus with an activated
molecular sieve catalyst at process conditions effective to produce
light olefins, i.e., an effective temperature, pressure, WHSV (weight
hour space velocity) and, optionally, an effective amount of diluent,
correlated to produce light olefins. Typically, the oxygenate feed
is contacted with the catalyst when the oxygenate is in a vapor
phase. However, the process may be carried out in a liquid or a
mixed vapor/liquid phase. When the process is carried out in a liquid
phase or a mixed vapor/liquid phase, different conversions and selectivities
of feed-to-product may result depending upon the catalyst and reaction
conditions.
Olefins can generally be produced at a wide range of temperatures.
An effective operating temperature range can be from about 200.degree.
C. to 700.degree. C. At the lower end of the temperature range,
the formation of the desired olefin products may become markedly
slow. At the upper end of the temperature range, the process may
not form an optimum amount of product. An operating temperature
of at least 300.degree. C., and up to 500.degree. C. is preferred.
Owing to the nature of the process, it may be desirable to carry
out the process of the present invention by use of the molecular
sieve catalysts in a dynamic bed system or any system of a variety
of transport beds rather than in a fixed bed system. It is particularly
desirable to operate the reaction process at high space velocities.
The conversion of oxygenates to produce light olefins may be carried
out in a variety of large scale catalytic reactors, including, but
not limited to, fluid bed reactors and concurrent riser reactors
as described in "Free Fall Reactor," Fluidization Engineering,
D. Kunii and O. Levenspiel, Robert E. Krieger Publishing Co. NY,
1977 incorporated in its entirety herein by reference. Additionally,
countercurrent free fall reactors may be used in the conversion
process. See, for example, U.S. Pat. No. 4068136 and "Riser
Reactor", Fluidization and Fluid-Particle Systems, pages 48-59
F. A. Zenz and D. F. Othmo, Reinhold Publishing Corp., NY 1960
the descriptions of which are expressly incorporated herein by reference.
Any standard commercial scale reactor system can be used, including
fixed bed or moving bed systems. The commercial scale reactor systems
can be operated at a weight hourly space velocity (WHSV) of from
1 hr.sup.-1 to 1000 hr.sup.-1. In the case of commercial scale reactors,
WHSV is defined as the weight of hydrocarbon in the feed per hour
per weight of silicoaluminophosphate molecular sieve content of
the catalyst. The hydrocarbon content will be oxygenate and any
hydrocarbon which may optionally be combined with the oxygenate.
The silicoaluminophosphate molecular sieve content is intended to
mean only the silicoaluminophosphate molecular sieve portion that
is contained within the catalyst. This excludes components such
as binders, diluents, inerts, rare earth components, etc.
It is highly desirable to operate at a temperature of at least
300.degree. C. and a Temperature Corrected Normalized Methane Sensitivity
(TCNMS) of less than about 0.016 preferably less than about 0.012
more preferably less than about 0.01. It is particularly preferred
that the reaction conditions for making olefin from oxygenate comprise
a WHSV of at least about 20 hr.sup.-1 producing olefins and a TCNMS
of less than about 0.016.
As used herein, TCNMS is defined as the Normalized Methane Selectivity
(NMS) when the temperature is less than 400.degree. C. The NMS is
defined as the methane product yield divided by the ethylene product
yield wherein each yield is measured on, or is converted to, a weight
% basis. When the temperature is 400.degree. C. or greater, the
TCNMS is defined by the following equation, in which T is the average
temperature within the reactor in .degree. C.: ##EQU1##
The pressure also may vary over a wide range, including autogenous
pressures. Effective pressures may be in, but are not necessarily
limited to, pressures of from about 0.1 kPa to about 10 MPa. Preferred
pressures are in the range of about 5 kPa to about 5 MPa, with the
most preferred range being of from about 50 kPa to about 0.5 MPa.
The foregoing pressures are exclusive of any oxygen depleted diluent,
and thus, refer to the partial pressure of the oxygenate compounds
and/or mixtures thereof with feedstock. At the lower and upper end
of the foregoing pressure ranges, the rate of selectivity, conversion
and/or reaction may not be optimum.
One or more inert diluents may be present in the feedstock, for
example, in an amount of from 1 to 99 molar percent, based on the
total number of moles of all feed and diluent components fed to
the reaction zone (or catalyst). Typical diluents include, but are
not necessarily limited to helium, argon, nitrogen, carbon monoxide,
carbon dioxide, hydrogen, water, paraffins, alkanes (especially
methane, ethane, and propane), alkylenes, aromatic compounds, and
mixtures thereof. The preferred diluents are water and nitrogen.
Water can be injected in either liquid or vapor form.
The process may be carried out in a batch, semi-continuous or continuous
fashion. The process can be conducted in a single reaction zone
or a number of reaction zones arranged in series or in parallel.
The level of conversion of the oxygenates can be maintained to
reduce the level of unwanted by-products. Conversion can also be
maintained sufficiently high to avoid the need for commercially
undesirable levels of recycling of unreacted feeds. A reduction
in unwanted by-products is seen when conversion moves from 100 mol
% to about 98 mol % or less. Recycling up to as much as about 50
mol % of the feed is commercially acceptable. Therefore, conversions
levels which achieve both goals are from about 50 mol % to about
98 mol % and, desirably, from about 85 mol % to about 98 mol %.
However, it is also acceptable to achieve conversion between 98
mol % and 100 mol % in order to simplify the recycling process.
Oxygenate conversion may be maintained at this level using a number
of methods familiar to persons of ordinary skill in the art. Examples
include, but are not necessarily limited to, adjusting one or more
of the following: the reaction temperature; pressure; flow rate
(i e., WHSV); level and degree of catalyst regeneration; amount
of catalyst re-circulation; the specific reactor configuration;
the feed composition; and other parameters which affect the conversion.
If regeneration is required, the molecular sieve catalyst can be
continuously introduced as a moving bed to a regeneration zone where
it can be regenerated, such as for example by removing carbonaceous
materials or by oxidation in an oxygen-containing atmosphere. In
a preferred embodiment, the catalyst is subject to a regeneration
step by burning off carbonaceous deposits accumulated during the
conversion reactions.
The oxygenate feedstock comprises at least one organic compound
which contains at least one oxygen atom, such as aliphatic alcohols,
ethers, carbonyl compounds (aldehydes, ketones, carboxylic acids,
carbonates, esters and the like), and the feedstock may optionally
contain at least one compound containing a halide, mercaptan, sulfide,
or amine, as long as the optional components do not significantly
impede the performance of the catalyst. When the oxygenate is an
alcohol, the alcohol can include an aliphatic moiety having from
1 to 10 carbon atoms, more preferably from 1 to 4 carbon atoms.
Representative alcohols include but are not necessarily limited
to lower straight and branched chain aliphatic alcohols, their unsaturated
counterparts and the nitrogen, halogen and sulfur analogues of such.
Examples of suitable oxygenate compounds include, but are not limited
to: methanol; ethanol; n-propanol; isopropanol; C.sub.4 -C.sub.20
alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl
ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic
acid; and mixtures thereof Preferred oxygenate compounds are methanol,
dimethyl ether, or a mixture thereof.
The method of making the preferred olefin product in this invention
can include the additional step of making these compositions from
hydrocarbons such as oil, coal, tar sand, shale, biomass and natural
gas. Methods for making the compositions are known in the art. These
methods include fermentation to alcohol or ether, making synthesis
gas, then converting the synthesis gas to alcohol or ether. Synthesis
gas can be produced by known processes such as steam reforming,
autothermal reforming and partial oxidization.
One skilled in the art will also appreciate that the olefins produced
by the oxygenate-to-olefin conversion reaction of the present invention
can be polymerized to form polyolefins, particularly polyethylene
and polypropylene. Processes for forming polyolefins from olefins
are known in the art. Catalytic processes are preferred. Particularly
preferred are metallocene, Ziegler/Natta and acid catalytic systems.
See, for example, U.S. Pat. Nos. 3258455; 3305538; 3364190;
5892079; 4659685; 4076698; 3645992; 4302565; and 4243691
the catalyst and process descriptions of each being expressly incorporated
herein by reference. In general, these methods involve contacting
the olefin product with a polyolefin-forming catalyst at a pressure
and temperature effective to form the polyolefin product.
A preferred polyolefin-forming catalyst is a metallocene catalyst.
The preferred temperature range of operation is between 50 and 240.degree.
C. and the reaction can be carried out at low, medium or high pressure,
being anywhere within the range of about 1 to 200 bars. For processes
carried out in solution, an inert diluent can be used, and the preferred
operating pressure range is between 10 and 150 bars, with a preferred
temperature range of between 120 and 230.degree. C. For gas phase
processes, it is preferred that the temperature generally be within
a range of 60 to 160.degree. C., and that the operating pressure
be between 5 and 50 bars.
This invention will be better understood with reference to the
following examples, which are intended to illustrate specific embodiments
within the overall scope of the invention as claimed.
EXAMPLE 1
Samples of SAPO-34 containing a morpholine template were heated
in order to remove the template. One sample was heated for 5 hours
at 650.degree. C. in N.sub.2 followed by 3 hours at 650.degree.
C. in air in a closed furnace to remove the template. The sample
was stored for 4 days over silica gel (relative humidity <20%
at 20.degree. C.). A second sample was heated in the same manner,
but was stored for 3 days at 80% relative humidity and 20.degree.
C. In a third sample, the template was removed in the same manner.
However, the third sample was transferred at 150.degree. C. into
a fixed bed, continuous reactor immediately after template removal.
The third sample was designated as the "0 days aged" sample
portion. Each portion was then individually evaluated in a fixed
bed, continuous reactor. Reaction temperature was maintained at
450.degree. C. Pressure in the reactor was held at 25 psig. Methanol
feed was continuously fed to the reactor at a WHSV of 25 hr.sup.-1.
Reaction products were analyzed with an on-line GC equipped with
a FID and TCD detector. FIG. 1 shows wt % of methanol converted
as a function of TOS (in minutes). The methanol conversion of the
first sample at a TOS of 5 minutes was approximately 27 wt. %. The
methanol conversion of the second sample at a TOS of 5 minutes was
approximately 0 wt. %. the methanol conversion of the third sample
at TOS of 5 minutes was approximately 100 wt. %.
EXAMPLE 2
Samples of SAPO-34 containing a morpholine template were heated
in order to remove the template. One sample was heated for 5 hours
at 650.degree. C. in N.sub.2 followed by 3 hours at 650.degree.
C. in air in a closed furnace to remove the template. The sample
was stored for 1 day under ambient conditions. A second sample was
heated in the same manner, but was stored for 5 days under ambient
conditions. In a third sample, the template was removed in situ
(i.e., in a fixed bed, continuous reactor) under nitrogen at 650.degree.
C. for 5 hours, followed by air at 650.degree. C. for 3 hours. The
third sample was designated as the "0 days aged" sample
portion. The sample portions having the templates removed were measured
for methanol uptake according to the following procedure:
About 5 mg of sample was introduced into a Perkin Elmer TGS-2 thermogravimetric
analyzer (TGA). The sample was subjected to heat treatment, which
included: (1) heating from room temperature to 450.degree. C., with
a heat up rate of 20.degree. C./min. in air; (2) holding at 450.degree.
C. for 40 min. in air; and cooling to 30.degree. C. in air. After
the sample reached 30.degree. C., the air flow was switched to a
methanol containing nitrogen flow with a methanol partial pressure
of 0.09 atm. The sample was then contacted with this nitrogen/methanol
mixture for 180 minutes, and the methanol adsorption capacity was
calculated as the weight percent increase after the 180 minutes
contact with the methanol vapor. FIG. 2 shows methanol adsorption
capacity or methanol uptake as a function of time. The horizontal
line fragments indicate the saturation level.
The methanol uptake index was calculated based on the saturation
level aged samples versus the saturation level of the "0 days
aged/in situ" sample. The 0 days aged/in situ sample was defined
as the base case, having a methanol uptake index of 1. The methanol
uptake index for the 1 day lab aged sample was calculated as approximately
0.85 and the methanol uptake index for the 5 days lab aged sample
was calculated as approximately 0.65.
EXAMPLE 3
A sample of SAPO-34 containing a DPA/TEAOH template was dried,
with the template being left in place. The sample was stored for
25 days, then mixed with SiC. (0.36 g SAPO/5 g SiC). The template
was then removed in situ (i.e., in a fixed bed, continuous reactor)
under nitrogen at 625.degree. C. for 1 hour. After template removal,
methanol was continuously fed to the reactor at a WHSV of 25 hr.sup.-1
while maintaining the reaction temperature at 450.degree. C. and
the reactor pressure at 23 psig. Reaction products were analyzed
with an on-line GC equipped with a FID and TCD detector for ethylene
and propylene yield. Methanol conversion was calculated as: 100-(wt
% methanol+DME) left in product. The data are shown in FIG. 3 with
the label "fresh, calcined sieve."
EXAMPLE 4
A sample of SAPO-34 containing a DPA/TEAOH template was filtered
and stored in a wet filter cake form, with the template being left
in place. After storing for 44 days, the filter cake was dried,
then the template was then removed and methanol conversion calculated
according to the procedure of Example 3. The data are shown in FIG.
3.
EXAMPLE 5
A sample of SAPO-34 containing a DPA/TEAOH template was filtered
and stored in a wet filter cake form, with the template being left
in place. After storing for 73 days, the filter cake was dried,
then the template was then removed and methanol conversion calculated
according to the procedure of Example 3. The data are shown in FIG.
3.
EXAMPLE 6
A sample of SAPO-34 containing a DPA/TEAOH template was dried,
with the template being left in place. The sample was stored for
132 days under ambient conditions. The template was then removed
and methanol conversion calculated according to the procedure of
Example 3. The data are shown in FIG. 3.
EXAMPLE 7
A detemplated SAPO-34 was aged under ambient conditions for 18
months. Methanol adsorption capacity was determined according to
the procedure of Example 2. From the initial methanol adsorption
capacity and the methanol adsorption capacity at feed contact, the
methanol uptake index was calculated as 0.12.
Methanol conversion of a sample of the 18 months aged SAPO-34 was
evaluated according to the procedure of Example 3. At a TOS of 2
minutes, the methanol conversion was 26.97 wt. %. At a TOS of 5
minutes, the methanol conversion had significantly dropped to 0.63
wt. %.
The data indicate that a silicoaluminophosphate molecular sieve
having a methanol uptake index of 0.12 is catalytically active for
a brief period of time. However, the catalytic activity of such
a molecular sieve drops off very quickly, indicating that a molecular
sieve having such a low methanol uptake index is less desirable
for reaction systems, as it will tend to increase the frequency
at which the molecular sieve will need to be regenerated.
Having now fully described this invention, it will be appreciated
by those skilled in the art that the invention can be performed
within a wide range of parameters within what is claimed, without
departing from the spirit and scope of the invention. |