Molecular sieve abstract
In a process for the oligomerization of light olefins (e.g. propylene)
to more valuable higher olefins (e.g. hexene), the use of a molecular
sieve (e.g. SAPO-11) that has been acid washed has shown to provide
superior selectivity to, and yield of, the desired product. These
benefits are attributed to a reduction in non-selective acid sites
as well as an increase in phosphorous content (in the case of SAPO
molecular sieves) at the molecular sieve crystallite surface, a
probable result of removing amorphous silicon aluminum phosphate
acid, silicophosphorous acids, and/or phosphoric acids. Furthermore,
this removal is likely accompanied by an increased accessibility
of olefinic feed components to the so-called "selective"
acid sites within the sieve pores. Post-synthesis acid washing of
the molecular sieve has not only demonstrated a narrower carbon
number distribution of olefin oligomers, but also improved the linearity
of the oligomerized olefinic product slate in general. Such increases
in linearity generally correspond to overall higher product values,
when downstream applications such as plasticizer intermediates are
considered.
Molecular sieve claims
What is claimed is:
1. A process for oligomerizing an olefinic feed, the process comprising:
a) providing a catalyst comprising a crystalline silicon aluminophosphate
molecular sieve characterized by a 3-dimensional framework structure
type of SAPO-11 or SAPO-41 and having a chemical composition on
an anhydrous basis expressed by an empirical formula of:
where "x" is the mole fraction of Si and has a value
of at least 0.005; "y" is the mole fraction of Al and
has a value of at least 0.01; "z" is the mole fraction
of P and has a value from greater than 0 to about 0.6; the molecular
sieve having been washed at washing conditions with a strong acid
having a pK.sub.a of less than 2.0 and;
b) contacting the light olefin feed with the catalyst at oligomerization
conditions to yield a higher olefin product.
2. The process of claim 1 where the molecular sieve has a phosphorous
surface/bulk ratio of greater than 1.3.
3. By The process of claim 2 where the silicon aluminophosphate
has the crystal structure of SAPO-11.
4. The process of claim 1 where the catalyst further comprises
a binder selected from the group consisting of alumina, silica,
aluminum phosphate, silica alumina, zirconia, titania, and mixtures
thereof.
5. The process of claim 1 where the light olefin feed comprises
propylene.
6. The process of claim 5 where the higher olefin product comprises
hexene present in an amount of at least 30% by weight, relative
to the light olefin feed weight.
7. The process of claim 5 where the higher olefin product comprises
a C.sub.7 olefin product containing linear heptene and methyl hexenes
present in an amount of at least 40% by weight, relative to the
C.sub.7 olefin product weight.
8. The process of claim 1 where the oligomerization conditions
comprise a temperature from about 40.degree. C. to about 250.degree.
C., an absolute pressure from about 0.5 to about 100 atmospheres,
and a feed WHSV from about 0.1 to about 20 hr.sup.-1.
9. The process of claim 1 where the acid is an aqueous solution
selected from the group consisting of aqueous HNO.sub.3 HCl, H.sub.2
SO.sub.4 HBr, HI, and mixtures thereof.
10. The process of claim 1 where the aqueous solution has a pH
from about 1 to about 4.
11. The process of claim 1 where the washing conditions comprise
a temperature from about 0.degree. C. to about 100.degree. C. and
a residence time from about 0.5 to about 48 hours.
12. A process for oligomerizing a light olefin feed to a higher
olefin product, the process comprising contacting the feed at oligomerization
conditions with a catalyst having been treated with a strong acid
having a pK.sub.a of less than about 5.0 the catalyst comprising
a crystalline silicon aluminophosphate molecular sieve characterized
by a 3-dimensional framework structure type of SAPO-11 or SAPO-41
and having a chemical composition on an anhydrous basis expressed
by an empirical formula of:
where "x" is the mole fraction of Si and has a value
of at least 0.005; "y" is the mole fraction of Al and
has a value of at least 0.01; "z" is the mole fraction
of P and has a value from greater than 0 to about 0.6.
Molecular sieve description
FIELD OF THE INVENTION
The present invention relates to a process for the oligomerization
of olefins (e.g. propylene) to higher value products (e.g. hexene)
using a molecular sieve catalyst. In such a process, acid washing
of the molecular sieve prior to use results in increased selectivity
of the oligomerization reaction to the desired product.
BACKGROUND OF THE INVENTION
Processes for the oligomerization of light olefins (e.g. ethylene,
propylene, and butylene) to produce higher carbon number olefin
products (e.g. C.sub.6.sup.+ olefins) are well known. Oligomerization
processes have been employed to produce high quality motor fuel
components as well as petrochemicals from ethylene, propylene, and
butylene. These oligomerization processes are also referred to as
catalytic condensation and polymerization, with the resulting motor
fuel often referred to as polymer gasoline. In the refining area,
methods have been continually sought to improve the octane number
of the gasoline boiling range oligomerization products. This octane
enhancement is generally realized through the improvement of the
oligomerization reaction selectivity to enhance the representation
of high octane blending components (e.g. branched olefins) in the
product slate. The ability of the process to better target specific
carbon number species is also a primary consideration when highly
purified chemical grade products are desired. In any case, the enrichment
of product slate to the targeted species, in addition to providing
a higher quality and quantity of useable products, also benefits
catalyst life. This is due to the reduction in non-selective heavy
oligomers that condense into coke which ultimately covers the catalyst.
Known catalysts for effecting the oligomerization reaction include
heterogeneous catalysts such as solid acids and homogeneous catalysts,
in particular boron trifluoride as described, for example, in U.S.
Patent No. 3981941. Other catalysts fall within the description
of mild protonic acids, generally having a Hammett acidity function
of less than -5.0. Particularly preferred among these are solid
phosphoric acid (SPA) catalysts having as a principal ingredient
an acid of phosphorous such as ortho, pyro, or tetraphosphoric acid.
Details of SPA catalysts are provided in the prior art, for example
in U.S. Pat. No. 5895830.
The use of zeolites, particularly those of the medium pore consideration,
as oligomerization catalysts is also described, along with various
catalyst treatment methods designed to improve performance. U.S.
Pat. No. 4547613 discloses the use of a ZSM-5 type catalyst that
has been conditioned by treatment with a light hydrocarbon gas at
low pressure and elevated temperature. U.S. Pat. No. 4520221 describes
a process for providing high yields of lubricating oils from the
conversion of light olefins such as propylene using ZSM-5 catalyst.
The results are achieved through removing the surface acidity of
the catalyst by treatment with a bulky amine. U.S. Pat. No. 4642404
is directed to the conversion of C.sub.2.sup.+ olefins to C.sub.5.sup.+
olefins over a catalyst of a bound, high silica zeolite having a
constraint index of 1-12. The catalyst is modified, while it is
at least partially in the hydrogen form, by steaming to improve
activity.
Finally, U.S. Pat. No. 5284989 discusses the use of a constrained
intermediate pore siliceous acidic zeolite (e.g. ZSM-22 -23 or
-35) having Bronsted acid activity and wherein the zeolite surface
is rendered substantially inactive for acidic reactions. The zeolite
can be inactivated by contact with dicarboxylic acid (e.g. oxalic
acid). The reference states that treatments with strong acids such
as HCl, HNO.sub.3 and H.sub.2 SO.sub.4 "are limited, in many
cases, . . . by the onset of crystal degradation and loss of sorption
capacity."
In contrast to the prior art, and specifically the teachings of
the '989 patent, the present invention is based on the realization
that treatment of solid oligomerization catalysts with a strong
acid provides significant benefits in terms of product selectivity
and yield. The unexpected improvements in process performance associated
with the present invention are believed to directly result from
changes in catalyst surface properties stemming from the acid treatment.
SUMMARY OF THE INVENTION
In a broad embodiment the present invention is a process for oligomerizing
an olefinic feed, the process, comprising:
a) providing a catalyst comprising a crystalline silicoaluminate
or metalloaluminophosphate molecular sieve characterized by a 3-dimensional
framework structure and having a chemical composition on an anhydrous
basis expressed by an empirical formula of:
where EL is an element selected from the group consisting of silicon,
magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures
thereof; "x" is the mole fraction of EL and has a value
of at least 0.005; "y" is the mole fraction of Al and
has a value of at least 0.01; "z" is the mole fraction
of P and has a value from 0 to about 0.6; EL is silicon when z=0;
and x+y+z=1 the molecular sieve having been washed at washing conditions
with a strong acid having a pK.sub.a of less than about 2.0 and;
b) contacting the light olefin feed with the catalyst at oligomerization
conditions to yield a higher olefin product.
In a more specific embodiment the present invention is as described
above, where the light olefin feed comprises propylene, the catalyst
is a silicon aluminophosphate having the crystal structure of SAPO-11
and the higher olefin product comprises hexene present in an amount
of at least 30% by weight, relative to the light olefin feed weight.
DETAILED DESCRIPTION OF THE INVENTION
The feed for the process of the present invention generally comprises
light olefin components, typically C.sub.2 to C.sub.5 olefins, although
olefins with higher carbon numbers may also be used. Sources of
the olefin feeds normally include: light gas streams recovered from
the gas separation section of a refinery fluid catalytic cracking
(FCC) process, C.sub.4 streams from steam cracking and coker off
gas, or the effluents from light paraffin (e.g. LPG) dehydrogenation
zones. In most operations, the combined C.sub.3 and C.sub.4 olefins
will account for at least 50% by weight of the total feed olefins.
Certain situations may also warrant the oligomerization of feeds
having, with respect to their olefin content, exclusively ethylene,
propylene, or butylene (either pure isomers or mixed normal and
branched isomers) to obtain relatively high yields of a given carbon
number product. If a feed comprising predominantly propylene (or
predominantly propylene with an inert diluent such as propane) is
processed, the dimer (hexene) and trimer (nonene) are generally
the desired products and their yields can be maximized using the
acid-washed molecular sieve catalyst of the present invention, combined
with optimizing the reaction conditions. Likewise, a butylene feed
may be incorporated to target primarily an octene product or a product
containing octenes as well as dodecenes.
The acid-washed molecular sieve provides exceptional results for
both liquid- and gas-phase operation, although maintaining the feed
in the liquid phase is generally preferred. Furthermore, it is certainly
within the scope of the present invention to combine the feed with
a number of diluents known in the art, such as heavy paraffins.
The use of these additives, as described in U.S. Pat. Nos. 6080903;
6072093; 5990367; and 5895830 provides a number of benefits
including catalyst performance enhancement and promotion of liquid-phase
conditions in the reaction zone.
The catalyst comprises a molecular sieve, which refers to a broad
class of crystalline materials understood in the art to include
both aluminosilicates (i.e. zeolites) and metalloaluminophosphates
(e.g. SAPOs). While a zeolite is a crystalline aluminosilicate,
a metalloaluminophosphate contains phosphorous cations (P.sup.+5)
in addition to aluminum (Al.sup.+3) and silicon (Si.sup.+4) situated
within tetrahedral sites of an extensive three-dimensional network
of oxygen ions. Types of materials classified as molecular sieves
are explained in detail in Molecular Sieves, Principles of Synthesis
and Identification by R. Szostak (Van Nostrand Reinhold, 1989) at
pages 2-4. These include silicas, metalloaluminates, aluminophosphates,
and others.
In focusing on the zeolite and metalloaluminophosphate molecular
sieves found to be useful in the oligomerization process of the
present invention, the range of suitable materials is encompassed
by those having a three-dimensional microporous framework structure
having the empirical formula:
where EL is an element selected from the group consisting of silicon,
magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures
thereof, "x" is the mole fraction of EL and has a value
of at least 0.005 "y" is the mole fraction of Al and
has a value of at least 0.01 "z" is the mole fraction
of P and has a value from 0 to about 0.6. In all cases, x+y+z=1.
In the specific case where z=0 EL is necessarily silicon and the
molecular sieve is a zeolite material, or a crystalline aluminosilicate.
When EL is a mixture of elements, "x" represents the total
amount of the element mixture present. The molecular sieve therefore
contains tetrahedral units of AlO.sub.2 and ELO.sub.2 (as well as
units of PO.sub.2 when z>0) in its framework structure. Preferred
elements (EL) are silicon, magnesium and cobalt with silicon being
especially preferred.
The preparation of these various "ELAPO" molecular sieve
materials as defined above is well known in the art and may be found
in U.S. Pat. Nos.: 4554143 (FeAPO); 4440871 (SAPO); 4853197
(MAPO, MnAPO, ZnAPO, CoAPO); 4793984 (CAPO), 4752651 and 4310440
all of which are incorporated by reference. Generally, the ELAPO
molecular sieves are synthesized by hydrothermal crystallization
from a reaction mixture containing reactive sources of EL, aluminum,
phosphorus and a templating agent. Reactive sources of EL are the
salts of the EL element such as the chloride and nitrate salts.
When EL is silicon a preferred source is fumed, colloidal or precipitated
silica. Preferred reactive sources of aluminum and phosphorus are
pseudo-boehmite alumina and phosphoric acid. Preferred templating
agents are amines and quaternary ammonium compounds. An especially
preferred templating agent is tetraethylammonium hydroxide (TEAOH).
After the initial synthesis, the ELAPOs will usually contain some
of the organic templating agent in its pores. In order for the ELAPOs
to be active catalysts, the templating agent in the pores must be
removed by heating the ELAPO powder in an oxygen-containing atmosphere
at a temperature of about 200.degree. to about 700.degree. C. until
the template is removed, usually a few hours.
A preferred embodiment of the invention is one in which the element
(EL) content of the ELAPO molecular sieve varies from about 0.005
to about 0.05 mole fraction. If EL is more than one element, then
the total concentration of all the elements is between about 0.005
and 0.05 mole fraction. An especially preferred embodiment is one
in which EL is silicon (usually referred to as SAPO). The SAPOs
that can be used in the present invention are any of those described
in U.S. Pat. Nos. 4440871; 5126308; and 5191141. SAPO catalysts
that are suitable for the present invention include SAPO-11 and
SAPO-41. Of the specific crystallographic structures described in
the '871 patent, the SAPO-11 i.e., structure type 11 is preferred.
The SAPO-11 structure is characterized in that it has elliptically
shaped pores with an average diameter of about 5.1 .ANG.. In this
case, the average pore opening is defined as the average length
of the major and minor axes across the elliptically shaped pore
formed by a 10-membered ring structure. In terms of the kinetic
diameters of molecules, SAPO-11 adsorbs cyclohexane (kinetic diameter
of 6.0 .ANG.) but does not adsorb neopentane (kinetic diameter of
6.2 .ANG.) to a significant extent. Another SAPO, SAPO-41 as exemplified
in Example 54 of the '871 patent, is also preferred. The SAPO-41
structure is also characterized in that it adsorbs cyclohexane but
not neopentane.
An important feature that characterizes the preferred SAPO-11 and
SAPO-41 materials described above is their "intermediate"
pore diameters. In determining the appropriate pore size for a given
application, the Pore Size Index, or value of the product of the
two major axes (in angstroms) of the crystallite pores is a useful
measure. Values of the Pore Size Index of various crystalline materials
are provided in the "Atlas of Zeolite Structure Types"
by W. M. Meier and D. H. Olson, Butterworths, publisher, Third Revised
Edition, 1992. For purposes of the present invention, the molecular
sieve will preferably have a Pore Size Index in the Range from about
20 to about 40. In general, this range of crystallite pore size
is desirable for the oligomerization of light olefins, because the
proper selectivity is provided for reacting specifically shaped
and sized molecules, as dictated by the configuration of the microporous
channels through which they must diffuse. In the case of the process
of the present invention, the above range of pore sizes has been
determined to provide shape-selectivity to the highly valued C.sub.6
-C.sub.9 olefinic products by allowing passage of both linear feed
and product molecules, while rejecting highly branched and ring
structures and molecules of high molecular weight (e.g. C.sub.12.sup.+
olefins).
The proper pore size is most often found in zeolites and SAPOs
having micropores defined by 10-membered ring structures, as opposed
to those molecular sieves with frameworks characterized by either
8- or 12-membered rings. Appropriately, therefore, the aforementioned
SAPO-11 and SAPO-41 are both examples of 10-membered ring structures.
The 10-membered zeolites or aluminosilicates preferable for use
in the present invention include ZSM-5 defined in U.S. Pat. No.
3702886 incorporated by reference; ZSM-11 defined in U.S. Pat.
No. 3709979 incorporated by reference; ZSM-22 defined in European
Pat. No. 102716 incorporated by reference, ZSM-23 defined in U.S.
Pat. No. 4076842 incorporated by reference; ZSM-35 defined in
U.S. Pat. No. 4016245 incorporated by reference; some of the
silicalite materials, defined in U.S. Pat. No. 4061724 incorporated
by reference; and ferrierite, defined in U.S. Pat. No. 3933974
incorporated by reference. A further method of classifying the pore
size openings of the zeolites and SAPO materials preferred in the
present invention is the "Constraint Index" (CI). This
term is defined specifically for 10-membered ring structures by
J. Weitkamp (Host/Guest Chemistry and Catalysis in Zeolites, proceedings
at the 9.sup.th International Zeolite Conference, Montreal 1992
edited by R. von Ballmoos et al., 1993 by Butterworth-Heinemann)
according to the relative uptake of the n-decane isomerization products,
2-methylnonane and 5-methylnonane. For purposes of the present invention,
the molecular sieve catalyst will have a CI preferably in the range
from about 5 to about 14.
An essential feature of the oligomerization process of the present
invention is associated with the realization that the molecular
sieve catalyst exhibits substantially improved performance after
it has been acid washed with a strong acid. By strong acid is meant
one having a pK.sub.a value of less than about 2 and preferably
less than about 0 where pK.sub.a is understood in the art to be
defined as -log K.sub.a, and K.sub.a is the equilibrium dissociation
constant of the acid in water. Acids that may be used therefore
include, but are not limited to, aqueous solutions of HNO.sub.3
HCl, HBr, HI, H.sub.2 SO.sub.4 and mixtures thereof. A preferred
acid is HNO.sub.3. The acid concentration used in the washing procedure,
as outlined in detail hereinafter, depends on the stability of the
particular molecular sieve employed. In general, degradation of
the crystallite structure is substantially avoided when the acid
solution has a pH value from about 1 to about 4. A simple test to
determine whether a specific acid molecular sieve combination is
suitable comprises subjecting a sample of the molecular sieve to
the acid solution under acid washing conditions for a period of
24 hours. Any substantial dissolution of the molecular sieve framework
appears as a cloudy precipitate in the liquid.
The acid washing procedure comprises contacting the molecular sieve
crystallites with any of the aforementioned acids in either a batch
or continuous mode. Preferably, the molecular sieve is maintained
in a bed over which aqueous acid solution is circulated for a specified
residence time. In this continuous washing process, it is usually
desirable to add sufficient concentrated acid (e.g. a 20 wt-% HNO.sub.3
stock solution) to maintain a pH within the preferred range from
about 1 to about 4. An equivalent amount of solution (based on the
water content of the added acid) may then be withdrawn from the
system. Of course, recirculation of the acid wash solution is not
essential, and it is also possible to simply pass the acid solution,
once through, over a bed of the molecular sieve. This type of operation,
however, tends to result in either discarding an unnecessary amount
of acid at high flow rates or achieving sub-optimal liquid/solid
contact at low flow rates. At the other extreme, batch-wise acid
washing is also possible, but some make-up acid addition may still
be required to maintain the proper pH.
Regardless of the use of either batch or continuous washing, the
residence, or contact, time between the acid and the molecular sieve,
as calculated according to methods well known in the art, is preferably
from about 0.5 to about 48 hours, and more preferably from about
1 to about 24 hours. Pressure is not a critical variable for the
acid washing, and ambient pressure is generally chosen for convenience.
The temperature used is preferably from about 0.degree. C. to about
100.degree. C., where higher temperatures generally expedite the
acid washing effects at the expense of increased corrosivity of
the acid to the molecular sieve structure. While the acid washing
is generally employed after formation of the molecular sieve crystallites,
it is also possible to achieve the desired catalytic performance
benefits even when acid washing is saved until after an optional
binding step, where the molecular sieve is bound into larger particles
suitable for commercial applications. In this case, the stability
of the binder in the acid solution must be considered.
After completion of the acid wash, the molecular sieve is then
typically filtered to separate any fine particles dislodged in the
washing step and thereafter flushed with water until the effluent
pH is at least 5. The resulting acid-washed molecular sieve is then
dried with or without the application of heat. Multiple washings
may be employed to achieve the desired final catalyst properties.
The washing may also be preceded or followed by steaming that is
sometimes used to de-aluminate the molecular sieve.
In a light olefin oligomerization process, the performance benefit
of using a catalyst comprising the acid-washed molecular sieve is
quantified in terms of increased selectivity in the reaction product
slate to desired species. Surprisingly, this selectivity benefit
does not come at the expense of reduced olefin conversion, as is
normally the case for conventional non acid-washed molecular sieves.
Rather, acid-washed materials can be used to achieve substantially
improved selectivity to a desired species (e.g. hexene), where this
selectivity enhancement corresponds to an increase, rather than
a decrease, in conversion (e.g. an increase from 85% to 90% conversion).
Thus, the overall yield of desired product is markedly improved
using the simple acid washing technique of the present invention.
The use of an acid-washed catalyst comprising SAPO-11 for example,
provides a product slate, compared to that obtained using non acid-washed
material, that is greatly enriched in propylene dimer and trimer,
but depleted of the less-valuable tetramer. The dimer (hexene) and
trimer (nonene) are known to be valuable intermediates for co-monomer
and for platicizer production, respectively. Furthermore, the selectivity
of the acid-washed molecular sieve catalyst can favor propylene
dimer production to an extent unrealized with conventional oligomerization
catalysts, including those comprising zeolites and solid phosphoric
acid (SPA) catalysts. In a preferred embodiment, when the feed stock
excluding any diluents comprises substantially pure C.sub.3 olefins,
the hexene present in the product stream (i.e. hexene yield) will
account for at least 20% by weight, and more preferably at least
30% by weight, of the feed olefins processed. The acid-washed molecular
sieve catalyst likewise shows a significant benefit in terms of
activity and product selectivity for the oligomerization of C.sub.4
olefins to their corresponding dimers and trimers.
An additional benefit of the acid treatment or wash, which will
become more apparent from the following explanation regarding the
improvement in catalyst shape selectivity, is a smaller degree of
branching (i.e. a greater degree of linearity) achieved for the
entire product slate of oligomers produced. For example, the oligomerization
of propylene is accompanied to some extent, through cracking and
rearrangement, by the co-production of heptene. Again using the
specific comparative data for propylene oligomerization over art
acid-washed and non-acid washed SAPO-11 the linear and mono-branched
C.sub.7 olefin byproduct as a fraction of the total heptene produced
is significantly greater for the acid-washed material. Preferably,
therefore, using the catalyst of the present invention and a feed
comprising propylene, the combined contribution of linear heptene
and methyl hexenes (i.e. 2-methyl-hexene and 3-methyl-hexene) to
the total C.sub.7 olefin byproduct is at least 40%, and more preferably
at least 50% by weight. As is known in the art, the value of the
C.sub.7 olefin byproduct for its main downstream commercial uses
as either a plasticizer or a precursor for linear alkylbenzene (LAB)
production is determined primarily from its degree of branching.
Therefore, the reduction in more highly branched C.sub.7 species
(i.e. dimethyl pentenes) resulting from the acid wash is a further
important advantage associated with the present invention. Correspondingly,
major oligomerization products, for example hexene and nonene, are
similarly less branched and consequently more valuable intermediates.
Without limiting the scope of the invention according to any particular
theory, a mechanism by which the previously unrecognized and significant
benefits of acid washed molecular sieves in oligomerization processes
is proposed. Namely, it is thought that acid washing influences
access of the feed components to the molecular sieve by removing
debris remaining from the synthesis thereof. Possibly, this debris
itself, similar to the molecular sieve, has acidic properties capable
of catalyzing oligomerization reactions. Thus, the removal of this
material can provide the beneficial result of removing non-selective
acid sites, unlike those within the pores of the crystalline molecular
sieve, which are characterized as being shape-selective. Removal
of such debris also generally improves traffic entering and exiting
crystallite pores, allowing reaction products to escape prior to
undergoing subsequent degradation.
Overall, therefore, the post-synthesis acid treatment of the molecular
sieve used in the present invention is likely to remove amorphous
silicon aluminum phosphate acid, silicophosporous acids, and/or
phosphoric acids (all of which are undesired acid catalysts), while
in the process increase the accessibility of the crystallite pores
of the molecular sieve to olefin feeds and products. Thus, a greater
amount of shape-selective acid sites residing with the molecular
sieve pores is available for reaction. This explanation therefore
accounts for not only the observed increase in product selectivity,
but also in catalyst activity, resulting from the acid treatment.
Additionally, the acid washing can even remove non-selective acid
sites from the external surface of the molecular sieve crystallites
themselves. As evidence of this, x-ray photoelectron spectroscopy
X(PS) measurements of both non acid-washed and acid-washed SAPO-11
molecular sieve crystallites show a decrease, as a result of acid
washing, in surface concentrations of silicon and aluminum with
a corresponding increase in phosphorous.
In considering changes, resulting from the acid wash, in acid properties
of SAPO molecular sieves in particular, it should be noted that
pure aluminum phosphate is a non-acidic material. However, the replacement
of phosphorous (+5 oxidation state) with silicon (+4 oxidation state),
to form a SAPO material necessarily involves the addition of acidic
protons to balance the charge offset. Thus, the observed removal
of surface silicon due to acid washing is consistent with the hypothesis
that external acidity is reduced, leading to even greater shape
selectivity. If a SAPO molecular sieve is employed, therefore, a
significant property resulting from acid washing is the difference
in composition between the outer surface and bulk framework structure
of the material. This difference may be conveniently characterized
according to the relative contributions of external and internal
phosphorous, where the former can be determined experimentally (e.g.
by XPS) and that latter may be either measured or deduced from the
molecular sieve empirical formula. For un-washed molecular sieves,
the surface and bulk phosphorous content (measured as a mole fraction)
should be identical, so that the phosphorous "surface/bulk
ratio" is 1. In contrast, preferred acid-washed SAPO molecular
sieves of the present invention preferably have a phosphorous surface/bulk
ratio of greater than about 1.3 and more preferably greater than
about 1.5.
The acid-washed molecular sieve is preferably incorporated into
solid catalyst particles where the molecular sieve is present in
an amount effective to promote the desired conversion of light olefins
to heavier olefin products. Thus, these solid particles comprise
a catalytically effective amount of the molecular sieve and either
an inorganic oxide binder, a filler, or both to provide a desired
level of mechanical strength or attrition resistance of the bound
catalyst.
The total amount of binder and filler material preferably contributes
from about 20% to about 80% of the total catalyst weight. In addition
to enhancing the catalyst strength properties, the binder and/or
filler materials allow the molecular sieve crystallite powder to
be bound into larger particle sizes suitable for commercial catalytic
processes. The molecular sieve/binder composite may be formed into
a wide variety of shapes including, for example, extrudates, spheres,
pills, and the like. The binder and/or filler material is often,
to some extent, porous in nature and may or may not be effective
to promote the desired oligomerization of light olefins through,
for example, the provision of acid sites. The binder and filler
materials may also promote conversion of the feed stream and often
provide reduced selectivity to the desired product or products relative
to the catalyst.
Examples of preferred binder materials include, but are not limited
to, alumina, silica, aluminum phosphate, silica-alumina, zirconia,
titania, and mixtures thereof. Filler materials can include, for
example, synthetic and naturally occurring substances such as clays,
metal oxides, silicas, aluminas, silica-aluminas, and mixtures thereof.
In referring to the types of binders and fillers that may be used,
it should be noted that the term silica-alumina does not mean a
physical mixture of silica and alumina but means an acidic and amorphous
material that has been cogelled or coprecipitated. This term is
well known in the art and is described, for example, in U.S. Pat.
Nos. 3909450; 3274124; and 4988659. In this respect, it is
possible to form other cogelled or coprecipitated amorphous materials
that will also be effective as either binder or filler materials.
These include silica-magnesias, silica-zirconias, silica-thorias,
silica-berylias, silica-titanias, silica-alumina-thorias, silica-alumina-zirconias,
aluminophosphates, mixtures of these, and the like. Preferably,
the filler is a clay, since clays are known to be essentially inert
under a wide range of reaction conditions. Suitable clays include
commercially available products such as kaolin, kaolinite, montmorillonite,
saponite, and bentonite. These clays can be used as mined in their
natural state, or they may also be employed in highly active forms,
typically activated by an acid treatment procedure. Commercial suppliers
of these clays include Thiele Kaolin Company (Sandersville, Ga.),
American Colloidal Co. (Arlington Heights, Ill.,), GSA Resources,
Inc. (Tucson, Ariz.), Albion Kaolin Co. (Hephzibah, Ga.), and others.
In preparing a bound catalyst of the present invention, a slurry
of the acid-washed crystalline molecular sieve powder, the inorganic
oxide binder, and the filler (if used) is formed. The slurry will
contain an appropriate sol, or carrier material, of the inorganic
oxide binder used for suspending the molecular sieve. In the case
of incorporating alumina, silica, magnesia, zirconia, or titania
binders into the bound catalyst composition of the present invention,
it is appropriate to use a hydrosol. For example, any of the transitional
aluminas can be mixed with water and an acid to give an aluminum
sol. Acids for this application may include inorganic acids such
as nitric, hydrochloric, and sulfuric, or organic acids, especially
carboxylic acids such as formic, acetic, propionic, and the like.
Alternatively, an aluminum sol can be made by for example, dissolving
aluminum metal in hydrochloric acid and then mixing the aluminum
sol with the alumina powder. When an alumina binder is desired,
it is also possible to use a solution of boehmite, which may be
available from commercial sources (e.g. Versal.TM., available from
UOP, LLC, Des Plaines, Ill.) or aluminum nitrate in place of the
aluminum sol.
Types of silica sols used to form a bound catalyst for use in the
oligomerization process are commercially available as aquasols or
organosols (e.g. Cab-O-Sil.TM., available from Cabot Corp., Boston,
Mass. or HiSil.TM., available from PPG Industries, Inc., Pittsburgh,
Pa.) containing dispersed colloidal silica particles. Sodium silicate
can also be used as a silica sol and combined with an acidic aluminum
sol to ultimately yield a silica-alumina binder in the final catalyst.
Otherwise, a silica gel, such as commercially available Ludox.TM.
(Aldrich Chemical Co., Milwaukee, Wis.) may also be used to provide
a silica binder in the molecular sieve catalyst. Silicic acid is
another possible source of silica. If a magnesia binder is desired,
the starting slurry will contain hydrolyzed magnesium alkoxide.
When a zirconia binder is used for the catalyst preparation, the
preferred starting acidic sol is an aqueous zirconium acetate solution,
which is preferably combined with a urea gelling agent. When a titania
binder is used, the acidic sol is preferably a solution of titanyl
oxychloride, which is also preferably combined with a urea gelling
agent. Acidic colloidal suspensions of various inorganic oxides
are also available from commercial suppliers such as Nano Technologies,
Inc. (Ashland, Mass.). The amount of sol added to the slurry is
based on the desired amount of binder in the finished catalyst.
Those skilled in the art will readily appreciate the relationship
between the molecular sieve:sol weight ratio of the slurry and the
resulting molecular sieve:binder ratio in the catalyst.
As discussed, the acid-washed molecular sieve-containing catalyst
may also incorporate a filler in addition to an inorganic oxide
binder. The filler may itself be an inorganic oxide (e.g. alumina)
that is added into the synthesis slurry in a powdered form rather
than a sol. Preferably, the filler is a clay selected from the group
of suitable clays provided previously. In some cases, the addition
of a clay filler may improve the overall strength of the bound catalyst,
where this improvement is measured by the amount of finished catalyst
material lost during a standard attrition test (i.e. attrition loss).
Loss of the catalyst by attrition can be measured by fluidizing
the catalyst in air for a given period of time, collecting and weighing
the fines generated, and then calculating an attrition loss as an
average percent of the initial catalyst weight per hour.
It is also within the scope of the present invention to include
other components in the slurry that may have an impact on the final
catalyst properties. For example, International Publication WO 99/21653
discloses the use of an external phosphorous source, which may have
a favorable impact on the catalyst and process of the present invention.
The teachings of this reference relating to potential sources of
phosphorous and relative amounts desired in the catalyst composition
are hereby incorporated by reference.
Depending on the average particle size of agglomerated molecular
sieve crystallites present in the slurry, it may be desired to mill
the slurry in order to break these agglomerates apart, thereby reducing
the agglomerate particle size and/or giving a narrower particle
size distribution. Milling can be done by means known in the art
such as ball milling for times from about 30 minutes to about 5
hours and preferably from about 1.5 to about 3 hours. It is believed
that using a slurry with a particle size distribution that has been
adjusted in this manner improves the structural characteristics
of the bound molecular sieve catalyst. Care must be taken, however,
not to mill the slurry so extensively as to destroy the crystallite
structure of the molecular sieve.
It should also be noted that, in addition to the molecular sieve
powder, sol of the inorganic oxide binder, and filler (if used),
the slurry will often contain water. The amount of water is often
adjusted after any milling operation in order to obtain a viscosity
of the milled slurry in the range from about 30 to about 600 centipoise.
Prior to drying, it is generally preferred that the slurry components
are well mixed to ensure a uniform slurry composition. A period
of high shear mixing of about 15 minutes, for example, is effective
in most cases for obtaining the proper uniformity. It is important
to initiate the subsequent drying step prior to the onset of gelling
of the slurry, usually about 1 hour after mixing.
The well-mixed slurry, either with or without prior milling, is
then dried at a temperature from about 50.degree. C. to about 300.degree.
C. for a time from about 1 to about 24 hours to form dried, shaped
particles. These particles may or may not be subsequently milled
or otherwise reduced in size at this point to provide catalyst physical
properties that in turn lead to the ultimately desired pressure
drop characteristics, fluidization velocity, diffusion resistance,
and other properties. The dried, shaped particles have an average
effective diameter generally from about 1 to about 10 millimeters,
although considerably smaller catalyst particles could be used if
a fluidized bed process is employed. By effective diameter is meant,
for non-spherical shapes, the diameter that the shaped particle
would have if it were molded into a sphere. In a preferred embodiment,
the dried, shaped particles are substantially cylindrical in shape
(i.e. extrudates).
Finally, the dried, shaped catalyst particles are calcined at a
temperature from about 400.degree. C. to about 900.degree. C. in
an air environment for a time from about 1 to about 10 hours to
effectively set the inorganic oxide binder. The calcination step
also removes any remaining template material that may be present
within the crystalline molecular sieve. In some cases, the catalyst
may be activated in a modified calcination step wherein the organic
template is first decomposed in a flow of pure nitrogen. The oxygen
concentration is then gradually increased to combust any residual
hydrocarbons in the molecular sieve. It is also possible to combine
the drying and calcining operations into a single step.
The oligomerization of light olefins is effected by contacting
the light olefin feed with the acid-washed molecular sieve-containing
catalyst at oligomerization conditions, thereby forming the desired
heavier olefin products. As mentioned, the feed can be either in
the liquid or vapor phase with the liquid phase being preferred.
Contacting the light olefin with the acid-washed molecular sieve
catalyst can be done in a continuous mode or a batch mode with a
continuous mode being preferred. The amount of time that the light
olefin is in contact with the acid-washed molecular sieve catalyst
must be sufficient to oligomerize the feed to the proper extent
or average degree of polymerization. When the process is carried
out in a batch process, the contact time varies from about 0.001
to about hours and preferably from about 0.01 to about 1.0 hour.
The longer contact times are used at lower temperatures while shorter
times are used at higher temperatures, assuming all other process
variables are equal. Further, when the process is carried out in
a continuous mode, the weight hourly space velocity (WHSV) based
on the total feed (including any diluents) can vary from about 0.1
hr.sup.-1 to about 20 hr.sup.-1 and preferably from about 0.5 hr.sup.-1
to about 10 hr.sup.-1. As is understood in the art, the weight hourly
space velocity is the weight flow of the feed divided by the catalyst
weight. This term provides a measure of how many equivalent weights
of the catalyst inventory are processed every hour as methanol.
Generally, the process must be carried out at elevated temperatures
in order to form light olefins at a fast enough rate. Thus, the
process is preferably carried out at a temperature from about 40.degree.
C. to about 250.degree. C. The process may be carried out over a
wide range of pressure including autogenous pressure. Preferably,
therefore, the oligomerization conditions include an absolute pressure
from about 0.5 to about 100 atmospheres (about 7.4 to about 1500
psig).
Optionally, the light olefin feed may be diluted with an inert
or slightly reactive diluent in order to more efficiently oligomerize
the olefin while reducing the production of undesired side products.
Examples of the diluents which may be used are paraffinic hydrocarbons
such as propane, butane, pentane, hexane, heptane, octane, nonane,
decane, etc., or gaseous components such as helium, argon, nitrogen,
carbon monoxide, carbon dioxide, hydrogen, and steam. The amount
of diluent used can vary considerably and usually represents from
about 5 to about 90 weight percent of the feedstock and preferably
from about 25 to about 75 weight percent.
The actual configuration of the oligomerization reaction zone may
be any well-known catalyst reaction apparatus known in the art.
Thus, a single reaction zone or a number of zones arrange d in series
or parallel may be used. When multiple reaction zones are used,
one or more molecular sieve catalysts may be used in series to produce
the desired product mixture In addition to a fixed bed, a dynamic
bed system, e.g., fluidized or moving, may be used. Such a dynamic
system would facilitate any regeneration of the acid-washed molecular
sieve catalyst that may be required. If regeneration is required,
the catalyst can be continuously introduced as a moving bed to a
regeneration zone where it can be regenerated by means such as oxidation
in an oxygen-containing atmosphere to remove carbonaceous materials.
The following examples are presented in illustration of this invention
and are not intended as undue limitations on the generally broad
scope of the invention as set forth in the appended claims.
EXAMPLE 1
SAPO-11 molecular sieve was prepared according to the prior art
as taught in U.S. Pat. No. 4440871. A reaction mixture was prepared
by combining 85% orthophosphoric acid with distilled water, then
adding a precipitated pseudo-boehmite form of alumina as Versal.TM.
250 and mixing) until homogeneous. Silica as HiSil.TM. 250 and di-n-propylamine
were added and blended into provide a reaction mixture composition
in molar oxide ratios of:
An amount of SAPO-11 corresponding to 2 mass-% of the Al and P
oxides in the reaction mixture was added as seed and mixed until
homogeneous. The reaction mixture was heated with stirring for 12
hours at 195.degree. C. at autogenous pressure, then cooled to ambient
temperature to form a SAPO product slurry. SAPO crystallite product
was recovered and washed by centrifugation, dried, and calcined
at 650.degree. C. The recovered SAPO-11 was designated as Molecular
Sieve R. or the reference molecular sieve.
EXAMPLE 2
Acid-washed SAPO-11 molecular sieve of the invention was prepared
by acid washing the reference Molecular Sieve R from Example 1.
For each 100 grams of SAPO-11 400 cc of distilled water was added.
HNO.sub.3 solution in a concentration of 17.7 mass-% was added to
obtain a pH of about 2. The acidic solution was continually added
to the slurry of the SAPO powder and water to maintain pH of 2 for
about 18 hours. The acid-washed SAPO-11 was filtered, washed repeatedly
with distilled water until the wash water showed a neutral pH and
dried at 110.degree. C. The sample was designated Molecular Sieve
A. The surface composition (in mole-%)of Molecular Sieve A was measured
using x-ray photoelecton spectroscopy (XPS), for comparison with
that of Molecular Sieve R:
Molecular Sieve % Al % Si % P surface/bulk ratio R (SAPO-11) 66.8
9.7 23.5 A 57.1 5.8 37.1 1.58
The accompanying x-ray photoelectron spectroscopy measurements
(XPS) show a decrease, upon acid washing, in surface concentrations
of silicon and aluminum and an increase in surface concentration
of phosphorous. The change in phosphorous is the basis for determining
the "surface/bulk ratio" characteristic of acid washed
SAPO materials, as defined previously. As demonstrated by the above
example, this value is >1.5 due to the acid wash employed for
Molecular Sieve A. The changes in surface characteristics are, in
fact, consistent with the hypothesis that external acidity is removed.
EXAMPLE 3
The acid-washed Molecular Sieve A and the standard (un-washed)
SAPO-11 crystallites (Molecular Sieve R) were bound according to
the procedures described previously to provide extrudates, or cylindrically-shaped
catalyst particles having a diameter of about 1-2 mm. The binder
used was silica and contributed to 35% of the total finished catalyst
weight. The catalysts made using Molecular Sieve R and Molecular
Sieve A were designated, respectively, Catalyst R and Catalyst A.
Samples of Catalyst R and Catalyst A were tested in a pilot plant
to compare their performance in terms of light olefin oligomerization
activity and selectivity. The experiment was performed using oligomerization
conditions within the preferred commercial operating ranges as defined
previously; namely, a pressure of about 900 psig (61 atmospheres,
gauged and a maximum catalyst bed temperature of 218.degree. C.
were used in each case. The feed was a blend of 50/50 propylene/propane
by weight. Since the catalysts R and A exhibited different oligomerization
activity, the catalyst WHSV was adjusted to achieve 80% propylene
conversion for each test. The olefin product slate obtained using
the two catalyst samples, and measured at the reactor effluent using
gas chromatography (GC) is provided below:
Feed: 50/50 w/w Propylene/Propane
Maximum Catalyst Temperatures: 218.degree. C.
Pressure: 900 psig
Propylene Conversion: 80%
Catalyst R Catalyst A Product Selectivity, wt-% (Reference) (Acid
washed SAPO-11) hexene (dimer) 12 32 nonene (trimer) 37 48 dodecene
(tetramer) 23 5 others 28 15
These results show the improvement in selectivity to the valuable
hexene and nonene dimer and trimer products using the acid-washed
molecular sieve catalyst, compared to the reference. Industrially,
hexene is valued for its use in co-monomer applications, whereas
nonene is important for its use as a plasticizer intermediate. The
yield enhancement of both of these products, therefore, carries
very important commercial implications.
EXAMPLE 4
Catalyst A was tested under conditions identical to those used
in Example 3 except that the WHSV was reduced to provide an increase
in propylene conversion from 80% to 95% The GC results are summarized
below and compared to those obtained for the same catalyst in Example
3:
Feed: 50/50 w/w Propylene/Propane
Maximum Catalyst Temperature: 218.degree. C.
Pressure: 900 psig
Catalyst: Catalyst A (acid-washed, bound 65% SAPO-11)
80% Propylene 95% Propylene Product Selectivity, wt-% Conversion
Conversion hexene (dimer) 32 42 nonene (trimer) 48 35 dodecene (tetramer)
5 5 others 15 18
Thus, at the higher conversion the dimer selectivity actually increased
using the acid-washed SAPO-11. This observation is totally unexpected
and cannot be explained by historic oligomerization mechanisms that
do not include shape selective catalysis. Such mechanisms predict
the higher oligomers such as trimers and tetramers being directionally
favored at higher conversions, since oligomerization is a sequential
process of monomer additions to growing chains.
EXAMPLE 5
Catalysts R and A were tested again under the conditions described
in Example 3 except that, in both cases, the identical process
conditions were used in order to more directly examine the effect
of acid washing the molecular sieve component of the catalyst. The
oligomerization was performed again using a feed stock of 50/50
propylene/propane by weight, a maximum catalyst bed temperature
of 218.degree. C., and a reaction pressure of 90.degree. C. psig.
In this experiment, however, the same feed WHSV of 1.1 hr.sup.-1
was used for testing both the acid-washed (Catalyst R) and un-washed
(Catalyst A) molecular sieve catalysts. The G.C. results, showing
the C.sub.3 C.sub.6 and C.sub.7 olefin product distribution, as
well as the breakdown of the C.sub.7 olefin isomers, are given below:
Feed: 50/50 w/w Propylene/Propane
Maximum Catalyst Temperature: 218.degree. C.
Pressure: 900 psig
WHSV: 1.1 hr.sup.-1
Catalyst R Catalyst A (Reference) (Acid-washed SAPO-11) Propylene
Conversion, % 81 90 Product Selectivity, wt % hexene (dimer) 14.4
37.9 nonene (trimer) 40.3 38.9 dodecene (tetramer) 19.4 4.1 heptene
4.0 5.7 others 21.9 13.4 Heptene Isomers, wt % n--C.sub.7 1.2 3.1
Methyl C.sub.6 29.7 52.2 Dimethyl C.sub.5 68.9 44.3 Trimethyl C.sub.4
0.2 0.4
These results further show the benefits of acid washing the molecular
sieve used for the oligomerization catalyst. Under identical reaction
conditions, the acid washed molecular sieve catalyst demonstrated
not only a higher conversion, but also a shifting of the product
slate toward the lower molecular weight oligomers. Thus, a significantly
greater amount of hexene was produced, coupled with a large decrease
in byproduct make. Furthermore, considering the composition of the
C.sub.7 olefin product, which accounted for 4-6% by weight of the
converted propylene, the heptenes resulting from the use of the
acid washed SAPO-11 were considerably less branched than those produced
with the unwashed catalyst. The increase in linearity (i.e. the
reduction in branching) achieved using the acid washing procedure
of the present invention is evident from the higher makes of n-heptene
and methyl-hexene with a corresponding reduction in the amount of
poly-methyl olefins produced. It is known that a decrease in the
branching degree of this material benefits its ability to undergo
downstream hydroformylation for plasticizer production.
EXAMPLE 6
Catalyst A was tested in a plant with a feed containing a major
amount of isobutylene. Specifically, the feed was composed of 0.6
wt-% propylene, 1.9 wt-% propane, 0.6 wt-% butene-1 25.0 wt-% isobutylene,
33.1 wt-% isobutane, 1.3 wt-% normal butane, and 37.5 wt-% isooctane.
Very high isobutylene conversions were possible even at elevated
feed space velocities. Furthermore, exceptional isobutylene timer
selectivities were noted, significantly higher than those expected
from conventional acid catalysts such as phosphoric acid. For instance,
at a WHSV of 5.6 hr.sup.-1 a catalyst bed maximum temperature of
194.degree. C., and a plant pressure of 900 psig, the following
results were obtained:
isobutylene conversion, % 95.4 octene selectivity, wt % 62.2 dodecene
selectivity, wt % 36.0
Within the octene fraction, 99.0% was highly-branched trimethylpentenes,
with, therefore, very little dimethylhexanes. Within the trimethylpentene
fraction 84% of the isomers had the 224 trimethylpentene backbone,
12% of them had the 234 backbone, and the remainder had the 223
and 233 backbones.
At the even higher WHSV of 14.5 hr.sup.-1 with catalyst bed maximum
temperature of e215.degree. C. and a plant pressure of 900 psig,
the results were as follows:
isobutylene conversion, % 88.6 octene selectivity, wt % 71.2 dodecene
selectivity, wt % 27.6
Under these conditions, within the octene fraction of the product,
99.1% was highly-branched trimethylpentenes. Within the trimethylpentene
fraction 86% had the 224 trimethylpentane backbone, 11% had the
234 backbone, and the remainder had the 223 and 233 backbones.
At the high WHSV of 5.6 hr.sup.-1 but at a much lower plant pressure
of 300 psig and with a feed having double the isooctane as in the
previous cases (and correspondingly lower levels of all other components),
only a moderate reduction in isobutylene conversion was observed
(compared to the example above using a WHSV of 5.6 hr.sup.-1). The
results in this case were:
isobutylene conversion, % 87.4 octene selectivity, wt % 78.6 dodecene
selectivity, wt % 20.8
Within the octene fraction of the product, 98.9% was trimethylpentenes.
Within the trimethylpentene fraction, 87.6% of the isomers had the
224 trimethylpentane backbone, 8.9% had the 234 backbone and
the rest had the 223 and 234 backbones.
An extraordinary space velocity and an isooctane-free feed (feed
composition: 0.96 wt-% propylene, 3.03 wt-% propane, 0.96 wt-% 1-butene,
40.0 wt-% isobutene, 52.98 wt-% isobutane, and 2.07 wt-% normal
butane) did substantially reduce the isobutylene conversion. For
instance, at a WHSV of 14.7 hr.sup.-1 a 203.degree. C. catalyst
bed maximum temperature, and 300 psig plant pressure, the following
results were obtained:
isobutylene conversion, % 55.0 octene selectivity, wt % 66.0 dodecene
selectivity, wt % 32.1
Acid-washed SAPO-11 prepared according to the present invention,
was therefore not only higher active for the oligomerization of
isobutylene, but also very selective for the dimer and trimer oligomers.
Furthermore, the catalyst of this example, using an acid-washed
molecular sieve, performed well with a solvent such as isooctane,
which maintained the reactor contents in a substantially liquid
state. |