Molecular sieve abstract
Described are catalyst mixtures comprising a HAMS-1B crystalline
borosilicate molecular sieve incorporated into an inorganic matrix
component and a molybdenum on silica component. These mixtures when
used to isomerize unextracted xylene streams containing ethylbenzene
to mixtures rich in paraxylene demonstrate improved paraffins and
naphthenes conversion to light hydrocarbons and convert most of
the ethylbenzene by a hydrodeethylation mechanism to benzene and
ethane.
Molecular sieve claims
What is claimed is:
1. A vapor phase process comprising isomerizing in the presence
of hydrogen an unextracted xylene stream containing a major amount
of xylene and a minor amount of ethylbenzene to a mixture rich in
paraxylene over a catalyst mixture containing a supported HAMS-1B
crystalline, borosilicate molecular sieve incorporated into alumina
component and a molybdenum on silica component, said molybdenum
on silica component containing between about 1 and about 20 weight
percent molybdenum calculated as the metal, said catalyst mixture
containing between about 5 and about 95 percent by weight of said
molybdenum on silica component based upon the total weight of said
mixture, and said HAMS-1B crystalline, borosilicate molecular sieve
incorporated in alumina component containing between about 40 and
about 95 weight percent alumina.
2. The process of claim 1 wherein said molybdenum on silica component
contains between about 2 and about 15 weight percent molybdenum.
3. The process of claim 2 wherein said catalyst mixture contains
between about 25 and 75 percent by weight of said molybdenum on
silica component.
4. The process of claim 3 wherein said HAMS-1B crystalline borosilicate
molecular sieve incorporated in alumina component contains between
about 40 and about 90 weight percent alumina.
5. The process of claim 1 wherein said molybdenum on silica component
contains between about 3 and about 10 weight percent molybdenum.
6. The process of claim 5 wherein said catalyst mixture contains
between about 35 and 60 percent by weight of said molybdenum on
silica component.
7. The process of claim 6 wherein said HAMS-1B crystalline borosilicate
molecular sieve incorporated in alumina component contains between
about 60 and about 90 weight percent alumina.
Molecular sieve description
BACKGROUND OF THE INVENTION
This invention relates to xylene isomerization catalyst mixtures
based upon supported AMS-1B crystalline, borosilicate molecular
sieve catalyst compositions, and particularly, to isomerization
of an unextracted, ethylbenzene-containing xylene stream using such
mixtures, which process converts ethylbenzene to benzene and ethane
primarily by hydrodeethylation and has improved paraffins and naphthenes
conversion. More particularly, it relates to catalyst mixtures comprising
an AMS-1B crystalline, borosilicate molecular sieve incorporated
into an inorganic matrix and silica-supported molybdenum and to
processes for using these catalyst mixtures to isomerize an unextracted,
ethylbenzene-containing xylene stream to a mixture rich in paraxylene
in a process which shows improved paraffins and naphthenes conversion
to light hydrocarbons and converts ethylbenzene primarily by hydrodeethylation
to benzene and ethane.
Typically, paraxylene is derived from mixtures of C.sub.8 aromatics
separated from such raw materials as petroleum naphthas, particularly
reformates, usually by isomerization followed by, for example, lower-temperature
crystallization of the paraxylene with recycle of the crystallizer
liquid phase to the isomerizer. Principal raw materials are catalytically
reformed naphthas and petroleum distillates. The fractions from
these sources that contain C.sub.8 aromatics vary quite widely in
composition but will usually contain 10 to 35 weight percent ethylbenzene
and up to about 10 weight percent primarily C.sub.9 paraffins and
naphthenes with the remainder being primarily xylenes divided approximately
50 weight percent meta, and 25 percent each of the ortho and para
isomers. The primarily C.sub.9 paraffins and naphthenes can be removed
substantially by extraction to produce what are termed "extracted"
xylene feeds, however, the extraction step adds to processing costs.
Feeds that do not have the primarily C.sub.9 paraffins and naphthenes
removed by extraction are termed "unextracted" xylene
feeds.
The ethylbenzene in a xylene mixture is very difficult to separate
from the other components due to similar volatility, and, if it
can be converted during isomerization to products more readily separated
from the xylenes, buildup of ethylbenzene in the recycle loop is
prevented and process economics are greatly improved. The primarily
C.sub.9 paraffins and naphthenes present in unextracted feeds unless
removed also build up in the recycle loop and are usually extracted
prior to isomerization as most commercial isomerization processes
do not provide a catalyst which effectively converts them to easily
separable-by-distillation products. Thus, it would be valuable to
have a catalyst/process for xylene isomerization which would effectively
convert both the ethylbenzene and primarily C.sub.9 paraffins and
naphthenes to easily separable products without affecting the isomerization
efficiency. In addition, the catalyst should minimize xylene loss
via hydrogenation and cracking.
Xylene isomerization catalysts can be classified into three types
based upon the manner in which they convert ethylbenzene: (1) naphthene
pool catalysts, (2) transalkylation catalysts, and (3) hydrodeethylation
catalysts.
Naphthene pool catalysts are capable of converting a portion of
the ethylbenzene to xylenes via naphthene intermediates. These catalysts
contain a strong hydrogenation function, such as platinum, and an
acid function, such as chlorided alumina, amorphous silica-alumina,
or a molecular sieve. The role of the hydrogenation function in
these catalysts is to hydrogenate the C.sub.8 aromatics to establish
essentially equilibrium between the C.sub.8 aromatics and the C.sub.8
cyclohexanes. The acid function interconverts ethylcyclohexane and
the dimethylcylohexanes via cyclopentane intermediates. These C.sub.8
cycloparaffins form the so-called naphthene pool.
It is necessary to operate naphthene pool catalysts at conditions
that allow the formation of a sizable naphthene pool to allow efficient
conversion of ethylbenzene to xylenes. Unfortunately, naphthenes
can crack on the acid function of the catalyst, and the rate of
cracking increases with the size of the naphthene pool. Naphthene
cracking leads to high xylene loss, and the by-products produced
by naphthene cracking are low-valued paraffins. Thus, naphthene
pool catalysts are generally less economic than the transalkylation-type
and hydrodee- thylation-type catalysts.
The transalkylation catalysts generally contain a shape selective
molecular sieve. A shape selective catalyst is one that prevents
some reactions from occurring based on the size of the reactants,
products, or intermediates involved. In the case of common transalkylation
catalysts, the molecular sieve contains pores that are apparently
large enough to allow ethyl transfer to occur via a dealkylation/realkylation
mechanism, but small enough to substantially suppress methyl transfer
which requires the formation of a bulky biphenylalkane intermediate.
The ability of transalkylation catalysts to catalyze ethyl transfer
while suppressing methyl transfer allows these catalysts to convert
ethylbenzene while minimizing xylene loss via xylene disproportionation.
When ethyl transfer occurs primarily by dealkylation/realkylation,
it is possible to intercept and hydrogenate the ethylene intermediate
involved with this mechanism of ethyl transfer by adding a hydrogenation
function to the catalyst. The primary route for converting ethylbenzene
then becomes hydrodeethylation, which is the conversion of ethylbenzene
to benzene and ethane. It is desirable to selectively hydrogenate
the ethylene intermediate without hydrogenating aromatics (without
establishing a naphthene pool) to prevent the cracking of the naphthenes
that occurs over the acid function of the catalyst. Commercial hydrodeethylation
catalysts selectively hydrogenate ethylene without substantial hydrogenation
of aromatics at reported commercial conditions.
In order to form a hydrodeethylation catalyst, it is essential
to use an acidic component that behaves as a shape selective catalyst,
i.e., one that suppresses the formation of the bulky biphenylalkane
intermediate required for transmethylation, because transethylation
can occur via a similar intermediate. For catalysts with pores large
enough to allow the formation of these biphenylalkane intermediates,
transethylation appears to occur primarily via these intermediates.
In this case, ethylene is not an intermediate for transethylation,
and the addition of a hydrogenation component cannot produce a hydrodeethylation
catalyst
Molecular sieves such as the AMS-1B crystalline, borosilicate molecular
sieves have shown great utility in the isomerization of xylenes
to make primarily paraxylene Such sieves when supported on an oxide
carrier like alumina effectively produce equilibrium amounts of
paraxylene and dispose of ethylbenzene largely by transalkylation
without serious loss of xylenes. However, such sieves are not very
effective in removing paraffins and naphthenes during the isomerization
of xylenes and they are generally used with extracted feeds.
Periodic Group VIb elements including molybdenum have shown utility
in the past for various hydrocarbon conversions including hydrogenation.
In particular, in U.S. Pat. Nos. 4420467; 4532226; and 4655255
molybdenum is said to be incorporated into or on a molecular sieve
framework, which sieve is useful for hydrocarbon conversions including
isomerization. In U.S. Pat. No. 4202996 hydrocarbon isomerization
is carried out over a catalytic composite having a nickel component,
a molybdenum component, a platinum component in combination with
a zeolitic carrier. In other work, the activity of supported molybdenum
compounds useful for hydrogenation/dehydrogenation has been found
to depend upon the oxidation state of molybdenum with the lower
molybdenum oxidation states being more effective.
Now it has been found that by adding molybdenum on silica to an
alumina-supported HAMS-1B crystalline, borosilicate molecular sieve
catalyst composition, a catalyst mixture is formed which, when used
for xylene isomerization of unextracted xylene streams, removes
ethylbenzene primarily by the hydrodeethylation mechanism to benzene
and ethane and can substantially increase the removal of paraffins
and naphthenes by cracking to light hydrocarbons. These results
are obtained, moreover, without otherwise substantially affecting
the isomerization effectiveness of the supported molecular sieve
catalyst composition. Unexpectedly, other common molybdenum supports
such as alumina do not produce a supported molybdenum which is as
effective in removing paraffins and naphthenes when made into a
catalyst mixture with the borosilicate sieve.
SUMMARY OF THE INVENTION
Described herein is a vapor phase process comprising isomerizing
in the presence of hydrogen an unextracted xylene stream containing
a major amount of xylene and a minor amount of ethylbenzene to a
mixture rich in paraxylene over a catalyst mixture containing a
HAMS-1B crystalline, borosilicate molecular sieve incorporated into
alumina component and a molybdenum on silica component, said molybdenum
on silica component containing between about 1 and about 20 weight
percent molybdenum calculated as the metal, said catalyst mixture
containing between about 5 to about 95 percent by weight of said
molybdenum on silica component based upon the total weight of said
mixture, and said HAMS-1B crystalline, borosilicate molecular sieve
incorporated in alumina component containing between about 40 and
about 95 weight percent alumina.
More specifically, the material useful in the present invention
is prepared by mixing a base, a boron oxide source, and an organic
template compound in water (preferably distilled or deionized).
The order of addition usually is not critical, although a typical
procedure is to dissolve base and boric acid in water and then add
the template compound. Generally, the silicon oxide compound is
added with intensive mixing such as that performed in a Waring Blender
and the resulting slurry is transferred to a closed crystallization
vessel for a suitable time. After crystallization, the resulting
crystalline product can be filtered, washed with water, dried, and
calcined.
During preparation, acidic conditions should be avoided. When alkali
metal hydroxides are used, the values of the ratio of OH.sup.- /SiO.sub.2
shown above, should furnish a pH of the system that broadly falls
within the range of about 9 to about 13.5. Advantageously, the pH
of the reaction system falls within the range of about 10.5 to about
11.5 and most preferably between about 10.8 and about 11.2.
Examples of materials affording silicon oxide useful in this invention
include silicic acid, sodium silicate, tetraalkyl silicates and
Ludox, a stabilized polymer of silicic acid manufactured by E. I.
DuPont de Nemours & Co. Typically, the oxide of boron source
is boric acid although equivalent species can be used such as sodium
borate and other boron-containing compounds.
Cations useful in formation of AMS-1B crystalline borosilicate
include hydrogen ion, the cationic form of the organic template,
alkali metal and alkaline earth metal cations such as sodium, potassium,
lithium, calcium, and magnesium. Ammonium cations may be used alone
or in conjunction with such metal cations. Since basic conditions
are required for crystallization of the molecular sieve of this
invention, the source of such cation can be a hydroxide such as
sodium hydroxide. Alternatively, AMS-1B can be prepared directly
and more preferably in the hydrogen form by replacing such metal
cation hydroxides with an organic base such as ethylenediamine as
described in Published European Application No. 68796.
Organic templates useful in preparing AMS-1B crystalline borosilicate
include alkylammonium cations or precursors thereof such as tetraalkylammonium
compounds, especially tetra-n-propylammonium compounds. A useful
organic template is tetra-n-propylammonium bromide. Diamines, such
as hexamethylenediamine, can be used.
In a more detailed description of a typical preparation of this
invention, suitable quantities of sodium hydroxide and boric acid
(H.sub.3 BO.sub.3) are dissolved in distilled or deionized water
followed by addition of the organic template. The pH may be adjusted
between about 11.0 .+-.0.2 using a compatible acid or base such
as sodium bisulfate or sodium hydroxide. After sufficient quantities
of a silica source such as a silicic acid polymer (Ludox) are added
with intensive mixing, preferably the pH is again checked and adjusted
to a range of about 11.0.+-.0.2.
Alternatively and more preferably, AMS-1B crystalline borosilicate
molecular sieve can be prepared by crystallizing a mixture of sources
for an oxide of silicon, an oxide of boron, an alkylammonium compound
and ethylenediamine such that the initial reactant molar ratios
of water to silica range from about 5 to about 25 preferably about
5 to about 20 and most preferably from about 10 to about 15. In
addition, preferable molar ratios for initial reactant silica to
oxide of boron range from about 4 to about 150 more preferably
from about 5 to about 80 and most preferably from about 5 to about
20. The molar ratio of ethylenediamine to silicon oxide should be
above about 0.05 typically below 5 preferably between about 0.1
and about 1.0 and most preferably between about 0.2 and 0.5. The
molar ratio of alkylammonium compound, such as tetra-n-propylammonium
bromide, to silicon oxide can range from 0 to about 1 or above,
typically above about 0.005 preferably about 0.01 to about 0.1
more preferably about 0.01 to about 0.1 and most preferably about
0.02 to about 0.05.
The resulting slurry is transferred to a closed crystallization
vessel and reacted usually at a pressure at least the vapor pressure
of water for a time sufficient to permit crystallization which usually
is about 0.25 to about 20 days, typically is about one to about
ten days and preferably is about one to about seven days, at a temperature
ranging from about 100.degree. C. to about 250.degree. C., preferably
about 125.degree. C. to about 200.degree. C. The crystallizing material
can be stirred or agitated as in a rocker bomb. Preferably, the
crystallization temperature is maintained below the decomposition
temperature of the organic template compound. Especially preferred
conditions are crystallizing at about 165.degree. C. for about five
to about seven days. Samples of material can be removed during crystallization
to check the degree of crystallization and determine the optimum
crystallization time.
The crystalline material formed can be separated and recovered
by well-known means such as filtration with aqueous washing. This
material can be mildly dried for anywhere from a few hours to a
few days at varying temperatures, typically about 50.degree.-225.degree.
C., to form a dry cake which can then be crushed to a powder or
to small particles and extruded, pelletized, or made into forms
suitable for its intended use. Typically, materials prepared after
mild drying contain the organic template compound and water of hydration
within the solid mass and a subsequent activation or calcination
procedure is necessary, if it is desired to remove this material
from the final product. Typically, mildly dried product is calcined
at temperatures ranging from about 260.degree. C. to about 850.degree.
C., and preferably from about 425.degree. C. to about 600.degree.
C. Extreme calcination temperatures or prolonged crystallization
times may prove detrimental to the crystal structure or may totally
destroy it. Generally, there is no need to raise the calcination
temperature beyond about 600.degree. C. in order to remove organic
material from the originally formed crystalline material. Typically,
the molecular sieve material is dried in a forced draft oven at
165.degree. C. for about 16 hours and is then calcined in air in
a manner such that the temperature rise does not exceed 125.degree.
C. per hour until a temperature of about 540.degree. C. is reached.
Calcination at this temperature usually is continued for about 4
to 16 hours.
The original cation in the AMS-1B crystalline borosilicate, if
not hydrogen, can be replaced all or in part by ion exchange with
other cations including other metal ions and their amine complexes,
alkylammonium ions, ammonium ions, hydrogen ions, and mixtures thereof
The preferred AMS-1B cation is hydrogen ion to form the HAMS-1B
component of the catalyst mixture of this invention.
The HAMS-1B crystalline borosilicate useful in this invention is
admixed with or incorporated with an alumina binder. Typically,
the borosilicate is incorporated within the binder by blending with
a sol of the alumina material and gelling the resulting mixture.
These supported compositions are then dried at 100.degree. to 200.degree.
C. and thereafter generally calcined at 500.degree.-600.degree.
C. The crystalline borosilicate content of the supported compositions
can vary anywhere from about 5 to 60 weight percent of the total
composition. Preferably they contain about 10 to about 60 weight
percent of sieve and more preferably, contain about 10 to about
40 weight percent sieve.
The silica used to support the molybdenum compound which is the
second component of the catalyst mixture can be obtained from any
one of a number of different sources. Preferably, the silica used
has a surface area above about 30 sq m/g.
The amount of molybdenum placed on the silica can vary from about
1 to about 20 weight percent, more preferably about 2 to about 15
weight percent, and most preferably from about 3 to about 10 weight
percent molybdenum, calculated as the metal. Soluble compounds of
molybdenum such as ammonium molybdate may be used to impregnate
the silica, and are generally dissolved in water and used to impregnate
the silica by the incipient wetness or other technique as may be
understood by one skilled in the art. The resulting molybdenum-containing
silica is then dried at about 100.degree. to about 200.degree. C.
and calcined at about 500.degree. to about 600.degree. C. before
use.
The catalyst mixtures containing alumina-supported HAMS-1B crystalline,
borosilicate molecular sieve and molybdenum on silica component
can be made by several different methods. The two components can
be physically mixed in a mixer with a little distilled water to
form a paste which may then be dried at elevated temperature and
formulated into catalyst particles of appropriate shape and size.
Alternatively, the sieve component and the molybdenum on silica
component may be added to an alumina sol and the catalyst mixture
gelled with, for example, concentrated ammonia after which it is
dried, calcined and formulated into catalyst particles of the appropriate
size and shape. The gellation technique of forming the catalyst
mixtures is preferred. Preferably, the catalyst mixture contains
between about 5 and about 95 weight percent of the molybdenum on
silica component based upon the total weight of the mixture, more
preferably between about 25 and 75 percent, and most preferably,
about 35 and 60 percent.
Isomerization of xylene in the presence of the above-described
catalyst mixtures is effected by contact at a temperature between
about 300.degree. and about 650.degree. C., and preferably between
about 350.degree. and about 600.degree. C. The reaction can take
place at atmospheric pressure, but the total pressure is preferably
within the approximate range of about 1 atm to about 1000 psig.
Reaction is suitably accomplished utilizing a weight hourly space
velocity of between about 0.2 and about 50 and preferably between
about 1 and about 25. The space velocity is calculated on the basis
of the weight of HAMS-1B sieve on alumina present in the catalyst
mixture.
Hydrogen is used in the isomerization process and is generally
present in a mol ratio, hydrogen to hydrocarbon, between about 0.5
and about 7 and more preferably between about 1 and about 6.
The following Examples will serve to illustrate certain specific
embodiments of the hereindisclosed invention. These Examples should
not, however, be construed as limiting the scope of the novel invention
as there are many variations which may be made thereon without departing
from the spirit of the disclosed invention, as those of skill in
the art will recognize.
EXAMPLES
General
Isomerization results were obtained using a 2 ft stainless steel
reactor with an i.d. of 0.5 in placed in a salt bath. The catalyst
was loosely packed in the reactor with glass beads on either side
of the catalyst charge.
EXAMPLE 1
In this Example a physical mixture of molybdenum on silica and
alumina-supported HAMS-1B was prepared by physically mixing the
two separate portions.
The supported HAMS-1B sieve was prepared as follows. A 120.0 g
portion of distilled water was added to 40.0 g of the hydrogen form
of AMS-1B. A 1985 g portion of PHF alumina sol from American Cyanamid
(8.06 wt % solids) was added and the mixture blended in a homogenizer
for approximately 5 min. A 160 ml amount of concentrated ammonium
hydroxide was added to gel this mixture, and the gel was blended
in a mixmaster for about 5 min. The gelled AMS-1B on alumina was
dried at 165.degree. C. for 16 hr. The resulting cake was ground
to a powder fine enough to pass through a 100 mesh sieve.
An impregnation solution was prepared by adding 33.13 g of ammonium
heptamolybdate to 800.0 g of distilled water. This solution was
added to 200.0 g of Cab-O-Sil brand silica in a mixmaster. The impregnated
Mo/SiO.sub.2 was dried 8 hr at 165.degree. C. and then calcined
12 hr at 82.degree. C. The resulting cake was ground to a powder
fine enough to pass through a 100 mesh sieve. This results in a
9.0% Mo/SiO.sub.2.
The catalyst mixture was prepared by adding 31.25 g of 9.0% Mo/SiO.sub.2
to 93.75 g of alumina-supported HAMS-1B and mixing in a mixmaster.
A 154.0 g portion of distilled water was added slowly to the result
while stirring them with the mixer until a thick paste was formed.
The result was then dried at 165.degree. C. for 16 hr and calcined
at 482.degree. C. for 12 hr. |