Process for producing aromatic compounds from aliphatic hydrocarbons

A dehydrocyclo-oligomerization process is provided for converting aliphatic hydrocarbons to aromatics by contacting the feedstock under conversion conditions which a zeolite bound zeolite catalyst. The zeolite bound zeolite catalyst comprises first zeolite crystals which are bound together by second zeolite crystals. If the zeolite bound zeolite catalyst is selectivated, the process can produce greater than equilibrium amounts of paraxylene.

FIELD OF THE INVENTION 
The present invention relates to a process using a zeolite bound zeolite 
catalyst for the production of aromatic hydrocarbons by the 
dehydrocyclo-oligomerization of aliphatic hydrocarbons. 
BACKGROUND OF THE INVENTION 
Dehydrocyclo-oligomerization is a process in which aliphatic hydrocarbons 
are reacted over a catalyst to produce a high yield of aromatics and 
hydrogen and certain byproducts. This process is distinct from the more 
conventional reforming where C.sub.6 and higher carbon number reactants, 
primarily paraffins and naphthenes, are converted to aromatics. The 
aromatics produced by conventional reforming contain the same or a lesser 
number of carbon atoms per molecule than the reactants from which they 
were formed, indicating the absence of reactant oligomerization reactions. 
In contrast, the dehydrocyclo-oligomerization reaction results in an 
aromatic product that almost always contains more carbon atoms per 
molecule than the reactants, thus indicating that the oligomerization 
reaction is an important step in the dehydrocyclo-oligomerization process. 
Typically, the dehydrocyclo-oligomerization reaction is carried out at 
temperatures in excess of 260.degree. C. using dual functional catalysts 
containing acidic and dehydrogenation components. 
Aromatics, hydrogen, a C.sub.4 + nonaromatics byproduct, and a light ends 
byproduct are all products of the dehydrocyclo-oligomerization process. 
The aromatics are the desired product of the reaction as they can be 
utilized as gasoline blending components or for the production of 
petrochemicals. Hydrogen is also a desirable product of the process. The 
hydrogen can be efficiently utilized in hydrogen-consuming refinery 
processes such as hydrotreating or hydrocracking processes. The least 
desirable product of the dehydrocyclo-oligomerization process is light 
ends byproducts. The light ends byproducts consist primarily of C.sub.1 
and C.sub.2 hydrocarbons produced as a result of the hydrocracking side 
reactions. 
Zeolites are crystalline microporous molecular sieves comprised of a 
lattice of silica and optionally alumina combined with exchangeable 
cations such as alkali or alkaline earth metal ions. Although the term 
"zeolites" includes materials containing silica and optionally alumina, it 
is recognized that the silica and alumina portions may be replaced in 
whole or in part with other oxides. For example, germanium oxide, tin 
oxide, phosphorous oxide, and mixtures thereof can replace the silica 
portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and 
mixtures thereof can replace the alumina portion. Accordingly, the terms 
"zeolite", "zeolites" and "zeolite material", as used herein, shall mean 
not only materials containing silicon and, optionally, aluminum atoms in 
the crystalline lattice structure thereof, but also materials which 
contain suitable replacement atoms for such silicon and aluminum, such as 
galliumsilicates, silicoaluminophosphates (SAPO) and aluminophosphates 
(ALPO). The term "aluminosilicate zeolite", as used herein, shall mean 
crystalline zeolite materials consisting essentially of silicon and 
aluminum atoms in the crystalline lattice structure thereof. 
Zeolites have been used in the past as a catalyst for the production of 
aromatic hydrocarbons by the dehydrocylco-oligomerization of aliphatic 
hydrocarbons. For example, U.S. Pat. No. 4,654,455 involves the production 
of aromatic hydrocarbons by the dehydrocylco-oligomerization of aliphatic 
hydrocarbons using a zeolite catalyst which contains gallium and an 
alumina binding material. 
Synthetic zeolites are normally prepared by the crystallization of zeolites 
from a supersaturated synthesis mixture. The resulting crystalline product 
is then dried and calcined to produce a zeolite powder. Although the 
zeolite powder has good adsorptive properties, its practical applications 
are severely limited because it is difficult to operate fixed beds with 
zeolite powder. Therefore, prior to using in commercial processes, the 
zeolite crystals are usually bound. 
The zeolite is typically bound by forming a zeolite aggregate such as a 
pill, sphere, or extrudate. The extrudate is usually formed by extruding 
the zeolite in the presence of a non-zeolitic binder and drying and 
calcining the resulting extrudate. The binder materials used are resistant 
to the temperatures and other conditions, e.g., mechanical attrition, 
which occur in various hydrocarbon conversion processes. Examples of 
binder materials include amorphous materials such as alumina, silica, 
titania, and various types of clays. It is generally necessary that the 
zeolite be resistant to mechanical attrition, that is, the formation of 
fines which are small particles, e.g., particles having a size of less 
than 20 microns. 
Although such bound zeolite aggregates have much better mechanical strength 
than the zeolite powder, when such a bound zeolite is used for the 
dehydrocyclo-oligomerization of aliphatic hydrocarbons, the performance of 
the catalyst, e.g., activity, selectivity, activity maintenance, or 
combinations thereof, can be reduced because of the binder. For instance, 
since the amorphorous binder is typically present in an amount of up to 
about 50 wt. % of zeolite, the binder dilutes the adsorptive properties of 
the zeolite aggregate. In addition, since the bound zeolite is prepared by 
extruding or otherwise forming the zeolite with the binder and 
subsequently drying and calcining the extrudate, the amorphous binder can 
penetrate the pores of the zeolite or otherwise block access to the pores 
of the zeolite, or slow the rate of mass transfer to the pores of the 
zeolite which can reduce the effectiveness of the zeolite when used in 
hydrocarbon conversion processes. Furthermore, when such a bound zeolite 
is used in catalytic conversions processes such as the 
dehydrocyclo-oligomerization of aliphatic hydrocarbon, the binder may 
affect the chemical reactions that are taking place within the zeolite and 
also may itself catalyze undesirable reactions which can result in the 
formation of undesirable products. 
SUMMARY OF THE INVENTION 
The present invention is directed process for producing aromatic 
hydrocarbons by the dehydrocyclo-oligomerization of aliphatic 
hydrocarbons. The process comprises contacting a feedstream containing 
aliphatic hydrocarbons under aromatization conditions with a zeolite bound 
zeolite catalyst which contains first crystals of an acidic intermediate 
pore size first zeolite and second crystals of a second zeolite which 
binds together the first zeolite crystals. 
In another embodiment of the present invention, there is provided a process 
for the dehydrocyclo-oligomerization of aliphatic hydrocarbons using the 
zeolite bound zeolite catalyst to produce a product which includes xylenes 
which are enriched in paraxylene. 
DETAILED DESCRIPTION OF THE INVENTION 
The zeolite bound zeolite catalyst used in the process of the present 
invention comprises first crystals of a acidic intermediate pore size 
first zeolite and a binder comprising second crystals of a second zeolite. 
The use of second zeolite crystals as a binder results in a catalyst which 
provides a means for controlling undesirable reactions taking place on or 
near the surface of the first zeolite crystals and can have improved mass 
transfer of reactants and greater access to and from the pores of the 
zeolite. 
Unlike zeolite catalysts bound with amorphous material such as silica or 
alumina to enhance the mechanical strength of the zeolite, the zeolite 
bound zeolite catalyst used in the process of the present invention does 
not contain significant amounts of non zeolitic binders. Preferably, the 
zeolite bound zeolite catalyst contains less than 10 percent by weight 
based on the total weight of the first and second zeolite of non-zeolitic 
binder, more preferably contains less than 5 percent by weight, and, most 
preferably, the first and second zeolite are substantially free of 
non-zeolitic binder. Preferably, the second zeolite crystals bind the 
first zeolite crystals by adhering to the surface of the first zeolite 
crystals thereby forming a matrix or bridge structure which also holds the 
first crystals particles together. More preferably, the second zeolite 
crystals bind the first zeolite by intergrowing so as to form a coating or 
partial coating on the larger first zeolite crystals and, most preferably, 
the second zeolite crystals bind the first zeolite crystals by 
intergrowing to form an attrition resistant over-growth over the first 
zeolite crystals. 
Although the invention is not intended to be limited to any theory of 
operation, it is believed that one of the advantages of the zeolite bound 
zeolite catalyst when used in the process of the present invention is 
obtained by the crystals of the second zeolite controlling the 
accessibility of the acid sites on the external surfaces of the first 
zeolite to reactants. Since the acid sites existing on the external 
surface of a zeolite catalyst are not shape selective, these acid sites 
can adversely affect reactants entering the pores of the zeolite and 
products exiting the pores of the zeolite. In line with this belief, since 
the acidity of the second zeolite can be carefully selected, the second 
zeolite does not significantly adversely affect the reactants exiting the 
pores of the first zeolite which can occur with conventionally bound 
zeolite catalysts and may beneficially affect the aromatic selectivity of 
a dehydrogenation process and also the reactants exiting the pores of the 
first zeolite. Still further, since the second zeolite is not amorphous 
but, instead, is a molecular sieve, hydrocarbons have increased access to 
the pores of the first zeolite during the aromatization process. 
The terms "acidity", "lower acidity" and "high acidity" as applied to 
zeolite are know to persons skilled in the art. The acidic properties of 
zeolite are well known. However, with respect to the present invention, a 
distinction must be made between acid strength and acid site density. Acid 
sites of a zeolite can be a Bronstead acid or a Lewis acid. The density of 
the acid sites and the number of acid sites are important in determining 
the acidity of the zeolite. Factors directly influencing the acid strength 
are (i) the chemical composition of the zeolite framework, i.e., relative 
concentration and type of tetrahendral atoms, (ii) the concentration of 
the extra-framework cations and the resulting extra-framework species, 
(iii) the local structure of the zeolite, e.g., the pore size and the 
location, within the crystal or at/near the surface of the zeolite, and 
(iv) the pretreatment conditions and presence of co-adsorbed molecules. 
The amount of acidity is related to the degree of isomorphous substitution 
provided, however, such acidity is limited to the loss of acid sites for a 
pure SiO.sub.2 composition. As used herein, the terms "acidity", "lower 
acidity" and "higher acidity" refers to the concentration of acid sites 
irregardless of the strength of such acid sites which can be measured by 
ammonia adsorption. 
The first zeolite used in the zeolite bound zeolite catalyst is an 
intermediate pore size zeolite. Intermediate pore size zeolites have a 
pore size from about 5 to about 7 .ANG. and include, for example, AEL, 
MFI, MEL, MFS, MEI, MTW, EUO, MTT, HEU, FER, and TON structure type 
zeolites. These zeolites are described in "Atlas of Zeolite Structure 
Types", eds. W. H. Meier and D. H. Olson, Butterworth-Heineman, Third 
Edition, 1992, which is hereby incorporated by reference. Examples of 
specific intermediate pore size zeolites include ZSM-5, ZSM-11, ZSM-12, 
ZSM-22, ZSM-23, ZSM-34, ZSM-35, ZSM-38, ZSM-48, ZSM-50, and ZSM-57. 
Preferred first zeolites are SAPO-11, galliumsilicate zeolites having an 
MFI structure, and aluminosilicate zeolites having an MFI structure. 
The term "average particle size" as used herein, means the average diameter 
of the crystals, e.g., number average of the major axis and minor axis. 
The average crystal size of the crystals of the first zeolite is preferably 
from about 0.1 micron to about 15 microns, more preferably from about 1 to 
about 6 microns. 
Procedures to determine crystal size are know to persons skilled in the 
art. For instance, crystal size may be determined directly by taking a 
suitable scanning electron microscope (SEW) picture of a representative 
sample of the crystals. 
Intermediate pore size first zeolites will generally comprise a composition 
having the following molar relationship: 
EQU X.sub.2 O.sub.3: :(n) Y0.sub.2, 
wherein X is a trivalent element such as aluminum and gallium and Y is a 
tetravalent element such as silicon, tin, and/or germanium; and n has a 
value greater than 12, said value being dependent upon the particular type 
of zeolite. When the intermediate pore size zeolite is a MFI structure 
type zeolite, n is preferably greater than 20. 
As known to persons skilled in the art, the acidity of a zeolite can be 
reduced using many techniques such as by steaming. In addition, the 
acidity of a zeolite is dependent upon the form of the zeolite with the 
hydrogen form having the highest acidity and other forms of the zeolite 
such as the sodium form having less acidity than the acid form. 
Accordingly, the mole ratios of silica to alumina and silica to gallia 
disclosed herein shall include not only zeolites having the disclosed mole 
ratios, but shall also include zeolites not having the disclosed mole 
ratios but having equivalent catalytic activity. 
When the first zeolite is an aluminosilicate zeolite, the first zeolite 
will preferably have a silica to alumina mole ratio from 20:1 to 300:1. 
More preferably, the first zeolite will have a silica to alumina mole 
ratio of from about 30:1 to about 150:1. 
When the first zeolite is a gallium silicate zeolite, the zeolite 
preferably comprises a composition having the following molar relationship 
: 
EQU Ga.sub.2 O.sub.3 :ySiO.sub.2 
wherein y is between about 24 and about 500. The zeolite framework may 
contain only gallium and silicon atoms or may also contain a combination 
of gallium, aluminum, and silicon. When the first zeolite is a MFI 
structure type gallium silicate zeolite, the second zeolite will 
preferably be an intermediate pore size zeolite having a silica to gallia 
mole ratio greater than 100. The second zeolite can also have higher 
silica to gallia mole ratios, e.g., greater than 200, 500, 1000, etc. 
The second zeolite will usually have an intermediate pore size and have 
less acid activity then the first zeolite. Preferably, the second zeolite 
will be substantially non-acidic and will have the same structure type as 
the first zeolite. The preferred second zeolites are ALPO-11 and 
aluminosilicate zeolites having a silica to alumina mole ratio greater 
than 100 such as low acidity ZSM-5. If the second zeolite is an 
aluminosilicate zeolite, the second zeolite will generally have a silica 
to alumina mole ratio greater than 200:1, e.g., 500:1; 1,000:1, etc., and 
in some applications will contain no more than trace amounts of alumina. 
The second zeolite can also be silicalite, i.e., a AM type substantially 
free of alumina, or silicalite 2, a MEL type substantially free of 
alumina. The second zeolite is usually present in the zeolite bound 
zeolite catalyst in an amount in the range of from about 10% to 60% by 
weight based on the weight of the first zeolite and, more preferably, from 
about 20% to about 50% by weight. 
The second zeolite crystals preferably have a smaller size than the first 
zeolite crystals and more preferably will have an average particle size of 
less than 1 micron, and most preferably will have an average particle size 
from about 0.1 to about 0.5 micron. The second zeolite crystals, in 
addition to binding the first zeolite particles and maximizing the 
performance of the catalyst will preferably intergrow and form an 
over-growth which coats or partially coats the first zeolite crystals. 
Preferably, the crystals will be resistant to attrition. 
The zeolite bound zeolite catalyst used in the process of the present 
invention will usually contain a metal component selected from the 
elements of Groups IIB through IVB of the Periodic Table of the Elements 
(IU). Examples of such metals include gallium, zinc, and tin. The metal 
component may be present in any form including elemental metal, oxide, 
hydroxide, halide, oxyhalide, or in chemical combination with one or more 
of the other ingredients of the zeolite bound zeolite catalyst. It is 
believed that the best results are obtained when the metal component is in 
the zero valency state. The metal component can be used in any amount 
which is catalytically effective. Generally, the zeolite bound zeolite 
catalyst will contain on an elemental basis from about 0.5 to about 5% of 
the metal based on the weight of the zeolite bound zeolite catalyst. The 
metal component preferably comprises gallium. 
The metal component may be incorporated into the zeolite bound zeolite 
catalyst in any suitable manner known to the art such as by ion exchange 
or impregnation. Additionally, the metal component may be surface 
impregnated such that the majority of the metal is located on the outer 
portion of the zeolite by means such as physical mixing by chemical 
complexing, or by pore blockage prior to impregnation of the metal. It is 
intended to include within the scope of the present invention all 
conventional methods for incorporating and simultaneously distributing 
either uniformly or non-uniformly a metallic component in zeolite bound 
zeolite catalyst. One method of incorporating the metal involves ion 
exchange of the active catalytic component with a soluble, decomposable 
compound of the metal such as, with respect gallium, gallium tribromide, 
gallium perchlorate, gallium trichloride, gallium hydroxide, gallium 
nitrates, gallium oxalate, and the like compounds. Another method of 
incorporating the gallium into the zeolite bound zeolite catalyst is to 
convert the silica of a silica bound aggregate containing said first 
zeolite and the metal to the second zeolite. 
The zeolite bound zeolite catalyst used in the process of the present 
invention is preferably prepared by a three step procedure which is 
described for a first ZSM-5 zeolite and a high silica MFI structure type 
second zeolite. The first step involves the synthesis of the first zeolite 
crystals prior to converting it to the zeolite bound zeolite catalyst. 
Processes for preparing the first zeolite are known in the art. For 
example, with respect to the preparation of a MFI type aluminosilicate 
zeolite, a preferred process comprises preparing a solution containing 
tetrapropyl ammonium hydroxide or bromide, alkali metal oxide, an oxide of 
aluminum, an oxide of silicon and water, and then heating the reaction 
mixture to a temperature of 80.degree. C. to 200.degree. C. for a period 
of from about four hours to eight days. The resulting gel forms solid 
crystal particles which are separated from the reaction medium, washed 
with water and dried. The resulting product may then be optionally 
calcined in air at temperatures of 400-550.degree. C. for a period of 
10-40 hours to remove tetrapropylammonium (TPA) cations. 
Next, a silica-bound aluminosilicate zeolite can be prepared preferably by 
mixing a mixture comprising the aluminosilicate zeolite crystals, a silica 
gel or sol, water and optionally an extrusion aid and, optionally alumina 
or gallium and, optionally, the metal component until a homogeneous 
composition in the form of an extrudable paste develops. The silica binder 
used in preparing the silica bound zeolite aggregate is preferably a 
silica sol and preferably contains only very minor amounts of alumina or 
gallium, e.g., less than 2,000 ppm. The amount of silica used is such that 
the content of the zeolite in the dried extrudate will range from about 40 
to 90% by weight, more preferably from about 50 to 80% by weight, with the 
balance being primarily silica, e.g. about 20 to 50% by weight silica. 
The resulting paste can be molded, e.g. extruded, and cut into small 
strands, e.g., approximately 2 mm diameter extrudates, which can be dried 
at 100-150.degree. C. for a period of 4-12 hours and then calcined in air 
at a temperature of from about 400.degree. C. to 550.degree. C. for a 
period of from about 1 to 10 hours. 
Optionally, the silica-bound aggregate can be made into a very small 
particles which have application in fluid bed processes such as catalytic 
cracking. This preferably involves mixing the zeolite with a silica 
containing matrix solution so that an aqueous solution of zeolite and 
silica binder is formed which can be sprayed dried to result in small 
fluidizable silica-bound aggregate particles. Procedures for preparing 
such aggregate particles are known to persons skilled in the art. An 
example of such a procedure is described by Scherzer (Octane-Enhancing 
Zeolitic FCC Catalysts, Julius Scherzer, Marcel Dekker, Inc. New York, 
1990). The fluidizable silica-bound aggregate particles, like the silica 
bound extrudates described above, would then undergo the final step 
described below to convert the silica binder to a second zeolite. 
The final step in the three step catalyst preparation process is the 
conversion of the silica present in the silica-bound catalyst to a second 
zeolite which serves to bind the first zeolite crystals together. The 
first zeolite crystals are thus held together without the use of a 
significant amount of non-zeolite binder. To prepare the zeolite bound 
zeolite catalyst, the silica-bound aggregate can be first aged in an 
appropriate aqueous solution at an elevated temperature. Next, the 
contents of the solution and the temperature at which the aggregate is 
aged should be selected to convert the amorphous silica binder into the 
second zeolite. It is preferable that the second zeolite be of the same 
type as the first zeolite. The newly-formed zeolite is produced as 
crystals. The crystals may grow on and/or adhere to the initial zeolite 
crystals, and may also be produced in the form of new intergrown crystals, 
which are generally much smaller than the initial crystals, e.g., of 
sub-micron size. These newly formed crystals may grow together and 
interconnect. 
The nature of the aluminosilicate zeolite formed in the secondary synthesis 
conversion of the silica to zeolite may vary as a function of the 
composition of the secondary synthesis solution and synthesis aging 
conditions. The secondary synthesis solution is preferably an aqueous 
ionic solution containing a source of hydroxyl ions sufficient to convert 
the silica to the desired zeolite. 
The zeolite bound zeolite catalyst is usually in the acidic or partially 
neutralized acidic form. One method of obtaining the acidic form is to ion 
exchange the zeolite to produce the ammonium salt form. As a result of 
calcination, the acid form of the zeolite bound zeolite catalyst is 
produced. 
The zeolite bound zeolite catalyst can be selectivated to improve its 
paraxylene selectivity to thereby produce a resulting xylene para- xylene 
rich fraction. Compounds suitable for selectivating to the zeolite bound 
zeolite catalyst include silicon compounds. 
The silicon compounds may comprise a polysiloxane including silicones, a 
siloxane, and a silane including disilanes and alkoxysilanes. 
Silicone compounds which can be used in the present invention can be 
characterized by general formula: 
##STR1## 
wherein R.sub.1 is hydrogen, fluoride, hydroxy, alkyl, aralkyl, alkaryl or 
fluoro-alkyl. The hydrocarbon substituents generally contain from 1 to 10 
carbon atoms and preferably are methyl or ethyl groups. R.sub.2 is 
selected from the same group as R.sub.1, and n is an integer of at least 2 
and generally in the range of 2 to 1000. The molecular weight of the 
silicone compound employed is generally between 80 and 20,000 and 
preferably 150 to 10,000. Representative silicone compounds included 
dimethylsilicone, diethylsilicone, phenylmethylsilicone, methyl 
hydrogensilicone, ethylhydrogensilicone, phenylhydrogensilicone, 
methylethylsilicone, phenylethylsilicone, diphenylsilicone, methyltri 
fluoropropylsilicone, ethyltrifluoropropylsilicone, tetrachlorophenyl 
methyl silicone, tetrachlorophenylethyl silicone, tetrachloro 
phenylhydrogen silicone, tetrachlorophenylphenyl silicone, 
methylvinylsilicone and ethylvinylsilicone. The silicone compound need not 
be linear but may be cyclic as for example hexamethylcyclotrisiloxane, 
octamethylcyclotetrasiloxane, hexaphenyl cyclotrisiloxane and 
octaphenylcyclotetrasiloxane. Mixtures of these compounds may also be used 
as well as silicones with other functional groups. 
Useful siloxanes or polysiloxanes include as non-limiting examples 
hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethyl 
cyclopentasiloxane, hexamethyldisiloxane, octamethytrisiloxane, 
decamethyltetrasiloxane, hexaethylcydotrisiloxane, octaethylcyclo 
tetrasiloxane, hexaphenylcyclotrisiloxane and octaphenylcyclo 
tetrasiloxane. 
Useful silanes, disilanes, or alkoxysilanes include organic substituted 
silanes having the general formula: 
##STR2## 
wherein R is a reactive group such as hydrogen, alkoxy, halogen, carboxy, 
amino, acetamide, trialkylsilyoxy, R.sub.1, R.sub.2 and R.sub.3 can be the 
same as R or can be an organic radical which may include alkyl of from 1 
to 40 carbon atoms, alkyl or aryl carboxylic acid wherein the organic 
portion of the alkyl contains 1 to 30 carbon atoms and the aryl group 
contains 6 to 24 carbon which may be further substituted, alkylaryl and 
arylalkyl groups containing 7 to 30 carbon atoms. Preferably, the alkyl 
group for an alkyl silane is between 1 and 4 carbon atoms in chain length. 
Mixtures may also be used. 
The silanes or disilanes include, as non-limiting examples, 
dimethylphenylsilane, phenytrimethylsilane, triethylsilane and hexa 
methyldisilane. Useful alkoxysilanes are those with at least one 
silicon-hydrogen bond. 
The zeolite bound zeolite catalyst can be preselectivated with the 
organosilicone compounds by depositing the compounds on the catalyst or 
the catalyst can be selectivated with the organosilicone compound by 
feeding the selectivating agent simultaneously with the feed stream at 5 
conditions of a temperature from 100.degree. C. to 600.degree. C., a 
pressure of 1 to 2000 psig, a weight hour space velocity of 0.1 to 100. 
The dehydrocyclo-oligomerization conditions employed in the process of the 
present invention will vary depending on such factors as feedstock 
composition and desired conversion. A desired range of conditions for the 
dehydro-cyclodimerization of the aliphatic hydrocarbons to aromatics 
include a temperature from about 350.degree. C. to about 650.degree. C., a 
pressure from about 1 to about 20 atmospheres, and weight hour space 
velocity from about 0.2 to about 5. The preferred process conditions are a 
temperature in the range from about 850.degree. F. to about 1250.degree. 
F., a pressure in or about the range from atmospheric to 400 psig, and a 
WHSV of 1 to 5. It is understood that, as the average carbon number of the 
feed increases, a temperature in the lower end of temperature range is 
required for optimum performance and conversely, as the average carbon 
number of the feed decreases, the higher the required reaction 
temperature. 
The feed stream used in the dehydrocyclo-oligomerization process of the 
present invention will preferably contain at least one aliphatic 
hydrocarbon containing 2 to about 6 carbon atoms. The aliphatic 
hydrocarbons may be open chain, straight chain, or cyclic. Examples such 
as hydrocarbons include ethane propane, propylene, n-butane, n-butenes, 
isobutane, straight and branch hexanes, and hexenes. Preferably, the 
hydrocarbons C.sub.3 and/or C.sub.4 are selected from isobutane, normal 
butane, isobutene, normal butene, isopropane, propane, and propylene. 
Diluents, refractory or reactant in nature, may also be included in the 
feed stream. Examples of such diluents include hydrogen, nitrogen, helium, 
argon, neon, CO, CO.sub.2, H.sub.O or a water precursor, i.e., compounds 
which liberate H.sub.O when heated to dehydrocyclo-oligomerization 
reaction temperatures. Methane may also be a component of the feedstock of 
the present invention. 
It is anticipated that the C.sub.2 -C.sub.6 aliphatic hydrocarbon 
feedstream utilized in the process of the instant invention may originate 
as a product or by-product of a refinery or petrochemical process. The 
light aliphatic hydrocarbons produced and recovered in a cracking or a 
reforming process would be examples of such process derived feed streams. 
The products of a synthesis gas production process is another potential 
source of feed for the instant process. Another anticipated source of feed 
is the light aliphatic hydrocarbons recovered at the well head at oil 
production facilities. 
The following examples illustrate the invention.

EXAMPLE 1 
Preparation Procedure for Catalyst A-D 
I. Catalyst A 
A catalyst comprising ZSM-5 (75:1 silica to alumina mole ratio) which was 
bound by 30% by weight amorphorous silica was treated three times at 
70.degree. C. with a 5 fold weight excess of 1.0 normal aqueous ammonium 
nitrate. The treated extrudates were washed until the wash water had a 
conductivity of less than 10 .mu.S/cm, dried overnight at 100.degree. C. 
and then cooled with nitrogen. The extrudates were impregnated with 20 wt. 
percent aqueous gallium nitrate solution until incipient wetness was 
achieved. The product was maintained at room temperature for 4 hours in 
air. Next, the product was dried in air for 1 hour at 88.degree. C. in air 
and then calcined by heating from 35.degree. C. to 510.degree. C. in 
31/2hours and holding at 510.degree. C. for 10 hours. The atomic gallium 
content of the catalyst was 1.92 wt. %. The catalyst is identified in the 
tables as Catalyst A. 
II. Catalyst B 
A catalyst comprising H-ZSM-5 core crystals having a silica to alumina mole 
ratio of 75:1 and bound by 30% by weight ZSM-5 binder crystals having a 
silica to alumina mole ratio of 900:1 was prepared by first mixing the 
ZSM-5 core crystals with amorphous silica containing a small amount of 
alumina and then forming the mixture into a silica bound extrudate. Next, 
the silica binder of the extrudate was converted to the second zeolite by 
aging the aggregate at elevated temperatures in an aqueous solution 
containing a template and a source hydroxy ions sufficient to convert the 
silica to the binder crystals. The resulting zeolite bound zeolite was 
then washed, dried, and calcined. Prior to use, the catalyst was dried 
overnight at 100.degree. C. and then cooled to room temperature under 
nitrogen. The resulting material was impregnated with gallium in the 
manner as described in above. The atomic gallium content of the catalyst 
was 1.95 wt. %. 
III. Catalyst C 
A catalyst comprising Na-ZSM-5 core crystals having a silica to alumina 
mole ratio of 75:1 and bound by 30% by weight MFI structure type zeolite 
crystals having a silica to gallia mole ratio of 70:1 was prepared by 
first mixing the ZSM-5 core crystals with amorphous silica containing 
gallia and then forming the mixture into a silica bound extrudate . Next, 
the silica binder of the extrudate was converted to the second zeolite by 
aging the aggregate at elevated temperatures in an aqueous solution 
containing a template and a source hydroxy ions sufficient to convert the 
silica to a binder crystal. The resulting zeolite bound zeolite was then 
washed, dried, and calcined. Prior to use, the catalyst was treated three 
times at 70.degree. C. with a 5 fold weight excess of a 1.0 normal aqueous 
ammonium nitrate solution. The resulting product was washed until the 
spent wash water had a conductivity of less than 10 micro-Siemens. The 
product was dried overnight in air and calcined by heating. The atomic 
gallium content of the catalyst was 0.87 wt. %. 
IV. Catalyst D 
Gallium was incorporated into a catalyst comprising ZSM-5 (80:1 silica to 
alumina mole ratio) bound by 20% by weight amorphous alumina using the 
procedure described in I. The atomic gallium content of the catalyst was 
2.37 wt. %. 
EXAMPLE 2 
Catalysts A-D were tested for the aromatization of propane. The catalysts 
were crushed and sized between 30 U.S. mesh and +40 U.S. mesh sieves. An 
amount of 1.5 grams of each catalyst was mixed with 3 grams of 14/20 mesh 
sized quartz chips and packed into a tubular reactor. The catalysts were 
treated for 1 hour at 584.degree. C. with 50 mole/h H.sub.2 diluted with 
95% vol. N.sub.2. The aromatization conditions used in the tests were 
550.degree. C., a propane weight hour space velocity (WHSV) of 2 and a 
pressure of 0.7 psig. Gas chromatography was used for analysis of the 
resulting products. The propane conversion and yield of C.sub.6 -C.sub.8 
aromatics and C.sub.1 -C.sub.2 are shown below in Table I. 
TABLE I 
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C.sub.6 -C.sub.8 
Time on Aromatics C.sub.1 -C.sub.2 Yield 
Catalyst Stream (hr) C.sub.3 Conv (%) Yield (%) (%) 
______________________________________ 
A. 8.67 63.5 35.5 11.5 
B. 3.92 51.6 49.2 29.5 
C. 6.68 46.1 35.7 34.8 
D. 3.33 89.2 15.6 2.2 
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Catalysts B and C were able to achieve high aromatics conversion. The 
performance of Catalyst C was achieved even though it contained about 50 
wt. % less gallium than Catalysts B and C. 
Example 3 
Catalysts A-D were tested for deactivation using the conditions of Example 
2, except that for Catalysts B and C, the WHSV was 1.0 and the pressure 
was 0.2 psig. The propane conversion was monitored over time and a first 
order rate constant for catalyst deactivation was calculated as the slope 
of the line from plotting -1n (1n (1/1-x)) versus time, where x is the 
fractional propane conversion. The wt. % of coke in-run time was 
determined by dividing the amount of coke in the catalyst by catalyst run 
time. The run times were: Catalyst A--43 hours, Catalyst B--114 hours, 
Catalyst C--52 hours, and Catalyst D--91 hours. The results are shown in 
Table II. 
TABLE II 
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Deactivation 
Wt. % Coke per 
Catalyst k obs (1/hr) hour run time 
______________________________________ 
A 0.026 0.082 
B 0.017 0.057 
C 0.031 0.142 
D 0.055 0.132 
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One of the benefits of the present invention is that of the zeolite bound 
zeolite catalyst deactivates slower than conventionally bound zeolite 
catalysts with an equivalent amount of gallium. The data shows that the 
deactivation rate for Catalyst B was at least 35% less than Catalyst A. 
Example 4 
Catalyst B was selectivated with hexamethyldisiloxane A S) by heating the 
catalyst to 500-599.degree. C. and then contacting it with a feed 
containing toluene and 1 wt. % HMDS at a WHSV of 4.1 and pressure of 1 
psig. The feed had a hydrogen to toluene mole ratio of 2:1. Toluene 
conversions are shown below in Table III: 
TABLE III 
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Benzene to 
p-Xylene 
Tol Conv Xylene Mole Selectivity 
Time (hr) Temp (.degree. C.) (%) Ratio (%) 
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1.38 500 3.35 0.63 62.0 
3.13 525 4.48 0.58 63.2 
5.18 599 11.6 0.63 60.6 
8.12 599 7.1 0.74 69.4 
10.59 599 5.8 0.72 76.1 
13.05 599 4.87 0.71 78.8 
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*PX selectivity = (PX/[PX + MX + OX]) .times. 100 
After selectivation, treated Catalyst B (identified in Table IV below as 
B-T) and untreated Catalyst B were tested for propane aromatization. The 
conditions were 550.degree. C., WHSV of 1.0 and a pressure of 0.2. The 
results of these tests are shown below in Table IV. 
TABLE IV 
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Time on C.sub.3 
C.sub.6 -C.sub.8 p-Xylene 
Stream Conv Aromatics C.sub.1 -C.sub.2 Xylene Selectivity 
Catalyst (hr) (%) Yield (%) Yield (%) Yield (%) (%) 
______________________________________ 
B 4.75 69.8 40.9 26.3 7.2 28.3 
B-T 0.52 37 41.7 26.9 7.9 59.5 
B-T 10.05 32.3 42.5 24.4 9.1 55.5 
B-T 19.58 28.3 46.7 20.4 10.0 52.6 
B-T 29.17 20.5 48.1 24.3 10.6 55.7 
B-T 38.72 17.3 47.1 24.8 10.6 56.7 
B-T 48.3 13.8 47.2 27.3 10.1 59.5 
B-T 61.05 11 46.3 29.4 10.0 61.9 
______________________________________ 
As shown in the tests, the zeolite bound zeolite catalyst exhibits good 
C.sub.6 -C.sub.8 aromatics selectivity and when selectivated, the process 
can produce a product stream having enriched amounts of para-xylene with 
respect to other isomers of xylene.