CA2047979C - Catalysts for the aromatization of light paraffins and olefins - Google Patents

Abstract

In accordance with the present invention, there is now provided a hybrid catalyst suitable for the aromatization of light paraffins and olefins, comprising a mixture of a pentasil type zeolite having the structure of ZSM-5 orZSM-11, and a cocatalyst consisting of gallium oxide supported by an oxide selected from the group consisting of silica, alumina particles and chromium oxide particles.

Description

20 17g79 CATAI,YSTS FOR THF APOM~T17~T~ON OF
!.IGHT PARAFFINS ~D O~ FFrNS

BACKGROUND OF T~IF I~VF.~TION
s This invention relates to catalysts suitable for arom~ti7~tion of paraffins and olefins, a method of making such catalysts, and an arom~ti7~tion process using same.

Aromatic hydrocarbons are important chemicals in the petroleum industry. The most commercially valuable aromatics are the BTX compounds such as ben_ene, toluene, ethylbenzene and xylenes. Aromatics are ~;u~nlly produced by catalytic cracking of n~phth~s, catalytic reforming of various petroleum feedsto~c etc. They can also be produced by catalytic cG~ ion of alcohols, particularly 15 meth~n~l or olefins. The catalysts used in these processes, like methanol-to-gasoline or MTG developed by Mobil Oil, or olefins-to-gasoline-and-dictill~te (MOGD) or M2 Forming, also dcvcloped by Mobil Oil, belong to the pentasil zeolite family whose most important member in terms of industrial appli~tionc is the ZSM-5 zeolite structure. The latter is a tr~ mpn~ion~l crystalline aluminocilicate having 20 strong acid sites and whose interme~ te pore (or channel) system displays a - reaction shape and size selectiviq which leads to the production of substantial amounts of monoaromatics. In the M2 Forming and MOGD proc~ss~, the ZSM-5 zeolite is used in its acid form alone without any cocatalyst.

Pentasil zeolites have been known for a number of years. llle ZSM-S
zeolite is described and claimed in U.S. Patent 3,702,886, while ZSM-11 is described in U.S. Patent 3,709,979.

Zinc o~ide and gallium oxide are known as modiSers of zeolite catalysts for the aromatization of hydrocarbons, particularly light alkanes such as ethane, propane and butane. In particular, the so called ~yclar process developed jointly by British Petroleum and United Oil Products, is commercially employed for the co.lve.sion of propane and butanes to aromatics. The catalyst used is a gallium ZSM-S zeolite. Such catalysts are usually prepared by wet impre~ation of a gallium salt onto the acidic surface of the zeolite, or by ion~Yrh~nge.

. -2- 2~7979 In US 4,97S,402 (Le Van Mao et al.), it has been observed that ZnO
species did not require to be adjacent to the zeolite acid sites to be effi~ient as arom~ti7~tion cocatalysts. Indeed, by simply setting the zinc bearing cocatalyst in physical contact with the zeolite particles and ~ g the two catalyst 5 components in an inert carrier, hybrid catalysts exhibiting high pelro.,..ance in the arom~ti7~tion of light olefins and paraffins were obtained.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is now provided a hybrid catalyst suitable for the arom~ti7~tion of light paraf_ns and olefins, conl~ ing a mixture of a pentasil type zeolite having the ~llu~u~e of ZSM-5 or ZSM-11, and a cocatalyst. More specifically, the hybrid catalyst of the present invention co.,.pr;ses a pentasil zeolite having the structure of ZSM-5 and ZSM-11, 15 in admixture with a cocatalyst con~;cting of gallium oxide supported by an oxide selected from the group consii,ling of silica, ~ min~ particles and chromium oxide particles. The term silica indudes colloidal silica, silica geL quartz, and the like.
Preferably, the silicon/aluminium atom ratio in the zeolite is ranging from about 25 to about 50. A weight ratio of zeolite/gallium oxide cocatalyst of about 4.7 to about 20 26.7 is preferred.

In another aspect of the present invention, the hybrid catalyst is embedded in from about 3 to about 20% by weight of an inert carrier, preferably,bentonite.
In further aspe~t of the invention, there is provided a method for preparing a hybrid catalyst suitable for the arorn~ti~tion of light paraffins and olefins.

30 DETAII,ED DESCRIPrlON OF THE INVENT~ON

The hybrid catalysts of the present invention containing the ZSM-5 zeolite have been found to give greater yields of aromatics and particularly BlXaromatics than a non-modified ZSM-5 zeolite in the acid form or a Cyclar-type 35 catalyst. Because of its structural similarities with ZSM-5, ZSM-11 behaves in the -3- 2Q ~ 7~79 same manner as ZSM-S. As used in this spe~;firation, the expression "zeolite having the structure of ZSM-S or ZSM-11~ is meant to include zeolites in which the ~lllmini~m atoms are ,~ ;nJ~ed totally or in part by other atoms, e.g. gallium.

S It is believed that the ~UppGI ~ed gallium cocatalyst, in the pr~nce of a pentasil zeolite, behaves as a hydrogen "trap", thus scavenging the h~J,o~nspecies released during the aromqti7at;on of paraffinic and olefinic col,lpounds. Th aromatization reaction occurs within the zeolite channels and the generated hydrogen species migrate to the cocatalyst particles located on the outside of the zeolite particles.

The method of preparation of aromatization catalysts of the present invention comp.ises.
(a) d-~ol.ing a water-soluble gallium salt in water; the sollltinn thus obtained is added to colloidal silica, and the res~ ine mixture is thoroughly stirred for a few minutPs;
(b) evaporating to dryness the obtained solution on a hot plate, and the resulting solid was further dried and activated in air at elevated temperature; preferably, the temperature of drying is about 120-C and the temperature of activation is about SS0 ~ C;
(c) mixing the gallium containing cocatalyst of step (b) with a pentasil type zeolite and Pmbed~ e the solid mixture in an inert carrier (weightbalance). Extrudates are then formed from this solid mixture by adding water to obtain a malleable paste; and (d) drying the extrudates and activation in air at elevated temperature, provides the desired hybrid catalyst of the present invention.

Colloidal silica can be replaced as the gallium oxide support by silica, such as quartz or silica geL ~IIlminq or chromium oxide particles. The method ofpreparation of the hybrid catalyst then comp"~es;
pre~.ating a gallium salt solution onto the silica, alumina or chromium oxide particles;
- drying and activating the resulting solid at an elevated temperature;

- mixing the obtained gallium o%ide cocatalyst with a pentasil type zeolite having the structure of ZSM-S or ZSM-II, and c~..~d~;ng the solid mixture in an inert carrier; and ~ activating the res~ltin~ hybrid catalyst at an elevated temperature.

PREPAR~TION OF 7.FO~.ITF.

The ZSM-S zeolite was synthec;7ed ac~rd,ng to U.S. Patent 3,702,886 and the ZSM-11 ao~ord-ng to US 3,709,979. The co...pos;tion of the synthesis gel10 and the e.~cl"..ental parameters were selected so that the resulting zeo!ites had a Si/AI atomic ratio ranging from about 25 to about S0.

C~TALYTIC TESTI~ G

The catalyst was loaded in the form of e~trudates in a tubular quarlz reactor and heated by a digitally controlled ele~l.ical furnace. A chromel-alumdthe,l..ocolJple was placed in the catalyst ~oed and was used in conjunction with a digital thermo~eter unit to m- nitor the temperature of the catalyst bed. n-butane (paraffin used as feed) and nitrogen, used as carrier gas were ,-.~p!i~4 from 20 cylinders. The control of flow-rates was done through automqt~r devices. Thc gaseous mi~ture flowing out of the reactor ran through a series of condercPrs m~int~ined at S - 10-C, to a liquid collector i,.. ersed in an ice bath follo.. ~l by a dynamic gas sampling bulb from which gas ~a1nr!irlg was carried out. During the reaction, while the liquids were being collerte~, the gases were analyzed periodically 25 by gas chromatography using a S m long column packed with Chromosorb P coate4with 20 % by weight of squalane connected in series with a 2.S m long column TM
packed with Carbopack C graphite modified with picric acid (0.19% by weight). The GC used was a dual FID Hewlett-Packard Model 5790 equipped with a 3392A
Model integrator. It was also equippet w;th a capillary column (length S0 m; PONA
30 type fused silica coated with a crosslinked polymer) which was used for accurate analysis of the liquid &actions after a completed run.

., -s 20~7~7~

The reactor parameters were as follows: temperature ~ 540-C, W.H.S.V. (weight hourly space ~el~;ly or g of injected paraffin per hour and pergram of catalyst) = O.S h-1, flow rate of nitrogen = 10 ml/min, weight of catalyst =
4 g and duration of a run = 4 h.
s The total conversation of n-butane is defined as follows:
(NC)F- (NC)P
Ct (C atom %) = - % 100 (NC)F
10 wherein (NC)F and (NC)P are the numbers of carbon atoms of n-butane in the feed and in the reactor outstream, respectively.

The yield of aromatics is defined as follows:
(NC)A,~5 YAr (C atom %) = x 100 (NC)F
wherein (NC)A, is the number of carbon atoms of aromatic products.

The following examples are provided to illustrate the present 20 invention rather than limiting its scope.

EXAMPLE I

The ZSM-5 zeolite was converted into the acid form (H-ZSM-5) by ion-P~r~h~nee with a S wt % ammonium chloride solution followed by drying at 120 ~ C for 10 h and activating by calcination in air at 550 ~ C for 10 h. The res~ltinp H-ZSM-5 zeolite was characterized by various techniques i.e. X-ray powder diffraction, atomic absorption, S~nning electron miclosco~y, BET surface area measurements, and exhibited the following physico-chemical characteristics:

Si/Al atomic ratio = 36, degree of crystallinity = 100%, BET surface area = 413 m2/g, Na20 = less than 0.2 wt% and average particle size = 3 ,um.

20~7979 Plocec1;~g in the same manner, the ZSM-11 zeolite was converted to its acid form (H-ZSM-11).

To prepare the final catalyst, H-ZSM-5 or H-ZSM-11 (80 wt%) and 5 bentonite (20 wt%) were mixed. Water was added dru~..;se to the mixture until a malleable paste was obtained. The latter was extruded into 1 mm O.D. ~spaghettis~.
The final extrudates were dried at 120 ~ C for 10 h and activated in air at 550 ~ C for 10 h, giving the desired H-ZSM-5 or H-ZSM-11 catalysts.

The H-ZSM-5 catalyst was tested in the reactor and the catalytic results are reported in Table 1.

EXA~PLE2 A gallium bearing catalyst was prepared in a similar manner to that of the Cydar process. Briefly, gallium was incorporated to the H-ZSM-5 zeolite powder by refluxing for 24 h a 0.06 M solution of gallium nitrate (5.6 ml per gram of zeolite). The resulting solid was then recovered by evaporating the gallium nitrate solution and finally activated in air for 10 h at 550 ~ C. The gallium oxide content was 3.0 weight %.

The final extrudates were prepal ed by using the method previously described in Example 1 (weight % of bentonite: 20). The catalytic results obtained with this H-ZSM-5 (Cy) sample are reported in Table 1.

EX~PLE3 GALLnnM OXlDE COLLOnDALSlUCA COCATALYST

2.3 g of gallium nitrate (13 H20) were dissolved in 5 ml of water.
lllis solution was added to 4.5 g of LudoxT~ (AS-40) colloidal silica m~mlf~ctllred and sold by DuPont Corp., and the resulting mixture was stirred for a few minutes Then the solution was gently evaporated to dryness on a hot plate. The resulting 7 2~s7~7~

solid was further dried at 120-C for 10 h and activated in air at 550- C for 10 h.
This cocatalyst will be referred to as Ga/LuSi.

GALLIUM OXIDE/Cr203 COCATALYST

1.6 of gallium nitrate were dissolved in 4.0 ml of water. This solution was added under gentle stirring to 1.3 g of Cr2O3 powder. The suspension was allowed to stand overnight at room temperature. The resulting wet solid was dried at 120 ~ C for 10 h and activated in air at 550 ~ C for 10 h. This cocatalyst will be referred to as Ga/Cr.

EXAMPLE S
GALLIUM OXIDE - SILICA GEL, GALLIUM OXIDE-OUARTZ, AND GALLIUM
OXIDE - ALUMINA COCATALYSTS

Ga was incorporated onto the surface of silica or alumina in the same manner as in Example 4, except that the silica and the alumina replaced the chromium oxide. The silicas used in the preparation of the present cocatalysts were:
silica gel manufactured and sold by Grace, having a BET surface area = 580 m2/g,and crushed quartz manufactured and sold by Fisher, with a surface area = 0.4 m2/g. The aluminas used were: activated aluminas acid and basic powders, manufactured and sold by Strem Chem., with a surface area of 146 and 171 m2/g respectively, and neutral activated alumina, 70 - 230 mesh, manufactured and sold by Merck, with a surface area = 133 m2/~

These cocatalysts will be referred to as Ga/SiGel (Silica gel), Ga/QU
(Quartz), Ga/AlAc (acidic alumina), Ga/AlBa (basic alumina) and Ga/AlNe (neutral alumina) respectively.

2~ 1~7~7 The H-ZSM-5 zeolite powder prepared in F~mple 1 (80 weight %) and the Ga/LuSi cocatalyst (16 weight %) prepared in EYample 3 were mixed at 5 room temperature. Then, the solid mixture was extruded with bentonite (4 weight %) in the presencc of water, dried and activated at high temperature as described in Example 1. The final hybrid catalyst which is referred to as H-ZSM-5tGa/LuSi contains 3 wt% of gaDium oxide, and its catalytic prope, lies are reported in Table 1.
By su~ostituting the H-ZSM-5 with H-ZSM-11, the H-ZSM-11/Ga/LuSi catalyst was also obtained.

The H-ZSM-5 zeolite powder prepared in EYample 1 (80 weight %) and the Ga/Cr cocatalyst (16 weight ~o) prepared in FY~mp'e 4 were miYed at roomtemperature. Then, the solid mixture was extruded with bentonite (4 weight %), in ~ 20 the presence of water, dried and activated at high temperature as previously described in EY~ample 1. The final hybrid catalyst which is referred to as H-ZSM-5/Ga/Cr, is treated at 540-C for 2 h under hydrogen atmosphere before the catalytic testing. The catalytic properties of such a hybrid catalyst, which contains 3 wt% of gaDium oxide, are also reported in Table 1.
By su~stituting the H-ZSM-5 with H-ZSM-11, the H-ZSM-1 1/Ga/Cr catalyst was also obtained.

The H-ZSM-S zeolite powder prepared in Example 1 (80 weight %) and the Ga/SiGel cocatalyst (16 weight %) prepared in Example 5 were mixed at room temperature. The solid mixture was then extruded with bentonite (4 weight 35 %) in the prcsellce of water, dried and activated at high temperature as described 2~7~9 .9.

previously in aample 1. The final hybrid catalyst which is referred to as H-ZSM-5/Ga/SiGel contains 3 wt% of gallium o ide, and its catalytic prope.lies are reported in Table 1.

S By ~lb~l;luti~e the H-ZSM-5 with H-ZSM-11, an H-ZSM-11/Ga/SiGel catalyst was obtained.

The H-ZSM-5 zeolite powder prepared in Example 1 (80 weight %) and the GA/SiQU (quartz particle size = 90 microns) cocatalyst (16 weight %) prepared in EYample 5 were mixed at room temperature. The solid mixture was eYtruded with be~to~ite (4 weight %) in the presence of water, dried and activated at high temperature as descrl~ed in FY~mple 1. The final hybrid catalyst which is referred to as H-ZSM-5/Ga/SiQU, contains 3 wt% of gallium oxide, and its catalytic properties are reported in Table 1.

By s~Jbi~l;lul;ng the H-ZSM-5 with H-ZSM-11, an H-ZSM-11/Ga/SiQU catalyst was obtained. -The H-ZSM-S zeolite powder prepared in Example 1 (80 weight %) and the Ga/AlAc cocatalyst (16 weight %) prepared in Example S were mixed at room temperature. The solid mixture was extruded with bentonite (4 weight %) in the presence of water, dried and activated at high temperature as described in Example 1. The final hybrid catalyst which is referred to as H-ZSM-5/Ga/AlAc, contains 3 wt% of gallium oxide, and its catalytic propenies are reponed in Table 1.

By suh~ ;n~ the H-ZSM-5 with H-ZSM- 1 1, an H-ZSM-11/Ga/AlAc catalyst was obtained.

~o. 20~7~ ~

The H-ZSM-S zeolite powder prepared in FY~mple 1 (80 weight %) and the Ga/AlBa cocatalyst (16 weight %) prepared in FY~mrle 5 were mixed at 5 room temperature. The solid mixture was extruded with bentonite (4 weight %) in the presence of water, dried and activated at high temperature as descrl~ed in _xample 1. The final hybrid catalyst which is referred to as H-ZSM-5/Ga/AlBa, cont~inC 3 wt% of gallium oYide, and its catalytic properties are reported in Table 1.

By sub~ ing the H-ZSM-5 with H - ZS M - 11 , a n H - ZS M-11/Ga/AlBa catalyst was obtained.

15 EXA~SPLE 12 The H-ZSM-5 zeolite powder prepared in Example 1 (80 weight %) and the Ga/AlNe cocatalyst (16 weight %) prepared in Example S were mixed at room temperature. The solid mixture was extruded with bentonite (4 weight %) in 20 the presence of water, dried and activated at high temperature as described in Example 1. The final hybrid catalyst which is referred to as H-ZSM-5/Ga/AlNe, contains 3 wt% of gallium oxide, and its catalytic properties are reported in Table 1.

By substituting the H-ZSM-5 with H-ZSM- 11, an H-ZSM-11/Ga/AlNe catalyst was obtained.

2~ 1~ 7 ~ ~

TABLE 1 ~romatizatlon acth~lb of the rererenoe catalysts and of the hybrid catalysts or the present In~entlon which contaln , rr led galllum oxlde ~s cocatalyst.
s Example number Catalyst Total co"~_.s;on Aromatic Yield, of n-butane, Ct YAr (C atom %) (C atom %) 1 H-ZSM-5 81.4 16.6 2 H-ZSM-5 (Cy) 823 40.5 6 H-ZSM-S/Ga/LuSi 98.7 643 7 H-ZSM-5/Ga/Cr 993 68.1 8 H-ZSM-5/Ga/SiGel %9 51.6 9 H-ZSM-5/Ga/SiQU %.9 50.1 H-ZSM-5/Ga/AlAc 983 45.6 11 H-ZSM-5/Ga/AlBa 98.4 43.7 12 H-ZSM-5/Ga/AlNe 98.1 42.9 In these experiments, the analysis of the mixture of aromatic products obtained by the use of the catalysts of the present invention revealed a BTX
aromatics content as follows:
benzene 35 - 38 %;
toluene 38 - 42 %;
xylenes 9 - 13 %;
ethylbenzene :~ 1 %;
styrene ~ 0.2 %; and the rern~ining being aromatics of 9 to 11 carbon atoms.

2 ~ 7 9 EX~IPLE 13 Ludox~ colloidal silica was evaporated to d~mess on a hot plate. The resulting solid was dried at 120 ~ C for 10 h and activated at 550 ~ C for 10 h. Such powder particles (13 weight %) and the H-ZSM-5 zeolite powder prepared in Example 1 (80 weight %) were mixed at room temperature. The solid mixture was extruded with bentoni~e (7 weight %) in the presence of water, dried and activated at high temperature as de~l~d in Example 1. The final hybrid catalyst which is referred to as H-ZSM-5/LuSi, showed the catalytic properties reported in Table 2.
A similar H-ZSM-1 1/LuSi catalyst was also obtained by replacing H-ZSM-5 by H-ZSM-ll.

In these examples, particles of the following~inert" oxide materials are used as cocata~ysts: quartz (particle size = 90 microns), silica geL acidic alumina, basic alumina and neutral ~ nin~ respectively.
The H-ZSM-5 zeolite powder prepared in Example 1 (80 weight %) and the inert oxide particles (13 weight %) were mixed at room temperature. The solid mixture was then extruded with bentonite (7 weight %) in the presence of water, dried and activated at high temperature as described in Example 1. The final hybrid catalysts which are respectively referred to as H-ZSM-5/SiQU, H-ZSM-S/SiGeL H-ZSM-5/AlAc, H-ZSM-5/AlBa and H-ZSM-5/AlNe, showed the catalytic properties reported in Table 2.

Similar H-ZSM-11/SiQU, H-ZSM-ll/SiGeL H-ZSM-11/AlAc, H-ZSM-11/AlBa and H-ZSM-11/AlNe were also obtained by replacing the H-ZSM-5 by H-ZSM-11.

~ Q ~ 7 ~ rt 9 TABLE 2 Aromatization aath~lty of the rererenoe catalysts and the hybrid catalysts of thls In~ention which contain partlcles of "inert" oxide materials s Example number Catalyst Total co~ ;on Aromatic Yield, of n-butane, Ct YA, (C atom %) (C atom %) 1 H-ZSM-5 81.4 16.6 2 H-ZSM-5 (Cy) 82.3 40.5 13 H-ZSM-5/LuSi 90.5 37.7 14 H-ZSM-5/SiQU 96.7 28.6 H-ZSM-5/SiGel 94.7 31.5 16 H-ZSM-5/AlAc 97.3 32.6 17 H-ZSM-5/AlBa 97.7 31.0 18 H-ZSM-5/AlNe 97.7 33.4 20 DISCUSSION OF THE CATALYT~C RESULTS

All the hybrid catalysts prepared according to the procedure of the invention and containing supported gallium oxide, display yields of aromatics higher than the catalyst prepared according to the cl~scic~l concept of bifunctional catalysis, 25 such as Cyclar-type catalysts (H-ZSM-5 (Cy)). Although the aromatization promoting effect of the gallium species is essentiaL the support itself plays also a very important role. Since it is assumed that the active Ga species have an oxidation degree lower than +3 and are formed during the reaction itself, the dispersion of gallium oxide on the support surface is believed to be the key factor. This is the 30 reason why the Ga/LuSi cocatalyst obtained by coevaporation of a mixture of gallium salt and colioidal silica is among the most preferred cocatalysts of theinvention: it presents the highest dispersion of Ga oxide and consequently, the in situ conversion of Ga species into its active form is very rapid, the co,.velsion being completed after a few minutes of reaction. In the case of the Ga/Cr cocatalyst, a 35 reduction operation is necessary prior to the catalytic testing probably because of the -14- 2~7~

oxidizing nature of Cr203 surface, which pr~,ients the gallium oxide to reach rapidly its most active form.

The inert carrier plays also an important role. It helps the hybrid 5 catalysts to m~in~in the configuration which is most suitable for maximum interactions between the zeolite component and the cocatalyst, thus providing m~imlln aromatization activity.

The importance of the contact surface between the zeolite and the 10 cocatalyst particles is demonstrated by the following series of ~_~ycrilllents. Particles of pure ~-quartz were used as cocatalysts. The amount and the size of the quartzpartides within the hybrid catalysts were changed and the results are reported in Table 3. As it can be seen therein, even in the absçnce of gallium, higher conversion of n-butane and higher production of aromatics and molecular hydrogen are lS obtained with pure quartz used as cocatalyst. Such hybrid catalysts exhibit higher production of aromatics and hyJIo~n when higher amounts of quartz are embedded within the hybrid catalyst and when the particle size of quartz is smaller.

The hybrid catalyst, H-ZSM-5/LuSi which colllylLses particles 20 obtained by the evaporation of the Ludox colloidal silica exhibits an aromatization activity very close to that of the Cyclar-type catalyst, H-ZSM-5 (Cy) (see Table 2).

2Q~7~7~
- lS-TABLE 3 Influence of the amount and size of the quartz pd~ltc!.es on the aromatization activity of the hybrid catalyst (H-ZSM-5/Quartz).
Si/~l atomic ratio of the zeolite = 30.

Quartz cocatalyst Quartz cocatalyst Aromatic yield Hydrogen yield ~ wt % (1) Particle size inYAr (C atom %) (2) microns O - 19.1 0.059 2 90 19.6 O.OSS

4 90 21.6 0.064 7 90 25.7 0.077 29.3 0.083 13 90 32.8 0.101 13 165 29.5 0.083 13 375 29.4 0.079 13 675 29.1 0.079 (1) zeolite component = 80 wt %, quartz = "x" wt % and bentonite =
25 (20 - x) wt %
(2) mole of hydrogen produced per C atom of n-butane fed Although it is premature to propose a theoretical explanation to these 30 phenomena, it is clear that hybrid catalysts with supported gallium oxide used as cocatalyst are, surprisingly, more active and selective in the aromatization of light paraffins and olefins than gallium catalysts prepared according to the known procedures.

Claims (18)

1. A hybrid catalyst for the aromatization of paraffins and olefins, comprising a mixture of a pentasil type zeolite having the structure of ZSM-5 orZSM-11 and a cocatalyst consisting of a gallium oxide supported by an oxide selected from the group consisting of silica, alumina, and chromium oxide.

2. A hybrid catalyst according to claim 1, wherein said zeolite has the structure of ZSM-5.

3. A hybrid catalyst according to claim 1, wherein the silica is selected from the group consisting of silica gel, colloidal silica and quartz.

4. A hybrid catalyst according to claim 1, further comprising an inert carrier.

5. A hybrid catalyst according to claim 1, wherein the pentasil type zeolite Si/Al atomic ratio is from about 25 to about 50.

6. A hybrid catalyst according to claim 1, wherein the weight ratio of the zeolite to the gallium cocatalyst is from about 4.7 to about 26.7.

7. A hybrid catalyst according to claim 4, wherein the weight percentage of the carrier in the catalyst is from about 3 to about 20, based on the total weight of the catalyst.

8. A hybrid catalyst according to claim 1, wherein the zeolite component is in the acid form.

9. A hybrid catalyst according to claim 1, wherein the content of gallium oxide is from about 0.5 to about 7 %, based on the weight of the final catalyst.

10. A hybrid catalyst according to claim 4, wherein the inert carrier is bentonite.

11. A method for the preparation of a hybrid catalyst according to claim 1, comprising:
- evaporating a mixture of a gallium salt and a colloidal silica;
- separating, drying and activating the resulting solid at an elevated temperature;
- mixing the obtained gallium oxide - silica cocatalyst with a pentasil type zeolite having the structure of ZSM-5 or ZSM-11, and embedding the solid mixture in an inert carrier; and - activating the resulting hybrid catalyst at an elevated temperature.

12. A method for the preparation of a hybrid catalyst according to claim 1, comprising:
- impregnating a gallium salt solution onto silica, alumina or chromium oxide support;
- drying and activating the resulting solid at an elevated temperature;
- mixing the obtained gallium oxide cocatalyst with a pentasil type zeolite having the structure of ZSM-5 or ZSM-11, and embedding the solid mixturein an inert carrier; and - activating the resulting hybrid catalyst at an elevated temperature.

13. A method according to claim 12, wherein the silica support is silica gel.

14. A method according to claim 12, wherein the silica support is quartz.

15. A method according to claim 12, wherein the alumina support is an activated alumina.

16. A method according to claim 12, wherein the alumina support has a neutral, acidic or basic surface.

17. A method according to claim 12, wherein the chromium oxide is Cr2O3.

18. A method according to claim 17, wherein the hybrid catalyst is activated under hydrogen atmosphere at an elevated temperature prior to the catalytic operation.