Applied Catalysis A: General, 86 (1992) 127-138 Elsevier Science Publishers B.V., Amsterdam

127

APCAT A2290

Gallium containing hybrid catalysts for the aromatization of n-butane*

Raymond Le Van Mao and Jianhua Yao

Catalysis Research Laboratory and Laboratories for Inorganic Materials, Department of Chemistry&Biochemistry, Concordia University, 1455 De Maisonneuve Blvd. W., Montreal (Quebec) H3G 1 M8 (Canada)

and

Riccardo Carli

Department of Physical Chemistry, University of Milan, Via Golgi 19,20133 Milan (Italy)

(Received 28 January 1992, revised manuscript received 22 April 1992)

Abstract

The enhancement of the aromatization activity of the gallium containing hybrid catalysts may partly be ascribed to the formation, upon reduction, of gallium species having a lower oxidation state and their subsequent transfer/redistribution within the catalyst mass. Within the hybrid catalysts which contains the cocatalyst prepared by evaporation of a gallium salt/colloidal silica mixture, some transfer of gallium onto the external surface of the zeolite particles effectively occurs. However, the high aromatization performance of such a hybrid catalyst results mainly from the physical presence of the cocatalyst which exhibits strong hydrogen spillover effects.

Keywords: butane aromatization, gallium, hydrogen spillover, zeolites, ZSM-5.

INTRODUCTION

The Cyclar process developed jointly by BP and UOP is now commercially available for the conversion of propane and butanes to aromatics [ 11. The catalyst used is a gallium ZSM-5 zeolite which is prepared according to the classical bifunctional catalysis [ 2-41. In our laboratory, it was observed that metal oxide species (ZnO or gallium oxide) did not require to be adjacent to

Correspondence to: Prof. R. Le Van Mao, Catalysis Research Laboratory and Laboratories for

Inorganic Materials, Department of Chemistry and Biochemistry, Concordia University, 1455 De

Maisonneuve Blvd. W, Montreal, Que. H3G lM8, Canada. Tel. ( + l-514) 8483343, fax. (+ l-

514) 8483494. *Presented in part at the ACS Symposium, Petroleum Chemistry Division, New York City, 25- 30 August 1991.

0926-3373/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

128 R. Le Van Mao et al. /Appl. Catal. A 86 (1992) 127-138

the ZSM-5 zeolite acid sites to be efficient as aromatization cocatalysts. In- deed, highly active and selective hybrid catalysts can be prepared by mechan- ical mixing of the two components (zeolite and metal oxide containing coca- talyst) followed by embedding in an inert clay matrix [5-131. The hydrogen transfer concept, also called hydrogen back (or reverse ) spillover (HBS ) , has been formulated. This concept has been strongly supported by recent experi- mental results [ 11,12,14,15]. In fact, it was shown that even pure a-quartz used as cocatalyst could enhance the aromatization activity of the ZSM-5 zeo- lite. The two compounds whose production was significantly increased by the presence of the quartz [ 11,12,14] or other irreducible oxide particles [ 11,121 within the hybrid catalyst, were aromatics and hydrogen. A clear relationship was established: the aromatic yield increased with increasing amount of quartz or with decreasing size of the quartz particles [ 141.

Recently, gallium species with a degree of oxidation lower than 3 + and ob- tained by reduction with hydrogen, have been identified [ 16-181. It was thus suggested that these species, which could be Ga’, were the active species for the aromatization of light alkanes. On the other hand, GazO and similar species are known to be relatively mobile [ 191.

Thanks to the clear separation between the zeolite acid sites and the metal oxide surface in our hybrid catalysts, results of the investigations on the effects of sequential reduction/oxidation steps on the catalyst performance may allow the identification of the role played by the reduced form of gallium oxide species.

Another objective of this work is to assess the influence of the physical pres- ence of the cocatalyst in the promotion of the aromatization activity, in accord with the concept of hydrogen back spillover (HBS).

EXPERIMENTAL

Preparation and characterization of the catalysts for redox studies

The parent ZSM-5 zeolite was synthesized according to the known tech- nique of Argauer and Landolt [ 201. The acid (powder) form was obtained by ion exchange with ammonium ions and subsequent activation in air at elevated temperature [ 5-141. Its structure and degree of crystallinity were determined according to the procedures described in previous papers [ 5-141. The Si/Al atomic ratio of the resulting zeolite, also called HZ, as determined by atomic absorption spectroscopy, was 36 and its Na,O content was lower than 0.2 wt.-%.

Hybrid catalysts which comprised a ZSM-5 zeolite component (HZ, powder form, 80 wt.-%) and a cocatalyst (mesh size> 120, 16 wt.-% ), were obtained by mixing mechanically these two powders and then extruding with bentonite clay (4 wt.-%) in the presence of water as described in refs. 5-9, which also reported the drying and activation procedure of the catalyst extrudates.

R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138 129

1.6

1.2

0.8

0.4

Fig. 1. Pore size disiribution of the (Ga+LuSi) cocatalyst. F= dV/dlog (D) in cm3/g versus D (pore diameter) in A.

Two hybrid catalysts were prepared for the reduction and reoxidation (Re- dox) studies.

(a) Ga,O, was obtained by calcining gallium nitrate (13 Hz0 from &rem Chem.) overnight at 550°C. The hybrid catalyst prepared according to the recipe described in the previous section, contained 5 wt.-% of GasO, and was called HZ// (Ga ox). This hybrid catalyst was similar in conception to that used by Price and Kanizarev [ 161.

(b ) 2.34 g of gallium nitrate were dissolved in 5 ml of distilled water. 4.5 g of Ludox colloidal silica (DuPont, AS-40, 40 wt.-% in silica) were added to the previous solution. The mixture was stirred thoroughly for a few minutes and evaporated to dryness on a hot plate. The resulting solid was dried overnight at 120” C and activated in air at 550” C overnight. The final hybrid catalyst prepared using this solid as cocatalyst, [also called (Ga + LuSi) 1, contained 3 wt.-% of Ga,O, and was thereafter called HZ// (Ga$ LuSi).

The textural properties of the (Ga + LuSi) cocatalyst were determined by adsorption/desorption of nitrogen (Micromeretics ASAP 2000 apparatus). Such a solid had a surface area of ca. 100 m2/g. Its pores were all concentrated in the mesopore region and the pore size distribution, determined by the BJH desorption method [ 211, exhibited a sharp maximum at ca. 20 nm or 200 A (Fig. 1).

Reduction and reoxidation procedures In a series of tests, these three catalysts (final forms) were submitted to a

reduction in hydrogen (flow-rate = 30 ml/min) for two hours at 540’ C in the same system as reported in the following section. Reoxidation consisted of heating in air (flow-rate= 10 ml/min) the reduced catalysts at 550°C for 12 h.

The fresh, reduced and reoxidized catalysts were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. For that purpose,

130 R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138

the catalyst extrudates were gently crushed in order to be suitable for loading on the sample holders.

A Jeol JSM-820 scanning electron microscope was used to measure the av- erage particle size of the zeolite and cocatalyst aggregates. This instrument was also equipped with an energy dispersive X-ray (EDAX) analysis micro- probe (volume of the “surface” portion irradiated = 1 ,um3 ) which allowed the determination of the Ga/Si ratio (arbitrary units related to the EDAX signals) on the surface of the (Ga+ LuSi) cocatalyst aggregates at various stages of the treatment.

The contamination of the external surface of the zeolite particles within the hybrid catalysts, HZ// (G a ox) and HZ// (Ga + LuSi), was studied by using a X-ray photoelectron ESCALABB MS11 (VG Scientific) instrument. The scan- ning was 3 x 4. 10-2cm2 area. This allowed the determination of the surface Ga/Si atom ratio of the hybrid catalysts at various stages of the treatment.

The thermal gravimetric analysis (TGA) on the cocatalysts was carried out using a PL Thermal Sciences STA 1500 instrument. Hydrogen was used as carrier gas and reducing medium.

Preparation and characterization of the Ga contaminated zeolite catalyst

In order to assess the influence of the gallium species which are transferred to the zeolite particle upon reduction of the HZ// (Ga+ LuSi), a gallium con- taminated zeolite sample was prepared according to the following procedure. (Ga+LuSi) cocatalyst particles with very well-defined particle size (particle diameter between 500 and 250. 10m6 m, hereafter called CO-46) were prepared. 0.98 g of CO-46 and 4.77 g of HZ zeolite powder were intimately mixed. The solid mixture was heated in a quartz reactor under hydrogen (flow rate: 17.5 ml/min) at 540°C for 3 h. The contaminated HZ zeolite particles were re- covered from the solid mixture by sieving by means of a sieve having a mesh size of 106*10-6 m. These particles contained less than 0.2 wt.-% of GazO, (as determined by atomic absorption), corresponding to less than 5% of the total amount of gallium incorporated into the cocatalyst. The final catalyst, ob- tained by extrusion with bentonite, is called HZ*.

Catalytic testing

The experimental set-up for the catalytic testing was identical to that de- scribed in refs. 5-9. The reaction parameters were as follows: tempera- ture = 540’ C, WHSV (weight hourly space velocity or g of injected n-butane per hour and per g of catalyst) = 0.5 h-‘, flow-rate of nitrogen (used as carrier gas) = 10 ml/min, weight of catalyst=4 g and duration of a run= 3.5 h. The quartz reactor had the following dimensions: I.D. = 2.0 cm, O.D. = 2.4 cm, length of the catalyst bed= 2.5 cm (diameter of the catalyst extrudates = 0.1 cm).

R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138 131

The procedures for catalyst testing and analysis of products were very sim- ilar to those described in refs. 9-12.

The total conversion of n-butane, Ct, is calculated as follows: C,(C atom- %)=lOO*[(NC),-(NC),]/(NC)rwhere (NC),and (NC),are thenumbers of carbon atoms of n-butane in the feed and in the reactor outstream, respectively.

The yield in product hydrocarbons is defined as follows: Yi(C atom- %) = [ (NC)i/(NC)f] -100, where (NC)i is the number of carbon atoms of hy- drocarbon (i) in the reactor outstream.

The production of molecular hydrogen is determined as the volume percent of product hydrogen in the outlet stream (V,) or as mole of product hydrogen per C atom of n-butane converted (yield, Yn) .

RESULTS AND DISCUSSION

Gallium transfer within the HZ//(Ga ox) sample

Fig. 2 shows the aromatic yields, YAr, obtained with the HZ// (Ga ox) hybrid catalyst. Catalytic runs were performed on the fresh catalysts, the same after reduction with hydrogen and then after reoxidation in air. The HZ// (Ga ox) hybrid catalyst experienced a sharp increase in aromatic yield upon reduction in hydrogen atmosphere. However, if a lower degree of oxidation of the gallium species were the main cause of such an enhancement as assumed previously [ 16-181, the subsequent reoxidation would decrease the aromatic yield of such catalysts. This was not verified in our experiments which showed, instead, a slight increase of YAr upon reoxidation. XPS investigation on the HZ// (Ga ox) did show a significant increase in the (Ga/Si) surface atom ratio upon reduction (Table 1) . It is worth noting that the silicon atoms involved in this case were those of the external surface of the zeolite particles. Therefore, an increase in such a (Ga/Si) ratio for the HZ// (Ga ox) sample indicates that

Y Ar A

B

Fresh Catalyst

Reduced Catalyst

Reoxidized Catalyst

Fig. 2. Aromatic yields obtained with the HZ// (Ga+ LuSi) [A] and HZ// (Ga ox) [B] catalysts.

132

TABLE 1

R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138

Ga/Si surface atom ratio of the hybrid catalysts as determined by XPS technique

Catalyst

HZ// (Ga ox)

Catalyst form Ga/Si

Fresh 6.9*10-3 Reduced 1.0~10-2 Reoxidized 1.3.10-Z

HZ//(Ga+LuSi) Fresh 2.2*10-3 Reduced 1.7.10-3 &oxidized 2.0.10-3

some gallium had moved from the gallium oxide particles to the external sur- face of the zeolite particle during the reduction phase with hydrogen. This resulted in a larger number of exposed gallium atoms. This gallium migration was confirmed by the SEM analysis which showed that upon such a reduction, the gallium oxide particles underwent dramatic decrease in size: the average value decreased from 70 pm down to 20 pm. However, the subsequent step of oxidation did not significantly change the size of the gallium oxide particles. This indicates that gallium transfer did not occur to the same extent as during the previous reduction phase. Thus, the increase in the Ga/Si ratio obtained by reoxidation (Table 1) was to be ascribed to the redistribution of the gallium species on the external surface of the zeolite particles rather than to an addi- tional and important transfer of gallium. Therefore, HZ// (Ga ox) catalyst was an evolutive system in terms of aromatization performance. The continuous evolution of such a system was mainly due to the mobility of gallium species. The HZ// (Ga ox) sample was similar to the catalyst studied by Price and Kanizarev [ 161 and Joly et al. [ 221.

Gallium transfer within the HZ//(Ga + L&i) sample

SEM analysis on the HZ// (Ga+LuSi) sample shows that there was no change in the size of the (Ga + LuSi) cocatalyst aggregates submitted to such reduction and oxidation operations (average size = 70 ,um). On the other hand, XPS results (Table 1) indicate that there was practically no change of the Ga/ Si ratio value for the HZ// (Ga+LuSi) hybrid catalyst upon reduction and reoxidation. However, the EDAX microprobe focussed on the aggregates showed a significant decrease in the surface Ga/Si atom ratio upon reduction (Table 2) whereas there was practically no change of such a ratio in the reoxidation step. This means that during the reduction operation, some gallium species [less than 5%) see Experimental section) ] migrated, in the atom or small clus-

R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138 133

TABLE 2

Gallium concentration on the surface of the cocatalyst aggregates within the HZ// (Ga + L&i) hybrid catalyst as determined by the EDAX technique (see Experimental section)

Catalyst form EDAX Ga/Si intensity ratio (arbitrary units)

Fresh Reduced Reoxidized

0.031 0.011 0.014

ter form, from the surface of the aggregates to the external surface of the zeolite particles. The XPS data reflected the fact that the numbers of uncovered sili- con atoms of the aggregates and the covered silicon atoms of the zeolite parti- cles were equivalent. This migration also explains the decrease of the Ga/Si ratio on the aggregate surface, as determined by EDAX.

Catalytic performance of the (fresh) HZ//(Ga + LuSi) hybrid catalyst

It is important to find out why the fresh HZ// (Ga+ LuSi) catalyst has the same performance as its reduced form. In our opinion, this may stem from the following.

(i ) the reduction of gallium species of the (Ga + LuSi) cocatalyst was faster than that of the gallium oxide as shown by the TGA results (Fig. 3). The weight loss upon reduction at high temperature, which occurred already at 540°C for both cocatalysts, was due to the volatility of the reduced species, probably Ga’. This easiness to undergo reduction might be attributed to the homogeneous mesopore texture of the (Ga+ LuSi) aggregates and the Ga (high) dispersion on the cocatalyst surface.

(ii) the catalytic run of the fresh catalyst lasted 3.5 h. As shown in Fig. 4, less than one hour on-stream was needed for the hydrogen, produced by the reaction itself, to reduce the gallium species to a more active form. Fig. 4 shows that the pre-reduced form of the HZ// (Ga + LuSi) reached the maximum aro- matization activity instantaneously. Such a small difference in the aromatic yield (calculated on the entire 3.5-h run) between these two forms was not detectable. In any event, the fresh hybrid catalyst exhibited already more than 80% of the aromatizing activity (much higher than that of the parent zeolite) during the very first minutes of the reaction (Fig. 4). In the case of the HZ// (Ga ox) catalyst, more than 12 h “on-stream” were necessary to bring the aromatization activity of the fresh form to the activity level of the reduced form.

Table 3 reports the n-butane conversion and the product selectivities of the HZ// (Ga ox) and the HZ// (Ga+LuSi) samples in the fresh, reduced and reoxidized forms, respectively. This shows the evolution, during different re-

134 R. L,e Van Mao et al. /Appl. Catal. A 86 (1992) 127-138

14.5

14.4

14.2

14.0

13.8

13.5

is.4

13.2

- 11.0

- 10.5

- 10.0

-w w - B*5 - 9.0

- 0.5

- 5.0

I- 7.5 I4 1 I I I I

100 200 300 400 500 I

500 I 700 800 &a i&o loo

Fig. 3. TGA spectra of the (Ga ox) [A] and the (Ga+LuSi) [B] cocatalysts. Weight loss, W, expressed in mg versus temperature, Z’, in ’ C.

40.

vii 30.

J

0 20 40 60 60 100 120 140

Fig. 4. “On-stream” reduction of the HZ// (Ga+LuSi) hybrid catalyst. Production of hydrogen, V, (volume-%), versus reaction time, t (in min): without (A) and with hydrogen pretreatment (B).

dox treatments, of the performance of the HZ// (Ga ox) catalyst towards that of the HZ// (Ga + LuSi).

(iii) The cocatalyst plays the key role in determining the aromatization ac- tivity, irrespectively to the oxidation state or location of the gallium species. In fact, in both cases (reduced and reoxidized forms), the amount of gallium species transferred to the zeolite particles was very small (much below the detection limit of the EDAX technique which is 0.5 wt.-% ). The HZ//

R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138

TABLE 3

135

Catalytic results obtained with catalysts used for redox studies

Catalyst C, Hydrocarbon yields Hydrogen yield

YIi Y1-3 yi4 y4= yn, y.4r

HZ//(Ga ox) (i) fresh (ii) reduced (iii) oxidized

HZ//(Ga + LuSi) (i) fresh (ii) reduced (ii) reoxidized

85.0 87.0 92.9

99.7 99.8 99.4

50.4 2.2 2.6 1.7 28.1 0.144 37.4 1.3 1.1 1.5 45.7 0.261 37.3 0.8 1.0 1.2 52.6 0.298

31.5 0.1 0.6 0.2 67.3 0.338 31.8 0.2 0.2 0.2 67.4 0.359 31.4 0.3 0.3 0.2 67.2 0.361

YIm3: yield in C1-Cs hydrocarbons; Y,,: in isobutane; Y4=: in butenes; Y,,: in liquid aliphatics; YAr: in aromatics. YaTX/ YAr = 0.9 (BTX: benzene, toluene, xylenes, ethylbenzene ) .

TABLE 4

Catalytic performance of different catalysts studied

Catalyst C, Aromatic yield ( YAr) Yn

HZ 95.7 30.8 0.085 HZ* 96.0 45.4 0.147 HZ/Ga(R,E) 87.8 48.6 n.a.

HZ/Ga(D) (2.5wt.-%) 83.3 37.0 n.a.

HZ/Ga(D) (5 wt.-%) 91.3 50.3 n.a. HZ//(Ga+LuSi) 99.7 67.3 0.338

na. = not available

(Ga+LuSi) hybrid catalyst was much more efficient than the HZ// (Ga ox) sample (Fig. 2)) although its Ga/Si ratio as determined by XPS was much lower.

Table 4 reports the catalytic results obtained with the HZ, the HZ* and the HZ// (Ga + LuSi) samples. It is clear that the transfer of gallium species onto the HZ zeolite particles increased the aromatization activity. However, the HZ// (Ga + LuSi) catalyst was much more active than the HZ*. Therefore, the enhancement of the aromatization activity of our hybrid catalyst with respect to the parent zeolite, is also due to the physical presence of the cocatalyst. The latter statement is supported by our recent results [ 11,12,14], and also by re- sults from other research groups [l&23,24]. En passant, Table 4 also reports the catalytic performance of the Cyclar-type and the gallium dry-impregnated

136 R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138

catalysts of ref. 9. The aromatic yields of such catalysts, as in the case of the HZ* sample, are much lower than that of the HZ// (Ga+ L&i) catalyst.

On-stream stability

The presence of the cocatalyst in our HZ// (Ga+LuSi) sample might also be the key factor for the exceptional “on-stream” stability. In fact, recently, Bayense and van Hooff [25] observed a relatively rapid “on-stream” deacti- vation rate of propane aromatization (a few minutes) over ZSM-5 zeolite which was physically mixed with Ga203. This was attributed to an increasing rate of coke formation due to the location of gallium species on the outer surface of the zeolite particles. These results are substantially in agreement with our re- sults obtained with the HZ// (Ga ox) where there were large amounts of Ga,O, particles on the zeolite external surface. However, with the HZ// (Ga + LuSi) hybrid catalyst, the n-butane conversion and the aromatic selectivity were high and did not decrease significantly even after 8 h of reaction (Fig. 5 showing the conversion only). Fig. 5 also shows the conversion of the Cyclar-type cat- alyst, the gallium and zinc dry impregnated catalysts of ref. 9, whose catalytic decay is much faster than the HZ// (Ga+ LuSi) sample. This is also shown in Table 5 which reports the values of first order deactivation rate constants for these catalysts [26,27]. In terms of aromatic yield, there was no significant change for the HZ// (Ga+LuSi) during this series of tests: the value of YAr varied between 67 and 65 C atom-%.

Agreement with the hydrogen spillover concept

In our opinion, the highly dispersed gallium species on the cocatalyst surface (with some actually transferred to the zeolite particles upon reduction) and the cocatalyst body itself were playing the role of hydrogen remover (concept of hydrogen back or reverse spillover [5-151) and there was no evidence of a

Fig. 5. Total n-butane conversion, C!, (C atom-% ) versus time-on-stream, t (in min). (v ) = HZ/ /(Ga+LuSi); (V)=HZ/Ga(R,E); (O)=HZ/Ga (D) and (O)=HZ/Zn (D).

R. Le Van Mao et al. / Appl. Catal. A 86 (1992) 127-138 137

TABLE 5

First order deactivation rate constants for different catalysts studied

Catalyst Metal oxide content (wt.-%) Kd ( 10e4 min-‘)

HZ/Zn(D) 5.0 25.7 HZ/Ga (R,E) 4.4 7.2 HZ/Ga(D) 2.5 9.7 HZ// (Ga+LuSi) 3.0 4.9

direct intervention of gallium on the reaction mechanism, as proposed by Mer- iaudeau et al. [ 281, which could lead to a rapid coke build-up as observed by Bayense and Van Hoff [ 251. The latter phenomenon could be due to the large sized gallium particles which had some polymerization activity, thus resulting in coke formation.

Finally, the transferred gallium atoms (or small clusters) on the external surface of the zeolite particles, actually adjacent to the cocatalyst surface, may have their own promoting effects which are however far below those of the cocatalyst itself.

CONCLUSION

(i) To promote the aromatization activity, Ga203 in the HZ//(Ga ox) cat- alyst needs to be dispersed onto the zeolite particle surface. Reduced form of gallium is not per se the active species. It is only a compulsory passage for having higher gallium mobility and finally, higher dispersion of such species.

(ii) The cocatalyst prepared by evaporation of a mixture of a gallium salt and a colloidal silica provides a hybrid catalyst with high aromatizing perform- ance and good “on-stream” stability. The key role in such catalytic properties is played by the highly dispersed gallium species on the cocatalyst surface, irrespectively to the state of oxidation of the gallium component. Upon reduc- tion, some gallium species may be transferred to the zeolite particles.

ACKNOWLEDGEMENTS

We thank the following granting agencies: NSERC of Canada and Quebec’s Action Structurante Program for their financial support. We also thank Mr. B. Sjiariel of this laboratory, Mrs. Suzie Poulin and the CM2 of the Ecole Po- lytechnique of Montreal for technical assistance.

138 R. Le Van Mao et al. /Appl. Catal. A 86 (1992) 127-138

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