Applied Catalysis A: General 238 (2003) 279–287 Development of a polyfunctional catalyst for benzene production from pyrolysis gasoline Rafig Alibeyli, Ali Karaduman∗ , Hasip Yeniova, Ayten Ateş, Ali Y. Bilgesü Department of Chemical Engineering, Engineering Faculty, Ankara University, Tandoğan, 06100 Ankara, Turkey Received 26 November 2001; received in revised form 17 June 2002; accepted 18 June 2002 Abstract The concurrence of hydrodealkylation, hydrocracking, and hydrodesulfurization reactions necessitates a polyfunctional catalyst in the production of pure benzene from pyrolysis gasoline or other industrial mixtures containing various alkylaromatic and nonaromatic hydrocarbons. A chromium catalyst was developed in this study with comparable performance to the commercial catalyst Pyrotol H-9430 (Houdry, USA). The new catalyst contained 15 wt.% Cr and was doped with KF. When tested utilizing a hydrocleaned BTX fraction of pyrolysis gasoline, it preserved its benzene selectivity and hydrocracking activity for 3000 h, but lost about 10% of its hydrodealkylation activity. The deactivated catalyst completely recovered its original surface properties and hence activity when subjected to oxidative regeneration. The developed catalyst maintained its Cr and KF content in regeneration as it did in drying, calcination, and reaction. The characteristics of the catalyst were examined by BET, XRD, DTA, TGA and electron microscope. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cr/Al2 O3 catalysts; Hydrodealkylation; Hydrocracking; Pyrolysis gasoline; Benzene 1. Introduction Hydrodealkylation of alkylaromatics is a common route in the petrochemical industry. Benzene for example is produced from either pyrolysis gasoline or pure alkylaromatic hydrocarbons, mostly toluene, thermally [1–4] or catalytically [5–7]. As is known, thermal processes require relatively high temperatures (700–725 ◦ C) compared to catalytic ones which usually take place at 600–625 ◦ C and high temperatures increase the heavy aromatic hydrocarbon content such as naphthalene and decrease the benzene selectivity. Moreover, catalytic processes are of course more ∗ Corresponding author. Tel.: +90-312-2126720; fax: +90-312-2232395. E-mail address: akrduman@eng.ankara.edu.tr (A. Karaduman). economic [8] provided a highly selective catalyst can be found. Various catalysts are given in the literature [9–13] for the hydrodealkylation reactions. Most of these lack industrial applications. Commercial catalysts are usually Cr/Al2 O3 catalysts. Their synthesis method are generally undisclosed. This is an impediment to further development. The composition can of course be analyzed if not published. Coke formation is a common problem in catalytic hydroprocessing. As stated by Afanasyev and Buyanev [14], there are two different mechanisms for coking. The first is via dehydrogenation, cyclization, aromatization, and condensation. The second mechanism is decomposition to carbon and light hydrocarbons on the active centers. When the reaction medium is rich in aromatic compounds, coking probably follows the first mechanism. 0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 3 7 4 - 5 280 R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 This study was carried out to develop and characterize a chromium catalyst for the production of pure benzene from industrial products, chiefly from pyrolysis gasoline, containing alkylaromatic hydrocarbons, various nonaromatic hydrocarbons, e.g. alkanes and cycloalkanes and sulfur compounds. Polyfunctional activity was required from the catalyst, i.e. hydrodealkylation of alkylbenzenes, hydrocracking of nonaromatic hydrocarbons and hydrodesulfurization of sulfur compounds. In order to test this activity, two model mixtures were utilized. The main steps of this research were the selection of support, preliminary activity tests to determine an appropriate amount of chromium and its compound type, further support modification to improve conversion and selectivity values, experiments to find a suitable chromium amount for longer operation periods, and performance comparison against a Pyrotol H-9430 catalyst (Houdry, USA). Finally, operation time without deactivation was determined for the developed catalyst utilizing a hydrocleaned BTX fraction of pyrolysis gasoline. in composition to a BTX fraction of pyrolysis gasoline: 39.7 wt.% benzene, 33.3 wt.% toluene, 14.0 wt.% C8 aromatics (ethylbenzene and xylenes), 13.0 wt.% C6–C8 nonaromatic hydrocarbons (alkanes and cycloalkanes) and 0.05 wt.% thiophene. Feed III was a hydrocleaned BTX fraction of pyrolysis gasoline, i.e. 25.4 wt.% benzene, 33.9 wt.% toluene, 7.4 wt.% C8 aromatics, 33.3 wt.% nonaromatic hydrocarbons and 0.04 wt.% total sulfur. The liquid and gas products were analyzed by GC. The characteristics of the catalyst were investigated by BET, DTA, TGA, XRD, electron microscopy. Acid sites of the ␥-Al2 O3 samples were examined using a Gamete Indicator for total acidity and aryl methanol for proton acidity. 3. Results and discussion The results are presented and discussed in the following sections in the order of the main steps given in Section 1. 3.1. Selection of catalyst support 2. Experimental The catalyst activity was tested in a continuous experimental system which consisted of a stainless steel reactor with 30 mm inner diameter with 100 cm3 volume, an electrical furnace surrounding it, a cooler, a gas–liquid separator, and a high pressure liquid pump. The reactor temperature was under automatic control. The hydrogen gas was supplied from a high pressure commercial cylinder. Another reactor with 500 cm3 volume was used in long tests. Catalyst preparation went through the following stages: the support was calcined, impregnated by adsorbing an aqueous solution of the chromium compound to its surface, dried, and recalcined. An amount of 19 g catalyst (127 g in the long tests) was placed in the isotherm section of the reactor, quartz beads filling the remaining empty volume. The catalyst was activated by passing hydrogen gas through the reactor at 550 ◦ C. The hydrogen feed was electrolytic and free from oxygen and moisture. Three different liquid feeds were used. Feed I consisted of toluene and n-octane in the molar ratio of 80:20 and 0.05 wt.% thiophene. Feed II was similar Available industrial supports were Al2 O3 , Al2 O3 SiO2 , and SiO2 . They came from Gorkiy Catalyst Company, Russia. These are stable at temperatures above 550 ◦ C. The prepared catalysts contained about 5 wt.% Cr as the active compound. The Cr precursor was analytical grade CrO3 . Experimental runs were conducted at 550 ◦ C and 4.0 MPa, the space velocity (WHSV) and the H2:feed molar ratio being 1.3 h−1 and 10, respectively. Feed I was employed. The catalyst properties and activities are given in Table 1. It was found that ␥-Al2 O3 rendered the highest conversions of toluene, n-octane and thiophene as well as the highest benzene selectivity. Thus, the research continued with ␥-Al2 O3 support. 3.2. Effect of Cr concentration and its compound type on the catalyst activity Attention was focused on the effects of changing first the amount and then the type of the active compound. The pertinent experimental runs were carried out at 600 ◦ C and 4.0 MPa, WHSV and the H2:feed molar ratio being 1.3 h−1 and 3, respectively. Feed I was employed. R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 281 Table 1 Effect of support type on activity Catalyst Support Type Particle size (mm) ␥-Al2 O3 5 × 6a 4–5b 4–5b 4–6b , c 4–6b , d Al2 O3 -SiO2 Al2 O3 -SiO2 SiO2 SiO2 Cr (wt.%) Surface properties 5.3 5.5 5.0 5.2 5.4 189 376 290 260 460 Surface area Pore volume (m2 g−1 ) (cm3 g−1 ) 0.56 1.01 0.36 0.77 0.28 Liquid product composition (wt.%) Conversion (%) Benzene selectivity Benzene Toluene n-Octane C5–C7 Toluene n-Octane Thiophene (mol%) nonaromatics 9.3 5.3 6.5 6.2 6.4 82.6 83.1 82.5 82.3 84.8 5.6 10.3 9.6 9.6 7.3 2.5 1.3 1.4 1.9 1.6 13.0 8.3 11.0 9.2 9.3 72.1 46.0 50.8 49.6 62.9 66.0 47.5 58.5 45.3 52.4 91.4 84.3 75.3 88.0 86.8 T = 550 ◦ C, P = 4.0 MPa, WHSV = 1.3 h−1 , H2 :feed = 10 (mol/mol) (Feed I). a Cylindrical. b Spherical. c Large pore. d Small pore. The weight of Cr was varied from 0.8 to 24 wt.% with the main polyfunctional activity results depicted in Fig. 1. The benzene selectivity was only slightly affected. Conversion of n-octane to C1–C4 hydrocarbons was about 95% throughout. The thiophene conversion rose from 82 to 100% with Cr amount, reaching a plateau at 10% Cr. The toluene conversion was below 30% initially, almost doubling when the Cr amount reached to 4.3 wt.%, remaining the same thereafter. Catalysts prepared from different Cr compounds, namely CrO3 , Cr(NO3 )3 and (NH4 )2 Cr2 O7 , all containing 3.5 wt.% Cr, indicated (Table 2) slight superiority of CrO3 . The other two compounds emitted Fig. 1. Effect of Cr concentration on polyfunctional activity of catalyst: T = 600 ◦ C, P = 4.0 MPa, WHSV = 1.3 h−1 , H2 :feed = 3 (mol/mol) (Feed I). 282 R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 Table 2 Effect of Cr compound type on activity Chromium compound CrO3 (NH4 )2 Cr2 O7 Cr(NO3 )3 Liquid product composition (wt.%) Benzene Toluene n-Octane C5–C7 nonaromatics Heavy aromatics 40.9 57.1 0.3 1.3 0.4 38.7 59.3 1.1 0.6 0.3 36.8 61.0 0.5 1.2 0.5 Conversion (%) Toluene n-Octane Thiophene 46.8 98.5 96.3 44.3 96.7 94.8 43.0 97.4 95.6 92.7 92.4 91.8 Benzene selectivity (mol%) T = 600 ◦ C, P = 4.0 MPa, WHSV = 1.3 h−1 , H2 :feed = 3 (mol/mol) (Feed I). nitrogen oxides and ammonia during the calcination of the catalyst. 3.3. Support modification to improve benzene selectivity A Cr2 O3 -Al2 O3 catalyst containing 3.5 wt.% Cr achieved in a previous study [15] approximately 100% conversions of nonaromatic hydrocarbons and sulfur compounds but a rather low benzene selectivity (about 90%) when a model mixture of toluene, n-octane and thiophene or a BTX fraction of pyrolysis gasoline was the feed. The presence of methane in the gas products, polycondensed aromatics in the liquid products, and coke deposits on the catalyst indicated benzene decomposed to some extent due to secondary reactions. A possible cause for these reactions was acid sites on the catalyst surface. Thus, the current research proceeded with neutralization of these. The use of NaOH or KOH indeed raised the selectivity albeit significantly lowering the conversions. The results of employing KF confirmed (Table 3), this effect of acid sites and pointed to the catalyst comprising 1.0 wt.% KF as the best, the pertinent selectivity being 98.7 mol%. Conversions of nonaromatic hydrocarbons and sulfur compounds were at the level of the previous work. It did not make any difference whether the KF and Cr precursor were impregnated on the support together or separately. 3.4. Suitable chromium amount for longer operation periods Long operation without regeneration is an advantage for a heterogeneous catalyst. Having selected the support, the Cr compound, and the appropriate neutralization came the stage where Al-Cr-KF catalysts with 5, 10, and 15 wt.% Cr were put through stability tests lasting 300 h. These experiments used Table 3 Effect of KF content on the activities of Cr/Al2 O3 Components (wt.%) Liquid product composition (wt.%) Conversion (%) Cr KF Benzene Toluene n-Octane C8 aromatics C5–C7 nonaromatics Heavy aromatics Toluene n-Octane Thiophene 3.8 3.8 3.9 0.5 1.0 2.0 42.5 42.8 40.6 55.6 55.8 58.5 0.38 0.25 0.15 0.31 0.25 0.26 0.61 0.50 0.18 0.60 0.40 0.35 47.9 47.8 45.3 98.3 99.6 99.4 97.5 98.8 97.4 T = 600 ◦ C, P = 4.0 MPa, WHSV = 1.3 h−1 , H2 :feed = 3 (mol/mol) (Feed I). Benzene selectivity (mol%) 96.6 98.7 98.2 R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 283 Fig. 2. Activity comparison of Pyrotol H-9430 and Al-Cr-KF catalysts containing 5, 10, and 15 wt.% Cr: T = 625 ◦ C, P = 6.0 MPa, WHSV = 0.67 h−1 , H2 :feed = 4 (mol/mol) (Feed II). Feed II and the Pyrotol Process conditions, i.e. 625 ◦ C, 6.0 MPa, and a WHSV of 0.67 h−1 . The gas input was 96–97 mol% H2 , the rest being CH4 . The mole ratio of H2 :feed was 4. The C7/C8 alkylaromatic conversion results are depicted in Fig. 2 where Pyrotol H-9430 catalyst is also included for comparison. The developed catalyst with 15 wt.% Cr displayed a clear lead over the other three throughout the operation time. It yielded a stable conversion over 85%. Lower Cr contents showed conversions which monotonically (disregarding the small fluctuations that are quite justifiable in long tests) declined with time due to deactivation. All four catalysts rendered benzene selectivities in the range 96–98 mol% and converted almost all the nonaromatics in the feed to C1–C4 gases. 3.5. Catalyst deactivation and regeneration The next step was to test the 15% Cr catalyst in a very long (3000 h) run using Feed III and the Pyrotol Process conditions. As seen in Fig. 3, neither the benzene selectivity nor the hydrocracking activity altered. On the other hand, the hydrodealkylation activity dropped about 10% in the first 900 h, probably due to coking mentioned above, remaining stable thereafter. Since this was a significant deactivation it would be worthwhile to investigate its causes and find out whether regeneration was possible. The deactivated catalyst samples from the 300 h runs were subjected to oxidative regeneration. Table 4 comprises the surface areas and pore volumes of fresh, used and regenerated catalysts. The degrees of deteriorations and recoveries are clear. Differential thermal analyses of the catalysts used in 300 h runs gave information on the coke oxidation route. Each catalyst displayed two peaks (Fig. 4), as previously [16], the first corresponding to the oxidation of the low valency chromium and the second to coke oxidation. It can be postulated that the higher valency Cr later catalyzed the coke oxidation. The exo peaks appeared at 500 ◦ C for all catalysts. Thermogravimetric analyses at 500 ◦ C indicate (Fig. 5) that coking decreased as the Cr content of the developed catalyst increased from 5 to 10–15 wt.%, the last being very close to the H-9430. Electron 284 R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 Fig. 3. Activities of the Al-Cr-KF catalyst containing 15 wt.% Cr and 1 wt.% KF in a 3000 h test at hydrodealkylation process conditions: T = 625 ◦ C, P = 6.0 MPa, WHSV = 0.64 h−1 , H2 :feed = 4 (mol/mol) (Feed III). microscopy by UZMV-100K (with a magnification of 17,000) gave the results in Fig. 6. Evidently, within the working range, the more the Cr amount, the better its dispersion and the less the coking. Fig. 7 gives the XRD spectra of the developed catalyst, H-9430, and the selected support. These were obtained from a Dron-2 diffractometer, using the Cu K␣ signal. The range of 20–70◦ of 2θ was scanned at a step size of 2◦ of 2θ. The main diffraction lines of ␥-Al2 O3 are at about 32, 37, 47, and 67◦ . These also appear in the other two curves. These catalyst spectra have the extra diffraction lines at about 24, 36, 41, 50, 56, and 61◦ , all probably due to Cr2 O3 . The spectrum similarity indicates that the new catalyst and H-9430 largely agree in composition. On the other hand, chromium-oxide and alumina may form various solid compounds [17]. Chemical analyses ascertained that developed catalyst preserved its Cr and KF in drying, calcination, activation, reaction, and regeneration. Table 4 Surface properties of the prepared and the commercial catalysts Catalysts Al-Cr-KF (1 wt.% KF) 5 wt.% Cr 10 wt.% Cr 15 wt.% Cr H-9430 Surface area (m2 g−1 ) Pore volume (cm3 g−1 ) Fresh Used Regenerated Fresh Used Regenerated 196 167 154 175 150 144 194 162 152 0.65 0.63 0.51 0.32 0.37 0.47 0.60 0.50 0.49 95 89 94 0.23 0.21 0.23 R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 Fig. 4. DTA curves of catalysts used at hydrodealkylation process conditions: (1) Al-Cr-KF (5 wt.% Cr), (2) Al-Cr-KF (10 wt.% Cr), (3) Al-Cr-KF (15 wt.% Cr), (4) Pyrotol H-9430. 3.6. Effects of calcination and drying temperatures The chosen catalyst was prepared using ␥-Al2 O3 support which was calcined in an air atmosphere in an oven heated at a rate of 200 ◦ C h−1 to different temperatures ranging between 600 and 1100 ◦ C. It is known that ␥-Al2 O3 begins to transform to the θ form at 900 ◦ C and then to α form at 1000 ◦ C. Table 5 shows the surface areas and pore volumes of the support and of the catalyst. It is seen that these properties keep their values at the calcination temperature of 600–800 ◦ C 285 Fig. 5. TGA curves of catalysts used at hydrodealkylation process conditions: (1) Al-Cr-KF (5 wt.% Cr), (2) Al-Cr-KF (10 wt.% Cr), (3) Al-Cr-KF (15 wt.% Cr), (4) Pyrotol H-9430. dropping considerably thereafter. Loading the support with active components causes a general fall in these values. When tested with Feed II at 600 ◦ C and 4.0 MPa, the WHSV and the H2:feed molar ratio being 1.34 h−1 and 3, respectively, this catalyst manifested activity decline after 800 ◦ C in harmony with the surface properties (Table 6). Benzene selectivity seemed independent of the calcination temperature. Table 6 also contains in brackets activity and selectivity values pertaining to calcination applied to the loaded catalyst instead of the support. Activity appeared in this case to decrease after Fig. 6. Electron microscope results of Al-Cr-KF catalysts containing 5, 10, and 15 wt.% Cr. 286 R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 Fig. 7. XRD analyses of support and catalysts: (1) developed catalyst (15 wt.% Cr and 1 wt.% KF), (2) Pyrotol H-9430, (3) ␥-Al2 O3 . Table 5 Effects of support calcination temperature on surface properties Calcination temperature (◦ C) Surface properties Surface area Support Catalyst 600 700 800 900 1000 1100 199 154 200 159 197 157 133 114 88 70 34 26 (m2 g−1 ) Pore volume (cm3 g−1 ) Support Catalyst 0.74 0.51 0.73 0.53 0.72 0.52 700 ◦ C while benzene selectivities remained almost the same. When the catalyst was dried in an air atmosphere at 100, 150, 200, and 250 ◦ C for 8 h conversions were not much altered. 0.59 0.44 0.49 0.40 0.17 0.16 3.7. Effect of H2 activation conditions on catalytic activity The freshly prepared catalyst contains high valency Cr in the form of CrO3 to be converted to its activated Table 6 Effect of calcination temperature on activity Calcination temperature (◦ C) The liquid product composition (wt.%) Nonaromatics Benzene Toluene C8 aromatics Nonaromatics C7/C8 aromatics Benzene selectivity (mol%) 600 700 800 900 1000 1100 2.1 2.0 2.0 2.6 3.2 10.9 54.9 54.6 54.4 53.9 49.3 39.8 32.9 33.5 34.3 34.2 35.0 36.8 10.1 9.9 9.3 9.3 11.6 12.5 91.2 91.5 91.4 89.0 85.9 48.4 24.8 24.3 24.0 23.7 16.0 5.7 96.9 97.3 96.8 97.0 98.0 98.3 (2.8) (3.0) (4.3) (4.9) (55.7) (55.3) (51.4) (48.3) Conversion (%) (33.8) (33.7) (34.4) (35.6) (7.7) (8.0) (9.9) (11.2) T = 600 ◦ C, P = 4.0 MPa, WHSV = 1.34 h−1 , H2 :feed = 3 (mol/mol) (Feed II). (86.2) (84.9) (78.2) (75.1) (27.7) (26.6) (20.4) (15.2) (96.5) (96.9) (97.8) (98.4) R. Alibeyli et al. / Applied Catalysis A: General 238 (2003) 279–287 287 Table 7 Effect of hydrogen activation conditions on catalyst activity Parameter The liquid product composition (wt.%) Conversion (%) Temperature (◦ C) Period (h) Aromatics Benzene Toluene C8 aromatics Nonaromatics C7/C8 aromatics Benzene selectivity (mol%) 350 350 400 500 500 650 500a 2 6 6 2 6 6 6 2.4 2.2 2.1 3.7 4.2 5.4 5.4 55.9 58.8 57.9 51.2 48.4 47.8 49.4 31.6 27.4 28.0 33.3 34.1 34.9 33.3 10.3 11.6 11.3 11.8 13.3 11.9 11.9 88.0 89.2 91.3 81.4 78.6 72.5 72.6 26.7 31.8 30.6 20.7 14.7 15.2 18.0 96.8 97.8 98.1 97.3 98.0 97.6 97.4 T = 600 ◦ C, P = 4.0 MPa, WHSV = 1.34 h−1 , H2 :feed = 3 (mol/mol) (Feed II). a Catalyst was activated under 4.0 MPa hydrogen pressure. form Cr2 O3 by passing hydrogen gas through the reactor at a rate of 600 h−1 . Different activation temperatures and periods were tried with the aim of determining the effects of these parameters on catalyst activity. The heating rate was 50 ◦ C h−1 . Feed II was used at conditions stated in Section 3.6. Table 7 indicates that the highest hydrodealkylation and hydrocracking activities were attained after treatment at 350 ◦ C for 6 h. Above 400 ◦ C, activities considerably decreased probably because of partial agglomeration due to accelerated valency reduction and re-crystallization. Benzene selectivity remained the same. Increasing the hydrogen pressure from atmospheric to 4 MPa seemed to affect positively hydrodealkylation conversion. 4. Conclusions A polyfunctional Cr-KF/␥-Al2 O3 catalyst was developed for the manufacture of pure benzene from pyrolysis gasoline or other industrial mixtures containing various alkylaromatic and nonaromatic hydrocarbons. Tests with a hydrocleaned BTX fraction of pyrolysis gasoline showed that the new catalyst maintained its high hydrodealkylation–hydrocracking activity and benzene selectivity for 3000 h. Oxidative regeneration completely restored original surface properties and activity of the used catalyst. The new catalyst was found to be similar in both composition and overall activity to the commercial catalyst Pyrotol H-9430. The developed catalyst has a higher surface area and hydrodealkylation activity. Acknowledgements We are thankful and gratefully appreciate the fellowship support of this work by NATO and TUBİTAK. References [1] S. Masamune, J. Fukuda, S. Katada, Hydrocarbon Process. 46 (1967) 155. [2] S. Masamunu, T.S. Kowatani, Hydrocarbon Process. 47 (1967) 111. [3] W.U. Luyben, Ind. Eng. Chem. Res. 39 (2000) 2384. [4] P. Torregrosa-Rodriguez, M. Martinez-Escandell, F. Rodriguez-Reinoso, H. Marsh, C.E. De Salazar, E.R. Palazon, Carbon 38 (2000) 535. [5] Detol Hydrodealkylation, Hydrocarbon Process. 46 (11) (1967) 188. [6] Pyrotol, Hydrocarbon Process. 47 (9) (1968) 209. [7] Pyrotol, Hydrocarbon Process. 49 (9) (1970) 223. [8] E.G. 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