Applied Catalysis A: General, 83 (1992) 75-86 Elsevier Science Publishers B.V., Amsterdam 75 APCAT 2227 Hydrodealkylation reaction of ethylbenzene over a supported nickel-tungsten catalyst Yosoon Songl, Gon Seo and Son-Ki Ihm Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusung-dong, Yusung-94, Taejeon 305-701 (South Korea) (Received 14 August 1991, revised manuscript received 5 November 1991) Abstract The hydrodealkylation reaction of ethylbenxene over a supported nickel-tungsten catalyst has been studied. Over a supported nickel catalyst, the dominant ring opening reaction of benzene over 400 C led to high methane selectivity. On the other hand, over a cosupported nickel-tungsten catalyst, the ring opening reaction was suppressed, thus resulting in high benzene selectivity. The activity and selectivities of the hydrodealkylation reaction were dependent on the degree of reduction of tungsten. Using an oxygen titration method, the presence of nickel in the cosupported catalysta was confirmed to promote the reduction of tungsten. The cosupported nickel-tungsten catalysts suppressed the decomposition of dioxane in temperature-programme desorption experiments more than the supported nickel catalyst. Keywords: ethylbenzene, hydrodealkylation, nickel-tungsten. INTRODUCTION Since the demand for benzene in the petrochemical industry far exceeds its production by distillation of crude oil, processes for the disproportionation of to luene and hydrodealkylation of alkylbenzene mixtures have been developed to fill the gap [ 11. Alkylbenzene mixtures are produced from catalytic reforming or light alkene production processes as by-products. The separation process of the alkylbenzene mixture into each component is not feasible economically because the mixture is comprised of too many components and their boiling points are too close. Therefore, the production of benzene by hydrodealkylation of the alkylbenzene mixture has an economic edge. Selectivity to benzene is a crucial criterion in selecting a catalyst for the hydrodealkylation process of the alkylbenzene mixture. The catalyst should Correspondence to: Dr. S.-K. Ihm, Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusung-dong, Yusung-94, Taejeon 305-701, South Korea. Present address: Department of Chemical Technology, Chonnam National University Kwang-ju 500-757, South Korea.
76 Y. Song et al./appl. Catal. A 83 (1992) 75-86 activate hydrogen in order to remove the alkyl groups from the alkylbenzene, but this should not go so far as to destroying the benzene ring. Supported nickel catalysts are reported to show good performance for hydrodealkylation of toluene due to their ability to activate hydrogen [ 2-41. The hydrodealkylation of ethylbenzene is also studied using supported nickel catalysts on silica, alumina and silica-alumina [ 51. However, excessive activation of hydrogen results in the hydrocracking of a benzene ring, so it is common practice to cosupport other metals like chromium, cobalt, tungsten or molybdenum with nickel in order to control the selectivity. It has also been reported that the acidity of the support has a substantial effect on the selectivity. Results are available for silica-alumina, zeolite, and Lewis-type acid catalysts [ 11. Among the catalysts, supported nickel-tungsten catalysts showed not only high activity for hydrodealkylation but also good benzene selectivity [ 51. The selectivity varies depending on the ratio of the amount of nickel impregnated to that of tungsten. Tungsten has varying states of oxidation (e.g. 6 +,5 +,4 + and 0), and in general tends to defy reduction. The reduction state of tungsten supported on y-alumina is reported to depend on the amount of tungsten oxide impregnated, when impregnation exceeded the thickness of a monolayer, an intermediate state W4+ is observed [ 61. In cosupported nickel-tungsten catalysts, spillover of hydrogen from nickel to tungsten is known to promote the reduction of tungsten [ 71. This study aims to investigate the hydrodealkylation of ethylbenzene over supported nickel-tungsten catalysts, focusing on the effect of tungsten upon the activity and the selectivity of the reaction. The extent of the reduction of the catalysts are examined by oxygen titration, and compared for differing reduction treatments. The catalytic activity of nickel for decomposition is investigated through temperature-programmed desorption (TPD ) (decomposition) of dioxane. These results are discussed in relation to the characteristics of catalysts for hydrodealkylation. EXPERIMENTAL Preparation of catalysts The catalysts were prepared by a conventional incipient wetness technique using aqueous solutions of nickel nitrate (Wako, GR) and/or phosphotungstic acid (Fluka, AG.). Solutions containing the required amount of metals to be impregnated were absorbed into a silica-alumina support (Davidson Chemicals, 400 m2/g, alumina content 13%) and a silica support (150 m /g). They were slowly dried in a rotary evaporator at 6O C, and further in an oven at 120 C for one day. The catalysts were calcined at 550 C for 7 h in an air environment to decompose the metal salts to either NiO or W03. The decom-
Y. Song et al.fappl. Catal. A 83 (1992) 75-86 17 position was confirmed by measuring the weight loss during the calcination process using a thermal gravimetric analyzer (Rigaku, Model CN807El). The naming convention for the prepared catalysts is as follows. The names of the impregnated metals are followed by their weight percents in parentheses with respect to the support. The support name is represented by /SA, which is an abbreviation of silica alumina (for example, Ni-W (5 : 15) /SA). Reaction The hydrodealkylation reaction of ethylbenzene was carried out in a conventional fixed-bed flow reactor. The reactor was made of a l/4 in O.D. stainless steel tube with catalysts packed in the middle. A thermocouple was inserted in the reactor to monitor and control the temperature of the catalyst bed using a PID controller (Research Industry, Model 63911). All the flow paths were heated to 200 C. Ethylbenzene (Wako GR. ) was introduced by a metering pump (Sage Instruments, Model 341A) at atmospheric pressure into a preheated gasifier, where the evaporated vapor was mixed with hydrogen carrier gas. The partial pressure of ethylbenzene vapor was kept low (hydrogen-to-ethylbenzene ratio = 130 (in mols), W/F= 135 gcat*h/mol) in order to prevent rapid deactivation of the catalysts. The reaction was also carried out at an elevated pressure up to 10 atm using a back pressure control valve installed after the reactor exit. In this case, the reactant was fed using a high pressure metering pump (Eldex Model A) at a rate of 1.0-2.0 ml/h. The hydrogen-to-ethylbenzene molar ratio was controlled at 6.5, and W/F at 6.11 gcat * h/gmol. Catalyts were reduced with hydrogen before introducing the reactants. The reduction temperature was 500 o C for the supported nickel catalyst, and 600 C for catalysts involving tungsten. The hydrogen pressure was maintained at the same level as the reaction pressure. The effluent product stream was introduced via a Valco six-port sampling valve to a Varian 1420 gas chromatograph with a dual thermal conductivity detector (TCD ) system. The separation column was 10% Pentorial-A on Uniport B. The activity and selectivity to a given product for the hydrodealkylation reaction were defined as follows. Conversion = [ 1 - (number of mols of ethylbenzene consumed) / (number of mols of ethylbenzene fed) ] - 100. Selectivity to a given product = (number of mols of the given product/number of mols of ethylbenzene reacted) 100. Generally the hydrodealkylation reaction is accompanied by a ring opening reaction of benzene. Since every mol of hydrocracked ethylbenzene produces 8 mols of methane and ethane was not detected in the product stream, the total mols of ethylbenzene converted can be calculated as follows: Number of mols of ethylbenzene reacted = number of mols of benzene produced (a) +number of mols of toluene produced (b) + (number of mols of methane produced - 2a-b )/8. Thus the selectivity to ring opening
78 Y. Song et al./appl. Catal. A 63 (1992) 75-66 is calculated from the last term of the above equation divided by the number of mols of ethylbenzene reacted. Characterization of catalysts Oxygen titration Oxygen uptake by catalysts reduced in the hydrogen flow was measured using a conventional TPD apparatus equipped with a TCD cell. The catalyst (0.1 g) packed in a Vycor tube (O.D. 6 mm) was reduced under the same conditions as for the reaction experiments. The reduced catalyst was flushed with helium at the same temperature for an hour, and at an elevated temperature of 625 C for another hour. Then, oxygen was injected as a pulse, and the exit stream was analyzed using a TCD cell. Though the oxygen may react slowly with tungsten at as low a temperature as 400 C, the reactor was maintained at 625 C to shorten the experiment time. TPD of dioxane TPD of dioxane was performed using a l/4 in. O.D. stainless steel tube. After 0.2 g of catalyst was reduced with hydrogen at 600 C for three hours and cooled in helium flow to 125 C, liquid dioxane was injected for adsorption on the catalyst at 125 C. Desorption and/or decomposition of the dioxane was then followed with the temperature rising at the rate of lo C/min to 650 C. The flow-rate of helium gas was maintained over 200 cm3/min to minimize dioxane readsorption. The effluent stream from the catalyst bed was first detected by a TCD cell, and then a part of the stream was introduced via a six-port sampling valve into a gas chromatograph (Shimadzu 7AG) for composition analysis. The analysis was performed using a separation column of Porapak Q programmed from 60 to 180 C. RESULTS AND DISCUSSION Hydrodealkyhtion of alkylbenzene Table 1 summarizes the effect of temperature on the conversion and product distribution of the hydrodealkylation reaction of ethylbenzene and m-xylene over Ni (5)/SA catalyst. The conversion of each reactant steadily increased with the rising reaction temperature. Up to 400 C, the major product was toluene with little sign of the ring-opening reaction taking place. Above 425 C, however, the ring opening proceeded to an appreciable extent for ethylbenzene, and finally at 450 C all the rings were hydrocracked to form methane for both ethylbenzene and toluene. Fig. 1 shows the effect of tungsten coimpregnated with nickel on the conversion and the selectivity of ethylbenzene at 550 o C and 1 atm. In contrast to
Y. Song et al. fapp1. Catal. A 83 (1992) 75-86 79 TABLE 1 Conversion and product composition from ethylbenzene and m-xylene over a Ni (5) /SA catalyst at various temperatures Catalyst: 0.04 g; W/F= 100 gcat*h/mol Temp. ( C) Ethylbenzene Conv. Mol fraction m-xylene Conv. Mol fraction (%) Benz. Tol. RO. (%) Benz. Tol. RO. 300 0-0 - 325 5 0.0 0.05 0.0 2 0.0 0.02 0.0 350 20 0.01 0.19 0.0 5 0.0 0.05 0.0 375 58 0.06 0.52 0.0 15 0.01 0.14 0.0 400 90 0.2 0.69 0.01 35 0.06 0.29 0.0 425 98 0.41 0.34 0.25 65 0.27 0.36 0.03 450 100 0.0 0.0 1.00 100 0.0 0.0 1.00 Benz., Tol. and RO. represent benzene, toluene and ring opening products respectively. 0 0.2 0.4 0.6 0.8 1.0 W/(Ni+W) Fig. 1. Hydrodealkylation reaction of ethylbenzene over Ni-W (5)/SA catalysts with various tungsten contents as mol at 550 C. Conversion ( 0 ); selectivities to benzene ( 0 ) ; to toluene ( 0 ), and to ring opening (A ). Reaction pressure: 1 atm. W/F= 135 gcat*h/mol. Hydrogen-to-ethylbenzene ratio (as mol) = 134.
80 Y. Song et al./appl. Catal. A 83 (1992) 75-86 the complete ring opening over a nickel catalyst at this temperature, a tendency for the preservation of the benzene ring is apparent over nickel-tungsten catalysts. The tendency becomes more conspicuous with increasing tungsten contents, but only at the expense of decreasing conversion. Table 2 compares the performance of nickel-tungsten catalysts for hydrodealkylation of ethylbenzene obtained under two different reaction pressures. Over a Ni-W (1.25 : 3.75) /SA catalyst used at 1 atm, severe coking occurred on its surface unless the partial pressure of hydrogen is pretty high. Thus a very diluent feed stream (ethylbenzene-to-hydrogen= l/134 in mols) was used to avoid deactivation of the catalyst. At higher pressure, on the other hand, one may use catalysts with more metal loading and a feed richer in ethylbenzene, and still need not worry about any significant deactivation. In a reaction performed at 10 atm and 6OO C, a Ni-W (5: E)/SA catalyst, which has the same Ni-to-W mol ratio as Ni-W (1.25: 3.75)/SA catalyst but four times more metal, was used with a much more concentrated feed stream (ethylbenzene-to-hydrogen ratio = 1: 6.5)) and showed a much better performance in respect to conversion and benzene selectivity than at 1 atm. The experiments using a Ni-W (5: 15)/SA catalyst at 10 atm as described above were repeated for various reaction temperatures. The conversion and the selectivity to each product species are plotted against the temperature in Fig. 2. The conversion of ethylbenzene showed a steady increase with rising temperature. The selectivity to benzene showed a slight decrease due to the increased ring opening at higher temperatures, but it still maintained 60% even at 650 C. The selectivity of the hydrodealkylation reaction over cosupported nickeltungsten catalysts depends on the reduction state of the metals. Table 3 shows the conversion and the selectivities of ethylbenzene at 600 C over Ni- W (5: 15)/SA catalysts which were reduced at the same temperature under a hydrogen flow of 10 atm for varying periods of reduction time. Over the catalyst TABLE 2 Conversion and product distribution of ethylbenzene over a Ni-W ( 1.25 : 3.75 )/SA catalyst and a Ni-W(5:15)/SAcatalystat600 C Catalysts Ni-W(1.25:3.75)/SA Ni-W(5:15)/SA Pressure 1 atm W/F(gcat*h/mol) 13.5 Product distribution (mol fraction) Benzene 0.064 Toluene 0.032 RO 0.060 Conversion (% ) 16 10 atm 6.11 0.27 0.09 0.07 43
Y. Song et al.fappl. Catal. A 83 (1992) 75-86 61 100 a 80-500 550 600 650 Reaction TemperaturelocI Fig. 2. Conversion and selectivities to benzene, toluene, and ring opening in the hydrodealkylation reaction of ethylbenzene over a Ni-W (5: 15)/SA catalyst at various reaction temperatures. Conversion (0 ). Selectivities to benzene ( 0 ), to toluene ( 0 ), and to ring opening ( A ). Reaction pressure: 10 atm. Pretreatment at 600 C under a hydrogen flow of 10 atm for 3 h. W/F=6.11 gcat. h/mol. Hydrogen-to-ethylbenzene ratio (as mol) =6.5 TABLE 3 Conversion and selectivities in the hydrodealkylation reaction of ethylbenzene at 600 C over a Ni-W (5 : 15 ) /SA catalyst reduced at 600 C under a hydrogen flow of 10 atm for various reduction times Reduction time (h) Conversion of ethylbenzene (%) Selectivity (% ) RO products Benzene Toluene 1 77 91 5 4 3 43 17 62 21 6 74 19 34 47 reduced for one hour, high conversion was observed, but the product mainly consisted of methane as a result of the ring opening reaction. Having been reduced for three hours, the catalyst showed a high selectivity to benzene at the expense of a decreased conversion. When reduction was prolonged up to six hours, the conversion was again high with little ring opening, but toluene showed an edge over benzene in the product distribution. The above results indicate the strong dependence of the catalytic behavior of nickel-tungsten on reduction conditions. While nickel oxide is susceptible
82 Y. Song et al./appl. Catal. A 83 (1992) 75-86 to reduction around 5OO C, tungsten oxide needs a higher temperature to be reduced, and passes through stages of 5 + and 4 + and then finally to metal. Hence one may reasonably assume that nickel would be sufficiently reduced at 600 C, and accordingly that the degree of reduction of tungsten dictates the selectivity behavior of cosupported nickel-tungsten catalysts. Unless reduced long enough, nickel-tungsten catalysts were found to behave as if only nickel were impregnated. Tungsten reduced for three hours had a negative influence upon the hydrocracking activity of nickel for the benzene ring, thus leading to a high benzene selectivity. Having been exposed to high pressure hydrogen for six hours, the catalysts yielded toluene more than benzene. Preservation of the benzene ring over cosupported nickel-tungsten catalyst was ascribed to the suppression of the catalytic activity of nickel for hydrocracking in the presence of reduced tungsten. Though interaction between reduced tungsten and nickel might be the cause of this modification, the effective oxidation state of tungsten in enhancing benzene selectivity is not clear. One may recall another mechanism proposed in the literature which refers to the promotion of o-adsorption of ethylbenzene in the presence of tungsten [ 4,8,9], but the mechanism seems to need further verification. Churacterization of the catalysts Oxygen titration experiments were performed for supported metal catalysts to examine the degree of reduction in relation to the treatment conditions. Fig. 3 shows the variation of oxygen uptake (oxygen-to-tungsten atomic ratio) of the W (10) /SA catalyst with reduction temperature and time. After reduction in hydrogen flow, the catalyst was titrated with oxygen at 625 C. For the cat- Time lh) Fig. 3. Variation of oxygen uptake (oxygen-to-tungsten atomic ratio) of tungsten of a W (lo)/sa catallyst with reduction temperature and time. Reduction temperature 725 C ( 0 ), 625 C ( 0 ), 525 C (A).
Y. Song et al/appl. Catal. A 83 (1992) 75-86 83 alyst reduced at 525 C, the oxygen uptake was very small (less than 3% ) regardless of the duration. On the other hand, the uptake significantly increased for catalysts treated at 625 C, reaching approximately 24% after three hours reduction. Using this result, one might state, on a rough basis, that 24% of the impregnated WO, was reduced to WOz. Similar results were obtained by Soled et al. [lo], who reported that about 22% of WO, impregnated on an alumina support was reduced to WOz at 600 C. Also, no diffraction peak was observed corresponding to tungsten in the X-ray diffraction (XRD) pattern of the catalyst surface. Thus, it seems to be reasonable to postulate intermediate metastable W4+ and W5+ states including a small amount of metal as representing the reduction state of the catalyst reduced with hydrogen around 625 C. When the reduction temperature rose to 725 C, a much higher level of oxygen uptake was observed. After reduction for 36 h, the presence of metal in the supported tungsten catalyst was confirmed by the observation of a W (110) diffraction peak in the XRD pattern [ 111. Similar titration experiments were conducted for the Ni(5)/SA catalyst. The results are summarized in Table 4. Reduced at a comparatively low temperature of 450 C, the catalyst showed 90% oxygen uptake. This indicates that 90% of the nickel oxide was converted to the metallic state by reduction. Comparing with the results of Fig. 3, it is clear that nickel oxide is more easily reduced to its metallic state than tungsten oxide. Table 5 shows the oxygen uptake by cosupported nickel-tungsten catalysts after reduction. Earlier results for nickel or tungsten catalysts are included to facilitate the comparison. When reduced at low temperatures up to 500 C, the nickel-tungsten catalysts took up almost the same amount of oxygen as the nickel catalysts. This is the expected result taking into account the resistance of tungsten oxide to reduction at low temperature. For a high reduction tem- TABLE 4 Amount of oxygen uptake at 625 C of supported, reduced nickel catalysts Catalyst: Ni( 5)/S& pulse injection with a helium carrier gas (60 cm3/min) Reduction treatment Temp. Time ( C) (h) Oxygen-to-nickel (atomic ratio) 450 3 0.90( ko.05) 500 3 0.90 ( f 0.03 ) 500 12 l.oo( ko.03) 550 3 0.95 ( f 0.03) 660 3 1.00 (?I 0.03) With hydrogen (60 cm3/min).
84 Y. Song et al./appl. Catal. A 83 (1992) 75-86 TABLE 5 Amount of oxygen uptake of supported nickel and tungsten catalysts after reduction Reduction treatment temperature ( C) Amount of oxygen upt.akeb ( 10H5 mol) Ni-W(B:lO)/SA Ni(5)/SA W(lO)/SA 450 3.83 3.83 0.0 500 3.77 3.83 0.01 600 5.36 4.26 0.54 With hydrogen (60 cm3/min) and for 3 h. bpulse injection with a helium carrier gas (60 cm3/min). perature of 6OO C, the tungsten catalyst also took up a modest amount of oxygen. It is noteworthy that the oxygen uptake of the nickel-tungsten catalyst was larger than that of the nickel catalyst and the tungsten catalysts combined. This seems to follow from the promoting action of reduced nickel metal toward the reduction of tungsten oxide. The rest of this section is concerned with temperature-programmed desorption of dioxane performed to investigate the catalytic role of tungsten in cosupported nickel-tungsten catalysts. Ethylbenzene did not adsorb to a sufficient extent enough to analyze the effluent using a gas chromatograph. In addition, the adsorbed ethylbenzene left carbon deposits, making it difficult to examine the interaction with active sites on the surface. On the other hand, being a ring compound without double bonds, dioxane adsorbs on the surface to a significant extent. Furthermore, the adsorbed dioxane is readily desorbed from the nickel catalyst without affecting the active sites. Hence dioxane appeared to be a suitable compound for investigating the effect of tungsten addition. Having oxygen atoms, dioxane molecules were expected to adsorb readily on the empty sites once occupied by lattice oxygen of unreduced tungsten oxides. However, it was not easy to compare the differing reduction treatments in respect to the amount of dioxane adsorbed. Fig. 4a shows the results of TPD of dioxane over a reduced Ni (5) /SA catalyst. Each TPD curve was separated into two peaks on the basis of the GC composition analysis of the effluent from the TCD cell. The first peak corresponds to desorption without decomposition, and the second to desorption with decomposition. They are referred to respectively as the desorption peak and the decomposition peak in the following sections. The desorption peak extended up to 300 C. Decomposition started around 200 C with the peak maximum located near 350 C. The effluent gas from decomposition was trapped using liquid nitrogen, and then analyzed using an NMR (JEOL PMXGOsi). The decomposition product was found to consist mainly of acetaldehyde and a trace amount of hydrocarbons.
Y. Song et al./appl. Catal. A 83 (1992) 75-86 85 %_ 100 200 300 400 500 600 Temperature( C) -. Fig. 4. TPD profile of dioxane from supported nickel and nickel-tungsten on silica-alumina. (a) Ni(5)/SAcatalystreducedat600 Cfor3h; (b) Ni-W(5:15)/SAcatalystreducedat600 Cfor 3 h; (c) silica-alumina support. Desorption peak (-); decomposition peak (---). Compared with Fig. 4a, Fig. 4b shows the effect of adding tungsten to nickel catalysts. A larger desorption peak extending to a higher temperature was obtained than when nickel was impregnated. The decomposition peak was also observed starting from 25O C, but the total amount of products was much smaller than in Fig. 4a. These differences can be ascribed to the suppression of the decomposition activity of nickel in the presence of tungsten. Fig. 4c shows the results obtained over a silica-alumina support without any metal loading. On the whole, both peaks have smaller areas than over supported metal catalysts. The desorption peak extended as far as 500 C, and the decomposition peak started only above 300 C. The TPD experiments of dioxane were also carried out using metal catalysts supported on silica. Similar trends were observed except that the peak areas were smaller compared with the cases of Fig. 4. Dioxane decomposed to a significant extent near 320 C over the nickel supported catalysts, while the decomposition was weakened when tungsten was added. CONCLUSIONS The present results on the hydrodealkylation reaction of ethylbenzene over supported metal catalyst on silica-alumina can be summarized as follows.
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