Ind. Eng. Chem. Fundam. 1985, 24, 433-438
433
Kinetics of Dehydrogenation of Methylcyclohexane over a Platinum-Rhenium-Alumina Catalyst in the Presence of Added Hydrogen K. Jothlmurugesan, Subhash Bhatla, and Rameshwar D. Srlvastava*
Ind. Eng. Chem. Fund. 1985.24:433-438. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/16/19. For personal use only.
Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Kanpur 208016, India
The kinetics of dehydration of methylcyclohexane to produce toluene over a Pt-Re-Al203 catalyst in the presence of added hydrogen have been investigated at atmospheric pressure and over the temperature range of 598-698 K. The experimental results were analyzed on the basis of Langmuir-Hinshelwood kinetics with statistical data interpretation to show the significance of mechanism determination when precise experimental data are used. The rate of methylcyclohexane adsorption was the rate-controlling step in the overall kinetics. The effect of toluene inhibition on the reaction rate was discussed. The activation energy for the reaction was 51.9 kJ/mol.
Introduction
single-site surface reaction (Zengel, 1967) and the adsorption of methylcyclohexane (Lander, 1971) have also been reported to control the rate. The deactivation kinetics of platinum-reforming catalyst have been reported by Wolf and Petersen (1977) and Jossens and Petersen (1982a). Recently, Jossens and Petersen (1982b) studied changes in the fouling characteristics from addition of rhenium to the platinum-reforming catalyst using MCH dehydrogenation to probe the changes in the metallic function. From this brief literature review, it is evident that there has been no prior detailed kinetic study for the methylcyclohexane dehydrogenation over platinum-rheniumalumina catalyst. The present study appears needed since the reforming of MCH should provide considerable information about the Pt-Re-AI203 catalyst because of the diverse nature of this reaction network. The objective of the work presented in this paper is a kinetic study of a catalytic dehydrogenation of MCH using the platinum-rhenium-alumina catalyst (Pt-Re-Al203) in a differential, fixed-bed reactor at atmospheric pressure in the temperature range 598-698 K. We analyzed the experimental results on the basis of assumed LangmuirHinshelwood kinetics, using statistical interpretation to show the real significance of the mechanism determination with precise experimental data.
The dehydrogenation of methylcyclohexane (MCH) is an important industrial operation in catalytic reforming. Reforming plants process ~108 tons of this compound annually because of the significant increase of octane number that is obtained. In the past decade bimetallic catalyst systems have been widely introduced in reforming (Kluksdahl, 1968; Sinfelt, 1981). Prior to this, platinumon-alumina catalysts were used throughout the industry during the 1950s and 1960s. Pt-Re-Al203 catalysts generally contain an amount of rhenium comparable to the amount of platinum present (often about 0.3 wt % of each). A major advantage of this mixture over catalysts containing only platinum is its lower rate of activity decline during a reforming operation. More recently, the MTH (methylcyclohexane, toluene, hydrogen) system with methylcyclohexane as a hydrogen carrier is commercially exploited for automotive application (Taube et al., 1983; Cresswell et al., 1984). This requires the catalytic production of hydrogen from an onboard reactor. The microreactor and pilot reactor tube studies (Cresswell et al., 1984) suggested the bimetallic catalyst, Pt-Re-Al203, to be the most promising, offering the best combination of initial activity and resistance to deactivation. Characterization of the kinetics of this reaction, however, is the basis for more accurate reactor design.
Catalyst Preparation and Characterization
Experimental studies of dehydrogenation of MCH over both Pt-Al203 (Sinfelt et al., 1960; Rohrer and Sinfelt, 1962; Ritchie and Nixon, 1966; Marlin, 1967; Zengel, 1967; Lander, 1971; Wolf and Petersen, 1977; Jossens and Petersen, 1982a) and Pt-Re-Al203 (Jossens and Petersen, 1982b) have been made for some time; however, kinetic analysis has been slow to develop. It has been reported that excess hydrogen was required to maintain good catalyst stability and had little effect on the dehydrogenation rate (Zengel, 1967; Lander, 1971). Sinfelt et al. (1960) reported the kinetics of MCH over a Pt-Al203 catalyst and found the rate to be nearly zero order with respect to MCH and hydrogen. Their derived rate expression assumed the rate of desorption of toluene to be rate controlling. In contrast, Marlin (1967) investigated the same system at 10 atm and concluded that desorption of hydrogen was the rate-controlling step. A *
The platinum-rhenium catalyst on alumina support was prepared by an impregnation (incipient wetness) technique. The chemicals used were chloroplatinic acid (Johnson Matthey Chemical Limited, London), Re207 (Riedel-De Haen G Seelze-Hannover, West Germany), and -alumina with a BET surface area of about 220 m2/g. After impregnation, the catalyst was dried at 373 K and calcined in an air stream for 5 h at 723 K. The catalyst composition was 0.3 wt % Pt, 0.3 wt % Re, and the remainder alumina. For comparison, individual metal catalysts containing 0.3 wt % (Pt or Re) on -alumina were also prepared under the same conditions except for Re where the calcination was not done. A detailed characterization of the catalyst by means of electron microscopy, proton-induced X-ray emission (PIXE), Rutherford backscattering spectrometry (RBS), and gas chemisorption has been described elsewhere (Jothimurugesan, 1984). All the kinetic runs in this study were performed exclusively with 0.3 wt % Pt-0.3 wt % Re (Pt-Re-Al203)
To whom correspondence should be addressed. 0196-4313/85/1024-0433$01.50/0
©
1985 American Chemical Society
434
Ind. Eng. Chem. Fundam., Vol. 24, No. 4, 1985 100
d
_Pt-Re : .. Pt
^&=—
80
60
„
