Ind. Eng. Chem. Fundam. 1985, 2 4 , 433-430
433
Kinetics of Dehydrogenation of Methylcyclohexane over a Platinum-Rhenium-Alumina Catalyst in the Presence of Added Hydrogen K. Jothimurugesan, Subhash Bhatla, and Rameshwar D. Srivastava" Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Kanpur 2080 16, India
The kinetics of dehydration of methylcyclohexane to produce toluene over a Pt-Re-Al,O, 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 methylcyclohexaneadsorption 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 The dehydrogenation of methylcyclohexane (MCH) is an important industrial operation in catalytic reforming. Reforming plants process -los 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-Al,O, catalysts generally contain an amount of rhenium comparable to the amount of platinum present (often about 0.3 w t % 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-Al,O,, 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. Experimental studies of dehydrogenation of MCH over both Pt-A1,03 (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-A1,03 (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-A1,0, 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 a t 10 atm and concluded that desorption of hydrogen was the rate-controlling step. A
* To w h o m
correspondence s h o u l d be addressed.
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-A120, 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-A1,O.J in a differential, fixed-bed reactor a t atmospheric pressure in the temperature range 598-699 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. Catalyst Preparation a n d Characterization 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 y-alumina with a BET surface area of about 220 m2/g. After impregnation, the catalyst was dried a t 373 K and calcined in an air stream for 5 h at 723 K. The catalyst composition was 0.3 wt 70Pt, 0.3 wt % Re, and the remainder alumina. For comparison, individual metal catalysts containing 0.3 wt '70 (Pt or Re) on y-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 w t % Pt-0.3 wt % Re (Pt-Re-Al,O,)
0196-4313/05/1024-0433$01.50/00 1985 American Chemical Society
434
Ind. Eng. Chem. Fundarn.. Vol. 24, No. 4, 1985
T -673°K t ~ / ~ = 2 6h iqm o l H2/M C H = 5
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Figure 2. Activity of Pt and Pt-Re catalysts with t,ime on stream, Table I. Experimental Feed Composition Figure
1.
Schematic diagram of t h e expermenial w i u p r u n no.
catalyst (metal area 0.35 m'/g). Pt-A120? and Re--A1,03 hereafter represent 0.3 wt 70Pt (metal area 0.29 m2/g) and 0.3 wt '70 Re (metal area 0.23 m'ig), respectivelv. iinlew otherwise stated Apparatus and Procedure Reactions were carried out in a fixed-bed type reactor with a continuous flow system at atmospheric pressure in the presence of added hydrogen. A general schematic diagram of the experimental apparatus is shown in Figure 1. Hydrogen gas flow from a high-pressure cylinder was measured by a calibrated rotameter and controlled by a needle valve. The MCH was fed by the calibrated metering pump into a preheater. The MCH feed was vaporized in 1.27-cm-0.d. tube wrapped with insulating heating tape. The vapor was then led to the reactor containing the catalyst bed by the incoming hydrogen as well as the diluent nitrogen gas. Prior to introduction of the MCH feed, the catalyst had been reduced with flowing hydrogen for 5 h at 698 K (Selman and Voorhies, 1975). The reactor was stainless steel tubing (2.54 cm in diameter and 15.24 cm long) placed into a salt bath. Temperatures a t various points in the reactor, including the catalyst bed and preheater. were measured with the help of chromel-alumei thermocouples. 'The average temperature was the mean of the temperatures measured at a point in the reactor at different time intervals and was controlled within f 0 . 5 of the desired value. The main stream of the effluents from the reactor wa5 cooled by an ice condenser. Reaction periods of 60 min were employed in all of the runs to ensure the attainment of steady-state conditions. After steady state had been attained, a minimum of seven runs were made in 20-min intervals. The average of the last two values of these analyses was used tor the estimates of conversion. Fresh catalyst was used in each experiment. The average size of the catalyst granules was 1.5 mm in diameter by 3 mm in length. The liquid condensate was analyzed by a gas chromatograph. A 2-m-long, 3.2-mm-diameter column of 10% SE-30 on Chromosorb W was used to separate the various components in each sample. Before the bulk of the experimental work was undertaken, certain preliminary studies were made to aid in an analysis of the results. The experimental conditions were so chosen that the reaction rate was not influenced by external and internal diffusion (Ross and Walsh, 1961). In the course of conducting these exploratory studies. i t was found that the major products from the dehvdroge-
I , 20. 39, 58
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methylcyclohexane. hydrogen, toluene, nitrogen. mol 70 mol % mol o/o mol % 16.7 83.3 16 1.7 10 14 i0 16 65 13 22 12 28 60 11 55 J4 10 50 40 9 4,i 46 52 8 40 -I 35 58 A4 6 30 a 25 70 4 20 76 :i I
