Alkyl Transfer Steps in the Catalytic Alkylation of Benzene, Toluene, and Cyclohexane
Total Page：16
File Type：pdf, Size：1020Kb
JOURNAL OF CATALYSIS 24, 233-240 (1972) Alkyl Transfer Steps in the Catalytic Alkylation of Benzene, Toluene, and Cyclohexane G. PARRAVANO Drpcrrtment oj Chemical and hfelallurgical Btcgi~~eering. University of Michignn, A& Arbor. Michignn @lob Received April 6. 1971 The rate of the catalytic redistribution of isotopic carbon in binary mixtures of benzene and toluene, benzene and ethylbenzene. toluene and xylene, toluene and ethylbenzene, cyclohexane and methylcyclohexane was investigated over supported Pt, Ir, Ru, and Au. The influence of the following experimental variables was as- sessed: hydrocarbon partial pressure ratios, 3 X 10e3 to 30; temperature, 185 to 400°C; catalyst support, ALO,, MgO; catalyst acidity and basicity, and method of catalyst preparation. The results are discussed in the framework of processes of com- petitive and reactive chemisorption of hydrocarbon mixtures. The dependence of the reaction rate upon t.he hydrocarbon partial pressure ratio is considered in detail. Displacement effects were found when xylene isomers were present simultaneously. The insight tha.t the carbon redistribution reaction may provide in investigations on the thermodynamic and kinetic asprcts of alkyl transfer strps at catalytic surfaces is pointed out. -4lkyl t.ran~fcr steps figure prominently catalytic surfatc sirnultanttouxly with the in catalytic alkylations. Their character- following step: istics are often indirectly gleaned from in- format.ion on the rate of net alkylation rc- *A(g) + R(s) --* *AI:(g). Ilb) actions. However, individual alkyl transfer The requcnce of reaction steps (la) and steps cannot be observed easily in a net (lb) corresponds to the exchange reaction: alkylation process operating at steady state. It is more convenient and direct to All(g) + *A(g) + A(g) + *AI<(g). (1) follow alkyl transfer at catalytic surfaces The asterisk in reactions (1) anti (lb) by suitable isotopic labeling of molecular represents an isotopic carbon atom. Since species differing in composition by a CH, there is no change in gas phase composition or higher alkenyl group. Consider the re- during the occurrence of reaction (1) and action between a CH, donor, ,4R, and a neglecting kinetic isotopic effects, the rates (‘H, acceptor, A, at :I catalytic surface, of reaction steps (la) and (lb) are equal to that of the overall reaction (1). Consc- AH(g) + A(g) + It(s), (18) quently, the rate of reaction (I) gives di- rectly that of the CH, transfer (la). These lvllere R. is a CH, group and the suffixes g considerations indicate that a convenient and s indicate gas and surface, respcctivcly. manner to study the rate of catalytic trans- Reaction (la) represents a key step in fcr of R between ,4R and A is to contact a catalytic alkylations. To assessits role in mixture of AR, “,4R, and A with the cata- the net reaction, the influence of catalyst lytic surface and follow the rate of appear- and reaction variables on the step rate ance of *A. We have carried out these cx- must be measured. To this end, consider 1,eriments with R being a methylene or transfer step (la1 taking place at the same ethylene group. In particular, tlic transfer Copyright @ 1972 by .4cadcmip Press. Inc.. 234 G. PARRAVANO was investigated between the hydrocarbons amounts of the various hydrocarbon com- of the following binary mixtures: ben- binations were flowed in a stream of He zene (BE) -toluene (TO) ; BE-ethyl benzene through the catalyst bed (l-5 mlj. The WEI ; TO-xylene (XE ) ; TO-EBE ; exit stream was analyzed and fractionated cyclohexane (CHA) - methylcyclohexane by gas chromatography; and radioactivity (MCHA). In the case of XE, the three counting on each fraction was carried out isomers were investigated independently by liquid scintillation techniques. Details and also in binary combinations. The range of the experimental setup and procedure of temperature employed was between 185 have been published previously (2). The and 400°C and that of the ratio of hydro- treatment of t’he experimental results fol- carbon partial pressure varied between lowed the method already described (2). 3 X lo-” to 30. Metal catalysts used were The rate of reaction (,I b) is given by: Pt, Ir, Au, and Ru supported on Al,O, and MgO. The possibility of following the rate of reaction (1) was suspected on the basis of previous studies showing that when where w catalyst weight, n&moles of *AR l”CH,TO was catalytically hydrodealkyl- and k,, k’,. rate coefficients of the forward ated, the BE produced was free from and reverse step of reaction (lb). Integra- radioactivity (1) . This was taken as an in- tion of Eq. (2) for a flow reactor yields: dication of a selective splitting of the CH,, group without skeletal rearrangements. EXPERIMENTAL METHODS where $‘, volumetric flow rate, p = p&p.,, Reagent grade hydrocarbons were used and the reaction conversion without further purification and high purity He gas was used as a carrier. Stock solu- tions of W-labeled BE, TO, and CHA were made from milligram portions of radioactively concentrated samples (0.5 to The subscripts e,O refer to equilibrium and 3 mCi). The catalysts tested included: initial conditions, respectively. The possi- Pt (0.4 wt %)-ALO,, Pt (0.4 wt %)-Al,O, bility of studying the rate of the transfer fluorided, Ir (0.7 wt %)--ALO,, Ru (2 wt step (la), by following the rate of the ex- %)-Al,O,, Au (0.3 wt %)-A1203, Au (2 change reactions (1) implied that no inter- wt %)-MgO. The Pt catalysts were com- ference from side reactions took place. The mercial samples, in the form of IA6 in. experimental conditions were specifically diameter spheres. Ir, Ru, and Au catalysts chosen to fulfill this point. The absence of were prepared by impregnation of high products was checked in each run. The area powdered A1,O3 with an H,O solu- former appeared at temperature or partial tion of the metal chloride, followed by pressure ratios higher than those reported. thermal decomposition. Details on these C&H, desorption was found absent. This preparations have been described previ- may have been due to the low equilibrium ously (2, 3). Part of the Ir catalyst was pressure corresponding to the temperature treated with sufficient amounts of NaHC03 and hydrocarbon ratios employed [ +lO+ solution to add to it 1% NaHCO,. Au atm for (pPTO/pBE)= I at 38O”C] and/or catalysts were also prepared by reduc- to low desorption rate. The conditions gen- tion of the Au salt with HCHO. Catalyst erally employed for the catalytic alkylation pretreatment included in situ heating in a of BE (4) and TO (5) are somewhat dif- stream of H, at 370°C for 1 hr, except for ferent from those of the present study. Pt-fluorided Al,O, for which the H, treat- ment was carried out at 450°C. The rate of EXPERIMENTAL RESULTS reaction (1) was studied in a flow system A direct check for the establishment of at a total pressure of 1 atm. Appropriate equilibrium during the course of reaction ALKYL THANSFEH. STEPS 235 LOG R FIG. 4. k, versus P~O/PBE = p for reaction (1) I I I I I catalyzed by Ru (2~wt C$)pAl&n: (0) 336°C; (0) -2 -I 0 I 390°C. LOG 4 FIG. 1. k, versus PTO/PHE = @ for reaction (1): presents the results obtained from experi- Pt (0.4 wt y0))-A1203 (0); *BE + EB ou Pt (1 ments on Au catalysts. In Fig. 6, we show wt Ojo)-fluorided A1203 (a), 390°C. the experimental results of the hydrocarbon (1) was not possible due to the smallness mixture TO-SE on Pt-Al,&, at 39O”C, of the equilibrium pressure of possible ole- while results at 210, 327, and 390°C for finic intermediates. The former was as- p-XE on a similar catalyst arc collected in sumed on the basis of the reversibility and Fig. 7. To obtain a direct indication of the reproducibility of the results. Reaction con- versions up to 50% were obtained with con- tact times of the order of 1 to 10 set at -I 0 I LOS p I I I I I -90’ -2 -I 0 I FIG. 5. k, versus ~TO/~HE = @ for reaction (1): LOG 4 (0 ) Au (2 wt s,)-MgO prepared by reduction; (A) Au (0.3 wt %)-A1~03 prepared by reduction; (0) FIG. 2. k, versus PTO/PBE = 6 for reaction (1) catalyzed by Pt(0.4 wt yG)-fluorided Al&: (0) Au (0.3 wt ‘j;)-AltO~ prepared by impregnation; 185°C; (A) 255°C; (0) 338°C; (0) 390°C. 386°C. 390°C. The effect of ,!3 on k, for the mix- relative rate of reaction between SE tures BE-TO and BE-EBE on Pt catalysts isomers, experiments on mixed XE feeds in the temperature range 187 to 393”C, is were carried out by introducing to the re- shown in Figs. 1 and 2. Figures 3 and 4 actor a mixture of TO + “TO + XE iso- summarize the observation on Ir and Ru mers. The results on the mixture o-XE + catalysk for BE-TO mixtures in the tem- VL-XE are reported in Fig. 8, while Fig. 9 perature range 276 to 39O”C, while Fig. 5 -6 -2 -8L---+J-I LOG B LOGoB FIG. 3. k, versus PTO,/PRE = fi for reaction (1): Fig. 6. k, versus ~TO/~BE = @ for reaction (1) (0) Ir (0.7 wt %)-AM&; (A) Ir (0.7 wt %)-Al& + catalyzed by Pt-Al&, 390°C: (@) O-XR; (0) lyc NaHCOr; 390°C. rn-xlz; CO) p-xl?. 236 G. PARRAVANO LOG /3 3 LOG 4 FIG.