Ind. Eng. Chem. Res. 1994,33, 1657-1664
1657
Isomerization of n-Heptane on an Oxygen-Modified Molybdenum Carbide Catalyst Edd A. BlekkanJ Cuong Pham-Huu, Marc J. Ledoux,’ and Jean Guillet Laboratoire de Chimie des Matkriaux Catalytiques, E.H.I.C.S., Universitk Louis Pasteur, 1, rue Blaise Pascal, F-67000 Strasbourg, France
The isomerization of n-heptane in the presence of hydrogen has been carried out over a molybdenum carbide catalyst modified by an oxygen treatment. Heptane was isomerized selectively to isoheptanes, a reaction which is difficult over traditional bifunctional catalysts due to extensive cracking. The C, products were mainly monomethylhexanes, 2-methylhexane and 3-methylhexane, in close to equilibrium ratios. A typical bifunctional catalyst (Pt supported on an acidic zeolite) gave similar isomerization products, but mostly propane and isobutane as the cracked products. The selectivity over the oxidized carbide was found to be a function of pressure but independent of the conversion; increased the hydrogen pressure led to a decrease in the C7 selectivity. This was found to be different from the Pt/zeolite catalyst, over which the selectivity was a function of the conversion; a high selectivity was only obtained a t low conversions. The active carbide-based catalyst was probably an oxycarbide of molybdenum. The results obtained over the oxidized carbide catalyst are discussed in terms of a bond-shift mechanism via a metallocyclobutane intermediate.
Introduction The use of transition metal carbides as catalysts has received a great deal of interest since the discovery of their catalytic activity and their similarity with noble metals in heterogeneous catalysis (Muller and Gault, 1970; Boudart and Levy, 1973; Boudart et al., 1981). The development of techniques to produce high surface area transition metal carbides (Volpe and Boudart, 1985; Lee et al., 1987,1988; Ledoux et al., 1989a, 1992a, 1993a) has renewed the interest in these materials as catalysts and supports. One area where these materials have shown a particular potential is as catalysts in hydrocarbon reactions. Clean carbide surfaces obtained by reductive treatment after the synthesis yield mostly hydrogenolysis products. It has however been shown that oxidation leads to an active and selective material in the isomerization of alkanes (Ledoux et al., 1993a). On WC and @-W2C prepared by temperature programmed carburization of WO3 in CHJH2 mixtures, Iglesia and co-workers (Ribeiro et al., 1991a,b; Iglesia et al., 1991) reported that oxidation gave a surface capable of alken isomerization without excessive hydrogenolysis. They explained their results by a bifunctional mechanism, assuming that the oxidation forms surface tungsten oxide with acidic sites, and showed by 13C-tracingthat heptene was more reactive than heptane in isomerization. Pham Huu et al. (Pham-Huu et al., 1993) reported a similar effect with high surface area Mo2C oxidized at 623 K in the isomerization of n-hexane and other C g hydrocarbons, but they attributed the effect to the formation of a new, catalytically active phase formed during the first hours of the hydrocarbonreaction, possibly an oxycarbide consisting of a Mo2C lattice where oxygen replaces some carbon atoms in the interstitial voids. In isomerization, high selectivities are easily obtained with CSand CS as feedstock, as opposed to C, or heavier hydrocarbons which give a high selectivitytoward cracking.
* Author to whom correspondence should be addressed.
+ Permanent address: Department of Industrial Chemistry, The Norwegian Institute of Technology (NTH), The University
of Trondheim, N-7034 Trondheim, Norway. Permanent address: Groupe des MatBriaux Inorganiques, IPCMS, EHICS, 1rue Blaise Pascal, 67000 Strasbourg, France.
*
This has been identified as a challenge in catalysis for cleaner fuels, as nonaromatic octane enhancement of gasoline becomes more important (Maxwell and Naber, 1992). Belloum et al. (Belloum et al., 1991) reviewed the literature on isomerization of CrC7 alkanes. The modern processes are performed in the gas phase, using combinations of acidic zeolites (usually mordenite or Y)and noble metals (Pt or Pd), and the feedstocks used in commercial processing are c4-C~hydrocarbons. The authors reported no commercial process for isomerization of C,. Sie (Sie, 1992,1993aIb)explained the tendency for C7+ hydrocarbons to crack in acid-catalyzedreactions by the protonated cyclopropane mechanism; hydrocracking and hydroisomerization share a common intermediate, a protonated cyclopropane. This species can crack easily if the chain length is seven or longer,but can only isomerize with chains of five and six C-atoms. With four C-atoms, only interconversion of the carbon atoms in the linear chain is catalyzed by this mechanism, which is also found experimentally, as reported by Belloum et al. (Belloum et al., 1991). The purpose of this paper is to report catalytic results in n-heptane isomerization on a Mo2C catalyst (Ledoux et al., 1993b), formed from the high surface area carbide synthesized according to the method of Ledoux and coworkers (Ledoux et al., 1989a, 1992b) followed by an oxidation and an activation period. Results obtained at atmosphericpressure and at pressures up to 1700 kPa will be reported and compared to experimentsperformed over a zeolite-supported Pt catalyst.
Experimental Section Atmospheric Pressure Apparatus. n-Heptane isomerization activity measurements were carried out in an all glass microreactor, described in detail elsewhere (Ledoux et al., 1992b). The system was equipped with greaseless valves, a flowmeter, a U-shaped silica reactor, a double thermal conductivity detector (TCD) system recording the constant pressure of reactant (provided by a saturator) before the reaction and the pressure of the products after the reaction, a system to extract the products for gas chromatography analysis, and a needle valve to regulate the gas flow. The catalyst was kept on a silica fritted disk,
QS88-5885/94/2633-1657$04.50/0 0 1994 American Chemical Society
1658 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
and the reactor was operated as a fixed bed system, at constant pressure (1 bar) and temperature. n-Heptane was introducedonto the catalyst at a constant partial pressure of 12 Torr, with H2 as complement to the atmospheric pressure. Medium-/High-pressureApparatus. Isomerization of n-heptane at higher pressures (200-1700 kPa) was performed in a reactor unit described in detail elsewhere (Ledoux and Djellouli, 1989b). Briefly, the reactor consisted of a copper-lined stainless steel tube with an inside diameter of 4 mm. The liquid feed was dosed with a high-pressure liquid chromatography (HPLC) pump (Gilson Model 302) and injected into the H2 stream (the H2 flow was regulated by Brooks 5850 TR flowmeters linked to a Brooks 5876 control unit) and subsequently vaporizedat the top of the reactor. The catalyst wasplaced between quartz wool wads in the center of the reactor, allowing preheating of the gases. The reactor was heated in a vertical electric furnace, controlled by a thermocouple. The reactor wall temperature was monitored by a separat thermocouple. The reactor pressure was regulated by a Grove membrane regulator, and samples were analyzed off line. Analysis. The products from both reactor units were analyzed by gas chromatography with a flame ionization detector (GC-FID), using a HP-PONA capillary column coated with methyl siloxane (50 m X 0.2 mm i.d., film thickness 0.5 mm), allowing the separation of hydrocarbons from C1 to C7. Reference data and pure component injections were used to identify the major peaks; some minor C7 peaks, including traces of olefins in the reactor exit gas, were not identified and were omitted from the data analysis. Calculations. The conversion and product distribution were calculated from the GC analysisof the exit gas. Kinetic data are reported as reaction rates (mol g1s-l),calculated assuming a differential reactor (conversion< 10%) or using pseudo-first-order kinetics from experiments with higher conversions. Selectivities are reported on a C7 basis. Materials. n-Heptane (Prolabo) was used as received. GC analysis indicated a purity of 99.3 wt %, the major organic impurities being methylcyclohexane (0.3% 1, 3methylhexane (3MHex) (0.1% 1, ethylcyclopentane (0.06%1, 2-methylhexane (2MHex) (0.04% ), and 3-ethylpentane (0.04 ?6 ), which were subtracted from the exit gas analysis before calculating the product distribution. According to the producer the n-heptane also contained sulfur (10 ppm), and some traces of inorganic materials. HZ(Air Liquide, grade U) was purified in a train consisting of a copper catalyst and a molecular sieve drier before metering. Catalysts. The high surface area molybdenum carbide (Mo2C) was synthesized by a high-temperature reaction between Moo3 vapor and active charcoal as described before (Ledoux et al., 1989a, 1992b). After analysis the material had a surface area of 125 m2/g (BET, liquid N2) and X-ray diffraction (XRD) showed only peaks corresponding to MozC. The apparent bulk density was 770 kg/m3, close to the value reported by Oyama (759 kg/m3) for Mo2C synthesized by temperature-programmed reduction (TPR) (Oyama, 1981). From the line broadening of the XRD carbide peaks the actual carbide surface area has been estimated to be at least 40 m2/g,the remainder being surface area attributed to unreacted charcoal (PhamHuu, 1991). This estimate of the actual carbide surface area is however very conservative, since spherical particles are assumed. Scanning electron microscope (SEM) analysis indicates that the unreacted charcoal forms the core of the carbide particles, which have a very rugged
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Figure 1. Activationperiod in isomerizationof n-heptane over MsC0,328 at 1 bar, 623 K,H2:HC = 62.
appearance (Pham-Huu, 1991). Before catalytic use the carbide was oxidized in a stream of flowing air at 623 K for 14h in order to activate the carbide as described before. These oxidized catalysts developed their catalytic activity and selectivityover a period of several hours under reaction conditions. Detailed characterization of these materials after the various activation treatments and especiallyXPS and XRD have already been reported (Ledouxetal., 1993a) and new high-resolution transmission electron microscopy interpretations will be published later (Delporte et al., personal communication). A typical bifunctional catalyst was also used for comparison. A 8-zeolite support with a Si/A1ratio of 12.5 was ion-exchanged 3 times with NH4N03 (aqueous, 1M) before drying, calcining in air (813 K, 3 h), and impregnation with (NH4)atClz to give a Pt loading of 0.8% using the incipient wetness technique. After the impregnation the catalyst was dried and subsequently calcined in air (573 K, 3 h). Before use the catalyst was reduced in situ at 773 K for 3 h and then cooled to the reaction temperature in flowing H2 before the hydrocarbon feed was introduced.
Results Activation of M0&!-0623 at Atmospheric Presssure. Figure 1 shows the development of the activity (n-C7 conversion)and the selectivity (to C7 isomers)of the MeC0 6 2 3 material with time on stream at 1bar. The activity is initially low, increases with time on stream, and levels off a t a conversion rate of 3.0 mol g l &after about 4 h of reaction. The selectivity to C7 isomers is initially low, but also increases with time on stream. The selectivity takes longer to reach its "steady-state" value. No deactivation of the isomerization activity is observed during 25 h under the n-heptanelhydrogen mixture at 623 K, 1 bar. There is however a slight decrease in the cracking rate, leading to a slight drop in the conversion with time on stream. The main products from the isomerization of n-heptane are 2-methylhexane (2MHex) and 3-methylhexane (3MHex); only traces of cyclic molecules are detected among the reaction products (
