Selective n-Butane Isomerization over High Specific Surface Area

Oct 1, 1997 - Reaction of MoO3 with a hydrogen and n-butane mixture at low temperature allows the formation of a high specific surface area MoO3-carbo...
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Ind. Eng. Chem. Res. 1997, 36, 4166-4175

Selective n-Butane Isomerization over High Specific Surface Area MoO3-Carbon-Modified Catalyst Pascal Del Gallo, Fre´ de´ ric Meunier,† Cuong Pham-Huu, Claude Crouzet, and Marc J. Ledoux* Laboratoire de Chimie des Mate´ riaux Catalytiques, ECPM-ULP, 1 rue Blaise Pascal, 67008 Strasbourg Cedex, France

Reaction of MoO3 with a hydrogen and n-butane mixture at low temperature allows the formation of a high specific surface area MoO3-carbon-modified catalyst, which is active and very selective for n-butane isomerization. The reaction is performed via a methyl shift mechanism probably involving a metallacyclobutane intermediate; among the products some homologation molecules are found to confirm this mechanism. I. Introduction Large quantities of n-butane are removed from the gasoline pool, and it is of interest to selectively isomerize it into isobutane which is a key molecule as well as to produce highly branched C7 and C8 paraffins for olefin alkylation or to form by dehydrogenation isobutene, a starting material for the synthesis of methyl tert-butyl ether (MTBE). Two types of catalysts are currently used industrially to perform the isomerization of n-butane, i.e., Pt-based catalysts on chlorinated alumina and Pt-based zeolitic catalysts on mordenite. The latter have the advantage of being easy to use and much less sensitive to the presence of poisons such as sulfur and water in the feedstock. However, deactivation still occurs, and sulfur and nitrogen tolerance is not well resolved. n-Butane isomerization studies (Corma, 1995) have reported that n-butane exhibits a low reactivity compared to nalkanes having a higher molecular weight. Hino et al. (Hino et al., 1979; Hino and Arata, 1980) have reported that Pt/SO42--ZrO2 catalyzes n-butane isomerization at 20 °C. Similar results have also been reported by Hsu et al. (1992) for n-butane isomerization over sulfated zirconia modified with iron and manganese oxides. Low-temperature isomerization is obviously of interest to the petroleum industry. However, zirconiasulfate catalysts rapidly deactivate (Ward and Ko, 1994; Cheung et al., 1995; Yori et al., 1995; Yori and Parera, 1995). There is a need to develop new processes using a novel, cheap and stable catalyst for the production of isobutane. Recently, it has been reported by Ledoux and coworkers (Ledoux et al., 1992a, 1993, 1996; Blekkan et al., 1994) that high specific surface area molybdenum oxycarbide catalyst could be prepared from oxidative treatment of high specific surface area Mo2C or low specific surface area MoO3. This catalyst exhibits a high activity and a very high selectivity for isomerization of n-alkanes between n-C6 and n-C8 into their corresponding mono- and dibranched isomers. The molybdenum oxycarbide phase exhibits properties different from those of MoO3 and also different from those of the clean carbide, which only gives hydrogenolysis when exposed to hydrocarbons and hydrogen (Ledoux et al., 1992b). * To whom all correspondence should be addressed. Telephone: (33) 03 88 60 24 72. Fax: (33) 03 88 41 68 09. E-mail: [email protected]. † Present address: Chemistry and Environmental Sciences Department, University of Limerick, Ireland. S0888-5885(97)00072-9 CCC: $14.00

Over molybdenum oxycarbide catalyst isomerization is performed via a bond-shift mechanism involving a metallacyclobutane intermediate (Pham-Huu et al., 1993; Blekkan et al., 1994). The isomers formed are mainly the mono- and dibranched molecules, and no trace of cyclic, aromatic, or unsaturated hydrocarbons is observed. In addition, isomerization of n-C6, n-C7, and n-C8 can be carried out over this catalyst at high total conversion (80%) without significant hydrogenolysis (less than 10%) (Ledoux et al., 1996). The molybdenumbased catalysts are very stable, and no deactivation occurs as a function of time on stream. The MoO3carbon-modified catalyst also exhibits a very high resistance toward poisons such as sulfur or basic nitrogen compounds in the feed (York et al., 1996; Del Gallo et al., 1996). On the classical bifunctional catalyst nitrogen compounds, even at very low concentrations (e2 ppm), drastically deactivate the catalyst by acid site titration. The aim of this paper is to report the selective isomerization of n-butane over the high specific surface area MoO3-carbon-modified (molybdenum oxycarbide) catalyst formed from reaction at atmospheric pressure between an unsupported MoO3 and the reactant mixture in a flow system. The isobutane selectivity is also followed as a function of time on stream, the n-butane/ hydrogen ratio, and the reaction temperature. After each treatment the catalyst is characterized by powder X-ray diffraction (XRD), temperature-programmed reduction (TPR), surface area measurements by BET, pore size distribution, and scanning electron microscopy (SEM). II. Experimental Section II.1. Catalysts. The high surface area MoO3-carbonmodified catalyst was synthesized by a low-temperature (350 °C) reaction between low surface area (1 m2 g-1) MoO3 and the hydrocarbon and hydrogen mixture (Ledoux et al., 1996; York et al., 1996). The activity and the selectivity of the catalyst developed after a period of several hours (activation period) under reaction conditions. Detailed characterization of this material, especially XPS, XRD, and high-resolution transmission electron microscopy (HRTEM), has already been reported (Delporte, 1995; Delporte et al., 1995). II.2. Apparatus. n-Butane 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 © 1997 American Chemical Society

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thermal conductivity detector (TCD) system recording the constant pressure of reactant before the reaction and the pressure of the products after the reaction, and a system to extract the products for gas chromatography analysis. The catalyst was placed on a silica fritted disk, and the reactor was operated as a fixed-bed system, at constant pressure (1 bar) and temperature. The reactor tube (inner diameter of 0.8 cm) was loaded with 0.3-1.0 g of catalyst, which resulted in a catalyst bed of ca. 1.0 cm in height. Premixed gases were taken from pressurized gas cylinders without further purification. The n-butane and the H2 flow were regulated by Tylan FC 260 flowmeters linked to a Tylan RO 28 control unit. In catalysis experiments, pure n-butane (Air Liquide, N35) was mixed with H2 (Air Liquide, U grade), resulting in a H2:n-C4H10 molar ratio of 9:1. The total volumetric flow rate was 70 ( 1 mL min-1 (NTP). II.3. Materials. n-Butane was used as received. GC analysis indicated a purity of 99.95 wt %, the major organic impurities being methane (0.01%), butene (0.02%), and isobutene (0.02%), which were subtracted from the exit gas analysis before calculating the product distribution. II.4. Analyses. The products were analyzed by offline GC-FID (HP5890 series II), using a HP-PONA capillary column coated with methylsiloxane (50 m × 0.2 mm id., film thickness 0.5 mm), allowing the efficient separation of hydrocarbons (saturated and olefinic) from C1 to C4 isothermally at -15 °C. II.5. Calculations. The conversion and product distribution were calculated from the GC analysis of the exit gas. Kinetic data are reported as reaction rates (mol g-1 s-1), calculated assuming a differential reactor (conversion e 10%) or using pseudo-first-order kinetics for experiments with higher conversions. Selectivities are reported on a C4 basis. The turnover frequency (TOF) per active site could not be evaluated because of the lack of knowledge on the nature of the sites which can be titrated by CO or other molecules. II.6. X-ray Diffraction (XRD). X-ray powder diffraction (XRD) was used for structural characterization of the samples. XRD measurements were performed on a Siemens Model D-5000 diffractometer with Cu KR monochromatic radiation (λ ) 1.540 56 Å). The measurements were performed with steps of 0.05° of 2θ and 5 s/step. The sample was crushed in an agate mortar in a glovebox kept under dry nitrogen in order to minimize the air pollution. II.7. Surface Area Measurements. The specific surface area (Sg) was obtained from nitrogen physisorption at liquid nitrogen temperature using a Coulter SA 3100 surface area and pore size analyzer. All the catalyst samples were sensitive to air after activation or reaction and therefore were transferred into the adsorption cell via a glovebox maintained under dry N2. The cell was equipped with a valve in order to avoid air contact during the transfer. The standard pretreatment consisted of heating the sample under dynamic vacuum at 300 °C for 3 h in order to remove the adsorbed water and other impurities. The measurement was made at liquid-nitrogen temperature with nitrogen (Air Liquide, U grade) as the adsorbate gas. The porosimeter allowed the measurement of different kinds of surface area contributions: SBET is the surface area of the sample calculated from the nitrogen isotherm using the BET method, SBJH is the surface area of all the pores except micropores calculated from the N2 desorption isotherm,

Figure 1. n-Butane isomerization activity and selectivity over MoO3-carbon-modified catalyst (350 °C, atmospheric pressure, H2/ n-C4 ) 9, catalyst weight ) 0.5 g, total flow rate ) 70 mL min-1).

Smicropore is the surface area of the micropores calculated using the t-plot method (De Boer, 1958; Mikhail et al., 1968). II.8. Temperature-Programmed Reduction (TPR). TPR analyses were performed in situ under pure hydrogen flow (10 mL min-1) in the same apparatus used for catalytic tests. The detailed description of the technique and the apparatus has been given elsewhere (Ledoux et al., 1987; Peter, 1991). Typically, the sample (0.02 g) after reaction, in a powder form, was cooled under H2 to room temperature and then heated from room temperature to 800 °C at a linear rate of 20 °C min-1. Water formed from TPR was detected by TCD interfaced with a NCR computer. The TPR profiles could be quantitatively analyzed by measuring the area under the peaks and comparing these values with a calibrated peak area. II.9. Scanning Electron Microscopy (SEM). The morphology of the MoO3-carbon-modified catalyst was observed by SEM using a Jeol Model JSM-850 operated at 20 kV and 10 mA. The observed samples were covered with gold in order to avoid the charge effect during the analysis. III. Results and Discussion III.1. Time on Stream Behavior for n-Butane Isomerization. III.1.1. Isomerization Activity and Selectivity. Figure 1 shows the development of the activity and selectivity of the MoO3-carbon-modified catalyst with time on stream at atmospheric pressure under the hydrocarbon and hydrogen mixture of 1:9 mole ratio at 350 °C. The activity was initially low but increased with time on stream to reach a steady state (8 × 10-7 mol g-1 s-1) after about 20 h of reaction. The results obtained here mean that the upper layers of MoO3 were not active for the isomerization reaction, which was in agreement with what was reported on the same catalyst during isomerization reactions carried out with heavier hydrocarbons (n-C6-n-C8) (Blekkan et al., 1994; Ledoux et al., 1996; York et al., 1996; Del Gallo et al., 1996). The activation period during which the activity and selectivity for n-butane isomerization increased was attributed to the time needed to transform the inactive low specific surface area MoO3 into an active molybdenum oxycarbide phase with high specific surface area by oxygen vacancy formation and by carbon atom incorporation as reported elsewhere (Delporte et al., 1995). The activation period observed here is directly comparable to the activation of a high surface

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Figure 2. Evolution of the oxygen and carbon content of MoO3, during the n-butane reaction, as a function of time on stream (350 °C, atmospheric pressure, H2/n-C4 ) 9, catalyst weight ) 0.5 g, total flow rate ) 70 mL min-1).

area Mo2C with n-hexane or n-heptane, in otherwise similar conditions (Ledoux et al., 1993; Pham-Huu et al., 1993; Blekkan et al., 1994; Pham-Huu et al., 1995). The transformation occurred through the reduction process which induced the formation of oxygen vacancies in the oxide matrix. These oxygen vacancies were rapidly filled by carbon atoms coming from the gas phase to form the oxycarbide phase, which was the active and selective isomerization catalyst (Ledoux et al., 1995; Delporte et al., 1995). During this transformation a part of the oxygen vacancies formed underwent rearrangments to form a more stable suboxide structure, MoO2, which is an inactive catalyst for isomerization. The amount of oxygen removed and carbon incorporated was monitored using absorption atomic spectroscopy, and the results are presented in Figure 2. It can be seen that the carbon content increased regularly with the reaction duration even after the isomerization activity had reached its steady state. The results observed suggested the formation of some carbonaceous residues on the catalyst surface which were responsible for the small drop in the isomerization activity as shown in Figure 1. The increase in the carbon content of the catalyst as a function of time on stream could also be attributed to the formation, besides the oxycarbide phase, of some more reduced phases (carbide-like phase), as the isomerization selectivity showed a slight decrease during the reaction time. However, the concentration of the carbide-like phase was negligible, as the isomerization selectivity remained high even after more than 48 h of reaction (88%). An extensive study of this new phase had been made by Delporte et al. (Delporte, 1995; Delporte et al., 1995, 1997) using different physical techniques. XPS analyses clearly showed that different sorts of carbon were detected on the surface besides the carbon of contamination. One of the carbon peaks could only be attributed to an intermediate product, an oxycarbide, between pure carbon and molybdenum carbide. Detailed product distributions are given in Table 1 as a function of time on stream. Small traces of olefins were observed among the reaction products, together with n-pentane and isopentane (the last two being homologation products). It has been reported (Pham-Huu et al., 1993; Blekkan et al., 1994; Ledoux et al., 1996) that isomerization of nalkanes over molybdenum oxycarbide catalyst occurred via a metallacyclobutane mechanism (Figure 3). Metathesis products, isopentane and n-pentane, contributed to about 1-2.5% of the reaction products. The

Figure 3. Mechanistic scheme for the n-butane isomerization via a bond-shift mechanism involving a metallacyclobutane intermediate.

Figure 4. Evolution of C5 products (isopentane and n-pentane) as a function of time on stream during the n-butane reaction over the MoO3-carbon-modified catalyst (350 °C, atmospheric pressure, H2/n-C4 ) 9, catalyst weight ) 0.5 g, total flow rate ) 70 mL min-1).

formation of C5 compounds occurred in parallel to the isomerization activity of the catalyst, meaning that sites which were able to perform homologation were formed as a function of the reaction duration (Figure 4). In the reaction of n-butane, the metallacyclobutane intermediate dissociated into methylidene and π-bond propylene, which was subsequently replaced either by but-1-ene or by but-2-ene to give n-pentane and isopentane. Butenes were detected in small quantities among the C4 products, meaning that some dehydrogenation functions were active on the catalyst. Among the cracked products the C3 + C1 fraction was predominant (6070% of the total cracked fraction) and only traces of extensive hydrogenolysis were observed (1-3% of the total cracked fraction) (Table 1). In conclusion this showed that under the reaction conditions employed in this study only single-bond scission occurred and that there was no multiple hydrogenolysis of n-butane. The C3 + C1 fraction detected in the reaction products was

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4169 Table 1. n-Butane Isomerization over MoO3-Carbon-Modified Catalyst at Atmospheric Pressure and 350 °C (Reaction Conditions: Mass of the Catalyst ) 0.5 g, H2/n-C4H10 Ratio ) 9:1, WHSV ) 2 h-1, Total Flow Rate ) 70 mL min-1) time on stream/h 2

6

18

22

28

47

conversion/% 0.4 1.4 9.4 9.6 9.7 8.6 0.4 1.4 8.7 8.8 8.9 8.0 rate/10-7 mol g-1 s-1 C4 selectivitya/% 100.0 95.0 93.0 92.0 91.0 87.0 homologation selectivityb/% 0.0 2.3 1.2 1.0 1.1 1.0 cracking selectivity/% 0.0 2.7 5.8 7.0 7.9 12.0 Distribution of the Reaction Products/% C4 products/% iso-C4 76.0 90.5 98.6 98.6 98.7 98.8 iso-C4d 0.0 0.0 0.0 0.0 0.0 0.0 n-C4d 24.0 9.5 1.4 1.4 1.3 1.2 cracked products/% C3 + C1 67.0 66.2 69.7 68.6 67.0 64.2 2C2 32.0 32.3 29.3 30.7 31.6 33.3 4C1 1.0 1.5 1.0 0.7 1.4 2.5 a Total C selectivity; iso-C ) isobutane, iso-C 4 4 4d ) isobutene, n-C4d ) n-butene. b Total C5 homologation selectivity: iso-C5 and n-C5.

Figure 6. TPR spectra of (a) the starting MoO3 and (b) MoO3carbon-modified catalyst after 48 h of reaction at atmospheric pressure in the presence of a n-butane and H2 mixture at 350 °C (catalyst weight ) 0.01 g, H2 flow ) 10 mL min-1, heating rate ) 20 °C min-1).

Figure 5. XRD pattern of (a) the starting MoO3 and (b) the MoO3carbon-modified catalyst after 48 h of reaction at atmospheric pressure in the presence of a n-butane and H2 mixture at 350 °C.

attributed to the hydrogenation of the intermediate methylidene and propylene fragments before their recombination into n-butane or isobutane. This mechanism is well illustrated in the literature and is based on the alkene metathesis reaction mechanism (Rappe´ and Goddard, 1982; Kazuta and Tanaka, 1990; Ribeiro et al., 1991a). III.1.2. Powder X-ray Diffraction (XRD). The XRD pattern of the MoO3-carbon-modified catalyst after >50 h of reaction at 350 °C is given in Figure 5B. All of the MoO3 (Figure 5A) had disappeared, and the diffraction pattern only showed the presence of MoO2 and diffraction lines corresponding to the molybdenum oxycarbide phase (Delporte, 1995; Delporte et al., 1995). No other crystalline phase was detected. MoO2 was believed to be formed as a side product along with the oxycarbide during the course of the transformation. Similar results have already been reported by Volpe and Boudart (Volpe and Boudart, 1985a,b) during topotactic synthesis of Mo2N from MoO3.

Since MoO2 stayed in the form of large crystallites giving narrow XRD peaks, it hardly contributed to the increase in the specific surface area of the material (see below). The mean particle size of the oxycarbide estimated from diffraction line broadening was around 8 nm. The XRD pattern also exhibited a high background noise, meaning that a significant amorphization had occurred on the sample during the reaction. Highresolution transmission electron microscopy performed on the material after reaction showed the presence of a significant amount of amorphous phase inside the MoO3-carbon-modified material (Delporte et al., 1995). III.1.3. Temperature-Programmed Reduction. TPR spectra of the starting MoO3 and the MoO3-carbonmodified catalyst after 48 h of reaction at 350 °C are presented respectively in parts a and b of Figure 6. The starting MoO3 exhibited two well-resolved reduction peaks located at 522 and 647 °C, which could be attributed to the sequence of reduction MoO3 f MoO2 f Mo, according to the results published in the literature (Oyama, 1992; Ledoux et al., 1993). On the MoO3carbon-modified sample several reduction peaks located at 548, 631, and 703 °C were observed. The peaks located at 548 and 631 °C were attributed to the reduction of MoO3 f MoO2 and MoO2 f Mo as for the starting MoO3 material. The peak located at high temperature (703 °C) was attributed to the reduction of the molybdenum oxycarbide phase as already proposed over MoO3-carbon-modified catalyst during the

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Figure 7. Evolution of the catalyst surface area (Sg) as a function of time on stream (350 °C, atmospheric pressure, H2/n-C4 ) 9, catalyst weight ) 0.5 g, total flow rate ) 70 mL min-1).

n-hexane isomerization (Delporte, 1995). The MoO3carbon-modified phase was very sensitive to oxygen, as shown by the strong decrease in the TPR peak of the sample after air exposure at room temperature for a few seconds (Figure 6c). The area of the reduction peak due to MoO2 was higher compared to the peak from the MoO3-carbonmodified phase, which meant that during the course of the reaction a higher amount of MoO2 was formed compared to that of the MoO3-carbon-modified phase, and this is in close agreement with the XRD results shown in Figure 5. III.1.4. Surface Area and Pore Size Measurements. Sg of the catalyst after reaction steadily increased from 1 m2 g-1 to around 100 m2 g-1 (Figure 7). The increase in Sg could be attributed to the simultanous formation of different phases from the starting MoO3 which allowed the breaking of the matrix, forming pores and channels which contributed to the increase in the surface area from the low surface area nonporous MoO3 (Delporte, 1995). The pore size distribution of the MoO3 and MoO3-carbon-modified catalyst is presented in Figure 8a and shows a significant increase in the number of small pores located between 2.5 and 10 nm, which explains the increase in the sample surface area. However, it is significant to note that the pore size distribution only took into account pores with a radius larger than 3 nm, while the narrow pores (95%). The analysis of the product distribution led to the conclusion that the isomerization reaction over MoO3-carbon-modified catalyst was performed via a nonbifunctional mechanism; the bond-shift mechanism involving a metallacyclobutane intermediate was proposed. The catalyst was stable as a function of time on stream (48 h), meaning that deactivation due to carbonaceous residues was negligible under the reaction conditions used. As a function of time on stream, a slight decrease in the isomerization selectivity was observed (88% instead of 100% after 45 h of reaction). This phenomenon was attributed to the formation of a superficial carbide-like phase, which is an active catalyst for the hydrogenolysis reaction (Boudart and Levy, 1973; Lee et al., 1990; Ledoux et al., 1992b). III.2. Kinetic Study. III.2.1. Reaction Order with Respect to the Hydrogen Partial Pressure. The effect of the hydrogen partial pressure on the isomerization activity and selectivity is shown in Figure 10. It should be noted here that the total pressure for this set of experiments was held constant at atmospheric pressure and the partial pressure of the nbutane was held at 76 Torr. As the H2 pressure was varied, the balance was made up with helium. The results obtained show that the hydrogen partial pressure has a negative effect on the n-butane isomerization reaction. The C4 selectivity remains high at 92% and unchanged as a function of the H2 partial pressure. However, in the absence of hydrogen in the reactant mixture, the isomerization activity and selectivity dras-

b

Figure 11. (a) Reaction order with respect to H2 over the MoO3carbon-modified catalyst at atmospheric pressure and 350 °C. (b) Reaction order with respect to n-butane over the MoO3-carbonmodified catalyst at atmospheric pressure and 350 °C.

tically decreased, which means that the H2 was necessary to avoid heavy dehydrogenated fragment formation which can poison the catalyst surface. The partial order of the reaction with respect to H2 is presented in Figure 11a: -0.27 for isomerization and -0.1 for hydrogenolysis. III.2.2. Reaction Order with Respect to the n-Butane Partial Pressure. In this study, the balance was held with helium and the H2 partial pressure was held constant at 690 Torr. The partial order of the reaction with respect to n-butane is presented in Figure 11b: 0.76 for isomerization and 0.59 for hydrogenolysis, quite close to 1, meaning that the hypothesis made for the rate calculation was acceptable.

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III.2.3. Summary and Conclusion. The negative order of the reaction with respect to the hydrogen partial pressure can be explained by the fact that at high H2 partial pressure a competitive adsorption of hydrogen and n-butane for the same sites lowered the amounts of n-butane to be transformed. Similar results have already been reported by Liu et al. (1995). III.3. Effect of the Reaction Temperature. In heterogeneous catalysis the reaction temperature is an important parameter which must be carefully controlled in order to obtain the highest activity, compatible with high selectivity, without significant deactivation as a function of the reaction duration. A series of experiments were performed on samples at different reaction temperatures ranging from 350 to 400 °C and under the hydrogen and hydrocarbon ratio of 9:1. The catalyst after reaction was characterized by different techniques such as XRD, TPR, and surface area and pore size distribution. III.3.1. Isomerization Activity and Selectivity. At reaction temperatures up to 400 °C the isomerization activity exhibited behavior similar to that observed on the sample activated at 350 °C (Figure 12a,b): after an activation period during which the isomerization activity quickly increased, the catalyst reached steady state. When the reaction temperature was increased to 450 °C, the activity was increased almost linearly as a function of time on stream (Figure 12c) and no steady state was observed for the duration of the experiment. This difference could be explained by the mechanism of the transformation involved: for reaction temperatures e 400 °C, the higher rate of formation of the oxygen vacancies resulted in the rapid collapse of the structure to form the stable molybdenum dioxide phase (MoO2), preventing the formation of a bulk carbide phase; the steady state observed in the experiment was attributed to the activity of the superficial carbide phase formed during the first hours of reaction; for reaction temperatures g 450 °C, the MoO2 formed was continuously transformed into the corresponding carbide, which is known to be an active catalyst for the hydrogenolysis reaction, and thus the activity increases as a function of the reaction duration. At a temperature of 375 °C the selectivity increases to reach a maximum (95%) and then monotonously decreases, and after 48 h of reaction the cracked products contribute to about 40% of the reaction products, meaning that the catalyst surface containing oxycarbide was partly transformed into carbide (Figure 12a). Increasing the reaction temperature to 400 °C results in a decrease of isobutane selectivity to 19% after 48 h of reaction (Table 3). Increasing reaction temperature also leads to the formation of butenes among the C4 products because of the endothermicity of the dehydrogenation reaction. Delporte et al. (1995) using X-ray photoelectron spectroscopy (XPS) have reported that the MoO3-carbon-modified surface undergoes carburization at a reaction temperature of 375 °C. The catalyst surface carburization was directly connected with the loss of the isomerization selectivity and the appearance of extensive hydrogenolysis products among the cracked products. III.3.2. XRD. The XRD patterns of the different catalysts treated at various temperatures are presented in Figure 13. For all the catalysts activated at temperatures higher than 350 °C the XRD background was

a

b

c

Figure 12. Isomerization activity and selectivity of the n-butane reaction over the MoO3-carbon-modified catalyst at atmospheric pressure and under different reaction temperatures: (a) 375 °C, (b) 400 °C, (c) 450 °C.

greatly decreased, meaning that the samples were becoming more crystalline than the sample activated at 350 °C. The XRD pattern of the catalyst after reaction at 375 °C for 48 h (Figure 13a) showed diffraction lines corresponding to MoO2 and the oxycarbide. At 400 °C the XRD pattern of the catalyst after 48 h of reaction only presented the diffraction lines corresponding to MoO2 (Figure 13b). No traces of oxycarbide or carbide were observed. Apparently, the reaction temperature was too high and favored the direct reduction of MoO3 to MoO2, which was stable in a bulk form, at the expense of the transformation of MoO3 into molybdenum oxycarbide. The observed results mean that the reaction temperature used here was not high enough to form bulk carbide, and it is thought that the carbide is only formed at the surface of the catalyst. At a reaction temperature of 450 °C the XRD pattern of the sample

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4173 Table 3. n-Butane Isomerization over MoO3-Carbon-Modified Catalyst at Atmospheric Pressure at Different Reaction Temperatures (Reaction Conditions: Mass of the Catalyst ) 0.5 g, H2:n-C4H10 Ratio ) 9:1, WHSV ) 2 h-1, Total Flow Rate ) 70 mL min-1)a reaction temperature/°C conversion/% rate/10-7 mol g-1 s-1 C4 selectivityb/% homologation selectivityc/% cracking selectivity/%

350

375

400

450

8.7 8.0 87.0 1.0 12.0

9.4 8.7 62 1.6 36.4

11.2 10.2 18.5 1.4 80.1

10.6 9.7 12.3 0.7 87.0

Distribution of the Reaction Product/% C4 products/% iso-C4 98.9 95.0 53.2 iso-C4d 0.0 0.0 0.0 n-C4d 1.1 5.0 46.8 cracked products/% C3 + C1 32.0 28.0 24.0 2C2 32.0 35.0 34.0 4C1 36.0 37.0 42.0

23.1 0.0 76.9 14.0 22.0 64.0

a The results presented are obtained on the catalyst after 48 h of reaction for the given temperature. b Total C4 selectivity: isoC4 ) isobutane, iso-C4d ) isobutene, n-C4d ) n-butene. c Total C5 homologation selectivity: iso-C5 and n-C5.

Figure 14. TPR spectra of the MoO3-carbon-modified catalyst at atmospheric pressure and under different reaction temperatures: (a) 375 °C, (b) 400 °C, (c) 450 °C.

Figure 13. XRD patterns of the MoO3-carbon-modified catalyst at atmospheric pressure and under different reaction temperatures: (a) 375 °C, (b) 400 °C, (c) 450 °C.

showed the presence of diffraction lines corresponding to MoO2 along with some broad peaks which can be attributed to the Mo2C phase (Figure 13c). It has been reported by Lee et al. (Lee et al., 1987) during the topotactic synthesis of Mo2C from MoO3 with a CH4/H2 mixture that the intermediate product, MoO2, started to be transformed into Mo2C at a temperature of around 623 °C. In the present study, the transformation of MoO2 into Mo2C was minimized because of the low temperature used (kinetic limitation). III.3.3. TPR. TPR spectra of the different samples at steady-state as a function of the reaction temperature are presented in Figure 14. Reduction peaks located around 640 and 700 °C were observed over the catalysts treated at 375 and 400 °C, respectively (Figure 14a,b). The peak located around 640 °C was attributed to the

reduction of MoO3 f MoO2. The peak located at high temperature (700 °C) was attributed to the reduction of the molybdenum oxycarbide phase. The relative surface of this peak decreased as a function of the reaction temperature as shown in Figure 14, meaning that at high temperature (>350 °C) the formation of the oxycarbide is lowered compared to the formation of the side products, MoO2, and the superficial Mo2C phase. It is probable that at temperatures higher than 350 °C, the process of diffusion of hydrogen to reduce MoO3 was more rapid than that of carbon, resulting in the formation of more MoO2 phase at the expense of MoOxCy. On the TPR spectra almost no peak of CH4 due to Mo2C decarburization (Ledoux et al., 1992b) was observed whatever the reaction temperature, meaning that the Mo2C phase was only present on the sample as a superficial phase which could not be detected by the TPR technique used. III.3.4. Surface Area and Pore Size Measurements. The Sgs at steady state of the catalyst after reaction at temperatures g 350 °C were much lower compared to that obtained at 350 °C. The surface area decreased from 100 m2 g-1 at 350 °C to around 40 m2 g-1 at 400 °C (Figure 15). At high reaction temperature the MoO2 (compact structure) formation was probably rapid, which resulted in the decrease in the overall surface area. The high rate of oxygen removal also increased the water formation inside the sample, which could have induced the surface area loss by hydrothermal sintering. The results observed were in close

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superficial molybdenum carbide phase when isomerization was carried out at reaction temperatures g 375 °C (Delporte et al., 1995, 1997). IV. Conclusion

Figure 15. Evolution of the surface area of the MoO3-carbonmodified catalyst as a function of the reaction temperatures. The surface area was measured after 48 h of reaction.

Figure 16. Cracked product distribution of the n-butane reaction over the MoO3-carbon-modified catalyst at atmospheric pressure and under different reaction temperatures.

agreement with the results obtained from XRD, which showed a significant increase in the sample crystallinity. III.3.5. Summary and Conclusion. At temperatures higher than 350 °C the oxycarbide phase is progressively transformed by incorporating more and more carbon atoms into a carbide phase which exhibits a very high extensive hydrogenolysis to form methane (Lee et al., 1987; Ribeiro et al., 1991a,b; Iglesia et al., 1992). It is interesting to note that the isomerization rate only slightly increases, from 7.9 to 9.1 × 10-7 mol g-1 s-1, when increasing the reaction temperature from 350 to 400 °C. These results could be explained as follows: at 350 °C, reaction under the hydrogen and hydrocarbon mixture led to an increase in the surface area from 1 to 100 m2 g-1 and consequently an increase in the number of active sites for the isomerization reaction. At temperatures ranging from 375 to 400 °C a slow increase in the isomerization rate was observed due to the fact that the decrease in the surface area was compensated by the increase in isomerization rate by increasing the reaction temperature. Isomerization selectivity also strongly decreased kinetically with increasing reaction temperature due to thermal cracking and to the modification of the nature of the catalyst surface, i.e., more carbidic. At 350 °C isomerization was accompanied by single C-C bond scission as reflected by the near absence of C1 products in the exit gas (Figure 16). At higher reaction temperatures, the propane formed by the rupture of a terminal C-C bond might, in turn, undergo hydrogenolysis to yield ethane and methane. XPS analysis performed on an oxidized molybdenum metal foil showed the formation of a

Reaction of MoO3 with a hydrogen and n-butane mixture at low temperature allows the formation of high specific surface area (100 m2 g-1) MoO3-carbon-modified catalyst which is active and very selective for n-butane isomerization: isobutane contributes about 95% to the reaction products. Over the MoO3-carbon-modified catalyst the isomerization is performed via a methyl shift mechanism involving a metallacyclobutane intermediate. Among the reaction products some homologation molecules are also observed and confirm once again the nature of the mechanism. Almost no deactivation was observed after 48 h of reaction at 350 °C whereas a slight decrease in the C4 selectivity was observed, meaning that during the course of the reaction some carbidic phase was formed on the catalyst surface. The lack of deactivation was attributed to the nature of the reaction mechanism which prevents the formation of carbonaceous residues. The MoO3-carbon-modified phase is stable in the range of temperature around 350-365 °C; at higher temperatures the amount of carbon incorporated increases and the oxycarbide phase is transformed into a carbide phase which exhibits a high reactivity toward the cracking reaction, especially for extensive hydrogenolysis. The surface area of the catalyst also decreases with increasing reaction temperature due to the formation of greater amounts of MoO2, which exhibits a more compact structure than that of the MoO3-carbonmodified phase. Acknowledgment This work was supported by the Pechiney Co. and the European Community (Brite Euram Program). SEM observations were performed in the Groupe des Mate´riaux Inorganiques of the Institut de Physique & Chimie des Mate´riaux de Strasbourg (CNRS, UMR 46). Literature Cited Blekkan, E. A.; Pham-Huu, C.; Ledoux, M. J.; Guille, J. Isomerization of n-Heptane on an Oxygen Modified Molybdenum Carbide Catalyst. Ind. Eng. Chem. Res. 1994, 33, 1657-1664. Boudart, M.; Levy, R. Platinum-like Behavior of Tungsten Carbide in Surface Catalysis. Science 1973, 181, 547-549. Cheung, T. K.; d’Itri, J.; Gates, B. C. Cracking of n-Butane Catalyzed by Iron- and Manganese-Promoted Sulfated Zirconia. J. Catal. 1995, 153, 344-349. Corma, A. Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon Reactions. Chem. Rev. 1995, 95, 559-614. De Boer, J. H. In The Structure and Properties of Porous Materials; Everett, D. H., Stone, F. S., Eds.; Butterworths: London, 1958. Del Gallo, P.; Pham-Huu, C.; York, A. P. E.; Ledoux, M. J. Comparison of the Effects of Nitrogen Poisoning on Molybdenum Oxycarbide and Pt/β-zeolite Catalysts in the Isomerization of n-Heptane. Ind. Eng. Chem. Res. 1996, 35, 3302-3310. Delporte, P. Etude d’un oxycarbure de molybde`ne catalytiquement actif: caracte´risation physique et activite´. Ph.D. Dissertation, University of Strasbourg, France, 1995. Delporte, P.; Meunier, F.; Pham-Huu, C.; Ve´nne´gues, P.; Ledoux, M. J.; Guille, J. Physical Characterization of Molybdenum Oxycarbide Catalyst; TEM, XRD and XPS. Catal. Today 1995, 23, 251-267. Delporte, P.; Pham-Huu, C.; Ledoux, M. J. Effect of the Reaction Temperature and Hydrocarbon Partial Pressure on the Activity of Carbon-Modified MoO3 for n-Hexane Isomerization. Appl. Catal. A 1997, 149, 151-180.

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Received for review January 24, 1997 Revised manuscript received May 13, 1997 Accepted July 22, 1997X IE9700728

X Abstract published in Advance ACS Abstracts, September 15, 1997.