Oxidative dehydrogenation of isobutane at short contact times $

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1 Applied Catalysis A: General 179 (1999) 93±106 Oxidative dehydrogenation of isobutane at short contact times $ Lisa S. Liebmann, L.D. Schmidt * Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Received 30 October 1997; received in revised form 17 August 1998; accepted 21 August 1998 Abstract The oxidative dehydrogenation of isobutane over noble metal coated ceramic foam monoliths selectively produces isobutylene with high conversions and yields at short contact times in an autothermal reactor at atmospheric pressure and temperatures of 800±9008C. Maximum selectivity of 71% to total ole ns is achieved with O 2 and reactant preheat of 3508C at a fuel/oxygen ratio of 1.7. Operation at a constant fuel/oxygen ratio of 1.2 while varying the space velocity from to h 1 increases selectivities to C 4 -ole ns and decreases selectivities to C 2 -ole ns by 8%, although the fuel conversion drops from 80% to 40%. To investigate the effects of the physical and chemical nature of the catalyst, Pt, Rh, Ir, Pd, and Pt±Sn catalysts were examined, with Pt being optimal for ole n production and catalytic activity decreasing in the order Pt>Pd>Rh>Ir. Pt±Sn deactivated by carbon formation and metal loss while Pd deactivated by rapid carbon formation. Monolithic catalysts with various pore sizes (20, 45, 80 ppi) show that in the fuel lean regime, smaller pores (80 ppi), as compared to catalysts with more open channels (20 ppi), lead to 20% higher fuel conversion but 10±15% lower selectivity to ole ns. Results indicate that either a purely catalytic b-elimination mechanism or a heterogeneously assisted homogeneous reaction mechanism can explain the product distribution. The causes of secondary reactions, oxygen breakthrough, and the effect of increased ow rate, and increased mass transfer are discussed. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Isobutane oxidation; Millisecond reactors; Partial oxidation; Monolith catalysts; Ole n synthesis 1. Introduction Isobutylene, a key component of methyl tert butyl ether (MTBE), is a chemical used as an oxygenate for reformulated gasoline, and much research has been undertaken on its synthesis by oxidative [2,4±6], nonoxidative [1,3], as well as steam cracking [7,8] reaction conditions. *Corresponding author. Tel.: ; fax: $ This research was supported by NSF CTS and by CR & L, Pasadena, TX. Currently, isobutylene is produced industrially by the endothermic dehydrogenation of isobutane. Using acr 2 O 3 ±Al 2 O 3 catalyst at 900 K at a residence time of 1 s, the selectivity to isobutylene is nearly 100%, but fuel conversion is less than 5% [1]. Higher temperatures favor coke formation, requiring frequent catalyst regeneration. A large amount of process heat is required, and the process is limited by thermodynamic constraints. The kinetics and mechanism of isobutane dehydrogenation on Pt±Sn, Pt±K, and Pt±In have been extensively studied [9,10]. Similar studies have been done for these metals on various supports, such as chromia supported on Al 2 O 3, SiO 2, and ZrO 2 [3] X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S X(98)

2 94 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93±106 Alternative reactor con gurations for non-oxidative conditions include cross- ow moving bed reactors and membrane reactors [1]. Oxidative dehydrogenation of isobutane is potentially much more economical since this reaction is exothermic: i-c 4 H O 2! i-c 4 H 8 H 2 O; H ˆ 103 kj=mol Several pyrophosphate catalysts have been used, with Ni 2 P 2 O 7, giving an isobutylene selectivity of 82% at 10% isobutane conversion at 5508C [4]. The oxidative dehydrogenation of lower alkanes (C 2 ±C 5 ) has been demonstrated on vanadium oxide-based catalysts [5] as well as over catalysts containing SnO x and PO x, where Sn tetrahalides or tetraalkyl and phosphorous pentahalides have been shown to be the catalyst [6]. Recent research in this laboratory has shown that ole ns can be produced in high selectivities and yields by the catalytic oxidative dehydrogenation of C 2 ±C 4 alkanes over monolithic catalysts [11±13]. Autothermal operations and millisecond contact times produce non-equilibrium products without carbon formation. Reactors are 10±100 times smaller than those currently used for the same production rate, and the autothermal operations require no heat input. Preliminary research into the oxidative dehydrogenation of isobutane showed slightly higher selectivities of isobutylene as compared to recent work [13]. The objective of this work is to extend the previous work by examining the oxidative dehydrogenation of isobutane to isobutylene in terms of different operating conditions (preheat, air vs. oxygen, ow rate) and the physical and chemical nature of the catalyst (geometry, support material, noble metal) in order to: (1) explore safe operating regimes, (2) maximize yields, (3) determine the roles of heterogeneous and homogeneous chemistry, and (4) provide insight into the nature of the reaction mechanism. 2. Experimental The apparatus used is similar to that described previously for the partial oxidation of light alkanes [11±13]. It consists of a quartz tube reactor (18, 11, 5 mm diameter), 40 cm long, with a noble metal coated ceramic foam monolith 10 mm long as the catalyst. Uncoated monoliths before and after the catalyst serve to reduce radiation losses. Before insertion into the quartz tube, the catalyst and heat shields are wrapped in Fiberfrax (silica±alumina) cloth for insulation and to prevent reactant bypass of the catalyst. Additional high temperature insulation around the reactor provided operation within 508C of the predicted adiabatic reaction temperature. Noble metal coated ceramic foam monoliths (surface area <70 cm 2 /g, void fraction 0.8 and pore sizes of 20, 45, and 80 pores per linear inch) were prepared by impregnating the a-al 2 O 3 monoliths with a saturated solution of the appropriate metal salt. After the monolith was dried in air, it was calcined in O 2 at 6008C for 1 h and reduced in 10% H 2 /Ar at 6008C for 4 h. This procedure resulted in metal loadings of 5 wt%. High purity O 2,N 2, and i-c 4 H 10 (99.5%) ows were controlled with mass ow controllers with an accuracy of 0.05 SLPM. Because reactant gases were premixed before entering the reactor, check valves inserted into the lines prevented propagation of ames or explosions into the gas handling system. Heated product gas lines prevented water condensation. A fraction of the product gases were sent to a HP 5890 Gas Chromatograph for analysis. The GC used a Haysep D packed column, with helium as the carrier gas, and a thermal conductivity detector. Since nitrogen is inert in this system, it was used for calibrations of mass balances. Selectivity, conversion and yield, de ned as described previously, are on a carbon atom basis to account for mole number changes [11±13]. An oxygen mass balance was used to calculate the amount of water produced. The results shown are believed to be accurate to within 2%. Carbon and hydrogen mass balances closed to 5%. The reported reaction temperature was measured using a Pt±Pt/13% Rh thermocouple inserted between the back of the catalyst and the downstream radiation shield. For some experiments, a signi cant temperature difference existed between the front and the back of the monolith. Thus, results are presented in terms of space velocity (SV): SVˆv o /F where v o is the reactor void volume and F is the volumetric ow rate. Ignition. Reactions are ignited by preheating the reactants to the heterogeneous ignition temperature (200±2508C). After ignition the heat is removed, the

3 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93± heterogeneous reaction sustains itself, and the operating parameters are set to the desired conditions. The reaction reached steady state within a few minutes, and except where noted, no deactivation or change was observed for operation over many hours. 3. Results Unless otherwise noted, the results presented were for isobutane oxidation in air with feed gases preheated to 3508C over a 17 mm diameter, 45 ppi catalyst at 5 SLPM. For most of these experiments, the same 5.7 wt% Pt catalyst was used. Selectivities and conversions are reported as a function of fuel/ oxygen ratio. Experiments were performed over the range of i-c 4 H 10 /O 2ˆ0.5 (syngas stoichiometry) to 2.0 (oxidative dehydrogenation stoichiometry). Running leaner approaches the upper ammability limit of i-c 4 H 10 in air (i-c 4 H 10 /O 2ˆ0.44), while operation further into the rich regime risks extinguishing the catalyst. Most experiments were repeated with at least three different catalysts with all results consistent with those shown Air oxidation Fig. 1 shows isobutane oxidation in air without preheat of feed gases. Presented are: (a) carbon atom selectivities, (b) selectivities for H 2,H 2 O, and total hydrocarbons, and (c) the fuel and O 2 conversion and catalyst temperature. Over the range examined, considering isobutane oxidation (Eqs. (1)±(3)), one would expect that as one runs progressively further into the Fig. 1. Oxidative dehydrogenation of isobutane in air. Carbon atom (a) and hydrogen atom (b) selectivities, and (c) fuel, oxygen conversion and reaction temperature for the partial oxidation of i-c 4 H 10 over a Pt-coated, 45 ppi a-al 2 O 3 foam monolith. Total flow rate of 5 SLPM in air with feed preheat of 258C.

4 96 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93±106 fuel rich regime, the product distribution would shift from production of CO 2 and H 2 O towards CO and H 2 and eventually towards production of ole ns and H 2 O. Experimentally, we see higher total ole ns, with selectivity to CO and H 2 decreasing and CO 2 and H 2 O increasing. All the observed trends follow what is expected with the exception of the increase in CO 2 selectivity. It is thought that since the oxygen is not completely converted (95±90%) in large fuel excess, oxygen downstream of the catalyst may be reacting with products to form the additional CO 2 [13]: i-c 4 H O 2! 4CO 2 5H 2 O (1) i-c 4 H 10 2O 2! CO 5H 2 (2) i-c 4 H O 2! i-c 4 H 8 H 2 O (3) The total ole n selectivity (i-c 4 H 10 C 3 H 6 C 2 H 4 ) has a maximum at 60% (30% C 3 H 6, 30% i-c 4 H 10 )at a ratio of 1.5. However, the conversion drops to 30% at this point. At a ratio of 0.5, stoichiometric for syngas, essentially all the fuel is converted. CO and H 2 are selectively produced, with the product distribution containing only 20% ole ns (15% C 2 H 4,5%C 3 H 6 ). It is also interesting to note that, except in the very fuel lean regimes, C 3 H 6 and CH 4 are produced in nearly a 2:1 ratio, corresponding to one mole each of C 3 H 6 and CH 4. This observation supports a single cracking reaction: i-c 4 H 10! C 3 H 6 CH 4 (4) 3.2. Preheat Shown in Fig. 2 are the results of isobutane in air at 5 SLPM with reactant preheat to 3508C. The most important differences compared to results without Fig. 2. Oxidative dehydrogenation of isobutane in air with feed preheat of 3508C. Carbon atom (a) and hydrogen atom (b) selectivities, and (c) fuel, oxygen conversion and reaction temperature for the partial oxidation of i-c 4 H 10 over a Pt-coated, 45 ppi a-al 2 O 3 foam monolith. Total flow rate of 5 SLPM.

5 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93± preheat are that fuel conversion is 20% greater and the temperature is 508C higher. Fuel conversion is complete at the lower ratios until a fuel/oxygen ratio of 0.8. The reaction continues until well into the fuel rich regime, where the fuel conversion drops to 40% at a ratio of 1.9. Oxygen conversion is fairly constant at 90±95%. Trends in product distribution are similar to those without preheat. Total ole n selectivity reaches a maximum of 65%, although maximum i- C 4 H 8 yields are found at a ratio of 1.2, with total ole n selectivity 60% (6% C 2 H 4, 23% i-c 4 H 8, 31% C 3 H 6 ). Fuel conversion is 70% and oxygen conversion is 93% at this composition Diluent These systems operate nearly adiabatically, and decreasing the N 2 diluent should strongly increase the reaction temperature. Experiments were run in O 2, with 20% N 2 serving for GC calibrations. Fig. 3 presents the results of isobutane in enriched air at 5 SLPM, with preheat of feed gases to 3508C. Leaner compositions were not explored because the homogeneous ammability limits widen as the amount of diluent is decreased. Temperatures are very similar to the reaction run in air, although fuel conversions increase by 20%, varying from 100% (i-c 4 H 10 /O 2ˆ1.2) to 70% (i-c 4 H 10 / O 2ˆ1.7). The trends seen in product selectivity are consistent with those reported earlier [13]. Maximum yields to total ole ns and i-c 4 H 10 and maximum total ole n selectivity, 70% (35% C 3 H 6, 27% i-c 4 H 10,8% C 2 H 4 ), is found at a ratio of 1.7 with fuel conversion at 71%. Increasing the reaction temperature by preheating reactant gases and decreasing the amount of diluent in Fig. 3. Oxidative dehydrogenation of isobutane in O 2 with 20% N 2 dilution, feed preheat of 3508C. Carbon atom (a) and hydrogen atom (b) selectivities, and (c) fuel, oxygen conversion and reaction temperature for the partial oxidation of i-c 4 H 10 over a Pt-coated, 45 ppi a-al 2 O 3 foam monolith. Total flow rate of 5 SLPM.

6 98 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93±106 the feed stream substantially increases conversion in these processes. The mass transfer coef cient increases with temperature, thus increasing conversion. The higher temperatures provide the necessary heat for the less exothermic or endothermic ole n producing reactions. At higher temperatures, however, more of the products can be lost to secondary reactions. This is evident in the increase in selectivity to CH 4,C 2 H 4, and decrease of i-c 4 H 8 and C 3 H 6 with increased temperature at the leaner regimes Residence time In these experiments, the space velocity was varied by changing the volumetric ow rate through the reactor from 3 to 10 SLPM and by using catalysts of various diameters (17, 11, and 5 mm). In these experiments, the feed was i-c 4 H 10 in air over a 5.7 wt% Pt-coated monolith at a fuel/oxygen ratio of 1.2. Previous work at 5 SLPM gave maximal yields of i-c 4 H 8 at this composition. Fig. 4 shows the results for selectivity, conversion, and temperature plotted vs. space velocity. It is seen that as the space velocity increases, selectivities of all major products change slightly. Total ole n selectivity remains essentially constant as does C 3 H 6 ; however, i-c 4 H 8 increases and C 2 H 4 and CH 4 decrease with increasing space velocity. This is indicative of decomposition of the product ole n molecules in the reaction zone. The oxygen conversion drops somewhat (95±80%) as the space velocity increases and fuel conversion drops from 80% at h 1 (3 SPLM, dˆ18 mm) to 40% at h 1 (10 SLPM, dˆ11 mm). Experiments at higher space velocities with a 5 mm diameter reactor give very low fuel conversion, 8%, Fig. 4. Oxidative dehydrogenation of isobutane in air with feed preheat of 3508C, at a fuel to oxygen ratio of 1.2. Carbon atom (a) and hydrogen atom (b) selectivities, and (c) fuel, oxygen conversion and reaction temperature for the partial oxidation of i-c 4 H 10 over a Pt-coated, 45 ppi a-al 2 O 3 foam monolith, vs. space velocity.

7 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93± with the majority of the products being syngas and total oxidation products. This occurs because by increasing the ow rate, conduction of heat from the back of the monolith, where most of the reactions occur, to the front, is much more dif cult, especially for the small diameter (5 mm) catalyst. The exothermicity of reactions at the back generally keeps the entire monolith ignited by conducting heat to the front of the monolith. The catalyst, however, can ``blow out,'' if the gases are owing so fast that all the heat generated by the reaction is convected away from the monolith. The catalyst is unable to maintain stable, autothermal operation. This is observed with the 5 mm diameter monolith where the low conversion and the selectivities to only oxidation products (CO CO 2 ) indicate that very little reaction is occurring Different metals The same experiments described previously on Pt were also run using several different noble metal coated monoliths. Pt, Rh, Ir, and Pd catalysts were prepared on 45 ppi a-al 2 O 3 ceramic foams, as well as a Pt catalyst promoted with Sn. The experimental conditions were isobutane oxidation in air, at a total ow rate of 5 SLPM, with preheat of feed gases to 3508C. As expected, Pt is the optimal catalyst for ole n production. As shown in Fig. 5, for Pt the fuel conversion was 100% out to a ratio of 0.9, where it began to drop and decreased to 50%. Pd, Rh, and Ir all had 100% conversion at 0.5, but began to drop more quickly than on Pt, with relative conversions of Rh>Ir>Pd. Beyond a ratio of 0.9, however, Pd quickly deactivated and Fig. 5. Oxidative dehydrogenation of isobutane on Pt, Rh, Ir, Pd: (a) total olefin and i-c 4 H 8 yields, (b) selectivity to total olefins and i-c 4 H 8, (c) fuel conversion, and (d) catalyst temperature for the partial oxidation of i-c 4 H 10 over noble metal coated, 45 ppi a-al 2 O 3 foam monoliths. Total flow rate of 5 SLPM, in air, with preheat of feed gases to 3508C.

8 100 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93±106 extinguished. Pt±Sn quickly deactivated due to carbon deposition and we were never able to maintain stable operation. At lower ratios, both total ole n and i-c 4 H 8 selectivities were very similar for all the metals. Pt gave the best results richer than 1.0. Both total ole n and i-c 4 H 8 yields followed the same trend as seen with selectivities, with Pt>Rh>Ir, with Pd operating similarly to Pt until deactivation. For all the metals, the total ole n yield maximized at a fuel/ oxygen ratio of 1.0 while i-c 4 H 8 yield maximized at a ratio of 1.1. Measured temperatures for all three metals were within 508C of each other, indicating that the differences in product distribution are mostly the result of surface chemistry, not reaction temperature Support pore size Mass transfer is a major catalyst design consideration for this partial oxidation reaction system. We wish to optimize the catalyst by maximizing mass transfer using a monolith with tortuous paths, while minimizing product ole n decomposition with a monolith with open channels and ample opportunity for the product gases to leave the reaction zone. Monoliths of various pore size were tested, again at the same experimental conditions as previously described: isobutane oxidation in air, at a total ow rate of 5 SLPM, with preheat of feed gases to 3508C. Catalysts loaded with Pt were prepared with ceramic foam monoliths of different porosity: 20, 45, and 80 ppi (pores per linear inch). Identical methods were Fig. 6. Oxidative dehydrogenation of isobutane over Pt/Al 2 O 3 foam monoliths with different cell sizes: (a) fuel conversion, (b) total olefin yield, (c) isobutane yield, and (d) catalyst temperature for the partial oxidation of i-c 4 H 10. Total flow rate of 5 SLPM, in air, with preheat of feed gases to 3508C.

9 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93± used with nearly identical loadings of 4.06, 5.65, and 5.15 wt%, respectively. It was shown previously that Pt loading has no signi cant effect upon selectivities as long as the metal coats the monolith as a lm [17]. The small differences in metal loading should therefore have no bearing upon the product distribution. Fig. 6 shows carbon atom selectivities to total ole ns and to i-c 4 H 8, yields to i-c 4 H 8, fuel and oxygen conversions, and catalyst temperature. The selectivity to total ole ns and i-c 4 H 8 was found to increase in the order 80<45<20, with the 20 ppi 5± 10% higher than the 45 ppi, which was 3% higher than the 80 ppi. However, at ratios <1.4, conversion increased in the opposite order: 20<45<80. The 80 and 45 ppi maintained near 100% fuel conversion until a ratio of 0.9, after which they decreased and leveled off to 45% and 35%, respectively. The fuel conversion for the 20 ppi monolith fell and steadied out more rapidly, eventually leveling at 50%. In terms of yields, it is clear that for i-c 4 H 8, the 20 ppi monolith operates the best. i-c 4 H 8 yields of at least 15% are achieved for all but the lowest compositions, while the 45 ppi levels out at 13% and the 80 ppi at 10%. Other noteworthy aspects are that the selectivity to CO CO 2 increased in the order 20<45<80. Also, the 20 ppi monolith produced less C 2 H 4 and CH 4 than the 45 and 80 ppi and more higher ole ns Support material In order to determine the impact of the support material, identical experiments were run using a Pt/ ZrO 2 monolith and Pt/Al 2 O 3 monolith with the same weight loading. The experimental conditions were isobutane oxidation in air, at a total ow rate of 5 SLPM, with preheat of feed gases to 3508C. Fig. 7 shows that using a Pt/ZrO 2 catalyst results in a slightly smaller selectivity to ole ns and slightly higher conversions Product quenching As an attempt to determine to what extent postcatalyst homogeneous reactions are occurring and simulate the results of only the heterogeneous chemistry occurring with the system, an additional 5 SLPM of N 2 was introduced into the reactor 5 mm downstream of the catalyst in order to quench any possible gas phase reactions. As seen in Fig. 8(a), for isobutane oxidation, the fuel conversion is 3% lower and oxygen conversion is 5% lower with the N 2 quench. The product distribution is very similar to that without quench, and most importantly, temperatures do not change with the quenching. Some reactions, therefore, must be occurring post-catalyst in the downstream section of the reactor tube. It is interesting to note that similar experiments for lower alkanes [21] showed no differences between the quenched and unquenched experiments, while for higher alkanes (C 5 's, C 6 's), fuel conversion is 10± 20% lower and oxygen conversion is 5±15% lower with the quench [15]. See Fig. 8(b). This is indicative of post-catalyst homogeneous reactions occurring in C 4 partial oxidation systems. Fig. 7. Comparison of Pt/Al 2 O 3 and Pt/ZrO 2 foam monoliths for the oxidative dehydrogenation of isobutane in air with feed preheat of 3508C.

10 102 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93±106 Fig. 8. Quenching experiments for: (a) i-c 4 H 10 oxidation, and (b) C 5 H 12 oxidation. For C 5 H 12, fuel conversion is 10±20% lower and oxygen conversion is 5±15% lower with the quench while for i-c 4 H 10, fuel and oxygen conversion are only 3% and 5% lower. 4. Discussion The notable features of these experiments include the following observations: 1. Up to 70% selectivity to ole ns are formed under non-equilibrium conditions with contact times of 5 ms. 2. Catalyst deactivation due to carbon formation was not found with Pt, Ir, Rh catalysts with this reaction system over many hours of operation (>25) even though it is predicted thermodynamically and is observed at longer residence times. 3. Some oxygen breakthrough was observed with isobutane oxidation at all flow rates, while with comparable reactions with methane, ethane, propane, and n-butane, oxygen was completely consumed (>99%) [11±14]. At very high fuel/oxygen ratios, considerable oxygen breakthrough is observed and some oxygenates and C 5 species form (1% total). 4. Increasing the flow rate at a constant composition increases the i-c 4 H 8 selectivity and decreases the C 2 H 4 selectivity by 8% while isobutane conversion drops from 80% to 40%. 5. Different metals result in very different product distributions. Pt is optimal for olefin production, while Rh and Ir are better for CO and H 2.Pd shows high activity initially but quickly deactivates due to carbon deposition at higher fuel/oxygen ratios. 6. The porosity of the monolith and the support material has a significant impact on the product distribution with larger pores resulting in greater selectivity to olefins but lower conversion. 7. Quenching the reaction post-catalyst with N 2 diluent to suppress homogeneous reactions results in a decrease in both fuel and oxygen conversion by several percent under some conditions, while the product distribution and catalyst temperature remain nearly constant. Two different mechanisms have been used to explain partial oxidation reaction systems similar to the one described here and we will attempt to explain these observations in terms of these mechanisms. One is a heterogeneous b-elimination mechanism based purely on surface chemistry [11±13], and the other is a heterogeneously assisted homogeneous mechanism based on gas phase pyrolysis [15] b-elimination Huff and Schmidt [11±13] proposed a simpli ed reaction scheme to explain the product distribution for C 2 ±C 4 alkane oxidation based on the catalytic oxidative dehydrogenation with -elimination from an adsorbed species. The Pt surface is covered with adsorbed surface oxygen atoms. Atomic oxygen abstracts a hydrogen, forming an adsorbed alkyl species and an adsorbed hydroxyl species. Formation of an ole n from an adsorbed alkyl involves abstraction of a b-hydrogen from the alkyl, which then forms the ole n C 2 H 2n 2! C 2 H 2n 1 s! C 2 H 2n

11 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93± Propane, butane and isobutane product distributions can be explained by the relative probability of abstracting a b-hydrogen or a b-carbon from the adsorbed alkyl. A weaker b-c±c bond will preferentially break over a stronger b-c±h bond, so for the case of n-butane, the C 2 and C 3 ole ns are selectively produced with little C 4 ole n. In isobutane oxidation, abstraction of a hydrogen by atomic oxygen leads to an adsorbed isobutyl group and an adsorbed hydroxyl species. The isobutyl group can be adsorbed at either a tertiary or a primary carbon. Based on the ratio of hydrogens on the primary carbons to those on tertiary carbons (9:1), and the relative strength of a primary vs. tertiary C±H bond (410 vs. 375 kj/mol), 75% of the isobutyl groups should be adsorbed at the tertiary position. At the tertiary position, the only way to eliminate a b-hydrogen is to form isobutylene. At the primary position, the available ways are through b-hydrogen elimination to form isobutylene or b-methyl elimination to form propylene: CH 3 3 C s! i-c 4 H 8 CH 3 2 CHCH 2 s! C 3 H 6 ; i-c 4 H 8 Using statistical and thermodynamic arguments, this system should produce three times as much isobutylene as propylene, assuming the contributions of competing mechanisms and secondary reactions are unimportant [13]. This simpli ed scheme provides a basic understanding of the most important reaction pathways in the system. However, we do not see as high selectivities to isobutylene as this model predicts. This suggests that there may be a contribution by secondary reactions. Its contribution and how it ts into the b- elimination mechanism will be discussed later Surface-initiated gas phase pyrolysis There has always been a question regarding the contribution of homogeneous chemistry to these partial oxidation reaction systems because temperatures are very high, 10008C. It is worth considering that, instead of a purely heterogeneous mechanism, there could be a heterogeneous±homogeneous reaction mechanism. It is proposed that oxidation reactions produce CO and CO 2 on the surface. Because of the Fig. 9. Selectivity in partial oxidation and thermal pyrolysis vs. isobutane conversion. Open markers show the results for thermal pyrolysis. heat generated by the oxidation reactions, and if radical species are not quenched by the monolith, the unreacted fuel and oxygen could undergo gas phase pyrolysis above the catalyst surface [15]. To test this hypothesis, ole n selectivities and conversions for isobutane partial oxidation on Pt are compared to that for thermal pyrolysis. As seen in Fig. 9, selectivities and conversions are very similar in trend and magnitude. It should be noted that the selectivities in this gure for thermal pyrolysis do not include CO CO 2 production for this comparison. The only O 2 present in thermal pyrolysis is from the steam co-feed, and as CO CO 2 are only produced by steam reforming, selectivities for major products in this partial oxidation system must be re-scaled to not include CO CO 2. Another characteristic of gas phase pyrolysis is the signi cant production of acetylene, butadiene, and aromatics (5% for isobutane pyrolysis). Under certain reaction conditions of these experiments, acetylene and unidenti ed C 5 were indeed formed, although in smaller amounts (1%) and mostly at the higher fuel/oxygen ratios. It has been shown, however, that in propane pyrolysis, adding even small amounts of oxygen leads to the production of more ole ns and less aromatics [16]. It can be assumed that this would be the case for isobutane as well. It should also be noted that no butadiene was observed in this laboratory at atmospheric pressure, although it has been observed in a pilot plant running this process at

12 104 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93±106 higher pressures, where the effects of homogeneous reactions would be magni ed Secondary reactions Varying the catalyst contact time controls the time available for series reactions to occur within the monolith, both oxidation reactions and secondary cracking reactions. In this system, signi cant secondary reactions exist, as selectivities to major products change as residence time changes. The most important of these secondary reactions is product ole n cracking reactions (Eqs. (5)±(7)), especially in the leaner regimes, where the product distribution shows a signi cant amount of C 2 and C 3 ole ns. At higher ow rates, total ole n and C 3 H 6 selectivities do not change, while C 2 H 4 production decreases and i-c 4 H 8 production increases. These observations are in accord with the secondary cracking reactions shown below: i-c 4 H 8! C 3 H 6 CH 2;s (5) i-c 4 H 8! C 2 H 4 2CH 2;s (6) C 3 H 6! C 2 H 4 CH 2;s (7) In this reaction system, it has been suggested that steam reforming of product ole ns may contribute to the product distribution. In separate experiments with this reaction system, however, it was found that the addition of up to 25% H 2 O results in a negligible increase in CO [13], suggesting that steam reforming of product ole ns is unimportant. It is also important to recognize the reverse of CO disproportionation (Eq. (8)) and the reverse steam reforming of C s (Eq. (9)), which may serve to remove coke from the catalyst: CO 2 C s! 2CO (8) C s H 2 O! CO H 2 (9) 4.4. Catalyst chemical properties Experiments with various noble metals make it clear that heterogeneous chemistry is crucial in determining the product distribution. Differentiating between a purely heterogeneous b-elimination mechanism or a heterogeneously assisted homogeneous mechanism may be dif cult. Conclusive evidence about the differences between the metal surfaces and their interactions with the gas molecules, however, must come from further experimentation. Using laser induced uorescence (LIF) to determine the activation energies of the intermediate species and computer simulations together to explain the different product distributions has been done previously in similar systems [14]. Some conclusions can be inferred with this information for isobutane oxidation although signi cant insights must come from further experimentation Catalyst physical properties It has been well established in this research group that mass transfer to the surface of the monolith limits the rate. To maintain high conversion, it is crucial to have high mass transfer rates. Increasing the porosity, a catalyst with more tortuous paths, will increase turbulence, improve mixing, and increase mass transfer. Increasing the porosity, however, makes it more dif cult for the product gases to exit the reaction zone without further decomposition. The net effect should be an increase in conversion with a reduction in the selectivity to the desired intermediate product. The data presented here substantiate these ideas. Higher mass transfer occurs in the 80 ppi monolith with higher fuel conversion and lower selectivity to ole ns. Both reaction mechanisms explain these experimental observations. For a catalytically assisted homogeneous reaction mechanism, where CO and CO 2 are produced on the surface, a catalyst of lower porosity (decreased mass transfer) will show lower selectivities to CO CO 2 and an increase in ole n production. If the surface mechanism is dominant, however, a higher porosity catalyst (higher mass transfer) will show higher fuel conversion and a greater selectivity to ole ns than oxidation products (CO CO 2 ). Secondary product ole n cracking reactions, however, will occur more, resulting in less total ole ns. If this is the case, this should occur only at the leaner compositions while at richer compositions, ole n cracking reactions are minimal and the selectivity to ole ns should be the same for the various porosity catalysts. Fig. 6 shows this experimentally Site blockage For the oxidation of lower alkanes over Pt and Rh monoliths, it was proposed that mass transfer of O 2 to

13 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93± the surface limits the reaction rate [17,18]. Since at similar operating conditions this should be no different for the various systems, this must also be the case for higher alkane fuels. For the lower alkanes, adsorbed O s quickly oxidizes C s to CO and CO 2 and complete oxygen conversion is observed. For higher fuels, oxygen conversion is always incomplete. It is proposed that the larger alkanes block surface sites normally occupied by adsorbed oxygen because they can form more surface carbon than the lower alkanes, and because sticking coef cients of larger alkanes are larger than smaller alkanes [15]. Observations of increased oxygen breakthrough with increased fuel/ oxygen ratio (increased amounts of adsorbed fuel) and decreased oxygen breakthrough with more porous catalytic supports (higher surface area for molecules to adsorb) are substantiated by this explanation Homogeneous vs. heterogeneous chemistry Homogeneous chemistry almost certainly plays a role in this partial oxidation reaction system. Results with increasing ow rate show that secondary product ole n cracking reactions are occurring, changing the porosity of the monolith shows that one can minimize these secondary reactions, and quenching experiments show the existence of post-catalyst homogeneous reactions. The issue still remains that the detailed reaction mechanism occurring in this system is unknown. Observation of the product distribution will not help to distinguish between homogeneous chemistry and heterogeneous chemistry. The thermodynamically favored products, those with the lowest free energy, will be formed independent of the path. Most likely, both mechanisms are operating, with one or the other dominating in different regimes due to reaction temperatures and the amount of oxygen downstream of the catalyst. The proposed surface pyrolysis mechanism, which would involve complete pyrolysis of the adsorbed isobutyl on the surface, involves a-elimination reactions. As has been discussed previously, however, b-elimination reactions are favored over a-elimination reactions on the more noble metal surfaces [19,20]. The rate of b-elimination reactions was over 6 orders of magnitude greater than a-elimination reactions. Also, residence time in the partial oxidation system is of the order of milliseconds, with over 95% of the fuel being converted in this time. Thermal pyrolysis, however, has a contact time of the order of 1 s. Thermal pyrolysis produces signi cant amounts of coke, while in this partial oxidation system none occurs. Finally, only small amounts of coupled and aromatic products were seen in this system, much less than thermal pyrolysis produces. Although this indicates that thermal pyrolysis is not the dominant mechanism, it does not preclude the possibility of surface generated radicals, product ole- ns and unreacted fuel reacting homogeneously within the reaction and post-catalyst zones. This is the key difference between the C 1 ±C 3 partial oxidation systems and the higher alkanes. With light alkanes as the fuel, O 2 conversion was complete, while for C 4 's, O 2 conversion was 90% and for C 5 's and C 6 's it was even less. Quenching of downstream gases indicated the existence of little post-catalyst homogeneous chemistry for the lower alkanes with more occurring as the carbon chain length of the fuel increased. It is likely that oxygen breakthrough allows homogeneous chemistry. In this way, i-c 4 H 10 is a transition molecule between the purely heterogeneous reaction systems of C 1 ±C 3 partial oxidation and the heterogeneously assisted homogeneous reactions of C 5 reaction systems. 5. Conclusions We nd that isobutylene and propylene can be produced with reasonable selectivity and yields through the catalytic partial oxidation of isobutane over noble metal coated foam monoliths at very short contact times. Maximum total ole ns selectivities of 70% (27% i-c 4 H 8 ) with 70% fuel conversion are found with a Pt/Al 2 O 3 catalyst, with reactant preheat to 3508C, running in enriched air (20% N 2 ) at a fuel/ oxidant ratio of 1.7. Experiments at higher space velocities over Pt monoliths show that selectivity to i-c 4 H 8 increases and selectivity to C 2 H 4 decreases by 8%, indicating the presence of secondary cracking reactions at the lower space velocities. Catalyst porosity signi cantly affects the product distribution, especially at lower fuel/oxygen ratios, where a monolith with larger pores and more open channels minimizes the product ole n decomposition reactions. The catalytically active

14 106 L.S. Liebmann, L.D. Schmidt / Applied Catalysis A: General 179 (1999) 93±106 noble metal signi cantly affects the product distribution, with Pt optimal for ole n production. The existence of oxygen breakthrough and the quenching of post-catalyst homogeneous reactions can be explained by blockage of the surface by adsorbed fuel species and/or surface carbon. A heterogeneously assisted thermal pyrolysis mechanism explains the product distribution due to thermodynamic considerations. However, it is more likely that a purely heterogeneous b-elimination mechanism is operating with homogeneous reactions occurring mainly in the form of unreacted fuel and product ole n oxidation and cracking reactions which occur within and after the monolith. Comparable experiments with C 5 and C 6 alkanes will be conducted later. Acknowledgements We would like to acknowledge the NSF under grant CTS , CR & L, Pasadena, TX, and Dr. Albert G. Dietz III. References [1] T. Matsuda, I. Koike, N. Kubo, E. Kikuchi, Appl. Catal. A 96 (1993) 3±13. [2] J.J.H.M. Font Freide, M.J. Howard, T.A. Lomas, EP Patent A2 (to The British Petroleum Company), [3] S. De Rossi, G. Ferraris, S. Freminotti, V. Indovina, A. Cimino, Appl. Catal. A 106 (1993) 125. [4] Y. Takita, K. Kurosaki, Y. Mizuhara, T. Ishihara, Chem. Lett. 2 (1993) 335. [5] E.A. Mamedov, V.C. Corberan, Appl. Catal. A 127 (1995) 1± 40. [6] P.G. Harrison et al. (Euro. Pat. Appl. EP A1), [7] G. Yaluris, J.E. Rekoske, L.M. Aparicio, R.J. Madon, J.A. Dumesic, J. Catal. 153 (1995) 54±64. [8] S.R. Mirzabekova, A.Kh. Mamedov, Ind. Eng. Chem. Res. 34 (1995) 474±482. [9] L.C. Loc, N.A. Gaidai, S.L. Kiperman, H.S. Thoang, P.B. Novikov, Kinet. Catal. 36 (1995) 504±510. [10] L.C. Loc, N.A. Gaidai, B.S. Gudkov, S.L. Kiperman, H.S. Thoang, N.M. Podkletnova, V.Yu. Georgievskii, Kinet. Catal. 35(6) (1994) 903±906. [11] M. Huff, L.D. Schmidt, J. Phys. Chem. 97 (1993) 11815± [12] M. Huff, L.D. Schmidt, J. Catal. 149 (1994) 127±141. [13] M. Huff, L.D. Schmidt, J. Catal. 155 (1995) 82±94. [14] D. Hickman, L.D. Schmidt, Science 259 (1993) 343±346. [15] A.G. Dietz III, L.D. Schmidt, J. Catal. 176 (1996) 459± 473. [16] Pyrolysis: Theory and Industrial Practice, Academic Press, New York, [17] D.A. Hickman, L.D. Schmidt, J. Catal. 138 (1992) 267±282. [18] D. Hickman, L.D. Schmidt, AICHE J. 39 (1993) 1164± [19] F. Zaera, Acc. Chem. Res. 25 (1992) 260. [20] F. Zaera, Surf. Sci. 219 (1989) 453. [21] A. Bodke, K. Hohn, Unpublished results.