Beta for Isoparaffin Productio - American Chemical Society

Ind. Eng. Chem. Res. 2005, 44, 7329-7336

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Insights into a Multifunctional Hybrid Catalyst Composed of Co/SiO2 and Pd/Beta for Isoparaffin Production from Syngas Zhong-Wen Liu,†,‡ Xiaohong Li,*,† Kenji Asami,† and Kaoru Fujimoto† Department of Chemical Processes and Environments, Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Fukuoka 808-0135, Japan, and College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China

The direct production of gasoline-range isoparaffins from synthesis gas (CO + H2, syngas) via the Fischer-Tropsch (FT) route was investigated over a hybrid catalyst composed of Co/SiO2 and Pd/β under the conditions of P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1 and reaction temperatures of 503-513 K. Results indicate that the contact state between the Co/SiO2 and the Pd/β particles in the hybrid catalyst (powdery mixture and granular mixture) had a clear effect on both the activity toward CO hydrogenation and the product distribution. Interestingly, the granular hybrid catalyst showed much higher performance in the case of isoparaffin selectivity than the powdery hybrid catalyst at the initial stage of the reaction. Furthermore, irrespective of the reaction conditions applied, the powdery hybrid catalyst gave apparently lower CO conversion and obviously higher CH4 selectivity than the granular hybrid catalyst. Irrespective of the catalyst used, the total selectivity of isoparaffins (I/C4+) noticeably decreased with TOS due to the increase of selectivity of normal paraffins, the decrease of selectivity of isoparaffins, and less cracking of long-chain FT hydrocarbons. In the case of the powdery hybrid catalyst, a synergistic effect of the FT reaction over Co/SiO2 and the hydroconversion reaction over Pd/β was suggested from the reaction results. A mechanistic model based on the migration of hydrogen species at the interfaces of the different particles was developed, and the reaction results were extensively discussed based on the proposed model. 1. Introduction The FT synthesis still continues to attract considerable research attention due to the efficient utilization of syngas derived from abundant resources such as natural gas, coal, and/or biomass.1-3 However, the FT products composed mainly of normal alkanes and alkenes with a wide carbon chain length are nonselective to any specific hydrocarbons due to the limitation of Anderson-Schulz-Flory (ASF) polymerization kinetics. Therefore, the key for improving the efficiency of FT synthesis is to break the ASF law such that desired products, e.g., high-quality gasoline or diesel, can be selectively produced. Presently, this has received significant attention.4-9 The most obvious strategy to achieve this target is to load active metal for FT synthesis directly on acidic supports such as zeolites. Indeed, results indicate that product distribution over zeolite-supported Co or Fe catalysts was significantly shifted from that over traditional Co- or Fe-based FT catalysts.10-12 However, besides the problems that emerged from zeolite support, such as difficult reduction of Co or Fe entrapped in the framework of the zeolite,12 high selectivity to aromatics is not suitable in comparison with branched paraffins under the present environmental legislations on liquid fuels. RuKY and Pt/SO42-/ZrO2 catalysts loaded consecutively in separate catalyst beds have been examined for enhancing the selectivity to branched paraffins.13 * To whom correspondence should be addressed. Tel.: +81-93-695-3286. Fax: +81-93-695-3378. E-mail: [email protected]. † The University of Kitakyushu. ‡ Nanjing University of Technology.

However, a significant increase of undesirable CH4 was observed.13 In previous works,8,9 a fundamental concept of combination of Co/SiO2 FT catalyst and Pd/zeolite catalyst was proposed to selectively synthesize gasolinerange isoparaffins. Preliminary results indicate that isoparaffin selectivity could be significantly increased without an apparent increase in CH4 selectivity. In recent years, the application of mechanical mixtures of catalysts with different functions has gained some attention.14,15 Seemingly, the mechanically physical mixture is simple. However, a profound phenomenon has been found, while the mechanistic explanation is still under debate.14 Moreover, synergistic effects between the ingredients of the hybrid catalysts for syngas to dimethyl ether have been suggested.16,17 In considering the potential synergistic effects between different components in hybrid catalysts, the possible effect of the contact state between individual components on the performance of the hybrid catalyst can be reasonably imagined. On the basis of this understanding, a preliminary study of the contact state effect on the performance of isoparaffin synthesis over Co/SiO2 and Pd/β hybrid catalyst has recently been carried out and reported in a short communication.18 Unexpectedly, well-mixed hybrid catalyst composed of small particles of Co/SiO2 and Pd/β showed poorer performance than that composed of bigger particles with the same composition. To better understand the interfacial phenomena, in this investigation a detailed analysis of this effect is given based on extensive experimental results. Moreover, a reasonable mechanistic model was proposed based on the analysis of the interfacial phenomena and the concept of hydrogen spillover.

10.1021/ie050645d CCC: $30.25 © 2005 American Chemical Society Published on Web 08/13/2005

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Figure 1. Diagram of the hybrid catalysts loaded in the reactor.

2. Experimental Section 2.1. Catalysts. The Co/SiO2 catalyst (20 wt % Co) was prepared by incipient wetness impregnation of an aqueous solution of cobalt nitrate on silica gel (Fujisilicia Q-15). After drying overnight at 393 K, the catalyst was calcined at 473 K for 2 h. The zeolite β (Tosoh, SiO2/ Al2O3 ) 25.6) was ion-exchanged at 353 K for 6 h in a solution of Pd(NH3)4Cl2 for the preparation of Pd/β catalysts containing 0.5 and 1.0 wt % Pd. After filtration and washing with distilled water until negative with a AgNO3 test, the final catalysts were dried overnight at 393 K and then calcined at 723 K for 2 h. To study the effect of the contact state between the individual catalyst on the performance of the mechanical physical mixture, the hybrid catalyst was prepared in two ways. One way was to cogrind the fine particles of Co/SiO2 and Pd/β catalysts with equal weight in an agate mortar for sufficient time such that the two kinds of catalysts could be further pulverized and uniformly mixed. After this, the mixture was subject to pressuremolding, crushing, and sieving. Finally, particles with sizes between 355 and 710 µm were selected for the reaction test. We define the physically mixed catalysts prepared in this method as powdery hybrid catalyst. Alternatively, each component, i.e., Co/SiO2 and Pd/β catalysts, was ground, pressure-molded, crushed, and sieved to prepare particles with sizes of 355-710 µm, respectively. Then, the particles of each component with a weight ratio of 1 were thoroughly mixed. The hybrid catalyst prepared in this way was named granular hybrid catalyst. The characteristics of the two kinds of hybrid catalysts loaded in the reactor are illustratively shown in Figure 1. The most obvious difference between the two kinds of hybrid catalysts lies in the particle size and contact state of the Co/SiO2 and Pd/β particles. In the case of the powdery hybrid catalyst, smaller particles of each component (much smaller than 355 µm) contacted each other closely, while only larger particles (355-710 µm) of Co/SiO2 and Pd/β were loosely touched in the granular hybrid catalyst. 2.2. Experimental Procedure. The reaction was carried out in a conventional fixed-bed reactor (SUS tube, 8 mm i.d.), in which 0.8 g of hybrid catalyst was loaded in the reactor for each experiment. Before reaction, the catalyst was atmospherically reduced at 673 K for 3 h in a flow of hydrogen. After cooling to room temperature, the gas line was switched to syngas. Finally, the experiment started when the predetermined temperature and pressure were achieved. The operating conditions were P ) 1.0 MPa, H2/CO ) 2.0, W (hybrid catalyst)/F (syngas) ) 7.9 g‚h‚mol-1. The hydrocarbons in the effluent after cooling at 453 K were analyzed by an on-line GC (GC 353, GL Sciences) equipped with an NB-1 capillary column and an FID detector. The CO,

Figure 2. CO conversion and CH4 selectivity over the powdery mixture of Co/SiO2 + 1.0 wt % Pd/β (A), the granular mixture of Co/SiO2 + 0.5 wt % Pd/β (B), and the granular mixture of Co/SiO2 + 1.0 wt % Pd/β (C) at different reaction temperatures. (Reaction conditions: P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1.)

CH4, and CO2 in the effluent after cooling in an icewater trap were analyzed on-line by a GC (Shimadzu, GC-8A) with a packed active carbon column and a TCD detector. The selectivity of the hydrocarbons was calculated on the basis of carbon number, and the carbon balance for majority of the experiments was within 100 ( 5%. For comparison purposes, one experiment in a consecutive dual reactor was carried out under similar reaction conditions. Granular hybrid catalyst composed of 0.4 g of Co/SiO2 and 0.1 g of 0.5 wt % Pd/β was loaded in the upper reactor and 0.3 g of 0.5 wt % Pd/β catalyst was charged in the lower reactor. The reaction conditions were P ) 1.0 MPa, H2/CO ) 2.0, W (weight of catalyst loaded in the two reactors)/F (syngas) ) 7.9 g‚h‚mol-1, T (upper reactor) ) 508 K, T (lower reactor) ) 503-513 K. 3. Results and Discussion 3.1. CO Conversion and CH4 Selectivity. The CO conversion and CH4 selectivity over different hybrid catalysts at varied reaction temperatures were very stable during the 8 h reaction. The averaged results are shown in Figure 2. In the case of granular hybrid catalysts, with increasing reaction temperature CO conversion increased significantly, while only an appreciable increase of CH4 selectivity was observed. Because CH4 is an undesired product, it is necessary to point out that, under the same operating conditions, the CH4 selectivity over the granular hybrid catalyst was quite similar to that over Co/SiO2. This indicates that CH4 produced from the reactions over Pd/β was negligible in comparison with that desorbed from Co/SiO2 during the FT reaction. In addition, the granular hybrid catalysts composed of either 0.5 or 1.0 wt % Pd/β with Co/SiO2 gave almost the same CO conversion and CH4 selectivity at a given temperature when the experimental errors were taken into account. Unexpectedly, at the reaction temperatures of 503 and 508 K, CO conversion over the powdery hybrid catalyst was significantly lower

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Figure 3. Carbon number distribution over the granular mixture of Co/SiO2 + 1.0 wt % Pd/β catalyst at temperatures of 503 K (A) and 513 K (B). (Reaction conditions: P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1, TOS ) 2 h.)

than that over the granular hybrid catalyst. However, at the reaction temperature of 513 K, quite similar CO conversion could be observed irrespective of the hybrid catalysts. Independent of reaction temperatures, the powdery hybrid catalyst always showed obviously higher CH4 selectivities than did the granular hybrid catalyst. Moreover, only a slight effect of reaction temperature on CH4 selectivity was observable in the case of the powdery hybrid catalyst. Under the same operating conditions, blank tests showed that maximum CO conversion was less than 5% when only the Pd/β catalyst was loaded in the reactor, indicating that the contribution of CO hydrogenation over the Pd/β catalyst was negligible. Therefore, the above observations must be related to the effect of interfacial contact between Pd/β particles and the Co active sites on the CO hydrogenation reaction. This will be further discussed in a later part by combining the results of the carbon number distribution and isomer selectivity. 3.2. Product Distribution. In the typical FT synthesis over Co/SiO2 catalyst, the products are composed mainly of n-alkanes with a wide range of carbon numbers. However, as shown in Figure 3, the product distribution over the granular hybrid catalyst containing 1.0 wt % Pd/β was cut off at a carbon number of about 10. This indicates that the carbon chain length was significantly impeded by the presence of the Pd/β catalyst. Moreover, the carbon number distribution was significantly changed from the ASF law, in which much higher selectivities of C4 to C7 hydrocarbons were observed. By further examining the selectivities of the different isomers, it is a fact that the selectivity of isoparaffins with varied carbon numbers was much higher than those of the corresponding n-alkanes and olefins, which was different from the composition of typical FT products. In addition, selectivity of isoparaffins strongly decreased with the increase of carbon number from 4 to 8 independent of the reaction temperatures. Because hydrogen was always present in the reaction system, from these results it could be concluded that both hydrocracking and hydroisomerization reactions occurred prominently over the bifunctional Pd/β catalyst. With the increase of reaction temperature from 503 to 513 K, as shown in Figures 3A and 3B, the

Figure 4. Carbon number distribution over the powdery mixture of Co/SiO2 + 1.0 wt % Pd/β catalyst at temperatures of 503 K (A), 508 K (B), and 513 K (C). (Reaction conditions: P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1, TOS ) 2 h.)

selectivity of normal paraffins slightly increased while the selectivity of isoparaffins insignificantly decreased. Moreover, a very tiny shift of carbon chain length toward light hydrocarbons was observable. Thus, it can be summarized that the carbon number distributions at different temperatures were very similar in considering the slight variation of carbon balance. It is worthy to note that, irrespective of the reaction temperature, the selectivities of C1 to C3 hydrocarbons over the hybrid catalyst were comparable to those obtained over the Co/ SiO2 catalyst under the same operating conditions, suggesting that C1 to C3 hydrocarbons, except those desorbed from Co sites in the FT reaction, were not apparently produced in the subsequent hydrocracking reactions. In comparison with the results over the granular hybrid catalyst containing 1.0 wt % Pd/β, the granular hybrid catalyst containing 0.5 wt % Pd/β showed slightly higher olefin selectivity at 503 K. Moreover, the difference in olefin selectivity was insignificantly enhanced with the increase of reaction temperature. Except for this difference, almost the same carbon number distribution and changing trend with reaction temperature were observed over granular hybrid catalysts containing Pd/β with different Pd loadings. Figure 4 illustrates the product distribution over the powdery hybrid catalyst containing Co/SiO2 and 1.0 wt % Pd/β. Comparing the results over the powdery and granular hybrid catalysts under the same operating conditions (Figures 3 and 4), one can see that the effect of reaction temperature on the product distribution followed the same pattern independent of the preparation method of the hybrid catalysts. However, if the product distribution at a given temperature was considered, the following points could be recognized. In

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Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 Table 1. i/n Molar Ratios of C4-C6 Paraffins in the Products over a Hybrid Catalyst Composed of Co/SiO2 and Pd/Betaa catalysts

items

Figure 5. I/C4+ molar ratios vs reaction temperature over different hybrid catalysts. (Reaction conditions: P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1, TOS ) 2 h.)

comparison with the results over the granular hybrid catalyst, the powdery hybrid catalyst showed (1) much evener selectivities to C4-C7 hydrocarbons, (2) a slightly higher selectivity to longer-chain hydrocarbons with carbon numbers of 9 and 10, especially at lower reaction temperature, (3) apparently higher olefin selectivities at a given reaction temperature, and (4) obviously lower selectivity to isoC4-C6 alkanes. Because the two hybrid catalysts were exactly the same in composition, it is reasonable to propose that these differences must result from the variation of the contact state between Co/SiO2 and Pd/β in the hybrid catalysts. 3.3. Selectivity of Total Isoparaffins. To characterize and compare the performance of the hybrid catalyst for isoparaffin production at different reaction temperatures, an index of molar ratio of C4-C10 isoparaffins to C4+ hydrocarbons (I/C4+) was defined, and the calculated results over different hybrid catalysts are provided in Figure 5. Among the reaction temperatures applied in this work, slightly higher I/C4+ ratios were always obtained at lower temperatures irrespective of the catalyst used. This could be explained by the combination of temperature effects on CO activation over Co sites and hydroconversion of hydrocarbons over Pd/β. There is no doubt that the CO hydrogenation reaction was speeded up at higher temperature as indicated from the increase of CO conversion with temperature. As a result, a higher load to Pd/β catalyst for the hydroconversion of FT hydrocarbons was expected, which may decrease the I/C4+ ratio. On the contrary, the increased activity of Pd/β at higher temperature kinetically favored the hydrocracking and hydroisomerization reactions, leading to an increase of the I/C4+ value. Therefore, it is rational to say that the temperature effect put on the Co catalyst slightly overweighed that on the Pd/β catalyst, leading to an insignificant decrease of I/C4+ with increase of reaction temperature. Irrespective of reaction temperatures, it was apparent that I/C4+ over the granular hybrid catalysts containing either 0.5 or 1 wt % Pd/β was almost the same, indicating that there is no obvious influence of Pd loading in the Pd/β catalyst on the selectivity of total isoparaffins under the present operating conditions. When the powdery hybrid catalyst was taken into account, independent of the reaction temperatures, the most noticeable observation was that the I/C4+ ratio was always significantly lower than that over the granular hybrid catalyst. This must be caused by the difference in the contact state between Co/SiO2 and Pd/β

powdery granular granular T 1 wt % 0.5 wt % 1 wt % (K) Pd/β Pd/β Pd/β equilibrium

i-butane/n-butane 503 i-pentanes/n-pentane 503 i-hexanes/n-hexane 503

2.44 3.48 3.72

6.68 10.09 8.46


2.48 3.61 4.12

6.32 9.46 7.95


2.39 3.62 3.86

5.66 8.69 7.44

5.42 8.38 7.27

1.44 4.21 6.05 1.41 4.11 5.89

4.33 6.94 4.31

1.38 4.02 5.75

a Reaction conditions: P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1, and TOS ) 2 h. The thermodynamic values are calculated based on the isomerization of normal paraffins.

in the hybrid catalysts as their composition is exactly the same. As a matter of fact, small particles of Co/SiO2 and Pd/β were cogrinded, and inevitably mutual coverage of the two components must be much richer in the powdery hybrid catalyst than that in the granular hybrid catalyst. Because the function of the Pd/β catalyst was only to hydroconvert the hydrocarbons produced over the Co/SiO2 catalyst, taking into account the hydroconversion reactions, it is easy to imagine that the active sites on Pd/β covered fully or partially by Co/ SiO2 will not be as efficient as those not covered. Therefore, here we can tentatively say that the mutual coverage of Pd/β and Co/SiO2 may cause the lower performance of the powdery hybrid catalyst for the production of isoparaffins. This will be further extended in a later discussion. 3. 4. Selectivity of Isomers with Varied Carbon Numbers. To examine the isomerization extent of different n-alkanes, C4, C5, and C6 paraffins, which were abundant in the products, were selected. The molar ratios of i/n for C4, C5, and C6 paraffins at different reaction temperatures were calculated and are given in Table 1. As expected from the I/C4+ ratio, molar ratios of i/n for C4, C5, and C6 alkanes decreased with the increase of reaction temperature. However, it should be noted that the experimental results exceeded the values predicted from the thermodynamic equilibrium, especially at low temperature. To explain this, the reaction mechanism must be examined, and this will be discussed in the following part. Moreover, the molar ratios of i/n showed a decreased sequence of i-C5/n-C5 > i-C6/ n-C6 > i-C4/n-C4 over the granular hybrid catalyst while i-C6/n-C6 > i-C5/n-C5 > i-C4/n-C4 was observed over the powdery hybrid catalyst. 3.5. Time-on-Stream Results. To probe the stability of the hybrid catalyst for the production of isoparaffins, experiments were carried out for an extended time of 8 h, and the results are given in Figure 6. Irrespective of the hybrid catalyst, I/C4+ obviously decreased with time-on-stream (TOS), indicating a clearly visible deactivation of the Pd/β catalyst for hydroconversion reactions. It was observable that the granular hybrid catalyst containing 1 wt % Pd/β showed a slightly more stable performance than the one containing 0.5 wt % Pd/β, suggesting that higher Pd loading favors the longterm stability of the catalyst for the production of isoparaffins. Over the powdery hybrid catalyst, due to low initial values, I/C4+ versus TOS was more stable


Figure 6. Time-on-stream I/C4+ molar ratios over the powdery hybrid catalyst composed of Co/SiO2 + 1.0 wt % Pd/β at 508 K (4) and 513 K (0), the granular hybrid catalyst containing 1 wt % Pd/β at 513 K (O) and the granular hybrid catalyst containing 0.5 wt % Pd/β at 508 K (]) and 513 K (x) (P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1) and results obtained in the dual reactor system (9).

Figure 7. Carbon number distribution over the granular mixture of Co/SiO2 + 1.0 wt % Pd/β catalyst at 513 K. (Reaction conditions: P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1, TOS ) 6 h.)

in comparison with that over the granular hybrid catalyst. Comparing the product distributions shown in Figures 3B and 7, which were obtained over the granular mixture of Co/SiO2 + 1.0 wt % Pd/β at 513 K and a TOS of 2 and 6 h, respectively, the noticeable decrease of I/C4+ versus TOS was ascribed to the increase of normal paraffin selectivity, the decrease of isoparaffin selectivity, and less cracking of the longchain FT hydrocarbons. As shown in Figure 6, the dual reactor system showed the most stable I/C4+. Thus, the deactivation of Pd/β for hydroconversion reactions may be related to the contact states between Co/SiO2 and Pd/β, and this will be discussed based on the reaction mechanism later. Moreover, because coke formation always accompanied the conversion of hydrocarbons over solid acid, this may compose another reason for the deactivation of the catalyst for the titled reaction. Alternatively, water, which has deteriorative effect on solid acids and was stoichiometrically produced in the FT reaction, may also contribute to the decrease of hydroconversion function of the catalyst, leading to the decrease of I/C4+ with TOS. 3.6. Examination of the Reaction Mechanism. The results presented above indicate that the product composition over hybrid catalysts was significantly deviated from the typical ASF distribution over Co/SiO2 catalyst, i.e., cutting off the carbon number at about 10, the sharp increase of isoparaffin selectivity, and the decrease of selectivities to n-alkanes and olefins. This indicates that hydrocracking and hydroisomerization of the primary FT products significantly occurred over the Pd/β contained in the hybrid catalyst. Although hydro-

Figure 8. Mechanistic diagram of the reactions that occurred over the hybrid catalyst containing Co/SiO2 and Pd/β.

conversion of pure normal alkanes such as pentane, hexane, and heptane has been intensively investigated over solid acid supported Pt or Pd catalysts, the reaction mechanism, especially the function of metal, is still under debate.19 It has been reported that hydroconversion of n-paraffins on Pt-supported solid acids proceeds through the bifunctional mechanism composed of the following steps:19,20 (1) dehydrogenation of paraffins to olefins on the Pt surface, (2) transfer of the olefins to the acidic site for protonation to carbenium ion, (3) isomerization and cracking of the carbenium ion and deprotonation on the acidic site to olefins, and (4) transfer of the intermediate olefin to the Pt site for hydrogenation leading to paraffins. In this mechanism, the most important function of Pt is for dehydrogenation of the reactant and hydrogenation of the intermediate olefin to the final product. However, besides other challenges such as very low equilibrium olefin content at the lower temperature of around 513 K, it is commonly believed that Pd is not a good catalyst for hydrogenation reactions although it can effectively activate hydrogen.21 Furthermore, it was found that the distance between the Pt particles and the acidic sites was important in determining the activity and selectivity of Pt/β for hexane hydroconversion,22 which cannot be explained based on the above bifunctional mechanism. As a matter of these facts, alternative mechanisms have been suggested.22,23 Based on the hydroconversion results of different hydrocarbons, Fujimoto’s research group has proposed a hydrogen spillover mechanism.21,24-29 In this mechanism, the following steps were suggested: First, gaseous hydrogen dissociatively adsorbs on the surface of Pt or Pd to create Hsp; after that, Hsp migrates from the metal surface to the surface of the solid acid leading to H+sp and H-sp. The H-sp promotes desorption of the carbenium intermediate as an alkane, while the H+sp regenerates and/ or enhances the strength of the Bronsted acidic sites on the surface of solid acid. To explain the present results, a mechanistic scheme that followed the hydrogen spillover concept together with the FT reaction mechanism was developed based on the above results and discussion, as shown in Figure 8. CO and H2 dissociatively adsorbed on Co sites to produce surface species such as H, C, O, and CHx for the initiation of the FT reaction. The adsorbed CH2‚‚‚CH2CH3 intermediate either desorbed from the Co sites with a certain probability to give the straight-chain paraffin

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or olefin or continued the chain growth with the insertion of a surface CHx group. Simultaneously, H2 dissociatively adsorbed on Pd sites to form Hsp, and subsequently the Hsp spilled over to the surface of the zeolite β leading to H+sp and H-sp. With the existence of Pd/β particles near the Co/SiO2 particles, the desorbed FT straight-chain hydrocarbon moved from Co/SiO2 to zeolite β, where H+sp acted as an acid and abstracted a H- from the hydrocarbon or attacked one of the carbon atoms in the hydrocarbon to promote the isomerization or cracking reactions. As a result of the spilled-over H+sp, the strength of the acidic sites on the zeolite β was enhanced. Thus, higher hydrocarbons (wax) that diffused from Co/SiO2 to Pd/β were exclusively cracked. This is consistent with and supported by the cutoff phenomenon in the product distribution. With the stabilization of H-sp on the carbonium or carbenium ions, the isomerization reaction was enhanced. Consequently, the selectivity of isoparaffin was increased. According to the concept of hydrogen spillover, it is implied that hydrogen spillover was enhanced with the increase of either hydrogen partial pressure or Pd loading in the Pd/β catalyst. This is well agreed with the slightly more stable performance of the granular hybrid catalysts containing 1.0 wt % Pd/β than that containing 0.5 wt % Pd/β. Because the molar ratios of i/n for C4, C5, and C6 alkanes exceeded the thermodynamic equilibrium values, examination of the sources for C4-C6 hydrocarbons is necessary before giving any explanations. As shown in Figure 8, on the one hand, normal C4-C6 paraffins were produced from the FT reaction. These paraffins may partly be isomerized to branched counterparts over Pd/β with the limitation of thermodynamic equilibrium. On the other hand, because of the significant increase of selectivity to C4-C6 hydrocarbons over the hybrid catalyst, C4-C6 hydrocarbons must mainly be produced from a series of isomerization and cracking reactions of the long-chain FT hydrocarbons. In this case, if branched C4-C6 alkanes were primary products of a series cracking reactions, with the limitation of reaction kinetics and residence time, these branched alkanes may less be isomerized to the corresponding normal alkanes. If it is so, it will not be surprising anymore that the molar ratios of i/n for C4, C5, and C6 alkanes exceeded the equilibrium values. The decrease of i/n ratios for C4, C5, and C6 alkanes with increase in temperature supports this explanation. Because the reaction of branched C4C6 alkanes to normal C4-C6 alkanes was both thermodynamically and kinetically favored with an increase of temperature, correspondingly, the molar ratios of i/n for C4, C5, and C6 decreased with an increase of temperature. Therefore, the apparently higher i/n ratios of C4, C5, and C6 alkanes than those predicted from thermodynamics is caused by the contribution of branched alkanes from alternative reactions, i.e., the cracking and isomerization of long-chain FT hydrocarbons. In studying the hydroconversion of n-heptane over Pt/zeolite catalysts, it has been reported that the skeletal isomerization occurred before the cracking reactions.30 As a matter of fact, long-chain FT hydrocarbons are mainly of normal paraffins. Thus, it is reasonable to propose that the skeletal isomerization of the long straight-chain carbonium or carbenium ions to the branched counterpart occurred before the cracking reaction as schematically shown in Figure 8. The question may arise on the influence of CO

competitive adsorption with H2 over Pd on its activity for hydrogen activation. Indeed, as reported in many works, it is conclusive that CO can be molecularly adsorbed on Pd while dissociative adsorption cannot proceed.31 This is also supported by the results of a blank test, in which negligible CO conversion was observed when only Pd/β catalyst was loaded in the reactor. Moreover, it was reported that separate CO and H islands over Pd (111) were formed during the coadsorption of mixed gases of CO and H2 (see ref 32 and references therein). Therefore, much less influence of CO adsorption on the hydrogen spillover function of Pd can be reasonably expected, which is consistent with the present experimental results. 3.7. Insights into the Interfaces between Co/SiO2 and Pd/Beta. As mentioned above, it is straightforward that smaller particles of Co/SiO2 and Pd/β were closely in touch in the case of the powdery hybrid catalyst while bigger particles were loosely contacted in the granular hybrid catalyst. Unexpectedly, the powdery hybrid catalyst showed poorer performance for the production of isoparaffins than that of the granular hybrid catalyst at the initial stage of the reaction. Moreover, irrespective of the reaction temperatures, an appreciably higher selectivity of CH4 was always observed in the case of the powdery hybrid catalyst. To explain these facts, the interfaces between the Co/SiO2 and Pd/β particles must be examined. It is no doubt that Pd has a higher activity than Co for the dissociative adsorption of hydrogen. In addition, the FT reaction over Co sites consumed more hydrogen than the hydroconversion reactions over Pd/ β. These facts indicate that the surface concentration of hydrogen species over Pd/β must be higher than that over Co/SiO2. Thus, driven by the concentration difference, it is very possible that hydrogen species may migrate from Pd/β to Co/SiO2. As a result of this migration, the surface H/C ratio over Co/SiO2 was locally modified in comparison with the situation without the existence of Pd/β. To keep this in mind, the difference in the results of the powdery and granular hybrid catalysts is easily understandable. In the case of the powdery hybrid catalyst, it is obvious that the interfaces of Co/SiO2 and Pd/β were much richer than those in the granular hybrid catalyst. Moreover, the distance between Pd/β and Co/SiO2 particles was shorter in the case of the powdery hybrid catalyst as a result of the pressure-molding. Consequently, the migration of hydrogen species from Pd/β to Co/SiO2 was much pronounced in the case of the powdery hybrid catalyst. Because the higher surface H/C ratio over Co favors the formation of CH4, the obvious higher CH4 selectivity over the powdery hybrid catalyst supports this explanation. Additionally, as a result of the migration of hydrogen species from Pd/β to Co/SiO2, decrease of the promotional effect of spilled-over hydrogen on cracking and isomerization reactions over zeolite β could be reasonably expected. As the contact between Co/SiO2 and Pd/β in granular hybrid catalyst was very loose and there was basically no physical contact between particles of Co/SiO2 and the majority of Pd/β in the dual reactor system, the migration of hydrogen from Pd/β to Co/SiO2 was very limited, leading to higher isoparaffin selectivity. The above explanation may compose one of the main reasons why the powdery hybrid catalyst showed lower performance for the production of isoparaffins at the early stage of the reaction.


4. Conclusion

Figure 9. Carbon number distribution in the consecutive dual reactor system (P ) 1.0 MPa, W/F ) 7.9 g‚h‚mol-1, T (upper reactor) ) 508 K, T (lower reactor) ) 503-513 K, TOS ) 4 h).

According to the above explanation, higher CO conversion over the powdery hybrid catalyst should be expected because it is commonly known that higher surface concentration of hydrogen over Co favors CO hydrogenation leading to higher CO conversion. On the contrary, our experimental data showed the reverse result. Therefore, the lower CO conversion over the powdery hybrid catalyst must be caused by other reasons. It is known that the reduction of Co or Fe ionexchanged zeolite such as ZSM-5 using gaseous hydrogen becomes difficult,33,34 which is caused by the metal ions having a large negative electrochemical potential and a strong bond to the zeolite framework. In the present case, zeolite β very closely interacted with Co/ SiO2, and possibly electronic effects of zeolite β on cobalt may exist. Potentially, the reduction of cobalt oxide may be slightly hindered from the electronic effect of zeolite β in the proximity. As a result of this effect, a lower reduction degree of cobalt in comparison with the case without zeolite β can be reasonably expected, leading to a slightly lower CO conversion in the case of the powdery hybrid catalyst. The comparable CO conversion over the powdery and granular hybrid catalysts at the higher temperature of 513 K supports the above explanation. 3.8. Synergistic Effects in Hybrid Catalyst. As observed from Figure 3, product selectivities sharply decreased from C4 to C7 hydrocarbons over the granular hybrid catalyst. Furthermore, product distribution over the granular catalyst (Figure 3) was quite similar to that obtained in a dual reactor system (Figure 9), in which there is basically no physical contact between particles of Co/SiO2 and the majority of Pd/β. Thus, it is easy to understand this phenomenon, because distribution of FT products follows the ASF law and the cracking of FT products over Pd/β gives lower hydrocarbons. However, in the case of the powdery hybrid catalyst, much evener selectivities of C4 to C7 hydrocarbons were observed irrespective of the reaction temperatures, implying that there may exist a synergistic effect between the reactions over Co/SiO2 and Pd/β. From the results of I/C4+ versus TOS shown in Figure 6, it was concluded that the I/C4+ was more stable if there was less contact between Co/SiO2 and Pd/β particles since the dual reactor system showed the most stable I/C4+. This indicates that the more pronounced decrease of I/C4+ over the hybrid catalyst may be from the mutual effect of the FT reaction over Co/SiO2 and hydroconversion reactions over Pd/β. Combining the above points in carbon number distribution and time-on-stream I/C4+, it can be concluded that there possibly exists stronger synergistic effects between the FT reaction and the hydroconversion reaction over the powdery hybrid catalyst.

Without an apparent increase of CH4 selectivity, isoparaffin selectivity over a hybrid catalyst containing Co/SiO2 and Pd/β was significantly increased as a result of the hydrocracking and hydroisomerization functions of the Pd/β. At the initial stage of the reaction, the powdery hybrid catalyst showed much lower isoparaffin selectivity than that of the granular hybrid catalyst with the same composition. Moreover, the product was cut off at a carbon number of about 10, and very low selectivity of hydrocarbons with a carbon number equal to or larger than 11 was only observed at a later stage of the reaction. Additionally, at temperatures of 503 and 508 K, CO conversion was noticeably lower over the powdery hybrid catalyst than that over the granular hybrid catalyst. On the contrary, CH4 selectivity in the case of the powdery hybrid catalyst was always higher than that of the granular hybrid catalyst irrespective of the catalyst composition and temperatures. The experimental molar ratios of i/n for C4, C5, and C6, which decreased with the increase of temperature, exceeded the values predicted from the thermodynamic equilibrium at the same conditions. Irrespective of the hybrid catalysts, the time-on-stream I/C4+ indicates an obvious deactivation of Pd/β for the production of isoparaffins, which was explained as the synergistic effect of FT reaction over Co/SiO2 and hydroconversion reaction over Pd/β, by comparing the results over hybrid catalysts with different contact states. The experimental results were extensively examined based on the interfacial behavior, the potential physical and electronic interactions of Co/SiO2 and Pd/β particles. A mechanistic scheme that followed the concept of hydrogen spillover together with FT reaction was developed to explain the phenomena in the interfaces between Co/SiO2 and Pd/ β. The reaction results were well explained based on the mechanistic model. Acknowledgment The authors are thankful for the financial support of this work from the Japan Oil, Gas and Metals National Corporation (JOGMEC). Literature Cited (1) Schulz, H. Short History and Present Trends of FischerTropsch Synthesis. Appl. Catal., A 1999, 186, 3. (2) Iglesia, E. Design, Synthesis, and use of Cobalt-based Fischer-Tropsch Synthesis Catalysts. Appl. Catal., A 1997, 161, 59. (3) Sellmer, C.; Decker, S.; Kruse, N. CO Hydrogenation over Co/SiO2: Catalytic Tests and Surface Analysis of Adsorbed Hydrocarbons. Catal. Lett. 1998, 52, 131. (4) Tsubaki, N.; Yoneyama, Y.; Michiki, K.; Fujimoto, K. Threecomponent Hybrid Catalyst for Direct Synthesis of Isoparaffin via Modified Fischer-Tropsch Synthesis. Catal. Commun. 2003, 4, 108. (5) Jothimurugesan, K.; Gangwal, S. K. Titania-supported Bimetallic Catalysts Combined with HZSM-5 for Fischer-Tropsch Synthesis. Ind. Eng. Chem. Res. 1998, 37, 1181. (6) Tsubaki, N.; Michiki, K.; Yoneyama, Y.; Fujimoto, K. Direct Isoparaffin Synthesis from Syngas by Hybrid Catalysts System. J. Pet. Inst. Jpn. 2001, 44, 338. (7) Bi, Y.; Dalai, A. K. Selective Production of C4 Hydrocarbons from Syngas Using Fe-Co/ZrO2 and SO42-/ZrO2 Catalysts. Can. J. Chem. Eng. 2003, 81, 230. (8) Li, X.; Luo, M.; Asami, K. Direct Synthesis of Middle Isoparaffins from Synthesis Gas on Hybrid Catalysts. Catal. Today 2004, 89, 439.

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Received for review June 2, 2005 Revised manuscript received July 5, 2005 Accepted July 12, 2005 IE050645D

Beta for Isoparaffin Productio - American Chemical Society

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