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Formation of Isoparaffins through Pd/β Zeolite Application in Fischer-Tropsch Synthesis 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, People’s Republic of China Received March 15, 2005. Revised Manuscript Received May 12, 2005
The direct synthesis of isoparaffins from synthesis gas (H2/CO ) 2) was carried out in a consecutive dual reactor system under the conditions of P ) 1.0 MPa and W/F ) 4.97 g‚h‚mol-1. In the upper reactor, Co/SiO2 was used for Fischer-Tropsch (FT) reaction. To primarily decompose the heavy FT hydrocarbons, zeolite β (4:1 weight ratio) was physically mixed with Co/SiO2. The Pd/β catalyst with a Pd loading of 0.5 wt % and different Si/Al ratios in zeolites β was loaded in the lower reactor for hydroconverting the products from the upper reactor. Irrespective of the Si/Al ratio of zeolite β, the results indicate that the final products showed a sharp carbon number distribution in the range of C4-C6 and very high selectivity to branched isomers of C4-C6 alkanes. An apparent change of selectivity to isoparaffins was observed with the variation of the Si/Al ratio in Pd/β catalyst. Moreover, the stability of the catalyst was also influenced by the Si/Al ratios. The effect of the Si/Al ratio on the selectivity of isomers at different times on stream was correlated with the acidity and crystallization of the β zeolite, which were characterized by ammonia TPD and XRD methods.
Introduction With the increasing public concern about the environment, more and more stringent legislations have been and will be implemented on transportation fuels such as gasoline. Presently, an aromatic is used as one of the main components for boosting the octane properties of gasoline. However, the aromatic has been identified as one of the carcinogens. Therefore, its content in gasoline must be decreased accordingly. To date, it seems that isoparaffins are more environmentally acceptable to boost the octane number of gasoline. Indeed, the branched paraffins have a high octane rating without the drawbacks arising from the presently used additives such as tetraethyllead, an aromatic, and an oxygenated compound of MTBE. However, within the limits of the existing technologies in isomerization and alkylation units, branched paraffins cannot be made through isomerization and alkylation in sufficient amounts to replace the aromatic below 30%.1 Thus, it is more attractive if branched paraffins can be directly produced from resources rather than petroleum. As a matter of fact, the Fischer-Tropsch (FT) synthesis is an effective route to convert coal, natural gas, or biamass-derived synthesis gas (syngas, CO + H2) to liquid fuels and high-value-added fine chemicals.2-5 * To whom correspondence should be addressed. Phone: 81-93-6953286. Fax: 81-93-695-3378. E-mail:
[email protected]. † The University of Kitakyushu. ‡ Nanjing University of Technology. (1) Song X.; Sayari, A. Energy Fuels 1996, 10, 561-565. (2) Vannice, M. A. Catal. Rev.sSci. Eng. 1976, 14, 153-191. (3) Tsubaki, N.; Sun, S.; Fujimoto, K. J. Catal. 2001, 199, 236-246. (4) Schulz, H. Appl. Catal., A 1999, 186, 3-12.
However, as a result of the Anderson-Schulz-Flory (ASF) polymerization kinetics, the FT products which are composed mainly of normal paraffins are nonselective to any specific product.2 To selectively synthesize desired products such as diesel or high-octane gasoline, it is essential to circumvent the ASF distribution such that high selectivity to the desired product can be achieved. Several groups have practiced for this purpose by utilizing FT-active components supported on acidic zeolites.6-8 However, acidic zeolite is not a stable support under FT reaction conditions, and typical results showed low activity and high methane selectivity. In our previous investigations,9,10 a fundamental concept by using a physical mixture of an FT catalyst to synthesize long-chain hydrocarbons and a Pd-supported solid acid catalyst to hydroconvert the FT products into isoparaffins was developed and experimentally evaluated both in one reactor and in a consecutive dual reactor system. Results showed that high selectivity to isoparaffins can be conveniently achieved in a dual reactor system by using Pd/β catalyst in the second reactor.10 Although the synthesis of zeolite β was reported in 1967, studies for the potential applications in petroleum (5) Sun, S.; Tsubaki, N.; Fujimoto, K. Appl. Catal., A 2000, 202, 121-131. (6) Jothimurugesan K., Gangwal, S. K. Ind. Eng. Chem. Res. 1998, 37, 1181-1188. (7) Koh, D. J.; Chung, J. S.; Kim, Y. G. Ind. Eng. Chem. Res. 1995, 34, 1969-1975. (8) Guczi, L.; Kiricsi, I. Appl. Catal., A 1999, 186, 375-394. (9) Li, X.; Luo, M.; Asami, K. Catal. Today 2004, 89, 439-446. (10) Li, X.; Asami, K.; Luo, M.; Michiki, K.; Tsubaki, N.; Fujimoto, K. Catal. Today 2003, 84, 59-65.
10.1021/ef050065t CCC: $30.25 © 2005 American Chemical Society Published on Web 06/23/2005
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Table 1. Summary of the Zeolite β Used in This Study Si/Al molar ratio crystal size (Å) source
a
b
c
d
e
8.1 92 Tosoh
12.8 86 Tosoh
19.5 108 Tosoh
27.0 101 homemade
75.0 95 Sud
chemistry, refining, and fine chemical production began only in recent years partly because of the late revelation of its framework structure.11 Zeolite β has a threedimensional, interconnected channel system with 12membered elliptical openings having mean diameters of 0.64 × 0.76 nm, which could be of great industrial interest. As a matter of fact, zeolite β has been reported to be a good catalyst for several reactions such as cracking, hydrotreating, and benzene alkylation with light olefins. It is a common fact that the acidic properties of zeolite are strongly related to its Si/Al ratio and crystallization. Consequently, the catalytic properties of zeolite β may be affected by the variation of Si/Al ratios. The present investigation aims at comparatively studying the effect of Si/Al ratios and crystal properties of zeolite β on the performance of Pd/β catalysts for the selective production of isoparaffins in a consecutive dual reactor system, i.e., FT synthesis over Co/SiO2 in the upper reactor to produce long-chain hydrocarbons and a Pd/β catalyst in the lower reactor to hydroconvert the FT products into isoparaffins. Experimental Section Catalysts and Preparation Procedures. The Co/SiO2 catalyst with a metallic cobalt loading of 20 wt % was prepared by the incipient impregnation method. The nitrate salt was used as the cobalt precursor and silica gel (Fujisilicia Q-15) as the support. After being dried overnight at 393 K, the catalyst was calcined at 473 K for 2 h. Zeolite β prepared in this laboratory followed the hydrothermal crystallization method under typical conditions. The properties of zeolites β from different sources are summarized in Table 1. The 0.5 wt % Pd/β catalysts were prepared by ion-exchanging different zeolites β, respectively, at 353 K for 6 h in the solution of Pd(NH3)4Cl2. The final catalysts were dried overnight at 393 K and then calcined at 723 K for 2 h. Catalyst Characterization. The XRD patterns of Pd/β catalysts were recorded using a Rigaku diffractometer (Nifiltered Cu KR radiation, 50 kV, 36 mA) with a scan speed of 2 deg (2θ)/min. The NH3-TPD of different zeolites β was measured with TPDRO 1100 (ThermoQuest Corp.). To minimize the physical adsorption, the adsorption of NH3 was performed at 423 K, and subsequently, it was purged for 12 h in a flow of helium at the same temperature. Then the TPD was carried out from 423 to 873 K with a heating rate of 10 K/min under a helium flow of 30 mL/min. Reaction System and Operating Procedure. The reaction was carried out in dual fixed-bed reactors (SUS tube, 8 mm i.d.), which were consecutively arranged in the vertical direction. As mentioned in the Introduction, the purpose of this investigation is to study the effect of the Si/Al ratio in the Pd/β catalyst. Therefore, catalyst in the upper reactor, i.e., 0.4 g of Co/SiO2 physically mixed with 0.1 g of β (Tosoh, Si/Al ) 19.5) and diluted with 0.5 g of quartz sands, was kept the same for all of the experiments. In the lower reactor, 0.5 g of 0.5 wt % Pd/β catalyst diluted with the same amount of quartz sands was loaded for each experiment. Before reaction, the catalysts were pretreated at 673 K for 3 h in a flow of (11) Wang, Z. B.; Kamo, A.; Yoneda, T.; Komatsu, T.; Yashima, T. Appl. Catal., A 1997, 159, 119-132.
Figure 1. XRD patterns of different Pd/β catalysts (the assignments of a-e are the same as those in Table 1). hydrogen. The operating conditions were P ) 1.0MPa, H2/CO ) 2.0, W/F ) 4.97 g‚h‚mol-1, and reaction temperature 508 K (upper reactor) and 573 K (lower reactor). To probe the reaction mechanism, in one experiment, hydrogen with a flow rate 3 times that of CO in syngas before reaction was fed to the lower reactor from a separate line. Analysis of the Products. The effluent hydrocarbons cooled at 453 K were analyzed with an on-line gas chromatograph (GC-353, GL Science) equipped with an NB-1 capillary column and an FID detector. The CO, CH4, and CO2 in the effluent after it was cooled in an ice-water trap were on-line analyzed with a gas chromatograph with a packed active carbon column and a TCD detector. The selectivity of the hydrocarbons was calculated on the basis of the carbon number.
Results and Discussion XRD Analysis. It is a common fact that the crystallite properties of zeolite β may be changed, to a certain degree, after ion exchange in the solution of Pd(NH3)4Cl2 at elevated temperature. Therefore, XRD investigations of the Pd/β catalysts were undertaken, and the results are given in Figure 1. As shown in Figure 1, it is clear that the characteristic crystal structure of zeolite β was still reserved after ion exchange in the alkaline solution. It is noteworthy that diffractions from palladium species were invisible due to its low content in the sample. Furthermore, the slight difference of d values for the main diffraction peaks was apparent, which can be attributed to the variation of the Si/Al ratio in the sample. The diffraction line at a 2θ of about 7.5° is stronger and well separated from the other diffraction lines. Thus, the crystal size of zeolite β was estimated on the basis of this diffraction by using the DebyeScherrer equation Dhkl ) 0.9λ/(βhkl cos θ), where Dhkl is the crystallite size, λ is the wavelength of Cu KR radiation, βhkl is the peak width at half-maximum, and θ is the Bragg diffraction angle. The results are summarized in Table 1. The crystal size of zeolite β was in the range of 80-110 Å; catalyst b gave the smallest value of 86 Å, while sample c showed the highest one. Moreover, from the strongest XRD diffraction at about 22.5°, the peak intensity of Pd/β catalyst decreased in the sequence e > d > c > b > a, indicating the crystallinity of zeolite β decreased in the same sequence. Acidic Properties of Zeolites β. Figure 2 shows ammonia TPD patterns of zeolite β with various Si/Al ratios. Irrespective of the sources of zeolite β, the amount of desorbed ammonia, which defines the total amount of acid over the sample, approximately de-
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Table 2. Product Distribution over Different Catalysts in One Reactor Systema product distribution (%) catalyst
CH4
C2-C3
C4-C10
C11-C15
C16+
iso-C4-C10 alkanes
C4-C10 alkenes
I/C4+ (%)
Co/SiO2 Co/SiO2 + β (4:1)
14.0 11.8
6.0 5.2
35.0 45.3
13.2 13.0
2.3 0.0
2.0 8.1
7.1 20.4
4.0 13.9
a
Reaction conditions: T ) 510 K, P ) 1.0 MPa, W/F ) 4.97 g‚h‚mol-1, and results obtained at a time on stream of 2 h.
Figure 2. NH3-TPD results of zeolites β with different Si/Al ratios (the assignments of a-e are the same as those in Table 1).
creased with the increase of the Si/Al ratio as revealed from the peak area of TPD. However, the TPD patterns were significantly changed from one sample to another. Generally, two peaks with temperatures at the peak maximum of about 573 and 700 K were observed. Moreover, the two peaks became very well separated with the increase of the Si/Al ratio. Considering the small peak at around 873 K, it possibly arose from the dehydroxylation of zeolite β as reported by other researchers.12 In the case of samples a and c, only the lower temperature peak was clearly shown and the higher temperature peak appeared as a small shoulder. Moreover, comparing the peak maxima of samples a and c, it is clear that the peak maximum shifted to higher temperature with the increase of the Si/Al ratio. This indicates that the strength of the acidic sites in sample c was stronger than that in sample a, which is in agreement with the observation that the acidic strength increases with the decrease of the alumina content in zeolite β.8 On the contrary, sample e showed only the higher temperature peak and the lower temperature peak disappeared, which was probably due to the very low alumina content. Furthermore, the peak maximum was shifted to higher temperature in comparison with those of samples a and c. When sample b was considered, its TPD pattern showed a very broad peak and the lower temperature peak was only a small shoulder, indicating that acidic sites with wide strengths are present. In the case of sample d, two well-separated peaks were apparently observed and the intensity of the higher temperature peak was much higher than that of the lower temperature peak. Effect of the Addition of Zeolite β to Co/SiO2. Our basic idea to physically mix a small amount of zeolite β with Co-based FT catalyst is intended to crack the very heavy hydrocarbons (wax) produced from FT synthesis. The results of product distribution over Co/SiO2 and Co/ (12) Camiloti, A. M.; Jahn, S. L.; Velasco, N. D.; Moura, L. F.; Cardoso, D. Appl. Catal., A 1999, 182, 107-113.
Figure 3. Typical carbon number distribution for the FT synthesis in a dual reactor system (lower reactor, 0.5 g of 0.5 wt % Pd/β (Tosoh, Si/Al ) 12.8), T ) 573 K, TOS ) 6 h).
SiO2 + β are given in Table 2. In comparison with the results over Co/SiO2, the addition of 20 wt % zeolite β slightly increased the selectivity of isoparaffins, which can be reasonably ascribed as an isomerization reaction over acidic sites of the zeolite. Furthermore, the very heavy hydrocarbons produced from FT synthesis disappeared as a result of the cracking function of zeolite β. Moreover, comparing the methane selectivity for FT synthesis over Co/SiO2 under the present conditions, it can be inferred that no appreciable amount of methane was produced in the presence of the acidic component together with FT catalyst. These results indicate that the idea to mix a small amount of zeolite β with FT catalyst is practically viable for cracking of the heavy FT hydrocarbons. Effect of the Si/Al Ratio in Zeolite β. To comparatively study the effect of the Si/Al ratio in Pd/β catalyst in the dual reactor system, catalyst in the upper reactor and the reaction conditions were kept the same for each experiment. Under the present operating conditions, CO conversion was at about 60%, methane selectivity was around 12%, and CO2 selectivity was less than 5%. Irrespective of the Si/Al ratio in Pd/β catalysts, results in the dual reactor system showed that the products gave a sharp carbon number distribution mainly ranging from C4 to C6 and quite high selectivity of branched C4-C6 alkanes. The typical carbon number distribution is given in Figure 3. Under the present conditions, as shown in Figure 3, the selectivity of alkanes with carbon number from 4 to 6 decreased sharply. For the purpose of gasoline ranged isoparaffins, optimization of the reaction conditions is necessary such that selectivity of butane and pentane could be decreased and selectivity of C6-C10 alkanes might be increased. In comparison with the results in Table 2, it should be noted that there was no obvious change of methane selectivity in the dual reactor system, indicating that additional methane was not produced or consumed in the lower reactor. Moreover, further cracking of the products from the upper
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Table 3. Effect of Si/Al Ratios of Zeolites β in Pd/β Catalysts for Isoparaffin Synthesis in a Consecutive Dual Reactor Systema TOS (h)
β zeolite
Si/Al in 0.5% Pd/β
2.0 a 8.1 2.0 b 12.8 2.0 c 19.5 2.0 d 27.0 2.0 e 75.0 7.0 a 8.1 7.0 b 12.8 7.0 c 19.5 7.0 d 27.0 8.0 e 75.0 thermodynamic equilibriumb
molar ratio i-C4/n-C4 i-C5/n-C5 i-C6/n-C6 2.2 3.2 1.9 2.6 1.9 2.0 2.6 1.8 2.3 1.8 1.3
2.4 3.5 2.1 2.8 1.9 1.9 2.7 1.7 2.2 1.5 3.2
2.1 3.6 1.9 2.6 1.7 1.7 2.6 1.5 2.1 1.5 4.5
I/C4+ (%) 65.4 76.1 61.5 71.2 59.9 61.6 71.5 57.7 66.9 57.6
a Reaction conditions: P ) 10 atm, W/F ) 4.97 g‚h‚mol-1, physical mixture of 0.4 g of Co/SiO2 and 0.1 g of zeolite β (Tosoh, Si/Al ) 19.5), T ) 508 K (upper reactor), T ) 573 K (lower reactor). b The molar ratios of i-C /n-C , i-C /n-C , and i-C /n-C at ther4 4 5 5 6 6 modynamic equilibrium were respectively calculated on the basis of the isomerization of butane, pentane, and hexane at 573 K.
reactor significantly occurred over Pd/β catalyst in the lower reactor as the much narrower carbon number distribution in the dual reactor system. This can also be illustrated by the slightly higher selectivity of propane in the dual reactor system than that without Pd/β catalyst in the lower reactor. It is important to note that selectivities of olefins with different carbon numbers were extremely low for the products obtained in a dual reactor system while they were quite high in a one reactor system. Because CO conversion was about 60%, an appreciable amount of hydrogen was available in the lower reactor for hydrocracking and hydroisomerization reactions over bifunctional Pd/β catalyst, leading to much lower olefin selectivity and higher isoparaffin selectivity. The time-on-stream (TOS) results by using different Pd/β catalysts in the lower reactor are summarized in Table 3. By comparing the isoparaffin content in C4+ hydrocarbons (I/C4+) at the initial reaction stage, it can be found that catalyst b showed the highest I/C4+ while catalyst e gave the lowest one. When iso to normal ratios of butane, pentane, and hexane were considered, as shown in Table 3, they followed the same changing trend as those of I/C4+. Comparing the results at a later stage of the reaction (TOS ) 7 h) with those at the beginning (TOS ) 2 h) of the reaction, it is apparent that the isoparaffin selectivity slightly decreased, indicating a minor deactivation of the catalyst occurred. Among all of the catalysts, sample e showed a relatively lower decrease in I/C4+ value with TOS than the remaining catalysts, which gave almost the same decrease in I/C4+ ratios. Because coke formation always accompanied the conversion of hydrocarbons over solid acid or metal/solid acid catalysts, the slight deactivation of Pd/β catalyst may be from coke laydown on the catalyst. In hydroconversion of alkanes over bifunctional catalyst, a higher ratio of hydrogen to hydrocarbon is applied for suppressing coke formation such that the lifetime of the catalyst can be extended. In the present case, the hydrogen to hydrocarbon ratio in the lower reactor may be too low for the suppression of coke formation over Pd/β catalyst as a majority of hydrogen was consumed for FT reaction in the upper reactor. To confirm this, additional hydrogen was fed to the lower reactor in one experiment, and the results are given in
Figure 4. Time-on-stream isoparaffin selectivities over 0.5 g of 0.5 wt % Pd/β (Tosoh, Si/Al ) 12.8) in the lower reactor (0.4 g of Co/SiO2 + 0.1 g of 0.5 wt % Pd/β (Tosoh, Si/Al ) 12.8) (upper reactor), T (upper reactor) ) 508 K, T (lower reactor) ) 503 K, additional H2 fed to the lower reactor with a flow rate 3 times that of CO in the syngas before reaction).
Figure 4. Even at a much lower temperature of 503 K in the lower reactor, the I/C4+ was very stable during the 10 h test. Therefore, it can be concluded that the main reason for the slight deactivation of Pd/β catalyst for the titled reaction was from coke formation on the catalyst as a result of low hydrogen partial pressure in the lower reactor. Alternatively, because water was produced in FT synthesis and the activity of the watergas shift reaction over Co-based FT catalyst was low, steam with a relatively higher partial pressure was always present in the lower reactor. The deteriorative effect of steam on the acidic sites over zeolite β may also contribute to the slight decrease of isoparaffin selectivity. On the basis of this idea, the most stable performance of catalyst e may be related to its highest crystallinity as revealed from the peak intensity of XRD. Because the octane property of paraffins strongly depends on molecular structures, the composition of hexane isomers was calculated, and the results are given in Table 4. For comparison, the thermodynamic equilibrium composition together with the octane number of different isomers is also provided in Table 4. Independent of the Pd/β catalyst, the content of monobranched hexanes with a poor octane rating was the highest, which is the same as that predicted from thermodynamic calculations. Moreover, the content of different isomers slightly changed with the increase of TOS from 2 to 7 h. The interesting observation in the case of dibranched hexanes is that the content of 2,3dimethylbutane approached the equilibrium value while the content of 2,2-dimethylbutane was near zero, which is far below the corresponding equilibrium value. Therefore, to improve the octane rating, the catalyst and operating conditions must be optimized such that the content of dibranched isomers, especially 2,2-dimethylbutane, can be increased. As shown in Table 3, irrespective of the Si/Al ratio in zeolite β, isobutane to n-butane ratios far exceeded the value at thermodynamic equilibrium. As a matter of fact, butane (mainly n-butane) was produced from FT reaction in the upper reactor. These n-butanes may isomerize to isobutane over Pd/β catalyst in the lower reactor under the control of thermodynamic limitation. However, because of the significant increase of isobutane selectivity in the dual reactor system, butane must mainly be produced from a series of cracking and/or
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Table 4. Composition of Hexanes under Different Conditions in the Dual Reactor Systema TOS (h) 2.0
β zeolite
Si/Al in 0.5% Pd/β
a 8.1 b 12.8 c 19.5 d 27.0 e 75.0 7.0 a 8.1 b 12.8 c 19.5 d 27.0 thermodynamic equilibrium octane number a
2,2-dimethylbutane
composition of hexanes (%) 2,3-dimethylbutane 2-methylpentane
0.0 1.4 0.0 0.9 0.0 0.0 1.0 0.0 0.0 22.7 91.8
9.1 10.7 8.1 9.7 7.8 7.9 9.8 6.3 9.3 10.6 104.3
39.7 40.3 38.9 39.8 37.7 36.0 40.0 34.8 37.3 32.3 73.4
3-methylpentane
n-hexane
18.8 25.8 18.7 21.7 17.3 18.6 21.7 18.6 20.7 16.1 74.5
32.4 21.9 34.3 28.0 37.2 37.4 27.5 40.3 32.7 18.3 24.8
Reaction conditions are the same as those in Table 3. Equilibrium data were calculated on the basis of isomerization of hexane at 573
K.
isomerization reactions of the long-chain FT hydrocarbons. In this case, if isobutane was a primary product of a series of cracking reactions, with the limitation of reaction kinetics and residence time, the isobutane may be less isomerized to the n-butane. Therefore, the apparently higher isobutane to n-butane ratio than that predicted from thermodynamics may be caused by the contribution of the series cracking of long-chain FT hydrocarbons. As a matter of fact, long-chain FT hydrocarbons over Co/SiO2 catalyst are mainly 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. The above explanation is further supported by examining the molar ratio of isohexanes to n-hexane. Because isohexane has a higher propensity for cracking reactions than isobutane, the molar ratio of isohexanes to n-hexane is lower than that calculated at thermodynamic equilibrium. For the hydroconversion of normal paraffins over metal/solid acid catalyst, it is generally accepted that acidic sites are responsible for cracking or isomerization reactions although the action mechanism of metal is still controversial.13 Therefore, the difference in activity for hydroconversion reactions over Pd/β catalyst with varied Si/Al ratios must be from the acidic properties of zeolites β because the same amount of Pd was introduced into zeolite β under equivalent conditions. From the above TPD results, the amount of acidic sites over zeolites β approximately decreased with the increase of the Si/Al ratio. However, the selectivity to isoparaffin of the titled reaction did not coincide with the amount of acidic sites over the zeolites. This may be contributed by different properties of acidic strength distribution over zeolites β as revealed by the TPD pattern. Therefore, catalyst b showing the highest I/C4+ is probably due to the relatively uniform distribution of the acidic sites with (13) Fujimoto, K. Stud. Surf. Sci. Catal. 1999, 127, 37-49.
different strengths, as reflected by the very broad peak of TPD. Additionally, from the XRD results shown in Table 1, it seems that zeolite β with a smaller crystal size favors isomerization reaction leading to a higher I/C4+. This is consistent with the idea that a smaller crystal size decreased the probability for secondary reactions due to the short diffusion length. Therefore, it can be summarized that, except the Si/Al ratio, both the crystal size and crystallinity of zeolite β have an apparent influence on the performance of the catalyst for the titled reaction. Conclusions The isoparaffins can be directly produced from synthesis gas through an FT route in a consecutive dual reactor system by the combination of Co/SiO2 FT catalyst to synthesize long-chain hydrocarbons and Pd/β catalyst to hydroconvert the FT products. Under the present operating conditions with a CO conversion of about 60%, irrespective of the Si/Al ratio in Pd/β catalysts in the lower reactor, the products showed a sharp carbon number distribution and quite high selectivity of branched C4-C6 alkanes. Both the activity and stability of Pd/β catalyst used in the lower reactor were obviously influenced by the zeolite β used. Combining the XRD, ammonia TPD, and reaction results, it can be concluded that the performance of the Pd/β catalyst for the titled reaction is closely related to the acidic properties, crystallinity, and crystal size of zeolite β. The slight deactivation of the Pd/β catalyst can be mainly ascribed to coke laydown on the catalyst surface due to lower partial pressure of hydrogen in the lower reactor. Acknowledgment. We thank Japan Oil, Gas and Metals National Corp. (JOGMEC) for financial support of this project. EF050065T
