Selective Synthesis of Isobutane from CO2-Containing Synthesis Gas

Jan 24, 2014 - Efficient conversion of carbon dioxide to non-methane light hydrocarbons — Two stage process with intercooler. Congming Li , Kaoru Fu...
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Selective Synthesis of Isobutane from CO2‑Containing Synthesis Gas Congming Li and Kaoru Fujimoto* Japan Gas Synthesis Company, Limited, 5-20, Kaigan 1-Chome, Minato-Ku, Tokyo 105-8527, Japan ABSTRACT: A stable and selective process for the synthesis of isobutane-rich light hydrocarbon from CO2-containing synthesis gas was developed using a Cu−Zn-containing hybrid catalyst. The CO2/COx ratio (COx = CO + CO2) in feed was found to show a significant effect on the activity, selectivity, and stability. It was revealed that the stability of the catalyst operated with the syngas of CO2/COx = 0.25 showed almost constant activity and selectivity over 600 h. In combination with the reaction and characterization results, the CO2/COx ratio in feed has obvious effects on the activity and selectivity and every composition of the hybrid catalyst. Under suitable conditions, CO conversion was as high as 80%, hydrocarbon selectivity was near 70%, and C4 paraffin was 85% [while keeping the dry gas (C1 + C2 paraffins) selectivity of 1−2%] with the isobutane/n-butane ratio of 30−35. first time, experimental evidence based on the stability test and characterization that the stability for LPG synthesis is related to the CO2/COx ratio of the feedstock used. The consequences of such an influence on stability of LPG synthesis are also addressed. These effects were discussed based on the formed water. Another subject should be noticed that the main product in the present system is not the straight-chain hydrocarbon, as is the case for Fischer−Tropsch synthesis, but branched paraffin. This phenomenon was also discussed.

1. INTRODUCTION Liquefied petroleum gas (LPG) is drawing more and more attention as a clean fuel because of its benign characteristics.1−3 LPG can serve as not only a high-quality household fuel, but it can also be used as a good substitute for the spark-ignition engines or as a replacement for aerosel propellants and refrigerants in place of fluorocarbons. At present, LPG is commercially produced through natural gas purification and crude oil production. Recently, an original technique named STL (synthesis gas to LPG) was developed for the direct synthesis of LPG from synthesis gas in a single reactor on a hybrid catalyst composed of a methanol synthesis catalyst and zeolite.4−12 This process greatly improved the overall reaction performance and overcomed the thermodynamic restriction of methanol synthesis by in situ transformation of formed products, driving the chemical system forward. This process gives much higher selectivity of LPG (as high as 80%) than that predicted by the Anderson−Schulz−Flory (ASF) distribution. LPG synthesis from synthesis gas is a complicated reaction system, involving many reactions: CO hydrogenation, CO2 hydrogenation, or parallel hydrogenation of CO and CO2, water−gas shift (WGS) reaction or reversed water−gas shift (RWGS) reaction, methanol dehydrogenation, oligomerization, isomerization, etc. The combination of these reactions results in a synergistic effect; one of the products of each step is a reactant for another, which creates a strong driving force for the overall reaction, allowing for very high syngas conversion in one single pass. In our previous study, β zeolite exhibits the best LPG selectivity among the investigated zeolites, such as ZSM-5, USY, SAPO-x, β zeolites, etc.4−12 Apart from the selection of the catalyst, the catalytic performance depends upon the various reaction factors, such as the temperature, pressure, space velocity, H2/COx ratio, CO/CO2 ratio, etc., for LPG synthesis from synthesis gas. The optimization of reaction variables has also been enormously investigated to find out the proper reaction conditions and the catalyst preparation method. However, to the best of our knowledge, there are no previous studies aimed at addressing the correlation between the reaction performances with the feedstock composition in terms of stability. In the present work, we provide, for the © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The hybrid catalyst was prepared by mechanically mixing the two catalysts: methanol synthesis catalyst CZZA (Cu/ZnO/ZrO2/Al2O3) and Pd-β catalyst. The methanol synthesis catalyst was prepared in accordance with the procedure set forth.13,14 The resulting catalyst has the composition of CZZA (Cu/ ZnO/ZrO2/Al2O3 = 4:3:1.5:1.5) by weight. Finally, the resultant powder was crushed into granules and sieved to 20−40 mesh for reaction. A Pd-modified β zeolite (NH4-β; Si/Al = 38) (Pd/β) was prepared by ion exchange with nitric acid solution of PdNO3. The loading amount of Pd was 0.1 wt %. The methanol synthesis catalyst and modified β zeolite (Pd-β) were independently pressure-molded, crushed, and sieved to the particles of 0.36−0.71 mm, respectively. The two kinds of catalyst were then physically mixed well to form the hybrid catalyst CZZA + Pd-β. 2.2. Catalyst Characterization. X-ray diffraction (XRD) patterns of the catalyst samples were collected at an ambient atmosphere on a RIGAKU X-ray diffractometer equipped with Cu Kα radiation. The specific surface area of the catalysts was determined by the Brunauer− Emmett−Teller (BET) method using a Micromeritics ASAP 2010. For the temperature-programmed reduction (TPR) carried out on the BELCAT-B instrument, the sample (50 mg) was previously treated in He flow up to 350 °C and kept for 2 h to remove adsorbed water and other contaminants, followed by cooling to 50 °C. The 10% H2/ He mixture was passed over the samples at a flow rate of 30 mL/min with a heating rate of 10 °C/min up to 400 °C. The effluent gas was passed over a molecular sieve trap to remove the generated water and then analyzed by gas chromatography (GC) equipped with a thermal conductivity detector (TCD). Received: December 3, 2013 Revised: January 16, 2014 Published: January 24, 2014 1331

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Table 1. Comparison of the Reaction Performancea CO2/COx ratio

0

reaction time (h) CO conversion (%) HC yield (%) CO2 yield (%) HC selectivity (%) HC distribution (%) C1 C2 C3 C4 C5 C6 C7+ LPG (%) i-C4/n-C4

0.125

0.25

0.5

1.0

50

600

50

600

50

600

82.8 45.5 37.2 55.0

80.2 45.7 34.5 57.0

69.9 38.8 31.0 55.5

73.9 44.6 29.3 60.4

67.7 40.3 27.1 59.5

72.9 49.3 23.6 67.6

62.5 39.1 23.1 62.6

33.9b 21.4 11.6c 63.1

0.1 0.5 5.8 85.2 6.4 1.2 0.7 91.0 46.7

0.1 0.6 6.9 83.9 6.8 1.2 0.4 90.8 39.3

0.2 0.5 5.7 80.8 8.6 2.6 1.6 86.4 36.9

0.2 0.9 8.2 81.6 7.1 1.5 0.6 89.8 34.8

0.2 0.7 6.8 79.5 8.7 2.4 1.7 86.3 31.8

0.2 1.2 9.6 79.9 7.4 1.5 0.3 89.5 27.5

0.3 1.1 8.0 75.2 10.3 3.1 2.0 83.2 23.5

1.3 3 12.8 64.8 12.7 3.9 1.5 77.6 16.0

CO2/COx is the percent of CO2 in total COx (COx = CO + CO2). Reaction conditions: T, 260 °C; P, 2.0 MPa; and W/F, 20 g of catalyst h mol−1. All data are based on a carbon atom. bCO2 conversion. cCO yield.

a

The temperature-programmed desorption of ammonia (NH3-TPD) measurement was performed in a similar reactor with TPR. The sample (50 mg) was previously degassed at 400 °C in a helium flow, cooled to 50 °C, and then saturated with NH3 for 30 min. After saturation, the sample was purged with He for 30 min to remove weakly adsorbed NH3 on the surface of the catalyst. The temperature of sample was then raised at a heating rate of 10 °C/min from 100 to 600 °C, and the amount of ammonia in the effluent was measured via a TCD and recorded as a function of the temperature. 2.3. Catalytic Reaction. A pressurized flow-type reaction apparatus with a fixed-bed reactor was used for this study. The apparatus was equipped with an electronic temperature controller for a furnace, a tubular reactor with an inner diameter of 8 mm, thermal mass flow controllers for gas flows, and a back-pressure regulator. A thermocouple was set at the axial center of the tubular reactor. A total of 1 g of hybrid catalyst was placed in the reactor with inert quartz sands above and under the catalyst. The catalyst was reduced in a flow of 5% H2 in nitrogen at 250 °C for 4 h before the reaction. The steadystate activity measurements were taken after 50 h on the stream. All of the products from the reactor were introduced in a gaseous state and analyzed by online GC. All data are calculated on a carbon basis.

suppresses the CO conversion. Contrary to the CO conversion, selectivity of hydrocarbons increased with an increase in the CO2 concentration in the feed. For example, at CO2/COx = 0.5, CO2/COx = 0.25, and CO2/COx = 0.125, selectivity of hydrocarbons are 67.6, 60.4, and 57.0%, respectively. An additional finding of significance is that selectivity changes with the synthesis gas composition. When the CO2/ COx ratio decreased, C4 paraffin increased and others decreased. LPG selectivity [(C3 + C4)/hydrocarbons] slightly increased from 89.5 to 89.8 to 90.8. Furthermore, it is observed that the selectivity of dry gas (CH4 and C2H6) only occupied about only 1% in the hydrocarbons, which is quite favorable from the industrial standpoint as well as its reaction mechanism. Furthermore, the conspicuous feature is the high C4 concentration in the hydrocarbon product, which is near or more than 80.0% for the investigated syngas. As shown in the right panel of Figure 1, iso-C4H10 was the main product in the

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance of the Hybrid Catalyst for the Reaction of Syngas to LPG. The catalytic performances of the catalysts over different kinds of syngas are summarized in Table 1, in which the selectivity of hydrocarbons is also presented. The activity and product selectivity of the hybrid catalysts were markedly affected by the investigated syngas. For comparison, the CO hydrogenation and CO2 hydrogenation to LPG are also carried out here. It can be seen from Table 1 that the catalyst exhibits higher catalytic activity with a decreasing CO2/COx ratio in the feed. Under the conditions listed in Table 1, the catalyst showed a maximum CO conversion at 82.8% for CO hydrogenation and a relative low CO 2 conversion at 33.9% for CO2 hydrogenation. For the syngas, the catalyst showed the highest CO conversion of 80.2% for the CO2/COx = 0.125 syngas, being higher than that of CO2/COx = 0.25 syngas by about 7.8% and that of CO2/COx = 0.5 syngas by about 9.1%. A relatively high CO2 selectivity reveals that the WGS reaction takes place very fast over the tested syngas because the detected SCO2 is more than 30%. However, with an increase in the CO2 concentration in the feed, reversed WGS

Figure 1. (Left) Equilibrium conversion of CO to different products as a function of the temperature. Synthesis gas: H2/CO/CO2 = 8:3:1 (molar ratio). Reaction conditions: P, 2.0 MPa; T, 260 °C. (Right) Evolution of the iso-C4H10/n-C4H10 ratio as a function of the CO2/ COx ratio.

C 4 composition and the iso-C4 H10/n-C4 H10 ratio was extraordinary high. The value increased with a decreasing CO2/COx ratio from 16 (CO2/COx = 1) to 47 (CO2/COx = 0), whose data are much higher than that of equilibrium data (dotted line). The isobutane/n-butane ratio in C5 paraffin was lower than that of C4, but it is also much higher than that of the equilibrium one. The formation of isobutane is favorable for 1332

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initial activity; the CO conversion of the catalyst on CO2/COx = 0.125 syngas decreased by 12.8%; and the CO conversion of the catalyst on CO2/COx = 0.5 syngas decreased by 14.3%. The stability of the catalyst operated in the syngas with CO2/COx = 0.25 was much better than that of the syngas of other compositions. Consequently, these findings suggest that some level of CO2 content in syngas lead to an obvious decrease in either activity or selectivity of the catalyst. The reason will be discussed later. From the change of the product distribution shown in Figure 3, it could be learned that selectivity to C1−C4 decreases, while those of C5 and C6+ increased with time on stream. During 600 h of reaction, the hybrid catalyst exhibits better catalytic stability over the CO2/COx = 0.25 syngas compared to other two kinds of syngas. Table 1 compared the catalytic activity and the product distribution at 50 and 600 h. These results are consistent with the aforementioned analysis. The results indicate that an optimum CO2/COx in the feedstock is observed. When the CO2 content is too low, the reduced and inactive form of the active site dominated, whereas the competitive adsorption of CO2 blocked the active site for the high CO2 content. It is also noteworthy that the formed water has an obvious negative effect on the methanol synthesis catalyst for the high CO2 content in the feed. On the contrary, the low CO2 content in the feed is not preferable for the zeolite. As a result, the stability of the catalyst operated in the syngas with CO2/COx = 0.25 was superior to other syngas. In addition, because time on stream is longer, a slight amount of DME was detected and increased with the increase of the CO2/ COx ratio. With the deactivation occurring, the iso-C4H10/nC4H10 ratio simultaneously decreased. The difference in catalytic activity and hydrocarbon distribution is supposedly due to the different influences on the methanol synthesis catalyst and zeolite by the feed composition. These results also illustrate that the effect of feed composition is important for optimizing the activity, selectivity, and stability. This subject will be discussed. 3.3. Effect of Operation Parameters. Figures 4−6 show effects of reaction parameters, such as reaction temperature, H2/COx molar ratio, and contact time, respectively. As shown in Figure 4, as the reaction temperature increased, the product distribution was strongly affected. The selectivity of C4, C5, and C6+ hydrocarbons decreased, while C1, C2, and especially C3 hydrocarbons increased. At the same time, the isobutane/nbutane ratio decreased from 33 at 260 °C to 16 at 300 °C. The H2/COx ratio also affected the reaction, as shown in Figure 5; the selectivity of C4 paraffin was almost constant (∼80%), irrespective of the change in the H2/COx ratio from 2.0 to 4.0. However, the isobutane/n-butane ratio slightly decreases from 33 to 30. As shown in Figure 6, it should be noted that the C4 selectivity and the isobutane/n-butane ratio increase with an increasing contact time (W/F). It is strange that the isobutane/ n-butane ratio, which is much higher than that of the equilibrium level, still increases with an increasing W/F. It can be pointed out that the favorable conditions for the production of isobutane are as follows: (a) lower reaction temperature, (b) low CO2 content in the feed gas, and (c) longer contact time. These characteristic features are discussed later. 3.4. Catalyst Changes during Reaction Operation. 3.4.1. Specific Surface Area. The change of the BET surface area of the hybrid catalyst CZZA + Pd-β was studied. As shown in Table 2, the surface area of the fresh CZZA and Pd-β was

improving the syngas conversion. As seen in the left panel of Figure 1 , when butane is synthesized from synthesis gas with the ratio H2/CO/CO2 of 8:3:1, the equilibrium conversion of CO is near 100% at lower temperatures than 350 °C, while the formation of methanol or dimethyl ether (DME) is limited. 3.2. Stability Test. The long-term stability and activity of the catalyst is vital for industrial application. Considering the obvious difference of syngas composition on the reaction performance, the fundamental difference about the effect of composition on stability and the reactivity of the catalyst were compared under otherwise identical reaction conditions. In all cases, the reaction was carried out at 2 MPa and 260 °C for 600 h. The hydrogen concentration relative to carbon oxides (H2/ COx) was fixed at 2. It took more than 50 h to reach a steady state for the LPG synthesis. Hence, the steady-state rate data reported in this paper were those obtained after 50 h on stream for different compositions of the CO2/COx ratio in the feed. Because the process of syngas-to-LPG was carried out over a hybrid catalyst provided with the two functions, consisting of synthesis of methanol and hydroconversion of methanol to hydrocarbons, the activity of the hybrid catalyst depends upon the synergistic effect between the two kinds of catalysts, which, in turn, depends upon the composition of the gas mixture. Catalytic activity and product selectivity as a function of the reaction time for the hybrid catalyst operated on various syngas are displayed in Figure 2. In comparison to CO and H2 only,

Figure 2. Catalytic stability of the hybrid catalysts with different CO2/ COx syngas. Reaction conditions: T, 260 °C; P, 2 MPa; H2/C, 2 (molar ratio); and time on stream, 600 h.

the deactivation of CO conversion was more rapid and decreased by 7.4% of its initial activity only after 70 h of stability test. These data were interpreted assuming that the active-site catalyst undergoes a redox reaction with gas-phase CO and CO2. The catalyst was deactivated by over-reduction at lower CO2 concentrations. This observation further emphasized the importance of CO2 in the conversion of syngas.15−17 For syngas containing CO2, the trend was clear: CO conversion dropped with time on stream; however, the decrease rate was related to the composition of syngas. The CO conversion of the catalyst on CO2/COx = 0.25 syngas decreased by 6.2% of its 1333

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Figure 3. Evolution with time on stream of the selectivity of hydrocarbon. Reaction conditions are the same as listed in Figure 2

Figure 4. Effect of the reaction temperature on hydrocarbon selectivity and isobutane/n-butane ratio over the CZZA + Pd-β catalyst. Synthesis gas: H2/CO/CO2 = 8:3:1 (molar ratio). Reaction conditions: P, 2.0 MPa; W/F, 20 g of catalyst h mol−1.

Figure 5. Effect of H2/C on hydrocarbon selectivity and isobutane/nbutane ratio over the CZZA + Pd-β catalyst. Synthesis gas: CO/CO2 = 3:1 (molar ratio). Reaction conditions: T, 260 °C; P, 2.0 MPa; and W/ F = 20 g of catalyst h mol−1.

106.3 and 419 m2/g. After the stability test, the surface area of CZZA was marginally reduced to 65.2 m2/g for CO2/COx = 0.5 syngas, whereas for CO2/COx = 0.25 and 0.125, the area decreased to 84.4 and 84.8 m2/g, respectively. It is interesting

that the change of the BET surface area of Pd-β was contrary to that of CZZA. The surface area of Pd-β decreased to 325 m2/g (CO2/COx = 0.5), 289 m2/g (CO2/COx = 0.25), and 273 m2/g (CO2/COx = 0.125) after the reaction. These results suggested 1334

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Figure 7. XRD curves of the fresh CZZA and Pd-β, the hybrid catalyst used in the LPG synthesis with different syngas: (a) fresh CZZA, (b) reduced CZZA, (c) used CZZA + Pd-β (CO2/COx = 0.125), (d) used CZZA + Pd-β (CO2/COx = 0.25), and (e) used CZZA + Pd-β (CO2/ COx = 0.5).

Scherrer equation, the particle size of CuO could be estimated to be 4.1, 4.4, 4.5, and 5.6 nm for fresh, CO2/COx = 0.25 syngas, CO2/COx = 0.125 syngas, and CO2/COx = 0.5 syngas, respectively. As shown in Figure 8, the water concentration was also calculated in the reaction time of 200 h. Although the calculated

Figure 6. Effect of W/F on hydrocarbon selectivity and isobutane/nbutane ratio over the CZZA + Pd-β catalyst. Synthesis gas: H2/CO/ CO2 = 8:3:1 (molar ratio). Reaction conditions: T, 260 °C; P, 2.0 MPa.

Table 2. Comparison of Textural Properties between Fresh and Used Catalysts BET surface area (m2/g) catalyst

CZZA

Pd-β

fresh CO2/COx = 0.125 (used) CO2/COx = 0.25 (used) CO2/COx = 0.5 (used)

106.3 84.8 84.4 65.2

419 273 289 325

Figure 8. Comparison of the water content in the product distribution in the reaction time of 200 h. Reaction conditions are the same as listed in Figure 2.

that the CO/CO2 ratio in the feed has an obvious different effect on every composition of the hybrid catalyst. A low CO2/ COx ratio was found to greatly influence the zeolite, and a high CO2/COx ratio showed significant negative effects on the methanol synthesis catalyst. Further, the impact of the CO2/ COx ratio on the used zeolite was also assessed by the thermogravimetric analysis (TGA) method. The total weight loss was about 5.0, 4.6, and 3.4% for CO2/COx = 0.125 syngas, CO2/COx = 0.25 syngas, and CO2/COx = 0.5 syngas, respectively (not shown). This means that the coke easily formed at a low CO2/COx ratio. This efficient limitation of deactivation as a consequence of the larger amount of formed water is explained by water competence with coke precursors on the adsorption on acid sites of the zeolite for the high CO2/ COx syngas.18 The results are in good agreement with that of the BET measurement. As a result, the catalyst exhibited the optimum catalytic stability for the CO2/COx = 0.25 syngas compared to the other two kinds of syngas. 3.4.2. Crystal Size, Reducibility, and Acid Strength. To obtain more information about the effect of different syngas on the reaction performance of the hybrid catalyst, post-analysis of the tested catalysts by XRD, TPR, and TPD was carried out. As shown in Figure 7, XRD post-analysis of CZZA + Pd-β displayed that there were some similarities among the catalysts used over three kinds of tested syngas in their XRD features regarding the CuO component. According to the well-known

water concentration was probably subjected to significant errors, Figure 8 clearly shows that the concentration of water increases with an increasing CO2/COx ratio in the feedstock. This suggests that the presence of water must be taken into consideration as a possible detrimental factor during the reaction process. The formed water not only adsorbed on the active site of the catalyst and inhibited the next catalytic reaction but also led to larger crystallite sizes.19 As the crystallite size increased, the methanol synthesis ability of CZZA decreased, which resulted in the significant decrease of CO2/COx = 0.5 syngas. These results reveal the good catalytic stability of the dual catalyst, which is indeed due to the existence of a chemical synergetic effect between two functions of it. H2-TPR test of the catalyst in the oxidation state provided useful information about the reducibility. Figure 9 shows the TPR curves of the catalyst in reactions with different CO2/COx molar ratios in the feed. It is noteworthy that the position and amplitude depend upon the investigated syngas; as the CO2/ COx ratio increased, the TPR curve shifts slightly toward lower temperatures. This is somewhat surprising because a higher CuO crystallite size should, in principle, increase its reduction temperature.20,21 This facilitated reduction of the CuO phase 1335

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largely improved the catalytic activity and selectivity of the hybrid catalysts. Second, the synergistic effect directly related to the selected feed composition. The obtained results indicate that the CO2/COx ratio in the feed has a significant effect on the catalytic activity, stability, selectivity, and physical and chemical states of different catalysts. It is clear from the above obtained results that a suitable CO2/COx ratio syngas can harmoniously adjust the two catalytic functionalities to effectively couple the methanol synthesis and LPG synthesis reactions.

4. DISCUSSION ON SELECTVE SYNTHESIS OF ISOBUTANE-RICH PARAFFIN The overall reaction path of the hydrocarbon synthesis can be shown in Figure 11a. As previously reported, the methanol

Figure 9. TPR curves of the fresh and used CZZA in the LPG synthesis with different syngas.

operated in CO2/COx = 0.5 syngas (concomitant more water is formed) is ambiguously attributed to a “synergetic effect” arising from the interaction between the metallic and acidic functions, which was influenced to some degree by the composition of syngas. It also indicated that the catalyst was easily oxidized with a relatively high CO2/COx molar ratio in the feed. It is conceivable that a high concentration of CO2 would promote the growth of CuO crystallite, the quantity of which on the surface increased, resulting in the reduction of the temperature shifting to low temperatures.22 The above order of reducibility was almost consistent with the order of the CuO size obtained by XRD. The findings also illustrated that a high CO2/COx ratio showed significant negative effects on the methanol synthesis catalyst. The NH3-TPD profile of the catalyst provided useful information about the intensity and concentration of surface acid sites of the catalyst. Figure 10 showed the NH3-TPD

Figure 11. Reaction path of CO2 hydrogenation to hydrocarbons over hybrid catalysts.

formation from CO or CO2 on the CZZA catalyst was assumed to pass through two routes: one is the direct hydrogenation of CO2 to methanol, and the other is the route that passes through the CO formation, while the conventional methanol synthesis catalyst (CZA) passes through the CO formation.13 Then, methanol is converted to DME, which is followed by the direct conversion to C1−C7 paraffin on the acidic sites of the Pd-β zeolite according to the mechanism of chain growth with the strong promotion effect of the spillover of hydrogen.23 All of these reactions are accompanied by the formation of H2O, especially the reaction of CO2. As already pointed out, the water molecule is strong poison for the methanol synthesis catalyst as well as acitivity.13 It means that the CO2-rich syngas shows lower reactivity and also the conversion of syngas is strongly suppressed by the product (H2O). The reaction on zeolite is also affected by the water molecule, which has been strongly adsorbed on the acid site to suppress the adsorption of the proton (H+) or carbenium ion (R+). The spillover of hydrogen makes it possible to convert methanol into paraffinic hydrocarbons at the temperature as low as 260 °C. The essential factors for the selective synthesis of LPG are as follows: (1) excellent activity of methanol formation and resistance of CZZA to steam, (2) easy transfer of methanol from CZZA to Pd-β zeolite, (3) large pores in β zeolite (∼0.7 nm) enabling the quick diffusion of methanol and product hydrocarbon, which suppresses the secondary reactions within the interior of the pore system, and (4) introduction of H species (H+ and or H−) to Pd-β zeolite, by the hydrogen spillover.24

Figure 10. TPD curves of the fresh and used Pd-β in the LPG synthesis with different syngas.

profile of the used catalyst and the reference Pd-β. There were mainly two desorption peaks of NH3 for the fresh and used Pdβ catalysts. It is noted that the first peak shifts to low temperatures compared to the fresh Pd-β, which results in the decrease of the activity of the catalyst. Generally, the observed differences of TPD profiles were not obvious, which also indicates the modified β zeolite fitting for industry application if regeneration is possible and will be worth studying in future works. From these results, the effect of the CO2/COx ratio in the feed on LPG synthesis over the hybrid catalyst is complicated. First, the process was carried out by the hybrid catalyst composed of a methanol catalyst and zeolite, the interaction of which was important for improving the catalytic activity. A synergistic effect between the two different kinds of catalysts 1336

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temperature, and long contact time are essential for the high selectivity of isobutane.

The high selectivity of C3 and C4 paraffin, especially isobutane, is interpreted as follows: the active species of the methanol conversion to hydrocarbons are accepted as carben (CH2) and carbenium (CnH2n+1+) ions. The thermodynamical stability of the carbenium ion is the ascending order: CH3+ ≪ C2H5+ < sec-C3H7+ < sec-C4H9+ ≪ tert-C4H9+. These carbenium ions come out of the zeolite through two possible routes (as shown in Figure 11b). One route is the deprotonation for making olefin. +

C4 H 9 → C4 H8 + H

+

■

Corresponding Author

*Telephone: +81-695-3202. Fax: +81-695-3309. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

(1)

ACKNOWLEDGMENTS This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).

The other route is the hydrogenation to make parraffin. C4 H 9+ + H− → C4 H10

AUTHOR INFORMATION

(2)

It is clear that reaction 1 is much more endothermic than reaction 2. The olefin formation from methanol [methanol to gasoline (MTG) reaction, which includes reaction 2] required high temperatures (>350 °C). On the other hand, reaction 2 is a largely exothermic reaction and, thus, proceeds at lower temperatures. The difference in the thermodynamics makes the large temperature difference for the hydrocarbon formation with Pd (for hydrogen activation) containing zeolite and Pdfree zeolite catalyst.25,26 Thus, the reactivity of three active species with carben is in the reverse order. Only a small amount of active species of C1 (CH3+) may come out of the zeolite as methane because of its high reactivity to higher carbon species.27 Active species of C3 or higher may have a chance to come out of the zeolite surface as hydrocarbons. In the case of C4 species, two types of carbenium ions possibly exist, that is, secondary butyl carbenium ion and tertiary butyl carbenium ion. The latter species is by about 14 kcal/mol more stable than the former species and, thus, may be predominant in its concentration over the former species. Its predominance is demonstrated by the fact that the amount of isobutane in the product is by about 10 times higher than that of n-butane. The tertiary butyl carbenium ion would be very low in its reactivity for chain growth, and thus, most of them come out of the catalyst surface as branched C4 hydrocarbons without growing to C5+ species. The isobutane/n-butane ratio is quite sensitive against reaction conditions, such as the CO2/CO ratio, reaction temperature, contact time, etc. The phenomena are left for further research.

REFERENCES

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5. CONCLUSION In this paper, we studied, for the first time, the effect of feed composition on catalytic activity and stability of the CZZA + Pd-β hybrid catalyst. Under our experimental conditions, the main results can be summarized as follows: (1) With a decreasing CO2/COx ratio in the studied range, the conversion of CO increased up to 80% but the selectivity of hydrocarbons decreased and LPG (C3 + C4 paraffins) selectivity slightly increased. In hydrocarbon distribution, the C4 paraffin occupied near or more than 80.0% in the hydrocarbon product. The isoC4H10 is the main product in the C4 paraffin, and the isoC4H10/n-C4H10 ratio is more than 25 and decreased when the CO2/COx ratio increased. (2) It is worth noting the distinctive difference of stability tested over the three kinds of syngas containing different ratios of CO/CO2; CO2/COx = 0.25 syngas showed excellent stability over the 600 h LPG synthesis reaction. (3) A low CO2 concentration in the feed, low reaction 1337

dx.doi.org/10.1021/ef402393j | Energy Fuels 2014, 28, 1331−1337