Direct Conversion of Syngas-to-Hydrocarbons over Higher Alcohols

Aug 18, 2014 - Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States...
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Direct Conversion of Syngas-to-Hydrocarbons over Higher Alcohols Synthesis Catalysts Mixed with HZSM‑5 Vanessa M. Lebarbier Dagle,*,† Robert A. Dagle,*,† Jinjing Li,‡ Chinmay Deshmane,† Charles E. Taylor,§ Xinhe Bao,‡ and Yong Wang*,†,∥ †

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China § National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States ∥ Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States ‡

ABSTRACT: Direct syngas conversion to hydrocarbons was investigated with HZSM-5 physically mixed with either a methanol synthesis catalyst (5Pd/ZnO/Al2O3) or a higher alcohols synthesis (HAS) catalyst. Reactivity measurements show a definitive advantage in using HAS catalysts. Undesired durene formation is negligible with HAS catalysts but it represents 50% of the C5+ fraction for 5Pd/ZnO/Al2O3. Furthermore, the desired C5+ hydrocarbons yield is twice higher with selected HAS catalysts. The 0.5Pd/FeCoCu (HAS) catalyst was found the most promising due to higher C5+ fraction and lower durene formation. When 0.5Pd/FeCoCu and HZSM-5 are operated sequentially (two-step process), the CO conversion and the C5+ hydrocarbons fraction are lower. The C5+ hydrocarbons yield is thus twice higher for the one-step process. The main advantage of the one-step process is that higher syngas conversion is achieved as the equilibrium-driven conversion limitations for methanol and dimethyl ether are removed since they are intermediates to the final hydrocarbons product. they are intermediates to the final hydrocarbon product leading to high syngas conversion. However, the maximum yield of C5+ liquid hydrocarbon product was only about 10% for a conversion equal to 44%. Based on these results a technoeconomic modeling of the S2GD process was conducted.2 It indicates that its simplification as compared to conventional MTG processes can lead to reduction in both capital and fuel production cost, assuming certain levels of catalyst productivity and selectivity. Recent work related to the conversion of alcohols over HZSM-5 suggests that the liquid hydrocarbons productivity (i.e., C5+ yield) can be improved with higher alcohol (i.e., ethanol, propanol, and butanol) as compared to methanol.3−5 For example, Gujar et al. observed in a batch reactor that the gasoline yield increases by a factor of 5 as the length of alcohol carbon chain increases from 1 to 4.4 Hence, there might be some advantage in applying the S2GD process to higher alcohols synthesis catalysts. Several catalysts recently reported for the conversion of syngas to higher alcohols have attracted our attention. For CO hydrogenation over a CoZrLa/activated carbon catalyst, exceptionally high C2+ alcohol fraction of 96% was obtained at reasonable conversion (59%) and alcohol selectivity (42%).6 For 0.5 wt % Pd/Fe−Co−Cu catalyst, although the amount of CO2 product was not reported, high syngas conversion (81.7%) and high oxygenates (i.e., alcohols + DME) selectivity of 79.8%

1. INTRODUCTION Syngas (i.e., H2 + CO), derived from natural gas, biomass, or coal, can be used to synthesize a variety of fuels and chemicals. Domestic transportation and military operational interests have driven continued focus on domestic syngas-based fuel production. Liquid transportation fuels may be made from syngas via methanol-to-gasoline (MTG) technology. The MTG process converts methanol to a mixture of hydrocarbons in the C2 to C10 range, including paraffins, aromatics, and olefins. Essentially only gasoline-type fuels are made in the conventional MTG process. The typical catalyst is a form of ZSM-5, a pentasil zeolite. Despite the successful demonstration of the feasibility of the MTG process, it has not been widely implemented. Because of the high capital cost of synthetic fuel plants, the production cost of the finished fuel cannot currently compete with petroleum-derived fuel. We have recently evaluated another way to potentially reduce capital cost and overall production cost for MTG by combining the methanol, dimethyl ether (DME), and MTG syntheses in a single reactor (S2GD process). Details of the S2GD process schematic can be found in our previous report.1 In this report it was shown that high syngas conversion (up to 86% at 340 °C, gas hourly space velocity (GHSV) = 740 h−1, and 20 bar) can be achieved using a Pd/ZnO/Al2O3 methanol synthesis catalyst (metal sites) physically mixed with HZSM-5 (acid sites). The metal sites of the methanol synthesis catalyst enable the conversion of syngas to alcohols, whereas HZSM-5 provides acid sites required for methanol dehydration, and DME-tohydrocarbons reactions. When metal and acid sites are in close proximity, such as for the S2GD process, the equilibrium-driven conversion limitations for methanol and DME are removed as © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13928

June 16, 2014 August 11, 2014 August 18, 2014 August 18, 2014 dx.doi.org/10.1021/ie502425d | Ind. Eng. Chem. Res. 2014, 53, 13928−13934

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Table 1. One-Step Conversion of Syngas to Fuels: CO Conversion, Products Selectivity and Hydrocarbons Product Distribution of the Alcohols Synthesis Catalysts Mixed with HZSM-5 (T = 300 °C; P = 70 bar; GHSV = 700 h−1; Zeolite:Alcohols Synthesis Catalyst Ratio = 3:1 by Weight) hydrocarbons product distribution (%)

selectivity (%)

a

catalysts mixture

CO conversiona (%)

hydrocarbons

CO2

oxygenatesb

C1

C2−C4

C5+

C5+ yield (%)

durene content in C5+ fraction (%)

5Pd/ZnO/Al2O3 + HZSM-5 15Co1Zr0.5La/AC + HZSM-5 5Rh2.5Mn/SiO2 + HZSM-5 5Rh2.5Mn/MWCNT + HZSM-5 Co3Cu1-11%MWCNT + HZSM-5 0.5Pd/FeCoCu + HZSM-5 FeCoCu + HZSM-5

45.0 28.0 44.0 40.6 45.0 50.0 40.6

54.7 86.4 56 78.4 76.7 61.8 63.6

45.3 11.7 2.5 10.8 23.0 38.0 35.9

1.9 41.5 10.8 0.3 0.2 0.5

9.3 56.2 74.6 54.1 35.0 29.7 48.4

49.4 17.6 23.6 25.2 32.0 31.5 37.1

41.3 26.2 1.8 20.7 33.0 38.8 14.5

10.2 6.3 0.4 7.7 11.4 12.0 3.7

48 3.2 2.3 2.7 1.0

Data recorded after 50 h on stream. bOxygenates: acids, ketones, aldehydes, alcohols, and esters.

were observed.7 High oxygenates selectivity up to 74% has also been reported for a Co3Cu1-11% CNT catalyst although the syngas conversion was relatively low and equal to 38.5% (for H2/CO = 1).8 In addition, extensive research conducted at PNNL has demonstrated that Rh-based-type catalysts such as RhMn/SiO2 are very selective toward C2+ oxygenates as compared to Cu-based methanol synthesis catalyst.9,10 In the present study, we report on the direct conversion of syngas to hydrocarbons over higher alcohols synthesis catalyst physically mixed with HZSM-5. First, we compared the performance of HZSM-5 mixed with selected HAS catalysts and HZSM-5 mixed with a Pd/ZnO/Al2O3 methanol synthesis catalyst. Then, we investigated the effect of operating temperature, pressure, gas hourly space velocity, and HZSM5:HAS catalyst weight ratio on the catalytic activity for HZSM5 mixed with 0.5Pd/FeCoCu catalyst. Finally, the efficiency of this one-step process was compared to the one for a more conventional two-step process operated under similar reaction conditions.

product distribution. The 5Pd/ZnO/Al2O3 catalyst containing 5 wt % Pd (precious metal) and a Pd:Zn molar ratio of 0.25:1 was synthesized by incipient wetness impregnation of Al2O3 (Engelhard, AL-3945E) according to ref 12. All of the catalysts were compressed into tablets and then crushed and sieved to produce 60−100 mesh particles. In the next sections the term “alcohols synthesis catalyst” will refer to either the methanol synthesis catalyst or to any of the higher alcohols synthesis catalysts. For the conversion of alcohols into hydrocarbon fuels, a commercial HZSM-5 zeolite (Si/Al = 40) was purchased from Zeolyst International. The zeolite material was pretreated ex situ under air flow at 400 °C for 3 h and then compressed into tablets, crushed, and sieved to produce 60−100 mesh particles. 2.2. Reactivity Measurements. Catalyst performance was evaluated using a packed bed, down-flow reactor system equipped with Brooks mass flow controllers, TESCOM backpressure regulators, and a BIOS DryCal for measurement of the outlet flow. A dual thermocouple was placed in the catalyst bed, and the temperatures were continuously monitored via a dedicated computer process control interface developed in LabVIEW 8.0. For the one-step conversion of syngas to fuels, the reactor was typically loaded with a physical mixture of 0.1 g of alcohols synthesis catalyst and 0.3 g of commercial zeolite resulting in a zeolite to alcohols synthesis catalyst ratio of 3:1. For a few experiments, the zeolite to alcohols synthesis catalyst ratio was modified as specified in the text. The efficiency of the one-step process was compared to a more conventional two-step approach. To mimic a two-step process, the reactor was loaded with 0.1 g of alcohols synthesis catalyst on top of 0.3 g of zeolite. The alcohols synthesis catalyst and the zeolite material were separated by ∼2.5 cm of quartz wool, and the temperature in the middle of each bed was recorded using a dual thermocouple. Before each experiment the catalyst bed was activated in situ under 10% H2/N2 flow for 12 h at 370 °C and atmospheric pressure. After that, the reactor was cooled to 300 °C and the system was pressurized. As soon as the system reached the desired pressure, the reaction started by introduction of the gas mixture containing 48% H2, 48% CO, and 4% N2 and the 10% H2/N2 reducing gas mixture being turned off. An atypical H2/ CO ratio = 1 was chosen to study the catalysts under severe conditions. Product gas was analyzed online using a 3000 micro-GC gas analyzer, equipped with thermal conductivity detectors (TCD)

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. A variety of precious metals and non-precious metals catalysts reported active for the synthesis of higher alcohols from syngas were synthesized according to protocols described in the literature. The 0.5Pd/ FeCoCu catalyst with 0.5 wt % Pd was synthesized by impregnation of Pd precursor to FeCoCu support for which details of the preparation method are given in ref 7. Two other precious metals catalysts, namely, 5Rh2.5Mn/SiO2 and 5Rh2.5Mn/MWCNT with 5 wt % Rh and 2.5 wt % Mn were synthesized by co-impregnation of Rh and Mn precursors on SiO2 (Davison) support and multiwall carbon nanotubes (MWCN, US Research Nanomaterials, Inc.) support, and the synthesis method can be found in a previous report from Hu et al.9 The 15Co1Zr0.5La/AC catalyst with 15 wt % Co, 1 wt % Zr, and 0.5 wt % La was prepared by co-impregnation of an activated carbon (from coconut shell, PICA) according to ref 11. The second cobalt catalyst labeled Co3Cu1-11%MWCNT (66.5.wt % Co, 23.5 wt % Cu, and 10 wt % MWCNT) was prepared by constant pH co-precipitation method according to the method described in ref 8. A 5Pd/ZnO/Al2O3 catalyst active for the synthesis of methanol from syngas was also synthesized in order to investigate the effect of the nature of the alcohols intermediate products on the catalytic performance and the hydrocarbons 13929

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hydrocarbons was observed (i.e., 86.5%). However, undesired methane is the main hydrocarbon product, and the yield toward the preferred C5+ hydrocarbons is quite low and equal to 6.3%. Among the different HAS catalysts evaluated, the highest C5+ yield and lowest methane formation was observed for the HZSM-5 + 0.5Pd/FeCoCu mixture. In addition, as shown in Figure 1 the conversion is fairly stable with time on stream for

and four columns (Mol Sieve 5A, Plot U, Alumina and OV-1). The liquid fraction of the product stream was accumulated in a refrigerated trap installed between the reactor and backpressure regulator and was collected every 24 h. The liquid fraction was analyzed off-line using a gas chromatograph (Agilent7890A) equipped with a DB-5 column and an flame ionization detector (FID). Conversion of CO was calculated as the ratio of the difference of the mass flow of CO in the feed and CO in the product, to the mass flow of CO in the feed. Selectivity of all carbon-bearing products was determined from the carbon appearing in any species divided by CO converted, on a carbon mole basis. Furthermore, for simplicity, selectivity to the many products is grouped in terms of three general product classes: hydrocarbons, oxygenates, and CO2. Further breakdown of the hydrocarbon components is made according to carbon number.

3. RESULTS AND DISCUSSION 3.1. One-Step Conversion of Syngas to Fuels. Catalytic Performance of the Methanol Synthesis Catalyst. In a previous study we have described a Pd/ZnO/Al2O3 catalyst that efficiently produces methanol and DME at temperatures up to 400 °C with excellent stability relative to a commercial Cu-based methanol catalyst.12 We have thus chosen to determine the performance of the 5Pd/ZnO/Al2O3 methanol synthesis catalyst physically mixed with HZSM-5 for the onestep conversion of syngas to hydrocarbons at 300 °C, 70 bar, and a GHSV = 700 h−1. The results presented in Table 1 show that the CO conversion is equal to 45% and the hydrocarbons selectivity is equal to 54.7%. The hydrocarbons selectivity is lower than that of the higher alcohols synthesis catalysts (see next section) due to the high reactivity of the catalyst for the water gas shift reaction.13 Indeed, the CO2 selectivity is high and equal to 45.3%. Note that the C5+ hydrocarbons represent 41.3% of the hydrocarbons product. However, half of the C5+ fraction consists of unwanted durene in gasoline products. Durene is undesired because of its high melting point (79 °C) and its tendency to crystallize out of solution at temperatures below 79 °C.14 Catalytic Performance of the Higher Alcohols Synthesis Catalysts. Promising HAS catalysts identified in the open literature were prepared. The performance of the HAS catalysts physically mixed with HZSM-5 was evaluated under the same conditions as for the methanol synthesis catalyst, and the results are presented in Table 1, as well. For all of the catalysts mixtures, the CO conversion is similar (between 40 and 50%) except for 15Co1Zr0.5La/AC. A lower conversion equal to 28% was observed for the 15Co1Zr0.5La/AC catalyst. Among the HAS catalysts selected for evaluation, the 5Rh2.5Mn/SiO2 and 5Rh2.5Mn/MWCNT catalysts distinguish themselves from the others. The selectivity toward the oxygenated compounds with the supported Rh catalysts is higher and equal to 41.4 and 10.0%, respectively. The Rh catalysts are known to be very active for the conversion of syngas into oxygenates (i.e., alcohols, acids, aldehydes, ketones, and esters).9,10 However, a large fraction of theses oxygenates consists of species such as acetic acid and acetaldehyde that are not easily converted into hydrocarbons over HZSM-5 under the present reaction conditions. Note that the quantity of acids and aldehydes produced with the Rh catalyst supported on SiO2 is higher as compared to the Rh catalyst supported on MWCNT which can explain the difference in oxygenates selectivity. For the HZSM5 + 15Co1Zr0.5La/AC mixture, a high selectivity toward

Figure 1. CO conversion as a function of time on stream for the HZSM-5 + 0.5Pd/FeCuCo mixture: H2/CO = 1, T = 300 °C, P = 70 bar, GHSV = 3,000 h−1, and HZSM-5:0.5Pd/FeCuCo ratio = 3:1 by weight.

the HZSM-5 + 0.5Pd/FeCoCu mixture. The yield toward the C5+ hydrocarbons is quite comparable for HZSM-5 + 5Pd/ ZnO/Al2O3 (yield = 10.2%) and for HZSM-5 + 0.5Pd/ FeCoCu (yield = 12%). However, as explained earlier, for the methanol synthesis catalyst undesirable durene represents half of the C5+ fraction. On the contrary, the production of durene is negligible with any HAS catalyst, and it is only ∼1% of the C5+ hydrocarbons product for the 0.5Pd/FeCoCu catalyst. A detailed analysis of the aromatics product also showed that the overall methyl substitution is reduced by ∼30% with the 0.5Pd/ FeCoCu catalyst as compared to the methanol synthesis catalyst. These results suggest that durene and other methylbenzene compounds are produced via benzene alkylation with methyl species from methanol. The production of desired C5+ gasoline-type hydrocarbons is thus higher with the 0.5Pd/FeCoCu catalyst, and it appears as the most promising candidate for the one-step conversion of syngas to fuels. These preliminary findings encouraged us to further investigate the HZSM-5 + 0.5Pd/FeCoCu combination and determine the effect of processing parameters such as temperature, pressure, gas hourly space velocity, and the HZSM-5:0.5Pd/FeCoCu weight ratio. Reactivity of the HZSM-5 + 0.5Pd/FeCoCu Catalysts Mixture: Effect of the Operating Conditions. The catalytic performance of the HZSM-5 + 0.5Pd/FeCoCu mixture was investigated at temperatures between 300 and 370 °C, 70 bar, HZSM-5:0.5Pd/FeCoCu catalysts ratio = 3:1, and GHSV = 3,000 h−1. As illustrated in Figure 2, the CO conversion increases from ∼36% to 87% when the temperature increases from 300 to 350 °C. When the temperature was increased from 350 to 370 °C, thermodynamic constraints for the methanol synthesis portion of the reaction scheme likely inhibited further CO conversion. Note that the higher alcohols formation from syngas is not an equilibrium-limited reaction but the 0.5Pd/ 13930

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to 15% when the temperature increases from 300 to 370 °C. These results highlight the importance of operating at lower temperature (i.e., 300 °C) in order to obtain higher yield of C5+ hydrocarbons. The catalytic performance of the HZSM-5 + 0.5Pd/FeCoCu mixture was then determined for T = 300 °C, GHSV = 3000 h−1, HZSM-5:0.5Pd/FeCoCu catalysts ratio of 3:1, and operating pressures of 20, 45, and 70 bar. As shown in Figure 4, with increasing reaction pressure from 20 to 70 bar the conversion increases from 18 to 36%. Higher alcohols synthesis being favored at high pressure over Pd/FeCoCu catalysts,7 the one-step conversion of syngas to hydrocarbons is expected to increase with reaction pressure. A selectivity variation can also be observed with increasing pressure. Indeed, increasing pressure from 20 to 70 bar resulted in an increased CO2 selectivity from 19.5% to 31% and decreased hydrocarbons selectivity from 80.5 to ∼68%. Although the total hydrocarbon selectivity decreases with the increase of pressure, operating at higher pressure is preferred since the C5+ hydrocarbons fraction increases with the increase of pressure, as shown in the inset of Figure 4. A comparable trend for methanol to gasoline process studies has been observed by Chang et al.16 who showed that the C5+ selectivity increases from 66.2 to 79.7% when the pressure increases from 1 to 50 bar. Oxygenates are almost fully converted at any pressure between 20 and 70 bar since the oxygenated compounds selectivity is minor and below 1%, as shown in Table 2. This is in comparison with the MTG process for which methanol and DME are fully converted for pressure of 20−30 bar.17 Figure 5 presents the catalytic performance results obtained while operating at 300 °C, 70 bar, HZSM-5:0.5Pd/FeCoCu = 3:1, and GHSV = 700, 3,000, 5,650, and 10,000 h−1. As anticipated, there is a decrease of CO conversion from 50% to 20% when the GHSV increases from 700 to 10,000 h−1. The hydrocarbons selectivity increases from ∼62% to 73% while the CO2 selectivity decreases from 38 to 27% when the GHSV increases from 700 to 10,000 h−1. From Table 2, one can see that both oxygenates selectivity and C5+ hydrocarbons product fraction show little variability when changing the GHSV. Since the CO conversion increases at low GHSV and the C5+ hydrocarbons product fraction is not significantly affected by the GHSV, conducting the one-step process at lower GHSV leads to higher C5+ yield. The results reported in the sections above were all conducted with a HZSM-5:0.5Pd/FeCoCu ratio of 3:1 by weight. For these experiments, the oxygenated compounds selectivity was negligible indicating that there was a sufficient amount of HZSM-5 to convert the alcohols to hydrocarbons. These results suggested that it might be possible to lower the HZSM5:0.5Pd/FeCoCu ratio while keeping the same catalytic performance. To determine the impact of the HZSM5:0.5Pd/FeCoCu ratio on the reactivity, two additional experiments were thus conducted at 300 °C, 70 bar, GHSV = 3,000 h−1, and HZSM-5:0.5Pd/FeCoCu ratio equal to 1:1 and 1:3. The results displayed in Figure 6 clearly show similar conversion and selectivity for all three HZSM-5:0.5Pd/FeCoCu ratios. However, one can see from the inset of Figure 5 that the C5+ fraction of the hydrocarbons product increases with the HZSM-5:0.5Pd/FeCoCu ratio. In addition, detailed analysis of the C5+ fraction indicated that the amount of aromatic compounds increases with the HZSM-5:0.5Pd/FeCoCu ratio from 29.2% to 41.8%. By increasing the HZSM-5:0.5Pd/ FeCoCu ratio, the contact time of the intermediates products

Figure 2. CO conversion and selectivities as a function of the temperature for the HZSM-5 + 0.5Pd/FeCuCo mixture: H2/CO = 1, P = 70 bar, GHSV = 3,000 h−1, and HZSM-5:0.5Pd/FeCoCu ratio = 3:1 by weight. Inset: C5+ fraction among the hydrocarbons product as a function of the temperature under the same reaction conditions.

FeCoCu catalyst produces mainly methanol, as shown Figure 3, and the conversion of syngas to methanol is an equilibrium-

Figure 3. Alcohol product distribution for the conversion of syngas to alcohols over 0.5Pd/FeCoCu catalyst: H2/CO = 1, T = 300 °C, P = 70 bar, and GHSV = 3,000 h−1.

limited reaction. As shown in Table 2, the methane formation increases significantly from ∼26 to 59% due to increased CO methanation activity with temperature. As for methane, CO2 selectivity increases with the temperature and it is attributed to increased water gas shift activity. The selectivity to oxygenates is negligible within the range of temperature studied. It indicates that the HZSM-5 converts efficiently the oxygenated compounds into hydrocarbons over the range of temperatures studied, even at low temperature (i.e., 300 °C). One could have expected the HZSM-5 to be less active at temperature equal to 300 °C since the methanol-to-gasoline process is typically conducted at 350−400 °C to ensure total conversion of methanol + DME.15 Although the hydrocarbons product selectivity decreases only from ∼68% to 54% when the temperature increases from 300 to 370 °C, the diminution of the C5+ hydrocarbons fraction is quite drastic. Indeed, as shown in the inset of Figure 2, the C5+ fraction decreases from ∼49% 13931

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Table 2. Effect of Reaction Parameters (i.e., GHSV, Temperature, Pressure, and HZSM-5:0.5Pd/FeCoCu Weight Ratio) on the CO Conversion, Selectivities, Hydrocarbons Distribution, and C5+ Yield for the HZSM-5:0.5Pd/FeCoCu Mixture and Reactivity of the 0.5Pd/FeCoCu HAS Catalysts without HZSM-5 Addition hydrocarbons product distribution (%)

selectivity (%) GHSV (h−1)

temp (°C)

pressure (bar)

HZSM-5:0.5Pd/FeCoCu ratio (weight)

700 3,000 5,600 10,000 3,000 3,000 3,000 3,000 3,000 3,000 3,000 3,000b

300 300 300 300 325 350 370 300 300 300 300 300

70 70 70 70 70 70 70 20 45 70 70 70

3:1 3:1 3:1 3:1 3:1 3:1 3:1 3:1 3:1 1:1 1:3 0:1

CO conversiona (%) hydrocarbons 50.0 35.6 28.0 20.0 41.0 86.7 80.0 18.0 31.0 30.0 38.0 16.5

61.8 68.1 70.1 73.2 68 55 54.1 80.5 72.6 72.5 73.8 57.7

CO2

oxygenates

CH4

C2 − C4

C5+

C5+ yield (%)

38.0 31.0 29.7 26.7 31.8 45.0 45.9 19.5 27.2 27.1 25.5 13

0.2 0.9 0.2 0.1 0.1 0.0 0.0 0.1 0.2 0.4 0.7 29.3

29.7 26.0 28.6 28.5 41.4 50.0 58.6 28.6 39.3 36.7 33.9 53.9

31.5 25.5 33.0 35.6 32.6 25.0 26.8 36.1 20.0 36.1 33.5 41.8

38.8 48.5 38.4 35.9 26.1 24.7 14.7 35.3 40.6 27.2 32.6 4.3

12.0 11.8 7.5 5.3 7.3 11.8 6.3 5.1 9.1 2.9 9.1 0.4

Data recorded after 50 h on stream. bSame syngas flow rate and same amount of 0.5Pd/FeCoCu catalyst as for HZSM-5:0.5Pd/FeCoCu weight ratio =3:1.

a

Figure 4. CO conversion and selectivities as a function of the pressure for the HZSM-5 + 0.5Pd/FeCuCo mixture: H2/CO = 1, T = 300 °C, GHSV = 3,000 h−1, and HZSM-5:0.5Pd/FeCoCu ratio = 3:1 by weight. Inset: C5+ fraction among the hydrocarbons product fraction as a function of the pressure under the same reaction conditions.

Figure 5. CO conversion and selectivities as a function of GHSV for the HZSM-5 + 0.5Pd/FeCuCo mixture: H2/CO = 1, T = 300 °C, P = 70 bar, and HZSM-5:0.5Pd/FeCoCu ratio = 3:1 by weight. Inset: C5+ fraction among the hydrocarbons product fraction as a function of the GHSV under the same reaction conditions.

with HZSM-5 is increased which favors oligomerization and aromatization. Hence, these results suggest that a higher HZSM-5:0.5Pd/FeCoCu ratio is preferred to enhance the production of aromatic compounds. 3.2. One-Step Process vs Two-Step Process for the Conversion of Syngas to Fuels. In the present study, the conversion of syngas to fuels was conducted in one-step by loading a reactor with a physical mixture of HZSM-5 and 0.5Pd/FeCoCu. However, as explained in the Introduction, the conversion on syngas to fuels has been commercially performed in two separate steps. In order to simulate a two-step process and compare its efficiency to the present one-step process, a reactor was loaded with the 0.5Pd/FeCoCu catalyst on top of the HZSM-5 catalyst (downflow operation). The 0.5Pd/ FeCoCu and HZSM-5 catalysts were separated by 2.5 cm of quartz wool and a dual thermocouple recording the temperature in the middle of each catalyst bed was used. This two-step process test was conducted under the same reaction conditions

as those for the one-step process for a direct comparison. Interestingly, the results presented in Table 3 show a significant difference in CO conversion between the two processes. The CO conversion is equal to ∼36% and ∼20% for the one-step process and two-step process, respectively. For the two-step process, a low CO conversion of ∼20% was anticipated. Indeed, as reported in Table 2, when the 0.5Pd/FeCoCu catalyst was tested in the absence of HZSM-5, the CO conversion was equal to ∼17%. CO conversion is known to occur over the metals sites of the HAS catalyst, whereas HZSM-5 is not expected to be active for the conversion of CO under the present reaction conditions. It is very likely that CO conversion is low for the two-step process because of thermodynamic constraints. Indeed, the syngas conversion is believed to be equilibriumlimited since, as shown Figure 3, methanol is the main product and syngas to methanol conversion is an equilibrium-limited 13932

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(i.e., 68.1%), detailed analysis of the hydrocarbon product distribution shows that the C5+ hydrocarbon production is superior for the one-step process. Another interesting finding is that aromatization is favored for the one-step configuration. The aromatics content among the C5+ fraction is equal to ∼42% for the one-step process and equal to only ∼21% for the two-step process. While further investigation is still needed to understand this behavior, one could speculate that it might be due to the conversion of olefins intermediates into aromatics for the one-step process because of the proximity of the olefins and the zeolite whereas for the two-step process the olefins are converted into hydrocarbons. Also, for the one-step process the conversion of oxygenates is higher which might lead to higher aromatics production, as well. For the conversion of methanolto-hydrocarbons Chang and Silvestri18 have observed an increase of the aromatic fractions from 6.6 to 41.1 wt % when the methanol + DME conversion increases from 47.5% to 100%. Finally, the desired C5+ yield is equal to 11.8% for the one-step process and is higher than that for the two-step process with a yield of 4.6% indicating a higher efficiency of the one-step process. It is worth mentioning that commercial twostep processes produce significantly higher C5+ yield (∼85− 90%)19 than in the present study. However, for commercial two-step processes syngas-to-methanol and methanol-to-hydrocarbons steps are operated under different conditions to maximize the yield of intermediates and final products. For comparison purposes, it is necessary to operate the one-step process and the two-step process under the same reaction conditions as in the present study. Hence, this work shows that a one-step process could be advantageous if CO conversion would be further increased while minimizing undesired water gas shift and methanation reactions leading to high CO2 and CH4 formation.

Figure 6. CO conversion and selectivities as a function of the HZSM5:0.5Pd/FeCoCu ratio: H2/CO = 1, T = 300 °C, P = 70 bar, and GHSV = 3,000 h−1. Inset: C5+ fraction among the hydrocarbons product fraction and aromatic content among the C5+ fraction as a function of the pressure under same reaction conditions.

Table 3. Comparison of the Catalytic Performance of the HZSM-5 + 0.5Pd/FeCoCu Mixture for the One-Step Process and the Two-Step Process (T = 300 °C; P = 70 bar; GHSV = 3000 h−1; H2/CO = 1; HZSM-5:0.5Pd/FeCoCu = 3:1)

CO conversion (%) selectivity (%): hydrocarbons CO2 oxygenates hydrocarbons product distribution (%): CH4 C2−C4 C5+ aromatics in C5+ (%) C5+ yield (%)

one-step process

two-step process

35.6

19.9

68.1 31 0.9

80 15 5

26.0 25.5 48.5 41.8 11.8

35.4 34.2 30.4 20.5 4.6

4. CONCLUSION In this work, the one-step conversion of syngas to hydrocarbons was investigated over HZSM-5 physically mixed with either a methanol synthesis catalyst or a higher alcohol synthesis (HAS) catalyst. The major difference can be found in the durene production. While durene formation is insignificant with any of the HAS catalysts evaluated, it represents half of the C5+ fraction for the HZSM-5 + 5Pd/ ZnO/Al2O3 mixture. Since durene is an undesired product, the HAS catalysts present definitively an advantage as compared to the methanol synthesis catalyst. Among all the HAS catalysts tested, the 0.5Pd/FeCoCu catalyst appears as the most promising. The highest C5+ yield was produced with the HZSM-5 + 0.5Pd/FeCoCu mixture. By varying process conditions such as temperature, pressure, GHSV, and HZSM5:0.5Pd/FeCoCu weight ratio, an optimal C5+ yield of 12% was obtained. Although the C5+ yield seems low when compared to the one obtained with commercial two-step processes, the onestep process shows some potential. Indeed, in this study we show that when operated under the same reaction conditions, the one-step process is more efficient than a two-step process for the production of the C5+ fraction. The C5+ yield was equal to ∼12% for the one-step process and only ∼5% for the twostep process. This can be explained by an improved CO conversion for the one-step process. A major benefit of the onestep process is that conversion of CO into methanol over the 0.5Pd/FeCoCu catalyst is no longer equilibrium-limited since methanol is rapidly converted into DME over HZSM-5 in close contact with the HAS catalyst. However, selectivity to C5+

reaction. For the one-step process, because of the proximity between 0.5Pd/FeCoCu and HZSM-5, methanol produced over 0.5Pd/FeCoCu catalyst is directly converted into DME over the acid sites of HZSM-5. In that case syngas conversion to methanol is no longer equilibrium-limited and higher quantities of CO are converted. One can also see from Table 3 some differences in selectivity between the two processes. The CO2 selectivity is equal to 31% for the one-step process, and it is twice higher than that of the two-step process. CO2 is produced via water gas shift reaction over the metals sites of the 0.5Pd/FeCoCu catalyst and requires the presence of CO and H2O. For the one-step process configuration, the 0.5Pd/ FeCoCu catalyst is likely to be surrounded by a higher quantity of H2O because of its proximity with HZSM-5. Indeed, H2O is partly produced from methanol conversion into DME over the acid sites of HZSM-5. Since more H2O is available, enhanced water gas shift occurs for the one-step process configuration. Although the hydrocarbons selectivity is equal to 80% for the two-step process and is higher than that of the one-step process 13933

dx.doi.org/10.1021/ie502425d | Ind. Eng. Chem. Res. 2014, 53, 13928−13934

Industrial & Engineering Chemistry Research

Article

(14) Chang, C. D.;Lang, W. H. Conversion of methanol to gasoline with minimum durene production. U.S. Pat. US4013732 A, Pennington, NJ, USA, 1977. (15) Mokrani, T.; Scurrell, M. Gas Conversion to Liquid Fuels and Chemicals: The Methanol Route-Catalysis and Processes Development. Catal. Rev. 2009, 51, 1. (16) Chang, C. D.; Kuo, J. C. W.; Lang, W. H.; Jacob, S. M.; Wise, J. J.; Silvestri, A. J. Process Studies on the Conversion of Methanol to Gasoline. Ind. Eng. Chem. Process Des. Dev. 1978, 117, 255. (17) Keil, F. J. Methanol-to-hydrocarbons: Process technology. Microporous Mesoporous Mater. 1999, 29, 49. (18) Chang, C. D.; Silvestri, A. J. The Conversion of Methanol and Other O-Compounds to Hydrocarbons over Zeolite Catalysts. J. Catal. 1977, 47, 249. (19) http://www.exxonmobil.com/Apps/RefiningTechnologies/ files/sellsheet_09_mtg_brochure.pdf (Last accessed: Aug. 15, 2014).

products is difficult to control as metal sites necessary for higher alcohol synthesis are also active for the formation of CH4, CO2, and light hydrocarbons.

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AUTHOR INFORMATION

Corresponding Authors

*(R.A.D.) E-mail: [email protected]. *(Y.W.) E-mail: [email protected]. *(V.M.L.D.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support for this work provided by the U.S. Department of Energy’s Office of Fossil Fuels. PNNL funding was provided under Contract PNNL-1158159.

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REFERENCES

(1) Dagle, R. A.; Lizarazo-Adarme, J. A.; Lebarbier, V. M. C.; Gray, M. J.; White, J. F.; King, D. L.; Palo, D. R. Syngas conversion to gasoline-range hydrocarbons over Pd/ZnO/Al2O3 and ZSM-5 composite catalyst system. Fuel Proces. Technol. 2014, 123, 65. (2) Zhu, Y.; Jones, S. B.; Biddy, M. J.; Dagle, R. D.; Palo, D. R. Singlestep syngas-to-distillates (S2D) process based on biomass-derived syngas-a techno-economic analysis. Bioresour. Technol. 2012, 117, 341. (3) Mentzel, U. V.; Shunmugavel, S.; Hruby, S. L.; Christensen, C. H.; Holm, M. S. High Yield of Liquid Range Olefins Obtained by Converting i-Propanol over Zeolite H-ZSM-5. J. Am. Chem. Soc. 2009, 131, 17009. (4) Gujar, A. C.; Guda, V. K.; Nolan, M.; Yan, Q.; Toghiani, H.; White, M. G. Reactions of methanol and higher alcohols over H-ZSM5. Appl. Catal. 2009, 363, 115. (5) Ramasamy, K. K.; Wang, Y. Catalyst activity comparison of alcohols over zeolites. J. Energy Chem. 2013, 22, 65. (6) Jiao, G.; Ding, Y.; Zhu, H.; Li, X.; Dong, W.; Li, J.; Lu, Y. Effect of the Reduction Temperature of Co-La-Zr/AC on the Synthesis of Higher Alcohols from Syngas. Chin. J. Catal. 2009, 30, 92. (7) Yang, X.; Zhu, X.; Hou, R.; Zhou, L.; Su, Y. The promotion effects of Pd on Fe-Cu-Co based catalyst for higher alcohols synthesis. Fuel Process. Technol. 2011, 92, 1876. (8) Dong, X.; Liang, X.l.; Li, H. Y.; Lin, G. D.; Zhang, P.; Zhang, H. B. Preparation and characterization of carbon nanotube-promoted CoCu catalyst for higher alcohol synthesis from syngas. Catal. Today 2009, 147, 158. (9) Hu, J.; Wang, Y.; Cao, C.; Elliott, D. C.; Stevens, D. J.; White, J. F. Conversion of biomass-derived syngas to alcohols and C2 oxygenates using supported Rh catalysts in a microchannel reactor. Catal. Today 2007, 120, 90. (10) Gerber, M. A.; White, J. F.; Stevens, D. J. Mixed Alcohols Synthesis Catalyst Screening, PNNL-16763. Pacific Northwest National Laboratory: Richland, WA, USA, 2007. (11) Lebarbier, V. M.; Mei, D.; Kim, D. H.; Andersen, A.; Male, J. L.; Holladay, J. E.; Rousseau, R.; Wang, Y. Effects of La2O3 on the Mixed Higher Alcohols Synthesis from Syngas over Co Catalysts: A Combined Theoretical and Experimental Study. J. Phys. Chem. C 2011, 115, 17440. (12) Lebarbier, V. M.; Dagle, R. A.; Kovarik, L.; Lizarazo-Adarme, J. A.; King, D. L.; Palo, D. R. Synthesis of methanol and dimethyl ether from syngas over Pd/ZnO/Al2O3 catalysts. Catal. Sci. Technol. 2012, 2, 2116. (13) Dagle, R. A.; Platon, A.; Palo, D. R.; Datye, A. K.; Vohs, J. M.; Wang, Y. PdZnAl catalysts for the reactions of water-gas-shift, methanol steam reforming, and reverse-water-gas-shift. Appl. Catal., A 2008, 242, 63. 13934

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