Elsevier

Fuel Processing Technology

Volume 110, June 2013, Pages 206-213
Fuel Processing Technology

Synthesis of ethanol from methanol and syngas through an indirect route containing methanol dehydrogenation, DME carbonylation, and methyl acetate hydrogenolysis

https://doi.org/10.1016/j.fuproc.2012.12.016Get rights and content

Abstract

An indirect route containing methanol dehydrogenation, DME carbonylation, and methyl acetate hydrogenolysis was used for ethanol synthesis from methanol and syngas. DME was produced stably from the methanol dehydrogenation with a 90% yield over H-ZSM-5 at 453 K under 1 MPa. A cold trap was set behind the methanol dehydrogenation to collect the formed water and unreacted methanol. Rh/Cs2H2SiW12O40 was an excellent catalyst for the DME carbonylation to methyl acetate at 473 K. The selectivity for methyl acetate was very high (> 90%) and raising CO/DME ratio improved the DME conversion. A reaction intermediate similar to the homogeneous Rh-I system could be formed on the Rh/Cs2H2SiW12O40 surface. Cu/CeO2 was an excellent catalyst for the methyl acetate hydrogenolysis to methanol and ethanol at 523 K. Both active Cu0 species and active Cu+ species could be stabilized in the Cu/CeO2 catalyst. When three fixed-bed reactors and one cold trap were linked together to achieve the ethanol synthesis from methanol and syngas, an ethanol selectivity of 91.1% was obtained at a methanol conversion of 47.2% under 1 MPa.

Highlights

► Ethanol was produced by an indirect method using three reactors and one trap. ► Trap was set behind the methanol dehydration to collect the formed water. ► Rh/Cs2H2SiW12O40 was found as an excellent catalyst for the DME carbonylation. ► Cu/CeO2 was found as an excellent catalyst for the methyl acetate hydrogenolysis. ► Ethanol selectivity of 91.1% was obtained at a methanol conversion of 47.2%.

Introduction

In recent years, the use of biomass for generating energy has become increasingly important because of the global climate change and the depletion of fossil fuel resources. The production of ethanol has attracted great attention in the worldwide because ethanol can be used as a fuel additive, a hydrogen carrier, and a chemical feedstock [1]. Two main processes have been used for the production of ethanol from biomass: a biological fermentation process through a sugar platform, and a chemical BTL (biomass to liquid) process through a syngas (mixture of CO and H2) platform [2]. The fermentation process is limited in its application only to selected biomass components (such as starch and cellulose) for ethanol production because some biomass components (such as lignin) cannot be converted to sugar for the fermentation process. In contrast, because syngas can be obtained from all kinds of biomass components by the gasification, all biomass can be converted to ethanol by the BTL process through a syngas platform.

From the view of economy, the direct synthesis of ethanol from syngas is the most desirable method for the ethanol production using the BTL process. The chemical formula of the direct synthesis of ethanol from syngas is described in Eq. (1).2CO + 4H2 = C2H5OH + H2O

Four kinds of catalysts have been reported for the direct synthesis of ethanol from syngas: modified F–T synthesis (Fe-based or Co-based) catalysts [3], [4], modified Cu-based catalysts [5], [6], [7], sulfide Mo-based catalysts [8], [9], and Rh-based catalysts [10], [11]. However, the direct synthesis of ethanol from syngas has not been achieved in a commercial available process because the selectivity for ethanol was still low over each catalyst. Modified F–T synthesis catalysts produce hydrocarbons as the main products. Modified Cu-based catalysts produce C1–C6 alcohols mixtures in which methanol and isobutanol are the main products. Mo-based catalysts produce methanol and ethanol as the main oxygenates but also form large amounts of methane and CO2 as by-products. Rh-based catalysts have the highest selectivity for ethanol among various catalysts but the selectivity for methane is still large (> 30%). As a result, although the direct synthesis of methanol from syngas has been commercialized in a huge scale in industry (using Cu-based catalysts), the direct synthesis of ethanol from syngas cannot be commercialized till now due to a shortage of highly selective catalysts.

Recently, two indirect routes through dimethyl ether (DME) or dimethyl oxalate (DMO) have been suggested for the synthesis of ethanol from syngas [12], [13], [14], [15], [16], [17]. DME is an important platform because it can be produced from syngas and also can be converted to ethanol [16], [17]. Tsubaki and co-workers investigated the synthesis of ethanol from DME and syngas using the indirect route [13–16]. At first, methyl acetate is synthesized by the DME carbonylation (as described in Eq. (2)) over H-mordenite. Then, ethanol and methanol are produced by the hydrogenolysis of methyl acetate (as described in Eq. (3)) over Cu/ZnO/Al2O3. After combining Eqs. (2), (3), the total reaction can be described in Eq. (4), which is the formula of the synthesis of ethanol and methanol from DME and syngas. Because H-mordenite selectively catalyzes the DME carbonylation and Cu/ZnO/Al2O3 selectively catalyzes the hydrogenolysis of methyl acetate, the new indirect method is promising for the commercialization of ethanol synthesis from DME and syngas [13–16].CH3OCH3 + CO = CH3COOCH3CH3COOCH3 + 2H2 = C2H5OH + CH3OHCH3OCH3 + CO + 2H2 = C2H5OH + CH3OH

Actually, DME can be synthesized from the dehydrogenation of methanol (as described in Eq. (5)). After combining Eq. (5) with Eq. (2) and Eq. (3), the total reaction can be described in Eq. (6), which is the formula of the synthesis of ethanol from methanol and syngas.2CH3OH = CH3OCH3 + H2OCH3OH + CO + 2H2 = C2H5OH + H2O

The direct synthesis of ethanol from methanol and syngas (Eq. (6)) is known as the homologation of methanol and the reaction has been investigated using the Cu-based catalysts [18], [19]. However, the homologation of methanol forms mixed alcohols and the fraction of ethanol in total alcohols was low because the reaction intermediate in the homologation of methanol was same as that in the synthesis of mixed alcohols from syngas over Cu-based catalysts [18], [19]. As a result, there is not a highly selective catalyst till now to achieve the direct synthesis of ethanol from methanol and syngas.

In the present study, we designed a continuous flow reaction system (containing three fixed-bed reactors and one cold trap) to achieve the selective synthesis of ethanol from methanol and syngas (Eq. (6)) through an indirect route containing Eq. (2), Eq. (3), and Eq. (5). Moreover, we developed a highly active Rh/Cs2H2SiW12O40 catalyst for the carbonylation of DME (Eq. (2)) and developed a highly active Cu/CeO2 catalyst for the hydrogenolysis of methyl acetate (Eq. (3)).

Section snippets

Reagents

Na-ZSM-5 and Na-mordenite (denoted as Na-MOR) were purchased from Tosoh Co., Japan. The Si/Al ratio was 30 in Na-ZSM-5 and was 23 in Na-Mor, respectively. Heteropolyacids H3PW12O40 and H4SiW12O40 were purchased from Wako Pure Chem. Ind. Co. with purities higher than 99%. Rh(NO3)3nH2O was purchased from Soekawa Chem. Co. and the assay of Rh was 32.5%. The other chemical reagents were purchased from Wako Pure Chem. Ind. Co. with purities higher than 99%. Gas cylinders were purchased from

Investigation of the methanol dehydration to DME

Table 1 shows the reaction results of the methanol dehydration to DME over various solid acid catalysts at 453 K under 1 MPa. The methanol dehydration to DME is an acid-catalytic reaction which needs the existence of acid sites on the catalyst surface. Both acidic zeolites (H-ZSM-5 and H-MOR) and heteropolyacids (Cs2HPW12O40 and Cs2H2SiW12O40) are strong Brønsted-type solid acids. As shown in Table 1, each solid acid catalyst produced DME with a very high selectivity (> 99%) from the methanol

Conclusions

Ethanol was produced from methanol and syngas by connecting in series of three fixed-bed reactors and one cold trap. The three fixed-bed reactors were used to achieve methanol dehydrogenation, DME carbonylation, and methyl acetate hydrogenolysis, respectively. The cold trap was used to eliminate the water formed from the methanol dehydrogenation. The methanol dehydrogenation is an easy step. A DME yield of 90% was obtained over H-ZSM-5 at 453 K. The DME carbonylation is a very difficult step. We

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