Novel Ethanol Synthesis Method via C1 Chemicals without Any

May 3, 2010 - common alternative to gasoline in some parts of the world. Ethanol is ... Furthermore, ethanol fuel produced from agricultural feedstock...
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Ind. Eng. Chem. Res. 2010, 49, 5485–5488

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Novel Ethanol Synthesis Method via C1 Chemicals without Any Agriculture Feedstocks Yi Zhang,*,† Xiaoguang San,‡ Noritatsu Tsubaki,*,‡,§ Yisheng Tan,| and Jianfeng Chen† Research center of the Ministry of Education for High GraVity Engineering and Technology, Beijing UniVersity of Chemical Technology, 15 Beisanhuan East Road Beijing 100029, PR China, Department of Applied Chemistry, School of Engineering, Toyama UniVersity, Gofuku 3190, Toyama 930-8555, Japan, JST, CREST, Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan, and State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan, 030001, PR China

A new method of ethanol synthesis from dimethyl ether (DME) and syngas was developed, which is via combined carbonylation of dimethyl ether and hydrogenation of methyl acetate. The conversion of DME is up to 100% and the selectivity of ethanol is as high as 48.5%. On the other hand, methanol as another main product of this study is formed as 47.6%, which can be recycled as a reactant of DME synthesis (MTD), leading to lower the cost of this new ethanol synthesis method. 1. Introduction Ethanol can be used directly as a fuel. It is an increasingly common alternative to gasoline in some parts of the world. Ethanol is also blended with gasoline, to reduce consumption of petroleum fuels and in attempts to reduce air pollution. And in the U.S., most ethanol is sold as a blend of 10% ethanol and 90% gasoline. Presently, the production of ethanol by fermentation of carbohydrates is the primary technology for the generation of liquid fuels from renewable biomass resources. However, ethanol fuel production depends on availability of land area, soil, and water, which is very limited for most countries. Furthermore, ethanol fuel produced from agricultural feedstocks such as starch will cause a global food shortage, contributing to starvation in the third world. There is a concern that as demand for ethanol fuel increases, food crops are replaced by fuel crops, driving food supply down and food prices up. Growing demand for ethanol in the United States has been discussed as a factor in the increased corn prices. Therefore, an environmentally friendly and effective synthesis method of ethanol is strongly required. Selective synthesis of ethanol directly from synthesis gas using metallic catalysts such as Rh or Mo based catalyst is studied for a long time, but not practical at the present stage. Low conversion, very high pressure, and excessive formation of CO2 make the formation rate of ethanol very slow. Especially hydrocarbons from the Fischer-Tropsch reaction chain growth mechanism are formed at the same time and alcohols other than ethanol, such as methanol and C3OH-C6OH exhibit high selectivity. All these byproducts lower ethanol selectivity severely. Dimethyl ether (DME) is a clean-burning alternative to liquified petroleum gas, liquified natural gas, diesel, and gasoline. It can be easily made from natural gas, coal, or biomass different forms starch, such as lignin, cellulose, via synthesis gas (CO + H2). Recent advances in the direct synthesis of

dimethyl ether from synthesis gas make DME a cheaper and less toxic feedstock for the synthesis of chemicals that currently produced from methanol. Carbonylation provides a convenient route to functionalize C1 intermediates via formation of carbon-carbon bonds at low temperatures1 and DME is more favorable for carbonylation than methanol from the economic viewpoint because it can be produced more effectively from syngas.2 Catalysts suggested for the halide-free carbonylation are based on the acid form of various types of zeolites, such as mordenite, Y, and H-ZSM-5.3,4 We consider that DME and CO can be converted to methyl acetate by halide-free carbonylation over zeolite, and then, the formed methyl acetate will be hydrogenated to methanol and ethanol. The formed methanol can be recycled as reactant of DME synthesis, which forms water as only byproduct. Therefore, DME and syngas can form the ethanol simply and effectively with unique the byproduct of water, namely, the new ethanol synthesis would be completed in an environmentally friendly manner by C1 chemicals without any agriculture feedstocks (Scheme 1). Scheme 1. Novel Ethanol Synthesis Method via DME and Syngas by Combination of Carbonylation and Hydrogenation

* To whom correspondence should be addressed. Tel.: 86-1064447274. Fax: 86-10-64423474. E-mail: [email protected] (Y.Z.). Tel. & Fax: 81-76-445-6846. E-mail: [email protected] (N.T.). † Beijing University of Chemical Technology. ‡ Toyama University. § JST, CREST. | Chinese Academy of Science. 10.1021/ie901882s  2010 American Chemical Society Published on Web 05/03/2010

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Here, we explore a novel and simple method of ethanol synthesis from dimethyl ether and syngas, using carbonylation and hydrogenation catalysts simultaneously. 2. Experimental Section The catalyst of carbonylation was H-MOR (Si/Al ) 9.9). Before reaction, the H-MOR was treated in muffle at 773 K for 2 h to remove water and organic chemicals attached on the H-MOR. The Cu/ZnO catalyst was used as hydrogenation catalyst. The Cu/ZnO (Cu/Zn in molar ratio ) 1:1) catalyst was prepared by conventional coprecipitation method. An aqueous solution containing metal nitrate (Cu, Zn) salts and an aqueous solution of Na2CO3 were added simultaneously to 300 mL deionized water with constant stirring. The precipitation temperature and pH value were maintained at 333 K and 8.5, respectively. The slurry was aged overnight. The precipitate was filtrated and repeatedly washed with deionized water to remove the excessive sodium cations, followed by drying at 393 K for 12 h and calcining in air at 623 K for 1 h. The obtained catalysts were reduced by a flow of 100% hydrogen at 573 K for 10 h and passivated by 1% oxygen diluted by nitrogen.5 DME carbonylation and ethanol synthesis were tested using a packed-bed stainless steel reactor (9.5 mm OD) equipped with a multipoint thermocouple held within a 1.6 mm outer thermowell aligned along the tube center. For DME carbonylation, the 0.5 g H-MOR was introduced into reactor. For ethanol synthesis, the 0.5 g Cu/ZnO catalyst was first introduced into reactor, followed by 0.5 g H-MOR, keeping the hydrogenation catalyst bed under carbonylation catalyst bed inside the same reactor. The reactant mixtures with different ratio were applied to DME carbonylation and ethanol synthesis. Effluent gas from the reactor was first analyzed by gas chromatograph equipped with TCD detector (Porpak Q column for DME, CO, CO2, and Ar), and then was introduced into the ice-water-cooled trap, where liquid products were collected. Ar was an inner standard for TCD online analysis. The liquid products (ethanol, methanol, methyl acetate) in the cold trap were collected and then analyzed by FID with a methanator. All liquid products were confirmed by GC-MS (Shimadzu GCMS 1600). The selectivity of various products is based on molecular moles of products. 3. Results and Discussion Carbonylation of alkenes and alcohols to carboxylic acids via Koch-type reactions6 is catalyzed by strong acids without metal cocatalysts.7 Acidic zeolites and sulfated zirconia catalyze the carbonylation of alcohols and alkenes to carboxylic acids via Koch-type pathways, which involve CO insertion into C-O bonds within tertiary surface-bound alkoxides and subsequent hydrolysis of the bound acetyl intermediates formed.8,9 Fujimoto and co-workers first reported methanol carbonylation to acetic acid on zeolites and proposed an intermediate role of surface methyl groups.10 Similar reactions of methanol and DME were later reported on acidic zeolites and polyoxometallate clusters, but with significant homologation side reactions and catalyst deactivation.11,12 On the other hand, Iglesia et al. reported that on H-MOR zeolite, DME carbonylation was stable and highly selective.3 In the present study, carbonylation of DME and CO (DME/CO ) 1/49) was carried out on H-MOR with different reaction temperature. As shown in Figure 1, conversion of DME is increased with the increased reaction temperature. At low reaction temperature, 423 K, DME is hard to be converted to methyl acetate, even though the selectivity of methyl acetate is

Figure 1. Carbonylation reaction of DME under different reaction temperature: (reaction conditions) P ) 1.5 MPa; F(CO + DME + Ar) ) 20 mL/ min; weight of catalysts ) 0.5 g; DME/CO ) 1/49.

Figure 2. DME carbonylation with different ratio of CO to DME: (reaction conditions) P ) 1.5 MPa; F(CO + DME + Ar) ) 20 mL/min; weight of catalysts ) 0.5 g; reaction temperature ) 493 K.

as high as 99.5%. At 513 K, the conversion of DME reaches almost 100%, however, the selectivity of methyl acetate is significantly low, due to the homologation and oligomerization side reaction which form hydrocarbons at high temperature. On the basis of these results, at 493 K, the DME carbonylation realizes relatively high DME conversion and methyl acetate selectivity as 41% and 95% respectively, indicating that 493 K is the best reaction temperature for DME carbonylation in this study. The DME conversion in Figure 1 is relatively high, due to the lower ratio of DME to CO. However, the lower ratio of DME in feedstock would result in very small amount of products for one pass of reaction process. In this study, we increase the DME content in feed gas to try to enlarge the STY of DME carbonylation reaction. The reaction results are illustrated in Figure 2. As shown in Figure 2, the DME conversion decreases with the increased DME ratio. When the ratio of DME/CO is up to 1/10, the DME conversion is as low as 1.8%. As reported, the methyl acetate synthesis rates do not depend on DME pressure and have a direct correspondence with proton concentration of zeolite surface.3 Therefore, increasing the DME partial pressure would not promote the reaction rate of DME, resulting in low reaction efficiency of DME carbonylation. On the other hand, as the high reaction temperature would result in low selectivity of methyl acetate as illustrated in Figure 1, the reaction rate of DME carbonylation should be increased via introducing subsequent reaction to consume the product of DME carbonylation, which is methyl acetate. Generally, methyl acetate could be easy to hydrogenate to methanol and ethanol via a

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Table 1. Reaction Performance of CO/H2 and DME/H2 via Cu/ZnO Catalysts conversion (%) reaction gas

DME

a

CO/H2 DME/H2b

CO 4.1

selectivity (%) methanol ethanol methyl acetate CO2 95.4

4.6

a CO/H2/Ar ) 47.5/50/2.5. b DME/H2/N2 ) 1/49/50: (reaction conditions) reaction temperature ) 493 K; P ) 15 bar; F ) 20 mL/min; catalyst Cu/ZnO ) 0.5 g.

Figure 3. Ethanol formation via combined DME carbonylation and methyl acetate hydrogenation: (reaction conditions) reaction temperature ) 493 K; P ) 15 bar; F(CO, DME, H2, and Ar) ) 40 mL/min; catalyst H-MOR ) 0.5 g, Cu/ZnO ) 0.5 g.

hydrogenation catalyst, such as Cu/ZnO. As is well-known, ethanol is an important bio fuel, as it can be used as fuel directly or blended with gasoline. And methanol can be dehydrated to DME again. Therefore, combined DME carbonylation with hydrogenation reaction would form the important biofuel, ethanol, and reactant of DME formation, methanol, as well as promote the reaction rate of DME carbonylation. The whole reaction of this combined process is described as follows: CO + CH3OCH3 f CH3COOCH3

(1)

CH3COOCH3 + 2H2 f CH3CH2OH + CH3OH

(2)

(2CH3OH f CH3OCH3 + H2O)

(3)

Therefore, this new ethanol synthesis process, which combined DME carbonylation and methyl acetate hydrogenation, would form ethanol via C1 chemicals, DME, and syngas, without any agriculture feed stocks, and produce only one byproduct, water, after methanol dehydration to form DME. The ethanol formation reaction via DME and syngas was carried out via H-MOR and Cu/ZnO as DME carbonylation and hydrogenation catalyst respectively, at different a DME/CO ratio, with constant CO/H2 ratio as 1. The reaction performance is illustrated in Figure 3. As shown in Figure 3, the DME conversion at DME/CO as 1/49 is as high as 98%, and for the DME/CO as 1/10, the DME conversion is up to 48%, which is 26 times higher than that of DME carbonylation without combination of hydrogenation, indicating that hydrogenation of methyl acetate, which is formed by DME carbonylation at first step of ethanol formation, rapidly removes the product of DME carbonylation and accelerates the reaction rate of DME carbonylation, resulting in extremely enhanced DME conversion. Concerning the products of this ethanol synthesis, the main products are methanol and ethanol, those are above 80% in all products. Unreacted methyl acetate and CO2 are also slightly formed. Because the methanol could be easily recycled to produce DME via dehydration, the ethanol is the only main product. The selectivity of methanol is slightly higher than that of ethanol, as shown in Figure 3.

As reported by Cheung et al.,13 in the induction period of DME carbonylation, DME molecules first react with an acidic proton to form a methanol molecule and a methyl group; the methanol thus formed can then react with another Brønsted acid site to form H2O and a second methyl group. During steadystate DME-CO reactions, some CH3OH molecules leave the reactor before they react with another proton, because of competing reaction of DME reactants with such protons. In this study, because the DME carbonylation is accelerated by hydrogenation reaction, the induction period might appear frequently, and/or DME would be more prior to reacting with protons of zeolite surface, resulting in slightly higher methanol selectivity. On the other hand, in the second reaction, a small amount of methanol and CO2 could be formed due to the methanol synthesis reaction of syngas on Cu/ZnO catalysts, as shown in Table 1. Meanwhile, the frequently appearing induction period of DME carbonylation would form more water, which would react with CO to form CO2 on Cu/ZnO catalyst. For comparison, the reaction of DME and H2 is tested on Cu/ ZnO under the same reaction conditions of this ethanol synthesis reaction. As shown in Table 1, no products are formed from DME and H2 over Cu/ZnO catalyst. Therefore, ethanol mainly formed with high selectivity by this new ethanol synthesis reaction, which combined DME carbonylation and methyl acetate hydrogenation. 4. Conclusion In summary, a new method of ethanol synthesis is developed. The ethanol is efficiently formed from DME and syngas via combination of DME carnbonylation and methyl acetate hydrogenation. This new ethanol synthesis is environmentally friendly completed by C1 chemicals, DME and syngas, without any agriculture feed stocks with the only byproduct of water, as the formed methanol could be recycled to produce DME and water, using Al2O3 or zeolite catalyst. The study on combining zeolite and Cu/ZnO into a single catalyst pellet is now in progress. Acknowledgment Financial support from the National Natural Science Foundation of China (No. 20821004 and No. 20990221), National “863” Program of China (No. 2009AA033301), and the Foundation of State Key Laboratory of Coal Conversion (No. 10-11-902-1) is greatly appreciated by Y.Z. and J.C. Literature Cited (1) Sunley, G. J.; Watson, D. J. High productivity methanol carbonylation catalysis using iridium: The Cativa process for the manufacture of acetic acid. Catal. Today 2000, 58, 293. (2) Shikada, T.; Ohno, Y.; Ogawa, T.; Ono, M.; Mizuguchi, M.; Tomura, K.; Fujimoto, K. Direct synthesis of dimethyl ether form synthesis gas. Stud. Surf. Sci. Catal. 1998, 119, 515. (3) Cheung, P.; Bhan, A.; Sunley, G. J.; Iglesia, E. Selective Carbonylation of Dimethyl Ether to Methyl Acetate Catalyzed by Acidic Zeolites. Angew. Chem., Int. Ed. 2006, 45, 1617.

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(4) Ellis, B.; Howard, M. J.; Joyner, R.; Reddy, W. K. N.; Padley, M. B.; Smith, W. J. Heterogeneous catalysts for the direct, halide-free carbonylation of methanol. Stud. Surf. Sci. Catal. 1996, 101, 771. (5) San, X.; Zhang, Y.; Shen, W.; Tsubaki, N. New Synthesis Method of Ethanol from Dimethyl Ether with a Synergic Effect between the Zeolite Catalyst and Metallic Catalyst. Energy Fuels 2009, 23, 2843. ¨ ber neuere bei der Synthese verzweigter Carbonsa¨uren (6) Koch, H. U erzielte Ergebnisse. Fette Seifen Anstrichmittel 1957, 59, 493. (7) Bagno, A.; Bukala, J.; Olah, G. A. Chemistry in superacids. 8. Superacid-catalyzed carbonylation of methane, methyl halides, methyl alcohol, and dimethyl ether to methyl acetate and acetic acid. J. Org. Chem. 1990, 55, 4284. (8) Luzgin, M. V.; Stepanov, A. G.; Sassi, A.; Sommer, J. Formation of Carboxylic Acids from Small Alkanes in Zeolite H-ZSM-5. Chem.sEur. J. 2000, 6, 2368. (9) Li, T.; Tsumori, N.; Souma, U.; Xu, Q. Highly active and stable performance of catalytic vapor phase Koch-type carbonylation of tert-butyl alcohol over H-zeolites. Chem. Commun. 2003, 2070.

(10) Fujimoto, K.; Shikada, T.; Omata, K.; Tominaga, H. Vapor phase carbonylation of methanol with solic acid catalysts. Chem. Lett. 1984, 2047. (11) Sardesai, A.; Lee, S.; Tartamella, T. Synthesis of methyl acetate from dimethyl ether using group VIII metal salts of phosphotungstic acid. Energy Sources 2002, 24, 301. (12) Wegman, R. W. Vapour phase carbonylation of methanol or dimethyl ether with metal-ion exchanged heteropoly acid catalysts. J. Chem. Soc. Chem. Commun. 1994, 947. (13) Cheung, P.; Bhan, A.; Sunley, G. J.; Law, D. J.; Iglesia, E. Site requirements and elementary steps in dimethyl ether carbonylation catalyzed by acidic zeolites. J. Catal. 2007, 245, 110.

ReceiVed for reView November 30, 2009 ReVised manuscript receiVed March 29, 2010 Accepted April 23, 2010 IE901882S