Dimethyl Ether Synthesis from CO 2 Hydrogenation on a CuO-ZnO-Al 2 O 3 -ZrO 2 /HZSM-5 Bifunctional Catalyst Xin An, Yi-Zan Zuo, Qiang Zhang, De-zheng Wang, and Jin-Fu Wang* Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua UniVersity, Beijing, 100084, China A CuO-ZnO-Al 2 O 3 -ZrO 2 + HZSM-5 physical mixture bifunctional catalyst with a high activity for dimethy ether (DME) synthesis was used for CO 2 hydrogenation. Various factors that affect catalyst activity, including the reaction temperature, pressure, and space velocity, were investigated. CO 2 conversion reached 0.309, and DME and methanol yields were 0.212 and 0.059 with a stoichiometric ratio of H 2 to CO 2 of 3 at 523 K, 5 MPa, and a space velocity of 6000 mL/(g cat · h). Well-studied kinetic models for methanol synthesis and methanol dehydration, respectively, were used to fit the experimental data and the kinetic parameters in the rate equations for DME synthesis were obtained by regression. A simulated process for CO 2 hydrogenation indicated that a higher DME yield can be obtained with CO recycle that will also give a CO-free product. 1. Introduction Recently, the greenhouse effect has become a threat to the living environment of mankind. The transformation of CO 2 into useful chemicals, e.g. methanol, dimethyl ether (DME), urea, salicylate, is an attractive way to protect the global environment since CO 2 is an important greenhouse gas. 1,2 Among them, DME is drawing more and more attention as a clean fuel because of the worldwide existence of serious air pollution and limited crude oil reserves. 3–7 It has the excellent properties of easier compression ignition combustion, lower NO x and CO emission, smokeless combustion, and less engine noise. Engine tests indicate that with minor fuel system modifications, engines can be operated with a thermal efficiency equivalent to that of traditional diesel and emissions below the strict criteria pre- scribed by the California ULEV standard. However, there are also some disadvantages for DME as a fuel. These include the loss of lubricating ability in car engines and the fact that it gets liquefied by high pressure at room temperature. As a household fuel, DME possesses better combustion performance than liquified petroleum gas (LPG). In addition to its use as fuel, DME is being contemplated as a raw material alternative for methanol for producing olefins (methanol to olefins process). DME also can be reformed to hydrogen and is considered an alternative automobile fuel for fuel cell. 8,9 Therefore, DME will be an important clean fuel and an effective way to use coal resources cleanly in the 21st century. Until now, there are two routes for the production of DME from CO 2 hydrogenation: 5–11 a two-step process (methanol synthesis on a metallic catalyst and subsequent dehydration of methanol on an acid catalyst) and a single-step process using both catalysts in the same reactor to perform the two steps simultaneously. The two steps in route 1 have been studied separately for understanding methanol synthesis and methanol dehydration (DME synthesis), respectively. The latter step, methanol dehydration, is actually also an intermediate step in the transformation of methanol into hydrocarbons. Route 2 has the merit that it is a one-step process (the two reactions are carried out in the same reactor). The main merit of DME synthesis in a single step is that it is not subject to the thermodynamic limitation that exists for methanol synthesis from CO 2 . With the two catalysts being used together, the subsequent methanol dehydration on the HZSM-5 catalyst continuously removes the product of methanol synthesis; thus, the CO 2 conversion can be higher than that determined by the thermo- dynamics of methanol synthesis. Also, CO 2 incorporation in the reactant is more feasible than in the synthesis of methanol because the process can be operated at a lower pressure. Research works on processes for DME synthesis have been directed at discriminating catalysts and getting data on the effect of the operating conditions. The first step in DME synthesis is a methanol synthesis reaction, and a CuO-ZnO based catalyst is commonly used. Various modified catalysts, including CuO-ZnO-Al 2 O 3 , CuO-ZnO-CrO 3 , and CuO-ZnO-ZrO 2 have been developed. 12–15 It is believed that the ZnO adsorbs CO 2 and Cu adsorbs H 2 , and the reaction takes place on the surface of the Cu catalyst. A catalyst with highly dispersed CuO-ZnO is a key factor for high yield and selectivity. CuO-ZnO-Al 2 O 3 catalyst has been commonly used in various studies, and many modified CuO-ZnO-Al 2 O 3 based catalysts were reported recently. 14–16 By making use of a phase separation effect of nanoparticles on the catalyst surface, 17,18 a fibrous CuO-ZnO-Al 2 O 3 -ZrO 2 catalyst that is active for methanol production from CO 2 (in place of CO) hydrogenation was reported by our group recently. 17 A 5% Zr addition gave a methanol space time yield 80% higher than that from a commercial catalyst. This fibrous CuO-ZnO-Al 2 O 3 -ZrO 2 was chosen as one component of the bifunctional catalyst in this work. The consequent step in DME synthesis is the selective dehydration of methanol, which is catalyzed by an acidic catalyst. Various catalysts, such as γ-Al 2 O 3 , NaHZSM-5, and HZSM-5, were used in previous studies. 6,19–24 In contrast tor CO hydrogenation, in CO 2 hydrogenation, more water is produced. Since HZSM-5 is not sensitive to the concentration of water, this was chosen as the other component of the bifunctional catalyst. Thus, a physical mixture of CuO-ZnO- Al 2 O 3 -ZrO 2 and HZSM-5 was the bifunctional catalyst chosen for the CO 2 hydrogenation process in this work. In CO 2 hydrogenation, there is competition between the reverse water gas shift reaction and methanol and DME syntheses reactions. 22–28 The water gas shift reaction reaches thermodynamic equilibrium fast, while the methanol and DME syntheses reactions are much slower. CO, which is a product of the reverse water-gas shift reaction, will be present in the * To whom correspondence should be addressed. Tel: +86-10- 62797490. Fax: +86-10-62772051. E-mail: wangjf@flotu.org. Ind. Eng. Chem. Res. 2008, 47, 6547–6554 6547 10.1021/ie800777t CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008