Dimethyl Ether Synthesis from CO2 Hydrogenation on a CuO−ZnO

Ind. Eng. Chem. Res. 2008, 47, 6547–6554

6547

Dimethyl Ether Synthesis from CO2 Hydrogenation on a CuO-ZnO-Al2O3-ZrO2/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-Al2O3-ZrO2 + HZSM-5 physical mixture bifunctional catalyst with a high activity for dimethy ether (DME) synthesis was used for CO2 hydrogenation. Various factors that affect catalyst activity, including the reaction temperature, pressure, and space velocity, were investigated. CO2 conversion reached 0.309, and DME and methanol yields were 0.212 and 0.059 with a stoichiometric ratio of H2 to CO2 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 CO2 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 CO2 into useful chemicals, e.g. methanol, dimethyl ether (DME), urea, salicylate, is an attractive way to protect the global environment since CO2 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 NOx 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 prescribed 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 CO2 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 * To whom correspondence should be addressed. Tel: +86-1062797490. Fax: +86-10-62772051. E-mail: [email protected].

CO2. 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 CO2 conversion can be higher than that determined by the thermodynamics of methanol synthesis. Also, CO2 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-Al2O3, CuO-ZnO-CrO3, and CuO-ZnO-ZrO2 have been developed.12–15 It is believed that the ZnO adsorbs CO2 and Cu adsorbs H2, 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-Al2O3 catalyst has been commonly used in various studies, and many modified CuO-ZnO-Al2O3 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-Al2O3-ZrO2 catalyst that is active for methanol production from CO2 (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-Al2O3-ZrO2 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-Al2O3, NaHZSM-5, and HZSM-5, were used in previous studies.6,19–24 In contrast tor CO hydrogenation, in CO2 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-ZnOAl2O3-ZrO2 and HZSM-5 was the bifunctional catalyst chosen for the CO2 hydrogenation process in this work. In CO2 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

10.1021/ie800777t CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

6548 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008

Figure 1. Experimental setup for DME synthesis on the CuO-ZnO-Al2O3-ZrO2 + HZSM-5 physically mixed bifunctional catalyst. 1 - Gas cylinders; 2 - depressed valve; 3 - mass flowmeter; 4 - valve; 5 - reactor; 6 - heater furnace; 7 - fixed temperature chamber; 8 - cold trap; 9 - liquid collection container; 10 - backpressure valve; 11 - flowmeter; 12 - gas chromatograph; 13 - computer.

product due to the fast rate of the water gas shift reaction. From a consideration of carbon source utilization, the CO in the outlet stream should be separated from the DME and methanol and recycled back into the reactor. Thus, it should be useful to simulate a process with this recycle step, for which it is first necessary to understand the catalytic behavior and establish the reaction kinetics. There have been previous reports on the kinetics for DME synthesis from CO2 hydrogenation,6,26–34 but with a CuO-ZnO-Al2O3/γ-Al2O3 catalyst. This CuO-ZnOAl2O3 catalyst is less active compared with the novel fibrous CuO-ZnO-Al2O3-ZrO2 catalyst used here for methanol synthesis, and the γ-Al2O3 catalyst has less activity than the HZSM-5 used here under the high water concentration situation that exists during CO2 hydrogenation. In this work, a physical mixture of CuO-ZnO-Al2O3-ZrO2 fibrous methanol synthesis catalyst and HZSM-5 methanol dehydration catalyst was used as a bifunctional catalyst for CO2 hydrogenation to DME at various reaction temperatures, pressures, and space velocities. Kinetics data that are specific to it were obtained as a basis for the understanding of the catalytic behavior and design of a process of CO2 hydrogenation. The experimental data were fitted to Graaf et al.’s and Tao et al.’s kinetic models for methanol synthesis and methanol dehydration, respectively. On the basis of the kinetic parameters determined, process simulations were performed and a simulated CO2 hydrogenation process that recycles CO was suggested because this does not waste CO and the presence of CO in the reactor inlet gives an increased space time yield of DME. CO is present in the reactor outlet because the Cu-based catalyst will give CO2 and CO concentrations that are determined by the water-gas reaction thermodynamics at the reactor outlet. 2. Experimental Details The tested catalyst was a physical mixture containing a methanol synthesis catalyst and a methanol dehydration catalyst. The methanol synthesis catalyst was a fibrous CuO-ZnO-

Al2O3-ZrO2 catalyst that was prepared by a coprecipitation procedure.16,17 Solutions of Cu(NO3)2 · 6H2O, Zn(NO3)2 · 6H2O, Al(NO3)3 · 6H2O, and ZrOCl2 with a concentration of 0.6 mol/L were mixed in the ratio 6:3:0.5:0.5. The mixture was precipitated using Na2CO3 solution at 353 K and vigorous stirring for 1 h. After filtration and washing, the catalyst was dried for 12 h at 120 °C and calcined for 4 h at 350 °C. The methanol dehydration acid catalyst was ZSM-5 with a Si/Al ratio of 38 purchased from the Catalyst Plant of Nankai University. It had been synthesized using tetrapropylammonium (TPA) as the structure directing agent. First, 20 g TPABr (Fluka 98%) was dissolved in 20 g deionized water, which was followed by the addition of 25 g Ludox LS to give solution A. Solution B was prepared by dissolving 0.25 g Al(OH)3 (Aldrich), 0.11 g NaOH (Merck 99.98%), and 0.41 g KOH (Merck). Solution A was then added to solution B and the pH was adjusted to 11 by the dropwise addition of 2 M H2SO4 while stirring for 2 h at room temperature. The gel was transferred to a 250 mL Teflon lined steel autoclave, and the zeolite was crystallized under static conditions at 170 °C for 14 days. The solid product obtained was filtered and washed with deionized water until no Br- could be detected in the washing water (no precipitation with Ag+). The final molar gel composition was 78 SiO2:1.0 Al2O3:2.3 K2O: 0.86 Na2O:47 TPABr:4000 H2O (Si/Al ratio 38). The gel was soaked in 0.1 M HNO3 solution for 1 h, and filtrated further. After calcined at 600 °C for 4 h, the HZSM-5 methanol dehydration catalyst was obtained. The catalysts were physically mixed together using a CuO-ZnO-Al2O3-ZrO2 catalyst and HZSM-5 ratio of 2.0. The prepared catalyst mixture was ground into a powder, mixed with quartz particles, and placed in the fixed bed. The quartz particles, with a diameter of 1 mm, were used to dilute the catalyst concentration in the fixed bed so that the heat production density was decreased in order to give a more uniform temperature in the reactor. Blank experiments were carried out to confirm that the quartz particles were inactive for CO2 hydrogenation.

Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6549

The catalytic reaction on the CuO-ZnO-Al2O3-ZrO2 + HZSM-5 catalyst was carried out in a fixed bed reactor shown in Figure 1. The reactor had a diameter of 12 mm and a length of 500 mm. The catalyst and quartz particles were distributed in the center part (about 100 mm long) of the reactor where the temperature was uniform, Hydrogen, nitrogen, and carbon dioxide used were of purity above 99.999%. Impurities that can be present in industrial gases, such as CO, H2S, COS, were probably present in negligible concentrations; thus, the experiments here did not provide data about the sulfur resistance of the catalyst. Before reaction, the catalyst was reduced with a 5% H2/95% N2 mixture at atmospheric pressure by raising the temperature slowly to the reaction temperature over 10 h. Then, the reduction gas was switched to the reaction gas and the pressure was raised to the reaction pressure to start the reaction. The first sample of the effluent was taken 2 h after steady reaction conditions were established, and then samples were taken every 30 min for online analysis of the effluent composition. The reaction temperature was controlled by furnace heating. The structure of the catalysts was studied by scanning electron microscopy (SEM), temperature programmed desorption (TPD), and N2 adsorption methods. The morphology of the CuOZnO-Al2O3-ZrO2 + HZSM-5 catalyst was characterized by a JSM 7401F high resolution scanning electron microscope (SEM) operated at 5.0 kV. The BET surface area was obtained with a high resolution BET equipment described in Wang et al.35,36 Ammonia temperature programmed desorption (NH3TPD) was employed for the measurement of acid amounts of the catalysts. After saturated adsorption of NH3, NH3-TPD was started at a heating rate of 15 °C/min from 100 to 650 °C. The reactor was connected to an online GC 7890II gas chromatograph with a thermal conductivity detector (TCD) and a Porapak T (5 m) column connected in parallel with a TDX01 (3 m) column. The Porapak T column was used for the separation of DME, MeOH, and H2O. The TDX-01 column was used for the separation of H2, CO, and CO2. The concentrations of these six gases were analyzed. CO2 conversion, DME yield, MeOH yield, and CO yield were defined as follows: XCO2 )

nCO2,in - nCO2,out

YDME )

nCO2,in 2nDME,out nCO2,in

YMeOH ) YCO )

nCH3OH,out nCO2,in nCO,out nCO,in

(1)

(2)

(3)

(4)

where nDME, nCO, and nCO2 were the molar flow rates of DME, CO, and CO2. In and out denotes inlet and outlet molar flow rates. The space time yields of DME and MeOH, which gave the amounts of DME and MeOH produced per gram catalyst per second, were defined and calculated using the following relations: STYDME ) YDME × SVCO2 × MDME × 0.001 (kg/g)/22.4 (L/mol)/2 (5) STYMeOH ) YMeOH × SVCO2 × MMeOH × 0.001 (kg/g)/22.4 (L/mol) (6)

where YDME and YMeOH were the yields of DME and MeOH, SVCO2 was the space velocity of CO2, and MDME and MMeOH were the molar masses of DME and MeOH. Data on catalyst stability are given in the Supporting Information. The conversion of CO2 changed from 0.268 to 0.255 during 145 h of DME synthesis. In this period, the carbon mass balance over the reactor closed to within experimental error, that is, there was no obvious carbon deposition on the catalyst. The thermodynamics of DME synthesis and methanol synthesis reactions were also studied. The equilibrium constants at different temperatures were calculated using Van Hoff equation, and these were used to calculate the equilibrium conversions and yields for given initial conditions. The details of the calculation have been given in the work of An et al.37 3. DME Synthesis from CO2 Hydrogenation For DME synthesis from CO2 and H2, there are four independent stoichiometric reactions, namely Methanol synthesis from CO2 CO2 + 3H2 ) CH3OH + H2O

(7)

Methanol synthesis from CO CO + 2H2 ) CH3OH

(8)

CO + H2O ) CO2 + H2

(9)

Water gas shift Methanol dehydration 2CH3OH ) CH3OCH3 + H2O

(10)

Equations 7–9 comprise methanol synthesis, for which the fibrous CuO-ZnO-Al2O3-ZrO2 catalyst was active. For methanol dehydration, shown as eq 10, HZSM-5 was the catalyst. As shown in Figure 2a, the mechanical mixed (with a ratio of 2.0) catalyst contained two kinds of particles. HZSM-5 particles with a regular crystal morphology was distributed among the fibers of the CuO-ZnO-Al2O3-ZrO2 catalyst. The high resolution SEM image showed that the CuO-ZnO-Al2O3-ZrO2 (12:6:1:1) catalyst particles were fibrous, with an aspect ratio over 25 (Figure 2b). The CuO-ZnO-Al2O3-ZrO2 catalyst was fibrous due to Zr incorporation, which had two effects on the metallic function used for methanol synthesis.17,38 One effect was a phase separation effect, to which was attributed the fibrous agglomerate morphology and the slow rate for Cu/Zn sintering. The second effect, to which was attributed the high activity of a Cu/Zn/Al/Zr catalyst, was an effect of ion doping and valence compensation. Zr4+ dissolved in ZnO crystal caused the formation of positive ion defects on the surface of Cu-ZnO. These defects can adsorb Cu+ and form and stabilize more active sites, Cu0-Cu+-O-Zn2+, on the catalyst surface. Thus, although the catalyst differed from a conventional methanol synthesis catalyst in the Zr incorporation, they are chemically similar in terms of the active sites. This Cu-based catalyst had a BET surface area of 70.9 m2/g and existed as regular one-dimensional nanomaterial agglomerates, some of which were attached to the surface of the HZSM-5 particles (Figure 2a). Since water is produced in DME synthesis, HZSM-5, which is a more hydrothermal stable catalyst than Al2O3, was used in this work. The HZSM-5 catalyst had a BET surface area of 450.0 m2/g. The amount of Bronsted sites was measured by NH3-TPD to be 0.630 mmol/g. From Figure 2, it can be deduced that the two kinds of catalyst were mixed at the micrometer scale. The pore size distribution and activity stability data of the bifunctional catalyst can be

6550 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008

Figure 3. (a) Relationship between the reaction temperature and CO2 conversion, DME yield, and methanol yield. (b) Comparison of CO2 conversion with the methanol synthesis thermodynamic limit, DME synthesis thermodynamic limit, and experimental results for DME synthesis. The space velocity was 6000 mL/(g cat · h), the ratio of H2 to CO2 was 3, and the pressure was 5 MPa.

Figure 2. (a) SEM image of the CuO-ZnO-Al2O3-ZrO2 + HZSM-5 physically mixed bifunctional catalyst. Regular HZSM-5 catalyst particles were distributed among the CuO-ZnO-Al2O3-ZrO2 particles. (b) High resolution image of the fibrous CuO-ZnO-Al2O3-ZrO2 catalyst in part a.

found in the Supporting Information. The bifunctional catalyst had a BET surface area of 167 cm2/g, pore volume of 0.351 m3/g for pores between 1.7 and 300 nm, and average pore diameter of 12.1 nm. The pore volume data probably includes a contribution from the mesopores that were present between the fibrous CuO-ZnO-Al2O3-ZrO2 catalyst particles. To evaluate the performance and kinetics of DME synthesis by CO2 hydrogenation, various factors, including the temperature, pressure, and space velocity, considered the main factors for DME synthesis were tested and found to have a large effect on methanol synthesis. The reaction condition intervals used have been chosen to be similar to those used previously in the literature.39,40 A stoichiometry ratio of CO2 and H2 with 3:1 was used. The details of the dependence on these factors are given below. 3.1. Effect of Reaction Temperature. DME synthesis takes place at temperatures from 483 to 543 K. Methanol synthesis,

water gas shift, and methanol dehydration occurred in the same reactor. The relationship between the reaction temperature and CO2 conversion or DME or methanol yield is shown in Figure 3a. The reaction rate increased when the temperature was higher, and more CO2 was converted into DME and methanol. At 523 K, CO2 conversion reached 0.256, and it increased to 0.280 at 543 K. With the bifunctional catalyst used here, methanol was dehydrated on the HZSM-5 catalyst which continuously removed the product of methanol synthesis; thus, when the temperature was higher than 523 K, the CO2 conversion was higher than that determined by the thermodynamics of methanol synthesis. The thermodynamics limited conversion in methanol synthesis is shown in Figure 3b. The DME yield and methanol yield, which are the middle and lower lines in Figure 3a, showed trends similar with CO2 conversion. DME yield increased from 6.44% at 483 K to 15.8% at 543 K, which is 146% higher. The methanol yield increased from 1.48% at 483 K to 5.38% at 543 K, which is 263% higher. As the temperature was increased, the DME yield increased faster than the methanol yield. This was because the activity of the HZSM-5 catalyst increased faster with temperature than did the Cu-based catalyst. The process did not reach thermodynamic equilibrium under these conditions, so it was controlled by the kinetics, and a high temperature, which would increase the reaction rates, was needed for a high conversion of CO2. Previous experiments with the Cu-based catalyst used here have shown that it can be

Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6551

Figure 4. Relationship between the reaction pressure and CO2 conversion, DME yield, and methanol yield. The space velocity was 6000 mL/(g cat · h), the ratio of H2 to CO2 was 3, and the temperature was 523 K.

deactivated at temperatures above 523 K by sintering. Thus, 523 K was the maximum temperature that could be used. 3.2. Effect of Reaction Pressure. From the thermodynamics, a high pressure is beneficial for DME production from CO2. Experimental CO2 conversion and DME and methanol yields are compared in Figure 4. At 2.0 MPa, the CO2 conversion was just 0.195. When the pressure was increased to 5.0 MPa, the CO2 conversion was increased to 0.255. The yield of methanol and DME also showed similar trends: methanol yield was 0.025 and 0.046 at 2.0 and 5.0 MPa, respectively. DME yields were 0.026 and 0.149 at 2.0 and 5.0 MPa, respectively. Since the process was controlled by the kinetics, a high pressure, which meant that the concentrations were increased, was needed for a high conversion of CO2. The figure also indicated that the DME yield increased faster than methanol yield, which showed that the HZSM-5 activity increased more sensitive than the CuO-ZnO-Al2O3-ZrO2 activity with pressure. Too high a reaction pressure has a higher requirement for the material of the facility and also poses a safety problem. A pressure no more than 5.0 MPa is recommended from our tests. 3.3. Effect of Space Velocity. The space velocity, which is a parameter that reflects the reactor efficiency, was also tested with a H2/CO2 ratio of 3 at the pressure of 5.0 MPa. Space velocities of 1000 to 10000 mL/(g cat · h) were used to test the catalytic behavior. The results are shown in Figure 5. At the space velocity of 1000 mL/(g cat · h), the conversion of CO2 was about 0.309. When this was increased to 10000 mL/(g cat · h), the conversion of CO2 decreased to 0.203. At a higher space velocity, the residence time was shorter. The DME yield decreased faster than the methanol yield, which showed that the reactions on HZSM-5 were more affected than CuO-ZnO-Al2O3-ZrO2 by the space velocity or residence time. The CuO-ZnO-Al2O3-ZrO2 + HZSM-5 catalyst had its highest DME and methanol yield at 523 K at a space velocity of 1000 mL/(g cat · h) and a pressure of 5 MPa. 4. Kinetics for Methanol Synthesis from CO2 Hydrogenation 4.1. Kinetic Modeling. Various kinetic models have been proposed for this process. From the reported kinetic data, it can be deduced that reaction 9 is fast and would be at thermodynamic equilibrium. Although the studied reaction was CO2 hydrogenation, due to reaction 9 both CO and CO2 coexist in the reactor. From studies on CuO-ZnO-Al2O3 catalyst, it is commonly believed that, due to this, the rate will be increased.

Figure 5. Relationship between the space velocity and CO2 conversion, DME yield, and methanol yield. The ratio of H2 to CO2 was 3, the temperature was 523 K, and the pressure was 5 MPa.

Among the various reported kinetic models for methanol synthesis, Graaf’s model 41,42 gave the best fit with our experimental data. Although Graaf’s model was developed using a CuO-ZnO-Al2O3 catalyst, it is probable that there was no significant difference in the reaction mechanism with the CuO-ZnO-Al2O3-ZrO2 catalyst used here. The difference between the catalysts was that the catalyst here contained Zr; it was noted above that the effect of Zr incorporation was to increase the number of active sites, but this did not change the nature of the active sites. In this work, the form of the kinetic expressions used was the same, but the kinetic parameters in these expressions were fitted with our experimental data. In Graaf’s kinetic model, the assumption was made that the rate limiting steps are CO hydrogenation, reversed water-gas shift reaction, and CO2 hydrogenation, respectively. The details have been given in the work of Graaf et al.41,42 The kinetic rate equations for 7–9 were as follows: r1 )

k1KCO fCOfH23/2 - fCH3OH/(fH21/2Kf1)

[

]

(1 + KCOfCO + KCO fCO )[fH 2

2

1/2 2

+ (KH2O/KH21/2)fH2O

] (11)

r2 )

k2KCO2(fCO2fH2 - fH2OfCO/Kf2)

(1 + KCOfCO + KCO fCO )[fH 2

2

1/2 2

+ (KH2O/KH21/2)fH2O

] (12)

r3 )

k3KCO2 fCO2fH23/2 - fCH3OHfH2O/(fH23/2Kf3)

[

(1 + KCOfCO + KCO fCO )[fH 2

2

1/2 2

+ (KH2O/KH2

] )fH O]

1/2

2

(13) The second reaction was methanol dehydration. Kinetic models for methanol dehydration have shown that the kinetics can depend on the catalyst. Considering that the same HZSM-5 catalysts were used in the kinetics developed by Tao et al.,43 this was the kinetic model adopted for this work. In the Tao kinetic model for methanol dehydration, the rate limiting step was assumed to be the surface reaction between two adjacent methanol molecules. The kinetic rate equation for 10 was as follows: r4 )

k4(fCH3OH2 - fDMEfH2O/Kf4) 1 + K′H2OfH2O + KCH3OHfCH3OH

(14)

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Figure 6. Comparison of the kinetic model results and experimental results of CO2 conversion and DME and methanol yields. Table 1. Regression Parameters for the Kinetic Model (Graaf and Tao’s Model) on the CuO-ZnO-Al2O3-ZrO2/HZSM-5 Bifunctional Catalyst Ki ) Ai exp(Bi/RT) ki ) Ai exp(-Ei/RT) parameters KCO KCO2 KH2O/KH20.5 KH2O′ KCH3OH k1 k2 k3 k4

Bi or -Ei

Ai -11

8.3965 × 10 1.7214 × 10-10 4.3676 × 10-12 8.3503 9.4630 4.0638 × 10-6 9.0421 × 108 1.5188 × 10-33 1.8966 × 10-3

1.1827 × 105 8.1287 × 104 1.1508 × 105 3.8867 × 10-5 1.1273 × 10-3 -1.1695 × 104 -1.1286 × 105 -2.6601 × 105 4.0841 × 103

where Kf1, Kf2, Kf3, and Kf4 are the equilibrium constants of the four reactions, respectively. They were determined by the thermodynamic model proposed by Wang.41 KCO, KCO2, KH2O/ KH20.5, KH2O′, and KCH3OH are the adsorption equilibrium constants of CO, CO2, H2O, H2, and CH3OH.44 The regression analysis was based on the data shown in Figures 3–5. The expressions, ki ) Ai exp(-Ei/RT), are the rate constants of each individual reaction. Experimental data from the fixed bed reactor was used to fit the kinetic model, and the fugacity in 11–14 was assumed equal to the partial pressure. Parameter estimation was performed using the Matlab software using a program based on the Nelder-Mead algorithm. The goodness of the fit was checked with the χ2 statistics and consistency tests provided in the software. The objective function in the parameter estimation was the value of F defined as follows: M

F)

∑ [(X j)1

CO2,exp - XCO2,cal

)2 + (YDME,exp - YDME,cal)2 + (YCO,exp - YCO,cal)2] (15)

where X and Y were conversions. The conversions were calculated by integrating the kinetic rate expressions. The conversions calculated with the kinetic model are compared with the experimental results in Figure 6. The fit was good with an average deviation of about 6%. The optimized kinetic parameters are listed in Table 1. Actually, the number of adjustable parameters was large, and thus, although the regressed fit was good, this did not prove the kinetic models. In this work, the main concern was on reactor modeling, and the reaction conditions of the simulated reactor were not very different from those of the reactor used in the experiments.

Figure 7. Effect of 3% CO substitution for CO2 on DME synthesis. The space velocity was 6000 mL/(g cat · h), the pressure was 5 MPa, and the ratio of H2 to CO2 and CO was 3. The yield of methanol and DME increased with the addition of CO.

4.2. Simulation of CO Cycle Process with the Kinetic Model. Since the kinetics over the CuO-ZnO-Al2O3-ZrO2 + HZSM-5 catalyst had been quantified, a process for DME synthesis can be simulated. Above, it was noted that although the studied reaction was CO2 hydrogenation, due to the fast water gas shift reaction, CO will also be present in the reactor. It was of interest to see if the rates were affected if a feed also contained CO. An example was performed with a calculation where 3% of the CO2 was substituted with CO, and the calculation result was shown in Figure 7. The value of 3% was chosen arbitrarily. It can be seen that both the DME and methanol yields increased. With a higher temperature, the increase was more obvious. At 540 K, the yields of DME and methanol were increased 37% and 6.5%, respectively (Figure 7). It is probable that CO addition into the reactant feed decreased the water concentration by the water gas shift reaction, and the reduced water competition for adsorption sites enabled more CO2 and H2 molecules to absorb and the yield of methanol was increased. Also, DME production is from methanol dehydration, in which overall reaction would be faster with a lower water concentration. Since CO is inevitably produced during DME production in the one-step single reactor, the cofeed CO that would increase the product yield can be obtained from the reactor effluent by recycling back into the reactor to accelerate DME production. Thus, a process with CO recycle was designed as shown in Figure 8. The gases CO, and unreacted H2 and CO2, can be separated in the first distiller and recycled to mix with the process feed. DME is obtained at the cooler. Methanol is obtained at the second distillation tower. Although no experimental data are available yet on the recycling process, the simulation results indicated that CO addition will increase the space time yields of methanol and DME, and are an incentive to actually perform it. There was no CO in the product in the simulated results, but this was because this was specified by the simulation. From the reaction kinetics and the reactor size, the product composition can be calculated for a fixed (yCO + yCO2)/yH2 value, temperature, and pressure. The calculation was performed by iteration by requiring that the CO amount that exits the reactor be equal to the CO amount that enters the reactor. The simulated reactor had a size the same as that used in the catalyst evaluation, a space velocity of 6000 mL/(g cat · h), ratio of H2 to CO2 and CO of 3, and ratio of CO to CO2 of 3:22. From the reactor simulation, the relationship between the reaction pressure and temperature can be obtained and this is

Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6553

Figure 8. Proposed CO2 hydrogenation process with CO recycle in the system. The inlet gases of the process were CO2 and H2, and the output gases of the process were methanol and DME.

and DME were also increased. From the simulation results, CO recycle is important for increasing the space time yield of DME, and this is a reason for it in addition to the conventional reason to avoid the unnecessary loss of a feed material. 5. Conclusion A CuO-ZnO-Al2O3-ZrO2 + HZSM-5 physical mixture bifunctional catalyst was used to catalyze CO2 hydrogenation. High CO2 conversion, DME yield, and methanol yield were obtained. The kinetic parameters were obtained by regression from experimental data. A process with CO recycle for CO2 hydrogenation was proposed. Acknowledgment

Figure 9. Relationship between pressure and temperature for a process with CO recycle for DME synthesis obtained from process simulation with the kinetic model.

The authors gratefully acknowledge the financial support by the Chinese National Science Foundation (Nos. 20576060 and 20606021) and by the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20050003030). Notation DME ) dimethyl ether fj ) fugacity of component j, atm Kfi ) equilibrium constant for reaction i Kj ) equilibrium constant for adsorption of component j LPG ) liquefied petroleum gas M ) mole mass, g/mol MeOH ) methanol n ) molar flow rate, mol/s P ) pressure, MPa r ) reaction speed, mol/(g cat · s) STY ) space time yield, g/(g cat · h) SV ) space time velocity, ml/(g cat · h) T ) temperature, °C/K X ) conversion Y ) yield

Figure 10. Space time yields of DME and methanol from CO2 hydrogenation with/without CO recycle. With CO recycling, the space time yields of DME and methanol were much increased.

shown in Figure 9. When the temperature increased, the operating pressure was also increased. The DME yield and methanol yield from the proposed operation is shown in Figure 10. The bottom two lines show the yields obtained without CO recycle and the upper two lines show the yields with CO recycle. The yields when the CO was recycled in the process were much higher. At the same time, the space time yields of methanol

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ReceiVed for reView January 6, 2008 Accepted June 6, 2008 IE800777T