Zeolite-Y Catalyst for One-Step Synthesis

Energy & Fuels 2008, 22, 2877–2884

2877

Characterization of Cu-Mn/Zeolite-Y Catalyst for One-Step Synthesis of Dimethyl Ether from CO-H2 Xiu-Juan Tang, Jin-Hua Fei,* Zhao-Yin Hou, Xiao-Ming Zheng, and Hui Lou* Institute of Catalysis, Key Lab of Applied Chemistry of Zhejiang ProVince, Zhejiang UniVersity, Hangzhou, 310028, Zhejiang, People’s Republic of China ReceiVed August 24, 2007. ReVised Manuscript ReceiVed June 11, 2008

Cu-Mn/zeolite-Y catalysts prepared using a coprecipitation impregnation procedure have been investigated to develop active Cu-based catalysts for the dimethyl ether synthesis from syngas. A great enhancement of CO conversion and DME selectivity is observed on Mn-containing catalysts compared to its Mn-free counterpart. However, an excess addition of manganese suppresses the syngas-dimethyl ether (STD) activity. Manganese has been found to be interacting with copper to form copper manganese mixed oxides phase (Cu1+xMn2-xO4) upon calcinations, and the excess addition of copper or manganese will induce the segregation of copper or manganese as the isolated CuO or Mn2O3 phase, respectively. The segregates CuO phases enhanced the reducibility of the Cu1+xMn2-xO4 phase and the Mn2O3 phase by reducing spill-over species. Harder reduction of the mixed oxides leads to the formation of much smaller metallic Cu particles, and MnO remaines closely associated with copper particles after reduction treatment, acting as a site for stability against CO2 oxidation of the surface copper. Furthermore, it is found that addition of Mn to Cu/zeolite-Y catalyst decreases the average crystalline size of copper to the same extent. Combining the TPR and XRD analysis, it suggests that catalysts with the excess CuO phases in conjunction with copper manganese mixed oxides are much more active than that with excess Mn2O3 phases together with copper manganese mixed oxides in the hydrogenation of CO to DME.

1. Introduction Conventionally, DME has been produced by methanol dehydration, but in the 1980s, a novel method called the STD process (synthesis gas-dimethyl ether) was developed for the direct synthesis of DME from syngas in a single reactor with hybrid catalysts (methanol synthesis catalyst and methanol dehydration catalyst).1 In comparison to the conventional method, the STD process is attracting more and more academic and industrial attention for its dramatically theoretical and economic significance.2,3 The principal reactions involved in the STD process consist of methanol synthesis, methanol dehydration, and water-gas shift (WGS) methanol synthesis reaction: CO + 2H2 T CH3OH ∆H ) -90.29 kJ/mol (1) methanol dehydration reaction: 2CH3OH T CH3OCH3+H2O ∆H ) -23.41 kJ/mol (2) water-gas shift reaction: H2O + CO T CO2+H2 ∆H ) -41.2 kJ/mol (3) Driven by reaction 2, reaction 1 can proceed more completely. Water produced by reaction 2 is consumed immediately by reaction 3, and both reactions 2 and 3 can be promoted directly or indirectly. Reactions 1 and 3 are catalyzed by a methanol * To whom correspondence should be addressed. E-mail: fjh210@ zju.edu.cn (J.-H.F.); [email protected] (H.L.). (1) Fujimoto, K.; Asami, K.; Shikada, T.; Tominaga, H. Chem. Lett. 1984, 2050. (2) Gogate, M. R.; Lee, S.; Kulik, C. J. Fuel Sci. Technol. Int. 1990, 10, 281. (3) Lee, S.; Gogate, M. R.; Kulik, C. J. Fuel Sci. Technol. Int. 1995, 13, 1039.

synthesis catalyst, and reaction 2 is catalyzed by a methanol dehydration catalyst.4,5 These synergetic effects of reactionreaction result in high once-through conversion of syngas and low cost of DME production.6 Mechanistic studies of the water-gas shift reaction7 and methanol synthesis8 have indicated that the oxidation of carbon monoxide to carbon dioxide can be an elementary step in these processes. Moreover, the most widely used commercial catalyst for the oxidation of carbon monoxide is the mixed copper manganese oxide catalyst, CuMn2O4 (hopcalite). This prompted us to consider the use of mixed copper manganese oxide as a potential catalyst for the one-step synthesis of dimethyl ether from CO hydrogenation. There has been a great deal of fundamental work devoted to clarifying the role played by each component;9–11 however, it is clear that many controversial issues remain to be resolved. For instance, the reducibility of copper oxides is promoted or hindered by the existence of manganese and which phase after calcination is beneficial for various reactions. Herein, we report the characterization and catalytic evaluation of Cu-Mn/zeolite-Y catalysts for the direct synthesis of (4) Aguayo, A. T.; Erenã, J.; Sierra, I.; Olazar, M.; Bilbao, J. Catal. Today 2005, 106, 265. (5) Kim, J. H.; Park, M. J.; Kim, S. J.; Joo, O. S.; Jung, K. D. Appl. Catal., A 2004, 264, 37. (6) Wang, Z. L.; Wang, J. F.; Diao, J.; Jin, Y. Chem. Eng. Technol. 2004, 24, 507. (7) Rhodes, C.; Hutchings, G. J.; Ward, A. M. Catal. Today 1995, 23, 43. (8) Waller, D.; Stirling, D.; Stone, F. S.; Spencer, M. S. J. Chem. Soc., Faraday Discuss. 1989, 87, 107. (9) Woellner, A.; Lange, F.; Schmelz, H.; Knoezinger, H. Appl Catal. 1993, 94, 181. (10) Buciumana, F. C.; Patcasa, F.; Hahn, T. Chem. Eng. Proc. 1999, 38, 563.

10.1021/ef800259e CCC: $40.75  2008 American Chemical Society Published on Web 08/06/2008

2878 Energy & Fuels, Vol. 22, No. 5, 2008

Tang et al.

Figure 2. Nitrogen adsorption isotherms of (A) zeolite-Y, (B) Cu2Mn1/ zeolite-Y, and (C) Cu1Mn2/zeolite-Y.

catalysts were prepared using a coprecipitation impregnation procedure to obtain the most active one. 2. Experimental Section

Figure 1. XRD patterns of calcined (a) Cu1Mn2/zeolite-Y, (b) Cu1Mn1/ zeolite-Y, (c) Cu2Mn1/zeolite-Y, (d) Cu3Mn1/zeolite-Y, and (e) Cu/ zeolite-Y catalysts. Table 1. Characterization Data of the CuxMny/Zeolite-Y Catalysts sample Cu/ Cu3Mn1/ Cu2Mn1/ Cu1Mn1/ Cu1Mn2/ zeolite-Y zeolite-Y zeolite-Y zeolite-Y zeolite-Y CuO wt % (ICP-AES) dCuO XRD (nm)

72.53

60.32

50.34

33.63

20.22

51

29

26

22

nd

dimethyl ether from syngas. The nature and interaction between copper and manganese as well as surface compositional analysis was determined by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). The activity of copper-manganese mixed oxide catalysts is substantially dependent upon the preparation methods, with the most active being the one prepared by a coprecipitation procedure.12 In the present work, the hybrid

(11) Morales, M. R.; Barbero, B. P.; Cadu´s, L. E. Appl. Catal., B 2006, 67, 229.

2.1. Catalyst Preparation. All of the catalysts tested in this study were prepared using a coprecipitation impregnation procedure. Aqueous solutions of Cu(NO3)2 · 3H2O and Mn(NO3)2 · 6H2O were premixed and dropped with Na2CO3 (1 mol/L) solution at the same time into a zeolite-Y slurry of water, which was continuously stirred while the temperature was maintained at 60 °C. The pH value of the slurry was controlled at about 7 during the coprecipitation. The resulting precipitate was then aged in the medium for 2 h. The precipitate was then filtered and washed several times with distilled water until no further Na+ was observed in the washings. The precipitate was dried at 160 °C for 4 h and subsequently calcined in air at 450 °C for 4 h to give the final catalyst. Cu-Mn/zeolite-Y catalysts with various Cu/Mn ratios were denoted as CuxMny/zeoliteY, where x and y represent the atomic ratio of Cu and Mn, respectively. The CuxMny/zeolite-Y catalyst for DME synthesis was obtained with methanol synthesis components and the zeolite-Y at a weight ratio of 2.5:1. 2.2. Characterization. Copper content was determined in an inductively coupled plasma mass spectrometer (Leeman Prodigy 4015). XRD analysis was performed on a Rigaku automated power X-ray diffractometer system (Cu KR radiation, 45 kV, 40 mA) (Rigaku RINT 2005, Japan). The average crystallite size of CuO was determined by the Sherrer equation: Dhkl ) kλ/β cos θ (where θ is the angle of diffraction, constant k ) 0.89, λ ) 0.1541 nm, and β is the half-peak width (radian) from the full-width at halfmaximum of the most intensive (111) peak. The morphology of the samples was examined by scanning electron microscopy (SEM515, Philips). Nitrogen adsorption and desorption isotherms were determined at -196 °C by means of an automated adsorption apparatus (OMNIISORP 100CX) from which Brunauer-Emmett-Teller (BET) surface areas were calculated. Samples were pretreated in high vacuum at 200 °C for 2 h. A temperature-programmed reduction with H2 (H2-TPR) experiment was performed on a catalyst characterization system (AMI200, Pittsburgh, PA), which was equipped with an online mass spectrum. The catalyst (10 mg, 60-80 mesh) was charged in a U-shaped quartz cell. After pretreatment in argon at 400 °C for 1 h, the sample was reduced under a 5% H2/Ar stream (40 mL/ min) from 50 to 700 °C with a ramp of 10 °C/min. The amount of H2 in the effluent was analyzed by an online mass spectrum (m/e 2) and recorded as a function of the temperature. H2-CO2-H2 (12) Puckhaber, L. S.; Cheung, H.; Cocke, D. L.; Clearfield, A. Solid State Ionics 1989, 32/33, 206.

Characterization of Cu-Mn/Zeolite-Y Catalyst

Energy & Fuels, Vol. 22, No. 5, 2008 2879

Table 2. Chemical Compositions and Pore Structures of the Catalysts CuO and MnO loading (wt %)

porosity characteristics

catalysts

CuO

MnO

BET surface area (m2/g)

zeolite-Y Cu2Mn1/zeolite-Y Cu1Mn2/zeolite-Y

50.34 20.22

23.48 47.96

302.1 132.1 90.6

redox cycles were carried out in the same characterization system (AMI-200, Pittsburgh, PA). A redox cycle consisted of three consecutive steps: (i) the sample (20 mg) was reduced under a 10% H2/Ar stream (50 mL/min) from 50 to 500 °C with a ramp of 10 °C/min and maintained at this temperature for 0.5 h (TPR1); (ii) the sample was cooled down to room temperature in the flow of He and then re-oxidized under a stream of CO2 (30 mL/min) at 300 °C for 1 h; and (iii) the sample was cooled to room temperature under He flow and a second standard TPR as described in step i (TPR2). The acidity and acidic strength of the samples were determined by temperature-programmed desorption of ammonia (NH3-TPD). About 0.10 g of the pelletized samples (60-80 mesh) was activated in the reactor at 400 °C in a flow of helium gas for 40 min. After the pretreatment, the sample was cooled to 100 °C and saturated with NH3 gas. As soon as the baseline in the gas chromatograph

micropore volume (cm2/g)

meso/macropore volume (cm3/g)

0.1708 0.0605 0.0249

0.137 0.286 0.127

was stable, the NH3-TPD was carried out under a constant flow of He (50 mL/min) from 100 to 500 °C at a heating rate of 10 °C/min. The concentration of ammonia in the exit gas was monitored continuously by a gas chromatograph with a TCD detector. XPS experimenets were carried out on a VG ESCALAB 2201XL spectrometer. Nonmonochro Mg KR radiation (hν ) 1253.6 eV) are used as a primary excitation. In general, the X-ray anode was run at 250 W, and the high voltage was kept at 14.0 kV with a detection angle at 54°. The binding energies are calibrated with the C1s level of adventitious carbon (284.6 eV) as the internal standard reference. 2.3. Catalytic Testing. The catalysts tested for the synthesis of dimethyl ether from syngas were carried out in a high-pressure microreactor (MCRS8400, China). The pressure in the reactor was maintained by means of a back-pressure regulator (the total pressure of the reactor is adjusted by a back-pressure regulator on the exit line), and the flow rates of feed gases were controlled using mass flow controllers. For security reason, there is a release valve on the inlet gas lines in case the exit was blocked by powder. The catalyst (20-40 mesh, 2 mL, about 1.6 g) was charged into a stainless-steel fixed-bed reactor and then reduced in H2 at 270 °C for 3 h under atmospheric pressure. After the reactor was cooled to 160 °C, the reactant gases, H2 and CO, at a controlled mole ratio of 3:2, were purged to 2.0 M Pa pressure. Typical reaction conditions were at 250 °C, 2.0 M Pa, 1500 h-1, and n(H2)/n(CO) ) 3:2. The products in the effluent were analyzed by an online gas chromatograph with a thermal detector (HP5890, series II, Raleigh, NC).

3. Results and Discussion

Figure 3. TPR profiles of CuxMny/zeolite-Y catalysts with different ratios of Cu/Mn: (a) 0.5, (b) 1, (c) 2, (d) 3, and (e) Cu/zeolite-Y.

Figure 4. In situ XRD patterns of sample Cu1Mn1/zeolite-Y during reducing treatment: (b) CuO, (3) Mn2O3, (0) Cu1.5Mn1.5O4, (]) MnO, and (/) Cu.

3.1. Catalyst Composition and Nitrogen Porosimetry. The X-ray diffraction results for the CuxMny/zeolite-Y with different Cu/Mn molar ratios are shown in Figure 1. The Cu1Mn2/ zeolite-Y catalyst comprises copper-manganese oxide (Cu1.5Mn1.5O4), together with Mn2O3 as a minor phase. The spinel-like Cu1.5Mn1.5O4 phase is a nonstoichiometric form of CuMn2O4 (also called hopcalite), and it is written as Cu1+xMn2-xO4 or CuxMn3-xO4.13 With the increase of the Cu/

Figure 5. TPR profile of the re-oxidized Cu2Mn1/zeolite-Y catalyst.


Tang et al.

Figure 6. XPS spectra of (A) Cu 2p lines, (B) Mn 2p lines, and (C) O 1s lines. From bottom to top, samples are Cu1Mn2/zeolite-Y and Cu2Mn1/ zeolite-Y catalysts. Table 3. XPS Binding Energy (eV) of the Core Electron and Atomic Ratio in Fresh Catalysts sample Cu2Mn1/zeolite-Y Cu1Mn2/zeolite-Y

BE (O 1s) BE (Cu 2p3/2) BE (Mn 2p3/2) (eV) (eV) Mn/Cu (eV) 529.9 532.8 530.2 532.9

933.5

641.8

0.89

931.3 934.2

642.4

3.52

Mn ratio up to 1:1, the reflections from the CuO phases are apparently observed with respect to the Cu1Mn2/zeolite-Y catalyst. With a further increase of the Cu/Mn ratio, the diffraction lines of Mn2O3 disappear and the CuO phase becomes predominant in addition to the minor copper-manganese mixed oxide (Cu1.5Mn1.5O4). It is found that a segregation of

CuO or Mn2O3 as an independent phase is expected when the content of copper or manganese is in excess for the formation of Cu1.5Mn1.5O4. It is expected that the intensity of reflection lines from the CuO phase will be much higher and the reflections characteristic of Cu1.5Mn1.5O4 will be much lower with a relatively increasing content of copper and relatively decreasing content of manganese. X-ray diffraction was also used to estimate the mean CuO crystalline size. The reflection at 2θ ) 38.8° corresponds to the CuO (111) plane, and it was used to calculate the average metal oxide crystalline size by the Scherrer equation. From the results listed in Table 1, it suggests that the average crystalline size of CuO is remarkably dropped with the addition of manganese and a further decrease in the mean


Figure 7. NH3-TPD profiles of CuxMny/zeolite-Y catalysts with different ratios of Cu/Mn: (a) 0.5, (b) 1, (c) 2, and (d) 3.

crystalline size is observed with a gradually increasing content of manganese. The N2 adsorption-desorption isotherms of the catalysts are presented in Figure 2. From the isotherms of calcined Y-typed zeolite, it can be seen that new pores formed in the range of meso- and macropores. These secondary meso/macropores resulted from the reorganization of the zeolitic framework during dealumination and also from the generation of larger aggregates from small primary zeolitic particles capable of inducing textural porosity (interparticle voids). The adsorption isotherm of the Y-typed zeolite-supported samples still displays the characteristic features of microporous materials (sharp knee at P/P0 lower than 0.1 because of the filling of micropores) but shows, in addition, a steep nitrogen uptake increase at P/P0 ∼ 0.8-1.0, which is indicative of the increase of the meso/macropores because of a higher loading amount of metallic oxide components. Table 2 gives the material properties measured by N2 adsorption/desorption analysis and ICP-AES analysis, which gives the Cu and Mn loading amount, BET surface areas, and micropore volumes of zeolite-Y, Cu2Mn1/zeolite-Y, and Cu1Mn2/ zeolite-Y. The N2 adsorption data presented in Table 2 show that the micropore volume decreases markedly from 0.1708 cm2/g for zeolite-Y to 0.0605 cm2/g for Cu2Mn1/zeolite-Y and 0.0249 cm2/g for Cu1Mn2/zeolite-Y. The decrease in micropore volume could be ascribed to the Cu and Mn species supported inside the surpercage or sodalite cage of Y-type zeolite. The specific surface area of zeolite-Y is found to be 302.1 m2/g and decreases rapidly to 214.2 and 130.7 m2/g for Cu2Mn1/zeolite-Y and Cu1Mn2/zeolite-Y, respectively. In addition, the decrease in specific surface area is also because some of the micropores are blocked by particles of Cu and Mn species. Incidentally, it can be found that the Cu2Mn1/zeolite-Y catalyst has much more micropore and meso/macropore volumes than Cu1Mn2/zeoliteY, indicating the micropores are less blocked when the relative content of copper is higher. 3.2. Reducibility of the Calcined Catalysts. The TPR profiles of CuxMny/zeolite-Y catalysts with different Cu/Mn molar ratios were used in identifying the reducible species presented (seen Figure 3). Under the reducing conditions including a large amount of H2, the CuO and mixed copper (13) Hutchings, G. J.; Mizaei, A. A.; Joyner, R. W.; Siddiqui, M. R. H.; Taylor, S. H. Appl. Catal., A 1998, 166, 143.


manganese oxides are reduced to Cu/MnO.14 The CuO particle size has a considerable influence on the reduction behavior.15 For larger CuO particles, the reduction of near-surface CuII species can occur at lower temperatures, but the reduction of the bulk then appears at higher temperatures. As for the Cu/ zeolite-Y catalyst, the particle size of CuO is larger than the other Mn-doped catalyst; thus, a main reduction peak (namely, β) appearing at higher temperatures is expected. However, for the CuxMny/zeolite-Y catalysts with Cu/Mn ) 3, 2, and 1, a main reduction peak centered at 240 °C (namely, β) with a minor shoulder peak at 188 °C (namely, R) and another main reduction peak at 264 °C (namely, γ) can be observed. The intensity of the β peak decreased with the decrease of the ratio of Cu/Mn. However, the intensity of the γ peak appears with the opposite trend. With the further decrease of the ratio of Cu/Mn to 0.5, a new major reduction peak at 326 °C (namely, δ) appears and γ peak became minor. According to the XRD analysis results from Figure 1, the β peak can be attributed to the reduction of bulky CuO, and the R peak is the reduction of the small grains and amorphous CuO or near-surface CuII species, which cannot be detected by XRD. The γ peak can be assigned to the reduction of mixed copper and manganese oxide, and the δ peak is the reduction of Mn2O3. To confirm the attribution of the TPR peaks, in situ XRD measurements of Cu1Mn1/zeolite-Y catalyst reduced under a 5% H2/Ar stream at different reducing stages were carried out (Figure 4). Evidently, mixed copper and manganese oxide and Mn2O3 phases are not reduced until most of the small CuO grains and bulk CuO are reduced. In addition, the phases identified after reduction correspond to MnO and Cu0, as reported previously.16,17 It is interesting to find that the position of the β peak relative to the bulky CuO remains unchanged; on the other hand, the γ peak shifts to a lower reduction temperature with the relative increase of the copper content. At the same time, it should be noted that the reduction temperature of δ and γ peaks are much higher than the others at the absence of the CuO phase. Thus, it can be deduced that the reduction of oxide phases related to manganese, such as copper-manganese mixed oxide (Cu1.5Mn1.5O4) and Mn2O3, was promoted in the presence of CuO. It is well-known that copper oxides are reduced much more easily than manganese oxides under the same reduction conditions.14,18 As soon as CuO is reduced to metallic copper, it will enhance the reducibility of manganese species by reducing spill-over species. However, manganese will not affect the reduction of copper species with the exclusion of the strong interaction between copper and manganese to form a mixed oxide phase, especially the spinellike phase. In the case of the copper-manganese mixed oxide (Cu1.5Mn1.5O4) phase, the TPR results show that manganese retards the reduction of copper, as indicated by the shift of the corresponding peak to higher temperatures with an increasing manganese content. It can be inferred that the β and γ peaks will overlap to one peak with the further increase of copper loading. (14) Tanaka, Y.; Utaka, T.; Kikuchi, R.; Takeguchi, T.; Sasaki, K.; Eguchi, K. J. Catal. 2003, 215, 271. (15) Guo, Y.; Meyer-Zaika, W.; Muhler, M.; Vukojevic, S.; Epple, M. Eur. J. Inorg. Chem. 2006, 23, 4774. (16) Tanaka, Y.; Utaka, T.; Kikuchi, R.; Sasaki, K.; Eguchi, K. Appl. Catal., A 2003, 242, 287. (17) Reddy, A. S.; Gopinath, C.; Chilukuri, S. J. Catal. 2006, 243, 278. (18) Alonso, L.; Palacios, J. M.; Garcıá, E.; Moliner, R. Fuel Process. Technol. 2000, 62, 31. (19) Fierro, G.; Lo Jacono, M.; Inversi, M.; Porta, P.; Cioci, F.; Lavecchia, R. Appl. Catal., A 1996, 137, 327. (20) Meliań-Cabrera, I.; Granados, Lo´pez, M.; Fierro, J. L. G. Catal. Lett. 2002, 79, 165.


Tang et al.

Figure 8. SEM images of (A) Cu2Mn1/zeolite-Y and (B) zeolite-Y. Table 4. Activity of Different CuxMny/Zeolite-Y Catalysts for DME Synthesis via CO Hydrogenationa selectivity (mol %) Cu/Mn (molar ratio)

conversion of CO (mol %)

DME

CH3OH

CO2

HCs

∞ (Cu/zeolite-Y) (3:1) 2 (2:1) 1 (1:1) 0.5 (1:2)

7.7 25.8 25.6 25.4 13.7

5.9 53.4 56.9 62.6 63.0

10.9 3.9 3.3 3.7 3.6

22.8 35.3 34.3 30.0 30.2

60.4 7.4 5.5 3.7 3.2

a Reaction conditions: pressure, 2.0 MPa; temperature, 245 °C; GHSV, 1500 h-1; n(H2)/n(CO), 3:2.

H2/CO2/H2 cycles have been used by Fierro and co-workers19,20 as a tool to determining the resistance of reduced Cu against

oxidation. These cycles were also carried out in our experiments to determine the influence of Mn on the redox properties of the copper phase in the reduced catalysts. The fact that the oxidized Cu species, produced by treatment with CO2 in the H2/CO2/H2 cycles, can be related to similar oxidized species found under reaction conditions (CO/H2/CO2/H2O) gives additional relevance to these kinds of experiments. The TPR profiles of calcined and re-oxidized Cu2Mn1/ zeolite-Y catalysts at 300 °C after the preliminary measurement, which are denoted as TPR1 and TPR2, are compared in Figure 5. As for TPR1, three reduction peaks can be observed (namely, R, β, and γ, respectively). The R peak is the reduction of the small CuO particles; the β peak can be attributed to the reduction


of bulky CuO; and the γ peak is the reduction of Cu1.5Mn1.5O4 as originally assigned. However, for TPR2, three kinds of reducible species in the calcined catalyst, which is characterized by three reduction peaks, have transformed to two kinds, which are represented by a two-peak reduction at much lower temperature after heat treatment at 300 °C (i.e., first reduction followed by oxidation at 300 °C for 1 h). The two peaks (λ and η) in TPR2 might be attributed to the reduction of small CuO particles and bulky CuO particles, respectively. Apparently, application of heat treatment enhanced the reducibility of Cu species in the CuxMny/zeolite-Y catalyst. The reduction temperature and the peak width are an indication of the ease of the reduction and the degree of interaction between the different species, respectively. A high reduction temperature indicates difficulty in reduction; a wide peak indicates a high degree of interaction between species; and the area of reduction peak indicates the extent of reduction.21 It can be seen that the peak area of reducible copper species associated with the small particles becomes larger, and the β peak in the TPR1 is sharper and larger than that in the TPR2 (η). This phenomenon demonstrates that copper and manganese atoms redistributed because new sites of low energy became available and MnO may migrate to the surface during reduction-oxidation processes. Therefore, the amount of re-oxidized copper by CO2 was significantly lower, which leads to a decrease in the reduction peak area in TPR2. Meanwhile, it can be deduced that manganese and a portion of copper are in physical contact after the reduction process, and the reduction of coppermanganese mixed oxides, such as Cu1.5Mn1.5O4, which has low reducibility, gives rise to small CuO particles. This is probably because Mn dispersed atomically near the Cu sites in the spinel lattice can retard Cu agglomeration under mild reduction conditions.14 From the analysis of H2/CO2/H2 cycles, it suggests that Mn stabilizes the copper particles against CO2 oxidation and this is beneficial to the catalytic performance, because it has been reported that CO2 tends to oxidize the active copper metal during the reaction.22The same reasoning could be applied to the H2O byproduct. 3.3. XP Spectroscopic Analysis. Chemical states of surface atoms in the catalysts were investigated by XPS as depicted in Figure 6. Meanwhile, the binding energies of Cu 2p3/2 and Mn 2p3/2 and O 1s and the derived atomic ratios in the different catalysts are summarized in Table 3. The Cu2Mn1/zeolite-Y catalyst calcined in air shows a Cu 2p3/2 transition with a symmetric main peak (EB ≈ 933.5) and a typical intense satellite peak on the higher binding energy side, pointing to the Cu2+ chemical state. The binding energy of the main Mn 2p3/2 line appears at ca. 641.8 eV, intermediate between that characteristic of Mn3+ species and that characteristic of Mn4+ species.23 This agrees with previously reported results that suggest that compounds of similar composition (spinel-like Cu1+xMn2-xO4 phase) contain both Mn3+ and Mn4+.24 As for the Cu1Mn2/zeolite-Y catalyst, the Cu 2p3/2 spectrum is characteristic by a relatively narrow line at 931.3 eV and a broader line at 934.3 eV. An evident shakeup satellite structure is observed at 941.2 and 944.2 eV. The narrow peak at a lower binding energy can be associated with the presence of Cu+, (21) Yeragi, D. C.; Pradhan, N. C.; Dalai, A. K. Catal. Lett. 2006, 112, 139. (22) Chinchen, G. C.; Waugh, K. C.; Whan, D. A. Appl. Catal. 1986, 25, 101. (23) Gautier, J. L.; Rıós, E.; Gracia, M.; Marco, J. F.; Gancedo, J. R. Thin Solid Films 1997, 51, 311. (24) Kukuznyak, D. A.; Han, S. -W.; Lee, M. -H.; Omland, K. A.; Gregg, M. C.; Stern, E. A.; Ohuchi, F. S. J. Vac. Sci. Technol., A 2001, 19, 1923.


while the broader peak and the satellite structure are indicative of Cu2+.24 The level Mn 2p3/2 appears as a single peak at 642.4 eV assigned to Mn3+ in Mn2O3.18 According to XRD analysis, it is composed of the Cu1+xMn2-xO4 phase, whose surface is covered with a Mn-rich oxidic coat. The XPS spectra of the O 1s core level for the catalysts are shown in Figure 6. Level O 1s appears split in two peaks, one at 529.8-531.0 eV assigned to O2- ions in oxides and another at 532.7-533.0 eV assigned to OH- ions. As can be observed in Table 2, the Mn/Cu atomic ratio was 0.89 and 3.52 for Cu2Mn1/zeolite-Y and Cu1Mn2/zeolite-Y, respectively. In the two cases, the surface Mn/Cu ratio is slightly higher than that in bulk, with the effect being particularly striking for the Cu1Mn2/zeolite-Y sample. The result suggests that Mn has a tendency for surface segregation. 3.4. NH3-TPD Measurements. The NH3-TPD spectra of the hybrid catalysts are shown in Figure 7. All of the catalysts show only one main similar desorption peak at 280 °C and have a long tailing, which indicates that the number and strength of the acid sites on the bifunctional catalysts are almost independent of the adjustment of the ratio of Cu/Mn. Moreover, the hybrid catalyst is shown to be moderately acidic. Actually, it can be expected that the strength and distribution of the acid sites will not be significantly influenced by just altering the ratio of Cu/ Mn, while the loading amount of metallic components are all kept constant. It is well-known that the acidity of the catalysts for dimethyl ether synthesis derives from the support, such as Y zeolite in the CuxMny/zeolite-Y catalysts. During the preparation process, some amount of the metallic ions might be exchanged into the inner pore and/or supercage of Y zeolite, just as confirmed by nitrogen adsorption-desorption isotherm analysis. However, most of the precipitants will cover mainly on the outside of the support because of the higher loading amount of methanol synthesis components, which is also supported by the morphology characterization of the samples as shown in Figure 8A. It can be clearly seen from the SEM images that the morphology of the dual catalyst is a bit more different from that of Y zeolite because of the loading of metallic components. 3.5. CO Hydrogenation Test. Table 4 lists the catalytic activity of CuxMny/zeolite-Y catalysts for DME synthesis via CO hydrogenation. It is known that, if the solid acid catalyst is active enough to convert the methanol to DME, then the overall catalytic performance will be influenced by the nature of methanol synthesis component; in reverse, if the acidity of the solid acid catalyst is not strong enough to convert effectively the methanol to DME, then the CO conversion and the selectivity for DME will both be affected.25,26 According to the analysis of acidity of the hybrid catalysts, it is a fact that the acidity of the catalysts is independent of the change in the ratios of Cu/Mn. Therefore, the overall catalytic properties will be controlled by the methanol synthesis component. It can be seen from Table 3 that the conversion of CO is very low and the selectivity of HCs predominates for the Cu/zeolite-Y catalyst. Apparently, the conversion of CO is significantly promoted with the addition of manganese with respect to the Cu/zeolite-Y catalyst. It presents better catalytic performance, and the conversion of CO reaches 25.4% over catalyst Cu/Mn ) 3. The catalytic activity remains unchanged within a certain content of manganese. However, the conversion of CO over catalyst Cu/Mn ) 0.5 is dramatically dropped with an excess addition (25) Mao, D.; Yang, W.; Xia, J. J. Catal. 2005, 230, 140. (26) Wang, L. G.; Qi, Y.; Wei, Y. X.; Fang, D.; Meng, S. H.; Liu, Z. M. Catal. Lett. 2006, 106, 61.


of manganese. At the same time, it shows that the selectivity to hydrocarbons (HCs) is slightly higher when the content of copper is higher. From the foregoing discussion on catalyst characterization of N2 adorsption-desorption isotherms and NH3-TPD measurements, it can be inferred that the production of hydrocarbons is related to the acidity of the catalysts as well as the porosity of the catalysts, in a manner analogous to the synthesis of hydrocarbons through hydrogenation of carbon dioxide/monoxide on hybrid catalysts consisting of a hydrogenation component with zeolite.27 It is well-known that the selectivity of CO2 is originated from WGSR. Tanaka et al. have found that the Cu-Mn spinel oxide showed excellent WGSR activity comparable to that of conventional Cu/ZnO/Al2O3 despite its low surface area.16 One of the mechanisms proposed for WGS involved the transfer of an oxygen from water to carbon monoxide via the catalytic surface. This redox mechanism can thus be written as two reactions28 H2O + M ) MO + H2 CO + MO ) CO2 + M Results from study of Idem and Bakhshi29 showed that the Cu0-Mn2O3 pair are the active sites on which H could be obtained by dissociation of water. This is consistent with the observation of Cybulski.30 According to this author, the presence of oxidized active sites (such as Cu0-ZnO and Cu0-MnO pairs) promotes the dissociation of adsorbed water. Therefore, it is reasonable to deduce that manganese oxide catalyzes the water-gas shift reaction, which is responsible for the higher CO conversion and CO2 selectivity compared to the manganesefree catalyst. It has been confirmed by TPR measurements of the calcined catalysts and heat treatment of the Cu2Mn1/zeolite-Y catalyst discussed earlier that manganese stays in physical (27) Jeon, J.-K.; Jeong, K.-E.; Park, Y.-K.; Ihm, S.-K. Appl. Catal., A 1995, 124, 91. (28) Wainwright, M. S.; Trimm, L. L. Catal. Today 1995, 23, 29. (29) Idem, R. O.; Bakhshi, N. N. Ind. Eng. Chem. Res. 1995, 34, 1548. (30) Cybulski, A. Catal. ReV. Sci. Eng. 1994, 36, 557.

Tang et al.

contact with copper even after the reduction treatment. Therefore, Cu0-MnO pairs play a significant role in catalyzing the conversion of CO to CO2 through the WGS reaction. The reason why the catalytic activity of the Cu1Mn2/zeolite-Y catalyst is much lower than the other Mn-containing catalyst is probably that the reduced catalyst is depleted of metallic copper species on the surface, which supply atomic hydrogen to all of the hydrogenation steps. In other words, manganese would migrate to the surface during reduction treatment and some of the active sites become blocked by excessive MnO coverage results in a lowering of the activity, as evidenced by XPS measurement. 4. Conclusions The XRD measurements have shown that the interaction between CuO and Mn2O3 results in the formation of the spinellike Cu1+xMn2-xO4 phase, and the excess addition of copper or manganese will induce the segregation of copper or manganese as the isolated CuO or Mn2O3 phase, respectively. Meanwhile, the TPR analysis indicates that the spinel-like Cu1+xMn2-xO4 phase has a lower reducibility than segregated CuO species. Interestingly, the presence of segregated CuO enhanced the reducibility of spinel-like Cu1+xMn2-xO4 and Mn2O3 phases. It is found that the segregated CuO phase in addition to the copper-manganese mixed oxide (Cu1.5Mn1.5O4) phase in the calcined Cu-Mn/zeolite-Y catalysts contributes to the catalytic performance of DME from CO hydrogenation. In other word, Cu0-MnO pairs in the reduced Cu-Mn/zeolite-Y catalysts may promote the dissociation of adsorbed water, which is essential for catalyzing the water-gas shift reaction. Acknowledgment. The authors are indebted to the Hi-Tech Research and Development Program of China (2007 AA 05Z415) and the National Basic Research Program of China (2007 CB 210207) for financial support. EF800259E

Zeolite-Y Catalyst for One-Step Synthesis

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