Energy Fuels 2010, 24, 804–810 Published on Web 01/05/2010
: DOI:10.1021/ef901133z
Enhanced Catalytic Performance for Dimethyl Ether Synthesis from Syngas with the Addition of Zr or Ga on a Cu-ZnO-Al2O3/γ-Al2O3 Bifunctional Catalyst Suk-Hwan Kang,† Jong Wook Bae,* Hyo-Sik Kim, G. Murali Dhar, and Ki-Won Jun* Petroleum Displacement Technology Research Center, Korea Research Institute of Chemical Technology (KRICT), Post Office Box 107, Yusong, Daejon 305-600, South Korea. †Present address: Plant Engineering Center, Institute for Advanced Engineering (IAE), Suwon, Kyonggi-do 443-749, Korea. Received October 6, 2009. Revised Manuscript Received December 10, 2009
Direct synthesis of dimethyl ether (DME) from synthesis gas was investigated on bifunctional catalysts, containing Cu-ZnO-Al2O3 promoted with Zr or Ga on γ-Al2O3 as a methanol dehydration catalyst. In comparison to an unpromoted bifunctional catalyst, promoted catalysts with Zr or Ga showed a higher catalytic performance and stability. The facile reducibility of copper particles with high dispersion and an appropriate acidity and electronic state of copper species are mainly responsible for the superior catalytic performance with the help of structural promoting effects of Ga or Zr components on Cu-ZnO-Al2O3. The high formation rate of methanol by CO hydrogenation on the Zr-promoted Cu-ZnO-Al2O3 and the dehydration rate of methanol to DME on the modified acidic sites of γ-Al2O3 are mainly responsible for the superior catalytic activity and stability.
mechanically mixing the above two components7,8 or coprecipitation with Cu-ZnO-Al2O3 in a slurry of acidic components of γ-alumina.6 Although the commercial production of DME from syngas is composed of two steps involving synthesis of methanol from syngas and dehydration of methanol to DME,6,8-11 a single-step synthesis of DME from syngas on a bifunctional catalyst is receiving attention because of the simplicity of the process by just using one fixed bed or a slurry reactor. Many researchers have reported bifunctional catalytic systems by developing sophisticated preparation/ pretreatment methods, and catalytic functionality is usually explained by adopting the dispersion and crystallinity of the Cu particle, the acidic properties of the solid-acid catalyst, and the adsorption behavior of H2O and CO2.9-11 Promoters such as Ga and Zr12-15 on a Cu-ZnO-based methanol synthesis catalyst have been widely investigated, and their contribution to enhance the catalytic activity has been reported mainly by the increased dispersion of copper speices. In the present investigation, we report the single-step synthesis of DME from syngas over a bifunctional catalyst that is modified properly to enhance DME selectivity by adding Zr or Ga promoters on a Cu-ZnO-Al2O3 methanol synthesis catalyst during a co-precipitation step in a slurry of
1. Introduction Conversion of synthesis gas (syngas) derived from coal, biomass, and natural gas to dimethyl ether (DME) is one of the promising methods to obtain a renewable clean energy source. The application of DME as an alternative diesel fuel has been proposed recently because of its superior character of low NOx emission and near-zero smoke evolution compared to traditional diesel fuel.1 In addition, many investigators have devoted their attention to this process, owing to the potential use of DME as a useful intermediate for the production of chemicals such as dimethyl sulfate, methyl acetate, hydrogen, and light olefins.2 For the direct synthesis of DME from syngas, there are three important reactions involved: (i) methanol formation by CO hydrogenation and/or CO2 hydrogenation, (ii) dehydration of methanol to DME and H2O, and (iii) reaction of H2O with CO to form CO2 and H2 by the water-gas shift (WGS) reaction.3 In the case of using a bifunctional catalyst that is usually containing simultaneously two active sites for methanol synthesis and subsequent dehydration to DME, reactions of (i) and (iii) occur simultaneously on the methanol synthesis component of Cu-ZnO-Al2O3 and reaction (ii) occurs for methanol dehydration on solidacid catalysts, such as γ-alumina, ZSM-5, and modified alumina.4-6 The bifunctional catalyst could be prepared by
(7) Takeguchi, T.; Yanagisawa, K. I.; Inui, T.; Inoue, M. Appl. Catal., A 2000, 192, 201–209. (8) Peng, X. D.; Wang, A. W.; Toseland, B. A.; Tijm, P. J. A. Ind. Eng. Chem. Res. 1999, 38, 4381–4388. (9) Pokrovski, K. A.; Bell, A. T. J. Catal. 2006, 241, 276–286. (10) Kang, S. H.; Bae, J. W.; Jun, K. W.; Potdar, H. S. Catal. Commun. 2008, 9, 2035–2039. (11) Sai Prasad, P. S.; Bae, J. W.; Kang, S. H.; Lee, Y. J.; Jun, K. W. Fuel Process. Technol. 2008, 89, 1281–1286. (12) Suh, Y. W.; Moon, S. H.; Rhee, H. K. Catal. Today 2000, 63, 447–452. (13) Saito, M.; Fujitani, T.; Takeuchi, M.; Watanabe, T. Appl. Catal., A 1996, 138, 311–318. (14) Liu, X. M.; Lu, G. Q.; Yan, Z. F.; Beltramini, J. Ind. Eng. Chem. Res. 2003, 42, 6518–6530. (15) Toyir, J.; de la Piscina, P. R.; Fierro, J. L. G.; Homs, N. Appl. Catal., B 2001, 34, 255–266.
*To whom correspondence should be addressed. Telephone: þ82-42860-7383 (J.W. Bae); þ82-42-860-7671 (K.-W. Jun). Fax: þ82-42-8607388 (J.W. Bae); þ82-42-860-7388 (K.-W. Jun). E-mail: finejw@ krict.re.kr (J.W. Bae);
[email protected] (K.-W. Jun). (1) Ng, K. L.; Chadwick, D.; Toseland, B. A. Chem. Eng. Sci. 1999, 54, 3587–3592. (2) Ge, Q.; Huang, Y.; Qiu, F.; Li, S. Appl. Catal., A 1998, 167, 23–30. (3) Wang, L.; Fang, D.; Huang, X; Zhang, S.; Qi, Y.; Liu, Z. J. Nat. Gas Chem. 2006, 15, 38–44. (4) Xia, J.; Mao, D.; Zhang, B.; Chen, Q.; Zhang, Y.; Tang, Y. Catal. Commun. 2006, 7, 362–366. (5) Moradi, G. R.; Nosrati, S.; Yaripor, F. Catal. Commun. 2007, 8, 598–606. (6) Bae, J. W.; Potdar, H. S.; Kang, S. H.; Jun, K. W. Energy Fuels 2008, 22, 223–230. r 2010 American Chemical Society
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The catalytic activity was measured in a tubular fixed-bed reactor [12.7 mm outer diameter] with a catalyst loading of 1.0 g, as shown in Figure 2. Prior to the reaction, the bifunctional catalyst was reduced in a flow of 5% H2 balanced with helium and the synthesis gas (H2/CO=2) including Ar as an internal standard gas was simultaneously fed into the reactor. The reaction conditions were T= 250 °C, P=5.0 MPa, and space velocity (SV)=4000 mL gcat-1 h-1. The products were analyzed on an online gas chromatograph (Donam model DS 6200) using a thermal conductivity detector (TCD; analysis for Ar, CO, and CO2) and a flame-ionized detector (FID; analysis for hydrocarbons, DME, CH3OH, etc.). The CO conversion and product distribution are calculated using the following equations based on mol %: conversion of CO ðmol %Þ ¼ ½ðinlet moles of CO outlet moles of COÞ=inlet moles of CO 100
ð1Þ
product distribution ðmol %Þ ¼ ðmoles of produced components; such as DME,
Figure 1. Preparation scheme of the bifunctional catalyst.
CH3 OH, etc:=total moles of productsÞ 100
γ-Al2O3. In our previous investigation,6 a bifunctional catalyst containing γ-Al2O3 and Cu-ZnO-Al2O3 components was found to show high DME selectivity compared to the simply admixed bifunctional catalyst. However, the additional effect of incorporated Zr or Ga on a Cu-ZnO-Al2O3 catalyst to the catalytic performance of a bifunctional catalyst has not been established well to date. The objectives of this investigation are to understand the additional influence of a Zr or Ga promoter on a Cu-ZnO-Al2O3/γ-Al2O3 bifunctional catalyst with respect to the high catalytic performance and stability for a single-step synthesis of DME from syngas at the similar reaction conditions of methanol synthesis.
ð2Þ
yield of DME ðmol %Þ ¼ ðCO conversion production distribution of DMEÞ=100 ð3Þ 2.2. Catalyst Characterization. The Brunauer-EmmettTeller (BET) surface area and pore size distribution of the bifunctional catalyst were determined by N2 physisorption using a Micromeritics ASAP2400 apparatus at the liquid-N2 temperature (-196 °C). The pore volume was determined at a relative pressure (P/Po) of 0.99. The calcined catalyst was degassed at 300 °C in a He flow for 2 h before measurement. The pore size distribution of the bifunctional catalyst was calculated using the Barett-Joyner-Halenda (BJH) model from the data of the desorption branch of nitrogen isotherms. The temperature-programmed desorption of ammonia (NH3-TPD) was carried out to determine the surface acidity of the bifunctional catalyst. About 0.1 g of sample were flushed previously with He flow at 250 °C for 2 h, cooled to 100 °C, and saturated with NH3. After NH3 exposure, the sample was purged with He flow until reaching equilibrium and then the TPD experiment was carried out from 100 to 700 °C at a heating rate of 10 °C/min with TCD. The metallic copper surface area was measured using the N2O surface titration method. Prior to N2O titration, the sample was reduced at 250 °C for 4 h with 5% H2/N2 flow. The consumption of N2O and the evolution of N2 on the metallic copper sites (N2O þ 2Cu=Cu2O þ N2) were analyzed by TCD. The surface area of copper was calculated by assuming 1.46 1019 Cu atoms/m2 and a molar stoichiometry N2O/Cus (Cu atom on surface) of 0.5. The powder X-ray diffraction (XRD) pattern for the calcined bifunctional catalyst was obtained with a Rigaku diffractometer using Cu KR radiation to identify the phases of the bifunctional catalyst and their crystallinity. To carry out temperature-programmed reduction (TPR) experiments with hydogen, the catalyst was previously pretreated in He flow up to 250 °C and kept for 2 h to remove the adsorbed water and other contaminants, followed by cooling to 50 °C. The 5% H2/He mixture was passed over the sample at a flow rate of 30 mL/min, with a heating rate of 10 °C/min up to 500 °C. The effluent gas was passed over a molecular sieve to remove the water generated and analyzed by GC equipped with TCD. The Fourier transform infrared (FTIR) experiment to observe the adsorbed CO species was carried out using Nicolet 6700, which was equipped with a Ge/KBr beam splitter and a Quantum detector (Thermo Electron GmbH, Germany).
2. Experimental Section 2.1. Catalyst Preparation and Activity Measurement. To prepare a bifunctional Cu-ZnO-Al2O3-(Ga or Zr)/γ-Al2O3 catalyst for single-step synthesis of DME from syngas, γ-Al2O3 as a solid-acid catalyst for methanol dehydration to DME was previously prepared from aluminum isopropoxide (Al[OCH(CH3)2]3, AIP), acetic acid (AA), and 2-propanol as starting precursors by the sol-gel method, as precisely described in our previous works.6,16 Initially, AIP was dissolved in 2-propanol under continuous stirring conditions. The molar ratio of AA/ AIP was fixed to 0.1, whereas that of H2O/AIP was fixed to 6. The product was washed several times with 2-propanol and finally dried at 80 °C in vacuum for 12 h. Finally, the product was calcined in a flow of air at 500 °C for 5 h with a heating rate of 2 °C/min. The final surface area of γ-Al2O3 is found to be around 412.8 m2/g. The bifunctional catalyst was prepared subsequently by a co-precipitation method in a slurry of γ-Al2O3 at 70 °C. An aqueous solution containing precursors of Cu, Zn, Al, and promoter (copper acetate, zinc acetate, aluminum nitrate, and zirconyl nitrate or gallium nitrate, with a weight ratio of CuO/ZnO/Al2O3/promoter=71.9:21.1:3.3:0 or 3.7) was used to precipitate with Na2CO3 solution at a final pH of ∼7. The precipitate was further aged for 3 h at 70 °C after coprecipitation followed by calcination at 300 °C for 5 h. The preparation scheme of the bifunctional catalyst is summarized in Figure 1. The final catalyst contains 83.3 wt % Cu-ZnO-Al2O3-promoter and 16.7 wt % Al2O3. The bifunctional catalysts are denoted as CZA-X, where C, Z, and A represent CuO, ZnO, and Al2O3 with the solid-acid component of γ-Al2O3, respectively, and X denotes the type of promoter, such as Zr or Ga species. (16) Kim, S. M.; Lee, Y. J.; Bae, J. W.; Potdar, H. S.; Jun, K. W. Appl. Catal., A 2008, 348, 113–120.
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Figure 2. Schematic diagram of the reaction system.
Table 1. Textural Properties, Particle Size, and Cu Surface Area of Bifunctional Catalysts Cu surface areaa (m2/g)
acidic sites (mmol of NH3/g) notationb
surface area (m2/g)
pore volume (cm3/g)
average pore diameter (nm)
T1
T2
T3
total
particle sizec (nm)
before reaction
after reaction
CZA CZA-Zr CZA-Ga
107 139 157
0.273 0.509 0.513
10.5 13.8 11.1
0.185 0.190 0.271
0.285 0.357 0.398
0.975 2.108 1.967
1.445 2.655 2.636
15.2 11.3 10.2
3.6 12.0 14.4
2.5 7.1 6.9
a The Cu surface area [defined as SCu = exposed metallic copper area (m2/g)] was measured by the N2O surface titration method. b The bifunctional catalysts are denoted as CZA-X, where CZA represents CuO, ZnO, and Al2O3 co-precipitated on a solid-acid component of γ-Al2O3, respectively, and X denotes the type of promoter, such as Zr or Ga. c The particle size of copper oxide was calculated from fwhm of the XRD diffraction peak at 2θ = 35.5° for CuO.
The self-supported pellet of the bifunctional catalyst was reduced at 250 °C for 4 h under H2 flow and, subsequently, purged with He for 40 min. IR spectra of adsorbed CO species at 50 °C were collected at that temperature after purging with He flow for 10 min. The characters of surface copper species and their electronic state after reaction were further characterized using X-ray photoelectron spectroscopy (XPS, ESCALAB MK-II) analysis. During the experiments, the AlKR monochromatized line (1486.6 eV) was adopted and the vacuum level was kept around 10-7 Pa. The used bifunctional catalyst after passivation with 1 vol % O2 balanced with N2 was characterized, and the binding energy (BE) was corrected with a reference of C1s (284.4 eV). The surface morphology of the bifunctional catalyst after reaction was also characterized using scanning electron microscopy [SEM, JEOL (JSM6700F)]. Figure 3. Pore size distribution of the calcined bifunctional catalyst.
3. Results and Discussion
co-precipitated bifunctional catalyst at steady state.17 All bifunctional catalysts show a bimodal pore size distribution, with the average pore diameter ranging from 10.5 to 13.8 nm, as shown in Figure 3, and the average pore diameter is largest on the CZA-Zr bifunctional catalyst. The largest average pore diameter on CZA-Zr is mainly attributed to the possible contribution for the structural promoting effect of the Zr component than that of Ga. Furthermore, these characteristics are beneficial for easy mass transfer of reactants and products and eventually resulted in showing high catalytic performance. The presence of a larger pore on CZA-Zr is also attributed to the facile formation of the intragrain structure of Cu-ZnO-Al2O3 with a small particle
3.1. Textural Properties of the Bifunctional Catalyst. The BET surface area, pore volume, and average pore diameter of bifunctional catalysts are shown in Table 1. The BET surface area of the bifunctional catalysts is found to be in the range of 107-157 m2/g with the following order: CZAGa > CZA-Zr > CZA. Previous studies reveal that there is no direct relationship between the BET surface area and catalytic activity of the bifunctional catalyst;5,10 however, it is strongly related to the copper surface area in the (17) (a) Bae, J. W.; Kang, S. H.; Lee, Y. J.; Jun, K. W. Appl. Catal., B 2009, 90, 426–435. (b) Bae, J. W.; Kang, S. H.; Lee, Y. J.; Jun, K. W. J. Ind. Eng. Chem. 2009, 15, 566–572.
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Figure 4. TPR profiles of the calcined bifunctional catalyst.
size because of the strong ZrO2-CuO interaction originating from oxygen vacancies of ZrO2.6,15 The average pore diameter is found to be on the order of CZA-Zr > CZAGa > CZA, and the presence of a large pore above 7 nm is found to be similar on bifunctional catalysts. 3.2. Reducibility and Particle Size of Copper Species. The activity of the bifunctional catalyst strongly depends upon the reducibility of copper species as well as acidity.17 TPR is a useful technique to reveal the reduction behavior of copper species on a bifunctional catalyst. In Figure 4, the TPR profile exhibits only one broad reduction peak, appearing in the range of 100-500 °C. In general, a clear single reduction peak, without any shoulder, indicates that CuO species are distributed homogeneously in the CZA matrix on the γ-Al2O3 surface.10 Whereas the low reducibility with the presence of a shoulder peak at a higher temperature region could be the main reason for exhibiting lower catalytic activity, which could be due to the strong interaction of the CZA particle with the γ-Al2O3 surface. The CZA catalyst shows the highest reduction temperature of Tmax at 336 °C, which implies difficult reducibility of copper species. In the case of promoted CZA-Zr and CZA-Ga catalysts, Tmax shows 301 and 317 °C, respectively. This suggests that the copper oxides with promoters are reduced more easily to metallic copper than that on the CZA catalyst because of the formation of smaller particles. The promoters, such as Zr or Ga, are known to enhance the catalytic activity on the methanol synthesis reaction because of the increased dispersion of copper,14,15 and its superior performance is attributed to the geometric effect of the promoter by forming a strong ZrO2-CuO interaction originating from oxygen vacancies of ZrO2.15 In addition, a series of Cu-ZnO-Ga-supported catalysts for CO2 hydrogenation to methanol were previously invetigated, and the Ga promoter is found to have some catalytic property for the inverse spillover of hydrogen.13 The XRD patterns of the calcined bifunctional catalyst revealed the sharp diffraction peaks corresponding to CuO and ZnO phases, as shown in Figure 5. The peaks of CuO were observed in the range of 35° < 2θ < 37° and 47° < 2θ < 49°. The peak at 31° < 2θ < 32° could be assigned to a ZnO particle. The particle sizes of CuO are calculated from Scherrer’s equation [the value of full width at half-maximum (fwhm) at 2θ=35.5] and summarized in Table 1. In addition, the copper surface area measured by the N2O surface titration method is also shown in Table 1. The particle size of the unpromoted CZA catalyst is found to be 15.2 nm for CuO, and the particle sizes of the promoted CZA-Zr and CZA-Ga catalysts are found to be 11.3 and 10.2 nm,
Figure 5. XRD patterns of the calcined bifunctional catalyst.
Figure 6. NH3-TPD of the calcined bifunctional catalyst.
respectively. It could be noted that, the smaller particle size of CuO, the higher the metallic surface area of copper.18 To verify the correlation, the metallic surface area of copper before and after reaction on a bifunctional catalyst was further evaluated by the N2O titration method. As shown in Table 1, an almost 3 times higher metallic surface area on the promoted bifunctional catalysts was observed around 2.5 m2/g for CZA, 7.1 m2/g for CZA-Zr, and 6.9 m2/g for CZA-Ga. Therefore, the promoted bifunctional CZA-Zr and CZA-Ga catalysts possessing high copper dispersion could have favorable characteristics for methanol synthesis compared to the unpromoted CZA catalyst. 3.3. Acidity of Bifunctional Catalysts. To elucidate surface acidity, a NH3-TPD experiment was carried out on the calcined bifunctional catalysts. As shown in Figure 6, the pattern displays three distinct peaks in the region of 100230 °C (T1), 230-320 °C (T2), and above 320 °C (T3). According to our previous investigations,11,17 T1 corresponds to the acidity contributed by the support matrix alone and T2 is due to catalyst particles present on the surface of γ-Al2O3. However, T3 corresponds to water produced by the combination of the inherent hydroxyl groups of γ-Al2O3 with oxygen of the bifunctional catalyst or by the presence of strong acidic sites. From the summarized NH3-TPD results, as shown in Table 1, the amount of acidic sites of T1 and T2 (18) Topsøe, N. Y.; Topsøe, H. Top. Catal. 1999, 8, 267–270.
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Figure 7. FTIR spectra of adsorbed CO on the reduced bifunctional catalyst.
Figure 8. XPS spectra of the used bifunctional catalyst.
could play an important role for the methanol dehydration to DME4 and these appropriate acidic sites are abundant on the promoted bifunctional catalyst. The quantitative amount of acidic sites for T1 and T2 is around 0.470, 0.547, and 0.669 mmol of NH3/g for CZA, CZA-Zr, and CZA-Ga, respectively, and these values are smaller than that of bare γ-Al2O3 (around 1.000 mmol of NH3/g, as reported in our previous work16) because of the possible blockage of acidic sites by CZA particles. Therefore, the observed high acidity on the promoted bifunctional catalyst is mainly attributed to the formation of small CZA particles by adding the promoters, such as Zr or Ga,14,15 with low possibility of γ-Al2O3 surface blockage.17 A higher dehydration activity of methanol on CZA-Zr and CZA-Ga, which are possessing large acidic sites (assigned to T1 and T2 peaks), not only increased DME selectivity by overcoming thermodynamic limitation but also helped to enhance the overall CO conversion by increasing WGS activity.11 3.4. Studies by FTIR and XPS Analyses. The variation of the CO adsorption strength on Cu/ZnO measured by FTIR provides useful information about different surface structures and electronic states of metallic copper.18 As shown in Figure 7, adsorbed CO molecules at different band positions in the frequency range of 2000-2100 cm-1 reveal differently adsorbed CO species. There are two distinct bands at 2100 and 2150 cm-1 on the CZA-Ga catalyst; however, there is only band with a high intensity on the CZA-Zr catalyst at the 2100 cm-1 region compared to that of low intensity on the unpromoted CZA catalyst. The band at around 2100 cm-1 is generally attributed to a linearly bonded CO molecule on Cu0 species, and its significant intense band on CZA-Zr and CZA-Ga catalysts implies the presence of an abundant active CO species that is responsible for exhibiting high CO conversion. Interestingly, the high intensity of adsorbed CO at 2101 cm-1 on the CZA-Zr catalyst is mainly attributed to the increased dispersion of Cu species because of the strong ZrO2-CuO interaction originating from oxygen vacancies of ZrO2.15 XPS analysis is a powerful technique to identify the surface phase and electronic state present in Cu-ZnO-based bifunctional catalysts. The Cu2p spin-orbit doublet after reaction is shown in Figure 8. It could be seen that Cu2p3/2 and Cu2p1/2 peaks are clearly resolved, and the presence of satellite peaks are observed in all bifunctional catalysts.
The binding energies (BEs) on the bifunctional catalyst were in the range of values that have been reported earlier.19 The spectra of used bifunctional catalysts are expected to contain metallic Cu, CuO, Cu2O, and Cu(OH)2 because of the oxidation by water during the reaction or passivation by oxygen before measurement. The distinguishing features of Cu2þ(d9) and Cuþ(d10) species on the catalyst surface could be verified by the presence of satellite peaks. The satellite peaks are absent for Cu2O species, while they are prominent for CuO or Cu(OH)2 species.19 In the present investigation, Cu2p3/2 spectra have shown a very broad line for CZA-Zr and CZA-Ga catalysts and they are discernible from a comparable sharp peak on the CZA catalyst. Furthermore, the BE of Cu2p3/2 is higher for the CZA catalyst. The changes in BE could be attributed to the presence of Zr or Ga promoters and γ-Al2O3, which could alter the BE through a metal-support or metal-promoter interaction. On the basis of the above discussion, there is evidence for the abundant presence of Cu2þ species on the used bifunctional catalysts, although Cu metal and Cu2O may also be present to various degrees because of the oxidation of metallic copper. However, the shift in BE of Cu2p3/2 to a lower value and the suppression of satellite peak intensity on the promoted bifunctional catalysts are mainly attributed to the presence of the Cuþ and/or Cu0 phase. In addition, the synergistic effect on methanol synthesis could be obtained by the co-presence of a two-dimensional Cu0-Cuþ layer, which causes a strong molecular adsorption instead of dissociative adsorption of CO (not active for methanol synthesis) through electron back-donation from metal to CO 2π*. This synergistic effect is further enhanced by the presence of Zn(2-δ)þ because of the stabilization of Cuþ.12,20-22 These studies indicate that the bifunctional catalysts promoted with Zr or Ga are more suitable for methanol synthesis because of the co-existence of the Cuþ and Cu0 layer with a small particle size of copper during the reaction. It was also confirmed by the shift to lower BE and the line broadening of Cu2p3/2 peaks. These promoting effects are much more prominent on CZA-Zr and CZA-Ga catalysts than that (20) Sun, K.; Lu, W.; Qiu, F.; Liu, S.; Xu, X. Appl. Catal., A 2003, 252, 243–249. (21) Okamoto, Y.; Fukino, K.; Imanaka, T.; Teranishi, S. J. Phys. Chem. 1983, 87, 3740–3747. (22) Nakamura, J.; Uchijima, T.; Kanai, Y.; Fujitani, T. Catal. Today 1996, 28, 223–230.
(19) Chwla, S. K.; Sankarraman, N.; Payer, J. H. J. Electron Spectrosc. Relat. Phenom. 1992, 61, 1–18.
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Kang et al. Table 2. CO Conversion and Product Distribution on the Bifunctional Catalystsa product distribution (mol %) yield of DME CO conversion (mol %) notation (mol %) CH3OH DME CO2 BPsb CZA CZA-Zr CZA-Ga CZA(Zr)c
61.1 72.3 63.9 34.9
23.5 18.1 19.9 95.3
44.2 55.7 53.1 0.9
29.5 25.2 25.1 3.0
2.8 1.0 1.9 0.8
27.0 40.3 33.9 0.3
a
Catalytic performance was measured under the following reaction conditions: P = 5.0 MPa, SV = 4000 mL gcat-1 h-1, and T = 250 °C. b Byproducts (BPs) mainly include CH4 and a small quantity of C2 hydrocarbons. c The CZA(Zr) (Cu-ZnO-Al2O3-ZrO2) catalyst without the addition of the solid-acid γ-Al2O3 component was compared to verify catalytic performance.
Table 2. In general, the activity of the bifunctional catalyst for DME synthesis from syngas strongly depends upon the dispersion of metallic copper and the number of acidic sites. In the case of the unpromoted CZA catalyst, the Cu surface area and acidity assigned to the peaks of T1 and T2 in NH3-TPR are lower than those of promoted bifunctional catalysts and resulting in showing a low DME yield around 27.0% and CO conversion of 61.1%. The higher CO conversion and yield to DME around 72.3 and 40.3%, respectively, were observed on the CZA-Zr catalyst. In comparison to that of the CZA(Zr) catalyst, which is prepared without the addition of the solid-acid γ-Al2O3 component (CO conversion of 34.9%), the observed higher CO conversion and DME selectivity on bifunctional catalysts are mainly attributed to the high activity of methanol dehydration to DME. Eventually, this high activity overcomes the thermodynamic limitation of the methanol synthesis reaction by making a low concentration of methanol during the reaction and a high concentration of hydrogen by the WGS reaction. The product distribution is consistent with the previously reported results based on the thermodynamics of DME synthesis by CO and CO2 hydrogenation.23 The selectivity and yield of DME on the promoted bifunctional catalysts are found to be much higher than those of the unpromoted CZA catalyst, especially the CZA-Zr catalyst. The appropriate amount of acidic sites by forming smaller CZA particles with a proper electronic state of copper species on the CZA-Zr catalyst and less blocking of acidic sites on γ-Al2O3 could contribute to obtaining high selectivity to DME. The Zr promoter largely increased the dispersion of Cu-ZnO-Al2O3 particles on γ-Al2O3 by forming a strong ZrO2-CuO interaction originating from oxygen vacancies of ZrO2. The Ga promoter also enhanced catalytic property by the inverse spillover of hydrogen,15,24 with a less promoting effect compared to the Zr promoter. Furthermore, all bifunctional catalysts promoted with Zr or Ga showed a higher catalytic performance because of the co-existence of the Cuþ and Cu0 layer on the methanol synthesis Cu-ZnO-Al2O3 catalyst. The catalytic performance with time on stream fro 16 h on the bifunctional catalysts is shown in Figure 10. Although CO conversion decreased with time on stream, especially for the CZA catalyst, DME selectivity is not much altered during the reaction for 16 h on stream. The CZA-Zr catalyst showed the best catalytic stability
Figure 9. SEM images of the used bifunctional catalyst.
of the CZA catalyst because of the presence of a lower BE of the Cu2p3/2 peak with broad lines. The surface morphology of bifunctional catalysts is also characterized by SEM analysis, as shown in Figure 9. The SEM images suggest that nano-sized spheres with a smaller particle size are homogeneously dispersed on the plate-like γ-Al2O3 on CZA-Ga and CZA-Zr bifunctional catalysts. The sphere-type granules are mainly composed of the CuZnO-Al2O3 catalyst and plate-like materials of the solid-acid γ-Al2O3 catalyst, as previously reported from our study.6 With an increasing dispersion of sphere-type granules on plate-like γ-Al2O3, the appropriate acidic sites (T1 and T2 assigned in NH3-TPD) for the methanol dehydration reaction increased, as shown in Table 1 and Figure 6. The surface acidic sites and their strength could be directly correlated with the formation rate of DME. 3.5. Catalytic Performance on Bifunctional Catalysts. The catalytic performance at steady state, with averaged values for 2 h after reaction of 14 h on stream, is summarized in
(23) Jia, G.; Tan, Y.; Han, Y. Ind. Eng. Chem. Res. 2006, 45, 1152– 1159. (24) Atake, I.; Nishida, K.; Li, D.; Shishido, T.; Oumia, Y.; Sano, T.; Takehira, K. J. Mol. Catal. A 2007, 275, 130–138.
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: DOI:10.1021/ef901133z
Kang et al.
Cu-ZnO-Al2O3/γ-Al2O3 catalyst during the co-precipitation step. 4. Conclusions The Cu-ZnO-Al2O3/γ-Al2O3 bifunctional catalyst promoted with Zr or Ga showed a higher catalytic performance with respect to high CO conversion and DME selectivity than that of an unpromoted catalyst. The characterization results indicated that a copper species in the promoted bifunctional catalyst is well-dispersed with a small particle size and results in showing a high surface area of metallic copper. The Zrpromoted Cu-ZnO-Al2O3 catalyst possessed a proper electronic state because the strong ZrO2-CuO interaction is responsible for increasing the adsorption of active CO molecules. Furthermore, the small particle size of Cu-ZnO-Al2O3 is beneficial for maintaining high acidic sites on γ-Al2O3, which resulted in showing high activity for methanol dehydration especially for the CZA-Zr catalyst. The promoters, such as Zr and Ga, play an important role in augmenting the dispersion of copper species with an appropriate electronic state and acidity of γ-Al2O3, which results in showing a high formation rate for methanol and consecutive dehydration rate to DME.
Figure 10. Catalytic performance with time on stream.
compared to the other catalysts because of the mentioned superior characteristics previously. Therefore, the application of the CZA-Zr bifunctional catalyst is beneficial for obtaining high CO conversion and stability by overcoming the thermodynamic limitation of methanol synthesis. Therefore, to prepare an efficient bifunctional catalyst for DME synthesis from syngas directly, the formation of small copper particles with an appropriate electronic state and the proper amount of acidic sites is simply justified by adding Zr or Ga promoters on a bifunctional
Acknowledgment. G. Murali Dhar thanks the Korea Federation of Science and Technology (KOFST) for the award of a visiting research fellowship under the Brain Pool program.
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