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Study on the deactivation phenomena of Cu-based catalyst for methanol synthesis in slurry phase
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Fuel 87 (2008) 430–434 www.fuelfirst.com
Study on the deactivation phenomena of Cu-based catalyst for methanol synthesis in slurry phase Xufang Zhai a
a,b
, Jun Shamoto c, Hongjuan Xie a, Yisheng Tan a, Yizhuo Han Noritatsu Tsubaki c
a,*
,
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate University of The Chinese Academy of Sciences, Beijing 100049, China c Department of Applied Chemistry, School of Engineering, Toyama University, Toyama 930-8555, Japan Received 28 November 2006; received in revised form 29 June 2007; accepted 4 July 2007 Available online 7 August 2007
Abstract A commercial Cu-based catalyst for methanol synthesis was studied using a stirred autoclave reactor system in the present study. The synthesis reactions were conducted for different time under the same reaction conditions in order to get catalyst samples with different deactivation degrees. The composition and morphology of the catalyst samples before and after reaction were characterized by the means of temperature programmed reduction (TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), elemental analysis (EA) and nitrogen adsorption techniques. The experimental results indicated that Cu composition of the catalyst had not changed significantly during the reaction, and sintering of Cu particles of the catalysts was the main cause of the catalyst deactivation with time on stream. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Cu-based catalyst; Slurry phase; Catalyst deactivation; Methanol synthesis
1. Introduction Methanol synthesis employing Cu–Zn–Al2O3 has been widely used in industrial methanol synthesis since the invention of low temperature and low pressure Cu–Zn– Al2O3 catalyst by ICI in 1966. Recently, methanol has attracted great attention because of its importance in chemical industries and its potential as an environmentally friendly energy carrier [1,2]. Moreover, methanol is a cheap and clean liquid alternative fuel, which can be produced from the widely available materials such as coal, natural gas, as well as biomass [3–5]. The economical efficiency of the traditional gas phase methanol synthesis is unsatisfied [6] because the conversion rate of the syngas (CO + H2) must be controlled at low level in order to avoid possible overheating of the catalyst *
Corresponding author. Tel.: +86 351 4049747; fax: +86 351 4041153. E-mail address: [email protected] (Y. Han).
0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.07.008
bed, because methanol synthesis is a highly exothermic reaction (DH 298 ¼ 90:97 kJ mol1). Comparing to the traditional gas phase methanol synthesis, the slurry phase synthesis of methanol has many advantages such as easy control of the reaction temperature, high reaction heat transfer rates, low investment, low production cost, etc. [7]. American Air Products and Chemicals exploited the three phase bubble column reactor for methanol synthesis with CuO–ZnO–Al2O3 catalysts in the 1980s and built an industrial demonstration plant in 1997 which showed economic advantages. Although the slurry phase synthesis of methanol is superior to the gas phase synthesis and the Cu-based catalysts which have the characteristic of high activity at low temperatures etc., the deactivation of the catalyst in slurry phase methanol synthesis is still a problem for commercialization [8]. To obtain catalysts with high activity and stability, previous works have focused on the study of methanol synthesis mechanism and catalyst activity site [9–12]. Few studies have been carried out on
X. Zhai et al. / Fuel 87 (2008) 430–434
the detailed deactivation mechanism of this reaction. In this study, methanol synthesis from syngas over a CuO– ZnO–Al2O3 catalyst prepared by co-precipitation was carried out in a stirred slurry autoclave reactor. Particular attention has been paid to the catalyst structure and properties change with reaction time by using the methods of temperature programmed reduction (H2-TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nitrogen adsorption.
2. Experimental
431
flame ion detector. Under above reaction conditions, several synthesis reactions with different durations were conducted in order to obtain the catalyst samples with different deactivation degrees. CO conversion is calculated as follows: C CO % ¼ ðF i X CO;i F o X CO;o Þ=ðF i X CO;i Þ 100 where Fi is the syngas flow-rate in, mmol/h; Fo is the tail gas flow-rate out, mmol/h; XCO,i is the CO molar concentration in the feed gas; XCO,o is the CO molar concentration in products stream. The carbon balance for all experiments were 95–98%.
2.1. Synthesis reaction 2.2. Characterization of the catalysts The experimental set-up for methanol synthesis in slurry phase is illustrated in Fig. 1, which includes feed gas purification, autoclave reactor and chemical analysis section. The Cu-based catalyst used in the present study is a commercial catalyst (Research Institute of Nanjing Chemical Industry Group, CuO:ZnO:Al2O3 60:35:5). First, the Cu-based catalysts (<250 mesh, 10 g) and paraffin were blended according to a definite ratio (10 wt% catalyst concentration) and placed into a 0.5 L stirred autoclave (stainless steel made) reactor. The catalyst was then reduced with mixed gas (10% H2 + 90% N2) for 6 h at 260 °C and 0.3 MPa. After reduction, the feed gas was switched to syngas, which contained approximately 66% H2, 29% CO, 4% CO2 and 1% CH4. The methanol synthesis reaction was conducted under the conditions of 260 °C, 5 MPa and GHSV = 1100 ml g1 h1 (STP). The tail gas was analyzed by two on-line gas chromatographs (GC4000A): one was equipped with carbon molecular sieve column and thermal conductivity detector, the other one was equipped with a GDX403 column and flame ionization detector. The chromatographic analytic results from two GSs were connected by CH4 concentration. The sample of liquid methanol from the condenser was analyzed with a GDX403 column and
The textural properties (BET specific surface area, pore volume and pore diameter) of the catalysts were evaluated using a Micromeritics Tristar 3000 nitrogen physisorption apparatus. X-ray diffraction (XRD) determinations were performed on a D8 Advance, Bruker. Axs was equipped to identify the crystalline species of the catalysts. A Cu tube serving as the X-ray source was employed to estimate the active site phase, and the working voltage and current was 40 kV and 40 mA. Transmission electron microscope (TEM) observations were obtained with a H-600-2 Microscope. Scanning electron microscope (SEM) analysis was conducted with a JSM-6360 V equipped with an energy dispersive (EDS). Temperature programmed reduction (TPR) experiments were conducted in a laboratory-made set-up consisting of a quartz reactor and a thermal conductivity detector (TCD). For each TPR experiment, 100 mg of catalyst was packed into the reactor and reduced in a stream of H2/N2 (5/95) at a flow-rate of 60 mL/min, from room temperature to 464 °C. Also, prior to each measurement the sample was
Fig. 1. Description of the experimental set-up for methanol synthesis in slurry reactor: 1, Syngas; 2 and 3, gas purifiers II; 4, condenser; 5, gas–liquid separator; 6, filter; 7, YT-2 pressure regulator; 8, mass-flow controller; 9, YT-4 pressure regulator; 10, autoclave reactor; 11, gas–liquid separator; 12, back pressure regulator; 13, sampling valve of FID; 14, sampling valve of TCD; 15, wet flow-meter and 16, rotameter.
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preheated in a stream of N2 at 464 °C for 120 min to remove the adsorbed water.
Table 2 Textural properties of the Cu-based catalysts
3. Results and discussion 3.1. Methanol synthesis reaction Table 1 shows the results of 4 reaction runs conducted for different time on stream. CO conversion listed in the table is the final conversion of each run. As time on stream increased, the conversion of CO and the space time yield (STY) of methanol decreased. The initial CO conversion of 52.37% was obtained after the reaction started for 6 h. The decrease rate of CO Conversion was comparatively slow during the period of 0–50 h, became fast in 50– 300 h, and slowed down again after 300 h. The CO conversion with the catalysts experienced different reaction time gave similar decrease tendency. The deactivation rate for the catalysts was not constant in the whole reaction time. The selectivity of methanol was about 99% and did not vary significantly.
Catalyst
Pore volume (cm3/g)
Average pore diameter (nm)
BET surface area (m2/g)
Un-reduced After reduction and before reaction After 300 h reaction After 500 h reaction
0.24 0.22
16.4 19.6
58.6 56.7
0.12 0.08
40.0 34.6
12.4 9.2
when the reaction temperature reaches the Tamman temperature (TTamman = 0.5 TMelting), atoms in the bulk will exhibit mobility, and this will lead to sintering [16]. The Cu-based catalysts are sensitive to high temperature since the melting point of Cu is comparatively low (1356 K). High temperature might result in the growth of the Cu particle size and hence in, the decrease of Cu dispersion. Crystalline forms identified by the XRD analyses of the Cu-based catalysts before and after reaction are shown in Fig. 2. Only the diffraction peaks of ZnO and CuO were predominant and comparatively dispersive for the catalysts before reaction, indicating the good dispersion of ZnO and CuO in the catalysts as well as the strong interaction. This is beneficial to high activity of the catalysts. But the diffraction peaks of Al2O3 were hardly detected, which indicated that Al2O3 dispersion in Cu-based catalyst was high. The peaks of Cu appeared for the catalysts after reduction
3.2. Catalyst characterization The reaction temperature was kept at 260 ± 1 °C for all experiments. After the reaction, the reactor was firstly cooled down to room temperature and then the pressure was reduced to the atmospheric pressure. The catalyst was taken out from the reactor with the liquid wax solvent together, which could keep the catalyst free from the oxidation by air. Table 2 shows the textural properties of the Cubased catalysts before and after reaction. It can be seen that there are evident changes in surface area, pore volume and pore diameter before and after reaction. On the other hand, the catalysts varied little before and after the reduction. The surface area of the catalyst decreases with time on stream. Comparing to the results of Tables 1 and 2, a correlation between the loss of the catalytic activity and the decrease of the catalysts surface area is found. These results are in agreement with Roberts et al.’s report [13–15]. The so-called Tamman and Hu¨ttig temperatures, indicative of the temperatures at which sintering may occur are directly related to the melting temperature. High temperature accelerates the mobility of the Cu atoms. First, when the temperature reaches the Hu¨ttig temperature (THu¨ttig = 0.3 TMelting), atoms at defects will become mobile. Later,
Fig. 2. The XRD spectra of the Cu-based catalysts: (a) un-reduced, (b) after reduction and before reaction, (c) after 300 h reaction and (d) after 500 h reaction.
Table 1 Performance of the Cu-based catalysts after different reaction times on stream Run No.
Total reaction time (h)
Final CO conversion (%)
Deactivation rate (%h1)
Final MeOH STY (mol kg1 h1)
Selectivity (C mol%) MeOH
C1
C2
C3
DME
C4
C5+
1 2 3 4
50 169 300 500
51.0 43.1 33.8 24.7
3.1 6.6 7.8 4.6
6.8 5.7 4.2 3.1
100 99.6 98.8 99.5
0 0 0 0
0 0.08 0.27 0.20
0 0.20 0.21 0.26
0 0 0 0
0 0.12 0.26 0.13
0 0 0.46 0.30
Reaction conditions: 533 K, 5 MPa, GHSV = 1100 ml g1 h1, H2/CO molar ratio of 2:1.
X. Zhai et al. / Fuel 87 (2008) 430–434
and before reaction, which revealed the complete CuO reduction of the catalysts. ZnO peaks kept almost unchanged after the reduction, indicating that ZnO was not reduced during the reduction. As seen in Fig. 2, the diffraction peaks of Cu and C (graphite) appeared in the catalysts after reaction. The peak of CuO emerged after 300 h reaction, indicating that the catalysts might be partly oxidized by the oxidant gas during the reaction. Meantime, the diffraction peaks of CuO gradually weakened with the proceeding of reaction and disappeared at last. The existent of CuO peak was due to 3–5% CO2 contained in the syngas. The slight change of diffraction peaks of ZnO proved that there was no significant change of morphology and structure of the catalyst after reaction. Furthermore, the diffraction peaks of Cu became sharper. This illustrates the growth of Cu particles which restrain the synergism of Cu and ZnO and leads to the decrease of the catalyst activity. Tan Yisheng et al. [11] considers the cooperation of Cu and ZnO is an important factor which determined the activity of the catalysts, the other references have the same conclusions [17–19]. This viewpoint is consistent with the experimental results of Table 1. At the same time, the carbon deposition became more serious with the reaction time going on. The deposed carbon might cover the surface of the catalysts, narrow the pores, decrease the surface area of the catalysts etc. All of these resulted in the catalyst deactivation. The TEM images of the catalysts are shown in Fig. 3. The images indicate that the crystallite size of the unre-
433
duced catalyst is small (about 10–30 nm). After reduction, the catalyst has similar particle size with the unreduced one. Comparing samples c, d to sample b, it is found that the significant aggregation of Cu took place during the reaction, leading to less active sites and low activity. The aggregation of sample d is more serious than sample c, due to the longer reaction time. Fig. 4 shows the SEM photos of the Cu-based catalysts before and after reaction. The carbon deposition on the catalyst surface during the reaction can be observed from these photos. It is believed that the CO decomposition to carbon and CO2 is easily caused by overheating during the catalytic conversion of syngas to hydrocarbons or alcohols, especially in gas phase reaction for its insufficient heat removal capability. For the slurry phase reaction of methanol synthesis, the carbon deposition should be depressed for the uniform temperature profile in both of reactor scale and the catalyst particle scale. Fig. 4 indicates that the carbon deposition could not be completely avoided by using inert liquid for enhancing the heat transfer. This is further revealed by the analysis results of SEM–EDX shown in Table 3. 5.03% of C content in the catalyst before reaction was from the graphite added during the catalyst preparation [18]. Comparing the fresh catalyst to the used catalysts, the content of Zn, Al, O varied little, whereas the content of C increases evident. The rate of carbon deposition became slow after 300 h reaction, as is in agreement with the results of Table 1 and Fig. 2. Furthermore revealed carbon deposition is one of the causes of the
Fig. 3. The TEM micrographs of the Cu-based catalysts: (a) un-reduced, (b) after reduction and before reaction, (c) after 300 h reaction and (d) after 500 h reaction.
Fig. 4. The SEM micrographs of catalyst: (a) un-reduced catalyst, (b) after reduction and before reaction, (c) after 300 h reaction and (d) after 500 h reaction.
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Table 3 SEM–EDS results of the Cu-based catalysts Catalyst
Un-reduced After 300 h reaction After 500 h reaction
Mass composition (%) C
O
Al
Cu
Zn
Cu/Zn
Cu/(O + Al + Cu + Zn)
5.03 14.0 18.3
21.4 20.1 18.8
2.00 2.48 1.82
47.6 39.4 38.5
23.9 24.0 22.6
1.99 1.64 1.70
50.2 45.9 47.1
4. Conclusions The characterization of the catalysts used for slurry phase methanol synthesis shows that the main cause of the catalyst deactivation is sintering and the particle aggravation during the reaction. Carbon deposition occurred during the reaction and resulted in low surface area. Acknowledgment This work is supported by the Abroad Outstanding Scholar Foundation of Chinese Academy of Sciences (2005-2-4). References Fig. 5. The TPR profiles of the Cu-based catalysts: (a) un-reduced, (b) after 50 h reaction, (c) after 169 h reaction (d) after 300 h reaction, (e) after 500 h reaction.
catalyst deactivation. The phenomena of Cu loss are not evident after 500 h reaction. The element analysis also proved the same conclusion, as is different from the previous reports [20,21]. The H2 TPR profiles of the catalysts are shown in Fig. 5. All of the catalysts before and after reaction have two peaks. Yang et al. [22] proposed that the lower temperature peak is in line with Cu+2 ! Cu+, and the main peak of higher temperature is in line with Cu+1 ! Cu0 With timeon-stream, the reduction peaks of the used catalyst became wider and the re-reduction became difficult. Moreover, we have measured the amount of hydrogen consumed during TPR by weighting the paper of the TPR area. The hydrogen consumption areas of the catalysts after 50 h, 169 h, 300 h, 500 h of reaction were 0.9002, 0.0651, 0.0551, 0.0526, respectively. This means that the hydrogen consumption of the used catalysts became smaller with time on stream. And at the same time, the re-reduction temperature of the used catalysts became higher. The results of TPR study also indicate that the catalyst crystal became larger and the rereduction became more difficult with the increase of time on the stream due to the large particle size of CuO caused by the gradual growth with reaction time going on.
[1] Chinchen GC, Denny PJ, Jennings JR, Spencer MS, Waugh KC. Appl Catal 1988;36:1–65. [2] Klier K. Adv Catal 1982;31:243–313. [3] Liao SY, Jiang DM, Huang ZH, Zeng K. Fuel 2006;85:1346–53. [4] Lin TC, Chao MR. Sci Total Environ 2002;284:61–74. [5] Heinrich W, Marquardt KJ, Schaefer AJ. SAE Paper No. 861581; 1986. p. 998–1010. [6] Diao XY, Zhang JB, Li GW, Gao HW. Hua gong Ke ji 2000;8:49–52. [7] Zhang K, Song HS, Sun DK, Li SF, Yang XG, Zhao YL, et al. Fuel 2003;82:233–9. [8] Zhao YL, Wang JH. J Fuel Chem Technol 1989;17:262–6. [9] Nalamura J, Choi Y, Fujitani T. Top Catal 2003;22:277. [10] Agrell J, Boutonnet M, Melia´n-Cabrera I, Fierro Jose´ LG. Appl Catal A: Gen 2003;253:201–11. [11] Tan YS, Xie HJ, Cui HT, Han YZ, Zhong B. Catal Today 2005;104:25–9. [12] Pan WX, Cao R, Roberts DL, et al. J Catal 1988;114:440–6. [13] Roberts GW, Brown DM, Hsiung TH, Lewnard JJ. Ind Eng Chem Res 1993;32:1610–21. [14] Robinson WRAM, Mol JC. Appl Catal 1990;63:165–79. [15] Kurtz M, Wilmer H, Genger T, et al. Catal Lett 2003;86:77–80. [16] Moulijn JA, van Diepen AE, Kapteijn F. Appl Catal A: Gen 2001;212:3–16. [17] Chen HY, Lau SP, Chen L, et al. Appl Surf Sci 1999;152:193–9. [18] Cen YQ, Li XN, Liu HZ. Chin J Catal 2006;27:210–6. [19] Figueiredo RT, Martinez Arias A, Lopez Granados M, et al. J Catal. 1998;178:146–52. [20] Wang CW, Ding BQ, Zhu BC, Fang DY. J Wuhan Inst Chem Tech 2001;23(3):1–4. [21] Liu HW, Nie ZG, Liu DH, Ying WY, Fang DY. J East China Univ Sci Technol (Natur Sci) 2006;32:365–9. [22] Yang YQ, Fang XM, Li C. J Xiamen Univ (Natur Sci) 2001;40:1251–5.
Fuel 87 (2008) 430–434 www.fuelfirst.com
Study on the deactivation phenomena of Cu-based catalyst for methanol synthesis in slurry phase Xufang Zhai a
a,b
, Jun Shamoto c, Hongjuan Xie a, Yisheng Tan a, Yizhuo Han Noritatsu Tsubaki c
a,*
,
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b Graduate University of The Chinese Academy of Sciences, Beijing 100049, China c Department of Applied Chemistry, School of Engineering, Toyama University, Toyama 930-8555, Japan Received 28 November 2006; received in revised form 29 June 2007; accepted 4 July 2007 Available online 7 August 2007
Abstract A commercial Cu-based catalyst for methanol synthesis was studied using a stirred autoclave reactor system in the present study. The synthesis reactions were conducted for different time under the same reaction conditions in order to get catalyst samples with different deactivation degrees. The composition and morphology of the catalyst samples before and after reaction were characterized by the means of temperature programmed reduction (TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), elemental analysis (EA) and nitrogen adsorption techniques. The experimental results indicated that Cu composition of the catalyst had not changed significantly during the reaction, and sintering of Cu particles of the catalysts was the main cause of the catalyst deactivation with time on stream. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Cu-based catalyst; Slurry phase; Catalyst deactivation; Methanol synthesis
1. Introduction Methanol synthesis employing Cu–Zn–Al2O3 has been widely used in industrial methanol synthesis since the invention of low temperature and low pressure Cu–Zn– Al2O3 catalyst by ICI in 1966. Recently, methanol has attracted great attention because of its importance in chemical industries and its potential as an environmentally friendly energy carrier [1,2]. Moreover, methanol is a cheap and clean liquid alternative fuel, which can be produced from the widely available materials such as coal, natural gas, as well as biomass [3–5]. The economical efficiency of the traditional gas phase methanol synthesis is unsatisfied [6] because the conversion rate of the syngas (CO + H2) must be controlled at low level in order to avoid possible overheating of the catalyst *
Corresponding author. Tel.: +86 351 4049747; fax: +86 351 4041153. E-mail address: [email protected] (Y. Han).
0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.07.008
bed, because methanol synthesis is a highly exothermic reaction (DH 298 ¼ 90:97 kJ mol1). Comparing to the traditional gas phase methanol synthesis, the slurry phase synthesis of methanol has many advantages such as easy control of the reaction temperature, high reaction heat transfer rates, low investment, low production cost, etc. [7]. American Air Products and Chemicals exploited the three phase bubble column reactor for methanol synthesis with CuO–ZnO–Al2O3 catalysts in the 1980s and built an industrial demonstration plant in 1997 which showed economic advantages. Although the slurry phase synthesis of methanol is superior to the gas phase synthesis and the Cu-based catalysts which have the characteristic of high activity at low temperatures etc., the deactivation of the catalyst in slurry phase methanol synthesis is still a problem for commercialization [8]. To obtain catalysts with high activity and stability, previous works have focused on the study of methanol synthesis mechanism and catalyst activity site [9–12]. Few studies have been carried out on
X. Zhai et al. / Fuel 87 (2008) 430–434
the detailed deactivation mechanism of this reaction. In this study, methanol synthesis from syngas over a CuO– ZnO–Al2O3 catalyst prepared by co-precipitation was carried out in a stirred slurry autoclave reactor. Particular attention has been paid to the catalyst structure and properties change with reaction time by using the methods of temperature programmed reduction (H2-TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nitrogen adsorption.
2. Experimental
431
flame ion detector. Under above reaction conditions, several synthesis reactions with different durations were conducted in order to obtain the catalyst samples with different deactivation degrees. CO conversion is calculated as follows: C CO % ¼ ðF i X CO;i F o X CO;o Þ=ðF i X CO;i Þ 100 where Fi is the syngas flow-rate in, mmol/h; Fo is the tail gas flow-rate out, mmol/h; XCO,i is the CO molar concentration in the feed gas; XCO,o is the CO molar concentration in products stream. The carbon balance for all experiments were 95–98%.
2.1. Synthesis reaction 2.2. Characterization of the catalysts The experimental set-up for methanol synthesis in slurry phase is illustrated in Fig. 1, which includes feed gas purification, autoclave reactor and chemical analysis section. The Cu-based catalyst used in the present study is a commercial catalyst (Research Institute of Nanjing Chemical Industry Group, CuO:ZnO:Al2O3 60:35:5). First, the Cu-based catalysts (<250 mesh, 10 g) and paraffin were blended according to a definite ratio (10 wt% catalyst concentration) and placed into a 0.5 L stirred autoclave (stainless steel made) reactor. The catalyst was then reduced with mixed gas (10% H2 + 90% N2) for 6 h at 260 °C and 0.3 MPa. After reduction, the feed gas was switched to syngas, which contained approximately 66% H2, 29% CO, 4% CO2 and 1% CH4. The methanol synthesis reaction was conducted under the conditions of 260 °C, 5 MPa and GHSV = 1100 ml g1 h1 (STP). The tail gas was analyzed by two on-line gas chromatographs (GC4000A): one was equipped with carbon molecular sieve column and thermal conductivity detector, the other one was equipped with a GDX403 column and flame ionization detector. The chromatographic analytic results from two GSs were connected by CH4 concentration. The sample of liquid methanol from the condenser was analyzed with a GDX403 column and
The textural properties (BET specific surface area, pore volume and pore diameter) of the catalysts were evaluated using a Micromeritics Tristar 3000 nitrogen physisorption apparatus. X-ray diffraction (XRD) determinations were performed on a D8 Advance, Bruker. Axs was equipped to identify the crystalline species of the catalysts. A Cu tube serving as the X-ray source was employed to estimate the active site phase, and the working voltage and current was 40 kV and 40 mA. Transmission electron microscope (TEM) observations were obtained with a H-600-2 Microscope. Scanning electron microscope (SEM) analysis was conducted with a JSM-6360 V equipped with an energy dispersive (EDS). Temperature programmed reduction (TPR) experiments were conducted in a laboratory-made set-up consisting of a quartz reactor and a thermal conductivity detector (TCD). For each TPR experiment, 100 mg of catalyst was packed into the reactor and reduced in a stream of H2/N2 (5/95) at a flow-rate of 60 mL/min, from room temperature to 464 °C. Also, prior to each measurement the sample was
Fig. 1. Description of the experimental set-up for methanol synthesis in slurry reactor: 1, Syngas; 2 and 3, gas purifiers II; 4, condenser; 5, gas–liquid separator; 6, filter; 7, YT-2 pressure regulator; 8, mass-flow controller; 9, YT-4 pressure regulator; 10, autoclave reactor; 11, gas–liquid separator; 12, back pressure regulator; 13, sampling valve of FID; 14, sampling valve of TCD; 15, wet flow-meter and 16, rotameter.
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X. Zhai et al. / Fuel 87 (2008) 430–434
preheated in a stream of N2 at 464 °C for 120 min to remove the adsorbed water.
Table 2 Textural properties of the Cu-based catalysts
3. Results and discussion 3.1. Methanol synthesis reaction Table 1 shows the results of 4 reaction runs conducted for different time on stream. CO conversion listed in the table is the final conversion of each run. As time on stream increased, the conversion of CO and the space time yield (STY) of methanol decreased. The initial CO conversion of 52.37% was obtained after the reaction started for 6 h. The decrease rate of CO Conversion was comparatively slow during the period of 0–50 h, became fast in 50– 300 h, and slowed down again after 300 h. The CO conversion with the catalysts experienced different reaction time gave similar decrease tendency. The deactivation rate for the catalysts was not constant in the whole reaction time. The selectivity of methanol was about 99% and did not vary significantly.
Catalyst
Pore volume (cm3/g)
Average pore diameter (nm)
BET surface area (m2/g)
Un-reduced After reduction and before reaction After 300 h reaction After 500 h reaction
0.24 0.22
16.4 19.6
58.6 56.7
0.12 0.08
40.0 34.6
12.4 9.2
when the reaction temperature reaches the Tamman temperature (TTamman = 0.5 TMelting), atoms in the bulk will exhibit mobility, and this will lead to sintering [16]. The Cu-based catalysts are sensitive to high temperature since the melting point of Cu is comparatively low (1356 K). High temperature might result in the growth of the Cu particle size and hence in, the decrease of Cu dispersion. Crystalline forms identified by the XRD analyses of the Cu-based catalysts before and after reaction are shown in Fig. 2. Only the diffraction peaks of ZnO and CuO were predominant and comparatively dispersive for the catalysts before reaction, indicating the good dispersion of ZnO and CuO in the catalysts as well as the strong interaction. This is beneficial to high activity of the catalysts. But the diffraction peaks of Al2O3 were hardly detected, which indicated that Al2O3 dispersion in Cu-based catalyst was high. The peaks of Cu appeared for the catalysts after reduction
3.2. Catalyst characterization The reaction temperature was kept at 260 ± 1 °C for all experiments. After the reaction, the reactor was firstly cooled down to room temperature and then the pressure was reduced to the atmospheric pressure. The catalyst was taken out from the reactor with the liquid wax solvent together, which could keep the catalyst free from the oxidation by air. Table 2 shows the textural properties of the Cubased catalysts before and after reaction. It can be seen that there are evident changes in surface area, pore volume and pore diameter before and after reaction. On the other hand, the catalysts varied little before and after the reduction. The surface area of the catalyst decreases with time on stream. Comparing to the results of Tables 1 and 2, a correlation between the loss of the catalytic activity and the decrease of the catalysts surface area is found. These results are in agreement with Roberts et al.’s report [13–15]. The so-called Tamman and Hu¨ttig temperatures, indicative of the temperatures at which sintering may occur are directly related to the melting temperature. High temperature accelerates the mobility of the Cu atoms. First, when the temperature reaches the Hu¨ttig temperature (THu¨ttig = 0.3 TMelting), atoms at defects will become mobile. Later,
Fig. 2. The XRD spectra of the Cu-based catalysts: (a) un-reduced, (b) after reduction and before reaction, (c) after 300 h reaction and (d) after 500 h reaction.
Table 1 Performance of the Cu-based catalysts after different reaction times on stream Run No.
Total reaction time (h)
Final CO conversion (%)
Deactivation rate (%h1)
Final MeOH STY (mol kg1 h1)
Selectivity (C mol%) MeOH
C1
C2
C3
DME
C4
C5+
1 2 3 4
50 169 300 500
51.0 43.1 33.8 24.7
3.1 6.6 7.8 4.6
6.8 5.7 4.2 3.1
100 99.6 98.8 99.5
0 0 0 0
0 0.08 0.27 0.20
0 0.20 0.21 0.26
0 0 0 0
0 0.12 0.26 0.13
0 0 0.46 0.30
Reaction conditions: 533 K, 5 MPa, GHSV = 1100 ml g1 h1, H2/CO molar ratio of 2:1.
X. Zhai et al. / Fuel 87 (2008) 430–434
and before reaction, which revealed the complete CuO reduction of the catalysts. ZnO peaks kept almost unchanged after the reduction, indicating that ZnO was not reduced during the reduction. As seen in Fig. 2, the diffraction peaks of Cu and C (graphite) appeared in the catalysts after reaction. The peak of CuO emerged after 300 h reaction, indicating that the catalysts might be partly oxidized by the oxidant gas during the reaction. Meantime, the diffraction peaks of CuO gradually weakened with the proceeding of reaction and disappeared at last. The existent of CuO peak was due to 3–5% CO2 contained in the syngas. The slight change of diffraction peaks of ZnO proved that there was no significant change of morphology and structure of the catalyst after reaction. Furthermore, the diffraction peaks of Cu became sharper. This illustrates the growth of Cu particles which restrain the synergism of Cu and ZnO and leads to the decrease of the catalyst activity. Tan Yisheng et al. [11] considers the cooperation of Cu and ZnO is an important factor which determined the activity of the catalysts, the other references have the same conclusions [17–19]. This viewpoint is consistent with the experimental results of Table 1. At the same time, the carbon deposition became more serious with the reaction time going on. The deposed carbon might cover the surface of the catalysts, narrow the pores, decrease the surface area of the catalysts etc. All of these resulted in the catalyst deactivation. The TEM images of the catalysts are shown in Fig. 3. The images indicate that the crystallite size of the unre-
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duced catalyst is small (about 10–30 nm). After reduction, the catalyst has similar particle size with the unreduced one. Comparing samples c, d to sample b, it is found that the significant aggregation of Cu took place during the reaction, leading to less active sites and low activity. The aggregation of sample d is more serious than sample c, due to the longer reaction time. Fig. 4 shows the SEM photos of the Cu-based catalysts before and after reaction. The carbon deposition on the catalyst surface during the reaction can be observed from these photos. It is believed that the CO decomposition to carbon and CO2 is easily caused by overheating during the catalytic conversion of syngas to hydrocarbons or alcohols, especially in gas phase reaction for its insufficient heat removal capability. For the slurry phase reaction of methanol synthesis, the carbon deposition should be depressed for the uniform temperature profile in both of reactor scale and the catalyst particle scale. Fig. 4 indicates that the carbon deposition could not be completely avoided by using inert liquid for enhancing the heat transfer. This is further revealed by the analysis results of SEM–EDX shown in Table 3. 5.03% of C content in the catalyst before reaction was from the graphite added during the catalyst preparation [18]. Comparing the fresh catalyst to the used catalysts, the content of Zn, Al, O varied little, whereas the content of C increases evident. The rate of carbon deposition became slow after 300 h reaction, as is in agreement with the results of Table 1 and Fig. 2. Furthermore revealed carbon deposition is one of the causes of the
Fig. 3. The TEM micrographs of the Cu-based catalysts: (a) un-reduced, (b) after reduction and before reaction, (c) after 300 h reaction and (d) after 500 h reaction.
Fig. 4. The SEM micrographs of catalyst: (a) un-reduced catalyst, (b) after reduction and before reaction, (c) after 300 h reaction and (d) after 500 h reaction.
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Table 3 SEM–EDS results of the Cu-based catalysts Catalyst
Un-reduced After 300 h reaction After 500 h reaction
Mass composition (%) C
O
Al
Cu
Zn
Cu/Zn
Cu/(O + Al + Cu + Zn)
5.03 14.0 18.3
21.4 20.1 18.8
2.00 2.48 1.82
47.6 39.4 38.5
23.9 24.0 22.6
1.99 1.64 1.70
50.2 45.9 47.1
4. Conclusions The characterization of the catalysts used for slurry phase methanol synthesis shows that the main cause of the catalyst deactivation is sintering and the particle aggravation during the reaction. Carbon deposition occurred during the reaction and resulted in low surface area. Acknowledgment This work is supported by the Abroad Outstanding Scholar Foundation of Chinese Academy of Sciences (2005-2-4). References Fig. 5. The TPR profiles of the Cu-based catalysts: (a) un-reduced, (b) after 50 h reaction, (c) after 169 h reaction (d) after 300 h reaction, (e) after 500 h reaction.
catalyst deactivation. The phenomena of Cu loss are not evident after 500 h reaction. The element analysis also proved the same conclusion, as is different from the previous reports [20,21]. The H2 TPR profiles of the catalysts are shown in Fig. 5. All of the catalysts before and after reaction have two peaks. Yang et al. [22] proposed that the lower temperature peak is in line with Cu+2 ! Cu+, and the main peak of higher temperature is in line with Cu+1 ! Cu0 With timeon-stream, the reduction peaks of the used catalyst became wider and the re-reduction became difficult. Moreover, we have measured the amount of hydrogen consumed during TPR by weighting the paper of the TPR area. The hydrogen consumption areas of the catalysts after 50 h, 169 h, 300 h, 500 h of reaction were 0.9002, 0.0651, 0.0551, 0.0526, respectively. This means that the hydrogen consumption of the used catalysts became smaller with time on stream. And at the same time, the re-reduction temperature of the used catalysts became higher. The results of TPR study also indicate that the catalyst crystal became larger and the rereduction became more difficult with the increase of time on the stream due to the large particle size of CuO caused by the gradual growth with reaction time going on.
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