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SiO2 catalyst
Catalysis Communications 6 (2005) 633–637 www.elsevier.com/locate/catcom
Vapor phase hydrogenation of furfural to furfuryl alcohol over environmentally friendly Cu–Ca/SiO2 catalyst Jing Wu
b
a,b,*
, Yanming Shen a,b, Changhou Liu a,*, Haibin Wang b, Caijun Geng b, Zhenxiang Zhang b
a School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China School of Chemical Engineering, Shenyang Institute of Chemical Technology, Shenyang 110142, PR China
Received 4 February 2005; accepted 14 June 2005 Available online 28 July 2005
Abstract The Cu–Ca/SiO2 catalysts were prepared by sol–gel and impregnation methods, respectively. A comparative study on their structures has been carried out by XRD, BET, chemisorption of nitrous oxide and XPS techniques. The catalytic performance of two catalysts for vapor phase hydrogenation of furfural to furfuryl alcohol was tested. Compared with the Cu–Ca/SiO2 catalyst prepared by impregnation method, the Cu–Ca/SiO2 catalyst prepared by sol–gel technique showed higher copper dispersion and higher activity. In addition, both catalysts revealed high selectivity in hydrogenation of furfural to furfuryl alcohol. 2005 Elsevier B.V. All rights reserved. Keywords: Cu–Ca/SiO2; Furfural; Hydrogenation; Furfuryl alcohol
1. Introduction Furfural is an important compound and is usually obtained by acidic hydrolysis of corn core. Although both vapor and liquid phase hydrogenation of furfural can produce furfuryl alcohol, the vapor phase hydrogenation is usually preferred because it can be carried out at atmospheric pressure. Furfuryl alcohol is mainly used in the production of various synthetic fibers, rubbers, and resins. Besides, it is widely used as a solvent for pigment or phenolic resins [1,2]. As a catalyst with moderate activity, copper chromite has been used in the furan industry for the selective hydrogenation of furfural to furfuryl alcohol for decades [1,3,4]. But the greatest disadvantage for this catalyst is its high toxicity, which causes severe environmental pollution. Recently, some
*
Corresponding authors. E-mail addresses: [email protected] (J. Wu), liuchanghou@ 163.com (C. Liu). 1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.06.009
environmentally friendly catalysts with different composition or prepared by different methods have been reported. Platinum catalysts on oxide supports covered with a monolayer of a transition metal oxide [1], and Cu/MgO catalysts prepared by three methods [2] were used for the vapor phase hydrogenation of furfural. Copper-based catalysts dispersed on three forms of carbon were prepared by a wet impregnation technique and the vapor phase kinetic studies of furfural hydrogenation were performed at atmospheric pressure [3]. Raney nickel catalysts modified by impregnation of salts of heteropolyacids for the liquid phase selective hydrogenation of furfural to produce furfuryl alcohol have been reported by Liu et al. [5]. With the development of amorphous and nanometer materials, some ultra-fine amorphous powders such as Ni–P, Ni–B, Ni–P–B, Co–B and Ni–Fe–B amorphous alloy powders have been used for the liquid phase selective hydrogenation of furfural [6–9]. In this work, the Cu–Ca/SiO2 environmentally friendly catalysts were prepared by sol–gel and impregnation methods, respectively. XRD, BET,
634
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
chemisorption of nitrous oxide and XPS techniques were utilized to investigate the influence of the preparation method on the structure of the catalysts. The catalytic performance of the catalysts for selective hydrogenation of furfural to furfuryl alcohol was tested.
2. Experimental Cu–Ca/SiO2 catalysts were prepared by sol–gel (SG) and impregnation (IM) methods. The SG catalyst was prepared as follows: copper and calcium nitrate were dissolved in the colloidal silica (acidic non-stabilizer type, Qingdao Haiyang Chemical Co. Ltd., China). The solution was then added into an aqueous solution of sodium carbonate with vigorous stirring. Whilst forming silica gel, the precipitates of copper and calcium were deposited onto the gel. The precursor of the catalyst was allowed to age continuously for 2 h prior to the filtration. The filtered precursor was washed thoroughly with deionized water, and then dried for 2 h at 383 K and calcined for 4 h at 673 K. The SiO2 support for the impregnation method was also obtained by gelling the colloidal silica as described in SG preparation. The IM catalyst was prepared by impregnating the dry supports overnight with an aqueous solution containing copper and calcium nitrate, and then dried and calcined as described in SG preparation. The CuO loadings of two catalysts were 20 (wt%). XRD patterns were obtained with a Rigaku D/max2500PC X-ray powder diffractometer operated at 50 kV and 250 mA, using Cu Ka radiation. The BET specific surface areas and average pore diameter were measured according to nitrogen adsorption at 77 K using a Micromeritics ASAP 2400 instrument. The copper dispersion in the reduced samples was determined by chemisorption of nitrous oxide. The procedures were performed in turn for complete oxidation, TPR, surface oxidation and TPR again [10–12]. Among them, the complete oxidation of the samples was performed in air (30 cm3 min1) at 723 K for 60 min. The surface oxidation (from Eq. (1)) was initiated by dissociative adsorption of nitrous oxide (5% N2O/N2, 30 cm3 min1) to the reduced catalysts at 363 K for 30 min. After the complete bulk oxidation and the surface oxidation, TPR were carried out by heating the sample from ambient temperature to 723 K at 10 K min1 in a 5% H2/N2 flow (30 cm3 min1). The total amount of copper was provided by the hydrogen consumption X after the complete oxidation (from Eq. (2)). The number of copper surface atoms was provided by the hydrogen consumption Y after the surface oxidation (from Eq. (3)). With X and Y, copper dispersion (D%), specific copper surfaces areas (S) and average volume-surface diameter of copper particles (dvs) can be calculated according to the formulae described by Guerreiro et al. [12]:
2Cu þ N2 O ! Cu2 O þ N2
ð1Þ
CuO þ H2 ! Cu þ H2 O
ð2Þ
Cu2 O þ H2 ! 2Cu þ H2 O
ð3Þ
XPS and AES spectra as well as the surface relative atom ratio of catalyst samples were obtained using an ESCALAB MKII spectrometer equipped with a small reactor in a pretreatment chamber (V.G. Scientific Ltd., UK). The instrument was operated at 2 · 108 Pa and Al Ka radiation (1486.6 eV). After the original XPS and AES spectra were recorded, the samples were transferred into the small reactor and reduced in hydrogen atmosphere at 523 K for 2 h. After being cooled to room temperature, the samples were transferred into the analyzer chamber for XPS and AES measurements. The binding energies of all spectra were calibrated using the value of contaminant carbon (C1s = 284.6 eV) as a reference. The surface relative atomic ratios of elements of the samples were calculated according to the peak area of the spectra. The area sensitivity factors of related spectra for relative elements were chosen from the data calculated by Scofield [13]. The vapor phase hydrogenation of furfural was carried out in a stainless fixed-bed reactor (300 mm long and 8 mm i.d.) at atmospheric pressure. The catalyst was reduced in a 10% H2 and N2 flow at 523 K for 3 h. The temperature of the bed was adjusted with a temperature controller. After the reduction, the reactor was cooled down to reaction temperature 403 K. Furfural was then injected into the vaporizer by a microfeeder and mixed with hydrogen, and then introduced into the reactor. The H2 flow was monitored by using a mass flow meter with H2/furfural molar ratio 5:1. Liquid space velocity of furfural was maintained at 0.33 ml h1 ml1 catalyst . The products were condensed in an ice-cooled trap and analyzed by a SP3420 gas chromatograph equipped with a GDX-102 column and thermal conductivity detector.
3. Results and discussion The XRD patterns for two fresh samples as shown in Fig. 1 indicate that the IM catalyst has strong CuO and weak CaO crystallite signals. On the contrary, in the SG sample only weak CuO signals were detected whereas no CaO signals were observed. The peaks at 2h = 26.2, 27.2, 33.2, 45.8 can be attributed to CaCO3. These results show that the copper particle sizes of the SG sample are smaller than those of the IM catalyst but with the same composition, which is clearly due to the different preparation methods. For the SG catalyst, the deposition of Cu and Ca ions was simultaneously carried out with the gelatination of colloidal silica. This not only led to the high dispersion of Cu and Ca on the support
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
1
Intensity / a.u.
surfaces areas of the SG catalyst are obviously higher than those of the IM catalyst. Based on the average volume-surface diameter of the copper particles in Table 1 and the results of the XRD analysis in Fig. 1, it is likely that the high Cu dispersion in the SG catalyst is due to the smaller Cu particles. The smaller D% in the IM catalyst is the result of occupation of larger Cu particles on the supports surface [2]. The XPS spectra of two catalysts are shown in Fig. 2. Since the binding energy values of Cu2p3/2 in metallic copper, Cu2O and CuO, are 932.4, 932.2 and 933.5 eV, respectively [14], it is believed that the copper species at the surface of two fresh catalysts (Fig. 2(1)) are mainly CuO. For the SG catalyst, the binding energy of Cu2p3/2 is slightly lower than that of the IM catalyst, which indicates there may be small amount of low valent copper present in the SG catalyst due to some strong interaction between CuO and the support or the promoter [12,15]. The binding energy values of Cu2p3/2 for the reduced samples (Fig. 2(2)) indicate that the copper existed as Cu0 or Cu2O. In order to distinguish the copper species in the reduced samples, AES spectra of CuL3VV for two reduced samples were recorded. The kinetic energy values of CuL3VV and the surface relative atomic ratio are shown in Table 2. It is evident that the copper in the reduced samples existed mainly as Cu0 (kinetic energy in metallic copper, Cu2O and CuO is 918.6, 917.4 and 917.9 eV, respectively [14]). Fig. 2(3) shows that there is little change in the binding energy values of Ca2p3/2 on the surface of two catalysts for the fresh and reduced samples. Based on the results of XRD
1---CuO 2---CaO 3---SiO2 4---CaCO3
1
3 2 2 1 3 4 4
4
20
1 1
11
IM
1
1 1
11
SG
1 1 4
10
1
30
4 1
40
50
60
70
635
80
2 theta Fig. 1. The XRD patterns of the samples.
but also might result in the formation of interaction between CuO and the supports or the promoter. According to the results of chemisorption of nitrous oxide, copper dispersion (D%), specific copper surfaces areas (S) and average volume-surface diameter of copper particles (dvs) for the reduced samples can be calculated and are given in Table 1. BET specific surface area (SBET) and average pore diameter for fresh catalysts are also summarized in Table 1. The results indicate that two catalysts have similar specific surface area and average pore diameter but rather different metallic dispersion. The copper dispersion and specific copper Table 1 Metallic dispersion and specific surface area of Cu–Ca/SiO2 catalysts Catalyst
D (%)
S ðm2 Cu g1 catalyst Þ
˚) dvs (A
SBET (m2 g1)
Mean pore diameter (nm)
SG IM
61 30
65.8 32.2
16.4 33.7
166.1 161.0
10.0 8.2
933.7
Cu2p 3/2
(1)
Cu2p 3/2
Intensity / a.u.
933.5
347.5
932.8
(2)
Ca2p3/2
932.5
(3) 347.5 347.2 347.0
Reduced IM Fresh IM
Fresh IM
Reduced IM Reduced SG Reduced SG
Fresh SG
950
945
940
935
930
925
940
936
932
928
924
Binding energy ( eV ) Fig. 2. XPS patterns of the catalysts.
Fresh SG 352
348
344
340
636
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
Table 2 Kinetic energy of CuL3VV and the surface relative atomic ratio for reduced samples Catalyst
SG IM
Kinetic energy (eV)
Surface relative atomic ratio
CuL3VV
Cu/Si
Ca/Si
918.6 918.7
0.0612 0.0249
0.0329 0.0395
Conversion and Selectivity (%)
analysis, it can be concluded that the species of calcium for the IM catalyst should be CaO whereas it is CaCO3 in the SG catalyst (The binding energy values of CaO and CaCO3 are 346.3 and 346.8 eV, respectively [14]). The slight drift in the binding energy may be due to the strong interaction between Ca and Cu or the support, which might be helpful for improving the adsorption and thermal stability of the catalysts. From the surface relative atomic ratio listed in Table 2, it can be found that the number of surface copper atoms for the SG catalyst is higher than that of the IM catalysts, which further confirms the results obtained by chemisorption of nitrous oxide. Hydrogenation of furfural can generate many compounds including furfuryl alcohol, 2-methylfuran, tetrahydro-furfuryl alcohol and other ring-opening products, depending on the catalysts employed and the reaction conditions [1,3]. In this work, furfuryl alcohol was obtained as the main product along with a small amount of 2-methylfuran. The catalytic performance of the SG and IM catalysts is shown in Fig. 3. The measurement results of Cu–Cr/c-Al2O3 catalyst prepared by impregnation method [16] (designated as Cu–Cr) are also shown in Fig. 3. The results of Fig. 3 indicate that the SG catalyst gave high catalytic activity (conversion 100%) and selectivity (about 98.7%), but showed no loss of activity after 80 h reaction. These were similar to that of Cu–Cr catalyst (conversion 99.5%, selectivity 99.0%).
On the contrary, the IM catalyst lost the activity significantly, decreasing from 93.5% to 66.0% after only 25 h although the selectivity was 100%. The difference of the catalytic performance between the SG and IM catalysts is mainly due to the different dispersion of the copper on the supports surface. The interaction between the copper and the supports or the promoter, which may be helpful for the adsorption of reactants [1,2], might be another factor. With high active site dispersion and the strong interaction, both the SG and Cu–Cr catalysts gave similarly high catalytic performances. As a structural promotor, basic metal calcium is usually used to improve the stability of catalysts. Jing et al. [17] have found that there is no evident effect on the initial activity for different Ca loadings, but the loading of Ca has a great effect on the stability of catalyst in reforming of methane with CO2. Hou et al. [18] have confirmed that Ca not only can improve the dispersion of Ni, strengthen the interaction between Ni and Al2O3, but also retard the sintering of Ni in Ca-promoted Ni/a-Al2O3 catalysts. In this work, both SG-1 and IM-1 catalyst (free of calcium) were prepared by the sol–gel and impregnation methods, respectively. Their catalytic performance for the hydrogenation of furfural (FFR) to furfuryl alcohol (FFA) as shown in Fig. 3 demonstrates that the conversion of furfural with the IM catalyst is obviously higher than that with the IM-1 catalyst. This reconfirms that promotor calcium can improve the dispersion of Cu, and enhance the ability to resist agglomeration of Cu crystal particles, therefore significantly improving the stability of the catalyst [17,18]. A similar result was also observed on the SG catalyst but in a much less extent, thanks to smaller crystal particles and higher dispersion. In addition, it has been noticed that the selectivity of furfural alcohol for the SG catalyst is higher than that for the SG-1 catalyst. This might be due to the fact that promotor calcium can prevent the resinification reaction of furfuryl alcohol
100 90
100
100
80
IM
SG
90
Cu-Cr
70
95
60
Conv. of FFR for SG Sel. of FFA for SG Conv. of FFR for SG-1 sel. of FFA for SG-1
50
95 Conv. of FFR for Cu-Cr Sel. of FFA for Cu-Cr
Conv. of FFR for IM Sel. of FFA for IM Conv. of FFR for IM-1 Sel. of FFA for IM-1
90
40 0
20
40
60
80
0
10
20 Time (hours)
30
0
20
40
60
80
Fig. 3. Catalytic performance of the catalysts for hydrogenation of furfural to furfuryl alcohol. Reaction conditions: reaction temperature 403 K, H2/furfural ratio 5:1, liquid space velocity of furfural 0.33 ml h1 ml1 catalyst .
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
[19] and improve the ability of the catalysts for resisting coking [20].
4. Conclusion The results discussed above demonstrate that the sol– gel technique is a very economical and effective method for preparing highly dispersed non-chromium catalysts. Since the copper dispersion obtained by the gelatination of colloidal silica is higher than that by the impregnation method, the SG catalyst has demonstrated high activity and excellent selectivity in the vapor phase hydrogenation reaction of furfural to furfuryl alcohol. In addition, Ca can improve the stability of the catalysts and increase the selectivity of the SG catalyst for the hydrogenation of furfural to furfuryl alcohol as a structural promotor.
Acknowledgement This work was supported by the Natural Science Foundation of Liaoning province Science and Technology Committee of PR China (9810301301).
References [1] J. Kijenski, P. Winiarek, T. Paryjczak, A. Lewicki, A. Mikolajska, Appl. Catal. A 233 (2002) 171.
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[2] B.M. Nagaraja, V. Siva Kumar, V. Shasikala, A.H. Padmasri, B. Sreedhar, B. David Raju, K.S. Rama Rao, Catal. Comm. 4 (2003) 287. [3] R.S. Rao, R.T.K. Baker, M. Vannice, Catal. Lett. 60 (1999) 51. [4] R. Rao, A. Dandekar, R.T.K. Baker, M.A. Vannice, J. Catal. 171 (1997) 406. [5] B.J. Liu, L.H. Lu, B.C. Wang, T.X. Cai, K. Iwatani, Appl. Catal. A 171 (1998) 117. [6] S.P. Lee, Y.W. Chen, Ind. Eng. Chem. Res. 38 (1999) 2548. [7] S.P. Lee, Y.W. Chen, Ind. Eng. Chem. Res. 40 (2001) 1495. [8] X.F. Chen, H.X. Li, H.S. Luo, M.H. Qiao, Appl. Catal. A 233 (2002) 13. [9] H.X. Li, H.S. Luo, L. Zhuang, W.L. Dai, M.H. Qiao, J. Mol. Catal. A 203 (2003) 267. [10] G.C. Bond, N. Namijo, J. Catal. 118 (1989) 507. [11] C.J.G. van der Grift, A.F.H. Wielers, B.P.J. Joghi, J. van Beijnum, M. de Boer, M. Versluijs-Helder, J.W. Geus, J. Catal. 131 (1991) 178. [12] E.D. Guerreiro, O.F. Gorriz, G. Larsen, L.A. Arru´a, Appl. Catal. A 204 (2000) 33. [13] J.H. Scofield, J. Elec. Spec. 8 (1979) 129. [14] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin–Elmer Corporation, USA, 1979. [15] Y. Cong, X.H. Bao, T. Zhang, X.Y. Sun, D.B. Liang, K.C. Tin, N.B. Wang, Chinese J. Catal. 21 (2000) 314. [16] H.B. Yin, Y.M. Shen, L.P. Gao, B.Q. Zhao, Zh.X. Zhang, Chinese J. Catal. 17 (1996) 333. [17] Q.Sh. Jing, H. Lou, L.Y. Mo, J.H. Fei, X.M. Zheng, React. Kinet. Catal. Lett. 83 (2004) 291. [18] Zh.Y. Hou, O. Yokota, T. Tanaka, T. Yashima, Appl. Catal. A 253 (2003) 381. [19] J. Yang, H.Y. Zheng, Y.L. Zhu, Chem. Res. (China) 15 (2004) 16. [20] L.X. Shang, W.G. Xie, Sh.J. Lu, F.L. Qiu, J. Fuel Chem. Thch. (China) 29 (2001) 422.
Vapor phase hydrogenation of furfural to furfuryl alcohol over environmentally friendly Cu–Ca/SiO2 catalyst Jing Wu
b
a,b,*
, Yanming Shen a,b, Changhou Liu a,*, Haibin Wang b, Caijun Geng b, Zhenxiang Zhang b
a School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China School of Chemical Engineering, Shenyang Institute of Chemical Technology, Shenyang 110142, PR China
Received 4 February 2005; accepted 14 June 2005 Available online 28 July 2005
Abstract The Cu–Ca/SiO2 catalysts were prepared by sol–gel and impregnation methods, respectively. A comparative study on their structures has been carried out by XRD, BET, chemisorption of nitrous oxide and XPS techniques. The catalytic performance of two catalysts for vapor phase hydrogenation of furfural to furfuryl alcohol was tested. Compared with the Cu–Ca/SiO2 catalyst prepared by impregnation method, the Cu–Ca/SiO2 catalyst prepared by sol–gel technique showed higher copper dispersion and higher activity. In addition, both catalysts revealed high selectivity in hydrogenation of furfural to furfuryl alcohol. 2005 Elsevier B.V. All rights reserved. Keywords: Cu–Ca/SiO2; Furfural; Hydrogenation; Furfuryl alcohol
1. Introduction Furfural is an important compound and is usually obtained by acidic hydrolysis of corn core. Although both vapor and liquid phase hydrogenation of furfural can produce furfuryl alcohol, the vapor phase hydrogenation is usually preferred because it can be carried out at atmospheric pressure. Furfuryl alcohol is mainly used in the production of various synthetic fibers, rubbers, and resins. Besides, it is widely used as a solvent for pigment or phenolic resins [1,2]. As a catalyst with moderate activity, copper chromite has been used in the furan industry for the selective hydrogenation of furfural to furfuryl alcohol for decades [1,3,4]. But the greatest disadvantage for this catalyst is its high toxicity, which causes severe environmental pollution. Recently, some
*
Corresponding authors. E-mail addresses: [email protected] (J. Wu), liuchanghou@ 163.com (C. Liu). 1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.06.009
environmentally friendly catalysts with different composition or prepared by different methods have been reported. Platinum catalysts on oxide supports covered with a monolayer of a transition metal oxide [1], and Cu/MgO catalysts prepared by three methods [2] were used for the vapor phase hydrogenation of furfural. Copper-based catalysts dispersed on three forms of carbon were prepared by a wet impregnation technique and the vapor phase kinetic studies of furfural hydrogenation were performed at atmospheric pressure [3]. Raney nickel catalysts modified by impregnation of salts of heteropolyacids for the liquid phase selective hydrogenation of furfural to produce furfuryl alcohol have been reported by Liu et al. [5]. With the development of amorphous and nanometer materials, some ultra-fine amorphous powders such as Ni–P, Ni–B, Ni–P–B, Co–B and Ni–Fe–B amorphous alloy powders have been used for the liquid phase selective hydrogenation of furfural [6–9]. In this work, the Cu–Ca/SiO2 environmentally friendly catalysts were prepared by sol–gel and impregnation methods, respectively. XRD, BET,
634
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
chemisorption of nitrous oxide and XPS techniques were utilized to investigate the influence of the preparation method on the structure of the catalysts. The catalytic performance of the catalysts for selective hydrogenation of furfural to furfuryl alcohol was tested.
2. Experimental Cu–Ca/SiO2 catalysts were prepared by sol–gel (SG) and impregnation (IM) methods. The SG catalyst was prepared as follows: copper and calcium nitrate were dissolved in the colloidal silica (acidic non-stabilizer type, Qingdao Haiyang Chemical Co. Ltd., China). The solution was then added into an aqueous solution of sodium carbonate with vigorous stirring. Whilst forming silica gel, the precipitates of copper and calcium were deposited onto the gel. The precursor of the catalyst was allowed to age continuously for 2 h prior to the filtration. The filtered precursor was washed thoroughly with deionized water, and then dried for 2 h at 383 K and calcined for 4 h at 673 K. The SiO2 support for the impregnation method was also obtained by gelling the colloidal silica as described in SG preparation. The IM catalyst was prepared by impregnating the dry supports overnight with an aqueous solution containing copper and calcium nitrate, and then dried and calcined as described in SG preparation. The CuO loadings of two catalysts were 20 (wt%). XRD patterns were obtained with a Rigaku D/max2500PC X-ray powder diffractometer operated at 50 kV and 250 mA, using Cu Ka radiation. The BET specific surface areas and average pore diameter were measured according to nitrogen adsorption at 77 K using a Micromeritics ASAP 2400 instrument. The copper dispersion in the reduced samples was determined by chemisorption of nitrous oxide. The procedures were performed in turn for complete oxidation, TPR, surface oxidation and TPR again [10–12]. Among them, the complete oxidation of the samples was performed in air (30 cm3 min1) at 723 K for 60 min. The surface oxidation (from Eq. (1)) was initiated by dissociative adsorption of nitrous oxide (5% N2O/N2, 30 cm3 min1) to the reduced catalysts at 363 K for 30 min. After the complete bulk oxidation and the surface oxidation, TPR were carried out by heating the sample from ambient temperature to 723 K at 10 K min1 in a 5% H2/N2 flow (30 cm3 min1). The total amount of copper was provided by the hydrogen consumption X after the complete oxidation (from Eq. (2)). The number of copper surface atoms was provided by the hydrogen consumption Y after the surface oxidation (from Eq. (3)). With X and Y, copper dispersion (D%), specific copper surfaces areas (S) and average volume-surface diameter of copper particles (dvs) can be calculated according to the formulae described by Guerreiro et al. [12]:
2Cu þ N2 O ! Cu2 O þ N2
ð1Þ
CuO þ H2 ! Cu þ H2 O
ð2Þ
Cu2 O þ H2 ! 2Cu þ H2 O
ð3Þ
XPS and AES spectra as well as the surface relative atom ratio of catalyst samples were obtained using an ESCALAB MKII spectrometer equipped with a small reactor in a pretreatment chamber (V.G. Scientific Ltd., UK). The instrument was operated at 2 · 108 Pa and Al Ka radiation (1486.6 eV). After the original XPS and AES spectra were recorded, the samples were transferred into the small reactor and reduced in hydrogen atmosphere at 523 K for 2 h. After being cooled to room temperature, the samples were transferred into the analyzer chamber for XPS and AES measurements. The binding energies of all spectra were calibrated using the value of contaminant carbon (C1s = 284.6 eV) as a reference. The surface relative atomic ratios of elements of the samples were calculated according to the peak area of the spectra. The area sensitivity factors of related spectra for relative elements were chosen from the data calculated by Scofield [13]. The vapor phase hydrogenation of furfural was carried out in a stainless fixed-bed reactor (300 mm long and 8 mm i.d.) at atmospheric pressure. The catalyst was reduced in a 10% H2 and N2 flow at 523 K for 3 h. The temperature of the bed was adjusted with a temperature controller. After the reduction, the reactor was cooled down to reaction temperature 403 K. Furfural was then injected into the vaporizer by a microfeeder and mixed with hydrogen, and then introduced into the reactor. The H2 flow was monitored by using a mass flow meter with H2/furfural molar ratio 5:1. Liquid space velocity of furfural was maintained at 0.33 ml h1 ml1 catalyst . The products were condensed in an ice-cooled trap and analyzed by a SP3420 gas chromatograph equipped with a GDX-102 column and thermal conductivity detector.
3. Results and discussion The XRD patterns for two fresh samples as shown in Fig. 1 indicate that the IM catalyst has strong CuO and weak CaO crystallite signals. On the contrary, in the SG sample only weak CuO signals were detected whereas no CaO signals were observed. The peaks at 2h = 26.2, 27.2, 33.2, 45.8 can be attributed to CaCO3. These results show that the copper particle sizes of the SG sample are smaller than those of the IM catalyst but with the same composition, which is clearly due to the different preparation methods. For the SG catalyst, the deposition of Cu and Ca ions was simultaneously carried out with the gelatination of colloidal silica. This not only led to the high dispersion of Cu and Ca on the support
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
1
Intensity / a.u.
surfaces areas of the SG catalyst are obviously higher than those of the IM catalyst. Based on the average volume-surface diameter of the copper particles in Table 1 and the results of the XRD analysis in Fig. 1, it is likely that the high Cu dispersion in the SG catalyst is due to the smaller Cu particles. The smaller D% in the IM catalyst is the result of occupation of larger Cu particles on the supports surface [2]. The XPS spectra of two catalysts are shown in Fig. 2. Since the binding energy values of Cu2p3/2 in metallic copper, Cu2O and CuO, are 932.4, 932.2 and 933.5 eV, respectively [14], it is believed that the copper species at the surface of two fresh catalysts (Fig. 2(1)) are mainly CuO. For the SG catalyst, the binding energy of Cu2p3/2 is slightly lower than that of the IM catalyst, which indicates there may be small amount of low valent copper present in the SG catalyst due to some strong interaction between CuO and the support or the promoter [12,15]. The binding energy values of Cu2p3/2 for the reduced samples (Fig. 2(2)) indicate that the copper existed as Cu0 or Cu2O. In order to distinguish the copper species in the reduced samples, AES spectra of CuL3VV for two reduced samples were recorded. The kinetic energy values of CuL3VV and the surface relative atomic ratio are shown in Table 2. It is evident that the copper in the reduced samples existed mainly as Cu0 (kinetic energy in metallic copper, Cu2O and CuO is 918.6, 917.4 and 917.9 eV, respectively [14]). Fig. 2(3) shows that there is little change in the binding energy values of Ca2p3/2 on the surface of two catalysts for the fresh and reduced samples. Based on the results of XRD
1---CuO 2---CaO 3---SiO2 4---CaCO3
1
3 2 2 1 3 4 4
4
20
1 1
11
IM
1
1 1
11
SG
1 1 4
10
1
30
4 1
40
50
60
70
635
80
2 theta Fig. 1. The XRD patterns of the samples.
but also might result in the formation of interaction between CuO and the supports or the promoter. According to the results of chemisorption of nitrous oxide, copper dispersion (D%), specific copper surfaces areas (S) and average volume-surface diameter of copper particles (dvs) for the reduced samples can be calculated and are given in Table 1. BET specific surface area (SBET) and average pore diameter for fresh catalysts are also summarized in Table 1. The results indicate that two catalysts have similar specific surface area and average pore diameter but rather different metallic dispersion. The copper dispersion and specific copper Table 1 Metallic dispersion and specific surface area of Cu–Ca/SiO2 catalysts Catalyst
D (%)
S ðm2 Cu g1 catalyst Þ
˚) dvs (A
SBET (m2 g1)
Mean pore diameter (nm)
SG IM
61 30
65.8 32.2
16.4 33.7
166.1 161.0
10.0 8.2
933.7
Cu2p 3/2
(1)
Cu2p 3/2
Intensity / a.u.
933.5
347.5
932.8
(2)
Ca2p3/2
932.5
(3) 347.5 347.2 347.0
Reduced IM Fresh IM
Fresh IM
Reduced IM Reduced SG Reduced SG
Fresh SG
950
945
940
935
930
925
940
936
932
928
924
Binding energy ( eV ) Fig. 2. XPS patterns of the catalysts.
Fresh SG 352
348
344
340
636
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
Table 2 Kinetic energy of CuL3VV and the surface relative atomic ratio for reduced samples Catalyst
SG IM
Kinetic energy (eV)
Surface relative atomic ratio
CuL3VV
Cu/Si
Ca/Si
918.6 918.7
0.0612 0.0249
0.0329 0.0395
Conversion and Selectivity (%)
analysis, it can be concluded that the species of calcium for the IM catalyst should be CaO whereas it is CaCO3 in the SG catalyst (The binding energy values of CaO and CaCO3 are 346.3 and 346.8 eV, respectively [14]). The slight drift in the binding energy may be due to the strong interaction between Ca and Cu or the support, which might be helpful for improving the adsorption and thermal stability of the catalysts. From the surface relative atomic ratio listed in Table 2, it can be found that the number of surface copper atoms for the SG catalyst is higher than that of the IM catalysts, which further confirms the results obtained by chemisorption of nitrous oxide. Hydrogenation of furfural can generate many compounds including furfuryl alcohol, 2-methylfuran, tetrahydro-furfuryl alcohol and other ring-opening products, depending on the catalysts employed and the reaction conditions [1,3]. In this work, furfuryl alcohol was obtained as the main product along with a small amount of 2-methylfuran. The catalytic performance of the SG and IM catalysts is shown in Fig. 3. The measurement results of Cu–Cr/c-Al2O3 catalyst prepared by impregnation method [16] (designated as Cu–Cr) are also shown in Fig. 3. The results of Fig. 3 indicate that the SG catalyst gave high catalytic activity (conversion 100%) and selectivity (about 98.7%), but showed no loss of activity after 80 h reaction. These were similar to that of Cu–Cr catalyst (conversion 99.5%, selectivity 99.0%).
On the contrary, the IM catalyst lost the activity significantly, decreasing from 93.5% to 66.0% after only 25 h although the selectivity was 100%. The difference of the catalytic performance between the SG and IM catalysts is mainly due to the different dispersion of the copper on the supports surface. The interaction between the copper and the supports or the promoter, which may be helpful for the adsorption of reactants [1,2], might be another factor. With high active site dispersion and the strong interaction, both the SG and Cu–Cr catalysts gave similarly high catalytic performances. As a structural promotor, basic metal calcium is usually used to improve the stability of catalysts. Jing et al. [17] have found that there is no evident effect on the initial activity for different Ca loadings, but the loading of Ca has a great effect on the stability of catalyst in reforming of methane with CO2. Hou et al. [18] have confirmed that Ca not only can improve the dispersion of Ni, strengthen the interaction between Ni and Al2O3, but also retard the sintering of Ni in Ca-promoted Ni/a-Al2O3 catalysts. In this work, both SG-1 and IM-1 catalyst (free of calcium) were prepared by the sol–gel and impregnation methods, respectively. Their catalytic performance for the hydrogenation of furfural (FFR) to furfuryl alcohol (FFA) as shown in Fig. 3 demonstrates that the conversion of furfural with the IM catalyst is obviously higher than that with the IM-1 catalyst. This reconfirms that promotor calcium can improve the dispersion of Cu, and enhance the ability to resist agglomeration of Cu crystal particles, therefore significantly improving the stability of the catalyst [17,18]. A similar result was also observed on the SG catalyst but in a much less extent, thanks to smaller crystal particles and higher dispersion. In addition, it has been noticed that the selectivity of furfural alcohol for the SG catalyst is higher than that for the SG-1 catalyst. This might be due to the fact that promotor calcium can prevent the resinification reaction of furfuryl alcohol
100 90
100
100
80
IM
SG
90
Cu-Cr
70
95
60
Conv. of FFR for SG Sel. of FFA for SG Conv. of FFR for SG-1 sel. of FFA for SG-1
50
95 Conv. of FFR for Cu-Cr Sel. of FFA for Cu-Cr
Conv. of FFR for IM Sel. of FFA for IM Conv. of FFR for IM-1 Sel. of FFA for IM-1
90
40 0
20
40
60
80
0
10
20 Time (hours)
30
0
20
40
60
80
Fig. 3. Catalytic performance of the catalysts for hydrogenation of furfural to furfuryl alcohol. Reaction conditions: reaction temperature 403 K, H2/furfural ratio 5:1, liquid space velocity of furfural 0.33 ml h1 ml1 catalyst .
J. Wu et al. / Catalysis Communications 6 (2005) 633–637
[19] and improve the ability of the catalysts for resisting coking [20].
4. Conclusion The results discussed above demonstrate that the sol– gel technique is a very economical and effective method for preparing highly dispersed non-chromium catalysts. Since the copper dispersion obtained by the gelatination of colloidal silica is higher than that by the impregnation method, the SG catalyst has demonstrated high activity and excellent selectivity in the vapor phase hydrogenation reaction of furfural to furfuryl alcohol. In addition, Ca can improve the stability of the catalysts and increase the selectivity of the SG catalyst for the hydrogenation of furfural to furfuryl alcohol as a structural promotor.
Acknowledgement This work was supported by the Natural Science Foundation of Liaoning province Science and Technology Committee of PR China (9810301301).
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