hydrogenation of furfural to furfuryl alcohol over co-b amorphous catalysts prepared by chemical...

Chinese Journal of Chemistry, 2006, 24, 1704—1708 Full Paper

* E-mail: [email protected]; Tel.: 0086-21-64322272; Fax: 0086-21-64322272 Received January 5, 2006; revised March 22, 2006; accepted August 22, 2006. Project supported by the 973 Program (No. 2005CCA01100), Shanghai Leading Academic Discipline Project (No. T0402), the Shanghai Science and

Technology Committee (Nos. 0452nm070, 05QMX1442 and 0552nm036) and the Shanghai Eduction Committee (No. 05DZ20).

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Hydrogenation of Furfural to Furfuryl Alcohol over Co-B Amorphous Catalysts Prepared by Chemical Reduction in

Variable Media

LI, Hui(李辉) CHAI, Wei-Mei(柴伟梅) LUO, Hong-Shan(骆红山) LI, He-Xing*(李和兴)

Department of Chemistry, Shanghai Normal University, Shanghai 200234, China

Five Co-B amorphous alloy catalysts were prepared by chemical reduction in different media, including pure water and pure ethanol as well as the mixture of ethanol and water with variable ethanol content. Their catalytic properties were evaluated using liquid phase furfural hydrogenation to furfuryl alcohol as the probe reaction. It was found that the reaction media had no significant influence on either the amorphous structure of the Co-B catalyst or the electronic interaction between metallic Co and alloying B. This could successfully account for the fact that all the as-prepared Co-B catalysts exhibited almost the same selectivity to furfuryl alcohol and the same activity per surface area ( S

HR ), which could be considered as the intrinsic activity, since the nature of active sites remained un-changed. However, the activity per gram of Co ( m

HR ) of the as-prepared Co-B catalysts increased rapidly when the ethanol content in the water-ethanol mixture used as the reaction medium for catalyst preparation increased. This could be attributed to the rapid increase in the surface area possibly owing to the presence of more oxidized boron species which could serve as a support for dispersing the Co-B amorphous alloy particles.

Keywords solvent effect, Co-B amorphous catalyst, furfural hydrogenation, ethanol-water solution

Introduction

The hydrogenation of furfural to furfuryl alcohol is an important industrial reaction owing to the wide ap-plication of furfuryl alcohol in polymers, fine chemicals and agro chemicals.1,2 In industry, the Cu-Cr catalysts are probably the most frequently employed catalysts in the frufural hydrogenation.3-6 Since Cu-Cr catalysts have high toxicity and thus cause severe environmental pollution, many attempts have been made to develop new catalysts, such as Raney Ni,5 Raney Co,7,8 Raney Cu,9 and etc. However, their lower selectivity to furfuryl alcohol seems to be a problem. In our previous papers, we reported that the Co-based amorphous catalysts (Co-B, Co-Ce-B and Co-Mo-B etc.) exhibited good ac-tivity and selectivity in the hydrogenation of furfural to furfuryl alcohol.10-12 All these Co-based catalysts were prepared by chemical reduction of Co2＋ ions by 4BH－ in aqueous solution. As well known, the reaction media play very important roles in determining the structural and electronic characteristics of the catalysts and thus, their catalytic performance.13 Although the effect of solvents on the structure of the Ni-B,14,15 Co-B,14 Ni-P-B16 amorphous alloys has been reported, the sub-ject has not been studied systematically, and further-more, only pure water and water-alcohol (V∶V, 1∶/1)

solution have been used as reaction media for compari-son. In this paper, we employed pure water and pure ethanol as well as the mixture of water and ethanol as reaction media during catalyst preparation. Various characterizations were performed with the aim to eluci-date the solvent effects on catalytic behaviors of the as-prepared Co-B amorphous alloy catalysts in details.

Experimental

Catalyst preparation

At room temperature, 2.0 mol•L－1 KBH4 solution was added dropwise within 2.0 h into the solution con-taining a desired amount of CoCl2. Mixtures of ethanol and water with various volume ratios were used as sol-vents for both KBH4 and CoCl2. The KBH4 was greatly excessive to ensure the complete reduction of the Co2＋ ions. The solution was kept stirring for another 1 h until the reaction reached completion. The as-prepared Co-B sample was washed thoroughly with distilled water until pH＝7. Then, it was further washed with absolute etha-nol for 5 times and kept in ethanol solution until use.

Catalyst characterization

The amorphous character of the Co-B catalysts was determined by both X-ray powder diffraction (XRD,

Furfuryl alcohol Chin. J. Chem., 2006 Vol. 24 No. 12 1705


Rigaku Dmax-3C with Cu Kα radiation) and selected area electron diffraction (SAED, JEM-2010). X-ray photoelectron spectroscopy (XPS) was performed on a Perkin Elmer PHI 5000C ESCA system to determine the surface electronic states. The surface morphology and the particle size were observed by means of transmis-sion electron micrography (TEM, JEM-2010). The composition of the Co-B catalysts was analyzed by means of inductively coupled plasma (ICP, Jarrell-As Scan 2000). BET surface areas (SBET) were measured by nitrogen adsorption at 77 K after degassing the catalyst at 373 K for 2.0 h.

Activity test

Liquid phase furfural hydrogenation was carried out in a 200 mL stainless steel autoclave containing 1.0 g of catalyst, 10 mL of furfural and 30 mL of ethanol. The air in the autoclave was excluded completely by repeti-tively filling H2. Then the autoclave was filled with H2 up to 1.0 MPa, followed by heating slowly (80 K•h－1) until 383 K. Once the pressure reached a steady state, the hydrogenation reaction was initiated immediately by stirring the reaction mixture vigorously. The stirring effect was preliminarily investigated and a stirring rate of 1200 r/min was employed, this turned out to be suffi-cient to eliminate the diffusion limit. According to the drop of H2 pressure within the first 0.5 h, the initial hy-drogenation rate (the H2 uptake rate per gram of cobalt,

mHR in mmol•h－1•gCo

－1) was calculated according to the ideal gas equation. The areal activity (the H2 uptake rate per m2 of the surface area, S

HR in mmol•h－1•m－2) was also calculated, which could be roughly considered as the intrinsic activity. The product analysis was con-ducted on a GC 1102 equipped with OV 101 column and FID. The oven temperature was 393 K and N2 flow was used as the carrier gas.

Results and discussion

Structural and electronic characteristics of the as-prepared catalysts

By changing φEtOH in the ethanol-water mixture used as the reaction medium in the chemical reduction of CoCl2 by KBH4, five Co-B amorphous alloy catalysts were prepared, designated as Co-B-0, Co-B-1/3, Co-B-2/3, Co-B-3/4 and Co-B-1, respectively, where the number attached to Co-B represents φEtOH. It is worthy to be noted that Co-B-0 was obtained with pure water as the solvent (φEtOH＝0), while Co-B-1 was ob-tained with pure ethanol as the solvent (φEtOH＝1).

As shown in Figure 1, the XRD patterns of as-prepared Co-B samples displayed one broad peak around 2θ＝45° regardless of the reaction medium, in-dicating a typical amorphous character of all those sam-ples.17 The amorphous structure could be further con-firmed by the selected area electronic diffraction (SAED). As shown, the fresh Co-B-0 and the fresh Co-B-1 are present in the amorphous structure, because

only a diffraction halo was observed in the SAED pat-terns (inset in Figure 3).18 These results implied that the change of reaction medium had very little or even no effects on the amorphous structure. However, from the peaks 2θ＝20° corresponding to the amorphous oxi-dized boron species,17 it can be seen that the amount of oxidized boron species present in the sample increased with the increase of φEtOH in reaction medium.

Figure 1 XRD patterns of (a) Co-B-0, (b) Co-B-1/3, (c) Co-B-2/3, (d) Co-B-3/4 and (e) Co-B-1 catalysts.

Figure 2 shows the XPS spectra of the Co-B amor-phous alloy catalysts obtained at various reaction media. Only the metallic Co was found in Co-B-0 and Co-B-1/3 samples, corresponding to the binding energy (BE) of 778.4 eV in Co 2p3/2 level. However, besides the metallic Co, the oxidized Co was also observed in Co-B-2/3, Co-B-3/4 and Co-B-1 samples corresponding to the BE of 781.3 eV. From the XPS spectra of Co 2p3/2 level for those Co-B samples, it can be seen that the surface density of oxidized Co increased slightly with the increase of the ethanol content in reaction me-dium. This could be easily understood since the reduc-ing ability of KBH4 in ethanol was much poorer than that in water due to the inhibition effect of alcohol solu-tion on the hydrolysis of 4BH－ .18 Concerning the XPS spectra in B 1s level, it was found that all the as-prepared Co-B samples contained significant amount of the elemental B species alloying with metallic Co, corresponding to BE of 188.5 eV, which was 1.3 eV higher than that of the pure boron.19 Thus, one can con-clude that the change of reaction medium had no sig-nificant effect on the electronic interaction between me-tallic Co and elemental B in the as-prepared Co-B amorphous alloys, i.e., partial electrons transferred from B to Co in all the Co-B amorphous alloys, making Co electron-enriched while B electron-deficient. The failure in observing the BE shift of the metallic cobalt can be justified by the following argument, which is similar to that observed in the Ni-B amorphous alloy.20 The B 1s BE shift is about ＋6 eV from elemental B to B3＋, while only ＋2 eV BE shift can be observed in the Co 2p level from metal Co to Co2＋. Combining the B 1s BE shift of ＋1.3 eV in the as-prepared Co-B samples with their bulk composition (listed in Table 1), a rough esti-mation gave a Co 2p BE shift (∆BE) of ＜－0.14 eV in

1706 Chin. J. Chem., 2006, Vol. 24, No. 12 LI et al.


amorphous Co-B alloy, which is well within the ex-perimental error of ±0.2 eV. From the B 1s level, it was also found all Co-B samples contained a pro-nounced portion of oxidized B species besides the ele-mental B alloying with metallic Co. These oxidized B species were produced from the hydrolysis of 4BH－ accompanying its reaction with Co2＋. It is very clear that the peak area ratio of oxidized B to alloying B in the as-prepared Co-B samples increased significantly with the increase of φEtOH in reaction medium, which was in good accordance with the aforementioned XRD characterization. Furthermore, it could be found that most oxidized B species in Co-B-0 sample were present in the forms of Na2B4O7 and 2BO－ , corresponding to the BE of 192.4 eV. However, the BE of oxidized B species in the Co-B samples prepared in media contain-ing ethanol shifted positively by 0.5 eV, which could be attributed to the presence of B2O3 with the BE of 192.9 eV.19 The different amount of oxidized B species re-maining in the as-prepared Co-B samples could be at-tributed to the different solubility of fresh B2O3•H2O in various reaction media. Very little B2O3 species were left in the Co-B-0 sample since the fresh B2O3•H2O was soluble in water, and so more B2O3 species could be dissolved into reaction solution and less quantity of oxi-dized B was incorporated into the as-prepared catalyst. The presence of ethanol in reaction medium may possi-bly lower the solubility of B2O3 species, making it dif-ficult to dissolve the oxidized boron species into reac-tion mixture. Therefore, more B2O3 species remained in the Co-B samples prepared in mixture media.

Figure 2 XPS spectra of the as-prepared catalysts. (a) Co2p, (b) B 1s.

As shown in Figure 3, the TEM morphologies dem-onstrated that both the Co-B-0 and Co-B-1 samples were present in spherical particles. However, the parti-cle size of Co-B-1 was about only 20 nm, much smaller than that of the Co-B-0 (≈70 nm). One possible reason was that the reaction between Co2 ＋ and 4BH－ in aqueous solution was vigorous, which might cause the agglomeration of Co-B particles since the reaction was exothermic. But the reaction between Co2＋ and 4BH－ in ethanol solution was relatively smooth owing to the poorer reducing ability of 4BH－ in ethanol, which might decrease the agglomeration of Co-B particles. Another reason might be ascribed to the presence of oxidized B species which could serve as a support for Co-B particles. As mentioned above, more oxidized B species were present in the Co-B-1 sample than that in the Co-B-0, which could effectively inhibit the particle gathering, resulting in well dispersion of the Co-B amorphous alloy particles, as shown in Figure 3.

Figure 3 TEM and SAED pictures (inset) of (a) Co-B-0 cata-lyst and (b) Co-B-1 catalyst.

Preliminarily kinetic studies

As all the as-prepared Co-B amorphous alloys are nonporous, the mass-transfer limitation relating to dif-fusion within the catalyst particles can be neglected.21 Over the fresh Co-B-0 amorphous catalysts, the initial rate of furfural hydrogenation ( m

HR ) first increased with the increase of the stirring speed up to 1000 r/min and then remained constant when the stirring rate increased further. Further experiments also revealed that the m

HR was proportional to the mass of the catalyst at stirring

Furfuryl alcohol Chin. J. Chem., 2006 Vol. 24 No. 12 1707


speed of 1200 r/min. These results demonstrated that the stirring rate of 1200 r/min was high enough to exclude the external diffusional limitation during liquid phase furfural hydrogenation.

Figure 4 shows the kinetic study results obtained from the hydrogenation of furfural over Co-B-0 amor-phous alloy catalyst. As shown, the m

HR increased linearly with p(H2) from 0.5 to 3.0 MPa while the m

HR remained nearly constant when the furfural concentra-tion (φ) varied from 15% to 30%. Similar kinetic results were obtained by using all the as-prepared Co-B amor-phous alloys as catalysts. Thus, it could be concluded that, under the present reaction conditions, the reaction was first order with respect to hydrogen pressure and

Figure 4 Dependence of the initial rate ( m

HR ) on hydrogen pressure p(H2) (a) and furfural concentration (φfurfural) (b) over Co-B-0 catalyst. Reaction conditions: 10 mL of furfural, 30 mL of ethanol, 1.0 g of catalyst, T＝353 K, p(H2)＝1.0 MPa, stirring rate＝1200 r/min.

zero order with respect to furfural concentration. A pos-sible reason is that the furfural was stronglyadsorbed by

the catalyst and could reach saturated adsorption even at very low furfural concentration. Thus, the increase of furfural concentration in the solution would not increase the adsorbed furfural on the catalyst surface and thus would not increase the hydrogenation rate. However, the adsorption of hydrogen on the Co-B catalyst was much weaker than the furfural adsorption and could not reach saturated adsorption under present conditions. Increase of p(H2) resulted in the increase of adsorbed hydrogen on the catalyst surface and in turn increased the hydrogenation rate.22

Roles of reaction media on the activity of Co-B amorphous alloy catalysts

Table 1 summarizes the catalytic properties of vari-ous catalysts in liquid phase hydrogenation of furfural, including the composition, the BET surface area, the initial reaction rate, and the selectivity to furfuryl alco-hol. From Table 1, the following results and conclusions could be obtained.

(1) According to ICP analysis, the B content in the Co-B bulk composition decreased with the increase of φEtOH. This could be attributed to the poorer reducing ability of 4BH－ in ethanol than that in water. Thus, nearly all 4BH－ species were consumed during its re-action with Co2＋ in aqueous solution. However, only a part of 4BH－ species was consumed during its reaction with Co2＋ in ethanol solution and a lot of 4BH－ spe-cies were left in the solution. Since the unreacted 4BH－ species could be washed away easily, the Co-B-1 sam-ple contained lower B content than the Co-B-0, taking into account that both the metallic Co and the oxidized Co could not be washed away in the ethanol solution, as confirmed by the above XPS spectra.

(2) The BET surface area (SBET) of the Co-B samples increased from 29.8 to 63.7 m2•g－1 with the increase of φEtOH from 0 to 1. The higher SBET of the Co-B-1 than that of the Co-B-0 could be mainly attributed to its higher dispersion degree.

(3) From the mHR values, one can see that the initial

activities of the Co-B amorphous alloy catalysts during the furfural hydrogenation increased with the increase of φEtOH. Whereas, the selectivity to furfuryl alcohol remained at 100%, regardless of φEtOH. Since the S

HR remained almost constant for all the Co-B samples re-gardless of φEtOH, it was reasonable to conclude that the

Table 1 Structural characteristics and catalytic activities of the as-prepared Co-B catalysts a

xatom/% Catalysts φEtOH

Co B SBET/(m2•g－1) m

HR /(mmol•h－1•gCo－1) S

HR /(mmol•h－1•m－2) Selectivity to furfuryl alcoholb/%

Co-B-0 0 76 24 29.8 41.4 1.4 100

Co-B-1/3 1/3 78.3 21.7 35.2 51.4 1.5 100

Co-B-2/3 2/3 79.1 20.9 43.1 64.6 1.5 100

Co-B-3/4 3/4 81.2 18.8 52.2 71.3 1.4 100

Co-B-1 1 83.8 16.2 63.7 95.2 1.5 100 a Reaction conditions: 10 mL of furfural, 30 mL of ethanol, 1.0 g of catalyst, T＝353 K, p(H2)＝1.0 MPa, stirring rate＝1200 r/min. b All the selectivities were detected when the furfural conversion reached 100%.

1708 Chin. J. Chem., 2006, Vol. 24, No. 12 LI et al.


increase in the activity of the Co-B amorphous alloy obtained in ethanol solution was mainly attributed to the increase of dispersion degree. While the nature of the active sites remained unchanged, which could also ac-count for the fact that all the Co-B amorphous catalysts exhibited 100% selectivity to furfuryl alcohol.

Conclusion

The above experimental results demonstrated that, the increase of ethanol content in the water-ethanol mixture used as reaction medium for Co-B catalyst preparation caused a rapid increase in the surface area and thus, enhanced the hydrogenation activity ( m

HR ) in liquid phase furfural hydrogenation to furfuryl alcohol. However, no significant change in both the intrinsic activity ( S

HR ) and the selectivity to furfuryl alcohol was observed since the nature of the active sites of the as-prepared Co-B catalysts (i.e., the amorphous charac-teristics and the electronic interaction between metallic Co and alloying B) remained nearly unchanged

References

1 Bauer, K.; Garbe, D. Common Fragrance and Flavor Mate-rials, VCH, Kyoto, Japan, 1985.

2 March, J. In Advanced Organic Chemistry, McGraw-Hill Kogakusha, Tokyo, 1977, p. 829.

3 Rao, R. S.; Baker, R. T. K.; Vannice, M. Catal. Lett. 1999, 60, 51.

4 Liu, B.; Lu, L.; Wang, B.; Cai, T.; Katsuyoshi, I. Appl. Catal. A: General 1998, 171, 117.

5 Thomas, D. US 4153578, 1979 [Chem. Abstr. 1979, 91, 28068].

6 Huang, Z. Z.; Qiu, L. J. Petrochem. Tech. 1992, 21, 35 (in Chinese).

7 Erzhanova, M. S.; Beisekov, T. B.; Elemesov, E.; Idirisov, A.; Bakhanov, E.; Kabulekov, K. SU 468648, 1975 [Chem. Abstr. 1975, 82, 145820].

8 Beisekov, T. B.; Musaeva, S. A.; Kuatbekov, A. M.; Pili-penko, S. V.; Utebaeva, A. A. Khim. Prom-st. 1992, 9, 507.

9 Plyusnin, L. D.; Beisekov, T. B.; Erzhanova, M. S.; Daurenbekov, B. D. Khim. Prom-st. (Moscow) 1988, 11, 672.

10 Luo, H. S.; Li, H. X.; Zhuang, L. Chem. Lett. 2001, 138. 11 Chen, X. F.; Li, H. X.; Luo, H. S.; Qiao, M. H. Appl. Catal.

A: Gen. 2002, 233, 13. 12 Li, H. X.; Luo, H. S.; Zhuang, L.; Dai, W. L.; Qiao, M. H. J.

Mol. Catal. A: Gen. 2003, 203, 267. 13 Reichardt, C. Solvents and Solvent Effects in Organic

Chemistry, VCH, Weinheim, 1990. 14 Maybury, P. C.; Mitchell, R. W.; Hawthorne, M. F. J. Chem.

Soc., Chem. Commun. 1974, 534. 15 Rei, M. H.; Sheu, L. L.; Chen, Y. Z. Appl. Catal. 1986, 23,

281. 16 Lee, S. P.; Chen, Y. W. J. Nanopart. Res. 2001, 3, 133. 17 Wonterghem, J. V.; Morup, S.; Koch, C. J. W.; Charles, S.

W.; Wells, S. Nature 1986, 322, 622. 18 Li, H. X.; Li, H.; Dai, W. L.; Qiao, M. H. Appl. Catal. A:

Gen. 2003, 238, 119. 19 Li, H.; Li, H. X.; Dai, W. L.; Wang, W. J.; Fang, Z. G.;

Deng, J. F. Appl. Surf. Sci. 1999, 152, 25. 20 Li, J.; Qiao, M. H.; Deng, J. F. J. Mol. Catal. A: Gen. 2001,

169, 295. 21 Molnar, A.; Smith, G. V.; Bartok, M. Adv. Catal. 1989, 36,

329. 22 Gallezot, P.; Cerino, P. J.; Blanc, B.; Flèche, G.; Fuertes, P.

J. Catal. 1994, 146, 93. 23 Yamashita, H.; Yoshikawa, M.; Funabiki, T.; Yoshida, S. J.

Chem. Soc., Faraday Trans. 1 1985, 81, 2485.

(E0601052 ZHAO, C. H.)

hydrogenation of furfural to furfuryl alcohol over co-b amorphous catalysts prepared by chemical...

Documents