Hydrogenolysis of cellulose into polyols over Ni/W/SiO2 catalysts
Graphical abstract
Introduction
Increasing fossil fuel prices because of limited reservoirs and the increase in the atmospheric CO2 concentration owing to fossil fuel consumption have stimulated a greater interest in utilizing biomass for sustainable production of energy and chemicals. Cellulose, one of the most abundant resources of biomass, typically comprises 40–50% of the terrestrial biomass and does not directly compete with the food supply chain. Cellulose has a crystalline and robust structure and cannot be easily hydrolyzed to its monomer as compared to starch because of the β-1,4 linkages of d-glucopyranose monomers and hydrogen bond [1]. A key challenge for effective cellulose utilization is its transformation into valuable chemicals, including glucose, polyols, 5-hydroxymethylfurfural, oil, organic acids, and gaseous hydrocarbons [1], [2], [3].
Recently, much attention has been paid to the one-pot conversion of cellulose into polyols (alcohols containing multiple hydroxyl groups), which are important intermediates in the manufacture of perfumes, beer, pharmaceuticals, polyesters, polyethers, and polyurethanes. The pioneering work of Fukuoka and Dhepe [4] has led to the investigation of various catalyst systems for hydrogenolysis of cellulose [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26] such as Pt/γ-Al2O3 [4], Pt/H-ZSM-5 [5], Pt/carbon black [6], Pt/MCM-48 [7], Ru/C [8], Ru/carbon nanotubes [9], [10], Ru/Cs3PW12O40 [11], heteropolyacids combined with Ru/C [12], [13], mineral acid combined with Ru/C [14], [15], mineral acid combined with Ru/USY [16], and Ru/C combined with WO3 and tungstic acid [17], [18]. Besides the expensive noble metal catalysts, nonnoble metal catalysts such as Ni- and W-based catalysts have also been reported to be quite active in the cellulose hydrogenolysis reaction [19], [20], [21]. These include Ni supported on ZnO, Al2O3, SiO2, TiO2, activated carbon, MgO, and H-ZSM-5 [22], [23], Ni-Ir/activated carbon [24], Ni2P/activated carbon [25], Ni/W/SiO2-Al2O3 [26], Ni-W2C/activated carbon [19], Ni-WxC/CMK-3 [20], and Ni-W/SBA-15 [21]. Zhang and co-workers reported that up to 62% of ethylene glycol yield could be obtained using W-based catalysts under optimized reaction conditions (518 K, 6 MPa H2) [19], [20], [21]. However, despite their outstanding catalytic performance, limited work has been reported on the supported NiW catalysts [21], [26].
In this work, the effect of certain factors (physical properties of the support and reduction degree of supported metals) on the catalytic performance for the reaction over NiW/SiO2 is investigated. The average pore diameter of SiO2 support and the reduction temperature affect the crystalline particle size of W, resulting in different catalytic activities. Furthermore, the oxidation states of Ni and W also determine the product distribution.
Section snippets
Catalyst preparation
Silica with various average pore diameters (denoted as SiO2, pore size in Å) viz. SiO2 (Zeochem, ZEOprep 40, SBET = 629 m2/g), SiO2 (Zeochem, ZEOprep 60, SBET = 542 m2/g), SiO2 (Zeochem, ZEOprep 90, SBET = 434 m2/g), and SiO2 (Zeochem, ZEOprep 300, SBET = 116 m2/g) were purchased and utilized as supports after calcination in air at 973 K. The Ni/W/SiO2 catalysts were prepared using a sequential incipient wetness impregnation method. First, the support was impregnated with an aqueous solution of ammonium
Physicochemical properties of prepared catalysts
Table 1 shows the physicochemical properties of the prepared Ni/W/SiO2 catalysts with different average pore sizes (40 Å, 60 Å, 90 Å, and 300 Å). The Ni and W loadings in each catalyst were confirmed as 5 wt.% and 25 wt.%, respectively. The BET surface area of the prepared catalyst decreased to approximately half of that of each support. However, as shown in Fig. S1, the pore-size distribution of the prepared catalyst did not undergo much change compared to that of each support.
The XRD patterns of
Conclusion
The primary crystalline size of W increased on increasing the average pore size of support and reduction temperature for the Ni/W/SiO2 catalysts containing 5 wt.% Ni and 25 wt.% W. However, the amount of surface acid site decreased on increasing the average pore size of support and reduction temperature. A certain fraction of Ni and W can be oxidized in the passivation step at room temperature. Therefore, the bulk crystalline W metal is covered with a certain fraction of WO2 and WO3 in Ni/W/SiO2
Acknowledgements
This work was financially supported by a grant from the Industrial Source Technology Development Programs (10033099) of the Ministry of Knowledge Economy (MKE), Republic of Korea.
References (35)
- et al.
Bioresour. Technol.
(2010) - et al.
Catal. Today
(2011) - et al.
Bioresour. Technol.
(2012) - et al.
Thermochim. Acta
(2008) - et al.
Catal. Today
(2003) - et al.
Appl. Catal. B: Environ.
(2011) - et al.
J. Catal.
(2010) - et al.
Chem. Rev.
(2006) - et al.
Energy. Environ. Sci.
(2009) - et al.
Angew. Chem. Int. Ed.
(2006)