Efficient conversion of furfural into cyclopentanone over high performing and stable Cu/ZrO2 catalysts
Graphical abstract
Introduction
With the decrease of global fossil resources, energy crisis caused by the increasing consumption of fossil fuels (coal and oil) and the resulting environmental pollution are becoming more and more serious all over the world. As a kind of renewable and sustainable resource and the best alternative to fossil fuels, biomass has attracted broad research attention, because its efficient transformation into important chemicals and high value-added liquid biofuels can not only decline the dependence on fossils but also reduce environmental pollution [[1], [2], [3]]. For example, furfural (FFA), a most important and significant biomass-derived platform compound from hemicellulose and xylose [[4], [5], [6], [7], [8], [9]], can be converted through hydrogenation/hydrogenolysis processes to produce various chemicals, nonpetroleum-derived liquid fuels, and fuel additives (e.g. furfuryl alcohol (FAL), tetrahydrofurfuryl alcohol (THFA), 2-methylfuran, tetrahydrofuran, γ-valerolactone, and pentadiol) [[10], [11], [12], [13], [14], [15], [16], [17]].
Cyclopentanone (CPO), as an important intermediate, is used to manufacture a wide range of chemicals (e.g. medicines, herbicides, pesticides, and perfumes) [18], fuel precursors and high-density fuels through condensation-hydrogenation process [19,20]. In industry, CPO is mainly produced by the pyrolysis of adipic acid and its derivatives or the direct oxidation of cyclopentene [[21], [22], [23]]. However, the former simultaneously produces large numbers of pollutants, while the latter always affords low CPO yields under harsh reaction conditions including high reaction temperatures and/or high pressures. Fortunately, it was recently reported that CPO could be obtained by the hydrogenation of biomass-derived FFA in neat water over various supported noble metal catalysts [[24], [25], [26], [27], [28]], especially, along with a high yield of 92.1% over the Pd-Cu/C catalyst under a hydrogen pressure of 3.0 MPa at 160 °C [26], and even a quantitative conversion of FFA into CPO over the Au/TiO2 under a hydrogen pressure of 4.0 MPa [27]. As for bimetallic Pd-Cu/C catalyst, it was shown that the synergy between Pd°-Cu+ species contributed to a good catalytic activity [26]. Very recently, Ru-based catalyst has been reported to efficiently catalyze the FFA hydrogenation to generate CPO under a hydrogen pressure of 1.0 MPa [28]. Taking into consideration high costs of precious metals, non-precious copper, nickel and bimetallic Cu-Ni catalysts also were developed for the above reaction [[29], [30], [31], [32], [33], [34]]. For non-precious metal catalysts, however, relatively high reaction temperatures and/or hydrogen pressures adopted greatly limit their large-scale industrial application. Therefore, developing high performing non-precious metal catalytic systems for the synthesis of CPO has important implications for practical applications.
To achieve the high CPO selectivity in the FFA hydrogenation, it is necessary for catalytically active sites on catalysts to be able to significantly promote the rearrangement of formed FAL intermediate to obtain CPO. It was reported that interfacial Cu+ species formed in the case of Cu-based catalysts could function as active sites activating the CO bond in FFA to some extent [29,35], while surface Lewis acidic sites favored the FAL rearrangement [36]. On the other side, it is well known that the metal-support interactions can greatly affect the catalytic performance of catalysts. For example, the strong interactions between copper species and metal oxides could improve the catalytic performances in large numbers of hydrogenation reactions [[37], [38], [39], [40]]. Recently, we also reported that supported Cu catalysts showed good catalytic performances in the transformation of γ-valerolactone to valerate esters with the help of surface Cu+ species [41,42].
In this contribution, we synthesized a series of Cu/ZrO2 catalysts by our developed one-pot reduction-oxidation route for the transformation of FFA to produce CPO, and especially investigated the effect of the calcination temperature for catalyst precursors on the surface structures and catalytic performances of the resulting copper-based catalysts to unravel the role of metal-support interactions in affecting the catalytic performance. It was shown that as-formed Cu/ZrO2 catalyst at a calcination temperature of 500 °C exhibited an excellent catalytic performance under mild reaction conditions (e.g. a quite low hydrogen pressure of 1.5 MPa and150 °C), along with a quite high CPO yield of 91.3%, indicative of the superiority to other metal oxides supported copper catalysts prepared by the conventional impregnation. High catalytic efficiency of the catalyst mainly resulted from the occurrence of strong metal-support interactions (SMSIs) facilitating the formation of surface Cu+ species, as well as favorable surface acidity. To the best of our knowledge, there is no report about using such high performing and stable Cu/ZrO2 catalyst for the conversion of FFA to produce CPO up to now.
Section snippets
Preparation of Cu/ZrO2 catalysts
A series of Cu/ZrO2 catalysts were prepared by our developed one-pot reduction-oxidation method [41]. Firstly, Zr(NO3)4·5H2O (0.01 mol, 4.293 g) and Cu(NO3)2·3H2O (0.01 mol, 2.416 g) were dissolved into 80 ml of deionized water to form a salt solution A, while NaBH4 (0.2 mol, 7.59 g) was dissolved into 80 ml of deionized water to obtain a solution B. Subsequently, above two solutions were simultaneously added to a colloid mill, and at once mixed rapidly at a rotor speed of 3000 rpm for 3 min.
Structural characterization
Fig. 1A presents the XRD patterns of different CuO/ZrO2 catalyst precursors. In each case, four diffractions appearing at 2θ of about 30.2°, 35.2°, 50.4° and 60.2° are indexed as the (011), (110), (020) and (121) planes of tetragonal ZrO2 (t-ZrO2) phase (JCPDS 50-1089), respectively. As the calcination temperature is elevated, no obvious change in diffractions assignable to t-ZrO2 is found, indicative of the formation of stable t-ZrO2 phase in all precursors. Interestingly, for CuO/ZrO2-R
Conclusions
In summary, series of Cu/ZrO2 catalysts synthesized by our developed one-pot reduction-oxidation route were employed in the conversion of FFA to produce CPO in water. By adjusting the calcination temperature of catalyst precursors, surface structures and catalytic performances of Cu/ZrO2 catalysts could be delicately regulated. It was found that the Cu/ZrO2 catalyst obtained at the calcination temperature of 500 °C exhibited a superior catalytic performance with a high CPO yield of 91.3% to
Acknowledgements
This study was funded through National Natural Science Foundation of China (21776017; 21521005) and Fundamental Research Funds for the Central Universities (buctrc201528).
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