Glycerol oxidehydration into acrolein and acrylic acid over W–V–Nb–O bronzes with hexagonal structure
Graphical abstract
Highlights
► W–V–Nb oxides with the hexagonal tungsten bronze structure were prepared hydrothermally. ► These systems possess both acid and redox features, for the oxidehydration of glycerol into acrylic acid. ► The best yield of 34% to acrylic acid was obtained with W–V–Nb–O bronze. ► The selectivity to acrolein and acrylic acid was greatly affected by the reaction conditions.
Introduction
Within the context of the Biorefinery concept, implying the use of bio-based building blocks for the production of fuels and chemicals, the transformation of glycerol into valuable compounds plays an important role [1], [2], [3], [4], [5]. This is also demonstrated by the impressive number of papers published in recent years, dealing with several options for the biochemical and chemical catalytic transformations of glycerol. Amongst the latter, a great deal of attention has been paid to the dehydration of glycerol into acrolein [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], and more recently also to the oxidehydration of glycerol into acrylic acid [32], [33], [34], [35], [36], [37], [38], [39], [40]. Indeed, the latter reaction can be carried out either using a single bifunctional catalyst [34], [35], [36], [37], [38], or using an in-series two-bed configuration [40]. In fact, the reaction involves the two steps of glycerol dehydration into acrolein, and of acrolein oxidation into acrylic acid; therefore, the reaction can be carried out either with the one-pot approach or with the two steps separately, each one with the optimal catalyst designed for that step.
In a previous work, we reported that W–V–O bronzes with hexagonal tungsten bronze (HTB) structure [38] are catalysts for the one-pot transformation of glycerol into acrylic acid; the highest acrylic acid yield of 25% was obtained using a feed containing 2 mol% glycerol, 4% oxygen and 40% steam. The hexagonal tungsten oxide was an excellent system for glycerol dehydration into acrolein, both in the presence and in the absence of molecular oxygen, as also confirmed from literature data [17], whereas the V species, incorporated inside the WO3 structure, were the active sites for the oxidation of the intermediately formed acrolein into acrylic acid. The optimal atomic ratio between the two elements in the bronze was obtained at V/W ratios between 0.1 and 0.2; higher V contents led to a lower selectivity to acrylic acid because of the enhanced formation of carbon oxides, whereas lower V contents led to a less efficient transformation of acrolein into acrylic acid and to an higher selectivity to heavy compounds [38].
Here, we report about the study of catalytic behavior of W–Nb–O and W–Nb–V–O mixed oxides with HTB structure. In this case, we used Nb as a dopant of the W–V–O bronze, to enhance the selectivity to acrolein and hence to acrylic acid. In fact, Nb oxide and Nb-containing mixed oxides have been recently reported as catalysts giving good performance in glycerol dehydration into acrolein [13], [29], [30].
Section snippets
Catalyst preparation and characterization
W–V–Nb–O mixed oxides, with hexagonal tungsten bronze (HTB) structure, were prepared hydrothermally, from aqueous solution of ammonium metatungstate, vanadyl sulphate and niobium oxalate [38]. The gels were loaded in Teflon-lined stainless-steel autoclaves and heated at 175 °C for 48 h. The solid obtained was filtered off, washed and dried at 100 °C for 16 h. Finally, the solids were heat-treated at 600 °C during 2 h in a N2-stream. For comparison tungsten oxide and a W–V oxide bronzes, both with HTB
Chemical-physical characteristics of catalysts
Table 1 compiles the samples prepared, and summarizes their main characteristics. We prepared samples based on hexagonal WO3: (a) bi-component systems made of mixed oxides of either W–V or W–Nb, and (b) tri-component systems of W, Nb and V. In a previous work, we found that W–V–O bronzes with HTB structure showed a good catalytic performance in glycerol oxidehydration to acrylic acid only for specific W/V atomic ratios [38]. Therefore, we maintained a similar W/V ratio also in tri-component
Conclusions
Tri-component W–V–Nb mixed oxides with the hexagonal tungsten bronze structure can be prepared hydrothermally, and thermal treatment at 600 °C in nitrogen atmosphere. These monophasic systems possess both acid and redox features, which are needed for the one-pot oxidehydration of glycerol into acrylic acid, via intermediate formation of acrolein. The incorporation of Nb5+ leads to a lower surface density of acid sites compared to both WO3 and the bi-component W–V–O system, but to a greater
Acknowledgements
CIRI (Centro Interdipartimentale per la Ricerca industriale, Università di Bologna) Energia e Ambiente is acknowledged for the research grant assigned to Alessandro Chieregato. MIUR (Ministero dell’Università e della Ricerca Scientifica) is acknowledged for the PhD grant to Stefania Guidetti (Progetto Giovani). MIPAAF (Ministero delle Politiche Agricole Alimentari e Forestali) is acknowledged for partially financing the project (Progetto Extravalore, PNR 2005–2007). Financial support from
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