Development of niobium oxide based catalysts

Abstract

One of the most promising molecules in biomass valorization is the xylose (XYL), a five carbon sugar, that once dehydrated in acid media, can produce furfural (FUR). In turn, FUR is an aldehyde that may produce furfuryl alcohol (FA) by hydrogenation. It is known that the presence of Lewis and Brønsted acid sites may transform XYL to FUR, and the presence of a metal phase can reduce FUR to FA. Among many researched materials, the hydrated niobium oxide (Nb 2 O 5 •nH 2 O) may provide both sites, due to its acidity. Besides, it is possible to modify Nb 2 O 5 structure with an acid treatment to provide new groups.Then, it is feasible to produce a catalyst supported on different niobium oxides to try the one-pot production of FA from XYL and to test the influence of the acid treatment. On this work, Co, Ni and Ru were supported using incipient wetness impregnation on two types of niobium oxides calcined at 400 o C for 4 h: calcined Nb 2 O 5 -NB400 or Nb 2 O 5 treated with H 3 PO 4 at 1 mol/L -NBAc400. For the base metals, the nominal amount was 10 wt.% (CoNB10, CoNBAc10, NiNB10 and NiNBAc10) whereas for the noble metal the content was 5 wt.% (RuNB05 and RuNBAc05). Our solids were characterized by N 2 adsorption, SEM/EDX, XRD, H 2 -TPR, XPS and NH 3 -TPD. The catalysts supported on NB400 showed diminution of its area, up to 60% in case of RuNB05. After the activation on hydrogen at 400 o C and 2 h, the TPR and XPS showed that the Co active phase was a majority of oxides, Ni samples were a mixture of oxides and reduced metal and the Ru solid was reduced. Furthermore, the acidity distribution of the sites was modified due to the acid treatment, with the increase of medium sites. After 6 h of reaction on water at 160 o C and 50 bar of H 2 pressure, the CoNB10 solid presented 14% of FA yield, while the treated catalyst CoNBAc10 had 8%. The same trend was reported on Ni and Ru solids -16% and 17% of FA yield on NiNB10 and RuNB05, whereas 2% and 0% on NiNBAc10 and RuNBAc05. However, Ni and Ru solid showed superior yields for xylitol (XOL) and tehtrahydrofurfuryl alcohol (THFA). The RuNB05 solid had 39% and 20% of XOL and THFA yields, as the content of metal sites was higher than Ni and Co solids. Besides, after one cycle of reaction, the treated catalysts showed P and metal leaching up to 30%.Therefore, the acid treatment did not improve neither the selectivity for hydrogenation products nor the stability of metal phases on the catalysts.