The production of biofuels additives on sulphonated MCF materials modified with Nb and Ta—Towards efficient solid catalysts of esterification

Abstract

The aim of this work was to compare the properties of niobium (Si/Nb=118) and tantalum (Si/Ta=121) containing mesostructured cellular foams (MCF) modified with (3-mercaptopropyl) trimethoxysilane (MPTMS) in one-pot synthesis and the application of materials obtained in acetic acid esterification with glycerol. TaMCF materials were prepared for the first time. Catalysts were characterized by different techniques: N2 adsorption, XRF, elemental and thermal analyses, FTIR+pyridine adsorption and titration of acid sites. New catalysts have showed an excellent catalytic activity in the acetic acid esterification with glycerol to yield triacetylglycerol, also known as triacetin (a valuable additive for biodiesel composition). The influence of modifiers on the number and strength of acidic centres in the materials obtained was found and discussed in details in the paper. The strength of Brønsted acid centres (BAS) was estimated by the interaction of organosilane with transition metal via the use of methoxy groups from MPTMS. The highest concentration of acidic sites determined by NaOH titration was obtained on tantalum modified MCF, whereas the highest strength of BAS was observed on MP-NbMCF sample. The catalytic behaviour of MPTMS and Nb modified MCF samples was comparable to that observed for Amberlyst 15 and better than that of Nafion SAC 13. The materials obtained could be reused and showed stable catalytic features in subsequent cycles. The addition of tantalum allows obtaining a new catalyst with high efficiency of thiol species oxidation (higher than reported before for other catalysts). MPTMS modified TaMCF material appeared to be attractive for further study in different reactions requiring high concentration of acidic centres.