No abstract

No abstract

The main objective of this work is to determine which kind of acidic sites, Brønsted (BAS) or Lewis (LAS), are more active and stable in the glycerol acetalization to solketal by means of Raman-monitoring. For this purpose, the mesoporous cellular foams (MCFs) were modified to obtain materials containing exclusively BAS occurring in different number and strength (samples MP-MCF (MP = (3mercaptopropyl)trimethoxysilane) and CS-MCF (CS = 2-(4chlorosulfonylphenyl)ethyltrimethoxysilane)) or the material containing exclusively LAS (NbMCF). Moreover, the materials containing both types of centers in different ratios (MoMCF and TaMCF) were also studied. Real-time Raman monitoring of the catalytic reaction allowed us to observe not only changes in substrates concentration and product yield but also the transformation of acetone−glycerol adduct (O•••CO vibrations) formed in the presence of a catalyst. The activity of the catalysts containing BAS depends on the number and strength of these centers, and the best performance was observed for CS-MCF. The maximum of solketal yield and the equilibrium state were achieved after 2 min from the start of the Raman monitoring. NbMCF containing exclusively LAS exhibited both the lowest conversion of glycerol and stability. The role of different recycling treatments was probed.

No abstract

The acetalization of glycerol with acetone over modified mesoporous cellular foam materials has been widely investigated using in situ Raman spectroscopy during reaction. Mesoporous cellular foams (MCFs) modified by niobium or tantalum and (3-mercapto­propyl)­tri­methoxy­silane (MP) followed by H2O2 treatment were used as catalysts in the acetalization of glycerol with acetone. The influence of the type of catalyst, which determines the solid texture and number of Brønsted acid sites, and different reaction parameters, such as reaction time, reaction temperature, glycerol/acetone ratio, and catalyst amount on acetalization reaction, were investigated. The results obtained in the characterization of the catalysts show that the materials obtained differ in the number of Brønsted acidic sites. Raman spectroscopy provides noninvasive insight during acetalization of glycerol with acetone in the presence of acid heterogeneous catalysts. The progress of the acetalization reaction was monitored following the variation in intensity of characteristic Raman bands and using chemometric analyses. The results obtained by real-time Raman monitoring confirm the mechanism proposed for the reaction, which proceeds via the formation of the 3-(2-hydroxy­propan-2-yloxy)­propane-1,2-diol intermediate, whose presence is confirmed by Raman spectroscopy. Under optimal reaction conditions, the 5-membered ring ketal 2,2-di­methyl-1,3-di­oxolane-4-yl methanol (solketal) was obtained with the highest selectivity (99%). Raman monitoring enables real-time control of the reaction, thus enabling the optimization of reaction conditions for a more efficient reaction. Raman monitoring illustrates the reversibility of the reaction upon evaporation of acetone, even under reflux.