دانلود رایگان مقاله نقش کاتالیزور نوع Keggin heteropolyacids برای استیله گلیسرول

ABSTRACT

 The heterogeneously catalyzed esterification (acetylation) of glycerol toward triacetin in batch mode in presence of toluene as entrainer was studied. Silicotungstic acid, tungstophosphoric acid and phosphomolybdic acid as heteropolyacids (HPAs) supported on silica, alumina or silica-alumina were used as catalysts. The course of the reaction was found to be very sensitive to the nature of the HPA as well as the support. Solid characterization by Raman spectroscopy, XRD, and pyridine-FTIR revealed that only combinations of tungsten-based HPAs and silica support were able to preserve the structure of active component throughout the preparation process, which was essential to obtain active and selective catalysts. The interaction between HPA and support was decisive for stability and dispersion of the catalytically active species. With the best performing catalyst H4SiW12O40/SiO2, selectivity to triacetin reached 71% at complete conversion within 24 h. The high selectivity to triacetin is attributed the Brønsted acidic sites originated from stabilized Keggin structure and continuous removal of water during course of reaction. Toluene is able to form azeotropic mixtures with water and acetic acid and keeps the reaction temperature below the boiling point of acetic acid. Thus, water-free reaction conditions can be established. The catalyst was reusable; however, the activity and selectivity towards triacetin slightly decreased in a repetition run due to loss of active sites.

4. Conclusions

The combination of various HPAs and common support materials led only to few supported catalysts with remarkable performance that were able to produce triacetin (TAG) at high yield (71%) within acceptable time periods (24 h). Such catalysts were exclusively composed of tungsten-containing HPAs supported on silica. To achieve good performance, high Brønsted acidity is required, which seems to be linked first of all to the preservation of the Keggin structure of the HPAs with its high number of reactive protons. Other prerequisites to get effective catalysts of this type are the thermal stability of HPAs and a tuned strength of their interaction with the support. This is ensured if supports like silica without any Brønsted and Lewis sites are used and the deposited amount of HPA is rather high, otherwise, surface reactions of silanol groups with Keggin units lead to undesired surface species. Beyond that, it seems to be detrimentalto use supports that possess remarkable intrinsic acidity, independent whether this is of Brønsted or Lewis type. This is typical for the alumina containing supports in this study. In such case, the acid sites of HPA and support may react with each other partially during the calcination at elevated temperature.