Synthesis of Heterogeneous Catalysts for the Transformation of Glycerol

Abstract

In the present thesis, three different heterogeneous catalysts Li/ZrO2, modified CaO, and K/ZIF-8, were prepared to transform glycerol, the side product of the biodiesel industry, into glycerol carbonate. Out of all the literature-reported pathways, the transesterification route of dimethyl carbonate with glycerol was adopted to carry out glycerol carbonate synthesis. All the catalysts were prepared by the simple wet impregnation method. The structural properties of the prepared catalysts were analyzed by XRD, FTIR, and XPS techniques. Catalysts morphology and their surface properties were studied via SEM, HRTEM, and BET techniques. The basicity of all the prepared catalysts was determined through the CO2-TPD experiment. Considering the merits of the quantitative 1H-NMR technique, the same was applied for the first time to quantify glycerol carbonate. Two quantification equations were proposed based on the 1H-NMR spectra obtained from the analysis of mixtures having varied molar ratios of glycerol and glycerol carbonate (GLC). The results obtained from both the quantification equations were in agreement with the actual molar ratios of the glycerol carbonate taken. Moreover, the results attained from the qHNMR technique were in line with that of the HPLC technique. Nevertheless, the quantification formula applied to the real sample provided promising results. In view of the requirement of the basic catalyst for the transesterification reaction of dimethyl carbonate (DMC) with glycerol (GL), basic properties of the alkali and alkaline earth metals (Li, Na, K, Ca, and Mg) and thermal stability of the ZrO2 were tuned together by the wet impregnation method forming metal impregnated ZrO2 catalysts. The application of the prepared catalysts for the glycerol carbonate synthesis revealed that 100 % GLC selectivity was achieved with all the prepared catalysts; however, 20 wt% Li-loaded ZrO2 catalysts provided better GLC yield (91 %). The better catalytic activity of 20-Li/ZrO2 catalyst was attributed to the formation of a single active Li2ZrO3 phase which was dependent upon the calcination temperature and LiOH concentration employed for the catalyst preparation. A decline in the catalytic activity during reusability was observed owing to the structural changes but selectivity remained unchanged. Besides determining the catalytic activity, a kinetic study was also conducted, revealing that the transesterification reaction followed second-order kinetic equation. To find out the solution to the active site poisoning of the CaO by atmospheric moisture, attempt was made to make its surface hydrophobic. Various organic modifiers (aliphatic and aromatic) were grafted over the CaO surface to restrain its moisture sensitivity. Besides imparting stability to the native CaO, the surface modification also increased the basic strength of the native catalyst. Besides providing a good glycerol conversion level (82 %), the benzyl bromide modified CaO catalyst was found to impart better moisture resistance up to 5 wt% H2O content in the glycerol. Even though the catalytic activity of the modified CaO catalyst got reduced upon repeated use, but was better than native CaO in every catalytic cycle. Further efforts were made to enhance the reusability and moisture resistibility of the catalyst without sacrificing its activity. For this purpose, the stable crystalline framework and hydrophobic properties of ZIF-8 were explored. The basicity of ZIF-8 was improved by loading KOH onto its surface following the wet impregnation method. Prepared K/ZIF-8 catalyst demonstrated high catalytic activity towards the transesterification reaction of DMC with GL (95 % glycerol conversion) and stability up to 3 consecutive reaction cycles.