Selective esterification of glycerol with oleic acid using tungstophosphoric acid supported on functionalized mesoporous silica

Abstract

A series of protonated amine-functionalized SBA-15 materials was successfully prepared. Each sample of mesoporous SBA-15 was first functionalized with each of the three types of amino-organosilanes: APTES (N), AAPTMS (NN), and DETTMS (NNN). Each of the resulting materials was then protonated with Keggin-type tungstophosphoric acid (HPW) at two different concentrations: 20 wt.% (H20) and 40 wt.% (H40). The six types of materials obtained were labelled as S-N1-H20, S-N1-H40, S-NN1-H20, S-NN1-H40, S-NNN1-H20, and S-NNN1-H40. The materials were fully characterized by XRD, N2 adsorption-desorption, FT-IR, SEM/EDX, TEM, and elemental analysis. The characterization results show that the synthesized materials exhibited highly ordered hexagonal mesoporous rope-like structure, indicating that the structure of the SBA-15 support was preserved after the modification with amino-organosilanes and HPW. The surface area, pore diameters, and acidity of the synthesized materials were in the range of 55 to 299 m2 g-1, 6.18 to 7.05 nm, and 0.18 to 0.49 mmole g-1, respectively.

The synthesized materials were tested as catalysts for the esterification of glycerol with oleic acid to produce monoolein using a glycerol/oleic acid molar ratio of 4:1 at 160 °C for 3 h and with 2.5 wt.% of catalyst loading. The results indicate that the conversion of oleic acid and the yield of monoolein were influenced by the acidity and pore characteristics of the catalysts. In particular, S-NN1-H40 exhibited the highest oleic acid conversion (95%) and monoolein yield (56%). Functionalizing SBA-15 with aminosilanes prior to HPW addition was also found to enhance the distribution of HPW throughout the support and, in turn, improved the catalytic efficiency. The best-performing catalyst in this study, S-NN1-H40, also exhibited good reusability whereby no significant loss in catalytic activity was observed for at least six catalytic cycles. The kinetics of the esterification catalyzed by S-NN1-H40 was first order with an activation energy of 35.45 kJ mol-1.