Abstract
Abstract We report the preparation of pore expanded and sulfonic acid functionalized mesoporous silica catalysts by using different kinds of swelling agents such as 1,3,5-trimethyl benzene (TMB), n-decane (D), n-tridecane (TD), and n-hexadecane (HD) at two different ratios of swelling agent/surfactant. Non-swelled mesoporous silica catalyst, SBA-15-SO3H, was also prepared for comparison. The performance of the sulfonic acid functionalized solid acid catalysts by using one-pot synthesis procedure was evaluated in terms of their catalytic activity in the esterification of propionic acid with methanol. The catalysts have been characterized by X-ray diffraction to investigate the phase transition between the hexagonally packed arrangement of cylindrical pores and mesocellular silica foams (MCF), and N2 adsorption/desorption technique in terms of pore size and distributions. It was concluded from the XRD and N2 adsorption/desorption analyses that hexagonally packed and cylindrical pore structure is maintained only for low ratio of swelling agent/surfactant and the increase of the dimension of a template micelle is accompanied by an increase in structural disruption, which was attributed to the phase transition from highly ordered hexagonal arrangement to mesocellular foam (MCF) phase with large nodded pore structures rather than cylindrical pores. Among the different swelling agents, TMB and D were found to be effective for the increase in pore diameter (up to 8 and 7.8 nm), whereas, TD and HD (6.6 nm) are effective to maintain the ordered X-ray diffraction patterns resulting mesoporous materials without giving enlargement of pore diameter in comparison with TMB and D. Pore expanded catalysts, SBA-15-SO3H-TMB-0.1 and SBA-15-SO3H-D-0.1, exhibit 68 and 43% enhancement in turnover frequency toward propionic acid methanol esterification, respectively, over non-swelled SBA-15-SO3H, despite possessing similar acid strengths. The increased activity especially for SBA-15-SO3H-TMB-0.1 and SBA-15-SO3H-D-0.1 likely reflects the accessibility to the active sulfonic acid sites as well as acidity.
Highlights
Neuroenhancement? (TD), and n-hexadecane (HD) at two differentasraptoiosssibolef anldikseulycceressfl-eocritesnttehde aapcpcleicsastiiboinl,itiynstioghthtseaancdtipvreofsituslfwointhicouatctihde swelling agent/ surfactant
Upon increasing the ratio to 0.5, the increase of the dimension of a template micelle is accompanied by an increase in structural disruption. It was concluded from the XRD results that the increasing trimethyl benzene (TMB)/Pluronic-123 ratio results in phase transition from highly ordered hexagonal arrangement to mesocellular foam (MCF) phase with large nodded pore structures rather than cylindrical pores [2]
The pore-expanded SBA-15-SO3H acid catalysts have been synthesized with the addition of TMB, D, TD, and HD swelling agents by using the one-pot modification method for the first time and applied to the esterification of propionic acid with methanol
Summary
4 g of Pluronic-123 triblock copolymer (Aldrich) was dissolved in 125 mL of 1.9 M HCl at 40oC. There are some disadvantages about direct functionalization, that is: the degree of mesoscopic order decreases with the increasing concentration of trialkoxyorganosilane due to the homocondensation instead of condensation with silica precursor and care must be taken not to destroy the organic functionality during the removal of the surfactant, which is why commonly only extractive methods can be used, and calcination is not suitable in most cases. For this reason, the surfactant and the micelle expanders were removed by refluxing the materials in 90:10% (v/v) ethanol/HCl mixture for 24 h
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