Environmentally Benign Bifunctional Solid Acid and Base Catalysts

Abstract

ABSTRACT:Solid bi-functional acid-base catalysts were prepared in two ways on an amorphous silica support: 1) by grafting mercaptopropyl units (followed by oxidation to propylsulfonic acid) and aminopropyl groups to the silica surface (NH 2 -SiO 2 -SO 3 H), and 2) by grafting only aminopropyl groups and then partially neutralising with phosphotungstic acid, relying on the H 2 PW 12 O 40 -ion for surface acidity (NH 2 -SiO 2 -NH 3 + [H 2 PW 12 O 40 -], denoted as NH 2 -SiO 2 -PTA).Surface acidity and basicity were characterised by adsorption calorimetry, using SO 2 as a probe for surface basicity and NH 3 for surface acidity. Catalytic activities were compared in a two-stage cascade: an acid-catalysed deacetalisation followed directly by a base-catalysed Henry reaction. Overall, the NH 2 -SiO 2 -SO 3 H catalysts showed higher concentrations and strengths of both acid and base sites, and higher activities than NH 2 -SiO 2 -PTA. Both catalysts showed evidence of cooperative acid-base catalysis. Importantly, the bi-functional catalysts exhibited catalytic advantage over physical mixtures of singly functionalised catalysts. 3 INTRODUCTIONMany liquid phase processes in fine chemicals synthesis require acid or base catalysts. Conventionally, homogeneous acids and bases dissolved in the reaction mixtures are used. In most cases, replacing these with solid acids and bases brings substantial environmental benefit by reducing waste and simplifying product separation. 1-3Almost inevitably, however, solid acids and bases are less active than their homogeneous counterparts. Despite this, one case where they might be able to offer particular advantage is where both acid and base catalysis is required, either in sequential steps or through a cooperative catalytic mechanism. A difficulty associated with preparation of the aminopropyl/propylsulfonic acid bi-functional catalysts (NH 2 -SiO 2 -SO 3 H) is that the acid group is grafted to the silica using (3-mercaptopropyl)trimethoxysilane (MPTMS) and an oxidation step is required to convert the thiol (-SH) to sulfonic acid (-SO 3 H). The two main routes reported are based on hydrogen peroxide/sulfuric acid, and on nitric acid (as both oxidant and acidifier). 11,12 The method used must be chosen to effectively oxidise thiol 12 but with minimal reaction with the base groups. Nitric acid has been shown to be the more effective reagent under these conditions 13 so nitric acid is used as the oxidant in the work reported here.An amorphous silica gel has been used as the catalyst support, rather than the ordered SBA-15 silica used in our previous work. Silica gel does not exhibit the friability and low bulk density of SBA-15 14-16 while the surface properties and stability are similar, and it can be functionalised in the same way. The silica gel used here has been chosen to have an average pore diameter similar to that of SBA-15. The tandem deacetalisation-Henry reaction shown in Scheme 2 has been used. The first step is the acid-catalysed deacetalisation of benzaldehyde dimethyl acetal i...

Hydrothermally Stable, Conformal, Sulfated Zirconia Monolayer Catalysts for Glucose Conversion to 5-HMF

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