Acidity tuning of montmorillonite K10 by impregnation with dodecatungstophosphoric acid and hydroxyalkylation of phenol

Abstract

Acidity tuning of montmorillonite K10 (mont K10) was achieved by impregnating with dodecatungstophosphoric acid (DTP). The effect on the hydroxyalkylation of phenol was studied at 353 K with phenol to formaldehyde molar ratio of 5. The nature and strength of acid sites were determined by NH₃-TPD measurement while the distribution of Brønsted and Lewis acid sites expressed as B/L ratio, was determined by pyridine IR technique. Among various loadings of DTP (5–60%) studied for the hydroxyalkylation of phenol, 20% DTP/mont K10 showed the highest catalyst activity (90% selectivity to bisphenol F with 28% conversion of phenol). Both total concentration of acid sites and the distribution of acid sites in a high temperature region were required for the high bisphenol F selectivity. Our catalyst (20% DTP/Mont K10) could be recycled three times.

Introduction

Due to their specific catalyst activity and recyclability as compared to the conventional acidic reagents, the use of solid acid catalysts has tremendously increased in the fine chemical industry (Corma et al., 2001, Okuhara, 2002, Garade et al., 2009a, Garade et al., 2009b, Garade et al., 2009c, Modrogan et al., 2009, Yadav and Kirthivasan, 1997, Rode et al., 2009). Owing to its inherent advantages like low toxicity and corrosive nature over conventional reagents, several industrial processes based on the use of acidic reagents in the stoichiometric amount were replaced by the solid acid catalyzed processes (Bhure et al., 2007, Bhure et al., 2008, Kozhevnikov, 1998, Misono, 2001). Hydroxyalkylation of phenol to bisphenol F is one such important reaction widely used in the fine chemical industry (De Angelis et al., 2004, Jana et al., 2004, Jana et al., 2005). Since the synthesis of bisphenols is a consecutive reaction, controlling the product selectivity particularly to bisphenol F is a challenging task in the field of catalysis. For the catalyst having lower acidity and acid strength, the predominant formation of carbinol was observed which decreases the bisphenols selectivity while for the catalyst having higher acidity and acid strength, the predominant formation of trimers was observed (Garade et al., 2009a). Hence, developing the catalyst with appropriate acidity and strength to give higher activity and selectivity to bisphenols is highly desirable. Several solid catalysts such as zeolites, clay minerals, ion exchange resins and amorphous aluminosilicates have been reported for the hydroxyalkylation reaction (Perego et al., 2000, Barthel et al., 2000, Alvaro et al., 1998, Alvaro et al., 2003, Yadav and Kirthivasan, 1997, Reincker and Gates, 1974, Das et al., 2001). In this context, heteropolyacids are also a good alternative due to its stronger acidity (Mizuno and Misono, 1998, Hou and Okuhara, 2003). However, heteropolyacids have very low (< 8 m²/g) surface area and very high acidity which generally decreased bisphenols selectivity due to the formation of unwanted side products (trimers and its higher homologue) (Garade et al., 2009a). Hence, better dispersion of heteropolyacids on a suitable support is highly imperative for the hydroxyalkylation of phenol. Although, mesoporous silica is widely used as support (Kuang et al., 2003, Izumi et al., 1995, Yadav and Manyar, 2003), montmorillonite clays are also a good alternative due its high surface area. In this paper, we report the effect of acidity tuning of montmorillonite K10 (mont K10) on the hydroxyalkylation of phenol to bisphenol F (Scheme 1). The acidity of mont K10 was tuned by dispersing dodecatungstophosphoric acid (DTP) on mont K10 support by wet impregnation method. Among various solid acid catalysts screened, 20% DTP/Mont K10 was found to be the most active catalyst for the hydroxyalkylation of phenol. Ammonia TPD studies of various catalysts showed that the acid sites present in a high temperature region are highly desirable for the high catalyst activity and bisphenol F selectivity. The catalyst was successfully recycled for three times.