Synthesis and evaluation of stable polymeric solid acid based on halloysite nanotubes for conversion of one-pot cellulose to 5-hydroxymethylfurfural

Abstract

Based on halloysite nanotubes (HNTs), precipitation polymerization and Pickering emulsion polymerization were firstly adopted to synthesize two composites i.e. HNTs–polystyrene(PSt)–polydivinylbenzene(DVB)(I) and HNTs–PSt–PDVB(II), respectively. After sulfonation by 98% H₂SO₄, two polymeric solid acid catalysts i.e. HNTs–PSt–PDVB–SO₃H(I) and HNTs–PSt–PDVB–SO₃H(II) were successfully prepared for conversion of one-pot cellulose to 5-hydroxymethylfurfural (HMF) in an ionic liquid 1-ethyl-3-methyl-imidazolium chloride ([EMIM]-Cl). Characterization of two catalysts showed that HNTs–PSt–PDVB–SO₃H(I) possessed superior hydrophobicity, content of –SO₃H groups, and total acidic amounts to those of HNTs–PSt–PDVB–SO₃H(II), but with less very strong acidic sites and non-uniform morphology. Then the amount of the catalysts, reaction time and reaction temperature were optimized for cellulosic conversion over the two catalysts, and a maximum yield of 28.52% for HNTs–PSt–PDVB–SO₃H(I) and 32.86% for HNTs–PSt–PDVB–SO₃H(II) under the optimized conditions was obtained, indicating very strong acidic sites played a key role in cellulose conversion. In addition, the two as-prepared catalysts could be very easily recycled at least five times without significant loss of catalytic activity.