Catalytic valorization of industrial grade sugarcane molasses to 5-hydroxymethylfurfural in water

Developing methods to transform industrial biomass waste into platform chemicals and materials is an important part of the EU circular bio-based economy. Thus, the present work focuses on the valorisation of crude sugarcane molasses (SCM) to 5-hydroxymethylfurfural (HMF) over starch-derived catalysts in water. A series of monofunctional catalysts having sulfonic, phosphate, or oxygen functionalities as Brönsted acid (BA) sites and a bifunctional catalyst containing phosphate groups as BA and SnO2 as Lewis acid (LA) sites was prepared via one-pot hydrothermal carbonisation of starch. The effect of the varied active sites on these catalysts was tested in the cascade conversion of SCM to HMF in water using a microwave reactor. The results revealed that the maximum yield of HMF obtained from SCM depends solely on the chemistry of the catalyst's surface. It is independent of the catalyst's texture and is not proportional to the total acidity (Atot). A promising HMF yield of around 30% was obtained from crude SCM in pure water over sulfonated hydrochar catalyst containing –SO3H, along with –OH and –COOH functionalities. The sulfonic groups were found to be the most selective towards fructose dehydration to HMF out of all of the BA sites tested here. On the other hand, introducing LA sites to the catalyst increased the conversion of glucose and selectivity to HMF compared to the monofunctional counterpart. However, the sugar conversion rate was lower over the bifunctional catalyst than that using sulfonated hydrochar. The catalytic system studied here was found to be suitable for high loadings of SCM. However, the best catalyst suffered a drop in activity after the first re-use. In general, obtaining high yields of HMF from SCM in water was hindered by a lack of effective isomerization of glucose to fructose and a high rate of side reactions producing humin, which were catalysed by impurities present in SCM. The transformation of SCM to HMF with sulfonated hydrochar occurred at a far slower rate than that of a solution of synthetic SCM (neat mixture of sugars) under similar conditions, which pointed to the negative effect of the impurities in the SCM matrix. Thus, it was shown that even though SCM is an interesting alternative feedstock for biorefineries, its purification prior to catalytic valorization might be necessary.