One-pot fructose conversion into 5-ethoxymethylfurfural using a sulfonated hydrophobic mesoporous organic polymer as a highly active and stable heterogeneous catalyst

Abstract

We report a sulfonated hydrophobic mesoporous organic polymer (MOP-SO₃H) as a highly efficient heterogeneous catalyst for one-pot 5-ethoxymethylfurfural (EMF) production from fructose in ethanol solvent. MOP-SO₃H was fabricated by co-polymerization of divinylbenzene (DVB) and sodium p-styrene sulfonate (SPSS) followed by ion exchange with dilute H₂SO₄, and its pore structure and acid density could be tuned easily by varying the mole ratio of SPSS to DVB. ³¹P MAS NMR analysis using trimethylphosphine oxide as a base probe molecule indicated that MOP-SO₃H possessed a weaker Brønsted acid site than conventional cation-exchange resins. The superhydrophobic properties of MOP-SO₃H were retained even after incorporating a greater number of sulfonic acid groups into the polymer framework, while conventional solid acid resins exhibited hydrophilic properties. MOP-SO₃H exhibited a superior catalytic performance in comparison with conventional acid resins, a mesoporous acid catalyst, and homogeneous acid catalysts in EMF production from fructose. After optimization of various reaction conditions using MOP-SO₃H, a high EMF yield of 72.2% at 99.3% fructose conversion was achieved at 100 °C in a very short reaction time of 5 h. Notably, MOP-SO₃H showed a much higher EMF formation rate than the Amberlyst-15 catalyst (53.5 vs. 6.1 μmol g⁻¹ min⁻¹). This superior performance of the MOP-SO₃H catalyst was attributed to its unique feature of large surface area containing a large quantity of readily accessible acid sites distributed throughout the hydrophobic polymer framework. In addition to its high catalytic activity, the notable stability of the MOP-SO₃H catalyst was also confirmed by leaching and recyclability tests. Thus, owing to its excellent catalytic performance and easy scalability, MOP-SO₃H can potentially be used as an industrial heterogeneous catalyst to produce EMF from various fructose-containing biomass.