Carbon Nanotube/Zeolite Hybrid Catalysts for Glucose Conversion in Water/Oil Emulsions

The isomerization of glucose to fructose and its subsequent dehydration to hydroxymethylfurfural (HMF) have been investigated on nanohybrid catalysts that stabilize emulsions comprising aqueous and organic phases. Significant improvement in catalyst stability was observed when NaX faujasite catalysts were functionalized with multiwalled carbon nanotubes (MWCNT-NaX), with a large fraction of the initial activity and selectivity preserved after several recycles. The combination of MWCNT-NaX, containing Lewis acid sites, and MWCNT-SO₃H, containing Brønsted acid sites, enables glucose isomerization and fructose dehydration at high conversion and HMF selectivity. The use of a water/oil biphasic emulsion favors the continuous separation of the HMF product into the organic phase. Furthermore, selective conversion of HMF into added-value products can be accomplished in the same emulsion by incorporating a metallic function on the amphiphilic nanohybrids in the presence of hydrogen. Depending on the metal used, different final products can be obtained. For example, when Ru was added, the main product was 2,5-hexanedione (47.8 mol %), followed by 2,5-bis(hydroxymethyl)furan (15 mol %) and γ-hydroxyvaleric acid (7.8 mol %). When Pd was used, γ-hydroxyvaleric acid (84 mol %) dominated the product distribution, with only small amounts of 2,5-bis(hydroxymethyl)furan (2.9 mol %).