Three-step cascade over a single catalyst: synthesis of 5-(ethoxymethyl)furfural from glucose over a hierarchical lamellar multi-functional zeolite catalyst
The synthesis of hierarchical lamellar zeolites with a controlled meso-/microporous morphology and acidity is an expanding area of research interest for a wide range of applications. Here, we report a one-step synthesis of a hierarchical meso-/microporous lamellar MFI–Sn/Al zeolite (i.e., containing both Lewis acidic Sn- and Al-sites and a Brønsted acidic Al–O(H)–Si site) and its catalytic application for the conversion of glucose into 5-(ethoxymethyl)furfural (EMF). The MFI–Sn/Al zeolite was prepared with the assistance of a diquaternary ammonium ([C22H45–N+(CH3)2–C6H12–N+(CH3)2–C6H13]Br2−, C22-6-6) template in a composition of 100SiO2/5C22-6-6/18.5Na2O/xAl2O3/ySnO2/2957H2O (x = 0.5, 1, and 2; y = 1 and 2, respectively). The MFI–Sn/Al zeolites innovatively feature dual meso-/microporosity and dual Lewis and Brønsted acidity, which enabled a three-step reaction cascade for EMF synthesis from glucose in ethanol solvent. The reaction proceeded via the isomerization of glucose to fructose over Lewis acidic Sn sites and the dehydration of fructose to 5-hydroxymethylfurfural (HMF) and then the etherification of HMF and ethanol to EMF over the Brønsted acidic Al–O(H)–Si sites. The co-existence of multiple acidities in a single zeolite catalyst enabled one-pot cascade reactions for carbohydrate upgrading. The dual meso-/microporosity in the MFI–Sn/Al zeolites facilitated mass transport in processing of bulky biomass molecules. The balance of both types of acidity and meso-/microporosity realized an EMF yield as high as 44% from the glucose reactant.