Cooperative Catalysis for Multistep Biomass Conversion with Sn/Al Beta Zeolite

Lewis acid Snβ-type zeolites with varying amounts of Brønsted acid Al in the framework were synthesized using a simple two-step procedure comprising partial dealumination of β zeolite under action of acid, followed by grafting with SnCl₄·5H₂O in dry isopropanol. Characterization of the thus-prepared Al-containing Snβ (Sn/pDeAlβ) zeolites with ICP, (pyridine probed) FTIR, and ²⁷Al MAS NMR demonstrates the presence of Brønsted acid framework Al^III. Tetrahedral Lewis acidic Sn^IV is present, as ascertained by a combination of techniques such as EPMA, ¹¹⁹Sn Möβbauer, XPS, (pyridine probed) FTIR, and UV–vis. A closed Sn^IV configuration was implied by comparing of ¹¹⁹Sn solid-state MAS NMR and deuterated acetonitrile probed FTIR spectra with literature. The catalytic activity of the Al-containing Snβ was tested for the conversion of 1,3-dihydroxyacetone (DHA) into ethyl lactate (ELA), proceeding via pyruvic aldehyde (PAL). Despite the difference in synthesis between the classic hydrothermal Snβ reference and Sn/pDeAlβ, the activity of Sn for the Lewis acid-catalyzed hydride shift of PAL to ELA is similar. Yet, the overall reaction rate of DHA into ELA is faster with Sn/pDeAlβ because Brønsted acidity of the remaining framework Al^III facilitates the rate-determining dehydration of DHA into PAL. Materials containing moderate amounts of Al (0.3 wt % Al) show the highest ELA productivities, leading to a record value of 2113 g ELA·kg catalyst^–1·h^–1 at 363 K. The cooperative effect of Lewis Sn^IV and Brønsted Al^III acid sites is verified by comparing catalytic data with physical mixtures of partially dealuminated β zeolite and Al-free Snβ