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

Abstract

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 SnCl 4 ·5H 2 O in dry isopropanol. Characterization of the thus-prepared Al-containing Snβ (Sn/pDeAlβ) zeolites with ICP, (pyridine probed) FTIR, and 27 Al 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, 119 Sn Möβbauer, XPS, (pyridine probed) FTIR, and UV–vis. A closed Sn IV configuration was implied by comparing of 119 Sn 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.