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

Lewis acid Sn beta-type zeolites with varying amounts of Bronsted acid Al in the framework were synthesized using a simple two-step procedure comprising partial dealumination of beta zeolite under action of acid, followed by grafting with SnCl4 center dot 5H(2)O in dry isopropanol. Characterization of the thus-prepared Al-containing Sn beta (Sn/pDeAl beta) zeolites with ICP, (pyridine probed) FTIR, and Al-27 MAS NMR demonstrates the presence of Bronsted acid framework Al-III. Tetrahedral Lewis acidic Sn-IV is present, as ascertained by a combination of techniques such as EPMA, Sn-119 Mo beta bauer, XPS, (pyridine probed) FTIR, and UV-vis. A closed Sn-IV configuration was implied by comparing of Sn-119 solid-state MAS NMR and deuterated acetonitrile probed FTIR spectra with literature. The catalytic activity of the Al-containing Sn beta 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 Sn beta reference and Sn/pDeAl beta, 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/pDeAl beta because Bronsted 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 Bronsted Al-III acid sites is verified by comparing catalytic data with physical mixtures of partially dealuminated beta zeolite and Al-free Sn beta.