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with 31% selectivity at 61% 5-(hydroxymethyl)furoic acid conversion. 4-(Hydroxy- methyl)benzoic acid could then be oxidized to produce terephthalic acid. Note that using Brønsted acidic H-beta as the catalyst significantly decreased the yield of Diels-Alder dehydration products, indicating the crucial role of Lewis acid sites in these reactions. In another study, the conversion of tetrose sugars to a-hydroxy acid esters (such as methyl vinyl glycolate, methyl-4-methoxy-2-hydroxybutanoate, and a-hydroxy-g-butyrolactone), which were useful building blocks for degradable functional polyesters, was realized over Lewis acidic zeolites.15 With Sn-beta as the catalyst, 80% selectivity of a-hydroxy acid esters at 95% conversion of tetrose sugar was achieved. In particular, a very high turnover frequency of 330 mol molSn1 hr1 was attained, which was a more than 6-fold increase in compar- ison with homogeneous SnCl4$5H2O. In addition, the pore confinement of Lewis acidic catalysts strongly controlled the product selectivity. Microporous Sn-beta and Sn-MFI favored the formation of methyl vinyl glycolate, whereas mesoporous catalysts, such as Sn-MCM-41 and Sn-SBA-15, preferred the formation of more bulky methyl-4-methoxy-2-hydroxybutanoate. Another recent example of Lewis acidic zeolite catalysis was the synthesis of unsaturated diacid esters via aldol condensa- tion of keto esters.16 Traditional strategies for production of diacids and diesters from biomass-derived molecules suffered from poor selectivity and inefficient carbon utilization. With zeolites Sn-, Zr-, and Hf-beta as the catalysts, ethyl pyruvate was condensed into diethyl 2-methyl-4-oxopent-2-enedioate and diethyl 2-methylene-4-oxopentanedioate. In particular, Zr- and Hf-beta exhibited the best catalytic performance, giving the highest ethyl pyruvate conversions (>80%) with comparable selectivities (>64%) toward diesters. The heteroatoms at the Lewis acid sites in zeolite frameworks were crucial to this reaction. When Sn-beta, H-beta, and pure-silica beta were used as the catalysts, much lower conversions were observed. Multifunctional Catalysis The transformation of biomass into chemicals and fuels often undergoes multistep reactions, each of which might require a distinct catalyst. Zeolite catalysts can be fine-tuned with combined active sites to allow multistep reactions occurring in a ‘‘one-pot’’ way. For instance, zeolite Sn-Al-beta contains both Brønsted and Lewis acid sites because of the presence of both tetrahedral AlIII and SnIV, respectively, which can be used for the cooperative catalysis of multistep conversion of 1,3-dihy- droxyacetone into ethyl lactate (Figure 2C).17 During this multistep reaction, the Brønsted AlIII acid sites accelerated the dehydration of dihydroxyacetone to form pyruvic aldehyde, and the Lewis SnIV acid sites catalyzed the hydride shift of pyruvic aldehyde into ethyl lactate with ethanol. In comparison with Sn-beta containing only Lewis acid sites, the Brønsted acid sites in bifunctional Sn-Al-beta facilitated the rate-determining step of dihydroxyacetone dehydration, giving a record high ethyl lactate production rate (2,113 g kgSn-Al-beta1 hr1). Moreover, the use of a physical mixture of Sn-beta and Al-beta showed modest lactate yield, implying a cooperative effect between Brønsted and Lewis acid sites in Sn-Al-beta. Besides combining different acid sites, zeolites can form composite catalysts with noble-metal nanoparticles, exhibiting both the high activity of noble metals and the shape selectivity of zeolites. For instance, oxidizing bioethanol produced by fermentation, which contains up to 90% water, into high-value chemicals is very appealing. Metal/zeolite composite catalysts can be used for the catalytic oxidation of bioethanol to acetaldehyde with high activity and selectivity. In particular, a Au/silicalite-1 (MFI type) composite could be prepared by encapsulating hundreds of dispersed Au nanoparticles with a diameter of 2–3 nm in zeolite Chem 3, 928–949, December 14, 2017 933PDF Image | Applications of Zeolites in Sustainable Chemistry
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