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species (e.g., H2O) in the pores of zeolites can be removed, leaving void space for the selective entrance of guest species of suitable size, shape, and polarity. This is known as the molecular-sieving effect of zeolites. Moreover, the spatial confinement of zeolite pores coupled with catalytically active sites in their frameworks endows zeolites with unique shape-selective catalysis toward the formation of specific products. Finally, zeolites can act as host matrices to encapsulate and stabilize metal clusters or nanoparticles, forming multifunctional composite materials with superior properties. Because of these unique characteristics as well as their high hydrother- mal stability and low production costs, zeolites have been widely used as highly efficient catalysts, detergents, adsorbents, and ion exchangers in many chemical processes.4,5 In particular, zeolites are currently the most important heterogeneous catalysts in the oil-refining and petrochemical industries, producing the majority of the world’s gasoline from crude oil via fluid catalytic cracking.6 Besides their traditional applications in the chemical industry, zeolites are playing an increasingly important role in many sustainable processes, particularly in the fields of renewable energy and environmental improvement, such as biomass conversion, fuel cell, thermal energy storage, CO2 capture and conversion, air-pollution remedi- ation, and water purification. In this review, we present the recent progress in zeolite applications in sustainable chemistry, and the key challenges in related fields are also briefly discussed. BIOMASS CONVERSION In recent decades, biomass has become a promising sustainable feedstock to substitute non-renewable fossil resources. Zeolites have been widely used as the catalysts to convert biomass feedstock into high-value biofuels and platform chemicals. There are already a number of comprehensive reviews on biomass conversion over zeolites.7,8 In this context, we present only a few examples to demonstrate the recent applications of zeolites in biomass conversions over their unique active sites, including Brønsted acid sites, Lewis acid sites, and multifunc- tional active sites (Figure 2). Brønsted Acid Catalysis Brønsted acid sites are highly polarized hydroxyl groups in zeolite frameworks, which are very active to many catalytic reactions as proton donors. As applications in the oil-refining and petrochemical industries, Brønsted acidic zeolites are playing an important role in many processes of biomass conversion (Figure 2A).8,9 For instance, Brønsted acidic zeolites can be used to catalyze the conversion of microbially produced lactic acid (LA) into lactide, which is the key building block for biodegrad- able and renewable polylactic acid. The traditional method for lactide synthesis from LA requires a time- and energy-intensive two-stage process involving polycon- densation and transesterification at high temperatures in vacuo. Moreover, different lengths of LA oligomers form as the side products (Figure 3A). Taking advantage of their capability to facilitate condensation reactions and their suitable confinement effect toward cyclic lactide over growing oligomers, Dusselier et al.10 utilized Brønsted acidic large-pore zeolites as the catalysts for a fast and selective lactide synthesis directly from LA. Record lactide space-time yields of >250 g L1 hr1 were attained with this one-step direct process. In particular, zeolite H-beta (BEA type) achieved 79% selectivity toward the formation of lactide at full LA conver- sion. In comparison, other acidic catalysts without suitable confinement effect mainly produced oligomers (Figure 3B). This Brønsted acid zeolite catalysis was also applicable to other a-hydroxy acids of interest in polyester chemistry, facilitating 1State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China 2International Center of Future Science, Jilin University, 2699 Qianjin Street, Changchun 130012, China *Correspondence: jihong@jlu.edu.cn https://doi.org/10.1016/j.chempr.2017.10.009 Chem 3, 928–949, December 14, 2017 929PDF Image | Applications of Zeolites in Sustainable Chemistry
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