The Computational Understanding and Design of Zeolites, pp. 1-21
Authors: (Gang Yang, Jing Guan, Danhong Zhou, Xiuwen Han, Xinhe Bao - Key Laboratory of Forest Plant Ecology, Northeast Forestry University, P.R. China and others)
Abstract: In the past few decades, the advent and development of sophisticated computational techniques have witnessed the prosperity of zeolites in heterogeneous catalysis. In this chapter, we will review the recent achievements on acidity characterization, hydrothermal stability, active sites and reaction mechanisms as well as computational designs of defect sites and surface catalysis. The ab initio and QM/MM calculations indicated that the three-valent cations (X = Al, B, Fe, Ga) are distributed unequally in the various crystallographic T sites of zeolites. The Brönsted acidities of all the T sites were determined, and their interacting information with adsorbents greatly helps to understand the first and crucial step of acid catalysis by zeolites. As the vital factor restricting the industrial applications, the hydrothermal stability of zeolites can be improved by certain additives with the mechanisms understood by the combined solid-state NMR and density functional theory studies. Moreover, the electrostatic potential was found to be the good criterion to measure the hydrothermal stabilities of various modified zeolites. In contrast to the conventional expoxidation routes of alkenes, the oxidation of styrene on the mixture of TS-1/H2O2 proceeds in a radical mechanism, starting from the η2-TiOO• active site to the hemiacetal species, and then under acidic conditions to the product phenylacetaldehyde. Two types of Lewis acidic sites (TiLAI and TiLAII) were identified in TS-1 zeolite, corresponding to the 31P MAS NMR peaks at -34.2 and -32.0 ppm, respectively. The oxidation of trimethylphosphine is a Lewis-acid assisted process, and the calculated Mulliken charge and geometries satisfactorily elucidated the better catalytic performance of TiLAI rather than TiLAII. A novel defect site was designed in TS-1 zeolite settling numerous experimental puzzles, which is responsible for the weakest Brönsted acidity among all the found zeolites. With the qualitative increases of computational precisions, the unequal acidities and reaction selectivities along different zeolite surfaces were thus predicted. In this way, the computational techniques have been used to design better catalysts. More fruitful results on the computational designs are expected on basis of the in-depth understanding on structures and properties.