A first-principles evaluation of the stability, accessibility, and strength of Brønsted acid sites in zeolites

Abstract

Catalytic reactions of zeolites occur mainly at their acid sites; however, there is a lack of comprehensive method to characterize the acid sites by multiple aspects due to their complexity associated with both the host structures and the origins of formation. Here we propose a comprehensive computational approach, from both thermodynamic and kinetic points of view, to evaluate jointly the energetic stability, space accessibility, and acid strength (SAS) critical to the catalytic performance of acid sites in zeolites. In this work, we evaluated the SAS systematically for various potential Brønsted acid sites in zeolites, using density functional methods and surface area calculations. The total energy was used to compare the relative stabilities of various acid sites of the same type. The deprotonation energy (DPE) was calculated to represent the acid strength. Moreover, we proposed a concept of local accessible surface area (LASA) to measure the accessibility of a localized surface region of molecular sites of complex shape. The SAS were evaluated comprehensively for various Brønsted acid sites including bridging hydroxyl (BH), nest hydroxyl (NH), and terminal silanol (TS) in the internal pores and on the external surfaces of MFI-type zeolite. We found the most stable acid sites are the Al2/H2, Al9/H2, and Al1/H1 sites, respectively, for intra-crystalline BH, and BHs on the (010) and (100) external surfaces. The most stable NH sites are located at the T4, T7, and T5 defective sites, respectively, in the internal pores, and on the (010) and (100) external surfaces. The accessibility of an acid site depends on the OH orientation at the studied acid sites. Most of the NH sites are difficult to access due to the connected hydrogen bonds. The acid strength of the studied Brønsted acid sites decreases as BH>NH>TS. Both the pore geometry and hydrogen bonds can enhance the acid strength. The first-principles computation of the SAS provides a comprehensive approach to characterize acid sites that facilitates evaluation of catalytic performance for the design of zeolite catalysts.