Acidic zeolites are the work horses of today’s chemical industry. Brønsted acid sites are typically generated by substituting a framework Si atom by Al and compensating the resulting effective negative charge on the framework by a proton. Instead of Al, also other metal substitutions can be used to tune the acid strength of the catalyst. Very recently, research in zeolite catalysis is more and more focused on unraveling trends in kinetics of certain types of reactions as a function of acid strength of the material.[1,2] The knowledge of so-called scaling relations (see Figure) should – on the longer term – pave the way for high throughput screening studies to select the most promising catalyst material for a certain catalytic application. To construct such scaling relations, an accurate measure for the acid strength is required. Although the acid strength of Brønsted acid sites is known to have a major influence on product selectivity and catalyst lifetime, it is extremely difficult to uniquely quantify a zeolite’s intrinsic acid strength. Both from a theoretical and experimental viewpoint, a lot of techniques can be used, but there is currently no rigorous way to perform these measurements. From a computational point of view, more knowledge of the Brønsted acidity can be gained with advanced MD simulations, which fully take into account the role of solvent molecules occluded in the zeolite pores. This method, called insertion/deletion scheme, has been proved to successfully estimate the pKa for solvated dye molecules, and this would be the first time it is applied to zeolite systems

Objectives

The objective of this project is to apply an innovative molecular dynamics based scheme to accurately quantify the acid strength of silicate and aluminophospate AFI type zeolites with various metal substitutions encompassing Al, Mg, Co, Zn, Zr, Ti, and Si. For these materials, experimental results for the influence of acid strength on methanol-to-hydrocarbons and alkene cracking reactions are available and the student will be involved in discussions with our collaborators. Our MD approach, based on the insertion/deletion scheme, will be benchmarked against the more conventional methods including the calculation of deprotonation energies and probe molecule adsorption enthalpies.

The Center for Molecular Modeling has built up vast expertise in these advanced simulation techniques and collaborates on the subject with leading experimental and theoretical partners. The student will be actively coached to get acquainted with the plethora of techniques needed to tackle the proposed problem. The CMM has access to sufficient computational resources to execute this research project. The proposed topic is challenging and requires technical skills, creativity and chemical insight.

Aspects
Chemical aspect: controlling the acid strength of zeolite catalysts
Engineering aspect: Application to the catalytic properties of nanoporous materials