Reactive molecular simulations of protonation of water clusters and depletion of acidity in H-ZSM-5 zeolite

Abstract

Using reactive molecular dynamics (RMD), we present an atomistic insight into the interaction between water molecules and acidic centers of H-ZSM-5 zeolite. The reactive force field method, ReaxFF, was used to evaluate the adsorption and diffusion of water as well as to study the protonation of water molecules inside zeolite channels. The existing Si/Al/O/H parameters were refitted against DFT calculations to improve the ReaxFF description of interaction between water molecules and the acidic sites of zeolites. The diffusion coefficient of water in the zeolite obtained from refitted parameters is in excellent agreement with experimental results. The molecular dynamics (MD) simulations indicate that protonation of water molecules and acidity of the zeolite catalyst depend on water loadings and temperature and the observed trends compare favorably with existing experimental and theoretical studies. At higher water loadings, protonation of water molecules is more frequent leading to formation and growth of protonated water clusters inside zeolite channels. From the analysis of various reaction channels that were observed during the simulations, we found that such water clusters have relatively short life due to frequent interchange of protons and water molecules among the water clusters. Such proton hopping events play a key role in moving the protons between different acidic centers of zeolite. These simulations show the capability of ReaxFF in providing atomistic details of complex chemical interactions between the water phase and solid acid zeolites.

Collective action of water molecules in zeolite dealumination

“…The water loading in the zeolite at realistic operating conditions (T = 450°C and atmospheric steam pressure) is determined using grand canonical Monte Carlo (GCMC, Section S3 †). Using the ReaxFF force field 23,24 the GCMC simulation constitutes a realistic model of the equilibrium between water vapor and water adsorbed in the zeolite pores, including specific interactions with the Brønsted acid sites. We reach equilibrium at 3-3.5 water molecules per unit cell.…”

“…The simulations used a recent ReaxFF force field, shown to have realistic water-zeolite interactions. 23,24 The simulations employed 3 × 3 × 3 unit cells (2943 atoms).…”

Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy

“…These findings are also in line with other theoretical studies based on static and dynamical approaches, stating that two methanol molecules are already able to deprotonate a zeolitic BAS, [80][81][82][83][84] whereas for water the formation of larger clusters is required. [85][86][87][88] Also note that the absolute values of the calculated probabilities depend on the acid site density of the H-SAPO-34 material (see section S1 of the Supporting Information). To connect the observation of framework deprotonation with methanol reactivity, the probability of methanol protonation once the framework is deprotonated was probed and the results are also depicted in Figure 1a.…”

Atomistic Simulations of Geopolymer Models: The Impact of Disorder on Structure and Mechanics

“…29,30 The force field parameters used in this work are based on the Si/O/H 31 set, extended with Al/O/H 32 parameters for the study of aluminosilicate frameworks, and improved with a posteriori specific parameterisation of the Si-O-Al angles and proton stability on aluminosilicate rings. 33 These parameters have been previously used to investigate silicalite and H-ZSM-5 aluminosilicate zeolites, their thermal stability, and their acid site chemistry. [32][33][34] In addition, the Si/O/H subset has been shown to reproduce the structural features of amorphous (sodium) silicate glasses.…”

“…33 These parameters have been previously used to investigate silicalite and H-ZSM-5 aluminosilicate zeolites, their thermal stability, and their acid site chemistry. [32][33][34] In addition, the Si/O/H subset has been shown to reproduce the structural features of amorphous (sodium) silicate glasses. 35,36 To verify the ability of the chosen parameters to produce realistic elastic properties of aluminosilicates, the elastic tensor of faujasite has been computed, a sodalite-based zeolite with structural resemblance to geopolymers.…”