A new method for the generation of realistic atomistic models of siliceous MCM-41

Christopher D Williams,Karl P Travis,Neil A Burton,John H Harding

doi:10.1016/j.micromeso.2016.03.034

Abstract

A new method is outlined for constructing realistic models of the mesoporous amorphous silica adsorbent, MCM-41. The procedure uses the melt-quench molecular dynamics technique. Previous methods are either computationally expensive or overly simplified, missing key details necessary for agreement with experimental data. Our approach enables a whole family of models spanning a range of pore widths and wall thicknesses to be efficiently developed and yet sophisticated enough to allow functionalisation of the surface – necessary for modelling systems such as self-assembled monolayers on mesoporous supports (SAMMS), used in nuclear effluent clean-up.The models were validated in two ways. The first method involved the construction of adsorption isotherms from grand canonical Monte Carlo simulations, which were in line with experimental data. The second method involved computing isosteric heats at zero coverage and Henry law coefficients for small adsorbate molecules. The values obtained for carbon dioxide gave good agreement with experimental values.We use the new method to explore the effect of increasing the preparation quench rate, pore diameter and wall thickness on low pressure adsorption. Our results show that tailoring a material to have a narrow pore diameter can enhance the physisorption of gas species to MCM-41 at low pressure.

Highlights

Ever since it was first synthesized by Mobil, in 1992 [1,2], MCM41, a silica-based porous material, has attracted widespread interest from both industry and the academic community
We have investigated the variation of isosteric heat with the models prepared at different quench rates but all with approximately the same pore diameter (3.16 nm) and wall thickness
This research demonstrates the ability of molecular simulation to optimize the physical adsorption process at very low pressure by modifying structural parameters

Summary

Introduction

Ever since it was first synthesized by Mobil, in 1992 [1,2], MCM41, a silica-based porous material, has attracted widespread interest from both industry and the academic community. MCM-41 contains well-defined cylindrical pores arranged in a hexagonal configuration. These pores have diameters that typically vary from 1.5 to 10 nm [1,3e7], classifying MCM-41 as a mesoporous material. The potential of MCM-41 as an effective material for difficult separation problems has been recognized, especially in the case of CO2 removal from gas mixtures where the selectivity and adsorbent capacity of zeolites and activated carbons can be poor in the high temperature conditions encountered in flue gas streams [17]

Methods

Results

Conclusion