Alcohols react with MCM-41 at room temperature and chemically modify mesoporous silica

Sebastian Björklund,Vitaly Kocherbitov

doi:10.1038/s41598-017-10090-x

Abstract

Mesoporous silica has received much attention due to its well-defined structural order, high surface area, and tunable pore diameter. To successfully employ mesoporous silica for nanotechnology applications it is important to consider how it is influenced by solvent molecules due to the fact that most preparation procedures involve treatment in various solvents. In the present work we contribute to this important topic with new results on how MCM-41 is affected by a simple treatment in alcohol at room temperature. The effects of alcohol treatment are characterized by TGA, FTIR, and sorption calorimetry. The results are clear and show that treatment of MCM-41 in methanol, ethanol, propanol, butanol, pentanol, or octanol at room temperature introduces alkoxy groups that are covalently bound to the silica surface. It is shown that alcohol treated MCM-41 becomes more hydrophobic and that this effect is sequentially more prominent going from methanol to octanol. Chemical formation of alkoxy groups onto MCM-41 occurs both for calcined and hydroxylated MCM-41 and the alkoxy groups are hydrolytically unstable and can be replaced by silanol groups after exposure to water. The results are highly relevant for mesoporous silica applications that involve contact or treatment in protic solvents, which is very common.

Highlights

Silica with ordered and accessible mesopores, such as MCM-411 and SBA-152, were successfully synthesized in the early 1990s by utilizing the principle of self-assembly of surfactants as structure-directing agents
The presented results are highly relevant for applications that are based on silica or mesoporous silica materials and involve any contact ortreatment in protic solvents, which is a very common combination
From this analysis the surface area of untreated MCM-41 was determined to 786 m2/g, which is in satisfactory agreement with reported values on MCM-41 synthesized according to similar protocols as in this work[10]

Summary

Introduction

Silica with ordered and accessible mesopores, such as MCM-411 and SBA-152, were successfully synthesized in the early 1990s by utilizing the principle of self-assembly of surfactants as structure-directing agents. The Si-OH groups constitute the anchor onto which organic molecules can covalently bind by post-synthesis modification steps, such as silylation[8] This is one example of why it is important to characterize the silanol chemistry of mesoporous materials to completely understand and control the surface modification process. In many cases the influence of this type of treatments on the properties of the mesoporous silica is largely overlooked, in spite of the fact that the surface chemistry may change significantly One illustration of this issue is formation of Si-OR groups at the silica surface during treatment in alcohols at elevated temperatures, which has been reported previously[9, 13]. Since this material is hydrolytically unstable, as compared to Si-O-Si and Si-C linked phases, this chromatographic support is only relevant for non-aqueous mobile phases[14]

Methods

Results

Conclusion