Studying Proton Mobility in Zeolites by Varying Temperature Infrared Spectroscopy

Pit Losch,Wolfgang Schmidt,Hrishikesh Joshi,Niklas Stegmann,Olena Vozniuk

doi:10.3390/molecules24173199

Abstract

We report a varying temperature infrared spectroscopic (VTIR) study with partial deuterium isotopic exchange as a method for characterizing proton mobility in acidic materials. This VTIR technique permits the estimation of activation energies for proton diffusion. Different acidic materials comprising classical proton-conducting materials, such as transition metal phosphates and sulfonated solids, as well as different zeolites, are tested with this new method. The applicability of the method is thus extended to a vast library of materials. Its underlying principles and assumptions are clearly presented herein. Depending on the temperature ranges, different activation energies for proton transfer are observed irrespective of the different materials. In addition to the well-studied transition metal phosphates, Si-rich zeolites appear to be promising proton-transfer materials (with Eact < 40 kJ mol−1) for application in high-temperature (>150 °C) PEM fuel cells. They significantly outperform Nafion and sulfonated silica, which exhibit higher activation energies with Eact ~ 50 and 120 kJ mol−1, respectively.

Highlights

We were motivated to characterize the acidity of zeolites
Unlike transition metal phosphates (TMPs) or sulfonated solids, which both have direct acidic groups [2,3], the Brønsted acidic character of zeolites results from trivalent atoms substituting silicon atoms in tetrahedral positions in a crystalline silicate framework. This substitution leads to local negative charges compensated by metal cations or protons. It is still debated whether or not these Brønsted bridging (Si-(OH)-Al) acid sites (BAS) might exhibit different intrinsic acid strengths depending on their concentration and the local chemical environment; zeolite topology-related parameters, such as Si-(OH)-X bond lengths and angles; and, the nature of X itself (with Si-(OH)-X, where X = B, Al, Ga, Fe etc.) [4,5]
A set of standard materials for proton conduction membranes, such as sulfonated solids (Nafion and SBA15-SO3 H) and TMPs (ZrP2 O7 and TiPO4 ), have been studied using the newly developed method relying on varying temperature infrared spectroscopic (VTIR)

Summary

Introduction

We were motivated to characterize the acidity of zeolites. Achieving an reliable experimental and theoretical characterization of zeolites intrinsic acid strength as an isolated parameter remains challenging. Unlike transition metal phosphates (TMPs) or sulfonated solids, which both have direct acidic groups [2,3], the Brønsted acidic character of zeolites results from trivalent atoms (mostly Al and B, Ga, or Fe) substituting silicon atoms in tetrahedral positions in a crystalline silicate framework This substitution leads to local negative charges compensated by metal cations or protons. It is still debated whether or not these Brønsted bridging (Si-(OH)-Al) acid sites (BAS) might exhibit different intrinsic acid strengths depending on their concentration and the local chemical environment (i.e., spatial separation to the nearest Al site); zeolite topology-related parameters, such as Si-(OH)-X bond lengths and angles; and, the nature of X itself (with Si-(OH)-X, where X = B, Al, Ga, Fe etc.) [4,5]. These hypotheses have been extensively studied with the advent of increasingly sophisticated spectroscopic and theoretical techniques, yet it is still difficult to gather independent parameters describing zeolites acidity [6,7,8]

Methods

Results

Discussion

Conclusion