Probing the acid sites of zeolites with pyridine: Quantitative AGIR measurements of the molar absorption coefficients

Highlights

• Integrated FTIR-gravimetry methodology for quantitative acidity measurements is established.

• The validity of Beer-Lambert-Bouguer law for IR characterisation of solid acids is demonstrated.

• The molar absorption coefficients of Py complexes with acid sites in zeolites are determined.

• The key effects of experimental design on FTIR measurements are quantified.

Abstract

This study presents a detailed methodology, which combines high-precision thermogravimetry and FTIR spectroscopy, aiming to establish the most accurate and reliable means of measuring the molar absorption coefficients of adsorbed species. As the integrated molar absorption coefficients of Py complexes with Brønsted and Lewis acid sites, ɛ(Py-B) and ɛ(Py-L), are determined and the validity of the Beer-Lambert-Bouguer law for IR characterisation of solid acids is demonstrated, this work is setting a benchmark for the quantitative acidity measurements in zeolites and related materials. The following values of ɛ(Py-B) have been obtained at 150 °C (band at ~1545 cm⁻¹): 1.09 ± 0.08 cm µmol⁻¹ for ZSM-5; 1.12 ± 0.16 cm µmol⁻¹ for BEA; 1.29 ± 0.04 cm µmol⁻¹ for MOR and 1.54 ± 0.15 cm µmol⁻¹ for FAU. The value of ɛ(Py-L) (band at ~1455 cm⁻¹, which refers to different cations) measured at the same temperature is 1.71 ± 0.1 cm µmol⁻¹. Values of ɛ(Py-B) depend on the zeolite structure, in contrast to that for ɛ(Py-L). Clear correlations are presented between the temperature of the FTIR measurements and ɛ values for Py complexes and other species, which decrease by ~10% as the temperature increases by 100 °C. In addition, the effects of key experimental procedures, instrumentation design and sample preparation are established and quantified.

Introduction

Zeolites, particularly crystalline aluminosilicates, are essential for a host of major industrial processes. They are utilised as molecular sieves, for nuclear waste clean-up, as water softeners in washing powders, as vital catalysts in oil refining and petrochemical industries for the production of petrol and diesel, olefins and simple aromatic compounds, polymers and plastics, etc. The successful application of these materials in catalysis is associated with their microporous structure and highly effective acid sites. Hence, characterisation of the acidic properties of zeolites is of significant fundamental and practical importance. Over several decades, pyridine (Py) has been the most frequently utilised infrared probe molecule for the characterisation of active sites in solid acids (see review [1] and references therein). Indeed, (i) Py complexes with acid sites can be easily discriminated in the spectra, thus Brønsted and Lewis acid sites (BAS and LAS) can be evaluated; (ii) Py is a rather stable molecule at a wide range of characterisation conditions, allowing for instance, high-temperature experiments, such as TPD; (iii) Py and its derivatives can be used to examine the accessibility of the acid sites in a variety of zeolite structures. In addition, FTIR spectra of adsorbed Py can be used for quantitative measurements of the number of acid sites present in a solid catalyst via the application of the Beer-Lambert law (this should be referred to as the Beer-Lambert-Bouguer law to acknowledge the original contribution by Pierre Bouguer). A wide range of molar absorption coefficient values can be found in the literature (Table 1) for Py adsorbed on BAS and LAS, ε(Py-B) and ε(Py-L). However, there are significant differences among the data published by different researchers: the ε values vary by six orders of magnitude, the ε(Py-B)/ε(Py-L) ratios are between 0.47 and 8.8, and different units of ε are reported (Table 1), suggesting that such variations may be associated with significant errors [1]. Furthermore, the details of experimental procedures, which differ a great deal, and of the data analysis are frequently unclear or incomplete. In addition, the application of the classic Beer-Lambert-Bouguer law to the FTIR characterisation of solid catalysts is challenging, and the accuracy of quantitative data reported in the literature may be limited by the equipment available and the methodology utilised in such research. Indeed, the physical parameters and properties of the sample (its thickness, pressure used to prepare the disc, particle size, etc.) and the experimental design (spectral resolution, temperature of the FTIR measurements, using a vacuum system or a gas flow) may have a large influence on the ε values obtained [2]. It should be noted that for other probe molecules there are quantitative data in the literature, including the calculation of ε values for ammonia, quinoline, acetonitrile and substituted pyridines [1], [3], [4], [5], [6], [7], [8], but, these are not discussed in the present work.

Significant advances in the design of new instrumentation for the accurate quantitative analysis of the FTIR spectra have been made in the last few years [7], [29], [40], such as the combination of high-precision thermogravimetry and FTIR spectroscopy (AGIR, combining gravimetric analysis and IR). This technique, in which a microbalance is integrated with an FTIR in situ cell within a single set-up, allows simultaneous monitoring of the weight changes of the sample along with its IR spectra during adsorption or desorption of probe molecules, and consequently, highly accurate quantitative data, e.g. the molar absorption coefficient values, can be obtained directly, as demonstrated in [41], [42], [43]. The aim of this work is to carry out quantitative analysis of the number of acid sites in different zeolites, to determine the value of the integrated molar absorption coefficients of Py adsorbed on Brønsted and Lewis acid sites and to examine the role of experimental conditions that are essential for the reliable characterisation of the acidic properties of zeolite-based catalysts. This study also examines the validity of the Beer-Lambert-Bouguer law in solid materials. The ɛ(Py-B) and ɛ(Py-L) values have been determined using the AGIR technique, which, we believe, is the most accurate and reliable method of measuring the molar absorption (extinction) coefficients of adsorbed species. Overall, the optimisation of the experimental procedures is imperative for the successful quantitative evaluation of different types of acid sites in zeolitic materials. With the new level of instrumentation available now, this work sets a benchmark for the quantitative acidity measurements in zeolites and related materials.