Characterization of a delaminated clay and pillared clays by adsorption of probe molecules

Abstract

Characterization of the textural and structural properties of a sodium form of a delaminated calcic montmorillonite, and of aluminium pillared materials prepared with and without amine pre-adsorption, was made using the adsorption of different probe molecules (nitrogen, toluene, methyl ethyl ketone and 1,1,1-trichloroethane). Due to the delaminated character of the prepared solids, the characterization by X-ray diffraction of the pillared materials was not possible. In this context, the adsorption of probe molecules revealed to be informative since, although the prepared materials were mainly mesoporous solids in consequence of their delaminated nature, when the amine pre-adsorption was used before the pillaring, microporosity was also formed.

Introduction

Pillared clays (PILCs) are materials prepared by the intercalation of metal oxide precursors between the silicate layers of smectite type clays. Upon heating, oxide pillars are formed, building a stable zeolite-like structure. The dimensions and the characteristics of the pores produced in this way are usually related to the nature, size and shape of the intercalated species and the precursor polymerization parameters (e.g. concentration of the metal ions, alkalinity, temperature of hydrolysis, time and temperature of aging) [1], [2], [3], [4], [5] and to the chemical and crystallographic parameters of the clay (the magnitude of the layer charge, the location and the distribution of the charge and the nature of the octahedral sheet) [6], [7]. Some authors tried to investigate the relation between the microporosity of the obtained material and the pillaring process [8], [9]. In this way, it was shown by X-ray diffraction (XRD) measurements that contrarily to the direct intercalation, by adsorbing amine in the clay prior to the intercalation step, an increased orientation of the clay layers is observed and that the pre-adsorption of amine compounds is also a method to obtain a more regular stacking of the clay layers and higher micropore volumes. Additionally, amines have also a positive influence on the face-to-face stacking of the clay layers, resulting in more regularly ordered pillared clays. These improvements are extremely valuable in case of selective adsorption purposes and catalysis applications. Furthermore, the need for PILCs with wide pores, that could be used in catalytic or adsorption processes, where large molecules are involved, seems not to be entirely achieved with the more usual pillaring methodologies.

The approach used in this work involves the preparation of pillared clays from delaminated clays, an aspect that was first pointed out by Pinnavaia et al. [10]. These materials do not exhibit long-range layer stacking as shown by the absence of the (0 0 1) X-ray diffractions. However, it is believed that there is some degree of short-range stacking and the pillared structure is described as a “house of cards” [11]. These clays contain pores with a wide range of widths, from micro to macropores [12].

It is well known that the physical adsorption of gases and vapors is a very useful technique for the characterization of porous solids, and especially, microporous solids. Usually, low temperature nitrogen adsorption is used to characterize microporous solids, but molecules of different sizes and polarities can also be used as adsorptives in order to obtain information about the textural properties of the solids [13], [14]. This approach is particularly valuable in the present work due to the difficulties in the characterization of materials with low degree of organisation and a wide range of porosities. In this way, the porosity of the samples was studied by adsorption of probe molecules at subcriticle temperatures. Under these conditions, micropore filling takes place and the micropore volumes can be estimated [14], [15]. For the characterization of microporous solids, several equations have been proposed for materials with a non-homogeneous microporous structure [16], [17], [18], [19], [20], [21]. One of them is the well-known Dubinin–Astakhov equation [16], [22], which has been applied to the adsorption of a large variety of organic molecules in different adsorbents [23], [24], [25]. The adsorbates studied were toluene, methyl ethyl ketone and 1,1,1-trichloroethane that, in spite of being used in this work mainly for textural characterization, are also relatively common air pollutants.