Acid-activated vermiculites as catalysts of the DeNO_x process

Abstract

Vermiculite was treated with 0.8 M solutions of hydrochloric and sulphuric acids for 2, 8, and 24 h. The obtained materials were characterized with respect to their composition (XRF), structure (XRD, FTIR, UV–vis-DRS), texture (BET), surface acidity (NH₃-TPD) and catalytic properties. Modification of vermiculite with acids significantly increased its surface area and pore volume and led to partial leaching of iron, aluminium and magnesium ions from its octahedral sheets. Efficiency of the leaching process depended on its duration and the nature of acid used. Acid treatment strongly modified the vermiculite structure leading to a distinct increase of its catalytic activity in the selective catalytic reduction of nitric oxide (SCR NO) process. In a series of the studied samples the best catalytic performance was found for vermiculite treated with sulphuric acid for 8 h.

Introduction

Cationic layered clays are excellent raw materials for the catalysts preparation [e.g. 1]. Various methods for the catalytic activation of clay minerals, including intercalation of metal oxides into their interlayer spaces [2], [3], were proposed. One of the cheapest and most effective methods of the clay activation is based on their acid treatment [4]. For example, acid-activated montmorillonites were used as commercial catalysts for cracking process of heavy oil fractions [5]. More recently, also other important reactions based on acid treated clays, including alkylation (e.g. alkylation of benzene with 1-dodecane), acylation (e.g. acylation of diisobutylene with acetic anhydride), dimerization (e.g. dimerization of α-methylstyrene), oligomerization (e.g. oligomerization of 1,3-diisopropenylbenzene), izomerization (e.g. izomerization of α-alkylnaphtalenes into β-alkylnaphtalenes), etherification (e.g. synthesis of methyl-tert-butyl ether from tert-butanol and methanol), estrification (e.g. estrification of myristic acid with propylene) and condensation (e.g. aldol condensation of cyclohexanone) were reported [6]. Acid treated montmorillonites were reported to be the most often used clays in various catalytic processes [6] and their catalytic activation was related to the modification of the surface acidic properties. The other cationic layered clays, especially in the form of acid treated minerals, were significantly less intensively studied as potential catalysts. However, there is a plethora of natural clay minerals which vary in their chemical composition, density of layer charge, texture, morphology, surface acidity and cation exchange capacity. Thus, such a broad variety of clays of different properties gives a rare possibility to produce catalysts for dedicated applications.

Vermiculite is a cationic layered clay which contains a significant amount of iron as natural component. On the other hand, iron was reported to be catalytically active in the SCR NO_x reaction [7], [8]. Unfortunately, iron ions are located mainly in the octahedral sheets of vermiculite and therefore, their accessibility is rather limited. Thus, as it was shown in our previous studies, the catalytic activity of raw vermiculite in the SCR NO_x process is very low [4]. However, treatment of vermiculite with a solution of nitric acid effectively activated this clay in the SCR NO_x reaction [4]. Acid treatment resulted in a partial leaching of cations, including Fe³⁺ ions, from the octahedral sheets of vermiculite and their consecutive deposition in the form of both isolated cations or aggregated metal oxide clusters on the clay surface. Thus, acid treatment resulted in transfer of iron species into positions accessible for reactants. Additionally, acid treatment significantly increased surface area and porosity of vermiculite due to its partial transformation into amorphous silica and possibly also deposition of metal oxide clusters into interlayer space of this clay mineral. Therefore, such simple and cheap method used for the vermiculite modification resulted in a production of active, selective and non-toxic catalyst of the SCR NO_x process [4]. It should be also noted that, according to the knowledge of the authors, the mentioned above studies were the first one reporting the application of acid treated vermiculites in the role of catalysts for the SCR NO_x process.

This paper is a continuation of the previously presented studies [4] and reports the catalytic performance of vermiculite treated with solutions of sulphuric and hydrochloric acids. The aim of this study is a comparison of vermiculite treated with various acids in the role of the catalyst for the SCR NO_x process.