Vanadium incorporation from aqueous NH₄VO₃ solution into siliceous Beta zeolite determined by NMR with formation of V-single site zeolite catalysts for application in SCR of NO

Highlights

• ⁵¹V static NMR allowed determining the V speciation in initial NH₄VO₃ solution.

• Different rate of incorporation of Vanadium at pH 2.5 and 6 was observed.

• NMR, XPS, DR UV–vis and EPR identified state of V in VSiBeta.

• High catalytic activity of V-single site VSiBeta catalysts evidenced in SCR of NO.

• Low catalytic activity of VSiBeta catalysts was observed with polynuclear V(V).

Abstract

The speciation of vanadium in initial aqueous NH₄VO₃ solutions as a function of pH and concentration, in supernatant and in wet solids were investigated by ⁵¹V static and MAS NMR. Two series of VSiBeta zeolite catalysts were prepared by a two-step postsynthesis procedure at pH = 2.5 and 6. ⁵¹V static and MAS NMR, ⁵¹V 3Q MAS NMR, DR UV–vis, XPS and EPR allowed determining the state of vanadium in both series of catalysts. The catalytic activity of VSiBeta catalysts in SCR of NO strongly depended on the state of the vanadium present. The V-single site V_1.0SiBeta(I) and V_1.4SiBeta(I) catalysts with isolated pseudo-tetrahedral V(V) were active in SCR of NO with NH₃ process, with maximum NO conversion about 75 % at 773 K for V_1.4SiBeta(I). In contrast, V_1.0SiBeta(II) and V_7.5SiBeta(I) catalysts containing pseudo-octahedral V(V) were much less active in SCR of NO and high amount of undesired N₂O was produced.

Introduction

It is well known that depending on the preparation method used for incorporation of vanadium into zeolites, different vanadium species can be formed: extra-framework V species, VO_x oligomers and/or vanadium oxides and finally isolated framework V species [1]. Several studies have aimed at assigning the activity and/or selectivity of vanadium-containing zeolites to one of these species [[2], [3], [4], [5], [6], [7]]. The question of metal speciation in the precursor solutions at different preparation conditions as well as in the zeolites after preparation of these zeolite systems have been mentioned [[8], [9], [10], [11]]. It has been shown that well defined catalysts with a single type of vanadium species could be prepared by a post-synthesis method developed earlier by Dzwigaj et al. [[12], [13], [14]]. This method is composed of two steps. In the first step the vacant T-atom sites are created in the zeolite Beta structure by its dealumination with concentrated nitric acid and in the second step the resulting SiBeta zeolite is containing with metavanadate ammonium aqueous solution. The two-step post-synthesis method allowed incorporating vanadium in SiBeta zeolite mainly as isolated pseudo-tetrahedral V species for low vanadium content (lower than 2 V wt %) without formation of VO_x oligomers, as shown earlier by XRD, DR UV–vis and FTIR [15].

As shown earlier [[16], [17], [18], [19], [20], [21], [22]], the two-step post-synthesis method also allowed incorporating copper, iron and cobalt cations in the framework of the SiBeta zeolite thus obtaining Cu-, Fe- and Co-single site Beta zeolites active in selective catalytic reduction of NO into N₂. Such catalytic materials are of highly relevant importance considering that the problematic of exhaust gas cleaning is very important from many decades and has thus been the subject of many reviewers [[23], [24], [25], [26], [27], [28], [29]].

In this work, the speciation of vanadium in initial aqueous NH₄VO₃ solutions as a function of pH and concentration, in the supernatant after contacting siliceous Beta zeolite with aqueous NH₄VO₃ solutions and in wet solid prepared in these conditions was investigated by ⁵¹V static and MAS NMR. Two series of V-containing Beta zeolite catalysts were prepared by a two-step post-synthesis procedure. The first series with V_0.1SiBeta(I), V_0.5SiBeta(I), V_1.0SiBeta(I), V_1.4SiBeta(I) and V_7.5SiBeta(I) samples was prepared at pH = 2.5 and the second series with V_0.6SiBeta(II), V_1.0SiBeta(II) and V_1.6SiBeta(II) samples was prepared at pH = 6.

The vanadium state in both series of solids was determined by ⁵¹V MAS NMR, DR UV–vis, EPR and XPS. The incorporation of V ions into the vacant T-atom sites of the framework of SiBeta zeolite as mononuclear pseudo-tetrahedral V(V) species was evidenced by combined use of XRD, FTIR, NMR and DR UV–vis. Reducibility of vanadium in V-containing Beta zeolites was investigated by EPR. The catalytic activity of V_xSiBeta(I) and V_xSiBeta(II) materials in selective catalytic reduction (SCR) of nitric oxide with ammonia as a reducing agent was investigated. It was found that SCR catalytic properties of these materials strongly depended on the state of vanadium in the Beta structure.