Stable iron-incorporated mesoporous silica materials (MFS-9) prepared in strong acidic media

Abstract

Iron-incorporated mesoporous silica materials (MFS-9) have been successfully prepared in strong acidic media by a two-step synthesis procedure, which has an hexagonal structure with uniform mesopores and exhibits superior hydrothermal stability and high catalytic activities for the hydroxylation of phenol and also of bulky trimethylphenol. X-ray diffraction data show that mesoporous ferrisilicate (MFS-9) can retain its mesoporous structure in boiling water up to 100 h. Characterization of MFS-9 by a variety of spectroscopic techniques, including electron spin resonance, UV–vis and UV–Raman spectroscopies, suggests that Fe species in the framework of MFS-9 are mainly tetrahedrally coordinated even after calcination at 823 K.

Introduction

A novel mesoporous silica material denoted as SBA-15 has been successfully prepared using triblock copolymer as template [1]. Due to its larger pores, thicker walls and much higher hydrothermal stability than those of MCM-41, SBA-15 shows potential catalytic application and therefore attracts much attention. However, it is very difficult to prepare SBA-15 containing heteroatoms in the framework because of the strong acidic synthesis conditions (pH<1). Under such conditions, some metals will only exist in the cationic form other than their corresponding oxo species and therefore cannot enter the framework of SBA-15. As we know, the active centers in the molecular sieve catalysts are often from the presence of various kinds of heteroatoms. Therefore, it is important to introduce heteroatoms into the framework of SBA-15.

Notably, many efforts have been devoted to the incorporation of Al, Ti and V into SBA-15 [2], [3], [4], [5], [6], including “post-synthesis” grafting procedures [2], [3], [4] and “direct synthesis” [5], [6]. The “post-synthesis” way always needs complex experimental conditions, especially for Ti and V (inside a glovebox under flowing of nitrogen). Another drawback of “post-synthesis” is that metal oxides (such as Al₂O₃, TiO₂) tend to appear in the channel or external surface of the products as byproducts, which would jam the channels or play a negative role in catalysis [7]. The direct synthesis of Al–SBA-15 was reported by adjusting the pH value of the synthesis system up to 1.5 [5]. Komarneni and coworkers directly prepared Ti–SBA-15 under microwave hydrothermal conditions [6]. However, the incorporation of Fe into the framework of mesoporous silica prepared in strong acidic media has not been reported yet, although Fe–zeolites and Fe–MCM-41 prepared in basic media have been widely investigated [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Furthermore, it has been reported that Fe species in the framework of mesoporous materials such as MCM-41 [18], MCM-48 [9], and HMS [19] are tetrahedrally coordinated but easily escape from the framework and turn to iron oxide when these materials are calcined at high temperature to remove the organic template. In order to obtain template-free Fe-containing mesoporous silicas, Echchahed et al. used solvent extraction to remove the organic template from Fe–MCM-48 [9]. Tuel et al. also found that the extraction of a template by solvent before final calcination is helpful for the reservation of Fe species in the framework of Fe–HMS [14].

In our previous work [23], the heteroatoms of Al and Ti were effectively introduced into ordered mesoporous silica materials templated from triblock polymers in strong acidic media (pH<0) by a two-step procedure and the products are designated as mesoporous aluminosilicate (MAS-7) and mesoporous titanosilicate (MTS-9), respectively. First, precursors containing zeolite nanoclusters were prepared. Secondly, the pre-formed precursors were used to assemble with triblock copolymers in strong acidic media. In this way, the heteroatoms were fixed in the framework of the zeolite nanoclusters in the first step and then directly introduced into the mesoporous structure along with the nanoclusters in the second step. We have also reported the catalytic hydroxylation of phenol by H₂O₂ over various catalysts [24], [25], [26], [27]. Here we report that iron could also be effectively introduced into the framework of ordered mesoporous silica materials templated from triblock copolymers in strong acidic system through the aforementioned way. The obtained iron-incorporated mesoporous silica was denoted as MFS-9 (mesoporous ferrisilicate), whose stability and catalytic activities in hydroxylation by H₂O₂ were investigated.