Manganese-containing MCM-41 for epoxidation of styrene and stilbene

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Abstract

Mn-MCM-41 is found to be the most effective heterogeneous catalyst for the epoxidation of styrene with tert-butyl hydroperoxide (TBHP) among several metal ion-containing mesoporous molecular sieves including Mn-, V-, Cr-, Fe-, and Mo-MCM-41. ESR, XANES, diffuse reflectance UV–VIS, UV–Raman and XPS are used to characterize the Mn-MCM-41 synthesized by both direct hydrothermal (DHT) and template ion exchange (TIE) methods. The results suggest that Mn2+ and Mn3+ coexist in the Mn-MCM-41 samples synthesized by both methods and a large part of manganese atoms could be incorporated into the framework of MCM-41 obtained by the DHT method. The oxidation of either styrene or stilbene with TBHP as the oxidant over the Mn-MCM-41 produces corresponding epoxide as the main product; the reaction probably proceeds through a radical intermediate. The TIE catalyst shows higher activity, while the DHT catalyst gives higher TBHP efficiency for the epoxidation reactions.

Mn-MCM-41 is the most effective catalyst for the epoxidation of styrene and stilbene with tert-butyl hydroperoxide (TBPH) among M-MCM-41 (M=Fe, V, Mo, and Mn).

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Introduction

Since MCM-41 possesses large surface area (∼1000 m2 g−1) and uniform mesopore with controllable diameter of 2–10 nm, it is expected as a desirable material for catalytic applications. However, in most cases, siliceous mesoporous materials do not have sufficient intrinsic activities as catalysts, and thus many studies have concentrated on introducing catalytically active sites such as metals, metal ions and metal complexes into mesoporous silica [1], [2]. The most popular method for the introduction of active sites is the direct hydrothermal (DHT) method, i.e. direct addition of the metal ion precursors to the synthesis gel before hydrothermal synthesis. The conventional impregnation method has also been used to deposit active component onto MCM-41; however this method cannot ensure the incorporation of active species into the mesopore of MCM-41, and some contractions of channels seem to occur during wet impregnation [3]. Several groups reported grafting methods [4], [5], [6], i.e. grafting of organometallic complexes or metal ions onto the surface of mesoporous silica by using surface silanol groups as anchor sites. Metal ions can also be implanted into MCM-41 with the template ion exchange (TIE) method [7], i.e. by exchanging the template cations embraced in the channels of the as-synthesized MCM-41 with the metal ions in solution.

It can be expected that different synthetic methods would result in different environments of the active sites introduced to MCM-41. Oldroyd et al. [8] reported that Ti-MCM-41 prepared by surface grafting method was more active than that prepared by the DHT method for the epoxidation of cyclohexene with TBHP. We have shown that V-MCM-41 prepared by the TIE and DHT methods possesses different coordination environment of vanadium and behaves differently in the partial oxidation of lower alkanes [9], [10], [11]. Vanadium species introduced by the TIE method are mainly dispersed on the surface of channel, preferring the oxidative dehydrogenation of ethane and propane, whereas vanadium sites introduced by the DHT method are mainly incorporated inside the framework of MCM-41, capable of oxidizing propane to acrolein with moderate selectivity. Recently, we found that Fe-MCM-41 prepared by the TIE and DHT methods showed remarkably different coordination environment of iron, resulting in different catalytic performance for epoxidation of styrene with H2O2 [12].

Manganese complexes are well known catalysts for epoxidation reactions [13]. Recently, many research groups worked on the immobilization of Mn complexes onto the mesoporous material. Although ion-exchange method was applied to Al-MCM-41 [14], only lower loading amount can be obtained by this method. Caps and Tsang [15] prepared Mn-MCM-41 with a molecular organic chemical vapor deposition (MOCVD) method. Burch et al. [16] prepared surface-grafted manganese-oxo species on the walls of MCM-41 channels and applied it for the complete oxidation of propene. Iwamoto and coworkers [7] firstly developed the TIE method for the synthesis of Mn-MCM-41, and they argued that manganese was highly dispersed in the mesopore of MCM-41 and only existed as Mn2+. The Mn-MCM-41 thus prepared exhibited high activity for the epoxidation of stilbene with TBHP [17].

In the present paper, several metal ion-containing mesoporous molecular sieves including Mn-, V-, Cr-, Fe- and Mo-MCM-41 are screened for the epoxidation of styrene using TBHP as an oxidant. With the most effective Mn-MCM-41, the influences of synthetic methods (the DHT and TIE methods) on the nature of manganese species and catalytic properties in the epoxidation of either styrene or stilbene are subsequently investigated.

Section snippets

Catalyst preparation

MCM-41 was prepared by hydrothermal synthesis at 120 °C for 96 h using sodium silicate and hexadecyltrimethylammonium bromide (C16H33(CH3)3NBr) as the silicon source and the template, respectively. For the TIE synthesis of Mn-MCM-41, 2 g of the as-synthesized MCM-41 containing ca. 50 wt.% template cations was added to an aqueous solution of manganese(II) nitrate (e.g. 48.7 mg Mn(NO3)2·6H2O in 40 ml of H2O for the preparation of the Mn-MCM-41, TIE, Si/Mn=124). The mixture was stirred vigorously at

Epoxidation of styrene over several metal ion-containing MCM-41

Table 1 shows the catalytic results of several metal ion-containing MCM-41 in the epoxidation of styrene using TBHP as an oxidant. It should be noted that XRD measurements of all these samples showed four diffraction lines at 2θ degrees of 2–6° ascribed to the (1 0 0), (1 1 0), (2 0 0) and (2 1 0) of MCM-41, suggesting that the mesoporous structure was retained after the introduction of each metal ion. The oxidation of styrene gave two main products, i.e. styrene oxide and benzaldehyde (abbreviated as

Conclusions

Mn-MCM-41 was found to be the most effective catalyst for the epoxidation of styrene using TBPH as an oxidant among Mn-, V-, Cr-, Fe- and Mo-containing MCM-41. As suggested by the characterizations with ESR, XANES, UV–VIS and UV–Raman, both Mn2+ and Mn3+ coexisted on the Mn-MCM-41 synthesized by both DHT and TIE methods, and some Mn cations might be incorporated into the framework of MCM-41 in the DHT samples. The epoxidation of both styrene and stilbene using TBHP as the oxidant afforded the

Acknowledgements

The X-ray absorption experiments were performed under the approval of the Photon Factory Program Advisory Committee (Proposal 2002G097).

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