Microwave synthesis, characterization and transesterification activities of Ti-MCM-41

doi:10.1016/j.micromeso.2012.02.011

Microporous and Mesoporous Materials

Volume 156, 1 July 2012, Pages 22-28

https://doi.org/10.1016/j.micromeso.2012.02.011 Get rights and content

Abstract

A series of Ti-MCM-41 mesoporous materials with different titanium content (Si/Ti ratio varying from 100 to 5) were prepared using a microwave irradiation method. Physicochemical and structural properties of the materials were investigated using small angle X-ray powder diffraction (SAXRD), N₂ adsorption–desorption, Fourier transform infrared (FT-IR) spectroscopy, transmission electron micrograph (TEM), inductively coupled plasma optical emission spectrometry (ICP-OES), diffuse reflectance UV–visible (DRUV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), FT-IR measure of adsorbed pyridine and ammonia temperature programmed desorption (NH₃-TPD) techniques. Merging results of those characterizations mentioned above, it indicates that, with the decrease of Si/Ti ratio from 100 to 5, Ti-MCM-41 samples exhibit typical long-range order of hexagonal structure with a certain decrease of structure ordering; titanium species incorporate into the framework entirely and are mainly in tetrahedral coordination. As the titanium content increases gradually, the amount of framework Ti(IV) sites, which could result in the formation of weak Lewis acid sites on the surface, also rises correspondingly. The rich weak Lewis acid sites and mesoporous structure with large pore diameter are significant factors affecting the catalytic properties of Ti-MCM-41 materials for transesterification of dimethyl oxalate and phenol.

Graphical abstract

For Ti-MCM-41, titanium species incorporate into the framework entirely and mainly exist in tetrahedral coordination. As the titanium content increases gradually, the amount of framework Ti(IV) sites, which could result in the formation of weak Lewis acid sites on the surface, are also increased correspondingly. The rich weak Lewis acid sites and mesoporous structure with large pore diameter are significant factors affecting the catalytic properties of Ti-MCM-41 materials.

Highlights

► A series of Ti-MCM-41 were prepared by a microwave irradiation method with different titanium content. ► Ti-MCM-41 samples exhibited typical long-range order of hexagonal structure. ► Titanium species incorporated into the framework and mainly existed in tetrahedral coordination. ► The increase of framework Ti(IV) sites resulted in the formation of more weak Lewis acid sites. ► The rich weak Lewis acid sites and mesoporous structure are favorable for the catalytic performances.

Introduction

The M41S family of mesoporous molecular sieves with regular pore diameters and large surface areas has aroused great interests in the scientific community since their appearance in 1990s [1], [2]. Among them, MCM-41 molecular sieve with a hexagonal arrangement of mono-dimensional pores has been widely noted and extensively studied. However, the application of MCM-41 molecular sieve as heterogeneous catalysts is limited to some extent due to the lacking of active sites in the neutral purely siliceous framework. The incorporation of heteroatoms as extra-framework nanoscale oxide clusters or in their appropriate valence state as tetrahedral framework species may generate active sites [3], [4], [5], [6]. Among them, Ti-MCM-41 is one of the most attractive heteroatoms submitted mesoporous molecular sieves for its good acid and redox properties and has been extensively used in catalytic fields, e.g., selective oxidation [7], [8], [9], isomerization [10], [11], epoxidation [12], [13], [14], photocatalysis [15], and transesterification [16], [17].

For the preparation of Ti-MCM-41, as reported in the literatures, the most commonly applied method is hydrothermal synthesis [10], [11], [18], [19], [20], [21], [22], [23]. At present, microwave irradiation method has been widely applied for the synthesis of mesoporous materials [24], [25], [26], [27], [28], [29]. Compared with the hydrothermal synthesis method, the microwave irradiation technique performs more homogeneous nucleation and rapid crystallization. Therefore, Ti-MCM-41 materials could be prepared under a microwave irradiation condition to obtain uniform mesoporous texture in a shorter crystallization period [9], [12].

In this paper, a series of titanium-containing mesoporous molecular sieves, with Si/Ti atomic ratio from 100 to 5, were prepared under a microwave irradiation condition. This work is designed to study the influence of titanium content on physicochemical properties, structural features, and catalytic efficiency of Ti-MCM-41 materials prepared by microwave irradiation method. A number of characterizations have been utilized in the present investigation including small angle X-ray powder diffraction (SAXRD), Fourier transform infrared (FT-IR) spectroscopy, N₂ adsorption–desorption, transmission electron micrograph (TEM), inductively coupled plasma optical emission spectrometry (ICP-OES), diffuse reflectance UV–visible (DRUV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), FT-IR measure of adsorbed pyridine and ammonia temperature programmed desorption. We employed transesterification of dimethyl oxalate (DMO) with phenol, which is usually catalyzed by the acid catalysts, as a probe reaction to test the functionality of these catalytic materials.

Section snippets

Materials

Tetraethoxysilane (TEOS), tetrabutyl titanate (TBOT), cetyltrimethylammonium bromide (CTAB), sodium hydroxide (NaOH), and isopropyl alcohol (i-PrOH) were all purchased from Kermel Fine Chemical Corporation (Tianjin, China). Dimethyl oxalate and phenol were obtained from Tianjin No. 1 Chemical Reagent Factory (Tianjin, China). All of these chemicals are analytically reagent-grade. Acetonitrile is HPLC grade acquired from Fisher Scientific Corporation (NJ, USA).

Synthesis of Ti-MCM-41

The titanium-containing mesoporous

Chemical and structural properties

The XRD patterns of series of Ti-MCM-41 samples with different Si/Ti atomic ratio, which are prepared by microwave irradiation method, are shown in Fig. 1. All the samples exhibit a shape reflection peak corresponding to the (1 0 0) plane at a low angle (2θ = 2–2.5°), characterizing the typical hexagonal M41S structure. Besides the main reflection peak, three reflection peaks which can be indexed with (1 1 0), (2 0 0), (2 1 0) planes are clearly visible at the region of 2θ = 3–7° in XRD patterns of the

Conclusions

We prepared Ti-MCM-41 successfully by microwave irradiation method with Si/Ti atomic ratio varying from 100 to 5. SAXRD patterns, N₂ adsorption–desorption analysis and TEM images indicate that the refined hexagonal arrangement will be slightly disordered by the increasing titanium content of materials. However, the amount of acid sites rises greatly with more titanium incorporating into MCM-41. By means of FT-IR, DRUV–vis, and XPS, it is confirmed that titanium atoms are entirely incorporated

Acknowledgments

Financial supports by Natural Science Foundation of China (NSFC) (Grant No. 20506018), the Program of Introducing Talents of Discipline to Universities (Grant B06006), the National Key Project for the 11th Five Year Plan (Grant No. 2006BAE02B00), and the Program for New Century Excellent Talents in University (NCET-04-0242) are gratefully acknowledged.

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Microwave synthesis, characterization and transesterification activities of Ti-MCM-41

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

Synthesis of Ti-MCM-41

Chemical and structural properties

Conclusions

Acknowledgments

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