Toluene oxidation on titanium- and iron-modified MCM-41 materials

https://doi.org/10.1016/j.jhazmat.2009.02.018Get rights and content

Abstract

Iron- and titanium-modified MCM-41 materials, prepared by direct synthesis at ambient temperature or wet impregnation technique, were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), UV–vis diffuse reflectance, Mössbauer and FT-IR spectroscopies. Their catalytic behavior was studied in total oxidation of toluene. Materials with high surface area and well-ordered pore structure were obtained. The increase of the titanium content (up to 50%) in the bisubstituted, iron and titanium containing materials leads to partial structure collapse of the silica matrix. Finely dispersed anatase particles were also formed during the impregnation procedure. The catalytic activity of the bisubstituted materials was influenced by the method of their preparation, but higher catalytic stability could be achieved, compared to iron monosubstituted one. The nature of the catalytic active sites is discussed.

Introduction

Volatile organic compounds (VOCs) are the main class of air pollutants, emitted from various industrial processes [1], [2], [3], [4], [5], [6], [7], [8]. They include over 300 compounds, such as oxygenates, aromatic and halogen hydrocarbons. The strict regulations on the environmental standards in several countries require a 40% reduction in VOCs emission by 2010. The catalytic total oxidation has been considered as the most appropriate method for VOCs removal and many efforts have been made to design catalysts with good activity and selectivity [1], [2], [3], [4], [5], [6], [7], [8]. The commercial catalysts for oxidation of VOC can be classified into three categories: (1) supported noble metals [3], [9], [10]; (2) metal oxides or supported metals [1], [2], [4], [6], [8], [11], [12]; (3) mixtures of noble metals and metal oxides [13], [14]. The noble metals used in practice are Pt and Pd usually supported on oxides (Al2O3 or SiO2), zeolites (BEA and FAU) [3], [9], [10]. These catalysts generally show higher activity and selectivity toward total oxidation. Transition metal oxides are one alternative to the noble metal-containing catalysts due to their resistance to halogens, low cost and high catalytic activity and selectivity [2], [4], [6], [8]. Transition metal oxides have been found to be very active, both in total and selective oxidation of hydrocarbons and their catalytic properties can be related to the kind of oxide species involved in the oxidation process [2], [4], [6]. The nature of the support is also important factor as the surface area and functionality determine the nature and dispersion of the metal oxide particles and therefore their catalytic behavior [1], [8]. MCM-41 silica materials, with their uniform mesoporous channel structure and high specific surface area, are of particular interest, as catalyst support [15], [16], [17], [18], [19], [20], [21], [22]. A wide variety of metal ions (Fe, Ti, V, Cr, Cu, etc.) has been introduced into the silica matrix to obtain modified mesoporous materials with tunable catalytic properties [23], [24], [25]. It has been established, that the applied method of modification strongly influences the state (localization, dispersion and oxidative state) of the loaded metal species [26], [27], [28], [29]. Their introduction in the host matrix could be realized during the silica synthesis procedure as well as by various postsynthesis techniques (impregnation and grafting) [26], [27], [28], [29], [30]. However, the preparation of stable, modified materials by conventional hydrothermal synthesis is usually possible at low metal loading, which strongly restricts the application as catalysts. It has been reported that the sol–gel technique reveals much higher ability in this aspect and the preparation of stable monocomponent titanium [31]- or iron [26], [27]-modified MCM-41 materials has been achieved. Recently, high catalytic activity of mixed iron and titanium oxide bulk materials in oxidative decomposition of chlorobenzene has been reported [32]. Catalytic oxidation of aromatics are in main importance because they are emitted from diverse sources, e.g. from printing, pressing, petrochemical industries, automobile exhaust and traffic-related processes. Total oxidation of toluene has been studied using different metals (Pt, Cu, Fe, V, Co and Cr) on different types of supports (Al2O3, CeO2 and TiO2 activated carbon) [3], [6], [8], [24]. The Fe-containing catalysts are active in toluene oxidation but the main problem remains the stabilization of the iron species during the catalytic process [8]. To the best of our knowledge no data are available for the catalytic properties of iron- and titanium-modified mesoporous silicas. That is why in this study we focus our attention on the catalytic behavior of iron and titanium substituted mesoporous MCM-41 for oxidative VOCs elimination. Two techniques of samples preparation (conventional impregnation and sol–gel synthesis) were applied. Toluene, which is the main air pollutant was tested as a probe VOC molecule.

Section snippets

Synthesis

The silica MCM-41, the monosubstituted TiMCM-41 and FeMCM-41 materials and the bisubstituted titanium- and iron-containing sample (TiFeMCM-41) were prepared at room temperature by the procedure described in our previous papers [27], [33]. Tetraethyl orthosilicate (TEOS, Aldrich) and tetraethyl orthotitanate (Ti(OEt)4, Aldrich) were used as silica and titania source, respectively. Typically, 5.2 ml of TEOS was mixed with 10.4 ml propan-2-ol, 1 ml of Ti(OEt)4 and 0.9 g of Fe(NO3)3·9H2O. The clear

Physico-chemical characterization of the samples

Small angle XRD patterns (Fig. 1) of parent silica material exhibit reflections typical of hexagonally arranged pore structure, which is almost preserved after the modification. The shift of the position of the (1 0 0) reflection for the modified materials in comparison with the parent one could be assigned to the incorporation of the metal ions in the silica framework. Higher degree of titanium incorporation could be concluded in the case of bisubstituted directly synthesized material, where

Conclusion

The method of MCM-41 modification with iron and/or titanium influences the state of the metal species in the silica matrix. Formation of Fe3+ and Ti4+ ions in tetrahedral position is registered for all materials prepared by direct sol–gel synthesis. Partial structure collapse of the silica matrix is observed with the increase in the titanium content (up to 50%). The introduction of titanium in the iron-modified MCM-41 by impregnation technique leads to additional formation of finely dispersed

Acknowledgements

Financial support by BY–X–305/07, Hungarian Research Fund, OTKA (grant F 61972), National Office for Research and Technology (NKTH, GVOP project no. 3.2.1. 2004-04-0277/3.0) and the Bulgarian–Hungarian Inter-academic Exchange Agreement are greatly acknowledged.

References (42)

  • S.K. Gangwal et al.

    Kinetics and selectivity of deep catalytic oxidation of n-hexane and benzene

    Appl. Catal.

    (1988)
  • Y. Wang et al.

    Characterizations of iron-containing MCM-41 and its catalytic properties in epoxidation of styrene with hydrogen peroxide

    J. Catal.

    (2002)
  • Ch. Wu et al.

    Synthesis, characterization and catalytic performance for phenol hydroxylation of Fe-MCM41 with high iron content

    Micropor. Mesopor. Mater.

    (2008)
  • T. Kawabata et al.

    Iron-containing MCM-41 catalysts for Baeyer–Villiger oxidation of ketones using molecular oxygen and benzaldehyde

    J. Mol. Catal.

    (2005)
  • M.S. Hamdy et al.

    Synthesis, characterization, and unique catalytic performance of the mesoporous material Fe-TUD-1 in Friedel–Crafts benzylation of benzene

    Catal. Today

    (2005)
  • R.S. Araujo et al.

    Al and Ti-containing mesoporous molecular sieves: synthesis, characterization and redox activity in the anthracene oxidation

    J. Mol. Catal.

    (2008)
  • A. Taguchi et al.

    Ordered mesoporous materials in catalysis

    Micropor. Mesopor. Mater.

    (2005)
  • I. Sobczak et al.

    Cu state and behaviour in MCM-41 mesoporous molecular sieves modified with copper during the synthesis––comparison with copper exchanged materials

    Micropor. Mesopor. Mater.

    (2004)
  • A. Szegedi et al.

    Spherical mesoporous MCM-41 materials containing transition metals: synthesis and characterization

    Appl. Catal. A

    (2004)
  • M. Kang et al.

    Synthesis and characterization of Al-, Bi-, and Fe-incorporated mesoporous titanosilicate (MPTS) materials and their hydrophilic properties

    Appl. Catal. A

    (2005)
  • M. Zhou et al.

    Effects of Fe-doping on the photocatalytic activity of mesoporous TiO2 powders prepared by an ultrasonic method

    J. Hazard. Mater. B

    (2006)
  • Cited by (72)

    • Co<inf>3</inf>O<inf>4</inf>/α-Fe<inf>2</inf>O<inf>3</inf> catalyzed oxidative degradation of gaseous benzene: Preparation, characterization and its catalytic properties

      2019, Solid State Sciences
      Citation Excerpt :

      Benzene which mainly derived from industrial processes, the cement concrete, furniture manufacturing and fossil fuel combustion is the representative pollutant in Volatile organic compound (VOCs) and is one of the most numerous found in indoor air [1,2].

    View all citing articles on Scopus
    View full text