Aerogel and xerogel WO3/ZrO2 samples for fine chemicals production
Graphical abstract
Highlights
► WO3–ZrO2 solid acids were designed by a sol–gel approach. ► Aerogel and xerogel catalysts were deeply characterised. ► Two reactions of industrial interest were investigated: anisole and veratrole acylations. ► The aerogel sample shows the best catalytic results for both conversion and yield. ► The aerogel sample attains the complete restoration of the catalytic activity even after three runs.
Introduction
Solid acid catalysts present many advantages if employed instead of traditional liquid (H2SO4, HCl, etc.) or mineral acid (AlCl3, BF3, etc.). In particular, they possess strength and concentration of active sites that can be tailored for particular applications, they present high surface area and adsorption capacity, thermal and chemical stability and above all they are environmentally friendly. Examples of these materials are zeolites [1], heteropoly acids [2], clays and metal oxides. The latest have many applications in organic synthesis either as supports for catalytically active materials or as catalysts themselves. The promotion of metal oxides (Fe2O3, TiO2, ZrO2, etc.) with sulfated, phosphated and heteropoly-anions leads to solid acids with a wide range of strength [3]. Mixed metal oxides, with two or more components, are of significant scientific and commercial interest [3], [4], [5]. In fact these systems exhibit enhanced chemical–physical properties (higher surface area, higher pore volume, superior thermal and chemical stability) and higher catalytic activity than their individual components [6], [7].
In this context, WO3/ZrO2 (hereafter referred to as ZW) are very attractive because they display high catalytic activity and stability in processes requiring medium–high acidity or red-ox properties, such as acylation and isomerization reactions [7], [8]. Recently the catalytic activity of this system has been evaluated also in catalytic processes concerning bio diesel production, as for example in the esterification of fatty acids [9], [10]. Tungstated zirconia catalysts offer several advantages compared to sulfated zirconia catalysts, such as higher thermal stability, lower deactivation rates, easier regeneration [11], [12]. The physico-chemical properties of ZW catalysts and, in particular, their acidity strength are strictly correlated to the synthetic procedure and mainly to the relative amount of each oxide. Usually ZW materials are prepared by impregnation of zirconium oxyhydroxide with ammonium metatungstate and subsequent high temperature oxidation or by co-precipitation of the zirconium and tungsten precursors [13], [14]. Unfortunately, conventional mixed oxides synthesis techniques do not usually produce homogeneous materials with high surface area. On the contrary, the sol–gel synthesis [15], [16], [17], [18] reduces the number of preparation steps and allows the strict control of the physico-chemical properties and of the homogeneity of the final materials. In particular, the sol–gel method stabilizes the surface area and the pore volume of the catalysts with respect to the traditional synthetic procedures. Despite these remarkable advantages, only few investigations concerning the sol–gel preparation of ZW systems are present in literature [15], [19], [20].
This work deals with the synthesis, the characterization and the reactivity of ZW systems obtained via sol–gel. The samples were synthesized by means of different procedures for the solvent extraction: (i) evaporation in vacuum at room temperature (xerogel) and (ii) evaporation in supercritical conditions (aerogel). In order to study the influence of the synthetic parameters on both structure and texture of the materials, different characterization techniques were employed: XPS, N2 physisorption, FT-IR spectroscopy, TPR and TPO analyses. The catalytic behaviour of the systems was investigated towards two reactions of industrial interest [21], [22]: (i) the acylation of veratrole with acetic anhydride, (ii) the acylation of anisole with benzoic anhydride.
Section snippets
Synthesis of catalysts
The catalysts were synthesized by a one-step sol–gel method [15]. Briefly, the gel was prepared by mixing the metal alcoxides precursors Zr(OC3H7)4 (18.4 mL) and W(OC3H7)6(40 mL) with a mixture of i-PrOH and water (60 mL) in presence of HNO3 (68%, 0.6 mL). Two different extraction procedures were used for the solvent extraction. The xerogel sample (termed XZW) sample was obtained by simply removing the solvent from the wet gel under vacuum (10−2 Torr) at room temperature. The aerogel sample (termed
Results and discussion
The catalytic activity in liquid phase was studied in the acylation of anisole and veratrole (Scheme 1) under mild reaction conditions. Friedel–Crafts acylation of aromatic compounds is widely used in the synthesis of aromatic ketones, that are important intermediates for the production of pharmaceuticals, dyes, agrochemicals and fragrances. In particular, acylation of anisole and veratrole (1,2-dimethoxy benzene) leads to the synthesis of p-acetoanisole and acetoveratrole, precursors of a sun
Conclusions
WO3/ZrO2 catalysts were synthesized using a reliable sol–gel method. All samples were found to be active in acylation reaction of veratrole and anisole with good conversions and yields despite the mild reaction conditions. The synthetic parameters and, in particular, the solvent extraction step causes some important variations in the physico-chemical features of the investigated systems and in their catalytic activity. The best performance was achieved by the aerogel sample obtained by the
Acknowledgement
We thank Mrs. Tania Fantinel for technical assistance.
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