Abstract
Mesoporous TiO2/H3PW12O40 composites were synthesized by sol–gel reactions using urea as a low-cost template, and adding tungstophosphoric acid (TPA) at the same time as the template. The TPA concentration was varied in order to obtain TPA contents of 0, 10, and 20 (w/w) in the solid. The samples presented mesopores with a diameter higher than 3.0 nm. The specific surface area of the solids decreased with both the increase of the TPA content and the calcination temperature. From Fourier transform infrared and 31P magic angle spinning-nuclear magnetic resonance studies it was observed that the main heteropolyoxometallate species present in the composites is the [PW12O40]3− anion, which was partially transformed into the [P2W21O71]6− and [PW11O39]7− anions during the synthesis and drying step. The X-ray diffraction patterns of the modified samples only exhibited the characteristic peaks of the anatase phase of titanium oxide. The point of zero charge decreased with both the increase of TPA content in the solids and the calcination temperature. The materials were found to be efficient and recyclable catalysts for the synthesis of a series of flavones. The reaction was carried out in different reaction media: heterogeneous and solvent-free conditions. The solvent-free conditions represent the best green conditions. Initially, we optimize the reaction condition to obtain 6-chloroflavone by direct reaction of the cyclodehydration of 1-(2-hydroxy-5-chlorophenyl)-3-phenyl-1,3-propanodione in the presence of a catalytic amount of mesoporous titania modified with tungstophosphoric acid catalyst. Reactions were performed in two conditions: low volume of toluene, at 110 °C, typically 24 h, and solvent-free at the same temperature, 1 h. In all cases the product (6-chloroflavone) was obtained with high selectivity. Conversions up to 76 and 92 % were obtained respectively, using the supported catalyst (TiTPA10). Optimal reaction conditions were applied to the preparation of six substituted flavones in both conditions. Open image in new window
Highlights
The search of new acid solids that can replace the classical acids for their use as catalysts in organic synthesis in liquid phase is constant due to the need for developing clean processes with environmental advantages [1]
In the present work we describe the preparation and characterization of mesoporous titania modified with tungstophosphoric acid; we present its use as a recyclable acid catalyst in the heterogeneous and solvent-free synthesis of flavones
The samples are mesoporous materials with a Dp higher than 3.0 nm and it decreased in the following way TiTPA00 (5.1 nm) [ TiTPA10 (4.2 nm) [ TiTPA20 (3.6 nm) when tungstophosphoric acid (TPA) content was increased
Summary
The search of new acid solids that can replace the classical acids for their use as catalysts in organic synthesis in liquid phase is constant due to the need for developing clean processes with environmental advantages [1]. Different HPA have been used to modify TiO2 and these materials are widely used as acid catalyst as well as oxidant. More recently they have been used as effective homogeneous photocatalyst in the oxidation of organic compounds, and in the degradation of pollutants in water [7]. Compounds containing the chromone skeleton (4H-benzopyran-4-one) are widely distributed in the plant kingdom; they constitute a group of compounds in the flavonoid family [8] These compounds have multiple biological properties, for example anti-inflammatory, antibacterial, antitumor [9], antioxidant [10], anti-HIV [11], vasodilator, antiviral, and antiallergenic [12]. All the starting 1,3-diketones were prepared following a method described elsewhere [27]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
