Advanced Materials Research Vol. 925 (2014) pp 38-42 (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/amr.925.38 Nanoclustered Gold: A Promising Green Catalysts for the Oxidation of Alkyl Substituted Benzenes Md. Eaqub Ali a*, Md.Motiar Rahman b and Sharifah Bee Abd Hamid c Nanotechnology and Catalysis Research Center (NANOCAT), 50603 Kuala Lumpur, University of Malaya, Malaysia. eaqubali@gmail.com a, motiar.rahman28@gmail.com b, sharifahbee@um.edu.my c Key-words: Gold-skeleton; green catalyst; Mesoporous silica; Catalytic oxidation; Nanoclustered catalysts. Abstract: Catalytic oxidation of alkyl substituted benzenes is an essential route for the synthesis of a number of important chemicals, perfumes, drugs and pharmaceuticals. The oxidation products of ethyl benzene are important precursors for a wide range of pharmaceuticals and synthetic materials. Acetophenone and 1-phenylethanol are two oxidation products of ethyl benzene which are the precursors of optically active alcohol, benzalacetophanones, hydrazones and so on. However, the oxidations of alkyl substituted benzenes have been remaining a challenging task. This is because of the limitations of an appropriate catalyst and requirement of corrosive chemical treatments (potassium permanganate/dichromate and ammonium cerium nitrate) which are hazardous and environmentally unfriendly. The current industrial practice in the oxidation of ethyl benzene unfortunately involves high temperature thermal autoxidation in the absence of catalysts. Although few catalysts have been tested for the oxidation of ethyl benzene, many of them found to be inefficient. For example, cobalt (II) oxide-immobilized on mesoporous silica (Co/SBA-15) was used to catalyze oxidation of alkyl benzene at high temperature (125-150 o C) but only 70% conversion was obtained after prolong treatment at 150 o C. Additionally, the catalyst formed mixed uncontrolled oxidation products like 1-phenylethyl hydro peroxide, benzoic acid, acetophenone and phenyl ethanol. Carbon/silica/metal oxide supported nanoporous gold is a promising green catalyst for heterogenous molecular transformation. This is because of their three dimensional open pore network structures, high surface to volume ratio, high reusability, distinct optolectronic and physio-chemical properties. Mesoporous carbon/silica/metal oxide thin film supports provide increase dispersion of metal nanocatalysts and facilitate transport of molecules, ions or electrons through the nanopores/nanochannels, enhancing product yields with minimum cost and time. This paper has reviewed various gold-skeleton green catalysts and their preparation and mechanistic schemes for the selective oxidation of alkyl substituted benzenes. Introduction Oxidation reaction play very essential role in chemical industry, and thus make the basis for the production of many crucial compounds [1]. Especially, supported gold catalysts play an important role to the conversion a variety of molecules with high specificity and productivity [2]. Various supported gold catalyst to be considered as an ideal catalyst because these types of catalysts are environmentally friendly and provide a consistent and well-isolated environment to the well deposition of active components and well access for the reactants [3]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 202.185.77.171-17/04/14,05:35:09)
Advanced Materials Research Vol. 925 39 Fig 1. (1) Glucose to gluconic acid; (2) Dimethylphenylsilane to dimethylphenylsilanol; (3) Hydrogen to hydrogen peroxide; (4) Carbon monoxide oxidation; (5) Glycol (EG) to glycolate; (6) Epoxidation of propene by gold catalysts. Preparation of mesoporous carbon supported gold catalyst: Here, synthesis of mesoporous carbon supported gold catalyst has been shown by two steps. The Fig. 2 represents the synthesis of hollow silica and Fig. 3 demonstrates the synthesis of mesoporous carbon supported gold catalyst using hollow silica template [4]. Fig 2. Schematic representation of fabrication of hollow silica nanospheres (Adapted with permission from ACS [4]).
40 Micro/Nano Science and Engineering Fig-3: Synthesis of Mesoporous carbon supported gold using hollow silica template. Catalytic oxidation of alkyl substituted benzene by gold catalyst: Fig 4. Oxidation of alkyl substituted benzene by gold catalyst.
Advanced Materials Research Vol. 925 41 Application: The gold catalyzed oxidation of alkyl substituted benzene can provide many important chemicals as well as perfumes, drugs and pharmaceuticals. The oxidation products of ethylbenzene are used as precursors for the synthesis of the process acetophanone, 1- phenylethanol, optically active alcohols, benzalacetophanones (chalcones), hydrazones, pharmaceuticals, resins, tear gas and so on (Fig. 5) [2, 5]. Fig 5. Some products of alkyl benzene oxidation Conclusion Until recently, various method have been applied to synthesize mesoporous materials supported gold catalyst such as (a) Deposition-Precipitation Method, (b) Etching, (c) Impregnation, (d) Anion adsorption method, (e) Co-precipitation and (f) Dispersion. Of them, the deposition-precipitation and anion adsorption methods require washing step to remove the chloride of chloroauric acid. In addition, maximum 1% gold can be loaded by these processes. In etching, the prepared gold catalysts work only at low temperature. Impregnation allows the deposition of gold onto solid supports irrespective of the gold loading and the nature of the support. However, the synthesized catalysts, in this process, contain large amount of chlorides as a result large gold particles present at the end of calcination treatment. In Coprecipitation method, the synthetic catalysts contain relatively low surface area and therefore, significant amount of gold deposited within the catalysts particles which is not suitable for the reaction. In this perspective, the dispersion is the best alternative method for the easy synthesis of supported gold catalyst and the oxidation of various compounds with high conversion rate expecting 90%. Acknowledgements: The authors would like to acknowledge the University of Malaya fund no. RP005A-13 AET to M.E. Ali.
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