Article pubs.acs.org/IECR
Catalytic Performance of Silica-Supported Silver Nanoparticles for Liquid-Phase Oxidation of Ethylbenzene Raji, Vadakkekara, Mousumi Chakraborty,* and Parimal A. Parikh* Department of Chemical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat 395 007, Gujarat, India ABSTRACT: In this study silver nanoparticles were prepared by chemical reduction method using silver nitrate as metal precursor, starch as protecting agent, and sodium borohydride (NaBH4) as a reducing agent. Formation of silver nanoparticles was monitored using UV−vis absorption spectroscopy and dynamic light scattering (DLS). They were supported on silica by dispersing silica powder in the suspension of destabilized silver nanoparticles. Samples containing different proportions of silver were thus prepared. This method is at variance from the conventionally employed method, i.e., impregnation of silver salt from its solution on support. Ag/SiO2 samples were characterized by UV−vis absorption spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductive coupled plasma optical emission spectroscopy (ICP-OES), and N2 adsorption−desorption. Superior catalytic performance of the catalyst prepared by the present method could be observed in a test reaction of ethylbenzene oxidation affording high selectivity to acetophenone as compared to the catalyst prepared by the conventional reported methods. The 5 wt % Ag/SiO2 catalyst was found not much susceptible to sintering as could be inferred from the comparable performance of the regenerated and fresh catalysts.
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oxidation of alcohols21 is quite unique; no other catalyst has yet been discovered promoting these reactions. These studies employed Ag loading exceeding 8 wt % with Ag cluster size reaching up to 70 nm. Acetophenone (AP) is an important raw material for the synthesis of some pharmaceuticals, fragrances, chewing gum, resins, alcohols, esters, aldehydes, and tear gas and is used as test substrate for asymmetric transfer of hydrogen, as a flavoring agent in many sweets and drinks, and as a solvent for cellulose ether.36 AP and α-methyl benzyl alcohol are the main products of the liquid-phase ethylbenzene (EB) oxidation reaction in the presence of the oxidizing agent, tertiary butyl hydrogen peroxide (TBHP). Of these two products the former has the higher commercial value; hence, it is desirable to increase selectivity to AP in the EB oxidation reaction. Beier et al.11 reported the oxidation of alkyl aromatics (toluene, xylene, ethylbenzene, and cumene) in the presence of Ag/SiO2 and Ag/SiO2−CeO2 as catalysts. It was found that α-methyl benzyl alcohol, AP, and additionally ethylbenzene hydroperoxide were produced by the oxidation of EB. A 10 wt % amount of Ag/ SiO2 produced 11.8% yield of AP under oxygen atmosphere and 3 h reflux. They found that the catalytic activity of silver catalysts mainly depended on silver loading, the amount of promoter, and the calcinations procedure. Kanjina and Trakarnpruk36 used metal complex [(n-C4H9)4N]4HPW11Co(H2O)O39·H2O as catalyst using oxidant hydrogen peroxide and acetonitrile as solvent. They observed 92% selectivity to AP with 33% yield. When EB oxidation was carried out using Mn− MCM-41, Mn−SBA-15, Co−MCM-41, and CNCr at 80 °C for
INTRODUCTION Heterogeneous catalysts are convenient to use on a large scale, and they are used in almost all the major catalytic processes. Their applications in liquid-phase reactions afford milder conditions, which preserves complex and temperature-sensitive structures. Heterogeneous catalysts present a high surface area of the catalytically active phase to the reactants and have separation and recycling advantages compared to homogeneous catalysts.1,2 A high surface area of active catalyst is achieved by dispersing it on a high surface area support preferably in the form of very small crystallites, maybe of nanometer size. Compared to other supports, silica support proved to offer high metal dispersion and, additionally, resistance to sintering of metal particles.3 Many researchers have studied metal nanoparticle synthesis and reported their high activity arising from their low coordination number. Synthesis and a variety of applications of nanoparticles of group IB metal silver have been widely studied due to its relatively low cost. Impregnation of silver salt from its solution on support has remained a common practice to synthesize silver catalysts, i.e., following the incipient wetness impregnation.4−12 Supported nanoparticles offer great potential as catalysts for the synthesis of fine chemicals, and they can contribute to the sustainable development of chemical processes.13,14 Though silver is not the best oxidation catalyst in terms of activity, it can be utilized as an active component of catalysts for some deep or partial oxidation. Ag° is an active species at low temperatures (
