Chemical Engineering Journal 165 (2010) 658–667 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej A study on the oxidation of phenol by heterogeneous iron silica catalyst Farook Adam ∗ , Jeyashelly Andas, Ismail Ab. Rahman School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia article info Article history: Received 19 June 2010 Received in revised form 1 September 2010 Accepted 10 September 2010 Keywords: Iron catalyst Silica Rice husk Oxidation of phenol Catechol abstract A series of iron silica catalyst with (5–20) wt.% Fe 3+ were prepared by means of a simple solvent extraction and sol–gel technique. These catalysts were probed for the oxidation of phenol employing hydrogen peroxide. Catalytic performance increased up to 10 wt.% Fe 3+ loading. Further increase in the iron content was found to reduce the phenol conversion rate. Higher Fe 3+ loading (>10% Fe 3+ ) resulted in smaller pore size and exhibited extra framework Fe 3+ in the catalyst, which lead to catalytic deficiency in phenol oxidation. Phenol oxidation by RH-10Fe gave 95.2% conversion at 343 K with selective formation of 61.3% catechol (CAT) and 38.7% hydroquinone (HQ). Reusability studies with RH-10Fe resulted in only 16% loss in catalytic activity. However, no leaching of iron was detected. The CAT/HQ ratio was found to be constant during the reaction which suggested a non-free radical catalytic mechanism to be operative. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Phenol is a foremost pollutant in UESEPA list with limits of discharge less than 0.5 mg L -1 [1]. This is a lethal and hazardous contaminant which is believed to be carcinogenic. Many inves- tigators are currently engaged in finding ways to eliminate this pollutant from the environment. Specific chemical processes are necessary to convert this carcinogenic compound into industrially benign products such as diphenols. Diphenols, namely catechol (CAT) and hydroquinone (HQ) are used for diverse applications such as photographic chemicals [2], polymerization inhibitors, antioxidants and flavoring agents [3,4]. In the oxidation of phenol, hydrogen peroxide appears to be an appropriate and potential oxi- dant due to high oxygen content and yielding water as the only by-product [5,6]. Hydrogen peroxide is known to be less of an environmental pollutant [7,8]. Many attempts to oxidize phenol by aqueous hydrogen peroxide had been reported [9–12] with a variety of heterogeneous catalysts. The invention of titanium silicate, TS-1, by Enichem has indeed opened a remarkable application of heterogeneous catalyst in the oxidation of phenol. However, some characteristics exhibited by TS-1, such as complex preparation method and small pore size restrict its use in the oxidation of large molecules. It also has an inherent difficulty during separation due to its small size particles which has limited its function in the oxidation of phenol [13]. This has stimulated much research to develop cheaper and better cat- alysts which are capable of converting this toxic compound into more benign and useful products. ∗ Corresponding author. Tel.: +60 4 6533567; fax: +60 4 6574854. E-mail addresses: farook@usm.my, farook dr@yahoo.com (F. Adam). Rice husk (RH), which is a by-product from the rice process- ing industry, is found abundantly in most rice producing countries. Burning seems to be an easy way of disposing the husks. However, this can lead to environmental problems [14]. Biodegradation of the RH is also a slow process due to the high silica content. Therefore, utilization of RH as an alternative source of silica had gained some interest. Recently, silica from RH had been used as catalyst support for important industrial applications such as oxidation of cyclohexane [15,16], cyclohexene and cyclohexanol [16], oxidation of phenyl- methanol [17] and decomposition of cyclohexanol [18]. The high catalytic behavior of these silica supported catalysts is due to the high surface area of the resulting catalysts. Several publications have appeared on iron supported rice husk ash silica. Adam et al. [19] had reported the catalytic activity of iron incorporated rice husk ash silica for benzylation of toluene. The synthesis of iron sup- ported rice husk ash silica as a catalyst for the benzylation of xylene had also been reported [20]. However, in both of these cases, the rice husk was pyrolyzed into ash which required high consumption of energy. To overcome this, Adam and Andas [21] found a cheaper way to extract the silica by solvent extraction and incorporated the metal via a sol–gel technique at room temperature. In this present studies, we extend the use of the iron loaded rice husk silica catalyst in the oxidation of pollutants such as phenol. Various sources of silica have been used as catalyst support for the oxidation of phenol [22–25]. However, there is no published literature of silica from rice husk being used as catalyst support for the oxidation of phenol. While there is no real advantage on using silica from RH, however, it certainly creates added value to the otherwise useless waste material. An added advantage of this procedure will also result in the husk becoming more amenable to biological degradation and ease the disposal problem for RH. 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.09.054