Synthesis of a nano-crystalline solid acid catalyst from ﬂy ash and its catalytic performance Chitralekha Khatri, Ashu Rani * Environmental Chemistry Laboratory, Department of Chemistry, Government P.G. College, Kota 324001, Rajasthan, India article info Article history: Received 13 January 2008 Received in revised form 16 April 2008 Accepted 17 April 2008 Available online 16 May 2008 Keywords: Fly ash Solid acid catalyst Esteriﬁcation abstract The synthesis of nano-crystalline activated ﬂy ash catalyst (AFAC) with crystallite size of 12 nm was car- ried out by chemical and thermal treatment of ﬂy ash, a waste material generated from coal-burning power plants. Fly ash was chemically activated using sulfuric acid followed by thermal activation at 600 °C. The variation of surface and Physico-chemical properties of the ﬂy ash by activation methods resulted in improved acidity and therefore, catalytic activity for acid catalyzed reactions. The AFAC was characterized by X-ray diffraction, FT-IR spectroscopy, N 2 -adsorption–desorption isotherm, scanning electron microscopy, ﬂame atomic absorption spectrophotometry and sulfur content by CHNS/O elemen- tal analysis. It showed amorphous nature due to high silica content (81%) and possessed high BET surface area (120 m 2 /g). The catalyst was found to be highly active solid acid catalyst for liquid phase esteriﬁca- tion of salicylic acid with acetic anhydride and methanol giving acetylsalicylic acid and methyl salicylate respectively. A maximum yield of 97% with high purity of acetylsalicylic acid (aspirin) and a very high conversion 87% of salicylic acid to methyl salicylate (oil of wintergreen) was obtained with AFAC. The sur- face acidity and therefore, catalytic activity in AFAC was originated by increased silica content, hydroxyl content and higher surface area as compared to ﬂy ash. The study shows that coal generated ﬂy ash can be converted into potential solid acid catalyst for acid catalyzed reactions. Furthermore, this catalyst may replace conventional environmentally hazardous homogeneous liquid acids making an ecofriendly; sol- vent free, atom efﬁcient, solid acid based catalytic process. The application of ﬂy ash to synthesize a solid acid catalyst ﬁnds a noble way to utilize this abundant waste material. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The coal-burning power plants, which consume pulverized solid fuels, produce large amounts of ﬂy ash as a residue. Every year a crude estimation of more than 300 billion tones of ﬂy ash is gener- ated in the world [1] and is being consumed in the production of building construction materials, in agriculture, metal recovery, in water and atmospheric pollution control, etc. [2]. These applica- tions could succeed up to some extent to consume the huge amount of ﬂy ash. Nevertheless, the search of new applications of the ﬂy ash as either catalyst or as catalyst support material is still ongoing. Literature reports the use of ﬂy ash as adsorption cat- alyst for the removal of dyes [3], heavy metals [4] etc. However, studies related to use of ﬂy ash as heterogeneous catalyst in organ- ic transformations are scanty. Fly ash is a silico-aluminate material consisting of SiO 2 , Al 2 O 3 , Fe 2 O 3 as the major constituents and varying amount of CaO, MgO, SO  3 with unburned carbon. Besides these, some minor ele- ments such as Hg, As, Ge, Ga, and traces of heavy metals (Cr, Co, Cu, Pb, Mn, Ni, Zn) and rare earths may also be present in ﬂy ash [5]. The glassy (amorphous) siliceous spherical particulates are the active portion of ﬂy ash. Typically, ﬂy ash is 30–50% glass and higher glass content in the form of quartz is also present in the ﬂy ash [5]. Other metal oxides such as Mn 2 O 3 , TiO 2 , etc. [6] and minerals like mullite, hematite, magnetite, ferrite, and rutile in ﬂy ash [7] are desirable from the point of view of reactivity. In addition negatively charged sulfur species such as sulfate and sul- ﬁde are also present in ﬂy ash [8] and the surface area of the ﬂy ash generally remains in the range of 40–115 m 2 /g [9]. The ﬂy ash is chemically activated by acid treatment, by dis- solving in acid [2]. Furthermore, the silica content of the ﬂy ash is increased by using strong acids, as most of the elements such as alumina and iron are easily eliminated via acid leaching [10]. The chemical activation at boiling temperature of acid has resulted in ﬂy ash similar in behavior to silica fume with higher silica contents 60–70% and also increased the BET surface area upto 173 m 2 /g surpassing the values of silica fume (22 m 2 /g) [10] conferring a great degree of reactivity in the ﬂy ash. Fly ash can also be used as catalyst and catalyst support as it 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.04.011 * Corresponding author. Present address: 2-m-1, Rangbari Scheme, Kota 324005, Rajasthan, India. Tel.: +91 9352619059. E-mail addresses: chitra_cool81@yahoo.com (C. Khatri), ashugck@rediffmail.com (A. Rani). Fuel 87 (2008) 2886–2892 Contents lists available at ScienceDirect Fuel journal homepage: www.fuelﬁrst.com