879 Review Received: 30 November 2009 Revised: 12 January 2010 Accepted: 16 January 2010 Published online in Wiley Interscience: 9 March 2010 (www.interscience.wiley.com) DOI 10.1002/jctb.2371 Oxidative processes of desulfurization of liquid fuels J.M. Campos-Martin, * M.C. Capel-Sanchez, P. Perez-Presas and J.L.G. Fierro Abstract Environmental concerns have introduced a need to remove sulfur-containing compounds from light oil. As oxidative desulfurization is conducted under very mild reaction conditions, much attention has recently been devoted to this process. In this contribution, the developments in selective removal of organosulfur compounds present in liquid fuels via oxidative desulfurization, including both chemical oxidation and biodesulfurization, are reviewed. At the end of each section, a brief account of the research directions needed in this field is also included. c  2010 Society of Chemical Industry Keywords: oxidative desulfurization; biodesulfurization; liquid phase; catalysis; clean fuels; selective oxidation INTRODUCTION Liquid fuels contain a large variety of sulfur compounds (thiols, sulfides, disulfides and thiophenes), which generate SO 2 and airborne particulate emissions during combustion. Therefore, desulfurization of light oil is extremely important in the petroleum- processing industry. Several processes have been proposed in the past to deal with the problem of removing these compounds from light oil. The most important and most common industrial process is that of treating the fuel under high temperatures and high pressures with hydrogen. This process is called hydrodesulfurization (HDS) and has received extensive attention since its discovery in the 1930s. The literature describing this technology is vast, amounting to thousands of patents and scientific and engineering papers. 1–4 HDS is a process in which light oil is heated, mixed with hydrogen, and fed to a reactor packed with a pelleted catalyst. Temperatures in the reactor typically range from 300 to 380 ◦ C. At these temperatures, some or all of the feed is vaporized, depending on the boiling range of the feed and the pressure in the unit. For heavier feeds it is common for the majority of the feed to be liquid. Reaction pressures range from as low as 15 to as high as 90 bar depending on the difficulty of removing the sulfur. 4 In the production of light oil such as diesel or jet fuel, pressures higher than 30 bar are commonly used. 2 In the earlier processes the feed and hydrogen mixture flow downward through the reactor, passing around and through the particulate catalyst, however in newer reactor configurations hydrogen flows upward from the bottom of the reactor (Synsat process). 4 Upon leaving the reactor, the mixture of treated fuel and hydrogen flows through a series of mechanical devices to separate and recycle the hydrogen, remove the H 2 S generated in the reaction, and recover the desulfurized product. HDS catalysts slowly lose activity during the operation, and must be removed and replaced after approximately 2 years of on-stream operation. As used in large integrated refineries, HDS is very effective and relatively inexpensive. However, the HDS is limited in treating benzothiophenes (BTs) and dibenzothiophenes (DBTs), especially DBTs having alkyl substituents on their 4 and/or 6 positions (Scheme 1). 1–3 The production of light oil with very low levels of sulfur-containing compounds, inevitably requires the application of severe operating conditions, i.e. very low space velocities, high temperatures and high pressures, as well as the use of highly active catalysts. 1–4 An alternative process, able to be operated under moderate conditions and without requirements for H 2 and catalysts, is required. 1,2 Organic sulfur compounds are divided into two groups: a portion that can be removed by cost-effective procedures such as HDS, and a second part, which is recalcitrant. In order to meet the ultra-low sulfur content regulations, the refractory part should also be desulfurized. 4 Hydrodesulfurization is able to remove the refractory sulfur; however, for these compounds, the process is very expensive and it is performed under very high temperature and pressure conditions. 1 Considering the growing trend of fuel consumption and exploiting petroleum resources, the remaining part of petroleum is more viscous and has a high sulfur content whose desulfurization will make the final desulfurized fuel product even more expensive. 2 Taking advantage of an efficient mild desulfurization process will provide several benefits. It lowers the final fuel price, economizes the desulfurization energy consumption and produces lower amounts of pollutants. Removing sulfur by alternative technologies is one of the approaches that can potentially be used after the HDS unit in the near future and replace HDS in the distant future. OXYDESULFURIZATION (ODS) PROCESS Oxidative desulfurization (ODS) has been considered as a further new technology for deep desulfurization of light oil. This desulfurization process includes two stages: (i) oxidation in a ∗ Correspondence to: J.M. Campos-Martin, Instituto de Cat´ alisis y Petroleoqu´ ımica, CSIC, Marie Curie 2, Cantoblanco, E-28049 Madrid, Spain. E-mail: j.m.campos@icp.csic.es Instituto de Cat´ alisis y Petroleoqu´ ımica, CSIC, Marie Curie 2, Cantoblanco, E-28049 Madrid, Spain. http://www.icp.csic.es/eac/ J Chem Technol Biotechnol 2010; 85: 879–890 www.soci.org c  2010 Society of Chemical Industry