1. Field of the Invention
The present invention relates to a hydrodesulfurization catalyst and hydrodesulfurization process for gasoline fractions.
2. Related Background Art
Emission standards are currently being established throughout the world for greenhouse gases such as carbon dioxide, and reduction measures for such gases are becoming an increasingly important topic. One such reduction measure is to control carbon dioxide emissions from transportation internal combustion engines which run on gasoline, and improved engine fuel efficiency is an essential condition for achieving this. It is believed that new systems such as direct injection engines or lean burn engines, which offer relatively enhanced fuel efficiency, will become even more prevalent in the future.
Nevertheless, such new systems pose a problem in that NOx gases cannot be sufficiently removed by using conventional three-way catalysts as NOx removal catalysts for exhaust gas. Although several novel NOx removal catalysts have been proposed to deal with this problem, such NOx removal catalysts are generally contaminated by sulfur, which lowers the NOx removal activity. This has required further removal of sulfur of the gasoline fuel itself, and in recent years some discussion has regarded the need for gasoline with a sulfur content of no greater than 10 ppm by weight (“sulfur-free gasoline”).
Incidentally, processes for production of gasoline stocks include appropriate treatment of gasoline fractions obtained by reforming straight run gasoline fractions yielded from distillation of crude oil, or treatment of gasoline fractions obtained by cracking heavy oil. Of the product oils yielded by such processes, catalytically cracked gasoline fractions obtained by fluid catalytic cracking (FCC) are characterized by having rich olefin and aromatic component contents compared to straight run gasoline, as well as a high octane number.
However, when it is attempted to accomplish hydrodesulfurization of gasoline fractions containing olefin components, hydrogenation of the olefins occurs simultaneously with the hydrodesulfurization reaction. This has led to a serious problem in which the octane number of the gasoline fraction obtained by the hydrodesulfurization process is reduced. The hydrodesulfurization process for gasoline fractions including catalytically cracked gasoline fractions must be selective by preventing hydrogenation of the olefin components while promoting only the hydrodesulfurization reaction. Such a process is also known as “selective hydrodesulfurization process”.
Technologies such as the following have been disclosed to meet the demands for selective hydrodesulfurization. For example, Japanese Translation of PCT publication No. HEI 6-509830 describes a process for restoring the octane number reduced by hydrogenation treatment, by carrying out a catalyst reforming step or isomerization step after the hydrogenation treatment. Also, Japanese Patent Application Laid-Open No. 2000-239668 describes a process for production of gasoline fractions which comprises a step of hydrogenation of the unsaturated sulfur-containing compounds and a step of decomposing the saturated sulfur-containing compounds. There have also been proposed a process of using a molybdenum and cobalt catalyst having controlled support volume and support surface area (see Japanese Translation of PCT publication No. 2000-505358), a process for preventing octane number reduction by combination with a zeolite catalyst (see U.S. Pat. No. 5,352,354) and a process of using a catalyst subjected to specific pretreatment (see U.S. Pat. No. 4,149,965).