Recently, regulations on the sulfur contents of fuel oils have been tightened from environmental viewpoint. In particular, the sulfur contents of gasoline and gas oil have been subjected to more severe regulations. Under these circumstances, catalysts with a high desulfurization performance have been developed so as to cope with these regulations.
The active sites of a desulfurization catalyst are attributable to the sulfide of active metals supported on the support and presumably exist mostly edge portions of the crystal structure molybdenum disulfide taking a laminated structure (hereinafter referred to as “molybdenum disulfide layer”). For example, Patent Literature 1 discloses that a hydrotreating catalyst having molybdenum disulfide layers with an average number of laminated layer of 2.5 to 5 and an average value (average length) of 1 to 3.5 nm exhibits a high desulfurization performance for gas oil.
It is known that a titania support exhibits a higher desulfurization performance than an alumina support, and thus a hydrotreating catalyst including a titania support have been expected to be a catalyst meeting the demands. However, titania has a problem that it has a small specific surface area and low thermal stability at elevated temperatures. In order to solve this problem, a porous titania has been developed, which is produced by adding a growth inhibitor that inhibits particles from growing when calcined, to hydrosol or hydrogel of an aqueous titanium oxide or a dried product thereof and then drying and calcining the mixture (for examples, see Patent Literature 2). However, there is a problem that the sole use of the porous titania as a support increases the cost of the resulting catalyst. Therefore, a hydrotreating catalyst has also been developed, which comprises an alumina-titania support prepared by loading a water-soluble titania compound on an alumina support (for example, see Patent Literature 3). However, this hydrotreating catalyst can be less expensive but has a drawback that it can support titania only in an amount corresponding to the water absorption rate of titania and thus is poor in catalyst performances. Alternatively, a hydrotreating catalyst has also been developed, which comprises an alumina-titania support prepared by mixing titania in alumina so as to be highly dispersed therein upon preparation of the alumina (for example, see Patent Literature 4). However, this catalyst enables titania to be highly dispersed in alumina but has disadvantages that as the content of titania increases, crystallization thereof is likely to accelerate, resulting in a catalyst with a decreased specific surface area and a deteriorated sharpness of the pore distribution, causing a deterioration in catalyst performances.