Development of High-performance Hydrodesulfurization Catalysts for Diesel Production
The nitrogenous compounds (NOX) and particulate matter in diesel engine exhaust are a source of environmental pollution. Cars and trucks are therefore equipped with devices that clean up the exhaust gas by removing these harmful substances. But because these devices are damaged by sulfur, the amount of sulfur allowed in automotive diesel fuel is strictly regulated.
In Japan, a law enacted in 2007 limits the sulfur content of diesel to 10 ppm or less. Fuels that meet this standard are referred to as sulfur-free diesel.
Manufacturing Technology for Sulfur-Free Diesel
The main component of sulfur-free diesel is straight-run gas oil, which typically has a boiling point between 260°C and 360°C and is separated from crude oil by distillation. This straight-run gas oil will contain 1–2% sulfur. In order to reduce the sulfur content to 10 ppm or less, at refineries, the gas oil is combined with hydrogen in the presence of a catalyst in a reaction that takes place under high pressure (hydrodesulfurization, see Fig. 1). This process of hydrodesulfurization is performed not only for diesel but for many other fuels as well.
Difficulty of Diesel Desulfurization
Sulfur is found in organic sulfur compounds with a variety of molecular structures. Dibenzothiophenes are typical of the sulfur compounds found in diesel. Some of these compounds can be very difficult to desulfurize, depending on their structure. Of the dibenzothiophene derivatives shown in Fig. 2, those with substituents in the 4- and 6- positions are most resistant to reaction. But in order to meet the definition of sulfur-free, these sulfur compounds must also be removed.
Higher Performance Desulfurization Catalysts Needed
In addition, crude diesel contains nitrogen and aromatic compounds that can also deactivate the catalyst, in concentrations that vary depending on the feedstock. To enable use of a greater variety of crude oils, high-performance desulfurization catalysts are needed now.
Points for Catalyst Development
The active species of most desulfurization catalysts is molybdenum sulfide (MoS2). The MoS2 is supported in a highly dispersed state on a carrier having a high surface area, such as alumina. Molybdenum sulfide has a layered structure, in which a plane of molybdenum atoms (Mo) is sandwiched between planes of sulfur atoms (S). We can see the molybdenum layers in a sulfurized catalyst using a transmission electron microscope (Photo 1). Increasing the number of active sites on the carrier is considered crucial to improving desulfurization activity. Also, because the catalyst will be used for prolonged periods after being loaded into the reactor, it is important that the catalyst have not only high activity but stable activity.
Catalyst Development and Introduction to Refineries
Our researchers discovered a special microparticulation process that brings down the size of the molybdenum sulfide particles to 10 nm or smaller and results in formation of a higher number of active sites, allowing us to successfully increase catalytic activity.
We have had success at practical application of many desulfurization catalysts. One particular desulfurization catalyst, put into use in 2011, has earned the company recognition for its contribution toward increasing productivity in sulfur-free diesel production. The technology related to development of this catalyst received the Japan Petroleum Institute Award for Technological Progress in FY2014 and the Catalysis Society of Japan Award for Technology in FY2016.