Hydrodesulfurization 500 Subject

Today s society asks for technology that has a minimum impact on the environment. Ideally, chemical processes should be clean in that harmful byproducts or waste are avoided. Moreover, the products, e.g. fuels, should not generate environmental problems when they are used. The hydrogen fuel cell (Chapter 8) and the hydrodesulfurization process (Chapter 9) are good examples of such technologies where catalysts play an essential role. However, harmful emissions cannot always be avoided, e.g. in power generation and automotive traffic, and here catalytic clean-up technology helps to abate environmental pollution. This is the subject of this chapter. 

It appears that the high molecular weight species originally present in the feedstock (or formed during the process) are not sufficiently mobile (or are too strongly adsorbed by the catalyst) to be saturated by the hydrogenation components and, hence, continue to condense and eventually degrade to coke. These deposits deactivate the catalyst sites and eventually interfere with the hydrodesulfurization process. Thus, the deposition of coke and, hence, the rate of catalyst deactivation, is subject to variations in the asphaltene (and resins) content of the feedstock as well as the adsorptive properties of the catalyst for the heavier molecules. 

Many of the catalysts for the hydrodesulfurization process are produced by combining (Table 5-5) a transition metal (or its salt) with a solid support. The metal constituent is the active catalyst. The most commonly used materials for supports are alumina, silica, silica-alumina, kieselguhr, magnesia (and other metal oxides), as well as the zeolites. The support can be manufactured in a variety of shapes or may even be crushed to particles of the desired size. The metal constituent can then be added by contact of the support with an aqueous solution of the metal salt. The whole is then subjected to further treatment that will dictate the final form of the metal on the support (i.e., the metal oxide, the metal sulfide, or even the metal itself). 

In contrast to the lighter feedstocks that may be subjected to the hydrodesulfurization operation, the heavy oils and residua may need some degree of pretreatment. For example, the process catalysts are usually susceptible to poisoning by nitrogen (and oxygen) compounds and metallic salts (in addition to the various sulfur-compound types) that tend to be concentrated in residua (Chapter 3) or exist as an integral part of the heavy oil matrix. 

A subject of technological interest is the bond between thiophene and a transition metal M. This is due to the hydrodesulfurization of petroleum, which is performed in the presence of a transition metal catalyst [74]... 

The subject of Raney nickel desulfurization has been exhaustively dealt with in CHEC-I. A few further interesting applications of this procedure will be presented in this section, along with the use of some nickel(O) complexes and nickel boride for this purpose. The major goal of understanding the mechanism of hydrodesulfurization by studying metal complexes as models is briefly dealt with in Section 2.10.6. 

The study of hydrodesulfurization (HDS) has had tremendous impact on the petroleum industry <88ACR387, 88ACR394). The mechanism of hydrodesulfurization has theoretical and commercial importance and has been the subject of numerous reports investigating mechanism, variation of catalyst and substrate, and product composition. In general, the role of the metal catalyst as well as the structures of thiophene-metal coordination complexes will be discussed in Section 2.12.11.2. In this section, we focus on specific desulfurization to produce isolable products. 

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