Sulphides, hydrotreating

Alumina-supported Co- and Ni-promoted molybdenum sulphide hydrotreating catalysts are the main workhorses in many refineries and have, therefore, attracted a lot of attention from catalytic chemists. They are usually prepared via co-impregnation, i.e. pore-volume impregnation with both Mo and the promoter atom present in solution. After drying and calcining, the catalyst manufacture is complete, but it has to be sulphided before use. Traditionally, this is done in situ... 

Specifications for reformulated gasoline have meant less aromatics and olefins and constraints on light hydrocarbons and sulphur. New legislation for diesel requires deep desulphurisation to 10-50 ppm S. This is done by reacting the sulphur compounds with hydrogen into hydrogen sulphide (hydrotreating), which is removed from the hydrocarbon stream. The requirement for sulphur removal may be accompanied by a wish to remove aromatics. 

Transition metal sulphides are able to catalyze a very large number of reactions. The most important utilization concerns catalytic hydrotreating, but many others can be foreseen due to the resistance of these catalysts towards sulphur. For example, recent studies have demonstrated the interest of such catalysts for the selective conversion of carbon monoxide into hydrocarbons [1] or alcohols [2]. Until now, only few papers and patents report on the utilization of sulphides for fine chemical applications [3-6]. Nevertheless, this type of solids fits well to catalyze the reactions dealing with sulphur containing molecules. 

Hydrodesulphurisation, hydrodenitrogenation, hydrotreating CoMo/Al203 or NiMo/Al203, sulphided (adiabatic, fixed beds with interstage cooling)... 

Alumina supported molybdenum oxide catalysts are extensively used in hydrotreating reactors. Under reaction conditions the oxides are not stable because they are transformed into molybdenum sulphides. In practice this step is carried out in a so-called sulphiding step. In this step the catalyst is pretreated with a mixture containing a sulphur compound. The sulphiding step was investigated by carrying out a TPS study. TPS is strictly analogous to TPR, except for the gas mixture which is here a H2/H2S mixture. 

Figure 11.18 shows an application of EXAFS on sulphided catalysts as used in hydrotreating processes [39]. The results in the corresponding table have been obtained by computer fitting. The great disadvantage of EXAFS is that data analysis is complicated and time consuming considerable expertise is required... 

Assuming that the vanadium deposits consist mainly of vanadium sulphides, it is then possible to extend the arguments of Tamm et al [20] to calculate possible effects of deposition of coke and metals on pore closure. Accepting that 200 mV is a typical surface area for hydrotreating catalysts, it is known that ca 50% of the surface area is lost during the initial deactivation [19-21], It is also known that V deposition occurs in the outermost part of the pellet [20,31,37,38]. As a result, the amount of V needed to provide a monolayer over the surface area available can be calculated to be between 0.034 and 0.079g cm ... 

A catalytic reaction involving the deposit as the catalyst seems more likely. Hydrotreating reactions have been suggested to be favoured by Ni and V sulphides, albeit at a lower rate of reaction [35]. Welters et al [40] have shown that nickel sulphides in or on a zeolite are active hydroprocessing catalysts and vanadium sulphides have also been found to promote hydrotreating [27], as least to some extent. 

It is probable that a combination of a lower temperatures (450 C) and higher pressures to those employed in this initial study will represent the optimum conditions for maintaining catalyst surface area. In order to prevent possible reduction of the promoter to the corresponding metal (Ni/Co), hydropyrolysis should be carried out in the presence of a small amount of hydrogen sulphide to help maintain the catalyst to remain in a reasonably fiilly sulfided form. Hydropyrolysis for the virtual complete carbon removal would need to be carried out off-line since the combination of temperature, pressure and flow rate required cannot be achieved in hydrotreating units. In terms of potential applications, carbon-supported catalysts may represent the major area since these cannot be regenerated oxidatively. 

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