Co-Mo hydrogenation catalysts

During ammonia synthesis, the major reactions of production and purification of synthesis gas and the synthesis of ammonia, all are carried out over different catalysts. At least eight kinds of catalysts are used in the whole process, where natural gas or naphtha is used as feedstock and steam reforming is used to produce synthesis gas. These catalysts are Co-Mo hydrogenation catalyst, zinc oxide desulfurizer, primary- and secondary-steam reforming catalysts, high- and low-temperature shift catalysts, methanation catalyst and ammonia synthesis catalyst etc (Table 1.1). 

The eight kinds of catalysts may be roughly classified as protective catalysts and economic catalysts . Co-Mo hydrogenation catalyst and zinc oxide desulfurizer are the protective catalysts for the primary steam reforming catalysts. The high-temperature shift catalyst protects the low-temperature shift catalyst, and the methanation catalyst are the protective catalyst for ammonia synthesis catalyst. The catalysts for primary- and secondary-steam reforming, low-temperature shift and ammonia synthesis are responsible for the conversions of raw materials and the yield of products, and have direct effect on economic benefits of the whole plant, and are thus called as economic catalysts. The amount of catalysts used depends on the process and raw material. Table 1.2 represents the amount of the eight kinds of catalysts used in the different processes. The total volume of the catalysts is about 330 m in every plant, while there are only two kinds of catalysts with the volume of about 100-140 m when heavy oil or coal is used as raw material. Both shift... 

It is known that alumina is chlorinated exothermically at above 200° C by contact with halocarbon vapours, and hydrogen chloride, phosgene etc. are produced. It has now been found that a Co/Mo-alumina catalyst will generate a substantial exotherm in contact with vapour of carbon tetrachloride or 1,1,1-trichloroethane at ambient temperature in presence of air. In absence of air, the effect is less intense. Two successive phases appear to be involved first, adsorption raises the temperature of the alumina then reaction, presumably metal-catalysed, sets in with a further exotherm. 

The mechanism proposed involves hydrogenation of the C2 C3 double bond, formation of 2-vinylthiophenol by an E2 elimination, and hydrocarbon elimination by homolysis of the S—Caryi bond. This pathway rationalizes the primary formation of (104) observed in some HDS reactions of (102) over Co/Mo/S catalysts, as well as the kinetic evidence that the rate-determining step on real catalysts is the removal of surface sulfur.158-160... 

The objective of the present work is to evaluate the effect of a wide range of process or reaction variables— reaction temperature, hydrogen partial pressure, catalyst loading, and reaction time—on hydrodesulfurization and hydrogenation of filtered liquid product (coal-derived liquid) obtained from the coal dissolution stage in the presence of a commercial presulfided Co-Mo-Al catalyst. The selectivity for desulfurization over hydrogenation (Se) is used to rate the effectiveness of the above mentioned process variables. Se is defined as the fraction of sulfur removal per unit (g) of hydrogen consumed, that is,... 

HDN with Metallophthalocyanines. The above considerations clearly show that the catalytic site geometry is important in HDN. It is difficult to control, or even to identify, the catalyst site in conventional catalysts such as the commercial Co-Mo-alumina catalyst. On the other hand, homogeneous catalysts with transition metal complexes provide a well-defined catalytic site. Unfortunately, most homogeneous catalysts are not sufficiently stable to be used at the temperatures required for the hydrogenation of hetero compounds. A class of catalysts that are thermally stable are the metallophthalocyanines,... 

Figure 2 Effect of hydrogen sulfide on hydrodesulfurization of dibenzothiophene (DBT) over a Co/Mo/Alumina catalyst. (Based on experimental data from Ref. 2.)...

Mercaptans, RSH, have been synthesized over sulphided Co-Mo/AI2O3 catalysts by hydrogenation of COS (422-589 K, 6,8 atm), reaction of aldehydes, ketones, carboxylic acids, and esters with H2/H2S (450-589 K, 12-31 atm),and reaction of organic sulphides R2S with H2S (533 K, 31 atm). 

In hdn of indole and quinoline and hdo of benzofuran N- and O-heterocyclics of the type occurring in petroleum residua, shale oils, and coal) over a sulphided Co-Mo/AI2O3 catalyst (617 K, 48 atm) ring hydrogenation preceded C-N or C-0 fission and was rate-determining. Selectivity to alkylcyclohexanes over alkylbenzenes was > 90%. 

Attempts have been made to estimate the concentration of active sites on a series of Co-Mo/AI2O3 catalysts by irreversible oxygen adsorption at 195 K on the reduced catalysts. There was a linear relationship between areal rate constants for cyclohexene hydrogenation and thiophen hds over the sulphided catalysts (623 K, 1 atm). For the most active catalyst [M0O3, 10.4% (Co + Mo), 0.10 Co/(Co + Mo), 0.17] the concentration of active... 

As the adsorption of hydrogen is rather weak under reaction conditions, the term (1+(A 2H2) ) in the denominator of (3.41) may be omitted. The rate expression shows that the reaction is suppressed by H2S. Catalysts that are less sensitive to H2S poisoning are usually the most active. Although the active Co-Mo/AI2O3 catalyst has extensively been studied and detailed models of its structure have been proposed (Figure 3.13), the molecular mechanism by which the catalyst operates is still not completely understood. 

Hydrogenation. Gas-phase catalytic hydrogenation of succinic anhydride yields y-butyrolactone [96-48-0] (GBL), tetrahydrofiiran [109-99-9] (THF), 1,4-butanediol (BDO), or a mixture of these products, depending on the experimental conditions. Catalysts mentioned in the Hterature include copper chromites with various additives (72), copper—zinc oxides with promoters (73—75), and mthenium (76). The same products are obtained by hquid-phase hydrogenation catalysts used include Pd with various modifiers on various carriers (77—80), Ru on C (81) or Ru complexes (82,83), Rh on C (79), Cu—Co—Mn oxides (84), Co—Ni—Re oxides (85), Cu—Ti oxides (86), Ca—Mo—Ni on diatomaceous earth (87), and Mo—Ba—Re oxides (88). Chemical reduction of succinic anhydride to GBL or THF can be performed with 2-propanol in the presence of Zr02 catalyst (89,90). 

Reduction and Hydrodesulfurization. Reduction of thiophene to 2,3- and 2,5-dihydrothiophene and ultimately tetrahydrothiophene can be achieved by treatment with sodium metal—alcohol or ammonia. Hydrogen with Pd, Co, Mo, and Rh catalysts also reduces thiophene to tetrahydrothiophene [110-01-0] a malodorous material used as a gas odorant. 

The activity of all catalysts were evaluated for the CO hydrogenation reaction. The histogram shown in Fig. 8 reveals that the bimetallic Co-Mo nitride system has appreciable hydrogenation activity with exception of samples 2 and 4. This apparent anomaly was probably due to the relatively high heat of adsorption for these two catalysts, which offered strong CO chemisorption but with imfavourable product release. 

---