Deactivation of the Ni-Mo and Co-Mo catalysts in the HDO is caused by:

• desulfurization of the catalyst

• coke formation

• water inhibition

• catalyst poisons

Desulfurization of the catalyst is one of the main reasons of deactivation. Sulfur compounds could be removed from the surface of the catalyst by reactions with compounds from the reaction mixture [11, 12, 14] or hydrogen [19]. Commonly accepted way to inhibit deactivation is addition of the sulfur-containing compounds to the feedstock. When H2S and CS2 were added to the feedstock no significant decrease of deactivation of the catalyst was observed. Sulfur content in the Ni-Mo catalyst was not affected when H2S or CS2 were added. In the case of Co-Mo catalyst the addition of different amounts of sulfur agents did not stop desulfur­ization of the catalyst. Moreover, addition of CS2 caused lower yield of hydro­carbons, compared to H2S, but increase of fatty acid production from esters [12].

Coke formation was observed over Ni-Mo and Co-Mo catalyst in the deoxy­genation of the aliphatic esters (Table 6.1). It can be partially responsible for deactivation of the catalyst by blocking the catalyst active sites. The Ni-Mo and Co-Mo catalysts on y-Al2O3 can be regenerated by burning away the coke deposits with hot air, but there is no published data available for the influence of regen­eration on the sulfided catalysts properties.

The water inhibition was studied with aliphatic esters, showing deactivation of Ni-Mo and Co-Mo catalysts. Water inhibition is an important issue because it can be created in different stages during the reaction (Fig. 6.2a). Increase of water in the reaction mixture causes not only deactivation but it also affects product dis­tribution by decreasing strongly decarboxylation/decarbonylation reaction rate, compared to hydrogenation rate, increasing the ratio between n-carbon and n-1- carbon hydrocarbons [20]. A negative effect of water can be diminished by addition of H2S but that will promote decarboxylation/decarbonylation reaction, as mentioned before (see Sect. 6.2.1.2).

The negative effect of phosphorus and alkali, impurities from vegetable oils feedstock, was studied over sulphided Co-Mo/y-Al2O3 catalyst at 310°C and

3.5 MPa of hydrogen pressure [21]. Alkali metals block/poison active sites leading to deactivation of the catalyst. When alkalis and phosphorus were present, deac­tivation of the catalyst was even higher than in the absence of them because of the formation of phosphates locating above charge-compensating alkalis. In the absence of alkalis, phospholipids produced phosphoric acid which catalyzed oligomerization reactions and lead to deactivation of the catalyst by carbonaceous deposits.