The development of highly active metal catalysts is of prime importance to reduce the tendency for repolymerisation during catalytic hydrotreatment. All data presented in this chapter so far are based on a Ru/C catalyst. Ru, however, is an expensive noble metal and there is an incentive to identify not only more active but also cheaper catalysts for the hydrotreatment reaction. A possibility is the use of cheaper bimetallic metal catalysts based on Ni. Ni is known to have high hydrogenation activity for a variety of organic functional groups and particularly for reactive ketones and aldehydes, and as such is a potential active metal for hydrotreatment reactions. However, monometallic Ni catalysts (on silicon oxide, y — or 8-alumina, or other supports) at the typical temperature and pressures applied here are not suitable to be used as a hydrogenation catalyst. There are basically two reasons: 1) Ni requires high reduction temperature (typically 700 oC) for complete reduction, and 2) Ni catalysts are known to deactivate rapidly at elevated conditions by char deposition ("coking"). The carbon deposition can block the nickel surface, or the pore mouths, and, eventually leading to a strong reduction in the reaction rates. These two drawbacks regarding the use of Ni were solved a. o. by using another element (metal or non-metal), also designated as a promoter. One of these proprietary catalysts was studied in detail and will be referred to in the following as catalyst D.

Figure 12 shows the liquid phase after a hydrotreatment over Cat D versus the severity of the process, showing the original oil (left) and an oil derived at the most severe conditions tested here on the right. Interestingly, the product oils obtained over cat D are much more transparent than those derived from the Ru/C catalyst.

A van Krevelen plot gives valuable insights in the difference in performance between catalyst D and Ru/ C (Figure 13). A similar pattern for both is observed as a function of severity but the curve for Cat D is shifted to higher H/ C values. Thus, at a similar oxygen content, the H/C ratio is higher for catalyst D. This is indicative for a higher hydrogenation rate for cat D and is known to be favorable regarding product properties.

Repolymerisation reactions appear to occur to a limited extent when using cat D instead of Ru/C. This is evident when comparing the average molecular weight of the final products (Figure 14a), as determined by GPC, for both Cat D and Ru/C. For Ru/C the average molecular weight shows a significant increase from 400 up to 1000 Da at low severities, but a constant value over the oxygen content interval of 400-450 Da for catalyst D is observed.

TGA residues of the product oils using cat D (Figure 14b) show carbon residues of around 5%. Surprisingly, and not expected on basis of test carried out using other catalysts, already at less severe operating conditions, a significant reduction in the TGA residues is achieved. Thus, products with a higher H/C ratio and a lower carbon residue were obtained with cat D indicating that the rate of the hydrogenation/hydrodeoxygenation reactions over catalysts D are higher than for Ru/C.

An important product property for the upgraded oils is the viscosity. In Figure 15, the viscosity profile of the product oils versus the oxygen content is compared for conventional catalysts (Ru/C and NiMo, CoMo) and catalyst D. Clearly, the viscosity in the mid range of oxygen contents is much lower for cat D. Further testing at the extreme of low oxygen content will be required to grab the full picture but it is clear that cat D gives upgraded products with a lower viscosity than for conventional catalysts. The lower viscosity is likely the result of a lower average molecular weight of the products, as shown earlier and the result of higher hydrogenation/hydrodeoxygenation rates for Cat D compared to Ru/C.

On the basis of the product properties of the upgraded oils obtained with cat D, we can conclude that repolymerisation is not occurring to a considerable extent. As a result, product oils with a lower molecular weight and a concomitant lower viscosity, lower TGA residue is obtained. Thus, the reaction pathway for catalyst D may be simplified considerably, see Figure 16 for details.

Oxygen content (wt%)

◄—- Process severity