NiMo system

In the case of the NiMo system, additionally to the diffusion limitation with increasing current, the convection, induced by the rate of rotation of the working electrode, has to be taken into account. An increasing rate of rotation decreases the thickness of the diffusion layer. In a rough approximation the diffusion layer is inversely proportional to the rate of rotation r. Thus, one obtains for diffusion limited molybdenum concentration... 

There is, nevertheless, some evidence (35, 36), based in NiNaY and NiMo/ alumina/Y model catalyst systems, that the amount of coke formed is reduced with increasing intimacy of mixing of the two functions at the submicron level. This concept is further supported by the reported relatively high performance of NiW/ASA (amorphous silica-alumina) cogel HC catalysts which, it is claimed, exhibit an excellent distribution of the NiW hydrogenation function throughout the catalyst particles (37). 

Abbattista et al. (26) found that phosphorus addition prevents crystallization of the y-alumina phase and the transformation from y- to a-alumina in the system AI2O3 —AIPO4 (Fig. 23). More precisely, Morterra et al. (77) reported that phosphates do not affect the phase transition from low-temperature spinel alumina (y-alumina) to high-temperature spinel aluminas 8 and 6 phases) but delay the transition of 8 and 9 to a-alumina (corundum). Stanislaus et al 46) also reported that phosphorus significantly improves the thermal stabihty of the y-alumina phase in P/Al catalysts. However, the same authors found that the positive effect of phosphorus seems to be canceled in the presence of molybdenum due to the formation of aluminum molybdate. Thermal treatments of MoP/Al catalysts at temperatures >700°C result in a considerable reduction of SSA and mechanical strength. The presence of phosphorus does not prevent the reaction between the molybdenum oxo-species and alumina since the interaction between molybdates and phosphates is weak. The presence of nickel does not obviously affect the positive effect of phosphorus in terms of thermal stability 46). On the other hand, Hopkins and Meyers 78) reported that the thermal stability of commercial CoMo/Al and NiMo/Al catalysts is improved by the addition of phosphorus. 

After adding 1 to 10 ppm H2S to the feed, both Ni and NiMo catalysts deactivate in time, although not at the same rate. In all cases the catalysts become completely deactivated, which means that the Ni based systems show almost no thiotolerance for H2S concentrations above 1 ppm. However, an important increase was noticed in the thioresistance of Mo-modified catalysts, such as is shown in Table 2, where the value of is considerably higher than that of the mcmometallic systems. 

This paper presents a catalyst that is able to incorporate a higher fraction of heavier feedstock to the conversion system. The NiMo-Alumina catalyst permits process scheme adaptations to achieve these objectives in an economic way, depending upon the particular refinery and its product slate to satisfy the market requirements. 

The characterization of Rh phases supported on alumina has been carried out by DRS. Although the spectra present less intense bands than those of pure phases, it is possible to observe the bands of load transference of octahedral Mo ( 300 nm) as well as a band of wide transition and lower resolution attributed to Rh(III), around 400 nm. The surface of catalytic systems based on CoMoe/y-AbOs and NiMoe/y-AlzOs, sulfided and untreated, has also been recently studied by XPS and EXAFS [4, 24], and the corresponding RhMo6/7-Al203 study is in course. 

The thiophene hydrodesulfurization and cyclohexene hydrogenation activity of the catalysts have been measured in a high pressure reactor, the operative conditions having been selected according to the recent experience about the CoMoe and NiMoe based systems because of the common structural and physical-chemical properties [3, 4]. Table 3 shows chemical data and conversion obtained for selected catalysts based on RhMoe. In addition, the data for CoMoe Anderson, CoMo commercial and Rh commercial 7-AI2O3 supported catalysts are included for comparative purposes. 

Recently, we observed the promotional effects of Pt added to the sulfided M0/AI2O3 catalyst in the HDS of thiophene and HDN of pyridine [4]. An interesting property of the promoted Pt-Mo(S)/Al203 catalyst was the high HDN activity, exceeding the activities of conventional CoMo and NiMo/Al203 catalysts. The Pt-Mo(S) system was deposited now on a mesoporous silica-... 

NiMo/CB catalysts were 0.84 and 0.28, respectively. This resulted from poisoning of the HDS sites on the NiMo/AhOs catalyst by pyridine and N-containing intermediates. The diminished poisoning was the reason for a high HDS rate and low HDN/HDS ratio on the NiMo/CB catalyst. The experiments were conducted in a continuous system at 593 K and 2 MPa of H2. 

AI2O3 and NiMo/Al203 catalysts with the corresponding AC-supported catalysts. The HDN activity of the latter catalysts was superior compared with that of the alumina-supported catalysts. Because of the diminished interaction of N-bases with AC support, the coke deposition and catalyst deactivation associated with it were much less evident on the carbon-supported catalysts. The HDS activity was similar for both AC-supported and alumina-supported catalysts. In a similar study, Hubaut et compared the FeMo catalyst supported on AC with that supported on 7-AI2O3. The experiments were conducted in a continuous system at 623 K and 7 MPa using a heavy VGO as the feed. Among several catalysts prepared by different methods, the... 

Alonso and collaborators have reported the synthesis of CoMo, NiMo and NiW catalysts using tetraalkylammonium thiometallate precursors using either ex situ activation method under flow of H2/H2S or through in situ decomposition under dibenzothiophene hydrodesulfurization conditions (52-55). In situ activation led systematically to more efficient catalysts. However, a balance between structural carbon (see below) and the formation of carbon in excess blocking active sites must be optimized. This situation depends on the reducibility of the initial sulfide phase. Use of tetraalkylammonium precursors is then beneficial for M0S2-based systems, particularly promoted by Co or Ni. The situation is reversed for WS2-based catalysts due to the more difficult reducibility of tungsten sulfide. Alonso and co-workers have more recently developed a modified ex situ activation method, that generates active catalysts (56-58). The method consists of the partial hydrothermal decomposition of the precursor at 523 K followed by complete activation in a tubular furnace at 673 K. 

The second-most studied bimetallic catalyst is sulfided NiMo/Al Oj. An NiMo/ AljOj (Ni 3 wt %, 15 wt % Mo) catalyst converts guaiacol to benzene and toluene as major products [82], Since benzene and toluene are oxygen-free, these products are preferred over phenol and catechol, making this a more attractive catalytic system. 

Conventional hydrocracking takes places over a bifunctional catalyst with acid sites to provide isomerization/cracking function and metal sites with hydrogenation-dehydrogenation function. Platinum, palladium, or bimetallic systems (ie, NiMo, NiW, and CoMo in the sulfided form) supported on oxidic supports (eg, silica-aluminas and zeolites) are the most commonly used catalysts, operating at high pressures, typically over 10 MPa, and temperatures above 350°C. 

Catalyst system chosen based on activity, selectivity, and cycle life and is generally composed of CoMo/NiMo alumina supported catalysts designed for specific objectives such as hydrodesulfurization (HDS), hydrodemetallization (HDM), hydrodenitrogenation (HDN), hydrodeasphaltenization (HDAs), HCR, and Conradson Carbon removal (CCR)... 

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