Phosphorus-Containing Hydrotreating Catalysts

The presence of phosphorus in catalysts significantly affects their physicochemical properties, such as pore structure, dispersion of active phases, acidity, thermal stability, and rcducibility or sulfidability. In this section, relationships between phosphorus content and physicochemical properties 

The influence of phosphorus on catalyst textural parameters, such as specific surface area (SSA), pore diameter (PD), and pore volume, has been thoroughly investigated (25, 30, 38, 60, 62, 68, 69, 72, 73). The SSA decreases with phosphorus loading, irrespective of the preparation procedures. In particular, it was reported that NiMoP catalysts obtained by coimpregnation have greater SSA decreases than those prepared by sequential impregnation 74). 

The effect of phosphorus on PD depends on the catalyst preparation method. The PD of catalysts prepared by impregnation decreases with phosphorus loading, whereas that of catalysis derived from the hydrogel or sol-gel methods tends to increase in some cases. Introduction of phosphorus compounds in the sol-gel procedure may affect the hydrolysis and condensa- 

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. 

In summary, it is difficult to conclude whether the thermal stabihty of 

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VII. Structural Models of Phosphorus-Containing Hydrotreating Catalysts... 

Structural models of phosphorus-containing hydrotreating catalysts have been proposed from catalyst characterization data and results characterizing the reactions of model compounds. Poulet et al. (79) reported a structural model of MoP/Al catalysts (Fig. 35) in which the phosphorus-containing species exist in three different states (i) isolated phosphorus oxo-species interacting with the alumina support, (ii) phosphorus directly incorporated into the M0S2 slab, and (iii) phosphorus oxo-species bound to both M0S2 and the alumina surface. 

Following Section 11. J, which reported some characteristic vibrational and NMR data representing phosphorus-based reference compounds (Tables VI-Vlll), it is interesting to propose similar compilations concerning phosphorus-containing alumina catalysts in the dried and calcined forms. These vibrational and NMR data (Tables IX-XII) are more informative in characterizing hydrotreating catalysts than are XRD data because the catalysts are amorphous. 

The phosphorus contents of hydrotreating catalysts are typically <10 wt%, with the remaining components including the catalytic components (molybdenum and promotors) and the aluminum oxide support. However, aluminum orthophosphates, which contain —25.4 wt% phosphorus from the formulation AIPO4, are commonly reported in hydrotreating catalysts. Therefore, a detailed examination of the aluminum orthophosphate struc-... 

To identify the phosphorus-containing compounds described in the previous sections and the related species containing aluminum, molybdenum, cobalt, or nickel which might be present in hydrotreating catalysts, it is convenient to use techniques such as NMR, IR, UV. and Raman spectroscopies and XRD. XRD is useful for characterizing crystalline bulk compounds, and other techniques are appropriate for well-dispersed species and amorphous phases. Typical IR, Raman, and NMR data presented in Tables VI, VII, and VIII, respectively, could be the basis for such identifications. 

Ml. Preparation of Alumina-Based Hydrotreating Catalysts Containing Phosphorus, Molybdenum, and Cobalt or Nickel... 

Fig. 13. Procedures for the preparation of alumina-based hydrotreating catalysts containing phosphorus, molybdenum, and cobalt or nickel, a. Impregnation or equilibrium adsorption method (coimpregnation) b, impregnation or equilibrium adsorption method (sequential impregnation) c, precipitation or hydrogel method d, sol-gel method [adapted from Iwamoto and Grimblot 40). ...

Hydrotreating Tests Conducted with Phosphorus-Containing Catalysts... 

Tungsten-based hydrotreating catalysts have been studied much less than the classical molybdenum-based catalysts. It is expected, however, that phosphorus addition should lead to similar effects in both cases since tungsten is chemically similar to molybdenum. Atanasova et al. (101) reported that phosphorus increases the thiophene HDS activity, especially that of a sequentially impregnated NiW—P/Al catalyst. Halachev et al (135) found that a maximum hydrogenation activity for naphthalene conversion is attained when the catalyst contains 0.6 wt% P2O5. Cruz Reyes et al (58) reported that phosphorus on a W/Al catalyst notably enhances gas oil HDS and pyridine HDN. 

The advantages of phosphorus addition to catalyst formulations found in patents can be approximately categorized as follows (i) optimization of the catalyst pore structure by addition of phosphorus to be applied with certain types of feedstocks such as residual oil, (ii) optimization of the dispersion of Co(Ni) I Mo-containing phases by the presence of phosphorus, (iii) optimization of synergistic effects resulting from complex chemical combinations of phosphorus and other incorporated elements, (iv) optimization of catalyst preparation by use of specific phosphorus precursors, and (v) the use of phosphorus-containing catalysts under specific reaction conditions or processes as well as their use in combination with other hydrotreating catalysts. 

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