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Effect of Phosphorus on the Activity of Ni-Mo/Alumina Coal-Liquid Upgrading Catalysts Richard E. Tischer,* Nand K. Narain, Gary J. Stiegel, and Daniel L. Cillo Engineering Research Division, Pittsburgh Energy Technology Center, US Department of Energy, Pittsburgh, Pennsylvania 15236
T h e effect of phosphorus additions on the initial activity of coal-liquids upgrading catalysts of the Ni-Mo/Al2O3 type were determined. The catalysts contained 3 wt 9% NiO and 19.8 wt % Moo3. They were tested in a 50/50 mixture of coal-liquid residuum and process-derived recycle solvent a t 390 " C and 2000 psig in a batch autoclave test. The addition of phosphorus has a significant effect on the ability of the catalyst to convert heavy coal liquids into products soluble in cyclohexane. The initial addition of phosphorus pentoxide causes an increase in this conversion, while addition of amounts in excess of 8 wt 90 reverses the trend and causes the activity to decline to values less than the activity of the phosphorus-free catalyst. However, the phosphorus addition to the catalyst did not improve its hydrogenation activity and caused only a very slight improvement in the hydrodenitrogenation and hydrodesulfurization (HDN and HDS) activities. Likewise, the phosphorus addition did not affect the activity of the catalyst for converting the +340 " C fraction of the coal liquids into lighter material. These results are in contrast t o those reported in the literature for use of a phosphorus-containing catalyst for hydrotreating light petroleum feedstocks. They also indicate that if phosphorus has any useful role to play in the performance of coal-liquid upgrading catalysts, it will have to be in the area of catalyst deactivation and not improvement of initial activity. New catalysts are needed for the processing of coal and coal liquids that contain high concentrations of fairly large aromatic molecules, relatively high quantities of heteroatoms, and significant quantitites of mineral matter. In current liquefaction work, emphasis has shifted from single-stage processes, where the catalyst is directly in contact with the coal, to two-stage processes involving a thermal dissolution step followed by a coal-liquid upgrading step. In these processes, the hydroprocessing catalyst is used to upgrade the primary liquefaction product to provide a high-quality solvent for coal dissolution. As a result, the severity in the liquefaction reactor can be reduced because the removal of heteroatoms can be performed more efficiently in the catalytic hydroprocessing reactor. The performance of the catalyst is a key factor in the overall operation of the two-stage process. Commercial hydrotreating catalysts, such as Shell 324M, have been used in current studies on the two-stage process. The determination of the relative importance of those properties of hydrotreating catalysts that affect their performance during the two-stage liquefaction process is a continuing research effort a t the Pittsburgh Energy Technology Center. Previous studies indicate that high metals' loadings improve hydrogenation activity, while both high metals' loadings and large pore diameter enhance denitrogenation and the conversion of the +450 "C fraction of the feedstock (Stiegel et al., 1983). Catalytic activity is also affected by variations in the pore structure, with bimodal catalysts being slightly more effective than unimodal catalysts (Tischer et al., 1985a). Work nearing completion on unimodal catalysts shows that increasing the pore size improves the effectiveness of the catalyst provided that the surface area of the catalyst can be maintained. The relative activities of Ni-Mo/alumina and Ni-W/alumina catalysts have also been compared, as well
as the effect of changing the Ni/Mo or Ni/W ratio (Tischer et al., 1985b). This work showed that the Ni-Mo/alumina catalysts have a greater initial activity than the Ni-W/ alumina catalysts. Although the nickel promoter effect is greater in the Ni-W system than in the Ni-Mo system, in general the effect is not very large in either system. These studies indicate that none of the obvious changes in pore structure or active component have a large effect on the initial activity or short-term stability of supported molybdenum hydrotreating catalysts when they are used with coal-liquid residuum. One of the principle limitations in the use of conventional supported hydrotreating catalysts in coal-liquids upgrading is their rapid rate of deactivation. Analysis of spent catalysts obtained from long-term runs made in developmental coal liquefaction and second-stage hydrotreating units shows that the deactivation is caused by carbon and metal deposition (Stiegel et al., 1985; Stohl et al., 1984; Stephen and Stohl, 1984; Thakur and Thomas, 1983). Since increasing the active metal loading causes a decrease in coke deposition in coal-liquid upgrading (Stiegel et al., 1983), it is possible that the nature of the support is having a significant effect on the amount of coke deposition. Therefore, this work was initiated to determine the effect of modifying the alumina support on catalytic activity and carbon deposition on the catalyst. Two approaches can be taken in an attempt to improve the coking characteristics of cod-liquid upgrading catalysts. The first approach would be to prepare Ni-Mo catalysts on various types of supports and determine their activity and amount of coke deposition. This approach would necessitate the development of non-alumina supports having suitable surface area, pore structure, and thermal stability, which is quite a considerable undertaking. The second and more limited approach is to modify the surface
This article not subject to U S . Copyright. Published 1987 by the American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 3, 1987 423 characteristics of a suitable alumina by adding a modifier. One such promoter for Co-Mo and Ni-Mo catalysts is phosphorus, which has been investigated and recommended over a period of 3 decades and is now used in a number of commercial catalysts. Indeed, the base-line catalyst (Stiegel et al., 1985) currently being used in the upgrading of coal extracts, Shell 324M, contains phosphorus. While the rationale behind the presence of phosphorus in this catalyst is proprietary, it suggests that phosphorus may have a beneficial effect on coal liquefaction and upgrading catalysts. Orthophosphoric acid or ammonium phosphate salts are known to react quickly with y-alumina at moderate temperatures to yield amorphorus or crystalline AIPOl (Gonzalez and Halloran, 1980). This effect can be used to modify the surface properties of the alumina or even to modify the pore structure during preparation of precipitated alumina to form large-pore supports (Marcelin et al., 1983). Phosphate anions in Mo03-A1203-based catalysts were found to strongly promote hydrogenationldehydrogenation activity while concurrently reducing the isomerization activity attributable to surface acid sites (Gishti et al., 1984). Cyclohexene was used as a probe molecule in this study. In a study involving the hydrotreating of petroleum naphtha (Stanulonis and Pedersen, 1980), phosphorus-containing catalysts showed improved hydrodenitrogenation activity (HDN). A similar result (Fitz and Rase, 1983) was found where a high-metalslhighphosphorus catalyst gave the best performance and lowest hydrogen consumption for nitrogen-containing feeds. In addition, the phosphorus-containing catalysts were less susceptible to coking and produced a more hydrogen-rich coke. T o follow up on this lead, a series of Ni-Mo catalysts was prepared on a y-alumina support that was modified by the addition of increasing amounts of phosphorus. The work reported in this paper is a more systematic follow up on previously reported work (Cillo et al., 1986) where deactivation characteristics of four commercial catalysts were examined. Since these catalysts varied in their pore size distribution type, as well as composition, it was felt that more systematic studies were needed to determine the relative contributions of the various physical and chemical characteristics of the catalyst on initial activity and deactivation. This work is limited to the effect of phosphorus on the initial catalytic activity and coke deposition on these catalysts. If significant differences in initial activity are found to exist, then more extensive deactivation studies will be undertaken. Experimental Section A. Catalyst Preparation. The catalysts were prepared by impregnating commercial 0.7-mm extrudates with the appropriate salts. The commercial support was a y-alumina having a surface area of 212 m2/g, a pore volume of 0.825 cm3/g, and an average pore diameter of 15.0 nm, as determined by nitrogen adsorption using a Micromeritic DigiSorb 2500. The alumina contains less than 0.09 wt % Na, K, or Fe, as determined by AA analysis. The molybdenum and phosphorus were coimpregnated into the alumina support first. For this impregnation, reagent-grade ammonium heptamolybdate and ammonium dihydrogen phosphate were used. The salts were dissolved in water and diluted to yield enough solution to impregnate the supports to incipient wetness. After impregnation, the extrudates were tumble-dried in a rotary evaporator at 90 "C. The dried extrudates were then calcined to 400 "C in a tube furnace with a 5 std L/min air purge, which ensured rapid removal of water vapor during the calcination step.
Table I. Batch Reactor Feedstocks
material carbon, wt % hydrogen, wt % nitrogen, wt % sulfur, wt % oxygen (by diff.), wt % ash, wt % H/C atomic ratio +340 "C fraction, wt %
Wilsonville deashed residuum 88.3 6.5 1.2
0.6 3.4
