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The SEM micrographs in Figure 1 show radial distribution profile of Mo, Ni and P elements through the cross section of the catalyst extrudates. The Ni radial distribution is uniform for all prepared catalysts, however significant differences in Mo and P radial distribution profile can be observed for the different prepared catalyst batches. For both, the reference and the NiMoP-2 catalysts, Mo and P are uniformly distributed through the cross section of the catalyst extrudates.
The above results clearly showed how the textural and mechanical properties, the chemical composition, the surface active metal dispersion, and the elemental radial distribution through the cross section of the alumina extrudates are strongly influenced by the catalyst preparation procedure employed. Co-impregnation of alumina in two successive steps with Ni, Mo and P diluted solutions NiMoP-2 gave a catalyst with the following features:.
To facilitate the discussion of our results, let us first describe the observed behavior during the preparation of the catalyst by impregnation in one single step NiMoP Subsequently, we will try to explain the obtained results by taking into account fundamental studies published in the literature that deal with the chemistry of Ni, Mo and P species in solution and their mechanism of adsorption on alumina. The results of Tables 1 — 4 and Figure 2 clearly indicated that this procedure gave catalysts with lower specific surface area and porous volume, mechanical strength properties, chemical composition, surface dispersion and catalytic activity properties than the other catalyst preparation procedures.
SEM analysis of Figure 1 revealed the formation of a reaction front where phosphate was preferentially adsorbed at the outer surface of the alumina extrudates, molybdenum radial distribution was not uniform while Ni showed a flat radial distribution profile. By breaking some catalyst extrudates in two halves, it was observed that some particles showed a light green color and others a darker color at the core of the particles confirming the heterogeneity of the impregnated material. After spraying a significant amount of solution on the solid, it was observed that the extrudates began to stick with each other which could indicate a loss of the solution uptake capacity of the alumina support.
A small volume of the metal solution remained adhered to the rotary drum walls which could explain the lower chemical composition and XPS intensity ratio observed for this catalyst batch. In view of these results, we decided to determine the factors affecting the preparation of the NiMoP-1 catalyst batch with the aim to design a new catalyst preparation strategy.
For this purpose, we proceeded to determine the properties of the impregnating solution and its stability as a function of the aging time. The turbid green aspect NiMoP-1 solution showed a density of 1. Instable metallic solutions represent a high technical and economic risk at the moment of producing commercial catalysts. For such reasons, catalyst makers first check these properties before deciding to prepare large volumes of metallic solutions in their facilities. Heat transfer limitations occurring during the impregnation are rarely considered in the literature, because the heat released when preparing catalysts at lab scale is very low.
However, at larger production scales, heat transfer limitations become critical and this requires immediate attention in the operation to avoid modifications in textural and mechanical properties of the material, changes in the kinetics of adsorption of the metal ions in solution, metallic salts decomposition and mass transfer limitations intra-granular or extra-granular.
The rapid increase of the temperature in the solid observed at the beginning of the impregnation step can be explained by two simultaneous effects: i the heat released by the wetting of the alumina, which is a typical behavior of catalyst supports having large internal surface area. These supports can burst at the moment of wetting under the effect of capillary forces and ii the heat released by the neutralization reaction between the phosphoric acid molecules in solution and the surface basic hydroxyl groups of the alumina.
The heat release due to the neutralization reaction is higher than the heat of wetting of the alumina support. If the local temperature is not well controlled during the solution addition, the porosity of the material could be affected by an increase of the pressure inside the pores. To avoid thermal effects during the impregnation step, the practice of different techniques can be employed. The simplest technique is to spray a certain volume of DI.
Some catalyst makers pretreat the alumina support with steam to reduce the tendency to burst [ 3 ]. Another technique is to pretreat the support with a wetting agent, for instance a solvent less polar such as alcohols, pentane, cyclohexane, etc. Among the benefits provided by the wetting agents are: i it improves the solubility of the metallic salts, ii it modifies the solution surface tension by decreasing liquid-solid contact angle facilitating in this manner the diffusion of the liquid inside the pores and iii it reduces significantly the impregnation and aging times.
Other fundamental aspects to consider when producing catalysts at pilot or commercial scales are the mass transfer limitations. In porous catalyst supports diffusional limitations intra-granular capillary diffusion or extra-granular take place. The rate of diffusion depends on the porous structure of the support, the solution properties density and viscosity , solution-solid contact angle, and surface ion-exchange reactions.
Very often, ion exchange is very rapid, in particular when it is an acid-base exchange reaction.
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In such case, the rate of diffusion becomes limiting for the overall process, and a reaction front takes place at the outer surface of the extrudates [ 11 ]. For the catalyst batch prepared according to the NiMoP-1 procedure, it was evidenced by SEM analysis a reaction front between P and Mo ions with the hydroxyl sites of the alumina surface.
This satisfactorily would explain the Ni flat radial distribution profile observed by SEM analysis for all prepared catalyst batches. Let us now examine some works published in the literature dealing with the behavior of the chemistry of the Ni, Mo and P species in solution and their proposed adsorption mechanism on alumina.
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When phosphoric acid is added to a solution containing ammonium heptamolybdate; pentamolybdodiphosphate complex is formed [ 12 , 13 ]. In these compounds, two phosphate groups in tetrahedral coordination with oxygen atoms are located above and below the planar of 5-members ring of MoO 6 octahedral. The phosphate groups in pentamolybdodiphosphate complex, to differences of other types of P—Mo heteropolycompounds, are accessible to interact with the alumina surface. At low solution pH, the predominant phosphomolybdate species is the protonated form [ 14 , 15 ].
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According to equation 1 , the decomposition of phosphomolybdate complex into molybdate and phosphate is favored by a rise of solution pH, which would shift the chemical equilibrium to the left. These researchers also observed that pentamolybdodiphosphate complex decomposes to phosphate and molybdate upon contact with alumina [ 11 ]. Equilibrium between heptamolybdate ions and molybdate ions is also affected by rise in pH Eq.
When complex decompose by contact with alumina surface, competition between phosphate and molybdate ions takes place for the same adsorption sites. Since phosphate ions interact more strongly with the alumina surface than molybdate ions, phosphate ions are preferentially adsorbed at the outer surface of the alumina while molybdate ions diffuse and adsorb inside the extrudates [ 17 ]. The adsorption mechanism of phosphate species on alumina was investigated by Morales et al.
When phosphorus is adsorbed on the alumina surface, the authors observed similar effects to that showed in Table 1 where the surface area and pore volume decreased significantly.
Two explanations were proposed; i phosphorous might behave as a corrosive agent breaking some micropores of the support with a consequent increase in macroporosity. The thermal effects produced during the initial addition of the metal solution on the alumina extrudates could amplify some physico-chemical processes, not observable when preparing catalysts at laboratory scale.
For instance; the rise in temperature in the solid during impregnation could favor; i the decomposition of phosphomolybdate complex on the alumina surface accelerating in this manner the kinetics of adsorption of phosphate ions, ii the pore blockage by phosphate adsorbed species can inhibit other ions to enter inside the pores and therefore the solution uptake capacity of the support can be affected, iii the digestion of some alumina particles could take place by contact with the hot strong acid solution. In summary, the preparation of the NiMoP-1 batch represents a typical example where the rate of capillary diffusion intra-granular and extra-granular is limiting the overall process.
The identified factors that caused a non-homogeneous metal distribution and significant changes in textural properties of the catalyst support are related to: i the high density and viscosity of the metallic solution, ii high liquid-solid contact angle and iii the rise of the temperature during the initial impregnation step which facilitated the decomposition of the phosphomolybdate complex and increased the reactivity between the phosphate species and the alumina surface causing a reaction front.
Metallic solutions containing both nickel or cobalt and molybdate salts are unstable, thus NiMoO 4 or CoMoO 4 phases start to precipitate when the solution pH is higher than 3. For such reasons, sequential impregnations are used in the practice where the alumina support is first impregnated with the molybdenum solution and then the nickel or cobalt promoter is supported. Additives such as phosphoric acid, citric acid, hydrogen peroxide, chelating agents, etc. We concluded that co-impregnation procedure gives catalysts with improved textural properties, surface acidity, mechanical strength, metal dispersion and homogeneous elemental radial distribution profile than sequential impregnation method [ 17 ].
As discussed above, co-impregnation procedure leads to the Mo-P heteropoly-compound formation, while sequential impregnations with phosphorus induce to the formation of bulk MoO 3 species. Jian and Prins [ 15 ] and Chadwick et al. Lopez Cordero et al. The density and viscosity values of the MoP solution were in this case lower than the NiMoP-1 solution 1. The colorless MoP solution remained stable for several days. In this case, the spraying of the metallic solution into the alumina extrudates was controlled to avoid an excessive temperature rise.
However, this catalyst showed lower mechanical strength and metal surface dispersion values.
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SEM images of Figure 1 , showed a substantial improvement of the elemental radial distribution profile as compared with the NiMoP-1 catalyst. Both Ni and Mo showed uniform radial distribution profile, while P still showed some heterogeneities.
The MoP solution density and viscosity and the phosphorus reactivity with the alumina surface are still high. According to our observations, there is high probability to minimize the impact over the metals diffusion inside the particles produced by the high solution density and viscosity and the high reactivity of the phosphate species if the alumina is co-impregnated with diluted metallic solutions NiMoP-2 procedure.
The aspect of this solution was translucent green color; it remained stable for several days, its density and viscosity were 1. A good solution uptake capacity was observed during the impregnation. Our hypothesis can be confirmed by the results of Tables 1 — 4 and Figure 1 , where the NiMoP-2 preparation procedure gave a catalyst with improved physico-chemical, surface, and catalytic properties.
When phosphorus is uniformly distributed through the cross section of the alumina extrudates, the beneficial effect of this additive for improving the mechanical properties of the catalyst support becomes more noticeable. Some relevant studies from the literature concerning the stability of phosphomolybdate species during their surface adsorption on different catalyst supports may help in the interpretation of our results. The stability of phosphomolybdate by contact with alumina was investigated by several researchers [ 14 — 17 , 21 , 22 ]. These compounds were supported by incipient wetness impregnation method.
The pentamolybdodiphosphate complex decomposes slowly upon contact with alumina surface into phosphate and molybdate species giving a catalyst phosphorus-rich shell and a molybdenum-rich core. The authors concluded that an excess of phosphorus inhibits the decomposition of pentamolybdodiphosphate complex. They also observed that molybdophosphate and the dimeric 9-molybdophosphate adsorbs on the alumina intact.