Sequential impregnation

Fig. 18. Relationship of preparation method to Co(Ni)-Mo-S/Al203 structure and activity. (a) Catalyst structure (MES) (61) (b) catalyst activity (61). SEQ = sequential impregnation COIM = coimpregnation. Modified and reproduced with the permission of Ref. 61.

Recent studies have shown that, by careful selection of the starting reagents and solvents, it is now possible to synthesize Co(Ni)-Mo-S species on any support without the requirement of sequential impregnations (61). 

In the result of the sequential impregnation pore volume of precursor was diminished almost twice and surface area by 30 %. However in this series of experiments hydrothermal treatment gave results quite reverse result. In this case small increase in pore volume and specific surface area was observed while increase in average pore radius size was negligible. 

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). ...

The impregnation method is further classified into coimpregnation and sequential impregnation. When this impregnation method is used, the sequence of introduction of phosphorus and metal-containing species and the pH of the impregnating solutions are important because they affect the resulting physicochemical properties and consequently the catalyst performance (Sections V.D and VI). 

The adsorption of molybdates in the presence of phosphorus oxo-species in solution (or those already deposited on an alumina support) or vice versa (i.e., adsorption of phosphate on a Al—Mo support) has also been explored. For example, Fig. 20 shows the isothermal adsorption on alumina supports of solutions containing different concentrations of ammonium heptamolybdate (AHM). From Fig. 20a, which corresponds to the sequential impregnation steps whereby phosphorus is adsorbed first (before molybdenum), it is clear that molybdenum is adsorbed in lower amounts on the Al—P support than on the phosphorus-free alumina. However, the amount of molybdenum adsorbed on the Al—P support also depends on the pH of the AHM solution (Fig. 20b). Molybdenum oxo-species tend to adsorb in greater amounts on the Al—P support under acidic condition. 

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). 

Bouwens et al. (100), Eijsbouts et al. (94), and Iwamoto and Grimblot (40) reported that phosphorus has no detectable promotion effect on thiophene HDS catalyzed by Mo—P/Al and MoP/Al. On the other hand, Fierro et al. (124) indicated that the thiophene HDS activity of coimpregnated MoP/ Al catalysts reaches a maximum for a loading of 4 wt% P2O5, whereas the activities of sequentially impregnated Mo—P/Al catalysts are nearly independent of the phosphorus content. Kim and Woo (84) reported that... 

Note. ( ). estimated value iik. unkno-wn CO, coimpregnation SI, sequential impregnation EA, equilibrium adsorption IlG, hvdrogel method SG, sol-gel method NS. no supported AP, atmospheric pressure PS. presulfiding PR, prereduction DBT, dibenzothiophene DIPB, diisoprq ylbenzene DHQ, decahydroquinoline OPA. ortho-propylaniline DMDS. dimethyldisulfide THQ, 5-tetrahydroquinoline. 

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. 

Impregnation is one of the most used techniques to incorporate an active phase in a support. It can also be used to deposit active phase to a monolith [85]. Usually, a high-surface-area monolith is dried, evacuated, and dipped in a solution containing a precursor of the active phase. After drying and calcination a monolithic catalyst is obtained. Often, an activation step is necessary to convert the precursor of the active phase into the active phase, e.g., the transformation of a metal oxide in the corresponding metal or metal sulfide. Monolithic catalysts with complex compositions of active phases can be prepared by sequential impregnations with suitable solutions or with a conunon solution containing various precursors of the components. 

Giorgio S, Henry CR (2002) Core-shed bimetallic particles, prepared by sequential impregnations. Eur Phys J Appl Phys 20 23... 

About 30 mg of a Pt-Sn/AlaOs catalyst were used in each run. The particle size were chosen small enough to avoid internal mass transfer limitations. The catalyst was prepared by sequential impregnation of a commercial y-alumina support with aqueous solutions of SnCl2 and HaPtClfi [9]. Characterization data are summarized in Table 1. 

After drying and reduction steps identical to those employed in the preparation of samples B, C, and D, the sequentially impregnated material (sample B-lr) had the same elemental composition as sample D. Another sample prepared in exactly the same manner, except that water was substituted for the chloroiridic acid solution, served as a reference material (designated as sample B-H20). 

All catalysts in this study were prepared via incipient wemess. The alkali promoters were introduced in the samples via a co-impregnation or a sequential impregnation technique. The details of the preparation was described elsewhere [1]. 

The presence of Na promoters decreased the amount of the weakly bound hydrogen states significantly. The weakly bound p state present on unpromoted catalysts [5-7] was not observed on the Na promoted catalysts (Fig. 4). The P state of hydrogen was unobservable on both co-impregnated and sequentially impregnated catalysts. This weakly bound state unobservable on a NAJMIO catalyst appeared after the Na promoter was eliminated from the catalyst by washing with boiling water several times. 

Mesoporous alumino silicate molecular sieves with MCM-41 type structure synthesized using various A1 sources (i.e. aluminum sulphate, aluminum isopropoxide, pseudo boehomite and sodium aluminate) have been used as supports for Ni - Mo catalysts. The HDN of o-toluidine tmd cyclohexylamine was studied in a fixed bed flow reactor at 450°C and PH2 = 1 atm. The activity per unit of weight of the MCM - 41 supported catalysts was evaluated and compared to that of supported catalysts preptu ed by sequential impregnation method. The XRD and DRS data have been used to explain the observed trend in catalytic activity towards HDN reaction... 

The catalysts were prepared by an incipient wetness impregnation technique, using aqueous solutions of RuCls and Fe(NOs)3. Solutions of predetermined concentrations were added dropwise to silica (HS-5 Cab-O-Sil from Cabot Corp.) with constant mixing. Sequential impregnations of ruthenium and then iron were used for the bimetallic catalysts, and all samples were dried overnight at 383 K after any impregnation step. The iron nitrate solution was 93% isotopically enriched in the Te Mossbauer isotope. 

Sequential impregnation of support, optimization of composition by genetic algorithm... 

---