Catalysts preparation, phosphorus

The advances that have taken place over the past five years in catalytic, enantioselective aldol addition reactions is evident in a number of important respects. The types of transition metals and their complexes that function competently as catalysts have been expanded considerably. Thus, in addition to B(III), Ag(I), Au(I), Sn(II), and La(III), chiral catalysts prepared from Cu(II), Ti(IV), Ln(III), Si(IV), Pt(II) and Pd(II) have been introduced. The expansion in the use of transition metals has taken place hand-in-hand with the design and synthesis of new bidentate and tridentate organic ligands based on nitrogen, oxygen, and phosphorus donors. Additionally, whereas the older methods primarily relied on the use of Lewis-acids for the activation of... 

The above procedures for catalyst preparation have generally provided excellent results. Especially important are surface-sensitive reactions. With supported catalysts in which the active components have a narrow particle-size distnbution, the optimum particle size for a demanding reaction can be established. Major improvements of supported catalysts, e.g. with respect to carbon deposition and ammonia decomposition, can be achieved by preparing catalysts with a narrow par-ticle-size distribution. Also, the preparation of catalysts in which the active components have a uniform chemical composition is highly important One instance is the preparation of supported vanadium oxide phosphorus oxide (VPO) catalysts for the selective oxidation of w-butane to maleic anhydride, which has been carried out using vanadium(III) deposition onto silica [31]... 

Zazhigalov et al. (209) investigated cobalt-doped vanadium phosphate catalysts prepared by coprecipitation and impregnation methods. The performance of catalysts prepared by both methods was improved as a consequence of the promotion. The cobalt is thought to have been present as cobalt phosphate, which is considered to stabilize excess phosphorus at the surface, which has previously been foimd to be an important characteristic of active catalysts. 

The mechanism of the amination reactions of 1,3-dienes assisted or catalyzed by nickel and palladium species has often not been elucidated. Catalysts prepared from nickel or palladium salts or complexes, Lewis acids, and phosphorus ligands were all used since amine telomers are mainly produced, it is plausible that zr-allyl complexes are the reactive intermediates21 24. The reaction carried out with amine hydroiodide or hydrochloride, or in the presence of trifluo-roacetic acid, is presumed to proceed via the following mechanism24 25. 

In this chapter we will discuss how the behavior of the catalyst is influenced by a number of factors including the method of preparation of the precursor, the oxidation state of the catalyst, the phosphorus/vanadium raho of the catalyst and the activation conditions. A variety of techniques have been used to characterize the morphology and nature of the achve sites of the catalysts and gain insight into the mechanism. Furthermore, the effect of these preparatory techniques will be discussed, with emphasis on the partial oxidation of n-butane. We will pay particular attention to industrially relevant examples where possible and attempt to describe the current state of the art. 

Theories and principles of the characterization techniques are not described here. For consistenc), all the catatysts described in this review are referred to with the same nomenclature, although a different nomenclature is sometimes used in the cited publications. Each catalyst component (element) separated by the symbol indicates the sequence of its introduction into the catalyst formulation from right to left. Those separated by the symbol 7 between right and left belong to the support material and the elements on the support, respectively. For example, NiMo-P/Al refers to a catalyst prepared such that the phosphorus-containing precursor is loaded on the alumina support first, followed by nickel and molybdenum, which are introduced simultaneously. CoMo/Al — P refers to a catalyst in which cobalt and molybdenum are introduced simultaneously onto an alumina support doped with phosphorus-containing species. Each element may represent its oxide or sulfide forms. In all cases, A1 refers to the alumina-based support or to its hydroxide precursor. 

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). ...

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-... 

The pH value of the impregnation solutions for depositing the active elements on alumina is an important variable in catalyst preparation. For example, Jian and Prins (66) reported that the content of bulk molybdenum oxide (detected by XRD) increases in MoP/Al catalysts when the pH of the impregnation solution decreases from 9 to 1 as a result of addition of phosphorus (Fig. 27). 

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. 

Fig. 38. Schematic diagram representing the different phosphorus oxo-species present during catalyst preparation.

The catalytic activity of fine nickel metal is very much reduced and modified when prepared by SHOP or the cation-exchange method, as shown in the previous sections. The partial poisoning of the nickel catalyst with phosphorus compounds brings about almost the same effect. These facts suggest the possible formation of specific and mild active sites of nickel, if nickel metal makes an alloy with phosphorus. 

The difficulties encountered in the synthesis of 2-alkyl- and 2-aryl-substituted selenazoles lie principally in the preparation of the corresponding selenoamides. In this respect, a method is worthy of note in which the use of selenoamides is dispensed with. For this, a nitrile, a hydrogen selenide, and an a-halogenoketone are reacted together in the presence of a condensation catalyst.5 Phosphorus oxychloride, alone or mixed with zinc chloride or phosphorus trichloride, is specially suitable. The yields of the corresponding 2-alkylselenazoles are up to a maximum of 25%. 

It has been established that the properties of vanadyl pyrophospate are strongly dependent on its biography, i.e. the preparation method, presence of overstoichiometric phosphorus and additives [1-4], Therefore, considerable effort of the researchers was directed to optimization of the synthesis technique and in recent studies also, non-traditional methods for the catalysts preparation were considered [5-10]. 

After conventional butane oxidation (450°C, dry feed) the ISS-P V ratio of the surface layer in used catalysts exceeds 2.0 indicating both an excess of phosphorus and a marked presence of vanadium in the utmost layer. This is in contrast to the pyrophosphate termination model of Thompson and Ebner [6] from which primarily the presence of phosphorus on the surface follows. P V ratios ranging from 1.3 up to 1.4 were measured by XPS in samples which were obtained after conventional butane oxidation. These ratios are higher than those of Okuhara et al. [21] (1.10 0.04) and Coulston et al. [3] (1.08) who also used reliable sensitivity factors of XPS obtained by calibration. The reason for this difference might be that both the precursor synthesis and the catalyst preparation of the authors differ from the methods applied in the present work. 

Catalysts used in this work had a composition of 3 wt% nickel, 8 wt% molybdenum and 0 or 2 wt% phosphorus. They were prepared by incipient wetness impregnation, followed by drying at 393 K and calcination at 773 K. Details of the catalyst preparation can be found elsewhere [2], The HDN reactions were carried out in a continuous-flow microreactor. A sample of 0.1 g catalyst diluted with 9.5 g SiC was used for each reaction. The catalyst was sulfided in situ with a mixture of 10% (mol) H2S and H2 at 643 K and 1.5 MPa for 4 h. After sulfidation, the pressure was increased to 3.0 MPa and liquid reactant was fed to the reactor by means of a high pressure pump, with n-octane as the solvent. The catalyst was stabilised at 643 K and 3.0 MPa for 100 h before samples were taken. The initial reactant partial pressure (P°) of Q, THQ5 and OPA was usually 4.76 kPa, and that of H2S was 6.5 kPa by adding dimethyldisulfide to the liquid reactant. n-Nonane as well as n-dodecane were used as internal standards. 

The above results can be compared with spectra published more recently on phosphorus-promoted y-alumina catalysts, prepared via the incipient wetness impregnation techniques [85].The general findings in the P and Al MAS-NMR results on these materials are quite similar, although a somewhat different interpretation is given for the compositional evolution of the P chemical shifts. Compared to the co-precipitates, in these impregnated samples bulk AIPO4 seems to be detectable at much lower phosphorus concentrations. 

The preparation of mixed oxide compositions of vanadium and phosphorus and the use of these as catalysts for the oxidation of hydrocarbons such as n-butane to maleic anhydride is known in the art. In U.S. Pat. No. 4,111,963 the importance of reducing the vanadium used in a vanadium/phosphorus oxide (V/P/O) catalyst to the four oxidation state is described. Preferred is the use of concentrated hydrochloric acid as the reaction medium to bring about this reduction and preferred catalysts have a phosphorus to vanadium atom ratio of 1 2 to 2 1 and a porosity of at least 35%. In U.S. Pat. No. 3,864,280 the reduction of the vanadium in such a catalyst system to an average valence state of 3.9-4.6 is emphasized the atomic ratio of phosphorus to vanadium is 0.9-1.8 1. Isobutyl alcohol is used as a solvent for the catalyst preparation, with the indication that an increase in catalyst smface area, over that obtained from use of an aqueous system, is achieved. The addition of promoters to the V/P/O catalyst compositions used for the oxidation of hydrocarbons to maleic anhydride is also disclosed in the art. Thus, in U.S. Pat. Nos. 4,062,873 and 4,064,070 are disclosed vanadium/phosphorus/silicon oxide catalyst compositions made in an organic medimn. In U.S. Pat. Nos. 4,132,670 and 4,187,235 are disclosed processes... 

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