Catalyst by adsorption

Ommen, J. G. van, K. Hoving, H. Bosch, A. J. van Hengstum and P. J. Gellings. 1983. The preparation of supported oxide catalysts by adsorption of metal acetylacetonates, M(AcAc)n on different supports. Z. Phys. Chemie Neue Folge 134 99-106. 

Hutchings el al. (102,103) synthesized enantioselective Mn catalysts by adsorption of chiral salen ligands on Mn-exchanged Al-MCM-41 and used the materials in selective epoxidation of dv-stilbcnc with PhlO. Although the simplicity of this approach is appealing, reuse of the material in a second run led to a dramatic decline of the epoxide yield from 69 to 18%, and of the ee of the trans epoxide from 70 to 30%. 

Figure 9 Simplified model for the formation of active centres of Ziegler-Natta catalysts by adsorption ofTiCl4 on solid MgCl2, reduction ofTi(IV) to Ti(III) chloride-alkyl exchange with aluminium alkyls and polyethylene (PE) formation by coordination and insertion of ethylene molecules.

Different mbted Ir-Mo/alumina catalysts were prepared. The details of procedures and catalyst characterization were published elsewhere [10,11]. In the first series, the Mo was deposited first. The samples were prepared from Mo03/alumina or MoS2/alumina catalyst by adsorption of Ir4(CO)i2 from a cyclohexane solution. This procedure was the same as for the Ir catalysts. In another series, an inverse order of impregnation was used. The Ir was deposited first and then Mo was deposited from an aqueous solution of AHM. One catalyst was prepared by coimpregnation of alumina by an aqueous solution of AHM and (NH4)2lrCl6. The samples containing Ir were sulfided or reduced without calcination in air in order to avoid Ir sintering. The surface areas of mixed catalysts varied between 200-208 m /g. 

Tartronic acid was oxidised to mesoxalic acid on Pt(5.7%)Bi(2.4%)/C at pH 1.5 (Figure 6(a)). The high initial selectivity was not maintained due to deactivation of the catalyst, by adsorption of the acid product, and over-oxidation (maximum yield 29% at 53% conversion). Higher initial rates of conversion were obtained at pH 5 (Figure 5(b)) to give higher yields (39% at 79% conversion). However, the rate of over-oxidation was also increased. 

Zeng J, Lee JY, Chen J, Shen PK, Song SQ. Increased metal utilization in carbon-supported Pt catalysts by adsorption of preformed Pt nanoparticles on colloidal silica. 

Pharmaceutical. Ion-exchange resins are useful in both the production of pharmaceuticals (qv) and the oral adrninistration of medicine (32). Antibiotics (qv), such as streptomycin [57-92-17, neomycin [1404-04-2] (33), and cephalosporin C [61-24-5] (34), which are produced by fermentation, are recovered, concentrated, and purified by adsorption on ion-exchange resins, or polymeric adsorbents. Impurities are removed from other types of pharmaceutical products in a similar manner. Resins serve as catalysts in the manufacture of intermediate chemicals. 

After epoxidation a distillation is performed to remove the propylene, propylene oxide, and a portion of the TBHP and TBA overhead. The bottoms of the distillation contains TBA, TBHP, some impurities such as formic and acetic acid, and the catalyst residue. Concentration of this catalyst residue for recycle or disposal is accompHshed by evaporation of the majority of the TBA and other organics (141,143,144), addition of various compounds to yield a metal precipitate that is filtered from the organics (145—148), or Hquid extraction with water (149). Low (<500 ppm) levels of soluble catalyst can be removed by adsorption on soHd magnesium siUcate (150). The recovered catalyst can be treated for recycle to the epoxidation reaction (151). 

The mother Hquoi obtained from the crystallization, or the raffinate after removal by adsorption, is isomerized using an acidic catalyst to convert xylene to the o- and -isomers (Unit K in Fig. 8). 

The flux of flie adsorbed species to die catalyst from flie gaseous phase affects die catalytic activity because die fractional coverage by die reactants on die surface of die catalyst, which is determined by die heat of adsorption, also determines die amount of uncovered surface and hence die reactive area of die catalyst. Strong adsorption of a reactant usually leads to high coverage, accompanied by a low mobility of die adsorbed species on die surface, which... 

The initiator usually constitutes less than 1% of the final product, and since starting the process with such a small amount of material in the reaction vessel may be difficult, it is often reacted with propylene oxide to produce a precursor compound, which may be stored until required [6]. The yield of poloxamer is essentially stoichiometric the lengths of the PO and EO blocks are determined by the amount of epoxide fed into the reactor at each stage. Upon completion of the reaction, the mixture is cooled and the alkaline catalyst neutralized. The neutral salt may then be removed or allowed to remain in the product, in which case it is present at a level of 0.5-1.0%. The catalyst may, alternatively, be removed by adsorption on acidic clays or with ion exchangers [7]. Exact maintenance of temperature, pressure, agitation speed, and other parameters are required if the products are to be reproducible, thus poloxamers from different suppliers may exhibit some difference in properties. 

Highly active catalysts have been produced by adsorption of lipases onto macroporous acrylate beads, polypropylene particles and phenol-formaldehyde weak anion exchange resins. Protein is bound, presumably essentially as a monolayer, within the pores of the particles. The large surface area of the particles (10m2 g 1) means that substantial amounts of protein can be adsorbed, and the pores are of sufficient size to allow easy access of reactants to this adsorbed protein. 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

The only example of immobilization of a bis(oxazoline) complex by adsorption onto silica was published very recently [70]. The complexes 6a-Cu(OTf)2 and 6b-Cu(OTf)2 were adsorbed onto a chromatographic grade of silica gel and the resulting solids used as catalysts in two Diels-Alder reactions. 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Fig. 4 TOC removal with y-Al203 and y-AljOj supported catalysts during catalytic ozonolysis and ozonolysis followed by adsorption.

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

The long-term stability of the Ru/Ti02 catalyst was studied under various reaction conditions and the spent catalysts were characterized for assessing the reasons of deactivation. It was observed that the rate exhibits a rapid reduction at the initial several hours of reaction, followed by a slow and continuous deactivation. Analysis of the spent catalyst, by H2 adsorption after removing surface carbon, showed that the initial rapid reduction of activity is mainly due to metal sintering, while the continuous and slow deactivation is related to the occurrence of the SMSl phenomenon at the later part of the catalyst bed, where reducing conditions prevail. In order to avoid these processes which lead to catalyst deactivation, Ti02... 

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