Reuse, catalyst

The polymer-bound catalyst was recyclable by filtration and showed just slightly decreased activity when reused. Catalyst (33) also promotes asymmetric Reissert-type reactions [106]. 

The catalyst obtained via anion exchange with the commercial resin Amberlite lRA-900 showed excellent selectivity in epoxidation of acid-sensitive natural terpenes and allylic alcohols [73-75]. The selectivity of 92% at 83% conversion was attained in epoxidation of a-pinene and limonene. The catalytic activity of the reused catalyst was completely maintained after several catalytic cycles, and the filtrate was catalytically inactive [75]. 

The catalytic activity of a lanthanum (R)-BINOL complex tethered either on silica (62a) or MCM-41 (62b) was evaluated for the enantioselective nitroaldol reaction of cyclohexanecarboxaldehyde (Se), hexanal (Sf), iso-butyraldehyde (Sg) and hydro-cinnamaldehyde (Sh) with nitromethane inTHF (Scheme 12.22) [166]. The silica-anchored lanthanum catalyst 62a gave 55-76% e.e. and yields up to 87%, while the PMS-immobilized catalyst 62b revealed slightly higher e.e.s (57-84%) for the same aldehydes. The homogeneous counterparts showed similar catalytic performance, albeit within a shorter reaction time. The increased enantioselectivity observed for the MCM-41 hybrid catalyst 62b was explained by transformations inside the channels, which is also reflected by lower yields due to hindered diffusion. The recyclability of the immobilized catalysts 62b was checked with hydrocin-namaldehyde (Ph). It was found that the reused catalyst gave nearly the same enantioselectivities after the fourth catalytic run, although the time period for achieving similar conversion increased from initially 30 to 42 h. 

The unconsumed reactants and products were both easily removed from the reaction mixture by extraction with K-hexane, which is not miscible with [BMIMJPFg. The brown-red ionic liquid phase containing the catalyst was reused five times with PhI(OAc)2 as the oxidant. The recovered catalyst gave catalytic activity comparable to that of the original. In epoxidation of styrene and of cyclohexene, both catalytic activity and selectivity fell slightly after five reuses. In the conversion of hept-l-ene, the reused catalyst showed the same activity and selectivity as the fresh catalyst, and the catalyst was shown to be unchanged after the reaction. 

Moreover, the catalytic activity of a reused catalyst is completely preserved after several cycles, and the filtrate is catalytically inactive. This supported and reusable P-W catalyst has also been applied for the epoxidation of a series of terpene olefins (356). The productivity of the catalyst for the conversion of these bulky molecules is superior to that reported for Ti-MCM materials, for example. The stereochemical and regioselective characteristics of W catalysis are preserved in the supported P-W catalyst. For instance, geraniol is epoxidized at the 2,3 position, affording the tram product. 

Physico-chemical characterisations of caesium impregnated catalysts and reused catalysts. 

CsF, the starting acid fluoride, and tetraglyme may be reused as catalyst. Better product yields are obtained with reused catalyst than with virgin catalyst. The last reaction (step 6) is best carried out using a fluidized bed of dried sodium carbonate at 270°-300° C. 

As can be seen, the maximum yield of p-MAP is reached at longer times when the reaction is performed with the reused catalyst. To gain an insight into this behavior, the spent AITPApva-peg catalyst was refluxed in dichloromethane... 

A second nucleophilic catalyst supported by PtBS is the polymer-bound di-methylaminopyridine analog that was also used in latent biphasic catalysis with the poly(JV-alkylacrylamide) support 129 [131]. This example of a nucleophilic catalyst (133) was used to catalyze formation of a t-Boc derivative of 2,6-di-methylphenol (Eq. 70). In this case, the extent of recovery of the catalyst and the yields of product were directly comparable to those seen with thermomorphic systems. The isolated yield for the first five cycles of this reaction were 34.3, 60.9,82.2,94.6, and 99%. In this case we reused catalyst 133 through 20 cycles. Yields after the first few cycles were essentially quantitative (ca. 93% average for each of 20 cycles). Separation of the polymer from the aqueous ethanol phase was quantitative as judged by either visual observation or UV-visible spectroscopic analysis. 

The repeated use of Pt-alumina modified by dihydrocinchonidine was studied by Balazsik and Bartok for the enantioselective hydrogenation of ethyl pyruvate under mild experimental conditions in toluene and AcOH. In toluene, depending on the reaction conditions, an increase of ee by 10-20% was observed on the reused catalysts. The same effect, however, was not found in AcOH. The phenomenon was attributed to an intrinsic feature of the Pt-alumina-cinchona system, in which the restructuring of the Pt surface may play an important role. 

A potential method to make synthetic chemistry more environment-friendly would be to reuse catalysts, such as ion-exchanging resins. Amberlite-IRA 402 (OH) could be employed as the... 

Hankari and collaborators studied the catalytic efficiency of hybrid zwitterionic acids (CAT-17) in the Biginelli reaction (Table 3) [27]. In this work, the authors showed that reusing catalyst CAT-17 is possible without losing its catalytic ability for at least for five cycles. The non-catalyzed reaction produced Biginelli adducts in less than 10% yield [2. ... 

The same MOF shows a moderate catalytic activity in wet catalytic oxidation with hydrogen peroxide using p-coumaric acid as a model phenolic compound [143]. Structural and morphological changes of the Cu-MOF were observed during the catalytic reaction. Note that the activity of the reused catalyst is higher than that of the fresh sample. 

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