Mass transfer promoters

We frequently do not have reliable reaction rate expressions for chain reactions, but we can compensate for this lack by designing and operating the reactor to manage the overall course of the reaction by properly dealing with mixing, mass transfer, promoters and inhibitors, and the presence of surfaces. 

The hydroformylation of co-alkene carboxylic acid methyl esters catalysed by a Rh/TPPTS system (Scheme 1.22) in a biphasic medium does not require additives with low molecular substrates such as methyl 4-pentenoate, whereas methyl esters of higher co-alkene carboxylic acids such as methyl 13-tetra-decenoate require the presence of surfactants as mass-transfer promoters. Surfactants, indeed, decrease the interfacial tension, forming aggregates above the critical micellar concentration that speed up the catalytic process by increasing the interfacial area. 

O. Kurokawa, H. Matsusaki and T. Takahashi, Characteristics of net-type spacer as mass transfer promoter in electrodialysis, Kagaku Kogaku Ronbunshu, 1983, 9, 142. 

Water-soluble Receptor as Mass Transfer Promoter... 

Hydroformylation of -alkene carboxylic acid methyl esters catalyzed by a Rh/ TPPTS system was initially investigated by Fell et al. (Eq. (1)]. As expected in a biphasic medium, low molecular (o-alkene carboxylic acid methyl esters such as methyl 4-pentenoate can be hydroformylated efficiently without any additives whereas methyl esters of higher (<>-alkene carboxylic adds such as methyl 13-tetra-decenoate require the presence of mass-transfer promoters such as surfactants [3] or chemically modified y9-cyclodextrins [4]. 

As organic substrates are generally poorly soluble in water, a phase transfer agent is needed in addition to the use of a water-soluble catalyst. In this context, CDs have been efficiently used as mass transfer promoters and are still the subject of intensive research [11], It is known that CDs and modified CDs can be efficiently used as mass transfer agents because an inclusion complex can be formed between the CDs and the organic substrate [12], This has been done with a broad scope of reactions as alkylation [13], metathesis [14],... 

In this part, we will focus on the synthesis of CD-based polymers and their use in mass transfer promoters. Their beneficial impact on the catalytic performance is especially described through relevant examples of the literature. 

The hydrocarboxylation of higher olefins has not been completely solved because it requires mass transfer promoters due to their low water solubilityP For instance, the selectivity obtained during the hydrocarboxylation of 1-decene reached 90% with per(2,6-di-o-methyl)-p-cyclodexUin, versus 53% for l-methyl-2-pyrrolidinone (one of the most suitable cosolvents) and 20% without a mass transfer promoter. 

Such a mass transfer promotion by ultrasonication provides an increase in the current efficiency for a variety of electrosyntheses. For example, Atobe et al. reported that a significant ultrasonic effect on the current... 

Aqueous two-phase hydroformylation of methyl oleate and a technical linseed oil was conducted by anchoring the Rh(TPPTS) catalyst either on silica [2] or on activated carbon (Nuchar WV-B) [47]. Especially, the latter proved highly efficient as a mass transfer promoter. 

Therefore, CDs are employed as carrier for biologically active substances [104], enzyme models [105], separating agents [106], catalysts [107], mass transfer promoters [108], additives in perfumes, cosmetics elements and food [109], environmental protection agents or sensors for organic molecules [110]. 

PTA and the A-benzylated derivative 7-phospha-l-benzyl-1,3,5-triazaada-mantanyl chloride (A-Bz-PTA)Cl were used as ligands for Rh(l) precursors and tested for the hydroformylation of long-chain terminal aldehydes such as 1-decene in water, in the presence of randomly methylated -cyclodextrin (RAME- -CD, Fig. 7.6) as mass transfer promoters [47]. 

Cyclodextrin is another type of mass-transfer promoter. Given that cyclodextrins are able to form inclusion complexes with hydrophobic substrates, it was proposed that they may carry the substrates into the aqueous phase, faciUtating catalysis with the water-soluble complexes. Monflier [38] first reported that, by using per(2,6-di-0-methyl)-P-cyclodextrin, the rate of hydroformylation of 1-decene increased up to 10 times than that observed without the cyclodextrin. 

A drawback of the Suzuki coupling in water is the phase-transfer limitation arising from water-insoluble substrates. To circumvent this problem, the use of surfactants or mass-transfer promoters has been explored. Cyclodextrins [127,133] and cahxarenes [134] were reported to be useful inverse-phase-transfer promoters for the reaction in aqueous medium the latter was better than the former in inducing rate enhancement, but troublesome in separation of the aqueous layer from the organic phase. 

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