Supported liquid phase catalysis SLPC

In supported-liquid-phase catalysis (SLPC) a metal complex is dissolved in a thin film of polar solvent (usually water) adsorbed on a high-surface-area hydrophilic support such as silica or controlled-pore glasses [40]. The resulting catalyst is placed in contact with a water-immiscible liquid phase containing the reactants and the reaction occurs at the aqueous-organic interface. The catalyst can be recovered by filtration. 

Supported aqueous-phase catalysts (SAPC) can be seen as a special case of adsorption, whereby a water-soluble catalyst dissolved in a very polar solvent is adsorbed on a hydrophilic support forming a water film on the inner surface of the support [30,31]. In the case of supported liquid-phase catalysis (SLPC),the water film on the inner surface is replaced by a solvent of low vapor pressure (e.g.jphthalic acid esters) [2]. The reaction itself takes place in the supportedUq-uid or at the interface of the supported liquid film, or in the gas phase or organic phase when dealing with SLPC or SAPC, respectively. The use of SLPC catalysts is generally restricted to the synthesis of low-boiling compounds. 

Abbreviations AD, asymmetric dihydroxylation BPY, 2,2 -bipyridine DMTACN, 1,4-dimethyl-l,4,7-triazacyclonane EBHP, ethylbenzene hydroperoxide ee, enantiomeric excess HAP, hydroxyapatite LDH, layered double hydroxide or hydrotalcite-type structure mCPBA, meta-chloroperbenzoic acid MTO, methyltrioxorhenium NMO, A-methylmorpholine-A-oxide OMS, octahedral molecular sieve Pc, phthalocyanine phen, 1,10-phenantroline PILC, pillared clay PBI, polybenzimidazole PI, polyimide Por, porphyrin PPNO, 4-phenylpyridine-A-oxide PS, polystyrene PVP, polyvinylpyridine SLPC, supported liquid-phase catalysis f-BuOOH, tertiary butylhydroperoxide TEMPO, 2,2,6,6-tetramethyl-l-piperdinyloxy TEOS, tetraethoxysilane TS-1, titanium silicalite 1 XPS, X-ray photoelectron spectroscopy. 

Supported liquid-phase catalysts (SLPCs) combine the salient features of both homogeneous and heterogeneous catalysis for enhanced catalytic and/or process efficiency (337). SLPC catalysts, in which a liquid-phase (homogeneous) catalyst is dispersed within a porous support, have been used in Wacker-type ethylene oxidation for acetaldehyde and vinyl acetate production (337, 338). In the former case, a traditional homogeneous Wacker catalyst (vide supra) consisting of a chlorinated solution of Pd and Cu chlorides retained on a support with monomodal pore size distribution... 

The transitions of supported liquid-phase catalysts (SLPC) and supported aqueous-phase catalysts (SAPC) are dealt with in Section 3.1.1.3, while special aspects of clusters and colloids are discussed in Sections 3.1.1.4 and 3.1.1.5 and those of aqueous-phase, re-immobilized catalysts in Section 3.1.1.6. The combination of heterogeneous catalysis with aqueous (biphasic) techniques is also under investigation, e. g., [209]. 

An interesting approach to overcome these limits and thus combine the advantages of homogeneous and heterogeneous catalysis is that of supported liquid phase catalysts (SLPC or SLP). In SLPC the organometallic complex active components are dissolved in a small quantity of liquid phase dispersed in the form of an isle or film on the surface of supports. A SLPC has been applied successfully for several chemical transformations [113], particularly in the Wacker-type ethylene oxidation to acetaldehyde and vinyl acetate production by ethylene acetoxylation [114], and in other reactions catalyzed by Pd-complexes such as the Heck reaction [115]. 

As an alternative to the heterogenization of homogeneous catalysis, there are some proposals to realize a solid catalyst with an immobilized species in aqueous/organic media. This concept, a continuation of the SLPC as mainly published and highlighted by Scholten et al., consists of a thin film of catalytic material that resides on a high-surface-area support such as controlled-pore glass, silica, zeolites. Thus this concept of supported aqueous phase catalysis (SAPC) contains both a hydrophilic liquid and a hydrophilic organometallic catalytic complex on a solid support as shown in Fig. 12.15. "... 

SLPC or SAPC (supported liquid [or aqueous] phase catalysis [9,10,62,64] see also Section 5.2.5) provide no improvement, probably because of the tremendous stress on the support/transition metal bond during the repeated change between tetrahedral and trigonal-bipyramidal metal carbonyls over the course of a single catalyst cycle. Only recent publications [11,21,26b,28h] report on successful realization of supported homogeneous hydroformylation catalysts, but so far there is no confirmation by practise-soriented tests -not to mention by commercial applications. 

Those inunobilization procedures generally yielding supported solid-phase catalysts (SSPCs) have already been described in the preceding sections this section deals with catalytically active species (e. g., Wilkinson s or Vaska s complex) that are dissolved in liquids (therefore the catalyst is exactly the same as in homogeneous catalysis) or are even liquids themselves supported on porous solids. The principal structure of such an SLPC and the principal difference from an SAPC (Supported Aqueous-Phase Catalyst) is shown in Figure 2. 

Supported ionic liquid catalysis is one of the main examples of SLPC adopted [120] to take advantage of ionic liquid properties without the drawbacks evidenced in Section 2.3.6. The viability of this concept has been confirmed by several studies that have successfully confined various ionic phases to the surface of support materials and explored their potential catalytic applications. Although most of the evaluated supports were silica based, several studies have focused on polymeric materials, including membranes. These materials were prepared by using two different immobilization approaches. The first involves the covalent attachment of ionic liquids to the support surface whereas the second simply deposits the ionic liquid phases containing catalytically active species on the surface of the support. 

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