Basic characteristics of heteropolyacids as green catalysts are overviewed, focusing on the various reaction fields in which the heteropolyacid catalysts function as acid and oxidation catalysts, and then two industrial processes which were made very green by the use of heteropolyacid catalysts in liquid biphase systems are described. In this context an interesting new reaction field formed at the interfacial layer of a solid heteropolyacid and liquid has also been reported.
The direct methyl esterification reaction for MMA was commercialized. Precisely synthesized Pd 3 Pb 1 catalyst and the catalysis at the proposed reductive oxidation reaction condition (PROC) realized and guaranteed high MMA yield and long catalyst life. Controlled Pd distribution in a catalyst particle drastically decreased Pd loss from the catalyst during reaction. Unit processes are introduced as well.
Polyoxytetramethyleneglycol (PTMG) can be directly prepared from tetrahydrofuran U H F ) and water in the presence of heteropolyacids (HPA), without the hydrolysis step. The reaction is carried out by mixing two liquid phases: the THF phase and a heteropolyacid catalyst phase. In this reaction the molar ratio of water to a heteropolyacid (H,OIHPA) in the catalyst phase plays an important role in the THF polymerization activity and the molecular weight of PTMG. IR spectrometric studies revealed that THF is coordinated to a heteropolyanion through either a water molecule or a proton in the catalyst phase. The latter type of coordination brings about the activation of THF capable of initiating the ring-opening polymerization even in the presence of water at a lower acid strength. The PTMG prepared by this method has a narrow molecular weight distribution ( M J M , , = 1.5 or less) and a number average molecular weight of which are requisite for the production of polyurethane elastomers. A new polymerization mechanism named "Phase Transfer Polymerization" is proposed for elucidating a narrow molecular weight distribution.
The extraction behavior of Am(VI) was studied for a nitric Cl:cid solution containing ammonium dihydrogenphosphate using tri-n-butylphosphate (TBP) and other extractants. TBP and HDEHP solutions diluted with n-dodecane do not reduce Am(VI) in the extraction procedure, whereas CMPO and TOA do. In TBP extraction, phosphate anion raises the separation factor of Am from Nd as high as 120 with 1 M TBP solvent from 3 M HN0 3-1 M N~H2P04, although it suppresses the extraction of both. A distribution ratio value of 6.6 was obtained using undiluted TBP. Dissociated phosphate anion H 2 P0 4forms a complex that lowers the distribution ratio of Am even in highly concentrated nitrate solution at low acidity. Americium(VI) is extracted as Am02(N03)2•2 TBP. The stability constant, log {3, for Am(VI)-H 2 P0 4-complex was estimated as 1.2.
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