The hydrogenation of benzaldehyde and cinnamaldehyde has been studied with a 5% Pt/C catalyst in compressed CO(2). The effect of CO(2) pressure on the total conversion was found to be different between the two aldehydes. The total conversion of benzaldehyde merely decreases with increasing CO(2) pressure, while that of cinnamaldehyde shows a maximum at a certain pressure. High-pressure FTIR measurements indicate the existence of interactions of CO(2) with the aldehydes. The absorption of nu(C=O) red-shifts at increasing CO(2) pressure, and this red-shift is more significant for cinnamaldehyde than for benzaldehyde, indicating that the C=O bond of the former becomes more reactive than the latter. The difference in the mode of interactions of CO(2) with these aldehydes has also been indicated by changes of nu(C=O) of CO(2). Thus, the conversion of benzaldehyde will decrease with increasing CO(2) pressure because of a simple dilution by introducing a larger quantity of CO(2). For cinnamaldehyde, the conversion will increase at low pressures because of increasing interactions with CO(2) molecules (increasing the reactivity of the C=O bond) but decrease at high pressures because of the simple dilution effect, similar to the case of benzaldehyde. The dense CO(2) molecules are not likely to change the catalytic activity of supported Pt particles, which was previously suggested from optical absorption of supported fine metal (Au) particles in a compressed CO(2) medium.

Although previous researchers have found that FSM-16 (#16 Folded Sheet Mesoporous material) doped with chromium and related Cr-doped silica catalysts has shown great activity for the oxidative dehydrogenation of isobutane to isobutene, information on the nature of these catalysts is insufficient. For this study, three types of Crdoped silica catalysts were prepared by applying the template ion exchange method. CrOx/FSM-16 and CrOx/SiO 2 were used as references. These catalysts were used for oxidative dehydrogenation, which was then characterized via various techniques. The most active catalyst was Cr-doped silica, which did not have the hexagonal structure that is characteristic of mesoporous FSM-16. Various characterizations showed that the catalytic activity of the Cr-species, stemmed from a weak acidic nature and a redox nature that originated from the combination of silicate and a Cr cation, as opposed to the hexagonal structure and strong acidic nature of FSM-16.

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The catalytic conversion of 1,2-propandiol to propanal is examined using FSM-16 particles (0.85-1.70 mm) molded by wet-treatment and pressurization. When FSM-16 was molded with 0.6 g of pressurization and supplied to the catalytic conversion of 1,2-propandiol at 673 K, this system resulted in a 94.8% conversion of 1,2-propandiol and 90.5% selectivity to propanal at 0.25 h on-stream, which was the maximum amount of activity. However, at 4.50 h on-stream, the activity decreased extremely to deactivation 19.9% conversion of 1,2-propandiol and 84.7% selectivity to propanal. In contrast, when FSM-16 molded with wettreatment (0.15 g) was used for the conversion at 573 K, activity was greatly increased and stable 98.6% conversion of 1,2-propandiol and 56.2% selectivity to propanal at 0.25 h on-stream followed by 91.9% and 52.5%, respectively, at 4.50 h on-stream. The hexagonal structure of FSM-16 was suggested to have contributed to the suitable conversion of 1,2-propandiol to propanal.

Hydroformylation of 1-hexene was carried out in supercritical CO 2 (scCO 2 ) and in organic solvents (toluene and ethyl acetate) using polymer-supported rhodium catalysts, which were prepared from polystyrene bound triphenylphosphine (TPP) and dicarbonylacetylecetonato rhodium. Preparation variables such as TPP/Rh ratio, time of rhodium precursor fixation on the support and time of syngas pretreatment do not indicate significant effects on the reaction. The product distribution slightly changes with CO 2 pressure. It increases appreciably as H pressure is raised in scCO but CO retards the reaction. The influence of H 2 2 2 pressure in scCO 2 is slightly different from that in toluene. Changes of the structure of rhodium complexes on the polymer during the catalyst preparation and the reaction were investigated by diffuse reflectance FT-IR. It should be noted that the catalyst is recyclable for the reaction in scCO 2 and the reaction rate and selectivity of the hydroformylation are much higher than those in the organic solvents.

The recyclability of water-soluble ruthenium -phosphine complex catalysts was investigated in water -toluene and in water -pressurized carbon dioxide systems for selective hydrogenation of trans-cinnamaldehyde (CAL). For the first hydrogenation run, the selectivity for cinnamyl alcohol (COL) is high for both toluene and dense CO 2 , because of interfacial catalysis in which the reaction mainly occurs at the interface between the aqueous phase and the other toluene or dense CO 2 phase. The total CAL conversion and the COL selectivity decrease on the second run, more significantly with dense CO 2 than toluene. On the subsequent runs, however, less significant changes were observed. During the first run, the active metal complexes should change to much less active ones such as Ru(H) 2 L n (TPPTS) m (L=COL) by accumulation of the main product of COL. This structural change may occur more easily in multiphase hydrogenation with dense CO 2 than that with toluene, probably because the solubility in the dense CO 2 gas phase is even smaller than that in toluene. For homogeneous reaction of COL in aqueous phase, Ru(H) 2 L n (TPPTS) m catalyzes the isomerization to HCAL compared with the hydrogenation to hydrocinnamyl alcohol. With those complexes, however, the selectivity for COL is still comparable to that for HCAL for multiphase hydrogenation reactions because the hydrogenation of an ampholytic substrate of CAL occurs mainly at interface between water and toluene or dense CO 2 gas phase. Interactions of CO 2 molecules with CAL would also increase the reactivity of carbonyl group of the substrate.

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The optimization of the oxidative esterification of propionaldehyde to methyl propionate using a supported palladium catalyst in methanol under heavy-metal-free and pressurized-oxygen conditions, which we recently reported in a previous paper, were carried out together with a study of the reaction route, the nature of the catalytic active sites, and the effect of the support. In our previous paper, we reported significantly improved activity for oxidative esterification using commercially available 5%Pd/Al 2 O 3 at 1.5 MPa of O 2 gas and 333 K and emphasized that the doping of 5%Pd/Al 2 O 3 with lead was not needed for the reaction system, but we could not improve the activity that was reported when using 5%Pd/-Al 2 O 3 doped with 5% Pb (a 93.2% conversion of propionaldehyde, 76.8% selectivity for methyl propionate and a 71.6% yield of methyl propionate) at 0.3 MPa of O 2 gas and 353 K, as reported by another laboratory. In the present study, however, we exceeded those values and obtained a 98.3% conversion of propionaldehyde, 75.3% selectivity for methyl propionate and a 74.0% yield of methyl propionate using 5%Pd/-Al 2 O 3 undoped with Pb at 1.5 MPa of O 2 gas and 333 K. It should be noted that, in the preparation of the present 5%Pd/-Al 2 O 3 , Pd was doped onto Al 2 O 3 that had been previously treated with aqueous NaOH. Another active alumina support,-Al 2 O 3 , prepared from boehmite, afforded activity that was substantially lower than that of -Al 2 O 3 and depended on the calcination temperature of boehmite to -Al 2 O 3. When using various concentrations of CH 3 OH in the aqueous reaction solution, the oxidative esterification proceeded through the formation of propionic acid. To determine why the Al 2 O 3 support afforded better activity than the active carbon support, Pd/Al 2 O 3 and Pd/C catalysts were examined by XAFS (X-ray absorption fine structure). XAFS revealed that Pd on Al 2 O 3 shows a better redox nature than Pd on C, which resulted in activity on Pd/Al 2 O 3 that was better than that on Pd/C.