Kyutae Na

Skeletal isomerization of n-butane over caesium hydrogen salts of 12-tungstophosphoric acid

Skeletal isomerization of n-butane to isobutane has been studied with a flow reactor, mainly at 573 K, over caesium hydrogen salts of 12-tungstophosphoric acid (trihydrogen phosphododecatungstate), CsxH3–xPW12O40. The activity was highly dependent on the caesium content, being a maximum for x= 2.5 where the number of acid sites on the surface was greatest. The initial rate was first order with respect to the butane pressure (ca. 0.05–0.5 atm) at both 423 and 573 K on Cs2.5H0.5PW12O40. Cs2.5H0.5PW12O40 was found to be much more active and selective (83%cf. 60%) than SO42–/ZrO2 at the steady state at 573 K. H-ZSM-5 showed a higher activity than Cs2.5H0.5PW12O40 and SO42–/ZrO2 but the selectivity was only 14%. Cs2.5H0.5PW12O40 is a promising catalyst for this reaction and the low deactivation of Cs2.5H0.5PW12O40 is probably responsible for the high catalytic activity.

4.8 Novel Catalysis of Cesium Salt of Heteropoly Acid and its Characterization by Solid-state NMR

Acid-catalyzed reactions by acidic cesium salts of 12-tungstophosphoric acid (H3PW12O40) have been studied in relation to the acidities and microstructure of the particles. Liquid-phase alkylation of 1,3,5-trimethylbenzene with cyclohexene, decomposition of cyclohexylacetate and gas-phase skeletal isomerization of n-butane were performed over heteropoly compounds and other typical solid acids such as zeolites, Nafion-H, SO42-/ZrO2, etc. It was found that Cs2.5H0.5PW12O40 showed very high activities for these reactions and the excellent selectivity for the skeletal isomerization. XRD, TEM and N2 adsorption revealed that Cs2.5H0.5PW12O40 consists of ultrafine primary particles (∼8 nm) and therefore has a high surface area. High resolution solid-state NMR demonstrated that the chemical shift of 31P (central atom of the polyanion) is determined by the number of proton which is directly attached to the polyanion and all protons are distributed randomly through the whole bulk of the particles. Owing to this, Cs2.5H0.5PW12O40 possesses a high surface protonic acidity, which is a reason for its high activity. The quantity of acid sites of Cs2.5H0.5PW12O40 was less than those of zeolites and SO42−/ZrO2 and the acid strength was lower than that of SO42−/ZrO2. It is presumed, therefore, that the acid-base bifunctional action together with the high surface acidity brought about the high activity of Cs2.5H0.5PW12O40.

Skeletal isomerization of n-pentane over Pt-promoted cesium hydrogen salts of 12-tungstophosphoric acid

Abstract Two kinds of Pt-promoted Cs2.5H0.5PW12O40 (Cs2.5H0.5PW12O40 will be denoted by Cs2.5), Pt directly impregnated on Cs2.5 (Pt/Cs2.5) and a physical mixture of Cs2.5 and Pt/Al2O3 (Pt+Cs2.5), were used to catalyze the isomerization of n-pentane in the presence of hydrogen at 453–573 K. Cs2.5 showed a high initial activity but deactivated rapidly. Addition of Pt greatly suppressed the deactivation and increased the selectivity to isopentane. High stationary conversion (34.8%) and selectivity (96.9%) were obtained by using Pt+Cs2.5 at a relatively low temperature (453 K) and a low hydrogen pressure (0.05 atm, hydrogen/pentane=1). Under these reaction conditions, the stationary activity and selectivity of Pt+Cs2.5 were significantly higher than those of Pt-promoted H–ZSM-5 or SO42−/ZrO2. It was deduced that the remarkable effect of Pt in suppressing the catalyst deactivation was brought about by activated hydrogen, which were formed on Pt, transferred to Cs2.5, and utilized to remove carbonaceous deposits or their precursors. Increase in the hydrogen pressure decreased the initial activity probably due to a decrease in the concentration of pentenes or pentyl carbenium ion.

Molecular design of solid acid catalysts. Isomerization of n-butane catalyzed by acidic cesium salts of 12-tungstophosphoric acid combined with platinum

Abstract Our recent efforts made for the molecular design of solid acid catalysts utilizing heteropoly compounds are described first and, then, as an example, the isomerization of n-butane in the presence of hydrogen catalyzed by a bifunctional catalyst consisting of Pt and Cs2.5H0.5PW12O40 was examined mostly at 573 K. Presence of Pt suppressed substantially the catalyst deactivation and gave much higher stationary activity, although the initial activity was lowered a little due to a decrease in the concentration of butenes produced. It was further demonstrated that the activity of Pt for hydrogenolysis was nearly completely suppressed by the protonic acid sites present near Pt, giving a very high selectivity for isomerization.

Michael addition in the pseudoliquid phase of heteropoly compounds

Catalytic behaviors of various Keggin- and Dawson-type heteropolyacids for the Michael addition of alcohols to cyclohexenone have been investigated and the results were compared with those of other acid catalysts. The heteropolyacids, particularly H3PW12O40 and H4SiW12O40, showed much higher catalytic activities than Cs2.5H0.5PW12O40, H-ZSM-5, and SO42−/ZrO2 and high selectivities (ca. 100%). Quantitative analysis revealed that both cyclohexenone and alcohol molecules were absorbed in the catalyst bulk phase at the early stage of the reaction and their amounts remained nearly constant during the reaction. Furthermore, the reaction rates of various heteropolyacids were approximately in proportion to the quantity of reactants absorbed in the catalyst phase, showing that the reaction proceeded in the pseudoliquid phase. The catalyst bulk phase gradually turned from a solid to a very viscous liquid lump, but this was called pseudoliquid in the present work from its appearance and the quantities of absorbed (or dissolved) molecules.

Isomerizations of n-pentane and n-hexane over cesium hydrogen salt of 12-tungstophosphoric acid promoted by platinum

Abstract Isomerizations of n -pentane and n -hexane were carried out in the presence of hydrogen over Cs 2.5 H 0.5 PW 12 O 40 (Cs2.5) promoted with Pt by direct impregnation and mechanical mixing, and the results were compared with similarly Pt-promoted H-ZSM-5 and sulfated zirconia. The mechanical mixture of Pt/Al 2 O 3 and Cs2.5 showed the highest stationary activity and selectivity and the least deactivation for both reactions at a relatively low temperature (453 K) and a low H 2 pressure (pentane:H 2 :N 2 =0.05:0.05:0.9, hexane:H 2 :N 2 =0.05:0.2:0.75).

Isomerization of n-butane over bifunctional platinum-heteropoly compounds in the presence of hydrogen

Bifunctional Pt-Cs2.5H0.5PW12O40 efficiently catalysed n-butane isomerization at 300 °C under a low H2 pressure (0.05 atm); the activity was comparable to or higher than that of Pt-H-ZSM-5 or Pt-SO42–/ZrO2, and the selectivity was higher than that of either of the other two catalysts.