Toshio Okuhara

Insoluble heteropoly compounds as highly active catalysts for liquid-phase reactions

Abstract Liquid-phase alkylation of m -xylene or trimethylbenzene with cyclohexene was studied using heteropoly compounds and other typical solid acids as the catalysts; the results are discussed in relation to the surface properties of the catalysts. Cs 2.5 H 0.5 PW 12 O 40 (abbreviated as Cs 2.5 -salt) was remarkably more active than H 3 PW 12 O 40 , zeolites, Nafion-H, SO 4 2− /ZrO 2 , H 2 SO 4 , and AlCl 3 HCl. Solid-state 31 P NMR revealed that Cs 2.5 -salt mainly consists of polyanions with one proton (Cs 2 HPW 12 O 40 ) and those without a proton (Cs 3 PW 12 O 40 ). The quantity of acid sites of Cs 2.5 -salt was determined to be 161 μmol g −1 by temperature-programmed desorption (TPD) of NH 3 , which was much greater than the estimated quantity of surface acid sites of H 3 PW 12 O 40 , and less than that of zeolites and SO 4 2− /ZrO 2 . The acid strength of Cs 2.5 -salt measured by Hammett indicators and TPD of NH 3 was smaller than that of SO 4 2− /ZrO 2 . It is presumed that the high activity of Cs 2.5 -salt is brought about by its large number of surface acid sites and its acid—base bifunctional nature, in which a caesium ion possibly increases the basicity of the heteropoly anion.

Catalysis by heteropoly compounds. VI. The role of the bulk acid sites in catalytic reactions over NaxH3 − xPW12O40

Abstract Several acid-catalyzed reactions were investigated over partial Na salts of 12-tungstophosphoric acid (Na x H 3 − x PW 12 O 40 ). Thermal desorption of pyridine absorbed in the bulk combined with in-frared measurement demonstrated that the Na salts were purely protonic acids and their “bulk” acidity (strength and amount) decreased as the Na content increased. On the basis of the absorptivity of reactants into the bulk of catalysts and the activity patterns for several acid-catalyzed reactions, it has been proposed that catalytic reactions are divided into “bulk-type” and “surface-type” reactions. In the case of the “bulk-type” reactions such as dehydration of 2-propanol or conversion of methanol to hydrocarbons, the reactants were readily absorbed in the bulk and the catalytic activities for these reactions were well correlated with the bulk acidity of the Na salts measured by the thermal desorption of pyridine. It was very probable that these reactions proceeded mainly in the bulk. On the other hand, reactions of hydrocarbons like butene and cumene, which were adsorbed only on the surface, take place only on the surface (“surface-type” reactions).

Catalysis by hetropoly compounds. Part XXVI. Gas phase synthesis of methyl tert-butyl ether over heteropolyacids

Abstract Gas phase synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutylene has been studied with several heteropolyacids at 303–383 K. It was found that a Dawson-type heteropolyacid, H6P2W18O62, was much active than Keggin-type heteropolyacids, HnXW12O40 (X = P, Si, Ge, B, and Co), and other solid acids such as SO 2− 4 ZrO 2 , SiOue5f8Al2O3 and H-ZSM-5 at 323 K. Since the acid strength of H6P2W18O62 was weaker than H3PW12O40 and H4SiW12O40, factors other than the acid strength are important for the catalytic activity. Pseudoliquid phase behavior was demonstrated for H6P2W18O62 and H3PW12O40 by the measurements of the absorption of methanol during the reaction and by the unique pressure dependencies of the rate of synthesis. From the absorption data (the amount and rate), it is concluded that the high catalytic activity of H6P2W18O62 is brought about by a high-activity state of the pseudoliquid phase in which controlled amounts of molecule are absorbed and the absorption-desorption is rapid. On the other hand, the pseudoliquid phase of H3PW12O40 is in a low-activity state absorbing excessive amounts of molecule.

Catalysis by heteropoly compounds x. Synthesis of lower olefins by conversion of dimethyl ether over 12-tungstophosphates

Conversion of dimethyl ether and methanol into hydrocarbons was studied over several salts of 12-tungstophosphoric acid, H3PW12O40, at 563 K by a flow method. For acid salts of Na, the catalytic activity decreased in parallel with the acidity as the Na content increased (NaxH3−xPW12O40, x = 0–3). However, the carbon number distribution and the olefin to alkane ratio of the product hydrocarbons were almost independent of the Na content. These results indicate that the acidity determines the activity but its effect on the selectivity is not significant. On the other hand, when the salts of organic bases, Cs and NH3, which showed a low ability to absorb dimethyl ether, were used, the selectivity towards lower olefins was much improved. It was concluded that the olefin to alkane ratios were not correlated with the acidity, but with the ability to absorb dimethyl ether; the ratios increased markedly as this ability decreased. This was explained on the basis of “pseudo-liquid” behaviour of heteropoly compounds.

Effect of potassium and phosphorus on the hydrogenation of CO over alumina-supported ruthenium catalyst

Abstract The effects of additives such as K, B, and P to Ru Al 2 O 3 catalysts were investigated with regard to the adsortion and the catalytic hydrogenation of CO. Thermal desorption of CO and infrared bands of CO adsorbed indicated that the addition of K increased the strength of CO adsorption, by enhancing the back-donation from Ru to CO. As for the catalytic hydrogenation of CO, the addition of K significantly increased the olefin/paraffin ratios in product hydrocarbons and suppressed the methane formation, while the catalytic activity decreased by the addition. On the other hand, the addition of P or B brought about opposite changes in both CO adsorption and CO hydrogenation. The relationships between the electronic state of Ru and the activity and selectivity for this reaction have been interpreted on the basis of the inhibiting effect of strongly adsorbed CO for the hydrogenation.

High catalytic activity of an insoluble acidic caesium salt of dodecatungstophosphoric acid for liquid-phase alkylation

The use of solid acid catalysts is often desirable for liquid-phase reactions [1] in the place of homogeneous catalysts such as H2SO4 [2] and HA12CI: [2,3]. Among the various solid acids that were tested for liquid-phase alkylation of aromatic hydrocarbons [4-8], perfluorinated resinsulfonic acid (Nation-H) has been reported as being a good catalyst [4,5]. The authors have previously found that solid heteropoly compounds, H3PW1204o and their acidic caesium salt, where highly active catalysts for the dehydration of alcohols in the gas phase [9-12] and for the decomposition of ester in the liquid phase [ 13 ]. Here we wish to report on the high activities of Ho.sCs2.~PW12040 for the alkylation of aromatic compounds with cyclohexene. This catalyst was found to be much more active than HY, H-ZSM-5, and Nafion-H. Ho..~Cs2.sPW1204o (abbreviated as Cs2.5-salt) was obtained as a precipitate by the titration of an aqueous solution of H3PWI2QO (Nippon Inorganic Colour and Chemicals) with an aqueous solution of Cs2CO3 as described before [9-13 ]. Carbon-supported H3PW~204o [14] was prepared with an activated carbon (Wako Pure Chemicals) and an aqueous solution of H3PW~204o. The amount of H3PWI204o loaded was 18 wt.-%. This is denoted by H3PW~204o/ AC. SiO2-A12Os (SA-1, Catalysts and Chemicals Industries; A1203 : 13 wt.-% ), Nafion-H (NR-50, Mitsui Toatsu Fine Chemicals), H-ZSM-5 (25H, Mobil Catalysts Company of Japan), HY-zeolite (prepared from SK-40, Linde), aluminium exchanged montmorillonite (prepared from Kunipia-F, Kunimine Kogyo), H2SO4-treated ZrO~ (abbreviated as SO42-/ZrO2) [15,16], and HA12C17 [3] were used as reference catalysts.

In situ diffuse reflectance IR of catalytic reduction of nitrogen oxides by propene in the presence of oxygen over silica-supported platinum

Abstract The reaction mechanism of catalytic reduction of nitrogen oxides (NO or NO 2 ) by propene in the presence of O 2 has been studied by means of in situ diffuse reflectance IR, combined with transient response analysis. Three surface species, organic nitro ( I ) (1565 cm −1 ), carbonyl ( II ) (1740 cm −1 ), and isocyanate species ( III ) (2174 cm −1 ) were the main species detected during the reaction. Organic nitro ( I ) and isocyanate species ( III ) were highly reactive toward NO 2 and O 2 to form N 2 and N 2 O, suggesting that these are the intermediates.

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.

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.

Catalysis by heteropoly compounds. IX: Role of water in catalytic dehydration of 2-propanol over copper salts of H3PW12O40

Abstract The catalytic and acidic properties of copper salts of 12-tungstophosphoric acid have been studied. An induction period was observed in the dehydration of 2-propanol and it became longer as the water content decreased. In addition the catalytic activity was enhanced by the addition of water to the feed gas. This catalytic behavior has been explained on the basis of the “pseudo-liquid phase” model as follows. The absorption of 2-propanol into the bulk of the copper salts was accelerated by the presence of water. Thus, the increase of the rate is ascribed to the expansion of the effective reaction zone caused by water-promoted absorption of 2-propanol. Infrared spectroscopy of adsorbed pyridine revealed that the copper salts were strong protonic acids, and the amounts of proton were about one-third that of the acid form. Based on the infrared data of absorbed pyridine and electron spin resonance of Cu 2+ , the mechanism for the generation of acidity is discussed.