A. Micek-Ilnicka

Oxide supports for 12-tungstosilicic acid catalysts in gas phase synthesis of MTBE

Abstract A series of simple and mixed oxides: SiO 2 , AlPO 4 , SiO 2 -Al 2 O 3 , TiO 2 , kaolin and γ-Al 2 O 3 differing by their basicities as characterised by effective negative charge on oxygen calculated according to Sanderson’s theorem was used for the preparation of supported H 4 SiW 12 O 40 catalysts. Using gas phase synthesis of MTBE as the catalytic test reaction it has been shown that on the supports of highest basicity, i.e. γ-Al 2 O 3 and kaolin the heteropolyacid was decomposed resulting in the catalyst which exhibited only low activity. On the other hand, the least basic oxide SiO 2 gave the most active catalysts. However, the sequence of activities differed from that of basicities by the fact that the activity of TiO 2 -supported catalyst was at the coverage with H 4 SiW 12 O 40 Θ =0.25 nearly as high as SiO 2 supported one and at Θ =1.0 TiO 2 -supported catalyst was the best. This indicates that the basicity of the support is only one of the factors determining the activity of supported heteropolyacid catalysts in acid–base type reactions. On the other hand good correlation was obtained between the catalytic activity and the neutralisation heat of the acid sites on the surface of catalyst which was determined using thermometric titration with n -butylamine solution in toluene.

FTIR study of hydration of dodecatungstosilicic acid

The dehydration of H4SiW12 O40·15.6 H2O was studied in situ in the IR chamber. On evacuation at room temperature the departure of most loosely bonded water characterized by bands at 3550 and 1616 cm−1 was observed. In the remaining hexahydrate the band at 3445 cm−1 was ascribed to the hydrogen bond between the Od oxygen atom of the Keggin unit and dioxonium H5O2+ ion, the presence of which is manifested by the 1710 and 1100 cm−1 vibrations. All these bands vanish in the case of anhydrous H4SiW12O40, in which the band at 3106 cm−1 ascribed to the hydrogen bond between neighbouring HPA anions Od−H+−Oc is still present. The dehydration of hexahydrate is accompanied by splitting of the W=Od band into 987 and 1010 cm−1 reflecting the change of the kind of hydrogen bond in which the Od oxygen atom is involved. Based on the above results it was concluded that protons forming oxonium ions in hydrated solid heteropoly acid are more strongly bonded than those in anhydrous one which are forming hydrogen bonds between neighbouring Keggin units.

The formation of MTBE on supported and unsupported H4SiW12O40

Abstract Sulphonated resins used in the industry as the catalysts in the synthesis of methyl tert -butyl ether (MTBE) until now, the most important anti-knocking additive to the gasoline, are not satisfactory because of environmental reasons and catalyst’s not adequate stability. This stimulates the search for new catalysts for this process. Among others heteropoly acids (HPA) are considered to be prospective ones. The present paper is the review of the authors’ investigations on physical chemistry of the MTBE synthesis in gas phase on dodecatungstosilicic acid H 4 SiW 12 O 40 (HSiW). It has been preceded by the study of the interaction of both substrates, methanol and iso -butylene and the product MTBE, with crystalline HSiW comprising classical sorption experiments, FTIR and calorimetric measurements. Together with reaction kinetic measurements it enabled to propose the mechanism of the catalytic reaction according to which iso -butylene not penetrating the bulk of HSiW crystallites gets protonated at the surface with the loosely bonded protons (protons forming hydrogen bond between HSiW anions) and subsequently reacts with methanol from next-to-surface layer or from the gas phase. Methanol penetrating into the bulk is forming protonated clusters thus diminishing the concentration of loosely bonded protons. At high enough methanol concentration this leads to the diminishing of the formation rate of carbocation. In the result reaction order positive at the initial state of catalytic process (low bulk concentration of methanol and high concentration of loose protons) turns into negative at the reaction steady state. A set of kinetic equations has been proposed describing all these processes. A series of oxide supports: SiO 2 , TiO 2 , SiO 2 -Al 2 O 3 , γ-Al 2 O 3 and AlPO 4 with increasing basicity (characterised by the net oxygen negative charge calculated according to Sanderson’s theorem) was used for the preparation of HSiW supported catalysts. It has been shown that the activity decreases with the increasing basicity as a result of strong bonding protons by the surface of the support.

Gas phase synthesis of MTBE on dodecatungstosilicic acid as the catalyst

Abstract Catalytic synthesis of methyl-tert-butyl ether (MTBE) in gas phase on solid H4SiW12O40 was studied at 40 and 80°C using differential constant flow reactor. Independently sorption of substrates, methanol and isobutene, as well as the product MTBE was studied using sorption balance. From the fact that methanol was easily sorbed by the whole volume of the solid (depending on pressure its uptake at 40°C reached up to 12 CH3OH molecules per 1 Keggin unit (KU)) while isobutene remained only adsorbed at the surface of heteropolyacid both pristine and saturated with methanol it was concluded that catalytic reaction occurs at the surface of H4SiW12O40 and reaction scheme has been proposed including the formation of tert-butyl carbenium ion with the participation of protons supplied by the catalyst. The assumption that reaction of this carbocation with methanol supplied also from the bulk or next-to-surface layer is the rate determining step led to the kinetic equations enabling to interpret the observed negative reaction order with respect to methanol at the steady state at 40°C but about one at initial period of the run. Reaction was characterised by the apparent activation energy as low as 25 kJ mol−1. At 80°C reaction order with respect to isobutene was one but that with respect to methanol decreased to about 0.5 indicating that reaction was diffusion controlled.

Heteropolyacid encapsulation into the MOF: influence of acid particles distribution on ethanol conversion in hybrid nanomaterials

Hybrid nanomaterials comprising tungsten heteropolyacid (HPW) and iron-based MOF Basolite™ F 300 used as a support were obtained by post-synthesis combination of the two components. Samples with increasing HPW loading (20, 50, and 80 wt%) were characterised by appropriate physicochemical methods, including nitrogen sorption, electron microscopy imaging, FT-IR spectroscopy, sorption microbalance and catalytic tests. The acidic protons of the tungsten-based heteropolyacid catalysed ethanol conversion depending on location of HPW at either internal or external surfaces of the commercial Basolite™ F 300.

Calorimetric study of ammonia sorption on H4SiW12O40

Using a Microscal flow microcalorimeter, integral sorption and desorption enthalpies of ammonia on silicotungstic acid H4SiW12O40 dehydrated at 433 K and 483 K were determined. The quantitative coverage determination enabled us to estimate the amounts of the irreversibly as well as (much smaller) reversibly bonded ammonia at its equilibrium state in the gas phase (pNH3 = 1.01 kPa). It has been shown that irreversible sorption at 373 K results in the formation of NH4+ ions while at 298 K two NH3 molecules are adsorbed per one proton in heteropolyacids. This suggests the formation of an NH4+·NH3 adduct which is also present in N2H7I, the only known solid salt of a diammonium ion.

Sorption of methanol on tungstosilicic acid

The sorption of methanol by anhydrous H4SiW12O40 (HPA) was investigated using classical sorption investigations, thermometric titration, FTIR spectroscopy and DTA and TG analyses. Methanol is sorbed by the whole volume of HPA crystallites. Sorption is governed by diffusion laws and is partially reversible. The existence of a series of methanolates, H4SiW12O40·4CH3OH, H4SiW12O40·12CH3OH and H4SiW12O40·16CH3OH has been stated. The FTIR investigation showed that in anhydrous HPA, anions [Keggin units (KU)] are bonded by hydrogen bonds between neighbouring KUs and in methanolates the bonds joining KUs are due to the interactions between protonated methanol clusters and KUs. The relatively high enthalpy of formation of methoxylate ion, CH3OH2+ (58.9 kJ mol-1), shows that protons are more loosely bonded in anhydrous HPA than in methanolate. The latter fact is in good accordance with the model of catalytic synthesis of methyl tert-butyl ether formation assuming different activities of protons depending on the amount of methanol in the quasi-liquid phase.

The role of protons in acid–base type reactions on heteropolyacid catalysts: gas-phase MTBE synthesis on H4SiW12O40

Basing on the kinetic and sorption experiments a mechanism of catalytic formation of methyl-tert-butyl ether (MTBE) on H4SiW12O40 was proposed. A reaction intermediate, carbenium ion C4H9+, is forming at the catalyst surface from isobutene supplied from gas phase and protons supplied by the solid. Subsequently C4H9+ reacts with methanol absorbed by the solid heteropolyacid. The model satisfactorily explains the anomalous reaction order with respect to methanol which under the applied conditions is positive at the initial stage of the catalytic process and becomes negative at its steady state.

Characterization of Acid Catalysts by Thermometric Titration

Thermometric titration as a method suitable for the characterization of the acidity of catalysts is presented. The determination of the acid strength, the end point of titration and the total heat of neutralization by thermometric titration is tested using samples of benzoic acid, zeolite NaHY and unsupported and supported (TiO2, SiO2, SiO2· Al2O3) tungstosilicic acid. The determination of the end point of titration acidity is discussed. The method is recommended for the determination of the total neutralization enthalpy as well as the enthalpy of the neutralization of the strongest acid centers in solid catalysts.

Thermal stability of giant wheel type polyoxomolybdates and their derivatives

The paper compares the thermal behaviour of hydrated giant ring polyoxomolybdates: sodium and ammonium salts as well as sodium polyoxomolybdates containing methanol. Two stages of dehydration are observed: release of water of crystallization reaching its maximum rate at about 70 °C and release of water of constitution above 280 °C leading to a collapse of the ring structure and formation of a new solid phase.