Applications of adsorption microcalorimetry in heterogeneous catalysis

Novel heterogeneous bi-functional catalysts bearing tin or zinc inserted as single sites within the silica architecture acting as acid centres and decorated with imidazolium moieties as the nucleophile source were successfully synthesized. The materials were extensively characterized via various techniques including N2 physisorption, solid state nuclear magnetic resonance, X-ray photoelectron spectroscopy, transmission electron microscopy and adsorption microcalorimetry. The solids were tested as catalysts for the conversion of carbon dioxide, selecting the synthesis of styrene carbonate as the target reaction. Both materials exhibited improved performances compared to the analogous solids functionalized with the sole imidazolium salt as well as to other materials reported in the literature. The Sn-based catalyst displayed excellent conversion also in the presence of various epoxides. In all experiments the bi-functional solid allowed reducing the reaction temperature below 150 °C. In the presence of glycidol the temperature was decreased down to 30 °C. The short synthesis protocol of the heterogeneous catalysts, together with the 100% atom economy of the target reaction and the low reaction temperature, make the entire process highly sustainable. Moreover, the Sn-based catalyst was stable under the selected reaction conditions and reusable for multiple catalytic cycles.

Applications of Adsorption Microcalorimetry to the Study of Heterogeneous Catalysis

Adsorption microcalorimetry was applied to determine heats of adsorption of ammonia on zeolites Y, mordenite, ZSM-5, heteropolyacid H3PW12O40, as well as silica gel and amorphous aluminosilicates. The plots of differential heats against coverage served to construct the acidity spectra and, in this way, to determine the number of acid sites with different acidity strengths. The behavior of these materials in acid-catalyzed reactions, primarily, in the transformations of hydrocarbons is discussed. Evidence is presented that heats of adsorption of ammonia can be used to obtain correlation plots that describe relations between acidic and catalytic properties of zeolite catalysts.

Chapter 11 Heterogeneous catalysis on solids

The aim of this thesis is to investigate heterogeneous catalysis for the dehydration of methanol and ethanol at a gas-solid interface over a wide range of solid Bronsted acid catalysts based on Keggin-type heteropoly acids (HPAs), focussing on the formation of dimethyl ether (DME) and diethyl ether (DEE), respectively. The dehydration of methanol to dimethyl ether (DME) was studied over a wide range of bulk and supported HPAs and was compared with the reaction over HZSM-5 zeolites (Si/Al = 10−120). Turnover rates for these catalysts were measured under zero-order reaction conditions. The HPA catalysts were demonstrated to have much higher catalytic activities than the HZSM-5 zeolites. A good correlation between the turnover rates and catalyst acid strengths, represented by the initial enthalpies of ammonia adsorption, was established. This correlation holds for the HPA and HZSM-5 catalysts studied, which indicates that the methanol-to-DME dehydration occurs via the same (or a similar) mechanism with both HPA and HZSM-5 catalysts, and that the turnover rate of methanol dehydration for both catalysts is primarily determined by the strength of catalyst acid sites, regardless of the catalyst pore geometry. Dehydration of ethanol was also studied over a wide range of solid Bronsted acid catalysts based on Keggin-type HPAs in a continuous flow fixed-bed reactor in the temperature range of 90-220 oC. The catalysts included H3PW12O40 (HPW) and H4SiW12O40 (HSiW) supported on SiO2, TiO2, Nb2O5 and ZrO2 with sub-monolayer HPA coverage, as well as bulk acidic Cs salts of HPW (Cs2.5H0.5PW12O40 and Cs2.25H0.75PW12O40) and the corresponding core-shell materials with the same total composition (15%HPW/Cs3PW12O40 and 25%HPW/Cs3PW12O40, respectively) comprising HPW supported on the neutral salt Cs3PW12O40. The ethanol-to-DEE reaction was found to be zero order in ethanol in the range of 1.5-10 kPa ethanol partial pressure. The acid strength of the catalysts was characterised by ammonia adsorption microcalorimetry. A fairly good correlation between the catalyst activity (turnover frequency) and the catalyst acid strength (initial enthalpy of ammonia adsorption) was established, which demonstrates that Bronsted acid sites play an important role in ethanol-to-DEE dehydration over HPA catalysts. The acid strength and the catalytic activity of core-shell catalysts HPW/Cs3PW12O40 did not exceed those of the corresponding bulk Cs salts of HPW with the same total composition, which contradicts the claims in the literature of the superiority of the core-shell HPA catalysts.

Effect of the reduction–preparation method on the surface states and catalytic properties of supported-nickel particles

Layered double hydroxides were prepared by pH-controlled co-precipitation method with a Mg/Al molar ratio between 1.5 and 4.0 and used as precursors for obtaining, through calcination, a series of MgAl mixed oxides, which were used as catalysts for soybean oil transesterification with methanol. The mixed oxide with the highest Mg/Al ratio was doped with potassium for obtaining highly basic catalysts. Three different potassium salts (K2CO3, KNO3, CH3COOK) were used for loading potassium on the support (K loading ca. 3 mass%) by two different techniques, namely conventional incipient wetness impregnation and innovative mechanical milling. All the catalysts were characterized as to their chemical composition, structure and texture by inductively coupled plasma atomic emission spectroscopy, X-ray diffraction and N2 physisorption, respectively. Their basic and acid features were assessed by adsorption microcalorimetry, using CO2 and NH3 as probe molecules, respectively. Catalytic testing was carried out in a slurry batch reactor operated at 343 K and atmospheric pressure. The occurrence of potassium leaching into the liquid phase was checked for the K-doped catalysts. The initial activity of the heterogeneous catalysts was interpreted in terms of surface basicity, by taking into account a possible role of acidity in determining the mechanism.

Structural and acidity analysis of heteropolyacids supported on faujasite zeolite and its effect in the esterification of oleic acid and n-butanol

Efficient and sustainable cellulose conversion into chemicals requires the design of heterogeneous catalysts with controlled acid–base properties, mesoporosity and excellent stability in hydrothermal conditions. To this end, modified zirconia with anchored W, Mo, Ta and Nb oxoanions were prepared by ionic exchange using peroxo precursors of the selected transition metals. Modified zirconia samples were characterized by X-ray diffraction, N2 isotherms, FTIR of pyridine adsorption and microcalorimetry of ammonia adsorption. If the nature of the oxoanion tunes slightly the catalysts Lewis acid strength or density, it influences strongly their mesoporosity with mesopores in the range of 10 nm for ZrTa and ZrNb. Upon Pt loading, all the Pt-modified zirconia are effective to catalyze C–C and C-O cleavage during cellulose hydrogenolysis leading to PG as main short polyols with the co-formation of 2,5-hexanedione over ZrNb and ZrTa. Most important, ZrNb and ZrTa exhibit excellent water tolerance without any leaching nor structure modification. Combined to their mesoporosity, these features make ZrNb and ZrTa, true solid Lewis acids with high potential in the frame of bulky biomass conversion in hot waterierGraphical Abstract

Applications of adsorption microcalorimetry for the characterization of metal-based catalysts

Sulfonic acid-functionalized mesoporous silicas: Microcalorimetric characterization and catalytic performance toward biodiesel synthesis

Comparing the performance of density functionals in describing the adsorption of atoms and small molecules on Ni(111)

Metal oxides such as Nb2O5, Cr2O3, and especially a ZnIICrIII mixed oxide are demonstrated to be highly active and recyclable heterogeneous catalysts for Prins condensation, which provides a clean, high-yielding route for the synthesis of nopol through the condensation of biorenewable β-pinene with paraformaldehyde. ZnCr mixed oxide with an optimum Zn/Cr atomic ratio of 1:6 gave 100 % nopol selectivity at 97 % β-pinene conversion, with the catalyst easily recovered and recycled. The acid properties of Nb2O5 and ZnCr mixed oxide were characterized by the diffuse reflectance IR Fourier transform spectroscopy of adsorbed pyridine and ammonia adsorption microcalorimetry. An appropriate combination of acid–base properties of ZnCr mixed oxide is believed to be responsible for its efficiency.