Wacław Makowski

Nickel doped hydrotalcites as catalyst precursors for the partial oxidation of light paraffins

Abstract A series of Ni, Mg and Al containing hydrotalcite-type precursors was synthesised (Ni X Mg 72− X Al 28 , X =10–72) by coprecipitation at low supersaturation at pH 8±0.2 at 60–70°C. Their calcination at 900°C in air resulted in the formation of solids consisting of spinel and divalent metal oxide phases. The catalyst activation was studied by temperature programmed reduction (TPR). Ni 2+ in the divalent metal oxide phase could be reduced in all samples, with the reduction temperatures increasing with decreasing Ni content, but only in the sample without Mg the Ni 2+ in the spinel phase were reducible. The Ni dispersion determined from the TPR results was higher for the catalysts with a lower Ni content and decreased for higher Ni contents. Catalytic tests were carried out at 700°C and at 800°C for 240 min using a C 3 H 8 /O 2 /He-mixture at a gas hourly space velocity (GHSV) of 900 dm 3 g −1 h −1 . High values for the conversion of C 3 H 8 (≥73%) as well as for the selectivities to CO (≥88%) and H 2 (≥69%) were observed for all the catalysts at both reaction temperatures. At lower temperatures and with higher Ni content, these values were lower, whereas the conversion of O 2 was found to be almost complete in all experiments. The carbon content of the spent catalysts was determined by temperature programmed combustion (TPC) using thermogravimetry (TG). The tendency towards carbon formation was found to be higher at lower temperatures and for higher Ni contents. On the basis of activity, selectivity and resistance against coke formation, the Ni containing catalyst with the lowest Ni content was found to be the best catalyst.

Activity enhancement of zeolite MCM-22 by interlayer expansion enabling higher Ce loading and room temperature CO oxidation

Layered zeolite precursor MCM-22P can be stabilized as interlayer expanded structure MWW-IEZ with enlarged pores by silylation producing –O–Si(OH)2–O– bridges. It adsorbs two times more cerium than MCM-22 and becomes activated for CO oxidation to CO2 at room temperature. This represents one of the most notable activity enhancements upon modification of layered zeolites.

Active state of model cobalt foil catalyst studied by SEM, TPR/TPO, XPS and TG

Four states of the cobalt foil catalyst, corresponding to different redox treatment and activity, were defined: oxidised, reduced, active and deactivated. They were investigated by scanning electron microscopy (SEM), temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS) and thermogravimetric (TG) methods and in the hydrogenation of ethylene used as a test reaction. Particular emphasis was laid on the study of the active state, achieved after the catalyst reduction at moderate temperatures. It was shown that the catalyst preactivated by a series of redox cycles is built of a cobalt oxide layer of a characteristic size and dispersion, which is stuck to the metallic bulk. Reduction at a moderate temperature, prolonged even to several hours, converts only a small fraction of the oxide layer into metallic Co. XPS, TPR and TPO methods distinguished various states of oxygen and cobalt on the surface of the activated or partially activated samples. The results were interpreted in terms of the mechanism of autocatalytic reduction. The deactivation was associated with the structural reconstruction of the surface, taking place either in the reaction mixture during the hydrogenation of ethylene or in hydrogen atmosphere. Formation of the inactive carbon deposit was experimentally excluded.

Catalytic dehydration of ethanol over hierarchical ZSM-5 zeolites: studies of their acidity and porosity properties

The catalytic activity of novel micro/mesoporous ZSM-5 in the dehydration process of alcohols has been studied with respect to their acidity and porosity. Samples characterized by similar Si/Al ratios in the range between 23 and 36 were chosen to ensure the ability to properly monitor the changes in catalytic performance related to the concentration of acid sites, as well as their accessibility and strength. Special attention has been paid to porosity studies involving low-temperature nitrogen sorption and the quasi-equilibrated temperature programmed desorption of hydrocarbons. Thus, the wide range of materials from typically microporous ZSM-5 to micro/mesoporous analogues obtained by different methods, but presenting many similar properties, allowed us to investigate the influence of porosity character on catalytic performance. Both the acidity and porosity properties, which were significantly perturbed by various synthesis and modification strategies, were clearly reflected in the catalytic performance of hierarchical zeolites, described as volcano curve dependence.

Fast and slow methanation pathways in hydrogenation of carbon dioxide on FeMn oxide catalysts

Abstract The hydrogenation Of CO2 on FeMn oxide catalysts (pure or doped with Rh and La) was investigated in pulse and continuous flow experiments. The maximum methanation rate observed in the short initial period of reaction on freshly reduced catalyst was attributed to the fast reaction pathway, connected with reactive chemisorption of CO2. The slow methanation pathway was related to the formation of carbonaceous deposits on the catalysts (observed even in pulse experiments). A correlation between maximum methanation rate and the content of the reactive deposit in continuous flow was established. Another, less reactive deposit, which required a higher temperature to be hydrogenated, was also observed. The effect of Rh and La on activity and selectivity of the catalysts was investigated. Rh enhances fast methanation while La substantially decreases rate of both reaction pathways. The same effects were observed during hydrogenation of carbon deposits.

Framework-substituted cerium MCM-22 zeolite and its interlayer expanded derivative MWW-IEZ

Framework-substituted cerium MCM-22 zeolites were synthesized with both TEOS and solid silica sources. The Si/Al ratio in the gel was 15/1 with two different Si/Ce ratios equal to 100/1 and 30/1. The products showed high Bronsted acid site concentrations, above 700 μmol g−1, that diminished slightly as the Ce content increased. The state of Ce was evaluated by IR and UV-vis spectroscopy. There was no indication of a separate Ce phase and extraframework atoms. Both oxidation states, 3+ and 4+, were present. In contrast to the interlayer expanded form MCM-22-IEZ with Ce exchanged into it, the present MCM-22 did not show oxidation of adsorbed CO. This indicates fundamental differences in the location of Ce, which is outside the framework in the exchanged form. The framework-substituted Ce-MCM-22 became active for CO oxidation after its conversion into the IEZ form and additional exchange of Ce. Textural properties were evaluated by nitrogen adsorption and quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) of hydrocarbons.

Application of quasi-equilibrated thermodesorption of hexane and cyclohexane for characterization of porosity of zeolites and ordered mesoporous silicas

Quasi equilibrated temperature programmed desorption and adsorption (QE-TPDA) of hexane and cyclohexane was applied for characterization of zeolites 5A, ZSM-5, 13X, Y, NaMOR and ordered mesoporous silicas MCM-41, MCM-41/TMB, SBA-15 and HMS. Similar QE-TPDA profiles of hexane and cyclohexane with a single desorption maximum were observed for the wide pore zeolites. No adsorption of cyclohexane for zeolite 5A and a single desorption maximum for ZSM-5 were found, while two-step desorption profiles of hexane were observed for these zeolites. Similar values of the adsorption enthalpy and entropy of hexane and cyclohexane were obtained by fitting the Langmuir model functions for the zeolites X and Y. For NaMOR and ZSM-5 larger differences in these parameters were found. A single desorption peak found at low temperatures in the QE-TPDA profiles of hexane and cyclohexane for the studied silicas was attributed to the multilayered adsorption on their mesopore surface. The adsorption isobars calculated from the thermodesorption profiles were fitted with the BET function. This way values of the specific surface area and the adsorption heat were calculated. Additionally values of the initial heat of adsorption were found by fitting the Henry’s law to the high-temperature sections of the linearized isobars. The largest deviations from the BET and Henry functions and the largest values of the adsorption heats found for SBA-15 indicated the greatest heterogeneity of the adsorption sites on its surface.

Water-Stable Metal–Organic Framework with Three Hydrogen-Bond Acceptors: Versatile Theoretical and Experimental Insights into Adsorption Ability and Thermo-Hydrolytic Stability

A new microporous cadmium metal-organic framework was synthesized both mechanochemically and in solution by using a sulfonyl-functionalized dicarboxylate linker and an acylhydrazone colinker. The three-dimensional framework is highly stable upon heating to 300 °C as well as in aqueous solutions at elevated temperatures or acidic conditions. The thermally activated material exhibits steep water vapor uptake at low relative pressures at 298 K and excellent recyclability up to 260 °C as confirmed by both quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) method as well as adsorption isotherm measurements. Reversible isotherms and hysteretic isobars recorded for the desorption-adsorption cycles indicate the maximum uptake of 0.19 g/g (at 298 K, up to p/p0 = 1) or 0.18 g/g (at 1 bar, within 295-375 K range), respectively. The experimental isosteric heat of adsorption (48.9 kJ/mol) indicates noncoordinative interactions of water molecules with the framework. Exchange of the solvent molecules in the as-made material with water, performed in the single-crystal to single-crystal manner, allows direct comparison of both X-ray crystal structures. The single-crystal X-ray diffraction for the water-loaded framework demonstrates the orientation of water clusters in the framework cavities and reveals their strong hydrogen bonding with sulfonyl, acyl, and carboxylate groups of the two linkers. The grand canonical Monte Carlo (GCMC) simulations of H2O adsorption corroborate the experimental findings and reveal preferable locations of guest molecules in the framework voids at various pressures. Additionally, both experimental and GCMC simulation insights into the adsorption of CO2 (at 195 K) on the activated framework are presented.

TPR and TPD studies of vanadia/silica catalysts for selective oxidation of methane to formaldehyde

High selectivity of V2O5/SiO2 catalysts prepared from fumed silica, observed in oxidation of CH4 to HCHO at 550-590°C, was attributed to the content of highly dispersed, easily reducible surface vanadia species.

Stability and Reactivity of the Active Assemblies in Modified Graphites Characterized by TPD and TPH

Synthetic graphites, as prepared and modified by sulphonation or acetylation, were doped by Fe-, Co-, Ni- and Ca-nitrates. Temperature Programmed Desorption (TPD) and Temperature Programmed Hydrogenation (TPH) were applied to characterize thermal stability and reactivity of the active assemblies formed on the matrix surface. The functional groups lowered the reactivity of the graphites. Thermally less stabile carboxylic groups decomposed with formation of secondary groups giving more reactive material. Fe containing as well as sulphonated graphites showed a much lower reactivity than the others. Synergistic effects of Co/Ca and Ni/Ca were confirmed in the graphite materials.