A. Parmaliana

A characterization study of the surface acidity of solid catalysts by temperature programmed methods

Abstract The surface acidic properties of various solid catalysts (i.e., clay montmorillonite, acid modified smectite clay, HY zeolite, sulphate-promoted ZrO 2 (SO 2− 4 /ZrO 2 ) and Cs-exchanged dodecatungstophosphoric acid (H 0.5 Cs 2.5 PW 12 O 40 )) have been comparatively evaluated by temperature programmed desorption (TPD) measurements of adsorbed NH 3 , C 5 H 5 N and C 6 H 6 . Mathematical analysis of TPD spectra of adsorbed ammonia indicates the presence of weak, medium and strong acid sites in all the catalysts. On the basis of the TPD patterns of NH 3 , C 5 H 5 N and C 6 H 6 , the nature (Bronsted and Lewis) of the acid sites has been ascertained. Catalytic measurements in CH 3 OH dehydration and benzene alkylation indicate that besides the acidic properties, the diffusional phenomena also play a key role on the reactivity of solid acid catalysts. The superior catalytic performance of the H 0.5 Cs 2.5 PW 12 O 40 system in the above reactions has been explained in the light of a pseudo-liquid behaviour.

Selective hydrogenation of acetylene in ethylene feedstocks on Pd catalysts

Abstract Pumice supported palladium catalysts were compared with Pd SiO 2 and Pd Al 2 O 3 in the hydrogenation of acetylene using typical industrial ethylene feedstocks: front-end and tail-end cuts. Pd/pumice catalysts exhibit good activity and excellent selectivity and stability in the title reaction. Their activity/selectivity pattern is controlled by the composition of the reaction mixture. The turnover frequency (TOF) increases, and the apparent activation energy (Ea) decreases, with the H 2 C 2 H 2 ratio, but they are not affected by the C 2 H 2 C 2 H 4 ratio. The selectivity to ethane (SE) does not change with acetylene conversion at low H 2 C 2 H 2 ratio (tail-end cut) and increases at high H 2 C 2 H 2 ratio (front-end cut). No dependence of SE with acetylene conversion was found, at any H 2 C 2 H 2 ratio, on varying the metal dispersion. TOF and SE change with metal loading depending on the reaction mixtures; low metal loaded catalysts gave the best activity/selectivity pattern. Pd/pumice catalysts do not show aging phenomena even when used in drastic conditions (high space velocities and front-end cuts). The catalytic behavior of Pd/pumice catalysts, as a function of the composition of reaction mixture, elucidated by using the isokinetic relationship (IKR) approach shows that the relative amount of hydrogen in the feedstocks plays an important role in activation/deactivation processes which occurs on the metal surface. By considering the presence of different centers on the catalyst and by performing data analysis with a suitable mathematical model, the microscopical mechanism and the role of surface deposits can be explained. A similar reaction mechanism is applicable to the other palladium catalysts, at least in the case of tail-end mixtures.

Magnesia-supported nickel catalysts. I : Factors affecting the structure and morphological properties

Abstract The effects of calcination and reduction temperature on the metal dispersion of MgO-supported Ni in the ranges 400–800°C and 300–800°C, respectively, have been systematically evaluated by hydrogen chemisorption measurements and comparison has been made with a NiOMgO physical mixture. The role of the bulk Ni x Mg (1 − x ) O solid solution in controlling NiO reducibility and the metal dispersion of Ni/MgO has been ascertained by XPS and ISS investigations. A mechanistic model accounting for the nickel oxide dispersion across the MgO support and the consequent formation of the bulk Ni x Mg (1 − x ) O solid solution has been proposed. A linear relationship between the mean particle size and the reduction temperature, the slope of which increases with Ni loading (2.8–18.0 wt%), has been found. A “multilayer arrangement” of Ni precursor on the support is discussed.

Factors controlling the selectivity of V2O5 supported catalysts in the oxidative dehydrogenation of propane

Abstract The surface properties of a series of V 2 O 5 catalysts supported on different oxides (Al 2 O 3 , H–Na/Y zeolite, MgO, SiO 2 , TiO 2 and ZrO 2 ) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH 3 adsorption. In the case of the V 2 O 5 /SiO 2 system TEM images evidenced the presence of V 2 O 5 crystallites, whereas such segregated phase was not observed for the other samples. VO x species resulted widely spread on the surface of Al 2 O 3 , H–Na/Y zeolite, MgO and SiO 2 , whereas on TiO 2 and ZrO 2 they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Bronsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V 2 O 5 /TiO 2 and V 2 O 5 /ZrO 2 catalysts.

Hydrogenation of acetylene in ethylene rich feedstocks: Comparison between palladium catalysts supported on pumice and alumina

Abstract The activity–selectivity patterns of Pd/pumice catalysts are compared with industrial and home prepared Pd/Al 2 O 3 catalysts in the hydrogenation of acetylene in ethylene rich feedstocks (front-end and tail-end cuts). The iso-kinetic relationship (IKR) approach and a new mathematical model, surface site evolution model (SSEM), are employed in this comparison. Pumice and alumina supported Pd catalysts show different metal redox properties. A similar reaction mechanism is adequate to describe the reaction pathway independently by the catalysts and the gas mixtures considered. This mechanism involves the formation of surface polymers during the catalytic reactions and different catalytic sites which are discriminated on the basis of the different steric interactions of the reagents with the surface species. Although the surface mechanism appears to be analogous for all the Pd catalysts considered, industrial Pd/Al 2 O 3 and Pd/pumice catalysts show a different activity–selectivity pattern as a consequence of their different electron density and ensemble size.

How oxide carriers affect the reactivity of V2O5 catalysts in the oxidative dehydrogenation of propane

The catalytic pattern of several oxide carriers (MgO, Al2O3, ZrO2, TiO2, SiO2, HY zeolite) and supported V2O5 (4.7–5.3 wt%) catalysts in the oxidative dehydrogenation of propane to propylene (PODH) has been comparatively investigated. The fundamental role of the oxide support on both reducibility and reactivity of vanadia catalysts has been assessed. A direct relationship between the specific surface activity of oxide carriers and that of vanadia catalysts is discussed. The inverse relationship between the specific activity and the onset temperature of reduction marks the prevailing redox behaviour of V2O5 catalysts in the PODH reaction.

FTIR spectroscopic investigation of the active sites on different types of silica catalysts for methane partial oxidation to formaldehyde

Several commercial silica samples showing different catalytic activities in the partial oxidation of methane (MPO) to formaldehyde have been investigated using FTIR technique. Two IR absorption bands at 893 and 909 cm−1, observed upon dehydroxylation of the silica catalysts and assigned to reactive siloxane sites on the surface (“strained siloxane bridges”), were found to disappear upon heating in methane at high temperature. The catalytic activity increases together with the intensity of the bands due to such “strained” sites in the different SiO2 samples.

The role of Ni2+ diffusion on the reducibility of NiO/MgO system: A combined TRP-XPS study

The progressive dissolution of nickel ions in the lattice of MgO matrix, promoted by the calcination temperature, has been monitored using XPS. The reducibility of Ni/MgO catalysts is affected by the modification of the surface chemical composition. TPR peaks have been correlated with several NiO forms. Both XPS and TPR results indicate a different behaviour pattern for the supported and physically mixed NiO/MgO systems, and this has been attributed to the different available “contact area” between NiO and MgO. The formation of both “NiO rich” and “MgO rich” NiO-MgO solid solutions, with the tendency to evolve towards the “bulk” solid solution, has been invoked to explain the substantial changes in the TPR pattern of the different air calcined systems.

Effect of vanadia loading in propane oxidative dehydrogenation on V2O5/SiO2 catalysts

The influence of V2O5 loading on the catalytic behaviour of V2O5/SiO22 catalysts in the oxidative dehydrogenation of propane to propylene (POD) has been investigated. The different activity-selectivity pattern of low (5 wt%) and highly (>10 wt%) loaded V2O5/SiO2 catalysts is explained in terms of different surface vanadia species.

Effect of the metal oxide loading on the activity of silica supported MoO3 and V2O5 catalysts in the selective partial oxidation of methane

Precipitated silica catalysts loaded with either MoO3 (0.2–4.0 wt%) or V2O5 (0.2–5.3 wt%) have been studied in the selective partial oxidation of methane to formaldehyde with molecular oxygen at 520 °C. The functionality of the SiO2 surface towards the formation of HCHO is significantly promoted by V2O5, while it is depressed by the MoO3.