Aline Auroux

Infrared, Microcalorimetric, and Electron Spin Resonance Investigations of the Acidic Properties of the H-ZSM-5 Zeolite

The infrared spectra of the H-ZSM-5 zeolite calcined at temperatures up to 1173 K, and the corresponding electron spin resonance and microcalorimetric data are discussed. Two types of hydroxyl groups are characterized by absorption bands at 3720 and 3605 cm −1 , a small shoulder being present at 3665 cm −1 . Infrared spectra were also recorded after pyridine adsorption, showing the presence of Bronsted acid sites and Lewis acid sites. After calcination at increasing temperatures, dehydroxylation of the zeolite is observed: above 675 K, the number of Bronsted acid sites decreases, while that of strong Lewis acid sites increases; however, a small dealumination occurs as shown by chemical analysis measurements and XPS data. Moreover, dehydroxylation enhances the constraint character of this zeolite, as observed by pyridine inability to titrate the total Lewis sites. Electron spin resonance studies of trapped hydrogen atoms, of adsorbed NO, and of adsorbed benzene radical cations formed on H-ZSM-5 at various calcination temperatures are discussed in terms of the number and strength of the acid sites. It is suggested that the acid sites which are present are very similar to those of H-mordenite although slightly stronger. A microcalorimetric study of ammonia adsorption confirms the very strong acidic character of the acid sites and shows their dependence in strength and heterogeneity upon calcination temperature.

Methanol conversion on acidic ZSM-5, offretite, and mordenite zeolites: A comparative study of the formation and stability of coke deposits

The deposition of carbonaceous residues, leading to aging and modifications in the acidic properties of three zeolite samples differing by the size and shape of their interconnecting channel networks (ZSM-5, offretite, and mordenite), has been investigated during the conversion reaction of methanol to hydrocarbons. Catalytic tests, thermogravimetry, and microcalorimetry are used as complementary techniques. For zeolite ZSM-5, it is observed that carbonaceous residues are primarily formed on the outer surface of the crystallites, resulting only in a slight modification of its molecular shape-selective properties and producing a high resistance to aging. For offretite and mordenite, by contrast, the channels are large enough to accommodate carbonaceous residues, which leads to a drastic loss of catalytic activity and a very fast aging. The intimate nature of the channel network is shown to play a determining role in the coke formation and in the ease of removal of carbonaceous deposit. The role played by the shape-selective properties of the zeolites on their behavior towards coking and aging and also the effect of coking on the availability of Bronsted acidic sites have been particularly emphasized.

Catalytic and physical properties of phosphorus-modified ZSM-5 zeolite

H-ZSM-5 zeolite modified by phosphorus (1.1 and 2.0 wt%) was studied by means of infrared spectroscopy and microcalorimetry for acidity characterization, by ESCA for determining elementary composition in the outer layers (depth ∼- 50 A) with respect to the bulk (φ ∼- several μm), and lastly by catalytic reaction analysis of methanol conversion. It was found that phosphorus neutralizes acidic sites primarily at the entrance of the channels of the zeolite particles in the same manner as do carbonaceous residues after some time of reaction. However, the strongest acid sites remain unmodified, which leads to the suggestion that the aluminium distribution and subsequently the acid site strength distribution along the zeolite channels is heterogeneous. Phosphorus-modified zeolites gave a higher yield of light olefins (C2=C4=) and subsequently a smaller yield of saturated aliphatics and aromatics than the parent zeolite in the methanol conversion reaction. The phosphorus-modified zeolite gave similar yields of the meta + para isomers of xylene and of ethyltoluene and a smaller yield of the ortho isomers. Lastly the yield of heavier aromatics A9+ was greatly decreased by the addition of phosphorus. The changes in selectivity were tentatively assigned to slight modifications in channel size and to increased tortuosity due to phosphorus compounds bonding to the zeolite framework, rather than to changes in acid strength.

Study of the acidic character of modified metal oxide surfaces using the test of isopropanol decomposition

The evolution of the acid/base properties of a series of oxide supports (alumina, magnesia and silica) modified by increasing loadings of additive ions (Li+, Ni2+, and SO42-) from 1 to 50% of the support surface coverage is reported using the catalytic test of isopropanol decomposition, studied as a function of the reaction temperature. The calculated kinetic parameters Ea, A, and ΔS≠ permit interpretation of the reaction mechanism with relation to the acidity/basicity of the modified surfaces. The series of alumina oxides, due to the amphoteric properties of the surfaces, decomposed isopropanol through an E2 mechanism leading to propene and di-isopropyl ether formation. The selectivity to the two products was dependent on the strength of the basicity (addition of lithium and nickel) or on the acidity (addition of sulfate) of the surfaces. Magnesia series oxides dehydrated isopropanol through an E1b mechanism due to the presence of very strong basic surfaces possessing some weak acid sites. The very weak amphoteric character of silica was strengthened by the loading of the three additives; the modified silica surfaces displayed enhanced decomposition activity with respect to pure silica.

Effect of particle size on the reactivity of oxygen-adsorbed platinum supported on alumina

Abstract A Pt/Al2O3 catalyst was aged in a methane-oxygen-nitrogen mixture at 873 K with an oxygen-to-methane ratio of 4. The metal particle size increased from 2 to 12 nm. The catalytic activity for methane oxidation was slightly increased for reaction temperature below 700 K and not modified at higher temperatures. As a consequence, the turnover number (i.e., the activity per surface platinum atom) is much higher on the large platinum particles. Microcalorimetric measurements showed a decrease in the heat of oxygen chemisorption when the platinum particle size increased. Titration of chemisorbed oxygen with gaseous hydrogen at 195 K indicated changes in the adsorbed oxygen reactivity: the reactivity of chemisorbed oxygen increased with increasing metal particle size. Changes in the strength of the PtS O bond are proposed to take into account the increase in turnover number that accompanies a decrease in the dispersion of the metallic phase.

Role of surface acidity on vanadia/silica catalysts used in the oxidative dehydrogenation of ethane

Abstract The oxidative dehydrogenation of ethane in ethene has been investigated in the 753–863 K temperature range over V 2 O 5 /SiO 2 catalysts prepared by grafting or wet impregnation methods. The acidic features of fresh and used catalysts were studied by ammonia and pyridine adsorption using microcalorimetry and DRIFT spectroscopy. V 2 O 5 /SiO 2 catalysts can be regarded as medium acidic materials. At the same conversion level of ethane, selectivity to ethene was favored by low V 2 O 5 loadings. Ethane conversion increased with the vanadium content and the ammonia uptake of the V 2 O 5 /SiO 2 catalysts. Some Bronsted-type sites disappeared in the course of the catalytic reaction, leading to minor changes in the acidic character of the catalysts.

Microcalorimetry Methods to Study the Acidity and Reactivity of Zeolites, Pillared Clays and Mesoporous Materials

This short review summarizes some of the most recently published results concerning the applications of microcalorimetry to the study of the acid/base sites of zeolites, pillared clays and mesoporous materials. The efficiency of thermal analysis techniques for the characterization of the acid/base strength of zeolite materials and their ability to provide information consistent with the catalytic data are discussed.

Properties of boron-substituted ZSM-5 and ZSM-11 zeolites

Using NMR and IR spectroscopy boron-substituted pentasil zeolites are found to contain four-coordinated B in the hydrated state and three-coordinated B in the dehydrated state. In addition a new tetrahedral B site assigned to (HO)2B(H2O)(OSi) was characterized by its NMR quadrupole coupling constant and asymmetry parameter. Ammonia adsorption microcalorimetry gave heats of adsorption of ~65 kJ/mol for HBZSM-11 and showed that B-substituted pentasils have only very weak acidity. Calcination at 800 °C increased the heat of NH3 adsorption to ~170 kJ/mol by creation of strong Lewis acid sites. The lack of strong Bronsted acid sites in HBZSM-11 was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation With methanol.

Nature and strength of acid sites in HY zeolites: a multitechnical approach

Abstract The combination of several physico-chemical techniques such as temperature programmed desorption, microcalorimetry, infrared spectroscopy and photoelectron spectroscopy, together with structural data (by XRD and NMR), has allowed a detailed study of the acidity of commercial HY faujasites, both non-dealuminated and dealuminated by hydrothermal treatment. These techniques have been used with different basic probe molecules such as ammonia, pyridine and 2,6-lutidine (2,6-dimethylpyridine). They have demonstrated the existence of three different regions of acid strength. The weak sites associated to NH 3 adsorption heats between 70 and 130 kJ mol −1 are of Lewis and Bronsted types, the former being predominent in the dealuminated zeolites. The sites of intermediary strength are essentially the framework bridged hydroxyl groups (heats between 130 and 150 kJ mol −1 ). The strong sites (heats above 160 kJ mol −1 ) contain a significant number of extra-framework Lewis-type sites, but also Bronsted sites. The strength of these sites seems to be due to mutual interactions between Lewis and Bronsted sites. The use of 2,6-lutidine has furthermore allowed us to differentiate between two different populations of Lewis sites.

The aldol condensation of lower aldehydes over MFI zeolites with different acidic properties

Various MFI type zeolites were synthesized taking into account two factors affecting zeolitic acidity “by synthesis”, namely the nature of the substituted heteroelement and the Si/Me3+ ratio. The surface properties with respect to chemical composition and acidity were investigated by temperature-programmed reduction, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia, Fourier transform infrared spectroscopy and microcalorimetry. Vapor phase aldol condensation of acetaldehyde and formaldehyde over these catalysts has been carried out, and it has been observed that the structural features of the catalysts do play an important role in controlling conversion and selectivity. A reasonable acidity and activity relationship is established. A mechanism for cross-aldol condensation involving both Bronsted and Lewis acid sites is proposed.