Abstract: The interaction between carbon monoxide (CO) and Na+ extra-framework cations has been studied at low temperature (80 K) by two adsorption procedures. Besides the classic one, CO adsorption on pyridine precovered NaY zeolite has been investigated and allows new IR spectral features to be assigned. Depending on the different pyridine precoverage level, three distinct C-O stretch vibrational bands of varying intensity and with maxima at 2175, 2163, and 2149 cm-1 have been observed. Beyond the CO equilibrium pressure introduced, the bands' appearance and intensities depend on the amounts of preadsorbed pyridine. At low pyridine coverage, only the classical, prominent band at 2175 cm-1 is present, while increasing pyridine coverage causes the appearance of the two additional bands. Furthermore, at higher pyridine coverage, the prominent band at 2175 cm-1 completely vanishes. After recalling the different spread of experimental results and their assignments, arguments are presented to show that these three distinct C-O stretch vibrational bands result from the formation of weakly bound (CO)(n) complexes, n = 1-3 through a C-end interaction. Moreover, this study provides the first experimental evidence of the formation of tricarbonyl Na+(CO)(3) after CO adsorption at low temperature on NaY. This last result would confirm the three coordinative vacancies of Na+ cations in S-II positions. These carbonyl adducts are stabilized and underwent specific conditions that are discussed.
Abstract: The acido-basicity of Mg-Ni-Al mixed oxides generated from the calcination of layered double hydroxides
(with the atomic ratios Mg/Ni variable and Mg + Ni/Al equal to 2) has been investigated by adsorption
experiments, using FTIR with pyrrole and pyridine as probe molecules and XPS with ammonia and sulfur
dioxide. The spectroscopic data were compared with microcalorimetric data and with the results of catalytic
tests (cyclohexanol conversion). The acidity, comparable to that of alumina, is mainly of Lewis type. It increases
with the nickel content. The FTIR spectra of chemisorbed pyrrole indicate the presence of strong, medium
and weak basic sites. The bulk basicity of trimetallic oxides is stronger than that of the bimetallic ones. The
effective basicity of the bimetallic Mg-Al oxide is minimized by the presence of numerous remaining carbonate
ions. The XPS analyses show that the surface basicity increases with the Mg content, as indicated by the
reaction test. Moreover it appears that the majority of the basic surface sites are hydroxyl OH- groups.
Abstract: Starting from a prepared series of NaHY zeolites with different number of sodium ion per unit cell (u.c.), the corresponding series of steamed HNaY(S) zeolites were obtained by a modified method of steaming in order to obtain less acidic but nevertheless thermally stable zeolites.
All prepared solids exhibit large amounts of unusual OH groups with stretch frequency at 3700 cmK1 which are thermally stable and characterized by their soft acid strength measured by CO adsorption at low temperature: Dn(OH)Zca. 145 cmK1, i.e. H0ZK5.
All the solids are highly crystalline (80%). Chemical and FTIR analysis of the two series denoted HNaY and HNaY(S) allowed the determination of their molar formula after calcination at 650 K. The concentration of the unusual (3700)OH groups depends of the level of Na exchange and reaches a maximum for 34 Na
per u.c. as deduced from the intensity of the corresponding IR absorption band. This study gives precise details about the role of Na in forming these unusual (3700)OH groups.
Abstract: Several zeolites (cubic and hexagonal faujasites, steamed or not) are tested as acid catalysts. The strength and number of Brönsted acid sites are determined from FTIR studies of adsorbed probe molecules (CO, pyridine).
Direct benzene hydrogenation leading to cyclohexane is observed only at low temperature (500 K) with the less acid zeolites (H0 = −8.5).
Side-reactions grow progressively with the acid strength of the catalyst and, in some cases, with increasing reaction temperature (in other cases, a drastic deactivation is observed). Coke is generated by the more acid hydroxyl groups (H0 = −12).The occurrence of the various steps of the scheme describing the benzene hydroconversion process depends on the strength of Brönsted acid sites and reaction temperature.
Abstract: We describe a series of steamed HNaY zeolites containing large amounts of unusual (3700)OH groups. The method of steaming and the solids obtained are well described. These hydroxyl groups are thermally stable and characterized by their weak acid strength measured by CO adsorption at low temperature: Δν(OH)≈145 cm−1 i.e. H0=−5, slightly dependent on the coverage level. Their number is estimated at 11.8 per unit cell. Interesting catalytic properties are expected for these (3700)OH groups since the unusual soft acidity would prevent cracking and coke formation in some acid-catalyzed reactions. Moreover, the (3670)OH groups are obtained in significant amount after soft leaching of the steamed zeolite with NH4+ solutions. This solid is highly crystalline (≈85%), therefore, after CO adsorption, the shift of the (3670)OH groups is unambiguous: Δν(OH)=185 cm−1 i.e. H0=−6.
Abstract: Some properties of hexagonal and cubic, highly crystalline faujasites are compared by means of two sorbed probes (pyridine andCO) studied by FTIR spectroscopy. Whatever the structural variety, the acid strength of the HF and LF hydroxy groups and the(LF)OH contribution to the pyH` species increase with the Si/Al ratio.
A l(CO/LF) band detectable between 2160 and 2167 F cm~1 is assigned to the formation of the (LF)OH...COcomplex, its wavenumber value depends on the dealumination and coverage levels. The molar absorption coefficients, e, of the HF and LF bands of HEMT zeolite are similar to those of HF and LF bands of HY zeolite, respectively.
No structural e†ect is detectable. The progressive CO adsorption confirms the e(HF) value
previously determined by pyridine adsorption, and the results obtained for e(LF) are discussed.
Abstract: Progressive CO adsorption has been studied by IR spectroscopy, at low temperature on acid-leached steamed Y zeolites and on two silica–alumina samples with different Si/Al ratios. The IR spectra of the steamed Y zeolites are complex in the (OH) range. After CO adsorption only some perturbed (OH) and (CO) bands had previously been assigned. This study showed that some specific Brønsted acid sites observed in steamed Y zeolites were also present in silica–alumina samples, allowing us to relate them to silica–alumina debris. On the silica–alumina samples, the more acidic site strength [characterized by a perturbed (OH) band near 3450 cm-1] was found to be close to that of the high-frequency (HF)OH groups of non-dealuminated HY zeolites. A quantitative estimation of such sites was obtained by determination of the molar absorption coefficient e(CO) of the corresponding perturbed (CO) band.
