Valérie Norberg

Characterization of the Brønsted acid properties of H(Na)-Beta zeolite by infrared spectroscopy and thermal analysis

Infrared spectroscopy and thermal analysis have been used to characterize the Bronsted acidity, the hydroxyl groups of H-Beta zeolite, and the interaction of hydroxyls with benzene molecules. After pretreatment under dry oxygen and then under vacuum at 723 K, three hydroxyl groups at 3,789, 3,745, and 3,612 cm−1, superimposed on a broader absorption (3,800–3,200 cm−1), have been detected. These three peaks are assigned to AlOH species near to one or more SiOH groups generated when Al leaves the framework, terminal silanol groups, and framework bridged SiOHAl species, respectively. The broader absorption band should be attributed to the internal silanol group. The present work shows that the hydroxyl group at 3,789 cm−1 can be generated also after a mild treatment. This observation is contrary to previously published results by other research groups. Furthermore, the present study shows that all three hydroxyl groups situated at 3,789, 3,745, and 3,612 cm−1 can interact completely with benzene. This is in contrast with the case of pyridine. It seems that some framework protons (or Na+ cations), located in the small cavities and being initially inaccessible, can be attracted toward the 12-R channels in the presence of benzene and finally become accessible for benzene. The acid strength of each hydroxyl group is evaluated by the shift ΔνOH of the hydroxyls upon their interaction with benzene and is compared to other proton zeolites such as HZSM-5, HY, and HEMT zeolites. It indicates that the framework-bridged SiOHAl groups of H(Na)-Beta zeolite have an intermediate Bronsted acidity. The amount of Bronsted acid sites as well as the total adsorption capacity for benzene have been determined from the study of the changes of the absorbance of the CH out-of-plane vibrations with benzene coverage. The number of TEA species associated with tetrahedral aluminum and with SiO− structural defects or OH− ions has been also determined using thermobalance coupled with ammonia titration.

Toward a Better Understanding on the Adsorption Behavior of Aromatics in 12R Window Zeolites

Benzene adsorption behavior in a large family of 12R window zeolites (X, Y, EMT, Beta and LTL) has been examined by means of in-situ FTIR spectroscopy and correlated with the zeolite structure, the type and number of counter-ions, and the negative charge on framework oxygen atoms of zeolites. The effect of coadsorption of HCl, NH3 and CH3NH2 on the benzene location has also been studied. The present work illustrates that besides the benzene adsorption on counter ions of zeolites, the 12R windows could also be the adsorption sites for benzene. Upon adsorption of coadsorbates such as HCl, NH3 and CH3NH2, the migration of preadsorbed benzene molecules from one type of adsorption sites towards another, i.e. from 12R windows towards the cations for HCl and opposite direction for NH3 and CH3NH2, has been evidenced. The lack of adsorption of benzene on 12R windows of NaBeta even upon coadsorption of a series of basic molecules reveals that benzene adsorption on 12R windows is most likely governed by a molecular recognition effect where benzene molecule and 12R window should have the adapted chemical and structural properties like in enzyme-substrate system and zeolites can be referred to as solid enzymes or zeo-enzymes. This paper indicates also that the adsorption properties of zeolites can be modified and accommodated by introduction of a co-adsorbate.

Quantitative characterisation of H-Mordenite zeolite structure by infrared spectroscopy using benzene adsorption

Abstract Infrared spectroscopy has been used to characterise the Bronsted acidity, the hydroxyls of H-Mordenite (HM) zeolite and the interaction of hydroxyls with benzene molecules. After pre-treatment under dry oxygen and then under vacuum at 723 K, three hydroxyl groups at 3749, 3660 and 3608 cm −1 , assigned to terminal silanol, extraframework Al–OH and framework bridged Si–OH–Al species, respectively, have been detected. It shows that all the silanols, but only part of framework bridged protons and not the extraframework Al–OH groups can interact with benzene molecules. The number of framework bridged protons located in the main 12R channels, i.e. those accessible to benzene molecules, and that in the small channels are determined by a quantitative study of the changes of the adsorbance of the CH out-of-plane vibrations with benzene loading. It reveals that although the studied HM zeolite contains 4.6 framework bridged protons per unit cell, only around 1.5 of them are located in the main 12R channels. Others are in the small channels and are not accessible to benzene molecules. The acid strength of hydroxyl groups is evaluated by the shift of the hydroxyls (Δ ν OH ) upon their interaction with benzene and compared with other protonic zeolites such as HZSM-5, HY, HBeta and HEMT. It indicates that the framework bridged OH groups of HM have a strong Bronsted acidity comparable with that of HZSM-5 zeolite.

Location of benzene and cations in Si-rich NaY zeolites: a comparative and quantitative infrared spectroscopic study

Abstract The location of benzene molecules and Na+ ions in two Si-rich NaY (NaYs and NaYd) zeolites (Si/Al≈3.5), prepared by different methods, has been investigated using FTIR spectroscopy and compared with that in NaY (Si/Al=2.3), and NaEMT (Si/Al=3.6) zeolites. Two adsorption sites for benzene, Na+ ions and 12R windows, have been evidenced in two Si-rich NaY and in NaY zeolite instead of one, i.e. Na+ ions, in NaEMT. The desorption experiments have revealed that the interaction of benzene with Na+ ions is stronger than that with 12R windows and there are various strengths of interaction of benzene with Na+ ions in NaEMT. The total benzene adsorption capacity and the selectivity of benzene adsorbed have been quantitatively analyzed. The adsorption behavior of benzene has been correlated with the Al distribution, the location of Na+ ions, the average negative charge of oxygen atoms, the zeolite structure and the interaction strength of benzene with one site or another.

Behavior of benzene molecules in large pore zeolite structures as studied by FTIR and 2H NMR techniques

The location and motion of benzene molecules in NaEMT and KL zeolites have been studied by in-situ FTIR and 2H NMR techniques and correlated with zeolite structures, benzene loading and negative charge of framework oxygen atoms. The FTIR study indicates that almost all benzene molecules are facially coordinated to cations located in large cages of NaEMT and KL zeolites at any benzene loading at room temperature. 2H NMR study reveals that below 183 K for KL and 263 K for NaEMT, only a rapid spinning of benzene molecules facially adsorbed on cations around its 6-fold axis is observed. Above these temperatures, a pseudo-isotropical motion of benzene molecules, indicating the jump of benzene molecules from one adsorption site to another, occurs.

Effect of ammonia and methylamine as coadsorbates on the adsorption behavior of benzene in NaBeta zeolite studied by FTIR spectroscopy

Abstract The single component (benzene, ammonia and methylamine) adsorption and the effect of coadsorbates such as ammonia and methylamine on the adsorption of benzene in NaBeta zeolite have been investigated by FTIR. The changes in the framework upon adsorption of different molecules have been checked and discussed. The benzene location in NaBeta zeolite with or without the presence of coadsorbates has been correlated with the structural and chemical properties of zeolite and with the interaction strength of ammonia, benzene and methylamine with NaBeta zeolite. It reveals that only cations, but not 12R windows of NaBeta are preferentiel adsorption sites for benzene even in the presence of coadsorbates such as ammonia and methylamine. The quantitative analysis on the amount of benzene adsorbed on cations when benzene is present alone suggests that all the cations present in NaBeta zeolite are accessible to benzene molecules. Ammonia and methylamine can interact not only with Na+ ions via the lone pair on nitrogen atoms but also with the large number of silanols present in NaBeta zeolite. The interaction strength of ammonia, methylamine and benzene with NaBeta ranks in the order of methylamine/NaBeta>benzene/NaBeta>ammonia/NaBeta. The interaction of ammonia and methylamine with NaBeta causes an important modification of lattice parameter, indicating the deformation of zeolite framework. However, no significant deformation is observed upon adsorption of benzene alone. In spite of the deformation of zeolite framework, only adsorption of benzene in Na+ ions has been observed and no change in the location of benzene in NaBeta zeolite is detected upon coadsorption of either ammonia or methylamine, being contrary to what we observed in KL zeolite upon coadsorption of methylamine and in NaEMT upon coadsorption of ammonia.

Improved synthesis of (Ga)- and (Ga,Al)-faujasites

A direct and efficient method to prepare in one step mixed (Ga,Al)-faujasites as well as the pure (Ga)- and (Al)-end members is proposed. Slow depolymerization of silica governs the formation of adequate gallosilicate precursors that probably undergo a solid state restructuration yielding crystalline Ga-faujasites. The true incorporation of gallium in the FAU framework tetrahedral sites was ascertained by measuring the unit cell expansion and confirmed by 29 Si-NMR. Both trivalent ions are incorporated with a 100% efficiency and their total amount is fairly constant (about 72 per unit cell). 129 Xe-NMR suggests that all samples are true X zeolites despite the average Si/M 3+ molar ratios close to, or larger than 1.6, and shows that their pore size is independent on the Ga content. Rutherford Back Scattering profiles reveal, along with 29 Si-NMR, that Ga is homogeneously distributed throughout the lattice and that it remains stable and well dispersed when the Ga faujasites are calcined at 500°C.

Mobility of benzene molecules in NaEMT and KL zeolitic nanostructures studied by 2H NMR technique

Abstract The molecular motions, diffusion coefficients and activation energies of deuterated benzene in NaEMT (Ea=18.0 kJ/mol) and KL (Ea=7.3 kJ/mol) zeolites have been examined by 2 H NMR spin-lattice relaxation experiments. It was observed that two types of movement of C 6 D 6 exist. Below 183 K for KL and 263 K for NaEMT, benzene molecules adsorbed on cations and rapidly rotate around their sixfold molecular symmetry axis. Above these temperatures, benzene molecules are in isotropic motion and jump from one adsorption site to another. The mobility of benzene molecules in NaEMT and KL zeolites was compared with that in faujasite zeolites.