Bao-Lian Su

Synthesis of nanostructured mesoporous zirconia using CTMABr–ZrOCl2·8H2O systems: a kinetic study of synthesis mechanism

Abstract A systematic kinetic study of mesoporous zirconia formation has been performed in order to optimize the synthesis conditions without addition of structure stabilizing agents such as sulfate or phosphate anions. We have investigated in particular the effect of synthesis time and temperature. On the basis of TEM, SEM, XRD and N 2 adsorption–desorption results, a synthesis mechanism has been proposed. It is observed that at low temperature or for short durations at higher temperatures, the obtained materials are first supermicroporous, then if hydrothermal treatment is prolonged, a breakdown of the walls separating adjacent pores allows the transformation to mesopores. The obtained materials have a uniform pore size and their surface can reach 300 m 2 /g. The channel array is, at least part of samples, wormlike. However, if hydrothermal treatment is performed at too high temperature or for too long durations, mesoporous compounds are no longer obtained, but thermodynamically more stable crystalline zirconium oxides with very low specific surface area, namely the tetragonal and monoclinic forms, are the final phases.

Alkylation of benzene by propene on a series of Beta zeolites: toward a better understanding of the mechanisms

Abstract Alkylation of benzene with propene has been investigated on a series of H-Beta zeolites with different Si/Al ratio: 10, 35 and 66, using different techniques such as FTIR, 13 C NMR and catalytic testing. It has been shown that H-Beta zeolite (Si/Al ratio=10) is a very active catalyst for this reaction with propene, but it leads also to fast oligomerization even at lower reaction temperatures. 13 C NMR experiments have demonstrated for the first time that the reaction of alkylation is already complete at −80°C on these catalysts. Dealuminated H-Beta zeolites are less active but lead to less aromatic by-products (especially n -propylbenzene) and the deactivation due to the coke formation is reduced. The high ratio of benzene/propene favors the formation of cumene and suppresses the oligomerization of propene. Triisopropylbenzene (TIPB) can also be formed at low temperatures ( 130°C). The present work reveals that the alkylation of benzene by propene occurs via an Eley–Rideal type mechanism. The apparent activation energy for this reaction has been determined and shows to decrease as the Si/Al ratio of H-Beta zeolites increases. The relatively low apparent activation energies obtained for this reaction on a series of H-Beta zeolites studied suggests that it exists a diffusion step of reactants and products between the silanols of the Beta zeolite.

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.

Kinetic study of MCM-41 synthesis

Abstract A kinetic study of MCM-41-type materials formation has been made to optimize the synthesis conditions in particular the synthesis time and temperature. The changes in morphology and textural properties of materials as a function of hydrothermal synthesis time at four different temperatures have been followed. From the characterization results, the synthesis mechanism is postulated. The present work shows clearly that the thickness of the wall separating two adjacent pores increases with hydrothermal synthesis time and temperature while pore size remains constant. The increase in the wall thickness, indicating the enhancement of polycondensation of silica around the micelles of surfactant, should be very important for strengthening the thermal stability of MCM-41 materials.

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.

Pure propane as alkylating agent in the formation of cumene from benzene on gallium-modified H-MFI and H-Beta zeolites

Pure propane was rarely used as an alkylating agent of benzene for the formation of cumene. However, by means of FTIR spectroscopy, Ga-modified MFI zeolite was shown to be a very active and selective catalyst for this reaction compared with H-MFI, H-Beta and Ga-modified Beta zeolites which lead to more coke formation.

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.

Pore size engineering of mesoporous silicas using alkanes as swelling agents

Abstract Alkanes have been used as swelling agents and incorporated during the micellar solution preparation in order to synthesize large pore mesoporous materials. The introduction sequence of the swelling agent, the effect of the number of carbon atoms of the alkane on the structural and textural properties of products were investigated. Final compounds were intensively characterized by several techniques (XRD diffraction, SEM, TEM, nitrogen adsorption-desorption analysis).

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.