Authors:Hanene Najar, Mongia Zina, and Abdelhamid Ghorbel
The aim of the present work was to combine different physicochemical techniques, namely X ray diffraction, IR spectroscopy,
and multinuclear (29Si and 27Al) solid-state NMR spectroscopy, to study the aluminum distribution and to determine the silicon–aluminum ordering in the
Y-zeolite framework when the latter was submitted to acid leaching (HCl, HNO3, H2SiF6). It was shown that all acids were effective in removing Al from the framework. Moreover, the extra-framework Al extraction
from the lattice was dependent on the nature and the concentration of the acid. After the dealumination treatment, different
species (silanol nest, six-coordinated non-framework Al) were detected. The investigation showed that the breakdown of the
parent Y-zeolite mainly depends on the degree of dealumination.
Authors:Rita Sattler, W.-D. Einicke, and B. Hunger
Temperature-programmed desorption (TPD) of water was applied to characterize short-time dealuminated HZSM-5 zeolites. Using
a regularization method, distribution functions of the effective desorption energy of water were calculated. The results clearly
show that during dealumination a new adsorption site is formed which can be attributed to non-framework or transient aluminium
species. The highest concentration of these sites was observed for a dealumination time of 25-30 min. NO adsorption studies
support this result. Furthermore, it could be concluded that the heterogeneity and the average acid strength of the remaining
Si-OH-Al groups of the dealuminated samples do not change compared to the Si-OH-Al groups of the parent HZSM-5 zeolite.
Authors:V. Rakić, V. Dondur, R. Hercigonja, and V. Andrić
The active sites of hydrogen-exchanged Y zeolite (HY) and dealuminated (HDY) zeolites are investigated by TPD of carbon monoxide.
Only the high temperature TPD spectra of CO (TM620-690C) were observed, meaning that CO molecules interact with very strong
acid sites. The amounts of CO bonded on these sites are small (less than 1 molecule per unit cell). The strong influence of
dealumination on the coverage degree is found. The calculated values for kinetic parameters indicate chemisorption of CO in
the investigated systems (Edes240 kJ mol-1, A1011 s-1).
Authors:M. Földesová, P. Lukáč, P. Dillinger, E. Klosová, Z. Málek, E. Večerníková, and V. Balek
Zeolites from Nižný Hrabovec (Slovak Republic) were, modified with solutions of NaOH. The changes of zeolites in the temperature
range 20–1200 °C were studied by thermal analysis (DTA, TG, ETA), X-ray analysis and REM analysis. Thermal analysis showed
that the process of dehydration started between temperatures 20 and 600 °C, over this temperature the dealumination and structural
changes have taken place. X-ray analysis and REM analysis showed the structural changes of natural zeolites and gradual loss
of cristallinity of the chemically modified zeolites.
A derivatograph was used to follow the modification of the skeleton structure and thermal stability of zeolites in sodium and ammonium forms. Analogous investigations were carried out by independent methods such as IR and X-ray. Comparison showed good agreement between the results. The controlled dealumination of the Y-type zeolite enhanced the thermal stability
Authors:A. Godelitsas, P. Misaelides, A. Filippidis, D. Charistos, and I. Anousis
The uranium sorption from aqueous solutions (concentration range 50–20.000 mg/l) by the sodium-form (Na-form) of HEU-type zeolite crystals (particle-size <20 m) has been investigated by means of a batch-type method. The INAA, RI-XRF, powder-XRF, SEM-EDS and FT-IR techniques were used for the study of the experimental products. The absolute uranium uptake by the material reached the value of 11.68 mg/g in the case of initial concentraton 20,000 mg/l. On the other hand theKd-values indicated that the relative uranium uptake, and consequently the percentage of removal, is higher for concentrations below 100 mg/l. The uranium uptake by the zeolite is attributed to different sorption processes such as ion-exchange, adsorption and surface precipitation, taking place both to the interior and the surface of the crystals and strongly depending on the pH of the solutions. The investigated zeolitic material was sufficiently resistant at the low initial pH of the solutions with dealumination phaenomena only observed in the case of the most acidic solution used.
Authors:E. Alsdorf, M. Feist, H. Fichtner-Schmittler, H. Jerschkewitz, U. Lohse, and B. Parlitz
H, Li, Na, Cs, NH4, Mg and Al-ZSM 5 zeolites and H and NH4-mordenites were studied by DTA, X-ray diffraction, TPD and adsorption measurements. The stability relating to dealumination and structure destruction depends on the nature of the cation.
Authors:Lin Sun, Xiangsheng Wang, Jichun Li, An Ma, and Hongchen Guo
that coke deposits occur prior to strong acid sites, which is related to Al species in the framework of the zeolite. Steaming is a very popular dealumination technique, which can modify the acidity and catalytic performance of zeolites [ 14 – 21
Authors:Cleanio L. Lima, Hélvio S. A. de Sousa, Santiago J. S. Vasconcelos, Josué M. Filho, Alcemira C. Oliveira, Francisco F. de Sousa, and Alcineia C. Oliveira
dealumination of the molecular sieves, resulting in an increase of the Si/Al ratio ( Table 1 ) by comparing with the pure counterparts; this dealumination led to structural and textural changes in the solids by creating mesopores, in agreement with previous
Carbon deposition on the surface and dealumination would cause the activity of H-ZSM-11 to decline. The weight ratio of deposited carbon amount to used catalyst amount was determined by thermogravimetry. The weight ratios of W-modified catalyst were 0