Search

The dealumination of nanosized zeolites is an important scientific problem, which should be solved to improve the activity of catalysts based on this zeolite in a broad range of heterogeneous catalytic reactions, particularly in commercial processes. However, the smaller the required size of the synthesized crystals, the lower the Si/Al ratio and the lower the degree of dealumination of this material can be achieved. In this study, the dealumination of zeolites Y with a crystal size of 50–1100 nm by treatment with ammonium hexafluorosilicate and steam heat treatment is discussed. It is shown that the dealumination with ammonium hexafluorosilicate is a “gentler” method in terms of structure preservation, whereas the dealumination by steam heat treatment provides a higher Si/Al ratio in the products; however, this method is inapplicable for crystals smaller than 500 nm, because it leads to the complete degradation of the structure. However, nanosized crystals can be dealuminated by treating with ammonium hexafluorosilicate. In this case, the degree of dealumination is close to 40%. A significant disadvantage of this method is the formation of a SiO2 film on the crystal surface; this feature substantially restricts the use of the ammonium hexafluorosilicate treatment in the synthesis of cracking catalysts.

Acidic zeolites are solid aluminosilicate catalysts whose utility arises from Bronsted sites that predominately reside in their crystalline framework structure. Data described herein indicate that extra-framework aluminum (EFAl) moieties, often proposed as important species in overall catalyst activity via Bronsted–Lewis synergies, can themselves contribute protons that are also reactive Bronsted acid centers. While the MFI family of zeolites is a relatively simple channel-structure type, the quantitative spectroscopic detection of all protons shows that the distribution of reactive Bronsted acid site protons arising from framework and extra-framework moieties can be complex. Experiments show that postsynthetic treatments can be used to modify this distribution, in theory enabling routes to HZSM-5 catalysts with only one type of reactive Bronsted site. Quantitative spin-counting NMR experiments combined with chemical washing using ammonium hexafluorosilicate (AHFS) show that the number of framework bridgi...

In the present work the solid-state dealumination of Y zeolite with ammonium hexafluorosilicate for various reaction times was studied. A detailed characterization of the porosity of the samples, and their hydrophobic-hydrophilic properties, was made. The characterization techniques used, besides the low temperature nitrogen adsorption and the room temperature water adsorption, include also the FTIR, the powder XRD and the 27Al and 29Si MAS NMR. These techniques showed evidence for the complex mechanism of dealumination that for sufficiently long times of reaction allows the partial reconstitution of the structure. Due to this mechanism the evolution of the Si/Al ratio, as well as the porosity and the hydrophobic-hydrophilic properties, do not run monotonically with the time.

The dealumination of zeolite H-β by ammonium hexafluorosilicate (i.e., (NH4)2SiF6, AHFS) treatment was investigated by 27Al and 19F solid-state NMR, combined with 27Al/19F double resonance NMR. The NMR results demonstrated that the experimental conditions of AHFS dealumination, that is, in the presence and absence of ammonium acetate (NH4OAc), strongly affected the amount, state, and nature of extraframework aluminum (EFAl) species. Different aluminum fluoro-complexes after dealumination were detected. 19F and 27Al NMR results revealed that the essential part of the dealumination process carried on with AHFS in the presence of NH4OAc was that most of the extracted Al3+ reacted with F- to form (NH4)3AlF6 species that were evident from the signals at 0 ppm in the 27Al spectrum and −143 ppm in the 19F spectrum, respectively. In the absence of NH4OAc, a resonance at 13 ppm and a broad pattern spread from −20 to −90 ppm in the 27Al spectrum were observed, indicating the presence of two different forms of alumi...

The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite in chlorinated VOC oxidative destruction. A series of Y zeolites with various Si/Al ratios was prepared from a commercial sample and then tested for the catalytic decomposition of 1,2-dichloroethane as a model reaction. In general, these modified Y zeolites exhibited a higher activity than that of the parent material, the zeolite subjected to 50% dealumination resulting the most active catalyst. This increase in activity was associated with the development of strong acidity due to dealumination. Likewise, 50% dealuminated sample showed an improved catalytic behaviour for the destruction of other typical chlorinated pollutants, namely, dichloromethane (DCM) and trichloroethylene (TCE). The ease of destruction was found to follow this trend: 1,2-dichloroethane > dichloromethane > trichloroethylene.

The treatment of MFI zeolites with ammonium hexafluorosilicate causes a selective dealumination of their outer surface. This dealumination improves the selectivity to the para isomer of m-xylene isomerization.

The effect of dealumination using ammonium hexafluorosilicate and steaming has been studied on a series of HY zeolites. The structure of the zeolites after dealumination was studied using XRD, 29Si NMR, 27Al NMR, and IR of the hydroxyl region. The effect of dealumination on the pore structure was investigated using 129Xe NMR experiments. Steam dealumination tends to result in the formation of a highly dispersed extralattice aluminum species within the zeolitic supercage. It is suggested that this extralattice species contributes to enhanced acidity.

Disproportionation of methylnaphthalene(MN) was carried out on ZSM-5 catalysts. The external acid sites were eliminated by dealumination with (NH4)2SiF6. The dealuminated ZSM-5 gave 2,6- and 2,7-dimethylnaphthalene(DMN) from disproportionation of 2-methylnaphthalene(MN), due to its shape selective property.

The present study focuses on the effect of the metal-modification method (ion-exchange or impregnation) and of the type of dealumination procedure (steaming, AHFS) of zeolite-Y on the quantity, quality and properties of the carbon nanotubes that are formed on the supported iron or cobalt oxides. Severe dealumination of zeolite-Y by steaming, which induced secondary meso/macropores, resulted in significant decrease in the carbon nanotube formation activity of the metal-impregnated zeolite-Y catalysts, while moderate dealumination by ammonium hexafluorosilicate had a less pronounced effect. The micro-Raman spectra revealed the presence of high quality as-grown CNTs with low degree of disordered.

The dealumination of zeolite H-β by ammonium hexafluorosilicate (i.e., (NH4)2SiF6, AHFS) treatment was investigated by 27Al and 19F solid-state NMR, combined with 27Al/19F double resonance NMR. The NMR results demonstrated that the experimental conditions of AHFS dealumination, that is, in the presence and absence of ammonium acetate (NH4OAc), strongly affected the amount, state, and nature of extraframework aluminum (EFAl) species. Different aluminum fluoro-complexes after dealumination were detected. 19F and 27Al NMR results revealed that the essential part of the dealumination process carried on with AHFS in the presence of NH4OAc was that most of the extracted Al3+ reacted with F- to form (NH4)3AlF6 species that were evident from the signals at 0 ppm in the 27Al spectrum and −143 ppm in the 19F spectrum, respectively. In the absence of NH4OAc, a resonance at 13 ppm and a broad pattern spread from −20 to −90 ppm in the 27Al spectrum were observed, indicating the presence of two different forms of alumi...

The number of washes with boiling water of a Y zeolite dealuminated with ammonium hexafluorosilicate strongly affects its stability under hydrothermal conditions. Traces of hexafluorosilicate, difficult to eliminate completely through washing, would be responsible for this effect. Hydrothermal treatments of mixtures of NH4NaY zeolite and of ammonium hexafluorosilicate (0.2-2 wt%) allow confirmation of this point.

This paper reports a comparison of the structure and catalytic properties of zeolite-Y whendealuminated by hydrothermal and ammonium hexafluorosilicate treatments. Initial results have shown that dealumination using ammonium hexafluorosilicate is highly sensitive to many of the reaction parameters such as reaction temperature, rate of AHFS addition, reaction time, pH, framework Si/Al ratio and also to the extent of washing of the final product. Further, degrees of dealumination beyond 50% are difficult to obtain without causing decreases in framework crystallinity. Structural characterisation of the samples has been performed using X-ray diffraction, X-ray fluorescence spectroscopy, nitrogen adsorption and 27Al MAS NMR. The catalytic properties of the samples were studied using the n-butane cracking reaction. In future work, analysis of the cracking data will be performed as well as a detailed comparison of the deactivation processes occurring within the samples.