Alkaline treatment zeolite acidity

The zeolitic materials obtained by the alkaline treatment and pillaring process, produced new materials with a specific surface area higher than the raw materid and also with a higher acidity. 

Table 3, Effect of Alkaline Treatment on Zeolite Acidity.

The desilication process of mordenite was also evaluated by FTIR spectroscopy [lOPlJ. A shift of lattice vibration bands toward smaller wave numbers was shown upon alkaline treatment. Consequently, this evolution can be taken as evidence of the silicon removal. A linear correlation between the number of framework Si per unit cell, Nsi, and the asymmetric stretching wave nmnber, Vi, was observed vi [cm ] = 20.2 Nsi+ 193.0 [lOPl], The desilicated samples exhibit the same acidic properties as the parent H-MOR zeolite. 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

Infrared spectra of alkaline earth-exchanged Y sieve containing adsorbed pyridine showed that the bands at 3650 and 3550 cm-1 arise from hydroxyl groups which function as Brpnsted acids (151, 209, 211). The zeolites undergo dehydroxylation upon treatment at temperatures in the range of 400° to 600°C. Experiments with Ca—Y zeolite by Eberly (151) indicated that extensive dehydroxylation occurred upon heating to 427°C. There was little evidence of hydroxyl groups in the 3800 to 3500 cm-1... 

Alkalinity Bicarbonate (HCOa" ), carbonate (C03 2), and hydroxyl (OH ), expressed as CaCOa Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

It was early realised that the acidity of zeolites is affected by their composition. Zeolites with a low aluminium content, hence a low density of lattice anions, present isolated acidic sites, which strength is not diminished by mutual interaction [30]. In this way, catalytic applications usually demand zeolites with a lower aluminium content than applications in water softening or in the separation of air gases, for which a high density of cations is demanded. The aluminium content of zeolites can be controlled by post-synthesis treatment, like in the dealumination treatments which lead to the ultra-stable Y (USY) used in the TCC catalysts [31]. The aluminium content can also be controlled by modifying the conditions of synthesis, albeit each zeolite structure presents a preferential field of composition. It was early shown that the Si/AI ratio is affected by the pi I of the synthesis system, the lowest Si/Al ratio being obtained in the most alkaline systems, in which silica is largely depolymerised and is incorporated as isolated tetrahedra [32],... 

Mesopores were created in MFI zeolite by alkali treatment technique without deterioration of zeolitic microporous structure. The size of formed mesopore is ca. 4 nm and more uniform than that of MCM-41. Mesopores are formed along a boundary of MFI crystallite twinning, which shows a weak quality against alkalinity, apart from the microporous structures. This unique structure causes superiority in acid catalysis because of the combination of its strong acidity originated by ZSM-5 with newly created mesoporosity. 

The relationship between cation radius and hydrated cation radius, at 298 K, is depicted in Fig. 2 [59N1, OlPl]. The radius of a fully hydrated ion is proportional to the ionic charge, but not proportional to the ionic radius without H2O shell. Cs rm- 1-69 A, rhyd= 3.29 A) is exchanged more easily thanUE (rion= 0.60 A, rhyd= 3.82 A) or Be = 0.31 A, rhyd = 4.59 A) [59N1, 88M2]. The elevated temperatures normally enhance the ion-exchange reactions, while the pH is also critical, particularly in the case of acidic solutions, because small H" cations are very mobile and reactive over many other cations. Other factors concern the solid/liquid ratio, the zeolite particle sizes, and the treatment time. As mentioned above, this discussion is valid in case of alkali or alkaline-earth element cations which are practically stable in aqueous solutions. 

Beznis, N., van Laak, A., Weekhuysen, B., et al. (2011). Oxidation of Methane to Methanol and Formaldehyde over Co-ZSM-5 Molecular Sieves Tuning the Reactivity and Selectivity by Alkaline and Acid Treatments of the Zeolite ZSM-5 Agglomerates, Micropor. Mesopor. Mat., 138, pp. 176-183. 

Leaching post-treatment with NaOH solution was recently applied to create mesopores in MFI zeolite [22-25]. In contrast to acid treatment, which preferentially removes framework A1 atoms, alkaline post-treatment was found to extract framework Si atoms selectively, leading to substantial formation of mesoporosity [26,27]. Ogura et al. concluded from TEM observation that mesopores were created by dissolving the portion of ZSM-5 with poor crystallinity and a... 

The stability of carbenium ions deereases in the order tertiary>seeondary>primary, meaning that the yield of ethylene is generally much less than that of propylene. Zhang et al. found that by adjusting the acid type, aeid strength and aeid distribution, the ratio of ethylene to propylene ean be altered in favor of ethylene. A modified MFI t e zeolite PMZ was formed by treatment of ZRP-1 with alkaline earth metal ions. When PMZ is used as the eatalyst for middle distillate cracking at 520 °C, the ratio... 

Alkalinity Bicarbonate (HCOji, carbonate expressed as CaCOj Foaming and carry over of solids with steam. Embrittlement of boiler steel. Bicarbonate and carbonate produce CO, in steam, a source of corrosion in condensate lines. Lime and lime soda softening. Acid treatment. Hydrogen zeolite softening. Demineralization. Dealkalization by anion exchange. 

He et al. [ 116] and Wan and Shu [117] reported on the influence of calcination and hydrothermal treatment on compositional characteristics and thermal stability of rare earth containing Y zeolites and their performance in catalytic cracking. The alkaline and hydrothermal stability of Y zeolites dealuminated via hydrothermal treatment and by the SiCl4 technique was studied by Lutz et al. [118]. Hydrothermal treatment was found to increase the chemical resistance of Y zeoHte to superheated water at 200°C as well as to alkaline solutions due to the formation of a protective layer of extra-lattice oxidic aluminum species on the external surface of the zeoHte crystals. The removal of this layer by acid leaching resulted in significantly less stable products. 

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