Zeolite acid-resistant

PuraSiv N A process for removing nitrogen oxides from the tail gases from nitric acid plants, using an acid-resistant zeolite molecular sieve. Developed by the Union Carbide Corporation in 1971. Not to be confused with PuraSiv HR, Type N (see previous entry). 

Pollution Control. Zeolite adsorbents can effectively remove pollutants such as S02, H2S, and NO from industrial off-gas streams at near ambient temperature (54-57). Since water vapor usually exists along with these acidic compounds, an acid-stable or acid-resistant zeolite adsorbent is necessary for a long service life. Union Carbide announced three new processes for pollution control recently. They are the PuraSiv-Hg process for mercury vapor removal, the PuraSiv-N process for NO removal from nitric acid plant off-gas, and the PuraSiv-S process for S02 removal from... 

Another acid-resistant zeolite with a high potential applicability in acidic environment reactions is mordenite, which has been successfully prepared as a membrane on tubular... 

The mechanism of acid-site development in Y-zeolites was described in the previous section, but it is essenticdly same for other zeolites. In the case of more acid-resistant zeolites such as ZSM-5 or mordenite, proton can be introduced by direct ex-chEmge with dilute hydrochloric acid, though this often causes dealumination of the zeolite framework. The reason why NaY must be converted into NH4Y to obtain HY is the instability of Y-type zeolites in acidic solutions. 

DEALUMINATIQN OF ZEOLITE Y Dealumination is an important process to improve the thermal stability and resistance to acid of zeolite. This is one of the main techniques for preparing zeolite catalysts (US-Y). New pores (mesopores) have been introduced during hydrothermal treatment (Fig.4), which were directly confirmed by electron microscopy. The density of mesopores depended on the degree of dealumination and the size distribution of mesopores... 

TTigh silica zeolites attract great attention since they are characterized by relatively high thermal stability and considerable acid resistance. Physicochemical properties of high silica zeolites, despite a number of investigations, have not been sufficiently studied. The same is true for L- and clinoptilolite zeolite. The data on synthesis, structure, adsorption properties, decationization, dealuminization, adsorption heats, and other properties of the above-mentioned zeolites have been given (1-15). Results of studies of physicochemical properties of L zeolites and of natural and modified clinoptilolite are given here. 

A number of inoiganic pollutants are removable by TSA processes. One of the major pollutants requiring removal is S02 from flue gases and from sulfuric acid plant tail gases. The Sulfacid and Hitachi fixed-bed processes, the Sumitomo and BF moving-bed processes, and the Westvaco fluidized-bed process all use activated carbon adsorbents for proven S02 removal (58). Zeolites with high acid resistance, such as mordenite and clinoptilolite, have proven to be effective adsorbents for dry S02 removal from sulfuric acid tail gas (59), and special zeolite adsorbents have been incorporated into the UOP PURASIV S process for this application (54). 

Zeolites (of which there are currently more than 200 types, 50 of which are naturally occurring) are differentiated by the Si/Al ratio in their anionic structure. This varies between 1 (in zeolite A) and °° in silicalite, an aluminum-free crystalline silica-modification. The industrially most important synthetic zeolites, apart from zeolite A, are zeolites X and Y with Si/Al ratios of 1 to 3, synthetic mordenite at ca. 5 and ZSM 5 at > 10. The thermal stability and acid resistance of zeolites increa.se as the Si/Al ratio increases. 

Usually, inorganic and organic acids can be used for framework dealumination of zeolites, and the acids include hydrochloric acid, nitric acid, formic acid, acetic acid, and so on. According to its acid-resistance ability, hydrochloric acid can be used for high-silica zeolites such as mordenite, clinoptilolite, erionite, etc. We will take mordenite as an example to describe this dealumination method (Table 6.4). The first step in the treatment of mordenite using hydrochloric acid is to convert the zeolite into H-type, and further acid treatment can then enlarge the pore diameter through dealumination. After partial dealumination, the Si/Al ratio of the zeolite is increased and the heat-resistance, water-resistance, and acid-resistance abilities are enhanced. 

Fig. 6 Illustration of Nafion-acid functionalized zeolite Beta nanocomposite membranes helping to increase the proton conductivity and decrease the methanol crossover (A) H2O and CH3OH diffusion reduced by zeolite flow resistance (B) sulphonic acid functionalized zeolite nanoparticles increases proton conductivity of composite membranes. (View this art in color at www. dekker. com.)...

Thus the weakly Bronsted acidic boron zeolites allow acid-catalyzed reactions to be carried out with high selectivity. Gallium substitution gives effective, sulfur-resistant catalysts for the synthesis of aromatics from lower alkanes, without the need for noble metal doping [8], The nonacidic titanium siUcalite exhibits very interesting properties in selective oxidation reactions with H2O2 [T32]. 

NaA/polyelectrolyte multilayer-pervaporation membrane showing a greater stability under acidic conditions in comparison with a pure zeolite A membrane and maintaining a high selectivity for water over alcohols. For the same purpose, Kita et al. [181-183] proposed a zeolite T membrane, prepared by ex situ crystallization, for the per-vaporation-aided or vapor-permeation-aided esterification of acetic acid with ethanol. This membrane has a higher acid resistance and can be directly immersed in the liquid-phase reaction mixture. The conversions achieved exceed the equilibrium limit and reached to almost 100% after a stabilization period of 8 h. 

Zeolite T membranes are hydrophilic and also acid resistant and they would be preferred for the pervaporation dehydration in the presence of organic acid, such as the esterification hybrid process. However, there are fewer reports about the zeolite T membranes preparation. Cui et reported the... 

The catalyst was an acidity-adjusted zeolite that was selective for BTX and resistant to coking. The process was used hy Nippon Oil hut abandoned hy them in 2008 because the product yield was too low. 

Natural zeolites such as Clinoptilolite (i.e., popularly known as Clino zeolites) and Chabazite have applications in various diversified fields such as water treatment, fertilizer application for soil amendment and plant growth by establishing better retention of nutrients. The Clinoptilolite has been broadly accepted for its usage in agriculture, soil amendment and feed additives because of its higher acid resistant silica content (viz., Si/Al = 1-5) [14]. However, such zeolites are contaminated by other minerals (e.g., Fe ", S04, Quartz, other zeolites, and amorphous glass) and hence they may not be suitable for several important commercial apphcafions where uniformity and purity are essential [8]. 

Si/Al molar ratio in the activated fly ash, zeolites can be classified/graded as low silica zeolites , intermediate silica zeolites and high silica zeolites , as listed in Table 2.2. In general, for zeolites, an increase in this parameter (i.e., Si/Al from 0.5 to infinity) [5] can significantly result in the increase in various other parameters (viz., acid resistivity, thermal stability and hydrophobicity) except few parameters (viz., hydrophilicity, acid site density and cation concentration) which get decreased [5, 8, 10,40, 41]. In general, synthetic zeolites hold some key advantages over their counterparts i.e. natural zeolites. Zeolites type A, X, Y, P and Na-Pl are well known synthetic zeolites synthesized from fly ash which have a wider range of industrial applications than the natural zeolites [1, 8, 20, 22, 36, 42, 43]. 

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