Nitrogen adsorption by mordenite

A detailed discussion of adsorption onto mesoporous solids is beyond the scope of this text, but certain features relevant to microporous solids should be described. Firstly, microporous solids can themselves contain mesoporosity. The most important example of this is observed in zeolites such as Y or mordenite that have been treated after synthesis to remove aluminium from the framework (Section 6.2.3). The migration of silica leaves mesopores that are evident from nitrogen adsorption isotherms and directly visible by electron microscopy. The presence of secondary mesopores enhances diffusion and catalytic properties. Conversely, mesoporous solids that are well ordered on the mesoscale can contain disordered micropores in their walls. The mesoporous channels of calcined SBA-15, for example, are connected by micropores that result from removal of block copolymer chains that run between the large channels in the as-synthesised material. This is observed from nitrogen... 

Another important physical property of the ash zeolites is their pore radius Rp. This parameter helps in studying the adsorption properties of zeolites as an adsorbent. Rp can be correlated with the specific surface area SSAbet, which can be determined by nitrogen adsorption technique (i.e., by employing BET method and the relationship, Rp = 2 VpISSAbet> where Vp is the pore volume) [44]. The pores are assumed to be cylindrical in shape for natural zeolites Clinoptilolite and Mordenite, for which SSAbet generally lies between 11-16 m and 115-120 m /g, respectively. The trend depicted in Fig. 2.4 exhibits an initial increase in Rp with an increase in SSAbet, up to 20 m /g, beyond which it decreases sharply [8]. This trend violates the inverse relationship between the two parameters as mentioned above. 

Early work of Barrer (3), McKee (4) and Domine and Hay (5) showed that calcium A, calcium X, mordenite and several types of natural zeolites could be used to enrich air by a selective adsorption of nitrogen. Several pressure-swing-adsorption processes utilizing zeolite adsorbents have been developed which yield a product containing up to 95% oxygen at rates of 20 tons per day (6,7). 

Zeolites have also proven applicable for removal of nitrogen oxides (NO ) from wet nitric acid plant tail gas (59) by the UOP PURASIV N process (54). The removal of NO from flue gases can also be accomplished by adsorption. The Unitaka process utilizes activated carbon with a catalyst for reaction of NO, with ammonia, and activated carbon has been used to convert NO to N02, which is removed by scrubbing (58). Mercury is another pollutant that can be removed and recovered by TSA. Activated carbon impregnated with elemental sulfur is effective for removing Hg vapor from air and other gas streams the Hg can be recovered by ex situ thermal oxidation in a retort (60). The UOP PURASIV Hg process recovers Hg from clilor-alkali plant vent streams using more conventional TSA regeneration (54). Mordenite and clinoptilolite zeolites are used to remove HQ from Q2, clilorinated hydrocarbons, and reformer catalyst gas streams (61). Activated aluminas are also used for such applications, and for the adsorption of fluorine and boron—fluorine compounds from alkylation (qv) processes (50). 

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