Condensation zeolites

In this study, we have shown that both alcohol and D20 have an Important effect on the nucleation and crystal growth of zeolites with Si/Al ratios between 1-2. In the case of alcohol, the formation of large pore zeolites such as zeolites X or Y is markedly accelerated at low alcohol levels. We attribute this to a stabilization of the cation-water complex and structured H20 which act as templates. However, at high alcohol levels, the structure of water disintegrates and leads to the formation of more condensed zeolites such as sodalite or cancrinite. Synthesis of zeolite A in D20 is slower than that in water, which primarily arises from the primary and secondary isotope effect during the condensation polymerization reactions necessary for zeolite growth. 

The adsorption isotherms are often Langmuirian in type (under conditions such that multilayer formation is not likely), and in the case of zeolites, both n and b vary with the cation present. At higher pressures, capillary condensation typically occurs, as discussed in the next section. Some N2 isotherms for M41S materials are shown in Fig. XVII-27 they are Langmuirian below P/P of about 0.2. In the case of a microporous carbon (prepared by carbonizing olive pits), the isotherms for He at 4.2 K and for N2 at 77 K were similar and Langmuirlike up to P/P near unity, but were fit to a modified Dubninin-Radushkevich (DR) equation (see Eq. XVII-75) to estimate micropore sizes around 40 A [186]. 

Below the critical temperature of the adsorbate, adsorption is generally multilayer in type, and the presence of pores may have the effect not only of limiting the possible number of layers of adsorbate (see Eq. XVII-65) but also of introducing capillary condensation phenomena. A wide range of porous adsorbents is now involved and usually having a broad distribution of pore sizes and shapes, unlike the zeolites. The most general characteristic of such adsorption systems is that of hysteresis as illustrated in Fig. XVII-27 and, more gener-... 

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

Angell (1) has investigated the Raman spectra of acetonitrile, propylene, and acrolein on a number of zeolites and found that physical adsorption occurred. There are sufficient differences between the spectrum of the liquid and of the adsorbed species (e.g. the carbon-carbon double bond stretching in the case of propylene and the carbon-nitrogen triple bond stretching in the case of acetonitrile) to make it quite clear that it was not merely a case of condensation in the pores of the solid adsorbent. 

Ammonium salts of the zeolites differ from most of the compounds containing this cation discussed above, in that the anion is a stable network of A104 and Si04 tetrahedra with acid groups situated within the regular channels and pore structure. The removal of ammonia (and water) from such structures has been of interest owing to the catalytic activity of the decomposition product. It is believed [1006] that the first step in deammination is proton transfer (as in the decomposition of many other ammonium salts) from NH4 to the (Al, Si)04 network with —OH production. This reaction is 90% complete by 673 K [1007] and water is lost by condensation of the —OH groups (773—1173 K). The rate of ammonia evolution and the nature of the residual product depend to some extent on reactant disposition [1006,1008]. 

Study [23] Jacobsen s complex was entrapped in the final step of the zeohte synthesis (method C). This process was possible because MCM-22 zeohte is prepared by condensation of a layered precursor, which is exchangeable by the catalytic complex. Leaching of Mn was not observed in these systems, which is not unexpected bearing in mind that the complex is also bovmd to the zeolite structure through an electrostatic interaction. 

Figure 5.19 shows an idealized form of the adsorption isotherm for physisorption on a nonporous or macroporous solid. At low pressures the surface is only partially occupied by the gas, until at higher pressures (point B on the curve) the monolayer is filled and the isotherm reaches a plateau. This part of the isotherm, from zero pressures to the point B, is equivalent to the Langmuir isotherm. At higher pressures a second layer starts to form, followed by unrestricted multilayer formation, which is in fact equivalent to condensation, i.e. formation of a liquid layer. In the jargon of physisorption (approved by lUPAC) this is a Type II adsorption isotherm. If a system contains predominantly micropores, i.e. a zeolite or an ultrahigh surface area carbon (>1000 m g ), multilayer formation is limited by the size of the pores. 

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

Zeolites have been shown to catalyse a variety of related reactions (Downing et al., 1997), e.g. zeolite beta catalyses the synthesis of coumarins via the Pechmann condensation. For example, condensation of resorcinol with ethyl acetoacetate over zeolite beta in refluxing toluene gave methylumbelliferone, a perfumery ingredient, in 70-80% yield (Fig. 2.24) (Gunnewegh ef a/., 1996). 

M.T. Telly, Where zeolites and oxides merge semi-condensed tetrahedral frameworks. J. Chem. Soc. Dalton Trans. 2000, 4227. 

Five common desiccant materials are used to adsorb water vapor montmorillonite clay ([(Na,Cao.5)o.33(Al,Mg)2Si40io(OH)2 H20], silica gel, molecular sieves (synthetic zeolite), calcium sulfate (CaS04), and calcium oxide (CaO). These desiccants remove water by a variety of physical and chemical methods adsorption, a process whereby a layer or layers of water molecules adhere to the surface of the desiccant capillary condensation, a procedure whereby the small pores of the desiccant become filled with water and chemical action, a procedure whereby the desiccant undergoes a chemical reaction with water. 

The initial transition of dissolved silicate molecules into solid nanoparticles is perhaps the least explored step in the synthesis of zeolites. One impediment to understanding this mysterious step is the poorly elucidated molecular composition of dissolved particles. The major mechanistic ideas for the formation of zeolites approach these structures differently i) many researchers believe that secondary building units (SBU) must be present to form initial nanoslabs [1,2] ii) some others prioritize the role of monomers to feed artificially introduced crystal nuclei or assume that even these nuclei form via appropriate aggregation of monomers [3] iii) silicate solutions are also frequently viewed as random mixtures of various siloxane polymers which condense first into an irregular gel configuration which can rearrange subsequently into a desired crystal nucleus at appropriate conditions [4,5],... 

Table 2. Variation in cooling COP of a zeolite - water regenerative cycle with evaporating, condensing, adsorption rejection and maximum desorption temperatures 0°C, 40°C, 50°C and 350°C respectively.

Haines A process for recovering sulfur from natural gas, using a zeolite adsorbent. The hydrogen sulfide in the gas is adsorbed on the zeolite when the bed is saturated, hot sulfur dioxide is passed through it. The zeolite catalyzes the reaction between hydrogen sulfide and sulfur dioxide to fonn elemental sulfur, which sublimes out and is condensed. The process was invented by H. W. Haines in 1960 it was developed by Krell Associates and piloted in Canada from 1961 to 1962, but not commercialized because of problems caused by fouling of the zeolite with heavy hydrocarbons. 

---