Microporous frameworks

Quaternary ammonium ions have long been known to act as stmcture-directing agents in the crystallization of zeolites <1961JCS971 >. Millini and co-workers have more recently employed the azoniaspiro compounds 154,9, and 155 for this reason to produce novel microporous frameworks <1998MI199>. 

The importance of framework density and molar volume is evident also for large pore, mesoporous silica [33] and for AIPO4 polymorphs [34], Data for the latter are included in Figure 7.19. For mesoporous silica a transition from a regime where cages and pores affects the energetics to one in which the large pores act as inert diluent is reported. A further increase in pore diameter does not appear to increase the enthalpy of the compound [33], The similarity in enthalpy of many different structures shows that the synthesis of metastable microporous framework... 

In the metal aluminophosphate (MeAPO) family the framework composition contains metal, aluminum and phosphorus [27]. The metal (Me) species include the divalent forms of Co, Fe, Mg, Mn and Zn and trivalent Fe. As in the case of SAPO, the MeAPOs exhibit both structural diversity and even more extensive composihonal variation. Seventeen microporous structures have been reported, 11 of these never before observed in zeoUtes. Structure types crystallized in the MeAPO family include framework topologies related to the zeolites, for example, -34 (CHA) and -35 (LEV), and to the AIPO4S, e.g., -5 and -11, as well as novel structures, e.g., -36 (O.Snm pore) and -39 (0.4nm pore). The MeAPOs represent the first demonstrated incorporation of divalent elements into microporous frameworks. 

Other Framework Compositions GrystaUine microporous frameworks have been reported with composihons of beryUophosphate [43], aluminoborate [44], alumi-noarsenate [45], galloarsenate [46], gallophosphate [47], antimonosilicate [48] and germanosilicate [49],... 

Two major new classes of molecular sieve type materials were reported in the 1990s (i) microporous frameworks based on mixed octahedral-tetrahedral frameworks in contrast to the previously described tetrahedral frameworks and (ii) mesoporous molecular sieves with pore sizes ranging from about 2 nm to greater than 10 nm. 

Xenon has been considered as the diffusing species in simulations of microporous frameworks other than faujasite (10-12, 21). Pickett et al. (10) considered the silicalite framework, the all-silica polymorph of ZSM-5. Once again, the framework was assumed to be rigid and a 6-12 Lennard-Jones potential was used to describe the interactions between Xe and zeolite oxygen atoms and interactions between Xe atoms. The potential parameters were slightly different from those used by Yashonath for migration of Xe in NaY zeolite (13). In total, 32 Xe atoms were distributed randomly over 8 unit cells of silicalite at the beginning of the simulations and calculations were made for a run time of 300 ps at temperatures from 77 to 450 K. At 298 K, the diffusion coefficient was calculated to be 1.86 X 10 9 m2/s. This... 

With the help of Equation 5.107, as was previously done with Equation 5.86, we obtain a transport or chemical diffusion coefficient that is a result of Fick s laws. We now interpret the meaning of this coefficient if we consider diffusion in a microporous solid, as a special case of binary diffusion, where A is the mobile species and the diffusivity of the microporous framework atoms is zero, then, the frame of reference are the fixed coordinates of the porous solid consequently, we have a particular case of interdiffusion where the diffusion coefficient is simply the diffusivity of the mobile species [12,20],... 

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

The third breakthrough concerns the mesoporous compounds [13,14] with pore sizes in the 20-100 A range by using surfactant micellar templates. Very recently, the concept of microporous frameworks, which was primarily reserved to inorganic skeletons took a new dimension by the discovery of porous mixed inorganic-organic frameworks [15]. 

Holland B.T., Abram L., Stein A., Dual templating of macroporous silicates with zeolitic microporous frameworks, J. Am. Chem. Soc. 121 (1999) pp. 4308-4309. 

Figure 3.13 The crystal structure of [Tb2(BDC)3-(H20)4] shown approximately down the crystallographic b-axis, where aqua ligands are found to point toward the center of the ID-channels (Tb, black O, grey C, white H, omitted). (Redrawn from the GIF file of T.M. Reineke et al, From condensed lanthanide coordination solids to microporous frameworks having accessible metal sites, Journal of the American Chemical Society, 121, 1651-1657, 1999 [72].)...

Highly sihceous zeoHtes (with Si/Al ratios 1) are microporous framework alumino-sihcate materials. Discussion of the framework skeletal vibrahons of highly sihceous zeolites is similar to that reported above for silicas. The addition of aluminum in the framework causes shifts in the positions of the sole band. In particular, the asymmetric Si—O—Si stretching modes of framework silicates, usually observed as a complex very strong absorption in the region 1200-1000 cm" , tend to shift down a litde with A1 for Si subshtution. 

Zeolites are microporous frameworks, and all of the ET chemistry that we have discussed is with molecules smaller than 13 A. The unique features of zeolites are their ion-exchanging ability, a stable structure upon dehydration and a pore/chan-nel structure that allows for a well-defined arrangement of molecules in space and the fact that redox-active atoms can be substituted on the framework. In most cases, the zeolite is an active host, influencing ET reactions via electrostatic fields or steric effects, a feature that is not found with the mesoporous and sol gel materials. Packing of molecules/ions in the intrazeolitic space with very high densities is also possible and was found to be important in charge propagation and electrochemistry. 

The MeAPO family described here marks the first demonstrated incorporation of the divalent forms of cobalt (CoAPO), iron (FAPO), magnesium (MAPO), manganese (MnAPO), or zinc (ZAPO) into microporous frameworks during synthesis. 

The commonly used approach to drive off the guest molecules from the microporous frameworks is high-temperature (550 °C in air) calcination, which oxidizes and decomposes the organic molecules. However, this process is highly exothermic, and if inappropriately handled, the zeolite structures would be destroyed by the calcination. For instance, Da[1] found that calcination of zeolite BEA at 550 °C removes the template... 

The application of solvent extraction to removal and recovery of templates or SDAs from zeolite channels was initiated by Whitehurst[7] in the 1990s for the extraction of surfactant from mesoporous M41S materials. This method or its improved analogs have become one of the most important techniques to recover surfactants from mesoporous molecular sieves. However, it is still difficult to use this technique to remove and to recover the SDAs from microporous molecular sieves because, first, the size of SDA molecules is similar to that of the channel openings and the molecules are not able to diffuse out from the channels, and, secondly, there are usually strong interactions between the microporous frameworks and the SDA molecules that prevent the SDA molecules from being extracted solely by solvents. Modification of the conventional solvent-extraction technique, such as addition of chemical agents which can adjust the... 

The modification of zeolites mainly relies on secondary synthesis methods. The aim of modification is to reprocess the zeolites using suitable techniques to improve the properties and functions such as (1) acidity, (2) thermal and hydrothermal stability, (3) catalytic performance such as redox catalytic and coordination catalytic properties, etc., (4) channel structures, (5) surface properties and microporous frameworks and charge-balancing ions. Modification is also called secondary synthesis and can lead to new properties that cannot be achieved through direct synthesis. Let us consider the case of faujasite (FAU), the main component of the cracking catalyst, and its catalytic performance (represented by the catalytic activity K/K Std for n-hexane cracking). From Table 6.1 it is seen that the secondary synthesis affects the catalytic performance to a considerable degree. 

D net sheet structures which are closely related to the 3-D microporous frameworks show rich structural diversity. For example, 2-D 3.4-connected layered aluminophosphates with Al3P40i63 stoichiometry exhibit various sheet structures.154-671 Except for... 

Thus, this methodology will serve as a powerful tool for the rational synthesis of target materials with specified sheets and microporous frameworks. 

Very recently, Fedin, Kim and colleagues synthesized a homochiral metalorganic polymeric material, [Zn2(bdc)(L-lac)(dmf)] (DMF) (36), by using a one-pot solvothermal reaction of Zn(NO3)2, L-lactic acid (L-H2lac) and 1,4-benzenedicarbox-ylic acid (H2bdc) in DMF [53]. This 3-D homochiral microporous framework exhibited permanent porosity and enantioselective host-guest sorption properties towards several substituted thioether oxides. Although 36 could catalyze the oxidation of thioethers to sulfoxides with size and chemoselectivity, no asymmetric induction was observed. 

Acidity of the materials has been determined by ir and thermodesorption of ammonia and their catalytic properties for toluene alkylation with methanol have been studied. Relations between acidic strength, diffusion rate, catalytic shape selectivity and structural features are discussed. It is suggested that the microporous framework has to be considered as a "living material" under catalytic reaction conditions. 

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