All-silica zeolite

Marquez, F., Zicovich-Wilson, C.M., Corma, A., Palomares, E. and Garcia, H. (2001). Naphthalene included within all-silica zeolites influence of the host on the naphthalene photophysics. J. Phys. Chem. B 105, 9973-9979... 

Bates et al. (172-174) considered the energetics, locations, and conformations of alkanes ranging from butane to decane in a variety of different all-silica zeolites. Calculations similar to those described already were performed for alkanes in mordenite, zeolite rho, faujasite, ferrierite, and zeolite A. A linear increase in the calculated heat of adsorption with increasing carbon number was found for all zeolites. Less experimental information is available to compare with the calculated heats of adsorption, and thus the performance of the technique and parameters cannot be subjected to quite the same scrutiny as the results for silicalite (111). Nonetheless, where... 

Hydrophobic zeolites, as well as the all-silica zeolites or zeolites with a very small aluminum content, possess high capacity for adsorbing organic compounds dissolved in water. Some recent studies demonstrated that hydrophobic, dealuminated zeolites adsorbed organic compounds from water as effectively as activated carbon [2,37,88,89,214], The hydrophobicity of zeolites is controlled basically by changing the Si/Al ratio in the framework by synthesis conditions and postsynthesis modification treatments [215],... 

The high Si Al ratio of the material, typically around 10, makes the material inherently hydrophobic. Changing the Si Al ratio of the material has, however, a marked influence on the hydrophobicity. The increased hydrophobicity of siliceous Beta has been demonstrated by competitive adsorption of toluene and water. The so-called hydrophobicity index32, the amount of toluene adsorbed divided by the amount of water adsorbed at 25 °C amounts to 1.4 to 2.2 for a Si Al = 10 sample and increases to 10.8 to 66 for the all-silica material (see Table 2). The large difference observed for the all-silica zeolites is most likely due to differences in the amount of defects in the material. These defects are essentially silanol pairs required for template charge compensation during synthesis as shown by van der Waal et al.12... 

De Vos Burchart et al. have recently developed a force field for modeling zeolites.21 The model originally was intended for all-silica zeolites but was quickly extended to aluminum-containing zeolites. The parameters were derived from several sources. Standard bond dissociation energies were used, and the force constants were refined to fit the structure of ZSM-5, the structure and frequencies of a-quartz, in addition to unit cell dimensions of other zeolites. With the all-silica model, the authors were able to calculate heats of formation... 

In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems Table 6). 

A Consistent Molecular Mechanics Force Field for All-Silica Zeolites. 

Astala, Auerbach, and Monson277 used plane wave LDA calculations to examine structural characteristics of five all-silica zeolites SOD, LTA, CHA, MOR, and MFI. The first four of these materials can be examined in calculations with 36-72 atoms in a periodic unit cell, but the MFI structure is considerably larger and requires 288 atoms in a single unit cell. Astala et al. argue that LDA calculations give more accurate results for the structure and energies of silica polymorphs than GGA calculations. Demuth et al. used LDA and GGA calculations to examine the properties of Al substitutions in morde-nite both in a protonated form and in the presence of Na cations.278... 

F was used for the first time as mineralizer in the synthesis of zeolites under neutral or acidic conditions by Flanigen and Patton1941 Subsequently, Guth and Kessler further studied this new synthetic method.[95] Many high- or all-silica zeolites such as ZSM-5, ZSM-23, Theta-1, ferrierite, / , MTN, AST, UTD-1, ITQ-3, ITQ-4, and heteroatom (B, Al, Fe, Ga, and Ti)-substituted high-silica zeolites could be synthesized from an F synthetic system. In addition, most of the open-framework gallophosphates could be hydrothermally synthesized under the influence of F. ... 

D.P. Serrano, R. Van Grieken, P. Sanchez, R. Sanz, and L. Rodriguez, Crystallization Mechanism of All-silica Zeolite Beta in Fluoride Medium. Microporous Mesoporous Mater., 2001, 46, 35-46. 

In electrides, electrons behave as anions. In a typical organic electride Cs+ (15-crown-5)2 e , the electrons occupy cavities connected by narrow channels. The first electride stable at room temperature was made by ionizing Cs into Cs+ and e within the channels of an all-silica zeolite. Petkov etaZ confirmed that Cs+ and a low-density electron gas are present in the one-dimensional nanopores of the siliceous zeolite lTQ-4 (8132054). The PDF technique provided direct structural evidence that Cs intercalated in the form of Cs+ ions arranged in short zigzag chains (see Figure 10). This structural model... 

Figure A2.3 (a) Structure of an all-silica zeolite silicalite, MFI framework (b) Speed-up of the total energy and wavefunction calculation with the number of processors in a cluster of AMD Athlon 1.9-GHz single-processor, 1-GB RAM, 512-KB cache, EIDE disks. The code was compiled with PGE90 v4.2 and -01 -tp athlon options.

We consider the scaling of PCRYSTAL with the number of active processors for silicalite (MFI framework see Figure A2.3a), an all silica zeolite with 288 atoms in the unit cell (4416 AOs, 8-point symmetry operations 8 k-points). Figure A2.3b shows that an almost linear correspondence exists between the number of active processors and execution time. The CPU time decreases from about 23 to 3 hours when distributing the job to eight proces-... 

Shah M, Tsapatsis M, Siepmann JI, Ilja J (2015) Monte carlo simulations probing the adsorptive separation of hydrogen sulfide/methane mixtures using all-silica zeolites. Langmuir. Ahead of Print... 

Simple potentials based on equation (2) have been used to describe the interaction of adsorbed hydrocarbon molecules with zeolites see, for example, Kiselev et al. For all-silica zeolites the electrostatic interaction is frequently neglected in such simulations. Further simplification is achieved when the two-body terms are calculated for whole CH groups instead of for individual atoms. Many of these applications assume that both the zeolite and the molecules are rigid. Some allow at least for torsions about C-C bonds in saturated hydrocarbons. More advanced approaches combine the intermolecular potentials with some type of force fields for the adsorbed organic molecules and/or for the zeolite framework. ... 

The all-silica zeolites, both BEA and MFI, are in general characterized by a variable amount of internal defects consisting of hydroxyl nests made up of H-bonding interacting Si-OH species, located within the zeolite nanopores. A virtually perfect (i.e. defect-free) MFl-Silicalite was also investigated for comparison purposes. See Ref. [24] and references therein for details on both defective and perfect all-silica specimens. 

NH3 gas (from Praxair) was used as molecular probe of moderate basic strength (PA = 854kJ moP ), [66]. in order to characterize the acidic strength of Brpnsted (and Lewis, if any) acidic sites in protonic and all-silica zeolites. 

NH3 is adsorbed on all-silica zeolites and on hydroxylated amorphous silica with a zero-coverage adsorption enthalpy as high as 80 kJ mol [24] whereas on dehydroxylated silica — (Afl/f)o is only = 60kJ mol [28]. 

Conversely, the heat of adsorption of CO at polar sites located in the all-silica nanopores (Si-OH nests) was much lower (q llk] mol ) than on H-BEA, and virtually constant over the whole coverage examined in agreement with the formation of very weak H-bonding adducts, as described in Refs. [73, 105], The adsorption capacity of the all-silica zeolite was dramatically lower than that of the proton-exchanged zeolite, as witnessed by the correspondent isotherms reported in the inset... 

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