Zeolite, adsorbed molecules

Zeolites are high-capacity, selective adsorbents because they separate molecules based on the size and configuration of the molecule relative to the size and geometry of the main apertures of the structures zeolites adsorb molecules, in particular those with permanent dipole moments which show other interaction effects, with a selectivity that is not found in other solid adsorbents. 

Zeolites form a unique class of oxides, consisting of microporous, crystalline aluminosilicates that can either be found in nature or synthesized artificially [J.M. Thomas, R.G. Bell and C.R.A. Catlow in Handbook of Heterogeneous Catalysis (Ed. G. Ertl, H. Knbzinger and J. Weitkamp) (1997), Vol. 1, p. 206, VCH, Weinheim.]. The zeolite framework is very open and contains channels and cages where cations, water and adsorbed molecules may reside and react. The specific absorption properties of zeolites are used in detergents, toothpaste, and desiccants, whereas their acidity makes them attractive catalysts. 

Fig. 2 Calculated low energy conformation of the protonated dithiane oxide cation (R=H) in zeohte Y (Si/Al = 1). The bottom view shows a view through the twelve ring containing e deprotonated framework oxygen, the top view is perpendicular to this. For clarity the zeolite framework is shown using a stick model and the adsorbed molecule is drawn in space filled form represented by tlie Van der Waals radii for the atoms being in the order S>0>C>H.

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

As a conclusion of this section, it can be said that the method used has to be carefully chosen according to the sample studied and/or the expected results. Conventional XRD may be sufficient to localise a single cation species in a dehydrated zeolite whereas for bicationic zeolites more elaborate techniques like anomalous XRD or MAS and MQMAS NMR may be necessary. If the focus of the study is more on the influence of adsorbed molecules on the distribution of the cations, neutron scattering may be needed to complete the work. Finally, highly dealuminated zeolites may be difficult to study with diffraction techniques, in this case NMR techniques may be the best available option. 

The transmittance IR spectra of propene adsorbed by the Ca and Mg forms of Y zeolite correspond to a superposition of the bands of weakly adsorbed molecules with those of molecules that are more strongly specifically adsorbed by the bivalent cations. Upon reducing the propene pressure, the bands from the weaker form were eliminated, while the stronger forms of adsorption predominate. Therefore, to separate the spectral contributions of the species strongly adsorbed by Ca2+ or Mg2+ cations from those of the weakly adsorbed species, we recorded spectra at very low propene pressures of 0.5 mbar. For propene adsorption by NaY the adsorbed species were homogeneous and all of the bands in the IR spectra increased in intensity with increasing pressure up to 15 mbar without changing their positions. 

The interaction of CO and acetonitrile with extra-framework metal-cation sites in zeolites was investigated at the periodic DFT level and using IR spectroscopy. The stability and IR spectra of adsorption complexes formed in M+-zcolitcs can be understood in detail only when both, (i) the interaction of the adsorbed molecule with the metal cation and (ii) the interaction of the opposite end of the molecule (the hydrocarbon part of acetonitrile or the oxygen atom of CO) with the zeolite are considered. These effects, which can be classified as the effect from the bottom and the effect from the top, respectively, are critically analyzed and discussed. 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

The model based on formal kinetics was used to model the TPD curves of adsorbed CO molecules, based on the model previously reported [4], The desorption is strongly affected by the fast readsorption of CO on unoccupied Cu+ ions, thus, a quasiequilibrium state is a suitable approximation for the description of adsorption. A Langmuir type of adsorption isotherm was assumed for the CO adsorption on the Cu+ sites in zeolite, without considering lateral interactions among adsorbed molecules. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

Ion transport in charged ion-exchange resins 4. Advance of an adsorbate molecule from one cage to another within a zeolite crystal... 

Upon selective absorption of analyte molecules from the ambient environment, the zeolite thin film increases its refractive index. Correspondingly, release of adsorbed molecules from the zeolite pore results in the decrease of its refractive index. The absorption/desorption of molecules depends on the molecule concentration in the environment to be monitored. Therefore, monitoring of the refractive index change induced phase shift in the interference spectrum can detect the presence and amount of the target analyte existing in the environment. 

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

The first comparison is based on the T values of gaseous, liquid and adsorbed molecules. Unfortunately, no measurements are available for butenes in the gas or liquid phase. Nevertheless a reasonable parallel can be drawn with propylene where the three different phases were investigated (35) at 295 K for the gas (1 atm) Tj of C2 is 0.095 s in the liquid state (2.6. M in CDCI3) 59.9, 58.7 and 65.2 s for Cj, C2 and C3 respectively adsorbed on NaY zeolite 0.81, 1.6 and 0.81 s. The shortest relaxation times characterize the gas phase where the spin-rotation mechanism (NOE factor n = 0) is very effective (30,35). In the liquid, dipole-dipole and spin-rotation mechanisms both play a role and the total relaxation rate is about three orders of magnitude lower than in the gas phase. The adsorbed molecules show therefore an intermediate behaviour between gas and liquid, as it was also suggested by chemical shift data. 

Ab initio methods, 147-49 Acetate ion, decomposition, 135 Acetylene, interaction with palladium, tunneling spectroscopy, 435,437f Acid-dealuminated Y zeolites catalytical properties, 183 sorption, 175-78 Acid sites, on zeolites, 254 acidification effects, 266 Acoustic ringing, in NMR, elimination, 386 Active sites, nature, 104 Activity measurements, Co-Mo catalysts, 74 Adsorbed molecules,... 

The determination of the specific surface area of a zeolite is not trivial. Providers of zeolites typically give surface areas for their products, which were calculated from gas adsorption measurements applying the Brunauer-Emmet-Teller (BET) method. The BET method is based on a model assuming the successive formation of several layers of gas molecules on a given surface (multilayer adsorption). The specific surface area is then calculated from the amount of adsorbed molecules in the first layer. The space occupied by one adsorbed molecule is multiplied by the number of molecules, thus resulting in an area, which is assumed to be the best estimate for the surface area of the solid. The BET method provides a tool to calculate the number of molecules in the first layer. Unfortunately, it is based on a model assuming multilayer formation. Yet, the formation of multilayers is impossible in the narrow pores of zeolites. Specific surface areas of zeolites calculated by the BET method (often termed BET surface area) are therefore erroneous and should not be mistaken as the real surface areas of a material. Such numbers are more related to the pore volume of a zeolite rather than to their surface areas. 

Overall the period since the 1980s can be described as a period of explosion in the discovery of new compositions and structures of molecular sieves. This can perhaps be seen most vividly by comparing the numbers of structure types contained in the various editions of the Atlas of Zeolite Structure Types [4]. The first edition (1978) contained 38 structure types, the second edition (1987) 64, the third edition (1992) 85 and the most recent edition (2007) 176. Thus 112 new structure types have been discovered since 1978. However, the reader should be cautioned that a significant number of the structure types included in the Atlas are not truly microporous or molecular sieve materials (i.e., they are not stable for the removal of as-synthesized guest species, typically water or organic templates) and therefore carmot reversibly adsorb molecules or carry out catalytic reactions. Unfortunately, the Atlas gives only limited information on the stability of the structures described. 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

Adsorbed water molecules on a zeolite adsorbent are polarizable due to a strong electrostatic field between the exchanged cations and alumina framework [26]. 

In discussing gas phase separations, a few definitions will help in understanding the subject matter. Adsorbents, sometimes referred to here as sorbents, are solid chemical substances that possess micro-porous surfaces that can admit molecules to the interior surface of the structure. Zeolites in particular are solid, micro-porous, alumino-silicates with adsorption and or ion exchange capability. They affect separations by adsorbing molecules into their micro-structures. 

As the aforementioned example demonstrates, for strong-strong separations the selectivity can be dependent on the loading of the membrane. When the size of the adsorbed molecule is similar to the zeolite pore, at loadings near saturation the zeolite framework atoms will adjust to allow for entropically favorable packing. Under these conditions, constituents must compete with one another for adsorption sites and molecule-molecule interactions play a dominant role [33]. 

Onida, B., Geobaldo, F., Borello, L., and Garrone, E. (2001) Addity of lTQ-2 zeolite as studied by FT-IT spectroscopy of adsorbed molecules in comparison with that of MCM-22. Stud. Surf Sd. Catal., vol. 135, Elsevier, Amsterdam, pp. 1984-1990. 

At a given temperature adsorption isotherms measure the number of adsorbed molecules as a function of pressure for the fluid that is in contact with the zeolite. The simplest form is the Langmuir isotherm which treats the zeolite as a collection of equivalent adsorption sites in the absence of adsorbate-adsorbate... 

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