Zeolites studied adsorption

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... 

Silver Zeolite Iodide Adsorption Studies. The U.S. Atomic Energy Commission, Idaho Operations Office, Under Contract AT(10-1)-1230, Nov. 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

We studied adsorption properties of L-zeolites with respect to water and benzene vapors. The experimental data are given as isotherms for... 

Carbon dioxide adsorption causes changes in the sizes of all the zeolites studied similar to the variation observed for faujasite-type zeolites after water adsorption (Figure 6). For all zeolites, an increase in the adsorption of carbon dioxide leads to contraction this reached a minimum in the adsorption range 3-5.5 mM/gram. The final length of the pellets is below the initial value up to a relative pressure of p/ps 0.7 for zeolites CaA, CaY, and NaY while for NaA and NaX the contraction passes to an expansion, reaching 0.11% of the initial length at p/ps = 0.66 for NaX and 0.32 for NaA. 

Paramagnetic ions are now being used quite extensively to study adsorption phenomena. Mn ions have been used as probes for studying molecular motion in synthetic zeolites, (350) Co and Ni ions have been used for studying the complexation of molecular hydrogen on the surface of zeolites, (351) and these same ions have been used in a variety of studies of adsorption on Aerosil surfaces. (352-358) Adsorbed molecules studied include olefins, saturated hydrocarbons, alcohols, and benzene. From the measured line-shifts the number of active surface sites can be deduced in favourable cases. (357, 358)... 

Eic M., Micke A., KoCirik M., Jama M. and Zik6nov6 A. Diffusion and immobilization mechanisms in zeolites studied by ZLC chromatography. Adsorption... 

Because zeolites are much used as catalysts, there have been many studies on the interaction of molecules with the framework. Recent studies (2005-2007) include the effect of incorporation of impurity anions into the zeolite DSP, adsorption of NO in iron-exchanged ferrierite, " adsorption of CO in Li -FER, adsorption of CO on Cu in Cu-FER,adsorption of methane in zeolites A and methane... 

J. Turkevich (Princeton University, Princeton, N. J. 08540) What practical advice could you give us for the preparation of the zeolite before adsorption studies, and how long must one wait for equilibrium to be established ... 

The composition of the zeolites studied, which were kindly supplied by S. P. Zhdanov, is shown in Table I. Differential heat of adsorption of water was measured with an isothermal constant heat exchange calorimeter (17) at 23 °C. The heat of adsorption of the alcohols was measured with a variant of this calorimeter (J). The heat of adsorption of ethane and ethylene was determined from isosteres. The zeolites were pretreated by evacuating at 400°C for 30 hours. 

Acidity Studies. Adsorption of pyridine on Bronsted acid sites results in the formation of pyridinium ions with a characteristic absorption band near 1540 cm" Adsorption on Lewis acid sites produces a band at about 1451 cm" while interaction with the exchangeable cations produces bands at 1444 and 1448 cm for the calcium and magnesium forms, respectively. Typical spectra of pyridine adsorbed on magnesium hydrogen Y zeolite have been reported previously (22). Spectra of calcium... 

The sensitivity of lattice modes to structural changes is illustrated by the recent study of Mueller and Connor [25] on the effects of cyclohexane adsorption on the structure of MFI zeolites. The adsorption of molecules such as paraxylene and benzene into MFI zeolites causes a structural transition from monoclinic to orthorhombic symmetry, which has been characterized by X-ray powder diffraction and 29 si NMR spectroscopy [26]. Cyclohexane has a slightly larger kinetic diameter than benzene or paraxylene (0.60 nm compared with 0.585nm), but does not cause the same structural transition. Cyclohexane adsorption does however affect the zeolite lattice mode vibrational frequencies. Figure 7 shows spectra taken from reference 25 before and after (upper spectrum) adsorption of cyclohexane in a low aluminium MFI zeolite. 

Table VII presents a summary of calorimetric measurements of the differential heat of adsorption of ammonia, water, and carbon dioxide on the sodium form of ZSM-5 zeolite. Ammonia adsorption at 416 K (97.147) shows that NaZSM-5 zeolite is weakly acidic, whereas CO adsorption (147) indicates that in addition there are some weak basic sites. It should be noted that of the two samples studied with ammonia adsorption one was 70% H exchanged and the sodium content of the other was not given. Water adsorption on NaZSM-5 displayed unusual behavior, with a steep increase in the differential heat of adsorption at high surface coverages (166). An adsorption mechanism was proposed to explain these findings in which adsorption occurs first on the hydrophilic sites, consisting of sodium cations and framework anions where water molecules are bound by dipole-field interactions. Further adsorption takes place near these sites through weak interaction with zeolite surfaces, and when the number of water molecules close to these sites exceeds a certain value, they tend to reorient by forming clathrate-like struc-...

Although there are many ways to describe a zeolite system, models are based either on classical mechanics, quantum mechanics, or a mixture of classical and quantum mechanics. Classical models employ parameterized interatomic potentials, so-called force fields, to describe the energies and forces acting in a system. Classical models have been shownto be able to describe accurately the structure and dynamics of zeolites, and they have also been employed to study aspects of adsorption in zeolites, including the interaction between adsorbates and the zeolite framework, adsorption sites, and diffusion of adsorbates. The forming and breaking of bonds, however, cannot be studied with classical models. In studies on zeolite-catalyzed chemical reactions, therefore, a quantum mechanical description is typically employed where the electronic structure of the atoms in the system is taken into account explicitly. 

So far, only very little attention has been focussed on the use of zeolites in biocatalysis, i.e., as supports for the immobilization of enzymes. Lie and Molin [116] studied the influence of hydrophobicity (dealuminated mordenite) and hydrophilicity (zeolite NaY) of the support on the adsorption of lipase from Candida cylindracea. The adsorption was achieved by precipitation of the enzyme with acetone. Hydrolysis of triacylglycerols and esterification of fatty acids with glycerol were the reactions studied. It was observed that the nature of the zeolite support has a significant influence on enzyme catalysis. Hydrolysis was blocked on the hydrophobic mordenite, but the esterification reaction was mediated. This reaction was, on the other hand, almost completely suppressed on the hydrophilic faujasite. The adsorption of enzymes on supports was also intensively examined with alkaline phosphatase on bentolite-L clay. The pH of the solution turned out to be very important both for the immobilization and for the activity of the enzyme [117]. Acid phosphatase from potato was immobilized onto zeolite NaX [118]. Also in this study, adsorption conditions were important in causing even multilayer formation of the enzyme on the zeolite. The influence of the cations in the zeolite support was scrutinized as well, and zeolite NaX turned out to be a better adsorbent than LiX orKX. 

Adsorption of Propene, Benzene, their Mixtures and Cumene on H-beta Zeolites studied by IR and UV-VIS Spectroscopy... 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

In the case of microporous solids with a unique pore size, like the zeolites subject of this study, the characteristic curve should be a straight line. However, Figures 5 and 6 present two aspects that must be noted a) the characteristic curves obtained by N2 adsorption remain beneath the obtained by CO2. This behaviour can be easily explained considering that the pore size of the zeolites is close to, or lower than, 0.7 nm that is a pore size in which limitations to the accessibility of N2 molecules at 77 K start to be important, b) the characteristic curve of the zeolite NaY has a pronounced negative deviation for the highest region of adsorption potentials which is not present for any other zeolite studied. 

There are three remarkable aspects to be emphasised from Table 3. Firstly, all the zeolites studied are highly microporous materials, in accordance with their open structure. Secondly, the micropore volumes obtained from CO2 adsorption at 273K are, in all the cases, higher than those obtained from N2 adsorption at 77K. The extreme case is zeolite NaA in which N2 adsorption at 77 K does not occur. Finally, gas adsorption is sensitive to the changes in volume produced by the presence of cations with different sizes. 

We are concerned with the kinetics of zeolite-catalyzed reactions. Emphasis is put on the use of the results of simulation studies for the prediction of the overall kinetics of a heterogeneous catalytic reaction. As we will see later, whereas for an analysis of reactivity the results of mechanistic quantum-chemical studies are relevant, to study adsorption and diffusion, statistical mechanical techniques that are based on empirical potentials have to be used. 

The striking affinity of dehydrated zeolites for adsorption of a wide variety of guest molecules in a highly selective manner was recognized at an early date by Barrer and others (3-5). Such studies have ultimately led to the development of several important commercial separation, drying, and purification processes using zeolites. 

Different solid acid catalysts like zeolite Y [2-6], beta [7-9], MCM-22 [10], solid superacids [11-13], sulphonic acid resins [14], etc. have been proposed as potential alkylation catalysts and some of them are being tested at a pilot plant scale. Zeolites and solid superacids of sulfated zirconia type were found to be the most active but they suffer rapid deactivation after an initial period. Among different zeolites studied large-pore zeolites are prefered over medium-pore type because the former favors the formation and diffusion of bulkier tri-methylpentane isomers. Beside pore size and zeolite structure, the fiamework composition (Si/Al ratio) and acid strength distribution also play an important role on the activity, selectivity and deactivation of the catalysts. It is known that the adsorption behavior of the zeolite and the extent of hydrogen transfer capacity (a crucial factor of alkylation activity) both depend on the aluminium concentration in the framework [15-16]. 

ZSM-5 and mordenite (both zeolites) showed adsorption capacities for MTBE similar to the carbonaceous resins [54]. Mordenite s superiority over activated carbon was documented by various studies [20,28,47,52,54,57]. The aperture size of the zeoHtes is of significant influence for the adsorp-... 

Methane has been proposed as an alternative probe for cationic forms of zeolites [159,160]. As with probes of metal oxides (Section 3.3), the low adsorption affinity of CH4 and its similarity to other hydrocarbon molecules involved in catalytic reactions are considered particularly advantageous. We studied adsorption complexes of CH4 with cationic sites of alkali (Li, Na, K, Rb, Cs) and aUcaline-earth (Mg, Ca, Sr, Ba) forms of zeolites [161]. The goal was to rationalize structure and bonding of methane complexes with metal forms of high-silica zeolites as well as their measured vibrational features. We aimed also at clarifying the origin of opposite trends observed in the adsorption-induced alteration of the vibrational frequency and the IR intensity of the symmetric stretching mode... 

As shown by studies on various zeolites (ZSM-5, mordenite, H-Y) [44,55], it is difficult to compare adsorptions of ammonia and pyridine, because of their difference in basicity and the specific interactions of each of these molecules with the host zeolite. The adsorption heats should be correlated, not to the basicity of the probe molecules in liquid phase or in aqueous solutions (where NH3 is more basic dian pyridine by about 20 kJ mol", because the pKa values are 9.3 and 5.2, respectively), but to their basicity in the gas phase, which can be expressed in terms of the proton affinities (PA). In the gas phase, isopropylamine is a stronger base than pyridine and tiian ammonia the PA value of isopropylamine (pKa = 10.7) is about 924 U mol, and tiiat of pyridine is about 922.2 kJ mol", while that of ammonia is only about 857.7 kJ mol". This feature is illustrated in Figure 3, which represents the differential heats of ammonia, pyridine and isopropylamine vs uptake on a H-mordenite zeolite [54]. Other parameters that have to be taken into account are size (the kinetic diameters of pyridine and ammonia are 0.533 and 0.375 nm, respectively) and the secondary interactions between these adsorbates and the zeolite structure (hydrogen bonds may lead to the formation of a monodentate bond with pyridine and a bidentate bond with ammonia), as shown by Parillo et al. [55-57], These differences are reflected in the heats of adsorption, which are usually 20-30 kJ mof higher in the case of pyridine, than in the case of ammonia. A similar difference is observed for the chemisorption limit [44,54]. 

Mid-IR spectroscopy (400 000 cm ) has been shown by the group of Zecchina to be very informative when studying adsorption at different adsorption sites in zeolites.The spectroscopy is performed at low temperatures using small diatomics such as CO, N2 and H2 as probe molecules that have suitable IR characteristics and that can gain access to the entire pore space. As well as being unreactive, these probe molecules have IR characteristics that... 

Datka, J. (1981) "Adsorption of benzene and toluene on Y zeolites studied by in-... 

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