Zeolite unique adsorption properties

Vne of the major industrial applications of zeolites is in the area of ad-sorption processes. Zeolite adsorbents are not only the most important adsorbents today, but their importance is increasing, mainly because of the following unique adsorptive properties (a) selective adsorption of molecules based on molecular dimensions, (b) highly preferential adsorption of polar molecules, (c) highly hydrophilic surface, and (d) variation of properties by ion exchange. 

Zeolites exhibit many unique adsorption properties, mainly because of their unique surface chemistry. The surface of the framework is essentially oxygen atoms, because Si and A1 are buried or recessed in the tetrahedra of oxygen atoms, so they are not exposed and cannot be accessed by adsorbate molecules. Also, the anionic oxygen atoms are much more polarizable... 

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

The polarizabilities of N2, O2, and Ar are nearly the same (1.74, 1.58, and 1.63 in units of 10 " cm, respectively), and are all nonpolar. Consequently, they adsorb nearly the same on all sorbents except zeolites. The fact that zeolites can distinguish between N2 and O2 was observed as early as 1938 (Barrer, 1937 1938). Barrer reported values for heats of adsorption of N2 on chabazite as high as 8 kcal/mol. The high heats of adsorption were subsequently explained quantitatively in terms of the quadrupole-electric field gradient interactions (Drain, 1953 Kington and Macleod, 1959). The unique adsorption properties of zeolites derive from the fact that their surfaces are composed of negatively charged oxides with isolated cations that are located above the surface planes. Despite... 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

The potential catalytic application of mesoporous molecular sieves is addressed to the adsorption and transformation of bulky compounds which are not capable of diffusing in the micropores of zeolites. In the case of Nb-containing mesoporous molecular sieves, their unique oxidizing properties [S] make them interesting also for the reactions of smaller molecules. 

Zeolites are crystalline materials with a well-defined system of micro-pores. Zeolitic materials are used in a variety of applications, one of the majors is in the area of separation processes because of their unique porous properties. The size and structure of the pores as well as the molecular structure of the adsorbate determines the adsorption capacity and selectivity [5]. 

A high degree of hydrophobic character is an almost unique characteristic of silicon-rich or pure-silica-type microporous crystals. In contrast to the surface of crystalline or amorphous oxides decorated with coordinatively unsaturated atoms (in activated form), the silicon-rich zeolites offer a well-defined, coordinatively saturated sur ce. Such surfrces, based on the strong covalent character of the silicon-oxygen bond and the absence of hydrophilic centers, display a strong hydrophobic character unmatched by the coordinativeiy unsaturated, imperfect surfaces. Also, hydrophobic zeolite crystals have been reported to suppress the water affinity of transition metal cations contained in the zeolite pores. This property permits the adsorption of reactants such as carbon monoxide or hydrocarbons in the presence of water. 

Zeolites [64] are crystalline aluminosilicates with a three dimensional micro-porous framework formed by corner sharing SiO and AIO4. (i.e. TO4) tetra-hedra. A framework with Si02 composition is stoichiometrically neutral. The substitution of Si by Al in such a silicate framework, results in an excess negative charge, which is compensated by cations or protons. Zeolites have unique adsorption and catalytic properties. Their diversity in framework composition and structure type leads to almost unlimited design opportunities. 

Chlorobenzenes are well known as important precursors of PCDD/Fs and are suitable model compounds for the complete oxidation of chlorinated POPs. Noble metal-based catalysts such as Pt/Al203 show high efficiencies but promote the formation of polychlorinated benzenes. Zeolites have unique properties for the deep oxidation of chlorinated compounds thanks to their well-defined framework and the presence of acid sites or transition metal cations. As stated by Corma, zeohtes present interesting properties of reactant/product partitioning and of molecules preactivated by the molecular confinement effect. Moreover, their adsorptive properties can be modulated by modifying the nature of the extra framework cations and the Si/Al ratio. 

The surface for adsorption is essentially entirely internal due to the channels and cavities which uniformly penetrate the entire volume of the adsorbent. The molecular sieving properties of the zeolites are uniquely determined by their pore diameters, the magnitude of which determines what size molecules are totally excluded from the interior of the zeolite. 

In contrast, recent work (4-12) has shown that Raman spectroscopy can be used to study Ti) adsorption on oxides, oxide supported metals and on bulk metals [including an unusual effect sometimes termed "enhanced Raman scattering" wherein signals of the order of 10 - 106 more intense than anticipated have been reported for certain molecules adsorbed on silver], (ii) catalytic processes on zeolites, and (iii) the surface properties of supported molybdenum oxide desulfurization catalysts. Further, the technique is unique in its ability to obtain vibrational data for adsorbed species at the water-solid interface. It is to these topics that we will turn our attention. We will mainly confine our discussion to work since 1977 (including unpublished work from our laboratory) because two early reviews (13,14) have covered work before 1974 and two short recent reviews have discussed work up to 1977 (15,16). 

In a relatively few years zeolites were promoted from simple adsorption agents to catalysts of wide spread use in all fields of chemistry. Apart from their acidic properties generated by exchanging their Na+ or K+ starting forms by ammonium ions and subsequent decomposition of the latter, their unique properties as supports for various precious metals and their solution behaviour attracted much of the attention devoted to catalysis. 

---