Basics of Zeolites

Lercher, J.A., Jentys, A., and Brait, A. (2008) Catalytic test reactions for probing the acidity and basicity of zeolites. Mol. Sieves, 6,153-212. 

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. 

Researchers attempted to find correlations between the composition of the unit cell and the selectivity for paraxylene. In this respect, D. Barthomeuf (9) proposed an approach based on Sanderson s intermediate electronegativity which allows us to estimate the basicity of zeolite oxygens, and hence the strength of the acid-base interaction between xylene molecules and zeolites. It should be noted that these calculations provide an insight into the interactions between the zeolite structure and the molecules at low loading only, i.e. when the interactions between adsorbed molecules are negligible. 

We have recently studied systematically the Lewis basicity of zeolites using pyrrole as a probe molecule. Pyrrole has H-donor properties and adsorbs on basic zeolites forming NH—O bridges. The NH stretching vibration was shown by Barthomeuf [51-53] and others [54] to be a measure of the Lewis base strength of 0 in zeolites. 

In view of the means available to vary acidity and basicity of zeolites, studies of mechanistic detail using model alcohols might be very rewarding. 

The basicity of zeolite catalysts can be increased even further by occluding... 

Alkaline or alkaline earth metal oxides are well known basic catalysts and much of work has been devoted to their characterization and to the study of their catalytic activity [64,65]. Taking into account the strong basic character of these oxides it is possible to enhance the basicity of zeolites by over-exchanging them with alkali [66-69] or alkaline earth metals [70-72] and producing after thermal de-... 

The use of nitromethane as a probe of basicity of zeolites (NaX, CsX, CsX 9Cs) and mixed oxides, Mg(Al)0, is discussed. Various species (physisorbed nitromethane, aci-anion nitromethane, and methazonate salt analogue) formed upon nitromethane adsorption were characterized by C MAS NMR spectroscopy. Heterogeneous base catalysis of the Michael addition of nitromethane on cyclohex-2-en-1-one was also studied. Low rates were obtained for catalysts showing only nitromethane physisorption. Formation of aci-anion nitromethane was observed for solids of medium efficiency correlation of the chemical shift with the initial rate was established. Finally, the decrease of Lewis acidity and concomitant increase of basicity led to methazonate formation and to the more efficient catalysts. 

The number of probe molecules suitable for studying the basicity of zeolites by application of IR spectroscopy is much more limited than in the case of acidity. The sites of basicity are most likely oxygen atoms of the framework or basic extra-framework species such as CS2O or alkali metal clusters (cf. Volume 3, Chapters 5 and 6 of the present series). 

As Rymsa et al. [813] have shown, the utilization of deuterochloroform as a probe is another possibility of characterizing the basicity of zeolitic systems by FTIR spectroscopy. The C-D stretching band of CDCI3 was shifted to lower wavenumbers when the probe was adsorbed on basic materials such as M, Na-X, M, Na-Y (M=K, Rb, Cs), Cs-Beta, and the shift increased with increasing size of the cation exchanged into the zeolite. 

For cationic forms of zeolites, the observed trends in (mild) basicity are clear. Zeolites with higher aluminium contents in the framework are more basic than high silica zeolites. Similarly, the basicity of zeolites exchanged with cations increases in the order Na[Pg.293]

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [59]. Moreover, a difficulty stems from the fact that some acidic probe molecules may interact simultaneously with cations (such as Na" ). The ideal probe molecule should be specific to basic sites and should not be amphoteric. It should not interact with several types of basic sites or give rise to chemical reactions [59]. For instance CO2 (pKa = 6.37) is commonly chosen to characterize the basicity of zeolites but it may either be adsorbed on the cations or physisorbed, or may react with hydroxyls and framework oxide ions to give carbonated species. However, the energetic aspect of the adsorption of CO2 on various molecular sieves in a large domain of temperature and pressure can provide interesting information on the nature of the adsorbate-adsorbent interactions [66]. The heats of adsorption of the quadrupolar molecule CO2 increase in the order silicalite < H-ZSM-5 < Na-ZSM-5 < Na-X [67]. 

Catalytic Test Reactions for Probing the Acidity and Basicity of Zeolites... 

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