FTIR and EPR of adsorbed probe molecules were used to study Lewis acid sites in FAU and MFI zeolites with different nonframework Al content. Infrared spectra of adsorbed CO at low temperatures give information about all types of sites. The absorption bands were assigned to CO fixed on Brönsted sites, cations, and true Lewis acid sites with different strengths. The frequency shifts and the relative intensities of these peaks were correlated with the structure type and the nonframework Al amount. In contrast, EPR of adsorbed NO is probing only the sites strong enough to quench the orbital magnetic moment of NO molecule. However, the method can provide an estimation of the strength of the detected Lewis acid sites, via the g zz parameter. A correlation was established between the amount of nonframework Al and the spin concentrations of adsorbed NO. These absolute concentrations are lower than the expected number of Lewis acid sites. Several methods were proposed for creating Lewis acid sites in a controlled way. Among them, alumination and mild steaming were the most efficient, leading to well-distributed nonframework Al species, which act as strong Lewis acid sites. Despite its high nonframework Al content, the severely steamed zeolite Y has weak sites, only partially accessible for probe molecules, such as CO and NO.

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One of the greatest challenges of experimentalists working in the field of molecular sieve science is the understanding of the principles that determine how porous crystalline materials are formed starting from a precursor gel under hydrothermal conditions. [1] This is far from easy because hydrothermal crystallizations take place in a closed vessel, where many interactions, equilibria, and chemical processes continuously change with crystallization time. [2] The lack of knowledge about these phenomena means that a rational a priori design of novel molecular sieves is still impossible and, consequently to date, the synthesis of such materials requires a systematic and intelligent screening of the n-dimensional reaction. Recently, the concepts of combinatorial chemistry [3] and experimental design [4] have been successfully explored in order to speed up this screening process. Clearly, a more detailed understanding of the processess occurring during the synthesis of these materials is required, which can lead to a more rational approach towards zeolite syntheses. Experimental SectionReaction of N-methyldihydropyridine with carbene complexes (general procedure): A solution of N-methyldihydropyridine (3 equiv) in CH 2 Cl 2 (0.5 mL per mmol of amine) was added dropwise to a solution of carbene complex (1 equiv) in CH 2 Cl 2 (25 mL per mmol of carbene) at À 10 8C under argon . After 15 min, the ice bath was removed and the mixture was stirred at room temperature for 24 h. The solution slowly turned dark red. The solvent was evaporated under vacuum, and the residue purified by chromatography on silica gel with petroleum ether (PE)/Et 2 O as eluent.

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