On the interaction of light amines and alcohols with alkali metal exchanged X zeolites R. Schenkel, R. Olindo, J. Kornatowski, J.A. Lercher * Institut fu ¨r Technische Chemie, Technische Universita ¨t Mu ¨nchen, Lichtenbergstr. 4, D-85747 Garching, Germany Available online 18 April 2006 Abstract Adsorption of ammonia, methylamine and 1-propylamine on alkali cation exchanged X zeolites was investigated using in situ IR spectroscopy and compared to sorption of water, ethanol and 1-propanol. Amines and alcohols formed similar primary adsorption complexes in the supercages of alkali cation exchanged zeolite X. Three elements contribute to the bonding, i.e., (i) the polar sorbate interacts with alkali metal cations via the lone electron pair of its electronegative atom, (ii) the protons of the functional groups of the sorbate form hydrogen bonds with the negatively charged oxygen atoms of the framework, and (iii) the hydrogen of the alkyl groups interact with the lattice oxygen. Amines form only primary adsorption complexes and do not exhibit sorbate–sorbate interactions as observed for alcohols. # 2006 Elsevier B.V. All rights reserved. Keywords: Alkali cation exchanged zeolite X; Alcohol adsorption; Amine adsorption; IR-spectroscopy 1. Introduction Small polar molecules such as water, ammonia, short chain alcohols and amines are important reactants for chemical syntheses involving zeolite or molecular sieve catalysts, as well as frequently used probe molecules. The efficiency of catalyzed reactions with these reactants depends critically on the sorbate– sorbent interactions. Therefore, the detailed understanding of the adsorption behavior is essential for the rational design of new catalysts or catalytic reaction routes. While ‘‘acidic’’ zeolites have Brønsted acid hydroxyl groups as main active sites, alkali metal exchanged X zeolites exhibit primarily electron pair acceptor and electron pair donor (Lewis acid–base pairs) [1], in which the exchangeable cations act as Lewis acidic sites and the negatively charged framework oxygens as Lewis basic sites [2]. The base strength of the framework oxygens adjacent to the counter cations can be controlled by the cations and it increases from Li + to Cs + [3]. The acidic and the basic sites of the ion exchanged zeolites are simultaneously involved in the adsorption of short polar molecules [4–7]. In particular, the adsorption structure of alcohols on alkali metal exchanged X zeolites [8] depends on the acid–base properties of the zeolite. In a primary interaction, the alcohol coordinates via a free electron pair on its oxygen atom to the exchangeable cation. In addition, the protons of the alcohol OH groups form hydrogen bonds with negatively charged oxygen atoms of the zeolite [7]. Similar to alcohols and water, alkylamines and ammonia have high dipole moments and form hydrogen bonds in liquid state. However, amines and alcohols differ in polarity because of the lower electronegativity of nitrogen compared to oxygen (see Table 1). The NH bond is longer, the dipole of amines is weaker, and the acidity of amines is about 20 times lower than that of comparable alcohols. Thus, amine groups form hydrogen bonds, which are, however, generally weaker than those of alcohols. Additionally, the amines have only one free electron pair, while the alcohols have two. In consequence, the boiling points of amines are significantly lower than those of the analogous alcohols, e.g., 321 K for propylamine and 390 K for propanol. In general, if a hydrogen atom of ammonia is replaced by an alkyl group (a stronger electron-donating group than hydrogen) the electron density on the nitrogen atom of the alkylamine molecule increases leading to higher base strength [8]. Therefore, alkylamines are stronger bases than ammonia. Similarly, the base strength of the oxygen of alcohols increases with the chain length. www.elsevier.com/locate/apcata Applied Catalysis A: General 307 (2006) 108–117 * Corresponding author. Tel.: +49 89 28913540; fax: +49 89 28913544. E-mail address: johannes.lercher@ch.tum.de (J.A. Lercher). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.03.012