ABSTRACT
Acidic properties of solid surfaces are characterized by the type (Brønsted or Lewis), amount, strength, and origin or location of acid sites. The amount is usually expressed by the number of acid sites per surface area or per weight of the catalyst. Type of acid sites is classified into Brønsted acid for proton donor and Lewis acid for electron pair acceptor. In most cases, Brønsted acid sites are surface hydroxyl groups and Lewis acid sites are coordinatively unsaturated surface metal cations. The strength is expressed by either protonating ability or the energy of the interaction of the acid site with a basic molecule. Adsorption energy of basic molecule can be a measure of the strength. The strengths of Brønsted acid sites and Lewis acid sites vary with the coordination number of the O atom of hydroxyl groups and the coordination number of the metal cations, respectively. Accordingly, the strengths of Brønsted acid sites and Lewis acid sites depend on the location of the acid sites.It should be noted that the order of interaction strength of different Brønsted acid sites does not change much with the type of the probe basic molecule, whereas that of different Lewis acid sites may vary with the type of the probe molecule. Suppose that there are two Brønsted acid sites, B1 and B2, and that the interaction
energy with a basic molecule is larger for B1 than for B2. The interaction energy with the other basic molecule would be larger for B1 than for B2. Accordingly, we can say that B1 is stronger Brønsted acid site than B2. On the other hand, the situation is complex for Lewis acid sites. According to the HSAB principle, soft Lewis acid sites interact more strongly with soft basic molecules than with hard basic molecules, and hard Lewis acid sites interact more strongly with hard basic molecules than with soft basic molecules [1]. The order of the interaction strength of Lewis acid sites measured with a soft probe molecule may be different from that measured with a hard probe molecule. Suppose that there are two Lewis acid sites, a soft Lewis acid L1 and a hard Lewis acid L2. A soft probe molecule will interact more strongly with the soft Lewis acid L1 than with the hard Lewis acid L2, whereas a hard probe molecule will interact more weakly with the soft Lewis acid L1 than with the hard Lewis acid L2. Accordingly, we cannot say which Lewis acid site is stronger without specifying the probe molecule.
