Direct Observation of Bronsted Acid Sites

Infrared spectroscopy of the O-H stretching vibration is more sensitive to differences between hydroxyl group enviromnents, but uncertainty in the extinction coefficient makes quantification difficult. Also, there is not a strong 

2 Interaction of Bronsted Acid Sites with Probe Molecules 

In order to study the interactions of molecules at these sites without the complication of subsequent reaction, much use has been made of probe molecules with different basicities, and therefore with different strengths of interaction at acid sites. The interactions have been monitored by many different techniques, including microcalorimetry and TPD, particularly of amines, and infrared spectroscopy and solid state NMR of the adsorbate-sorbate complex. This work has primarily aimed to establish a measure of acid strength that can be linked to catalytic activity in hydrocarbon transformations. 

This describes the protonation of an adsorbed molecule B and the attendant formation of an ion pair at the zeolite s internal surface (1). The protonation is known from IR studies to occur with bases such as ammonia or pyridine. The 

Whereas calorimetry measures a heat of adsorption, infrared spectroscopy is able to follow changes in bonding within probe molecules of different basicities adsorbed at Bronsted acid sites. The work of Zecchina s group in Turin, in 

---

Kentgens, A.P.M., luga, D., Kalwei, M., and Koller, H. Direct observation of Bronsted acidic sites in dehydrated zeolite H-ZSM5 using DFS-enhanced Al MQMAS NMR spectroscopy. J. Am. Chem. Soc. 2001,123, 2925-2926. 

---