BRONSTED ACID SITES AND SURFACE-STRUCTURE IN ZEOLITES - A HIGH-RESOLUTION SI-29 NMR REDOR STUDY

In well-crystallized zeolites, the distribution of silicon atoms among Q(4)(nAl) clusters containing up to n = 4 framework aluminum (FAl) is well documented. Upon replacement of the alkali charge-balancing cati ons by protons and thermal activation of the original sieve, a solid a cid is obtained. There are Lewis sites in nanoparticles of nonframewor k alumina (NFAl) created by the thermal treatment. The Bronsted sites are OH groups bridging framework silicon and aluminum. First, it is de monstrated that it is possible to estimate semiquantitatively heteronu clear second moments M(2)(Si-H) in acid zeolites having chemisorbed am monia using an adapted Si-29 CP REDOR (cross-polarization rotational e cho double resonance) technique. Then we show that, on this basis and with comparison with simple models, two kinds of bridging OH groups ar e distinguishable: (i) one donating its proton to NH3, forming NH4+ re orienting isotropically rapidly, and (ii) one operating as a proton do nor to a strongly hydrogen-bonded ammonia. The strong Bronsted sites o f the first kind are mostly located on Q(4)(nAl) with n = 1 while the hydrogen-bonded species as mostly associated with Q(4)(nAl) with n > 1 . The higher the rate of polarization transfer is, the larger M(2)(Si- H). The largest M(2)(Si-H) is associated with the sites on which NH3 i s hydrogen bonded. It is suggested that a noticeable fraction of the a cidic OHs react with, and therefore are neutralized by, nonframework a lumina. Moreover, preliminary information on an exchange process invol ving NH4+ is presented.