Functionalization of zeolitic cavities: Grafting NH2 groups in framework T sites of B-SSZ-13 - A way to obtain basic solids catalysts?

Insertion of B atoms into an Al-free zeolitic framework with CHA topology results in the formation of B-SSZ-13 zeotype with Si/B = 11. B K-edge NEXAFS testifies that B forms [B(OSi)(4)] units in a T-d-like geometry (sp(3)-hybridized B atoms). According to B K-edge NEXAFS and IR, template burning results in the formation of [B(OSi)(3)] units in a D-3h-like geometry (sp(2)-hybridized B atoms) with a break of a B-O-Si bond and the formation of a Si-OH group. The activated material contains B(III) Lewis acid centers able to specifically coordinate bases like NH3. Such [B(OSi)31 units are reactive toward ammonia, resulting in the formation of B-NH2 surface functionality inside the pores of B-SSZ-13 already under mild conditions, i.e., 35 mbar of NH3 at 373 K for 30 min and without crystallinity degradation. A minor fraction of Si-NH2 cannot be excluded owing to the presence of two IR doublets at 3500 and 3430 cm(-1) and at 1600 and 1550 cm(-1). Ab initio B3LYP/6-31+G(d,p) calculations on a cluster model, supported by a single-point MP2 on 133LYP/6-31 +G(D,P) optimized structures, found the break by NH3 of a 13-0-Si bond of the [B(OSi)(3)] unit with formation of [SiOH] and [H2N-B(OSi)(2)] species to be energetically favored. Comparison between experimental and computed frequency shifts shows them to be in semiquantitative agreement. The high stability of the B-NH2 surface functionality is probed by N K-edge NEXAFS spectra collected under UHV conditions. These findings can open a new route in the preparation of shape selective solid basic catalysts.