文摘
Insertion of B atoms into an Al-free zeolitic framework with CHA topology results in the formationof B-SSZ-13 zeotype with Si/B = 11. B K-edge NEXAFS testifies that B forms [B(OSi)4] units in a Td-likegeometry (sp3-hybridized B atoms). According to B K-edge NEXAFS and IR, template burning results inthe formation of [B(OSi)3] units in a D3h-like geometry (sp2-hybridized B atoms) with a break of a B-O-Sibond and the formation of a Si-OH group. The activated material contains B(III) Lewis acid centers ableto specifically coordinate bases like NH3. Such [B(OSi)3] units are reactive toward ammonia, resulting inthe 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-NH2cannot be excluded owing to the presence of two IR doublets at 3500 and 3430 cm-1 and at 1600 and1550 cm-1. Ab initio B3LYP/6-31+G(d,p) calculations on a cluster model, supported by a single-point MP2on B3LYP/6-31+G(D,P) optimized structures, found the break by NH3 of a B-O-Si bond of the [B(OSi)3]unit with formation of [SiOH] and [H2N-B(OSi)2] species to be energetically favored. Comparison betweenexperimental and computed frequency shifts shows them to be in semiquantitative agreement. The highstability of the B-NH2 surface functionality is probed by N K-edge NEXAFS spectra collected under UHVconditions. These findings can open a new route in the preparation of shape selective solid basic catalysts.