Functionalization of Zeolitic Cavities: Grafting NH2 Groups in Framework T Sites of B-SSZ-13 - A Way to Obtain Basic Solids Catalysts?

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• 作者：Laura Regli ; Silvia Bordiga ; Claudia Busco ; Carmelo Prestipino ; Piero Ugliengo ; Adriano Zecchina ; Carlo Lamberti
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：October 10, 2007
• 年：2007
• 卷：129
• 期：40
• 页码：12131 - 12140
• 全文大小：379K
• 年卷期：v.129,no.40(October 10, 2007)
• ISSN：1520-5126

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 T_d-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 D_3h-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.