Abstract
In this work, we have studied the catalytic activity of different silicates (MFI, MCM-41, and Beta) containing Lewis acid sites (including Sn, Ti, Zr, and Hf) for the tandem N-alkylation reaction of aniline with benzyl alcohol. The Hf- and Zr-Beta were the most active catalysts for this transformation, showing in both cases selectivities toward the corresponding N-benzylaniline higher than 97%. FTIR and DFT analyses confirm that the active sites in the Hf-Beta catalyst for this process are the open sites where one of the four Hf–O bonds is hydrolyzed. Moreover, the amount of these active species could be notoriously increased with previous thermal treatment of the Lewis acid zeolite with benzyl alcohol. Isotopically labeled experiments and theoretical mechanistic studies reveal that the N-alkylation reaction occurs through a hydrogen borrowing pathway, in which in situ Hf-hydride species were generated. Finally, the Hf-Beta zeolite was reused several times in the N-alkylation reaction without any appreciable deactivation detected. This catalytic system could be expanded to a variety of amines, including aliphatic and biomass-derived amines.
Highlights
Transition-metal hydrides have been recognized for many years as intermediates or catalysts in industrially relevant processes, such as hydroformylation, olefin isomerization, or hydrogen exchange in alkanes, among others.[1]
Group 4 transition metal sites can be incorporated in high-silica zeolites in dilute amounts, generating MIV isolated atoms with unique Lewis acid properties.[32−36] In order to study if these M-zeolites could act as adequate alternative catalysts for the base-free N-alkylation reaction of amines with alcohols, we have initially incorporated different transition metals in framework positions of a large-pore Beta zeolite
The UV Raman spectra of all Lewisacid-containing Beta zeolites show a band centered at ∼696 nm that is not present in the Si-Beta spectrum and that is assigned to M atoms incorporated in the Beta framework.[37,38]
Summary
Transition-metal hydrides have been recognized for many years as intermediates or catalysts in industrially relevant processes, such as hydroformylation, olefin isomerization, or hydrogen exchange in alkanes, among others.[1]. A common strategy to in situ generate reactive transition-metal hydrides is via a borrowing hydrogen (BH) approach from a donor molecule.[3] The borrowing hydrogen methodology, called hydrogen autotransfer, is a powerful strategy that combines transfer hydrogenation with one or more intermediate reactions, avoiding the direct use of molecular hydrogen The key of this concept is that the hydrogen from a donor molecule is stored by a catalytic metal site to be released in a final hydrogenation step.[3] This approach has been tentatively applied in the tandem N-alkylation reaction of amines using alcohols as hydrogen donors and either noble-metal-based (i.e., Ru, Pd, Ir, Rh, and Au)[4−8] or, more recently, non-noble transition metals (i.e., Mn, Co, Ni, and Fe)[9−12] complexes as catalysts. These solids still present some key disadvantages, including metal leaching and/or uncertain stability and recyclability
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