Cellulose Nanocrystals as Catalysts Supports
Cellulose nanocrystals (CNCs) are nano whiskers (150 x 5 nm) composed of highly crystalline cellulose. This material can be generated from wood and has found applications as coating or reinforcing material. Since 2012 CNC is produced at the ton scale per day in Windsor, Quebec. We showed that this material could be used to support Pd NPs (Cirtiu, Green Chem. 2011). A well-defined hybrid material, PdNPs@CNCs, consisting of monodisperse PdNPs evenly deposited onto colloidal CNCs, was fabricated and applied with success towards the hydrogenation of phenol to cyclohexanone under mild conditions and the Heck coupling of iodobenzene and styrene. We then explored the enantioselective version of the hydrogenation reaction. Cellulose has been long sought as a chiral inducer in enantioselective catalysis, unfortunately it only allows marginal enantiomeric excesses (ees). With ultra-thin Pd patches deposited onto CNCs used as catalysts for the hydrogenation of prochiral ketones in water at room temperature and 4 bar H2 we achieved an unprecedented ee of 65% with 100% conversions. Cryo-electron microscopy, high-resolution transmission electron microscopy, and tomography were used for the first time to study the 3D structure of a metal-functionalized CNC hybrid. The presence of sub-nanometer-thick Pd patches at the surface of CNCs was established and provided insight into the chiral induction mechanism (Kaushik, J. Am. Chem. Soc. 2015). With the same CNCs and RuCl3, we easily accessed an extremely active and recyclable catalyst for the difficult reduction of arenes under mild conditions (4 bars H2, RT, 24 hours - Kaushik, RSC Adv. 2015). At last, we devised a simple and atom-economical synthetic method to access Ag NPs directly from bulk metal. A water suspension of cellulose nanocrystals was exposed to an Ag wire, under air and light exposure. In 2 weeks, Ag NPs of 1.3 nm ± 0.3 nm had grown onto the biopolymer. These species were active for the hydrogenation of aldehydes, 4-nitrophenol, alkenes and alkynes. Interestingly, this paper describes as well the first example of the metal catalyzed reduction of aromatic nitro groups to amines using H2 pressure in place of the classic NaBH4 with Ag NPs in water (Kaushik, Green Chem. 2016). We reviewed the field of CNCs used as catalyst support (Kaushik, Green Chem. 2016b).