This work explores the use of hexagonal boron nitride (h-BN), a graphite-like compound, as a novel catalyst with base and acid functionalities. For use as a solid catalyst, the layered structure of h-BN was disrupted by ball-milling, exposing boron and nitrogen edge sites as well as increasing the surface area from 3 to ca. 400 m2 g−1. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and proton magic-angle spinning nuclear magnetic resonance spectroscopy (1H MAS NMR) indicated simultaneous and adjacent formation of amino and hydroxyl groups by milling, which function as Brønsted base and acid sites, respectively. Analysis using color indicator reagents and pyrrole-adsorbed 1H MAS NMR results revealed that the ball-milled h-BN had basic sites of strength +9.3 > H− ≥ +7.2, comparable to those of KY zeolite. Measurements of 31P MAS NMR of adsorbed trimethylphosphine oxide indicated that the ball-milled h-BN had weak acid sites, comparable to those in HY zeolite. Despite its weak basicity, the ball-milled h-BN showed high activity and selectivity toward β-nitroalkenes for the nitroaldol reaction (Henry reaction) and the Knoevenagel condensation, whereas nontreated h-BN did not show activity. The nitroaldol reaction was considered to proceed in two steps: the abstraction of a proton from nitromethane by the amino group and the formation of an imine followed by a nucleophilic attack of the deprotonated nitromethane. Kinetic isotope effect experiments using D-substituted nitromethane revealed that the first step was the rate-determining step. Several nitroaldol reactions using a variety of monosubstituted benzaldehydes indicated that electron-donating groups enhanced the activity, suggesting that the formation of adjacent base and acid sites is responsible for it. This study shows the high catalytic activity of BN, a solid catalyst with moderate basicity and weak acidity.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry