Spatially Directed Pyrolysis via Thermally Morphing Surface Adducts.

Abstract

Combustion is often difficult to spatially direct or tune associated kinetics - hence a run-away reaction. Coupling pyrolytic chemical transformation to mass transport and reaction rates (Damköhler number), however, we spatially directed ignition with concomitant switch from combustion to pyrolysis (low oxidant). This establishes a 'surface-then-core' order in ignition with concomitant change in burning rate. Herein, alkysilanes grafted onto cellulose fibers are pyrolyzed into non-flammable SiO2 terminating ignition propagation and further pyrolysis. Sustaining high temperatures, however, triggers ignition in the bulk of the fibers but under restricted gas flow (oxidant and/or waste) hence significantly reduced ignition propagation and pyrolysis compared the surface (Liñán's equation). This leads to inside-out degradation and, under felicitous choice of conditions, formation of graphitic tubes. Given the dependence on temperature, imbibing fibers with an exothermically oxidizing synthon (MnCl2) or a heat sink (KCl) abets or inhibits pyrolysis tuning tube-wall thickness. We apply this approach to create magnetic, paramagnetic, or oxide containing carbon fibers. The magnetic fibers are used for rapid filtration of oil from water.