Autonomous flow control by a responsive poly(2-hydroxyethyl acrylate) organogel-valve during the aerobic oxidation of benzyl alcohol

Macroscopic poly(2-hydroxyethyl acrylate) organogel beads were synthesized and applied as a chemoresponsive component in a smart valve. Monodisperse round beads of about 3 mm diameter were obtained by cryogenic dripping and subsequent UV photopolymerization of the frozen aqueous monomer solution. The synthesis yields dimensionally stable, soft spheres with a macroporous architecture. The elastic modulus increases from the soft, swollen state in benzyl alcohol to over 250 kPa for high-density networks in the collapsed state in benzaldehyde. They have a degree of swelling in benzaldehyde lower by a factor of 3–7 to that in benzyl alcohol, in dependence of the amount of polyethylene glycol diacrylate cross-linker in the precursor solution. Upon exposure to benzaldehyde, the beads become opaque, indicating a phase transition. Kinetic analysis of the bead volume change for swelling in benzyl alcohol and shrinkage in benzaldehyde gives half-life times in the range of 2–5 min. Beads exhibiting a macroscopic volume change of up to 71% were confined in a casing and acted as a smart valve in a reactor system. The aerobic oxidation of benzyl alcohol to benzaldehyde with a TEMPOL/copper acetate catalyst mixture demonstrated the high efficiency and practical applicability of the self-regulating flow-control component.