Abstract:
Unconventional approaches to driving chemical reactions using light, ultrasound, plasma, mechanical forces, and microwaves, are having a renaissance with the expected electrification of the chemical industry. Among these approaches, ultrasound irradiation is particularly interesting, given its ability to generate free radicals (oxidants) at 300 K and 1 atm. pressure. Such oxidants that are generated in sonochemical reactors enable chemistries that otherwise do not occur at 300 K and 1 atm. Ultrasound irradiation is already used in the industry for mixing processes, cleaning materials, and wastewater treatment. Recent studies have shown the promise of ultrasound irradiation for fine chemical synthesis. Ultrasound irradiation depolymerizes cellulose to glucose, and selectively oxidizes glucose to high-value acids.
Notwithstanding decades of progress in ultrasound mediated chemical reactions, we do not yet have an atomistic understanding of how free radicals, generated in situ by cavitating bubbles, enable selective chemistries in the liquid phase. To address this knowledge gap, we present a multiscale framework that unites quantum chemistry calculations at the nanometre scale, heat/mass/momentum transport models for cavitating bubbles at the micrometre scale, and kinetics of reaction networks in batch reactors at the metre scale. This multiscale framework across three different length scales is applied to glyoxal oxidation. Glyoxal oxidation is selected as the probe because this reaction network is a subset of larger reaction networks of polyethylene terephthalate depolymerization (plastic waste), phenol degradation (pollutants), and classical oxidation reactions like benzyl alcohol oxidation. Our computational studies reveal that glyoxal oxidation pathways are initiated by OH radicals generated in cavitating Ar/O2 bubbles. The OH radical formation is confirmed using EPR spectroscopy. Glyoxal oxidation pathways in sonochemical reactors mirror elementary steps observed in atmospheric chemistry studies for similar species, as reflected by quantum chemistry calculations. This finding indicates a nexus between radical mediated oxidation mechanisms taken from two different contexts. Microkinetic modelling shows that the selectivity between different oxidation products like glyoxylic acid, oxalic acid, and formic acid is dependent on conversion through a dynamically evolving ionic equilibrium in the solution. Experimental concentration profiles and selectivity to various oxidation products in sonochemical batch reactors operating at low frequency (20 kHz) and high frequency (500 kHz) are faithfully determined by our quantum chemistry-derived microkinetic model.
Taken together, this study reveals the power of using quantum chemistry derived reaction pathways and microkinetic modelling to create an atomistic picture of how selective oxidation chemistries occur in sonochemical reactors. Such atomistic insights will enable the future optimisation and scale-up of sonochemical reactors.
Speaker Bio:
Tej Choksi is an Assistant Professor at the School of Chemistry, Chemical Engineering and Biotechnology at Nanyang Technological University (NTU), Singapore having joined in December 2019. His group employs first principles methods and kinetic modelling to understand how catalysts operate at the atomic scale. These atomistic insights are thereafter used to design improved catalysts. Tej graduated with a B.Chem.Engg. from the Institute of Chemical Technology (formerly known as UDCT) in 2012. He obtained his PhD in chemical engineering from Purdue University in December 2017 and completed postdoctoral training at Stanford University in November 2019. He is a fellow of the Renaissance Engineering Programme, NTU’s flagship engineering programme for nurturing next-generation leaders and entrepreneurs. He is also a co-Investigator at the Cambridge Centre for Carbon Reduction in Chemical Technology, Singapore and is the Vice President of the Singapore Catalysis Society. Tej is an early career editorial advisory board member for ChemSusChem and the Journal of Catalysis.