Intensified ozonolysis of lignins in a spray reactor: insights into product yields and lignin structure†
We demonstrate a simple spray reactor for an ozonolysis pretreatment step to cleave carbon–carbon double bonds in grass lignins to conveniently recover vanillin and p-hydroxybenzaldehyde (∼5 wt% of the lignin), two of the most value-added monomers. Lignin dissolved in an acid solution is sprayed into an ozone containing gas stream at ambient temperatures with contact times of less than 10 seconds. The production rate of these valuable species is between one to two orders of magnitude greater than that previously reported in a CSTR in which ozone is bubbled through a liquid phase containing dissolved lignin. The spray reactor thus significantly intensifies the reaction alleviating liquid-phase ozone starvation experienced in the CSTR. Gel permeation chromatography of the ozone-pretreated lignin reveals a decrease in the molecular weight distribution of macromolecular components corresponding to the recovered monomers. Scanning electron microscopy of the ozone-pretreated lignin reveals that the lignin superstructure is largely preserved as evidenced by the presence of globular assemblages in both the untreated and ozone-pretreated lignin. Thus, a mild ozone pretreatment of grass lignins in a spray reactor allows for the recovery of valuable aromatics without overoxidation of the products or unwanted bleaching of the bulk lignin.