Regarding the production and recovery of value-added aldehydes from lignin — containing raw materials, Fig. 12.12 shows a simplified flow sheet proposed by the research group of LSRE, working with lignin-based biorefining since the 1990s [133].The strategy is to combine reaction engineering and efficient separation processes for converting lignin from pulping spent liquors into value-added aldehydes. A portion of the by-product streams is processed to extract lignosulf — onates or lignin (acidification/precipitation, UF or LignoBoost process). The subsequent processes are based on three main steps. The first step consists on the alkaline lignin oxidation in a structured bubble column reactor as reported in Sect. 12.4.5 [120]. Then, the reactor stream follows to an ultra-filtration process leading to the separation of high molecular weight fraction of degraded lignin from

the lower molecular weight species, which goes preferentially to the permeate [135]. The permeate flows through a packed bed on acid resin in H+ form to protonate the phenolates [134]. At the end, vanillin is recovered from solution by using crystallization process.

The production of lignin-based polyurethanes elastomers and foams could be also explored. The high molecular weight fraction retained in the UF process can be considered as raw material for lignin-based polyurethanes. The production of polymers from lignin is undoubtedly an attractive approach since it can take advantage of its functional groups and macromolecular proprieties. This applica­tion has been the topic of intense research and materials with quite promising properties were already obtained [80, 83, 175].

This complete process (reaction and separation) could be integrated in a pulp and paper mill, with the possibility of diverting a fraction of liquor lignin for oxidation, producing vanillin and syringaldehyde. The unreacted lignin (after oxidation and separation of added-value chemicals) can be reintroduced in the liquor stream to be burned, recovering by this way part of the energy lost by the deviated fraction. Alternatively, this lignin could be the raw material for polymers production [174]. Moreover, this process perfectly fits into the scope of new emerging lignocellulosic-based biorefineries to valorize lignin. This concept is entirely related to the development strategies and policies regulated by Agenda 21 program, offering a framework to enable the smooth transition toward a Bio-based Economy supported by innovation and sustainable growth.

Acknowledgments Authors are grateful to Dr. Detlef Schmiedl, Fraunhofer Institute for Chemical Technology, Germany and Dr. Daniel Araujo, Faculty of Engineering, University of Porto, Portugal, for kindly providing figures and data.