Solar energy-driven lignin-first approach to full biomass valorization using CdS quantum dots
The lignin-first approach, i.e., the catalytic valorization of native lignin in biomass in the first step, offers an opportunity to utilize the entire lignocellulosic biomass, but still has problems of harsh reaction conditions and difficulties in catalyst recovery.
In a recent article published in Nature Catalysis, we report our success in developing a lignin-first strategy based on solar energy-driven β-O-4 ether bond cleavage, which can be performed under mild conditions, and colloidal CdS quantum dot (QD) catalysts enabling us to overcome the difficulty in catalyst recovery.
The efficient utilization of lignocellulosic biomass, which represents >90% of all plant biomass, is of great importance in the pursuit of a sustainable future for chemical production and bio-based economy. Traditional biorefineries have focused on the utilization of carbohydrate part of lignocellulose with lignin either released as a waste or burnt to generate power. To increase the profitability and sustainability of biorefinery, much recent effort has been put into utilizing efficiently all the three components of lignocellulose including lignin, which is the most abundant source of renewable aromatics. Although progress has been achieved in the conversion of lignin model compounds and Organosolv lignin into aromatics in recent years, the direct transformation of native lignin in biomass is still a challenging task. The conventional methods for the fractionation of lignocellulose, such as Kraft and Organosolv processes, usually lead to changes in lignin structure, in particular condensation to form more C-C linkages at the expense of β-O-4 bonds, making the extracted lignin less reactive and more difficult to be transformed.
The lignin-first concept, i.e., the catalytic valorization of native lignin in biomass in the first step, offers an opportunity to utilize the entire lignocellulosic biomass in a more efficient manner and has attracted much recent attention. However, the current lignin-first method relies on hydrogenolysis of native lignin by supported metal catalyst at high temperatures (typically ≥ 200 ºC), usually leading to low-functionalized products and unavoidable loss of polysaccharides. Furthermore, the difficulty in separation of solid catalyst with solid cellulose/hemicellulose residue requires new strategy. Thus, a method that can activate the dominant linkage in lignin, i.e., the β-O-4 ether bond with a bond dissociation energy (BDE) of 54-72 kcal mol-1, under milder conditions and allow for a facile separation of the catalyst from the remaining cellulose/hemicellulose would be promising.
In a recent article published in Nature Catalysis, we report our success in developing such a lignin-first strategy based on solar energy-driven β-O-4 ether bond cleavage, which can be performed under mild conditions, and colloidal CdS quantum dot (QD) catalysts enabling us to overcome the difficulty in catalyst recovery.
We have discovered that CdS QDs are excellent catalysts for the selective cleavage of the β-O-4 bond in lignin model compounds. The unique feature of CdS QDs is the high selectivity without breaking other chemical bonds so that the functional groups in lignin model compounds can be maintained. More significantly, the CdS QDs can successfully catalyze the direct conversion of native lignin in birch woodmeal, providing functionalized monomeric aromatics with a yield of 27 wt%, which is 84% of the theoretical maximum based on cleavage of the β-O-4 linkages. Although a few photocatalysts including metal complexes and semiconductors have been applied to the conversion of lignin model compounds or Organosolv lignin, so far no photocatalyst has been reported for the direct conversion of native lignin in biomass. The key to success is that we have harnessed the colloidal character of CdS QDs, which allows the intimate contact with the solid biomass substrate, enhancing the accessibility of β-O-4 linkages by catalyst, and thus enables efficient transformation of native lignin into aromatic monomers at room temperature. Our control experiments confirm that other particulate semiconductors only show very low activity due to the poor solid-solid contact.
The separation of catalyst from aromatic products (for homogeneous catalyst) or the remaining solid cellulose/hemicellulose (for heterogeneous catalyst) is a big challenge in the lignin-first approach. We have devised a novel reversible aggregation-colloidization strategy to solve this challenge. After photocatalysis, the aggregation of colloidal CdS QDs by addition of acetone allows the separation of catalyst from liquid aromatic products. The re-dispersion of CdS QDs in aqueous solution enables facile separation of catalyst from the solid cellulose/hemicellulose residue. 84 wt% xylose and 91 wt% glucose have been obtained in the subsequent acidolysis and enzymolysis of solid residues, respectively. Therefore, our solar energy-driven lignin-first approach provides a promising way to maintain hemicellulose/cellulose during lignin transformation and can be integrated with the current biorefinery system without compromising carbohydrate fractions.
Our mechanistic studies via both experimental and computational approaches have pointed out a new electron-hole coupled (EHCO) mechanism for precisely cleaving the C−O (β-O-4) bond. Briefly speaking, the Cα−H bond first breaks upon oxidation by holes, forming Cα radical intermediate. We have demonstrated that the formation of this Cα radical markedly decreases the bond dissociation energy (BDE) of the β-O-4 bond from 58 to 4.8 kcal mol-1. Therefore, when accepting an electron, the Cα radical undergoes facile β-O-4 bond breaking. We believe that the remarkably lower BDE of β-O-4 bond in the Cα radical intermediate is the key reason for the high activity and selectivity of our system.
The present work not only offers a promising tool to cleave precisely the β-O-4 bond but also opens a new avenue for full utilization of lignocellulosic biomass by the lignin-first approach under mild conditions. Furthermore, we believe that our work can inspire the application of QDs for catalytic transformations of macromolecules.
A full story of this work can be found in: http://www.nature.com/articles/s41929-018-0148-8