Critical Techniques for Overcoming the Diffusion Limitations in Heterogeneously Catalytic Depolymerization of Lignin

Abstract

Heterogeneously catalyzed depolymerization of lignin to valueadded chemicals is increasingly attractive but highly challengeable. Particularly, the diffusion limitation of lignin macromolecule to the solid catalyst surface is a big barrier, which significantly decreases the yield of monomer while increasing char formation. Therefore, for the potential industrial utilization of lignin, new knowledge focused on the size of lignin particles is of great importance to offer guidance for promoting lignin depolymerization and suppressing condensation in the hetero-geneously catalytic systems. In this Review, the size of lignin particles and macromolecules are summarized. Previous approaches for improving the mass diffusion including enhancing the solubility of lignin and exploitation of hierarchical and "solubilized" materials are also discussed. Based on these, a constructive perspective is proposed. Thus, this work provides a new insight on the rational design of heterogeneous catalytic techniques for efficient utilization of the aromatic polymer of lignin.

Mechanistic Insights into Formation of Residual Solid in Lignin Depolymerization

“…Based on above mentioned mechanism, we can safely deduce that the lignin char would be significantly suppressed by rational design of novel depolymerization strategies. For example, catalytic hydrogenolysis in the presence of a hierarchical porous support would be a promising method because of the enhanced mass transfer of lignin and phenolic oligomer, [1b] which consequently reduces the condensation of phenolic compounds. Catalytic oxidative depolymerization, which generally is highly reactive on the conversion of G lignin, [39] is also a good choice for avoiding char formation.…”

Greener starch oxidation : Exploring challenges in designing catalytic processes for polydisperse feedstocks

“…However, biopolymers from biobased feedstocks can be several times that size, ranging from ~10 to ~250 nm (table 1). As a result of the different size scale of the substrate, catalytic modification of biopolymers through heterogeneous catalysis faces additional challenges in the form of mass transfer limitation though hindered diffusion and size exclusion, and substrate the influence of molecule size (distribution) thereupon (Li et al, 2023;. This is especially relevant for depolymerisation reactions of the substrate where the substrate is processed from its native form, or conversions that aim to retain some of the original structure of the biopolymer intact, which use biobased feedstocks as substrate that have undergone minimal depolymerisation.…”

“…As a result, the wider the distribution of the feedstock, the larger the potential error of neglecting the influence of size on a process. Some recent studies recognise this problem like (Li et al, 2023) on the potential of heterogeneously catalysed depolymerisation of lignin, and some studies try to address this challenge like , who took substrate polydispersity into account in modelling the precipitation behaviour of asphaltenes. However, studies that integrate the knowledge on size dependency in multiple processing steps and quantitatively evaluate the cumulative size dependent effects are lacking.…”

“…Catalytic conversion of these biomacromolecules is a common strategy to introduce these modifications. However, the size and complexity of these substrates introduces a challenge in designing and optimising catalytic processes for the conversion of these macromolecules (Li, Cui, Wang, & Long, 2023;. Currently in industry, heterogeneous catalysis is applied in approximately 80-90% of the catalytic processes , because of the simple separation and re-use of the catalyst compared to homogeneous catalysis (Glotov & Karakhanov, 2021).…”

“…Therefor values given in table 2 are to be used as an estimation rather than precise data. 10-20 1.8 -2.1 Cellulose 20-50 2-7 Hemicellulose 34-57 1.37-1.61 Lignin 8.4 -33.5 2-50 (Li et al, 2023) 1 76 -97 1.04 -1.27 >100 2.22-3.39 Pectin 9 (Gu & Catchmark, 2013) ~ 22 (Masuelli, 2011) 1.15 (Gu & Catchmark, 2013) 3 The large molecular size of biopolymers and the resulting diffusion, pore blocking of large molecules are contributing causes that current processing/modification techniques for biopolymers first break them down into platform molecules before further catalytic upgrading. However, maintaining some of the original structure of the biopolymers is beneficial for specific applications.…”