Catalytic Processes for The Valorization of Biomass Derived Molecules

1. Introduction

Industrial chemistry is changing its fossil distinctiveness into a new green identity by using renewable resources. Biomasses, produced and used in a cyclical way, constitute an important environmentally friendly resource for the production of energy, chemicals, and biofuels [1,2]. To this regard, abundant and inedible lignocellulosic biomasses have attracted a lot of attention being not in competition with agricultural land and food production and, therefore, representing renewable feedstocks for modern biorefineries. The three main components of lignocellulosic biomasses are cellulose, hemicellulose, and lignin, which can be converted into energy (biogas and H₂), liquid biofuels, and into a pool of platform molecules including sugars, polyols, alcohols, aldehydes, ketones, ethers, esters, acids, and aromatics compounds [3,4,5,6,7]. However, in order to develop efficient catalytic processes for the selective production of desired products from lignocellulose, a deep understanding of the molecular aspects of the basic chemistry and reactivity of biomass derived molecules is still necessary.

This Special Issue aims to cover recent progresses in the catalytic valorization of cellulose, hemicellulose and lignin model molecules promoted by novel heterogeneous systems for the production of energy, fuels and chemicals. In this context, it is worth to highlight some recent research advances. Among many, hydrogenation/hydrogenolysis [8,9] and transfer hydrogenolysis [10,11,12] of cellulose, hemicellulose, lignin, C6-C3 polyols, furan derivatives and aromatic ethers represent a core technology of modern biorefineries. Glycerol (C3 polyol) and glycidol (2,3-epoxy-1-propanol) can be converted into hydrogen (via APR process), C3-C1 alcohols, as well as to cyclic acetals and ketals [13,14,15,16]. Accordingly, reforming processes are surely a new way for the production of H₂ and liquid hydrocarbons from lignocellulosic biomasses or their derived molecules [17,18,19,20]. At the same time, biomasses can be directly converted into liquid fuels and bio-oils via pyrolysis (thermal degradation process in absence of added oxygen) [21] or used as starting materials for the production of vegetable oils that are efficiently used in several energetic application [22,23]. Finally, a recent trend is surely the production of aromatics, including BTX compounds (benzene-toluene-xylene), starting from lignin, sugars and aromatic ethers, and esters in the framework of the so-called “lignin-first biorefinery” [24,25,26,27,28].

2. The Contents of the Special Issue

In this special issue, several fields of research on the catalytic valorization of biomasses and their relative molecules are covered. Therefore, we would like to sincerely thank all the authors who contributed with their excellent contributions to this special issue that includes six articles (three reviews among them).

Martín et al. propose a natural zeolite (Chilean) as an innovative catalyst for bio-oil upgrade processes [29]. The results clearly show that Chilean-zeolites efficiently increase both quality and stability of the bio-oil obtained from the catalytic pyrolysis of the Chilean native oak. In particular, zeolite acid sites allow the decrease of oil viscosity as a consequence of the increase of the concentration of hydrocarbons, alcohols and aldehydes during the storage practice. At the same time, the presence of Brønsted acid sites on Chilean-zeolites promotes the reduction of carbonyl and alcoholic groups of bio-oil, even after storage.

The hydro-isomerization upgrading of vegetable oil-based insulating oil was presented by Dieu-Phuong Phan and Eun Yeol Lee [30]. In their review, they demonstrated that vegetable oils can be a valid feedstock of insulating oils for electric transformers presenting the effect of (i) metal phase, (ii) acid sites, and (iii) pore structure on the catalytic hydroisomerization processes. At the same time, the influence of blending processes on the physico-chemical properties of these alternative oils are also presented. Moreover, in their contribution, authors pointed out some of the drawbacks related to vegetable oil-based insulating oils (e.g., high pour points, minor aging, and higher viscosity).

Prof. Jose A. Lopez-Sanchez and co-workers demonstrated the influence of alkaline treatment of H-ZSM-5 catalyst for the production of p-xylene and other aromatic compounds starting from bioderived sugars and ethylene [31]. Authors show that the alkaline treatment allows obtaining a series of catalysts characterized by a mesoporous structure preserving the typical crystallinity of the H-ZSM-5 based zeolites. The key factor in driving the production of p-xylene from 2,5-dimethylfuran in high activity/selectivity was found to be the right compromise between acidity and mesoporosity of the alkaline H-ZSM-5 catalyst.

The use of an acid heterogeneous catalyst (Nafion NR50) for the synthesis of solketal from bio-glycidol was presented by Cucciniello et al. [32]. The Nafion NR50 system was found to be very efficient even at a very low catalyst loading, allowing a quantitative acetalization of glycidol into solketal under relative mild reaction conditions. Moreover, the catalyst can be re-used several times without any significant decrease in activity/selectivity.

The research group of Prof. Cavani and co-workers contributed a review on the use of mixed-oxide catalysts for the chemical-loop reforming (CRL) of bioethanol [33]. Authors pointed out how the different preparation methods drive the reactivity of spinels in the CLR of bioethanol. In particular, M-Fe₂O₄ ferrospinels (M = Cu, Co, Mn) were found to be very active in H₂ production using several building block chemicals that can be obtained from ethanol (acetone, acetaldehyde, and C4 compounds) in the second loop. Furthermore, an easy recovery and reuse of the initial M-Fe₂O₄ can be done.

In another review, the use of transfer hydrogenolysis approach for the reductive upgrading of lignocellulosic biomasses is presented [12]. The reductive valorization of cellulose, hemicellulose, lignin, and their relative derived/model molecules in absence of added H₂ allows the production of several added value chemicals including acids, ethers, esters, aromatics, polyols, and alcohols. Furthermore, the use of simple organic molecules such as methanol, ethanol, 2-propanol, and formic acid as an indirect H-source may solve most of the problems related to the classic use of high-pressure molecular hydrogen (including safety hazards, expensive infrastructures, and costs related to the transport/storage of pressurized H₂).