polymers
Sources: Werpy et al. [44], Bozell et al. [45], Gallezot [46], Isikgor et al. [47].
For instance, by taking the case study of plastic production from biomass, a variety of options can be imagined [47], giving strong environmental benefits. If traditional plastics (e.g. PE, polyamides, and PET) are produced starting from biomass, a possible reduction of ca. 310 Mt of CO2‐equiv per year could be achieved with the substitution of less than 66% of the current fossil‐based plastics [48]. This footprint reduction is solely based on the process and not on the product as the degradation characteristics of these plastics (i.e. nonbiodegradable) are the same regardless of the feedstock type (e.g. biomass and petroleum). At the same time, new and innovative platform chemicals can be produced with fewer chemical steps (e.g. furanics as opposed to aromatics), opening new opportunities in the production of bio‐based plastics. For instance, a furan‐based plastic was synthesized via the polymerization of 2,5‐furandicarboxylic acid (FDCA), an oxidized product of 5‐hydroxymethylfurfural (HMF) (see Table 1.1), and ethylene glycol [49,50]. This plastic, known as polyethylene furanoate (PEF), can be conceived as the bio‐based parallelism of the common PET, where the C‐6 aromatic functionality is substituted by a furan ring. Further, the furan‐based polymer was found to perform better compared to PET in terms of physical properties such as gas barrier [49]. The production of PEF relies on the acid‐catalyzed dehydration of lignocellulosic biomass and/or sugars (e.g. glucose) into HMF with subsequent oxidation to FDCA (Figure 1.2) (see further Chapter 13).
However, this process is hindered by the coproduction of substances known as humins (at times, referred to as coke) in the dehydration step [51–53]. The lower resource efficiency of the first step increases the overall cost of PEF, limiting its competition with fossil‐based PET regardless of the improved properties. This low atom economy is common to many bio‐based processes.
Generally, when producing chemicals from whole lignocelluloses, the yields of conversion processes are lower compared to the same synthesis starting from the sugar (e.g. fructose, xylose, and glucose); hence, process costs tend to increase. Predominantly, the differences of lignin content and feedstock density depending on the considered biomass (e.g. grasses vs. softwood vs. hardwood) cause variation on the process yields as well as the amount of volumes to be processed (e.g. grasses require bigger volumes). In particular, the hydrogen bonding between the different components (i.e. lignin, hemicellulose, and cellulose) reduces the available surface to processing, increasing the structural complexity and the recalcitrant nature of lignocellulosic feedstocks. Furthermore, the inorganic metals (e.g. Mg and Ca) naturally present in plants may induce reactor fouling by induced precipitation of salts or heterogeneous catalyst (e.g. zeolites) deactivation by ion exchange [54,55]. Above all, the aforementioned large presence of heteroatoms (particularly oxygen) increases the moieties' reactivity, leading to low atom efficiency and occurrence of undesired side reactions, such as the synthesis of humins that act as a catalyst deactivator (in a similar way to coke) and promote reactor fouling.
Figure 1.2 Parallelism between the production of PET (left) and PEF (right).
In order to overcome these challenges, strategies include the use of unconventional solvents, milder conditions, and various pretreatment methods in order to separate the single components (e.g. decompose cellulose to glucose) and allow targeted valorization.
1.5 Solvents
Solvent selection is an important factor in the viability of biomass processes. Aqueous solvents are advantageous for high solubility with most biomass feedstocks. Other desirable factors of aqueous solvents are low cost and low environmental impact, especially when compared to other solvents. Water, for instance, is the ideal solvent for biocatalytic routes as they have fewer chances to denature the used microorganism [56]. However, a downside of using water is usually the higher energy‐intensive separation processes, hence higher cost [57]. From the chemical point of view, aqueous solvents typically facilitate many side reactions that lower yields, including decomposition and polymerization [58–61]. In fact, many of the mechanisms of humin formation (i.e. sugar platform chemical polymer with low current market value) are attributed to ring‐opening
