**1. Introduction**

Air quality deterioration, environmental, health and safety issues have raised serious concerns over continued processing of fossil-based feedstocks in producing chemical products such as fuels and solvents. As such, many efforts are being made to reduce the use of hazardous substances (particularly volatile organic solvents (VOCs)) and to eliminate or minimize waste generation in chemical processes. Switching from the currently widely used fossil-based solvents to greener ones derived from renewable resources constitutes a key strategy to drive sustainability as well as clean and safer chemical procedures in both industry and academia [1, 2].

Solvents are central to many chemical processes as they dissolve reagents and ensure sufficient interactions, at a molecular level, for chemical transformations to take place. For instance, the pharmaceutical industry is heavily reliant on solvents for

#### **Figure 1.**

*The various sustainable solvents derived from biomass.*

drug discovery, process development and drug manufacturing processes [3]. They are also used to extract and separate compounds from mixtures or natural products. Solvents are also important for performing compound purifications and form part of many manufacturing protocols as well as consumer products including fragrances, cleaning agents, cosmetics, paints, flavors, adhesives and inks, to name a few [1].

Many organic solvents are volatile, flammable and toxic; therefore, their use poses safety and health risks and impacts negatively on the environment. In order to address these issues, 'solvent-free' chemistry has been proposed, to mitigate against solvent exposure risks, and for chemical manipulations and formulations that absolutely require solvents, a system of ranking them by their environmental, safety and health (ESH) attributes has been introduced. This system aims to aid in selecting and using those solvents with minimum ESH risks and good green profiles [4–6].

Making use of renewable resources in producing solvents is a promising and important strategy to move towards sustainable chemical processing and to replace organic solvents derived from fossil raw materials. To this end, bio-based feedstocks such as carbohydrates, carbohydrate polymers, proteins, alkaloids, plant oils and animal fats have been used to produce bio-based solvents (**Figure 1**). This often requires prior processing of the raw materials (typical by thermochemical and biochemical conversion methods) to give familiar solvents or to provide completely new and innovative solvent entries [7, 8]. There are some, such as essential oils extracted from citrus peels (which are rich in terpenes), that are used directly.

The main processing methods include biochemical and thermochemical conversion [7]. Using one or a combination of these processing techniques, several classes of bio-based solvents (including alcohols [9–11], esters [12, 13], ethers [14], alkanes [15], aromatics [16] and neoterics [17, 18]) can be manufactured (**Figure 1**).
