In historical retrospect, the relationship between man and nature is viewed differently. At times, man saw himself as subject to the uncontrollable, superhuman forces of nature; at other times, the use and subjugation of nature by man became the leitmotif, and in many a retrospective the lost unity of man and nature is deplored. Renewable raw materials (RRM) have accompanied humanity since its beginnings. Linseed oil, creosote, tar, turpentine and pine oil, natural resins, and gums, amber and copal, egg white, and bone glues as renewable resources have been used for centuries, even millennia, as paints, writing, drawing, and printing inks, sealants, or adhesives.

Renewable raw materials (RRM) are relatively new as a segment of chemistry.¹  The term arose in the 1970s in connection with the first oil crisis, and in the discussions on Sustainable Development and the Limits to Growth.² 

In the 1960s, relevant textbooks had described chemistry as, so to speak, static, as the teaching of materials and the transformation of materials, classified as inorganic chemistry and organic chemistry.³ 

The Swedish physician and lecturer on chemistry Jöns Jacob Berzelius, who coined the term Organic Chemistry in 1807 as opposed to Inorganic Chemistry, like all other scientists of his time still assumed that organic compounds only can be built in the living organism as only the living organism had the life force (Latin. vis vitalis) necessary for the synthesis of organic matter. It was his alum Friedrich Wöhler (from 1832 until his death Professor of medicine, chemistry and pharmacy at the University of Göttingen) who disproved this hypothesis through the synthesis of urea starting from (inorganic) ammonium cyanate:

With urea, an endogenous (organic) metabolite had been made for the first time from an inorganic material.

That even amino acids, as building blocks for proteins and thus for reproduction-enabled life, might arise from inorganic matter under the conditions of the early Earth's primordial atmosphere was shown for the first time in the laboratory of the University of Chicago (Stanley Miller and Harold Urey, 1953).⁴  The experiment (Figure 1.1) used water, methane, ammonia, and hydrogen. The liquid water was heated to add water vapour to the chemical mixture and the resulting gases were circulated and submitted to electrical discharges to simulate lightning storms (believed to be common on the early earth). Under these conditions, the mixture components in a first step can produce hydrogen cyanide, formaldehyde, and other active intermediate compounds. Miller and Urey observed that 2% of the carbon in their mixture had reacted to form amino acids that are used to make proteins in living cells, with glycine and alanine as the most abundant species.

In some ways, the concept of RRM has introduced a new perspective into the world view of chemistry. The term makes clear that not only the properties of materials and the processes of their transformation are important, but that, in addition, substances also have, so to speak, a dynamic component; that their finiteness and their temporal, local and regional availability are crucial aspects. The term renewable raw materials appears, as opposed to fossil fuels. Also, fossil fuels are derived from plant and animal materials (i.e. from biomass), but in millions of years of ongoing geological processes. With a regeneration time of several million years, fossil resources today are more rapidly extracted and consumed than they are produced. Thus, their availability is limited. They are considered as non-renewable. Furthermore, the conversion of fossil fuels into carbon dioxide is considered a big threat to our climate.

In contrast, renewable raw materials regenerate in reasonable periods of time. With their usage, a synthesis capability inherent in nature comes into play, i.e. the conversion of carbon dioxide by green plants using solar energy into energy-rich organic compounds. In view of the reorientation of raw material and energy policies, resource conservation and climate protection, and thus in terms of sustainability, their increased use and promotion is at the centre of attention. RRM can contribute to reducing CO₂ emissions and carbon footprints. Their use is one of the principles of green chemistry.⁶  The lifestyle of health and sustainability is firmly anchored in the population and determines consumer behaviour more and more. However, issues such as competition with food, land use and biodiversity also play an important part in the debate about RRM. Furthermore, some of the admirable properties in nature are tied to the living organism and cannot simply be imitated. Just think of the resilience of a living coconut palm tree against the forces of a typhoon or hurricane⁷  (Figure 1.2).

Accordingly, biomimicry, literally the imitation of the living, has inspired innovation, research, and engineering to create new products, processes, and applications by emulating nature's time-tested patterns. Some examples are climbing pads mimicking the biomechanics of gecko feet and thus capable of supporting human weight, Lotus effect paints, shark skin swimming suits, and riblet surface coatings for aircraft fuselages.⁸ 

Without doubt, the growth of the global economy since the beginning of the industrial age has only been possible because of easily available, abundant, and, hence, cheap fossil raw materials. This is particularly true for the growth of the chemical industry. However chemical synthesis processes use only a minor proportion (approximately 10%) of fossil resources, whereas 90% is used for energy and transport. Regarding the usage of renewable resources in traditional and in more recently developed applications, the chemical industry in Germany, as an example, in 2013 used 21.2 million tonnes (Mio t) of feedstock out of which 13% (2.7 million tonnes) are RRM⁹  compared with 18.5 million tonnes of fossil petroleum, natural gas, and coal (Figure 1.3).

The untreated fossil raw materials, however, are virtually useless. Only in a refinery can the complex mixture of hydrocarbon molecules in crude oil or natural gas be separated and converted by unit operations such as fractionation, cracking, reforming, isomerisation, and hydrotreating (gasification or coking in the case of coal) into low molecular weight intermediates (Figure 1.4), which can be used as fuels, lubricants, and as feedstock for the chemical industry.

At first glance renewable resources appear to be infinite compared with fossil fuels. About 170 billion tons of biomass are growing every year¹⁰  and have been used by humans through the millennia as food, to feed domesticated animals, to clothe themselves, as an energy source, as construction materials, for chemical and biochemical conversion, and to make articles for daily use.

The composition of biomass differs significantly from that of fossil raw materials. Instead of a complex mixture of hydrocarbons, photosynthesis and biosynthetic conversion make a wide variety of medium and high molecular weight molecules available – with amazing high precision and reproducibility regarding their chemical composition and molecular structures. Aliphatic structures can be found in vegetable oils, cycloaliphatic molecules in carbohydrates and terpenes, and aromatics in lignin. Moreover, all these renewable resources display manifold functional groups, which make them easily suitable for chemical modifications or polymerisation. Biomolecules, such as cellulose, starch or castor oil, can be used directly and unchanged or can be physically, chemically, or enzymatically modified.

The different groups of renewable raw materials¹¹  (Figure 1.5), including:

• fatty oils from plants (such as linseed oil),

• proteins from milk or animal tissue (casein, gelatine),

• gums and polysaccharides,

• starch and starch derivatives,

• rosin, tall oil, turpentine and cellulose from wood pulping,

• natural rubber latex from the Pará rubber tree (Hevea brasiliensis),

are important parts of many surface coating, ink, and adhesive formulations. On the other hand, cellulose, starch, or sucrose can be enzymatically depolymerised to yield glucose syrup which then can be submitted to different fermentation processes. An entire branch of biotechnology, known as white biotechnology, is devoted to use living cells – from yeast, moulds, bacteria and plants – and enzymes to synthesise building-block chemicals from renewable carbon sources. Industrial processes using white biotechnology methods have made significant progress during the last decade. Bio-based manufacturing processes for 1,3-propanediol, isobutanol, succinic acid and 1,4-butanediol, used as intermediates for the manufacture of resins for surface coatings, inks, and adhesives, were still at the laboratory level in 2006 and have since entered world-scale production.¹²  The first of its type in the USA, Croda's manufacturing site at Atlas Point, New Castle, Delaware, including an ethylene oxide plant using naturally derived feedstocks, produces a new 100 percent bio-based and 100 percent renewable range of non-ionic surfactants certified to meet the criteria of the United States Department of Agriculture (USDA) BioPreferred^® program.¹³  The plant will, inter alia, use sorbitol and fatty acids to produce sorbitan esters and ethylene oxide derived from bioethanol for the corresponding polysorbates.

Further research and development efforts are underway in the chemical industry for expanding the use of renewables as raw materials in existing fields of application and beyond.

Basically, photosynthesis, i.e., the conversion of carbon dioxide and water to sugar molecules, liberating oxygen by action of sunlight

is the origin of both fossil and renewable resources, with – as already stated – the slight difference of a few hundred million years, which have completely deoxygenated primordial biomass, forming solid coal or liquid hydrocarbons by geochemical processes under enormous pressures in a sedimentary sequence.¹⁴ 

However, the millions of years of formation of liquid hydrocarbon fuels through geochemical processes can be shortcut by submitting RRM to pyrolysis liquefaction and/or hydroprocessing technologies.

Already widely deployed in existing petrochemical refineries and thus not a new field of reaction engineering, their application to RRM has gained a new momentum first in the context of alternative fuels and then in carbon footprint and greenhouse gas emission control and in particular as a basis for the circular economy.¹⁵  In this sense, RRM and resources used in the technical cycle link the concepts of bioeconomy and circular economy.¹⁶ 

By hydroprocessing, vegetable oils, including waste cooking oils, biodiesel [Fatty acid methyl esters (FAMEs)], and tall oil, are converted in a two-step process into second generation green diesel, i.e., zero sulphur, zero aromatics and high cetane, pure paraffinic hydrocarbons with chain lengths typical for fossil diesel fuel (Figure 1.6).¹⁷ 

In the first step of hydroprocessing (also called hydrotreatment), hydrogen is added to saturate the double bonds of the unsaturated vegetable oil triglycerides (refer to fat hardening; in Chapter 5). In the second hydroprocessing step (called hydrocracking) the saturated vegetable oil triglycerides are decomposed into fatty acids and propane (or methane when hydrotreating FAMEs). Finally, the fatty acids either undergo hydrodeoxygenation (by addition of more hydrogen the oxygen leaves as H₂O), decarboxylation (oxygen leaves as CO₂ without further addition of hydrogen), or decarbonylation (by addition of hydrogen, the oxygen leaves as CO) or a combination of these. During hydrodeoxygenation the alkyl chain length and the distribution pattern of the fatty acids in the original triglyceride is typically preserved, whereas during decarboxylation and decarbonylation alkyl chains are shortened due to the loss of carbon atoms as CO₂ or CO. During the final process stage, the unbranched long chain alkanes are also partially cracked and hydroisomerised in a process referred to as dewaxing. The result is a mixture of straight chain, isomerised branched chain, and cyclic paraffinic hydrocarbons (hydrotreated vegetable oils–hydroprocessed esters and fatty acids HVO–HEFA).

In biomass gasification RRM, particularly lignocellulosic biomass, is reacted under pressures of 1–40 bar and at temperatures of about 850–900 °C and in the presence of regulated amounts of oxygen to produce bio-synthesis gas that then is converted by the Fischer–Tropsch (FT) process to renewable liquid hydrocarbons with chain lengths in the diesel fuel range [Biomass to liquid, (BtL), technology, Carbo-V-process, bioliq^® pilot plant at Karlsruhe Institute of Technology (KIT)^17a,c,18 ].

Unlike biomass gasification, fast pyrolysis liquefaction is carried out at temperatures of 500 °C under atmospheric pressure in the absence of oxygen. Short reactor times and rapid cooling or quenching from the pyrolysis temperature is required. The resulting pyrolysis oil is submitted to a two-stage hydrotreatment to produce hydrotreated pyrolysis oils (HPO). However, chemically, pyrolysis oils derived from lignocellulose biomass still contain about 40% oxygen compared with the formerly described bio-oils and compared with the typical maximum amount of 2% oxygen found in crude oil. Their chemical composition is derived from the decomposition (depolymerisation and fragmentation reactions) of the main biomass components of lignin, cellulose, and hemicellulose.

Hydrothermal liquefaction (HTL) is another thermochemical process which produces a crude-like bio-oil. However, it is distinct from pyrolysis as it converts biomass to low oxygen bio-oil (5–20% oxygen) and, unlike pyrolysis and gasification, it can utilise wet biomass. The HTL process uses high pressures (e.g., 50–250 bar or more) and moderate temperatures (around 250–550 °C) as well as catalysts for 20–60 min to liquefy and deoxygenate biomass.

Roughly simplified, one can say that in a circular economy, unlike the linear economy from cradle to grave,¹⁹  the grave does not mean the end of a material's life. A circular economy renews materials at the end of their life cycle. Ideally, in chemical terms, the oxygen content is converted into water and the carbon is made accessible again for new reaction chains in the form of small hydrocarbon molecules. De facto, this reduces the industry's need for fossil carbon.

Between the hydrothermal treatment technologies, gasification yields the smallest possible building blocks, syngas, i.e., hydrogen and carbon monoxide, which can be converted by FT-synthesis into fuels or chemicals.

Hydrothermal liquefaction is characterised by its ability to use wet raw materials, typically sewage sludge, food industry, and sorted household organic waste. Grosso modo, bio-oils are obtained with carbon contents up to 76% and remaining oxygen of about 12%.

Fast pyrolysis needs dry biomass feedstock and, in comparison, leads to bio-oils with up to 58% carbon but about 35–40% remaining oxygen. Many kinds of lignocellulosic biomass, including agricultural residues, forestry waste, and energy crops, as well as waste tyres and plastics, including multi-layered plastic food packaging films, and electromobility components, have been used as feedstock.²⁰  Bio-oil, prepared by biomass pyrolysis, can be directly used as a substitute for petrochemical-based bitumen in bitumen or bitumen-based coatings for its similar properties of good adhesion and anti-corrosion characteristics.²¹  Fast pyrolysis oil can further be upgraded via esterification²²  (mainly using methanol), hydrocracking, or catalytic hydrodeoxygenation.²³