Contributed by Max J. Hülsey, Ph.D. Candidate; Ning Yan, Assistant Professor, Department of Chemical and Biomolecular Engineering, National University of Singapore
The word ‘refinery’ evokes imagery of huge chemical plants spitting out steams and flames in the process of producing gasoline, tar or other chemicals for our everyday life. Fossil fuels are still the major driving force of chemical industries — a paradigm that could be changing in the not-too-distant future.
Analogous to chemical refineries, the concept of a biorefinery has
been proposed and developed, referring to a facility that converts biomass into bio-fuels and value-added chemicals. Sugars and starch are largely utilized as starting materials, while woody biomass is becoming a more popular feedstock. In both cases, ethanol obtained via the fermentation of sugars serves as the primary product. It is then directly used as a biofuel or further processed into other chemicals.
There are new trends in biorefining. For example, a series of non-conventional biomass starting materials, ranging from waste kitchen oils to crustacean shells, have been considered. In the design of new processes for biomass valorization, increasingly more attention is being paid to harnessing the structural uniqueness of certain types of biomass. That is, to preserve some of the functional groups and/or structural motifs that originally exist in biomass and transfer them into the product. This minimizes the required steps for transformation, enables the direct production of value-added products from biomass, and acts in accordance with the principles of green chemistry.
Though one major limitation of the current biomass refinery is that it is not as economically competitive as the petroleum refinery, further technological developments along with the implementation of improved biomass refinery schemes, focused on the direct generation of value-added chemicals, may address the problem.
Several examples below illustrate some new advances in biorefining:
Waste Shell Biorefinery
Despite their sheer abundance, resources from oceans are generally underutilized. Among these is chitin, a major component of crustacean waste, such as shrimp, crab and lobster shells. It is also the major component in the exoskeleton of insects and the cell walls of fungi. Its structure resembles cellulose, but it has an additional amino group instead of one of the sugar hydroxyl groups. In nature, this amino group is commonly derivatized by an acetyl group that can be easily cleaved to yield chitosan. It is estimated that some million tons of those shells are dumped into landfills and back into the ocean every year. Besides chitin, the shells contain calcium carbonate and proteins, both of which are useful chemicals.
Processes for the conversion of chitin and chitosan into materials for a range of medical and environmental applications exist, but little work was done previously to demonstrate at industrial scale how to convert chitin into valuable chemicals. In 2014, the first study on the production of a N-containing furan-derivative from chitin was presented. The furan-derivative represents an intermediate in the synthesis of several proximicin derivatives – an important class of antibiotics and anticancer agents.
Following that, a variety of other chemicals, such as derivatives of the monomeric sugar units, acetic acid, pyrrole and pyrazine-derivatives, were obtained from chitin. Most of those compounds are not directly obtainable from fossil fuels, as nitrogen normally is not a significant constituent thereof. Therefore, the Haber-Bosch process – highly redox ineffective and energy-intensive – is required to produce ammonia, which is the most common industrial source of nitrogen. Employing a starting material that contains the crucial element is thus beneficial.
Fuels from Kitchen Grease
Fuels such as gasoline or diesel represent one of the biggest fractions of our daily consumption of chemicals. Although processes for the production of biodiesel exist, they mostly rely on the use of food-grade oils. The high oxygen content of the oil renders it corrosive and thus incompatible with current motors.
Used kitchen oil has been shown to be an excellent source of alkanes with chain lengths in the common fuel range. A recently developed conversion process does not require stoichiometric reagents or solvents, but relies on the use of various nickel salts, where nickel acetate proved to be the best, producing up to 60 percent fuel from common fatty acids. The elimination of reagents and the absence of oxygen in the product may make this process attractive compared to existing biorefinery schemes for biodiesel production.
Aromatic Chemicals from Lignin
Lignocellulosic biomass, the major component of a plant’s dry weight, primarily contains three components: cellulose, hemicellulose and lignin. The first two are normally utilized in the production of paper or bioethanol. Lignin is the only abundant biopolymer that contains aromatic building blocks, but currently, we are not able to exploit its potential, and for the most part, it is used as a fuel for steam production in the biorefinery.
Compared to most biopolymers, the structure of lignin is very complex, and it contains many aromatic ether bonds that are rather difficult to break. It has been shown that bimetallic metal catalysts containing nickel and other metals, such as ruthenium, palladium, rhodium or gold, work exceptionally well compared to their monometallic counterparts. A common problem is the hydrogenation of the aromatic ring by noble metals, whereas nickel is more selective in the cleavage of ether bonds, but possesses a lower activity. The combination of both can lead to a highly active catalyst that yields aromatic monomers from lignin. It remains to be seen whether or not the use of such catalysts can be sustainably employed at industrial scale and further development is warranted.
Transformation of biorefinery 2.0 from a concept into reality requires substantial efforts from multiple parties. A series of projects should be launched to enable new chemistry and processes. These projects should be supported jointly by government funding agencies and major chemical producers. Researchers with multidisciplinary backgrounds should work together to solve various scientific and technical challenges using the state-of-the-art advances in green chemistry, catalysis and materials science. Various media should advertise the new progresses to the public to increase general awareness and to get their support for the production of high value, renewable chemicals.
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