Green Chemistry Blog

Japanese scientists have discovered a method to convert tree hemicellulose (xylan) in a tunable fashion to either one of three chemical product streams, depending on the combination of catalysts and solvent phases employed. Hemicellulose is 20-30% of the structural lignocellulose component of biomass, with cellulose (40-50%) and lignin (20-30%) accounting for the remainder. As part of a biorefinery, it is important to be able to advantageously develop all of the feedstock into products in order to reduce waste and value the economic and material investment made during biomass production.

First, rhenium was introduced into an iridium impregnated silica. The resulting catalyst was capable of producing xylitol from xylan (79% yield) if used in combination with sulphuric acid in an aqueous reaction. Some success in achieving a partial reduction to pentanol was then observed when an organic solvent was introduced to the reaction. Finally, full hydrogenolysis to give pentane (in 70% yield) was accomplished by adding a HZSM-5 zeolite to the biphasic reaction.

The heterogeneous catalyst is recoverable and can be reactivated to continue providing a good yield of pentane. Thus a new flexibility to woody biomass processing has been demonstrated, complementing the more prevalent studies on cellulose and lignin.

This work is included in a online collection showcasing work presented at the 3rd International Symposium on Green Chemistry held in La Rochelle, France on 3rd-7th May 2015. Access the full collection of articles here.

Selective transformation of hemicellulose (xylan) into n-pentane, pentanols or xylitol over a rhenium-modified iridium catalyst combined with acids
Sibao Liu, Yasuyo Okuyama, Masazumi Tamura, Yoshinao Nakagawa, Akio Imai and Keiichi Tomishige
Green Chem., 2016, Advance Article. DOI: 10.1039/C5GC02183A

Membrane technology is a major contributor to present day desalination and other water purification processes. Ironically, the benefits to water quality that membranes provide are counteracted by the actual membrane fabrication procedure, which collectively emits billions of litres of wastewater, contaminated with reprotoxic organic solvents, every year. Only 31% of membrane manufacturing companies surveyed treat the waste water, either themselves or using a waste management company. The rest dispose of this contaminated water down the sink.

This study, carried out by researchers at the University of Manchester, UK, sought to identify reusable absorbents for the purpose of purifying the aqueous wastestreams from membrane production. Several absorbents were found to be suitable for extracting DMF and NMP from wastewater. These include imprinted polymers, zeolites, and graphene based materials, all of which can be regenerated and reused. The purified waste water could also be reused in the membrane fabrication process, vastly reducing water use.

This article is open access and available to everyone to read for free:

Sustainable wastewater treatment and recycling in membrane manufacturing

Razali et al., Green Chem., 2015, advanced article. DOI: 10.1039/c5gc01937k

A new study comparing life cycle assessments has shown that changing from a fossil derived feedstock to biomass is not necessarily beneficial to the environment. A reliance on food crops and inefficient conversion into chemical products will cause impact the environmental and result in an expensive product, also limiting its commercial opportunities.

Taking the example of hexamethylenediamine, the precursor to nylon-66, scientists based at Solvay in China have shown that the benefit of adopting a renewable feedstock (starch, in this instance) is counteracted by eutrophication concerns and greenhouse gas emissions. The issue largely stems from the cultivation of the biomass and the energy required to process the starch into chemical intermediates. The renewable route requires several high energy transformations to remove the original functionality of the biomass and obtain the conventional nylon monomer.

One lesson to be taken from this study is to give greater consideration to novel bio-based polymers that can be produced advantageously from bio-based feedstocks. Ideally, these would include wastes and agricultural by-products, thus making use of the reactive chemical groups inherent to the biomass rather than removing them.

Hexamethylenediamine (HMDA) from Fossil vs. Bio-based Routes: An Economic and Life Cycle Assessment Comparative Study

A. B. Dros et al., Green Chem., 2015, advanced article. DOI: 10.1039/C5GC01549A

High purity silicon is essential for manufacturing solar panels. Unfortunately this prerequisite conversion of silica to elemental silicon requires a lot of energy, and the associated greenhouse gas emissions are significant. It has now been demonstrated that the ashes from burning biomass (rice hulls in this case) can provide a rich source of silica than can be reduced to give solar grade silicon.

For the preparation of the silica from rice hull ash only dilute acid and hot water are required. The energy requirement to then produce 99.9999% pure silicon is an order of magnitude less than the conventional process and is actually lower than the energy created by burning the rice hulls in the first place. As the carbon dioxide generated by burning biomass is originally fixed from the atmosphere by plants, the carbon footprint for the production of this sustainable silicon is very low.

Read the advanced article in Green Chemistry online now:

A low cost, low energy route to solar grade silicon from rice hull ash (RHA), a sustainable source

Julien C. Marchal, David J. Krug III, Patrick McDonnell, Kai Sun and Richard M. Laine

Green Chem., 2015, Advance Article. DOI: 10.1039/C5GC00622H

The metal contamination introduced into the environment by mine tailings is a strikingly visual concern. As global demand for precious metals increases, many reserves are now regarded as critical. Pollution could be reduced and the effect of demand on resources lessened if effective reclamation of metals from mine tailings could be performed.

In work conducted jointly by scientists in the UK and in Spain, a bio-based mesoporous carbon material has been found to selectively absorb gold and platinum group metals from acidic solutions containing a mixture of metal salts representative of wastes typical of mining operations. The absorbed metals go on to create nanoparticles in the carbonaceous material. The spontaneous formation of nanoparticles on renewable carbon supports has potential applications in catalysis, or the metal could be isolated for other uses.

TEM image showing absorbed metal as nanoparticles and the Starbon® monolith (inset)

Read the advanced article in Green Chemistry online now:

Starch-derived carbonaceous mesoporous materials (Starbon®) for the selective adsorption and recovery of critical metals

Andrea Muñoz García, Andrew J. Hunt,* Vitaliy L. Budarin, Helen L. Parker, Peter S. Shuttleworth, Gary J. Ellis and James. H. Clark

Green Chem., 2015, Advance Article. DOI: 10.1039/C5GC00154D

The land use issues associated with biomass production points to marine biomass as a promising alternative. Seaweed is a rich resource, abundant in the world’s oceans. In addition to its potential for biofuel production, it is also important to obtain a stream of renewable chemicals from any seaweed bio-refinery to create an economically viable and sustainable process.

The present development, led by an Indian research team prominent in this field, creates a valuable side stream of chemical products to supplement the production of bio-ethanol from cellulose. Lipids, pigments and agar are all obtainable from the red algae feedstock by way of sequential extraction processes that improves the quality of the agar produced and significantly reduces the amount of auxiliary chemicals required compared to previous methods.

Read the advanced article in Green Chemistry online now:

Open access communication: One-pot triangular chemoenzymatic cascades for the synthesis of chiral alkaloids from dopamine

Researchers at UCL, UK, have developed a novel use of one-pot, multi-step enzymatic catalysis for the synthesis of chiral alkaloids. Transaminase (TAm) and norcoclaurine synthase (NCS) were employed as catalysts. In the synthesis of the tetrahydroprotoberberine product two carbon-carbon bonds are created under mild conditions with good enantiomeric purity (ee >95%), all in a short duration (3.5 hours).

The successive Pictet-Spengler cyclisation reactions provide an atom economic approach and high enantioselectivity to the formation of this valuable class of products. The authors of this work recognise the “remarkable potential of in vitro biocatalysis for the formation of complex chiral compounds“, which is increasingly important to contemporary green chemistry.

This article is open access and available to everyone to read for free:

One-pot triangular chemoenzymatic cascades for the synthesis of chiral alkaloids from dopamine

B. R. Lichman, E. D. Lamming, T. Pesnot, J. M. Smith, H. C. Hailes and J. M. Ward

Green Chem., 2015, Advance Article. DOI: 10.1039/C4GC02325K

Efficient and selective conversion of lactic acid into acetaldehyde using a mesoporous aluminum phosphate catalyst

Acetaldehyde is needed by the chemical industry for many diverse applications, such as paint and cosmetic formulations, plastics and construction materials. As such there is a need for renewable acetaldehyde within the bio-based economy and bio-ethanol can be oxidised to give acetaldehyde for this purpose. Now an alternative process using lactic acid as a feedstock has been developed.

The catalyst for this transformation is a mesoporous aluminium phosphate, facilitating full conversion of the lactic acid and yields of acetaldehyde exceeding 90%. While the use of aluminium is very favourable compared to ethylene oxidation catalysts based on silver for example, the longevity of phosphorus reserves are a concern. However the catalyst is robust and can be used for over 200 hours. Also, the process is efficient at lower temperatures than are often needed for transformations of lactic acid.

Read the advanced article in Green Chemistry online now:

Efficient and selective conversion of lactic acid into acetaldehyde using a mesoporous aluminum phosphate catalyst

Congming Tang, Jiansheng Peng, Xinli Li, Zhanjie Zhai, Wei Bai, Ning Jiang, Hejun Gao and Yunwen Liao

Green Chem., 2015, Advance Article. DOI: 10.1039/C4GC01779J

The separation of lignocellulosic biomass into its three component polymers; cellulose, hemicellulose, and lignin, is an important field of research relevant to biorefineries and the production of bio-based products. The chemical valorisation of polysaccaharides (to bio-ethanol for example) and the paper industries leave the lignin behind as waste.

Rich in aromatic functionality, it is unfortunate that because of the poor solubility of lignin up to 40% of lignocellulosic biomass feedstock is consigned to low value applications.

The use of ammonia as a solvent for lignin has now been revisited by a team of Dutch scientists in order to resolve this solubilisation bottleneck. Ammonia was shown to readily dissolve most varieties of lignin at room temperature and 7-11 bar, and can be removed simply by releasing the pressure.

The mild conditions make ammonia an attractive solvent for biorefineries looking to maximise lignocellulosic biomass utilisation. Furthermore the necessary apparatus is already incorporated into biorefineries for the ammonia fibre explosion (AFEX) process.

This article is open access and available to everyone to read for free:

Lignin solubilisation and gentle fractionation in liquid ammonia

Zea Strassberger, Pepijn Prinsen, Frits van der Klis, Daan S. van Es, Stefania Tanasea and Gadi Rothenberg

Green Chem., 2014, Advance Article. DOI: 10.1039/C4GC01143K