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Catalysing biomass conversion
Biomass conversion is expected to make a massive contribution to liberating the world from its dependence on fossil fuels over the next several decades. However, if biomass is to fulfil its true long-term potential there will need to be rapid progress in the discovery and development of more effective catalysts – both chemical and biological – for biomass processing. Indeed, the US Department of Energy (DoE) has stated that catalysis will be ‘a primary tool for industry to produce valuable fuels, chemicals and materials from biomass’.
In a report published earlier in 2011, the International Energy Agency (IEA) forecast that biofuels consumption would grow almost 14 times from the current 55m t of oil equivalent (Mtoe) to 750Mtoe in 2050. This would mean that their share of the global transport fuel market would increase from 2% today to 27%.
To achieve these levels of growth, catalyst developers and producers will have to raise the performance and effectiveness of biomass catalysis to similar levels seen in the petrochemicals sector. ‘Catalysts in petrochemicals production are tremendously efficient,’ says Jonathan Holladay, head of biomass research at the Pacific Northwest National Laboratory (PNNL), Richland, Washington, a major R&D centre in catalysis funded by the US government.
‘But the petrochemicals in industry has been around for 80 years and in 40 of those years it did most of its catalyst development,’ he continues. ‘We can’t take that long. What took the petrochemicals sector decades to do we will have to do in a much shorter time.’
The global catalysts market for biomass conversion will be worth at least tens of billions of US dollars at current value, if not more. The IEA estimates that total expenditure on biofuels over the next 40 years will amount to $11-13 trillion, depending on the actual production costs.
In a recent report on the production in Europe of ethanol and biochemicals, Bloomberg New Energy Finance estimated that the conversion and capital costs of second generation production of ethanol from biomass would be more than double that of the conversion and capital costs of the first generation production of ethanol from food crops like wheat.
Some analysts believe that catalysis could account for as much as 10-15% of the capital and operating costs of second generation production.
A major source of biomass will be the waste and residues of food crop cultivation and of food processing. Around 3bn t of biomass will be needed annually by 2050 to meet the world’s demand for biofuel, according to IEA estimates. About 1m t will come from residues and waste. Energy crops covering 100m ha of land – around 2% of the total agricultural area – will produce much of the rest. Also, yields of biofuels from biomass will, with the help of innovations in catalysis, have to increase by a factor of 10, according to the IEA.
Currently only around 200m t of a total of nearly 60bn t of biomass is recycled annually. Innovative catalysis technologies will have to be developed to process the substances and materials in the highly complex biological structures within plants that evolution has made deliberately difficult to break down.
Lignocellulose comprises 70-95% of terrestrial plant biomass. Cellulose, which makes up 40-50% of lignocellulose, is a homopolymer of glucose while hemicellulose, comprising 16-33%, contains a variety of pentoses, mostly xylose, arabinose and hexoses. Lignin which takes up the remaining 15-30% of lignocellulose is a copolymer of aromatic lignols.
The crystalline structure of cellulose, in which its glucose monomers are linked by glycosidic bonds, is hard to degrade while hemicellulose with a branched chain structure is much easier. Lignin is even more difficult to break down than cellulose.
With most processes, the biomass has to be pretreated, sometimes with the aid of catalysis, in order to loosen the structure of the lignocellulose and make the bonds more accessible to cleaving chemicals. After pre-treatment, the main processing route is currently catalytic-supported hydrolysis to produce pure sugars that can be fermented into fuels and chemicals.
Another major route is pyrolysis of the biomass into oils that can be catalytically cracked into a variety of fuels and chemical feedstocks. The biomass can also be gasified into syngas – comprising carbon monoxide and hydrogen – for further catalytic synthesis into fuels and chemicals.
Substitute natural gas (SNG) can also be produced via catalysis from biomass to provide a gas rich in methane. This can be used as an energy source or a means to make methanol for the production of fuel and chemicals.
For catalysis developers, one of the biggest challenges in the early stages of biomass processing is dealing with its high oxygen content. ‘Oxygen can account for around half the content of biomass, which is a level much higher than that in crude oil,’ Carl Mesters, Shell’s chief scientist for chemistry and catalysis, told a recent press briefing at Shell Technology Centre Amsterdam (STCA).
‘As a company whose main experience is in the processing of crude oil, this makes biomass a completely new area for us,’ he explained.
Among the other big technological problems with biomass catalysis are the intricacies of gaining access to key parts of the structure of lignocellulose and the need for the catalyst to be able to remain active in the large quantities of water used in biomass processing.
Producers of biocatalysts and chemical catalysts compete for business in some of the key stages of biomass conversion. Enzyme makers claim to have a dominant share of hydrolysis routes to fermentation of biomass sugars into bioethanol now that they have managed to reduce the costs of enzymes.
‘We have got enzyme costs down to $0.50/gal on cellulosic bioethanol and are now aiming at lowering this even further,’ says Claus Fuglsang, head of biomass research at the US R&D centre of Novozymes, the Danish-based enzyme specialist, in Davis, California. ‘We are able to do more with less enzymes by shortening the hydrolysis times. We also made the fermentation stage in which enzymes are also used more efficient.’
As chemical catalysts become more effective in all the stages of biomass processing, they are starting to be introduced in areas in which biocatalysis has the ascendancy. Researchers at Haldor Topsoe, the Danish-based catalyst producer, and the Technical University of Denmark, have shown that lactic acid, usually made by fermentation of sugar from biomass carbohydrates, can be produced from common sugars through a non-fermentive route by Lewis acidic zeotype catalysts – with a similar structure to zeolites.
Increasingly, however, catalyst businesses are combining expertise in both chemical and biocatalysts to take a hybrid approach to resolving problems with biomass processing. Shell, which among leading oil and gas companies has been one of the heaviest investors in biofuels, has been forming partnerships with US-based biocatalysis specialists like Virent Energy Systems and Codexis. It has also formed a $12bn joint venture with Cosan of Brazil, a major producer of ethanol from sugarcane bagasse or residues.
‘There is more advantage in co-operation through partnership than trying to do everything ourselves,’ said Mesters. ‘But we are also building our own team in biocatalysis because we need our own people with biological knowledge and expertise to communicate with our partners.’
Ineos Group, the petrochemical and chlor-akali producer, is due to open its first biowaste-to-energy plant using a gasification and fermentation process in Florida, US, in 2012. A second plant using the technology is likely to be built at Seal Sands, Teesside, UK, to convert 100,000t/year of biodegradable household and commercial waste into bioethanol and electricity.
After thermochemical gasification of the waste into syngas, the microorganism Clostridium ljungdahlii, is used as a biocatalyst suspended in water in a fermenter to produce the biofuel.
‘The combination of thermochemical and biochemical processing offers the best of both worlds,’ says Graham Rice, business development manager at Ineos Bio. ‘The thermochemical gasification step provides feedstock flexibility, allowing us to use household waste, while the biocatalytic microorganism is very selective, producing just ethanol, and the yield of ethanol is very high. Conventional catalytic processes, using, for example, a Fischer-Tropsch catalyst require high temperatures and high pressures and have a relatively low yield and selectivity. Furthermore these catalysts are easily poisoned.’
Nonetheless, chemical catalysis in biomass processing has become far more productive and efficient with the development of more sophisticated multifunctional heterogeneous catalysts. Hydrolysis and hydrogenation to achieve deoxygenation can take place along the same catalytic pathway, with hydrolysis being activated by acid sites on the catalyst and hydrogenation on metal sites, usually containing platinum or ruthenium or nickel.
The development of carefully structured multiphase catalysts, increasingly with nanoscale reactive sites, requires highly precise design work. With hydrolysis and hydrogenation catalysts there has to be an exact equilibrium between the acid and metal sites. Otherwise unwanted polymeric byproducts could be generated that would undermine the efficiency of the whole process.
PNNL has recently developed a technology for converting bioethanol into isobutene which opens the way for bioethanol to become a building block for making rubbers, solvents and a range of other chemicals instead of mainly an energy source.
The technology is based on an heterogeneous catalyst that combines zinc oxide with zirconium oxide. Separately, the zinc oxide creates acetone and zirconium oxide ethylene but together they made isobutene. In addition to the right reaction residence times and temperatures, a delicate balance has to be obtained between surface acid-base chemistry in the mixed oxides.
‘The research on this conversion of bioethanol to isobutene is a great example of the need for a fundamental understanding of how a catalyst works and what exactly influences its selectivity,’ says Holladay.
At PNNL’s Institute of Integrated Catalysis, which brings together expertise in imaging technologies and computational modelling tools, new equipment is showing how catalyst materials work at the atom level. Recently, researchers discovered that particles in a platinum and iridium catalyst used in biomass conversion had merged into an alloy rather than operating separately.
‘Without this detailed imaging we would not have known how the catalyst operates and in particular how the alloying of the two metals made the catalyst perform so well,’ says Chuck Peden, head of the institute. ‘It is helping us to make catalysts that are more effective.’
New equipment at the institute includes an aberration-corrected microscope, which has a spatial resolution at the atom level, and an ultra-high field nuclear magnetic resonance (NMR) spectroscope for determining physical and chemical properties of atoms and molecules. The PNNL has also acquired a scanning/transmission electron microscope (S/TEM), which gives a 3D image at the nanoparticle level. ‘This enables us to see whether metal particles are located inside or outside micropores in the oxide supporting materials of catalysts, which can be especially important to know with substances with possible poisonous properties,’ says Peden.
The imaging technologies and computational backup being used by leading catalysis research laboratories like PNNL are providing a platform for the development of future highly selective biomass catalysts. Their discoveries should ensure the production of added value fuels and chemicals from biomass, which will be essential to make its conversion on huge scale economically viable in the long term.
Sean Milmo is a freelance writer based in Braintree, Essex, UK.