Platinum Metals Rev., 2013, 57, (4), 322
Final Analysis: Platinum Group Metal Catalysts for the Development of New Processes to Biorenewables
- By Andrew D. Heavers* and Michael J. Watson
- Johnson Matthey Plc, PO Box 1, Belasis Avenue, Billingham TS23 1LB, UK
- Andrew Steele
- Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK
- Jeanette Simpson
- Johnson Matthey Plc, Orchard Road, Royston, Hertfordshire SG8 5HE, UK
There is a growing move away from so called first generation biorenewables (which use food crops as the feedstock) towards second generation biorenewables which use non-food sources of biomass. Biorenewable products have the potential to support growing resource needs while addressing concerns regarding climate change and energy security.
Examples of second generation biorenewable feedstocks include:
Wood from natural forests and woodlands;
Forestry plantations and residues;
Agricultural residues such as straw and corn stover;
Municipal solid wastes (MSWs);
Industry processing wastes.
Process developers have been looking to utilise lower cost biofeedstocks, including those traditionally classified as waste streams. The use of cheaper biomass feedstocks and waste streams, which are often accompanied by higher impurity levels, do however present additional purification and processing challenges such as the need for new feedstock pretreatment steps and the solving of catalyst deactivation issues. Catalysis is playing a leading role in addressing and solving these conversion and purification challenges.
Key Reactions for Biorenewables Processes
Some of the key reactions being targeted in biorenewables processes include hydrogenation, dehydration, decarbonylation, dehydrogenation and oxidation. Each of these processes covers a number of important potential reaction steps which are necessary to achieve efficient renewable processes. For some processes, the removal of oxygen and water and addition of hydrogen are key to success, and there are a range of base and precious metal catalysts to address this.
As can be seen from Figure 1, each of these conversions can lead to important products for industries and end consumers. Bioplastics, for example, find many applications in consumer goods and household items. Biochemicals can themselves be consumed in or converted into products for industry, such as fibres, coatings, automotive components and many more. Therefore by integrating biorenewable analogues of chemical intermediates into manufacturing processes through the use of chemical catalysis or otherwise, the sustainability of all products in the supply chain can be enhanced.
In many cases, using low cost biorenewable raw materials reduces the overall production costs of the end chemical product. However, it is important to note that many of the processing steps required in this expanding field are new and this presents exciting challenges for the development of new, custom designed catalysts.
The unique properties of the platinum group metals (pgms) can be applied to these complex challenges, such as catalyst deactivation and low-temperature operation. One of the problematic features of biorenewable feedstocks is the level and type of impurities they contain. Consequently, work is being focused on exploiting the resistance of pgms to common poisons, such as sulfur, encountered in biomass processing and seeking to take advantage of their high activity to enable milder operating conditions (1).
Bio-Based Routes to Valuable Chemicals
There is interest in using biomass as a raw material for the production of a range of valuable chemical building blocks including diols, oxygenates such as ethanol and biosyngas (a mixture of hydrogen and carbon oxides). Three main processes can be identified:
Conversion of lignocellulose (dry plant matter) to high value chemicals. The biochemical conversion of biomass, such as sugars, via fermentation is well established but thermochemical conversion of cellulosics may offer alternative pathways to valuable chemicals such as diols. ln particular the focus on lignocellulosic feedstocks such as wood and agricultural waste avoids the ‘food to fuel’ dilemma.
Conversion of bioderived syngas to oxygenates. Although oxygenates (such as ethanol) are already commonly produced from biomass via fermentation routes, the thermochemical synthesis of C2 oxygenates from syngas using rhodium-on-silica-based catalysts with a range of promoters is being investigated as an alternative route.
Hot conditioning of syngas derived from gasified biomass. Gasification of biomass consists of reacting biomass with steam/oxygen to produce a mixture of carbon monoxide, carbon dioxide and hydrogen. This route offers a more efficient way of producing power (as an alternative to combustion) or syngas (which can be used as a building block for many other chemical products). Biomass gasification at a relatively small scale produces tar (aromatic condensable hydrocarbons) and light hydrocarbons, in particular methane. These hydrocarbons make up a significant proportion of the carbon content of the gas and need to be converted or ‘reformed’ into syngas to improve the economics of the process and prevent downstream fouling (2). Biomass derived syngas also contains sulfur as an impurity and promoted rhodium catalysts have been found to work better for reforming methane than conventional nickel-based steam reforming catalysts in this environment.
Conversion of Biomass to Liquid Fuels
With continued focus on the availability of oil and gas and interest in alternative energy sources it is not surprising that the feasibility of producing hydrocarbon fuels from biomass is a topic of much research.
Triglycerides and fatty acids are naturally occurring molecules that have attracted much interest. They have the potential to be catalytically converted into long chain hydrocarbons which can be used as a drop-in replacement for traditional fuels. These compounds occur in feedstocks such as palm oil, soya oil and algae. The molecular structure of the triglycerides and acids found in algae in particular makes them an excellent starting point for the production of long chain hydrocarbons, such as those found in diesel fuel. This conversion can be carried out catalytically in high yield, over pgm-based catalysts, as illustrated in Figure 2 for a palmitic acid feed.
Recent catalyst developments have demonstrated the potential of more complex, bifunctional catalysts for the single step conversion of fatty acid feeds to the branched and aromatic molecules that are important constituents of aviation fuel. Figure 3 compares the performance of a 5% palladium-on-carbon catalyst with a novel bifunctional catalyst, 0.3% platinum-on-ferrierite (3), for the conversion of a palmitic acid feed. The palladium on carbon catalyst gives high yields of linear hydrocarbons suitable for diesel fuels, while the bifunctional catalyst gives a product that contains good levels of branched and aromatic hydrocarbons in the C9 to C16 chain length range, in addition to linear hydrocarbons.
This technology is being investigated within projects that are developing the biology and fermentation systems to produce a fatty acid feed from CO2 and H2 which can then be converted to an infrastructure-compatible fuel using catalysts. An example of the composition of the product from one project is shown in Figure 4 where complete conversion of the fatty acid feed has been achieved with high selectivity to a range of linear hydrocarbons in the range C11 to C18, as would be found in diesel fuel.
The move to second-generation biorenewables will require innovative solutions in terms of both catalyst and process technology capabilities. Catalysts based on pgms can provide both high selectivity and good resistance to the poisons typically found in biomass. They are set to play a leading role in providing cost effective solutions to manufacturing biorenewable products. By opening up access to milder operating conditions and pathways to new, valuable products, they will contribute to environmental stewardship and sustainability.
This article is adapted from the 2013 edition of Assay, Johnson Matthey's in-house technology magazine.
- ‘Alternative Catalytic Solutions Take Centre Stage’, Biofuels Int., 2013, 7, (2), 54 LINK http://www.biofuels-news.com/content_item_details.php?item_id=656
- A. Steele, S. Poulston, K. Magrini-Blair and W. Jablonski, Catal. Today., 2013, 214, 74 LINK http://dx.doi.org/10.1016/j.cattod.2013.02.007
- D. Davis, C. M. Lok, M. J. Watson and A. Zwijnenburg, Johnson Matthey Plc, World Appl. 2009/095,711
Dr Andrew Heavers is a Business Development Director for New Technologies in Johnson Matthey's Process Technologies Division. He has a central role across the Johnson Matthey group leading biomass related projects.
Dr Michael Watson is a Project Manager in Johnson Matthey's Technology Centre at Billingham, UK. His current research activities are focused on the development of heterogeneous catalysts for a wide range of industrial processes, including the use of biorenewable feedstocks.
Dr Andrew Steele is a Principal Scientist at Johnson Matthey's Technology Centre at Sonning Common, UK. His current interests are in research and development of new commercial opportunities for the platinum group metals with particular focus on rhodium.
Jeanette Simpson is Business Development Manager for the Chemical Catalyst business in Johnson Matthey's Process Technologies Division. She identifies and assesses new opportunities in the field of chemical catalysis, with a particular interest in biorenewable chemical processes.