Heterogeneous catalysts for biodiesel production

The combination of dwindling oil reserves and growing concerns over carbon dioxide emissions and associated climate change is driving the urgent development of clean, sustainable energy supplies. Biodiesel is non-toxic and biodegradable, with the potential for closed CO2 cycles and thus vastly reduced carbon footprints compared with petroleum fuels. However, current manufacturing routes employing soluble catalysts are very energy inefficient and produce copious amounts of contaminated water waste. This review highlights the significant progress made in recent years towards developing solid acid and base catalysts for biodiesel synthesis. Issues to be addressed in the future are also discussed including the introduction of non-edible oil feedstocks, as well as technical advances in catalyst and reactor design to ensure that biodiesel remains a key player in the renewable energy sector for the 21st century.

Designer catalysts for biodiesel synthesis

The combination of dwindling oil reserves and growing concerns over carbon dioxide emissions and associated climate change is driving the urgent development of clean, sustainable energy supplies. Biodiesel is a non-toxic and biodegradable fuel, with the potential for closed CO2 cycles and thus vastly reduced carbon footprints compared with petroleum. However, current manufacturing routes employing soluble catalysts are very energy inefficient, with their removal necessitating an energy intensive separation to purify biodiesel, which in turn produces copious amounts of contaminated aqueous waste. The introduction of non-food based feedstocks and technical advances in heterogeneous catalyst and reactor design are required to ensure that biodiesel remains a key player in the renewable energy sector for the 21st century. Here we report on the development of tuneable solid acid and bases for biodiesel synthesis, which offer several process advantages by eliminating the quenching step and allowing operation in a continuous reactor. Significant progress has been made towards developing tuneable solid base catalysts for biodiesel synthesis, including Li/CaO [1], Mg-Al hydrotalcites [2] and calcined dolomite [3] which exhibit excellent activity for triglyceride transesterification. However, the effects of solid base strength on catalytic activity in biodiesel synthesis remains poorly understood, hampering material optimisation and commercial exploitation. To improve our understanding of factors influencing solid base catalysts for biodiesel synthesis, we have applied a simple spectroscopic method for the quantitative determination of surface basicity which is independent of adsorption probes. Such measurements reveal how the morphology and basicity of MgO nanocrystals correlate with their biodiesel synthesis activity [4]. While diverse solid acids and bases have been investigated for TAG transesterification, the micro and mesoporous nature of catalyst systems investigated to date are not optimal for the diffusion of bulky and viscous C16-C18 TAGs typical of plant oils. The final part of this presentation will address the benefits of designing porous networks comprising interconnected hierarchical macroporous and mesoporous channels (Figure 1) to enhance mass-transport properties of viscous plant oils during biodiesel synthesis [5]. References: [1] R.S. Watkins, A.F. Lee, K. Wilson, Green Chem., 2004, 6, 335. [2]D.G. Cantrell, L.J. Gillie, A.F. Lee and K. Wilson, Appl. Catal. A, 2005, 287,183. [3] C. Hardacre, A.F. Lee, J.M. Montero, L. Shellard, K.Wilson, Green Chem., 2008, 10, 654. [4] J.M. Montero, P.L. Gai, K. Wilson, A.F. Lee, Green Chem., 2009, 11, 265. [5] J. Dhainaut, J.-P. Dacquin, A.F. Lee, K. Wilson, Green Chem., 2010, 12, 296.

Geographical variation in carbon dioxide fluxes from soils in agro-ecosystems and its implications for life-cycle assessment

1. Exchange of carbon dioxide (CO2) from soils can contribute significantly to the global warming potential (GWP) of agro-ecosystems. Due to variations in soil type, climatic onditions and land management practices, exchange of CO2 can differ markedly in different geographical locations. The food industry is developing carbon footprints for their products necessitating integration of CO2 exchange from soils with other CO2 emissions along the food chain. It may be advantageous to grow certain crops in different geographical locations to minimize CO2 emissions from the soil, and this may provide potential to offset other emissions in the food chain, such as transport. 2. Values are derived for the C balance of soils growing horticultural crops in the UK, Spain and Uganda. Net ecosystem production (NEP) is firstly calculated from the difference in net primary production (NPP) and heterotrophic soil respiration (Rh). Both NPP and Rh were estimated from intensive direct field measurements. Secondly, net biome production (NBP) is calculated by subtracting the crop biomass from NEP to give an indication of C balance. The importance of soil exchange is discussed in the light of recent discussions on carbon footprints and within the context of food life-cycle assessment (LCA). 3. The amount of crop relative to the biomass and the Rh prevailing in the different countries were the dominant factors influencing the magnitude of NEP and NBP. The majority of the biomass for lettuce Lactuca sativa and vining peas Pisum sativum, was removed from the field as crop; therefore, NEP and NBP were mainly negative. This was amplified for lettuces grown in Uganda (-16·5 and -17 t C ha-1 year-1 compared to UK and Spain -4·8 to 7·4 and -5·1 to 6·3 t C ha-1 year-1 for NEP and NBP, respectively) where the climate elevated Rh. 4. Synthesis and applications. This study demonstrates the importance of soil emissions in the overall life cycle of vegetables. Variability in such emissions suggests that assigning a single value to food carbon footprints may not be adequate, even within a country. Locations with high heterotrophic soil respiration, such as Spain and Uganda (21·9 and 21·6 t C ha-1 year-1, respectively), could mitigate the negative effects of climate on the C costs of crop production by growth of crops with greater returns of residue to the soil. This would minimize net CO2 emissions from these agricultural ecosystems.

Vulnerability of exporting nations to the development of a carbon label in the United Kingdom

Carbon labels inform consumers about the amount of greenhouse gases (GHGs) released during the production and consumption of goods, including food. In the future consumer and legislative responses to carbon labels may favour goods with lower emissions, and thereby change established supply chains. This may have unintended consequences. We present the carbon footprint of three horticultural goods of different origins supplied to the United Kingdom market: lettuce, broccoli and green beans. Analysis of these footprints enables the characterisation of three different classes of vulnerability which are related to: transport, national economy and supply chain specifics. There is no simple relationship between the characteristics of an exporting country and its vulnerability to the introduction of a carbon label. Geographically distant developing countries with a high level of substitutable exports to the UK are most vulnerable. However, many developing countries have low vulnerability as their main exports are tropical crops which would be hard to substitute with local produce. In the short term it is unlikely that consumers will respond to carbon labels in such a way that will have major impacts in the horticultural sector. Labels which require contractual reductions in GHG emissions may have greater impacts in the short term.