New simple, energy-efficient process for the direct production of renewable diesel from biomass waste
|Non-edible carbohydrates are converted to renewable diesel-range liquids via two consecutive catalytic steps. Credit: Wiley-VCH, Corma et al. Click to enlarge.|
A team at the Universidad Politécnica de Valencia (Spain) has designed a new simple, energy-efficient process (that also does not require any organic solvents) for the production of renewable diesel from biomass waste. A paper on their work is published in the journal Angewandte Chemie International Edition.
The process converts 2-methylfuran (2MF)—which can be obtained from biomass wastes such as corncobs, oat hulls, bagasse, and sunflower husks—into diesel-range hydrocarbons through two consecutive catalytic steps that involve hydroxyalkylation/alkylation and hydrodeoxygenation, with an overall yield of 87%.
A number of routes have been and are being developed for the conversion of biomass into renewable fuels, including (but not limited to):
- Gasification of biomass followed by Fischer-Tropsch synthesis;
- Fast pyrolysis and upgrading of bio-oil;
- Hydrolysis of biomass followed by the fermentation of the sugars by genetically modified microorganisms to hydrocarbons;
- Dehydration of hydrolyzed sugars to 5-hydroxymethylfurfural (HMF) or into furfural (FUR) when starting from hexoses or pentoses, respectively, followed by aqueous phase processing;
- Production of γ-valerolactone from biomass-derived carbohydrates via levulinic acid, followed by decarboxylation to produce butene and CO2, the former then being oligomerized to octenes and hexadecenes in a second step.
This list of processes to produce second-generation biofuels can be further expanded, but the extent to which one of these technologies will play an active role in the future biofuel industry will depend on economics, energy efficiency, and environmental issues. Surplus energy consumption and process limitations can be detected in most of the processes proposed to date. For example, the excessive cleavage of carbon–carbon bonds and subsequent reformation leads to energy losses. Extractions of products with organic solvents are energy and cost-intensive steps that change, to the worse, the overall energy balance of the process. Organic solvents as reaction medium should be avoided, as they enlarge process volumes with a negative impact on process economics and environment. A crucial point for the optimization of the overall process economics is the perfect overlap of the boiling point range of the product mixture, with diesel range C9 to C24 hydrocarbons.
By considering all of the above described limitations, we have designed a sustainable process, based on reactions other than those reported before, which involves hydrophobic intermediates and produces nonpolar alkane products suitable for high-quality diesel fuel. This strategy implies that, owing to the polarity of the products, water separation by distillation is not required, but an automatic, energy-neutral physical separation of (intermediate) products from water will occur reducing energy consumption in the process.—Corma et al.
The first step is the conversion of biomass into furfural—an established industrial process. In an adaptation of another current process, furfural can be converted with high selectivity into 2-methyl-furfural (2MF), a ring consisting of four carbon atoms and one oxygen atom, with a side chain consisting of a methyl group (-CH3).
Three molecules of 2MF are linked together. This requires water and an acid catalyst. This reaction causes one third of the rings to open and each to link to two other rings (hydroxy alkylation/alkylation). The aqueous phase, which also contains the catalyst, separates from the organic phase, which contains the intermediate product, on its own. It can easily be removed and the catalyst recycled. In a second reaction, the two other rings must also be opened and their oxygen atoms removed. This reaction uses a special platinum-containing catalyst (hydrodeoxygenation).
In the end we obtain 87% of the diesel fraction in the form of branched hydrocarbon chains with nine to 16 carbon atoms. This is the best yield reported in the literature thus far for biodiesel synthesis.—Avelino Corma
The process is very stable at lab levels (more than 140h). Gas-phase and lower molecular weight byproducts can be used to produce heat. The resulting renewable hydrocarbon liquids are of excellent quality (cetane number 71, pour point -90 °C) and can be mixed directly with conventional diesel fuels.
The process described herein opens new routes for producing high quality diesel from waste biomass. Indeed, we can envisage two other processes that we are currently working out in which 5-hydroxymethylfurfural (HMF) and 5- methylfurfural derived from hexoses are reacted with 2MF in the presence of an acid catalyst under similar conditions as employed before for butanal and 2MF.—Corma et al.
Corma, A., Torre, O. d. l. , Renz, M. and Villandier, N. (2011) Production of High-Quality Diesel from Biomass Waste Products. Angewandte Chemie International Edition, doi: 10.1002/anie.201007508