http://www3.interscience.wiley.com/journal/121359280/abstract Direct, High-Yield Conversion of Cellulose into Biofuel**
Mark Mascal* and Edward B. Nikitin
These are days of great incentive in the field of bioenergy research. The stakes, of course, are immense—economic independence from politically unstable, petroleum-exporting countries, the remediation of greenhouse gas levels in the atmosphere and their potential effect on the climate, and mitigation of the economic consequences of our imminent arrival at the Peak Oil point, particularly in light of the pace of industrialization of emerging economic superpowers in Asia. It may be said that the final answer to the global energy issue will lie most credibly in ultraclean technologies based on hydrogen and solar energy.1 However, few would deny a more immediate future to carbon-based fuels, in view of the prevailing automotive infrastructure based on the internal combustion engine, as well as the fact that the chemical industry will always require feedstocks for the production of organic materials and chemicals, regardless of what is being used for energy.2
The challenge to a new carbon-based fuel economy, as it emerges, is twofold : First, the carbon source must ultimately be atmospheric carbon dioxide, which is most practically harvested by the photosynthetic production of cellulose, hemicellulose, starch, and simple sugars, and second, these saccharides must be efficiently converted into molecules which are ambient temperature liquids of low volatility and high energy content. To some extent, the above challenge is currently being met by the production of ethanol from either starch-derived glucose or cane sugar, but this has largely been an issue of expediency, making use of mature technologies (agriculture and brewery/distillery) that were established long before energy became an issue, and the approach is now considered by many to be transitional.3
Since cellulose is by far the major form of photosynthetically fixed carbon, it can be argued that it should be the principal focus of any emerging carbon-fuel technology. The difficulty, from the point of view of ethanol production, is that fermentable sugars are not easily liberated from this material. The current model for cellulose utilization involves saccharification with immobilized enzymes, but despite recent advancements, this remains a slow and expensive process.
Our own interest in this area had less to do with the problems of cellulose hydrolysis than the poor carbon economy of glucose fermentation. Glucose is utilized by microorganisms according to the equation C6H12O6 → 2 C2H5OH + 2 CO2. Even assuming quantitative efficiency both in the derivation of glucose from cellulose as well as the fermentation process, one third of the available carbon is expelled as carbon dioxide, 9.6 g of which is produced for every 10 g of ethanol. Effective approaches to biomass utilization which avoid fermentation altogether and exploit all of the available carbon present would thus be extremely valuable. Certainly, this point has not escaped the attention of researchers at the forefront of biomass conversion chemistry.4 One promising direction this research has taken is towards “furanics”, that is, high-energy, furan-based organic liquids. A high-profile contribution by Dumesic and co-workers in this area showed that fructose could be efficiently converted, via 5-hydroxymethyfurfural (HMF, 1), into a range of substituted furan and tetrahydrofuran products.5 If, however, this approach is to find broader application, it cannot rely on fructose as the source of HMF (1). Interestingly, a concurrent publication by Zhang and co-workers described the conversion of glucose into HMF (1) in record yield,6 and taken together these two studies point towards a workable nonfermentive process for the conversion of glucose into biofuel. However, closer inspection of the latter of these papers shows that the expensive 1-ethyl-3-methyl-imidazolium chloride ionic liquid is used as the solvent which, along with chromium(II) catalyst, produces HMF in about 70 % yield, determined not by isolation but HPLC analysis. Along the same lines, Dumesic and co-workers have published a study in which HMF (1) is derived from glucose with 53 % selectivity at high conversion in 60 % aqueous DMSO in a biphasic reactor. While also promising, the separation of DMSO from HMF (1) remains an issue.7
…http://www.sciencedaily.com/releases/2008/08/080808114928.htm Fuel From Cellulose, Cheaper And With Better Yields Than Ever Before
ScienceDaily (Aug. 8, 2008) — Independence from fossil fuel exporting nations, a reduction in the release of greenhouse gases, conservation of dwindling resources: there are any number of reasons to stop the use of fossil fuels. Hydrogen technology and solar energy will very probably provide the solution to our global energy problem—in the long term.
For an initial quick remedy we may look to bioenergy. Biomass can be used to generate alternative carbon-based liquid fuels, allowing the continued use of current automotive combustion engine technology and existing infrastructure. At the same time, the chemical industry would continue to be supplied with the carbon compounds it requires as raw materials for plastics, textiles, etc. Mark Mascal and Edward B. Nikitin at the University of California, Davis (USA) have now developed an interesting new method for the direct conversion of cellulose into furan-based biofuels. Their simple, inexpensive process delivers furanic compounds in yields never achieved before.
Atmospheric carbon dioxide is viewed as the ultimate carbon source of the future. It is most efficiently “harvested” by plants via photosynthesis. Currently, biofuel producers primarily use starch, which is broken down to form sugars that are then fermented to give ethanol. Cellulose is however the most common form of photosynthetically fixed carbon. The problem is that the degradation of cellulose into its individual sugar components, which could then be fermented, is a slow and expensive process. “Another problem is that the carbon economy of glucose fermentation is poor,” explains Mascal, “for every 10 g of ethanol produced, you also release 9.6 g CO2.”
Could we avoid the breakdown of cellulose and fermentation? Mascal and Nikitin demonstrate that we can indeed. They have developed a simple process for the conversion of cellulose directly into “furanics”, which are furan-based organic liquids. Furans are molecules whose basic unit is an aromatic ring made of one oxygen and four carbon atoms. The main product the researchers obtain under the conditions they have been developing is 5-chloromethylfurfural (CMF).
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