Chemistry: The Future of Cellulose
I am not a big believer in a commercial future for the biochemical conversion of cellulose into fuels. There are many big hurdles in place that are going to have to be overcome before cellulose is commercially converted to ethanol. In a nutshell, one is the logistical problem, which I have covered before. Beyond the logistical problem is the issue that biochemistry often starts to malfunction as the conditions in a reactor change, and with cellulosic ethanol that means that if you get a 4% solution of ethanol in water, you are doing well. But from an energy return point of view, a 4% solution is about like the trillions barrels of oil shale reserves we have. If it takes over a trillion barrels of energy to extract and process them, that largely defeats their usability.
Chemistry is a different matter, which is why I favor gasification processes over fermentation processes. But even beyond gasification, I have wondered about chemically processing cellulose in a refinery. I used to have a guy who e-mailed me all the time and told me he had invented a chemical process for reacting cellulose to hexane, which can then be turned into gasoline. If you look at cellulose (there is a graphic of a segment of cellulose at the previous link), you can envision that it could be done. (Whether he had actually done it is a different story).
But the chemistry pathway isn’t limited to fuels. With that preface, I want to thank a reader for bringing this story to my attention. In a recently published story in Applied Catalysis A: General (available online at Science Direct), scientists at Pacific Northwest National Laboratory have reported on a new process for converting cellulose directly into an important chemical building block (e.g., for plastics and fuel):
Now we all know that you can do lots of neat things in the lab that can’t really be done on a larger scale. But this particular process does not appear to be overly complicated. The abstract from the paper explains what they are doing:
The ability to use cellulosic biomass as feedstock for the large-scale production of liquid fuels and chemicals depends critically on the development of effective low temperature processes. One promising biomass-derived platform chemical is 5-hydroxymethylfurfural (HMF), which is suitable for alternative polymers or for liquid biofuels. While HMF can currently be made from fructose and glucose, the ability to synthesize HMF directly from raw natural cellulose would remove a major barrier to the development of a sustainable HMF platform. Here we report a single-step catalytic process where cellulose as the feed is rapidly depolymerized and the resulting glucose is converted to HMF under mild conditions. A pair of metal chlorides (CuCl2 and CrCl2) dissolved in 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) at temperatures of 80–120 °C collectively catalyze the single-step process of converting cellulose to HMF with an unrefined 96% purity among recoverable products (at 55.4 ± 4.0% HMF yield). After extractive separation of HMF from the solvent, the catalytic performance of recovered [EMIM]Cl and the catalysts was maintained in repeated uses. Cellulose depolymerization occurs at a rate that is about one order of magnitude faster than conventional acid-catalyzed hydrolysis. In contrast, single metal chlorides at the same total loading showed considerably less activity under similar conditions.
So they take cellulose and react it with two metal chlorides at 80–120°C for a direct conversion of cellulose into HMF – which can be easily converted to fuel or plastics. I would think then the important considerations would be 1). What happens to the lignin and hemicellulose in the biomass?; and 2). How much energy does it take? The second item is particularly important if fuel is the objective.
While it is too early to tell whether there is a fatal flaw, this one certainly bears watching. It also strengthens my conviction that in the long-run, the right way to process cellulose is chemically.