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New one-pot catalytic process efficiently converts biomass to liquid alkanes under mild conditions

Debeeck1
Conversion of microcrystalline cellulose to liquid alkanes with the biphasic system in function of time and temperature. Yield insoluble products (%) = cellulose conversion (%) - total yield dissolved products (%). de Beeck et al. Click to enlarge.

A team from KU Leuven, Belgium, together with colleagues at the Leibniz Institute for Solid State and Materials Research in Germany, have designed a novel one-pot biphasic catalytic system that is able directly to transform cellulose into straight-chain alkanes (mainly n-hexane) with high yields.

The carbon-based yields are high (up to 82%) and the process completes in less than 6 hours at a comparatively mild 220 ˚C. The resulting bio-derived light naphtha fraction is a green feedstock suited for existing processes that produce aromatics, gasoline or olefins. With low-cost cellulosic residue and the absence of required pretreatment for this process, the researchers said, this approach seems highly promising en route to more sustainable chemicals and fuels. A paper on the work is published in the RSC journal Energy & Environmental Science.

There are elaborate examples in literature describing the production of new generation biofuels from sugars, sugar alcohols or other platform molecules such as HMF and levulinic acid, but research on the direct route from low cost cellulose to alkanes is still in its infancy. Although high temperature hydropyrolytic routes from biomass towards mixtures of gasoline and other compounds are promising, there is room to improve the carbon efficiency to liquid alkanes. Due to its high natural abundance and uniform chemical structure with repeating C6 sugar units, cellulose should be the ideal precursor for selectively making C6 alkanes (and thus light naphtha) as C–C bond breaking and forming are not required. The main challenge is to selectively break C–O in presence of C–C bonds.

…This paper reports a direct, fast and selective conversion of cellulose into liquid straight-chain alkanes, mainly n-hexane, by tuning the hydrogenation selectivity of a commercial Ru catalyst in a biphasic liquid system. The surface modification steers the reaction via a novel pathway, forming liquid alkanes through intermediate HMF.

—de Beeck et al.

The catalytic reaction occurs under hydrogen pressure in the presence of tungstosilicic acid, dissolved in the aqueous phase, and modified Ru/C, suspended in the organic phase. Tungstosilicic acid is primarily responsible for cellulose hydrolysis and dehydration steps, while the modified Ru/C selectively hydrogenates intermediates en route to the liquid alkanes.

Debeeck2
Overview of the process. de Beeck et al. Click to enlarge.

The dominant route to the liquid alkanes proceeds via 5-hydroxymethylfurfural (HMF), whereas the more common pathway via sorbitol appears to be less efficient, they found.

High liquid alkane yields were possible through:

  1. selective conversion of cellulose to glucose and further to HMF by gradually heating the reactor;

  2. hydrothermal modification of commercial Ru/C to tune its chemoselectivity to furan hydrogenation rather than glucose hydrogenation; and

  3. the use of a biphasic reaction system with optimal partitioning of the intermediates and catalytic reactions.

In contrast to other recently reported hydroprocessing processes, the authors said, their biphasic liquid approach at moderate temperatures mainly produces straight- chain alkanes with n-hexane and n-pentane as the major components. Recuperation of alkanes—floating on top of a separate water phase—is easy, while hydrogen selectivity is high as almost no gaseous products are formed as the C-C bonds are not broken. (Breaking those bonds results in gaseous ethanes such as methane and ethane.)

The catalytic system proved appreciably reusable and was applicable on raw softwood sawdust (almost 40% n-hexane yield). Future improvement in n-hexane yield is envisioned through a more selective formation of 2,5-DMTHF (or HMMF and DMF) to circumvent the n-pentane production. Identification of the modifying role of TSA on Ru/C, optimization of the stability of the catalytic biphasic system and decreasing the carbon content in the water phase are several focus points for future research.

—de Beeck et al.

This work was carried out within the EU FP7 project BIOCORE supported by the European Commission through the Seventh Framework Program.

Resources

  • Beau Op de Beeck, Michiel Dusselier, Jan Geboers, Jensen Holsbeek, Eline Morré, Steffen Oswald, Lars Giebeler and Bert F. Sels (2014) “Direct catalytic conversion of cellulose to liquid straight-chain alkanes” Energy Environ. Sci. doi: 10.1039/c4ee01523a

Comments

mahonj

Looks interesting - wood straight to hexane+pentane would be a boon.

SJC

The BioForming process converts aqueous carbohydrate solutions into mixtures of “drop-in” hydrocarbons.

http://www.virent.com/technology/bioforming/

gorr

Nice, now can I bring them raw materials that I can sell to them like wood, paper, trash, used cooking oil, farming surplus, sewage, etc. I need to quit my job for a more paying one.

Roger Pham

Great, similar to what I've been suggesting here in GCC, the process of adding H2 to the pyrolysis process to obtain bio-cruide oil ready for the refinery. The H2 can come from solar and wind power in order to keep the process 100% renewable. Soon, America's MidWestern Farms, with abundance of waste biomass, solar, and wind energy, will be Saudi America!

Soon, it will be cheaper to produce liquid hydrocarbon fuels from Renewable Energy rather than from Petroleum! This means that HEV's and PHEV's will be 100% green when that day will come.

SJC

Roger,

They are not doing pyrolysis, they are doing aqueous phase. Pyrolysis creates way too many acids and other compounds. Gasification is THE proven method.

Roger Pham

@SJC,
If heat is used, it is pyrolysis, usually in aqueous phase. Heat is used here, 220 degrees C, so yes, it is pyrolysis,or rather hydropyrolysis. The acids and aldehydes and other corrosive oxidated compounds will be reduced by addition of H2 using proper catalysts, turning these compounds into hydrocarbons. That's why the addition of H2 is critical for the success of this method. This is called HydroPyrolysis.

Gasification and subsequent F-T synthesis is too inefficient and costs too much of investments into facilities. That's why the gasification and F-T route is not used much, but HydroPyrolysis will be far more efficient and low in cost.

SJC

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen.

Gasification has oxygen and hence is NOT pyrolysis.

Roger Pham

Gasification turns organic matters into gases CO and H2. O2 may not be needed if heat is supplied externally. All carbon-carbon bonds are broken, requiring high heat energy. Later steps of F-T synthesis, these bonds are reformed, releasing heat but at much lower temps to be useable, hence very inefficient processes. Hardware investment is high, hence expensive products.

Pyrolysis leaves liquids behind by preserving carbon-carbon bonds. The entire process is done in one step requiring little energy input and low investment of hardware, hence low cost products.

Engineer-Poet

Hydrothermal processes are not pyrolysis.  The temperatures in this process are not even sufficient for torrefaction (250-275°C), let alone pyrolysis.

The nomenclature issue aside, if the catalysts are recycled easily/cheaply enough to make this work, there's a lot of feedstock out there just waiting to be turned into fuels or chemicals.  A biomass-based source of polymers and such allows the products to actually sequester carbon.  Imagine landfills as climate protection schemes!

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