Cellulosic Carbon

April 30, 2014

The saying, "cheap as dirt," needs some revision. A quick scan of the Internet reveals a "wonder soil" sold for more than five dollars a pound. This patented dirt has a five star rating and its own material safety data sheet (MSDS). Its unique properties arise from a water-retentive, cross-linked acrylamide potassium acrylate copolymer. There's more to high-technology than just tablet computers. Silicon Valley, meet Grimy Gulch.

The unique properties of this dirt arise from an additive, but it's even nicer when you can harvest a material from nature and use it with little or no processing. A prime example of this is kieselguhr (a.k.a., diatomaceous earth), the fossilized remains of diatoms.

Beam me up to the mothership! Nature routinely organizes matter into objects, like this diatom shell, that resemble the finest of human craftsmanship. (Photomicrograph by George Swan, via Wikimedia Commons.)

The shells of these algae are about 80-90% silica, with a few percent alumina and iron oxide. You might be brushing your teeth with diatoms, since this material is used as an abrasive in toothpaste. It's used, also, as a filler for plastics, a thermal insulator, and a filtration medium. When impregnated with nitroglycerin, it becomes dynamite, the invention that funded the Nobel Prizes.

Another useful natural material is wood. If you exclude its concrete block foundation, my house is likely 80% wood by weight, and an additional 10% would be the gypsum (calcium sulfate dihydrate, CaSO4⋅2H₂O) wall board. The many books in my home library are printed on paper derived from wood.

Although wood is a combination of several chemical compounds, its major constituent (about 42%) is cellulose, a crystalline polymer. Hydrogen bonding between straight, parallel chains of cellulose polymer gives wood its strength (see figure). A carpenter might use a saw on wood in the course of his work, but chemists are applying some of their laboratory tools to cellulose to make this abundant material more useful.

Polymer chains of cellulose. The polymer chains are straight, and the strength of wood derives from hydrogen bonding, not the coiling or branching found in other polymers. (Illustration by Luca Laghi, via Wikimedia Commons.)

Cellulose has been researched for many years, mostly with the goal of making better paper. Some important principles were discovered, such as the detrimental affects of acid on paper stability, and how the lignin in wood pulp undergoes a photochemical reaction with oxygen to cause a yellow coloration.

Since useful materials are sometimes more useful in nanoscopic form, scientists and engineers from Purdue University (West Lafayette, IN), the General Motors Research and Development Center (Warren, MI), and the United States Forest Service Forest Products Laboratory (Madison, WI) have been modeling the mechanical properties of cellulose nanocrystals.[1-2]

As Pablo D. Zavattieri, a Purdue University assistant professor of civil engineering, explains,

"It is very difficult to measure the properties of these crystals experimentally because they are really tiny... For the first time, we predicted their properties using Quantum mechanics. This is important for the design of novel cellulose-based materials as other research groups are considering them for a huge variety of applications, ranging from electronics and medical devices to structural components for the automotive, civil and aerospace industries."[2]

Cellulose nanocrystals are needle-shaped, about 3 nanometers wide by 500 nanometers long, so it's nearly impossible to measure their mechanical properties directly, but they can be computed.[2] Not surprisingly, the mechanical properties of the cellulose nanocrystals are highly anisotropic. The computed Young's modulus along the [001] (c-axis) is 206 GPa, which is comparable to that of steel. The [010] (b-axis) Young's modulus was 98 GPa, and the Young's modulus along [100] (a-axis) was only 19 GPa. The average Poisson's ratio was also found to be extremely anisotropic.[1]

Left, structural details of cellulose nanocrystals. Right, transmission electron micrograph of cellulose nanocrystals. (Left image, and right image by Pablo Zavattieri/Purdue University and the Purdue Life Sciences Microscopy Center.)[2]

These cellulose nanocrystals, which are inherently renewable, carbon-neutral, biodegradable and sustainable, might be a potential green alternative to carbon nanotubes for applications such as polymer and concrete reinforcement.[2] The Purdue team is also looking at alpha-chitin, a material with mechanical properties similar to that of cellulose. Chitin is contained in the shells of insects, lobsters and crabs; and, it's an abundant material that's now waste in the food industry. Funding for this work was provided by the U.S. Department of Agriculture and the National Science Foundation.[2]

Chemists at Oregon State University have found that heating cellulose in a furnace in the presence of ammonia will produce nitrogen-doped, nanoporous carbon membranes.[3-4] This environmentally benign process, whose only byproduct is methane, produces material suitable for use as electrodes for supercapacitors.[4] Xiulei (David) Ji, an assistant professor of chemistry at Oregon State, and principal investigator for this research, says that it's surprising that this basic reaction wasn't discovered earlier. "The ease, speed and potential of this process is really exciting."[4]

These nanoporous carbon membranes have a surface area in a single gram of nearly 2,000 square meters. The process stars with cellulose filter paper, which is much like the filter paper used in coffeemakers. The exposure to high heat and ammonia converts the cellulose to the nanoporous carbon material useful for supercapacitors.

A one-step process for production of nanoporous carbon membranes from cellulose paper. (Oregon State University image.)[4]

Cellulose pyrolysis at 700 °C or above in the presence of NH₃ produces nitrogen doping up to 10.3 atomic-% and a surface area up to 1973.3 m²/g).[3] Methane (CH₄) is produced as a product.[3] Activated carbon, now used as the supercapacitor electrode material, was measured to have a surface area of 1533.6 m²/g. The nitrogen-doped nanoporous carbon showed more than double the unit area capacitance of activated carbon, 90 versus 41 mF/m².[3] Aside from the supercapacitor application, nanoporous carbon can be used as a gas absorber material, or as a water filter.[4]

References:

1. Fernando L. Dri, Louis G. Hector Jr., Robert J. Moon and Pablo D. Zavattieri, "Anisotropy of the Elastic Properties of Crystalline Cellulose Iβ from First Principles Density Functional Theory with Van der Waals Interactions," Cellulose, vol. 20, no. 6 (December 2013), pp 2703-2718 .
2. Cellulose nanocrystals possible 'green' wonder material, Purdue University Press Release, December 16, 2013.
3. Wei Luo, Bao Wang, Christopher G. Heron, Marshall J. Allen, Jeff Morre, Claudia S. Maier, William F. Stickle and Xiulei Ji, "Pyrolysis of Cellulose under Ammonia Leads to Nitrogen-Doped Nanoporous Carbon Generated through Methane Formation," Nano Letters Article ASAP (March 28, 2014), DOI: 10.1021/nl500859p.
4. Trees go high-tech: process turns cellulose into energy storage devices, Oregon State University Press Release, April 7, 2014.