Cellulosic ethanol is being commercially generated right now. Burning it releases less carbon dioxide—ideally, no carbon dioxide—into the
atmosphere, and producing it requires no new drilling for fossil fuels
underground. All that needs to happen is for the chemistry to advance
enough to make it as economically viable as petroleum, and your future
fill-up at a gas pump may have a sticker that says “Contains 10% Petroleum” in your ethanol.
Ethanol Facts: Engine Performance. Renewable Fuels Association, 2014:
Colaneri, K. AP: Environmental Impacts of Ethanol May Outweigh the Benefits.
State Impact: A Reporting Project of National Public Radio Member Stations,
Nov 12, 2013: https://stateimpact.npr.org/pennsylvania/2013/11/12/ethanol/
[accessed Jan 2016].
Overview of Greenhouse Gases: Carbon Dioxide Emissions. U.S. Environmental
Protection Agency: http://www3.epa.gov/climatechange/ghgemissions/gases/
co2.html [accessed Jan 2016].
Jonathan Sherwood is a science writer who lives in Rochester, N. Y. This is his
first article in ChemMatters.
molecules that work with cellulose to create sturdy plant cell walls
(Fig. 4). Lignin encloses the cellulose and hemicellulose molecules,
making them difficult to reach. To access cellulose, it must be separated
from the other molecules.
Chemists have several ways of extracting the glucose from cellulose,
either biochemically or thermochemically (Fig. 5).
Cellulosic ethanol holds promise
There is still a great deal of research to be done to find the best way
to make cellulosic ethanol. The prize for the individual who finds an economical way of converting plants to a biofuel, such as ethanol, is estimated to be a $1.4 trillion consumer market within a decade, according
to the Renewable Fuels Association.
Currently, only about 3 million gallons of cellulosic ethanol is produced in the United States each year, compared to 15 billion gallons of
corn ethanol and 290 billion gallons of petroleum. However, researchers
estimate there are enough plant materials—such as forest trimmings,
lumber mill scraps, and agricultural waste—to supply enough cellulosic
ethanol to replace half of the country’s petroleum needs in the near
future. These plant materials are currently left to rot, are burned, or are
sent to landfills. No new crops need to be planted, and no conversion of
rainforests to farmland needs to occur. U . S
18 ChemMatters | APRIL/MAY 2016 www.acs.org/chemmatters
Six- to seven-year-old poplar
trees grown for biofuels
Figure 5. Glucose can be extracted from a plant’s waste materials by one of two methods, which are shown here. (A)
In the biochemical method, the plant’s waste material is dried and ground up and then soaked in sulfuric acid, which
removes the lignin and releases the hemicellulose and cellulose; enzymes break the cellulose into individual glucose
molecules, and a mix of microbes is added to ferment the sugars into ethanol. (B) In the thermochemical method, the
plant material is treated with hot steam to break it down into “syngas,” or “synthesis gas,” which is mainly a mixture
of carbon monoxide (CO) and hydrogen (H2); the carbon monoxide and hydrogen are passed over a metal catalyst,
and they combine to form ethanol and other alcohols.
other alcohols Syngas
Figure 4. Plant cell walls are made
of cellulose molecules (yellow)
surrounded by lignin (red) and
hemicellulose (blue), two groups of
molecules that are very difficult to
break down. This is the main reason
why ethanol from cellulose
is difficult to produce.