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1 ~
Opportunities for Catalysis Research in
Energy and Transportation
R. Thomas Baker,
Los Alamos National Laboratory
INTRODUCTION
Catalytic processes, key to a variety of efficient transformations, are of
enormous importance and are responsible in many ways for the high standard of
living we enjoy today. Approximately one-third of the Gross Domestic Product
of the United States today can be traced to the success of one or more catalytic
processes. In the chemical industry approximately 80 percent of the processes
use catalysts, and the percentage is expected to grow as concern over waste dis-
posal grows. By definition, catalysis saves energy and plays a key role in diverse
areas in energy and transportation. Future opportunities for catalysis in energy
and transportation will rely on the development of more efficient catalytic
processes that take place under milder conditions (lower temperature and pres-
sure) along with the discovery and development of selective new catalysts for
transformations that either are not known or are too inefficient today.
A catalyst simply provides a reaction pathway that is not possible in its
absence. A perfect catalyst would catalyze a reaction forever with no activation
energy and 100 percent selectivity. Although many catalysts have turnover num-
bers in the millions, most catalyst lifetimes are limited by side reactions that lead
to catalyst deactivation. Modern study of chemical catalysis is an interdisciplinary
enterprise that involves scientists from every chemical discipline collaborating
with each other and with material scientists, theoreticians, and chemical engineers.
The largest area of catalysis in industry today is heterogeneous catalysis.
Heterogeneous catalysts have the advantage of being insoluble and often rela-
tively robust. Therefore, gaseous or liquid reactants and products readily pass
through or over a heterogeneous catalyst, and the product can be separated from
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OPPORTUNITIES FOR CATALYSIS RESEARCH IN ENERGY AND TRANSPORTATION 71
the catalyst readily. Disadvantages of heterogeneous catalysts include the
inability to control activity by systematically altering the surface structure, the
difficulty of identifying precisely the type of site that promotes a given reaction,
and competing side reactions at other catalytic sites on the surface. It is also
difficult to guarantee that a high percentage of a metal in a heterogeneous catalyst
is participating in the catalytic reaction, thereby requiring either higher tempera-
tures or high metal loading for practical operation.
Homogeneous catalysts, reactants, and products are all in the same phase,
usually liquid. Homogeneous catalysts often employ metal complexes with well-
defined ligand coordination environments and reactivities, which lead to a high
percentage of active catalyst sites, and predictable and reproducible activity that
depends on a 'single site' being present. The main disadvantage of homogeneous
catalysts is separation of the product from the catalyst, a problem that has been
solved in some cases using phase transfer catalysis, or by 'tethering' a homoge-
neous catalyst to an insoluble support. A second disadvantage is the widespread
use of hydrocarbon solvents, which has led to an exploration of the possible use
of other solvents (water, supercritical carbon dioxide, and ionic liquids) in many
reactions.
In the future new metal catalysts and catalytic reactions will be perfected
primarily as a consequence of fundamental studies of reactions at a metal center.
One must be able to control the timing of a sequence of steps in the primary
coordination sphere of a metal with exquisite delicacy and thereby direct the
metal through a maze of possible outcomes, including no reaction at all. The
challenge is to control the outcome of a desired catalytic reaction, either through
ligand design in homogeneous catalysts or surface design in heterogeneous
catalysts.
OPPORTUNITIES FOR CATALYSIS RESEARCH
Below are some examples of catalysts and areas of catalyst research that
could contribute significantly in the future in terms of energy savings and im-
proved transportation.
De-Nitrogen Oxide Catalysts
The transportation sector accounts for 68 percent of all of the petroleum used
and one-third of the anthropogenic carbon dioxide emissions in the United States.
In addition, the utilization of internal combustion engines for transportation results
in a significant amount of nitrogen oxides and particulate emissions.
Technology exists today to manufacture much more thermally and fuel effi-
cient 'lean burn' engines (compression ignition, direct injection engines such as
diesels or homogeneous charge compression ignition engines) that operate under
lean conditions. Existing noble metal-based catalytic converter technology for
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ENERGY AND TRANSPORTATION
stoichiometric gasoline engines is not suitable for the oxygen-rich exhaust of a
lean-burn engine, so there is no suitable existing technology for emissions control
for the lean-burn engine. The scientific challenge is to catalytically reduce
nitrogen oxides under oxidizing conditions over a wide operating temperature
range, from below 200 °C up to 500 °C.
To achieve this goal, new catalysts are being sought that could affect the
selective catalytic reduction of nitrogen oxide in the presence of oxygen by
exhaust hydrocarbons or by introduction of ammonia from the hydrothermal
decomposition of urea. Molecular sieve zeolites and other oxide supports, ion-
exchanged with transition metals and other ions, are the best of the catalysts
known to affect this chemistry. However, the mechanism of the selective cata-
lytic reduction is still not understood. In addition to discovering catalysts of high
activity, the challenge is to maintain the activity of the catalyst under conditions
of high temperature, steam, sulfur, and the long operating times necessary to
make these materials viable catalysts for use in lean-burn engines.
The past decade saw additional major advances in automotive catalysts. The
amount of costly precious metals such as palladium, platinum, and rhodium in
catalytic converters has been reduced by more than 20 percent through the use of
precision coating onto increasingly sophisticated mixed metal oxide supports. To
reduce existing "smog" levels, Engelhard's PremAir technology uses a platinum-
based coating on the radiator and air-conditioner condenser to convert ground-
level ozone into oxygen and carbon monoxide into carbon dioxide.
Clean Coal Technologies
The fact that a massive supply of coal still exists in this country argues for
technologies that allow for more efficient use of coal and that burn coal with
fewer emissions and less carbon dioxide. One of the technologies presently being
pursued is to combine anaerobic hydrolysis of coal with the capture of carbon
dioxide by calcium oxide. The thermodynamics of these combined processes
indicate that coupling these reactions leads to a thermodynamically neutral over-
all reaction.
The benefit of this type of process is that all potential emissions could be
handled at once. However, burning coal releases not just small molecule emis-
sions but sulfur and heavy metals in addition to carbon formation and radioactive
emissions. Therefore, it is necessary to perform in situ diagnostics to receive
feedback on transient concentrations in order to maintain the most efficient coal
burning.
New Catalysts for Refining Applications
Over the past 60 years catalysis has transformed the refinery from a distilla-
tion plant to a sophisticated chemical-processing plant. One major challenge
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OPPORTUNITIES FOR CATALYSIS RESEARCH IN ENERGY AND TRANSPORTATION 73
facing the refining industry is the cost-effective manufacture of ultra low sulfur
transportation fuels to enable vehicles to meet stringent reduction in emissions.
To enable the production of less than 10 ppm of sulfur gasoline, a number of new
catalysts and processes have been developed to selectively desulfurize part or all
of the FCC (Fluid Catalytic Cracking) naphtha and preserve the olefins that pro-
vide octane value. There are a variety of selective desulfurization options avail-
able that preserve octane value. These include IFP's Prime G+, BP's OATS
process (Olefin Alkylation of Thyophenic Sulfur), Phillips' S-Zorb process, and
ExxonMobil's SCANfining process.
A breakthrough distillate hydrotreating catalyst, jointly developed by
ExxonMobil and AkzoNobel, was recently commercialized and deployed in three
refineries. This catalyst has three to ten times the activity of the most active cur-
rent HDS/HDN catalysts, depending on pressure. This catalyst also exhibits novel
and unique pressure sensitivity from 400 psi Hydrogen-2 pressure up to 2,000 psi
Hydrogen-2. It represents a major advance in catalyst performance, composition,
structure, and morphology. The product is stripped not only of sulfur but also
nitrogen and achieves substantial aromatic saturation.
Short contact time FCC has significantly improved the yield and quality of
naphtha. A major improvement in cracking catalyst design came about from the
recognition of bifunctionality of FCC catalysts. An example of a new catalyst is
one where detrital alumina deposited on the active zeolite doubled activity and
increased selectivity. The ADA catalyst family has been developed in a joint
research program by Grace Davison and ExxonMobil. Much higher product value
has been generated by significant improvements in the ZSM-5 catalyst system to
produce olefins in FCC units, specifically ethylene and propylene. Major im-
provements in lube hydroprocessing have come about through the use of the new
MSDW-2 isodewaxing catalyst.
Selective Reactions of Methane and Other Alkanes
Currently a great deal of energy is expended to obtain starting materials,
usually alkenes, for the chemical industry from petroleum or coal. These alkenes
are then converted into polymers, alcohols, and other downstream chemical
products. In view of the large reserves of methane (natural gas), it would be
highly desirable to use it as a source of products for the chemical industry.
Although methane can be reformed with steam and oxygen to produce a mixture
of carbon oxide and hydrogen ('synthesis gas'), which is then reacted over a
catalyst to produce a variety of hydrocarbons, this 'Fischer-Tropsch' chemistry
involves high pressure and temperatures.
It would be highly desirable to be able to convert methane directly into value-
added products, most importantly into methanol by direct oxidation with oxygen
at low temperatures and pressures. Selective direct oxidation of methane to
methanol would allow plants to be built at the sources of methane, thereby elimi-
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ENERGY AND TRANSPORTATION
eating problems associated with transportation of methane. Methanol at present
is the starting point for the synthesis of multicarbon commodity chemicals and
can be converted into alkalies using zeolite catalysts. Significant progress has
been made in selectively oxidizing methane to methanol with oxygen and plati-
num catalysts, although an efficient catalytic process has yet to be perfected. It
also would be desirable to directly functionalize higher alkalies, for example to,
alcohols, or to dehydrogenate them selectively to alkenes.i
Catalysts for Photocatalytic Water Splitting
One of the grand challenges is to develop catalysts that will split water into
hydrogen and oxygen using sunlight as the source of energy for this uphill
reaction. This would be the artificial counterpart of photosynthesis in green
plants. Hydrogen and oxygen could then be recombined in fuel cells, thereby
effectively utilizing the energy of the sun to provide electricity with an efficiency
that is not inherently limited theoretically to a small yield, as in silicon-based
solar cells. Photocatalytic splitting of water is an ambitious project with a huge
and important payoff. Complex new transition metal complexes must be designed
that operate in water and allow the production and separation of hydrogen and
oxygen using photochemistry. These catalysts are likely to be either homoge-
neous or tethered to a support that doubles as a hydrogen-permeable membrane,
thereby allowing hydrogen to be separated from oxygen. The most ambitious
approach would be to link photocatalytic splitting of water with a fuel cell in
order to effectively convert sunlight into electricity in a single device. Although
progress has been made in driving unfavorable reactions photochemically, we are
not close to a solution to the problem. The solution will require long term sup-
port, research effort, and patience.
Linking Catalysis Theory with Experiment
With advancements in computer power and the advantage of reasonable
scalability to larger systems, density functional theory has made computational
chemistry widely accessible in the chemical sciences, permitting direct compari-
sons to be made between theory and a wide variety of experiments. Examples of
the application of density functional theory for prediction, understanding, and
interpretation in surface science and heterogenous catalysis include:
· understanding active site structures and structures of key intermediates in
acid catalysis of hydrocarbon conversion by zeolites,
iAs hydrocarbons will most likely be used for the foreseeable future, the development of hydro-
carbon anode catalysis is another area of research that could potentially provide significant break-
throughs.
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OPPORTUNITIES FOR CATALYSIS RESEARCH IN ENERGY AND TRANSPORTATION 75
.
understanding the importance of dynamic changes of the catalyst surface
in ammonia synthesis,
identification of new catalytic intermediates in selective oxidation;
prediction of reaction energetics and the kinetics of selective and non-
selective catalytic pathways,
· predictions leading to the discovery of a new Diels-Alder reaction for
organic functionalization of semiconductor surfaces, and
· prediction leading to invention of new steam reforming catalyst.
Although computational chemistry is far from predicting the outcome of a
chemical reaction through full-scale modeling of the catalytic reaction in question,
it can be used in conjunction with experiments to guide the experimentalist in
potentially fruitful directions. Efforts should be made to formalize collaborations
between experimentalists and theoreticians as much as possible in the future.
Representative terms from entire chapter:
catalytic reaction
