3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
TEACHING SUSTAINABILITY THROUGH CATALYSIS
Gregory S. Yablonsky1*, John T. Gleaves2, Rebecca Fushimi3
1
Saint Louis University, Parks College of Engineering, Aviation and Technology, Department of
Chemistry, 3450 Lindell Blvd, St. Louis MO 63103, USA
2
Washington University in St. Louis, Department of Energy, Environmental and Chemical
Engineering, One Brookings Drive, Campus Box 1180, St. Louis MO 63130, USA
3
The Langmuir Research Institute, Saint Louis, MO 63367, USA
Abstract: Catalysis is the essential chemical phenomenon that underlies all living systems, and
is key to creating sustainable processes and a greener environment (Armor 1999, Centi 2008).
Catalysts accelerate chemical reactions, and efficiently channel energy into building complex
molecular structures. Catalyst's ability to perform specific reactions with great precision through
millions of cycles is the basis of sustainable processes. Thus the concepts of sustainability can be
clearly illustrated using examples of natural and manmade catalytic processes.
Natural catalytic cycles, such as the photosynthetic production of carbohydrates, are made
possible through enzymes. The efficient conversion of oil, gas, coal and biomass into fuels and
chemicals is made possible by modern catalytic technology. In this discussion, the catalytic cycle
is presented to facilitate the discussion of sustainability. Catalysis can be used to address some
of the key problems facing the 21st century; in particular the production of fuels and chemicals in
absence of petroleum is discussed here as an example.
On the cutting-edge of catalysis research is the Temporal Analysis of Products (TAP) reactor
system. The TAP technique is a unique tool for capturing kinetic features (e.g. rates of
transformation and energetic properties) of the fundamental molecular transformations occurring
on catalytic surfaces. These experiments promote inquiry based learning where the outcome of
one experiment will determine the conditions for setting up the next. This demonstration of the
catalytic cycle in action will reinforce classroom learning of sustainable processes.
Keywords: catalysis, sustainability, TAP reactor, catalytic cycles, internet experiments,
engineering education.
*Correspondence to: Gregory Yablonsky, Saint Louis University, Parks College of Engineering,
Aviation and Technology, Department of Chemistry, 3450 Lindell Blvd, St. Louis MO 63103,
USA, Email: [email protected]
1. INTRODUCTION
1.1 Catalysis and Sustainability
As tomorrow’s scientists and engineers take on the challenges of pollution, climate change
and energy needs, the interdisciplinary nature of the environment, human health and the systems
that support our modern life will require that their choices in the development of new
technologies be made within the framework of sustainability. Taking the example of catalysis
and catalytic transformations can be used to integrate the principles of sustainability in the
curriculum learning. Catalysts play a key role in improving and maintaining air, water and soil
quality and are central to the production of fuels, food, chemicals and pharmaceuticals. The
program described here captures the central role of catalysis as an enabling science to instill the
3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
ideals of sustainability in the next generation of entrepreneurs, educators, scientists and
professionals.
2. NATURAL CATALYTIC PHENOMENA
The complexity of natural cycles such as the geophysical carbon, water and nitrogen cycles
provide an excellent starting point for describing the natural basis of sustainable processes.
Although they are global in scale, the action of these processes are evident in everyday life and
thus more relevant to the student. Observation of these phenomena outside of the classroom will
strengthen the student’s understanding of how the natural systems in place are inherently
sustainable.
The chemical processes of photosynthesis and the conversion of these
carbohydrates into energy output and material substance in the animal species are two
mechanisms relying on biocatalysts or enzymes. These cyclical processes are depicted in Fig. 1.
Figure 1. Coupled Catalytic Cycles - The natural basis of sustainable processes.
Explaining the earth’s natural process in terms of these coupled catalytic cycles can also help to
emphasize the consequences when human activity disturbs these often delicately balanced
processes. Prior to the industrial revolution man’s impact on the environment was insignificant.
With the discovery of new fuels and chemical process for making materials a modern way of life
emerged. At first little attention was paid to the efficiency or waste since fuels and raw materials
were abundant. As the scale of these processes increased along with the explosion of population
growth the consequences of irresponsible consumer and industrial practices are now clearly
apparent with repercussions throughout the environment, economy and social fabric. The
solution to making most industrial processes more efficient, less wasteful, more environmentally
benign and more economical are to employ catalysts; this philosophy is the basis of
sustainability.
3. THE CATALYTIC CYCLE
A catalyst facilitates a molecular transformation that would otherwise proceed at a very slow
rate. A chemical reaction may be thermodynamically possible but too slow to be useful. For
example, many of the transformations we pursue for the production of fuels and chemicals are
readily performed in biological systems; and at ambient conditions. In nature, enzymes easily
convert methane and CO2 to functionalized products (alcohols, sugars and carboxylic acids). For
example, methantropic bacteria containing an enzyme methane monooxygenase, will selectively
produce methanol from methane and oxygen at ambient conditions using an iron center
(Labinger 2004). In the laboratory, efficient catalytic conversion of methane to higher order
3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
oxygenates remains a major unsolved scientific challenge.
These reactions are
thermodynamically possible but presently there are no catalytic materials available for an
efficient and economic process. Perhaps the biggest impediment is the energy required to
activate methane’s very stable C-H bond (425 kJ/mol), a much stronger bond than any potential
products. Thus products are more reactive and selectivity suffers if thermal routes alone are
employed.
The comparison is not entirely fair however amongst biocatalytic (enzymes), homogeneous
(atomic, molecular, and metal complexes), and heterogeneous (simple or complex solids)
processes. Enzymatic processes offer precise control and selectivity but lower rates. For
example, the absolute rate of an industrial catalytic process is typically 100 times higher than that
of a biochemical process (Grima 2009). Classical descriptions of enzyme kinetics typically do
not include the role of transport in the intracellular environment which slows the observed rate of
the process. Homogeneous processes offer superior selectivity while heterogeneous catalytic
processes are useful in controlling heat transfer, minimize the use and need to separate solvents,
and often generate less by products.
The generic catalytic cycle is depicted in figure 2. The catalyst in state 1 (for example an
oxidized state) adsorbs molecules A and B on the surface. With the application of energy, either
thermal or radiative, reaction occurs and products C and D are formed. The goal is to find a
catalyst that maximizes the amount of the desired product and minimizes waste products but
without sacrificing the rate of reaction where the process would no longer be economical.
During the reaction step the catalyst may supply electrons and thus be rendered to a reduced
state. Regeneration of the catalyst through oxidation then completes the cycle and the catalyst
can be reused. The catalyst offers precise spatial and temporal control of chemical reactions, can
operate billions of cycles and performs the storage and release of energy.
Figure 2: A generic catalytic cycle.
Engineers aim to design processes that mimic the complete catalytic cycle as closely as
possible. In reality however the catalytic materials and reaction technologies are not ideal. With
the demonstration of catalytic technologies used on an industrial level, students can examine the
consequences of shortcomings at each step of the cycle. For chemical engineering students, the
chemistry and reactor engineering (mass balance, materials contacting, heat transfer, etc.) are
3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
part of the core curriculum learning. Students however are less often introduced to the
extraneous processes that support these operations and the consequences of deviations from
ideality. If we examine the catalytic cycle with the ideals of sustainability in mind we must
consider first where the reactants come from, how much energy is needed to extract and purify
them, what is the impact on the economy, environment and social stability of the locale
producing these raw materials? The environmental fate of the raw materials can be presented in
the form of a life cycle analysis and generally will demonstrate the interconnectedness of
engineered systems with the environment, economy and social fabric. Catalysts in particular are
often composed of rare earth and noble metals, the mining of which can be very damaging to the
environment.
Now that the scale of industrial processes has highlighted the limits of raw materials
available the engineering must consider if reactants A and B are not completely adsorbed on the
catalyst surface the reactor feed may be recycled. Even recycling requires separation prior to
reintroduction to prevent dilution of the reactant stream. At what point does feed recycling no
longer have an economic advantage? What might be the result of having unreacted reactants in
the product stream?
Many advances in efficiency can be made in the supply of energy to the reaction. The
student must consider the processes that go into producing electricity, thermal oxidation or other
alternative heat sources. Different modes of heat transfer can be evaluated for their benefit to
energy efficiency but also one must consider their effect on catalytic performance. Heat
integration from other parts of the process should be considered. Equally important is the
consideration of the cost of cooling a process; what is the fate of the waste heat? This can be
made as another example of life cycle analysis.
The catalytic material must be designed to maximize the production of selective products but
not at the expense of the reaction rate. Students can examine the tradeoff between separation of
byproducts and loss of reaction rate. What will become of the byproducts; disposal, thermal
oxidation or use in another process? What is the cost of regenerating the catalyst after reaction?
What are the advantages to using air as opposed to pure oxygen? How many cycles can the
catalyst perform? Students can examine the consequences of catalyst deactivation due to
incomplete reaction (e.g. coking), trace impurities in the feed stream (e.g. poisoning due to
sulphur) and sintering at high temperatures.
A case study of several different industrial processes will highlight all of these deviations
from the ideal catalytic cycle and help students to understand that the needs of designing
sustainable processes go much further than maximizing product yield. Examples of catalytic
processes in place that would be useful include the production of maleic anhydride from butane
over the VPO catalyst, auto-exhaust catalysis, fuel cell catalysis, and the production of nylon, Ldopa and aspartame. From these examples of catalytic processes, students can calculate
estimates of the ecological footprint for producing these materials.
Some of the technological problems scientists and engineers are now working on should be
introduced for open-ended discussion. Suggestions include:
1)
Manufacturing fuels and chemicals from alternative sources, this topic expanded in the
next section as an example.
2)
CO2 capture and large scale conversion to CO, CO conversion to fuels and chemicals
using Fischer-Tropsch chemistry,
3)
Hydrogen production in fuel cells with on-board reforming of ethanol,
3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
4)
Enzymatic conversion versus catalytic reforming of biomass for the production of
ethanol,
5)
Catalysis for mitigating radioactive pollutants, mercury, sulfur dioxide, and nitrogen
oxides in coal processing, clean coal technology for reduction of CO2 emissions.
The reader is referred to several articles that highlight the importance of catalysis for creating
sustainable solutions to today’s environmental and energy challenges (Armor 1999, Centi 2008,
Vlachos 2010). Key drivers for invention of new catalysts include the development of advanced
materials and more efficient processes with negligible emissions and a reduced environmental
footprint. New technology to replace existing outdated processes must be safer, produce
negligible emissions, require lower capital, and demonstrate a lower cost of manufacturing.
These projects are still open-ended and will be the challenges that today’s student will take on
after their studies. Introducing these problems now is useful to help students envision their role
in providing tomorrow’s solutions.
4. EXAMPLE DISCUSSION
4.1 Beyond petroleum, manufacturing fuels and chemicals from small molecules
Current world oil consumption is 80 million barrels a day, and at this rate the proven world
oil reserves will be depleted in 40 years (Davis 2003). Today's consumption rate is more than 8
times the 1950 rate, and is expected to grow to at least a 120 million barrels a day by 2025
(Davis 2003). As oil reserves decline, natural gas will begin to replace oil as the major feedstock
for fuels and industrial chemicals. During the period of decrease the demand for oil will exceed
the rate of production (Gai 2003). By 2050 it is projected that the world population will reach 9
to 10 billion (Fontes 2002), and current reserves of both oil and natural gas will be exhausted
(Davis 2003).
How to supply the vast quantities of fuels and chemicals when oil is no longer readily
available is one of the most challenging and important problems now facing humanity. Students
should consider the consequences that come with each alternative feedstock. For example, at
present, natural gas and coal are the only viable alternatives to oil, but reserves are also finite.
Like oil, burning gas or coal emits CO2 (a greenhouse gas). At current rates of production, the
CO2 concentration will more than double the preindustrial concentration by 2050 (Bradley
2004). Coal burning also produces radioactive pollutants, mercury, sulfur dioxide, and nitrogen
oxides. Biomass is the only renewable, emissions neutral source of carbon, and hydrogen
(produced from sunlight) the only renewable carbon-free energy carrier. The long-term solution
to the problems of dwindling oil and gas resources and global warming is switching to these
renewable sources. However, regardless which feedstock (natural gas, coal, or biomass) is used,
there are currently no economically viable processes for producing many of the materials now
obtained from oil. For the next-generation the development of efficient, economical, and
environmentally benign processes for producing fuels and chemicals from gas, coal, and biomass
will have a tremendous societal impact.
Current developments in catalytic technology are aimed at addressing these concerns. This
topic presents an excellent opportunity for students to carry out a research report. After
researching a specific catalytic technology addressing this issue students may present their
evaluation of the sustainability of the approach. For example, the synthesis of liquid fuels and
chemicals from natural gas or coal starts with the direct conversion of CH4, or the production of
synthesis gas (aka syngas), a mixture of CO and H2 (Holmen 2009, Vora 2009). Fischer-Tropsch
synthesis is currently used on the industrial scale to convert syngas to long-chain hydrocarbons,
aka the gas to liquids (GTL) process (Schanke 2004). With the current technology, more than
3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
60% of the capital costs of GTL plants arises from the energy intensive reforming of methane
(Holmen 2009). An enormous economic impact could be made if current efforts in reforming
catalyst technology are successful. Even though methane and coal resources are more plentiful
than petroleum, their use results in a net positive addition of carbon to the atmosphere.
Many of these issues can serve as a starting point for research and discussion amongst
students. Learning the example of a catalytic process will reinforce the ideals of sustainability.
More importantly, participation in the cutting-edge of catalysis research will motivate and
empower students with the skills to take on these challenges.
5. TAP REACTOR STUDIES OF CATALYTIC PROCESSES
Following the lecture-based and research-based case studies, the project can culminate with
students participating in cutting-edge catalysis research by performing long-distance experiments
using the Temporal Analysis of Products (TAP) reactor system, described in the next section.
One benefit of using the TAP reactor system is the execution and analysis of experiments can be
demonstrated during a lecture using a remote automation feature of the TAP automation and
control software.
5.1 TAP experiment and theory
The TAP reactor provides a unique method for characterizing heterogeneous catalytic
reactions using realistic catalytic materials (Gleaves 1988, 1997, 2010). Figure 3 depicts the
TAP reactor system. The catalysts particles are packed in a thin layer between two zones of inert
material. The microreactor is heated and a thermocouple monitors the catalyst bed temperatures.
The reactor is continuously pumped by an oil diffusion pump with a liquid nitrogen cryotrap to
maintain ultra high vacuum conditions (pressure ≈ 10-9 torr). A small pulse of molecules is
injected into the evacuated reactor and the gas molecules travel the reactor via Knudsen
diffusion. That is to say there is no plug flow and the concentration is low enough so that
molecules are not influenced by collisions with other molecules in the gas phase. The narrow
pulse of molecules injected into the reactor will naturally diffuse through the reactor packing and
interact with the catalyst particles resulting in a broadening of the pulse shape as seen at the exit
of the reactor. Thus the diffusion is well defined and any time delay of the molecules exiting the
reactor can be attributed to kinetic processes occurring on the catalyst surface. The exit flow of
the product or reactant molecules is monitored by a quadrupole mass spectrometer directly
beneath the exit of the microreactor.
5.2 Real time experiments over the internet
One feature recently developed in the TAP system is the ability to perform real-time
experiments remotely using special software and an internet connection (Fushimi 2010).
Integrating this feature into the research equipment makes state-of-the-art science available to
students and makes this program easily portable to other institutions. A TAP pulse response
experiment is very fast, usually between 1 and 10 seconds. This makes the remote
demonstration of the experiment possible during a classroom lecture. To perform an experiment
the catalyst must first be loaded manually by an operator. The lecturer can then, from a remote
site, increase the temperature and begin pulsed experiments. After quickly calculating the
conversion with the help of a ready-made analysis algorithm, the experiment conditions may be
changed, either temperature, pulse size or reactant molecules (e.g. oxygen as opposed to a
hydrocarbon). In this manner, conversion and selectivity data could be quickly complied at
several temperatures in one hour’s time. Students could use this data to calculate a first order
3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
Figure 3. Simplified schematic of the TAP reactor.
reaction rate constant, x/(1-x), where x represents conversion, followed by construction of an
Arrhenius plot to determine activation energy. This simple demonstration will create a clear
impression on students the meaning behind the rate constant and activation energy. The
chemistry and catalysts used will show how the clever engineering of materials can lead to more
efficient molecular transformations.
6. REFERENCES
Armor, J.N. (1999) “Striving for catalytically green processes in the 21st century”, Applied
Catalysis A: General, 189 153-162
Bradley, J., G. Schmid (2004) “Noble Metal Nanoparticles” Nanoparticles, Wiley.
Centi, G., S. Perathoner (2008) “Catalysis, a driver for sustainability and societal challenges”,
Catalysis Today, 138,1-2 (2008), 69-76
Davis, M., D. Tilley (2003) “NSF Workshop report on Future Directions in Catalysis: Structures
that Function on the nanoscale” Arlington, Virginia, June 19-20.
Farrell, A., R. Plevin, B. Turner, A. Jones, M. O’Hare, D. Kammen (2006) “Ethanol can
contribute to energy and environmental goals” Science 311 506-508.
Fontes, E., P. Bosander (2002) “Process Catalysis: computer modeling catalytic processes and
experimentation provide one of the most efficient and cheapest tools for R&D” Chemistry and
Industry 2 18-20.
3rd International Symposium for Engineering Education, 2010, University College Cork, Ireland
Fushimi, R., X. Zheng, P. Mills, G. Yablonsky, J. Gleaves (2010) “Internet kinetics: Remote
control of temporal analysis of products (TAP) reactor system” Industrial & Engineering
Chemistry Research, In Press.
Gai, P., E. Boyes (2003) Electron Microscopy in Heterogeneous Catalysis, Institute of Physics
Publishing, Bristol, 2003.
Gleaves, J.T., J.R. Ebner, T.C. Kuechler (1988) “Temporal Analysis of Products (TAP) – A
unique catalyst evaluation system with submillisecond time resolution” Catalysis Reviews
Science and Engineering 30 49-116.
Gleaves, J.T., G.S. Yablonskii, P. Phanawadee and Y.Schuurman. (1997) TAP-2. Interrogative
Kinetics Approach, Applied Catalysis A: General, 160(1) 55-885.
Gleaves, J.T., G. Yablonsky, X. Zheng, R. Fushimi, P.L. Mills (2010) “Temporal Analysis of
Products (TAP) - Recent Advances in Technology for Kinetic Analysis of Multicomponent
Catalysts”, Journal of Molecular Catalysis A: Chemical 315 108-134.
Grima, R. (2009) “Investigating the robustness of the classical enzyme kinetic equation in small
intracellular compartments” Genome Biology 3 101-116.
Hashaikeh, R., I. Butler, J. Kozinski (2006) “Selective promotion of catalytic reactions during
biomass gasification to hydrogen” Energy & Fuels 20 2743-2747.
Holmen, A. (2009) “Direct conversion of methane to fuels and chemicals” Catalysis Today 142
2-8.
Labinger, J. (2004) “Selective alkane oxidation: hot and cold approaches to a hot problem”
Journal of Molecular Catalysis A: Chemical 220 27-35.
Ragauskas, A., C. Williams, B. Davison, G. Britovsek, J. Cairney, C. Eckert, W. Frederick, J.
Hallet, D. Leak, C. Liotta, J. Mielenz, R. Murphy, R. Templer, T. Tschaplinski (2006) “The path
forward for biofuels and biomaterials” Science 311 484-489.
Schanke, D., R. Rytter, F. Jaer (2004) “Scale-up of Statoil’s Fischer-Tropsch process” Studies in
Surface Science and Catalysis 147 43-48.
Tanksale, A., J. Beltramini, G. Lu (2010) “A review of catalytic hydrogen production processes
from biomass” Renewable and Sustainable Energy Reviews 14 166-182.
Vlachos, D., S. Caratzoulas (2010) “The roles of catalysis and reaction nengineering in
overcoming the energy and the environment crisis” Chemical Engineering Science 65 18-29.
Vora, B., J. Chen, A. Bozzano, B. Glover, P. Barger (2009) “Various routes to methane
utilization – SAPO-34 catalysis offers the best option” Catalysis Today 141 77-83.