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OCR for page 21
CHAPTER III
Control of
Chemical Reactions
IlI-A. New Processes
A prime reason for wishing to understand and control chemical reactions is so
that we can convert abundant substances into useful substances. When this can
be done on an economically significant scale, the reaction (or sequence of
reactions) is called a chemical process.
A significant number of employed chemists are engaged in perfecting existing
chemical processes and developing new ones. Their past success is attested by
the vitality and strength of the U.S. chemical
industry. It has manufacturers' shipments total-
ling $175 billion (see Table A-2), a $12 billion
positive international balance of trade (in both
1980 and 1981, see Table A-3) and more than a
million employees. The industry makes billions of
pounds of organic chemicals at Tow cost, in high
yield, and with minimum waste products. For
example, we produce 9.S billion pounds of syn-
thetic fibers (such as polyesters), 28 billion L1J
pounds of plastics (such as polyethylene), and 4.4 <' $20B
billion pounds of synthetic rubber.
Continued competitiveness in this multifaceted
industry depends upon readiness to improve ex-
isting processes and to introduce new ones. Our
current position of world leadership can be attrib-
uted to our strength in the field of chemical
catalysis. The major role of industrial catalysis is
signalled by estimates that 20 percent of the gross
national product is generated through the use of
catalytic processes that assist in satisfying such diverse societal needs as food
production, energy conversion, defense technologies, environmental protection,
$200 B
IOOB
E IOB
LL
IIJ O
C'
J
-BOB
~CD ~
. 1
. _ ~m
J 1~;
Lid
=
1
CHEMICALS: SECOND LARGEST
POSITIVE TRADE BALANCE
21
OCR for page 22
22
CONTROL OF CHEMICAL REACTIONS
and health care. On the horizon, the extensive use of catalysts will tap new
energy sources (the subject of Section IlI-B). Our ability to remain in the
forefront of the research frontiers of chemical catalysis will figure strongly in
the health of our chemical industry and, hence, in the buoyancy of the U.S.
economic condition.
A catalyst accelerates chemical reactions without being consumed. Such
accelerations can be as much as 10 orders of magnitude. A selective catalyst can
have the same dramatic erect but on only one of many competing reactions. A
stereoselective catalyst not only controls the product composition, it also favors
a particular molecular shape, often with remarkable effects on the physical
properties (such as tensile strength, stiffness, or plasticityJ and, for biologically
active substances, on the potency. Catalysis can be subdivided according to the
physical and chemical nature of the catalytic substance.
· In heterogeneous catalysis, the catalyzed reaction occurs at the interface
between a metal, metal oxide, or other solid and either a gaseous or liquid
mixture of the reactants.
· In homogeneous catalysis, reaction occurs in either gas or liquid phase in
which both catalyst and reactants are dissolved.
· In electrocatalysis, reaction occurs at an electrode surface in contact with a
solution but assisted by a flow of current. Electrocatalysis includes the advan-
tages of catalytic rate control, including specificity, and adds the opportunity to
inject or extract electrical energy.
· In photocatalysis, reaction may take place at an interface (including
electrode surfaces) or in homogeneous solution, but in these reactions energy
encouragement is provided by absorbed light.
· In enzyme catalysis, some characteristics of both heterogeneous and homo-
geneous catalysis appear. Whether natural or artificial enzymes are considered,
large molecular structures are involved that can be seen to provide an
"interface" upon which a dissolved reactant molecule can be immobilized,
awaiting reaction (as in heterogeneous catalysis). In addition, the structure
incorporates at the site of immobilization a suitable chemical environment that
facilitates the desired reaction when a suitable reaction partner arrives (as in
homogeneous catalysis).
We discuss below the aspects of each of these catalytic situations that are
relevant to the development of new chemical processes. Then they will be
revisited in Section IlI-B because of their importance in the development of new
energy sources.
Heterogeneous Catalysis
A heterogeneous catalyst is a solid material prepared with large surface area
(1-500 m2/g) upon which a chemical reaction can occur at extremely high rate
and selectivity. Some major new commercial processes based on heterogeneous
catalyst developments in recent years are shown in Table IlI-1. The potential
OCR for page 23
III-A. NEW PROCESSES
TABLE III-1 New Processes Based on Heterogeneous Catalysis
Feedstocks Catalyst
Product
Used to Manufacture
1982 U.S.
Production
(metric tons)a
Ethylene Silver, cesium chlo-
ride salts
Bismuth molybdates
Propylene,
NH3, O2
Ethylene
Propylene
Chromium titanium
Titanium, magne-
. . .
SlUm OX1C .es
Ethylene oxide Polyesters, textiles, lubricants
Acrylonitrile
High-density
polyethylene
Polypropylene
2,300,000
Plastics, fibers, resins 925,000
Molded products
2,200,000
Plastics, fibers, films 1,600,000
a Production by all processes, including the innovative process; U.S. Tariff Commission Report.
economic significance is displayed in the last column, the total U.S. production
by all processes.
Surface science is developing rapidly and now gives us experimental access to
this two-dimensional reaction domain. Because of the unused bonding capabil-
ity of the atoms at the surface, chemistry here can
be qualitatively different from that of the same
reactants brought together in solution or in the
gas phase. However, when chemists are able to
"see" what molecular structures are on the sur-
face, our knowledge of reactions in conventional
settings becomes applicable and opens the door to
understanding and control of chemistry in this
surface domain. There are five areas of heteroge-
neous catalysis where this understanding will
have major impact on new chemical technologies.
Molecular Sieve Synthesis and Catalysis
HOW DOES CARBON MONOXIDE
BIND TO A METAL SURFACE?
Molecular sieves are natural or synthetic crys-
talline aluminosilicates containing pores or channels within which chemical
reactions can be initiated. They offer unparalleled efficiency both for cracking of
petroleum and for conversions such as methanol to gasoline. We need to know
better how to synthesize molecular sieves with controlled molecular pore size,
and how to determine the elementary reaction steps and intermediates that
account for their efficacy.
Metal Catalysis
Finely dispersed transition metals are increasingly coming into use to
catalyze hydrocarbon conversions and ammonia synthesis for fertilizers. Other
such applications and improved performance will follow from intensive research
into the control of surface structures, oxidation states, residence time of reaction
intermediates, and resistance to catalyst "poisons" (such as lead and sulfur).
23
OCR for page 24
24
HIGH
H: 1'H
HI
H
Para-xylene
CONTROL OF CHEMICAL REACTIONS
Substitutes for Precious Metal Catalysts
Many of the most effective catalysts are rare metals of limited availability in
the United States, including cobalt, manganese, nickel, rhodium, platinum,
palladium, and ruthenium. Their strategic value requires a concerted research
effort to find more accessible substitutes, such as other transition-metal oxides,
carbides, sulfides, and nitrides.
Conversion Catalysts
We must find catalysts to convert abundant substances to useful fuels and
industrial feedstocks. These reactions include conversion of atmospheric nitro-
gen to nitrates, methane to methanol, carbon dioxide to formate, and depoly-
merization of coal and biomass to useful hydrocarbons.
Catalysts to Improve the Quality of Air and Water
We have many environmental pollution problems for which we need solutions
that will match the success of the catalytic converter used in cleaning automo-
bile exhaust gases. To begin, we need catalysts that remove sulfur oxides from
smoke plumes, that purify water, and that prevent acid rain.
As we learn more about molecular structures at the solid-gas interface
(reactants, intermediates, and products), a better understanding of surface
chemical bonding will follow. We can look forward to understanding additives
that modify catalyst performance ("promoters" and "poisons". Then, the chal-
lenge of synthesis of the designed catalyst can be addressed. All this fundamen-
tal knowledge will underlie and facilitate the development of new and more
selective heterogeneous catalysts.
Homogeneous Catalysis
Homogeneous catalysts are soluble and active in a liquid reaction medium.
Often they are complex, metal-containing molecules whose structures can be
modified to tune reactivity in desired directions to achieve high selectivities. In
this respect, homogeneous catalysts can be supe-
rior to heterogeneous ones. The largest industri-
al-scale process using homogeneous catalysis is
the partial oxidation of para-xylene to tereph-
thalic acid with U.S. production of 6.2 billion
pounds in 1981. The process uses salts of cobalt
and manganese dissolved in acetic acid at 215°C
as the catalyst system. Most of the product ends
up copolymerized with ethylene glyco] to give us
polyester clothing, tire cord, soda bottles, and a
host of other useful articles. The strength of the
chemical industry in the United States has been
repeatedly enhanced by the introduction of new processes based upon homoge-
l
catal yet
Co, Mn salts AH
O~C`o
Terephthalic Acid
U.S. PRODUCTION (19SI), $2.3 BILLION!
OCR for page 25
III-A. NEW PROCESSES
TABLE III-2 New Processes Based on Homogeneous Catalysis
25
Start- 1982 U.S.
up Production
Feedstocks Catalysta Product Used to Manufacture Date (metric tons)
Propylene, oxi- MoV' complexes Propylene oxide Polyurethanes (foams) 1969 303,000
dizer Polyesters (plastics)
Methanol, CO [Rh(CO)2I2] Acetic acid Vinyl acetate (coatings) 1970 495,000
Polyvinyl alcohol
Butadiene, HCN Ni(L~)4 Adiponitrile Nylon (fibers, plastics) 1971 220,000
a-Olefins PhH(CO)(L2)3 Aldehydes Plasticizers 1976 300,000-350,000
Lubricants
Ethylene Ni(L3)2 cr-Olefins Detergents 1977 150,000-200,000
CO, H2 (from [Rh(CO)2I2] Acetic anhy- Cellulose acetate 1983 [225,000, Capacity]
coal) dride (films)
a L = Ligand; Lo = Triaryl Phosphite, L2 = PPh3, L3 = 0OCCH2PPh2, Ph = C6H5
neons catalysts. Table IlI-2 lists six such processes, whose 1982 production
figures were valued at over $1 billion.
An important branch of homogeneous catalysis has developed from research
in organometallic chemistry. For example, in the second reaction in Table IlI-2,
rhodium dicarbony] diiodide catalyzes the commercial production of acetic acid
from methanol and carbon monoxide. With this catalyst present, the reaction
economically gives more than 99 percent preference for acetic acid over other
products. Almost a billion pounds of acetic acid is so produced, a large part of
which is used to manufacture such polymeric materials as vinyl acetate
coatings and polyvinyl alcohol polymers.
There are three areas of homogeneous catalysis where increased understand-
ing has potential for major impact on new chemical technologies.
Activat~on of Inert MoZecules
Several relatively inert molecules are enticing as reaction feedstocks because
of their abundance, including nitrogen, carbon monoxide, carbon dioxide, and
methane. One way to facilitate their use might be through homogeneous
organometallic catalysis, and quite promising examples are beginning to
appear. For examnIe. soluble comcounds of tun~sten and molybdenum with
~ , ,
molecular nitrogen have been prepared and induced to produce ammonia under
mild conditions. The carbon-hydrogen bonds in normally unreactive hydrocar-
bons have been split by organorhodium, organorhenium, and organoiridium
complexes. Hope for build-up of complex molecules from one-carbon molecules,
such as carbon monoxide and carbon dioxide, is stimulated by recent demon-
strations of carbon-carbon bond formation at metal centers bound in soluble
metal-organic molecules. Synthesis of compounds with multiple bonds between
carbon and metal atoms has had major impact in cIarifying the catalytic
interconversion (metathesis) of olefins. While there is much to learn, the stakes
are high and the odds favor success.
OCR for page 26
26
Cys-S
CONTROL OF CHEMICAL REACTIONS
Metal Cluster Chemistry
An adventurous frontier of catalysis involves the expanding capability of
chemists to synthesize molecules built around several metal atoms in proxim-
ity. Many of these "metal cluster" compounds consist of several metal atoms
bound to each other in the "core" of the molecule with carbon monoxide
molecules chemically attached on the periphery. These metal carbonyIs have
formulas MX(CO)Y' and x can be made very large. The worId's record as of this
writing is a platinum compound with x = 3S, Pt3~(CO)24.
At the same time, very low temperature techniques are revealing the
structures and chemistry of small aggregates containing only metal ions or
atoms ("naked clusters". In still another direction, cubical units of four metal
atoms and four sulfur atoms are now known for
iron, nickel, tungsten, zinc, cobalt, manganese,
and chromium. This "cubane" structure, which
involves three metal-sulfur bonds to each metal
atom, has its own characteristic chemistry. This
is demonstrated by the iron example, which
proves to be a functional unit in the ferrodoxin
iron proteins that catalyze electron transfer reac-
tions in biological systems.
These cluster compounds, bound or "naked,"
furnish a natural bridge between homogeneous
catalysis and bulk metal, heterogeneous cataly-
sis. What makes it intriguing is that many of the metals that are most active as
heterogeneous catalysts also form cluster compounds (e.g., rhodium, platinum,
osmium, ruthenium, and iridium). Now the chemistry of these elements can be
studied as a function of cluster size. There is much to be gained from better
understanding because all the elements mentioned above are derived from
imported strategic mineral ores.
ll
--- Fe—s-cvs
Cys-S'
S-Cys
THE BIOLOGICAL ENZYME FERRODOXIN: AN
I RON-SULFUR "CUBANE " STRUCTURE
Stereoselective Catalysts
Another frontier full of promise involves the development of homogeneous
stereoselective catalysts. Many biological molecules can have either of two
geometric structures connected by mirror-image (chiral) relationships, and
generally only one of these structures is functionally useful in the biological
system. If a complex molecule has seven such chiral carbon atoms and a
synthetic process produces all the mirror-image structures in equal amounts,
there would be 27 = 128 structures, 127 of which might have no activity or,
worse, might have some undesired effect. Thus, the ability to synthesize
preferentially at every chiral center the desired structure with the desired
geometry is essential.
OCR for page 27
OCR for page 28
28
CONTROL OF CHEMICAL REACTIONS
PhotocataZysis
An electrochemical cell can be built with one or both electrodes made of
semiconductor materials that absorb incident light. In such a cell, the light
absorbed by the electrode can be used to promote catalytic oxidation-reduction
chemistry at the electrode-solution interface. The same sort of chemistry can be
induced in solutions containing suspensions of semiconductor materials but now
at the particle-solution interface. Such oxidation-reduction chemistry has
significant scientific interest and, without doubt, practical importance as well.
For example, photodestruction of toxic waste material, such as cyanide, has
been demonstrated at titanium dioxide surfaces. A more popularized and
conceivably feasible concept is that such photocatalytic chemistry, driven by
solar energy, could give a process for producing massive amounts of hydrogen
and oxygen from water. It is an intriguing prospect: to convert from diminish-
ing, polluting petroleum fuels to a renewable fuel hydrogen that burns to
water and that is made from water using energy from the Sun.
Electrocatalysis
Apart from light-initiated processes, electrode surfaces with catalytic activity
offer a new domain for chemical synthesis. In a field with a long heritage, recent
developments have shown that electrode surfaces can be chemically tailored to
promote particular reactions. For example, electrocatalytic control of electron
flow opens new synthetic pathways that require one-electron transfer in
preference to two. Furthermore, this research area has benefitted from tech-
niques used by the semiconductor industry, such as chemical-vapor deposition,
by coupling them with imaginative synthetic chemical techniques for surface
modification.
An example is the electrocatalyst family developed for use in chlorine
generation in chIor-alkali cells. A successful case is based upon a thin layer of
ruthenium dioxide—the catalys~eposited on a base-metal electrode. This
electrocatalyst has dramatically improved energy efficiency and reduced cell
maintenance in the chIor-alkali industry, an industry representing billions of
dollars in sales. The cumulative savings are enormous because this crucial
industry consumes up to 3 percent of all electric power produced in the United
States.
Chemistry at the Solid/Liquid Interface
Before the technological potentialities of any of the above can be realized, we
must have a much better understanding of the nature of chemistry at the
semiconductor/liquid interface. Most of the extraordinary instrumentation so
far developed for surface science studies is applicable only at solid/vacuum
interfaces. We need comparable capability at the solid/liquid boundary, and we
will gain it from fundamental research in solid state chemistry, electro-
chemistry, surface analysis, and surface spectroscopy. There is already reason
for optimism. The surprising discovery of the million-fold intensification in-
OCR for page 29
III-A. NEW PROCESSES
volved in the surface-enhanced Raman effect provides a technique with appli-
cability yet to be determined. Perhaps even more promising is the demonstra-
tion of surface-enhanced second harmonic generation. This method, which
depends upon the intrinsic asymmetry at a phase boundary, has become
possible because of the high intensity of laser light sources. It may have general
applicability and, because pulsed lasers are used, temporal (kinetic) measure-
ments can be expected.
The potential gains from these areas are considerable. We need to know how
to catalyze multielectron transfer reactions at an electrode surface. That is the
chemistry required, for example, to photogenerate a liquid fuel like methanol
from carbon dioxide and water. Multi-electron transfer catalytic electrodes for
oxygen reduction in electrochemical cells would find a welcoming home in the
fuel cell industry.
It is also likely that research on semiconductor electrode surface modification
will reflect beneficially on the field of electronics. Thus, the integrated circuit
technology based upon gallium-arsenide may depend upon control of its surface
chemistry. Already scientists concerned with photoresist/chemical etching tech-
niques are recognizing the importance of the chemistry involved in surface
modification, as shown by the active pursuit of "anisotropic chemical etching."
In summary, our evolving understanding of the electrode/solution interface,
buttressed by concepts based on semiconductor electrodes and the development
of new methods for modifying electrode surfaces, has opened novel approaches
to both photocatalysis and electrocatalysis. Future advances will benefit syner-
gistically from progress in heterogeneous and homogeneous catalysis, increased
understanding of mass and charge transport within the electrode surface layers,
and continued development of experimental methods and theoretical models for
the interface.
Artificial-Enzyme Catalysis
The most striking benefit of our expanding knowledge of reaction pathways
and the analytical capacity of modern instrumentation has horn the develon-
ment of our ability to deal with molecular systems ot extreme complexity. With
the synthetic chemist's prowess and such diagnostic instruments as nuclear
magnetic resonance, X-ray spectroscopy, and mass spectroscopy, we can now
synthesize and control the structure of molecules that approach biological
complexity. This control includes the ability to fix the molecular shape, even
extending to the mirror-image properties so crucial to biological function.
There is no application of these capabilities more intriguing than that of
coupling them with our growing knowledge of catalysis in an attempt to
synthesize artificial enzymes. In Nature, enzymes are the biological catalysts
that accelerate a wide variety of chemical reactions at the modest temperatures
at which living organisms can survive. An appropriate enzyme selects from a
system with many components a single reactant molecule and transforms it to
a single product with prescribed chiral geometry.
Without catalysts, many simple reactions are extremely slow under ambient
29
OCR for page 30
30
CONTROL OF CHEMICAL REACTIONS
conditions. Raising the temperature speeds things up, but at risk of a variety of
possible undesired outcomes, such as acceleration of unwanted reactions,
destruction of delicate products, and waste of energy. Hence, there are compel-
ling reasons to develop synthetic catalysts that work like enzymes. First,
natural enzymes do not exist for most of the chemical reactions in which we
have interest. In the manufacture of polymers, synthetic fibers, medicinal
compounds, and many industrial chemicals, very few of the reactions used could
be catalyzed by naturally occurring enzymes; even where there are natural
enzymes, their properties are not ideal for chemical manufacture. Enzymes are
proteins, sensitive substances that are easily denatured and destroyed. In
industries that do use enzymes, major effort is devoted to modifying them to
make them more stable.
Controlled Molecular Topography ancl Designec! Catalysts
We have a pretty good idea of how enzymes work. Nature contrives a
molecular surface suited to a specific reactant. This surface attracts from a
mixture the unique molecular type desired and immobilizes it in a distinct
shape that facilitates reaction. When the reaction partner arrives, the scene is
set for the desired reaction to take place in the desired geometry.
Organic chemists who have taken up this challenge are making notable
progress. Without special control, large molecules usually have exclusively
convex surfaces (ball-like shapes). So a first step has been to learn to synthesize
large molecules with concave surfaces, after
which the concave surfaces could be shaped to
accommodate a desired reactant. Cyclodextrins,
which are toroidal in shape, provide examples.
The crown ethers, developed over the last 15
years, have a quite different surface topography.
For instance, 18-crown-6 consists oftweIve carbon
atoms and six oxygen atoms evenly spaced in a
cyclic arrangement. In the presence of potassium
ions, the ether assumes a crown-like structure in
Top View which the six oxygen atoms point toward and bind
a potassium ion. Lithium and sodium ions are too
small, and rubidium ions too large, to fit in the
crown-shaped cavity; so this ether preferentially
extracts potassium ions from a mixture. More
ornate examples now exist. Chiral binaphthy]
units can be coupled into cylindrical or egg-shaped
Side View cavities. With benzene rings, enforced cavities
have been made with the shapes of bowls, pots,
saucers, and vases.
We are clearly moving toward the next step,
which is to build into these shaped cavities a catalytic binding site, such as a
transition metal complex that is already known to have catalytic activity in
Br
Br p:Br
Br
air
CAVITANDS
WHAT SHAPE DO YOU NEED?
OCR for page 31
III-A. NEW PROCESSES
solution. The earliest successes are likely to be patterned after natural enzymes,
but there is no doubt that, in time, artificial, enzyme-like catalysts will not be
limited to what we find already known in nature.
Biomimetic Enzymes
A short-cut approach to improved catalysis is to pattern artificial enzymes
closely after natural enzymes sometimes called "biomimetic chemistry." For
example, mimics have been prepared for the enzymes that biologically synthe-
size amino acids. Artificial enzymes that incorporate the important catalytic
groups of a natural enzyme, such as vitamin B-6, can show good selectivity and
even the correct chirality in the product. Mimics have been prepared for several
of the common enzymes involved in the digestion of proteins, and substances
that catalyze the cleavage of RNA have been synthesized based upon the
catalytic groups found in the enzyme ribonuclease. Mimics have also been
synthesized that imitate the class of enzymes called cytochromes P-450, which
are involved in many biological oxidations, and the oxygen carrier hemoglobin.
In addition, mimics have been prepared for biological membranes and for those
molecules that carry substances through membranes. These have potential
applications in the construction of organized sys-
tems to perform selective absorption and detec-
_ Ho CH=CH2
tion, as in living cells. It is important for the HC^CH
United States to build on its early lead in this H3C ~ I JU'CH3
field. Although most of the work mentioned above O '~N--Ff+-N~
has been done in the United States, the Japanese Ho>CCH2CH2 Hi - N. - l H CH=CH2
have also become quite active and have specifi- O>CCH2C/ - CH3 c'-
cally targeted b~om~met~c chemistry as an area HO
of special opportunity. Research on synthetic or- HEMIN: THE ACTIVE PART OF
HEMOGLOBIN
ganic chemistry develops novel methods to con-
struct the required molecule and elaborates new kinds of structures. The
study of detailed reaction mechanisms in organic and biological chemistry
permits a rational approach to catalyst design. It is an area ripe for develop-
ment, and it deserves encouragement as a part of this program in chemical
catalysis.
Conclusion
A significant share of our economy is built upon the chemical industry. The
long-range health of this critical industry will depend upon our ability to
innovate, developing new processes that increase energy and cost efficiency, and
creating new products for new markets, all the while enhancing our protection
of the environment. Today's basic research in all facets of catalysis will provide
the source of such innovation. It will also produce the cadre of young scientists
working at the frontiers of knowledge with the state-of-the art instrumental
skills needed to recognize and implement the potentialities. Research in
catalysis, viewed in the broad sense presented here, is one of the research fields
of chemistry that deserves high priority.
31
OCR for page 95
III-E. INSTRUMENTATION
TABLE III-7 Research Areas Utilizing the Laser
Relative
Number Laser Used
Area of Users Application (see Table III-6)
95
A. Subpicosecond kinetics Very few Vibrational relaxation in Passively mode-locked dye
condensed phase laser 1, 6
Fast semiconductor de- Amplifier 3, 7
cay
B. Several picosecond ki- Few Fast electronic state re- High ~ Mode-locked
netics taxation rep. ~ pump—1 or 2
Decay of coherent pro- rate J Synchronously pumped
cesses in the condensed dye laser 6, 14
phase High l Mode-locked
power J Solid state 3, 14
C. Nanosecond Many
D. Microsecond kinetics Many
E. Photochemistry
F. Isotope separation
G. Materials science
H. Microprobe analysis
I. Raman spectroscopy
J. Atomic absorption/
Many
Few
Many
Few
Few
Lifetimes of excited
states
Characterization of fast
reactions
Gas phase decay
processes; non-
thermal chemical reac-
tions
UV or visible photolysis
Pure element isolation
Controlled melting and
crystallization
Source for mass spec-
trometer or atomic
emission
Very many Routine sample charac-
terization and analysis
Trace element analysis
Visible 3 or 4, 7
IR 8
Visible—11
IR- 8 or 9
4 or 11
4 or 12, 7
1, 3, or 8
3
1, 6
1 optionally 6
fluorescence
K. Combustion diagnostics Few Probing reaction cham- 3, 7
hers
L. Atmospheric gas Sam- Many Monitoring industrial 10
pling processes
M. Ultrahigh resolution Few Linewidth measurement; Visible 1, 6, 13
spectroscopy excitation of single IR—10
quantum states in the
gas phase
N. Tunable cw spectroscopy Very many Condensed phase absorp- 1, 6
tion and fluorescence
excitation
0. High power experiments Many Saturation of transitions Visible 3 or 4, 7
IR 8 or 9
P. Generation of extreme Few Frequency conversion Visible 3 or 4, 7, 8
frequencies stimulated Raman IR range .03 to 300 m
scattering or frequency
sum/difference
Q. Cells sorting Many Discrimination based on 1
fluorescent tags
R. Cellular bleaching Few Photochemistry within 1, 6
microscopic structures
S. Nonlinear spectroscopy Few Saturation spectroscopy; 3 or 4, 7
CARS and related
methods
OCR for page 96
96
TABLE III-8 Relative Computing Speeds of
Computer Levels
Computing
Superminicomputers
Mainframes
Supercomputers
Example
DEC VAX 11/780
IBM 3033
CRAY IS
(1)
10-15
80-120
CONTROL OF CHEMICAL REACTIONS
This can be contrasted with a
recent study of decamethyl
Relative ferrocene, which involved 501
Speed basis functions. Because such
studies require computing ef-
fort proportional to the fourth
power of the number of basis
functions, the decamethyl fer-
rocene computation involves
(501/1614 or 1 million times more computation than the prototype ethane
problem!
Superminicomputers
This level of computer has become a workhorse in chemistry. Instruments
like the DEC VAX 11/780 are comparable to the largest mainframe computers
available in the late sixties. They have revolutionized computing in chemistry
because of their substantial capacity, high speed, and lower cost, which is now
in the range of $300K to $600K.
The last 20 years have also seen three important development phases in the
use of computers in chemical experiments. In the first, the computerization
phase, advances in both hardware and software greatly enhanced data acqui-
sition. Then an automation phase increased the possibilities for experiment
control through real-time monitoring of critical parameters. Finally, a "knowI-
edge engineering" phase ushered in an era in which computers perform
high-level tasks in interpreting collected information
An excellent example is the Fast Fourier Transform algorithm, which
permits us to record spectral data in the time domain and then to transform the
results to the frequency domain. Because this allows detection of quite weak
signals, the algorithm is now routinely used to record i3C NMR signals and to
transform infrared interferograms. This is accomplished by building into the
instrument a dedicated computer of adequate capability and speed. Because of
the success of these instruments, the Fourier transform algorithm is now being
; ~ ~~ ~~ ~ ~ ~ 1 : ~ ~ ~ ~ 1 ~ ~1~ ~ ~1~ ~ _ 1 ~ _ · ~ 1
.
lil~Vlp~l=~U lilLU ~l~-~il~lnl~l, microwave, ion cyclotron resonance, dielec-
tric, and solid state NMR equipment.
Now, microprocessors have come down sufficiently in cost to motivate chem-
ists and instrument designers to integrate them into their scientific instru-
ments. Such microprocessor-based equipment can monitor data from several
sensors simultaneously to provide a multidimensional perspective that is
difficult to obtain otherwise. Research examples include excitation-emission
fluorescence and mass spectrometry, and practical applications include Com-
puterized Axial Tomography (CAT) and NMR imaging as diagnostic tools in
. .
mec 1c1ne.
Despite their capability, superminicomputers are barely practical for the
OCR for page 97
Ill-E. lNSTRUMENTATlON
larger chemical computations because of Tong turn-around times (e.g., many
days of continuous computing). However, addition of an attached processor
greatly enhances computational speed, while retaining the convenience of the
local superminicomputer but with a price/performance ratio better than those of
many large mainframe computers by several-fold.
Mainframe and Supercomputers
Even so, the potential for some computation in chemistry can be met only
with the greater capacity and capability of the largest scientific computers
(Cray/M and X-MP or CYBER 205) coupled with specialized resources, such as
software libraries and graphics systems. This is true most notably for ab initio
electronic structure studies.
The entire development of ab initio electronic structure studies has been
closely linked with parallel developments of computer hardware and software to
facilitate manipulation of massive amounts of high-precision numbers. In some
cases, computational quantum chemists have interacted closely with hardware
manufacturers in the area of design and performance standards. For example,
the Hitachi vector processor was designed in consultation with quantum
chemists at the Institute for Molecular Science in Okazaki, Japan. In its April
1984 report, the Task Force on Large Scale Computing of the Committee on
Science (ComSci) of the American Chemical Society recommended that ". . . the
ACS take initiative to establish interaction between the scientists who are users of
large-scale computation and the designers of new supercomputers."
Another area that would be stimulated by increased access to supercomputers
is computational biochemistry. Most dynamical simulation procedures applica-
ble to biological molecules require energy solutions for the simultaneous
motions of many atoms. A conventional 100-picosecond molecular dynamics
simulation of a small protein in water would require about 100 hours on a DEC
VAX 11/780 or 10 hours on an IBM 3033. Calculations of the rate constant for
a simple activated process require a sequence of dynamical simulations to
determine the free energy barrier and additional simulations to determine the
nonequilibrium contributions; times required can now reach 1000 hours on a
DEC VAX 11/780. More complicated processes or longer simulations become
impossible without extensive access to supercomputers.
Costs
It is plain that computers have already exerted a strong and beneficial impact
on chemistry and that this will continue. At present, the progress that can be
made using computers in chemistry is limited exclusively by lack of resources.
Existing computers at all levels could increase enormously the research produc-
tivity of chemists if they had access to the computing equipment of the ap-
propriate capability (whether dedicated, locally shared, or networked) and to
the support infrastructure needed for cost-effective use of that capability. For
chemistry, the problem is less the need for a next-generation development than
97
OCR for page 98
98 CONTROL OF CHEMICAL REACTIONS
35M it is for ability to use, when-
LARGE COMPUTERS ever applicable, the computa-
~ tional capabilities that al-
/ ready exist.
S4M
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1 950 1 960 1 970 1 980 1 990
YEAR
Molecular Beams
When the NRC report
Chemistry: Opportunities and
Needs was written in 1965,
the powerful crossed molecu-
lar beam studies on reaction
dynamics relied entirely on
the surface ionization method
for the detection of scattered
products, and systems inves-
tigated were limited to those
containing alkali metals. The
situation has changed drasti-
cally over the past 17 years.
AS COMPUTING POWER GROWS, COSTS ESCALATE The development of univer-
sally applicable crossed mo-
lecular beam systems and high-intensity beam sources has made this method a
powerful experimental too] for the investigation of elementary chemical reac-
tions, energy transfer processes, and intermolecular potentials.
Capabilities
A typical, crossed molecular beam apparatus can contain as many as eight
differentially pumped regions provided by various high-speed and ultrahigh
vacuum pumping equipment. It may be necessary to maintain a pressure
differential of 14 orders of magnitude, from 1 atm of pressure behind the nozzle
of the molecular beam source to 10- torr at the innermost ionization chamber
of the detector. What is glibly called the detector is likely to be an extremely
sensitive, ultrahigh-vacuum electron-bombardment mass spectrometer detector
with which to measure the velocity and angular distributions of products. With
optimum design, an angular resolution better than 1° and velocity resolution
better than 3 percent can be achieved for scattering processes that provide a
steady state concentration of scattered molecules of less than 100 molecuTes/sec
(~lo-~5 torr) in the ionization region of the detector chamber. With such
sensitivity and versatility, many new experiements become possible. For
example, by replacing one of the beams by a high power laser, molecular beam
systems are now giving new kinds of information on the dynamics and
mechanism of primary photochemical processes.
In the past 5 years, molecular beam experiments have played a crucial role in
OCR for page 99
III-E. INSTRUMENTATION
advancing our fundamental understandings of elementary chemical reactions
at the microscopic level. The advances provide deeper insights with which to
build our explanations of macroscopic chemical phenomena from the informa-
tion gathered in microscopic experiments. In view of this success, more than 10
new advanced crossed molecular beam systems have been constructed abroad in
the last ~ years, in Germany, France, England, ~Japan, Italy, Australia, and
other countries. China and Taiwan are contemplating building comparable
molecular beam laboratories in the coming years. But the high cost and the
complexity of setting up and operating a crossed molecular beam apparatus
have limited the general availability of this powerful too] in chemistry commu-
. .
nines.
Costs
A state-of-the-art crossed molecular beam apparatus equipped with high-
intensity beam sources and data acquisition electronics will cost $350K to
construct and $40K/year to maintain. If a tunable laser is also used for the
excitation of reagent atoms or molecules to specific quantum states, an addi-
tional $100K will be needed. In addition, auxiliary general supporting equip-
ment costing in the neighborhood of $100K will generally be needed in a
molecular beam laboratory.
Thus about half-a-million dollars in equipment funds is needed to establish a
new, state-of-the-art, molecular beam laboratory. Such an amount has become
almost beyond reach in the United States, particularly for scientists in aca-
demic institutions. Only two new crossed molecular beam systems equipped to
measure angular and velocity distributions of products have been constructed in
the United States during the past 5 years. If the current trend were to continue
for another decade, we would certainly fall significantly behind other countries
in this area of research.
As the experimental sophistication of crossed molecular beams methods
continues to increase, a broader range of investigations come within reach.
:Laser technology is playing a more important role. Thus we can expect crossed
molecular beam techniques to have great impact on chemistry provided re-
sources are made available to exploit the potentialities.
Synchrotron Light Sources
Existing Characteristics of Synchrotron Sources
The most intense, currently available source of tunable radiation in the
extreme ultraviolet and X-ray region is synchrotron radiation, which is pro-
duced when energetic electrons are deflected in a magnetic field. Current
capabilities and future needs for synchrotron radiation were recently reviewed
by the NAS/NRC Major Materials Facilities Committee. As described in detail
in their 1984 report, the principal current use of tunable synchrotron radiation
99
OCR for page 100
100
CONTROL OF CHEMICAL REACTIONS
TABLE III-9 U.S. Synchrotron Light Sources
Beam Critical
Energy Energy Spectral
Facility Name (Location) Yeara (GeV) (keV) Brilliancef
Dedicated
Tantalus (U. Wisconsin) 1968 0.24 0.05 4. 10~°
SurfII (NBS) 1974 0.28 0.06 5 10~i
NSLS (Brookhaven, N.Y.) 1981 0.75 0.5 3 10~3
NSLS (Brookhaven, N.Y.) b 2.5 5.0 1 ~ 1014
Aladdin (U. Wisconsin) b,C 0.75 0.45 (2 10~3)
"Hard X-ray" ring ~ 6 10 (1 ~ 1Oi8)e
"Soft X-ray" ring ~ 1-2 0.010 (3 1Oi8)e
Parasitic
SPEAR (Stanford U., Calif.) 1974 3-4 5 6 10~2
CESR (Cornell U., N.Y.) 1980 5.5 11.5 5 10~2
Year first experiments were conducted.
b Not yet operational.
c Construction interrupted.
Proposed, NRC Report "Major Facilities for Materials Research and Related Disciplines," D. E. East-
man and F. Seitz (1984).
e Proposed brilliance using undulator insertion devices.
f Spectral brilliance is the number of photons per square millimeter per square radian per unit band-
width.
falls in the photon energy range of 1 to 100 keV as provided at several, dedicated
facilities and as a parasitic activity at a few high energy particle-physics
accelerator facilities here and abroad. Table IlI-9 shows that in the United
States, sychrotron radiation is currently in research use at three dedicated
synchrotron laboratories and parasitically at two particle-physics accelerator
laboratories. By comparison, there are in foreign countries seven dedicated
synchrotrons operating (in France, Germany, Great Britain, Japan (three), and
the Soviet Union) and five under design or construction.
Over the past decade, there have been important advances as attention has
turned from synchrotrons as accelerators (with radiation seen as an undesired
energy dissipation) toward synchrotrons as sources of light. Insertion devices
were designed to place sharp bends in the electron trajectories to increase the
radiative properties (bends, "wigglers," or "undulators"~. These devices show
potential for intensity increases by several powers of 10.
Such a quantum jump in brightness will surely lead to new types of
experiments. For many chemical applications, the intensity will be of particular
importance if the design specifications attempt to optimize radiation in the
vacuum ultraviolet spectral region rather than regard it as a parasitic use. It is
important to note that the pulsed nature of synchrotron radiation (tens of
picosecond pulse durations) and its high repetition rate (108-109 pulses per
second) can be of particular value in chemical kinetic investigations if the
wavelength range is appropriate.
OCR for page 101
III-E. INSTRUMENTATION
Applications of Synchrotron Sources in Chemistry
Scientific use of synchrotron radiation in the United States has been building.
There were some 210 individual users in 1976, and the number had grown to
about 600 by 1982. Statistics are not available on the fraction of these users who
would identify themselves as chemists, but the number is not negligible. A
recent estimate at the NSES facility indicated that less than 20 percent of the
research was placed in chemistry. The range of problems under study at our
synchrotron light sources is illustrated in the examples below.
Extended X-ray Absorption Fine Structure (EXAFS) has been one of the more
fruitful applications of synchrotron radiation to solid substances. When one of
an atom's inner-shell electrons is excited above its X-ray edge energy, the atom
emits light that is diffracted by neighboring atoms. The result is a diffraction
pattern that contains information about the interatomic spacings of these
neighbors. Much attention has been directed toward crystal structures of
inorganic solids, some of it seeking information on oxidation state when other
methods are not definitive. Because heavy atoms are most readily detected,
EXAFS has been usefully employed to learn the immediate chemical environ-
ment of transition metal atoms as they occur in biologically important mole-
cules, including manganese in chlorophyll. The method is also applicable to
dilute species (as Tow as one part in 104-105) using fluorescence intensity as a
function of incident photon energy. Applicability to surface corrosion studies
can be achieved through measurement of photon-induced Resorption. Time-
resolved EXAFS is also feasible through the use of dispersive techniques in
which the entire absorption spectrum is measured simultaneously.
As synchrotron intensities are increased, photoelectron emission intensity
can be measured instead of absorption coefficient. A promising development in
this area is called Angle Resolved Photoemission Extended Fine Structure
(ARPEFS). The diffraction patterns so obtained are sensitive to bond distances
and bond angles of atoms located beyond the closest neighbors. Still another
possible application of brighter synchrotron X-ray sources would be time-
resolved X-ray scattering. Time dependence of puIsed-laser surface damage
(annealing) could be measured in real time. Time-resolved, small angle scat-
tering experiments on muscles have already shown changes in repeat distances
as muscles undergo contraction and relaxation.
Synchrotron Radiation Costs
The costs of synchrotron radiation are highly varied because such facilities
range from relatively small, in-house facilities to large, user facilities. However,
the recently proposed hard X-ray (optimum, 1.2 A wavelength) 6-GeV synchro-
tron light source (which would find some limited use in chemical applications)
has been priced at about $160M for construction, including its proposed 10
insertion devices and associated beam lines. A European counterpart proposal
101
OCR for page 102
102
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CONTROL OF CHEMICAL REACTIONS
considering a 5-GeV storage ring has an estimated construction cost of $200M.
Softer X-ray synchrotron light sources, for which there may be more general
chemical applicability, might cost in the range $60M to $80M.
At the storage rings at the University of Wisconsin ancI at the Brookhaven
NSL:both clesignec! specifically as radiation sources once the synchrotron is
in place, the acTditional cost for a new, cleclicatec! beam line falls in the range
$.8M to $1.5M. Of course, once built, such an installation requires anc! warrants
a substantial, continued annual investment for operation anc! maintenance. For
example, the 1985 operating budget at NSL`S is about $15M.
~ I, — ~ ~ . ~
Free Electron Lasers
When a beam of electrons with velocities near the speec! of light moves
through a static, periodically alternating magnetic fielcI (a "wiggler"), light is
emitter! in the direction of electron beam propagation. The wavelength of the
light is cleterminecI by the period of the wiggler field ancl the energy of the
electrons. This provides a "gain medium" that, if placed between the mirrors of
a conventional laser, can emit coherent laser light. Such a crevice is called a free
electron laser (FEL).
Potential Capabilities
Experience to date indicates that high-e~ciency, wavelength tunability anc3
high average ancl peak power will all be forthcoming over a wavelength range
extending from microwave fre-
....................... . . ~ .
quenches turoug n t. He ~ntrarec .
BW = BAND WIDTH ancI visible to the vacuum ul-
traviolet spectral ranges. If the
hopes and expectations of the
most enthusiastic FEL propo-
nents are realizecI, average
brightnesses several powers of
10 greater than those proviclecI
by conventional tunable lasers
or synchrotron sources may be
possible, particularly in the
ultraviolet. Furthermore,
short wavelength perform-
ance may be extenclec! beyond
the limit of present mirror
reflectivity, e.g., to the 10- to
30-eV range, either with
multilayer mirrors now under
development, or with "single
pass" high-gain FEL's that do
not use mirrors.
////
LASERS
,,,,,,,,,,,,,,,,
_ ~(,.,,~,,,,4,X,,B,,W,?9
1-2 GeV
SYNCHROTRON (.1XBW)
x~'~\=
BENDS
. 1 , . 1 , . 1
1 0 1 00 1 000 1 0,000
PHOTON ENERGY (eV)
DESIGN GOALS ARE AMBITIOUS - AND PROMISING
OCR for page 103
III-E. INSTRUMENTATION
Possible Costs
The optimum design (and cost) may vary greatly, depending upon the
wavelength to be produced. Theoretical estimates of energy efficiency range up
to 20 percent for linear accelerator-driven FEL'S, a performance most readily
achieved in the microwave and infrared spectral regions. Such devices, which
might cost from $10K to $30M, have proposed performance characteristics of 30-
picosecond pulse duration and peak powers comparable to those of laboratory
CO2 lasers (tens of megawatts peak power). An FEI~ with approximately these
characteristics has already been operated at Los Alamos National Laboratory.
It is based on a linear accelerator 2 or 3 meters long to produce electron energies
of 10 to 25 MeV. The device provides a train of pulses of tunable infrared
radiation currently in the 9- to 11-micron range with 30-picosecond pulses,
peak power of 5 megawatts, 50-nanosecond spacing between pulses, and a pulse
train duration of 80 microseconds once a second. Such performance extended
over the mid-infrared spectral region (4 to 50 microns) would open the way to
many novel applications in chemistry. Examples are vibrational relaxation,
multiphoton excitation, nonlinear processes in the infrared region, fast chemi-
cal kinetics, infrared study of adsorbed molecules, and light-catalyzed chemical
reactions. As the wavelength is moved through the visible and toward the
ultraviolet, not only the electron energy, but also the current density of the
electron beam must be raised. Some researchers (but not all) fee] that the
current density needs are best met by the larger (and more costly) storage rings
like those proposed for use as synchrotron light sources. In fact, free electron
laser emission has been demonstrated at visible wavelengths making use of
existing storage rings not initially designed for FEL use. It seems likely that a
synchrotron-type storage ring with electron energies in the range of .5-1.5 GeV
could provide an effective electron beam gain medium for FEL use. At this
energy range, the synchrotron could be suitable both as a soft X-ray source and
as a tunable FEL source. If designed with the FEL use as a primary application,
extremely high brightness might be achieved in the ultraviolet and vacuum
ultraviolet spectral ranges. Such a synchrotron might cost between $30 and
$80M. Again, a variety of novel chemical applications could be explored in
photochemistry and fast chemical kinetics, as well as multiple photon and other
nonlinear processes. Instrument developers should be aware that most of these
applications could be productively pursued in the visible and normal ultraviolet
spectral ranges.
103
OCR for page 104
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OCR for page 105
W. . . .... ~ . - ..
hipping a wlcRecl weed .
The plant Striga asiatica is one of the most devastating destroyers of grain crops
in the world. This wicked weed~restricts the food supply of more than ;400 million
people in Asia and Attica. It is a parasite that~no~shes itself by latching onto
and draining the vitality~of a nearby gain plant. The results are stunted grain,
a meager harvest, and hungry people. ~
Basic ~research~on Striga asiatica by chemists~and biologists has revealed one:
of the plant world's incredible host-parasite adaptations. The:~parasite~seed lies
in wait until it detects the proximity ofthe host plant by using an uncanny chemical
_
rat ar. '1' He give-away is provided by specific chemical compounds exuded by the
host. Striga asiatica can recognize the exuded compounds and use~them to trigger
its own growth c role. Then the parasite has an independent growth period of 4 days,
Wring which it must locate~the nearby host. ~ ~ ~ ~ ~ ;
_ . · ~ ~ . ~ ~ ~ · ~ ~ . ~ ·
nesearcners trying to solve tne mystery ot tnls recognition system tacea:tormldable
. . . ~ ~ . . .
O. Stan .es; t Hey were see mug un gnome comp ex molecu es proc ucec on y ln tmy
amounts. But, by extending~the sensitivity ofthe most~modern~instruments, chemists
have been able to Educe the chemical structures of these host-recogr~ition sub-
stances, even though the agricultural~scientist could~accumulate~the~active chem-
icals in amounts no larger than a few bits of trust (a few micrograms). One method
used, nuclear magnetic resonance (NMR), dopers upon the fact thank the nuclei
of madams have magnetic fields that respond measurably to Tithe Presence of
other such nuclei nearby.:Thus precise NMR~measurements reveal molecular go-
: ~ _ : : : Got _
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· ~ ~ ~ : the ~ ~ ~ : ~ ~ : t ~ · ~ · ~ ~
ometrles,::evel~ or ornate mo-~lecu es. ~ secono' equally sop ~~sUcatea~approach~s~h~gh-
reso: utlon mass ppectrome~y.~ n a 1lg ,~vacuum, mo .ecu es~are~g~ven angled ectrlc
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c ~arge,:then;;accelerated with~a known energy. By Leasing the velocities ~ which
these molecules ar~ffagments from them are traveling (or heir curved paths in
magnetic~fields), chemist can~measure~;the masses and decide~the ~atomic~o~ings
present.These~are~critical~cluestothe~molecul~identities. ~~ ~ ~~ ;~ ~~ ~~ :~:
~ ~ ~ AT ~ :i ::: if: I: :i: : I ~ ~ · o : : · · ~ , : ~ :~: ~ ~ · ~ · ~ ~
~ ~ l~OW, me complex nosr-reco~Itlon xenoglstlc} ;sunst~ces cam neen 1uentlilea
, . . . . . . ~ ~ ~ . ~ . ~ ~ , . hi .: ~ . . ~ . . . ~ .
t1 1 ant I. 1elr c etallea~structures are Known. :Wlth thls:~lr~ormatlon In hand, we moor
]] be able to beat this wicked weed at its own game. Chemists can; now synthesize the:
V substances Andes give agricultural scientists enough material for f eld tests designed
to trick the parasite into beginning its 4-day growth cycle. It will die on never
having found its host. A few days later, grain can be planted safely. ~
AT' ~ ~ l al · ^ ~ · ~ - · ~ ~
Olin Miss success Ior gulc~ance, similar host~araslte r~ionships are being sought—
. ~ . . . .
and round—nere In tne;United~States. Tn. addition to grains, bean crops have similar
parasite enemies. Thus in collaboration with agricultural and biological scientists,
chemists play a crucial role indoor efforts to increase the world's food supply and
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Representative terms from entire chapter:
molecular beam
