SPECIAL REPORT
in
Lasers Chemical Processing
Richard L. Woodin, Norton Co., David S. Bomse, Southwest Sciences Inc., and Gary W. Rica, Engelhard Corp.
The unique properties of laser radiation—including its high intensity, narrow spectral width, exceptional focusability, and temporal resolution—promised a decade ago to revolutionize chemical processing. Lasers already have found wide use in several segments of heavy industry. Robot-guided laser beams now weld entire automobile frames in a few seconds; industrial lasers also cut materials ranging from diamond to silk. Early predictions that the chemical industry would be making many laser-synthesized products that were unavailable from existing technologies were overly enthusiastic, however; lasers are not yet common in chemical operations. Although the laser revolution in chemical processing certainly has been delayed and transformed, recent developments in laser chemistry nevertheless will surely have an impact on the chemical industry, not only through new process technology but also in the ways that laser-based analytical methods may influence process design and process control. Laser applications will affect the chemical industry in three broad areas. Laser-based processes that offer lower costs, improved yields, higher product purity, and fewer unwanted side products have been developed. Laser-based analytical methods, too, have achieved the ultimate in sensitivity: detection of single atoms in the gas phase and single molecules in solution. These new analytical methods will change the way product specifications are defined and will provide new opportunities for process monitoring and control. In addition, reaction diagnostics using lasers provide the detailed 20
December 17, 1990 C&EN
mechanistic and kinetic information required to help research scientists design new processes. These diagnostic methods also may mature into new analytical techniques for process monitoring. The laser revolution already has had wide impact in the chemical research laboratory, where laser radiation is ideal for controlled initiation of reactions and for highly selective and sensitive detection of both stable and transient molecules. The ability to control both the spatial and temporal properties of energy input to chemical systems and to probe selected molecules (both stable and reactive) at low concentrations is providing unprecedented insights into the microscopic details of chemical reactions. By controlling reactions, lasers have been used to synthesize novel solid materials, "ideal" powders, and small clusters of bare metal atoms. The narrow bandwidth of laser radiation has led to routine use of high-resolution spectroscopy for selectively identifying chemical constituents. Laser-surface interactions, too, are being used to induce rapid desorption of gases and liquids to probe the kinetics of reactions on surfaces. In contrast to these varied and valuable laboratory applications, laser chemistry on an industrial scale has been slow in coming to fruition. At present, to our knowledge, only one laser-driven chemical process is operating commercially: photoablation for drilling holes in epoxy-resin circuit boards. Use of lasers to separate isotopes (of uranium, for example) was an early and well-publicized potential industrial application of laser chemistry. However, efforts at developing this technology now are stymied (except for research in the national laboratories) by reduced demand for nuclear fuel. As a result, large-scale use of lasers to separate iso-
topes is unlikely. Recent advances in photochemical etching of semiconductors may provide the next com mercial application of laser chemistry. Nevertheless, lasers already are used commercially for a wide variety of other materials-processing opera tions, such as drilling, cutting, welding, and surface heat treating. Such applications demonstrate that lasers are economically competitive with—indeed, often even superior to—many conventional mechanical tools in se lected uses. These advances should encourage the use of the laser's superior properties of energy control, en ergy density, and energy intensity to replace conven tional energy sources in chemical processing. Laser ra diation also has clear advantages over other sources of photons, such as mercury lamps, for many traditional photochemical applications. Although the fundamental chemistry is no different, laser light often can improve process efficiency because of its high power, greater se lectivity (narrow wavelength), the spatial control it af fords, and its energy efficiency. One example is the work by Peter A. Hackett at Can ada's National Research Council on the photochemical synthesis of a precursor to vitamin D. Judicious selec tion of wavelengths discriminates against undesired side products, improving the yield of the desired pre cursor from 30% (in a single-color process) to as much as 80% (using a two-color process). In addition to being well suited for conventional photochemistry, laser radiation, because of its high in tensity, has the potential to open the way to previously impossible photochemical reactions, known as multiple photon, or multiphoton, reactions. In conventional photochemistry, the flux of radiation is small and a molecule that has absorbed a photon will react or be come deactivated before it can absorb more energy. Hence, only single-photon excitation occurs. With IR radiation, for example, the energy per photon is small compared with the energy required for reaction—1 to 10 kcal per mole compared with the 70 to 100 kcal per mole required for typical chemical reactions. This nor mally precludes use of low-intensity IR radiation for inducing chemical change. If the photon flux is high enough (on the order of megawatts per square centimeter), however, a molecule that has absorbed one quantum of energy has a high probability of absorbing additional quanta, so that it ac cumulates enough energy to undergo reaction. For ra diation at a wavelength of 10 Mm, this may require ab sorption of more than 30 quanta of radiation. Because IR absorption excites vibrational modes, it is analogous to thermal excitation—but with the advan tages of homogeneous excitation, species selectivity, and rapid (submillisecond) heating and cooling. Pulsed IR lasers provide the peak intensities that are necessary to drive multiphoton processes, thereby opening up en tirely new areas for research during the past 10 years. The intensities available from pulsed visible and UV lasers also are sufficient for inducing multiphoton pro cesses. Photon energies are much larger than in the IR (on the order of 20 to 80 kcal per mole), and fewer quanta of energy are required to initiate reactions. In addition to inducing chemical reaction, multiphoton
Quantum yield is critical to economy Price per mote,$ 10*
Cost œ r mote Droduct. $ 10*
2% efficiency
10*
10» 3% Enriched uranium^
Uranium separation
1
1 6% efficiency
Vinyl chloride —
to*
Vinyl chloride r\ process
*«* ·:.:* - - ^m ^^èiM^mÊÊmmmmÊm^ *ÉF ΈΕ^ι„^ΑΪυ2«ύάΜ L·J^L·à luà^tUlM.
The various forms of laser-based chemical production can \ be put on common ground by considering the cost per mole \ of photons as a function of quantum yield (the number of product nrwlecules per photon absorbed). The two lines are for efficiencies of 6 % and 2 % , respectively. The point in- \ dicated on the 6 % efficiency curve is for an IR two-color ; uranium isotope separation process. The point on the 2 % j efficiency curve is for laser-assisted vinyl chloride produc- ; tion. Laser costs are calculated assuming $100 per watt per j year for a 6%-efficient carbon dioxide IR laser and $300 \ per watt per year for a 2%-efficient krypton fluoride UV laser. Costs include gases, electricity, and capital, assuming j 5 0 % utilization. In terms of conventional processing, the cost of energy for a 6%-efficient C 0 2 laser then is $11.40 ! per kWh (which can be compared to electrical energy costs ' of 5 to 10 cents per kWh). ί
visible and UV excitation can be used to ionize mole cules. Multiphoton ionization often gains spectroscopic selectivity through intermediate electronic states spe cific to the excited molecule. The ease of ion collection and sensitivity of measurement make multiphoton ion ization a powerful laser-based diagnostic technique. As an unconventional energy source, lasers provide additional benefits for chemical processing. Because the energy comes from a remote source, one laser can be directed to whichever one of several reactors are oper ating at any given time. In locations where not all pro cesses are running continuously, a "turntable" laser can be switched rapidly from process to process, reducing capital costs and allowing the laser to be used more fre quently. This approach also may simplify reactor de sign and reduce reactor costs. Chemical synthesis using lasers, however, suffers from the general problem that plagues all photochemi cal processes: the high cost of photons when used as December 17,1990 C&EN
21
Special Report
Use of lasers is increasing The laser—light amplification by stimulated emission of radi ation—is a scientific breakthrough that now has become an accepted technology of modem life. Lasers are being used increasingly in manufacturing, communications, surveying, printing, and now even home entertainment (for example, in audio compact disc players), as well as in scientific research and for a wide range of scientific instruments. The laser ex cels as a controlled, monochromatic, and intense light source. Laser radiation, with its narrow spectral bandwidth and ability to be tightly focused, is many orders of magnitude brighter than solar radiation. Maximum available energy den sity (radiant omittance) from a conventional black body source at 6000 Κ (for example, solar radiation) is much less bright than that from HeNe or C 0 2 lasers (opposite page, top). The first requirement for lasing is to have a material (or medium) that can store energy. Typical materials are crystals such as ruby and neodymium, yttrium, aluminum, garnet (Nd:YAQ); gases lice carbon dioxide, helium, neon, argon, and krypton; and liquids, including such dyes as rhodamine-6G. Energy is stored in an excited electronic or vibrational state (state 3 in diagram top right) of these lasing media by optical pumping (using a conventional flashlamp or another laser), by electrical energy from a discharge, or by collisions with other excited molecules (in gases). Relaxation from the upper to a lower state (state 2) produces the upper state for the lasing transition from state 2 down to state 1. Light of a specific fre quency passing through the lasing medium will stimulate emission at the same frequency from the state 2 to state 1 transition, thereby amplifying the radiation. Enclosing the lasing medium in an end-mirrored optical resonator (below ri£it), which reflects the radiation back and forth through the medium, causes a buildup of high levels of radiation within the resonator by continued stimulated emis sion. The laser radiation then is emitted from the cavity through the partially transparent output mirror at one end. A long-standing problem with laser radiation is a lack of
reagents. Lasers, because of their monochromaticity, can be substantially more efficient than conventional lamps in producing a particular wavelength. But for producing chemicals that sell for relatively low prices, photon costs remain prohibitive. Nevertheless, the high intensity and monochromaticity of laser radiation, plus its availability at wavelengths impossible with conventional light sources, suggest that in some areas of commercial synthesis it may be used successfully. IR lasers and IR multiphoton dissociation provide a bridge between highly selective IR spectroscopy and synthetic photochemistry. Use of their properties to enhance se lectivity and alter reaction pathways may yet provide an industrial niche for laser photochemistry.
Laser processing costs Capital costs and operating expenses related to lasers vary with the efficiency and complexity of each type of device. High-pressure gas discharge lasers, such as carbon dioxide or excimer (rare gas halide) units, gen 22
December 17, 1990 C&EN
Absorption band or levels
Fast decay σ> k. Φ
c
UJ
State 2 Excitation
Laser transition
hv 21 State 1 Fast decay
Ground state
Excitation (flashlamp, electric discharge, for example)
ΓΛ
Laser medium
n\ Laser output
u/ Mirror
\AJ Partially transmitting output mirror
erally are least expensive because they cost less to make, produce more light, and operate at high efficien cy compared with solid-state or dye lasers. For applica tions requiring high levels of power, as in commercial chemical processes, use of one or a few large lasers is likely to be more cost effective than use of a larger number of small ones. Operating costs (cost per mole of photons) can be calculated from the efficiency (optical power produced relative to electrical power consumed) and photon wavelength. IR radiation from a carbon dioxide laser of 6% efficiency (at a wavelength of 10.6 μιη) costs from 10 to 50 cents per mole of photons, and UV radiation from a 2% efficient krypton fluoride laser (at 193 nm) costs from $10 to $70 per mole of photons, depending on capitalization, utilization, and maintenance. Compared with commodity chemicals, therefore, photons are ex pensive reagents. Ethylene, for example, typically sells for less than 1 cent per mole (8 to 15 cents per lb). For photons to be practical reagents, their cost must
broad wavelength coverage (bottom). Because individual la sers do not have extensive wavelength tunabilfty, coverage of the entire optical spectrum requires many separate devices. Common IR lasers (2 to 10 μπ\) include carbon dioxide, carbon monoxide, alkali halide crystal (F-center), and diode devices. Nd:YAQ lasers generate light at 1.06 μηι, and Alex andrite or communications diode lasers are tunable in the near IR (at wavelengths from 2000 to 700 nm). Argon ion, krypton ion, helium-neon (HeNe), dye, and titanium:sapphire lasers cover the visible spectrum (700 to 400 nm). Exclmer (rare gas halide), nitrogen, frequency-doubled dye, and fre quency-multiplied Nd:YAG lasers operate in the UV wave lengths (400 to 200 nm).
Radiant emittance, Watt per cm 2 per μηι
10 1 6
10 1 2
10 8
104 1 ηm
ni
1 Wavelength, μ m
Titanium: sapphire
-tn
Optical parametric oscillator
-inn
1 2 3 4 5
Excimer Md:YAG ^r Ion Kr Ion Mitrogen
Semiconductor diode lasers
0.1
1 UV
Vis
Wavelength, μ m
NearIR
10
100 IR
be low relative to the product's value. Either the quan tum yield (number of molecules of product produced per photon absorbed) must be high, or the product must support a high selling price because it is new, unique, or has a high-value-added content. Some specialty chemi cals, such as complex biomolecules or vitamin D, are valuable enough to tolerate such high reagent prices, es pecially when the volume of product to be processed is such that small-scale laser devices can be used. Production of transient intermediates for synthetic applications also may be feasible with the high intensi ties available from lasers. R. Marshall Wilson and his coworkers at the University of Cincinnati have used ra diation from an argon laser to synthesize 1,3-cyclopentadienyl biradicals, which add molecular oxygen to yield analogs of prostaglandin endoperoxides. High-volume products like commodity chemicals, on the other hand, require processes with high quantum yields to be economical. For laser-driven vinyl chloride synthesis to be viable, for example, quantum yields in
Current laser research is pushing tunability further into both the IR and UV ranges. Candidates include Raman-shifted vis ible dye lasers for broad IR tunability and new frequency mul tiplication schemes for wavelengths from 200 to 80 nm. Laser radiation is easier to control than conventional light sources as a result of its spatial coherence and temporal properties. The coherence of laser radiation provides low di vergence (spreading of the beam), which simplifies the design of reactor vessels suitable for laser photochemistry or diag nostics. Many lasers also are amenable to pulsed operation, with pulse durations ranging from femtoseconds to more than one second. Pulsed operation makes possible novel kinetic control of chemical reactions and provides high intensity (en ergy/area/time). Because light has a finite velocity, optical radar has been developed to allow distance-resolved remote sensing of chemical species. Commercial lasers now available for industrial applica tions, largely as a result of materials processing opportunities in marking, welding, or drilling, include NdiYAG, carbon diox ide, and, most recently, exclmer lasers. Development is still under way on very large exclmer lasers—those with power levels of a kilowatt or more—which would be most suited for chemical processing. Current research activity on the Strate gic Defense Initiative ("Star Wars") program is likely to fur ther spur the development of high-power UV tunable lasers.
excess of 104 are necessary to compensate for the rela tively low price of the product.
Separation and purification Selected isotopes, such as those of uranium, often command a substantial value. The recent discovery by Thomas R. Anthony and coworkers at the General Elec tric Research Center of greatly improved thermal con ductivity of diamond using isotopically enriched carbon is one example of a use for such enriched mate rial. Isotopes may be separated economically by laser processes even with quantum yields well below one. Because different atomic isotopes have slightly differ ent absorption frequencies, narrow-band laser sources can preferentially excite one isotope. Multiple photon excitation using lasers at several wavelengths leads to selective ionization, so that isotopically enriched mate rial can be collected from either the reacted or unreacted fraction. Although a device for laser isotope enrich ment is more complex than a centrifuge or diffusion December 17, 1990 C&EN
23
Special Report
Laser chemistry produces a variety of high-value-added products The high cost of laser radiation may be justified if it leads to the formation of a high-value-added reaction product such as a complex biomolecule. One example (below) is the synthe sis of analogs of prostaglandin endoperoxide (an intermediate for the drugs prostacyclin and thromboxane) with benzophenone as photosensitizer and using 351-nm radiation from an argon ion laser. R. Marshall Wilson and his coworkers at the University of Cincinnati have used high-intensity laser radia tion to produce triplet 1,3-cyclopentadiyl biradicals, which then add oxygen to form the desired endoperoxides. Another laser-driven synthesis involves the further photo-
C6H5
Ο
p6H5
hv, UV
OH
OH
+
^Y^C6H5
^ s
XH3 1-Methylbenzophenone
CH 2
\ A c H
2
hv, UV-vis/ hv,UV
Anthrone OR ΝC5H1 5n11
H Prostaglandin endoperoxide hv, Benzophenone OR C«H. '5n11 ! H Triplet 1,3-cyclopentadiyl biradical
o2 /
γ2
OR O^
r n CsH. 5 11
Endoperoxides
C5H
chemistry of a photolytically generated intermediate to pro duce anthrone from 1-methylbenzophenone (top, right). The two initial photoproducts formed using a conventional lamp producing 350-nm radiation rapidly convert back to methylbenzophenone and form a variety of dimerization products. Wilson finds that irradiation of the methylbenzophenone with focused argon ion laser radiation at 334 to 364 nm enhances the production of the dimerization products, increasing the production of anthrone from a yield of 0.5% to 2 4 % ; adding a second laser operating in the visible spectrum at 458 to 514
cell, the overall process can be simpler because the en richment is much higher per pass. Molecular separation is similar to isotope separation in this regard. Separation processes succeed when a laser with a narrow bandwidth can excite selected compo nents of a mixture of molecules because of differences in the components' absorption spectra. For example, the large absorptivity of hydrogen sulfide relative to that of 24
December 17, 1990 C&EN
nm further promotes the formation of the dimerization prod ucts, increasing the yield of anthrone to 3 4 % . A third example is the application of wavelength selectivity to improve the synthesis of the precursor for vitamin D shown in the center of the reaction scheme below. Choosing wave lengths based on the absorption spectra of the isomers, Peter A. Hackett of the National Research Council in Canada achieves a photostationary state that optimizes production of the provitamin. Radiation at 248 nm from a krypton fluoride laser promotes formation of the bottom isomer; because it is the only component that absorbs at 337 nm, radiation at that wavelength from a nitrogen laser converts only it, in turn, to the provitamin D. The provitamin then can be converted to vi tamin D (shown at the right of the reaction scheme), which also is a high-value-added product.
Vitamin D3
KrF laser
γ
*
°
\
Vitamin D
N 2 laser
synthesis gas (a mixture of carbon monoxide and hy drogen that can be catalytically converted to petro chemicals) permits an argon-fluoride excimer laser to selectively remove hydrogen sulfide (which poisons the catalyst) from a gas stream. Because catalysts can be poisoned by feedstocks with less than part-per-million levels of hydrogen sulfide, adequate purification is dif ficult by conventional chemical processes.
Laser photochemistry economically removes trace contaminants Even though the cost of laser photons may be high on an individual molecule basis, the cost as a process can be low if the laser reacts with only a relatively small number of molecules, as in a pu rification process. Removal of only a small amount of impurity can increase greatly the value of a chemical in cer tain applications. The laser operating costs per mole of purified products are shown in the graph below as a function of impurity level. Laser efficiencies and costs similar to those discussed in the box on page 21 are assumed. Also as sumed is a quantum yield for impurity removal of 10"4 for the infrared process and a quantum yield of 1 for the ultravi olet process.
Impurity removal often is needed be cause the impurities reduce perfor mance of the resulting product (as in semiconductor manufacturing) or be cause they affect subsequent process ing (for example, by poisoning a cata lyst). At levels greater than 1 ppm, sul fur-bearing molecules can rapidly poison the catalysts used in synthesisgas processes. Hao-Lin Chen and Charles Borziliers of Lawrence Livermore Laboratory have used the output of a Raman-shifted argon fluoride excimer laser to selectively remove hydro gen sulfide from a mixture of synthesis gas (H 2 , CO, and small amounts of CH 4 , C 2 H 2 , C 0 2 , and 0 2 ). The diagram at right shows the absolute absorption
Cost, $ per mole purified product 10°
10-1 2% efficiency (CO2 |R laser)
10-2
6% efficiency (KrF UV laser)
10*3
10
100 Impurity level, ppm
10*
Chemical purification by laser radiation is economi cally feasible when removal of a trace impurity sub stantially enhances the value of the product (as is the case, for example, with gases used for semiconductor processing) and the process has a high effective quan tum yield, so that relatively few photons are needed to produce a mole of purified material. When impurity levels are low (less than 100 ppm), a laser-based purifi cation scheme may compete favorably, even for bulk chemicals, with such energy-intensive processes as cryogenic distillation or when repetitive processing would be necessary because separation factors are low. IR laser chemistry usually is the method of choice for separations because of its species selectivity. However, further development will be necessary before IR lasers can operate satisfactorily under such process conditions as high pressure or high material throughput.
1000
Absorption cross section, M& 10-17 Ramanshifted ArF laser H2S^
10*o
CO (Cameron band)
10* 1 10r«
1 5 Synthesis gas
10-58
10•24 10*•25 160
-ί
180
α. 200
JL
J .
220
240
Wavelength, urn
cross sections for hydrogen sulfide, the carbon monoxide Cameron bands (which dominate ultraviolet absorption by synthesis gas), and a synthesis gas mixture that does not contain hydrogen sulfide. The syngas cross sections were measured with three discrete la ser wavelengths, as indicated in the bottom half of the diagram. Removal of hydrogen sulfide from syngas is possi ble because of the high absorption cross section of hydrogen sulfide rela tive to syngas. Maximum removal effi ciency occurs where the absorption cross section ratio a(H2S)/a(syngas) Is at a maximum. This is 10 7 between 210 and 220 nm. In demonstration ex periments, the laser purification pro cess has proved as economical as conventional processes for removing sulfur at hydrogen sulfide levels below 1 ppm.
Chain reactions initiated by lasers High-volume products like commodity chemicals re quire processes with high quantum yields. One reac tion for which high quantum yield offsets the cost of laser photons is photochemical initiation of free-radical chain reactions. Photoinitiation of a chain reaction pro duces many product molecules for each photon ab sorbed by decoupling initiation of the reaction from subsequent chain propagation. Because initiation is usually the mechanistic step requiring the most energy, photoinitiation eliminates the need to start the reac tion by using high temperatures, allowing it to be run at a temperature best suited for chain propaga tion and reducing side reactions. As a result, yields of desired products and rates of product formation are increased while formation of undesired by-products December 17, 1990 C&EN
25
Special Report
Laser-initiated chain reactions are fast and form few by-products Η
«y
'CI Ή
Η 1,2-Dichloroethane
Concentration, parts per million 1500[
H
hv
Laser initiation
Thermal. 480 °C Laser (XeCI, 80Hz), 350 °C
1000 H
H
H ^ ~ ^ * < CI H H
H
Vinyl chloride
1,2-Dichloroethane
Free-radical chain reaction
φ c φ
500 c
>
CL
S
υ
» LU
radical chain reaction can be operated at reduced tempera ture when a UV laser is used for photoinitiation. This results in reduced formation of the undesired reaction products (as shown in the graph above). Using a laser to initiate a chain reaction also can drastical ly increase the reaction rate by overcoming an initial induc tion period. This is shown for production of fe/t-butylhydroperoxide by HBr-catalyzed oxidation of isobutane (in the reac tion sequence below left). For both reaction temperatures shown (below, right), laser initiation increases the reaction rate by more than a factor of three. It is clear from the graph that laser initiation significantly decreases the reaction time needed for any chosen yield. % conversion to fert-butylhydroperoxide Ίϋϋ
f-BuO*
Laser Thermal
80
HBr
160°C
60
Isobutane •Br*
HBr
t-Bu02 Η
20
December 17, 1990 C&EN
0 10
0 2
