HANDBOOK OF HETEROGENEOUS CATALYSIS

Edited by G. Ertl, H. Kniizinger, and J. Weitkamp

Part A, Chapter 8.6

SONOCATALYSIS

Professor Kenneth S. Suslick

School of Chemical Sciences

University of Illinois at Urbana-Champaign

505 S. Mathews Ave.

Urbana, Illinois 61801

tel: 217-333-2794 f&x: 217-333-2685 mail: ?auslick@uiuc.edu

1. Introduction and the Origins of Sonochemistry p . 2

2. Effects of Ultrasound on Heterogeneous Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 .1 . MetalPowders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1.1. Hydrogenation and Hydrosilation Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.2. Fischer-Tropsch Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.3. Amorphous and Nanostmctured Metal Catalysts . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.. Metal Oxides as Oxidation Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3. Silica, Alumina, and Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.4. Supported Metal Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

2.5. Polymerization Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1

3. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3

5. Figure Captions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6

8.6 Sonocatalysis

K. S. SUSLICK

8.6.1 Introduction and the Origins ofSonochemistry

Research on the chemical effects of ultrasound has un-dergone a renaissance during the past decade and hashad a significant impact in a variety of areas [l, 2].Applications of sonochemistry have been developed invirtually all areas of chemistry and related chemicaltechnologies [3, 4]. We can conceptually divide theeffects of ultrasonic irradiation on heterogeneous cat-alysis into those that alter the formation of heteroge-neous catalysts, those that perturb the properties ofpreviously formed catalysts, and those that affect cata-lyst reactivity during catalysis. In practice, these threeclasses of effects are often deeply intertwined in re-ported experimental results.

No direct coupling of the acoustic field with chem-ical species on a molecular level can account for sono-chemistry. Ultrasound spans the frequencies of roughly2OKHz to lOMHz, with associated acoustic wave-lengths in liquids of roughly 100 to 0.15 mm: these annot on the scale of molecular dimensions. Instead, thechemical effects of ultrasound derive from several nonlinear acoustic phenomena, of which cavitation is themost important. Acoustic cavitation is the formation,growth, and implosive collapse of bubbles in a liquidirradiated with sound or ultrasound. When soundpasses through a liquid, it consists of expansion (neg-ative pressure) waves and compression (positive pres-sure) waves. These cause bubbles (which are filled withboth solvent and solute vapor and with previously dis-solved gases) to grow and recompress. Under properconditions, acoustic cavitation can lead to implosivecompression in such cavities. Such implosive bubblecollapse produces intense local heating, high pressures,and very short lifetimes. As discussed elsewhere, thesehot spots have temperatures of roughly 5300K, pres-sures of about 1720 bar, and heating and cooling ratesabove lo9 KS-’ [5-7]. Cavitation is an extraordinarymethod of concentrating the diffuse energy of soundinto a chemically useable form.

When a liquid-solid interface is subjected to ultra-sound, cavitation still occurs, but with major changesin the nature of the bubble collapse. If the surface

is significantly larger than the cavitating bubble(~ 100~ at 20 kHz), spherical implosion of the cavityno longer occurs, but instead there is a markedlyasymmetric collapse which generates a jet of liquiddirected at the surface, as seen directly in high speedmicrocinematographic sequences shown in Fig. 1 [8].The tip jet velocities have been measured by Lauter-born to be greater than IOOms-‘. The origin of this jetformation is essentially a shaped-charge effect: the rateof collapse is proportional to the local radius of curva-ture. As collapse of a bubble near a surface begins, itdoes so with a slight elliptical asymmetry, which is self-reinforcing, and generates the observed jet. The im-pingement of this jet can create a localized erosion (andeven melting), surface pitting, and ultrasonic cleaning.A second contribution to erosion created by cavitationinvolves the impact of shock waves generated by cav-itational collapse. The magnitude of such shock wavesis thought to be as high as lo4 bar, which will easilyproduce plastic deformation of malleable metals [9].The relative importance of these two effects dependsheavily on the specific system under consideration.

Enhanced chemical reactivity of solid surfaces isassociated with these processes. The cavitational ero-sion generates unpassivated, highly reactive surfaces; itcauses short-lived high temperatures and pressures atthe surface; it produces surface defects and defor-mations; it forms fines and increases the surface areaof friable solid supports; and it ejects material in un-known form into solution. Finally, the local turbulentflow associated with acoustic streaming improves masstransport between the liquid phase and the surface,thus increasing observed reaction rates. In general,all of these effects are likely to be occurring simulta-neously.

In contrast, the effects of ultrasound on slurries offine particles does not come from microjet formationduring cavitation. Distortions of bubble collapse de-pend on a surface several times larger than the reso-nance. bubble size. Thus, for ultrasonic frequenciesof ~2OkHz, damage associated with jet formationcannot occur for solid particles smaller than ~200 m.In these cases, however, the shockwaves createdby homogeneous cavitation can create high-velocityinterparticle collisions, with impact speeds of severalhundred ms-’ and local effective transient impacttemperatures of roughly 3OOOK [l0]. The turbulentflow and shockwaves produced by intense ultrasoundcan drive metal particles together at sufficiently highspeeds to induce effective melting at the point of colli-sion as shown in Fig. 2. The high-velocity interparticlecollisions produced in slurries of malleable materials

cause smoothing of individual particles and agglomer-ation of particles into extended aggregates. Surfacecomposition depth profiles of sonicated powders showthat ultrasonic irradiation effectively removes surfaceoxide coatings. The removal of such passivating coat-ings dramatically improves reaction rates for a widevariety of reactions. With larger flakes of brittle mate-rials, interparticle collisions causes shock fragmenta-tion instead, which can increase surface areas dramati-cally and contribute to the increased activity [ll-13].

The term “sonocatalysis” should be restricted inits use to refer only to the creation of a catalyticallycompetent intermediate by ultrasonic irradiation. Oneshould not refer to a simple sonochemical rate en-hancement of a reaction by this term, just as one woulduse the term photochemistry, and not photocatalysis,to describe a stoichiometric reaction caused by light. Inthis chapter, the symbol below (eq 1) will be used toindicate ultrasonic irradiation or “sonication” of asolution leading to a sonochemical reaction.

8.6.2 Effects of Ultrasound onHeterogeneous Catalysts

Ultrasonic irradiation can alter the reactivity observedduring the heterogeneous catalysis of a variety of re-actions. In addition to the more recent work describedin this chapter, there is an extensive (but little recog-nized) past literature in this area, particularly fromEastern Europe [14].

The effects of ultrasound on catalyst formation canbe far reaching; changes in patterns of crystallization,dispersion, and surface properties are all possible. Al-teration of properties of preformed catalysts can alsohave substantial effects. Oxide or other passivatingcoatings can be removed, and increased dispersion canoccur. sometimes from the fracture of friable solid

‘The use of ultrasound for syntheses involving liquid-solid heterogeneous organometallic reactions has beena matter of intense current investigation [15 16]. Ingeneral, ultrasonic treatment of these metals promotesreaction pathways favoring single-electron transfers

[17], probably through the removal of thin oxide coat-ings which are often dominated by acid-base activity.The ultrasonic activation of commercial transitionmetal powders has also received substantial attention[18-20].

A Hydrogenation and Hydrosilation CatalystsThe most heavily studied sonochemical systems forhydrogenation involve nickel catalysts and has a longhistory [21-23]. Impressive accelerations have beenreported recently. For example, the hydrogenation ofalkenes by ordinary Ni powder is enormously en-hanced (>lOs-fold) by ultrasonic irradiation [18]. Thesurface area of the catalyst did not change significantlyeven after lengthy irradiation. Both surface smoothing(shown in Fig. 3) and particle agglomeration occur,probably due to interparticle collisions caused by cav-itation-induced shockwaves, as discussed earlier. Augerelectron spectroscopy reveals that there is a strikingdecrease in the thickness of the oxide coat after ultra-sonic irradiation. The removal of this passivating layeris probably responsible for the >105-fold increase ob-served in catalytic activity.

Hydrogen-deuterium exchange by Raney Ni powderis also improved by ultrasonic irradiation, as first dis-closed in a 1986 patent [24] Selective introduction ofhydrogen isotopes into aromatic compounds wasaccomplished by the reaction of haloaromatic com-pounds with basic deuterated (or tritiated) aqueoussolutions over Raney metal catalysts under ultrasoundat m320K. Others have extended this work to deu-terate carbohydrates and glycosphingolipids, with rateincreases of roughly threefold upon sonication [25].These workers concluded that the increased activityoriginates from fresh Ni surfaces produced daringcrack propagation during ultrasonic irradiation [26].

Of special value to the synthetic organic chemist, isthat a highly efficient enantioseleotlve catalyst can besonochemically prepared from tartaric acid treatmentof Raney Ni for the hydrogenation of 1,3-diketones to1,3-diols [27]. Here, the Raney Ni/Al alloy is sonicatedin water to remove the Al before exposure to the tar-taric acid treatment. Although the mechanism of im-provement is obscure, in the best cases, enantiomericexcesses of 90% are observed.

The allylation of ketones and aldehydes by allylicalcohols (eq 2) has been improved using ultrasonicirradiation of a palladium/tin dichloride catalyst in lesspolar solvents [28]. Inverted regioselectivity is observedcompared to homogeneous carbonyl allylation in polarsolvents.

H&CH=CHCH20H + H.&&HO

There have been several studies of the effects ofultrasound on hydrosilation reactions, primarily byBoudjouk [29], who extended the use of ultrasoundwith nickel to the hydrosilation of alkenes [30].Whereas there are many catalysts for hydrosilation ofsimple alkenes, relatively few work well for functional-ized substrates, such as acrylonitrile. The use of ultra-sound to prepare hydrosilation catalysts for the hydro-silation of acrylonitrile with silanes has met withsuccess. Using ultrasound to create a high surface areadispersion of Ni from NiI2 reduction with Li powder,various alkenes were hydrosilated. Even the reactionof acrylonitrile occurred in yields >95% at 273K.Induction periods of lo-30min were observed beforerapid hydrosilation began. The Ni catalyst can berecycled many times without loss of activity. By com-parison, Ni powder from commercial sources is notactive even after extensive sonication.

B Fischer-Tropsch (Hydrogenation of CO) CatalystsThere have been four recent reports on the use ofultrasound for catalyst preparation for Fischer-Tropsch synthesis. Liquid-phase hydrogenation ofcarbon monoxide was accomplished with ultrafineparticles (

micrograph of Fig. 4 [34-361. Amorphous metallicalloys lack long-range crystalline order and haveunique electronic, magnetic, and catalytic properties.The production of amorphous metals (see SectionA4.4) is difficult because extremely rapid cooling(> lo6 K s-‘) of molten metals is necessary to preventcrystallization. As discussed earlier, acoustic cav-itation can induce extraordinary local heating in oth-erwise cold liquids and can provide enormous coolingrates (>109Ks-I), which provides a new syntheticroute to amorphous metal powders using the sono-chemical decomposition of volatile organometallics.From recent work on the sonolysis of volatile Co, MO,and W precursors [37], it appears that this is a generalphenomenon, and extension to the synthesis of amor-phous intermetallic alloys is also proving successful.

The sonochemically synthesized amorphous powdersmay have important catalytic applications, especiallygiven their very high surface areas and nanometercluster size. For example, sonochemically preparednanophase iron powder is an active catalyst for theFischer-Tropsch hydrogenation of CO and for hydro-genolysis and dehydrogenation of alkanes, in large partdue to its high surface area (>12Om*g-I). Rates ofconversion of CO and Hz to low molecular weight al-kanes were approximately 20 times higher per gram ofFe than for tine particle (5~ diameter) commercialcrystalline iron powder at 523 K, and more than 100times more active at 473 K. Selectivities were not sub-stantially different. The reactions of cyclohexane provean interesting case because of its inherent catalyst sur-face-structure sensitivity. In this manner, the nature ofthe catalytic process can be useful as a chemical probeof the effect of ultrasound on the catalytically activesurface. Catalytic studies were carried out in a con-tinuous flow microreactor on nanophase Fe/Co alloysproduced sonochemically. The ratio of cyclohexanedehydrogenation to hydrogenolysis depended on alloycomposition. The 1: 1 alloys gave nearly exclusivelybenzene, in stark contrast to either pure metal [3X].

8.6.2.2 Metal Oxides as Oxidation Catalysts

There are several reports on the effects of ultrasoundon metal oxides catalyst preparation, but in no caseshave substantial improvements in rates or selectivitiesbeen observed. Mixed Cr/Mo and Cr/Fe oxide cata-lysts have also been prepared with ultrasonic treatmentand examined for the oxidation of methanol to form-aldehyde [39, 401. Recent studies by Mokry and co-workers [41] have examined the vapor-phase oxidationof a number of organic compounds after ultrasonicactivation of Fe/Te/Mo oxide and Cs/Pb/Mo oxidecatalysts. Vapor-phase oxidation of isobutylene, meth-anol, and ethanol were examined. Although modest

increases in specific areas and catalytic activity couldbe obtained, selectivity toward the desired productsdecreased.

An entirely separate use of ultrasound for catalystpreparation involves the use of ultrasonic nebuliiation,followed by secondary gas-phase reaction to prepareTiOz photocatalysts [42]. Tic4 and diisopropoxy-bis-(acetylacetonato)titanium(IV) were employed for thepreparation of TiOz. Products obtained using thismethod gave photocatalytic activities for the degra-dation of 1,4-dichlorobenzene, similar to that of thecommercially available Degcssa P25 titanium(N)oxide.

8.6.2.3 Slllca, Alumhs, and Zeolites

There have been several recent reports by Zany&i andco-workers on the “sonocatalytic” production of silica“sonogels” [43-45]. Whereas the term “sonogel” isnew and appears to mean either a solve&containinggel or a gas-containing gel (a xerogel) made in thepresence of ultrasound, the processes used to makethem are similar to those tried 20 years ago, and theterm “sonocatalytic” is misused in this context. Silicasonogels have been obtained by the hypercritical dryingof gels from the hydrolysis of (EtO)&/HzO mixtureirradiated with ultrasound. In general, the xerogelsformed in this manner appear to have a liner porosityand greater reticulation of the network than un-sonlcated gels. Very recently zirconia sols and gels havealso been prepared sonochemically and studied bysmall-angle X-ray scattering (SAXS) with a synchro-troll source [46].

It is well known that alumina itself, acting as a solidacid or base, can be an active catalyst for a variety oforganic transformations. Perhaps as a consequence,considerably more work has been done on the effects ofultrasound on catalysis by alumina. Much of the workin this area has been from the group of Ando duringthe past decade [47]. Ando’s initial discovery was theimprovement made by ultrasonic irradiation of theliquid-solid two-phase synthesis of aromatic acyl cya-nides from acid chlorides and solid KCN in acetocitrile[48]. The extension of this reaction to benzyl bromidesled to an unusual observation of reaction pathwayswitching [49]. With mechanical agitation (i.e. stirring),the reaction of benzyl bromide and KCN in aromaticsolvents, catalyzed by alumina, yields diarylmethaneproducts from Friedel-Crafts attack on the solvent (eq3); whereas with ultrasonic irradiation, one obtainsbenzyl cyanide (eq 4). Apparently, the ultrasonic irra-diation of alumina deactivates the Lewis acid sitesnormally present that are responsible for the Friedel-Crafts reactivity. It is thought that this poisoning isaccomplished by the added solid basic salts (e.g. KCN)

with ultrasound, perhaps through solid-solid contactsor through increased access of dissolved bases to thealumina surface.

C6HsCH2Br + C~HSCH~ + KCN

3 CaHsCHz-C.&W& (3)

CsHsCHzBr + CaH&H3 + KCN

--$ c&CH+2N

This is a generalizable class of reactions catalyzed byalumina in the presence of ultrasound. For example,another application is the sonocatalysis of aldol con-densations by alumina. Substantial improvements inyields were observed, with greatly diminished reactiontimes, for several ketones 1471. In the same vein, a use-ful synthesis of cc-aminonitriles, which are importantintermediates in amino acid synthesis, has been re-ported that sonicated carbonyl compounds with salts ofamines and potassium cyanide iti acetonitrile in thepresence of alumina under solid-liquid two-phase con-ditions [50].

Although some investigations on the effects of ultra-sound on almninosilicates and their syntheses havebeen published, this area remains relatively unexplored.The best characterized study is that of Lindley [Sl],who examined the sonochemical effects on syntheses ofzeolite NaA. Severalfold reductions in nucleation timeand rates of formation during hydrothermal synthesiswere monitored by X-ray diffraction. Scanning electronmicrographs showed significant changes in morphol-ogy, as well, with ultrasound producing a more agglo-merated product made up of much finer crystallites inthe few-micron region.

Finally, the effect of ultrasound on gas-solid heter-ogeneous catalytic decomposition of cmnene to ben-zene and propylene was examined with a silica/aluminacracking catalyst where the entire reaction bed wassubjected to ultrasound [52]. Rate improvements of160% were observed. Because cavitation cannot occurin such a system, these results must come simply fromimproved mass transport between the gas and surface.

8.6.2.4 Supported Metal Catalysts

The use of ultrasound on the preparation of supportedmetal heterogeneous catalysts has been examined pri-marily for hydrogenation and related catalysts, usuallyinvolving the noble metals. For example, ultrasonic ir-radiation during the deposition of Pt on silica producesan 80% increase in Pt dispersion [53]. Several recentJapanese patents make we of ultrasound to improvethe dispersion and reliability of supported noble metal

for fuel cells [54, 551. The general process describedinvolves the reduction of H#tCls in the presence ofa carbon carrier, often colloidal, in the presence ofultrasonic irradiation.

The use of ultrasound to improve the formation ofmetal-containing gels has been reported. A patent wasissued for a sonochemical process for preparing Zr-containing aluminoxanes [56]. Upon sonication of atoluene solution of (CH&Al and CpzZrClz with water(Cp = cyclopentadienyl), an aluminoxane gel wasformed that was an active catalyst for oligomerizationof 1-octcne. Rhodium catalysts have been dispersed ontitania/silica aerogels [57]. Ultrasound was utilized intwo different preparations. In the tirst case, a “sono-gel” was obtained by hydrolysis of Ti and Si alkoxidesin the absence of alcohol with ultrasonic irradiation,which was then impregnated with a rhodium nitratesolution. In the second, a mixture of the alkoxides anda rhodium nitrate aqueous solution were exposed toultrasound, thus leading to a ternary Rh/TiOzSiOz so-nogel. The behavior of these catalysts was comparedwith that of a Rh/TiO#iOz system obtained by con-ventional impregnation methods, starting from a com-mercial silica support. The samples prepared by im-pregnation of preformed aerogels give high levels of Rhdispersion and show an increase in catalytic activity ofroughly tenfold for the hydrogenation of benzene.

Another application of ultrasound to supportedmetal catalysts alters the reactivity of already formedcatalysts. Boudjouk and co-workers examined the ac-celeration of hydrosilation reactions of alkenes andalkynes catalyzed by Pt/C [58, 591. Various substrates,including l-hexene, styrene, and phenylacetylene, workeffectively even at 243 K with various silanes (includingHSiC13, HSiClzMe, HSi(OGH&, and HSi(GH&).Catalyst concentrations of O.Olmol% are s&icient.Because the effect of the ultrasound on the carbonsupport is the generation of a very fine colloidal SWpension, separation of products from catalyst by filtra-tion is not possible, which defeats one of the primaryadvantages of heterogeneous catalysis.

These researchers extended the we. of this system forhydrogenation of alkenes using formic acid as a hy-drogen transfer agent [60]. Palladium on carbon wasused to catalyze hydrogenations of various alkenes, in-cluding terminal and internal alkenes, a diene, a vinylether, and an @unsaturated ketone, in very highyields at room temperature and atmospheric pressure.In this case, liltration was still effective for removal ofthe catalyst. Sonic&ion of the reaction mixtures awel-erated the reactions; the effects were not quantified, butseemed no better than the effects of heating to reflex.Han continued this work using hydrazine in place offormic acid as the redo&ant [61]; again, yields werecomparable to those obtained by refloxing the mixture.

Loss of activity during extensive use is a cotmn~n

industrial problem with any catalyst, but especiallywith supported metal catalysts. The deactivation pro-cess will vary depending on the catalyst and conditionsof its use, and includes the deposition of carbonaceouscontaminants, coking, the oxidation of metal surfaces,neutralization of surface acid sites, etc. There is anextensive patent literature over the past 20 years de-scribing the use of ultrasound to regenerate spent cata-lysts. Although the mechanism of action has not beenexamined, it is likely that improved mass transportand increased tie-pore penetration arc significantcontributors.

An early disclosure of the use of ultrasound to re-activate a deactivated hydrocarbon conversion catalystgoes back to Exxon Research and Engineering in 19781621. Highly deactivated hydrocracklng catalysts, whichhad resulted from successive use and conventional re-generation, could be reclaimed by oxidizing the cata-lyst at elevated temperatures followed by ultrasonic ir-radiation of the catalyst in a nonreactive liquid. In anexample, a conventional Ni/Mo hydrocracking catalyston an amorphous support was treated with air at783K, then suspended in low-viscosity white oil andtreated with ultrasound.

A variety of similar applications of ultrasound toclean or reactivate various catalysts have also beenreported. The most common carrier/cleaning liquidphase has been either aqueous [63] or standard feed-stock flow. Commercial noble metal catalysts sup-ported on alumina used either for NO, removal or hy-drogenation of hydrocarbons have been regeneratedefficiently with ultrasound [64,65] with nearly completerestoration of specific surface arca, porosity, and ac-tivity. In the same manner, substantial regenerationhas been disclosed for deactivated industrial oxideTiO2/VzOs catalysts for oxidation of o-xylene tophthalic anhydride [66] and for flue gas denitration[671. Ultrasonic reactivation is also useful for a par-tially &activated BF&apbite intercalatc catalystused in an alkylation process [68].

8.6.2.5 Polymerization Catalysts

There are only a few examples of the use of ultrasoundto modify catalysts for polymerization. The first was ina 1961 patent [69], which found a substantial decreasein catalyst particle size and a consequent increase inactivity due to diminished aggregation [70]. A furtherinvestigation of Ziegler-Natta polymerization underhigh-intensity ultrasound of styrene using a TiCb/I&Al catalyst has been recently published [71]. Thepolymers are produced in better yield and with morecontrol over the molecular weight distribution than inthe conventional, unsonicated process. For example,polystyrene produced under the same conditions with

stirring yields polydispersities above 10, while withultrasound they are 2.5, with comparable mean mo-lecular weights of ~50000. In part, this may be due topreferential cleavage of the longer chains by the ultra-sound [72].

A European patent has recently described anotherapplication of ultrasound to polymerization catalystpreparation [73]. By spraying catalyst precursor sol-utions through an ultrasonic nozzle into an inert gas,small droplets are formed which solidify into catalystmicroparticles. Thus, molten MgClz 3.5CzHsOH at403 K was fed through an ultrasonic atomizer at5 kg h-’ into a cooled Nz atmosphere, yielding uniformoval or spherical particles with 3040 q diameters.

8.6.3 Concluding Remarks

In principal, ultrasound is well suited to industrialapplications. Since the reaction liquid itself carries thesound, there is no barrier to its .use with large vol-umes. In fact, ultrasound is already heavily used in-dustrially for the physical processing of liquids, suchas emulsification, solvent degassing, solid dispersion,and sol formation. It is also extremely important insolids processing including cutting, welding, cleaning,and precipitation.

ultrasound has already become a common labora-tory tool for nearly any case where a liquid and a solidmust react. The production of heterogeneous catalystsinvolves high value-added materials, where processingcosts are not always economically limiting. In thiscontext, ultrasound is a viable method for the prepa-ration and treatment of heterogeneous catalysts. Theability of ultrasound to create highly reactive surfaces,to improve mixing even in viscous media, and to in-crease mass transport makes it a particularly promisingtechnique to explore for catalyst preparation, activa-tion, and regeneration.

References

1. K. S. &slick, Science 1990,247, 1439.2. K. S. Suslick, in Encyclopedia of of MaleridsMalerids Science Science andand En-

oineerina 0T.d.: R. W. Calm). Peramon Press, Oxford, 1993,5, Su&:, pp. 2093-20?%. ” -

3. K. S. Swlick (ed.), [Ntraso~ Ifs Chemical, Physical, andBio/ogica/ E&c@, VCH, New York, 1988.

4. T. J. Mason (ed.), Aduances in Sonochemistry; JAI Press,New York, 1990-1993 Vols 1-3.

5. K. S. Suslick. D. A. Hammerton, R. E. Cline Jr., J. Amer.Chem. SK. 1986,108, SMl.

6. E. B. Flint, K. S. Suslick, Science 1991,253, 1397.7. K. S. Swlick. K. A. Kemper in Bubble Dynamics and Inter-face face Phenomem (Ed.: J. R. Blake, N. Thomas) Kluwer,Dordrecht, 1994, pp. 31 I-320.

8. W. Iauterbom, Ii. Belle, J. Fluidh’ech 1975, 72, 391.

9. C. M. plrecc. I. L Ham.wn, Ado. Me& Phys. SW/: 1981,1,

10. K. S. Suslick, S. J. Doktycz, Science 1990,247, 1067.Il. K. S. &lick S. J. Doktyn in Advances in Sonochemisrry

(Ed.: T. 1. ifam) JAI Press: New York, 1990; vol. 1,pp. 197-230.

199.

12. K. S. Suslick, M. L. H. Green, M. E. Thompson, K. Chata-kondu. J. Chem. Sot.. Chew. &mm. 1987,900.

13. J. Lb&y, T. J. M&n, J. P. Lorbner, (Ntrasonics 1987, 25,45.

14. A. N. Z. M&w, Fiz Khim, 1976950, 1641-52.IS. K. S. Suslick, Modern Synthetic Methods 1986, 4, 1.16. C. &horn, J. Einhom, J. L. Luche, Synthesis 1989,787.17. J. L. Luche, Ulm.wdcs 1992,30, 156.18. K. S. Suslick, D. J. Casadonte, J. Am. Chem. Sm. 1987,109,

3459.19. K. S. Swliok, S. J. Doktya, J. Amer. Chem. Sm. 1989, 111,

2342.20. K. S. Suslick, S. J. Doktya, Chemistry o/ M&rials 1989,

l(I), 6.21. H. B. Wiener, P. W. Young, J. AppL Gum. 19% 336.22. G. Samcm, F. Ammo, Chim Ind 1968,50, 314.23. K. J. Moulton Sr. S. Koritala, K. Warner, E. N. Frankel, J.

A m Oil Chem. SW 1987,64,542.24. M. Tashiro, M. Nakayama. II. Nakamum, Y. Aoki, A.

Takigawa, K . Maeda, I . Ta80, M . Yoshida, Jpn KokaiTokkyo Koho, Jpn Patent 61053228, A2, 17 Mar 1986.

25. E. A. Cioffi, W. S. Willis, S. L. Suib, Langmuir, 1988,4,697.26. E. A. CiotB, W. S. Willis, S. L. Suib, Langmuir 1990, 6, 404.27. A. Tai, T. Kikukawa, T. Sugimura, Y. Inow, T. Osawa,

S. Fuiii. J. Cbem. Sot.. Chem. Comma 1991. 795.28. Y. tiaktyama, A. Hayakawa, Y. Kurtw, J. Chem Sm..

them. commm 1992, 1102.29. P. Boudjouk, Commenrs Inorg. Chem 1990.9.123.30. P. D. Boudjouk, Eur. Pat. Appl.. EP 306177, Al, 8 Mar

1989.31. J. Ito, M. Sakumi, Jpn Khi Tokkyo Koho; Jpn Patent

01227360, A2, 11 Sep 1989; H. Itch, I!. Kikwhi. Y. Morita,Sekiyu Gokkalrhi 1987,30, 324.

32. E. Kikwhi, H. Itoh, Sekiyu Gakkuishi 1591.34, 407.33. B. T. Chang, S. J. Kim, P. B. Hong, Chm Left. 1989,805.34. K. S. &lick, S. B. Choe, A. A. Cichowlas. M. W. Grinstaff,

Nalure 1%X,353,414.35. M. W. GrinstaE. M. B. Salamon. K. S. Suslick, Phys. Rev. B

1993.48, 269.36. R. EIeUissent, G. Galli, M. W. Grinstaff, P. Migliardo, K. S.

Sdic!c, Phys. Rev. B 1993,48, 15 797.37. K. S. Suslick, M. Fang, T. Hyeon, A. A. Cichowlas, M&c-

ularly Designed Nanosrructured MaIeriais (Ed: K E. Gon-sabm) Marls. Rex Sac., Vol. 351. Pittsburgh, 1994,201.443.

38. K. S. &slick, M. Fang, T. Hyeon, A. A. Cichowlq Mat/.Sci Eng. A, 1995, in press.

39. T. S. Popov, D. G. Klisurski, K. I. Ivanov. J. Pesheva, StwLSurjY sci. Catal. 1987,31, 191.

40. A. V. Romenskii, I. V. Popik, A. Y. Loboiko, V. I. As-troshenko, Kbbn. Tekchnol. (Kiev) 1985,3, 17, 21, 23.

41. E. N. M&y, V. L. Starchevskii, Adv. Sonochem 1993, 3,257.

42. W. Lee, Y. M. Gao, K. Dwight, A. Weld, Mafer. Res. Bull.

43. E. Blanw, N. De la Rosa-Fox, L. Esquiviaa, A. F. Craievich,J. Non-Cryst Solids 1992, 1471148, 296.

44. N. De la Rosa-Fox, L. Esquivias, A. F. Craievich, J. Zar-zycki, J. Non-Cryst Solids 1989, 121,211.

45. J. Zarzycki U/tras:ruct. Processing Ado. Mafl (Pd.: D. R.Ubbmum, D. R. Ulrich,) Wiley, NY, pp. 135-148.

46. D. Cbaumont, A. Craievich, J. Zany&i. J. Non-Cryst Solids1992, 1471148, 41.

1992.27, 685.

4 1 . 4 1 . T. Ando, Adv. Sonochem. 1991,2,211.48. 48. T. Ando. T. Kawate. J. Yamawaki. T. Hanafusa. Svnthesis

1983,63i.1983,63i.49. T. Ando, S. Simm, T. Kawate, J. Icbihara, T. Hanafusa, J.

Chew. Sot., Chem Commun 1984,439.50. T. Hanafusa, J. Icbiham, T. Ashida, Chm LetI. 1981. 4,

Ia7__,51. J. Lb&y, Uimmnics 1992,30, 163.52. W. Linmer. D. Han&m, Ulr*osonics 1977, 15, 21.53. V. I. Shekhobalova, L. V:Voronova, Vesm. h4osk Univ.,

Ser. 2: 2: Khtm. 19% 27, 321.321.54. T . Nakamura. Jon. K&al Tokkvo Koho, Jm P a t e n t

63319050, A2, ilil &c 1988.55. N. Yanagihm, I. Kunio, U. Makoto, T. Mieko, Jpn. Kokai

Tokkyo K&o, Jpn Patent 02051865, A2,21 Feb 1990.56. G. W. Schcentbal, L. H. Slaugh, US Patent; US 4 1 3 0 0 1 1 , 84 1 3 0 0 1 1 , 8

MarMar 1 9 8 8 . 1 9 8 8 .5 1. 51. M. A. Cauqui, J. J. calvino, G. Cifredo, L. Esquivim, J. M.

Rodriguez-Iquierdo, J. Non-Crysf. Solids 1 9 9 2 , 1471148,1 5 8 .1 5 8 .

5 8 . 5 8 . B. II. Han, P. Boudjouk, Organwet. 1983,2,169.59. D. G. Jam. D. H. Shin. B. H. Han. J. Korean Chem. Sm.

1991,35,1991,35, G5.G5.60. P. Boucjouk, B. H. Han, J. Catal. 1983, 79,489.6 1 . 6 1 . D. H. Shin, B. Ii. Han, Bull. Korean Chem. Sot. 1985,6,X7.62. H. C. Henry, R. Rimganathan, US Patent, US 4086184, 25

63. 63. M. Honda. Jon. Kokni Tokkvo Koho, Jpn Patent 03000135Apr 1918.

AZ,AZ, 1 1 Jan i961.64. A. V. Romenskii, A. Ya. Loboiko, V. V. Tsimbmvskii, Kh*n

Tekhnol. (Kiev) 1989,6,94.65. 65. G. Wachholz. G. Thelen. II. W. Voecs. Eur. Pal. AmL. EP

4 1 2 8 5 3 , 4 1 2 8 5 3 , Al, 4 Mar 1992:- . .

6 6 . 6 6 . S. Slavov, V. Niiolov, Gewog. Kafai.: Srh, Pt. I, 1983, 81.67. Mitsubishi Heavy Industries Ltd., Jpn Kokai Tokkyo Koho,

Jpn Patent 58186445, AZ, 31 Get 1983.68. P. G. Rodewald, USPatent, US 4914256, 3 Apr 1990.69. T. S. M&es, US Patent, US 2968652, 1962; Chem Absfr. 57,

ZA”6C.10. 10. N. Kobayashi, K. Abe, Jpn K&i Tokkyo Koho, Jpn Patent

62291302,A2,2462291302,A2,24 DeeDee 1981. 1981.1 1 . 1 1 . G. J. price, M. R. Daw, N. J. Newcomte, P. F. Smith, Brir.

Polymer 1. 1990,23,63.12. A. M. Basedow. K. H. Ebcrt, Adv. Polym Sci.. 1977,22,83.13. 13. J. Koskinen, I. Patti, Eur. Pat. AppL, EP 424049. AZ, 24

Apr 1991._I _.._.. -...

Figure 1. Cavitation near B liquid-solid interfam. High-@microcinematographic sequence of laser-induced cavitation near asolid surface, showing the formation of a microjet impact; 15 000frames 8-I. The sequence is from left to right, top to bottom; thesolid boundary is at the bottom of each frame. Photograph cowtesy of W. Lauterbom; reproduced with permission [8].

FigureFigure 2. 2. scanning electron micrograph of 5m diameter znpowder. Neck formation from localized melting is cawed byhigh-velocity interparticle collisions. Similar micrographs andelemental composition maps (by Auger electron spectmscopy)of mixed metal collisions have also been made. Reproduced withpermission [lo].

Figure 3. 3. The elikt of ultmsonic irmdiation on the surfacemorphology and particle size of Ni powder. Initial particle diam-eters before ultrasound were Q 160~; after ultrasound, * 80 flm.

High-velocity interparticle collisions caused by ultrasonic irradi-ation of slurries are responsible for the smoothing and removal ofpassivating oxide coating. Reproduced with permission [11].

Figure 4. Scanning electron micrograph of amorphous nano-structured iron powder produced from the ultrasonic irradiationof Fe(CO)5. Reproduced with permission [34].