Langmuir 1988,4,697-702
697
Ultrasonically Induced Enhancement of Isotope-Exchange Catalysts: Surface Analysis of Raney Nickel Alloys Eugene A. Cioffi* Department of Chemistry, Yale University, New Haven, Connecticut 06511
William S. Willis Institute of Material Science, University of Connecticut, Storrs, Connecticut 06268
Steven L. Suib* Departments of Chemistry and Chemical Engineering and Institute of Material Science, University of Connecticut, Storrs, Connecticut 06268 Received July 10,1987. In Final Form: December 29,1987 UltrmnicatedNi-AI catalysts stereoselectively promote rapid 2Hincorporation into carbohydrates and glycoephingolipids under very mild conditions; the enhanced rates of isotopic exchange have been shown in a previous comparative kinetic investigation to result from ultrasonically induced alterations of the catalysts. An extensive examination of nonsonicated and sonicated Raney nickel alloys was conducted using X-ray photoelectron spectroscopy, Auger electron spectroscopy, Ar+ ion-sputtering, static secondary ion mass spectroscopy, and surface area techniques. The changes in the surface and bulk properties of thw catdpta by ultrasonic irradiation include a negligible increase in surface area, exposure and definition of distinct catalytic sites, removal of passivating impurities, and an elemental redistribution within the catalyst bulk.
Introduction Promotion of the exchange between an isotopic hydrogen donor and organic substrates remains an important goal in biosynthetic and synthetic organic chemistry and continues to engender major efforts in homogeneous and heterogeneous catalysts.' "Deuteriated" Raney nickel catalysts, prepared by the digestion of nickel-aluminum alloys with base followed by repeated washing with H 2 0 and final preexchange with D20, have been used extensively2to incorporate *H (and to a lesser extent tritium) into simple carbohydrates.3-6 Unfortunately, the site selectivity of this nickel-catalyzed exchange technique often varies considerably, being dependent upon the reaction conditions, the substrates employed, the amounts and age of the catalyst used, etc. Further, the high temperatures and long incubation times frequently result in the formation of undesired racemized and degradated byprodu~ts.~ A recent investigation" has shown that ultrasonic irradiation affords a significant amelioration in the ability of Raney nickel to promote selective hydrogen-deuterium exchange. Under the influence of ultrasound, deuterium is rapidly and stereoselectively incorporated into a model carbohydrate (1-O-methyl-&D-gala&pyranoside;c.f. 1 2) and a thermally sensitive glycosphingolipid at carbon
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HOCH,
H
1
HOCH,
OH
-2
atoms bearing vicinal hydroxyl groups. Further, the very mild reaction conditions of this technique aid in suppression of any attendant byproduct formation (in contrast to other investigationss). A subsequent kinetic investiga-
* Author to whom correspondence should be addressed.
tion? (employing 1 as a model compound) has shown that ultrasonication of the catalyst, either prior to or during the reaction course, substantially enhances the initial and overall rates of H/D exchange (as compared to that of a conventional stirred-only reaction). Noteworthy are the observations of a normal temperature dependency and the fact that individual acceleration factors are not equal at all molecular sites.? Furthermore, the kinetic behavior of the presonicated catalyst in a 'H 2Hexchange reaction is nearly identical with the behavior of reactions involving continuous ultras~nication.~ The successful application of ultrasonic irradiation to chemical systems has recently experienced a dramatic increase.8 In heterogeneous reactions, the efficacy of ultrasonication is well documentedgand is hypothesized to arise from a variety of sources. In particular, acoustic cavitation, increased bulk transport, localized "hot-spots", cavitational erosion (with large concomitant increases in surface area), and induced lattice deformations have all been proposed'O as being responsible for ultrasonically enhanced reactivity. On the basis of recent kinetic evidence,' the sonolytidy enhanced H/D exchange activity of Raney nickel was shown to be most closely associated
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(1) Cameron, D. G.; Martin, A.; Mantach,H. H. Science (Washington, D.C.) 1983,219, 180. (2) Koch, H. J.; Stuart, R. 9. Carbohydr. Res. 1977,59, C1. (3) Koch, H. J.; Stuart, R. S. Carbohydr. Res. 1978,64,127. Koch, H. J.; Stuart, R. S. Carbohydr. Res. 1978,67, 341. (4) Lockley, W. J. S. J. Chem. SOC.,Chem. Commun. 1986, 723. (5) Balza, F.; Perlin, A. S. Carbohydr. Res. 1982,121,270. Balza, F.; Cyr, N.; Hamer, G. K.;Perlin, A. S.; Koch,H. J.; Stuart, R. S.Carbohydr. Res. 1977,59, C7. Angyal, S. J.; Odier, L. Carbohydr. Res. 1983, 123, 13. (6) Cioffi, E. A.; Prestegard, J. H. Tetrahedron Lett. 1986, 27, 415. (7) Cioffi, E. A.; Prestegard, J. H. J. Org. Chem., submitted for pub-
lication. (8) Bremner, D. Chem. Br. 1986,633. Boudjouk, P. J. Chem. E d w .
1986, 63, 427. (9) Suslick, K. S. Mod. Synth. Methods 1986,4,1. Mason,T. J. Lab. &act. 1984, 33, 13. (10) Boudjouk, P.; Thompson, D. P.; Ohrbom, W. H.; Han, B-H Organometallics 1986,5,1257. Suslick, K. S.; Goodale,J. W.; Schubert,P. F.; Wang, H. H. J. Am. Chem. SOC.1983, 105, 5781. Suslick, K. S.; Johnson, R. E.J. Am. Chem. SOC.1984,106,6856. Ando, T.; Kawate, T.; Ichihara, J.; Hanafusa, T. Chem. Lett. (Jpn.) 1984, 725.
0143-7463/88/ 2404-0697$01.50/0 0 1988 American Chemical Society
698 Langmuir, Vol. 4 , No. 3, 1988
Cioffi et al.
with morphological alterations of the catalyst itself. In order to gain insight into the nature of the ultrasonically induced changes of the Raney nickel catalysts, we have undertaken an extensive examination of nonsonicated and sonicated catalyst samples typically used for 2Hlabeling by surface analysis techniques. X-ray photoelectron spectroscopy (XPS),Auger electron spectroscopy (AES), static secondary ion mass spectroscopy (SSIMS), and BET surface area measurements were used in this study to evaluate the surface composition and chemical states of the alloys. Argon ion sputtering was coupled with the Auger experiments to obtain depth profiles. XPS and Auger are surface-sensitive techniques, probing approximately the upper five atomic layers of the samples. Static SIMS sputters the surface with ions, yielding mass information of atomic and molecular fragments; with a probing depth of only one or two atomic layers, this technique is complementary to XPS and Auger spectrosCOPY. Experimental Section Materials. All solvents and reactants were purified as neceasary prior to use. b e y nickel (W-2 grade, purchased as a slurry from Aldrich Chemical Co.) was washed repeatedly to neutral pH and preexchanged with D20 as outlined.6 Instrumentation. Ultrasonication of the sample alloy was conducted by using a Bransonic Model W-200-P sonicator equipped with a titanium microtip immersed 1cm directly into the reaction mixture, as described previously? The acoustic intensities were -30 W cmm2at 20 kHz applied in a continuous mode. X-ray photoelectron spectra were collected on a Leyboldusing either Mg K a or A1 K a raHeraeus 10 system (LHS-10) diation (dual Mg/Al anode) a t a pressure < 5 X kPa. An initial wide-band survey of each sample was conducted in the constant relative resolution mode to qualitatively determine the overall surface composition." Subsequently,narrow-band surveys were performed in the constant absolute resolution mode to determine semiquantitative amounts of surface elements and accurately assign the binding energy positions of the respective surface elements. The spectrometer was calibrated to the Au 4f7/2 and the Cu 2pSl2transitions, set at binding energies of 830.8 and 932.4 eV., respectively. Auger electron spectroscopymeasurements were performed in the static mode utilizing the LHS-10 system; primary electron energies of 5 keV were employed in the spectroscopy. The sample chamber residual vacuum was less than 5 X kPa prior to all analyses. Depth-profiling experiments were conducted by repetitively monitoring the appropriate Auger transitions with simultaneous 5-keV Ar+ ion beam bombardment of the sample surface at an Ar partial pressure of -4 X lo4 kPa. Initially, the ion beam was rastered over a 8 mm X 8 mm area for 0.5 h and then incremented twice (for 0.5 h each) over 6 mm X 6 mm and 4 mm X 4 mm areas, respectively. Finally, the 4 mm X 4 mm surface area was sputtered for 1.5 h. Peak-to-peak heights of the Ni LaM,M, and the A1 KL2LBderivative peaks were measured to calculate relative intensities. Relative surface concentrations were calculated by using published elemental sensitivity factors; data acquisitionand manipulation were performed as described." Static secondary ion mass spectrometry(SSIMS)was performed by using a Balzers quadrupole mass spectrometer directly interfaced to the LHS-10 system. Ion beam voltages and emission currents were 5 KeV and 3 X A, respectively. An Ar+ ion source was used, with an ion beam current density dose < 2 x lo4 A cm-, at a pressure of 4 X kPa. the ion beam was rastered over a 6 mm X 6 mm surface area, and the mass spectrum was scanned at a rate of 3.0 s amu-'. Data processing was performed as described." Surface area determinations of the catalyst were conducted by measurement of adsorbed N2using the three-point Brunauer-
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(11)For a detailed summary of the surface analysis procedures, see: Willis, W. S.; Suib, S. L. J. Am. Chem. SOC.1986, 108,5657.
Table I. Kinetic Rates of 'H-*H Incorporationn relative rates condition site rates x lo6, s-lCsd (kilkz)' stirred CZ 1.2 f 0.6 1.0 f 0.0 25.2 f 15.6 21.0 f 3.3 c3 c4 25.0 f 20.6 20.8 f 9.0 presonicated c2 4.8 1.0 9.4 c3 45.3 c4 65.8 13.7 cont sonicb cz 6.2 f 0.1 1.0 f 0.0 c3 41.7 f 2.6 6.7 f 0.5 c4 41.2 f 1.8 6.6 f 0.1 Data reproduced from ref 7. Rates subsequent to induction period. CThecalculated initial rates utilizing the same catalyst batch. dThe deviations in the initial rates are representative of consecutive experiments run utilizing different catalyst batches. e Rates of individual exchangeable sites relative to the rates at C2. Emmett-Teller (BET)method." Typically, N, was adsorbed onto the catalysts at low temperature and the physisorbed Nz removed under vacuum. The chemisorbed N2 was then determined by heating the catalyst sample and monitoring the evolved N2 by chromatographic methods. Proton NMR spectra (for kinetic rate deter mint ion^^,^) were recorded at either 490 MHz on a home-built instrument or at 500 MHz on a Bruker WP-500spectrometer. Pulse widths of 90"were used as well as a pulse delay sufficiently long to allow for complete T1 relaxation between scans (-20 8). Typically, 64 scans were acquired per NMR experiment, and three experiments were performed on each sample. Average integrated peak areas6s7 internally referenced to the average areas of nonexchanging protons are considered accurate to a t least f3%. Procedures. Comparative kinetic rate determinations (involving stirred-only, presonicated, and stirred and continuously sonicated experiments) were performed as ~ u t l i n e d .All ~ sample manipulations were conducted under a dry Ar atmosphere. All surface area determinations and surface analyses were performed on the same catalyst batch. Sample washing typically invoIved addition of 15 mL of solvent mixture, swirling of the catalyst suspension for -3-5 min, centifugation, and careful removal of the bulk solvent supernatant by pipet (leaving 1mm of solvent mixture over the packed catalyst). The "deuteriated" catalyst (-20 g) was washed 3 times with a degassed 5 1 tetrahydrofuran-deuterium oxide (THF-D20) solvent mixture and divided equally into two test tubes. Each sample was washed 1additional time with the solvent mixture, both tubes were placed into a constant-temperature bath, and the samples were equilibrated to 40.0 "C under a dry, continuous Ar purge. One sample was irradiated for 1 h a t 40.0 "C under Ar as described previ~usly.~ After 1h, both samples were washed 3 more times with additional solvent mixture and kept cold (
