Enantioselective Chemisorption and Reactions on Model Chirally

Mar 21, 2008 - Mausumi Mahapatra , Luke Burkholder , Yun Bai , Michael Garvey , J. Anibal Boscoboinik , Carol Hirschmugl , and Wilfred T. Tysoe...
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J. Phys. Chem. C 2008, 112, 6145-6150

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Enantioselective Chemisorption and Reactions on Model Chirally Modified Surfaces: 2-Butanol on L-Proline Templated Pd(111) Surfaces Feng Gao, Yilin Wang, and Wilfred. T. Tysoe* Department of Chemistry and Biochemistry, and Laboratory for Surface Studies, UniVersity of WisconsinsMilwaukee, Milwaukee, Wisconsin 53211 ReceiVed: January 16, 2008

The adsorption of R- and S-2-butanol is studied on L-proline modified Pd(111) surfaces. It is found that L-proline modifies the chemistry of 2-butanol such that it predominantly adsorbs and desorbs reversibly with a few percent reacting to form 2-butoxide species. The chemisorptive enantioselectivity ratio, defined as the S-2-butanol coverage divided R-2-butanol coverage, varies with the amount of L-proline on the surface and reaches a maximum of ∼1.2 at an L-proline coverage of ∼0.2. The enantioselectivity of 2-butoxide hydrogenation by adsorbed deuterium is measured and reaches a maximum of ∼1.9, also at an L-proline coverage of 0.2 indicating that both reaction and adsorption occur enantioselectively.

1. Introduction We have shown previously that enantioselective chemisorption occurs on Pd(111) in ultrahigh vacuum (UHV) when the surface is chirally modified by R- or S-2-butanol, where enhanced chemisorption of propylene oxide of the same chirality as the modifier is found over a narrow 2-butanol coverage range.1,2 More recently, similar behavior was found on Pt(111).3 In such studies, since PO adsorbs and desorbs molecularly, our primary goal was to investigate the enantioselective chemisorption behavior of R- and S-PO on chiral 2-butanol templated surfaces. The ultimate goal is to understand enantioselective catalysis, for which enantioselective chemisorption of the reactant is only one step and is followed by other, possibly enantioselective, reactions steps. In this case, the total reaction enantioselectivity will be a product of the enantioselectivities of adsorption and subsequent reactions. Coadsorbed 2-butanol and propylene oxide have been studied on a Au/Pd(111) alloy where alloy formation modifies the surface chemistry of both propylene oxide and 2-butanol, allowing enantioselective reactions to be followed.4 However, since both adsorbates decompose, it is difficult to cleanly follow enantioselective reactions. To address this problem, 2-butanol is used as a probe on a surface templated by an R-amino acid, L-proline. The adsorption, desorption, and reaction of L-proline have recently been investigated on Pd(111) using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS).5 Briefly, proline is stable on Pd(111) to slightly above room temperature and is present almost exclusively in its zwitterionic form following room temperature adsorption. In addition, adsorption at 300 K leads to adsorption only into the first monolayer and the proline coverage can be readily determined using CO titrations due to the fact that CO selectively adsorbs on the bare palladium surface but not on top of adsorbed proline. Since 2-butanol reactions occur below room temperature, proline + 2-butanol provides an ideal combination for exploring both enantioselective chemisorption and subsequent reactions. 2-Butanol can undergo a number of reactions initiated by deprotonation to form 2-butoxide.1,2 This can, in principle, * To whom correspondence should be addressed. E-mail: [email protected].

undergo β-hydride elimination to generate 2-butanone, complete dissociation to form H2, CO, and deposit carbon on the surface, or rehydrogenation to reform 2-butanol. 2. Experimental Section Temperature-programmed desorption (TPD) data were collected in an ultrahigh vacuum chamber operating at a base pressure of 8 × 10-11 Torr that has been described in detail elsewhere,6 where desorbing species were detected using a Dycor quadrupole mass spectrometer placed in line of sight of the sample. The temperature ramp and data collection were controlled using LabView software. All TPD spectra were recorded using a heating rate of 6.5 K/s in this study. This chamber was also equipped with a double-pass cylindrical mirror analyzer for Auger spectroscopy measurements and an ionsputtering gun for sample cleaning. The Pd(111) substrate (1 cm diameter, 0.5 mm thick) was cleaned using a standard procedure, which consisted of cycles of argon ion bombardment (2 kV, 1 µA/cm2) and annealing in 4 × 10-8 Torr of O2 at 1000 K.1 The cleanliness of the sample was judged using Auger spectroscopy and oxygen titration, where O2 instead of CO desorbs following O2 adsorption when the sample is carbon free. Following each TPD experiment, the surface is briefly annealed once again in O2 to regain the cleanliness. L-Proline was adsorbed on the Pd(111) surface using a homebuilt source, which was differentially pumped using a turbomolecular pump that can be isolated from the main chamber. Proline powder (Aldrich, 99% purity) was stored in a stainless steel vial. The whole evaporation source was warmed by means of a heating tape, and temperature was measured by a K-type thermocouple attached to the outer wall of the vial. Proline was typically outgassed for at least 2 h at 350 K before adsorption at the same temperature. Proline was dosed onto Pd(111) using a glass tube that was placed ∼1 cm from the sample to avoid contaminating other parts of the chamber. A dosing rate of 0.022 ML/min was measured using CO titration when the Pd sample is held at room temperature.5 2-Butanol enantiomers (Aldrich, 99%) were further purified by cycles of freeze-pump-thaw before use. D2 (Matheson, g99.5%) was used as received.

10.1021/jp800428m CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008

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3. Results 3.1. 2-Butanol Adsorption on L-Proline Templated Surfaces. Since both the surface chemistry and the enantioselectivity of 2-butanol can be modified by the presence of L-proline, experiments were carried out to measure both the reaction products and the way in which the adsorption or reaction is modified when adsorbing both R- and S-2-butanol on L-prolinemodified Pd(111) surfaces. Previous studies have revealed that a variety of desorption products are formed from L-proline decomposition on Pd(111)5 including hydrogen (2 amu), carbon monoxide (28 amu), carbon dioxide (44 amu), and a number of nitrogen-containing molecules due to the total thermal decomposition of the adsorbate. It is emphasized that molecular proline and its dissociation products desorb exclusively at above 300 K. A survey experiment performed following co-adsorption of L-proline and 2-butanol revealed that the only additional desorption product was 2-butanol. Especially, no 2-butanone formation was detected since no desorption signal was measured at 72 amu (due to 2-butanone) below 300 K. At high L-proline coverages, weak desorption at 72 amu does occur between 400 and 500 K, but that is due to ring-opening products of proline (probably butanamine).5 Figure 1 displays the resulting 2-butanol (59 amu) profiles measured following R/S-2-butanol adsorption on surfaces precovered with various amounts of L-proline adsorbed at room temperature, where the proline coverages are displayed adjacent to each spectrum. As mentioned above, L-proline coverages were measured using 13CO titrations.5 2 L of R/S-2-butanol was adsorbed at a sample temperature of 200 K to saturate the (partially covered) surfaces and to completely exclude multilayer adsorption1-3 thus facilitating the following peak area integrations to measure the adsorbate coverages on the surface. Figure 1, panels a and b, displays results with S- and R-2-butanol, respectively. Several general trends are noted for these desorption profiles. First, the desorption peak areas decrease with increasing proline exposure, indicating that L-proline blocks the adsorption of 2-butanol, and second, a slight desorption temperature decrease is found with increasing proline exposure. Figure 2a presents the resulting enantioselectivity ratio plotted as a function of proline coverage where each data point is measured from the ratio of the desorption peak areas of S- and R-2-butanol, and the solid line is a Gaussian fit to the experimental data as a guide to the eye. It is found that at every L-proline coverage more S-2-butanol desorbs from the surface than R-2-butanol, and the largest enantioselective ratio of ∼1.2 occurs at a L-proline coverage of ∼0.2 ML. Shown as an inset is the corresponding desorption curves of R- and S-2-butanol at a L-proline coverage of 0.17 ML. This highlights the fact that both the desorption peak areas and desorption temperatures of S-2-butanol are higher than for R-2-butanol. Figure 2b displays the temperatures of the desorption peak maxima for R/S-2-butanol versus L-proline coverage. The adsorption of 2-butanol is weakened slightly by the presence of L-proline, but the effect is slightly less for S-2-butanol than R-2-butanol, in accord with the measured enantioselectivity (Figure 2a). Similar results were found for 2-butanol adsorbed at 150 K (data not shown). In this case some multilayer adsorption occurs on the surface so that the monolayer and multilayer features must be separated during data analysis but these results serve to emphasize the reproducibility of this behavior. As noted above, the adsorbates thermally decompose on the surface. In order to follow the total decomposition products, desorption at 2 (H2), 28 (CO), and 44 (CO2) amu was monitored following co-adsorption of L-proline and R/S-2-butanol. Figure

Figure 1. (a) S-2-Butanol and (b) R-2-butanol (both at 59 amu) temperature-programmed desorption profiles collected following exposure of Pd(111) surfaces covered with L-proline to 2 L (1 L ) 1 × 10-6 Torr s) of 2-butanol as a function of L-proline coverage, where the coverages are displayed adjacent to the corresponding spectra.

3 plots an example of such results at an L-proline coverage of ∼0.2 ML (which exhibits the largest chemisorptive enantioselectivity). For these desorption profiles, features below 300 K are due to mass spectrometer ionizer fragmentation of 2-butanol, and only the high-temperature features are due to decomposition products. By comparing the desorption yields for R- and S-2butanol at each mass, less than 5% difference in peak areas is found. Identical results are found at other L-proline coverages. This strongly suggests that the chemisorptive enantioselectivity found above is not due to enantiospecific R-2-butanol decomposition. Moreover, the desorption peak areas are almost

2-Butanol on L-Proline Templated Pd(111) Surfaces

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Figure 3. 2 (hydrogen), 28 (CO), and 44 (CO2) amu desorption profiles following exposure of 0.17 monolayers of L-proline on Pd(111) to 2 L of S-2-butanol.

Figure 4. 2 (hydrogen), 3 (HD), and 4 (D2) amu desorption profiles following exposure of various coverages of L-proline on Pd(111) to 2 L of S-2-butanol, where the L-proline coverages are displayed adjacent to the corresponding spectra.

Figure 2. (a) Plot of the enantioselectivity ratio, defined as Θ(S-2butanol)/Θ(R-2-butanol), where Θ indicates the coverage measured from the integrated area of the TPD profile, plotted as a function of L-proline coverage. (b) Corresponding plot of peak desorption temperatures for R-2-butanol (9) and S-2-butanol (b) from L-proline-covered Pd(111) as a function of L-proline coverage.

identical to those for 0.17 ML L-proline alone. In particular, carbon dioxide is not found for 2-butanol decomposition on clean Pd(111) so that this arises exclusively from L-proline.5 This suggests that the decomposition of chemisorbed 2-butanol, even if exists, must be extremely weak. These results indicate that the surface chemistry of 2-butanol is modified by the presence of even relatively low L-proline coverages and that the major desorption product is 2-butanol.

3.2. Effects of Coadsorbed Deuterium. There are two possible pathways for 2-butanol formation, either by desorption of chemisorbed 2-butanol itself or by hydrogenation of 2-butoxide species. It has been shown previously that 2-butanol deprotonates to form 2-butoxide on Pd(111) at 150 K, and this has been verified using reflection-absorption infrared spectroscopy (RAIRS).1,2 This question is addressed by coadsorbing D2 on the surfaces, where, in principle, 2-butoxide reacting with adsorbed deuterium to form C2H5CH(OD)CH3 will provide information regarding the 2-butoxide formation pathway. This also allows us to explore enantioselective reactions in a controlled manner. Figure 4 presents the desorption profiles at 2 (H2), 3 (HD), and 4 (D2) amu from surfaces precovered by various amounts of L-proline at room temperature, following which 10 L of D2 and then 2 L of S-2-butanol are adsorbed at 150 K. Note that almost identical results are obtained when using R-2-butanol. Single D2 desorption peaks (at 4 amu) are evident at all L-proline coverages, where D2 starts to desorb at ∼220 K and the desorption peak maximum is found at ∼320 K. Note also that the D2 yield decreases rapidly with increasing L-proline

6148 J. Phys. Chem. C, Vol. 112, No. 15, 2008 coverage suggesting that proline strongly blocks the adsorption of D2. In contrast, HD (3 amu) starts to desorb between 250 and 290 K (and the desorption temperature increases with increasing L-proline coverage) with a desorption peak maximum located at between ∼345 and 370 K. Note also that weak HD desorption is also found above 500 K. Bear in mind that there are three possible sources of surface H for HD formation: background adsorption, 2-butanol, or L-proline decomposition. Finally, the weak H2 (2 amu) desorption profiles display three regimes. A signal below 300 K is due to 2-butanol fragmentation in the mass spectrometer ionizer, and signals also are evident between 340 and 400 K, and above 500 K. It should be emphasized that the most intense hydrogen desorption occurs above 300 K. Figure 5a depicts the desorption profiles of 2-butanol (measured at 45 amu, also due exclusively to 2-butanol and the most intense fragment) for surfaces covered by both L-proline and D2 in an experiment identical to that shown above, that is, following adsorption of L-proline at room temperature where the Pd sample is allowed to cool to 150 K and exposed to 10 L of D2 to ensure saturation, and then to 2 L of R- or S-2butanol. It should be noted that adsorbing 2-butanol at 150 K would inevitably allow a small amount of multilayer to adsorb (manifest by the broadening of desorption profiles at high proline coverages). However, as will be shown below, this does not profoundly affect the enantioselectivity measurements. Again the data in Figure 5a reveal that, for both 2-butanol enantiomers, the desorption yield and peak temperatures decrease with increasing proline coverage. Figure 5b displays the resulting enantioselective ratio on the deuterium-covered surface as a function of proline coverage. The close similarity between these data and Figure 2a immediately reveals that co-adsorption of deuterium has no effect on enantioselective adsorption of 2-butanol. An inset to Figure 5b displays the desorption profiles of 2-butanol enantiomers at a proline coverage of 0.17 ML and reveals the enrichment of 2-butanol with the same chirality by ∼20%. This result suggests that the data in Figure 2 are for 2-butanol adsorption and are not affected by subsequent surface reactions. To further address whether enantioselective reactions occur on these surfaces, 2-butoxide hydrogenation is studied by examining the formation of C2H5CH(OD)CH3. The most intense mass spectrometer ionizer fragment of C2H5CH(OD)CH3 is at 46 amu (due to DO-+CH-CH3 formation). However, hydrogenated 2-butanol itself has a fragment at 46 amu with an intensity that is 2.7% of the 45 amu signal measured using our mass spectrometer. Thus, in order to monitor only C2H5CH(OD)CH3, the 2-butanol contribution must be subtracted as shown in Figure 6a. Here, the 45 amu profile is reduced to 2.7% of its original intensity (spectrum 1) and subtracted from the 46 amu desorption profile (spectrum 2), resulting in a profile due only to C2H5CH(OD)CH3 desorption (spectrum 3). Note that desorption at these masses is not due to L-proline and/or its dissociation products. The resulting C2H5CH(OD)CH3 desorption profiles are plotted in Figure 6b as a function of L-proline coverage showing that 2-butoxide hydrogenation does, in fact, occur but signal intensities, indicated by the vertical bars on each spectrum, are only a few percent of the total desorption (Figure 5a). The data reveal that the C2H5CH(OD)CH3 yield decreases drastically even for 0.06 ML of proline, and by a coverage of 0.33 ML, the signal intensity has almost disappeared due to the blocking of deuterium adsorption by L-proline (Figure 4). The results also show that more C2H5CH(OD)CH3 is formed

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Figure 5. (a) S-2-Butanol and R-2-butanol (both at 45 amu) temperature-programmed desorption profiles collected following exposure of Pd(111) surfaces covered with L-proline to 2 L of 2-butanol as a function of L-proline coverage, where the coverages are displayed adjacent to the corresponding spectra. (b) Plot of the enantioselectivity ratio, defined as Θ(S-2-butanol)/Θ(R-2-butanol), where Θ indicates the coverage measured from the integrated area of the TPD profile, as a function of L-proline coverage.

from S-2-butanol than R-2-butanol and the reactive enantioselectivity ratio for C2H5CH(OD)CH3 formation is plotted in Figure 7. For L-proline coverages between 0.12 and 0.22 ML, enantioselectivity ratios of greater than ∼1.9 are obtained. Note that these ratios are much larger than those presented in Figures 2a and 5b, even though the largest ratio always occurs at the same proline coverages. 4. Discussion Previous work has shown that L-proline adsorbs on Pd(111) at ∼300 K only in the first monolayer and that no second layer adsorption occurs. X-ray photoelectron spectroscopy results suggest that it forms zwitterions, but no information is available on the adsorption geometry or the distribution of L-proline on

2-Butanol on L-Proline Templated Pd(111) Surfaces

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Figure 7. Plot of the enantioselectivity ratio, defined as Θ(S-2-butanol)/ Θ(R-2-butanol), where Θ indicates the coverage measured from the integrated areas of the TPD profiles shown in Figure 6b, as a function of L-proline coverage.

Figure 6. (a) Illustration of the method used to obtain the d1-2-butanol desorption profiles from a hydrogen- and L-proline-covered surface exposed to 2 L of S-2-butanol. (b) S-2-Butanol and R-2-butanol temperature-programmed desorption profiles, obtained using the subtraction method shown in panel a, collected following exposure of Pd(111) surfaces covered with L-proline and deuterium to 2 L (1 L ) 1 × 10-6 Torr s) of 2-butanol as a function of L-proline coverage, where the coverages are displayed adjacent to the corresponding spectra.

the surface.5 It is stable to a little above 300 K so that 2-butanol desorption, either of intact 2-butanol (Figures 1 and 5a) or by 2-butoxide hydrogenation (Figure 6a), occurs on an intact, L-proline covered surface. The overall surface chemistry of 2-butanol is strongly affected by the presence of coadsorbed L-proline compared to clean Pd(111),1,2 where the vast majority adsorbs and desorbs reversibly and only a few percent forms 2-butoxide species, which rehydrogenate to form 2-butanol (Figure 6). Clearly, 2-butanol dehydrogenation to form 2-butoxide species is likely to be affected by the presence of pre-adsorbed hydrogen or deuterium. However, the similarity in the 2-butanol desorption profiles in the absence (Figure 1) and presence of (Figure 5a) coadsorbed hydrogen, and their similar enantioselectivity ratios (compare

Figures 2a and 5b), suggest that this is not a large effect. Finally, total decomposition pathways of 2-butanol are completely suppressed (Figure 3). This implies that dehydrogenation pathways are, in general, suppressed on the L-proline covered surface and that 2-butanol adsorbs almost completely reversibly with a small proportion forming 2-butoxide species that react with adsorbed deuterium to reform 2-butanol (Figure 6). This effect is in accord with the suppressed hydrogen adsorption on L-proline-covered surfaces (Figure 4). Since L-proline adsorbs as a zwitterion on Pd(111),5 no additional hydrogen adsorbs on the surface following exposure to L-proline so that this effect cannot be due to site blocking by hydrogen and must be due to an electronic modification of the Pd(111) surface by L-proline. Figure 2a shows that, at an L-proline coverage of ∼0.2 ML, the enantioselective ratio is ∼1.2 and that this is not substantially modified by the presence of coadsorbed deuterium (Figure 5). As argued above, this is due predominantly to the adsorption of molecular 2-butanol so that the chemisorptive enantioselective ratio is ∼1.2. This is in general accord with the desorption temperature differences found for R- and S-2-butanol (Figure 2b), although this difference persists for L-proline coverages greater than ∼0.4, while the enantioselectivity decreases. Such desorption temperature differences have been detected previously for intrinsically chiral single-crystal surfaces both experimentally7,8 and theoretically using density functional theory,9-12 and on chiral-molecule templated surfaces due to a stronger hydrogen-bonding interaction of one probe enantiomer (propylene oxide) with the (2-butanol templated) surface compared to the other.14 The reactive formation of C2H5CH(OD)CH3 (Figure 6b) occurs with a larger enantioselectivity ratio (∼1.9, Figure 7) than for the chemisorption of 2-butanol (∼1.2, Figure 2a) showing that enantioselective reactions can be probed using very simple model systems under UHV conditions. This indicates that the subsequent reaction between hydrogen and 2-butoxide species also occurs enantioselectively, with an enantioselectivity ratio of ∼1.6, giving rise to the overall ratio of ∼1.9. A possible

6150 J. Phys. Chem. C, Vol. 112, No. 15, 2008 origin for the larger enantioselectivity ratio for reaction than for chemisorption may be due to the stronger interaction between S-2-butanol and L-proline (Figure 2b) so that it remains on the surface to higher temperatures than R-2-butanol, facilitating hydrogenation to S-C2H5CH(OD)CH3. Certainly the maximum difference in desorption activation energy occurs over the same L-proline coverage range as the maximum in enantioselectivity ratio, in support of this notion. There may be other origins of such differences, such as the proline affecting the access of hydrogen to the 2-butoxy species. 5. Conclusions It is found that L-proline only relatively weakly modifies the chemisorption of 2-butanol probe molecules on Pd(111) surfaces showing an enantioselectivity ratio of ∼1.2. This is accompanied by slight differences (∼3 K) in 2-butanol desorption temperatures. In contrast, much higher enantioselectivity ratios are found for 2-butoxide hydrogenation, where values of ∼1.9 are found. A likely origin of this effect is the stronger heat of adsorption referred to above, which enantioselectively holds the reactant on the surface, thus enhancing the yield. Acknowledgment. We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of

Gao et al. Chemical Sciences, Office of Basic Energy Sciences, under Grant No. DE-FG02-03ER15474. References and Notes (1) Stacchiola, D.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2002, 124, 8984. (2) Stacchiola, D.; Burkholder, L.; Tysoe, W. T. J. Mol. Catal. A: Chemical 2004, 216, 215. (3) Lee, I.; Zaera, F. J. Phys. Chem. B 2005, 109, 12920. (4) Gao, F.; Wang, Y.; Li, Z.; Furlong, O.; Tysoe, W. T. J. Chem. C., in press. (5) Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. Surf. Sci. 2007, 601, 3579. (6) Kaltchev, M. G.; Thompson, A.; Tysoe, W. T. Surf. Sci. 1997, 391, 145. (7) Horvath, J. D.; Gellman, A. J. J. Am. Chem. Soc. 2001, 123, 7953. (8) Horvath, J. D.; Gellman, A. J. J. Am. Chem. Soc. 2002, 124, 2384. (9) Sholl, D. S. Langmuir 1998, 14, 862. (10) Sholl, D. S.; Asthagiri, A.; Power, T. D. J. Phys. Chem. B 2001, 105, 4771. (11) Power, T. D.; Asthagiri, A.; Sholl, D. S. Langmuir 2002, 18, 3737. (12) Power, T. D.; Sholl, D. S. Top. Catal. 2002, 18, 201. (13) Lee, I.; Zaera, F. J. Am. Chem. Soc. 2006, 128, 8890. (14) Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240.