Increase in Activity and Selectivity in Catalysis via Surface

Imke Schrader , Sarah Neumann , Anda Å ulce , Fabian Schmidt , Vladimir Azov , and Sebastian Kunz. ACS Catalysis 2017 7 (6), 3979-3987. Abstract | Ful...
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Increase in Activity and Selectivity in Catalysis via Surface Modification with Self-Assembled Monolayers Zhihuan Weng and Francisco Zaera* Department of Chemistry, University of California, Riverside, Riverside, California 92507, United States S Supporting Information *

ABSTRACT: Selectivity has become a critical consideration in the design of catalysts, but increases in selectivity are typically achieved at the expense of overall activity. Here we report on a unique case where the addition of alkyl thiol self-assembled monolayers to colloidal platinum nanoparticles leads to significant improvements in both activity and selectivity during the hydrogenation of α-keto esters with cinchonidine as a chiral modifier. These catalytic improvements may be explained by a kinetic effect in which the cinchonidine residence time on the surface is increased by the thiol self-assembled layers. More nuanced compromises between activity and selectivity are involved when using supported catalysts, but reasonable performances are still possible with Pt/Al2O3 catalysts treated with cinchonidine-derivatized thiols. Indeed, these all-heterogeneous catalysts can promote the α-keto ester hydrogenation reaction with almost the same enantioselectivity as the best naked supported Pt catalyst without the need to add cinchonidine to the reaction mixture. They are among the most enantioselective all-heterogeneous catalysts reported to date.

1. INTRODUCTION As the use of catalysis is extended to more complex chemical systems, the issue of chemical selectivity often prevails over that of overall chemical activity.1,2 Avoiding the formation of undesirable side products prevents the waste of chemicals, minimizes the need of purification steps, and makes the processes more environmentally friendly. Therefore, it is often worth sacrificing speed for the sake of selectivity in chemical industrial processes. In order to improve selectivity, undesirable sites on the surfaces of heterogeneous catalysts are often blocked via pretreatments with specific chemicals; this is what is done, for instance, with hydrocarbon reforming catalysts, which are exposed to small amounts of H2S before introducing the crude oil feeds. In that case, it has been established that sulfur selectively blocks the surface sites for hydrocarbon hydrogenolysis, affecting those for skeletal isomerization to a much lesser extent.3 It has also been shown recently that selectivity can be improved for other reactions by using self-assembled (SAMs) capping monolayers.4−8 Unfortunately, as stated above, selectivity in those cases comes at the expense of reactivity. Here we report on a unique case where both activity and selectivity can be enhanced by surface modification, for the enantioselective hydrogenation of α-keto esters promoted by platinum catalysts.9,10 This is accomplished by combining the selective site-blocking power of alkyl thiol SAMs with the enantioselectivity provided by cinchona alkaloids as surface comodifiers.9,11 In colloidal solutions, SAM modification of Pt nanoparticles leads to significant improvements in both activity and enantioselectivity. With supported catalysts a trade-off is required between activity and selectivity depending on the concentrations used for the surface modifiers, but even there © 2014 American Chemical Society

conditions were identified leading to total activities and enantiomeric excesses quite close to those obtained with the best cinchonidine-modified Pt/Al2O3 platinum catalysts. In fact, the performance of our cinchonidine-derivatized thiol-SAMsmodified catalysts ranks among the best ever reported for allheterogeneous catalysts in terms of enantioselectivity.

2. MATERIALS AND METHODS 2.1. Materials. Cinchonidine (>98% purity), 1-propanethiol (99%), 1-hexanethiol (95%), 1-nonanethiol (95%), 1-hexadecanethiol (92%), 1,3-propanedithiol (99%), 1,6-hexanedithiol (96%), 1,9-nonanedithiol (95%), ethyl pyruvate (98%), and all solvents were purchased from Sigma-Aldrich and used as supplied. 1 wt% Pt/SiO2 and 1 wt% Pt/Al2O3 powder catalysts were also purchased from Sigma-Aldrich and used as supplied. Hydrogen gas (>99.995%) was purchased from Liquid Carbonic and used as received. 2.2. Catalyst Preparation. 2.2.1. “Naked” Pt Nanoparticles (NPs).12 A 0.5 M NaOH solution in ethylene glycol/H2O (12.5/2.5 mL) was added to a second ethylene glycol solution (12.5 mL) containing H2PtCl6·6H2O (24.6 mg, 50 μmol) while stirring under an inert atmosphere until obtaining a transparent yellow solution; the solution was heated to 433 K for 3 h under a N2 flow to remove all water and organic byproducts, after which a transparent dark brown homogeneous colloidal solution of the Pt NPs was obtained. These “unprotected” Pt NPs were dispersed in ethanol, without Received: December 17, 2013 Revised: January 23, 2014 Published: February 6, 2014 3672

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Figure 1. Activity (in %conversion after 24 h of reaction) and enantioselectivity (%enantiomeric excess, %ee) of Pt-nanoparticle-based catalysts for the hydrogenation of ethyl pyruvate in the presence of cinchonidine (Cid), which in these experiments was added in the reaction mixture for chiral modification. (a) Contrast in performance among “naked”, sodium sulfide-treated, and alkyl thiol-treated nanoparticles. Three alkyl thiols were usedC3H7SH, C9H19SH, and C16H33SHand constant 1:3:9 S:Cid:Pt ratios were used in all cases. (b) Activity and enantioselectivity of C9H19SH-treated Pt nanoparticles as a function of the ratio of cinchonidine to platinum used. A constant 1:27 thiol:Pt ratio was used in all samples. (c) Activity and enantioselectivity of C9H19SH-treated Pt nanoparticles as a function of the ratio of the thiol (C9H19SH) to platinum used. A constant 1:3 Cid:Pt ratio was used in all samples. Reaction conditions: 1.25 mmol of ethyl pyruvate, 20 bar of H2, 5 μmol of Pt, 10 mL of toluene (solvent), 300 K.

CH2Cl2 or toluene while stirring under N2; the thiol (alkanethiol or cinchonidine-derivatized thiol, amount determined by the desired S:Pt ratio) was added and let to react for 2 h; the mixture was filtered, washed with CH2Cl2 several times, and dried under vacuum overnight. 2.3. Infrared Absorption, TEM, and XPS Characterization. Infrared absorption spectra of the catalysts were taken in transmission mode by using a Bruker Tensor 27 Fourier transform IR (FT-IR) instrument. Some samples were diluted with KBr powder to facilitate the making of the solid sample disks. Transmission electron microscopy (TEM) images were taken using a Tecnail T12 instrument. X-ray photoelectron spectroscopy (XPS) data were acquired by using a Kratos analytical AXIS instrument equipped with a 165 mm mean radius semihemispherical electron energy analyzer and a 120element delay line detector. A monochromatized Al-anode Xray gun was used as the excitation source, and an electron flood source was employed as needed to compensate for sample charging. 2.4. Catalytic Tests. The hydrogenation of ethyl pyruvate was tested by using a 300 mL high-pressure liquid-phase Parr reactor. The Pt catalyst (5 μmol) was suspended in 10 mL of toluene, to which the rest of the compounds, namely, the ethyl pyruvate (1.25 mmol) and cinchonidine (Cid; amount determined by desired Cid:Pt ratio), were added. The reactor was closed and pressurized with 20 bar of H2; the reaction mixture was continuously stirred, and an external buret was used to keep the internal pressure constant during reaction. The reactions were carried out at room temperature. Liquid aliquots were taken periodically for analysis by gas chromatography (Agilent 6890N equipped with a CP Chirasil-Dex CB, 25 m × 0.25 mm × 0.25 μm, chiral column) to follow the progress of the reaction. The conversion and enantioselectivity excess values reported here were reproducible within 10% and 5%, respectively. The ethyl pyruvate (Sigma-Aldrich, 98% purity)

using any stabilizer, and the solvent removed prior to their use for the hydrogenation reactions. 2.2.2. Derivatized Alkyl Thiols. 0.1764 g of cinchonidine (0.6 mmol) was dissolved in 15 mL of CHCl3 and refluxed under N2 for 0.5 h; a solution of AlBN (60 μmol, 9.9 mg) and 1,x-alkanedithiol (1,3-propanedithiol, 1,6-hexanedithiol, or 1,9nonanedithiol, 6 mmol) in 6 mL of CHCl3 was added over 1 h via a dropping funnel while refluxing, which was continued for 24 h; the solvent was then removed under reduced pressure, and pentane (15 mL) was added to the residual yellow sticky solid to precipitate the crude product. The solid was collected and washed with pentane, dried under vacuum overnight, and purified by silica chromatography. Product identification was corroborated by 1H NMR, 13C NMR, FT-IR, and liquid injection field desorption ionization-mass spectrometry (LIFDI-MS). 2.2.3. Thiol-Covered Pt Nanoparticles. An ethylene glycol solution of “naked” Pt nanoparticles (50 μmol) was mixed with a 20 mL of toluene solution of the thiol (1-propanethiol, 1hexaethiol, 1-nonanethiol, or 1-hexadecanethiol, amount determined by the desired S:Pt ratio) and stirred overnight; 50 mL of water was added, after which a black precipitate (thiol-coated Pt NPs) appeared in the two-phase liquid; the black solid was collected via centrifugation, washed with toluene and ethanol to remove the excess thiol and residual glycol, and dried under vacuum overnight at ambient temperature. 2.2.4. Homemade Supported Catalysts. Catalysts were made by using both silica and alumina supports. Pt-NPs/SiO2 was prepared by dispersing the “naked” Pt nanoparticles (1 wt %) in ethanol and adding the Aerosil 200 support, filtering the mix, and drying under reduced pressure. A similar procedure was followed to disperse the thiol-covered Pt nanoparticles. 2.2.5. Thiol-Covered Supported Catalysts. A fixed amount of the Pt/SiO2 or Pt/Al2O3 supported catalysts was dispersed in 3673

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catalyst.19,20 In this context, we propose that the enhancement in reaction rate seen here upon the addition of thiol SAMs to the surface of the Pt nanoparticles may be due to an increase in the adsorption equilibrium constant of cinchonidine, which may be held in the vicinity of the surface for longer times by the alkyl chains of the thiol adsorbates. Unfortunately, this is a hypothesis difficult to test because it is not easy to measure the cinchonidine coverages on the surface in situ in catalysts covered with SAMs layers and immersed in the liquid reaction solution. Moreover, our explanation is an oversimplification because it has also been established that the performance of cinchonidine as a chiral modifier also depends on the adsorption configuration it adopts on the surface21 and on the freedom it has to rearrange around the quinoline− quinuclidine linking moiety.11 Regardless, the important empirical observation here is that the thiol SAMs improve both the activity and the enantioselectivity of this reaction, by about an order of magnitude each, with solution-suspended Pt nanoparticles. Incidentally, it could be thought that the role of the thiols in these systems may be to stabilize the platinum nanoparticles in solution, since the initial catalysts are based on unprotected particles. However, no aggregation of those particles was ever seen in our experiments, not even in the absence of the thiol SAMs (Supporting Information, Figure S1); the “naked” nanoparticles seem to be stable by themselves under the conditions of the reactions probed here. The behavior of the thiols responsible for the improvement of catalytic activity and enantioselectivity reported above was investigated in more detail. It was determined that the observed catalytic performance enhancement cannot be due exclusively (or even primarily) to the addition of sulfur on the surface, since treatment of the platinum nanoparticles with sodium sulfide does not lead to any appreciable changes in catalytic performance (Figure 1a). In fact, short-chain thiols, C3H7SH in particular, are also not effective modifiers. It is clear that the alkyl chains play an important role in the changes in catalytic performance. This is in contrast with the recent case of the use of thiol-modified Pd catalysts for the selective hydrogenation of unsaturated epoxides, where the sulfur atoms were shown to be the main responsible elements for the selectivity changes.4 It is suggested that a different mechanism is operative here, perhaps the reason why the increase in selectivity is not obtained at the expense of activity (which is enhanced as well). Some information on the collective ordering of the alkyl chains in the SAMs may be deduced from infrared (IR) absorption spectroscopy data. Figure 2 displays IR data in the C−H stretching region, together with Gaussian fits using wellestablished frequency assignments,22 for a number of thioltreated Pt nanoparticles, and Table S2 summarizes the main parameters extracted from analysis of the data. In Figure 2a, a study is presented of the evolution of the IR spectra for the C9H19S−Pt case as a function of dilution of the SAMs (expressed as a molar ratio between the thiol and the Pt precursor used in the sample preparation). The main observation is that an increase in thiol dilution leads to a decrease in the ordering of the alkyl chains. This is particularly evident by the almost stepwise change in frequency seen for the peaks associated with the methyl symmetric and Fermi resonance modes (νs(CH3) and νFR(CH3), respectively) between the 1:1 and 1:3 S:Pt cases: the positions of the peaks switch from approximately 2871 and 2895 cm−1 to 2875 and 2897 cm−1, respectively. These shifts are small but consistent, and outside the experimental error of our IR

was distilled under vacuum prior to its use to minimize any possible interferences from impurities.

3. RESULTS AND DISCUSSION 3.1. Pt Nanoparticles in Solution. The initial experiments were carried out by using platinum nanoparticles suspended in solution, to which cinchonidine was added for chiral modification. It was found that the original “naked” (or “unprotected”) Pt nanoparticles, prepared by reduction of chloroplatinic acid with ethylene glycol (TEM, XPS, and IR characterization provided in the Supporting Information in Figures S1, S2, and S3, respectively),12 display quite poor performance for the promotion of the hydrogenation of ethyl pyruvate to ethyl lactate (Figure 1a). The activities are much lower that those reported for other similar systems, although this may be due, at least in part, to the fact that we have here used lower H2 pressures,13,14 or perhaps because we have used the total number of Pt atoms, not only the surface Pt atoms, in our turnover frequency (TOF) calculations. In any case, both activity and enantioselectivity could be significantly enhanced by pretreating those nanoparticles with alkyl thiols (Figure 1a). Under similar reaction conditions, the reaction initial TOF (molecules converted per total number of platinum atoms per second, calculated at conversions below 20%) could be increased from about 0.004 s−1 with the naked Pt nanoparticles to approximately 0.03 s−1 with the nanoparticles treated with C16H33SH. A 100 %conversion is shown in Figure 1, but total conversion is reached in this case at a much earlier time; to better be able to compare activities for the more active samples, TOFs for all the samples in Figure 1a are provided in Table S1 of the Supporting Information. Enantioselectivity was improved as well, from less than 5 %enantiomeric excess (%ee) to more than 65 %ee with C9H19SH. The concentrations of both the cinchonidine (Figure 1b) and the thiol (Figure 1c) required for the catalytic hydrogenation reaction were then optimized by using the C9 thiol. Intermediate S:Cid:Pt molar ratios of 1:3:9 were found to be the best, leading to complete ethyl pyruvate conversion by 24 h of reaction at room temperature and an enantiomeric excess of 66.3 %ee. One possible explanation for the low activity of the naked Pt nanoparticles is that they may become poisoned early on in the reaction because of the deposition of EtPy, cinchonidine, or any products resulting from reactions of those on the surface of the metal. With Na2S, it may be that sulfur is the agent that poisons that surface. It could then be argued that the thiols may protect the surface from such poisons and therefore afford the retention of catalytic activity. However, thiols also bind through their sulfur atoms, and therefore sulfur poisoning similar to that seen with Na2S would be expected (but not seen) in those cases. Moreover, the changes in enantioselectivity observed in the series of catalyst reported in Figure 1 are not easy to explain purely on the basis of surface poisoning. An additional effect must be in play in these samplesone that we believe relates to a synergistic interaction between the thiol SAMs and the cinchonidine modifier. It is already well established that the addition of cinchonidine to the reaction mixture in these α-keto ester hydrogenation reactions not only adds enantioselectivity but also accelerates the reaction.15−18 Much discussion has been published on the reasons for this effect, and the subject is still a matter of some debate. Nevertheless, most explanations rely on kinetic arguments, and advocate for an effect that depends on the equilibrium of cinchonidine on the surface of the platinum 3674

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1a, it is concluded that a compromise may be needed in terms of the nature of the SAMs in order to induce sufficient alkyl chain disorder to allow for the penetration of the reactants while at the same time retaining enough interchain interactions to help shape particular adsorption sites on the surface of the metal. 3.2. Supported Pt Catalysts. The catalysts discussed above consist of platinum nanoparticles suspended in solution. In order to facilitate their handling and separation from reactants and products, it is usually desirable to use solid catalysts instead. With that in mind, a number of experiments were designed to prepare heterogeneous catalysts based on the same type of platinum nanoparticles covered with thiol SAMs. Several parameters were considered to optimize performance. In general, it was found that the use of silica as a support yields worst results than when using alumina. Regular Pt/SiO2 catalysts displayed low activities and only moderate enantioselectivities (when modified with cinchonidine added in solution), and that performance was found to deteriorate significantly upon the addition of small amounts of thiols (Figure S4). Better results were obtained when dispersing the platinum nanoparticles pretreated with the thiols on a fresh silica support, but even then the reaction conversion rates were low. Interesting, an optimum intermediate ratio of thiol to platinum of about 1:27 was identified for maximum performance, in qualitative agreement with the behavior seen with the colloidal Pt nanoparticles (even if the best ratio is lower in this case, Figure S5). Much better behavior was observed with Pt/Al2O3 catalysts. Regular supported Pt catalysts are in fact much more active than the free Pt nanoparticles for the α-keto ester hydrogenation reaction, displaying conversions close to 100% after ∼1 h of reaction under the conditions used in our experiments (approximately one day is required for comparable conversions with the Pt nanoparticles). Typical kinetic curves for these samples are provided in Figure S6. Moreover, the addition of cinchonidine to the reaction mixture also increases both activity and enantioselectivity, as discussed above for the case of the platinum nanoparticles (Figure 3, green bars). Other studies with colloidal Pt nanoparticles dispersed on alumina supports have yielded similar results as well.13,14,23 On the other hand, it was found that, although both activity and enantioselectivity can be modified on these Pt/Al2O3 catalysts upon the formation of thiol SAMs, it is not possible to increase both at the same time as with the free Pt nanoparticles (Figure 3, center rows). The main effect of adding SAMs to the platinum surface (and cinchonidine to the reaction solution) is to decrease the catalytic activity, an effect that is more significant with longer alkyl chains. Enantioselectivity follows the same trend, although the effect is less pronounced. In the preceding examples, the catalysts are not truly heterogeneous, because a chiral modifier (cinchonidine) still

Figure 2. Infrared (IR) absorption spectra in the C−H stretching region for thiol-treated Pt nanoparticles. Shown are the raw traces together with fits to five Gaussian peaks centered at the frequencies expected for thiol-based self-assembled monolayers (the assignment is provided in the right panel).22 (a) Spectra for C9H19S−Pt as a function of the ratio of C9H19SH to Pt. (b) Spectra for CxH2x+1S−Pt samples as a function of alkyl chain length (x = 3, 9, and 16). Data are also provided for the free C9H19SH and C16H33SH thiols for reference.

instrument. Similar frequency increases have been seen with alkyl thiols when transitioning from crystalline to liquid phase, and have also been ascribed to an increase in alkyl chain disorder.22 In addition, most of the peaks in Figure 2a become broader, by up to ∼50% in some cases, around the same dilution values. These observations correlate well with the sharp increases in activity and enantioselectivity reported in Figure 1c. It appears that a degree of alkyl chain disorder is required to provide access to the metal surface for catalysis. A similar increase in alkyl chain disorder is seen as a function of decreasing alkyl chain length in Figure 2b. This becomes particularly clear in the evolution of the peak for the methylene symmetric mode (νs(CH2)), the width of which transitions from 17 cm−1 with C16H33SH to 27 cm−1 with C3H7SH. However, the story may be more complex here, because there may also be changes in the orientation of those chains. To note in particular is the relative increase in the peak intensity ratio between the symmetric and asymmetric CH2 stretching modes with decreasing chain length. Those features, centered at 2853 and 2924 cm−1, respectively, have dynamic dipole vectors perpendicular to each other but both in the plane of the CH2 moieties. The symmetric CH2 stretching peak is usually weak in SAMs because the dynamic dipoles from the individual methylene moieties cancel out as the alkyl chains adopt an all-trans configuration. In our case, however, it appears that such all-trans arrangement is less stable with the shorter chains. Based on the catalytic performance trends reported in Figure Scheme 1

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Figure 4. The %conversion (top) and %ee (bottom) for the hydrogenation of ethyl pyruvate with thiol-treated 1 wt% Pt/Al2O3 catalysts after 5 min of reaction. Shown are data for the original catalyst as is, promoted by the addition of cinchonidine in solution (blue), and after being treated with cinchonidine-derivatized thiols ((Cid)CxH2xS−Pt) of three different alkyl chain lengths (x = 3, red, x = 6, green, and x = 9, purple). Data are also provided for various Cid:Pt molar ratios, displayed in the x-axis. No thiol-modified catalyst fully matches the best Cid-modified Pt/Al2O3 catalysts, but reasonable compromises can be found to allow for the use of a fully heterogeneous (Cid)CxH2xS−Pt/Al2O3 catalyst.

Figure 3. The %conversion (top) and %ee (bottom) for the hydrogenation of ethyl pyruvate with thiol-treated 1 wt% Pt/Al2O3 catalysts after 5 min of reaction. Shown are data for the original catalyst as is (green) and after being treated with regular (CxH2x+1SH, two left rows) and cinchonidine-derivatized ((Cid)CxH2xSH, right row) thiols of two alkyl chain lengths (x = 3, blue, and x = 9, red). Experiments with the regular thiols were carried out both with (center row) and without (left row) adding cinchonidine to the reaction mixture. A compromise needs to be reached between activity and enantioselectivity upon SAM modification of the catalyst, but acceptable performances are possible with some heterogeneous catalysts prepared this way.

pronounced, but again, increasing chain lengths were seen to lead to decreases in %ee. Interestingly, optimum enantioselectivity requires higher Cid:Pt ratios than those needed for high activity, around 1:6; the intermediate case of Cid:Pt = 1:12 with (Cid)C3H6S−Pt/Al2O3 may represent a compromise where activity is reduced by ∼50% while the loss in enantioselectivity amounts to only ∼20%. On the other hand, preserving enantioselectivity may be more important in many applications, in which case the 1:6 Cid:Pt ratio would be preferred. No (Cid)thiol-covered supported Pt sample in this family of catalysts can improve on the performance of the regular Pt/Al2O3 catalyst modified with cinchonidine added in solution, but some come close to that performance and offer the advantage of having the chiral modification already added to the solid. The molecular details of the catalysts prepared by adding cinchonidine-derivatized thiols were also characterized by infrared absorption spectroscopy. One concern in this case is that the organic ligands may bind to the metal via either of their two ends. It was determined that binding through the thiol moiety, as needed for the formation of the SAMs, still occurs in these cases. This is evidenced by the IR spectra in the S−H stretching region reported in Figure 5. The broad peak for that S−H stretching mode, seen around 2570 cm−1 with all free

needs to be added to the solution. As a final step, the original thiols were derivatized in order to add the cinchonidine to the end of their alkyl chains. For this, cinchonidine was appended via its vinyl moiety to one end of the appropriate dithiol according to a published protocol, shown in Scheme 1 (details are provided in the Materials and Methods section).24,25 Pt nanoparticles covered with SAMs made out of these Cidderivatized thiols proved to be poor catalysts for the ethyl pyruvate reaction (Figure S6). Catalysts made out by adsorbing Cid-thiol SAMs on Pt/Al2O3, on the other hand, displayed reasonable performance. Typical values for their activity and enantioselectivity are contrasted with those from the other types of catalysts in Figure 3 (the new data are provided in the right row), and results from a more detailed study of the effects of chain length and (Cid)CxH2xSH:Pt molar ratio are summarized in Figure 4. The main effect in these systems is seen in the conversion rate, which decreases rapidly when deviating from the ideal conditions, in particular when longer alkyl chains are used. Also, for a given Cid-derivatized thiol, optimum catalytic activity was seen for Cid:Pt ratios around 1:24. In terms of enantioselectivity, the effects were not as 3676

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Figure 6. IR absorption spectra in the hydrocarbon deformation mode region for catalysts prepared by using cinchonidine-derivatized (Cid)C9H18SH thiols. Reference spectra are also provided for comparison. The traces correspond, from bottom to top, to free C9H19SH (dark green), C9H19SH adsorbed on colloidal platinum nanoparticles (green), solid cinchonidine (blue-green), cinchonidine adsorbed on Pt/SiO2 (blue), cinchonidine adsorbed on silica (darker blue), cinchonidine adsorbed on a Pt foil (darkest blue), free (Cid)C9H18SH (dark red), (Cid)C9H18SH adsorbed on Pt nanoparticles (red), and (Cid)C9H18SH adsorbed on Pt/SiO2 (brick color). The cinchonidine interacts with both silica and Pt surfaces, as indicated by the shifts and relative intensity changes seen in the main peaks associated with the quinoline ring (see details in text).

Figure 5. IR absorption spectra in the S−H stretching mode region for catalysts prepared by using cinchonidine-derivatized alkyl thiols. Reference spectra are also provided for comparison. (a) IR traces for (Cid)C9H18SH by itself (bottom, blue) and adsorbed on colloidal Pt nanoparticles (second from bottom, magenta), on silica-supported Pt (third from bottom, purple), and on pure silica (fourth from bottom, red). Additional traces are shown for cinchonidine-free C9H19SH alone (second from top, green) and adsorbed on Pt nanoparticles (top, brown). (b) IR traces for Cid-derivatized thiols of three different lengths, (Cid)CxH2xSH (x = 3, bottom green and purple traces; x = 6, middle blue-green and dark red traces; x = 9, top blue and red traces), by themselves (bottom set) and adsorbed on Pt/ SiO2 (top set). In all cases, the disappearance of the S−H stretching mode peak at 2570 cm−1 corroborates the formation of thiol selfassembled layers on the Pt surfaces.

modes within the quinuclidine moiety, but those sometimes interfere with similar modes from the alkyl chains of the thiols (Figure 6, bottom two traces). All the cinchonidine vibrational modes tend to blue-shift upon adsorption on surfaces, and that is what is seen here with the (Cid)CxH2xSH thiols. However, the interaction between cinchonidine and the solid seems to be more moderate than that seen on pure platinum foils,21,26 and affected by the presence of the silica. For instance, the reference data in Figure 6 show that the quinoline deformation modes shift to 1599, 1581, and 1516 cm−1 on the Pt foil (Figure 6, sixth trace from bottom), but only to 1594, 1577, and 1513 cm−1 on Pt/SiO2 (Figure 6, fourth trace from bottom). Moreover, cinchonidine adsorbed on pure silica also displays peaks at 1593, 1577, and 1513 cm−1 (Figure 6, fifth trace from bottom). In a similar fashion, the values for those modes with the thiol-modified platinum catalysts used in this study (using C9 alkyl chains) are 1590, 1572, and 1509 cm−1 with the Pt nanoparticles (Figure 6, second trace from top) and 1513 cm−1 with the silica-supported catalyst (Figure 6, top trace; only the latter mode is visible in this case). It could be concluded from the discussion above that cinchonidine adsorption in our systems occurs primarily on the silica surface. However, clear evidence can be extracted from Figure 6 to conclude that the metal plays an important role in this interaction as well. The observation to highlight for this is the change in the relative intensities of the key quinoline IR peaks observed in the samples containing platinum. A surface dipole selection rule that applies to adsorption on metal surfaces indicates that only the component of the dynamic dipoles of molecular vibrations oriented perpendicular to the surface can be detected by IR absorption spectroscopy.27 Application of that rule to adsorbates on metal surfaces affords an estimation of their adsorption geometry.28,29 In our case, the orientation of the quinoline ring may be described qualitatively

thiols, disappears almost completely upon adsorption on the metal surfaces. (Some peaks are seen in that region of the IR spectrum for the RS-Pt samples, but those are not reproducible and quite weak.) The data in the left panel show that this is true for the colloidal platinum nanoparticles as well as for platinum particles dispersed on a silica support. Even on pure silica thiol adsorption leads to the loss of some of the S−H bonds, although the interaction in that case is weaker, and a small amount of intact S−H bonds can be detected still in spite of the large surface area of the oxide. The spectra reported in the right panel of Figure 5 indicate that thiol attachment via the scission of the S−H bond occurs with all thiols regardless of their alkyl chain length, although it may be that the reactivity with the shorter chains may be lower. (A small remnant of the IR signal for the S−H stretching mode is observed with (Cid)C3H6SH upon adsorption on the Pt/SiO2 catalyst.) The data in Figure 5 were obtained with the silica-based catalysts, but similar behavior was observed with those made with alumina supports. In order for the catalyst to display enantioselectivity, the cinchonidine moiety also needs to interact with the surface of the metal, and our IR spectroscopic studies show that this is indeed the case. It should be said that the interpretation of the data in these studies is not straightforward and that the observations obtained from ex-situ characterization experiments may not fully reflect the performance of these catalysts. Nevertheless, a number of lessons can be extracted from the appropriate IR spectra, some of which are reported in Figure 6. The most characteristic vibrational modes from cinchonidine in these spectra are those associated with the in-plane deformation of the quinoline aromatic ring, specifically the peaks seen at 1591, 1569, and 1508 cm−1 with solid cinchonidine (Figure 6, third trace from bottom).26 Additional peaks are also seen at 1463 and 1453 cm−1, associated out-of-plane CH2 scissoring 3677

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by comparing the relative intensities of the 1569 cm−1 (Ag) and 1508 (B3u) cm−1 modes, which have dynamic dipoles aligned with the short and long axis of the aromatic ring, respectively.30 The fact that the former peak is not visible in the samples involving the supported Pt particles (Figure 6, fourth trace from bottom and first trace from top) indicates that the ring in those cases interacts with the metal and that it is oriented with the long axis perpendicular to the Pt surface. This is not the case with the pure silica (Figure 6, fifth trace from bottom), and also not seen with the catalysts based on the colloidal Pt nanoparticles (Figure 6, second trace from top). Ultimately, the IR data show that the supported Pt nanoparticles may still become covered with SAMs when using the cinchonidinederivatized thiols but that the cinchonidine in those cases also interacts with the surface, most likely at the interface between the platinum metal and the silica support. Similar results were obtained with other chain lengths and with platinum catalysts dispersed on alumina (Figure S7).

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of Pt nanoparticles, XPS of C9H19S−Pt nanoparticles, IR spectra for thiol adsorption on Pt nanoparticles and for catalysts prepared using cinchonidinederivatized (Cid)C6H12SH thiols, initial TOFs from the samples in Figure 1a, summary of data from Figure 2, C9H19S−Pt/SiO2, (C9H19S−Pt)/SiO2, and (Cid)CxH2xS−Pt catalyst performances, kinetic data for EtPy conversion on C3H7S−Pt/Al2O3 and (Cid)C3H7S−Pt/Al2O3 catalysts. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (F.Z.). Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In summary, the data reported here show that it is possible to improve on the performance of platinum catalysts in terms of both activity and selectivity upon the deposition of alkyl thiol self-assembled monolayers. In the particular case of the use of platinum nanoparticles in liquid suspension for the hydrogenation of ethyl pyruvate, the optimum initial turnover frequencies obtained with the naked nanoparticles are ∼0.004 s−1, and the enantioselectivities (under the reaction conditions used in our experiments and when adding cinchonidine to the solution) are approximately 5 %ee. Addition of alkyl thiol SAMs to those nanoparticles was found to lead to significant increases in both values, to 0.02 s−1 and 66 %ee, respectively (with C9H19SH). Of course, the free Pt nanoparticles display quite a poor performance for this reaction, but our results still show how improvements in selectivity may be accompanied by concomitant improvements in activity in some cases where thiol SAMs are used to modify catalysts. This is not the case with supported Pt catalysts, where the initial performance is much better: the TOFs and enantioselectivities of regular Pt/Al2O3 catalysts (plus cinchonidine in solution) exhibit values around 60 s−1 and 74 %ee in our studies. Nevertheless, reasonable compromises can still be reached with supported Pt catalysts treated with cinchonidinederivatized thiols. For instance, most of the enantioselectivity can be preserved upon addition of (Cid)C3H6SH at a 1:6 Cid:Pt ratio (73 %ee), although the TOF does decrease in that case by a factor of about 4, to a value of 13 s−1. If activity is an important consideration, using a 1:24 Cid:Pt ratio can increase the TOF to approximately 55 s−1 with only a moderate decrease in enantioselectivity, to 58 %ee. In no case studied here the thiol-treated catalysts could outperform the regular Pt/ Al2O3 sample (plus cinchonidine in the liquid phase) in terms of both activity and enantioselectivity, but the modest losses associated with the examples cited above may be a reasonable price to pay for having an all-heterogeneous catalyst, where cinchonidine does not need to be added to the reaction mixture (and separated from the products). Certainly, these are among the most enantioselective all-heterogeneous catalysts ever reported.31,32 In more general terms, it has been shown here that the addition of two functionalities to solid catalysts via the deposition of specifically derivatized self-assembled monolayers can lead to improvements in both activity and selectivity.

ACKNOWLEDGMENTS Financial support for this research was provided by a grant from the U.S. National Science Foundation (NSF), Contract CBET1063595. XPS data were acquired with an instrument purchased with funds from NSF, Contract DMR-0958796.

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