Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

Abstract

•Thiol-treated ultrathin Pd nanosheets are used as a model catalyst•The catalyst exhibits both high activity and selectivity in alkyne semihydrogenation•Both steric and electronic effects contribute to the enhanced selective hydrogenation•Practical Pd catalysts for semihydrogenation of internal alkynes are prepared The development of next-generation catalytic materials requires a methodological shift from trial-and-error to mechanism-directed design. It is highly desirable to build model catalyst systems with simplified structures to ensure maximized utilization of both state-of-the-art characterization tools and computational chemistry methods. In this work, thiol-treated palladium nanosheets are adopted as a model catalyst for the selective semihydrogenation of internal alkynes. Unexpectedly, thiol treatment created highly selective palladium catalysts with high activity toward the semihydrogenation reaction. The ultrathin nature of the as-prepared catalysts allows for the application of a variety of surface science and computational methods to resolve the complexity of metal-organic interfaces and thus elucidate the underlying mechanism. Driven by atomic-level understanding, we have realized practical, lead-free catalysts for semihydrogenation. Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes. Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes. Selective hydrogenation lies at the heart of industrial manufacture of fine chemicals, pharmaceuticals, nutraceuticals, and agrochemicals.1Swiegers G.F. 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Such a sulfide/thiolate modification creates both steric and electronic effects to prevent the hydrogenation of alkene intermediates during alkyne hydrogenation, endowing the modified Pd surface with high catalytic selectivity toward alkenes. With such an understanding, we have developed a facile strategy for preparing highly selective and stable Pd catalysts for semihydrogenation of internal alkynes by simply treating commercial Pd catalysts with thiols. To create a model metal catalyst with metal-thiol interfaces that can be characterized in full, we chose ultrathin two-dimensional Pd NSs as the metal substrate for thiol surface modification in this work.40Huang X. Tang S. Mu X. Dai Y. Chen G. Zhou Z. Ruan F. Yang Z. Zheng N. Freestanding palladium nanosheets with plasmonic and catalytic properties.Nat. Nanotechnol. 2011; 6: 28-32Crossref PubMed Scopus (1279) Google Scholar The ultrathin feature offers a high fraction of surface Pd atoms and thus allows the extraction of Pd-thiol interface structure information by spectroscopic techniques that normally collect both surface and bulk signals. In this study, uniform hexagonal Pd NSs (1.8-nm thick) with a diameter of 80 nm (Figure S1A) were prepared by a CO-assisted method. The Pd NSs were then mixed with 3,4-difluorothiol (HSPhF2) in N,N-dimethylformamide (DMF) to induce thiol surface modification. The molar ratio of HSPhF2/Pd was 1:1. The mixture was kept at 60°C for 12 hr before being cooled down to room temperature and centrifuged for collection of the product (see Supplemental Information). The as-modified Pd NSs were denoted as [email protected]2 (1:1). As revealed by transmission electron microscopy (TEM) (Figure 1A), the hexagonal shape of the original Pd NSs was heavily deformed after the thiol treatment. The structure of the thiol-modified Pd NSs was characterized by various techniques. Lattice fridges of Pd 1/3(422) were clearly observed in the high-resolution TEM (HRTEM) image of Pd NSs (inset of Figure S1A), but it became impossible to distinguish lattice fringes on [email protected]2 (1:1) from random noise because of their poor crystalline nature. After the reaction with thiol, the color of the Pd NSs solution changed from blue to dark gray with the disappearance of the broad peak at 1,100 nm in the ultraviolet-visible (UV-vis) spectrum (Figure S1B), demonstrating that their ultrathin metallic feature was altered. Moreover, as shown in the X-ray diffraction pattern of [email protected]2 (1:1), the Pd(111) peak at 40° was dramatically broadened, and the other diffraction peaks became negligible as a result of the formation of palladium sulfide on the surface of Pd NSs (Figures S1C and S1D). Simultaneously, the poor crystallinity of [email protected]2(1:1) was observed in the HRTEM analysis. High-angle annular dark field scanning transmission electron microscopy and energy-dispersive X-ray (EDX) elemental mapping (Figures 1B and 1C) revealed that both Pd and S were uniformly distributed throughout the [email protected]2 (1:1). All these results demonstrate that the thiol modification heavily modulated the surface structure of Pd NSs. Because of the ultrathin nature of Pd NSs, such a structure modulation readily induced an obvious morphological change that was detectable by TEM. It would be impossible to directly visualize the structure changes caused by surface ligand modification if Pd nanoparticles were used as the metal substrate. The change in the structure and composition of Pd NSs is expected to influence their catalysis. Surprisingly, after the thiol treatment, Pd NSs readily served as a highly selective catalyst for the semihydrogenation of internal alkynyl compounds. When 1-phenyl-1-propyne was chosen as the model substance, as shown in Figures 1D and 1E, [email protected]2(1:1) exhibited an excellent selectivity of 98.1% toward 1-phenyl-1-propene at a conversion of 100% that was achieved within 50 min. In comparison, when the conversion of 1-phenyl-1-propyne reached 100%, the selectivity of the semihydrogenation product was only 30.5% over unmodified Pd NSs. In contrast, the Lindlar catalyst showed better selectivity than Pd NSs. But the main problem for the Lindlar catalyst is its low conversion rate. The conversion of 1-phenyl-1-propyne only reaches 25% within 70 min of the reaction. More impressively, no obvious decay in semihydrogenation selectivity was observed even after full conversion when [email protected]2(1:1) was used. The selectivity was maintained at ∼97% in the ten cycles of catalysis in which very little decay in the activity was observed (Figure 1F). In comparison, Pd NSs maintained good activity in the five catalysis cycles, but displayed poor selectivity toward the semihydrogenation product (Figure S2A). When the Lindlar catalyst was used, an obvious decay in activity was observed although the selectivity was high (Figure S2B). Overall, [email protected]2(1:1) exhibited much better performance in the semihydrogenation of 1-phenyl-1-propyne than both unmodified Pd NSs and the Lindlar catalyst. The enhanced catalytic selectivity motivated us to understand how the surface thiol modification modulates hydrogenation catalysis. The morphological transformation process of Pd NSs caused by surface thiol modification was first investigated by analysis of Pd NSs collected at different reaction times by TEM (Figures S3A–S3E). It was clearly revealed that the deformation process was initiated at the edges of the Pd NSs. Although only the edges became thicker at 0.5 hr, edge distortion was observed at 1.5 hr. The distortion process continued until 3 hr, beyond which no further morphological change was observed. Similarly, the optical absorption peak of the Pd NSs gradually decreased with the reaction time and disappeared after 3 hr (Figure S3F), consistent with the TEM observations. As revealed by the time-dependent EDX analysis (Figure S4), the sulfur content in the NSs increased with the reaction time and eventually reached ∼25%. Moreover, the final morphology of Pd NSs depended on the SPhF2/Pd ratio used in the modification process. As revealed by TEM and UV-vis analyses (Figure S5), whereas only edge deformation was observed with SPhF2/Pd ratios of 0.2 and 0.5, extensive distortion took place with the SPhF2/Pd ratio > 1. When the SPhF2/Pd ratio increased from 0.2 to 0.5 to 1 to 5 to 10, the S/Pd ratio in the modified NSs increased by 12%, 21%, 26%, and 30%–32% (Figure S6). The sulfur content was not linearly increased with the SPhF2/Pd ratio used in the modification. When the SPhF2/Pd ratio was increased beyond 1, the content of S did not increase much. The catalytic performance of different ratios of SPhF2/Pd is given in Figure S7. With an increased amount of thiol, the activity of the catalyst decreased and the selectivity increased. Interestingly, [email protected]2(1:1) exhibited both high reactivity and high selectivity. Considering the large size of SPhF2, its coverage on Pd cannot be high. The rather high S/Pd ratio on [email protected]2(1:1) suggested that S species should not be limited to SPhF2 only. In order to study the nature of S, we extensively characterized the as-obtained [email protected]2 NSs by various means. Although the presence of SPhF2 on Pd was confirmed by Fourier transform infrared spectroscopy (Figure S8), a temperature-programmed decomposition-mass spectrometry (TPD-MS) study illustrated that SPhF2 on Pd underwent S−C bond cleavage to release PhF2 upon thermal treatment (Figure S9). Considering that temperature could have a great influence on S–C bond cleavage, the reaction was also conducted at room temperature for analysis of the temperature effect. Compared with the product obtained at 60°C, only the edge of Pd NSs was crimped at room temperature (Figure S10A). Moreover, when Na2S was used to replace thiol, Pd NSs were heavily distorted as well (Figure S10B), indicating that S2− generated by C–S cleavage should be responsible for the deformation of the Pd NSs. The low activity and high selectivity of [email protected]2S(1:1) (Figure S11) suggested that the sulfide-treated Pd NSs were overpassivated by S2−. The X-ray photoelectron spectroscopic (XPS) data confirmed the co-presence of S2− and thiolate on the thiol-treated Pd NSs. The overall S/Pd ratio of [email protected]2(1:1) was estimated to be 27%, consistent with the EDX result. In the XPS spectra of S 2p (Figure 2A), two main sulfur components at 162.9 and 163.8 eV were assigned to S2− and SR−, respectively. The S2−/SR− ratio was estimated to be 3.6. The presence of S2− verified the S–C bond cleavage in the thiol modification process. Moreover, the binding energy of Pd 3d in distorted Pd NSs displayed a slight shift toward higher binding energy than that of unmodified NSs, indicating that Pd had been partial oxidized (Figure 2B). The peaks at 336.7 and 342.2 eV were assigned to Pd2+ 3d5/2 and Pd2+ 3d3/2, respectively (Figure S12). Moreover, the XPS spectra of different ratios of SPhF2/Pd showed that, with the increased amount of SPhF2, the ratio of S2−/SR− was increased and eventually converged to about 1:5 (Figure S13). We proposed that the cleavage of C–S began at the edge and then the generated S2− entered into the lattice of Pd NSs to form PdxS. With increasing HSPhF2, Pd NSs were eventually fully converted into PdxS, whereas their surfaces were saturated by thiolate groups such that the ratio of SPhF2/Pd reached a constant. To better understand the local environment of Pd, we further characterized the distorted Pd NSs by X-ray absorption fine structures (XAFS). Although Pd K-edge X-ray absorption near-edge structure spectrum (XANES) of [email protected]2(1:1) revealed the slightly oxidized nature of Pd (Figure 2C), the extended XAFS spectrum demonstrated that the coordination environment of Pd experienced a dramatic change upon thiol treatment (Figure 2D). Although the Pd-Pd coordination number was decreased from 9.7 to 4.7, a Pd-S scattering path appeared with the coordination number of 1.7. The Debye-Waller factor (σ2) of Pd-Pd correlation increased from 6.2 to 12.3 after the thiol treatment, indicating the increase of local disorder in the Pd lattice (Figure S14 and Table S1). These data demonstrate that sulfur atoms entered the first coordination shell around Pd atoms after the thiol treatment, yielding PdxS species. To further characterize the 3D distribution of Pd and S elements, we used high-sensitivity low-energy ion scattering spectroscopy (Figure S15). Even with the increased depth detected, enabled by Ar ion sputtering, the presence of sulfur species was still revealed, indicating the incorporation of S2− into the inner lattice of Pd. To gain insight on how the thiol treatment enhanced hydrogenation selectivity, we performed periodic density functional theory (DFT) calculations (see Supplemental Information for computational details). 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To be consistent with the experimental nomenclature, HSPhF2-modified Pd4S(110) and Pd3S(100) surfaces are denoted as Pd4[email protected]2 and Pd3[email protected]2, respectively. These two models had similar composition and chemical environment to those of [email protected]2(1:1) (Table S5). We used both models to explore the observed high catalytic selectivity in the hydrogenation of internal alkynes (Figure 3A). To understand why the catalytic selectivity of thiol-treated Pd NSs was better than the unmodified ones, we compared the hydrogenation of PhC≡CCH3 over Pd4[email protected]2, Pd3[email protected]2 and bare Pd NSs. We assumed that all sides of the slabs were saturated with hydrogen under the hydrogenation conditions.44Zhao X. Zhao Y. Fu G. Zheng N. Origin of the facet dependence in the hydrogenation catalysis of olefins: experiment and theory.Chem. Commun. 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