Control experiments and mechanistic studies
A tentative mechanism for the dehydrogenation and hydrogenation reaction, i.e., interconversion between 1a and 2a, using the Pd3Au1/CNT catalyst is illustrated in Fig. 2A. For the dehydrogenation reaction, 1a is adsorbed on the Pd3Au1(111) surface (I in Fig. 2A), activating the N─H and C─H bonds and leading to the liberation of the first H2 and the formation of intermediate 1a″ (II in Fig. 2A). Next, the intermediate in 1a″ undergoes tautomerization to afford 2a″. A second dehydrogenation step takes place, involving the same N─H and C─H bonds, to complete the process with the release of the second H2 and the formation of 2a (III in Fig. 2A and fig. S4F).
To substantiate the proposed mechanism, a number of control experiments were performed. No conversion was observed under dehydrogenation conditions using 1,2,3,4-tetrahydronaphthalene, 1-methyl-1,2,3,4-tetrahydroquinoline, or 2,2,4,7-tetramethyl-1,2,3,4-tetrahydroquinoline as the starting materials (Fig. 2B). These experiments all indicated that a N─H bond is essential for the reaction to proceed and indirectly supports the tautomerization step. Isomerization of C═N to C═C appears to be rapid, as 1a″ was not observed and the dehydrogenation of 1,2-dihydroquinoline (2a″) was very fast (fig. S4C). The dehydrogenation of indoline-d2, used as a model compound, was studied in situ by 1H NMR spectroscopy, confirming that the tautomerization step as indoline-d2 was quantitatively converted to indole with 20% of the N-D product resulting from the tautomerization process (Fig. 2C and scheme S2). Moreover, the formation of HD was also observed (33).
To further understand the differences in catalytic performance between the Pd and Pd3Au1 NP catalysts, DFT calculations were carried out. The (111) surface of both catalysts was chosen for all simulations as the structural data strongly suggest that it is the main facet orientation [Fig. 1 (B and G) for Pd3Au1 and fig. S2A for Pd]. The mechanism for the hydrogenation of quinoline involves sequential addition of H to the nitrogen and carbon atoms of quinoline. However, there are several possibilities for the first H addition, i.e., to the N atoms or to the C atoms of quinoline. We therefore calculated the energy barriers of the first H addition to N and the different C atoms and found that the formation of the N─H bond is the most favorable pathway with the lowest energy barrier. Next, for the addition of the second H to the partially hydrogenated quinoline, we calculated the energy barriers for H addition on the C atoms, allowing the most favorable pathway for the second H addition to be identified. Following this procedure, we identified the most favorable pathway for the hydrogenation. Other potential pathways include ring opening via C─C bond scission, which was not considered in this work because no such products were detected experimentally.
Figure 3A depicts the potential-free energy diagrams for the dehydrogenation of 1a at 140°C. The Pd3Au1(111) surface leads to lower-energy barriers for each dehydrogenation step compared to the Pd(111) facet. The energy barriers of the four dehydrogenation steps are 0.91, 0.57, 0.85, and 0.25 eV on Pd3Au1(111) compared to values of 1.15, 0.92, 1.07, and 0.36 eV on the Pd(111) surface. Here, the energetic span model (34) was applied to compare the differences in activity of the two catalysts, which provides an elegant way to evaluate the TOF for a reaction. In this model, only one transition state, i.e., the TOF-determining transition state (TDTS), and one intermediate, i.e., the TOF-determining intermediate (TDI), determine the TOF in catalytic cycles. The TDTS-TDI energy difference and the reaction driving force (∆Gr) define the energetic span (δE) of the reaction, and a lower δE indicates higher activity. On the basis of the Gibbs free energy diagram in Fig. 3A, the δE values for the dehydrogenation of 1a on the Pd3Au1(111) and Pd(111) surfaces are 2.04 and 2.69 eV, respectively. This indicates the superior activity of the Pd3Au1 catalyst in dehydrogenation reactions, which is in line with the experimental data.
In addition, the reaction mechanisms for the selective hydrogenation of 2a to 1a on the Pd3Au1 (111) and Pd(111) surfaces were also calculated, and the associated free energy profile is shown in Fig. 3B. As expected, initial hydrogenation takes place at the C═N bond to afford intermediate 2a′, which is subsequently converted to 2a″. The energy barrier for the hydrogenation of the C═C in 2a″ is much lower than that of the C═N bond in 2a, and in a control experiment, it was shown that 2a″ is quantitatively hydrogenated to 1a in 3 hours (fig. S4D). Notably, the Pd3Au1(111) surface leads to lower-energy barriers than those observed on the Pd(111) surface for each of the elementary step, due to modulation of the electronic properties of the active Pd atoms by the Au atoms and the CNT surface. Furthermore, the δE values for the hydrogenation of 2a to 1a on Pd3Au1(111) and Pd(111) surfaces are 1.59 and 1.63 eV, respectively. This indicates the better activity of Pd3Au1 compared to the Pd catalyst in this hydrogenation reaction. To better understand the differences in selectivity of these two catalysts, we further calculated the reaction mechanism for the further hydrogenation of aromatic ring in 1a. As shown in the potential-free diagram (fig. S5), the δE values for hydrogenation of aromatic ring of 1a to 1a4H on the Pd3Au1(111) and Pd(111) surfaces are 1.58 and 1.45 eV, respectively, which shows that Pd is more active in the hydrogenation of aromatic ring than the Pd3Au1 catalyst. In this respect, the calculations indicate that the Pd3Au1 catalyst has higher activity and selectivity for the selective hydrogenation of 2a to 1a, whereas the Pd(111) catalyst tends to over-hydrogenate the product, lowering the selectivity to 1a. All these results are in good agreement with our experimental findings.
To establish the origin of the superior performance of the Pd3Au1 catalyst compared to the Pd1Au1, Pd1Au3, Pd, and Au systems, we considered the Sabatier principle (35), in which the interactions between the adsorbate and catalyst should be neither too strong nor too weak but ideally poised to activate the substrate and release the product. As shown in Fig. 3A, the adsorption of quinoline (2a*) corresponds to the TDI of this reaction, which also indicates its important role in determining the performance of the catalysts. We therefore chose the adsorption energy of 2a as the descriptor to compare the intrinsic differences among the catalysts. As shown in table S4, Pd(111) and Au(111) have the strongest (−3.03 eV) and weakest (−0.91 eV) adsorption energies for 2a, indicating their less promising performance in dehydrogenation reactions based on the Sabatier principle. However, the adsorption energy of 2a in the Pd3Au1(111) surface is −2.14 eV, stronger than that of Pd1Au1(111), Pd1Au3(111), and Au(111) but weaker than that of Pd(111). Combined with the catalytic performance, the Pd3Au1(111) displays optimal adsorption energies of 2a, leading to the superior performance of this catalyst. In this respect, the reason for the superior performance of Pd3Au1(111) to other Pd-Au–based catalysts could be attributed to its optimal adsorption strengths of adsorbates as intrinsically driven by the Sabatier principle.
In summary, we described the development of a highly efficient bimetallic Pd3Au1/CNT catalyst for the dehydrogenation and hydrogenation of N- and O-containing compounds. The mechanism of the interconversion between 1,2,3,4-tetrahydroquinoline and quinoline was probed as a model reaction using experimental methods and consolidated by DFT calculations, which show that the Pd3Au1(111) facet leads to lower-energy barriers than those observed on the Pd(111) surface for all of the elementary steps in both dehydrogenation and hydrogenation reactions. The high selectivity of the Pd3Au1/CNT catalyst allows it to be used in the synthesis of a wide range of N- and O-containing compounds offering a practical strategy for the synthesis of N-heterocycles, amines/imines, and alcohols/ketones substrates, tolerating a wide range of functional groups via dehydrogenation and hydrogenation reactions, in the absence of additives.