Acetylene semi-hydrogenation catalyzed by Pd single atoms sandwiched in zeolitic imidazolate frameworks via hydrogen activation and spillover

Abstract

The semi-hydrogenation of alkynes into alkenes rather than alkanes is of great importance in the chemical industry, and palladium-based metallic catalysts are currently employed. Unfortunately, a fairly high cost and uncontrollable over-hydrogenation impeded the application of Pd-based catalysts on a large scale. Herein, a sandwich structure single atom Pd catalyst, Z@Pd@Z, was prepared via impregnation exchange and epitaxial growth methods (Z stands for ZIF-8), in which Pd single atoms were stabilized by pyrrolic N in a zeolitic imidazolate framework (ZIF-8). Semi-hydrogenation of acetylene was performed and Z@Pd@Z achieved 100% acetylene conversion at 120 °C with an ethylene selectivity of more than 98.3% at an extra low Pd concentration. Z@Pd@Z exhibited a specific activity of 1872.69 mL_C2H4 mg_Pd⁻¹ h⁻¹, surpassing most of the reported Pd-based catalysts. The existence of Pd single atoms coordinated by nitrogen (Pd–N₄) was verified by XAS (synchrotron X-ray absorption spectroscopy), which provided active sites for H₂ dissociation and the dissociated hydrogen quickly spilled over the surface of the outer ZIF layer to hydrogenate alkyne to ethene; besides, the catalytic activity could be controlled by adjusting the thickness of the outer ZIF layer. The confinement of the ZIF on Pd single-atom sites and the high energy barrier of ethylene hydrogenation were found to be responsible for the superior C₂H₂ semi-hydrogenation activity. This work opens up valuable insights into the design of ZIF-derived single-atom catalysts for efficient acetylene selective hydrogenation.

---

New concepts

Catalytic selective hydrogenation of acetylene is a universally adopted and effective method to purify ethylene feedstock in the polyethylene industry. To date, Pd-based catalysts have been widely studied due to their superior hydrogenation activity, but still face a major challenge in overcoming the trade-off between conversion and selectivity as well as increasing atomic utilization efficiency to improve sustainability and reduce costs. Here, we developed a novel sandwich structure single atom Pd catalyst, Z@Pd@Z, which stabilized Pd single atoms by pyrrolic N in the zeolitic imidazolate framework (ZIF-8) to realize maximum atomic utilization. We pioneer the use of a ZIF shell to limit the direct contact of Pd sites with acetylene and ethylene to inhibit over-hydrogenation. Experiments and mechanism studies confirm that the synergy between the spatial restriction effect and hydrogen spillover endows the Z@Pd@Z catalyst with excellent acetylene conversion and ethylene selectivity simultaneously in acetylene semi-hydrogenation reaction with an extremely low Pd content. We anticipate that this work will provide new insights into the precise design of efficient selective hydrogenation catalysts.

1. Introduction

Semi-hydrogenation of trace acetylene from ethylene stream prior to the production of polyethylene has attracted much attention in both laboratory research and industrial production.^1–7 Pd-based catalysts are commonly regarded as efficient catalysts for this process due to their excellent hydrogenation activity.^8–12 However, besides their relatively high cost, high acetylene conversion is often accompanied by over-hydrogenation and polymerization, resulting in the formation of undesirable by-products ethane and green oils that could deactivate the catalyst.⁸ Improving ethylene selectivity as much as possible without the loss of acetylene conversion to overcome the trade-off between conversion and selectivity as well as increasing atomic utilization efficiency is the central issue in current research.¹³

In general, acetylene hydrogenation selectivity regulation mainly focuses on the following key factors, such as the modification of metal dispersion,^14,15 the geometry and electronic structure of the active metal,^16–18 and the adsorption of reactants and intermediates at the active sites.^4,11,15 Encapsulating active metals with microporous materials to restrict the direct contact between the reactants and active sites provides an effective strategy to control product selectivity. Gong's group has reported a sodalite zeolite (SOD) confined Pd nanocluster catalyst, Pd@SOD, to realize acetylene semi-hydrogenation. Pd@SOD achieved 94.5% ethylene selectivity, much higher than that of Pd/SOD (21.5%).¹⁹ Improving the dispersion of active sites to the single-atom level is also an effective way to improve ethylene selectivity, single metal atom active sites make the intermediates adsorb on the surface in the form of π-bond adsorption, which is conducive for ethylene desorption. Moreover, dispersing the active metal to the single-atom level allows for 100% atom utilization, high atom utilization efficiency could significantly save costs and open up the possibility of industrialization. However, atomically dispersed metal atoms are prone to migrate and agglomerate due to their high surface energy, thus leading to catalyst deactivation. The preparation of efficient and stable single-atom catalysts still faces great challenges.

Metal–organic frameworks (MOFs), also called porous coordination polymers (PCPs), have emerged as a new promising class of porous materials with the inherent advantages of high porosity and well-defined and tailorable structures.^20–24 MOFs exhibit immense potential as substrate materials for anchoring individual atoms by coordinating with the surrounding atoms like C, N, S, and others.^25–29 As one of the most stable MOFs, zeolitic imidazole framework-8 (ZIF-8) with small pores and high surface areas has been pioneered in various applications due to its exceptionally high stability, especially catalysis.^30–34 Yang et al. prepared Pd@H-Zn/Co-ZIF derived from core–shell ZIF-67@ZIF-8 as an acetylene semi-hydrogenation catalyst.³⁵ The unique hollow structure and ZIF-8 coating endowed the catalyst with enhanced gas storage capacity, resulting in superior catalytic performance for acetylene semi-hydrogenation; >80% C₂H₂ conversion and >80% C₂H₄ selectivity were achieved at 50 °C, and the catalyst remained stable for 10 h. Hu et al. reported a ZIF-confined intermetallic PdZn nanoparticle catalyst, PdZn-sub-2@ZIF-8C, for acetylene semi-hydrogenation.³⁶ Ultrasmall PdZn intermetallic particles exhibited better catalytic properties than the larger-sized ones due to the more favorable acetylene hydrogenation reaction path and ethylene desorption over ultrasmall particles as revealed by DFT calculations. PdZn-1.2@ZIF-8C achieved 70% acetylene conversion with 80% ethylene selectivity at 115 °C, which could be maintained well in the 12 h catalytic stability test. Though exquisitely designed ZIF-derived Pd-based catalysts have been applied in acetylene semi-hydrogenation, the activity–selectivity trade-off is still challenging.

Besides, it is worth noting that the aperture size of ZIF-8 is 0.34 nm,³⁷ between the kinetic diameter of H₂ (0.289 nm) and C₂H₂ (0.38 nm)/C₂H₄ (0.39 nm),¹⁹ which should make it possible to serve as a cage to separate C₂H₂/C₂H₄ and active metal sites based on size exclusion. The small pores in ZIF-8 could impede the direct contact of C₂H₂, C₂H₄ and Pd single atoms, and only H₂ could diffuse into the channels and dissociate on the encapsulated Pd single atoms. The dissociated active hydrogen species would spill over ZIF-8 surfaces,³⁸ hydrogenate acetylene to ethene and avoid over-hydrogenation due to the spatial restriction effect.

In light of the above discussion, a sandwich structure ZIF-confined Pd single atom catalyst, Z@Pd@Z, with an extremely low Pd content was successfully constructed by impregnation exchange and epitaxial growth methods (Z stands for ZIF-8). The Pd single atom was anchored by pyrrolic-N in the ZIF, forming Pd–N₄ coordination bonds to stabilize the Pd atom. Due to the high dispersion of Pd, the appropriate ethylene adsorption and hydrogen spillover, the Z@Pd@Z catalyst showed outstanding activity in the selective hydrogenation reaction of acetylene. The ethylene selectivity was 98.3% with complete acetylene conversion, and STY_C2H4 was 1872.69 mL_C2H4 mg_Pd⁻¹ h⁻¹, which was higher than most of the reported Pd-based catalysts. In addition, because of the protection of the ZIF-8 shell which inhibits excessive hydrogenation and coordination protection, the catalyst was able to remain stable during the 100 h stability test.

2. Materials and methods

2.1. Catalyst preparation

2.1.1. Synthesis of ZIF-8. ZIF-8 was prepared according to the previously reported method with some modifications.³⁹ In a typical synthesis, Zn(NO₃)₂·6H₂O (5.95 g) and 2-methylimidazole (2-meim, 6.16 g) were dissolved in 150 mL of methanol, respectively, and then, 2-meim solution was rapidly added into the solution of Zn(NO₃)₂·6H₂O under vigorously stirring and kept stirring for another 24 h. The white powder was washed with methanol 3 times and dried at 80 °C in a vacuum. The ZIF-8 powder was obtained after grinding (yield ≈ 42%, based on Zn).

2.1.2. Synthesis of Pd/Z. Pd/Z was prepared by the impregnation exchange method. Pd(PPh₃)₄ (0.02 g) was ultrasonically dispersed in 6 mL of methanol, and ZIF-8 (0.20 g) was added into the above solution, ultrasonicated for 20 min and stirred for 24 h at room temperature. Next, the product was centrifuged and dried in a vacuum at 80 °C overnight.

2.1.3. Synthesis of Z@Pd@Z_x. Pd/Z (0.20 g) was dispersed in 30 mL of methanol solution of Zn(NO₃)₂·6H₂O (0.025 M) and stirred vigorously for 20 min. Then, 30 mL of the methanol solution of 2-meim (0.0058 M) was added under stirring and kept stirring for another 24 h. After this, the sample was centrifuged and washed with methanol three times and finally dried in a vacuum at 80 °C overnight to obtain Z@Pd@Z. Z@Pd@Z_x was prepared using the same method as Z@Pd@Z except that Pd/Z was replaced by Z@Pd@Z_x−1 as the precursor (x = 2 and 3).

2.2. Catalytic test

The hydrogenation of acetylene was performed using a quartz tube fixed-bed reactor. Typically, the catalyst (0.1 g) was mixed with 1.0 g of quartz sand, activated in N₂ at 200 °C for 1 h and cooled down to 40 °C for the reaction. Subsequently, the reactant gas mixture containing 0.515% C₂H₂ and 5.15% H₂ (N₂ balance) was fed into the quartz reactor. The total flow rate was controlled to 50 mL min⁻¹, corresponding to a weight hourly space velocity (WHSV) of 30 000 ml g_cat⁻¹ h⁻¹. The effluent was analyzed using an online gas chromatograph GC-7820 equipped with a Plot Q capillary column and a flame ionization detector (FID). The conversion of C₂H₂ and the selectivity of C₂H₄ were calculated according to the following equations:

3. Results and discussion

3.1. Characterization of Z@Pd@Z

The powder X-ray diffraction (PXRD) pattern confirmed the successful synthesis of ZIF-8 (Fig. 1a). After the impregnation and epitaxial growth of the outer layers of the ZIF, the PXRD patterns of Pd/Z and Z@Pd@Z showed characteristic diffraction peaks identical to those of ZIF-8. No prominent peaks were attributed to Pd crystalline indicating the high dispersion of Pd in these samples. The presence of Pd species could be confirmed by EDS mapping (Fig. 1f and j) and ICP-OES (Table S2, ESI†), and the Pd content decreased with the growth of the outer ZIF layer.

Fig. 1 (a) PXRD patterns of ZIF-derived catalysts. (b) High-resolution XPS spectra of Pd 3d for Pd/Z and Z@Pd@Z. (c) N 1s for Pd/Z and Z@Pd@Z. The representative (d) SEM, (e) TEM, (f) corresponding EDS mapping, and (g) STEM images of the Z@Pd@Z catalyst. The representative (h) SEM, (i) TEM, (j) corresponding EDS mapping, and (k) STEM images of the Pd/Z catalyst.

SEM images displayed a typical rhombic dodecahedron shape of Pd/Z with an average diameter of 59 nm (Fig. 1h). With the epitaxial growth of the ZIF layer, the overall morphology of the sample was maintained while the particle size gradually increased to 88 nm for Z@Pd@Z (Fig. 1d). With the increase of epitaxial growth times, the thickness of the outer ZIF layer increased from 14.50 nm for Z@Pd@Z to 71.35 nm for Z@Pd@Z₃, and the average shell thickness is summarized in Table S2 (ESI†). Detailed information about the constitution of Z@Pd@Z is given in Fig. 1f, showing that Pd, Zn, C and N are distributed uniformly in the catalyst. The uniformity of the metal element might give credit to the fine structure of ZIF-8. The absence of the Pd lattice in HRTEM indicated no aggregation of Pd, which also proved that Pd existed atomically in Z@Pd@Z. Furthermore, the HAADF-STEM images of Z@Pd@Z and the corresponding EDS mapping confirmed the uniform distribution of Pd as single atoms. Due to the larger atomic number of Pd compared to Zn, Pd atoms would exhibit brighter intensity in HAADF-STEM images as denoted by red circles in Fig. 1g. The structure of Z@Pd@Z was further described by the FTIR spectrum (Fig. S1, ESI†). Pd/Z and Z@Pd@Z exhibited obvious characteristic peaks of ZIF-8. For instance, the peak at 1584 cm⁻¹ could be attributed to C=N stretching, the signal at 685 cm⁻¹ was ascribed to the out-of-plane bending of 2-meim. No obvious changes were found in the structure of Pd/Z and Z@Pd@Z compared to ZIF-8, which was consistent with the PXRD results. It should be noted that the Pd–P stretching vibration peak at 407 cm⁻¹ of Pd(PPh₃)₄ disappeared while the HAADF-STEM images revealed the distribution of Pd single atoms on ZIF-8, indicating that Pd single atoms were anchored on ZIF-8.

XPS measurements were employed to probe the surface compositions and electronic structures of the samples. As shown in Fig. 1b, the Pd 3d peaks of Pd/Z could be clearly observed, and peaks at 335.7 eV and 337.6 eV indicate that both oxidized and metallic Pd species exist in Pd/Z.¹³ In contrast, no Pd signal was observed on Z@Pd@Z, which indicated that Pd did not exist on the surface of the Z@Pd@Z catalyst, and Pd was successfully encapsulated inside the ZIF layer. The valence state of Pd in Z@Pd@Z would be analyzed by XANES later. To further confirm the sandwich structure, TEM was conducted (Fig. S4, ESI†). It could be seen from quantitative analysis that the existence of Pd could be confirmed in the whole Z@Pd@Z, while no Pd was detected on the edge of Z@Pd@Z. Thus, Pd existed inside the ZIF, rather than on the outer surface of the ZIF. In addition, in the high-resolution N 1s spectrum of Pd/Z and Z@Pd@Z (Fig. 1c), the peaks with high intensity centered at 398.8 eV and peaks at 400.2 eV belong to C–N and C=N of 2-meim in ZIF-8, which are classified into pyrrolic-N and graphitic-N, respectively.⁴⁰

To further prove the existence of Pd single atoms and disclose the electronic and coordination structures of Pd atoms in Z@Pd@Z, X-ray absorption spectroscopy (XAS) was performed at the Pd K-edge, Pd foil, PdO and Pd(PPh₃)₄ were studied comparatively. The X-ray absorption near-edge structure (XANES) spectra (Fig. 2a) showed that the Pd K-edge absorption edge position for Z@Pd@Z was between those of PdO and Pd foil, indicating that Pd existed in the form of positively charged Pd species.^41,42 Fourier transformed R-space curves of the Pd K-edge EXAFS spectra clearly revealed the bonding environment of Pd in Z@Pd@Z (Fig. 2b). The Pd–Pd scattering feature at 2.5 Å was absent in both Pd(PPh₃)₄ and Z@Pd@Z, confirming the atomic dispersion of Pd in the catalyst, which was consistent with the previous HAADF-STEM results. However, Z@Pd@Z and Pd(PPh₃)₄ exhibited completely distinct peaks in the EXAFS spectrum for Pd. EXAFS spectroscopy of Pd(PPh₃)₄ exhibited a peak at 1.75 Å, which was attributed to Pd–P bonds. EXAFS spectroscopy of Z@Pd@Z consisted of a feature at 1.5 Å in the R-space spectrum, which could be ascribed to Pd–N coordination, and no significant feature attributed to the Pd–P (1.75 Å) scattering path was observed.^15,43,44 These results proved that Pd–P bonds were broken during the impregnation exchange process and Pd–N bonds formed, which agreed well with the results of FTIR. The EXAFS result confirmed that the single atom Pd in Z@Pd@Z involved Pd^δ+ coordination by N and the EXAFS fitting of Z@Pd@Z (Fig. 2) indicated the coordination numbers of Pd with N is 4. The Pd–N₄ coordination geometry is thermodynamically stable; thereby, the Z@Pd@Z catalyst may exhibit high catalytic stability as could be seen later in this article. Accordingly, it could be concluded that the single atom Pd^δ+ in Pd(PPh₃)₄ was coordinated to pyrrolic-N in ZIF-8 after impregnation exchange to form the Pd–N₄ coordination geometry, and no Pd–P ligand was retained. The interaction between pyrrolic-N and Pd atoms was found to be beneficial to improving the hydrogenation activity of the catalysts.⁴⁵

Fig. 2 (a) the comparison of Pd K-edge XANES spectra. (b) The comparison of Pd K-edge EXAFS, as shown in k² weighted R-space. The wavelet transform of (c) Pd(PPh₃)₄, (d) PdO, (e) Z@Pd@Z and (f) Pd foil.

3.2. Catalytic acetylene hydrogenation

To gain further insight into the ZIF layer confined Pd single-atom catalyst on catalytic performance, selective hydrogenation of acetylene was systematically evaluated on Pd/Z and Z@Pd@Z_x catalysts. The acetylene conversion and ethene selectivity are shown in Fig. 3. Pd/Z enabled the complete conversion of acetylene at 100 °C, while with the encapsulation of the outer ZIF layer, acetylene conversion decreased gradually, indicating that the outer ZIF layer had a negative effect on hydrogenation activity.

Fig. 3 Catalytic performance of Pd/Z and Z@Pd@Z_x catalysts. (a) Acetylene conversion, (b) ethylene selectivity, (c) ethylene yield and space time yield in the hydrogenation of acetylene. (d) Acetylene hydrogenation stability test over the Z@Pd@Z catalyst at 120 °C for 100 h. (e) Ethylene space time yield of the reported Pd-based acetylene hydrogenation catalysts (reference numbers are listed in Table S3, ESI†).

On the other hand, much higher ethylene selectivity was achieved on Z@Pd@Z_x than on the nonconfined Pd/Z catalyst in the whole reaction temperature range. Pd/Z had the highest acetylene conversion, and it preferred catalyzing the over-hydrogenation of acetylene to ethane, especially at high acetylene conversion. The best-performance catalyst Z@Pd@Z achieved 100% acetylene conversion with an ethylene selectivity of more than 98.3% at 120 °C, and traces of ethane and no long chain were detected. Besides, it is worth mentioning that thanks to the confinement effect of the ZIF, the Z@Pd@Z catalyst showed a wide-operating temperature window (120–200 °C), with no significant reduction in C₂H₄ selectivity.

In addition to achieving high activity and considerable selectivity simultaneously, stability is also a vital evaluation factor. The stability of Z@Pd@Z was investigated at 120 °C. The ethylene selectivity remained almost constant (>98%) with unchanged conversion for a 100 h test period. The outer ZIF layer might serve as “cages” for confining the sintering of Pd single atoms (Fig. S4, ESI†) and inhabiting catalyst poisoning during the reaction, hence improving the catalyst lifetime. This result indicates the favorable stability and potentially long lifetimes of Z@Pd@Z in catalyzing C₂H₂ semi-hydrogenation.

3.3. Mechanism investigation

The activation of hydrogen is crucial for controlling product selectivity. According to the aforementioned characterization, Pd single atoms were anchored by pyrrolic-N in the ZIF to form a sandwich structure. The outer ZIF layer served as “cages” to separate acetylene/ethylene and Pd sites based on size exclusion, and only H₂ could diffuse into the channels and dissociate on the encapsulated Pd single atom. Thus, acetylene hydrogenation needs to be performed on the surface of the outer ZIF layer. It has been confirmed that ZIF-8 favored hydrogen spillover from active metals in them to form reactive hydrogen species;^38,46 a hypothesis could be proposed that hydrogen spillover occurred on the Z@Pd@Z catalyst and the dissociated active hydrogen species spilled over the surface of the outer ZIF layer to hydrogenate acetylene to ethylene. To check for the hydrogen spillover on Z@Pd@Z, WO₃ was mixed with a series of Pd/Z and Z@Pd@Z_x catalysts to diagnose their activation ability of hydrogen because the dissociated H can react with the yellow WO₃ to form dark blue H_xWO₃.⁴⁷ As can be seen in Fig. 4a, white powder catalysts mixed with WO₃ appeared light yellow. Due to the absence of metal Pd to activate H₂, no color change was observed after H₂ treatment on ZIF-8. In contrast, Pd/Z exhibited the darkest coloration of tungsten species, indicating that H₂ dissociated at the Pd site. For Z@Pd@Z and Z@Pd@Z₂, there were also significant color changes, but the discoloration was not as significant as that on Pd/Z. After wrapping one or two layers of ZIF-8 on the outer layer of Pd, hydrogen activated by Pd could migrate out, confirming the occurrence of hydrogen spillover on the catalysts. But Z@Pd@Z₃ did not change color after hydrogen treatment, which might be due to the thick shell of the ZIF exceeding the hydrogen spillover range, which is also the reason for no acetylene hydrogenation activity of the catalyst. Combining the size distribution (Fig. S6 and Table S2, ESI†) with the hydrogen spillover experiment together, it was inferred that active *H species generated from H₂ dissociation at Pd single atoms could travel through the outer ZIF layer and hydrogenate WO₃ on the surface of Z@Pd@Z catalysts, the outer ZIF layer of which was more than 14.5 nm. As the thickness of the outer ZIF layer was increased to 71.35 nm (Z@Pd@Z₃), a negligible color change was observed, suggesting that *H could hardly migrate through the ZIF layer to participate in the hydrogenation reaction with such a thickness of the outer ZIF layer. Thus, the hydrogen spillover limitary distance on the ZIF is beyond 37.94 nm but less than 71.35 nm. This phenomenon is consistent with the performance test that acetylene conversion decreased with the increase of the thickness of the outer ZIF layer. In order to further prove the occurrence of hydrogen spillover on the Z@Pd@Z catalyst, the catalytic performance of phenylacetylene hydrogenation on Pd/Z and Z@Pd@Z was tested. The molecular size of phenylacetylene is 0.74 nm, much larger than the channel size of ZIF-8, and no conversion would be observed over Z@Pd@Z if hydrogen spillover did not exist. Fig. S3 (ESI†) demonstrates the conversion of phenylacetylene on both Pd/Z and Z@Pd@Z, suggesting the spillover of active hydrogen from the confined Pd single atom to the surface of the catalyst.

Fig. 4 (a) Photographs of ZIF-derived catalysts mixed with WO₃ after treatment with H₂. (b) C₂H₂-TPD profiles of Pd/Z, Z@Pd@Z and Z@Pd@Z₂. (c) FTIR spectra of the Z@Pd@Z catalyst in a reaction atmosphere at different temperatures.

In order to further understand the critical role of ZIF encapsulation in acetylene selective hydrogenation, C₂H₂-TPD experiments were performed. The thermal stability of ZIF-derived catalysts was evaluated prior to the test, and the TG results showed that the samples remained stable up to 500 °C without decomposition (Fig. S6, ESI†). In order to avoid the interference caused by ZIF decomposition during the programmed heating process and ensure the complete desorption of C₂H₂, samples were heated up to 400 °C and maintained for 30 min. As shown in Fig. 4b, Pd/Z had an obvious desorption peak at around 400 °C, which could be attributed to the strong interaction of acetylene species with exposed Pd sites. This indicated that it had obvious adsorption to acetylene, which could explain its quite high acetylene conversion activity. After encapsulating the outer ZIF layers, the acetylene desorption peak was significantly reduced on Z@Pd@Z. The weaker adsorption of C₂H₂ also explained the relatively lower C₂H₂ conversion on Z@Pd@Z when compared to the Pd/Z catalyst. This result was in line with our hypothesis of limiting the connection of acetylene and active Pd sites by ZIF encapsulation. As the diameter of acetylene is larger than the aperture size of the ZIF, the ZIF layer will hinder the diffusion of C₂H₂ into the ZIF to contact Pd sites. The encapsulation of the outer ZIF layer only allowed H₂ diffusion, and the H₂ dissociated on the single-atom Pd sites to form active hydrogen species that spilled over the ZIF layer surfaces and thus avoided over-hydrogenation to ethane.

The hydrogenation process of acetylene on Z@Pd@Z was characterized by in situ FTIR technology, and the results are shown in Fig. 4c. When the reaction temperature was 50 °C, the infrared characteristic peaks of C₂H₄ molecules in the gas phase of 3013, 2987 and 2968 cm⁻¹ appeared,¹⁶ indicating that the catalyst could produce ethylene products during acetylene hydrogenation at 50 °C. The strong absorption peak at 2937 cm⁻¹ belongs to the symmetric stretching vibration of CH₂, which probably came from the reaction intermediate C₂H₃*, and further hydrogenation of C₂H₃ could produce the ethylene product. The weak absorption peak at 2870 cm⁻¹ is attributed to the symmetric stretching vibration of the CH₃ species, which was derived from the reaction intermediate C₂H₅*, and further hydrogenation resulted in the formation of ethane.⁴⁸ From the strength of these two peaks, it could also be seen that on the Z@Pd@Z catalyst, the ethylene product was mainly generated, accompanied by a very small amount of ethane molecules at high temperatures, which was very consistent with the experimental results of the previous catalyst performance evaluation.

DFT calculations were employed to explain why the Z@Pd@Z catalyst exhibited considerable C₂H₄ selectivity. Since C₂H₂ hydrogenation occurred on the surface of ZIF-8, according to the PXRD result, the ZIF-8 (110) facet with exposed Zn sites was considered to simulate the exposed ZIF-8 surface. The corresponding configurations of C₂H₂ and C₂H₄ hydrogenation on the ZIF-8(110) surface are shown in Fig. 5a and Fig. S10 (ESI†). Previous research has proved that acetylene is prone to partially hydrogenate to ethylene rather than over-hydrogenate to ethane when the ethylene hydrogenation barrier is higher than the desorption energy of ethylene.¹⁵ It could be seen from Fig. 5a that C₂H₂ had a lower hydrogenation barrier than C₂H₄ on the (110) surface of ZIF-8; therefore, acetylene more easily reacted with H* than ethylene. Fig. 5b shows that the desorption energy of ethene on the ZIF-8 (110) surface was 0.43 eV, and the hydrogenation barrier for ethene was 0.92 eV, which revealed that ethene was prone to desorption rather than hydrogenation on the (110) surface of ZIF-8, that is, ethylene hydrogenation to ethane was highly unlikely to proceed on the Z@Pd@Z catalyst.

Fig. 5 (a) The energy profile of C₂H₂ and C₂H₄ hydrogenation on the ZIF-8 (110) surface. (b) The desorption energy of C₂H₄ and the energy barrier of C₂H₄ hydrogenation. (c) Schematic illustration of acetylene hydrogenation on the Z@Pd@Z catalyst.

It is worth noting that the Pd/Z catalyst was agglomerated to form Pd nanoparticles after the reaction (Fig. S8, ESI†). The C₂H₂ hydrogenation activity was also conducted on the Pd_NPs/Z catalyst, and similar C₂H₄ selectivity was obtained as Pd/Z (Fig. S9, ESI†) at high C₂H₂ conversion. Previous studies have confirmed that Pd nanoparticles would lead to over-hydrogenation, which was not conducive to C₂H₄ selectivity.¹⁵ Therefore, the decreased C₂H₄ selectivity of Pd/Z might result from the agglomeration of Pd sites. Z@Pd@Z exhibited higher C₂H₄ selectivity than Pd/Z at the same C₂H₂ conversion, and the improved selectivity mainly originated from the hydrogen spillover effect. The sandwich structure in the Z@Pd@Z catalyst formed stable Pd–N₄ sites and avoided the direct contact between C₂H₂/C₂H₄ and Pd sites. The dissociated H* spilled over to the surface of the outer ZIF layer, and C₂H₂ was prone to hydrogenation to produce C₂H₄, while C₂H₄ was inclined to desorb on the Zn sites of the ZIF-8 (110) surface. The confinement effect, hydrogen spillover and high ethylene hydrogenation energy barrier were crucial to avoid over-hydrogenation and improve the C₂H₄ selectivity in the Z@Pd@Z catalyst. Besides, the Pd single atoms in Z@Pd@Z ensure the maximum atomic utilization, thus, Z@Pd@Z exhibited a superior space-time yield of 1872.69 mL_C2H4 mg_Pd⁻¹ h⁻¹.

4. Conclusions

In summary, we reported a strategy to construct Pd single atoms sandwiched in ZIF-8 using impregnation exchange and epitaxial growth methods. HAADF-STEM and XAS analyses confirmed the atomic distribution of Pd with Pd–N₄ coordination in Z@Pd@Z. The Z@Pd@Z catalyst containing single-atom Pd sites and an outer ZIF layer displayed remarkable acetylene semi-hydrogenation catalytic performance, achieving complete acetylene conversion with >98.3% ethylene selectivity at 120 °C. The STY_C2H4 was 1872.69 mL_C2H4 mg_Pd⁻¹ h⁻¹, outperforming those of most of the previously reported Pd-based acetylene hydrogenation catalysts. The atomically dispersed Pd–N₄ sites and outer ZIF layer encapsulation contributed to the excellent catalytic performance of the Z@Pd@Z catalyst. The confinement effect of the ZIF layer inhibited the direct contact of C₂H₂/C₂H₄ with the Pd sites. Additionally, the high hydrogenation energy barrier for ethylene on Zn sites of the ZIF-8 (110) surface suppressed the over-hydrogenation of ethene to ethane, thus explaining the high C₂H₄ selectivity and wide operating temperature window (at least 80 °C). Furthermore, the Z@Pd@Z catalyst displayed superb stability during the durability test at 120 °C, which might be explained by the coordination of Pd atoms by pyrrolic nitrogen in the ZIF, which prevents the migration and aggregation of Pd single atoms. The strategy presented here provides a promising approach for the rational design of selective hydrogenation catalysts.

Author contributions

Yan-Ting Li: conceptualization, methodology, visualization, investigation, and writing – original draft. Wen-Gang Cui: conceptualization, methodology, visualization, investigation, and writing – original draft. Ying-Fei Huo: DFT calculations. Lei Zhou: DFT calculations. Wei Li: DFT calculations. Xinqiang Wang: formal analysis. Fan Gao: formal analysis. Qiang Zhang: visualization. Tong-Liang Hu: supervision, writing – review and editing, conceptualization, project administration, and funding acquisition.

Conflicts of interest

The authors declare no conflicts of interests.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22275102 and 52201274), the NCC Fund (NCC2022FH01), the Project of Education Department of Shaanxi Province (22JK0419), and the Haihe Laboratory of Sustainable Chemical Transformations.

References

1. Y. Yue, B. Wang, C. Jin, K. Huang, Q. Zhou, R. Chang, S. Wang, Z. Pan, J. Zhao and X. Li, ACS Catal., 2024, 14, 3900–3911.
2. F. Huang, Y. Deng, Y. Chen, X. Cai, M. Peng, Z. Jia, J. Xie, D. Xiao, X. Wen, N. Wang, Z. Jiang, H. Liu and D. Ma, Nat. Commun., 2019, 10, 4431.
3. S. Zhou, C. Lu, Y. Bi, C. Zhou, Q. Li, L. Tan and L. Dong, Chem. Eng. J., 2023, 476, 146594.
4. L. Zhang, Y. Ding, K. H. Wu, Y. Niu, J. Luo, X. Yang, B. Zhang and D. Su, Nanoscale, 2017, 9, 14317–14321.
5. L. Zhang, M. Zhou, A. Wang and T. Zhang, Chem. Rev., 2019, 120, 683–733.
6. M. R. Mian, L. R. Redfern, S. M. Pratik, D. Ray, J. Liu, K. B. Idrees, T. Islamoglu, L. Gagliardi and O. K. Farha, Chem. Mater., 2020, 32, 3078–3086.
7. W. Xue, X. Liu, C. Liu, X. Zhang, J. Li, Z. Yang, P. Cui, H.-J. Peng, Q. Jiang, H. Li, P. Xu, T. Zheng, C. Xia and J. Zeng, Nat. Commun., 2023, 14, 2137.
8. Y. Cao, Z. Sui, Y. Zhu, X. Zhou and D. Chen, ACS Catal., 2017, 7, 7835–7846.
9. S. Dong, Y. Niu, Y. Pu, Y. Wang and B. Zhang, Chin. Chem. Lett., 2024, 35, 109525.
10. S. J. Wei, A. Li, J. C. Liu, Z. Li, W. X. Chen, Y. Gong, Q. H. Zhang, W. C. Cheong, Y. Wang, L. R. Zheng, H. Xiao, C. Chen, D. S. Wang, Q. Peng, L. Gu, X. D. Han, J. Li and Y. D. Li, Nat. Nanotechnol., 2018, 13, 856–861.
11. R. Li, Y. Yue, X. Chen, R. Chang, J. Zhang, B. Zhao, J. Zhang, D. Cai, Y. Zhu, D. Han, J. Zhao and X. Li, Nano Res., 2022, 16, 6167–6177.
12. J. Xu, W. Huang, R. Li, L. Li, J. Ma, J. Qi, H. Ma, M. Ruan and L. Lu, J. Colloid Interface Sci., 2024, 655, 584–593.
13. C. Lu, S. Zhou, W. Zhou, C. Zhou, Q. Li, A. Zeng, A. Wang, L. Tan and L. Dong, Chem. Eng. J., 2023, 464, 142609.
14. F. Huang, Y. Deng, Y. Chen, X. Cai, M. Peng, Z. Jia, P. Ren, D. Xiao, X. Wen, N. Wang, H. Liu and D. Ma, J. Am. Chem. Soc., 2018, 140, 13142–13146.
15. S. Zhou, S. Lu, Y. Zhao, R. Shi, G. I. N. Waterhouse, Y.-C. Huang, L. Zheng and T. Zhang, Adv. Mater., 2019, 31, 1900509.
16. M. R. Ball, K. R. Rivera-Dones, E. B. Gilcher, S. F. Ausman, C. W. Hullfish, E. A. Lebrón and J. A. Dumesic, ACS Catal., 2020, 10, 8567–8581.
17. F. Huang, M. Peng, Y. Chen, Z. Gao, X. Cai, J. Xie, D. Xiao, L. Jin, G. Wang, X. Wen, N. Wang, W. Zhou, H. Liu and D. Ma, ACS Catal., 2021, 12, 48–57.
18. G. X. Pei, X. Y. Liu, A. Wang, A. F. Lee, M. A. Isaacs, L. Li, X. Pan, X. Yang, X. Wang, Z. Tai, K. Wilson and T. Zhang, ACS Catal., 2015, 5, 3717–3725.
19. S. Wang, Z. J. Zhao, X. Chang, J. Zhao, H. Tian, C. Yang, M. Li, Q. Fu, R. Mu and J. Gong, Angew. Chem., Int. Ed., 2019, 58, 7668–7672.
20. Y. Liu, H. Chen, T. Li, Y. Ren, H. Wang, Z. Song, J. Li, Q. Zhao, J. Li and L. Li, Angew. Chem., Int. Ed., 2023, 62, e202309095.
21. W.-G. Cui, G.-Y. Zhang, T.-L. Hu and X.-H. Bu, Coord. Chem. Rev., 2019, 387, 79–120.
22. X. Han, W. Zhang, Z. Chen, Y. Liu and Y. Cui, Mater. Horiz., 2023, 10, 5337–5342.
23. R.-R. Liang, Z. Han, P. Cai, Y. Yang, J. Rushlow, Z. Liu, K.-Y. Wang and H.-C. Zhou, J. Am. Chem. Soc., 2024, 146, 14174–14181.
24. T. Zurrer, K. Wong, J. Horlyck, E. C. Lovell, J. Wright, N. M. Bedford, Z. Han, K. Liang, J. Scott and R. Amal, Adv. Funct. Mater., 2021, 31, 2007624.
25. W. Qu, C. Chen, Z. Tang, H. Wen, L. Hu, D. Xia, S. Tian, H. Zhao, C. He and D. Shu, Coord. Chem. Rev., 2023, 474, 214855.
26. P. Yin, T. Yao, Y. Wu, L. Zheng, Y. Lin, W. Liu, H. Ju, J. Zhu, X. Hong, Z. Deng, G. Zhou, S. Wei and Y. Li, Angew. Chem., Int. Ed., 2016, 55, 10800–10805.
27. A. M. Abdel-Mageed, B. Rungtaweevoranit, S. Impeng, J. Bansmann, J. Rabeah, S. Chen, T. Haring, S. Namuangrak, K. Faungnawakij, A. Bruckner and R. J. Behm, Angew. Chem., Int. Ed., 2023, 62, e202301920.
28. L. Zhou, P. Zhou, Y. Zhang, B. Liu, P. Gao and S. Guo, J. Energy Chem., 2021, 55, 355–360.
29. Y. Chen, H. Kang, M. Cheng, G. Zhang, G. Wang, H. Liu, W. Xiao, Y. Liu and H. Jiang, Adv. Funct. Mater., 2024, 34, 2309223.
30. L. Yang, Y. Liu, F. Zheng, F. Shen, B. Liu, R. Krishna, Z. Zhang, Q. Yang, Q. Ren and Z. Bao, ACS Nano, 2024, 18, 3614–3626.
31. A. L. Gu, Y. X. Zhang, Z. L. Wu, H. Y. Cui, T. D. Hu and B. Zhao, Angew. Chem., Int. Ed., 2022, 134, e202114817.
32. Y. Yun, H. Sheng, K. Bao, L. Xu, Y. Zhang, D. Astruc and M. Zhu, J. Am. Chem. Soc., 2020, 142, 4126–4130.
33. D. Cao, W. Kang, W. Wang, K. Sun, Y. Wang, P. Ma and D. Sun, Small, 2020, 16, 1907641.
34. S. Wang, L. Luo, A. Wu, D. Wang, L. Wang, Y. Jiao and C. Tian, Coord. Chem. Rev., 2024, 498, 215464.
35. J. Yang, F. J. Zhang, H. Y. Lu, X. Hong, H. L. Jiang, Y. Wu and Y. D. Li, Angew. Chem., Int. Ed., 2015, 54, 10889–10893.
36. M. Hu, S. Zhao, S. Liu, C. Chen, W. Chen, W. Chen, W. Zhu, C. Liang, W.-C. Cheong, Y. Wang, Y. Yu, Q. Peng, K. Zhou, J. Li and Y. Li, Adv. Mater., 2018, 30, 1801878.
37. Q. Hou, Y. Wu, S. Zhou, Y. Wei, J. Caro and H. Wang, Angew. Chem., Int. Ed., 2018, 58, 327–331.
38. Q. Liu, W. Xu, H. Huang, H. Shou, J. Low, Y. Dai, W. Gong, Y. Li, D. Duan, W. Zhang, Y. Jiang, G. Zhang, D. Cao, K. Wei, R. Long, S. Chen, L. Song and Y. Xiong, Nat. Commun., 2024, 15, 2562.
39. Y. Zhang, L. Jiao, W. Yang, C. Xie and H. L. Jiang, Angew. Chem., Int. Ed., 2021, 60, 7607–7611.
40. J.-L. Qi, D. Zhou, Q.-Q. Xu, J.-Z. Yin, K.-P. Yu and H.-X. Sun, Microporous Mesoporous Mater., 2022, 346, 112287.
41. D. Zhou, C. Chen, Y. Zhang, M. Wang, S. Han, X. Dong, T. Yao, S. Jia, M. He, H. Wu and B. Han, Angew. Chem., Int. Ed., 2024, 63, e202400439.
42. Y. Guo, Y. Huang, B. Zeng, B. Han, M. Akri, M. Shi, Y. Zhao, Q. Li, Y. Su, L. Li, Q. Jiang, Y. T. Cui, L. Li, R. Li, B. Qiao and T. Zhang, Nat. Commun., 2022, 13, 2648.
43. Q. He, J. H. Lee, D. Liu, Y. Liu, Z. Lin, Z. Xie, S. Hwang, S. Kattel, L. Song and J. G. Chen, Adv. Funct. Mater., 2020, 30, 2000407.
44. B. Wang, Y. Yue, C. Jin, J. Lu, S. Wang, L. Yu, L. Guo, R. Li, Z. T. Hu, Z. Pan, J. Zhao and X. Li, Appl. Catal., B, 2020, 272, 118944.
45. Z. Li, M. Hu, J. Liu, W. Wang, Y. Li, W. Fan, Y. Gong, J. Yao, P. Wang, M. He and Y. Li, Nano Res., 2021, 15, 1983–1992.
46. G. Zhan and H. C. Zeng, Nat. Commun., 2018, 9, 3778.
47. C. T. Wang, E. Guan, L. Wang, X. F. Chu, Z. Y. Wu, J. Zhang, Z. Y. Yang, Y. W. Jiang, L. Zhang, X. J. Meng, B. C. Gates and F. S. Xiao, J. Am. Chem. Soc., 2019, 141, 8482–8488.
48. Y. Chai, G. Wu, X. Liu, Y. Ren, W. Dai, C. Wang, Z. Xie, N. Guan and L. Li, J. Am. Chem. Soc., 2019, 141, 9920–9927.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mh01787k

‡ Y.-T. Li and W.-G. Cui contributed equally to this work.

---