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Dynamic metal-polymer interaction for the design of chemoselective and long-lived hydrogenation catalysts – Science Advances

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Synthesis and structures of supported Pd catalysts

For comparative studies, monodisperse Pd particles with a diameter of ~5 nm were synthesized using oleylamine as a stabilizer (13) and then deposited on three different supports (Fig. 1A). A commercially available PPS powder, mesoporous silica SBA-15 (SiO2) (14), and a thermally cured PPS at 823 K (c-PPS) were used as soft organic, hard inorganic, and hard organic support, respectively. According to differential scanning calorimetry (DSC), the pristine PPS showed clear melting and crystallization behaviors at 569 and 524 K, respectively, whereas c-PPS did not (fig. S1). This means that PPS is composed of discrete mobile polymer chains, whereas c-PPS has a highly cross-linked framework. c-PPS showed a somewhat lower S/C elemental ratio (0.12) than the pristine PPS (0.17), indicating that parts of the sulfide linkages were removed during curing. To support 0.3 weight % (wt %) Pd, the support materials were dispersed in a hexane solution of Pd particles and sonicated for 12 hours at room temperature. After collecting the sample by filtration and washing with hexane, the remaining oleylamine on the Pd surface was carefully removed by the treatment with concentrated acetic acid (13). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed a uniform dispersion of Pd particles with a surface-averaged mean particle diameter (dEM) of 5.0 to 5.1 nm on all support materials (Fig. 1, B to D). Because all samples have the same Pd loading and particle size distribution, they can serve as the ideal model catalysts to unequivocally understand the catalytic effects of different metal-support interactions.

(A) Synthesis scheme. (B to D) HAADF-STEM images of Pd/PPS (B), Pd/SiO2 (C), and Pd/c-PPS (D). (E and F) Side-view TEM images and elemental mapping of Pd particles on PPS (E) and c-PPS (F) precoated on globular -Al2O3 particles.

H2 chemisorption on the Pd catalysts was carried out at 343 K to avoid the formation of -hydride phase (15). Pd/SiO2 showed an H/Pd ratio of 0.23, which corresponded to spherical Pd particles with a diameter (dchem) of 4.9 nm (15). The good consistency between the Pd particle diameters determined by electron microscopy (dEM = 5.1 nm) and chemisorption (dchem = 4.9 nm) confirmed the successful removal of oleylamine stabilizer from the Pd surface (13). In contrast, Pd/PPS showed undetectable chemisorption (H/Pd < 0.01), although oleylamine was removed by the same acetic acid treatment. This implied that the Pd surface was still covered by some strongly binding species. We postulated that even the mild sample pretreatment temperature for chemisorption experiments (373 K) induced the surface coverage of Pd particles with mobile PPS chains. The possible formation of bulk palladium sulfide phases via polymer decomposition could be excluded, because extended x-ray absorption fine structure (EXAFS) analysis showed only the presence of Pd-Pd coordination (coordination number: 9.24) and negligible Pd-S coordination (fig. S2 and table S1). Pd/c-PPS exhibited a slightly smaller chemisorption (H/Pd = 0.19) than Pd/SiO2 (0.23) presumably due to a minor Pd surface coverage with the polymer framework.

High-resolution transmission electron microscopy (TEM) was used to directly prove the Pd surface coverage by the polymer frameworks. To efficiently obtain the side-view images of Pd particles supported on PPS and c-PPS surfaces, we first coated PPS and c-PPS on globular -Al2O3 particles via melt coating and then supported the premade Pd particles (see Methods). The samples were treated similarly with acetic acid and then thermally treated in H2 at 373 K. As shown in TEM image and elemental mapping (Fig. 1E), the Pd particles supported on the PPS-coated -Al2O3 were fully covered with a thin layer of the polymer. On the other hand, the Pd particles supported on the c-PPScoated -Al2O3 (Fig. 1F) were mainly positioned on top of the polymer layer, showing only minor polymer coverage on the periphery of Pd particles. These results indicate that the mobile polymer chains of PPS could fully cover the surface of Pd particles at 373 K, while the cross-linked rigid framework of c-PPS exhibited only limited coverage. These results are consistent with the earlier chemisorption data.

To understand the temperature effect on the surface coverage of Pd particles with PPS chains, temperature-programmed H2-D2 exchange experiments were performed (Fig. 2A). Pd/SiO2 showed monotonically increasing HD formation with increasing temperature. Compared to Pd/SiO2, Pd/PPS showed substantially lower HD formation at the lowest temperature (313 K). This indicates that part of the Pd surface was already covered by PPS chains right after the catalyst preparation (note that final drying temperature during catalyst preparation was also 313 K). The HD formation over Pd/PPS slightly increased up to 353 K and then decreased to almost zero. The result indicates that the remaining fresh Pd surface was completely covered by PPS above 353 K. This temperature is close to the generally known glass transition temperature of PPS (358 K), where the polymer chains start having sufficient mobility. Pd/c-PPS showed monotonically increasing HD formation as in the case of Pd/SiO2. However, its HD formation was always smaller than that of Pd/SiO2, confirming that the Pd surface was partly covered by the polymer framework.

(A) Temperature-programmed H2-D2 exchange over Pd catalysts. The H2/D2 gas mixture (10-kPa H2, 10-kPa D2, 80-kPa Ar) was flowed over Pd/PPS, Pd/SiO2, and Pd/c-PPS during a temperature ramp of 2 K min1. a.u., arbitrary units. (B) MD simulation results showing that PPS chains can cover the Pd particle surface at 360 K (~glass transition temperature of PPS). Pd, C, H, and S atoms are shown in dark cyan, gray, white, and yellow, respectively.

To get insight into the Pd-PPS interaction at the atomistic level, molecular dynamics (MD) simulation was carried out for a Pd particle supported on the surface of crystalline PPS. Force-field (FF) parameters were carefully prepared to reproduce key physical parameters of PPS: experimental lattice parameters of crystalline PPS, quantum mechanical interchain interaction energy from density functional theory (DFT) calculations (fig. S3), and a glass transition temperature (fig. S4). FF parameters for Pd-PPS interaction was fitted using the DFT energetics (fig. S5), while interatomic interaction of the metal particle was described using the embedded-atom method (EAM) (16). When the MD simulation was performed at 300 K, PPS remained crystalline and the Pd particles still positioned on top of it (fig. S6). At 360 K (~glass transition temperature of PPS), the sulfide groups of PPS strongly interact with the Pd surface (interaction energy of ~22 kcal/mol per S), which provides a thermodynamic driving force to make PPS chains climb up the Pd particle surface (Fig. 2B). Sufficient thermal energy near the glass transition temperature appears to allow the PPS chains to overcome the interchain interactions and migrate to the Pd surface. When the temperature was cooled to 300 K again, the Pd surface still remained covered by PPS chains (fig. S6) because of strong Pd-PPS interactions.

Thermochemical stabilities of the metal-free PPS and Pd/PPS catalyst were investigated by thermogravimetric analysismass spectrometry (TGA-MS) under H2 atmosphere (fig. S7). The pristine PPS showed weight loss only above 673 K along with the generation of H2S, indicating its very high thermal stability compared to those of typical organic polymers. On the other hand, Pd/PPS showed a weight loss from a somewhat lower temperature (523 K). A small evolution of H2S was also detected at this temperature. This result indicates that the supported Pd particles can accelerate the degradation of PPS chains by catalyzing the desulfurization of sulfide linkages in the polymer backbone. Nevertheless, this result shows that Pd/PPS can be safely used up to 523 K under H2 atmosphere, which is adequate for various selective hydrogenation reactions (8, 17).

The prepared Pd catalysts were investigated for the partial hydrogenation of acetylene in an ethylene-rich stream (ethylene/acetylene = 82, H2/acetylene = 1.5) at 373 K. This reaction is very important in the petrochemical industry for removing acetylene impurity from ethylene, which would otherwise poison the downstream ethylene polymerization catalysts (8). In this reaction, high ethylene selectivity at complete acetylene conversion (>99%) is important for minimizing the ethylene loss. In industry, Pd-based catalysts have been widely used because of their high catalytic activity and good ethylene selectivity (9).

As shown in Fig. 3 (A to C), all Pd catalysts showed similar acetylene conversion as a function of contact time [1/weight hourly space velocity (WHSV)] and required the same minimum contact time (1/WHSV) of 4.83 hours for achieving full acetylene conversion (>99%). Pd/PPS exhibited substantially higher ethylene selectivity than Pd/SiO2 and Pd/c-PPS. Even when the contact time (1/WHSV) was excessively increased above 4.83 hours, high ethylene selectivity could be maintained (>65%) and H2 consumption did not increase noticeably above 80% (Fig. 3A). This means that ethylene was not substantially hydrogenated even after full acetylene conversion. In contrast, Pd/SiO2 (Fig. 3B) and Pd/c-PPS (Fig. 3C) showed a steadily decreasing ethylene selectivity (down to 30 and 34%, respectively) and increasing H2 consumption with increasing 1/WHSV. These results indicated that both Pd/SiO2 and Pd/c-PPS substantially hydrogenated ethylene when acetylene was largely consumed.

(A to C) Acetylene/hydrogen conversions and product selectivities over Pd/PPS (A), Pd/SiO2 (B), and Pd/c-PPS (C) as a function of 1/WHSV (reaction conditions, 373 K; 0.9-kPa H2, 0.6-kPa acetylene, 49.3-kPa ethylene, 0.6-kPa propane, 48.6-kPa N2; WHSV = 0.031 to 1.9 gacetylene gcatalyst1 hour1). (D to F) Long-term reaction data for Pd/PPS (D), Pd/SiO2 (E), and Pd/c-PPS (F) at the 1/WHSV of 4.83 hours.

Long-term reaction data at the 1/WHSV of 4.83 hours (i.e., minimum contact time enabling full acetylene conversion) are shown in Fig. 3 (D to F). Pd/PPS (Fig. 3D) showed complete acetylene conversion and stable ethylene selectivity (>65%) for 200 hours. In contrast, Pd/SiO2 (Fig. 3E) showed a rapid decrease in acetylene conversion (<75%) and ethylene selectivity (<22%) during the same period. This catalyst showed increasing selectivity to the full hydrogenation product, ethane, despite decreasing acetylene conversion. This means that the active sites for acetylene hydrogenation were deactivated, while the sites for preferential ethylene hydrogenation were newly generated during the reaction. To explain such behaviors, it was proposed that coke deposited on the catalyst surface can preferentially hydrogenate ethylene via hydrogen spillover (9). Pd/c-PPS (Fig. 3F) also showed a substantial decrease in acetylene conversion with reaction time, indicating gradual catalyst deactivation. Compared to Pd/SiO2, Pd/c-PPS showed a slower decrease in acetylene conversion and a more stable ethylene selectivity, indicating improved catalyst stability.

As explained earlier, the reaction temperature for acetylene hydrogenation (373 K) was high enough to induce the full coverage of the Pd surface with mobile PPS chains in Pd/PPS. Therefore, the superior ethylene selectivity and long-term stability of Pd/PPS are likely to originate from the formation of a unique Pd-PPS interface. Another important question regarding Pd/PPS is how the catalyst with no apparent hydrogen activation capability (i.e., negligible H2 chemisorption and H2-D2 exchange activity) could hydrogenate acetylene to ethylene in a similar rate to those of Pd/SiO2 and Pd/c-PPS with fresh Pd surfaces.

To understand the reaction mechanism over Pd/PPS, we carried out H2-D2 exchange experiments with and without co-injection of acetylene or ethylene at 373 K (Fig. 4). When a simple H2/D2 mixture was flowed over Pd/PPS (Fig. 4A), no HD formation was detected, confirming the absence of hydrogen activation capability. When acetylene was co-injected into the H2/D2 stream, a sudden formation of HD was observed along with the formation of ethylene (either deuterated or nondeuterated). These results indicate that H2/D2 cannot be activated (or dissociatively adsorbed) alone on the Pd surface but can be activated in the presence of co-adsorbed acetylene, thereby converting them into ethylene. This is a strong evidence indicating cooperative adsorption of acetylene and H2/D2 at the Pd-PPS interface. When ethylene was co-injected into the H2/D2 stream (Fig. 4D), no formation of HD and ethane was observed. This implied that cooperative adsorption of ethylene and H2/D2 did not occur.

(A to C) H2-D2 isotope exchange at 373 K over Pd/PPS (A), Pd/SiO2 (B), and Pd/c-PPS (C) with and without co-injection of acetylene. (D to F) H2-D2 isotope exchange at 373 K over Pd/PPS (D), Pd/SiO2 (E), and Pd/c-PPS (F) with and without co-injection of ethylene. Products were analyzed with a quadrupole mass spectrometer. (G) MD simulation of acetylene adsorption on Pd/PPS, which shows that acetylene penetrates the void space between PPS chains and is adsorbed on the Pd surface, lifting the PPS chains and enlarging the pocket beneath them (C2H2 is indicated in magenta, and all other color codes are the same as in Fig. 2B).

These results can be interpreted that the acetylene-Pd interaction is strong enough to locally detach the PPS chains from the Pd surface, thereby providing accessible Pd sites for the adsorption of H2/D2. In contrast, ethylene-Pd interaction might be too weak to disturb the Pd-PPS interface. To support this postulation, we carried out DFT calculations to understand the adsorption thermodynamics of acetylene, ethylene, and hydrogen on the Pd surface (fig. S8), compared to that of diphenyl sulfide (a basic building unit of PPS; as an estimate of Pd-PPS interaction). The binding free energy (GB) of diphenyl sulfide was calculated to be 22.05 kcal/mol, which was between that of ethylene (GB of 7.75 kcal/mol) and acetylene (GB of 37.58 kcal/mol). H2 dissociative adsorption exhibited GB of 17.57 kcal/mol, indicating a less favorable adsorption than that of diphenyl sulfide. These DFT energetics confirmed that only acetylene can effectively compete with the diphenyl sulfide units of PPS for adsorption on the Pd surface. This was further confirmed by the fact that the Pd/SiO2 catalyst modified with diphenyl sulfide as a molecular promoter showed similar H2-D2 exchange behaviors to those of Pd/PPS (fig. S9). In addition to such thermodynamic aspects, it is also possible that the PPS overlayer further kinetically hindered the access of ethylene to the Pd surface. The kinetic diameter of ethylene (0.39 nm) is substantially larger than that of acetylene (0.33 nm), and its penetration through the PPS overlayer should be much slower. The gas permeability measurements through a commercial PPS film (100 m thickness) revealed that ethylene permeability (1.3 1011 mol m m2 s1 Pa1) is five times smaller than acetylene permeability (6.7 1011 mol m m2 s1 Pa1).

To better understand the structural reorganization of PPS chains on the Pd surface during acetylene adsorption, we additionally performed MD simulations. Acetylene adsorption on Pd was modeled using a Lennard-Jonestype pairwise potential between Pd and C of acetylene and selecting LJ parameters to reproduce the adsorption energy from DFT (fig. S10). From MD trajectories, we sampled an instance when the gas-phase acetylene was adsorbed onto the PPS-covered Pd surface. As shown in Fig. 4G, (i) a small acetylene molecule first penetrates the void space between PPS chains, (ii) is adsorbed on the exposed Pd surface while enlarging the void space further, and then (iii) is loosely covered by PPS chains to maximize van der Waals (vdW) interactions. This result implies that the adsorption of small acetylene molecules can generate accessible Pd sites for H2 adsorption (i.e., sites for H2-D2 exchange) by lifting the PPS chains and widening the pocket beneath them.

In the H2-D2 exchange experiments with Pd/SiO2 (Fig. 4, B and E) and Pd/c-PPS (Fig. 4, C and F), a substantial amount of HD was immediately formed during the flow of a H2/D2 mixture, indicating the presence of fresh Pd surface. When acetylene was co-injected into the H2/D2 stream, HD formation was substantially reduced, while ethylene and ethane were produced (Fig. 4, B and C). When ethylene was co-injected into the H2/D2 stream, HD formation was again decreased, while the production of ethane was detected (Fig. 4, E and F). The reduced HD formation with the co-injection of acetylene/ethylene is in clear contrast to the case of Pd/PPS (i.e., HD formation was markedly enhanced with the co-injection of acetylene; Fig. 4A). Such behaviors can be interpreted that the surface coverage of Pd by acetylene/ethylene reduces the number of available sites for H2-D2 exchange (hydrogen chemisorption). This means that the adsorption of H2/D2, acetylene, and ethylene is all competitive on the fresh Pd surface, which is in line with the classical Langmuir-Hinshelwood mechanism (1).

Repeated acetylene and ethylene hydrogenation cycles were additionally carried out under (i) 0.6-kPa acetylene/0.9-kPa H2 in Ar and (ii) 0.6-kPa ethylene/0.9-kPa H2 in Ar, respectively. Pd/PPS (Fig. 5A) showed complete acetylene hydrogenation in step (i), while showing negligible ethylene hydrogenation (<2%) in step (ii). Such behaviors could be fully reproduced over the three repeated cycles. The result indicates that the PPS layer covering the Pd catalyst surface reversibly switches the hydrogenation activity on and off, enabling exclusively the hydrogenation of acetylene. On the other hand, Pd/SiO2 (Fig. 5B) showed high conversion of not only acetylene (>90%) in step (i) but also ethylene (>70%) in step (ii). This result indicates that the fresh Pd surface of Pd/SiO2 allows unconstrained ethylene adsorption and its subsequent hydrogenation to ethane. Pd/c-PPS (Fig. 5C) showed intermediate behavior. Our experimental and theoretical investigations reveal that the PPS chains decorating the Pd surface can act like a membrane that selectively allows the cooperative adsorption of acetylene and hydrogen but not ethylene and hydrogen. Once acetylene is fully consumed, ethylene in the gas phase cannot be adsorbed on Pd, because PPS chains re-cover the Pd surface. Such a unique action of the PPS overlayer (Fig. 5D) can explain why Pd/PPS exhibited suppressed ethylene hydrogenation even after full consumption of acetylene in the ethylene-rich stream.

(A to C) Acetylene/ethylene conversions and product selectivities over Pd/PPS (A), Pd/SiO2 (B), and Pd/c-PPS (C) during repeated acetylene and ethylene hydrogenation cycles (reaction condition, 373 K; 0.9-kPa H2, 0.6-kPa acetylene or ethylene, 98.5-kPa N2; WHSV = 0.25 gacetylene or ethylene gcatalyst1 hour1). (D) Proposed scheme for the selective acetylene partial hydrogenation over Pd/PPS. (i) In the initial stage, PPS chains cover the entire surface of supported Pd catalysts due to strong Pd-PPS interaction. (ii) Acetylene, a strongly binding species on the Pd surface, can disturb the Pd-PPS interface and induce cooperative adsorption of H2. (iii) Once acetylene is hydrogenated to ethylene (i.e., weakly binding species on Pd), PPS chains are readsorbed on the Pd surface while repelling ethylene into the gas stream. (iv) After full conversion of acetylene, ethylene and H2 cannot be adsorbed on the Pd surface due to the stable Pd-PPS interface, thereby inhibiting the formation of a fully hydrogenated product, ethane.

In the long-term reactions (Fig. 3, D to F), Pd/PPS exhibited much higher catalytic stability than Pd/SiO2 and Pd/c-PPS. According to EXAFS and HAADF-STEM investigations (figs. S2 and S11 and table S1), all the catalysts did not show substantial chemical state change or sintering of Pd after the reaction. Catalyst deactivation in this reaction is known to occur due to the deposition of coke (i.e., unsaturated species, insoluble in organic solvent) and/or polymeric species, so-called green oil (i.e., saturated species, soluble in organic solvent), via hydropolymerization of acetylene (18). Earlier studies showed that coke is much more harmful than green oil for catalyst deactivation (18). In this regard, we collected the used catalysts at different time on streams and separately determined the amounts of green oil and coke (see Methods). As shown in Fig. 6A, Pd/PPS only accumulated less harmful green oil; more harmful coke deposition was not observed. Even the rate of green oil accumulation markedly slowed down after 100-hour reaction. On the other hand, Pd/SiO2 (Fig. 6B) and Pd/c-PPS (Fig. 6C) showed fast and steady accumulation of both green oil and coke. These results explain why Pd/PPS showed superior catalyst stability to those of Pd/SiO2 and Pd/c-PPS. The suppressed green oil and coke deposition in Pd/PPS implies that green oil/coke precursors (e.g., polyunsaturated olefins) might be repelled from the Pd surface before their polymerization/dehydrocyclization, because of the strong interaction between Pd and PPS chains (i.e., similar to the ethylene exclusion from the Pd surface).

(A to C) Amounts of green oil and coke deposited in Pd/PPS (A), Pd/SiO2 (B), and Pd/c-PPS (C) as a function of time on stream in acetylene hydrogenation (reaction conditions, 373 K; 0.9-kPa H2, 0.6-kPa acetylene, 49.3-kPa ethylene, 0.6-kPa propane, 48.6-kPa N2; 1/WHSV of 4.83 hours).

Elemental analysis of the used Pd/PPS after washing with dichloromethane to remove green oil showed undetectable sulfur loss, indicating the very high thermochemical stability of PPS. It is noteworthy that the Pd/SiO2 catalyst modified with diphenyl sulfide as a molecular promoter showed improved ethylene selectivity, similar to Pd/PPS at the beginning of acetylene hydrogenation (fig. S12). However, the catalyst showed rapidly decreasing acetylene conversion and ethylene selectivity with time on stream. Even after relatively short reaction time of 50 hours, no sulfur was detected in the spent catalyst by elemental analysis, whereas the fresh catalyst contained 0.45 wt % of sulfur. This could be attributed to the rapid leaching of diphenyl sulfide via evaporation at the reaction temperature (373 K; fig. S13). These results implied that monomeric diphenyl sulfide can play a similar catalytic role to that of polymeric PPS, although the former has a much lower thermochemical stability under the reaction conditions.

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Dynamic metal-polymer interaction for the design of chemoselective and long-lived hydrogenation catalysts - Science Advances

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