Adsorption Behavior and Electron Structure Engineering of Pd-Based Catalysts for Acetylene Hydrochlorination

Adsorption and activation for substrates and the stability of Pd species in Pd-based catalysts are imperative for their wider adoption in industrial and practical applications. However, the influence factor of these aspects has remained unclear. This indicates a need to understand the various perceptions of the structure–function relationship that exists between microstructure and catalytic performance. Herein, we revisit the catalytic performance of supported-ionic-liquid-phase stabilized Pd-based catalysts with nitrogen-containing ligands as a promoter for acetylene hydrochlorination, and try to figure out their regulation. We found that the absolute value of the differential energy, |Eads(C2H2)-Eads(HCl)|, is negative correlated with the stability of palladium catalysts. These findings imply that the optimization of the electron structure provides a new strategy for designing highly active yet durable Pd-based catalysts.

Recent studies have shown that Pd supported catalysts could be an efficient, low-cost, mercury-free alternative for acetylene hydrochlorination [29,[36][37][38][39][40][41][42][43]. However, the inherent volatilization of Pd component limits the use of Pd-based catalysts for producing VCM [36][37][38][39][40][41][42][43]. We have previously reported a supported-ionic-liquid-phase (SILP) Pd catalysts (Pd-IL/AC) for the acetylene hydrochlorination [29]. The presence of ionic liquids proved to be formed a more active and durable catalyst, which furnishes significant promotion on Pd species dispersion, valence states and structure stability, deposition reduction and thermal stability. Recently investigators have examined the effects of NH 4 Cl on Pd catalyst, which greatly enhanced the adsorption behavior for reactants via the construction of a micoelectronic environment [30]. It is now well established that adsorption and reaction behavior can be controlled by ionic liquids and NH 4 Cl ligands. However, the influence of these additives on the active site structures and reaction process have remained unclear. This indicates a need to understand the various perceptions of the structure-function relationship that exists between the microstructure and catalytic performance of Pd-based catalysts. Herein, we synthesized a series of N-containing ligands Pd-based catalysts with ionic liquids as medium. The addition of N-containing ligands can selectivity modify the adsorption energy for HCl and C 2 H 2 on Pd active sites. Among the various N-containing ligands, NH 4 Cl is able to furnish Pd-based catalysts in a higher catalytic activity and durability. Furthermore, by designing differential adsorption energy, |E ads (C 2 H 2 )-E ads (HCl)|, which features the difference of adsorption intensity for HCl and C 2 H 2 , a bridge between |E ads (C 2 H 2 )-E ads (HCl)| and Pd catalysts performance can effectively be established to achieve concurrently high catalytic durability. Following this trail, we further explored the effect of nitrogen ligands on the electron structure of Pd-based catalysts. Results illustrate that nitrogen-containing ligands, along with ionic liquids strongly control the electron density of metal center and obviously improve the catalytic properties.

Catalytic Performance of Pd-Based Catalysts
First, NH 4 Cl was used as the representative N-ligands to evaluate the catalytic performance of Pd-N-IL/AC catalysts in acetylene hydrochlorination reaction. The investigated catalysts, including AC, Pd/AC, Pd-IL/AC Pd-A/AC, A/AC, IL/AC, A-IL/AC, Pd-0.5A-IL/AC, Pd-1A-IL/AC, Pd-2A-IL/AC, and Pd-3A-IL/AC have been performed at a temperature of 160 • C and the GHSV of 1000 h −1 . From Figure 1a, we note that catalysts without ionic liquids exhibits almost no acetylene conversion after 10 h. Although in the presence of NH 4 Cl promoter the Pd-A/AC catalyst exhibits a relatively high initial activity than Pd/AC catalyst, the deactivation rate was extremely fast. However, when we used the Pd-IL/AC catalyst, which loaded with 20% IL, the conversion of acetylene rises to 80%, meanwhile, furnishes an excellent stability. The further used Pd-A-IL/AC catalyst with 0.5NH 4 Cl promoted the conversion rises to 85.3%. With the rising loading of NH 4 Cl, the conversion of acetylene increases obviously. The catalysts, including Pd-1A-IL/AC, Pd-2A-IL/AC, and Pd-3A-IL/AC (0.5, 1, 2 and 3 are the molar multiple of Pd) achieve the acetylene conversion to 88%, 91.1%, and 89.3% at 10 h, respectively. The conversion of acetylene reaches a maximum as the amount of NH 4 Cl is up to 2, indicating that 2 is the optimal addition amount of NH 4 Cl for Pd-A-IL/AC catalyst. However, for the metal-free catalysts, A/AC and IL/AC, the conversion of acetylene is only 2.28% and 1.21%. Thus, the excellent catalytic performance over Pd-A-IL/AC is attributed to the synergistic effect among NH 4 Cl, ILs and Pd components. All these catalysts furnished almost 100% selectivity to VCM.
The catalysts with other three kinds N-ligands additive were also performed in the reaction of acetylene hydrochlorination. As is shown in Figure 1b, the catalytic performance of Pd-2A-IL/AC, Pd-2EDA-IL/AC, Pd-2PR-IL/AC, and Pd-2PD-IL/AC were performed. The initial conversion of acetylene over these catalysts achieves at 97.2%, 97%, 90.3%, and 91.1%, respectively. Interestingly, all these catalysts exhibit higher stability than Pd-IL/AC catalyst, indicating that these N-containing ligands indeed can enhance the stability of supported-ionic-liquids-phase Pd catalyst.
Different space velocity tests were also performed to further to investigate the catalytic ability of Pd-2A-IL/AC catalyst in the reaction of acetylene hydrochlorination. As is shown in Figure 1c, the acetylene conversion attained 99% and 82% as the GHSV increases from 50 h −1 to 1480 h −1 at 160 • C. We have demonstrated that the designed Pd-A-IL/AC catalyst is adequate for acetylene hydrochlorination in a wide space velocity range. To comprehensive illustrate the catalytic performance evolution of the active phase for Pd-2A-IL/AC catalyst, the space time yield (STY) were discussed for relevant literature reported (Table S1) acetylene hydrochlorination catalysts [1,21,27,[29][30][31], and the results were displayed in Figure 1d. In the case of catalysts with ionic liquids, the results show that Pd-IL/AC and Pd-2A-IL/AC have better catalytic activity and higher STY, while the presence of N-containing ligands and ionic liquids together result in an even greater enhance in catalytic activity. The enhancement of catalytic ability is attributed to the improvement on the intrinsic electron properties of the catalyst, which implies a synergic effect for Pd active sites by N-containing ligands and ionic liquids that results in a highly active catalyst. Surprisingly, the STY of Pd and was less active than the Au-Cu-IL/AC catalyst.

Characterization of Pd-Based Catalysts
A lot of studies have made great progress towards increased resistance against active component leach of Pd catalysts. Wang et al. introduced K and NH 4 F promoters in Pd/HY catalyst. The presence of K and NH 4 F promoters exhibit a good effect on the acidity on the surface of catalyst and effectively inhibit the run off of Pd component [38][39][40][41][42][43]. Hu et al. reported an (NPs)/surfactant ionic liquids system and constructed an effective redox cycle in catalytic system [61]. The presence of ionic liquids and redox cycle have an instructive effect on the stabilization of Pd component. Zhao et al. synthesized a Pd-IL/AC catalyst by adopting SILP system [29]. The catalyst exhibits a stable catalytic performance within 500 h. He et al. reported a NH 4 + promoted palladium catalyst which can enhance the stability of Pd component and improve the adsorption ability of HCl molecule [30]. Obviously, ionic liquids and nitrogen-containing ligands play an important role in improving the catalytic performance of Pd catalyst.
To get a further understanding of the effects of N-containing ligands on structure and stability, fresh and used Pd-2A-IL/AC catalysts are characterized by TEM, XRD, and XPS. Figure 2 displays the HAADF-STEM images of fresh and used Pd-based catalysts. As is shown, for fresh Pd-IL/AC (Figure 2b), the small amount of dispersed particles was detected on support, less than that on Pd/AC ( Figure S1) and Pd-2A-IL/AC (Figure 3a) catalysts can also prove that the co-addition of ionic liuids and ligands could further enhance the stability of Pd species during the reaction process. For the used Pd/AC and Pd-IL/AC catalysts both presented distinguishable diffraction peaks in XRD profiles as is shown in Figure S1, however, the XRD profile of used Pd-2A-IL/AC catalysts still exhibited amorphous peaks that corresponded to activated carbon. Meanwhile, the co-addition of ionic liuids and ligands improves the dispersion of active components (Pd-A-IL) on support compared with Pd-IL/AC catalysts, which can be confirmed by the BET results in Table 1, as the surface area and pore volume of Pd-A-IL/AC series catalysts (>688 m 2 g −1 ) were higher than Pd-IL/AC catalyst (653 m 2 g −1 ). These results show that NH 4 Cl can not only improve the dispersion of Pd species in the catalysts, but also inhibit to some extent the aggregation of Pd species during the reaction, which can be further verified by EDS mapping in Figure 3. To further investigate the enhancement of NH 4 Cl on Pd catalyst stability. XRD and XPS analysis were also performed. Figure 3a shows the XRD results, we can see that no significant metallic Pd crystals were formed in the Pd-2A-IL/AC catalyst. Supplementary Figure S2 compares the XPS results of fresh and used Pd/AC and Pd-Il/AC catalysts, the fresh and used Pd/AC catalysts both exhibited Pd 0 peaks at 334.6 eV, and the Pd 0 species fraction used in Pd/AC were slightly higher than in fresh Pd/AC catalyst. For the Pd-IL/AC catalyst, both fresh and used catalysts exhibited Pd 2+ peaks and no Pd 0 peaks were detected, which was conflicted with that of TEM and XRD observations, this may ascribe to that the Pd 0 fraction was too low to detect by XPS or the fraction difference between Pd 2+ and Pd 0 was too large. The XPS spectra for the Pd (Figure 3b) of fresh and used Pd-2A-IL/AC samples showed a main peak at 337.4 eV, suggesting that Pd species exhibit positive charges (Pd 2+ ) rather than Pd 0 . Zhao et al. has reported that the introduce of ionic liquids (BMImCl) can dramatically enhance the structure of Pd species and valence states in acetylene hydrochlorination stability [29]. This is indicative of the fact that Pd-2A-IL/AC catalyst also has an excellent stability on the Pd valence states as well as Pd dispersion.  Previous research has proved that cationic Pd species works as catalytic active site in hydrochlorination catalysts [29,30,36]. However, the leaching of Pd active component (in the form of PdCl 2 -C 2 H 2 ) and the reduction of Pd 2+ species during acetylene hydrochlorination reaction has also been widely accepted [36,[62][63][64]. Surprisingly, the addition of ionic liquids and N-containing additives can not only improve the distribution of Pd complex, but also enhance the thermal and chemical stability of Pd species in reaction. For one hand, Pd species exists entirely in the form of Pd 2+ in the fresh and used Pd-2A-IL/AC catalyst, indicating that the synergistic effect of N-containing ligands and ionic liquids can greatly enhanced the stability of Pd valence as well as dispersion. For another hand, as listed in Table 2  High angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and energy-dispersive spectrum (EDS) mapping revealed that Palladium, Nitrogen, and Chlorine elements were homogeneously distributed over the fresh and used Pd-A-IL/AC catalysts (Figure 3c,d) [21,27,29,30,65]. Furthermore, the EDS-determined N content on the used Pd-A-IL/AC was ca. 2.07 wt% (Table 3), which is, almost consistent with that of fresh Pd-2A-IL/AC catalyst (2.05 wt%), confirming that the N-ligand exist steadily during the reaction. On the basis of the above discussion, unambiguously, the stability of Pd component was greatly improved as expected. It is speculated that the microelectronic environment of active Pd 2+ center was changed and the adsorption behavior of Pd species was enhanced via the synergistic effect of NH 4 Cl and ionic liquids, which will be further discussed in the following part.

Stability and Coke Deposition of Pd-2A-IL/AC Catalyst
As one of the main reasons for catalyst deactivation, coke deposition especially occurred on metal-based catalyst like Pd catalyst. Aiming to study the coke deposition for the Pd-2A-IL/AC catalyst, we performed TGA measurements. Figure 4a shows the TGA results, we can see that both the fresh and used Pd-2A-IL/AC have a slight weight loss before 150 • C, due to the desorption of water. Generally, in the temperature range of 150-400 • C, the weight loss can be ascribed to the burning of coke. However, the decomposition temperature of 1-butyl-3-methylimidazolium chloride is about 254 • C. Therefore, in the temperature range of 150-254 • C, the weight loss for the used catalyst is ascribed to the coke deposition. According to the profile shown in Figure 4b, the coke deposition amount for the used Pd-2A-IL/AC catalyst is 1.16%. Table 4 lists the coke deposition amounts on the used Pd/AC, Pd-IL/AC and various Pd-A-IL/AC catalysts. For the Pd-A-IL/AC catalysts, the amount of coke deposition decreases with the increase doping content of NH 4 Cl, with the lowest amount of 1.09% on the used Pd-1A-IL/AC catalyst. All N-containing ligands doping supported ionic liquids Pd-based catalysts show lower coke deposition amount than that of Pd/AC and Pd-IL/AC catalysts. It illustrates that the co-addition of ionic liquids and N-containing ligands can efficiently inhibit coke deposition on the Pd based catalyst.

DFT Calculation
The strong adsorption capacity for acetylene leads to the serious deactivation of metal catalysts in the form of coke deposition, reduction and active components leaching [36,62,63]. To surmount the intrinsic limitations of typical palladium catalysts in acetylene hydrochlorination (i.e., leaching of Pd component), studies have focused on the modulation of the catalyst formulation, including carrier screening/doping and additive adding, to improve their durability. However, the intrinsic properties that influence the activity and stability of Pd catalysts in acetylene hydrochlorination have not been studied yet. Following this trail, we studied above discussed four nitrogen-containing-ligands ( Figure 5) modified Pd-based catalysts via DFT calculation to figure out their regulation principle and the results were shown in Figure 6. From Figure 6a and Table S2, we notice that the addition of NH 4 Cl or ionic liquids both can decreased the acetylene adsorption energy and the co-addition of nitrogen-containing ligands and ionic liquids can further modulated the adsorption behavior for acetylene. In particular, the addition of N-ligands can flexibly adjust the absorption properties for hydrogen chloride. It is well known that the strong adsorption of acetylene often leads to the loss and reduction of metal species, therefore, the relative low acetylene adsorption energy (E ads (C 2 H 2 )) and moderate hydrochloride adsorption energy (E ads (HCl)) would beneficial to the catalytic performance and durability of catalysts. Figure 6a also shows a correlation between adsorption energies and catalytic activity of Pd-based catalysts. Without exception, compared with Pd/AC and Pd-A/AC, the catalysts with ionic liquids furnish higher VCM formation rate, and the co-addition of nitrogen-containing ligands and ionic liquids further improves the formation rate.  Moreover, we evaluated the catalytic properties utilizing |E ads (C 2 H 2 )-E ads (HCl)| as a unified parameter to explain the intrinsic catalytic stability. The |E ads (C 2 H 2 )-E ads (HCl)| of the obtained Pd-based catalysts were carefully examined by DFT. Surprisingly, |E ads (C 2 H 2 )-E ads (HCl)| is very significanttly correlated with the durability of investigated Palladium catalysts as shown in Figure 6b. Rises in |E ads (C 2 H 2 )-E ads (HCl)| paralleled increase in deactivation rates, which implies that the higher durability came from the similar adsorption intensity for acetylene and hydrochlorination. Based on the information discussed above, the approximate adsorption strength to acetylene and hydrogen chloride is beneficial to Pd-based catalyst.
To further understand the intrinsic properties of Pd catalysts, we likewise calculated the Hirshfeld charge of Pd and N atoms in Pd-based catalysts, for it can provide the properties and feasible active sites for atoms in catalysts to reserve or release electrons in reaction. Hirshfeld charge on Pd atoms (C H (Pd)) all increased with the addition of N-containing ligands (Figure 6c and Table S3), this implies that the capacity of the Pd atoms to accept electrons is enhanced, thus, the adsorption strength (energy) to acetylene may also enhance. However, the adsorption energy of acetylene was not increased in all N-containing ligands catalysts, in contrast, all C H (Pd) were enlarged after the adsorption for acetylene. The increase in local Hirshfeld charge of Pd atoms probably comes from the presence of electron receptor effect brought about by the N-containing ligands, which can be confirmed by the Hirshfeld charge on the N atom (C H (N), where N is derived from nitrogen ligands). As is shown in Figure 6c and Supplementary Table S3, all C H (N) decreased after adsorbing acetylene in N-containing ligands Pd-based catalysts, indicating that partial of electrons were transferred from Pd atoms to N atoms during the adsorption. Pd atoms can serve as an electron transfer medium, which electronic properties can be tailored by doping with various N-containing ligands and can transfer the electron that from acetylene to nitrogen ligands during the activation process ( Supplementary Figures S3-S6). Taken together, our data provide a new insight into possible strategies for fabricating a stable Pd-based acetylene hydrochlorination catalyst involved in modifying the electronic structure and regulating adsorption behavior.

Insights into Pd-2A-IL/AC Catalyst
It is worth mentioning that Pd-2A-IL/AC catalyst exhibit the highest activity and durability during the reaction. Therefore, we further investigate the Pd-2A-IL/AC catalyst for insight into the mechanism.
As is shown in Figure 7c,d, the adsorption energy for acetylene and hydrogen chloride by different Pd active sites were investigated. Clearly, C 2 H 2 can strongly be adsorbed by PdCl 2 due to the endothermic nature of the adsorption process with the adsorption energy Eads = −125.02 kJ mol −1 , which was almost 2 times of HCl (Eads = −64.65 kJ mol −1 ). As for Pd-IL/AC catalyst, the adsorption energy for both acetylene and hydrogen chloride were greatly decreased. However, the adsorption energy for acetylene (Eads = −29.75 kJ mol −1 ) by the Pd active center is still far greater than that of hydrogen chloride (Eads = −14.46 kJ mol −1 ). Therefore, the possibility of reduction or loss of Pd species still exists in long-term catalytic reactions. Zhao et al. reported that about one tenth of the initial Pd component was ran off from Pd-IL/AC catalyst during the reaction process [29], which can be well explained by above calculation results. Encouragingly, with the further modification by N-containing ligands, i.e., Pd-A-IL/AC catalyst, the adsorption ability for HCl were markedly enhanced (Eads = −42.75 kJ mol −1 ), which was three times of Pd-IL/AC catalyst (Eads = −14.46 kJ mol −1 ). Meanwhile, the adsorption energy for hydrogen chloride was higher than acetylene (Eads = −26.28 kJ mol −1 ). These results imply that, theoretically, the catalyst adsorbs hydrogen chloride preferentially in the reaction, which avoiding the formation of sublimated PdCl 2 -C 2 H 2 , thereby greatly inhibiting the leaching of active components. Since the adsorption properties of Pd-A-IL/AC for substrates is well illustrated, we further studied the electronic properties of the Pd active center. The electron density distribution of Pd-2A-IL active center after interacting with acetylene and hydrogen chloride were shown in Figure 7a,b, respectively. From Figure 7a, we can see that the NH 4 + ligands adopt part of the electrons from PdCl 4 2complexes via Cl-bridges. In typical Pd catalyzed acetylene hydrochlorination reaction, acetylene adsorbed on PdCl 2 active sites, d-cavitation of Pd 3d accept electrons that donated from the triple bond of acetylene, thus acetylene was greatly activated. However, the electrons transferring from acetylene to Pd were excessive, which leads to an unexpected coordination between them. Surprisingly, in the presence of NH 4 + , the interaction was efficiently regulated. The NH 4 + cations act as electron acceptors, which accept excessive electrons. This kind of electron transfer mechanism has an instructive effect on the regulation of the adsorption strength for acetylene. In addition, the activation path of acetylene was completely changed, as is shown, the activation was accomplished under the synergistic effect of (NH 4 ) 2 PdCl 4 and ionic liquids. On the basis of the same principle, the adsorption ability of hydrogen chloride on active center is also being strongly controlled, the addition of NH 4 + along with PdCl 4 2− create a favorable environment for adsorption and activation of hydrogen chloride. Moderately activated acetylene molecules and enhanced hydrogen chloride transition state make the reaction easier, as could be confirmed by TPD results in Figure 7e.

Materials and Methods
Pd-based catalysts that loading with N-containing ligands were prepared by impregnation. For example, the (NH 4 ) 2 PdCl 4 complex was synthesized according to the literature [30,66,67]. First of all, 35.3 mg H 2 PdCl 4 and a certain amount of NH 4 Cl was dissolved in 8 mL water at 80 • C using ultrasound for 60 min to obtained yellow-brown (NH 4 ) 2 PdCl 4 crystals. Then, a certain amount of the (NH 4 ) 2 PdCl 4 and 1-butyl-3-methylimidazolium chloride ionic liquids were added in muriatic acid. In the end, 3 g activated carbon (AC) was mixed with the above prepared liquid accompanying agitation. Then the mixture was dried at 110 • C for 10 h to remove water, the prepared catalyst was named as Pd-xA-IL/AC (where A is the NH 4 + ligand, x is the mole ratio between NH4 + and Pd). Similar methods were also applied to prepare A/AC, IL/AC, Pd-EDA-IL/AC, Pd-PR-IL/AC, Pd-PD-IL/AC catalysts replacing NH 4 + with ethylenediamine (EDA), pyrrole (PR) and pyridine (PD), respectively.
Pd/AC catalyst was directly prepared from H 2 PdCl 4 . Without specifically mentioned, the loading of Pd in all catalysts were fixed at 0.5 wt% and the mole ratio of Pd:N-ligands were 1:2, the loading of IL in IL-containing catalysts were fixed at 20 wt%. All chemical reagents were bought from Sigma-Aldrich (St. Louis, MO, USA). Catalytic performance were tested in a heterogenous fixed-bed reactor. At first, nitrogen (N 2 ) was used to sweep the reactor for at least half hour to remove impurities, such as water and air. Then, C 2 H 2 and HCl were controlled by mass flow controllers and mixed in a gas mixer after cleaning and drying. The obtained products were washed with NaOH solution to remove the excess HCl and then analyzed by gas chromatography (GC-9790A, Fuli, Hangzhou, China).
The valence states of Pd were determined by X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS Ultra DLD apparatus (Shimadzu, Kyoto, Japan), selecting Al Kα as X-ray source, with a pass-energy of 40 eV. Inductively coupled plasma mass spectroscopy (ICP-MS, PerkinElmer Elan DRC-e, Shelton, CT, USA) was taken to determine the Pd loadings in catalysts. The dispersion and particles size of Pd were analyzed by X-ray diffraction (XRD) on a PANalytical X'pert diffractometer (Bruker, Karlsruhe, Germany). HAADF-STEM characterization was carried out on a Tecnai G2 F30 S-Twin electron microscope (FEI, Hillsboro, OR, USA) to investigate the element distribution of catalysts. The TPD was carried out on an AutochemII 2920 (Micromeritics Instrument Ltd., Atlanta, GA, USA) and recorded by gas chromatography. Thermogravimetric analysis (TGA) was conducted with a thermogravimetric differential scanning calorimetry simultaneous analyzer (NETZSCH STA449F5, NETZSCH, Selb, Germany).
In order to investigate the electron structures and adsorption properties of the investigated catalysts, geometry optimization was calculated in the AMS-ADF package, together with the B3LYP-D3 (BJ) functional and TZP corrections [65,[68][69][70].

Conclusions
In summary, a general strategy is developed to stabilize Pd active components in activated carbon via the synergistic effect of nitrogen ligand and ionic liquids. Meanwhile, a standard for modifying the catalyst electron structure and regulating the adsorption behavior of reactants is also established. In this approach, the addition of N-containing ligands not only changes the adsorption properties of catalysts to substrates, but also modifies the electronic structure of the Pd active center microenvironment. Additionally, the differential energy, |E ads (C 2 H 2 )-E ads (HCl)|, can accurately predict the durability of Pd-based catalyst. Despite its exploratory nature, this study offers some insight into the design principle of highly catalytic active and durable Pd-based catalysts, which reveal the enormous potential of Pd-based catalysts as promising candidates for Chlor-alkali (chlorine alkali) industry industrial applications.

Conflicts of Interest:
The authors declare no conflict of interest.