Immobilization of Peroxo-Heteropoly Compound and Palladium on Hydroxyapatite for the Epoxidation of Propylene by Molecular Oxygen in Methanol

Peroxo-heteropoly compound PO4[W(O)(O2)2] was synthesized on calcium-deficient hydroxyapatite using a reaction of surface [HPO4]2− groups on hydroxyapatite with a Na2[W2O3(O2)4] aqueous solution. The vibration of [HPO4]2− at 875 cm−1 became very weak, and the vibration of the peroxo-oxygen bond [O–O]2− at 845 cm−1 appeared in the FT-IR spectrum of the solid product, indicating that PO4[W(O)(O2)2] was formed on the surface of hydroxyapatite. The formed solid sample was further reacted with PdCl2(PhCN)2 in an acetone solution to fix PdCl2 between the O sites on the hydroxyapatite. Elemental analyses proved that the resultant solid contained 1.2 wt.% Pd, implying that PdCl2 molecules were immobilized on the surface of hydroxyapatite. The hydroxyapatite-based hybrid compound containing Pd and PO4[W(O)(O2)2] was used as a heterogeneous catalyst in a methanol solvent for propylene epoxidation by molecular oxygen in an autoclave batch reaction system. A propylene conversion of 53.4% and a selectivity for propylene oxide of 88.7% were obtained over the solid catalyst after reaction at 363 K for 8 h. The novel catalyst could be reused by a simple centrifugal separation, and the yield of propylene oxide did not decrease after the reaction for five runs. By prolonging the reaction time to 13 h, the highest yield of propylene oxide at 363 K over the solid catalyst was obtained as 53.8%, which was almost the same as that of the homogeneous catalyst containing PdCl2(PhCN)2 and [(C6H13)4N]2{HPO4[W(O)(O2)2]2} for the propylene epoxidation. Methanol was used as a solvent as well as a reducing agent in the propylene epoxidation by molecular oxygen. Small particles of Pd metal were formed on the surface of the hybrid solid catalyst during the reaction, and acted as active species to achieve the catalytic turnover of PO4[W(O)(O2)2] in the propylene epoxidation by molecular oxygen in methanol.


Introduction
Propylene oxide (denoted as PO), obtained from the epoxidation of propylene, is an important industrial intermediate to produce polyurethanes, unsaturated resins, and other products. The chlorohydrin process and hydroperoxide process are conventional industrial methods for producing PO from propylene. However, they are not environmentally benign methods due to the formation of a large amount of co-product CaCl 2 or t-butyl alcohol. Hydrogen peroxide (H 2 O 2 ) is an environmentally benign oxidant and exhibits a high selectivity for PO in propylene epoxidation [1]. However, H 2 O 2 is too expensive to achieve an economically viable process. Molecular oxygen (O 2 ) is the best oxidant from views of cost and environment. The catalytic oxidation of propylene by O 2 mainly forms acrolein, and the selectivity for PO is very low due to the high activity of allylic C−H bonds of propylene molecules [2,3]. The selectivity for PO, in the propylene oxidation by O 2 , greatly increased in the presence of a reducing agent, such as H 2 [4] and N 2 O [5]. In an industrial process, it is hard to handle the gaseous reducing agent H 2 or N 2 O in the presence of O 2 . We developed a homogeneous catalytic system containing Pd and peroxo-heteropoly compound for the propylene epoxidation by O 2 in methanol [6]. Methanol was used as a solvent as well as a reducing agent in the reaction. The catalytic system is promising in the industry because methanol is a tractable and reusable liquid-reducing agent [7]. Compared to homogeneous catalysts, heterogeneous catalysts are desirable in the industry because they are easily separated and reused [8][9][10]. We have designed two kinds of heterogeneous catalysts containing Pd and peroxo-heteropoly compound for the propylene epoxidation by O 2 in methanol [11,12].
Hydroxyapatite (denoted as HAP) has been used as a kind of useful material in various fields [34]. HAP attracts considerable attention as an effective catalyst and support in organic synthesis [35]. HAP-supported heteropolyacids have been reported as excellent catalysts in the epoxidation of olefin using H 2 O 2 as an oxidant [36][37][38]. Stoichiometric HAP has a formula of Ca 10 (PO 4 ) 6 (OH) 2 , giving a Ca/P molar ratio of 1.67 [34]. Nonstoichiometric HAP is a kind of calcium-deficient HAP with a formula of Ca 10−x (HPO 4 ) x (PO 4 ) 6−x (OH) 2−x , where x ranges from 0 to 2, giving a Ca/P molar ratio ranging from 1.67 to 1.33 [39][40][41][42]. The surface of calcium-deficient HAP contains the groups of HPO 4 , which can be used for surface modification [43]. On the other hand, PdCl 2 can be immobilized on the HAP surface and the PdCl 2 -immobilized HAP has been used as a highly effective catalyst in alcohol oxidation by O 2 [44][45][46][47][48].
This study reported a kind of novel hybrid compound prepared by immobilizing peroxo-heteropoly compound and PdCl 2 on the HPA surface at the same time. The hybrid compound showed high catalytic ability and reusability in the epoxidation of propylene by O 2 in methanol. Figure 1 shows the FT-IR spectra of various samples. HAP showed main peaks at around 1060, 875, 680, and 630 cm −1 in the FT-IR spectrum. These peaks coincided with the FT-IR spectrum of HAP in the literature [49]. The large peak at around 1060 cm −1 and its shoulder peaks were assigned to the vibrations of tetrahedral [PO 4 ] 3groups, and the peak at 875 cm −1 was assigned to the vibrations of [HPO 4 ] 2− groups in the calcium-deficient hydroxyapatite HAP [50][51][52]. The FT-IR spectrum of K 2 [W 2 O 3 (O 2 ) 4 ] showed peaks at around 980, 960, 845, 775, and 615 cm −1 , coinciding with the literature data [53]. The peak at 845 cm −1 was assigned to the stretching vibration of the peroxy-oxygen bond [O-O] 2− [54]. In the FT-IR spectrum of [(C 6 H 13 ) 4 N] 2 {HPO 4 [W(O)(O 2 ) 2 ] 2 } (denoted as THA-PW 2 ), the big peak at around 1070 cm −1 , and its shoulder peaks, were assigned to the stretching vibration of P−O, and the peak at 930 cm −1 was assigned to the stretching vibration of W=O nearby P−O bonds [53]. Moreover, the peak at 845 cm − could be observed in the spectrum in the PW-HAP. The FT-IR spectrum of PdCl 2 did not show an obvious peak in the range from 600 to 1400 cm −1 . On the contrary, PdCl 2 (PhCN) 2 showed many peaks correlating with the organic PhCN groups in the FT-IR spectrum. The IR spectrum of PdCl 2 -immobilized PW-HAP (denoted as Pd-PW-HAP) was almost the same as the IR spectrum of PW-HAP. The peaks of PhCN groups could not be observed in the IR spectrum of Pd-PW-HAP.    Figure S1 shows the GC charts of gas and liquid products after reaction at 363 K for 8 h over Pd-PW-HAP. Table 2 shows the results of propylene epoxidation by O 2 in methanol over various catalysts. PW-HAP showed high selectivity for PO of 90.3%, but the C 3 H 6 conversion was low (2.3%) after reaction at 363 K for 8 h. PW groups in PW-HAP contain peroxy-oxygen bonds [O-O] 2− which can stoichiometrically epoxidize olefin to olefin oxide [25]. However, the catalytic turnover of peroxy-oxygen bonds in PW-HAP could not be achieved by O 2 . PdCl 2 (PhCN) 2 showed a very low C 3 H 6 conversion of 2.1%, and a low selectivity for PO of 9.5% for the oxidation of propylene by O 2 . The solid catalyst Pd-PW-HAP showed a propylene conversion of 53.4% and a selectivity for PO of 88.7%, after reaction at 363 K for 8 h. The by-products were acrolein, propionaldehyde, acetone, propane, and other products (mainly ring-opened compounds of PO). The homogeneous catalyst Pd+PW 2 (with 0.24 g [(C 6 H 13 ) 4 N] 2 {HPO 4 [W(O)(O 2 ) 2 ] 2 } and 0.02 g PdCl 2 (PhCN) 2 ) was designed to contain the same amounts of W and Pd as the heterogeneous Pd-PW-HAP catalyst. Pd+PW 2 showed a propylene conversion of 57.2% and a selectivity for PO of 88.5% for the reaction. Pd-HMS-PW 2 and PdMgAl-PW 4 are two kinds of solid catalysts we reported previously for the propylene epoxidation by O 2 in methanol [11,12]. As shown in Table 2 4 Other products containing ring-opened compounds and C 1 -C 6 hydrocarbons except for C 3 H 8 .

Comparison of Catalytic
Performance between Pd-PW-HAP and Pd+PW 2 Figure 2 shows the effect of reaction time on the propylene epoxidation by O 2 in methanol at 363 K over Pd-PW-HAP and Pd+PW 2 . The homogeneous catalyst Pd+PW 2 exhibited a C 3 H 6 conversion higher than that of the heterogeneous catalyst Pd-PW-HAP at the same reaction time. On the other hand, the PO selectivity over Pd+PW 2 was higher than that over Pd-PW-HAP from 1 to 7 h, but became lower than that over Pd-PW-HAP when the reaction time was more than 8 h. The increase in by-products from the open-ring and over-oxidation decreased the PO selectivity in a long-time reaction. Pd+PW obtained the highest PO yield after the reaction for 10 h, and then the PO yield decreased with increasing reaction time. On the other hand, the highest PO yield over Pd-PW-HAP was obtained after the reaction for 13 h. The highest PO yield over Pd-PW-HAP (53.8%) was almost the same as that of Pd+PW 2 for the propylene epoxidation at 363 K by O 2 in methanol. when the reaction time was more than 8 h. The increase in by-products from the openring and over-oxidation decreased the PO selectivity in a long-time reaction. Pd+PW obtained the highest PO yield after the reaction for 10 h, and then the PO yield decreased with increasing reaction time. On the other hand, the highest PO yield over Pd-PW-HAP was obtained after the reaction for 13 h. The highest PO yield over Pd-PW-HAP (53.8%) was almost the same as that of Pd+PW2 for the propylene epoxidation at 363 K by O2 in methanol.
(a) (b)  Table 2. Figure 3 shows the stability of the solid catalyst Pd-PW-HAP for the propylene epoxidation by O2 in methanol. The experiment was carried out using a method reported previously [11]. A portion of 0.5 g fresh Pd-PW-HAP catalyst was used in the first run for catalyzing the propylene epoxidation at 363 K for 8 h. After the reaction, the autoclave reactor was cooled down to room temperature. The used solid catalyst was obtained by centrifugal separation, and was then put into another reactor containing reactants and solvent. The second run was started by heating the reactor containing the used Pd-PW-HAP to 363 K. In the meantime, the liquid after the first run, by eliminating out Pd-PW-HAP, was charged with reaction gases for a further reaction at 363 K for 8 h. As shown in Figure 3, the PO yield over the used Pd-PW-HAP in the second run did not decrease in comparison with that over the fresh catalyst in the first run. On the contrary, the liquid without Pd-PW-HAP did not show a further reaction at 363 K, and the PO yield slightly decreased with reaction time due to the increase in ring-opened products.  Table 2. Figure 3 shows the stability of the solid catalyst Pd-PW-HAP for the propylene epoxidation by O 2 in methanol. The experiment was carried out using a method reported previously [11]. A portion of 0.5 g fresh Pd-PW-HAP catalyst was used in the first run for catalyzing the propylene epoxidation at 363 K for 8 h. After the reaction, the autoclave reactor was cooled down to room temperature. The used solid catalyst was obtained by centrifugal separation, and was then put into another reactor containing reactants and solvent. The second run was started by heating the reactor containing the used Pd-PW-HAP to 363 K. In the meantime, the liquid after the first run, by eliminating out Pd-PW-HAP, was charged with reaction gases for a further reaction at 363 K for 8 h. As shown in Figure 3, the PO yield over the used Pd-PW-HAP in the second run did not decrease in comparison with that over the fresh catalyst in the first run. On the contrary, the liquid without Pd-PW-HAP did not show a further reaction at 363 K, and the PO yield slightly decreased with reaction time due to the increase in ring-opened products. Figure 4 shows the time courses of solid catalyst Pd-PW-HAP in the propylene epoxidation by O 2 in methanol for five runs. The used catalyst was obtained through centrifugal separation, and then dried under vacuum at room temperature for 3 h. When the amount of the used catalyst after drying was less than 0.5 g, due to loss in the operation, a small amount of fresh catalyst was added to the used catalyst to keep the catalyst amount at 0.5 g for the next run. The propylene conversion greatly increased, but the selectivity for PO slightly decreased with reaction time in each run. The catalytic activity of Pd-PW-HAP was almost kept in the reaction for five runs.   Figure 4 shows the time courses of solid catalyst Pd-PW-HAP in the propylene epox idation by O2 in methanol for five runs. The used catalyst was obtained through centrifu gal separation, and then dried under vacuum at room temperature for 3 h. When th amount of the used catalyst after drying was less than 0.5 g, due to loss in the operation a small amount of fresh catalyst was added to the used catalyst to keep the catalyst amoun at 0.5 g for the next run. The propylene conversion greatly increased, but the selectivit for PO slightly decreased with reaction time in each run. The catalytic activity of Pd-PW HAP was almost kept in the reaction for five runs.  Table 2. Table 3 shows the reusability of solid catalyst Pd-PW-HAP for the propylene epox dation by O2 in methanol. The fresh catalyst showed a C3H6 conversion of 53.4% and selectivity for PO of 88.7% after the reaction at 363 K for 8 h. The used catalysts showe   Figure 4 shows the time courses of solid catalyst Pd-PW-HAP in the propylene epoxidation by O2 in methanol for five runs. The used catalyst was obtained through centrifugal separation, and then dried under vacuum at room temperature for 3 h. When the amount of the used catalyst after drying was less than 0.5 g, due to loss in the operation, a small amount of fresh catalyst was added to the used catalyst to keep the catalyst amount at 0.5 g for the next run. The propylene conversion greatly increased, but the selectivity for PO slightly decreased with reaction time in each run. The catalytic activity of Pd-PW-HAP was almost kept in the reaction for five runs.  Table 2. Table 3 shows the reusability of solid catalyst Pd-PW-HAP for the propylene epoxidation by O2 in methanol. The fresh catalyst showed a C3H6 conversion of 53.4% and a selectivity for PO of 88.7% after the reaction at 363 K for 8 h. The used catalysts showed very similar C3H6 conversion and selectivity for PO to the fresh catalyst after reaction at 363 K for 8 h. The PO yield obtained in the fifth run was 47.3%, which was the same as those obtained in the first run.  Table 2. Table 3 shows the reusability of solid catalyst Pd-PW-HAP for the propylene epoxidation by O 2 in methanol. The fresh catalyst showed a C 3 H 6 conversion of 53.4% and a selectivity for PO of 88.7% after the reaction at 363 K for 8 h. The used catalysts showed very similar C 3 H 6 conversion and selectivity for PO to the fresh catalyst after reaction at 363 K for 8 h. The PO yield obtained in the fifth run was 47.3%, which was the same as those obtained in the first run. Figure S2 shows XRD patterns of HAP and Pd-PW-HAP before reaction. In the range from 5 to 70 degrees, the samples showed characteristic reflections of the HAP phase according to the Rigaku PDXL2 database (No 1011242). The strong reflections of HAP and Pd-PW-HAP existed in the range of 25-45 degrees in the XRD patterns [55,56].  Figure 5 shows the XRD patterns of Pd-PW-HAP and Pd+PW 2 before and after reaction. The sample of Pd+PW 2 before reaction showed the reflections of the peroxo-heteropoly compound in the XRD pattern. For the sample of Pd+PW 2 after reaction at 363 K for 8 h, a weak peak at 40.1 degrees corresponding to the reflection of (1 1 1) for Pd 0 metal species could be observed in the XRD pattern. The XRD pattern of Pd-PW-HAP, after reaction at 363 K for 8 h, was almost the same as that of Pd-PW-HAP before reaction. Because a peak at 39.9 degrees corresponding to the (3 1 0) reflection of HAP appeared in the XRD pattern, the reflection of Pd (1 1 1) at 40.1 degrees could not be ensured in the XRD pattern of Pd-PW-HAP after reaction, due to overlapping by the (3 1 0) Figure S2 shows XRD patterns of HAP and Pd-PW-HAP before reaction. In the range from 5 to 70 degrees, the samples showed characteristic reflections of the HAP phase according to the Rigaku PDXL2 database (No 1011242). The strong reflections of HAP and Pd-PW-HAP existed in the range of 25-45 degrees in the XRD patterns [55,56]. Figure 5 shows the XRD patterns of Pd-PW-HAP and Pd+PW2 before and after reaction. The sample of Pd+PW2 before reaction showed the reflections of the peroxo-heteropoly compound in the XRD pattern. For the sample of Pd+PW2 after reaction at 363 K for 8 h, a weak peak at 40.1 degrees corresponding to the reflection of (1 1 1) for Pd 0 metal species could be observed in the XRD pattern. The XRD pattern of Pd-PW-HAP, after reaction at 363 K for 8 h, was almost the same as that of Pd-PW-HAP before reaction. Because a peak at 39.9 degrees corresponding to the (3 1 0) reflection of HAP appeared in the XRD pattern, the reflection of Pd (1 1 1) at 40.1 degrees could not be ensured in the XRD pattern of Pd-PW-HAP after reaction, due to overlapping by the (3 1 0) reflection of HAP. 2.8. TEM Images of Pd-PW-HAP before and after Reaction Figure 6 shows the TEM images of Pd-PW-HAP before reaction and after reaction. For the sample of Pd-PW-HAP before reaction, the metal particles could not be observed in the TEM image. On the hand, for the sample of Pd-PW-HAP after reaction at 363 K for 8 h, the metal particles were observed as small black spots in the TEM image. The diameter of metal particles ranged from 1 to 2 nm in the TEM image of Pd-PW-HAP after reaction 2.8. TEM Images of Pd-PW-HAP before and after Reaction Figure 6 shows the TEM images of Pd-PW-HAP before reaction and after reaction. For the sample of Pd-PW-HAP before reaction, the metal particles could not be observed in the TEM image. On the hand, for the sample of Pd-PW-HAP after reaction at 363 K for 8 h, the metal particles were observed as small black spots in the TEM image. The diameter of metal particles ranged from 1 to 2 nm in the TEM image of Pd-PW-HAP after reaction, and the size distribution of metal particles was uniform. Moreover, the particle size of Pd metal did not increase after reaction at 363 K for 8 h for five runs.

XRD Patterns of Various Samples
Molecules 2023, 28, 24 8 of 18 and the size distribution of metal particles was uniform. Moreover, the particle size of Pd metal did not increase after reaction at 363 K for 8 h for five runs.  Table S1 shows the results of elemental analyses of Pd-PW-HAP before reaction by ICP and EDS. Three points on the Pd-PW-HAP particle in the TEM image of Pd-PW-HAP before reaction ( Figure 6A) were analyzed using the EDS instrument attached to the electron microscope. The three points by EDS analyses gave a similar composition of each element, indicating that PW and Pd were uniformly distributed on the surface of Pd-PW-HAP before reaction. Because EDS mainly analyzed the solid surface of Pd-PW-HAP, but ICP analyzed the HNO3 aqueous solution of Pd-PW-HAP, the EDS analyses gave lower values of Ca and P and higher values of W and Pd in comparison with the results of ICP analyses. Figure 7 shows the EXAFS functions of various samples. The sample of Pd-PW-HAP before reaction showed a Pd-Cl interaction peak at 1.85 Å, coinciding with the Pd-Cl interaction peak in the PdCl2 sample. As for the sample of Pd-PW-HAP after reaction at 363 K for 8 h, the Pd-Cl interaction peak disappeared and an interaction peak at 2.3 Å was observed in the EXAFS function. In comparison with the EXAFS function of Pd foil, the interaction peak at 2.3 Å was assigned to the interaction of the Pd-Pd bond in the Pd metal. The PdCl2 molecules immobilized on the Pd-PW-HAP were reduced to the Pd 0 metal species during the reaction.  Table S1 shows the results of elemental analyses of Pd-PW-HAP before reaction by ICP and EDS. Three points on the Pd-PW-HAP particle in the TEM image of Pd-PW-HAP before reaction ( Figure 6A) were analyzed using the EDS instrument attached to the electron microscope. The three points by EDS analyses gave a similar composition of each element, indicating that PW and Pd were uniformly distributed on the surface of Pd-PW-HAP before reaction. Because EDS mainly analyzed the solid surface of Pd-PW-HAP, but ICP analyzed the HNO 3 aqueous solution of Pd-PW-HAP, the EDS analyses gave lower values of Ca and P and higher values of W and Pd in comparison with the results of ICP analyses. Figure 7 shows the EXAFS functions of various samples. The sample of Pd-PW-HAP before reaction showed a Pd-Cl interaction peak at 1.85 Å, coinciding with the Pd-Cl interaction peak in the PdCl 2 sample. As for the sample of Pd-PW-HAP after reaction at 363 K for 8 h, the Pd-Cl interaction peak disappeared and an interaction peak at 2.3 Å was observed in the EXAFS function. In comparison with the EXAFS function of Pd foil, the interaction peak at 2.3 Å was assigned to the interaction of the Pd-Pd bond in the Pd metal. The PdCl 2 molecules immobilized on the Pd-PW-HAP were reduced to the Pd 0 metal species during the reaction. Figure 8 shows the XPS spectra of Pd-PW-HAP before and after reaction. The binding energy of the Pd(3d 5/2 ) peak was 338.2 eV in the XPS spectrum of Pd-PW-HAP before reaction, indicating that the Pd species remained in the oxidation state of +2 in the sample. On the other hand, the sample of Pd-PW-HAP after reaction showed the binding energy of the metallic Pd 0 species with 3d 5/2 of 334.9 eV and 3d 3/2 of 340.2 eV in the XPS spectrum. These results proved that all Pd 2+ species in the sample of Pd-PW-HAP before reaction were reduced into the metallic Pd 0 species during the epoxidation of propylene by O 2 in methanol at 363 K for 8 h.  Figure 8 shows the XPS spectra of Pd-PW-HAP before and after reaction. The binding energy of the Pd(3d5/2) peak was 338.2 eV in the XPS spectrum of Pd-PW-HAP before reaction, indicating that the Pd species remained in the oxidation state of +2 in the sample. On the other hand, the sample of Pd-PW-HAP after reaction showed the binding energy of the metallic Pd 0 species with 3d5/2 of 334.9 eV and 3d3/2 of 340.2 eV in the XPS spectrum. These results proved that all Pd 2+ species in the sample of Pd-PW-HAP before reaction were reduced into the metallic Pd 0 species during the epoxidation of propylene by O2 in methanol at 363 K for 8 h.    Figure 8 shows the XPS spectra of Pd-PW-HAP before and after reaction. The binding energy of the Pd(3d5/2) peak was 338.2 eV in the XPS spectrum of Pd-PW-HAP before reaction, indicating that the Pd species remained in the oxidation state of +2 in the sample. On the other hand, the sample of Pd-PW-HAP after reaction showed the binding energy of the metallic Pd 0 species with 3d5/2 of 334.9 eV and 3d3/2 of 340.2 eV in the XPS spectrum. These results proved that all Pd 2+ species in the sample of Pd-PW-HAP before reaction were reduced into the metallic Pd 0 species during the epoxidation of propylene by O2 in methanol at 363 K for 8 h.   Table 4 shows the results of catalytic propylene epoxidation by O 2 over Pd-PW-HAP in various solvents. Pd-PW-HAP obtained a propylene conversion of 53.4%, and a selectivity for PO of 88.7% after reaction at 363 K for 8 h in the methanol solvent. On the other hand, in the chloroform solvent, Pd-PW-HAP showed a low propylene conversion of 2.2% although the selectivity for PO was high (91.5%) after reaction at 363 K for 8 h. Hence, the peroxo-heteropoly compound in Pd-PW-HAP stoichiometrically epoxidizes propylene to PO using the peroxy-oxygen bonds, but the catalyst turnover could not be achieved in the chloroform solvent. Methanol is a necessary solvent for the epoxidation of propylene by O 2 over Pd-PW-HAP.  Table 5 shows the consumed amounts, and formed amounts, in the propylene epoxidation in methanol over various catalysts at 363 K for 8 h. PW-HAP consumed a small number of C 3 H 6 (0.7 mmol) and selectively converted them to PO (0.6 mmol). The methanol medium was not consumed over PW-HAP during the reaction. These results indicated that PW-HAP stoichiometrically converted C 3 H 6 to PO using the peroxy-oxygen bonds in PW, but the peroxy-oxygen bonds were not recovered to achieve a catalytic turnover. On the other hand, Pd-PW-HAP consumed 18.8 mmol of C 3 H 6 to form 17.0 mmol of PO after reaction at 363 K for 8 h. Hence, Pd is a necessary component in the catalysts containing peroxo-heteropoly compounds for the epoxidation of propylene by O 2 in methanol. As shown in Table 5, an amount of 10.1 mmol of methanol was consumed to form co-products (mainly CO and CO 2 ) over Pd-PW-HAP after reaction at 363 K for 8 h. O 2 was consumed for both oxidizing propylene and oxidizing methanol during the reaction. The consumed methanol was almost converted to the co-products involving CO, CO 2 , and small amounts of HCHO and HCH(OH) 2 .  Table 2. 2 By-products: formed from propylene oxidation, including acetone, acrolein, propionaldehyde, propane, hydrocarbons, and ring-opened compounds. 3 Co-products: formed from methanol oxidation, mainly CO and CO 2 (denoted as CO x ), with a small amount of HCHO. The first step in the synthesis route is grafting peroxotungstic ions on the HAP surface to form PW-HAP by using the reaction of surface HPO4 2-groups. Calcium-deficient HAP has a structure of stoichiometric HAP (Ca/P = 1.67), but involves Ca and OH defects The first step in the synthesis route is grafting peroxotungstic ions on the HAP surface to form PW-HAP by using the reaction of surface HPO 4 2− groups. Calcium-deficient HAP has a structure of stoichiometric HAP (Ca/P = 1.67), but involves Ca and OH defects and HPO 4 2− groups [44][45][46][47]. The surface HPO 4 2− groups have activity for some chemical reactions, and thus have been used for surface modification to graft other groups [38]. On the other hand, the HPO 4 2− groups on the solid surface have a reaction activity with peroxotungstic ions to form peroxo-heteropoly ions in an aqueous solution [11,27,28]. As shown in the FT-IR spectra (Figure 1) The second step in the synthesis route is fixing PdCl 2 molecules between the O atoms of PO 4 3groups on the HAP surface. It has been reported that PdCl 2 molecules can be fixed on the HAP surface, and the resultant compounds are excellent catalysts for alcohol oxidation by O 2 [44,47,48]. As shown in the FT-IR spectra (Figure 1), the peaks of PhCN groups disappeared in the FT-IR spectrum of Pd-PW-HAP, indicating that the organic PhCN groups dropped out from Pd-PW-HAP during the synthesis process. Because the IR spectrum of Pd-PW-HAP was almost the same as that of PW-HAP, the peroxo-heteropoly compound PW remained on the surface of HAP during the PdCl 2 immobilization. Using the two steps of surface modification, both peroxo-heteropoly compounds and PdCl 2 molecules were immobilized on the HAP surface in Pd-PW-HAP simultaneously.

Synthesis Route of Pd-PW-HAP
The molecular formulas of the samples in Table 1 can be obtained using the elemental compositions and the general formulas. At first, the total molecular weight of each compound was calculated using the content of Ca from elemental analysis, and the chemical stoichiometry of Ca in the general formula. Then, the chemical stoichiometries of various groups were calculated using their proportions in the sample, total molecular weight, and the general formula. The deficiency between the summed percentages of various elements and 100% was calculated as adsorbed water. By calculation using the method described above, the formulas of HPA, PW-HAP, and Pd-PW-HAP were Ca 4 (HPO 4 4 ]. However, the HPO 4 groups in the inner structure of HAP remained in the PW-HAP. On the other hand, the actual Pd amount in Pd-PW-HAP (as shown in Table 1) was 1.2 wt.%, which was lower than the designed Pd amount in Pd-PW-HAP (2 wt.%) in the synthesis step. This implied that a suitable distance and angle between two PO 4 groups were needed to fix PdCl 2 molecules on the HAP surface.

Strong Points of Pd-PW-HAP
As shown in the XRD pattern Pd-PW-HAP before reaction (in Figure 5), the reflections of palladium and peroxo-heteropoly compounds did not appear. The crystals of PW and PdCl 2 did not form on the Pd-PW-HAP, implying that PW and PdCl 2 were uniformly immobilized on the surface of HAP. Because palladium and peroxo-heteropoly compounds were fixed on the surface of HAP by chemical bonds, the active components (PW and Pd) did not leach from Pd-PW-HAP in the solvent during the reaction (results in Figure 3). Not only the Pd 2+ complex could be fixed on the surface of HAP [44], but also the Pd 0 nanocluster formed from the reduction of Pd 2+ during the reaction had a strong interaction with the surface of HAP [48]. This is the reason why Pd-PW-HAP could be used for five runs without a decrease in the catalytic activity (as shown in Table 3). Because the homogeneous catalyst Pd+PW 2 easily diffuses in the methanol solvent, Pd+PW 2 showed a higher C 3 H 6 conversion than that of Pd-PW-HAP (as shown in Table 2). However, the only possible method for recovering the homogeneous catalysts was vacuum distillation of the mixture after the reaction. The vacuum distillation consumed a larger amount of energy compared with the centrifugal separation. On the contrary, the solid catalyst Pd-PW-HAP can be reused by simple centrifugal separation in the propylene epoxidation by O 2 in methanol. Hence, Pd-PW-HAP is desirable in industry manufacture because of its ease of separation and renewability. As shown in Figure 2, there was only a small difference in the PO yield between Pd-PW-HAP and Pd+PW 2 because PW and PdCl 2 were uniformly distributed on the HAP surface. Moreover, the highest PO yield over Pd-PW-HAP, obtained by increasing reaction time, was equal to the highest PO yield over Pd+PW 2 for the reaction at 363 K (as shown in Figure 2b). HAP-immobilized Pd is a kind of highly efficient catalyst for oxidation reactions using O 2 as an oxidant [44,47,48]. This is the reason that Pd-PW-HAP showed a catalytic activity higher than those of PdMgAl-PW 4 and Pd-HMS-PW 2 for the propylene epoxidation by O 2 in methanol (as shown in Table 2).

Active Pd Species in Pd-PW-HAP
As shown in the XRD pattern of Pd+PW 2 after reaction ( Figure 5), the peak at 40.1 degrees, corresponding to Pd 0 species, could be observed. The metal Pd 0 species was the active species in the propylene epoxidation by O 2 in methanol over the homogenous catalyst Pd+PW 2 [7]. On the other hand, the peak of Pd 0 species at 40.1 degrees could not be confirmed in the XRD pattern of Pd-PW-HAP after reaction (as shown in Figure 4). It is necessary to confirm the active Pd species in the propylene epoxidation by O 2 in methanol over the heterogeneous catalyst Pd-PW-HAP. The EXAFS function confirmed that the PdCl 2 species were converted to Pd 0 metal species after the reaction (as shown in Figure 7). Furthermore, the Pd metal particles were observed in the TEM image of Pd-PW-HAP after the reaction (as shown in Figure 6). The size of Pd 0 metal particles ranged from 1-2 nm in the TEM image of Pd-PW-HAP after the reaction. The metal particle with a size lower than 3 nm hardly exhibited their reflections in the XRD pattern [57]. Hence, the reflections of Pd 0 metal did not appear in the XRD pattern of Pd-PW-HAP after reaction because the size of the formed Pd 0 metal particles was too small. The small Pd 0 particles were formed after reaction because the precursor PdCl 2 molecules were uniformly fixed on the surface of Pd-PW-HAP before reaction. As a result, Pd 0 metal was the species for the propylene epoxidation by O 2 in methanol over the solid catalyst Pd-PW-HAP.

Roles of Methanol in the Reaction
Scheme 1 shows the co-oxidation of methanol in the system containing Pd and O 2 . HAP-immobilized palladium is a kind of highly efficient catalyst for the oxidation of alcohols by O 2 [44][45][46][47][48]. Methanol has the highest reducibility among various alcohols, and thus can be oxidized to a peroxy intermediate HOCH 2 OOH by O 2 in the presence of a Pd catalyst [58,59]. The aliphatic organic peroxide HOCH 2 OOH can achieve the catalytic turnover of the peroxo-heteropoly compound in propylene epoxidation [15,25]. Some HOCH 2 OOH molecules turn back to HCH(OH) 2 for the next catalytic cycle after giving their peroxo-bonds to heteropoly compounds. On the other hand, some HOCH 2 OOH molecules decompose to CO x and H 2 O in the catalytic system because HOCH 2 OOH is not stable [60]. Hence, methanol was not only a solvent but also a co-reactant or reducing agent in the propylene epoxidation by O 2 in methanol, over Pd-PW-HAP. Because the reaction temperature was low (mainly at 363 K) in this study, only a small amount of methanol underwent co-oxidation by O 2 during the reaction.
HOCH2OOH molecules turn back to HCH(OH)2 for the next catalytic cycle after giving their peroxo-bonds to heteropoly compounds. On the other hand, some HOCH2OOH molecules decompose to COx and H2O in the catalytic system because HOCH2OOH is not stable [60]. Hence, methanol was not only a solvent but also a co-reactant or reducing agent in the propylene epoxidation by O2 in methanol, over Pd-PW-HAP. Because the reaction temperature was low (mainly at 363 K) in this study, only a small amount of methanol underwent co-oxidation by O2 during the reaction. Scheme 1. Co-oxidation of methanol in the system containing Pd and O2.

Reaction Mechanism for the Reaction
Scheme 2 shows the reaction mechanism for propylene epoxidation by O2 in methanol. The peroxo-heteropoly compounds in PW-HAP stoichiometrically epoxidize propylene to PO by O-O peroxo-bonds in methanol, but the O-O peroxo-bonds cannot be recovered by O2 (results in Table 2). Moreover, the O-O peroxo-bonds in the peroxo-heteropoly compounds on Pd-PW-HAP cannot be recovered in a chloroform solvent (results in Table  4). Hence, Pd on HAP oxidizes CH3OH to the peroxy intermediate HOCH2OOH by O2, and the formed HOCH2OOH acts to recover the O-O peroxo-bonds of peroxo-heteropoly compounds. Pd and methanol convert the weak oxidant O2 to the strong oxidant HOCH2OOH in situ during the reaction. As shown in Table 2, PdCl2(PhCN)2 showed a very low PO yield (0.2%) for the propylene epoxidation in methanol. This indicates that the concentration of HOCH2OOH formed in the system was low, and could not directly result in epoxidation of propylene to PO. The peroxo-heteropoly compounds are necessary for the catalytic system to convert propylene to PO using their peroxy-oxygen bonds. As a result, peroxo-heteropoly compounds achieve the catalytic conversion of propylene to PO, and Pd achieves the catalytic conversion of methanol and O2 to peroxy intermediate HOCH2OOH, which are used for recovering peroxo-heteropoly compounds in situ.

Scheme 2.
Reaction mechanism for propylene epoxidation by O2 in methanol.  Table 2). Moreover, the O-O peroxo-bonds in the peroxo-heteropoly compounds on Pd-PW-HAP cannot be recovered in a chloroform solvent (results in Table 4). Hence, Pd on HAP oxidizes CH 3 OH to the peroxy intermediate HOCH 2 OOH by O 2 , and the formed HOCH 2 OOH acts to recover the O-O peroxo-bonds of peroxo-heteropoly compounds. Pd and methanol convert the weak oxidant O 2 to the strong oxidant HOCH 2 OOH in situ during the reaction. As shown in Table 2, PdCl 2 (PhCN) 2 showed a very low PO yield (0.2%) for the propylene epoxidation in methanol. This indicates that the concentration of HOCH 2 OOH formed in the system was low, and could not directly result in epoxidation of propylene to PO. The peroxo-heteropoly compounds are necessary for the catalytic system to convert propylene to PO using their peroxy-oxygen bonds. As a result, peroxo-heteropoly compounds achieve the catalytic conversion of propylene to PO, and Pd achieves the catalytic conversion of methanol and O 2 to peroxy intermediate HOCH 2 OOH, which are used for recovering peroxo-heteropoly compounds in situ. agent in the propylene epoxidation by O2 in methanol, over Pd-PW-HAP. Because the reaction temperature was low (mainly at 363 K) in this study, only a small amount of methanol underwent co-oxidation by O2 during the reaction. Scheme 1. Co-oxidation of methanol in the system containing Pd and O2.

Reaction Mechanism for the Reaction
Scheme 2 shows the reaction mechanism for propylene epoxidation by O2 in methanol. The peroxo-heteropoly compounds in PW-HAP stoichiometrically epoxidize propylene to PO by O-O peroxo-bonds in methanol, but the O-O peroxo-bonds cannot be recovered by O2 (results in Table 2). Moreover, the O-O peroxo-bonds in the peroxo-heteropoly compounds on Pd-PW-HAP cannot be recovered in a chloroform solvent (results in Table  4). Hence, Pd on HAP oxidizes CH3OH to the peroxy intermediate HOCH2OOH by O2, and the formed HOCH2OOH acts to recover the O-O peroxo-bonds of peroxo-heteropoly compounds. Pd and methanol convert the weak oxidant O2 to the strong oxidant HOCH2OOH in situ during the reaction. As shown in Table 2, PdCl2(PhCN)2 showed a very low PO yield (0.2%) for the propylene epoxidation in methanol. This indicates that the concentration of HOCH2OOH formed in the system was low, and could not directly result in epoxidation of propylene to PO. The peroxo-heteropoly compounds are necessary for the catalytic system to convert propylene to PO using their peroxy-oxygen bonds. As a result, peroxo-heteropoly compounds achieve the catalytic conversion of propylene to PO, and Pd achieves the catalytic conversion of methanol and O2 to peroxy intermediate HOCH2OOH, which are used for recovering peroxo-heteropoly compounds in situ.  Although the aliphatic organic peroxide HOCH 2 OOH formed in situ was used as an oxidant for propylene epoxidation, the catalytic system in propylene epoxidation by O 2 in methanol, over Pd-PW-HAP, is different from the conventional hydroperoxide process. In comparison with the t-butyl alcohol co-product formed in the conventional hydroperoxide process, the co-product CO x can be used to synthesize methanol in the industrial process over Cu-based catalysts, which achieves the recycling of methanol solvent in the catalytic system. Pd-PW-HAP is a kind of heterogeneous catalyst that can be reused by simple centrifugal separation after the reaction. Because Pd-PW-HAP fixed Pd and PW on the HAP surface by chemical bonds, Pd-PW-HAP did not leach the active components to the solvent during the reaction.

Reagents
Inorganic reagents were purchased from Wako Pure Chemical Industries Ltd. (Tokyo, Japan) with purities higher than 99%. Organic reagents were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) with purities higher than 99.5%. Gas cylinders were purchased from Sumitomo Seika Chemicals Co., Ltd. (Tokyo, Japan) with purities higher than 99.995%.

Catalyst Preparation
According to the literature, calcium-deficient HAP was prepared using a wet method [41]. A 1.0 M Ca(NO 3 ) 2 aqueous solution and a 0.6 M (NH 4 ) 2 HPO 4 aqueous solution were simultaneously added to 500 mL distilled water in a three-necked flask while stirring. The pH was kept at around 11 during the process by adding an ammonia solution. The resultant milky solution was further stirred at 353 K for 1 h. Then, the precipitate was filtered and washed with distilled water. The resulting solid was dried at 383 K for 24 h and calcined at 773 K for 3 h. The designed Ca/P molar ratio was 1. 33 [54]. The precipitate was filtered, dried in the air at room temperature, and stored in a refrigerator at 278 K.
PW-HAP was prepared using a surface reaction, according to the method in the literature [11,29]. Two grams of HAP powder were added to 20 mL of Na 2 [W 2 O 3 (O 2 ) 4 ] solution with vigorous stirring at room temperature for 6 h. Then, the solid was isolated by filtration, dried in the air at room temperature, and stored in a refrigerator at 278 K.
Pd-PW-HAP was synthesized by fixing PdCl 2 molecules on PW-HAP, according to the method in the literature [44]. Two grams of PW-HAP were stirred in a 150 mL acetone solution of PdCl 2 (PhCN) 2 (2.5×10 −3 M) at room temperature for 3 h. The solid product Pd-PW-HAP was isolated by filtration, dried in the air at room temperature, and stored in a refrigerator at 278 K. The designed Pd loading was 2 wt.%.
THA-PW 2 , with a molecular formula of [(C 6 H 13 ) 4 N] 2 {HPO 4 [W(O)(O 2 ) 2 ] 2 }, was prepared according to the method in the literature [54]. A portion of 2.5 g H 2 WO 4 was added to 7 mL 30% H 2 O 2 . After stirring at 323 K for 40 min, 0.85 mL of 6 M H 3 PO 4 was added to the supernatant. After further stirring for 5 min, 7.8 g of tetrahexylammonium chloride dissolved in 10 mL of water was added to the clear solution. Further stirring was carried out at room temperature for 10 min. Then, the solid product was filtered, dried in the air at room temperature, and stored in a refrigerator at 278 K.
Pd-HMS-PW 2 , a mesoporous silica containing peroxo-heteropoly compound and Pd, was prepared according to the literature [11]

Catalyst Characterization
The elemental analyses were measured by a method of inductively coupled plasma (ICP) using a Thermo Jarrel Ash IRIS/AP instrument. The samples for the ICP analyses were obtained by dissolving HAP-based samples in the HNO 3 aqueous solution.
The X-ray diffraction (XRD) patterns were measured by a RINT-2500HLR diffractometer (Rigaku Corp.) with Cu Kα radiation (λ = 0.154 nm). The operation voltage was 40 kV, and the operation current was 50 mA in the XRD analyses.
Fourier transform infrared (FT-IR) spectra were carried out using a KBr pellet method by a JASCO FT/IR-7000 spectrometer. The KBr pellet, containing 0.5 wt.% of each sample, was measured at room temperature under atmospheric conditions. Transmission electron microscopy (TEM) images were taken using a JEOL JEM 2010FX electron microscope equipped with a Hitachi/Keves H-8100/Delta IV EDS at 200 kV. The samples were supported on carbon-coated copper grids before the TEM measurement.
X-ray absorption fine structure (XAFS) of Pd K-edges was measured at the beamline 10B of the Photon Factory in the National Laboratory for High-Energy Physics, Tsukuba, Japan. Extended X-ray Absorption Fine Structure (EXAFS) data were examined using the Rigaku EXAFS analysis program. Fourier transformation of k 3 -weighted normalized EXAFS data was carried out in the range of 3.5 Å < k/Å −1 < 11 Å.
X-ray photoelectron spectra (XPS) were carried out using a Phi-5500 ESCA spectrometer equipped with Mg Kα radiation (1253.6 eV). The chamber pressure was 10 −9 Torr. The binding energy of Pd was calibrated by the C 1s of 284.6 eV.

Reaction Procedure and Analyses
The reaction was performed in a 100 mL stainless steel autoclave reactor. In a typical reaction, 0.5 g Pd-PW-Hap and 20 mL methanol were added to the autoclave. A nitrogen gas with a pressure of 0.4 MPa was charged in the autoclave, and then charged out from the autoclave to change the air. After purging with nitrogen five times, the autoclave contained 0.1 MPa of nitrogen at room temperature. Then, 1.0 MPa of propylene and 1.0 MPa of oxygen were sequentially charged in the autoclave at room temperature. Then, the autoclave was immersed in an oil bath with temperature control. The reaction started with vigorous stirring (500 revolutions per minute) after the autoclave reactor was heated to the reaction temperature.
After the reaction, the autoclave was cooled down to room temperature. The gas in the autoclave was collected in a plastic gas bag. Then, a certain amount of 1,4-dioxane was added to the autoclave as an internal standard [61]. Inorganic gases (CO 2 , CO, O 2 , and N 2 ) were analyzed by a Shimadzu 2014 TCD-GC equipped with a Shincarbon-ST packed column in He carrier gas. Organic gaseous compounds were analyzed by a Shimadzu 2014 FID-GC equipped with an RT-QPLOT capillary column. The factors of various gaseous compounds were obtained using a standard mixed gas (with a known concentration for each component) from a cylinder. On the other hand, the liquid and the solid were separated using a centrifuge. The liquid compounds were analyzed by an Agilent 6890 N FID-GC equipped with a PoraPLOT U capillary column.
The propylene conversion was calculated from the difference in its molar amounts before and after reactions using Equation (1): The selectivity for each product was expressed in terms of carbon efficiency and was calculated using Equation (2): S = n i C i /Σ(n i C i ) × 100 (2) Herein, n i denotes the number of carbon atoms in each product.

Conclusions
The novel hybrid compound Pd-PW-HAP was synthesized by immobilizing PdCl 2 molecules and peroxo-heteropoly compounds on the surface of calcium-deficient hydroxyapatite. Pd-PW-HAP was an effective heterogeneous catalyst for propylene epoxidation by O 2 in methanol. The propylene conversion over Pd-PW-HAP was slightly lower than that over the homogeneous catalyst in the epoxidation of propylene by O 2 in methanol. By increasing the reaction time, Pd-PW-HAP could obtain the highest PO yield, comparable to that of the homogeneous catalyst at 363 K. Pd-PW-HAP could be used five times, without a decrease in the catalytic performance in the epoxidation of propylene. During the reaction, the immobilized PdCl 2 formed small particles of Pd 0 metal species on the Pd-PW-HAP surface by the reduction of methanol. During the reaction, a small part of the methanol solvent was oxidized to the peroxy intermediates HOCH 2 OOH by O 2 in the presence of Pd. The peroxy intermediates recovered the peroxy-oxygen bonds, and achieved the catalytic turnover of peroxo-heteropoly compounds for the epoxidation of propylene.
Funding: This research received no external funding.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28010024/s1, Table S1: Results of elemental analyses of Pd-PW-HAP before reaction by ICP and EDS; Figure S1: GC charts of products after reaction at 363 K for 8 h over Pd-PW-HAP; Figure S2: XRD patterns of HAP and Pd-PW-HAP before reaction.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.