C-C Coupling on Single-Atom-Based Heterogeneous Catalyst

Transcription

1 Supporting Information C-C Coupling on Single-Atom-Based Heterogeneous Catalyst Xiaoyan Zhang,,, Zaicheng Sun, Bin Wang, ǁ Yu Tang, Luan Nguyen, Yuting Li and Franklin Feng Tao*, Department of Chemical Engineering and Department of Chemistry, University of Kansas, Lawrence, KS, 66045, United States School of Chemistry, Beijing University of Technology, Beijing, , China State Key Laboratory of Photocatalysis on Energy and Environment and College of Chemistry, Fuzhou University, Fuzhou, , China ǁ School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, OK 73019, United States S1

2 1. Control experiments for checking whether Pd atoms of 0.20 wt% Pd/TiO2 detached or not The area ratios of Pd 3d /Ti 2p on surface of catalyst Pd1/TiO2 before catalysis (called a fresh catalyst) and that of the Pd1/TiO2 after catalysis (called a used catalyst) were measured with XPS in UHV. For preparing sample to measure the Pd 3d /Ti 2p area ratio on surface of used catalyst with XPS, a clean solution containing the used catalyst Pd1/TiO2 was prepared by centrifugation of the solution after catalysis of 3 hours at 60 o C. In this way, the K2CO3 was precipitated. Notably, some of these Pd1/TiO2 nanoparticles remained in the solution after centrifugation since these nanoparticles are small (<50 nm). Notably, Pd 2+ of 0.20 wt% Pd/TiO2 catalyst nanoparticles could have detached to solution to form Pd nanoclusters (Figure 6c); thus, the solution obtained through centrifugation could contain both Pd/TiO2 nanoparticles and freestanding Pd nanoclusters as schematically shown in Figure 6c if Pd atoms could have detached from the catalyst nanoparticle during catalysis. After drying the obtained solution and calcinating the powder in air at 200 o C, the small Pd nanoclusters and even the left free-standing Pd cations (Figure 6c) should have been oxidized to PdO nanoclusters as schematically shown in Figure 6d. Compared to the quite low surface density of Pd atoms on TiO2, the formed PdOx nanoparticles have much higher surface density of Pd atoms; if some Pd atoms of some Pd1/TiO2 nanoparticles could have detected to solution and formed PdOx nanoparticles after calcination, the surface density of the left Pd atoms on TiO2 would be lower that of the catalyst before catalysis. XPS was used to measure (1) the Pd 3d /Ti 2p arearatio of the catalyst formed from centrifugation to generate the solution, drying the solution to form powder and calcination the powder and (2) the Pd/Ti ratio of the catalyst before catalysis. As XPS is a surface-sensitive technique, the Pd 3d /Ti 2p area ratio S2

3 of the catalyst formed from centrifugation, drying and calcination would be definitely lower than that before catalysis. Figures 6e and 6f are XPS spectra of Pd 3d and Ti 2p of catalyst before catalysis, respectively. Figures 6g and 6h are XPS spectra of Pd 3d and Ti 2p of the used catalyst after the above configuration to generate the solution, drying the solution to form powder and calcination of the powder in air. The calculated ratios of peak areas of Pd 3d to Ti 2p of the fresh catalyst and the treated used catalyst were listed in Figure 6i. Obviously, the two ratios are in fact very close. Thus, the detachment of Pd atoms from surface of catalyst nanoparticles was excluded here. In other words, the preservation of Pd 3d /Ti 2p area ratio after catalysis of C-C coupling at 60 o C for 3 hours suggests that majority of these anchored Pd atoms of Pd1/TiO2 nanoparticles during catalysis at 60 o C are robust. 2. Catalytic activity for C-C coupling on Pd1/TiO2 in solvent benzene Another set of control experiments was performed to check whether the anchored Pd atoms on TiO2 are active sites for C-C coupling at 60 o C in another solvent benzene. One concern on the experiment using ethanol as a solvent is that Pd cations could potentially detach from surface of TiO2 and then were reduced to Pd atoms and immediately form Pd nanoparticles. In the following two types of experiments, benzene instead of ethanol was used as the solvent. Compared to the experiments using ethanol, there is no contribution from Pd nanoparticles since benzene cannot reduce Pd cations to Pd(0). When benzene is used as solvent and Pd1/TiO2 as catalyst (experiment #1), only the anchored Pd1 atoms on TiO2 or detached Pd 2+ in solution could be active phase for C-C coupling. To test whether Pd 2+ in solvent benzene could be active for C-C coupling or not, another experiment using Pd(NO3)2 and benzene (experiment #2) was performed. If product could S3

4 be observed from experiment #2, the Pd 2+ in benzene would be active for the C-C coupling. Then, all or a part of the activity in experiment #1 should contributed from the Pd 2+ in benzene. In this series, the above experiments #1 and #2 were performed in solvent benzene instead of ethanol. The difference between the two experiments is the catalyst. One experiment used Pd1/TiO2 and the other Pd(NO3)2. 50 mg of 0.20 wt% Pd/TiO2 (experiment #1) or 0.26 μmol of Pd(NO3)2 (experiment #2) was used as catalyst here. The sources of Pd atoms (Pd1/TiO2 or Pd(NO3)2) used in the two experiments have the same number of Pd atoms ( mol). Other than the catalysts, 2.0 mmol K2CO3, 1.0 mmol iodobenzene, 1.0 mmol phenylacetylene, 0.3 mmol dodecane and certain amount of benzene were added to make the total volume of the reaction mixture to 10 ml before catalysis. The two experiments using solvent benzene were performed under exactly same catalytic condition of C-C coupling as that used for C-C coupling on Pd1/TiO2 in ethanol described in the main text. We found that the conversion of phenylacetylene on the 50 mg 0.20 wt% Pd/TiO2 ( mol of Pd cations) in benzene at 60 o C is 24 % after catalysis of 3 hours and the yield of diphenylacetylene is 17% (experiment #1). Unfortunately, the yield of ideal product diphenylacetylene on 0.26 μmol of Pd(NO3)2 is zero although 10% of phenylacetylene was converted (experiment #2). This distinct difference between experiment #1 using Pd1/TiO2 and the other using Pd(NO3)2 (experiment #2) clearly showed that the Pd cations in the form of Pd(NO3)2 dispersed in benzene are not active for the C-C coupling (experiment #2). On the other hand, this observed activity in conversion of phenylacetylene and formation of diphenylacetylene in the experiments Pd1/TiO2 using benzene as solvent (experiment #2) shows that the Pd1-based single sites are definitely active for Sonogashira C-C coupling. Thus, the two control experiments supported our conclusion that the anchored Pd-based sites of Pd1/TiO2 are active for C-C coupling. S4

5 3. Computational studies Optimized configuration of the anchored Pd atoms Density functional calculations were carried out using the VASP package [1]. The Perdew- Burke-Ernzerh of generalized gradient approximation exchange-correlation potential (PBE- GGA) [2] was used, and the electron-core interactions were treated in the projector augmented wave (PAW) method [3]. Structures were optimized until the atomic forces were smaller than 0.02 ev Å - 1 with a kinetic cut off energy of 300 ev. Van der Waals interactions were taken into account by incorporating the DFT-D3 semi-empirical method [4]. Reaction barriers were determined with the Nudged Elastic Band method [5]. The optimized lattice constants of the anatase TiO2 unit cell is Å Å Å, which is in good agreement with experimental results [6]. We modeled the TiO2 (101) surface using a slab with four TiO2 layers and a rectangular surface cell with dimensions of 20.7 Å 19.1 Å. The vacuum layer is at least 16 Å in all the calculations to eliminate interactions between adjacent cells. The bottom 3 layers were fixed at their bulk positions while the top TiO2 layer is fully relaxed. To understand catalytic mechanism of C-C coupling on Pd1/TiO2, we performed DFT calculations to optimize surface structure of Pd1/TiO2 and then simulate reaction pathway by using the Anatase (101) surface which has a low formation energy. As shown in Figure 3a, a Pd atom bond with two surface lattice oxygen atoms of TiO2 (101); the Pd atom in Figure 3a is highly unstable. Compared to the structure in Figure 3a, we found that one oxygen atoms (Oad) can be added between Pd1 and its near 5-coordinted Ti atoms (Ti5c) of the TiO2 (101) surface, forming a different structure (Figures 3b) in which three O atoms coordinate with the Pd atom. In this structure (Figure 3b), the Oad comes from the dissociation of molecular O2 on surface; the other S5

6 two oxygen atoms bonding to the Pd1 atom are surface lattice oxygen atoms. In addition, Pd1 can bond with three surface lattice oxygen atoms and one Oad as shown in Figure 3c; in Figure 3c, there are four oxygen atoms in total bonding to Pd1 atom. Different from Figure 3c, the Pd1 atom bonds with two surface lattice oxygen atoms and two Oad which come from dissociation of molecular O2; in Figure 3d, the coordination number of oxygen atoms to Pd1 is 4, the same as Figure 3c. In Figure 3e, Pd1 atom bonds with three surface lattice oxygen atoms and two Oad which come from dissociation of molecular O2; in this case, Pd1 coordinates with five oxygen atoms. Compared to Figure 3b, the structure in Figure 3c is more stable than the one in Figure 3b by about 30 kj/mol. The structural optimization with DFT shows that the average Pd-O distance in structure of Figure 3c is 2.06 Å, in good agreement with the experimental values, 2.01 Å measured with EXAFS (Figure 2e). As shown in Table S3 in Supplementary Materials, formation of this structure (Figure 3c) is highly energetically favorable with an energy gain of 128 kj/mol. Although Pd1 atom in Figure 3d has the same coordination environment as that in Figure 3c, its energy is higher since it has two Oad atoms. Thus, Figure 3d is not a favorable structure. As the coordination number of O atoms to a Pd atom and the optimized distance between Pd and O in the structure of Figure 3c are quite consistent with the measured values of catalyst 0.20 wt% Pd/TiO2 (Figure 2e), we took the structure in Figure 3c as active site of Pd1/TiO2 in the following simulation of reaction pathway of C-C coupling reaction on Pd1/TiO2. Calculated energy profile As shown in the energy profile (Figure 3f), the step of dehydrogenation of phenylacetylene (from IV to V in Figure 3f) is barrier-less and highly exothermic with a large energy gain of 170 kj/mol. The last step is the coupling between the phenyl group adsorbed on the Pd atom and the S6

7 phenylacetylenyl bound on the Oad site due to their close proximity as shown in Figure 3f-V. Although this step has an activation barrier of 72 kj/mol, the gained energy, 170 kj/mol in the C- H dissociation of phenylacetylene (from IV to V) makes the intermediate (Figure 3f-V) have enough energy to across the barrier of 72 kj/mol for coupling between phenyl and phenylacetylenyl (Figure 3f-VI). Thus, we can consider that the steps from adsorption of phenylacetylene (Figure 3f-IV) to formation of product diphenylacetylene (Figure 3f-VII) is barrierless. From this point of view, the rate-limiting step of this pathway is thus the dissociative chemisorption of iodobenzene; activation barrier of this step is 52 kj/mol (Figure 3f-II) if referenced to energy of gaseous iodobenzene (the black dashed line aligned to 0 kj/mol). It agrees reasonably with the experimental value derived from the Arrhenius plot of kinetics studies of C-C coupling on Pd1/TiO2, 28.9 kj/mol (Figure 4d). In summary, the theoretical simulation of C-C coupling on Pd1/TiO2 suggested that the unique site Pd1-Oad-Ti5c provides multifunctional atoms Pd, Oad, and Ti5c for activating C-I of iodobenzene, activating C-H of phenylacetylene and then performing C-C coupling to form diphenylacetylene. 4. XANES and EXAFS studies The X-ray Absorption near Edge Structure (XANES) and Extended Xray Absorption Fine Structure (EXAFS) measurement were performed at the Beamline 2-2 at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory. The Pd K-edge spectra of the catalyst (0.20 wt% Pd1/TiO2) was measured in fluorescence mode under ambient conditions. The Pd K-edge spectrum of a metallic Pd foil was collected at the same time in reference mode for X- ray energy calibration and data alignment. XANES and EXAFS data were processed and fitted with ARTEMINS and ARTHENA software [7]. S7

8 Intensity (a.u.) (a) (b) Theta (degree) Figure S1. XRD patterns of (a) as-prepared 0.20 wt% Pd/TiO2 catalyst and (b) pure TiO2. (a) 3.0 Phenylacetylene 2.5 A Phenylacetylene /A Dodecane Amounts (mmol) (b) A Iodobenzene /A Dodecane Iodobenzene Amounts (mmol) Figure S2. Standard curves of quantitative analyses of (a) phenylacetylene, (b) iodobenzene with GC. For each chemical, five solutions with different amounts were made. To prepare a solution, the amount of the chemical (volume of X-axis) was added to 0.30 mmol dodecane and then ethanol was added to make volume of the solution to 10.0 ml. S8

9 Figure S3. Photos of solution containing (a) 1μmol Pd (NO3)2 2H2O, (b) 50 mg 0.20 wt% Pd/TiO2 or (c) 0.26 μmol Pd (NO3)2 2H2O under different conditions. (a) Solution of 1μmol Pd (NO3)2 2H2O dissolved in 10 ml ethanol. (a1) 25 o C. (a2) after heating to 60 o C and remain for one hour. (a3) after heating to 60 o C and remain for two hours. (a4) after heating to 60 o C and remain for three hours. (b) Solution of 50 mg 0.20 wt% Pd/TiO2 in solution consisting of 1 mmol phenylacetylene, 1 mmol iodobenzene, 0.3 mmol dodecane, and 10 ml ethanol. (b1) 25 o C. the yellow color of the solution is the clear yellow of phenylacetylene. (b2) after heating to 60 o C and remain for one hour. (b3) after heating to 60 o C and remain for two hours. (b4) after heating to 60 o C and remain for three hours. (c) Solution of 0.26 μmol Pd (NO3)2 2H2O in 10 ml ethanol. (c1) 25 o C. (c2) after heating to 60 o C and remain for one hour. (c3) after heating to 60 o C and remain for two hours. (c4) after heating to 60 o C and remain for three hours. (d) 50 mg 0.20 wt% Pd/TiO2 in 10 ml ethanol. (d1) 25 o C. (d2) after heating to 60 o C and remain for one hour. (d3) after heating to 60 o C and remain for two hours. (d4) after heating to 60 o C and remain for three hours. S9

10 Intensity (a.u.) Time (min) Figure S4. GC spectrum of the solution after reaction between 1-Iodo-4-nitrobenzene and Phenylacetylene in ethanol, refluxed at 60 o C for 3h. Retention time at 2.87 minute is Dodecane, 4.30 minute is 1-Iodo-4-nitrobenzene and 7.11 minute is 1-Nitro-4-(phenylethynyl) benzene. This spectrum corresponds to the C-C coupling in entry 1 of Table S Intensity (a.u.) Time (min) Figure S5. GC spectrum of the solution after reaction between 4-Iodotoluene and Phenylacetylene in ethanol, refluxed at 60 o C for 3h. Retention time at 2.61 minute is 4-Iodotoluene, 2.87 minute is Dodecane and 5.81 minute is 1-Methyl-4-(phenylethynyl) benzene. This spectrum corresponds to the C-C coupling in entry 2 of Table S2. S10

11 Intensity (a.u.) Time (min) Figure S6. GC spectrum of the solution after reaction between 4-Iodoanisole and Phenylacetylene in ethanol, refluxed at 60 o C for 3h. Retention time at 2.87 minute is Dodecane, 3.47 minute is 4- Iodoanisole and 6.52 minute is 1-Methoxy-4-phenylethynyl-benzene. This spectrum corresponds to the C-C coupling in entry 3 of Table S Intensity (a.u.) Time (min) Figure S7. GC spectrum of the solution after reaction between Iodobenzene and 1-Hexyne in ethanol, refluxed at 60 o C for 3h. Retention time at 1.97 minute is Iodobenzene, 2.87 minute is Dodecane and 3.65 minute is 1-Phenyl-1-hexyne. This spectrum corresponds to the C-C coupling in entry 4 of Table S2. S11

12 Intensity (a.u.) Time (min) Figure S8. GC spectrum of the solution after reaction between Iodobenzene and 4-Ethynyltoluene in ethanol, refluxed at 60 o C for 3h. Retention time at 1.97 minute is Iodobenzene, 2.87 minute is Dodecane and 5.81 minute is 1-Methyl-4-(phenylethynyl) benzene. This spectrum corresponds to the C-C coupling in entry 5 of Table S Time (min) Figure S9. GC spectrum of the solution after reaction between 1-Bromo-4-Nitrobenzene and Phenylacetylene in ethanol, refluxed at 60 o C for 3h. Retention time at 1.16 minute is Phenylacetylene, 2.87 minute is Dodecane, 3.65 minute is 1-Bromo-4-nitrobenzene and 7.11 minute is 1-Nitro-4-(phenylethynyl) benzene. This spectrum corresponds to the C-C coupling in entry 6 of Table S2. S12

13 5.26 Intensity (a.u.) Time (min) Figure S10. GC spectrum of the solution after reaction between Iodobenzene and Phenylacetylene in DMSO, refluxed at 100 o C for 3h. Retention time at 2.87 minute is Dodecane, 5.26 minute is Diphenylacetylene and 8.74 minute is side product. This spectrum corresponds to the C-C coupling in entry 7 of Table S2. Intensity (a.u.) Time (min) Figure S11. GC spectrum of the solution after reaction between Bromobenzene and Phenylacetylene in DMSO, refluxed at 100 o C for 3h. Retention time at 2.87 minute is Dodecane, 5.26 minute is Diphenylacetylene, 6.80 min is homocoupling product of Phenylacetylene and 8.74 minute is side product. This spectrum corresponds to the C-C coupling in entry 8 of Table S2. S13

14 2.87 Intensity (a.u.) Time (min) Figure S12. GC spectrum of the solution after reaction between 1-Bromo-2-nitrobenzene and Phenylacetylene in DMSO, refluxed at 100 o C for 3h. Retention time at 2.87 minute is Dodecane, 3.65 minute is 1-Bromo-2-nitrobenzene and7.11 minute is 1-Nitro-2-(phenylethynyl) benzene. This spectrum corresponds to the C-C coupling in entry 9 of Table S2. Intensity (a.u.) Time (min) Figure S13. GC spectrum of the solution after reaction between 1-Bromo-4-nitrobenzene and Phenylacetylene in DMSO, refluxed at 100 o C for 3h. Retention time at 2.87 minute is Dodecane, 3.65 minute is 1-Bromo-4-nitrobenzene and7.11 minute is 1-Nitro-4-(phenylethynyl) benzene. This spectrum corresponds to the C-C coupling in entry 10 of Table S2. S14

15 m/z Figure S14. Mass Spectrum of reactant 1-Iodo-4-nitrobenzene, retention time is 4.30 minute m/z Figure S15. Mass Spectrum of reactant 4-Iodotoluene, retention time is 2.61 minute. S15

16 m/z Figure S16. Mass Spectrum of reactant 4-Iodoanisole, retention time is 3.47 minute m/z Figure S17. Mass Spectrum of reactant 1-Bromo-2-nitrobenzene or 1-Bromo-4-nitrobenzene, retention time is 3.65 minute. S16

17 m/z Figure S18. Mass Spectrum of product 1-Nitro-4-(phenylethynyl) benzene, retention time is 7.11 minute m/z Figure S19. Mass Spectrum of product 1-Methyl-4-(phenylethynyl) benzene, retention time is 5.81 minute. S17

18 m/z Figure S20. Mass Spectrum of product 1-Methoxy-4-phenylethynyl-benzene, retention time is 6.52 minute m/z Figure S21. Mass Spectrum of product 1-Phenyl-1-hexyne, retention time is 3.65 minute. Table S1. Conversion of phenylacetylene and selectivity for production of diphenylacetylene in the first three cycles. Cycle Catalyst Conversion (a) Yield (mmol) (b) Selectivity 1 1st cycle 99% % 2 2 nd cycle 92% % 3 3 rd cycle 91% % (a) Conversions based on unreacted Phenylacetylene and yields were determined by GC with an internal standard Dodecane. (b) Yield of isolated product is the average of two runs. S18

19 Table S2. Catalytic performance of C-C coupling of difference substrates on the same catalyst (50 mg 0.20 wt% Pd/TiO2). Entry Aryl halide Terminal alkyne Product Conversion (%) Yield (%) NO 2 1 I NO 2 99 a 91 a CH 3 2 I 100 a 90 a OCH 3 3 I OCH3 100 a 89 a I 4 C 4 H 9 C 4 H a 86 a I CH a 88 a NO 2 6 Br NO2 27 a 25 a I 7 N/A 85 b Br 8 N/A 76 b NO 2 NO 2 9 Br 90 b 72 b NO 2 10 Br NO2 92 b 83 b a Aryl halide (1.00 mmol), terminal alkyne (1.00 mmol), 50 mg Catalyst, K2CO3 (2.00 mmol) in Ethanol (10 ml) at 60 o C refluxed for 3 h, conversions based on unreacted terminal alkyne and yields were determined by GC with an internal standard dodecane. S19

20 b Aryl halide (1.00 mmol), terminal alkyne (1.00 mmol), 50 mg Catalyst, K2CO3 (2.00 mmol) in dimethyl sulfoxide (10 ml) at 100 o C refluxed for 3 h, conversions based on unreacted aryl halide and yields were determined by GC with an internal standard dodecane. Table S3. DFT-calculated formation energy of oxygen adatoms for structures in Figure 3 of the main text i. Calculations were performed using the experimental O2 dissociation energy 5.19 ev (NIST) and DFT-calculated total energy of an O atom in this work (-1.67 ev). No entropy was taken into account. The other reference was the 3a structure where no oxygen adatom exists. ii. Calculations were performed using the experimental values of enthalpy and entropy of an O atom and a O2 molecule in gas phase at 330 K. The other reference was the 3a structure where no oxygen adatom exists. The formation energies (-100 kj/mol, -128 kj/mol, -52 kj/mol and -52 kj/mol for Figure 3b, 3c, 3d and 3e, respectively) calculated by the method ii which used exerpimental values of enthalply and entropy of an O atom and a O2 molecule in gas phase at 330 K, were used for comparison. Through this comparison, we chose the strucure of Figure 3c as surface of the catalyst to simulate the reaction pathway. iii-iv: similar to i, but referenced to 3c where one oxygen adatom already exists. In other words, these values are the formation energy of the second oxygen adatom. v-vi. similar to ii, but referenced to 3c where one oxygen adatom already exist. In other words, these values are the formation energy of the second oxygen adatom. Note: The formation energies of oxygen adatoms (Oad) on all the structures (Figures 3b- 3e) have been calculated by referencing to gas-phase oxygen molecule (either the DFT-calculated value or the experimental one shown in Table S3) and the clean Pd1/TiO2 surface. The structure 3c, in which the Pd1 atom is coordinated with three lattice oxygen atoms and one oxygen adatom, is the most stable one, evidenced by the most negative formation energy (-128 kj/mol). Addition of one more Oad atoms leading to structure 3d and 3e gave the adsorption of -52 kj/mol for Figure 3d and -55 kj/mol for Figure 3e. Therefore, in the calculations of the reaction activation barriers, the structure 3c has been chosen as the catalyst structure. S20

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