Cationic Zn-Porphyrin Immobilized in Mesoporous Silicas as

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Cationic Zn-porphyrin immobilized in mesoporous silicas as bifunctional catalyst for CO2 cycloaddition reaction under co-catalyst free conditions Sanjeevi Jayakumar, He Li, Lin Tao, Chunzhi Li, Lina Liu, Jian Chen, and Qihua Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01548 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Sustainable Chemistry & Engineering

Cationic Zn-porphyrin immobilized in mesoporous silicas as bifunctional catalyst for CO2 cycloaddition reaction under co-catalyst free conditions ‡

‡

Sanjeevi Jayakumara,b , He Lia , Lin Taoa,c, Chunzhi Lia,c, Lina Liua,c, Jian Chena,c, Qihua Yanga,* a

State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b

International College, University of Chinese Academy of Sciences, Yuquan Road 19A,

Shijingshan District, Beijing 100049, China c

University of Chinese Academy of Sciences, Yuquan Road 19A, Shijingshan District, Beijing

100049, China Corresponding Author Qihua Yang: [email protected]. KEYWORDS Zinc-porphyrin complex, SBA-15, CO2 cycloaddition reaction, bifunctional catalyst, cyclic carbonate.

ABSTRACT Immobilization of metalloporphyrin onto solid support is highly desirable in chemical industry. Herein, we reported for the first time the efficient immobilization of cationic

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zincporphyrin complexes on mesoporous SBA-15 by a simple one-pot method by refluxing 5,10,15,20-tetrakis(4-pyridyl)porphyrin

zinc

(II)

(Zn-TPy),

SBA-15

and

3-

(trimethoxysilyl)propyl bromide in toluene or DMF or NMP or THF. Studies suggest that solid catalyst with high Zn content, stiochiometric Br-/Zn ratio and uniform distribution of active sites could be obtained using DMF as solvent, possibly related with its suitable polarity. The bifunctional solid material with both Lewis acid site and nucleophile acts as an efficient catalyst for catalyzing cycloaddition of CO2 and epoxides under co-catalyst free conditions. Compared with homogeneous counterpart, the solid catalyst is more active (TOF: 1686 h-1 versus 370 h-1). This suggests the closely connected Lewis acid and Br- in solid material could enhance their cooperation during catalytic process. This work provides an efficient method for the synthesis of bifunctional solid catalyst exhibiting enhanced cooperation activation effect for CO2 cycloaddition reaction.

INTRODUCTION

CO2 cycloaddition reaction is of great importance not only because cyclic carbonates are valueadded fine chemicals, but also conversion of CO2 to fuels and chemicals has attracted much research attention due to rapid consumption of fossil energy and environment problems.1-8 Many heterogeneous catalysts have been developed and used in CO2 cycloaddition reactions due to the advantages of easy handling, facile separation of catalysts and products, and possible catalysts recycling.9-37 However, most reported heterogeneous catalysts afforded desirable activity only in the presence of nucleophilic co-catalyst, such as immobilized metal complexes,9-12 metal-free catalysts,13-14 metal-organic frameworks (MOFs)15-18 and porous organic polymers (POPs),19-25 etc. It is still a significant challenge to recover molecular co-catalyst from the reaction solution.


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To make CO2 cycloaddition reaction a real heterogeneous process, different strategies have been reported.26-37 Ionic liquids (ILs) based porous polymers could efficiently catalyze CO2 cycloaddition reaction under co-catalyst free conditions.26-28 But in comparison with Lewisnucleophile system, ILs-based polymers still are not active enough. The bifunctional solid catalysts incorporated with both Lewis acid sites and nucleophilic sites were reported.29-37 Ding and co-workers reported the synthesis of bifunctional polymers containing magnesium porphyrin and phosphonium salt to serve as an efficient catalyst in CO2 cycloaddition reaction.29 Ma’s group successfully encapsulated linear ionic polymer within the channels of a covalent organic framework (COF), the composite exhibited high flexibility and enriched concentration of the active sites thus leading to good activity in cycloaddition reaction without using any cocatalyst.30 Inspired by high catalytic activities of homogeneous bifunctional porphyrin catalysts,38-41 cationic metalloporphyrin (M-Py) based polymers have been designed to act as bifunctional catalysts for CO2 cycloaddition reaction under co-catalysts free conditions.42-44 One of the advantages of cationic M-Py polymers is the close contact of Lewis acid sites and nucleophiles, which may benefit their cooperation during CO2 cycloaddition process. However, the BrunnerEmmet-Teller (BET) surface area of most cationic M-Py polymers is not very high, which results in low exposure degree of active sites. To increase exposure degree of active sites, cationic M-Py polymers was successfully supported on carbon nanotubes (CNTs).43 However, high exposure degree of active sites could be achieved only with low polymer content. Consequently, the product yield of per gram catalyst is not very high. Immobilization of cationic M-Py complexes on ordered mesoporous silicas is a good choice for the synthesis of solid catalysts with high content and high exposure degree of active sites


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considering the high BET surface area, tunable pore diameter and large pore volume of mesoporous silicas.45-46 Previously, M. Fukushima et al. reported the immobilization of cationic Fe-porphyrin on SBA-15 by a two-step method.46 In a first step, SBA-15 was functionalized with chloropropyl group by grafting. Then, the reaction of Fe(III) N-pyridylporphyrin complex (FeTPy) with chloropropyl functionalized SBA-15 results in the formation of cationic Fe-porphyrin functionalized SBA-15. However, only low amounts of Fe-TPy (0.023 mmol/g) were immobilized in SBA-15. Most works relating with immobilization of cationic M-Py complexes on silica employed the two-step method. It still lacks efficient method to immobilize high content of cationic M-Py complexes on solid materials. Herein, we reported an efficient one-pot method for immobilization of high concentration of cationic pyridyl zinc-porphyrin (Zn-TPy) on SBA-15 by heating a mixture of SBA-15, Zn-TPy and 3-(trimethoxysilyl)propyl bromide. By using DMF as solvent, the solid catalyst with high concentration of cationic Zn-TPy (0.18 mmol/g) and high BET surface area (395 m2/g) was obtained, which is closely related with the matched grafting rate and formation rate of ionic bond between propyl bromide and Zn-TPy. The bifunctional solid catalyst affords high activity in CO2 cycloaddition reaction under co-catalyst free conditions due to efficient cooperation of Zn-TPy (Lewis acid) and Br- (nucleophile). EXPERIMENTAL SECTION. Chemicals and Reagents. All chemicals used in this study were of analytical grade and used as received unless otherwise specified.

5,10,15,20-Tetrakis(4-pyridyl)-21H,23H-porphine,47

5,10,15,20-Tetrakis(4-


4

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pyridyl)porphyrin zinc(II) (Zn-TPy)48 and SBA-1549 were synthesized according to the literature procedures. Characterization: The contents of C, H and N were quantified by using an Oxygen/Nitrogen/Hydrogen Analyzer (EMGA-930, HORIBA, Japan) and a Carbon/Sulfur Analyzer (EMIA-930, HORIBA, Japan). Zn and Si content of the samples was measured by PLASAM-SPEC-II inductively coupled plasma atomic emission spectrometry (ICP-AES). Thermal gravimetric analyses (TGA) were performed on a NETZSCH STA 449F3 analyzer, with the temperature range from 30 to 900 oC and a heating rate of 10 oC min-1 in air atmosphere. X-ray powder diffraction (XRD) patterns were recorded on a Rigaku RINT D/Max-2500 powder diffraction system, equipped with Cu Kα radiation (λ = 1.54 Å). Transmission electron microscope (TEM) characterization was undertaken using a HITACHI HT7700 microscope at an acceleration voltage of 100 kV. Before measurement, the samples were fully dispersed in ethanol and then deposited on a holey carbon film on a Cu grid. Nitrogen sorption characterization was performed on an automatic volumetric adsorption analyzer (Micromeritics ASAP2020). Before analysis, all the samples were carefully degassed at 100 oC for 6 hours under vacuum of 10 mmHg. The BET surface area was calculated from the adsorption data in a relative partial pressure (P/P0) range of 0.04 to 0.20. The total pore volume was calculated at P/P0 of 0.99 using a single-point adsorption value. The pore diameter was determined from the adsorption branch by using the Barret-Joyner-Halender (BJH) method. The Fourier-transform infrared spectroscopy (FT-IR) spectra were recorded from 400-4000 cm-1 on a Nicolet Nexus 470 IR spectrometer by using KBr pellets. Solid-state 13C and 29

Si CP-MAS NMR spectra were performed on a Bruker 500 MHz spectrometer.

13

C and

29

Si


5


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signals were referenced to tetramethylsilane (TMS). UV/Vis diffuse-reflectance spectra were recorded on SHIMADZU UV-Vis 2550 spectrophotometer. Synthesis of SBA-Zn-TPy+PBr⁻: 600 mg of SBA-15 (evacuated at 120 °C for 3 h) was dispersed in 15 mL solvent (toluene, THF, DMF or NMP), then Zn-TPy (145 mg, 0.21 mmol) and 3-(trimethoxysilyl)propyl bromide (310 mg, 1.27 mmol) were added. The mixture was refluxed at 140 ºC for 48 h under nitrogen atmosphere. After filtration, the obtained material was fully washed by DMF, THF, water and ethanol, and then dried under vacuum at 60 ºC for 6 h. The samples were denoted as SBA-ZnTPy+PBr-. After fully washed the obtained SBA-Zn-TPy+PBr⁻DMF with DMF, we collected the filtration and the amount of 3-(trimethoxysilyl)propyl bromide in the filtration was checked by 1

H NMR and ICP analysis of Si. From the results of 1H NMR and ICP analysis, the amount of

unreacted 3-(trimethoxysilyl)propyl bromide was ca. 2%, indicating most of silane precursors were immobilized in SBA-15. Synthesis of SBA-Zn-TPy+PI⁻: The synthesis procedure was similar to that of SBA-Zn-TPy+PBr-DMF, with the exception that 3-(trimethoxysilyl)propyl iodide was used instead of 3-(trimethoxysilyl)propyl bromide and the reaction temperature of 120 oC. Synthesis of Zn-TPy+PBr-/SBA-15: Zn-TPy+PBr-/SBA-15 was prepared with a modified two-step method.42 In a 100 mL round flask, 600 mg of SBA-15 (evacuated at 120 °C for 3 h) was dispersed in 15 mL of toluene. Then 3-(trimethoxysilyl)propyl bromide (310 mg, 1.27 mmol) was added. The mixture was refluxed


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for 24 h under nitrogen atmosphere. After filtration, the obtained material was fully washed by ethanol, and then dried under vacuum at 60 ºC for 6 h. The obtained material was denoted as SBA-PBr. 300 mg of SBA-PBr (evacuated at 120 °C for 3 h) was dispersed in 15 mL DMF, followed by the addition of Zn-TPy (73 mg, 0.105 mmol). The mixture was refluxed for 48 h under nitrogen atmosphere. After filtration, the obtained material was fully washed by DMF, THF, water and ethanol, and then dried under vacuum at 60 ºC for 6 h. The obtained material was denoted as Zn-TPy+PBr-/SBA-15. General procedure for the cycloaddition reaction of CO2 and epoxides. In a typical experiment, propylene oxide (PO) (770 mg, 13.27 mmol) and catalyst (0.05 mol% based on metal content) were put into a 15 mL stainless steel reactor equipped with a magnetic stirrer. After purging the autoclave with CO2 for 3 times, the reactor was pressurized to 1.5 MPa of CO2 and kept in a pre-heated oil bath. After reacting at 120 °C for 3.5 h with a stir speed of 550 rpm, the reactor was taken out from the oil bath and cooled to room temperature. After slowly releasing the CO2 in the reactor, a certain amount of n-butyl acetate was added to serve as the external standard. The organic solvent contain liquid products were analysed by gas chromatograph (Agilent 7890 GC) equipped with a PEG-column (30 m × 0.25 mm × 0.25 mm). For other substrates, a similar procedure was used with the exception that epichlorohydrin, 1,2-epoxyhexane, butyl glycidyl ether, styrene oxide and cyclohexene oxide was used instead of PO. For the reaction conducted under mild condition, a similar procedure was used with the exception that the reaction temperature was 40 °C and CO2 pressure was 0.5 MPa. For catalyst recycle experiments, the solid catalyst was recovered by centrifugation and washed for 3 times (each time 10 mL) with dichloromethane. After being dried under vacuum at


7


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60 oC for 3 h, the recovered catalyst was reused for the next run. The recycling experiment was performed at 120 °C under CO2 pressure of 1.5 MPa, S/C of 1000, 1 h.

RESULTS AND DISCUSSION

Cationic Zn-TPy was grafted in the nanopore of SBA-15 by a one-pot grafting method as illustrated in Figure 1A, in which the grafting and formation of ionic bonds occurred simultaneously. Toluene, tetrahydrofuran (THF), N,N-dimethylformamide (DMF) and N-methyl pyrrolidone (NMP) were used as solvents for grafting. The SBA-15 functionalized with cationic Zn-porphyrin was denoted as SBA-Zn-TPy+PBr-x, where x refers to solvent. The elemental analysis data show that all SBA-Zn-TPy+PBr- samples give N/Zn molar ratio of ca. 8, verifying that no leaching of metal from porphyrin ring occurs during the grafting process (Table 1). The Zn content of SBA-Zn-TPy+PBr- depends strongly on the type of solvents (Figure 1B). With toluene, DMF and NMP as solvent, the Zn content of solid catalysts varied in the range of 0.21 to 0.16 mmol/g. With THF as solvent, the Zn content in the solid catalyst decreases to 0.08 mmol/g. For comparison, Zn-TPy+PBr- was also supported on SBA-15 using previously reported two-step method (the sample was denoted as Zn-TPy+PBr-/SBA-15), the Zn content of ZnTPy+PBr-/SBA-15 is 0.08 mmol/g. The above results suggest that one-pot synthesis method using appropriate solvent is effective in obtaining solid catalyst with high content of ZnTPy+PBr-.


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Figure 1. (A) Schematic illustration for the one-pot synthesis of SBA-Zn-TPy+PBr- and (B) the Zn content and Br-/Zn-TPy+ molar ratio of SBA-Zn-TPy+PBr- prepared with different types of solvent. The zinc content was measured by ICP analysis and Br- content was calculated with the formula: mmol Br- = [Total weight loss – mmol Zn × MZn-TPy – weight loss (125 to 300 °C)]/MPBr, where total weight loss in the range of 125 to 800 °C, MZn-TPy is the molecular weight of Zn-TPy and MPBr is the molecular weight of propyl bromide anions. Table 1. Physiochemical parameters of SBA-Zn-TPy+PBr⁻ prepared with different types of solvents. S Sample

Dp

BET

(m2/g)

Na

Vp (cm3/g)

(nm)

(mmol/g)

Weight lossb (%)


9


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SBA-Zn-TPy+PBr-Toluene

358

0.44

3.4 & 5.4

1.74

25.8

SBA-Zn-TPy+PBr-DMF

395

0.34

3.5

1.48

25.2

SBA-Zn-TPy+PBr-NMP

429

0.44

3.5

1.32

23.2

SBA-Zn-TPy+PBr-THF

469

0.61

5.3

0.64

13.8

SBA-Zn-TPy+PI-DMF

541

0.62

4.6

1.0

14.2

SBA-PBr

519

0.7

6.4

-

10.6

a

Measured with C, H, N elemental analysis. b Weight loss in the range of 125 to 800 °C based on TG curves. The thermal gravimetric curves of SBA-Zn-TPy+PBr- prepared using toluene, THF, DMF and NMP are quite different (Figure 2). Propyl bromide group grafted sample, SBA-PBr (prepared by grafting (MeO)3SiCH2CH2CH2Br on SBA-15) decomposes from 170 to 300 °C (Figure S1). According to our previous report, the cationic Zn-TPy could be stable up to 300 °C.43 Thus, the weight loss below 300 °C is from free propyl bromide group and the weight loss

above 300 °C derives from the decomposition of Zn-TPy+PBr-. The TG curves of SBA-ZnTPy+PBr-THF and SBA-Zn-TPy+PBr-NMP show sharp weight loss below 300 °C, indicating the existence of unreacted propyl bromide group. For SBA-Zn-TPy+PBr-Toluene and SBA-ZnTPy+PBr-DMF, only slight weight loss was observed at temperature below 300 °C, indicating that almost all propyl bromide groups are reacted with Zn-TPy. Based on Br-/Zn-TPy+ molar ratio summarized in Figure 1B, SBA-Zn-TPy+PBr-Toluene and SBA-Zn-TPy+PBr-DMF have Br-/Zn-TPy+ molar ratio respectively of 3.9 and 3.8, suggesting that one Zn-TPy+ is almost connected with four Br-, reaching the stoichiometric ratio. SBA-Zn-TPy+PBr-NMP and SBA-Zn-TPy+PBr-THF have Br-/Zn-TPy+ molar ratio respectively of 3.5 and 2.6, indicating incomplete reaction of propyl bromide groups with Zn-TPy.


10

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100 95

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90 85 -

80 75

SBA-Zn-TPy+PBr THF -

SBA-Zn-TPy+PBr DMF -

SBA-Zn-TPy+PBr NMP -

70

SBA-Zn-TPy+PBr Toluene 200

400 600 o Temperature ( C)

800

Figure 2. The thermal gravimetric curves of SBA-Zn-TPy+PBr⁻Toluene, SBA-Zn-TPy+PBr⁻THF, SBA-Zn-TPy+PBr⁻DMF and SBA-Zn-TPy+PBr⁻NMP. The chemical composition of representative SBA-Zn-TPy+PBr-DMF was characterized using FT-IR and NMR spectroscopy (Figure 3). The FT-IR spectrum of SBA-Zn-TPy+PBr-DMF clearly exhibits characteristic vibration peaks at 993, 996 and 1007 cm-1 assigned to pyrrole rings of Znporphyrins. The peaks at 1636 cm-1 and 1458 cm-1 assigned respectively to pyridine iminium ion (-C=N+-) and CH2 bending vibration indicate the reaction of propyl bromide with Zn-TPy for the formation of pyridine iminium ion (-C=N+-). In comparison with Zn-TPy, the red shift of C=N bond of pyridine moieties from 1593 to 1636 cm-1 observed in the FT-IR spectrum of SBA-ZnTPy+PBr-DMF gave an extra proof for the formation of pyridine iminium ion (-C=N+-). The immobilization of Zn-TPy on SBA-15 was further confirmed by UV-vis spectroscopy (Figure 3B). The absorption peaks for Zn-TPy at 426 nm and 558 nm refer to B band and Q band, respectively.50 SBA-Zn-TPy+PBr-DMF gave similar UV-vis spectrum to Zn-TPy. The red shift of Q/B band was observed for SBA-Zn-TPy+PBr-DMF compared with that of Zn-TPy, possibly due


11


to the variation in molecular structure of zincporphyrin ring caused by interaction of immobilized zincporphyrin with hydroxyl group of SBA-15.51

a

A

Absorbance

b

B

Zn-TPy 1.2

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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+

-

SBA-Zn-TPy PBr DMF 426

0.8

0.4 570 558

0.0

3000

1500

1000

300

500

Wavenumber (cm-1)

400

500

C

b 3

Q

f

250

200

150

D

4

Q

g e

* *

700

a

c

h

600

Wavelength (nm)

2

T T3

i

d *

100

50

Chemical shift (ppm)

0

-50

0

-50

-100

-150

-200

Chemical shift (ppm)

Figure 3. (A) FT-IR spectra of (a) Zn-TPy, (b) SBA-Zn-TPy+PBr-DMF, (B) UV-vis spectra of ZnTPy and SBA-Zn-TPy+PBr-DMF, (C) 13C CP-MAS and (D) 29Si MAS NMR spectrum of SBA-ZnTPy+PBr-DMF.. The 13C CP-MAS NMR spectrum of characteristic SBA-Zn-TPy+PBr-DMF clearly shows the signals at 64, 25, and 8 ppm assigned to C3, C2, and C1 carbon species of Si-C1H2C2H2C3H2, respectively (Figure 3C).52 The signal at 116 ppm (d) could be assigned to the porphyrin macro cycle ring carbon connected with pyridyl ring. The peak at 160 ppm (h) is from the carbon of pyridyl ring (p-C) attached with porphyrin macro cycle. The signals at 132 (e) and 143 ppm (f) correspond to carbons of pyrrole ring of porphyrin and carbon of pyridyl ring (β-C). The


12

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resonance peaks at 149 ppm (g) is assigned to carbons adjacent to pyridyl-N (α-C). The signal at 53 ppm (i) corresponds to unhydrolyzed methoxy carbon of 3-(trimethoxysilyl)propyl bromide. 29

Si NMR spectrum of SBA-Zn-TPy+PBr⁻DMF clearly shows T and Q silicon sites (Figure 3D).

The signals at −66 ppm (T3) and −57 ppm (T2) are assigned to silicon atom connected with organic group and the signals at −110 ppm (Q4) and −101 ppm (Q3) arise from [Si(OSi)4] and [Si(OH)(OSi)3], respectively.53 The NMR characterizations confirm the successfully grafting of Zn-TPy+PBr- on SBA-15 by one-pot reaction method. The combined FT-IR, UV-vis and NMR characterizations confirm the successful incorporation of cationic Zn-TPy+PBr- in SBA-15.

Figure 4. a) XRD pattern, b) TEM image and c) N2 sorption isotherm of SBA-Zn-TPy+PBr-DMF. The structural properties of SBA-Zn-TPy+PBr⁻ samples were characterized with XRD, TEM and N2 sorption isotherms (Figure 4, Figure S2-S5 and Table 1). The XRD pattern of all SBAZn-TPy+PBr⁻ samples clearly shows (100) diffraction peak, implying that all samples have ordered mesostructure similar to SBA-15 (Figure S2). The fading of (110) and (200) diffraction peaks may be due to the occupation of organic component in the mesopore, which decreases the contrast between the pore and wall.54 The TEM image of representative SBA-Zn-TPy+PBr⁻DMF clearly shows the ordered arrangement of 2-D hexagonal mesopore, further confirming that SBA-Zn-TPy+PBr⁻ has ordered mesoporous structure.


13


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The N2 sorption isotherms of SBA-Zn-TPy+PBr⁻ samples are of type IV, characteristic of mesoporous structure (Figure S4). The pore blocking was observed for SBA-Zn-TPy+PBr⁻ prepared with toluene as solvent due to the existence of two desorption steps in the N2 isotherm pattern.55 This was further confirmed by two sets of pore distribution at 3.4 and 5.4 nm shown in Table 1 and Figure S5. In comparison with pristine SBA-15, the BET surface area, pore volume and pore diameter of all SBA-Zn-TPy+PBr- decrease due to the occupation of Zn-TPy+PBr- in the nanopore of SBA-15. The above results suggest that the solvent has big influence on the physical parameters, Zn content and Br⁻ to Zn-TPy+ of SBA-Zn-TPy+PBr-. In the one-pot synthesis process, the following competitive reactions occur almost at the same time, the reaction of Zn-TPy with (MeO)3SiCH2CH2CH2Br for the formation of (MeO)3SiCH2CH2CH2Br-Zn-TPy+, the grafting of (MeO)3SiCH2CH2CH2Br and (MeO)3SiCH2CH2CH2Br-Zn-TPy+ on SBA-15 and the reaction of grafted propyl bromide with Zn-TPy. Once propyl bromide is grafted onto SBA-15, its reaction with Zn-TPy is slowed down due to the loss of mobility and restriction of the nanopore. Thus, the successful grafting of Zn-TPy+PBr⁻ on SBA-15 requires fast reaction of Zn-TPy with (MeO)3SiCH2CH2CH2Br. The above results show that high content of Zn-TPy+PBr- could be grafted in SBA-15 with toluene, DMF and NMP as solvent, possibly due to the rapid reaction rate of Zn-TPy with (MeO)3SiCH2CH2CH2Br in these solvents. The diffusion of (MeO)3SiCH2CH2CH2Br-Zn-TPy+ in the nanopore determines the uniformity of Zn-TPy+PBr- in SBA-15. The pore blockage of SBA-Zn-TPy+PBr-Toluene is possibly related with low diffusion rate of (MeO)3SiCH2CH2CH2Br-Zn-TPy+ in toluene with low polarity. The above results suggest that DMF is a desirable solvent for one–pot synthesis of SBA-ZnTPy+PBr- in view of content of Zn-TPy+PBr- and textural properties of the final product. Thus,


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SBA-Zn-TPy+PI-DMF was prepared in a similar method to SBA-Zn-TPy+PBr-DMF with the exception that (MeO)3SiCH2CH2CH2I was used and the reaction temperature was decreased to 120 ºC. The FT-IR spectrum of SBA-Zn-TPy+PI-DMF was similar to that of SBA-Zn-TPy+PBrDMF

(Figure S6). The Zn content of SBA-Zn-TPy+PI-DMF is only slightly lower than that of SBA-

Zn-TPy+PBr-DMF (0.12 versus 0.18 mmol/g). The molar ratio of I-/Zn-TPy+ is calculated to be only 2.0 based on ICP and TG analysis (Table 1 and Figure S1). This is possibly due to the larger size of I- than Br-, which impedes the stoichiometric connection of Zn-TPy with I-. Table 2. Catalytic performances of SBA-Zn-TPy+PBr- and SBA-Zn-TPy+PI-DMF in catalyzing cycloaddition of CO2 and propylene oxide (PO).a

Entry

Catal.

Time

PCO2

T

Sel.

Yield

TOF

(h)

(MPa)

(°C)

(%)

(%)

(h-1)

S/C

1

SBA-15

-

3.5

1.5

120

0

0

0

2

SBA-PBrb

-

3.5

1.5

120

0

0

0

3

SBA-Zn-TPy+PBr-Toluene

2000

3.5

1.5

120

>99

73

734

4

SBA-Zn-TPy+PBr-THF

2000

3.5

1.5

120

>99

42

240

5

SBA-Zn-TPy+PBr-NMP

2000

3.5

1.5

120

>99

50

600

6

SBA-Zn-TPy+PBr-DMF

2000

3.5

1.5

120

>99

91

892

7

SBA-Zn-TPy+PI-DMF

2000

3.5

1.5

120

>99

99

1535

8

SBA-Zn-TPy+PBr-DMF

5000

6

1.5

120

>99

74

927

9

SBA-Zn-TPy+PI-DMF

5000

6

1.5

120

>99

91

1686

10

SBA-Zn-TPy+PI-DMF

500

36

0.5

40

>99

94

14

13

ZnTPy+PBr-/SBA-15c

2000

3.5

1.5

120

>99

17

51

14

Zn-TPy + TBAB

5000

6

1.5

120

>99

38

370


15


Page 16 of 31

15

NH3I-Zn/SBA-1556

-

12

3

150

>99

99

326

16

ZnTPy-BIM4/CNTs-343

7100

6

1.5

120

>95

95

2602

17

BIO-1C42

1000

6

1.7

120

>99

90

-

a

770 mg of PO, TOF was calculated with conversion less than 35%. b Prepared by grafting of (MeO)3SiCH2CH2CH2Br onto SBA-15. c Prepared by a two-step method (SBA-PBr was reacted with Zn-TPy.) The catalytic performance of SBA-Zn-TPy+PBr- was tested in CO2 cycloaddition reaction under co-catalyst free conditions using propylene oxide (PO) as substrate (Table 2). With SBA15 or SBA-PBr as catalyst, almost no cyclic carbonates could be detected. In the presence of SBA-Zn-TPy+PBr-, the cycloaddition reaction of CO2 and PO goes smoothly with selectivity to cyclic carbonates higher than 99%, implying that cationic Zn-TPy+PBr- are active sites for cycloaddition reaction. Though solid catalysts with M-porphyrin as active site have been used for cycloaddition reaction, co-catalysts are generally needed during the catalytic process. In this work, SBA-Zn-TPy+PBr- with bifunctional nature of M-TPy+ activating epoxide and Br- as nucleophiles could efficiently catalyze the cycloaddition reaction under co-catalysts free conditions. In comparison with homogeneous Zn-TPy with tetrabutylammonium bromide (TBAB) as co-catalyst, SBA-Zn-TPy+PBr-DMF gives much higher activity (TOF: 370 versus 927 h-1), indicating the enhanced cooperation of closely connected Br- and Zn-TPy in SBA-15. Also, SBA-Zn-TPy+PBr-DMF is more active than Zn-TPy+PBr-/SBA-15 (prepared by a two-step method), which reveals the advantage of the one-pot synthesis method.


16

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CO2 R Br

O Br

Br-

-

Br-Zn Br-

Br- ZnBrBrBr-

O

Br-

Zn Br-

Br

O

BrO

O

R

Br-

R

O O R

O

Scheme 1. Plausible mechanism pathway of cycloaddition of CO2 and propylene oxide catalyzed by SBA-Zn-TPy+PBr-DMF. The plausible cooperative activation mechanism for CO2 cycloaddition reaction is shown in Scheme 1. The porphyrin Zn(II) plays a Lewis acid role and the Br– act as a nucleophile. At the beginning, the oxygen atom of epoxide coordinates with Zn to form Zn-O bond for activation of epoxide. Later, the Br– anion attacks to the less sterically hindered β-carbon atom of epoxide to form the Br-alkoxide intermediate. Simultaneously, the CO2 insertion occurs to produce Zncarbonate intermediate, which further proceeds via an intramolecular ring-closure step to release cyclic carbonate followed by the regeneration of the catalyst. SBA-Zn-TPy+PBr- samples prepared with different types of organic solvent show quite different catalytic activity (entries 3-6, Table 2). In 3.5 h, SBA-Zn-TPy+PBr-DMF and SBA-ZnTPy+PBr-Toluene respectively give 91% and 73% yields. Under similar reaction conditions, the yields for SBA-Zn-TPy+PBr-THF and SBA-Zn-TPy+PBr-NMP are less than 50%. The TOF decreases in the order of SBA-Zn-TPy+PBr-DMF > SBA-Zn-TPy+PBr-Toluene > SBA-Zn-TPy+PBrNMP

> SBA-Zn-TPy+PBr-THF. Previous results suggest that Br-/Zn ratio is very important for


17


Page 18 of 31

cycloaddition reaction and higher Br-/Zn ratio generally results in higher activity.43 The low activity of SBA-Zn-TPy+PBr-THF and SBA-Zn-TPy+PBr-NMP is due to their low Br-/Zn-TPy+ ratio. SBA-Zn-TPy+PBr-DMF with similar Br-/Zn-TPy+ ratio to SBA-Zn-TPy+PBr-Toluene shows higher TOF (892 versus 734 h-1) than the later. This is mainly related with heterogeneously distributed Zn-TPy+PBr- on SBA-Zn-TPy+PBr-Toluene. Zn-TPy+PBr- plugged in the nanopore of SBA-ZnTPy+PBr-Toluene cannot be accessed by the reactants, which does not benefit for obtaining high activity. Thus, both Br-/Zn-TPy+ ratio and uniform distribution of active sites should be optimized for the synthesis of high performance solid catalysts. The type of nucleophiles plays an important role in Lewis acid catalyzed cycloaddition reactions. The influence of nucleophiles on the catalytic performance of solid catalysts was investigated by comparing SBA-Zn-TPy+PBr-DMF with SBA-Zn-TPy+PI-DMF (entries 6-9, Table 2). SBA-Zn-TPy+PI-DMF is more active than SBA-Zn-TPy+PBr-DMF with S/C ratio of 2000 or 5000. The TOF of SBA-Zn-TPy+PI-DMF is ca. 1.7 to 1.8-fold that of SBA-Zn-TPy+PBr-DMF. The enhanced activity is due to higher nucleophilicity of iodine anion compared with bromide anion. Even under mild condition (40 ºC, 0.5 MPa CO2), SBA-Zn-TPy+PI⁻DMF could smoothly catalyze the cycloaddition reaction to afford 94% yield. In comparison with previously reported solid catalysts with cationic Zn-Py as active sites, SBA-Zn-TPy+PI-DMF shows comparable high activity under co-catalyst free conditions.42-43 And the activity is much higher than that of the SBA-15 based bifunctional catalyst NH3I-Zn/SBA-15.56 Table 3. The catalytic performances of SBA-Zn-TPy+PBr-DMF for cycloaddition of various epoxides with CO2.


18

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Conv.: 99%

Conv.: 99%

Conv.: 84%

Conv.: 96%

Conv.: 23%

TOF: 685 h-1

TOF: 597 h-1

TOF: 293 h-1

TOF: 619 h-1

TOF: 115 h-1

Time: 3.5 h

Time: 4.5 h

Time: 5 h

Time: 4.5 h

Time: 5 h

Reaction conditions: epoxide, 6.63 mmol; SBA-Zn-TPy+PBr-DMF (0.1 mol %); CO2, 1.5 MPa; 120 ºC. TOF was estimated at conversion less than 35% and SBA-Zn-TPy+PBr⁻DMF (0.05 mol %) was used. Furthermore, a wide range of substrates could be efficiently converted into corresponding cyclocarbonates with SBA-Zn-TPy+PBr-DMF as catalyst (Table 3). SBA-Zn-TPy+PBr-DMF affords high activity and selectivity towards epichlorohydrin (EPH), styrene oxide (SO) and 1,2epoxyhexane. A moderate conversion of 84% was found for butyl glycidyl ether as a substrate, this is possibly due to the long alkyl chain which affects the diffusion of substrate. Even using a challenging sterically hindered substrate, cyclohexene oxide, SBA-Zn-TPy+PBr-DMF could also exhibits a conversion of 23% with TOF value of 115 h-1. The above results suggest that SBA-ZnTPy+PBr-DMF is an efficient solid catalyst for various kinds of substrates in cycloaddition reaction under solvent free conditions. The stability of SBA-Zn-TPy+PBr⁻DMF was tested in cycloaddition of CO2 and PO (Figure 5). No obvious decrease in activity and selectivity could be observed for SBA-Zn-TPy+PBr-DMF under co-catalyst free conditions after five cycles. The reused catalyst was characterized by elemental analysis and ICP, the N and Zn content was 1.44 mmol/g and 0.18 mmol/g, respectively. This result indicates almost no leaching of N and Zn, showing the robustness of SBA-Zn-TPy+PBr⁻DMF during the recycling process.


19


Yield (%)

100

Yield & Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

Selectivity (%)

80 60 40 20 0 1

2

3

4

5

Run

Figure 5. Recycle stability of SBA-Zn-TPy+PBr⁻DMF in cycloaddition of CO2 and propylene oxide (Reaction conditions: SBA-Zn-TPy+PBr⁻DMF, 0.1 mol%; PO, 13.27 mmol; 120 ºC; 1.5 MPa; 1 h).

CONCLUSIONS

In summary, an efficient one-pot method was developed for immobilization of cationic ZnTPy complex on SBA-15. Compared with two-step method, high amount of cationic Zn-TPy complex could be immobilized on SBA-15. Studies suggest that DMF is an appropriate solvent for one-pot grafting method in view of high grafting content and uniform distribution of active sites and almost stoichiometric formation of ionic bond. SBA-Zn-TPy+PBr-DMF could efficiently catalyze the cycloaddition of CO2 and epoxides under solvent-free and co-catalyst-free conditions and is more active than the homogeneous system with Zn-TPy as Lewis acid and TBAB as co-catalyst. This indicates the closely connected Zn-TPy and Br- in solid catalyst could enhance their cooperation. The one-pot method provides a new approach for the immobilization


20

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of cationic metalloporphyrin on solid supports for CO2 cycloaddition reactions under co-catalystfree conditions. ASSOCIATED CONTENT Supporting Information. Additional TG curves, XRD patterns, TEM images, N2 sorption isotherms curves, pore size distributions and FT-IR spectra. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; ORCID ID: 0000-0002-1118-3397. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key R&D Program of China (2017YFB0702800), the National Natural Science Foundation of China (21733009, 21621063) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200). J.S. thanks the International College, University of Chinese Academy of Sciences (UCAS), and the World Academy of Sciences (TWAS) for the award of CAS-TWAS President’s Research Fellowship.


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Table of Contents

Synopsis Cationic zincporphyrin complex was successfully immobilized on mesoporous SBA-15 by a simple one-pot method. The resulting hybrid materials exhibit enhanced cooperation activation effect towards CO2 cycloaddition reaction under co-catalyst-free conditions.


31

Cationic Zn-Porphyrin Immobilized in Mesoporous Silicas as

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