State-of-the-Art Multifunctional Heterogeneous POP Catalyst for

May 9, 2017 - A single-component multifunctional catalyst (denoted as Mg-por/pho@POP) based on a magnesium porphyrin and phosphonium salt-integrated p...
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State-of-the-Art Multifunctional Heterogeneous POP Catalyst for Cooperative Transformation of CO2 to Cyclic Carbonates Wenlong Wang,†,∥ Yuqing Wang,†,§,∥ Cunyao Li,† Li Yan,*,† Miao Jiang,† and Yunjie Ding*,†,‡ †

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ‡ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China § University of Chinese Academy of Sciences, 19 Yuquan Road, Shijingshan District, Beijing 100049, China S Supporting Information *

ABSTRACT: A single-component multifunctional catalyst (denoted as Mg-por/pho@POP) based on a magnesium porphyrin and phosphonium salt-integrated porous organic polymer (POP) was afforded via a solvothermal synthetic technique for cyclic carbonate production which uses epoxides and CO2. In consequence of the cooperative (or synergistic) effect of a phosphonium salt and homogeneously dispersed magnesium porphyrin moiety, which is possibly reinforced through the flexible frameworks and confined microporous structure, this powerful catalyst offered the highest activity of a heterogeneous catalyst within the context of cyclic carbonates synthesis from epoxide and CO2 (turnover frequencies up to 15,600 h−1) without the addition of co-catalysts. More surprisingly, very promising turnover numbers (TONs) of 14,400 and 4200 were realized at very mild temperatures of 25 and 40 °C. Moreover, Mg-por/pho@POP can be simply recovered and reused at least five times. KEYWORDS: Carbon dioxide, Polymeric catalyst, Heterogeneous catalyst, Cooperative catalysis, Cyclic carbonates



INTRODUCTION Carbon dioxide, a main kind of greenhouse gas, has increased more than 37% since preindustrial times, from 280 ppm by volume (ppmv) to more than 400 ppmv today, and has resulted serious environmental problems, such as global warming and sea-level rise.1 On the other hand, CO2 is an abundant, lowpriced, nontoxic, and sustainable carbon source for the modern chemical industry.2−4 Therefore, recently, there has been an explosion in the growth of catalytic reactions that use CO2 as a feedstock.5−10 In particular, the catalytic cycloaddition of CO2 with epoxides to produce cyclic carbonates, which can be widely used as polar aprotic solvents, chemical intermediates, and battery electrolytes, was proved to be an effective way for the transformation of CO2 into value-added chemicals.11−14 Various homogeneous or heterogeneous catalytic systems have been established for the cycloaddition of CO2 to epoxide,15−19 and some catalytic systems exhibit good to excellent activity, especially for the porphyrin, Salen metal complexes as well as amino-phenolate-coordinated complexes.20−33 However, organic ammonium salts or ionic liquids acting as separate co-catalysts are usually required, which increases the cost and calls for a cumbersome purification process. As an improved catalyst model, integrating porphyrin or a Salen-metal complex and organic ammonium into one © 2017 American Chemical Society

complex to form a bifunctional catalyst exhibits very high efficiency.34−40 Nevertheless, these homogeneous catalysts suffered from complicated synthesis and intricate recycling problems, while the use of porous heterogeneous catalytic materials could overcome this problem.41 Porous organic polymers (POPs) have attracted a lot of attention due to their porous structures comprising various organic functional groups which, in theory, could bring numerous combinational structures. The combination of numerous secondary synthons into a microporous threedimensional grid structure using very robust organic covalent bonds, definitely, can afford stable polymeric materials with exceptional chemical and thermal stability, high surface areas, and versatile chemical functionalities.42−45 A wide range of POPs materials have been developed for requests in many areas, especially for catalysis and gas storage.46−48 More recently, a few works of employing metal-functionalized porous organic polymers which integrate valued physical and chemical properties, for example, both of gas storage and catalytic efficiency, have been published. For instance, biomimetic Received: March 29, 2017 Revised: May 1, 2017 Published: May 9, 2017 4523

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porphyrin and MgBr2·OEt2 (Scheme S2), whereas vinylfunctionalized phosphonium salt was synthesized from vinylfunctionalized PPh3 and bromoethane (Scheme S3).54 The synthesized Mg-por/pho@POP is stable in an air atmosphere, and their structure and composition information were determined through solid-state 13C, 31P nuclear magnetic resonance (NMR), and inductively coupled plasma (ICP) analysis. As shown in the spectrum of solid-state 13C NMR (Figure 1), the signals from 131 to 150 ppm show the carbon peak of

catalytic metalloporphyrin-based POPs for the thiols oxidation by using oxygen gas was developed by Jiang’s group, and this catalyst exhibits excellent activity.49 Highly competent Rh/PPh3 POPs for the hydroformylation process have been successively reported by Xiao’s and our group.50−52 Similar integration of the CO2-philic group and metal−organic species in porous organic polymers may result in this kind of catalyst possessing both the ability of CO2 sorption and its simultaneous catalytic conversion. Considering the reported high efficiency of a porphyrin metal complex in the reaction of epoxides and CO2 to produce cyclic carbonates and the CO2 capture ability of N and P atom-doped POPs,53−55 herein, in this work, we demonstrate a singlecomponent multifunctional catalyst based on a magnesium porphyrin and phosphonium salt-integrated porous organic polymer (Mg-por/pho@POP) prepared through solvothermal technique. This kind of polymerization is a free radicaltriggered polymerization between a vinyl-functionalized magnesium porphyrin and phosphonium salt moiety under hydrothermal similar solvothermal conditions without stirring (Scheme 1). This is the first example of POP that contains multifunctional groups of metalloporphyrin and phosphonium salt. Scheme 1. Synthesis of Mg-por/pho@POP

Figure 1. Solid-phase represents sidebands.

13

C NMR spectra of Mg-por/pho@POP, “∗”

aromatic groups (including benzene and pyrrole rings), and the peaks at 29 and 42 ppm are related to those of polymerized vinyl groups. In addition, the peak at about 7 ppm could be ascribed to the ethyl group of a phosphonium salt monomer. The solid-state 31P NMR spectrum of Mg-por/pho@POP shows a peak of 24 ppm which is attributed to the P element of the phosphonium salt monomer (Figure S1). The inductively coupled plasma (ICP) atomic emission spectrometer showed that the Mg content of Mg-por/pho@POP was 0.38 wt %, which was close to theoretic value of 0.45 wt %. The N2 adsorption isotherm and pore width distribution plots which were calculated from nonlocal density functional theory (NLDFT) of Mg-por/pho@POP are shown in Figure 2. The synthesized porous organic polymer displays a moderately high Brunauer−Emmett−Teller (multipoint BET, P\P0 = 0.1− 0.3, Figure S2) surface area of 558 m2/g and a total pore volume of 0.55 cm3/g. The pore width distribution plot (Figure 2B) indicates that pore sizes of the POP material are mainly distributed in a micropore region (98%. bPressure was consistent and realized by pressure regulating valve. cYield was determined by GC analysis with an internal standard of n-butyl alcohol. dNo catalyst. eCatalyst = poly(phosphonium salt). fIsolated yields. g2 h. 4525

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ACS Sustainable Chemistry & Engineering between 1 and 3 MPa at 120 °C (Table 1, entry 1−3), and the yields and turnover frequencies (TOF) were increased from 48% to 62% and 9600 to 12,400 h−1, respectively, while increasing the CO2 pressure from 1 to 3 MPa at 120 °C. With the optimized pressure of 3 MPa, the temperature conditions of 100 and 140 °C were further evaluated. When the temperature was reduced from 120 to 100 °C at 3 MPa, the yields were reduced from 62% to 55% (Table 1, entry 6). When the reaction temperature was increased to 140 °C at 3 MPa, an ultrahigh TOF value of 15,600 h−1 was acquired (Table 1, entry 7), which is the state-of-the-art TOF value of the heterogeneous catalytic system. Prolonging the reaction time to 2 h, a high yield of 90% was achieved (Table 1, entry 11). Under these reaction conditions, without catalysts, no cyclic carbonates were formed (Table 1, entry 4). When the control catalyst of poly(phosphonium salt) was used, only 3% yield of cyclic carbonates was obtained (Table 1, entry 5), which implies the necessity of porphyrin-Mg(II) species. The recovered Mg-por/pho@POP showed nearly the same reactivity as the original, and the porous polymeric catalyst can be simply recovered and reused at least five times by simple centrifugation (Figure 4). Also, an ICP test of the recovered

Deng reported a relatively high efficient catalyst, and a TON result around 200 was achieved at 25 °C concerning this reaction.25 More recently, Ema and co-workers described a bifunctional homogeneous porphyrin-Zn(II) complex, which obtained a TON value of 1640 at 20 °C and 1 atm.57 Hereby, our catalyst was tested in this conversion at 25 and 40 °C (Table 2). A very high TON value of 14400 was achieved when Table 2. Catalytic Activities of Mg-por/pho@POP in Conversion of CO2 to Cyclic Carbonates at 25 and 40 °Ca

entry

PCO2b [MPa]

T [°C]

yieldc [%]

TOF [h−1]

TON

1 2 3 4 5

1 1 0.5 0.5 0.1

40 25 40 25 25

72 21 56 17 12

300 87.5 233 71 50

14400 4200 11,200 3400 2400

a

Reaction conditions: epoxide (160 mmol), catalyst Mg-por/pho@ POP (50 mg), substrate/catalyst = 20,000 (catalyst amount equal to the amount to magnesium porphyrin complex, which is calculated from Mg content of ICP test). The selectivities for the cyclic carbonate products of all results are >98%. bPressure was consistent and realized by pressure regulating valve. cYield was determined by GC analysis with an internal standard of n-butyl alcohol.

the conversion was performed at a very mild temperature of 40 °C (Table 2, entry 1), a TON value of 4200 was attained at an even low temperature condition of 25 °C (Table 2, entry 2). When the pressure was decreased from 1 to 0.5 MPa at 40 and 25 °C, the TON value were reduced to 11200 and 3400, respectively (Table 2, entry 3 and 4). Finally, when the reaction was carried out under ambient conditions (25 °C, 1 atm), a TON value of 2400 was still obtained (Table 2, entry 5). These investigations clearly show that Mg-por/pho@POP could be a real energy-saving and CO2 alleviating catalyst. On the basis of our experimental observations and literature results, a mechanism of cooperative activation process was suggested for this catalytic transformation of CO2 to cyclic carbonates. During the catalysis process, the Mg-porphyrin moiety in Mg-por/pho@POP acted as a Lewis acid, and the bromine anion served as a nucleophile, whereas the P and N atoms play a role of “CO2 catcher”. As shown in Scheme 2, an epoxide could be activated by the Mg-porphyrin site and generate an intermediate with a strong δ− charge in the porous polymer’s confined micropores, thereupon a nucleophilic attack at the less steric carbon atom (which was from the Br−) generates an alcoholate; following one more nucleophilic attack from the alcoholate intermediate toward CO2, another intermediate of acyclic carbonate can be produced. Then, the acyclic carbonate goes through an intramolecular substitution reaction, finally producing a cyclic carbonate product, and the integral Mg-por/pho@POP catalyst was liberated. In summary, a novel multifunctional porous organic polymer Mg-por/pho@POP-integrating porphyrin-Mg moiety and phosphonium salt was first afforded through solvothermal synthesis of free radical polymerization. Without any cocatalyst, the porous heterogeneous catalyst exhibited ultrahigh activity (TOF up to 15,600 h−1 with a high yield of 78%), high selectivity, and good recyclability for the cycloaddition of CO2

Figure 4. Recyclability test of Mg-por/pho@POP. Reaction conditions: propylene oxide (160 mmol), 140 °C, 3 MPa, 50 mg of catalyst, substrate/catalyst = 20,000 (catalyst amount equal to the amount to magnesium porphyrin complex), 1 h.

catalyst of the fifth run displayed 0.37 wt % of Mg content which is very close to the value of a fresh sample, 0.38 wt %. Other substrates of epichlorohydrin, styrene oxide, and 1,2epoxyhexane were also examined, and epichlorohydrin could be transformed into cyclic carbonate in high efficiency (TOF = 13,400 h−1, Table 1, entry 8). However, styrene oxide and 1,2epoxyhexane displayed a comparatively low reactivity (TOF = 7200 and 6000 h−1, respectively, Table 1, entries 9 and 10), which is most probably a result of the steric hindrance effect and electronic effect56 of styrene oxide and a big molecular size of 1,2-epoxyhexane. Worth noting is that all the selectivities of these reactions are greater than 98%, even for the high temperature of 140 °C, which is probably ascribed to the high concentration of CO2 in the micropores of POPs materials. Although our heterogeneous POPs catalyst displays exceptionally high efficiency in cyclic carbonates synthesis from CO2 at high temperatures, it is very attractive that the catalyst used in the process be capable of converting CO2 at very low temperature with heat from the neighboring environment to circumvent the production of extra CO2. Unfortunately, up to now, few promising outcomes have been described. In 2013, 4526

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Chinese Academy of Sciences (XDB17020400), and China Postdotoral Science Fundation (2016M590239).

Scheme 2. Possible Dual-Activation, Cooperative Reaction Mechanism

Notes

The authors declare no competing financial interest.



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and epoxides to produce cyclic carbonates. More importantly, very promising turnover numbers of 14,400 and 4200 were achieved at 25 and 40 °C, respectively. The synthetic strategy of the multifunctional POPs material discussed in this communication holds potential for designing other multifunctional catalysts for highly efficient catalytic processes.



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00947. Synthesis of vinyl-functionalized porphyrin-Mg(II) complex, synthesis of Mg-por/pho@POP, synthesis of poly(phosphonium salt), general synthetic procedure of cyclic carbonates and NMR of product (PDF). (PDF)



REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Li Yan. Fax: +86 411-8437-9143. E-mail: [email protected]. *Yunjie Ding. Fax: +86 411-8437-9143 E-mail: [email protected]. ORCID

Yunjie Ding: 0000-0001-8894-9648 Author Contributions

∥ Wenlong Wang and Yuqing Wang have contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding

National Science Fundation of China (21403258, 21273227, and 21503218), Strategic Priority Research Program of the 4527

DOI: 10.1021/acssuschemeng.7b00947 ACS Sustainable Chem. Eng. 2017, 5, 4523−4528

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