Honeycomb MetalâOrganic Framework with Lewis Acidic and Basic

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Honeycomb Metal−Organic Framework with Lewis Acidic and Basic Bifunctional Sites: Selective Adsorption and CO2 Catalytic Fixation Xiu-Yuan Li,† Li-Na Ma,† Yang Liu,† Lei Hou,*,† Yao-Yu Wang,† and Zhonghua Zhu‡ †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China ‡ School of Chemical Engineering, The University of Queensland, Brisbane 4072, Australia S Supporting Information *

ABSTRACT: Carrying out the strategy of incorporating rod secondary building units and polar functional groups in metal−organic frameworks (MOFs) to accomplish the separation of CO2 and C2 hydrocarbons over CH4 as well as CO2 fixation, an oxalamide-functionalized ligand N,N′-bis(isophthalic acid)-oxalamide (H4BDPO) has been designed. The solvothermal reaction of H4BDPO with the oxophilic alkaline-earth Ba2+ ion afforded a honeycomb Ba-MOF, {[Ba2(BDPO)(H2O)]· DMA}n (1). Due to the existence of Lewis basic oxalamide groups and unsaturated Lewis acid metal sites in the tubular channels, the activated framework presents not only high C2H6, C2H4, and CO2 uptakes and selective capture from CH4, but also efficient CO2 chemical fixation as a recyclable heterogeneous catalyst. Grand canonical Monte Carlo simulations were combined to explore the adsorption selectivities for C2H6−CH4 and C2H4−CH4 mixtures as well as the interaction mechanisms between the framework and epoxides. KEYWORDS: metal−organic framework, crystal structure, gas adsorption, chemical conversion, heterogeneous catalysis

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The Ba2+ ion is the last nonradioactive element of alkalineearth metal; in particular, it possesses almost the largest ionic radius in common metal ions, which enables it not only to be easily coordinated by more atoms to form cluster or rod SBUs in MOFs but also to attract the target adsorbate molecules favorably.23−26 As is well known, isophthalic acid is one of the most representative organic ligand building complexes containing various cluster or rod SBUs.27−29 Furthermore, a variety of bi- and tri-isophthalic acid organic linkers were designed to create some impressive MOFs.30−32 Comparably, an oxalamide-functionalized bis-isophthalic acid ligand, N,N′bis(isophthalic acid)-oxalamide (H4BDPO), intrigued us. H4BDPO contains four carboxyls and one Lewis basic acidamide unit, which make it an ideal candidate to coordinate with oxophilic alkaline-earth metal ions to produce oxalamidefunctionalized MOFs. Moreover, the immobilization of active sites of oxalamide groups together with UMSs enables the MOFs to reach the dual requirements of gas adsorption and heterogeneous catalysis.33,34 Typically, these active sites boost the interactions between MOFs and certain gas molecules, such as CO2, C2H4, and C2H6,35−39 which uncover the very important application of MOFs not only for the separation of CO2, C2H4, or C2H6 over CH4 in light hydrocarbon (C2/C1)

INTRODUCTION

Metal−organic frameworks (MOFs) are a group of crystalline porous materials composed of metal centers and organic linkers, being in favor of by chemists and material scientists because of their intrinsic merits of ordered and designable structures1,2 and extensive applications.3−5 Rationally selecting organic ligands and creating secondary building units (SBUs) are vital for building MOFs with desired functions and properties,6,7 even though the oriented synthesis is still a challenge.8 In this context, metal−organic multinuclears have been emerged as one kind of successful SBU to fabricate functional MOFs with versatile topologies.9−12 Compared to discrete cluster SBUs, the infinite onedimensional (1D) rodlike metal−organic chain SBUs possess more advantages, such as not only preventing framework interpenetration and vacating more free voids in MOFs but also generating the predicable shapes of triangular, tetragonal, or hexagonal channels.13−15 Meanwhile, the rod SBUs also contain dense potential unsaturated metal sites (UMSs), which will be the crucial sites for no matter adsorption or catalysis processes.16−18 In general, most MOFs with cluster or rod SBUs are centered on employing d-block and lanthanide elements, leaving the inexpensive and environment-friendly alkaline-earth metal ions relatively largely unexplored.19−21 The recent strategy of using alkaline-earth Sr2+ ions to build rod SBUs forming porous frameworks was proved to be effective.22 © XXXX American Chemical Society

Received: January 23, 2018 Accepted: March 9, 2018

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DOI: 10.1021/acsami.8b01291 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Coordination environment of the Ba2+ ions in 1 (symmetry codes: #1 x − y + 1, −y + 1, −z; #2 x − y + 1, x + 1, z + 1/6; #3 y, x + 1, −z + 1/3; #4 y, −x + y, z − 1/6; #5 x, x − y + 1, −z + 1/6; #6 −x + y, −x + 1, z − 1/3; #7 −y + 1, −x + 1, −z − 1/6; #8 −y, −x, −z − 1/6), (b) lefthanded 21 helical rod SBUs, (c) tubular structure, (d) porous surface, and (e) porous framework viewed along the c axis.

Figure 2. Gas sorption isotherms of 1a for CO2 and CH4 at 195 K and N2 at 77 K (a), CO2, C2H6, C2H4, and CH4 at 273 and 298 K (b), and comparison of adsorbed amounts at 1 atm and different temperatures (c). Filled and open symbols represent adsorption and desorption curves, respectively.

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mixtures, but also for the utilization of CO2 through catalytic conversion processes.40−42 Herein, the assembly of H4BDPO and Ba2+ ions produced a three-dimensional (3D) honeycomb MOF, {[Ba2(BDPO)(H2O)]·DMA}n (1), which possesses the Ba-carboxylate rod SBUs and contains the Lewis basic oxalamide motifs and exposed Lewis acidic Ba2+ ions located on the walls of hexagonal channels. This MOF shows good stability and achieves efficient separation of C2H6, C2H4, and CO2 over CH4 as well as high catalytic conversion of CO2 with epoxides under ambient conditions.

EXPERIMENTAL SECTION

Materials and General Methods. All reagents were commercially available, except H4BDPO, which was synthesized by a reported method.35 Fourier transform infrared spectrum was measured with a Nicolet FT-IR 170 SX spectrophotometer (4000−400 cm−1). The contents of C, H, and N were determined with a PerkinElmer 2400C Elemental Analyzer. A NETZSCH TG 209 thermal analyzer was used to perform thermogravimetric analysis (TGA) in N2 atmosphere with a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) pattern was recorded on a Bruker D8 ADVANCE X-ray powder diffractometer. Gas sorption was tested with a Micromeritics ASAP 2020M apparatus. Inductively coupled plasma (ICP) spectroscopy was B


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Figure 3. IAST adsorption selectivities of 1a for CO2/CH4, C2H6/CH4, and C2H4/CH4 in diverse CH4 partial pressure at 298 K (a), 313 K (b), and a total pressure of 1 atm.

Figure 4. Density contours of C2H6 and CH4 (a), and C2H4 and CH4 (b), in 1a simulated from equimolar C2H6−CH4 and C2H4−CH4 mixtures, respectively. performed using an Agilent 725 ICP-OES spectrometer. The products of catalysis reaction were monitored by gas chromatography (GC) with a Shimadzu GC-14CPTF. Catalytic Cycloaddition of CO2 with Epoxides. Batch catalytic reactions were performed by adding epoxide substrate (20 mmol), activated 1 (0.5% mmol), and tetra-n-tert-butylammonium bromide (TBAB, 2% mmol) in a 15 mL Schlenk tube or a 10 mL stainless-steel autoclave. The reaction was carried out under 1 or 10 atm CO2 pressure at room temperature with continuous stirring. The conversion of the reaction was monitored by GC. Crystallography. Crystal structure was determined at 296(2) K by a Bruker SMART APEX II CCD diffractometer with a Mo Kα radiation source. The structure was solved by direct methods and refined on F2 by full-matrix least-squares methods using the SHELXTL program package.43 The non-H atoms were refined anisotropically, whereas the H atoms fixed to their geometrically ideal positions were refined isotropically. We failed to determine the right model of solvent molecules from the difference Fourier map and therefore the SQUEEZE routine of PLATON program was used in structural refinement.44 The refinement results and selected bond distances/ angles are given in Tables S1 and S2, respectively. Synthesis of {[Ba2(BDPO)(H2O)]·DMA}n (1). H4BDPO (20.8 mg, 0.05 mmol) and Ba(NO3)2 (26.1 mg, 0.1 mmol) were mixed in DMA (3 mL) and distilled water (1 mL). The mixture was sealed in a glass vial (10 mL) and heated at 95 °C for 48 h. Cooling the vial to room temperature at a rate of 5 °C h−1 yielded colorless block-shaped crystals in 55% yield. Anal. Calcd for C22H19Ba2N3O12: C, 33.36; H, 2.42; N, 5.31. Found: C, 33.48; H, 2.30; N, 5.23. IR data (KBr, cm−1): 3454(m), 3199(w), 3059(w), 2925(w), 1684(s), 1616(s), 1520(s),

1433(m), 1367(s), 1194(w), 1099(w), 1012(w), 922(w), 843(w), 781(m), 714(m), 538(w), 478(w), 430(w).

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RESULTS AND DISCUSSION Crystal Structure. Compound 1 crystallizes in a chiral P6122 space group (Flack parameter, 0.11(7)) and exhibits a 3D honeycomb framework with tubular channels. The asymmetric unit contains three Ba2+ ions (50, 50, and 100% site occupancies), one BDPO ligand, and one coordinated water molecule (Figure 1a). All Ba2+ ions with the distorted dodecahedron coordination geometries are ligated by eight O atoms. Ba1 and Ba2 atoms possess similar coordination environments, which are bonded to six carboxylate O atoms and two oxalamide O atoms from six BDPO. In contrast, Ba3 atom is coordinated by seven carboxylate O atoms from five BDPO and one water O atom. Distinguishing from the previously reported Sr-BDPO MOF, in which BDPO links 10 Sr2+ ions and 4 carboxylates adopt the same μ2-η2:η1 mode, however, in Ba-MOF, one BDPO links 11 Ba2+ ions by 2 oxalamide O atoms and 4 carboxylates with μ2-η2:η1 and μ3η2:η2 coordination modes (Figure S1). In addition, the documented Sr-BDPO MOF has a different P6222 space group.22 Notably, along the a and b axes, the coordination of carboxylates and Ba2+ ions gives rise to two identical lefthanded 21 infinite helical chains (Figure 1b), which are very sporadically reported in alkaline-earth metal ions compared to C


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ACS Applied Materials & Interfaces Table 1. Different Carbonates Formed from CO2 and Various Epoxides Catalyzed by 1a

a

Reaction conditions: epoxides (20 mmol), catalyst (0.5% mmol), CO2 (1 atm), TBAB (2% mmol), room temperature, 48 h. bThe same conditions but CO2 pressure rises to 10 atm and reaction time reduces to 6 h. cThe same conditions but temperature rises to 50 °C. dThe same conditions but temperature rises to 70 °C. The conversion was determined by GC. eTON is the turnover number (product (mmol)/catalyst (mmol)).

Scheme 1. Catalytic Mechanism for Chemical Fixation of CO2 into Cyclic Carbonates

transition-metal ions. In 1, the chains as rod SBUs are linked by BDPO to generate a 3D framework, which features the 1D

Figure 5. Cyclic experiments for the cycloaddition of CO2 with epichlorohydrin. D


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Figure 6. Interactions between the framework and (a) propylene oxide, (b) 1,2-butylene oxide, (c) 1-bromo-2,3-epoxypropane, (d) epichlorohydrin, (e) styrene oxide, and (f) glycidyl phenyl ether.

maximum loading of CO2 (213.8 cm3 cm−1) at 1 atm. From CO2 adsorption curve, the calculated Brunauer−Emmett− Teller and Langmuir surface areas are 487 and 551 m2 g−1, respectively. Meanwhile, the pore size distribution was found to be within the range of 8.3−9.8 Å, and a median pore width of 8.7 Å was also obtained using the Horvath−Kawazoe model, agreeing with the single crystal structure analysis (Figure S4). Notably, the channel in 1a presents a high total concentration of amide groups and UMSs (7.5 mol L−1), which are Lewis basic and acidic sites, respectively, and would be crucial active sites for gas adsorption and catalysis applications. The gas adsorption performance of 1a was further assessed at ambient temperature. The volumetric uptake is herein adopted, which is more significant than the gravimetric uptake for practical use to appraise gas capture behaviors of adsorbents. At 273 and 298 K, 1a shows remarkable CO2 volumetric absorption amounts of 134.7 (14.9 wt %) and 93.8 (10.4 wt %) cm3 cm−3 at 1 atm, respectively (Figure 2b). This CO2 absorption amount at 298 K is higher than that reported for Ba-based MOFs.23,46,47 The initial adsorption heat (Qst) calculated by virial equation is 32.4 kJ mol−1, indicating the high CO2-binding affinity of 1a (Figure S5). Compared to amide and UMSs containing MOFs, the Qst value of 1a for CO2 is lower than that of NJU-Bai3 (36.5 kJ mol−1),48 but superior to that of PCN-124 (26.3 kJ mol−1),34 NOTT-125 (25.4 kJ mol−1),35 and PCN-124-stu(Cu) (26.0 kJ mol−1).49 In contrast, 1a has low CH4 uptake capacity at 298 K and 1 atm (19.1 cm3 cm−1 or 0.8 wt %) (Figure 2b,c). The significant uptake of CO2 in 1a originates from the highly active channels decorated by

hexagonal channels (Figure 1c−e). The channels and rod SBUs effectively promote the rigidity of the skeleton and prevent the occurrence of interpenetration as well. The channel has an effective window size of 8.2 Å and is decorated by oxalamide groups and exposed Ba2+ centers formed by excluding coordinated waters. TGA and PXRD. Compound 1 shows a weight loss of 13.3% before 280 °C (calcd 13.3%), resulting from the departure of one coordinated water and one DMA molecules. It reveals a high thermal stability up to 430 °C and then collapses (Figure S2). Generally, high coordination number and rod SBUs would endow MOFs with higher thermal stability.45 As expected, after 1 was heated for 5 h at 350 °C in air or at 400 °C in N2, 1 remains intact, as evidenced by PXRD patterns (Figure S3). Meanwhile, 1 is stable in common organic solvents and laboratory environment with some humidity after a long time exposure. Sorption Properties. In view of the permanent porosity of 1, the experimental samples were soaked in CH2Cl2 for 72 h and then heated at 160 °C under vacuum for 5 h to exclude the coordinated water and guest solvent molecules, as demonstrated by TGA (Figure S2). Thus, the guest-free phase 1a with open Ba2+ ions on the porous walls was acquired. The skeleton intactness of 1a was supported by PXRD (Figure S3). The gas sorption measurement of 1a for N2 at 77 K and for CO2 and CH4 at 195 K all showed the reversible typical type-I isotherms, as expected for micropore (Figure 2a). For the three gases, 1a possesses the minimum uptake of N2 (110.3 cm3 cm−1) but the E


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cell. The obtained selectivity (C2H6/CH4 = 17.2, C2H4/CH4 = 10.9) is in agreement with the calculated value through experiment data. Cycloaddition of CO2 with Epoxides. In 1a, the UMSs embedded in the channel wall serve as not only main CO2 binding sites but also potential Lewis acid catalytic centers, which in conjunction with the CO2-philic oxalamide groups would be very beneficial to CO2 chemical fixation. The catalytic capability of 1a toward the transformation of various epoxides to cyclic carbonates with CO2 was investigated. First, 1a together with TBAB cocatalyst that provides the nucleophile Br− ion was tested for cycloaddition of propylene oxide (PO) with CO2 into propylene carbonate at room temperature and 1 atm. As shown in Table 1, 1a demonstrates efficient catalytic activity for cycloaddition of PO with CO2, with the conversion of 98.1% over 48 h as well as a TON of 196.2 (Table 1, entry 1). This conversion is very high and comparable to the excellent conversions reported in some MOFs with Lewis acid catalytic sites, such as Hf-Nu-1000 (100%)56 and MMCF-2 (95%),57 under the same condition. However, at 10 atm, the conversion reaches 99.2% in only 6 h (Table 1, entry 2). The remarkable catalytic ability of 1a could be attributed to not only the high density of Lewis acid UMSs and CO2-philic oxalamide sites but also tubular channels that facilitate the diffusion of reactants and products. The highly efficient catalytic activity of 1a for cycloaddition of PO with CO2 under ambient condition allowed us to explore CO2 chemical transformation with other epoxide substrates with different sizes/types of substituted groups (−C2H5, −CH3−Cl, −CH3−Br, −Ph, and −O−Ph) under similar conditions (Table 1, entries 3−7). It showed that 1a also reveals high catalytic efficiency at 1 atm for converting 1,2butylene oxide, 1-bromo-2,3-epoxypropane, and epichlorohydrin with CO2 to produce corresponding cyclic carbonates with conversions of 95.7, 94.5, and 90.2% over 48 h, respectively (Table 1, entries 3−5). However, a striking decline in the cycloaddition reaction conversion of styrene oxide (19.8%) and glycidyl phenyl ether (52.3%) was observed (Table 1, entries 8 and 9). Notably, although glycidyl phenyl ether is bigger than styrene oxide, it has a higher conversion; this phenomenon is attributable to the lower reactivity at β-carbon of styrene oxide and higher polarity of glycidyl phenyl ether matching polar pore surfaces. Furthermore, the conversions of epichlorohydrin ascend to 94.1 and 97.5% as the temperature is increased to 50 and 70 °C, respectively (Table 1, entries 6 and 7). The reusable nature of the catalyst is significant for practical application. Cyclic experiments were performed by taking the cycloaddition of CO2 with epichlorohydrin as an example at room temperature and 1 atm. For five runs of reactions, the conversions of epichlorohydrin keep nearly constant with the original one (Figure 5). The PXRD pattern evidences the structural integrity of 1a after five runs of experiments (Figure S10). Moreover, only trace amounts of Ba2+ leach from the framework into reaction of filtrate, as confirmed by ICP analysis (0.0891 ppm). Overall, the high efficiency and recoverable performance enable 1a to be an outstanding heterogeneous catalyst for the chemical fixation of CO2 into cyclic carbonates. The catalytic mechanism was further inspected on the basis of literature reports.58−61 As illustrated in Scheme 1, epoxides first interact with the open Lewis acidic Ba2+ sites by O atoms, and CO2 are adsorbed on the framework in the meantime. Next, the Br− ions from TBAB acting as nucleophiles attack the less sterically hindered C atoms of epoxides, which cause the

high density of UMSs and oxalamide groups, which generate strong dipole−quadrupole interactions with CO2 molecules owing to the larger quadrupolar moment (4.30 × 10−26 esu cm2 for CO2; 0 for CH4) and higher polarizability value (29.1 × 10−25 cm3 for CO2; 25.9 × 10−25 cm3 for CH4) of CO2 than CH4. The adsorption selectivity of 1a for CO2 in CO2−CH4 binary mixtures was estimated by the ideal adsorbed solution theory (IAST) (Figure S8). For an equimolar ratio of CO2− CH4 mixture, 1a shows a significant CO2/CH4 adsorption selectivity at 298 K, the calculated selectivity at 1 atm is 12.3 (Figure 3a), which surpasses that of JUC-199 (9.0)50 and UiO66 (6.87).51 At a higher temperature of 313 K, 1a also reveals the significant difference in the adsorption amounts for CO2 and CH4, and the CO2/CH4 selectivity is 9.5 for an equimolar mixture at 1 atm (Figures 3b and S9). Moreover, at a total pressure of 1 atm, for the various CO2 partial pressures from 0.05 to 0.95 atm in CO2−CH4 mixtures, the CO2/CH4 selectivities of 1a at 298 and 313 K hold high values of 10.0−16.1 and 8.4−11.0, respectively (Figure 3). Compound 1a was also applied to explore its potential in C2H6 and C2H4 adsorption. At 1 atm, differing from the low CH4 uptake, significant C2H6 and C2H4 volumetric uptakes of 96.6 (7.3 wt %) and 103.9 (7.3 wt %) cm3 cm−3 at 273 K and 71.5 (5.4 wt %) and 83.5 (5.9 wt %) cm3 cm−3 at 298 K are reached, respectively (Figure 2b). Even more striking is the uptakes at 313 K, although the temperature increases from 298 to 313 K, the uptakes are reduced by only 1.3 and 7.7% for C2H6 and C2H4, respectively (Figure 2c). The undisturbed adsorption capacities in the scope of 298−313 K are scarce to date. The Qst values at zero coverage were computed by the viral method to be 29.0 and 28.5 kJ mol−1 for C2H6 and C2H4, respectively (Figures S6 and S7). The selectivities of C2H6 and C2H4 with respect to CH4 were also calculated using the IAST method. At 298 K and 1 atm, the C2H6/CH4 selectivity for an equimolar C2H6−CH4 mixture is 21.4 (Figure 3a), which is almost equal to that of Sr-MOF (22.5)22 and surpasses that of ZJNU-62 (13.2)52 and JLU-Liu5 (17.6),53 whereas this value is 19.1 as the temperature increases to 313 K (Figure 3b). CH4 partial pressures changed from 0.05 to 0.95 atm for C2H6−CH4 mixtures, and the C2H6/CH4 selectivities of 1a are in the ranges of 20.7−23.5 and 20.7−15.6 at 298 and 313 K, 1 atm, respectively (Figure 3). As for an equimolar C2H4−CH4 mixture, the C2H4/CH4 selectivities are 16.2 and 14.4 at 298 and 313 K, respectively (Figure 3). The value of selectivity at 298 K is higher than that of UTSA-33a (12)54 but lower than that of FJI-C4 (22.1).55 For a C2H4−CH4 mixture, the C2H4/ CH4 selectivity changed from 19.7 to 13.6 and from 16.3 to 12.6 as the various partial pressures of CH4 increased from 0.05 to 0.95 atm at 298 and 313 K, respectively (Figure 3). The excellent abilities of adsorption/separation highlights 1a as a promising material for high CO2 and C2 hydrocarbons uptake and selective capture from CH4. Furthermore, grand canonical Monte Carlo (GCMC) simulations were further carried out at 298 K to assess the adsorption selectivity of 1a for C2H6 and C2H4 over CH4 in C2H6−CH4 and C2H4−CH4 mixtures, respectively (see the Supporting Information). As shown in Figure 4a,b, the obtained density contours at 1 atm clearly suggested the major C2H6 and C2H4 molecules, but the minor CH4 molecules were adsorbed in the channels of 1a for equimolar C2H6−CH4 and equimolar C2H4−CH4 mixtures. Meanwhile, on the basis of the above density contours, it found that 1a adsorbs 22.2 C2H6 and 1.3 CH4 molecules, and 27.2 C2H4 and 2.5 CH4 molecules per unit F


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opening of epoxide rings, and are then inserted by adsorbed CO2 and cyclized to yield cyclic carbonates. Moreover, the interaction mechanism between UMSs and epoxides has also been explored by GCMC simulations. The O atoms from epoxide substrates bind with exposed Ba2+ centers through strong coordination interactions (Figure 6).

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CONCLUSIONS In conclusion, on the basis of an oxalamide-functionalized ligand and Ba-carboxylate rod SBUs, a honeycomb MOF has been fabricated. The MOF shows good stability and features tubular porous surface modified by high density of polar Lewis basic oxalamide groups and Lewis acidic UMSs. Benefitting from numerous active sites, it shows not only high C2H6, C2H4, and CO2 adsorption capabilities but also good adsorption selectivities for C2H6, C2H4, and CO2 over CH4. The high catalytic activity to chemical fixation of CO2 into cyclic carbonates under ambient conditions was also achieved. In addition, GCMC simulations confirm the significant C2H6/ CH4 and C2H4/CH4 selectivity of the MOF for the binary mixtures as well as strong interactions between the framework and different epoxide substrates. The combination of experiment results and theoretical simulations in this work offers an important foundation to design and fabricate porous frameworks with efficient separation of CO2 and C2 hydrocarbons from CH4 as well as CO2 fixation capacity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01291. Structural figures, detailed calculations on sorption, and bond length/angle tables (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiu-Yuan Li: 0000-0002-3508-9864 Lei Hou: 0000-0002-2874-9326 Yao-Yu Wang: 0000-0002-0800-7093 Zhonghua Zhu: 0000-0003-2144-8093 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21471124 and 21531007), NSF of Shaanxi Province (15JS113), the Postgraduate Innovation Foundation of Northwest University (YZZ17115), and the Australian Research Council Future Fellowship (FT12010072).

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