Strong Cation Exchange with Innocence: Synthesis and

Article pubs.acs.org/Macromolecules

Strong Cation Exchange with Innocence: Synthesis and Characterization of Borate Containing Resins and Macroporous Monoliths Francesca Liguori,† Serena Coiai,‡ Elisa Passaglia,‡ and Pierluigi Barbaro*,† †

Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organo Metallici, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Firenze, Italy ‡ Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organo Metallici, UOS Pisa, via Moruzzi 1, 56100 Pisa, Italy S Supporting Information *

ABSTRACT: Strong cation-exchange resins featured by a low loading of noncoordinating BF3− or B(C6H5)3− anions in styrene−divinylbenzene matrices at various cross-linking degrees were synthesized via free-radical bulk copolymerization with the appropriate borate monomers. The polymerization mixture was tailored to obtain swellable materials for use as insoluble carriers of single-site heterogeneous catalysts. The parent macroporous monolith was also prepared for direct use in continuous flow applications showing excellent hydrodynamic properties and high chemical and mechanical stability. All resins were carefully characterized in the solid state, and their ability to immobilize molecular metal complexes was demonstrated.

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polymers were the first used to produce monoliths for application to flow synthesis of fine chemicals via catalyst immobilization on the laboratory scale.14,15 Problems to be solved in the field concern chemical stability, shrinking phenomena, and back-pressure evolution at high flow rate due to limited porosity of polymer-based monoliths.16 With the aim to develop innovative materials to support heterogenized catalysts, herein we report on the synthesis and characterization of a novel series of strong cation-exchange polymers featuring a low loading of borate anions in crosslinked styrene−divinylbenzene matrices.17 The choice was motivated by the potential offered by the combination of ionexchange and innocent (noncoordinating) functional group to obtain isolated catalytic entities, firmly and uniformly anchored to a solid support, without catalyst poisoning effect from the matrix.18 Having established an effective protocol for the synthesis of these materials, we also prepared and characterized the parent macroporous monolith for direct use in column applications. A mesofluidic reactor built on this monolith is also described.19 The efficiency of the materials devised to immobilize molecular complexes was also demonstrated.

INTRODUCTION Ion-exchange resins are among the earlier established and most widely used class of functional polymers.1 They find broad application in chromatography, water purification, metal recovery, and ion substitution processes and as acid−base catalysts, sensors, or solid electrolytes.2 Their use as insoluble support materials for catalytically active species, either metal nanoparticles3 or molecular complexes,4 to produce single-site heterogeneous catalysts via noncovalent binding,5,6 has also received increasing interest over the years.7 A number of industrial processes are in operation, and various large-market fine chemicals are presently manufactured using ion-exchange resin-supported catalysts (IESC).8 Drawbacks in use of resins, particularly cation exchangers, for catalysts heterogenization include loss of catalytic activity upon reuse, catalyst leaching, and limited resistance under catalytic conditions.9 Key issues to be addressed in the near future are the development of friendly synthetic methods for innovative reins, stable in a wide range of solvents and conditions, and showing effective and reproducible immobilization of metal catalysts, without adversely affecting their performance. Moreover, most of large-scale processes based on IESC are still performed using conventional batch methods, with serious limitations from an environmental and economic point of view.10 Hence, the adoption of catalytic technologies under continuous flow may represent an excellent alternative due to the considerable benefits in terms of efficiency, work-up, safety, waste emission, automation, space, and energy consumption.11 Particularly, use of monolithic reactors may be extremely helpful because of the improved mass and heat transfer, lower pressure drop, and uniform residence times distribution, compared to conventional packed-bed systems.12,13 Organic © 2013 American Chemical Society

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EXPERIMENTAL SECTION

Materials and Methods. All reactions and manipulations were routinely performed under nitrogen by using standard Schlenk techniques unless otherwise stated. Tetrahydrofuran and diethyl ether were distilled from sodium benzophenone, styrene, and divinylbenzene were distilled under vacuum at rt prior to use. All the other reagents were reagent-grade commercial products and were Received: May 29, 2013 Revised: June 26, 2013 Published: July 11, 2013 5423

dx.doi.org/10.1021/ma401120v | Macromolecules 2013, 46, 5423−5433

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(B−F), 1394.28 (=C−H), 1606.66 (CC), 1629.88 (CC), 3020.0 (C−H). Elem Anal. Calcd for C8H7BF3K (%): C, 45.8; H, 3.4; B, 5.2; K, 18.6. Found: C, 45.5; H, 3.4 B, 5.4; K, 18.2. Synthesis of the Monomer Sodium Triphenyl-4-vinylphenylborate (2). The synthesis was modified from the reported procedure.23 Magnesium turnings (1.0 g, 41.6 mmol) were transferred into a three-necked round-bottom flask equipped with a water condenser, a septa, and a dropping funnel, and they were heated at 100 °C under high vacuum for 1 h. After cooling to rt under a nitrogen flow, dry THF (45 mL) and a small I2 crystal were added, and the suspension was degassed with three cycles vacuum/nitrogen. 4Bromostyrene (4.9 mL, 37.4 mmol) in THF (4 mL) was added dropwise without stirring until a gas evolution was detected, and then the addition was continued with stirring in order to maintain a light reflux. After the addition was complete, the reaction mixture was stirred for further 1 h. A small amount of black precipitate was allowed to settle, and the yellow-green solution was added via cannula at 0 °C to a 0.25 M solution of triphenylborane in THF (100 mL, 25 mmol). The mixture was stirred for 30 min at 0 °C and for 18 h at rt to give a milky emulsion. An aqueous solution of sodium carbonate (0.3 M, 250 mL) was then added, causing the precipitation of a white solid. The organic phase was separated, and the aqueous phase was extracted with THF (2 × 100 mL). The organic phases were combined, washed with a saturated solution of NaCl, dried over anhydrous sodium sulfate, and filtered. After the solvent was removed in vacuum, a white powdery solid was obtained (yield: 9.11 g, 98.8%). 1H NMR (300.13 MHz, acetone-d6, 294 K): δ (ppm) 7.35−7.32 (m, 8H, Ar−H), 7.08−7.05 (m, 2H, Ar−H), 6.96−6.91 (m, 6H, Ar−H) 6.81−6.76 (m, 3H, Ar− H), 6.63 (dd, 1H, 3Jtrans = 17.6, 3Jcis = 10.9 Hz, CH2CH−), 5.56 (dd, 1H, 3Jtrans = 17.6, 2J = 1.5 Hz, CH2CH−), 4.92 (dd, 1H, 3Jcis = 10.9 2 J = 1.5 Hz, CH2CH−). 13C{1H} NMR (75.47 MHz, acetone-d6, 294 K): δ (ppm) 163.5 (m, CAr−B), 138.9 (CH2CH−), 136.2 (Ar− C), 136.09 (Ar−C), 130.8 (Ar−C−CHCH2), 125.1 (Ar−C), 123.3 (Ar−C), 121.4 (Ar−C), 108.2 (CH2CH−). 11B NMR (96.29 MHz, D2O, 294 K): δ (ppm) −6.56 (bm, Ar−B). 13C CP-MAS NMR (100.61 MHz, 294 K): δ (ppm) 161.2 (m, CAr−B), 138.6 (CH2 CH−), 137.4 (Ar−C), 135.49 (Ar−C), 131.3 (Ar−C−CHCH2), 125.6 (Ar−C), 123.0 (Ar−C), 121.4 (Ar−C), 108.7 (CH2CH−). 11 B CP-MAS NMR (128.38 MHz, 294 K): δ (ppm) −6.96 (Ar−B). ATR (cm−1) 740.0, 826.5, 905.3, 995.4, 1152.0, 1258.9 (B−C), 1428.0 (B−C), 1478.1 (CC), 1625.29 (CC), 3000.59 (C−H), 3057.53 (C−H). Elem Anal. Calcd for C26H22BNa (%): C, 84.8; H, 6.0; B, 2.9; Na, 6.2. Found: C, 85.0; H, 6.0; B, 2.7; Na, 6.0. General Procedure for the Synthesis of the Polymers Containing Potassium 4-Vinylphenyltrifluoroborate (3−5). Monomer 1 was dissolved in warm cyclohexanol (ca. 60 °C). The other monomer components in a chosen molar ratio, styrene and/or divinylbenzene, and AIBN (1 mol % total monomers) were then added in an autoclave. The autoclave was closed, and the solution was degassed with three cycles vacuum/nitrogen. The reactor was heated using an oil bath (Text 80 °C) with mechanical stirring (80 rpm) for 48 h. After that time, the autoclave was depressurized and opened without cooling. The resulting cloudy reaction mixture was dropped in methanol under vigorous stirring. A flocculent white solid precipitated, which was collected, filtered, and dried under vacuum. The solid was washed using a Soxhlet apparatus with the following solvents in the order MeOH (2 h), toluene (rt), acetone (1 h), and THF (1 h) and finally dried under high vacuum (48 h at 40 °C) to obtain a white powdery solid. The polymer was characterized by 1H NMR, 11B NMR, 19 F NMR, 13C CP-MAS NMR, 11B CP-MAS NMR, EDS, ICP-OES, ATR, TGA, and DSC. General Procedure for the Synthesis of the Polymers Containing Sodium Triphenyl-4-vinylphenylborate (6−8). Monomer 2 was dissolved in warm cyclohexanol (ca. 60 °C). The other comonomers in a chosen molar ratio, styrene and/or divinylbenzene, and AIBN (1 mol % total monomers) were then added in an autoclave. The reactor was heated using an oil bath (Text 80 °C) with mechanical stirring (80 rpm) for 48 h. After that time, the autoclave was depressurized and opened without cooling. The resulting cloudy reaction mixture was dropped in n-pentane under

used as received. Solution NMR spectra were recorded on Bruker Avance DRX-400 or Avance DRX-300 spectrometers. Solid-state 13C and 11B CP-MAS NMR spectra were recorded on a Bruker Avance DRX-400 spectrometer operating at 100.61 and 128.38 MHz, respectively, using a 4 mm rotor at a spinning rate of 10 kHz. FTIR and ATR spectra were recorded using a PerkinElmer Spectrum One instrument equipped with a reflectance accessory with diamond crystal for the film surface analysis. Full optimization of the molecular geometries and calculation of IR spectra were carried out using the 08 version of Spartan computer program, running on a Linux workstation. The minima and the spectra were characterized by performing a DFT vibrational frequency analysis. ESEM measurements were performed on a FEI quanta200 microscope operating at 20 keV accelerating voltage in the low-vacuum mode (1 Torr) and equipped with an EDAX-EDS spectrometer. Images were collected in high-vacuum mode after coating with gold using a sputter-coater. ICP-OES measurements were performed with a Varian 720ES instrument. TGA analyses were performed with a Seiko EXSTAR TG/DTA 7200 thermogravimetric analyzer. DSC analyses were performed with DSC 7 PerkinElmer. N2 sorption porosimetry experiments were carried out using a Micromeritics ASAP 2020 analyzer, and the data were manipulated using the software supplied with the instrument. Hg intrusion porosimetry experiments were performed on a Micromeritics Autopore IV 9500 V1.09. An X-ray microcomputer aided tomography, and 3D data extraction was performed using a Skyscan 1172 highresolution MicroCT system. The 3D image of the objects internal structure has been reconstructed using a modified Feldkamp algorithm for cone-beam acquisition geometry realized in Nrecon v.1.6.3.3 software. Swelling measurements were carried out using reported procedures.20 Batch reactions in autoclaves were performed using a stainless steel PARR4565 instrument equipped with a PARR 4842 temperature and pressure controller, a mechanical stirrer, and the inner walls covered with Teflon. Use of solvents under continuous flow was achieved by an Alltech model 426 HPLC pump in PEEK. Synthesis of the Monomer Potassium 4-Vinylphenyltrifluoroborate (1). The synthesis was modified from the literature procedure,21 using a lithiation−boronation sequence.22 Thus, a solution of n-BuLi in n-hexane (1.6 M, 34.5 mL, 55.10 mmol) was added dropwise to a solution of 4-bromostyrene (6.4 mL, 49.2 mmol) in dry THF (120 mL) at −78 °C, and the mixture was stirred for 1 h at the same temperature. After that time, the solution was added dropwise via cannula over 45 min to a solution of trimethylborate (6.6 mL, 59.4 mmol) in THF (38 mL) at −78 °C. The cloudy mixture was stirred for 30 min before being slowly warmed to rt over a 1 h period. Solid KHF2 (19.20 g, 246 mmol) was then added in one portion under stirring at 0 °C. H2O (33 mL) was added dropwise, then the cold bath was removed, and the white suspension obtained was stirred for 30 min. After the solvent was evaporated under high vacuum, the solid was extracted with portions of hot acetone (8 × 100 mL). The extracts were collected and concentrated under high vacuum to afford a white solid (8.5 g). The solid was purified by dissolution in refluxing acetone (600 mL), filtration, evaporation to small volume (50 mL), and addition of Et2O in small portions (200 mL) at rt. The suspension obtained was allowed to stand for 1−2 h. The solid obtained was then collected on a sintered-glass frit before being washed with Et2O (30 mL) and dried overnight under a stream of nitrogen to give white microcrystals (yield: 5.20 g, 51.0%). 1H NMR (300.13 MHz, acetoned6, 294 K): δ (ppm) 7.44 (d, 2H, 3J = 7.7 Hz, Ar−H), 7.19 (d, 2H, 3J = 7.7 Hz, Ar−H), 6.66 (dd, 1H, 3Jtrans = 17.7, 3Jcis = 10.8 Hz, CH2 CH−), 5.65 (dd, 1H, 3Jtrans = 17.7, 2J = 1.38 Hz, CH2CH−), 5.03 (dd, 1H, 3Jcis = 10.8 2J = 1.38 Hz, CH2CH−). 13C{1H} NMR (75.47 MHz, acetone-d6, 294 K): δ (ppm) 141.6 (CAr−B), 138.2 (CH2 CH−), 134.2 (Ar−C−CHCH2), 131.8 (Ar−C), 124.2 (Ar−C), 110.1 (CH2CH−). 11B NMR (96.29 MHz, acetone-d6, 294 K): δ (ppm) 3.42 (bm, −BF3K). 19F NMR (376.42 MHz, acetone-d6, 294 K): δ (ppm) 142.73 (m, BF3K). 13C CP-MAS NMR (100.61 MHz, 294 K): δ (ppm) 141.0 (CAr−B), 138.0 (CH2CH−), 137.4 (Ar−C− CHCH2), 129.7 (Ar−C), 124.2 (Ar−C), 113.5 (CH2CH−). 11B CP-MAS NMR (128.38 MHz, 294 K): δ (ppm) 4.16 (BF3K). ATR (cm−1) 749.2, 838.4, 922.2, 968.5 (B−F), 1202.18 (B−C), 1225.33 5424


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vigorous stirring. A rubbery white solid precipitated which was washed several times with n-pentane in order to obtain a powdery solid. The solid was dissolved in refluxing EtOH, precipitated with n-pentane at rt, filtered, washed with n-pentane, and dried under vacuum. The product obtained was allowed to swell in propan-2-ol for 24 h, and then the solid was filtered off and dried under high vacuum (48 h at 40 °C). The polymer was characterized by 1H NMR, 11B NMR, 13C CPMAS NMR, 11B CP-MAS NMR, EDS, ICP-OES, ATR, TGA, and DSC. Synthesis of the Styrene−Divinylbenzene−Sodium Triphenyl-4-vinylphenylborate Monolith (11). Styrene (26.5 mg, 0.25 mmol), divinylbenzene (16.5 mg, 0.12 mmol), 2 (6.6 mg, 0.02 mmol), and 1-dodecanol (0.116 g, 0.142 mL) were transferred in a glass vial, and the mixture was homogenized by sonication. After addition of AIBN (0.5 mg), the solution was sonicated for further 5 min and then transferred via a syringe in a commercial tubular glass column (Omnifit, 3 mm i.d. × 25 mm length) sealed at one end with a PEEK screw-cap and equipped at the other end with a PEEK connector, a 0.2 μm PE frit, and a two-way PFA valve. The column was connected to a vacuum/N2 line, degassed by three cycles vacuum/N2, and heated at 60 °C for 30 min, then from 60 to 85 °C over a period of 1.5 h, and at 85 °C for a further 24 h, using an heating device for NMR tubes. After that time, the column was cooled to rt, and it was connected to an HPLC pump through appropriate fittings and pipes, and ethanol, THF, and n-pentane were flushed through the column at a flow rate of 0.5 mL/min for 1 h in that order to provide a white solid material in the column. The as-prepared monolithic column could be used without any other treatment for further application, provided it was stored in THF for long periods. In order characterize the solid monolith by 13C CP-MAS NMR, 11B CP-MAS NMR, ICP-OES, ESEM, EDS, ATR, TGA, DSC, X-ray microtomography, N2 sorption, and Hg-intrusion porosimetry, the column was connected to a N2 line, and a stream of nitrogen flowed at a rate of 0.2 mL/min for 24 h. The monolith was then carefully removed from the column and dried under vacuum at 70 °C for 72 h. The monolith can be regenerated by flushing with a 0.1 M NaCl water solution at 0.2 mL min−1 for 1 h at room temperature using an HPLC pump. Immobilization of Rhodium Complexes on Borate Resins. In a typical procedure, the complex [((R)-Monophos)2Rh(NBD)]PF6 (10)24 was immobilized onto resin 8 as follows. The dry resin 8 (100 mg, 0.074 mequiv of B(C6H5)3−Na+ groups) was allowed to swell in degassed 2-PrOH (10 mL) for 1 h. A solution of 10 (15.0 mg, 0.015 mmol) in 2-PrOH (10 mL) was then added under nitrogen, and the mixture was stirred at room temperature for 24 h. The orange resin obtained was decanted, washed with warm 2-PrOH (4 × 20 mL, 50 °C) and with diethyl ether (3 × 20 mL, rt), dried in a stream of nitrogen overnight, and stored under nitrogen. ICP-OES and EDS analyses showed the rhodium content to be 0.83% (w/w) (average value over three samples), corresponding to the immobilization of 60% the rhodium used. Immobilization of Rhodium Complexes on Borate Monoliths. In a typical procedure, a degassed solution of the complex [((R)Monophos)2Rh(NBD)]PF6 (10) in MeOH (0.0017 M) flowed through the monolithic column 11 using an HPLC pump at a 0.2 mL min−1 rate for 1 h. The monolith was then washed with MeOH (at 0.5 mL min−1 rate for 1 h) and dried under a nitrogen flow (0.3 mL min−1 for 18 h). ICP-OES and EDS analyses showed the rhodium content to be 1.29% (w/w) (average value over three samples). Cation Exchange Experiments. In a typical experiment, the dry polymer 8 (50.3 mg, 0.037 mequiv) was added to a solution 0.0037 M of KPF6 in methanol (10 mL), and the mixture was stirred at room temperature for 24 h. After that time, the polymer obtained was decanted, washed with methanol (5 × 10 mL), filtered, and dried under high vacuum. EDS analysis of the polymer gave a molar ratio K/ Na of 2.2, corresponding to a potassium uptake of 68.8% of the total ion exchange capacity. A selectivity constant kK/Na = 4.86 was calculated from the following equation:1 kK/Na = [K+]S[Na+]M/ [Na+ ]S[K+]M, where the subscripts S and M refer to the “stationary phase” and to the “mobile phase”, respectively.

Article

RESULTS AND DISCUSSION Synthesis of Polymers. Strong cation-exchange resins containing borate functional groups, and having the general formula shown in Scheme 1, were synthesized via free-radical Scheme 1. Sketch of Cation-Exchange Borate Terpolymers Synthesis and Labeling Scheme Adopted

bulk copolymerization of the functional monomers potassium 4-vinylphenyltrifluoroborate (1) or sodium triphenyl-4-vinylphenylborate (2) with styrene (STY) and divinylbenzene (DVB), respectively. 1 and 2 were selected being easily accessible vinyl monomers bearing innocent, i.e., noncoordinating, borate anions,25 which were obtained in good yields by a slight modification of the literature procedures (Scheme 2). Scheme 2. Scheme of Potassium 4Vinylphenyltrifluoroborate (1) and Sodium Triphenyl-4vinylphenylborate (2) Synthesis

With the aim to prepare robust, insoluble carrier materials suitable for chromatographic or catalytic applications, the polymerization reaction mixtures were adjusted to obtain geltype resins. Indeed, gel resins, typically featured by a low crosslinkage degree,26 develop a microporous structure when swollen in the appropriate solvent, which ensures an enhanced 5425


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type resins in the dry state.32 Size exclusion chromatography was hampered by the scarce solubility of the polymers. The 1H NMR solution spectra and the solid-state 13C and 11 B CP-MAS NMR spectra of 3 are reported in Figures 1 and 2,

mass transfer inside the polymer and a good active-sites accessibility to all soluble reactants.27 Thus, the cross-linked terpolymers 3 and 6−8 (Table 1) were synthesized using 2.5− Table 1. Compositiona and Ion-Exchange Capacityb of the Polymers Synthesized polymer

1

3 4 5 6 7 8 9 11

10.0 10.0 100.0

2

10.0 10.0 10.0 3.3

STY

DVB

exchange capacity

87.5 90.0

2.5

87.5 85.0 82.0 97.3 65.1

2.5 5.0 8.0 2.7 31.6

0.80 0.81 4.90 0.73 0.74 0.74 0.27

Molar fraction % in the dry resin. bDry weight capacity in mequiv g−1 of resin. Data from ICP-OES analysis of boron. a

8.0 mol % of DVB, AIBN as initiator and cyclohexanol (20:1 v/ v of monomers) as most the appropriate porogen among the solvents examined.28 Loading of the ion-exchanging monomers 1 and 2 was limited to 10 mol %, in order to minimize potential interactions between functional groups and to favor the formation of well-dispersed, single-site heterogeneous catalysts when the resins are used as solid support.29,30 Use of lowdensity charged solid polyelectrolytes is also beneficial in terms of activity of immobilized catalysts thanks to reduced siteinhibition effect that may be caused by either electrostatic or coordination interactions.7a,9a The fluoroborate homopolymer 5 and the copolymers 4 (1-co-STY) and 9 (STY-co-DVB), having analogous monomer molar ratios as in the terpolymers, were also prepared to aid in the characterization of the terpolymers. The solid polymers were isolated in satisfactory yields as white powders af ter extensive washings to eliminate unreacted monomers, residual soluble oligomers, and/or the porogen. In any case, the composition of the solids obtained from elemental and ICP-OES analyses agreed with those calculated from the original reaction mixture (Table 1), thus indicating a coherent polymerization process (see also TGA analysis below).31 The BF 3 − K+ group-containing resins (3−5) showed negligible solubility in all common solvents, being slightly soluble only in 1,1′,2,2′-tetrachloroethane at 373 K (maximum ca. 2 mg mL−1), with the homopolymer 5 showing the lower solubility in the series. B(C6H5)3−Na+-based resins (6−8) were satisfactory insoluble only if containing a minimum of 8 mol % DVB. In this latter case (8), a solubility of ca. 1 mg mL−1 was estimated in methanol, chloroform (at room temperature) and in ethanol (reflux temperature). Comparable polymers containing monomer 2 and styrene, without DVB, were previously reported to form latices in water or in methanol, with an average particle size of 90−150 nm.23a For the abovementioned solubility reason, no homopolymers containing the B(C6H5)3−Na+ group were synthesized. Characterization of Terpolymers. All resins were characterized in the solid state by a combination of spectroscopic, thermoanalytical, and microscopic techniques. In no case ESEM microscopy analysis showed the materials to have a macroporous structure, neither in the dry nor in the swollen state, whereas N2 sorption porosimetry experiments were not possible due to the limited accessibility of gases to gel-

Figure 1. 1H NMR spectra of 3, 4, 9 (300.13 MHz, 1,1′,2,2′tetrachloroethane-d2, 373 K), and 1 (300.13 MHz, acetone-d6, 294 K). X = residual solvent/water resonances.

respectively, together with those of 1, 4, 5, and 9 for comparison. Consistently with the polymeric structure, the 1 H, 13C, 11B, and 19F signals of 3 were significantly broader compared to the corresponding resonances of the monomers.

Figure 2. Solid-state 13C CP-MAS NMR (100.61 MHz, left column) and 11B CP-MAS NMR (128.38 MHz, right column) spectra of 3, 4, 5, and 1 (10 kHz, 294 K). X = spinning side-bands. 5426


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The NMR spectra of 3 showed resonances consistent with the presence of methine and methylene chain groups and pendant BF3− units. No signals attributable to residual vinyl substituents were detected. The 1H aromatic resonances in the polymers were observed in the range 6.4−7.4 ppm, while methine and methylene protons were detected at 2.4−1.8 and 1.8−1.1 ppm, respectively. The signals due to the vinyl protons observed at 5.65 and 5.03 ppm in 1 were not detected in the spectra of resins.33 Integration of proton resonances were consistent with the composition obtained from elemental analysis, as indicated in Scheme 1 and in Table 1. The solid-state 13C CP-MAS NMR spectra of 3−5 showed peaks attributable to methylene and methine carbons in the range 35−50 ppm, to protonated aromatic carbons at ca. 130 ppm, and to quaternary aromatic carbons at ca. 145 ppm. In no case were resonances due to vinyl carbons observed, as assigned to the peaks at 138.0 and 113.5 ppm in monomer 1. If one assumes that all carbon atoms were equally detectable in the NMR spectra,34 comparison of integrals of carbon signals gave an estimate of polymers composition consistent with those indicated in Table 1. The 11 B CP-MAS NMR spectra of resins 3−5 were in agreement with those recorded in solution and consistent with that of monomer 1 in the solid state, taking into account the lower concentration of 11B nuclei, thus indicating the presence of intact BF3− groups in the polymers. Analogous results were obtained from the NMR spectra of the resins containing the B(C6H5)3− group. The 1H NMR solution spectrum and solid-state 13C and 11B CP-MAS NMR spectra of 8 are reported in Figure 3 as representative examples.

Figure 4. Simulated (a) and experimental (b) ATR spectra of 1. Experimental ATR spectra of 3, 5, and 9.

stretching, respectively, on the basis of the spectra of the monomer.37 Medium intensity bands in the range 1402−1452 cm−1 were attributed to B−C ring vibrations. Similar spectra were recorded for 6−8 in which B−C stretching was observed at 1452 cm−1. In no case were bands attributable to hydrolysis of the borate groups detected. In order to evaluate the thermal stability and the composition of the resins, TGA experiments under nitrogen and under nitrogen/air were carried out, respectively. Representative TGA plots for terpolymers under nitrogen are reported in Figure 5,

Figure 5. TGA plots recorded under N2 (left) and DSC curves (right) for terpolymers 3 and 8 and for styrene−divinylbenzene copolymer 9.

left, while significant thermogravimetric data and the residue yields (%) at 900 °C under air (R900), used to calculate the resins composition, are summarized in Table 2.38 Compared to

Figure 3. NMR spectra of 8: (a) 1H NMR in solution (300.13 MHz, methanol-d4, 294 K); (b) solid-state 13C CP-MAS NMR (100.61 MHz, 10 kHz, 294 K); (c) solid-state 11B CP-MAS NMR (128.38 MHz,10 kHz, 294 K). X = residual solvent/water resonances.

Table 2. TGA (Nitrogen, Air) and DSC Data of the Polymers Synthesizeda,b

1

Very broad signals were observed in the H NMR solution spectrum at room temperature, while the solid-state 11B CPMAS NMR spectrum showed resonances at −6.77 ppm consistent with that of B(C6H5)3− in monomer 2. For all resins obtained, the 1H and 13C NMR spectra were in line with those previously reported for comparable STY-DVB polymers.35,36 The ATR spectra of resins were diagnostic for the presence of unmodified borate functional groups, particularly in the case of BF3− containing polymers. The ATR spectra of 3 and 5 are reported in Figure 4, together with those of the unfunctionalized polymer 9 and the experimental and simulated spectra of monomer 1. Compared with that of 9, the spectra of 3 and 5 showed strong additional bands at 958−964 and 1218−1222 cm−1 that were attributed to symmetric and asymmetric B−F

nitrogen

air

polymer

T5% (°C)

Tmaxc (°C)

R700 (%)

R900 (%)

3 4 5 6 7 8 9 11

156 347 67 256 266 259 332 352

418 416 419 425 427 424 411 429

6.4 4.8 41.7 4.6 4.3 8.6 0.3 8.8

6.5 4.4 35.0 3.6 4.0 5.9 0.8 3.9

Tg (°C) 105.9

101.7

a

No glass transition (Tg) was observed unless specified. bResidue of 1 at 900 °C under air 35.5 wt %; residue of 2 at 900 °C under air 19.1 wt %. cTemperature of the maximum degradation rate. 5427


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the boron-free STY-DVB copolymer 9 (Tmax 411 °C), all boron-containing polymers showed higher decomposition temperatures (Tmax 416−427 °C), particularly for the B(C6H5)3−-containing resins 6−8, as well as higher residues at 700 °C, consistent with the presence of Na and K. The loading of borate monomers calculated from the R900 values were in any case in agreement with those obtained from elemental and ICPOES analyses, within the experimental errors. For example, data for resin 8 gave a 27.9% (w/w) content of monomer 2 (corresponding to 0.8% B; 1.7% Na w/w) that well matches the value of 27.8 % computed from the data in Table 1. The thermal transitions of the resins were examined via DSC. No glass transitions (Tg) were observed below 250 °C, except for copolymer 4 at 105.9 °C and for STY-DVB copolymer 9 at 101.7 °C (Table 2). The DSC curves of all other polymers were flat in the temperature range 40−250 °C. This behavior is attributable to strong intermacromolecular interactions in the cross-linked borate resins.39 DSC curve of resins 3 and 8 are reported in Figure 5, right, together with that of 9 for comparison. The solvent uptake properties of resins were investigated by measuring their weight change after absorption of various solvents. Representative data for terpolymers 3 and 8 are reported in Table 3 as milliliters of solvent absorbed per unit

of a preformed molecular catalyst was explored, the resin was allowed to swell in propan-2-ol, and an amount of the cationic rhodium complex [((R)-Monophos)2Rh(NBD)]PF6 (10),24 corresponding to 20% of the ion exchange capacity of the resin, was added in the same solvent (Scheme 3). After 24 h stirring Scheme 3. Noncovalent Immobilization of Cationic Molecular Catalysts via Ion Exchange

and accurate washing, the dry resin was analyzed by ICP-EOS and EDS to give a Rh loading of 0.83% (w/w), corresponding to an uptake of ca. 60% of the starting rhodium used. A representative EDS spectrum of 8 recorded after cation exchange is shown in Figure 6. The amount of rhodium

Table 3. Weight Swelling Properties of Resinsa polymer

toluene (2.4)

DCEb (10.4)

2-BuOH (17.3)

2-PrOH (20.2)

3 8

7.7 35.5

4.7 14.1

1.2 6.5

1.0 5.4

a

In mL of solvent absorbed per 1 g of dry resin at room temperature. Dielectric constant in parentheses, data from ref 40. b1,2-Dichloroethane.

weight of resin. While being clearly hydrophobic, the resins showed significant swelling in common organic solvents with increasing swellability upon solvent polarity decrease. This finding can be justified by the low charge density in the overall hydrocarbon polymer backbone. The stability of all resins was tested in the pH range 0−14, showing no significant signs of damage or degradation by EDS, ESEM, and ATR analysis. The ion-exchange capacity of each resin was measured by titration of boron via ICP-OES analysis. Relevant data are reported in Table 1 in mequiv g−1. In agreement with the resins composition, exchange capacities of ca. 0.8 mequiv g−1 were obtained for all 10% borate containing polymers and 4.9 mequiv g −1 for homopolymer 5. Capacity values for terpolymers are about 1 order of magnitude lower compared to those of commercial polystyrene−DVB-based strong cationexchange resins.41 Ion-exchange experiments were carried out to ascertain the exchange ability of the resins prepared and their potential for use as solid carriers of molecular catalysts. Thus, in a typical experiment, the B(C6H5)3−Na+ containing resin 8 was stirred in the presence of a measured amount of KPF6 in methanol, and the resulting solid was analyzed for the Na and K content to afford a selectivity constant kK/Na = 4.86 and a complete ion substitution when an excess of K+ was used. This result, while in line with the affinity values reported for commercial strong cation-exchange resins,41 demonstrates a full site accessibility under the experimental conditions tested. When the anchoring

Figure 6. Typical EDS surface area spectrum of resin 8 containing the immobilized complex 10 (1 Torr, 20 keV).

immobilized onto the resin was comparable with that found under similar conditions using the commercial, sulfonated resins DOWEX 50WX2-100, having a much greater exchange capacity (4.8 mequiv g−1),4a and indicated the effective anchoring of the complex cation onto the resin. The 31P CPMAS NMR solid-state spectrum of the tethered complex was consistent with its noncovalent anchoring onto the support, while maintaining intact the overall cation structure. Thus, the spectrum of 10 onto 8 showed a significant broadening and a small downfield shift (ca. 1 ppm) of the 31P resonances, compared to the unsupported complex 10, due to the interaction with the ionic matrix. No signals due to PF6− anions were detected. It is worth noticing that a larger shift (ca. 5 ppm) was observed using the DOWEX resin under analogous 5428


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conditions, which can be attributed to the relative excess of charged groups in the latter polymer.42 Synthesis of Monoliths. Having established and effective protocol for the synthesis of insoluble styrene−divinylbenzene borate cation-exchange resins, we turned our attention to the preparation of the parent porous monoliths. The synthetic procedure was tailored to the obtainment of macroporous polymers for on-column applications, particularly for use in continuous-flow catalytic devices. Engineering issues in the area are related to polymer shrinking, wall effects, need of pressureresistant encasings, and reactor manufacture.43 In order to circumvent these drawbacks, a method was adopted in which the polymerization reaction was directly carried out into a commercial glass column resistant up to 1200 psi internal pressure.44 Thus, a mixture of STY, DVB, and 2 (64, 31, and 5 mol %, respectively), AIBN, and 1-dodecanol (3:1 v/v of monomers) was slowly heated from 60° to 85 °C in a 3 mm internal diameter and 25 mm length column without stirring, followed by extensive washing of the resulting solid using a conventional HPLC system (Scheme 4).

Figure 7. Images of the monolithic column (left) and dry monolith (right).

common organic solvent, nor in water, and fairly lower swelling degree with typical values of 13.1 and 3.2 mL g−1 in THF and in methanol, respectively. Quantitative analyses showed that the composition of the monolith did not correspond to that of the original polymerization reaction mixture in this case. Indeed, a reproducible content of monomer 2, STY, and DVB corresponding to 3.3, 65.1, and 31.6 mol %, respectively, could be calculated from elemental analysis and ICP-OES data (see also TGA discussion below). A ion-exchange capacity of 0.27 mequiv g−1 was measured from ICP-OES determination of boron (Table 1). 11B and 13C CP-MAS NMR spectra were similar to those previously described for 6−8, while integration of the 13C resonances gave values consistent with the composition above. Characteristic medium-intensity B−C vibrations were observed at 1451.5 cm−1 in the ATR spectrum. Careful inspection by SEM showed the monolith to be featured by a homogeneous network of interconnected macropores of average cavity 10 μm forming the monolith skeleton of ca. 6 μm thickness. A representative image is reported in Figure 8, left. The porous morphology of 11 was

Scheme 4. Sketch of the Procedure for the In-Column Preparation of the Monolithic Resin 11

The approach was chosen to reduce the possibility of a steep radial temperature gradient leading to an inhomogeneous polymerization inside the column.45 All manipulations (except washings) were performed under nitrogen using a metal-free equipment to avoid any possible metal contamination. The asprepared monolithic column could be directly used for subsequent flow applications provided that the column did not dry out, to avoid polymer shrinking (Figure 7, left). Choice of monomer 2 was motivated by its larger accessibility and by the higher stability of the polymers generated, compared to those obtained from 1, as shown by TGA data. Characterization of Monoliths. The in situ prepared solid monolith 11 could be safely removed from the glass column after carefully drying, resulting in typical ca. 2.5 × 20 mm (diameter × length) and 25 mg rods, before being characterized in the solid state (Figure 7, right). Consistent with the properties of the parent less cross-linked polymer 8, the material obtained showed no detectable solubility in any

Figure 8. SEM image (left, secondary electrons, 1600 magnifications, 25 kV, after Au coating) and mercury porosimetry (right) of monolith 11.

thus accurately investigated by X-ray tomography, mercury porosimetry, and N2 sorption experiments. X-ray microtomography confirmed the material to be isotropic with a uniform radial distribution of voids (average diameter 9.97 μm) and struts (6.8 μm), corresponding to a total porosity of 64.2% (Figure 9). Mercury intrusion evidenced the high mechanical strength of 11. The macropore size was calculated from the sharp step in the isotherm revealing a narrow distribution of pore diameters with a mean value of 7.28 μm (Figure 8, right), corresponding to a total intrusion volume of 2.53 cm3 g−1, a pore area of 34.97 m2 g−1, and a total porosity of 67.6%. The pore size distribution curve obtained from nitrogen sorption measurements by the BJH method was indicative of the 5429


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contrast with the theory that the pressure drop through a column at a given flow rate is proportional to the viscosity of the solvent for a given porosity,50 the back-pressure of the monolith followed the reverse order THF > MeOH > water, thus indicating a significant swelling contribution to the backpressure (Figure 10). However, swelling propensity factors computed for THF and methanol (4 and 1, respectively) were low and comparable with those of other STY−DVB monoliths, thus confirming the overall rigidity of the material.51 We took advantage of the scarce flow resistance exerted by 11 to test its ion-exchange ability and to achieve the immobilization of catalytically active specie in an easy and efficient manner. Thus, anchoring of the cationic rhodium complex 10 onto the monolith by ion exchange was possible by pumping a degassed methanol solution of 10 (0.0017 M) through the monolith at a flow rate of 0.2 mL min−1 over a period of 1 h. The monolith was then washed with methanol (1 h at 0.5 mL min−1) and dried under nitrogen to give the supported complex with a 1.29% (w/w) Rh loading (EDS, ICPOES). EDS measurements carried out on sections of the monolith containing 10 showed the complex to be evenly distributed within the solid support, as shown by the uniform radial and longitudinal loading of rhodium and phosphorus. EDS maps of the supported complex are shown in Figure 11 in which carbon, sodium, rhodium, and phosphorus signals are reported for comparison.

Figure 9. X-ray tomography 3D-reconstruction image of 11.

presence of macropores and a very low content of mesopores and micropores.46 Accordingly, the BET surface area measured was 9.72 m2 g−1. The open-cell structure of 11 thus resembles that of ceramic foams, but 2 orders of magnitude smaller.47 TGA and DCS data for monolith 11 were in line with those of the parent B(C6H5)3− resins and evidenced the high thermal stability of the material (Tmax 429 °C, Table 2). Loading of monomer 2 calculated from the value of residue R900 under air (12% w/w) agreed with the data obtained from elemental and ICP-OES analyses (10.1%), within the experimental errors. The in situ prepared monolith could be directly used for applications under a continuous flow of liquid by simple connection to a conventional HPLC pump. The hydrodynamic properties of the monolith mesoreactor 11 (i.d. 3 mm, length 25 mm) showed to be very favorable in that circumstance, exhibiting very low flow resistance and pressure drops lower than 4.5 MPa up to 5 mL min−1 flow rate (methanol). Under the same conditions, the reactor of analogous dimensions packed with a 50−75 μm sieved fraction generated a ca. 3 times higher back-pressure. The pressure drop dependence on the flow rate for the monolithic column was examined using water, methanol, and THF, showing high linearity in any case (R2 better than 0.97), thus indicating the high mechanical stability and good rigidity of the material.48 A typical plot of pressure drop versus flow rate in the range 0.2−1.0 mL min−1 is reported in Figure 10. The above striking properties of 11 can be attributed to its homogeneous microstructure featuring a narrow size distribution of interconnected flow-through pores that is maintained in the swollen state. The rigidity of the monolith is ascribable to its high cross-linking degree, which usually reflects in the incompressibility and in the low swelling volume of organic polymers which prevents pore blockage.49 In

Figure 11. ESEM image and EDS maps of a equatorial section of monolith 11 with supported complex 10 (1.29% Rh w/w). Top left: secondary electrons image; C map (C Kα1), Na map (Na Kα1), P map (P Kα1), Rh map (Rh Lα1).

■

CONCLUSIONS Use of heterogeneous metal catalysts in the fine-chemical industry may significantly contribute to the development of green and economical large-scale production routes, due to the considerable benefits in terms of ease of reuse of precious catalysts, clean catalyst separation, and integration in existing reactor equipment.52 This is particularly true if catalysts can be effectively reused with no efficiency decay and/or implemented into continuous flow (monolithic) systems. Ion-exchange resins are excellent candidates to support catalytic entities because of their potential to provide a variety of single-site catalyst with ease and at low costs. However, they often suffer from catalyst deactivation due to coordination of the exchanging functional group to the active site. In order to circumvent this limitation, borate materials may be particularly useful thanks to the weak coordination ability and satisfactory chemical inertness.53 Nonetheless, only two examples were reported so far of bulk cation-exchange resins bearing borate anions, and both were limited to linear trispentafluorophenyl-4-vinylphenylborate-costyrene polymers.54,55

Figure 10. Plot of pressure drop versus flow rate for monolithic column 11: i.d. 3 mm, length 25 mm, room temperature. Viscosity η in mPa s at 21 °C from ref 40. 5430


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(3) For a review, see: (a) Corain, B.; Zecca, M.; Canton, P.; Centomo, P. Philos. Trans. R. Soc., A 2010, 368, 1495−1507. (b) Zecca, M.; Centomo, P.; Corain, B. In Metal Nanoclusters in Catalysis and Materials Science; Corain, B., Schmid, G., Toshima, N., Eds.; Elsevier: Amsterdam, 2008; pp 201−232. (4) For a review, see: (a) Barbaro, P.; Liguori, F. Chem. Rev. 2009, 109, 515−529. (b) Barbaro, P. Chem.Eur. J. 2006, 12, 5666−5675. (c) Gelbard, G. Ind. Eng. Chem. Res. 2005, 44, 8468−8498. (5) (a) Fraile, J. M.; García, J. I.; Mayoral, J. A. Chem. Rev. 2009, 109, 360−417. (b) McMorn, P.; Hutchings, G. J. Chem. Soc. Rev. 2004, 33, 108−122. (6) Thomas, J. M.; Raja, R.; Lewis, D. W. Angew. Chem., Int. Ed. 2005, 44, 6456−6482. (7) (a) Heterogenized Homogeneous Catalysts for Fine Chemicals Production; Barbaro, P., Liguori, F., Eds.; Springer: London, 2010. (b) Catalyst Separation, Recovery and Recycling; Chemistry and Process Design; Cole-Hamilton, D. J., Tooze, R. P., Eds.; Springer: Dordrecht, 2006. (c) Fine Chemicals Through Heterogeneous Catalysis; Sheldon, R. A., van Bekkum, H., Eds.; Wiley-VCH: Weinheim, 2001. (8) (a) Preparation of methyl isobutyl ketone: Vandersall, M. T.; Weinand, R. A. (Rohm and Haas Company), Pat. EP 1321450A2, 2003. Schmitt, K.; Disteldorf, J.; Flakus, W.; Hubel, W.; Eickel, W. (Veba-Chemie Aktiengesellchaft) Pat. US 3953517, 1976. (b) Preparation of methyl-tert-butyl ether: Harland, C. E. Ion-Exchange, 2nd ed.; Royal Society of Chemistry: Cambridge, 1994. (c) Chiyoda/UOP process for the production of acetic acid: Yoneda, N.; Minami, T.; Weiszmann, J.; Spehlmann, B. In Studies in Surface Science and Catalysis; Hattori, H., Otsuka, K., Eds.; Kodansha: Tokyo, 1999; Vol. 121, pp 93−98. (f) Yoneda, N.; Minami, T.; Hamato, K.; Shiroto, Y.; Hosono, Y. J. Jpn. Petrol. Inst. 2003, 46, 229−234. (d) For a review see: Weissermel, K.; Arpe, H. P. Industrial Organic Chemistry, 3rd ed.; VCH: Weinheim, 1997. (9) (a) Barbaro, P.; Bianchini, C.; Giambastiani, G.; Oberhauser, W.; Morassi Bonzi, L.; Rossi, F.; Dal Santo, V. J. Chem. Soc., Dalton Trans. 2004, 1783−1784. (b) Kralik, M.; Kratky, V.; Centomo, P.; Guerriero, P.; Lora, S.; Corain, B. J. Mol. Catal. A: Chem. 2003, 195, 219−223. (c) Seki, T.; Grunwaldt, J. D.; van Vegten, N.; Baiker, A. Adv. Synth. Catal. 2008, 350, 691−705. (10) (a) Lucarelli, C.; Vaccari, A. Green Chem. 2011, 13, 1941−1949. (b) Sheldon, R. A.; Arends, I.; Hanefeld, U. Green Chemistry and Catalysis; Wiley-VCH: Weinheim, 2007. (c) Clark, J. H. Green Chem. 2006, 8, 17−21. (d) German Catalysis Society, Roadmap for catalysis research in Germany, March 2010. (11) For recent reviews see: (a) Wegner, J.; Ceylan, S.; Kirschning, A. Adv. Synth. Catal. 2012, 354, 17−57. (b) Irfan, M.; Glasnov, T. N.; Kappe, C. O. ChemSusChem 2011, 4, 300−316. (c) Webb, D.; Jamison, T. F. Chem. Sci. 2010, 1, 675−680. (d) Mak, X. Y.; Laurino, P.; Seeberger, P. H. Beilstein J. Org. Chem. 2009, 5 (no. 19). (e) Luis, S. V.; García-Verdugo, E. In Polymeric Chiral Catalyst Design and Chiral Polymer Synthesis; Itsuno, S., Ed.; Wiley: New York, 2011; Chapter 5. (12) (a) Sachse, A.; Galarneau, A.; Coq, B.; Fajula, F. New J. Chem. 2011, 35, 259−264. (b) Fletcher, P. D. I.; Haswell, S. J.; He, P.; Kelly, S. M.; Mansfield, A. J. Porous Mater. 2011, 18, 501−508. (c) Linares, N.; Hartmann, S.; Galarneau, A.; Barbaro, P. ACS Catal. 2012, 2, 2194−2198. (d) El Kadib, A.; Chimenton, R.; Sachse, A.; Fajula, F.; Galarneau, A.; Coq, B. Angew. Chem., Int. Ed. 2009, 48, 4969−4972. (13) The term “monolith” to which we refer in the present paper complies with the definition given by IUPAC: “a shaped, fabricated intractable article with a homogeneous microstructure that does not exhibit any structural components distinguishable by optical microscopy” (“Definitions of terms relating to the structural and processing of sols, gels, networks and inorganic−organic hybrid materials”, IUPAC Recommendations, 2007, p 1812). This “singlepiece” material shall not be confused with the conventional monoliths found in the chemical engineering literature as inert carrier honeycomb-type materials or foams, with millimeter size parallel channels or cavities, onto which a layer of catalytically active material is deposited. See, e.g.: (a) Cybulski, A.; Moulijn, J. A. Structured Catalysts and Reactors, 2nd ed.; Marcel Dekker Ltd.: New York, 2005, p 19.

We showed that phenyltrifluoroborate and tetraphenylborate anions can be easily incorporated into cross-linked styrene− divinylbenzene matrices via radical copolymerization to give gel-type cation-exchange resins. Parent monolithic materials featuring an homogeneous microstructure of interconnected macropores were also synthesized in situ into conventional glass columns by an analogous procedure. The as-prepared monolith could be directly used for large-scale applications under a continuous flow of fluids showing very low flow resistance and satisfactory mechanical stability. To the best of our knowledge, this is the f irst example of ion-exchange polymeric monoliths based on tethered borate anions. Previous examples of polymeric ionexchange monoliths are mostly related to chromatographic applications and include sulfonated resins prepared either by removal of water from water-in-oil STY−DVB emulsions followed by postfunctionalization56 or by sulfonation of preformed poly(STY-co-DVB) or poly(GMA-co-EDMA) monoliths in capillary silica columns57 and quaternary ammonium polymers obtained by amine treatment of the corresponding chlorides15g or by copolymerization of 2(acryloyloxy)ethyltrimethylammonium chloride and PEGDA.58 The ability of all resins obtained to immobilize preformed molecular catalysts via cation exchange was demonstrated by the effective anchoring of the rhodium complex [((R)-Monophos)2Rh(NBD)]+. Preliminary findings obtained in our lab using the immobilized catalyst in enantioselective hydrogenation reactions under continuous flow have shown the catalysts performance to be comparable with that of the corresponding homogeneous-phase system. Full results will be communicated in due course.

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ASSOCIATED CONTENT

S Supporting Information *

Details of experimental procedures, synthesis, and characterization of polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Dr. Marco Carlo Mascherpa (ICCOMCNR) for ICP-OES analyses, Dr. Francesca Loglio (Centro di Crystallografia University of Florence) for X-ray tomography, Dr. Orazio Russo (Micromeritics Italia) for Hg intrusion measurements, and Dr. Calogero Pinzino (ICCOM-CNR) for IR simulations.

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REFERENCES

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Strong Cation Exchange with Innocence: Synthesis and

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