Metathesis@MOF: Simple and Robust Immobilization of Olefin

Research Article pubs.acs.org/acscatalysis

Metathesis@MOF: Simple and Robust Immobilization of Olefin Metathesis Catalysts inside (Al)MIL-101-NH2 Artur Chołuj, Adam Zieliński, Karol Grela,* and Michał J. Chmielewski* Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Ż wirki i Wigury 101, 02-089 Warszawa, Poland S Supporting Information *

ABSTRACT: Despite their record-breaking sorption capacities, metal−organic frameworks (MOFs) have rarely been used for the immobilization of homogeneous catalysts by simple absorption from solution. Here we demonstrate that this simple strategy allows successful immobilization of olefin metathesis catalysts inside MOFs. Ruthenium alkylidene complexes bearing ammonium-tagged NHC ligands were successfully supported inside (Al)MIL-101-NH2·HCl. The materials thus obtained are true heterogeneous catalysts, active toward various substrates with TONs up to 8900 (in batch conditions) or 4700 (in continuous flow). Although the catalysts were held inside the MOF by noncovalent forces only, leaching was not observed and heavy metal contamination of the products was found to be below the detection limit of ICP MS (0.02 ppm). The robustness of the catalyst attachment allowed their use in a continuous flow setup. KEYWORDS: metal−organic frameworks, olefin metathesis, supported catalysts, microporous materials, green chemistry

M

lysts are not suitable for the above heterogenization methods and must be tailored by often-lengthy synthetic procedures. Given the extremely high specific surface areas of MOFs and their superior sorption capacities, direct heterogenization by simple absorption of homogeneous catalysts inside porous MOFs can be envisaged. Nonetheless, this possibility seems to have been largely overlooked, despite prior successful implementation in mesoporous silicas.9 To the best of our knowledge, the precedents in MOFs are limited to polyoxometalates10 and enzymes.11 Here we use the simple impregnation strategy to successfully immobilize, for the first time, olefin metathesis catalysts inside MOFs and demonstrate the robustness of their attachment in a continuous flow setup. Catalytic olefin metathesis (OM) is a gentle yet powerful synthetic method that allows the formation of carbon−carbon double bonds.12 A number of well-defined molybdenum and ruthenium OM catalysts were developed, with the latter class showing higher stability toward air and moisture.13,14 Despite undeniable benefits related to it, homogeneous OM catalysis exhibits some disadvantages, such as difficult postreaction separation and contamination of products with spent catalyst residues that can in addition cause some unwanted side reactions during reaction workup.15 All of these factors are highly undesirable, especially when an industrial process is considered. In contrast, well-defined heterogeneous catalysts can be applied and removed easily, even in a continuous flow

etal−organic frameworks (MOFs) are increasingly being recognized as highly promising materials for heterogeneous catalysis.1 Their crystalline, highly porous, and easily tunable structures offer unique environments for the immobilization of catalytically active species. In contrast to some more conventional solid supports, such as activated carbons, amorphous silicas, and polymers, catalytic centers in MOFs are not only anchored to their surface but are also located inside well-defined nanoscopic voids in a crystalline framework. The voids in MOFs can be made large enough to easily encompass typical homogeneous catalysts and their substrates without significant transport problems. On the other hand, spatial restrictions imposed by nanocavities of MOFs offer new opportunities to control shape selectivity, regioselectivity, and enantioselectivity of the immobilized catalysts. Furthermore, confinement of different catalytic species in the same cavity opens exciting avenues in tandem catalysis. Clearly, MOFs have much more to offer than the facile separation and potential reusability of supported catalystscommon advantages of all heterogeneous catalysts. Motivated by the prospects outlined above, intense scientific effort is currently directed toward grafting of various homogeneous catalysts inside MOFs. Among various strategies that have been used to incorporate discrete catalysts inside MOFs, postsynthetic covalent attachment to the framework,2 formation of coordination bonds,3 and ion exchange4 are the most prominent.5 In an alternative approach, catalysts based on salen,6 BINOL,7 and NHC8 ligands have been synthetically elaborated to become struts of metal−organic frameworks. In general, however, commercially available homogeneous cata© XXXX American Chemical Society

Received: April 12, 2016 Revised: August 10, 2016

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DOI: 10.1021/acscatal.6b01048 ACS Catal. 2016, 6, 6343−6349

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The (Cr)MIL-101 was successfully obtained according to the literature method,23 with a high specific surface area of 2965 m2/g (BET model). Its aluminum analogue, (Al)MIL-101, is unknown, because the direct reaction of terephthalic acid with aluminum salts yields the more thermodynamically stable MOF (Al)MIL-53.24 Therefore, we opted for amino-substituted (Al)MIL-101-NH2, which is easily available from inexpensive 2-aminoterephthalic acid, while good-quality (Cr)MIL-101NH2 could only be obtained by postsynthetic nitration and reduction of (Cr)MIL-101.25 The literature procedure24 for the synthesis of (Al)MIL-101-NH2 gave partially formylated material, containing ca. 8% of NHCHO groups, so that the formylamido groups had to be converted back to NH2 by our recently developed method.26 Deformylation yielded very pure material with more than 98% of the free −NH2 groups (according to 1H NMR of digested sample) with a BET surface area of 2340 m2/g, slightly higher than the 2100 m2/g reported in the original procedure. Sorption. Having two highly porous and well-activated MOFs, we investigated their sorption properties with respect to six readily available OM catalysts: two EWG-activated (1 and 2)27 and four tagged (3−6,28 Figure 2) Hoveyda−Grubbs second-generation complexes. The catalysts were dissolved in toluene (1−3) or in DCM (the cationic complexes 4−6 are insoluble in toluene) followed by the addition of the MOF in such an amount that there was on average one mesoporous void in the MOF for each molecule of catalyst. The progress of absorption was followed spectrophotometrically, measuring the catalyst concentration in the supernatant. The method was independently validated using ICP MS (see the Supporting Information). Very efficient sorption was observed in all cases, even for the catalysts bearing bulky SIPr ligands (Table 1 and Figure 3). The four most polar complexes 3−6 were absorbed almost quantitatively, giving materials with very high catalyst loadings (one molecule per cage corresponds to ca. 7−9 w/w %, depending on the molecular weight of a particular catalyst). Note that, in contrast to most conventional supports, even at this high loading the catalysts in MOFs may still remain siteseparated, because each catalytic species may occupy a separate cavity. The two less polar catalysts (1 and 2) were also efficiently absorbed. Despite the high loading of catalysts, the materials remained highly porous, with BET values around 1800 m2/g (see the Supporting Information for details). In order to estimate the maximal loading capacity of (Al)MIL-101-NH2, the MOF was equilibrated with a 10-fold excess of various catalysts with respect to MOF cavities. Very high loadings were achieved, corresponding to 4.1, 2.3, and 7.1 molecules per cage, for complexes 1, 2, and 4 respectively (Table S5 in the Supporting Information). Although cohabitation of two or more molecules of Ru catalyst in one compartment is likely to lead to bimolecular deactivation29 and hence is not advantageous for catalysis, these results suggest that the catalysts easily penetrate the whole volume of crystals and do not just bind to their outer surface. These numbers highlight also the huge size of the MIL-101 cavities (see the Supporting Information for details and illustrations). Similar, extremely efficient absorption of cationic dyes in the same MOF has been reported recently and attributed to electrostatic interactions with amino groups.30 Desorption. As the catalysts are held inside the MOFs by reversible noncovalent interactions, their desorption was an

mode. Several methodologies for heterogenization of OM catalysts have therefore been developed.16 They include immobilization via various ligands present in a catalyst structure, using sometimes very sophisticated linkers and strategies. A simpler approach is to support an unmodified homogeneous OM catalyst on a solid support. This was tested by Sels and Jacobs,17 and more recently by Limbach18 and Kirschning,19 who proved that some commercially available Hoveyda−Grubbs catalysts can be successfully immobilized on silica gel. Since these catalysts are quite soluble in methylene chloride (DCM) and toluene, this strategy, although easy and economical, has a drawback of being limited to solvents (usually pentane or hexane) in which the system stays heterogeneous.17−19 Most recently, Skowerski and Balcar immobilized Ru catalysts bearing polar quaternary ammonium groups on siliceous mesoporous molecular sieves SBA-15 and MCM-41 and used the resulting materials in toluene.20 However, thus far there has been no precedent for immobilization of OM catalysts inside MOFs.21 As OM catalysts are unlikely to survive the typical solvothermal conditions of MOF synthesis, we decided to try the simplest immobilization strategy of physisorption inside MOFs.

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RESULTS AND DISCUSSION Ruthenium OM catalyst molecules are relatively large (for example, a Hoveyda−Grubbs second-generation catalyst is ca. 1.6 nm × 1.2 nm × 0.9 nm), so that only few MOFs have pores and pore windows large enough to accommodate them. Also, frameworks with large voids tend to be fragile. Some of the few MOFs that combine high chemical stability and a spacious pore system belong to the MIL-101 family (Figure 1).22 The voids in

Figure 1. Visualization of an olefin metathesis catalyst inside a void of MIL-101-NH2 structure.

the MIL-101 structure form a 3D network, which facilitates free diffusion of substrates, products, and solvents. Considering this, we chose the two most easily available MOFs of this family, (Cr)MIL-101 and (Al)MIL-101-NH2, as our model supports. Both can be constructed from inexpensive terephthalic acids, have very large pores (ca. 2.9 and 3.4 nm), and importantly, have wide pore windows of 1.2 and 1.5 nm, respectively. 6344


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Figure 2. Structures of Ru complexes investigated for their absorption in MOFs.

methanol and isopropyl alcohol (Figure 4). Static desorption (equilibration of the absorbed catalyst with a portion of fresh

Table 1. Sorption of Various Catalysts on Two Model MOFsa catalyst absorbed from soln (%) catalyst

solvent

(Cr)MIL-101

(Al)MIL-101-NH2

1 2 3 4 5 6

toluene toluene toluene DCM DCM DCM

81.0 78.5 97.9 99.0 99.8 99.9

98.3 88.8 99.7 99.4 99.9 99.9

a

Conditions: 2.0 mL of 1.0 mM solution of catalyst was added to a vial containing the MOF (16.0 mg of (Al)MIL-101-NH2 or 16.3 mg of (Cr)MIL-101; 2.0 μmol of cages) and was stirred for 1 h at room temperature. The absorption degree was determined spectrophotometrically.

Figure 4. Desorption of selected catalysts from (Al)MIL-101-NH2 upon washing with various solvents.

solvent) proved to be very ineffective (see the Supporting Information). Thus, we decided to put the materials to a more stringent test of dynamic desorption: various solvents were slowly filtered through a pad of catalyst@MOF on a sinteredglass frit and the effluents were monitored by UV−vis. Even under dynamic conditions none of the absorbed catalysts could be significantly washed out of the (Al)MIL-101-NH2 with hexanetheir concentration in the eluate was below the UV− vis detection limit (0.5 nmol/mL). Toluene washed out uncharged catalysts 1−3, but cationic complexes 4−6 remained intact. The more polar DCM very quickly washed out the uncharged complexes 1−3, but again, the loss of cationic complex 4 from the MOF was negligible, in contrast to literature precedents, where catalysts readily desorb in DCM.17−19 The cationic complexes could be removed from the MOF only by alcohols, as exemplified below for catalyst 4. Interestingly, methanol rapidly washes out also complexes 1 and 2, despite the fact that they are very insoluble in MeOH, which suggests that the main driving force for desorption is the strong affinity between the MOF and MeOH, and not the particularly good solvation of the catalysts. Some 10−30% of

Figure 3. Sorption of 4 (6.4 mg, 8.0 μmol solution in 4.0 mL of DCM) on MIL-101-NH2 (100 mg). The catalyst was completely absorbed on the first ca. 15% of the MOF layer. The effluent was completely colorless and contained no catalyst according to UV−vis.

obvious possibility. Therefore, we tested the robustness of the catalysts’ attachment by washing them with solvents of increased polarity: hexane, toluene, DCM, and the alcohols 6345


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immobilization gave completely inactive materials. It seems therefore that perhaps chloride anions are more important in recovering catalytic activity than the protons. In accordance with this hypothesis, treatment of the MOF either with tetrabutylammonium chloride in dichloromethane or with NaCl in H2O suffices to recover the catalytic activity of 7 in (Al)MIL-101-NH2, although dry HCl is more efficient and aqueous NaCl causes amorphization of the MOF (Table 2). It is well-known that both MOFs used in our study contain in their structure anions which can be poisonous for olefin metathesis catalysts:31 fluoride and OH− in the case of (Cr)MIL-10122 (each Cr cluster contains either one OH− or one F− anion) and OH− in the case of (Al)MIL-101-NH2 (each Al cluster contains either one OH− or one Cl− anion).24 Furthermore, each metal cluster contains two Lewis acidic sites with high affinity to chloride anion, which are occupied by weakly bound neutral molecules (water, MeOH, etc.) in the assynthesized materials but are emptied by thermal activation applied before the catalyst immobilization.3,22,24 Sequestration of the Ru-bound chloride anions by the Lewis acidic MOFs may lead to much less active and less stable catalysts.32 Pretreatment of the MOFs with HCl is likely to prevent both inhibition mechanisms by (1) OH− or F− exchange for Cl− 33 and (2) saturation of the Lewis acidic sites at nodes by Cl−, at least in (Al)MIL-101-NH2, where the electroneutrality is maintained by the protonated amino groups. Having solved the problem of catalyst deactivation by the MOFs, we tested the performance of 4@(Al)MIL-101-NH2· HCl with a set of metathesis substrates at 2 and 0.5 mol % loading (Table 3). Excellent results were obtained in all RCM reactions tested, allowing various carba- and heterocycles to be obtained smoothly. A model cross-metathesis reaction also gave the expected product as a mixture of E and Z isomers in the ratio 5.5:1, slightly higher than that obtained under homogeneous conditions using nonimmobilized catalyst 4 (4.3:1). Furthermore, we found that in most cases the catalyst loading could be lowered to 0.5 mol % with only a minor decrease of conversion. In light of expected large-scale applications of OM,34 considerable attention must be given to technological and economical aspects (use of precious-metal catalysts at low loadings, low-level metal contamination, green solvents, minimal waste production, equipment costs, etc.). Thus, we decided to test if the Ru@MOF catalysts could be applied at even lower loadings and under continuous flow conditions.35 This requires an extremely robust catalyst, being able to survive increased number of catalytic cycles in the presence of ethylene.36 Such a system has been very recently disclosed by Skowerski et al.37 and consists of catalyst 7an analogue of 4, bearing a sterically enlarged NHC ligand. As expected, the catalyst 7, when entrapped in (Al)MIL-101NH2·HCl and tested in RCM of 9 in toluene, gave >99% conversion even at 0.1 mol % loading, while the split test shows that the system is completely heterogeneous (Tables 4 and 5). Even better results were obtained upon “dilution” of 7 in the MOF matrix from the initial concentration of one molecule per cage (ca. 9% w/w) to one molecule per 10 cages (ca. 1% w/w). With this latter material, 0.01 mol % of 7 (100 ppm) was sufficient to yield 64% conversion after 24 h or 89% after 4 days (Table 4). It is important to note that a glovebox was not used for these reactions and that the reaction mixtures were actually open to air (vials vented via a thin needle).13

each catalyst cannot be recovered from MIL-101-NH2, even after extensive washing with polar solvents. Catalysis. The activity of the 12 catalytic materials in ringclosing olefin metathesis (RCM) was investigated using 8 as a model diene, toluene as a solvent, and 2 mol % loading of each catalyst (Table S7 in the Supporting Information). The initial results were disappointing: all six catalysts incorporated in (Cr)MIL-101 were completely inactive, whereas good conversions observed for the uncharged catalysts 1 and 2 in (Al)MIL-101-NH2 were found to result from leaching followed by homogeneous reaction, as proved by split tests. However, moderate activity and no leaching was observed for the cationic species 4−6 immobilized on the aluminum-based MOF. The highest activity among the catalysts incorporated in (Al)MIL101-NH2 was observed for 4: 22% after 5 h and 75% after 24 h. The poor performance of the heterogenized catalysts suggested that they were somehow deactivated by the MOFs: more strongly by the (Cr)MIL and less strongly by the (Al)MIL-NH2. To resolve this problem, we treated the MOFs with water, base (N,N-diisopropylethylamine), or acid (dry HCl in Et2O), before or after the sorption of the model catalyst 4. Only the HCl treatment led to dramatic improvement of catalytic activity, no matter if it was applied before or after the absorption of the catalyst: the conversion jumped from 60% to 99% for 4@(Al)MIL-101-NH2 and from 1% to 60% for 4@(Cr)MIL-101. The frameworks remain intact upon HCl treatment, except protonation of the NH2 groups in (Al)MIL101-NH2, as inferred from elemental analysis, NMR of digested samples, PXRD, BET, and SEM (see the Supporting Information for details). To shed some light on the role of HCl in material activation, we performed additional experiments, focusing on the most active and stable catalyst: 7@(Al)MIL-101-NH2 (see below). First, we found that dry HCl has a minor effect on the catalyst activity in homogeneous solution: even in the presence of 5 equiv of dry hydrogen chloride 7 gives >90% conversion already after 30 min (see the Supporting Information for details). Next, we checked if other strong acids are able to recover the activity of the catalyst in (Al)MIL-101-NH2 (Table 2). However, treatment of the MOF with anhydrous CH3SO3H in DCM or CF 3 SO 3 H in Et 2 O followed by catalyst Table 2. Influence of Pretreatment of (Al)MIL-101-NH2 with Various Chemicals on the Catalytic Activity of 7@(Al)MIL-101-NH2 in RCM Reaction of 9 treatment untreated untreated HCl/Et2O HCl/Et2O CH3SO3H/DCM CF3SO3H/Et2O TBACl/DCM NaCl/H2O

Ru (mol %) a

1 0.1b 1a 0.1b 1a 1a 0.1b 0.1b

conversn (%)c 74 23 100 100 0 0 70 74

a

Reaction conditions unless specified otherwise: 7@(Al)MIL-101NH2 (containing ca. 2% w/w of 7 and 1 μmol of Ru), pretreated with various chemicals before the sorption of Ru complex, and 9 (0.1 mmol, 2 mL of 0.05 M toluene solution) were stirred at room temperature for 24 h. bAs in footnote a except that 1.0 mmol of 9 (2 mL of 0.5 M toluene solution) was used. cConversion values were calculated by GC using authentic samples as standards. 6346


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ACS Catalysis Table 3. Catalytic OM Activity of 4@(Al)MIL-101-NH2·HCl with Model Olefins in Toluene at Room Temperature

Table 5. Split Tests for Catalyst@MOF in RCM of 9 at Room Temperature in Various Solventsa conversn (%) catalyst

solvent

4 7 4 7 4 7

toluene toluene DCM DCM EA EA

at split 55 (3 49 (3 37 (16 19 (16 50 (3 32 (6

h) h) h) h) h) h)

in filtrate

in suspension

55b 49b 37c 19c 50b 33b

99b 100b 93c 100c 96b 87b

a Conditions: 4 or 7@(Al)MIL-101-NH2·HCl (2.0 μmol of Ru, 1 mol % with respect to 9), 9 (200 μmol), and toluene (4 mL) stirred at room temperature. A split test was performed after the time specified in parentheses. Part of the reaction mixture was filtered through a 0.2 μm PTFE syringe filter, and both samples were stirred further. Conversion values were calculated by GC using authentic samples. b Conversion after 24 h. cConversion after 72 h.

must be stressed here that the compatibility of the immobilized Ru catalysts with such polar solvents is quite rare, as most of the commercial homogeneous Ru complexes immobilized on silica by physisorption can be used only in pentane or hexane.17−19 On the other hand, the ammonium-tagged38 complexes, including 4 and 7, recently immobilized on various supports, such as zeolites, silica, alumina, and even cotton wool,20,39,40 were found to stay heterogeneous in EtOAc, but they still could be washed out from these supports by DCM.40 This shows that MOFs can offer some significant advantages over other, more conventional supports. In an attempt to compare the performance of homogeneous catalysts 4 and 7 with their MOF-supported versions, we set up model reactions in toluene. Unfortunately, however, both unsupported catalysts are so insoluble in toluene that the respective homogeneous reactions fail completely. It is interesting to note, therefore, that immobilization inside a MOF allows the catalysts to work very well in a medium where the unsupported catalysts cannot operate. Such a comparison could be made in DCM, where the cationic catalysts are readily soluble, but this solvent is clearly disadvantageous for Ru@ MOFheterogeneous reactions in DCM are much slower than in toluene, for instance. Continuous Flow. Development of new heterogeneous catalysts that can work under continuous flow (CF) conditions is crucial for further application of OM in industry.34,35 Unfortunately, while a few catalysts have proved to be effective (TON up to 104) in metathesis of internal alkenes under CF, none of them have shown good activity for more elaborate substrates, especially in OM reactions that proceed with the evolution of ethylene.41 Recently, Skowerski et al. introduced the system that was shown to work satisfactorily in RCM of 8 under CF conditions where ethylene was formed during the process.37 Encouraged by the performance of 7@MOF in batch reactions, we decided to test it also in CF. A prototypical flow reactor was made from a short Teflon tube of 1.5 mm internal diameter, filled with 27 mg of (Al)MIL-101-NH2·HCl containing 1% of 7 (0.30 μmol, 120 ppm with respect to the total amount of substrate passing through), forming ca. 5 cm thick catalyst bed. The reactor was connected to a syringe pump. After some optimization of the reaction conditions (see the Supporting Information), a 0.5 M solution of 9 was forced through the bed at 40 °C at a rate of 10 μL/min for 9 h. The retention time of the substrate on the bed was less than 7 min.

a Conditions: 2.0 μmol of catalyst (2 mol %) supported on 23 mg of (Al)MIL-101-NH2·HCl, 100 μmol of substrate, and 4.0 mL of toluene were stirred for 24 h at room temperature. Conversion values were estimated by GC using authentic samples. bConversion for 0.5 mol % of the catalysts: 2.0 μmol of 4@(Al)MIL-101-NH2·HCl and 400 μmol of substrate.

Table 4. Comparison of MOF-Supported 4 and 7 at Decreasing Loading in RCM of 9 in Toluene at Room Temperature catalyst

loading of catalyst (mol %)

conversn (%) after 24 h

TON after 24 h

conversn (%) after 48 h

4 4 4 4 7 7 7 7 7a 7

0.5 0.25 0.1 0.05 0.5 0.25 0.1 0.05 0.05 0.01

94 80 73 53 99 99 99 88 99 27

7a

0.01

64

188 320 730 1060 198 396 990 1760 1980 2700 4000b 6400 8900b

98 91 80 61 100 100 100 95 100 33 40b 77 89b

a

1 wt % of catalyst in MOF. bReaction time: 96 h.

Moreover, both tagged catalysts 4@MOF and 7@MOF remain truly heterogeneous even in the quite polar solvents DCM and ethyl acetate, as proven by split tests (Table 5). It 6347


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Large bubbles of ethylene were immediately observed (see video material in the Supporting Information), and the conversion was >90% after the first 1 h. The conversion steadily dropped over the time span of the experiment, giving an average value of 57% after 9 h. This corresponds to a very high TON of 4700 (Figure 5). Most importantly, however, no

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed by the Polish budget for science and education for the years 2013−2017. A. C. and M.J.C. thank the Polish Ministry of Science and Higher Education for IDEAS PLUS grant no. IdP 2012/0002/62. A.Z. and K.G. are grateful to the National Science Centre (Poland) for the NCN OPUS Grant No. UMO-2014/15/B/ST5/02156. The study was carried out at the Biological and Chemical Research Centre, University of Warsaw, established within the project cofinanced by the European Union from the European Regional Development Fund under the Operational Programme Innovative Economy, 2007−2013.

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Figure 5. Turnover number and conversion over time in continuous flow (CF) experiment. The curves are a visual aid only.

ruthenium could be detected in the crude product by inductively coupled plasma mass spectrometry (ICP-MS), meaning that there was less than 0.02 ppm of Ru in the product. Such surprisingly low leaching42 suggests a very strong bonding of the catalyst by noncovalent forces.

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CONCLUSIONS The first successful immobilization of olefin metathesis catalysts inside MOFs has been achieved by absorption of a new generation of tagged, commercially available,28 Hoveyda− Grubbs type complexes inside an easily available, inexpensive, and nontoxic MOF, (Al)MIL-101-NH2. HCl treatment of the MOF proved to be critical for its high activity in OM reactions (TONs up to 8900 in RCM reactions without glovebox). Such linker-free immobilization, consisting only of mixing MOFs with catalyst solutions at room temperature, is experimentally simple, economical, and can be easily scaled up. This system is stable even under continuous flow conditions in the presence of ethylene, which was manifested by the low Ru content in the RCM product (below 0.02 ppm) and a high cumulated TON (4700). We believe that the method may be generally applicable to other homogeneous catalysts.

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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/acscatal.6b01048. Detailed experimental procedures for synthesis of MOFs, sorption experiments, catalytic tests, and characterization of materials including 1H NMR spectra, PXRD traces, and N2 adsorption isotherms (PDF) Continuous flow experiment (AVI)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for K.G.: [email protected]. *E-mail for M.J.C.: [email protected]. 6348


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ACS Catalysis

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