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A MoVI grafted Metal Organic Framework: Synthesis, characterization and catalytic investigations
Journal of Catalysis 316 (2014) 201–209
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A MoVI grafted Metal Organic Framework: Synthesis, characterization and catalytic investigations Karen Leus a,1, Ying-Ya Liu b,1, Maria Meledina c, Stuart Turner c, Gustaaf Van Tendeloo c, Pascal Van Der Voort a,⇑ a b c
Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024 Dalian, China Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium
a r t i c l e
i n f o
Article history: Received 18 February 2014 Revised 1 May 2014 Accepted 18 May 2014
Keywords: Metal Organic Framework Oxidation Catalysis Molybdenum Post-modification
a b s t r a c t We present the post-modification of a gallium based Metal Organic Framework, COMOC-4, with a Mocomplex. The resulting Mo@COMOC-4 was characterized by means of N2 sorption, XRPD, DRIFT, TGA, XRF, XPS and TEM analysis. The results demonstrate that even at high Mo-complex loadings on the framework, no aggregation or any Mo or Mo oxide species are formed. Moreover, the Mo@COMOC-4 was evaluated as a catalyst in the epoxidation of cyclohexene, cyclooctene and cyclododecene employing TBHP in decane as oxidant. The post-modified COMOC-4 exhibits a very high selectivity toward the epoxide (up to 100%). Regenerability and stability tests have been carried out demonstrating that the catalyst can be recycled without leaching of Mo or loss of crystallinity. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction It is well known that compounds containing certain transition metals, such as Mo, W, Ti and V, are able to catalyze the liquidphase epoxidation of olefins using alkyl hydroperoxides as oxidants in homogeneous solution. The catalysis is most effective when these transition metals are in their highest oxidation state. During the last decade, several molybdenum (VI) complexes have shown to be successful catalysts for various reactions ranging from Lewis acid catalyzed transformations to oxidation and reduction reactions [1]. Treatment of [MoO2X2] species (X = halide, OR, OSiR3) with monodentate or bidentate Lewis bases (L or L2), such as pyridine and 2,20 -bipyridine, in the presence of a donor solvent, form a series of dioxomolybdenum complexes with the composition of [MoO2X2L]. Such dioxomolybdenum(VI) complexes are well documented as efficient oxo-transfer catalyst for various organic transformations, such as acylation reaction, hydrosilylation of aldehydes and ketones, oxidation of alcohols and thiols [1]. In the epoxidation of olefins, many reports on such homogeneous molybdenum complexes have been described. Within this context, Kuhn et al. reported that MoO2Br2L2 complexes with ⇑ Corresponding author. 1
E-mail address: [email protected] (P. Van Der Voort). Both authors contributed equally to this work.
http://dx.doi.org/10.1016/j.jcat.2014.05.019 0021-9517/Ó 2014 Elsevier Inc. All rights reserved.
bidentate nitrogen donor ligands exhibit very high selectivities (90%) in the epoxidation of cyclooctene applying TBHP in decane as oxidant [2,3]. Amaranta et al. examined Mo-complexes of the type MoO2Cl2L with L = bipyridine-based ligands as catalysts for the epoxidation of the biorenewable olefins DL-limonene and methyl oleate [4]. The Mo catalysts rendered the epoxide monomers in high selectivity and high conversions (89% selectivity for 96% limonene and 99% selectivity for 94% methyl oleate conversion). Whereas the previous studies employed TBHP as oxidant, others report on the use of H2O2 in combination with NaHCO3 as cocatalyst to study the Mo-complexes as catalyst in the epoxidation of various olefins showing high conversions and selectivities at room temperature [5,6]. However, as these homogeneous Mo-complexes have some well known disadvantages (difficulties with separation and recyclability), many attempts to immobilize these Mo-complexes have been performed. In the paper of Kühn et al., the different supports and immobilization techniques ranging from direct grafting to tethering via a functionalized spacer ligand are described [7]. Different supports have been applied, for example, silica-based supports e.g. MCM-41 [8–10], polymers [11,12], hybrid materials [13] or ionic liquids [14,15]. However, it should be noted that several reports point out that octahedral coordinated MoVI sites cannot be easily incorporated into the tetrahedral positions of porous silicas (zeolites/mesoporous materials) [16,17].
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Alternatively, attempts have been made to synthesize active metal containing organic frameworks as well. This is another efficient way to heterogenize the homogeneous metal complexes. Metal Organic Frameworks (MOFs) are crystalline porous materials consisting of metal ions held in place by multidendate organic linkers to build up a framework. MOFs have been examined for many potential applications, for example in gas sorption and separation [18–20], sensing [21–23], luminescene [23–25], and catalysis [26,27]. In the latter field, for instance, MIL-47(VIV)-, MIL-125(TiIV), and MoIII-based MOFs have been reported and have shown promising catalytic activity in epoxidation reactions. However, synthesizing MOFs with active metal centers in their highest oxidation state is very difficult. This limits the application in oxidation catalysis. Up to now, no MOFs with MoVI centers have been reported. In this study, we report on the immobilization of a homogeneous MoVI catalyst on a MOF support to obtain a well dispersed single-site catalyst. Yaghi’s group synthesized an Al-based MOF, Al(OH)(bpydc) (bpydc2 = 2,20 -bipyridine-5,50 -dicarboxylate) denoted as MOF-253 [28]. This MOF has free 2,20 -bipyridine sites which form excellent anchoring points for the grafting of metal complexes. The successful post-modification of MOF-253 has been demonstrated by Zou and co-workers for the incorporation of RuCl3 on MOF-253 [29] and by Li et al. [30], who anchored Cu+ ions onto MOF-253 to catalyze the cross-coupling of phenols and alcohols with aryl halides. The post-modification, however, resulted in a significant reduction in surface area and pore volume, which is much higher than the volume taken by the complex, suggesting a partial collapse of the framework during the post-modification. In this work, we used a novel Ga-based MOF, denoted as COMOC-4, which is isostructural to MOF-253 and which is stable in air and water (50 °C for 24 h) [31]. COMOC-4 was applied as host matrix for the preparation of a MOF supported MoVI catalyst. To the best of our knowledge, this is the first post-modified MOF possessing MoVI active centers. So far, there is only one MoII-based MOF, denoted as TUDMOF-1 which is isotopic to Cu3(BTC)2 [32]. However, this MOF is very air sensitive which limits its practical use. In another report, [Ni2(dhtp)], a member of the isostructural CPO-27 or MOF-74 series, was post-modified with Mo via Mo(CO)6 sublimation however, during the process, the MoVI was reduced to MoIV [33]. In this paper, we employed MoO2Cl2(THF)2 as a Mo precursor to anchor different MoVI loadings on the COMOC-4 framework. The important feature of this MOF support is that the chelating bipyridine ligand binds strongly to the metal ion, and therefore, leaching of the metal ion is expected to be reduced. The resulting Mo@COMOC-4 materials were evaluated in the epoxidation of cyclohexene, cyclooctene and cyclododecene. We also investigated the recyclability and stability of the heterogeneous catalyst. Furthermore, (scanning) transmission electron microscopy ((S)TEM) provided additional valuable structural information on this supported catalyst. The (S)TEM images demonstrate that even at high Mo-complex loadings on the COMOC-4 framework, no aggregation or any Mo or Mo oxide species are formed. 2. Materials and methods 2.1. General procedures All chemicals were purchased from Sigma-Aldrich or TCI Europe and used without further purification. Nitrogen adsorption experiments were carried out at 196 °C using a Belsorp-mini II gas analyzer. Prior to analysis, the samples were dried under vacuum at 120 °C to remove adsorbed water. X-ray powder diffraction (XRPD) patterns were collected on a ARL X’TRA X-ray diffractometer with Cu Ka radiation of 0.15418 nm wavelength and a solid-state
detector. X-ray fluorescence (XRF) measurements were taken on a NEX CG from Rigaku using a Mo-X-ray source. All XPS measurements were recorded on an X-ray photoelectron spectroscopy S-Probe XPS spectrometer with monochromated Al (1486 eV) exciting radiation from Surface Science Instruments (VG). The catalyst powder was positioned on conducting carbon tape. In order to compensate for charging of the sample, a nickel grid was used, placed 2 mm above the sample. A low-energy electron flood gun 3 eV was used as a neutralizer. All measurements were calibrated toward a value for the C 1s peak of adventitious carbon at 284.6 eV. Calculation of the atomic concentrations and peak fittings was performed using a linear background subtraction. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were recorded on a Thermo Nicolet 6700 FT-IR spectrometer equipped with a N2 cooled MCT-A (mercury– cadmium–tellurium) detector and a KBr beam splitter. An ultra-fast GC equipped with a flame ionization detector (FID) and a 5% diphenyl/95% polydimethylsiloxane column, with 10 m length and 0.10 mm internal diameter was used to follow the conversions of the products during the catalytic tests. Helium was used as carrier gas and the flow rate was programmed as 0.8 mL/min. The reaction products were identified with a TRACE GC GC (Thermo, Interscience), coupled to a TEMPUS TOF-MS detector (Thermo, Interscience). The first column consists of a dimethyl polysiloxane package and has a length of 50 m, with an internal diameter of 0.25 mm, whereas the second column has a length of 2 m with an internal diameter of 0.15 mm. The package of the latter is a 50% phenyl polysilphenylene-siloxane. Helium was used as carrier gas with a constant flow (1.8 mL/min). 2.2. Synthesis of COMOC-4 COMOC-4 was synthesized according to our optimized procedure published elsewhere [31]. In general, 4.4 mmol Ga(NO3)3H2O and 5 mmol H2bpydc were added to 120 mL DMF and stirred at 150 °C for 48 h. Hereafter, the precipitate was filtered and washed, respectively, with DMF, methanol, and acetone. To remove the unreacted linker from the pores, an extraction in DMF was performed at 80 °C for 2 h. In addition, a soxhlet extraction in methanol was carried out during 48 h to obtain a complete exclusion of the organic species. The resulting COMOC-4 material was activated prior to use. 2.3. Synthesis of Mo@COMOC-4 In a first step, the MoO2Cl2(THF)2 complex was prepared according to the procedure reported by Kühn et al. [3]. It should be noted that this complex must be used immediately after synthesis as this complex is very unstable. All the manipulations to obtain Mo@COMOC-4 were carried out under an oxygen and water-free atmosphere with standard Schlenk techniques. Typically, 0.18 g MoO2Cl2 was added to 7.5 mL THF and stirred for 10 min at room temperature. The yellow solution was removed from the non-dissolved residue by employing a combined nylon membrane filter and evaporated to dryness to obtain the MoO2Cl2(THF)2 complex. In a second step, the obtained MoO2Cl2(THF)2 complex was redissolved in THF and COMOC-4 was added. Typically, 3 mL of the MoO2Cl2(THF)2 solution was added to 0.25 g COMOC-4 material suspended in 37 mL of THF to obtain a high Mo loading material (25 mol% Mo, equals to 25% occupation of the bipyridine sites), namely Mo0.25@COMOC-4. As for low Mo loading material, 1.5 mL of the MoO2Cl2(THF)2 solution was added to 0.25 g COMOC-4 material suspended in 38.5 mL of THF to obtain a low Mo loading (14 mol% Mo, equals to 14% occupation of bipyridine sites), namely Mo0.14@COMOC-4. The Mo loading was determined by means of XRF measurements, indicating a loading efficiency of 63% for high Mo loading and 59% for low Mo loading. After stirring
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at RT for 1.5 h, the solid material was filtered off and washed several times with acetone to remove unreacted Mo-complex. The obtained air stable solid was dried overnight under vacuum and activated prior to use. 2.4. Synthesis of MoO2Cl2 [Me2bpydc] All the manipulations were carried out under an oxygen and water-free atmosphere. The dioxomolybdenum(VI) complex MoO2Cl2[Me2bpydc] was synthesized according to the reported literature procedure [34]. 0.17 g (0.5 mmol) MoO2Cl2(THF)2 was dissolved in 10 mL THF and treated with 0.15 g (0.55 mmol) Me2bpydc ligand. The mixture was stirring at RT for 30 min, afterward, filtered off under Ar protection. The resulting filtrate was evaporated to dryness under vacuum at room temperature, and the product was washed with diethyl ether (2 times 5 mL) and dried under vacuum. Calcd for C14H12N2Cl2MoO6: C:35.69; H:2.57; N:5.95 Found: C:35.83; H:2.49; N:6.07. 2.5. Catalytic setup In a typical catalytic test, a Schlenk flask was loaded with 30.0 mL chloroform as solvent and 9 mL of tert-butyl hydroperoxide dissolved in decane employed, respectively, as oxidant and internal standard. The substrates examined in this study are cyclohexene, cyclooctene and cyclododecene (25 mmol). The substrate/ oxidant molar ratio was 1/2. The Mo@COMOC-4 sample having 0.18 mmol of Mo active sites was employed as catalyst. When COMOC-4 was examined, 0.18 mmol of Ga sites was used in the catalytic tests. The molar ratio of substrate:oxidant:catalyst is 140:280:1. All the catalytic tests were performed at a temperature of 50 °C, a temperature at which all the blank tests showed no conversion of substrate. During each test, aliquots were gradually taken out of the mixture and diluted with 500 lL ethylacetate and subsequently analyzed by GC-FID. All the fresh catalysts were activated under vacuum at 120 °C for 3 h prior to catalysis. After each catalytic run, the catalyst was recovered by filtration, washed with acetone, and dried at RT overnight under vacuum to reuse it in another run. 2.6. Transmission electron microscopy The samples were prepared for investigation by crushing the as-received powder in a mortar and placing the powder onto a holey carbon grid. Bright-field transmission electron microscopy (BF-TEM) investigations were performed using a FEI Tecnai G2 electron microscope operated at 200 kV and a JEOL 3000F electron microscope operated at 300 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), annular dark-field scanning transmission electron microscopy (ADF-STEM), and energy dispersive X-ray spectroscopy (EDX) investigations were performed on aberration-corrected FEI Titan ‘‘cubed’’ microscope, operated in STEM mode at 120 kV acceleration voltage. The convergence semi-angle a for STEM was 22 mrad, the inner acceptance semi-angle b for ADF-STEM imaging was 22 mrad, and the acceptance semi-angle b for HAADF-STEM imaging was 90 mrad.
Scheme 1. Schematic representation of the structure of the homogeneous Mocomplex(MoO2Cl2[Me2bpydc]) and the heterogeneous Mo@COMOC-4 material (MoO2Cl2@COMOC-4).
a second metal site to coordinate to them. A schematic view of the obtained Mo@COMOC-4 structure is shown in Scheme 1. The MoO2Cl2(THF)2 complex is known to be a highly active but unstable catalyst. This complex gains stability when it coordinates to the organic ligands with donor functionalities, such as bipyridine sites, while the catalytic activity is maintained. As this complex is such a strong oxidation catalyst, the stability of the support toward oxidation is an important issue. Besides COMOC4, MOF-253, the aluminum 2,20 -bipyridine-5,50 -dicarboxylate framework was also examined as a host matrix to graft the MoVI complex. However, at a loading of 25% of the Mo-complex, the surface area dropped drastically of the latter material (6 times lower compared to the parent MOF). The same behavior was observed by Zou’s group as well [29]. In contrast to MOF-253, the COMOC4 material shows only a minor decrease in surface area when 25% of the Mo species is loaded onto the material (Table 1). 3.2. Characterization of Mo@COMOC-4 3.2.1. X-ray diffraction, nitrogen adsorption and thermogravimetric analysis Two Mo loadings of supported COMOC-4 catalyst were investigated; each has a different percentage of the bipyridine moieties that are functionalized with the Mo-complex: one with a high Mo loading (25% occupation of the bipyridine sites, Mo/Ga molar ratio is 0.25) and one with a low loading of Mo (14% occupation of the bipyridine sites, Mo/Ga molar ratio is 0.14), respectively, denoted as Mo0.25@COMOC-4 and Mo0.14@COMOC-4. XRPD patterns were collected of all the Mo@COMOC-4 materials. As can be seen from Fig. 1, the crystallinity of all the functionalized materials is well preserved after the post-modification process.
Table 1 Properties of the catalytic materials used in this work.
3. Results and discussion
Catalyst
Mo (wt%)a
Slang (m2 g1)
Pore volume (cm3 g1)
3.1. Synthesis and structural information
Mo0.14@COMOC-4 Mo0.25@COMOC-4 COMOC-4
3.9 6.8 –
630 500 770
0.67 0.62 0.69
The parent MOF (COMOC-4), is a gallium based 2,20 -bipyridine5,50 -dicarboxylate framework with 1D microporous channels. The bipyridine sites are located on the walls of the channels, allowing
a The wt% of Mo loading is calculated based on the empirical formula of the catalysts, C12H7N2Cl0.28GaMo0.14O5.28 for Mo0.14@COMOC-4 and C12H7N2Cl0.5GaMo0.25O5.5 for Mo0.25@COMOC-4.
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Fig. 1. XRPD pattern of COMOC-4, Mo0.25@COMOC-4 and Mo0.14@COMOC-4. The diffraction signal (indicated as a *) is due to the background of the sample holder.
The introduction of metals slightly decreased the intensity of the reflections, especially the diffraction peak at 6.5°, which is related to the (1 0 1) planes that are parallel to the linkers (both directions are equivalent because of symmetry). The incorporation of the Mo-complex induced slight changes in the shape and angle of the linkers. This results in a decrease in the intensity and in a slight broadening of the peaks as observed in the XRPD patterns. However, as can be seen from Fig. 1, these effects are rather minor. In Fig. 2, the nitrogen adsorption isotherms of COMOC-4 and the functionalized Mo@COMOC-4 materials are presented. After functionalization, the surface area (Langmuir) of COMOC-4 drops very moderately for the Mo0.14@COMOC-4 and Mo0.25@COMOC-4, respectively. The corresponding pore volumes of the Mo@COMOC-4 materials are slightly lower than the starting material, which is clearly due to the incorporation of the Mo-complex. Moreover, the thermal stability of Mo@COMOC-4 in comparison with the parent COMOC-4 material has been examined (see Fig. S1, Supporting information). From this figure, one can clearly see that the thermal stability of the Mo@COMOC-4 is only slightly reduced. In comparison with COMOC-4 which starts to decompose from 300 °C, the decomposition temperature of Mo@COMOC-4 is 250 °C.
Fig. 2. Nitrogen adsorption isotherms of COMOC-4, Mo0.25@COMOC-4 and Mo0.14@COMOC-4.
3.2.2. Spectroscopy analysis In Fig. 3 the DRIFT spectrum is presented of COMOC-4 and Mo0.25@COMOC-4. As can be seen, the characteristic vibrations of the framework are still present in the post-modified material. Vibrations due to the benzene ring can be observed in the region at 1510–1450 cm1 (aromatic ring stretch), at 1225–950 cm1 (aromatic C–H in plane bend), and at 900–670 cm1 (aromatic C–H out of plane bend). The vibrations in the region 1597– 1616 cm1 and 1415–1463 cm1 can be attributed respectively to the asymmetric and symmetric –CO2 stretching vibrations. Besides the characteristic vibrations which are present in both materials, the Mo0.25@COMOC-4 material exhibits two extra vibrations at 910 cm1 and 947 cm1, which can be assigned to the msym(O@Mo@O) and masym(O@Mo@O) vibrations, respectively [35]. Additionally XPS measurements were taken to verify the oxidation state of the Mo in Mo@COMOC-4. The results are presented in Fig. 4. In both samples (before and after catalysis), Mo is present in the oxidation state +6. The molybdenum 3d peak was deconvoluted with a fixed energy split of 3.2 eV and a fixed area ratio of 3/2 for Mo3d5/2 versus Mo 3d3/2. Peaks are situated at an average value 232.38 and 235.56 eV, pointing to an oxidation state of +6.
Fig. 3. DRIFT spectrum of COMOC-4 and Mo0.25@COMOC-4.
Fig. 4. The Mo 3d3/2 and Mo 3d5/2 peak of Mo@COMOC-4 before and after catalysis.
K. Leus et al. / Journal of Catalysis 316 (2014) 201–209
The results show no indication at all for changes in the oxidation state of molybdenum. 3.2.3. High-resolution(scanning) transmission electron microscopy The unloaded COMOC-4 sample (Fig. S3 in Supplementary information) consists of agglomerated COMOC-4 nanoparticles with sizes ranging from 10 to 100 nm. After loading, the samples retain their initial morphology, as can be seen from the bright-field TEM image in Fig. 5a. The presence of Mo in the sample is confirmed by the EDX spectrum in Fig. 5e. No (large) Mo nanoparticles were detected in HRTEM images of the sample (Fig. 5b), indicating that the Mo should be present as either single atoms or ultra-small atomic clusters. In the high-resolution ADF-STEM (Fig. 5c) and HAADF-STEM images (Fig. 5d), atom-size bright-contrast features are clearly visible (examples marked by white arrows). As the image contrast in HAADF-STEM is proportional to Z 1.7, these bright features should correspond to either single heavy Mo (Z = 42) or Ga (Z = 31) atoms or few atom clusters. The limited difference in Z number between Ga and Mo makes a definitive distinction between Mo and Ga difficult. To clarify the image contrast observed in the HAADF-STEM images, an Al-based COMOC-4 analogue, MOF-253, with larger Z number difference between Mo and the Al (Z = 14) metal center, was also investigated. In this case (Fig. S4 in Supplementary info), the contrast between the Mo single atoms and few atom clusters and the Al MOF is much
Fig. 5. (a) Low magnification TEM image of Mo0.25@COMOC-4, (b) HRTEM image of the Mo0.25@COMOC-4 nanoparticles, (c) ADF-STEM image of a single Mo0.25@COMOC-4 nanoparticle, (d) simultaneously acquired HAADF-STEM image and (e) EDX spectrum acquired from the Mo0.25@COMOC-4 sample.
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higher than in the Ga case. The high-resolution HAADF-STEM image in Fig. S4 therefore provides clear evidence for the presence of single Mo atoms and few atom clusters as a result of the loading procedure used in this work. The bright-contrast features in the HAADF-STEM images in Fig. 5 can therefore also be interpreted as Mo single atoms or few atom clusters.
3.3. Catalytic tests As observed from the HRTEM micrographs of the COMOC-4 samples, the COMOC-4 material consists of nanoparticles, with particle sizes ranging from 10 nm to 100 nm. In this case, it should be noted that the surface catalysis must plays an important role in the catalytic reaction. The COMOC-4 material has 1D channels. After the grafting of the MoO2Cl2 complex, the estimated pore window is 1 nm. Two Mo loadings (14 mol% and 25 mol%) of supported COMOC-4 catalyst were investigated. In general, the catalytic performance of the Mo@COMOC-4 catalysts for the epoxidation reactions was investigated using cis-cyclooctene as a model substrate and t-BuOOH as the oxygen donor. In a control experiment carried out without catalyst, no reaction occurred at all, whereas in the presence of the Mo@COMOC-4 catalysts, the epoxide was obtained almost quantitatively during a reaction time of 24 h, indicating that these catalysts exhibit excellent product selectivity. In all the catalytic experiments, the Mo0.25@COMOC-4 sample was employed as a catalyst unless mentioned otherwise. In Fig. 6 left, the cyclooctene and cyclododecene conversion and the formed products are presented employing Mo0.25@COMOC-4 as catalyst. As can be seen from this figure, a cyclooctene conversion of 67.6% is observed after a reaction time of 24 h, whereas for cyclododecene, only 42.4% of conversion is noted. This difference in substrate conversion can be attributed to the different sizes of the substrates which will result in a significant difference in the reaction kinetics. It should be noted that for both cycloalkenes, a selectivity of almost 100% is observed toward the epoxide, which is consistent with the literature reports [7]. As shown in Fig. 6 right, the homogeneous catalyst MoO2Cl2[Me2bpydc] (structure shown in Scheme 1) shows full conversion of cyclooctene after 4 h of reaction with a selectivity of almost 100% toward the epoxide. Although the efficiency of the heterogeneous catalyst is somewhat lower than the homogenous catalyst, the selectivity is well maintained. We also tested the catalytic activity of the COMOC-4 material for the oxidation of cyclooctene. In this case, 24% conversion after a reaction time of 24 h is observed, which is due to the lewis acidity of the COMOC-4. Although cyclododecene has a significant larger kinetic diameter compared to cyclooctene (respectively 7.8 and 5.7 Å) Mo0.25@COMOC-4 is still able to epoxidize cyclododecene as can be seen from Fig. 6 left. This is probably due to surface catalysis on the Mo sites located on the exterior. To find additional proof, two Mo@COMOC-4 samples with different Mo loadings were evaluated as catalysts in the epoxidation of cyclododecene: Mo0.14@COMOC4 and Mo0.25@COMOC-4. It is important to note that in the catalytic experiments, the amount of catalyst is adapted, so that the reactor is charged with an equal amount of Mo sites. This implies that there is more Mo0.14@COMOC-4 added to the reactor than Mo0.25@COMOC-4. This means also that there is much more ‘‘surface’’ present in the reactor for the Mo0.14@COMOC-4 material. The conversion plots of both catalyst with equal amount of Mo sites and the epoxide formation are presented in Fig. 7. It is interesting to note that during the first 8 h of reaction, the Mo0.14@COMOC-4 sample exhibits a higher cyclododecene conversion in comparison with Mo0.25@COMOC-4. This might be due to the different surface coverage of the two catalysts. The active surface sites are certainly easier to reach than the active sites inside the channels. On the
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Fig. 6. Left: cyclooctene(Cy8) and cyclododecene(Cy12) conversion and formed products employing Mo0.25@COMOC-4 as catalyst. Right: Cyclooctene (Cy8) conversions and formed products employing the homogeneous MoO2Cl2[Me2bpydc] complex, Mo0.25@COMOC-4 and COMOC-4 as catalysts. Reaction conditions: substrate: cyclooctene or cyclododecene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C.
Table 2 Summary of the Mo0.25@COMOC-4 catalyst in oxidation of cycloalkenes.
Fig. 7. Cyclododecene conversion and epoxide formation for Mo0.25@COMOC-4 and Mo0.14@COMOC-4. Reaction conditions: substrate: cyclododecene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C.
other hand, after 24 h, the total conversion was not affected by the loading difference. The catalytic performance was further evaluated for the oxidation of cyclohexene. As shown in Fig. 8 and Table 2, entry 5,
Fig. 8. Cyclohexene conversion and epoxide formation for Mo0.25@COMOC-4 at two temperatures. Reaction conditions: substrate: cyclohexene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C and 80 °C.
Entry
Product
T (h)
T (°C)
Conversion/selectivity (%)
1 2 3 4 5 6
Cyclooctene oxide Cyclododecene oxide Cyclooctene oxide Cyclododecene oxide Cyclohexene oxide Cyclohexene oxide
24 24 7 7 7 7
50 50 50 50 50 80
67.6/>99.9 42.4/>99.9 30.3/>99.9 14.4/>99.9 21.2/66.5 80.8/>99.9
cyclohexene oxide is observed as the predominant product with a selectivity of 66.5% at 50 °C; however, 2-cyclohexene-1-one and 2-cyclohexene-1-ol are also detected, and the formation of the latter products is obtained by the allylic oxidation of cyclohexene. The epoxide selectivity increases with an increase in reaction temperature, and such behavior was also reported on a MoO2Cl2(OPMePh2)2 complex supported on silica (Table 3, entry 6) [9]. At 80 °C, cyclohexene oxide is the only product (Table 2, entry 6). As the temperature rises, there is an increase in conversion, and the cyclohexene conversion reached a value of 80.8% at 80 °C, while at 50 °C, only 21.2% cyclohexene conversion was observed. Sharpless [36] first carried out the epoxidation of cyclododecene by TBHP in the presence of Mo(CO)6 and 18O enriched water, and they concluded that no peroxo compound was formed in the reaction and that an intact OO-t-Bu group was the active species. To assure that the formation of the epoxide is not due to the presence of radical mechanisms, the same catalytic tests were repeated in the presence of a radical inhibitor, hydroquinone (ratio alkene:hydroquinone is 1:0.05). An equal conversion and product formation were observed for both substrates in comparison with the tests without the presence of the inhibitor which corroborates that the reaction occurs via a non-radical mechanism which has also been stated in the literature [37]. Compared to published Mo-based heterogeneous catalyst systems, as summarized in Table 3, all these Mo-based catalysts including Mo@COMOC-4 show good selectivity in epoxidation of cyclooctene. Mo@COMOC-4 is a highly active catalyst. For instance, Mo@COMOC-4-catalyzed cyclooctene epoxidation reaches similar epoxide conversion and selectivity (entry 2) than MoO2Cl2[4,40 dimethyl-2,20 -bipyridine] supported on MCM-41 under similar conditions (entry 3). However, a high Mo leaching was observed on the silica-based material. The reaction temperature has a great influence on the reaction kinetics in the epoxidation of cyclohexene, but the temperature also affects the selectivity. An allylic oxidation of cyclohexene was observed at low reaction temperature for Mo@COMOC-4 and MoO2Cl2(OPMePh2)2 supported on silica (entry 5 and 6).
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K. Leus et al. / Journal of Catalysis 316 (2014) 201–209 Table 3 Comparison of catalytic activity of Mo0.25@COMOC-4 with other Mo-based heterogeneous catalysts in the epoxidation of cycloalkenes.
a b c
Entry
Catalyst
Oxidant
Major product
Oxidant to substrate molar ratio
T (h)
T (°C)
Conversion/selectivity
Ref.
1 2 3 4 5 6 7
Mo0.25@COMOC-4 Mo0.25@COMOC-4 MoO2Cl2[L] on MCM-41b MoO2Cl2(OPMePh2)2 on silica Mo0.25@COMOC-4 MoO2Cl2(OPMePh2)2 on silica Polymer supported MoO2(ligand)na
TBHP TBHP TBHP TBHP TBHP TBHP H2O2
Cyclooctene oxide Cyclooctene oxide Cyclooctene oxide Cyclooctene oxide Cyclohexene oxide Cyclohexene oxide Cyclohexane-1,2-diol
2 2 1.5 1.5 2 1.5 2
7 24 24 8 7 7 6
50 50 55 80 50 55 80
30.3/>99.9 67.6/>99.9 74.1/100c 99/99 21.2/66.5 35/84 66/83.4
This work This work [38] [9] This work [9] [12]
Ligand: 2-thiomethylbenzimidazole. L = 4,40 -dimethyl-2,20 -bipyridine. Catalytic activity decreased drastically in the 2nd run due to the severe leaching.
It is well known that early transition metal ions in their highest oxidation state such as Mo(VI) tend to be stable toward changes in their oxidation state. Consequently, in epoxidation reactions with H2O2 or alkylhydroperoxides, they form adducts (M-OOH and MOOR) which are the key intermediates in the epoxidation, and the role of the metal ion is that of a Lewis acid site. The metal center acts as a Lewis acid by removing charge from the O–O bond, facilitating its dissociation, and activating the nearest oxygen atom (proximal oxygen) for insertion into the olefin double bond, whereas the distal oxygen constitutes a good leaving group in the form of –OtBu (see Scheme 2). In our catalytic system, the oxidizing agent TBHP is transformed to tert-butyl alcohol during the reaction, which can coordinate to the Mo(VI) center and consequently slows down the reaction. 3.4. Stability and regenerability tests A hot filtration was carried out. After 4 h of catalysis, Mo0.25@COMOC-4 was filtered off and the reaction mixture was followed on during 20 h. Fig. 9 shows that the epoxide formation
Fig. 9. Hot filtration experiment of Mo0.25@COMOC-4. Reaction conditions: substrate: cyclooctene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C.
Scheme 2. Accepted alkylperoxo mechanism of Mo-catalyzed epoxidation with hydroperoxides[37].
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Table 4 TON, TOF, selectivity toward the epoxide and leaching results for Mo@COMOC-4 for each run. Examined substrate: cyclooctene.
a b c
Run
TONa
TONb
TOFc
Selectivity epoxide (%)
Leaching Ga (%)
Leaching Mo (%)
1 2 3
48 35 42
82 76 65
12 6 8
>99.9 >99.9 >99.9
0.13 0.16 0.3
ND ND ND
The TON value was calculated after 7 h of catalysis. The TON value calculated after 24 h of catalysis. The TOF value was calculated after 0.5 h of catalysis.
stops after the removal of the catalyst, which indicates that the catalysis occurs truly heterogeneous. To examine the regenerability of the Mo0.25@COMOC-4, three consecutive runs were carried out. In Table 4, the TON, TOF, selectivity, and the leaching percentage of Mo and Ga are shown for each run. The TON and TOF values remain fairly constant for each run which demonstrates the regenerability of the catalyst. Additionally, the selectivity of the catalyst stays unaltered during these additional runs. No detectable leaching of the Mo-complex is observed which probably indicates that the Mo leaching is below the detection limit of the XRF. Furthermore, only a very small amount of Ga was leached out during these extra runs. Comparison of the XRPD pattern of Mo@COMOC-4 before catalysis and after each run shows that the framework integrity of the MOF is preserved during the three runs (see Fig. 10). Additional proof for the stability of the framework was seen in the nitrogen adsorption measurements (see Fig. S2). Comparison of the Mo@COMOC-4 material after the third run with the pristine Mo@COMOC-4 sample shows no loss in surface area. Moreover, SEM analysis has been carried out (see Fig. S5). As can be seen from
the SEM pictures, the morphology and size of the Mo@COMOC-4 material is preserved after catalysis. 4. Conclusions The Ga(OH)(bpydc) MOF (COMOC-4) was successfully grafted with the MoO2Cl2(THF)2 complex and examined as an epoxidation catalyst. The COMOC-4 framework maintains its volume and crystallinity during the post-modification process and during the catalytic tests. Moreover the Mo-complex does not leach into the solution during catalysis, making the Mo@COMOC-4 a stable and recyclable catalyst. The Mo@COMOC-4 is an efficient catalyst in the epoxidation of cycloalkenes with a selectivity up to 100% toward the epoxide. We believe that the COMOC-4 framework might be used as a host for the heterogenization of many other interesting metal complexes. Acknowledgments The authors acknowledge the financial support from the Ghent University BOF postdoctoral Grants 01P02911T (Y.Y.L) and 01P068135 (K.L.) and UGent GOA Grant 01G00710. Y.Y.L. acknowledges the Fundamental Research Fund for the Central Universities of China (DUT13RC(3)85). We acknowledge the Long Term Structural Methusalem Grant No. 01M00409 funding by the Flemish Government. S.T. gratefully acknowledges the FWO Flanders for a post-doctoral scholarship. This work was supported by funding from the European Research Council under the Seventh Framework Program (FP7), ERC Grant No. 246791 – COUNTATOMS. The Titan microscope used for this study was partially funded by the Hercules foundation of the Flemish Government. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2014.05.019. References
Fig. 10. XRPD pattern of Mo@COMOC-4 before catalysis and after each run. The diffraction signal (indicated as a *) is due to the background of the sample holder.
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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
A MoVI grafted Metal Organic Framework: Synthesis, characterization and catalytic investigations Karen Leus a,1, Ying-Ya Liu b,1, Maria Meledina c, Stuart Turner c, Gustaaf Van Tendeloo c, Pascal Van Der Voort a,⇑ a b c
Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024 Dalian, China Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium
a r t i c l e
i n f o
Article history: Received 18 February 2014 Revised 1 May 2014 Accepted 18 May 2014
Keywords: Metal Organic Framework Oxidation Catalysis Molybdenum Post-modification
a b s t r a c t We present the post-modification of a gallium based Metal Organic Framework, COMOC-4, with a Mocomplex. The resulting Mo@COMOC-4 was characterized by means of N2 sorption, XRPD, DRIFT, TGA, XRF, XPS and TEM analysis. The results demonstrate that even at high Mo-complex loadings on the framework, no aggregation or any Mo or Mo oxide species are formed. Moreover, the Mo@COMOC-4 was evaluated as a catalyst in the epoxidation of cyclohexene, cyclooctene and cyclododecene employing TBHP in decane as oxidant. The post-modified COMOC-4 exhibits a very high selectivity toward the epoxide (up to 100%). Regenerability and stability tests have been carried out demonstrating that the catalyst can be recycled without leaching of Mo or loss of crystallinity. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction It is well known that compounds containing certain transition metals, such as Mo, W, Ti and V, are able to catalyze the liquidphase epoxidation of olefins using alkyl hydroperoxides as oxidants in homogeneous solution. The catalysis is most effective when these transition metals are in their highest oxidation state. During the last decade, several molybdenum (VI) complexes have shown to be successful catalysts for various reactions ranging from Lewis acid catalyzed transformations to oxidation and reduction reactions [1]. Treatment of [MoO2X2] species (X = halide, OR, OSiR3) with monodentate or bidentate Lewis bases (L or L2), such as pyridine and 2,20 -bipyridine, in the presence of a donor solvent, form a series of dioxomolybdenum complexes with the composition of [MoO2X2L]. Such dioxomolybdenum(VI) complexes are well documented as efficient oxo-transfer catalyst for various organic transformations, such as acylation reaction, hydrosilylation of aldehydes and ketones, oxidation of alcohols and thiols [1]. In the epoxidation of olefins, many reports on such homogeneous molybdenum complexes have been described. Within this context, Kuhn et al. reported that MoO2Br2L2 complexes with ⇑ Corresponding author. 1
E-mail address: [email protected] (P. Van Der Voort). Both authors contributed equally to this work.
http://dx.doi.org/10.1016/j.jcat.2014.05.019 0021-9517/Ó 2014 Elsevier Inc. All rights reserved.
bidentate nitrogen donor ligands exhibit very high selectivities (90%) in the epoxidation of cyclooctene applying TBHP in decane as oxidant [2,3]. Amaranta et al. examined Mo-complexes of the type MoO2Cl2L with L = bipyridine-based ligands as catalysts for the epoxidation of the biorenewable olefins DL-limonene and methyl oleate [4]. The Mo catalysts rendered the epoxide monomers in high selectivity and high conversions (89% selectivity for 96% limonene and 99% selectivity for 94% methyl oleate conversion). Whereas the previous studies employed TBHP as oxidant, others report on the use of H2O2 in combination with NaHCO3 as cocatalyst to study the Mo-complexes as catalyst in the epoxidation of various olefins showing high conversions and selectivities at room temperature [5,6]. However, as these homogeneous Mo-complexes have some well known disadvantages (difficulties with separation and recyclability), many attempts to immobilize these Mo-complexes have been performed. In the paper of Kühn et al., the different supports and immobilization techniques ranging from direct grafting to tethering via a functionalized spacer ligand are described [7]. Different supports have been applied, for example, silica-based supports e.g. MCM-41 [8–10], polymers [11,12], hybrid materials [13] or ionic liquids [14,15]. However, it should be noted that several reports point out that octahedral coordinated MoVI sites cannot be easily incorporated into the tetrahedral positions of porous silicas (zeolites/mesoporous materials) [16,17].
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Alternatively, attempts have been made to synthesize active metal containing organic frameworks as well. This is another efficient way to heterogenize the homogeneous metal complexes. Metal Organic Frameworks (MOFs) are crystalline porous materials consisting of metal ions held in place by multidendate organic linkers to build up a framework. MOFs have been examined for many potential applications, for example in gas sorption and separation [18–20], sensing [21–23], luminescene [23–25], and catalysis [26,27]. In the latter field, for instance, MIL-47(VIV)-, MIL-125(TiIV), and MoIII-based MOFs have been reported and have shown promising catalytic activity in epoxidation reactions. However, synthesizing MOFs with active metal centers in their highest oxidation state is very difficult. This limits the application in oxidation catalysis. Up to now, no MOFs with MoVI centers have been reported. In this study, we report on the immobilization of a homogeneous MoVI catalyst on a MOF support to obtain a well dispersed single-site catalyst. Yaghi’s group synthesized an Al-based MOF, Al(OH)(bpydc) (bpydc2 = 2,20 -bipyridine-5,50 -dicarboxylate) denoted as MOF-253 [28]. This MOF has free 2,20 -bipyridine sites which form excellent anchoring points for the grafting of metal complexes. The successful post-modification of MOF-253 has been demonstrated by Zou and co-workers for the incorporation of RuCl3 on MOF-253 [29] and by Li et al. [30], who anchored Cu+ ions onto MOF-253 to catalyze the cross-coupling of phenols and alcohols with aryl halides. The post-modification, however, resulted in a significant reduction in surface area and pore volume, which is much higher than the volume taken by the complex, suggesting a partial collapse of the framework during the post-modification. In this work, we used a novel Ga-based MOF, denoted as COMOC-4, which is isostructural to MOF-253 and which is stable in air and water (50 °C for 24 h) [31]. COMOC-4 was applied as host matrix for the preparation of a MOF supported MoVI catalyst. To the best of our knowledge, this is the first post-modified MOF possessing MoVI active centers. So far, there is only one MoII-based MOF, denoted as TUDMOF-1 which is isotopic to Cu3(BTC)2 [32]. However, this MOF is very air sensitive which limits its practical use. In another report, [Ni2(dhtp)], a member of the isostructural CPO-27 or MOF-74 series, was post-modified with Mo via Mo(CO)6 sublimation however, during the process, the MoVI was reduced to MoIV [33]. In this paper, we employed MoO2Cl2(THF)2 as a Mo precursor to anchor different MoVI loadings on the COMOC-4 framework. The important feature of this MOF support is that the chelating bipyridine ligand binds strongly to the metal ion, and therefore, leaching of the metal ion is expected to be reduced. The resulting Mo@COMOC-4 materials were evaluated in the epoxidation of cyclohexene, cyclooctene and cyclododecene. We also investigated the recyclability and stability of the heterogeneous catalyst. Furthermore, (scanning) transmission electron microscopy ((S)TEM) provided additional valuable structural information on this supported catalyst. The (S)TEM images demonstrate that even at high Mo-complex loadings on the COMOC-4 framework, no aggregation or any Mo or Mo oxide species are formed. 2. Materials and methods 2.1. General procedures All chemicals were purchased from Sigma-Aldrich or TCI Europe and used without further purification. Nitrogen adsorption experiments were carried out at 196 °C using a Belsorp-mini II gas analyzer. Prior to analysis, the samples were dried under vacuum at 120 °C to remove adsorbed water. X-ray powder diffraction (XRPD) patterns were collected on a ARL X’TRA X-ray diffractometer with Cu Ka radiation of 0.15418 nm wavelength and a solid-state
detector. X-ray fluorescence (XRF) measurements were taken on a NEX CG from Rigaku using a Mo-X-ray source. All XPS measurements were recorded on an X-ray photoelectron spectroscopy S-Probe XPS spectrometer with monochromated Al (1486 eV) exciting radiation from Surface Science Instruments (VG). The catalyst powder was positioned on conducting carbon tape. In order to compensate for charging of the sample, a nickel grid was used, placed 2 mm above the sample. A low-energy electron flood gun 3 eV was used as a neutralizer. All measurements were calibrated toward a value for the C 1s peak of adventitious carbon at 284.6 eV. Calculation of the atomic concentrations and peak fittings was performed using a linear background subtraction. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were recorded on a Thermo Nicolet 6700 FT-IR spectrometer equipped with a N2 cooled MCT-A (mercury– cadmium–tellurium) detector and a KBr beam splitter. An ultra-fast GC equipped with a flame ionization detector (FID) and a 5% diphenyl/95% polydimethylsiloxane column, with 10 m length and 0.10 mm internal diameter was used to follow the conversions of the products during the catalytic tests. Helium was used as carrier gas and the flow rate was programmed as 0.8 mL/min. The reaction products were identified with a TRACE GC GC (Thermo, Interscience), coupled to a TEMPUS TOF-MS detector (Thermo, Interscience). The first column consists of a dimethyl polysiloxane package and has a length of 50 m, with an internal diameter of 0.25 mm, whereas the second column has a length of 2 m with an internal diameter of 0.15 mm. The package of the latter is a 50% phenyl polysilphenylene-siloxane. Helium was used as carrier gas with a constant flow (1.8 mL/min). 2.2. Synthesis of COMOC-4 COMOC-4 was synthesized according to our optimized procedure published elsewhere [31]. In general, 4.4 mmol Ga(NO3)3H2O and 5 mmol H2bpydc were added to 120 mL DMF and stirred at 150 °C for 48 h. Hereafter, the precipitate was filtered and washed, respectively, with DMF, methanol, and acetone. To remove the unreacted linker from the pores, an extraction in DMF was performed at 80 °C for 2 h. In addition, a soxhlet extraction in methanol was carried out during 48 h to obtain a complete exclusion of the organic species. The resulting COMOC-4 material was activated prior to use. 2.3. Synthesis of Mo@COMOC-4 In a first step, the MoO2Cl2(THF)2 complex was prepared according to the procedure reported by Kühn et al. [3]. It should be noted that this complex must be used immediately after synthesis as this complex is very unstable. All the manipulations to obtain Mo@COMOC-4 were carried out under an oxygen and water-free atmosphere with standard Schlenk techniques. Typically, 0.18 g MoO2Cl2 was added to 7.5 mL THF and stirred for 10 min at room temperature. The yellow solution was removed from the non-dissolved residue by employing a combined nylon membrane filter and evaporated to dryness to obtain the MoO2Cl2(THF)2 complex. In a second step, the obtained MoO2Cl2(THF)2 complex was redissolved in THF and COMOC-4 was added. Typically, 3 mL of the MoO2Cl2(THF)2 solution was added to 0.25 g COMOC-4 material suspended in 37 mL of THF to obtain a high Mo loading material (25 mol% Mo, equals to 25% occupation of the bipyridine sites), namely Mo0.25@COMOC-4. As for low Mo loading material, 1.5 mL of the MoO2Cl2(THF)2 solution was added to 0.25 g COMOC-4 material suspended in 38.5 mL of THF to obtain a low Mo loading (14 mol% Mo, equals to 14% occupation of bipyridine sites), namely Mo0.14@COMOC-4. The Mo loading was determined by means of XRF measurements, indicating a loading efficiency of 63% for high Mo loading and 59% for low Mo loading. After stirring
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at RT for 1.5 h, the solid material was filtered off and washed several times with acetone to remove unreacted Mo-complex. The obtained air stable solid was dried overnight under vacuum and activated prior to use. 2.4. Synthesis of MoO2Cl2 [Me2bpydc] All the manipulations were carried out under an oxygen and water-free atmosphere. The dioxomolybdenum(VI) complex MoO2Cl2[Me2bpydc] was synthesized according to the reported literature procedure [34]. 0.17 g (0.5 mmol) MoO2Cl2(THF)2 was dissolved in 10 mL THF and treated with 0.15 g (0.55 mmol) Me2bpydc ligand. The mixture was stirring at RT for 30 min, afterward, filtered off under Ar protection. The resulting filtrate was evaporated to dryness under vacuum at room temperature, and the product was washed with diethyl ether (2 times 5 mL) and dried under vacuum. Calcd for C14H12N2Cl2MoO6: C:35.69; H:2.57; N:5.95 Found: C:35.83; H:2.49; N:6.07. 2.5. Catalytic setup In a typical catalytic test, a Schlenk flask was loaded with 30.0 mL chloroform as solvent and 9 mL of tert-butyl hydroperoxide dissolved in decane employed, respectively, as oxidant and internal standard. The substrates examined in this study are cyclohexene, cyclooctene and cyclododecene (25 mmol). The substrate/ oxidant molar ratio was 1/2. The Mo@COMOC-4 sample having 0.18 mmol of Mo active sites was employed as catalyst. When COMOC-4 was examined, 0.18 mmol of Ga sites was used in the catalytic tests. The molar ratio of substrate:oxidant:catalyst is 140:280:1. All the catalytic tests were performed at a temperature of 50 °C, a temperature at which all the blank tests showed no conversion of substrate. During each test, aliquots were gradually taken out of the mixture and diluted with 500 lL ethylacetate and subsequently analyzed by GC-FID. All the fresh catalysts were activated under vacuum at 120 °C for 3 h prior to catalysis. After each catalytic run, the catalyst was recovered by filtration, washed with acetone, and dried at RT overnight under vacuum to reuse it in another run. 2.6. Transmission electron microscopy The samples were prepared for investigation by crushing the as-received powder in a mortar and placing the powder onto a holey carbon grid. Bright-field transmission electron microscopy (BF-TEM) investigations were performed using a FEI Tecnai G2 electron microscope operated at 200 kV and a JEOL 3000F electron microscope operated at 300 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), annular dark-field scanning transmission electron microscopy (ADF-STEM), and energy dispersive X-ray spectroscopy (EDX) investigations were performed on aberration-corrected FEI Titan ‘‘cubed’’ microscope, operated in STEM mode at 120 kV acceleration voltage. The convergence semi-angle a for STEM was 22 mrad, the inner acceptance semi-angle b for ADF-STEM imaging was 22 mrad, and the acceptance semi-angle b for HAADF-STEM imaging was 90 mrad.
Scheme 1. Schematic representation of the structure of the homogeneous Mocomplex(MoO2Cl2[Me2bpydc]) and the heterogeneous Mo@COMOC-4 material (MoO2Cl2@COMOC-4).
a second metal site to coordinate to them. A schematic view of the obtained Mo@COMOC-4 structure is shown in Scheme 1. The MoO2Cl2(THF)2 complex is known to be a highly active but unstable catalyst. This complex gains stability when it coordinates to the organic ligands with donor functionalities, such as bipyridine sites, while the catalytic activity is maintained. As this complex is such a strong oxidation catalyst, the stability of the support toward oxidation is an important issue. Besides COMOC4, MOF-253, the aluminum 2,20 -bipyridine-5,50 -dicarboxylate framework was also examined as a host matrix to graft the MoVI complex. However, at a loading of 25% of the Mo-complex, the surface area dropped drastically of the latter material (6 times lower compared to the parent MOF). The same behavior was observed by Zou’s group as well [29]. In contrast to MOF-253, the COMOC4 material shows only a minor decrease in surface area when 25% of the Mo species is loaded onto the material (Table 1). 3.2. Characterization of Mo@COMOC-4 3.2.1. X-ray diffraction, nitrogen adsorption and thermogravimetric analysis Two Mo loadings of supported COMOC-4 catalyst were investigated; each has a different percentage of the bipyridine moieties that are functionalized with the Mo-complex: one with a high Mo loading (25% occupation of the bipyridine sites, Mo/Ga molar ratio is 0.25) and one with a low loading of Mo (14% occupation of the bipyridine sites, Mo/Ga molar ratio is 0.14), respectively, denoted as Mo0.25@COMOC-4 and Mo0.14@COMOC-4. XRPD patterns were collected of all the Mo@COMOC-4 materials. As can be seen from Fig. 1, the crystallinity of all the functionalized materials is well preserved after the post-modification process.
Table 1 Properties of the catalytic materials used in this work.
3. Results and discussion
Catalyst
Mo (wt%)a
Slang (m2 g1)
Pore volume (cm3 g1)
3.1. Synthesis and structural information
Mo0.14@COMOC-4 Mo0.25@COMOC-4 COMOC-4
3.9 6.8 –
630 500 770
0.67 0.62 0.69
The parent MOF (COMOC-4), is a gallium based 2,20 -bipyridine5,50 -dicarboxylate framework with 1D microporous channels. The bipyridine sites are located on the walls of the channels, allowing
a The wt% of Mo loading is calculated based on the empirical formula of the catalysts, C12H7N2Cl0.28GaMo0.14O5.28 for Mo0.14@COMOC-4 and C12H7N2Cl0.5GaMo0.25O5.5 for Mo0.25@COMOC-4.
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Fig. 1. XRPD pattern of COMOC-4, Mo0.25@COMOC-4 and Mo0.14@COMOC-4. The diffraction signal (indicated as a *) is due to the background of the sample holder.
The introduction of metals slightly decreased the intensity of the reflections, especially the diffraction peak at 6.5°, which is related to the (1 0 1) planes that are parallel to the linkers (both directions are equivalent because of symmetry). The incorporation of the Mo-complex induced slight changes in the shape and angle of the linkers. This results in a decrease in the intensity and in a slight broadening of the peaks as observed in the XRPD patterns. However, as can be seen from Fig. 1, these effects are rather minor. In Fig. 2, the nitrogen adsorption isotherms of COMOC-4 and the functionalized Mo@COMOC-4 materials are presented. After functionalization, the surface area (Langmuir) of COMOC-4 drops very moderately for the Mo0.14@COMOC-4 and Mo0.25@COMOC-4, respectively. The corresponding pore volumes of the Mo@COMOC-4 materials are slightly lower than the starting material, which is clearly due to the incorporation of the Mo-complex. Moreover, the thermal stability of Mo@COMOC-4 in comparison with the parent COMOC-4 material has been examined (see Fig. S1, Supporting information). From this figure, one can clearly see that the thermal stability of the Mo@COMOC-4 is only slightly reduced. In comparison with COMOC-4 which starts to decompose from 300 °C, the decomposition temperature of Mo@COMOC-4 is 250 °C.
Fig. 2. Nitrogen adsorption isotherms of COMOC-4, Mo0.25@COMOC-4 and Mo0.14@COMOC-4.
3.2.2. Spectroscopy analysis In Fig. 3 the DRIFT spectrum is presented of COMOC-4 and Mo0.25@COMOC-4. As can be seen, the characteristic vibrations of the framework are still present in the post-modified material. Vibrations due to the benzene ring can be observed in the region at 1510–1450 cm1 (aromatic ring stretch), at 1225–950 cm1 (aromatic C–H in plane bend), and at 900–670 cm1 (aromatic C–H out of plane bend). The vibrations in the region 1597– 1616 cm1 and 1415–1463 cm1 can be attributed respectively to the asymmetric and symmetric –CO2 stretching vibrations. Besides the characteristic vibrations which are present in both materials, the Mo0.25@COMOC-4 material exhibits two extra vibrations at 910 cm1 and 947 cm1, which can be assigned to the msym(O@Mo@O) and masym(O@Mo@O) vibrations, respectively [35]. Additionally XPS measurements were taken to verify the oxidation state of the Mo in Mo@COMOC-4. The results are presented in Fig. 4. In both samples (before and after catalysis), Mo is present in the oxidation state +6. The molybdenum 3d peak was deconvoluted with a fixed energy split of 3.2 eV and a fixed area ratio of 3/2 for Mo3d5/2 versus Mo 3d3/2. Peaks are situated at an average value 232.38 and 235.56 eV, pointing to an oxidation state of +6.
Fig. 3. DRIFT spectrum of COMOC-4 and Mo0.25@COMOC-4.
Fig. 4. The Mo 3d3/2 and Mo 3d5/2 peak of Mo@COMOC-4 before and after catalysis.
K. Leus et al. / Journal of Catalysis 316 (2014) 201–209
The results show no indication at all for changes in the oxidation state of molybdenum. 3.2.3. High-resolution(scanning) transmission electron microscopy The unloaded COMOC-4 sample (Fig. S3 in Supplementary information) consists of agglomerated COMOC-4 nanoparticles with sizes ranging from 10 to 100 nm. After loading, the samples retain their initial morphology, as can be seen from the bright-field TEM image in Fig. 5a. The presence of Mo in the sample is confirmed by the EDX spectrum in Fig. 5e. No (large) Mo nanoparticles were detected in HRTEM images of the sample (Fig. 5b), indicating that the Mo should be present as either single atoms or ultra-small atomic clusters. In the high-resolution ADF-STEM (Fig. 5c) and HAADF-STEM images (Fig. 5d), atom-size bright-contrast features are clearly visible (examples marked by white arrows). As the image contrast in HAADF-STEM is proportional to Z 1.7, these bright features should correspond to either single heavy Mo (Z = 42) or Ga (Z = 31) atoms or few atom clusters. The limited difference in Z number between Ga and Mo makes a definitive distinction between Mo and Ga difficult. To clarify the image contrast observed in the HAADF-STEM images, an Al-based COMOC-4 analogue, MOF-253, with larger Z number difference between Mo and the Al (Z = 14) metal center, was also investigated. In this case (Fig. S4 in Supplementary info), the contrast between the Mo single atoms and few atom clusters and the Al MOF is much
Fig. 5. (a) Low magnification TEM image of Mo0.25@COMOC-4, (b) HRTEM image of the Mo0.25@COMOC-4 nanoparticles, (c) ADF-STEM image of a single Mo0.25@COMOC-4 nanoparticle, (d) simultaneously acquired HAADF-STEM image and (e) EDX spectrum acquired from the Mo0.25@COMOC-4 sample.
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higher than in the Ga case. The high-resolution HAADF-STEM image in Fig. S4 therefore provides clear evidence for the presence of single Mo atoms and few atom clusters as a result of the loading procedure used in this work. The bright-contrast features in the HAADF-STEM images in Fig. 5 can therefore also be interpreted as Mo single atoms or few atom clusters.
3.3. Catalytic tests As observed from the HRTEM micrographs of the COMOC-4 samples, the COMOC-4 material consists of nanoparticles, with particle sizes ranging from 10 nm to 100 nm. In this case, it should be noted that the surface catalysis must plays an important role in the catalytic reaction. The COMOC-4 material has 1D channels. After the grafting of the MoO2Cl2 complex, the estimated pore window is 1 nm. Two Mo loadings (14 mol% and 25 mol%) of supported COMOC-4 catalyst were investigated. In general, the catalytic performance of the Mo@COMOC-4 catalysts for the epoxidation reactions was investigated using cis-cyclooctene as a model substrate and t-BuOOH as the oxygen donor. In a control experiment carried out without catalyst, no reaction occurred at all, whereas in the presence of the Mo@COMOC-4 catalysts, the epoxide was obtained almost quantitatively during a reaction time of 24 h, indicating that these catalysts exhibit excellent product selectivity. In all the catalytic experiments, the Mo0.25@COMOC-4 sample was employed as a catalyst unless mentioned otherwise. In Fig. 6 left, the cyclooctene and cyclododecene conversion and the formed products are presented employing Mo0.25@COMOC-4 as catalyst. As can be seen from this figure, a cyclooctene conversion of 67.6% is observed after a reaction time of 24 h, whereas for cyclododecene, only 42.4% of conversion is noted. This difference in substrate conversion can be attributed to the different sizes of the substrates which will result in a significant difference in the reaction kinetics. It should be noted that for both cycloalkenes, a selectivity of almost 100% is observed toward the epoxide, which is consistent with the literature reports [7]. As shown in Fig. 6 right, the homogeneous catalyst MoO2Cl2[Me2bpydc] (structure shown in Scheme 1) shows full conversion of cyclooctene after 4 h of reaction with a selectivity of almost 100% toward the epoxide. Although the efficiency of the heterogeneous catalyst is somewhat lower than the homogenous catalyst, the selectivity is well maintained. We also tested the catalytic activity of the COMOC-4 material for the oxidation of cyclooctene. In this case, 24% conversion after a reaction time of 24 h is observed, which is due to the lewis acidity of the COMOC-4. Although cyclododecene has a significant larger kinetic diameter compared to cyclooctene (respectively 7.8 and 5.7 Å) Mo0.25@COMOC-4 is still able to epoxidize cyclododecene as can be seen from Fig. 6 left. This is probably due to surface catalysis on the Mo sites located on the exterior. To find additional proof, two Mo@COMOC-4 samples with different Mo loadings were evaluated as catalysts in the epoxidation of cyclododecene: Mo0.14@COMOC4 and Mo0.25@COMOC-4. It is important to note that in the catalytic experiments, the amount of catalyst is adapted, so that the reactor is charged with an equal amount of Mo sites. This implies that there is more Mo0.14@COMOC-4 added to the reactor than Mo0.25@COMOC-4. This means also that there is much more ‘‘surface’’ present in the reactor for the Mo0.14@COMOC-4 material. The conversion plots of both catalyst with equal amount of Mo sites and the epoxide formation are presented in Fig. 7. It is interesting to note that during the first 8 h of reaction, the Mo0.14@COMOC-4 sample exhibits a higher cyclododecene conversion in comparison with Mo0.25@COMOC-4. This might be due to the different surface coverage of the two catalysts. The active surface sites are certainly easier to reach than the active sites inside the channels. On the
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Fig. 6. Left: cyclooctene(Cy8) and cyclododecene(Cy12) conversion and formed products employing Mo0.25@COMOC-4 as catalyst. Right: Cyclooctene (Cy8) conversions and formed products employing the homogeneous MoO2Cl2[Me2bpydc] complex, Mo0.25@COMOC-4 and COMOC-4 as catalysts. Reaction conditions: substrate: cyclooctene or cyclododecene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C.
Table 2 Summary of the Mo0.25@COMOC-4 catalyst in oxidation of cycloalkenes.
Fig. 7. Cyclododecene conversion and epoxide formation for Mo0.25@COMOC-4 and Mo0.14@COMOC-4. Reaction conditions: substrate: cyclododecene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C.
other hand, after 24 h, the total conversion was not affected by the loading difference. The catalytic performance was further evaluated for the oxidation of cyclohexene. As shown in Fig. 8 and Table 2, entry 5,
Fig. 8. Cyclohexene conversion and epoxide formation for Mo0.25@COMOC-4 at two temperatures. Reaction conditions: substrate: cyclohexene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C and 80 °C.
Entry
Product
T (h)
T (°C)
Conversion/selectivity (%)
1 2 3 4 5 6
Cyclooctene oxide Cyclododecene oxide Cyclooctene oxide Cyclododecene oxide Cyclohexene oxide Cyclohexene oxide
24 24 7 7 7 7
50 50 50 50 50 80
67.6/>99.9 42.4/>99.9 30.3/>99.9 14.4/>99.9 21.2/66.5 80.8/>99.9
cyclohexene oxide is observed as the predominant product with a selectivity of 66.5% at 50 °C; however, 2-cyclohexene-1-one and 2-cyclohexene-1-ol are also detected, and the formation of the latter products is obtained by the allylic oxidation of cyclohexene. The epoxide selectivity increases with an increase in reaction temperature, and such behavior was also reported on a MoO2Cl2(OPMePh2)2 complex supported on silica (Table 3, entry 6) [9]. At 80 °C, cyclohexene oxide is the only product (Table 2, entry 6). As the temperature rises, there is an increase in conversion, and the cyclohexene conversion reached a value of 80.8% at 80 °C, while at 50 °C, only 21.2% cyclohexene conversion was observed. Sharpless [36] first carried out the epoxidation of cyclododecene by TBHP in the presence of Mo(CO)6 and 18O enriched water, and they concluded that no peroxo compound was formed in the reaction and that an intact OO-t-Bu group was the active species. To assure that the formation of the epoxide is not due to the presence of radical mechanisms, the same catalytic tests were repeated in the presence of a radical inhibitor, hydroquinone (ratio alkene:hydroquinone is 1:0.05). An equal conversion and product formation were observed for both substrates in comparison with the tests without the presence of the inhibitor which corroborates that the reaction occurs via a non-radical mechanism which has also been stated in the literature [37]. Compared to published Mo-based heterogeneous catalyst systems, as summarized in Table 3, all these Mo-based catalysts including Mo@COMOC-4 show good selectivity in epoxidation of cyclooctene. Mo@COMOC-4 is a highly active catalyst. For instance, Mo@COMOC-4-catalyzed cyclooctene epoxidation reaches similar epoxide conversion and selectivity (entry 2) than MoO2Cl2[4,40 dimethyl-2,20 -bipyridine] supported on MCM-41 under similar conditions (entry 3). However, a high Mo leaching was observed on the silica-based material. The reaction temperature has a great influence on the reaction kinetics in the epoxidation of cyclohexene, but the temperature also affects the selectivity. An allylic oxidation of cyclohexene was observed at low reaction temperature for Mo@COMOC-4 and MoO2Cl2(OPMePh2)2 supported on silica (entry 5 and 6).
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K. Leus et al. / Journal of Catalysis 316 (2014) 201–209 Table 3 Comparison of catalytic activity of Mo0.25@COMOC-4 with other Mo-based heterogeneous catalysts in the epoxidation of cycloalkenes.
a b c
Entry
Catalyst
Oxidant
Major product
Oxidant to substrate molar ratio
T (h)
T (°C)
Conversion/selectivity
Ref.
1 2 3 4 5 6 7
Mo0.25@COMOC-4 Mo0.25@COMOC-4 MoO2Cl2[L] on MCM-41b MoO2Cl2(OPMePh2)2 on silica Mo0.25@COMOC-4 MoO2Cl2(OPMePh2)2 on silica Polymer supported MoO2(ligand)na
TBHP TBHP TBHP TBHP TBHP TBHP H2O2
Cyclooctene oxide Cyclooctene oxide Cyclooctene oxide Cyclooctene oxide Cyclohexene oxide Cyclohexene oxide Cyclohexane-1,2-diol
2 2 1.5 1.5 2 1.5 2
7 24 24 8 7 7 6
50 50 55 80 50 55 80
30.3/>99.9 67.6/>99.9 74.1/100c 99/99 21.2/66.5 35/84 66/83.4
This work This work [38] [9] This work [9] [12]
Ligand: 2-thiomethylbenzimidazole. L = 4,40 -dimethyl-2,20 -bipyridine. Catalytic activity decreased drastically in the 2nd run due to the severe leaching.
It is well known that early transition metal ions in their highest oxidation state such as Mo(VI) tend to be stable toward changes in their oxidation state. Consequently, in epoxidation reactions with H2O2 or alkylhydroperoxides, they form adducts (M-OOH and MOOR) which are the key intermediates in the epoxidation, and the role of the metal ion is that of a Lewis acid site. The metal center acts as a Lewis acid by removing charge from the O–O bond, facilitating its dissociation, and activating the nearest oxygen atom (proximal oxygen) for insertion into the olefin double bond, whereas the distal oxygen constitutes a good leaving group in the form of –OtBu (see Scheme 2). In our catalytic system, the oxidizing agent TBHP is transformed to tert-butyl alcohol during the reaction, which can coordinate to the Mo(VI) center and consequently slows down the reaction. 3.4. Stability and regenerability tests A hot filtration was carried out. After 4 h of catalysis, Mo0.25@COMOC-4 was filtered off and the reaction mixture was followed on during 20 h. Fig. 9 shows that the epoxide formation
Fig. 9. Hot filtration experiment of Mo0.25@COMOC-4. Reaction conditions: substrate: cyclooctene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 °C.
Scheme 2. Accepted alkylperoxo mechanism of Mo-catalyzed epoxidation with hydroperoxides[37].
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Table 4 TON, TOF, selectivity toward the epoxide and leaching results for Mo@COMOC-4 for each run. Examined substrate: cyclooctene.
a b c
Run
TONa
TONb
TOFc
Selectivity epoxide (%)
Leaching Ga (%)
Leaching Mo (%)
1 2 3
48 35 42
82 76 65
12 6 8
>99.9 >99.9 >99.9
0.13 0.16 0.3
ND ND ND
The TON value was calculated after 7 h of catalysis. The TON value calculated after 24 h of catalysis. The TOF value was calculated after 0.5 h of catalysis.
stops after the removal of the catalyst, which indicates that the catalysis occurs truly heterogeneous. To examine the regenerability of the Mo0.25@COMOC-4, three consecutive runs were carried out. In Table 4, the TON, TOF, selectivity, and the leaching percentage of Mo and Ga are shown for each run. The TON and TOF values remain fairly constant for each run which demonstrates the regenerability of the catalyst. Additionally, the selectivity of the catalyst stays unaltered during these additional runs. No detectable leaching of the Mo-complex is observed which probably indicates that the Mo leaching is below the detection limit of the XRF. Furthermore, only a very small amount of Ga was leached out during these extra runs. Comparison of the XRPD pattern of Mo@COMOC-4 before catalysis and after each run shows that the framework integrity of the MOF is preserved during the three runs (see Fig. 10). Additional proof for the stability of the framework was seen in the nitrogen adsorption measurements (see Fig. S2). Comparison of the Mo@COMOC-4 material after the third run with the pristine Mo@COMOC-4 sample shows no loss in surface area. Moreover, SEM analysis has been carried out (see Fig. S5). As can be seen from
the SEM pictures, the morphology and size of the Mo@COMOC-4 material is preserved after catalysis. 4. Conclusions The Ga(OH)(bpydc) MOF (COMOC-4) was successfully grafted with the MoO2Cl2(THF)2 complex and examined as an epoxidation catalyst. The COMOC-4 framework maintains its volume and crystallinity during the post-modification process and during the catalytic tests. Moreover the Mo-complex does not leach into the solution during catalysis, making the Mo@COMOC-4 a stable and recyclable catalyst. The Mo@COMOC-4 is an efficient catalyst in the epoxidation of cycloalkenes with a selectivity up to 100% toward the epoxide. We believe that the COMOC-4 framework might be used as a host for the heterogenization of many other interesting metal complexes. Acknowledgments The authors acknowledge the financial support from the Ghent University BOF postdoctoral Grants 01P02911T (Y.Y.L) and 01P068135 (K.L.) and UGent GOA Grant 01G00710. Y.Y.L. acknowledges the Fundamental Research Fund for the Central Universities of China (DUT13RC(3)85). We acknowledge the Long Term Structural Methusalem Grant No. 01M00409 funding by the Flemish Government. S.T. gratefully acknowledges the FWO Flanders for a post-doctoral scholarship. This work was supported by funding from the European Research Council under the Seventh Framework Program (FP7), ERC Grant No. 246791 – COUNTATOMS. The Titan microscope used for this study was partially funded by the Hercules foundation of the Flemish Government. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2014.05.019. References
Fig. 10. XRPD pattern of Mo@COMOC-4 before catalysis and after each run. The diffraction signal (indicated as a *) is due to the background of the sample holder.
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