Journal of Catalysis 316 (2014) 201–209

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Journal of Catalysis

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A MoVI grafted Metal Organic Framework: Synthesis, characterizationand catalytic investigations

http://dx.doi.org/10.1016/j.jcat.2014.05.0190021-9517/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: pascal.vandervoort@ugent.be (P. Van Der Voort).

1 Both authors contributed equally to this work.

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 Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgiumb State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024 Dalian, Chinac 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 2014Revised 1 May 2014Accepted 18 May 2014

Keywords:Metal Organic FrameworkOxidationCatalysisMolybdenumPost-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 Mo-complex. 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 theframework, no aggregation or any Mo or Mo oxide species are formed. Moreover, the Mo@COMOC-4was evaluated as a catalyst in the epoxidation of cyclohexene, cyclooctene and cyclododecene employingTBHP in decane as oxidant. The post-modified COMOC-4 exhibits a very high selectivity toward the epox-ide (up to 100%). Regenerability and stability tests have been carried out demonstrating that the catalystcan be recycled without leaching of Mo or loss of crystallinity.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

It is well known that compounds containing certain transitionmetals, such as Mo, W, Ti and V, are able to catalyze the liquid-phase epoxidation of olefins using alkyl hydroperoxides as oxi-dants in homogeneous solution. The catalysis is most effectivewhen these transition metals are in their highest oxidation state.During the last decade, several molybdenum (VI) complexes haveshown to be successful catalysts for various reactions rangingfrom Lewis acid catalyzed transformations to oxidation and reduc-tion reactions [1]. Treatment of [MoO2X2] species (X = halide, OR,OSiR3) with monodentate or bidentate Lewis bases (L or L2), suchas pyridine and 2,20-bipyridine, in the presence of a donor solvent,form a series of dioxomolybdenum complexes with the composi-tion of [MoO2X2L]. Such dioxomolybdenum(VI) complexes arewell documented as efficient oxo-transfer catalyst for variousorganic transformations, such as acylation reaction, hydrosilyla-tion of aldehydes and ketones, oxidation of alcohols and thiols[1]. In the epoxidation of olefins, many reports on such homoge-neous molybdenum complexes have been described. Within thiscontext, Kuhn et al. reported that MoO2Br2L2 complexes with

bidentate nitrogen donor ligands exhibit very high selectivities(�90%) in the epoxidation of cyclooctene applying TBHP in decaneas oxidant [2,3]. Amaranta et al. examined Mo-complexes of thetype MoO2Cl2L with L = bipyridine-based ligands as catalysts forthe epoxidation of the biorenewable olefins DL-limonene andmethyl oleate [4]. The Mo catalysts rendered the epoxide mono-mers in high selectivity and high conversions (89% selectivityfor 96% limonene and 99% selectivity for 94% methyl oleateconversion). Whereas the previous studies employed TBHP asoxidant, others report on the use of H2O2 in combination withNaHCO3 as cocatalyst to study the Mo-complexes as catalyst inthe epoxidation of various olefins showing high conversions andselectivities at room temperature [5,6]. However, as these homo-geneous Mo-complexes have some well known disadvantages(difficulties with separation and recyclability), many attempts toimmobilize these Mo-complexes have been performed. In thepaper of Kühn et al., the different supports and immobilizationtechniques ranging from direct grafting to tethering via afunctionalized spacer ligand are described [7]. Different supportshave been applied, for example, silica-based supports e.g.MCM-41 [8–10], polymers [11,12], hybrid materials [13] or ionicliquids [14,15]. However, it should be noted that several reportspoint out that octahedral coordinated MoVI sites cannot beeasily incorporated into the tetrahedral positions of porous silicas(zeolites/mesoporous materials) [16,17].

202 K. Leus et al. / Journal of Catalysis 316 (2014) 201–209

Alternatively, attempts have been made to synthesize activemetal containing organic frameworks as well. This is another effi-cient way to heterogenize the homogeneous metal complexes.Metal Organic Frameworks (MOFs) are crystalline porous materialsconsisting of metal ions held in place by multidendate organic link-ers to build up a framework. MOFs have been examined for manypotential 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 shownpromising catalytic activity in epoxidation reactions. However,synthesizing MOFs with active metal centers in their highest oxi-dation state is very difficult. This limits the application in oxidationcatalysis. Up to now, no MOFs with MoVI centers have beenreported.

In this study, we report on the immobilization of a homoge-neous MoVI catalyst on a MOF support to obtain a well dispersedsingle-site catalyst. Yaghi’s group synthesized an Al-basedMOF, Al(OH)(bpydc) (bpydc2� = 2,20-bipyridine-5,50-dicarboxylate)denoted as MOF-253 [28]. This MOF has free 2,20-bipyridine siteswhich form excellent anchoring points for the grafting of metalcomplexes. The successful post-modification of MOF-253 has beendemonstrated by Zou and co-workers for the incorporation ofRuCl3 on MOF-253 [29] and by Li et al. [30], who anchored Cu+ ionsonto MOF-253 to catalyze the cross-coupling of phenols and alco-hols with aryl halides. The post-modification, however, resulted ina significant reduction in surface area and pore volume, which ismuch higher than the volume taken by the complex, suggesting apartial collapse of the framework during the post-modification.In this work, we used a novel Ga-based MOF, denoted asCOMOC-4, which is isostructural to MOF-253 and which is stablein air and water (50 �C for 24 h) [31].

COMOC-4 was applied as host matrix for the preparation of aMOF supported MoVI catalyst. To the best of our knowledge, thisis the first post-modified MOF possessing MoVI active centers. Sofar, there is only one MoII-based MOF, denoted as TUDMOF-1which is isotopic to Cu3(BTC)2 [32]. However, this MOF is veryair sensitive which limits its practical use. In another report,[Ni2(dhtp)], a member of the isostructural CPO-27 or MOF-74series, was post-modified with Mo via Mo(CO)6 sublimationhowever, during the process, the MoVI was reduced to MoIV [33].In this paper, we employed MoO2Cl2(THF)2 as a Mo precursor toanchor different MoVI loadings on the COMOC-4 framework. Theimportant feature of this MOF support is that the chelating bipyr-idine ligand binds strongly to the metal ion, and therefore, leachingof the metal ion is expected to be reduced. The resulting Mo@CO-MOC-4 materials were evaluated in the epoxidation of cyclohex-ene, cyclooctene and cyclododecene. We also investigated therecyclability and stability of the heterogeneous catalyst. Further-more, (scanning) transmission electron microscopy ((S)TEM)provided additional valuable structural information on thissupported catalyst. The (S)TEM images demonstrate that even athigh Mo-complex loadings on the COMOC-4 framework, no aggre-gation 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 Europeand used without further purification. Nitrogen adsorption exper-iments were carried out at �196 �C using a Belsorp-mini II gas ana-lyzer. Prior to analysis, the samples were dried under vacuum at120 �C to remove adsorbed water. X-ray powder diffraction (XRPD)patterns were collected on a ARL X’TRA X-ray diffractometer withCu Ka radiation of 0.15418 nm wavelength and a solid-state

detector. X-ray fluorescence (XRF) measurements were taken ona NEX CG from Rigaku using a Mo-X-ray source. All XPS measure-ments were recorded on an X-ray photoelectron spectroscopyS-Probe XPS spectrometer with monochromated Al (1486 eV)exciting radiation from Surface Science Instruments (VG). Thecatalyst powder was positioned on conducting carbon tape. Inorder to compensate for charging of the sample, a nickel gridwas used, placed 2 mm above the sample. A low-energy electronflood gun 3 eV was used as a neutralizer. All measurements werecalibrated toward a value for the C 1s peak of adventitious carbonat 284.6 eV. Calculation of the atomic concentrations and peak fit-tings was performed using a linear background subtraction. Diffusereflectance infrared Fourier transform spectroscopy (DRIFTS)measurements were recorded on a Thermo Nicolet 6700 FT-IRspectrometer equipped with a N2 cooled MCT-A (mercury–cadmium–tellurium) detector and a KBr beam splitter. Anultra-fast GC equipped with a flame ionization detector (FID) anda 5% diphenyl/95% polydimethylsiloxane column, with 10 m lengthand 0.10 mm internal diameter was used to follow the conversionsof the products during the catalytic tests. Helium was used as car-rier gas and the flow rate was programmed as 0.8 mL/min. Thereaction 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 polysiloxanepackage and has a length of 50 m, with an internal diameter of0.25 mm, whereas the second column has a length of 2 m withan internal diameter of 0.15 mm. The package of the latter is a50% phenyl polysilphenylene-siloxane. Helium was used as carriergas with a constant flow (1.8 mL/min).

2.2. Synthesis of COMOC-4

COMOC-4 was synthesized according to our optimized proce-dure published elsewhere [31]. In general, 4.4 mmol Ga(NO3)3�H2Oand 5 mmol H2bpydc were added to 120 mL DMF and stirred at150 �C for 48 h. Hereafter, the precipitate was filtered and washed,respectively, with DMF, methanol, and acetone. To remove the unre-acted linker from the pores, an extraction in DMF was performed at80 �C for 2 h. In addition, a soxhlet extraction in methanol was car-ried out during 48 h to obtain a complete exclusion of the organicspecies. 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 preparedaccording to the procedure reported by Kühn et al. [3]. It shouldbe noted that this complex must be used immediately after synthe-sis as this complex is very unstable. All the manipulations to obtainMo@COMOC-4 were carried out under an oxygen and water-freeatmosphere with standard Schlenk techniques. Typically, 0.18 gMoO2Cl2 was added to 7.5 mL THF and stirred for 10 min at roomtemperature. The yellow solution was removed from the non-dis-solved residue by employing a combined nylon membrane filterand evaporated to dryness to obtain the MoO2Cl2(THF)2 complex.In a second step, the obtained MoO2Cl2(THF)2 complex was redis-solved in THF and COMOC-4 was added. Typically, 3 mL of theMoO2Cl2(THF)2 solution was added to 0.25 g COMOC-4 materialsuspended 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 gCOMOC-4 material suspended in 38.5 mL of THF to obtain a lowMo loading (14 mol% Mo, equals to 14% occupation of bipyridinesites), namely Mo0.14@COMOC-4. The Mo loading was determinedby means of XRF measurements, indicating a loading efficiency of63% for high Mo loading and 59% for low Mo loading. After stirring

Scheme 1. Schematic representation of the structure of the homogeneous Mo-complex(MoO2Cl2[Me2bpydc]) and the heterogeneous Mo@COMOC-4 material(MoO2Cl2@COMOC-4).

Table 1

K. Leus et al. / Journal of Catalysis 316 (2014) 201–209 203

at RT for 1.5 h, the solid material was filtered off and washed sev-eral times with acetone to remove unreacted Mo-complex. Theobtained air stable solid was dried overnight under vacuum andactivated prior to use.

2.4. Synthesis of MoO2Cl2 [Me2bpydc]

All the manipulations were carried out under an oxygen andwater-free atmosphere. The dioxomolybdenum(VI) complexMoO2Cl2[Me2bpydc] was synthesized according to the reportedliterature procedure [34]. 0.17 g (0.5 mmol) MoO2Cl2(THF)2 wasdissolved in 10 mL THF and treated with 0.15 g (0.55 mmol)Me2bpydc ligand. The mixture was stirring at RT for 30 min, after-ward, filtered off under Ar protection. The resulting filtrate wasevaporated to dryness under vacuum at room temperature, andthe product was washed with diethyl ether (2 times 5 mL) anddried 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 with30.0 mL chloroform as solvent and 9 mL of tert-butyl hydroperox-ide dissolved in decane employed, respectively, as oxidant andinternal standard. The substrates examined in this study are cyclo-hexene, cyclooctene and cyclododecene (25 mmol). The substrate/oxidant molar ratio was 1/2. The Mo@COMOC-4 sample having0.18 mmol of Mo active sites was employed as catalyst. WhenCOMOC-4 was examined, 0.18 mmol of Ga sites was used in thecatalytic tests. The molar ratio of substrate:oxidant:catalyst is140:280:1. All the catalytic tests were performed at a temperatureof 50 �C, a temperature at which all the blank tests showed no con-version of substrate. During each test, aliquots were graduallytaken out of the mixture and diluted with 500 lL ethylacetateand subsequently analyzed by GC-FID.

All the fresh catalysts were activated under vacuum at 120 �Cfor 3 h prior to catalysis. After each catalytic run, the catalystwas recovered by filtration, washed with acetone, and dried atRT overnight under vacuum to reuse it in another run.

2.6. Transmission electron microscopy

The samples were prepared for investigation by crushing theas-received powder in a mortar and placing the powder onto aholey carbon grid. Bright-field transmission electron microscopy(BF-TEM) investigations were performed using a FEI Tecnai G2electron microscope operated at 200 kV and a JEOL 3000F electronmicroscope operated at 300 kV. High-angle annular dark-fieldscanning 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 accelera-tion voltage. The convergence semi-angle a for STEM was 22 mrad,the inner acceptance semi-angle b for ADF-STEM imaging was22 mrad, and the acceptance semi-angle b for HAADF-STEMimaging was 90 mrad.

Properties of the catalytic materials used in this work.

Catalyst Mo (wt%)a Slang (m2 g�1) Pore volume (cm3 g�1)

Mo0.14@COMOC-4 3.9 630 0.67Mo0.25@COMOC-4 6.8 500 0.62COMOC-4 – 770 0.69

a The wt% of Mo loading is calculated based on the empirical formula ofthe catalysts, C12H7N2Cl0.28GaMo0.14O5.28 for Mo0.14@COMOC-4 andC12H7N2Cl0.5GaMo0.25O5.5 for Mo0.25@COMOC-4.

3. Results and discussion

3.1. Synthesis and structural information

The parent MOF (COMOC-4), is a gallium based 2,20-bipyridine-5,50-dicarboxylate framework with 1D microporous channels. Thebipyridine sites are located on the walls of the channels, allowing

a second metal site to coordinate to them. A schematic view ofthe obtained Mo@COMOC-4 structure is shown in Scheme 1.

The MoO2Cl2(THF)2 complex is known to be a highly active butunstable catalyst. This complex gains stability when it coordinatesto the organic ligands with donor functionalities, such as bipyri-dine sites, while the catalytic activity is maintained. As this com-plex is such a strong oxidation catalyst, the stability of thesupport toward oxidation is an important issue. Besides COMOC-4, MOF-253, the aluminum 2,20-bipyridine-5,50-dicarboxylateframework was also examined as a host matrix to graft the MoVI

complex. However, at a loading of 25% of the Mo-complex, the sur-face area dropped drastically of the latter material (6 times lowercompared to the parent MOF). The same behavior was observedby Zou’s group as well [29]. In contrast to MOF-253, the COMOC-4 material shows only a minor decrease in surface area when25% 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 thermogravimetricanalysis

Two Mo loadings of supported COMOC-4 catalyst were investi-gated; each has a different percentage of the bipyridine moietiesthat are functionalized with the Mo-complex: one with a highMo loading (25% occupation of the bipyridine sites, Mo/Ga molarratio is 0.25) and one with a low loading of Mo (14% occupationof the bipyridine sites, Mo/Ga molar ratio is 0.14), respectively,denoted as Mo0.25@COMOC-4 and Mo0.14@COMOC-4. XRPD pat-terns were collected of all the Mo@COMOC-4 materials. As canbe seen from Fig. 1, the crystallinity of all the functionalizedmaterials is well preserved after the post-modification process.

Fig. 1. XRPD pattern of COMOC-4, Mo0.25@COMOC-4 and Mo0.14@COMOC-4. Thediffraction signal (indicated as a *) is due to the background of the sample holder.

Fig. 3. DRIFT spectrum of COMOC-4 and Mo0.25@COMOC-4.

204 K. Leus et al. / Journal of Catalysis 316 (2014) 201–209

The introduction of metals slightly decreased the intensity of thereflections, especially the diffraction peak at 6.5�, which is relatedto the (101) planes that are parallel to the linkers (both directionsare equivalent because of symmetry). The incorporation of theMo-complex induced slight changes in the shape and angle ofthe linkers. This results in a decrease in the intensity and in a slightbroadening of the peaks as observed in the XRPD patterns. How-ever, as can be seen from Fig. 1, these effects are rather minor.

In Fig. 2, the nitrogen adsorption isotherms of COMOC-4 and thefunctionalized Mo@COMOC-4 materials are presented. After func-tionalization, the surface area (Langmuir) of COMOC-4 drops verymoderately for the Mo0.14@COMOC-4 and Mo0.25@COMOC-4,respectively. The corresponding pore volumes of the Mo@CO-MOC-4 materials are slightly lower than the starting material,which is clearly due to the incorporation of the Mo-complex. More-over, the thermal stability of Mo@COMOC-4 in comparison withthe parent COMOC-4 material has been examined (see Fig. S1, Sup-porting information). From this figure, one can clearly see that thethermal stability of the Mo@COMOC-4 is only slightly reduced. Incomparison with COMOC-4 which starts to decompose from300 �C, the decomposition temperature of Mo@COMOC-4 is 250 �C.

Fig. 2. Nitrogen adsorption isotherms of COMOC-4, Mo0.25@COMOC-4 andMo0.14@COMOC-4.

3.2.2. Spectroscopy analysisIn Fig. 3 the DRIFT spectrum is presented of COMOC-4 and

Mo0.25@COMOC-4. As can be seen, the characteristic vibrations ofthe framework are still present in the post-modified material.Vibrations due to the benzene ring can be observed in the regionat 1510–1450 cm�1 (aromatic ring stretch), at 1225–950 cm�1

(aromatic C–H in plane bend), and at 900–670 cm�1 (aromaticC–H out of plane bend). The vibrations in the region 1597–1616 cm�1 and 1415–1463 cm�1 can be attributed respectively tothe asymmetric and symmetric –CO2 stretching vibrations. Besidesthe characteristic vibrations which are present in both materials,the Mo0.25@COMOC-4 material exhibits two extra vibrationsat 910 cm�1 and 947 cm�1, which can be assigned to themsym(O@Mo@O) and masym(O@Mo@O) vibrations, respectively [35].

Additionally XPS measurements were taken to verify the oxida-tion state of the Mo in Mo@COMOC-4. The results are presented inFig. 4. In both samples (before and after catalysis), Mo is present inthe oxidation state +6. The molybdenum 3d peak was deconvo-luted with a fixed energy split of 3.2 eV and a fixed area ratio of3/2 for Mo3d5/2 versus Mo 3d3/2. Peaks are situated at an averagevalue 232.38 and 235.56 eV, pointing to an oxidation state of +6.

Fig. 4. The Mo 3d3/2 and Mo 3d5/2 peak of Mo@COMOC-4 before and aftercatalysis.

K. Leus et al. / Journal of Catalysis 316 (2014) 201–209 205

The results show no indication at all for changes in the oxidationstate of molybdenum.

3.2.3. High-resolution(scanning) transmission electron microscopyThe unloaded COMOC-4 sample (Fig. S3 in Supplementary

information) consists of agglomerated COMOC-4 nanoparticleswith sizes ranging from 10 to 100 nm. After loading, the samplesretain their initial morphology, as can be seen from the bright-fieldTEM image in Fig. 5a. The presence of Mo in the sample is con-firmed by the EDX spectrum in Fig. 5e. No (large) Mo nanoparticleswere detected in HRTEM images of the sample (Fig. 5b), indicatingthat the Mo should be present as either single atoms or ultra-smallatomic clusters. In the high-resolution ADF-STEM (Fig. 5c) andHAADF-STEM images (Fig. 5d), atom-size bright-contrast featuresare clearly visible (examples marked by white arrows). As theimage contrast in HAADF-STEM is proportional to Z � 1.7, thesebright features should correspond to either single heavy Mo(Z = 42) or Ga (Z = 31) atoms or few atom clusters. The limited dif-ference in Z number between Ga and Mo makes a definitive dis-tinction between Mo and Ga difficult. To clarify the imagecontrast observed in the HAADF-STEM images, an Al-basedCOMOC-4 analogue, MOF-253, with larger Z number differencebetween Mo and the Al (Z = 14) metal center, was also investigated.In this case (Fig. S4 in Supplementary info), the contrast betweenthe 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 imageof the Mo0.25@COMOC-4 nanoparticles, (c) ADF-STEM image of a singleMo0.25@COMOC-4 nanoparticle, (d) simultaneously acquired HAADF-STEMimage and (e) EDX spectrum acquired from the Mo0.25@COMOC-4 sample.

higher than in the Ga case. The high-resolution HAADF-STEMimage in Fig. S4 therefore provides clear evidence for the presenceof single Mo atoms and few atom clusters as a result of the loadingprocedure used in this work. The bright-contrast features in theHAADF-STEM images in Fig. 5 can therefore also be interpretedas Mo single atoms or few atom clusters.

3.3. Catalytic tests

As observed from the HRTEM micrographs of the COMOC-4samples, the COMOC-4 material consists of nanoparticles, withparticle sizes ranging from �10 nm to �100 nm. In this case, itshould be noted that the surface catalysis must plays an importantrole in the catalytic reaction. The COMOC-4 material has 1D chan-nels. After the grafting of the MoO2Cl2 complex, the estimated porewindow is �1 nm. Two Mo loadings (14 mol% and 25 mol%) of sup-ported COMOC-4 catalyst were investigated. In general, the cata-lytic performance of the Mo@COMOC-4 catalysts for theepoxidation reactions was investigated using cis-cyclooctene as amodel substrate and t-BuOOH as the oxygen donor. In a controlexperiment carried out without catalyst, no reaction occurred atall, whereas in the presence of the Mo@COMOC-4 catalysts, theepoxide was obtained almost quantitatively during a reaction timeof 24 h, indicating that these catalysts exhibit excellent productselectivity. In all the catalytic experiments, the Mo0.25@COMOC-4sample was employed as a catalyst unless mentioned otherwise.

In Fig. 6 left, the cyclooctene and cyclododecene conversion andthe formed products are presented employing Mo0.25@COMOC-4 ascatalyst. As can be seen from this figure, a cyclooctene conversionof 67.6% is observed after a reaction time of 24 h, whereas forcyclododecene, only 42.4% of conversion is noted. This differencein substrate conversion can be attributed to the different sizes ofthe substrates which will result in a significant difference in thereaction kinetics. It should be noted that for both cycloalkenes, aselectivity of almost 100% is observed toward the epoxide, whichis consistent with the literature reports [7]. As shown in Fig. 6right, the homogeneous catalyst MoO2Cl2[Me2bpydc] (structureshown in Scheme 1) shows full conversion of cyclooctene after4 h of reaction with a selectivity of almost 100% toward theepoxide. Although the efficiency of the heterogeneous catalyst issomewhat lower than the homogenous catalyst, the selectivity iswell maintained. We also tested the catalytic activity of theCOMOC-4 material for the oxidation of cyclooctene. In this case,24% conversion after a reaction time of 24 h is observed, which isdue to the lewis acidity of the COMOC-4.

Although cyclododecene has a significant larger kinetic diame-ter compared to cyclooctene (respectively 7.8 and 5.7 Å) Mo0.25@-COMOC-4 is still able to epoxidize cyclododecene as can be seenfrom Fig. 6 left. This is probably due to surface catalysis on theMo sites located on the exterior. To find additional proof, twoMo@COMOC-4 samples with different Mo loadings were evaluatedas catalysts in the epoxidation of cyclododecene: Mo0.14@COMOC-4 and Mo0.25@COMOC-4. It is important to note that in the catalyticexperiments, the amount of catalyst is adapted, so that the reactoris charged with an equal amount of Mo sites. This implies thatthere is more Mo0.14@COMOC-4 added to the reactor than Mo0.25@-COMOC-4. This means also that there is much more ‘‘surface’’ pres-ent in the reactor for the Mo0.14@COMOC-4 material. Theconversion plots of both catalyst with equal amount of Mo sitesand the epoxide formation are presented in Fig. 7. It is interestingto note that during the first 8 h of reaction, the Mo0.14@COMOC-4sample exhibits a higher cyclododecene conversion in comparisonwith Mo0.25@COMOC-4. This might be due to the different surfacecoverage of the two catalysts. The active surface sites are certainlyeasier to reach than the active sites inside the channels. On the

Fig. 6. Left: cyclooctene(Cy8) and cyclododecene(Cy12) conversion and formed products employing Mo0.25@COMOC-4 as catalyst. Right: Cyclooctene (Cy8) conversions andformed products employing the homogeneous MoO2Cl2[Me2bpydc] complex, Mo0.25@COMOC-4 and COMOC-4 as catalysts. Reaction conditions: substrate: cyclooctene orcyclododecene, oxidant: TBHP in decane, solvent: chloroform, Temperature: 50 �C.

Fig. 7. Cyclododecene conversion and epoxide formation for Mo0.25@COMOC-4 andMo0.14@COMOC-4. Reaction conditions: substrate: cyclododecene, oxidant: TBHP indecane, solvent: chloroform, Temperature: 50 �C.

Table 2Summary of the Mo0.25@COMOC-4 catalyst in oxidation of cycloalkenes.

Entry Product T (h) T (�C) Conversion/selectivity (%)

1 Cyclooctene oxide 24 50 67.6/>99.92 Cyclododecene oxide 24 50 42.4/>99.93 Cyclooctene oxide 7 50 30.3/>99.94 Cyclododecene oxide 7 50 14.4/>99.95 Cyclohexene oxide 7 50 21.2/66.56 Cyclohexene oxide 7 80 80.8/>99.9

206 K. Leus et al. / Journal of Catalysis 316 (2014) 201–209

other hand, after 24 h, the total conversion was not affected by theloading difference.

The catalytic performance was further evaluated for theoxidation 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 twotemperatures. Reaction conditions: substrate: cyclohexene, oxidant: TBHP indecane, solvent: chloroform, Temperature: 50 �C and 80 �C.

cyclohexene oxide is observed as the predominant product witha selectivity of 66.5% at 50 �C; however, 2-cyclohexene-1-one and2-cyclohexene-1-ol are also detected, and the formation of the lat-ter products is obtained by the allylic oxidation of cyclohexene.The epoxide selectivity increases with an increase in reactiontemperature, and such behavior was also reported on aMoO2Cl2(OPMePh2)2 complex supported on silica (Table 3, entry6) [9]. At 80 �C, cyclohexene oxide is the only product (Table 2,entry 6). As the temperature rises, there is an increase in conver-sion, and the cyclohexene conversion reached a value of 80.8% at80 �C, while at 50 �C, only 21.2% cyclohexene conversion wasobserved. Sharpless [36] first carried out the epoxidation of cyclod-odecene by TBHP in the presence of Mo(CO)6 and 18O enrichedwater, and they concluded that no peroxo compound was formedin the reaction and that an intact OO-t-Bu group was the activespecies. To assure that the formation of the epoxide is not due tothe presence of radical mechanisms, the same catalytic tests wererepeated in the presence of a radical inhibitor, hydroquinone (ratioalkene:hydroquinone is 1:0.05). An equal conversion and productformation were observed for both substrates in comparison withthe tests without the presence of the inhibitor which corroboratesthat the reaction occurs via a non-radical mechanism which hasalso been stated in the literature [37].

Compared to published Mo-based heterogeneous catalyst sys-tems, as summarized in Table 3, all these Mo-based catalystsincluding Mo@COMOC-4 show good selectivity in epoxidation ofcyclooctene. Mo@COMOC-4 is a highly active catalyst. For instance,Mo@COMOC-4-catalyzed cyclooctene epoxidation reaches similarepoxide conversion and selectivity (entry 2) than MoO2Cl2[4,40-dimethyl-2,20-bipyridine] supported on MCM-41 under similarconditions (entry 3). However, a high Mo leaching was observedon the silica-based material. The reaction temperature has a greatinfluence on the reaction kinetics in the epoxidation of cyclohex-ene, but the temperature also affects the selectivity. An allylic oxi-dation of cyclohexene was observed at low reaction temperaturefor Mo@COMOC-4 and MoO2Cl2(OPMePh2)2 supported on silica(entry 5 and 6).

Table 3Comparison of catalytic activity of Mo0.25@COMOC-4 with other Mo-based heterogeneous catalysts in the epoxidation of cycloalkenes.

Entry Catalyst Oxidant Major product Oxidant to substratemolar ratio

T (h) T (�C) Conversion/selectivity Ref.

1 Mo0.25@COMOC-4 TBHP Cyclooctene oxide 2 7 50 30.3/>99.9 This work2 Mo0.25@COMOC-4 TBHP Cyclooctene oxide 2 24 50 67.6/>99.9 This work3 MoO2Cl2[L] on MCM-41b TBHP Cyclooctene oxide 1.5 24 55 74.1/100c [38]4 MoO2Cl2(OPMePh2)2 on silica TBHP Cyclooctene oxide 1.5 8 80 99/99 [9]5 Mo0.25@COMOC-4 TBHP Cyclohexene oxide 2 7 50 21.2/66.5 This work6 MoO2Cl2(OPMePh2)2 on silica TBHP Cyclohexene oxide 1.5 7 55 �35/�84 [9]7 Polymer supported MoO2(ligand)na H2O2 Cyclohexane-1,2-diol 2 6 80 66/83.4 [12]

a Ligand: 2-thiomethylbenzimidazole.b L = 4,40-dimethyl-2,20-bipyridine.c Catalytic activity decreased drastically in the 2nd run due to the severe leaching.

Fig. 9. Hot filtration experiment of Mo0.25@COMOC-4. Reaction conditions: sub-strate: cyclooctene, oxidant: TBHP in decane, solvent: chloroform, Temperature:50 �C.

K. Leus et al. / Journal of Catalysis 316 (2014) 201–209 207

It is well known that early transition metal ions in their highestoxidation state such as Mo(VI) tend to be stable toward changes intheir oxidation state. Consequently, in epoxidation reactions withH2O2 or alkylhydroperoxides, they form adducts (M-OOH and M-OOR) which are the key intermediates in the epoxidation, andthe role of the metal ion is that of a Lewis acid site. The metal cen-ter 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 theform of –OtBu (see Scheme 2). In our catalytic system, the oxidiz-ing agent TBHP is transformed to tert-butyl alcohol during thereaction, which can coordinate to the Mo(VI) center and conse-quently 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 wasfollowed on during 20 h. Fig. 9 shows that the epoxide formation

Scheme 2. Accepted alkylperoxo mechanism of Mo-catalyzed epoxidation with hydroperoxides[37].

Table 4TON, TOF, selectivity toward the epoxide and leaching results for Mo@COMOC-4 for each run. Examined substrate: cyclooctene.

Run TONa TONb TOFc Selectivity epoxide (%) Leaching Ga (%) Leaching Mo (%)

1 48 82 12 >99.9 0.13 ND2 35 76 6 >99.9 0.16 ND3 42 65 8 >99.9 0.3 ND

a The TON value was calculated after 7 h of catalysis.b The TON value calculated after 24 h of catalysis.c The TOF value was calculated after 0.5 h of catalysis.

208 K. Leus et al. / Journal of Catalysis 316 (2014) 201–209

stops after the removal of the catalyst, which indicates that thecatalysis occurs truly heterogeneous.

To examine the regenerability of the Mo0.25@COMOC-4, threeconsecutive runs were carried out. In Table 4, the TON, TOF, selec-tivity, and the leaching percentage of Mo and Ga are shown foreach run. The TON and TOF values remain fairly constant for eachrun which demonstrates the regenerability of the catalyst. Addi-tionally, the selectivity of the catalyst stays unaltered during theseadditional runs. No detectable leaching of the Mo-complex isobserved which probably indicates that the Mo leaching is belowthe detection limit of the XRF. Furthermore, only a very smallamount of Ga was leached out during these extra runs.

Comparison of the XRPD pattern of Mo@COMOC-4 before catal-ysis and after each run shows that the framework integrity of theMOF is preserved during the three runs (see Fig. 10). Additionalproof for the stability of the framework was seen in the nitrogenadsorption measurements (see Fig. S2). Comparison of theMo@COMOC-4 material after the third run with the pristineMo@COMOC-4 sample shows no loss in surface area. Moreover,SEM analysis has been carried out (see Fig. S5). As can be seen from

Fig. 10. XRPD pattern of Mo@COMOC-4 before catalysis and after each run. Thediffraction signal (indicated as a *) is due to the background of the sample holder.

the SEM pictures, the morphology and size of the Mo@COMOC-4material is preserved after catalysis.

4. Conclusions

The Ga(OH)(bpydc) MOF (COMOC-4) was successfully graftedwith the MoO2Cl2(THF)2 complex and examined as an epoxidationcatalyst. The COMOC-4 framework maintains its volume andcrystallinity during the post-modification process and during thecatalytic tests. Moreover the Mo-complex does not leach into thesolution during catalysis, making the Mo@COMOC-4 a stable andrecyclable catalyst. The Mo@COMOC-4 is an efficient catalyst inthe epoxidation of cycloalkenes with a selectivity up to 100%toward the epoxide. We believe that the COMOC-4 frameworkmight be used as a host for the heterogenization of many otherinteresting metal complexes.

Acknowledgments

The authors acknowledge the financial support from the GhentUniversity BOF postdoctoral Grants 01P02911T (Y.Y.L) and01P068135 (K.L.) and UGent GOA Grant 01G00710. Y.Y.L. acknowl-edges the Fundamental Research Fund for the Central Universitiesof China (DUT13RC(3)85). We acknowledge the Long Term Struc-tural Methusalem Grant No. 01M00409 funding by the FlemishGovernment. S.T. gratefully acknowledges the FWO Flanders for apost-doctoral scholarship. This work was supported by fundingfrom the European Research Council under the Seventh FrameworkProgram (FP7), ERC Grant No. 246791 – COUNTATOMS. The Titanmicroscope used for this study was partially funded by theHercules foundation of the Flemish Government.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2014.05.019.

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