The catalytic oxidation (or epoxidation) of olefins is an important reaction in chemical industry [1, 2, 3, 4]. Polyoxometalates (POMs) are a class of early-transition-metal oxides with controllable redox properties and have shown attractive catalytic performance in the selective oxidation of olefins [5, 6, 7, 8, 9, 10]. Much recent effort has been focused on achieving the heterogenization of homogeneous POM catalysts in order to overcome their drawbacks,separation and recycling problems. Different preparation strategies have been developed to obtain heterogeneous POM-based catalysts. These include immobilizing POMs on porous support surfaces [11, 12, 13, 14, 15, 16, 17, 18],incorporating POMs in silica or metal oxide matrices via sol-gel techniques [19, 20, 21, 22, 23],encapsulating POMs within nanocages of metal-organic frameworks [24, 25, 26],and self-assembling POMs with organic compounds to form supramolecular structures [27].
To obtain highly efficient and stable POM-based heterogeneous catalysts,an appropriate interaction between POM and the support (or host) is usually necessary. This may include covalent bonds,electrostatic binding,hydrogen bonds,and/or other interactions. The support should possess a sufficiently high surface area,a large pore size,and a suitable surface polarity,as these parameters influence the distribution and accessibility of catalytically active species. Optimizing preparation methods and conditions has led to significant recent progress on preparing heterogeneous POM-based catalysts. Kasai et al. [11] immobilized polyoxometalate anions of [g-1,2-H2SiV2W10O40]4- on the surface of dihydroimidazolium-cation modified SiO2 by anion exchange,and this catalyst heterogeneously oxidized a broad range of olefins and sulfides. Armatas et al. [19] immobilized 12-phosphomolybdic acid anions in a zirconia matrix by sol-gel copolymerization,and this catalyst exhibited high activity and stability in the oxidation of alkenes. Qi et al. [20] immobilized surfactant-encapsulated polyoxometalate complexes in a silica matrix through a sol-gel process. The resulting supramolecular hybrid catalyst was highly active and stable for the selective oxidation of various organic substrates. Zhao et al. [28] immobilized ionic 12-tungstophosphoric acid-(dihydro)imidazolium complexes in mesoporous SBA-15 using a one-pot procedure. The resulting hybrid exhibited high catalytic efficiency and reusability in selective alcohol oxidations with H2O2 as the oxygen source. We recently synthesized supramolecular assemblies based on octamolybdate and imidazole by a one-step hydrothermal method. The resulting hybrids exhibited excellent catalytic activity and stability in the epoxidation of olefins with tert-butyl hydroperoxide (tBuOOH) as the oxidant [27]. Designing and preparing efficient POM-based heterogeneous catalysts for selective oxidation and epoxidation remain an attractive research area.
Periodic mesoporous organosilica containing embedded imidazolium cations (PMO-IL) was recently synthesized by controlling the hydrolysis and co-condensation of a disilylated imidazolium based ionic liquid (IL) [29, 30, 31, 32, 33, 34]. Such materials combine the advantages of the mesoporous material and IL. The PMO-IL possessed a high alkyl imidazolium loading,high specific surface area,large uniform pore size,and outstanding hydrothermal stability and ion exchange properties. Various recent studies showed that PMO-IL could serve as a host for dispersing and stabilizing noble metal species/particles. The resulting materials were highly active and stable heterogeneous hybrid catalysts for coupling reactions [29,30],aerobic oxidation of alcohols [31,32],and olefin hydrogenation [33].
In the present study,PMO-ILs containing different imidazolium cation contents were used as supports for the electrostatic immobilization of 12-phosphomolybdic acid (PMA). The resulting materials (PMA@PMO-ILs) were characterized,and their catalytic performance was investigated in the liquid-phase epoxidation of cyclooctene with tBuOOH as the oxidant.
Sodium hydride (NaH),imidazole,3- iodopropyltrimethoxysilane,PEG-PPG-PEG (P123),tetraethoxysilane (TEOS),12- phosphomolybdic acid (H3PMo12O40·28H2O),cis-cyclooctene,n-dodecane,tert-butyl hydroperoxide (tBuOOH),and all solvents (dried by standard procedures) were purchased from Sigma-Aldrich. All manipulations involving air-sensitive materials were performed using the Schlenk line technique under N2 atmosphere.
N-(3-Propyltrimethoxysilane)imidazole and 1,3-di(3- propyltrimethoxysilane)imidazolium iodide (1) were synthesized according to literature procedures [33,34]. PMO-ILs with various imidazolium cation contents were prepared according to Wang et al. [33]. Typically,P123 (2 g) was dissolved in 2 mol/L HCl (60 ml). H2O (15 ml) was added,and the solution was stirred at room temperature for 2 h. A mixture of 1 and TEOS/ethanol was added dropwise to the above solution,leading to a composition of SiO2/1/P123/HCl/ethanol/H2O molar ratio of (1.0 - x):x:0.017:6.0:4.0:208 (x = 0.10 for a,0.15 for b). The mixture was stirred at room temperature for 3 h,then heated to 38 °C for 24 h and subsequently aged at 100 °C for 72 h. The obtained solid material was filtered and washed with deionized water,and the template was extracted with ethanol in a Soxhlet apparatus for 24 h. The resulting solids (a and b) were dried under vacuum at 50 °C. Anal. found for a (%): C 13.52; H 3.31; N 3.03. Anal. found for b (%): C 17.23; H 3.54; N 4.44.
PMO-ILs a or b (1 g) was vigorously stirred with H3PMo12O40·28H2O (0.4 g) in methanol (20 ml) at 30 °C for 24 h. The solid was filtered off,washed with excess methanol,and extracted with methanol in a Soxhlet apparatus for 12 h. The obtained solids (2a or 2b) were dried under vacuum at 50 °C (Scheme 1). The PMA loading was calculated by atomic adsorption spectroscopy (AAS),as shown in Table 1.
PMA-functionalized SBA-15 (2c) was also prepared following the above procedure,using mesoporous SBA-15 in place of PMO-IL [15]. The PMA loading calculated by AAS in 2c was 0.04 mmol/g.
Elemental analyses for C,H,and N were performed on a Carlo Erba instrument (model EA 1108). AAS was performed on a Perkin Elmer Analyst 300. N2 adsorption-desorption isotherms were measured at -196 °C using a Quantachrome Autosorb 1 sorption analyzer. All samples were degassed at 130°C for 12 h before measurement. Surface areas were calculated using the BET method on the adsorption data at p/p0 = 0.05-0.2. Total pore volumes were derived at p/p0 = 0.95,assuming full surface saturation with N2. Pore size distributions were estimated from the adsorption branch of isotherms using the BJH model. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer (40 kV,30 mA),using Ni-filtered Cu Kα radiation. Thermogravimetric and differential thermal analyses (TG-DTA) were carried out using a SETARAM Setsys 16/MS instrument,with an air flow and heating rate of 5 °C/min,from room temperature to 800 °C. Fourier transform infrared (FT-IR) spectra were recorded using a Nicolet AVATAR 370 DTGS spectrometer in the range 4000-500 cm-1. Diffuse reflectance ultraviolet-visible (UV-Vis) spectra of solid samples were recorded on a Shimadzu 3600 spectrometer in the range 200-800 nm. Liquid phase NMR spectra were recorded with Bruker Avance 400 or 600 spectrometers. Solid-state 13C,29Si,and 31P cross-polarization-magic angle spinning (CP-MAS) NMR spectra were recorded on a Bruker DSX Avance spectrometer at resonance frequencies of 100.6,79.5,and 162.0 MHz,respectively.
The catalytic epoxidation of cyclooctene was carried as follows. Cyclooctene (5.0 mmol),5.0 mmol of n-dodecane (internal standard),100 mg of solid catalyst,and 10 ml of CHCl3 were added to a 25 ml flask. The suspension was stirred at a specific temperature,and then 1.0 ml of 5.5 mol/L tBuOOH in decane was added to start the reaction. After conducting the reaction,the catalyst was filtered off,washed with CHCl3,dried under vacuum at 50 °C,and reused directly without further purification. The course of the reaction was monitored by quantitative gas chromatography. The catalytic activity for the epoxidation of cyclooctene was evaluated by the conversion of cyclooctene to cyclooctene epoxide.
The textural properties of a,b,2a,and 2b were evaluated by N2 adsorption-desorptionexperiments. These samples exhibit type IV isotherms according to the IUPAC classification,which is characteristic of mesoporous materials (Fig. 1). The appearance of H2 hysteresis loops with adsorption branches at a relative pressure of 0.8 and desorption branches at a relative pressure of 0.4 implies “ink-bottle” type pores [35]. Sample b possesses a smaller surface area and pore volume than a,which is attributed to a greater content of bulky imidazolium cations in the framework,and thus more strain in the mesostructure [32,33]. Samples 2a and 2b exhibit smaller surface areas,pore sizes,and pore volumes than a and b because of the incorporation of PMA into the PMO-ILs (Table 1) [29,36].
The XRD patterns of unsupported PMA,the PMO-IL supports a and b, and the PMA-based catalysts 2a and 2b are shown in Fig. 2. Unsupported PMA exhibits characteristic XRD peaks at 2θ = 18°-30° [37,38]. All hybrid materials exhibit negligible reflections at low-angle,suggesting the absence of long-range ordering. In the wide-angle region,a,b,2a,and 2b exhibit only one very broad peak centered at 22°,which is a characteristic of amorphous materials [35]. The absence of characteristic peaks of PMA in the XRD patterns of 2a and 2b suggests that PMA units are well-dispersed on the surface and in the channels of the PMO-IL supports [37,38].
TG-DTA was carried out to estimate the thermal stability of 2a (Fig. 3). A small weight loss below 100 °C is attributed to the desorption of adsorbed water and ethanol from pore channels. The obvious weight loss at 300-500 °C is due to the thermal dissociation of imidazolium moieties and PMA. This indicates that 2a isthermally stable up to at least 300 °C [14,33,39].
FT-IR spectra of a,2a,2b,and pure PMA are shown in Fig. 4. All hybrid materials exhibit absorptions at 1067 and 925 cm-1,which correspond to the stretching vibration of Si-O-Si. Absorptions at 1456,1561,and 1625 cm-1 correspond to C-H bending vibrations,or C=C and C=N stretching vibrations of the imidazolium ring,respectively [29,33]. In the spectrum of PMA,bands occur at 1064 (P-Oa stretching mode),959 (Mo=Ot terminal bonds),864 (Mo-Ob-Mo inter-bridge bonds),and 784 (Mo-Oc-Mo intra-bridge bonds) cm-1. For 2a and 2b,a significant shift in the Mo-Ob-Mo (882 cm-1) and Mo-Oc-Mo (810 cm-1) bands is observed,indicating the interaction between heteropoly acid anions and the PMO-IL support [15,19,40].
UV-Vis spectra of PMA,a,b,2a,and 2b are shown in Fig. 5. Neat PMA exhibits characteristic absorptions at ~217 and ~313 nm,which arise from charge transition from O2- to Mo6+ in Mo=O and Mo-O-Mo bonds,respectively. For 2a and 2b,these absorption peaks exhibit a small shift,indicating interactions between PMA anions and imidazolium cations in the mesoporous materials [12].
Solid-state 13C CP-MAS NMR was used to determine the nature of imidazolium compound incorporation in 2a and 2b (Fig. 6). 2a and 2b exhibit resonances typical of imidazolium rings (NCHN δ = 136.5,CHCH δ = 122.1) and propylene chains (SiCH2 δ = 5.9,CH2CH2CH2 δ = 24.2,CH2N δ = 51.8). These chemical shifts are comparable with those in the 13C NMR spectrum of the parent imidazolium IL. The absence of any further carbon signal indicates the preservation of the IL bridging structure during synthesis,extraction and PMA immobilization [29,32,33].
29Si CP-MAS NMR spectra provide direct evidence for the covalent bonding of silica species in the mesoporous materials (Fig. 7). The spectra of 2a and 2b exhibit resonances at δ = -110,-100,and -92,which correspond to the framework silica sites Q4 (silicon atoms without hydroxyl groups Si(OSi)4),Q3 (isolated hydroxyl group Si(OSi)3(OH)),and Q2 (germinal silandiols Si(OSi)2(OH)2),respectively. Signals at δ = -66 and -58 are due to T3 (R-Si(OSi)3) and T2 (R-Si(OH)(OSi)2) sites,respectively. The presence of T3 and T2 functionalities indicates that sufficient cross-linking occurs between the silylated IL and TEOS during condensation. This results in a strong linkage (predominantly three Si-O-Si covalent bonds) between the bridging imidazolium cations and silica matrix [32].
Solid-state 31P CP-MAS NMR spectra of PMA and 2a are shown in Fig. 8. The spectrum of 2a exhibits a resonance peak of structural P at around δ = -3.2,which is comparable with the P resonance in pure PMA. 31P NMR resonances are sensitive to the P atom coordination environment. The unchanged chemical shift of structural P in 2a suggests that the PMA structure is retained during immobilization [20]. The small half-width of the resonance indicates a highly symmetric environment about P in both free PMA and 2a.
The catalytic performance of 2a and 2b was investigated in the epoxidation of cyclooctene using tBuOOH as the oxidant and CHCl3 as the solvent. The catalytic properties of PMA-functionalized SBA-15 (2c) were also investigated under identical reaction conditions. Figure 9 shows that all catalysts are active for the epoxidation of cyclooctene. 2a and 2b give 90% and 72% yields of cyclooctene epoxide after 12 h of reaction at 50 °C,respectively. The reference catalyst 2c is more active,providing a 90% yield of cyclooctene epoxide after 7 h of reaction. All three PMA-based catalysts exhibit very high selectivity to cyclooctene epoxide (≥ 99%),and no detectable side products are observed under the test conditions.
The stability of the active species is extremely important in heterogeneous catalysts. Leaching tests were conducted for 2a,2b, and 2c (Fig. 10). In duplicate reactions,the solid catalysts were separated by hot filtration,and the filtrate was then further stirred. Continued activity at the reaction temperature was monitored. For 2a,cyclooctene is converted at a very low rate in the filtrate,which demonstrates that very little active material leached from the hybrid catalyst,and that the vast majority of catalysis is performed by the heterogeneous catalyst. For 2b,the epoxidation rate after removing the catalyst is marginally higher than that for 2a. For 2c,the yield of cyclooctene epoxide in the filtrate increases from 36% to 67% after 5 h,indicating that a significant amount of PMA leaching occurs during the reaction course. Thus,homogeneous catalysis is at least partly responsible for the high activity of 2c.
The recycling behavior of the solid catalysts was investigated using dried catalysts recovered by filtration and washed with CHCl3 (Table 2). 2a can be reused at least four times without obvious loss of activity and selectivity under identical reaction conditions. 2b exhibits a slight decrease after the initial cycle,and conversion of cyclooctene is largely retained after the second cycle,which is because of the loss of trace immobilized PMA. 2c exhibits an obvious decrease in catalytic activity with increasing cycling numbers,suggesting a poor reusability.
The influence of solvent and oxidant on the catalytic properties of 2a was also evaluated (Table 3). Under the tested conditions,the catalytic activity of 2a changes considerably with different solvents. The yield of cyclooctene epoxide decreases in the order of chloroform > acetonitrile > ethanol > toluene,indicating that the catalytic properties of 2a are solvent dependent. To estimate the influence of the oxidant,30% aqueous H2O2 was applied in the epoxidation of cyclooctene. As shown in Table 3,2a exhibits lower activity when using H2O2 as an oxidant,compared with the tBuOOH-based system. Obvious leaching of the PMA active species is also observed when H2O2 is used as the oxidant. These results indicate that tBuOOH is a more suitable oxidant than H2O2 for the PMA@PMO-IL catalytic system.
Polyoxometalates are reportedly active homogeneous catalysts for the epoxidation of olefins [7,41]. Hence,it is reasonable to expect that PMA@PMO-ILs catalysts should exhibit good activity in the epoxidation of cyclooctene. Similar to the homogeneous PMA catalyst,the supported PMA anions in 2a or 2b are the major active centers for epoxidation. These could activate the tBuOOH oxidant to form oxoperoxide intermediates and then facilitate the transfer of oxygen from the oxidant to the olefin [7,15,42,43].
The high stability and reusability of 2a and 2b are attributed to imidazolium cations in the PMO-IL support. Considering the ionic nature of the imidazolium group in the PMO-IL,it is believed that the PMO-IL nanostructure acts as an anion exchanger,allowing PMA anions to be immobilized on the PMO-IL through anion exchange [30]. An additional experiment was performed to prove the anion exchange reaction between PMA and Cl-of PMO-IL. A solution of H3PMo12O40 in methanol was gradually added to a suspension of a dispersed in methanol. The mixture was stirred at room temperature and then filtrated. The filtratewas tested using aqueous AgNO3 to verify the presence of Cl- as a consequence of anion exchange. The resulting cloudy precipitate was consistent with this assumption.
For the reference catalyst 2c,the terminal Si-OH groups of SBA-15 could also interfere with PMA anions via ion-dipole interactions or hydrogen bonds. However,the supported PMA anions are not very stable under the current test conditions. Karimi et al. [15] reported that an analogous PMA modified SBA-15 catalyst was active for the epoxidation of cyclooctene with H2O2 as an oxidant. Introducing aminopropyl groups on the SBA-15 surface reportedly further improved the catalytic activity and stability of the supported PMA catalyst. These results suggest that a relatively strong electrostatic interaction between PMA anions and imidazolium cations stabilizes the catalytic active units against leaching during reaction.
The characterization results show that a contains a lower amount of imidazolium cations within the framework than b. So why does the corresponding catalyst 2a stabilize a higher amount of ion-exchanged PMA than 2b? Further structural characterization suggests that the difference in the PMA loadings and catalytic properties of 2a and 2b may be associated with the different structural features of supports a and b. Sample a has a higher specific surface area,thus creating more accessible imidazolium cation sites for the electrostatic immobilization of PMA anions. The relatively large pore size of a should provide more space for introducing and stabilizing PMA units and further improve the stability of catalyst 2a.
Two PMA-functionalized catalysts were obtained through the electrostatic immobilization of PMA on PMO-ILs with different imidazolium contents. Both exhibited good activity and stability in the epoxidation of cyclooctene. The imidazolium cation present in the materials stabilized the PMA active units against leaching into the reaction medium. More efficient heterogeneous catalysts could be achieved by adjusting the surface properties of the PMO-IL support and/or introducing other types of polyoxometalate units into PMO-ILs.
2014, Vol. 35 Issue (4): 532-539
PDF (619KB)
2014, Vol. 35 Issue (4): 532-539
PDF (619KB)