Jeena Jyoti Boruah‡
*ab and
Siva Prasad Das‡*a
aDepartment of Chemistry, School of Science, RK University, Bhavnagar Highway, Kasturbadham, Rajkot-360020, Gujarat, India. E-mail: siva.spd@gmail.com; siva.das@rku.ac.in; jeena.jyoti@gmail.com; jeena.jyoti@rku.ac.in; Tel: +91-9678084296
bDepartment of Chemistry, Moridhal College, Moridhal, Dhemaji-787057, Assam, India
First published on 8th October 2018
Here, we have described the synthesis, characterization and catalytic activity of a dioxo-molybdenum(VI) complex supported on functionalized Merrifield resin (MR-SB-Mo). The functionalization of Merrifield resin (MR) was achieved in two-steps viz. carbonylation (MR-C) and Schiff base formation (MR-SB). The compounds, MR-C, MR-SB and MR-SB-Mo, were characterized at each step of the synthesis by elemental, SEM, EDX, thermal, BET and different spectroscopic analysis. The catalyst, MR-SB-Mo, efficiently and selectively oxidized a wide variety of alcohols to aldehydes or ketones using 30% H2O2 as an oxidant with reasonably good TOF (660 h−1 in case of benzyl alcohol). The catalyst acted heterogeneously under solventless reaction conditions and did not lead to over oxidized products under optimized conditions. The catalyst afforded regeneration and can be reused for at least five reaction cycles without loss of efficiency and product selectivity. A reaction mechanism for the catalytic activity of MR-SB-Mo was proposed and a probable reactive intermediate species isolated.
Traditional protocol for the oxidation of alcohol carried out by using stoichiometric inorganic oxidant such as permanganate, bromate, or Cr(VI) based reagents3 suffer from drawback such as large amount of heavy metal waste, generation of toxic by-products, difficulty in work up and requirement of larger amount of oxidizing agents.3,4 Moreover, in some cases, the oxidation reactions are performed under severe reaction conditions, such as high temperature and high oxygen pressure, in presence of environmentally undesired solvents, typically chlorinated hydrocarbons which are very toxic, corrosive and environmentally hazardous.3,4a,5 Therefore, search for alternative environmentally benign and safe protocols for the synthesis of different organic carbonyl compounds and their derivatives by alcohol oxidation continues unabated.
Although myriads of traditional user-friendly oxygen sources such as molecular oxygen, hydrogen peroxide, tert-butylhydroperoxide (TBHP) are used to eradicate such harmful waste for alcohol oxidation,5 but aqueous H2O2 constitutes a clean, waste avoiding, potentially green, and environmental-friendly ideal oxidant due to its cost effectiveness, easy availability, ecologically acceptable and it generates only water as the by-product.6 Also, the reaction parameters can be highly tuned by using H2O2 which play a vital role in oxidation reaction. We foresee that H2O2 and O2 (or air) will be complementary useful clean oxidants in practical chemical synthesis. Though molecular oxygen is an ideal oxidant, however, aerial oxidation has one or more of the major impacts such as difficult to control which result combustion and occasionally the reaction should have to be performed with a low conversion to circumvent from over oxidation limit their synthetic applications.6a Besides these, although both the oxygen atom present in O2 may be employed for oxidation to meet the metrices for measuring the ‘greenness’ i.e. 100% atom efficiency, but in most of the reactions, only one oxygen atom is used and show only 50% atom efficiency, consequently to meet the requirement, the oxidation reactions often need certain reducing agents to arrest the extra oxygen atom during the reaction.6a,7 The rate of oxidation towards H2O2 induced oxidation reaction including sulfide, olefin and alcohol oxidation is relatively slow or negligible owing to its weak oxidative nature and as a consequence, it has to be activated by using a suitable catalysts.1b,6b,8 As a result, immense number of transition metal based catalytic systems such as vanadium,9 osmium,10 palladium,11 ruthenium,12 manganese,13 tungsten,9f,g,14 molybdenum,9f,14a,e,f,15 rhenium,16 cobalt,11c,17 copper,11c,18 and iron19 have been developed towards oxidation of alcohols. Though the transition metal ion complexes are reported to have their catalytic activity with high selectivity, good efficiency and reproducibility, however several catalytic processes associate with one or more of the disadvantages such as requirement of additives, use of halogenated solvents, homogeneous in nature, production of huge waste materials, corrosion to the industrial materials and some of them are deposited on the reactor wall as well as disrupting the environmental and ecological stability. These shortcomings are fetching increasingly conspicuous in the light of growing ecological awareness in recent years.20 Interestingly, over the recent years, a new class of catalyst has been developed which utilizes light for alcohol oxidation.21 These photocatalysts absorbs light (visible or UV) and coming under “Greener” approaches. However, majority of these protocols also suffered from drawbacks of the requirement of organic solvents and higher reaction time.
Therefore, constructing a safer, efficient and highly selective heterogeneous catalyst towards alcohol oxidation with H2O2 as the oxidant is considered to be the better choice. Apart from these, heterogenization of homogeneous catalysts22 by immobilization of active soluble catalysts on insoluble polymeric matrix offers improved stability, increased product selectivity, easy separation and recycling to the precious metal complexes.22a,23 In this regard, the use of Merrifield resin, which is a chloromethylated polystyrene cross-linked with divinyl benzene,24 is quite reasonable as the resin is readily availability, easy to handle, cheap, mechanically and chemically robust and most importantly able to undergo facile functionalization as well as provide high-loading capacity.23a,25
Keeping these facts into our account, we presented here the synthesis and characterization of a Merrifield resin supported dioxomolybdenum(VI) compound which acts as a heterogenous catalyst for the efficient and selective oxidation of alcohols to aldehydes or ketones. Interestingly, the protocol worked under solventless conditions with aqueous H2O2 as an oxidant and achieved the product selectivity with a wide range of substrates viz. primary, secondary and benzylic alcohols.
Compound | Data obtained from elemental analysis (%) (data obtained from EDX analysis (%)) | Metal loadinga (mmol g−1 of polymer) | ||||
---|---|---|---|---|---|---|
C | H | N | Cl | Mo | ||
a Metal loading = (Observed molybdenum % × 10)/(atomic weight of molybdenum).b Data obtained from AAS. “—” stands for not determined.c Molybdenum content determined by AAS after 5th reaction cycle. | ||||||
MR | 83.20 | 6.98 | — | — | — | — |
(83.13) | — | (9.82) | ||||
MR-C | 87.10 | 7.31 | — | — | — | — |
(87.04) | — | (1.99) | ||||
MR-SB | 86.02 | 7.22 | 4.68 | — | — | — |
(68.14) | — | (4.66) | (1.69) | |||
MR-SB-Mo | 78.07 | 6.20 | 4.41 | — | 4.32b | 0.45 |
(78.21) | — | (4.36) | (4.60) | (4.28) | ||
4.29c |
The scanning electron micrographs (SEM) of the compounds obtained at different stages in the preparation of MR-SB-Mo are shown in Fig. 1. It is revealed from the micrograph that the virgin polymer beads, MR undergoes striking morphological changes to form MR-SB-Mo. The surface of the smooth and spherical beads of MR became lightly rough after carbonyl functionalization (MR-C, Fig. 1(b)) as well as Schiff base formation (MR-SB, Fig. 1(c)). The even roughening as seen in the images also suggest the uniform covalent functionalization on the surface of the MR beads. However, after molybdenum coordination, randomly oriented depositions were observed causing further roughening on the surface of the polymeric beads (Fig. 1(d)). The EDX spectra of MR-SB-Mo (Fig. 1(e)) showed the presence of carbon, nitrogen, oxygen, chlorine and molybdenum on the support.
Fig. 1 Scanning electron micrographs of (a) MR, (b) MR-C, (c) MR-SB, and (d) MR-SB-Mo. EDX spectra of (e) MR-SB-Mo. |
The powder XRD patterns of MR, MR-C, MR-SB and MR-SB-Mo are shown in Fig. 2 and were recorded at 2θ values between 5 and 70°. A broad peak cantered at 2θ value of ca. 20° was observed in pristine MR. This type of diffraction pattern is characteristic for the PS-DVB resin.28 Similar to MR, almost identical diffraction patterns were observed in MR-C, MR-SB and MR-SB-Mo. This type of broad peak indicates that the developed catalyst and its precursors, i.e., MR, MR-C and MR-SB are almost amorphous in nature. Moreover, the absence of sharp diffraction peaks in MR-SB-Mo indicated the amorphous nature of the attached MoO22− moiety. It is notable that during the synthesis of the MR-SB-Mo via different functionalization steps, there is no change in amorphousness of MR.
In order to gain the electronic properties for the surface of MR-SB-Mo, XPS spectrum of the compound was recorded and presented in Fig. 3. From the figure it is seen that the spectrum displayed two well resolved peaks at 232.5 and 235.7 eV in the Mo (3d) region.29 On the basis of available literature reports,29 these peaks correspond to Mo (3d3/2) and Mo (3d5/2), respectively which is attributable to the molybdenum centers in +6 oxidation state, i.e., diamagnetic. This diamagnetic behavior of MR-SB-Mo shown by XPS analysis is in agreement with the results obtained from magnetic susceptibility analysis.
Fig. 3 XPS Mo (3d3/2) and Mo (3d5/2) spectra of (a) MR-SB-Mo and (b) MR-SB-Mo after the 5th reaction cycle. |
The surface area, pore volume, and pore size of the synthesized compounds viz. MR, MR-C, MR-SB, and MR-SB-Mo, were investigated by N2 absorption and desorption measurements at liquid nitrogen temperature. The surface areas were measured by following the Brunauer–Emmett–Teller (BET) method30 and the pore volume was determined by following the Barrett–Joyner–Halenda (BJH) model in the nitrogen isotherms.31 The data are presented in Table 2. It is seen from the data that the surface areas, pore volumes and pore radius were decreased with increasing functionalization steps. This is because the functionalization may as well as metal loading blocked the pore of the polymeric beads. Similar types of observation were reported earlier with MR based catalysts.32 The nitrogen adsorption/desorption isotherms showed typical TYPE II adsorption (Fig. S1, ESI†) of an IUPAC standard33 which is the characteristics of macroporous or nonporous material.34
Vital information can be derived by comparing the infrared (IR) spectral data for the compounds at each synthetic step. After each synthetic step, characteristic new peaks may appear or shifted to a new position which confirmed the chemical transformations. The FT-IR spectra for the compounds are shown in Fig. 4 and important peaks are assigned in Table 3. On the basis of available reports, the change in intensity of the ν(C–Cl) peak is found to be the indicator of the functionalization of chloromethyl group.32a,35 It is evident by comparing the IR spectra of MR and MR-C that the strong peak appeared at 1262 cm−1 in MR which is attributed to ν(C–Cl)32a,35a,b was disappeared (or significantly decreased the intensity) after carbonyl functionalization in MR-C. It is pertinent here to mention that after carbonyl functionalization the concentration of chloromethyl group in MR was dropped nearly 80%. Thus, this observation is in consistent with the data obtained from elemental analysis. Besides this, a strong peak appeared at 1728 cm−1 which is due to ν(CO) of the aldehyde group.36 Additionally, two new weak intensity peaks appeared for ν(C–H)aldehydic at 2826 and 2721 cm−1.36b,c This further confirmed the conversion of chloromethyl group to aldehyde group.
Fig. 4 FTIR spectra for (a) MR, (b) MR-C, (c) MR-SB, (d) MR-SB-Mo, and (e) MR-SB-Mo after 5th reaction cycle. |
Compound | Peak position (cm−1) | Peak assignment | |
---|---|---|---|
IR | Raman | ||
a Py, pyridine; vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder; “—” stands for not seen. | |||
MR | 1262 (s) | 1260 (w) | ν(C–Cl) |
MR-C | 1261 (vw) | — | ν(C–Cl) |
1728 (vs) | 1730 (vw) | ν(CO) | |
2826 (m) | 2822 (w) | ν(C–H)aldehydic | |
2721 (m) | 2726 (w) | ||
MR-SB | 1261 (vw) | — | ν(C–Cl) |
1638 (vs) | 1640 (m, sh) | ν(CN)imine | |
1573 (s) | 1570 (m, sh) | ν(CN)py | |
609 (m) | 610 (m) | Py(in-plane ring deformation) | |
408 (m) | 407 (m) | Py(out-of-plane ring deformation) | |
MR-SB-Mo | 1263 (vw) | — | ν(C–Cl) |
1622 (vs) | 1625 (m, sh) | ν(CN)imine | |
1597 (s) | 1596 (m, sh) | ν(CN)py | |
964 (s) | 962 (s) | ν(OMoO)asym | |
911 (s) | 910 (s) | ν(OMoO)sym | |
640 (m) | 645 (m) | Py(in-plane ring deformation) | |
440 (m) | 441 (m) | Py(out-of-plane ring deformation) |
In MR-SB, a new strong peak appeared at 1638 cm−1 which is attributable to ν(CN)imine.36a,37 However, the peak at 1728 cm−1 due to ν(CO) in MR-C was disappeared in MR-SB. This confirmed the condensation of aldehyde group with the amine group of 2-aminomethylpyridine to form an imine. Additionally, new medium to strong intensity peaks were appeared at 1573 (s), 609 (m), and 408 (m) cm−1 which can be assigned to ν(CN)pyridine, Py(in-plane ring deformation), and Py(out-of-plane ring deformation), respectively of a pyridine moiety.37,38 Thus the IR spectral analysis clearly confirmed the formation of MR-SB as shown in Scheme 1. Interestingly, in MR-SB-Mo, new and strong peaks appeared at 964 and 911 cm−1 which were assigned for ν(OMoO)asym and ν(OMoO)sym, respectively.39 Further the peak due to ν(CN)imine appeared at 1638 cm−1 of MR-SB was shift to 1622 cm−1 in MR-SB-Mo. In view of the existing literature, such lower shifting of ν(CN)imine peak confirmed the coordination of imine group with molybdenum(VI) center.36a Apart from this, the peaks due to ν(CN)pyridine, Py(in-plane ring deformation) and Py(out-of-plane ring deformation) were shifted to higher wave no after molybdenum(VI) coordination. This type of shifting suggest the coordination of pyridine with the metal center.37,38a Thus the FTIR spectral analysis clearly showed the formation of MR-C, MR-SB and MR-SB-Mo.
The Raman spectral analysis further confirmed the formation of MR-C, MR-SB and MR-SB-Mo that shown by FTIR analysis. The spectral data of the samples are given in Table 3 and the spectrum for MR-SB-Mo is shown in Fig. 5. The strong peaks appeared commonly in each of the spectrum at ca. 1600 and 1000 cm−1 are attributed to the benzene skeletal vibrations arising from the polymeric backbone.32a,40 The weak peak appeared at 1260 cm−1 due to ν(C–Cl) mode of –CH2Cl group in MR was absent in MR-C.40a This observation is in consistent with the results obtained from IR spectral analysis. The very weak peak appeared in the spectrum of MR-C at 1730 cm−1 is attributed to the ν(CO) mode of aldehydic group. Similar to IR spectral analysis, this peak was vanished in the spectrum of MR-SB due to the Schiff base formation. The ν(CN)imine and ν(CN)pyridine peaks were appeared as medium intensity shoulder in the spectrum of MR-SB. However, after complex formation in MR-SB-Mo, the former peak was shifted to lower wave number and the latter was shifted to higher wave number that confirmed the coordination of the imine and pyridine moieties with the molybdenum(VI) center (through nitrogen atom). Further, the Raman spectrum of MR-SB-Mo displayed two strong peaks at 962 and 910 cm−1 for ν(OMoO)asym and ν(OMoO)sym, respectively. These two peaks are characteristic of dioxomolybdenum(VI) moiety.41 Thus, the Raman spectral analysis confirmed the formation of MR-C, MR-SB and MR-SB-Mo.
The diffuse reflectance UV–visible spectra of MR-SB and MR-SB-Mo were recorded by taking BaSO4 as reference. The spectrum of MR-SB display two well resolved peaks at ca. 264 and 345 nm. The relatively higher energy peaks at 264 nm corresponds to the π → π* transition of the pyridyl ring, benzene ring of polymeric support and imine functional group.37,42 On the other hand, the lower energy peak at 345 nm may be assigned to n → π* transition of the pyridyl ring and imine functional group.37,42 Interestingly, in MR-SB-Mo, the peak for n → π* transition was shifted to ca. 331 nm. But the peaks for π → π* transition were remained unaffected after complex formation. This blue shift of n → π* transition indicate the coordination of molybdenum(VI) via the pyridyl and imine nitrogen.37,42 Beside these, there was no LMCT bands present in the UV-Vis spectrum of MR-SB-Mo. This is because those bands generally appear at higher concentration of metal complexes.
The thermal stability of the compounds was studied with the help of TGA-DTG analysis. The thermograms are shown in Fig. 6 and data are presented in Table 4. The thermograms showed multiple stage of thermal degradation for each of the compounds. The pristine polymer, MR showed three weight loss steps in the temperature range of 291–366, 366–477, and 477–700 °C with corresponding weight loss of 14.23, 39.53, and 46.24%, respectively. On the basis of available literatures data, the first step of decomposition is attributed to the loss of chloromethylated group and the subsequent steps are attributed to the decomposition of the cross-linked polystyrene backbone.43 The MR was completely decomposed at ca. 700 °C. Similar to MR, MR-C also showed three stage thermal decomposing. The thermogram of MR-SB provided vital information regarding the Schiff base loading on the support. The decomposition at ca. 177–230 °C with weight loss of 20.77% matched exactly with the data obtained from elemental analysis for Schiff base loading (20.53% Schiff base, based on N content). Further evidence about the loss of Schiff base ligand at this stage was confirmed by recording IR spectrum of the sample at this temperature which did not show the presence of characteristic peak for ν(CN)imine. The subsequent decomposition steps are attributed to the decomposition of the polystyrene backbone. The thermogram of MR-SB-Mo showed four steps of thermal degradation at the temperature range of 195–234, 299–350, 350–486, and 486–700 °C with the corresponding weight loss of 18.86, 2.34, 29.90, and 39.27%, respectively. Interestingly, the compound never decomposed completely upto 700 °C which was due to the residual oxo-molybdenum species formed after complete decomposition of polymeric materials.32a,44 The thermal stability of upto 195 °C for MR-SB-Mo also provide additional evidence about the stability of the catalyst under different reaction temperatures.
Compound | Temperature, oC | Weight loss (%) |
---|---|---|
MR | 291–366 | 14.23 |
366–477 | 39.53 | |
477–700 | 46.24 | |
MR-C | 296–344 | 3.27 |
344–480 | 46.74 | |
480–700 | 48.92 | |
MR-SB | 177–230 | 20.77 |
294–340 | 2.46 | |
340–480 | 32.08 | |
480–700 | 43.47 | |
MR-SB-Mo | 195–234 | 18.86 |
299–350 | 2.34 | |
350–486 | 29.90 | |
486–700 | 39.27 |
On the basis of the above analysis, a structure for MR-SB-Mo is proposed as shown in Scheme 1 where a dioxomolybdenum(VI) species is anchored to the support via coordination with the nitrogen atom of imine and pyridine group.
In order to gain a standard reaction condition for product selectivity and better yield for oxidation of alcohols to ketones or aldehydes several reaction parameters such as solvents, amount of catalyst and oxidant, reaction temperature, etc. were screened using benzyl alcohol as model substrate.
In the initial investigation, the oxidation of benzyl alcohol was carried out with varying amount of 30% aqueous H2O2 as oxidant. We have conducted the reactions by keeping the molar ratios of benzyl alcohol:H2O2 at 1:0.5, 1:1.1, 1:1.5 and 1:2 and Mo: benzyl alcohol at 1:1000. The results are summarized in Table 5. The reaction at molar ratio of 1:0.5 was not completed even after 5 h [Table 5, entry 1]. So, we have increased the amount of H2O2 to 1:1.1, i.e., slightly excess than the substrate. At this molar ratio, the reaction comfortably completed in 90 min with benzaldehyde as the sole product [Table 5, entry 2]. With a view to decrease the reaction time, we have further conducted the reactions at 1:1.5 and 1:2, but product selectivity was lost and formed 6% and 9% of benzoic acid, respectively [Table 5, entries 3 and 4]. Thus, the molar ratio of substrate:H2O2 at 1:1.1 is found to be the optimum condition for selectively oxidize benzyl alcohol to benzaldehyde.
Sl. no. | Molar ratio | Solvent | Temperature (°C) | Time (min) | Isolated yield (%) | Selectivity (%) a:b | TOFb (h−1) | |
---|---|---|---|---|---|---|---|---|
Mo:Sc | S:H2O2c | |||||||
a All reactions were carried out with benzyl alcohol as substrate (2.5 mmol), MR-SB-Mo (5.6 mg for 0.0025 mmol of Mo) and 5 mL solvent (unless otherwise indicated).b TOF = (mmol of product)/[(mmol of catalyst) × (time)].c ‘S’ stands for substrate.d Reaction with 6% aqueous H2O2 as oxidant.e Reaction with 50% aqueous H2O2 as oxidant.f Reaction with 70% aqueous TBHP as oxidant.g Reaction conducted with MR-SB-Mo but no added oxidant.h Reaction conducted without MR-SB-Mo or blank reaction.i Reaction conducted under optimum condition with SB-Mo (1.5 mg, 0.0025 mmol) as catalyst. | ||||||||
1 | 1:1000 | 1:0.5 | Solventless | 65 | 300 | 47 | 100:0 | 94 |
2 | 1:1000 | 1:1.1 | Solventless | 65 | 90 | 99 | 100:0 | 660 |
3 | 1:1000 | 1:1.5 | Solventless | 65 | 70 | 97 | 91:6 | 831 |
4 | 1:1000 | 1:2.0 | Solventless | 65 | 60 | 98 | 89:9 | 980 |
5 | 1:1000 | 1:1.1 | Water | 65 | 90 | 87 | 100:0 | 580 |
6 | 1:1000 | 1:1.1 | Acetonitrile | 65 | 90 | 89 | 100:0 | 593 |
7 | 1:1000 | 1:1.1 | Chloroform | 65 | 90 | 24 | 100:0 | 160 |
8 | 1:1000 | 1:1.1 | Dichloromethane | 65 | 90 | 23 | 100:0 | 153 |
9 | 1:1000 | 1:1.1 | Toluene | 65 | 90 | 20 | 100:0 | 133 |
10 | 1:500 | 1:1.1 | Solventless | 65 | 80 | 98 | 100:0 | 368 |
11 | 1:100 | 1:1.1 | Solventless | 65 | 70 | 99 | 100:0 | 84 |
12 | 1:1000 | 1:1.1d | Solventless | 65 | 150 | 96 | 100:0 | 384 |
13 | 1:1000 | 1:1.1e | Solventless | 65 | 70 | 94 | 92:2 | 806 |
14 | 1:1000 | 1:1.1f | Solventless | 65 | 300 | 97 | 100:0 | 194 |
15 | 1:1000 | In air | Solventless | 65 | 90 | 0 | — | 0 |
16 | 1:1000 | In O2 | Solventless | 65 | 90 | 0 | — | 0 |
17 | 1:1000 | 1:1.1 | Solventless | RT | 300 | 98 | 100:0 | 196 |
18 | 1:1000 | 1:1.1 | Solventless | 50 | 120 | 99 | 100:0 | 495 |
19 | 1:1000 | 1:1.1 | Solventless | 90 | 90 | 91 | 100:0 | 607 |
20g | 1:1000 | — | Solventless | 65 | 90 | 0 | — | 0 |
21h | — | 1:1.1 | Solventless | 65 | 90 | 0 | — | 0 |
22i | 1:1000 | 1:1.1 | Solventless | 65 | 90 | 53 | 91:9 | 353 |
Keeping the molar ratio of substrate:H2O2 at 1:1.1, we have screened the oxidation of benzyl alcohol by using different solvents such as water, acetonitrile, chloroform, dichloromethane, toluene, etc. as well as under solventless condition. The highest activity in terms of TOF was found in solventless condition [Table 5, entry 2]. The reaction conducted in water decreased the TOF [Table 5, entry 5]. This may be due to dilution of the reaction mixture by the added water. In acetonitrile, the reactivity is not improved [Table 5, entry 6]. Further, reactions in chloroform and dichloromethane are very slow with poor TOF [Table 5, entries 7 and 8]. This may be due to the immiscibility of the oxidant with the solvents. Moreover, we do not intend to use halogenated solvents. Similar type of reactivity was also found while doing reaction in toluene [Table 5, entry 9]. The use of methanol and ethanol were avoided as these solvents may compete for oxidation. Indeed, the reactions were very slow and not completed (data not shown). Thus, solventless condition was found to be optimum for selectively oxidize benzyl alcohol to benzaldehyde.
The catalyst amount has a great impact on TOF. We have conducted reactions by keeping molar ratio of Mo:benzyl alcohol at 1:100, 1:500 and 1:1000. Each of the reactions were conducted by keeping benzyl alcohol: H2O2 at 1:1.1 and under solventless condition at 65 °C. There was decrease in reaction time with increased amount of catalyst but the TOF was significantly decreased [Table 5, entries 2, 10 and 11].
Reactions were conducted with 6%, 30% and 50% of aqueous H2O2 solutions [Table 5, entries 2, 12 and 13] at molar ratio of benzyl alcohol: H2O2 and Mo:benzyl alcohol at 1:1.1 and 1:1000, respectively under solventless condition at 65 °C. The slowest reaction and lowest TOF was found with 6% H2O2 whereas fastest was found with 50% H2O2 [Table 5, entry 13]. However, trace amount of carboxylic acid was formed with 50% H2O2, i.e., loss of product selectivity. The slowest reaction with 6% H2O2 may be due to the dilution of the oxidant in the reaction mixture as was found in case of reactions in water as solvent [Table 5, entry 5]. Therefore, we have chosen 30% H2O2 as optimum for this reaction.
We have also screened the reaction with different types of oxidant such as 30% H2O2 (aqueous), t-butyl hydroperoxide (70% aqueous solution, TBHP), air and O2 balloon. The highest TOF was found with 30% H2O2 [Table 5, entry 2] and no reaction with air or O2 balloon under identical reaction conditions [Table 5, entries 15 and 16]. There were 3 to 4-fold increase in reaction time with TBHP [Table 5, entry 14] as an oxidant. Moreover, it was reported that TBHP generates butanol as byproduct.45 Interestingly, the use of H2O2 is advantageous as it is cheap, environmentally clean, easy to handle and water is the only byproduct.6a–c,e,g
The reaction temperature has a remarkable impact on rate of reaction. So, we have conducted reactions at different temperatures viz. room temperature, 50, 65, and 90 °C [Table 5, entries 2, 17–19]. The reactions were conducted under solventless condition by keeping molar ratio of benzyl alcohol:H2O2 (30%):Mo at 1000:1100:1. It was found that with increasing reaction temperature, the TOF increases and reached a highest value at 65 °C. Thereafter, the TOF decreases which indicates that 65 °C is the optimum reaction temperature.
Thus the optimum reaction condition for the selective oxidation of benzyl alcohol to benzaldehyde was found to be substrate:H2O2 (30%):Mo at 1000:1100:1 under solventless condition at 65 °C as shown in Scheme 2.
Scheme 2 Optimum reaction condition for oxidation of alcohol catalyzed by MR-SB-Mo using 30% H2O2 as oxidant. |
Having gained the optimal conditions, we explored the substrate scope of the newly developed catalyst, MR-SB-Mo under the optimum condition, for the selective oxidation of a wide range of alcohols such as primary, secondary and benzyl alcohols to their corresponding aldehydes or ketones. The results are summarized in Table 6. It is seen from the table that each of the substrate oxidized in high yields with reasonably good TOF. Besides this, under same reaction conditions, benzylic [Table 6, entries 1–9] and secondary alcohols [Table 6, entries 10–15] were found to be oxidize relatively at faster rate than the primary alcohols [Table 6, entries 16–20]. The beauty of the protocol is that no overoxidation to carboxylic acid took place with all the studied substrates. In case of substituted benzyl alcohols, different types of substituents such as –F, –Cl, –Br, –OMe, –OH and –NO2 well-tolerate during the oxidation process [Table 6, entries 2–7], some of which could be utilized for further derivation. One of the notable aspect of the developed catalytic system is its ability to oxidize benzyl alcohol to benzaldehyde at relatively higher scale (10 g scale) without losing the catalytic efficiency and product selectivity [Table 6, entry 1d] which provides its potential application towards commercial processes. It is pertinent here to mentioned that in a separate blank reaction using benzyl alcohol as substrate, i.e., under identical optimum conditions without the added catalyst, the reaction did not progress within the stipulated time which indicate the active role of the catalyst in the oxidation processes (Table 5, entry 21). Similarly, the reaction was not successful without the added H2O2 (Table 5, entry 20).
Sl. no. | Substrate | Time (min) | Product | Isolated yield (%) | TOFb (h−1) |
---|---|---|---|---|---|
a Reaction conditions: unless otherwise stated, all reactions were performed solventless at 65 °C using 2.5 mmol of substrate, 5.6 mg of MR-SB-Mo (contain 0.0025 mmol of Mo) and 2.75 mmol of 30% aqueous H2O2.b TOF=(mmol of product)/[(mmol of catalyst) × (time)].c Yield at 5th reaction cycle.d Yield at 10 g scale reaction.e Reaction conducted with 2 mL acetonitrile. | |||||
1 | 90 | 99 | 660 | ||
653 | |||||
646 | |||||
98c | |||||
97d | |||||
90 | |||||
90 | |||||
2e | 100 | 97 | 582 | ||
3e | 100 | 96 | 576 | ||
4e | 100 | 97 | 582 | ||
5e | 105 | 97 | 554 | ||
6e | 100 | 99 | 594 | ||
7 | 100 | 98 | 588 | ||
8 | 120 | 98 | 490 | ||
9 | 105 | 97 | 554 | ||
10 | 150 | 97 | 388 | ||
11 | 135 | 96 | 426 | ||
12 | 165 | 98 | 356 | ||
13e | 180 | 98 | 327 | ||
14 | 165 | 96 | 349 | ||
15 | 180 | 97 | 323 | ||
16 | 240 | 99 | 248 | ||
17 | 270 | 96 | 213 | ||
18 | CH3(CH2)3CH2OH | 240 | CH3(CH2)3CHO | 97 | 243 |
19 | CH3(CH2)8CH2OH | 270 | CH3(CH2)8CHO | 98 | 218 |
20 | CH3(CH2)2CH2OH | 240 | CH3(CH2)8CHO | 96 | 240 |
In order to check the advantage of synthesizing the heterogeneous catalyst, MR-SB-Mo, over homogeneous catalyst, a neat dioxomolybdenum complex, SB-Mo was synthesized (detailed synthetic procedure is in ESI†). The ligand for the neat complex was designed in such a way that it provides almost identical coordination environment that present in MR-SB-Mo. Moreover, the catalytic oxidation reaction was conducted under identical optimum condition using benzyl alcohol as substrate. From the reaction it was seen that the SB-Mo could reached upto TOF = 353 h−1 within the stipulated time period (Table 5, entry 22) and produced benzaldehyde along with benzoic acid (9%). Thus, under identical optimum condition the heterogeneous catalyst, MR-SB-Mo showed superior catalytic activity in terms of product yield as well as product selectivity over the homogeneous catalyst, SB-Mo.
In order to compare the catalytic activity of MR-SB-Mo over the reported catalyst, a separate comparison table containing catalytic activity of reported molybdenum-based catalyst towards oxidation of benzyl alcohol is given in ESI (Table S1†). From the table it is seen that MR-SB-Mo exhibit superior activity over the reported catalysts.
The change of substituents in the ligand environment of the metal complexes lead to a dramatic change in their catalytic activity. In this regard, the Hammett relation47 can be used to predict the catalytic activity of MR-SB-Mo. The substituent constant, σ of Hammett equation, for the substituents with (+)ve and (−)ve values indicate the electron withdrawing and electron releasing group, respectively. Thus, choosing substituents from the series could help to design a better catalyst for their activity. From the available literature report,14f,47,48 it is found that oxidation of alcohol by the peroxometal complexes take place by electrophilic attack of the peroxo moiety on the oxygen atom of the alcohol group (–OH). Thus, adding substituents with (+)ve σ-value in the ligand environment of MR-SB-Mo will increase the electrophilicity of the peroxo moiety which in return increase the catalytic activity of the catalyst. The catalyst, MR-SB-Mo can be substituted in its both the aromatic rings viz. phenyl or pyridyl ring. So, the synthesis of MR-SB-Mo with electron withdrawing substituents anticipated a higher reaction rate.
Footnotes |
† Electronic supplementary information (ESI) available: N2 adsorption–desorption isotherms, IR spectra of the isolated peroxomolybdate(VI) intermediate, comparison table, and detailed synthetic procedure and characterization of SB-Mo. See DOI: 10.1039/c8ra05969a |
‡ Both the authors contributed equally to this work. |
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