b Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China;
c College of Environmental and Chemical Engineering, Heilongjiang University of Science and Technology, Harbin 150022, China
Methyl methacrylate (MMA), a raw material for the production of organic glass and higher alkyl methacrylate, is widely used in the fields of automobile, medicine, communication and architecture due to its outstanding physical and chemical characteristic, and has emerged as a key chemical product [1, 2, 3]. However, MMA is easy to self-polymerize in the process of separation, purification, storage as well as transportation even at room temperature due to the existence of highly active MMA monomers .
The self-polymerization of MMA usually resulted in the loss of MMA products or impurities. Therefore, a certain quantity of inhibitor should be added to prevent the polymerization reaction. At present, many polymerization inhibitors can be used for solving this problem, such as organic sulfur compound, 1, 1-diphenyl-2- picryl-hydrazyl (DPPH), quinones, polyphenol, aromatic amines and nitrates. Besides, there are also some inorganic inhibitors, for instance, ferric chloride, titanium trichloride, cupric chloride, elemental sulfur and so on [5, 6]. However, most of the above mentioned inhibitors usually result in environmental pollution and hard to be post-processed, which limited their application. Heteropoly acid is a promising green catalyst that can be used for both homogeneous and heterogeneous catalytic reaction systems. It is friendly to the environment when used as an inorganic inhibitor, and possesses better catalytic activity, selectivity and thermal stability [7, 8, 9]. Hence, heteropoly acid has been widely used as polymerization inhibitors for alkene monomers in recent years . H3 + xPMo12-xVxO40, as a derivative of heteropoly acid, possess both the structure of heteropolyanion and the quaternary ammonium cation. Therefore, it is a better phase transfer catalyst that could transfer reactants from one phase to another, accelerate the reaction rate in a heterogeneous system, and display a better inhibition effect.
Herein, in this work, a series of molybdovanadophosphoric heteropoly acid quaternary ammonium salts, H3 + xPMo12-xVxO40-T were successfully synthesized through reacting molybdovanadophosphoric heteropoly acid with tetrabutylammonium bromide, and utilized as inhibitors in the polymerization reaction of MMA. The influence factors and mechanism of the inhibition reaction were explored using differential scanning calorimetry (DSC) and cyclic voltammograms (CV). The results show that H3 + xPMo12-xVxO40-T is a satisfied self-polymerization inhibitor for MMA due to its outstanding redox properties.2. Experimental
Instruments and reagents: All chemicals were obtained from commercial sources and used without further purification. IR spectra were recorded on an FTIR IFS66 V/S spectrometer from KBr pellets; UV-vis absorption spectra were recorded on a SPECORD S600 spectrophotometer; XRD patterns were performed between 108 and 408 (2u) on Empyrean; The element analysis of H3 + xPMo12-xVxO40-T were carried out on a 7500 typed inductively coupled plasma mass spectrometry (ICP-MS); Cyclic voltammetry were recorded using a CHI660E electrochemical workstation; Differential Scanning Calorimeter were recorded on a 201F1 equipment.
Synthesis of H3 + xPMo12-xVxO40-T: Keggin type of molybdovanadophosphoric acid H3 + xPMo12-xVxO40(H4PMo11V1O40·25H2O, H5PMo10V2O40·30H2O, H6PMo9V3O40·24H2O, H7PMo8V4O40·24H2O) were firstly prepared according to the literature [11, 12]. Then, its quaternary ammonium salt H3 + xPMo12-xVxO40-T [(C16H36BrN)4PMo11V1O40·25H2O,(C16H36BrN)5 PMo10V2O40·30H2O,(C16H36BrN)6PMo9V3O40·24H2O,(C16H36BrN)7 PMo8V4O40·24H2O)] were prepared by a modified procedure described in the literature using the prepared H3 + xPMo12-xVxO40 ·nH2O [13, 14]. In a typical procedure, H3 + xPMo12-xVxO40 (0.2 mmol) was dissolved in deionized water (30 mL). After 5 min, TBAB (0.8 mmol) was slowly added to the solution under continuous stirring for 2 h at 40℃. The reactant was allowed to cool to room temperature and was stirred for 20 min followed by filtration and abstersion with deionized water. The obtained solid was dried in an oven at 50℃ for 1 h.
DSC test of inhibition effects of H3 + xPMo12-xVxO40-T on MMA: Differential scanning calorimetry device is a precision instrument that could accurately measure transition temperature and enthalpy. In order to analyze the inhibition effect of H3 + xPMo12-xVxO40-T on the MMA polymerization reaction, DSC was used to test the peak temperature (Tp) under different heating rates. 0.005 g of initiator benzoyl peroxide (BPO) and a certain amount of H3 + xPMo12-xVxO40-T were accurately weighted with the electronic weighting scales and dissolved in a 20 mL beaker containing 10 mL of MMA. Under vigorous stirring, 10 mg of the above solution was put into a pre-weighed empty crucible. Then crucible was weighed again with a micro-scale sampler and the quantity of added sample can be calculated. The initial and final temperatures were set to the room temperature and 150℃, respectively. The heating rate (B) was 5℃/min, 10℃/min, 15℃/min and 20℃/min, respectively, with the nitrogen flow rate of 20 mL/min being controlled by a flowmeter.
CV characterization of H3 + xPMo12-xVxO40-T on MMA: Cyclic voltammetry (CV) was recorded using a CHI660D electrochemical potentiostat. The measurements were carried out in a threeelectrode cell under nitrogen. The working electrode was a 213 type planar platinum electrode and the auxiliary electrode was a platinum wire. The reference electrode was an Ag/AgCl electrode. A solution of 0.1 mol/L tetrabutylammonium hexafluorophosphate (TBAPF6) in dry DMF was used as an electrolyte. Before the experiment, high purity nitrogen was flowed into the solution for 15 min with a scan rate of 50 mV/s.3. Results and discussion 3.1. Synthesis and characterization of H3 + xPMo12-xVxO40-T
The synthetic routes of H3 + xPMo12-xVxO40-T (x = 1, 2, 3, 4) are depicted in Eqs. (1) and (2). All of the compounds were obtained in good yields (ca. 68%-82%) as air stable yellow to red solids with an increased proportion of vanadium atoms. And all the compounds were characterized by IR and UV-vis. The successful synthesis of H3 + xPMo12-xVxO40-T was further confirmed by X-ray diffraction analysis.
The IR spectra of as-prepared H3 + xPMo12-xVxO40-T are shown in Fig. 1. As can be seen from Fig. 1, four characteristic peaks in the range of 700-1100 cm-1 are observed. These characteristic peaks indicate a Keggin structure of H3 + xPMo12-xVxO40-T, which is agreement with the literature [15, 16, 17]. The peak at 876 cm-1 is attributed to the antisymmetric stretching vibration absorption of Mo-Ob-Mo bond and the peak at 955 cm-1 is associated with antisymmetric stretching vibration of Mo55O and V55O. As the number of the vanadium atoms increased, the molecular symmetry decreased and the center ion was distorted, resulting in an unimodal split of V55O, and a red shift of the vibration peaks . As shown in Fig. 1, H4PMo11V1O40-T exhibits the smallest red shift of 2-3 cm-1 and C16H36BrN-PMo8V4 exhibits the maximum red shift of 4-6 cm-1. The characteristic peak at 1065 cm-1 is assigned to the antisymmetric stretching vibration absorption of P-O bond. The peaks at 2870 cm-1and 2960 cm-1 are the antisymmetric stretching vibration of -CH3 and the characteristic peak of -CH2- is at 2930 cm-1. The characteristic peaks at 1630 cm-1 and 3430 cm-1 illustrate that quaternary ammonium salt has been formed with a certain amount of crystal water. In as-synthesized H3 + xPMo12-xVxO40-T, Mo-Ob-Mo bond red shifts by 3-15 cm-1, P-O and Mo55O are blue shifted by about 18 cm-1, respectively. These shifts are attributed to the incoming of quaternary ammonium cations, whose cationic volume effect weakens the interaction between anions and the position of the absorption peak was fluctuated to different degrees. Therefore, the absorption peak position of tetrabutyl ammonium bromide shifts red or blue compared with the report in literature due to the interactions between quaternary ammonium cationics and heteropoly anions .
Fig. 2 presents the results of UV-vis absorption for the samples of H3 + xPMo12-xVxO40-T. The broad absorption peak in the wavelength range of 200-600 nm can be attributed to the intrinsic band gap of H3 + xPMo12-xVxO40-T. It was easy to observe that there are two peaks at 220 nm and 320 nm, which are characteristics adsorption peaks of H3 + xPMo12-xVxO40-T [14, 19]. In addition, the absorption band at 320 nm was enhanced with more vanadium atoms.
|Fig. 2.UV–vis absorption spectra of as-prepared H3 + xPMo12 - xVxO40-T, x = 1, 2, 3, 4.|
Fig. 3 shows the X-ray powder diffraction patterns of H3 + xPMo12-xVxO40-T. The XRD peaks at 2u = 128, 158, 18-258, 308, 338 and 358 in the spectra of H3 + xPMo12-xVxO40-T samples are easily identified as a relatively high crystallinity H3 + xPMo12-xVxO40-T, but not H3 + xPMo12-xVxO40 [14, 20].
|Fig. 3.XRD diffraction patterns of as-prepared H3 + xPMo12 - xVxO40-T, x = 1, 2, 3, 4.|
The element analysis of H3 + xPMo12-xVxO40-T was carried out on ICP-MS and the results are listed in Table 1. As shown in Table 1, the contents of Mo and V are almost consistent with the theoretical values, which further confirm the successful synthesis of H4PMo11V1O40-T, H5PMo10V2O40-T, H6PMo9V3O40-T and H7PMo8V4O40-T.
The inhibitory effect of H3 + xPMo12-xVxO40-T on the MMA polymerization reaction is expressed by the reaction rate constant of MMA polymerization reaction. A smaller reaction rate constant means better inhibitory effect on polymerization. According to the measurement of DSC and Arrhenius equation, the reaction rate constant can be calculated. In the Arrhenius equation k = Aexp(-Ea/RT), A is a pre-exponential factor, Ea is the activation energy, k is the reaction rate constants, and R and T are the gas constant and reaction temperature, respectively . On the other hand, in the Kissinger plot ln(B/T2) = -Ea/RT, where B is the heating rate, T is the specific temperature, R is the gas constant and Ea is the activation energy. By OT with Tp (peak temperature of DSC measurement) in ln (B/T2) = -Ea/RT and plotting lnðB=T2 p Þ against 1/Tp, a straight line is obtained. The slope of this straight line is the activation energy (Ea) and the intercept in Y axis is the preexponential factor (A) [22, 23, 24]. The lnðB=T2 p Þ as a function of 1/Tp based on H3 + xPMo12-xVxO40-T is plotted in Fig. 4 and the corresponding data calculated for different plots are listed in Table 2.
|Fig. 4.The ln(B=Tp2 ) as a function of 1/Tp based on H3 + xPMo12 - xVxO40-T. (a) H4PMo11V1O40-T, (b) H5PMo10V2O40-T, (c) H6PMo9V3O40-T, (d) H7PMo8V4O40-T.|
The inhibition effects of H3 + xPMo12-xVxO40-T are shown in Fig. 5. For comparison purpose, the inhibitory effects of hydroquinone (HQ) on MMA polymerization and the reaction system without inhibitors (blank) were also fabricated under the same experimental conditions. And all the results are listed in Fig. 5.
|Fig. 5.The inhibition effects of different samples measured at the temperature of 298 K.|
As is shown Fig. 5, the reaction rate constant of blank sample without any inhibitors surpassed that of samples with an inhibitor under all conditions. In other words, the reaction without adding inhibitor was fastest and the reaction rate constants was the highest, which also indicates the existence of MMA selfpolymerization. Both HQ and H3 + xPMo12-xVxO40-T play an important role in the inhibition of MMA polymerization. When the dosage of inhibitors is 0.01 g, it is found that the reaction rate constant of H3+xPMo12-xVxO40-T is higher than that of that of HQ, indicating a lower inhibitory effect of H3 + xPMo12-xVxO40-T than HQ. However, with the increasing of inhibitor dosage, the inhibitory effect based on different inhibitors is enhanced, and H6PMo9V3O40-T is equivalent to that of HQ and H7PMo8V4O40-T surpasses that of HQ when the dosage of inhibitor is 0.02 g. Moreover, when the dosage of inhibitor increased to 0.03 g, the inhibitory effect of both H6PMo9V3O40-T and H7PMo8V4O40-T surpass that of HQ. Meanwhile, the reaction rate constant reduces gradually with the increasing of vanadium atom in H3 + xPMo12-xVxO40-T at any dosages (0.01 g, 0.02 g or 0.03 g).
In conclusion, the self-polymerization of MMA could be effectively inhibited by as-synthesized H3 + xPMo12-xVxO40-T. The inhibition effect was enhanced with the increase dosage of H3 + xPMo12-xVxO40-T and the number of vanadium atoms in H3 + xPMo12-xVxO40-T also affects its inhibitory effect. The inhibitory effect of H3 + xPMo12-xVxO40-T containing more vanadium atoms is better than that of samples with fewer vanadium atoms.3.3. Inhibition mechanism of H3 + xPMo12-xVxO40-T on MMA
In order to investigate the inhibitory mechanism of H3 + xPMo12-xVxO40-T on MMA, CV tests of different H3 + xPMo12-xVxO40-T with and without MMA were performed. The CV curve of H4PMo11V1O40-T, H5PMo10V2O40-T, H6PMo9- V3O40-T, H7PMo8V4O40-T with and without MMA are shown in Fig. 6 and the corresponding data are listed in Table 3. As shown in Fig. 6a, there are two pairs of redox potentials in H4PMo11V1O40-T. After MMA was added, the first pair redox potential in H4PMo11V1O40-T is not changed, which indicates the absence of an inhibition reaction. However, the second pair redox potential of H4PMo11V1O40-T changes a lot, the reduction peak shifts negatively, and the oxidation peak shifts positively after MMA is added. These shifts indicate that the second pair redox potential relates closely to the inhibition reaction of MMA and it is further confirmed by the CV curves in Fig. 6b-d. From Fig. 6 and Table 3, it could be inferred that the inhibition mechanism of H3 + xPMo12-xVxO40-T on MMA should be as follows: In the reaction system, although electron donor ability of MMA is not strong, heteropoly anion contains various molybdenum atoms and vanadium atoms, which is benefit for electrons transfer, especially the synergistic effect of molybdenum atoms and vanadium atoms in H3 + xPMo12-xVxO40-T makes it more easily oxidized, and easier to accept more electrons to form heteropoly blue with a mixed state (Eqs. (3) and (4). Therefore, H3 + xPMo12-xVxO40-T could oxidize the MMA radicals and transfer itself to heteropoly blue, which cause the reduction peak potential of H3 + xPMo12-xVxO40-T to shift negatively in the presence of MMA (Fig. 6). Meanwhile, it is easy for heteropoly blue to be oxidized to H3 + xPMo12-xVxO40-T by giving its electrons to MMA ions (Eqs. (5) and (6)), which makes the oxidation peak potentials shift positively (Fig. 6).
|Fig. 6.Cyclic voltammograms of H3 + xPMo12 - xVxO40-T with and without MMA. (a) H4PMo11V1O40-T, (b) H5PMo10V2O40-T, (c) H6PMo9V3O40-T, (d) H7PMo8V4O40-T.|
In addition, as can be seen from Table 3, the difference between the reduction peaks of PMo12 - xVxO40-T with andwithoutMMAis gradually reduced from 0.045 V of H4PMo11V1O40-T to 0.009 V of H7PMo8V4O40-T, and the difference between the oxidation peaks of PMo12 - xVxO40-T with and MMA is gradually increased from 0.004 V of C16H36BrN-PMO11V1 to 0.072 V of C16H36BrN-PMO8V4. This is consistent with the results that the inhibitory effect of PMo12 - xVxO40-T increased with the increased number of vanadium atoms. It can be inferred that radical generated from MMA was gradually oxidized by PMo12 - xVxO40-T, and disappeared at last. Therefore, PMo12 - xVxO40-T has played an inhibitory role in preventing self-polymerization of MMA. As discussed above, the inhibitory mechanism of PMo12 - xVxO40-T is similar to that of the hydroquinone (HQ). Both are through electron shuffling to quench MMA monomer radicals to avoid self-polymerization [24, 25].4. Conclusion
In summary, as-synthesized H3 + xPMo12-xVxO40-T is a satisfactory self-polymerization inhibitor for MMA due to its outstanding redox properties. The inhibitory effect was improved with the increasing dosages of H3 + xPMo12-xVxO40-T, and was also affected by the number of vanadium atoms in the H3 + xPMo12-xVxO40-T. The investigation of mechanism indicates that the inhibition reaction is an oxidation-reduction reaction. The H3 + xPMo12-xVxO40-T can react directly with the MMA monomer radicals by accepting or loosing electrons, which eliminates the MMAmonomer radicals in the reaction system. Therefore, the selfpolymerization of the MMA can be effectively inhibited by H3 + xPMo12-xVxO40-T.Acknowledgment
This work was supported by the Research and Development Fund for the postdoctoral researchers of Heilongjiang Province (2012).
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