Diphenyl carbonate (DPC) is used to produce many fine chemicals and polymer materials,especially where it used as an intermediate for the phosgene-free synthesis of polycarbonate. Several processes have been developed for the synthesis of DPC,including the phosgene process [1],the oxidative carbonylation of phenol [2, 3, 4],and the transesterification of phenol with dimethyl oxalate [5] or dimethyl carbonate (DMC) [6]. The transesterification of DMC with phenol is the most environmentally attractive of these processes for commercial production of DPC [7].
A wide range of efficient homogeneous catalysts have been reported for the transesterification of DMC with phenol,including organic Sn,Ti,Fe and Sm complexes [8, 9, 10, 11]. The use of homogeneous catalysts for this transformation,however,has been limited because of problems associated with their separation from the product,and research towards the development of heterogeneous catalysts for this reaction has consequently attracted considerable attention. Several heterogeneous catalysts have been reported for this reaction,including transition metal oxides [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23],such as TiO2,MoO3,V2O5,V-Cu mixed oxide,PbO,TiO2/SiO2,and MoO3/SiO2. Although some of these catalysts performed with suitable activity,their selectivity and reusability properties were not satisfactory. For this reason,there is an urgent need for the development of a highly efficient heterogeneous catalyst for the synthesis of DPC.
Carbon nanotubes (CNTs) represent a promising catalyst support because of their unique and outstanding mechanical,electronic,and chemical properties. CNT-supported catalysts have been used in a wide range of chemical transformations,including hydrogenation reactions,Fischer-Tropsch synthesis,hydroxylation of benzene,and photo-catalysis [24, 25, 26]. We recently reported the preparation of TiO2 supported on unmodified multi-walled carbon nanotubes (TiO2/MWCNTs) in the presence of a surfactant and its use as a catalyst for the transesterification of DMC with phenol [27]. The results showed that the TiO2/MWCNTs exhibited excellent activity and selectivity. Unfortunately,its reusability was poor,with the separation of the catalyst from product by centrifugation being particularly time consuming. Furthermore,traces of surfactant were found in the reaction system,which made the purification of the product very difficult.
Herein,we develop a method for the preparation of a readily separable,efficient and reusable heterogeneous catalyst by the controlled hydrolysis of a Ti precursor to form TiO2 following deposition on the surface of an oxidative modified MWCNT (TiO2/o-MWCNT). Our results revealed that the species and concentration of the precipitants had a remarkable effect on the activity,stability and separability of the TiO2/o-MWCNTs for the transesterification of DMC with phenol.
DMC was obtained from Huasheng Co. Ltd.,Shangdong University of Petroleum,China. Phenol (AR),ethanol (AR),and ammonia water (25-28%,AR) were purchased from Guangdong Guanghua Sci-Tech Co. Ltd.,China. The MWCNTs (inner diameter: 4-8 nm,outer diameter: 10-30 nm) were obtained from Chengdu Organic Chemicals Co. Ltd.,Chinese Academy of Sciences,China. All of the materials and reagents were used directly without further purification.
The MWCNTs were initially treated with an acidic KMnO4 solution to remove any impurities and improve its solubility. Briefly,the MWCNTs (20 g) and solid KMnO4 (60 g) were added to sulfuric acid (6 mol/L,800 ml),and the resulting mixture was stirred at 100 °C for 1.5 h. The suspension was then cooled to room temperature and filtered,and the solid was collected and suspended in concentrated HCl to remove any of the MnO2 formed during the process. The suspension was then filtered and the filter-cake was washed with deionized water until neutral before being dried in an oven at 120 °C for 8 h.
Typical procedure for the preparation of the TiO2/ o-MWCNTs: The o-MWCNTs (1.2 g) and ammonium hydroxide (25%-28%,0.6-10 ml) were dispersed in ethanol (180 ml) with ultrasonic treatment for 1 h. Tetrabutyl titanate (0.5-1.5 ml) was dispersed in ethanol (20 ml) and added to the o-MWCNT suspension in a drop-wise manner under vigorous stirring. The resulting mixture was stirred at 45 °C for 12 h to complete the hydrolysis reaction before being cooled to ambient temperature and filtered. The filter-cake was then washed sequentially with deionized water and ethanol (3 times) before being collected and dried in an oven at 120 °C for 4 h. The dried material was calcined at 200 °C for 3 h in air to obtain the powdered TiO2/o-MWCNTs.
The TC-A-0.3,TC-A-0.4,TC-A-0.5,TC-A-0.6,and TC-A-7.5 samples represent the TiO2/o-MWCNTs prepared using 0.3%,0.4%,0.5%,0.6%,and 7.5% (based on ethanol) ammonium hydroxide (25%-28%) as the precipitant,respectively. The TC-W-0.4,TC-W-0.5,TC-W-0.6,and TC-W-7.5 samples represent the TiO2/o-MWCNTs prepared using 0.4%,0.5%,0.6%,and 7.5% (based on ethanol) deionized water as the precipitant,respectively.
X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (Philip XPERT PRO MPD) using a Cu Kα radiation source (λ = 0.154056 nm),operating at 40 kV and 45 mA. The system was operated in the continuous mode to allow for the collection of data at a scanning speed of 0.02°/s. The surface electronic structures of the catalysts were detected by X-ray photoelectron spectroscopy (XPS) on a Kratos Model XSAM 800 instrument using a monochromatic and focused (350W) Mg Kα (1253.6 eV) radiation source. Transmission electron microscopy (TEM) studies were conducted on a JEM-1000CX TEM system operating at an accelerating voltage of 80 kV. The specific surface area (ABET),pore size (BJH),and pore volume of the catalysts were calculated from low temperature N2 adsorption-desorption isotherms using a TriStar II 3020 system (Micromeritics Instrument Corporation). The Ti content of the filtrate was determined using a TS IRIS 1000 ICP-AES instrument.
The transesterification reaction was carried out in a 100 ml three-neck round-bottomed flask equipped with a magnetic stirring bar,a nitrogen inlet,a dropping funnel,and a fractionating column connected to a liquid dividing head. Phenol and the catalyst were added to the flask under N2 atmosphere,and the resulting mixture was heated in the range of 175-178 °C. DMC was then added to the mixture in a drop-wise manner under continuous stirring. During the reaction,a distillate composed of DMC and methanol was collected in a receiver flask attached to the liquid dividing head. Upon completion of the reaction,the mixture was cooled to room temperature and the catalyst was separated by centrifugation. The filtrate and distillate were quantitative analyzed by GC system (Agilent Technologies 7820A) equipped with an FID detector and a DB-35 capillary column (30 m x 320 mm x 0.25 mm). The GC results were calculated using a correction factor normalization method. The structure of the product was defined by GC-MS on a HP 6890/5973 system. In the experiment designed to test the reusability of catalyst,the catalyst was separated by filtration and washed with DMC before being dried in an oven and then reused in the next reaction.
TC-W-0.5 and TC-A-0.4 were analyzed by XPS to investigate the atomic composition and corresponding chemical states on the surfaces of these TiO2/o-MWCNT catalysts. The atomic surface compositions are shown in Table 1. The results revealed that the amount of surface Ti on the TC-A-0.4 catalyst was slightly higher than that of TC-W-0.5. The ICP-AES analysis,however,revealed that the TiO2 loading of TC-A-0.4 was much lower than that of TC-W-0.5 (Table 2). These results indicated that the dispersion of TiO2 on the surface of the o-MWCNTs was enhanced by the use of ammonia hydroxide as the precipitant instead of deionized water. Furthermore,there were about 1.28% N atoms on the surface of the TC-A-0.4 catalyst. The N atoms belonged to the amide that was formed by the reaction of ammonia with the carboxyl groups on the surface of the o-MWCNTs.
As shown in Fig. 1(a),the electron binding energy of Ti 2p3/2 and Ti 2p1/2 for TC-A-0.4 was 459.2 and 464.9 eV,respectively,representing a shift of 0.4 eV towards a higher binding energy compared with pure bulk anatase. This result indicated that the Ti on the surface of the TiO2/o-MWCNTs was in a different environment to that of pure anatase. As shown in Fig. 1(b),the electron binding energy of Ti 2p3/2 and Ti 2p1/2 for TC-W-0.5 was 458.9 and 464.6 eV,respectively,representing a shift of 0.3 eV towards a lower binding energy compared with TC-A-0.4. Taken together,these results suggested that the Ti in the TC-A-0.4 and TC-W-0.5 catalysts were not in the same chemical environment.
Figure 1(c) and 1(d) show the C 1s spectra of TC-A-0.4 and TC-W-0.5,respectively. Each C 1s spectra could be deconvoluted into four peaks. For TC-A-0.4,the peaks at 284.6,285.8,287.2,and 289.1 eV were attributed to the C-C,C-O,C=O,and O-C =O bonds,respectively. The presence of these oxygen containing functional groups confirmed that the surfaces of the MWCNTs had been successfully modified by the oxidation,which provided anchors for the immobilization of the TiO2. The O 1s spectra of TC-A-0.4 and TC-W-0.5 are shown in Fig. 1(e) and (f),respectively. Each O 1s spectra could be deconvoluted into three peaks. For TC-A-0.4,the peaks at 531.1,532.6,and 534.0 eV were attributed to the Ti-O,C-O,and C =O bonds,respectively. As shown in Fig. 1(g),the N 1s spectrum of TC-A-0.4 consisted of two peaks at 400.1 and 402.1 eV,which were attributed to pyrrole-like nitrogen and C-N of the amide group,respectively.
The TEM images of the different catalysts are shown in Fig. 2. It showed that the TiO2 particles were uniformly dispersed on the surfaces of TC-A-0.4 without any agglomeration [Fig. 2(a)]. The concentration of TiO2 particles on the tip of the o-MWCNTs was slightly higher than that on the sidewall of the o-MWCNTs,which was attributed to the higher density of oxygen containing functional groups on the tip of o-MWCNTs. This was in agreement with the reported results [24, 25, 26]. As shown in Fig. 2(b),some of the TiO2 particles agglomerated on the surface of the o-MWCNTs,indicating that the dispersion of TC-W-0.5 (0.5% deionized water as the precipitant) was less effective than that of TC-A-0.4 (0.4% ammonium hydroxide as the precipitant). These results were in agreement with the XPS result. When the concentration of precipitant was increased to 7.5%,the agglomeration of the TiO2 particles increased significantly on the surfaces of the o-MWCNTs,regardless of whether ammonium hydroxide (Fig. 2(c)) or deionized water (Fig. 2(d)) was used as the precipitant.
The BET surface area,pore size,and pore volume of the catalysts were summarized in Table 2. The results revealed that the specific surface area increased,whereas the average pore size decreased following the supporting of the TiO2 particles for all catalysts. According to the TEM and XRD results,the active TiO2 components were well dispersed on the surfaces of the o-MWCNTs in an amorphous state. The ABET of the porous TiO2 was larger than that of the o-MWCNTs,which resulted in the higher ABET of the TiO2/o-MWCNTs. These results were consistent with those previously reported in the literature [24, 25, 26, 27]. Although more TiO2 particles had been supported on the surface of TC-W-0.5,the BET surface area of TC-A-0.4 was higher than that of TC-W-0.5. These results indicated that the TiO2 particles were distributed more uniformly on the surface of TC-A-0.4,which was in good agreement with the XPS and TEM results. There are two possible reasons for the decrease in the average pore size of the TiO2/o-MWCNTs compared with the o-MWCNTs,including (1) the pore size of the TiO2 was smaller than that of the o-MWCNTs,and (2) partial TiO2 particles were introduced into the channels of the o-MWCNTs,resulting in a decrease in the diameter of o-MWCNTs. Furthermore,there were no discernible changes in the pore volumes of TC-W-0.5,TC-A-0.4,and TC-A-7.5 compared with the o-MWCNTs.
Figure 3 shows the XRD patterns of the o-MWCNTs and TiO2/o-MWCNTs samples. All samples showed three obvious diffraction peaks at 25.8°,42.7°,and 53.3°,corresponding to the (002),(100),and (004) diffractions of the hexagonal graphite in the o-MWCNTs,respectively. The peak intensity of the TiO2/o-MWCNTs was lower than that of the o-MWCNTs,because the partial surface of the o-MWCNTs was covered with TiO2. As expected,the characteristic diffraction peaks of the TiO2 crystals were not found in the TC-A-0.4,and TC-A-7.5 samples prepared by calcinating at 200 °C,because the calcination temperature was too low to transform the amorphous TiO2 into its crystal form. There were four diffraction peaks at 25.4°,37.8°,47.9° and 62.7° corresponding to the (101),(004),(200),and (204) of anatase TiO2 over TC-A-7.5 calcinated at 400 °C,respectively. However,the characteristic diffraction peaks of crystalline TiO2 were not found in the TC-A-0.4 calcinated at 400 °C. The difference could be due to that the TiO2 was well dispersed on the surface of the o-MWCNTs when a low concentration of the precipitant was used.
There are two steps involved in the synthesis of DPC from DMC and phenol,including the transesterification of DMC with phenol to form methyl phenol carbonate (MPC) (reaction 1),and the further transesterification of MPC with phenol (reaction 2) or the disproportionation of MPC (reaction 3,main process) to obtain DPC. The formation of MPC is the key step because the thermodynamic equilibrium constants for reactions 1,2,and 3 are 6.3 x 10-5,1.2 x 10-5,and 0.19,respectively. The properties of a catalyst for this transformation are usually evaluated by the total selectivity and yield of MPC and DPC. Furthermore,the total reaction of DMC with phenol is an equilibrium reaction. The methanol byproduct can be removed by azeotropic distillation with DMC to promote the reaction towards the formation of MPC and DPC. The amount of DMC used in the transformation is therefore usually greater than that of the theoretically required dosage. Based on our previous work [7, 9, 10, 11, 27],the optimum molar ratio of DMC to phenol was 1:1.
For comparison,the o-MWCNTs and TiO2 were initially used directly as the catalyst for the transesterification of DMC with phenol. As shown in Table 3,MPC and DPC were not detected in the reaction mixture when the o-MWCNT was used. Although TiO2 promoted the reaction,its activity was relatively low [28]. The catalytic properties of the TiO2/o-MWCNTs prepared under the same conditions,except for the concentration and/or type of precipitant,were also investigated,and the results are summarized in Table 3. As previously reported in the literatures [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21],this reaction would lead to the formation of anisole. The yield of anisole,however,was less than 0.03% for all of the catalytic reactions evaluated in the current study,which resulted in the very high total selectivity of MPC and DPC. The activity of the TiO2/o-MWCNTs could be varied by changing the concentration and/or type of precipitant during the preparation process. When deionized water was used as the precipitant,the conversion of phenol initially increased from 33.7% to 39.0% as the concentration of water was increased from 0.4% to 0.5%. The conversion of phenol decreased slightly to 36.9% when the concentration of water was further increased to 0.6%. When the concentration of water was increased to 7.5%,the phenol conversion decreased significantly to 28.2%. A similar result was obtained for the catalysts prepared using ammonium hydroxide. When the amount of ammonium hydroxide was raised from 0.3% to 0.6%,the conversion of phenol also increased initially and then decreased,with the highest phenol conversion being 39.5%. A phenol conversion of only 20.4% was observed when the amount of ammonium hydroxide was increased to 7.5%. Two possible explanations were provided for these results: (1) the loading of the TiO2 increased as the concentration of the precipitant increased; (2) the agglomeration of the TiO2 particles would increase as the concent ration of precipitant increased. It is noteworthy that most of the TiO2/o-MWCNT catalysts showed higher activity than TiO2,which could be attributed to the interaction between the active TiO2 component and the o-MWCNT support. These results were in agreement with those previously published in the literatures [20,27].
As expected,the calcination temperature had a remarkable effect on the activity of the TiO2/o-MWCNTs towards the transesterification of DMC with phenol. The conversion of phenol decreased quickly from 39.5% to 12.2% as the calcination temperature was increased from 200 to 400 °C. The reason for this decrease was that the morphology of TiO2 changed from the amorphous to the anatase state at 400 °C,and this process was confirmed in our previous study [27]. Although the characteristic peak of anatase TiO2 was not found in the TC-A-0.4 calcined at 400 °C,it was clearly present in the TC-A-7.5 calcined at 400 °C (Fig. 3). The characterization results also indicated that the TiO2 was well dispersed on the surfaces of the MWCNTs constructed with a low concentration of ammonium hydroxide.
In addition,we found that TC-W-0.4 and TC-W-0.5 prepared using water as the precipitant were difficult to separate from the mixture upon completion of the reaction. As shown in Fig. 4 (a) and (b),these catalysts could not be completely separated even under centrifugation at a speed of 4000 r/min for 2 h. Although they could be separated by high-speed centrifugation (10000 r/min),their application was limited by the requirement for such harsh separation conditions. In contrast,the TC-W-7.5 catalyst could be readily separated from the reaction mixture under centrifugation at a speed of 4000 r/min for 15 min. Furthermore,the catalysts prepared using ammonium hydroxide instead of water as a precipitant were readily separated from the reaction mixture,and centrifugation was not necessary in these cases because the catalysts were automatic deposited within 3-5 min,as shown in Fig. 4(c) and (d).
It is well known that oxidatively modified CNTs can be well dispersed in polar solvents because of the hydrophilic nature of the oxygen containing functional groups on their surface. The reason for the difficulties encountered during the separation of the TC-W-0.4 and TC-W-0.5 catalysts could be that large numbers of oxygen containing functional groups were still exposed on the surfaces of catalysts. Certainly,the number of oxygen containing functional groups on the surfaces of the CNTs reduced with increasing TiO2 loading,which resulted in the catalysts being more readily separated (e.g.,TC-W-7.5). When ammonium hydroxide was used as the precipitant instead of water to prepare the catalysts,the nucleation and growth of the metal oxides occurred prior to the formation of the oxygen containing functional groups of the CNTs,as reported in the literature [24, 25, 26]. The corresponding catalysts (TC-A-0.4 and TC-A-0.5) exhibited hydrophobic behavior since most of their oxygen containing functional groups had been covered by hydrophobic TiO2,which resulted in the catalyst being easy to separate from the product.
TiO2/o-MWCNT catalysts with different TiO2 loadings were prepared by using 0.4% ammonium hydroxide as the precipitant,in a manner similar to that of the preparation of TC-A-0.4. The effect of the TiO2 loading on the transesterification of DMC with phenol was investigated,and the results are shown in Fig. 5. The conversion of phenol increased from 35.0% to 39.5% as the TiO2 loading (actual loading,detected by ICP-AES) was increased from 4.5% to 10.8%. The conversion of phenol,however,decreased slightly when the TiO2 loading was increased further to 20.1%. The reason for this reduction in the conversion could be that the excess TiO2 loading would lead to the aggregation of TiO2 on the surface of the o-MWCNTs. Furthermore,the total yield of MPC and DPC exhibited a similar trend to the conversion of phenol,and the selectivity of the byproduct (anisole) was lower than 0.03% for all of the catalytic reactions.
The effect of TC-A-0.4 amount on the transesterification was also investigated. As shown in Fig. 6,the conversion of phenol increased from 31.1% to 42.5% as the catalyst amount was increased from 0.68% to 2.04% (based on the total raw material). Further increasing the catalyst amount to 2.72% led a slight increase in the phenol conversion to 44.2%. When the catalyst amount was further increased to 3.40%,no discernible difference was observed in the conversion of the phenol. Because the o-MWCNT catalytic support possessed good adsorption capability and low density,it was envisaged that an expansion phenomenon would be observed in the reaction system when a large amount of catalyst was used. The expansion of reaction mixture would lead to problems in terms of mass transfer and separation. Following comprehensive consideration,the optimum amount of catalyst was set at 2.04%.
The effect of the reaction time on the yield of MPC and DPC was also investigated because the transesterification of DMC with phenol is a reversible reaction. As shown in Fig. 7,the conversion of phenol increased as the reaction time was increased from 7 to 13 h using TC-A-0.4 as catalyst,with the highest phenol conversion reached being 49.2%. The phenol conversion effectively reached a plateau at this point,with further increases in the reaction time providing a similar conversion. According to the results of thermodynamic analyses reported previously in the literature [21,29],the conversion observed in our study was close to the theoretical equilibrium value. The yield of DPC increased continuously with increasing reaction time. The yield of MPC,however,initially increased as the reaction time was increased from 7 to 9 h,and then decreased as the reaction time was increased further to 15 h. This could be attributed to the rate of MPC formation being lower than that of MPC to DPC after 9 h. Given that most of the unreacted DMC would have been removed from the reaction system through azeotropic distillation with methanol after 9 h,the rate of MPC formation would be very low. During this period,the main reaction process would be the disproportionation of MPC to DPC,which would result in a decrease in the MPC yield and an increase in the DPC yield. Furthermore,the total selectivity was always higher than 99% even after an extended reaction time of 15 h.
It has been reported that the active component can be lost from the catalyst during the transesterification of DMC with phenol [20]. Owing to the strong interaction between TiO2 and the CNTs,the leaching of TiO2 could be greatly reduced from the surface of the CNTs [27]. With this in mind,we evaluated the reusability of TC-W-0.5 and TC-A-0.4 catalysts. Upon completion of the reaction,the catalyst was filtered,washed with DMC,dried under vacuum,and reused for the next run. As shown in Table 4,leaching of TiO2 still occurred from the TC-W-0.5 and TC-A-0.4 catalysts. The level of TiO2 leaching from TC-W-0.5,however,was much greater than that from TC-A-0.4. In the first run,the level of TiO2 leaching from TC-W-0.5 was up to 27%. Furthermore,the total TiO2 leaching from TC-W-0.5 reached about 51% after three runs,and was accompanied by a reduction in the conversion of phenol to only 19.2%. In comparison,TiO2 leaching from TC-A-0.4 was only 12% after the first run. Furthermore,the total TiO2 leaching from TC-A-0.4 was about 20% after three runs,resulting in a reduction in the conversion of phenol from 39.5% to 33.5%. These differences could be attributed to differences in the dispersions of TC-W-0.5 and TC-A-0.4. As shown in the TEM and XPS results,the agglomeration of TiO2 particles was observed on the surfaces of the o-MWCNTs for TC-W-0.5,whereas the TiO2 particles were well dispersed for TC-A-0.4. It was envisaged that the interaction between TiO2 and the o-MWCNTs would decrease as the thickness of the TiO2 particles increased. The level of TiO2 leaching from TC-W-0.5 was much higher than that from TC-A-0.4. It is noteworthy that there was no significant change in the catalytic activity of TC-A-0.4 between the third and fourth runs,and that the leaching of TiO2 was also very low (1%).
A TEM image of the reused TC-A-0.4 (second run) is shown in Fig. 8. Compared with fresh TC-A-0.4,the TiO2 amount on the side wall of the o-MWCNTs had obviously decreased,whereas the TiO2 density on the tip of the o-MWCNTs did not change for the reused TC-A-0.4. It is well known that the tip of CNTs possesses more oxygen containing functional groups than the side wall. These results indicated that the interactions between TiO2 and the CNTs could be enhanced by the introduction of oxygen containing functional groups,which was consistent with the reusability test results. Furthermore,the specific surface area of the reused TC-A-0.4 (second run,155 m2/g) was slightly lower than that of the fresh one (161 m2/g),which was attributed to the loss of TiO2.
Under the same reaction conditions,TC-A-0.4 showed much better reusability than the TiO2/CNTs,which were prepared by depositing TiO2 on the unmodified CNTs in the presence of a surfactant [27]. Furthermore,the reusability of TC-A-0.4 was better than that of other similar catalysts. For example,the activity of MoO3/SiO2 and TiO2/SiO2 was almost completely lost after they had been reused for four runs [20]. This result could be attributed to the interaction between TiO2 and the CNTs being stronger than that between TiO2 and the other inorganic supports.
An efficient and easy to separate heterogeneous catalyst,TiO2/o-MWCNT,has been prepared and used for the transesterification of DMC with phenol. The type and concentration of the precipitant had a remarkable effect on the activity,separability and reusability of TiO2/o-MWCNTs. During the preparation of the TiO2/o-MWCNTs,high levels of TiO2 dispersion could be obtained by controlling the speed of the hydrolysis of the Ti precursor to form TiO2 under a low concentration of the precipitant. The nucleation and growth of the TiO2 occurred prior to the formation of the oxygen containing functional groups of the CNTs when ammonium hydroxide was used instead of deionized water as the precipitant,and this resulted in much easily to separate from the reaction mixture. When TC-A-0.4 was used as a catalyst,the conversion of phenol reached 42.5% with over 99.9% selectivity for MPC and DPC over 9 h,which could be reused for four runs with only a slight loss in its catalytic activity.
We thank Professor Mei-zheng Qu from Chengdu Organic Chemicals Co. Ltd. for providing MWCNT and helpful suggestions.
2014, Vol. 35 Issue (4): 481-489
PDF (871KB)
2014, Vol. 35 Issue (4): 481-489
PDF (871KB)