Construction of a Br?nsted-Lewis solid acid catalyst La-PW-SiO2/SWCNTs based on electron withdrawing effect of La(III) on π bond of SWCNTs for biodiesel synthesis from esterification of oleic acid and methanol

Qing Shu,Xinyuan Liu,Yanting Huo,Yuhui Tan,Caixia Zhang,Laixi Zou

School of Chemical and Chemical Engineering,Jiangxi University of Science and Technology,Ganzhou 341000,China

Keywords:Single-wall carbon nanotubes (SWCNTs)H3PW12O40 La3+Solid Br?nsted-Lewis acid catalyst Biodiesel Esterification

ABSTRACT A novel solid Br?nsted-Lewis acid catalyst La-PW-SiO2/SWCNTs(single-wall carbon nanotubes)was synthesized from the synergistic modification of H3PW12O40(HPW)by single-walled carbon nanotubes functionalized with sidewall hydroxyl groups (SWCNTs–OH) and La3+ via sol–gel method.The freshly prepared catalyst was characterized by several methods,and the catalytic activity and stability of it were studied from the esterification of oleic acid and methanol.Results showed that the highest conversion of oleic acid was 93.1% (mass) and maintained as high as 88.7% (mass) after six cycles of La-PW-SiO2/SWCNTs.The high catalytic activity and stability of La-PW-SiO2/SWCNTs can be attributed to the strong electron withdrawing effect of La3+on π bond of SWCNTs,because it can facilitate the formation of a large number of strong Lewis acid sites.Therefore,the reduction of catalytic activity of a solid acid catalyst due to the fact that hydration reaction of its Br?nsted acid sites can be effectively reduced.La-PW-SiO2/SWCNTs can be an efficient and economical catalyst,because it shows good catalytic activity and stability.

1.Introduction

In the United States and Europe,pure vegetable oils are commonly used as feedstock oils for biodiesel production through transesterification reactions.However,it is not economical to do the same in China because most vegetable oils need to be imported at relatively high prices,and people usually use these vegetable oils in cooking.In addition,China produces more than 4.5 million tons of used kitchen cooking oil every year.The used kitchen cooking oil are usually converted into acidified oil first when they were used as feedstock oils for the production of biodiesel,and the FFA(free fatty acid) content in acidified oil can be higher than 90%(mass).Therefore,when acidified oil is used as feedstock oil to prepare biodiesel,the reaction process is mainly composed of esterification reaction.

In recent years,the using of easy-prepared and low-cost solid acid catalysts for the synthesis of biodiesel from low-cost raw materials with high amount of FFAs has become a new trend[1–4].Heteropoly acids (HPAs) is a solid acid catalyst with many advantages,such as strong Br?nsted acidity and low volatility,low corrosiveness and flexibility.Some researchers have reported that HPAs is an efficient catalyst for the synthesis of biodiesel via transesterification of vegetable oil and animal grease (comprised by glycerides with diversified composition in fatty acids)with methanol or ethanol,or esterification of FFA with methanol or ethanol [5–10].Currently,research was focused on the 12 series HPAs with a Keggin structure (molecular formula is HnAB12O40?xH2O),such as phospho tungstic acid (H3PW12O40,HPW),phospho molybdic acid (H3PMo12O40) and silico tungstic acid (H4SiW12O40).Among of them,HPW is the usual catalyst of choice because of its stronger acidity,higher thermal stability and lower oxidation [11,12].

However,HPAs generally exhibit strong Br?nsted acidity both in the liquid and solid state,so they will show an extremely high solubility in polar solvents.It will undoubtedly contribute to the overall reaction due to the existence of a homogeneous reaction system.However,it is not conducive to separate and recycle the HPAs catalyst from the liquid phase after the reaction has completed.At the same time,it also should be noticed that the catalytic activity is usually closely related to the surface acidity of a catalyst,while HPAs has a small specific surface area(<10 m2?g-1),which is not conducive to the formation of large number of available Br?nsted sites on the surface.Due to these above defects,the practicability of HPAs is not high.

Current research has found that different counter ions (such asK+and Cs+)can produce a synergistic effect between anions of HPAs and them,which can accelerate the migration of free oxygen on the surface of HPA and facilitate the dispersion of its catalytic active sites.The dispersion increases as the diameter of the counterbalance ions increases [13–16].Therefore,the specific surface area can be effectively extended by introducing a counterbalance ion into HPAs.In this study,La3+was used as counterbalance ion to modify HPW.Reasons for choosing La3+as the counter ion are as follows:the large diameter of La3+is beneficial to the extension of the specific surface area of HPW and the dispersion of its active centers.In addition,it must be noticed that the HPW possesses strong Br?nsted acidity.However,the Br?nsted acidic active site of a catalyst is prone to hydrate when it was in water-rich solutions,leading to the loss of catalytic.Fortunately,La3+also exhibits a strong electron withdrawing effect,which helps to form Lewis acid sites.Therefore,the catalytic stability of HPAs in a waterrich reaction medium environment can be enhanced after it was modified by La3+.

However,although La3+modification can effectively adjust the specific surface area and acidic active site type of HPW,but the separation and recovery of catalyst after being used is still a problem.The separation and recovery of HPW can be easily performed after immobilisation on a suitable support.A large number of supports with high surface areas have been used to load HPW,such as silica,titania and alumina,active carbon and MCM-41 [17–21].In general,the main function of the support is to ensure that the catalyst can be separated smoothly after the reaction.Therefore,if such a support can be found,it not only helps the separation of catalyst,but can also improve the catalytic activity and stability of a catalyst,which will have greater application value in biodiesel production.Single-wall carbon nanotubes (SWCNTs) are attracting increasing levels of interest because of their unique mechanical,electrical,thermal and structural properties [22–24].Some researchers found that SWCNT can also play a role in improving catalytic activity and stability when they tried to use it as a catalyst support[25,26].The possible reasons for this catalytic activity and stability enhancement can be summarized as follows:the carbon atoms in carbon nanotubes mainly exist in the form of sp2hybridization.At the same time,it should be noted that some carbon atoms exist in sp3hybridization due to the existence of a certain degree of bending in its hexagonal grid structure.Therefore,the formed chemical bond has a mixed hybrid state of sp2and sp3.In addition,p orbits overlap each other.All these advantageous structural features cause the formation of a highly delocalized large π bond outside the graphene sheet of SWCNTs.Therefore,it can be expected that the charge bond imbalance becomes more intense when HPW is modified by La3+and further loaded by SWCNTs than that of HPW catalyst when it is modified by La3+only due to the fact that a strong interaction will occur between La3+and highly delocalized large π bond.As a result,excess positive charge will appear in such a catalyst and which will further increase the number of Lewis acidic active sites,making such a catalyst to exhibit higher catalytic stability in a water-rich reaction medium environment.However,the surface of SWCNTs is chemically inert and does not easily functionalize with La3+.Fortunately,the sidewalls of SWCNTs are mainly composed of sp2hybridized carbon atoms.Hence,the sidewalls of SWCNTs are susceptible to the reaction with some reactive functional groups,such as halogens,carbene and hydroxyl groups.Therefore,sidewalls of SWCNTs can be firstly functionalized by hydroxyl group to form SWCNTs–OH,which can be expected to help the occurring of interaction between La3+and highly delocalized large π bond of SWCNTs.

Based on analysis above,La3+and SWCNTs–OH were used together to modify HPW for the preparation of a novel solid Br?nsted-Lewis acid La-PW-SiO2/SWCNTs catalyst via sol–gel method in this study.In order to determine whether the modification of HPW by SWCNS–OH and La3+can improve the catalytic activity and stability of it.The main component of biodiesel is a mixture of fatty acid methyl esters(C16–C22),including methyl oleate,methyl linoleate,methyl stearate,methyl palmitate,methyl linoleate,etc.And more,the main component of FFA is oleic acid.Hence,if it can be proved that the catalyst has a higher catalytic activity for the esterification reaction of oleic acid and methanol,it also means that it has a higher catalytic activity for the esterification reaction of acidified oil and methanol.Based on these above considerations,this study uses oleic acid instead of acidified oil and reacts with methanol to prepare biodiesel.The catalytic activity and stability of HPW and La-PW-SiO2/SWCNTs were compared when they were used to catalyze the esterification reaction of oleic acid and methanol respectively.

2.Experimental

2.1.Reagents and instruments

Reagents:single-wall carbon nanotubes(provided by Key Laboratory of Green Reaction Engineering and Technology,Department of Chemical Engineering,Tsinghua University,China),hydrochloric acid (37% (vol),AR,Sinopharm Chemical Reagent Co.,Ltd.,China),phosphotungstic acid (99% (mass),AR,Aladdin Reagent Co.,Ltd.,China),Niobium nitrate hexahydrate (99.9% (mass),AR,Aladdin Reagent Co.,Ltd.),Tetraethyl silicate (99% (mass),GC,Aladdin Reagents Co.,Ltd.),Absolute ethanol (99%,AR,Xishao Chemical Co.,Ltd.,China),Triton X-100 (99.5%(mass),CP,Sinopharm Group Chemical Reagent Co.,Ltd.),Methanol (99% (mass),AR,Tianjin Damao Chemical Reagent Factory,China),Oleic Acid (99% (mass),AR,Xishao Chemical Co.,Ltd.),Ether (99.5% (mass),AR,Xishao Chemical Co.,Ltd.) and Phenolphthalein (99% (mass),AR,Tianjin Guangfu Fine Chemicals Research Institute,China),Methyl oleate(99% (mass),GC,Aladdin Reagents Co.,Ltd.).

Instruments:microwave and ultrasonic combination reaction system (Nanjing Xianou Instrument Manufacturing Co.,Ltd.,China),computer numerical control (CNC) ultrasonic cleaning device(KQ-S00DE,Kunshan Ultrasonic Instrument Co.,Ltd.,China),heat-collecting magnetic heating stirrer(DF-101S,Jintan Baita Xin-Bao Instrument Factory,China),Electronic Balance (AR223CN,Ohaus Instrument Co.,Ltd.,China),Electric Thermostatic Drying Cabinet(DHG-9036A,Shanghai Jinghong Experimental Equipment Co.,Ltd.,China),Recycled Water Vacuum Pump (SHZ-D(III),Huayi Instrument Co.,Ltd.,Gongyi City,China),gas chromatograph(7820A,Agilent,USA).

2.2.Catalyst preparation

The preparation process of SWCNTs–OH is as follows:(1)a certain amount of SWCNTs was added together with AlCl3and they were placed into a 50 ml beaker under constant stirring with a glass rod for 1 h;(2) 20 ml ethanol was added to the mixed solution and sonicated for 5 min to make the SWCNTs evenly dispersed.(3) after ultrasonication treatment,1 ml concentrated hydrochloric acid was added to the mixture,and which was then placed into the microwave and ultrasonic combination reaction system for 30 min.(4)After the microwave treatment is complete,the residue was filtered and repeatedly washed with deionized water until the filter is neutral.The obtained residue was placed in an oven and dried to obtain SWCNTs–OH powder.

Preparation of La-PW-SiO2/SWCNTsviasol–gel method is as follows:(1) In a 50 ml beaker,1 ml of absolute ethanol,10 ml of 0.1 mol?L–1nitric acid,0.01 g SWCNTs–OH powder,and 2.0 g of HPW,0.1 g of lanthanum nitrate hexahydrate were added in turn and well stirred with a glass rod for 1 h.The mixture was then oscillated for 5 min by an ultrasonic oscillator;(2) after ultrasonication,1.15 g tetraethyl silicate and Triton X-100 were added into the solution;(3) The mixture was oscillated again for 5 min by an ultrasonic oscillator.It was allowed to stand until gelation occurred.(4) The gel was placed in an oven and calcined in air atmosphere at 110 °C for 3 h with a heating rate of 2oC?min-1.(5) The dried gel was ground into a fine powder with a mortar to obtain La-PW-SiO2/SWCNTs sample.

2.3.Catalyst characterization

The content of each element in the HPW and La-PW-SiO2/SWCNTs catalysts was analyzed by using an Axios max X-ray fluorescence spectrometer (Axios PW4400,PANalytical,Netherlands).The morphology of the La-PW-SiO2/SWCNTs catalysts was observed by transmission electron microscope (TEM,Tecnai G2 F20 S-TWIN,FEI,USA).The analytical test procedure was as follows:sample was dissolved in anhydrous ethanol and sonicated for 10 min.The diluted sample was placed on a copper grid coated with carbon and analyzed after the ethanol was evaporated completely,at an acceleration voltage of 200 kV and 0.24 nm pointto-point resolution.The crystal structures of HPW,SWCNTs and La-PW-SiO2/SWCNTs catalysts were analyzed by X-ray diffraction(XRD,Empyrean,PANalytical,Netherlands) using a tube voltage of 40 kV and a tube current of 40 mA with Cu Kα radiation.The incident X-ray wavelength λ was 0.15444 nm.The scanning range of the samples varied from 0° to 80°,at a scanning speed of 4 (°)?min-1.The weightlessness of HPW and La-PW-SiO2/SWCNTs at different temperatures was analyzed by means of a thermogravimetric instrument(TG/DTA 6300,Hitachi,Japan).Conditions used in a typical run were as follows:sample weight:6.527 mg;heating rate:20oC?min-1;atmosphere:O2;temperature range:25–900°C;reference:alumina 60 mesh.The surface molecular functional groups of SWCNTs,SWCNTs-OH and HPW,La-PW-SiO2/SWCNTs were analyzed by a Fourier transform infrared spectrometer (FTIR,Magna-IR 750,Nicolet,USA).The test procedure was as follows:2 mg powder sample and 200 mg dried KBr were ground in a mortar and pestle.The well ground powders were pressed and analyzed.Combined with the pyridine adsorption method,the acidic active site type and intensity of La-PW-SiO2/SWCNTs catalyst was further analyzed.The test procedure was as follows:The powder sample was pressed into a self-supporting piece with a diameter of 15 mm and weighed about 10–20 mg.The samples were pretreated at 350 °C for 1 h under vacuum (10–3Pa) prior to pyridine adsorption.Pyridine was adsorbed at room temperature with an argon flow that contains 2% (vol) pyridine.Then,the samples were heated to 350 °C and evacuated to remove physisorbed and weakly chemisorbed pyridine.Temperature-programmed desorption of the adsorbed pyridine starting at 100 °C was studied by stepwise heating of the sample under vacuum to characterize the strength of the acid sites.After deducting the background (base spectrum) of the unloaded sample,a pyridine adsorption infrared spectrum at 350°C was obtained.The strength of the acidic active site of the La-PW-SiO2/SWCNTs catalyst was analyzed by a temperature programmed 152 desorption(NH3-TPD)method using a program temperature monitor (AutoChem II 2920,Micromeritics,USA) at a temperature range of 0 to 800 °C.The bond type of the La-PW-SiO2/SWCNTs catalyst was analyzed by the multifunctional imaging electron energy spectrometer XPS(Escalab 250Xi,Thermo Scientific,USA) using a monochromatic Al Kα (hv=1486.6 eV),Power was 150 W,beam spot was 500 μm.

2.4.Study on the activity and stability of catalyst

The acid-catalyzed esterification of oleic acid with methanol is inspiring in the production of biodiesel due to the fact that oleic acid is present in different proportions in a variety of vegetable oils.Therefore,the catalytic activity and stability of HPW and La-PW-SiO2/SWCNTs catalysts were investigated from the esterification reaction of oleic acid and methanol.

The catalytic activity test was carried out as follows:a certain proportion of methanol was added together with catalyst and they were placed into a 250 ml four-necked round-bottomed flask(equipped with a reflux condenser,a thermometer and a mechanical stirrer),and then heated.The oleic acid was added into the reactor after the mixture has been heated to the required temperature,then the reaction was started with stirring at 240 r?min-1.The synthesis conditions that related to the catalytic activity had been optimized individually,including the molar ratio of methanol to oleic acid(5:1–12:1),the mass ratio of catalyst to reactants(1%–2%),reaction temperature (64–70°C).The reaction was stopped after 8 h,and the reaction mixture was poured into a separation funnel to allow phase separation for 12 h.

A continuous cycle of these above catalysts was performed six times under the optimum reaction conditions.During each cycle of the experiment,the reaction mixture was firstly poured into a separatory funnel and allowed to cool to ambient temperature.After the reaction mixture was cooled,diethyl ether was added to extract catalyst.The upper layer was collected,and then the ether was evaporated to precipitate catalyst.

The conversion of oleic acid was calculated by gas chromatographic analysis of the content of methyl oleate in the sample.The details were as follows:sample was taken every 1 h (take 1.5 ml of sample,then take an equal amount of methanol and oleic acid in a set molar ratio and add it to the reaction solution to ensure that the molar ratio is the same as the initial reaction state.).The test sample was placed in a drying oven and heated at 110°C for 30 min to distill off the residual methanol and water in the test sample.After cooling,the sample was filtered through a solid filter and the filtered sample was calibrated using a 5 ml volumetric flask.The calibrated sample was injected into a gas chromatograph for analysis.

2.5.Determination of methyl oleate content

The quantitative analysis of methyl oleate in the biodiesel samples was determined using gas chromatography equipped with a flame ionized detector (FID),a capillary column (DB-5) with 30 mm column length,0.25 mm inner diameter and 0.25 μm film thicknesses.Methyl oleate with different concentrations was used as the standard sample,and the external standard method was used to quantitatively analyze the concentrations of methyl oleate.The temperature program for the biodiesel samples started at 40°C and ramped to 150°C at 20°C?min-1.The temperature was held at 150°C for 15 min and ramped to 250°C at 4°C?min-1.The holding time at the final temperature (250°C) was 5 min.The injections were performed with 1 μl of the samples at splitting rate of 1/20 using nitrogen as gas carrier,and the flux was kept constant(2 ml?min-1).The injector and detector temperatures were set at 270 and 250°C,respectively.

3.Results and Discussion

3.1.Characterization of catalyst

Table 1 shows the contents of main elements in HPW and La-PW-SiO2/SWCNTs catalysts.

It can be seen from Table 1 that the content of Si in La-PW-SiO2/SWCNTs is 13.982% (mass).The high content of Si can be tried to explain as follows:TEOS is used in the sol–gel process,which hydrolyzes to form SiO2during the sol process.Thebond in SiO2network will coordinate with H+of HPW and form (≡Si–OH2+)(H2PW12)complexes that has strong electrostatic adsorption forces.The formed(≡Si–OH2+)(H2PW12)complex will further combine with–OH of SWCNTs–OH and lose H+,and which will finally bond with La3+.After calcination,La-PW-SiO2was formed and which can act as active substance.

Specific surface area,pore volume and pore size distribution are key properties that have been related to catalytic activity and stability.In order to understand the influence of the synergistic modification effect of La3+and SWCNTs–OH on the specific surface area,pore volume and pore size distribution of HPW,the values of the above structural parameters before and after modification of HPW were measured.Results were shown in Table 2.

It can be seen from Table 2 that the specific surface area has increased from 1.411 to 3.107 m2?g-1after HPW was modified by SWCNTs–OH and La3+.Hence,it can be concluded that La3+and SWCNTs–OH is favorable for dispersing of the active center of HPW and the extension of its specific surface area.

Table 1 The mass contents of main elements in HPW and La-PW-SiO2/SWCNTs catalysts (%).

Table 2 Specific surface area,pore volume and pore size of the HPW and La-PW-SiO2/SWCNTs catalysts.

For a catalyst,the surface functional groups play an extremely important role in both its catalytic activity and stability.Therefore,FT-IR spectra of SWCNTs,SWCNTs–OH and HPW,La-PW-SiO2/SWCNTs were measured.Results were shown in Fig.1.

In Fig.1(a),the absorption peak appearing at around 1575 cm-1corresponds to the C=C stretching vibration peak,which is generated by the infrared active characteristic vibration absorption mode E1uof graphite layer on the walls of the SWCNTs [27];the weak absorption peak appearing at around 2923 cm-1corresponds to the stretching vibration absorption of C–H band [28];the sharp adsorption peak appearing at 3430 cm-1comes from the hydroxy stretching vibration [29].It is worth noting that the FT-IR spectra of SWCNTs–OH showed two wider and sharper absorption peaks at 1575 and 1635 cm-1when it was compared with the IR spectra of SWCNTs,which are respectively attributed to the symmetrical vibration of -COO-and the C=O stretching vibration band[30].It also shows the formation of new groups on the surface of SWCNTs after microwave and sonication treatment.At the sametime,it can be seen that a very strong and rather sharp peak appeared at 3430 cm-1,which is caused by associative hydroxyl groups,indicating that the number of hydroxyl groups has increased significantly.A large amount of hydroxyl groups can make the interaction between the SWCNTs and other groups more malleable and easier.It can be concluded from Fig.1(b)that the FTIR spectra of HPW and La-PW-SiO2/SWCNTs catalysts shows no significant difference in the range of 800 to 1100 cm-1.Both of them retain four characteristic peaks of Keggin structure,peak appearing at 1082 cm-1corresponds to the P–O stretching vibration absorption peak,peak appearing at 981 cm-1corresponds to the W=O vibration absorption peak,peak appearing at 893 cm-1corresponds to the W–Oa–W common-point vibration absorption peak,and peak appearing at 813 cm-1corresponds to the W–Ob–W common-edge vibrational absorption peak [31].However,the infrared spectrum of the La-PW-SiO2/SWCNTs catalyst is different from that of the HPW catalyst.Two absorption peaks were appeared at 1090 and 465 cm-1,which are correspond to the Si–O–Si stretching vibration and Si-O stretching vibration absorption[32,33].Thus,it was revealed that the prepared La-PW-SiO2/SWCNTs catalyst was a compound formed by Si-O bond.

TEM is one of the most informative characterization technique due to the electrons can pass through the sample,so it can provide information not only about particle size and shape,but also about the lattice structure and the chemical composition of individual particles.Hence,SWCNTs and La-PW-SiO2/SWCNTs were characterized by TEM.Fig.2(a) and (b) are TEM images of SWCNTs and La-PW-SiO2/SWCNTs catalysts.

It can be seen from Fig.2(a) and (b) that La-PW-SiO2/SWCNTs maintained the tubular structure of SWCNTs,but the wall morphology has changed significantly.The surface of pure SWCNTs is very smooth and intertwines in bundles.However,the surface of La-PW-SiO2/SWCNTs is no longer smooth and shows dense spots.This may be due to the fact that some active sites (LaPW12O40-SiO2) have loaded on La-PW-SiO2/SWCNTs,which can produce bonding effect with–OH of SWCNTs–OH,thus changing the surface properties and overall morphology of SWCNTs.It also can be known from the Fig.2(a) and (b) that the diameters of SWCNTs and La-PW-SiO2/SWCNTs catalysts are all around 10 nm,which is a kind of hollow tube.However,in La-PW-SiO2/SWCNTs,LaPW12-O40-SiO2has boned with the–OH of SWCNTs–OH and formed new group,thus the outer tube wall of it is rougher than that of the SWCNTs.

In order to further determine the phase structure of the HPW,SWCNTs and La-PW-SiO2/SWCNTs catalysts,XRD analysis was performed.Results were shown in Fig.3.

It can be seen from Fig.3 that HPW exhibits three characteristic peaks at 2θ=10.3°,25.4° and 34.6° corresponding to (110),(222)and(332)crystal plane of the Keggin structure,and the peak position and peak height are in accordance with the cubic crystal system HPW?6H2O (JCPDS no.00–050-0304).In the XRD spectra of SWCNTs and La-PW-SiO2/SWCNTs,a strong and broad diffraction peak appeared at 2θ=26.16°,which corresponds to the(002)characteristic peak of SWCNTs [34].Besides,a characteristic peak corresponding to SiO2appeared at 2θ=42° to 49° in the XRD spectrum of the La-PW-SiO2/SWCNTs catalyst.Therefore,it can be concluded from the XRD spectrum that the La-PW-SiO2/SWCNTs catalyst maintains the Keggin structure of HPW and forms Si=O bond.

In order to understand the thermal stability of HPW and La-PWSiO2/SWCNTs catalysts,differential thermal analysis was performed on these two catalysts.Fig.4 shows the TG,DTA and DTG analysis results of HPW and La-PW-SiO2/SWCNTs catalysts.

It can be seen from Fig.4 that both HPW and La-PW-SiO2/SWCNTs catalysts have a large weight loss between 30 and 200 °C,and the weight loss in this temperature region is mainlyattributed to the loss of crystal water,protonated water and structure water.The temperature of exothermic peak in the DTA curve can be used to characterize the thermal stability of HPW and La-PW-SiO2/SWCNTs catalysts.The endothermic peaks and the first exothermic peak in the DTA curve of these above catalysts can be attributed to the processes of their dehydration and irreversible decomposition.Thus,the dehydration of HPW occurs at around 200°C.However,in the La-PW-SiO2/SWCNTs,no obvious exothermic peak was found,indicating that the water content is small.The exothermic peak appears at around 600°C,which is due to the irreversible decomposition of Keggin-type anion.It indicated that the stability of La-PW-SiO2/SWCNTs catalysts is higher than HPW.

Fig.1.FT-IR spectra of (a) SWCNTs and SWCNTs–OH and (b) HPW and La-PW-SiO2/SWCNTs.

Fig.2.TEM images of (a) SWCNTs and (b) La-PW-SiO2/SWCNTs catalysts.

Fig.3.XRD spectra of HPW,SWCNTs and La-PW-SiO2/SWCNTs.

In order to understand whether the valence state of the main elements in the La-PW-SiO2/SWCNTs catalyst changes,the photoelectron spectroscopy analysis was carried out and the results were shown in Fig.5.

Fig.4.TG,DTA,DTG curves of (a) HPW and (b) La-PW-SiO2/SWCNTs catalysts.

The C1s,La3d and W4f,P2p,Si2p and O1s XPS spectra are shown in Fig.5(a)–(f),respectively.After peaking treatment,it can be seen that the C1s of the La-PW-SiO2/SWCNTs catalyst mainly consists of three peaks,wherein the binding energy of 284.8 eV corresponds to C=C and C-C bonds.The two peaks at binding energies of 286.7 and 289.1 eV correspond to C-O and O=C-O bonds,respectively.The C1s map indicates that the surface of La-PW-SiO2/SWCNTs catalyst contains some oxygencontaining groups,such as hydroxyl group,epoxy group and carbonyl group.Fig.5(b) shows the photoelectron absorption peak of La3d.In this figure,the two peaks at binding energies of 837 and 840.1 eV are the photoelectron absorption peaks of La3d5/2;the three peaks at binding energies of 854.1,856.8,and 864.2 eV are the photoelectron absorption peaks of La3d3/2.It has reported that the photoelectron absorption peaks of La3d5/2and La3d3/2will respectively appear at 833.9 and 850.0 eV if the 4f orbit of La was not affected by any chemical environment [16].Hence,it can be seen that the binding energies of La3d5/2and La3d3/2are shifted to the high energy direction,and which is due to the occurring of electron transfer between La3+and the high delocalized π electrons that were exist on the SWCNTs surface.Fig.5(c) shows the W4f spectrum,and the two peaks appearing at binding energies of 36.6 and 38.7 eV correspond to W4f7/2and W4f5/2.However,the W4f7/2and W4f5/2photoelectron absorption peaks of pure HPW will appear at 35.8 and 37.8 eV,respectively [35].Therefore,the binding energies of W4f7/2and W4f5/2are also shifted to the high energy direction.The shifting of binding energy to the high energy direction can be explained as follows:HPW has some degree of oxygen vacancy,which can restore part of W6+to W5+during the process of sol–gel conversion.Fig.5(d) and (e) show the P2p and Si2p spectrums,two photoelectron absorption peaks appeared at the binding energies of 126.7 and 134.9 eV,respectively,corresponding to P0and P5+.After considering P2p with the absorption peak of Si2p at 104 eV,it can be concluded that Si has entered the keggin structure and formed a complex with P.It also validated that SiO2was bonded with LaPW12O40and formed active center of LaPW12O40-SiO2.Fig.5(f) shows the O1s spectrum,a photoelectron absorption peak appears at 533.4 eV,and which corresponds to P=O bond.

In order to understand the type of acidic active sites of the La-PW-SiO2/SWCNTs catalyst,the pyridine infrared absorption method was performed on them.Results were showed in Fig.6.

It can be seen from Fig.6 that the pyridine infrared adsorption spectrum of La-PW-SiO2/SWCNTs appeared two characteristic peaks at the wavenumbers of 1450 and 1540 cm-1,which can be respectively corresponded to Lewis acid sites and Br?nsted acid sites [36].Hence,La-PW-SiO2/SWCNTs catalysts are acidic catalysts with both Br?nsted acid sites and Lewis acid sites.It should be noticed that the intensity of the peaks at 1450 cm-1was much stronger than that at 1540 cm-1,which indicated that the content of Lewis acid sites was higher than that of Br?nsted acid sites.Hence,it can be concluded that the La-PW-SiO2/SWCNTs was mainly composed of Lewis acid sites.According to the peak area of the desorption peak(1540 cm-1)corresponding to the Br?nsted acid sites in the NH3-TPD analysis spectrum,the surface acid density of the sample was calculated.The Br?nsted acid site concentrations of the fresh catalyst was 1.24 mmol?g-1.

The acid strength of La-PW-SiO2/SWCNTs catalyst was measured by temperature-programmed desorption of ammonia(NH3-TPD).Result was shown in Fig.7.

It can be seen from Fig.7 that La-PW-SiO2/SWCNTs catalyst appeared two desorption peaks at 100–250 and 600–700°C,respectively.The desorption peak curves appeared at 100–250°C corresponds to weak acid sites,and the desorption peak appeared at 600–700°C corresponds to strong acid sites,respectively.A wide desorption peak appeared at 600–700°C,which indicates the La-PW-SiO2/SWCNTs catalyst has large amounts of strong acid sites.The Keggin structure has been decomposed at 600–700°C,the peak appearing here should not come from the Br?nsted acidic active site of the Keggin structure,but from the Lewis acidic active site formed by the interaction of the SWCNTs–OH with La3+.Pyridine adsorption infrared spectroscopy has also proved that the La-PW-SiO2/SWCNTs catalyst is dominated by Lewis acidic active sites.

In order to better describe the formation of Lewis acid centers in the La-PW-SiO2/SWCNTs catalyst.Schematic diagram of Lewis acid site formation mechanism of La-PW-SiO2/SWCNTs catalyst was shown in Fig.8.

The specific process is shown as follows:first,hydrolysis of TEOS occurred when it was used as silicon source material,and SiO2network was formed during the conversion process of sol–gel under acidic conditions;second,that exists in the SiO2network was coordinated with H+of HPW and formed(≡Si–OH2+)(H2PW12) complex with strong electrostatic adsorption force.Finally,(≡Si–OH2+)(H2PW12) will further bond with–OH of SWCNTs–OH and La3+,forming active component of LaPW12O40-SiO2after calcination.The strong electronwithdrawing effect of La3+will cause e–move to the surface of SWCNTs and disperse in its benzene-like ring structure.Thereby,the system charge imbalance tendency of the La-PW-SiO2/SWCNTs catalyst was intensified,which caused excess of positive charge in it.

3.2.Comparative study on catalytic activity and stability

Based on these above analysis of results of the physicochemical properties of HPW and La-PW-SiO2/SWCNTs catalysts,the catalytic activities of these above catalysts in the catalytic esterification of oleic acid with methanol to methyl oleate were compared.The experimental conditions were as follows:the molar ratio of methanol to oleic acid was 8:1,the amount of catalyst was 2% (mass) of the reactants,the reaction temperature was 65°C,and the reaction time was 8 h.The results are shown in Fig.9.

Fig.5.XPS spectra of La-PW-SiO2/SWCNTs catalyst.(a) C1s,(b) La3+,(c) W4f,(d) P2p,(e) Si2p,(f) O1s.

It can be seen from Fig.9 that the La-PW-SiO2/SWCNTs catalyst has higher catalytic activity than HPW.The conversion of oleic acid was 86%(mass)after 8 h of reaction with the using of HPW as catalyst.However,the conversion of oleic acid was 92% (mass) after 8 h of reaction when La-PW-SiO2/SWCNTs was used as the catalyst.From the results of temperature-programmed desorption of ammonia and pyridine infrared adsorption spectrums,it can be known that La-PW-SiO2/SWCNTs catalyst has a large amount of strong Lewis acid.Hence,it can be concluded that the influence of the surface acidity on the catalytic activity is very significant.The relative low catalytic activity of HPW should be should be because it is a Br?nsted acid type solid acid,and the Br?nsted acid site concentrations will be reduced due to the occurrence of–OH hydration after the water was formed from the esterification of oleic acid and methanol and existed in the reaction solution.

Based on the comparison of catalytic activity,the catalytic stability of HPW and La-PW-SiO2/SWCNTs catalysts was compared.The experimental conditions are the same as the reaction conditions used in the study of catalytic activity.The results were shown in Fig.10.

Fig.6.Pyridine infrared adsorption spectrum of La-PW-SiO2/SWCNTs catalysts.

Fig.7.Temperature-programmed desorption of ammonia of La-PW-SiO2/SWCNTs catalyst.

It can be seen from Fig.10 that the catalytic stability of La-PWSiO2/SWCNTs catalyst is the highest.After 6 cycles of use,the conversion of oleic acid can still be as high as 88.7% (mass).However,the final conversion of oleic acid is only 45%(mass)after 6 cycles of HPW.The reason why La-PW-SiO2/SWCNTs catalyst has higher catalytic stability can be explained as follows:the water generated during the esterification reaction of methanol and oleic acid will hydrate with Br?nsted acid sites and reduce the catalytic activity.Based on the NH3-TPD analysis results(Fig.7)and pyridine adsorption infrared spectroscopy (Fig.6),it can be seen that the La-PWSiO2/SWCNTs catalyst contains a large number of Lewis acidic active sites that are strong acid sites.XPS analysis (Fig.5) confirmed that a large number of Lewis acidic active sites belonging to strong acid sites were formed by the action of La3+and the high delocalized π electrons that were existed on the SWCNTs surface.Therefore,the catalytic stability of the Bronsted acid type HPW catalyst is much lower than that of the Br?nsted-Lewis acid type La-PW-SiO2/SWCNTs catalyst.Although La-PW-SiO2/SWCNTs has high catalytic stability,there is still a problem that the activity decreases to a certain extent after six cycles of use.In order to explore whether the decrease in the catalytic activity of La-PWSiO2/SWCNTs is caused by the partial hydration of the Br?nsted acidic active sites,In order to explore whether the decrease in the catalytic activity of La-PW-SiO2/SWCNTs is caused by the partial hydration of the Br?nsted acidic active sites,the concentration of the Br?nsted acidic active sites in the catalyst after six cycles of use was determined,and the result was 0.93 mmol?g-1.Therefore,the decreased catalytic activity of La-PW-SiO2/SWCNTs may be caused by the partial hydration of the Br?nsted acidic active sites during the recycling process.The synergic function of Br?nsted acid and Lewis acid on the esterification reaction of methanol and oleic acid was shown in Fig.11.

It can be seen from Fig.11 that the catalytic mechanisms of the Br?nsted acid sites and Lewis acid sites for esterification reaction are essentially different.Br?nsted acid sites catalyze the esterification reaction of oleic acid and methanol as follows:first,the carbonyl group of oleic acid is protonated;second,the methanol molecule conducts a nucleophilic attack on the protonated carbonyl group to form a tetrahedral intermediate;finally,the proton migration occurs and the tetrahedron disintegrates to form methyl oleate and water.Lewis acid sites catalyze the esterification reaction of oleic acid and methanol as follows:first,Lewis acid sites form complexes with oleic acid;second,methanol molecules form new esters through nucleophilic interaction;finally,the new esters leave Lewis acid sites.Therefore,the strength of Lewis acid sites has a great influence on the esterification reaction.If the Lewis acid sites are too weak,it is difficult to form a complex with oleic acid.Since La-PW-SiO2/SWCNTs catalyst contains a large number of strong Lewis acidic active sites,so it has high catalytic activity.

The optimal reaction conditions for the esterification of oleic acid with methanol by La-PW-SiO2/SWCNTs were studied by investigating the oleic acid conversion rates under different reaction temperatures,different catalyst dosages and different molar ratios of alcohol to oil.First,experimental studies were carried out at different reaction temperatures(61–69°C)under the condition that the mass ratio of catalyst to reactant was 0.5%,molar ratio of methanol to oleic acid was 15:1 and reaction time was 8 h,respectively.Results of oleic acid conversion at different reaction temperatures were shown in Fig.12.

As shown in Fig.12,the La-PW-SiO2/SWCNTs catalyst has the best catalytic effect when reaction temperature was 65°C.It is because the esterification reaction is an endothermic reaction.Thus,the reaction is not favorable to the positive reaction direction when the temperature is low,so the oleic acid conversion rate is lower at the reaction temperature of 61 and 63°C.Since the boiling point of methanol is 64.7°C(standard atmospheric pressure),part of methanol cannot fully react with oleic acid due to methanol gasification when the reaction temperature is higher than 65°C,so the optimal reaction temperature is 65°C.

The amount of catalyst is an important factor affecting the reaction,so it is necessary to optimize the percentage of the catalyst in the reaction system.Experimental studies were carried out at different mass ratio of catalyst to reactant(0.5%–2.5%)under the condition that the reaction temperatures was 65°C,molar ratio of methanol to oleic acid was 15:1 and reaction time was 8 h,respectively.The results of oleic acid conversion under different mass ratio of catalyst to reactant were shown in Fig.13.

As shown in Fig.13,the conversion of oleic acid was increased with the increasing of the amount of catalyst when the total amount of catalyst is in the range of 0.5% to 2.5%.However,the conversion of oleic acid is no longer increased after 3 h of reaction when the amount of catalyst is 1.5%.It is because the molar ratio of the alcohol to oil is 15:1,so the increased active site is mainly occupied by methanol due to the fact that the content of methanol is much higher than that of oleic acid,which is not favorable to promote the esterification reaction.From this,it can be determined that the optimum amount of catalyst is 1.5%.Since the esterification reaction is a reversible reaction,when the amount of methanol is increased,the reaction proceeds to the positive reaction direction.Therefore,an increase of the amount of methanol can lead to the increase of the conversion of oleic acid.Experimental studies were carried out at different molar ratio of methanol to oleic acid(6:1,9:1,12:1,15:1 and 18:1) under the condition that the reaction temperatures was 65°C,mass ratio of catalyst to reactant was 15:1 and reaction time was 8 h,respectively.The results of oleic acid conversion under different molar ratio of methanol to oleic acid are shown in Fig.14.

Fig.8.Schematic diagram of Lewis acid site formation mechanism of La-PW-SiO2/SWCNTs catalyst.

Fig.9.Comparison of catalytic activity.Experimental conditions were as follows:the molar ratio of methanol to oleic acid was 8:1,catalyst to reactants was 2%(mass),reaction temperature was 65°C and reaction time was 8 h.

Fig.10.Comparison of catalytic stability.Experimental conditions were as follows:the molar ratio of methanol to oleic acid was 8:1,catalyst to reactants was 2%(mass),reaction temperature was 65°C and reaction time was 8 h.

Fig.11.The synergic function of Br?nsted acid and Lewis acid on the esterification of oleic acid and methanol.

Fig.12.Diagram of reaction temperature versus oleic acid conversion.Reaction conditions:the La-PW-SiO2/SWCNTs catalyst to reactant was 0.5%(mass),the molar ratio of methanol to oleic acid was 15:1 and reaction time was 8 h.

Fig.13.Diagram of La-PW-SiO2/SWCNTs catalyst dosage and oleic acid conversion rate.Reaction conditions:reaction temperature was 65°C,molar ratio of methanol to oleic acid was 15:1 and reaction time was 8 h.

Fig.14.Diagram of molar ratio of methanol to oleic acid and conversion of oleic acid.Reaction conditions:the reaction temperature was 65°C,La-PW-SiO2/SWCNTs catalyst to reactant was 1.5% (mass) and reaction time was 8 h.

It can be seen from Fig.14 that the oleic acid conversion rate was increased when the molar ratio of methanol to oleic acid was increased from 6:1 to 18:1,which is because the reaction proceeds through the electron-withdrawing effect of the strong Lewis acid sites since it can cause the oleic acid to form carbon cations,which then attack the carbon cations to form methyl oleate.Therefore,when the amount of methanol increases,the probability of collision of carbon positive ions with methanol is increased,thereby increasing the conversion rate of oleic acid.It also can be seen from Fig.14 that when the molar ratio of methanol to oleic acid was increased from 15:1 to 18:1,the conversion of oleic acid increases by less than 1% (mass) after 8 h of reaction.Hence,increasing the amount of methanol does not significantly increase the conversion of oleic acid after the molar ratio of methanol to oleic acid has increased to 15:1.From an economic point of view,the amount of methanol should not be too high,so the optimum molar ratio of methanol to oleic acid is determined to be 15:1.It can be seen from Fig.12 to Fig.14 that the oleic acid conversion rate increases as the reaction time increases when the reaction time is in the range of 1 to 4 h,and the increasing of oleic acid conversion rate decreases when the reaction time reached 4 h,so the optimal reaction time is determined to be 4 h.Based on these above results,the optimal reaction conditions can be determined,and the conversion of oleic acid can reach 90.4% (mass) when the temperature is 65°C,the catalyst accounts for 1.5% (mass) of the total mass of reactants,the molar ratio of methanol to oleic acid is 15:1 and the reaction time is 4 h.

Table 3 shows different solid acid catalysts for biodiesel preparation that were collected from literatures.

Table 3 Comparison of solid acid catalysts for biodiesel preparation.

It can be seen from Table 3 that the catalytic activity of this study is higher than most solid acid catalysts listed in the table.Compared with other catalysts with higher activity listed in the table,it has the following advantages:it requires less catalyst addition,or lower alcohol-to-oil molar ratio,or lower reaction temperature to achieve the same catalytic effect.In addition,the conversion of oleic acid was still as high as 88.7% (mass) after six cycles of La-PW-SiO2/SWCNTs,so the recyclability is acceptable.Hence,La-PW-SiO2/SWCNTs can be an efficient and economical catalyst because it shows good catalytic activity and stability.

4.Conclusions

A novel solid Br?nsted-Lewis acid La-PW-SiO2/SWCNTs catalyst was synthesized from the synergistic modification of HPW by La3+and SWCNTs–OHviasol–gel method.The highest conversion of oleic acid was 93.1% (mass) when the mass ratio of methanol to oleic acid was 15:1,the amount of catalyst was 1.5% (mass) of the total mass of the reactants,the reaction temperature was 65°C after 8 h of reaction.After six cycles of La-PW-SiO2/SWCNTs,the conversion of oleic acid was still as high as 88.7% (mass).The high catalytic activity and stability of La-PW-SiO2/SWCNTs can be attributed as follows:due to the strong electron-withdrawing effect of La3+,so e-was moved to the surface of SWCNTs and dispersed in the benzene-like ring structure.Therefore,the La-PWSiO2/SWCNTs catalyst has a tendency to increase the charge imbalance of the system and lead to excessive positive charges.Due to the decreasing of Br?nsted acid sites and the increasing of Lewis acid sites have occurred in the catalyst,so the deactivation of the catalyst is greatly reduced,and the repeatability of the catalyst is also promoted.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(21766009),Program of Qingjiang Excellent Young Talents (Jiangxi University of Science and Technology).Thanks to Wang Tingting from the School of Foreign Languages,Jiangxi University of Science and Technology,for polishing the English grammar of this paper.