Wei Xu,Lijing Gao,Feng Jiang,Guomin Xiao*

School of Chemistry and Chemical Engineering,Southeast University,Nanjing 211189,China

Keywords:Hydrotalcite coating In situ synthesis Ceramic membrane Biodiesel Transesterification

ABSTRACT In situ surface synthesis of Ca–Mg–Al hydrotalcite(HT)on inorganic ceramic membrane(CM)was investigated with urea as precipitator.The effects of molar ratio of raw materials,crystallization time,and temperature on surface synthesis of HT were examined.The as-prepared HT/CM samples were characterized by XRD and SEM and an in situ growth mechanism of HT on CM was proposed.KF/HT/CM obtained by loading potassium fluoride(KF)on the HT layer by impregnation and calcination method was used as catalyst for transesterification between palm oil and methanol.The comparison of KF/HT/CM and pure KF/HT powder under identical reaction conditions shows that the production of fatty acid methyl ester is equivalent,which means that the use of inorganic catalytic membrane in the transesterification is a viable alternative.

1.Introduction

Hydrotalcite(HT)or hydrotalcite-like materials have been successful as catalysts or supports in the synthesis of a variety of fine chemicals and attracted much attention in the development of new environmentally friendly catalysts[1–4].Thermal decomposition of these materials by calcinations forms basic mixed oxides with high surface areas,which are also widely used as catalysts or supports.For example,calcined Mg–Al HT[5]and Ca–Al HT[6],Mg–Al–La HT[7],KF/Mg–Al HT[8]and KF/Ca–Al HT[9]were adopted for alcoholysis of vegetable oil to produce biodiesel,which is an alternative fuel for diesel engines.

In most cases,these HT materials are synthesized by constant pH co-precipitation method or hydrothermal method.The particle sizes are generally small.Direct use of materials with small particle size may cause many problems,such as high pressure drop and difficulty in separation and operation.A simple way to solve these problems is to make use of HT materials coated on or embedded in structured support materials.This approach can also extend the application of catalysts.For example,a new heterogeneous catalytic process for biodiesel production is the application of membrane as part of solid catalyst or the support of heterogeneous catalyst[10,11].This alternative is also used in membrane catalytic process and is a potential material in membrane separation and reaction processes[12,13].Guerreiro et al.have utilized a PVA embedded HT membrane to catalyze transesterification of soybean oil and the catalyst shows considerable activity and reusability[14].In these studies,organic membrane materials are commonly used for the preparation of organic membrane or organic–inorganic hybrid membrane,while fewer researches have been done on inorganic catalytic membranes for liquid reactions.

To form an inorganic catalytic membrane,a simple way is to coat the catalyst on inorganic membranes directly.However,due to the properties of HT,it is difficult to coat.Then,it is proposed to prepare the catalytic component from crystallites grown on the support surfaces,called in situ synthesis,to obtain more stable catalytic coatings.The main advantage of in situ synthesis over other coating techniques is that the support is used as a base for nucleation and that a chemical bonding between crystals and outer support layer is formed[15,16].Therefore,the active layers by in situ synthesis method have better stability and less release.In situ hydrothermal synthesis method has been successfully introduced to the coating of ZSM-5[17,18],TS-1[19]and mordenite[20]on ceramics or honey cordierites.

In the present study,Ca–Mg–Al HT is synthesized on ceramic membrane(CM)supports by an in situ hydrothermal method.The samples are characterized by XRD and SEM to investigate the surface synthesis of Ca–Mg–Al HT.Such a simple synthesis system will provide a better understanding of the growth of HT on supports and KF modified HT/CM are tested as catalysts for the transesterification between palm oil(PO)and methanol.

2.Experimental

2.1.Synthesis

CM manufactured by Foshan Ceramics Research Institute&Jingang Group was used as support and cut to cylinders.The length,inner diameter,outer diameter and the pore size of CM cylinder were 10 mm,6 mm,10 mm,and 0.2 μm,respectively.The commercial CM was cleaned in HNO3(5%,by mass)for 24 h,washed by deionized water,dried at 333 K,and calcined at 823 K.Then it was pretreated by immersing in aluminum sol for 30 min,dried at 333 K and calcined at 823 K for 3 h.

In the surface synthesis of Ca–Mg–Al HT,Ca(NO3)2·4H2O(AR),Mg(NO3)2·6H2O(AR),and Al(NO3)3·9H2O(AR)were used as the calcium,magnesium and aluminum sources,respectively.Urea(AR)was used as precipitator.The nitrates,urea and water were mixed at certain proportions to obtain aqueous solution with Ca2+,Mg2+,and Al3+concentration at 0.6,0.6,and 0.4 mol·L-1,respectively.The transparent liquid and pretreated CM were added into a flask with vigorous stirring,and then heated to 373 K and kept for 3 h.The suspension liquid and CM were put into a Te flon-lined stainless steel autoclave for static crystallization at certain fixed temperature for a period of time.Then the samples were taken out,washed in deionized water by ultrasonic waves to remove the unstable adherends on the CM,and dried at 333 K overnight.The precipitate was filtered out and dried at 333 K overnight to obtain HT powder.

For the preparation of KF/HT/CM,Ca–Mg–Al HT/CM was calcined at 823 K for 3 h,and then impregnated with different concentrations of KF aqueous solutions atroomtemperature.After a period oftime(typically 24 h),the catalyst was taken out and dried.Finally,KF/HT/CM was obtained after calcined at 873 K for 3 h.

For the preparation of alumina sol,aluminum powders(AR,0.075–0.15 mm),Al(NO3)3·9H2O and deionized water were mixed with a molar ratio of 2:3:50.The mixture was kept stirring at 373 K until aluminum powder was completely dissolved.Finally,a transparent and stable alumina sol was obtained after aged at 353 K for 2 h.

2.2.Characterization

To characterize the HT coating,quantitative filter paper was used instead of CM.Subsequent procedures were the same as those for modifying the CM.The obtained powder on the filter paper was characterized by powder X-ray diffraction(XRD)[21].XRD data were collected on a Bruker D8 instrument with Cu Kαradiation(λ =0.15418 nm)at 50 kVand 30 mA；the range of the scan was 2θ=5°–90°.The integral intensity and full width in half maximum(FWHM)of the(003)(～11.7°)diffraction line were evaluated in order to compare the crystallinity of samples treated hydrothermally under various conditions.

A HITACHI S-4800N scanning electron microscope(SEM)was used for examining the surface morphology of modified and unmodified CMs.SEM images were recorded on the samples covered with a thin layer of gold deposited by sputtering.

2.3.Catalytic activity test

Transesterification experiments were carried out in a 50 ml threenecked flask,with thermostat and mechanical stirring.A prescribed amount of PO,methanol and catalytic membrane( fixed on the two sides of the stirring paddle)was added into the flask,and then heated to refluxing temperature,with stirring.Other reaction conditions were as follows:methanol/oil(molar ratio)12:1,catalyst amount 5%(referred to catalytic coating,mass/oil mass),and transesterification time 3 h.The reactor was cooled to room temperature after the reaction.The mixture was filtered and the content of fatty acid methyl ester(FAME)was analyzed by the gas chromatograph(Ouhua GC 9160)equipped with a DB-5Ht capillary column(15 m×0.25 mm×0.25 mm)by a flame ionization detector.Nitrogen was used as a carrier gas with a flow rate of 2 ml·min-1.The temperatures of injector and detector were both 633 K.

3.Results and Discussion

3.1.In situ hydrothermal synthesis of HT

The XRD patterns of samples synthesized by hydrothermal method(denoted as HT-HM)in this work and co-precipitation method(denoted as HT-CPM)reported in reference[22]were shown in Fig.1.Re flections at 2θ=11.7°(003),23.5°(006),35°(009),60.8°(110),61.8°(113)are defined as the characteristic reflections of HT,which stand for the layer structure.The reflections assigned to(003)and(006)reflections can be used to calculate the basal spacing between the layers of hydrotalcite(d,nm)[23].The pattern and location of the reflections are almostthe same in the two XRDlines,butthe(003)reflection intensity of HT-HMis much stronger than that of HT-CPM.Moreover,Table 1 demonstrates that the FWHM data of HT-HM is much smaller than that of HT-CPM,which means that the primary particle of HT is bigger.This mainly attributes to the advantages of urea hydrolysis reaction combined with hydrothermal conditions for HT synthesis[24],as the growth of crystallites is connected with increasing HT content in the hydrothermally treated samples.

Fig.1.XRD patterns of HT samples synthesized by different methods.

Table 1 XRD data of samples synthesized by different methods

3.2.Effect of preparation condition on HT/CM

3.2.1.Effect of crystallization time on the synthesized HT

Fig.2 shows the XRD patterns of Ca-Mg-Al HT samples crystallized with different crystallization time ranging from 0 h(non-treated)to 10 h.The intensity of HT characteristic reflections increases with the crystallization time.Particularly,the intensity with 6 h increases dramatically compared to that with 4 h.The XRD patterns of samples crystallized for 6 h and 8 h are basically in accordance,and the reflections are sharp-pointed and symmetric,indicating that the synthesized Ca–Mg–Al HT possesses good crystalline and structure regularity.With the crystallization time of 10 h,reflections at 2θ=14.8°,15.3°,26.8°,30.1°,31.5°appear,attributed to NH4Al(OH)2CO3·H2O(JSPDS PDF#29-106).The reason for this phenomena mightbe thatwhen the hydrothermal time is longer than 8 h,some Al3+reacts with excessive,and OH-in the solution,which are the hydrolyzates of urea,forming NH4Al(OH)2CO3·H2O.

Fig.2.XRD patterns of HT samples crystallized with different crystallization times.

The FWHM values of samples with crystallization time from 0 to 10 h are 0.52,0.48,0.39,0.34,0.35 and 0.31.According to the crystal chemistry,the FWHM value decreases with the increase of primary particle size.The decreasing trend of FWHM value with crystallization time means that the primary particle size of in situ synthesized HT increases gradually.The FWHM value decreases markedly in a relatively short time(0–4 h)and then more slowly as the crystallization time increases.Although the FWHM value at 10 h is smaller than that at 6 h,unpredictable component forms according to the XRD pattern.Moreover,the FWHM values of the samples crystallized for 6 h and 8 h are almost the same.Therefore,taking the XRD patterns and FWHM data into consideration,6 h is chosen as a proper crystallization time.

3.2.2.Effect of crystallization temperature on the synthesized HT

Fig.3.XRD patterns of HT samples crystallized at different temperatures.

Fig.3 shows the XRD patterns of HT samples crystallized at different temperatures.The diffraction reflection of HT character peaks(003),(006),and(009)increases with crystallization temperature at temperatures lower than 443 K,so higher temperature benefits the formation of HT sample.At the crystallization temperature of 443 K,the(003)peak presents the highest intensity.At the crystallization temperature of 463 K typical reflections of HT disappear,indicating that the layer structure is destroyed at high temperature.The FWHM values of the HT samples are 0.40,0.38,0.34 and 0.29 with the crystallization temperature ranging from 383 to 443 K.The variation of FWHM values reflects a slightly different crystallinity of the synthesized samples.In general,the higher temperature in the interval 383–443 K is applied,the lower the FWHM value.These processes might be explained by the dissolution of an amorphous part in the samples and the growth of crystallites during HT synthesis[25].Although the crystallinity of samples aged at 423 K is not as good as that at 443 K,423 K is considered as the best temperature to obtain good crystallized HT under a relatively milder condition.

The load ratio of HT on CM with different crystallization times and crystallization temperatures is shown in Fig.4.The loading amount of HT on CM increases with crystallization time.It suggests a continuous process for HT to grow and crystallize on CM surface.As the crystallization time prolongs,the amorphous part dissolves and the crystallite grows,so the loading amount increases.Moreover,the load ratio of HT increases with crystallization temperature.The reason is similar with that described in the discussion of Figs.2 and 3.

Fig.4.Load ratio of HT on CM crystallized at different temperatures with different crystallization times.

3.2.3.Effect of molar ratio of raw materials on the synthesized HT

Fig.5 shows the XRD patterns of samples crystallized at 423 K for 6 h with the molar ratio of urea to total cations from 0.5 to 4.Layer structure formed at molar ratios larger than 2,while no obvious HT characteristic reflection was detected at the ratio of 0.5 or 1.The reason could be that the pH value of the system is provided by hydrolyzing urea,and at low urea molar ratios,pH is lower than the co-precipitation value of Ca–Mg–Al HT and it cannot provide the environment for formation of layer structure.It is proved by the pH tests for suspension liquids of the two cases after hydrothermal process,which was 7–8 and 8–9.The sample with a molar ratio of 3 shows the strongest typicalpeaks ofHT layerstructure,while the ratios of 4 and 2 present weaker ones.Their FWHM values also show the same tendency,0.36 at molar ratio of 3,0.43 at ratio of 2,and 0.38 at ratio of 4.This may be explained as follows.Plenty urea contributes to the relative yield of HT[26]while too much urea makes pH of the system change too fast in the dynamic crystallization process,leading to inhomogeneity of sedimentation of Ca2+,Mg2+and Al3+and partly destroying the layer structure or crystal texture.In order to synthesize HT with better layer structure and higher relative yield,the molar ratio of urea to total cations is chosen as 3.

Fig.5.XRD patterns of HT samples with different molar ratios of precipitator.

3.3.SEM characterization

Fig.6 shows the SEMimages for the surface ofblank CMand surfaces of HT/CM and KF/HT/CM after in situ synthesis and alkaline modification.CM support exhibits irregular porous structure[Fig.6(a)]；such structure is utilized efficiently for the crystallization process by in situ synthesis method.Loaded with γ-Al2O3[Fig.6(b)],the surface does notchange apparently.HT/CMsamples withouttreated hydrothermally[Fig.6(c)]form a little layer structure on the surface around the pores,while its XRD patterns in Fig.2 also show very weak character peaks of HT.The HT/CM treated hydrothermally for 6 h[Fig.6(d)]presents obvious layer structure grained with regular shapes completely and compactly covering the external surface of CM.Combined with the XRD pattern of HT powder synthesized under the same conditions,it is conclude that Ca–Mg–Al HT is coated on the CM surface by in situ synthesis.According to the above results and the possible growth mechanism of zeolites on cordierite supports proposed by Li et al.[17],a possible simple growth mechanism of HT on CM is proposed as follows.γ-Al2O3is introduced on the CM surface with dipcoating as the second support and part of aluminum source,and it is activated under the alkaline circumstances,building the layers with the cations in the solution.And then the crystals grow continuously under hydrothermal conditions in the synthesis gel.The aluminum atoms shared by the CM and HT layers act as a bridge connecting these two parts.In Fig.6(e),the layer structure on the surface of calcined HT/CM(sample d calcined at 823 K for 3 h)disappears and some small particles form.From the appearance of KF/HT/CM(KF load ratio of 92.1%based on the mass of HT coatings)[Fig.6(f)],it can be seen that the surface state of KF/HT/CM is completely different from those without KF loaded.This might be caused by the reaction between KF and calcined HT in the KF loading process.Rebuilding partial layer structure may be a consequence of fabric memory effect of HT,as the catalyst absorbs some water in the operation processes.

3.4.Catalytic activity of KF/HT/CM

Table 2 shows the FAME yield catalyzed by CM,KF/CM(γ-Al2O3/CM with load ratio of γ-Al2O3～8%impregnated by KF solution),HT/CM and KF/HT/CM,with the load mass ratio of HT/CM about 8%.It can be seen that the loading of KF endues HT/CM with catalytic activity for the biodiesel synthesis.With the same KF load ratio and reaction conditions,FAME yield obtained by KF/HT/CM is higher than that by KF/CM,which indicates that the HT layer on the CM surface also plays an active role in improving catalytic activity.

The results oftransesterification reactions are shown in Table 3,with powder KF/HT prepared by the HT synthesized under the optimumconditions impregnated by KF solution.The yield of FAME increased dramatically with the increase of KF loading and reached 93%at the loading of 76.5%(based on the loading mass of HT).With the same KF load ratio and reaction conditions,little difference was observed between the powder catalyst and the catalytic membrane when the KF loading was 92.1%and 110.9%.This might attribute to the high catalytic activity of catalysts and good dispersion of catalytic species on the CM surface,which could be proved by the SEM images of KF/HT/CM as shown in Fig.6(f).When the KF/HT/CM was used for the second time,the FAME yield reduced about15%.Deactivation could be a consequence of catalytic component leached by shear stress and covered by glycerol[22].For the catalysts(with KF loading 92.1%and 110.9%)used for the second time,a FAME yield of～80%indicates the reusability to some extent.

4.Conclusions

Ca–Mg–Al HT can be coated on the CM surface by in situ hydrothermal synthesis method.Some parameters in preparation of HT/CM such as crystallization time,crystallization temperature and molar ratio of raw materials affect the crystallinity and direct growth of HT on CM.KF can be loaded on HT/CM under proper conditions by impregnation and calcination.FAME yield of transesterification between palm oil and methanol is used to determine the catalytic activity of prepared KF/HT/CM.According to the XRD and SEM results for the HT/CM samples,crystallization at 423 K for 6 h with molar ratio of urea to cations of 3:1 is considered to be the proper condition of in situ synthesis of HT on CM.The catalytic test shows that the KF/HT/CM with the KF load ratio of 92.1%and 110.9%presents the best activity and reusability.For pure powder KF/HT,the as-coated KF/HT on the support has similar catalytic performance with relatively high load ratio of KF.

Fig.6.SEM images of surfaces of CM(a),γ-Al2O3/CM(b),HT/CM non-treated hydrothermally(c),HT/CM treated hydrothermally(423 K)for 6 h(d),calcined HT/CM(e),and KF/HT/CM(f).

Table 2 Yield(%)of FAME over different catalysts

Table 3 Yield(%)of FAME over catalysts with different KF load ratios