Novel Synthesis of Mesoporous Nanocrystalline Zirconia

CHE Hong-Wei HAN Shu-Hua,* HOU Wan-Guo,2,* LIU Ai-Feng

(1Key Laboratory of Colloid and Interface Chemistry,Ministry of Education,Shandong University,Jinan 250100,P.R.China;2College of Chemistry and Molecular Engineering,Qingdao University of Science and Technology,Qingdao 266042,Shandong Province,P.R.China)

Novel Synthesis of Mesoporous Nanocrystalline Zirconia

CHE Hong-Wei1HAN Shu-Hua1,*HOU Wan-Guo1,2,*LIU Ai-Feng1

(1Key Laboratory of Colloid and Interface Chemistry,Ministry of Education,Shandong University,Jinan 250100,P.R.China;2College of Chemistry and Molecular Engineering,Qingdao University of Science and Technology,Qingdao 266042,Shandong Province,P.R.China)

A novel strategy involving the combination of soft-templating and the solid-liquid method(CSSL)is presented for the synthesis of mesoporous nanocrystalline zirconia with a high specific surface area.The mesostructured zirconia hybrid is firstly synthesized by the soft-templating method using 1-hexadecyl-3-methylimidazolium bromide(C16mim+Br-)as the structure-directing agent and zirconium sulphate as an inorganic precursor.It is then ground in the presence of solid copper nitrate followed by heat-treatment in air.The resulting zirconia material,after calcination at 600℃,possesses a wormlike arrangement of mesopores surrounded by tetragonal ZrO2nanocrystallites of ca 2.50 nm diameter.The Brunauer-Emmett-Teller(BET)surface area is 240.0 m2·g-1and the pore size is 4.10 nm.However,no mesoporous structure exists in the obtained zirconia material that was produced using the simple soft-templating method at the same calcination temperature.The BET surface area is only 9.5 m2·g-1for this material.

Mesoporous;Zirconia;Nanocrystalline;Soft-templating method;Thermal stability

FollowingthediscoveryoforderedmesoporoussilicabyMobil scientists[1],mesoporous metal oxide materials such as TiO2[2-3],Al2O3[4-6],SnO2[7-8],WO3[9-11],and CeO2[12]have been synthesized successfully using various approaches for their applications in catalysis and energy conversion.The conventional soft-templating method[13-15]is generally used to fabricate mesoporous metal oxides in the presence of surfactants as structure-directing agents.Compared to the silicon alkoxides,the hydrolysis and polymerization of metal alkoxides are more difficult to be controlled.Therefore,the resulting metal oxides usually exhibit low thermal stability and poor crystallinity after the removal of the surfactant templates.On the other hand,the hard-templating pathway is also employed to synthesize mesoporous nanocrystalline metal oxide via the loading of metal salts into the preordered hard mesoporous silica or carbon templates[16-18].Subsequent heat treatment is carried out without the collapse of meso-pores,and nanocrystalline metal oxides with replicated mesopores can be obtained after the removal of hard templates by etching or calcination.However,this synthetic method suffers from multiple and tedious steps and low loading of metal precursors in porous channels.Therefore,the synthesis of mesoporous metal oxide materials possessing highly thermal stability and high crystallinity is still a huge challenge.

Herein,we make further attempt to fabricate mesoporous nanocrystalline zirconia via such a strategy combining the softtemplating method with solid-liquid method(CSSL).Firstly,the mesostructured zirconia hybrid is synthesized via the soft-templating method using 1-hexadecyl-3-methylimidazolium bromide(C16mim+Br-)asthe templating agent and zirconium sulphate as inorganic source.Secondly,the synthesized mesostructured zirconia hybrid(as host)is ground with solid copper nitrate hydrate(as guest)followed by heat-treatment in air.If the temperature is above its melting point(115℃),solid guest will transform into liquid and infiltrate into the pore channels occupied by templates inside the host.Finally,the mesoporous nanocrystalline zirconia calcined at high calcination temperature is obtained after the removal of CuO by etching with HCl solution.

1 Experimental

1.1 Materials

All the chemicals used in the present work are available from Shanghai Sinopharm Chemical Reagent Co.,Ltd.as analyticgrade reagents and used without further purification.Deionized water(18 MΩ·cm)is used for all the preparations.1-hexadecyl-3-methylimidazolium bromide(C16mim+Br-)is synthesized according to Ref.[19].1H-NMR data of the C16mim+Br-(CDCl3,δ: 10.44(s,1H),7.48(s,1H),7.34(s,1H),4.32(t,2H),4.13(s, 3H),1.92(m,2H),1.30(m,26H),and 0.88(t,3H).

1.2 Synthesis of the mesostructured zirconia hybrid material via the soft-templating method

Atypicalprocedurewasasfollows:2.0gofZr(SO4)2·4H2Owas dissolved in 50 g of deionized water at 35℃.Then,40.0 mL of 0.04 mol·L-1C16mim+Br-solution was slowly added to Zr(SO4)2· 4H2O solution(n(Zr(SO4)2·4H2O))/n(C16mim+Br-)=1∶0.3),immediately,a white solid was precipitated.After 1 h stirring,the mixture was aged for 72 h at 80℃.The final white product was recovered by centrifugation,washed with water and alcohol successively,and dried at 100℃ for 24 h.It was denoted as meso-ZrO2.

1.3 Synthesis of mesoporous nanocrystalline zirconia via the solid-liquid method

0.5 g of the sample meso-ZrO2was manually ground with 0.5 g copper nitrate(Cu(NO3)2·3H2O)in an agate mortar for 15 min. Afterwards,the mixture was put into one crucible and then placed in a muffle furnace.The calcination was carried out at two stages:first,the temperature was increased from room temperature to 120℃and kept for 3 h;subsequently,the temperature was further increased to 600℃at a ramping rate of 3℃·min-1in air for 5 h.The sample was then cooled down to room temperature and impregnated in 10.0%(w)HCl solution for 48 h. The final mesoporous zirconia materials were recovered by centrifugation and washed with deionized water for five times and alcohol for three times.According to the above calcination temperature,the as-made zirconia material was denoted as CSSLZrO2-600.In addition,the as-synthesized mesostructured zirconia material(meso-ZrO2)via single soft-templating approach was directly calcined at 600℃in air for 5 h for comparison.Accordingly,it was denoted as S-ZrO2-600.

1.4 Characterization

The small-angle X-ray diffraction(XRD)value of each sample was collected by using a Rigaku D/Max-rB diffractometer (Rigaku International Corp.,Japan)operated at 40 kV and 100 mA.The wide-angle XRD patterns were recorded by a Bruker AXSD8 Advance X-ray diffractometer(Bruker Corp.,Germany) operated at 40 kV and 40 mA.High-resolution transmission electron microscope(HRTEM)images were recorded using a JEM-2100 electron microscope(JEOL Electronics Co.,Ltd.,Japan)operating at 200 kV.Nitrogen adsorption-desorption isotherm was determined at 77 K by a conventional volumetric technique with a Coulter Omnisorp 100CX sorption analyzer(Coulter Electronics Ltd.,America).Mass changes of the products were measured out on a Thermal Analysis SDT Q600 analyzer(TA Instrument Corp.,America)from 25 to 800℃under an air atmosphere at a heating rate of 10℃·min-1.

2 Results and discussion

2.1 Powder XRD

Fig.1a exhibits the small-angle XRD pattern of the obtained zirconia material via the CSSL method after calcined at 600℃. Only one broad diffraction peak at 2θ≈1.35°with d spacing of 6.54 nm is exhibited for the sample CSSL-ZrO2-600,which is typical of wormlike mesopores.This result is further confirmed by the following HRTEM photograph.The corresponding wideangle XRD pattern(Fig.1c)exhibits the typical diffraction peaks ascribed to the tetragonal ZrO2crystal in the 2θ range from 26° to 70°.The crystallite size calculated by the Scherrer equation is 2.50 nm.Contrastingly,no diffraction peaks in the small-angleregion(Fig.1b)are displayed for the sample S-ZrO2-600 via single soft-templating method after calcination at the same temperature.In combination with the following N2sorption characterization,this result suggests the absence of mesopores for the sample S-ZrO2-600.And also,the wide-angle XRD pattern(Fig.1d) exhibits sharper diffraction peaks corresponding to tetragonal ZrO2crystal.The crystallite size is 7.80 nm,obviously higher than that of the obtained zirconia via the CSSL method.Therefore,it is concluded that fast growth of ZrO2nanocrystallites in the pore walls during calcination is regarded as a key reason for the collapse of mesopores in the resulting zirconia matrixes. Compared with the single soft-templating method,the CSSL method is favorable to retard the growing rate of ZrO2nanocrystallites.

2.2 N2adsorption-desorption

Fig.2 illustrates the N2adsorption-desorption isotherms together with the Barret-Joyner-Halenda(BJH)pore size distribution(PSD)plots of the samples CSSL-ZrO2-600 and S-ZrO2-600. Upon calcination to 600℃,type IV isotherm is shown for the sample CSSL-ZrO2-600(Fig.2A,a),similar to those of the mesoporous transition metal oxides synthesized through the hardtemplating method[20-21].This result clearly indicates the mesoporous nature in ZrO2matrix.The BET surface area is 240.0 m2· g-1and the pore volume is 0.26 cm3·g-1.The pore size distribution(PSD)calculated from the desorption branch with the BJH method is centered at 4.10 nm(Fig.2B,c).Whereas,as for the sample S-ZrO2-600(Fig.2A,b),the BET surface area is only 9.5 m2·g-1and the total volume is 0.0060 cm3·g-1,implying the absence of mesopores.Therefore,these results further prove that the CSSL method is more beneficial to possess mesopores than the single soft-templating method after high-temperature calcination.

2.3 HRTEM analysis

Fig.3 demonstrates the HRTEM images of the samples CSSLZrO2-600(A,B)and S-ZrO2-600(C,D).Wormlike pores distributed in the ZrO2matrix are observed for the sample CSSL-ZrO2-600.The clear lattice fringes of nanocrystalline ZrO2are displayed(Fig.3B),which have a lattice spacing of 0.293 nm,corresponding to the(101)planes of tetragonal ZrO2.This result is also confirmed by the corresponding electron diffraction image,indicating that such a synthesized sample presents a mesoporous nanocrystalline ZrO2framework.For the sample S-ZrO2-600,the wormlike mesopores completely disappear,replaced by more disordered porous network of aggregated ZrO2nanoparticles with higher crystallinity and larger sizes.These results are also in accordance with those deduced from the XRD and N2adsorption-desorption analyses.

2.4 Formation mechanism for mesoporous nanocrystalline zirconia

The CSSL method is an efficient method to synthesize mesoporous nanocrystalline zirconia with high surface area and thermal stability.It involves the following three procedures:(1)the formation of mesostructured zirconia hybrid;(2)the introduction of copper nitrate salt(as guest)into the pore channels occluded by templates inside the mesostructured zirconia hybrid(as host) via the solid-liquid method;(3)the removal of copper oxide by etching with HCl solution.

Firstly,in order to know whether the mesostructured zirconiais formed via the soft-templating method,the XRD analysis is carried out.As we can see from Fig.4,three diffraction peaks at 2θ of 2.02°,3.52°,and 4.11°with d-spacing values of 4.37,2.51, and 2.15 nm,respectively,indicate the existence of hexagonal mesostructure in the as-synthesized zirconia hybrid.The corresponding wide-angle XRD pattern indicates that the pore wall is composed of amorphous zirconia before calcination.

Secondly,it is well known that the content(guest)of 1.0%(w) was sufficient to result in the appearance of sharp peaks in the XRD patterns for crystalline metal compounds[22].So,in order to demonstrate that Cu(NO3)2·3H2O salt can infiltrate into the occupied pore channels through solid-liquid method,wide-angle XRD characterization is carried out.As we can see from Fig.5b, no diffraction peaks corresponding to Cu(NO3)2·3H2O crystal phase are detected if 50.0%(w/w)of Cu(NO3)2·3H2O salts are ground with the sample meso-ZrO2followed by a thermal treatment at 120℃for 3 h.In contrast,significant diffraction peaks corresponding to Cu(NO3)2·3H2O crystal phase exist in the wideangle XRD pattern(Fig.5a,marked with“*”)if the same weight fractionofCu(NO3)2·3H2Osaltsaregroundwiththesamplemeso-ZrO2at room temperature.These results indicate that Cu(NO3)2· 3H2O salts transforming into liquid can indeed infiltrate into the pore channels occluded by templates and be well dispersed in the mesostructured zirconia hybrid.

In order to investigate the effect of the infiltration of copper nitrate salts on the mesostructured zirconia during calcination, thermogravimetric differential scanning calorimetry analysis (TG-DSC)is carried out.As for the sample meso-ZrO2,the first exothermic peak centered at 370℃is revealed in the DSC curve (Fig.6A,a),accompanied by a significant mass loss of about 31.8%(w)from 300℃to 450℃(Fig.6B,c).The mass loss is attributed to the decomposition of the templating agent C16mim+Br-. However,the exothermic peak is decreased to 305℃as for the meso-ZrO2/Cu(NO3)2·3H2O mixture(1∶1,w/w)(Fig.6A,b).These results indicate that the introduction of copper nitrate contributes to the decomposition of templates at lower temperature due to the fact that the thermolysis of copper nitrate will provide the oxidative atmosphere inside the pore channels.Therefore,the possible redox reaction on the pore walls will be avoided,which is regarded as one possible reason for the collapse of mesoporous structure.Moreover,it is noted that the thermal effect for the meso-ZrO2/Cu(NO3)2·3H2O mixture is significantly lower than that for the sample meso-ZrO2in the temperature range of 500-700℃ accompanied by the crystallization of pore walls.We postulate that it is attributed to the“pillaring”effect generated by the existence of CuO in pore channels,which retards the growth of nanocrystalline zirconia particles in pore walls.

Finally,the BET surface area for the calcined meso-ZrO2/ Cu(NO3)2·3H2O(1∶1,mass fraction)is 8.5 m2·g-1.However,af-ter etching CuO with 10.0%(w)HCl,the BET surface area of the obtained zirconia is 240.0 m2·g-1,indicating the existence of mesopores.This result has been confirmed with HRTEM photograph.

3 Conclusions

In summary,mesoporous nanocrystalline zirconia material has been successfully synthesized via the CSSL method.The infiltration of inorganic salt into the pore channels inside mesostructured zirconia hybrid via solid-liquid method is believed to play a key role in avoiding the collapse of pore channels during the crystallization of pore walls.The CSSL method is expected to present a new strategy to fabricate the mesoporous nanocrystalline metal oxides.

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合成介孔納米晶體氧化鋯的新方法

車紅衛(wèi)1韓書華1,*侯萬國1,2,*劉愛鳳1

(1山東大學(xué)膠體與界面教育部重點(diǎn)實(shí)驗室,濟(jì)南 250100;2青島科技大學(xué)化學(xué)與分子工程學(xué)院,山東青島 266042)

通過一種新穎的方法,即軟模板-固液技術(shù)(CSSL)合成具有高比表面積的介孔納米晶體氧化鋯.首先,通過軟模板法以1-十六烷基-3-甲基咪唑溴(C16mim+Br-)為結(jié)構(gòu)導(dǎo)向劑,硫酸鋯為無機(jī)前驅(qū)物合成了介觀相氧化鋯雜化物,然后該雜化物與固體硝酸銅無機(jī)鹽研磨并進(jìn)行熱處理.在600℃焙燒后所得到的氧化鋯材料具有蠕蟲狀介孔結(jié)構(gòu),且孔壁由尺寸約為2.50 nm的四方相氧化鋯納米粒子組成.該材料的比表面積為240.0 m2·g-1,孔徑為4.10 nm.與之對應(yīng),使用單一的軟模板法在相同的溫度焙燒后,所得到的氧化鋯材料介孔結(jié)構(gòu)坍塌,比表面積僅為9.5 m2·g-1.

介孔;氧化鋯;納米晶體;軟模板法;熱穩(wěn)定性

O648

Received:March 26,2010;Revised:May 5,2010;Published on Web:May 25,2010.

*Corresponding authors.Email:shuhhan@sdu.edu.cn,wghou@sdu.edu.cn;Tel:+86-531-88564750,+86-531-88365450.The project was supported by the National Natural Science Foundation of China(50572057).

國家自然科學(xué)基金(50572057)資助項目

?Editorial office of Acta Physico-Chimica Sinica