Synthesis of texture-excellent mesoporous alumina using PEG1000 as structure-directing agent☆

Kui Zhang,Changming Li,Jian Yu,Shiqiu Gao,Guangwen Xu,*

1State Key Laboratory of Multiphase Complex Systems,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2University of Chinese Academy of Sciences,Beijing 100049,China

1.Introduction

Mesoporous alumina(MA)with suitable structure,large specific surface area and high pore volume has potential applications for being catalyst supports[1].Large surface area and high pore volume enable high loading of active catalytic matters or species[2].Several methods have been used to successfully synthesize MA,including precipitation,use of template,self-assembly and so on[3–14].Recently synthesis using non-ionic surfactants represents a facile route to prepare mesoporousalumina, and the prepared MA samples have always tunablestructure and excellent textural properties.

Nieszet al.[4]have reported one route to prepare orderedMA withhigh surface area and narrow pore size distribution by using P123 as the structure-directing agent.The surface area and pore volume of their made samples were 410 m2·g-1and 0.8 cm3·g-1,respectively.Different from this,lots of MA samples prepared by surfactants do not have long-range order structure and are often amorphous[5].Xuet al.[6,7]obtained amorphous MA with high specific area,narrow pore size distribution and excellent thermal stability through the sol–gel method using a selected group of saccharide molecules including glucose,sucrose,starch and β-cyclodextrin as surfactants.

Nonetheless,the textural property and thermal property of synthesized MA still need great improvement for its application,especially being used as catalyst supports.In the literature there are a few reports[15–17]investigating the role of polyethylene glycol with high molecular weight in synthesis of MA,but the polyethylene glycol with low molecular weight as structure-directing agent will work better for itshigher solubility. In this study, PEG1000 (polyethylene glycol,molecularweight=1000) is selected as the structure-directing agent, and theMAobtained is expected to have excellent textural properties and thermalstabilities. This article synthesized MA using aluminum isopropoxideas precursor to minimize the effect of alumina precursor on synthesis.By varying the concentration of PEG1000 this article further studiedthe effect of polyethylene glycol on synthesis of MA.

2.Experimental

Aluminum isopropoxide (Sinopharm Chemical Reagent Co., Ltd.)was used as aluminum precursor, and distilled water and absoluteethyl alcohol (Sinopharm Chemical Reagent Co., Ltd.) were used assolvents. PEG1000 (polyethylene glycol, molecular weight = 1000)(Sinopharm Chemical Reagent Co., Ltd.) was a nonionic template. Forsynthesis of MA, a solution A was prepared by dissolving a certainamount of aluminum isopropoxide into 40 ml absolute ethyl alcoholand further stirring the formed solution for 60 min at 50 °C. A solutionB was prepared by mixing a specified quantity of PEG1000 into 90 mlabsolute ethyl alcohol and 9 ml distilled water and further stirring for30min at 50 °C. Themass ratios between PEG1000 in solution B and aluminumisopropoxide in solution A were 0.0, 0.3, 0.9, 1.8, 3.6 and 7.2 inthis study. For each test, the solution A was doped into solution Bmade with different quantities of PEG1000, and then the mixture was stirred for 60 min at 50°C.The obtained mixture solution was dried for 48 h at 120°C,and the obtained gel was calcined at temperatures of 600 °C,750 °C and 850 °C in air under a heating rate of 1 °C·min-1and further held at each temperature for 3 h.The obtained MA samples were labeled as Al-Px-T,where Al,P,x and T refer to alumina,PEG1000,weight ratio of PEG1000 and aluminum isopropoxide and calcination temperature,respectively.

Powder X-ray diffraction(XRD)patterns for all MA samples were obtained on a Panalytical X'Pert Pro MPD X-ray diffractometer with Cu-Kα radiation(λ =0.15406 nm)of 40 kV and 40 mA.Wide-angle diffractions(2θ =5°–90°)and low-angle diffractions(2θ =0.5°–5°)were re cord edat as canning speed of10°·min-1and1°·min-1,respectively.Scanning electron microscopy(SEM)micrographs were taken on HitachiSU8020s canning electron microscope at an accelerationvoltage of 15 kV.Gold was vapor-deposited on the measured samples before the analysis.Transmission electron microscopy(TEM)images were recorded on a JEOL JEM 2100(UHR)electron microscope operated at 200 kV.N2adsorption–desorption isotherms were taken using a Quantachrome Autosorb-iQ apparatus working at-196°C.In prior to measurement the sample was degassed at 240°C in vacuum for 10 h.The specific surface area(SBET)of a sample was calculated according to the Multi-Point BET method based on the adsorption data inP/P0of 0.05–0.30.The pore size distribution(PSD)was derived from the adsorption branch of an isotherm following the BJH method.The total pore volume(Vp)was calculated from the amount of N2adsorbed on a sample at a relative pressure of about 0.99.The pore width(Dp)was determined at the maximum peak position of the PSD curve.

3.Results and Discussion

3.1.Synthesis of petal-like MA

Fig.2.TEM images of MA samples.(a)Al-P0.0-600,(b)Al-P0.3-600.

Figs.1 and 2 give us the SEM and TEM images of MA samples prepared.For Al-P0.0-600(see Experimental for indication),most crystallites are microspheres as shown in SEM image(Fig.1a)corresponding to the TEM image(Fig.2a).While the SEM images indicate that lots of petal-like crystallites are formed for Al-P0.3-600,Al-P0.9-600,Al-P1.8-600,Al-P3.6-600 and Al-P7.2-600.And there are smaller microspheres locating on the edge of petal-like crystallites in comparison with Al-P0.0-600.As the TEM image(Fig.2b)shown,the nanosheets clearly appear in the sample of Al-P0.3-600.Consequently,PEG1000 might be a structure-directing agent and its addition induced the formation of petal-like structure.During gelation,the surface of boehmite layer is surrounded and also adsorbed by PEG1000 molecules through hydrogen bond,and boehmite layers may greatly grow to form the petal-like structure under the action of PEG1000 molecules.Increasing temperature during calcination facilitated the removal of interlayer hydroxyls into H2O and the formation of petal-like crystallite[14].In addition,the petal-like crystallites existing on the samples Al-P0.3-600,Al-P0.9-600,Al-P1.8-600.Al-P3.6-600 and Al-P7.2-600 may cause their large specific surface area in comparison with the Al-P0.0-600 sample.

3.2.Effect of PEG1000 concentration

Fig.3 shows the low-and wide-angle XRD patterns for MA samples Al-P0.0-600,Al-P0.3-600,Al-P0.9-600,Al-P1.8-600,Al-P3.6-600 and Al-P7.2-600.There was no any XRD peak in low-angle region for Al-P0.0-600,Al-P0.3-600,Al-P0.9-600,Al-P1.8-600 and Al-P3.6-600.Even for Al-P7.2-600,only one peak was observed between 1°and 2°of 2θ and no other peak at 3°–10°,indicating either the absence of long-range ordered structure or the presence of small-sized crystals[18].Thus,all the obtained MA samples did not show the long-range ordered mesoporous structure.From wide-angle XRD patterns one can see that all MA samples were amorphous and their peaks appeared around the peaks characterizing γ-alumina.

Fig.3.Low-and wide-angle XRD patterns for MA samples.(a)Al-P0.0-600,(b)Al-P0.3-600,(c)Al-P0.9-600,(d)Al-P1.8-600,(e)Al-P3.6-600,(f)Al-P7.2-600.

Table 1 summarizes the textural properties of MA samples obtained from different weight ratios of PEG1000 to aluminum is opropoxide at 600°C.The Al-P0-600 sample without the addition of PEG1000 had a specific surface area of 337 m2·g-1and a pore volume of 1.2 ml·g-1,whereas the other samples with the addition of certain PEG1000 exhibited the larger specific surface areas and higher pore volumes than Al-P0.0-600 did.Especially,Al-P0.9-600 had a big specific surface area of464m2·g-1and ahigh porevolume of1.7ml·g-1.This indicates that adding PEG1000 into the solution can improve thetextural properties of MA samples.In addition,the specific surface area and porevolume increased with raising the mass ratio of PEG1000 to aluminumisopropoxide up to 0.9, and then they remained to change little at ratiosof 0.9 to 3.6.However, a significant drop appeared in such parameters asthe mass ratio of PEG1000 to aluminum isopropoxide exceeded 3.6.Adding PEG100 only slightly varied the pore width of MA samples in arange of 6.6 to 7.8 nm, except for Al-P0.3-600 that had a pore width of9.6 nm.

Table 1Textural properties of MA samples with different weight ratios of PEG1000 to aluminum isopropoxide obtained at 600°C

Fig.4 summarizes N2adsorption–desorption isotherms and PSD curves for MA samples Al-P0.0-600,Al-P0.3-600,Al-P0.9-600,Al-P1.8-600,Al-P3.6-600 and Al-P7.2-600.All displayed the classical IVa type isotherms[19]with H3 hysteresis loops.Compared to Al-P0.0-600,the other samples with the addition of PEG1000 during precipitation exhibited the narrower PSD.Especially,the Al-P0.3-600 sample had the narrowest PSD and its pore width was largest among all the samples.Increasing further the concentration of added PEG1000 in the solution caused the MA samples to have small pore width and wide PSD.Thus,appropriate concentration of PEG1000 is beneficial to the formation of mesoporous structure and uniform pore size.

Fig.4.N2adsorption–desorption isotherms and PSD curves for MA samples.(a)Al-P0.0-600,(b)Al-P0.3-600,(c)Al-P0.9-600,(d)Al-P1.8-600,(e)Al-P3.6-600,(f)Al-P7.2-600.

As solvent evaporates in drying, it brings about the formation ofliquid-like “organic zones” inside the alumina structure,which presumablyare the origin of pores [20]. According to this idea Fig. 5 shows thepotential effect and mechanism of added PEG1000 on synthesis of MA.When the concentration of PEG1000 is low, as a structure-directingagent PEG1000 uniformly disperses into the solvent in Step 1. Afterhydrolysis of aluminum isopropoxide, PEG1000 molecules can beadsorbed on the surface of the formed boehmite layers via hydrogenbonds (Step 2). The MA samples like Al-P0.3-600 exhibited thenarrower PSD because of the more uniform size pores formed byremoving PEG1000 than the MA sample without PEG1000 did. Furtherraising the concentration of PEG1000, a part of PEG1000 moleculestransfer to micelles by Step I, and only the left PEG1000 molecules acton Steps 1 to 3. The PEG1000micelles distribute among boehmite layersin Step II, and pores are formed by burning off PEG1000 micelles in StepIII. The formed pores thus caused thewide PSD curves shown in Fig. 4 forAl-P0.9-600, Al-P1.8-600, Al-P3.6-600 and Al-P7.2-600.

Fig.5.Proposed potential effect and mechanism of PEG1000 on the synthesis of MA.

3.3.High thermal stability of MA

Fig.6.Wide-angle XRD patterns for MA samples.(i)Al-P0.3-600,(ii)Al-P0.3-750 and(iii)Al-P0.3-850.

From the wide-angle XRD patterns of MA samples Al-P0.3-600,Al-P0.3-750 and Al-P0.3-850 shown in Fig. 6, one can see that thesethree MA samples were amorphous but the crystallinity of γ-aluminain theseMA samples increasedwith raising the calcination temperatureso that there were narrower XRD peaks at higher calcination temperature so that there were narrower XRD peaks at higher calcination temperature.The cause may be that increasing calcination temperature facilitated the growth of crystals.The same result was obtained also for the other MA samples with different mass ratios of PEG1000 to aluminum isopropoxide.Raising calcination temperature also varied the textural properties of MA samples.Table 2 summarizes the textural properties of MA samples with different mass ratios of PEG1000 to aluminum isopropoxide calcined at 750 °C and 850 °C.All exhibited good textural properties to have high specific area(＞210 m2·g-1)and largeporevolume(＞1.1cm3·g-1).The influence of PEG1000ontextural properties of MA became weak at high calcination temperatures of 750 °C and 850 °C.Compared to samples calcined at 600 °C(Table 1),the higher calcination temperature decreased specific surface area but increased pore width of MA samples.The crystals growing with calcination temperature negatively impacts on the specific surface area and accumulative pores also become big due to the formation of large crystals.Nonetheless,this negative impact due to calcination temperature on textural properties of MA is weaker for the sample with the narrower PSD.For example,Al-P0.3-850 displayed the higher specific surface area of 246 m2·g-1than the other MA samples(＜220 m2·g-1)made at the same calcination temperature.

Table 2Textural properties of MA samples with different mass ratios of PEG1000 to aluminum isopropoxide made by calcination at 750 °C and 850 °C

4.Conclusions

The study successfully synthesized MA samples with excellent textural properties by using PEG1000 as structure-directing agent.The following are main conclusions.

(1)Compared to the case without PEG1000 addition,lots of petallike crystallites were presented for the MA samples using PEG1000 as the structure-directing agent.These petal-like crystallites,compared to microspheres existing in the MA sample without PEG1000,contributed greatly to the specific surface area of the prepared MA.

(2)Adding PEG1000 did not change the crystalline state,and all MA samples were amorphous and their XRD peaks were around the peaks of γ-alumina.The MA samples with added PEG1000 also exhibited the larger specific surface areas and higher pore volumes,in comparison with the MA samples without PEG1000 addition.Especially,Al-P0.9-600 had the biggest specific surface area of 464 m2·g-1and the larger pore volume of 1.7 ml·g-1.Varying concentration of PEG1000 affected nitrogen adsorption–desorption isotherms and PSD curves for all prepared MA samples.We also proposed the potential effect and relatedmechanismof PEG1000 added in the precipitation of MA.

(3)At calcination temperatures of 750 °C and 850 °C,the prepared MA samples manifested high thermal stability to have large specific area(＞210 m2·g-1)and high pore volume(＞1.1 cm3·g-1).The influence of added PEG1000on textural properties of MA became weak at high calcination temperature,and this influence is also weaker for MA samples with narrower PSD.

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