One-step synthesis of three-dimensionally orderedmacro–mesoporous silica–alumina composites

Long Kou a, Youhe Wang a,b,n, Yan Dong a, Dezhi Han c, Bin Ni a, Zifeng Yan a,nn

a State Key Laboratory for Heavy Oil Processing, Key Laboratory of Catalysis, CNPC, China University of Petroleum, Qingdao 266555, Chinab School of Science, China University of Petroleum, Qingdao 266555, Chinac Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China

a r t i c l e i n f o

Article history:Received 19 August 2013Accepted 25 January 2014Available online 2 February 2014

Keywords:Amorphous materialsPorous materialsAcid solutionSilica–alumina composites

a b s t r a c t

In this work, three-dimensional (3D) ordered macro–mesoporous silica–alumina composites weresynthesized for the first time in an acid solution, using an effective dual-templating approach. Thematerial with a high surface (i.e., 509 m2 g�1) possesses spherical macropores with an ordered mesoporestructure wall. The strong diffraction peak and the pore size distribution curves reveal the presence ofrelatively uniform mesopores (3.5 nm pore width). The synthesized aluminosilicate materials withtailorable macro–mesoporous structure hold promise in select applications as catalysts, catalystsupports, or adsorbents. This synthesis method also has the potential to make other macro–mesoporouscomposites with different compositions.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Studies have been done to prepare ordered macro–mesoporoussilica monoliths due to their potential applications in catalysis, gasseparation and producing electrode materials [1–3]. The monolithsexhibit continuous macropore and interconnected mesopore dis-tributions in the skeleton walls. Compared with single-sized porematerials, the hierarchical materials combine the advantages ofboth macroporous and mesoporous structures, which will favormass transfer and reduce transport limitations [4]. Meanwhile,the well-ordered mesopores are ideally suitable for stabilizingnanoparticles in high dispersion and supplying abundant activesites [5].

However, in terms of catalytic reactions, silica monoliths showpoor chemical activity and could only be used as catalytic supports[5]. In order to solve this problem, aluminum is incorporated intothe porous silica framework to form active acidic sites, which willgreatly broaden the field of their catalytic applications in petro-chemical industry [6]. Extensive efforts have occurred withthe post-synthesis method to produce these monoliths [7], which

always lead to the extra-framework species and irregularly distrib-uted active sites [8]. In the direct synthesis method, it is observedthat the tetrahedral Al content in the framework is sensitive to thepH value of the reaction [6,9]. At high pH values, the molecularSi�O�Al link forms easily and is hard to cleave. As a result, a largenumber of aluminum content will be introduced, which may resultin the decrease of the mesoporous order degree in return [10]. Untilnow, research has demonstrated successful syntheses of macro–mesoporous silica–alumina composites in an alkaline environment[11–13]. On the other hand, in the acid solution, aluminum, whichexists in the form of Al3þ , is rarely incorporated into the frameworkto form very ordered macro–mesoporous materials [14]. In fact, thesynthesis of porous aluminosilicate materials with a homogeneousdistribution of the Si–O–Al linkages in the final framework at lowpH values still remains a great challenge. However, using triblockcopolymers as template, acidic condition is necessary for the synth-esis of ordered mesoporous. Under acidic conditions, the PPO block isexpected to display more hydrophobicity than the PEO block, therebyincreasing the tendency for mesoscopic ordering to occur [15]. Theaddition of P123 can lead to the wider modulation range and thickerwall of mesopores than the other template [11–13].

In this paper, we report, for the first time, the synthesis ofhierarchically ordered macro–mesoporous silica–alumina compo-sites in an acidic solution using custom laboratory-produced poly(methyl methacrylate) (PMMA) latex spheres and amphiphilictriblock copolymers (Pluronic P123) as hard template and softtemplate, respectively. The as-synthesized aluminosilica monolith

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.01.1440167-577X & 2014 Elsevier B.V. All rights reserved.

n Corresponding author at: State Key Laboratory for Heavy Oil Processing,Key Laboratory of Catalysis, CNPC, China University of Petroleum, Qingdao266555, China. Tel.: þ86 532 86981856.

nn Corresponding author. Tel.: þ86 532 86981296; fax: þ86 532 86981295.E-mail addresses: yhewang0168@Gmail.com (Y. Wang),

zfyancat@upc.edu.cn (Z. Yan).

Materials Letters 121 (2014) 212–214

is prepared in one step, and anhydrous aluminum chloride (AlCl3)is used as aluminum precursor.

2. Experimental methods

Chemicals: Ethanol (analytical reagent, AR), concentrated HCl(38 wt%, AR), potassium persulfate (AR), methyl methacrylate(MMA, AR), anhydrous aluminum chloride (AR) and tetraethylorthosilicate (TEOS, AR) were purchased from Sinopharm Chemi-cal Reagent Co. Amphiphilic triblock copolymer Pluronic P123(PEO20PPO70PEO20, AR) was purchased from Sigma-Aldrich Co.

Synthesis: Highly ordered PMMA colloidal crystal template wassynthesized using a modified emulsifier-free emulsion polymeriza-tion technique [16].The typical preparation procedure of hierarchi-cally ordered macro–mesoporous silica–alumina composites isdescribed as follows: 6.0 g of Pluronic P123 was dissolved in amixture of 50 g of ethanol, 5.2 g of distilled water and 0.6 g of 0.2 MHCl with stirring for 4 h at 40 1C in an airtight case, which wascalled solution A. At the same time, 0.32 g of anhydrous AlCl3 wasadded to 40 g of ethanol, which was called solution B. When AlCl3was completely dissolved, solution A was mixed with B. Then12.48 g of TEOS (SiO2/Al2O3¼50) was added dropwise into themixed precursor solution with stirring for 20 h at 40 1C. Finally,6.6 g of PMMA was added to the precursor solution and stored for10 h without stirring. Excess solution was removed from theimpregnated PMMA colloidal crystals by drying for 24 h at 50 1C.The dried sample was calcined in a muffle furnace at 550 1C for 6 h.

Characterizations: N2 sorption experiments were performedat 77 K (TRISTAR 3000 system) in static measurement mode. Thesamples were outgassed at 300 1C for 5 h before the measurement.Specific surface area of the samples was calculated using the BETmethod and the pore size distributions were obtained from theanalysis of the desorption branch of the isotherm by the Barrett–Joyner–Halenda (BJH) method. X-ray diffraction (XRD, Bruker Axsdiffractometer) measurements were completed at 0.011 s�1 with

CuKα radiation generated at 40 kV and 30 mA. Scanning electronmicroscopy (SEM) and energy dispersive X-ray spectroscopy (EDX)was completed using an S-4800 high resolution analytical fieldemission scanning electron microscope with an operating voltageof 5.0 kV. Transmission electron microscopy (TEM, JEOL JEM 2100)measurements were conducted with an acceleration voltage of200 kV.

3. Results and discussion

Fig. 1(a) is a representative SEM picture of the macro–meso-porous silica–alumina composite, in which the macropores arespherical with a diameter range of 90–110 nm. The macroporesgenerated by the removal of the hard template (PMMA spheres)during calcination show very good connectivity. A typical TEMimage of macro–mesoporous silica–alumina composite is providedin Fig. 1(b). Large orderly macropores are clearly observed,which is consistent with the SEM picture. At higher magnification(Fig. 1(b) inset), it can be seen that the walls of the macropores arefull of ordered mesopores (the mesopores size is 3.5 nm), due tothe initial addition and then removal of the Pluronic P123.

Evidence for the formation of mesostructures is provided bysmall-angle XRD patterns shown in Fig. 2(a). From the spectro-gram, a very strong diffraction peak at 11 can be observed, whichindicates that the sample possesses ordered mesopores in the porearrangement. It can be noticed that no distinct peak is observableon XRD spectrum (Fig. 2(b)), which indicates that aluminum andsilicon are amorphous.

The N2 adsorption–desorption isotherms and pore size distri-bution of the sample are shown in Fig. 3. The isotherms are type IVwith H2-shaped hysteresis loops, which is the characteristic of amesoporous material. The sharp capillary condensation stepsoccur at relative pressure of 0.4–0.6, indicating uniform mesopores[17]. The sample has a large BET surface area of 509 m2 g�1. Thepore size distribution derived from the adsorption branches using

Fig. 1. (a) SEM and (b) TEM images of macro–mesoporous silica–alumina composite.

1 2 3 4 50

20000

40000

60000

80000

Intensity

2 theta(degree)

0 10 20 30 40 50 600

500

1000

1500

2000

2500

Intensity

2 theta(degree)

Fig. 2. Small-angle (a) and wide-angle (b) XRD pattern of the macro–mesoporous silica–alumina composite.

L. Kou et al. / Materials Letters 121 (2014) 212–214 213

the BJH model further reveals the presence of relatively uniformmesopores (Fig. 3 inset). The mesopores size is 3.5 nm, which isentirely correspondent with the results of TEM.

The EDX pattern (Table 1) reveals the presence of Si, Al and Oelements in the sample, where C is the interference, which isgenerated due to the carbonization of the conducting resin underhigh voltage. The molar ratio of Si/Al¼23.14 from this EDX analysisis similar to the theoretic molar ratio of Si/Al¼25 calculated by thedosage of TEOS and AlCl3.

4. Summary and conclusions

We demonstrate, for the first time, a one-step synthesis ofhierarchically ordered macro-/mesoporous silica–alumina composite

with ordered mesoporous walls using an acid solution via a dual-templating approach. This technique combines colloidal crystalPMMA and amphiphilic triblock copolymer Pluronic P123. Calcina-tion of the sample resulted in (1) an amorphous phase, (2) highsurface area of 509 m2 g�1, (3) well connective macropores, and(4) ordered mesopores with a narrow and uniform pore width of3.5 nm. The resulting tailored meso-/macroporous material can beapplied to areas such as catalysis, catalyst supports, or adsorbents.This synthesis method also has the potential to be extended to makeother macro–mesoporous composites with different compositions.

Acknowledgments

This work was financially supported by the Petrochemical JointFunds of NSFC-CNPC (No. U1362202); Shandong Province NaturalScience Foundation of China (Grant no. ZR2011BQ014) and theFundamental Research Funds for the Central Universities (Grantno. 12CX04093A).

References

[1] Guo AR, Liu JC, Dong X, Liu MX. Mater Lett 2013;95:74–7.[2] Stein A. Adv Mater 2003;15:763–75.[3] Yuan ZY, Su BL. J Mater Chem 2006;16:663–77.[4] Yang XY, Léonard A, Lemaire A, Tian G, Su BL. Chem Commun

2011;47:2763–86.[5] Taguchi A, Schuth F. Microporous Mesoporous Mater 2005;77:1–45.[6] Corma A. Chem Rev 1995;95:559–614.[7] Morey MS, O’Brien S, Schwarz S, Stucky GD. Chem Mater 2000;12:898–911.[8] Han Y, Meng XJ, Guan HB, Yu Y, Zhao L, Xu XZ, et al. Microporous Mesoporous

Mater 2003;57:191–8.[9] Lemaire A, Su BL. Langmuir 2010;26:17603–16.[10] Li Y, Zhang WH, Zhang L, Yang QH, Wei ZB, Feng ZC, et al. J Phys Chem B

2004;108:9739–44.[11] Gundiah GB. Mater Sci 2001;24:211–4.[12] Lemaire A, Su BL. Microporous Mesoporous Mater 2011;142:70–81.[13] Lemairea A, Wang QY, Wei YX, Liu ZM, Su BL. J Colloid Interface Sci

2011;363:511–20.[14] Yang HQ, Liu Q, Liu ZC, Gao HX, Xie ZK. Microporous Mesoporous Mater

2010;127:213–8.[15] Zhao DY, Feng JL, Huo QS, Melosh N, Fredrickson GH, Chmelka BF, et al.

Science 1998;279:548–52.[16] Han DZ, Li X, Zhang L, Wang YH, Yan ZF, Liu SM. Microporous Mesoporous

Mater 2012;158:1–6.[17] Sun ZK, Deng YH, Wei J, Gu D, Tu B, Zhao DY. Chem Mater 2011;23:2176–84.

0.0 0.2 0.4 0.6 0.8 1.0

100

150

200

250

300Q

uant

ity A

dsor

bed(

cm3 /g

ST

P)

Relative Pressure(P/Po)

Fig. 3. N2 adsorption isotherms and BJH pore size distributions (inset) of macro–mesoporous silica–alumina composite.

Table 1EDX line scan pattern of the macro–mesoporous silica–alumina composite.

Element Wt% At%

C 2.29 3.65O 57.75 69.08Al 1.59 1.13Si 38.37 26.15

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