Chemical Engineering Journal 223 (2013) 670–677

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Chemical Engineering Journal

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Tailored ordered porous alumina with well-defined and uniformpore-structure

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.01.024

⇑ Corresponding author. Tel.: +86 10 89733396.E-mail address: slchen@cup.edu.cn (S.-L. Chen).

Zheng Zhou, Sheng-Li Chen ⇑, Derun Hua, Zhi-Gang Wang, Ai-Cheng Chen, Wen-Hao WangState Key Laboratory of Heavy Oil Processing and Department of Chemical Engineering, China University of Petroleum, Changping, Beijing 102249, PR China

h i g h l i g h t s

" Welldefined and uniform pore-structure (WDUPS) Al2O3 was prepared." The pore-size of the prepared WDUPS Al2O3 can be tailored." The acid properties of the WDUPS Al2O3 is the same to that of conventional c-Al2O3." The WDUPS Al2O3 is an ideal material for fundamental research in catalysis.

a r t i c l e i n f o

Article history:Received 16 September 2012Received in revised form 28 December 2012Accepted 7 January 2013Available online 16 January 2013

Keywords:Uniform pore-sizeOpal structureAlumina coatingTailored pore-sizeMonodisperse silica spheres

a b s t r a c t

Well-defined and uniform pore-structure (WDUPS) Al2O3 catalyst supports were prepared by coatingspecially treated SiO2 opals with Al2O3. The pore-size of the WDUPS Al2O3 can be tailored in the rangeof meso- to macro-size by using different-sized microspheres to fabricate the SiO2 opal. The optimalamount of Al2O3 coating, which keeps the pore structure of SiO2 template intact, was determined to beof the 1–2 atomic layers. The surface acid amount of WDUPS Al2O3 significantly increased with theAl2O3 coating until one atomic layer formed. When the SiO2 opals were coated with optimal amountof Al2O3, the obtained WDUPS Al2O3 showed a uniform acid density. With these characteristics, theWDUPS Al2O3 support is ideal material for fundamental research in catalysis. The method is promisingstrategy to obtain other catalytic materials with the same structure.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Al2O3 is the most widely used catalyst support. However, thepores of conventional Al2O3 are disordered and broadly distributed,which is unsuitable for pore-size-related studies such as internaldiffusion. Furthermore, the irregularity of internal surface of theconventional Al2O3 makes it difficult to observe the stack of activespecies (e.g. CoMoS for hydrodesulfurization catalyst) on the sur-face by electronic microscopy. Therefore, well-defined and uniformpore-structure (WDUPS) Al2O3 with regular internal surface isneeded. Moreover, to study the diffusion phenomenon of largemolecules, the WDUPS Al2O3 with larger and tunable pore size isindispensable [1].

To date, many efforts were made to prepare uniform pore sizedAl2O3. The Al2O3 synthesized by using either soft-templates [2–6]or hard-templates (nanocasting) [7] were said to have narrow poresize distribution. Actually, their pore size exhibits a hierarchical

distribution due to the pores from dehydration in Al2O3 crystalplanes [8]. These Al2O3 only have a relatively high ratio of meso-pore volume to total pore volume [9], and their pores are still ran-dom and not welldefined at all.

Coating Al2O3 on the surface of ordered mesoporous materials(most of them are silica) is another way to prepare uniform poresize Al2O3 [10–16]. This method is also used to prepare orderedmesoporous ZrO2 by coating ZrO2 on SBA-15 [17]. The pore struc-ture of the post-synthesized Al2O3 is dependent on its template.Unfortunately, it is also difficult to prepare the template with uni-form and well-defined pore structures.

Opal is a porous material with ordered face-centered cubicpacked microspheres array. The ordered compact with a high coor-dination number has very uniform pore size. Theoretically, its porediameter, specific surface area (SSA) and pore volume can be calcu-lated [18,19]. The pore diameter of an opal is expressed as thediameter of inscribed circle in the throat, being 0.155D (D is thediameter of microspheres), and the diameter of void inscribed ballis 0.225D. The specific surface area of opal is 6/(qD) (q is the den-sity of microspheres). The opal void fraction of 0.26 is independent

Z. Zhou et al. / Chemical Engineering Journal 223 (2013) 670–677 671

of the size of the microspheres composing the opal [20]. Thus, opalhas a welldefined pore structure, which can be tuned by changingthe diameter of microspheres.

In this paper, WDUPS Al2O3 catalyst supports were obtainedthrough coating the pretreated SiO2 opals with an appropriatethickness of Al2O3. The prepared WDUPS Al2O3 catalyst supportshave well-defined and uniform pore structures and regular internalsurface, and their surface acidity is comparable to that of c-Al2O3.By this method, many other catalyst materials with the same struc-ture, such as TiO2 and SnO2, can be obtained. They are ideal mate-rials for fundamental research in many heterogeneous catalysisand pore-size related study such as the internal diffusion ofreactants.

2. Materials and methods

2.1. Synthesis of SiO2 opal

The monodisperse SiO2 microspheres were synthesized throughhydrolysis and condensation of tetraethyl orthosilicate (TEOS) inalcohol and in the presence of water and ammonia by the seed par-ticle growth method. Detailed description of the synthesis proce-dures were reported in our previous papers [21,22]. In a typicalSiO2 opal assembly process, the suspension of SiO2 microsphereswas put into a glass beaker. The beaker was then placed into anoven and kept at 50 �C with a relative humidity of 90%. After sev-eral days, the SiO2 microspheres were automatically assembledinto opal when the suspension was dried. The as-prepared SiO2

opal was further dried at 100 �C for 24 h, and calcined in mufflefurnace at 800 �C for 2 h. Finally, the opal was immerged in water,put into an autoclave and then hydrothermally treated at 220 �Cfor 10 h to recover the surface silanol groups which were lost dur-ing the calcination.

2.2. Coating the internal surface of SiO2 opal with Al2O3

Al2O3 was coated onto the internal surface of SiO2 opal by theVIH (ammonia/water vapor induced internal hydrolysis) method[16]. A specific amount of precursor Al(NO3)3 9H2O was dissolvedin bi-distilled water with volume same as the pore volume of opal.The SiO2 opal was impregnated by using the Al(NO3)3 solution bythe incipient-wetness impregnation method and then stood overnight. Then Al(NO3)3 was deposited onto the inner surface of theSiO2 opal after drying the sample at 100 �C for 12 h in an air oven.The amount of Al(NO3)3 9H2O was calculated according to theatomic layer capacity of Al2O3 on SiO2 surface, [23] by the follow-ing equations:

mA1 ¼ 0:046�mSi � SSA=100 ð1Þ

mAINO ¼ 373:15�mA1=101:96 ð2Þ

Table 1The prepared samples and their preparation methods.

Sample Diameters of SiO2

microspheres composing theopal templates (nm)

CalcinationtemperatureSiO2 templat

SI53 53 700SI100 100 800SI223 223 800SI300 300 800OPA53-x 53 700OPA100-x 100 800OPA223-x 223 800OPA300-x 300 800

where mAl is the atomic mono-layer capacity of Al2O3 coating (g);mAlNO is the required quantity of precursor Al(NO3)3 9H2O (g); mSi

is the quantity of SiO2 opal (g); SSA is the specific surface area ofSiO2 opal (m2/g).

About 1.0 g dried sample was transferred into a 20 mL openglass vial. The vial was placed in a 100 mL autoclave (with a Tef-lon™ lining) filled with 10 mL 12.5 wt.% NH3–H2O solution, makingthe vial keep clear of the NH3–H2O solution. Then the sample wassealed tightly in the autoclave, and kept at 100 �C for 7 h. TheAl(NO3)3 on the opal surface was hydrolyzed to Al2O3 xH2O inammonia/water vapor during the hydrolysis treatment. Finally,the WDUPS Al2O3 was obtained after the sample was dried at100 �C for 6 h and calcined at 500 �C for 5 h. WDUPS Al2O3 andtheir SiO2 template were respectively labeled as ‘‘OPA’’ and ‘‘SI’’ fol-lowed by the diameter of microspheres composing the SiO2 opals.All the samples and their preparation conditions are listed inTable 1.

2.3. Control alumina

Aluminum nitrate (3.0 g) was dissolved in bi-distilled water.Then the solution was kept in an air oven at 100 �C until dryness.The dried sample was hydrolyzed in the ammonia–water vaporat 100 �C for 7 h, and then dried and calcined at the same conditionas WDUPS Al2O3 preparation mentioned above. The control alu-mina was denoted as ‘‘control-alumina’’.

2.4. Characterization

The morphology of samples was observed on a FEI quanta 200FSEM (FEI, Oregon, USA) using 20 kV accelerating voltage. Porestructure analysis of samples was conducted with a MicromeriticsASAP 2010 automatic N2 adsorption analyzer (Micromeritics, Nor-cross, GA) after the samples were degassed at 250 �C for 3 h. Dataof mercury intrusion porosimetry were obtained with an Auto-Pore IV 9500 mercury porosimeter (Micromeritics, Norcross, GA)using a contact angle of 140� and a mercury surface tension of0.473 N/m. The sample was dried at 120 �C for 4 h before test.For comparison, the pore volume of the samples was also deter-mined by the method of incipient-wetness impregnation of water.

27Al Nuclear Magnetic Resonance (NMR) experiments were per-formed on a 500 MHz Bruker Advance III NMR spectrometer, usinga 4 mm WVT double-resonance Bruker probe. The 27Al Larmor fre-quency of this spectrometer is 130.33 MHz. The acid types of sam-ples surface were identified by the Fourier transform infraredspectroscopy of pyridine adsorption (Py-FTIR). In the Py-FTIRstudy, 12 mg of sample were mixed with 38 mg KBr and pressedinto self-supported wafer. In situ pyridine adsorption over the wa-fer was carried out in a FTIR cell using a conventional glass adsorp-tion setup and performed on Nicolet 6700 spectrophotometer(Nicolet, Madison, USA). The sample wafer was vacuum-degassed

ofes (�C)

Hydrothermaltreatment of SiO2

templates

Al2O3 loading(number of theatomic layer)

VIHtreatment

No – NoYes – NoYes – NoYes – NoNo x YesYes x YesYes x YesYes x Yes

Table 2Effect of calcination and hydrothermal treatment on pore structure of opals.

SiO2 opal Pore diameter (nm) Pore volume of water impregnation (mL/g) BET specific surface area (m2/g)

DUC CAL HYD DUC CAL HYD DUC CAL HYD

SI53 13.1 15.3 – 0.24 0.20 – 77 53 –SI100 25.9 24.4 26.0 0.23 0.22 0.23 41 36 31SI223 42.0a 41.3a 41.8a 0.23 0.23 0.23 21 16 16SI300 58.8a 59.1a 58.3a 0.23 0.22 0.22 15 13 13

Notes: DUC, dried but uncalcined; CAL, calcined; HYD, hydrothermal treated.a Data from mercury intrusion.

672 Z. Zhou et al. / Chemical Engineering Journal 223 (2013) 670–677

(1 � 10�3 Pa) at 350 �C for 4 h, and then exposed to the vapor ofpyridine after cooling to room temperature. The pyridine-IR spec-tra of the sample were then recorded at 200 �C after the samplebeing evacuated for 15 min.

The acid sites amount and acid strength distribution of sampleswere determined by a temperature-programmed desorption ofammonia (NH3-TPD). The ammonia in the effluent gas was de-tected by a TCD detector, and then adsorbed with 0.02 mol/L HClsolution. The total acid amount was determined by back-titratingthe HCl solution using 0.01 mol/L NaOH solution, and bromocresolgreen/methyl red as indicator.

3. Results and discussion

3.1. Pretreatment of SiO2 opal template

During the SiO2 microspheres growth, micropores more or lessformed in them. The existence of micropores broadened the poresize distribution of opal. To eliminate the micropores, the as-pre-pared opals were calcined at 800 �C for 2 h. The skeleton densityof opals was measured by helium true density meter (MDMDY-300, Zhongshan Meidi Analysis Instrument Factory, Guangdong,China). The density of calcined opals with microspheres of 53,100, 223 and 300 nm was 2.362, 2.210, 2.227 and 2.215 g/cm3,respectively. These values are higher than that of dried as-preparedopals (2.348 g/cm3 for SI53 and 1.9–2.0 g/cm3 for others). It impliesthat the calcined SiO2 microspheres are dense and nonporous. The53 nm microspheres are originally dense and the skeleton densityof SI53 was almost unchanged after calcination.

Calcination also led to sintering between particles and 3–9%shrinkage of diameter. All of these resulted in a SSA decrease ofopals (Table 2). According to literature [24], most silanol groupson the SiO2 disappear during calcination, and this would be unfa-vorable to the subsequent Al2O3 grafting. Therefore, it is necessaryto reactivate the surface silanols of the calcined opal by hydrother-mal treatment in water [25–27]. After hydrothermal treatment, thepore structure of SI100, SI223 and SI300 is intact. However, thepore size of SI53 greatly increased from 13.1 nm to 20 nm, SSA re-duced from 53 m2/g to 42 m2/g. This is ascribed to the hydrolysis ofSi–O–Si bonds of small SiO2 particles with larger surface area [28].In addition, the acid density of calcined SiO2 opal with larger sizeparticles tested by NH3-TPD is about 0.15 lmol/m2, and then re-bounded to 0.61 lmol/m2 after hydrothermal treatment. It is com-parable to that of 700 �C calcined SI53 (0.57 lmol/m2). To avoidpore structure collapse and obtain the same surface silanol density,the opal with 53 nm microspheres calcined at 700 �C withouthydrothermal treatment was used as the template of OPA53.

1 For interpretation of color in Fig. 3, the reader is referred to the web version ofis article.

3.2. Pore structure of WDUPS Al2O3 with different amounts of Al2O3

layer

Al2O3 coating process is another key step for obtaining alumina-like porous materials with well-defined and uniform pore struc-ture. Therefore, the optimal Al2O3 loading for preserving the pore

size of template should be determined. To determine the appropri-ate Al2O3 coating, different amounts of Al2O3 was deposited on theSiO2 opals SI53 and SI300. The pore structure of WDUPS Al2O3 wasexamined by N2 adsorption apparatus and/or mercuryporosimeter.

As shown in Fig. 1B, samples OPA53-1.0, 1.5 and 2.0 have a uni-modal pore size distribution centered at 13.3 nm, similar to theSiO2 template (SI53). Their SSA remains almost unchanged (ca.64 m2/g). The WDUPS Al2O3 with less Al2O3 coating (OPA53-0,0.5) exhibit large pore-size (25.6 and 16.7 nm) and low SSA dueto ammonia corrosion to the SiO2 opal during the VIH process[29]. However, with more Al2O3 coating, the samples (OPA53-3.0,4.0) exhibit bimodal pore-size distribution, small pore-size (11.7and 10.3 nm) and high SSA. The pore volume of OPA53 (Fig. 1A) de-creased from 0.23 mL/g (SI53) to 0.18 mL/g (OPA53-4.0). This indi-cates that SiO2 opal with less atomic Al2O3 is out of protection ofammonia corrosion. While excessive Al2O3 on the SiO2 template re-sulted in un-uniform and porous Al2O3 coating layer.

Fig. 2A shows the pore size distribution of OPA300 with differ-ent Al2O3 coatings. OPA300-1.0 and OPA300-2.0 have a unimodaland narrow pore size distribution centered at 58.6 nm. WhenAl2O3 loading increased, the pore-size of WDUPS Al2O3 (OPA300-3.0 and OPA300-4.0) decreased and a shoulder peak appeared,indicating that Al2O3 unevenly coated on the opal surface. The porevolumes of OPA300 obtained either by water impregnation (ca.0.22 mL/g) or mercury intrusion (ca. 0.24 mL/g), were almost un-changed with the Al2O3 coatings.

The SSA of OPA300 remained unchanged until one atomic Al2O3

layer is loaded (Fig. 2B), then it increased slightly when Al2O3 in-creased to double atomic layers. However, the SSA of OPA300 in-creased significantly when Al2O3 loading is further increased. Itseems that, with the Al2O3 loading being more than two atomiclayers, the Al2O3 coating becomes porous. From the above experi-mental results, we can determine that the optimal Al2O3 coatingfor WDUPS Al2O3 is of 1–2 atomic layer. The WDUPS Al2O3 com-posed of 100 and 223 nm microspheres also have such an optimalAl2O3 coating value. The pore structure data of WDUPS Al2O3 withthe optimal Al2O3 coating are listed in Table 3.

3.3. Morphology of the WDUPS Al2O3

Opal materials made up of different size of microspheres exhibitdifferent opalescence. The opals and their WDUPS Al2O3 supportscomposed of 300, 223, 100 and 53 nm SiO2 microspheres showedpink–green1, bluish violet, white and semitransparent opalescence,respectively (Fig. 3). Microscopic morphology of WDUPS Al2O3 sup-ports was observed by electronic microscope.

Fig. 3 shows the images of WDUPS Al2O3 with one atomic layerof Al2O3 coating. It can be seen that the alumina-coated micro-spheres composing the WDUPS Al2O3 (A–C) are well-ordered,implying the pores of WDUPS Al2O3 are ordered and uniform.

th

OPA53-4.0

OPA53-3.0

OPA53-2.0

OPA53-1.5

OPA53-1.0

OPA53-0.5

OPA53-0

Pore diameter (nm)

B

SI 53

d V/d

LogD

(mL/

g)

64 m2/g

35 m2/g

52 m2/g

64 m2/g

64 m2/g

65 m2/g

72 m2/g

77 m2/g0.18 mL/g

0.18 mL/g

0.20 mL/g

0.20 mL/g

0.21 mL/g

0.20 mL/g

0.23 mL/g

0.20 mL/g 1

OPA53-0

SI 53

AOPA53-4.0

OPA53-3.0

OPA53-2.0

OPA53-1.5

OPA53-1.0

OPA53-0.5

Relative pressure (P/P0)

Volu

me

@ S

TP (m

L/g)

100

0.0 0.2 0.4 0.6 0.8 1.0 1 10 100

Fig. 1. Pore structure of mesoporous OPA53 with different Al2O3 coatings. (A) N2 adsorption–desorption hysteresis loops with pore volume; (B) pore-size distributions andBET surface area.

0 1 2 3 4 5 60

2

4

6

8

10

12

14

16

18

20

22

24

26

28

BET

Spe

cific

Sur

face

Are

a (m

2 /g)

silicaopal

Form

atio

n of

one

ato

mic

la

yer o

f Al 2O

3

Number of alumina atomic layers

0.2194 mL/g

0.23 mL/g

0.24 mL/g

0.24 mL/g

0.24 mL/g

0.2154 mL/g

0.2168 mL/g

0.2176 mL/g

Mercuryintrusionvolume

Waterimpregnationvolume

B

101 102 103 104 105

OPA300-1.0

OPA300-2.0

OPA300-3.0

Pore diameter (nm)

dV /d

LogD

(mL/

g)

A

OPA300-4.0

1

Fig. 2. Pore structure of macroporous OPA300 with different Al2O3 coatings. (A) Pore size distribution obtained from mercury porosimetry. (B) BET SSA obtained from N2

adsorption.

Z. Zhou et al. / Chemical Engineering Journal 223 (2013) 670–677 673

However, the microspheres of OPA53 (D) seem randomly packed.This can be attributed to the size effect of small SiO2 microspheresduring the assembly period. In neutral water, SiO2 microspheresare negatively charged and electric double layer formed aroundthem. Therefore, regarding to particle size, small SiO2 particles takemore charges and thicker double electric layer formed than largermicrospheres since their higher specific surface area [30]. Thus,they are more difficult to be assembled into ordered array due tothermal agitation of Brownian motion and strong repulsive force.Although these small SiO2 microspheres seem to be less ordered,

but they are nearly the most closely packed because the pore vol-ume is very close to that of the most-closely-packed materials(shown in Table 3).

3.4. Al2O3 coordination states

Solid-state 27Al NMR spectrum was used to analyze the coordi-nation of aluminum atom. Different thicknesses of Al2O3 wascoated onto the surface of SiO2 opal composed of SiO2 micro-

Table 3Pore structural data of control-alumina, WDUPS Al2O3 samples and their templates.

Sample Pore diameter (nm) Pore volume (mL/g) Water impregnation volume (mL/g) BET specific surface area (m2/g)

Template OPA Template OPA Template OPA Template OPA

Control-alumina 4.4 0.41 0.42 288OPA53-1.0 13.3 13.4 0.21 0.21 0.23 0.22 64 64OPA100-1.0 26.0 26.4 0.21 0.20 0.23 0.23 31 30OPA223-1.0 41.8a 41.7a 0.23 0.24a 0.23 0.23 16 17OPA300-1.0 58.3a 58.6a 0.23 0.24a 0.22 0.22 13 13

a Data from mercury intrusion.

Fig. 3. Images of WDUPS Al2O3 samples with one atomic layer of Al2O3 coating. SEM images of WDUPS Al2O3 with microspheres diameters of 300 nm for (A), 223 nm for (B),100 nm for (C) and 53 nm for (D), respectively. Insert pictures are optical photos of bulk samples.

Inte

nsity

Control alumina

OPA100-2.0

OPA100-1.0

OPA100-0.5

Chemical shift (ppm)

630

54

150 100 50 -50 -1000

Fig. 4. 27Al NMR spectra of OPA100 with different amount of Al2O3 coatings andcontrol alumina.

674 Z. Zhou et al. / Chemical Engineering Journal 223 (2013) 670–677

spheres of 100 nm diameter, and control alumina was tested forcomparison.

NMR spectra of the WDUPS Al2O3 (Fig. 4) show three signals at6, 30 and 54 ppm which are typically assigned to octahedral(AlOh), pentahedral (AlPd) and tetrahedral (AlTd) aluminumatoms, respectively [31]. The chemical shift of AlTd peak (54 ppm)of the WDUPS Al2O3 is lower than that of control-alumina (ca.65 ppm) and close to that of tetrahedral Al in zeolite and amor-phous aluminum silicate, suggesting the formation of Al–O–Sibonds resulting from incorporation of Al atoms within the frame-work. When Al2O3 reached one atomic layer, AlTd peak becomesweak and shows a little shift towards high chemical shift value.However, the nonframework AlOh signals at 6 ppm [32] becomesstrong, implying the fraction of Al–O–Si bonds decreased. The AlPd

peak (30 ppm) in aluminum silicate [33] is not obvious whateverthe number of coatings, also indicating the low fraction of Al–O–Si bonds in all the WDUPS Al2O3. The NMR data indicate thatAl2O3 was formed on WDUPS Al2O3 surface and cooperated withSiO2 by Al–O–Si bonds. But these Al–O–Si bonds are not so muchas aluminum silicate for the low silanol concentration of SiO2 opalafter pretreatment.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

OPA53-1.0 no Py OPA53-1.0

OPA100-1.0

OPA223-1.0

Abs

orba

nce

(a.u

.)

Wave number (cm-1)

L B+L

Contral-alumina

OPA300-1.0

1350 1400 1450 1550 1600 1650 17001500

Fig. 5. Py-IR spectra of control alumina and WDUPS Al2O3 samples.

Table 4Acid distribution (fraction) of control alumina and WDUPS Al2O3 samples.

Sample Weak acid Medium acid Strong acid110–220 �C 220–390 �C 390–550 �C

Control-alumina 0.25 0.53 0.22OPA53-1.0 0.26 0.51 0.23OPA100-1.0 0.25 0.54 0.21OPA223-1.0 0.24 0.50 0.26OPA300-1.0 0.24 0.51 0.25

0

20

40

60

80

100

120

140

160

180

Number of atomic layers of Al2O

3(x)

OPA100-x

OPA53-x

OPA223-x

OPA300-xAci

d am

ount

m

ol/g

)

0 1 2 3 4

(

Fig. 7. Acid amount of WDUPS Al2O3 samples with different amount of Al2O3

coatings.

Z. Zhou et al. / Chemical Engineering Journal 223 (2013) 670–677 675

3.5. Acid types of WDUPS Al2O3

The Py-IR spectra of pyridine-adsorbed WDUPS Al2O3 and con-trol alumina were recorded. The WDUPS Al2O3 samples were allcoated with one atomic Al2O3 layer. The Py-IR spectra in Fig. 5shows the adsorption bands of every type of acid sites becausethere is no any peak on the Py-IR spectrum of OPA53-1.0 withoutpyridine adsorption. The bands near 1455 cm�1 correspond to pyr-idine adsorption on Lewis site (PyL), and bands near 1490 cm�1

correspond to pyridine adsorption on both Brönsted and Lewissites. However, the typical bands of pyridinium ions (PyH+) onBrönsted acid sites at 1540 cm�1 are not presented [34]. Thismeans that Lewis acid sites are dominant ones on all the WDUPSAl2O3, same as the control alumina. The bands of WDUPS Al2O3

are relatively weak compared to that of the control alumina dueto the lower SSA of the OPAs.

3.6. Acid strength distribution and acid density of WDUPS Al2O3

The WDUPS Al2O3 samples with one atomic layer Al2O3 coatingare selected for comparing the acidity with the control alumina.The acid strength distribution of the samples was characterizedby NH3-TPD and the curves are presented in Fig. 6. The desorption

(°C)

Inte

nsity

(a.u

.)

Temperature

Control-alumina

OPA53-1.0

OPA100-1.0

OPA223-1.0

OPA300-1.0

kept 550 °C

5000

100 200 300 400 500 600 700

Fig. 6. NH3-TPD curves of control alumina and WDUPS Al2O3 samples. The testedsample is 0.5 g for control alumina and 1.0 g for WDUPS Al2O3 samples.

peaks of the WDUPS Al2O3 are similar to each other and to the con-trol-alumina. The peak of OPA53-1.0 is higher than other WDUPSAl2O3 samples due to its larger specific surface area. In order tocompare their acid sites distribution in detail, the fractions ofweak-, medium- and strong-acid to their total acid amount werecalculated from the integral peak area of curves (Table 4).

NH3-desorption peaks at 120–220 �C, 220–390 �C and 390–550 �C ranges represent weak, medium and strong acid, respec-tively. The area under the curves represents the relative amountof acid-sites. The fraction of acid-sites with different strength cal-culated from the area of the NH3-TPD peaks is listed in Table 4.

0

2

4

6

8

10

12

14

16

18

20 acid density acid amount

Pore diameter (nm)

Aci

d de

nsity

0

100

200

300

400

500

600

Aci

d am

ount

mol

/g)

mol

/m )

Control alumina OPA53-1.0 OPA100-1.0 OPA300-1.0

0 10 20 30 40 50 60

OPA223-1.0

2

Fig. 8. Acid amount and acid density of WDUPS Al2O3 samples with one atomiclayer of Al2O3 coating.

676 Z. Zhou et al. / Chemical Engineering Journal 223 (2013) 670–677

The acid distribution of the WDUPS Al2O3 with different sizes ofmicrospheres is similar to each other and to the control alumina.

The total acid amount of WDUPS Al2O3 increased with increas-ing Al2O3 coatings (Fig. 7). After one atomic Al2O3 layer formed, thetotal acid amount of OPAs increased very little with the further in-crease of Al2O3. The plateau between OPA-1.0 and OPA-2.0 pro-vides an evidence of the formation of complete Al2O3 layer onthe SiO2 microspheres surface. The further increase of total acidamount when the Al2O3 coating is more than two atomic layersdemonstrated the formation of porous Al2O3.

As shown in Fig. 8, the total acid amount of WDUPS Al2O3 sam-ples increases with decreasing pore size. However, the acid densityof WDUPS Al2O3, which was calculated by using the total acidamount divided by the surface area (Table 3), is almost unchangedand very similar to that of control alumina. In other words, the to-tal acid amount of WDUPS Al2O3 changes proportionally to theirsurface area. The surface acidity of the WDUPS Al2O3 is indepen-dent of their pore-size and almost identical to that of the controlAl2O3.

The WDUPS Al2O3 exhibits acidity similar to alumina ratherthan aluminum silicate. This might be attributed to the pretreat-ment of SiO2 opals, which resulted in a low silanol concentrationon the surface of SiO2 microspheres. Only small amount of thecoated Al2O3 combined with the limited residual silanols. Besides,WDUPS Al2O3 has a high thermal stability since their templateswere pretreated at high temperature. The aluminum coordinationstate and acidity of the WDUPS Al2O3 are unchanged below thepretreatment temperature of their SiO2 templates as our experi-ments proved.

3.7. Catalytic performance of WDUPS Al2O3

The catalytic activity of WDUPS Al2O3 (OPA100-1.0) comparingto the unimodal commercial Al2O3 (SSA = 169 m2/g, average poresize = 7.6 nm, PV = 0.32 mL/g) was conducted under dibenzothio-phene (DBT) hydrodesulfurization over NiMo/Al2O3 catalyst. Thiswork was performed on a bench scale trickle-bed reactor under6.0 MPa, 300 �C. Details have been published in our previous paper[35]. With increasing metal loading, an obvious Al2O3–MoO3 inter-action was observed on WDUPS Al2O3. The catalytic acitivity persurface area of NiMo/WDUPS Al2O3 is 1.5–2.7 lmol DBT/(h m2 cat-alyst), higher than that of unimodal NiMo/Al2O3 0.2 lmol DBT/(hm2 catalyst). However, catalytic activity tested on a Al2O3 with bi-modal macropores (SSA = 230.3 m2/g; average pore size = 14 nm;PV = 0.77 mL/g, 1–11 nm 44%, 12–120 nm 56%; Mo 5.42 lmol/m2

and Ni 2.71 lmol/m2), is as high as 6.5 lmol DBT/(h m2 catalyst).This result well exhibits the pore-diffusion resistance phenomenonamong different structured aluminas.

4. Conclusion

Three-dimensional ordered porous Al2O3 catalyst supports withwell-defined and uniform pore-size were prepared by coating thepretreated SiO2 opal with Al2O3. The pore size of Al2O3 can be tai-lored in the range of meso- to macro- by using opals with differentsized microspheres. The optimal Al2O3 coating was determined tobe of 1–2 atomic layers and more Al2O3 coating become porous.The surface acid amount of OPAs increased with the Al2O3 coatinguntil one atomic layer formed. The prepared Al2O3 catalyst sup-ports have regular internal surface; their acid types and densityare pore size-independent and comparable to the control alumina.These characteristics of the opal-like Al2O3 supports make themideal to be materials for fundamental research in catalysis, suchas the internal diffusion of reactant or observation of active speciesstacked on the Al2O3 support. Above all, this material gives a gen-

eral way to prepare WDUPS catalytic materials with different sur-face chemical properties, which can find their applications invarious heterogeneous catalysis systems.

Acknowledgements

This research was supported by CNPC (China National Petro-leum Corporation), the National Natural Science Foundation of Chi-na (Grant No. 20976192), and the PhD Program Foundation ofEducation Ministry, China (Grant No. 20090007110003).

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