Tailored ordered porous alumina with well-defined and uniform pore-structure

Chemical Engineering Journal 223 (2013) 670–677

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Tailored ordered porous alumina with well-defined and uniform pore-structure Zheng Zhou, Sheng-Li Chen ⇑, Derun Hua, Zhi-Gang Wang, Ai-Cheng Chen, Wen-Hao Wang State 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.

c-Al2O3.

" The acid properties of the WDUPS Al2O3 is the same to that of conventional " 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 2012 Received in revised form 28 December 2012 Accepted 7 January 2013 Available online 16 January 2013 Keywords: Uniform pore-size Opal structure Alumina coating Tailored pore-size Monodisperse silica spheres

a b s t r a c t Well-defined and uniform pore-structure (WDUPS) Al2O3 catalyst supports were prepared by coating specially treated SiO2 opals with Al2O3. The pore-size of the WDUPS Al2O3 can be tailored in the range of meso- to macro-size by using different-sized microspheres to fabricate the SiO2 opal. The optimal amount of Al2O3 coating, which keeps the pore structure of SiO2 template intact, was determined to be of the 1–2 atomic layers. The surface acid amount of WDUPS Al2O3 significantly increased with the Al2O3 coating until one atomic layer formed. When the SiO2 opals were coated with optimal amount of Al2O3, the obtained WDUPS Al2O3 showed a uniform acid density. With these characteristics, the WDUPS Al2O3 support is ideal material for fundamental research in catalysis. The method is promising strategy to obtain other catalytic materials with the same structure. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Al2O3 is the most widely used catalyst support. However, the pores of conventional Al2O3 are disordered and broadly distributed, which is unsuitable for pore-size-related studies such as internal diffusion. Furthermore, the irregularity of internal surface of the conventional Al2O3 makes it difficult to observe the stack of active species (e.g. CoMoS for hydrodesulfurization catalyst) on the surface by electronic microscopy. Therefore, well-defined and uniform pore-structure (WDUPS) Al2O3 with regular internal surface is needed. Moreover, to study the diffusion phenomenon of large molecules, the WDUPS Al2O3 with larger and tunable pore size is indispensable [1]. To date, many efforts were made to prepare uniform pore sized Al2O3. The Al2O3 synthesized by using either soft-templates [2–6] or hard-templates (nanocasting) [7] were said to have narrow pore size distribution. Actually, their pore size exhibits a hierarchical

⇑ Corresponding author. Tel.: +86 10 89733396. E-mail address: [email protected] (S.-L. Chen). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.01.024

distribution due to the pores from dehydration in Al2O3 crystal planes [8]. These Al2O3 only have a relatively high ratio of mesopore volume to total pore volume [9], and their pores are still random 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 pore size Al2O3 [10–16]. This method is also used to prepare ordered mesoporous ZrO2 by coating ZrO2 on SBA-15 [17]. The pore structure of the post-synthesized Al2O3 is dependent on its template. Unfortunately, it is also difficult to prepare the template with uniform and well-defined pore structures. Opal is a porous material with ordered face-centered cubic packed microspheres array. The ordered compact with a high coordination number has very uniform pore size. Theoretically, its pore diameter, specific surface area (SSA) and pore volume can be calculated [18,19]. The pore diameter of an opal is expressed as the diameter of inscribed circle in the throat, being 0.155D (D is the diameter of microspheres), and the diameter of void inscribed ball is 0.225D. The specific surface area of opal is 6/(qD) (q is the density of microspheres). The opal void fraction of 0.26 is independent

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of the size of the microspheres composing the opal [20]. Thus, opal has a welldefined pore structure, which can be tuned by changing the diameter of microspheres. In this paper, WDUPS Al2O3 catalyst supports were obtained through coating the pretreated SiO2 opals with an appropriate thickness of Al2O3. The prepared WDUPS Al2O3 catalyst supports have well-defined and uniform pore structures and regular internal surface, and their surface acidity is comparable to that of c-Al2O3. By this method, many other catalyst materials with the same structure, such as TiO2 and SnO2, can be obtained. They are ideal materials for fundamental research in many heterogeneous catalysis and pore-size related study such as the internal diffusion of reactants. 2. Materials and methods 2.1. Synthesis of SiO2 opal The monodisperse SiO2 microspheres were synthesized through hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in alcohol and in the presence of water and ammonia by the seed particle growth method. Detailed description of the synthesis procedures were reported in our previous papers [21,22]. In a typical SiO2 opal assembly process, the suspension of SiO2 microspheres was put into a glass beaker. The beaker was then placed into an oven and kept at 50 °C with a relative humidity of 90%. After several days, the SiO2 microspheres were automatically assembled into opal when the suspension was dried. The as-prepared SiO2 opal was further dried at 100 °C for 24 h, and calcined in muffle furnace at 800 °C for 2 h. Finally, the opal was immerged in water, put into an autoclave and then hydrothermally treated at 220 °C for 10 h to recover the surface silanol groups which were lost during the calcination. 2.2. Coating the internal surface of SiO2 opal with Al2O3 Al2O3 was coated onto the internal surface of SiO2 opal by the VIH (ammonia/water vapor induced internal hydrolysis) method [16]. A specific amount of precursor Al(NO3)3 9H2O was dissolved in bi-distilled water with volume same as the pore volume of opal. The SiO2 opal was impregnated by using the Al(NO3)3 solution by the incipient-wetness impregnation method and then stood over night. Then Al(NO3)3 was deposited onto the inner surface of the SiO2 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 the atomic layer capacity of Al2O3 on SiO2 surface, [23] by the following equations:

mA1 ¼ 0:046  mSi  SSA=100

ð1Þ

mAINO ¼ 373:15  mA1 =101:96

ð2Þ

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 of SiO2 opal (m2/g). About 1.0 g dried sample was transferred into a 20 mL open glass vial. The vial was placed in a 100 mL autoclave (with a Teflon™ lining) filled with 10 mL 12.5 wt.% NH3–H2O solution, making the vial keep clear of the NH3–H2O solution. Then the sample was sealed tightly in the autoclave, and kept at 100 °C for 7 h. The Al(NO3)3 on the opal surface was hydrolyzed to Al2O3 xH2O in ammonia/water vapor during the hydrolysis treatment. Finally, the WDUPS Al2O3 was obtained after the sample was dried at 100 °C for 6 h and calcined at 500 °C for 5 h. WDUPS Al2O3 and their SiO2 template were respectively labeled as ‘‘OPA’’ and ‘‘SI’’ followed by the diameter of microspheres composing the SiO2 opals. All the samples and their preparation conditions are listed in Table 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 vapor at 100 °C for 7 h, and then dried and calcined at the same condition as WDUPS Al2O3 preparation mentioned above. The control alumina was denoted as ‘‘control-alumina’’. 2.4. Characterization The morphology of samples was observed on a FEI quanta 200F SEM (FEI, Oregon, USA) using 20 kV accelerating voltage. Pore structure analysis of samples was conducted with a Micromeritics ASAP 2010 automatic N2 adsorption analyzer (Micromeritics, Norcross, GA) after the samples were degassed at 250 °C for 3 h. Data of mercury intrusion porosimetry were obtained with an AutoPore IV 9500 mercury porosimeter (Micromeritics, Norcross, GA) using a contact angle of 140° and a mercury surface tension of 0.473 N/m. The sample was dried at 120 °C for 4 h before test. For comparison, the pore volume of the samples was also determined by the method of incipient-wetness impregnation of water. 27 Al Nuclear Magnetic Resonance (NMR) experiments were performed on a 500 MHz Bruker Advance III NMR spectrometer, using a 4 mm WVT double-resonance Bruker probe. The 27Al Larmor frequency of this spectrometer is 130.33 MHz. The acid types of samples surface were identified by the Fourier transform infrared spectroscopy of pyridine adsorption (Py-FTIR). In the Py-FTIR study, 12 mg of sample were mixed with 38 mg KBr and pressed into self-supported wafer. In situ pyridine adsorption over the wafer was carried out in a FTIR cell using a conventional glass adsorption setup and performed on Nicolet 6700 spectrophotometer (Nicolet, Madison, USA). The sample wafer was vacuum-degassed

Table 1 The prepared samples and their preparation methods. Sample

Diameters of SiO2 microspheres composing the opal templates (nm)

Calcination temperature of SiO2 templates (°C)

Hydrothermal treatment of SiO2 templates

Al2O3 loading (number of the atomic layer)

VIH treatment

SI53 SI100 SI223 SI300 OPA53-x OPA100-x OPA223-x OPA300-x

53 100 223 300 53 100 223 300

700 800 800 800 700 800 800 800

No Yes Yes Yes No Yes Yes Yes

– – – – x x x x

No No No No Yes Yes Yes Yes

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Table 2 Effect of calcination and hydrothermal treatment on pore structure of opals. SiO2 opal

SI53 SI100 SI223 SI300

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

13.1 25.9 42.0a 58.8a

15.3 24.4 41.3a 59.1a

– 26.0 41.8a 58.3a

0.24 0.23 0.23 0.23

0.20 0.22 0.23 0.22

– 0.23 0.23 0.22

77 41 21 15

53 36 16 13

– 31 16 13

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

(1  103 Pa) at 350 °C for 4 h, and then exposed to the vapor of pyridine after cooling to room temperature. The pyridine-IR spectra of the sample were then recorded at 200 °C after the sample being evacuated for 15 min. The acid sites amount and acid strength distribution of samples were determined by a temperature-programmed desorption of ammonia (NH3-TPD). The ammonia in the effluent gas was detected by a TCD detector, and then adsorbed with 0.02 mol/L HCl solution. The total acid amount was determined by back-titrating the HCl solution using 0.01 mol/L NaOH solution, and bromocresol green/methyl red as indicator. 3. Results and discussion 3.1. Pretreatment of SiO2 opal template During the SiO2 microspheres growth, micropores more or less formed in them. The existence of micropores broadened the pore size distribution of opal. To eliminate the micropores, the as-prepared opals were calcined at 800 °C for 2 h. The skeleton density of opals was measured by helium true density meter (MDMDY300, 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-prepared opals (2.348 g/cm3 for SI53 and 1.9–2.0 g/cm3 for others). It implies that the calcined SiO2 microspheres are dense and nonporous. The 53 nm microspheres are originally dense and the skeleton density of 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 of opals (Table 2). According to literature [24], most silanol groups on the SiO2 disappear during calcination, and this would be unfavorable to the subsequent Al2O3 grafting. Therefore, it is necessary to reactivate the surface silanols of the calcined opal by hydrothermal treatment in water [25–27]. After hydrothermal treatment, the pore structure of SI100, SI223 and SI300 is intact. However, the pore size of SI53 greatly increased from 13.1 nm to 20 nm, SSA reduced from 53 m2/g to 42 m2/g. This is ascribed to the hydrolysis of Si–O–Si bonds of small SiO2 particles with larger surface area [28]. In addition, the acid density of calcined SiO2 opal with larger size particles tested by NH3-TPD is about 0.15 lmol/m2, and then rebounded to 0.61 lmol/m2 after hydrothermal treatment. It is comparable to that of 700 °C calcined SI53 (0.57 lmol/m2). To avoid pore structure collapse and obtain the same surface silanol density, the opal with 53 nm microspheres calcined at 700 °C without hydrothermal treatment was used as the template of OPA53. 3.2. Pore structure of WDUPS Al2O3 with different amounts of Al2O3 layer Al2O3 coating process is another key step for obtaining aluminalike porous materials with well-defined and uniform pore structure. Therefore, the optimal Al2O3 loading for preserving the pore

size of template should be determined. To determine the appropriate Al2O3 coating, different amounts of Al2O3 was deposited on the SiO2 opals SI53 and SI300. The pore structure of WDUPS Al2O3 was examined by N2 adsorption apparatus and/or mercury porosimeter. As shown in Fig. 1B, samples OPA53-1.0, 1.5 and 2.0 have a unimodal pore size distribution centered at 13.3 nm, similar to the SiO2 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 due to 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 (11.7 and 10.3 nm) and high SSA. The pore volume of OPA53 (Fig. 1A) decreased from 0.23 mL/g (SI53) to 0.18 mL/g (OPA53-4.0). This indicates that SiO2 opal with less atomic Al2O3 is out of protection of ammonia corrosion. While excessive Al2O3 on the SiO2 template resulted in un-uniform and porous Al2O3 coating layer. Fig. 2A shows the pore size distribution of OPA300 with different Al2O3 coatings. OPA300-1.0 and OPA300-2.0 have a unimodal and narrow pore size distribution centered at 58.6 nm. When Al2O3 loading increased, the pore-size of WDUPS Al2O3 (OPA3003.0 and OPA300-4.0) decreased and a shoulder peak appeared, indicating that Al2O3 unevenly coated on the opal surface. The pore volumes of OPA300 obtained either by water impregnation (ca. 0.22 mL/g) or mercury intrusion (ca. 0.24 mL/g), were almost unchanged 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 increased to double atomic layers. However, the SSA of OPA300 increased significantly when Al2O3 loading is further increased. It seems that, with the Al2O3 loading being more than two atomic layers, the Al2O3 coating becomes porous. From the above experimental results, we can determine that the optimal Al2O3 coating for WDUPS Al2O3 is of 1–2 atomic layer. The WDUPS Al2O3 composed of 100 and 223 nm microspheres also have such an optimal Al2O3 coating value. The pore structure data of WDUPS Al2O3 with the optimal Al2O3 coating are listed in Table 3.

3.3. Morphology of the WDUPS Al2O3 Opal materials made up of different size of microspheres exhibit different opalescence. The opals and their WDUPS Al2O3 supports composed of 300, 223, 100 and 53 nm SiO2 microspheres showed pink–green1, bluish violet, white and semitransparent opalescence, respectively (Fig. 3). Microscopic morphology of WDUPS Al2O3 supports was observed by electronic microscope. Fig. 3 shows the images of WDUPS Al2O3 with one atomic layer of Al2O3 coating. It can be seen that the alumina-coated microspheres composing the WDUPS Al2O3 (A–C) are well-ordered, implying the pores of WDUPS Al2O3 are ordered and uniform. 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.

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B

A OPA53-4.0

0.18 mL/g

OPA53-3.0

0.18 mL/g

2

OPA53-4.0

77 m /g

OPA53-3.0

72 m /g

OPA53-2.0

65 m /g

OPA53-1.5

64 m /g

OPA53-1.0

64 m /g

OPA53-0.5

52 m /g

OPA53-0

35 m /g

2

OPA53-1.5

0.20 mL/g

OPA53-1.0

0.21 mL/g

OPA53-0.5

0.20 mL/g

0.20 mL/g

SI 53 0.0

2

2

2

2

0.23 mL/g

OPA53-0

100

dV/dLogD (mL/g)

OPA53-2.0

0.2

0.4

2

1

Volume @ STP (mL/g)

2

0.20 mL/g

0.6

0.8

1.0

64 m /g

SI 53 1

10

Relative pressure ( P/P0)

100

Pore diameter (nm)

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

Mercury intrusion volume

Water impregnation volume

0.24 mL/g

0.2154 mL/g

26

B

24 22

2

OPA300-4.0

28

BET Specific Surface Area (m /g)

A

0.2168 mL/g

OPA300-3.0

0.24 mL/g

0.2176 mL/g

1

OPA300-2.0

18 16

silica opal

14 12 Formation of one atomic layer of Al2O3

dV /dLogD (mL/g)

0.23 mL/g

20

10 8 6 4

0.24 mL/g

0.2194 mL/g

OPA300-1.0 1

10

2

10

3

10

4

10

2 0

5

10

Pore diameter (nm)

0

1

2

3

4

5

6

Number of alumina atomic layers

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.

However, the microspheres of OPA53 (D) seem randomly packed. This can be attributed to the size effect of small SiO2 microspheres during the assembly period. In neutral water, SiO2 microspheres are negatively charged and electric double layer formed around them. Therefore, regarding to particle size, small SiO2 particles take more charges and thicker double electric layer formed than larger microspheres since their higher specific surface area [30]. Thus, they are more difficult to be assembled into ordered array due to thermal 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 volume 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 coordination of aluminum atom. Different thicknesses of Al2O3 was coated onto the surface of SiO2 opal composed of SiO2 micro-

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

Control-alumina OPA53-1.0 OPA100-1.0 OPA223-1.0 OPA300-1.0 a

Pore diameter (nm)

Pore volume (mL/g)

Water impregnation volume (mL/g)

Template

OPA

Template

OPA

Template

13.4 26.4 41.7a 58.6a

0.41 0.21 0.21 0.23 0.23

0.21 0.20 0.24a 0.24a

0.42 0.23 0.23 0.23 0.22

4.4 13.3 26.0 41.8a 58.3a

BET specific surface area (m2/g)

OPA

Template

OPA

0.22 0.23 0.23 0.22

288 64 31 16 13

64 30 17 13

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.

OPA100-0.5

Intensity

spheres of 100 nm diameter, and control alumina was tested for comparison. NMR spectra of the WDUPS Al2O3 (Fig. 4) show three signals at 6, 30 and 54 ppm which are typically assigned to octahedral (AlOh), pentahedral (AlPd) and tetrahedral (AlTd) aluminum atoms, 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 amorphous aluminum silicate, suggesting the formation of Al–O–Si bonds resulting from incorporation of Al atoms within the framework. When Al2O3 reached one atomic layer, AlTd peak becomes weak and shows a little shift towards high chemical shift value. However, the nonframework AlOh signals at 6 ppm [32] becomes strong, implying the fraction of Al–O–Si bonds decreased. The AlPd peak (30 ppm) in aluminum silicate [33] is not obvious whatever the number of coatings, also indicating the low fraction of Al–O– Si bonds in all the WDUPS Al2O3. The NMR data indicate that Al2O3 was formed on WDUPS Al2O3 surface and cooperated with SiO2 by Al–O–Si bonds. But these Al–O–Si bonds are not so much as aluminum silicate for the low silanol concentration of SiO2 opal after pretreatment.

OPA100-1.0

OPA100-2.0

6 54

30

Control alumina

150

100

50

0

-50

-100

Chemical shift (ppm) Fig. 4. 27Al NMR spectra of OPA100 with different amount of Al2O3 coatings and control alumina.

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Z. Zhou et al. / Chemical Engineering Journal 223 (2013) 670–677 Table 4 Acid distribution (fraction) of control alumina and WDUPS Al2O3 samples.

4.0 Contral-alumina

Absorbance (a.u.)

3.5 3.0

OPA300-1.0

2.5

L

2.0

B+L

Sample

Weak acid 110–220 °C

Medium acid 220–390 °C

Strong acid 390–550 °C

Control-alumina OPA53-1.0 OPA100-1.0 OPA223-1.0 OPA300-1.0

0.25 0.26 0.25 0.24 0.24

0.53 0.51 0.54 0.50 0.51

0.22 0.23 0.21 0.26 0.25

OPA223-1.0

1.5

180

OPA100-1.0

1.0

160

OPA53-1.0

OPA53-x

OPA53-1.0 no Py

1350

1400

1450

1500

1550

1600

1650

1700

-1

Wave number (cm ) Fig. 5. Py-IR spectra of control alumina and WDUPS Al2O3 samples.

3.5. Acid types of WDUPS Al2O3 The Py-IR spectra of pyridine-adsorbed WDUPS Al2O3 and control alumina were recorded. The WDUPS Al2O3 samples were all coated with one atomic Al2O3 layer. The Py-IR spectra in Fig. 5 shows the adsorption bands of every type of acid sites because there is no any peak on the Py-IR spectrum of OPA53-1.0 without pyridine adsorption. The bands near 1455 cm1 correspond to pyridine adsorption on Lewis site (PyL), and bands near 1490 cm1 correspond to pyridine adsorption on both Brönsted and Lewis sites. However, the typical bands of pyridinium ions (PyH+) on Brönsted acid sites at 1540 cm1 are not presented [34]. This means that Lewis acid sites are dominant ones on all the WDUPS Al2O3, same as the control alumina. The bands of WDUPS Al2O3 are relatively weak compared to that of the control alumina due to 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 coating are selected for comparing the acidity with the control alumina. The acid strength distribution of the samples was characterized by NH3-TPD and the curves are presented in Fig. 6. The desorption

140

Acid amount ( mol/g)

0.5

120 100 OPA100-x

80 60

OPA223-x

40 OPA300-x

20 0 0

1

2

3

4

Number of atomic layers of Al2O3 (x) Fig. 7. Acid amount of WDUPS Al2O3 samples with different amount of Al2O3 coatings.

peaks of the WDUPS Al2O3 are similar to each other and to the control-alumina. The peak of OPA53-1.0 is higher than other WDUPS Al2O3 samples due to its larger specific surface area. In order to compare their acid sites distribution in detail, the fractions of weak-, medium- and strong-acid to their total acid amount were calculated 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, respectively. The area under the curves represents the relative amount of acid-sites. The fraction of acid-sites with different strength calculated from the area of the NH3-TPD peaks is listed in Table 4.

20 Control-alumina

600 acid density acid amount

18

OPA100-1.0

5000

OPA223-1.0 OPA300-1.0

400

12 300 10 200

8 6

100 4 0

2

Control OPA53-1.0 alumina

0

100

200

300

400

Temperature (°C)

500

600 kept 550 °C

700

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

mol/g)

14

Acid amount

OPA53-1.0

Acid density

Intensity (a.u.)

2

mol/m )

500 16

0

10

20

OPA100-1.0

30

OPA223-1.0

40

OPA300-1.0

50

60

Pore diameter (nm) Fig. 8. Acid amount and acid density of WDUPS Al2O3 samples with one atomic layer of Al2O3 coating.

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The acid distribution of the WDUPS Al2O3 with different sizes of microspheres is similar to each other and to the control alumina. The total acid amount of WDUPS Al2O3 increased with increasing Al2O3 coatings (Fig. 7). After one atomic Al2O3 layer formed, the total acid amount of OPAs increased very little with the further increase of Al2O3. The plateau between OPA-1.0 and OPA-2.0 provides an evidence of the formation of complete Al2O3 layer on the SiO2 microspheres surface. The further increase of total acid amount when the Al2O3 coating is more than two atomic layers demonstrated the formation of porous Al2O3. As shown in Fig. 8, the total acid amount of WDUPS Al2O3 samples increases with decreasing pore size. However, the acid density of WDUPS Al2O3, which was calculated by using the total acid amount divided by the surface area (Table 3), is almost unchanged and very similar to that of control alumina. In other words, the total acid amount of WDUPS Al2O3 changes proportionally to their surface area. The surface acidity of the WDUPS Al2O3 is independent of their pore-size and almost identical to that of the control Al2O3. The WDUPS Al2O3 exhibits acidity similar to alumina rather than aluminum silicate. This might be attributed to the pretreatment of SiO2 opals, which resulted in a low silanol concentration on the surface of SiO2 microspheres. Only small amount of the coated Al2O3 combined with the limited residual silanols. Besides, WDUPS Al2O3 has a high thermal stability since their templates were pretreated at high temperature. The aluminum coordination state and acidity of the WDUPS Al2O3 are unchanged below the pretreatment temperature of their SiO2 templates as our experiments proved. 3.7. Catalytic performance of WDUPS Al2O3 The catalytic activity of WDUPS Al2O3 (OPA100-1.0) comparing to the unimodal commercial Al2O3 (SSA = 169 m2/g, average pore size = 7.6 nm, PV = 0.32 mL/g) was conducted under dibenzothiophene (DBT) hydrodesulfurization over NiMo/Al2O3 catalyst. This work was performed on a bench scale trickle-bed reactor under 6.0 MPa, 300 °C. Details have been published in our previous paper [35]. With increasing metal loading, an obvious Al2O3–MoO3 interaction was observed on WDUPS Al2O3. The catalytic acitivity per surface area of NiMo/WDUPS Al2O3 is 1.5–2.7 lmol DBT/(h m2 catalyst), higher than that of unimodal NiMo/Al2O3 0.2 lmol DBT/(h m2 catalyst). However, catalytic activity tested on a Al2O3 with bimodal 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 phenomenon among different structured aluminas. 4. Conclusion Three-dimensional ordered porous Al2O3 catalyst supports with well-defined and uniform pore-size were prepared by coating the pretreated SiO2 opal with Al2O3. The pore size of Al2O3 can be tailored in the range of meso- to macro- by using opals with different sized microspheres. The optimal Al2O3 coating was determined to be of 1–2 atomic layers and more Al2O3 coating become porous. The surface acid amount of OPAs increased with the Al2O3 coating until one atomic layer formed. The prepared Al2O3 catalyst supports have regular internal surface; their acid types and density are pore size-independent and comparable to the control alumina. These characteristics of the opal-like Al2O3 supports make them ideal to be materials for fundamental research in catalysis, such as the internal diffusion of reactant or observation of active species stacked on the Al2O3 support. Above all, this material gives a gen-

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