Preparation of Pseudoboehmite with a Large Pore Volume and a

Feb 24, 2011 - After calcinations of PB at 550 °C, the γ -Al2O3 powder obtained also yielded large pore volume and large average pore diameter. View...
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Preparation of Pseudoboehmite with a Large Pore Volume and a Large Pore Size by Using a Membrane-Dispersion Microstructured Reactor through the Reaction of CO2 and a NaAlO2 Solution Yujun Wang,†,* Dengqing Xu,‡ Haitao Sun,‡ and Guangsheng Luo† † ‡

The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Department of Chemistry, QuFu Normal University, Shandong 273165, China ABSTRACT: In this work, a membrane-dispersion microstructured reactor was specially designed to prepare PB (pseudoboehmite) with a large pore volume and a large pore size through the reaction of CO2 and an aqueous solution of NaAlO2. Tiny crystals of PB and a suitable template are necessary for generating large pore diameter and large pore volume. In this reactor, the tiny crystals could be obtained by increasing the mass transfer rate between the gas phase and the liquid phase through dispersing the CO2 gas into many microbubbles, and these microbubbles could act as a template to increase the pore volume. The effects of the final pH of the aqueous solution, the concentrations of raw materials, and the flow rate of the gas phase on pore properties of products were investigated. The results showed that when the final pH of the aqueous solution was between 10.5 and 9.4, pure PB could be obtained, and the crystal size of PB was about 2.3 nm. In addition, the specific surface area, the pore volume, and the average pore diameter of PB reached 548.5 m2/g, 2.22 mL/g, and 16.2 nm, respectively, under optimal conditions; these values were much higher than those prepared in the stirred tank. After calcinations of PB at 550 °C, the γ -Al2O3 powder obtained also yielded large pore volume and large average pore diameter.

1. INTRODUCTION Alumina, especially the porous γ-alumina (γ-Al2O3), is one of the supports most widely used in heterogeneous catalytic reactions, such as hydrocracking and hydrotreatments for heavy oil in the refinery industry, because of several attractive features.1-3 To reduce the internal diffusion resistance of large molecules, the preparation of porous γ-alumina with a large pore diameter and a large pore volume is becoming more and more important.4-6 It is well-known that γ-Al2O3 is usually prepared through the dehydration of PB (pseudoboehmite) under high temperature conditions; thus, the properties of the precursor-PB determine the properties of the γ-Al2O3 support.7-10 In recent years, the neutralization of NaAlO2 solution with CO2 gas has become one of the most economical techniques for the commercial preparation of PB, because the NaAlO2 solution is an intermediate product of the production process of alumina and the cost of CO2 is low. The procedure for the neutralization of NaAlO2 can be described by the following subreactions:11 2NaOHðaqÞ þ CO2 ðgÞ f Na2 CO3 ðaqÞ þ H2 OðaqÞ

ð1Þ

2NaAlO2 ðaqÞ þ CO2 ðgÞ þ 3H2 OðaqÞ f Na2 CO3 ðaqÞ þ 2AlðOHÞ3 ðsÞ ðamorphous and small size PBÞ

ð2Þ

NaAlO2 ðaqÞ þ 2H2 OðaqÞ f NaOHðaqÞ þ AlðOHÞ3 ðsÞ ðaluminum trihydroxide; BayeriteÞ

ð3Þ

Na2 CO3 ðaqÞ þ CO2 ðgÞ þ 2AlðOHÞ3 ðsÞ f 2NaAlðCO3 ÞðOHÞ2 ðsÞ þ H2 OðaqÞ ðDawsoniteÞ ð4Þ The products of reactions 2, 3, and 4 are PB, Bayerite, and Dawsonite, respectively. With the addition of CO2 into the r 2011 American Chemical Society

aqueous solution, the pH was decreased. When the pH is high, there are many OH- ions in the aqueous solution; since the reaction between the CO2 gas and the OH- ions is very fast, the individual mass transfer resistance in the aqueous phase can be omitted, as the mass transfer in the bubbles dominates the whole process. But when the pH is lower than 11.0, the hydrolysis of NaAlO2 is fast, and the trend of generating Bayerite becomes strong. There is competition between reaction 2 and reaction 3; unfortunately, under these conditions, the mass transfer resistance in the aqueous phase increases significantly, and therefore the total mass transfer resistance becomes strong. Although a large quantity of CO2 is added to the system, most of the CO2 does not react with the aqueous phase; it is necessary to intensify the mass transfer rate to make sure that the crystal form of the product is PB. Some other important factors are the pore diameter, the pore volume, and the specific area of PB. When stirred tanks are used to carry out the neutralization reaction, the PB powder prepared usually has a low pore volume (smaller than 0.45 mL/g) and a small pore size (basically ranging from 2 to 6 nm in pore diameter).12 Therefore, some researchers have made efforts to produce PB with a large pore volume by changing the reaction conditions. For example, Du Jianwei et al.13 prepared PB with a large surface area (331 m2/g) and a large pore volume (0.97 mL/g) using cyanuric acid as an expanding agent. Zeng Shuangqin et al.14 prepared PB with a larger pore volume (0.3-1.3 mL/g) by using a water-solution alkali with the raw Received: July 30, 2010 Accepted: January 14, 2011 Revised: January 3, 2011 Published: February 24, 2011 3889

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Figure 1. Schematic of the membrane-dispersion microstructured reactor and the experimental apparatus: (1) cylinder of the mixed gas; (2) pressure gauges; (3) gas flow meter; (4) microreactor; (5) membrane; (6) plunger pump; (7) pH meter; (8) paddle agitator; (9) vessel of NaAlO2 solution.

material of CO2 and a partial sodium aluminate solution. Yang Qinghe et al.11 contributed a method to prepare γ-Al2O3 with a large pore volume (1.00 mL/g) by keeping the pH below 9. According to relevant research, the average pore diameter and the pore volume should be larger than 15 nm and 1.2 mL/g, respectively, when PB is used as the support for hydrocracking and hydrotreatments; despite the value of these contributions, the PB products still could not meet these requirements. It is known that the pore properties of materials depend on the crystal size and the packing mode of the crystals. A small crystal diameter is suitable for forming many pores and increasing the specific area, and a suitable template can enlarge the pore diameter and the pore volume. In stirred tanks, the crystal diameter of PB was about 5 nm, which is too large; another issue is that in the reaction process, the gas can act as a template to enlarge the pore diameter and the pore volume, but in stirred tanks, the ratio of the gas volume and the liquid volume cannot reach a large value, so the pore volume is relatively small. The crystal size depends on the supersaturation of the aqueous solution, which is related to the mass transfer rate; the average diameter of bubbles in the tank is several millimeters, so the mass transfer specific surface area is low, which is not conducive to forming tiny crystals. A further disadvantage of stirred tanks is that the distribution of gas concentration is uneven from the

bottom of the tank to the top. When the reactor is very high, which could cause reaction 4 to occur in advance at the bottom when reactions 2 and 3 have not yet been completed, the products prepared in the tank always contain a small quantity of Dawsonite. In this work, a microreactor was designed and used to generate PB powder with a large pore volume and a pore diameter. The design process and apparatus is shown in Figure 1a. The gas was dispersed into the microchannel through a microporous membrane which was laid on the microchannels; the liquid quickly flowed through the microchannels and sheared the gas from the surface of the membrane. Through this special design, a large ratio of gas volume to liquid volume was reached. The neutralization reaction is instantaneous, and the reaction takes place at the interface between the gas phase and the liquid phase; the nanoparticles were formed along the bubbles’ surface. When the gas bubbles were just emerging from the membrane surface, reaction 2 was simultaneously started; the nanometer or submicrometer bubbles could act as the templates, which could improve the pore volume of products. In addition, a high mass transfer efficiency based on the microscale gas-liquid contact was obtained, which helped make the PB crystal size small. Some previous studies have been performed on preparing nanoparticles using such a device.15-17 Wang et al. studied the mixing and 3890

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Figure 2. The SEM image of porous membrane made of stainless steel fibers.

mass transfer progress and prepared CaCO3 nanoparticles with average diameters ranging from 34.3 to 110 nm.15 That study demonstrated that such a microreactor possessed a very high mixing efficiency and produced a high supersaturation. In this study, the final pH of the aqueous phase had a significant effect on the crystal form, and the concentration of the aqueous phase influenced the supersaturation; in addition, the flow rate and the concentration of the gas phase had a large effect on the mixing performance, supersaturation, and the amount of template. The influences of these factors on the pore properties of products were investigated.

2. MATERIALS AND METHODS 2.1. Experimental Device and Process. The experimental apparatus is shown in Figure 1a. A membrane dispersion microreactor device was fabricated out of poly(tetrafluoroethene) (PTFE) sample plates (30 mm  30 mm  0.5/2 mm) using a high-pressure thermal sealing technique. The PEFT sample plates were used to support or secure the membrane dispersion microreaction device. The active area of the microporous membrane (as shown in Figure 2) with pore size of about 5 μm was 19 mm2, and the geometric size of the microchannel was 10 mm  2 mm  2 mm (length  width  height). The raw materials used were mainly sodium metaaluminate (Guangfu Fine Chemical Research Institute, China), carbon dioxide, and ammonium nitrate (Beijing Modern Eastern Fine Chemical, China); only sodium metaaluminate was filtered out before using. A mixture of CO2 and N2 passed through the membrane before the sodium metaaluminate solution (continuous phase) entered the channel. A special apparatus was used to maintain the gas pressure of 0.9 MPa. The gas phase and the liquid phase were mixed and reacted in the membrane dispersion microreactor. The operation procedure is as follows: the aqueous phase which first flowed through the channel did not mix with the original sodium metaaluminate solution; after that, the circulation of the NaAlO2 solution was started. The volume of NaAlO2 aqueous phase was 0.5 L. The reaction occurred at the temperature of 25 °C, and the pH was recorded during the whole reaction process. After the pH reached a certain value, the reaction was stopped, and the product was aged for 1 h at a temperature of 70 °C and then

Figure 3. XRD results of PB and Al2O3 (with a flow rate of the aqueous phase of 0.34 L/min, an aqueous phase volume of 0.5 L, an NaAlO2 initial concentration of0.2 mol/L, a flow rate of the gas phase of 0.5 L/ min, and a CO2 concentration of 30%): (a) XRD spectrum of Bayerite with a final pH of 11.5. (b) XRD spectra of products prepared at various final pHs. (c) XRD spectra of Al2O3 obtained through the calcinations of PB.

washed by an ammonium nitrate concentration of 0.01% (amount of substance fraction). After that, it was leached out and dried at 70 °C for 10 h. After the calcination of the PB at 3891

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Figure 4. SEM morphology of products prepared at various final pHs (with a flow rate of the aqueous phase of 0.34 L/min, an NaAlO2 initial concentration of 0.2 mol/L, a flow rate of the gas phase of 0.5 L/min, and a CO2 concentration of 30%).

550 °C for 4 h in a muffle furnace, the microporous γ-Al2O3 could be obtained. 2.2. Characterization. To identify the crystal form, powder X-ray diffraction (XRD) patterns were recorded using the TTR with Cu KR (45 kV and 200 mA) between 10° and 70° at a scan speed of 10° min-1. In addition, the specific surface area was obtained by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the desorption branch of the isotherm measured by the Barrett-JoynerHalenda (BJH) method. The morphologies of particles and membrane were observed using transmission electron microscopy (TEM, JEM-2010 type) and scanning electron microscopy (SEM, JEM-6301F Japan).

3. RESULTS AND DISCUSSION 3.1. Effect of the Final pH of Solution on Crystal Form and Pore Characteristics. The neutralization reaction of NaAlO2

was a pH-decreasing process with the addition of the CO2 gas. The effect of the final pH of the solution on the crystal forms of the products is shown in Figure 3. The final pH of the aqueous solution had a significant effect on the crystal forms of the products. When the final pH of the aqueous solution was 11.5, the crystal form of the product was Bayerite, when the final pH was 10.8, it was a mixture of Bayerite and PB, and when the final pH was 10.5 or lower, the crystal form was pure PB. After the calcination of PB, pure γ-Al2O3 was obtained. The morphology of PB was observed by using TEM and SEM, as shown in Figure 4

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Figure 5. TEM morphology of products prepared at different final pHs (with a flow rate of the aqueous phase of 0.34 L/min, an NaAlO2 initial concentration of 0.2 mol/L, a flow rate of the gas phase of 0.5 L/min, and a CO2 concentration of 30%).

and Figure 5. As shown in Figure 4a and Figure 5a, when the final pH was 11.5, the particles of Bayerite were large and there were no pores, but when the final pH was dropped to 10.5, there were some pores, as shown in Figure 4b and Figure 5b. With the further decrease of the final pH to 10.5, the crystals of PB were fiber-like and the morphology was porous. Thus, the morphology and crystal form of aluminum hydrate were significantly influenced by the final pH of the solution. Table 1 shows that the final pH also had a great effect on pore properties. With the decrease of the pH, the pore volume and the average pore size of the PB increased, but the surface area first increased and then decreased. Just as mentioned in the Introduction, when the pH of the solution was low, the mass transfer resistance was large, and most of the CO2 added did not react with the aqueous solution, so the residual CO2 and N2 could act as a template for the PB. Table 1 also indicates that when the final pH was 9.4, the pore volume and the pore diameter was the largest, and after the calcinations, the pore volume and the specific surface area of the γ-Al2O3 reached 1.61 mg/g and 359 m2/g, respectively. 3.2. pH Change Curve in the Neutralization Process. Figure 6 shows that the neutralization of NaAlO2 solution with CO2 is a process in which the pH decreases. At first, the neutralization time in the minireactor was much shorter than that in the stirred tank. When the gas flow was 0.51 L/min, the neutralization time was about 6.5 min and the final pH was 9.79, while when Yang et al.18 measured the pH change with the time in the stirred tank, they found that the carbonization took 18 min to make the pH drop from 13.5 to 10.8; especially when the pH was lower than 11.8, the pH dropped very slowly, because the 3892

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Table 1. Effect of Final pH on the Crystal Form and Pore Properties (with a flow rate of the aqueous phase of 0.34 L/min, an initial NaAlO2 concentration of 0.2 mol/L, a flow rate of the gas phase of 0.5 L/min, and a CO2 concentration of 30% (v/v)) final pH crystal form

S (m2/g)

V (mL/g)

D (nm)

CS (nm)

SC (m2/g)

crystal form

VC (mL/g)

DC (nm)

CSC (nm)

11.5

B

121

0.16

4.8

63.9

η-Al2O3

538

0.41

3.0

5.1

10.8 10.5

BþPB PB

298 322

0.47 0.67

6.3 8.4

45.4 2.8

η-Al2O3 γ-Al2O3

427 384

0.44 0.77

4.1 4.0

4.4 2.5

10.3

PB

365

1.15

12.6

2.9

γ-Al2O3

331

0.78

9.5

2.4

9.9

PB

382

1.38

14.5

2.9

γ-Al2O3

302

1.57

20.8

2.3

9.4

PB

291

1.45

20.0

2.9

γ-Al2O3

359

1.61

17.9

3.2

Table 2. Effect of the Flow Rate of the Gas Phase on Pore Properties of PB (with a flow rate of the aqueous phase of 0.51 L/min, an NaAlO2 initial concentration of 0.2 mol/L, and a CO2 concentration of 30% (v/v)) gas flow final crystal (L/min) pH form 0.23 0.45 0.51 0.91 1.14

9.73 9.74 9.79 9.76 9.73

PB PB PB PB PB

S (m2/g)

V (mL/g)

D (nm)

CS (nm)

T (min)

201.6 259.3 548.5 403.4 175.2

0.87 1.06 2.22 1.73 0.77

17.2 16.4 16.2 17.0 17.5

2.5 2.4 2.3 2.8 2.5

14.00 7.16 6.50 7.00 5.25

Table 3. Effect of the CO2 Concentration on the Pore Properties of PB (with a flow rate of the aqueous phase of 0.51 L/min, an NaAlO2 initial concentration of 0.2 mol/L, and a flow rate of the gas phase of 0.51 L/min) CO2

Figure 6. pH change curves during the neutralization process (with a flow rate of the aqueous phase of 0.51 L/min, an NaAlO2 initial concentration of 0.2 mol/L, and a CO2 concentration of 30%).

mass transfer resistance became increasingly great with the decrease in pH. Stirred tanks could not generate small crystals; the intensification of mass transfer is necessary. According to our previous research work,15 in this microreactor, gas bubbles with an average diameter of about 0.9 mm were produced by the strong shear flowing of the continuous phase of NaAlO2 solution, and effective mixing was achieved due to the strong turbulence. Since the height of the mixing chamber is only 2 mm, the diffusion distance was short, and the mixing was very homogeneous. These factors are very important for producing pure PB. In addition, using the membrane-dispersion microstructured reactor, a short nucleation period could be obtained by increasing the ratio of the flow rate of gas to the flow rate of liquid, which made the PB nuclei smaller. The smaller the grain size, the larger the specific surface area.19 Table 1 shows that the crystallite sizes of PB were about 2.9 nm; however, the crystallite sizes of PB prepared using stirred tanks were about 5.8 nm.20 3.3. Effect of Flow Rate of the Gas Phase on Pore Properties. Table 2 indicates that the flow rate of CO2 had a great effect on the nucleation time and the growth period. It can be assumed that with an increase in the CO2 flow rate, the neutralization time becomes shorter. The short nucleation time caused the generation of fine particles, which was suitable for forming large pore diameter and large pore volume. In Table 2, the results show that when the suitable gas flow was 0.51 L/min, the specific surface area, the pore volume, and the average pore diameter of PB produced reached 548.5 m2/g, 2.22 mL/g, and 16.2 nm,

final pH concn, %

crystal form

S (m2/g) V (mL/g) D (nm) CS (nm)

9.79

30

PB

548.5

2.22

16.2

2.3

9.70

50

PB

237.5

1.05

17.7

2.5

9.60

70

PB

359.6

1.77

19.9

2.7

9.40

100

PB

237.9

1.14

19.1

2.3

respectively. The specific surface area was larger than that found in the PB prepared by Du Jianwei et al.12 (331 m2/g), Zeng Shuangqin et al.13 (350 m2/g), and Qinghe Yang et al.14 (272 m2/g); the pore volume also was larger than that found in the PB prepared in those studies: 0.97 mL/g, 1.3 mL/g, and 1.00 mL/g, respectively. In addition, Table 3also shows that when the final pH levels were almost the same, the pore diameter did not change significantly under varied flow rates; under this condition, the specific area was relevant to the pore volume. When the flow rate of CO2 increased, the diameter of the minibubbles formed at the membrane surface increased; the CO2 and N2 gas could have acted as a pore template, causing the pore volume to become enlarged. When the flow rate of CO2 was 0.51 L/min, the pore volume reached 2.22 mL/g. But when the flow rate of CO2 was further increased, the bubbles were aggregated together, which caused the collapse of pores, so the pore volume decreased. 3.4. Effect of the CO2 Concentration on Pore Properties. The effect of the CO2 concentration on the PB pore properties is shown in Table 3. Because the gas could act as a template, with the increase of the CO2concentration, less N2 was introduced into the aqueous phase, and the amount of template was decreased; thus, the pore volume became smaller. The most suitable concentration of CO2 was 30%. 3893

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Table 4. Effect of the NaAlO2 Initial Concentration on the Pore Properties of PB (with a flow rate of the aqueous phase of 0.51 L/min, a flow rate of the gas phase of 0.51 L/min, and a CO2 concentration of 30% (v/v)) NaAlO2 concn (mol/L)

final pH

0.1

9.82

0.2

9.79

0.3 0.4

crystal form

S (m2/g)

V (mL/g)

D (nm)

PB

317.1

1.30

16.4

PB

548.5

2.22

16.2

9.80

PB

310.1

1.38

17.9

9.70

PB

358.3

1.45

16.2

3.5. Effect of the Concentration of NaAlO2 Solution on Pore Properties. The effect of concentration of NaAlO2 solu-

tion on the pore properties is shown in Table 4. When the concentration of NaAlO2 was 0.1 mol/L, the amount of NaAlO2 was not sufficient to form a wall, so the pore volume was low. When the concentration was increased to 0.2 mol/L, the pore volume increased to 2.22 mL/g. But when the concentration was too high, the wall thickness of pores increased, resulting in a decrease of pore volume. Thus, the suitable concentration of NaAlO2 was 0.2 mol/L.

3. CONCLUSION In this paper, PB (pseudoboehmite) was prepared through the reaction of CO2 and an aqueous solution of NaAlO2 in a membrane-dispersion microstructured reactor. When the final pH of the aqueous solution was between 10.5 and 9.4, pure PB could be obtained, the crystal size of the PB was about 2 nm, and the specific surface area, the pore volume, and the average pore diameter of PB obtained were 548.5 m2/g, 2.22 mL/g, and 16.2 nm, respectively, under optimal conditions. These values were much higher than those prepared in the stirred tank. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 08610-62770304. Phone: 08610-62773017.

’ ACKNOWLEDGMENT We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (20976096, 20576058, and 21036002) and by the National Basic Research Program (973 plan, no. 2007CB714302). ’ NOMENCLATURE B = Bayerite CS = crystallite size [nm] CSC = crystallite size of products after calcinations [nm] D = average pore diameter of PB [nm] DC = average pore diameter of products after calcinations [nm] g = gas phase l = liquid phase PB = pseudoboehmite s = solid phase S = surface area [m2/g] SC = surface area of products after calcinations [m2/g] T = time [min] V = pore volume of PB [mL/g] VC = pore volume of PB after calcinations [mL/g]

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