Peptization Mechanism of Boehmite and Its Effect on the Preparation

May 25, 2014 - Among them, the acid/alumina molar ratio ([HCl]/[Al2O3]), ζ potential, and ..... We acknowledge National “Twelfth Five-Year” Plan ...
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Peptization Mechanism of Boehmite and Its Effect on the Preparation of a Fluid Catalytic Cracking Catalyst Yongsheng Zheng,† Jiaqing Song,*,† Xiangyu Xu,† Mingyuan He,‡ Qian Wang,§ and Lijun Yan§ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200241, China § Petrochemical Research Institute of Petrochina, Beijing 100195, China ‡

ABSTRACT: A peptization mechanism of boehmite was proposed in this work. The effects of peptization on the properties and performances of a fluid catalytic cracking (FCC) catalyst were explored. A boehmite colloid was prepared through peptization of boehmite with hydrochloric acid, and its particle size distribution was characterized during peptization. With an increase of the acid/alumina molar ratio from 0 to 0.11, the particle size of the boehmite colloid decreased to 3.5 nm. The particle size increased from 3.5 to 11 nm when the acid/alumina molar ratio was further increased to 0.16 and remained at 10 nm thereafter. The smallest dispersed boehmite particles were obtained in an acid/alumina molar ratio of 0.11. On the basis of many experiments, the dispersion and coalition mechanisms of boehmite during acid peptization were proposed. Boehmite particles absorb protons on the surface hydroxyl groups and repel each other to form colloidal particles when the acid/aluminia ratio is low. With more hydrochloric acid added, large amounts of chloride anions cause compression of the boehmite diffuse layer, thus resulting in boehmite coalition. A FCC catalyst was prepared by peptizing boehmite with an acid/alumina molar ratio of 0.11. The catalyst has a larger external surface area, a higher mesoporous volume, and better acidity distribution than the catalyst prepared with boehmite. Both conversion of residue oil and yields of diesel and gasoline over a peptized catalyst are higher than those over the catalyst without being peptized.

1. INTRODUCTION

In this paper, boehmite colloids were prepared by peptization of boehmite with hydrochloric acid under various conditions. A peptization mechanism of boehmite, including dispersion and coalition, was proposed. In addition, a FCC catalyst was prepared with peptized boehmite. The effects of boehmite peptization on the preparation and performance of a FCC catalyst were discussed based on the fix-fluidized bed, nitrogen adsorption−desorption characterizations, and Fourier transform infrared (FTIR) measurements.

Colloidal boehmite prepared by peptization of boehmite with mineral acid has been widely used for the fabrication of ceramic powders,1−4 hollow microspheres,5 porous membranes,6 dispersants,7,8 and photoluminescent and mesoporous alumina,9,10 pollution control,11 separation technology,12,13 paste,14 and so on. As a matrix and binder of a fluid catalytic cracking (FCC) catalyst, peptized boehmite can promote its catalytic activity for gas oil and residual oil cracking.15,16 In 2008, the catalytic cracking capacity was up to 600 million tons/year, which is about 23% of the total refining capacity in the petroleum industry.17 With increasing tension over and inferior quality of oil resources, the residual oil cracking ability of FCC catalysts needs to be improved a great deal. Currently, residual oil with a dynamic molecular diameter of 1−2 nm and a boiling point above 500 °C cannot be cracked in zeolite pores, so a FCC catalyst with larger surface area and better pore volume distribution is required. Because the properties of peptized boehmite in a spray dryer slurry of a FCC catalyst are difficult to observe, the mechanism of boehmite peptization has been studied individually by various parameters such as the viscosity,18,19 particle size distribution,20,21 acid ratio,22 phase diagram,23 anion type, anion concentration, alumina concentration, peptization time and temperature,24 dispersion,25 surface charges, sedimentation,26,27 nature of the colloid boehmite by NMR,28 etc. Among them, the acid/alumina molar ratio ([HCl]/[Al2O3]), ζ potential, and particle size distribution are the most widely used to study the properties of colloidal boehmite because of the simplicity and intuition of these methods. © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals used in this work, including hydrochloric acid (HCl; 36−38 wt %, AR), potassium chloride (AR), and ammonium dihydrogen phosphate (AR), were purchased from commercial vendors without further purification. Other chemicals, including zeolite REY, zeolite USY, alumina sol, and kaolin clay, were purchased from a commercial vendor (Shanxi Tengmao Technology Ltd.). The particle size of zeolites and kaolin clay is about 1 μm. The surface area and pore volume of boehmite (Shandong Aluminum Industry Co., Ltd.) are 332.92 m2/g and 0.29 cm3/g. 2.2. Boehmite Peptization Process. In a typical experiment, 7.74 g of boehmite (64.64 wt %) was mixed with 41.30 g of deionized water, and 0.97 g of HCl (18.56 wt %) was added dropwise to the solution under vigorous stirring at room temperature. The maximum acid/aluminum molar ratio was Received: Revised: Accepted: Published: 10029

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boehmite. The t-plot external surface area was measured by a TriStar II 3020 isothermal adsorption instrument. The primary particle size was calculated according to the following equation: R = 3000/Sρ, where R is the primary particle size of boehmite, S is the t-plot external surface area, and ρ is the unit cell density of boehmite, which is 3.014 g/cm3 here.

kept below 0.3. The pH of the mixture was then measured with a pH meter. The mixture was then centrifuged, and the particle size of the supernatant colloid was analyzed on a laser particle analyzer. 2.3. Catalyst Preparation. The FCC catalyst A was prepared with peptized boehmite. Briefly, zeolite REY, zeolite USY, peptized boehmite (PB), alumina sol (A), kaolin clay (KC), and deionized water were mixed and stirred for 30 min. The dry weight ratios of each reactant are shown in Table 1,

3. RESULTS AND DISCUSSION 3.1. Dispersion of Boehmite. The diffusion rate of protons into particles (Rp) and in the solution (Rs) and the adsorption rate of protons on the surface of boehmite (Ra) were defined as shown in Figure 1. For convenience, the

Table 1. Dry Weight Ratios of the Materials Mixed dry weight ratio (wt %)

REY

USY

PB

A

KC

18

10

18

3

51

and the solid content was 40 wt %. Under stirring, a certain amount of HCl (18.56 wt %) was dropwise added to the mixture and resulted in a [HCl]/[Al2O3] molar ratio of 0.11. The mixture was then spray-dried, calcined at 540 °C for 2 h, ion-exchanged by ammonium dihydrogen (5 wt %) phosphate, and hydrothermally aged for 17 h. Catalyst B was prepared with the same procedure except that boehmite instead of peptized boehmite was used. 2.4. Characterizations. The particle size and ζ potential were tested on a Zetasizer Nano ZS 90 laser particle analyzer (Malvern Instruments Ltd.). The pH values were measured with a Sartoruis PB-10 pH meter (Sartorius Stedim Biotech GmbH). Transmission electron microscopy (TEM) studies were carried out on a JEM-2100 electron microscope. Nitrogen adsorption−desorption characterizations were done by a TriStar II 3020 isothermal adsorption instrument. The parameters of a fixed fluidized bed are listed in Table 2.

Figure 1. Scheme for the diffusion and adsorption rates of protons.

magnitudes of Rp and Rs are only determined by the concentration of protons in solution, and the magnitude of Ra is controlled by the concentration of protons and the amounts of boehmite surface hydroxyl groups. Boehmite colloids were prepared by adding HCl into the boehmite solution either under stirring or static. As shown in Table 3, the way of adding HCl had no effect on the particle size of boehmite. This might be caused by the fact that Rs is much faster than Rp and Ra.

Table 2. Parameters of a Fixed Fluidized Bed item

parameter

feedstock oil catalyst weight (g) temperature (°C) catalyst/oil weight ratio WHSVc (h−1) material balanced (%)

Daqing FCC feedstock oil (ARa) + 45VRb 200 460−530 6 or 8 15 95−105

Table 3. Effect of the Ways of Adding HCl on the Particle Size of Boehmite Colloids

Atmospheric residue. bVacuum residue. cWeight hourly space velocity = feed speed (g/h)/catalyst weight (g). dMaterial balance = total product weight/feedstock weight.

acid alumina molar ratio

particle size (stirring)/ nm

particle size (static)/ nm

0.01 0.04

6.5 31.1

6.6 30.5

FTIR analysis was carried out using pyridine as the probe molecule. About 16 mg of a solid sample was finely ground, tableted, and heated to 450 °C, followed by evacuation under ca. 10−3 Pa for 2 h. The sample was then allowed to absorb pyridine at 90 °C for 30 min, evacuated for 30 min at 200 °C, and cooled to room temperature before FTIR analysis on a Nicolet 6700 Fourier transform spectrometer. 2.5. Primary Particle Size Measurement. The primary particle size was determined using the X-ray diffraction (XRD) method and t-plot external surface area. XRD measurement was conducted on a Bruker D8 Advance diffractometer with Cu Kα radiation. The Scherrer equation R = 0.45λ/(β cos θ) was used to calculate the primary particle size, where R is the primary particle size of boehmite, λ is the X-ray wavelength, which is equal to 0.154 nm, β is the width of the peak [full width at halfmaximum (fwhm)] after correcting for the instrumental peak’s broadening, and θ is the Bragg angle of the (020) peak for

Boehmite particles are the secondary particles that consist of numerous primary particles. In order to illustrate the dispersion of boehmite particles, the term of dispersed particles is defined as the boehmite particles dispersed. TEM images of the boehmite particles in the supernatant colloid before and after peptization are shown in Figure 2. Before the acid was added, the secondary particles aggregated together and showed a tight structure (Figure 2a). The boehmite particles were dispersed from the secondary particles to form small dispersed particles when the acid/alumina molar ratio increased to 0.02 and 0.11 (Figure 2b,c). To further explore the relationship between the boehmite particle dispersion and acid/alumina molar ratio, various amounts of HCl were added and size distributions of the dispersed particles were analyzed. Also, the dispersion of the boehmite particles affected by ammonium dihydrogen phosphate was investigated to understand the influence of the surface hydroxyl groups.

a

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Figure 2. TEM pictures of colloid particles at acid/alumina molar ratios of (a) 0, (b) 0.02, and (c) 0.11.

of 0.11. The ζ-potential measurement shows that the repulse force between boehmite particles increased when the acid/ alumina molar ratio was increased from 0 to 0.07. On the basis of these results, it can be determined that dispersion of supernatant colloidal particles happened in stage 1. A possible explanation of this boehmite particle dispersion is shown in Figure 5. With HCl added, the secondary boehmite particles

Figure 4 shows the effect of the acid/alumina molar ratio on the particle size distribution. The size change of the dispersed

Figure 3. XRD pattern of boehmite. Figure 5. Scheme for boehmite peptization from (A) a secondary boemite particle to (B−D) dispersed boehmite particles.

absorb protons and repulse each other because of electrostatic repulsion force. The higher the acid/alumina molar ratio is, the smaller the dispersed particles are. The absorption of protons on the particle surface causes a decreased proton concentration inside the dispersed or boehmite particle, which resulted in decreasing Rp and Ra. Rp and Ra determine the uniformity and size of the dispersed particles, respectively. The influence of dihydrogen phosphate ions on the particle size was investigated by adding ammonium dihydrogen phosphate to the boehmite solution before peptization. The phosphorus/aluminum molar ratios varied from 0 to 0.1, and the acid/aluminum molar ratio remained at 0.11. The particle sizes in the phosphorus/aluminum molar ratio range are listed in Table 4. Possible reactions occurred as follows when

Figure 4. Effect of the acid/alumina molar ratio on the particle size distribution and ζ potential by a laser particle analyzer.

particles went through three stages with increasing acid/ alumina molar ratio. In stage 1, where the acid/alumina molar ratio was increased from 0 to 0.11, the particle size decreased dramatically to 3.5 nm. According to XRD analysis (Figure 3) and the t-plot external surface area method, the primary particle sizes of boehmite are 3.99 nm (β = 0.018 rad; θ = 14.19°) and 2.99 nm (the t-plot surface area of boehmite is 332.92 m2/g and pore volume is 0.29 cm3/g), respectively, indicating that the boehmite secondary particles were dispersed to single primary particles. In stage 2, where the acid/alumina molar ratio was increased from 0.11 to 0.16, the particle size increased from 3.5 to 11 nm. When the acid/alumina molar ratio was further increased in stage 3, the particle size remained at 10 nm. The smallest particle size was found in an acid/alumina molar ratio

Table 4. Particle Size of Boehmite with Ammonium Dihydrogen Phosphate at an Acid/Alumina Ratio of 0.11 and Without Ammonium Dihydrogen Phosphate at Various Acid/Alumina Ratios

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phosphorus/aluminum ratio

particle size/ nm

acid/aluminum ratio

particle size/ nm

0 0.05 0.06 0.08 0.10

3.5 15.2 27.8 63.2 391.9

0.11 0.0125 0.01 0.005 0

3.5 16.2 30.5 49.7 396.6

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compresses and the interface of the diffusion and compact layers reduces to the original compact layer.29 The electrostatic repulsion force of dispersed particles is consequently reduced. The Brownian movement of the particles dominates their moving behaviors; thus, the dispersed particles collide with each other and aggregate. Although the particle size increased from 4.9 to 3.5 nm with increasing acid/alumina ratio from 0.07 to 0.11, the ζ potential decreased from 49.1 to 44.3 mV because of an increase of the chloride ion concentration (Figure 4). The further increasing chloride ion concentration causes an increase of the particle size and a decrease of the ζ potential, and eventually the colloidal solution gelled with acid/alumina molar ratios higher than 0.15. The ζ potential could not be tested in situ at this point. 3.3. Effect of Peptization on the Preparation of a FCC Catalyst. As discussed above, the process of peptization includes two stages, e.g., dispersion and coalition. However, how peptization affects the preparation of a FCC catalyst is still unclear. Because the peptization abilities of different boehmites vary depending on the properties of boehmite,30 only general effects of boehmite peptization on a FCC catalyst will be discussed in this work. Catalyst A was prepared by peptization of boehmite with a [HCl]/[Al2O3] ratio of 0.11 in the smallest particle distribution. Catalyst B was prepared without peptization. The performances of a FCC catalyst were evaluated via nitrogen adsorption−desorption, FTIR analysis, and characterization of a fixed fluidized bed. The performances of catalysts A and B were tested with a catalyst/oil weight ratio of 6 at 490 °C. The product yields are listed in Table 7. It can be seen that catalyst A gives higher

ammonium dihydrogen phosphate was added to the boehmite solution: AlOH + H2PO4− = AlHPO4− + H2O. It can be concluded that the outer surface hydroxyl groups were partially covered by dihydrogen phosphate ions, which could inhibit the absorption of protons to the outer surface of boehmite particles thereafter. Therefore, Rp increased, Ra decreased, and the dispersed particles became larger and more uniform. When the repulse force between adsorbed protons was insufficient to break boehmite particles, the particle size of boehmite remained 391.9 nm. In full, the particle size of dispersed boehmite was determined by the acid/alumina ratio and surface hydroxyl group distribution. 3.2. Coalition of Boehmite. Coalition of boehmite happened in stage 2 (Figure 4), where the size particles increased slightly with an increase of the acid/alumina molar ratio. To explore the underlying mechanism of this coalition, the following two experiments were conducted. Colloid prepared with a fixed acid/alumina molar ratio of 0.15 was gradually diluted with deionized water. As can be seen in Table 5, the particle size and ζ potential remained steady Table 5. Particle Size and ζ Potential of Diluted Supernatant Colloids at Different pH Values pH value

particle size/nm

ζ potential/mV

1.59 1.84 1.99 2.27 2.57

5.3 5.6 6.4 6.0 6.7

33.8 36.2 38.0 46.1 47.8

when the pH was changed from 1.57 to 2.57. This is caused by the steadiness of the protons absorbed on the boehmite particle surface. Chloride ions were added to the boehmite solution to get chlorine/alumina molar ratios of 0.15 and 0.2 in the colloid. The acid/alumina molar ratio was kept at 0.1. As shown in Table 6, the paricle size increased with an increase of the

Table 7. Product Yields of Catalysts A and B at WHSV of 15, Catalyst/Oil Weight Ratio of 6, and Temperature of 490 °C

Table 6. Particle Size of Boehmite at Chlorine/Alumina Molar Ratios of 0.15 and 0.2 and Acid/Alumina Molar Ratios of 0.15 and 0.2 a

chlorine/alumina ratio

particle size/ nm

acid/alumina molar ratio

particle size/ nm

0.15 0.2

6.9 9.9

0.15 0.2

6.4 11.0

product

catalyst A yield (wt %)

catalyst B yield (wt %)

dry gas (wt %) LPG (wt %) gasoline (wt %) diesel (wt %) heavy oil (wt %) coke (wt %) conversiona (wt %)

1.46 9.31 46.34 26.02 9.50 7.37 64.48

2.04 12.25 42.44 23.91 10.28 9.08 65.82

Conversion = dry gas + LPG + gasoline + diesel + coke.

yields of high-value products, including gasoline and diesel, than catalyst B. On the contrary, catalyst A produces fewer lowvalue products, such as dry gas, LPG, coke, and heavy oil, than catalyst B. The performances of catalysts A and B were tested with catalyst/oil weight ratios of 6 and 8 at various temperatures, as shown in Figure 7. It is clear that the yields of gasoline and diesel over catalyst A are higher than those over catalyst B at the same conversion rate. These results indicate that more feedstock is converted into valuable product and

chloride ion concentration. This indicates that chloride ions instead of protons contributed to the size increasing in stages 2 and 3. On the basis of these results, a possible mechanism of aggregation of the dispersed boehmite particle in stages 2 and 3 is proposed, as shown in Figure 6. In an environment with a high amount of anions, the diffuse layer of boehmite

Figure 6. Influence of the anions on coalition of boehmite. 10032

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Figure 7. Fixed fluidized bed results of catalysts A and B.

Figure 8. Pore size distributions of catalysts A and B.

better product distribution is acquired using catalyst A at the same WHSV values. As mentioned above, the diameter of residual oil is between 1 and 2 nm. According to the theory proposed by Spry and Sawyer, the desired diameter of the catalytic sites should be about 10−20 times the size of the diffusing molecules. Thus, a large external surface area and a pore volume of about 10−40 nm are required to get the desired conversion rate of residual oil and high yields of diesel and gasoline.17 The BET and t-plot results of catalysts A and B are listed in Table 8. Catalyst A has almost the same total surface, total pore Table 8. BET and t-Plot Results of Catalysts A and B catalyst A catalyst B BET (g/m2) t-plot micropore surface area (g/m2) t-plot external surface area (g/m2) single-point adsorption total pore volume of the pores (cm3/g) t-plot micropore volume (cm3/g) BJH adsorption cumulative volume of the pores between 17.000 and 3000.000 Å diameter (cm3/g)

103.69 24.21 79.49 0.18

102.00 44.76 57.24 0.17

0.005 0.16

0.014 0.16

Figure 9. IR spectra of pyridine-adsorbed catalysts A and B.

4. CONCLUSIONS Boehmite was subjected to dispersion and coalition during peptization with HCl. The mechanism of these phenomena was elucidated through a series of simple experiments. The boehmite particles were dispersed by the electrostatic repulsion between the protons on their surface absorbing to the surface hydroxyl groups. When a high acid/alumina ratio is used, the high concentration of anion from the acid can cause boehmite particle aggregation. A FCC catalyst prepared with peptized boehmite has a larger external surface area, a mesoporous volume, and a better zeolite acidity distribution, which can improve the residue oil conversion and yields of gasoline and diesel.

volume, and mesoporous volume as catalyst B. However, catalyst A has a smaller micropore surface area and thus a larger external surface area. Therefore, the zeolite activity in catalyst A is lower than that in catalyst B. The high zeolite activity could result in high coke and LPG yields, so the influences of HCl to zeolite are effective to produce high-value products. The mesoporous volume distributions of catalysts A and B are shown in Figure 8. The mesoporous volume of catalyst A with sizes ranging from 4 to 40 nm is twice higher than that of catalyst B in the same size range. It can be explained that dispersed boehmite particles in catalyst A form stacking pores with other materials, while the bulk boehmite particles in catalyst B sintered after hydrothermal aging, which could be attributed to the cracking of residual oil. As illustrated in Figure 9, the bands attributed to pyridine molecules coordinated to Lewis acidity sites and pyridinium ions formed by protonation of pyridine on Bronsted acidity sites are observed at 1450 and 1540 cm−1, respectively.31 Thus, it can be confirmed that both Lewis and Bronsted acidity sites are diminished because of dealumination of zeolite after peptization of catalyst A using HCl. Peptization gives a better acidity distribution, which reduces the formation of coke and cracking in gasoline and diesel products.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We acknowledge National “Twelfth Five-Year” Plan for Science & Technology Support (Grant 2012BAE05B00). REFERENCES

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