Zeolite Beta Precursors as Building Units toward Enhancing the

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Kinetics, Catalysis, and Reaction Engineering

Zeolite Beta Precursors as Building Units towards Enhancing Microporosity Fraction of Mesoporous Aluminosilicates Dekun Ji, Baojie Wang, Ting Yue, Xiaoping Li, Xiaotong Mi, Honghai Liu, Hongtao Liu, and Xionghou Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01821 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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Industrial & Engineering Chemistry Research

Zeolite Beta Precursors as Building Units towards Enhancing Microporosity Fraction of Mesoporous Aluminosilicates Dekun Ji,b Baojie Wang,c Ting Yue,a Xiaoping Li,a Xiaotong Mi,a Honghai Liu,c Hongtao Liu,a* Xionghou Gaoc a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China b

Beijing Institute of Petrochemical Technology, Beijing 102617, P. R. China

c

Petrochemical Research Institute, Petrochina Company Limited, Beijing, 100195, P. R. China

Corresponding author: [email protected]. 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Our group has reported the synthesis of hydrothermally stable mesoporous aluminosilicates (MAs) by self-assembly of triblock copolymers and zeolite Y precursors. However, degradation of zeolite Y precursors in acidic synthesis system will inevitably lead to low microporosity fraction of final products. Development of a facile route to obtain MAs simultaneously with high hydrothermal stability and microporosity fraction is still a great challenge. Herein, we employ zeolite Beta precursors as building units to solve this problem. In this strategy, zeolite Beta precursors were more stable than zeolite Y precursors in acidic synthesis system due to the presence of organic directing agent TEAOH, which would result in more microporosity implanted into the framework of MAs. By fine-turning the assembly time, the microporosity fraction could be adjusted in a range of 38.7%-52.9%. In addition, the synthesized MAs maintain well-resolved crystal patterns, high surface area and pore volume even after a severe hydrothermal treatment. Key words: Beta precursors; mesoporous aluminosilicates; microporosity fraction; hydrothermal stability

2


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INTRODUCTION Much attention is being given to fluid catalytic cracking (FCC) of heavy oil due to the processing difficulty of bulky molecules. 1-4 As we know, heavy oil cracking was carried out in FCC catalysts in a successive way: (1) The large molecules of heavy oil precrack in macrosopores of carriers, such as kaolin; (2) intermediate products of step 1 cracked in mesopores to moderate molecules; (3) Moderate molecules of step 2 cracked selectively to small molecules, such as gasoline and diesel. Therefore, it is still a great challenge to introduce mesoporosity into FCC catalysts. Tremendous effort has been made to grafting mesopores into catalysts. A typical way is combining mesoporous aluminosilicates with zeolites,5-8 and implanting the intracrystalline mesoporosity within the crystals of zeolite.9-13 However, phase separation is inevitable in this synthesis process. “Top-down” and “bottom-up” methods are two main approach to introduce mesoporosity into zeolite.14,15 Unfortuanately, these methods still suffer from zeolite crystallinity loss and poor repeatability.16 On the basis of above investigations, specially designed templates have been developed to obtain mesopores directly.

17-23

However, the intrinsic poor

hydrothermal stability of these aluminosilicates impedes their industrial application. Assembly of zeolite precursors was used to enhance the hydrothermal stability of mesophase. 24,25 For example, the authors of this article have obtained hydrothermally stabile MAs by self-assembly of zeolite Y precursors. The resultant MAs exhibited a hydrothermal stability comparable to that of USY.26 Nevertheless, one key problem remains unresolved for MAs in our previous literatures. That is, the microporosity fraction (which is defined as proportion of microporosity surface area to total surface area) in these MAs is relatively lower (lower than 30%) than that of zeolite with intracrystal mesopores.26-28 Therefore, development of MAs simultaneously with excellent hydrothermal stability and high microporosity fraction, is highly desirable. From microscopic aspects, zeolite Y precursors preformed in alkaline medium would partly dissolve in the acidic synthesis system. Generally, microporosity fraction 3



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of final products arises from retaining ratio of the precursors. For this reason, it is expected that microporosity fraction could be improved by enhancing the stability of precursors in acidic medium. On this basis, our group were inspired to to improve the stability of precursors in acidic medium to enhance the microporosity fraction of final products. Li et al. 29 investigated the aluminosilicate nanoclusters using density functional theory (DFT) method and concluded that five-membered ring structure was more stable than six-membered ring structure. Moreover, the presence of organic template TEAOH in Beta precursor favor the enhancement of the stability of micro-framework in acidic medium. 30-32 In addition, it has been shown that the condensation degree reaches its maximum at the isoelectric point.33,34 On the other hand, it can be concluded that the isoelectric point of zeolite Beta is lower than that of zeolite Y. Consequently, zeolite Beta precursors would be more stable than zeolite Y precursors. Here, we report the obtaining of hydrothermally stable MAs via assembly of zeolite Beta precursors. Due to the stability of zeolite Beta precursors in acidic synthesis system, the microporosity fraction of final products was improved greatly.

1. EXPERIMENTAL 2.1. Materials Triblock copolymer P123 (EO20PO70EO20) with the MW 5800 was from Sigma-Aldrich Co. LLC. Sodium hydroxide (NaOH), sulfuric acid (H2SO4), and aluminum sulfate (Al2(SO4)318H2O) were from Fuchen Company (Tianjin). Tetraethyl ammonium hydroxide, TEAOH was obtained from Xilong Chemical Company. Water glass (containing 27.9 wt % SiO2 and 8.9 wt % Na2O) was purchased from Hongxing Sodium Silicate Company (Beijing). 2.2. Synthesis of MAs. Beta precursors were synthesized according to the literature.28 The mixture of Al2(SO4)3,

Na2SiO3,

TEAOH,

and

NaOH

with

a

composition

of

Al2O3/SiO2/Na2O/TEAOH/H2O=1.0/60/2.5/22/800 (molar ratio) was prepared. A sticky solution was obtained after an ageing at 140 °C for 4 h, denoted as “zeolite Beta precursor”. 4


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Synthesis of Beta precursors: 4.0 g of EO20PO70EO20 (Pluronic P123) and 5 mL 10 M HCl were dissolved in 150 mL H2O. 30 mL Beta precursors and HCl were dropwise added to keep pH at 0.75 under stirring. After that the mixture was assembled at 40 °C for 14 h, 17 h, 20 h, 23 h, and 26 h, the products in this step were denoted as BMP-1, BMP-2, BMP-3, BMP-4, and BMP-5, respectively. Crystallization: The resultant mixture obtained in the step (b) was transferred into an autoclave to keep crystallization for 24 h at 100 °C. After filtration, washing and drying, the template was removed by a calcination at 550 °C for 5 h. The products obtained from BMP-1, BMP-2, BMP-3, BMP-4, and BMP-5 were denoted as BM-1, BM-2, BM-3, BM-4 and BM-5, respectively. 2.3. Characterization. A Rigaku D/Max 2500VB2+/PC diffractometer (Cu Kα radiation) was employed to obtain XRD patterns. JEM 100CX (an acceleration voltage of 200 kVt) was used to record the transmission electron microscopy images. Micromeritics ASAP 2405N system was used to measure the isotherms of nitrogen. The mesoporous structure was determined by the Barrett-Joyner-Halenda (BJH) method using the Halsey equation for multi-layer thickness.

3. RESULTS AND DISCUSSION 3.1 Microporosity fraction increase due to the employment of zeolite Beta precursors. Data obtained from Figure 1 and Figure 2 show microporosity fraction of samples assembled from zeolite Beta precursors with various assembly time. All samples exhibit microporosity fraction larger than 38%, which is much larger than that of MAs obtained from zeolite Y precursors.26 This result indicates that zeolite Beta precursors allows a large amount of microporosity to be retained in the crystallization process. It has been proposed that zeolite Y precursors with small size are favorable for the assembly with copolymer micelles to form the mesophase, leading to the low microporosity fraction of final product.23 However, it is clearly difficult to assembly of precursors with large size on normal P123 micelles to mesophase (resulting in enhanced microporosity fraction). Therefore, it is reasonably deduced that zeolite Beta precursors are more stable in acidic medium. 5



In addition, we observe a remarkable increase in both micropore surface area and micropore volume for BM-1-5 (Table 1), as compared with those of AlSBA-15 (82.6 m2/g and 0.02 cm3/g), further revealing that precursor assembly method is beneficial for the introduction of micropores into their walls.22,23

1500

Volume (cm /g)

e d

3

1000

c

b

500

a 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Figure 1. N2 adsorption-desorption isotherms of (a) BM-1, (b) BM-2, (c) BM -3, (d) BM -4, and (e) BM -5 40 e 30 d

3

dV/dlogd (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

c

b

10

a 0 0

20

40

Pore diameter (nm)

Figure 2. BJH pore distribution curves of (a) BM-1, (b) BM-2, (c) BM -3, (d) BM -4, and (e) BM -5 Table 1. d100, a0, D, d, SBET, SMic, SMes, VBJH, VMic, and VMes of BM-1, BM-2, BM-3, BM-4, BM-5 Sample

d100 /nm

a0

SBET

Smic

/nm

2

/m ·g

2

/m ·g

/m ·g

%

-1

Mic/SBET,

Smes -1

2

-1

VBJH -1

Vmic -1

Vmes -1

/ml·g

/ml·g

/ml·g

D

d

/nm

/nm

BM-1

10.1

11.6

460.4

243.4

217.0

52.9

0.50

0.15

0.35

6.1

5.5

BM-2

9.2

10.6

620

280.3

339.7

45.2

0.89

0.22

0.67

6.0

4.0

BM-3

10.9

12.6

711.0

303.2

407.8

42.6

1.17

0.14

1.03

6.6

6.0

BM-4

9.4

10.9

596

248.4

347.6

41.6

0.92

0.20

0.72

5.8

5.1

BM-5

9.5

11.0

478.4

185.1

293.3

38.7

0.60

0.13

0.47

5.7

5.3

a0, Unit cell parameters; D, Average Diameter; d, Pore wall thickness.

3.2 Competition of the formation of mesoporosity and microporosity. Beta precursors consisting of the primary and the secondary building units are firstly 6


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formed. According to Xiao’ s report, there are two types of precursors particles: 2.8 nm size primary particles and their aggregates (≈10 nm).35 The hydrodynamic diameter of micelles in this investigation is about 20 nm.36 During the second assembly step, 2.8 nm primary particles are the main building unites that assemble with the micelles of P123 to obtain the mesophase. However, aggregates with large size do not favor the assembly with P123 micelles. Figure. 3 show the assembly products with different assembly time. Interestingly, by gradually increasing the assembly time, the crystal plane spacing (d100) and unit cell parameter (a0) decreased from 10.24 nm and 11.82 nm (sample BMP-1) to 9.74 nm and 11.25 nm (sample BMP-5), respectively (Figure 3 and Table 2). The XRD patterns of corresponding final MAs are shown in Figure. 4. It has been shown that the ordering of product is dependent on the assembly time. With the extension of assembly time, the increase of the diffraction peaks was followed by the subsequent decreases. When the assembly time is up to 20 h, three well-resolved peaks are exhibited in XRD patterns of BM-3, indicative of high ordering of mesostructure. It has been suggested that precursors diminished (partially degraded) in acidic synthesis system favor the formation of mesoporotiy.24,25 As a result of this, the microporosity fraction would decrease accordingly. In addition, Figure 5 shows the wide-angle XRD pattern of BM-3, there is no diffraction peaks of Beta zeolite, indicating the no Beta crystalline phase is formed.

Table 2. XRD parameters of the assembly samples Samples

Degree of crystal plane(100)/°

d(100)/(nm)

a0/(nm)

BMP-1 BMP-2 BMP-3 BMP-4 BMP-5

0.86 0.87 0.88 0.90 0.91

10.24 10.22 10.03 9.83 9.74

11.82 11.80 11.58 11.35 11.25

7



5000

4000

Intensity

e 3000

d

2000

c

1000

b a

0

0

1

2

3

4

5

6

2 Theta/deg

Figure 3. XRD pattern of assembly products (a) BMP-1, (b) BMP-2, (c) BMP-3, (d) BMP-4, and (e) BMP-5 (small-angle).

4 500 4 000 3 500 3 000

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2 500

e

2 000

d

1 500

c

1 000

b

500

a 0 1

2

3

4

5

6

7

8

2 Theta/deg

Figure 4. XRD patterns of (a) BM-1, (b) BM-2, (c) BM-3, (d) BM-4, and (e) BM-5

Generally, the precursors diminished gradually in the acidic medium in assembly step. It is reasonable that precursors with large spatial volume will result in more microporosity.37 Scheme 1 proposed the mechanism for the formation of microporosity and mesoporosity. The normal P123 micelles did not match the precursors with relative large size. Assembling of precursors in P123 micelles to mesophase was not favored. As a result of this, BM-1 had low mesoporosity fraction and small pore volume. By prolonging the assembly time, precursors with decreased size enable the matching with P123 micelles. Therefore, the strengthened interaction between P123 micelles and diminished precursors promoted the formation of mesoporous structure.38

8


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Scheme 1. Proposed process for the construction of mesopores and micropores

160

120

Intensity

80

40

0 0

10

20

30

40

50

2-Theta/deg

Figure 5. XRD pattern of BM-3 (wide-angle) Figure 6 shows the FTIR spectra of calcined BM-1-5. Obviously, all the samples

show a peaks at 521 cm-1 and 571 cm-1 that are attributed to the four- and five-member rings of Beta zeolites. This is the direct evidence that zeolites Beta precursors have been implanted into the walls of the products, exactly consistent with other literatures results.38-41

Transimittance [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


463 e

463

d 462 c

461

798 b

571 521

951 460

a 1084 1200

800

Wavenumber cm

400 -1

9



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Figure 6. FTIR spectra of (a) BM-1, (b) BM-2, (c) BM-3, (d) BM-4, and (e) BM-5

Figure 7 gave the high-resolution TEM images of BM-1-5. The 2D hexagonal arrays of uniform mesoporosity exhibit that all the samples obtained by different assembly time showed the ordered hexagonal arrays of mesopores.26,42,43 TEM results are basically consistent with those of XRD.

a

b

d

e

c

Figure 7. TEM images of (a) BM-1, (b) BM-2, (c) BM-3, (d) BM-4, and (e) BM-5

3.3 Hydrothermal stability of samples. To study the hydrothermal stability of sample BM-3, hydrothermal treatments 100% water vapor at 800 °C for 4 h, 8 h, and 12 h are processed, and the obtained samples were denoted as BM-3-4h, BM-3-8h, and BM-3-12h. Figure 8 show the XRD patterns of the four samples. After the hydrothermal treatment for 12 h, it can be seen clearly that sample BM-3-12h still exhibits (100) diffraction peak with strong intensity, indicating the synthesis of mesophase with high hydrothermal stability. Moreover, the unit cell parameter (a0) decreased to 9.0 nm due to the further shrinkage of mesophase framework in the atmosphere of high temperature steam.44 10


5500 5000 4500 4000

d

Intensity

3500 3000

c

2500 2000 1500

b

1000 500

a

0 1

2

3

4

5

6

7

8

2 Theta/deg

Figure 8. XRD patterns of (a) BM-3 (b) BM-3-4h, (c) BM-3-8h, and (d) BM-3-12h

d

c -3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Volume(cm /g)

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b

a

0.2

0.4

0.6

0.8

1.0

Relative pressure P/P0

Figure 9. N2 adsorption-desorption isotherms of (a) BM-3 (b) BM-3-4h, (c) BM-3-8h, and (d) BM-3-12h

After the hydrothermal ageing for 12 h (Figure 9 and 10), its N2 adsorption-desorption curve still shows a typical II adsorption isotherms. Moreover, it is interesting that the mean pore size obtained from the desorption branch is even larger than that of the parent BM-3 (Table 3), which can be attributed to the disordered meso-tunnels formation during the hydrothermal treatment process. It is reasonable to deduce that these interconnected meso-tunnels are favorable for the diffusion and transportation of bulky molecules. As shown in Table 3, sample BM-3 retains 34.8% of the BET surface area and 38.3% of the total pore volume after hydrothermal treatment for 12 h. For comparison, B-MAS-4 obtained by Bao et al. had a retaining ratio of surface area 23.5% and pore volume 47.9% after a hydrothermal ageing for 2 h.23 AlSBA-15 exhibited retaining 11



ratio of 13.3% surface area and 25.9% pore volume after the same hydrothermal treatment.23 For comparison, after a hydrothermal ageing for 12 h, sample BM-3 retains 20% of the micropore surface area (303.2 to 60.2 m2·g-1) and 45.9% (407.8 to 187.0 m2·g-1) of the mesopore surface area, 21.4% (0.14 to 0.03 ml·g-1) of the micropore pore volume and 40.7% (1.03 to 0.42 ml·g-1) of the mesopore pore volume. These results were further verified by the TEM images. It is widely accepted that the stability of micropores is higher than that of mesopores, and the retaining ratio of mesoporosity is higher than that of microporosity when the products are hydrothermally aged. The present results are consistent with those of the investigation of the present authors.26

12

d c

8

Dv(d)/(ml/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b

4

a 0 0

5

10

15

20

25

30

35

40

45

50

Diameter/nm

Figure 10. BJH pore distribution curves of (a) BM-3 (b) BM-3-4h, (c) BM-3-8h, and (d) BM-3-12h

Figure 11 shows the FTIR spectra of samples after the hydrothermal ageing for different time (4 h, 8 h, and 12 h). Obviously, all the samples exhibit a peaks at 560 cm-1, which can be ascribed to the four- and five-member rings of Beta zeolites,35,38,39 which further verify that the Beta zeolite precursors have been maintained after the hydrothermal treatment. This result is consistent with those of XRD, BET, and TEM (Figure 12). Therefore, it can be inferred that the increased stability of mesopore is attributed to the introduced zeolite Beta precursors by this unique strategy. The significantly improved hydrothermal stability of mesoporosity make it a good 12


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candidate for catalytic cracking of heavy oil.26, 45 Table 3. d100, a0, D, d, SBET, SMic, SMes, VBJH, VMic, and VMes of BM-3 before and after hydrothermal treatment at 800 °C for 4 h, 8 h, and 12 h d100

a0

Stotal

Smic

Smeso

Vtotal

Vmic

Vmeso

/nm

/nm

/m2·g-1

/m2·g-1

/m2·g-1

/ ml·g-1

/ ml·g-1

/ ml·g-1

BM-3

11.0

12.7

711.0

303.2

407.8

1.17

0.14

BM-4h

9.1

10.5

283.2

64.0

219.2

0.51

BM-8h

9.0

10.4

256.9

55.6

201.3

BM-12h

9.3

10.7

247.2

60.2

10.05

11.61

716.3

238.8

Samples

B-MAS-423 23

B-MAS-4-2h

AlSBA-1523

d/nm

D/nm

1.03

5.1

6.6

0.03

0.48

3.6

6.9

0.46

0.03

0.43

3.2

7.2

187.0

0.45

0.03

0.42

3.2

7.5

477.5

0.73

0.13

0.60

5.2

6.4

0.02

0.79

4.0

6.3

168.6 8.90

10.3

0.35

695.0

AlSBA-15-2h23

82.6

612.4

0.81

92.3

0.21

a0, Unit cell parameters; D, Average Diameter; d, Pore wall thickness.

d c

Transimittance [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b a

1629 962

802

463

1080 1500

1000

Wavenumber cm

500 -1

Figure 11. FTIR spectra of BM-3 (a) BM-3 (b) BM-3-4h, (c) BM-3-8h, and (d) BM-3-12h

a

13



b

c

d Figure 12. TEM images of (a) BM-3 (b) BM-3-4h, (c) BM-3-8h, and (d) BM-3-12h

4. CONCLUSION In this study, MAs with enhanced microporosity fraction MF has been obtained by using zeolite Beta precursors as building units. By tuning the assembly time, BET surface area and microporosity area could be improved greatly. The MF can be changed at the microscopic level via altering the stability of precursors in acidic medium. The spatial volume is suggested to be the key factor in assembly of mesophase with high MF.

Acknowledgements 14


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We acknowledge the financial supports by the PetroChina Company Limited (Grants Nos. 2016E-0701, 2016A-1801, and 2016A-1804).

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Zeolite Beta Precursors as Building Units toward Enhancing the

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