Efficient synthesis of hydrothermally stable mesoporous

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Efficient synthesis of hydrothermally stable mesoporous aluminosilicates using trace amounts of anionic surfactant as co-template Xiaozheng Zhao, Xiaotong Mi, Han Chen, Jiang Li, Zhanggui Hou, Honghai Liu, Hongtao Liu, and Xionghou Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03879 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Efficient synthesis of hydrothermally stable mesoporous aluminosilicates using trace amounts of anionic surfactant as co-template Xiaozheng Zhao&, c Xiaotong Mi&,a, b Han Chen,a Jiang Li,a Zhanggui Hou,b Honghai Liu,c Hongtao Liu,a,* Xionghou Gaoc State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

a

Technology, Beijing 100029, P. R. China. bCNOOC

Research Institute of Refining and Petrochemicals, Beijing 102200, P. R. China

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

c

* Corresponding authors: Email: [email protected] (Liu H.)

& These authors contributed equally to this work and should be considered co-first authors 1


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Abstract Mesoporous aluminosilicates (MAs) with excellent hydrothermal stability have been a growing interest due to their potential application in FCC. How to improve the P123 utilization efficiency and reduce the water consumption is an important issue for the synthesis and application of MAs. In this study, trace amounts of AES being the cotemplate were introduced into high P123 concentration to synthesize MAs. The water consumption is reduced 77.3%, and P123 utilization efficiency is increased 1.2 times in comparison to that of conventional method. In high P123 concentration solution, micelle aggregates were formed without AES, resulting in the formation of less ordered MAs. However, well-dispersed micelles were formed in presence of AES, which is beneficial for the subsequent assembling process. In addition, the micellar state corresponding to different AES content and the influence on physicochemical properties of MAs were discussed.

Introduction Mesoporous aluminosilicates (MAs) with excellent hydrothermal have potential applications as catalyst or support.1-3 Our group has synthesizedMAs with excellent hydrothermal stability by incorporating zeolite precursors (Y and Beta) into the walls of MAs.4,5 After hydrothermal treatment under extremely strict consitions, above 30% of the total surface area are preserved. Moreover, the obtained samples dispalyed good catalytic activity in FCC. However, the relative low starting P123 concentration will lead to low synthetic efficiency and high dosage of water, which limits the practical application of MAs.6,7 Enormous effort has been made to overcome these problems. A typical way to reduce the water consumption is steam-assisted conversion route, in which only small quantity of water vapor is needed to make the aluminosilicate gel into zeolite.8,9 However, the preparation of aluminosilicate gel is still needed a large amount of water. It is suggested that microwave-assisted route is also an effective way to reduce the water consumption, but there is a long way to go before the method could be used in the industrial 2



process.10,11 It is reported that both crystal-seed method and mother liquor recycling method could simultaneously increase the P123 utilization efficiency and reduce the water consumption.12-15 In crystal-seed method, the as synthesized MAs can be used as crystal-seed to replace parts of P123 in synthesis of next generation of MAs. But this method still suffers from low P123 utilization efficiency. In mother liquor recycling (MLR) method, the reactants remaining in mother liquor can be reused. But complicated procedures or time consuming is necessary and the concentration of inorganic salts is needed to strictly control. Because the inorganic salts accumulation has a negative influence on the mesophase ordering. The author of present investigation proved that introducing urea into high P123 concentration is an effective method to synthesis of MAs.16 The urea molecules can substitute the water molecules around the P123 micelles, and interact with PEO blocks via hydrogen bonds, enhancing the hydrophilicity of micelles. Consequently, the inorganic species are easily assemble with P123 micelles. The water and P123 consumption is decreased a lot in comparison to conventional method. Unfortunately, it is still far from meeting the requirement of industrial application. It has been reported that the hydrophobic/hydrophilic character of the P123 micelles can be altered by introducing ionic surfactants.17-23 Due to the hydrophobic interactions, the ionic surfactant molecules can be incorporated into the P123 micelles, leading to the formation of mixed micelles. Under the driving of polar group, a number of water molecules migrate to the shell of the micelle. Consequently, the hydration degree of mixed micelles will be greatly improved. The specific state of mixed micelles depends on their concentrations. At low mole ratios of ionic surfactant to P123 (n), P123-rich micelles exist in solution and the micelle aggregation number of the P123 reduce gradually with an increase in ionic surfactants content; At relatively high n value, P123rich micelles are replaced by surfactant-rich micelles and free P123 monomers or multimers exist in solution. In this work, trace amounts of anionic surfactant sodium alcohol ether sulphate (AES) are introduced into high P123 concentration. Thus well-dispersed mixed micelles can 3


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be formed to direct the formation of hydrothermally stable MAs. The facile route shows remarkably increased P23 utilization efficiency and decreased water consumption.

2. EXPERIMENTAL SECTION 2.1. Synthesis Procedures of MAs. The preparation of zeolite Y precursors followed a previous literature.12 The molar ratio of the mixtures is 16Na2O: Al2O3: 15SiO2: 320H2O. After aging at 363-371 K water bath for a period of time with vigorous stirring. The gel named as zeolite Y precursors was obtained. Conventional MAs were prepared by the following procedures. 150 g Y precursors were mixed with 2.5 wt% P123 solution. 6 M H2SO4 worked as the pH adjusting additive to keep the pH value at the range of 1.5-1.8. After assembling at room temperature for a period of time, the gel was crystallized at 393 K for 24 h. The final products named M-0 are obtained after being filtered, washed, dried at 343 K overnight and calcined at 823 K for 5 h. The synthesis procedures of MAs at high P123 concentration (5.1 wt% P123 solution) are same to that of M-0, except that different amounts of AES (0, 1, 1.5, 2, and 4 g AES ) were added into P123 solution. The obtained sample without adding AES was name M-1, and the samples with adding AES were denoted as MAES-x (x being the addition amount of AES). 2.2. Characterization. XRD patterns were recorded using a Rigaku D/MAX 2500 diffractometer with Cu Kα radiation. TEM pictures were determined by a JEM 100CX instrument. The N2 isotherms were measured using a Micromeritics ASAP 2405N system. The SBET were calculated according to the Brumauer-Emmett-Teller (BET) method. VMIC and SMES were determined by the t-plot method.24 Pore size distributions were obtained using the desorption branches according to the BJH method. FT-IR spectra were obtained by using a Nicolet 8700 Infrared Spectrometer. Viscosity was measured using an Ubbelhode suspended level capillary viscometer and the flow times exceeded 150 s.23 Dynamic light scattering (DLS) was used to estimate the hydrodynamic diameter of the micelles. The 29Si MAS NMR spectra were recorded on 4



a Varian Unity Inova 300 spectrometer.

3 RESULTS AND DISCUSSION 3.1 The effect of AES content on the hydration degree and state of micelles Viscosity and DLS measurements are employed to study the effect of AES content on the hydration degree and state of micelles. The plots in Figure 1 show that the relative viscosity (ƞrel) increases with continuously increased P123 concentration. And the relative viscosity displays significant increase with a small amounts of AES, similar results were reported by Desai et al.25 It is also observed from Figure 1 that the relative viscosity increases slowly at lower AES content and then increase dramaticlly at higher AES content. According to the results of viscosity measurements, the intrinsic viscosity [ƞ], reflecting the hydration degree of micelles, can be calculated in the following relationship:

lim 𝐶→0

ƞrel ― 1 𝐶

= [ƞ]

where C is the P123 concentration. The calculated data [ƞ] are recorded in Table 1. The [ƞ] data shows an increasing trend with increase in AES content, indicating the enhanced hydration degree. Similar behavior has been reported by Ganguly et al that with increased in addition amounts of sodium dodecyl sulfate (SDS), which has similar molecule structure to AES.26 2.2

e d c b

2.0

Relative viscosity

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

1.8

1.6

a

1.4

1.2 2.5

3.0

3.5

4.0

4.5

5.0

5.5

P123 Concentration (g/dl)

Figure 1. Relatively viscosity versus P123 concentration in absence and presence of AES: (a) 0 g, (b) 1 g, (c) 1.5 g, (d) 2 g, and (e) 4 g

Figure 2 shows the hydrodynamic diameter (Dh) of P123 micelles in absence and presence of AES. The Dh of 5.1 wt% P123 solution in absence of AES is 24 nm, larger 5


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than that of the Dh of 2.5 wt% P123 solution, indicating the presence of large micelle aggregates. The enhanced hydration degree of micelles could induce the release of P123 molecules from the mixed micelles, giving rise to the decreasing of Dh. For example, with the addition of AES, the Dh obviously decreases from 24 nm to 20 nm, similar to that of P123-SDS/CTAB aqueous system. This result confirms that AES could suppress the aggregation of P123 micelles. It is noticed that the Dh reaches a value of 11 nm at 4 g of AES, much smaller than the Dh of 2.5 wt% P123 solution, implying that the micellar state was possibly changed. The decrease trend in Dh in case of other anionic surfactant SDS or polar additive urea demonstrating micellar state transformation with increase in addition amount were observed by Desai et al. For example, the DLS results exhibited that the Dh of 5 wt% triblock copolymers F127 (PEO106PPO70PEO106) solution is 30 nm. After addition of a certain amount of SDS, the Dh of F127 micelles decrease to 27±1 nm.25

5.1 wt% P123-AES-4g 5.1 wt% P123-AES-2g

5.1 wt% P123-AES-1.5g

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


5.1 wt% P123-AES-1g 5.1 wt% P123-AES-0g

2.5 wt% P123-AES-0g

0

10

20

30

40

50

60

70

Hydrodynamic Diameter (nm)

Figure 2. Dh of 2.5 wt% of P123 solution and 5.1 wt% P123 solution as a function of AES content Table 1. The intrinsic viscosity [ƞ] and Dh of P123 solution in with and without AES P123 concentration (wt%)

AES content (g)

[ƞ]

Dh (nm)

2.5 5.1 5.1 5.1

0 0 1 1.5

0.050 0.050 0.092 0.095

21 24 20 16

6



5.1 5.1

2 4

0.099 0.110

13 11

3.2 The change of mesopore ordering in absence and presence of AES. The evolution of structural ordering of MAs as a function of P123 concentration is shown in Figure 3. The M-0 synthesized by conventional method shows three well-resolved diffraction peaks with high intensity. In contrast, the three peaks intensity of M-1 shows remarkably reduction, implying lower mesophase ordering.16 Moreover, the 2-theta value of (100) for M-1 shows an apparently decreased, indicative of increased d100. This phenomenon may be due to the increased Dh of micelles. In presence of AES, the structural ordering of MAs changes with different AES content. For sample MAES-1, three well-resolved peaks can be seen and the intensity of (100) refection becomes stronger, which is almost equal to that of M-0. In addition, the AES amounts have significant impact on the position of (100) reflection peak of the samples. For example, when the content of AES are below 1.5 g, the (100) reflection peak shifts to wide angle, indicative of decreased d100. After the contents of AES rise up to 2 g or more, the (100) reflection peak shifts to low angle, indicative of increased d100. This behavior is closely related to the micellar state. At relative low AES amounts, P123-rich mixed micelles are formed, leading to reducing the hydrodynamic diameter of the micelles. Consequently, the value of d100 is decreased. At high AES content, the increased d100 may be due to the transformation of micellar state from P123-rich to P123-poor, which cannot be confirmed by current study.

16000

f e

12000

d Intensity

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8000

c 4000

b a

0 0

1

2

3

4

5

2-Theta/deg

Figure 3. XRD patterns of sample (a) M-0, (b) M-1, (c) MAES-1, (d) MAES-1.5, (e) MAES-2, 7


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and (f) MAES-4.

Figure 4 shows the N2 adsorption-desorption isotherms of samples that corresponding to IV type adsorption-desorption isotherms with H1 hysteresis loop.28 In absence of AES, the isotherms of M-1 present smaller hysteresis loop, implying that the mesostructural ordering decreases.29 Figure 5 displays that the M-1 has larger pore size in comparison to that of M-0. This is corresponding to the larger micellar size due to the aggregation of micelles.30,31 While, the inflection point decreases with increased AES content, indicating that the pore size of the resultant samples decreases, which is similar to the results reported by Tan et al and Pauly et al.32,33 Table 2 exhibits that the AES content has a remarkable effect on the structural parameters. For example, the SBET and SMIC of MAES-1 are 666 m2/g and 157 m2/g respectively, larger than that of sample M-1. But with a subsequent increase in AES content, the SBET and SMIC gradually decrease. It is suggested that the structural parameters closely depend on the assembly ability of P123 with inorganic species. At relatively low AES content, well-dispersed micelles are formed because of the enhanced hydration degree, leading to more zeolite Y precursors are incorporated into mesoporous walls. As a results of this, the SBET and SMIC increase. However, at relatively higher AES content, a lot of P123 monomers are released, resulting in that the current micelles structure cannot afford subsequent assembly process. Thus MAs with poor ordering are formed. 3000

f e

2000

Volume cm3/g

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d c

1000

b a

0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure P/P0

Figure 4. N2 adsorption-desorption isotherms of (a) M-0, (b) M-1 (c) MAES-1, (d) MAES-1.5, (e) MAES-2, and (f) MAES-4. 8



Table 2. Specific physicochemical properties of M-0, M-1, MAES-1, MAES-1.5, MAES-2, MAES-4, HM-0, HM-1, and HMAES-1. Sample M-0 M-1 MAES-1 MAES-1.5 MAES-2 MAES-4 HM-0 HM-1 HMAES-1

SBET (m2/g)

SMES (m2/g)

SMIC (m2/g)

VTotal (cm3/g)

VMES (cm3/g)

VMIC (cm3/g)

DBJH (nm)

651 662 666 641 625 577 218 149 235

543 584 509 568 531 514 176 134 190

108 78 157 73 94 63 42 15 45

0.84 0.93 0.91 0.82 0.74 0.68 0.34 0.19 0.48

0.65 0.61 0.62 0.66 0.53 0.56 0.25 0.19 0.35

0.19 0.32 0.29 0.16 0.21 0.12 0.09 0.13

6.6 7.7 6.6 6.1 5.1 5.2 8.8 8.4 7.8

40

f 30

dV/dlogD (cm3/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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e d

20

c 10

b a

0 0

10

20

30

40

50

Diameter (nm)

Figure 5. Pore size distribution curves of (a) M-0, (b) M-1, (c) MAES-1, (d) MAES-1.5, (e) MAES-2, and (f) MAES-4.

On basis of above analysis results, scheme 1 lists a possible micellar state change progress as a function of AES content is proposed. At high P123 concentration solution without AES, the P123 molecules tend to form large micelles aggregates, resulting in obtaining large pore size MAs with poor ordering. After introducing a small amounts of AES into the P123 solution, AES molecules are gradually entered into the P123 micelles accompany with the decreased micelle aggregation number because of the enhanced hydration degree, leading to the formation of well-dispersed P123-rich micelles. Thus, the mixed micelles could direct the formation of well-ordered MAs with 9


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relatively small pore size. When the addition amount of AES increases, the P123-rich mixed micelles are possibly transferred into P123-poor mixed micelles. There may be two pathway for the micellar state transformation: (1) AES molecules bound to P123 micelles will be rapidly saturated. AES micelles are formed accompany with the incorporation of P123 molecules, leading to the formation of P123-poor micelles; (2) AES molecules are gradually incorporated into the P123 micelles with the decreased micelle aggregation number, and P123-poor/AES-rich micelles are finally formed. Although there is ambiguity regarding the transformation of dominantly P123-rich mixed micelles to mainly P123-poor mixed micelles, less ordered MAs are inevitably obtained at high AES content.

Scheme 1 The transformation process of micelles as a function of AES content

It can be seen form the FT-IR spectra (Figure 6), no significant difference can be found from the three samples. The band at 570 cm-1 corresponding to the double sixmember ring of zeolite Y, implied that the primary and secondary building unites were introduced into the mesoporous walls of samples.34-36

10



300

Transmittance (%)

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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c

200

b 570

100

a

0 2000

1800

1600

1400

1200

1000

800

600

400

-1

Wavenumber (cm )

Figure 6. FT-IR spectra of (a) M-0, (b) M-3, and (c) MAES-1.

The P123 utilization efficiency and water consumption were listed in Table 3. AES acts as a critical role in reducing the water consumption and increasing the P123 utilization efficiency at high P123 concentration solution. For example, in comparison to those of M-0, the water consumption for per g product is reduced by 77.3% and the P123 utilization efficiency increases from 0.52 g (M-0) to 1.13 g MAs/g P123 (MAES1). Compared with the urea-assisted method, not only the synthetic efficiency is improved but also the consumption of additives for per g product is decreased a lot (0.04 g AES/product VS 0.17 g urea/g product), which is due to the stronger hydrophilicity of AES in comparison to urea. On one hand, the hydrophilic group SO4Na can attract the water molecules to surround the micelles; on the other hand, there are three ether bonds in the hydrophobic chain AES, which can easily interact with H2O molecules through hydrogen bonds. In contrast, urea molecules just interact with PEO blocks via hydrogen bonds. Thus the hydrophilicity of AES is much higher than that of urea.

11


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Table 3. P123 utilization efficiency and consumption of H2O of MAs Sample

MAs/P123 (g/g)

H2O/MAs (g/g)

Additive/MAs (g/g)

M-0 M-1 MAES-1 U-116

0.52 0.67 1.13 0.77

75 29 17 25

0.04 0.17

3.3 Hydrothermal stability of samples. Figure 7 shows the XRD patterns of samples after hydrothermal treatment. The treatment conditions of M-0 and MAES-1 are under 100% water vapor at 800 °C for 16 h, but the treatment time is 10 h for M-1. The obtained samples are donated as HM-0, HMAES-1 and HM-1. HMAES-1 and HM0 sample still show one strong (100) and two clearly distinguishable (110) and (200) diffraction peaks, indicative of well-maintained mesostructure. In contrast, only one weak diffraction peak can be observed for sample HM-1. In addition, it can be seen form Figure 8, HMAES-1 still shows IV isotherm with clearly observed hysteresis loop. Meanwhile, the relatively more sharpness of infection steep for HMAES-1 indicates presence of ordered mesostructure.32 For HMAES-1, 35.2% surface area and 52.7 % pore volume are preserved, higher than of HM-1. 5000

4000

c 3000

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


2000

b 1000

a

0 0

2

4

6

2-Theta/deg

Figure 7. XRD patterns of (a) HM-0, (b) HMAES-1, and (c) HM-1.

12



800

600

c

3

Volume 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

400

b 200

a 0

0.0

0.4

0.8

Relative Pressure P/P0

Figure 8. N2 adsorption-desorption isotherms of (a) HM-0, (b) HMAES-1, and (c) HM-1.

The enhanced hydrothermal stability can be reflected by the degree of framwor condensation.37,38

29Si

MAS NMR was performed to explore the nature of silicate

framework. Three bands centered at -91 ppm, -100 ppm and -108 ppm are shown in Figure 9, which is corresponding to Si(OSi)2(OH)2 or Si(OSi)2(OAl)2 (Q2), Si(OSi)3OH (Q3), and Si(OSi)4 (Q4). Most of the Si is in Q3 due to that a chemical environment Si(OSi)3Al can also contribute to the resonance at -100 ppm.37 MAs with high proportion of Q4 sites have excellent hydrothermal stability. The calculated value of Q4/(Q3+Q2) of sample MAES-1 is 0.29, larger than that of M-1 (0.17), indicative of higher hydrothermal stability of MAES-1. Q3

Q2

Q4

b

a -200

-150

-100

-50

0

Chemical Shift (ppm)

Figure 9. 29Si MAS NMR spectra of (a) M-1 and (b) MAES-1

TEM pictures of MAES-1 and HMAES-1 are shown in Figure 10. The highly ordered mesostructure for MAES-1 can be observed, which is consistent with XRD analysis 13


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results (Figure. 3). After hydrothermal treatment, ordered mesophase is well-preserved, indicative of excellent hydrothermal stability of MAES-1.

Figure 10. TEM images of (a) MAES-1 and (b) HMAES-1.

4. Conclusions In this study, the shortcomings of synthesis of MAs by conventional method are overcome via introducing trace amounts of AES into high P123 concentrations solution. The water consumption is reduced by 77.3%, and P123 utilization efficiency is increased by 1.2 times. Due to the addition of AES, the hydrophilicity of micelles can be improved a lot, thus the aggregated micelles are broken into well-dispersed micelles, which can realize highly efficient synthesis of MAs with ordered structure. The obtained MAs have large surface area, pore volume, and adjustable pore size. Significantly, after hydrothermal treatment in extremely strict conditions, the sample still preserves 35.3% SBET and 52.7 % Vtotal. This work provides a simple and practical approach to the synthesis of MAs, which lays a strong foundation for the large-scale applications of MAs.

14



Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge financial support from PetroChina Co. Ltd. (Grants 2016E0701, 2016A-1801, and 2016A-1804)

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Efficient synthesis of hydrothermally stable mesoporous

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