Hydrothermally Stable Bimodal Aluminosilicates with Enhanced

Article pubs.acs.org/IECR

Hydrothermally Stable Bimodal Aluminosilicates with Enhanced Acidity by Combination of Zeolite Y Precursors Assembly and the pH-Adjusting Method Hongtao Liu,† Lei Wang,† Wei Feng,† Li Cao,† Xionghou Gao,*,‡ Honghai Liu,‡ and Chunyan Xu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China Petrochemical Research Institute, Petrochina Company Limited, Beijing, 100195, P. R. China

‡

ABSTRACT: Through a combination of Y zeolite precursors and the pH-adjusting method, bimodal aluminosilicates (denoted as LFs) with strong acidity and excellent hydrothermal stability were synthesized. The hydrothermal stability of the resulting aluminosilicates was improved greatly by taking advantage of Y zeolite precursors (the retaining ratio of the total surface area was 33% after hydrothermal treatment in 100% water vapor at 800 °C for 15 h). When the mesostructure was basically formed in the first crystallization, the pH of the reaction system was adjusted from strong acid to neutral, followed by the second hydrothermal crystallization, during which a large amount of Al could be introduced into the mesophases. XRF and Al NMR showed that LFs possessed a high alumina loading (with SiO2/Al2O3 molar ratio of 24.4) and most of the Al was tetrahydrally coordinated. NH3TPD results indicated that the total amount of acid sites increased 12.9 times compared with that of ZF (without pH adjustment). The resulting aluminosilicates with simultaneously enhanced acidity and hydrothermal stability showed superior catalytic properties when used in heavy crude oil catalytic cracking. Our achievements have developed a general synthetic route of bimodal aluminosilicates with strong acidity and excellent hydrothermal stability.

1. INTRODUCTION Assembly of microporous zeolite precursors (the primary and secondary structural building units) such as Y, ZSM-5, and Beta in acidic medium by using P123 as the template has attracted considerable attention because of the obtaining of mesoporous aluminosilicates with bimodal structure and high hydrothermal stability.1−3 However, compared with the conventional microporous zeolites, these obtained mesoporous materials have much weaker acidity because the heteroatoms (the origin of active sites in zeolites4) such as Al3+ and Ti4+ exist only in the cationic form rather than their corresponding oxo species and thus they cannot be grafted into the mesoporous walls. Therefore, how to substantially introduce heteroatoms into the walls and thus improve the acidity of these mesoporous aluminosilicates has been the common focus of materials science.5−18 To date, many approaches to introducing heteroatoms into the walls of mesoporous aluminosilicates have been developed. For example, incorporation of heteroatoms can be achieved by “direct synthesis19” and “grafting or impregnation20”. The procedure of “direct synthesis” is relatively simple, but only a small part of the heteroatoms added into the initial gel can be grafted into the mesophases. By contrast, the procedure of “grafting or impregnation” is complicated and difficult to replicate. Moreover, there are always more 6-coordinated Al atoms (which are considered as the origin of coke formation) in the zeolites than 4-coordinated ones (which are considered as the Brønsted acid sites). The pH-adjusting method provided an alternative way to improve the content of Al species. For example, Xiao’s group4 has demonstrated that the incorporation of heteroatoms (Al and Ti) into the walls of mesoporous SBA-15 can be achieved by the pHadjusting method. Subsequently, they reported another pH adjusting route by urea to introduce heteroatoms into © 2013 American Chemical Society

mesoporous walls of SBA-15 and the lowest Si/Al molar ratio reached 1.0.21 By this method, Shah’s group22 prepared coppercontaining SBA-16 mesoporous zeolites and Bao’s group23 reported the synthesis of mesoporous Al-SBA-15 with a Si/Al ratio of 5.1. Although significant progress has been made on improving the hydrothermal stability and increasing the heteroatom content, it is still a great challenge to develop a general direct synthetic strategy of the mesoporous aluminosilicates with simultaneously enhanced acidity and hydrothermal stability. To address this issue, we report here how to enhance the acidity as well as hydrothermal stability of the bimodal micromesoporous aluminosilicate based on Y zeolite precursors by using the pH-adjusting method. In this strategy, zeolite Y precursors were first templated in acidic media by P123. When the mesophase was basically formed, the pH of the reaction system was adjusted from strong acid to neutral, followed by a second hydrothermal crystallization, during which a large amount of Al could be introduced into the mesophases. Therefore, the first crystallization of zeolite precursors in an acidic system aims at improving hydrothermal stability, and the second crystallization in a neutral system after pH adjusting aims at introducing Al into the framework of mesophases. A combination of pH adjusting and zeolite precursors, which introduce the Al species into the framework of the preformed mesophase in a neutral system, is believed to be the key in improving hydrothermal stability and avoiding Al loss. Received: Revised: Accepted: Published: 3618

September 27, 2012 January 21, 2013 February 10, 2013 February 11, 2013 dx.doi.org/10.1021/ie302604m | Ind. Eng. Chem. Res. 2013, 52, 3618−3627

Industrial & Engineering Chemistry Research

Article

Figure 1. An optimized procedure for synthesis of LFs.

2. EXPERIMENTAL SECTION 2.1. Synthesis Procedures. The triblock copolymer P123 (EO20PO70EO20, Mw = 5800) was purchased from Aldrich. Na2SiO3, NaOH, aluminum sulfate (Al2(SO4)3·18H2O), and aqueous ammonia (NH3·H2O) were obtained from Tianjin Guangfu company. (1) Preparation of zeolite Y precursors: The mixture of Na2SiO3, Al2(SO4)3, and NaOH solution with a molar ratio of Al2O3/SiO2/Na2O/H2O = 1.0/16/15/320 was prepared. After stirring for 40 min, it was aged at 98 °C for 8 h to obtain a sticky solution, called the “Y-precursor”. (2) Assembly of Y precursors: 4.05 g of P123 and 4 mL of 6 M H2SO4 solution were dissolved in 150 mL of H2O. After being stirred for 3 h at 30 °C, 30.88 g of Y-precursor solution and 10 mL of 6 M H2SO4 was simultaneoulsy added under stirring to maintain the pH of the whole system at 1.0. After that, it was aged at 30 °C for 20 h. (3) Precrystallization: The resulting gel obtained in step (2) was transferred into an autoclave for crystallization at 120 °C for 18 h. (4) pH adjusting and final crystallization: The resulting gel obtained in step (3) was further adjusted to pH 7.0 by adding ammonia solution. After being stirred for 2 h at room temperature, the mixtures were transferred into an autoclave for crystallization at 120 °C for 24 h. After being filtrated, washed, and dried, the powder was calcined at 550 °C for 5 h to remove the template to obtain the final Al-containing mesoporous zeolites. The samples synthesized at pH 5.5, 6.0, 6.5, 7.0, 7.5, and 8.5 were denoted as LF-1, LF-2, LF-3, LF-4, LF-5, and LF-6, respectively. The comparison sample synthesized with the pH-adjusting step was denoted as YF. The resulting gel of step (2) was further adjusted by adding ammonia solution. After being stirred for 2 h at room temperature, the mixtures were transferred into an autoclave for crystallization at 120 °C for 24 h. The comparison sample synthesized without the pH-adjusting step was denoted as ZF. The resulting gel obtained in step (2) was crystallized at 120 °C for 24 h, and then calcined at 550 °C for 5 h. The diagram of the synthesis procedure of LF is shown in Figure 1. 2.2. Catalytic Cracking Performance Tests. The La-form samples of LF-1, LF-2, LF-3, LF-4, LF-5, LF-6, and ZF were obtained by impregnation with La(NO3)3 solution and calcination at 550 °C for 2 h. La, Ce modified Y zeolites (La,Ce-Y) were obtained from Lanzhou Petrochemical Company. Cat-n was prepared from kaolin (65%), alumina gel (10%), La-LF-n (5%), and La,Ce-Y (20%). Cat-Z was prepared from

kaolin (65%), alumina gel (10%), La-ZF (5%), and La,Ce-Y (20%). Cat-Y was prepared from kaolin (65%), alumina gel (10%), and La,Ce-Y (25%). The three catalysts were crushed, sieved to 20−40 mesh, and finally steamed in 800 °C, 100% water vapor for 8 h before using. Heavy oil catalytic cracking tests were carried out in a MAT unit by the pulse method. The heavy oil feedstock used was a blend of 30 wt % Xinjiang vacuum residue (Lanzhou Petrochemical Company) and 70.0 wt % Xinjiang vacuum wax oil (Lanzhou Petrochemical Company). The physical and chemical properties of the feedstock are shown in Table 1. The Table 1. Properties of Xinjiang Crude Oil item Conradson carbon (m%) element analysis (m%) N C H components saturated hydrocarbon aromatic hydrocarbon colloid asphalt molecular weight

vacuum wax oil

vacuum residue

0.285

9.760

0.05 86.79 13.16

0.62 87.01 12.37

86.7 12.8 0.5

45.7 47.3 5.7 1.2 882

323

heavy crude oil catalytic cracking tests were performed under the following standard conditions: catalyst loading 4.0 g; reaction temperature 500 °C; 1.0 g of the heavy crude oil was introduced to the reactor through an injection tube within 30 s, then nitrogen purging was followed for 5 min. The reaction products were collected in a gas collector and a liquid collector, respectively, through a cooling bath. The component analyses of the cracking products were carried out using an Agilent gas chromatographer equipped with a flame ionization detector. 2.3. Characterization. X-ray diffraction (XRD) patterns of the synthesized aluminosilicate were obtained with a Rigaku D/ Max 2500VB2+/PC diffractometer using Cu Kα radiation. Transmission electron microscopy (TEM) images were recorded by JEM 100CX with an acceleration voltage of 200 kV. The isotherms of nitrogen were measured at the temperature of liquid nitrogen using a Micromeritics ASAP 2405N system. The pore-size distribution was calculated using the Barrett− Joyner−Halenda (BJH) model. 27Al NMR spectra were recorded on a Varian Unity Inova 300 spectrometer, using 6 mm Chemagnetics TM solid single-pulse double resonance probe sampling. UV−vis spectra were measured with a Nicolet 8700 spectrometer, and KBr was using as an internal standard sample. 3619

dx.doi.org/10.1021/ie302604m | Ind. Eng. Chem. Res. 2013, 52, 3618−3627


Article

Table 2. XRF Results of the LF Samples sample

Na2O (mass%)

Al2O3 (mass%)

SiO2 (mass%)

n(SiO2)/n(Al2O3)

ZF LF-1 LF-2 LF-3 LF-4 LF-5 LF-6

0.63 1.16 1.06 1.30 1.60 0.92 0.76

1.57 2.74 3.84 5.40 6.40 3.88 3.04

97.8 96.1 95.1 93.3 92.0 95.2 96.2

105 59.6 42.1 29.4 24.4 41.7 53.8

XRF was performed on a PANalytical B.V. X’Pert PRO X-ray fluorescence spectrometer (XRF).

3. RESULTS AND DISCUSSION The mole ratio of SiO2/Al2O3 (noted as η) is measured by XRF, and the corresponding results are shown in Table 2. The η value in the initial gel is calculated as 16, and that of the synthesized ZF is determined as 105, which indicated that there were many Al species lost during the assembly procedure in acidic medium. In contrast, all of the η values in samples LF-1−6 were even less than that of ZF (η = 105). These results are approximately consistent with those obtained by XRD and TEM, and confirm that the pH adjusting is a good method for the alumination of mesoporous siliceous mesophases.23 The η value decreased from LF-1 (η = 59.6) to LF-4 (η = 24.4) but increased in LF-5. These can be interpreted that Si−O−Si bands are formed in the acidic medium, and the bands of Si−O− Si formed in the precrystallization step will undergo the hydrolysis to Si−OH in neutral or alkaline medium, and then condensate with Al3+ to form more Si−O−Al bands.23 The authors of the present investigation have reported this condensation−hydrolysis equilibrium equation in the previous literature.24 The synthesis process in this article is quite different from that in other literature.23,25,26 In this process, the preformation of zeolite Y precursors in the precrystallization step in acidic medium plays an important role in this synthesis procedure. Y precursors are assembled by P123, and the ordered mesophase walls with Y precursors are formed. 23 In the second crystallization, the Si−O−Si undergoes the hydrolysis to Si− O−H and further condensation with Al−O−H to Si−O−Al. In strong acidic medium (pH 1.0), Al species are homogeneously dispersed in the synthesis system in the form of Al3+ rather than the Al oxo form,25,26 so little Si−O−Al is formed in strongly acidic medium. At pH 5.5, the Al species change into the oxo form and condense with the silicate. At pH 7.0, which can be regarded as the isoelectric point of silica, most of the Al3+ species are in the form of oxo species rather than cations, so most of the Al species are introduced into the walls of mesophase.27 The authors of the present investigation have synthesized La, Alcontaining mesoporous aluminosilicates, and the isoelectric point of silica is also at pH 7.0.24 Bao et al. have prepared Al-SBA15 with the isoelectric point of silica at pH 2.4.23 27 Al NMR of LF-4, YF, and ZF (as shown in Figure 2) shows a sharp and symmetric signal centered at 50 ppm, indicating that aluminum species have been grafted into the mesoporous framework in the tetrahedrally coordinated position. In strong acidic synthesis medium, Al species exist in cationic form rather than their oxo form, so free Al3+ cannot enter the framework of mesophases. In the present investigation, zeolite Y precursors were first crystallized in acidic medium by the P123 template. When the mesostructure was basically formed, the pH of the

Figure 2. Solid-state 27Al NMR spectrum of (a) ZF, (b) YF, and (c) LF4.

reaction system was adjusted from strong acid to neutral, followed by a second hydrothermal crystallization, during which a large amount of Al could be introduced into the mesophase. This result is quite different from those in Tan’s literature.28 Due to the fact that only the tetrahedral Al in the pore walls can be ion-exchanged and thus contribute to the Brønsted acidity, more tetrahedral Al species than octahedral ones are desired. These results suggest that the pH-adjusting method can lead to formation of tetrahedrally coordinated Al. The acidity of LFs and ZF was studied by NH3-TPD measurement, and the profiles are shown in Figure 3. In terms of desorption temperature, acid sites are classified as weak (100− 250 °C), medium (250−400 °C), and strong (>400 °C). ZF shows two desorption peaks at about 130 and 180 °C, corresponding to weak acidic sites arising from surface hydroxyl groups attached to Si. All of the LF samples display two desorption peaks at about 200 and 700 °C, indicating the presence of both weak and medium acid sites. Moreover, the intensity of desorption peaks shows that the number of total acid sites of LFs is much higher than that of ZF. Table 3 shows the qualitative analysis results of acid strength distributions calculated from NH3-TPD profiles of LFs and ZF. From Table 3, it is clear that the total amount of LF is 695.7 μmol/g, 13.9 times that of ZF (50 μmol/g). Compared with the results of other literature,29−31 the LF-4 we have synthesized has a much stronger acidity. It can be interpreted that the introduced Al species (as XRD and XRF analyses have suggested) by the pHadjusting method contribute to the greater acidity in the synthesized samples. Moreover, the proportion of strong acid centers increases from 0 to 54.6% from ZF to LF-1 and increases 3620



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increase of pH value, suggesting that the mesopore ordering decreases with the introduction of Al species. This is because the introduction of the heteroatoms (Al) decreases the interaction between surfactants and silicate species, leading to the disordering of zeolites.4 On the other hand, the mesostructure order of LF-6 is significantly reduced, as can be seen from the XRD pattern. This suggests that the alkaline medium does not allow the optimal interaction between the inorganic species and the surfactant array in the process of Al grafting. These results indicate that the pH-adjusting method allows the introduction of a large amount of Al species into the wall of zeolites while maintaining the well ordered mesostructures. These results are fully consistent with those of XRF studies. In addition, from Table 4, it can be seen that the d-spacing of the (100) plane is shifted to lower values with increasing aluminum content. Similar results have been observed for SBA15 (9.75 nm)32 and Al-SBA-15 (8.90 nm),28 and the same results are also reported by Xiao et al. in the pH-adjusting method.4 However, the contrary results have also been reported in other literature25,26 due the longer Al−O bond length compared to the Si−O bond.27 In this paper, the authors inferred that the decreased d-spacing was due to a small number of Al−O bonds and Si−O bonds involved in the unit cell in spite of the longer Al−O bond length. These results indicate that the pH-adjusting method allows the introduction of a large amount of Al species into the wall of zeolites while maintaining the well ordered mesostructure. The high-resolution TEM images of LF-4 were shown in Figure 5. The 2D hexagonal arrays of uniform mesopores indicate that the followed procedure of pH adjusting does not destroy the ordered structure formed in the former step. These results are basically consistent with those of XRD and support the conclusions that the pH-adjusting method is of favor for retaining the well ordered mesostructures even when a large amount of Al content is introduced. FTIR spectra of LFs, YF, and ZF are shown in Figure 6. All the samples show a peak at 564 cm−1 that is ascribed to the five- and six-member rings of Y zeolites,33−36 which is direct evidence that the precursors of Y zeolites have been introduced into the walls of the samples. Besides, three bands at 464, 806, and 1082 cm−1 can be observed for the ZF sample. The band at 464 cm−1 is ascribed to the bending vibration of Si−O−Si. The band at 806 cm−1 is the symmetric stretching vibration of Si−O−Si. The band at 1082 cm−1 is assigned to the antisymmetric stretching vibration of Si−O−Si in the tetrahedron skeleton.37 With the introduction of Al, a slight red shift for all the vibration bands is observed. Particularly, the Si−O−Si bending vibration of LF-4 has a red shift from 464 to 459 cm−1. The bending vibration of Si−O−Si is very sensitive to chemical environment, and it can be concluded that Al species enter the framework in the materials. Therefore, zeolite precursor assembly is a good method for the introduction of zeolite precursors into the walls of mesostructures. Figures 7 and 8 show the nitrogen adsorption−desorption isotherm of LFs, YF, and ZF. All of them show typical IV adsorption−desorption isotherms with a sharp inflection at a relative pressure of 0.7 < P/P0 < 0.85 and a well-defined hysteresis loop, indicating that well ordered mesoporous structures are formed in their framework. Interestingly, the remarkable increase in micropore surface area and micropore volume is observed for ZF and LFs compared with those of AlSBA-15 (82.6 m2/g and 0.022 cm3/g), suggesting the presence of microporous structures due to the introduction of zeolite Y

Figure 3. NH3-TPD profiles of (a) LF-1, (b) LF-2, (c) LF-3, (d) LF-4, (e) LF-5, (f) LF-6, and (g) ZF.

Table 3. The Result of NH3-TPD for the meso-Microporous Molecular Sieve sample

total acid (μmol/g)

proportion of weak acid (%)

proportion of strong acid (%)

ZF LF-1 LF-2 LF-3 LF-4 LF-5 LF-6

50.0 562.7 646.6 667.9 695.7 617.0 592.4

100 45.4 42.4 37.7 18.3 40.4 34.9

54.6 57.6 62.3 81.7 59.6 65.1

from 54.6 to 81.7% from LF-1 to LF-4. These results indicated that the introduced Al species contribute more strong acid centers than weak acid centers. These results confirm that the pH-adjusting method is favorable for generating more strong acid sites. The small-angle X-ray diffraction patterns of the samples of LFs and ZF were shown in Figure 4. Three clearly well-resolved diffraction peaks indexed as the (100), (110), and (200) associated with the p6mm hexagonal symmetry are present. Furthermore, the intensity of diffraction peaks lessens with the

Figure 4. Small-angle XRD of (a) LF-1, (b) LF-2, (c) LF-3, (d) LF-4, (e) LF-5, and (f) LF-6. 3621



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Table 4. d100 Values (d100), Unit Cell Parameters (a0), Average Diameter (D), Pore Wall Thickness (d), BET Surface Area (SBET), Micropore Surface Area (SMic), Mesopore Surface Area (SMes), Total Pore Volume (VBJH), Micropore Volume (VMic), and Mesopore Volume (VMes) of LF-1, LF-2, LF-3, LF-4, LF-5, LF-6, ZF, YF, SBA-15, and Al-SBA-15a

a

sample

d100

a0

SBET

Smes

Smic

SMic/SBET (%)

VBJH

Vmes

Vmic

D

d

LF-1 LF-2 LF-3 LF-4 LF-5 LF-6 ZF YF SBA-1532 Al-SBA-1528

10.48 9.64 9.98 10.43 10.22 10.74 10.74 10.26 9.75 8.90

12.05 11.09 11.48 11.99 11.75 12.35 12.40 11.85 11.3 10.30

590.5 528.1 488.1 429.4 485.8 402.4 728.2 431.1 780 695

495.8 438.1 398.1 343.5 382.8 312.7 538.0 365.2

94.7 90 90 85.9 103 89.7 187.2 65.9

19.1 20.5 22.6 25 26.9 28.7 34.8 18.1

0.79 0.81 0.89 0.97 0.76 0.87 1.13 0.94

0.05 0.04 0.05 0.03 0.04 0.10 0.09 0.11

612.4

82.6

13.5

0.84 0.85 0.94 1.00 0.80 0.97 1.22 1.05 0.80 0.810

0.788

0.022

6.89 6.46 7.71 8.22 6.56 8.77 6.72 9.91 6 6.3

5.16 4.63 3.77 3.77 5.19 3.58 5.68 1.94 5.3 4.0

D, pore size obtained from the Barrett−Joyner−Halenda (BJH) analysis model of the adsorption data; a0 = 2d100/31/2; wall thickness d = a0 − D.

Figure 5. The high resolution TEM images of LF-4.

Figure 7. N2 adsorption−desorption isotherms of (a) LF-1, (b) LF-2, (c) LF-3, (d) LF-4, (e) LF-5, (f) LF-6, (g) ZF, and (h) YF.

Figure 6. FTIR spectra of (a) LF-1, (b) LF-2, (c) LF-3, (d) LF-4, (e) LF5, (f) LF-6, (g) ZF, and (h) YF.

precursors into their walls.28,38 Since no bulky zeolite phase is found in ZF and LFs by the wide-angle XRD patterns, it can be reasonably deduced that YF, ZF, and LFs have a micromesoporous structure with micropores existing in the mesoporous walls. Interestingly, a change in pH value in the final crystallization of the preformed mesophases leads to a systematic change of BET textural properties. From Table 4, it can be seen that pH-

adjusting method leads to a decrease in BET surface area and pore volume. This is because neutral and alkaline medium is beneficial to the introduction of the Al species into the framework of mesophases and the grafting of Al leads to the disordering of zeolites. In addition, by increasing the pH value from 5.5 (LF-1) to 7.0 (LF-4), a decrease in micropore surface area from 94.7 to 85.9 m2/g and the micropore volume from 0.05 to 0.03 cm3/g is observed for the four LFs. Moreover, the BET 3622



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Figure 8. BJH mesopore distributions of (a) LF-1, (b) LF-2, (c) LF-3, (d) LF-4, (e) LF-5, (f) LF-6, (g) ZF, and (h) YF. Figure 9. XRD patterns for LF-4 before (a) and after hydrothermal treatment at 800 °C for (b) 5 h, (c) 10 h, and (d) 15 h.

surface area in LF-5 increases due to the decrease of Al content, and a remarkable decrease in LF-6 because the alkaline medium is unfavorable to the precursor assembly.32 These results are fully consistent with those of XRF and XRD. Although the microporosity and mesoporosity of LF-1−4 decrease with the increasing pH value, the ratio of Smic/Smeso and the density of microporosity in LF-4 (the density is obtained by dividing the micropore surface area by the mesopore surface area) increased. These results indicate that pH adjusting accounts for the increasing micropore density. It is generally accepted that the partial occlusion of hydrophilic poly(ethylene oxide) (PEO) chains into the silica walls contributes to the generation of micropores in SBA-15 zeolites.39,40 The synthesis parameters,41 including silica/ template ratio,42 crystallization time and temperature,42 and pH value of the mixture,43 would affect the microporosity of SBA-15 product due to the hydrophilicity of PEO chains. Tan’s results28 have indicated that the microporosity in B-MAS (ZF is synthesized by the method described in this literature) increases only with the increasing aging temperature of the preformed zeolite precursor. However, the results of the present investigation are quite different from those in other articles: the density of microporosity in LFs increases with the increasing pH value of the final crystallization system consisting of the preformed mesophases when other synthesis parameters such as crystallization time, temperature, and surfactant concentration are kept constant. Therefore, it is reasonable to conclude that the density of microporosity in LFs is attributed to the pH value of the final crystallization process. Interestingly, after the hydrothermal treatment in 100% water vapor at 800 °C for 5, 10, and 15 h, the XRD patterns for the four samples are shown in Figure 9. It can be seen clearly that the sample LF-4 after hydrothermal treatment for 15 h still shows strong (100) diffraction peak intensity, indicating the high hydrothermal stability. Therefore, a combination of zeolite precursors and pH adjusting is a good strategy for improving the hydrothermal stability of the mesophase. The N2 adsorption−desorption isotherms of LF-4 before and after steaming treatment at 800 °C and 100% vapor are shown in Figure 10. All the samples after hydrothermal treatment exhibit typical type II adsorption isotherms. However, high total surface area and total pore volumes are retained. The dimension and uniformity of mesopores in LF-4 before and after steaming treatment are also directly reflected by the pore size distribution

Figure 10. N2 adsorption−desorption isotherms of LF-4 before (a) and after hydrothermal treatment for 5 h (b), 10 h (c), and 15 h (d).

curves. From Figure 11, it can be seen that LF-4 possesses a relatively narrower pore size distribution, which is typical of well

Figure 11. BJH pore distribution curves of LF-4 before (a) and after hydrothermal treatment for 5 h (b), 10 h (c), and 15 h (d). 3623



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Table 5. d100 Values (d100), Unit Cell Parameters (a0), Average Diameter (D), Pore Wall Thickness (d), BET Surface Area (SBET), Micropore Surface Area (SMic), Mesopore Surface Area (SMes), Total Pore Volume (VBJH), Micropore Volume (VMic), and Mesopore Volume (VMes) of LF-4 before and after Hydrothermal Treatment for 5 h (LF-5h), 10 h (LF-10h), and 15 h (LF-15h)a

a

samples

d(100) (nm)

a0 (nm)

SBET (m2/g)

SMic (m2/g)

SMes (m2/g)

VBJH (cm3/g)

VMic (cm3/g)

VMes (cm3/g)

D (nm)

LF-4 LF-5h LF-10h LF-15h

10.22 9.43 9.43 9.23

11.80 10.84 10.84 10.61

392.6 208.7 148.3 129.5

96.2 42.9 20.7 20.0

296.4 165.8 127.6 109.5

1.36 1.00 0.75 0.71

0.042 0.017 0.007 0.007

1.318 0.983 0.743 0.703

13.02 18.56 20.12 21.73

D, pore size obtained from the Barrett−Joyner−Halenda (BJH) analysis model of the adsorption data; a0 = 2d100/31/2.

Figure 12. TEM images of LF-4 after hydrothermal treatment for 5, 10, and 15 h.

LF-4 samples. For example, higher mesoporosity, higher total surface areas, and total pore volumes (33 and 52%, respectively) were retained for the LF-4 sample after the hydrothermal treatment in 100% water vapor at 800 °C for 15 h, indicating the increased hydrothermal stability. For comparison, the sample B-

ordered mesostructures. It is interesting that great changes are observed in the N2 adsorption−desorption isotherm and pore size distribution curve of LF-4 after hydrothermal treatment. Table 5 shows that the change in the hydrothermal treatment time leads to a systematic change of the textural properties of the 3624



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MAS-4 prepared by Tan et al. retained 23.5% surface area and 47.2% pore volume after a hydrothermal treatment in 100% water vapor at 800 °C for 2 h.28 Surprisingly, by increasing the hydrothermal treatment time to 15 h, the mesopore surface area decreases from 296.4 to 109.5 m2/g, while the micropore surface area decreases from 96.2 to 20 m2/g. The retaining ratio (the retaining ratio is obtained by dividing the retained area by the primary area) of mesopore area and micropore area is 37 and 21%. This result was further verified by the TEM studies. The present results are quite different from those in the literature:1−3 the retaining ratio of mesopores is higher than that of micropores when the samples are hydrothermally treated. Thus, it is reasonable to deduce that the increased stability of mesopores in the LF-4 is inherited from the incorporated zeolite Y precursors, as discussed earlier in view of their unique synthesis route. The characteristics of the much higher stability of mesophases make it a good candidate for catalytic materials in cracking of large molecules, such as heavy oil. The TEM images of LF-4 after hydrothermal treatment for 5, 10, and 15 h are shown in Figure 12. It can be seen clearly that the sample LF-4 after hydrothermal treatment for 15 h still shows the ordered hexagonal arrays of uniform mesopores, indicating the high hydrothermal stability of LF-4. Catalytic Cracking Test. To explore the catalytic potentials of the LFs, six catalysts including La-LF-n, La-ZF, and commercial product La,Ce-Y (for comparison purposes) were selected and tested in a model reaction system of Xinjiang heavy oil catalytic cracking. The results are shown in Table 6.

and coke are 69.7, 14.7, 1.9, and 13.7%, respectively.28 Because the formation of dry gas and coke is mainly due to the secondary cracking and condensation polymerization reactions of product molecules, the coordination of acidity and pores accounts for the lesser yields of dry gas and coke. In this investigation, pH adjusting was carried out in the secondary crystallization of the preformed mesophase. As a result of this, Al species introduced into the shorter micropore channels in LF-4 account for the lesser yield of dry gas and coke.44−46 To evaluate the stability of the catalysts, Cat-4 after cracking catalytic reaction was characterized and the results were shown in Figures 13 and 14. It can be seen that the Cat-4 used (after

Table 6. Heavy Oil Catalytic Cracking Results

Figure 13. N2 adsorption−desorption isotherms of the Cat-4 used.

catalyst samples

dry gas (wt %)

liquefied gas (wt %)

light fraction oil (wt %)

heavy cycle oil (wt %)

coke (wt %)

Cat-1 Cat-2 Cat-3 Cat-4 Cat-5 Cat-6 Cat-Z Cat-Y

3.68 4.07 2.73 2.76 2.70 2.90 3.72 2.12

19.01 19.42 19.98 20.16 20.01 20.19 20.09 18.98

65.34 65.21 67.19 67.11 66.21 65.67 63.76 62.42

5.96 5.54 4.54 4.31 4.87 5.01 5.98 6.72

6.01 5.76 5.56 5.66 6.21 6.23 6.45 9.76

Surprisingly, the yields of light oil fraction of the LF-n derived catalysts are much higher than that of the Y derived catalyst, despite the fact that Y has much more acid sites than LF-n. Because the accessibility of catalytically active sites often plays a critical role in diffusion-limited reactions, the enhanced accessibility induced by the mesopores of the bimodal pore system in the LF-n can compensate the relatively lower acidity of their derived catalysts and thus improve the catalytic performance. From Table 6, it can be seen that the yield of the light fraction (gasoline + diesel) increases with the increase of the acidity of the catalysts. For example, the yield of the light fraction of the Cat-4 catalysts is 87.27 wt %. This is because the LF-4 derived catalyst has a much stronger acidity. It is considered that more acid sites will account for the higher dry gas and coke yield.28 Interestingly, the LF-4 derived catalyst has almost identical coke yields (5.66%) and dry gas yield (2.76%). For comparison, the catalyst B-MAS-5 prepared by Tan et al.28 is selected and the yields of light oil, heavy oil, dry gas, and coke are 38.8, 53.3, 1.0, and 6.9%, respectively. The Al-SBA-15 yields of light oil, heavy oil, dry gas,

Figure 14. BJH pore distribution curves of the Cat-4 used.

cracking catalytic reaction) has a much large BET surface area and total pore volume with the hierarchical macro-mesomicropores (Table 7). It is generally accepted that the presence of graded pores plays an important role in the cracking of large molecules,44−46 such as heavy oil. These results indicated that the catalysts prepared from LF-4 have excellent hydrothermal stability. Table 7. BET Parameters of the Cat-1 Catalyst Used

3625

SBET (m2/g)

SMic (m2/g)

SMes (m2/g)

VBJH (cm3/g)

VMic (cm3/g)

VMes (cm3/g)

D (nm)

234.3

91.1

143.2

0.417

0.043

0.374

6.4



Article

(6) Liu, Y.; Zhang, W.; Pinnavaia, T. J. Steam-Stable Aluminosilicate Mesostructures Assembled from Zeolite Type Y Seeds. J. Am. Chem. Soc. 2000, 122, 8791. (7) Liu, Y.; Pinnavaia, T. J. Assembly of Hydrothermally Stable Aluminosilicate Foams and Large-pore Hexagonal Mesostructures from Zeolite Seeds under Strongly Acidic Conditions. Chem. Mater. 2002, 14, 3. (8) Liu, Y.; Zhang, W.; Pinnavaia, T. J. Steam-Stable MSU-S Aluminosilicate Mesostructures Assembled from Zeolite ZSM-5 and Zeolite Beta Seeds. Angew. Chem., Int. Ed. 2001, 40, 1255. (9) Yu, C.; Yu, Y.; Zhao, D. Highly Ordered Large Caged Cubic Mesoporous Silica Structures Templated by Triblock PEO-PBO-PEO Copolymer. Chem. Commun. 2000, 7, 575. (10) Mokaya, R.; Jones, W. Synthesis of Acidic Aluminosilicate Mesoporous Molecular Sieves Using Primary Amines. Chem. Commun. 1996, 18, 981. (11) Kim, S. S.; Zhang, W. Z.; Pinnavaia, T. J. Ultrastable Mesostructured Silica Vesicles. Science 1998, 282, 1302. (12) Wu, P.; Tatsumi, T.; Komatsu, T.; Yashima, T. Postsynthesis, Characterization, and Catalytic Properties in Alvene Epoxidation of Hydrothermally Stable Mesoporous Ti-SBA-15. Chem. Mater. 2002, 14, 1657. (13) Shen, S. C.; Kawi, S. MCM-41 with Improved Hydrothermal Stability: Formation and Prevention of Al Content Dependent Structural Defects. Langmuir 2002, 18, 4720. (14) Wang, K. X.; Lin, Y. J.; Morris, M. A.; Holmes, J. D. Preparation of MCM-48 Materials with Enhanced Hydrothermal Stability. J. Mater. Chem. 2006, 16, 4051. (15) Petkov, N.; Holzl, M.; Metzger, T. H.; Mintova, S.; Bein, T. Ordered Micro/Mesoporous Composite Prepared as Thin Films. J. Phys. Chem. B 2005, 109, 4485. (16) Wang, H.; Liu, Y.; Pinnavaia, T. J. Highly Acidic Mesostructured Aluminosilicates from Surfactant-Mediated Zeolite Hydrolysis Products. J. Phys. Chem. B 2006, 110, 4524. (17) Choi, M.; Cho, H. S.; Srivastava, R.; Venkatesan, C.; Choi, D. H.; Ryoo, R. Amphiphilic Organosilane-Directed Synthesis of Crystalline Zeolite with Tunable Mesoporosity. Nat. Mater. 2006, 5, 718. (18) Fan, W.; Snyder, M. A.; Kumar, S.; Lee, P.-S.; Yoo, W. C.; Mccormick, A. V.; Penn, R. L.; Stein, A.; Tsapatsis, M. Hierarchical Nanofabrication of Microporous Crystals with Ordered Mesoporosity. Nat. Mater. 2008, 7, 984. (19) Zhang, W.; Lu, Q.; Han, B.; Li, M.; Xiu, J.; Ying, P.; Li, C. Direct Synthesis and Characterization of Titanium-Substituted Mesoporous Molecular Sieve SBA-15. Chem. Mater. 2002, 14, 3413. (20) Hoffmann, F.; Cornelius, M.; Morell, J.; Fröba, M. Silica-Based Mesoporous Organic-Inorganic Hybrid Materials. Angew. Chem., Int. Ed. 2006, 45, 3216. (21) Du, Y.; Liu, S.; Zhang, Y.; Li, D.; Xiao, F. Ordered Mesoporous Silica-based Materials with Very High Content of Substituted Heteroatoms from a pH-Adjustor of Urea. From Zeolites to Porous MOF Material-the 40th Anniversary of International Zeolite Conference; Elsevier B. V.: 2007, p 1734. (22) Shah, A. T.; Li, B.; Nagra, S. A. Preparation and Characterization of Copper-Substituted SBA-16 Type Mesoporous Materials by Modified pH-Adjusting Method. Can. J. Chem. Eng. 2011, 9999, 1. (23) Li, Y.; Pan, D.; Yu, C.; Fan, Y.; Bao, X. Synthesis and Hydrodesulfurization Properties of NiW Catalyst Supported on HighAluminum-Content, Highly Ordered, and Hydrothermally Stable AlSBA-15. J. Catal. 2012, 286, 124. (24) Liu, H.; Wang, J.; Feng, W.; Xu, C. Synthesis of La-Substituted Aluminosilicates with Hierarchical Pores by pH-Adjusting Method. J. Alloys Compd. 2013, 557, 223. (25) Calleja, G.; Aguado, J.; Carrero, A.; Moreno, J. Preparation, Characterization and Testing of Cr/AlSBA-15 Ethylene Polymerization Catalysts. Appl. Catal., A 2007, 316, 22. (26) Grieken, R.; Escola, J. M.; Moreno, J.; Rodríguez, R. Direct Synthesis of Mesoporous M-SBA-15 (M= Al, Fe, B, Cr) and Application to 1-Hexene Oligomerization. Chem. Eng. J. 2009, 155, 442.

In summary, the advantageous pore structure and acidity of LFs synthesized by combination of precursor assembly and pHadjusting make a great difference from the general mesoporous materials, such as SBA-15. The coordination of acidity and pore results in the higher light oil fraction yield, lower coke, and lower dry gas than that of Y, Al-SBA-15,28 and ZF zeolites.

4. CONCLUSIONS Through a combination of zeolite Y precursors and pH-adjusting method, bimodal aluminosilicates (denoted as LFs) with strong acidity and excellent hydrothermal stability were synthesized. The hydrothermal stability of the resulting aluminosilicates was improved greatly by taking advantage of Y zeolite precursors (the retaining ratio of the total surface area was 33% after hydrothermal treatment in 100% water vapor at 800 °C for 15 h). It was suggested that incorporation of zeolite Y precursors accounted for the increased stability of mesophases. Significantly, it is found that most of the Al is tetrahedrally coordinated and the total acidity is improved greatly due to the combination of pH adjusting and the precursors method. When used for cracking heavy crude oil, LF-4 derived catalysts showed a much better catalytic performance than ZF and Y derived catalysts because of the advantageous pore structure and moderate acidity. Clearly, our achievements have added new contributions to understanding the preparation of hydrothermally stable mesoporous aluminosilicates with strong acidity, which sheds light on the practical application of mesoporous zeolites in large molecule catalytic conversion.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.X.); gaoxionghou@ petrochina.com.cn (X.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the Natural Science Foundation of China (Grant No. 20606003) and Petrochina Limited Company (Grant Nos. 09-09-05-02, 11-0201-13, and 2012A-2102-01).

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REFERENCES

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Hydrothermally Stable Bimodal Aluminosilicates with Enhanced

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