A multiple-assembly/one-pot-crystallization strategy for a relatively more eco-friendly synthesis of hydrothermally stable mesoporous aluminosilicates

Abstract

Mesoporous aluminosilicates (MA) with high hydrothermal stability obtained by assembly of zeolite Y precursors are attractive for the potential application in heavy oil catalytic cracking. A relatively more eco-friendly synthesis of MA with low synthesis cost and waste water discharge is particularly appealing. We report on the green synthesis of MA via a multiple-assembly/one-pot-crystallization (MA/OC) process by recycling the non-reacted reagents in the assembly mother liquor after separating the assembly solid product. This approach was achieved by exact supplementary compensation of the consumed raw materials and correct pH adjustment for the assembly liquor after each cycle of assembly. The assembly solid products in 5 cycles were collected and crystallized in one-pot in the mother liquor. Characterization results indicated that consumption of P123 and water discharge of MAOC-5 could be reduced to 51.5% and 27.3% of those of conventional MA-1. Meanwhile, the product yield of MAOC-5 is 102.66 g L⁻¹, 4.8 times that of MA-1. This strategy suggests a relatively more eco-friendly and low cost route for the synthesis of hydrothermally stable MA.

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Mesoporous aluminosilicates (MA) with high hydrothermal stability have attracted substantial research efforts directed to the cracking of large heavy oil molecules in the fluid catalytic cracking (FCC) process.^1–4 For example, the authors of the present investigation have obtained MA with excellent hydrothermal stability by assembling of zeolite Y precursors into the walls of MA.^5–10 Such materials are expected to be capable of enduring the severe operations encountered in heavy oil FCC.^8–10 Two critical problems should be addressed for the practical application of such materials: one is the high synthesis cost due to the loss of large amount of P123 remaining in the mother liquors (utilization of P123 is 1.88 g per g MA), and the other is the discharge of a large amount of waste in the synthesis of MA (86 g waste water was discharged per g MA). Therefore, the design of an advanced process for the synthesis of MA with low cost and low waste water discharge remains a challenging goal.

During the past decades, considerable efforts have been focused on lowering synthesis cost and water discharge in the preparation of MA. A typical approach is the application of crystal seeds in the synthesis of MA, which was first reported by the authors of the present investigation.⁵ In this strategy, synthesis cost was greatly reduced by the introduction of crystal seeds. Unfortunately, this process still suffers from a large amount of waste water discharge, which limits the practical application of MA. One possible solution to this problem is to explore a novel process for the synthesis of hydrothermally stable MA with simultaneous low cost and low waste water discharge.

Mother liquor recycling (MLR) is one approach that can reuse the remaining species (such as Si, Al, and templates) in the mother liquor. MLR has been used in the synthesis of microporous zeolites, such as NaY,¹¹ ZSM-5,¹² TS-1 (ref. 13 and 14) and aluminophosphates.¹⁵ However, the mother liquor after crystallization in the synthesis of MA cannot be reused because zeolite Y precursors remaining in the liquor undergoes condensation and crystallization and are not suitable for introduction into the walls of the MA. In spite of this, recycling of assembly mother liquor can be considered as an alternative route since a large amount of non-reacted species remain in the assembly mother liquor.

In this article, we report, for the first time, the synthesis of MAs with a relatively more eco-friendly synthesis strategy by assembly mother liquor recycling. In this process, a mixture gel (including Si, Al, H₂O, and P123) was also assembled and then filtered to obtain the assembly mother liquor and the filter cake (termed the “assembly product”). In the following step, another assembly product is prepared with a green synthesis strategy by reusing the non-reacted species (including Si, Al, P123, and water) remaining in the assembly mother liquor, followed by a supplementary compensation of the consumed species and pH adjustment. The procedure was repeated 5 times and the assembly products (filter cake) obtained in 5 runs were crystallized in the assembly mother liquor. Therefore, assembly mother liquor recycling aims to decrease the amounts of P123 and water, and precursor assembly aims to improve hydrothermal stability. A combination of assembly mother liquor cycling and precursor assembly is believed to be the key to obtaining hydrothermally stable MAs with a significantly decreased amount of waste water and organic template use, and this route is considered to be eco-friendly.

As can be seen in the synthesis of MA-1 (MA synthesized by conventional method), large amounts of non-reacted reactants, including the expensive organic P123, H₂O, silica, alumina, and H₂SO₄ remain in the mother liquor. The discharge of mother liquor during the recovery of mesostructured particles will lead directly to high synthesis cost and a large amount of waste water. From Table 1, it can be seen that utilization of P123 and discharge of waste water is 1.88 g and 83.33 g per g MA-1, respectively.

Table 1 Synthesis yields and requirement of reagents of (a) MA-1, (b) MAOC-5 and (c) MA-CS

MA synthesis	MA-1^a	MAOC-5^b	MA-CS^c
a MA-1: conventional MA material.b MAOC-5: MA synthesized by MA/OC process for 5 cycles.c MA-CS: MA synthesized by crystal seed method according to literature.^5
Yield (g L^−1)	21.31	102.66	78.00
Requirement of reagents (g reagent/g MA)
P123	1.88	0.97	1.03
Water glass	6.36	5.55	5.20
NaOH	1.83	1.59	1.34
Al2(SO4)3·18H2O	1.22	1.07	1.00
H2SO4	3.42	2.14	2.36
H2O	83.33	22.75	73.44

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It should be noted that exact supplementary compensation of the consumed raw materials and correct pH adjustment for the assembly liquor will lead to the formation of assembly products in the next cycle of the assembly process. A scheme representing the process of conventional synthesis and multiple-assembly/one-pot crystallization (MA/OC) process is illustrated in Fig. 1. The total consumption of reagents is calculated based on the conventional synthesis (MA-1) and five recycles (MAOC-5). In the MA/OC strategy, it was found from Table 1 that consumption of P123 and water discharge could be reduced to 51.5% and 27.3% of those of the conventional MA-1 method. Moreover, zeolite product yield of MAOC-5 is 102.66 g L⁻¹, 4.8 times that of MA-1. For comparison, utilization of P123, discharge of waste water and product yield for crystal seed method (Sample MA-CS) are 1.03 g P123/g MA, 73.44 g H₂O/g MA, and 78.00 g L⁻¹, respectively. Therefore, the MA/OC process is a good strategy to reduce significantly the synthesis cost and waste water discharge.

Fig. 1 Flow chart of mother liquor recycling synthesis of MAOC-5.

The well-ordered mesoporosity of MAOC-5 obtained from 5 assembly cycles is confirmed by XRD analysis (Fig. S1†). XRD patterns of MAOC-5 exhibit three well resolved diffraction peaks, which can be indexed as the (100), (110) and (200) diffraction peaks, indicating that well-ordered mesopores can be obtained via MA/OC process. For comparison, MAs obtained in the 5 cycles exhibit similar characteristics in XRD patterns (Fig. S2†), indicating that MA/OC process is effective in obtaining well-ordered mesophase by recycling the assembly mother liquor. Interestingly, after a hydrothermal treatment at 100% vapor, 800 °C for 16 h, HMAOC-5 (MAOC-5 after hydrothermal treatment) still exhibited well-resolved (100) diffraction peaks, indicating the high hydrothermal stability of this material. These results revealed that the MA/OC procedure is a good strategy for synthesizing MA with high hydrothermal stability with low cost and high yield.

Fig. 2(1) gives the nitrogen adsorption–desorption isotherms of MA-1 and MAOC-5 before and after hydrothermal treatment for 16 h. Similar to MA-1, the isotherm of MAOC-5 shows the representative characteristics of type IV adsorption isotherm with a sharp inflection at relative pressure 0.60 < P/P₀ < 0.75 and a well-defined H1 type hysteresis loop, revealing the existence of mesopores.

Fig. 2 (1) N₂ adsorption–desorption isotherms and (2) pore size distribution curves of (a) MA-1, (b) MAOC-5, (c) HMA-1 and (d) HMAOC-5.

Table 2 shows that MAOC-5 had high surface area (635.7 m² g⁻¹) and pore volume (1.013 cm³ g⁻¹), which can be inferred from its high degree of order. This result is consistent with that of XRD studies. Both MAOC-5 and MA-1 showed a narrow pore distribution (Fig. 2(2)) with the largest pore size of 4.87 and 5.56 nm, respectively.

Table 2 d₁₀₀ values (d₁₀₀), unit cell parameters (a₀), BET surface area (S_BET), mesopore surface area (S_MES), micropore surface area (S_MIC), total pore volume (V_BJH), mesoporous volume (V_MES), micropore volume (V_MIC), average diameter (D_avg), and pore wall thickness (D_w) of (a) MA-1, (b) MAOC-5, (c) HMA-1 and (d) HMAOC-5

Sample	d100 (nm)	a0 (nm)	SBET (m^2 g^−1)	SMIC (m^2 g^−1)	SMES (m^2 g^−1)	VBJH (cm^3 g^−1)	VMIC (cm^3 g^−1)	VMES (cm^3 g^−1)	Davg (nm)	Dw (nm)
a	8.10	9.35	733.8	176.4	557.4	0.813	0.141	0.672	4.43	4.92
b	7.73	8.93	635.7	78.8	556.9	1.013	0.836	0.177	4.61	4.32
c	6.62	7.64	178.0	22.0	156.0	0.344	0.013	0.331	8.82	—
d	6.75	7.80	95.5	82.5	13.0	0.203	0.121	0.082	3.81	3.99

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After a long time (for 16 h) hydrothermal treatment in 100% water vapor at 800 °C, 15% surface area and 20% total pore volume is retained for HMAOC-5 (Table 2), indicating that MAOC-5 obtained by MA/OC strategy has excellent hydrothermal stability. For comparison, B-MAS-4 obtained in Tan's investigation¹⁶ retained 23.5% surface area and 47.2% pore volume after hydrothermal treatment for only 2 h.

The ordered mesoporosity and excellent hydrothermal stability of MAOC-5 obtained by 5 assembly cycles were further confirmed by TEM analysis. From Fig. S3,† it can be seen that the mesoporous channel is 2D hexagonal arrays. Interestingly, HMAOC-5 after hydrothermal treatment in 100% water vapor at 800 °C for 16 h still had hexagonal mesopores (Fig. S3(b)†).

The FT-IR spectra of MAOC-5 sample are shown in Fig. S4.† The band at 573 cm⁻¹, which is attributed to the double six-member rings,^17–21 indicated the introduction of zeolite Y primary and secondary building units into the walls of MAOC-5.

Conclusions

Through a multiple-assembly and one-pot crystallization strategy, hydrothermally stable MAs with low surfactant and water amount were obtained. The samples prepared by this method exhibit similar physicochemical properties and hydrothermal stability to that of MA synthesized by the conventional method. Significantly, the strategy based on combination of MA/OC and precursors assembly in this work provides a feasible approach for hydrothermally stable MA with low P123 amount, low waste water discharge, and greatly improved product yield. The achievements in this research have developed a relatively more eco-friendly synthesis route for hydrothermally stable MA, which sheds a light on the practical application of MA in FCC process.

Acknowledgements

The authors acknowledge Professor Kevin J. Smith of the University of British Columbia (Canada) for fruitful discussion. The authors also acknowledge the financial supports from the Natural Science Foundation of China (Grant no. 20606003), Petrochina Limited Company (Grant nos 11-02-01-1 and 2012A-2102-01), and China Scholarship Council (Grant no. 201306885022).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10408k

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