Enhancement of hydrothermal synthesis of FDU-12-derived nickel phyllosilicate using double accelerators of ammonium fluoride and urea for CO2 methanation

https://doi.org/10.1016/j.jcou.2021.101677Get rights and content

Highlights

  • Ni-phyllosilicate was synthesized by a modified hydrothermal method for CO2 methanation.

  • NH4F and urea were used as double accelerators to form Ni-phyllosilicate.

  • NH4F could improve the formation of important intermediate of H4SiO4.

  • Urea could facilitate the formation of intermediate of Ni(OH)2.

  • Ni-phyllosilicate showed high catalytic performance for CO2 methanation.

Abstract

Nickel phyllosilicate materials were usually prepared by the hydrothermal method under severe conditions with high hydrothermal temperature and long hydrothermal time. In order to obtain Ni-phyllosilicate under mild hydrothermal conditions, a modified hydrothermal method assisted by double accelerators of ammonium fluoride and urea was proposed in this work. The N/F-A100-12 catalyst was prepared by the modified hydrothermal method at 100 °C for 12 h with similar Ni content and morphology as N/F-H220-48 prepared by the conventional hydrothermal method at 220 °C for 48 h. The improvement derived from that ammonium fluoride accelerated the etching of FDU-12 to form the intermediate of H4SiO4, and urea could facilitate the formation of Ni(OH)2, resulting in the quick formation of Ni-phyllosilicate under mild conditions. N/F-A100-12 exhibited high TOFCO2 (220 °C) value of 3.05 × 10−3 s−1 and low activation energy of 96.5 kJ mol−1 for CO2 methanation due to the small Ni particle size and enhanced CO2 and H2 adsorption property. In addition, N/F-A100-12 also displayed high long-term stability and anti-sintering property. In all, the modified hydrothermal method was highly efficient to synthesize Ni-phyllosilicate material under mild hydrothermal conditions.

Introduction

To inhibit the increase of CO2 concentration in atmosphere, carbon capture, utilization and storage (CCUS) has been widely used to reduce the greenhouse gas emission [1,2]. As a reactant, CO2 can be converted to a variety of useful chemicals including methane, formic acid, methanol, methyl formate, etc. The hydrogenation of CO2 to CH4 (CO2 + 4H2 → CH4 + 2H2O), namely CO2 methanation, is considered to be the most promising “Power to Gas” technology, which can make the utmost of CO2 and renewable hydrogen to produce CH4, and transport the existing natural gas grid after a simple treatment [3]. CO2 methanation is an exothermic reaction and Ni/SiO2 catalysts are generally used for this reaction. However, there is a fatal disadvantage that Ni sintering easily occurs over Ni/SiO2 owing to the weak interaction between metal and support [4]. Thus, it is urgent to develop a SiO2-supported Ni catalyst with a strong metal-support interaction.

Various mesoporous silica materials, such as MSN [5], MCM-41 [6] and SBA-15 [7], have been extensively utilized as supports to fabricate a nickel-based catalyst owing to the competitive pore structure [3]. FDU-12 with three-dimensional mesopores, can be an effective support for the Ni/SiO2 catalyst due to its adjustable cage and window size as well as high surface area [8]. Ni/SiO2 catalyst can be obtained after the reduction of nickel phyllosilicates, which have been widely used in some high-temperature reactions with high catalytic performance due to the strong metal-support interaction [9,10]. For example, nickel phyllosilicates show excellent catalytic performance in CO or CO2 hydrogenation, CH4 dry reforming and nitrile hydrogenation reactions [11,12].

Nickel phyllosilicates can be prepared by different methods including hydrothermal method, ammonia evaporation method, and deposition-precipitation method [13]. For instance, Dong et al. successfully prepared a series of nickel phyllosilicate-derived Ni/3D-SBA-15 catalysts by hydrothermal method at the hydrothermal temperature of 180 °C [14]. Ye et al. synthesized the nickel phyllosilicate catalyst with a layered structure by an ammonia-evaporation method [15]. In addition, Ni-phyllosilicate can also be synthesized using a deposition-precipitation method through the reaction of silica, nickel salt and urea at 90 °C [16]. Compared with the ammonia-evaporation method and deposition-precipitation method, the hydrothermal method obtains some advantages including facile operation, environmentally friendly, high crystallinity of nickel phyllosilicate, etc. Therefore, the hydrothermal method has been widely used for synthesis of nickel phyllosilicate catalysts. For the conventional hydrothermal method, the required hydrothermal conditions to produce nickel phyllosilicate are usually severe with high temperature and long time. According to the reported literatures [14,17], the hydrothermal temperature can be above 200 °C for the synthesis of nickel phyllosilicate. Thus, it is a challenge to prepare Ni-phyllosilicate under mild condition via hydrothermal method. Yang et al. immersed nickel foam in a mixed solution of cobalt nitrate, ammonium fluoride and urea, and successfully synthesized a Co3O4 nanosheet@nanowire array structured catalyst under hydrothermal conditions of 100 °C for 6 h [18]. The hydrolysis of urea promoted the formation of Co(OH)2. Ammonium fluoride could effectively etch the foamed silicon and enhance the bond between the support and the nanosheet. Therefore, urea and ammonium fluoride may be the potential accelerators to enhance the hydrothermal synthesis of Ni-phyllosilicate.

In this work, a modified hydrothermal method using double accelerators of ammonium fluoride and urea is proposed to prepare FDU-12 derived nickel phyllosilicate. The effects of the conventional and modified hydrothermal method on the catalyst structure and catalytic performance were investigated in detail, which testifies that the modified hydrothermal method is highly efficient and competitive to synthesize Ni-phyllosilicate material.

Section snippets

Synthesis of FDU-12

FDU-12 was synthesized by a typical hydrothermal method according to the literature [19]. Firstly, triblock copolymer F127 (0.50 g), tetramethyl ammonium bromide (0.60 g) and potassium chloride (1.25 g) were dissolved in hydrochloric acid solution (120.00 mL, 1.00 mol·L−1) under stirring at 14 ± 0.1 °C. Tetraethyl silicate (2.08 g) was then added dropwise under stirring, and the mixture was hydrothermally treated at 140 °C for 24 h. After filtration, washing, drying, and calcination at 400 °C,

Results and discussion

As observed by SEM, the as-synthesized FDU-12 material exhibits a regular hexagonal prism morphology (Fig. 1a and b), which is consistent with its typical feature in literature [20]. After the hydrothermal treatment at 100 or 160 °C for 48 h, only sparse and tiny nanosheets can be observed on the surface of FDU-12 (Fig. S1), which attribute to the small amount of nickel phyllosilicate with layered structure [14]. In order to increase the amount of nickel phyllosilicate, a rather high

Conclusions

In order to address the severe operation conditions of synthesis of Ni-phyllosilicate using the conventional hydrothermal method, a modified hydrothermal method using double accelerators of ammonium fluoride and urea was proposed. Under a severe condition of 220 °C for 48 h, FDU-12 can react with nickel nitrate to form Ni-phyllosilicate (N/F-H220-48) with Ni content of 25.6 wt% using the conventional hydrothermal method. However, after addition ammonium fluoride and urea as double accelerators,

CRediT authorship contribution statement

Hai Li: Methodology, Investigation, Formal analysis, Data curation, Writing - original draft. Yaqi Chen: Methodology, Investigation, Formal analysis, Data curation, Writing - original draft. Shuqi Liu: Methodology, Investigation, Formal analysis, Data curation. Qing Liu: Methodology, Writing - review & editing, Supervision, Visualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the support Foundation of Division of Chemical Sciences of Qingdao University of Science and Technology (no. QUSTHX201912).

References (36)

Cited by (13)

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    Apparently, the reaction paths could affect the morphology of Ni-phyllosilicates. Hence, it is concluded that Ni(OH)2 intermediate (Ni/D-S) can be more efficient than H4SiO4 intermediate (Ni/D-A) for the synthesis of Ni-phyllosilicate [23,33]. However, increasing the amount of NH4F, the formation of both H4SiO4 and Ni-phyllosilicate could be also enhanced.

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    Inspired by the above result, hydroxylation treatment was used to increase the surface silanol group concentration of biomass-based silica, and more Ni phyllosilicate could be formed under the same hydrothermal conditions [10]. In addition, Chen et al. found the double accelerators (NH4F and urea) could promote the synthesis nickel phyllosilicates under a mild condition owing to the promotion of intermediates of H4SiO4 and Ni(OH)2 [13,14]. However, the effect of the concentration of Ni(NO3)2 solution on Ni-phyllosilicate synthesis has not been investigated in detail in literature.

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    For Ni0·5Mg2·5Al-HT, TOF is determined to be 0.033–0.119 s−1 at 200–250 °C (SV = 30000 mL gcat−1 h−1), which is similar to that of 10%Ni/CeO2 [56] and 8%Ni/ZrO2–P [57]. As for Ni2Mg1Al-HT, TOF is decreased to 0.009–0.071 s−1 (SV = 60000 mL gcat−1 h−1), but it is still higher than the value for Ni1·5Mg0·5Al [39], Ru/Ni1·5Mg0·5Al [39], 10%Ni/ZSM-5 [58], 24%Ni/FDU-12 [59], 2.9%Ni/TiO2 [60], 2.6%Ni‒4.9%Mn/TiO2 [60] and comparable to that for 6%Ni/MgO/ZrO2 [61]. Studies have indicated that CO2 methanation is a structure-sensitive reaction and TOF increases with the Ni dispersion [23,62,63].

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These authors contributed equally to this work.

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