Elsevier

Catalysis Today

Volume 352, 1 August 2020, Pages 183-191
Catalysis Today

Effects of nanosheet catalysts on synthesis of aromatics and light hydrocarbons from acetylene

https://doi.org/10.1016/j.cattod.2020.01.004Get rights and content

Highlights

  • The performance of nanosheet ITQ-2 was compared with MCM-22.

  • Nanosheet ITQ-2 zeolites slowed down the deactivation rate.

  • The yield of aromatics was proportional to the concentration of Brønsted acid sites.

  • More light aliphatics were produced from ITQ-2 than MCM-22.

Abstract

A nanosheet zeolite ITQ-2 formed by delamination of MCM-22 zeolite precursor was used and tested as a catalyst for the aromatization of acetylene. Along with MCM-22 (calcined form of MCM-22 zeolite precursor) zeolites, the resulting materials were successfully prepared and characterized in terms of pore structure and surface property. The performance of aromatization of ITQ-2 zeolites was comparable to that of MCM-22 zeolites. Due to the isolated and thin-layer structure, ITQ-2 zeolites had a significant fraction of external active sites. This feature seemed to result in relatively higher amount of C3–C8 aliphatic compounds rather than the C6–C8 aromatic compounds because of the shortened residence and, thus, reaction time. We first found that the cumulative C3–C8 aliphatic products in the case of ITQ-2 zeolites increased with the amounts of external surface area, while no clear relation was observed for the case of MCM-22 zeolites. Second, the initial selectivity towards aromatics was found to clearly show a linear relation with the concentration of Brønsted acid sites regardless of the used catalyst. In addition, the nanosheet ITQ-2 zeolites showed a much slower deactivation unlike the traditional microporous MCM-22 zeolites. Finally, to investigate the difference in the catalytic performance, we rebuilt the reaction pathways from acetylene to the aromatics and aliphatics.

Introduction

Due to the development of shale gas and other non-traditional natural gas resources, methane is drawing much attention as a new feedstock for chemical, petrochemical, and pharmaceutical industries. Methane can be easily seen in the forms of landfill gas, waste gas, and biogas generated from various sources. Because of this abundancy and cheap price, the conversion technology from methane to basic chemicals such as aromatics and olefins now became much more significant than ever [[1], [2], [3]]. However, the methane is so thermodynamically stable that for direct conversion, a great deal of energy is required to dissociate Csingle bondH bonds, aside from further synthesis steps to final value-added products. Alternatively, the researchers have developed two- or three-step methods via synthesis gas, hydrocarbon intermediates, and so on. For example, synthesis gas can be formed by using various reforming techniques, and then products such as methanol, olefins, synthetic oils, waxes, and even electricity can be produced over appropriate catalysts.

Similar to this approach, C2 chemicals such as acetylene and ethylene may be successful candidates for producing aromatic compounds. Especially, the process for acetylene production from methane gas has already been commercialized by employing pyrolysis and Hüels arc methods. In addition, acetylene is highly reactive for aromatization [4]. Zeolite catalysts for aromatization of acetylene are known to have high shape-selectivity, and many research papers have been widely published [[5], [6], [7], [8], [9], [10], [11]]. In the aromatization, it is very important to control the structure of pores and to adjust the concentration and the strength of acid sites to get high conversion and selectivity [12,13]. Despite the high performance of such traditional microporous zeolites, the diffusional limitation in the micropores is known to be a problematic issue [14,15]. Because of this, mesoporosity has been adopted by using various methods such as physical and chemical dealumination [16], and desilication [17]. The methods are traditional and basic, but may damage the structure and decrease the active specific surface area. To avoid the demerits and obtain a regular size of mesopores, a new type of hierarchical MCM-36 zeolite was prepared and tested for aromatization by Lee et al. [18]. The MCM-22 precursor was swollen and then silica material was used for pillaring between swollen layers. By doing so, 4−5 nm mesopores were prepared and showed a better performance in terms of BTX selectivity and coke formation.

In this study, we used a MCM-22 precursor as a mother material to prepare a nanosheet-structured zeolite, called ITQ-2 [[19], [20], [21], [22], [23], [24]]; as a reference, calcination of the precursor resulted in microporous MCM-22 (representative example of the MWW type zeolite). This catalyst retains original crystallinity of MCM-22 while preserving a well-defined thin intralayer zeolite structure. The delaminated zeolite material has a very high fraction of external surface area, allowing for high accessibility of reactants with large molecular sizes to the active sites on the outer surface and, concomitantly, high degree of interaction with reactants [25]. By conducting aromatization of acetylene over such catalysts along with the MCM-22 reference samples, the effects of pore structure on the catalytic performance could be elucidated in a rigorous way. For this, ITQ-2 catalysts were derived from the MCM-22 precursor synthesized with 3 different Si/Al ratios as well as their calcined forms of MCM-22 were tested, analyzed, and compared.

Section snippets

Synthesis of MCM-22 zeolites

The precursors of MCM-22 zeolites (called as MCM-22(P)) were synthesized with 3 different nominal Si/Al ratios around 53, 31, and 16 according to the previously reported method [22,26]. First, sodium aluminate (about 55% Al2O3 and about 45% Na2O, Sigma-Aldrich) and sodium hydroxide (98%, Sigma-Aldrich) were added to deionized (DI) water while stirring. Subsequently, hexamethyleneimine (HMI, 99%, Sigma-Aldrich) and fumed silica (CAB-O-SIL M5) were added to the solution. The final molar

Structural properties of MCM-22 and ITQ-2 zeolites

XRD patterns of MCM-22 and ITQ-2 zeolites with three different Si/Al ratios were acquired in order to investigate their crystal structures (Fig. 1). The XRD patterns of MCM-22 and ITQ-2 zeolites (Fig. 1) were in good agreement with those reported in previous studies [29,30]. Compared to MCM-22 zeolites, in general, three ITQ-2 zeolites had broader XRD patterns and lower signal to noise ratios (S/N ratios). Fig. S1 was added for precise comparison of ITQ-2(15) and ITQ-2(50). The pronounced peak

Conclusions

For the synthesis of BTX aromatics and C3–C8 aliphatics in the aromatization of acetylene, the nanosheet-type ITQ-2 zeolite was employed. By delamination process, such thin-layered zeolitic material was successfully prepared for the synthesis. Due to the isolated and thin-layer structure, ITQ-2 zeolites had a significant fraction of external active sites. This feature seemed to result in relatively higher amount of C3–C8 aliphatic compounds rather than the C6–C8 aromatic compounds because of

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

This work was supported by the Center for C1 Gas Refinery, grant funded by the Korean Ministry of Science and ICT (NRF-2018M3D3A1A01018004), the National Research Foundation of Korea (NRF-2019M3F4A1073044), and Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20174010201150).

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