(61d) Ultrafast Synthesis of High-Silica Erionite Zeolite As a Catalyst for NH3-SCR

Small-pore zeolites, a class of zeolites whose largest pores are limited to eight-membered rings, have recently received much attention due to their excellent performance in catalytic reactions such as the conversion of methanol to olefins (MTO) and the selective catalytic reduction (SCR) of NO_x.^[1] The synthesis of high-silica erionite (ERI), one of the small pore zeolites, was first reported by UOP researchers via the so-called charge density mismatch (CDM) approach.^[2] The CDM approach starts from a clear aluminosilicate solution (the CDM solution) that cannot crystallize by itself because of the mismatch between the high charge density of the aluminosilicate solution and the low charge density of the CDM SDA. This CDM barrier can be overcome by adding another SDA with a high charge density (called a crystallization SDA).^[3] With the CDM approach, usually it takes several days to weeks to synthesize ERI zeolite. Considering the great potential of high-silica ERI zeolite in catalytic applications, developing an efficient route to synthesize this small-pore zeolite is of great significance.

Recently, our group has reported an ultrafast route for zeolites synthesis, with which several industrially important zeolites have been synthesized within a few to tens of minutes.^[4,5] Generally, the conventional synthesis of ERI zeolites takes 5 days at 150 ^oC with CDM method. However it was found out that the crystallization rate of ERI zeolite can hardly be improved by conventional techniques employed by us to achieve the ultrafast synthesis, such as high-temperature synthesis, as seed-assisted method and even the combination of both due to the presence of CDM barrier.^[6] An ultrafast synthesis of high-silica ERI zeolite is reported here without employing the CDM approach, in which the CDM barrier was lifted, so that ERI zeolite could be directly synthesized by the simple seed-assisted method at high temperatures.^[7] Further optimization of the synthesis parameters led to the ultrafast synthesis of ERI in as short as 2 h at 210 ^oC. The fast-synthesized (ca. 700 nm) has a comparably larger particle size than that of the conventionally synthesized ERI product (ca. 100 nm). The N₂ adsorption–desorption data also confirmed the similar micropore volumes of the fast-synthesized ERI crystal and the ERI seed and (0.20 cm³/g and 0.21 cm³/g, respectively). Of particular interest is that the fast-synthesized ERI zeolite shows improved hydrothermal stability and catalytic performance in NH₃-SCR, which could not be achieved by the ERI products synthesized by the conventional method.

Reference

[1] M. Moliner, C. Martínez and A. Corma, Chem. Mater. 2014, 26, 246.

[2] M. A. Miller, G. J. Lewis, J. G. Moscoso, S. Koster, F. Modica, M. G. Gatter and L. T. Nemeth, Stud. Surf. Sci. Catal. 2007,170, 487.

[3] G. J. Lewis, M. A. Miller, J. G. Moscoso, B. A. Wilson, L. M. Knight and S. T. Wilson, Stud. Surf. Sci. Catal. 2004, 154, 364.

[4] Z. Liu, N. Nomura, D. Nishioka, Y. Hotta, T. Matsuo, K. Oshima, T. Okubo, T. Wakihara, Chem. Commun. 2015, 51, 12567.

[5] J. Zhu, Z. Liu, A. Endo, Y. Yanaba, T. Yoshikawa, T. Wakihara, T. Okubo, CrystEngComm 2017, 19, 632.

[6] J. Zhu, Z. Liu, K. Iyoki, C. Anand, K. Yoshida, Y. Sasaki, S. Sukenaga, M. Ando, H. Shibata, T. Okubo and T. Wakihara, Chem. Commun. 2017, 53, 6796.

[7] T. Moteki and T. Okubo, Chem. Mater. 2013, 25, 2603.