Kumar Prashant, Kim Dae Woo, Rangnekar Neel, Xu Hao, Fetisov Evgenii O, Ghosh Supriya, Zhang Han, Xiao Qiang, Shete Meera, Siepmann J Ilja, Dumitrica Traian, McCool Benjamin, Tsapatsis Michael, Mkhoyan K Andre
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, USA.
Department of Chemical and Biomolecular Engineering, YONSEI University, Seodaemun-gu, Seoul, Republic of Korea.
Nat Mater. 2020 Apr;19(4):443-449. doi: 10.1038/s41563-019-0581-3. Epub 2020 Feb 24.
Zeolite MFI is a widely used catalyst and adsorbent that also holds promise as a thin-film membrane. The discovery of nanometre-thick two-dimensional (2D) MFI nanosheets has facilitated methods for thin-film zeolite fabrication that open new horizons for membrane science and engineering. However, the crystal structure of 2D-MFI nanosheets and their relationship to separation performance remain elusive. Using transmission electron microscopy, we find that one- to few-unit-cell-wide intergrowths of zeolite MEL exist within 2D-MFI. We identify the planar distribution of these 1D or near-1D-MEL domains, and show that a fraction of nanosheets have high (~25% by volume) MEL content while the majority of nanosheets are MEL-free. Atomistic simulations show that commensurate knitting of 1D-MEL within 2D-MFI creates more rigid and highly selective pores compared to pristine MFI nanosheets, and permeation experiments show a separation factor of 60 using an industrially relevant (undiluted 1 bar xylene mixture) feed. Confined growth in graphite is shown to increase the MEL content in MFI nanosheets. Our observation of these intergrowths suggests strategies for the development of ultra-selective zeolite membranes.
MFI 型沸石是一种广泛使用的催化剂和吸附剂，也有望用作薄膜。纳米级厚度的二维（2D）MFI 纳米片的发现推动了薄膜沸石制造方法的发展，为膜科学与工程开辟了新视野。然而，二维 MFI 纳米片的晶体结构及其与分离性能的关系仍不明确。通过透射电子显微镜，我们发现二维 MFI 中存在一到几个晶胞宽度的 MEL 型沸石共生结构。我们确定了这些一维或近一维 MEL 域的平面分布，并表明一部分纳米片具有高 MEL 含量（约占体积的 25%），而大多数纳米片不含 MEL。原子模拟表明，与原始 MFI 纳米片相比，二维 MFI 中一维 MEL 的匹配编织产生了更刚性和高选择性的孔，渗透实验表明，使用工业相关（未稀释的 1 巴二甲苯混合物）进料时，分离系数为 60。结果表明，在石墨中受限生长可增加 MFI 纳米片中的 MEL 含量。我们对这些共生结构的观察为超选择性沸石膜的开发提供了策略。
