EMM-25: The Structure of Two-Dimensional 11 × 10 Medium-Pore Borosilicate Zeolite Unraveled Using 3D Electron Diffraction

Jung Cho,Yifeng Yun,Hao Wang,Karl G Strohmaier,Guang Cao,Allen W Burton,Junliang Sun,Tom Willhammar,Gene Terefenko,Hilda Vroman,Xiaodong Zou,Hongyi Xu,Mobae Afeworki

doi:10.1021/acs.chemmater.1c00892

Abstract

The structure of the novel medium-pore borosilicate zeolite EMM-25 has been determined by continuous rotation electron diffraction (cRED). EMM-25 crystallizes in the space group Cmcm with unit cell parameters a = 11.055, b = 22.912, and c = 24.914 Å and a composition of |C4H8(C11H25N)2|2[Si112.5B3.5O232]. The EMM-25 framework possesses a two-dimensional channel system composed of 10-ring channels connected via 11-ring windows. Its channel system is analogous to that of the medium-pore zeolite NU-87 framework but with 11- rather than 12-ring windows, suggesting a different shape selectivity. EMM-25 was first obtained using 1,4-bis(N-methyl-N,N-dihexylammonium)butane as an organic structure directing agent (OSDA). Based on a molecular docking study of the OSDA within the pores of the determined framework structure, a new ammonium dication OSDA with an improved fit was devised. By using this new OSDA, the synthesis time was reduced 80%, from 52 to just 10 days. Furthermore, cRED data revealed a structural disorder of the EMM-25 framework present as swinging zigzag chains. The introduction of the disorder, which is a consequence of geometry relaxation, was crucial for an accurate structure refinement. Lastly, the cRED data from as-made EMM-25 showed residual potential consistent with the location of the OSDA position determined from the Rietveld refinement, concluding a complete refinement of the as-made structure based on the cRED data.

Highlights

Zeolites are built from TO4 tetrahedra (T = Si, Al) arrangements forming versatile pore and channel systems responsible for their unique physicochemical properties that continue to impact industries with their exceptional performance in catalysis, ionexchange, sorption, and separation.[1]
JU-64 (JSR) and PKU-16 (POS) are gallogermanate and germanosilicate, respectively, which are unstable to calcination of the organic structure directing agent (OSDA), and only EMM-17, NU-86, and PST-31 are catalytically active aluminosilicates.[6−10] PST-31, does not contain 11-membered ring channels
The continuous rotation electron diffraction method was utilized for collection of three-dimensional electron diffraction (3DED) data from submicron-sized single crystals of the novel borosilicate material EMM-25.13−17 The structure determined from analysis of the cRED data possesses a 2D medium-pore channel system with 11-ring window intersections

Summary

INTRODUCTION

NU-86 was the first aluminosilicate zeolite with a proposed 11-ring channel system, and EMM-17 possesses the framework proposed for NU-86 and can be prepared in all-silica or higher-silica compositions It exhibits exceptional activity and selectivity for dewaxing and selective paraffin hydroisomerization. Powder X-ray diffraction (PXRD) bears inherent challenges for structure determination of complicated structures with large unit cells due to severe peak overlaps This challenge is especially more difficult for disordered structures.[12]. In this contribution, the continuous rotation electron diffraction (cRED) method was utilized for collection of three-dimensional electron diffraction (3DED) data from submicron-sized single crystals of the novel borosilicate material EMM-25.13−17 The structure determined from analysis of the cRED data possesses a 2D medium-pore channel system with 11-ring window intersections. The newly devised OSDA (Figure 1b) both curtailed the staggering synthesis time down to just 10 days and improved the reproducibility of complete crystallization

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES