Metal@Zeolite Hybrid Materials for Catalysis

Abstract

The fixation of metal nanoparticles into zeolite crystals has emerged as a new series of heterogeneous catalysts, giving performances that steadily outperform the generally supported catalysts in many important reactions. In this outlook, we define different noble metal-in-zeolite structures (metal@zeolite) according to the size of the nanoparticles and their relative location to the micropores. The metal species within the micropores and those larger than the micropores are denoted as encapsulated and fixed structures, respectively. The development in the strategies for the construction of metal@zeolite hybrid materials is briefly summarized in this work, where the rational preparation and improved thermal stability of the metal nanostructures are particularly mentioned. More importantly, these metal@zeolite hybrid materials as catalysts exhibit excellent shape selectivity. Finally, we review the current challenges and future perspectives for these metal@zeolite catalysts.

Figure 1

Metal@zeolite with encapsulated and fixed structures. (a) Isolated metal sites and (b) metal nanoclusters encapsulated in the micropores. (c) Metal nanoparticles fixed in the zeolite crystals.

Figure 2

(a–c) Models of zeolite LTL with (a) different pore and (b) [Pt(NH₃)₄]²⁺ and (c) PtO_x located in the 8-MR. Reprinted with permission from ref (80). Copyright 2014 Wiley-VCH. The models illustrate the positions of the Ir⁺ ions: (d) T5, three-hollow position, (e) T6, six-ring. (f) STEM images showing site-isolated Pt atoms in KLTL zeolite in the as-prepared samples. White features in dashed blue circles indicate Pt atoms. (g–i) Magnified views of the highlighted regions in (f), containing one Pt atom each at A/B sites in (g), at C/E sites in (h), and at D sites in (i). (j–l) Simulations of the LTL zeolite in the [110] direction superimposed on the magnified views in (g–i), showing Pt atoms (green) at A/B sites in (j), at C/E sites in (k) (purple), and at D sites in (l) (red). Pt atoms are located right at the edge of the 12-membered rings of site D; between the two 12-membered rings of sites C/E; and in the center of three 12-membered rings of sites A/B. Reprinted with permission from ref (80). Copyright 2014 Wiley-VCH. (m) Schematic illustration of the aggregation of Pt species during the redox treatment.

Figure 3

(a) Scheme showing the transformation of single-site Rh into Rh₂ clusters. Magnified view of the (b) Rh atom and (c) Rh₂ cluster in the HAADF-STEM image with (d, e) intensity surface plot and (f, g) three-dimensional intensity surface plot. Reprinted with permission from ref (86). Copyright 2016 American Chemical Society. (h) Metal@zeolite synthesis using bifunctional ligand of (3-mercaptopropyl)trimethoxysilane. Reprinted with permission from ref (39). Copyright 2010 American Chemical Society. (i) Interzeolite transformation route and conventional hydrothermal synthesis route for preparing metal@zeolite materials. Reprinted with permission from ref (40). Copyright 2014 American Chemical Society. (j) Preparation of Pt@MWW by inducing Pt species during the swelling process of layered zeolite precursors. Reprinted with permission from ref (91). Copyright 2017 Nature Publishing Group.

Figure 4

(a) Proposed growth mechanism of metal nanoparticles fixed in zeolite crystals. (b) Model of metal@zeolite hybrid materials. (c–g) TEM characterization of Pt@Beta. (c) STEM image characterizing the platinum nanoparticle distribution. Region I might be the zeolite seed with abundant Pt species, and region II should be the newly formed zeolite with negligible Pt. The dashed line shows the boundary of Region I. (d) HR-TEM image of Pt@Beta. (e) Enlarged view of the red square in (d). The white circle highlights the microdomains of polymorph B (BEB) of zeolite Beta, overlaid by a BEB structure model viewed along [110]. (f) STEM image of a Pt@Beta sample crystallized at 4 h. (g) Corresponding electron diffraction pattern of the circle in (d). Scale bars: 50 nm in (c), 20 nm in (d), 25 Å in (e), and 500 nm in (f). Reprinted with permission from ref (62). Copyright 2018 Nature Publishing Group.

Figure 5

Techniques for constructing a metal@zeolite structure. Models in the ultrafast encapsulation technique are reprinted with permission from ref (63). Copyright 2020 Wiley-VCH.

Figure 6

Catalysis on metal@zeolite hybrid materials.