Encapsulating subnanometric metal clusters in zeolites for catalysis and their challenges

Highlights

• First systemic review on zeolite encapsulation of subnanometric metal clusters.

• Firstly illustrating prerequisites for in situ hydrothermal encapsulation.

• The challenges and perspectives for such encapsulation are accentuated.

Abstract

Subnanometric metal clusters encapsulated within zeolites are of great interests for the industrial catalysis. Herein we review for the first time the encapsulation of subnanometric metal clusters in zeolites. The concepts of both subnanometric metal clusters and zeolites are briefly introduced. The recent advancements of synthesis methods, such as impregnation, ion-exchange followed by post-treatment, template-guidance approach, in situ hydrothermal synthesis and interzeolite transformation are summarized. Further, the encapsulation effects including metal loading, ligand property, zeolite type and calcination condition for subdividing subnanometric metal clusters and nanoparticles in zeolites using quite similar synthesis procedures during in situ hydrothermal synthesis are firstly reviewed to explore the underlying mechanism. The important catalytic applications mainly contained propane dehydrogenation, formic acid decomposition, ammonia borane hydrolysis, cyclohexane oxidation, water–gas shift reaction and hydrogenation reaction are demonstrated. This review concludes with the challenges and status of both the stability issue under high temperature and advanced characterization techniques as well as the industrial perspectives.

Introduction

Subnanometric metal clusters consisted of only several to tens of metal atoms have attracted great attention in catalysis [1], [2], [3]. There is still no accurate definition to discriminate the difference between subnanometric metal clusters and conventional nanoparticles at present, herein in this review, those with nanometer sizes below 1 nm are classified as subnanometric metal clusters by convention.Such unique sizes cause the catalytic behaviors of subnanometric metal clusters dramatically different from the conventional nanoparticles for the lower coordination or unsaturated atoms and higher fraction of surface-to-volume atoms as well as the ultra-small size comparable to the Fermi wavelength of electrons, thereby confining the electrons in molecular dimensions and discrete energy levels[1], [4], [5], [6]. Furthermore, benefiting from the significant advances in aberration-corrected electron microscopy, the subnanometric metal clusters greatly promote the fundamental understanding of the nature of catalysis [3], [7], [8]. Interestingly, subnanometric metal clusters exhibit heterogeneous “body” and homogeneous “soul”, indicating it may provide an opportunity to critically link hetero- with homogeneous catalysis.

However, encountering the dilemma same as metal nanoparticles, subnanometric metal clusters usually occur structural transformation or surface reconstruction under the reaction conditions, resulting in severe deactivation [9]. Anchoring subnanometric metal clusters on a solid support can effectively higher the stability by the interaction of subnanometric metal clusters and support. The surface metal atoms can be positioned at the supports interface via chemical bonding, while the supports may conversely modify the structure and electronic characteristics of the subnanometric metal clusters [10]. Compared to the supports with open structures, porous supports, such as zeolites [11], carbons [12], metal–organic framework materials [13] and oxides [14], the subnanometric metal clusters can be confined or encapsulated into the supports channels or cavities, bringing a higher thermal stability and resistance to sintering [11].

Zeolites and molecular sieves (herein we shortly name zeolites) are highly attractive to be applied as the supports in the varieties of porous materials [15], [16]. Zeolites are porous crystalline oxides consisting of tetrahedra (TO₄) linked together at the oxygen corners to form an uniform three-dimensional network [17]. The T atom centered in TO₄ is generally Si, but it usually can be substituted by Al, or even B, Ga, Be and other heteroatoms [18], [19]. And the isomorphous substitution of Al for Si generates a negatively charged framework, thus originating the ion exchange capacity. Furthermore, the Si/Al ratio can be tuned from 1:1 to infinity to affect the intrinsic properties, like hydrophilicity, ion exchange capacity, and acid stability [20], [21]. Besides that, zeolites are also highlighted as the high surface area and physicochemical host stability of catalysts. Accordingly, subnanometric metal clusters encapsulated within zeolite crystallites enable the cluster species to be confined in some or even exact locations inside the zeolite framework, and thus providing excellent thermal stability, remarkable catalytic selectivity as well as sulfur poisoning resistance [11]. Moreover, The nature of zeolite as Bronsted and Lewis acid catalyst structurally integrated with the subnanometric metal clusters may extend as multi-functional catalysts for the refining of petroleum and many other reactions.

Although the subnanometrial metal clusters encapsulated in zeolites are of great promise and notable progress has been made. Several critical challenges must be addressed for the practical applications: (1) The rational synthesis of novel catalysts with long-term stability especially at high temperatures. (2) The exploration of stabilization strategies and the fundamental understanding of their factors. (3) The development of advanced characterization technologies for the direct visualization of atomic-level structural evolution [22] and the exact location.

A series of reviews on zeolite confined catalysts [23], [24] have been published. However, previous reviews are mainly focused on metal nanoparticles encapsulation strategy and their excellent performance in various applications, without subdividing the differences between metal clusters and metal nanoparticles. Therefore, this review will focus on the zeolite encapsulated subnanometric metal clusters catalysts, including the latest progress of synthesis strategies, and their important applications in the field of catalysis.

This review will start from the encapsulation strategies of subnanometric metal cluster in zeolites, and the recent progresses are demonstrated. We firstly demonstrate the recent achievements of the synthesis approaches. Specifically, we also discuss the possible reasons for obtaining subnanometric metal clusters instead of nanoparticles during in situ hydrothermal synthesis for the first time. The important catalytic applications are also summarized. Finally, the challenges addressing stability and advanced characterization techniques as well as the perspectives for industry are given.