Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur Location of silver clusters confined in FAU skeleton of dehydrated bi-metallic Ag x M 96-x -LSX (M = Na + , Li + ) zeolite and resultant influences on N 2 and O 2 adsorption Hamida Panezai, Jihong Sun ⁎ , Xiaoqi Jin, Raza Ullah Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry and Chemical Engineering, Beijing University of Technology, Beijing 100124, PR China ARTICLE INFO Keywords: Bimetallic Ag x Li y Na 96-x-y -LSX zeolite N 2 /O 2 adsorption Isosteric heat of adsorption Silver clusters FAU structures ABSTRACT The location of Ag + -clusters in well-defined dielectric cavities of bi-metallic Ag x Na 96-x -LSX, Ag x Li y Na 96-x-y - LSX, and Li y Ag x Na 96-x-y -LSX zeolites was investigated by XRD, SEM, XPS, ESR, PESA and TG-DTG char- acterizations. The results showed that color of Ag x M 96-x -LSX zeolites changes to yellow, brown and even to black by heating up to 623 K, which may be related to the formation of Ag-Ag clusters produced by auto- reduction or interaction of Ag + ions with framework O 2 . During heat treatment the Ag + -clusters are migrated towards the two distinct possible cation sites (SII’ and SIII). Their ESR silent spectra indicated the diamagnetic clusters Ag 3 n+ formation. The ionization potentials of Ag x M 96-x -LSX further suggested that the electronic properties of Ag + -clusters depend strongly on space confinement and cation content. Moreover, N 2 and O 2 adsorption capacity, selectivity and isosteric heat show strong dependence on the location, nature, extent and order of cations exchange. Particularly, a marked decrease in isosteric heats of adsorption with an increasing N 2 loading suggests heterogeneous interactions between N 2 and Ag + -cluster. The strong influence of Ag + loading and host environment on the cluster formation in correlation with structural parameters is utilized in under- standing to find the most influential sites for N 2 and O 2 adsorption. 1. Introduction The location and environment of transition metal (TM) cations ex- changed in zeolites and the related metal–framework interaction characteristics play an important role in the adsorption and hetero- geneous catalysis [1]. Presently, most of the studies are devoted to the precise understanding of the proper control of reactivity, particularly for dehydrated TM-exchanged faujasites (X and Y zeolites) [1,2]. Fau- jasite zeolite framework structure, with large pore is made up of eight sodalite cages that are joined by oxygen bridges between the hexagonal faces and forming a large central cavity or supercage with a diameter of 11.8 Å and this supercage shares a 12-membered ring with an open diameter of 7.4 Å [2–4]. In hydrated TM-faujasites, it is well established that the preferential location of cation is inside the supercages having pore diameters of the order of 13 Å [1,2], thus offering a space to ac- commodate the cation with its hydration sphere made of different water molecules depending on the nature of cation. On complete dehydration, all water molecules are removed and the TM cation migrates to more confined environments in order to optimize its coordination, being then stabilized by its interaction with the framework oxygen atoms in zeo- lites [5]. Since 1970s, silver-exchanged zeolites have been extensively studied because of their promising catalytic properties. Likewise, small clusters of Ag + -zeolites are also of great interest for various applica- tions and using the zeolite cavity, nanoparticles and clusters are pro- duced simply and the particle size is easily controlled [6]. For instance, Ag-exchanged LTA or FAU-type zeolites have become fluorescent after calcination, due to the formation of small clusters, such as Ag 3 n+ and Ag 6 n+ , in the zeolite cages by an autoreduction process, in which the electrons necessary for silver reduction most probably originate from the expulsion of oxygen atoms from the zeolite framework or hydration water, which is oxidized to molecular oxygen [7,8]. In addition, it was reported that, upon dehydration of silver-containing zeolites, the color changed from white over yellow to brick red or black and these highly colored ionic silver complexes, however, have poor fluorescent prop- erties at room temperature [7,9,10]. Dehydration of the silver ions has been suggested to induce the reduction of silver ions with the formation of charged silver clusters by auto-reduction processes when the zeolites are exposed to a simple vacuum dehydration or heat treatment [7]. https://doi.org/10.1016/j.seppur.2018.01.027 Received 30 September 2017; Received in revised form 10 January 2018; Accepted 14 January 2018 ⁎ Corresponding author at: Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry and Chemical Engineering, Beijing University of Technology, 100 PingLeYuan, Chaoyang District, Beijing 100124, PR China. E-mail address: jhsun@bjut.edu.cn (J. Sun). Separation and Purification Technology 197 (2018) 418–431 Available online 20 February 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved. T