High-pressure, high temperature insertion of bismuth in the siliceous zeolite silicalite-1

Highlights

• Bismuth enters the pores of silicalite-1 under high-pressure, high-temperature conditions.

• The Bi-filled silicalite-1 structure is stabilized with respect to pressure-induced amorphization.

• DFT results show that Bi atoms form chains with short and long Bi-Bi distances linked to the framework by VDW interactions.

• The calculated eDOS show that the Bi-silicalite-1 composite material is a semiconductor with a band gap of 0.4 eV.

Abstract

A silicalite-1/bismuth composite was synthesized by insertion of liquid bismuth in the 5.5 Å diameter pores of the zeolite under high-pressure, high-temperature conditions. The insertion of bismuth stabilizes the structure with respect to pressure-induced amorphization. Transmission electron microscopy indicated the presence of chains of atoms, which correspond to the 5.5 Å diameter of the host silicalite-1 structure. Neutron powder diffraction also confirmed the insertion of Bi in the pores of silicalite-1. Density functional theory calculations indicate that the insertion of bismuth results in formation of chains with short and long Bi-Bi distances in the pores of the host silicalite-1 linked to the framework by van der Waal's interactions. The material is predicted to be a semiconductor with a band gap of 0.4 eV.

Introduction

Zeolites are microporous aluminosilicates with pore diameters typically less than 10 Å. These materials have a large number of applications as molecular sieves, in catalysis, as selective adsorbers among others. Confinement of guest species, in particular metal atoms, in zeolites opens the way to design new composite materials with novel electronic, optical and magnetic properties.

Previous studies [1,2] have shown that molten metals can be inserted into the nano- and subnanometric channels of porous materials at high pressure and high temperature. The interest of this method compared to more conventional methods of metal insertion, such as vapor adsorption, intrazeolite cation reduction and incorporation directly during hydrothermal synthesis, is that a much higher degree of pore filling can be obtained. Oxidization of the metal can also readily be avoided. The critical pressure for insertion of a liquid metal in a porous solid (P_c) is defined in the general case [3] by the Laplace-Washburn equation:

$$P_c=-4{\gamma }_L\cos \theta /D$$

where ${\gamma }_L$ is the surface tension of the nonwetting liquid, $\theta$ is the contact angle between the liquid and the solid and D is the pore diameter.

According to Bogomolov [1], an interface transition gap between channel surfaces and metallic layer should be considered in the case of metallic dielectric liquid due to its nonwetting properties and a new equation was thus proposed:

$$P_c=\frac{0.4\left({\gamma }_o-{\gamma }_c\right)}{D{\left(1-\frac{2{\delta }_o\left({\gamma }_o-{\gamma }_c\right)}{D{\gamma }_o}\right)}^2}$$

where γ_o is the surface tension of liquid, γ_c is the adhesion energy and δ_o is the Tolman length.

Pressures of 0.6–2.9 GPa are necessary to insert the molten metal in microporous zeolites with pores between 8.4 and 6.6 Å [1].

The electronic structures of nanowires were also investigated by different authors [2,4]. A model for the electronic structure for metals inserted into zeolite was established in order to calculate the density of states, the conductivity and even optical properties [4]. Bi quantum wires have been reported to be incorporated in natural mordenite with pore diameter of 6.7 Å under high pressure [2]. Based on a comparison between x-ray powder diffraction patterns of mordenite with that of mordenite/Bi, an enhancement of the intensity of the peak at 6.5° was proposed as proof of the wire synthesis; however, x-ray diffraction has a very low penetration depth in such a sample containing bismuth and no structure refinements were performed. This increase in intensity is however evidence of incorporation of Bi in the pores. The electronic structure of this composite material was determined based on optical absorption spectroscopy and its current-voltage characteristics. A double zigzag chain was proposed for the structure of the wires with a bond length of 3 Å, a 90° bond angle and a period of the chain of 4.2 Å. This proposition was not verified using x-ray diffraction data nor by theoretical calculations.

Little is known on how the insertion of metals affects the host zeolite nor on the quantity and location of the inserted metal atoms. In the present study, even a higher degree of confinement of metals (bismuth) inside the siliceous zeolite silicalite-1 with a monoclinic MFI (Mobil-Five) structure, space group P2₁/n and 5.5 Å diameter pores [5] is studied by injecting liquid bismuth in the host zeolite under high-pressure-high-temperature conditions. Up to now, liquid Bi has not been inserted in such small channels, which requires a pressure exceeding about 4 GPa based on equation (2). The use of a purely siliceous zeolite is an advantage compared to the use of a natural zeolite as no counter ions are present in the pores, thereby allowing a greater degree of filling with metal atoms and reducing any tendency to oxidation. A combination of transmission electron microscopy (TEM), synchrotron x-ray and neutron diffraction and density functional calculations are used to obtain information on the amount and the nature of the inserted Bi atoms in silicalite-1.