Synthesis of hydrophobic zeolite X@SiO₂ core–shell composites

Abstract

Core–shell structures of zeolite X coated with silicalite as well as mesoporous (MCM-41) have been synthesized. Furthermore, the surfaces of the silicalite and mesoporous silica shells were silylated using organosilanes. The materials were characterized by X-ray diffraction, nitrogen adsorption/desorption, scanning and transmission electron microscopy. The results show that the properties of zeolite 13X@silicalite and zeolite 13X@mesoporous silica core–shells composite structures are well maintained even after the modification. As expected, the shell thickness increased with increase in synthesis time, however, the micropore volume decreased. Silylation with smaller organosilanes (trimethyl chlorosilane) resulted in decrease in surface area as they diffused through the pores; however, bulkier silane reacted with surface hydroxyl groups and maintained the pore structure. Contact angle measurements revealed that hydrophobicity of zeolite 13X was enhanced by the microporous and mesoporous shell coating and was further improved by silylation.

Introduction

Zeolites are well known for catalytic, separation and adsorption properties due to their acidity, molecular sieving ability and large surface areas [1], [2], [3]. Aluminosilicates viz. zeolites are typically hydrophilic in nature as the substitution of Si for Al in the zeolite framework creates a negative charge which is neutralized by cations thus inducing a strong electrostatic field on the zeolite surface. Water molecules (or any polar molecules) adsorb on zeolite surface through dipole–field interaction as well as hydrogen bonds with residual hydroxyl groups depending on Si/Al ratio. As the Si/Al ratio increases, number of cations required to neutralize the charge decreases and the hydrophobicity increases. However, the properties of zeolite change with Si/Al ratio and high silica or totally siliceous zeolites have few applications in adsorption and separation [4], [5]. Surface of high silica zeolites or silicalite type structures contain a number of silanol groups which can still make the surface hydrophilic due to interaction with water molecules. To overcome this issue silylation of the silica surface using conventional silica coupling agents (viz. organosilanes) has been successfully attempted in the past and modification of mesoporous silicas by silylation is very useful for designing multifunctional materials [6], [7], [8]. Yamashita and co-workers [9] prepared hydrophobic Y zeolites by using triethoxy flurosilane as silylating agent and demonstrated that hydrophobic zeolite surfaces could be prepared, while the thermal stability and high surface area of the microporous material could be retained. Vuong and Do [10] demonstrated that Faujasite nano-crystals with hydrophobic external surface could be synthesized by single-phase synthesis method. Furthermore, hydrophobic films with micro and mesoporousity have been prepared by addition of organosilanes to silica precursors [11].

Core–shell zeolite composite materials are currently being extensively studied as the properties of two materials can be integrated into one for wide range of applications. For example, in catalysis or separation, a composite material with a microporous shell that could control the access to a microporous or mesoporous core would be of great advantage. Zeolite–zeolite microcomposites could be used for the simultaneous separation and storage of small molecules. Core shell microcapsules might be employed for controlled drug release or as catalytic microreactors [12], [13], [14], [15], [16].

Shell structures have been grown on various types of cores, which may be hollow, amorphous, microporous, mesoporous or solid (metal oxides). A number of techniques have been used to grow the shells for example, sol–gel approach (typically for mesoporous shells) [17], [18]; preliminary adsorption of seeds followed by their secondary growth or in some cases after seeding the cores were subjected to vapor phase transport synthesis (typically for microporous shells) [12], [19]. Recently Hao and co-workers [20] have demonstrated that hydrophobic hybrid micro–mesoporous materials could be used for benzene adsorption under dry and wet conditions. In this work we have attempted to prepare a composite material by integrating the properties of a microporous material (high alumina zeolite 13X) with silicas (microporous or mesoporous) and silylation process to design a hydrophobic zeolite which may find applications in liquid–liquid or gas–liquid separations. Shell structures with tunable coating thickness were successfully synthesized by adjusting the synthesis time and the corresponding influence on the properties of the materials was also studied. Finally these core–shell surfaces were exposed to trimethylchlorosilane vapor or dimethyl (3,3,3-trifluoropropyl) chlorosilane solution to achieve hydrophobicity which was evaluated by contact angle, water breakthrough as well as by water isotherm measurements.