Growth of hydrothermally stable meso-porous silica structure interconnected around micro-porous zeolite crystals

Abstract

The growth of hydrothermally stable ordered meso-porous silica with M41S type structure interconnected around zeolite crystals was achieved from seeds normally nucleating the crystallization of micro-porous zeolite. The solid state NMR and FT-IR showed species like Si(OSi)₃OH and Si(OSi)₂(OH)₂ (or Q₃ and Q₂ sites, respectively) are formed during first hydrothermal treatment with surfactant and NaOH. These species and surface silanol group of zeolite crystal condense to give meso-porous silica structure interconnected around zeolite crystal during re-crystallization at lower pH. Resulting meso-micro-porous hybrid material is found to be highly active, selective and stable in the synthesis of 2-(phenylamino) ethanol from aniline and ethylene carbonate compare to parent ZSM-5 and pure MCM-41.

Introduction

Zeolites with uniform micro-pore size, exceptional topology, adjustable and excellent thermal and hydrothermal structural stability are widely used in many petroleum and petrochemical industrial processes. These zeolites offer well defined micro-pore size of < 1 nm and exceptional properties for their applications in various shape selective catalytic reactions and separation processes. However, relatively narrow pores of micro-porous zeolites limit their use in conversion and separation of bulky molecules because of diffusional limitations in chemical reactions and mass transfer [1]. For this reason, meso-porous materials attracted extensive attention since their emergence in 1992 [2], [3]. Various efforts over the last decade [4], [5], [6], [7], were developed to fabricate meso-porous materials with high hydrothermal stability and adjustable acidity, which at present remain as one of the greatest challenges in material science. The studies were identified that poor hydrothermal stability and weaker acidity of meso-porous materials are due to the amorphous walls [8]. Consequently, new synthesis strategies were employed to prepare new materials which can combine the advantages of high surface area and large pore size of meso-porous materials with the thermal and hydrothermal stability of micro-porous materials such as zeolites. Thus, assembling primary and secondary structural building units (nanocluster or seeds) of micro-porous zeolites should provide an alternative way to synthesize meso-porous material with improved thermal and hydrothermal stability [9], [10], [11], [12], [13]. The available routes normally imply multi-step synthesis methods, that is, either by first preparing the meso-porous phase and then coating its pore walls with zeolite secondary building units [14], [15], [16], [17], [18], [19], [20], or by first preparing the zeolite seeds and using them to form the meso-porous phase [9], [10], [11], [12], [21], [22], [23]. Generally, to generate meso-porosity, micro-porous materials are treated with strong inorganic acid to remove aluminum from the framework. The effectiveness of this technique depends on zeolites used. In the case of mordenite, direct acid treatment was used [24], [25] and also in case of beta meso-porosity was created by direct attack of strong acid [26]. A few examples are known for meso-porosity generated in a zeolite by treatment with a base. Five member ring zeolite (ZSM-5) [27], [28] and 10-membered ring zeolite (Ferrierite) [29] crystals were treated with a NaOH solution to partial dissolve zeolite crystals. Higher NaOH concentrations and prolonged contact with the NaOH solution resulted in an increase of the external surface area and meso-porous volume. Therefore it is interesting to use low concentration of NaOH along with surfactant (structure directing agent) to obtain new class of hybrid materials with high surface area, meso-porous volume and hydrothermal stability.

We report here structurally ordered meso-micro-porous hybrid material with ordered hexagonal meso-porous silica is interconnected around micro-porous zeolite, synthesized by using precursors that generally nucleate the crystallization of micro-porous zeolites. The hydrothermal stability and its catalytic application in synthesis of 2-(phenylamino) ethanol are studied.