Synthesis of hybrid zeolites using a solvent-free method in the presence of different organosilanes

doi:10.1016/j.micromeso.2016.11.030

Microporous and Mesoporous Materials

Volume 241, 15 March 2017, Pages 98-106

https://doi.org/10.1016/j.micromeso.2016.11.030 Get rights and content

Highlights

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Hybrid sodalite zeolites produced by in situ solvent-free synthesis.
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Sodalite phase was obtained with high crystallinity without seed or template addition.
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Solvent-free route seems to tend to form several nuclei of crystallization.
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Only organosilanes with longer alkyl chains afforded hydrophobic characteristics.

Abstract

Hybrid sodalite zeolites were produced by grinding, homogenizing and heating the reactants, including different organosilanes, without the addition of solvent (water). Hybrid sodalite zeolites were produced in the presence of triethoxy(octyl)silane (C8Si), 3-chloropropyltrimethoxysilane, triethoxymethylsilane, hexamethyldisiloxane, octadecyltrimethoxysilane (ODS) and chlorotrimethylsilane. The resulting hybrid zeolites were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, water contact angle, small-angle X-ray scattering and thermogravimetric analysis. The presence of 6 wt.-% of organosilane introduced in the reaction medium caused no disturbance to the crystal growth. All materials exhibited dense, spherical and uniform particles, with slightly rough surfaces according to SAXS measurements. In the case of octadecylsilane, it appears that the alkyl chains are well packaged and organized, while for octylsilane, a more disordered structure seems to be present. Only organosilanes with longer alkyl chains (C8Si and ODS) show hydrophobic properties. The solvent-free method proved to be effective in synthesizing samples with good crystallinity and hydrophobic characteristics. This synthesis method was shown to be efficient, fast, economical and environmentally friendly.

Graphical abstract

Introduction

Recent studies [1], [2], [3], [4], [5], [6] have shown strong interest in hydrophobic zeolites due to their good catalytic performance in organic media compared to conventional zeolites. For instance, these materials can stabilize water/oil emulsions and catalyse reactions at the liquid−liquid interface. In general, hydrophobic zeolites have shown enhanced stability in water and selective interaction with the substrate [7].

Essentially, there are two means of generating hydrophobicity in zeolites: (i) by increasing the Si/Al ratio or (ii) through surface modification (for example, by grafting organosilanes) in a post-synthesis step. However, in the first approach, the acid character of the zeolite structure is altered, reducing the availability of Brönsted acid sites, which in turn may render these materials unsuitable for dehydration, catalytic oligomerization and alkylation reactions, for example [8], [9], [10]. Furthermore, the methods used to generate Al⁺³-free zeolitic structures are mostly based on environmentally aggressive media such as hydrofluoric acid [10].

An alternative approach to increasing the hydrophobicity of zeolites without reducing the density of the Brönsted acid sites is based on the silanization of the outer surface with organosilanes [1], [11]. The resulting materials exhibit high affinity for the liquid-liquid interface, allowing the stabilization of small-size emulsion droplets. In addition to improving the liquid-solid-liquid interface, salinization may facilitate the separation of molecules from the reaction medium [1]. Therefore, organosilanes containing hydrolysable groups (chloro, alkoxy, etc.) have been widely used to generate hydrophobic surfaces on inorganic materials by grafting reactions in many applications, such as in the preparation of chromatographic columns, biomedical implants, nanoscale devices, adsorbents and catalysts [11], [12]. In the particular case of zeolites, silanization has been used to increase the adsorption of organic molecules from aqueous media [13]. Likewise, the silanized zeolites have been employed in gas separation membranes with improved permeation of CO₂ [14]. They also have been applied in the selective dehydration of fructose to HMF [15]. In zeolites, the hydrophobicity improves thermal stability, preventing the structural collapse that usually occurs when zeolites are immersed in hot liquid water [16].

A typical silanization process involves an additional post-synthesis step [17]. An alternative process may consist of in situ synthesis, in which the zeolite is synthesized and functionalized with the desired organosilanes in a single step, thus conserving energy, time and reagents. Currently, the hydrothermal method is the main synthesis route for zeolitic materials. This method consists of promoting precipitation reactions above room temperature and pressure. Such a route involves potential incompatibilities between water and organosilanes, and therefore the risk of side reactions that may generate zeolitic and hybrid silica domains exists. An approach to overcome some of the drawbacks of the hydrothermal method may be based on solvent-free synthesis, which may prevent side reactions among organosilanes from occurring due to the absence of water (or any other solvent).

The success of dry ice conversion in zeolites hints that the presence of solvent and variation of pressure are not mandatory during crystallization. Based on these results, Xiao et al., [18], [19], [20] proposed a simple and general route to synthesize zeolites by mixing, grinding and heating the raw materials in the solid-state without using a solvent. The first successful solvent-free zeolite synthesis was reported in 2012 by Ren et al. [18], who obtained the ZSM-5 structure. For that synthesis, NaSiO₃ • 9H₂O (SiO₂, 20% by weight), silica Aerosil^®, tetrapropylammonium bromide and NH₄Cl were mixed in a mortar for 20 min. Thereafter, the resulting powder was transferred to an autoclave and sealed for crystallization at 180 °C. The resulting ZSM-5 exhibited a much larger particle size than the product obtained through the hydrothermal method. Meng et al. [21] published a review discussing the most recent developments regarding the solvent-free synthesis of zeolites, which can open the pathway for a highly sustainable zeolite synthesis protocol for industrial applications.

In the present manuscript, we discuss the synthesis of hybrid zeolites in a single step by means of a solvent-free synthesis method in the presence of different organosilanes, namely triethoxy(octyl)silane, 3-chloropropyltrimethoxysilane, triethoxymethylsilane, hexamethyldisiloxane, chlorotrimethylsilane and octadecyltrimethoxysilane. To our knowledge, no study has investigated the potentiality of this route using common (routine) reagents, without the use of templates or seeds. Thus, the synthesis of sodalite was shown to be a low cost and an environmentally friendly approach. The resulting hybrid zeolites were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), powder X-ray diffraction (XRD), water contact angle (WCA), small-angle X-ray scattering (SAXS), and thermogravimetric analysis (TGA).

Section snippets

Materials

Hydrophilic silica was supplied by Diatom (mined in Mogi das Cruzes, Brazil). Sodium hydroxide (Quimex ≥ 97%), sodium aluminate (Sigma-Aldrich ≥ 99%), triethoxy(octyl)silane (Dow Corning ≥ 98%), 3-Chloropropyltrimethoxysilane (Wacker 95%), triethoxymethylsilane (Aldrich 99%), hexamethyldisiloxane (Acros Organics ≥ 98%), chlorotrimethylsilane (Aldrich ≥ 99%) and octadecyltrimethoxysilane (Acros Organics ≥ 95%) were used as received.

Direct synthesis of zeolitic materials

In a typical reaction, SiO₂:NaAlO₂:NaOH was added in a

Results and discussion

In the following section, the results from the addition of different organosilanes are discussed. Fig. 1 shows the resulting XRD patterns of the samples prepared in the presence of organosilanes (6 wt.-% relative to the weight of the silicon used in the reaction). For comparative reasons, the XRD pattern from the resulting bare sodalite (without any organosilane) was also added.

The presence of the organosilanes in the reaction medium did not significantly hamper the type of the crystalline

Conclusions

In situ solvent-free synthesis was shown to be a feasible preparation method for hybrid zeolites. In this study, a sodalite phase was obtained with high crystallinity without the need of a seed or template, in an environmentally friendly method, in the absence of any solvent. Morphologically, the solvent-free route tends to form several crystallization nuclei. The presence of the 6 wt.-% of organosilane introduced in the reaction medium caused no disturbance to the crystal growth. Regarding

Acknowledgements

This project was partially funded by the CNPq. The authors wish to thank the LNLS (Project D11A-SAXS1-8691) for SAXS beamline measurements.

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Synthesis of hybrid zeolites using a solvent-free method in the presence of different organosilanes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Direct synthesis of zeolitic materials

Results and discussion

Conclusions

Acknowledgements

Catal. Commun.

Polyhedron

Fuel Process. Technol.

J. Catal.

Microp. Mesop. Mater.

J. Catal.

J. Catal.

J. Colloid. Interf. Sci.

Colloid. Surf. B

Waste Manag.

J. Electron. Spectrosc. Relat. Phenom.

J. Colloid Interface Sci.

Microporous Mater.

J. Colloid Interface Sci.

Polymer

Chem. Rev.

J. Am. Chem. Soc.

AIChE J.

Chem. Commun.