Synthesis of capillary titanosilicalite TS-1 ceramic membranes by MW-assisted hydrothermal heating for pervaporation application

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Abstract

Titanium silicalite-1 (TS-1) membranes were obtained on α-Al2O3 capillaries by secondary growth of silicalite-1 seeds under microwave irradiation. TS-1 membranes were grown at 180–190 °C either on the outer side or on both sides of the capillaries, from sols with a molar ratio Si/Ti = 16–75. All the derived membranes were gas-tight before template calcination (no macro-defects) although the highest Si/Ti ratios lead to higher membrane quality with N2/SF6 > 100 for single gases and separation factors up to 65 for EtOH/H2O mixture separation by pervaporation (EtOH flux up to 2.2 kg/h m2 at 65 °C). The reproducibility of membrane performance, with good balance between flux and selectivity, is highly attractive for further industrial applications of these TS-1 membranes which are currently developed on multi-capillary modules.

Research highlights

▶ Microwave-assisted synthesis is a reproducible method to grow thin TS-1 membranes ▶ The higher Si/Ti ratios lead to higher membrane quality. ▶ Double-sided TS-1 membranes have a good pervaporation performance. ▶ TS-1 capillary membranes with a good balance between PV flux and selectivity.

Introduction

Zeolite membranes have been considered for a number of applications in gas separation, pervaporation and catalysis because of their adsorption and molecular sieving properties, coupled with their higher thermal, chemical and structural stability compared with polymeric membranes [1], [2], [3], [4]. Pervaporation (PV) is a quite effective method for dehydration of organic substances, especially for separation of azeotropic mixtures. Pervaporation is a highly efficient separation processes, involving simple operation, low-energy consumption [5], no sweep fluid [6] and having low-environmental impact [7]. Processing parameters such as operating pressure, temperature and hydrodynamic conditions strongly impact the efficiency of the PV process, although membrane structure and properties are far-reaching issues. The PV process is economically more attractive when the preferentially permeable component is present at low concentration in the feed, as far as much lower evaporation heat consumption is required for the small fraction component. Therefore, this separation technique could be very effective to concentrate ethanol in fermentation broths with high (around 95 wt.%) water concentration.

Zeolite membranes are attractive for separating mixtures of components with differences in molecular sizes or in adsorption properties, but which are hardly or costly separated using conventional separation methods or polymeric membranes [3]. Hence, zeolite membranes have been widely used for pervaporation at both laboratory [3] and industrial [8], [9] scales for the dehydration of solvents. Although hydrophilic membrane structures, such as LTA and FAU, are the most largely developed for the dehydration of organics by PV, promising performance were recently published for the extraction of organics from aqueous mixtures with hydrophobic MFI type membranes [10], [11]. The most studied MFI hydrophobic membrane is silicalite-1 (S-1), which is composed of pure silica, but the titanium-substituted silicalite-1 (TS-1) has been reported to perform even better than pure S-1 for the separation of EtOH/H2O mixtures by pervaporation [11].

TS-1 zeolite is an excellent selective oxidation catalyst with hydrogen peroxide as the oxidant, for e.g. the selective oxidation of olefins to epoxies, alcohols to aldehydes or ketones, the hydroxylation of aromatic compounds, and the ammoximation of cyclohexanone [12]. Normally, TS-1 is crystallized by hydrothermal process that often lasts between 3 and 10 days [13]. The required long crystallization time often leads to large TS-1 zeolite crystals and formation of extra-framework titanium in form of anatase, decreasing the hydrophobicity of the zeolite. Also, in the case of membranes, the insertion of support species (e.g. Al by leaching of alumina supports) within the TS-1 structure during the long synthesis process could strongly impact the PV performance. Indeed if Al Lewis sites are formed in the TS-1 membranes (Ti-ZSM-5), the membrane becomes more hydrophilic and preferably adsorb/permeate water.

In order to prepare Ti-rich TS-1 membranes, free of both Al and extra-framework species, Motuzas et al. [14] employed a MW-assisted secondary growth hydrothermal method, using tubular α-Al2O3 supports. Microwave (MW) heating is a fast, simple, uniform and energy efficient method, which can reduce significantly the synthesis time of zeolite membranes [16] and then decrease the chance of support etching. Uniform sol heating and good control of the chemical integrity of the support helps providing reproducible and good quality MFI zeolite membranes from MW derived S-1 nanoseeds [15], [16]. When growing TS-1 membranes by MW-assisted heating a competition is expected between: (i) MFI network distortion due to Ti incorporation in the growing zeolite and (ii) selective MW absorption by Ti species which are potentially overheating centers due to their high coordination ability (higher than Si) which attracts H2O molecules and charged species [15].

Most of the zeolite membranes reported in literature have either a flat plate or tubular geometry, although monolithic supports have also been used [10]. The flat plate geometry is advantageous for the homogeneity in seeding and synthesis on the small disks used for academic studies, but it has a low surface area to volume ratio (S/V). The S/V ratio of tubular membranes is larger and easier to scale-up, consequently, commercially available zeolite membranes for pervaporation have a tubular geometry [8], [9].

In order to achieve high ethanol fluxes and selectivities in the ethanol/water separation, it is important to optimize all the transport steps occurring during the pervaporation process: (i) adsorption, in this sense it is important to maximize the hydrophobicity of the zeolite layer, (ii) transport resistance in the zeolite layer, by minimizing its thickness, and (iii) transport through the porous support, by decreasing its thickness and, eventually, by synthesizing the zeolite layer on both sides of the support for limiting concentration–polarization phenomena [17]. In this work a series of TS-1 zeolite membranes have been prepared under MW irradiation from a series of sols with different Si/Ti molar ratios on ceramic capillary supports with a high membrane surface area-to volume ratio (>1000 m2 m−3), which is twice higher than that of classical tubular supports (≪500 m2 m−3) [17]. The selective layer was deposited either outside or on both sides of the capillaries. Membrane series have been characterized by both XRD and SEM methods, their quality was evaluated by single gas permeance with both N2 and SF6, and their performance has been compared for the separation of EtOH/H2O mixture by pervaporation.

Section snippets

Membrane synthesis by secondary growth

The ceramic α-Al2O3 capillary supports (Hyflux CEPAration Technologies, The Netherlands) have a symmetric pore structure that corresponds to a pore diameter of 200 nm. The capillary has an inner–outer diameter of 2.0–3.0 mm and a total length of 50 mm, in order to properly fit in the microwave autoclave.

The Microwave (MW) oven used in this study was a computer controlled Milestone ETHOS 1600, with a fixed frequency of 2.45 GHz (length of a single full wave is 12.2 cm). One of the MW Teflon lined

Results and discussion

TS-1 film synthesis by the secondary growth method was selected because it is currently recognized as one of the most attractive and flexible methods for orienting the formation of thin consolidated membranes and films. This method involves pre-seeding of the support prior to film growth from seeds. Therefore, the nucleation and crystal growth are carried out in separate steps and may thus be controlled separately, which could be an advantage for membrane reproducibility.

It is common to prepare

Comparison with literature data

The main drawback in EtOH/H2O pervaporation is that diffusion favors water permeation because EtOH kinetic diameter (0.43 nm) is larger than that of water (0.26 nm). Hydrophobic zeolites overcome this drawback by preferentially adsorbing organics, but non-zeolite pores and structural defects in the membranes may be a more significant disadvantage to hydrophobic membranes than to hydrophilic ones because of silanol groups on the zeolite surface. The presence of these silanol groups increases the

Conclusions

A microwave-assisted synthesis method was used to grow reproducible TS-1 membranes on capillary alumina supports. The derived membranes revealed high flux and intermediate selectivity for ethanol/H2O separation by pervaporation.

Strong influence of synthesis conditions: temperature and duration, as well as Ti concentration in the mother sol, was evidenced on the membrane performance. The Si/Ti ratio was vital for membrane quality. Membranes prepared from sols with a molar ratio Si/Ti = 16–25

Acknowledgements

This work has been performed in Hyflux CEPAration Technologies (preparation of supports), IEM and Universidad Zaragoza, thanks to the financial support of both the European Commission (through the NanoMemPro Network of Excellence) and Hyflux CEPAration Technologies (Europe). Authors also warmly acknowledge the IEM technical staff and namely Emilie Gérardin and Didier Cot for their assistance in membrane characterization by SEM.

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