Binderless zeolite coatings on macroporous α-SiC foams

doi:10.1016/j.micromeso.2014.01.008

Microporous and Mesoporous Materials

Volume 188, April 2014, Pages 99-107

https://doi.org/10.1016/j.micromeso.2014.01.008 Get rights and content

Highlights

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ANA and MFI zeolite crystals were grown without any binder use over α-SiC supports.
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The novelty consists of using the Si-contained in α-SiC substrate to allow the zeolite growth.
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The size of ANA crystals was controlled, thanks to the SiC duplex macroporosity.
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A support surface coverage of 55g_zeolite/m²_SiC was achieved.

Abstract

Several syntheses have been performed to allow the growth of zeolite crystals on α-silicon carbide supports (α-SiC). Silicon-carbide foams exhibit a duplex macroporous structure. MFI and ANA zeolites have been successfully coated on this relatively inert material. While the synthesis of MFI/SiC required the presence of an additional Si-containing source, in contrast, ANA/SiC composites have been unexpectedly obtained through the self-recrystallization of the silicon contained in the α-SiC substrate. In addition, the size of icositetrahedral ANA crystals was controlled to nearly 35 μm, thanks to SiC open cells templating effect.

The different composite materials were thoroughly characterized by SEM, complete textural analysis and XRD. The amount of zeolite coating as well as the coverage by ANA zeolite crystals on SiC surface were determined by both SEM observations and adsorption measurements.

Graphical abstract

Introduction

The microporous structure as well as the defined pore architecture and size of zeolites are well established. In addition, these crystalline aluminosilicates have found widespread applications as ionic exchangers [1], molecular sieves [2], petrochemicals cracking catalysts [3] and more recently as confinement media [4]. Hence, zeolites are considered as green catalysts. A proper control of their chemical composition and microstructure is ensured during the synthesis. In contrast, zeolites are mainly used in catalytic fixed beds in the forms of randomly packed microgranules or extrudated pellets having few millimeters in size. Their macrostructure remains therefore fairly undefined.

Although zeolites exhibit many advantages, their industrial application can be hindered by the following drawbacks in a fixed bed-use; (a) high-pressure drop, (b) limited heat and mass transport rates, (c) axial dispersion leading to loss of selectivity, and (d) susceptibility to fouling by dust [5].

During the past decades, a deeper attention has been paid to the development of zeolite coatings on various macro-shaped supports [6]. Studying new active phase formulations, numerous researches have been focused on the development of structured composites (catalyst + support), which could provide an easier access of the reactants to the active sites and also avoid mass transport limitations [7]. There have been various attempts to allow the growth of zeolite crystals on macroscopic inert supporting materials. One can cite for instance, both meso- and macroporous glass monoliths [8], alumina [9], metals [10], ceramic foam [5], [11], sacrificial biological supports [6](d), [12], which have been used as supports for the development of structured catalytic beds. Among those aforementioned materials, silicon carbide (SiC) appears as a promising support due to its robustness. Indeed, these porous ceramics exhibit the required intrinsic properties to become valuable candidates as zeolite support. Silicon carbide materials possess high thermal conductivity, high mechanical strength, excellent resistance to oxidation, chemical inertness and ease of shaping [13]. In contrast to β-polymorph, α-SiC demonstrates a higher thermal conductivity and better resistance to attrition [14].

SiC porous substrates have been widely used as filters, membranes, catalyst supports, thermal insulator, gas-burner media, and refractory materials owing to their superior properties, such as: low bulk density, high permeability, high temperature stability, and erosion resistance [15]. Silicon carbide possesses a low density which significantly reduces the fraction of useless weight in the overall composite weight. Finally, β-SiC synthesized according to the shape-memory-synthesis [13] exhibits a medium surface area, rendering it suitable for the dispersion of zeolite crystals on its surface [5], [11](e).

The aim of the present study is to use our experience in the zeolite coating techniques [5], [8](b), [11](e) to allow the growth of zeolite crystals on barely “receptive” α-SiC material. The two main routes for the synthesis will be investigated using alkaline medium or fluoride anions as mineralizer. Furthermore, the addition or not of external silica source will be studied. Fig. 1 schematically describes the two different strategies followed: depending on the conditions, one may expect the formation of different zeolite structures. One further objective is to produce composite materials having both a duplex macroporosity (giant pores: diameter ≈ 38 μm and macropores: diameter ≈ 7 μm) along with microporosity and mesoporosity gained by the zeolite coating.

Section snippets

Experimental

All the chemicals were used as received from the suppliers.

Results and discussion

The main objective of our study is to develop structured catalytic beds for an easier application in real reactors. Accordingly, ZSM-5 zeolite was chosen as catalyst to be coated on extremely robust α-SiC material.

Table 1 summarizes the syntheses performed to obtain composite materials along with respective operating conditions. It is noteworthy that the addition of TEOS as an external silica source led to the sole formation of MFI zeolite crystals onto macroporous SiC foam. Indeed, both

Tentative mechanism of ANA crystals growth on SiC surface

The experimental factors which govern the self-assembly of zeolite crystals onto support surfaces in thin films or other coatings are not well understood yet. Nevertheless, the T–O–T bond making and bond breaking (where T = Si or Al), based upon nucleophilic reactions, facilitate structural modification catalyzed by hydroxyl ions. Furthermore, it appears that this strategy of using the self-transformation of pristine α-SiC support, allows the formation of ANA zeolite crystals, which can grow with

Conclusion

Several zeolite coatings have been successfully achieved over α-SiC support. This study demonstrates that depending on the presence (or not) of an external Si-source, different zeolites can be synthesized on α-SiC foam surface. MFI crystals could be grown following a conventional procedure. In contrast, ANA zeolite structure was formed without any additional Si-source. The surface coverage and the size of ANA crystals have been tailored in view of future application of these composites.

Merging

Acknowledgements

The authors are grateful to the Agence Nationale de la Recherche (ANR) for supporting the ANR-10-JCJC-0703 project (SelfAsZeo). K.S. and B.L. are grateful to the Barrande program for funding 26551RE project. The present project is supported by the National Research Fund, Luxembourg (PL PhD Grant). The technical assistance of Thierry Romero (ICPEES) was highly appreciated.

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Binderless zeolite coatings on macroporous α-SiC foams

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results and discussion

Tentative mechanism of ANA crystals growth on SiC surface

Conclusion

Acknowledgements

Appl. Catal. A

J. Phys. Chem. C

J. Phys. Chem. B

Catal. Today

Cattech

Ind. Eng. Chem. Res.

Microporous Mesoporous Mater.

Mater. Sci. Eng., C

Angew. Chem. Int. Ed.

Cattech

Adv. Ceram. Mater.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Chem. Eur. J.

J. Am. Chem. Soc.

Allg. Chem.

J. Phys. Chem. B

J. Phys. Chem. B

J. Phys. Chem. B

J. Phys. Chem. B

Langmuir

J. Am. Chem. Soc.

Microporous Mesoporous Mater.

Microporous Mesoporous Mater.

Microporous Mesoporous Mater.

J. Memb. Sci.

Zeolites in Industrial Separation and Catalysis

Chem. Rev.

Top. Catal.

Appl. Catal. A

Chem. Eur. J.