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Carbon NANOTUBE MEMbranes by Templated Growth in Oriented Molecular Sieve Films

Final Report Summary - NANOTUBEMEM (Carbon NANOTUBE MEMbranes by Templated Growth in Oriented Molecular Sieve Films)

Project objectives

The project aims at the development of 1-D pore system membranes consisting of orientated and uniformly-sized microporous channels, including their use as templates to fabricate ultra-thin carbon nanotubes (CNTs) inside these channels and target highly efficient (high flux - high selectivity) membranes for energy-efficient, high-throughput mixture separation. The main study is the aluninophosphate (AlPO4-5 or substituted AlPO4-5) molecular sieve, which will be optimised, as it consist of sub-nanometer thin pores that are extended parallel to the c- axis of the crystal. As preferred orientation and film thickness are major characteristics that control membrane performance, particular attention is paid to the optimisation of the host materials concerning fabrication of orientated, thin and well-intergrown films. To this extent, performance of the fabricated AlPO4 films will be evaluated because, based on modelling data by other groups, it is anticipated that they may exhibit significantly higher transport rates compared to typical MFI films, due to lack of pore tortuosity and interconnectivity. Once successful, CNT growth on the porous host materials will be studied resulting in CNT membranes and composites.

Work performed

Detailed investigation of the growth behaviour of AlPO4-5 and metal-substituted AlPO4-5 films was performed. Cobalt-substituted AlPO4-5 (CoAPO-5) was also examined as Co incorporation into the AFI framework is anticipated to catalytically enhance CNT growth inside the channels. Using a constant mixture composition and hydrothermal temperature, emphasis was given to pre-growth processes and specifically to precursor mixture preparation and manipulation in an attempt to affect growth. The seeded growth process was employed by involving deposition of AlPO4-5 particles (seeds) on appropriately functionalised substrates, followed by secondary growth under proper conditions in order to decouple nucleation from growth and achieve a better control over film quality. In parallel to AFI film optimisation, experiments on CNT growth behaviour on various supports, including AlPO4-5 in powder form, were performed in order to investigate how support properties and growth conditions affect the resulting CNTs. Furthermore, fabrication of CNT membranes on commercially available supports consisting of 1-D pores with larger diameters, such as anodised alumina (AAO), was attempted and characterisation of the resulting membranes, mainly aiming at the evaluation of the internal CNT configuration, was performed. These experiments revealed that the properties of the resulting CNTs (morphology, size, monodispersity, density, growth rate, yield) are strongly affected by the porosity and chemical composition of the support, the structure, composition and morphology of the catalyst, and the CVD conditions (temperature, reaction time and gas precursor flow rates). In addition, experiments using AAO supports yielded templated growth of CNTs inside the channels, and the resulting CNT/AAO membranes were evaluated by a combination of inside-pore techniques including sorption, and single-phase and relative permeability, providing useful insights on the overall internal CNT morphology, which is crucial in membrane performance.

Experiments on film growth on porous alumina supports took place next for membrane fabrication. A series of growth conditions were examined and systematically manipulated. The fabricated AlPO4-5 films were tested for high temperature stability and optimised accordingly, targeting films that are resistant (in terms of cracking, phase alterations, etc.) to template removal processing, CNT growth and application in a wide range of conditions. Comparative stability experiments were performed on unsupported crystals and films grown on macroporous a-alumina, silicon substrates, and quartz glass using different calcination methods and conditions, including rapid thermal processing (RTP). Growth using microwaves was also investigated on home-made asymmetric supports consisting of a mesoporous titania layer on macroporous a-alumina disks. Pervaporation experiments were performed on the optimal AFI membranes to evaluate performance in separation of binary mixtures. CNT growth experiments took place using a variety of techniques and experimental conditions. Structure directing agent (SDA) pyrolysis inside the AFI channels in vacuum and at atmospheric pressure was examined, as well as CVD using external carbon precursors. The composites and membranes were characterised and their properties were evaluated, with particular attention paid to structural features and performance.

Main results

1. Development of the microporous and CNT films and membranes laboratory;
2. Fabrication of orientated AlPO films on various supports;
3. Tuning of growth direction, thickness, orientation and continuity;
4. Stability performance evaluation - optimisation of growth and calcination;
5. Evaluation of separation performance by pervaporation. The first orientated homogeneous 1-D AlPO4 membrane (exhibiting high separation factors);
6. CNT growth investigation - optimisation in AFI and other orientated supports (templates), such as anodized alumina and other porous support;
7. Parametric investigation of conditions and parameters affecting CNT growth and the resulting composite product - establishment of optimal conditions;
7. AFI/CNT composite membranes. Growth inside the AFI channels - characterisation.

Potential impact and use

The AFI and CNT membranes developed in this project have the potential to substantially contribute toward more energy-efficient separation processes, focusing on applications such as sustainable energy purification, conversion and separation multifunctional membranes, water purification, desalination and removal of pollutants. Also, new perspectives can open up concerning the development of a scalable and economic fabrication technique that results in densely-packed, well-aligned CNTs in the sub-nanometer size range, which could be a breakthrough in membrane technology, in particular with respect to hydrocarbon, hydrogen and other gas separations, and water purification. Although the main focus of the project is the development of membranes for energy-efficient, high-throughput separations, the CNT arrays that were fabricated can also have a multidisciplinary impact on applications in the fields of electronic, optical and electrochemical device fabrication (organised, uniform and ultra-thin single-wall carbon nanotube arrays), hydrogen storage, and photocatalysis.

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