(183w) Thin and Highly Selective Molecular Sieving Membranes for Cost Effective Hydrogen Purification

Most of the energy today is provided from fossil fuels, which threatens our energy supplies and puts enormous constrains on the environment. A promising alternative to fossil fuels is hydrogen, which is a clean and energy intensive fuel with high specific energy (119 MJ/kg) and zero emission combustion, with water being the only product. However, even though hydrogen is abundant, it does not occur in nature as the fuel H₂. H₂ needs to be produced from chemical compounds like water or hydrocarbons.

To achieve the benefits of the hydrogen economy, hydrogen should be produced from non-fossil resources, such as biomass or water, using renewable energy sources (e.g. sunlight). However, even using the cheapest source of hydrogen, steam reforming of methane, H₂ is more expensive than gasoline for the same amount of energy.[1] Until hydrogen can be produced economically in quantities and at costs competitive with fossil fuels, H₂ will come mainly from fossil fuels. While industry players have already started the market introduction of hydrogen fuel cell systems, including fuel cell electric vehicles, the use of hydrogen at grid scale requires the challenges of clean hydrogen production, bulk storage and distribution to be resolved.[1]

Majority of hydrogen is produced by steam reforming of methane and water vapor into hydrogen and carbon monoxide. This is followed by the water-gas shift (WGS) reaction to convert carbon monoxide and steam to more hydrogen and carbon dioxide. The product contains impurities such as CO₂, CO, hydrocarbons, H₂O and H₂S. Therefore, to ensure sustainable hydrogen production from fossil fuels, carbon dioxide should be captured.[2] That kind of dilemma is confronted in virtually all of the proposed routes for hydrogen production. Research is particularly needed on hydrogen purification (to separate hydrogen or CO₂ from gas mixtures and to produce high purity H₂ suitable for fuel cells). This involves the development of novel catalysts, adsorption materials and gas separation membranes.[2]

Commercial technologies for H₂ purification include cryogenic distillation and pressure-swing adsorption, PSA, (using zeolites, activated carbon, activated alumina and silica gel) which are energy intensive, accounting for around 30% of the total plant capital and operating cost. Membranes have also been commercialized but they are either expensive (Pd) or cannot meet the H₂ purity required by fuel cells (polymer).

Fundamental advances in catalysis, membranes, and gas separation could enable more efficient, lower-cost fossil hydrogen technologies.[3] Efficient, high-volume gas separations by semipermeable membranes are an attractive option to achieve this goal. This works addresses the challenge of hydrogen purification from water gas shift reactions using membrane technology, where highly selective and permeable membranes are prepared and tested for hydrogen purification.

The objective of this project is the preparation of thin and highly selective molecular sieving membranes on polymer supports for cost effective hydrogen separation. The focus will be on zeolites LTA and Chabazite, as well as other zeolites of similar pore size or multilayer membrane. The tasks are i) seed or gel synthesis, ii) membrane preparation and characterization, and iii) membrane permeation testing.

Zeolite membranes are prepared either by in situ hydrothermal synthesis, secondary (seeded) growth synthesis, or vapor phase transport synthesis.[4]–[6] Often, pre-treatment of supports and post-treatment of membranes for pore size reduction and defects removal are carried out to improve the quality of the as-synthesized membranes.[4], [7] Dry gel conversion and seeding followed by secondary growth are attempted for the preparation of zeolite layers on polymer supports

Nanosized zeolite LTA (< 50 nm) are synthesized from low cost inorganic routes (not reported yet) to allow the preparation of sub 100 nm zeolite films on polymer supports. Avoiding organic additives, which until recently were needed for the reduction of zeolite crystal sizes, reduces cost and eliminates the need for a calcination step at the end to burn the organic which adds safety and environmental concerns and can create defects in the membrane. The size reduction is attempted by systematic changes in synthesis compositions and parameters including heating temperature and time.[8] Synthesis approaches are optimized to lower the cost.

Polyethersulfone (PES) or polybenzimidazole (PBI) are used as supports. Both are very stable polymers at 200 °C. Initial studies are performed on flat supports that will be synthesized in-house by phase inversion.[9]. Successful membranes prepared from the previous tasks are subjected to permeation testing.

We acknowledge support for this work from Khalifa University of Science and Technology.

References

[1] G. W. Crabtree, M. S. Dresselhaus, and M. V. Buchanan, “The Hydrogen Economy,” Phys. Today, vol. 57, no. 12, pp. 39–44, 2004.

[2] “Hydrogen Production and Storage: R&D Priorities and Gaps,” 2006.

[3] “Basic Research Needs for the Hydrogen Economy,” Feb. 2004.

[4] N. Rangnekar, N. Mittal, B. Elyassi, J. Caro, and M. Tsapatsis, “Zeolite membranes - a review and comparison with MOFs,” Chem. Soc. Rev., vol. 44, no. 20, pp. 7128–7154, 2015.

[5] M. Matsukata, K. Kizu, M. Ogura, and E. Kikuchi, “Synthesis of EMT Zeolite by a Steam-Assisted Crystallization Method Using Crown Ether as a Structure-Directing Agent,” Cryst. Growth Des., vol. 1, no. 6, pp. 509–516, 2001.

[6] S. Alfaro, M. Arruebo, J. I. In Coronas, M. Men E Endez, and J. U. Us Santamar I Ia, “Preparation of MFI type tubular membranes by steam-assisted crystallization,” Microporous Mesoporous Mater., vol. 50, pp. 195–200, 2001.

[7] F. Gallucci, E. Fernandez, P. Corengia, and M. van Sint Annaland, “Recent advances on membranes and membrane reactors for hydrogen production,” Chem. Eng. Sci., vol. 92, pp. 40–66, 2013.

[8] M. Khaleel, W. Xu, D. A. Lesch, and M. Tsapatsis, “Combining Pre- and Post-Nucleation Trajectories for the Synthesis of High FAU-Content Faujasite Nanocrystals from Organic-Free Sols,” Chem. Mater., vol. 28, no. 12, pp. 4204–4213, Jun. 2016.

[9] K. E. Kinzer, D. R. Lloyd, M. S. Gay, J. P. Wightman, B. C. Johnson, and J. E. McGrath, “Phase inversion sulfonated polysulfone membranes,” J. Memb. Sci., vol. 22, no. 1, pp. 1–29, Jan. 1985.