(5p) Fundamental Understanding of Zeolites towards Engineering of Zeolite Membranes

Separation processes currently consume 15 % of global energy (60 vs. 400 1015 Btu/Year). With the global commodity production expected to increase six-fold by 2040, a business as usual scenario is not sustainable and an order of magnitude increase in separation efficiency is a necessary step toward sustainable global prosperity. Active nanoporous (10-9 ~10-7 m pore size) adsorbents (e.g., zeolites) and membranes enabling separations with molecular resolution to replace thermally driven processes like distillation and crystallization meet this goal and are re-emerging as an area of nanotechnology research. These highly selective zeolite membranes could not only efficiently purify products but also be integrated with chemical reactors to increase the yield of the desired product eliminating or reducing the need for energy intensive separation steps. Zeolites are crystalline inorganic oxides built up by connected TO4 (T= Silicon or Aluminum) tetrahedra resulting in unique pore sizes and structures. Zeolites are used in a number of applications such as detergents, ionic exchangers for water softening and purification, and catalysts in the specialty chemical and petrochemical industry. A breakthrough technology for energy efficiency in the field of separations is that of molecular sieve or zeolite membrane technology.

Through my doctoral (Dept. of Chemical Engineering & Materials Science, University of Minnesota with Professor Michael Tsapatsis) and postdoctoral (Dept. of Chemical Engineering, University of California, Berkeley with Professor Enrique Iglesia) work, we have focused on resolving several issues in the field of zeolite technology including: (1) controlling morphology of zeolite particles via an appropriate choice of structure directing agents, (2) developing a methodology for oriented molecular-sieve zeolite and composite films, (3) engineering defects present on most polycrystalline films, and (4) characterizing transport inside zeolite crystals using frequency modulation flow protocols.

1. Methodology development for oriented zeolite film formation (Ph.D. work with Prof. Tsapatsis).

Zeolite membranes are physical barriers that can separate molecules based on even minute differences in molecular size and/or shape. As a result, much research has been focused on the field of zeolite membranes with respect to control of pore orientation and film thickness on diverse supports. However, the use of zeolite membranes in industry has been limited largely because the reproduction of defect-free zeolite membranes on a large scale is difficult. In the first efforts to make zeolite films on porous supports, many researchers followed the so called in-situ method in which the supports were placed in direct contact with the growth solution. Although researchers contrived to find optimal synthesis conditions by exhaustive investigation of all possible parameter combinations, the success was limited because it was not easy to control both nucleation and crystal growth at the same time.

Therefore, nucleation and crystal growth need to be decoupled for practical controllability. One possible way is the so called secondary growth method.[1, 2] That is, a seed layer is formed on supports and layered crystals are inter-grown by closing the gaps between crystals. The key for successful secondary growth for continuous oriented films is to promote in-plane (parallel to the support) growth of the deposits, while minimizing out-of-plane (perpendicular to the support) growth and avoiding new nucleation of grains. We found that the secondary growth conditions need to be optimized to induce comparable in-plane and out-of-plane growth rates.[1] This requirement is satisfied mainly by an appropriate choice of SDAs. We have shown conclusively that the SDAs can play a dual role in the synthesis of membranes: they template the desirable crystal structure and they allow for systematic control of growth rates along different crystallographic directions.[1]

Along with secondary growth, we also demonstrated that composite films consisting of zeolite particles and a matrix between them (MCM-22/silica films [3] and MFI/PTMSP films [4]) showed promise in gas and vapor separation applications. In particular, no requirement of hydrothermal growth, known as a time- and energy-consuming step, during film fabrication in such methods made these membranes attracted for practical use at the large scale.

2. Engineering grain boundary defects on c-oriented MFI Membranes (Ph.D. work with Prof. Tsapatsis).

The overarching goal of my research was to develop a hierarchical manufacturing process for the production of high performance zeolite membranes. At the more applied level, the goal was to develop scalable processing strategies that allow the fabrication of high performance zeolite membranes on practical industrial supports like cylindrical porous stainless steel tubes.

Defects frequently present between and/or on polycrystalline zeolite films could not be well controlled by any means up to now, though poor separation performance was already attributed to non-selective pathways provided by these defects. Rather, they were concomitantly generated during membrane synthesis and/or calcinations steps. Rapid thermal processing (RTP) was introduced for the first time to calcine c-oriented MFI membranes, because any efforts to reduce defect formation based on a slow heating process did not work. This RTP resulted in the dramatic improvement of separation performance for both aromatic compounds and linear/branched alkanes.[5] The remarkable improvement was attributed to the lowered density of grain boundary defects on RTP-treated c-oriented MFI membranes, as envisaged by Fluorescence Confocal Optical Microscopy at the molecular level. Despite the virtually identical microstructures (e.g., out-of-plane orientation, film thickness, grain size, Si/Al ratio, etc.) to conventionally calcined c-oriented MFI membranes, reduced defects could lead to approaching towards the expected high separation performance. The easy, but robust fabrication protocol for c-oriented MFI films makes RTP-treated ones feasibly adopted as alternative or supplemental to industrially important, but energy-intensive separation system.

3. Characterization of transport inside zeolites by frequency modulation flow protocols (Postdoctoral work with Prof. Iglesia).

Through post-doctoral research, my knowledge of separations during Ph.D. course is being complemented with experiences in catalysis. The effective integration of separation and reaction systems has the potential to be of great industrial value. For instance, the combination of reaction and separation can lead to the realization of the membrane reactor concept which could save even more energy than a separation membrane alone.

The current project is aimed to perform the measurement of diffusion rates and isotopic exchange rates for probing the role of intra-zeolite channels on the traffic and reactive encounters of hydrogen-containing molecules. Prior to an intensive work on this ultimate goal, initially we are attempting to prove the concept of the frequency response technique by employing a diffusion system without a reaction being involved. As an interesting example, the separation of CO2 and CH4 during natural gas extraction attracts researchers because of its technical and economical effectiveness. By doing so, one can eliminate or at least reduce the need of additional CO2 gas separation resulting in energy saving. Concomitantly, this approach will help to lower CO2 concentration in the atmosphere as global climate change becomes intensively concerning to many. We believe that the frequency response technique has a strong potential to contribute to understanding the separation of CO2 and CH4 in powder-based approaches. More specifically, we could characterize the diffusion coefficients of these and find optimal operating conditions for maximizing separation performance. Now, we are measuring the diffusion coefficients of CH4 and CO2 single gas components and the corresponding mixture on MFI type zeolites using the frequency response technique. This approach will be easily extended to the examination of other types of zeolites and direct a guideline to seek out appropriate materials for this specific CO2/CH4 separation. Given that a model is well established that reflects this system, one could take advantage of this technique to characterize the properties of diverse materials systematically and further select proper ones for a specific purpose.

References

1. J. Choi, S. Ghosh, Z. Lai, and M. Tsapatsis, Angew. Chem., Int. Ed., 2006. 45(7): p. 1154-1158.

2. J. Choi, S. Ghosh, L. King, and M. Tsapatsis, Adsorption, 2006. 12(5-6): p. 339-360.

3. J. Choi, Z. Lai, S. Ghosh, D.E. Beving, Y. Yan, and M. Tsapatsis, Ind. Eng. Chem. Res., 2007. 46(22): p. 7096-7106.

4. M. Woo, J. Choi, and M. Tsapatsis, Micropor. Mesopor. Mater., 2008. 110(2-3): p. 330-338.

5. J. Choi, H.-K. Jeong, M.A. Snyder, J.A. Stoeger, R.I. Masel, and M. Tsapatsis, submitted.