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The proposed research seeks to rationally design highly selective zeolite membranes for CO₂ separation. This research has the potential to be quite significant from a scientific as well as from a technological perspective. If successful, it will establish the viability of novel self-assembly concepts as general synthesis methods for a broad range of functional zeolite membranes with tunable structures, compositions, and porosities for applications in relevant industrial gas separations. It will represent an important advance in fundamental understanding of the structure/separation relationships of the resulting zeolite ordered porous structures. In particular, the proposed research has practical implications in environmental and energy issues, which are areas of great strategic importance. For the targeted application of carbon dioxide purification from methane, the proposed work may have an important economic impact in reducing considerably the separation costs associated to natural gas pretreatment. It is anticipated that this work could serve as a model for the rational design of nanocrystalline zeolite membranes for other important relevant gas separations, such as CO₂ capture from flue gas, and the purification of synthesis gas to produce hydrogen for fuel cells. 

Impact on my career

This ACS-PRF DNI grant has positively impacted the initial stages of my academic career in two main fronts. First, in the research component, it has enormously helped me to propel my research activities related to the development of nanocrystalline membranes for carbon dioxide purification. It has allowed me to attend scientific meetings, including the recent Gordon Research Conference on Membranes: Materials & Processes, where I had for the first time the unique opportunity to informally discuss and exchange ideas with experts in the field. Furthermore, the research outcomes resulted from this grant, have enlarged my research vision by learning about other exciting energy related materials, such as metal organic framework membranes. In the teaching component, I disseminate on a continuous basis the results of the research in my Materials Science lectures. Therefore my undergraduate and graduate Engineering students have benefited indirectly from this grant, by having the great opportunity to be exposed to the state-of-the art in this field of membrane and gas separation science.

Impact on students

My former PhD student (Surendar Venna, now postdoctoral fellow in the Chemical Engineering Department at Georgia Tech) , and three undergraduate students (Sera Kim, Juan Merizalde, Joey Bohrman) participated actively participated in this project. Currently, another PhD student (Minqi Zhu) is working in the project. The students have greatly benefited from this project by learning fundamental aspects on Materials Chemistry (synthesis of seeds and membranes), Materials Characterization (use of different characterization techniques including XRD. SEM, TEM, FTIR, porosimetry, ICP analysis), and Engineering/Separation (evaluation of the separation performance of the membranes in a gas continuous separation system). Dr. Venna presented research results related to this grant in Scientific National Meetings including the 2009 Materials Research Society Meeting (Boston, MA; December 2009) and in the North American Membrane Society Meeting (Washington, DC; July 2010). Ms. Minqi Zhu presented in the North American Membrane Society Meeting (Las Vegas; June 2011). 

Summary of Results

We have prepared SAPO-34 seeds and membranes employing one of the proposed self-assembly approaches, namely the mesoporous approach. We have incorporated different structure directing agents in the gel solution such as Brij-35, polyethylene glycol and P123. The resultant seeds displayed specific surface areas in the 640-700m²/g range. Typical SAPO-34 surface areas are only in the 450-500 m²/g range. Our seeds displayed higher surface areas, most likely due to their smaller crystal size and due to the incorporation of extra microporosity in the SAPO-34 framework. The average crystal size of seeds was in the ~0.5-0.7 µm range, a highly desirable size to prepare thin membranes. Furthermore, the seeds showed particle narrow size distribution. The SAPO-34 seeds were employed to prepare ~5 micron membranes on porous tubular alumina and stainless steel supports by secondary seeded approach. The resultant membranes have been tested in the separation of equimolar CO₂/CH₄ gas mixtures. The membranes displayed CO₂/CH₄ selectivities of ~180 and CO₂ permeances as high as 1x10^-6 mol/m² s Pa at 295 K at 0.22 MPa. The SAPO-34 membranes have been functionalized with organic amino cations, such as ethyl diamine, hexylamine and octyl amine to promote CO₂ preferential adsorption. CO₂ a Lewis acid molecule adsorbs preferentially on the amino cation basic sites. The functionalized membranes showed CO₂/CH₄ selectivities as high as ~250.^[1] Recently, we have developed novel nanocrystalline seeds ^[2] and membranes ^[3] with zeolitic imidazolate framework compositions. These membranes (ZIF-8) displayed unprecedented high CO₂ permeances up to ~ 2.4 x 10^-5 mol/ m²·s·Pa and CO₂/CH₄ selectivities from ~ 4 to 7 at 295 K and a feed pressure of 0.14 MPa. To the best of our knowledge, these membranes represents one of the first examples of the preparation of continuous, thin, and reproducible zeolitic imidazolate framework membranes for a functional gas mixture separation.

Our current research efforts are focused on the synthesis of other zeolite membrane compositions (mainly DD3R) for the effective separation of CO₂/CH₄ gas mixtures.

I have acknowledged ACS-PRF for funding in the following three refereed publications:

• [1] S.R. Venna, M.A.Carreon*, Langmuir 2011, 27, 2888-2894.
• [2]S.R. Venna, Jacek B. Jasinski, M.A.Carreon*,  Journal of the American Chemical Society 2010, 132, 18030–18033.
• [3] S.R. Venna, M.A.Carreon*,  Journal of the American Chemical Society 2010, 132, 76-78.