Date on Master's Thesis/Doctoral Dissertation

12-2010

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Carreon, Moises A.

Author's Keywords

Membranes; Gas separation; Zeolites; Metal organic framework; Carbon dioxide separation; Material characterization

Subject

Gas separation membranes; Carbon dioxide

Abstract

The separation of C02 from light gases is a very important environmental and energy issue. The state-of-the-art process for the purification of C02 uses amine adsorption, which is a complex, and costly. Membrane technology is far less expensive and requires less energy consumption. Although polymeric membranes can separate CO2, high pressures plasticize them and decrease their separation ability considerably. Zeolite membranes have significant advantages over traditional polymeric membranes, such as high thermal, mechanical, and chemical stability. Furthermore, the development of superior performance membranes for gas mixture separations requires novel materials with fundamentally different structural, adsorption and transport properties than those of polymers and zeolites. In this respect, zeolitic imidazolate frameworks (ZIFs) a subclass of metal organic frameworks, have emerged as a novel crystalline porous materials which combine highly desirable properties, such as uniform pores and exceptional thermal and chemical stability, making them ideal candidates for molecular separations. This work demonstrates the development of continuous zeolite (SAPO-34) and metal organic framework (ZIF-8) membranes able to separate C02 from CH4 and N2. The membranes were prepared on tubular porous supports by secondary seeded growth. Therefore, first we focused on the synthesis of small homogeneous crystals (both SAPO-34 and ZIF -8) with high surface area and used as “seeds" for membrane nucleation and growth. Crystal growth inhibitors, and microwave heating were used to prepare SAPO-34 seeds (~ 0.5 µm). Solvothermal synthesis was employed to prepare ZIF-8 seeds displaying (~ 50 µm). The entire process from gel formation, nucleation, crystallization and growth of ZIF -8 at room temperature was followed. The resultant SAPO-34 membranes were functionalized with organic amino cations to promote CO2 preferential adsorption and evaluated for the separation of Co2/CH4 and Co2/N2 gas mixtures. CO2/CH4 selectivities as high as 245 with CO2 permeances of ~5 x10-7 mol/m2 s Pa at 295K and 138 KPa were observed. To our best knowledge, our SAPO-34 membranes display one of the best (if not the best) overall separation performance for the separation of CO2/CH4 gas mixtures. Moreover, we demonstrate the successful synthesis of novel ZIF-8 membranes for CO2/CH4 gas separation. This work represents one of the first examples (and the only example on CO2/CH4 separation) of the successful preparation of continuous, thin, and reproducible zeolitic imidazolate framework membranes for a functional gas mixture separation. Our ZIF-8 membranes displayed unprecedented high CO2 permeances up to ~ 2.4 X 10-5 mol/m2·s·Pa and CO2/CH4 selectivities from ~4 to 7. The central intellectual thrust of this work is the rational design of SAPO-34 and ZIF-8 membranes, which offer the possibility of demonstrating high separation performance for CO2 purification from light gases, and other functional gas separations. The specific research objectives are: 1. Rational design of small SAPO-34 and ZIF-8 crystals, displaying narrow size distribution and enhanced CO2 sorption properties. 2. Development of continuous SAPO-34 and ZIF-8 membranes for CO2/ CH4 and CO2/N2 gas separations. 3. Elucidate and understand the basic information mechanisms governing the transformation of precursor solutions into ZIF phases. 4. Establish the fundamental structure/separation relationships of SAPO-34 and ZIF-8 membranes in relevant functional gas separations such as CO2/ CH4 and CO2/N2. This work represents an important advance in the rational design of zeolite and metal organic framework membranes and in basic fundamental understanding of its structure/separation relationships. In particular, the proposed research has practical implications in energy and environmental issues, which are areas of great societal importance. For the targeted applications of carbon dioxide purification from methane and nitrogen, the proposed work may have an important economic impact in reducing considerably the separation costs associated to natural gas pretreatment, and reduction of greenhouse gases emissions respectively. It is anticipated that this work could serve as a model for the rational design of zeolitic imidazolate framework membranes for other important relevant molecular gas separations, such as hydrogen purification from synthesis gas.

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