Artificial membranes made from sand-like materials known as silica are potentially more energy efficient than other separation processes such as distillation (change in phase from liquid to gas) because there is no phase change required to perform the separation. In addition, the opportunity exists for combining reaction and separation within a single unit using membrane reactors, thereby increasing yield on thermodynamically-limited reactions. However, the fabrication of high-quality silica membranes with pore size control and surface chemistry control remains challenging because of the inherent limits of existing synthetic approaches used to fabricate silica membranes. The researchers at the University of Maine have achieved promising preliminary results on pore size control and surface chemistry control using new synthetic approaches toward fabricating silica membranes. These techniques are based on highly controlled catalyzed surface chemistry reactions that are used to modify mesoporous silica membranes. The reactions are atomically controlled at the surface to provide a self-limited pore size reduction and the functionalization of the mesoporous matrix. In this CAREER plan, the university of Maine will use the new synthesis technique, known as catalyzed-atomic layer deposition, to prepare silica membranes with controlled pore sizes in the pore size range of 10-20 angstroms and create new hybrid organic/inorganic membranes. This will be achieved using both vapor phase deposition and supercritical fluid CO2 deposition techniques. This will provide a new class of silica materials that may find application in the separations of higher molecular weight compounds as well as a new class of hybrid organic/inorganic silica-based membranes for gas/vapor separations. The research will focus upon understanding chemical, microstructural, permeation, and separation properties of the new materials while quantitatively linking the synthesis procedure to material performance. The proposed synthesis techniques offer a level of atomic control during the materials preparation that is not known today. The applications for these membranes are diverse and include separations of heavy distillates in petroleum processing, separations of organic compounds from lighter gases, separators for lithium-ion batteries, and bio-separations. These new synthetic techniques are expected to spur application towards different classes of materials, including adsorbents or even different inorganic membranes. The proposed education activities will affect all chemical engineering undergraduates at the University of Maine and a significant number of high school students, including those in some of Maines poorest and most geographically remote communities.
The objective of this work was to investigate methods to prepare membranes that would result in a higher degree of control over the membrane properties. Membranes are important tools for engineers and are used to separate mixtures containing larger structures like particulates, all the way down to individual molecules. For the separation of small molecules, membranes have some advantages over traditional separation methods because they use less energy. The ability of a membrane to separate molecules from a mixture depends on its physical and chemical properties. Membranes are made up of tiny pores, roughly the size of the molecules in the mixture. A fine level of control over the size of these pores is needed to sieve the molecular mixture. For typical molecules, this size can be near one nanometer. Also, the affinity for the molecules in the mixture can also affect the separation. Molecules with a higher affinity for the membrane material will likely pass through the membrane easier and enhance separation. Therefore, a good membrane synthesis strategy needs to address controlling the final pore size and the surface chemistry of the membrane. This work resulted in generating fundamental knowledge regarding a new method for controlling membrane properties that involved carrying out controlled chemical reactions within the tiny pores of a membrane. These chemical reactions left behind an atomic layer of material within the pores. Since the reactions were highly controlled, several atomic layers could be placed within the pores, resulting in a controlled reduction in membrane pore size. The chemical reactions favored larger pores due to the ease with which the chemical reactants could get into the pores. In addition, the surfaces of the pores could be terminated with different chemicals that changed their affinity for certain molecules. For example, the pores could be terminated with something like a plastic that would repel water and attract hydrocarbons. Understanding how to synthesize membranes using these types of reactions is the first step in developing new membranes that could impact many aspects of society. For example, energy savings in the chemical processing industry could be very large, reducing our dependence on foreign oil. In the emerging biofuels production, there are many needs for separations that membranes could provide a unique solution. In addition, having the ability to finely tune membranes is important in health care. Highly specified membranes can offer the ability to control how drugs and therapies are administered in the body, to target specific cells for example. This grant also had an educational impact. Several students received doctoral degrees in Chemical Engineering and are currently working in research and development positions that will generate new knowledge and jobs. The grant was used to mentor undergraduate engineers as well as high school students to encourage them to pursue careers in engineering.
