Membranes are well suited to desalinate or purify water, while using little power. Innovation is required, however, as scarcity of pure, fresh water is a growing concern. To guide the design of new membranes, we propose to learn from protein channels in cell walls, some of which are remarkably efficient for separations such as water desalination. This EAGER proposal focuses primarily on the high-risk, high-reward endeavor that consists of the synthesis of nanoporous silica membranes with tunable, controlled pore sizes and functional groups of the desired chemistry and charge, tethered to the surface. These will form the basis for realizing a proof-of concept bio-inspired membrane. We will extend our preliminary molecular simulations of biological nanopores to obtain fundamental understanding in the physico-chemical principles that are responsible for their superior performance. These principles will then guide the design of the artificial membranes. Permeance and selectivity of this proof-of-concept will be tested to validate the methodology, and as a basis for further performance and optimization studies.
Despite a vast literature on membrane separations, innovative chemical synthesis and molecular simulation-based design are rarely integrated. To date there is no validated, simulation-aided methodology to design artificial membranes based on the critical mechanisms underlying the performance of biological membranes. In a cell membrane, each aquaporin protein channel of nanoscale dimensions allows for the passage of three billion water molecules per second, while blocking protons from entering the cell. Discovering the principal molecular and cooperative interactions leading to the high permeation and selectivity of such pores is relevant to biology, but our main aim is to use the insights offered by these systems to implement an artificial membrane design for a purpose that mirrors that of the biological pores. To synthesize such a design is a non-trivial, high-risk effort, yet tremendous progress and our own experience in nanoscopically precise materials synthesis should allow us to implement structured designs, including ordered arrays of functionalized nanopores, as guided by the computations. This project merges innovation in materials science with state-of-the-art computational methods and guidance from biology to rationally design membranes for pressing water needs, using an approach that could extend to many other separation problems.
More than a billion people live without access to safe drinking water, making more effective water desalination and purification one of the National Academy of Engineering Grand Challenges. This research is a step toward high-performance membranes that seek to address those needs in a transformative way. The simulation-based design methodology is applicable to membranes for other separation processes, in pharmaceutics, biochemical processing or the energy field. In addition, this project offers a multidisciplinary educational experience for undergraduates. The New Visions Math, Engineering, Technology and Science (METS) program will continue to be used to involve high school students and attract them to studies in science and engineering. International exchanges will be strengthened, including collaboration with colleagues in Germany and at the National Institute of Materials Science in Japan.
Lack of fresh water, not only in the developing world, but with increasing shortages in developed nations as well, prompts us to seek ways to desalinate and purify water while consuming less useful energy. Membranes are an excellent way to do so, however the performance of current membranes is far surpassed by that of biological membranes. For example, certain protein channels in cell membranes (aquaporins) are extremely selective to water, and have orders of magnitude higher permeation rates than currently used artificial membranes. Hence, the aim of our research is to design and synthesize an artificial membrane, guided by the fundamental mechanisms behind these superior, biological membranes – those membranes have channels of nano-scale (10-9 m) size, while the surface structure (chemistry and charges) modulates the transport of desired molecules (like water), while repelling undesired species (like protons and salt, in the case of aquaporins). A first step in the creation of these membranes, and the purpose of the NSF EAGER grant, was to realize a proof-of-principle, based on nanoporous silica – a material with the same chemical composition as sand, but with pores of well-controlled nano-scale diameter. Silica is easy to functionalize with different chemical groups (charged or not), as our previous research has shown, and thus the mechanism of a biological membrane could be implemented. Intellectual Merit In the project, silica nanoporous silica was grown inside the wider channels of anodic alumina, which was used as a template (see figure). These wider channels help to guide the orientation of the nanoporous in the silica. However, the results of this study illustrate that it is not trivial to obtain a mesoporous silica-alumina hybrid membrane. Filling the anodic alumina channels tightly with silica and without cracks is difficult. Guaranteeing uniformity over macroscopic distances as well as reproducing ‘good’ samples is also challenging. In order to fabricate a nanoporous membrane capable of effective separation, several methods were tried, including sol-gel based methods and aspiration (under suction); the aspiration technique is determined to be the most effective. With this method, an overgrowth of silica on the external surface of the alumina membrane can be avoided. In addition, parameters that can enhance the growth of silica within the alumina channels can be more effectively controlled. These results will provide guidance in the ongoing study of synthesizing a mechanically stable, functionalized hybrid molecular sieve-template, as the basis for the synthesis of membranes inspired by biological membranes. Broader Impacts This research has led to a successfully defended Rensselaer MSc thesis in December 2012 for PhD student Silo Meoto, who is originally from Cameroon, where water quality is a serious everyday issue, especially in rural areas. A paper is in preparation. Silo is continuing her research as a PhD student on this topic at UCL (University College London), where the PI (Prof. Marc-Olivier Coppens) has moved as Head of Department of Chemical Engineering, and now directs a Centre for Nature Inspired Engineering, funded by the UK’s National Science Foundation, EPSRC. Silo also presented her results at the North American Membrane Society Conference in New Orleans, in June 2012. Silo and Marc-Olivier also advised a talented high school student over Summer 2011 and 2012, Alex Frieder from Half Hollow Hills high school; Alex presented his work in a report ("Bioseparations using a Composite SBA-15 Inorganic Membrane"), a poster presentation at Rensselaer (see photo) and numerous fairs, and he qualified for a National Merit Scholarship. Reactor setups were designed and built by Prof. Marc-Olivier Coppens and his graduate students, including Silo Meoto, to illustrate chemical engineering concepts to high school students in the Summer@Rensselaer program (2011 and 2012); this demo was successful enough to encourage the design and building of a robust reactor experiment, which was used to illustrate the Chemical Reactor Design class taught by Prof. Coppens, as well as used as a module for the senior undergraduate laboratory.
