Cation-Controlled Gating for Selective Gas Adsorption over Adaptable Zeolites

Adsorption technology by which gas streams can be purified and separated is essential for many key industries, including those in the oil and gas, chemicals, manufacturing and medical sectors. As a result, solid adsorbents are worth £2.4 billion per year, some 10% of the total industrial gas market. Furthermore, adsorption can offer green, energy-efficient routes to environmental applications, including carbon capture from power generation and other industrial sources. Typically, adsorption is achieved via pressure swing and temperature swing adsorption processes with cycle times of minutes or more. New kinetic-based adsorption technologies, using rotating valves, rotary wheel adsorbers and novel thin layer adsorbent structures can reduce the equipment footprint and increase the efficiency of these processes of gas separation so that, for example, pure gas can be generated on site rather than centrally, with the distribution costs associated with that.
Zeolites, microporous aluminosilicates, make up over 30% of industrial adsorbents by value. Their well ordered and robust framework structures impart high selectivity by both molecular sieving and thermodynamics-based separation. Although over 200 zeolitic structure types are known, only a very few find widespread application as adsorbents, in part due to the economics of their synthesis. In our recent EPSRC-funded research, we have indentified two new mechanisms by which very high adsorption selectivity can be achieved. The first mechanism is via a chemoselective 'trapdoor' effect, in which cations occupying window sites only permit diffusion of molecules (such as CO2) that interact strongly with them. The second mechanism makes use of the flexibility of some structures in response to the composition of their extra-framework cations, so that their structure and cation composition can be modified to fine-tune molecular sieving via 'cation-controlled molecular sieving'.
In this ambitious project we will develop gas separation by these two mechanism by zeolites not commonly used as adsorbents, including some recently reported by us as CO2 adsorbents in 'Nature'. Their potential advantages of new zeolites in kinetic-based separations (including a requirement for an order of magnitude less material) can enable much higher specific production costs to be tolerated. Consequently, the number of potential zeolite candidates for adsorption is increased. To develop these new materials and make possible this step change in adsorbent technology, we have assembled a research team comprising materials chemists, computational modellers and chemical engineers as well as industrial partners in zeolite adsorption and gas adsorption. Materials chemistry will be used to modify and optimise the chemical structure of chosen zeolite frameworks and also their texture (particle size, hierarchical porosity) for target gas separations, and the performance of these new compositions will be measured and modelled macroscopically by chemical engineers. Multiscale computational modelling (via a range of techniques of different levels of theory) will give a detailed picture of the mechanisms and so provide feedback to inform the experimental studies. This will result in greater understanding of the relationship of chemical structure and dynamics to the adsorption properties. In concert with this, ongoing discussion with industrial project partners at project meetings will enable the practical development and exploitation of a new generation of zeolite-based adsorbents for industrial and environmental applications.

(a) Adsorption technology impacts a wide range of sectors, providing pure gases for the industrial, energy, semiconductor, food and medical sectors. The total adsorbent market is ca. $2.4 billion, some 10% of the value of the total industrial gas market, and zeolites make up about one-third of this in monetary terms. Existing adsorption technology is currently centred on pressure and temperature swing equilibrium based technologies, and only two main types of zeolites, but future developments will see faster, kinetics-based processes, including those using structured adsorbents, having a major impact. This is more energy-efficient and also allows generation of gases on site using equipment with a smaller footprint. It will provide a market for the advanced zeolite adsorbents of the type detailed in this proposal. These are likely to be useful to separate gases for carbon capture (and enhanced oil recovery), hydrogen generation, energy-related separations (natural and biogas upgrading) and other industrial and medical applications.
Industrial beneficiaries will include those working in the energy and chemicals sector requiring the energy efficient and highly selective separation of gases. There are many examples: e.g. (i) Natural gas contains N2 and CO2 as well as hydrocarbons such as methane and these reduce its calorific value. The N2 content transported by pipeline should be less than 3%, (ii) Biogas contains high concentrations of CO2 that must be removed to increase it calorific value (iii) Hydrogen produced by gasification is separated from a mixture of CO and CO2. The current proposal has an emphasis on recovering CO from CO2 to increase the efficiency of the conversion to H2 by recycling the CO, (iv) In steel making, CO is produced in large quantities mixed with other gases and could usefully be separated and used as a chemical. These and other technologies benefit from the preparation of gases of high purity non-cryogenically via adsorption-based processes. If successful, the work will build demand for different zeolites to be scaled up by zeolite manufacturers. Furthermore, it is likely that if these zeolite adsorbents can be prepared as small particles, they can find application in future membrane technologies, such as those being developed to separate CO2 from CH4 and water from alcohols.

(b) These studies will increase the knowledge and understanding of adsorption, and in particular of the two newly discovered mechanisms of adsorption over zeolite explained in the proposal. They will therefore inform those chemists and chemical engineers in academia and industry who use zeolites as adsorbents and catalysts. There will be particular relevance for the use of materials where structural dynamics influence their properties, which are expected to be much more widespread than previously thought, and so this work will give researchers a new way of looking at many existing processes.
The work will enable development of the tools of computer simulation of flexible structures, so that new separation technologies can result from the understanding of structural flexibility, enabling adsorbents to be tailored for specific molecules. While of direct importance in this project for zeolites, this information will also have an impact in the examination of metal organic frameworks, which are very widely studied in EPSRC-funded research and also internationally.

(c) The project will also have an impact in the interdisciplinary training of early career researchers at the interface of Chemistry and Chemical Engineering, who will then have expertise and understanding of practical and modelling methods at both the atomic and the macroscopic scale.

(d) The project will strengthen the interactions of the principal and co-investigators, building on the strength of a UK Chemistry-Chemical Engineering collaboration that has already yielded sixteen highly cited publications, including recent Nature, JACS and Angew. Chem. papers.