This approach is inspired in natural methane hydrates that grow in the deep sea and in permafrost under demanding pressure and temperature conditions. Research using TOSCA at the ISIS neutron source has paved the way towards using metal-organic frameworks (MOFs) as a potential platform to promote the nucleation and growth of methane hydrates.
Last year the researchers, from the University of Alicante, Universidad Politécnica de Valencia-CSIC, ISIS and ALBA Light Source, used TOSCA to show that confinement effects in the cavities of high-surface area activated carbons can promote the nucleation and growth of artificial methane hydrates with faster kinetics than nature and with a stoichiometry that mimics natural hydrates [Nature Commun. 6, 6432 (2015)].
Based on these results, the group extended these hydrates studies to more versatile porous solids, in terms of porous structure and chemical composition, including the well-known metal-organic-frameworks (MOFs). By choosing their surface chemistry (hydrophobic/hydrophilic character of the linkers), the group were able to design the nucleation and growth of the methane hydrates. While hydrophilic MOFs (e.g., MIL-100) promote gas hydrate nucleation in the inner cavities with a relatively low water to hydrate yield, hydrophobic materials (e.g., ZIF-8) promote nucleation in the external surface of the MOF, leaving behind the inner cavities of the MOF fully available for further methane storage. This dual system is fully reversible, exhibits a very narrow pressure window for methane hydrate formation, and has faster kinetics (less than 4 hours for nucleation and growth) than bulk artificial hydrates.
Dr Joaquín Silvestre-Albero from the University of Alicante says, “The high sensitivity of inelastic neutron scattering to hydrogen atoms has made it possible to identify the isolated CH4 trapped inside the hydrate cages. The methane molecules in the cavities of a hydrate-like deuterated water give rise to two well-defined elastic peaks at 2.3 meV and 7.2 meV. The peaks can only be attributed to the different rotational transitions of methane behaving as an almost free rotor in methane hydrate, in close agreement with the INS spectra obtained for natural hydrates from the Pacific seafloor. Furthermore, synchrotron X-ray powder diffraction measurements performed at ALBA Light source in Spain have identified the sI structure of the methane hydrates.”
This investigation, led by Dr. Mirian E. Casco, Dr. Ir. Enrique V. Ramos-Fernandez and Dr. Joaquín Silvestre-Albero from the University of Alicante, opens the door towards the application of these MOFs in short and long-term methane storage applications based on a proper design of the host metal-organic framework in terms of porous structure (cavities must be able to allocate some unit cells) and chemical nature (to promote water-gas interactions rather than water-framework interactions).
Chemical Science, DOI: 10.1039/C6SC00272B
Joaquín Silvestre-Albero
Further Information
For more information please contact Joaquín Silvestre-Albero (email: joaquin.silvestre@ua.es) or Enrique V. Ramos-Fernández (Email: enrique.ramos@ua.es).
