Acetylene is a small molecule with immense utility to society as fuel for welding and cutting or as a carbon source for heat treatment. However, acetylene cannot be safely compressed to pressures over 2 bar due to its high reactivity. The current solvent-based storage technologies allow safe transportation at high pressures (typically over 15 bar in either acetone or DMF). Although these solvents are vital to ensuring acetylene's secure storage and transport, they suffer serious demerits, such as bulky and cumbersome cylinders, environmental pollution, and a limited acetylene flow and delivery rate.
Owing to these limitations, the group. S. Kitagawa of Kyoto University (Japan) and the Material Science group at Innovation Campus Tokyo of Air Liquide have collaboratively developed suitable porous materials capable of storing large quantities of acetylene at low and safe pressure (below 2 bar) while being able to release it under standard operating conditions at atmospheric pressure. To date, no such porous materials have been reported.
This study focused on porous coordination polymers (PCP) or metal-organic frameworks (MOF) with structural flexibility. MOFs are crystalline materials composed of organic molecules linking metal ions, forming a porous framework with a pore size ranging from 0.1 to a few nanometres, similar to that of nanoporous sponges. For the tuneable sorption of acetylene, we developed a solution based on flexible MOFs, a unique type of adsorbent pioneered by the Pr. S. Kitagawa Laboratory at Kyoto University, which can spontaneously release the gas stored in its nanopores, resulting in typical S-shaped adsorption curves.
The porous MOFs developed in this study comprised a structure with two jungle-gym-like networks that interpenetrated each other. We succeeded in enabling the precise and predictive control of the pressure required for gate opening and closing using a solid solution approach, thereby introducing an arbitrary ratio of selected functional groups (amino or nitro) of the organic ligands in the framework. We also elucidated the detailed mechanism that enables accurate control of the gate-opening pressure, using a combination of in situ X-ray diffraction measurements and theoretical calculations.
Applying this knowledge, we have successfully developed a material suitable for the target application, which is to store large amounts of acetylene loaded at a low pressure (less than 2 bar) to avoid the risk of explosion while releasing it under near-ambient conditions (1 bar). The ability to release the stored gas under ambient conditions is essential because it presents an opportunity for the recovery and use of the stored gas without the need for heat or vacuum as triggers for its release from the storage substrate. The performance of this material was demonstrated using a prototype cylinder on a dedicated test bench developed at the Air Liquide Innovation Campus Tokyo. Using the porous material as a filler in acetylene gas cylinders, it is possible to fill large volumes of acetylene (90 cm3 cm-3) at a safe pressure with high recovery rates of > 75%, which is not achievable with conventionally-used adsorbents.
The success of this material can pave the way for more efficient technology for the storage and transportation of acetylene. This technology can contribute not only to industrial fields where acetylene is used but also to fields where acetylene has not been used because of concerns regarding solvent contamination and risks. The gate-open pressure control method demonstrated in this study presents opportunities to develop novel materials for improving the storage and transport efficiency of volatile gases other than acetylene, which can be adapted for numerous practical applications.
These exciting results culminate almost five years of contributions to this collaborative project by Dr. C. Lavenn and Dr. M. Bonneau, co-managed by Prof. S. Kitagawa (Kyoto University) and Dr. C. Lavenn (Air Liquide Innovation Campus, Tokyo). This study also involved the participation of an international team of members from Japan – Kyoto University, Air Liquide Innovation Campus Tokyo and France - Air Liquide, CNRS/SMOLAB international project.
Additional information regarding our research can be found in our paper published in Nature Chemistry, DOI: 10.1038/s41557-022-00928-x.