New Metal–Organic Frameworks
Two new types of metal−organic frameworks (MOFs) have been made by researchers in Vietnam working with colleagues in the United States and Saudi Arabia. The MOFs called M-VNU-74-I and -II (where M = Mg, Ni, Co and VNU stands for Vietnam National University) were designed with polar amide functionalities to boost the methanol uptake capacities of this class of materials. Ultimately, the aim is to use such highly porous compounds in adsorption-driven heat pump applications.
MOFs usually comprise a three-dimensional network of metal ions or clusters coordinated to organic ligands. They are a sub-class of coordination polymers and many are porous because of the spacing effect of the ligands. They have been investigated extensively for their ability to adsorb various small molecules and as such have been studied for hydrogen storage applications for vehicle fuel cells, as sponges to soak up the greenhouse gas carbon dioxide, and for various other uses, e.g., as molecular sieves, catalysts, and sensor compounds.
Hiroyasu Furukawa, University of California and Lawrence Berkeley National Laboratory, Berkeley, USA, as well as King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, and colleagues hope to exploit the porosity of their two new series of MOFs in a heat-pump application. Heat pumps essentially move thermal energy (heat) from one place to another, commonly through a cycle of evaporation and condensation. Air-conditioners and freezers all use heat pumps that rely on heating a volatile compound electrically in a closed system and cooling the air or the interior through the loss of latent heat of evaporation to the outside. The process can be used in reverse to heat a site, of course.
Finding new and more efficient low-energy alternatives to current technology is the aim of much research aimed at improving sustainability and reducing the environmental impact of our endless heating and cooling of living and work spaces, food and other storage, as well as our vehicles.
Thermal Batteries for Air Conditioning
In their article in Chemistry of Materials, the researchers allude to the US Department of Energy’s targets of reducing residential building primary energy usage by 40 % from the 2010 value until 2025. Unfortunately, energy use is increasing, so alternative approaches are urgently needed if these targets, of which there are commercial counterparts and equivalents in other countries, are to be met. The researchers assert that we most likely need to completely overhaul how we currently heat and cool buildings, which is where their new MOFs might be able to help.
Thermal batteries have already been proposed for use in air-conditioned buildings as an alternative to the conventional machines. Heating and cooling are driven by the adsorption and evaporation of water, respectively, due to water’s high latent heat, the team explains. Unfortunately, there is a significant flaw in such an approach, in that water also has a rather low vapor saturation pressure. “This presents a conspicuous disadvantage for realizing an effective thermal battery system, as strong interactions between water and the adsorbent are required to achieve high uptake capacities,” the team explains. One workaround might have been to use zeolites as the adsorbent. However, regenerating the system would require heating the zeolite with its aqueous load to 250 °C, which would negate the energy benefits of using a thermal battery.
Methanol as an Energy-Saving Alternative to Water
Instead, methanol, with its higher saturation vapor pressure and lower boiling point than water, might be the answer. “It is expected that the overall efficiency of heat release, when utilizing methanol-based thermal batteries, has the potential to outperform its water-based counterparts if suitable adsorbent materials are available,” the team explains. However, an alternative adsorption material for methanol is needed to replace zeolites. The two new series of designer MOFs developed by the team are the near-perfect sponges for methanol and could be used in a thermal battery heat pump to heat or cool through methanol evaporation and condensation.
In the charging cycle, methanol evaporates from a reservoir, leading to cooling of the lower part of the system. The methanol is then absorbed by the MOF in the upper part of the system and the thermal energy is released as the methanol molecules are adsorbed. The reverse process releases the methanol to regenerate the empty pores of the MOF and the methanol is condensed at the bottom, ready for the next cycle. With this methanol system, regeneration can be driven by a relatively low temperature of 80 °C as opposed to the much higher temperature needed for zeolite regeneration. Such temperatures would be achievable using solar energy, for instance. The team’s judicious choice of metal ions and spacer ligands, as well as the decision to use amide functionalities, means they can optimize the methanol capacity of their MOFs.
“It is still necessary to increase the storage capacity,” Furukawa told ChemViews Magazine. “More specifically, the step position of the methanol isotherm is still a bit high, which should be controlled by either modification of MOFs or design of new MOFs. In addition, it would be preferable if the isotherm showed a sigmoid shape (i.e. minimize the uptake in the very low-p region) so that we can increase the working capacity.” He adds that, from an engineering perspective, the next step will be to prove the concept and actually evaluate the capacity as a heat pump.
Expert in MOF design and synthesis Omar Farha, Northwestern University, Evanston, IL, USA, says that, “This is a very nice piece on an important issue that needs to be addressed in the future.” He points out that, “Using MOFs for heat pumps is not a new idea, but using methanol instead of water is very smart.”
- High Methanol Uptake Capacity in Two New Series of Metal–Organic Frameworks: Promising Materials for Adsorption-Driven Heat Pump Applications,
Binh T. Nguyen, Ha L. Nguyen, Tranh C. Nguyen, Kyle E. Cordova, Hiroyasu Furukawa,
Chem. Mater. 2016.
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