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Accelerated Synthesis of Nanoporous Photocatalysts for Indoor Air Purification

Periodic Reporting for period 1 - ASPAir (Accelerated Synthesis of Nanoporous Photocatalysts for Indoor Air Purification)

Reporting period: 2016-09-01 to 2018-08-31

The primary objective of the Marie Curie Fellowship was to enhance the scientific knowledge and personal development of the researcher by joining the LSMO team at EPFL Valais, and developing a project based on the design and synthesis of Metal Organic Frameworks (MOFs), for their use in indoor air purification.
Perhaps surprisingly, indoor air contains a greater number of Volatile Organic Carbons (VOCs), and at concentrations higher than outdoor air. Although the concentrations of these compounds are in the parts per billion (ppb) range, and with people spending up to 80% of their time indoors, continued exposure to VOCs, many of which are mutagenic or carcinogenic, can lead to ill health. Commercial photocatalytic air purifiers utilise UV light and titanium dioxide to purify air, however the effectiveness of this can be questioned, due to the challenge in measuring such low concentrations of pollutants; it could be that instead of destroying the pollutant, the air purifier is simply transforming one pollutant to another. There is therefore an excellent opportunity to improve this technology by fabricating next generation air purification photocatalysts. The project directly addresses this challenge through the investigation of next-generation nanoporous materials, namely, metal-organic frameworks (MOFs). MOFs consist of metal ions or clusters coordinated to multidentate organic ligands to form one-, two-, or three-dimensional porous structures, depending on the strategic choice of building blocks, and the preferred coordination geometry of the metal centres. Following rapid progress in the field over the past decade, MOFs are now widely regarded as having exceptional promise across a range of technological areas. `
The project delivered a number of novel MOFs, and protocols were developed for activating (removing guest molecules from the pores of high surface area MOFs) and monitoring the degradation of organic pollutants under UV and visible light.
The project aimed to synthesise novel MOFs based on reducing metal centres and clusters, and photoactive ligands such as porphyrins. With this in mind, a plethora of MOFs were synthesised, utilising the high throughput robotic synthesis apparatus (RoSy), with 3 new MOFs in particular showing appealing properties that warranted further study. All 3 MOFs were based on zinc as the metal, and porphyrin as the ligand. Two MOFs named SION-17 and SION-18 demonstrated visible light absorption, with SION-18 additionally being porous to N2 at 77K. Upon heating, SION-17 collapses, while SION-18 undergoes phase transition. Both materials demonstrate visible-light absorption which is enhanced compared to the free porphyrin ligand used; using several techniques such as UV-Vis spectroscopy, fluorescence and transient absorption spectroscopy and UV-Vis provided us with insights into the structure-to-property relationship; this is work is currently being written up. SION-13 is a 3-dimensional material based on porphyrin ligand as well and in addition to its visible light absorbance, it shows outstanding stability under heating and in aqueous and organic media. The properties of SION-13 prompted us to investigate its potential toward water splitting, and was shown to be active in reducing water into H2 with a rate of 70 µmol g-1 h-1; this manuscript is currently at the last stages of preparation. RoSy was also used to streamline the synthesis conditions of SION-13, with the aim of synthesising the material in the shortest time, lowest temperature and most environmentally friendly solvent. This was successfully achieved, with the powder x-ray diffraction carried out on a high throughput well plate assembly, in collaboration with EPFL Valais Technical staff.
The project aimed to use MOFs for indoor air purification applications; the challenge being the ppb concentrations of typical indoor air pollutants. A protocol was developed to monitor ppb concentrations of indoor air pollutants using a gas analyser, and to activate (remove guest molecules from the pores of high surface area MOFs) sufficiently to allow the sensitive gas analyser to monitor the ppb level concentrations of pollutants such as 2-propanol. A protocol was also developed to calculate the loading of the pollutants into the MOF at these ppb concentrations. Finally, a protocol was developed to monitor the photocatalytic degradation of the pollutants under visible and UV light with the MOF as the photocatalyst. SION-13 was utilised for this application due to its stability and light-responsiveness but it was shown to be unsuitable for this application, due to low selectivity for 2-propanol at ppb level concentrations. Al-PMOF, a known porphyrin MOF was demonstrated to degrade 2-propanol under UV light, however acetone was also detected by the gas analyser, implying complete degradation was not achieved, however with the protocol now setup, further work is underway to screen additional MOFs.
The fellowship allowed the scientific knowledge and expertise of the researcher to exponentially develop by introducing him to the fascinating science of metal-organic frameworks. With the experimental part of the research group just starting up, the researcher was extensively involved in the purchasing and installation of laboratory equipment. A number of new MOFs were synthesised and characterised, and protocols were developed both for the synthesis of materials using the High Throughput Robotic Synthesis setup, and the monitoring of indoor air concentration pollutants, leading to the potential benefits of better quality materials for improving indoor air quality
Metal organic framework (MOF) formation, and the robotic equipment used to synthesise the MOF