1:45 PM - 3:15 PM
[SGL21-P08] Investigation on the thermal stability of radical species in silica clathrate for ESR dating of chibaite: a case study using synthetic isopropylamine silica clathrate
Keywords:chibaite, synthetic silica clathrate, radical species, thermal stability, electron spin resonance
Silica clathrates have a SiO2 framework structure of cage-like voids occupied by guest species such as hydrocarbons and a similar framework structure to those of gas hydrates which contain molecular compounds enclosed within cage-like structures of water molecules. Chibaite is a natural analogue of gas hydrate structure II and has larger cages than melanophlogite, isostructural to gas hydrate structure I. It suggests that the cages in chibaite store propane and isobutane together with methane and ethane as guest molecules. Chibaite was found in marine sediments of Early Miocene age (Hota Group) at Arakawa, Minami-boso, City, Chiba prefecture, Japan (Momma et al., 2011). It occurs in quartz veins as euhedral ranging from a few mm to 1 cm thick in tuffaceous sandstone and mudstone. Although chibaite crystallized after marine sedimentation, it is not clear when and how it formed in nature.
In the natural chibaite, methyl and tert-butyl radicals were observed at room temperature. If organic radical species in chibaite are thermally stable in geological time scale, electron spin resonance (ESR) dating could be applied to evaluate the formation age. To establish the ESR dating method of chibaite using organic radicals, we investigated radiation-induced radicals and their thermal stabilities (Isogai et al., 2023). The ESR signal intensity of tert-butyl radical in chibaite increased by gamma irradiation, whereas the intensity of methyl radical almost unchanged. In annealing experiments, tert-butyl radical was thermally as stable as the defects such as Al center and Ti center in quartz (Toyota and Ikeya, 1991). However, the amounts of methyl and tert-butyl radicals were inversely correlated around 200 ºC. This implies reactions occur between these radicals and other guest molecules. The thermal behavior of radical species in silica clathrates is still not understood and we need to investigate the behavior of radicals for ESR dating of chibaite. Mainly because of multiple gaseous molecules enclathrated in chibaite, the physicochemical process of these radicals would not be easily understood using chibaite. To understand the behavior of radical species in silica clathrates, it is desired to use a silica clathrate with a single gaseous molecule. However, it is difficult to synthesize a silica clathrate with propane or isobutane because of almost no solubility of alkane in water.
In past study, silica clathrates with several amines were successfully synthesized with orthosilicic acid solution (Gunawardane et al., 1987). If isopropylamine is enclathrated in silica clathrate instead of hydrocarbon in chibaite, the cage structure might be the same as that in chibaite because this molecular size is similar. In this study, isopropylamine silica clathrates was synthesized and the gamma-ray-induced radicals were investigated in the silica clathrate. Therefore, we compared the behavior of radicals in synthetic silica clathrate and chibaite.
Orthosilicic acid solution (0.50 mol/L) was prepared by hydrolyzing tetraethyl orthosilicate (TEOS) with ultrapure water. We mixed orthosilicic acid solution with isopropylamine aqueous solution (4.7 mol/L). The mixed solution was sealed in a PTFE inner container and the container was set in a stainless pressure vessel. The vessel was kept at 180ºC for 1.5 months. The synthetic silica clathrate particles were irradiated by 6 kGy gamma-rays at 77 K using a source of 60Co. To investigated thermal stability of isopropylamine radicals, annealing experiments were performed. In isochronal annealing, the ESR sample was annealed from 360 to 690 K with a step of 30 degree for 15 minutes each temperature. ESR measurements were performed at room temperature using the X-band ESR spectrometer.
The carbon-centered radical (CH3)3C·(NH2) was observed at room temperature in the sample. It decreased over 480 K and almost disappeared at 690 K. In this presentation, we will discuss the thermal stability of radical species and compare it with the result of the study at chibaite.
In the natural chibaite, methyl and tert-butyl radicals were observed at room temperature. If organic radical species in chibaite are thermally stable in geological time scale, electron spin resonance (ESR) dating could be applied to evaluate the formation age. To establish the ESR dating method of chibaite using organic radicals, we investigated radiation-induced radicals and their thermal stabilities (Isogai et al., 2023). The ESR signal intensity of tert-butyl radical in chibaite increased by gamma irradiation, whereas the intensity of methyl radical almost unchanged. In annealing experiments, tert-butyl radical was thermally as stable as the defects such as Al center and Ti center in quartz (Toyota and Ikeya, 1991). However, the amounts of methyl and tert-butyl radicals were inversely correlated around 200 ºC. This implies reactions occur between these radicals and other guest molecules. The thermal behavior of radical species in silica clathrates is still not understood and we need to investigate the behavior of radicals for ESR dating of chibaite. Mainly because of multiple gaseous molecules enclathrated in chibaite, the physicochemical process of these radicals would not be easily understood using chibaite. To understand the behavior of radical species in silica clathrates, it is desired to use a silica clathrate with a single gaseous molecule. However, it is difficult to synthesize a silica clathrate with propane or isobutane because of almost no solubility of alkane in water.
In past study, silica clathrates with several amines were successfully synthesized with orthosilicic acid solution (Gunawardane et al., 1987). If isopropylamine is enclathrated in silica clathrate instead of hydrocarbon in chibaite, the cage structure might be the same as that in chibaite because this molecular size is similar. In this study, isopropylamine silica clathrates was synthesized and the gamma-ray-induced radicals were investigated in the silica clathrate. Therefore, we compared the behavior of radicals in synthetic silica clathrate and chibaite.
Orthosilicic acid solution (0.50 mol/L) was prepared by hydrolyzing tetraethyl orthosilicate (TEOS) with ultrapure water. We mixed orthosilicic acid solution with isopropylamine aqueous solution (4.7 mol/L). The mixed solution was sealed in a PTFE inner container and the container was set in a stainless pressure vessel. The vessel was kept at 180ºC for 1.5 months. The synthetic silica clathrate particles were irradiated by 6 kGy gamma-rays at 77 K using a source of 60Co. To investigated thermal stability of isopropylamine radicals, annealing experiments were performed. In isochronal annealing, the ESR sample was annealed from 360 to 690 K with a step of 30 degree for 15 minutes each temperature. ESR measurements were performed at room temperature using the X-band ESR spectrometer.
The carbon-centered radical (CH3)3C·(NH2) was observed at room temperature in the sample. It decreased over 480 K and almost disappeared at 690 K. In this presentation, we will discuss the thermal stability of radical species and compare it with the result of the study at chibaite.