Decrease of aliphatic CHs from diatoms by in situ heating infrared microspectroscopy

Abstract

In situ heating IR microspectroscopy at 260–300°C under air and N2 conditions has been conducted on diatom frustules to examine aliphatic CH losses during heating, simulating their changes with burial‐diagenesis. Assuming a reaction model made up of two first‐order kinetic relations, reaction rate constants k1 and k2 and activation energies (Ea) were evaluated for aliphatic CHs. The rate constants for loss of aliphatic CHs of diatom frustules under air and N2 flow are much larger, with much smaller activation energies (57–109 kJ/mol: air; 14–44 kJ/mol: N2), than those for conventional hydrocarbon generation reactions from kerogens (170–370 kJ/mol) studied at higher temperatures (350–450°C). The CH decrease rates are somewhat different from the amide I decrease (protein degradation) rates. The obtained results suggest that organic transformation reactions including degradation of aliphatic CHs inside the diatom silica frustules might be quite different from those of kerogens separated from the biological structures.

Hydrothermal Transformation of Inorganic and Biogenic Silica as Studied Using in Situ Hydrothermal Infrared Microspectroscopy

“…Centric diatoms (diameters ¼ 100-350 mm) were taken from red algae at Kurosakinohana, near Misakiguchi station, Miura Peninsula, Kanagawa Prefecture, Japan (May 2009). These diatoms can be classified into Arachnoidiscus ornatus after the description of Kobayashi et al 6,22,26 Dried diatoms attached on red algae without washing were picked up by a tatting shuttle for lacework and used for IR microspectroscopy.…”

“…The IR spectra for diatoms show bands around 2930 cm À1 due to aliphatic CHs and a band around 1650 cm À1 due to C¼O stretching of peptide bonds (amide I) and a band around 1550 cm À1 due to C-N-H bending and C-N stretching of peptide bonds (amide II). 22 Since these organic bands show also decreasing trends during dry heating experiments, 22 these organic components might affect the present Si-OH decrease rates. Although behavior of these organic IR bands during the hydrothermal heating experiments will be reported separately, the present results with a lower E a for diatoms than silica gels suggest some ''catalytic'' activities of organic components in the silica dehydration-condensation.…”

“…2c). 6,22 Bands around 2930 cm À1 are due to aliphatic CHs. A band around 1650 cm À1 includes C¼O stretching of peptide bonds (amide I) together with H-O-H bending of water molecules.…”

“…Moreover, organic molecules contained in these biogenic sediments such as diatoms are considered to generate hydrocarbons at temperature ranges of 80-240 C, called an oil and gas window (petroleum and natural gas). 3,[17][18][19][20][21] Alipour and Nakashima 22 conducted transformation experiments of diatoms in a heating stage under air and N 2 flow at 260-300 C to examine decrease rates of aliphatic CHs and proteins by using in situ infrared (IR) microspectroscopy. They evaluated faster and slower CH and protein decrease rates from diatom silica frustules showing much lower activation energies than thermal cracking reactions of organic molecules called kerogens.…”

“…Therefore, in order to study transformation of silica in the amorphous state under hydrothermal conditions, we have developed an in situ hydrothermal cell, which can be placed under an IR microscope. A commercial silica gel has been used as a representative inorganic amorphous silica, while the same present-day diatoms as those studied in Alipour et al 6 and Alipour and Nakashima 22 are employed as bio-silica including organic components. Decreases of IR peak heights at 950 cm À1 due to the Si-O stretching of Si-OH groups were observed and analyzed by kinetic equations for determining silica transformation and dissolution rates.…”