Recent technologies in the production of chemicals and bio-materials products are focusing on lignocellulosic resources since it is the world's most abundant material, low cost, as well as sustainable. Lignocellulosic biomass consists of three main compounds: cellulose, hemicellulose, and lignin. Productions of carbon fiber from lignin as its precursor are proposed to reduce the usage of fossil fuel based materials. However, the difficulties on recovering lignin from biomass are widely known. Therefore, several studies were conducted to explore possible technologies to isolate lignin from the complex lignocellulosic biomass in simple and minimal cost. One of the potential technologies is by using sub-and supercritical fluids. The polymer made of from phenylpropane units (p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol), the building block of lignin in plants can be converted to low molecular weight compounds using sub-and supercritical fluid technology with diverse applications including carbon fiber production.Hence, this paper aims to review on the lignin extraction from lignocellulosic biomass using sub-and supercritical fluid technology as precursor for carbon fiber production.

Carbon fiber is a well-established high value-added material. However, its price is considerably high. Lignin obtained from oil palm frond (OPF) is a compatible precursor for carbon fiber production. This study aims to determine the feasibility of OPF lignin obtained under subcritical phenol conditions by focusing on its fundamental properties (purity and particulate matter content) as a precursor for carbon fiber production. This study also focuses on the effect of temperature (260-300 °C), time (5-30 min), and solid loading (6 and 10 g) on these properties. From the results obtained, the highest lignin purity of 95.1% was obtained at 260 °C, 5 min, and 10 g solid loading. Meanwhile, the lowest particulate content of 15.6% was obtained at 300 °C, 30 min, and 6 g solid loading. It can be further concluded that at higher temperature and longer reaction time, the low molecular weight compounds are dissolved in the liquid product, resulting in a decrease in the lignin purity as well as the particulate matter content.

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