## 1 Introduction  
The increasing severity of environmental problems and global climate change due to excessive fossil fuel consumption has led to a new era in which the urgent need for a structural change in global energy strategy is recognized. That is, the global energy structure is changing, with a focus on limiting fossil energy consumption and developing new and renewable energy sources removed. Among them, cellulosic ethanol, which can be produced from the prevalent biomass, is considered the best alternative. Lignocellulosic biomass can be supplied at an affordable price through a variety of biological resources. These can be classified as forest resources, municipal solid waste, agricultural residues, marine resources, etc., depending on where they are produced. Although lignocellulosic biomass varies somewhat depending on its type, it contains 60-80% polysaccharides (cellulose and hemicellulose), which can be hydrolyzed to produce fermentable sugars such as glucose and xylose. Fermentable sugars can also be used as raw materials for bio-based products in biorefineries to replace petrochemicals [1]. Among the various biomass sources mentioned above, agricultural residues, which are generated in abundance every year, comprise raw biomass that is of particular interest in the era of biorefinery industrialization [2]. Among the many types of agricultural biomass, rice husk (RH) has received attention from many researchers as a promising energy resource. The reason is that rice, as a major food in Southeast Asia and elsewhere, is one of the most cultivated crops in the world, and RH, which is produced in proportion to rice production, accounts for about 20% _w/w_ of rice. In addition, although RH is the world's most predominant agricultural by-product, its nutrient content is very low, making it unsuitable for animal feed or plant compost, so its utilization value is very low [3; 4; 5]. Notably, the constituents of RH include a large amount of ash, consisting mainly of silica, and lignin, thus imparting it with high resistance to biological and thermochemical fractionation. In order to effectively utilize RH, its structure must be destroyed through the disruption of inter- and intramolecular bonds, allowing RH components to be easily fractionated [6]. For this purpose, various pretreatment processes for RH have been reported: alkalis and acids [7; 8], dilute acid [9], dilute alkali [6; 10], alkaline peroxide [11], ionic liquid [12], etc. Although attempts have been made to fractionate the components of RH using various catalytic solvents, there is a high preference for alkaline fractionation, which has been suggested to substantially increase the susceptibility of RH to enzymatic hydrolysis [13; 14]. These reports are also in good agreement with our previous findings, which demonstrated that, in the case of barley straw, a low-acid-catalyzed hydrothermal (LAH) pretreatment was able to obtain a fairly high fractionation efficacy, but an alkali fractionation was required for rice straw [15]. The greater need for alkaline pretreatment of RH compared to other biomass sources is thought to be due to the high ash content (mainly silica) in RHs, so silica must be dissolved in advance in the form of silicate for an effective fractionation process. In addition, alkaline fractionation is an effective delignification process that can produce cellulose-rich solid biomass while minimizing the loss of carbohydrates and thus the generation of decomposition products [16]. Fractionation with alkali causes the biomass to swell, which increases the surface area accessible to enzymes, and hemicellulose and amorphous cellulose are solubilized by cleaving the lignin-carbohydrate complex. Hemicellulose consists mainly of pentosan, which is highly reactive and can be degraded into monomeric sugars under acidified reaction conditions [17]. Pentose (mainly xylose) can be further dehydrated to furfural, which is considered an important bio-based platform chemical and is a precursor to furfuryl alcohol, tetrahydrofuran, ferric acid, etc.