**1. Introduction**

The depletion of fossil fuel reserves and climate change issues have raised concerns about renewable petroleum alternatives with the increment of global energy demand [1,2]. Biomass has received increasing attention in recent decades due to it being widespread, abundant, diverse, and inexpensive. Moreover, it has been intensively investigated as a highly sustainable carbon-containing source for the production of bioplatform molecules and biochemicals. The unique property of lignocellulosic biomass as the only renewable carbon carrier makes it an attractive source for bioenergy production. The conversion of lignocellulosic biomass to useful chemicals and biofuels via green and efficient approaches is one of the most popular topics in recent years [3]. However, problems such as its high pretreatment cost and difficulty in catalyst recovery hinder their utilization. Therefore, the development of new reaction techniques, including novel catalysts, novel pretreatment methods, or reaction media, is crucial for biomass-to-bioenergy industries [4].

The cell wall of lignocellulose mainly consists of lignin, cellulose, and hemicellulose [5]. The amount of each constituent is related to the type of plant species and their age [6]. Although lignocellulose is rich in cellulose and hemicellulose, its high lignin content, and tough and strong physical structure make it difficult to convert into chemicals and biofuels. Chemical methods such as acid pretreatment are commonly used to prepare reducing sugars. Compared to concentrated acid pretreatment, dilute acid pretreatment needs a higher reaction temperature and longer reaction time. Although the required reaction time and temperature of concentrated acid pretreatment are milder than that of dilute acid pretreatment, disadvantages such as a high equipment loss rate and environmental pollution impede its competitiveness [7]. In order to achieve low-cost and green sustainable production, solid acid catalysts had been proposed and applied to the production of reducing sugars [8].

The most common used solid acid catalysts include silica solid acids [9], biopolymer-based solid acids [10], ion-exchange resin solid acids [11], zirconia solid acids [12], and hydroxyapatite solid acids [13]. Nowadays, green catalysts, which are based on the use of renewable raw materials, are getting more and more attention [14]. Hence, more environmentally and economically-friendly solid acids were proposed. Biomass-based magnetic solid acid is prepared by using lignocellulose as a carbon carrier. It was regarded as a porous solid with a large surface area. Biomass-based magnetic solid acid was widely used in a large amount of reactions, such as the separation and purification of gases, and the removal of organic pollutants from water, refrigeration, and electrochemical devices [15]. Compared to traditional heterogeneous acid catalysts, the biomass-based magnetic solid acid has the characteristics of simple preparation, better recovery, convenient material selection, and low cost. Li et al. used a corn straw biomass-based solid acid to catalyze the hydrolysis of corn straw. The results showed that the prepared catalyst exhibited high catalytic activity for the conversion of corn straw into levulinic acid, and the most favorable values of catalyst dosage, hydrolysis temperature, hydrolyzation duration, and the maximum yield of LA were 3 g, 249.66 ◦C, 67.3 min, and 23.17%, respectively [15]. Chen et al. prepared a series of carbonaceous solid acids from biorenewable feedstock and used them as catalysts for the direct conversion of carbohydrates into 5-ethoxymethylfurfural (EMF) [16]. The results showed that the prepared catalysts presented a porous structure, high acid density, and easy separation. An EMF yield of 63.2% could be obtained from fructose at 120 ◦C. Lignocellulose contains a large amount of hemicellulose. It has been reported that the yield of xylan-based activated carbon (mainly based on hemicellulose) was much lower than that of cellulose and lignin due to the instability of hemicellulose [17]. Moreover, the presence of hemicellulose decreases the specific surface area of the solid acid when the carbonized temperature was up to 400 ◦C, which may reduce the catalytic performance of the solid acid. Simultaneously, hemicelluloses decompose at low temperatures and produce waste gas, which lead to environmental pollution. Therefore, in order to achieve a better carbon precursor, the biomass needs to be treated with suitable pretreatment methods [18]. Dilute acid and dilute alkali pretreatments are usually employed to destroy the complex structure of biomass. Dilute acid pretreatment can not only effectively remove hemicellulose, but can also minimize the damage of lignin and cellulose [19]. Meanwhile, dilute alkali pretreatment can effectively expand the biomass, leaving carbohydrates (cellulose and hemicelluloses) behind, thus increasing the contact area of the solid acid [20].

Bamboo is a fast-growing perennial herbaceous plant with a large phytomass, which is widely distributed in China [21]. Bamboo has many excellent properties that make it a suitable carbonized material for catalyst preparation, such as its porous structure and high thermal stability. Corncob is one of the abundant lignocellulose sources in China, and the annual global corncob production exceeds 1.03 billion metric tons, which can be used as a substrate for the production of platform products [8]. In this study, a bamboo-derived carbonaceous magnetic solid acid with a unique magnetic core–shell and high acid content was prepared by the impregnation-incomplete carbonization–sulfonation method and used as a magnetic solid acid to catalyze the hydrolysis of corncob to produce reducing sugar. The effects of dilute acid and dilute alkaline pretreatments on the catalytic performance of the as-prepared catalysts were investigated. The pretreated bamboo-derived carbonaceous support is expected with a porous structure for the introducing of –SO3H, forming layers of adsorbate molecules that can interact with reactants. This process mainly comprises two steps: the preparation of bamboo-based magnetic solid acid and the catalytic hydrolysis of corncob by solid acid to prepare reducing sugars.
