**1. Introduction**

The depletion of fossil fuels along with the environmental problems associated with their utilization promote new processes for the generation of fuels based on renewable sources [1]. The use of biomass, typically lignocellulosic biomass, in the production of fuels, fuel additives or added-value chemicals has attracted considerable interest, becoming a potential research area [2]. Biomass can be converted into biofuels and valuable chemicals via chemical or thermochemical processes; among them, aqueous phase reaction is an effective method to convert lignocellulose into biochemicals [3]. Lignocellulosic biomass is principally constituted of cellulose, hemicellulose and lignin [4,5]. Due to the complex nature of biomasses and their chemical compositions, they have different reactivities, and conversion processes can occur via various reaction pathways. Thus, researchers often prefer to use typical

model components focusing on special reaction stages in order to study the conversion process [6,7]. Glucose, the most plentiful and approachable monosaccharide unit in the lignocellulosic biomass, is the most desirable feedstock for the production of valuable biochemicals [8]. One of the most significant reaction pathways is the isomerization of glucose to fructose, followed by dehydration of fructose to 5-hydroxymethylfurfural (5-HMF) and rehydration of 5-HMF to levulinic acid and formic acid [9]. Moreover, retro aldol condensation may occur with glucose, yielding glycolaldehyde and erythrose, or with fructose to give dihydroxyacetone and glyceraldehyde. Glyceraldehyde can be hydrogenated to glycerol or rearranged to lactic acid, which can be further hydrogenated with propionic acid or decarboxylated to yield ethanol. Another pathway involving C-O and C-C cleavage leads to acetic acid, which can be obtained from glycerol [10–14].

The competing reaction pathways happening through biomass conversion lead to different bioproducts with relatively low yields and difficulties in separation. It is important to underline one of the major drawbacks of working with sugars: the formation of humins, i.e., insoluble, heavy compounds that form under glucose transformation conditions. Therefore, catalysts and the reaction conditions play a crucial role in the control of reactions or the production of the desirable bioproduct, higher feedstock conversion and also in avoiding byproducts [15]. Compared to homogeneous catalysts, heterogeneous catalysts can be used in viable greener methods and approaches for efficient biomass transformation. High thermal and mechanical stability, recovery and recyclability are the main advantages of heterogenous catalysts [16]. In recent years, a wide variety of heterogeneous catalysts has been utilized for biomass transformation, such as carbon-based materials, mesoporous silica, zeolites, metal oxide supported metals, organic polymers and ionic liquids [17,18].

In particular, mesoporous silica materials such as SBA-15, KIT-6, and MCM-41 have been investigated as supports owing to their high surface area and large pore volume, flexible and tunable properties, and easy diffusion of large molecules, enabling efficient transformation [2,7,19]. For instance, Qing Xu et al. reported the effect of a Sn-containing silica mesoporous framework (Sn-MCM-41) on the conversion of glucose to 5-HMF in ionic liquid. The results showed that Sn can act as a highly active Lewis acid center in conjunction with the silanol group of MCM-41, which catalyzes both the isomerization of glucose into fructose and the dehydration of fructose to HMF without the addition of a mineral Brønsted acid catalyst [20]. Other authors reported the use of hybrid catalysts such as CrCl<sup>3</sup> and HY zeolite for the production of lactic acid (LA) acid from glucose, with better performance compared to parent HY catalyst because of their higher acidity [21,22]. Cao Xuefei et al. used various transition metal sulfates (Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) to achieve glucose, fructose and cellulose conversion into several chemicals; the authors pointed out that different metal salts showed different reactivities regarding the conversion of sugars. Among these metal ions, Zn2<sup>+</sup> and Ni2<sup>+</sup> were more selective towards LA, whereas Cu2<sup>+</sup> and Fe3<sup>+</sup> showed high levels of efficiency for the conversion of glucose and cellulose into LA and formic acid at high temperature [23].

Materials possessing high surface areas and large pore sizes with more accessible acidic moieties are crucial for catalyst preparation. To the best of our knowledge, no studies have reported the use of mesoporous-supported, bimetallic catalysts for the aqueous phase transformation of glucose into biochemicals. Furthermore, using metals such as Fe, Co, Mo and Mn, which are cheaper and more widely available, as the active phase may be interesting and of crucial importance.

Here, we report on four catalysts including, i.e., Co, Co-Mn, Co-Mo, and Co-Fe supported MCM-41 (hexagonal mesoporous silica) for the aqueous phase formation of valuable chemicals from glucose under different reaction conditions. The aim of this study is to benefit from the high surface area, large pore size and acidity of the silanol group in MCM-41 and the metal active phase in order to facilitate the penetration of substrates into the catalyst pores.
