**1. Introduction**

In recent years, much attention is being paid to the development of environmentally and economically viable synthetic routes and technologies for producing chemicals and fuels from non-fossil carbon sources as alternative to fossil raw materials [1]. In this context, biomass is emerging as a very promising sustainable feedstock, being the only widely available and renewable carbon source [2,3]. Lignocellulosic biomass, mainly composed by lignin, cellulose and hemicellulose, with an estimated annual production about 2 × 10<sup>11</sup> metric tons, is the most abundant source of carbohydrates, but physico-chemical treatments are required for its use as a raw material [4]. Although lignocellulose is a sustainable resource for production of biofuels and chemicals, it is necessary that this does not interfere with the food chain. The hydrolysis of cellulose and hemicellulose leads to monomeric C5 and C6 sugars, which can be converted into important platform molecules, such as furfural and 5-hydroxymethylfurfural (5-HMF), respectively, which are the starting point for the

synthesis of a large variety of biofuels and chemicals [5,6]. For instance, 5-HMF can be transformed into 2,5-dimethylfuran [7] or levulinic acid [8,9], among others, which are key intermediates for the synthesis of pharmaceuticals, polymers or biofuels.

Although the dehydration of fructose to 5-HMF has largely been reported in the literature, glucose is preferred due to its abundance and low price [10]. There is not a general consensus about the mechanism of glucose dehydration to 5-HMF, even though a generally accepted route based on: (i) isomerization of glucose to fructose, and (ii) dehydration of fructose to 5-HMF [4]. The first step is considerably di fficult and requires Lewis acid or basic sites, being the limiting factor for 5-HMF production. The reaction may be performed either in water, organic solvents or ionic liquids, in particular polar aprotic solvents. Homogeneous catalysts such as sulphuric or hydrochloric acids can be e ffective for the hydrolysis of cellulose to glucose, and even for the dehydration of fructose to 5-HMF. However, due to their corrosive properties which are hazardous for equipment, they are gradually replaced by heterogeneous catalysts. Besides, heterogeneous catalysts allow their easy separation from solution, recovery and reuse [11–13]. Di fferent solid acid catalysts have been tested for dehydration of glucose to 5-HMF, such as γ-Al2O3 [14], zeolites [15], metal oxides like TiO2 or ZrO2 [12–15], mesoporous solids [16,17], inter alia.

Nevertheless, both solvent and catalyst must be considered as two key factors to attain high 5-HMF yields from C6 carbohydrates. A common strategy for 5-HMF production is the use of biphasic systems because this approach gives higher 5-HMF yields than systems employing only water. Usually, the biphasic medium is formed by the addition of organic solvents (toluene, methyl isobutyl ketone, among other) to an aqueous solution, or the addition of miscible organic solvents like tetrahydrofuran (THF) [18], to a saturated salt solution, which allows to extract the 5-HMF formed from the aqueous phase, preventing its further degradation and condensation [4].

In order to prevent these side reactions, Román-Leshkov et al. [19] employed inorganic salts in a biphasic system for dehydration of fructose to 5-HMF and concluded that the salting-out e ffect leading to a higher partition coe fficient, limiting the individual cationic or anionic contributions, so then it is feasible to correlate to the interaction of all ionic species. In this context, it has also been reported that divalent cations interact more strongly with saccharides than the monovalent ones [20]. Thus, Combs et al. [21] observed that alkaline earth metal cations can form bidentate complexes with glucose, which accelerated its transformation. Later, our research group studied the beneficial e ffects of CaCl2 on glucose dehydration to 5-HMF in the presence of Al2O3 as catalyst, in such a way that the addition of CaCl2 to the reaction medium notably improved the catalytic performance, even at very short reaction times, due to the interaction between Ca2+ ions and glucose molecules, which favored the α-D-glucopyranose formation [14].

Concerning the use of zeolites for glucose dehydration, di fferent acidic ZSM-5-zeolites (H-, Feand Cu-ZSM-5) were prepared and studied by modifying several experimental variables [15]. It was demonstrated the positive e ffect of the addition of inorganic salt (NaCl) to a biphasic water/methyl isobutyl ketone (MIBK) system, since a glucose conversion of 80%, with a HMF yield of 42% was attained at 195 ◦C, after 30 min, by using a H-ZSM-5-zeolite, which had the lowest Lewis/Brönsted ratio among the studied zeolites. Later, the H-ZSM-5-zeolite was compared with H-Y and H-β-zeolites, in order to assess the influence of the textural properties on the catalytic performance in glucose dehydration [22]. Under similar experimental conditions, by using a H-β-zeolite, the highest 5-HMF yield (56%) was reached, thus demonstrating the benefit of mesoporosity in this catalytic process.

Recently, by using a bifunctional Cr/β zeolite, a high selectivity to 5-HMF with a yield of 72%, was found at 150 ◦C, after 90 min, by adding NaCl to a biphasic H2O/THF system [23]. After three consecutive catalytic cycles, the catalytic activity slightly decreased, but after a thermal treatment was almost recovered.

Morphological or textural characteristics play a highly important role when discussing catalytic activity in a chemical reaction [24]. Nevertheless, the morphological roles of zeolite in glucose dehydration are still not completely understood, hence further investigation to reveal this e ffect in the dehydration reaction is of the utmost importance. The aim of this work is a thorough study of glucose dehydration for 5-HMF production (Scheme 1) using protonated L-type (H-LTL) zeolites with different morphologies, which have been characterized and their catalytic performance has been correlated with their textural and acid-base properties.

**Scheme 1.** Reaction pathway of glucose to 5-hydroxymethylfurfural.
