*2.2. Catalytic Study*

These zeolites have been evaluated as acid catalysts in the dehydration of glucose to 5-hydroxymethylfurfural, an important platform molecule for the synthesis of biofuels and high value-added chemicals [33]. Although the mechanism involved in glucose dehydration to 5-HMF seems to depend on type of catalyst, nature of solvent, among other parameters, it is broadly accepted that the rate-determining step is the isomerization of glucose to fructose [4]. For this reason, most of studies have used fructose as starting sugar. The isomerization process is catalyzed by Lewis acid or basic sites, whereas dehydration of fructose to 5-HMF requires the participation of Brönsted acid sites. The production of 5-HMF from glucose has two main drawbacks associated to the catalytic process, that is, the formation of by-products, such as soluble/insoluble polymers and humins, and the rehydration of 5-HMF for giving rise to levulinic and formic acids. These processes decrease the 5-HMF yield and, in some cases, provoke the catalyst deactivation. In this sense, the use of organic co-solvents together with the aqueous phase containing carbohydrates has been usually reported as a suitable strategy for improving 5-HMF yield. Among these organic solvents, MIBK is one of the

most solvents often used due to their benign physico-chemical properties [34]. In this work, a biphasic water-MIBK system was employed to study the dehydration of glucose to 5-HMF.

Moreover, previous studies have demonstrated that the addition of inorganic salts to biphasic phases ameliorates the extraction of 5-HMF from biphasic systems [14,16,19]. In particular, CaCl2 exerts a positive e ffect on the catalytic performance, even at very short reaction times [14,35]. This was explained by the interaction between Ca2+ ions and carbohydrates (glucose and xylose), modifying the corresponding anomeric equilibrium towards the anomer α more prone for the dehydration process, as was concluded from 1H-NMR spectroscopy data.

In this sense, the catalytic activity of the LTL-zeolites was studied with and without CaCl2 addition (Figure 7). The positive e ffect is clearly observed, since at 150 ◦C, after 60 min of reaction and in the absence of inorganic salt, the conversion of glucose is lower than 20% and in this case the formation of 5-HMF is barely detected (5-HMF yield < 1%). However, the addition of CaCl2 outstandingly improves the glucose conversion until values higher than 95% for ROD-LTL, with a 5-HMF yield of 58.5%, whereas the rest of catalysts attain conversion and 5-HMF yield values higher than 75% and 25%, respectively. Therefore, these data confirm previous results attained with other families of catalysts in the presence of inorganic salts [15,19].

**Figure 7.** Effect of the addition of CaCl2 on the catalytic performance (C: glucose conversion; Y: yield) (Experimental conditions: 0.15 g glucose, 0.05 g catalyst, 1.5 mL water, 3.5 mL MIBK, Temperature = 150 ◦C; time = 60 min, CaCl2 = 0.65 g gH2O<sup>−</sup>1).

The improving e ffect of CaCl2 addition on the catalytic performance of this series of LTL-based catalysts is evident. However, an additional catalytic study was performed in order to evaluate the contribution of CaCl2 to the overall catalytic activity. For this, CaCl2 (0.65 g gH2O<sup>−</sup>1) was put in contact with glucose in the biphasic system, and compared with the same system to which the NEEDLE-LTL catalyst was added. The data reveal that, in the absence of catalyst, after 3 h at 150 ◦C, the glucose conversion was 47.1%, with a 5-HMF yield of 16.8% (Figure 8).

However, under similar experimental conditions, this catalyst exhibits better catalytic performance, attaining a conversion of 86.9% and a 5-HMF yield of 44.0%. The catalytic activity in the presence of CaCl2 might be explained by the formation of α-anomer in the presence of this salt, as it has been demonstrated in previous studies [14], which could easily be dehydrated due to the thermal contribution (non-catalyzed process) to the overall catalytic performance.

**Figure 8.** Catalytic performance of CaCl2 and CaCl2/NEEDLE-LTL (C: glucose conversion; Y: yield) (Experimental conditions: 0.15 g glucose, straight line: 0.65 gH2O<sup>−</sup><sup>1</sup> and 0.05 g NEEDLE-LTL and dash line: 0.65 g gH2O<sup>−</sup>1, 1.5 mL water, 3.5 mL MIBK, Temperature = 150 ◦C).

The kinetics of the dehydration process with LTL-based catalysts were studied at 150 ◦C (Figure 9), and similar conversion curves were obtained for the three catalysts, in spite of their different acid properties (Table 2) and morphologies. However, at intermediate reaction times, the 5-HMF yield values are lower for CYL-LTL, which could be explained by considering its lower acidity, although its mesoporous character could compensate the lower concentration of acid sites in this catalyst. In this study, the highest 5-HMF yield (50.3%) was attained after 300 min with the ROD-LTL catalyst.

**Figure 9.** Kinetics of glucose dehydration (C: glucose conversion; Y: yield) (Experimental conditions: 0.15 g glucose, 0.05 g catalyst, 1.5 mL water, 3.5 mL MIBK, Temperature=150 ◦C; CaCl2 = 0.65 g gH2O<sup>−</sup>1).

By increasing the temperature to 175 ◦C, the reaction time for attaining the highest 5-HMF yield is shortened, making it possible to obtain a 5-HMF yield of 63.1% for a glucose conversion of 87.9% with the NEEDLE-LTL catalyst, after only 90 min (Figure 10). The figure shows that glucose conversion increases progressively with the reaction time, and values of 100% are already reached after 60 min of reaction time. However, 5-HMF yield values attain a maximum, and then, despite the raise of conversion, degradation processes lead to a decrease in 5-HMF yield.

**Figure 10.** Kinetics of glucose dehydration (C: glucose conversion; Y: yield) (Experimental conditions: 0.15 g glucose, 0.05 g catalyst, 1.5 mL water, 3.5 mL MIBK, Temperature=175 ◦C; CaCl2 = 0.65 g gH2O<sup>−</sup>1).

Another experimental parameter that has been evaluated is the glucose:catalyst weight ratio. The ratio has been varied between 1:1 and 10:1, by maintaining the amount of glucose and adding different amounts of NEEDLE-LTL. It can be observed that the conversion of glucose rises with the catalyst loading until a weight ratio of 3:1, which can be explained by the increment of available acid sites for glucose dehydration (Figure 11). Nevertheless, a higher amount of catalyst (ratio of 1:1) does not improve the conversion, although a slightly higher 5-HMF yield is attained. This could be explained by considering diffusional problems associated to the location of active sites in micropores.

**Figure 11.** Influence of the glucose:catalyst (NEEDLE-LTL) weight ratio (C: glucose conversion; Y: yield) (Experimental conditions: 0.15 g glucose, 1.5 water, 3.5 mL MIBK, Temperature: 150 ◦C; time: 60 min, CaCl2 = 0.65 g gH2O<sup>−</sup>1).

A key aspect in heterogeneous catalysis is the recovery of the solid catalyst to be used in successive catalytic runs. In order to carry out this study, a glucose:catalyst weight ratio of 1:1 was used, since a higher catalyst loading would minimize the loss of catalyst between cycles, produced by catalyst handling. The reaction was studied at 150 ◦C, for 60 min (Figure 12).

**Figure 12.** Reusing study of NEEDLE-LTL after (1) drying at 65 ◦C and (2) washing with water/acetone (C: glucose conversion; Y: yield) (Experimental conditions: 0.15 g glucose, 0.15 g catalyst, 1.5 water, 3.5 mL MIBK, Temperature: 150 ◦C; time: 60 min, CaCl2 = 0.65 gH2O-1).

After the first catalytic cycle, the solid catalyst was filtered and dried at 65 ◦C. The catalytic data reveal a decrease in glucose conversion from 53.4 to 43.4%, although active sites involved in glucose dehydration to 5-HMF seem to be almost deactivated, since 5-HMF yield drastically diminishes from 15.5 to 3.7%. After the second run, a water/acetone washing and filtration of the used solid was also carried out, recovering the glucose conversion, but the 5-HMF yield is still lower. Finally, the used catalyst was calcined until 550 ◦C for 2 h after the first catalytic reaction cycle in order to remove the organic fraction. However, the initial catalytic performance continued without recovery, with values of glucose conversion and 5-HMF yield of 100% and 4.52%, respectively. In order to explain the reason of this behavior, the thermogravimetric analyses of used and fresh NEEDLE-LTL zeolite catalysts were performed (Figure 13). The fresh zeolite shows a weight loss of 10% between room temperature and 200 ◦C, associated to the removing of adsorbed water, without any remarkable weight variation onwards until 900. However, the used catalyst exhibits a high weight loss in the same temperature range, but with two distinguishable steps, which could be associated to the elimination of adsorbed water and MIBK, account for 20%. Then, a progressive weight loss is observed, but with an important sloop change between 700 and 900 ◦C. Thus, a weight loss of 22% is observed between 200 and 900 ◦C, and the high temperature required to remove organic molecules could be explained by the strong interaction between the organic gues<sup>t</sup> molecules with the catalyst surface inside the micropores. In this sense, a regeneration temperature of 550 ◦C was not enough to remove organic species, which cover the active sites responsible of glucose dehydration to 5-HMF. The covering of active sites by carbonaceous deposits, as main cause of the decrease in catalytic activity, has been already reported for zeolites [23].

Therefore, it can be concluded that the LTL-zeolites can be used as heterogeneous acid catalysts for the dehydration of glucose to 5-hydroxymethylfurfural, attaining a maximum 5-HMF yield of 63.1% for a glucose conversion of 87.9%, at 175 ◦C after 90 min, with the NEEDLE-LTL catalyst, formed by nanoparticles with a length of 4.46 μm and a width of 0.63 μm. The diffusion of glucose molecules to the active sites present in LTL zeolites is favored by using needle and rod morphologies, but it is necessary to prepare hierarchical zeolites, where mesoporosity is a key feature for facilitating the access of reactants and the regeneration of active sites, thus delaying the catalyst deactivation.

**Figure 13.** TG curves of fresh (black line) and used (blue line) (glucose:catalyst weight ratio = 1:1, T= 150 ◦C, time = 60 min) NEEDLE-LTL-zeolite.
