*2.1. Catalyst Characterization*

The choice of a LTL zeolitic framework to evaluate the influence of the morphology (short rod, cylinder and needle) on the catalytic performance in glucose dehydration to HMF arises from the dimensions of pore mouth and channels of this crystallographic structure (Figure 1). These allow the entrance of glucose molecules to reach active acid sites, and 5-hydroxymethylfurfural can easily go out, leaving active sites available for new sugar molecules.

**Figure 1.** (**a**) Structure of LTL-zeolite view along [001] plane illustrating its hexagonal framework. (**b**) An LTL channel view normal to [001] plane that consists of 0.75 nm unit cells with a pore opening of 0.71 nm. (**c**) Schematic illustration of diffusion of glucose as reactant and HMF as product in the 12-membered ring channel of LTL zeolite.

The X-ray diffraction patterns of H-LTL-zeolites exhibit many narrow diffraction peaks at 2θ = 5.55◦ (100), 11.10◦ (200), 11.77◦ (001), 18.89◦ (210), 15.23◦ (111), 19.31◦ (220), 20.09◦ (310), 20.47◦ (301), etc. which can be assigned to the hexagonal LTL-type framework (PDF 98-007-4170) (Figure 2). In all cases, no additional crystalline phase is detected and the crystallinity of the solids is preserved after ion-exchange and calcination treatments.

**Figure 2.** XRD patterns of (**a**) ROD-LTL, (**b**) NEEDLE-LTL and (**c**) CYL-LTL zeolites.

Concerning the chemical composition of LTL-zeolites, the Si/Al molar ratio values obtained from ICP-OES are close to 3, which are lower than that used in the synthesis gel (10) (Table 1). The comparison between the bulk and surface chemical composition data, deduced from ICP-OES and XPS, respectively, points an enrichment of Si on the surface of LTL-zeolites. This could be explained by the higher Si/Al molar ratio (10) used in the synthesis of these zeolites. However, the inherent error associated with the semi-quantitative analysis by XPS must be taken into account. After the ion exchange process between K<sup>+</sup> and NH4+ ions, not all the alkaline cations are removed, which can be due to the existence of strong acid sites remaining dissociated and neutralized by K<sup>+</sup> cations, even in the presence of an excessive ammonium solution. Therefore, the K/Al molar ratio is lower than 1 for NEEDLE-LTL and ROD-LTL, indicating that, although acid sites are protonated, a fraction of sites is still occupied by K<sup>+</sup> ions. This fraction is even higher for the CYL-LTL, where its value closed to 1 would point out that this morphology is the less favorable for the ion-exchange process.


**Table 1.** XPS and ICP-OES (\* in parentheses) data of LTL-zeolites.

X-ray photoelectron spectroscopy (XPS) has been used to ge<sup>t</sup> insights into the surface nature of LTL- zeolites. In all cases, the binding energies of Si 2p (102.8–103.2 eV), Al 2p (74.3–74.6 eV) and O 1s (532.1–532.4 eV) are typical of these elements forming part of microporous aluminosilicates [25]. As regards the K 2p spectra, they exhibit the characteristic doublet with the K 2p3/2 at 293.3-293.7 eV, and a spectral separation of 2.8 eV, typical of K<sup>+</sup> ions [26].

On the other hand, the textural properties reveal that all zeolites maintain high surface area values, being the largest one for short rod morphology (Table 2). As expected, these zeolites are mainly microporous solids, with a percentage of microporous surface area higher than 95%.


**Table 2.** Textural and acid properties of LTL-zeolites.

\* Determined from NH3-TPD.

The microporous nature of these LTL-based zeolites can be easily confirmed by the shape of their adsorption-desorption isotherms of N2 at −196 ◦C, which are Type I in the IUPAC classification, typical of microporous solids (Figure 3). The slight hysteresis loop could be associated to some mesoporosity generated during treatments used for synthesizing their protonated forms, probably associated to interparticular voids [27,28], which is corroborated by the pore size distribution curves (Figure 3, right).

**Figure 3.** N2 adsorption-desorption isotherms (**left**) and pore size distributions (**right**) of LTL-zeolites at −196 ◦C.

The di fferent morphologies are more clearly appreciated in the scanning electron micrographs (Figure 4), where micrometric particles are observed, whose dimensions and shapes fit in really well with short rods, needles and cylinders (Table 2).

On the other hand, the chemical environment of aluminium has been analyzed by 27Al MAS-NMR spectroscopy (Figure 5). Extra-framework octahedral Al species are associated to a resonance signal at a chemical shift near 0 ppm, whereas tetrahedral Al in crystallographic sites in the zeolite framework appears at about 60 ppm. This latter signal is observed in all cases, and the absence of broadening effect at lower chemical shift values could discard the existence of pentacoordinated or distorted tetrahedral aluminium, reported in other microporous aluminosilicates, which give rise to signals at 30 and 47 ppm, respectively [29–31]. The small contribution at 0 ppm is associated to extra-framework Al species, but this is absent in the MAS-NMR spectrum of CYL-LTL, where all Al seems to be in tetrahedral coordination.

**Figure 4.** SEM images (left) and particle size distribution histograms (right) of LTL-zeolite crystals synthesized of (**a**) ROD-LTL, (**b**) NEEDLE-LTL and (**c**) CYL-LTL. Scale bar = 2 μm.

**Figure 5.** Solid state 27Al-NMR spectra of LTL-zeolites.

The total acidity of catalysts was determined from ammonia temperature-programmed desorption (NH3-TPD), whereas the nature (Brönsted or Lewis) of acid sites was studied from pyridine adsorption coupled to FTIR spectroscopy. Figure 6 displays the amount of ammonia desorbed in different temperature ranges, which have been assigned to weak (100-200 ◦C), medium (200-300 ◦C) and strong (300–550 ◦C) acid sites. The total acidity follows the order: ROD-LTL (1874 μmol g<sup>−</sup>1) > NEEDLE-LTL (1672 μmol g<sup>−</sup>1) > CYL-LTL (1065 μmol g<sup>−</sup>1), being strong acid sites predominant in all cases. This acidity order is the same found for Langmuir surface area values (Table 2).

**Figure 6.** NH3 desorption of LTL-based zeolites, as a function of the strength: weak (100–200 ◦C), medium (200–300 ◦C) and strong (300–550 ◦C).

The nature of acid sites (Brönsted and/or Lewis) has been studied by pyridine adsorption coupled to FTIR spectroscopy. The concentration of Lewis and Brönsted acid sites after adsorption and subsequent desorption of pyridine at 150 and 300 ◦C is given in Table 3. The lowest concentration of both Lewis and Brönsted acid sites is found for the ROD-LTL-zeolite, which after evacuation at 300 ◦C does not exhibit Lewis acidity. The other two zeolites (CYL- and NEEDLE-LTL) exhibit similar concentrations of strong Brönsted and Lewis acid sites, calculated from pyridine amount remaining on catalysts after evacuation at 300 ◦C. Moreover, these two zeolites possess a similar B/L molar ratio (4.76 and 4.86, respectively).

It has been previously reported that Brönsted acid sites could be associated to distorted tetrahedral Al species located in the zeolite framework, whereas distorted pentacoordinated Al species have been associated to Lewis acid sites in zeolites [32].


**Table 3.** Surface acidity of LTL-zeolites with different morphology at different temperature.
