2.1. Preparation of Fluorohectorites
Our first attempt to reproduce the results of Barrer and Jones [30
] consisted of performing the synthesis using commercial reagents. The XRD pattern of the sample prepared at 800 °C for 24 h showed a complex mixture of crystalline phases, which could be identified as quartz, magnesium silicates and lithium silicates. In contrast, the fluorohectorite phase was obtained together with quartz, antophyllite (Mg7
) and some other phases for the sample prepared at 850 °C for 2 h. The presence of high amounts of quartz in both samples can be explained by the non-homogeneous mixture of the reagents. These results were not in agreement with those previously reported [30
To improve the synthesis, we decided to increase the solid homogeneity by mixing the starting solids reagents, grounding them, and suspending the resulting solid mixture in a few milliliters of acetone. Then, the suspension was submitted to an ultrasound bath for 20 min and, finally, the solvent was evaporated. With this procedure, the fluorohectorite phase increased significantly for the sample heated at 850 °C for 2 h, but a mixture of a large number of phases still remained. For the sample prepared at 800 °C for 24 h, the fluorohectorite phase again was not detected. The thinner particles of the reagent powders should be more reactive, and therefore, the fluorohectorite phase can be easily obtained. However, the use of longer synthesis times can favor the decomposition of the fluorohectorite into other more thermodynamically favored phases.
To increase the fluorohectorite content, the behavior of the system with time was studied in more detail, and the reactivity of the reagents was controlled by sintering them before using when performing the synthesis at 800 °C. For this reason, we prepared four samples with sintered reagents at 800 °C for 3, 6, 12 and 24 h and three samples with non-sintered reagents at 850 °C for 0.5, 1 and 2 h. XRD patterns of the FH800s samples are shown in Figure 1
. The main crystalline phases of all samples synthesized at 800 and 850 °C are summarized in Table 1
Fluorohectorite was the main crystalline phase in sample FH800s3, although anthophyllite and quartz were also present. This allowed us to conclude that the use of sintered reagents together with short synthesis times (3 h) at 800 °C are crucial factors in forming higher amounts of the fluorohectorite phase. Anthophyllite seems to be the most thermodynamically favored phase, since it was the main phase obtained at longer synthesis times, but it also appeared at shorter times in lower amounts. Quartz was always present: after 3 h as residual amount from the SiO2 reagent, and after 6 h as the main phase, probably as a product of the decomposition of fluorohectorite. With respect to the experiments at 850 °C, the fluorohectorite phase was only observed in sample FH850ns2 together with other phases, such as quartz, anthophyllite, lithium silicates and lithium fluoride. The best results were achieved for FH800s3 and FH850ns2 samples. In any case, FH850ns2 showed higher complexity in its composition, indicating less purity of the material.
Based on these results, obtaining pure fluorohectorite by synthesis appears to be very difficult. Taking into account the differences in the size of the components detected in the optimized samples, we tried to purify the material after synthesis (FH800s3
) by centrifugation. Thus, quartz, identified by XRD, was partially separated after centrifugation at 600 rpm for 6 min. After subsequent centrifugation at 4000 rpm for 30 min, we obtained the best results for the fluorohectorite isolation. TEM image and XRD pattern of the purified material are shown in Figure 2
. The characteristics of the resulting purified fluorohectorite (Li-FH) are summarized and compared with Na-delaminated hectorite (Na-DH) and H-β zeolite in Table 2
XRD pattern of Li-FH showed a very well defined 001 reflection at 2θ = 7° corresponding to a lamellar material with high ordering in the stacking direction (Figure 2
). However, some amounts of anthophyllite and a very low amount of quartz were also detected. Fluorohectorite formation was also confirmed by TEM, since well-defined but sintered lamellae with sizes between 100 and 400 nm were visualized (Figure 2
). The expected presence of fibrous crystals due to anthophyllite was not observed.
EDS-SEM analysis of the Li-FH sample allowed us to confirm the presence of fluorine. Si/F atomic ratio in the purified material was 1.8 (Table 2
), near the expected ratio for a totally fluorinated hectorite (theoretical value Si/F = 2). Additionally, the Si/Mg ratio of 1.5 matched the theoretical stoichiometry of the fluorohectorite Li0.7
]. Lithium cannot be observed by this technique due to its small atomic number. The amount of Li was estimated in consideration of the Si/Mg ratio.
The BET surface area of Li-FH, determined by nitrogen physisorption, was low (16 m2
/g), which is in agreement with its high crystallinity and sinterization. The adsorption isotherm of this sample was mainly of type II, corresponding to non-porous materials, but with some contribution of mesoporosity related to the presence of hysteresis of type H3 according to IUPAC classification (Figure 3
]. The hysteresis shape can be associated with aggregates of plate-like particles with slit-shaped pore formation. In fact, the porosity presented a bimodal distribution in which the majority of the pores had a radius of around 20 Å, but a minor part had a radius of around 105 Å (Figure 3
b). The C.E.C. value of Li-FH was 105 meq/100 g, which is close to the theoretical value of 95 meq/100 g, considering the stoichiometry and expected fluorination.
The porosity, BET surface area and C.E.C. value of Na delaminated hectorite (Table 2
) were similar to those obtained for delaminated hectorites previously prepared in the research group [27
]. Fluorohectorite had much lower surface area but higher C.E.C. value than Na-DH and H-β. This will later be correlated with the catalytic results. The textural properties of the protonated forms of Li-FH and Na-DH (H-FH and H-DH) were similar to those obtained for their corresponding precursors, but significant differences in acidity should be expected.
XPS of the fluorohectorite Li-FH was performed in order to obtain information regarding the surface composition and electronic characteristics of this sample. Figure 4
shows the 2p XPS spectra for Si and Mg, and the 1s for O and F. The atomic concentration (%), binding energy values and atomic ratios are shown in Table 3
After deconvolution, we can observe only one contribution for the F 1s peak, two contributions for Si and Mg 2p and three for O 1s. For the elements with several peak contributions, the main one was always that with the highest binding energy. No peak due to Li was observed.
The most significant information from Figure 4
and Table 3
is that O and F were present in lower amounts on the surface than the expected bulk stoichiometric values (see values *). Additionally, Li was not detected. This could be correlated with the minor peak contribution observed from deconvolution of the Si and Mg spectra in the 2p region, with binding energies (B.E) of 101.3 and 48.6 eV, respectively, associated with higher electron density. These results could be related to an initial decomposition of hectorite with loss of Li as LiF and Li2
O. This is in agreement with the decrease of the fluorohectorite phase observed when the synthesis time was longer than 3 h. The presence of defects on surface could have a potential influence on catalysis.
The acidity of the materials, which will be used as acid catalysts for the transformation of xylose to furfural, was characterized by NH3
-TPD (Figure 5
, Table 4
By examining the acidity results of the five catalysts compared in the current study, the order of the total amount of acid sites was H-β > Na-DH > H-DH > H-FH > Li-FH (Table 4
). It is important to note that for H-FH, the acidity was very strong, since the maximum of the desorption temperature for the main desorption peak was 721 °C. This confirms that fluorination increased the strength of acidity, as expected. Additionally, although the acidity strength of H-β was higher than that observed for the hectorites Na-DH, H-DH and Li-FH, it was lower enough than that obtained for H-FH. The acidity of H-β, as described in the literature, can be assigned to Brønsted acid sites with some contribution of Lewis acid sites due to the presence of structural defects [32
]. Another question to note is the higher number of acid sites of Na-DH compared to H-DH. Acidity should be mainly due to the Lewis and Brønsted acid sites. However, the acidity of H-DH was stronger than the acidity of Na-DH, since the NH3
desorption took place at higher temperature (224 °C compared to 190 °C). The total acidity of Li-FH was the lowest, but presented several contributions, mainly due to Lewis acid sites, but probably also with a certain amount of Brønsted acid sites due to the hydrolysis of the Li+
cation. Interestingly, fluorinated hectorites presented much higher amount of acid sites by m2
, especially H-FH, than the rest of catalysts (Table 4