(2is) Effect of the Concentration of Brönsted and Lewis Acidic Sites on the Main Reaction Pathways during the Conversion of Fructose over Sn-KIT-6-PrSO3h Bifunctional Catalyst

I have focused on synthesizing and characterizing solid catalysts for the generation of high value-added products. I am also very interested in investigating reactive pathways influenced by the various types of acidic and basic nature present within these catalysts and modeling, analyzing, and simulating catalytic reactors.

Due to the limited availability of fossil resources and the increasing environmental issues related to their emissions, the scientific community has focused on developing more sustainable routes to produce high value-added chemicals. 5-hydroxymethylfurfural (HMF) is a promising molecule, serving as a key intermediate between biomass and a wide range of bioproducts such as precursors, resins, polymers, solvents, fuels, and additives. However, a number of studies have demonstrated that the chemistry of the formation of HMF is very complex, including multiple side-reactions like the decomposition to levulinic acid and the polymerization to humins, which influence strongly on the efficiency of the process [1]. Previous studies have reported HMF yields of approximately 40 % for conversions of 70 % to 80 %, achieved through the dehydration reaction of fructose in the presence of a Brönsted acid catalyst [2,3]. Therefore, bifunctional acid catalysts have been designed to promote the dehydration of fructose to HMF. Generally, these catalysts consist of thioether, and sulfonic acid groups supported on silica or alumina that promote the tautomerization of fructose and catalyze its dehydration, respectively [4–6]. In addition, only a few studies have analyzed the effect of Lewis acid sites on this reaction, confirming through experimental and computational studies that there are reaction pathways catalyzed by Lewis acid sites which exhibit a higher yield compared to non-catalytic reactions [7]. However, these Lewis acid sites are also responsible for unwanted polymerization reactions, so it is necessary to find an appropriate balance of Brönsted and Lewis acid sites to favor the production of HMF.

Recently, the novel amorphous silica material KIT-6 has gained attention due to its particular three-dimensional mesoporous structure with cubic symmetry (Ia3d), consisting of highly interconnected enantiomeric pairs of channels [8]. Furthermore, the KIT-6 material possesses a large surface area and pore diameter, thermal stability, and the capability to be modified through the incorporation of transition metals (Al, Ti, Zr, Sn, V, for example) and organic groups (-PrSO3H) in order to modulate its acidic properties[9–13].

In this study, the effect of the concentration of Brönsted and Lewis acid sites in the catalyst on the main reaction pathways and the selectivity towards valuable products during fructose conversion was analyzed.

A set of four catalysts were synthesized through sol-gel method, KIT-6, Sn-KIT-6, KIT-6-PrSO₃H and Sn-KIT-6-PrSO₃H. Tin was used as the source of Lewis acidic sites (LAS), while sulfonic groups were used as the source of Brönsted acidic sites (BAS). The catalysts synthesized preserved the well-ordered, typical cubic bicontinuous mesostructure of KIT-6 after the incorporation of Sn and -PrSO₃H groups [14]. Furthermore, specific surface areas between 800-950 m²/g and average pore size between 5.5-8 nm were determined from nitrogen physisorption experiments. The modified catalysts exhibited pore size contractions with respect to the KIT-6 material, which are distinctive of the incorporation of the metal heteroatom and sulfonic groups into the silica framework [13,15]. The presence of Sn⁴⁺ species in a tetrahedral arrangement, poly-hydrated Sn⁴⁺ species with high coordination number and SnO₂ in an octahedral arrangement was also confirmed by UV-VIS-DRS [16,17]. Moreover, the presence of sulfonic groups was confirmed by FTIR spectroscopy [18]. The concentration of acidic sites was quantified and characterized using the pyridine adsorption technique. The metal component was responsible for generating Lewis acidic sites within the mesoporous silica framework, while the organic groups were responsible for generating Brönsted acidic sites. Sn-KIT-6 catalyst exhibited an increase in the intensity of the signals corresponding to Lewis acidic sites in comparison with the KIT-6 sample. These sites could be related to close and open Sn⁴⁺ species in a tetrahedral arrangement, coordinated to four oxygen atoms (LAS) [15,19]. The sulfonic acid functionalized KIT-6-PrSO₃H exhibited a significant increase in the intensity associated with the bond between the pyridinium ion and the acid sites, also identified as Brönsted acid sites. Furthermore, the bifunctional catalyst Sn-KIT-6-PrSO₃H showed the presence of both acidic sites, Brönsted and Lewis, with a BAS/LAS ratio of 14.4, in comparison with Sn-KIT-6 (BAS/LAS = 0.3) and KIT-6-PrSO₃H (BAS/LAS = 651.6) [20].

In order to study the effect of the concentration of Bronsted and Lewis acid sites in the catalysts on the activity and selectivity during fructose conversion, reaction experiments were performed in a stainless-steel batch reactor, heated at 453 K at different reaction times (from 15 to 120 min) [4,5]. In a typical run, 50 mg of catalyst, 1.5 g of fructose, 3.5 g of deionized water and 3 g of a mix of methyl isobutyl ketone and 2-butanol (mass ratio of 7:3) were mixed together in the reactor. The organic phase of products was separated from the aqueous phase by centrifugation and then analyzed by gas chromatography to determine HMF, furfural (FUR) and levulinic acid (LA) concentrations. The conversion of fructose over time was determined by fitting a kinetic model using the gamultiobj function in MATLAB^®. The minimized objective functions are Σ(Ci* - Ci)², where Ci is the concentration of component i = HMF, FUR and AL.

Sn-KIT-6-PrSO₃H showed the highest fructose conversion (76 %) and yield to HMF (32.5 %), after 30 min of reaction, these results agree with previous values reported in the literature [3–5]. The catalyst with the highest BAS/LAS ratio, KIT-6-PrSO₃H, also performed a high conversion of fructose (66.5 %) and yield to HMF (32.5 %), however, this material also exhibited the lowest turnover frequency (TOF) for HMF production due to its high concentration of Brönsted acidic sites. This suggests that a catalyst with an appropriate balance of BAS/LAS may show better fructose conversion performance compared to one where Brönsted acid sites predominate. The aforementioned result is further supported when comparing the conversion obtained with the Sn-KIT-6 catalyst (43.7%), which is significantly higher than that obtained with the KIT-6 sample (27.7%). This could indicate that tin incorporated catalysts promoted the formation of HMF. However, propyl sulfonic acid functionalized catalysts performed their respective maximum HMF yield at shorter reaction times, due to the fastest consumption of fructose achieved by the catalysts with the higher concentration of Brönsted acid sites.

Sn-KIT-6-PrSO₃H materials were successfully synthesized by incorporating Sn⁴⁺ and -PrSO₃H into the KIT-6 structure using the sol-gel method. Tin incorporation into SBA-15 led to relatively low BAS/LAS ratios of 0.23 and 14.45, due to the formation of Lewis acid sites, whereas sulfonic groups mainly increased the concentration of Brönsted acid sites, resulting in a high BAS/LAS ratio of 651.56. The reaction study revealed that with a low BAS/LAS ratio in the catalyst, the maximum yield of HMF from fructose dehydration is increased, while a higher BAS/LAS ratio accelerated the fructose consumption. Therefore, among the studied catalysts, the Sn-KIT-6-PrSO3H catalyst exhibited the highest yield, achieving an HMF yield of 36.5 % in a reaction time of 20 min.

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