Synthesis and characterization of mesoporous materials with acid properties

VERGARA LOURDES; GUSTAVO MENDOW; C.A. QUERINI; BÁRBARA SÁNCHEZ

Santa Fe

Workshop; French-Argentinian Workshop on Heterogeneous Catalysis; 2023

Synthesis and characterization of mesoporous materials with acidpropertiesLourdes Vergara1*, Gustavo Mendow1, Carlos Querini1, Bárbara Sánchez11INCAPE, Colectora Ruta 168 Km 0 Pje El Pozo, Santa Fe 3000 (Argentina),*lvergara@fiq.unl.edu.arIntroductionMesoporous materials functionalized with sulfonic groups are interesting for use inbiomass valorization reactions. These types of reactions generally involve bulky reagentsand/or products and also require different levels of acidity to be catalyzed. The solids studiedin this work can be modified according to the requirements of the reaction: they have theparticularity of allowing the type of acid (Brønsted and/or Lewis) anchored to the surface andthe density of the sites to be varied. This study presents the most important results of thesynthesized mesoporous materials, which were characterized with various techniques withthe aim of correlating the textural and structural properties, chemical composition and acidityof the catalysts.Materials and MethodsThe synthesized mesoporous materials were KIT-6, FDU-12, Silica Nanotubes (NTS),SBA-15, SBA-15 large pore, SBA-16, mesoporous SnO2 and MCM-48 silicas. In all cases,Tetraethylorthosilicate (TEOS) was used as silicon source, and mecaptopropyltrimethoxysilane (MPTMS) as a precursor of the sulfonic groups. In some materials thePluronic P-123 copolymer was used as template, and in others Pluronic F-127 and CTAB.The synthesis conditions (temperature, hydrothermal treatment) were different for eachsupport, following procedures published in the literature.For the incorporation of propylsulfonic groups with the objective of generating acidity,three alternatives were explored:(i) Co-condensation 1 (C1): Functionalization with mercapto groups (-SH) during thesynthesis and subsequent oxidation to sulfonic groups (-SO3H) [1].(ii) Co-condensation 2 (C2): Functionalization with -SH groups and oxidation to -SO3H inthe same step [2].(iii) Grafting (G): Functionalization with -SH groups post-synthesis followed by oxidation[3].With the three methods, catalysts with different contents of functional groups wereprepared: 10 and 15%, expressed as %mol/mol S/Si.Textural properties were determined by N2 adsorption-desorption isotherms. The specificsurface area was estimated by BET method and the pore size distribution was calculatedusing the Broekhoff-de Boer (BdB) method for adsorption branches. The S/Si ratios on thesilicas were measured by energy dispersive X-ray fluorescence (XRF) (EDX). Aciditymeasurements were made by potentiometric titration with butylamine using acetonitrile assolvent. The efficiency of the mercapto groups oxidation treatment was evaluated withthermogravimetric analysis (TGA) and qualitatively, by N2 stripping at programmedtemperature.Results and DiscussionIn general, it was observed that all of the catalysts had good levels of acidity, but thosesynthesized by co-condensation showed better efficiency in the oxidation of mercapto tosulfonic groups, compared to those prepared by grafting method, which had lower oxidationefficiency. As a counterpart, in the latter, the mesoporous structure of the starting solid wasbetter preserved. As the sulfonic groups content increases, the acidity increases and the BETarea and pore diameter decreases.The FDU-12(C2)-15% catalyst reported very good textural properties. The structure ofthe starting mesoporous solid was preserved, decreasing the BET area from 602 m2/g for thenon-functionalized support to 395 m2/g, with a pore diameter of 5.5 nm. In addition, thehighest acidity of the series of catalysts prepared was obtained and the –SH groups weretotally oxidized. In comparison, for the NTS(C1)-15% catalyst, only 33% of the incorporatedgroups were completely oxidize to sulfonic, while for the KIT-6 support functionalized with thesame method, this value was 63%. For those materials in which the oxidation was notcomplete, a two stages oxidation treatment was explored, achieving an improvement in thefinal acidity.The effectiveness in the incorporation of sulfur was evaluated by means of the FRXtechnique, which allows determining the % S/Si that was actually incorporated, definingeffectiveness as the ratio between the measured value and the theoretical or nominal value.It was found that for all of the catalysts it was between 0.7 and 1. However, the lower valuescorrespond to the catalysts prepared by grafting. This is consistent with the result obtained byTGA.KIT-6 catalysts synthesized by the three methods were particularly studied in theesterification reaction of glycerin with free fatty acids (FFA). It was found that with the KIT-6(C2)-15% catalyst, at 180 min of reaction, a FFA conversion of 93% was achieved, and 80%selectivity, calculated as the sum of mono, di and triglycerides (products of interest). This is agood result considering what has been reported in the bibliography for this reaction, usingheterogeneous catalysis and similar reaction conditions.Significance or Main ConclusionsMore than nine mesoporous supports functionalized with sulfonic groups to generateacidity were synthesized. It should be highlighted that these solids have been little explored inthe literature and therefore, they were characterized by several techniques and evaluated indifferent reactions in which the presence of acid sites and mesoporosity are required. Thecatalysts were evaluated in the esterification reaction of glycerin with free fatty acids, in theobtention of 5-ethoxymethylfurfural from fructose and in the C-C coupling reaction ofhydroxyalkylation-alkylation of furfural with 2-methylfuran.