Propylsulfonic Acid-Functionalized Mesostructured Natural Rubber/Silica Nanocomposites as Promising Hydrophobic Solid Catalysts for Alkyl Levulinate Synthesis

Abstract

Organosulfonic acid-functionalized mesoporous silica is a class of heterogeneous acid catalysts used in esterification processes due to its high surface area, shape-selective properties, and strongly acidic sites. Since water is generated as a by-product of esterification, the surface of mesostructured silica is modified to enhance hydrophobicity and catalytic performance. In this study, a series of propylsulfonic acid-functionalized nanocomposites based on natural rubber and hexagonal mesoporous silica (NRHMS-SO3H) with different acidities were prepared via an in situ sol-gel process using tetraethyl orthosilicate as the silica source, dodecylamine as the nonionic templating agent, and (3-mercaptopropyl)trimethoxysilane as the acid-functional group precursor. Compared with conventional propylsulfonic acid-functionalized hexagonal mesoporous silica (HMS-SO3H), NRHMS-SO3H provided higher hydrophobicity, while retaining mesoporosity and high surface area. The catalytic activity of synthesized solid acids was then evaluated via batch esterification of levulinic acid (LA) with alcohols (ethanol, n-propanol, and n-butanol) to produce alkyl levulinate esters. NRHMS-SO3H exhibited higher catalytic activity than HMS-SO3H and ultra-stable Y (HUSY) zeolite owing to the synergistic effect between the strongly acidic-functional group and surface hydrophobicity. The activation energy of the reaction over the NRHMS-SO3H surface was lower than that of HUSY and HMS-SO3H, suggesting that tuning the hydrophobicity and acidity on a nanocomposite surface is a compelling strategy for energy reduction to promote catalysis.

Levulinic Acid Esterification with n-Butanol over a Preyssler Catalyst in a Microwave-Assisted Batch Reactor: A Kinetic Study

“…Therefore, results suggest an inverse relationship between the levulinic acid conversion and the chain length of primary alcohols (C4−C10), which is attributed to a decrease in the nucleophilic nature of the alcohols with increased molecular size, inhibiting their ability to donate electron pairs to the carbonyl atom of the levulinic acid molecule. Similar conclusions have been reported for levulinic acid esterification using organosulfonic acid-functionalized mesoporous silica 34 and PW 12 /MCM-22 31 as heterogeneous catalysts.…”

Adenine‐Functionalized Dendritic Fibrous Nanosilica as Bifunctional Catalyst for CO2 Fixation into Cyclic Carbonate

“…The ICPTES spacers, owing to their C 3 hydrocarbon, also render hydrophobicity to the final material DFNS@ICPTES@adenine (DAD), thereby boosting the selective adsorption of CO 2 . [51] This was confirmed by comparing the contact angle measurements obtained from DFNS and DAD-coated glass surfaces (Figure S1). The study reveals that the contact angle for DAD (57°) is much higher than DFNS (39°), confirming the more hydrophobic nature of DAD compared to DFNS.…”

“…The adenine base was attached to DFNS@ICPTES ( DI ) through carbamide linkage formation, resulting from the reaction of −N=C=O and −NH 2 groups of ICPTES and adenine, respectively. The ICPTES spacers, owing to their C 3 hydrocarbon, also render hydrophobicity to the final material DFNS@ICPTES@adenine ( DAD ), thereby boosting the selective adsorption of CO 2 [51] . This was confirmed by comparing the contact angle measurements obtained from DFNS and DAD ‐coated glass surfaces (Figure S1).…”

“…In all the materials, a broad peak centered around 3400 cm −1 (ν O−H of H 2 O and Si−OH groups) and a sharp peak at ≈1660 cm −1 (δ O−H of water) indicate the presence of adsorbed water in their porous structure. The FTIR spectrum of DFNS reveals the peaks at 959 cm −1 , 802 cm −1 , and 1126 cm −1 , corresponding to the vibration of silanol groups (Si−OH) and symmetric and asymmetric vibration of Si−O−Si bond, respectively [51] . In the case of DI , the cross‐linking with ICPTES leads to the appearance of the new peaks at 1638 cm −1 (stretching vibration of C=N (ν C=N )) and at 2929 cm −1 and 2983 cm −1 (C−H stretching vibration (ν C−H ) of C 3 aliphatic spacer) [52,53] .…”

“…The FTIR spectrum of DFNS reveals the peaks at 959 cm À 1 , 802 cm À 1 , and 1126 cm À 1 , corresponding to the vibration of silanol groups (SiÀ OH) and symmetric and asymmetric vibration of SiÀ OÀ Si bond, respectively. [51] In the case of DI, the cross-linking with ICPTES leads to the appearance of the new peaks at 1638 cm À 1 (stretching vibration of C=N (ν C=N )) and at 2929 cm À 1 and 2983 cm À 1 (CÀ H stretching vibration (ν CÀ H ) of C 3 aliphatic spacer). [52,53] The FT-IR spectrum of DAD almost resembles that of DI because its characteristic NÀ H stretching peaks are obscured by the broad OÀ H peak of moisture.…”