Hydrothermal degradation of model sulfonic acid compounds: Probing the relative sulfur–carbon bond strength in water

Hydration of phenylacetylene on sulfonated carbon materials: active site and intrinsic catalytic activity

A series of sulfonated carbon materials (sulfonated glucose-derived carbon, carbon nanotubes, activated carbon and ordered mesoporous carbon, denoted as Sglu, SCNT, SAC and SCMK, respectively) were synthesized and applied as acid catalysts in phenylacetylene (PA) hydration reactions. The sulfonic acid groups (-SO3H) were identified to be the only kind of active sites and were quantified with XPS and a cation exchange process. Mechanistic studies revealed that the catalytic PA hydration reaction follows pseudo first order reaction kinetics. Sglu exhibits a higher reaction rate constant (k) and lower apparent activation energy (E a) in the hydration reactions than SCNT catalysts. NH3-temperature programmed desorption measurement results revealed that the relatively high catalytic activity of Sglu was attributed to both the stronger acidity and larger number of -SO3H active sites. This work exhibited the performance of carbon materials without any extra acidic additives in PA hydration reaction and investigated the intrinsic catalytic activity by kinetics. The present work provides the possibility for acid catalytic applications of carbon materials, which sheds light on the environmentally friendly and sustainable production strategy for aldehyde ketone compounds via the catalytic alkyne hydration reactions.

Pyrolyzing Renewable Sugar and Taurine on the Surface of Multi-Walled Carbon Nanotubes as Heterogeneous Catalysts for Hydroxymethylfurfural Production

Conversion of biorenewable feedstocks into transportation fuels or chemicals likely necessitates the development of novel heterogeneous catalysts with good hydrothermal stability, due to the nature of highly oxygenated biomass compounds and the prevalence of water as a processing solvent. The use of carbon-based materials, derived from sugars as catalyst precursors, can achieve hydrothermal stability while simultaneously realizing the goal of sustainability. In this work, the simultaneous pyrolysis of glucose and taurine in the presence of multi-walled carbon nanotubes (MWCNTs), to obtain versatile solid acids, has been demonstrated. Structural and textural properties of the catalysts have been characterized by TEM, TGA, and XPS. Additionally, solid state nuclear magnetic resonance (ssNMR) spectroscopy has been exploited to elucidate the chemical nature of carbon species deposited on the surface of MWCNTs. Al(OTf)3, a model Lewis acidic metal salt, has been successfully supported on sulfonic groups tethered to MWCNTs. This catalyst has been tested for C6 sugar dehydration for the production of HMF in a tetrahydrofuran (THF)/water solvent system with good recyclability.

Deactivation and Regeneration of Sulfonated Carbon Catalysts in Hydrothermal Reaction Environments

The deactivation pathways of sulfonated carbon catalysts prepared from different carbons were studied during the aqueous-phase hydrolysis of cellobiose under continuous-flow conditions. The sulfonation of carbon materials with a low degree of graphitization introduced sulfonic acid groups that are partially stable even during prolonged exposure to harsh hydrothermal treatment conditions (180 °C). The physicochemical characterization of hydrothermally treated materials coupled with the treatment of model compounds for sulfonic acids demonstrated that the stability is related to the presence of activating and deactivating substituents on the aromatic system. Besides sulfonic acid group leaching, a hitherto unknown mode of deactivation was identified that proceeds by the ion exchange of cations contained in the aqueous feed and protons of the sulfonic acid groups. Proton leaching is a fully reversible mode of deactivation by the treatment of the spent catalysts with strong Brønsted acids. Through a combined approach of physicochemical characterization, catalytic testing, and hydrothermal treatment, a methodology for the preparation of catalytically stable carbon materials that bear sulfonic acid groups was established.

Versatile design and synthesis of mesoporous sulfonic acid catalysts

Mesoporous sulfonic acid catalysts (MSAC) are widely used in acid-catalyzed reactions, including biomass conversions with plenty of polar solvents and precursors. The catalytic efficiency of MSAC is greatly affected by the microenvironment around the sulfonic acid sites. In this review, the progress on modification of microenvironment of MSAC is reviewed over the past decade. Hydrophobic modification allows MSAC prevent the adhesion of water molecules onto sulfonic acid sites, to abate the risk of reduced acid strength and catalytic efficiency. In comparison, hydrophilic properties can bring positive effect on acid-catalyzed reactions with the aid of hydrophilic interaction between polar functional groups on MSAC and hydrophilic groups of specific substrates. Amphiphilic MSAC with tunable wettability for specific substrates and solvents tend to improve the efficiency in certain reactions with mixed solvents or reactants of different polarity, especially for biphasic systems of immiscible liquids. Furthermore, much attention has been attracted on modification of surface to simulate the microenvironment of homogeneous solvents and enzyme biocatalysts in recent research. New trends of this field are also highlighted.

Importance of Internal Porosity for Glucan Adsorption in Mesoporous Carbon Materials

To better understand the adsorption of long-chain poly(1 → 4)-β-D-glucans on carbon surfaces as well as interactions responsible for this adsorption, we use a comparative study involving mesoporous carbon-silica composite materials that have been etched to varying degrees and all-atom molecular dynamics simulations. The materials synthesized as part of this etching study consist of an as-synthesized composite material (MCN-MSN), MCN-MSN-0.5 (composite materials consisting of 50% carbon by mass), MCN-MSN-0.3 (composite materials consisting of 70% carbon by mass), and MCN, in which silica etching was conducted using an aqueous ethanolic solution of either NaOH or HF. Data for the adsorption of long-chain glucans to these materials from concentrated aqueous HCl (37 wt %) solution demonstrate a direct relationship between the amount of β-glu adsorption and the magnitude of exposed carbon mesopore surface area, which systematically increases and is also accompanied by an increase in the mesopore size during silica etching. This demonstrates β-glu adsorption as occurring on internal carbon mesopores rather than exclusively on the external carbon surface. These experimental data on adsorption were corroborated by molecular dynamics (MD) simulations of β-glu adsorption to a graphene bilayer separated by a distance of 3.2 nm, chosen to correspond to the carbon mesopore diameter of the experimental system. Simulation results using a variety of β-glu solvent systems demonstrate the rapid adsorption of a β-glu strand on the graphitic carbon surface via axial coupling and are consistent with experimentally observed trends in fast adsorption kinetics. Solvent-mediated effects such as small-scale hydrophobicity and preferential interactions with ions are shown to play important roles in modulating glucan adsorption to carbon surfaces, whereas experimental data on hydrophobically modified silica demonstrate that hydrophobicity in and of itself is insufficient to cause β-glu adsorption from concentrated aqueous HCl solution.