Structure of Chemisorbed CO(2) Species in Amine-Functionalized Mesoporous Silicas Studied by Solid-State NMR and Computer Modeling.

Two-dimensional (2D) solid-state nuclear magnetic resonance (SSNMR) experiments on samples loaded with 13C-labeled CO2, "under controlled partial pressures", have been performed in this work, revealing unprecedented structural details about the formation of CO2 adducts from its reaction with various amine-functionalized SBA-15 containing amines having distinct steric hindrances (e.g., primary, secondary) and similar loadings. Three chemisorbed CO2 species were identified by NMR from distinct carbonyl environments resonating at δC ≈ 153, 160, and 164 ppm. The newly reported chemisorbed CO2 species at δC ≈ 153 ppm was found to be extremely moisture dependent. A comprehensive 1H-based SSNMR study [1D 1H and 2D 1H-X heteronuclear correlation (HETCOR, X = 13C, 29Si) experiments] was performed on samples subjected to different treatments. It was found that all chemisorbed CO2 species are involved in hydrogen bonds (HBs) with either surface silanols or neighboring alkylamines. 1H chemical shifts up to 11.8 ppm revealed that certain chemisorbed CO2 species are engaged in very strong HBs. We effectively demonstrate that NMR may help in discriminating among free and hydrogen-bonded functional groups. 13C{14N} dipolar-recoupling NMR showed that the formation of carbonate or bicarbonate is excluded. Density functional theory calculations on models of alkylamines grafted into the silica surface assisted the 1H/13C assignments and validated various HB arrangements that may occur upon formation of carbamic acid. This work extends the understanding of the chemisorbed CO2 structures that are formed upon bonding of CO2 with surface amines and readily released from the surface by pressure swing.

Sulfonated graphene oxide as effective catalyst for conversion of 5-(hydroxymethyl)-2-furfural into biofuels.

The acid-catalyzed reaction of 5-(hydroxymethyl)-2-furfural with ethanol is a promising route to produce biofuels or fuel additives within the carbohydrate platform; specifically, this reaction may give 5-ethoxymethylfurfural, 5-(ethoxymethyl)furfural diethylacetal, and/or ethyl levulinate (bioEs). It is shown that sulfonated, partially reduced graphene oxide (S-RGO) exhibits a more superior catalytic performance for the production of bioEs than several other acid catalysts, which include sulfonated carbons and the commercial acid resin Amberlyst-15, which has a much higher sulfonic acid content and stronger acidity. This was attributed to the cooperative effects of the sulfonic acid groups and other types of acid sites (e.g., carboxylic acids), and to the enhanced accessibility to the active sites as a result of the 2D structure. Moreover, the acidic functionalities bonded to the S-RGO surface were more stable under the catalytic reaction conditions than those of the other solids tested, which allowed its efficient reuse.

Dimensionality transformation through paddlewheel reconfiguration in a flexible and porous Zn-based metal-organic framework.

The reaction between Zn and a pyrene-based ligand decorated with benzoate fragments (H(4)TBAPy) yields a 2D layered porous network with the metal coordination based on a paddlewheel motif. Upon desolvation, the structure undergoes a significant and reversible structural adjustment with a corresponding reduction in crystallinity. The combination of computationally assisted structure determination and experimental data analysis of the desolvated phase revealed a structural change in the metal coordination geometry from square-pyramidal to tetrahedral. Simulations of desolvation showed that the local distortion of the ligand geometry followed by the rotation and displacement of the pyrene core permits the breakup of the metal-paddlewheel motifs and the formation of 1D Zn-O chains that cross-link adjacent layers, resulting in a dimensionality change from the 2D layered structure to a 3D structure. Constrained Rietveld refinement of the powder X-ray diffraction pattern of the desolvated phase and the use of other analytical techniques such as porosity measurements, (13)C CP MAS NMR spectroscopy, and fluorescence spectroscopy strongly supported the observed structural transformation. The 3D network is stable up to 425 °C and is permanently porous to CO(2) with an apparent BET surface area of 523(8) m(2)/g (p/p° = 0.02-0.22). Because of the hydrophobic nature, size, and shape of the pores of the 3D framework, the adsorption behavior of the structure toward p-xylene and m-xylene was studied, and the results indicated that the shape of the isotherm and the kinetics of the adsorption process are determined mainly by the shape of the xylene isomers, with each xylene isomer interacting with the host framework in a different manner.