One key route for controlling reaction selectivity in heterogeneous catalysis is to prepare catalysts that exhibit only specific types of sites required for desired product formation. Here we show that alkanethiolate self-assembled monolayers with varying surface densities can be used to tune selectivity to desired hydrogenation and hydrodeoxygenation products during the reaction of furfural on supported palladium catalysts. Vibrational spectroscopic studies demonstrate that the selectivity improvement is achieved by controlling the availability of specific sites for the hydrogenation of furfural on supported palladium catalysts through the selection of an appropriate alkanethiolate. Increasing self-assembled monolayer density by controlling the steric bulk of the organic tail ligand restricts adsorption on terrace sites and dramatically increases selectivity to desired products furfuryl alcohol and methylfuran. This technique of active-site selection simultaneously serves both to enhance selectivity and provide insight into the reaction mechanism.

Surface chemical reactions of highly functional biomass derivatives such as furans with oxygenated ligands are often considered in terms of the chemistry of their individual functional groups, with little focus on how multifunctionality affects surface chemistry. To probe these effects on functionalized furans, temperature-programmed desorption (TPD) experiments and density functional theory (DFT) calculations were used to study the thermal chemistry of furfural, C4H3(CHO)O, and furfuryl alcohol, C4H3(CH2OH)O on Pd(111). The TPD results indicate that furfural undergoes decomposition to produce furan, propylene, carbon monoxide, and hydrogen. Furfuryl alcohol forms the same products but also undergoes an unexpected C–O scission process that yields methylfuran and water. Together with DFT calculations, these results indicate that furfuryl alcohol can decompose through a surface furfural intermediate, similar to the reaction pathway observed for simple alcohols such as ethanol. The additional methylfuran pathway, however, is not observed for simple alcohols. In addition, the production of propylene suggests that substitution of the furan ring strongly affects the available reaction pathways, since TPD of furan does not show any propylene evolution. TPD experiments conducted with coadsorbed deuterium provide additional information on the reaction mechanism and suggest that methylfuran formation may be assisted by interactions between adsorbates. Furthermore, observed trends in the isotopic product distribution together with a thermochemical reaction pathway constructed using DFT indicate that the presence of oxygenated pendant groups on the furan ring strongly influences the chemistry of the ring. The importance of these mechanisms for catalytic reactions of sugar derivatives such as 5-hydroxymethylfurfural are discussed.

ZIF-8 is a crystalline microporous material that has been widely studied because of its thermal and chemical stability relative to other metal−organic frameworks (MOFs) and its potential for use in a number of gas adsorption, separation, and catalysis applications. However, most studies focus on characterization of the bulk structure of ZIF-8, ignoring the potential effect of the particle shape and the external surface on these applications. This report describes studies that examined the stability of the {110} and {100} crystallographic facets of ZIF-8 under mildly acidic conditions. Though the {110} facet is more thermodynamically stable than the {100} facet, it is found to be more susceptible to degradation by acid exposure. It is hypothesized that the mechanism of particle degradation follows a shrinking-core model, with surface imidazolates being replaced by hydroxyls, as suggested by complementary X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy characterization. Computational investigations support this hypothesis and suggest that the reaction energy for insertion of water into the Zn− MeIm bond is more favorable on the {110} facet than on the {100} facet because of steric constraints. Additionally, the reaction energies are more favorable on both surfaces in the presence of SO 2 . These findings have implications for the formation of surface barriers that can affect mass transfer into or out of the ZIF crystals. This study represents the first comprehensive investigation of differences between exposed surface facets of a MOF.

Aminopolymer-based solid sorbents have been widely investigated for the capture of CO from dilute streams such as flue gas or ambient air. However, the oxidative stability of the widely studied aminopolymer, poly(ethylenimine) (PEI), is limited, causing it to lose its CO capture capacity after exposure to oxygen at elevated temperatures. Here, we demonstrate the use of linear poly(propylenimine) (PPI), synthesized through a simple cationic ring-opening polymerization, as a more oxidatively stable alternative to PEI with high CO capacity and amine efficiency. The performance of linear PPI/SBA-15 composites was investigated over a range of CO capture conditions (CO partial pressure, adsorption temperature) to examine the tradeoff between adsorption capacity and sorption-site accessibility, which was expected to be more limited in linear polymers relative to the prototypical hyperbranched PEI. Linear PPI/SBA-15 composites were more efficient at CO capture and retained 65-83 % of their CO capacity after exposure to a harsh oxidative treatment, compared to 20-40 % retention for linear PEI. Additionally, we demonstrated long-term stability of linear PPI sorbents over 50 adsorption/desorption cycles with no loss in performance. Combined with other strategies for improving the oxidative stability and adsorption kinetics, linear PPI may play a role as a component of stable solid adsorbents in commercial applications for CO capture.

Studies on aminopolymer/oxide composite materials for direct CO capture from air have often focused on the prototypical poly(ethylenimine) (PEI) as the aminopolymer. However, it is known that PEI will oxidatively degrade at elevated temperatures. This degradation has been ascribed to the presence of secondary amines, which, when oxidized, lose their CO capture capacity. Here, we demonstrate the use of small molecule poly(propylenimine) (PPI) in linear and dendritic architectures supported in silica as adsorbent materials for direct CO capture from air. Regardless of amine loading or aminopolymer architecture, the PPI-based sorbents are found to be more efficient for CO capture than PEI-based sorbents. Moreover, PPI is found to be more resistant to oxidative degradation than PEI, even while containing secondary amines, as supported by FTIR, NMR, and ESI-MS studies. These results suggest that PPI-based CO sorbents may allow for longer sorbent working lifetimes due to an increased tolerance to sorbent regeneration conditions and suggest that the presence of secondary amines may not mean that all aminopolymers will oxidatively degrade.

Zeolitic imidazolate frameworks (ZIFs) are a set of nanoporous metal−organic frameworks (MOFs) with tunable porosity and functionality. Among MOFs, they also show relatively good stability with respect to temperature and humidity. These characteristics lead to their possible applications in separation processes. In many practical separation processes, adsorbents are exposed to a variety of molecular species including acid gases. However, there is little knowledge of the effects of such acid gas exposure on the adsorption and separation properties of ZIFs. Here, the stability of a model ZIF material (ZIF-8) under SO 2 exposure in dry, humid, and aqueous environments has been investigated in detail. Combined characterization by several techniques (PXRD, N 2 physisorption, EDX, XPS, and FTIR) allowed us to track the structural and compositional properties of ZIF-8 before and after SO 2 exposure. ZIF-8 is stable after prolonged exposure in dry SO 2 and in humid air without SO 2 . However, exposure to 10−20 ppm concentrations of SO 2 in the presence of high relative humidity led to its irreversible structural degradation over time as evidenced by substantial losses in crystallinity and textural properties. Exposure to similar concentrations of aqueous SO 2 did not lead to bulk degradation. Humid SO 2 exposed ZIF-8 showed a significant presence of sulfur (S) even after reactivation, with vibrational characteristics corresponding to (bi)sulfite and (bi)sulfate groups. A mechanism of ZIF-8 degradation combining the synergistic effects of SO 2 and humidity is proposed. Attack by sulfuric and sulfurous acid species (generated in humid SO 2 ) leads to protonation of nitrogen in the imidazole ring, resulting in cleavage of metal−linker (Zn−N) bonds. Our detailed experimental findings serve as a starting point for developing a generalized mechanism of acid gas interactions with ZIF materials.

Direct CO2 capture from atmospheric air is gaining increased attention as one of the most scalable negative carbon approaches available to tackle climate change if coupled with the sequestration of CO2 geologically. Furthermore, it can also provide CO2 for further utilization from a globally uniform source, which is especially advantageous for economies without natural sources of carbon-based feedstocks. Solid-supported amine-based materials are effective for direct air capture (DAC) due to their high CO2 uptakes and acceptable sorption kinetics at ambient temperature. In this work, we describe the application of polymer/silica fiber sorbents functionalized with a primary amine-rich polymer, poly­(ethylenimine) (PEI), for DAC. Monolithic fiber sorbents composed of cellulose acetate and SiO2 are synthesized via the dry-jet, wet quench spinning technique. These fibers are then functionalized with PEI (M w 800 Da) in a simple and scalable postspinning infusion step and tested for CO2 capture under pseudoequilibrium conditions as well as under breakthrough conditions. An investigation to study the effect of feed flow rate, adsorption temperature, and presence of moisture in the feed on the CO2 breakthrough performance of a densely packed fiber sorbent module is conducted to highlight the potential application of this class of structured contactors in direct air capture. The pressure drop of these contactors at high gas velocities is also evaluated. Finally, a vacuum-assisted desorption step is demonstrated for production of high-purity CO2 from both dry and humid ambient air mixtures.

Catalyst characterization and performance Figure S1. Infrared spectra of the hydrocarbon stretching region for C18 and BDT monolayers on 5 wt% Pd/Al 2 O 3 catalysts. The feature found at 2923 cm -1 for the C18 catalyst was attributed to a CH 2 methylene stretch. The feature at 3050 cm -1 for BDT was attributed to a CH stretch of the phenyl moiety.