College of Chemistry

The capture and separation of carbon dioxide (CO2) has been the focus of a plethora of research in order to mitigate its emissions and contribute to global development. Given that CO2 is commonly found in natural gas streams, there have been efforts to seek more efficient materials to separate gaseous mixtures such as CO2/CH4. However, there are only a few reports regarding adsorption processes within pressurized systems. In the offshore scenario, natural gas streams still exhibit high moisture content, necessitating a greater understanding of processes in moist systems. In this article, a metal-organic framework synthesis based on zirconium (MOF-808) was carried out through a conventional solvothermal method and autoclave for the adsorption of CO2 and CH4 under different temperatures (45–65 °C) and pressures up to 100 bar. Furthermore, the adsorption of humid CO2 was evaluated using thermal analyses. The MOF-808 synthesized in autoclave showed a high surface area (1502 m2/g), a high capacity for CO2 adsorption at 50 bar and 45 °C and had a low selectivity to capture CH4 molecules. It also exhibited a fine stability after five cycles of CO2 adsorption and desorption at 50 bar and 45 °C − as confirmed by structural post-adsorption analyses while maintaining its adsorption capacity and crystallinity. Furthermore, it can be observed that the adsorption capacity increased in a humid environment, and that the adsorbent remained stable after adsorption cycles in the presence of moisture. Finally, it was possible to confirm the occurrence of physisorption processes through nuclear magnetic resonance (NMR) analyses, thus validating the choice of mild temperatures for regeneration and contributing to the reduction of energy consumption in processing plants.

Circular plastics thrive on the ability to chemically recycle polymers into reusable monomers, ideally closing the loop from manufacturing to the end of life. Mechanisms for polymer deconstruction are complex, involving diffusion and transport of reagents to reactive sites in a material continuously undergoing chemical transformations. A deeper understanding of the deconstruction phenomena would better inform the molecular basis of circularity. Here, we show how nuclear magnetic resonance (NMR) spectroscopy, relaxometry, and diffusometry enable monitoring of the heterogeneous deconstruction of a model elastomer with acid-cleavable diketoenamine bonds. In chaotropic aqueous HBr, polydiketoenamine (PDK) deconstruction is fast, enabled by macro- and microscale swelling, which facilitates acid penetration and protonation of reaction sites deep within the polymer. We observe a previously unrecognized hydrogen-bond-stabilized amine intermediate that is persistent throughout deconstruction. In kosmotropic aqueous H2SO4, PDK deconstruction is notably slower. Here, swelling occurred at a more gradual pace, characterized by low polymer chain mobility, thereby trapping the acid in matrix pores and modifying the activity of the reaction medium under confinement in the process. We find that polymer swelling, chain mobility, and deconstruction kinetics are strongly linked, requiring a multifaceted NMR characterization tool box for in-depth analysis.

Cryogenic transmission electron microscopy (cryo-TEM) combined with single particle analysis (SPA) is an emerging imaging approach for soft materials. However, the accuracy of SPA-reconstructed nanostructures, particularly those formed by synthetic polymers, remains uncertain due to potential packing heterogeneity of the nanostructures. In this study, the combination of molecular dynamics (MD) simulations and image simulations is utilized to validate the accuracy of cryo-TEM 3D reconstructions of self-assembled polypeptoid fibril nanostructures. Using CryoSPARC software, image simulations, 2D classifications, ab initio reconstructions, and homogenous refinements are performed. By comparing the results with atomic models, the recovery of molecular details is assessed, heterogeneous structures are identified, and the influence of extraction location on the reconstructions is evaluated. These findings confirm the fidelity of single particle analysis in accurately resolving complex structural characteristics and heterogeneous structures, exhibiting its potential as a valuable tool for detailed structural analysis of synthetic polymers and soft materials.

The capture of carbon dioxide (CO₂) is crucial for reducing greenhouse emissions and achieving net-zero emission goals. Metal-organic frameworks (MOFs) present a promising solution for carbon capture due to their structural adaptability, tunability, porosity, and pore modification. In this research, we explored the use of a copper (Cu(II))-based MOF called m CBMOF-1. After activation, m CBMOF-1 generates one-dimensional channels with square cross sections, featuring sets of four Cu(II) open metal sites spaced by 6.042 Å, allowing strong interactions with coordinating molecules. To investigate this capability, m CBMOF-1 was exposed to ammonia (NH₃) gas, resulting in hysteretic NH₃ isotherms indicative of strong interactions between Cu(II) and NH₃. At 150 mbar and 298 K, the NH₃-loaded (∼1 mmol/g) material exhibited a 106% increase in CO₂ uptake compared to that of the pristine m CBMOF-1. Carbon-13 solid-state nuclear magnetic resonance spectra and density functional theory calculations confirmed that the sequential loading of NH₃ followed by CO₂ adsorption generated a copper-carbamic acid complex within the pores of m CBMOF-1. Our study highlights the effectiveness of sequential pore functionalization in MOFs as an attractive strategy for enhancing the interactions of MOFs with small molecules such as CO₂.

Manganese-based materials have tremendous potential to become the next-generation lithium-ion cathode as they are Earth abundant, low cost and stable. Here we show how the mobility of manganese cations can be used to obtain a unique nanosized microstructure in large-particle-sized cathode materials with enhanced electrochemical properties. By combining atomic-resolution scanning transmission electron microscopy, four-dimensional scanning electron nanodiffraction and in situ X-ray diffraction, we show that when a partially delithiated, high-manganese-content, disordered rocksalt cathode is slightly heated, it forms a nanomosaic of partially ordered spinel domains of 3-7 nm in size, which impinge on each other at antiphase boundaries. The short coherence length of these domains removes the detrimental two-phase lithiation reaction present near 3 V in a regular spinel and turns it into a solid solution. This nanodomain structure enables good rate performance and delivers 200 mAh g-1 discharge capacity in a (partially) disordered material with an average primary particle size of ∼5 µm. The work not only expands the synthesis strategies available for developing high-performance Earth-abundant manganese-based cathodes but also offers structural insights into the ability to nanoengineer spinel-like phases.

Capturing carbon dioxide from diluted streams, such as flue gas originating from natural gas combustion, can be achieved using recyclable, humidity-resistant porous materials. Three such materials were synthesized by chemically modifying the pores of metal-organic frameworks (MOFs) with Lewis basic functional groups. These materials included aluminum 1,2,4,5-tetrakis(4-carboxylatophenyl) benzene (Al-TCPB) and two novel MOFs: Al-TCPB(OH), and Al-TCPB(NH2), both isostructural to Al-TCPB, and chemically and thermally stable. Single-component adsorption isotherms revealed significantly increased CO2 uptakes upon pore functionalization. Breakthrough experiments using a 4/96 CO2/N2 gas mixture humidified up to 75% RH at 25 °C showed that Al-TCPB(OH) displayed the highest CO2 dynamic breakthrough capacity (0.52 mmol/g) followed by that of Al-TCPB(NH2) (0.47 mmol/g) and Al-TCPB (0.26 mmol/g). All three materials demonstrated excellent recyclability over eight humid breakthrough-regeneration cycles. Solid-state nuclear magnetic resonance spectra revealed that upon CO2/H2O loading, H2O molecules do not interfere with CO2 physisorption and are localized near the Al-O(H) chain and the -NH2 functional group, whereas CO2 molecules are spatially confined in Al-TCPB(OH) and relatively mobile in Al-TCPB(NH2). Density functional theory calculations confirmed the impact of the adsorbaphore site between of two parallel ligand-forming benzene rings for CO2 capture. Our study elucidates how pore functionalization influences the fundamental adsorption properties of MOFs, underscoring their practical potential as porous sorbent materials.

The functioning of a wide variety of charged macromolecules, from DNA to fuel cell membranes, is dependent on how the counterions surrounding them are arranged. In order to decrease Coulombic repulsion, some of the fixed charges on these molecules are neutralized by a fraction of the counterions─this phenomenon is called counterion condensation. The nature of counterion condensation can be only be inferred indirectly from traditional experiments such as X-ray scattering and modern experiments such as single molecule electrometry. The prevalent conclusion in the literature, based on both theory and experiment, is that the distribution of counterions is peaked right next to the macromolecule, i.e., condensation results in the formation of contact ion pairs. In this study, cryogenic electron microscopy (cryo-EM) was used to study the arrangement of condensed halide counterions near a positively charged polypeptoid nanofiber. The locations of both condensed and fixed charges were determined directly from atomic-scale images. Our experimentally determined counterion distributions were peaked at distances of about 5 Å away from the fixed positive charge, indicating the presence of a layer of water molecules between condensed ion pairs. We posit that this distribution is driven by the entropy of the condensed ions.

The copy number of a plasmid is linked to its functionality, yet there have been few attempts to optimize higher-copy-number mutants for use across diverse origins of replication in different hosts. We use a high-throughput growth-coupled selection assay and a directed evolution approach to rapidly identify origin of replication mutations that influence copy number and screen for mutants that improve Agrobacterium-mediated transformation (AMT) efficiency. By introducing these mutations into binary vectors within the plasmid backbone used for AMT, we observe improved transient transformation of Nicotiana benthamiana in four diverse tested origins (pVS1, RK2, pSa and BBR1). For the best-performing origin, pVS1, we isolate higher-copy-number variants that increase stable transformation efficiencies by 60-100% in Arabidopsis thaliana and 390% in the oleaginous yeast Rhodosporidium toruloides. Our work provides an easily deployable framework to generate plasmid copy number variants that will enable greater precision in prokaryotic genetic engineering, in addition to improving AMT efficiency.

The photochemical generation of nicotinamide cofactor 1,4-NADH, facilitated by inorganic photosensitizers, emerges as a promising model system for investigating charge transfer phenomena at the interface of semiconductors and bacteria, with implications for enhancing photosynthetic biohybrid systems (PBSs). However, extant semiconductor nanocrystal model systems suffer from achieving optimal conversion efficiency under visible light. This study investigates quasi-one-dimensional CdS nanorods as superior light absorbers, surface modified with catalyst Cp*Rh(4,4'-COOH-bpy)Cl₂ to produce enzymatically active NADH. This model subsystem facilitates easy product isolation and achieves a turnover frequency (TOF) of 175 h^-1, one of the highest efficiencies reported for inorganic photosensitizer-based nicotinamide cofactor generation. Charge transfer kinetics, fundamental for catalytic solar energy conversion, range from 10⁶ to 10⁸ s^-1 for this system highlighting the superior electron transfer capabilities of NRs. This model ensures efficient cofactor production and offers critical insights into advancing systems that mimic natural photosynthesis for sustainable solar-to-chemical synthesis.

Abstract: We incorporate Se into the 3D halide perovskite framework using the zwitterionic ligand: SeCYS (+NH3(CH2)2Se−), which occupies both the X− and A+ sites in the prototypical ABX3 perovskite. The new organoselenide‐halide perovskites: (SeCYS)PbX2 (X=Cl, Br) expand upon the recently discovered organosulfide‐halide perovskites. Single‐crystal X‐ray diffraction and pair distribution function analysis reveal the average structures of the organoselenide‐halide perovskites, whereas the local lead coordination environments and their distributions were probed through solid‐state 77Se and 207Pb NMR, complemented by theoretical simulations. Density functional theory calculations illustrate that the band structures of (SeCYS)PbX2 largely resemble those of their S analogs, with similar band dispersion patterns, yet with a considerable band gap decrease. Optical absorbance measurements indeed show band gaps of 2.07 and 1.86 eV for (SeCYS)PbX2 with X=Cl and Br, respectively. We further demonstrate routes to alloying the halides (Cl, Br) and chalcogenides (S, Se) continuously tuning the band gap from 1.86 to 2.31 eV–straddling the ideal range for tandem solar cells or visible‐light photocatalysis. The comprehensive description of the average and local structures, and how they can fine‐tune the band gap and potential trap states, respectively, establishes the foundation for understanding this new perovskite family, which combines solid‐state and organo‐main‐group chemistry.