Abstract Researchers from all sectors of homogeneous catalysis convened in response to concerns regarding reproducibility in science to analyze the issue and provide recommendations. In addition to an in-person workshop, the group engaged the broader homogeneous catalysis community through a webinar series and virtually during the workshop. Results of the project affirm that homogeneous catalysis is not in a reproducibility crisis, as evidenced by the field’s current and past contributions to society that have led to economic growth and advances in a range of industries from agriculture to consumer goods to human health. However, it is not uncommon for researchers to encounter obstacles related to reproducibility. Ensuring reproducibility remains the responsibility of the community, both in current work and in training future researchers. This report is intended to engage key stakeholders, including disciplinary societies, publishers, employers, research leaders, and researchers, in practices that maximize reproducible homogeneous catalysis and ensure continued innovation and translatable discoveries. Recommendations made herein are also framed to be applicable beyond homogeneous catalysis, empowering the broader chemical if not scientific community.

The demand for plastic has made nonbiodegradable plastic waste a global environmental problem. Chemical processes for polymer upcycling aim to convert plastic waste into value-added products. Many research efforts draw insights about the kinetics and mechanism of depolymerization reactions from the evolving molecular weight distribution (MWD). However, MWDs are reported in several different representations: as fractional-mass distributions or fractional-molar distributions, and on linear or logarithmic molecular mass scales. Conversion from one representation to another can dramatically change the appearance of the MWD. This tutorial discusses some common techniques for molecular weight characterization and quantitative interpretations for the raw data. We provide formulas to transform between representations of the MWD data, to change variables within population balance equations that predict time evolution of the MWD, and to create composite MWDs (with uncertainty estimates) from experiments in different but overlapping molecular weight ranges.

The rotation frequencies of amido ligands are highly sensitive to the electronic structure of d0 transition metal complexes and have been used to study ligand donor properties. While attempting to study the donor properties of silanolate ligands in a silica-supported Cr complex, we observed highly restricted motions due to the added steric hindrance from the support, with only approximately half of the amides rotating on a 50 ms time scale. Surprisingly, when the same species is grafted into narrow 2.2 nm pores, all amido ligands are able to rotate. Density functional theory calculations suggest that confinement may limit the possible coordination sites and the configuration of the formed surface species, potentially enabling the formation of conformationally homogeneous surface site populations.

AlMe3-treatment of the borylation precatalyst La(BH4)2(THF)2.5-Ph3Si-HY30 affords La(BH4)2(AlMe3)-Ph3Si-HY30, which is the superior lanthanum-based precatalyst for benzene borylation with pinacolborane (HBpin), giving higher turnovers (>285) and improved yields (up to 42%) of phenylpinacolborane (PhBpin). Solid-state NMR spectroscopy, X-ray adsorption spectroscopy, and theoretical studies characterized the precatalytic sites in La(BH4)2(AlMe3)-Ph3Si-HY30 as κ2-O,O-{≡SiO(=Al)≡SiO}La(BH4)2(AlMe3), revealing that AlMe3 had displaced the THF ligands. In contrast to the expected high reactivity of THF-free organolanthanum, the turnover frequency (TOF) for PhBpin formation catalyzed by La(BH4)2(AlMe3)-Ph3Si-HY30 (2.7 h-1) is slightly lower than that of untreated La(BH4)2(THF)2.5-Ph3Si-HY30 (3.6 h-1), implying that AlMe3 is a stronger inhibitor than THF for lanthanum. On the other hand, AlMe3-treatment of inactive La(BH4)2(THF)2.2-SiO2 generates an active benzene borylation catalyst. AlMe3 also desorbs surface-O-BxHy species and quenches residual Brønsted acid sites (BAS) and silanols. Alumination of the BAS inhibits HBpin degradation, while alumination of silanols creates sites for that reaction. The 8-fold inhibition of the BAS-catalyzed HBpin decomposition rate by AlMe3 treatment gives a kinetic advantage to the lanthanum-catalyzed C-H borylation, leading to increased yields and turnovers. Knowledge of the competing roles of sites in La(BH4)2(AlMe3)-Ph3Si-HY30 and the catalytic rate law law enables identification of favorable conditions of low [HBpin] to maximize turnovers or PhBpin yield. Sterics affect the selectivity in borylation of substituted arenes and heteroarenes, which can proceed without the precoordination of a donor. These steric effects, as well as the AlMe3 treatment having an opposite effect on the activity of lanthanum in HY vs SiO2, point to confinement-activated sites.

Quadrupolar nuclei with a nuclear spin I > 1/2 account for ∼73 % of all NMR-active nuclei. The quadrupolar interaction broadens solid-state NMR spectra, frequently resulting in low resolution and poor sensitivity. Here, we present a theoretical and experimental investigation of the use of magic angle spinning (MAS) 1H{X} double-echo resonance-echo saturation-pulse double-resonance (DE-RESPDOR) pulse sequences for the indirect detection of NMR spectra of half-integer quadrupolar nuclei with spin >3/2 (spin 5/2, 7/2, or 9/2 nuclei). In these experiments, a dephasing profile for the quadrupolar nucleus is created by plotting the observed dephasing of the detected spin as a function of the transmitter offset of the indirectly detected spin. Simulating the dephasing profile allows the quadrupolar coupling constant (CQ) and the EFG tensor asymmetry parameter (ηQ) to be estimated. The achievable dephasing levels and the lineshapes of dephasing profiles of the indirectly detected nuclei were predicted using numerical simulations. We demonstrate 1H detection of 127I (I = 5/2), 139La (I = 7/2), and 115In (I = 9/2) nuclei in BaI2.2H2O (barium iodide dihydrate), La(BH4)3(THF)3 (tris(borohydride)tris(tetrahydrofuran)lanthanum(III)), and In(OH)3 (indium(III) hydroxide), respectively. The observed improvements or reductions in sensitivity with indirect detection are related to the proportion of 1H T1 to quadrupolar nucleus T1, alongside the quadrupolar nucleus's spin quantum number and gyromagnetic ratio (γ). Additionally, the indirect detection experiments confirm the existence of dipolar or scalar couplings between the detected nucleus and the quadrupolar nucleus of interest, providing important structural information. Numerical simulations suggest these methods are also potentially applicable to quadrupolar nuclei having CQ larger than 100 MHz.

Solid-state NMR experiments were used to investigate the dynamics of supported complexes grafted to a series of silica gel materials of varied pore sizes. Through dipolar recoupling measurements, we found that ligand dynamics were impeded in the more confined environments, as would be expected. A new form of motion involving the complex as a whole, however, appeared in the most restricted environment consisting of 22 & Aring; diameter pores. These motions persisted down to -100 degrees C at which point the ligands were frozen on the NMR timescale. The newly observed dynamics could only result from the breaking of secondary dative metal-siloxane interactions that otherwise lock the complex in a preferred orientation on the surface. Crucially, these results show that confinement effects alone can be sufficient to reduce a grafted metal's effective coordination number in direct analogy to the synthesis of undercoordinated complexes using bulky ligands. This finding could have important implications in the synthesis of more active heterogeneous catalysts.

Many polymer upcycling efforts aim to convert plastic waste into high-value liquid hydrocarbons. However, the subsequent cleavage of middle distillates to light gases can be problematic. The reactor often contains a vapor phase (light gases and middle distillates) and a liquid phase (molten polymers and waxes with a suspended or dissolved catalyst). Because the catalyst resides in the liquid phase, middle distillates that partition into the vapor phase are protected against further cleavage into light gases. In this paper, we consider a simple reactive separation strategy, in which a gas outflow removes the volatile products as they form. We combine vapor-liquid equilibrium models and population balance equations (PBEs) to describe polymer upcycling in a two-phase semibatch reactor. The results suggest that the temperature, headspace volume, and flow rate of the reactor can be used to tune selectivity toward the middle distillates, in addition to the molecular mechanism of catalysis. We anticipate that two-phase reactor models will be important in many polymer upcycling processes and that reactive separation strategies will provide ways to boost the yield of the desired products in these cases.

Biomass-derived ethanol (EtOH) and acetaldehyde (AcH) conversion to 1,3-butadiene (1,3-BD) is an alternative process for 1,3-BD production. The present investigation reports the preparation and characterization of isolated La sites introduced into the silanol nests in DeAlBEA as well as detailed studies of the mechanism and kinetics for the conversion of an EtOH-AcH mixture to 1,3-BD. La sites supported on DeAlBEA are found to be present as (Xi SiO)(2)La-OH groups that are H-bonded with adjacent Si-OH groups, possessing high C-C coupling activity and stability, superior to state-of-the-art Y-DeAlBEA. La sites supported on silica (La-SiO2) with a similar chemical structure but no H-bonding interaction with Si-OH groups were prepared for comparison. Lewis acid La sites promote AcH aldol condensation, and the activity of such sites is nearly identical for both La-DeAlBEA and La-SiO2. The rate of C-4 product formation increases by a factor of 4.8 upon addition of EtOH to the feed of AcH over La-DeAlBEA, whereas that over La/SiO2 remains unchanged. Investigation of the mechanism and kinetics of AcH aldol condensation and EtOH-AcH conversion to 1,3-BD revealed two C-C bond forming pathways Xi AcH aldol condensation by Lewis acid La sites and direct coupling of EtOH-AcH over H-bonded (Xi SiO)(2)La-OHHO-Si Xi sites. This study provides important information about the role of the local environment of isolated Lewis acid sites and their effects on the direct coupling of EtOH and AcH to form 1,3-BD.

Supported platinum nanoparticle catalysts are known to convert polyolefins to high-quality liquid hydrocarbons using hydrogen under relatively mild conditions. To date, few studies using platinum grafted onto various metal oxide (M x O y ) supports have been undertaken to understand the role of the acidity of the oxide support in the carbon-carbon bond cleavage of polyethylene under consistent catalytic conditions. Specifically, two Pt/MxOy catalysts (MxOy = SrTiO3 and SiO2-Al(2)O3; Al = 3.0 wt %, target Pt loading 2 wt % Pt similar to 1.5 nm), under identical catalytic polyethylene hydrogenolysis conditions (T = 300 degree celsius, P(H2) = 170 psi, t = 24 h; M-w = similar to 3,800 g/mol, M-n = similar to 1,100 g/mol, D = 3.45, N-branch/100C = 1.0), yielded a narrow distribution of hydrocarbons with molecular weights in the range of lubricants (M-w = < 600 g/mol; M-n < 400 g/mol; D = 1.5). While Pt/SrTiO3 formed saturated hydrocarbons with negligible branching, Pt/SiO2-Al2O3 formed partially unsaturated hydrocarbons (<1 mol % alkenes and similar to 4 mol % alkyl aromatics) with increased branch density (N-branch/100C = 5.5). Further investigations suggest evidence for a competitive hydrocracking mechanism occurring alongside hydrogenolysis, stemming from the increased acidity of Pt/SiO2-Al2O3 compared to Pt/SrTiO3. Additionally, the products of these polymer deconstruction reactions were found to be independent of the polyethylene feedstock, allowing the potential to upcycle polyethylenes with various properties into a value-added product.

Copper(I) iodide catalyzes the disproportionation of aromatic and heteroaromatic carboxylic acids to give dicarboxylic acids. Potassium furoate heated to 280 - 300 degrees C in the presence of 10 mol % CuI under carbon dioxide (40 bar) forms dipotassium 2,5-furandicarboxylate (2,5-FDCK), which is isolated as pure 2,5-dicarboxylic acid (2,5-FDCA) in up to 75 % isolated yield and in excellent selectivity over the regioisomeric 2,4-furandicarboxylate (92 % selectivity). Starting materials are recovered in experiments lacking CuI or with pressurized N 2 replacing CO 2 , whereas high conversion and low yields of 2,5-FDCA are obtained with moderate CO 2 pressures, suggesting several roles for carbon dioxide in the transformation. Similarly, potassium benzoate heated to 320 - 350 degrees C in the presence of 10 mol % CuI under carbon dioxide (40 bar) produces terephthalic acid after workup in up to 60 % yield.

Atomically dispersed first-row transition metals embedded in nitrogen-doped carbon materials (M-N-C) show promising performance in catalytic hydrogenation but are less well-studied for reactions with more complex mechanisms, such as hydrogenolysis. Their ability to catalyze selective C-O bond cleavage of oxygenated hydrocarbons such as aryl alcohols and ethers is enhanced with the participation of ligands directly bound to the metal ion as well as longer-range contributions from the support. In this article, we describe how Fe-N-C catalysts with well-defined local structures for the Fe sites catalyze C-O bond hydrogenolysis. The reaction is facilitated by the N-C support. According to spectroscopic analyses, the as-synthesized catalysts contain mostly pentacoordinated FeIII sites, with four in-plane nitrogen donor ligands and one axial hydroxyl ligand. In the presence of 20 bar of H2 at 170-230 °C, the hydroxyl ligand is lost when N4FeIIIOH is reduced to N4FeII, assisted by the H2 chemisorbed on the support. When an alcohol binds to the tetracoordinated FeII sites, homolytic cleavage of the O-H bond is accompanied by reoxidation to FeIII and H atom transfer to the support. The role of the N-C support in catalytic hydrogenolysis is analogous to the behavior of chemically and redox-non-innocent ligands in molecular catalysts based on first-row transition metal ions and enhances the ability of M-N-Cs to achieve the types of multistep activations of strong bonds needed to upgrade renewable and recycled feedstocks.

Deconstruction of polyolefins by catalytic hydrogenolysis is typically accompanied by the generation of undesired light gases. At reaction temperatures, the desired liquid products also tend to be volatile. Secondary cleavage of these liquid products contributes to light gas formation. The latter process was mitigated by reactive separation, continuously separating the liquid products from the catalyst throughout the experiment. At equivalent conversion, the yield and selectivity for oligomeric liquid species are increased under reactive separation, even though the carbon-carbon bond cleavage rate is slower than that in sealed experiments. More light gas is formed in the sealed reactor. Under 1 atm H-2 partial pressure, alkenes accompany the typical alkane hydrogenolysis products. The alkene yield is higher, with greater selectivity for valuable alpha-olefins under reactive separation. These results provide the mechanistic insight that terminal alkenes are primary products of carbon-carbon bond cleavage during hydrogenolysis under experimental conditions, and secondary deconstruction of these species produces light gases.

In isomerizing ethenolysis, tandem double bond isomerization and olefin metathesis catalysts convert polyethylene and ethylene coreactants into propylene. Isomerizing ethenolysis is particularly interesting among polymer upcycling strategies because of its potentially high selectivity to a specific value-added product. Following a theoretical analysis by Guironnet and Peters [J. Phys. Chem. 124, 3935 (2020)], Conk et al. [Science, 377, 1561 (2022)] demonstrated isomerizing ethenolysis in experiments using an iridium pincer dehydrogenation catalyst, a dimeric Pd(I) bromide isomerization catalyst, and a second-generation Hoveyda-Grubbs metathesis catalyst. This paper compares model predictions to the two-stage dehydrogenation and isomerizing ethenolysis experiments of Conk et al. In a model that accounts for the initial dehydrogenation and subsequent evolution of the chain length distribution, we show that the experimental propylene generation rates are consistent with an isomerizing ethenolysis rate that is zeroth order in the concentration of long chain ends. In contrast, Guironnet and Peters assumed a first order dependence on chain ends. To understand the discrepancy, we developed and solved a microkinetic model for the isomerizing ethenolysis reaction. Rate parameters in the microkinetic model are estimated from prior experiments and known equilibria. We find that the experiments of Conk et al. are performed near conditions of the theoretical maximum propylene production rate, where the kinetics are saturated with respect to both chain end and ethylene concentrations. For long chains, not preshortened by initial dehydrogenation and ethenolysis steps as in Conk et al., the model predicts lowered chain end concentrations and smaller propylene production rates that can become inhibited by high ethylene pressure.

Electrification to reduce or eliminate greenhouse gas emissions is essential to mitigate climate change. However, a substantial portion of our manufacturing and transportation infrastructure will be difficult to electrify and/or will continue to use carbon as a key component, including areas in aviation, heavy-duty and marine transportation, and the chemical industry. In this Roadmap, we explore how multidisciplinary approaches will enable us to close the carbon cycle and create a circular economy by defossilizing these difficult-to-electrify areas and those that will continue to need carbon. We discuss two approaches for this: developing carbon alternatives and improving our ability to reuse carbon, enabled by separations. Furthermore, we posit that co-design and use-driven fundamental science are essential to reach aggressive greenhouse gas reduction targets. To achieve net-zero carbon emissions, we must close the carbon cycle for industries that are difficult to electrify. Developing the needed science to provide carbon alternatives and non-fossil carbon will accelerate advances towards defossilization.

Scandium borohydride grafted into the micropores of the faujasite zeolite HY30 catalyzes the C-H borylation of benzene, whereas silica-grafted species are inactive. This catalytic activity may originate from grafting at a Br & oslash;nsted acid site leading to a more electron-deficient rare earth center. Herein, we apply multinuclear double-resonance nuclear magnetic resonance (NMR) experiments to probe the structure and dynamics of zeolite- and silica-bound scandium borohydride complexes. The experiments reveal that scandium centers located within the zeolite micropores, in proximity to Al-created Br & oslash;nsted sites, are more dynamic than rigid scandium sites grafted on silanols. Through a combination of NMR and molecular dynamics simulations, we show that the coordination of the scandium in the zeolite is labile, with the metal exchanging between two binding sites. The weak electron donation from the support that enables the movement of the Sc center leads to the formation of an undercoordinated metal center that cannot exist on silica, ultimately leading to the new catalytic activity of the species.

Due to the plastic waste crisis, selective chemical upcycling of polyolefins into value-added products is a topic of intense interest, demanding polymer deconstruction processes that afford control over the product chain lengths. Recently, a catalytic architecture was synthesized in which a polyolefin melt infiltrates a porous support, and its chains are cleaved by a metal nanoparticle catalyst at the bottom of the pores, yielding a narrow distribution of alkane products. Although the influence of various parameters of these catalytic materials, including the effects of the nanoparticle size and pore diameter on product chain length, has been examined before, here, we investigate the role of the pore length in the cleavage process through the first study that combines catalytic hydrogenolysis and coarse-grained modeling to gain insights not available by experiment alone. We show that the pore length can permit control over the average product length with qualitative agreement between experiment and simulation. We go beyond this observation to uncover the dynamic phenomenon responsible for the pore-length dependence of the cleavage products.

Carbon–carbon bond cleavage reactions, adapted to deconstruct aliphatic hydrocarbon polymers and recover the intrinsic energy and carbon value in plastic waste, have typically been catalysed by metal nanoparticles or air-sensitive organometallics. Metal oxides that serve as supports for these catalysts are typically considered to be inert. Here we show that Earth-abundant, non-reducible zirconia catalyses the hydrogenolysis of polyolefins with activity rivalling that of precious metal nanoparticles. To harness this unusual reactivity, our catalytic architecture localizes ultrasmall amorphous zirconia nanoparticles between two fused platelets of mesoporous silica. Macromolecules translocate from bulk through radial mesopores to the highly active zirconia particles, where the chains undergo selective hydrogenolytic cleavage into a narrow, C 18 -centred distribution. Calculations indicated that C–H bond heterolysis across a Zr–O bond of a Zr(O) 2 adatom model for unsaturated surface sites gives a zirconium hydrocarbyl, which cleaves a C–C bond via β-alkyl elimination.

Pt/SrTiO3nanoparticle catalysts have been synthesized by surface organometallic chemistry in solution on a 5 g scale. Pt/SrTiO3selectively and repeatedly upcycles isotactic polypropylene into uniform liquid products withMn∼ 200 Da.