Group Additivity and Modified Linear Scaling Relations for Estimating Surface Thermochemistry on Transition Metal Surfaces: Application to Furanics

We assess the applicability of DFT-parametrized group additivity (GA) and extended linear scaling relations (ELSRs) to the adsorption properties of biomass furanic derivatives on close-packed metal surfaces. We find that previous schemes are inadequate for larger furanic derivatives and even small unsaturated species, such as ethylene, and identify sources of error. We propose an interpolative GA-LSR scheme that accounts for (1) dispersion forces, (2) a modified valence model for unsaturated compounds, which depends on the number of metal atoms occupied by heteroatoms of an adsorbate, and (3) hydrogen–metal interactions, captured using a secondary atomic binding energy descriptor. This new model requires only the atomic carbon and oxygen adsorption energies on a metal surface to estimate the thermochemistry of a given furanic derivative. It predicts the preferred adsorption mode (di-σ_CC vs π_CC) of unsaturated species and the conformation (flat, tilted, vertical) of furanics and greatly reduces the error in the estimated species enthalpy of formation <i>H</i>_f,298

Coverage-Induced Conformational Effects on Activity and Selectivity: Hydrogenation and Decarbonylation of Furfural on Pd(111)

Adsorption, hydrogenation, and decarbonylation of furfural on hydrogen-covered Pd(111) was investigated using density functional theory calculations. It was found that both the energy and the conformation of adsorbed furfural vary with increasing coverage of hydrogen or furfural. Furfural lies flat at low coverage but becomes tilted on crowded surfaces. The energy profiles of hydrogenation and decarbonylation reactions on a hydrogen-covered Pd(111) change profoundly compared to those on bare Pd(111). The energy span theory shows that the furfural hydrogenation and decarbonylation effective barriers exhibit a maximum with increasing hydrogen coverage. In contrast, the selectivity to hydrogenation toward furfuryl alcohol over decarbonylation is favored with increasing hydrogen coverage. Microkinetic modeling suggests that the conformation change with increasing H coverage has a significant effect on reaction rates (up to orders of magnitude) and induces a selectivity reversal from furan as the main product (low-H coverage limit) to furfuryl alcohol (high-H coverage limit). Our results may rationalize different selectivity trends seen experimentally under typical reactor and UHV conditions. Importantly, this study underscores the potential importance of operating conditions on hydrodeoxygenation activity and selectivity due to conformational changes of multifunctional biomass derivatives

Group Additivity for Estimating Thermochemical Properties of Furanic Compounds on Pd(111)

We analyze the adsorption of 101 furan chemistry-related adsorbates, including intermediates with varying levels of hydrogenation of the furan ring as well as ring-open compounds, on Pd(111). The standard heat of formation (<i>H</i>_f,298), standard absolute entropy (<i>S</i>₂₉₈), and heat capacity (<i>C</i>_{<i>p</i>,<i>T</i>}) are estimated using dispersion-corrected density functional theory (DFT-D3) and statistical mechanics formulas, and all <i>H</i>_f,298 values are referenced to a single set of gas-phase molecules to ensure consistency. We further estimate the dispersion contribution, Δ<i>E</i>_disp, to the heat of adsorption. Subsequently, we develop a group additivity scheme to quickly estimate thermochemical properties of furanic molecules. For the proposed model, the vibrational and rotational contributions of gas-phase-like groups are estimated using G4-level calculations. We find that the group additivity scheme developed for open-chain molecules is inaccurate for furanics. We report 17 new groups, involving heteroatom–nearest neighbor interactions and four new corrections that account for furan ring deformation and the level of ring hydrogenation. The mean deviations from DFT-computed values are 1.8 kcal/mol in <i>H</i>_f,298, 0.3 kcal/mol in Δ<i>E</i>_disp, 0.9 cal/(mol K) in <i>S</i>₂₉₈, and 0.7 cal/(mol K) in <i>C</i>_<i>p</i>,800. The largest deviations are observed in highly saturated adsorbates with multiple gas-phase-like surface groups. We further introduce a nine-parameter heteroatom-based model for estimating Δ<i>E</i>_disp, resulting in a mean deviation from DFT-computed values of 1.4 kcal/mol

DFT Study of Furfural Conversion to Furan, Furfuryl Alcohol, and 2‑Methylfuran on Pd(111)

Dispersion-corrected density functional theory calculations were performed to investigate the adsorption of furan, furfural, furfuryl alcohol, and 2-methylfuran as well as the reaction barriers for their interconversion. The most stable configuration for furan, furfural, furfuryl alcohol, and 2-methylfuran entails the furan ring lying flat on the surface, centered over a hollow site. We performed an elementary step analysis for the reaction of furfural to furan, furfuryl alcohol, and 2-methylfuran. Thermodynamics favors the production of furan and CO. The activation energy for furfural reduction to furfuryl alcohol is lower than that for its decarbonylation to furan. The formation of 2-methylfuran occurs via dehydration of furfuryl alcohol or a dehydrogenation pathway through a methoxy intermediate. Our findings are in agreement with recently reported experimental results

Brønsted–Evans–Polanyi and Transition State Scaling Relations of Furan Derivatives on Pd(111) and Their Relation to Those of Small Molecules

Brønsted–Evans–Polanyi (BEP) and transition state scaling (TSS) linear free energy relations are extended to C–H, O–H, C–O and C–C bond breaking reactions occurring on the ring and the functional groups of furan (hydrofuran, dihydrofuran, trihydrofuran, and tetrahydrofuran) and furfural derivatives (e.g., furfural, furfuryl alcohol, methyl furan, etc.) on Pd(111). The relations perform statistically as well as those for small molecules reported previously. Hydrogenation/dehydrogenation reactions have smaller deviations compared to C–C and C–O bond breaking ones. This is in line with the degree of structural change during reaction and agrees with observations in previous works. We conclude that BEP relations developed for small molecules are not statistically different from those developed for furanics. A universal BEP relation is not statistically different from most of the BEP relations developed for furanics, with the exception of C–O, O–H and C–H scission reactions at the functional group, for which only the intercept is statistically different. Small-molecule BEP relations perform adequately for exploring biomass-relevant chemical kinetics on metal surfaces with higher accuracy than the universal BEP relation but lower accuracy than the BEPs of furanics. Finally, we make general observations about the effect of structural change and reaction energy on the accuracy of linear free energy relations on metal surfaces

Ab Initio Surface Phase Diagrams for Coadsorption of Aromatics and Hydrogen on the Pt(111) Surface

Supported metal catalysts are commonly used for the hydrogenation and deoxygenation of biomass-derived aromatic compounds in catalytic fast pyrolysis. To date, the substrate–adsorbate interactions under reaction conditions crucial to these processes remain poorly understood, yet understanding this is critical to constructing detailed mechanistic models of the reactions important to catalytic fast pyrolysis. Density functional theory (DFT) has been used in identifying mechanistic details, but many of these works assume surface models that are not representative of realistic conditions, for example, under which the surface is covered with some concentration of hydrogen and aromatic compounds. In this study, we investigate hydrogen-guaiacol coadsorption on Pt(111) using van der Waals-corrected DFT and ab initio thermodynamics over a range of temperatures and pressures relevant to bio-oil upgrading. We find that relative coverage of hydrogen and guaiacol is strongly dependent on the temperature and pressure of the system. Under conditions relevant to ex situ catalytic fast pyrolysis (CFP; 620–730 K, 1–10 bar), guaiacol and hydrogen chemisorb to the surface with a submonolayer hydrogen (∼0.44 ML H), while under conditions relevant to hydrotreating (470–580 K, 10–200 bar), the surface exhibits a full-monolayer hydrogen coverage with guaiacol physisorbed to the surface. These results correlate with experimentally observed selectivities, which show ring saturation to methoxycyclohexanol at hydrotreating conditions and deoxygenation to phenol at CFP-relevant conditions. Additionally, the vibrational energy of the adsorbates on the surface significantly contributes to surface energy at higher coverage. Ignoring this contribution results in not only quantitatively, but also qualitatively incorrect interpretation of coadsorption, shifting the phase boundaries by more than 200 K and ∼10–20 bar and predicting no guaiacol adsorption under CFP and hydrotreating conditions. The implications of this work are discussed in the context of modeling hydrogenation and deoxygenation reactions on Pt(111), and we find that only the models representative of equilibrium surface coverage can capture the hydrogenation kinetics correctly. Last, as a major outcome of this work, we introduce a freely available web-based tool, dubbed the Surface Phase Explorer (SPE), which allows researchers to conveniently determine surface composition for any one- or two-component system at thermodynamic equilibrium over a wide range of temperatures and pressures on any crystalline surface using standard DFT output

Ru-Sn/AC for the Aqueous-Phase Reduction of Succinic Acid to 1,4‑Butanediol under Continuous Process Conditions

Succinic acid is a biomass-derived platform chemical that can be catalytically converted in the aqueous phase to 1,4-butanediol (BDO), a prevalent building block used in the polymer and chemical industries. Despite significant interest, limited work has been reported regarding sustained catalyst performance and stability under continuous aqueous-phase process conditions. As such, this work examines Ru-Sn on activated carbon (AC) for the aqueous-phase conversion of succinic acid to BDO under batch and flow reactor conditions. Initially, powder Ru-Sn catalysts were screened to determine the most effective bimetallic ratio and provide a comparison to other monometallic (Pd, Pt, Ru) and bimetallic (Pt-Sn, Pd-Re) catalysts. Batch reactor tests determined that a ∼1:1 metal weight ratio of Ru to Sn was effective for producing BDO in high yields, with complete conversion resulting in 82% molar yield. Characterization of the fresh Ru-Sn catalyst suggests that the sequential loading method results in Ru sites that are colocated and surface-enriched with Sn. Postbatch reaction characterization confirmed stable Ru-Sn material properties; however, upon a transition to continuous conditions, significant Ru-Sn/AC deactivation occurred due to stainless steel leaching of Ni that resulted in Ru-Sn metal crystallite restructuring to form discrete Ni-Sn sites. Computational modeling confirmed favorable energetics for Ru-Sn segregation and Ni-Sn formation at submonolayer Sn incorporation. To address stainless steel leaching, reactor walls were treated with an inert silica coating by chemical vapor deposition. With leaching reduced, stable Ru-Sn/AC performance was observed that resulted in a molar yield of 71% BDO and 15% tetrahydrofuran for 96 h of time on stream. Postreaction catalyst characterization confirmed low levels of Ni and Cr deposition, although early-stage islanding of Ni-Sn will likely be problematic for industrially relevant time scales (i.e., thousands of hours). Overall, these results (i) demonstrate the performance of Ru-Sn/AC for aqueous phase succinic acid reduction, (ii) provide insight into the Ru-Sn bimetallic structure and deactivation in the presence of leached Ni, and (iii) underscore the importance of compatible reactor metallurgy and durable catalysts