DFT Insights into the Competitive Adsorption of Sulfur- and Nitrogen-Containing Compounds and Hydrocarbons on Co-Promoted Molybdenum Sulfide Catalysts

The adsorption of 20 nitrogen-/sulfur-containing and hydrocarbon compounds on the sulfur edge of cobalt-promoted molybdenum sulfide (CoMoS) catalyst was studied using density functional theory, accounting for van der Waals interactions, to elicit comparative structure–property trends across different classes of molecules relevant to hydrotreating. Unhindered organosulfur compounds preferentially adsorb on a “CUS-like” site formed by the dimerization of two neighboring sulfur atoms on the edge to create a vacancy. Nitrogen-containing compounds and 4,6-dimethyldibenzothiophene, however, prefer the brim sites. Binding energy trends indicate that nitrogen-containing compounds will inhibit hydrodesulfurization on the brim sites and, relatively weakly, on the CUS-like sites. Edge vacancies are, therefore, likely to be essential for hydrodesulfurization of unhindered organosulfur compounds. Further, van der Waals forces contribute significantly to the binding energy of compounds (up to 1.0 eV for large compounds such as alkyl-substituted acridines) on CoMoS

Sequential-Optimization-Based Framework for Robust Modeling and Design of Heterogeneous Catalytic Systems

We present a general optimization-based framework for (i) ab initio and experimental data driven mechanistic modeling and (ii) optimal catalyst design of heterogeneous catalytic systems. Both cases are formulated as a nonlinear optimization problem that is subject to a mean-field microkinetic model and thermodynamic consistency requirements as constraints, for which we seek sparse solutions through a ridge (L₂ regularization) penalty. The solution procedure involves an iterative sequence of forward simulation of the differential algebraic equations pertaining to the microkinetic model using a numerical tool capable of handling stiff systems, sensitivity calculations using linear algebra, and gradient-based nonlinear optimization. A multistart approach is used to explore the solution space, and a hierarchical clustering procedure is implemented for statistically classifying potentially competing solutions. An example of methanol synthesis through hydrogenation of CO and CO₂ on a Cu-based catalyst is used to illustrate the framework. The framework is fast, is robust, and can be used to comprehensively explore the model solution and design space of any heterogeneous catalytic system

Automated Generation and Optimal Selection of Biofuel-Gasoline Blends and Their Synthesis Routes

Biomass can be converted into a plethora of compounds through different chemical transformations, thus leading to a complex network of possible synthesis steps. In this work, we propose a novel strategy that simultaneously identifies (a) the most desirable biomass-derived products for an application of interest and (b) the corresponding synthesis routes. The strategy consists of i) constructing an exhaustive network of reactions consistent with an input set of chemistry rules and ii) using the network information to formulate and solve an optimization problem that yields an optimal product distribution and the sequence of reactions that synthesize them. We use this strategy to identify potential renewable oxygenates and hydrocarbons obtained from heterogeneous catalysis of biomass that can be blended with gasoline to satisfy ASTM specifications. Multiple objectives (energy loss, catalyst requirement, and absolute heat duty) are considered, and multiple alternative solutions are found in each case. We identified that both oxygenates and hydrocarbons are components of optimal blends for the energy loss objective, but the other two objectives produce only oxygenates. The proposed strategy is flexible enough to be applicable for any problem involving concurrent product and chemistry selection

Kinetics and Thermochemistry of C₄–C₆ Olefin Cracking on H‑ZSM‑5

C₄–C₆ olefin β-scission rate constants were inferred from experimental studies at 773–813 K and <15% conversion by considering every C₄–C₆ olefin isomer and all available β-scission modes: 2° to 2° (C), 1° to 3° and 3° to 1° (E), 1° to 2° and 2° to 1° (D), and 1° to 1° (F). Group contribution methods were implemented to assess adsorption enthalpies and entropies of C₄–C₆ olefin isomers on H-ZSM-5 via the development of group correction terms for surface alkoxides; a linear dependence of enthalpy (or entropy) of formation difference between a surface alkoxide and a gas-phase alkane on carbon number was considered. Tertiary alkoxides have the smallest adsorption constants among surface adsorbates, and the resulting low coverage of highly substituted alkoxides restricts their contribution to alkene cracking pathways. Intrinsic β-scission rate constants (<i>k</i>_E:<i>k</i>_C:<i>k</i>_D:<i>k</i>_F ratio of 1094:21:8:1 at 783 K) and activation energies (<i>E</i>_inE < <i>E</i>_inC < <i>E</i>_inD < <i>E</i>_inF) from experimentally observed effluent compositions of C₄–C₆ olefin cracking consistent with computational studies were derived after rigorously accounting for adsorption constants and surface coverages of each C₄–C₆ olefin isomer. These results demonstrate that shape selectivity constraints prevent equilibration of surface alkoxides on surfaces under reaction conditions relevant for alkene cracking and present a quantitative description of C–C bond cracking reactions of olefins catalyzed by solid acids

Adsorption of Small Alkanes on ZSM‑5 Zeolites: Influence of Brønsted Sites

The adsorption of a series of small alkanes was studied experimentally on H-ZSM-5 zeolites using calorimetric measurements in order to determine their interactions with the Brønsted sites. Differential heats measured on four ZSM-5 samples with different Si/Al₂ ratio and with different defect concentrations were found to depend strongly on the Brønsted-site density but not on the presence of defects. The interactions for CH₄ with the Brønsted sites were minimal but the effect was significant (up to 11 ± 2 kJ/mol extra heats) for larger alkanes, such as <i>n</i>-C₆H₁₄. The affinity of the alkanes with the Brønsted sites increased with the gas-phase proton affinity of the alkanes and the calculated affinity of the alkanes for the strong acid, fluorosulfonic acid. The extra heats of adsorption in H-ZSM-5 over its siliceous counterparts can therefore be associated with the strength of hydrogen bonding between the adsorbed alkane and the Brønsted sites, which in turn increases with molecular size. Specifically, extra heats were found to vary linearly with acid affinity corrected for dispersion interactions. The comparison of the experimental and computational results, therefore, indicates that the hydrogen bonded interaction theory describes the effect of Brønsted sites for alkane adsorption on zeolites

Resolving the Oxygen Species on Ozone Activated AgAu Alloy Catalysts for Oxidative Methanol Coupling

Bimetallic alloy catalysts frequently demonstrate distinct performances that are superior to their monometallic counterparts, yet their surface chemistry needs to be carefully studied to understand their structure–activity relationships. The nanoporous Ag0.03Au0.97 alloy catalyst becomes highly active and selective for oxidative methanol coupling to methyl formate after O3 activation. HS-LEIS reveals the O3 treatment results in enrichment of Ag (>30%) on the outermost surface layer, while oxygen treatment additionally leads to segregation of a larger portion of Cu impurity on the surface. A series of characteristic Raman bands at 395, 577, 867, and 904 cm–1 only form under oxidative methanol coupling reaction on O3-activated AgAu catalyst. These bands correspond to Ag3–O* (395 cm–1), M–O* on O–Au(111) and AgAu alloy (577 cm–1), CH3OH* (867 cm–1), and HOOH* (904 cm–1), as revealed by DFT calculations. The cyclic in situ Raman and reactivity studies indicate the detected oxygen species could be related to a “memory effect” of the catalyst upon pretreatment. The current study highlights the importance of applying surface-specific techniques for investigation of compositions of outermost surface layers of alloy catalysts, as well as integration of in situ spectroscopies and computational investigations for understanding surface structures at the molecular level under reaction conditions

Elucidating Local Structure and Positional Effect of Dopants in Colloidal Transition Metal Dichalcogenide Nanosheets for Catalytic Hydrogenolysis

Tailoring nanoscale catalysts to targeted applications is a vital component in reducing the carbon footprint of industrial processes; however, understanding and controlling the nanostructure influence on catalysts is challenging. Molybdenum disulfide (MoS2), a transition metal dichalcogenide (TMD) material, is a popular example of a nonplatinum-group-metal catalyst with tunable nanoscale properties. Doping with transition metal atoms, such as cobalt, is one method of enhancing its catalytic properties. However, the location and influence of dopant atoms on catalyst behavior are poorly understood. To investigate this knowledge gap, we studied the influence of Co dopants in MoS2 nanosheets on catalytic hydrodesulfurization (HDS) through a well-controlled, ligand-directed, tunable colloidal doping approach. X-ray absorption spectroscopy and density functional theory calculations revealed the nonmonotonous relationship between dopant concentration, location, and activity in HDS. Catalyst activity peaked at 21% Co:Mo as Co saturates the edge sites and begins basal plane doping. While Co prefers to dope the edges over basal sites, basal Co atoms are demonstrably more catalytically active than edge Co. These findings provide insight into the hydrogenolysis behavior of doped TMDs and can be extended to other TMD materials