The discovery of the dual aromatic- and olefin-based catalytic cycles in methanol-to-hydrocarbons (MTH) catalysis on acid zeolites has given a new context for rationalizing structure–function ...relationships for this complex chemistry. This perspective examines six major chemistries involved in the hydrocarbon pool mechanism for MTHolefin methylation, olefin cracking, hydrogen transfer, cyclization, aromatic methylation, and aromatic dealkylationwith a focus on what is known about the rate and mechanism of these chemistries. The current mechanistic understanding of MTH limits structure–function relationships to the effect of the zeolite framework on the identity of the hydrocarbon pool and the resulting product selectivity. We emphasize the need for assessing the consequences of zeolite structure in MTH in terms of experimentally measured rates and activation barriers for individual reaction steps and in terms of speciation preferences within the dual olefin- and aromatic-catalytic cycles to alter their relative propagation. In the absence of individual reaction rates, we propose using ethene/isobutane selectivity as a measure to describe the relative rates of propagation for the aromatic- and olefin-based cycles.
In zeolite pores large enough to accommodate ethanol dimers, ethanol preferentially dehydrates via a bimolecular pathway to generate diethyl ether since the formation of ethanol dimeric species is ...energetically more favorable than the formation of ethanol monomers. In zeolite channels too small to accommodate ethanol dimers, ethanol is selectively dehydrated via a unimolecular reaction pathway to generate ethylene.
The effects of zeolite topology on the dehydration of oxygen-containing molecules were probed in steady-state and isotopic chemical reactions of ethanol over proton-form zeolite materials (FER, MFI and MOR) at low temperatures (368–409
K). The measured rate of diethyl ether (DEE) synthesis was largely independent of ethanol partial pressure on all proton-form zeolites (FER, MFI, and MOR), indicating that DEE formation involves the activation of ethanol dimers. The measured rate of DEE synthesis over H-FER increased with increasing ethylene pressure in experiments done with ethanol–ethylene mixtures, reflecting the weaker adsorption of ethanol dimers on the FER framework compared to that on MFI and MOR materials, thereby resulting in the co-adsorption and reaction of ethylene with ethanol on FER materials. Ethylene production was only observed on H-MOR because the small eight-membered ring side pockets protect ethanol monomers from forming bulky ethanol dimers. Secondary kinetic isotopic effects measured for ethylene synthesis rates using C
2D
5OH reactants imply that the kinetically relevant step involves the cleavage of C–O bonds via a carbenium-ion transition state.
The extent to which spatial constraints influence rates and pathways in catalysis depends on the structure of intermediates, transition states, and active sites involved. We aim to answer, as we seek ...insights into catalytic mechanisms and site requirements, persistent questions about the potential for controlling rates and selectivities by rational design of spatial constraints around active sites within inorganic structures useful as catalysts. This Account addresses these matters for the specific case of reactions on zeolites that contain Brønsted acid sites encapsulated within subnanometer channels. We compare and contrast here the effects of local zeolite structure on the dynamics of the carbonylation of surface methyl groups and of the isotopic exchange of CD4 with surface OH groups on zeolites. Methyl and hydroxyl groups are the smallest monovalent cations relevant in catalysis by zeolites. Their small size, taken together with their inability to desorb except via reactions with other species, allowed us to discriminate between stabilization of cationic transition states and stabilization of adsorbed reactants and products by spatial constraints. We show that apparent effects of proton density and of zeolite channel structure on dimethyl ether carbonylation turnover rates reflect instead the remarkable specificity of eight-membered ring zeolite channels in accelerating kinetically relevant steps that form *COCH3 species via CO insertion into methyl groups. This specificity reflects the selective stabilization of cationic transition states via interactions with framework oxygen anions. These findings for carbonylation catalysts contrast sharply the weak effects of channel structure on the rate of exchange of CD4 with OH groups. This latter reaction involves concerted symmetric transition states with much lower charge than that required for CH3 carbonylation. Our Account extends the scope of shape selectivity concepts beyond those reflecting size exclusion and preferential adsorption. Our ability to discriminate among various effects of spatial constraints depends critically on dissecting chemical conversions into elementary steps of kinetic relevance and on eliminating secondary reactions and accounting for the concomitant effects of zeolite structure on the stability of adsorbed reactants and intermediates.
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•Mo2C catalyzes propane-CO2 conversion via dehydrogenation, hydrogenolysis, and reforming.•Equilibrated RWGS implies O∗ coverage of Mo2C can be calculated using ...CO2/CO.•Dehydrogenation occurs on O∗O∗, O∗∗, and ∗∗ site pairs; O∗∗ has highest rates.•O∗ removal transients can be modeled using two-site dehydrogenation mechanistic model.•All CC scission events have similar site requirements that are inhibited by O∗
Mo2C catalyzes propane dehydrogenation and hydrogenolysis at 823 K; carbon selectivity can be tuned to >95% propylene via dehydrogenation in absence of H2, >95% CH4 via hydrogenolysis with H2 co-feed, or >80% CO via reforming pathways with H2 and CO2 co-feed. The changes in selectivity are mediated by an evolution in the coverage of oxidized (O∗) and carbidic (∗) surface sites which results in an evolution of O∗O∗, O∗∗, and ∗∗ site pairs that catalyze propane dehydrogenation. The fraction of O∗ in relation to ∗ was assessed from measured CO2/CO ratios because reverse water gas shift equilibrium exists under H2/CO2 co-feed steady state reaction conditions. Kinetic models based on the two-site dehydrogenation mechanism could be used to quantitatively describe measured rates of propane dehydrogenation at steady state with or without H2 and/or CO2 co-feed and the transient evolution in dehydrogenation rates upon removing H2 or CO2 in the influent stream to note that O∗∗ site pairs exhibit the highest rate per gram. This model also provides a rationale for O∗ inhibition of H-activated hydrogenolysis pathways and for promotion of oxidative dehydrogenation rates with the introduction of hydrogen into CO2-propane influent streams. This study extends concepts developed for examining the catalytic effects of O∗ coverage on oxidative light alkane conversion from transition metal catalysts to also include carbidic formulations.
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•Total turnovers increase with decreasing methanol concentrations local to active centers.•Methanol undergoes dehydrative disproportionation to methane and formaldehyde.•Increased ...concentration of methanol dehydrogenation derivatives accelerates catalyst deactivation.
The effects of methanol space velocity and inlet methanol partial pressure on lifetime and selectivity of methanol-to-olefins catalysis are examined and interpreted to elucidate reaction parameters and propose intermediates and reactions relevant to catalyst deactivation. The propensity of active centers in HSSZ-13 to turn over for methanol-to-olefins catalysis increases when the methanol partial pressure local to organic co-catalysts confined within the inorganic chabazite cages is lower either by decreasing methanol space velocity or inlet methanol partial pressure. High initial methane selectivity reveals methanol disproportionation, to methane and formaldehyde, a primary reaction, and continual methane formation implicates persistent participation of methanol in bimolecular hydrogen transfer reactions throughout the catalyst lifetime. Methane selectivity correlates positively with inlet methanol partial pressure reflecting enhanced relative rates of formaldehyde formation with increasing methanol partial pressure. Subsequent alkylation reactions of olefins- and aromatics-based CC chain growth carriers by formaldehyde accelerate the relative rates of hydrogen transfer and proliferate, apparently, the precursors mediating the transformation of active hydrocarbon pool participants to those inducing catalyst deactivation.
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•Contact-time variation shows ethane is the sole primary product of methane DHA.•Kinetic and thermodynamic formalisms reveal acetylene is a key intermediate.•Degree of reversibility ...control quantifies thermodynamic influence of each step.•Forward rate calculations require spatial-averaging of reaction reversibility.•DHA forward rate is invariant with contact-time and sets upper bounds to net rate.
Acetylene is identified as a key intermediate in methane dehydroaromatization (DHA) reactions present in concentrations O(1) Pascal. The rank of acetylene and other C2 hydrocarbon intermediates is determined by conversion-selectivity profiles collected from 0.01% to 8% methane conversion varied by extent of “non-selective” deactivation of Mo/H-ZSM-5 catalysts. Ethane is shown to be the sole primary product of methane pyrolysis and is sequentially dehydrogenated to ethylene and acetylene – which aromatizes to benzene with rates similar to direct acetylene aromatization measured in the absence of methane. The influence of CH cleavage and CC coupling events to control the rate and reversibility of DHA is assessed by the degree of reversibility control, introduced here for the first time, and the degree of rate control. The approach to equilibrium of the methane to benzene synthesis reaction is length averaged and affinity averaged by the degree of reversibility control of each intervening elementary step to rigorously calculate forward rates of benzene synthesis by use of De Donder relations. Forward rates are found to be invariant along the catalyst bed once the DHA network reaches a pseudo-steady state and methane, ethane, and ethylene form an equilibrated pool.
Single non-heme Fe(II) ions present as structural moieties in several metal–organic frameworks (e.g., MIL-100, MIL-101, and MIL-808, where MIL indicates Materials of Institute Lavoisier) are ...identified by Kohn–Sham density functional calculations as promising catalysts for C–H bond activation, with energetic barriers as low as 40 kJ mol–1 for ethane and 60 kJ mol–1 for methane following the oxidative activation of iron. The rate-determining step is the activation of N2O and has a barrier of 140 kJ mol–1. Through consideration of the full reaction profile leading to the corresponding alcohols, ethanol and methanol, we have identified key changes in the chemical composition of the node that would modulate catalytic activity. The thermal and chemical stabilities of these materials, together with the scalability of their syntheses, make them attractive catalysts for the selective low-temperature conversion of light alkanes to higher-value oxygenates.
A reaction network detailing the formation and consumption of all C1–C3 products of propylene oxidation on Bi2Mo3O12 at 623 K is developed to show that acrolein, acetaldehyde, acetone, and acetic ...acid are direct oxidation products of propylene while acrylic acid and ethylene are secondary products. Coprocessing acetaldehyde, acetone, acrylic acid, and acetic acid, separately, with propylene, oxygen, and water revealed (i) the existence of overoxidation reactions of acrolein to acrylic acid and ethylene and oxidation pathways from acetone to acetaldehyde and acetic acid, (ii) the promotional effects of water on the synthesis rates of acetaldehyde from acrolein, acetone from propylene, and acetic acid from acetaldehyde and acrylic acid, and (iii) the inhibitory effects of water on the decomposition of acetic acid to CO x and acrylic acid to acetaldehyde and ethylene. A kinetic model is developed to quantitatively capture the kinetic behavior of all species using pseudo-first-order rate expressions in the organic reactant for all reaction pathways; additional promotional and inhibitory dependencies on water pressure were added to describe the kinetics of reaction rates affected by water. Based on the proposition that multiple types of active sites exist on the mixed metal oxide surface during propylene oxidation, a detailed mechanistic network is postulated that describes all molecular transformations with relevant surface intermediates and provides critical insights into the underlying pathways involved in overoxidation and C–C bond scission reactions.
Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in ...petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores. The house-of-cards arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites.
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