Vapor-phase m-cresol hydrodeoxygenation rates on oxygenate-modified Mo2C catalysts prepared by pretreating fresh Mo2C catalysts in 1 kPa of O2, H2O, and CO2 at 333 K showed that (i) molecular oxygen has a higher propensity to deposit oxygen (O/Mobulk before HDO = 0.23 ± 0.02) on fresh Mo2C, compared to CO2 and H2O (O/Mobulk before HDO ≈ 0.036), as assessed from temperature-programmed surface reaction with H2, and (ii) oxygen adsorbed in amounts exceeding ∼0.06 ± 0.01 of O/Mobulk poisons the metal-like sites for toluene synthesis as inferred from a 10-fold decrease in toluene synthesis rate per gram on the O2–1 kPa (333 K)–Mo2C compared to that on fresh Mo2C, H2O–1 kPa (333 K)–Mo2C, and CO2–1 kPa (333 K)–Mo2C catalysts. Invariant turnover frequencies of toluene synthesis measured from in situ CO titration among the O2-, H2O-, and CO2-modified samples demonstrate that the effect of adsorbed oxygen is independent of the oxygen source.Keywords: biofuels; hydrodeoxygenation; lignin; m-cresol; oxygenate-modification;

•Methanol partial pressure controls propylene-to-ethylene ratio during MTH.•Formaldehyde co-feed with methanol or propylene results in enhanced aromatics production.•Formaldehyde-olefins Prins condensation reactions are likely involved in aromatics synthesis during MTH.A monotonic increase (2–18) in the effluent propylene-to-ethylene molar ratio as inlet methanol pressure is varied from 52.5 to 0.6 kPa during methanol-to-hydrocarbons catalysis (∼30%C conversion) on H-ZSM-5 at 673 K reveals methanol pressure as the salient process parameter that allows control over the relative rates of propagation of the olefins- and aromatics-based methylation/cracking events. The enhanced propagation of the olefins-based cycle over its aromatics-based counterpart and consequently, decoupling of the two catalytic cycles at low influent methanol pressures is observed to persist irrespective of the reaction temperature (623–773 K). Reactions involving formaldehyde co-feeds (3–20 Pa or 0.5–5%C) with low-pressure (0.6 kPa) methanol at 623 K result in a monotonically decreasing trend in propylene-to-ethylene molar ratio from 24.7 in the absence of formaldehyde to 0.8 in the presence of 20 Pa formaldehyde implicating suppressed formaldehyde production from methanol transfer dehydrogenation events at low methanol pressures as the mechanistic basis for the observed effect of enhanced olefin cycle propagation. Co-reacting formaldehyde (11 Pa or 3%C) with propylene (0.1 kPa) on H-ZSM-5 at 623 K results in a 5.5-fold increase in aromatics selectivity suggesting Prins condensation reactions between formaldehyde and olefins are likely involved in aromatics production during methanol-to-hydrocarbons catalysis over H-ZSM-5.Download high-res image (236KB)Download full-size image

•A detailed mechanistic network for the formation and consumption of C1 to C6 products during acrolein synthesis from propylene on a mixed metal oxide catalyst.•Transient kinetic measurements, co-feed experiments, and isotopic tracer studies enable elucidation of CC bond cleavage and formation mechanisms.•This study can improve acrolein manufacturing process by describing the formation mechanisms of deleterious byproductsWe report a reaction network detailing the mechanistic origins of 20 C2–C6 byproducts observed during the oxidation of propylene to acrolein at 623 K on a molybdenum-based catalyst promoted with cobalt and nickel used in the industrial production of acrolein. The carbon backbone of propylene is preserved in the sequential oxidation of propylene to allyl alcohol, acrolein, and acrylic acid, as well as propylene oxidation to acetone and propanal via water-mediated pathways. Transient kinetic measurements in conjunction with co-feed experiments of C2 and C3 aldehydes and carboxylic acids show that decarbonylation and decarboxylation reactions, reactions of organic compounds with surface-adsorbed oxygen species, and total combustion reactions are the three mechanisms for CC bond cleavage. CC bond formation reactions that result in C4–C6 byproducts occur via: (i) the addition reaction of a propylene-derived surface allyl species with formaldehyde to form C4 products and with propylene and allyl alcohol to form C6 products, or (ii) the addition reaction of an acrolein (acrylic acid)-derived surface ethenyl intermediate with propylene to form pentadiene and with acrolein to form C5 cyclic oxygenates.Download high-res image (218KB)Download full-size image

Metallic–acidic bifunctionality of molybdenum carbidic catalytic formulations can be tuned by oxygen cofeed and reductive pretreatments to control carbonyl hydrodeoxygenation (HDO). Acetone is deoxygenated over activated Mo2C via sequential hydrogenation of acetone to equilibrium and subsequent dehydration of isopropyl alcohol (IPA). Dehydration to propylene occurs over Brønsted acid sites with an intrinsic activation energy of 103 ± 1 kJ mol–1 and a rate-determining step of β-hydrogen scission, as inferred from a kinetic isotope effect (KIE) of 1.85. HDO rate-determining dehydration rates were kinetically independent of H2 and oxygenate pressure from 10 to 82 kPa and from 0.05 to 4 kPa, respectively. Both the kinetics and the deoxygenation reaction pathway were shown to be similar for the aldehyde group of propanal. Oxygen treatment of activated carbides via 13.5 kPa O2 cofeed was shown to decrease the catalyst surface area from 68 to 9 m2 g–1 and to suppress metallic hydrogenation of acetone to IPA. IPA dehydration rates per gram of catalyst could be altered by a factor of ∼60; a longer activation time under H2 flow at 773 K increased dehydration activation energies from 103 to 140 kJ mol–1, decreased Brønsted acid site densities, and decreased 2,6-di-tert-butylpyridine-normalized dehydration turnover frequencies, indicative of a reduction-induced change in the number and nature of the acid sites.Keywords: acidic; bifunctional catalysis; biomass; hydrodeoxygenation; metallic; molybdenum carbide; oxygen modification

Cofeeding acetaldehyde (1–4 C%) with dimethyl ether (DME) and methanol (DME:methanol ∼9:1, on a carbon basis) on two MFI-type zeolites: a conventional (Conv) MFI zeolite (SiO2/Al2O3 ∼80, diffusion length ∼250 nm) and a self-pillared pentasil (SPP) MFI zeolite (SiO2/Al2O3 ∼150, diffusion length ∼1.5 nm) at 673 K resulted in a monotonic increase in selectivity toward ethene (from 9.3 to 15 C% on Conv MFI and from 1.4 to 6.4 C% on SPP MFI) and methylbenzenes (from 4.9 to 7.8 C% on Conv MFI and 2.6 to 5.3 C% on SPP MFI). The mechanistic basis for this increase in ethene and methylbenzene (MB) selectivity is acetaldehyde undergoing multiple aldol-condensation reactions to form higher homologues (e.g., sorbaldehyde) that subsequently undergo ring-closure followed by dehydration to form aromatics (e.g., benzene). Cofeeding acetaldehyde, therefore, increases the concentration of aromatics inside the zeolite pores, which in turn enhances the propagation of the aromatics-based methylation/dealkylation cycle and consequentially results in higher ethene production. In an isotopic experiment where 13C2-acetaldehyde (∼4 C%) was coreacted with unlabeled DME and methanol (DME:methanol ∼9:1, on carbon basis) on Conv MFI and SPP MFI at 673 K, ethene present in the effluent was enriched with two 13C labels and the net 13C-content in ethene (11–12% on Conv MFI and 45–52% on SPP MFI) was higher than the 13C-content in MBs (5–6% on Conv MFI and 9–17% on SPP MFI). Ethene, therefore, besides being formed via aromatic-dealkylation, is also being produced from acetaldehyde or its aldol-condensation products via a direct synthesis route.Keywords: acetaldehyde cofeed; aldol-condensation; ethene selectivity; HZSM-5; methanol-to-hydrocarbons; methanol-to-olefins; MFI; oxygenate cofeed

Mesoporous metal carbides are of particular interest as catalysts for a variety of reactions because of their high surface areas, porous networks, nanosized walls, and unique electronic structures. Here, two ordered mesoporous metal carbides, Mo2C and W2C, were synthesized using a nanocasting approach coupled with a simultaneous decomposition/carburization process under a continuous methane flow. The as-synthesized mesoporous Mo2C and W2C have three dimensionally ordered porous structures, large surface areas (70–90 m2 g–1), and crystalline walls. In vapor-phase anisole hydrodeoxygenation (HDO) reactions, they exhibited turnover frequencies of approximately 9 and 2 × 10–4 mol molCO–1 s–1, respectively, at relatively low reaction temperatures (423–443 K) and ambient hydrogen pressures. Notably, the ordered mesoporous W2C catalyst showed a greater than 96% benzene selectivity in anisole HDO, the highest benzene selectivity reported to date.Keywords: biomass; hydrodeoxygenation; mesoporous; metal carbide; vapor-phase reaction

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 COx 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.Keywords: acrolein; bismuth molybdate; byproducts; kinetic model; propylene oxidation; reaction network

The steady state rates of ethene and diethyl ether formation in parallel ethanol dehydration reactions at 573 and 623 K are mechanistically and kinetically described by the same rate expression on different alumina materials (α-, γ-, and η-Al2O3), implying that alumina materials have similar surface sites under reaction environments. In situ chemical titration using pyridine as a titrant elucidates similar site densities (∼0.12 sites nm−2 and ∼0.07 sites nm−2 for ethene formation and ∼0.14 sites nm−2 and ∼0.09 sites nm−2 for diethyl ether formation on γ- and η-Al2O3, respectively) on γ- and η-Al2O3 indicating that similar surface features exist on both γ- and η-Al2O3. Pyridine-ethanol co-feed experiments show that pyridine inhibited the formation of ethene to a greater extent than diethyl ether suggesting that the two parallel dehydration reactions are not catalyzed by a common active site.

We discuss the evolution of catalytic function of interstitial transition metal formulations as a result of bulk and surface structure modifications via alteration of synthesis and reaction conditions, specifically, in the context of catalytic deoxygenation reactions. We compare and contrast synthesis techniques of molybdenum and tungsten carbides, including temperature programmed reaction and ultra-high vacuum methods, and note that stoichiometric reactions may occur on phase-pure materials and that in situ surface modification during deoxygenation likely results in the formation of oxycarbides. We surmise that catalytic metal–acid bifunctionality of transition metal carbides can be tuned via oxygen modification due to the inherent oxophilicity of these materials, and we demonstrate the use of in situ chemical titration methods to assess catalytic site requirements on these formulations.

Steady-state rates of ether formation from alcohols (1-propanol, 2-propanol, and isobutanol) on γ-Al2O3 at 488 K increase at low alcohol pressure (0.1–4.2 kPa) but asymptotically converge to different values, inversely proportional to water pressure, at high alcohol pressure (4.2–7.2 kPa). This observed inhibition of etherification rates for C2–C4 alcohols on γ-Al2O3 by water is mechanistically explained by the inhibiting effect of surface trimers composed of two alcohol molecules and one water molecule. Unimolecular dehydration of C3–C4 alcohols follows the same mechanism as that for ethanol and involves inhibition by dimers. Deuterated alcohols show a primary kinetic isotope effect for unimolecular dehydration, implicating cleavage of a C–H bond (such as the Cβ–H bond) in the rate-determining step for olefin formation on γ-Al2O3. Bimolecular dehydration does not show a kinetic isotope effect with deuterated alcohols, implying that C–O or Al–O bond cleavage is the rate-determining step for ether formation. The amount of adsorbed pyridine estimated by in situ titration to completely inhibit ether formation on γ-Al2O3 shows that the number of sites available for bimolecular dehydration reactions is the same for different alcohols, irrespective of the carbon chain length and substitution. 2-Propanol has the highest rate constant for unimolecular dehydration among studied alcohols, demonstrating that stability of the carbocation-like transition state is the primary factor in determining rates of unimolecular dehydration which concomitantly results in high selectivity to the olefin. 1-Propanol and isobutanol have olefin formation rate constants higher than that of ethanol, indicating that olefin formation is also affected by carbon chain length. Isobutanol has the lowest rate constant for bimolecular dehydration because of steric factors. These results implicate the formation and importance of di- and trimeric species in low-temperature dehydration reactions of alcohols and demonstrate the critical role of substitution and carbon chain length in determining selectivity in parallel unimolecular and bimolecular dehydration reactions.Keywords: alcohol dehydration; carbocation stability; kinetics and mechanism; multimer inhibition; site requirements; transition state; γ-alumina

The turnover frequency (TOF) of benzene synthesis from vapor phase anisole hydrodeoxygenation (HDO), estimated via in situ CO titration, was found to be invariant (1.1 ± 0.3 × 10–3 s–1) over molybdenum carbide (Mo2C) catalysts with varying CO chemisorption uptakes (∼70 to ∼260 μmol g–1, measured ex situ at 323 K). Accumulation of oxygen (∼0.29 monolayer) over Mo2C catalysts was determined by an oxygen mass balance during the transient of anisole HDO at 423 K under ambient pressure (H2/anisole molar ratio ∼ 110). Similar product selectivity, apparent activation energy, and TOF of benzene synthesis for an oxygen treated (with oxygen incorporation: O/Mobulk (molar ratio) = 0.075) and freshly prepared Mo2C catalysts (no exposure to air prior to kinetic measurements) demonstrate that the effect of oxygen at these low concentrations is solely to reduce the number of active sites for anisole HDO, resulting in a lower (∼3 times) benzene synthesis rate per gram of catalyst for the oxygen-modified material. The observed benzene synthesis rates per CO chemisorption site for bulk molybdenum oxide (MoOx) catalysts were found to be ∼10 times lower than those for Mo2C catalysts, suggesting that bulk molybdenum oxide phases are not associated with the dominant active sites for anisole HDO at 423 K under ambient pressure.Keywords: anisole; benzene synthesis; hydrodeoxygenation (HDO); in situ CO titration; molybdenum carbide; molybdenum oxide; oxidation; oxygen treatment; turover frequency (TOF)

An automated method is presented to identify energetically feasible mechanisms of heterogeneous catalytic systems comprising of several thousand reactions and species. Specifically, it combines automated rule-based network generation of complex networks with semi-empirical estimation of thermochemical properties to (a) generate large reaction networks, (b) calculate thermochemistry and activation barriers of each step on-the-fly, and (c) identify energetically feasible pathways and mechanisms to experimentally observed products. This method is applied to analyze the mechanism of glycerol conversion on transition metal catalysts for which a network of 3300 reactions and 500 species was generated. Using (i) group additivity methods for calculating reaction and species energies, (ii) linear free energy relationships for activation barriers, and (iii) linear scaling relationships for comparative assessment of energetics of species on different transition metals, the energetically feasible pathways for forming C–C scission and C–O scission products (syngas and propane diol, respectively) are identified on platinum, palladium, rhodium, and ruthenium catalysts. Our results indicate that (a) C–C scission products are preferred on platinum and palladium, (b) rhodium and ruthenium will have a comparatively higher selectivity for C–O scission products, and (c) glycerol tends to undergo several dehydrogenation steps prior to undergoing C–C scission, which are all consistent with experimental observations and DFT calculations. We propose that this method can be used to screen a large number of pathways in complex catalytic networks and to thereby identify the dominant modes in the system, because it is fast and scalable in handling larger networks, flexible in accommodating different types of semi-empirical correlations, and generic in handling any catalytic chemistry.

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 (kE:kC:kD:kF ratio of 1094:21:8:1 at 783 K) and activation energies (EinE < EinC < EinD < EinF) 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.Keywords: alkoxides; group additivity; group correction; olefin cracking; ZSM-5; β-scission

Steady state kinetics and measured pyridine inhibition of ethanol dehydration and dehydrogenation rates on γ-alumina above 623 K show that ethanol dehydrogenation can be described with an indirect hydrogen transfer mechanism to form acetaldehyde and ethane and that this mechanism proceeds through a shared surface intermediate with ethylene synthesis from ethanol dehydration. Ethane is produced at a rate within experimental error of acetaldehyde production, demonstrating that ethane is a coproduct of acetaldehyde synthesis from ethanol dehydrogenation. Steady state kinetic measurements indicate that acetaldehyde synthesis rates above 623 K are independent of co-fed water partial pressure up to 1.7 kPa and possess an ethanol partial pressure dependence between 0 and 1 (Pethanol = 1.0–16.2 kPa), consistent with ethanol dehydrogenation rates being inhibited only by ethanol monomer surface species. The surface density of catalytically active sites for ethylene and diethyl ether production were estimated from in situ pyridine titration experiments to be ∼0.2 and ∼1.8 sites nm–2, respectively, at 623 K. Primary kinetic isotope effects for ethylene and acetaldehyde are measured only when the C–H bonds of ethanol are deuterated, verifying that C–H bond cleavage is kinetically limiting for both products. The proposed indirect hydrogen transfer model for acetaldehyde synthesis is consistent with experimentally observed reaction rate dependences and kinetic isotope effects and highlights the complementary role of hydrogen adatom removal pathways in the formation of aldehydes on Lewis acidic systems.Keywords: acetaldehyde; diethyl ether; ethanol dehydration and dehydrogenation; ethylene; kinetics and mechanism; site requirements; γ-alumina

Vapor phase hydrodeoxygenation (HDO) of furfural over Mo2C catalysts at low temperatures (423 K) and ambient pressure showed high/low selectivity to CO bond/C–C bond cleavage, resulting in selectivity to 2-methylfuran (2MF) and furan of ~50–60% and <1%, respectively. Efficient usage of H2 for deoxygenation, instead of unwanted sequential hydrogenation, was evidenced by the low selectivity to 2-methyltetrahydrofuran. The apparent activation energy and H2 order for 2MF production rates were both found to be invariant with furfural conversion caused by catalyst deactivation, suggesting that (1) the measured reaction kinetics are not influenced by the products of furfural HDO and (2) the loss of active sites, presumably by formation of carbonaceous species observed by TEM analysis, is the reason for the observed catalyst deactivation. The observed half order dependence of 2MF production rates on H2 pressure at different furfural pressures (~0.12–0.96 kPa) and the 0–0.3 order dependence in furfural pressure support the idea of two distinct sites required for vapor phase furfural HDO reactions on Mo2C catalysts. The invariance of 2MF production rates normalized by the number of catalytic centers assessed via ex situ CO chemisorption suggests that metal-like sites on Mo2C catalysts are involved in selective HDO reactions.

Co-processing of H2O, CO2, and light (C1–2) oxygenates with CH4 at 950 K over Mo/H-ZSM-5 catalysts results in complete fragmentation of the oxygenate and CO as the sole oxygen-containing product. The C/Heff accounts for removal of O as CO and describes the net C6H6 and total hydrocarbon synthesis rates at varying (0.0–0.10) oxygenate and H2 to CH4 co-feed ratios.

The methylation of benzene, toluene, para-xylene, and ortho-xylene over MFI structured H-ZSM-5 and mesoporous self-pillared pentasil (H-SPP) with dimethyl ether (DME) at low conversions (<0.1%) and high DME:aromatic ratios (>30:1) showed linear rate dependencies on aromatic pressure and zero dependence on DME pressure for benzene and toluene. These results are consistent with studies performed for olefin methylation, and are indicative of a zeolite surface covered in DME-derived species reacting with benzene or toluene in the rate-determining step. Saturation in the reaction rate was observed in xylene pressure dependence experiments (at 473 K, <5 kPa xylene); however, enhancement in the reaction rate was not observed when comparing ∼1 μm crystallite H-ZSM-5 and 2–7 nm mesopore H-SPP, indicating that xylene methylation proceeds in the absence of diffusion limitations. Simultaneous zero-order rate dependencies on xylene and DME pressures are described by a model based on adsorption of xylene onto a surface methylating species. This model is consistent with observed secondary kinetic isotope effects (kH/kD = 1.25–1.35) and extents of d0, d3, and d6 DME formation in the effluent because of isotopic scrambling between unlabeled and d6 DME when co-fed with aromatics over H-ZSM-5. Post-reaction titration of surface species with water after desorption of physisorbed intermediates showed a 1:1 evolution of methanol to Al present in the catalyst, indicating the presence and involvement of surface methoxides during steady-state methylation of aromatics species.Keywords: aromatic methylation; Brønsted acid catalysis; H-ZSM-5; mesoporous zeolite; methanol-to-gasoline conversion

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 MTH—olefin methylation, olefin cracking, hydrogen transfer, cyclization, aromatic methylation, and aromatic dealkylation—with 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.Keywords: cracking; hydrocarbon pool; methanol-to-hydrocarbons; methanol-to-olefins; methylation; propene; zeolite

Steady state, isotopic, and chemical transient studies of ethanol dehydration on γ-alumina show unimolecular and bimolecular dehydration reactions of ethanol are reversibly inhibited by the formation of ethanol–water dimers at 488 K. Measured rates of ethylene synthesis are independent of ethanol pressure (1.9–7.0 kPa) but decrease with increasing water pressure (0.4–2.2 kPa), reflecting the competitive adsorption of ethanol–water dimers with ethanol monomers; while diethyl ether formation rates have a positive, less than first order dependence on ethanol pressure (0.9–4.7 kPa) and also decrease with water pressure (0.6–2.2 kPa), signifying a competition for active sites between ethanol–water dimers and ethanol dimers. Pyridine inhibits the rate of ethylene and diethyl ether formation to different extents verifying the existence of acidic and nonequivalent active sites for the dehydration reactions. A primary kinetic isotope effect does not occur for diethyl ether synthesis from deuterated ethanol and only occurs for ethylene synthesis when the β-proton is deuterated; demonstrating olefin synthesis is kinetically limited by either the cleavage of a Cβ-H bond or the desorption of water on the γ-alumina surface and ether synthesis is limited by the cleavage of either the C–O bond of the alcohol molecule or the Al–O bond of a surface bound ethoxide species. These observations are consistent with a mechanism inhibited by the formation of dimer species. The proposed model rigorously describes the observed kinetics at this temperature and highlights the fundamental differences between the Lewis acidic γ-alumina and Brønsted acidic zeolite catalysts.Keywords: diethyl ether; ethanol dehydration; ethylene; Lewis acid sites; parallel reactions; surface dimer species; γ-alumina

Co-processing of formic acid or carbon dioxide with CH4 (FA/CH4 = 0.01–0.03 and CO2/CH4 = 0.01–0.03) on Mo/H-ZSM-5 catalysts at 950 K with the prospect of kinetically coupling dehydrogenation and deoxygenation cycles results instead in a two-zone, staged bed reactor configuration consisting of upstream oxygenate/CH4 reforming and downstream CH4 dehydroaromatization. The addition of an oxygenate co-feed (oxygenate/CH4 = 0.01–0.03) causes oxidation of the active molybdenum carbide catalyst while producing CO and H2 until completely converted. Forward rates of C6H6 synthesis are unaffected by the introduction of an oxygenate co-feed after rigorously accounting for the thermodynamic reversibility caused by the H2 produced in oxygenate reforming reactions and the fraction of the active catalyst deemed unavailable for CH4 DHA. All effects of co-processing oxygenates with CH4 can be construed in terms of an approach to equilibrium.

Eight β-scission modes involving C6 and C8 olefin isomers are investigated using dispersion-corrected density functional theory (i.e., PBE-D) calculations. Potential energy surfaces are evaluated within an acidic H-ZSM-5 supercell containing a single, isolated active site. Minimum energy pathways are localized using the nudged elastic band method. The relative enthalpic barriers of β-scission steps can be described by the substitution order of the carbocationic carbon atom present in the reactant and transition states. Specifically, the total charge on the hydrocarbon fragment containing the β C atom increases going from the physi- or chemisorbed reactant state to the β-scission transition state; the magnitude of this change (+0.37e–0.97e) is found to correlate nearly monotonically with the activation energy (89–233 kJ mol–1). A comparison of 1° to 3° (E1) and 3° to 1° (E2) β-scission modes as well as 2° to 3° (B1) and 3° to 2° (B2) β-scission modes reveals that the barrier heights depend on the substitution order of the β C, indicating that a subcategorization of β-scission modes is required based on the substitution order of the β C atom. Isomerization reactions, which are fast with respect to β-scission, enable reactant hydrocarbons to explore and find low-barrier β-scission pathways. Selectivities predicted on the basis of the relative barrier heights of β-scission modes accessible to C6 and C8 species indicate agreement with experimental observations.

The relationship between polyol adsorption affinity and silanol defect density was investigated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials which vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes were confirmed as crystalline mordenite framework inverted (MFI) by SEM and XRD and were shown to contain ∼8.5–0 silanol defects per unit cell by 29Si MAS, 1H MAS, and 1H–29Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10–3P/P0 evolved with a decrease in silanol defects, and, through features in the XRD and 29Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic–monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption at ∼10–3 g/mL with an increase from ∼0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption isotherms on defect-free silicalite-1 at 333 K, both of which exhibit the Type-V character, indicates that water enhances adsorption by a factor of ∼2 in the Henry’s Law regime. Henry’s constants for aqueous C2–C4 polyol adsorption (concentrations below 0.004 g/mL) at 298 K are shown to have a linear dependence on the silanol defect density, demonstrating that these molecules preferentially adsorb at silanol defects at dilute concentrations. This systematic study of polyol adsorption on silicalite-1 materials highlights the critical role of defects on adsorption of hydrophilic molecules and clearly details the effects of coadsorption of water, which can guide the selection of zeolites for separation of biomass-derived oxygenates.

The systematic investigation of 1-butene, trans-2-butene, cis-2-butene, and isobutene methylation with dimethyl ether (DME) over acidic zeolites FER, MFI, MOR, and BEA at low conversions (<0.1%) and high DME:olefin ratios (>15:1) showed linear rate dependencies on butene pressure and no dependence on DME pressure. Such dependencies are consistent with the zeolite surface being predominantly covered by DME-derived species, which either directly reacts with butene species in the rate-determining step or through the formation and subsequent degradation of a coadsorbed complex. A comparison of rate constants for butene methylation across isomers over MFI and BEA shows that, in the absence of hydride shift, a 10-fold increase is observed for reactants capable of forming more substituted carbenium ion-like transition states, as predicted for carbocation mediated mechanisms. High cis-2-butene pressure experiments over BEA show linear dependencies of the butene methylation rate on butene pressure for olefin to DME ratios as high as a ∼1.5, indicating that surface-bound DME derived species react with butene in Eley–Rideal type kinetics at low temperatures. Titration with water after steady-state methylation of cis-2-butene over BEA results in DME-derived intermediates being removed as methanol in a 1:1 ratio with zeolite Al suggesting that surface methyl groups are involved in olefin methylation reactions.Keywords: Brønsted acid catalysis; butenes; methanol-to-gasoline conversion; olefin methylation; zeolites;

The role of zeolite topology in the stepwise methylation of ethene by surface methoxides was investigated. Density functional theory was employed in the determination of reaction mechanisms and energy barriers. Elementary steps were studied across multiple frameworks (i.e., BEA, CHA, FER, MFI, and MOR) constituting a wide variety of confinement environments. Surface methoxides were found to react with ethene through a transition state containing planar CH3 species, which was best stabilized at the intersection of the 10-membered ring channels of MFI. A cyclopropane reaction intermediate was found in all cases; its decomposition necessitated a transition state containing a primary carbocation, which was best stabilized within the 8-membered ring side pockets of MOR. The activation energies corresponding to each transition state geometry depend upon different aspects of the local pore topology, implying that confinement effects can not be simply correlated to pore size.

The catalytic behavior of Brønsted acid sites in three acidic zeolite materials (one MWW and two MFI zeolites) containing dual meso-/microporosity was studied using ethanol dehydration and monomolecular conversion of propane and isobutane as probe reactions. The meso-/microporous MWW zeolite, MCM-36 or pillared MWW, consists of a zeolitic layer structure, with independent microporosity and mesoporosity within the layers and between the layers, respectively. A meso-/microporous MFI (pillared MFI) zeolite also contains a zeolitic layer structure, but with interconnected micropore and mesopore systems. A different meso-/microporous MFI zeolite, three-dimensionally ordered mesoporous-imprinted (3DOm-i) MFI, contains nanometer-sized spherical elements forming an opaline structure, with highly interconnected meso- and micropores. The rate and apparent activation energy of the catalytic probe reactions in zeolites possessing dual meso- and microporosity was comparable to conventional microporous MCM-22 (MWW) and MFI materials. This similarity in kinetic behavior between materials possessing dual meso-/microporosity and their microporous analogues when assessed under conditions of strict kinetic control implies that the catalytic behavior of Brønsted acid sites in materials with dual meso-/microporosity is preferentially dominated by the microporous environment possibly because it provides a better fit for adsorption of small alkane or alcohol reactant molecules.Keywords (keywords): base titration; ethanol dehydration; hierarchical materials; isobutane activation; meso-/microporous zeolite; monomolecular alkane conversion; propane activation; shape selectivity

Henry’s constants (Kads) for adsorption of C3 polyfunctional molecules onto zeolites from aqueous solutions at 278 K were obtained and compared with the octanol–water partition coefficients, Kow, which were calculated using the prevalent ClogP group contribution method. Kads increases linearly with Kow for these adsorbates on H–ZSM-5 (MFI), FAU, BEA, and ITQ-1 (MWW). Kads values for C2–C6 diol adsorption at 278 K are also linearly correlated with Kow regardless of interactions in the bulk phase as measured by the solution activity coefficient. Exceptions to the correlation established between Kads and Kow are the adsorption of 1,2,ω-triols with carbon number greater than three on H–ZSM-5 and adsorption of all oxygenates studied on FER, which we postulate to be due to the effect of changing adsorption configuration with adsorbate/zeolite structure which cannot be captured by Kow alone. These results enable the prediction of separation selectivities of biomass-derived compounds on zeolite adsorbents.

The full catalytic cycle for the self-metathesis of ethane was studied by density functional theory (DFT). The active site was a Ta-dihydride grafted on a Brønsted acid site [(≡AlO)2Ta(H2)] of the internal pore surface of the FER zeolite. The transition state geometries and activation energies were determined through the nudged elastic band (NEB) method for each elementary step, and the complete cycle was found to be thermodynamically consistent. Investigated elementary steps include ethane C–H σ-bond activation, ethylene desorption through α and β hydrogen elimination mechanisms, Ta-ethylcarbene formation, olefin metathesis, and hydrogenation of olefin metathesis products. For the activation of ethane, as compared to catalytic systems involving zeolite-supported Ga and Zn, a low barrier (∼64 kJ mol–1) was observed. In the olefin metathesis step, where Ta-ethylcarbene reacts with ethylene, it was found that the Ta-metallacyclobutane has a relatively high stability (∼143 kJ mol–1) as compared to similar metallacyclobutane species and that the forward decomposition of the Ta-metallacyclobutane is the most energetically demanding step.

Biomass conversion to fuels and chemicals involves a multitude of oxygen-containing compounds and thermochemical reaction routes. A detailed elucidation of the process chemistry is, thus, a key step in understanding the reaction mechanisms and designing chemical processes in a biorefinery. In this paper, a computational tool, called Rule Input Network Generator (RING), is presented as a platform for modeling diverse homogeneous and heterogeneous chemistries in biomass conversion and automatically generating the underlying complex reaction networks. RING accepts a set of reaction rules and initial reactants as inputs and exhaustively generates the reactions of the system. The reaction center of an elementary step is represented by a SMARTS-like string and identified as a submolecular pattern in a reactant molecular graph using a pattern-matching algorithm. The reaction events are subsequently modeled as a graph transformation system. The generality of this framework was substantiated by the successful application of RING in reproducing the reaction mechanisms of different biomass conversion systems, such as acid-catalyzed dehydration of fructose, base-catalyzed esterification of triglycerides, and gas phase pyrolysis of fatty esters.

Ambient temperature adsorption isotherms have been developed for C2−C6 diols and triols on small (FER), medium (MWW, MFI, BEA), and large (MOR, FAU) pore zeolites as well as on ordered mesoporous materials (MCM-36, 3DOm-MFI, and SBA-15) using gravimetry. Henry’s constants for diol and triol adsorption on silicalite-1 increase exponentially with carbon number demonstrating that confinement of the adsorbate in the zeolite pores is the primary driving force for adsorption. This conclusion is supported by results for propylene glycol adsorption at low coverages on materials differing in topology and chemical composition. It is shown that adsorption decreases with an increase in the adsorbent pore size, and aluminum content only has a marginal effect. Comparison of diol and triol adsorption on silicalite-1 shows that increasing the number of hydroxyl groups causes a decrease in the Henry’s constant possibly due to a change of the configuration of the adsorbate in the zeolite pores, while the location of the hydroxyl groups does not have a significant effect. Overall, this study provides evidence that polyol adsorption is primarily a function of dispersion forces that are derived from the fit of the adsorbate in the adsorbent pores. These findings could have an impact on the separation and catalytic conversion of oxygenates in the processing of biomass to chemicals and fuels.

•Dehydroaromatization of methane to benzene with hydrogen and acetic acid co-feed.•Hydrogen pressure does not increase linearly with catalyst weight.•Forward rate of benzene synthesis is invariant with respect to hydrogen pressure and catalyst weight.•Coupling deoxygenation and dehydrogenation reactions on catalytic surfaces.The co-processing of acetic acid with methane (CH3COOH/CH4 = 0.04–0.10) on Mo/ZSM-5 formulations at 950 K and atmospheric pressure in an effort to couple deoxygenation and dehydrogenation reaction sequences results instead in a stratified reactor bed with upstream CH4 reforming with acetic acid and downstream CH4 pyrolysis. X-ray absorption spectroscopy and chemical transient experiments show that molybdenum carbide is formed inside zeolite micropores during CH4 reactions. The introduction of acetic acid oxidizes a fraction of these carbide moieties upstream while producing H2 and CO mixtures until completely consumed. Forward rates of CH4 pyrolysis are unperturbed in the presence of an acetic acid or hydrogen co-feed after rigorously accounting for the reversibility of pyrolysis rates and the fraction of molybdenum carbide oxidized by CH3COOH implying that all consequences of CH3COOH and H2 co-feeds can be interpreted in terms of an approach to equilibrium.Graphical abstractConcurrent deoxygenation of C1-C2 oxygenates and dehydrogenation of CH4 on Mo/ZSM-5 results in a stratified reactor bed and downstream CH4 pyrolysis rates can be interpreted in terms of an approach to equilibrium based on the hydrogen formed in the upstream oxidation zone.Download high-res image (78KB)Download full-size image

•Mo2C was used for vapor-phase anisole HDO at low temperatures (420–520 K) under ambient pressure.•High benzene selectivity (>90%) and high hydrogen efficiency (cyclohexane selectivity <9%).•Two distinct sites are required; metallic sites are involved in HDO chemistry over Mo2C.•The strong phenolic C–O bond was cleaved preferentially during anisole HDO.Vapor-phase hydrodeoxygenation (HDO) of anisole over Mo2C catalysts at low temperatures (420–520 K) and ambient pressure showed (1) remarkable selectivity for C–O bond cleavage, giving benzene selectivity >90% among C6+ products, (2) high hydrogen efficiency for the HDO reaction as indicated by low cyclohexane selectivity (<9%), and (3) good stability over ∼50 h. Methane selectivity increased at the expense of methanol selectivity as anisole conversion increased, suggesting that the phenolic C–O bond was cleaved preferentially. The concurrent near half-/zero-order dependence of benzene synthesis rates on H2/anisole pressure, and the preferential inhibition of benzene synthesis rates upon introduction of CO relative to isotopic HD exchange suggest that catalytic sites for H2 activation are distinct from those required for the activation of anisole. The involvement of metallic sites on Mo2C catalysts for this reaction was demonstrated by the nearly invariant benzene synthesis rate per CO chemisorption site.Download high-res image (143KB)Download full-size image

•Olefin isotopic composition based on paring, side-chain, and ring-expansion aromatic dealkylation mechanisms.•Aromatic dealkylation to form ethene and propene occurs through the paring mechanism on HZSM-5.•1,2,4,5-Tetramethylbenzene is the predominant aromatic precursor to light olefins in methanol-to-hydrocarbons conversion.Co-reactions of 7.5–9.3 kPa of DME with 4 kPa of toluene, p-xylene, and 4-ethyltoluene onH-ZSM-5 at 523–723 K at low conversions (<10 C%) with varying isotopic feed compositions of 13C/12C show that carbons originating from the aromatic ring are incorporated into ethene and propene. A comparison of the predicted 13C-contents of ethene and propene postulated on the basis of the paring, side-chain, and ring-expansion aromatic dealkylation mechanisms based on the experimentally observed isotopologue distribution of 1,2,4-trimethylbenzene, 1,2,4,5-tetramethylbenzene, and 4-ethyltoluene reveal that the predicted 13C-content of ethene and propene from 1,2,4,5-tetramethylbenzene via the paring mechanism most closely match the experimentally observed 13C-contents of ethene and propene (<10% mean relative error), compared to the other mechanisms and aromatic precursors examined. This work quantitatively shows that aromatic dealkylation to form ethene and propene occurs through the paring mechanism and that 1,2,4,5-tetraMB is the predominant aromatic precursor for light olefin formation for MTO conversion on H-ZSM-5 for a 200 K range in temperature.Aromatic dealkylation in methanol-to-hydrocarbons conversion on HZSM-5 occurs via the paring mechanism and 1,2,4,5 tetramethylbenzene is the predominant alkylbenzene precursor to ethylene and propene.Download high-res image (113KB)Download full-size image

Steady-state reaction studies of acetic acid and ethanol were used to identify co-adsorbed acetic acid/ethanol dimers as surface intermediates within specific elementary steps involved in the esterification of acetic acid with ethanol on zeolites. A reaction mechanism involving two dominating surface species, an inactive ethanol dimeric species adsorbed on Brønsted sites inhibiting ester formation and a co-adsorbed complex of acetic acid and ethanol on the active site reacting to produce ethyl acetate, is shown to describe the reaction rate as a function of temperature (323–383 K), acetic acid (0.5–6.0 kPa), and ethanol (5.0–13.0 kPa) partial pressure on proton-form BEA, FER, MFI, and MOR zeolites. Measured differences in rates as a function of zeolite structure and the rigorous interpretation of these differences in terms of esterification rate and equilibrium constants is presented to show that the intrinsic rate constant for the activation of the co-adsorbed complex increases in the order FER < MOR < MFI < BEA.Graphical abstractAcetic acid esterification by ethanol proceeds through an acetic acid/ethanol co-adsorbed complex on the zeolite surface. The mechanistic pathway accurately describes the ethyl acetate synthesis rate as a function of temperature, acetic acid, and ethanol partial pressure on proton-form BEA, FER, MFI, and MOR zeolitesDownload high-res image (108KB)Download full-size imageHighlights► Rate per proton in different zeolite structures for acetic acid esterification by ethanol. ► Mechanism proceeds through acetic acid/ethanol co-adsorbed complex. ► Rate is described as a function of temperature, zeolite structure, acetic acid, and ethanol pressure. ► Prevalent dimeric intermediates are in general more stable in larger pore zeolites.

The product selectivity of dimethyl ether (DME) conversion to hydrocarbons on H-ZSM-5 was systematically tuned by co-feeding small amounts of 13C-propene and 13C-toluene (4 kPa) with 12C-DME (70 kPa) under isoconversion conditions (20.8–22.7 C%) at 548 K. The selectivity to ethene (14.5–18 C%) and aromatics (7.1–33.7 C%) increased while selectivity to C4–C7 aliphatics (42.8–16.9 C%) decreased with increasing amounts of toluene (0–4 kPa) in the co-feed. Similar trends were also observed at lower conversions (4.6–5.1 C%) at 548 K and at higher temperatures (623 K), showing that the olefin-to-aromatic ratio can be used as a parameter to propagate the olefin- and aromatic-based carbon pools to varying extents within the range of conditions studied in this work. The co-reaction of 13C-propene with 12C-DME showed that C5–C7 olefins are formed almost exclusively from methylation reactions while butenes are formed from both olefin cracking and methylation reactions. The high fraction of propene (55.1%) with at least one 12C indicated that a large fraction of propene is a product of olefin cracking reactions. Under conditions in which the aromatic-based cycle is dominant (increasing amounts of toluene in the co-feed), both ethene and propene contained approximately 10% 13C atoms, showing that when the olefin-based cycle is suppressed, these light olefins primarily originate from the aromatic-based cycle. The 13C content of toluene in the effluent was unchanged compared to that in the 13C-toluene feed, implying that toluene is not formed as a significant product. Additionally, at least 9.8% of p-xylene, 1,2,4-trimethylbenzene, and 1,2,4,5-tetramethylbenzene isotopomers were entirely 12C-labeled, while less than 2% of toluene and o-xylene isotopomers were entirely 12C-labeled, showing that under the conditions studied in this work, cyclization reactions occur predominantly for C8+ aliphatics to form p-xylene and larger aromatics. Because the olefin- and aromatic-based cycles are not isolated from one another, understanding communication between the two cycles is an important step in controlling selectivity of MTH on H-ZSM-5.Graphical abstractDownload high-res image (71KB)Download full-size imageHighlights► Selectivity of MTH can be systematically tuned at isoconversion through the addition of an olefin or aromatic co-feed. ► Propene is formed by both cracking of olefins and from dealkylation of aromatics. ► C8+ aromatics are the predominant products of cyclization of aliphatics.

•Ethene and 2MB are representative products of the aromatic- and olefin-based cycles on H-ZSM-5.•Ethene/2MB yield describes relative propagation of the cycles on H-ZSM-5.•Selectivity and ethene/2MB yield can be tuned at iso-conversion conditions.The observed product distribution in methanol-to-hydrocarbons (MTH) catalysis can be rationalized based on the relative rates of propagation of the aromatic- and olefin-based cycles that operate on the zeolite catalyst. We report that the ratio of ethene to 2-methylbutane + 2-methyl-2-butene (ethene/2MB) yield can be used to describe the propagation of aromatic and olefin methylation/cracking cycles. The co-reaction of 12C-ethene with 13C-dimethyl ether (DME) shows that the rate of DME conversion (1.62 mol C (mol Al s)−1) is ∼20 times faster than ethene conversion (0.08 mol C (mol Al s)−1), suggesting that ethene can be considered as terminal product for MTH at 623 K. At iso-conversion conditions at 548 K, propene is co-fed with DME to increase propagation of the olefin-based cycle and correspondingly a 1.7-fold decrease in the ethene/2MB yield is observed. Similarly, the co-reaction of toluene with DME increases propagation of the aromatic-based cycle and a 2.1-fold increase in the ethene/2MB yield is observed. The ethene/2MB yield also increased by a factor of 2 as DME conversion increased from 5% to 62%, which is consistent with the observed concurrent increase in selectivity to ethene and methylbenzenes. For the reaction of DME alone, increasing the temperature from 548 K to 723 K increases the propagation of the olefin-based cycle and a corresponding decrease in the ethene/2MB yield from 4.7 to 1.3 is also observed. The ethene/2MB yield varies systematically with feed composition, conversion, and temperature, showing that this ratio describes the relative propagation of the aromatic to olefin methylation/cracking cycles in MTH conversion on H-ZSM-5.Download high-res image (152KB)Download full-size image

Ethylene and propylene methylation rates increased linearly with olefin pressure but did not depend on dimethyl ether (DME) pressures on proton-form FER, MFI, MOR, and BEA zeolites at low conversions (<0.2%) and high DME/olefin ratios (30:1) in accordance with a mechanism that involves the zeolite surface being predominantly covered by DME-derived species reacting with olefins. Higher first-order reaction rate constants for both ethylene and propylene methylation were observed over H-BEA and H-MFI compared with H-FER and H-MOR, indicating that olefin methylation reaction cycles involved in the conversion of methanol-to-gasoline over zeolitic acids are propagated to varying extents by different framework materials. Systematically lower activation barriers and higher rate constants were observed for propylene methylation in comparison with ethylene methylation over all frameworks studied, reflecting the increased stability of reaction intermediates and activated complexes with increasing olefin substitution. A binomial distribution of d0/d3/d6 in unreacted DME upon introduction of equimolar protium- and deuterium-form DME under steady-state reaction conditions of ethylene methylation over H-MFI suggests the presence and facile formation of reactive surface-bound methoxide species and the absence of C–H bond cleavage.Graphical abstractDimethyl ether methylation of olefins proceeds via the same mechanistic pathway on different zeolites, but with different rates of reaction showing that zeolites propagate olefin methylation reaction cycles prevalent in methanol-to-hydrocarbons conversion to varying extents. The formation and involvement of surface methoxide species is consistent with the secondary kinetic isotope effect observed using CD3OCD3 reactants and with olefin methylation rates having a zero-order dependence in DME pressures and a first-order dependence in olefin pressure.Download high-res image (52KB)Download full-size imageHighlights► Rate per proton in different zeolite environments for C2H4 and C3H6 methylation. ► Rate is zero-order in dimethyl ether pressure and first-order in olefin pressure. ► Same mechanism, but rates differ by a factor of 30 across zeolites. ► Surface methoxide species are formed during steady-state operation; no direct C–H activation.

•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.Download high-res image (106KB)Download full-size image

•Brønsted acid sites are generated on W- and Mo-carbides by O2-induced surface oxidation.•Bulk carbide structure is unaltered by surface oxidation.•Rate per proton on W- and Mo-carbides with and without O2 treatment is nearly invariant.Acidic properties of β-Mo2C, α-Mo2C, W2C, and WC were quantified by assessing the kinetics of isopropanol (IPA) dehydration at 415 K either (i) under inert He/Ar atmosphere or (ii) with 13 kPa O2 co-feed. Dehydration kinetics were zero-order with respect to IPA for all catalysts and under all reaction conditions. Intrinsic activation energies were similar across all catalysts (89–104 kJ mol−1). Acid site densities calculated via in situ 2,6-di-tert-butylpyridine (DTBP) titration were used to normalize dehydration turnover frequencies (TOF). O2 co-feed increased dehydration rates per gram by an order of magnitude for all catalysts tested, but TOF remained invariant within a factor of ∼2. Mo- and W-based carbides showed similar dehydration kinetics regardless of O2 co-feed, and O2 co-feed did not alter bulk carbidic structure as noted by X-ray diffraction. Brønsted acid site provenance results from the oxophilicity of Mo and W carbides.Download high-res image (63KB)Download full-size image

•Synthesis and structure of 3DOm-i and self-pillared zeolites are compared.•Site accessibility and diffusion in hierarchical zeolites are discussed.A long-standing synthetic challenge is to reduce the size of zeolitic domains to its absolute minimum, the unit or sub-unit cell dimension. Hierarchical zeolites with singe-unit or few-unit cell zeolite domains along with methods to assess adsorption, diffusion and reaction in these materials are reviewed.

•Interaction between MBs and active sites increases with increasing aluminum content.•MBs undergo multiple methylation/dealkylation reaction cycles and produce ethene.•Ethene selectivity increases with increasing aluminum content or decreasing Si/Al.Increasing crystallize size or aluminum content in MFI-type zeolites independently enhances the propagation of the aromatics-based methylation/dealkylation cycle relative to that of the olefins-based methylation/cracking cycle in methanol-to-hydrocarbons (MTH) conversion and consequentially results in higher ethene selectivity. Ethene selectivity increases monotonically with increasing aluminum content for HZSM-5 samples with nearly identical crystallite size consequent to an increase in the intracrystalline contact time analogous to our recent report detailing the effects of crystallite size (Khare et al., 2015) on MTH selectivity. The confected effects of crystallite size and site density on MTH selectivity can therefore, be correlated using a descriptor that represents the average number of acid sites that an olefin-precursor will interact with before elution.Download high-res image (166KB)Download full-size image

The measured kinetics of n-C6H14 hydroisomerization reactions is consistent with a bifunctional mechanism involving the facile dehydrogenation of n-hexane on the metal catalyst and a kinetically relevant step involving isomerization of n-hexene on zeolitic acidic sites. The measured activation entropy in small 8-MR pockets of MOR (−35 J mol−1 K−1) is similar to that in larger 12-MR channels of MOR (−37 J mol−1 K−1) and BEA (−33 J mol−1 K−1) but higher than that in medium pore FER (−86 J mol−1 K−1), suggesting that partial confinement of C6 olefinic reactants results in lower free energy for the isomerization reaction in 8-MR pockets of MOR. The hypothesis that a cyclopropane-like cationic transition state is not completely contained within the 8-MR pockets of MOR is consistent with the observed selectivity to 2-methylpentane and 3-methylpentane in the 8-MR pockets being identical to that measured in larger 12-MR channels of MOR and BEA. The lower activation energy measured in 8-MR pockets compared to larger 12-MR channels of MOR may arise due to greater electrostatic stabilization of the positively charged transition state by framework oxygen atoms located on the pore mouth of the smaller 8-MR pockets of MOR or due to the larger heat of adsorption caused by confinement in smaller 8-MR pockets. The lower activation energy in 8-MR channels and comparable loss in entropy mediated by partial confinement results in the rate per proton in 8-MR pockets being five times larger than the rate in 12-MR channels of MOR. These results provide another conceptual consideration for rigorous and quantitative understanding of local environment effects of zeolite channel size and connectivity on the rate and selectivity of acid-catalyzed reactions.Graphical abstractThe n-hexene molecule is partially confined in the 8-MR side pockets of MOR, which results in entropy of activation and selectivity for isomerization reactions in 8-MR pockets being similar to that in 12-MR channel zeolites; the lower activation energy in 8-MR channels results in catalytic rates per proton being five times higher than those in 12-MR channels of MOR.Download high-res image (66KB)Download full-size imageHighlights► Rate per proton in different zeolite environments for n-hexane hydroisomerization. ► Selectivity, entropy of activation similar in 8-MR pockets and larger 12-MR channels. ► Higher rates in 8-MR pockets than in 12-MR channels of MOR due to partial confinement. ► Pore size does not predict the occurrence of a particular reaction within zeolites.

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 C2D5OH reactants imply that the kinetically relevant step involves the cleavage of C–O bonds via a carbenium-ion transition state.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.Download high-res image (88KB)Download full-size image

Co-processing of formic acid or carbon dioxide with CH4 (FA/CH4 = 0.01–0.03 and CO2/CH4 = 0.01–0.03) on Mo/H-ZSM-5 catalysts at 950 K with the prospect of kinetically coupling dehydrogenation and deoxygenation cycles results instead in a two-zone, staged bed reactor configuration consisting of upstream oxygenate/CH4 reforming and downstream CH4 dehydroaromatization. The addition of an oxygenate co-feed (oxygenate/CH4 = 0.01–0.03) causes oxidation of the active molybdenum carbide catalyst while producing CO and H2 until completely converted. Forward rates of C6H6 synthesis are unaffected by the introduction of an oxygenate co-feed after rigorously accounting for the thermodynamic reversibility caused by the H2 produced in oxygenate reforming reactions and the fraction of the active catalyst deemed unavailable for CH4 DHA. All effects of co-processing oxygenates with CH4 can be construed in terms of an approach to equilibrium.

Vapor phase hydrodeoxygenation (HDO) of furfural over Mo2C catalysts at low temperatures (423 K) and ambient pressure showed high/low selectivity to CO bond/C–C bond cleavage, resulting in selectivity to 2-methylfuran (2MF) and furan of ~50–60% and <1%, respectively. Efficient usage of H2 for deoxygenation, instead of unwanted sequential hydrogenation, was evidenced by the low selectivity to 2-methyltetrahydrofuran. The apparent activation energy and H2 order for 2MF production rates were both found to be invariant with furfural conversion caused by catalyst deactivation, suggesting that (1) the measured reaction kinetics are not influenced by the products of furfural HDO and (2) the loss of active sites, presumably by formation of carbonaceous species observed by TEM analysis, is the reason for the observed catalyst deactivation. The observed half order dependence of 2MF production rates on H2 pressure at different furfural pressures (~0.12–0.96 kPa) and the 0–0.3 order dependence in furfural pressure support the idea of two distinct sites required for vapor phase furfural HDO reactions on Mo2C catalysts. The invariance of 2MF production rates normalized by the number of catalytic centers assessed via ex situ CO chemisorption suggests that metal-like sites on Mo2C catalysts are involved in selective HDO reactions.

We discuss the evolution of catalytic function of interstitial transition metal formulations as a result of bulk and surface structure modifications via alteration of synthesis and reaction conditions, specifically, in the context of catalytic deoxygenation reactions. We compare and contrast synthesis techniques of molybdenum and tungsten carbides, including temperature programmed reaction and ultra-high vacuum methods, and note that stoichiometric reactions may occur on phase-pure materials and that in situ surface modification during deoxygenation likely results in the formation of oxycarbides. We surmise that catalytic metal–acid bifunctionality of transition metal carbides can be tuned via oxygen modification due to the inherent oxophilicity of these materials, and we demonstrate the use of in situ chemical titration methods to assess catalytic site requirements on these formulations.

The steady state rates of ethene and diethyl ether formation in parallel ethanol dehydration reactions at 573 and 623 K are mechanistically and kinetically described by the same rate expression on different alumina materials (α-, γ-, and η-Al2O3), implying that alumina materials have similar surface sites under reaction environments. In situ chemical titration using pyridine as a titrant elucidates similar site densities (∼0.12 sites nm−2 and ∼0.07 sites nm−2 for ethene formation and ∼0.14 sites nm−2 and ∼0.09 sites nm−2 for diethyl ether formation on γ- and η-Al2O3, respectively) on γ- and η-Al2O3 indicating that similar surface features exist on both γ- and η-Al2O3. Pyridine-ethanol co-feed experiments show that pyridine inhibited the formation of ethene to a greater extent than diethyl ether suggesting that the two parallel dehydration reactions are not catalyzed by a common active site.