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Abstract: Selectivity control in heterogeneously catalyzed hydrogenation of conjugated hydrocarbons (ene-yne and diene compounds) is a challenging task. Available studies on the topic mainly encircle 1,3-butadiene as the substrate and palladium as the catalyst, while more elaborated playground molecules and other metals remain largely unexplored. This study investigates the gas-phase hydrogenation of valylene (2-methyl-1-butene-3-yne) and isoprene (2-methyl-1,3-butadiene) over Pd, Pb-poisoned Pd, CO-modified Pd, Cu, Ni, and bimetallic Custs. Chemoselectivity, regioselectivity, full hydrogenation, and Crmation/scission footprints of the catalytic systems at different inlet hydrogen-to-hydrocarbon ratios and conversion degrees have been rationalized. Complementary studies of 3-methylbutyne and 1-penten-4-yne hydrogenation were carried out in order to analyze (i) the impact of isomerization on the observed mono-olefin distribution in valylene/isoprene hydrogenation and (ii) the conjugation issue in partial ene-yne hydrogenation. Our results lead to an improved understanding of hydrogenation of polyunsaturated hydrocarbons and open doors to design more selective heterogeneous catalysts and related processes for this practically important class of reactions. [Copyright &y& Elsevier]

Abstract: We study the application of hierarchical ZSM-5 zeolites, combining micropores and intracrystalline mesopores, as carriers for lipase enzymes compared with purely microporous ZSM-5 and mesoporous MCM-41. Strategies to improve enzyme immobilization by modification of the support porosity and surface properties (e.g. by reaction with organosilane, or by treatment with the enzyme cross-linking agent glutaraldehyde) are also reported. Spectroscopic screening of catalyst activity and recyclability for the aqueous phase hydrolysis of p-nitrophenyl esters, permits evaluation of the influence of support properties and immobilization conditions on the performance of the resulting biocatalysts. An excellent correlation is observed between the mesopore surface area, the enzyme uptake, and the corresponding biocatalyst activity, demonstrating the functional character of mesopores in hierarchical zeolites. Modification of the mesopore walls prior to enzyme immobilization is essential to attain an active and recyclable biocatalyst. Enzymes immobilized on purely inorganic supports exhibit rapid loss of activity attributed to enzyme leaching. Despite the high mesopore surface area of surface-functionalized MCM-41, the mono-dimensionality of the mesopores results in restricted accessibility and a reduced enzyme uptake. In comparison, the interconnected mesopores of the hierarchical zeolites remain accessible after surface functionalization showing good adsorption properties. Lipase immobilized on thiol-functionalized mesoporous ZSM-5 was found to be the most efficient biocatalyst. [Copyright &y& Elsevier]

Abstract: The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary and secondary () interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N2O, and N2 are equivalent on the three noble metals. NO is a primary reaction product, while N2 and N2O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH3, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH3 conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of , both N2O and N2 production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage. [Copyright &y& Elsevier]

Abstract: Remarkable progress towards implementation of catalytic technology for N2O mitigation in nitric acid plants has been experienced in recent years. EnviNOx®, one of the commercial processes developed by Uhde, accomplishes the tail-gas abatement of N2O and NO x over a single reactor using iron-containing zeolite catalysts. Two variants of the process are available depending on the tail-gas temperature. This manuscript evaluates the low-temperature abatement process (<700K), where NO x and N2O are selectively reduced by NH3 and CH4, respectively. To this end, activity tests in mixtures with N2O, O2, NO, NH3, and CH4 at different temperatures and partial pressures of reactants were carried out over steam-activated FeZSM-5. Ammonia and nitric oxide strongly inhibit the selective catalytic reduction (SCR) of N2O by CH4, shifting the temperature for N2O conversion over 700K. Controlled ammonia dosing is required for the efficient operation of the low-temperature EnviNOx® variant over the particular iron zeolite used in this study. The optimal inlet NH3 concentration is determined by the NO concentration in the tail-gas and its temperature. In the absence of NO and NH3, the CH4-SCR of N2O over FeZSM-5 is effective above 623K. Still, this process cannot retrofit a number of existing plants with lower tail-gas temperatures. [Copyright &y& Elsevier]

Abstract: A temporal analysis of products (TAP) reactor was used to study the mechanism of ammonia oxidation at high temperature (1073 K) over Fe2O3, Cr2O3, and CeO2. The results were compared with those obtained over the industrially applied Pt95–Rh5 alloy. Analysis of characteristic times of the transient responses of N2 and NO made it possible to gain insight into the sequence of their formation. A common feature of both metal oxide and noble metal catalysts is that NO is a primary product of NH3 oxidation, whereas N2 results mainly from secondary transformations of NO. The amount of N2O formed over the oxides was minimal. Multipulse NH3 experiments in the absence of gas-phase O2 give unequivocal evidence of the participation of surface lattice oxygen of the metal oxides in the reaction of NH3 to NO. The degree of reduction of the oxide surface determines the product distribution but does not alter the catalytic activity for NH3 conversion. Greater reduction favors the reaction channel to N2 over that to NO. The desired reaction follows a Mars–van Krevelen-type scheme involving the participation of lattice oxygen in the NH3 conversion to NO and regeneration of the so-formed vacancies by gas-phase O2 and bulk lattice oxygen. Replenishment of surface vacancies by the diffusion of bulk oxygen occurs to a significant extent over Fe2O3. However, when NH3 and O2 are fed together, dissociative adsorption of gas-phase oxygen can be considered the main mechanism for vacancy regeneration. [Copyright &y& Elsevier]

Abstract: This manuscript concisely reviews the significance of the temporal analysis of products (TAP) technique in catalysis research from its introduction to the scientific community in the late 1980s. Evolutionary aspects of this time-resolved transient pulse method are presented, highlighting its relevance for elucidation of mechanistic and kinetic aspects of adsorption, diffusion, and reaction in gas–solid systems. The high-temperature ammonia oxidation over noble metal catalysts is used to underline key advantages of the TAP reactor in mechanistic studies, narrowing the often-encountered pressure and materials gaps between techniques operating at ambient pressure and in ultra-high vacuum. Perspectives to further enhance the capabilities of this technique are briefly put forward. [Copyright &y& Elsevier]

Mechanistic and kinetic aspects of the direct decomposition of N2O over steam-activated Fe-silicalite were investigated by transient experiments in vacuum (N2O peak pressure of ca. 10 Pa) using the temporal analysis of products (TAP) reactor in the temperature range of 773–848K. The transient responses of N2O, N2, and O2 obtained upon N2O decomposition were fitted to different micro-kinetic models. Through model discrimination it was concluded that both free iron sites and iron sites with adsorbed mono-atomic oxygen (*O) species are active for N2O decomposition. Oxygen formation occurs via decomposition of bi-atomic (*O2) oxygen species adsorbed over the iron site. This bi-atomic oxygen species originates from another bi-atomic oxygen species (O*O), which is initially formed via interaction of N2O with iron site possessing mono-atomic oxygen species (*O). Based on our modeling, the recombination of two mono-atomic oxygen (*O) species or direct O2 formation via reaction of N2O with *O can be excluded as potential reaction pathways yielding gas-phase O2. The simulation results predict that the overall rate of N2O decomposition is controlled by regeneration of free iron sites via a multi-step oxygen formation at least below 700K. [Copyright &y& Elsevier]

Abstract: European authorities consider that N2O emissions from nitric acid plants can be reduced to a large extent at a relatively low cost. Two regulation approaches can have major implications for the fertilizer industry in Europe. The EU integrated pollution prevention and control (IPPC) directive will be effective from October 2007 for existing plants. In this, emission permits will be based on best available techniques (BAT). Additionally, the EU commission will shortly consider whether the emission trading directive should be applicable to cover other greenhouse gases besides CO2. This paper discusses advantages and drawbacks of these approaches, with emphasis on how emission trading with N2O can be turned into a win–win situation both for European governments and for the fertilizer industry. [Copyright &y& Elsevier]

Abstract: Transient isotopic studies in the temporal analysis of products (TAP) reactor evidenced the importance of the lifetime of oxygen species generated upon N2O decomposition on extraframework iron sites of Fe-silicalite for methane oxidation at 723K. Fe-silicalite effectively activates CH4 when N2O and CH4 are pulsed together in the reactor. However, these oxygen species gradually become inactive for methane oxidation as the time delay between the N2O and CH4 pulses is increased from 0 to 2s. [Copyright &y& Elsevier]

Abstract: Various commercially available zeolite frameworks (MFI, FER, BEA, MOR, FAU) with iron impurities ranging from 85 to 780ppm and nominal Si/Al ratios of 10–15 have been tested in direct N2O decomposition after calcination and steam treatment. Our results show a significant impact of the framework type on the catalytic activity at these high degrees of iron dilution (Fe/Al<0.01). H-beta was the most effective material in the reaction, followed by the other pentasyl-type H-ferrierite and H-ZSM-5 zeolites. [Copyright &y& Elsevier]

Abstract: The high-temperature catalytic partial oxidation (CPO) of methane over a platinum gauze reactor was modeled by three-dimensional numerical simulations of the flow field coupled with heat transport as well as detailed gas-phase and surface reaction mechanisms. Model results agree well with data of CPO experiments over Pt-gauzes in the literature, confirming the presence of strong mass and heat-transport limitations. The conversions of CH4 and O2 increase with an increased contact time and were practically constant in the temperature range of 1000–1200K. The selectivity to CO linearly increases with temperature. H2 was only observed above 1200K, below this temperature H2O was the only hydrogen-containing product. The contribution of heterogeneous steps in the overall process is prominent, but in the later stages of the reactor, gas-phase reactions become significant at certain conditions of temperature, pressure, and residence time. For example, simulations predicted some gas-phase production of ethane and ethylene via methane oxidative coupling at elevated pressure and residence time. The study shows that today''s CFD tools allow the implementation of detailed homogeneous and heterogeneous reaction schemes even for complex catalyst geometries. [Copyright &y& Elsevier]

Abstract: Transient isotopic studies in the Temporal Analysis of Products (TAP) reactor evidenced a clear mechanistic regularity between methane and propane for N2O reduction over Fe-silicalite in the temperature range of 623–773K. The lifetime of the oxygen species generated by N2O decomposition on extraframework iron sites was found to be crucial for alkane oxidation. When N2O and CH4 or C3H8 were pulsed together over Fe-silicalite, oxygen species deposited on iron sites effectively activated the hydrocarbon. However, these oxygen species become inactive for alkane oxidation if the pulses of N2O and the alkane were separated by more than 0.1s. Transient studies have indicated that C3H8 is more efficient for N2O reduction than CH4, as expected from the facilitated activation of the Con increasing the carbon number in alkanes. The performance derived from the TAP reactor agrees well with experiments under flow conditions at ambient pressure. [Copyright &y& Elsevier]

Abstract: This paper reports the application of mixed conducting perovskite membranes for simultaneous in situ O2 separation and catalytic oxidation of ammonia to nitric oxide. Calcium and strontium-substituted lanthanum ferrite perovskites (La1−x A x FeO3−δ , with A=Ca, Sr and x =0.1–0.2) were prepared by a conventional wet complexation route with citric acid. Dense membrane disks were obtained by uniaxial pressing of the calcined powders followed by sintering. The oxidation of ammonia over these catalytic membranes in the temperature range of 1000–1333K leads to NO selectivities up to 98% with no N2O production, showing stable performance during several days on stream. This novel process demonstrates the great potential for intensification of nitric acid manufacture into miniaturized plants and is considered to be particularly attractive for on-site production of this important bulk chemical. [Copyright &y& Elsevier]

Abstract: The effect of oxygen on the activity and selectivity of the NO reduction by NH3 over Pt–Rh (95–5wt.%) alloy gauze at 1023–1073K has been investigated. To this end, the Temporal Analysis of Products (TAP) reactor in combination with isotopic tracers was applied. Single pulse experiments evidenced the rapid activation of NH3 over the catalyst surface covered by adsorbed oxygen. Contrarily, NO requires an essentially reduced surface in order to be dissociated. Adsorbed oxygen species in the O2-pretreated gauze accelerate the reaction of NH3 with NO with respect to the as-received gauze. N2 was the main reaction product and traces of N2O were comparatively formed (N2O/N2 ∼ 10−3). Pulsing of an equimolar O2–15NH3–NO mixture over the Pt–Rh gauze mainly produces 15NO, while the formation of N2 is largely suppressed. The selectivity to 15NO and N2O in the ternary O2–15NH3–NO system diminished upon decreasing the O2/(15NH3 +NO) ratio, in favor of N2. This ratio was qualitatively varied by changing the time delay between O2 and 15NH3–NO in sequential pulse experiments. Our results indicate that the NH3 oxidation by O2 to NO is much faster than the NO reduction by NH3 at similar concentrations of oxygen and nitric oxide. This explains the low production of N2 and N2O in ammonia burners within nitric acid manufacture. [Copyright &y& Elsevier]

Abstract: The mechanism of the reaction of N2O and 13CO over Fe-silicalite was investigated with the use of the temporal analysis of products (TAP) reactor and compared with that of the reaction of N2O and C3H8 previously reported (Appl. Catal. A 267 (2004) 181). Upon direct N2O decomposition at 523–573 K, Fe-silicalite stored ca. 1018 atoms of oxygen per gram, with a ratio of 1 O atom per each 30–60 Fe atoms in the sample. Only a small fraction of the deposited oxygen was reactive for CO oxidation. Pump-probe experiments at different time delays (0–2 s) between the pulses of nitrous oxide and the reducing agent indicated the markedly different mechanisms of the N2O–13CO and N2O–C3H8 reactions in the temperature range of 623–673 K. Fe-silicalite is active for propane oxidation in the presence of short-lived oxygen species, that are produced when N2O and C3H8 are pulsed simultaneously. Time delays between the N2O and C3H8 pulses greater than 0.1 s are sufficient to transform these active oxygen species for hydrocarbon conversion into inactive ones. In contrast, the oxidation of CO by N2O does not depend on the lifetime of the oxygen species in the range of time delays investigated. The mechanisms of the N2O-mediated 13CO and C3H8 oxidations differ as a consequence of the different interactions of the two reducing agents with iron species in the zeolite. Pulse experiments support the occurrence of the scavenging mechanism with both propane and carbon monoxide. In this mechanism, short-lived oxygen deposited by N2O is efficiently eliminated by the reductant. Distinctive to propane, the strikingly high affinity of carbon monoxide for isolated Fe3+ ions in the zeolite gives rise to an additional pathway for N2O reduction in the presence of chemisorbed CO species. These particular Fe3+–CO species were identified by in situ UV/vis and EPR spectroscopies (J. Catal. 223 (2004) 13). [Copyright &y& Elsevier]

Abstract: The performance of differently prepared Fe-ZSM-5 zeolites in the N2O-mediated propane oxidative dehydrogenation to propylene has been investigated at 723K using a tapered element oscillating microbalance (TEOM) coupled to on-line GC analysis. Catalysts were prepared by hydrothermal synthesis followed by calcination and steam treatment, liquid-ion exchange, and chemical vapor deposition, and contain molar Fe/Al ratios in the range of 0.26–1. Propylene yields in the range of 22–25% were attained over steam-activated Fe-ZSM-5, much higher over the iron zeolites prepared by post-synthesis methods (9–16%), with propylene selectivities around 40%. The deactivation behaviour due to coking differs among the catalysts investigated. Calcined H-ZSM-5 with impurities of iron (Fe/Al=0.007) leads to an initial yield of propylene of 8%, and is increased to 16% upon steam treatment, obtaining a selectivity to propylene of 90%. The turnover frequency for propylene production is up to three orders of magnitude higher in steamed H-ZSM-5 than in the synthesized iron zeolites. Our results suggest that traces of well-isolated (mononuclear) Fe species released from framework positions by steaming are extremely active in the oxidative dehydrogenation of propane to propylene, while large iron clusters enhance deep oxidation of important reaction intermediates to COx. [Copyright &y& Elsevier]

The relation between the intrinsic mechanism of various N2O conversions over FeMFI catalysts and the nature of the active iron site(s) has been analyzed. To this end, direct N2O decomposition and N2O reduction with CO in the absence or presence of NO were investigated using a combination of transient pulse and steady-state techniques over steam-activated FeMFI zeolites with a similar iron content (0.6–0.7 wt% Fe) and different framework compositions (Si–Al, Si–Ga, Si–Ge, and Si). The forms of iron in the catalysts were characterized by UV/vis and HRTEM. The intrinsic reaction mechanism determines the optimal iron site distribution, which can be modulated by tuning the steaming temperature during activation. Oligonuclear iron oxo clusters in the zeolite channels are essential in direct N2O decomposition due to a faster desorption of O2 as compared to isolated ions. Such forms of active iron can be achieved at a lower steam-activation temperature over FeAlMFI and FeGaMFI (900 K) than over FeGeMFI and FeMFI (1150 K). Contrarily, zeolites with a more uniform distribution of isolated iron species lead to higher activities in N2O reduction with CO as compared to highly clustered catalysts. In this case, O-removal as CO2 is strongly accelerated vis-à-vis O2 desorption in direct N2O decomposition. The dual role of NO as a promotor in N2O decomposition and as an inhibitor in N2O reduction also supports the participation of different sites in both types of conversions. NO selectively inhibits N2O reduction over isolated iron ions, further evidencing the essential role of oligonuclear iron clusters in the NO-assisted N2O decomposition. [Copyright &y& Elsevier]

An increasing number of contributions aiming at the development of advanced or modified porous materials for improved performance in various applications incorporate unreliable analyses of N2 adsorption data. As a result, well-defined peaks at 2 and 4 nm in the pore size distribution are attributed to physical mesopores, while being strictly associated with the nature of the adsorbed phase. These erroneous claims often comprise the main conclusion of related publications and should be firmly prevented. A better fundamental insight on the adsorption methods and related physical phenomena is required. [Copyright &y& Elsevier]

The effect of the iron distribution in steam-activated FeZSM-5 and Fe-silicalite on their catalytic performance for direct N2O decomposition and for the oxidative dehydrogenation of propane (ODHP) with N2O and O2 was investigated by means of steady-state and transient pulse experiments. Steam-activated FeZSM-5, containing a large fraction of iron as oligonuclear FexOy clusters, is more active and selective in ODHP and direct N2O decomposition than steam-activated Fe-silicalite, where isolated iron species predominate. This suggests that the same type of iron centers is involved in both conversions. The higher activity of steam-activated FeZSM-5 is associated to the higher mobility of atomic oxygen species attached to oligonuclear iron species, which lead to a faster oxygen desorption in direct N2O decomposition as well as to a faster transfer to propane in the ODHP reaction. Steam-activated FeMFI zeolites were found to be highly effective catalysts for ODHP with N2O, with initial propene yields up to 24%. Relatively high yields of propionaldehyde are also obtained, which mainly originates from the selective oxidation of propene. The unique ODHP performance of iron zeolites requires the use of N2O as monooxygen donor. Such species cannot be created by O2. Only a fraction of oxygen species deposited over iron sites of FeMFI upon N2O activation was found to be active towards propane oxidation under transient vacuum conditions. This is explained by a fast transformation of short-living highly reactive atomic oxygen species into less reactive ones. [Copyright &y& Elsevier]

Steam-activated Fe-zeolites (MFI and BEA) are efficient catalysts for the N2O-mediated oxidative dehydrogenation of propane, with initial propylene yields up to 25% at 723 K, but deactivate as a result of coke formation. A tapered element oscillating microbalance (TEOM) coupled to GC analysis has been applied to correlate activity and deactivation by simultaneous measurements of reaction and coking. The presence of extraframework iron species, generated during steam treatment, is essential for producing active catalysts. The nature and distribution of extraframework iron species, the zeolite host, and the catalyst acidity have no significant effect on the initial propene yield, but strongly influence the deactivation behavior. FeZSM-5 shows a remarkable resistance against deactivation, while a very rapid and complete deactivation occurred in FeBEA. The initial propene yield over FeZSM-5 and FeBEA was remarkably higher than over commercial H-ZSM-5 and H-BEA (with 30–40 times lower iron content), although the residual activities after 400 min on stream were very similar. [Copyright &y& Elsevier]

The reduction of N2O by CO was investigated over FeMFI zeolites prepared by different methods including sublimation, liquid ion exchange, and hydrothermal synthesis followed by steam activation. This leads to catalysts with different nature and distribution of iron species, as characterized by HRTEM, UV/vis, and EPR techniques. A common denominator in the FeZSM-5 samples is the heterogeneous iron constitution, with a significant degree of clustering in the form of iron oxide particles. Iron clustering was suppressed in steam-activated Fe-silicalite, presenting a remarkable uniform distribution of isolated iron species. In the presence of CO, the conversion of N2O over the catalysts is strongly accelerated with respect to direct N2O decomposition. The reaction rate increases linearly with the molar CO/N2O feed ratio and strongly depends on the preparation method applied. A correlation was found between the fraction of isolated Fe(III) species in the as-prepared catalysts and the activity for N2O reduction with CO. Steam-activated Fe-silicalite, containing mostly isolated iron ions in extraframework positions, shows the highest activity per mole of iron, while the highly clustered liquid-ion-exchanged catalyst presents the lowest activity. In situ UV/vis and EPR studies evidence the participation of mononuclear iron ions in the N2O–CO reaction, and also support the involvement of oligonuclear FexOy species. The reaction mechanism is iron site dependent. Over isolated sites, the reduction of N2O with CO occurs via coordinated CO species on Fe3+ ions. The reaction over oligonuclear sites proceeds via a redox Fe3+/Fe2+ process with intermediate formation of O− radicals. [Copyright &y& Elsevier]

Selective catalytic reduction (SCR) of N2O with C3H8 over Fe-zeolites (MFI, BEA, USY) under excess oxygen is strongly inhibited by NO. The assistance of the hydrocarbon in N2O reduction vanishes at high partial NO pressures, approaching the activity of the N2O+NO system at molar NO/C3H8 ratios of 1.5–2, with N2O/C3H8=1. This effect is opposite to the NO promotion in direct N2O decomposition over these catalysts at temperatures >625 K. At lower temperatures and in the absence of NO, propane is oxydehydrogenated to propene by N2O over the catalysts. Extraframework iron sites are essential for N2O and C3H8 conversions, in particular iron in isolated positions is most effective. Propane activates on N2O-oxidized iron sites, while acidic sites play a minor role in the overall process. In the absence of NO, N2O and C3H8 conversions are coupled in temperature. Addition of NO uncouples the N2O and C3H8 conversion profiles and shifts them to higher temperature. NO inhibits N2O decomposition over iron sites by competitive adsorption at temperatures where N2O reduction with C3H8 typically starts (>525 K). At these temperatures, adsorbed NO blocks the isolated Fe-sites for N2O activation. The relatively low activity for NOx reduction to N2 (<20%) and the lower optimal operation temperature for this process (550 K) compared to N2O reduction (725 K) makes the simultaneous removal of NO and N2O with hydrocarbons unfeasible over Fe-zeolites. [Copyright &y& Elsevier]

Preparation of steam-activated Fe-silicalite containing mainly isolated iron species in extraframework positions was essential for deriving structure-activity relationships in various N2O conversion reactions over iron zeolite catalysts. Characterization by UV–Vis/DRS revealed that any significant clustering of iron did not occur in this sample. Other steam-activated FeMFI zeolites, with different framework compositions or treated at higher temperatures, showed various degrees of clustering. The activity of the cluster-free Fe-silicalite was significantly higher in N2O reduction with C3H8 and CO. However, some level of association of iron species leads to higher activities in direct N2O decomposition. Due to the intrinsic reaction mechanism, this result demonstrates the sensitivity of reactions for the form of the iron species in Fe-zeolites, rather than the existence of a unique active site. [Copyright &y& Elsevier]

Selective catalytic reduction (SCR) of N2O with C3H8 over FeZSM-5 under excess oxygen is strongly inhibited by NO. The assistance of the hydrocarbon in N2O reduction vanishes at high partial NO pressures, approaching the activity of the N2O + NO system at molar NO/N2O ratios of 1.5–2, with N2O/C3H8=1. This effect differs from the NO promotion in direct N2O decomposition over Fe-zeolite catalysts. The negative effect of NO on the N2O reduction has a significant impact in the design and operation of catalytic reactors in tail-gases of nitric acid plants and other sources where NO comes along with N2O. [Copyright &y& Elsevier]

Recently discovered phosphate-derived Ni catalysts have opened a new pathway towards multicarbon products via CO2 electroreduction. However, understanding the influence of basic parameters such as electrode potential, pH, and buffer capacity is needed for optimized C3+ product formation. To this end, rigorous catalyst evaluation and sensitive analytical tools are required to identify potential new products and minimize increasing quantification errors linked to long-chain carbon compounds. Herein, we contribute to enhance testing accuracy by presenting sensitive 1H NMR spectroscopy protocols for liquid product assessment featuring optimized water suppression and reduced experiment time. When combined with an automated NMR data processing routine, samples containing up to 12 products can be quantified within 15 min with low quantification limits equivalent to Faradaic efficiencies of 0.1%. These developments disclosed performance trends in carbon product formation and the detection of four hitherto unreported compounds: acetate, ethylene glycol, hydroxyacetone, and i-propanol. Phosphate-derived nickel catalysts enable access to multicarbon products via CO2 electroreduction, but it remains unclear how operating conditions influence product formation. Here, refined 1H NMR spectroscopy protocols are introduced and combined with an automated NMR data processing routine, enabling quantification of carbon product formation as a function of various parameters. [ABSTRACT FROM AUTHOR]

Thermocatalytic CO 2 hydrogenation to methanol is an attractive defossilization technology to combat climate change while producing a valuable platform chemical and energy carrier. However, predicting the performance of catalytic systems for this process remains a challenge. Herein, we present a machine learning framework to predict catalyst performance from experimental descriptors. A database of Cu-, Pd-, In 2 O 3 -, and ZnO-ZrO 2 -based catalysts with 1425 datapoints is compiled from literature and subjected to data mining. Accurate ensemble-tree models (R 2 > 0.85) are developed to predict the methanol space-time yield (STY) from 12 descriptors, where the significance of space velocity, pressure, and metal content is revealed. The model prediction and its insights are experimentally validated, with a root mean squared error of 0.11 g MeOH h−1 g cat −1 between the actual and predicted methanol STY. The framework is purely data-driven, interpretable, cross-deployable to other catalytic processes, and serves as an invaluable tool for guided experiments and optimization. [Display omitted] • Machine learning framework for CO 2 hydrogenation to methanol devised. • Database for Cu-, Pd-, In 2 O 3 - and ZnO-ZrO 2 -based catalysts compiled. • Generalized model to predict methanol space-time yield (STY) developed. • Efficacy and fidelity of the model experimentally validated. • Model enhancements to aid the discovery of novel catalysts required. [ABSTRACT FROM AUTHOR]

This study assesses the impact of feed impurities on the design of a robust catalytic process for chlorine recycling via HCl oxidation (Deacon reaction). The influence of CO, SO 2 , and COS was investigated over stable catalysts, including RuO 2 /SnO 2 -Al 2 O 3 , CeO 2 /ZrO 2 , U 3 O 8 /ZrO 2 , IrO 2 /TiO 2 , and CuCrO 2 . Carbon monoxide is detrimental for RuO 2 - and IrO 2 -based catalysts, since the highly exothermic oxidation to CO 2 under Deacon conditions causes pronounced temperature rises leading to material and reactor instabilities. Advantageously, the other catalysts are practically unaffected by this impurity due to their inactivity for CO oxidation in the chlorinated state. Carbonyl sulfide and particularly sulfur dioxide severely poison all the catalysts investigated due to the strong adsorption of the sulfur compounds and active site blockage. However, the initial activity can be restored by treating the deactivated catalysts in HCl (without oxygen) at the reaction temperature. We discuss process strategies for feed purification and catalyst regeneration depending on the chosen catalytic system, which are of utmost important for the ultimate sustainability of the process. [ABSTRACT FROM AUTHOR]

A promising route to upgrade crude bio-oil prior to hydrotreating is via the catalytic esterification of intrinsic acids and alcohols. This process reduces the acidity and oxygen content of the bio-oil, thereby improving the stability and decreasing the hydrogen consumption in subsequent refining steps. Here, the applicability of microporous zeolite catalysts of different framework type and composition is explored in the liquid-phase esterification between the characteristic bio-oil constituents, acetic acid and o-cresol. Ester formation, which is significantly influenced by concurrent coke-forming reactions, is dependent on the reaction temperature and on the size of the zeolite micropores, with maximum yields observed over large-pore beta and faujasite catalysts. The benefits of introducing secondary mesoporosity through demetallation in alkaline media are remarkable for medium-pore ZSM-5 due to the alleviated diffusion constraints, while the enhanced performance of hierarchical faujasite is less pronounced, but still appreciable. Characterization of the spent catalysts by XRD and N2 sorption evidences the preserved crystallinity and microporosity of the zeolites, while IR spectroscopy of adsorbed pyridine reveals a small reduction in the concentration of Brønsted acid sites. The latter is detrimental for the ester formation upon reuse of ZSM-5 zeolites, while faujasite catalysts preserve their initial activity. These findings highlight both the potential of microporous zeolites and their hierarchical analogs for acid-catalyzed condensation reactions in the field of bio-oil upgrading and some of the possible challenges which could be faced. [ABSTRACT FROM AUTHOR]

[Display omitted] • Impregnated ZnO/ZrO 2 catalysts rival state-of-the-art benchmarks in CO 2 hydrogenation. • Stable methanol productivity > 0.7 g h−1 g cat −1 attained in CO 2 and CO 2 + CO feeds. • Tuning content to 5 mol% Zn on high-surface m -ZrO 2 support maximized Zn dispersion. • Facile vacancy generation and retention of isolated zinc sites during reaction. • Successful translation of performance and architecture from powder to technical form. While impregnation approaches are attractive, scalable routes for preparing supported oxide catalysts, achieving competitive methanol productivities over impregnated ZnO/ZrO 2 remains challenging as they underperform state-of-the art systems with isolated zinc sites. Herein, by targeting the optimal active site structure, ZnO/ZrO 2 systems prepared by impregnation achieve stable methanol space–time yields of 0.73 g MeOH h−1 g cat −1 during CO 2 and hybrid CO-CO 2 hydrogenation at suitable conditions. Notably, controlling the catalyst formulation using 5 mol% Zn and a high surface area m -ZrO 2 support fosters high zinc dispersion. In situ electron paramagnetic resonance and operando X-ray spectroscopy studies affirm the retention of isolated zinc species and facile generation of associated oxygen vacancies during the reaction. Analysis of pore structure and composition within the shaped bodies evidences abundant mesopores and a uniform zinc distribution, ensuring similar performance when translating from powder to technical forms. This work bridges fundamental understanding with practical demonstration of ZnO/ZrO 2 systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Prins cyclisation from isovaleraldehyde and isoprenol to tetrahydropyranol. • Zeolites with varying acidity and hierarchical materials as catalysts. • H-Beta-300 zeolite giving 72% selectivity to the desired product at 99% conversion. • The cis/trans ratio of tetrahydropyranols depends on porous structure. • Parallel formation of tetrahydropyranol and dihydropyran confirmed by DFT. In this work, several heterogeneous micro- and mesoporous, acidic catalysts were tested for the selective synthesis of Florol®, an industrial product formed via condensation and rehydration starting from isoprenol and isovaleraldehyde in dimethylcarbonate as a solvent. The results showed that a mildly acidic, microporous H-Beta-300 with SiO 2 /Al 2 O 3 ratio of 300 was the best catalyst, giving 72% selectivity with 99% conversion at 40 °C when using the molar ratio isoprenol to isovaleraldehyde of 1:5. More acidic zeolites gave slightly lower selectivity, while the lowest selectivity (up to 52%) was obtained with mesoporous catalysts exhibiting mild acidity and no strong Brønsted acid sites. Selectivity to pyranols was nearly constant when changing temperature, while a larger excess of isovaleraldehyde promoted formation of tetrahydropyranol. H-Beta-300 catalyst was successfully after calcination at 400 °C. DFT calculations pointed out on the parallel formation of tetrahydro pyranols and dihydropyrans, which are the corresponding dehydration products. [ABSTRACT FROM AUTHOR]

[Display omitted] • Metal nanostructures on nitrogen-doped carbon studied in CH 2 X 2 hydrodehalogenation. • Nanoparticle-based systems provide the highest catalytic activity and stability. • Selectivity to the target CH 3 X depends on the active phase size and halogen type. • Ir single atoms compete with the best reported CH 2 Cl 2 hydrodechlorination catalysts. • Adsorption and dissociation energies of CH 2 X 2 and H 2 govern catalytic performance. Nanostructuring metal catalysts has been demonstrated as an attractive strategy to enable selective hydrodehalogenation of CH 2 X 2 (X = Cl, Br) to CH 3 X, but active phase size effects of promising metals and the role of the halogen are still poorly understood. Herein, the impact of these parameters on performance (activity, selectivity, and stability) is systematically assessed by employing a platform of N–doped carbon–supported metal nanostructures (Ir, Pt, Ru, and Ni), ranging from single atoms (SA) with defined coordination environment to nanoparticles (NP) of ca. 3.0 nm. Catalytic tests reveal that when compared to single atoms, highest reaction rates are attained over NP–based systems, which also exhibit improved stability ranking as Ir ≈ Pt > Ru ≫ Ni, independent of the halogen. The product distribution was markedly affected by the nanostructure and speciation of the active center as well as the dihalomethane type. Specifically, CH 3 Cl is the main reaction product over SA in hydrodechlorination, achieving an exceptional selectivity over Ir (up to 95%). In contrast, NP mainly generated CH 4 or coke. Comparable patterns were observed in hydrodebromination, except over Ru, which exhibited an inverse structure–selectivity trend. Density Functional Theory simulations shed light on the speciation of the active phase and identified the adsorption and dissociation energies of CH 2 X 2 and H 2 as descriptors for catalytic reactivity. These findings elucidate hydrodehalogenation performance patterns, highlighting the impact of nanostructuring and the halogen type to advance future catalyst design. [ABSTRACT FROM AUTHOR]

Abstract: Microporous and mesoporous (hierarchical) ZSM-5 samples, prepared by desilication, dealumination and templating with carbon nanoparticles have been characterized by adsorbing benzene, cyclohexane and 1,3,5-trimethylbenzene (mesitylene) to probe the location, the strength and the accessibility of Brønsted acid sites. The mesoporosity introduced by the methods explored resulted in a statistical accommodation of benzene at the channel intersections. Cyclohexane showed the highest strength of physisorption in micro- and mesopores and, thus, probes the largest fractions of acid sites. Mesitylene adsorption showed that mesopores of hierarchical zeolites do not contain Brønsted acid sites. All results indicated that the mesoporosity influences only the accessibility of the acid sites in the ZSM-5 crystals mostly via shortening diffusion pathways, but not the strength of the interaction with the cyclic hydrocarbons. The enhanced transport rates observed for benzene are associated to the decreased pore length inside the mesoporous particles, rather than to changes of the pore structure or the generation of additional sites. [Copyright &y& Elsevier]

Abstract: The gas-phase hydrogenation of ethyne over Pd/Al2O3 is studied using the Temporal Analysis of Products (TAP) reactor, leading to valuable insights into the reaction mechanism. The exceptional time resolution of this transient technique enables to capture the hydrogenation of the carbon–carbon triple bond via the sequential addition of hydrogen. Hydrogen/deuterium exchange experiments demonstrate the dissociative and reversible adsorption of H2 on palladium. Conversion and selectivity change at early stages of the reaction as a consequence of the formation of a stable carbide layer. The alkene/alkane ratio was markedly higher in TAP compared to reported flow data, due to the mbar pressure operation of the technique. In this regime, the formation of subsurface hydrogen is unfavorable, enabling the assessment of the sole effect of reactants coverage on the product distribution, that is, in the absence of unselective hydride phases. Mechanistic aspects related to the use of carbon monoxide as a selectivity enhancer are also discussed. [Copyright &y& Elsevier]

Abstract: Alkyne hydrogenation, a widely used process in industry to purify olefin streams, comprises a prototype reaction to understand selectivity in heterogeneously catalyzed reactions. The selectivity of the reaction on palladium catalysts to the alkene, alkane, or oligomers strongly depends on the state of the (sub)surface; i.e., the occurrence of complex carbide/hydride phases. In practice, hydrogenation reactors in C2 and C3 cuts of steam crackers require continuous CO feeding in order to enhance the alkene selectivity of palladium-supported catalysts. In the present work, we have studied the impact of carbon monoxide on the formation of carbide and hydride phases as a standpoint to derive structure–performance relationships under realistic process conditions. For this purpose, catalytic tests on a standard 1wt.% Pd/Al2O3 and Density Functional Theory on Pd(111) were combined. The influence of: (i) the alkyne (ethyne and propyne), (ii) the hydrogen:alkyne ratio (1–10), (iii) the carbon monoxide:hydrogen ratio (0–0.2), and (iv) the catalyst pretreatment on the product distribution was assessed in a continuous flow fixed-bed reactor at ambient pressure. In absence of CO, subtle changes in the hydrogen:alkyne ratio generate undesired products. Carbon monoxide enables the external control of the catalyst state by suppressing the formation of subsurface hydride and carbide phases, thereby stabilizing a high alkene yield in a broad range of feed hydrogen:alkyne ratios. This scenario contrasts with the more fragile regime of the hydride–carbide phases under CO-free conditions. DFT calculations obtained a single Brønsted–Evans–Polanyi relationship independently of the state of the catalyst (carbide, hydride, CO-covered) and the alkyne–alkene–alkane set (C2, C3). [Copyright &y& Elsevier]

Abstract: The partial hydrogenation of propyne was studied over copper-based catalysts derived from Cu–Al hydrotalcite and malachite precursors and compared with supported systems (Cu/Al2O3 and Cu/SiO2). The as-synthesized samples and the materials derived from calcination and reduction were characterized by XRF, XRD, TGA, TEM, N2 adsorption, H2-TPR, XPS, and N2O pulse chemisorption. Catalytic tests were carried out in a continuous flow-reactor at ambient pressure and 423–523K using H2:C3H4 ratios of 1–12 and were complemented by operando DRIFTS experiments. The propyne conversion and propene selectivity correlated with the copper dispersion, which varied with the type of precursor or support and the calcination and reduction temperatures. The highest exposed copper surface was attained on hydrotalcite-derived catalysts, which displayed C3H6 selectivity up to 80% at full C3H4 conversion and stable performance in long-run tests at T ⩾473K. Both activated Cu–Al hydrotalcites (this work) and Ni–Al hydrotalcites [S. Abelló, D. Verboekend, B. Bridier, J. Pérez-Ramírez, J. Catal. 259 (2008) 85] exhibited a relatively high alkene selectivity under optimal operation conditions, but they present a markedly distinctive catalytic behavior with respect to temperature and hydrogen-to-alkyne ratio. The product distribution was assigned through Density Functional Theory (DFT) simulations to the different stability of subsurface phases (carbides, hydrides) and the energies and barriers for the competing reaction mechanisms. The behavior of Cu in partial alkyne hydrogenation resembles that of Au nanoparticles, while Ni is closer to Pd. [Copyright &y& Elsevier]

Abstract: The transesterification of ethylene carbonate with methanol to dimethyl carbonate, an alternative phosgene-free route to DMC, was investigated over aluminas derived from calcination at different temperatures of Na- and NH4-dawsonites. The influence of the monovalent cation (NH4 +, Na+) on the thermal decomposition of the starting dawsonites and derived crystallinity, morphology and porosity of the resulting aluminates was studied. The physico-chemical properties of the dawsonite precursors and the derived oxides were characterized by ICP, in situ XRD, TGA-MS, TEM, N2 adsorption, FTIR, and 27Al MAS-NMR. Phase transitions during thermal decomposition are dependent on the monovalent cation in the original dawsonite, i.e. the NH4-counterpart is transformed into finely dispersed and highly amorphous aluminas with well-developed porosity in the range 473–1073K, while Na-dawsonite exhibits an intermediate amorphous sodium-containing alumina phase at 523–773K, which crystallizes into sodium aluminate at 773–1073K. Tests were carried out in a parallel reactor system at 298–343K, methanol-to-ethylene carbonate ratios of 2–12, and 0.1–10wt.% catalyst amount. Tailoring the catalyst activation conditions is essential to optimize the transesterification performance, and consequently the production of DMC, a valuable environment-friendly chemical for many potential applications. Any of the oxides derived from NH4-dawsonite were in active in the reaction. The basicity of the Na-containing oxides, attained by calcination of Na-dawsonite at 973K, is required to obtain an active catalyst, which rendered a maximum DMC yield ca. 65%. Recycling experiments demonstrated that activated dawsonites can be successfully reused. [Copyright &y& Elsevier]

Abstract: Hierarchical zeolites combining micro- and mesoporosity were prepared by desilication of ZSM-5 (Si/Al=42) in aqueous solutions of tetraalkylammonium hydroxides (TPAOH, TBAOH). Similarities between the treatment mediated by organic hydroxides and the conventional NaOH leaching comprised (i) the requirement to work with a starting zeolite in the optimal Si/Al window of 25–50 and (ii) the minor influence of the counter-cation (H+, NH4 +, Na+) on the degree of mesoporosity developed. However, both desilication media presented distinctive kinetic and mechanistic characteristics. The use of organic hydroxides directly produced the protonic form of the mesoporous zeolite upon calcination, simplifying the final ion exchange with NH4NO3 characteristic of the NaOH treatment. Silicon dissolution in TPAOH (or TBAOH) was much slower than in NaOH, making the demetallation process highly controllable. The treatment in the organic base was less selective for silicon extraction, i.e. a higher amount of aluminum was leached to the solution compared to the NaOH treatment. The differences in porosity development and chemical composition of the final solid can be assigned to the effect of the bulky tetraalkylammonium cation. The optimal hierarchical ZSM-5 zeolite (treated in 1M TPAOH at 338K and 8h) had a mesopore surface area of 160m2 g−1 and the intrinsic zeolite properties were largely preserved. This sample displayed improved catalytic performance in the liquid-phase benzene alkylation with ethylene compared to the parent and NaOH-treated zeolites. [Copyright &y& Elsevier]

Abstract: Desilication of the medium-pore ferrierite zeolite in sodium hydroxide solutions was conducted to modify the porous structure and to create structural defects, resulting in enhanced accessibility and ultimately improved catalytic performance. Commercial H-ferrierite (Si/Al=29) with the characteristic plate-like morphology was used as starting material. The attack by NaOH induces deaggregation, exfoliation, fracture, and ultimately perforation of the ferrierite crystals, resulting in mesoporosity of combined inter- and intracrystalline nature. The parent and treated samples were characterized by ICP-OES, XRD, N2 adsorption, SEM, TEM, 27Al and 29Si MAS-NMR, FTIR, and NH3-TPD. Optimization of the treatment conditions (NaOH concentration, temperature, and time) is required to introduce substantial mesoporosity without significantly altering the micropore structure due to excessive Si leaching. Compared to other frameworks (e.g. MFI, MTW, MOR, and BEA), FER requires harsher conditions to extract silicon leading to mesoporosity. Under optimal conditions, the mesopore surface area of the NaOH-treated ferrierite increased by a factor of 3–4 with respect to the parent zeolite, while mostly preserving the native crystallinity and acidity. The ability of NaOH to induce porosity changes in FER largely exceeds conventional dealumination post-treatments. The benefit of the introduced porosity was demonstrated in the catalytic pyrolysis of low-density polyethylene. [Copyright &y& Elsevier]

Abstract: Several catalysts, including FeZSM-5, Co2AlO4, LaCoO3, and BaFeAl11O19, were evaluated for N2O decomposition under representative flue-gas conditions in fluidized-bed combustion (FBC). Closely related formulations proved active and stable catalysts for process-gas or tail-gas de-N2O in nitric acid plants. With this as starting point, their potential suitability for N2O abatement in stationary combustion was assessed. Tests were carried out in a fixed-bed reactor at ambient pressure and in the temperature range of 473–1123K using mixtures of N2O, O2, NO, CO, SO2, and H2O. The mixed oxide catalysts were strongly inhibited by water and sulfur dioxide and experienced fast deactivation in the simulated mixture containing all the gases. Bulk sulfate phases were detected by X-ray diffraction in the used perovskite and hexaaluminate, revealing insufficient chemical stability in the presence of sulfur and discouraging installation in the freeboard of the combustor. In great contrast, the activity of steam-activated FeZSM-5 in the model and simulated mixtures was comparable, rendering very stable performance during 30h on stream. The unique tolerance of this iron zeolite to the complex combination of feed components makes it prone to implementation after the cyclone of FBCs, where temperatures are typically 800–1100K. [Copyright &y& Elsevier]

Carbon templating was used to synthesize high-surface area LaFeAl11O19 hexaaluminates leading to improved catalytic performance. The oxide precursor–template composites were prepared by impregnation, complexation with citric acid, and coprecipitation routes varying the amounts of acetylene black incorporated. Calcination in static air at 1473 K led to single-phased hexaaluminates. The samples were characterized at different stages of the synthetic protocol by ICP–OES, in situ and ex situ XRD, TEM, N2 adsorption, He pycnometry, TGA, H2-TPR, and XPS. The templating effect of carbon originates fine oxide particles. Consequently, the specific surface area of the non-templated hexaaluminates (1–11 m2 g−1 depending on the preparation method) increased up to a factor of 25 upon incorporation of carbon in the synthesis. The stabilization of small particles occurs despite the temperature difference between the elimination of carbon (973 K) and the attainment of the hexaaluminate phase above 1373 K. The morphology of the oxide obtained upon carbon removal determines the textural properties of the hexaaluminate particles. Our results demonstrate that the confined space synthesis is not exclusively responsible for small oxide particles. The stabilization of small crystallites on the carbon surface in the composite and breakage of the oxide layer caused by template combustion are key aspects for attaining finely dispersed particles when the carbon is macroporous. The enhanced catalytic activity and time-on-stream stability of the templated hexaaluminates was demonstrated in the direct N2O decomposition using model and simulated feed mixtures. In this application, a quasi-linear relation between the reaction rate and the specific surface area of the samples was obtained. Characterization by H2-TRP and XPS indicated that the nature of iron in LaFeAl11O19 was not altered by carbon incorporation in the synthesis. [Copyright &y& Elsevier]

Abstract: A number of studies have shown that solid bases, among others activated hydrotalcites, are highly efficient catalysts for C–C bond formation reactions. A widely studied case is the aldol condensation of citral and acetone, where rehydrated Mg-Al hydrotalcite shows higher yields to pseudoionone compared to NaOH solutions. Despite this fact, the fine chemical industry still operates with the traditional liquid inorganic bases. This manuscript addresses technical aspects that can explain the limited implementation of activated hydrotalcites in aldol condensations. For this purpose, the catalyst stability in air, its reusability and regenerability after reaction, and the process scalability were investigated. The conciliation of activity data of the fresh hydrotalcite in batch laboratory (ml) and bench (l) reactors with on line ATR analysis is excellent, revealing that the citral–acetone reaction over hydrotalcite can be upscaled. However, the poisoning of the active Brønsted basic centers in the rehydrated hydrotalcite by CO2 is very fast (50% activity loss after 1h exposure to ambient). Besides, the catalyst after one run is inactive due to the presence of strongly adsorbed products and requires time-consuming (and not fully complete) regeneration. Basic centers of Lewis nature in calcined hydrotalcites (basically MgO) are more stable, but their activity is very low compared to the rehydrated counterpart. Both the technical disadvantages of current solid bases and the lack of stringent environmental regulations motivate the conservatism of industry to use alkaline solutions. [Copyright &y& Elsevier]

Abstract: The N2O-mediated oxidative dehydrogenation of propane over iron-containing MFI zeolites was studied by in situ diffuse reflectance Fourier transform infrared spectroscopy, complemented by analyses in a tapered element oscillating microbalance coupled with gas chromatography and characterization by X-ray diffraction, transmission electron microscopy, and N2 adsorption. Samples with different iron speciation and acidic properties, induced by the preparation method (ion-exchange and steam activation) and the framework composition (Fe–Al–Si, Fe–Ga–Si, and Fe–Ge–Si) were analyzed. Real-time monitoring of the reaction under operando conditions allowed us to gain insight into the mechanism and kinetics of deactivation and coke formation, as well as zeolite regeneration in air. Deactivation of iron zeolites in the oxidative dehydrogenation of propane is caused by coke deposition on active extra-framework iron species. The mechanism of coke formation and the nature of the deposits were similar over the samples, independent of the catalytic performance and deactivation pattern. Regeneration in air led to the disappearance of coke-related bands and the practically complete recovery of the original porosity, while the zeolite crystallinity remained intact. The initial propylene yield over the fresh and regenerated Fe–Ga–Si zeolites was the same, but the latter sample exhibited faster deactivation. This is attributed to a change of iron constitution in the first reaction–regeneration cycle. The alteration of the local environment of the iron species was indirectly concluded from the appearance of a band associated with Lewis centers in the infrared spectra of the regenerated zeolite. In support of this, TEM studies demonstrated an extensive degree of iron clustering as FeO x nanoparticles in the regenerated catalyst compared with the fresh and coked catalysts. [Copyright &y& Elsevier]

Abstract: DFT simulations have uncovered the origin of the highly selective character of gold nanoparticles in hydrogenation of triple bonds. This is ascribed to the better adsorption of Cedges of Au nanoparticles compared with Cthe barriers for hydrogenation of triple and double bonds on gold are comparable, selectivity is determined by the binding energy of the reactants. The situation is less favorable over palladium (commercial hydrorefining catalyst), because both Ce:glyph name="tbnd" />C bonds are adsorbed on the Pd surface. The outcome of the simulations was demonstrated experimentally in mixtures with both propyne and propylene over a Au/CeO2 catalyst, where C3H6 selectivities up to 95% were attained. Our finding opens a new path for chemoselective hydrogenation of molecules containing -yne and -ene groups on gold, with prospective application in petrochemical operations and in the fine chemical industry. [Copyright &y& Elsevier]

[Display omitted] • TiO 2 -anatase displays substantial activity and stability in gas-phase HBr oxidation. • Anatase and rutile exhibit similar kinetics, yet rutile is more active. • Self-doping with Br creates defect level enabling O 2 activation on both polymorphs. • O 2 activation is more facile on rutile due to shorter Ti cus distances. • Br 2 evolution is facilitated on rutile due to easier reduction of Ti4+ centers. This article investigates the activity of TiO 2 -rutile and TiO 2 -anatase polymorphs in the catalytic HBr oxidation, which is an enabling process to close the halogen loop in bromine-mediated transformation of natural gas to high-value chemicals and liquid fuels. The evaluation of rutile-, anatase-, and rutile-anatase TiO 2 catalysts, exhibiting the variable specific surface areas, revealed that anatase phase is also active in this reaction. Nonetheless, in contrast to photocatalytic processes in which anatase is typically more active than rutile, rutile exhibits ca. 2–5 times higher intrinsic rates of HBr oxidation than anatase. Thereby, the apparent activation energies and reaction orders with respect to HBr, O 2 , and H 2 O display similar values for the two polymorphs. The activity differences were rationalized by density functional theory analysis, which showed that HBr oxidation follows a similar defect-driven mechanism over the most stable rutile (110) and anatase (101) surfaces. Herein, HBr activates the catalyst through a self-doping mechanism that involves the substitution of surface oxygen by bromine with the concomitant reduction of Ti4+ to Ti3+ centers. This forms a defect level that is placed in the band gap and allows for the O 2 activation on the catalyst surface. While the HBr adsorption and H 2 O desorption display a similar energy profiles on both polymorphs, the O 2 activation and Br 2 evolution are more facile over rutile compared to anatase surface due to shorter distances between the coordinatively unsaturated Ti cus sites and easier reduction of Ti4+ centers upon product desorption, respectively. [ABSTRACT FROM AUTHOR]

The applicability of advanced composite materials with hierarchical structure that conjugate metal–organic frameworks (MOFs) with macroporous materials is commonly limited by their inferior mechanical properties. Here, a universal green synthesis method for the in situ growth of MOF nanocrystals within wood substrates is introduced. Nucleation sites for different types of MOFs are readily created by a sodium hydroxide treatment, which is demonstrated to be broadly applicable to different wood species. The resulting MOF/wood composite exhibits hierarchical porosity with 130 times larger specific surface area compared to native wood. Assessment of the CO2 adsorption capacity demonstrates the efficient utilization of the MOF loading along with similar adsorption ability to that of pure MOF. Compression and tensile tests reveal superior mechanical properties, which surpass those obtained for polymer substrates. The functionalization strategy offers a stable, sustainable, and scalable platform for the fabrication of multifunctional MOF/wood‐derived composites with potential applications in environmental‐ and energy‐related fields. [ABSTRACT FROM AUTHOR]

• The mechanism of ethane and ethene oxychlorination is assessed on iron phosphate. • Ethane yields ethene via ethyl chloride, while ethene directly forms vinyl chloride. • Ethane acts as a vinyl chloride suppression agent in ethene oxychlorination. • Active site competition favors ethane, depleting sites for ethene activation. • The diverging selectivity patterns are translatable to other catalytic systems. In the context of vinyl chloride monomer (VCM) production, an oxychlorination catalyst that allows direct VCM formation from gas-derived ethane instead of expensive oil-derived ethene is intensively sought after. A wide range of stable ethane oxychlorination catalysts for this purpose have recently been reported, yet they mainly yield ethene, while VCM remains a minor by-product. Strikingly, the same catalysts are active in ethene oxychlorination, resulting in selective VCM formation under equivalent reaction conditions. This work reveals the origin of these diverging selectivity patterns by combining quantitative catalytic tests, temporal analysis of products (TAP), and density functional theory (DFT) on iron phosphate. Ethane oxychlorination is found to proceed sequentially through ethyl chloride (EtCl) dehydrochlorination to ethene, while ethene oxychlorination directly yields VCM without formation of the intermediate dichloroethane (EDC) on iron phosphate. Furthermore, by co-feeding ethane in ethene oxychlorination, we demonstrate that the alkane suppresses the formation of VCM in ethene oxychlorination. The reason for this VCM inhibition is found in the hydrocarbon competition for a combination of the active, free and chlorinated iron centers. As ethane activation exhibits half of the barrier of ethene activation, the presence of ethane leads to active site depletion, hindering VCM formation. These observations are extended by ethane co-feeding tests in ethene oxychlorination over a wide range of known oxychlorination catalysts (EuOCl, LaOCl, CeO 2 , and CuCl 2 -KCl-LaCl 3 / γ -Al 2 O 3), and corresponding DFT calculations, indicating that the described phenomenon is material independent. The gathered molecular-level understanding explains the major hurdle of using ethane as feedstock for vinyl chloride production. [ABSTRACT FROM AUTHOR]

5-Methylfurfural (MF) is a critical commodity chemical used as food additive and synthetic intermediate. Opening a sustainable route towards highly selective synthesis of MF from 5-hydroxymethylfurfural (HMF) is of great significance, but has demonstrated extremely challenging. Herein, we report an efficient process for the selective production of MF from HMF over an anatase-supported gold catalyst (Au/a-TiO 2). During the hydrogenolysis of HMF, the desired C O bond is maintained and MF can be obtained in a high yield of 83.1 %, exhibiting excellent stability both in batch and flow operation. A combination of experimental and theoretical analyses attribute the superior performance of Au/a-TiO 2 to the synergistic effect of limited H access, low adsorption of reactants, and avoiding condensation, driven by the high concentration of oxygen vacancies in the support. This study highlights a novel approach of tailoring the coverage of active species for the selective transformation of renewable compounds into high-value-added chemicals. [Display omitted] • Selective production of MF from HMF via direct hydrogenolysis of the hydroxyl group using the Au/a-TiO 2 catalyst. • The C=O functional group was preserved under reductive conditions and MF can be obtained in a high yield of 83.1%. • The Au/a-TiO 2 catalyst shows high activity, remarkable selectivity, and excellent stability both in batch and flow operation. • The synergistic effect of limited H access, low adsorption of reactants, high concentration of oxygen vacancies. • This study paves the way for the design of new bifunctional catalysts toward selective hydrogenolysis of biomass. [ABSTRACT FROM AUTHOR]