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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: 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 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]

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]

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: 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: 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: As-synthesised and reconstructed NH4-Al-dawsonite and NH4-Ga-dawsonite, and the materials obtained by thermal treatment in the range 373–873K, were studied as heterogeneous catalysts for the epoxidation of cyclooctene with hydrogen peroxide at 353K and ambient pressure by means of High-Throughput Experimentation. The structural, morphological, and textural properties of the materials were investigated by elemental analysis, ICP-OES, XRD, FTIR, TEM, N2 adsorption, and TGA. The temperature at which the dawsonite precursors were treated proved to be crucial in determining the activity of the catalysts. The best catalyst identified in this work was obtained by calcination at 573K of reconstructed Ga-dawsonite. This material provides an epoxide yield of 51% and 99% selectivity after 4h at 353K. Remarkably, many of the catalysts derived from Al-dawsonite displayed an enhanced epoxidation activity upon recycling, reaching up to 30% epoxide yield with 98% selectivity over Al-dawsonite treated at 473K. [Copyright &y& Elsevier]

Abstract: The kinetics of the citral–acetone condensation to pseudoionone (PS) over activated Mg–Al hydrotalcite (meixnerite) was studied in a 16 parallel-batch reactor set-up varying the temperature, catalyst amount, and molar acetone-to-citral (A/C) ratio. Tests with different stirring speeds and particle sizes were conducted in order to exclude the presence of mass transport limitations. The A/C ratio is a key reaction descriptor. The initial rate of pseudoionone formation exhibits a maximum at A/C=3, leading to a PS selectivity of 100%. At higher or lower A/C ratios, diacetone alcohol is the main by-product and at A/C<2 the product of citral self-condensation is additionally formed. Due to this, the pseudoionone selectivity at extreme A/C ratios decreases to 70–80%. The amount of carbonaceous deposits was determined by thermogravimetry and increased as the A/C ratio decreased. A Langmuir–Hinshelwood model involving acidic Al3+ and basic OH− sites and the surface reaction as rate-determining step described well the experimental data. The position of the optimal A/C ratio is determined by the relative ratio of adsorption constants of the reactants on the catalyst, being one order of magnitude larger in citral than in acetone. This feature can be generalized to aldol reactions involving other substrates. [Copyright &y& Elsevier]

Abstract: The effect of reaction conditions and rhodium content in H2-pretreated Pt–Rh materials on their initial activity and time-on-stream behavior in direct N2O decomposition at 1023K was investigated by means of continuous-flow catalytic tests at ambient pressure and X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Reduced pure Pt (Pt100) gauze shows a slightly higher initial activity (after 10min on stream) in the reaction than reduced pure Rh (Rh100) wire. The initial N2O conversion over various Pt–Rh alloys (95–5, 90–10, and 80–20) was considerably lower than that of the pure metals. The following order of initial activity was obtained: Pt100 >Rh100 >Pt95–Rh5 >Pt90–Rh10 >Pt80–Rh20. However, Pt100 gauze rapidly deactivates with time-on-stream. This deactivation is tentatively attributed to the penetration of oxygen (generated upon N2O decomposition) into the bulk of the metal. In contrast to Pt100 gauze, Rh100 wire activates with time-on-stream leading to a significantly enhanced and stable N2O conversion. The activity trend after 900min on stream is changed to: Rh100 >Pt95–Rh5 >Pt90–Rh10 >Pt80–Rh20 >Pt100. The different catalytic performance of Rh is related to the gradual transformation of metallic Rh to Rh2O3 under reaction conditions as concluded from surface composition obtained by XPS analyses. Our results indicate that the Pt–Rh alloy gauzes are inactive for direct N2O decomposition in the presence of excess oxygen, as typically found in ammonia burners. [Copyright &y& Elsevier]