Ga-chabazite zeolites (Ga-CHA) have been found to efficiently catalyze propane dehydrogenation with high propylene selectivity (96%). In situ Fourier transform infrared spectroscopy and pulse ...titrations are employed to determine that upon reduction, surface Ga2O3 is reduced and diffuses into the zeolite pores, displacing the Brønsted acid sites and forming extra-framework Ga+ sites. This isolated Ga+ site reacts reversibly with H2 to form GaH x (2034 cm–1) with an enthalpy of formation of ∼−51.2 kJ·mol–1, a result supported by density functional theory calculations. The initial C3H8 dehydrogenation rates decrease rapidly (40%) during the first 100 min and then decline slowly afterward, while the C3H6 selectivity is stable at ∼96%. The reduction in the reaction rate is correlated with the formation of polycyclic aromatics inside the zeolite (using UV–vis spectroscopy) indicating that the accumulation of polycyclic aromatics is the main cause of the deactivation. The carbon species formed can be easily oxidized at 600 °C with complete recovery of the PDH catalytic properties. The correlations between GaH x vs Ga/Al ratio and PDH rates vs Ga/Al ratio show that extra-framework Ga+ is the active center catalyzing propane dehydrogenation. The higher reaction rate on Ga+ than In+ in CHA zeolites, by a factor of 43, is the result of differences in the stabilization of the transition state due to the higher stability of Ga3+ vs In3+. The uniformity of the Ga+ sites in this material makes it an excellent model for the molecular understanding of metal cation-exchanged hydrocarbon interactions in zeolites.
Nitrogen oxides (NO x ) are a major atmospheric pollutant produced through the combustion of fossil fuels in internal combustion engines. Copper-exchanged zeolites are promising as selective ...catalytic reduction catalysts for the direct conversion of NO into N2 and O2, and recent reports have shown the enhanced performance of Cu-CHA catalysts over other zeolite frameworks in the NO decomposition of exhaust gas streams. In the present study, Rietveld refinement of variable-temperature XRD synchrotron data obtained for Cu-SSZ-13 and Cu-SSZ-16 is used to investigate the location of copper cations in the zeolite pores and the effect of temperature on these sites and on framework stability. The XRD patterns show that the thermal stability of SSZ-13 is increased significantly when copper is exchanged into the framework compared with the acid form of the zeolite, H-SSZ-13. Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16. From the refined diffraction patterns, the atomic positions of atoms, copper locations and occupancies, and thermal displacement parameters were determined as a function of temperature for both zeolites. Copper is found in the cages coordinated to three oxygen atoms of the six-membered rings.
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► ZSM-5 for glucose, furan, and maple wood catalytic fast pyrolysis. ► Maximum aromatic yield from glucose pyrolysis on HZSM-5 at SiO2/Al2O3 ratio=30. ► Mesoporous HZSM-5 favors ...production of larger aromatics. ► Removing external surface acid sites does not affect product distribution.
The conversion of glucose, furan and maple wood has been investigated over different types of ZSM-5 catalyst in semi-batch and fixed-bed reactors. The aromatic yield from glucose conversion goes through a maximum as a function of the framework silica-to-alumina ratio (SAR) of ZSM-5 with an optimum at SAR=30. This suggests that the concentration of acid sites inside the zeolite is critical for maximizing aromatic yield. Creating hierarchical mesopores within the zeolite slightly increased of coke formation and decreased the formation of the monocyclic aromatics. Mesoporous ZSM-5 was also observed to favor the production of larger alkylated monoaromatics. The selective removal of external acid sites from the ZSM-5 catalysts only slightly increased the catalyst activity but also decreased the selectivity to the desired aromatic products.
Isomerization of sugars is used in a variety of industrially relevant processes and in glycolysis. Here, we show that hydrophobic zeolite beta with framework tin or titanium Lewis acid centers ...isomerizes sugars, e.g., glucose, via reaction pathways that are analogous to those of metalloenzymes. Specifically, experimental and theoretical investigations reveal that glucose partitions into the zeolite in the pyranose form, ring opens to the acyclic form in the presence of the Lewis acid center, isomerizes into the acyclic form of fructose, and finally ring closes to yield the furanose product. The zeolite catalysts provide processing advantages over metalloenzymes such as an ability to work at higher temperatures and in acidic conditions that allow for the isomerization reaction to be coupled with other important conversions.
H-ZSM-5 zeolite-supported Ga (Ga/H-ZSM-5) has been considered as a selective catalyst for nonoxidative propane dehydrogenation (PDH) for decades; however, the reaction mechanism remains a topic of ...considerable discussion. In particular, the correlation between various Ga species present on the catalyst at the reaction conditions and the PDH activity has yet to be established. In this work, intrinsic PDH rates and activation energies were determined on Ga+–H+ pair sites and isolated Ga+ sites on Ga/H-ZSM-5 samples with a wide range of Si/Al and Ga/Al ratios. The turnover frequency on Ga+–H+ pair sites in the PDH is higher than that of isolated Ga+ sites by a factor of ∼15. Experimental measurements combined with a dual-site model show the activation energy in the PDH on the Ga+–H+ pair sites and isolated Ga+ sites to be 90.8 ± 1.5 and 117 ± 4.7 kJ·mol–1, respectively. These results demonstrate that Ga+–H+ pair sites are much more active in the PDH than isolated Ga+ sites. The activation energy of GaH x decomposition to form H2 was determined to be 40–60 kJ·mol–1 higher than that of the PDH on Ga species, suggesting that the GaH x decomposition is unlikely to be part of the PDH mechanism. Although both Brønsted acid and Ga sites interact with propane, Fourier transform infrared spectroscopy results provide strong evidence suggesting that the alkyl mechanism is more likely in the PDH on Ga/H-ZSM-5 catalysts.
Porcine reproductive and respiratory syndrome virus (PRRSV) isolates are classified in two different genotypes, based on genomic heterogeneity: type 1, which comprises European type isolates, and ...type 2, which includes North American type isolates. It is believed that members of both genotypes differ in some biological properties including pathogenicity, however extensive studies comparing isolates of both genotypes have never been carried out. The objective of the present study was to compare the pathogenic properties of six different PRRSV isolates, three of type 1 and three of type 2, in a young pig infection model. For this purpose, a total of 105 3-week-old piglets were divided in 7 groups of 15 animals that were exposed on day 0 of the experiment to one of the six isolates tested or were mock infected (negative control group). Clinical signs and rectal temperatures were recorded daily and blood samples were taken on days 3, 6, 9, 12, 15, 18 and 21 of the experiment. On days 7, 14 and 21 post-inoculation five animals per group were sacrificed, macroscopic lung lesions were evaluated and different tissue samples were collected to determine viral organic distribution. The results obtained indicate that type 2 isolates are more pneumovirulent than type 1 isolates, as demonstrated by the recording of respiratory clinical signs only in pigs exposed to type 2 viruses and by the severity of macroscopic and microscopic lung lesions in those pigs. However, no clear differences could be established between genotypes in systemic clinical signs or viral load and viral distribution after challenge. These results support the general idea that type 2 isolates induce more severe respiratory disease than type 1 isolates.
The conversion of 2,5-dimethylfuran (DMF) to p-xylene is studied via DFT for both the uncatalyzed and acid-catalyzed (Brønsted and Lewis) reactions. The mechanism comprises the symmetry-allowed ...Diels-Alder cycloaddition of DMF and ethylene and the dehydration of the produced oxa-norbornene. Display omitted
► The conversion mechanism of 2,5-dimethylfuran and ethylene to p-xylene is explored. ► p-Xylene is obtained by dehydration of a Diels–Alder oxa-norbonene cyclo-adduct. ► Lewis acids are predicted to be very effective Diels–Alder catalysts. ► Brønsted acids have no catalytic effect on the Diels–Alder reaction. ► Brønsted acids are found to be very effective dehydration catalysts.
We present a detailed mechanism for the conversion of DMF (2,5-dimethylfuran) and ethylene to p-xylene, obtained by gas-phase DFT electronic structure calculations. The conversion consists of Diels–Alder cycloaddition and subsequent dehydration of the cycloadduct, an oxa-norbornene derivative. We present the energetics of both the uncatalyzed and acid-catalyzed (Brønsted and Lewis) reactions. Even though the DMF–ethylene cycloaddition is thermally feasible, we show that Lewis acids can further lower the activation requirements by decreasing the HOMO–LUMO gap of the addends. The catalytic effect may be significant or negligible depending on whether the Diels–Alder reaction proceeds in the normal or the inverse electron-demand direction. We also show that Brønsted acids are extremely effective at catalyzing the dehydration of the oxa-norbornene derivative, which, according to our calculations, cannot proceed uncatalyzed. On the other hand, we conclude that Brønsted acids do not catalyze the cycloaddition. Although strong Lewis acids like Li+ can catalyze the dehydration, our calculations indicate that relatively elevated temperatures would be required as they are not as effective as Brønsted acids. We argue that the specific synthetic route to p-xylene is kinetically limited by the Diels–Alder reaction when Brønsted acids are used and by the dehydration when a Lewis acid is used, with the latter being slower than the former. Finally, we adduce experimental data that corroborate the theoretical predictions: we observe no activity in the absence of a catalyst and a higher turnover frequency to p-xylene in the Brønsted acidic zeolite HY than in the Lewis acidic zeolite NaY.
AEI, CHA, and STT siliceous zeolites and RUB-41 (RRO) were prepared with low concentration of internal silanols and were characterized by X-ray diffraction, N2 adsorption isotherms, electron ...microscopy, and 29Si MAS NMR spectroscopy. Adsorption isotherms of methane, ethylene, ethane, propylene, propane, and CO2 were measured volumetrically on siliceous zeolites AEI, CHA, RRO, and STT at various temperatures ranging from 273 K to 323 K up to a pressure of 101.3 kPa. The Clausius-Clapeyron equation was used to calculate the isosteric heats of adsorption. The amount adsorbed depends strongly on adsorbate polarizability in the low-pressure region, and on the adsorbent micropore volume in the high-pressure region. Adsorption entropy losses are independent of temperature and increase with increasing adsorption enthalpy. RRO showed the highest pure-component selectivity of CO2 over CH4, and potential for the separation of the natural gas mixtures; AEI, CHA, and RRO-type materials showed potential for the kinetic separation of propylene/propane.
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•AEI, CHA, STT, and RRO siliceous zeolites were prepared with low silanol defects.•Small pore zeolites AEI, CHA, and RRO are promising for propane/propylene kinetic separations.•RRO showed high selectivity of CO2/CH4.
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