Effects of Treatment Conditions on Pd Speciation in CHA and Beta Zeolites for Passive NOx Adsorption
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SUBMITTER: Pace R
PROVIDER: S-EPMC8581994 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:In the current article, the effect of Si/Al ratio on the NOx adsorption and storage capacity over Pd/Beta with 1 wt% Pd loading was investigated. The XRD, 27Al NMR and 29Si NMR measurements were used to determine the structure of Pd/Beta zeolites. XAFS, XPS, CO-DRIFT, TEM and H2-TPR were used to identify the Pd species. The results showed that the NOx adsorption and storage capacity on Pd/Beta zeolites gradually decreased with the increase of Si/Al ratio. Pd/Beta-Si (Si-rich, Si/Al~260) rarely has NOx adsorption and storage capacity, while Pd/Beta-Al (Al-rich, Si/Al~6) and Pd/Beta-C (Common, Si/Al~25) exhibit excellent NOx adsorption and storage capacity and suitable desorption temperature. Pd/Beta-C has slightly lower desorption temperature compared to Pd/Beta-Al. The NOx adsorption and storage capacity increased for Pd/Beta-Al and Pd/Beta-C by hydrothermal aging treatment, while the NOx adsorption and storage capacity on Pd/Beta-Si had no change.
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Project description:Atomic dispersion of metal species has attracted attention as a unique phenomenon that affects adsorption properties and catalytic activities and that can be used to design so-called single atom materials. In this work, we describe atomic dispersion of bulk Pd into small pores of CHA zeolites. Under 4% NO flow at 600 °C, bulk Pd metal on the outside of CHA zeolites effectively disperses, affording Pd2+ cations on Al sites with concomitant formation of N2O, as revealed by microscopic and spectroscopic characterizations combined with mass spectroscopy. In the present method, even commercially available submicrosized Pd black can be used as a Pd source, and importantly, 4.1 wt % of atomic Pd2+ cations, which is the highest loading amount reported so far, can be introduced into CHA zeolites. The structural evolution of bulk Pd metal is also investigated by in situ X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), as well as ab initio thermodynamic analysis using density functional theory (DFT) calculations.
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Project description:The development of technologies that allow us to reduce CO2 emissions is mandatory in today's society. In this regard, we present herein a comparative study of CO2 adsorption over three types of materials: zeolites, layered double hydroxides (LDH), and zeolites coated LDH composites. The influence of the zeolite Si/Al ratio on zeolites sorption capacity along with the presence of mesopores was investigated. By comparing these results with the well-known performance of LDHs, we aim to provide insights on the factors that may influence the CO2 capture capacity over zeolites, thus providing useful tools for tuning their properties upon post-treatments.
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Project description:Small-pore silicoaluminophosphate (SAPO) zeolites with 8-ring pore windows and appropriate acidities/polarities, for example, SAPO-34 (CHA) and SAPO-56 (AFX), have proven to be potential adsorbing materials for selective adsorption of CO2. However, SAPO-35 zeolites (LEV framework topology) synthesized using conventional templates are less reported for highly selective CO2 adsorption which might be due to inappropriate Si contents and acidities in the framework. In this work, by using N-methylpiperidine (NMP) as a template, SAPO-35 zeolites with various Si contents were synthesized under hydrothermal conditions, which allowed SAPO-35 zeolites with modulated acidities and polarities. The CO2 adsorption and separation properties of SAPO-35_x (x: Si/(Si + P + Al) in molar ratio) were investigated, and a close relationship between the acidity, polarity and CO2 adsorption and separation capacity was revealed. SAPO-35_0.14 with the strongest acidity showed the highest CO2 uptake of 4.76 mmol g-1 (273 K and 100 kPa), and appeared to be one of the best SAPO materials for CO2 adsorption. Moreover, increased Brønsted acidity can significantly enhance the adsorption selectivity of CO2 over N2. At 298 K and 100 kPa, SAPO-35_0.14 showed the highest CO2/N2 selectivity of 49.9, exhibiting potential for industrial processes. Transient binary breakthrough experiments on SAPO-35_0.14 further proved the efficient separation performance and stable circulation. The results of this study prove that the framework Si content of SAPO-35 zeolites is essential for regulating their CO2 adsorption performance. This work demonstrates that modulating the silicon content and acidity in SAPO zeolites via a suitable choice of template, as well as polarity, is of great significance for the rational synthesis of zeolites with superior CO2 adsorption and separation abilities.
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Project description:The dynamic adsorption/desorption performances of modified hierarchical USY zeolites treated with an ammonia solution (NH4OH) at different concentrations were investigated using gas-phase toluene as an indicator. The characterization results indicated that the ammonia treatment could result in the expansion of microporous channels and the formation of a mesoporous structure without evident decrease in crystallinity. The experiment results regarding dynamic adsorption/desorption performances revealed that the mass transfer resistance of modified USY adsorbents were greatly reduced treating with NH4OH. Among the modified samples, the 0.1 mol L-1 NH4OH treated USY adsorbent exhibited large adsorptive capacity and highest desorption rate, which show good cyclic performance that could preserve its adsorbent capacity after 20 cycles. In contrast, pristine USY samples had lost around 28% of the initial adsorption capacity after 20 cycles. Moreover, the NaOH-treated sample showed great crystallinity decline compared to the NH4OH-treated samples due to excessive silicon atom leaching from the USY framework, and had lower adsorption capacity under humid conditions. Therefore, NH4OH-modified USY zeolites could be promising adsorbents for the adsorption/desorption process of volatile organic compounds (VOCs).
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Project description:The separation of xylenes is one of the most important processes in the petrochemical industry. In this article, the competitive adsorption from a fluid-phase mixture of xylenes in zeolites is studied. Adsorption from both vapor and liquid phases is considered. Computations of adsorption of pure xylenes and a mixture of xylenes at chemical equilibrium in several zeolite types at 250 °C are performed by Monte Carlo simulations. It is observed that shape and size selectivity entropic effects are predominant for small one-dimensional systems. Entropic effects due to the efficient arrangement of xylenes become relevant for large one-dimensional systems. For zeolites with two intersecting channels, the selectivity is determined by a competition between enthalpic and entropic effects. Such effects are related to the orientation of the methyl groups of the xylenes. m-Xylene is preferentially adsorbed if xylenes fit tightly in the intersection of the channels. If the intersection is much larger than the adsorbed molecules, p-xylene is preferentially adsorbed. This study provides insight into how the zeolite topology can influence the competitive adsorption and selectivity of xylenes at reaction conditions. Different selectivities are observed when a vapor phase is adsorbed compared to the adsorption from a liquid phase. These insight have a direct impact on the design criteria for future applications of zeolites in the industry. MRE-type and AFI-type zeolites exclusively adsorb p-xylene and o-xylene from the mixture of xylenes in the liquid phase, respectively. These zeolite types show potential to be used as high-performing molecular sieves for xylene separation and catalysis.
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Project description:Efficient adsorbents are critical to the purification of liquefied natural gas (LNG) by the adsorption method. In this study, the physiochemical properties of JLOX-500 and 13X were examined. JLOX-500 with more Al content had a more compact unit cell, a larger surface area and pore volume, a smaller average pore size, and more microchannels on the surface than 13X. The separation performance of the two adsorbents was evaluated by the adsorption experiment. The CO2 adsorption capacity of JLOX-500 was higher than that of 13X, while the equilibrium and ideal selectivity and separation factor of CO2/CH4 were also larger for JLOX-500. Especially in dynamic adsorption, the CO2 adsorption capacities at 50 ppm of the gas mixture at the outlet were 3.46 and 1.64 mmol/g for JLOX-500 and 13X, respectively. The adsorption heats of CO2 and CH4 on JLOX-500 were 40.50 and 18.77 kJ/mol, whereas these values were 31.49 and 18.50 kJ/mol for 13X, respectively. A better separation performance for JLOX-500 was observed because of fewer binders and a lower Si/Al ratio (1.34). The Toth adsorption isotherm model described best the experimental data. According to the results of this study, JLOX-500 was a more efficient adsorbent used in purification for LNG production at high pressure with low CO2 concentration.
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Project description:The water-stable material NOTT-401 was investigated for CO2 capture under humid conditions. Water adsorption properties of NOTT-401 were studied, and their correlation with CO2 sequestration at different relative humidities (RHs) showed that the CO2 capture increased from 1.2 wt % (anhydrous conditions) to 3.9 wt % under 5% RH at 30 °C, representing a 3.2-fold improvement.
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Project description:AimsIn atrial fibrillation, stroke risk is assessed by the CHA2 DS2 -VASc score. Heart failure is included in CHA2 DS2 -VASc, but the rationale is uncertain. Our objective was to test if heart failure is a risk factor for stroke, independent of other risk factors in CHA2 DS2 -VASc.Methods and resultsWe studied 300 839 patients with atrial fibrillation in the Swedish Patient Register 2005-11. Three definitions of heart failure were used in order to assess the robustness of the results. In the main analysis, heart failure was defined by a hospital discharge diagnosis of heart failure as first or second diagnosis and a filled prescription of a diuretic within 3 months before index + 30 days. The second definition counted first or second discharge diagnoses <1 year before index + 30 days and the third definition any heart failure diagnosis in open or hospital care before index + 30 days. Associations with outcomes were assessed with multivariable Cox analyses. Patients with heart failure were older (80.5 vs. 74.0 years, P < 0.001) and had higher CHA2 DS2 -VASc score (4.4 vs. 2.7, P < 0.001). The 1 year incidence of ischaemic stroke without warfarin was 4.4% with heart failure and 3.1% without. Adjustment for the cofactors in CHA2 DS2 -VASc eradicated the difference in stroke risk between patients with and without heart failure (hazard ratio 1.01 with 95% confidence interval 0.96-1.05). The area under the receiver operating characteristic curve for CHA2 DS2 -VASc was not improved by points for heart failure.ConclusionsA clinical diagnosis of heart failure was not an independent risk factor for stroke in patients with atrial fibrillation, which may have implications for anticoagulation management.
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