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A detailed kinetic model was proposed to analyze experimental data obtained from indole hydrodenitrogenation (HDN) over γ-Al2O3 and amorphous silica-alumina (ASA)-supported NiMo catalysts. The goal was to investigate the support acidity effects on indole HDN and compare with a recent study on quinoline HDN. Similarly to quinoline HDN, indole HDN occurred via hydrogenation of the aromatic ring, followed by N-ring opening and exocyclic C-N bond breaking. The high support acidity of NiMo(P)/ASA exhibited a promoting effect for N-removal steps and adsorption of nitrogen compounds. However, in contrast to quinoline HDN, it did not clearly induce a positive effect for the hydrogenation step. The acidic function of ASA also favored the formation of byproducts such as toluene, cyclohexane, dimer, and trimer of indole. Catalytic conversion of a quinoline and indole mixture revealed a strong inhibiting effect of quinoline on indole HDN, whereas the inhibiting effect of indole on quinoline HDN was weak. The inhibition was weaker over NiMo(P)/Al2O3 than over NiMo(P)/ASA. This result is in agreement with a relative ranking of apparent adsorption constants of quinoline, indole, and their products on NiMo(P)/Al2O3 and ASA. [ABSTRACT FROM AUTHOR]

The adsorption processes for postcombustion CO2 capture are usually based on a temperature or vacuum swing (TSA or VSA). In the present contribution an alternative concept is presented, which is based on the regeneration of the solid sorbent (an immobilized amine) by a purge gas, low pressure vapor, under almost isothermal conditions. Because of the close to isothermal operation, the process consumes significantly less thermal and mechanical energy than conventional TSA and VSA processes, respectively. We present a rough design of such an isothermal concentration swing process based on a simplified, analytical model of a 2-step PSA (pressure swing adsorption) process. The analytical model allows a definition of the range of operating conditions that lead to the best compromise between energy consumption and productivity (size and number of the adsorbers). Moreover, it is possible to define the equilibrium and mass transfer properties of the ideal solid sorbent. The feasibility of the concentration swing process was finally validated by numerical simulations of a full PSA cycle under adiabatic conditions. [ABSTRACT FROM AUTHOR]

In silico screening of CO2 adsorbents is a very powerful method for preselecting the most promising porous solids for experimental studies. Because of increased computational power, it is now possible to investigate a large number of adsorbents in a fairly short time. However, it remains difficult to rationalize structure-performance correlations because the complexity of such porous materials cannot be reduced to a few simple descriptors. In this paper, we present a different approach that was applied to the design of an optimal adsorbent for CO2 separation from gas mixtures. A CO2-CH4 mixture was used as the feed. We constructed a simplified model of the porous material by adopting a spherical pore geometry. From the dispersion-repulsion point of view, the spherical pore was modeled by homogeneously distributed Lennard-Jones sites. Besides, a charge distribution was introduced to mimic the electrostatic behavior. The selected charge distribution is constituted of negative charges homogeneously distributed over the surface of the pore. The total negative charge is counterbalanced by eight positive discrete charges placed on the corners of the cube inscribed in the sphere. This model can be characterized by two main descriptors: the pore size and the cation charge. For this model, the adsorbate-adsorbent interaction potential due to electrostatic and dispersion-repulsion interactions was determined. Then, the Henry constants of CO2 and CH4 are calculated from statistical thermodynamics and substituted in the Ruthven statistical model, which allows calculation of the binary adsorption isotherms. Finally, the adsorbed quantities are used to estimate the performance indexes of the pressure swing adsorption (PSA) process, such as the working capacity and separation factor. We show how these two parameters depend on the pore size and cation charge. For a medium-pressure PSA process, the optimal adsorbent has a large pore volume and is highly charged, which provides high working capacity and selectivity. The high cationic charge allows one to efficiently fill the large pore volume and assures high CO2 selectivity. [ABSTRACT FROM AUTHOR]

Thechoice of an appropriate adsorbent for CO2separationby pressure-swing adsorption remains a field of intense research.In this work, several FAU and LTA zeolites with different Na contents(Si/Al ratios) are studied for the separation of CO2frommixtures of CO2, CO, and CH4by means of breakthroughexperiments. The breakthrough experiments were carried out between1 and 5 bar at 303 K using two feed mixtures: 50/50 (v/v) CO2/CH4and 75/15/15 (v/v/v) CO2/CH4/CO. The most polar zeolites, i.e., those with high Na content, exhibitthe highest adsorption capacity and selectivity for CO2, but their regeneration is difficult; hence, their working capacityis low. The opposite is true for the least polar zeolites, i.e., thosewith low Na content. In order to quantify the trade-off between theselectivity and working capacity, the Ruthven statistical model (RSM)was used. It satisfactorily reproduced the experimental trends. We,therefore, used the RSM to identify the properties of the adsorbentthat lead to an optimal compromise between the working capacity andseparation factor. The critical parameter is the concentration ofextraframework cations, which, in turn, depends on the framework chargeof the zeolites FAU and LTA. The optimal trade-off zone is definedin terms of the Henry constant of CO2(KCO2). It is found that this zone is placedbetween KCO2= 5 × 10–3and 50 × 10–3molecules·bar–1·Å–3. This interval correspondsto a heat of adsorption of CO2at zero coverage between27 and 32 kJ·mol–1. In our study, this optimalrange of Henry constants was achieved for the zeolites Na-USY, SAPO-37,LTA (Si/Al = 5), and EMC-1. [ABSTRACT FROM AUTHOR]

The role of the oxide support on the structure of the MoS2 active phase (size, morphology, orientation, sulfidation ratio, etc.) remains an open question in hydrotreating catalysis and biomass processing with important industrial implications for the design of improved catalytic formulations. The present work builds on an aqueous-phase surface-science approach using four well-defined a-alumina single crystal surfaces (C (0001), A (1120), M (lOlO), and R (1102) planes) as surrogates for y-alumina (the industrial support) in order to discriminate the specific role of individual support facets. The reactivity of the various surface orientations toward molybdenum adsorption is controlled by the speciation of surface hydroxyls that determines the surface charge at the oxide/water interface. The C (0001) plane is inert, and the R (1102 ) plane has a limited Mo adsorption capacity while the A (1120) and M (1010) surfaces are highly reactive. Sulfidation of model catalysts reveals the highest sulfidation degree for the A (1120) and M (1010) planes suggesting weak metal/support interactions. Conversely, a low sulfidation rate and shorter MoS2 slabs are found for the R (1102) plane implying stronger Mo-O-Al bonds. These limiting cases are reminiscent of type I/type II MoS2 nanostructures. Structural analogies between α-and γ-alumina surfaces allow us to bridge the material gap with real Al203-supported catalysts. Hence, it can be proposed that Mo distribution and sulfidation rate are heterogeneous and surfacedependent on industrial γ-Al203-supported high-surface-area catalysts. These results demonstrate that a proper control of the γ-alumina morphology is a strategic lever for a molecular-scale design of hydrotreating catalysts. [ABSTRACT FROM AUTHOR]

We propose a method for analyticallypredicting single-componentadsorption isotherms from molecular, microscopic and structural descriptorsof the adsorbate–adsorbent system and concepts of statisticalthermodynamics. Expressions for Henry’s constant and the heatof adsorption at zero coverage are derived. These functions dependon the pore size, pore shape, chemical composition, and density ofthe adsorbent material. They quantify the strength of the solid–fluidinteraction, which governs the low-pressure part of the adsorptionisotherm. For intermediate and high pressures, the fluid–fluidinteractions must also betaken into account. Both solid–fluid and fluid–fluidinteractions are combined within the framework of the Ruthven statisticalmodel (RSM). The RSM thus constructs theoretical adsorption isothermsthat are entirely based on microscopic molecular and structural descriptors.The theoretical results that we obtained are compared with experimentaldata for the adsorption of pure CO2and CH4onall-silica zeolites. The developed methodology allows for the estimationof the optimum properties of a nonpolar adsorbent for the adsorptionof CO2in cyclic adsorption processes. [ABSTRACT FROM AUTHOR]

The objective of this work was to study the adsorption and separation of the most important families of hydrocarbon compounds on metal-organic frameworks (MOFs), in comparison with zeolites. For this purpose, we have selected four probe molecules, each of them representing one of these families, i.e., o- and p-xylene as aromatics, 1-octene as an alkene, and n-octane as an alkane. The separation of these four molecules was studied by binary breakthrough experiments. To represent the large diversity of MOF structures, the experiments were carried out with (i) two MOFs with coordinatively unsaturated metal sites (CUS), i.e., Cu-btc (HKUST-1) and CPO-27-Ni, (ii) a MOF with an anionic framework and extraframework cations, i.e. RHO-ZMOF, and (iii) two rather apolar zeolitic imidazolate framework (ZIF) materials with different pore sizes, i.e. ZIF-8 and ZIF-76. Zeolite NaY and zeolite β were used as polar and apolar reference adsorbents, respectively. The results can be briefly summarized as follows: ZIFs (not carrying any polar functional groups) behave like apolar adsorbents and exhibit very interesting and unexpected molecular sieving properties. CUS-MOFs behave like polar adsorbents but show the specificity of preferring alkenes over aromatics. This feature is rationalized thanks to DFT+D calculations. MOFs with extraframework cations behave like polar (cationic) zeolites. [ABSTRACT FROM AUTHOR]

The separation of paraffin isomers is a very importanttopic inthe petrochemical industry. Zeolite 5A is industrially used to sievealkane isomers, but its pore size does not allow the separation ofmonobranched and dibranched alkanes by a kinetic mechanism. In thispublication, we compare three ZIF materials in the separation of C6-paraffinisomers: ZIF-8, ZIF-76, and a new material called IM-22. The performanceof the materials is evaluated by a breakthrough curve of binary mixturesof n-hexane, 3-methylpentane, and 2,2-dimethylbutane.We show that ZIF-8 is a very attractive alternative to zeolite 5Abecause it exhibits a high (kinetic) selectivity for the adsorptionof linear alkanes and at the same time a high adsorption capacity.The new material IM-22, a ZIF with CHAtopology, seemsto be particularly suited for the separation of mono- and dibranchedparaffin isomers. [ABSTRACT FROM AUTHOR]

The present study attempts to understand the use of the flexible porous chromium terephthalate Cr(OH)(O2C--C6H4--CO2) denoted MIL-53(Cr) (MIL = Material from Institut Lavoisier) for the separation of mixtures of CO2 and CH4 at ambient temperature. The coadsorption of CO2 and CH4 was studied by a variety of different techniques. In situ synchrotron X-ray Powder Diffraction allowed study of the breathing of the solid upon adsorption of the gas mixtures and simultaneously measured Raman spectra yielded an estimation of the adsorbed quantities of CO2 and CH4, as well as a quantification of the fraction of the narrow pore (NP) and the large pore (LP) form of MIL-53. Quantitative coadsorption data were then measured by gravimetry and by breakthrough curves. In addition, computer simulation was performed to calculate the composition of the adsorbed phase in comparison with experimental equilibrium isotherms and breakthrough results. The body of results shows that the coadsorption of CO2 and CH4 leads to a similar breathing of MIL-53(Cr) as with pure CO2. The breathing is mainly controlled by the partial pressure of CO2, but increasing the CH4 content progressively decreases the transformation of LP to NP. CH4 seems to be excluded from the NP form, which is filled exclusively by CO2 molecules. The consequences in terms of CO2/CH4 selectivity and the possible use of MIL-53(Cr) in a PSA process are discussed. [ABSTRACT FROM AUTHOR]