ABSTRACT
The pore network architecture of porous heterogeneous catalyst supports has a significant effect on the kinetics of mass transfer occurring within them. Therefore, characterizing and understanding structure-transport relationships is essential to guide new designs of heterogeneous catalysts with higher activity and selectivity and superior resistance to deactivation. This study combines classical characterization via N2 adsorption and desorption and mercury porosimetry with advanced scanning electron microscopy (SEM) imaging and processing approaches to quantify the spatial heterogeneity of γ-alumina (γ-Al2O3), a catalyst support of great industrial relevance. Based on this, a model is proposed for the spatial organization of γ-Al2O3, containing alumina inclusions of different porosities with respect to the alumina matrix. Using original, advanced SEM image analysis techniques, including deep learning semantic segmentation and porosity measurement under gray-level calibration, the inclusion volume fraction and interphase porosity difference were identified and quantified as the key parameters that served as input for effective tortuosity factor predictions using effective medium theory (EMT)-based models. For the studied aluminas, spatial porosity heterogeneity impact on the effective tortuosity factor was found to be negligible, yet it was proven to become significant for an inclusion content of at least 30% and an interphase porosity difference of over 20%. The proposed methodology based on machine-learning-supported image analysis, in conjunction with other analytical techniques, is a general platform that should have a broader impact on porous materials characterization.
ABSTRACT
Mesoporous γ-alumina is used as an adsorbent in the decontamination of water from heavy metals (e.g., nickel and cobalt) and as a support for heterogeneous catalysts prepared by impregnation. In these cases, alumina extrudates are in contact with aqueous solutions containing precursors of the active metal phase to be deposited. The proton concentration (or pH) in the metal solution in contact with alumina can impact the adsorption efficiency of decontamination processes and the activities of catalysts. Yet, it is difficult to quantify the effect of the pH inside the pores since protons are not detected by classical imaging techniques. In this article, the effect of protons on nickel adsorption on alumina is evaluated using a novel technique coupling liquid analysis (pH, conductivity, and UV/vis) and laser-induced breakdown spectroscopy (or LIBS) analysis of concentration gradients inside the solid. Both methods are in excellent agreement. The results show a slow diffusion of protons inside alumina pores (diffusion continues even after 940 min), yielding high proton concentration gradients. On the other hand, the nickel species penetrate the extrudates faster but are slowly displaced by protons under certain operating conditions. As a result, different metal concentration profiles are obtained, depending on the initial pH and contact time. These findings are interesting in catalysis since they prove the possibility of controlling the deposition of the active metal on catalysts by regulating the operating conditions of impregnation. For typical industrial impregnation times (a few minutes to 1 to 2 h), protons do not have enough time to deeply penetrate inside extrudates, so the initial pH of the metal solution will have nearly no effect on the metal distribution. Conversely, decontamination processes have much longer contact times; therefore, lower initial pH values should have negative impacts on the adsorption efficiency due to the protons displacing the adsorbed nickel.
ABSTRACT
The present study attempts to understand the use of the flexible porous chromium terephthalate Cr(OH)(O(2)C-C(6)H(4)-CO(2)) denoted MIL-53(Cr) (MIL = Material from Institut Lavoisier) for the separation of mixtures of CO(2) and CH(4) at ambient temperature. The coadsorption of CO(2) and CH(4) 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 CO(2) and CH(4), 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 CO(2) and CH(4) leads to a similar breathing of MIL-53(Cr) as with pure CO(2). The breathing is mainly controlled by the partial pressure of CO(2), but increasing the CH(4) content progressively decreases the transformation of LP to NP. CH(4) seems to be excluded from the NP form, which is filled exclusively by CO(2) molecules. The consequences in terms of CO(2)/CH(4) selectivity and the possible use of MIL-53(Cr) in a PSA process are discussed.
