Metal–Organic Frameworks for Separations

The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts. © 2001 Elsevier Science B.V. All rights reserved.

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Mechanisms of catalyst deactivation

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

https://web.iitd.ac.in/~arunku/files/CEL311_Y13/Adsorption%20Theory%20to%20practice_Dabrowski.pdf

Adsorption — from theory to practice

Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

Progress in the synthesis and engineering of advanced porous materials demands better pore structure characterization. The analysis of pore structure is complicated by (1) the wide range in pore sizes observed, from molecular ( 1 mm) dimensions, (2) complex pore shapes and connectivities, (3) chemical and physical heterogeneities, and (4) pore structure changes that can occur during characterization.The required pore structure information varies with application. Bulk density and the pore-size distribution are needed for thermal insulation. In this case, the dimension of interest is the so-called hydraulic radius since, for small pores, the gas-phase conductivity is proportional to the mean hydraulic radius to the mean free path. A few large but isolated pores will significantly affect conductivity but will go undetected in typical gas-absorption methods. In contrast, for separations, bottlenecks control performance. For transport, such as migration through geologic formations, both the pore-size distribution and pore connectivity are important. For adsorption, surface area and pore size are the relevant factors. Finally, the conventional concepts of pore structure lose meaning as the pore size approaches molecular dimensions, typical of adsorbents and gas-separation membranes.

Characterization of Porous Solids

Sol-gel strategies for controlled porosity inorganic materials

A model is provided for predicting gel shrinkage during drying by combining the empirical observations that: (1) the bulk modulus of a gel increases with density, ρ, according to K p = K 0 ( ρ ρ y ) m (2.5 ≤ m ≤ 4) ; and (2) the variation in pore radius, r, is approximately proportional to pore volume (contrary to the dependence, r α ρ −1 3 , conventionally assumed). No allowance is made for viscoelastic relaxation, so the model applies only for drying from an inert solvent. The model may be used to guide processing efforts to yield either aerogel-like materials with high porosity or xerogels with high density and small pore size for adsorbents, membranes, etc. The extent of shrinkage is governed by two dimensionless parameters, P and m. The quantity P = As γ cos (θ)mρ y K 0 (where As is specific surface area, γ is surface tension, and θ is the contact angle) represents the relative magnitudes of capillary pressure and gel stiffness, and m describes the variation of stiffness with density. For P values less than 1, the shrinkage upon drying is less than 10%, and is reversible. For shrinkages greater than ⋍ 50%, the density increase is irreversible, and is proportional to P 1/(m - 1). Predicted shrinkage is compared with experimental results for silica gels dried from different surface tension pore fluids (2

Shrinkage during drying of silica gel

NMR measurements of the spin—lattice relaxation decay time for a fluid in a porous solid have been previously demonstrated as a significant tool for pore size/structure analysis. The analysis requires the solution of a Fredholm integral equation of the first kind to extract the desired T1, and therefore, pore size distribution from the relaxation measurements. Earlier work successfully used a nonnegative least-squares (NNLS) technique to solve this problem. However, for stability to be achieved with that approach, the problem must be significantly overconstrained (i.e., more relaxation measurements than pore size distribution points) which limits resolution. In addition, broad distributions are represented as several narrow peaks. In this work, the method of regularization is applied to extracting continuous pore volume distributions from relaxation measurements. The validity of the approach used to calculate the optimum value of the regularization smoothing parameter given an expected noise level and the data is demonstrated using “data sets” generated from known T1 distributions. Varying levels of random noise are added to these data sets and the T1 distribution is calculated for comparison to the starting distribution. For error levels in the range of 0.001 to 0.01, agreement between the starting and the calculated distributions is good. The method has also been applied to relaxation data obtained for water in several controlled pore glasses and “random packing of sphere” solids. Agreement to NMR/regularization and mercury porosimetry-derived pore size distributions was excellent with the only deviations being the result of porosimetry measuring pore neck size and NMR measuring the true pore volume to surface area ratio.

A NMR technique for the analysis of pore structure: Determination of continuous pore size distributions

Commercial development of aerogel granules, produced via ambient pressure processing, for advanced thermal insulation is discussed. By employing modeling and experimental verification, the thermal performance of granule compacts was studied. The application of a novel approach for surface modifying carbon black to the problem of adding opacifiers to silica aerogels is shown and improved thermal and mechanical properties were obtained. When opacified granules are produced with the correct density/strength, the application of a 1 bar load (as is naturally applied in vacuum panels) results in deformation/rearrangement such that a packing fraction approaching one is achieved. The thermal performance of these granule compacts, at ambient pressure and vacuum, is similar to that of aerogel monoliths without the cost and processing problems of monoliths.

Douglas M. Smith

Papers

Characterization of Porous Solids

Sol-gel strategies for controlled porosity inorganic materials

Shrinkage during drying of silica gel

A NMR technique for the analysis of pore structure: Determination of continuous pore size distributions

Aerogel-based thermal insulation