Extent Of Diffusion Limitations Research Articles

Effects of Internal Electron Donor on Distribution and Reactivity of Active Centers in Ethylene/1‐Hexene Copolymerization with MgCl2‐Supported Ziegler‐Natta Catalyst

AbstractEthylene/1‐hexene copolymerization with two MgCl2‐supported Ziegler‐Natta catalysts containing no internal electron donor or diethylphthalate (DEP) is conducted for different polymerization time . Effects of DEP on active center distribution are studied by fractionating each copolymer sample into boiling n‐heptane soluble (C7‐sol) and insoluble (C7‐ins) fractions, and counting the number of active centers in the copolymer fractions . The main effect of introducing DEP in the catalyst are reduction in the Ti content and significant increase in the proportion of active centers producing C7‐ins fraction. The propagation rate constants of ethylene insertion (kpE) and 1‐hexene insertion (kpH) are respectively estimated by linear fitting/extrapolating the change of apparent propagation rate constants (kpi)a with polymer yield according to a simplified multi‐grain particle model. In both catalysts, kpE in the C7‐ins fraction is 9–12 times larger than that in the C7‐sol fraction, and kpH in the C7‐ins fraction is 3–4 times larger than that in the C7‐sol fraction. The two groups of active centers have distinctly different catalytic properties. Introducing DEP reduced the kpE and kpH values and the extent of diffusion limitation . In summary, addition of electron donor in MgCl2‐supported Z‐N catalyst significantly changed the active center distribution and catalytic properties of its two groups of active centers.

Impact of zeolite and [formula omitted]-alumina intra-particle diffusion on the performance of a dual layer catalyst

Multi-layer catalytic converters reduce the cost and space of the system and improve the heat distribution for cold-start applications. However, the removal of pollutants can be limited by internal diffusion through several coated layers. In this paper we investigate the impact of diffusion limitation on the performance of a model dual-layer oxidation catalyst. The samples consist of bottom a Pt/γ-Al2O3 layer further coated by a top layer of either zeolite or alumina — both materials are of general interest for automotive exhaust gas aftertreatment. Zeolites find applications in ammonia slip catalysts, selective catalytic reduction of NOx and hydrocarbon adsorbers, and alumina is typically used in oxidation catalysts, three-way catalysts and lean NOx traps.The samples are tested in a lab reactor for CO oxidation as a probe reaction and the extent of diffusion limitation on CO conversion is evaluated. The gas transport in dual layer systems is further investigated by a 1D+1D model combined with a 3D digital reconstruction of pore space, and the contribution of micro-, meso- and macro-pores to the overall transport and conversion is quantified. The internal pores of zeolites are an order of magnitude smaller than those found in γ-Al2O3, which results in a substantially different intra-particle diffusivity. However, the analyzed samples and performed parametric study demonstrate that the macropores in the coated layers represent a key structural property that enable efficient transport. Relatively similar values of overall effective diffusivity are therefore achieved in macroporous zeolite and alumina layers, regardless of quite different sizes of small intra-particle pores.

Effects of diffusion limitation on binding of soluble ligand to avidin-coupled latex particles

The influence of particle diameter on the diffusion limitation of the interaction between a soluble ligand and its specific binder immobilized onto the surface of those particles was investigated in this study. To model a system experimentally, egg white avidin covalently immobilized onto three different diameter latex particles (0.31, 0.65, and 1.25 μm) and soluble biotin-horseradish peroxidase were used. After determination of affinity constants ( K) and the number of binding sites ( B 0) for each particle size to establish initial conditions, the relative diffusion limitation in each case was investigated by measuring the deviation of the reaction kinetics from the relationship ln ([A] M[B] ) = [A 0] – [B 0]) ft . The results were then compared to a mathematical model of the same binding reaction under completely homogeneous, soluble conditions. Larger particle size resulted in a greater deviation of the reaction kinetics from linearity. The extent of diffusion limitation however, was not found to be directly proportional to diameter. The variability in number of binding sites for each of the particle sizes did not bear on these conclusions.

Diffusion limitations in fischer‐tropsch catalysts

AbstractThe extent of diffusion limitations in the catalytic conversion of synthesis gas to hydrocarbons by the Fischer‐Tropsch reaction has been established for a number of iron‐ and cobalt‐based catalysts. The studies were performed in a fixed‐bed microreactor system at temperatures in the range 473–523 K. Variation of catalyst particle size in the range 0.2–2.6 mm shows that the conversion of synthesis gas decreases considerably when the average particle size is increased. The effects of variation of particle size and pore diameter have been quantified with the Thiele model for diffusion limitations. Evidence has accumulated that the limited mobility of reactant molecules in the liquid‐filled pores of Fischer‐Tropsch catalysts is the main cause of retardation of the reaction rates. The experimentally determined reaction rates with various catalysts operated under different conditions show an excellent fit with the theoretical model.

The reactions of 1-methyl-2-ethylbenzene: I. Exploring the structure of intracrystalline void space and the catalytic properties of molecular sieves and other catalysts

Reactions of dialkylaromatic hydrocarbons can tell one a great deal about properties of unknown catalysts by comparing the activities and selectivities over them with those over known catalysts. Minimum pore dimensions, the strength and spatial environment of acid sites, the approximate dimensions of the intracrystalline cavities in which reactions occur, the extent of diffusion limitation, and optimum activation conditions are examples of the type of information revealed by such tests. We determined methylethylbenzene reaction patterns over HY, diuturnal Y, HZSM-5, H-mordenite, and many other materials.