M.A.N.D.A. Lemos

The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

In this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammetts indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO2 or SO2 for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H2, etc.), MAS-NMR of 27Al, 29Si, 1H elements and of 1H, 13C, 31P, etc. of adsorbed probe molecules, and model catalytic reactions. Modeling the way the acid features of zeolites influence the catalytic activity of these catalysts toward acid-catalyzed reactions (relation between ammonia desorption activation energy values and catalytic activities, reaction mechanism, and kinetics) completes the general analysis of acidity and zeolite chemistry.

Analysis and Modelling of Multi-Site Acid Catalysts

Acid catalysis plays a major role in industrial applications of catalytic processes. In particular, the use of microporous solid acid catalysts has an enormous impact in the petroleum refining industry, where large-scale processes, such as the catalytic cracking of heavy oil fractions, are carried out over very active zeolite catalysts. In this paper we will try to give an overview of the way one can characterise the acidity of a solid catalyst and used this information to predict the activity of the catalyst for a given acid catalysed reaction.

15-P-16-Activity-acidity relationship in Y zeolite: An experimental and quantum-chemical study

Publisher Summary This chapter discusses a clear relation between the catalytic activity and the acidity for acid catalysis using Y zeolites and that this relationship has theoretical support through quantum chemical calculations on model acid sites. The study is centered on the transformation of ethene and propene over acidic Y and USY zeolites.

Using Digital Simulation to Study Hydroquinone Oxidation on Porous Electrodes by Cyclic Voltammetry

Digital simulation is a very useful tool to understand dynamic electrochemical processes. Composite porous electrodes, comprising a micro‐porous material provide electrodes with very high effective areas and can also make use of the molecular sieving properties of some of these materials, like zeolites. Porous electrodes also raise some difficulties for the analysis of their behaviour, due to internal diffusion/adsorption processes and high ohmic‐drops. In this paper we have applied digital simulation to analyse the cyclic voltammetric behaviour of a zeolite/graphite composite electrode used in the electro‐oxidation of phenolic compounds.

Ethylene Polymerization over Transition Metal Supported Catalysts. III. Vanadium

Abstract The polymerization of ethylene in the presence of vanadium catalysts supported on zeolite NaY, HY and charcoal was examined. The catalysts were prepared by the incipient wetness method and characterized by a variety of techniques to determine the oxidation state of the vanadium species inside the zeolite. In all the catalysts that were prepared vanadium is not in a single oxidation state. These catalysts were active in the polymerization of ethylene with activities of the order of magnitude 105 gPE/molV.[M].h for the zeolite catalysts and 103 gPE/molV.[M].h for the charcoal systems. No polymer could be obtained in the absence of alkylaluminum or aluminoxane in the experimental conditions used. The kinetics of ethylene polymerization reactions using the vanadium catalysts was studied. A kinetic model based on a proposed mechanism for these reactions was used to fit the experimental data. The application of this model resulted in very good fittings and the kinetic rate constants of each elementary step could be estimated. Since the polyethylene that was produced is closely bound to the zeolite catalysts, the thermal and mechanical properties of the PE/zeolite composites were measured, indicating that the presence of zeolite improved the mechanical properties of the polymers produced.

Microkinetic Model for Propane Activation over H-ZSM5

Upgrade of light saturated hydrocarbons has received great deal of attention, strongly enhanced by the widespread use of natural gas. Due to their chemical inertness, light alkanes are hardly reactive and therefore their use is most often confined to combustion chambers. Alternative pathways towards valuable products are highly desired [1] and require the use of catalysts able to promote those hard transformations in an economical feasible way. Efficient routes for catalyst design and development for activation of light alkanes require a better understanding of the surface phenomena taking place at the catalyst surface. The microkinetic approach should be able to provide guidance in the search and tuning of materials having specific catalytic properties, by studying the correlations between surface phenomena in terms of elementary reaction steps and physicochemical properties of the catalyst. Microkinetics differs from classical macrokinetics in that it provides detailed knowledge on the surface phenomena involving reactants, products and intermediates while the later is limited to a global rate expression which, although useful in reactor design, does not reflect the underlying chemistry of the surface processes. Knowledge on these surface processes supported by characterisation techniques will provide a better understanding on the origin of the catalytic

Acidity, Activity and Micro-Kinetics Studies in an H-ZSM5

An H-ZSM5 zeolite has been characterized by TPD of ammonia and its performance was tested with several olefins (ethylene and propylene) and n-hexane. The catalytic tests were performed at a wide range of temperatures (300 – 450°C) and molar fractions of reagent in the feed. The results from the TPD have been deconvoluted in order to obtain a distribution of the strength of the acidity of the active sites. A micro-kinetic model and a simplified kinetic equation have been developed and fitted to results for the conversion of n-hexane and the olefins, respectively.

MO and MM Study of Homogeneous Propylene Polymerization with a Zirconocene Catalyst

The stereoregulation in α-olefins polymerization is one of the most important capabilities of Ziegler-Natta catalysts. Homogeneous ansa-metallocene complexes of group 4 metals have been shown to produce isotactic [1–5], syndiotactic [6] and crystalline-amorphous stereoblock polypropylene [1, 7, 8]. Molecular mechanics and ab initio methods have been used to model the catalytic sites [1, 9–13]. The results of these calculations have been used not only to confirm experimental results but also to predict the tacticity obtained with certain catalyst precursors [12]. The design of stereochemical control based on these calculations can help the experimentalists to save time and efforts.