CATALYSIS ON FUEL CHEMISTRY BASIC INFORMATION AND TUTORIALS

CATALYSIS AND ITS APPLICATION IN FUEL CHEMISTRY
What is Catalysis?

The topic of catalysis recurs throughout fuel chemistry. A catalyst increases the rate of a chemical reaction without itself being permanently altered by the reaction, or appearing among the products. The key word is rate. Catalysts affect reaction kinetics.

A catalyst affects reaction rate by providing a different mechanism for the reaction, usually one that has a markedly lower activation energy than that of the non-catalyzed reaction. Catalysts do not change reaction thermodynamics; they do not alter the position of equilibrium [A], but they can help reach equilibrium much more quickly. And, they cannot cause a thermodynamically unfavorable reaction to occur.

Catalysts can be classified as homogeneous, in the same phase as the reactants and products, and heterogeneous, in a separate phase. Homogeneous catalysts mix intimately with the reactants. This good mixing often leads to enormous rate enhancements, in some cases by more than eight orders of magnitude.

But, because they are in the same phase as the reactants and products, industrial use would require a separation operation for catalyst recovery downstream of the reaction, unless one were willing to throw away the catalyst (possibly allowing it to contaminate the products) as it passes through the reactor. For many catalytic processes, the catalyst costs much more than the reactants do, so loss of the catalyst would result in a significant economic penalty.

Usually, heterogeneous catalysts have no major separation problems, thanks to their being in a separate phase from reactants and products. However, because of their being in a separate phase, mass-transfer limitations can hold up access of the reactants to the catalyst, or hold up departure of products. Heterogeneous catalysis can also be affected by various problems at the catalyst surface.

Large-scale industrial processing almost always favors use of heterogeneous catalysts, to avoid possibly difficult downstream separation issues. Nevertheless, steady progress is being made in finding ways to overcome separation problems with homogeneous catalysts, including, as examples, membrane separation, selective crystallization, and use of supercritical solvents.

While, by definition, a catalyst remains unchanged at the end of a reaction, it can, and often does, change during a reaction. Mechanisms of many catalytic reactions often involve many steps, which collectively comprise the catalytic cycle. The catalyst might undergo change during one or more of the elementary reaction steps of the mechanism, but at the end, when its action is complete, the catalyst must emerge in its original form, ready for another catalytic cycle.

Most homogeneous catalytic reactions occur in the liquid phase. Some reactions can be catalyzed in the gas phase by homogeneous catalysts (which, because they are homogeneous, must be gases themselves). Probably the most important example of homogeneous catalysis in the gas phase is the chlorine-catalyzed decomposition of ozone, the reaction responsible for the so-called ozone hole in the atmosphere [B].

Various parameters can be used to describe quantitatively the quality or “goodness” of a catalyst. Turnover number, and the related turnover frequency, compare the efficiency of different catalysts. Turnover number indicates the number of molecules of reactant that one molecule of catalyst can convert into product.

The term “turnover” comes from the notion that the catalytic conversion is “turning over” reactant molecules into product molecules. Turnover frequency is the turnover number expressed per unit time. Selectivity expresses the fraction of the desired product, usually in weight percent or mole percent, among all products of the reaction. Ideally, selectivity should be as close to 1, or 100%, as possible. Catalyst activity can be broadly defined in terms of rate of consumption of the reactant(s) or rate of formation of products.

MEMBRANE LIPIDS BASIC INFORMATION AND TUTORIALS

WHAT ARE MEMBRANE LIPIDS?
The Importance of Membrane Lipids

Membranes form boundaries around the cell and around distinct subcellular compartments. They act as selectively permeable barriers and are involved in signaling processes.

All membranes contain varying amounts of lipid and protein and some contain small amounts of carbohydrate.

In membranes the three major classes of lipids are the glycerophospholipids, the sphingolipids and the sterols. The glycerophospholipids have a glycerol backbone that is attached to two fatty acid hydrocarbon chains and a phosphorylated headgroup.

These include phosphatidate, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol and phosphatidylserine. The sphingolipids are based on sphingosine to which a single fatty acid chain is attached and either a phosphorylated headgroup (sphingomyelin) or one or more sugar residues (cerebrosides and gangliosides, the glycosphingolipids).

The major sterol in animal plasma membranes is cholesterol, while the structurally related stigmasterol and β-sitosterol are found in plants.

The fatty acid chains of glycerophospholipids and sphingolipids consist of long chains of carbon atoms which are usually unbranched and have an even number of carbon atoms (e.g. palmitate C16, stearate C18).

The chains are either fully saturated with hydrogen atoms or have one or more unsaturated double bonds that are in the cis configuration (e.g. oleate C18:1 with one double bond).
Membrane lipids are amphipathic since they contain both hydrophilic and hydrophobic regions. In the glycerophospholipids and the sphingolipids the fatty acid hydrocarbon chains are hydrophobic whereas the polar headgroups are hydrophilic.

In cholesterol the entire molecule except for the hydroxyl group on carbon-3 is hydrophobic. In aqueous solution the amphipathic lipids arrange themselves into either micelles or more extensive bimolecular sheets (bilayers) in order to prevent the hydrophobic regions from coming into contact with the surrounding water molecules.

The structure of the bilayer is maintained by multiple noncovalent interactions between neighboring fatty acid chains and between the polar headgroups of the lipids. In biological membranes there is an asymmetrical distribution of lipids between the inner and outer leaflets of the bilayer.
Lipids are relatively free to move within the plane of the bilayer by either rotational or lateral motion, but do not readily flip from one side of the bilayer to the other (transverse motion). Increasing the length of the fatty acid chains or decreasing the number of unsaturated double bonds in the fatty acid chains leads to a decrease in the fluidity of the membrane.

In animal membranes, increasing the amount of cholesterol also decreases the fluidity of the membrane.

The fluid mosaic model describes the structure of biological membranes, in which the membranes are considered as two-dimensional solutions of orientated lipids and globular proteins.
Within biological membranes lipids and proteins cluster together in discrete domains. Lipid rafts are domains of the plasma membrane that are enriched in cholesterol, sphingomyelin and glycosphingolipids, as well as lipid modified proteins.