Paraffin chain dimensions

Table VII (51). The relevant free dimensions are often similar for zeolite and nonzeolite. Urea (free diameter 5.2 A) is like Sieve A (free diameter of windows 4.3 A) in accommodating n- but not isoparaffins. Thiourea (6.1 A) and offretite (6.3 A) have channels with similar free diameters as do 0-cyclodextrin (7-8 A) and zeolite L (7.1 X 7.8 A). In thiourea the loose fit of n-paraffins in the tunnel appears to destabilize the adducts (85, 36). The same is true of disc-shaped molecules comprising only benzenoid rings. However, if suitably bulky saturated side chains are attached (cyclohexyl-benzene or fertf-butylbenzene), then adduction readily occurs. Heterocy-clics, like unsubstituted aromatics, do not readily form adducts. Thus flat molecules also exert a destabilizing effect upon the tunnels of a circular cross section. Such stability problems do not arise with the robust, permanent zeolite structures, and this constitutes an interesting distinction. Offretite, for example, readily sorbs benzene or heterocyclics with or without alkyl side chains, provided only that they are not too large to permeate the structure.

Kobayashi used molecular dimensions directly (101-4). For paraffins he defined an unstability factor which was a measure of approach to a sphere. A fair correlation was obtained between his factor and blending octane number. The calculated factors indicated the observed rise in knock rating with centralization of the double bond in a straight-chain olefin. Gaylor (69) applied Kobayashi s method to aromatics and compared calculations with both clear and blending octane numbers. It is difficult to select molecular dimensions of a flexible molecule representative of its configuration during reaction. 

The shape and size of a molecule are the factors which determine the form of the crystal lattice, and Bernal has distinguished between five main classes of organic molecules simple molecules, e.g. CH, GO(NH2)2 i molecules containing a long hydrocarbon chain, e.g. the higher paraffins and their derivatives molecules composed of planar rings, e.g. aromatic compounds complex molecules in three dimensions, e.g. the terpenes high polymeric compounds. 

It follows from the above results that small-pore molecular sieves yield principally hydrocarbons and very small amounts of Cg compounds. This may be related to diffusion constraints and cavity dimensions. Gorring has determined that the diffusion coefficients of n-paraffins in T-zeolite at 340 C decrease as the number of chain C-atoms increase from 2 to 8. Also, for most of the cases the length of zeolite cavities is less or equal to the length of the n-heptane molecule. Thus, it may be assumed that the cavity length imposes a restriction on the formation of Cg linear compounds. The combination of cavity dimensions and pore opening permits attaining high selectivities for C2-C4 linear hydrocarbons. 

Two vanadium siiicate moiecular sieves, VS-2 and V-NCL-1 with medium and iarge pore dimensions, respectively, have been synthesised and their cataiytic activity in oxidation reactions evaluated. Isolated vanadium ions, probably in framework positions, possesss unique catalytic activity and shape seiectivity in oxidation reactions. So far, such behaviour has been found oniy in the case of titanium silicaiites. Our studies demonstrate that vanadium siiicates aiso possess such features. The iatter differ from the former in their ability to oxidise even the primary carbon atoms (in paraffins and side chain aikyl groups of aromatic hydrocarbons), and effect further secondary oxidation to a greater extent. V-NCL-1, with its large pore dimensions, enables the oxidation of bulky molecules like o- and m-xylenes and 1,3,5-and 1,2,4-trimethylbenzenes. 

The unit cell dimensions calculated by these procedures for polyethylene are a = 0.740 nm, b = 0.493 nm and c = 0.254 nm and the density is (8.6 kg/m. Figure 8.3 shows the ab plane. The c-axis is parallel to the chain axis and its length of 0.254 nm is equal to the distance between alternate carbon atoms. This structure for polyethylene is identical with that determined for crystals of pure n-paraffins such as C32H66 - low molecular analogs of polyethylene. X-ray structures have been determined for many crystalline polymers. Miller [5] has published an extensive compilation of polymer unit cell data. 

The unit cell structure of polyethylene was first investigated by Bunn (20). A number of experiments were reviewed by Natta and Corradini (21). The unit cell is orthorhombic, with cell dimensions of a = 7.40, b = 4.93, and c = 2.534 A. The unit cell contains two mers (see Figure 6.5) (22). Not unexpectedly, the unit cell dimensions are substantially the same as those found for the normal paraffins of molecular weights in the range 300 to 600 g/mol.The chains are in the extended zigzag form that is, the carbon-carbon bonds are trans rather than gauche. The zigzag form may also be viewed as a twofold screw axis. 

---