N-alkane transformation

The mechanism of n-alkane transformation over bifunctional zeolite catalysts has traditionally been assumed to occur through a sequence of steps (41) ... 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

The transformation of n-Ci6, (Aldrich, > 99.9 % purity) was carried out in a fixed bed stainless steel reactor under the following conditions temperature = 220°C, total pressure = 30 bar, H2/n-alkane molar ratio = 20, WHSV (weight hourly space velocity) = 2-100 h 1. WHSV was changed by modifying the catalyst weight and/or the flow rates in order to obtain different conversion values. Before use, the catalysts were reduced in-situ under hydrogen flow at 450°C during 6h. 

The analysis of the literature data shows that zeolites modified with nobel metals are among perspective catalysts for this process. The main drawbacks related to these catalysts are rather low efficiency and selectivity. The low efficiency is connected with intracrystalline diffusion limitations in zeolitic porous system. Thus, the effectiveness factor for transformation of n-alkanes over mordenite calculated basing on Thiele model pointed that only 30% of zeolitic pore system are involved in the catalytic reaction [1], On the other hand, lower selectivity in the case of longer alkanes is due to their easier cracking in comparison to shorter alkanes. 

One of the initial spectroscopic methods applied to stationary-phase characterization was Fourier transform infrared spectroscopy (FTIR). This originated from several important studies of phase conformational order in crystalline n-alkanes conducted in the late 1960s and early 1980s by Snyder, Maroncelli, and coworkers [111-114], In this work, assignments of C—H bond wagging modes were associated with chain... 

A unique information with respect to the use Pd on HZSM-5 in a selective hydrogenolysis has been disclosed.501 The transformation of methylcyclohexane to n-alkanes with two or more carbon atoms is a useful transformation since these products are desirable components in synthetic steam-cracker feedstock. It was shown that these compounds are not obtained on catalysts with high (0.2 or 1%) Pd loading or without Pd. But on Pd on H-ZSM-5, with Pd content in the range of 10-100 ppm, the desired products are formed with high ( 78%) selectivity. 

The skeletal isomerization of straight-chain paraffins is important for the enhancement of the octane numbers of light petroleum fractions. The isomerization of H-butane to isobutane has attracted much attention because isobutane is a feedstock for alkylation with olefins and MTBE synthesis. It is widely believed that the low-temperature transformation of n-alkanes can be catalyzed only by superacidic sites, and this reaction has often been used to test for the presence of these sites. 

Reactant shape selectivity was the basis of the Selectoforming process previously mentioned. The n-alkanes of light gasoline (essentially n-pentane, n-hexane) enter the pores of the erionite catalysts and are transformed into propane and n-butane, whereas the branched alkanes are excluded from the pores and do not react (Figure 1.5a). 

Size Calibration. The -Styragel columns were calibrated to transform the elution data from the time domain to the size domain using both n-alkane and polystyrene (Pressure Chemicals) standards. The n-alkane sizes are related to the carbon number by Equation 1 (7). 

Figure 7. Schematic representation of the transformation from (a) NIF (four layers shown) to (c) mixed folded-extended (FE) forms (two triple layers shown) in long-chain n-alkanes with around 200 C atoms. The intermediate stage in panel b should not be taken literally, as the cilia are likely to crystallize simultaneously with their coalescence in the middle layer (from ref 61, by permission of Elsevier Science Publishers).

When either of the semicrystalline forms of the asymmetric alkane are cooled, they transform to a double-layer and a triple-layer crystalline structure, respectively. These 1-d superlattices are described as ABAB... and ABAABA... stacks of crystalline layers as depicted in Figure 25, panels c and e. Note that the structure in Figure 25e is related to the mixed integer folded-extended structure in some pure n-alkanes (Figure 7c) and in binary mixtures of long n-alkanes (Section II.J, Figure 30b). 

Molnar, S.P. and King, J.W. (1998) Parametric transform and moment indices in die molecular dynamics of n-alkanes. Int. J. Quant. Chem., 70. 1185-1194. 

The transformation of primary into secondary undecyl radicals and the dominance of secondary undecyl radicals at much higher concentrations unequivocally show that, as a rule, undecane aggregates in CCI3F are amorphous and this conclusion can reasonably be extended to all n-alkanes with Hq < 11 for concentrations below about 20 mol%. In the EPR spectra of y-irradiated CCl3F/undecane primary radical features reappear from about 7 mol%, however, and these features... 

Jovancicevic B, Polic P, Vrvic M, Sheeder G, Teschner T, Wehner H (2003) Transformations of n-alkanes from petroleum pollutants in alluvial ground waters. Environ Chem Lett 1, 73-81. 

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