Shape selectivity chain length

A (Figure 4.9). The diameter of such a neck, 2.3 A, is sufficiently large for a linear C-C chain to pass, but too small to also be an equilibrium adsorption position. The largest compound allowed inside the pores is a linear molecule limited in length to four carbon atoms due to the distance between two subsequent necks [103]. Another example of shape-selective behavior is found in a Zn-based MOF able to encapsulate linear hexane while branched hexanes are blocked [104]. 

Anhydrous zinc phosphonate Zn(03PCH3) is unreactive with water but reacts with primary amines (chain length up to Cg) and ammonia to form intercalation compounds. The intercalation is shape selective as amines with branching at the a-carbon cannot coordinate to the zinc. Different structures are noted for even and odd carbon chain lengths with odd numbers of carbons in the alkyl chain forming a more efficiently packed interdigitated structure.420... 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

Alkylglucosides are a class of valuable commercial surfactants, particularly for cosmetics applications because of their biocompatibility. They are obtained by acetalization of carbohydrates with fatty alcohols in the presence of acid catalysts. Zeolites and MCM-41 have been used as acidic catalysts to achieve the acetalization of glucose with alcohols of different chain lengths [45, 46]. Shape selectivity effects decrease the amount of oligomers formed and the activity and selectivity can be controlled with the Si/Al ratio. 

The influence of alkyl chain length on shape selectivity has been the topic of numerous investigations [98-105]. Through the use of SRM 869a test mixture, the shape... 

The exact mechanism controlling shape-selective retentive processes is not fully understood, although it is clear that the pure partitioning and adsorption models cannot account for differences in retention for isomer separations or the range of selectivityobserved for columns of various surface coverages and alkyl chain lengths. 

When the chain length is dramatically increased, as in (28), podands are available which could imitate the helical ion channels believed to be available in membranes and so give information on the selective transfer of cations across such barriers. This type of ligand shows pronounced transfer of inorganic salt into organic phases and NMR studies indicate a likely helical coordination of the cation.33,218 Complexes have been isolated and the structure of (29)2KNCS has been solved.219 In this molecule, the podand wraps, in an S-shaped manner, within the dinuclear complex, and contains a KNCS unit within each loop of the S shape. 

However, the reactivities of primary alcohols are much lower than the reactivities of secondary alcohols. While an increase in reactivity of 2-alcohols with increasing chain length can be expected on the basis of chemical reactivity, the decrease beyond C8 must have another origin, which may be reactant shape selectivity in the TS-1 catalyst. The 2-alcohol generally react faster than the 3-alcohol (Van der Pol et al., 1993b). 

The cage or window effect was proposed by Gorring (48) to explain the nonlinear effect of chain length observed in hydrocracking of various n alkanes over T zeolite, chabazite (CHA) and erionite (ERI). Thus, when a nC22 alkane is cracked over erionite, there are two maxima in the size distribution of the product molecules at carbon numbers of 4 and 11 and a minimum at carbon number of 8. The diffiisivities of n-alkanes also change in a similar periodic manner by over two orders of magnitude between the minimum at C8 and the maxima. This shows that for diffusion, and hence for shape selective effects, not only the size but also the structure of the reactant and product molecules need to be considered. 

The ZSM-5 family of zeolites show further interesting shape-selective effects. Both normal and methyl-substituted paraffins have access to interior sites, so both hexane and 3-methylpentane are cracked by ZSM-5, but steric constraints cause hexane to be cracked faster than 3-methylpentane. Further shape selectivity was found between 3-methylpentane and 2,3-dimethylbutane. No window effect with paraffin chain length was found with ZSM-5. In the conversion of methanol to hydrocarbons over ZSM-5 catalysts, the distribution 94,152,195 of aromatic products ends at Cio- The distribution of tetramethylbenzenes is not far from equilibrium, but has excess 1,2,4,5-tetramethylbenzene. Measurements of diffusion coefficients of alkyl benzenes show rapid decrease, by orders of magnitude, as ring substitution increases. 

This multicomponent catalytic system also made it possible to oxidize in high yields a wide range of straight-chain higher a-alkenes [24], However, the oxidation rate and the ketone selectivity were strikingly dependent on the nature of the cyclodextrin and on the chain length of the alkene (Figure 2). The alkene optimal size and shape was reached with 1-decene. Attempts to oxidize internal alkenes in biphasic medium with cyclodextrins modified in this way failed due to the lack of accessibility of the double bond in the inclusion complex. 

When passing a mixture of ammonia, acrolein and butanal (molar ratio 3 1 1) over a B-MPI-zeolite at 400 °C one obtains P-ethylpyridine with 72 % selectivity. For R = n-C tH and n-C6H 3 the selectivities are 78 % and 90 %, respectively. In all cases conversion is complete and catalyst lifetime > 48 h. The increasing selectivity with increasing chain length indicates shape selectivity the zeolite might induce the long chain alkanals to adopt - on average -favourable positions with respect to the other reactants. 

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