Probe Reactions of Alcohols and Alkanes for Understanding Catalytic Properties of Microporous Materials and Alumina Oxide Solid Acid Catalysts.

摘要

The measured kinetic effects of ethanol and ethylene pressure on diethyl ether (DEE) formation over three zeolite materials (H-MFI, H-FER and H-MOR) show that ethanol dimers are formed during reaction and that these dimeric species are subsequently dehydrated to form DEE. In zeolites, ethylene formation was only observed on zeolites possessing 8-MR channels (H-MOR). The 8-MR channels protect ethanol monomeric species and prevent the formation of ethanol dimeric species due to size restrictions. The results of ethanol dehydration reactions studied in this research imply that the design and selection of microporous catalysts for performing shape-selective reactions of oxygenates requires us to consider the size and stability of the corresponding surface intermediates as well as the location of Bronsted acid sites.;Hydroisomerization reactions on bifunctional metal-acid catalysts (Pt/Al 2O3 and acid zeolites) can convert n-hexane into 2-methylpentane (2MP) and 3-methylpentane (3MP). The measured rate of n-hexane isomerization was linearly proportional to the molar ratio of H2 to n-C6H14 over three zeolites (FER, MOR, BEA), consistent with a bifunctional mechanism involving the facile dehydrogenation of n-hexane into n-hexene on the metal catalyst and a kinetically-relevant step involving isomerization of n-hexene to 2MP and 3MP on zeolitic acidic sites. Sodium-exchanged MOR was used to study the rate of n-hexane isomerization in 12MR channel (MOR(12MR) ) and 8MR pockets (MOR(8MR)) in MOR. The measured rate of isomerization over zeolites increase in the order of FER < MOR(12MR) < MOR(8MR) < BEA, showing that pore size cannot be used to accurately predict the occurrence or exclusion of a particular reaction within zeolitic solids.;The mechanisms and site requirements for unimolecular and bimolecular dehydration reactions of ethanol on gamma-Al2O3 were investigated using steady state and isotopic kinetic studies and in situ titration. Pyridine exposure (3kPa) nearly shut down the rates of formation, showing that acid sites rather than basic sites are required for ethylene and DEE formation. The proposed mechanisms for ethylene and DEE formation proceed through desorption of surface-bound ethoxide species and the activation of surface ethanol dimeric species, respectively.