Probe reactions of alcohols and alkanes for understanding catalytic properties of microporous materials and alumina oxide solid acid catalysts.

Abstract

Steady-state chemical reactions of alkanol dehydration and alkane hydroisomerization were employed as probe reactions to evaluate the effects of structure and composition of zeolites and gamma alumina (γ-Al2O3) on measured catalytic rates and selectivity to establish the mechanistic cycles and catalytic site requirements for the corresponding reactions. 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 Brønsted acid sites. Hydroisomerization reactions on bifunctional metal-acid catalysts (Pt/Al2O3 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 γ-Al2O3 were investigated using steady state and isotopic kinetic studies and in situ titration. The rates of ethylene and DEE formation from ethanol dehydration on γ-Al2O3 at 488K were not inhibited by exposure to CO2 (0-47kPa) while 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. The proposed mechanisms are consistent with the measured pressure dependence in ethanol and water, and the measured kinetic isotope effects using isotopic labeling reactants. The rate and equilibrium constants in the kinetic models derived from these mechanisms were estimated using Athena Visual Studio software to assess the stability of surface species formed during ethanol dehydration reactions.