Nanocavity effects of various zeolite frameworks on n-pentane cracking to light olefins: combination studies of DFT calculations and experiments.

Better control of the product selectivity of light olefins (e.g., ethylene and propylene) obtained from the n-pentane catalytic cracking process has attracted considerable attention from both scientific and petrochemical industrial points of view. In this context, we report insights into the effects of the nanocavities of various zeolite frameworks, including H-FER, H-ZSM-5, and H-FAU, representing small, medium, and large cavities, on the reaction mechanism of n-pentane cracking to light olefins by using M06-2X/6-31G(d,p) density functional calculations, eventually leading to fine-tuning the product distribution of light olefins. The reaction mechanism consists of the following two main steps: (i) the protolytic cracking of n-pentane to form a pentonium intermediate; and (ii) the subsequent dissociation of the intermediate to either ethane-propylene or ethylene-propane. The key reaction pathways controlling the product distribution of light olefins relate to the dissociation of the pentonium intermediate, which can produce selectively either propylene (P) or ethylene (E), resulting in a controllable P/E ratio. The differences in the activation energies for ethylene production compared with those of propylene production over H-FER, H-ZSM-5, and H-FAU are 6.7, 5.0, and 0.5 kcal mol-1, respectively. Compared with H-ZSM-5 and H-FAU, the higher difference in the activation energy of these two pathways over H-FER implies that the preferable production of ethane-propylene compared with ethylene-propane is more pronounced. It is therefore reasonable to conclude that a smaller pore zeolite such as H-FER eventually leads to a high ratio of production of propylene to ethylene, in accordance with experimental observations.

Insights into the reaction mechanism of n-hexane dehydroaromatization to benzene over gallium embedded HZSM-5: effect of H2 incorporated on active sites.

The catalytic dehydroaromatization of alkanes to aromatics has attracted considerable attention from the scientific community, because it can be used for the upgrading of low-cost alkanes into high added-value aromatics, such as benzene, toluene, and xylene (BTX). In this context, we report the reaction mechanism of n-hexane dehydroaromatization to benzene over two different reduced gallium species embedded in HZSM-5, including univalent Ga+ embedded in HZSM-5 (Ga/HZSM-5) and dihydrido gallium complex (GaH2+) embedded in HZSM-5 (GaH2/HZSM-5) by using the M06-2X/6-31G(d,p) level of calculation. The reaction proceeds by following two main steps: (i) the dehydrogenation of hexane to haxa-1,3,5-triene; (ii) the dehydroaromatization of haxa-1,3,5-triene to benzene. For the univalent Ga+ embedded in HZSM-5, the first step of the hexane dehydrogenation is considered to be the rate-determining step, which requires a high activation energy of 76.6 kcal mol-1. In strong contrast to this, in the case of the GaH2/HZSM-5 catalyst the rate determining step is found to be the second hydrogen abstraction from n-hexane with a lower activation barrier of 11.1 kcal mol-1. The reaction is therefore preferentially taking place over the GaH2/HZSM-5 catalyst. These observations clearly confirm the existence of a dihydrido gallium complex (GaH2+) as one of the most active species for the dehydroaromatization of alkanes and it is obtained in the presence of hydrogen in the catalytic system. This example opens up perspectives for a better understanding of the effect of active species on the catalytic reaction.

Treatment of distillery wastewater by the nano-scale zero-valent iron and the supported nano-scale zero-valent iron.

The treatment of wastewater from the distillery industry was carried out by using nano-scale- and supported nano-scale zero-valent iron at a laboratory scale and ambient temperature. Effects of dilution, pH, mixing rate, zero-valent iron dosage, and amount of support for the zero-valent iron were investigated. All parameters had a significant effect on the removal efficiency of all investigated pollutants. Increasing the number of dilutions and the nano-scale zero-valent iron dosage led to the increase of removal efficiency of pollutants. Higher removal efficiency was achieved in an acidic initial pH of wastewater. The reduction of all pollutants was limited by the kinetics of the pollutant destruction/reduction by nano-scale zero-valent iron particles at a mixing rate greater than 170 rpm. At optimum condition, greater than 95, 94, and 64% of color, chemical oxygen demand, and biochemical oxygen demand were removed, respectively, within 6 hours. Additionally, the presence of a support had a significant effect on pollutant removal.