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

Competitive and sequence reactions of typical hydrocarbon molecules in diesel fraction hydrocracking - a theoretical study by DFT calculations

The molecular structures of hydrocarbon molecules determine the competitive and sequence reactions in the diesel hydrocracking process. In this study, the hydrocracking reactions of typical hydrocarbons with various saturation degrees and molecular weights in diesel fractions synergistically catalyzed by the Ni–Mo–S nanocluster and Al–Si FAU zeolite are investigated. The results show that the two major rate-controlling steps in saturated hydrocarbon hydrocracking are dehydrogenation on the Ni–Mo–S active sites and the cracking of the C–C bonds on the FAU zeolite acid center. Moreover, the major rate-controlling step in cracking the cycloalkyl aromatic hydrocarbons is the protonation of the aromatic ring. Moreover, the aromatic hydrocarbons presented an apparent advantage in competitive adsorption on the Ni–Mo–S active sites, whereas hydrocarbons with higher molecular weights demonstrated a moderate adsorption advantage on both Ni–Mo–S active sites and FAU zeolite acid centers.

The molecular structures of hydrocarbon molecules determine the competitive and sequence reactions in the diesel hydrocracking process.

Enhanced light olefins production via n-pentane cracking using modified MFI catalysts

n-pentane catalytic cracking was studied over a series of MFI zeolites with varying SiO₂/Al₂O₃ ratios (30, 80, 280, 500, and 1500) using a fixed-bed reactor operated at temperature 550–650 °C. Other MFI zeolites (SiO₂/Al₂O₃ = 280) with various crystal morphology and size (such as large crystal and nano size) were also synthesized and tested for n-pentane cracking. The effects of MFI zeolite modification with ammonia and phosphorus on its physiochemical properties and catalytic activity were investigated. Among the parent MFI zeolites, MFI (280) demonstrated high selectivity (51%) towards light olefins (C₃⁼/C₂⁼ = 0.7) at 650 °C with undesired C₁–C₄ alkanes (38%). Surface modified MFI (280) zeolites of different crystal size and morphology showed improvement towards propylene selectivity by suppressing undesired reactions. Phosphorous-modified MFI zeolite with a large crystal size was found to improve light olefin selectivity (52.2%) with C₃⁼/C₂⁼ = ∼1.3 and reduce undesired C₁–C₄ alkanes (8%) formation due to suppressed strong acidic sites. The characterization and evaluation results for the modified MFI (280) revealed that the incorporation of phosphorous created moderate acidic sites, which were stabilized by some non-framework aluminum species, thereby leading to suppressing the formation of undesired C₁–C₄ alkanes with improved light olefins selectivity.

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n-pentane cracking over MFI zeolites with various Si₂O₃/Al₂O₃ ratios (30, 80, 280, 500, and 1500) was investigated.

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MFI (280) zeolite at 650 °C showed 51% selectivity for light olefins (C₃⁼/C₂⁼ = 0.7) with 23.8% undesirable C₂-C₄ alkanes.

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Phosphorous-modified large crystal MFI catalyst with moderate acid sites inhibited hydrogen transfer reaction.

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P/HZ280-LC provide improved light olefins production (selectivity 52.2%, C₃⁼/C₂⁼ = 1.3).

Effects of single and double active sites of Cu oxide clusters over the MFI zeolite for direct conversion of methane to methanol: DFT calculations

In this work, we investigate the effect of various species of Cu oxide clusters including single and double active sites incorporated in the MFI zeolite framework for the direct conversion of methane to methanol. An M06-2X density functional calculation is employed to fine-tune the suitable number and species of active sites and to provide insights into the effect of the active sites on the reaction mechanism of methane to methanol. Two models, single and double active sites of Cu oxide clusters, have been chosen, in which the single active site of Cu oxide clusters, (mono(μ-oxo)dicopper(ii)), is located at the Al1'-Al12' pair ([Cu(μ-O)Cu]2+@Al1'-Al12'/MFI) or at the Al6-Al7 pair ([Cu(μ-O)Cu]2+@Al6-Al7/MFI) in the MFI framework. For the double active sites of Cu oxide clusters, two species of double active sites of Cu oxide are considered. The first one is the double active site of mono(μ-oxo)dicopper(ii) containingtwo Al-Al pairs (Al1'-Al12' and Al6-Al7 pairs) in the MFI framework (2[Cu(μ-O)Cu]2+/MFI) and the other is the double active site of trans-μ-1,2-peroxo dicopper(ii), which occupies two Al-Al pairs (Al1'-Al12' and Al6-Al7 pairs) in the MFI framework (2[Cu(μ-1,2-peroxo)Cu]2+/MFI). Furthermore, the activation energy for C-H bond dissociation in direct methane conversion to methanol is considered. Compared with the single active site of [Cu(μ-O)Cu]2+/MFI, the double active sites, in particular (2[Cu(μ-O)Cu]2+/MFI), exhibited the lowest activation energy, approximately 12.5 kcal mol-1. The high charge transfer between activated methane and Cu oxide active sites and also the high negative partial charge at the bridging oxygen of Cu oxide active sites, which directly interact with the methane molecule and abstracts its H atom, are considered as the important factors which affect the catalytic activity of Cu oxide clusters for direct methane conversion to methanol. These findings strongly support that the number and species of Cu oxide active sites incorporated in the MFI framework can highly affect the reaction mechanism of methane to methanol.