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In petroleum refining, middle distillates are produced via hydroisomerization/hydrocracking processes using bifunctional catalysts often containing ultrastable Y (USY) zeolite. Petroleum fractions serving as feedstock for hydrocracking processes contain a majority of (poly)cyclic hydrocarbon molecules. Few studies on reaction mechanisms of hydrocracking of such polycyclic compounds are available. In this study the tricyclic phenanthrene molecule was used as model compound. Phenanthrene was hydrogenated on a Pt/alumina pre‐catalyst to perhydrophenanthrene, and subsequently hydrocracked over Pt/H‐USY zeolite. The feed consisted of 3 wt.% phenanthrene dissolved in n‐heptane. The reaction was performed at 280 °C and 60 bar, with a molar H2 to total hydrocarbon ratio of 7 mol/mol. The reaction products were analyzed using GCxGC‐FID/MS. This detailed analysis enabled identification of the reaction scheme involving skeletal isomerization, ring opening and fragmentation. New mechanistic insight is gained on relative reactivities of bridged and non‐bridged tricyclic naphthenes. Preferential hydrocracking routes were identified. The study provides insight in the products to be expected from polycyclic hydrocarbon isomerization and hydrocracking. [ABSTRACT FROM AUTHOR]

Abstract: A zeolite beta–silicalite-1 composite was prepared by growing polycrystalline silicalite-1 shells on single micron-sized zeolite beta crystals. The obtained core–shell material was employed for the separation of hexane isomers. Gravimetric uptake measurements revealed that the silicalite-1 shell acted as a membrane that controlled the transport to zeolite beta core. The positive impact of the silicalite-1 membrane on the separation of mono- and di-branched hexane isomers was confirmed by breakthrough experiments. The core–shell composite material allowed more selective separation of mono- and di-branched isomers than either of its components alone and had a significantly higher adsorption capacity than silicalite-1. [Copyright &y& Elsevier]

The effect of pretreatment conditions on the N2O-decomposition activity of Fe-ZSM-5 prepared by chemical vapor deposition (CVD) was studied by steady-state kinetics and step-response experiments. Pretreatment at 873 K in H2 or He significantly increased the N2O-decomposition activity of Fe-ZSM-5 CVD. In addition to the increased steady-state activity, the catalyst exhibited a very high initial activity in the step-response experiments, which slowly decayed to steady state. This “transient” activity was particularly high after pretreatment of the samples in He, whereas it vanished completely after pretreatment in O2 or exposure to water vapor. This led to the conclusion that the transient activity is related to strongly dehydroxylated Fe2+ sites created by autoreduction at high temperatures. On the basis of isotope-labeling data a mechanism for N2O decomposition is proposed. The first step is the reaction of Fe2+ with N2O to give a Fe3+O- species, which quickly transforms into a Fe2+–peroxo complex. Desorption of O2 occurs upon reaction of the complex with another N2O molecule (mainly during the transient) or via the recombination of two peroxo species after their migration over the iron oxide cluster (mainly during steady state). [Copyright &y& Elsevier]

The reaction of Fe-zeolites prepared by sublimation of FeCl3 with N2O was studied by step- and pulse-response experiments. Steps from 0 to 5000 ppm N2O were performed at 673 K, after pretreatment of the catalyst in H2, He, or O2. When the catalysts were prereduced in H2, N2O was first consumed for the reoxidation of the catalyst, before steady-state N2O decomposition started. Also pretreatment in He reduced the catalyst. This autoreduction was not spontaneous, but caused by a small amount of hydrocarbons adsorbed on the catalyst. Prereduced catalysts had a higher steady-state activity, indicating that the incorporation of oxygen from N2O changed the structure of the active sites. Pulse responses proved that O2 desorption is the rate-limiting step of N2O decomposition. O2 desorption was relatively fast from a small fraction of sites, while a second oxygen pool desorbed only slowly from the catalyst. Only the former is responsible for catalytic activity. [Copyright &y& Elsevier]

Hydroisomerization and hydrocracking of octylcyclohexane (C14H28) were performed over Pt and NiMoS‐supported catalysts, at 300 °C and 60 bar, with a molar H2 to hydrocarbon ratio of 7 mol/mol. The feed, composed of 5 wt.% phenyloctane dissolved in n‐heptane, was initially hydrogenated in situ over a pre‐catalyst, Pt or NiMoS/Al2O3. The C14 naphthene underwent isomerization and cracking under high hydrogen pressure over the bifunctional catalysts, whose acid function was represented by large‐pore zeolities (USY, Beta) or amorphous silica‐alumina (SA). For the Pt‐catalysts, Beta was slightly more active than USY. Both zeolites produced a similar product pattern. Sulfide catalysts were less well equilibrated than Pt ones and hence less active. They led to some over‐cracking, but the cracking selectivity of our naphthene reactant was much less sensitive to the metal‐acid balance than the cracking selectivity of n‐alkanes. The comparison of reactivity of octylcyclohexane with n‐hexadecane and perhydrophenanthrene is also discussed. [ABSTRACT FROM AUTHOR]

We propose a method for analyticallypredicting single-componentadsorption isotherms from molecular, microscopic and structural descriptorsof the adsorbate–adsorbent system and concepts of statisticalthermodynamics. Expressions for Henry’s constant and the heatof adsorption at zero coverage are derived. These functions dependon the pore size, pore shape, chemical composition, and density ofthe adsorbent material. They quantify the strength of the solid–fluidinteraction, which governs the low-pressure part of the adsorptionisotherm. For intermediate and high pressures, the fluid–fluidinteractions must also betaken into account. Both solid–fluid and fluid–fluidinteractions are combined within the framework of the Ruthven statisticalmodel (RSM). The RSM thus constructs theoretical adsorption isothermsthat are entirely based on microscopic molecular and structural descriptors.The theoretical results that we obtained are compared with experimentaldata for the adsorption of pure CO2and CH4onall-silica zeolites. The developed methodology allows for the estimationof the optimum properties of a nonpolar adsorbent for the adsorptionof CO2in cyclic adsorption processes. [ABSTRACT FROM AUTHOR]

Abstract: The present work describes the adsorption and separation of xylene isomers by ZIF-8. Although the formal pore diameter of ZIF-8 is much smaller than the molecular diameter of the xylene isomers, ZIF-8 is able to separate the isomers by molecular sieving. A structural study indicates that the diffusion of the xylenes into the pore structure of ZIF-8 happens via a transitory deformation of the pore aperture which is based on a tilt of the imidazolate linkers, followed by a return to the initial conformation. The rate of adsorption depends on the size of the isomer, i.e. it decreases from para- to meta- and to ortho-xylene. The separation of the xylene isomers is good in the gas phase. In liquid phase breakthrough experiments, the quality of the separation is deteriorated. Moreover, as expected for a separation based on molecular sieving, para-xylene cannot be well separated from ethyl-benzene. [Copyright &y& Elsevier]

The objective of this work was to study the adsorption and separation of the most important families of hydrocarbon compounds on metal-organic frameworks (MOFs), in comparison with zeolites. For this purpose, we have selected four probe molecules, each of them representing one of these families, i.e., o- and p-xylene as aromatics, 1-octene as an alkene, and n-octane as an alkane. The separation of these four molecules was studied by binary breakthrough experiments. To represent the large diversity of MOF structures, the experiments were carried out with (i) two MOFs with coordinatively unsaturated metal sites (CUS), i.e., Cu-btc (HKUST-1) and CPO-27-Ni, (ii) a MOF with an anionic framework and extraframework cations, i.e. RHO-ZMOF, and (iii) two rather apolar zeolitic imidazolate framework (ZIF) materials with different pore sizes, i.e. ZIF-8 and ZIF-76. Zeolite NaY and zeolite β were used as polar and apolar reference adsorbents, respectively. The results can be briefly summarized as follows: ZIFs (not carrying any polar functional groups) behave like apolar adsorbents and exhibit very interesting and unexpected molecular sieving properties. CUS-MOFs behave like polar adsorbents but show the specificity of preferring alkenes over aromatics. This feature is rationalized thanks to DFT+D calculations. MOFs with extraframework cations behave like polar (cationic) zeolites. [ABSTRACT FROM AUTHOR]

The separation of paraffin isomers is a very importanttopic inthe petrochemical industry. Zeolite 5A is industrially used to sievealkane isomers, but its pore size does not allow the separation ofmonobranched and dibranched alkanes by a kinetic mechanism. In thispublication, we compare three ZIF materials in the separation of C6-paraffinisomers: ZIF-8, ZIF-76, and a new material called IM-22. The performanceof the materials is evaluated by a breakthrough curve of binary mixturesof n-hexane, 3-methylpentane, and 2,2-dimethylbutane.We show that ZIF-8 is a very attractive alternative to zeolite 5Abecause it exhibits a high (kinetic) selectivity for the adsorptionof linear alkanes and at the same time a high adsorption capacity.The new material IM-22, a ZIF with CHAtopology, seemsto be particularly suited for the separation of mono- and dibranchedparaffin isomers. [ABSTRACT FROM AUTHOR]

The texture of a series of dealuminated macrocrystalline beta zeolites has been investigated by N2 physisorption, transmission electron microscopy, and electron diffraction. Hydrothermal dealumination creates a secondary mesoporous system in zeolite BEA. N2 physisorption together with DFT calculations indicated the presence of pores with an estimated size of 2–4 nm after steaming. After subsequent leaching, TEM revealed the newly formed cylindrical mesopores. As determined by electron diffraction, the new porous network consists of pores lying in the (0 0 1) plane. [Copyright &y& Elsevier]