Nabil Al-Yassir

The effect of coke deposition on the activity and selectivity of the HZSM-5 zeolite during ethylbenzene alkylation reaction in the presence of ethanol

The alkylation of ethylbenzene (EB) with ethanol over HZSM-5 zeolite catalysts was carried out using a riser simulator reactor at different reaction temperatures and contact times. It was observed that the amount of coke deposited over the zeolite has a great influence on the reaction pathway. At higher temperature (400 °C), the spent catalyst was found to exhibit a much higher activity towards the alkylation products (DEB) as compared to the fresh catalyst. This difference between spent and fresh samples became less significant at lower temperature (250 °C). The highest yield of DEB products over the spent catalyst was obtained for intermediate temperatures (300–350 °C). The coke deposition was further analyzed using terahertz time-domain spectroscopy (THz-TDS), 27Al MAS NMR spectroscopy, TPO measurements and pyridine-FTIR. THz-TDS and TPO results revealed that the structure of coke formed on all catalysts is essentially the same, while pyridine-FTIR studies revealed that coke leads to a reduction in the acidity of the catalyst. 27Al MAS NMR results of spent samples suggested a relationship between alkylation activity and extra-framework aluminium species, which is possibly associated with the formation of very active Lewis sites. This work shows that an understanding and control of different types of catalytic sites over zeolite surfaces may improve and optimize reaction performances during alkylation of aromatics.

A new perspective on catalytic dehydrogenation of ethylbenzene: the influence of side-reactions on catalytic performance

The dehydrogenation of ethylbenzene to styrene is a highly important industrial reaction and the focus of significant research in order to optimise the selectivity to styrene and minimise catalyst deactivation. The reaction itself is a complex network of parallel and consecutive processes including cracking, steam-reforming and reverse water-gas shift (RWGS) in addition to dehydrogenation. The goal of this investigation is to decouple the major processes occurring and analyse how side-reactions affect both the equilibrium of ethylbenzene dehydrogenation and the surface chemistry of the catalyst. Studies have employed a CrOx/Al2O3 catalyst and reactions have been conducted at 500, 600 and 700 °C. The catalyst and reaction have been investigated using elemental analysis, temperature programmed oxidation (TPO), temperature-programmed desorption (TPD), Raman spectroscopy, THz time-domain spectroscopy (THz-TDS), X-ray photoelectron spectroscopy (XPS), in situ infrared spectroscopy and on-line gas chromatography and mass spectrometry. The reaction profile shows an induction time corresponding to a cracking regime, followed by a dehydrogenation regime. The cracking period involves the activation of CrOx/Al2O3 catalysts for dehydrogenation activity through a number of processes: cracking of ethylbenzene over acid sites; coke deposition; reduction of chromium from Cr(VI) to Cr(III); steam reforming activity over the reduced catalyst; and reverse water-gas shift reaction. Each of these processes plays a critical role in the observed catalytic activity. Notably, the presence of CO2 evolved from the reduction of chromium by ethylbenzene and from the gasification of the deposited oxygen-functionalised coke results in the dehydrogenation reaction becoming partially oxidative, i.e. selectivity to styrene is enhanced by coupling of ethylbenzene dehydrogenation with the reverse water-gas shift reaction. Ethylbenzene cracking, coke gasification, steam-reforming and reverse water-gas shift determine the relative quantities of CO2, CO, H2 and H2O and hence affect the coupling of the reactions. Coke deposition during the cracking period lowers the catalyst acidity and may contribute to chromium reduction, hence diminishing the competition between acid and metal sites and favouring dehydrogenation activity.

The enhancement of the catalytic performance of CrOx/Al2O3 catalysts for ethylbenzene dehydrogenation through tailored coke deposition

In previous work we have shown that ethylbenzene dehydrogenation over CrOx/Al2O3 catalysts proceeds sequentially via cracking and then dehydrogenation reactions. The present work reports how tailored coke deposition on the catalyst surface can suppress undesired reactions such as cracking to benzene and coke during ethylbenzene dehydrogenation. Additionally, this approach also provides insights into the precursor molecules involved in the formation of carbonaceous deposits, hence providing further understanding of coke formation. Pre-coked catalysts were prepared by adsorbing the products of the ethylbenzene reaction (i.e., benzene, toluene, styrene, ethylene) as single components, in a flowing system at 600 °C over the fresh catalyst. The resulting pre-coked catalysts were then evaluated in the ethylbenzene dehydrogenation reaction and their performance compared with that of the catalyst without exposure to pre-treatment. Characterisation of pre-coked catalysts by elemental analysis, temperature-programmed oxidation (TPO), temperature-programmed desorption (TPD), Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that ethylene is the main coke precursor during ethylbenzene dehydrogenation and that ethylene-derived coke is associated with a reduction in selectivity to styrene as compared to the fresh catalyst. Coke deposited after pre-coking with aromatic molecules, and in particular with benzene, was beneficial for dehydrogenation activity, as shown by the increase in styrene selectivity relative to the fresh catalyst. This enhancement of dehydrogenation activity was correlated with deactivation of acid sites and the reduction of chromium from Cr(VI) to Cr(III) (active species for dehydrogenation) as a result of the pre-coking procedure.

Structural changes in FeOx/γ-Al2O3 catalysts during ethylbenzene dehydrogenation

Abstract The structural changes that occur in a FeOx/γ-Al2O3 catalyst during the dehydrogenation of ethylbenzene in a fluidized CREC Riser Simulator have been investigated. Chemical and morphological changes are observed to take place as a result of reaction. Electron microscopy reveals the formation of needle-like alumina structures apparently enclosing iron oxide particles. The formation of such structures at relatively low temperatures is unexpected and has not previously been reported. Additionally, X-ray diffraction and Mössbauer spectroscopy confirmed the reduction of the oxidation state of iron, from Fe2O3 (haematite) to Fe3O4 (magnetite). Iron carbides, Fe3C and ɛ-Fe2C, were detected by electron microscopy through electron diffraction and lattice fringes analysis. Carbon deposition (coking) on the catalyst surface also occurs. The observed structural changes are likely to be closely correlated with the catalytic properties of the materials, in particular with catalyst deactivation, and thereby provide important avenues for future study of this industrially important reaction. Graphical abstract Fe2O3/Al2O3 catalyst undergoes chemical and morphological changes during ethylbenzene dehydrogenation forming Al2O3 needles which appear to contain reduced Fe3O4 particles. Fe3C also forms during reaction.