S. Al-Khattaf

Recent Advances in Reactions of Alkylbenzenes Over Novel Zeolites: The Effects of Zeolite Structure and Morphology

Alkylbenzenes form an important segment of petrochemical industry for the manufacture of widely used commodities and specialty products. Since the last review on this topic (8), numerous new zeolite-based catalysts have been synthesized, characterized and evaluated in various transformations of aromatic hydrocarbons. This comprehensive review covers major reactions of mono-, di-, and tri-alkylbenzenes such as disproportionation, alkylation, transalkylation, isomerization, etc., over different zeolite-based acid catalysts. During the last decade, significant progress was made in the synthesis and structure determination of novel zeolites, mesoporous single crystals, hierarchic zeolites and two-dimensional zeolites. These developments have enhanced the understanding of the role of zeolites (effects of structural type, morphology, acid sites, accessibility of acid sites, shape selectivity factors) in transformations of aromatics. In this review, the emphasis is on the influence of the type of acid sites, zeolite topology, and reaction conditions on the activity, selectivity and pathways of these reactions. Thermodynamics and reaction kinetics of transformations of aromatic hydrocarbons are also discussed. This article covers mostly literature published during the period of 2002–2013.

Diffusion and catalytic cracking of 1,3,5 tri-iso-propyl-benzene in FCC catalysts

Abstract The present study describes catalytic cracking experiments developed in a novel CREC Riser Simulator using 1,3,5-Tri-iso-propyl-benzene and two FCC catalysts with different crystal sizes (0.4 and 0.9 μm diameter). The experiments are modeled using an unsteady state model for both gas and catalyst phases. It is found that a quasi-steady state approximation can be used for the catalyst and changes in the gas phase can be accounted, under the allowed model simplifications, with a relatively simple unsteady state equation. The model is completed using two catalytic decay models, with one of them involving a decay function based on “reactant converted”. Experimental and modeling observations point towards an overall cracking reaction rate controlled by diffusion at 350–450°C with this rate shifting to one being controlled by the intrinsic cracking reaction at 500–550°C.

The influence of Y-zeolite unit cell size on the performance of FCC catalysts during gas oil catalytic cracking

FCC catalysts were prepared from Y-zeolite with different unit cell size (UCS). The matrix of the catalysts composed of kaolin filler and silica sol binder. The experimental data clearly show that UCS controls the product distribution of VGO cracking with Y-zeolite catalysts. Coke model was presented to predict coke formation as a function of VGO conversion and catalyst UCS. Gasoline yield was slightly higher at low UCS range (lower than 24.36 A) while, coke, LPG, and dry gas yields were higher at high UCS range (higher than 24.36 A). The effect of UCS on hydrogen transfer and isomerization reactions was also investigated.

Solvent-free iridium-catalyzed CO2 hydrosilylation: experiments and kinetic modeling

The iridium(III) complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN = bis(pyridine-2-yloxy)methylsilyl, coe = cyclooctene) has been demonstrated to be an active catalyst for the solvent-free hydrosilylation of CO2 with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) under mild reaction conditions (3 bar). The activity of this catalytic system depends on the reaction temperature. The best catalytic performance has been achieved at 75 °C. A kinetic study at variable temperature (from 25 °C to 75 °C) and constant pressure (3 bar) together with kinetic modeling has been carried out. The results from such a study show an activation energy of 73.8 kJ mol−1 for the process.

Hydroconversion of Triglycerides to Hydrocarbons Over Mo–Ni/γ-Al2O3 Catalyst Under Low Hydrogen Pressure

The hydroconversion of coconut oil to saturated hydrocarbons under low hydrogen pressure was demonstrated, using a sulfur-free Mo–Ni/γ-Al2O3 catalyst prepared by the co-impregnation of Ni and Mo species. The Mo–Ni/γ-Al2O3 catalyst exhibited remarkably high conversion of coconut oil as well as high selectivity for the generation of the hydrocarbon fraction associated with jet fuel. Examining variations in product distributions with contact time showed that hydrocarbons were produced primarily through the hydrogenolysis of triglycerides followed by hydrodecarboxylation of fatty acids. Increases in the contact time led to improvements in the proportion of hydrocarbons via the hydrodeoxygenation of fatty acids.Graphical Abstract

Effect of unit cell size on the activity and coke selectivity of FCC catalysts

Abstract The effect of the unit cell size of Y zeolites on their cracking performance has been investigated by X-ray diffraction, nitrogen adsorption, and VGO cracking reaction using a micro-activity test unit. Y zeolites with unit cell size in the range 24.21–24.60Awere used as materials for fluid catalytic cracking. The matrix of the catalysts composed of kaoline filler and silica binder is quite inactive for VGO cracking. The intrinsic activity of the catalysts, defined as the rate constant of VGO cracking divided by the zeolite surface area, increases with increasing unit cell size and then decreases through a maximum and correlates directly with the number of 0-NNN aluminum atoms. This indicates that only 0-NNN aluminum atoms in the zeolite are the active sites responsible for VGO cracking. The increased coke yield with larger unit cell sizes can be attributed to the increased number of 1,2,3,4-NNN aluminum atoms.

Pathway to Ethylbenzene Formation in Side-Chain Alkylation of Toluene with Methanol Over Cesium Ion-Exchanged Zeolite X

To control the styrene to ethylbenzene ratio in the products of side-chain alkylation of toluene with methanol over Cs-X-based catalysts, pathway to ethylbenzene was examined. Styrene underwent transfer hydrogenation with methanol much faster than hydrogenation with hydrogen to ethylbenzene. Addition of Cs2O and ZrB2O5 to Cs-X enhanced and suppressed, respectively, the transfer hydrogenation.Graphical Abstract

Aromatic transformations over aluminosilicate micro/mesoporous composite materials

Catalytic behavior of micro/mesoporous ZSM-5/MCM-41 composites were investigated in the transformation of 1,2,4-trimethylbenzene (TMB), meta-xylene transformation and in the cracking of 1,3,5-triisopropylbenzene (TIPB). The samples were characterized by XRD, TGA, SEM, nitrogen sorption and FTIR of pyridine adsorption. The composite materials exhibited exceptional catalytic performance compared with the microporous ZSM-5 in the transformation of 1,2,4-trimethylbenzene and m-xylene. In the cracking of 1,3,5-triisopropylbenzene, the composite materials showed higher activity as compared with the conventional Y-zeolite. The distinctive catalytic performance of these micro/mesoporous composite materials in the reactions studied was attributed to the excellent accessibility of the active sites provided by the mesopores for both reactant and product molecules. In the transformation of m-xylene, selectivity towards para-xylene over all catalysts under study follows the order: conventional ZSM-5 ≈ ZM41A1 < ZM41A2 < ZM41A3.

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.