While most papers focuses on catalysts improvement, this paper is about a kinetic model based on a new reaction scheme to explain the transformation of ethanol/acetaldehyde to butadiene and side products. The reaction scheme presented here is more complex than the route usually presented in the literature, i.e. Gorin-Jones route. In order to develop this kinetic model, the reaction scheme is studied by means of fixed bed type catalytic tests using Ta2O5-SiO2 catalysts with a temperature range from 320 degrees C to 370 degrees C and a constant pressure. The reactor (gas/solid, isothermal and isobaric) is modelled in steady state. It is concluded that the model that includes two different routes of butadiene production is in good agreement with the productions of major products and is necessary to explain the formation of some side products, highlighting a brand new reaction scheme for ethanol-to-butadiene transformation.

Successfully modeling the behavior of catalytic systems at different scales is a matter of importance not only for a fundamental understanding but also for a more rational design of catalysts and a more precise definition of the kinetic laws used as inputs in chemical engineering. We have developed here a multiscale modeling of the dehydration of isopropyl alcohol to propene and diisopropyl ether on gamma-alumina catalysts, which clearly evidences and explains the central character of cooperative effects between coadsorbates in the kinetic network. The evolution of partial pressures with contact time was simulated using an original DFT-based microkinetic model based on a "macro site" centered on the main active site located on the (100) planes of alumina and comprising several neighboring adsorption sites. The formation of isopropyl alcohol isopropyl alcohol or water isopropyl alcohol dimers on the surface was required to correctly simulate the production of the minor product, diisopropyl ether, and the evolution of the product partial pressures at high conversion. DFT calculations were used to identify the structure of these dimers. In addition to entropic effects, the selectivity to ether is ruled by (i) stabilizing interactions between coadsorbed isopropyl alcohol or water molecules and the nucleophilic alcohol molecule reacting with the alcoholate intermediate, (ii) the formation of alcoholate water dimers that selectively inhibit the formation of propene and increase the selectivity to ether at low conversion, and (iii) the reverse transformation of diisopropyl ether into propene and isopropyl alcohol that consumes ether at high conversion. The analytical expression of the reaction rate derived from this model and based on the existence of ensembles of interacting isopropyl alcohol and water molecules leads to a satisfactory modeling of the experimental kinetic measurements at all conversions.

The mechanism of isopropanol dehydration on amorphous silica-alumina (ASA) was unraveled by a combination of experimental kinetic measurements and periodic density functional theory (DFT) calculations. We show that pseudo-bridging silanols (PBS-Al) are the most likely active sites owing to the synergy between the Brønsted and Lewis acidic properties of these sites, which facilitates the activation of alcohol hydroxy groups as leaving groups. Isopropanol dehydration was used to specifically investigate these PBS-Al sites, whose density was estimated to be about 10-1  site nm-2 on the silica-doped alumina surface under investigation, by combining information from experiments and theoretical calculations.

Alcohol dehydration is of prominent relevance in the context of biomass conversion. This reaction can be efficiently catalyzed by alumina surfaces, but the nature of active sites, the mechanisms involved, and the key parameters to tune both the activity and the alkene/ether selectivity remain a matter of debate. In the present paper, isopropanol dehydration to propene and diisopropylether over gamma-alumina, delta-alumina, and sodium-poisoned gamma-alumina was investigated through a combined experimental and theoretical study. The experimental kinetic study shows that dehydration occurs following the same reaction mechanism on all materials, although gamma-alumina activated above 450 degrees C exhibits the highest density of active sites and the highest global activity. Results suggest that all the reaction pathways involved in dehydration require the same set of adjacent active sites located on the (100) facets of gamma-alumina. DFT transition-state calculations of the formation of propene and diisopropylether on the main terminations of alumina, (110) and (100), were also performed. The less activated pathways for both the formation of the olefin (E2 mechanism) and the formation of the ether (S(N)2 mechanism) were found on a Al-V Lewis acidic site of the (100) termination, with calculated activation enthalpies (125 and 112 kJ.mol(-1) for propene and diisopropylether formation, respectively) in good agreement with the experimental values (128 and 118 kJ.mol(-1), respectively). The higher or lesser selectivity toward propene or ether appears to originate from significantly different activation entropies. The effect of coadsorbed sodium on the reaction is linked to the poisoning of Al sites by neighboring, Na-stabilized OH groups, but no influence of sodium on distant sites is evidenced. Reaction temperature is identified as the main key parameter to tune alkene/ether selectivity rather than morphology effects, which in turn affect drastically the number of available active sites, and thus catalytic activity.

Na-doped Cu–Al catalysts are efficient catalysts for acetone condensation to linear trimers.

The catalytic performances of HZSM-5 zeolites with Si/Al ratios ranging from 16 to 500 were investigated for ethanol transformation into hydrocarbons. The fresh and used catalysts were characterized by a combination of nitrogen adsorption, pyridine adsorption followed by infrared spectroscopy (IR), gas chromatography–mass spectrometry (GC–MS) coupling and electron paramagnetic resonance (EPR). HZSM-5(Si/Al=40) was found to be the most stably and selective catalyst due to an optimum balance between the number of Brønsted acid sites and the amount of radicals, which are active sites for ethanol conversion into higher hydrocarbons. However, a change of radical species nature occurred with time-on-stream (TOS) which could be responsible for the deactivation of all catalysts leading to a decrease of C3+ hydrocarbons yield.

HZSM-5 (Si/Al ratio = 16) zeolite was found to be a very stable and efficient catalyst for ethanol transformation into hydrocarbons at 350 degrees C and 30 bar total pressure. Deactivation, in our operating conditions, was only observed after 16 h on stream, and for ethylene transformation into higher hydrocarbons only. Carbon deposit evolution with time-on-stream (TOS) was fully characterized using IR spectroscopy, GC-MS (after CH2Cl2 extraction and HF solubilization), and electron paramagnetic resonance (EPR) techniques. The carbon content was very high from the reaction beginning, leading to great losses, of microporosity and acidity. Nevertheless, C3+ hydrocarbons yield remained high even after 30 h on stream. EPR analysis allowed us to show. the existence of free radical species among the species adsorbed, from the reaction beginning. A decay period of the number of radical species, as well as a change in their chemical nature coincides with the moment of deactivation of the catalyst, leading to a decrease in the formation of C3+ hydrocarbons. The existence of reactive radical species could explain the high catalytic performances of the catalyst at 30 h TOS, considering the losses in acidity and microporosity. The apparent correlation between the formation of C3+ hydrocarbons and the existence of active radical species could indicate the existence of radical reactions, which should occur at pore mouth. The correlation between the analytical and the catalytic results should be instructive to a better understanding of the deactivation as well as ethanol's transformation reaction mechanism.

HZSM-5 zeolite was found to be a very stable catalyst for the ethanol transformation into hydrocarbons at 350°C and 30bar total pressure. It was found to maintain high activity for C3+ hydrocarbons formation with time-on-stream in spite of a near total loss of Brønsted acidity, 92% loss of microporosity and high coke content deposited inside its micropore volume. The same solid, passivated with TEOS was tested in the same conditions and it was found that the treatment slightly improved the catalytic performance of the zeolite, even if similar losses of acidity and microporisty were determined after reaction. This shows that C3+ hydrocarbons’ formation does not occur at the external surface. Alkyl aromatic hydrocarbons were found occluded in the zeolite structure after reaction, detected by IR spectroscopy analysis and by CH2Cl2 extraction after solubilization of the structure with HF solution. EPR-CW analysis of both coked samples proved existence of free radicals. This last technique could provide us further enlightening of the ethanol transformation mechanism.

Ethanol transformation into higher hydrocarbons in one step by heterogeneous acid catalysis was studied, under 350 degrees C and 30 bar of total pressure. A comparison was established among three zeolites (HFAU, HBEA and HZSM-5) having the same quantity of Bronsted acid sites but possessing different pore architectures. Large pore HFAU and HBEA zeolites gave mainly increasing yield of ethylene and diethyl ether with time-on-stream, due to the deactivation of the strongest acid sites and only a low quantity of C3+ hydrocarbons. This was explained by a faster deactivation of large pore zeolites due to fast coke formation which rapidly eliminates strong Bronsted acid sites, required for the transformation of ethylene into higher hydrocarbons. These coke molecules were identified as being polyaromatic compounds. Medium pore zeolite HZSM-5 showed an important formation of C3+ hydrocarbons (mostly C-5-C-11 compounds) and very small amounts of ethylene and diethyl ether. For this zeolite, after 16 h reaction, there was still complete ethanol transformation into C3+ hydrocarbons, even though a 55% loss of microporosity and 94% loss of Bronsted acidity were observed. On HZSM-5 the deactivation is slower and the formation of C3+ hydrocarbons was observed even when the catalyst was saturated with coke molecules (high activity for the hydrogen transfer reactions). It could be possible, that for this zeolite, reaction occurs at the pore mouth of the channel. (C) 2009 Elsevier B.V. All rights reserved.

Describes a catalyst comprising at least one IZM-2 zeolite and at least one matrix, said zeolite having a chemical composition expressed on an anhydrous basis, in terms of moles of oxides, by the following general formula: XO 2: aY2O3: BMNO wherein X represents at least one tetravalent element, Y represents at least one trivalent element and M is at least one alkali metal and / or alkaline earth metal, a and b respectively representing the number of moles of Y2O3 and MnO and is between 0.001 and 0.5, b is between 0 and 1 and n is between 1 and 2. Said catalyst is used in different processes for converting hydrocarbon feedstocks.