Rachid Oukaci

Geometric and electronic effects in the selective hydrogenation of α,β-unsaturated aldehydes over zeolite-supported metals

Abstract The hydrogenation of two α, β-unsaturated aldehydes, cinnamaldehyde and 3-methylcrotonaldehyde was investigated over Ru, Pt, and Rh. The use of zeolite supports was compared with activated carbon for Ru, and the effect of changing the neutralizing cation in the Y zeolite was compared for all three metals. Selectivity to the unsaturated alcohol could be influenced by both geometric and electronic effects, and the relative importance of these effects was found to depend on the nature of the organic substrate.

The effect of group IA cations on CO hydrogenation over Ru/Y-zeolites

Previous results for zeolite-supported Ru prepared by ion-exchange suggested a possible effect of the nature and concentration of the neutralizing cations in the zeolite on the catalytic properties of the metal. The interpretation of these results were complicated by the fact that a series of zeolites with different Si/Al ratio was used. This paper reports the results of a study of a series of RuY catalysts prepared from NH/sub 4/Y, LiY, NaY, KY, RbY, and CsY zeolites. The nature of the group IA cations was found to have little effect on chemisorptive properties and on the activity and chain growth probability in CO hydrogenation on Ru. However, the nature of the cations produced changes in secondary reactions of primary olefinic products as a result of differences produced during catalyst preparation in Ru distribution in the zeolite crystallites and of possible modifications of the acid sites generated during reduction of the Ru. 57 references.

Adsorption and reaction of CO and H2 on K-promoted Rh/SiO2 catalysts

The adsorption of CO and H2 on a series of alkali-promoted RhSiO2 catalysts was investigated by IR spectroscopy and volumetric chemisorption. The characteristics of the support as well as the method of addition of the alkali species were found to influence the adsorptive properties of the catalysts. Alkali species on wide-pore RhSiO2 tended to partition to the support and did not interact strongly with the Rh crystallites. When alkali and metal salts were coimpregnated onto a nonporous SiO2 support, intimate alkali-metal contact resulted in significant electronic interactions between the alkali species and the metal. When alkali species were added to a prereduced RhSiO2 (nonporous) catalyst, a chemical interaction between a tilted adsorbed CO and the alkali species was suggested. The nature and location of the alkali species were suggested to be important parameters in determining the effect of alkali promoters on RhSiO2 catalysts. The rate of CO conversion decreased substantially with promotion for all of the promoted catalysts. An unusually low apparent activation energy was found for the sequentially impregnated (nonporous SiO2) promoted catalyst, and it was suggested that this might be related to the unusually low frequency peak seen in the IR spectrum of adsorbed CO on this catalyst.

Effect of Si/Al ratio on secondary reactions during CO hydrogenation on zeolite-supported metal catalysts

Abstract The effect of zeolite acidity on the product distribution in CO hydrogenation over ion-exchanged zeolite-supported Ru catalysts was investigated using zeolites with different Si Al ratios. CO hydrogenation over RuNaX, RuNaY, RuKL, and RuNa-mordenite and the transformation of 1-butene on the zeolites without the metal, under conditions similar to those used for CO hydrogenation, were investigated in order to understand the effect of zeolite acidity on possible secondary reactions of the primary olefinic products of Fischer-Tropsch (FT) synthesis. The results of this study establish the importance of the bifunctional nature of the zeolite-supported FT catalysts in modifying catalyst selectivity. This modification is achieved as a result of secondary reactions of the primary olefinic FT products on the acid sites of the zeolite, generated during catalyst preparation. Depending on the strength of the acid sites, a function of the Si Al ratio, various competitive reaction paths can be observed for the transformation of the olefinic compounds.

Steady-state isotopic transient kinetic analysis on Pd-supported hexaaluminates used for methane combustion in the presence and absence of NO

Abstract This paper presents a study for the combustion of methane in the presence and absence of NO over hexaaluminates (La 1− X Sr X MnAl 11 O 19 ) and Pd-supported hexaaluminates (Pd/La 1− X Sr X MnAl 11 O 19 ). All the catalysts were subjected to a reaction cycle comprising of (1) CH 4 +O 2 , (2) CH 4 +O 2 +NO, and (3) CH 4 +O 2 reactions at temperatures between 300 and 700°C. Steady-state isotopic transient kinetic analysis (SSITKA) technique was employed to determine the surface lifetimes ( τ ), average site activities ( k ), and surface concentrations ( N ) of each relevant species during the reactions (1) and (2), as described above, on La 0.6 Sr 0.4 MnAl 11 O 19 and 0.9% Pd/La 0.6 Sr 0.4 MnAl 11 O 19 . The SSITKA experiments were performed at temperatures of 360–440°C. Steady-state results showed that the contribution of homogeneous gas phase reactions are appreciable in the presence of NO at temperatures above 650°C. Steady-state and SSITKA reactions on La 0.6 Sr 0.4 MnAl 11 O 19 suggested that methane and NO oxidation occur on the catalyst surface at 440°C. Steady-state and SSITKA reactions on Pd/La 0.6 Sr 0.4 MnAl 11 O 19 showed that the presence of NO 2 (formed via NO and O 2 ) boosted the site activity required for the formation of CO 2 . In the reaction where NO is involved, NO acts indirectly as chain initiator in the form of NO 2 , and also as chain terminator.

Selectivity behavior during the equilibrium-limited high temperature formation of MTBE on acid zeolites

The synthesis of MTBE was studied in the gas phase at elevated temperatures (up to 175°C) and low pressures (150 kPa) where the MTBE formation rate is limited by thermodynamic equilibrium, using various solid acid catalysts (Amberlyst-15 resin, silica-alumina, HY and H-ZSM-5 zeolites). All the zeolites studied were found to exhibit better selectivities to MTBE than the commercially used Amberlyst-15 resin catalyst. The formation of byproducts increased with increasing temperature and appeared to have a strong enhancing effect on catalyst deactivation. H-ZSM-5 seems to be more suitable for high temperature formation of MTBE because of its excellent selectivity towards MTBE and low deactivation behavior.

Prediction of the Gas Holdup in Industrial-Scale Bubble Columns and Slurry Bubble Column Reactors Using Back-Propagation Neural Networks

The total gas holdup and the holdup of large gas bubbles were predicted in bubble column reactors (BCRs) and slurry bubble column rectors (SBCRs) using two Back-Propagation Neural Networks (BPNNs). Over 3880 and 1425 data points for gas holdup and Large gas bubble holdup respectively, covering wide ranges of gas-liquid-solid physical properties, operating variables, reactor geometry, and gas sparger type/size, were employed to develop, train and validate the two neural networks. The developed BPNN for gas holdup has a topology of [14,9-7,1] and was able to predict the trained and untrained data with an average absolute relative error (AARE), standard deviation, and regression coefficient (R2) of 16, 19 and 90%, respectively. The developed BPNN for large gas bubble holdup has a topology of [14,8,1] and was capable of predicting the trained and untrained data with AARE, standard deviation, and R2 of 10, 14 and 93%, respectively. The BPNNs were then used to predict the effects of pressure, superficial gas velocity, temperature and catalyst loading on the total syngas holdup for Low-Temperature Fischer-Tropsch (LTFT) synthesis carried out in a 5 m ID SBCR. The predicted total syngas holdup appeared to increase with increasing reactor pressure, superficial gas velocity and the number of orifices in the gas sparger. The predicted syngas holdup, however, was found to decrease with increasing catalyst loading and reactor temperature. Also, under similar LTFT operating conditions (P = 3 MPa, T = 513 K, CW = 30 and 50 wt%), the total syngas holdup values predicted for H2/CO ratio of 2:1 and cobalt-based catalyst are consistently lower than those obtained for H2/CO ratio of 1:1 and iron oxide catalyst in the superficial gas velocity range from 0.005 to 0.4 m/s. These predictions are in perfect agreement with reported literature trends, which underscore the reliability and validity of the developed BPNNs in predicting the total syngas holdup and the holdup of large gas bubbles in large-scale bubble columns and SBCRs operating under industrial conditions.

Probe molecule studies of CO hydrogenation over Ru/SiO2

The effects of the addition of small amounts of CH{sub 3}NO{sub 2} to the reactants during CO hydrogenation were investigated under various reaction conditions. The major changes in the product distribution of CO hydrogenation on Ru/SiO{sub 2} caused by CH{sub 3}NO{sub 2} addition to the reactants included substantial reduction of CH{sub 4} production, increased production of hydrocarbons in the C3 to C6 carbon fractions, and enhanced selectivity toward linear olefinic products. A substantial incorporation of carbon originating from the added Ch{sub 3}NO{sub 2} into these hydrocarbon products was observed. Changes in the product distribution took place without significant disturbance of the main reaction pathways leading to hydrocarbon formation, offering great potential for the use of CH{sub 3}NO{sub 2} as a probe molecule in the study of the mechanism of CO hydrogenation.

In situ chemical trapping of CO/H2 surface species

Abstract CO hydrogenation reactions over RuKY catalysts were monitored in situ using a novel chemical trapping technique to identify surface species. Study of alterations in the product distribution upon addition of the trapping agent led to a good prediction of the type of reactive surface species thought to be present on the surface of these catalysts. This work represents the first evidence that this chemical trapping technique selectively traps active surface intermediate species. A number of other significant changes in the surface chemistry of the catalysts occurred upon addition of the trapping agent, indicating that the action of the trapping agent appears to be more complex than had previously been suggested. Although these results have uncovered interesting and revealing phenomena, it is evident that additional investigations are needed before the full value and import of this technique as a means of identifying reactive surface species can be fully assessed.

CH3NO2 addition to CO hydrogenation over a Ru/KY catalyst

CH{sub 3}NO{sub 2} addition to CO hydrogenation over Ru/KY led to the formation of partially dehydrogenated CH{sub x} groups, which became indistinguishable from CH{sub x} groups derived from CO hydrogenation. Random reaction of these groups provided hydrocarbon formation along the same reaction pathway that normally occurs during CO hydrogenation over Ru/KY. The additional CH{sub x} groups from CH{sub 3}NO{sub 2} enhanced the rates of chain initiation and propagation to similar extents, leading to increases in the rates of formation of C{sub 2}{sup +} hydrocarbons. Some of these CH{sub x} groups were further dehydrogenated and formed surface carbon, resulting in the incorporation of some carbon from CH{sub 3}NO{sub 2} into CO{sub 2}, an increase in the rate of catalyst deactivation, and suppression of secondary reactions on the support.