Samuel Mignard

Coking, aging, and regeneration of zeolites. III: Comparison of the deactivation modes of H-mordenite, HZSM-5, and HY during n-heptane cracking

The rates of deactivation by carbonaceous residues (coke) of HY, H-mordenite, and HZSM-5, which have similar initial activities in n-heptane cracking are quite different: HZSM-5 forms very little coke and is very stable, while HM deactivates much faster than HY. The deactivation mechanisms of these zeolites could be specified by comparing as a function of the coke content: (i) the cracking activity of these zeolites and their capacity for adsorption of n-hexane, (ii) the volume apparently and really occupied by coke, and (iii) the number of coke molecules and of sites on which NH3 can no longer be adsorbed. On HY, polyaromatic molecules are very rapidly formed on the strongest acid sites, these molecules obstructing partially or completely the access of the reactant to the acid sites. On HZSM-5 deactivation is initially due to the “coverage” of the acid sites, located at channel intersections, by alkylaromatics with 1 or 2 rings; later on (above 3% coke content) polyaromatic molecules, formed on the external surface, block access to a part of the pore volume. On HM deactivation is due to pore blockage: even at very low coke contents (1% coke), coke can block the access of n-heptane to a pore volume 10 times greater than the volume really occupied by the coke.

Coking, aging, and regeneration of zeolites: II. Deactivation of HY zeolite during n-heptane cracking

Heavy carbonaceous compounds (coke) are deposited very rapidly during n-heptane cracking on a stabilized HY zeolite at 450/sup 0/C, decreasing its activity, modifying its selectivity and its adsorption properties. Deactivation of the zeolite can be explained mainly by partial or total blocking of access to the active centers, even for low coke content. Indeed, while the decrease of the pore volume accessible to n-hexane (hence to the reactant n-heptane) is about 3 times less than the decrease of the activity, calorimetric study of NH/sub 3/ adsorption shows that the sites which no longer adsorb NH/sub 3/ are the strongest. Moreover, the diffusivity of n-hexane in the pore volume which remains accessible is definitely smaller than in the coke-free zeolite. Finally, the pore volume which has become inaccessible to n-hexane is, even at low coke content, about three times smaller than the volume really occupied by coke (estimated from its density). This deactivation through pore blockage is due to the fact that, even at low coke content, the coke molecules are sufficiently large to prevent access of the reactant to the supercages of the zeolite.

Tuning the hydrophobicity of mesoporous silica materials for the adsorption of organic pollutant in aqueous solution

The ability of various as-prepared and organically modified MCM-41 and HMS mesoporous silica materials to behave as efficient adsorbents for organic pollutants in aqueous solution was investigated by using different surface functionalization procedures, so as to adjust their hydrophilic/hydrophobic balance. The hydrophilic and organophilic properties of the parent silica materials and their corresponding surface functionalized counterparts were studied by using water and toluene adsorption isotherms. Their quantification was determined by the hydrophobic static index value (HI(static)), as well as by the silanol and organic group densities after the functionalization step. A clear correlation could be found between the HI(static) values and either the superficial silanol density, or the amount of organic moieties grafted or incorporated to the silica materials. For the highly organically functionalized samples, the residual superficial silanol groups (<50%) are sufficiently isolated from each other so as to prevent the water capillary condensation within the pores, thereby leading to an increased hydrophobic character of the resulting mesoporous silica. Those hydrophobic samples, for which the water liquid meniscus formation within the mesopores was minimized or avoided, exhibited a storage capacity for an organic pollutant (N,N-diethyl-m-toluamide, DEET) in aqueous solution more than 20 times higher than that of the corresponding unmodified sample, independently of the silica nature (MCM-41 or HMS). For all calcined and silylated samples, the DEET maximum adsorption capacities determined by the Langmuir model could be correlated with the silica surface coverage by trimethylsilyl groups and thus with the remaining silanol amount.

Coking, aging, and regeneration of zeolites. VIII: Nature of coke formed on hydrogen offretite during n-heptane cracking: mode of formation

Abstract The carbonaceous compounds (coke) responsible for the deactivation of hydrogen offretite during n -heptane cracking at 450 °C were analyzed in detail for different reaction times. With this zeolite, which has a double pore system, n -heptane cracking occurs partly in the gmelinite cages and partly in the large channels, but in both cases through the carbenium ion chain mechanism. It is shown that the coke formation begins in the gmelinite cages starting from branched cracking products (e.g., isobutene) too bulky to be desorbed. These products undergo reactions leading successively to monoaromatic, biaromatic, and polyaromatic products. The polyaromatic products, because of their size, cannot be located entirely in the gmelinite cages and hence overflow into the large channels. With long reaction times, highly condensed polyaromatics are formed by the growth of the coke molecules in the pores near the outer surface which they cover.

Tailoring the Hydrophobic Character of Mesoporous Silica by Silylation for VOC Removal

MCM-41 and non-ordered mesoporous silica were modified using hexamethyl-disilazane (HMDS). The hydrophobic and hydrophilic properties of grafted materials were studied and compared to a purely all silica BEA zeolite by using competitive and noncompetitive water toluene adsorption. A linear correlation between the silylation degree and the hydrophobicity measurements has been found for MCM-41 materials. Even if highly silylated MCM-41 material have more hydrophilic sites (silanol groups) than all silica zeolite, water molecule condensation is not observed because these sites are isolated. Thus, the highly silylated MCM-41 sample exhibits not only hydrophobicity 2.3 times higher than all silica BEA zeolite but also possesses a storage capacity for toluene and chlorobenzene 3 times higher than this zeolite. In competition with water, the organic molecule (toluene or chlorobenzene) adsorption is always favored even if water adsorption is enhanced by chlorobenzene polarity.

Separation of monobranched and dibranched Isomers of n-hexane on zeolitic molecular sieves: a thermodynamic study.

A thermodynamic study was realized by competitive adsorption over zeolitic adsorbents to determine the efficiency of these solids for the separation of monobranched and dibranched isomers of n-hexane. The effect of the zeolite structure was studied. The medium-pores ZSM-5-type zeolites were better than the large-pores BEA and MOR zeolites. The size and number of the extraframework cations had an important influence on the efficiency of the separation over ZSM-5 zeolites. The sodic Na(6)ZSM-5 sample was found to be the better adsorbent for the separation of the studied mixture because of steric hindrance induced by the presence of Na(+) cations in the zeolite structure. The initial composition of the mixture also had an important influence on the separation. In fact, when the initial mixture was equimolar the monobranched isomer was preferentially adsorbed, whereas when the molar percentages of the isomers were different in the initial mixture the adsorption of the majority isomer was favored. The temperature of the adsorption was another important parameter influencing the separation. Indeed, when the temperature of adsorption was low the separation was more effective. At an adsorption temperature of 333 K the Na(6)ZSM-5 sample was the most efficient by adsorbing 65% of the monobranched isomer and only 35% of the dibranched isomer.

n-Heptane hydrocracking over a sulfided NiMo/Y zeolite catalyst: reversible deactivation of the zeolite by sulfur - containing molecules

Abstract Hydrocracking of n-heptane was carried out over a sulfided NiMo/Y zeolite catalyst in the presence of ammonia and hydrogen sulfide. An increase in the hydrogen sulfide pressure decreased the activity of the catalyst and increased the isomerization/cracking ratio, which indicated a poisoning of the acid sites. This poisoning was accompanied by an increase in the formation of coke on the catalyst when the hydrogen sulfide pressure increased. The coke was composed mainly of sulfur-containing molecules, namely dimethyl polysulfides (tri-, tetra- and pentasulfides), and also pure sulfur: S 8 . The deactivation of the catalyst was due to a blockage of the porosity of the zeolite by the sulfur polymers because of the low volatility of these compounds. The higher the hydrogen sulfide pressure the greater the amount of sulfur polymers, this phenomenon being totally reversible.

The preferential adsorption sites of H2O on adsorption sites of CO2 at low temperature onto NaX and BaX zeolites

Study of preferential adsorption sites of H2O on NaX and BaX zeolites was carried out after adsorption/desorption of carbon dioxide (CO2). The profiles of CO2 desorption revealed two distinct zones. The first one, at low temperature (T < 533 K), corresponding to physisorbed CO2 where BaX zeolite has an energy of adsorption higher than that of NaX as well as strong interactions of adsorbent/adsorbate. The second one (533–623 K) corresponds to the chemisorbed CO2. The presence of water influences CO2 adsorption. The effect is more important for NaX than for BaX. For NaX, water occupies the adsorption sites of CO2 between 373 and 483 K. In contrast, BaX preserves a majority of adsorption sites of physisorbed CO2 in the presence of water, in particular the adsorption sites located in the interval of temperature 423–533 K.

Single and Competitive Adsorption of Linear and Branched Paraffins over Silicalite: Thermodynamic and Kinetic Study

A thermodynamic and kinetic study was realized by competitive and non-competitive adsorption for the separation of n-hexane isomers. Two mixtures of n-C6/3MP and 3MP/23DMB were studied. For the single component adsorption the maximal adsorption capacity were close for the different isomers while the diffusivity for the 23DMB isomer was outstandingly low due to steric hindrance. For the competitive adsorption we had from an initial proportion of 50% for each sorbate an adsorption of 34% and 66% respectively for 3MP and n-C6. For the 3MP/23DMB mixture we had an adsorption of 62% and 38% for 3MP and 23DMB respectively. For the two mixtures the selectivity of the zeolite increased with the adsorption capacity and decreased after the values of 0.63 mmol/g and 1.65 mmol/g respectively for the 3MP/23DMB and n-C6/3MP mixtures. The adsorption rates were lower in the case of competitive adsorption and a kinetic separation could be envisaged for the improvement of the octane number.

Adsorption of butane isomers on ferrierite: influence of the Si/Al ratio

The adsorption and the kinetics of diffusion of n-butane and i-butane on three HFER were studied as a function of the global Si/Al ratio from 6 to 19. The capacity of adsorption, the kinetics of diffusion and the heat of adsorption are higher for n-butane than for i-butane. N-butane is preferentially adsorbed into the micropores while i-butane is rather adsorbed into the micropores and mesopores. The sites of adsorption are energetically homogeneous for sample HFER-10 whereas they are heterogeneous for HFER-6 and HFER-19.