USHY Zeolite Research Articles

Dechlorination of molten chloride waste salt from electrorefining via ion-exchange using pelletized ultra-stable H-Y zeolite in a fluidized particle reactor

Dechlorination of eutectic LiCl–KCl based electrorefiner (ER) salt is reported via ion-exchange reaction with protonated ultrastable Y-type (USHY) zeolite bound into mechanically fluidized 45–250 μm diameter particles. Evidence of exchange of cations from the salt (Li+, K+, and fission product cations) into the zeolite lattice replacing H+ ions was found based on a change in unit cell size, ICP-MS, XRD and TEM–EDS in addition to detection of HCl off gas. Ion exchange reaction was carried out at 625 and 650 °C, temperatures above the melting point of eutectic LiCl–KCl. Experiments were carried out to optimize zeolite drying temperature, estimate maximum ion-exchange capacity, and determine the thermal stability of USHY zeolite. The results indicate over 90% dechlorination can be achieved without zeolite structure collapse at 625 °C. This provides a promising route to stabilizing waste from radioactive chloride salts into dechlorinated waste forms for permanent geologic disposal.

Destruction of PAH and dioxin precursors using selective oxidation over zeolite catalysts. Influence of the presence of ammonia in the flue gas

This work reports the influence of ammonia during 1-methylnaphthalene (1-MN) and 1-methylnaphthalene/1,2-dichlorobenzene mixture oxidation over USHY zeolite and 0.8%PtUSHY catalyst. The USHY zeolite was able to transform 1-MN into carbon dioxide from 400°C whatever the reaction conditions. However the presence of ammonia improves the transformation rate of 1-MN: 1-MN was totally transformed from 300°C when the reaction was carried out in the presence of ammonia. Whatever the reaction temperature, only carbon dioxide was formed from 1-MN and no aromatic by-products were observed. Over the PtUSHY catalyst, the same phenomenon was observed in the 200–300°C temperature range. Indeed, the 1-MN was totally transformed from 200°C when the reaction was carried out with ammonia against 250°C when 1-MN was alone. It was also shown that the presence of NH3 favoured the 1-MN transformation into adsorbed oxygenated polyaromatic compounds over the USHY zeolite. In the case of PtUSHY catalyst, the presence of NH3 improves the transformation rate of 1-MN into carbon dioxide by formation of NO2, which can participate in the oxidation reaction of 1-MN.The oxidation reaction of 1-methylnaphthalene/1,2-dichlorobenzene (1-MN/1,2PhCl2) mixture was also studied in the absence and presence of NH3 over PtUSHY catalyst. The presence of 1,2PhCl2 and 1,2PhCl2/NH3 had no influence over 1-MN transformation in the temperature range studied (250–400°C). Indeed, 1-MN was completely converted into CO2 from 250°C. 1,2PhCl2 appears as the most difficult compound to destroy. However, the presence of NH3 leads to a promoting effect over 1,2PhCl2 transformation rate. 1,2PhCl2 was totally transformed from 350°C in the presence of NH3 against 400°C in the absence of NH3. As for 1-MN this promoting effect can be explained by the NH3 oxidation over Pt atoms leading to NO2 formation. Some experiments carried out with NO confirm that NO2 formation (formed by the oxidation of NH3) participate in the oxidation reaction. However, for an initial concentration of NH3 up to 100ppm, an inhibitory effect over the 1,2PhCl2 transformation rate was observed.

Transformation of m-xylene over an USHY zeolite: Deactivation and regeneration

Abstract Deactivation of the USHY zeolite (Si/Al = 6.5) during m-xylene transformation under N 2 flow at 350 °C, is due to the rapid formation and retention of coke. In situ mx-transformation over regenerated coked zeolite samples after stripping at 500 °C under air, hydrogen, or nitrogen shows that the effectiveness of the stripping gas in coke removal follows the order air ⋙ H 2 ≫ N 2 . Coke is also characterized by temperature programmed oxidation (TPO) experiments. For the regenerated zeolite by TPO under O 2 , the micropore volume is completely restored while a small loss of total volume is noticeable. This is shown by the N 2 adsorption isotherms of the three fresh, coked and regenerated zeolites.

Effect of phenol addition on the performances of H–Y zeolite during methylcyclohexane transformation

The transformation of a mixture of methylcyclohexane and a small amount of phenol, as representative of a mixture of bio-oil and Fluid Catalytic Cracking (FCC) feedstock, was studied at various temperatures over a USHY zeolite. At 350 °C, the adsorption of phenol on both Brönsted and Lewis acid sites increases the zeolite deactivation. The increase of the reaction temperature allows limiting the effect of phenol addition on the zeolite activity and stability. The transformation of a mixture of methylcyclohexane and a small amount of phenol, as representative of the transformation of a mixture of biomass-derived oxygenates and Fluid Catalytic Cracking (FCC) feedstock was studied at 350 and 450 °C over a USHY zeolite. At 350 °C, the addition of phenol increases the deactivation of the zeolite catalyst, which is due to phenol adsorption on both Brönsted and Lewis acid sites, in addition to the formation of coke from methylcyclohexane. Whatever the contact time and the phenol amount, this deactivation is always partial, which suggests that a substantial portion of the active sites of the HY zeolite is not affected by deactivation. Finally, the increase of the reaction temperature (to 450 °C) allows limiting the effect of phenol addition on the zeolite activity and stability.

Role of Strong Zeolitic Acid Sites on Hydrocarbon Reactions

In the work reported in this article, 1-pentene reactions were carried out over USHY zeolite, whose strong-acid sites were selectively poisoned by hard coke to study the role of these strong zeolitic acid sites on hydrocarbon conversions. We show conclusively that strong-acid sites are responsible for cracking and hydride transfer reactions as well as strong coke formation, whereas weak-acid sites can only catalyze double-bond isomerization.

Acid Site Characterization of Coked USHY Zeolite Using Temperature Programmed Desorption with a Component-Nonspecific Detector

In this paper we describe a novel NH3-TPD methodology to study the amount as well as strength of acid sites of coked catalysts. Conventional temperature programmed desorption (TPD) of ammonia cannot be applied to coked catalyst as coke precursors removed during the temperature program falsify the ammonia signal. The effects of coke formation from different reactants, time-on-stream, and reaction temperatures on acid site deactivation were investigated. Besides acid site deactivation, characterization of coke precursors can also be revealed. The initial deactivation preferentially on strong acid sites is very fast. The concentration of free acid sites is inversely correlated well with the total concentration of coke rather than individual coke groups. Coke precursors tend to be more stable at higher reaction temperatures.

Interaction between Ni and V with USHY and rare earth HY zeolite during hydrothermal deactivation

The effect of Ni and V in the hydrothermal deactivation of an ultrastable Y zeolite with and without rare earth elements is presented and discussed. Although in the presence of steam, vanadium itself is the main factor responsible for zeolite loss of surface area, for the rare earth containing sample the presence of nickel increased the vanadium effect. Temperature-programmed reduction of these zeolites further support this observation by showing an increased amount of mobile readily reducible vanadium species while nickel and rare earth interact. The same deleterious effect of nickel was also observed in rare earth containing FCC catalysts after hydrothermal deactivation.

Evolution of coke composition during ageing under nitrogen

The ageing under nitrogen flow at 250–450 °C of coke formed during different model molecules ( m-xylene, n-heptane, propene) over a USHY zeolite is discussed. This ageing treatment causes a decrease in the zeolite coke content (essentially for low reaction temperature) and an increase in the size and aromaticity of coke molecules. Pyridine adsorption followed by IR spectroscopy, carried out on the coked samples before and after ageing, shows that ageing causes a significant decrease in the concentrations of protonic and Lewis acid sites able to retain pyridine adsorbed at 150 °C. The deactivating effect of coke is also more pronounced after than before ageing. A change in the mode of deactivation from site poisoning to pore blockage is proposed to explain these observations. The same trends were observed for ageing a commercial coked catalyst, sampled from the fluid catalytic cracking (FCC) unit.

Coke formation and coke profiles during the transformation of various reactants at 450°C over a USHY zeolite

Coking of a USHY was studied in a fixed-bed reactor at 450°C. Different model compounds were used: an alkane ( n-heptane), an alkene (propene), a naphthenic (methylcyclohexane) and aromatics ( m-xylene, 1-methyl-naphthalene). Those reactants present very different coking rates in the order: 1-methyl-naphthalene>propene⪢ m-xylene> n-heptane>methylcyclohexane. The coke profile along the reactor is a function of the coke precursors. By using a triple-bed reactor, it is shown that the coking process proceeds primarily with olefins and aromatic reactants and secondarily with alkanes and naphthenes. In the first case, coke is preferentially deposited on the first part of the bed, whereas in the second case, the coke deposit is rather concentrated on the last part of the bed (after formation of coke maker molecules). GC-MS analysis has shown that whatever the reactant, coke is mainly constituted by small polyaromatic compounds with four to six aromatic rings located in the zeolite pores. Also, highly polyaromatic compounds located either inside the pores or on the external surface of zeolite crystallite were observed.

M-Xylene Transformation over a USHY Zeolite at 523 and 723 K: Influence of Coke Deposits on Activity, Acidity, and Porosity

The modes of coking and deactivation of a USHY zeolite during m-xylene transformation at 523 and 723 K were established. At both reactions temperatures, coke molecules result mainly from transformation of the diarylmethane intermediates of transmethylation reactions, olefinic products formed only at 723 K also participating in coke formation at this temperature. From the effect of the number of coke molecules on the accessible micropore volume, on the hydroxyl bands, and on the number of protonic sites able to retain adsorbed pyridine, it can be concluded that at both temperatures, deactivation is initially due to poisoning of the protonic sites, pore blockage by coke deposits appearing at longer time-on-stream. The effect of this latter mode of deactivation is more significant at 723 than at 523 K. No interaction of Lewis acid sites with coke molecules is observed, which suggests that these sites play no role in m-xylene transformation and coke formation.

T-Butylation of 1,2-dihydroxybenzene over acidic zeolites

t-Butylation of 1,2-dihydroxybenzene (DHB) with isobutene as alkylating agent was carried out over various acidic zeolites such as USH-Y, H-beta, and H-ZSM-5. USH-Y zeolite exhibits the highest catalytic activity and considerable selectivity of 4- t-butylcatechol (4-TBC). The selectivity of 4-TBC is increased in the order of H-ZSM-5>USH-Y≅H-beta. 3- t-Butylcatechol (3-TBC) is well produced over catalyst with high SiO 2/Al 2O 3 ratio and large pore aperture. 3,5-Di- t-butylcatechol (3,5-DTBC) selectivity is maximum in zeolite which contains strong acidity and large pore channel. The influences of various reaction parameters such as reaction temperature, space velocity, reactant molar ratio are discussed. In order to improve 4-TBC formation and decrease in 3,5-DTBC selectivity simultaneously, USH-Y zeolite was silylated by tetraethylorthosilicate (TEOS) resulting in the decrease of 3,5-DTBC selectivity over modified catalysts by about 25%. USH-Y zeolite shows high stability in the t-butylation for at least 350 h. The coke formed during the reaction was identified by FT-IR and the USH-Y zeolite could be regenerated through oxidative thermal treatment.

Influence of coke on the acid properties of a USHY zeolite

The composition of coke and its effect on the acid sites were determined with samples of a USHY zeolite (with total and framework Si/Al ratios of 3.1 and 5.4, respectively) used for m-xylene transformation at 520 and 720 K. At low coke contents, coke is mainly constituted by methyl substituted polyaromatic compounds with three (520 K) or four (720 K) aromatic rings trapped in the zeolite pores. Coke causes a decrease in the intensity of the IR hydroxyl band. The most acidic hydroxyls, i.e., the bridging hydroxyls in interaction with extra-framework aluminum species are most affected by coke, whereas no interaction is observed between the non-acidic hydroxyls and the coke molecules. Pyridine adsorption shows that, while the number of protonic sites able to retain pyridine adsorbed decreases by coking, this was not the case for the Lewis sites. Pyridine adsorption facilitates the desorption under vacuum of coke molecules from the coked zeolites, the effect being more significant for coking at 520 K. This indicates that the retention of coke molecules is not only due to their low volatility (at 520 K) or to their steric blockage (720 K), but also to their adsorption on the acid sites.

Contrasting effects of γ-irradiation on the role of sorbed water assisting the protonic conduction in US-HY zeolite

Acid-dealumination of zeolites leaves a number of nest silanols as structural defects in the vacancies resulting from aluminum removal from the channel structure. The resultant siliceous zeolite is, therefore, unexpectedly hydrophilic and sorbs water for over 15% of its mass. The zeolite protons associated with the remaining aluminum gain more freedom. Sorbed water H-bonds to the formed nest silanols, and forms a lake streaming over the zeolite surface. This high mass of sorbed water assists greatly the proton mobility and hence the protonic conduction, so that the dealuminated zeolite gains a two order of magnitude rise in its electric conductivity. A regular rise in temperature should then result in a regular loss of sorbed water, unless other structural factors are involved. An irregular drop in water-assisted conduction is rather shown at the middle of the measurement, in the temperature range 293–373 K, at 333 K. This is associated with a concurrent channel contraction taking part around 333 K. Such a structural shift would force sorbed water faster out of the channel structure, which could explain the irregular drop in water-assisted protonic conduction. Complementary analysis for the results of γ-irradiation at integrated doses reveals contrasting effects. Low dose irradiation with ionizing γ-rays assists the proton mobility at the low temperature of 308 K, inducing an extra rise by one order of magnitude in conductivity in addition to the two orders gained due to sorbed water. At variance, high dose irradiation reduces the conductivity at the same temperature to even lower values than for the non-irradiated zeolite. This dramatic loss in protonic conduction is attributed to a relevant loss in the assisting role of sorbed water via two mechanisms: (i) general loss due to γ-induced dehydration; and (ii) specific loss due to γ-assisted contraction in the zeolite channels initiated at even lower temperatures. These observations are discussed in the light of a.c. conductivity analysis for the changes in frequency-dependent electric permittivity, impedance and phase angle with the zeolite water content and the measurement temperature.

Kinetics of Coke Formation for the Reactions of Light Olefins Ethene, Propene and 1-Butene Over Ushy Zeolite

A simplified three-parameter model for the interpretation of the kinetic data of coke formation during transformation of light olefins is presented. This model has been applied to the transformation of ethene, propene and 1-butene over fresh and regenerated zeolite USHY at 623 K. The proposed model covers both the coverage of the acid sites and the autocatalytic growth of coke.

Alkylation of Isobutane with 2-Butene over a HFAU Zeolite. Composition of Coke and Deactivating Effect

The transformation of an isobutane −2-butene mixture with a 40 weight ratio was carried out in liquid phase at 50 °C on an USHY zeolite with a framework Si/Al ratio of 4.5. The reaction products can be classified into seven families: (a) trimethylpentanes resulting from isobutane alkylation with 2-butene or from isobutane self-alkylation, (b) dimethylhexanes, (c) C8 alkenes, (d) heavy products (mainly C9−C12), (e) light products C5−C7 alkanes, (f) n-butane, and (g) nondesorbed products (coke). These latter products, which are responsible for deactivation, were removed from samples coked for different times, after dissolution of the zeolite in a hydrofluoric acid solution, and analyzed by GC, IR, HNMR, MS, and GC/MS coupling. The coke components were identified as alkyl bicyclic olefinic compounds with 12−28 carbon atoms. IR spectroscopy analysis of the coked zeolite samples shows that coke molecules interact with the protonic sites, this interaction being mainly responsible for deactivation. Large differences can be observed in deactivation of the various reaction steps. The steps which are the more demanding in number (e.g., hydride transfer) or in strength (e.g., isomerization) of the acid sites are preferentially deactivated.

The Consequences of Steam Dilution in Catalytic Cracking: II. Effect of Steam Dilution on the Selectivity and Mechanism in 2-Methylpentane Cracking over USHY

The effect of steam dilution in shaping the selectivity of 2-methylpentane cracking has been studied at 400, 450, and 500°C using a USHY zeolite. The results show that the presence of steam enhances isomerization of the feed and suppresses the production of cracked products. This and other evidence leads us to believe that steam dilution enhances the chain propagating reactions in the mechanism of catalytic cracking. At the same time, steam inhibits chain termination by desorption. The net result is that the presence of steam increases kinetic chain length and the paraffin-to-olefin ratio. The effects of steam dilution are surprisingly strong at lower temperatures but become less significant as temperature rises. All these effects are interpreted in terms of their influence on the elementary steps of the chain mechanism and are principally rendered in terms of a new measure: the reaction path probabilities (RPPs). These describe the likelihood that a feed molecule will be converted by a given reaction. Probabilities of all the reactions of the various ions present in the chain mechanism of 2-methylpentane cracking are quantified by formulating appropriate secondary functions using the RPPs derived from our experimental data. These various “internal” probabilities are used to quantify just how likely are each of the many alternative reactions of a given ion, as reaction conditions change. They allow us to track the effect of changes in reaction conditions, or in catalyst formulation, in unprecedented detail. Moreover, they give a detailed picture of the behavior of elementary processes in this reaction. We present and discuss examples of the quantitative information which our methods make available, and the mechanistic interpretations which flow from this information. In particular, we apply these measures in a study of the effects of steam when it is added to cracking 2-methylpentane in various proportions. It appears that catalytic cracking is much more susceptible to the effects of dilution than has previously been suspected.

Reactions involved in the alkylation of isobutane with 2-butene and with propene on a USHY zeolite

The transformation of an isobutane-2-butene mixture with a 41 molar ratio was carried out in liquid phase at 323 K on a USHY zeolite with a framework Si/Al ratio of 4.5. C 5–C 14 isoalkanes with about 70 wt.-% of C 8 and a small amount of C 8 alkenes were obtained. Moreover, about 7% of butene was transformed into undesorbed C 10–C 30 products containing three unsaturations or cycles. The C 5–C 14 products resulted from the following reactions: alkylation of isobutane with 2-butene, dimerization of butene, oligomerization-cracking and self-alkylation of isobutane. In order to determine the significance of this latter reaction, the transformation of an isobutane-propene mixture was investigated under similar operating conditions. Trimethylpentanes, and particularly the 2,2,4 isomer, were formed, which confirmed that isobutane self-alkylation was taking place. This reaction was about twice slower than the alkylation of isobutane with propene and twice faster than the dimerization of propene. From the hydrogen balance it was concluded that self-alkylation of isobutane supplied the main part of the hydrogen necessary for the saturation of the products of alkene dimerization and oligomerization-cracking. Hydride transfer would be the limiting step of the formation of isoalkanes. However, isomerization through hydride and methyl shifts of carbenium ion intermediates played also a significant role in the product distribution. This reaction occurred at a rate similar to that of hydride transfer to tertiary carbenium ions but much faster than hydride transfer to secondary carbenium ions.

The Protolysis of Hexanes over a USHY Zeolite

Detailed information gathered on the cracking of five hexane isomers has revealed major differences in the mechanisms of cracking in the individual isomers and suggests the possibility that synergistic effects play a role in the cracking of mixtures of hydrocarbons. We find that both n-hexane and 2,2-dimethylbutane do not crack via the chain process to any significant extent, presumably due to the absence of tertiary hydrogens in both molecules. On the other hand, 2,3-dimethylbutane, with two tertiary hydrogens, cracks readily via a chain process. The two monomethyl pentanes illustrate the fact that tertiary hydrogens are not all the same in terms of reactivity. 3-methylpentane has a much more reactive hydrogen than 2-methylpentane, with the result that more of this feed is converted by chain processes. However, the overall rate of conversion is higher in 2-methylpentane and lower in 2,3-dimethylbutane. It seems that, when it comes to maximizing the overall rate of reaction, the dominant influence is the rate of initiation by protolysis. In this work we examine the initiating protolysis reactions of the partent hexane molecules. These reactions start the chain processes which, in various measures, are responsible for the observed overall conversion in "catalytic cracking." We find that the rates and modes of the various protolysis reactions are profoundly different in the five isomeric hexanes. In discussing this fact we raise the issue of the significance of "test reactions" in catalyst evaluation.

The Cracking of Isobutane Interpreted as a Chain Mechanism

The effect of steam dilution in shaping the selectivity of 2-methylpentane cracking has been studied at 400, 450, and 500°C using a USHY zeolite. The results show that the presence of steam enhances isomerization of the feed and suppresses the production of cracked products. This and other evidence leads us to believe that steam dilution enhances the chain propagating reactions in the mechanism of catalytic cracking. At the same time, steam inhibits chain termination by desorption. The net result is that the presence of steam increases kinetic chain length and the paraffin-to-olefin ratio. The effects of steam dilution are surprisingly strong at lower temperatures but become less significant as temperature rises. All these effects are interpreted in terms of their influence on the elementary steps of the chain mechanism and are principally rendered in terms of a new measure: the reaction path probabilities (RPPs). These describe the likelihood that a feed molecule will be converted by a given reaction. Probabilities of all the reactions of the various ions present in the chain mechanism of 2-methylpentane cracking are quantified by formulating appropriate secondary functions using the RPPs derived from our experimental data. These various “internal” probabilities are used to quantify just how likely are each of the many alternative reactions of a given ion, as reaction conditions change. They allow us to track the effect of changes in reaction conditions, or in catalyst formulation, in unprecedented detail. Moreover, they give a detailed picture of the behavior of elementary processes in this reaction. We present and discuss examples of the quantitative information which our methods make available, and the mechanistic interpretations which flow from this information. In particular, we apply these measures in a study of the effects of steam when it is added to cracking 2-methylpentane in various proportions. It appears that catalytic cracking is much more susceptible to the effects of dilution than has previously been suspected.

Reactions of mixtures of light alkenes and n-heptane over USHY zeolite

The influence of ethene and propene on the catalytic cracking of n-heptane at 673 K over the USHY zeolite has been studied with mixtures of n-heptane with different concentrations of alkene. The study reveals that the presence of light alkenes, with an alkene-to-n-heptane molar ratio in the range of 0.1–1.2, results in a decrease of the initial cracking activity of n-heptane, and an increase in the yield of heptenes and amount of coke formed. So, for this range of relatively high concentrations of light alkenes studied, the cracking reaction of n-heptane is substantially inhibited by these species.