Unique Aspects of Mechanisms and Requirements for Zeolite Catalysis in Refining and Petrochemicals

Abrevaya, H.

doi:10.1002/9783527629565.ch13

Cited by 8 publications

(10 citation statements)

References 103 publications

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“…Zeolites are microporous aluminosilicates whose shape-selective properties are exploited extensively for the catalytic cracking of hydrocarbons. − The apparent kinetics of cracking are affected by the thermodynamics and kinetics of hydrocarbon adsorption, which are in turn affected by zeolite topology. For these reasons, many previous studies have been aimed at elucidating the effects of topology on adsorption behavior. − Transformations of hydrocarbons are often carried out at high pressures and surface coverages and are limited by diffusion.…”

Section: Introductionmentioning

confidence: 99%

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Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

et al. 2017

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Monte Carlo simulations are used to systematically investigate the effects of structural topology on the thermodynamics of n-alkanes adsorbed at Brønsted protons in zeolites having one-dimensional channel systems. In zeolites without cages, the enthalpy and entropy of adsorption (∆H ads-H+ and ∆S ads-H+ ) at fixed pore-limiting diameter (PLD) generally increase (become less negative) as the ratio of the minimum to maximum channel diameter decreases, and are lowest when this ratio equals 1 (corresponding to approximately circular cross-sections). The effect of a change in diameter ratio on the free energy of adsorption (∆A ads-H+ ) is weak because the changes in ∆H ads-H+ and T∆S ads-H+ largely cancel. The addition of cages having a largestcavity diameter (LCD) greater than the PLD increases both ∆H ads-H+ and ∆S ads-H+ . Replacing channels with cages of the same diameter does not change ∆S ads-H+ significantly when the PLD is similar to the alkane length, but decreases both ∆H ads-H+ and ∆A ads-H+ because of the greater surface area of cages relative to channels. The selectivity to adsorption via a central C-C bond vs. a terminal bond when cages are absent is smallest for PLDs near the alkane length, and when cages are present, is even lower when the LCD exceeds the alkane length. This effect is attributed to more rotation of the alkane in cages vs. channels. The results show that ∆A ads-H+ at 773 K can be tuned by manipulating a characteristic dimension (LCD, PLD) and topology (e.g., adding cages) simultaneously, in order to circumvent the compensating changes in T∆S ads-H+ and ∆H ads-H+ that occur upon changing only one structural parameter.3

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Section: Introductionmentioning

confidence: 99%

“…[1][2][3][4][5] The apparent kinetics of cracking are affected by the thermodynamics and kinetics of hydrocarbon adsorption, which are in turn affected by zeolite topology. For these reasons, many previous studies have been aimed at elucidating the effects of topology on adsorption behavior.…”

Section: Introductionmentioning

confidence: 99%

Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

et al. 2017

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“…The contraction of methylcyclohexane rings to form alkylcyclopentanes on bifunctional catalysts consisting of Brønsted acid and metal functions is often used as a probe of acid strength and of the effects of confinement within amorphous and crystalline solid acids [1][2][3][4], it is also of industrial importance because such chemistry is paramount for the effective conversion of ethylbenzene to xylene isomers [5,6]. Yet, connections between reactivity, selectivity and acid strength remain empirical because of the challenges inherent in assessing the number and strength of acid sites and, their consequential distances from the metal function, and the identity and reversibility of the elementary steps that mediate such reactions.…”

Section: Introductionmentioning

confidence: 99%

Acid strength and metal-acid proximity effects on methylcyclohexane ring contraction turnover rates and selectivities

Knaeble

Iglesia

2016

Journal of Catalysis

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Methylcyclohexane ring contraction is used here to assess the effects of acid strength and metal-acid site proximity on turnover rates and selectivities for bifunctional catalysts consisting of Keggin type polyoxometalates (POM)with different central atoms that act as Brønsted acids. Bifunctional catalysts with metal sites that fully equilibrate cycloalkanes and cycloalkenes give methylcyclohexene conversion rate constants that decreased exponentially with increasing deprotonation energy, a rigorous descriptor of acid strength, consistent with the ion-pair character of transition states that mediate the kinetically-relevant ring contraction of bound methylcyclohexoxide intermediates. The measured rates of formation of each alkylcyclopentane isomer, however, do not reflect their intrinsic formation kinetics, because of fast diffusionenhanced interconversions within acid domains; thus, isomer selectivities cannot be used to infer, even indirectly, the strength of acid sites, as often proposed in previous studies. Selectivities reflect instead diffusional effects that become more severe as the number and strength of acid sites and the size and diffusive resistances increase within these acid domains, which shift, in turn, the products formed from relative abundances dictated by kinetics to those prescribed by thermodynamics. A rigorous accounting of these diffusional effects using a kinetic-transport model leads to ratios of intrinsic rate constants for the formation of the different alkylcyclopentane isomers that do not depend on acid strength, because all isomerization routes are mediated by bicyclo[3.1.0]hexyl cation transition states similar in the amount and location of charge and which therefore benefit to the same extent from the more stable conjugate anions characteristic of stronger acids.

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“…Most of the catalytic processes in the petroleum refining industry use bifunctional catalysts comprising a combination of acid and hydrogenation-dehydrogenation functions. From a chemical point of view, bifunctional catalysts ability to generate carbenium ions at relatively low temperatures comparing to monofunctional acidic catalysts [4] explains their key role in industry. The acidity can be provided by a zeolite, while the hydrogenation-dehydrogenation activity is exerted by finely dispersed (noble or transition) metals.…”

Section: Introductionmentioning

confidence: 99%

Hydrogenating activity of Pt/zeolite catalysts focusing acid support and metal dispersion influence

Mendes

Lapisardi

Bouchy

et al. 2015

Applied Catalysis A: General

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International audienceToluene hydrogenation was studied over catalysts based on Pt supported on large pore zeolites (HUSY and HBEA) with different metal/acid ratios. Acidity of zeolites was assessed by pyridine adsorption followed by FTIR showing only small changes before and after Pt introduction. Metal dispersion was determined by H 2-O 2 titration and verified by a linear correlation with the intensity of Pt 0-CO band obtained by in situ FTIR. It was also observed that the electronic properties of Pt 0 clusters were similar for the different catalysts. Catalytic tests showed rapid catalyst deactivation with an activity loss of 80-95% after 60 min. of reaction. The turnover frequency of fresh catalysts depended both on metal dispersion and the support. For the same support, it changed by a 1.7-fold (HBEA) and 4.0-fold (HUSY) showing that toluene hydrogenation is structure-sensitive, i.e. hydrogenating activity is not a unique function of accessible metal. This was proposed to be due to the contribution to the overall activity of the hydrogenation of adsorbed toluene on acid sites via hydrogen spillover. Taking into account the role of zeolite acidity, the catalysts series were compared by the activity per total adsorbing sites which was observed to increase steadily with n Pt /(n Pt +n A). An increase of the accessible Pt atoms leads to an increase on the amount of spilled over 1 hydrogen available in acid sites therefore increasing the overall activity. Pt/HBEA catalysts were found to be more active per total adsorbing site than Pt/HUSY which is proposed to be due to an augmentation in the efficiency of spilled over hydrogen diffusion related to the proximity between Pt clusters and acid sites. The intervention of Lewis acid sites in a greater extent than that measured by pyridine adsorption may also contribute to this higher activity of Pt/HBEA catalysts. These results reinforce the importance of model reactions as a closer perspective to the relevant catalyst properties in reaction conditions

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Unique Aspects of Mechanisms and Requirements for Zeolite Catalysis in Refining and Petrochemicals

Cited by 8 publications

References 103 publications

Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

Acid strength and metal-acid proximity effects on methylcyclohexane ring contraction turnover rates and selectivities

Hydrogenating activity of Pt/zeolite catalysts focusing acid support and metal dispersion influence

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