Influence of the size, order and topology of mesopores in bifunctional Pd-containing acidic SBA-15 and M41S catalysts for n-hexadecane hydrocracking

The catalytic performance in n-hexadecane hydrocracking of a set of bifunctional catalysts composed of acidic mesoporous silicas, i.e., SBA-15, MCM-41, MCM-48, and amorphous silica-alumina (ASA), and Pd as acid and (de)hydrogenation components, respectively, was investigated. The selectivity to cracked products and the occurrence of secondary cracking depended on the pore topology, acidity, and Pd dispersion. The Si/Al ratio and the mesopore order of SBA-15 were modified by changing the pH in the synthesis step. Al was introduced in the M41S materials by post-synthesis grafting. All materials including ASA exhibited low acidity compared to crystalline zeolites. Increasing Al content led to a decrease of the order of mesopores. Secondary cracking of n-hexadecane was more pronounced for catalysts containing long one-dimensional cylindrical pores (SBA-15 and MCM-41) in comparison with catalysts containing three-dimensional ordered (MCM-48) or disordered (ASA) mesopores. The selectivity difference is attributed to differences in residence time of intermediates in the mesopores. The distance between acid sites located in mesopores and Pd nanoparticles primarily located outside these pores also influences the product distribution. Ideal hydrocracking operation is approached for ASA, MCM-48, and SBA-15 prepared at a high pH containing disordered mesopores

Mechanistic aspects of n-hexadecane hydroconversion:Impact of di-branched isomers on the cracked products distribution

The mechanism of n-hexadecane (n-C16) hydroisomerization/hydrocracking was studied over bifunctional catalysts employing Pd and Pt as (de)hydrogenation components and large-pore zeolites and (ordered) mesoporous materials as acidic supports. Zeolite Y and Beta supports were compared to amorphous silica-alumina as well as aluminium-containing ordered mesoporous MCM-41, MCM-48 and SBA-15 materials to cover a wide range of pore sizes and topologies. Products were analyzed in terms of mono- and di-branched C16 isomers and cracked products as a function of the n-C16 conversion. All samples followed a similar mechanism in which, at low conversion, di-branched isomers with methyl groups toward the ends of the tetradecane chain were observed. At higher conversion, the products shifted to isomers with methyl groups closer to the center at higher n-C16 conversion. Consistent with these differences, the typical ‘M’ shape distribution of cracked products was obtained at low conversion, which evolved into ideal symmetric cracking patterns at higher conversion. The unusual di-branched isomer distribution at low conversion and the corresponding ‘M’ shaped cracked products distribution have earlier been associated with pore mouth catalysis induced by restricted diffusion in medium-pore zeolites. Here we show that such reaction pathways also occur in catalysts with large enough pores to exclude diffusion restrictions

Hydrothermal synthesis and characterization of a layered zirconium silicate

A layered zirconosilicate composed of SiO4 tetrahedra and ZrO6 octahedra was hydrothermally synthesized in the presence of tetramethylammonium hydroxide. The layered material with a nominal composition Na2ZrSi4O11·xH2O was further characterized by XRD, SEM, thermogravimetric analysis, UV-Raman spectroscopy and single- and triple-quantum 23Na and 29Si MAS NMR spectroscopy. There are no Zr–O–Zr chains in the framework structure implying that the ZrO6 octahedra are isolated by SiO4 tetrahedra. 23Na MAS NMR indicates at least two different Na sites in the framework of SZS and the Na located in the interlayer can be exchanged by TMAOH, leading to an increase of the interlayer spacing. The layered zirconosilicate SZS has a high activity in the isomerization of glucose to fructose in water and can be reused.status: publishe

Shape selectivity in linear paraffins hydroconversion in 10-membered-ring pore zeolites

Pd/zeolite-catalyzed hydroconversion of n-hexadecane (n-C16) and n-heptane (n-C7) was studied for 10MR (ZSM-5, ZSM-22), 12MR (ZSM-12), and EMM-23 (21MR × 10MR) zeolites. The catalytic activity depended on the Brønsted acidity and the crystalline domain size. n-C16 hydroconversion benefited from short diffusion lengths in ZSM-5 nanosheets compared to bulk ZSM-5. In general, over-cracking is dominant in ZSM-5 with a cracked product distribution skewed to C4 products, to be explained by a snug fit of particular dibranched isomers at zeolite intersections. This effect is less pronounced for the 1D 10MR pores in ZSM-22, which lacks intersections. Although large pores in ZSM-12 offer relatively high activity, those in EMM-23 do not. Based on selectivity patterns, EMM-23 behaves like ZSM-5, probably because of the trilobe shape of its 21MR pores acting as 10MR pores. Only ZSM-12 offers operation in the ideal hydrocracking regime, in the sense of impediments neither by hydrogenation nor by diffusion. Faster intrazeolite diffusion of n-C7 in comparison to n-C16 leads to a higher yield of isomers for the nanostructured zeolites. Overall, the hydroconversion of the smaller alkane is more substantially impacted by variations in the crystalline zeolite domain size

Shape selectivity in linear paraffins hydroconversion in 10-membered-ring pore zeolites

Pd/zeolite-catalyzed hydroconversion of n-hexadecane (n-C16) and n-heptane (n-C7) was studied for 10MR (ZSM-5, ZSM-22), 12MR (ZSM-12), and EMM-23 (21MR × 10MR) zeolites. The catalytic activity depended on the Brønsted acidity and the crystalline domain size. n-C16 hydroconversion benefited from short diffusion lengths in ZSM-5 nanosheets compared to bulk ZSM-5. In general, over-cracking is dominant in ZSM-5 with a cracked product distribution skewed to C4 products, to be explained by a snug fit of particular dibranched isomers at zeolite intersections. This effect is less pronounced for the 1D 10MR pores in ZSM-22, which lacks intersections. Although large pores in ZSM-12 offer relatively high activity, those in EMM-23 do not. Based on selectivity patterns, EMM-23 behaves like ZSM-5, probably because of the trilobe shape of its 21MR pores acting as 10MR pores. Only ZSM-12 offers operation in the ideal hydrocracking regime, in the sense of impediments neither by hydrogenation nor by diffusion. Faster intrazeolite diffusion of n-C7 in comparison to n-C16 leads to a higher yield of isomers for the nanostructured zeolites. Overall, the hydroconversion of the smaller alkane is more substantially impacted by variations in the crystalline zeolite domain size.</p

Influence of polyvinylpyrrolidone as stabilizing agent on Pt nanoparticles in Pt/H-BEA catalyzed hydroconversion of n-hexadecane

Colloidal Pt particles prepared using polyvinylpyrrolidone (PVP) as a capping agent were loaded on acidic Beta zeolite, with the aim of obtaining bifunctional catalysts with controllable dispersion for the hydroconversion of n-hexadecane (n-C16). Among the different approaches to remove PVP, reduction was the most effective whilst maintaining the initial Pt particle size after deposition on the zeolite. The presence of small amounts of nitrogen- and carbon-containing products from PVP decomposition affected the acidity stronger than the Pt metal function. Therefore, it was not possible to establish the effect of Pt particle size on the performance of the Pt/Beta zeolites in n-C16 hydroconversion and the performance trends were dominated by the acidity differences. Small Pt particles on zeolite obtained by using high PVP/Pt ratios during the colloidal synthesis step presented lower Brønsted acidity than catalysts containing larger Pt particles. The resulting variations in Pt/H+ ratios led to a transition of observed ideal cracking behavior for weakly acidic catalysts (small Pt particles, larger amount of PVP residuals) to overcracking behavior for catalysts with stronger acidity. We find that the Pt/H+ ratio and the number of acid-catalyzed steps extrapolated to zero conversion are better indicators of ideal cracking behavior than the isomers yield

Competitive Adsorption of Xylenes at Chemical Equilibrium in Zeolites

The separation of xylenes is one of the most important processes in the petrochemical industry. In this article, the competitive adsorption from a fluid-phase mixture of xylenes in zeolites is studied. Adsorption from both vapor and liquid phases is considered. Computations of adsorption of pure xylenes and a mixture of xylenes at chemical equilibrium in several zeolite types at 250 °C are performed by Monte Carlo simulations. It is observed that shape and size selectivity entropic effects are predominant for small one-dimensional systems. Entropic effects due to the efficient arrangement of xylenes become relevant for large one-dimensional systems. For zeolites with two intersecting channels, the selectivity is determined by a competition between enthalpic and entropic effects. Such effects are related to the orientation of the methyl groups of the xylenes. m-Xylene is preferentially adsorbed if xylenes fit tightly in the intersection of the channels. If the intersection is much larger than the adsorbed molecules, p-xylene is preferentially adsorbed. This study provides insight into how the zeolite topology can influence the competitive adsorption and selectivity of xylenes at reaction conditions. Different selectivities are observed when a vapor phase is adsorbed compared to the adsorption from a liquid phase. These insight have a direct impact on the design criteria for future applications of zeolites in the industry. MRE-type and AFI-type zeolites exclusively adsorb p-xylene and o-xylene from the mixture of xylenes in the liquid phase, respectively. These zeolite types show potential to be used as high-performing molecular sieves for xylene separation and catalysis.Engineering Thermodynamic

Effects of Framework Flexibility on the Adsorption and Diffusion of Aromatics in MFI-Type Zeolites

We systematically study how the degree of framework flexibility affects the adsorption and diffusion of aromatics in MFI-type zeolites as computed by Monte Carlo simulations. It is observed that as the framework is more flexible, the zeolite structure is inherently changed. We have found that framework flexibility has a significant effect on the adsorption of aromatics in MFI-type zeolites, especially at high pressure. Framework flexibility allows the zeolite framework to accommodate to the presence of guest aromatic molecules. For very flexible zeolite frameworks, loadings up to two times larger than that in a rigid zeolite framework are obtained at a given pressure. We assessed the "flexible snapshot"method, which captures framework flexibility using independent snapshots of the framework. We have found that this method only works well when the loadings are low. This suggests that the effect of the guest molecules on the zeolite framework is important. Framework flexibility lowers the free-energy barriers between low energy states, increasing the rate of diffusion of aromatics in the straight channel of MFI-type zeolites for many orders of magnitude compared to a rigid zeolite framework. The simulations show that framework flexibility should not be neglected and that it significantly affects the diffusion and adsorption properties of aromatics in an MFI-type zeolite. Engineering Thermodynamic

Kinetics of zeolite-catalyzed heptane hydroisomerization and hydrocracking with CBMC-modeled adsorption terms: Zeolite Beta as a large pore base case

A reactor model that deconvolutes thermodynamics of adsorption of hydrocarbon in the pores of zeolite Beta, obtained by Configurational-bias Monte Carlo simulations, from intrinsic, intraporous kinetics of hydroisomerization and hydrocracking reactions, provides a good quantitative description of all significant reactions in the kinetic network for interconversion and cracking of different heptane isomers. Activation enthalpies obtained for intraporous reactions follow the expected order according to the carbenium ion formalism: methyl shift&lt; ethyl shift &lt; isom(B) ∼ crack(B2) &lt; crack(B1) &lt; crack(C) ∼ crack(D) &lt; crack(E) and apparently within each isomerization class, in terms of carbenium ions formally involved: sec → tert &lt; sec → sec ∼ tert → tert &lt; tert → sec. except for the ethyl shift reaction forming 3-ethylpentane. Cracking happens primarily through 2,4-dimethylpentane (type B2), regardless of the initial reactant. The model can be subsequently used to separate the effect of pore structure on selective adsorption and on intraporous reaction kinetics. Zeolite Beta will serve as a base case for a comparison of different zeolite structures.</p

RUPTURA: simulation code for breakthrough, ideal adsorption solution theory computations, and fitting of isotherm models

We present the RUPTURA code (https://github.com/iraspa/ruptura) as a free and open-source software package (MIT license) for (1) the simulation of gas adsorption breakthrough curves, (2) mixture prediction using methods like the Ideal Adsorption Solution Theory (IAST), segregated-IAST and explicit isotherm models, and (3) fitting of isotherm models on computed or measured adsorption isotherm data. The combination with the RASPA software enables computation of breakthrough curves directly from adsorption simulations in the grand-canonical ensemble. RUPTURA and RASPA have similar input styles. IAST is implemented near machine precision but we also provide several explicit mixture prediction methods that are non-iterative and potentially faster than IAST. The code supports a wide variety of isotherm models like Langmuir, Anti-Langmuir, BET, Henry, Freundlich, Sips, Langmuir-Freundlich, Redlich-Peterson, Toth, Unilan, O'Brian &amp; Myers, Asymptotic Temkin, and Bingel &amp; Walton. The isotherm model parameters can easily be obtained by the fitting module. Breakthrough plots and animations of the column properties are automatically generated. In addition to highlighting the code, we also review all the developed techniques from literature for mixture prediction, breakthrough simulations, and isotherm model fitting, and provide a tutorial discussing the workflows.</p