Hydrocarbon Conversion Processes Research Articles

Illuminating the impact of the proximity communication modes between redox metallic and acidic zeolite sites during hydro-conversion of n-heptane

The unique physicochemical properties of “redox-acid” bifunctional zeolites make them an essential tool in the petrochemical industry, especially in the hydro-conversion of linear alkanes to deliver high-octane fuels. The current research activities in this domain are primarily tailored towards evaluating the “intimacy/proximity” criteria among two catalytically active sites and minimizing the loading of noble metals (e.g., Pd or Pt) in catalysis. However, the chemistry behind the communication mode (i.e., “through-space” or “through-bond” interaction between metallic and acid sites) has remained vastly underexplored during the hydro-conversion of n-alkanes over Pt or Pd/zeolites. To dig deeper into this uncharted territory, this work demonstrates that both “through-space” (cf. metals in extra-framework position in/on zeolite) and “through-bond” (cf. metals on the zeolite framework) bifunctionality favors the hydroisomerization step, while the later partially also promote the hydro-cracking. Their working protocol was probed by the diverse material characterization techniques that improved the understanding of the zeolite-catalyzed hydrocarbon conversion processes.

Untangling Framework Confinements: A Dynamical Study on Bulky Aromatic Molecules in MFI Zeolites

In MFI zeolites, differentiating the molecular dynamics as a function of pore structures, that is, straight-channel versus zigzag-channel versus channel-intersection, has always been challenging, mainly due to small differences in pore size but is of great interest because these subtle structural differences can remarkably influence shape selectivity. Herein, 1,2,4-trimethyl benzene (1,2,4-TMB), a characteristic molecule larger than the 10-MR channel diameter, while smaller than the channel intersection, is chosen to probe the pore-confined dynamical behaviors in MFI via 2H NMR spectroscopy and density functional theory calculations. Our results show that in the absence of acid sites, that is, in the siliceous MFI silicalite-1, 1,2,4-TMBs can only diffuse along the straight channels; while at equilibrium, they incline to occupy the channel intersections with structure-defined orientations. Furthermore, a series of dynamic motions of 1,2,4-TMBs under different types of pore confinements are revealed and evaluated at a molecular level over a wide range of timescales, concluded, in short, as methyl C3-rotation > 112°-flip > 90°-flip > translational diffusion. With the presence of acid sites, that is, in H-ZSM-5; however, 1,2,4-TMBs are strongly adsorbed on Brønsted acid sites and the confined motions are further impeded. The findings in this work may provide insights to the catalytic roles of polymethyl-benzene intermediates, including 1,2,4-TMB, which usually serve as active centers or deactivation precursors in zeolite-based hydrocarbon conversion processes.

Binder Selection to Modify Hydrocarbon Cracking Properties of Zeolite-Containing Composites.

Activity, selectivity, and deactivation behavior of catalyst materials determine their efficiency in hydrocarbon conversion processes. For hydrocarbon cracking, the industrial catalyst is an important parameter in reaction technology to produce valuable compounds, e.g., light olefins (C3–C5) and gasoline from crude oil fractions with high molecular weight (C16+). One strategy to enhance the catalytic activity for precracking is increasing the matrix activity, which depends on the used binder and additives. In this work, three binders (water glass, aluminum chloride, and a mixture of colloidal silica with aluminum dihydrogen phosphate) were used in combination with active zeolite Y, kaolin as filler, and ZSM-5 as additive to produce composite materials. Specific surface area and surface acidity measurements were combined with catalytic testing of the formulated samples in order to find the relation between the catalyst morphology and its activity. In addition, constraint index was used as a control parameter for the determination of the shape-selective properties and their correlation with the catalytic activity. The results show that the binders determine the porosity of the matrix and so the accessibility to zeolite pores and active sites. Matrixes with low porosity and activity enhance coke production and deactivate faster than matrixes with mesopores. Furthermore, ZSM-5 modifies the individual morphological and catalytic effects of the binders. Everything considered, the small crystals of ZSM-5 together with mesopores increase the olefins yield, reduce coking, and therefore enhance the performance of the final grain.

Overcoming Stability Problems in Microwave-Assisted Heterogeneous Catalytic Processes Affected by Catalyst Coking

Microwave-assisted heterogeneous catalysis (MHC) is gaining attention due to its exciting prospects related to selective catalyst heating, enhanced energy-efficiency, and partial inhibition of detrimental side gas-phase reactions. The induced temperature difference between the catalyst and the comparatively colder surrounding reactive atmosphere is pointed as the main factor of the process selectivity enhancement towards the products of interest in a number of hydrocarbon conversion processes. However, MHC is traditionally restricted to catalytic reactions in the absence of catalyst coking. As excellent MW-susceptors, carbon deposits represent an enormous drawback of the MHC technology, being main responsible of long-term process malfunctions. This work addresses the potentials and limitations of MHC for such processes affected by coking (MHCC). It also intends to evaluate the use of different catalyst and reactor configurations to overcome heating stability problems derived from the undesired coke deposits. The concept of long-term MHCC operation has been experimentally tested/applied to for the methane non-oxidative coupling reaction at 700 °C on Mo/ZSM-5@SiC structured catalysts. Preliminary process scalability tests suggest that a 6-fold power input increases the processing of methane flow by 150 times under the same controlled temperature and spatial velocity conditions. This finding paves the way for the implementation of high-capacity MHCC processes at up-scaled facilities.

Quantification and Statistical Analysis of Errors Related to the Approximate Description of Active Site Models in Metal‐Exchanged Zeolites

AbstractMetal‐exchanged zeolites are complex catalytic systems with great potential for applications in hydrocarbon conversion processes. The numerous combinations of zeotypes and metal‐based Lewis acid motifs create a vast material space that can only be effectively navigated with robust structure‐function relationships. Exhaustive modeling of extraframework metal sites in all crystallographically distinct framework positions is not practical; thus, we quantified the uncertainties arising from the use of simplified active site representations using the C−H bond activation in methane as a probe reaction. Density functional theory calculations and statistical analysis of associated error ensembles suggest that the Lewis acid identity primarily determines the reactivity of the zeolite towards methane activation, with other factors such as zeolite model characteristics deemed secondary. The error ensemble distributions were used to predict the relative probability of creating active C−H bond scission fragments in metal‐exchanged zeolites, and thus can be used to efficiently screen various metal‐exchange candidates in their efficacy towards hydrocarbon conversion.

Ecological Consequences of the Development of Shale Formations Containing Toxic Elements

Abstract—The average concentrations of trace elements, including toxic elements, in shales from various shale basins of the world are reported. The concentrations of a number of elements significantly exceeding the Clarke numbers for clay rocks in the Kenderlyk shale of the Republic of Kazakhstan, in the Domanic sediments of the Volga–Ural oil and gas bearing basin, some American formations (Barnett), etc., were noted. It was demonstrated that the concentrations of potentially toxic elements in many deposits were higher than their average values in clays and other sedimentary rocks. The release of potentially toxic elements into the environment significantly increased as a result of thermal effects on the formation, which can occur on the development of similar deposits of fossil fuels and in some hydrocarbon conversion processes. This was confirmed by natural observations and laboratory studies. On the hydraulic fracturing, which is widely used in the development of shale formations by the horizontal drilling method, toxic elements from both shales and their naphthide components can be released into the environment. In the development of shale deposits, additional studies are required for the monitoring of the trace element composition of shale formations to exclude environmental risks.

Investigation of the Fluid Behavior of Asphaltenes and Toluene Insolubles by High-Temperature Proton Nuclear Magnetic Resonance and Rheometry and Their Application to Visbreaking

The fluid behavior of asphaltenes at elevated temperatures impacts coke formation in a number of hydrocarbon conversion processes, including visbreaking and delayed coking. In this study, the asphaltenes from a number of sources, namely, a vacuum residue, a petroleum source rock (Kimmeridge clay) bitumen obtained by hydrous pyrolysis, and bitumen products from a sub-bituminous coal and pine wood obtained by thermolytic solvent extraction using tetralin, have been characterized using high-temperature proton nuclear magnetic resonance (1H NMR), and the results correlated with those from small-amplitude oscillatory shear rheometry. Further for comparison, the coke (toluene insolubles) obtained from visbreaking the vacuum residue was also characterized. All of the asphaltenes became completely fluid by 300 °C, with hydrogen being completely mobile with coke formation, identified as a solid phase, not occurring to a significant extent until 450 °C. Extremely good agreement was obtained between high-temperature 1H NMR and rheometry results, which confirmed that the asphaltenes were highly fluid from 300 °C, with initial signs of resolidification being observed at temperatures of around 450 °C. During softening, extremely good correlations between fluid hydrogen and phase angle were obtained as the asphaltenes softened. The toluene insolubles however did contain some fluid material; thus, it cannot be regarded as strictly solid coke, but clearly, with increasing temperature, the fluid material did convert to coke. Under actual process conditions, this fluid material could be responsible for coke adhering to reactor surfaces.

Rol de la Química Orgánica en los procesos de conversión de hidrocarburos

Complex crude oil refining processes involve several organic reactions whose mechanisms, kinetic and thermodynamic phenomena determine the product distributions, therefore, these reactions constitute an important reference for setting operating conditions of many processes in the industry. Under the integral vision of the chemical engineering teaching, 3 hydrocarbon conversion processes are analyzed (combustion, alkylation and cracking) taking into account the organic reactions involved in each process and its influence to determine the type and quality of the products obtained.

Group Additivity Determination for Enthalpies of Formation of Carbenium Ions

Modeling of acid-catalyzed hydrocarbon conversion processes at the mechanistic level requires rate coefficients for a large number of reactions. The computational demand of finding activation energy barriers for each reaction is substantially reduced by employing structure–reactivity correlations such as the Evans–Polanyi relationship that correlates activation energy with the enthalpy of reaction. However, there are many species for which the enthalpies of formation are unknown. Therefore, group additivity methods to specify enthalpies of formation for each species involved in the reaction network are valuable. Quantum mechanical (QM) calculations and isodesmic reactions were used to calculate enthalpies of formation for a number of acyclic and cyclic carbenium ions, including allylic carbenium ions. These values compare favorably with experimental values, establishing Gaussian-4 as an accurate QM method for these calculations. Using these values, Benson-type group additivity values for enthalpies of for...

On the application of petroleum feedstock modeling techniques for developing molecule-based models of hydrocarbon conversion processes

The design of hydrocarbon conversion processes currently demands more realistic modeling tools, with the capability of predicting product distribution at the molecular level. This requires a detailed molecular characterization, which generally is not available for complex feeds. The implementation of modeling techniques to simulate the molecular composition of petroleum feedstocks from routine analyses represents an alternative route towards molecule-based kinetic modeling. Nevertheless, generating a useful molecular representation depends heavily on the scope of the input analyses. This study aimed at verifying the requirements to construct a molecular representation that suits a detailed kinetic model, in terms of input data and model formulation. Two straight-run naphtha samples were selected as basis, in order to compare the simulated composition against gas chromatography data, and afterwards the analysis was extended to a light cycle oil. It was confirmed that it is possible to generate a synthetic mixture that behaves like the actual petroleum fraction in terms of bulk properties and even carbon number distributions. The comparison against gas chromatography data on the other hand, revealed that there are significant differences at the molecular level primarily due to the inability to predict all possible structural combinations of alkyl groups, which increases exponentially with the carbon number. For practical reasons then, it is proposed to work with a reduced set of chemically relevant species, built from a main core structure (e.g. an arrangement of aromatic or naphthenic rings) and a reduced set of alkyl branch configurations, rather than considering a vast number of structural combinations.

Forty Years of Designing Catalytic and Adsorptive Sites in FAU Type Zeolites at K.U. Leuven

An overview of scientific achievements at K.U. Leuven on the design and characterization of active sites in X and Y zeolites and on the development of adsorptive and catalytic processes in which these active sites are operated is provided. The historical development of this research area and the scientific progress are presented. Zeolite Y and ultrastabilized specimen present Bronsted and Lewis sites that can be operated in vapor and liquid phase hydrocarbon conversion processes, bifunctional catalysis, epoxidation and ring opening of epoxides. Examples of molecular shape selective catalysis with Y zeolites are presented. The redox chemistry of transition metal exchanged faujasites is understood in great detail and has been exploited in Wacker chemistry and water splitting in a photochemical-thermal cycle. Selective adsorbents were designed based on knowledge of cation siting in X and Y zeolites. The occlusion of coordination compounds in the faujasite supercages is a means of creating unique active sites. Catalytic oxidative properties of the enzymes can be mimicked by embedding zeozyme in a hydrophobic membrane.

First Principles Study of the Stability and the Formation Kinetics of Subsurface and Bulk Carbon on a Ni Catalyst

Ni-based catalysts are widely used in hydrocarbon conversion processes. First principles based analysis of the stability of subsurface and bulk carbon on Ni catalysts indicates that they might be formed under typical reaction conditions. It was shown that carbon diffusion to the octahedral sites of the first subsurface layer is thermodynamically favorable until about 75% of the subsurface sites are occupied. Activation barriers for diffusion to the subsurface octahedral sites were found to decrease with on-surface carbon coverage, and typical barriers of less than 110 kJ/mol were calculated for moderate to high surface coverages. Diffusion from the subsurface sites to the Ni bulk was found to be thermodynamically favorable if more than 50% of the subsurface octahedral sites are occupied. Such diffusion is, however, kinetically limited, with typical barriers above 150 kJ/mol. For carbon concentrations above 50% in the first subsurface layer, calculations indicate that the Ni lattice is likely to expand. This expansion decreases the diffusion barriers to below 100 kJ/mol and opens up a diffusion path. Hence, carbon diffusion into Ni catalysts is proposed to be a gradual process, first partially filling the subsurface octahedral sites before diffusing to the bulk.

A quantum chemical study of tertiary carbenium ions in acid catalyzed hydrocarbon conversions over phosphotungstic acid

The formation and transformation of adsorbed alkoxide intermediates to active carbenium-ion species over solid acid catalysts are essential steps that determine the rate and selectivity of many hydrocarbon conversion processes. Herein, density functional theory is used to examine these processes for a t-butyl species adsorbed to phosphotungstic acid, a Keggin structured polyoxometalate. The tertiary carbenium ion is found to be a meta-stable intermediate state. The conversion of a physisorbed, π-bound isobutene state to the t-butyl alkoxide state occurs in a two-step process through formation of this tertiary carbenium-ion intermediate. Both the intermediate and transition states contain a positive charge and are properly termed carbenium ions, and the interaction with the catalyst surface is mainly Coulombic. The dehydrogenation of isobutane and the dehydration of 2-methyl-2-propanol are also examined. Tertiary carbenium ions are found to be transition states for both of these processes as well.

A Quantum Chemical Study of the Decomposition of Keggin-Structured Heteropolyacids

Heterpolyacids (HPAs) demonstrate catalytic activity for oxidative and acid-catalyzed hydrocarbon conversion processes. Deactivation and thermal instability, however, have prevented their widespread use. Herein, ab initio density functional theory is used to study the thermal decomposition of the Keggin molecular HPA structure through the desorption of constitutional water molecules. The overall reaction energy and activation barrier are computed for the overall reaction HnXM12O40-->Hn-2XM12O39+H2O. and subsequently used to predict the effect of HPA composition on thermal stability. For example, the desorption of a constitutional water molecule is found to be increasingly endothermic in the order silicomolybdic acid (H4SiMo12O40)<phosphomolybdic acid (H3PMo12O40)<silicotungstic acid (H4SiW12O40)<phosphotungstic acid (H3PW12O40), in agreement with the experimental ordering of their thermal stability. The presence of an adjacent Keggin unit may stabilize the structural defect created by the water desorption, thus suggesting that constitutional water loss is an initial step toward the decomposition into a bulk mixed oxide. The equilibrium concentration of defective Keggin units is determined as a function of temperature and water partial pressure. It is concluded that the loss of constitutional water molecules is a plausible deactivation mechanism of the acid catalyst. The intermediate structures along the decomposition path are proposed as possible active sites for oxidation catalysis. The results presented herein provide molecular level insight into the dynamic nature of the heteropolyacid catalyst structure.

The Relationship Between Adsorption and Solid Acidity of Heteropolyacids

AbstractFor Abstract see ChemInform Abstract in Full Text.

The relationship between adsorption and solid acidity of heteropolyacids

The adsorption energy of a base to a solid acid catalyst is often assumed to correlate with the acid strength of the catalyst. In this study, the influences of adsorbate type, binding configuration and solid acid composition on the adsorption energy are explored using quantum chemical methods. In particular, density functional theory is used to calculate the adsorption energies of functionalized hydrocarbons containing O, N, or S heteroatoms or CC to phosphotungstic (H3PW12O40) and phosphomolybdic (H3PMo12O40) acids. The adsorption energies of the different molecules bound to the same solid acid are not easily predicted by the proton affinity of the adsorbate because the stabilization of the protonated adsorbate also varies with composition. Bond order conservation helps to explain the relatively small variance in adsorption energies among reactants of widely varying base strength. The activation barriers to form carbenium-ion transition states from adsorbed olefins are also calculated over the two heteropolyacids. The stronger adsorption of propylene to phosphotungstic acid compared to phosphomolybdic acid results in a higher activation barrier to form the carbenium-ion transition state. These heteropolyacids are predicted to have higher activation barriers than zeolites for carbenium-ion formation, which is typically thought to be the rate controlling step in many hydrocarbon conversion processes. This contrasts with the ranking of acid strength based solely on the magnitude of the adsorption energy.

Nonoxidative Activation of Methane

Effective utilization of methane remains one of the long-standing problems in catalysis. Over the past several years, various routes, both direct and indirect, have been considered for the conversion of methane to value-added products such as higher hydrocarbons and oxygenates. This review will focus on the range of issues dealing with thermal and catalytic decomposition of methane that have been addressed in the last few years. Surface science studies (molecular beam methods and elevated-pressure reaction studies) involving methane activation on model catalyst systems are extensively reviewed. These studies have contributed significantly to our understanding of the fundamental dynamics of methane decomposition. Various aspects of the nonoxidative methane to higher hydrocarbon conversion processes such as high-temperature coupling and two-step low-temperature methane homologation have been discussed. Decomposition of methane results in the production of COx-free hydrogen (which is of great interest to state-of-the-art low-temperature fuel cells) and various types of carbon (filamentous carbon, carbon black, diamond films, etc.) depending on the reaction conditions employed; these issues will be briefly addressed in this review.

Atom-Resolved Imaging of Dynamic Shape Changes in Supported Copper Nanocrystals

In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports. These are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For zinc oxide-supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For copper nanocrystals supported on silica, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant atomic-scale insight.

Zeolite SSZ-37

A crystalline zeolite SSZ-37 is prepared using a N,N-dimethyl-4-azoniatricyclo [5.2.2.0.sup.(2,6) ] undec-8-ene cation as a template wherein said zeolite is used in hydrocarbon conversion processes.

Promising catalysts for hydrocarbon conversion processes

Promising catalysts for hydrocarbon conversion processes