Dehydrogenation Stages Research Articles

Unraveling the role of single-atom metal dopant over CeO2 catalyst in tuning catalytic performance of ethane oxidative dehydrogenation

Single-atom metal doped CeO2 (M1@CeO2) catalysts have exhibited better catalytic performance in methane and propane dehydrogenation, which may be applied to ethane dehydrogenation, revealing structure-activity relationship of M1@CeO2 catalysts is of great importance. In this work, DFT + U calculations together with kinetic Monte Carlo simulations were implemented to investigate the oxidative dehydrogenation stage (ODH) in ethane chemical looping-oxidative dehydrogenation (CL-ODH) over thirteen types of M1@CeO2 catalysts (M = Cr, Mn, Fe, Co, Ni, Ga, Mo, Ru, Rh, Pd, In, Ir and Pt). The results show that the doped single-atom metal and its adjacent O atoms act as active sites over four catalysts Rh, Pd, Ir and Pt doped CeO2, while only O atoms act as active sites over other nine catalysts M1@CeO2. Rh1@CeO2 was screened out to perform higher C2H4(g) selectivity of 98.8 % and C2H4(g) formation activity of 8.72×103 s−1 site−1 at 823.15 K with C2H6(g) partial pressure <0.2 bar, which is superior to the previously reported catalysts Pt, Pt3Sn, Pt3Ir, Ni3Mo, Co/g-C3N4 and Rh/ZrO2, and so on. A quantitative and quick descriptor, the binding strength of C2H5* intermediate is proposed to evaluate C2H4(g) formation activity. This study unravels the role of doped single-atom metal over CeO2 catalyst and help rationally design high-efficiency alkane dehydrogenation.

ISOLATION OF HIGH PURITY PIPERYLENE

Piperylene is a valuable raw material component for petrochemical synthesis, perfumery and cosmetic products, and is used as a solvent for petroleum products in the production of synthetic rubbers and latexes, and in the paint and varnish industry. Piperylene in the form of a fraction is produced in the production of isoprene by the two-stage dehydrogenation of isopentane, where it is a secondary product of the main synthesis. Isolation of commercial piperylene of high purity will allow the production of a value-added product and expand the sales market. Currently, restrictions on the production of commercial piperylene are associated with the high content of an impurity – cyclopentadiene. Previous studies by the author showed the feasibility of extracting cyclopentadiene at the stage of obtaining the catalyzate of the second stage of isoprene dehydrogenation. Simulation of the technological scheme in the application program showed that, together, the extraction of cyclopentadiene by a chemical method and a change in the technological operating mode makes it possible to isolate piperylene with a content of 99.45 % wt and CPD equal to 0.0046 % wt.

ISOLATION AND PURIFICATION OF ISOPRENE FROM CYCLOPENTADIENE

Isoprene is a basic monomer in the petrochemical industry. It is used in the synthetic rubber production for the manufacture of rubber products for the electrical, aviation and engineering industries, for the manufacture of food and medical products. In the Republic of Bashkortostan, Sintez-Kauchuk OJSC (Sterlitamak) produces isoprene by the two-stage dehydrogenation of isopentane with an annual product output of hundreds of thousands of tons. The disadvantage of this technology for producing isoprene is the significant content of impurities in raw isoprene, which seriously affect the kinetics of polymerization and the quality of the final product. Of all the impurities present in the monomer, cyclopentadiene is the most detrimental to the stereoregular polymerization process. Its content in isoprene should not exceed 0.0001 % wt. Due to the urgency of the problem, a technical solution was developed to remove cyclopentadiene impurities from monomers.Currently, there is a problem of accumulation of cyclopentadiene in the system of purification and treatment of isoprene at the operating production. The paper proposes to purify isoprene from cyclopentadiene before the stage of extractive distillation. This purification method allows to reduce the content of cyclopentadiene in the isoprene-isoamylene fraction from 0.10 % wt. to 0.00014 % wt, which reduces its concentration in all recycle flows of the isoprene isolation and purification system. To implement this solution, laboratory experiments were carried out to purify the isoamylene-isoprene fraction formed at the second stage of isopentane dehydrogenation. In order to assess the energy efficiency of the studied method for reducing cyclopentadiene, a computer model of the isoprene isolation and purification process was built in the Honeywell Unisim Design software environment.

Catalytic effects of magnesium-transition metal (Fe and Ni) hydrides on oxygen and nitrogen reduction: A case study of explosive characteristics and their environmental contaminants

The catalytic effects of magnesium-transition metal (Mg–TM) hydrides on O2 and N2 reduction were examined. The results indicated that magnesium hydrides (MHs) combustion process primarily occurred in three stages: dehydrogenation, active Mg combustion, and residual Mg combustion. After the dehydrogenation stage, the active Mg particles and released H atoms became more likely to be oxidised and nitrified in MgFeH than in MgH2. Moreover, a large quantity of NH3 was observed after the combustion of MgFeH. Fe has a rich electronic structure and an active surface position, which enables it to activate N2 and thus facilitate the catalytic reduction of N2. Due to the lower temperature and pressure requirements of the initial reaction in the MgFeH combustion or explosion process, compared to those of the Haber-Bosch process, the combustion and explosion of MgFeH can result in the release of significant amounts of NH3. However, the addition of Ni can prevent the activation of O2 and N2, thereby inhibiting their reduction. This means that Ni not only inhibits N2 reduction but also enhances the dehydrogenation capacity of MHs. Therefore, to develop and design hydrogen storage materials for MHs, Ni is a safer and more environmentally friendly catalyst than Fe.

Synthesis of Zn2TiO4 via solid-state method as a promising additive for dehydrogenation properties of NaAlH4

Sodium alanate (NaAlH4) has a high capacity for hydrogen, making it one of the most promising materials for storing hydrogen, but its slow dehydrogenation kinetics make it less practical. In this study, to speed up NaAlH4's dehydrogenation rate and improve its dehydrogenation properties, Zn2TiO4 synthesised through the method of solid-state was milled with NaAlH4. Zn2TiO4 additive remarkably reduces the initial temperature for the NaAlH4 to release hydrogen and fastens the dehydrogenation rate of NaAlH4. After adding different weight percentages of Zn2TiO4, the initial temperature for the NaAlH4 to release hydrogen was reduced to approximately 150 °C-165 °C (first dehydrogenation stage) and 190 °C-224 °C (second dehydrogenation stage), which is lower by 40 °C-55 °C than as-milled NaAlH4. The NaAlH4–Zn2TiO4 system also fastened the dehydrogenation rate by 10–15 times more than as-milled NaAlH4. The dehydrogenation activation energies of the NaAlH4 were also downshifted to 86.2 kJ/mol (first dehydrogenation stage) and 92.4 kJ/mol (second dehydrogenation stage) with the addition of Zn2TiO4. The activation energies were reduced by 26% (first dehydrogenation stage) and 24% (second dehydrogenation stage) illustrating the superior effect of Zn2TiO4. X-ray diffraction analysis discovered that TiZn2 and AlTi3 formed during the heating process are responsible for the enhanced performance of NaAlH4.

Liquid Organic Hydrogen Carriers Applied on Methane–Hydrogen-Fueled Internal Combustion Engines: A Preliminary Analysis of Process Heat Balance

One of the main issues that has limited the use of hydrogen as an energy vector for a long time is its low energy density per unit of volume. Alternative chemical storage methods have been developed in recent years to overcome the limitations associated with compressed or liquified hydrogen storage. One of these is the Liquid Organic Hydrogen Carrier (LOHC), which utilizes organic hydrocarbons that can capture hydrogen (through an exothermic hydrogenation reaction) and release hydrogen (through an endothermic dehydrogenation reaction). In this paper, a 0D model of an internal combustion engine fueled with a mixture of hydrogen and methane was used to investigate whether the enthalpy of the exhaust gases can balance the heat rate required to self-sustain the dehydrogenation stage. Two LOHC+ compounds were considered, namely, Perhydro-dibenzyltoluene and Perhydro-N-Ethylcarbazole. Four different hydrogen-to-methane ratios were considered, assuming an engine maximum brake power ranging from 500 to 6000 RPM. An energy balance was performed, balancing the dehydrogenation heat rate and the exhaust gas cooling heat rate, in order to establish the minimum temperatures of the exhaust gases required to self-sustain the LOHC+ dehydrogenation. We demonstrated that the minimum exhaust temperatures required to self-sustain the process in different running regimes and at different hydrogen-to-methane ratios are lower than literature and experimental exhaust temperatures.

Effects of pyrolysis temperature on the release characteristics of polycyclic aromatic hydrocarbons during pyrolysis of corn stover pellet

The release characteristics of polycyclic aromatic hydrocarbons during the pyrolysis of biomass remain poorly understood. This study investigated the effects of pyrolysis temperature on the polycyclic aromatic hydrocarbon release characteristics by analyzing both the transient vapor products and the physicochemical properties of corresponding biochar from the pyrolysis of corn stover pellets. The results revealed that the transient volatile compounds mainly included phenols, ketones, acids, furans, aldehydes, substances containing benzene ring, polycyclic aromatic compounds, and gaseous products. A range of 2 to 4 ring polycyclic aromatic hydrocarbons were generated at 400 to 700 °C with the peak at 560 °C, and the sum of relative content of polycyclic aromatic hydrocarbons ranged from 0.23% to 40.36%. For the biochar, the carbonization stage (400 to 700 °C) of corn stover pellets was further divided into three evolutionary stages, including the preliminary carbonization stage (380 to 480 °C), amorphous carbon structure stage (480 to 600 °C), and the stage of dehydrogenation and growth of aromatic rings (600 to 700 °C).The relationship between polycyclic aromatic hydrocarbon release in volatile compounds and H/C ratio of the biochar could be described by a power function.

In situ observation of the on-surface thermal dehydrogenation of n-octane on Pt(111).

The catalytic dehydrogenation of alkanes constitutes a key step for the industrial conversion of these inert sp3-bonded carbon chains into other valuable unsaturated chemicals. To this end, platinum-based materials are among the most widely used catalysts. In this work, we characterize the thermal dehydrogenation of n-octane (n-C8H18) on Pt(111) under ultra-high vacuum using synchrotron-radiation X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning tunneling microscopy, combined with ab initio calculations. At low activation temperatures, two different dehydrogenation stages are observed. At 330 K, n-C8H18 effectively undergoes a 100% regioselective single C-H bond cleavage at one methyl end. At 600 K, the chemisorbed molecules undergo a double dehydrogenation, yielding double bonds in their carbon skeletons. Diffusion of the dehydrogenated species leads to the formation of carbon molecular clusters, which represents the first step towards poisoning of the catalyst. Our results reveal the chemical mechanisms behind the first stages of alkane dehydrogenation on a platinum model surface at the atomic scale, paving the way for designing more efficient dehydrogenation catalysts.

Operando time-gated Raman spectroscopy of solid catalysts.

Operando Raman spectroscopy is a powerful analytical tool to provide new insights in the working and deactivation principles of solid catalysts. Intense fluorescence can obscure Raman spectra to the extent that they become uninterpretable. Time-gated Raman spectroscopy, based on pulsed excitation and time-gated detection, suppresses background fluorescence based on its slower time dynamics compared to Raman scattering. In this work, we demonstrate and quantify the benefit of time gating for operando Raman spectroscopy, using the propane dehydrogenation reaction over Pt-Sn-based catalyst materials as a case study. Experimental time-gated Raman spectroscopy data are fitted to a time-trace model that is used to optimize time gating for the maximum signal-to-background-noise ratio. Time-gated Raman spectra of a spent propane dehydrogenation catalyst material show lower background fluorescence compared to the time-integrated Raman spectra counterparts. Simultaneous operando time-gated and time-integrated Raman spectroscopy experiments demonstrate the benefit of time gating to obtain more distinct Raman features, especially in the early coking stages where spectra are dominated by background fluorescence.

Optimization of Thermal Conditions of Heat Recovery Boilers with Regenerative Heating in the High-Temperature Section of Isoamylene Dehydrogenation

Improving the efficiency of use of energy resources at large-capacity energy-consuming enterprises in the petrochemical industry in conditions of high internal and external competition is the priority for the development of the fuel and energy industry. This is confirmed by various legislative acts, including the energy strategy of the Russian Federation for the period up to 2035. This research focuses on a high-temperature section of dehydrogenation of isoamylenes into isoprene, the production of which relates (isoprene production relates to large petrochemical enterprises that consume a huge amount of energy resources) to large-capacity energy-consuming industries. To increase the thermodynamic efficiency of the research object, regenerative feedwater heating for heat recovery boilers is proposed due to deeper cooling of fuel and contact gas (the term "contact gas" is used in the technological regulations of an isoprene production company), which are secondary thermal energy resources in this technology. In accordance with the industry’s technology regulations, a block diagram of the initial and improved high-temperature section with the indication of material flows (The term "material flow" refers to the type of substances that are used in the high-temperature dehydrogenation stage of isoamylenes) was developed. The balance equations of the section under consideration are provided, and the thermal efficiency and exergy efficiency for systems utilizing fuel and contact gas are determined. The estimated economic effect was determined in physical terms; it was found to be 2008.58 toe/h. An exergy flow diagram is also provided to show how the system utilizes contact gas.

Effect of K2NbF7 on the hydrogen release behaviour of NaAlH4

We explored the influences of the addition of K2NbF7 by mechanical-milling on the dehydrogenation properties of NaAlH4. The results revealed that the addition of K2NbF7 to NaAlH4 decreases the dehydrogenation temperature and improves the dehydrogenation kinetic. The 10 wt% K2NbF7-doped NaAlH4 starts to desorb hydrogen at ∼100 °C and ∼204 °C for the first and second stages of dehydrogenation, respectively, which were ∼80 °C and ∼36 °C lower than the as-milled NaAlH4. For the dehydrogenation kinetic measurement at 150 °C, the K2NbF7-doped NaAlH4 releases ∼2.0 wt% hydrogen in 25 min, which is 10 times faster than as-milled NaAlH4. From the Kissinger calculation, the activation energies for the dehydrogenation process of K2NbF7-doped NaAlH4 are ∼84.6 kJ mol−1 for the first stage of dehydrogenation and ∼75.1 kJ mol−1 for the second stage of dehydrogenation. These activation energies are lower, about 33.3 and 44.9 kJ mol−1, compared with the undoped NaAlH4. The catalytic role of the active species (NbF4 and Al3Nb) that formed during the heating process and the reduction of particle size due to the milling process are the factors that presumably influence the reduction of the onset decomposition temperature and improvement in the dehydrogenation kinetic of the K2NbF7-doped NaAlH4 composite.

DFT исследование элементарных актов сверхкритического водного окисления на примере каталитической реакции радикальной диссоциации воды с участием модельного кластера Fe4O6

This report details aspects of the reactivity of the products of catalytic dissociation of water as applied to the description of the nature of supercritical water oxidation and other oxidative SCF processes in general. It has been shown quantum chemically that the products of thermodynamically nonequilibrium to one degree or another dissociation of water on the surface of a model Fe4O6 cluster of different multiplet nature can act as effective agents for interaction with atmospheric oxygen and organic compounds. The antiferromagnetic forms of iron oxides can actively participate in radical reactions. It is shown using a model cluster Fe4O6 as an example. The radical reactions are able to easily change their multiplet state by simple chemisorption of 3O2 on their surface. In combination with the ability of water to dissociate on the model Fe4O6 cluster by the ionic type, the latter can easily transform into a triplet system, which, accordingly, leads to the emergence of an equilibrium biradical water dissociation formally even at room temperature. Hydrate-oxygen interactions on the surface of the Fe4O6 cluster lead to the generation of either an extremely reactive iron-hydroperoxide complex or a hydroperoxyl radical capable of conducting oxidative processes under SCF conditions in the volume of the corresponding fluid. It is shown the dissociative chemisorption of a water molecule on a Fe4O6 cluster in a triplet state (initiated, for example, by chemisorption of 3O2), in contrast to the heterolytic dissociation of water (whose thermodynamic equilibrium is shifted towards the molecular form of water), is practically a thermodynamically equilibrium process. It is this fact allowed us to consider the hydride-hydroxyl form of the Fe4O6 triplet cluster as one of the real agents for the oxidation of propylene glycol by the example of its first stage of catalytic dehydrogenation.

Beta Titanium Alloys Produced from Titanium Hydride: Effect of Alloying Elements on Titanium Hydride Decomposition

The use of titanium hydride as a raw material has been an attractive alternative for the production of titanium components produced by powder metallurgy, due to increased densification of Ti compacts, greater control of contamination and cost reduction of the raw materials. However, a significant amount of hydrogen that often remains on the samples could generate degradation of the mechanical properties. Therefore, understanding decomposition mechanisms is essential to promote the components’ long life. Several studies on titanium hydride (TiH2) decomposition have been developed; nevertheless, few studies focus on the effect of the alloying elements on the dehydrogenation process. In this work, the effects of the addition of different amounts of Fe (5 and 7 wt. %) and Nb (12, 25, and 40 wt. %) as alloying elements were evaluated in detail. Results suggest that α→β transformation of Ti occurs below 800 °C; β phase can be observed at lower temperature than the expected according to the phase diagram. It was found that β phase transformation could take place during the intermediate stage of dehydrogenation. A mechanism was proposed for the effect of allying elements on the dehydrogenation process.

Grain refinement mechanism of Ti-55 titanium alloy by hydrogenation and dehydrogenation treatment

The average grain size of Ti-55 titanium alloy is refined by hydrogenation and dehydrogenation treatment from 5.96 μm to 0.5 μm. The refinement mechanism is discussed through microstructural observations. The main refining effects may derive from recrystallization and phase transformation. The micro-strain field in the matrix by the formed hydride during the hydrogenation process results in serious distortion and many dislocations. The recrystallization nucleation and growth during the dehydrogenation process are promoted. Additionally, the eutectoid transformation of βH → δ + α during hydrogenation process and the hydride decomposition of δ → α + H2↑ as well as the hydrogen release in the form of solid solution transformation of αH → α + H2↑ and βH → α + β + H2↑ during dehydrogenation stage break the matrix structure into smaller microstructure.

Software implementation of the dynamic model of the first stage of the reactor unit for styrene production in the MATLAB/SIMULINK environment

A mathematical description of the physicochemical processes occurring in the first stage of ethylbenzene dehydrogenation in a two-stage adiabatic continuous reactor is developed. A software implementation of the resulting model in Matlab/Simulink environment is presented, that allows to simulate the sequence and dynamics of the flow of internal processes during the equipment operation, taking into account the decrease in activity of the catalytic layer. The algorithm of functioning, structure and model of information flows of the created software, implementing a modular approach to the description of a series-parallel system of thermal, chemical processes and processes on the surface of the catalytic layer are given. The created software allows to simulate the operation of the reactor stage when process parameters change.

The Onset of Dehydrogenation in Solid Ammonia Borane: An Ab Initio Metadynamics Study.

The discovery of effective hydrogen storage materials is fundamental for the progress of a clean energy economy. Ammonia borane (H3 BNH3 , AB) has attracted great interest as a promising candidate but the reaction path that leads from its solid phase to hydrogen release is not yet fully understood. To address the need for insights in the atomistic details of such a complex solid state process, in this work we use ab-initio molecular dynamics and metadynamics to study the early stages of AB dehydrogenation. We show that the formation of ammonia diborane (H3 NBH2 (μ-H)BH3 ) leads to the release of NH4 + , which in turn triggers an autocatalytic H2 production cycle. Our calculations provide a model for how complex solid state reactions can be theoretically investigated and rely upon the presence of multiple ammonia borane molecules, as substantiated by standard quantum-mechanical simulations on a cluster.

The Onset of Dehydrogenation in Solid Ammonia Borane: An Ab Initio Metadynamics Study

AbstractThe discovery of effective hydrogen storage materials is fundamental for the progress of a clean energy economy. Ammonia borane (H3BNH3, AB) has attracted great interest as a promising candidate but the reaction path that leads from its solid phase to hydrogen release is not yet fully understood. To address the need for insights in the atomistic details of such a complex solid state process, in this work we use ab‐initio molecular dynamics and metadynamics to study the early stages of AB dehydrogenation. We show that the formation of ammonia diborane (H3NBH2(μ‐H)BH3) leads to the release of NH4+, which in turn triggers an autocatalytic H2 production cycle. Our calculations provide a model for how complex solid state reactions can be theoretically investigated and rely upon the presence of multiple ammonia borane molecules, as substantiated by standard quantum‐mechanical simulations on a cluster.

Modifying the hydrogen storage performances of NaBH4 by catalyzing with MgFe2O4 synthesized via hydrothermal method

In this study, the hydrogenation performance of NaBH4 was modified by the addition of 10 wt% MgFe2O4 as the catalyst. The NaBH4 + 10 wt% of MgFe2O4 sample was prepared by a ball milling technique. The onset decomposition temperature of MgFe2O4-doped NaBH4 was decreased to 323 °C and 483 °C for the first and second stage of dehydrogenation as compared to the milled NaBH4 (497 °C). The desorption kinetics study showed that the addition of MgFe2O4 caused the sample to had faster hydrogen desorption with a capacity of 6.2 wt% within 60 min while the milled NaBH4 had only released 5.3 wt% of hydrogen in the same period of time. For the isothermal absorption kinetics, the total amount of hydrogen absorbed by the milled NaBH4 was 3.7 wt% while the NaBH4 + 10 wt% MgFe2O4 sample showed better absorption characteristic with a total amount of 4.5 wt% of hydrogen within 60 min. The calculated desorption activation energy from the Kissinger plot of NaBH4 + 10 wt% MgFe2O4 sample was 187 kJ/mol which reduced by 28 kJ/mol than the milled NaBH4 (215 kJ/mol). The in-situ formation of MgB6 and Fe3O4 after the dehydrogenation process indicates that these new species were responsible for the improved hydrogenation performances of NaBH4.

Synthesis of a ternary amide Li K (NH2) and a novel Li3K(NH2)4–xMgH2 combination system for hydrogen storage

Abstract The ternary amides LiK2(NH2)3, LiK(NH2)2, and Li3K(NH2)4 are successfully synthesized by ball milling mixtures of LiNH2 and KNH2, and the hydrogen storage properties of Li3K(NH2)4–xMgH2 (x = 1, 2, 3, 4) are systematically investigated. The Li3K(NH2)4–2MgH2 sample displays optimized hydrogen storage properties, releasing 6.37 wt% of hydrogen in a two-stage reaction with an onset temperature of 60 °C. The first dehydrogenation stage exhibits good reaction kinetics and thermodynamic properties because of a lower activation energy and appropriate enthalpy change. After full dehydrogenation at 130 °C, the Li3K(NH2)4–2MgH2 sample absorbs 3.80 wt% of H2 below 160 °C in a variable temperature hydrogenation mode. Mechanistic investigations indicate that Li3K(NH2)4 reacts with MgH2 to produce Mg(NH2)2, LiH, and KH during ball milling. In the heating process, Mg(NH2)2 first reacts with LiH to form Li2Mg2N3H3 and LiNH2, while KH works as a catalyst, and then, KH reacts with Li2Mg2N3H3 and LiNH2 to generate a new K-containing compound.

Enhanced hydrogen storage properties of K2TiF6 doped Mg-Na-Al composite system

The present investigation has introduced the hydrogen storage properties of MgH2+NaAlH4 (4:1) composite system with the addition of 10 wt% K2TiF6 by ball milling method for the first time. It is found that the K2TiF6-doped MgH2+NaAlH4 composite has better hydrogen storage properties compared to that of undoped composite. From the temperature programmed desorption result, the decomposition temperatures for two-step dehydrogenation stages that attributed to the NaAlH4 and MgH2 were reduced by 70 °C and 225 °C respectively, as compared to the undoped composite. The apparent activation energy of the Mg-relevant decomposition for the 4MgH2+NaAlH4+ K2TiF6 composite was reduced to 96.4 kJ/mol as compared to the undoped 4MgH2+NaAlH4 composite (124.9 kJ/mol). The X-ray diffraction analysis displayed formation of new phases; the Al3Ti, the TiH2 and the KF in the doped composite after de/rehydrogenation processes. It is believed that these new phases may responsible as catalysts in the K2TiF6-doped MgH2+NaAlH4 composite.