Dehydrogenation Of Cyclohexane Research Articles

Cyclohexane dehydrogenation: Critical evaluation of parameter estimation procedures for kinetic modeling

In the present paper a comparison between different parameter estimation procedures commonly used for the kinetic modeling of chemical reaction is performed, based on experimental measurements of the cyclohexane dehydrogenation to benzene. The obtained results show that, when the Arrhenius equation parameters are estimated from estimates of the rate constant taken at different temperatures, larger parameter uncertainties and correlations are obtained, particularly when the variances of the experimental measurements are not considered during the estimation process. It is also observed that an apparent kinetic compensation effect occurs when the experimental data are separated according to the inlet partial pressure and catalyst mass in the reactor, mainly due to the existing and unavoidable experimental uncertainties and parameter correlations. Additionally, it is shown that larger uncertainties and correlations are obtained when the parameter estimates are computed through the differential method, which can also lead to poorer model predictions of the experimental data. Finally, it is shown that the simultaneous one-step estimation of all model parameters through the integral method and considering the available experimental uncertainties can provide the most accurate parameter estimates, making use of mathematical expressions that describe how variances of the experimental measurements depend on the experimental conditions.

Stable Pt/MgAl2O4 catalysts for efficient production of H2 from cyclohexane dehydrogenation

Cyclohexane as liquid organic hydrogen carriers has attracted considerable interest in the field of hydrogen storage technology. However, the innovation of efficient catalyst for producing H2 through cyclohexane dehydrogenation remains a substantial challenge. In this study, we have successfully developed Pt/MgAl2O4 catalysts exhibiting outstanding catalytic performance in cyclohexane dehydrogenation. Notably, the optimal catalyst Pt/MgAl2O4-700 achieves cyclohexane conversion of 89.1 % and benzene selectivity of over 99 % at 345 °C, resulting in a high hydrogen evolution rate of 933 mmol/gPt/min. Moreover, this catalyst exhibits remarkable long-term stability, with no evident loss of activity in 225-hour dehydrogenation reaction. The catalyst was characterized by XRD, N2 adsorption–desorption, STEM, XPS, NH3-TPD, CO-FTIR and H2-TPR techniques, which provides detailed structure information for elucidation of structure–activity relationship. The spinel structure and moderate acid density of the support enable highly dispersed Pt species at the surface of MgAl2O4. This results in the formation of ultra-small and sintering-resistant Pt particles that exhibit exceptional activity in cyclohexane dehydrogenation. Additionally, the presence of positively charged Ptδ+ species in Pt/MgAl2O4 facilitates the rapid desorption of benzene product. This effectively prevents the formation of coke arising from benzene dehydrogenation, which ensures to achieve high stability. These findings provide valuable knowledge for the rational design of efficient metal-based catalysts for liquid organic hydrogen carriers in hydrogen storage systems.

Liberating C-H Bond Activation: Achieving 56% Quantum Efficiency in Photocatalytic Cyclohexane Dehydrogenation.

The technology of liquid organic hydrogen carriers presents great promise for large-scale hydrogen storage. Nevertheless, the activation of inert C(sp3)-H bonds in hydrocarbon carriers poses formidable challenges, resulting in a sluggish dehydrogenation process and necessitating high operating temperatures. Here, we break the shackles of C-H bond activation under visible light irradiation by fabricating subnanometer Pt clusters on defective Ce-Zr solid solutions. We achieved an unprecedented hydrogen production rate of 2601 mmol gcat.-1 h-1 (turnover frequency >50,000 molH2 molPt-1 h-1) from cyclohexane, surpassing the most advanced thermo- and photocatalysts. By optimizing the temperature-dominated hydrogen transfer process, achievable by harnessing hitherto wasted infrared light in sunlight, an astonishing 56% apparent quantum efficiency and a 5.2% solar-to-hydrogen efficiency are attained at 353 K. Our research stands as one of the most effective photocatalytic processes to date, holding profound practical significance in the utilization of solar energy and the exploitation of alkanes.

Industrial Process and Modern Technical Adaptations for Nylon 6 Monomer Caprolactam: A Mini Review

Caprolactam is in high demand in the new materials industry as a monomer for nylon and polyamides. Although the schemes of traditional processes such as hydroxylamine production (hydroxylamine sulphate oxime process, hydroxylamine phosphate oxime process, nitric oxide reduction process) and cyclohexanone production were still involved in the caprolactam production industry, the modern technical adaptations achieved higher atomic utilisation and higher selectivity. In this review, the basic traditional schemes for the production of caprolactam are for the first time presented. The modern technical adaptation, the rectification dehydrogenation of the by-product cyclohexane in cyclohexanone production, was highlighted to achieve an increase in atomic utilisation from 78 % to 98 %. The higher selectivity achieved with membrane separation resulted in a conversion of cyclohexanone of > 99.6 % and a selectivity of cyclohexanone oxime of > 99.5 %. In addition, the progress of the catalysts used in the modern technical adaptation was briefly discussed. This review highlights the modern process with atom economy and high selectivity.

Low-Temperature Selective Oxidative Dehydrogenation of Cyclohexene by Titania-Supported Nanostructured Pd, Pt, and Pt-Pd Catalytic Films.

Films of titania-supported monometallic Pd, Pt, and bimetallic Pt-Pd catalysts made of metallic nanoparticles were prepared by magnetron sputtering and studied in the oxidative dehydrogenation (ODH) of cyclohexene. Pd/TiOx and Pt-Pd/TiOx were found active at as low temperature as 150 °C and showed high catalytic activity with high conversion (up to 81%) and benzene selectivity exceeding 97% above 200 °C. In turn, the Pt/TiOx catalyst performed poorly with the onset of benzene production at 200 °C only and conversions not exceeding 5%. The activity of bimetallic Pt-Pd catalysts far exceeded all of the other investigated catalysts at temperatures below 250 °C. However, the production of benzene significantly dropped with a further temperature increase due to the enhanced combustion of CO2 at the expense of benzene formation. As in situ NAP-XPS measurement of the Pt-Pd/TiOx catalyst in the reaction conditions of the ODH of cyclohexene revealed Pd surface enrichment during the first temperature ramp, we assume that Pd surface enrichment is responsible for enhanced activity at low temperatures in the bimetallic catalyst. At the same time, the Pt constituent contributes to stronger cyclohexene adsorption and oxygen activation at elevated temperatures, leading to changes in conversion and selectivity with a drop in benzene formation and increased combustion to CO2. Both the monometallic Pd and the Pt-Pd-based catalysts produced a small amount of the second valuable product, cyclohexadiene, and below 250 °C produced only a negligible amount of CO2 (<0.2%). To summarize, Pd- and Pt-Pd-based catalysts were found to be promising candidates for highly selective low-temperature dehydrogenation of cyclic hydrocarbons that showcased reproducibility and stability after the temperature activation. Importantly, these catalysts were fabricated by utilizing proven methods suitable for large-scale production on extended surfaces.

Gas phase oxidative dehydrogenation of cyclohexane over Alumina-Supported Copper-Manganese oxide Catalyst: in-situ DRIFTS studies

Oxidative dehydrogenation (ODH) is a promising technique for converting Cyclohexane (Cy-H) to Cyclohexene (Cy-ene). However, this reaction yields undesired byproducts (Benzene and CO2) due to deep-dehydrogenation and over-oxidation. Therefore, controlling reaction parameters, comprehending intermediate species, and optimizing catalyst design are crucial for enhancing selective Cy-ene formation. Copper-manganese oxide catalysts were synthesized via the incipient wetness impregnation technique. A solid solution (CuMnOδ) exhibited superior catalytic activity due to synergistic interaction and active site formation. The study elucidated that copper oxide facilitates the deep-dehydrogenation of Cy-H. While manganese oxide incorporation inhibits CO2 and benzene formation, promoting partial dehydrogenation to Cy-ene. The studies unveiled Cy-H activation via hydrogen abstraction, forming Cyclohexanolate intermediates interacting with surface hydroxyls, yielding Cy-ene via dehydration. Notably, Cy-one formation via Enolate intermediates was also observed. The catalyst 10Cu0.25Mn0.75Oδ/Al2O3 exhibited the highest Cy-ene yield (∼12.1 %), facilitated by the synergistic effect, with a Cy-H conversion (∼15.7 %) at 300 °C.

Reversible hydrogenation and dehydrogenation of benzene for hydrogen storage on highly dispersed Pd/γ-Al2O3 catalyst

The research and development of efficient catalyst is the key to achieving high-capacity hydrogen storage in liquid organic hydrogen carriers (LOHCs). The highly dispersed Pd/γ-Al2O3 catalysts with few-atom Pd were prepared by impregnation method using HNO3 as promoter. The hydrogen storage capacity of the benzene/cyclohexane hydrogen carriers was further investigated by vapor phase benzene hydrogenation and cyclohexane dehydrogenation reactions over the studied Pd/γ-Al2O3 catalysts. The results showed that the metal Pd was the active centers for the benzene hydrogenation/cyclohexane dehydrogenation reactions. The addition of HNO3 can effectively promote the metal Pd to be highly dispersed, thus improving the Pd atoms utilization and reducing the Pd dosage. Meanwhile, the strongly electronic effects between the highly dispersed Pd species and the Al2O3 support promoted the electron-deficient Pdδ+ sites to be formed, which enhanced the adsorption and activation ability for the reactants molecules. The benzene conversion on the Pd/γ-Al2O3 catalyst with a metallic Pd loading of 1.0 wt% reached 97.51 % at 200 °C. While the cyclohexane conversion reached 90.94 % at 400 °C with the actual hydrogen storage capacity of 6.54 wt%, which provided an effective idea for large-scale storage and transportation of H2 based on LOHCs.

Hydroxyapatite supported molybdenum oxide catalyst for selective dehydrogenation of cyclohexane to cyclohexene: studies of dispersibility and chemical environment

Highly dispersed MoOx/HAP catalyst shows high selectivity towards cyclohexene.

Cyclohexane Oxidative Dehydrogenation on Graphene-Oxide-Supported Cobalt Ferrite Nanohybrids: Effect of Dynamic Nature of Active Sites on Reaction Selectivity.

In this work, we investigated cyclohexane oxidative dehydrogenation (ODH) catalyzed by cobalt ferrite nanoparticles supported on reduced graphene oxide (RGO). We aim to identify the active sites that are specifically responsible for full and partial dehydrogenation using advanced spectroscopic techniques such as X-ray photoelectron emission microscopy (XPEEM) and X-ray photoelectron spectroscopy (XPS) along with kinetic analysis. Spectroscopically, we propose that Fe3+/Td sites could exclusively produce benzene through full cyclohexane dehydrogenation, while kinetic analysis shows that oxygen-derived species (O*) are responsible for partial dehydrogenation to form cyclohexene in a single catalytic sojourn. We unravel the dynamic cooperativity between octahedral and tetrahedral sites and the unique role of the support in masking undesired active (Fe3+/Td) sites. This phenomenon was strategically used to control the abundance of these species on the catalyst surface by varying the particle size and the wt % content of the nanoparticles on the RGO support in order to control the reaction selectivity without compromising reaction rates which are otherwise extremely challenging due to the much favorable thermodynamics for complete dehydrogenation and complete combustion under oxidative conditions.

Defect‐induced Synthesis of Highly Dispersed Hydroxyapatite‐Supported Vanadium Oxide for the Oxidative Dehydrogenation of Cyclohexane

AbstractHydroxyapatite (HAP) contains abundant defect sites and easily releases hydroxyl groups to produce new vacancies under calcination at high temperature. The highly dispersed VOx/HAP catalyst was prepared by an impregnation method using these defects as inducement. VOx species with different structures were analysed by XRD, XPS, H2‐TPR, Raman and UV–vis spectroscopy. At low calcination temperatures (500 °C and 600 °C), the V species are mainly V2O5 crystals. At high calcination temperatures (above 700 °C), VOx on the HAP surface fills these defect sites and strongly interacts with HAP to form Ca−O−V or P−O−V bands. These scattered defects improved the dispersion of V species. These highly dispersed VOx/HAP catalysts were used for oxidative dehydrogenation (ODH) of cyclohexane to cyclohexene. The highly dispersed VOx/HAP catalyst showed a high selectivity for cyclohexene, and the selectivity reached 48.2 % when the conversion of cyclohexane was 13.1 % at 410 °C.

Dehydrogenation of cycloalkanes over Pt/SBA-15 for endothermic cooling

Dehydrogenation of cyclohexane, methylcyclohexane, and decalin was studied on Pt/SBA-15 between 773 and 873 K for application to engine cooling in hypersonic aircraft. While the reactions were unstable at 1 bar, the reactions showed no evidence for deactivation at pressures of 20 and 60 bar for at least 4 h. Selectivities for dehydrogenation of cyclohexane to benzene and methylcyclohexane to toluene were greater than 90% in all cases. The reaction of decalin proceeded sequentially, forming primarily tetralin at low reactor space times and naphthalene at higher space times. The reaction of 50% mixtures of n-dodecane and decalin at 773 K showed negligible conversion of n-dodecane but the reaction of decalin was similar to what was observed for a pure decalin feed. Heat measurements gave endothermicities that were in good agreement with standard reaction enthalpies.

A metal-free carbon catalyst for oxidative dehydrogenation of aryl cyclohexenes to produce biaryl compounds

A metal-free route based on a carbon catalyst to synthesize biphenyls through oxidative dehydrogenation (ODH) of phenyl cyclohexene has been investigated. Among the samples examined, an air-oxidized active carbon exhibits the best activity with a 9.1 × 10-2 h-1 rate constant, yielding 74% biphenyl in 28 h at 140 °C under five bar O2 in anisole. The apparent activation energy is measured as 54.5 kJ⋅mol-1. The extended reaction scope, consisting of 15 differently substituted phenyl cyclohexenes, shows the wide applicability of the proposed method. The catalyst's good recyclability over six runs suggests this ODH method as a promising route to access the biaryl compounds. In addition, the reaction mechanism is investigated with a combination of X-ray photoelectron spectroscopy, functional group blocking, and model compounds of carbon catalysts and is proposed to be based on the redox cycle of the quinoidic groups on the carbon surface. Additional experiments prove that the addition of the catalytic amount of acid (methanesulfonic acid) accelerates the reaction. In addition, Hammett plot examination suggests the formation of a carbonium intermediate, and its possible structure is outlined.

Nitrogen-doped carbon supported PtPd alloy nanoparticles exhibiting high catalytic activity for cyclohexane dehydrogenation under low temperatures

While Pt-based heterogeneous catalysts have been extensively studied for cyclohexane dehydrogenation, the reaction efficiency under low temperatures remains to be resolved. In this paper, ultrafine PtPd alloy nanoparticles homogeneously anchored on nitrogen-doped carbon were synthesized and applied for cyclohexane dehydrogenation. The experimental results indicate that the synergistic effect of Pt-Pd in PtPd/CN catalyst enhances the activity and durability of the catalyst in cyclohexane dehydrogenation compared with Pt/CN and Pd/CN catalysts. The as-prepared catalyst (labeled as Pt0.75Pd0.25/CN) exhibits an excellent catalytic performance with an initial TOF value of 90.07 mmol·g−1metal·min−1 and a 100% H2 purity for cyclohexane dehydrogenation at 180 °C, exceeding reported values under comparable reaction conditions. Catalyst characterizations and DFT calculations reveal that the electronic coupling effect of Pd and Pt in PtPd/CN catalyst makes the d-band center of Pt active sites closer to the Fermi level, which promotes the specific adsorption/activation of cyclohexane on Pt active sites, thus improving the catalytic performance of cyclohexane dehydrogenation. Under this guidance, the synthesized PtRu/CN and PtIr/CN alloy catalysts exhibited similar enhanced activity in cyclohexane dehydrogenation. This work provides insights into the rational design of efficient and stable Pt-based bimetallic catalysts for cyclohexane dehydrogenation.

Comparative study of the oxidative dehydrogenation of cyclohexane over vanadium isomorphic-substituted hydroxyapatite and hydroxyapatite-supported vanadium oxide

Vanadium oxide-modified hydroxyapatite (HAP) with varying vanadium content was synthesized by the coprecipitation and impregnation methods. The structures of the two series of catalysts were compared, and the effects of vanadium content on catalytic properties in cyclohexane oxidative dehydrogenation (ODH) are evaluated. The XRD, UV‒Vis and XPS analyses indicate that the V species in the two series of samples exist in different chemical environments. The samples prepared by the impregnation method are mainly composed of V2O5 species, while the samples prepared by the coprecipitation method show a replacement of a part of the P in the HAP lattice by VO4. The influence of the preparation method and vanadium content on the reaction rate and reaction route for the ODH of cyclohexane is investigated. VHAP shows higher cyclohexene selectivity. The effect of VOx species on the ODH reaction of cyclohexane was investigated and the reaction mechanism was proposed.

Promotional effects of Ag on catalytic combustion of cyclohexane over PdAg/Ti-SBA-15

Recently, PdM bimetallic nanoparticle-supported catalysts have attracted considerable attention as superior catalysts for VOC combustion due to the promotional effect rendered by the added second metal. However, the promotional effect is rarely investigated. In this study, Ti-SBA-15-supported bimetallic PdAg nanoparticle catalysts with different molar ratios were prepared. Compared to Pd/Ti-SBA-15 and Ag/Ti-SBA-15, Pd13Ag1/Ti-SBA-15 with a loading amount of 0.63 wt% exhibited superior activity. Furthermore, Pd13Ag1/Ti-SBA-15 exhibited outstanding stability in the on-stream reaction. The excellent catalytic performance of Pd13Ag1/Ti-SBA-15 was mainly ascribed to the synergistic effect of Pd and Ag in promoting the key steps of cyclohexane combustion. The reaction pathways of cyclohexane over the samples were proposed on the basis of the analysis of the key intermediates generated during catalytic combustion by in situ DRIFTS and PTR-TOF-MS. Higher concentrations of cyclohexene, benzene, and light olefins were generated over Pd13Ag1/Ti-SBA-15 in comparison with those generated over Pd/Ti-SBA-15 at a temperature range of 180–220 °C, demonstrating that the addition of Ag to Pd/Ti-SBA-15 promotes the key steps of the oxidative dehydrogenation of cyclohexane to benzene and catalytic cracking of cyclohexane to olefins due to the increase of Lewis and Bronsted acid sites over Pd13Ag1/Ti-SBA-15. Revealing the promotional effect of bimetallic catalysts on catalytic VOC combustion would be beneficial for the design of bimetallic catalysts with high efficiencies and cost-effectiveness.

Nickel single atoms/cerium oxide hybrid for hydrogen production via solar-heating catalytic dehydrogenation of methyl Cyclohexane

Methylcyclohexane (MCH)-toluene-hydrogen cycle can realize the recycling of matter and energy, owning a wide foreground in industrial application. The MCH dehydrogenation process is highly endothermic, and a heating temperature is needed to stimulate this reaction. The combination of thermal catalytic reactor and solar-to-heat system can realize solar-heating catalytic MCH dehydrogenation reaction. Whereas, such a highly desirable advanced system has not yet been reported. In this study, a hybrid of single-atom nickel anchored on cerium oxide nanosheets (Ni SA/CeO2 NS) has been constructed successfully. The hybrid exhibits remarkable thermal catalytic activity in MCH dehydrogenation, owning an H2 generation rate of 2756 mmol g−1 h−1 at 400 °C. The stepwise MCH dehydrogenation mechanism on the hybrid is investigated via density functional theory calculations. Moreover, we have developed a novel type of solar-to-heat device based on Ti2O3 thin film/Cu. Based on this device and the Ni SA/CeO2 NS hybrid, a solar-heating catalysis mode has been developed, with the H2 generation rates of 404 and 2604 mmol g−1 h−1 from MCH dehydrogenation under 1 and 2 kW m−2, respectively. This strategy provides the possibility to realize scalable MCH dehydrogenation under solar irradiation without secondary energy input.

膜工学研究を振り返って– 膜を創る

Last time, the author described the study of membrane fouling and the transport equation for membrane permeation. This time, the author would like to introduce his research on creating membranes. Polymer membranes and inorganic membranes have been developed. The polymer membranes are negatively and positively charged ultrafiltration membranes, which are polysulfone membranes with sulfonic acid groups and quaternary ammonium groups. The charged membranes have a complex separation mechanism, that is combination of separation by membrane pores and by charge repulsion between the membrane and solute. It was possible to separate not only salts but also various substances such as amino acids, peptides, and proteins. The transport equations of charged membranes were also studied. Inorganic membranes are mainly silica membranes produced by chemical vapor deposition, and zeolite membranes have also been produced, but they will not be introduced here. The author started with low–temperature deposition using ozone, and finally deposited membranes at high temperature (600 ℃) using various silica sources, and was able to obtain high–performance membranes. Since the advantages of inorganic membranes can be applied to membrane reactors, membrane reactors have been applied to steam reforming of methane, dehydrogenation of cyclohexane and methylcyclohexane, and thermal decomposition of hydrogen sulfide. In particular, the cyclohexane dehydrogenation membrane reactor operated continuously for 1,054 hours, and a small membrane reactor equipped with six 50 cm membranes was also developed.

Surface chemistry and photochemistry of cyclohexane on rutile TiO2(110)

Cyclohexane is a high-valued chemical rece1vmg significant interest in liquid hydrogen storage technology. TiO2-based catalysts show high performance in the photocatalytic dehydrogenation of cyclohexane under mild conditions, but the detailed reaction mechanism is not well understood. With the surface science approaches, we have studied the adsorption and surface chemistry of cyclohexane on rutile TiO2(110). The thermal desorption spectroscopy and X-ray photoelectron spectroscopy results both demonstrate the molecular adsorption of cyclohexane on rutile TiO2(110). Upon the UV Hg light irradiation, photodesorption of cyclohexane occurs from both the chemisorbed monolayer and the multilayer. No decomposition nor dehydrogenation of cyclohexane occurs on rutile TiO2(110). These results deepen the fundamental understanding of the surface chemistry of cyclohexane on the TiO2 surface.

Oxidative dehydrogenation of cyclohexene on atomically precise subnanometer Cu4-nPdn (0 ≤ n ≤ 4) tetramer clusters: the effect of cluster composition and support on performance.

The pronounced effects of the composition of four-atom monometallic Cu and Pd and bimetallic CuPd clusters and the support on the catalytic activity and selectivity in the oxidative dehydrogenation of cyclohexene are reported. The ultra-nanocrystalline diamond supported clusters are highly active and dominantly produce benzene; some of the mixed clusters also produce cyclohexadiene, which are all clusters with a much suppressed combustion channel. The also highly active TiO2-supported tetramers solely produce benzene, without any combustion to CO2. The selectivity of the zirconia-supported mixed CuPd clusters and the monometallic Cu cluster is entirely different; though they are less active in comparison to clusters with other supports, these clusters produce significant fractions of cyclohexadiene, with their selectivity towards cyclohexadiene gradually increasing with the increasing number of copper atoms in the cluster, reaching about 50% for Cu3Pd1. The zirconia-supported copper tetramer stands out from among all the other tetramers in this reaction, with a selectivity towards cyclohexadiene of 70%, which far exceeds those of all the other cluster-support combinations. The findings from this study indicate a positive effect of copper on the stability of the mixed tetramers and potential new ways of fine-tuning catalyst performance by controlling the composition of the active site and via cluster-support interactions in complex oxidative reactions under the suppression of the undesired combustion of the feed.

Catalytic Dehydrogenation of Liquid Organic Hydrogen Carrier Model Compounds by CpM+ (M = Fe, Co, Ni) in the Gas Phase

Although there is much interest in developing liquid organic hydrogen carriers (LOHCs) and new metal catalysts to release hydrogen on demand for application in energy generation, few studies have provided mechanistic insights into the crucial metal-catalyzed dehydrogenation reactions. Here we use multistage mass spectrometry experiments and DFT calculations to examine the dehydrogenation of the LOHC model compounds cyclohexane, pyrrolidine, N-methylpyrrolidine, and piperidine by the half-sandwich cyclopentadienyl cations, [CpM]+ (M = Fe, Co, and Ni). The [CpM]+ ions were generated via collision-induced dissociation (CID) of precursor ions generated via electrospray ionization (ESI) of solutions of the compounds CpFe(CO)2I, Cp2Ni, and Cp2Co. The [CpFe]+ ion was found to catalyze triple dehydrogenation of cyclohexane to benzene via a combination of ion–molecule reactions (IMR) and CID experiments. Reactions of [CpM]+ (M = Co, Ni) were found to be more complex as dehydrogenation was accompanied by significant cyclohexane ring cleavage. Reactions of [CpFe]+ with the N-heterocyclic model compounds showed catalytic double dehydrogenation of pyrrolidine/N-methylpyrrolidine and triple dehydrogenation of piperidine. However, multiple side reactions were found, especially in the case of N-methylpyrrolidine and piperidine. They included proton transfer/hydride abstraction to/from the LOHC model methyl loss from the N-methylpyrrolidine complex, and NH2/NH3 extrusion from the piperidine complex. DFT calculations shed light on the structure of the key intermediates in all these systems.