Coke Precursors Research Articles

Revealing the Janus Character of the Coke Precursor in the Propane Direct Dehydrogenation on Pt Catalysts from a kMC Simulation

As the commercial catalyst in the propane direct dehydrogenation (PDH) reaction, one of the biggest challenges of Pt catalysts is coke formation, which severely reduces activity and stability. In this work, a first-principles DFT-based kinetic Monte Carlo simulation (kMC) is performed to understand the origin of coke formation, and an effective method is proposed to curb coke. The conventional DFT calculations give a complete description of the reaction pathway of dehydrogenation to propylene, deep dehydrogenation, and C–C bond cracking. The rate-limiting step is identified as the dissociative adsorption of propane. Moreover, a comparison between different exchange-correlation functionals indicates the importance of van der Waals corrections for the adsorption of propane and propylene. The lateral interactions between the surface adsorbates are significant, which implies that mean field microkinetic modeling might not adequately describe the reaction process. There are two distinct stages in PDH, which ar...

Selective ring opening of decalin on Rh-Pd/SiO2-Al2O3 bifunctional systems: Catalytic performance and deactivation

Ring opening of decalin was studied using Rh-Pd(x) catalysts supported on SiO2-Al2O3, prepared by coimpregnation with a variable total metallic content (x = 1, 1.5 and 2 wt%) while maintaining constant the Rh/Pd atomic ratio (equal to 1). The catalytic performances of these bimetallic systems were compared to those of monometallic Pd and Rh (1 wt%) catalysts. Bimetallic Rh-Pd(x) catalysts were unable to achieve decalin conversion and ring opening selectivity as high as those obtained using the Rh catalyst (~71 and ~67%, respectively, after 6 h reaction time). The principal cause was the reduction of the hydrogenolytic activity and acidity caused by the combination of Pd and Rh. In the case of the bimetallic Rh-Pd(x) samples, an increase in the metal content favored the formation of dehydrogenated products, reduced cracking (by reducing the concentration of strong acid sites) and increased coke deposition during decalin ring opening. Conversely, the Rh catalyst showed less deactivation since its efficient hydrogenolytic character eliminates coke precursors.

Roles of Al2O3 promoter for an enhanced structural stability of ordered-mesoporous Co3O4 catalyst during CO hydrogenation to hydrocarbons

A crucial contribution of Al2O3 pillar for an enhanced structural stability of a highly ordered mesoporous Co3O4 (Al/m-Co) for CO hydrogenation activity was verified in terms of newly formed stable phases with their interactions. At an optimal 5 wt%Al2O3 on the Al/m-Co, the activity and stability of Fischer-Trospch synthesis (FTS) reaction were largely enhanced by preserving the highly ordered original mesoporous Co3O4 structures due to the newly formed strong Al2O3-Co3O4 interactions with the partial formation of the catalytically inactive spinel-type CoAl2O4 phases in the matrices of the mesoporous Al/m-Co. The main roles of the Al2O3 pillar on the Al/m-Co were a structural promoter to maintain the highly ordered mesoporous Co3O4 structures by easily removing the heavy hydrocarbons formed during FTS reaction without significant coke depositions through those large mesopores. The homogeneously distributed Al2O3 pillaring material less than monolayer on the Al/m-Co surfaces played a crucial role to preserve the original ordered mesoporous Co3O4 structures through the partial formation of outer surface CoAl2O4 phases with less blockages of active metallic cobalt sites by heavy coke precursors formed.

Enhanced ethylene selectivity and stability of Mo/ZSM5 upon modification with phosphorus in ethane dehydrogenation

Nonoxidative conversion of ethane to ethylene and/or BTX (benzene, toluene, and xylene) suffers rapid deactivation due to coke deposition. We report here the effects of phosphorus modification on the stability and activity of Mo/ZSM5 for nonoxidative conversion of ethane. The results show that the ethylene and BTX yield and stability are significantly enhanced upon modification with 2.5 wt.% P. NH3 TPD, pyridine FTIR, 1H MAS NMR, 27Al MAS NMR, 31P MAS NMR, 129Xe NMR, XPS, UV–visible diffuse reflectance spectra (UV–vis DRS), and nitrogen physisorption were carried out to understand the effects of P on the structure of Mo/ZSM5 and its correlation with catalytic performance. The presence of P reduces the acid strength and density, changes the channel system of ZSM5 by forming thermally stable SAPO-like interfaces with the framework Al, and improves the dispersion of molybdenum. Rapid deactivation still occurs on Mo/ZSM5 with 1 wt.% P due to the existence of denser silanol groups, more isolated Mo species, and reduced aperture size with little change in effective micropore volume. A higher P loading (2.5 wt.%) leads to less dense silanol groups and less reduced but stable molybdenum species, and simultaneously reduces channel diameter and internal volume. Consequently, the ethylene selectivity is enhanced and the formation of coke precursors is restricted, resulting in improved stability.

PAH/Aromatic Tar and Coke Precursor Formation in the Early Stages of Triglyceride (Triolein) Pyrolysis.

There has been a limited understanding of high MW polycyclic aromatic hydrocarbon (PAH) product chemistry in the pyrolysis of triglycerides (TGs), though the subject has important implications for both fuel production from TGs and food science. Previous TG pyrolysis studies have been able to identify only relatively low MW GC-elutable aromatics occurring in the bulk liquid phase; products occurring in the solid phase have remained inaccessible to chemical analysis. In contrast, cold gas expansion molecular beam methods, where pyrolysis products are analyzed in real time as they are entrained in gas expansions, remove product collection difficulties, thereby allowing for analysis of coke/tar PAH precursors. In this study, the model TG triolein was heated and the ensuing products in the molecular beam were soft photoionized, enabling time-of-flight mass detection. Use of 266 nm pulses enabled selective photoionization of aromatic products. Unlike previous work on analysis of the liquid phase TG cracking products, a different and distinct pattern of rather large PAHs, up to 444 amu, was observed, at nontrivial relative product fractions. With an increase of temperature to ∼350 °C, a small number of PAHs with MW ≥ 276 amu increasingly dominated the aromatic product distribution. Surprisingly, PAH product detection ensued at rather low temperatures, as low as ∼260 °C. For tentative PAH product identification and product chemistry rationalization, we observed the product homology pattern and applied a stoichiometric analysis. The latter, combined with the known homology profiles of TG cracking products, indicated specific patterns of intermediate fragment association that facilitated large-MW PAH formation as a result of TG cracking.

2D COS analysis of m-xylene transformation over medium-pore zeolites

m-Xylene isomerisation, as well as secondary disproportionation reactions, were examined over the 10-ring zeolites with different pore topology. As recognised in Py-sorption IR studies, all the 3D zeolites tested, i.e. IM-5, TNU-9, and ZSM-5 delivered the same protonic functionality, both in the manner of the amount and strength of the Si(OH)Al groups. Only zeolite TNU-10 provided lower number of Brønsted acid sites due to the limited accessibility of the sites hidden in the 8-ring system. Two-dimensional correlation spectroscopy (2D COS) analysis was applied to investigate the dataset obtained from IR spectroscopic studies. Both diffusion characteristics, as well as the different size of internal cavities, were found as crucial parameters determining the catalytic effectiveness. The presence of intermediate species, whose the origin is related to the distortion of the substrate molecule geometry during the adsorption process was recognised for all the studied structures. Such species are believed to be represented by the 1485 cm−1 band and assigned to the intermediate arenium ions, e.g. methylbenzenium ions, CH3–C6H5+–CH3. Both m-xylene and intermediate were consumed for coke precursors in the most spacious zeolite TNU-9. The production of p-xylene was detected only in the first period of the reaction, then coke formation was proved to be the privileged process. The simultaneous loss of m-xylene and methylbenzenium ions was also detected for the zeolite IM-5. Both species underwent isomerisation to p-xylene. No formation of coke species was observed for less spacious structures, i.e. ZSM-5 and TNU-10. The generation of isomerization products was found to be the most rapid in TNU-10. The formation of the intermediate species correlates with other products in the first minutes of the reaction (1–6 min), then the consumption to other reaction products distort the correlation.

Catalytic Decomposition of Pyrolysis Fuel Oil over in Situ Carbon-Coated Ferrierite Zeolite for Selective Hydrogen Production

Catalytic decomposition of pyrolysis fuel oils (PFO) for selective production of hydrogen without any significant formation of greenhouse gas (CO2 and CH4) was investigated using ferrierite (FER) zeolites with different Si/Al molar ratios. The hydrogen production rate based on the feed moles of PFO was maximized on the FER having a Si/Al molar ratio of 10.4, and the hydrogen production rate on the FER zeolites was well correlated with their amounts of strong acid sites which easily form the active coke intermediates. In situ generated crystalline coke precursors on the acidic FER(10) surfaces having larger amounts of defect sites further played an important role as catalytic active sites for PFO decomposition and reforming reaction of CH4 generated as a main byproduct. The crystalline phases of the encapsulated graphitic carbon layers formed on the outer surfaces of the FER zeolites were strongly affected by their original acidic strengths, which simultaneously altered a steady-state hydrogen production r...

Catalytic aromatization of ethylene in syngas from biomass to enhance economic sustainability of gas production

Ethylene aromatization is a novel approach that utilizes gas produced from biomass or waste gasification to facilitate the removal of unsaturated hydrocarbons (precursors of coke in downstream synthesis catalysts) while simultaneously producing a valuable aromatic fraction. The recovery of valuable compounds (ethylene and benzene, toluene, and xylene (BTX)) from biomass-derived syngas effectively improves the economic benefits of the gasification process.This study determined the effects of reaction temperature and Ga loading on the conversion and selectivity of bifunctional Ga–zeolite catalysts for the conversion of ethylene to aromatic compounds. The activity and stability of the synthesized materials were analyzed under realistic gasification conditions by using the gas produced from an indirect gasification process. The maximum ethylene conversion (∼97%) was achieved when 0.025 g/g Ga–zeolite catalyst was used. However, 0.005 g/g Ga–zeolite catalyst exhibited the highest carbon selectivity to benzene (∼32%) and the highest total carbon selectivity to aromatics (73%). The temperature markedly influences the distribution of carbon selectivity to aromatics. Lower temperatures favor the production of ethylbenzene and xylenes, whereas benzene, naphthalene, and naphthalene derivatives are yielded at higher temperatures. The results suggest that benzene formation and toluene formation require different active sites. This study proposes a biomass gasification process to obtain methane for use in industrial applications. Substitute natural gas production ensures economic sustainability by generating in situ high-value-added by-products (aromatics).

Tuning the dimensionality of layered Srn+1Tin−xNixO3n+1 perovskite structures for improved activity in syngas generation

Interest in perovskite type oxides is growing due to their versatile catalytic applications. A series of Ruddlesden-Popper (RP) type layered perovskite oxides Srn+1Tin−xNixO3n+1 were prepared and evaluated for their catalytic activity in steam reforming, CO2 reforming and bi-reforming of methane. These materials, prepared through citrate gel method were characterized to understand their structure and Ni reducibility. Substitution of Ni in RP phases was established through refinement of XRD powder pattern. During methane reforming, the catalytic activity increased with the order of RP phase of SrTi1−xNixO3−δ (n = ∞). TPR results show variation in Ni reducibility with the order of RP phase, while TPD-O2 study helped to estimate oxygen vacancies. These vacancies seem to influence catalytic activity during methane reforming. Transient pulse experiments show that CO2 dissociates over oxygen vacancies to give CO and oxygen, with later replenishing lattice oxygen in SrTi0.8Ni0.2O3−δ (n = ∞) phases. Highly labile oxygen vacancies generated in the bulk of SrTi0.8Ni0.2O3−δ must be migrating to the surface, helping in the removal of coke formed. Characterization of catalysts after reaction helped in better understanding of coke precursors.

Upgrading of petroleum vacuum residue using a hydrogen-donor solvent with acid-treated carbon

Hydrocarbon/carbon systems can be used to perform the hydrogen transfer reaction in heavy oil upgrading, as alternatives to metal catalysts and molecular hydrogen. In this study, a system comprising a hydrogen-donor solvent (tetralin) and acid-treated activated carbon was established to evaluate its ability as a hydrogen transfer agent to upgrade a vacuum residue (VR). The use of tetralin substantially decreased coke formation from 32.7 wt% to a negligible amount in VR upgrading at 450 °C because the solvent could act as a hydrogen donor and diluent for the coke precursors. The acid-treated activated carbon accelerated dehydrogenation of tetralin through hydrogen transfer in the upgrading, resulting in a higher residue conversion and moderate coke formation. In view of the phase behavior, the hydrogen transfer reaction could also be promoted by increasing the contact between the hydrogen acceptor and donor in the supercritical medium. In the VR upgrading using the activated carbon in supercritical tetralin, complete residue conversion was achieved with 47 wt% light fractions (gas and light oil) and 6 wt% coke at 450 °C and 5.33 MPa. The results indicate that the streams available in oil refinery processes, which are rich in hydrocarbons with hydrogen-donor abilities, have great potential for use in heavy oil upgrading.

Diffusion of aromatic hydrocarbons in hierarchical mesoporous H-ZSM-5 zeolite

Hierarchical mesoporous zeolites exhibit higher catalytic activities and longer lifetime compared to the traditional microporous zeolites due to improved diffusivity of substrate molecules and their enhanced access to the zeolite active sites. Understanding diffusion of biomass pyrolysis vapors and their upgraded products in such materials is fundamentally important during catalytic fast pyrolysis (CFP) of lignocellulosic biomass, since diffusion makes major contribution to determine shape selectivity and product distribution. However, diffusivities of biomass relevant species in hierarchical mesoporous zeolites are poorly characterized, primarily due to the limitations of the available experimental technology. In this work, molecular dynamics (MD) simulations are utilized to investigate the diffusivities of several selected coke precursor molecules, benzene, naphthalene, and anthracene, in hierarchical mesoporous H-ZSM-5 zeolite. The effects of temperature and size of mesopores on the diffusivity of the chosen model compounds are examined. The simulation results demonstrate that diffusion within the microspores as well as on the external surface of mesoporous H-ZSM-5 dominates only at low temperature. At pyrolysis relevant temperatures, mass transfer is essentially conducted via diffusion along the mesopores. Additionally, the results illustrate the heuristic diffusion model, such as the extensively used Knudsen diffusion, overestimates the diffusion of bulky molecules in the mesopores, thus making MD simulation a powerful and compulsory approach to explore diffusion in zeolites.

Catalytic thermal cracking of Athabasca VR in a closed reactor system

Catalytic thermal cracking of the residue is less common, due to high susceptibility of catalysts to deactivation. In this work, cracking of Athabasca vacuum residue in an autoclave at 400 °C was investigated in presence of in situ prepared and commercial alumina nanoparticles as well as drill cuttings. A high liquid yield of ∼90 wt% was obtained. The nanoparticles and drill cuttings (at 30 wt%) contributed to relatively high coke and gas yields. Nevertheless, the catalytic runs had higher quality maltene product. Reducing drill cuttings concentration to 10 wt% gave the highest liquid yield with high quality maltene. Coke inhibition in presence of optimum drill cuttings concentration is attributed to a balance between the rate of coke precursor formation and adsorption onto the cuttings. Subsequently, at a higher temperature, 420 °C, the higher rate of precursor formation led to similar results between the 10 wt% drill cuttings and the control sample. Both toluene insolubles and produced asphaltenes displayed a lower H/C ratio than asphaltenes in the feed, in line with the thermal cracking activity. Thermogravimetry profiles for the produced asphaltenes was attained at higher temperatures than the toluene insolubles, and was initial-mass sensitive. Accordingly, thermogravimetry profiles should not be indefinitely used to identify the different oil fractions.

Spatiotemporal coke formation over zeolite ZSM-5 during the methanol-to-olefins process as studied with operando UV-vis spectroscopy: a comparison between H-ZSM-5 and Mg-ZSM-5

In this work, during the methanol-to-olefins (MTO) reaction, the formation of hydrocarbon pool species as well as the accumulation of coke and coke precursor molecules were monitored with operando UV-vis spectroscopy.

Energy Valorization by Continuous Pyrolysis of Straight Vegetable Oils (SVOs)

Researches have been undertaken to find a form of valorization of the surplus production of vegetable oils in C&#244te d’Ivoire for their use as a substitute diesel. The first tests of the use of crude oils-diesel blends by the company Palmindustrie faced enormous difficulties. We have therefore undertaken a campaign of pyrolysis of Tropical Straight Vegetable Oils: palm, copra, peanut, cotton, cabbage palm and shea, between 400°C and 600°C under atmospheric pressure. A silica support was used in co-catalysis either with water or with methylcyclohexane, which is a model compound of cetanes contained in gas oil. This compound has the advantage, unlike the gas oil itself, of not masking the peaks of the pyrolysis recombinates of oils in the chromatograms. The condensed organic phase consists mainly of hydrocarbons including paraffins, olefins, alkylbenzenes and styrenes. The yields of liquid hydrocarbons vary between 72% and 86%. A comparative study of coke precursors and gas production was carried out. A discussion on the parameters to be considered for a large-scale implementation was undertaken.

Synergistic effects of Nb2O5 promoter on Ru/Al2O3 for an aqueous-phase hydrodeoxygenation of glycerol to hydrocarbons

Synergistic effects of niobium oxide (Nb2O5) on the Ru/Al2O3, where the alumina had boehmite phases prepared by sol-gel method with subsequent co-impregnation of bimetallic Ru-Nb, were investigated for an aqueous-phase hydrodeoxygenation (APH) as well as aqueous-phase reforming (APR) reaction of glycerol to form gaseous hydrocarbons and hydrogen with useful liquid-phase glycols such as ethylene and propylene glycols. The promoting effects of the Nb2O5 on the prototype Ru/Al2O3 were mainly attributed to the stabilized boehmite structures with the help of an incorporation of niobium oxides, which largely increased the catalytic activity and stability through less aggregations of smaller Ru nanoparticles. At an optimal Nb/Ru molar ratio of ∼1, the much smaller Ru nanoparticles were highly dispersed on the Nb-incorporated alumina matrices on the hexa-coordinated Al3+ sites selectively even after hydrothermal aqueous-phase reactions. The highly dispersed Ru nanoparticles on the Lewis acid sites of the boehmite phase of Al2O3 surfaces showed a facile reducibility and larger metallic Ru surface area with less formations of inactive hard coke precursors. It was mainly originated from the preferential presence of the reduced metallic Ru nanoparticles on the surfaces of the acidic Nb2O5-modified boehmite, which can strongly anchor the active Ru metals with their smaller sizes.

Influence of the conditions for reforming HDPE pyrolysis volatiles on the catalyst deactivation by coke

Pyrolysis of high density polyethylene (HDPE) has been carried out in a conical spouted bed reactor (500°C) and the volatiles have been reformed in-line over a Ni commercial catalyst in a fluidized bed reactor. The evolution of reaction indices (conversion, H2 yield and other gaseous product yields) with time on stream has been studied under the following operating conditions: temperature, 600–700°C; space-time, 8.3–20.8gcatmingHDPE−1, and steam/plastic ratio, 3–5. High initial conversion (>94.5%) and H2 yields (>76.7%) are attained under all the operating conditions studied, with HDPE conversion and H2 yield increasing when the three variables are increased, which is explained by the enhancement of the reforming reaction. However, a significant effect of operating conditions on catalyst stability has been observed. Thus, an increase in temperature, space-time and steam/plastic ratio decreases catalyst deactivation as a consequence of a lower rate of coke deposition on the catalyst due to both a lower C5+ coke precursor amount in the reaction medium and a higher gasification rate of the coke deposited.

Kinetic Modeling of the Catalytic Steam Reforming of High-Density Polyethylene Pyrolysis Volatiles

The kinetics of the steam reforming of HDPE pyrolysis volatiles is studied on a Ni commercial catalyst in a fluidized bed reactor in-line with the pyrolysis reactor (a conical spouted bed reactor at 500 oC). Steam reforming reactions have been carried out under the following conditions: 600-700 oC and space time 0-16.7 gcat min gHDPE-1. Based on the composition of HDPE pyrolysis volatiles, a kinetic scheme is assumed with four reactions (C5+ hydrocarbon reforming, C2-C4 hydrocarbon reforming, CH4 reforming and WGS reaction). Moreover, the kinetics of the deactivation has been quantified with an expression dependant on C5+ hydrocarbon concentration (coke precursors). Calculation of the kinetic parameters is conducted by nonlinear multiple regression, fitting the experimental data to those calculated by the mass conservation equations for each component in the reaction medium. The overall kinetic model proposed describes accurately the evolution of the main products (H2, CO2, CO) with time on stream in the ...

Active and durable alkaline earth metal substituted perovskite catalysts for dry reforming of methane

Dry reforming of methane is an important process for the utilization of CO2 and to get valuable synthesis gas. Alkaline earth metal substituted MZr1-xNixO3-δ perovskites were synthesized by citrate gel method, characterized and evaluated for dry reforming methane. Characterization results show that the type of alkaline earth substituted at the A site of the perovskite oxide plays an important role in terms of structure, basicity, oxygen deficiency and Ni dispersion. Calcium substituted CaZr0.8Ni0.2O3-δ catalyst shows superior activity in terms of high CH4 and CO2 conversion, while maintaining the activity even after 500h of reaction. Mechanistic investigations were carried out using transient pulse experiments and insitu FTIR-diffuse reflectance spectroscopy. These experiments reveal that redox property and basicity play important role in activation and sustaining the reforming reaction. Insitu FTIR measurements show that surface hydroxyl groups of the support are vital for high activity and durability of CaZr0.8Ni0.2O3-δ catalyst. XRD and TGA analysis of catalysts after reaction show the structures are retained, but peaks pertaining to coke were observed on SrZr0.8Ni0.2O3-δ and BaZr0.8Ni0.2O3-δ catalysts. On the otherhand, CaZr0.8Ni0.2O3-δ catalyst had only amorphous carbon even after 500h of reaction. HRTEM studies revealed that SrZr0.8Ni0.2O3-δ and BaZr0.8Ni0.2O3-δ catalysts deactivated mostly due to the formation of carbon nanotubes with Ni embedded in them. Raman and XPS analysis helped in identifying types of coke precursors present on the catalysts. The investigation also illustrate that type of carbon formed depends on the basicity of perovskite oxide, metal to support interaction, Ni crystallite size, surface hydroxyl groups and oxygen defects. This study clearly demonstrated that CaZr0.8Ni0.2O3-δ is an excellent catalyst for dry reforming reaction with long life.

The Positive Role of Hydrogen on the Dehydrogenation of Propane on Pt(111)

Propane dehydrogenation on a Pt-based catalyst can be accelerated by cofeeding hydrogen. An extensive reaction network for propane dehydrogenation on Pt(111), including side reactions and deep dehydrogenation reactions, is proposed to explain the effect of cofeeding hydrogen. Simulations at 873 K and 1 bar total pressure reproduce the experimental trends at increasing H2/C3H8 inlet ratios and allow exploration of the origin of the positive effect of cofeeding hydrogen. Increasing hydrogen pressure leads to a lower coverage of deeply dehydrogenated coke precursors on the surface: in particular, CCH3 (ethylidyne) and CH (methylidyne). In addition, it increases hydrogen coverage, which decreases the propylene adsorption strength while the energy barriers for the further dehydrogenation of propylene increase. The combined effect of a decreasing coke precursor coverage, facilitated propylene desorption, and increasing deep dehydrogenation barriers explains the higher catalytic activity when hydrogen is cofed.

Conversion of Light Alkane to Value-Added Chemicals over ZSM-5/Metal Promoted Catalysts

Gallium–platinum promoted HZSM-5 is found to be a promising catalyst for ethane aromatization reaction. The influence of Pt as a promoter on the activity of Ga/HZSM-5 catalyst for ethane aromatization has been investigated. Comparative study was performed between bimetallic Ga–Pt based and Mo based HZSM-5, where the GaPt/HZSM-5 showed better aromatic and hydrogen selectivity. Pt promoted Ga/HZSM-5 catalyst exhibited higher activity compared to pure Ga/HZSM-5 catalyst. The presence of platinum in the gallium zeolite considerably accelerated dehydrogenation step in ethane aromatization. In addition, GaPt/HSZM-5 deactivated significantly slower than Mo/HZSM-5 and Ga/HZSM-5. TPO study of spent catalysts revealed that carbonaceous deposit on GaPt/HZSM-5 catalyst was burnt off at lower temperature compared to pure Ga/HZSM-5 catalyst, indicating the presence of Pt facilitated hydrogen spillover resulting in hydrogenolysis of coke precursors. The reaction mechanism associated with aromatic formation is postulated...