Coke Precursors Research Articles

Coke formation on the surface of Ni/HZSM-5 and Ni-Cu/HZSM-5 catalysts during bio-oil hydrodeoxygenation

Deactivation of zeolite catalysts due to coke deposition is a pronounced challenge in the hydrodeoxygenation (HDO) of bio-oil. This paper reports on the mechanism of coke formation and catalyst regeneration for two catalysts (Ni/HZSM-5, Ni-Cu/HZSM-5). FT-IR, XPS and Raman spectroscopy characterizations indicate that catalyst deactivation can be divided into three main stages: (1) Lewis acid sites in HZSM-5 support are rapidly covered by oxygenated hydrocarbons. The Brønsted acid sites of HZSM-5 donate protons to oxygenates, leading to the formation of carbocations - precursors of soluble coke on the catalyst surfaces. (2) Soluble coke is then stacked to form disordered filament-like carbon strands that are similar to carbon nanotubes or graphite-like structures. (3) Filament-like carbon evolves into graphite carbon. At the same HDO temperature, more graphite particles are found on the Ni/HZSM-5 catalyst than on the Ni-Cu/HZSM-5 catalyst, indicating higher resistance for the coke formation for Ni-Cu/HZSM-5 catalyst.

Coking characteristics and deactivation mechanism of the HZSM-5 zeolite employed in the upgrading of biomass-derived vapors

Abstract HZSM-5 zeolite was employed in the upgrading of biomass pyrolysis vapors in this study. The coking characteristics were investigated by means of TG, FT-IR, XRD, N2 adsorption–desorption, NH3-TPD, Py-IR, SEM and TEM methods. When the zeolite was used for three times about 120 min, its activity had been lost and the amount of coke was 12.15% composed of 9.90% of I type filamentous coke and 2.25% of II type graphite-like coke. The skeleton structure of zeolite was basically intact, which had not been seriously destroyed by coke. The surface area, pore volume and acidity of zeolite were all deteriorated vary degrees with the usage time. The zeolite granules became larger after deactivated, and the filamentous coke can be observed both on the surface and in the pore. The compositions of the coke precursors were analyzed by FT-IR and GC/MS. The precursors of coke deposited in the pore were mainly aromatic hydrocarbons, while the species of the precursors deposited on the outer surface was more including many long-chain saturated hydrocarbons. The deactivation of HZSM-5 zeolite began from inner, large molecular substances blocked the pores which resulted in the zeolite deactivation eventually.

Different Mechanisms of Coke Precursor Formation in Thermal Conversion and Deep Hydroprocessing of Vacuum Residue

Coke formation during the refining of heavy oils has attracted extensive attention as a result of the effects on the liquid yield, catalyst deactivation, and operating period. Polycyclic aromatic hydrocarbons (PAHs) generally have the strongest tendencies to form coke during the refining processes, which are considered as coke precursors. In this work, a vacuum residue was treated by thermal conversion and deep hydroprocessing. The feedstock and products were characterized by Fourier transform ion cyclotron resonance mass spectrometry. The detailed distributions of aromatic hydrocarbons of the products behaved with clear boundaries, which were described in limit lines. The slopes of the limit lines differed greatly between the two kinds of products, indicating different mechanisms for the growth of PAHs. Thermal conversion and deep hydroprocessing of model compounds were also conducted. Thermal conversion products of phenanthrene and pyrene proved that only condensation reactions occurred at a temperature...

Steam reforming of plastic pyrolysis model hydrocarbons and catalyst deactivation

Catalytic steam reforming of n-hexane, 1-hexene, tetradecane and toluene over a Ni commercial catalyst has been carried out in a fluidized bed reactor at 700°C. These compounds have been selected as model compounds of the volatiles formed in the pyrolysis of waste plastics in order to study in detail the performance of the catalyst in the pyrolysis-reforming of different plastic wastes. High carbon conversions and hydrogen yields are obtained at zero time on stream, with peak values being 96.5% and 82.8%, respectively, when n-hexane is used as model compound. Similar reactivity has been observed for tetradecane and 1-hexene, whereas lower carbon conversion (82%) and hydrogen yields (65%) are obtained for toluene. Concerning catalyst stability, olefinic compounds (1-hexene) and aromatic compounds (toluene) cause faster catalyst deactivation than paraffinic compounds (tetradecane and n-hexane). These disparities are explained by the different nature of the coke deposited and the different potential of the compounds to block Ni active sites, with olefins and aromatics being encapsulating coke precursors (amorphous and structured, respectively) and paraffins being filamentous and inert coke precursors.

Coke suppression of kerosene by wall catalytic steam reforming

A new method is developed to decrease coke formation during the thermal cracking of kerosene with applications in air-breathing jet engines. Endothermic catalytic steam reforming of kerosene is tested to investigate the potential for consuming excess heat and decreasing coke deposition rates. A plate reactor is used to simulate an engine-cooling channel and to compare the coke deposition with and without use of a reforming catalyst. The reactor temperature and the blending proportions of water fed to the reactor are varied across experiments. Spraying catalyst on the channel's inner surface prevents the production of filamentous coke, the production of which is catalyzed by the metal surface. The generation of a certain amount of hydrogen from the catalytic steam reforming of kerosene favors the reverse direction of the Diels–Alder and polymerization reactions, consuming the aromatic coke precursors and decreasing the coke formation rate. The effects of varying water content and reactor temperature on coke formation have been investigated during the thermal cracking of kerosene with catalysis. The coking rate decreases with an increase in water content. However, the suppression of coking become weaker as the fuel temperature is increased.

Promoter effect of Ce and Pr on the catalytic stability of the Ni-Co system for the oxidative steam reforming of ethanol

This article presents a study of the catalytic stability of Ni and Co mixed oxides promoted with 5wt% Ce or Pr in the oxidative steam reforming of ethanol (OSRE). The catalysts were obtained by the calcination of hydrotalcite-type precursors (HT) prepared by the co-precipitation method and the reconstruction of the HT phase in the presence of aqueous (Ce/Pr-EDTA) complexes. The solids were characterized by X ray-diffraction (XRD), surface area (BET), X-ray fluorescence (XRF), temperature-programmed reduction (TPR-H2), and temperature-programmed desorption of CO2 (TPD-CO2) and O2 (TPD-O2), and they were additionally evaluated in the OSRE for 100h. The stability tests indicate that the presence of rare earths promotes a reduction of the precursors of coke. These results evidence the fundamental role of the lattice oxygen and the reducibility of the oxide phases.

Coke Formation in a Zeolite Crystal During the Methanol‐to‐Hydrocarbons Reaction as Studied with Atom Probe Tomography

Understanding the formation of carbon deposits in zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the sub‐nm length scale in a single zeolite ZSM‐5 crystal, which has been partially deactivated by the methanol‐to‐hydrocarbons reaction using 13C‐labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with a median size of 30–60 13C atoms. These clusters correlate with local increases in Brønsted acid site density, demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation.

Coke Formation in a Zeolite Crystal During the Methanol‐to‐Hydrocarbons Reaction as Studied with Atom Probe Tomography

AbstractUnderstanding the formation of carbon deposits in zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the sub‐nm length scale in a single zeolite ZSM‐5 crystal, which has been partially deactivated by the methanol‐to‐hydrocarbons reaction using 13C‐labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with a median size of 30–60 13C atoms. These clusters correlate with local increases in Brønsted acid site density, demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation.

Features of model hydrocarbon fuel oxidation for channel flow in the presence of electrostatic field

A mathematical model of the hydrocarbon fuel channel flow taking into account coke precursor deposit formation on the walls of aeroengine cooling systems and chemical transformations in the gas phase is presented. The model is based on the Navier—Stokes and Maxwell equations in the electric potential form. The description in the form of a radical-chain mechanism is used for the partial liquid-phase oxidation of hydrocarbons. The effect of the electric field intensity (E = 100—2000 V m–1) on the formation of coke precursors at different temperatures of the channel walls T w is shown. The calculations in the range of wall temperatures 350 < T w < 600 K showed that the temperature was the major factor affecting the deposit intensity. The influence of the electrostatic field on the deposit intensity is significantly lower: the difference in the weights of coke deposits for the variants with E = 0 and E = 2000 V m–1 differs by 10% on the average.

Direct synthesis of hierarchical USY zeolite for retardation of catalyst deactivation

Hierarchically structured zeolite Y was hydrothermally synthesized using a water glass containing organosilane surfactant as a mesoscale template. The resulting hierarchical zeolite Y was characterized by a complementary combination of X-ray powder diffraction, Fourier transform infrared spectroscopy, N2 adsorption–desorption isotherms, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, 27Al MAS NMR spectroscopy and density functional theory (DFT) calculation. These results indicated that the hierarchical zeolite Y contained disordered intracrystal mesopores. In addition, quantum chemical calculation results showed that organosilane surfactants could be easily dispersed into the viscous gel used in the synthesis of zeolite Y, inhibiting the phase-separation phenomenon. Hierarchical zeolite Y exhibited remarkably higher resistance to deactivation in the aldol condensation reaction of benzaldehyde with n-butyl alcohol than that of commercial zeolite Y, which is attributed to the highly mesoporous structure that can improve access of reactant molecules to the active sites and minimize the diffusion length of coke precursors out of the zeolite matrix. Such hierarchically porous zeolite Y is very attractive for applications in adsorption and catalysis involving bulky molecules.

Hydrocarbons from Bio-oils: Performance of the Matrix in FCC Catalysts in the Immediate Catalytic Upgrading of Different Raw Bio-oils

Matrices of FCC catalysts with different content of alumina and the compound catalysts prepared with them including Y zeolite were used to upgrade bio-oils from pine wood sawdust and soybean shell. The biomasses were selected according to their different lignin content which results in very different proportions of phenolic compounds (coke precursors) in the respective bio-oils. The bio-oils were produced by fast pyrolysis and the vapours were immediately upgraded over a fixed bed of catalyst at 550 °C using a mass catalyst/bio-oil relationship of 3.5. In terms of hydrocarbon yield, the matrices were more effective in deoxygenating pine wood sawdust bio-oil and the compound catalysts in deoxygenating soybean shell bio-oil. These differences can be the consequence of the different compositions of the bio-oils which, in the case of pine wood sawdust, includes a significant concentration of phenolic ethers which form coke on the matrix and the external surface of the zeolite, thus preventing lighter compounds from accessing the micropore system of the compound catalysts. Pine sawdust bio-oil produced more coke than soybean shell bio-oil; in the case of compound catalysts, coke deposited preferentially on the catalyst matrix, thus decreasing its mesopore specific surface area up to 65 %.

Catalytic conversion of ethanol over ZSM-11 based catalysts

The catalytic performance of ZSM-11 zeolites was studied in ethanol conversion to produce light olefins and BTX. Compared to HZSM-5 with same SiO2/Al2O3 ratio, it was found that HZSM-11 possesses significant advantages, resulting in higher stability and lower selectivity to light paraffins. The superior catalytic performance was attributed to the larger size of the intersections and the milder acid strength. Higher SiO2/Al2O3 ratio of HZSM-11 led to lighter and more olefinic products, with less formation of aromatics. Higher (para+meta)/ortho-xylenes ratios were obtained over HZSM-11 with higher SiO2/Al2O3 ratio, while the ratio of toluene/xylenes decreased. The yield of trimethylbenzenes (TMBs) was slightly enhanced over HZSM-11 catalysts with weaker acidity and well correlated with catalyst deactivation. Weak acidity led to faster deactivation and selectivity decay, and the growth of coke precursors inside the intersections was suggested as the main deactivation mechanism. A significant change of the hydrocarbon distribution can be obtained by modifying HZSM-11 with Fe and Ga oxides. Fe doping resulted in an increased selectivity to light olefins and C4+ hydrocarbons; while Ga modification of HZSM-11 showed slightly higher selectivity toward aromatics. The stability of HZSM-11 can be significantly improved by Ga doping.

Attempts to produce blast furnace coke from Victorian brown coal. 3. Hydrothermally dewatered and acid washed coal as a blast furnace coke precursor

The products obtained by carbonization of a mixture of hydrothermally dewatered acid washed Victorian brown coal and coking coal tar pitch, which had been briquetted at 150 or 230°C and air cured were shown to approach blast furnace (BF) coke in many of their properties. The samples had higher strength than BF coke and showed surface areas and reactivities much closer to those of a typical BF coke than products from non-hydrothermally dewatered Victorian brown coal prepared under similar conditions. The reactivity showed a good correlation with the relative amount of graphitic structure determined by Raman spectroscopy.

Effect of supercritical water on the stability of WOX/TiO2 and NbOX/TiO2 catalysts during glycerol dehydration

The catalytic stabilities of WOX/TiO2 (WTi) and NbOX/TiO2 (NbTi) during glycerol dehydration in 400°C supercritical water at 25MPa and 33MPa were investigated. The effects of coking and active component leaching on catalytic activities are quantitatively discussed. The catalytic activity of NbTi did not decrease significantly with time at both pressures. At 25MPa, the coke precursors on the NbTi surface dissolved in the supercritical water because of the middle-strength acidity of NbTi, thus preventing loss of catalytic activities due to coking. At 33MPa, coking was more suppressed, most likely because the coke precursors were transferred from the catalyst pores more effectively than at 25MPa. Leaching of WOX from WTi occurred at the higher reaction pressure due to high ion products of water, but leaching of Nb from NbTi did not occur because of the high chemical stability of NbOX in acid/base solutions.

Analysis of coke precursor on deactivated catalyst in bio-oil upgrading process

ABSTRACTCharacterization of coke precursor on deactivated catalyst in bio-oil upgrading under CO atmosphere was analyzed with thermogravimetry, X-ray diffraction, infrared analysis, scanning electron microscopy, and nuclear magnetic resonance. The results showed that the deposition of coke precursor on a catalyst surface was mainly low-boiling point of saturated hydrocarbon compounds and the coke precursor of internal channels of deactivated catalyst were major aromatic hydrocarbon compounds. The coke precursor was in an amorphous state, which was highly dispersed on the catalyst surface or into the catalyst micropore. In addition, the catalyst deactivation mechanism was speculated that the small molecule aromatic compounds entered the internal channels of a catalyst and formed macromolecular aromatic hydrocarbons by aromatization gradually, further condensed to form coke and obstructed the channels of the catalyst.

Pentane transformations over sulfated alumina catalyst

The transformation of n-pentane at 295 K in the presence of catalysts of defined acid strength from −9.6 to −15 HO units were studied. The results were compared with those for $${{\text {Al}}_2}{{\text O}_{3}-}{\text{SO}}_{4}^{2 - }$$ (8 %) of acid strength equal to −11.5 < HO ≤ −10.8. It was found that: (a) for superacids of acid strength HO < −13, the reaction begins with the protonation of the C–H σ bond of a pentane and proceeds according to the bimolecular mechanism; (b) for less acidic catalysts such as sulfated alumina, pentane is oxidized to pentene, which is converted to the pentyl cation after protonation. The latter reacts in the presence of catalysts of acid strength of at least HO equal to −9.6 according to two pathways: (a) unimolecular at an excess of H− (in the presence of additional hydride donor pentane co-substrate) with selective formation of 2-methylbutane; (b) bimolecular in the absence of H− (pentane alone is the substrate). In the latter case, the transformation proceeds through the formation of a ten-carbon dimer cation state, which upon decomposition leads to 2-methylpropane, 2-methylbutane and unsaturated coke precursors.

Particular properties of the coke formed on nano-sponge *BEA zeolite during ethanol-to-hydrocarbons transformation

The impact of crystal size and textural properties of *BEA-type zeolites in the conversion of ethanol-to-hydrocarbons at 350°C and under 3.0MPa pressure is investigated. Three *BEA zeolites (molar ratio Si/Al of ca. 23) with important differences concerning their textural properties are synthesized: a material constituted of micron-sized crystals and two hierarchical porous zeolites with a mesoporous network created either by the aggregation of nanometer-sized crystals or by the use of a gemini-type quaternary ammonium surfactant given a nano-sponge zeolite. The micrometric zeolite favors the formation of polyaromatic molecules in same proportion than the concentration of Brønsted acid sites. The deactivation mode of acid sites is poisoning rather than pore blocking. The short thickness of crystal in the nano-sponge zeolite combined with its low acidity, limit the growth of coke molecules as well as their accumulation inside micropores. Coke precursors (alkylbenzenes) are adsorbed on many silanol group located in the ordered mesopores, but the extreme downsizing of the zeolite crystal leads to an unexpected pore blocking by these light molecules.

Numerical study of fluid flows and heat transfer of aviation kerosene with consideration of fuel pyrolysis and surface coking at supercritical pressures

A computational fluid dynamics (CFD) model has been developed and applied for numerical studies of fluid flows and heat transfer of the aviation kerosene RP-3 with consideration of both fuel pyrolysis and surface coking in a circular cooling tube at a supercritical pressure of 5MPa. A detailed pyrolytic chemical reaction mechanism, which contains 18 species and 24 elementary reactions, and a modified kinetic model for considering the surface coking reactions are incorporated in the CFD model. The effects of the surface heat flux and inlet flow velocity on the complex physicochemical process of supercritical-pressure turbulent heat transfer of RP-3 are examined. Results reveal that the endothermic fuel pyrolysis improves the convective heat transfer by two means: providing extra heat absorption through the endothermic chemical reactions and thus decreasing the bulk fluid temperature, and increasing the flow velocity and consequently increasing the convective heat transfer coefficient. The equivalent surface heat flux from the endothermic chemical reactions can contribute to around 70% of the total surface heat flux at the high fluid temperature region. As the aviation kerosene RP-3 is thermally decomposed, the main surface coking precursors, propene and aromatics, are produced. As a result, carbon deposition accumulates on the interior surface of the cooling tube, particularly at a low inlet flow velocity and/or a high surface heat flux.

Effects of Dealumination and Desilication of Beta Zeolite on Catalytic Performance in n-Hexane Cracking

Catalytic cracking of n-hexane to selectively produce propylene on Beta zeolite was carried out. The H-Beta (HB) (Si/Al = 77) zeolite showed higher catalytic stability and propylene selectivity than the Al-rich HB (Si/Al = 12), due to its smaller number of acid sites, especially Lewis acid sites (LAS). However, catalytic stability and propylene selectivity in high n-hexane conversions were still not satisfactory. After dealumination with HNO3 treatment, catalytic stability was improved and propylene selectivity during high n-hexane conversions was increased. On the other hand, catalytic stability was not improved after desilication with NaOH treatment, although mesopores were formed. This may be related to the partially destroyed structure. However, propylene selectivity in high n-hexane conversions was increased after alkali treatment. We successfully found that the catalytic stability was improved and the propylene selectivity in high n-hexane conversions was further increased after the NaOH treatment followed by HNO3 treatment. This is due to the decrease in the number of acid sites and the increase in mesopores which are beneficial to the diffusion of coke precursor.

Aromatic hydrocarbon production by catalytic pyrolysis of palm kernel shell waste using a bifunctional Fe/HBeta catalyst: effect of lignin-derived phenolics on zeolite deactivation

Lignin-derived phenolics are tightly bound with zeolite acid sites, and act as coke precursors. A bifunctional Fe/HBeta catalyst is efficient for upgrading of biomass materials with high lignin content.