Suppression Of Coke Formation Research Articles

Inhibiting the Dealkylation of Basic Arenes during n-Alkane Direct Aromatization Reactions and Understanding the C6 Ring Closure Mechanism

Pt/KL is the widely accepted catalyst for aromatizing n-hexane by 1,6 ring closure but encounters deactivation issues when aromatizing higher carbon-number feeds; undergoing extensive dealkylation to give unwanted CH4 and unselective products, as well as over-aromatization to form coke. Here, we report the use of a non-acidic MFI zeolite support, containing excess K+ beyond ion exchange capacity, well dispersed Pt, and high Pt presence inside the pores, for maximising direct n-alkane aromatization selectivity. TGA, catalyst deactivation studies, and characterizations show that the smaller pore sizes and lack of large cages in the MFI support sterically inhibit coke formation inside the pores (0% compared to 4.9% over Pt/KL for n-octane aromatization), which also reduces dealkylation of ethylbenzene and o-xylene under mild conditions to give a more selective product distribution, 86% selectivity by weight towards C6 ring closure compared to 27% for Pt/KL. Additionally, using NH3-TPD, XPS,CO-DRIFTS, and STEM, we show the contribution of excess K+ as an inhibitor of strong acid sites, an indirect Pt electron promoter through improving metal support interaction, and Pt dispersant. This work highlights the alternative use of well-understood zeolitic supports for the highly selective aromatization of n-heptane and n-octane by 1,6 ring closure, increasing the number of potential streams that can undergo direct aromatization, and providing a suitable alternative to Pt/KL.

Radicals and coking behaviors during thermal cracking of two vacuum resids and their SARA fractions

Inhibition of coke formation in cracking of distillation residues, such as the vacuum residue (VR), has attracted extensive attention for the development of better technologies. However, the radical coking mechanism was seldom studied in the past. This work studies coking and radical concentration during cracking of two VRs and their SARA fractions in sealed micro-reactors at 400, 420, 440 and 460 °C and elaborates the results with respect to the chemical composition of the starting materials and the kinetics of radicals formation. It is found that most of the radicals in VR locates mainly in the asphaltene fraction (Asp). The agglomeration of Asp to coke at low temperatures does not involve significant change in radical concentration. The coke yield includes that formed from the agglomeration of Asp and that from the condensation of radical fragments generated in cracking of all the fractions. The coke itself also undergoes cracking and condensation, resulting in a higher radical concentration and a lower coke yield. The interactions between the Asp and other SARA fractions are relatively large in coke formation by agglomeration, but relatively small in cracking and condensation due to limited amounts of small radicals or hydrogen radicals from the cracking of other fractions. The VR with a higher Asp fraction and richer poly-aromatic structures (QD-VR) is higher in radical generation and lower in activation energy.

The key to catalytic stability on sol–gel derived SnOx/SiO2 catalyst and the comparative study of side reaction with K-PtSn/Al2O3 toward propane dehydrogenation

Side reactions competing with propane dehydrogenation were studied on K-PtSn/Al2O3 and SnOx/SiO2 catalysts. The results showed that hydrogenolysis, hydrogenation, deep dehydrogenation, and coke formation occurred more pronouncedly on K-PtSn/Al2O3, contrasting with the catalytic cracking result. Because of the higher total acidity on SnOx/SiO2, measured by NH3-TPD, more catalytic cracking occurred on this catalyst. However, SnOx/SiO2 showed a lower coke deposition, although the acid site can promote the coke formation. This contrast was not due to the spillover effect, which can suppress coke formation on the acid site. Essentially, the propylene hydrogenation, H2-TPD, and FTIR results demonstrate that SnOx/SiO2 had a lower hydrogen spillover than that of K-PtSn/Al2O3. On the other hand, TPO results from propylene dehydrogenation and coke formation of propadiene illustrate that the key of distinguished SnOx/SiO2 catalyst stability was owing to a low ability of coke precursor generation.

Effects of CaO addition on Ni/CeO2–ZrO2–Al2O3 coated monolith catalysts for steam reforming of N-decane

A series of 10 wt%Ni/CeO2–ZrO2–Al2O3 (10%Ni/CZA) coated monolith catalysts modified by CaO with the addition amount of 1 wt%~7 wt% are prepared by incipient-wetness co-impregnation method. Effects of CaO promoter on the catalytic activity and anti-coking ability of 10%Ni/CZA for steam reforming of n-decane are investigated. The catalysts are characterized by N2 adsorption-desorption, XRD, SEM-EDS, TEM, NH3-TPD, XPS, H2-TPR and Raman. The results show that specific surface area and pore volume of as-prepared catalysts decrease to some extent with the increasing addition of CaO. However, the proper amounts of CaO (≤3 wt%) significantly enhance the catalytic activity in terms of n-decane conversion and H2 selectivity mainly due to the improved dispersion of NiO particles (precursor of Ni particles). As for anti-coking performance, reducibility of CeO2 in composite oxide support CZA is promoted by CaO resulting in providing more lattice oxygen, which favors suppressing coke formation. Moreover, the addition of CaO reduces the acidity of 10%Ni/CZA, especially the medium and strong acidity. But far more importantly, a better dispersion of NiO particles obtained by proper amounts of CaO addition is dominant for the lower carbon formation, as well as the higher catalytic activity. For the spent catalysts, amorphous carbon is the main type of coke over 10%Ni–3%CaO/CZA, while abundant filamentous carbon is found over the others.

Synergistic effects of CO2 on ex situ catalytic pyrolysis of lignocellulosic biomass over a Ni/SiO2 catalyst

This work laid emphasis on catalytic pyrolysis of Styrax japonicus to construct a biomass-to-energy conversion platform. CO2 was particularly chosen to enhance the catalytic pyrolysis of the biomass waste, in reference to N2-purged pyrolysis. The specific roles of CO2 on pyrolysis of S. japonicus in line with synergistic effect of Ni-based catalysts was investigated. It was shown that CO2 played as an oxidant for CO formation via homogeneous gas phase reaction of CO2 with volatile organic carbons (VOCs) liberated from the thermolysis of S. japonicus. Ni/SiO2 catalyst expedited dehydrogenation (DH) of the VOCs, resulting in significant H2 generation and alteration of aromaticity in VOCs. Each CO2 and Ni catalyst independently enhanced CO and H2 formation, respectively, and their synergistic effects were shown at ≥ 600 °C. Coke deposition on the Ni/SiO2 catalyst was confirmed after catalytic pyrolysis, while the suppression of coke formation on the catalyst surface was achieved under the CO2 environment.

Adsorption Site Regulation to Guide Atomic Design of Ni–Ga Catalysts for Acetylene Semi‐Hydrogenation

AbstractAtomic regulation of metal catalysts has emerged as an intriguing yet challenging strategy to boost product selectivity. Here, we report a density functional theory‐guided atomic design strategy for the fabrication of a NiGa intermetallic catalyst with completely isolated Ni sites to optimize acetylene semi‐hydrogenation processes. Such Ni sites show not only preferential acetylene π‐adsorption, but also enhanced ethylene desorption. The characteristics of the Ni sites are confirmed by multiple characterization techniques, including aberration‐corrected high‐resolution scanning transmission electron microscopy and X‐ray absorption spectrometry measurements. The superior performance is also confirmed experimentally against a Ni5Ga3 intermetallic catalyst with partially isolated Ni sites and against a Ni catalyst with multi‐atomic ensemble Ni sites. Accordingly, the NiGa intermetallic catalyst with the completely isolated Ni sites shows significantly enhanced selectivity to ethylene and suppressed coke formation.

Adsorption Site Regulation to Guide Atomic Design of Ni-Ga Catalysts for Acetylene Semi-Hydrogenation.

Atomic regulation of metal catalysts has emerged as an intriguing yet challenging strategy to boost product selectivity. Here, we report a density functional theory-guided atomic design strategy for the fabrication of a NiGa intermetallic catalyst with completely isolated Ni sites to optimize acetylene semi-hydrogenation processes. Such Ni sites show not only preferential acetylene π-adsorption, but also enhanced ethylene desorption. The characteristics of the Ni sites are confirmed by multiple characterization techniques, including aberration-corrected high-resolution scanning transmission electron microscopy and X-ray absorption spectrometry measurements. The superior performance is also confirmed experimentally against a Ni5 Ga3 intermetallic catalyst with partially isolated Ni sites and against a Ni catalyst with multi-atomic ensemble Ni sites. Accordingly, the NiGa intermetallic catalyst with the completely isolated Ni sites shows significantly enhanced selectivity to ethylene and suppressed coke formation.

Chemicals from heavy oils by ZSM‐5 catalysis in supercritical water: Model compound and reaction engineering

AbstractDodecane cracking and aromatization over ZSM‐5 was studied in the presence and absence of supercritical water (SCW). A group‐type model was used to determine five best‐fit rate constants to describe yields to aliphatics, aromatics, coke, and gases. SCW accelerated gas formation while suppressing coke formation. CO and CO2 were formed in the presence of SCW, but not in its absence; a new, low‐temperature coke gasification pathway was suggested to account for this observation. Similarly, a low‐temperature alkane reforming pathway was hypothesized to explain the increased relative rate constant for production of gases in the presence of SCW compared with its absence. Additional tests and analysis indicated that these effects could not be ascribed solely to zeolite degradation in the presence of SCW, implying that water directly influences the reaction mechanism. These results provide new insights into the role(s) of water during oil cracking under supercritical conditions.

Synthesis of hierarchical GaZSM-5 zeolites by a post-treatment method and their catalytic conversion of methanol to olefins

Abstract A number of hierarchical GaZSM-5 zeolite molecular sieves with diverse amounts of mesoporosity and acidity were prepared by post-synthetic desilication with different concentrations of alkali. The zeolite structure and acidity have an important effect on the catalyst lifetime and the product distribution of the methanol to olefins (MTO) reaction, which have been investigated. The changes in pores and acidity by post-treatment were characterized by XRD, SEM, TEM, NH3-TPD, FTIR, Py-IR, etc. An alkali treatment of an appropriate concentration can increase the mesoporosity and reduce the strong acid density, which improves catalytic stability by suppressing coke formation. However, excessive alkali concentrations cause low crystallinity and excessive acidity despite a high mesoporosity, which is conducive to the formation of aromatic products and coking. The desilication significantly changes acidity of GaZSM-5 zeolites and affects the product selectivity; the selectivities of C2 and C3 olefin are consistent with the change of Lewis acid amount.

Tetralin and Decalin H-Donor Effect on Catalytic Upgrading of Heavy Oil Inductively Heated with Steel Balls

The Toe-to-Heel Air Injection (THAI) combined with catalytic upgrading process in situ (CAPRI) has demonstrated it can simultaneously extract and upgrade heavy oil in situ. This paper reports the investigation of augmenting temperature deficit and suppressing coke formation in the CAPRI section through the incorporation of induction heating and H-donor solvents. An induction-heated catalytic reactor was designed and developed, heated with steel balls in a mixed bed of NiMo/Al2O3 catalyst (66% v/v) to 425 °C temperature, 15 bar pressure and 0.75 h−1 LHSV (Liquid Hourly Space Velocity). The catalyst surface area, pore volume and pore size distribution were determined by using nitrogen adsorption–desorption, while the location of coke deposits within the microstructure of the pelleted spent catalyst was analyzed with X-ray nano-Computed Tomography (X-ray nano-CT). Findings showed that induction heating improved the catalyst performance, resulting in a 2.2° American Petroleum Institute (API) gravity increase of the upgraded oil over that achieved with the conventional heating method. The increment in API gravity and viscosity reduction in the upgraded oils with nitrogen gas only, N2 and H-donor solvents, and hydrogen gas environments can be summarized as follows: decalin > H2 gas >= tetralin > N2 gas. Meanwhile, the improvement in naphtha fraction, middle distillate fractions and suppression of coke formation are as follows: decalin > H2 gas > tetralin > N2 gas. The X-ray nano-CT of the spent catalyst revealed that the pellet suffers deactivation due to coke deposit at the external surface and pore-mouth blockage, signifying underutilization of the catalyst interior surface area.

Fabrication of zeolite coatings onto stainless-steel capillary tubes for supercritical hydrocarbon cracking

In tubular reactors, the pressure drop can be decreased, and the heat transfer can be enhanced by coating the catalyst on its inner-wall when compared with the performance of fixed-bed types. This study focused on preparing catalyst coatings in capillary tubes for hydrocarbon cracking. The effect of pretreatment processes on the physicochemical properties of the catalyst was studied, and the influence was negligible. In addition, the preparation process was optimized by adjusting the coating method, binder types and contents. The coating prepared using 10 wt% organic binder with an interval coating method showed optimal adhesion. The mass loss was 3.92% after thermal shock and ultrasonic oscillation treatment. During n-dodecane cracking, the gas production rate (GPR) without coating was 8.27%, while that were 31.62% and 65.42% with ZSM-5 and MFI coatings, respectively. Above all, the coatings inhibited coke formation so that the reaction could be continuously performed at 600 °C.

Disentangling Reaction Processes of Zeolites within Single‐Oriented Channels

AbstractEstablishing structure–reactivity relationships for specific channel orientations of zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, a well‐defined model system was developed to build structure–reactivity relationships for specific zeolite‐channel orientations during various catalytic reaction processes, for example, the methanol‐ and ethanol‐to‐hydrocarbons (MTH and ETH) process as well as oligomerization reactions. The entrapped and effluent hydrocarbons from single‐oriented zeolite ZSM‐5 channels during the MTH process were monitored by using operando UV/Vis diffuse reflectance spectroscopy (DRS) and on‐line mass spectrometry (MS), respectively. The results reveal that the straight channels favor the formation of internal coke, promoting the aromatic cycle. Furthermore, the sinusoidal channels produce aromatics, (e.g., toluene) that further grow into larger polyaromatics (e.g., graphitic coke) leading to deactivation of the zeolites. This underscores the importance of careful engineering of materials to suppress coke formation and tune product distribution by rational control of the location of zeolite acid sites and crystallographic orientations.

Disentangling Reaction Processes of Zeolites within Single-Oriented Channels.

Establishing structure–reactivity relationships for specific channel orientations of zeolites is vital to developing new, superior materials for various applications, including oil and gas conversion processes. Herein, a well‐defined model system was developed to build structure–reactivity relationships for specific zeolite‐channel orientations during various catalytic reaction processes, for example, the methanol‐ and ethanol‐to‐hydrocarbons (MTH and ETH) process as well as oligomerization reactions. The entrapped and effluent hydrocarbons from single‐oriented zeolite ZSM‐5 channels during the MTH process were monitored by using operando UV/Vis diffuse reflectance spectroscopy (DRS) and on‐line mass spectrometry (MS), respectively. The results reveal that the straight channels favor the formation of internal coke, promoting the aromatic cycle. Furthermore, the sinusoidal channels produce aromatics, (e.g., toluene) that further grow into larger polyaromatics (e.g., graphitic coke) leading to deactivation of the zeolites. This underscores the importance of careful engineering of materials to suppress coke formation and tune product distribution by rational control of the location of zeolite acid sites and crystallographic orientations.

Эффективность использования гетерополисоединений типа (NH4)2[Co(H2O)4]2[Mo8O27]∙6H2O в качестве катализаторов для получения этилена

Carrying out heterogeneous acid catalysis with the use of heteropoly compounds has received considerable attention due to the great economic and environmental benefits. In spite of this, its industrial application is limited as there are difficulties in catalyst regeneration (settling) caused by its relatively low thermal stability. The aim of present work was to search and select catalysts related to the class of heteropoly compounds for propane cracking, to test the selectivity of the prosses as well as to discuss possible approaches for solving the problem of catalyst deactivation, that can contribute to achieve stable characteristics of solid heteropoly catalysts. Among these approaches are: the development of new catalysts with high thermal stability, the modification of catalysts to promote coke combustion, the inhibition of coke formation on heteropoly compound catalysts during the process, carrying out the reactions in supercritical media and also the cascade reactions using a multifunctional heteropoly catalyst. The obtained catalyst was also studied by physicochemical methods to get deep knowledge about which features of these compounds influence on the catalytic activity. A highly active and selective catalyst for ammonium octomolybdenocobaltate(II) ammonium (NH4)2[Co(H2O)4]2[Mo8O27]∙6H2O was synthesized for cracking associated petroleum gases. The qualitative, quantitative, and structural composition as well as the specific surface area of the obtained catalyst was established by the methods of X-ray diffraction, X-ray phase and fluorescence analysis. It was revealed that ammonium octomolybdenocobaltate(II) crystallizes in a triclinic syngony with cell parameters: а = 8.6292(9) Å b = 9.4795(10) Å c = 12.2071(13) Å α = 104.326(2)° β = 109.910(2)° γ = 100.820(2)°.

Inductive Heating Assisted-Catalytic Dehydrogenation of Tetralin as a Hydrogen Source for Downhole Catalytic Upgrading of Heavy Oil

The Toe-to-Heel Air Injection (THAI) combined with a catalytic add-on (CAPRI, CATalytic upgrading PRocess In-situ) have been a subject of investigation since 2002. The major challenges have been catalyst deactivation due to coke deposition and low temperatures (~ 300 °C) of the mobilised hot oil flowing over the catalyst packing around the horizontal well. Tetralin has been used to suppress coke formation and also improve upgraded oil quality due to its hydrogen-donor capability. Herein, inductive heating (IH) incorporated to the horizontal production well is investigated as one means to resolve the temperature shortfall. The effect of reaction temperature on tetralin dehydrogenation and hydrogen evolution over NiMo/Al2O3 catalyst at 250–350 °C, catalyst-to-steel ball ratio (70% v/v), 18 bar and 0.75 h−1 was investigated. As temperature increased from 250 to 350 °C, tetralin conversion increased from 40 to 88% while liberated hydrogen increased from 0.36 to 0.88 mol based on 0.61 mol of tetralin used. The evolved hydrogen in situ hydrogenated unreacted tetralin to trans and cis-decalins with the selectivity of cis-decalin slightly more at 250 °C while at 300–350 °C trans-decalin showed superior selectivity. With IH the catalyst bed temperature was closer to the desired temperature (300 °C) with a mean of 299.2 °C while conventional heating is 294.3 °C. This thermal advantage and the nonthermal effect from electromagnetic field under IH improved catalytic activity and reaction rate, though coke formation increased.

Analysis of coke formed during zeolite-catalyzed supercritical dodecane cracking: Effect of supercritical water

Coking behavior of ZSM-5 was studied in the absence and presence of supercritical water (SCW) using dodecane cracking as a model reaction. SCW suppresses coke formation and inhibits conversion of coke precursors into polycyclic aromatic hydrocarbons. Interestingly, the thermal and structural characteristics of coke formed in the presence and absence of SCW appear to follow similar reaction trajectories, with the primary difference being rate inhibition in the presence of SCW. Further experiments indicated that SCW must suppress coke by an indirect role of catalyst modification and by a direct role involving chemical or physical participation in the coke formation mechanism. These results provide new insight into the role of SCW on coke formation and motivate future work to identify strategies for stabilizing acid sites in the presence of liquid water.

Improving the physicochemical properties of Y zeolite for catalytic cracking of heavy oil via sequential steam-alkali-acid treatments

Abstract In this study, modified Y zeolite with high catalytic activity and long lifetime was successfully prepared using sequential steam-alkali-acid treatments. Different alkali agents (NaOH, Na2CO3 and CaCO3) with various concentrations were used in order to improve the catalytic performance of the synthesized Y zeolite in heavy oil conversion process. The physicochemical properties of the prepared samples were investigated using XRD, FESEM, EDX, solid-state29Si and 27Al NMR, BET, XRF, ICP, FTIR, and NH3-TPD techniques. In addition, the properties of the deposited coke on the used catalysts were studied by FTIR and TGA-DTA techniques. Al extraction via steam treatment created Si-rich regions in the Y zeolite samples. Then, the Si elements were easily extracted from these Si-rich regions by using alkali treatment. In addition, acid washing with weak EDTA solution resulted in removal of extracted elements from the zeolite structure and pore opening after steam and alkali treatments. Combination of these treatments stimulated mesopores formation and improvement in textural properties for the Y zeolite samples. Structural and textural properties of Y zeolite were found to be preserved after sequential treatments. Catalytic performance of the prepared catalysts was investigated for the heavy oil catalytic cracking process in a fixed bed reactor. Regardless of the type of alkali agent used for sequential treatments, the heavy oil conversion and gasoline selectivity were strongly improved for all the modified catalysts in reference to the parent catalyst. The catalytic activity tests showed that sample treated with 1 M CaCO3 solution could secure a high heavy oil conversion (70%) and high selectivity towards gasoline and kerosene (45%). Furthermore, modification with CaCO3 inhibits coke formation significantly and enhanced the catalytic lifetime.

Steam reforming of methanol, ethanol and glycerol over nickel-based catalysts-A review

Depletion of non-renewable energy sources such as coal and natural gas is paving the way to generate alternative energy sources. Hydrogen, a very promising alternative energy has the highest energy density (143 MJ/kg) compared to any known fuel and it has zero air pollution due to the formation of water as the only by-product after combustion. Currently, 95% of hydrogen is produced from non-renewable sources. Hydrogen production from renewable sources is considered a promising route for development of sustainable energy production. Steam reforming of renewable sources such as methanol, ethanol and glycerol is a promising route to hydrogen production. This review covers steam reforming of these three alcohols using Ni-based catalysts with different supports. Chemistry of the steam reforming reactions is discussed. Hydrogen yield depends on operating conditions, the nature of active metal and the catalyst support. Supports play an important role in terms of hydrogen selectivity and catalyst stability because of their basic characteristics and redox properties. Synthesis of suitable catalysts that can suppress coke formation during reforming is suggested.

Optimization of an Axial Catalyst Profile in Methane Dry Reformer: Suppression of Coke Formation

Methane dry reforming is a promising greenhouse gas reduction technology that can obtain CO and H2, which are high value-added materials utilizing CO2 and CH4. However, there is a significant coking problem during the operation of the dry reforming reactor. Because methane dry reforming is a strong endothermic reaction, the temperature of the reactor drops near the reactor inlet and causes coke formation. To solve this problem, it is important to ensure that the reaction takes place in a temperature range where coke production is minimized. In this study, we proposed a design method that can maintain the reaction temperature in the region where the coke is rarely generated using new configurations of packed catalysts. It was experimentally confirmed that a new catalyst configuration method could be used to obtain the desired temperature profile in the reactor to inhibit coke formation. The design method also optimizes the reactor by solving the optimization problem, which minimizes the reactor length for ...

Evaluation of a Ti-Base Alloy as Steam Cracking Reactor Material.

Low-coking reactor material technologies are key for improving the performance and sustainability of steam crackers. In an attempt to appraise the coking performance of an alternative Ti–base alloy during ethane steam cracking, an experimental study was performed in a jet stirred reactor under industrially relevant conditions using thermogravimetry (Tgasphase = 1173 K, Ptot = 0.1 MPa, XC2H6 = 70%, and dilution δ = 0.33 kgH2O/kgHC). Initially, a typical pretreatment used for Fe–Ni–Cr alloys was utilized and compared with a pretreatment at increased temperature, aiming at better surface oxidation and thus suppressing coke formation. The results revealed a decrease in coking rates upon high temperature pretreatment of the Ti–base alloy, however, its coking performance was significantly worse compared to the typically used Fe–Ni–Cr alloys, and carbon oxides formation increased by a factor of 30 or more. Moreover, the analyzed coupons showed crack propagation after coking/decoking and cooling down to ambient temperature. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy indicated that the prompt and unsystematic oxidation of the surface and bulk caused observable crack initiation and propagation due to alloy brittleness. Hence, the tested Ti–base alloy cannot be considered an industrially noteworthy steam cracking reactor alloy.