N-hexane Cracking Research Articles

Enhancing the selectivity of light olefins through synergistic VOx/ZSM-5 composite in the oxidative cracking of hexane

Oxidative-aided cracking of n-hexane is studied in a gas-phase oxygen-free environment over VOx/ZSM-5. The catalysts were prepared by impregnation of VOx on ZSM-5. Characterization of the prepared samples using XRD, SEM, and BET showed smooth, fine, highly crystalline, and highly dispersed VOx on ZSM-5. Raman spectroscopy revealed the presence of isolated moieties on the 1.5 and 2.5 % vanadia loading while a small number of polyvanadate species are detected on the 5 % VOx sample. TPR confirmed the ZSM-5 support contains highly reducible vanadia species. The amount of VOx loading also affected the activity and product distribution. The olefin and COx selectivities are affected by reaction time/temperature and amount/availability of lattice oxygen. VOx/ZSM-5(1.5) showed 99.9 % n-hexane conversion and 91.39 olefin selectivity at 575 °C and 25 s time-on-stream. This superior performance is attributed to the intermittent release of oxygen, optimum acidity, microporosity, and mesoporosity.

Optimization of ethylene and propylene yields from n-hexane catalytic cracking over H-ZSM-5 catalyst using model based design of experiments (DOE)

Catalytic cracking of n-paraffins of varied carbon chain length (C6-C11) to represent light naphtha and gasoline was studied using extrudates of H-ZSM-5 (zeolite: binder = 70: 30) as a catalyst in a fixed bed reactor. Optimization of ethylene and propylene yields using model-based design of experiments (DOE) was carried out for n-hexane cracking by varying the temperature (550–650℃), weight hourly space velocity (3.3–9.9 h−1) and carrier gas (N2) flowrates (3–10 L/h). These variables showed significant effect on the conversion of n-hexane and yield of ethylene and propylene under studied reaction conditions. The Box-Behnken design coupled with response surface methodology was used to determine the effect of reaction temperature, WHSV and N2 flow rate on the yield of ethylene and propylene. Empirical models were constructed for the response variables namely n-hexane conversion, ethylene (E) yield, propylene (P) yield and total ethylene and propylene (E+P) yield. Main effect plot showed that the average yield of ethylene and propylene increased with increase in reaction temperature and decreased with increase in WHSV. The optimization study of multiple responses showed maximum 94.7% conversion of n-hexane and 46.1 wt% yield of ethylene and propylene at 650℃ catalyst bed temperature, 3.3 h−1 WHSV and 8.3 L/h N2 flow rate. The model predicted ethylene and propylene yields matched closely with experimental data for catalytic cracking of naphtha feed at 550 and 650℃ cracking temperature.

Organosilane-assisted synthesis of EU-1 nanozeolite aggregates with improved activity in catalytic cracking of n-hexane

One-dimensional microporous zeolite, EU-1, exhibits more favorable selectivity of propylene in cracking of petroleum components. However, it faced the problem of low catalytic activity for the poor utilization of active sites. In this work, dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (TPOAC) was used to functionalize protozeolitic units and inhibit the excessive growth of crystals by the long hydrophobic alkyl chain. The introduction of TPOAC decreased the crystal sizes into nanoscale, enhanced the mesoporous volume of EU-1 zeolite, and exposed more external acid sites. Moreover, TPOAC could interact with Al species, and in this case, fewer Al atoms were incorporated into the zeolite framework, reducing the acid strength and amount. Intelligent gravitation analyzer test indicated that the zeolite prepared with the presence of 0.30 g TPOAC exhibited the largest accessible volumes in n-hexane adsorption, and the highest accessibility of acid sites, which synergistically promoted cracking of n-hexane for the production of light olefins.

Metal-assisted low-temperature cracking of n-hexane over Rh-encapsulated ZSM-5 catalysts

Naphtha cracking is the most common process for producing light olefins such as ethylene, propylene, and butene. This process consumes enormous amounts of energy, so decreasing the energy consumption and operating temperature is an urgent issue. To decrease the reaction temperature for naphtha cracking, we focused on the metal-assisted cracking reaction, in which paraffin is first dehydrogenated into the corresponding olefin on a metal catalyst, and the produced olefin is then decomposed into light olefins. To effectively realize metal-assisted cracking, we considered metal-encapsulated zeolite catalysts to be useful. In metal-encapsulated zeolite catalysts, the dehydrogenation reaction proceeds inside the zeolite particles, and the dehydrogenated intermediates can access the solid-acid sites frequently. In this study, Rh nanoparticle encapsulated ZSM-5 catalysts (Rh@ZSM-5) were employed for the metal-assisted cracking of n-hexane. Rh@ZSM-5 exhibited significantly high activity for n-hexane cracking below 450 °C owing to the metal encapsulation structure and close proximity between the metal and solid-acid sites. Furthermore, the effects of reaction conditions, reaction temperature, amounts of metal and solid-acid sites, and contact time on metal-assisted n-hexane cracking over the Rh@ZSM-5 catalysts were investigated. The highest light-olefin yield of 38.6 carbon mol% was achieved by the conversion of n-hexane over Rh@ZSM-5 using 0.3 wt% Rh loading, an Si/Al ratio of 100, temperature of 450 °C, reaction time of 0.5 h, and W/F of 2 h.

Enhancing the selectivity for light olefins through catalytic cracking of n-hexane by phosphorus doping on lanthanum-modified ZSM-5.

Naphtha, as the primary raw material in the production of light olefins, could well accommodate their increasing demand through the energy-efficient process of catalytic cracking with ZSM-5. In the current work, different amounts of lanthanum and phosphorous were loaded on ZSM-5 using the wet impregnation method to tune the acidic properties of ZSM-5 for selective catalytic cracking of n-hexane to produce light olefins. Various characterization techniques such as X-ray diffraction (XRD), Al nuclear magnetic resonance (NMR), temperature-programmed desorption of NH3 (NH3-TPD), Py-Fourier transform infra-red (Py-FTIR), inductively coupled plasma optical emission spectroscopy (ICP-OES), N2 adsorption-desorption, X-ray photoelectron spectra (XPS), and scanning electron microscopy were adopted to investigate the modified zeolites. It was found that adding La to ZSM-5 (0.25wt% to 1wt%) improved the catalytic life and increased the n-hexane conversion (to 99.7%), while the further addition had a negative impact, reducing the conversion rate and deviating the product selectivity towards a substantial, undesired benzene, toluene, and xylene (BTX) fraction (33%). On the other hand, a 64% selectivity for light olefins was achieved on phosphorous-doped ZSM-5 (at a loading amount of 1wt%) while reducing the BTX fraction (2.3%) and converting 69% of the n-hexane. A dual metal-modified ZSM-5 with optimal loading amount, 1P0.25LaZ5 (phosphorus 1wt% and La 0.25wt%), helped boost the light olefin selectivity to 62% in the tuned Lewis acid sites at an n-hexane conversion of about 77% while decreasing the undesired BTX selectivity to 3% by reducing the number of Brønsted sites. Thus, the current study reveals that tuning the acidic sites of ZMS-5 by dual metal augmentation with P.La is an effective way of controlling the amount of undesirable BTX produced at a stable n-hexane conversion rate and substantial olefin selectivity.

Improvement of catalyst activity in cracking of n-hexane via metal (Fe/Ga) impregnation over ZSM-12 catalyst

In this research, zeolite ZSM-12 with Si/Al ratio = 80 was synthesized by hydrothermal method. The synthesized zeolite ZSM-12 was modified using Fe and Ga metals and a combination of these two metals, 1 % iron metals (Z80-Fe) and 1 % gallium metals(Z80-Ga), and a 2 % combination of these two metals (Z80-Fe-Ga). The physicochemical properties of synthesized zeolites were evaluated and compared by XRD, EDX-dot-mapping, NH3-TPD, BET, FT-IR, and TGA analyses. The catalytic assessment of synthesized zeolites in the HTO (n-hexane to olefins) process in a fixed bed reactor under atmospheric pressure and Weight hourly space velocity (WHSV) equal to 4 h−1 at 550 °C was evaluated. Various parameters such as selectivity towards light olefins, propylene to ethylene (P/E ratio), production of light alkanes, and aromatic compounds (BTX) were investigated. The results show that using metals led to the improvement and adjustment of the acid sites of zeolite, and the highest amount of light olefin and the lowest amount of coke were obtained. The result of the n-hexane to olefin process showed that the yield of light olefins was significantly improved in all modified catalysts compared to parent zeolite MTW. Compared to other modified zeolites, Z80-Fe-Ga zeolite has the highest yield of light olefins, equal to 59 %. This zeolite performs better due to the presence of gallium and iron metals and shows the highest propylene selectivity (P/E = 2.4). In addition, according to the results of the TGA analysis, the content of coke on the Z80-Fe-Ga catalyst after the catalytic reaction is much less than that of other catalysts after the catalytic reactor test.

Preparation of hierarchical EU-1 zeolite via NH4F etching and the catalytic activity in cracking of n-hexane

One-dimensional microporous zeolite, EU-1, has tighter restriction for bulky intermediates, exhibiting more favorable selectivity of propylene than ZSM-5 in catalytic cracking. However, the diffusion limitation accelerates coke generation, passivates acid sites and deactivates the zeolite. In this study, a strategy was proposed to etch amorphous species between EU-1 nanocrystals using NH4F solution to improve the diffusion property. It was found that the etching process occurred on the crystal surface and mesopores were introduced. Catalytic performance evaluation in n-hexane cracking demonstrated that the maximum propylene selectivity over parent EU-1 was 33.6 wt% (vs. 5.7 wt% for ZSM-5). The n-hexane conversion was increased by 24 wt% over EU-1 etched in 20 wt% NH4F solution. Moreover, the introduction of mesopores provides accommodation space for coke. This study developed hierarchical EU-1 zeolite with favorable diffusion ability and reported a feasible strategy to improve the catalytic performance of EU-1 for propylene production.

L-glutamic monosodium amino acid-assisted approach to mordenite zeolite synthesis with application in the catalytic cracking of n-hexane

L-glutamic monosodium amino acid-assisted approach to mordenite zeolite synthesis with application in the catalytic cracking of n-hexane

CALCULATION OF THERMODYNAMIC AND KINETIC PARAMETERS OF CATALYTIC CRACKING REACTIONS ON LEWIS AND BRØNSTED ACID SITES

Link for citation: Nazarova G.Y., Ivashkina E.N., Maltsev V.V. Calculation of thermodynamic and kinetic parameters of catalytic cracking reactions on Lewis and Brønsted acid sites. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 7, рр. 214-225. In Rus. The relevance of the research is caused by the emerging necessity of developing a mathematical model to optimize the heterogeneous process of catalytic cracking. This tools should take into account both the chemical transformations of a wide range of hydrocarbon groups (different feedstock types), as well as the stages of adsorption, reactants diffusion, conversion of hydrocarbons on the catalyst surface, acid characteristics and pore size of the catalysts. The study of the hydrocarbon conversion patterns on Lewis or Brønsted acid sites using quantum-chemical modeling methods allow us to quantify the thermodynamic parameters of reactants adsorption, the kinetic parameters of carbocations formation and cracking on acid sites. These results are necessary to develop a mathematical model based on the of heterogeneous catalytic reaction mechanism. The aim of this work is to identify the level of quantum chemical theory and to determine the thermodynamic and kinetic parameters of reactants adsorption, carbenium ions formationand hydrocarbons cracking on Lewis and Brønsted acid sites. Methods: quantum-chemical modeling methods to optimize the molecular geometry of reactants and products of catalytic cracking reactions, calculate vibrational frequencies, thermodynamic parameters of adsorption and catalytic cracking of hydrocarbons and heteroatomic compounds with the participation of Bronsted and Lewis acid sites. Results. The chosen level of quantum-chemical theory allowed obtaining the results that are consistent with the laws of the process and the experimental reactivity of hydrocarbons in cracking reactions on acid catalysts. The thermodynamic parameters of the adsorption of C6–C16 hydrocarbons and thiophenes on Lewis and Brønsted acid sites were identified. We found that during the cracking of n-hexane on the Lewis acid site, the reaction is limited by the carbenium ion formation stage.The activation energy of this stage was 281,3 kJ/mol whereas the value for the cracking stage was 277,2 kJ/mol. Further study shows that the activation energy of carbenium ion formation from izohexane and C8–C10 alkanes with normal structure on the Lewis acid site was 257,6 and 279,2…277,9 kJ/mol. The most energetically favorable is the formation of carbocation from hexene at Brønsted acid sites (76,59 kJ/mol). The results of the work will be used to create a mathematical model of a heterogeneous process based on the Langmuir–Hinshelwood equations.

Organosilane-assisted synthesis of reduced a-axis ZSM-23 with enhanced performances in catalytic cracking of n-hexane

ZSM-23 zeolite with a reduced a-axis is synthesized using trimethoxy[3-(phenylamino)propyl]silane (TPPS) and applied in n-hexane cracking. Once nuclei are formed, TPPS primarily attaches to the ends of rod-like ZSM-23 crystals, effectively hindering elongation growth and significantly promoting the diffusivity of ZSM-23 zeolite. The carbonium pathway is favored over the carbenium pathway in n-hexane cracking when reduced a-axis ZSM-23 is applied, leading to higher ethene yield instead of propene. The fast diffusion also shifts deactivation from an inside-out process to an outside-in one due to the different precursors for carbon deposition. The obtained short a-axis ZSM-23 exhibits excellent catalytic performances in n-hexane cracking, displaying better activity (98.4% vs. 86.4%) and greater stability (2400 min vs. 120 min) than traditional ZSM-23. These findings deepen the understanding of organosilane’s role in unidimensional zeolite synthesis and unveil the structure-performance relationship and deactivation mechanism in n-hexane cracking.

Excellent Activity of Nb in Promoting Performance of ZSM-5–USY for the Catalytic Cracking of n-Hexane to Light Olefins

Excellent Activity of Nb in Promoting Performance of ZSM-5–USY for the Catalytic Cracking of n-Hexane to Light Olefins

Thermodynamic Insight into Thermal Cracking of n-Hexane in the H-FAU Zeolite

To understand catalytic cracking processes of alkanes in the petrochemical industry, the monomolecular cracking mechanism of n-hexane in the H-FAU zeolite has been examined using ab initio molecular dynamics simulations combined with enhanced sampling techniques. It has been found that the adsorption energies of short n-alkanes are linearly correlated with the chain length and contact areas with the zeolite framework, while no such correlation can be applied to the short n-alkenes. High-dimensional free energy landscapes characterizing four cracking modes of n-hexane at three different temperatures have been drawn. The electron transfer in the cracking process was analyzed by quantifying the charge evolution for carbon atoms. The formation of carbonium ions in each cracking mode is found to be the rate-limiting step. At high temperature, free energy differences are nonpositive values, suggesting that the cracking of n-hexane is spontaneous, which is also coincident with the experimental observation. The cracking of the central C–C bond of n-hexane is easier than the cracking of terminal C–C bonds, which is due to the lower free energy barrier. The present work sheds new insights into the cracking process of n-alkanes in the industrially used FAU zeolite catalysts.

Study on the deactivation mechanism of HZSM-5 in the process of catalytic cracking of n-hexane

The coke deactivation over HZSM-5 zeolites with different Si/Al ratios in the n-hexane cracking reaction was studied in 200 h of reaction. In the products, alkanes, C2=, C3=, C4= and aromatic compounds such as benzene, toluene, and xylene (BTX) were formed. The selectivity variations of C3= and C4= were inverse proportional to that of BTX, indicating that C3= and C4= intermediates were the precursors of BTX compounds. The internal coke was preferentially formed inside zeolite micropores in the initial reaction period (0–70 h), which significantly diminished the micropore volume, covered the strong acid sites, and strongly inhibited catalytic activity of the catalysts with lower Si/Al ratio. The external coke was principally formed after 70 h of reaction and had a less influence on the catalytic activity. The internal coke (density 1.03 g·cm−3 and a H/C ratio of 1.16) could be effectively removed by H2 treatment at 850 °C; while the external coke (density 1.46 g·cm−3 and a H/C ratio of 0.28) with graphite-like structure resisted H2 treatment. For the zeolites with low Si/Al molar ratio (25 and 50), the rapid coking stage (0–70 h) occurred at high n-hexane conversion followed by a slow deactivation stage at low conversion (70–200 h); for zeolites with high Si/Al molar ratio (85 and 130), a very slow deactivation stage took place in the reaction period (0–70 h), followed by a faster deactivation stage (70–200 h). This work demonstrated that zeolite deactivation behavior was correlated not only with the strong acid sites and microporosity of catalyst, but also affected by the location, amount, structure, and type of coke. The mechanisms for internal and external coke formation and H2 decoking strategy for the deactivated catalyst regeneration were discussed and proposed.

Seed-assisted synthesis of hierarchical zeolite ZSM-5 in the absence of organic templates

Hierarchical zeolite ZSM-5 was synthesized by template-free seed-assisted technique. The influence of different synthesis conditions (such as temperature (120–200 ⁰С) and time (12–72 h) of the hydrothermal treatment, alkali content, the amount (0–20 wt%) and size (100–1000 nm) of seed crystals) on crystallinity and texture of hierarchical zeolite ZSM-5 was studied. Materials have been characterized by X-ray diffraction, scanning electron microscopy, and nitrogen adsorption measurements. The catalytic activity of synthesized samples was tested in the n-hexane cracking process and compared with the one of commercial ZSM-5.

Effect of Transition Metal Nickel on the Selectivity of Light Olefins in n-Hexane Cracking of Ni/IM-5 Zeolite

The production of light olefins by catalytic cracking is a research hotspot in the petrochemical industry. Herein, nickel was used to modify IM-5 zeolite to improve the performance in catalytic cracking. Properties of the Ni/IM-5 zeolites with different nickel loadings were characterized. It was demonstrated that nickel species were mainly located on the external surface of impregnated IM-5 zeolite, only a few Ni2+ ions were distributed in the ion exchange site as compensation cations. Pyridine infrared results indicated that the introduction of nickel could modulate the acidity of IM-5 zeolite and increase the amount of Lewis acid sites. Compared with IM-5, Ni/IM-5 exhibited higher olefin selectivity, especially ethylene, in n-hexane cracking reactions. It was considered that nickel could provide dehydrogenation active sites and promote the formation of light olefins. Thus, the selectivity of light olefins can be improved by controlling the amount and distribution of nickel in IM-5 zeolite.

Insight into reaction path and mechanism of catalytic cracking of n-hexane in HZSM-5 zeolites

The n-hexane was used as a model compound to study the catalytic cracking behavior of light hydrocarbon in HZSM-5 zeolites, and the law of product selectivity of real acid-catalyzed reaction was investigated by analyzing the product distributions. The results showed that no pyrolysis reaction was found at 300°C. Only the acid catalytic reaction took place by the mechanism of carbocation, whose activity was positively correlated to the amount of Brønsted (B) acid sites. The selectivity of ethane, ethylene and propane was negatively correlated, while that of propylene was positively correlated with the Si/Al ratios and catalyst to oil ratios, suggesting that low acid density might be more favorable for monomolecular cracking reactions. It was worth nothing that the total selectivity of C4 products was much higher than that of C2 products. Combined with the quantum chemistry calculation results, it could be confirmed that the super-stability of C2H+ 5 carbenium ion from the monomolecular cracking of n-hexane made it difficult to produce ethylene and ethane through hydrogen transfer reaction. It’s easier to form a C8 carbenium ion (C8H+ 19) with another n-hexane molecule, and then to generate more C4 products. These results revealed the nature of the low selectivity of ethylene in light hydrocarbon catalytic cracking products. It could be concluded that the product selectivity of catalytic cracking of light hydrocarbons could be modulated by controlling reaction paths depending on the catalyst acid properties and the catalyst to oil ratios. This work will provide important theoretical support for the catalyst design and process development of naphtha catalytic cracking.

Synthesis of Hierarchical MOR-Type Zeolites with Improved Catalytic Properties.

Hierarchical MOR-type zeolites were synthesized in the presence of hexadecyltrimethylammonium bromide (CTAB) as a porogen agent. XRD proved that the concentration of CTAB in the synthesis medium plays an essential role in forming pure hierarchical MOR-type material. Above a CTAB concentration of 0.04 mol·L−1, amorphous materials are observed. These hierarchical mordenite possess a higher porous volume compared to its counterpart conventional micrometer crystals. Nitrogen sorption showed the presence of mesoporosity for all mordenite samples synthesized in the presence of CTAB. The creation of mesopores due to the presence of CTAB in the synthesis medium does not occur at the expense of zeolite micropores. In addition, mesoporous volume and BET surface seem to increase upon the increase of CTAB concentration in the synthesis medium. The Si/Al ratio of the zeolite framework can be increased from 5.5 to 9.1 by halving the aluminum content present in the synthesis gel. These synthesized hierarchical MOR-type zeolites possess an improved catalytic activity for n-hexane cracking compared to large zeolite crystals obtained in the absence of CTAB.

Synergistic effect of micro-meso-macroporous system and structural Al amount of ZSM-5 for intensification of light olefins production in n-hexane cracking

The effect of Si/Al ratio on the structural and acidic properties of hierarchical ZSM-5 was studied. The zeolites were obtained under the fluoride medium and optimal synthesis conditions. The microchannel reactor improved the activity and stability of the catalysts in n-hexane cracking. The performance of the optimum catalyst in the increasing of olefins production was compared in the fixed bed and microchannel reactors. Investigation of the performance of the catalyst with Si/Al ​= ​200, as the optimum catalyst in the fixed bed reactor, showed that the microchannel reactor improved the yield of light olefins by improving the transmission phenomena (34.2% vs 45.5%). The hierarchical factor of this catalyst was equal to that of the optimum of the desilicated zeolites, which was obtained in the absence of extra templates and post-treatment operations. The synthesized zeolites showed a 55–64% yield in the production of olefins in the microchannel reactor. Although the ZSM-5 with Si/Al ​= ​200 had a lower conversion, it was more selective to light olefins; especially propylene due to its optimum mesoporosity and acidity. Micro-meso-macroporous system seems to be a sustainable approach for the production of light olefins using microchannel reactors and hierarchical zeolites, which lead to the increasing of stability and activity of catalysts in the future technologies.

Kinetics of n-Hexane Cracking over Mesoporous HY Zeolites Based on Catalyst Descriptors

A simple kinetic model based on the zeolite acid strength, the number of Brønsted acid sites, and the catalyst efficiency was developed for the cracking of n-hexane. A series of HY zeolites with a mesopore volume from 0.04 to 0.32 cm3/g was synthesized and characterized by various physical-chemical methods and tested for n-hexane cracking. The generation of mesoporosity influenced several other important parameters, such as acidity and extra-framework aluminum. Zero-length column diffusion measurements for mesitylene showed a large decrease in the characteristic diffusion time upon the introduction of mesoporosity, which changed only slightly with a further increase in mesoporosity. Similar n-hexane physisorption enthalpies were measured for all samples. The highest initial activity for n-hexane cracking per catalyst volume was observed for the sample with an intermediate mesopore volume of 0.15 cm3/g. The three mesoporous H-USY zeolites showed the same value of the intrinsic rate constant and the same activation energy. The difference in initial activity of the mesoporous zeolites was caused by the difference in the number of Brønsted acid sites. The increase in initial activity for the mesoporous zeolites compared to a microporous zeolite was caused by an increase in the acid strength.

Vinylacetylene synthesis with a low power submerged carbon arc in n-hexane

A low power carbon arc (12–18 W) struck between two graphite electrodes submerged in n-hexane has produced selectively vinylacetylene as the main product. Vinylacetylene was identified through its electronic absorption spectrum, the HPLC-DAD analysis and its precipitation as silver vinylacetylide. The HPLC analysis has revealed that the main by-product accompanying vinylacetylene is 1,3-butadiene and small amounts of diacetylene and divinylacetylene. The formation of other important by-products detected using also GC-MS (e.g., indene, naphthalene and some others) were discussed also in an astrochemical perspective. In contrast, a higher power submerged carbon arc in n-hexane struck at 108–216 W, yields an abundant mixture of polyynes accompanied by PAHs by-products. In the two arc regimes two mechanism are effective in producing selectively different products. In the low power arc regime, vinylacetylene is formed by the pyrolysis of acetylene primarily produced from n-hexane cracking in the arc. Chemical thermodynamics arguments also explain the vinylacetylene formation in low power arc conditions and diacetylene (and higher polyynes) in a higher power arc conditions.