Cracking Of Hexane 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.

Effective hierarchical ZSM‐5 catalysts for the cracking of naphtha and waste tire‐derived oil to light olefins

AbstractHierarchical ZSM‐5 zeolite materials with different SiO2/Al2O3 molar ratios in the range of 60–300 were synthesized using soft templating and microemulsion methods to generate zeolite materials with narrow mesopore size distributions. The resulting materials were characterized by X‐ray flourescence (XRF), Fourier transform infra‐red (FTIR) spectroscopy, X‐ray diffraction (XRD), scanning electron microscopy (SEM), ammonia temperature programmed desorption (NH3‐TPD), thermo‐gravimetric analysis (TGA), Brunauer‐Emmett‐Teller (BET) surface area measurements, and products from the catalytic tests were analyzed by gas chromatography (GC). The XRF analysis determined that the Si/Al ratios for the synthesized ZSM‐5 were close to the batch ratios. The XRD and FTIR results revealed that the synthesized samples had crystalline ZSM‐5 zeolite structures. The small angle observed from the XRD patterns confirmed the presence of mesopores in the structure of the prepared materials. The SEM results showed that the ZSM‐5 synthesized materials had different morphologies and particle sizes, as well as worm‐like holes indicating that some macropores with average pore sizes ranging between 68 and 85 nm were successfully generated in these materials. NH3‐TPD results showed that the total acidity of the prepared materials decreased with an increase in the Si/Al (SA) ratio following this trend: SA = 71 > 177 > 345. This may be due to the decrease in the aluminum content, which is largely responsible for the formation of acidic sites in zeolites. The acid strength was found to increase with a decrease in the Si/Al ratio, highlighted by the peak shifts to higher temperatures. From the BET results, the SA = 71 was observed to have the highest SSA of 618 m2/g given by its micropore area of 103 m2/g and an external surface area of 515 m2/g. The high external surface area present in the material was due to the development of mesopores with narrow pore size distribution of approximately 6 nm indicating the success of the microemulsion method for the generation of hierarchical zeolites with well controlled pore sizes. The hierarchical ZSM‐5 catalysts were tested for the cracking of various hydrocarbon chain lengths. Low conversions, less than 10%, were obtained in the cracking of hexane; however, when cracking longer chain C12 hydrocarbons, the conversion increased remarkably to 100%. The catalyst with SA = 71 had the highest selectivity towards ethylene and propylene olefins due to its higher acidity in comparison with the other prepared catalysts. Improved activity, selectivity towards ethylene and propylene greater than 60%, and enhanced catalyst stability were demonstrated when using a hierarchical ZSM‐5 compared to the commercial catalyst in the cracking of dodecane. High selectivity towards light olefins was obtained for the cracking of waste tire‐derived oil. The results suggest that these hierarchical materials are suited to the cracking of long chain hydrocarbons and are promising materials for the valorization of waste tire‐derived oils.

Effect of simultaneous dealumination and metal incorporation of zeolite ZSM-5 on the catalytic performance in HTO process

The hierarchical Metal Fluorinate ZSM-5 zeolites, known as M-F-Z (M: Ga, Fe, Zr, La, Ce), were modified and dealuminated through the post-synthesis method, using an NH4HF2 + metal salt mixture. The zeolites were utilized in the catalytic cracking of normal hexane to produce light olefins in high yield and propylene to ethylene ratio. A unique post-synthesis method was used in this study without any structural collapse in which a significant amount of the metal was incorporated into the zeolite framework to provide effective interactions with the acidic sites. The catalytic cracking and investigation of the performance of the prepared catalyst were carried out in a fixed-bed microreactor system. The effect of zeolite modification during the dealumination process; especially acidity changes and hierarchical factors, on the performance of the modified ZSM-5 zeolite was studied. The modified catalysts showed superior acidic properties and higher mesoporosity volume compared with the parent zeolite. XRD, ATR-FTIR, FESEM, AAS, UV–visible (DRS), BET, EDX Dot-Mapping, H2-TPR, NH3-TPD, and TGA analyses were used to determine the physicochemical properties of the modified zeolites. Based on the results, the modified Ga-F-Z zeolite, with the minimum strong to weak acidic site ratio (0.1) and the highest hierarchical factor (0.192), had the maximum light olefin production yield and propylene to ethylene ratio of 63.26% and 3.13, respectively, which is a significant improvement compared to the parent zeolite.

Synergistic effect of Fe and Ga incorporation into ZSM-5 to increase propylene production in the cracking of n-hexane utilizing a microchannel reactor

Comprehensive investigation of the synergistic effect of incorporating Fe and Ga into ZSM-5 in cracking of hexane.

Dynamic and Steady State Evolution of Active Sites in H-ZSM5

Catalytic cracking of hexane over steamed ZSM-5 is studied under steady state and dynamic conditions to elucidate the role of the active sites on the product distribution. The product distribution from the riser simulator representing the dynamic state of the catalyst cannot be resembled from monocracking or bimolecular reactions by Bronsted acid sites alone. The catalyst promotes the hydride transfer function which controls the hexane conversion at 460–500 °C that flips into methanation function at 550 °C with a propene to ethene ratio of 1.04. In addition, hydrogen induction is observed in the first two pulses. Steady state data obtained from a fixed bed reactor, on the other side, shows that the product distribution is controlled by monomolecular cracking with low yield of methane and high propene to ethene ratio ranging from 4.3 to 3.3 depending on the temperature and conversion. We are not able to explain these data by considering the Bronsted acid sites alone and suggest that Lewis acid sites with short-lived activity are not inactive in the carbon-carbon activation before fading by coke deactivation. The reported findings are of importance to academia and industry and are very relevant to fluid catalytic cracking (FCC) processes.

Microwave assisted synthesis of MTT-TON intergrowth crystals for the catalytic conversion of naphtha to olefins

We present herein, the microwave assisted synthesis of SSZ-54, an intergrowth of MTT and TON frameworks using novel dual templates and the ability of the synthesized zeolites in the cracking of hexane to produce propylene was examined. SSZ-54 zeolite was successfully synthesized in a shorter time of 12 h via microwave heating as compared to 6–9 days by conventional oven. To tackle the inherent diffusion resistance in one-dimensional zeolite, we further reduced mass transfer limitation in the synthesized SSZ-54 zeolite by employing desilication and subsequent dealumination using a mixture of NaOH and TEAOH (for desilication) and nitric acid (for dealumination). N2 sorption of modified SSZ-54 samples presented improved mesopore volume, about 30% increment, with zero loss in micropore volume when compared to the parent sample. Furthermore, modified SSZ-54 zeolite showed enhanced catalytic activity and improved resistance to deactivation during hexane conversion. Parent and hierarchical zeolites displayed appreciable conversion for 10 h time on stream, with desilicated and dealuminated sample (DSA-50) showing the highest conversion of ca. 90% during the first 1 h and ca.60% selectivity to light olefins.

Hydrothermally stable acid-modified ZSM-22 zeolite for selective propylene production via steam-assisted catalytic cracking of n-hexane

To enhance the textural properties and the hydrothermal stability of porous catalysts, ZSM-22 zeolite samples were treated with different concentrations of HF aqueous solutions (0.7, 1.0 and 1.5 wt%). ZSM-22 samples that were treated with 0.7 and 1.0 wt% HF aqueous solutions showed a selective removal of aluminum atoms from TON framework. On the other hand, more concentrated HF (1.5 wt %) caused the leaching of silicon atom from the zeolites framework. The HF treated ZSM-22 samples showed smaller crystals size, better hydrothermal stability as reflected by intact textural properties as well as their mesoporosity. The modified samples also showed enhanced acidity with lower Brønsted acidity than the parent ZSM-22. ZSM-22 samples modified with low HF concentrations (0.7 and 1.0 wt%) showed better initial conversions in steam catalytic cracking (SCC) of hexane, which can be attributed to the enhancement in the acidity. However, deactivation rate was not enhanced after the HF treatment. The HF modified ZSM-22 samples showed more BTX selectivity which can be attributed to their larger external surface areas. The HF-treated samples lost more acidity, especially Brønsted acidity, after SCC reaction than the parents ZSM-22 zeolite.

Iron- and Cobalt-Doped Ceria–Zirconia Nanocomposites for Catalytic Cracking of Naphtha with Regenerative Capability

Series of nanosized iron- and cobalt-doped ceria–zirconia nanocomposites were prepared using a hydrothermal synthesis technique at 180 °C for 24 h, with the successful novel incorporation of both Co and Fe on ceria–zirconia, for n-hexane catalytic cracking. Effects of dopant ions on the improvement of intrinsic properties of ceria–zirconia nanocomposites were investigated using disparate characterization techniques. The synthesized ceria–zirconia nanocomposites exhibited similar X-ray diffraction (XRD) patterns, indicating full fusion of the metal ions into the ceria–zirconia lattice structure. The synthesized nanocomposite catalysts were tested for n-hexane cracking over 10 h time-on-stream, with no previous study or report for catalytic cracking of hexane via ceria–zirconia nanocomposites. Relatively high ethylene and propylene selectivity (both >62%) was obtained over CZ, FeCoCZa, and FeCoCZb over time-on-stream. Comparatively, the best catalytic activity and stability was exhibited by FeCoCZa with hig...

Mesoporous Aluminosilicate Catalysts for the Selective Isomerization of n-Hexane: The Roles of Surface Acidity and Platinum Metal.

Several types of mesoporous aluminosilicates were synthesized and evaluated in the catalytic isomerization of n-hexane, both with and without Pt nanoparticles loaded into the mesopores. The materials investigated included mesoporous MFI and BEA type zeolites, MCF-17 mesoporous silica, and an aluminum modified MCF-17. The acidity of the materials was investigated through pyridine adsorption and Fourier Transform-Infrared Spectroscopy (FT-IR). It was found that the strong Brönsted acid sites in the micropores of the zeolite catalysts facilitated the cracking of hexane. However, the medium strength acid sites on the Al modified MCF-17 mesoporous silica greatly enhanced the isomerization reaction. Through the loading of different amounts of Pt into the mesopores of the Al modified MCF-17, the relationship between the metal nanoparticles and acidic sites on the support was revealed.

Facile Fabrication of ZSM-5 Zeolite Catalyst with High Durability to Coke Formation during Catalytic Cracking of Paraffins

Post-synthetic HNO3 treatment of ZSM-5 zeolite synthesized in the absence of organic structure-directing agent removes framework Al selectively from the external surface, producing a unique ZSM-5 zeolite catalyst that has very few acid sites on its external surfaces. The resulting external acid sites can be readily deactivated at the early stage of the reaction, giving a potentially long-life catalyst as a result of a high durability to coke formation during the cracking of hexane or other paraffin molecules.

HZSM-5 treated with ammonia and water vapor: Characterization and cracking activity

HZSM-5 was treated with ammonia water vapor at 373 and 473K, and the catalytic activities for alkane cracking were evaluated. The treatment removed a fraction of silicon from the framework, but the meso-pore was not generated and the micro-pore volume was decreased. The solid acidity was characterized by infrared/mass spectroscopy-temperature programmed desorption (IRMS-TPD) of ammonia experiment. Two kinds of Brønsted acid sites were observed on the unmodified HZSM-5, and the solid acidity did not change largely by the treatment. Catalytic cracking of hexane and octane was enhanced by the treatment, and the second order reaction of cracking became accompanied with the first order reaction. From the products distribution in the propane cracking, the proceeding of bimolecular reaction was confirmed. Discussion was made on the morphology change of the modified HZSM-5 and the resulting enhanced cracking activity.

Isomerization and Cracking of Hexane over Beta Zeolites Synthesized by Dry Gel Conversion Method

Isomerization and Cracking of Hexane over Beta Zeolites Synthesized by Dry Gel Conversion Method

Facile control of crystallite size of ZSM-5 catalyst for cracking of hexane

Nano-sized H-ZSM-5 ( ca. ∼65 nm) was successfully prepared by mixing the mother gel under mild conditions before crystallization. Furthermore, a facile control of the crystallite size of the H-ZSM-5 catalyst was achieved merely by changing the amount of water in the mother gel. Three different-sized H-ZSM-5 catalysts were used as a catalyst for hexane cracking at 723–923 K. The crystallite size of H-ZSM-5 little affected the initial activity, but affected the deactivation rate considerably. The reduction in the crystallite size of the H-ZSM-5 catalysts contributes to the slow deactivation, probably because the H-ZSM-5 with a smaller crystallite size is less sensitive to deactivation by pore blocking by coke formed.

Production of C3/C4Olefins fromn-Hexane: Conceptual Design of a Catalytic Oxidative Cracking Process and Comparison to Steam Cracking

A conceptual design of the catalytic oxidative cracking (COC) of hexane as a model compound of naphtha is reported. The design is based on experimental data which are elaborated through a structural design method to a process flow sheet. The potential of COC as an alternative to steam cracking (SC) is discussed through comparing the key differences between the two processes. The presence of Li/MgO catalyst in the COC process (i) induces hexane cracking at lower operation temperatures (575 °C) than in SC (800 °C) and (ii) controls the olefin distribution by increasing the ratio of (butylene + propylene)/ethylene. The product distribution, and thus the separation train of both processes, is different. Catalytic oxidative cracking is designed to maximize propylene and butylene production, while steam cracking is designed to maximize ethylene production. In comparison to SC, the COC process is more energy efficient and consumes 53% less total duty for a production capacity of 300 kton/year of light olefins. However, a preliminary economic evaluation illustrates that the loss of valuable feedstock as a result of combustion of part of the naphtha feed makes the COC process economically less attractive than SC.

Pathway Study on Dielectric Barrier Discharge Plasma Conversion of Hexane

A plasma reactor based on dielectric barrier discharge has been developed for oxidative cracking of hexane to yield olefins at atmospheric pressure. Dissociation of hexane in the presence of oxygen with nonequilibrium plasma state represents complex chemistry, and both conversion and product selectivities differ significantly from the thermodynamic equilibrium state. In order to understand plasma chemistry initiated by electron impact processes, the Boltzmann equation is solved to determine the average electron energy and energy fractions in collision processes. Activation of oxygen in the plasma brings a new route with electron impact dissociation yielding atomic oxygen radicals and initiates oxidative cracking of hexane. Changes in certain features of the dissociation pattern of hexane to yield olefin products with varying parameters such as temperature, oxygen addition, and helium concentration are discussed.

ChemInform Abstract: Synthesis and Catalytic Properties of MAPO-36 Molecular Sieve.

The effects of the conditions of hydrothermal synthesis and the composition of the starting gels on MAPO-36 synthesis have been examined. Pure MAPO-36 was obtained from the gel with Mg/P = 0.08–0.25 after hydrothermal synthesis at 373 K for 44 h and then at 423 K for 22 h.The infrared spectrum of adsorbed pyridine confirmed that MAPO-36 has Bronsted acid sites in conformity with the expected substitution of Mg2+ for Al3+ in the framework. The strength of the acid sites on MAPO-36, as estimated by temperature-programmed desorption of ammonia, is as high as that of the sites on H-ZSM-5, though the number is smaller.The order of the catalytic activities for dehydration of ethanol and cracking of butane and hexane is as follows: H-ZSM-5 > MAPO-36 HY. The higher activity of MAPO-36 compared to that of HY may be caused by the greater strength of acid sites, as indicated by the temperature-programmed desorption of ammonia.The cracking of isobutane and 3-methylpentane is in the decreasing order: MAPO-36 HY > H-ZSM-5. This order is a result not only of the acidic nature of the catalysts, but also of the pore sizes. The lower activity of H-ZSM-5, in spite of the stronger acid sites, can be explained in terms of spatial constraints on the reactions. On the other hand, the high activity of MAPO-36 confirms that this material is a large-pore molecular sieve.

Catalytic oxidative cracking as a route to olefins: Oxidative conversion of hexane over MoO 3-Li/MgO

Li/MgO shows promise as catalyst for oxidative cracking of hexane. However, it suffers from low catalyst activity and slight deactivation. Aiming at higher yields of C 2–C 4 olefins during the oxidative cracking of hexane, modification of sol–gel Li/MgO with MoO 3 is reported. The influence of varying loadings of MoO 3 on performance of Li/MgO during the oxidative cracking of hexane has been studied. Catalyst with minimum MoO 3 loading (0.5 wt%) shows best yields to C 2–C 4 olefins. Promotion by MoO 3 maintains higher surface area upon calcination at T > 500 °C and improves catalyst stability significantly by minimizing surface carbonate formation.

Correlation of the cracking activity with solid acidity and adsorption property on zeolites

Activities for catalytic cracking of octane and hexane were measured on various zeolites, and the dependence of the turnover frequency and the activation energy on the characterization parameters was studied. The heats of ammonia and alkanes adsorption were measured to be correlated with the catalytic activities. From the relations between the heats of alkane and ammonia adsorption, zeolites were divided into two Groups I and II. In the zeolites of Group I on which a close relation between heats of alkane and ammonia adsorptions was observed, a compensation relation between the heat of alkane adsorption and activation energy was observed, but the so-called constant intrinsic activation energy was not confirmed. In the zeolites of Group II on which the relation between the heats of adsorption was not observed, the compensation relation did not exist. Thus, the dependence of the cracking activity upon characterization parameters was summarized: i.e., the heat of alkane adsorption cannot be utilized as a parameter to elucidate the activities of a few zeolites in Group II: on the other hand, the heat of ammonia adsorption can be totally correlated with these activities. Thus, the prevailing role of the solid acidity in the alkane cracking is proven. Finally, the cracking activity is discussed based on the solid acidity.

Catalytic oxidative cracking of hexane as a route to olefins

Catalytic oxidative cracking of naphtha is conceptually an alternative process to steam cracking. The performance of sol–gel synthesized Li/MgO in oxidative cracking of hexane as a model compound of naphtha, has been studied and compared to that of conventionally prepared catalyst. At a temperature as low as 575 °C, Li/MgO shows reasonable hexane conversions (28 mol%) and excellent selectivity to light olefins (60 mol%). It is proposed that hexane activation occurs on the catalyst surface via the Li +O − defect sites, where O − active sites abstract hydrogen from a secondary carbon atom. The formed hexyl radical in gas phase and in the presence of molecular oxygen will then undergo a complex radical chemistry resulting in a product mixture of C 1–C 5 hydrocarbons (paraffins and olefins) as well as combustion products. Presence of oxygen in the feed is crucial to prevent coking, and to regenerate the catalyst surface through reaction with adsorbed surface hydrogen atoms, thus maintaining catalyst activity. Oxygen also plays a significant role in accelerating radical chemistry in gas phase. Unlike steam cracking, catalytic oxidative cracking results in a relatively higher ratio of high olefins (butylenes + propylene) to ethylene. Thus presence of the catalyst provides a better control over product distribution. Promotion of Li/MgO with MoO 3 and Bi 2O 3 results in considerable improvements in catalyst activity and stability.

Development of a heterogeneous catalytic cracking reactor utilizing online mass spectrometry analysis

A laboratory system has been designed, constructed, and validated that reduces the complexity, time required, and data variability associated with catalytic microreactors that require post reaction steps prior to product analysis. In this work, a Varian (Walnut Creek, CA, USA) 3600 GC (gas chromatography) system coupled with a Saturn quadrupole ion trap mass spectrometer was used to perform mass spectral analysis in real-time catalytic cracking reactions. As this was an integrated reactor/analyzer, the GC column was exposed to temperatures beyond the degradation point of the column, and so selective ion storage RF waveform was used to remove the siloxane masses from the spectra. This produced lower detection limits and full scan data for identification. Mass/charge segmentation of the mass spectrometer allowed the complete product identification for electron impact spectra. Hexane was reacted over H-ZSM-5 catalyst for instrument validation. This produced a series of alkanes, alkenes, and aromatics with distributions consistent with that reported for the cracking of hexane.