Catalytic Cracking Of N-hexane Research Articles

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.

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

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.

Role of hierarchical micro-mesoporous structure of ZSM-5 derived from an embedded nanocarbon cluster synthesis approach in isomerization of alkenes, catalytic cracking and hydrocracking of alkanes

Hierarchical micro-mesoporous ZSM-5 zeolites synthesized via an embedded nanocarbon cluster approach were studied for isomerization of n-butene, catalytic cracking of n-hexane, catalytic cracking and hydrocracking of n-hexadecane. The introduction of mesoporosity into ZSM-5 zeolite framework was achieved by using carbon/SiO2 composite obtained from a carbonaceous gas deposition on silica gel as a mesoporogen. The synthesized hierarchical ZSM-5 zeolites with different Si/Al ratios were characterized by means of XRD, ICP-OES, N2 adsorption-desorption isotherm measurement, TEM, 27Al MAS NMR, NH3-TPD, FTIR of pyridine adsorption and constraint index (CI) test. Compared with the conventional H-ZSM-5, the hierarchical H-ZSM-5 exhibits an enhancement in the catalytic activity in terms of higher reactant conversion and product yield for isomerization of n-butene and catalytic cracking of n-hexadecane, as the mesopores provide larger pores as well as improved accessible active sites. Moreover, Pt loaded on hierarchical H-ZSM-5 exhibit a remarkably enhanced catalytic activity in hydrocracking of n-hexadecane compared to Pt loaded on conventional H-ZSM-5 due to a smaller and higher dispersion of Pt nanoparticles. For n-hexane cracking, the presence of mesopores does not improve n-hexane conversion on the fresh catalyst because this reaction proceeds via monomolecular pathway and the corresponding reaction species are smaller than 10-ring pore windows of ZSM-5 zeolite. However, the hierarchical structure improves the catalytic stability in n-hexane cracking due to the presence of mesopores, which can shorten the diffusion path length, suppressing the consecutive reactions and, thus reducing the coke formation.

Catalytic Cracking of n-Hexane in the Presence of Zeolite ZSM-5 Micro- and Nanocrystals

Results of a study on the synthesis of two sets of zeolites of the MFI structural type with a crystal size of a few microns (1–5 μm) in the first set and 120–140 nm in the second set and different Si/Al molar ratios of 20–220 in each of the sets have been described. The crystal sizes have been determined by dynamic light scattering and electron microscopy; the phase, textural, and acid properties of the samples have been studied by XRD, nitrogen sorption by the BET method, and ammonia TPD, respectively. The synthesized samples have been tested in the model n-hexane catalytic cracking reaction; the catalytic properties of the samples with respect to n-hexane conversion and the protolytic cracking, dehydrogenation, and hydride transfer reactions have been compared. It has been shown that a key role in the catalyst activity is played by the combination of the strong acid site concentration and the pore hierarchy factor (HF); the maximum conversion is observed at a strong acid site concentration of 0.2 mmol/g and HF = 0.26.

Catalytic Cracking of n-Hexane and n-Heptane over ZSM-5 Zeolite: Influence of SiO2/Al2O3 Ratio

The performance of two types of ZSM-5 zeolite catalysts (MFI-type zeolite, SiO2/Al2O3 = 50 and 300) was studied in the catalytic cracking of n-hexane and n-heptane as a model compound of light naphtha for production of light olefins at 500, 550, and 600°C. The physicochemical properties of ZSM-5 catalysts were characterized by means of XRD, BET, SEM and NH3-TPD. The influence of SiO2/Al2O3 molar ratio was investigated on conversion and product selectivity. ZSM-5 zeolite yielded higher conversion in the cracking of n-hexane compared to n-heptane and maximum conversion was achieved over ZSM-5(50) at 600°C. ZSM-5(50) showed higher alkane selectivity rather than olefins. It was found that ZSM-5(300) was more desirable in terms of having significant selectivity to light olefins as well as producing high propylene to ethylene ratio. The maximum propylene to ethylene ratio of 2.7 and 2.48 was observed over ZSM-5(300) at 500°C for n-hexane and n-heptane cracking, respectively.

Catalytic cracking of n-hexane over HEU-1 zeolite for selective propylene production: Optimizing the SiO2/Al2O3 ratio by in-situ synthesis

HEU-1 zeolites with different SiO2/Al2O3 ratios (60–530) were in-situ synthesized and systematically evaluated in n-hexane catalytic cracking reaction. The influencing factors including SiO2/Al2O3 ratio, weight hourly space velocity and reaction temperature were investigated to optimize the light olefin yield, especially propylene. Compared to HZSM-48 and HZSM-5 zeolites with similar SiO2/Al2O3 ratios, HEU-1-300 (SiO2/Al2O3=ca. 300), for example, displays much higher yield of ethylene plus propylene than HZSM-48 and comparable with HZSM-5. Furthermore, the propylene yield (35.6%) on it is higher than HZSM-5 by 9.0 percentage points, thus leading to higher propylene/ethylene ratio of 2.1. This excellent cracking performance of HEU-1-300 is ascribed to the combination of its moderate Brönsted acidity, due to the optimal SiO2/Al2O3 ratio, and unique framework topology. Moreover, both HEU-1 and HZSM-48 zeolites present much lower aromatics selectivity in the gas effluent than HZSM-5. For HEU-1 zeolite, it displays significant product shape selectivity due to its confined channel system, even though the large void space favors the formation of large intermediate species or aromatics precursors, while there is transition state shape selectivity for HZSM-48 structure which possesses no enough void space available to accommodate the large and complex hydrocarbons. HEU-1 topological structure also has pronounced effect on the formation and oxidation of coke deposition.

Dealumination of organic structure-directing agent (OSDA) free beta zeolite for enhancing its catalytic performance in n-hexane cracking

This work deals with the dealumination of organic structure-directing agent (OSDA) free beta (OFB) zeolite by different methods to change its acid amount and enhance its catalytic performance in n-hexane cracking. The OFB zeolite was pre-treated with ammonium hexafluorosilicate (AHFS) solution, steaming, or concentrated NH4NO3 (ca. 10 M) followed by calcination at high temperature. Then, thus pre-treated OFB samples can be effectively dealuminated by acid treatment with the structure retained. The H-form OFB (HOFB) and dealuminated OFB (DOFB) catalysts were characterized by using X-ray diffraction, N2 adsorption–desorption, SEM, ICP, NH3-TPD, 29Si and 27Al MAS NMR, and pyridine adsorption FT-IR. In the catalytic cracking of n-hexane, the DOFB catalysts showed a higher propylene selectivity and a longer catalytic life than the parent Al-rich HOFB catalyst. This would be attributed to the lower amount of acid sites, especially the Lewis acid sites, which suppressed the secondary reaction of propylene and butenes and the hydride transfer to form coke.

Effect of steam during catalytic cracking of n-hexane using P-ZSM-5 catalyst

Phosphorus-modified ZSM-5 (P-ZSM-5) catalyst was active for steam cracking of n-hexane to produce propylene and ethylene; however, the P-ZSM-5 catalyst deactivated because of both dealumination and coke deposition, which were strongly influenced by the steam to n-hexane ratio in the reactant flow. The amount of coke deposition increased with decreasing the steam to n-hexane ratio from the TG-DTA measurement. The steam could reduce the reactions of the light olefins to BTX and coke and enhance the yield of the light olefins; however, the excess steam deactivated the ZSM-5 catalysts irreversibly via dealumination.

Microwave-assisted hydrothermal synthesis of submicron ZSM-22 zeolites and their applications in light olefin production

There is an increased demand for facile and rapid synthesis of one-dimensional pore zeolites with controlled crystal size in the nanometer range. Here, we report for the first time, the rapid synthesis of ZSM-22 (TON) by using microwave-assisted hydrothermal synthesis (MAHyS) to control nucleation and growth. ZSM-22 crystals were synthesized rapidly in 24h (instead of 72h in conventional synthesis). Effect of synthesis time under microwave irradiation on the crystallinity of TON zeolites from 8 to 24h was investigated. The conventional hydrothermal synthesis was used for comparison. Under microwave-assisted synthesis, uniform crystal size with lower crystal aspect-ratio (length/width) was obtained and high agglomeration was prevented. Surface modification was applied by deposition of silica on the external surface of ZSM-22 using catalytic cracking of diphenyl silane. The protonated H-ZSM-22 prepared by the MAHyS and the surface-modified TON zeolites were applied in catalytic cracking of n-hexane. The modified MAHyS zeolites showed superior selectivity to propylene and lower selectivity to aromatics. Stability against deactivation was clearly observed over surface-modified ZSM-22 zeolites. The physico-chemical properties of modified zeolite-catalysts were characterized by using XRD, FE-SEM, N2 adsorption–desorption, 27Al MAS NMR and NH3-TPD.

N-Hexane cracking at high pressures on H-ZSM-5, H-BEA, H-MOR, and USY for endothermic reforming

Catalytic cracking of n-hexane was studied over H-ZSM-5, H-BEA, H-MOR, and USY zeolites at pressures between 1 and 137bar and temperatures between 573 and 673K, for application to endothermic reforming for cooling of aircraft engines. While the product distributions over each zeolite exhibited characteristics of a bimolecular mechanism, the product distributions were distinctly different on H-ZSM-5, exhibiting much less isomerization and more C-4 and C-5 products. Furthermore, in contrast to results on the larger pore zeolites, the reaction rates on H-ZSM-5 followed a simple Langmuir–Hinshelwood rate expression over the entire range of pressures and were stable for at least several hours at all pressures. Reaction rates on H-BEA, H-MOR, and USY were only weakly dependent on pressure and also exhibited an induction period at 137bar before decreasing with time. Implications of these results for endothermic reforming are discussed.

Location of coke on H-ZSM-5 zeolite formed in the cracking of n-hexane

Abstracts The catalytic cracking of n-hexane was carried out on H-ZSM-5 zeolites. The activity decreased with time on stream due to coke formation. Adsorption measurements with H-ZSM-5 used for n-hexane cracking revealed that coke is deposited on the external surface of H-ZSM-5 crystallites. At low coke amount, that is, at the initial stage of the cracking, only some pore openings are blocked. However, at high coke amount, that is, after a long-time run of the cracking, the pore openings are blocked completely in some crystallites resulting in the deactivation. Therefore, H-ZSM-5 with large crystallite size suffers from the fast deactivation because the number of pore openings is too small to survive the blockage by coke. The rate of deactivation of H-ZSM-5 depends not only on the coke formation rate but also on the crystallite size.

Application of ferrierite zeolite in high-olefin catalytic cracking

The potential use of pure ferrierite (H-FER) or H-FER/H-ZSM-5 blends in catalytic cracking to enhance the production of light olefins was evaluated in the catalytic cracking of n-hexane at 500°C and the high-olefin catalytic cracking (HOCC) of a Brazilian gasoil feedstock at 600°C. For the n-hexane cracking evaluation, only pure zeolites were employed. For the HOCC testing, both pure zeolites and the H-FER/H-ZSM-5 blends were mixed with a reference commercial FCC equilibrium catalyst to give a total zeolite content ranging from 0.5 to 8wt.%. In both catalytic tests, the use of ferrierite resulted in the improvement of olefin production. The H-FER zeolite exhibited higher propylene and butene selectivities than H-ZSM-5. Conversely, H-ZSM-5 resulted in higher ethylene selectivity. In the HOCC tests, pure H-ZSM-5 exhibited higher ethylene and propylene yields than H-FER. However, the use of the H-ZSM-5/H-FER blends caused higher yields of the total amount of light olefins (ethylene, propylene and C4 olefins) than when only pure H-ZSM-5 zeolite was employed. This synergetic effect of the combination of the H-FER and H-ZSM-5 zeolites is reported for the first time in the literature and can be explained by taking into account the fact that the H-FER zeolite has a lower hydrogen transfer ability.

Kinetics of n-hexane cracking over ZSM-5 zeolites – Effect of crystal size on effectiveness factor and catalyst lifetime

The catalytic cracking of n-hexane over ZSM-5 zeolite (MFI-type zeolite, Si/Al=150 and 240) catalysts was examined at reaction temperatures ranging from 823 to 923K under atmospheric pressure. The reaction rate constants and activation energies of n-hexane cracking over ZSM-5 zeolites with various crystal sizes and Si/Al ratios were evaluated. The catalytic cracking of n-hexane was first order with respect to the n-hexane concentration, and the activation energies of n-hexane cracking over ZSM-5 zeolites were found to be approximately 123–128kJmol−1. Compared with the macro-sized zeolite, the nano-sized zeolites exhibited high n-hexane conversion with stable activity for 50h. This is because the cracking reaction with nano-sized zeolite proceeded under reaction-limiting conditions, whereas the reaction with macro-sized zeolite proceeded under transition conditions between reaction- and diffusion-limiting conditions. As a result, the application of nano-zeolite to the catalytic cracking of n-hexane was effective and gave light olefins with high yield and excellent stable activity.

Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor

Kinetic analysis of burning regeneration of coked MFI-type zeolite obtained by catalytic cracking of n-hexane was studied using a multiple-reaction model. Hydrogen contained in the coke was oxidized faster than carbon, and subsequently carbon remaining after the hydrogen combustion was gradually oxidized. Reaction rates of carbon and hydrogen correlated with the Arrhenius equation; and activation energies for the combustion of carbon and hydrogen were 156kJ/mol and 140kJ/mol, respectively, regardless of the coke loading. Regeneration of coked MFI-type zeolite in the fixed-bed reactor was simulated numerically using reaction rates obtained from the kinetic analysis. The numerical result for changes in gas composition with time at the reactor outlet agreed well with the experimental results. Axial distribution of the temperature and concentration of water vapor in the catalyst bed was obtained numerically and results showed that the temperature and water vapor near the outlet were higher than other location in the reactor.

Selective formation of light olefin by n-hexane cracking over HZSM-5: Influence of crystal size and acid sites of nano- and micrometer-sized crystals

A series of HZSM-5 catalysts with different characteristics (size, texture, surface acid site density) was prepared by hydrothermal method and their performances were investigated in catalytic cracking of n-hexane (as a model compound of light naphtha) at 370 and 470°C. Uniform nano-sized HZSM-5 (ca. ∼0.1μm) was found to be a very attractive catalyst, showing a selective formation of propylene and light olefin (about 83% at weight hourly space velocity (WHSV)=3h−1) and a high resistance to the deactivation. Under similar conditions micro-size HZSM-5 (ca. ∼2.0μm) produced higher amount of other hydrocarbons i.e., C5+) and coke. This is because nano-size crystals have a larger external surface area, which offers a higher number of pore entrances and enhances the reaction occurring inside the pore channels compared with large micro-sized crystals. In addition, it is found that the selectivity to light olefin increases by decreasing the number of acid sites on the external surface. This leads to the conclusion that in n-hexane cracking to light olefin, the activity and selectivity are related to the crystal size and the ratio of surface acid sites to total acid sites. As shown, reducing crystal size and decreasing acid sites on the external surface are remarkable ways to enhance catalytic activity and selectivity, respectively.

Study of Coke and Coke Precursors During Catalytic Cracking of n-Hexane and 1-Hexene over Ultrastable Y Zeolite

In this paper we report about coke formation during catalytic cracking of n-hexane and 1-hexene over acidic ultrastable Y zeolite in a fixed-bed reactor. The study is based on a classification between coke precursors, whose amount is estimated through volatilisation by nitrogen purging, and coke, large formed coke molecules that remain during nitrogen purging even at 873 K and whose amount is estimated by oxidation at that temperature. The role of coke precursors on the coking process and catalytic cracking reaction is discussed as well as differences in coking tendencies between the two reactants. The results show that the amount of coke precursors and/or coke may decrease with reaction temperature resulting into a reversal of the coking dependence upon temperature.

Sulfated zirconia supported in mesoporous materials

SO 4 2−/ZrO 2 (SZ) supported on ordered mesoporous hexagonal materials were prepared by dispersion of ZrOCl 2·8H 2O into the mesopores, followed by the hydrolysis and sulfation. These materials have been characterized by X-ray diffraction, nitrogen adsorption isotherms, infrared spectroscopy, and catalytic cracking of n-hexane, cumene and 1,3,5-triisopropylbenzene. The results show that SZ was successfully loaded into the inner pores of MCM-41; the as-synthesized catalyst showed favorable catalytic properties. The factors in the preparative process that affected the final activity were discussed.

An explanation for the differences in catalytic hydrocarbon cracking activity between steam and chemically dealuminated Y zeolites

The effects of varying alkane and alkene partial pressures in the catalytic cracking of n-hexane over a chemically or a steam dealuminated Y zeolite at 673 K have been examined. Remarkably similar behavior was observed in terms of both product selectivities and catalytic activity at alkane partial pressures below 6 kPa, low conversions and before deactivation. This supports the earlier conclusion, based on a higher temperature reaction study, that the Brønsted acid sites of both Y zeolites are chemically similar. The contribution of bimolecular and oligomeric cracking to the overall reaction rate became increasingly dominant with increasing alkene partial pressure, whether it resulted from a higher conversion of alkane or a higher alkane pressure at constant conversion. Even at the lowest alkane partial pressures studied, reactions between the alkene products and the reactant contributed to the apparent cracking activity of the zeolite. The main difference observed between chemically dealuminated and steam dealuminated Y zeolites is that the former deactivated more quickly and to a greater extent. This was explained by the different porosities of the two zeolites. With less severe treatment of the zeolite structure, the chemically dealuminated zeolite is more sensitive to deactivation, and consequently, when rates are measured after deactivation has occurred, the steamed zeolite is much more active.

Physicochemical and catalytic properties of a new type of as-synthesized aluminium-deficient Y zeolite

Physicochemical (IR spectroscopy, 27Al and 29Si MAS NMR spectroscopy, XRD, adsorption measurements) and catalytic properties of a new type of low-alumina Y zeolite (LAY) prepared by direct synthesis (SiO 2/Al 2O 3 molar ratios 5.5–7.2) without using any templating agents or post-synthesis dealumination procedures were studied in comparison with the conventional or dealuminated Y zeolites. The low-alumina NaH-LAY zeolites are characterized by the higher thermal stability of acidic OH groups as compared with dealuminated Y zeolite. The unit cell parameter measured by XRD remains virtually unchanged and close to 2.463 nm for Na-LAY zeolite with SiO 2/Al 2O 3 ratios of 5.85–7.2. Extraframework aluminium was not found in decationized LAY zeolite samples by 27Al MAS NMR until the residual Na 2O content reached values below 1 wt.%. In addition, it is possible to tune the activity in the catalytic cracking of n-hexane by varying the extent of decationization. The content of isomer products increases with a growth of the residual Na 2O content in LAY zeolite. Introduction of this new zeolite into the cracking catalysts provides the maximum yield of gasoline at lower catalyst/feed ratios, compared to catalysts containing dealuminated Y zeolites. The case of the commercial use of the NaH-LAY zeolite is discussed.