Catalytic Cracking Of N-heptane Research Articles

Synthesis of self-pillared pentasil zeolites without organic templates and seeds.

Self-pillared pentasil (SPP) zeolites have received considerable interest due to their distinctive intergrowth structure, while the precise process and mechanism for the formation of SPP zeolites remain obscure. Herein, SPP zeolites (ZSM-5) have been successfully synthesized by pre-aging an Al-rich gel without employing any organic templates or seeds for the first time. The as-synthesized SPP zeolites possess a notably high external surface area while the micropores for Ar adsorption are partially blocked by excess Na+, which can be fully recovered by Mg2+ or H+ exchange. The crystallization process is monitored and the impacts of synthesis parameters are investigated. The results show that self-pillaring originates from the partial lattice distortion at the intersections of nanosheets, offering a new insight into the self-pillaring process. Typically, with decreasing SiO2/Al2O3 ratio, more crossovers could be observed in the crystals, hinting that self-pillaring predominately occurs at the (101) plane of twins in the ZSM-5 precursor due to Al-rich lattice distortion. Finally, in the catalytic cracking of n-heptane, H-SPP zeolites exhibit superior performance to commercial H-ZSM-5 zeolites due to their abundant Brønsted acid sites arising from a low framework SiO2/Al2O3 ratio of ∼21 and the short diffusion path originating from the house-of-cards structure.

Synthesis and catalytic performance of zeolite-Y supported on silicon carbide in n-heptane cracking

In this work, we demonstrate a facile approach for the synthesis of zeolite-Y crystals (size, ca. ∼400 nm) supported on silicon carbide (SiC) with the assistance of the cationic template (polydiallyldimethylammonium chloride, PDDA). The polymeric cationic template used to treat SiC particles induces a positive charge on SiC surface which electrostatically attracts negatively charged aluminosilicate seeds and promotes the growth of zeolite (ZY) particles over SiC, thus leading to the formation of stable ZY@SiC supported catalysts. The supported ZY catalysts with different weight ratio of ZY and SiC were synthesized and characterized by various techniques such as XRD, SEM, SEM-EDX, SEM-mapping, TEM, STEM, FT-IR, 27Al MAS NMR and N2 sorption. The characterization of the supported ZY catalysts suggests the uniform growth of ZY particles over SiC together with the creation of hierarchical micro-mesopores assembly. In the catalytic cracking of n-heptane, the catalyst ZY@SiC-50 displayed a remarkable improvement in reaction rate when compared to commercial zeolite-Y (CBV-600) amounting to 3.5 folds enhancement. Interestingly, the light olefins yield is also substantially improved. At WHSV of 8 h−1 and 475 °C, the highest light olefin yield (24–36 %) was achieved over ZY@SiC-50 whereas the reference catalyst, CBV-600 produced lower light olefins yield (7–17 %). Moreover, the supported ZY catalyst exhibited less deactivation rates. This improved performance is attributed to the hierarchical micro-mesopores assembly created by the homogeneous dispersion of zeolite crystals on SiC which offers fast diffusion pathways for the reactants and enhanced accessibility to active sites thus leading to higher observed reaction rates and fast diffusion of products thus minimizing the occurrence of side reactions.

CFD Simulation of Catalytic Cracking of n-Heptane in a Fixed Bed Reactor

This work aims to three-dimension computational fluid dynamics (CFD) simulation of n-heptane catalytic cracking in fixed bed reactor (L=0.80 m) and to promote the cracking model of n-heptane using CFD. The catalyst granules were located in middle section of the reactor. The reaction scheme of n-heptane catalytic cracking was involved one primary reaction and 24 secondary reactions. Catalytic cracking process with a model of 25 molecular reactions was simulated by Fluent 6.0 software. The ratio of tube-to-particle diameter was considered N=2. The contours of coke deposition rate, vorticity, velocity and coke precursors and their relations along the reactor were predicted and discussed.

Effect of hierarchical ZSM-5 zeolite crystal size on diffusion and catalytic performance of n-heptane cracking

Hierarchical ZSM-5 zeolite aggregates with different sizes of nanocrystals were synthesized using different amounts of the mesoporogen 3-aminopropyltriethoxysilane. The effect of the crystal size on the catalytic cracking of n-heptane was investigated and the Thiele modulus and effectiveness factor were used to determine the reaction rate-limiting step. The crystal size affected the textual properties of the catalysts but not the acidic properties of the catalysts. The reaction rate was first order with respect to the n-heptane concentration. Cracking over hierarchical zeolites with nanocrystal sizes larger than about 50 nm took place under transition-limiting conditions, whereas the reaction over hierarchical zeolites with nanocrystal sizes of 15 or 30 nm proceeded under reaction control conditions. Hierarchical ZSM-5 zeolite aggregates with smaller nanocrystals had better selectivity for light olefins which can be ascribed to the shorter diffusion path lengths and lower diffusion resistance in these catalysts. Furthermore, these catalysts had lower coking levels which can be attributed to the substantial number of mesopores which prevent the formation of coke precursors. Open image in new window

Dealumination kinetics of composite ZSM-5/mordenite zeolite during steam treatment: An in-situ DRIFTS study

Abstract To get a better understanding of structural deactivation of ZSM-5/MOR during the catalytic cracking of n-heptane in the steam atmosphere, a comprehensive mechanism of hydrothermal dealumination was proposed through in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) in this work. The mechanism can be divided into two steps: firstly, the hydrolysis of four Al O bonds, and secondly, the self-healing of Si OH bonds accompanied with partial condensation of the extra-framework Al species. Accordingly, the kinetics of dealumination process has also been fully discussed. In the IR spectra, the range of 3450–3850 cm− 1 could be deconvolved to distinguish the hydroxyl groups on the different position and calculate the consumption of each hydroxyl group during the reaction. Based on results from the in-situ DRIFTS, the kinetics of dealumination was hence developed and also in well agreement with the kinetics of deactivation of ZSM/MOR catalysts during the reaction in the presence of little coke deposits.

N-Heptane catalytic cracking on hierarchical ZSM-5 zeolite: The effect of mesopores

To investigate the effect of pore structure on catalytic cracking of n-heptane, hierarchical ZSM-5 zeolites with unchanged acidity were prepared by hydrothermal synthesis with organosilane ((3-aminopropyl) triethoxysilane, APTES) and tetrapropylammonium hydroxide (TPAOH) as double templates. Chemical and textural properties of samples were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), inductively coupled plasma atomic emission spectroscopy (ICP-OES) and NH3 temperature programmed desorption (NH3-TPD). The results revealed that the dual functions of APTES and increasing amount of TPAOH favored for forming higher intercrystalline mesopore volume in the hierarchical ZSM-5 zeolites while maintained the micropores. Additionally, the crystal size distribution of the hierarchical samples ranging from 250 to 330nm did not influence the diffusion rate of light olefins. The improved selectivity of both ethylene and propylene was then ascribed to the decreased diffusion resistance caused by the presence of secondary porosity, comparing with traditional ZSM-5. We also proposed a related model accordingly.

Fabrication of desilicated MTW zeolite and its application in catalytic cracking of n-heptane

Desilicated MTW zeolite was developed for long chain hydrocarbon cracking such as n-heptane cracking. The catalytic performance of desilicated ZSM-12 (MTW) zeolite was investigated for n-heptane cracking in a fixed bed reactor at 650°C and atmospheric pressure. Average conversion of 75.3% was achieved with a stability of 240min. Higher selectivity to olefins (63.51%) was observed over a mesoporous MTW catalyst (treated with 0.2M sodium hydroxide for 10min) with average propylene to ethylene ratio (P/E) of 0.73. Desilicated MTW zeolite was prepared by using alkaline treatment under microwave irradiation to obtain ZSM-12 with larger pore volume (mesopores), larger surface area and different strength of acid sites. Maximum mesopore volume was 0.53cm3g−1 with external surface area of 437m2g−1. This is a remarkable result for mesoporous MTW prepared by post-alkaline treatment.

Selective formation of light olefin by n-heptane cracking over HZSM-5 at high temperatures

The catalytic cracking of n-heptane has been performed over HZSM-5 catalysts with various Si/Al ratios at 723–923 K to form light olefins selectively. The HZSM-5 zeolites with various acid site densities exhibited almost the same selectivity at the same conversion. The ethylene + propylene selectivity increased, while the propylene/ethylene ratio decreased with an increase in reaction temperatures. It is found that a high temperature is required to obtain a high ethylene + propylene yield. The highest ethylene + propylene yield obtained in this study was 59.7 C-% with a propylene/ethylene ratio of ca. 0.72 at 99.6% conversion over HZSM-5 (Si/Al = 31) at 923 K. Moreover, it is concluded from the selectivities and activation energies that the monomolecular cracking is predominant at a high temperature as 923 K.

Catalytic cracking of n-heptane under activation of lattice oxygen in a circulating fluidized bed unit

A circulating fluidized bed unit was used to evaluate influences of lattice oxygen introduction over catalytic cracking of n-heptane over HZSM-5 catalyst in a continuous reaction fashion. The results demonstrated that V 2O 5/Al 2O 3, which functioned as lattice oxygen supplier, promoted conversion of n-heptane in a much larger extent, compared with fixed-bed reactor. It also improved yields of propylene and butylenes, and intensified formation of aromatics and coke. Besides lattice oxygen species, some adsorbed oxygen species also participated into the reaction. CO and H 2O were formed as main oxygenated products. Steam introduction improved selectivity of propylene and butylenes, promoted desorption of strong adsorbed species in V 2O 5/Al 2O 3, and thus formation of coke was depressed; however, the damage to HZSM-5 framework was aggravated simultaneously.

Catalytic cracking of n-heptane over HZSM-5 catalysts with the activation of lattice oxygen

Paraffin, one of the major components of straight-run naphtha, is relatively inert compared with olefin, which poses a difficulty for catalytic cracking of naphtha. The present study proposed introducing lattice oxygen into previous HZSM-5 catalyst system to improve conversion and light olefin selectivity. Vanadium oxide based catalyst was prepared separately as the lattice oxygen provider and then mixed with HZSM-5 catalyst for evaluation in a fixed-bed reactor using n-heptane as model compound. The results demonstrated that conversion of reactant increased from ∼51% to ∼59% by replacing 20% of HZSM-5 catalyst with V 2O 5/Al 2O 3; depending on different location of V 2O 5/Al 2O 3 relative to HZSM-5 catalyst in the reactor, conversion and product distribution varied significantly, while the addition of pure alumina did not have such an effect. Besides, water was formed in liquid products accompanied by reduction of V 5+ during the reaction, as revealed by FT-IR and XRD results, respectively; the oxidized V 5+ seemed to have greater capability in promoting conversion of n-heptane. Therefore, it was concluded that n-heptane could be activated by lattice oxygen provided by oxidized vanadia before further cracking over acidic sites in HZSM-5 catalyst.

Catalytic Cracking Behavior of n-Heptane over HZSM-5 Catalyst

The catalytic cracking of n-heptane, which was selected as a model compound for the paraffin of light straight-run naphtha, was studied over HZSM-5 catalyst in a fixed-bed microreactor. Its catalytic behavior was compared with that of 1-heptene and the effects of hydrothermal treatment and catalyst carrier were evaluated. The results showed that compared with the cracking of 1-heptene, the concentration of hydrogen, methane, and ethane was much higher in the cracked gas obtained from the cracking of n-heptane. We concluded that this mainly originated from the monomolecular cracking pathway while the content of propylene and butylene in liquefied petroleum gas (LPG) was much lower. Upon hydrothermal treatment, the total amount of acid decreased sharply, especially the strong Br[o]nsted acid (Bacid) sites, leading to a steep decline in catalytic activity. This was accompanied by the improved propylene/propane and butylene/butane molar ratios in the products. Meanwhile, the ratio between the C3 and C4 products decreased, suggesting a decrease in the occurrence of monomolecular cracking. The carrier also significantly influenced the cracking behavior of n-heptane. We found that the presence of Lewis acid (Lacid) sites in the carrier improved the n-heptane conversion and promoted the bimolecular cracking pathway. Generally, compared with the olefin reactant, paraffin usually shows much lower reactivity and light olefin selectivity and, therefore, it is not a desirable feed for the catalytic cracking reaction over the zeolite catalyst for the purpose of light olefin production.

Kinetic Models for the Coke Combustion on Deactivated ZSM-5/MOR Derived from n-Heptane Cracking

The combustion behavior of coked H−ZSM-5/H−mordenite (ZSM-5/MOR) derived from catalytic cracking of n-heptane with and without water steam has been studied by thermogravimetric analysis (TGA). Catalytic cracking under vapor atmosphere with a water−oil ratio of 2 at 650 °C created only one type of coke deposit on the catalyst. On the other hand, two types of coke can be found on the coked catalyst deactivated by catalytic cracking without water steam. These results indicated different combustion kinetic models of different types of coke. A one-coke combustion model for the regeneration of coked catalyst obtained under water steam and a two-coke combustion model for the regeneration of coked catalyst derived without water steam were conducted by using TGA, respectively. Finally, the relationship between the C/H ratios and activation energies in the combustion kinetic models were investigated by ultimate analysis.

Highly Active BEA Catalyst for Catalytic Cracking of n-Heptane

Homemade BEA(HTS) catalysts prepared by the hydrothermal synthesis method exhibited much higher activity than commercial BEA, USY and ZSM-5 in catalytic cracking of n-heptane. It is characteristic that BEA(HTS) has a much larger amount of Bronsted acid sites than other zeolites. This is related to the high activity.

Anin SituCIR-FTIR Study ofn-Heptane Cracking over a Commercial Y-Type Zeolite under Subcritical and Supercritical Conditions

Anin situcylindrical internal reflection infrared technique (CIR-FTIR) was used in order to understand the behavior of subcritical and supercritical hydrocarbons processing over a commercial Y-type zeolite (P-Octacat) at 475°C and pressures up to 41.6 bar. The changes in the concentrations of the active catalytic sites were monitoredin situby CIR-FTIR during the catalytic cracking ofn-heptane at subcritical and supercritical conditions. High-qualityin situspectra were acquired for the first time at such high temperatures and pressures. The results showed that the catalyst maintained higher activity when catalytic cracking was performed under the supercritical conditions of the hydrocarbon, whereas the subcritical conditions led to a rapid deactivation of the catalyst. The subcritical IR spectra showed dramatic reductions in the concentration of the acid sites and in the interactions of the acid sites with the olefinic and paraffinic products of the reaction. Supercritical fuel showed an extraordinary dense behavior within the pores of the zeolite, which was significantly higher than the values estimated by equations of state under the same conditions. Higher amounts of carbonaceous deposits under subcritical conditions were also observed. Finally, reactivation studies on a deactivated catalyst resulted in a partial regeneration of the catalyst under supercritical conditions, provided with a complete experimental proof by CIR-FTIR.

Cubic and hexagonal faujasites with varying [formula omitted] ratios II. Catalysis of n-heptane cracking

Faujasite (FAU) and its hexagonal analog (EMT) was investigated in the catalytic cracking of n-heptane. Measurements were conducted with Aerosil-200 bound catalysts in an integral fixed bed reactor at 723 K. Catalytic activity is related to the concentration of Brønsted acid sites of the zeolite framework. Catalytic behaviour of dealuminated and subsequently acid leached varieties evidences that extra-framework aluminium species formed during dealumination contribute to catalysis. The distribution within the spectrum of the products of reaction clearly demonstrates that pathways producing alkenes and free hydrogen are involved in the scission of the parent hydrocarbon.

Reactions of mixtures of light alkenes and n-heptane over USHY zeolite

The influence of ethene and propene on the catalytic cracking of n-heptane at 673 K over the USHY zeolite has been studied with mixtures of n-heptane with different concentrations of alkene. The study reveals that the presence of light alkenes, with an alkene-to-n-heptane molar ratio in the range of 0.1–1.2, results in a decrease of the initial cracking activity of n-heptane, and an increase in the yield of heptenes and amount of coke formed. So, for this range of relatively high concentrations of light alkenes studied, the cracking reaction of n-heptane is substantially inhibited by these species.