Hexane Cracking Research Articles

Effect of desilication of H-ZSM-5 by alkali treatment on catalytic performance in hexane cracking

The effects of external surface and acid properties of desilicated H-type ZSM-5 zeolites (H-ZSM-5) on their catalytic performance in hexane cracking were investigated. H-ZSM-5 with two different crystallite sizes of 100nm and 1μm were treated with NaOH solution of different concentrations. The external surface area (SEXT) was increased with an increase in the NaOH concentration, because of the formation of mesopores inside the H-ZSM-5 crystallites as a result of desilication. The increase in the SEXT of the H-ZSM-5 catalysts contributed to mitigating the catalyst deactivation during the hexane cracking. Although the amount of coke deposited on the alkali-treated H-ZSM-5 was larger than that on the parent H-ZSM-5, the micropore volume of the alkali-treated H-ZSM-5 decreased less due to coke deposition than that of the parent. The deactivation rate and the decrease in the micropore volume of the small-sized H-ZSM-5 catalysts were smaller than those of the large-sized catalysts, because they had shorter average diffusion path lengths. Thus the activity of the alkali-treated H-ZSM-5, especially small-sized one is less sensitive to coke deposition. Lewis acid sites (LASs) were generated by treating with NaOH of high concentrations. The selectivities to benzene, toluene and xylene (BTX) in the hexane cracking were increased with an increase in the LASs amount at high reaction temperatures (≥873K). The LASs on the alkali-treated H-ZSM-5 were selectively removed by acid treatment. The resultant H-ZSM-5 exhibited a slightly lower hexane conversion and a lower selectivity to BTX but a small amount of coke compared to the parent and alkali-treated H-ZSM-5 catalysts, suggesting that LASs on alkali-treated H-ZSM-5 accelerated the dehydrogenation including hydride transfer and aromatization, forming BTX, which would be precursors of coke.

Strong Brønsted acid site in HZSM-5 created by mild steaming

HZSM-5 was treated with water vapor of less than 0.2bar (mild steaming). The acidity of the treated HZSM-5 samples was characterized by usual and IRMS–TPD of ammonia, catalytic cracking activity of octane and hexane was measured, and a correlation between the solid acidity and the catalytic activity was studied. The mild steaming changed the Brønsted acidity; the number was decreased but the strength was increased by increasing the partial pressure of water vapor. The cracking activity showed the maximum conversion against the partial pressure of water vapor, as has already been reported. Therefore, the present study confirmed the principal of the mild steaming for the catalytic activity enhancement. The activation energy for hexane cracking was correlated linearly with the strength of Brønsted acidity; the larger the strength of acid site, the smaller the activation energy. First order dependence of the reaction rate of the cracking on the alkane pressure was confirmed irrespective of the treatment conditions. Thus, the catalytic activity of hexane cracking could be simply explained by proceeding of the mono-molecular reaction on the Brønsted acid site. Dislodged Al-containing species most probably enhances the Brønsted acidity, and accordingly the catalytic activity is enhanced, as long as number of the site is kept sufficiently.

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.

Selective formation of propylene by hexane cracking over MCM-68 zeolite catalyst

Dealumination of MCM-68 zeolite with MSE topology enhanced the selectivity to propylene (45-50%) and durability to coke formation during hexane cracking, regardless of the reaction temperature, which is attributed to the control of the position and density of acid sites originating from the framework Al atoms.

Indirect assessment of unknown zeolite structures through inference from zeolite synthesis comparisons coupled with adsorption and catalytic selectivity studies

This study focuses on an attempt to determine the internal pore architecture for two unknown zeolites through comparison with a selection of known zeolites. Unlike most studies, we also attempt to gain insight into structures by analyzing synthesis conditions. The details of structure-directing agents (SDAs) for zeolite formation and the inorganic context in which they do and do not work provide important data. In addition to synthesis comparisons, we show comparative adsorption data (2,2-dimethylbutane) and catalysis selectivity (methanol conversion and hexane cracking) for the unknown and selected known zeolites that should have relevant pore features. We focused on zeolites with large and intermediate pores, particularly with channel intersections, and those with portals opening into larger cavities. In these three analytical approaches, the unknowns IM-5 and SSZ-57 matched best as multidimensional 10-ring zeolites.

Surface and bulk investigation of ZSM5 and Al-MCM-41 using synchrotron XPS, XANES, and hexane cracking

We present a comparative study of ZSM5 and Al-MCM-41 catalysts using spectroscopic and chemical techniques. The analysis of conventional and synchrotron XPS spectra of these catalysts reveals the presence of a topmost surface-related Si peak in addition to the bulk peak. XANES results suggest structural modification upon heating Al-MCM-41 at 500 °C. Depth-resolved XPS data show Al depletion from the surface of Al-MCM-41 in contrast to surface enrichment of Al in ZSM5. These surface modifications could be one of the reasons for the weak acidity of Al-MCM-41 in chemical reactions such as hexane cracking at different temperatures.

An Explanation for the Enhanced Activity for Light Alkane Conversion in Mildly Steam Dealuminated Mordenite: The Dominant Role of Adsorption

This paper presents a catalytic, spectroscopic, calorimetric study of mildly steam-dealuminated mordenite (H-MOR). With increasing steam partial pressures at 673 K there is increasing dealumination, the loss of Brønsted acid sites and the presence of extra-framework Al (AlEF) likely in the zeolite pores. Additionally, the number of Lewis sites increases with increasing AlEF. As observed in previous studies, the catalytic activity for hydroisomerization of n-hexane and monomolecular cracking of n-butane and n-hexane initially increases and goes through a maximum with increasing steam severity. While the maximum increase in the rate of hexane cracking is about 5 times, the activation energies indicate that steaming does not significantly change the strength of the Brønsted acid sites. In the absence of AlEF, the enthalpy of alkane adsorption is constant at all surface coverages. However, in the presence of AlEF, the initial heat of alkane sorption increases by about 5–10 kJ/mol. After covering these adsorption sites, the heat of adsorption is identical to that on the unsteamed H-MOR until saturation coverage has been reached. The increased enthalpies of adsorption are suggested to occur on the AlEF, Lewis acid sites. Since the surface coverage of paraffins under typical reaction conditions is low, it is proposed that the enhanced rate is due to the increased initial heat of adsorption leading to a higher concentration of reactants in the steamed H-MOR zeolites.

Selective Removal of Surface Acidity in ZSM-5 Zeolite Using (NH4)2SiF6 Treatment

Ammonium hexafluorosilicate, (NH4)2SiF6, has been found to be partially selective for removing surface acidity in zeolite ZSM-5. Under static digestion conditions using HZSM-5 extrudate, the zeolite was found to lose approximately 30% of its acid sites as determined by NH3 desorption and hexane cracking results. However, tetrapropylammonium desorption and α-naphthyl red adsorption studies showed up to a 50% reduction in surface acid sites. This surface deactivation was conclusively demonstrated in propene oligomerization catalytic studies. Use of fluorosilicate-treated HZSM-5 in the surface acidity-sensitive reaction gave substantial viscosity index (VI) improvements in the C20+ fraction due to more linear oligomerized products. It is believed that the static digestion procedure necessary to treat the HZSM-5 catalysts in this study allowed for increased reaction between the zeolite surface and the SiF62− anion. If optimized, this method may be a useful technique to prepare catalysts for surface acidity-sensitive hydrocarbon conversion reactions.

On the Role of Small Amines in Zeolite Synthesis

A systematic study explored the ability of small amines to control the zeolites obtained at the high-pH levels typical of crystallizations at SiO2/Al2O3 ratios between 20 and 40. Experiments in the absence of amine showed that mordenite (MOR) was the preferred product at the lowest ratio, and ZSM-5 was the preferred product at the highest SiO2/Al2O3 ratio. However, the addition of C2-, C3-, and C4-alkylamines changed the product dramatically. On addition of ethylamine, ZSM-5 was produced at the lowest SiO2/Al2O3 ratio, and ZSM-35 was the preferred product with C3- and C4-amines. On addition of the largest of the group, tert-butylamine, the product reverted to MOR. With polyamines such as diethylenetriamine and triethylenetetramine, ZSM-5 samples were produced having framework SiO2/Al2O3 ratios below 20. The hexane cracking activity of these samples was consistent with their exceptionally high Al content. The cyclic C4- to C6-amines, at both SiO2/Al2O3 extremes, yielded ZSM-35 and MCM-22. Again, with the l...

Long-Range Interaction of Alkali Cations with the Acidic OHGroups in H-ZSM-5

The effect of alkali cations on the properties of the acidic OHgroups of H-ZSM-5 was studied by different methods. Thetemperature dependence of the line-width of the1H MAS NMRsignal attributed to acidic protons is greatly affected bysubstituting only 1% of Na+or NH4+ions for protons. Theline-width broadens significantly upon raising the temperatureof measurement from room temperature to 373 K. Theline-broadening was almost completely suppressed when only 1% ofprotons was replaced by Na+or NH4+ions. The rate of H–Dexchange between acidic OH groups and D2molecules in the gasphase is also greatly suppressed by a small introduction of Na+ions by ion exchange, though the whole of the protons behavehomogeneously for the exchange. These results indicate thepresence of a long-range interaction between alkali metalcations and the remaining acidic OH groups. The catalyticactivities for the acid-catalyzed reactions of cyclopropaneisomerization and hexane cracking are reduced greatly when asmall percentage of protons is replaced by alkali cations. K+ions are a more effective poison than Na+ions. These resultsfurther support the view that alkali cations in H-ZSM-5 exert along-range inductive effect on the activity of the acidic OH groups.

Acidity and cracking activity changes during coke deactivation of ultrastable Y zeolite

Changes in acidic ultrastable Y zeolite (H-USY) due to coke deactivation during hexane cracking were determined by ammonia TPD, microcalorimetry, IR spectroscopy of adsorbed pyridine, 13C NMR, powder XRD, TGA of hexane adsorption, and nitrogen and argon adsorption. Small amounts of coke produced a large decrease in activity; however, few acid sites were poisoned. Compared to a fresh USY zeolite, coke deactivated H-USY did not show significant changes in the acid strength distribution, number of acid sites, fraction of Brønsted and Lewis acid sites, pore size distribution, or hexane diffusivity that could account for the large decrease in the cracking activity. It is proposed that the rapid loss in activity resulting from a small decrease in the number of acid sites is consistent with a site poisoning deactivation model for a diffusion-limited reaction, i.e., the rate of hexane reaction is greater than the rate of diffusion. For a diffusion limited reaction, the initial hexane cracking and coke deposition occurs primarily at active sites near the external surface of the crystal with little reaction at the particle interior. As deactivation proceeds, the thickness of coked region grows and hexane diffuses further into the zeolite particle before reaction.

Comparison of the Acidic Properties of H-[Al]ZSM-5, H-[Fe]ZSM-5, and H-[Ga]ZSM-5 Using Microcalorimetry, Hexane Cracking, and Propene Oligomerization

Comparison of the Acidic Properties of H-[Al]ZSM-5, H-[Fe]ZSM-5, and H-[Ga]ZSM-5 Using Microcalorimetry, Hexane Cracking, and Propene Oligomerization

Diffusion and adsorption in zeolite Y and mordenite deactivated by propene oligomerization and hexane cracking

The bidisperse pore diffusion model applied to the pulse gas chromatography (PGC) technique was used to determine the effect of coke formation on the diffusion and adsorption of propane, n-butane and isobutane in the pores of zeolite Y (HY) and mordenite (HM) before and after deactivation during hexane cracking and propene oligomerization. The catalysts were characterized using TPD, TG/DTA, BET and pore volume measurements. Tracer diffusion measurements taken before and after reaction showed that in the case of hexane cracking over HY the diffusivity decreased by more than three orders of magnitude while the tracer adsorption equilibrium constant increased. In the case of HM, the diffusivity decreased by five orders of magnitude and the adsorption equilibrium constant also decreased by a factor of 10. In HY, deactivated during propene oligomerization, the diffusion was an order of magnitude faster than in the case of the catalyst deactivated by hexane cracking, while the adsorption equilibrium constant decreased. HM deactivated by oligomerization showed macropore behaviour. Proposals are made to explain the above observations.

A kinetic model for the hexane cracking reaction over H-ZSM-5

A kinetic model for the n-hexane cracking reaction over H-ZSM-5 has been developed. This model describes hydrocarbon transformations in the following reaction steps: protolytic cracking of n-hexane, hydrogen transfer between n-hexane and light olefins, oligomerization, and cracking of olefins. The activation energy of the protolytic cracking steps is 75.6–152.4 kJ/mol, depending on the reaction products, and that of the hydrogen transfer steps is 4.1–8.9 kJ/mol, depending on the olefin involved in the reaction step. It is demonstrated that a considerable contribution from hydrogen transfer to n-hexane cracking manifests itself in the autocatalytic character for the reaction at 400°C. At 538°C (the temperature of the α-test) hydrogen transfer does not play an appreciable role in hexane cracking over H-ZSM-5 zeolites.

Reactivity of zeolite hydroxy groups toward σ-donor bases. H–D exchange with 3-methylpentane

The H–D exchange of deuteriated Y zeolites, silica–alumina and γ-alumina with 3-methylpentane has been studied by IR spectroscopy. At 373 K exchange ocurred only in the zeolites. Silica–alumina required higher temperatures (523–623 K) and exchange did not occur in γ-alumina in the range of temperature studied. Ultrastable Y zeolite with non-framework alumina (NFA), showed a higher reactivity for H–D exchange and hexane cracking than Y zeolite dealuminated by (NH4)2SiF6. These findings were explained by the difference in acid strength between the zeolites, probably due to the presence of NFA as proposed in the literature, and stress the dependence of H–D exchange on acid strength. The IR study of H–D exchange between 3-methylpentane and Y zeolites having pyridine pre-adsorbed, neutralizing some of the hydroxy groups, showed that the high-frequency (hf) hydroxy group is responsible for the exchange. The low-frequency (If) and NFA hydroxy groups appear to exchange via hydrogen bridges with the hf hydroxy group, rather than by direct interaction with the hydrocarbon. The H–D exchange between solid acids and alkanes could be a qualitative test for acid strength, since only very strong acids would attack σ bonds.

The effect of nonframework aluminum on acidity in dealuminated mordenite

Despite its high crystalline stability, mordenite (MOR) is relatively easily dealuminated. Calcination of NH 4-MOR to H-MOR at 500°C results in 20% dealumination with the production of octahedral, nonstructural aluminum located within the zeolite pores. The octahedral aluminum is nonacidic and does not affect the acid strength of the remaining structural aluminum ions. Calcination at 735°C results in over 50% dealumination, creating internal, nonacidic pentacoordinate aluminum. In H-MOR (735), 60% of the acid sites are equivalent in strength to H-MOR (500); however, the remaining acid sites are reduced in strength. Pentacoordinated aluminum may interact with the portion of structural aluminum, reducing their acid strength. Dealumination by strong acid removes 75% of the structural aluminum; the aluminum is completely removed from the mordenite, leaving no nonframework species. Strong acid creates many SiOH defect sites; however, the SiOH defects did not affect the acid strength. High-temperature steam produces extensive dealumination. In the presence of steam, nonstructural aluminum migrates to the crystallite surface and restricts the pore aperture. Eventually, the pore aperture becomes sufficiently restricted that large molecules, e.g., n-hexane or pyridine, cannot diffuse to the zeolite interior. In steam-dealuminated mordenite, pentacoordinate aluminum also reduces the acid strength of 40% of the remaining acid sites. Qualitatively, the catalysts' activities for hexane cracking could be correctly ordered by either the number of strong acid sites (independent of the maximum acid strength) measured by NH 3 TPD or infrared spectroscopy.

Structure and acidity modification of mordenite through isomorphous Fe substitution

A series of Fe-modified mordenite were hydrothermally synthesized. The effects of substrate composition, aging time, reaction temperature, and reaction time upon the crystallization of the mordenite were investigated. Isomorphous substitution of Fe was confirmed by FT-IR and EPR analysis. Acid strength and acid sites distribution of each catalyst were measured by pyridine TPD, which showed that the number of strong acid sites in H-Fe-mordenite are smaller than in H-Al-mordenite. Structural characteristics and thermal stability were also examined using XRD, SEM and TG. Fe-substitution into the mordenite structure have resulted in substantial decrease in thermal stability. H-Fe-mordenite catalysts showed high selectivity towards p-xylene in toluene alkylation and in xylene isomerization reaction, compared with those obtained using H-Al-mordenite because of lower concentration of strong acid sites. In addition, these H-Fe-modified mordenite showed little hexane cracking activity. Deactivation in both toluene alkylation and xylene isomerization reaction and coke formation over H-Fe-mordenite in these reactions were negligible.

On the nature of the sites responsible for the enhancement of the cracking activity of HZSM-5 zeolites dealuminated under mild steaming conditions: Part 2

New data on n -hexane cracking catalyzed by HZSM-5 zeolites dealuminated under mild steaming conditions were discussed. They supported the earlier hypothesis on the dehydrogenating role of Lewis acidic sites in the initial stages of the cracking reactions. Higher yields of H 2 were found for dealuminated HZSM-5 zeolites as compared with the fresh catalyst. The addition of 1-hexene to n -hexane as a feed led to an enhancement of the activity of the fresh zeolite, whereas it had just the opposite effect on the activity of dealuminated samples. The mechanism of paraffin cracking that involved both Lewis and Brönsted acidic sites was discussed.

ChemInform Abstract: The Role of Polyvalent Cations in Developing Strong Acidity: A Study of Lanthanum‐Exchanged Zeolites.

AbstractLa3+ ions are introduced (ion‐exchange) into a series of faujasite and ZSM‐20 zeolites having a minimum amount of extraframework Al and a broad range of Si/Al ratios and the catalytic activity of the resulting materials in the hexane cracking reaction is measured.