Aromatization Of N-octane Research Articles

Selective regulation of Pt clusters inside KY zeolite using atomic layer deposition for n-octane reforming

Engineering the chemical environment of active metal phase plays vital role for the alkane reforming, yet great challenges remain. Herein, ABC-type atomic layer deposition (ALD) with methanol as surface modifier was considered to regulate the distribution of metallic Pt on KY zeolite to obtain high-efficient catalysts for n-alkane reforming. The pre-adsorbed methanol occupied the reactive sites of Pt precursors on the external surface of KY zeolite, and thus trapped Pt clusters inside the zeolite cages. The obtained catalyst (Pt/KYC2) exhibited enhanced n-octane aromatization performances with aromatics selectivity up to 78.9% and the proportion of C8 aromatics among total aromatics reached up to 97%. Systematic characterizations revealed that the addition of potassium eliminated the acidity of Y zeolite and induced the formation of electron-rich Pt clusters inside cages, which inhibited the side reactions effectively. Meanwhile, density functional theory (DFT) calculations and temperature programmed desorption (TPD) demonstrated that the Pt clusters inside cages contributed to the initial dehydrogenation and the adsorption of intermediates (1-octene and 2-octene), promoting the dehydrocyclization process. These findings paved a way for engineering the location of Pt clusters, and provided prospective catalytic system to obtain C8 aromatics (ethylbenzene and o-xylene) from naphtha reforming.

Insight into the performance of different Pt/KL catalysts for n-alkane (C6–C8) aromatization: catalytic role of zeolite channels

The weakened catalytic role of the zeolite channels might be a considerable reason for the low superiority of Pt/KL catalysts in n-octane aromatization.

Inhibiting the Dealkylation of Basic Arenes during n-Alkane Direct Aromatization Reactions and Understanding the C6 Ring Closure Mechanism

Pt/KL is the widely accepted catalyst for aromatizing n-hexane by 1,6 ring closure but encounters deactivation issues when aromatizing higher carbon-number feeds; undergoing extensive dealkylation to give unwanted CH4 and unselective products, as well as over-aromatization to form coke. Here, we report the use of a non-acidic MFI zeolite support, containing excess K+ beyond ion exchange capacity, well dispersed Pt, and high Pt presence inside the pores, for maximising direct n-alkane aromatization selectivity. TGA, catalyst deactivation studies, and characterizations show that the smaller pore sizes and lack of large cages in the MFI support sterically inhibit coke formation inside the pores (0% compared to 4.9% over Pt/KL for n-octane aromatization), which also reduces dealkylation of ethylbenzene and o-xylene under mild conditions to give a more selective product distribution, 86% selectivity by weight towards C6 ring closure compared to 27% for Pt/KL. Additionally, using NH3-TPD, XPS,CO-DRIFTS, and STEM, we show the contribution of excess K+ as an inhibitor of strong acid sites, an indirect Pt electron promoter through improving metal support interaction, and Pt dispersant. This work highlights the alternative use of well-understood zeolitic supports for the highly selective aromatization of n-heptane and n-octane by 1,6 ring closure, increasing the number of potential streams that can undergo direct aromatization, and providing a suitable alternative to Pt/KL.

Synthesis of nano-sized LTL zeolite by addition of a Ba precursor with superior n-octane aromatization performance

Nano-sized LTL zeolites obtained by the Ba-assisted method show improved catalytic performance in n-octane aromatization reaction.

Magnesium oxide as a catalyst for the dehydrogenation of n-octane

Magnesium oxide was prepared and used as a catalyst for the dehydrogenation of n-octane. The catalyst was characterized by ICP-OES, BET, powder XRD, IR, H2-TPR and SEM. The catalyst was tested for the oxidative dehydrogenation of n-octane using a continuous flow fixed bed reactor at a GHSV of 6000h−1 and n-octane/oxygen (O2) molar ratio of 0.8. Octenes were found to be the dominant dehydrogenation products, with 2-octenes being the dominant isomers. Among the C8 aromatics, ethylbenzene was the most-formed product, followed by styrene and o-xylene. The aromatization of n-octane was predominantly through the C1–C6 mode of cyclization.

Core of the Structural Characteristic of Natural Zeolite and its Catalytic Activity in the Aromatization of Light-Molecular Paraffin

The porous structure of Semeytau natural zeolite field, catalysts on its basis, and their catalytic activity in the course of aromatization of n-octane has been investigated. Higher activity of Ni, the Zn sample containing catalyst is defined by formation by more developed specific surface and porous structure.

Oxidative dehydrogenation and aromatization of n-octane over VMgO catalysts obtained by using different MgO precursors and different precursor treatments

VMgO catalysts with a V2O5 loading of 15% were prepared by the wet impregnation method. Catalysts synthesis was carried out by employing different MgO precursors (MgC2O4 and Mg(OH)2) and different precursors treatments (under vacuum and under static air). The synthesized catalysts were characterized by ICP-OES, BET, powder XRD, SEM, EDX, 51V NMR, H2-TPR, and NH3-TPD. The catalysts were tested for the oxidative dehydrogenation of n-octane using a continuous flow fixed bed reactor at GHSV of 8000h−1 and n-octane/O2 molar ratio of 0.8. Changing the above synthesis parameters led to the formation of catalysts with different morphologies, textural properties, and acidity. As a consequence, the results of the catalytic testing were also different, and treating the MgO precursors under vacuum induced a general positive effect on the catalytic performance. The aromatization of n-octane was predominantly via C1–C6 bonding.

Aromatization of n-octane over Pd/C catalysts

Gas-phase aromatization of n-octane was investigated using Pd/C catalyst. The objectives were to: (1) determine the effects of temperature (400–600°C), weight hourly space velocity (WHSV) (0.8–∞), and hydrogen to hydrocarbon molar ratio (MR) (0–6) on conversion, selectivity, and yield (2) compare the activity of Pd/C with Pt/C and Pt/KL catalysts and (3) test the suitability of Pd/C for aromatization of different alkanes including n-hexane, n-heptane, and n-octane. Pd/C exhibited the best aromatization performance, including 54.4% conversion and 31.5% aromatics yield at 500°C, WHSV=2h−1, and a MR of 2. The Pd/C catalyst had higher selectivity towards the preferred aromatics including ethylbenzene and xylenes, whereas Pt/KL had higher selectivity towards benzene and toluene. The results were somewhat consistent with adsorbed n-octane cyclization proceeding mainly through the six-membered ring closure mechanism. In addition, Pd/C was also capable of catalyzing aromatization of n-hexane and n-heptane.

In situ nanocrystalline HZSM-5 zeolites encaged heteropoly acid [formula omitted] and Ni catalyst for hydroconversion of n-octane

The heteropoly acid H 3 PMo 12 O 40 (PMo) and Ni in situ encaged in the secondary pore of nanocrystalline HZSM-5 zeolites was prepared from molybdenum oxide and phosphoric acid and nickel nitrate, in a slurry mixture of nanocrystalline HZSM-5 zeolite crystals and deionized water. Catalysts were characterized by ICP, FT-IR, XRD, 31P MAS-NMR, ESR, Py-IR, NH 3 -TPD, BET, SEM and TEM. PMo cannot enter the interior pores of the zeolite, but becomes encaged in the secondary pore of nanocrystalline HZSM-5 zeolite. The hydroconversion of n-octane over various catalysts was investigated in order to obtain light isomers of alkanes and aromatics products with high octane number. From the results presented in the paper, it is clear that the catalytic properties of the “in situ encaged” PMo–Ni and impregnated PMo–Ni properties are similar, and the number and distribution of Brønsted and Lewis acid sites in these two catalysts are similar, too. It is concluded that both the aromatization of n-octane and the ability of producing light iso-alkanes are enhanced over the PMo and Ni ‘in situ encaged’ and impregnated catalysts than other catalysts. The effects of the acidity and temperature on the activity of n-octane hydroconversion over investigated catalysts are demonstrated.

EFFECT OF ZEOLITE CRYSTALLITE SIZE ON Pt/KL CATALYSTS USED FOR THE AROMATIZATION OF n-OCTANE

The effects of varying zeolite crystallite size in n-octane aromatization over Pt/KL have been studied on a series of catalysts. Various KL zeolites were synthesized via microwave-hydrothermal treatment, which allows for good control of crystallite morphology. Zeolites with different crystallite sizes were prepared by varying aging time (17–24 h), amount of barium (0–445 ppm), and seeding (0–8 wt%). The results showed that higher aging time resulted in smaller zeolite crystallite size, whereas the addition of barium resulted in larger crystallite size. Moreover, the addition of seeding reduced the crystallite size from 1.47 to 0.94 μm. Pt supported on different zeolite catalysts (Pt/KL) was prepared by vapor phase impregnation (VPI). The fresh catalysts were characterized by DRIFTS of adsorbed CO and volumetric hydrogen chemisorption. The results indicated that Pt clusters are well dispersed inside the zeolite channel in all the catalysts prepared. The aromatization of n-octane was tested on the different catalysts at 500°C and atmospheric pressure. It was found that the catalytic activity of all catalysts dropped rapidly after about 200 min on stream due to coke plugging inside the pore of the KL zeolite. It was also observed that less ethylbenzene (EB) and o-xylene (OX) were obtained as the conversion increased because both EB and OX are converted to smaller molecules such as benzene, toluene, etc., by secondary hydrogenolysis. Furthermore, the EB/OX ratio increases with zeolite crystallite size due to an enhanced preferential conversion of the larger OX molecules compared to the narrower EB as their path through the pores is restricted.

N-Octane aromatization over Pt/KL of varying morphology and channel lengths

Various Pt/KL catalysts were prepared over a series of zeolites with different crystallite size and shape. To control the crystallite size and shape and consequently the channel lengths, different zeolite synthesis parameters were varied. X-ray diffraction (XRD) analysis of the synthesized KL zeolites indicated a high degree of crystallinity in all samples. The crystallite morphologies and the length/diameter (L/D) ratios in each sample were characterized by scanning electron microscopy (SEM). The Pt/KL catalysts were prepared by loading 1 wt% of Pt on these supports, using the vapor phase impregnation (VPI) method. They were characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, volumetric hydrogen chemisorption, and transmission electron microscopy (TEM). It was found that KL zeolites with cylindrical shape are effective catalyst supports for n-octane aromatization, but the effectiveness strongly depends on the channel length of the zeolite crystallite as well as the dispersion and location of the Pt clusters. The different Pt/KL catalysts in the series were compared in the aromatization of n-octane at 500 °C and atmospheric pressure. It was found that the catalysts with shorter channel length exhibited improved activity, selectivity, and catalyst life. Some of these differences can be ascribed to a different distribution of Pt clusters and different metal dispersions. That is, while the cylindrical-shape KL zeolites with short channels (i.e., cylindrical crystallites with low L/D ratio) favored a high dispersion of Pt inside the zeolite, those with longer channels (i.e., high L/D ratio) resulted in a large fraction of Pt cluster outside the zeolite. By contrast, KL zeolites synthesized with nanosize crystallites and having a high external-to-internal surface area ratio yield much lower Pt dispersions and exhibited a large fraction of Pt clusters located outside the pores, which can explain the differences in activity and selectivity. On the other hand, differences in aromatic product distribution observed for the different morphologies are related to a varying extent of secondary reactions. That is, for a given conversion level, the catalysts with shorter channels had a much lower extent of secondary hydrogenolysis. Consequently, more C8-aromatics are preserved in the product and less benzene and toluene are produced compared to the catalysts with longer channels.

N-Octane aromatization on Mo 2C-containing catalysts

The reaction pathways of n-octane were studied on various Mo 2C-containing catalysts. On unsupported Mo 2C prepared by C 2H 6/H 2 gas mixture the reaction of octane started at 573 K, and the conversion reached an initial value of ∼38% at 873 K. At lower temperature, 573–623 K, the main process is the dehydrogenation with some cracking. At 673–873 K, however, aromatics, xylene, toluene, benzene and ethylbenzene also formed with 58–30% selectivity. On Mo 2C/Al 2O 3 the product distribution was different. At 773–823 K the main aromatics were ethylbenzene and o-xylene, they formed with ∼62–51% selectivity at 23–50% conversion. In contrast, on Mo 2C/ZSM-5(80), which was found to be the most effective catalyst in the present work, toluene and benzene were the dominant aromatics. This suggests that the occurrence of the hydrogenolysis of C 8 aromatics on the acidic sites of ZSM-5. This catalyst exhibited a very stable activity: the conversion decayed only with few percent even after 10 h, and the aromatizing capability also remained high. The results are interpreted by the monofunctional (pure Mo 2C) and bifunctional mechanism (supported Mo 2C) of the aromatization of n-octane.

N-Octane aromatization on Pt-containing non-acidic large pore zeolite catalysts

Platinum catalysts supported on the potassium-form of different large-pore zeolites (i.e. K-LTL, K-BEA, K-MAZ, and K-FAU) have been tested for n-octane aromatization at 500 °C. All catalysts were prepared by the vapor phase impregnation (VPI) method. It was found that the Pt/K-LTL catalyst exhibit a better aromatization performance than the other zeolite catalysts. However, due to secondary hydrogenolysis, the C8 aromatics produced inside the zeolite are converted to benzene and toluene. By contrast, a non-microporous Pt/SiO2 catalyst did not present the secondary hydrogenolysis. Therefore, despite a lower initial aromatization activity, Pt/SiO2 results in higher selectivity to C8 aromatics than any of the other zeolite catalysts. All fresh catalysts were characterized by hydrogen chemisorption and FT-IR of adsorbed CO. In addition, the residual acidity of the supports was analyzed by temperature programmed desorption (TPD) of ammonia. In agreement with previous studies, it was found that after reduction at either 350 or 500 °C, the Pt/K-LTL showed much higher Pt dispersion than other catalysts. It is known that the structure of L zeolite can stabilize the small Pt clusters inside the zeolite channel. By contrast, FT-IR indicated that a large fraction of platinum clusters were located outside the zeolite channels in the case of Pt/K-BEA and Pt/K-MAZ catalysts.

N-Octane Dehydrocyclization on Cr2O3/La2O3/ZrO2Catalyst Elucidated by H–D-Tracer Experiments

H–D tracer experiments during n-octane aromatization on Cr2O3/La2O3/ZrO2 catalysts allow conclusions to be drawn on the reaction mechanism. The rate-determining step is the dissociative adsorption of the paraffin molecule. The following dehydrogenation steps seem to proceed predominantly reversibly.

N-Octane aromatization on a Pt/KL catalyst prepared by vapor-phase impregnation

The aromatization of n-octane was investigated on a Pt/KL catalyst prepared by vapor phase impregnation (VPI), a preparation method that in previous studies was found to result in the highest Pt dispersion and maximum incorporation of Pt inside the channels of the zeolite compared to any other method. Although the aromatization of n-octane on Pt/KL has been investigated previously, those studies were conducted on nonoptimized catalysts. Our results show that, even on the Pt/KL catalyst prepared by the VPI method that exhibited optimum performance for the n-hexane aromatization, the activity for n-octane aromatization at 500 °C and 1 atm was low and it quickly dropped after a few hours on stream. The product distribution obtained from the n-octane conversion showed benzene and toluene as the dominant aromatic compounds, with small quantities of ethylbenzene (EB) and o-xylene (OX), which are the expected products from the direct closure of the six-member ring. The analysis of the product evolution as a function of conversion indicated that the benzene and toluene are secondary products resulting from the hydrogenolysis of ethylbenzene and o-xylene. Diffusional effects play a significant role in determining this product distribution. Since ethylbenzene and o-xylene are produced inside the channels of the zeolite, they are hydrogenolyzed before they can escape. By contrast, on the Pt/SiO 2 catalyst used for comparison, ethylbenzene and o-xylene were the dominant aromatic products, although the overall aromatization activity was much lower than on the Pt/KL catalyst. The rapid deactivation found in the aromatization of n-octane on Pt/KL compared to that of n-hexane can also be explained in terms of the diffusional effects. The C8-aromatics produced inside the zeolite diffuse out of the system with much greater difficulty than benzene. Therefore, they form coke and plug the pores to a greater extent than benzene. Temperature programmed oxidation and sorption studies on spent samples demonstrate that the degree of pore blocking is much higher during n-octane aromatization than during n-hexane aromatization.

Transition metal oxide/carbon composite catalysts for n-alkane aromatization: structure and catalytic properties

Nanocrystalline particles of high temperature pretreated titania, zirconia or hafnium oxide, embedded in a carbon matrix, have been found to catalyze the aromatization of n-octane into ethylbenzene (EB) and o-xylene (OX) with high selectivity. The carbon matrix itself is catalytically not active, but seems to co-operate with the transition metal oxides in such a way that the various metal oxide/carbon composite materials exhibit equal selectivity patterns. In detail, the carbon component stabilizes a high dispersion of the oxides during the high temperature pretreatment procedure. This thermal treatment results in a destruction of surface acidity of the oxides, which would otherwise be responsible for undesirable consecutive and parallel reactions. Moreover, the carbon component is involved in the deep dehydrogenation of alkanes to multiple unsaturated alkenes. This is explained by the ability of surface carbon atoms to interact with hydrogen . The bulk and surface structure of the catalysts have been characterized by XRD, specific surface area measurements, XPS, UPS, Raman spectroscopy, in situ ESR and DRIFT spectroscopy.

Influence of Lanthana on the Nature of Surface Chromium Species in La2O3-Modified CrOx/ZrO2 Catalysts

Infrared spectroscopy of the adsorbed probe molecules CO, NO, and NH3, photoelectron spectroscopy, temperature-programmed desorption of NH3, X-ray diffraction, and BET surface analysis have been used to study surface and bulk characteristics of lanthana-modified CrOx/ZrO2 catalysts. La2O3 addition to CrOx/ZrO2 strongly modifies the structure of supported chromia species and stabilizes coexisting isolated Cr3+ and highly dispersed Cr2O3 particles. Unlike that, chromia in La2O3-free CrOx/ZrO2 undergoes considerable sintering at 823 K, which is the necessary reduction and reaction temperature for n-octane aromatization, applied as a test reaction. The strong interaction between chromia and La2O3-ZrO2 could be due to an increased number of nonacid hydroxyl groups, which are possibly responsible for a more efficient anchoring of chromia species on the La2O3-containing support rather than on pure zirconia. The performances of La2O3-promoted and La2O3-free CrOx/ZrO2 catalysts in n-octane aromatization do not differ so dramatically as would be expected from the large differences in the structure of surface chromia species.

In situ investigation of active sites in zirconia-supported chromium oxide catalysts during the aromatization of n -octane

Supported chromium oxide catalysts containing 0.5 and 4.0 wt% Cr, respectively, on a 7 wt% La2O3/ZrO2 carrier were studied by in situ EPR equipped with on-line GC during the catalyst pretreatment in flowing H2 and the aromatization of n-octane in the range 293<T<823 K. Active catalysts contain almost exclusively Cr3+ in the form of Cr2O3 clusters and isolated, coordinatively unsaturated Cr3+ species on the surface. The latter appear to be less sensitive to poisoning by coke deposition and govern residual catalytic activity.

Aromatization of n-Octane over Pt-Based Catalysts: Influence of the Support on the Product Distribution Among C8 Aromatics

Oxidative dehydrogenation (ODH) of n-octane was carried out over a vanadium–magnesium oxide catalyst in a continuous flow fixed bed reactor. The catalyst was characterized by ICP–OES, powder XRD and SEM. The catalytic tests were carried out at different gas hourly space velocities (GHSVs), viz. 4000, 6000, 8000, and 10,000 h−1. The best selectivity for octenes was obtained at the GHSV of 8000 h−1, while that for C8 aromatics was attained at the GHSV of 6000 h−1 at high temperatures (500 and 550 °C). The catalytic testing at the GHSV of 10,000 h−1 showed the lowest activity, while that at the GHSV of 4000 h−1 consistently showed the lowest ODH selectivity. Generally, the best ODH performance was obtained by the catalytic testing at the GHSVs of 6000 and 8000 h−1. No phasic changes were observed after the catalytic testing.

Carbon-14 tracer study of the conversion of labeled n-propylcyclopentane during n-octane aromatization with a Pt-zeolite L catalyst

n-Propylene cyclopentane or n-propylcyclopentane labeled in the ring with 14C was converted together with n-octane using a Pt-KL zeolite catalyst operating at 482°C and ca. 14 bar. The products indicate that hydrogenolysis to produce isooctanes, not ring expansion to produce aromatics, is the major reaction pathway for the alkyl cyclopentane compound. Dilution of the 14C activity in n-propylcyclopentane during the conversion shows that C 5 as well as C 6 cyclization occurs during the conversion of n-octane. The current data were obtained with a catalyst that has a Pt crystal size range that is similar to those reported earlier. Furthermore, the conversion data for n-octane and n-propylcyclopentane using the Pt-KL zeolite catalyst are very similar to data obtained with catalysts based on other nonacidic supports where the Pt crystals cannot be located in a zeolite type channel. Thus, for n-octane conversion, it appears that the Pt in L zeolite catalysts has selectivities that are similar to Pt on other nonacidic supports.