Aromatization Of N-heptane Research Articles

Enhanced stability of Pt@S-1 with the aid of potassium ions for n-hexane and n-heptane aromatization

As a typical high-efficient alkane aromatization catalyst, Pt/KL is prone to deactivation due to the sintering growth of Pt on the outer surface of KL and the channel blockage caused by carbon deposition, which is further exacerbated by the one-dimensional channel structure of KL. Therefore, a series of encapsulated catalysts combining the three-dimensional channel structure advantages of S-1 with the role of potassium are prepared and evaluated. It is shown that the encapsulated catalyst KPt@S-1-02 modified by moderate potassium exhibits extremely superior stability and no significant deactivation is observable after n-heptane aromatization for 180 h under harsh conditions. On the one hand, encapsulation structure enables a strong anti-agglomeration ability of Pt and the topological structure of S-1 endows the encapsulation catalysts less susceptible to channel blockage, which has been demonstrated by HAADF-STEM and N2-adsorption. On the other hand, characterizations involving NH3-TPD, CO-DRIFT and HAADF-STEM verify that the modification by an appropriate amount of K can inhibit acid sites of catalysts and improve Pt dispersion and electron density, thus enhance the stability of catalysts. However, excessive K can serve as weak acid sites, poison of active sites and inhibitor of H2 adsorption on Pt surface, leading to deactivation of the catalyst.

Development and catalytic mechanism of a highly efficient Pt/Kβ catalyst for n-Heptane aromatization

Development of a high-performance naphtha aromatization catalyst has been paid many attentions as this reaction is generally employed to elevate the octane number of FCC gasoline and Fischer-Tropsch-synthesized oil. Here, a hierarchical porous Pt/Kβ catalyst is developed. It shows much higher activity, selectivity and stability than conventional Pt/KL. The catalytic performance of Pt/Kβ depends on Brϕnsted acidity and mesoporosity of zeolite β and nanoparticles (NPs) size and electronic state of Pt. The catalytic mechanism was investigated by combining TPSR, GC–MS, reaction kinetic studies and DFT calculations. Pt NPs catalyze dehydrogenations of alkanes/cycloalkenes to alkenes and dienes, while Brϕnsted acid sites facilitate alkene cyclization and ring-expansion of cycloalkenes despite that they can also dehydrogenate alkanes/alkenes through hydrogen transfer reactions. The n-heptane is mainly aromatized by dehydrogenating to 1-heptene and 2-heptene, 1,6- and 1,5-cyclizing into methylcyclohexane and ethyl-cyclopentane, and successively ring-expanding and/or dehydrogenating to toluene that may be further dealkylated to benzene.

Engineering spatial locations of Pt in hierarchically porous KL zeolite by atomic layer deposition with enhanced n-heptane aromatization

Atomic layer deposition (ALD) is an efficient method to confine the Pt particles in the channels of KL zeolite to improve the durability of catalysts. However, the diffusion of metal precursors is impeded by the size of micropore. Herein, using polyethylene glycol (PEG) as zeolite growth modifier to limit the growth of KL zeolite in [001] direction, a unique KL zeolite with trimodal porosity (native and larger micropores, mesopores) was successfully synthesized. Hierarchically porous structure engineered the spatial location of Pt precursor during ALD process. The obtained micro/mesoporous structure favored the diffusion of the Pt precursor into the pores of KL zeolite, and led to the formation of highly dispersed Pt clusters in micropores, mesopores, and intersection of micro/mesopores of KL zeolite. The resultant Pt/KLP2 catalyst showed the enhanced n-heptane aromatization performances operated at low temperature (420 °C). Systematic characterizations reveal the synergetic effect of spatial environment of hierarchical porous KL zeolite promoted the n-heptane aromatization and molecular diffusion. Namely, micropores enhanced the n-heptane dehydrocyclization, and the mesopores with shortened diffusion length facilitated the mass transport of aromatics and inhibited secondary hydrogenolysis efficiently. The present work broadens the fundamental understanding of the role of zeolite channels for alkane reforming reactions.

Regulation of sub-nanometric platinum on BaKL zeolite for boosting n-heptane aromatization

Sub-nanometric Pt have attracted a great attention due to the advantageous to active CH bond of alkanes and trigger the cyclization at lower temperature. However, the regulation of sub-nanometric platinum on zeolite presents a significant challenge. Herein, by means of atomic layer deposition (ALD) technique, sub-nanometric Pt were fabricated on BaKL zeolite (Ba modified KL) and stabilized by the coordinated Ba atoms. The resultant catalyst (6Pt/BaKL) inhibited the hydrogenolysis and remarkably improve the aromatics selectivity (92 %) in n-heptane aromatization. The normalized turnover rate (TOR, ×10−2 moltoluene molPt−1 s−1) of 6Pt/BaKL catalyst is 3.6 times higher than that of 6Pt/KL catalyst. Density functional theory (DFT) calculation and kinetic experimental results revealed that sub-nanometric Pt decreased the energy barrier for the cleavage of primary CH bond of n-heptane, and the activation of terminal carbon atom of σC7= (2-C7=) promoted the formation of toluene via 2,7-ring closure of 2-C7= intermediate. These findings broaden the fundamental understanding the role of sub-nanometric Pt for the alkanes aromatization.

Promoting effect of alkali metal on the catalytic performance of hierarchical Pt/Beta in the aromatization of n-heptane

Previous research showed that hierarchical Beta zeolite supported Pt was an effective catalyst for the aromatization of alkanes and the modification with K could further enhance its catalytic performance; however, the intrinsic effect of alkali metal on the alkanes aromatization over Pt/Beta was still rather obscure. In this work, a series of hierarchical Pt/Beta-Me catalysts were prepared by modifying the Beta zeolite with different alkali metal nitrates (MeNO3, Me = Li, Na, K and Rb) through ion-exchange and used in the aromatization of n-heptane; the effect of alkali metal on the catalytic performance of Pt/Beta-Me was then systematically investigated. The results indicate that the alkali metals used to modify the Pt/Beta-Me catalysts can not only passivate the strong Brønsted acid sites of Beta zeolite, but also enhance the Pt dispersion and induce an electron-rich chemical environment for the supported Pt species. Accordingly, the modification with alkali metal can effectively suppress the cracking reactions and carbonaceous deposition and meanwhile promote the dehydroaromatization of alkanes, in favor of enhancing the catalytic stability and aromatization selectivity of Pt/Beta-Me. The extent of promoting effect of various alkali metals follows the order of Rb > K > Na > Li. In particular, the Pt/Beta-Rb catalyst exhibits excellent performance in the aromatization of n-heptane, with a much higher selectivity to aromatics (78.2%) and larger turnover number of aromatic products (Pt-based TON, 19.4 × 103) than the unmodified Pt/Beta-H one (27.7% and 4.8 × 103, respectively). These results help to clarify the role of alkali metal played in the catalysis of Pt/Beta-Me for n-heptane aromatization and pave the way for designing more efficient catalysts in the aromatization of alkanes.

Regulation of Sub-Nanometric Platinum on Bakl Zeolite for Boosting N-Heptane Aromatization

Regulation of Sub-Nanometric Platinum on Bakl Zeolite for Boosting N-Heptane Aromatization

Ce-introduced effects on modification of acidity and Pt electronic states on Pt-Sn/γ-Al2O3 catalysts for catalytic reforming

Catalytic reforming of n-heptane over a series of Pt-based catalysts modified by Sn and Ce has been conducted in a fixed-bed reactor. Introducing Sn and Ce resulted in changes in the electronic environment of the Pt particles and acidity properties of the γ-Al2O3 support, causing changes in product distribution. Results from in-situ Diffusion Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), X-ray Photoelectron Spectroscopy (XPS) and density functional theory (DFT) evidenced that the metal promoters altered the metal-support interaction, endowing the generation of negatively charged Pt particles which favored the adsorption of n-heptene, a commonly accepted intermediate for n-heptane aromatization. A mechanistic study on the interaction of n-heptene and the Pt clusters with different chemical states based on n-heptene- Temperature Programmed Desorption (TPD) and Density-function theory (DFT) calculations suggested that negatively charged Pt clusters (Ptδ−) were thermodynamically favorable for the adsorption of n-heptene and its further cyclization step.

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.

Elucidating the restructuring-induced highly active bimetallic Pt-Co/KL catalyst for the aromatization of n-heptane.

A bimetallic Pt-Co/KL catalyst was fabricated via the atomic layer deposition (ALD) method and demonstrated to be highly active and stable for the aromatization of n-heptane at a low temperature (420 °C), which is due to the restructured Pt-Co clusters providing unique catalytic sites and the enhanced electron-donating properties of the catalyst.

Highly Dispersed Single-Atom Pt and Pt Clusters in the Fe-Modified KL Zeolite with Enhanced Selectivity for n-Heptane Aromatization.

Conversion of straight-chain paraffins into aromatics is particularly attractive but extremely challenging in the oil refining industry. Constructing the Pt-supported catalysts with high aromatic selectivity is vital. Here, we report a strategy to use Fe-modified KL zeolites to improve the Pt atom utilization efficiency and anchor them inside KL zeolite channels via atomic-layer deposition technique. A combination of highly dispersed single-atom Pt and electron-rich Pt clusters is fabricated on the KL zeolite through the creation of proper nucleation sites. The resulted catalyst (PtFe-1/KL) exhibits excellent performance for the n-heptane aromatization (90.1% aromatic selectivity) with an apparent activation energy of 131 kJ/mol and much enhanced stability at a relatively lower temperature (420 °C). Experimental analysis and density functional theory calculation demonstrate that the single-atom Pt might play a key role in the initial dehydrogenation of n-heptane to 1-heptene, and the superior stable Pt clusters encapsulated inside Fe-decorated KL zeolite channels accelerate the 1-heptene dehydrocyclization to aromatics. The synergetic interaction between single-atom Pt and Pt clusters enables the PtFe-1/KL catalyst to be one of the most effective n-heptane aromatization catalysts reported to date.

Performance of MFI Zeolite Catalysts in n-heptane Conversion Reaction Test

The aromatization of n-heptane, one of the main components of light naphtha, has been investigated over HZSM-5 (Si/Al = 33.9) and Zn-HZSM-5: Zn1-HZSM-5 (0.86% Zn); Zn2-HZSM-5 (1.35% Zn) and Zn3-HZSM-5 (2.89% Zn) catalysts prepared by the ion exchange with 10M aqueous solution of Zn(NO3)2. The catalytic activity measurement was performed by the chromatographic pulse method in the temperature range of 673 - 823K. The presence of zinc in Zn-HZSM-5 catalysts provided a significant increase in total aromatics selectivity due to the improvement of dehydrogenation activity by Zn incorporation. The Zn species facilitated the aromatization by converting paraffin�s into corresponding olefins which, in turn, were the aromatics precursors. Therefore, both acidity and zinc incorporation played important roles in the aromatization of n-heptane. Over 773 K the activity and selectivity of the Zn-HZSM-5 catalysts decrease very probably due to the loss of zinc ions.

Tailoring Pt locations in KL zeolite by improved atomic layer deposition for excellent performance in n-heptane aromatization

New evidences have been collected to demonstrate that Pt localization plays a crucial role in modulating the catalytic performance of Pt/KL catalyst for n-heptane aromatization. Here, an improved atomic layer deposition (ALD) technique was applied to deliberately tune the locations of Pt nanoparticles (NPs) in the Pt/KL catalysts. Both scanning transmission electron microscopy (STEM) and CO diffuse reflectance infrared Fourier transform (CO-DRIFT) spectra suggest that Pt NPs can be gradually introduced into the depths of KL zeolite channels by the increment of exposure time of Pt precursor from 1/12 to 20 min in one ALD cycle. Moreover, once KL channels are blocked by some encapsulated species (AlOx as example), Pt NPs can only be localized on the orifices or external surface and grow larger. As a result, surprisingly high aromatics selectivity (84.3%) in n-heptane aromatization is performed for a Pt/KL catalyst with Pt dominantly localized in the depth of zeolite channels even at a low temperature of 420 °C. Comparatively, the aromatics selectivity decreases to 31.5% and heptene (mainly 1-C7=) selectivity is up to 51.3% for a Pt/KL catalyst with some of Pt deposited on the orifices or external surface of KL. A suggested mechanism of n-heptane cyclization based on density-functional theory (DFT) calculations was provided to delineate the optional reaction pathways on different Pt localizations. Pt sites inside the KL zeolite channels are more preferable for heptene cyclization than those near the zeolite orifices both from thermodynamically and kinetically. A heptene molecule absorbed on Pt inside zeolite cage was found to curve itself, reducing the reaction barrier for the breakage of CH bond to be only 1.37 eV, therefore activating the 6th carbon atom (the rate-determining step). The barrier comparison for that on Pt near the orifices is 1.84 eV. The calculated mechanism broadens the fundamental understanding of the role of zeolite channels for alkane reforming reactions.

Deactivation of Pt/KL catalyst during n-heptane aromatization reaction

The deactivation of Pt/KL catalyst during n-heptane aromatization reaction was investigated. The activity fell quickly, especially in the initial stage. The fresh, the used and the regenerated catalysts were characterized by N2 physisorption, HADDF-STEM, TGA, UV-Raman, etc. The rapid deactivation is attributed to declined accessibility to Pt active sites because of coke deposition. GC-MS analysis revealed that the coke species are polycyclic aromatic hydrocarbons (PAHs). The dimensions of coke molecules and the coke volumes of used catalysts were estimated via DFT calculations. The variation of the contents of different coke species with reaction time implies a shape-selective transformation under micropore constraints. This, together with the XPS-determined depth profile of the coke in the used catalyst, indicates most of the coke was internally encased near the micropore openings. Quantitative evaluation concerning the influence of coke deposition on micropore volume confirms severe channel blockage by coke.

Influence of Pt nanoparticles modified by La and Ce oxides on catalytic dehydrocyclization of n-alkanes

Catalytic reforming accounts for a large share of the world’s gasoline production, it is the most important source of aromatics for the petrochemical industry. In addition, reforming of hydrocarbon on the dual-function catalysts has been found to form fundamentally different products in hydrogen diluents. Typical catalysts employed for this reforming process are Pt/Al2O3 and Pt-M/Al2O3, M being the promoter. These solids are characterized by both acid and metal functions which catalyze dehydrocyclization, dehydrogenation, isomerization and cracking processes. In this regard, information about cerium and lanthanum, as promoters, is hardly revealed. The present work aims to study the performance of Pt/Al2O3 catalysts modified by lanthanum or cerium during the conversion of cyclohexane, n-hexane and n-heptane. Catalytic activities of the prepared catalysts were tested using a micro catalytic pulse technique. Physicochemical characterization of the solid catalysts such as, surface area (SBET), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hydrogen-temperature programed reduction (H2-TPR), hydrogen-temperature-programed desorption (H2-TPD), CO2-TPD, NH3-TPD, high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were depicted. Results indicated clearly that Pt/Al2O3 catalyst is selective toward dehydrogenation to benzene which could be explained as due to the decrease in the active acid sites and the comparative segregation of the alumina support especially at 3% load of CeO. The presence of La2O3 in the Pt/Al2O3 catalyst promotes aromatization of n-hexane and n-heptane, also the dehydrocyclization of n-hexane is more difficult than that of n-heptane. Thus, modification of the Pt/Al2O3 catalyst by La, resulted in a more active and selective reforming catalyst.

Effect of structural defects in alumina supports on the formation and catalytic properties of the active component of reforming catalysts

The pair distribution function (PDF method) was used to investigate the phase composition and local structure of supports and the model reforming catalysts Pt/γ-Al2O3. The structural modification of support during acidic peptization of aluminum hydroxide with the pseudoboehmite structure was suggested as a possible approach to obtaining highly defective alumina. This modification of γ-Al2O3 results in the formation of a greater number of cation vacancies in the octahedral positions of the spinel-like alumina structure. The effect of structural defects in the support on the oxidation state of the active component and catalytic properties of the catalysts was demonstrated. It was found that platinum ions are located in the vacant octahedral positions (defects) of the spinel-like structure of the support. These ionic platinum centers on alumina with higher defect density demonstrated a higher selectivity in the model reaction of n-heptane aromatization as compared to metallic centers which prevail in traditional Pt/γ-Al2O3 catalyst.

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.

Effect of Synthesis Methods and Modification Treatments of ZSM-5 on Light Alkane Aromatization

ZSM-5 samples were prepared by hydrothermal synthesis from template-free (TF) and templated (HZ) gel media. The HZ samples were hydrothermally dealuminated to different levels (from HZS-1 to HZS-4). The physicochemical properties of the samples were characterized by X-ray diffraction analysis (XRD), X-ray fluorescence, microcalorimetry, and 29Si and 27Al nuclear magnetic resonance, and their catalytic performance was evaluated for n-heptane aromatization. The samples exhibited considerable differences in catalytic properties such as acidity and aromatic product selectivities in n-heptane conversion. The TF sample exhibited slightly lower XRD crystallinity and surface area than HZ but possessed comparable aromatic selectivity, except for a slight variation in product distribution, including a greater selectivity for C9+ aromatics. Among the dealuminated samples, the sample steamed at 673 K (HZS-2), exhibited the presence of very strong acid sties, and enhanced aromatic yields when compared to parent HZ and other dealuminated catalysts. The performance of HZS-2 was also high in the aromatization of other n-paraffins, such as n-hexane, n-octane, and the feedstocks, natural gas liquids (NGL) and light naphtha (LN). The aromatization selectivity of the catalysts was found to be in the order, HZS-4 < HZS-3 < HZ < HZS-1 < HZS-2.

Aromatization of n-heptane on Mo 2C-containing catalysts

The reaction pathways of n-heptane were studied on various Mo 2C-containing catalysts characterized by X-ray photoemission spectroscopy and surface acidity measurements. Unsupported Mo 2C catalyzed the dehydrogenation and cracking of n-heptane even at 573–623 K. Above 723 K, the dehydroaromatization of heptane became the dominant route for yielding toluene and benzene, with selectivities of 43 and 8%, respectively, at conversions of 20–23%. The catalytic performance of Mo 2C was improved when Mo 2C was deposited in a highly dispersed state on various supports, like silica, alumina, and ZSM-5. The best yield of the formation of aromatics (48.7%) was obtained for 5% Mo 2C/ZSM-5 (SiO 2/Al 2O 3 = 80) at 873 K. Although the deposition of coke occurred on all samples during the reaction, the latter catalyst underwent only a slight deactivation even after 20 h of reaction. The results obtained were interpreted by the monofunctional (pure Mo 2C), and bifunctional mechanism (supported Mo 2C) of the aromatization of n-heptane.

Aromatization of n-heptane over H-AlMFI, Ga/H-AlMFI, H-GaMFI and H-GaAlMFI zeolite catalysts: influence of zeolitic acidity and non-framework gallium

Influence of zeolite acidity (strong acid sites measured in terms of the pyridine chemisorbed at 400 °C) and non-framework Ga-species present in the zeolite channels on the conversion (total and to aromatics) and selectivity in the aromatization of n-heptane at 500 °C over the H-AlMFI (H-ZSM-5), Ga/ H-AlMFI, H-GaMFI and H-GaAlMFI zeolite catalysts (GHSV=3400 and 13,600 h −1) has been investigated. Among the zeolite catalysts, the H-GaAlMFI (with high acidity and high non-FW Ga) showed the highest n-heptane conversion and aromatization activity. Both the zeolitic acidity and non-FW Ga-species were found to play important role in the aromatization process, producing a combined effect.

Studies on acidity, activity and coke deactivation of ZSM-5 during n-heptane aromatization

In order to understand the effects of acidity and acid strength distribution on the nature of coke formed over ZSM-5 zeolite in n-heptane aromatization reaction, a series of ZSM-5 samples dealuminated to different degrees are prepared. Microcalorimetric ammonia adsorption studies indicated the presence of very strong acid sites (Δ H>140 kJ/mol) in the catalysts steamed at 400 and 500°C. 13C CP/MAS NMR along with dipolar dephasing is used to estimate the fraction of aromatic carbon and variation in aliphatic components with change in acidity of the ZSM-5 zeolites. Maximum selectivity to aromatics in n-heptane conversion is observed in the case of partly dealuminated ZSM-5 catalyst, whereas the aromatic nature of the coke is continuously decreased with steaming temperature, i.e. dealumination.