Dehydrocyclization Of N-hexane Research Articles

Aromatics formation by cracking and dehydrocyclization of n-hexane using Zn ion-exchanged ZSM-5–Al2O3 hierarchical composite catalysts

Aromatics formation by cracking and dehydrocyclization of n-hexane using Zn ion-exchanged ZSM-5–Al2O3 hierarchical composite catalysts

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.

Dehydrocyclization of n-Hexane over Heteropolyoxometalates Catalysts

The catalytic dehydrocyclization of n-hexane was studied here for the first time using a number of compounds based on H3PMo12O40. The described catalysts were prepared by either replacing the acidic proton with counter-ions such as ammonium or transition metal cations (NH4+, Fe3+, K+), or by replacing Mo6+ with (Ni3+, Co3+, Mn3+) in the polyoxometalate framework, as reported earlier. For comparison purposes, the known (TBA)7PW11O39 catalyst system was used. All reactions were conducted at different temperatures in the range 200 - 450. The Keggin structure of these heteropolycompounds was ascertained by XRD, UV and IR measurements. 31P NMR measurements and thermal behaviour of the prepared catalysts were also studied. These modified polyoxometalates exhibited heterogeneous superacidic catalytic activities in dehydrocyclization of n-hexane into benzene, cyclohexane, cyclohexene and cyclohexadiene. The catalysts obtained by substituting the acidic proton or coordination atom exhibited higher selectivity and stability than the parent compound H3PMo12O40. Catalytic activity and selectivity were heavily dependent on the composition of the catalyst and on the reaction conditions. At higher temperatures, the catalyst exhibited higher conversion efficiency at the expense of selectivity. Using higher temperatures (>400) in the presence of hydrogen carrier gas, selectivity towards dehydrocyclization ceased and methane dominated. To explain the results, a plausible mechanism is presented, based on super-acidic nature of the catalyst systems.

The Nanoscience Revolution: Merging of Colloid Science, Catalysis and Nanoelectronics

The incorporation of nanosciences into catalysis studies has become the most powerful approach to understanding reaction mechanisms of industrial catalysts and designing new-generation catalysts with high selectivity. Nanoparticle catalysts were synthesized via controlled colloid chemistry routes. Nanostructured catalysts such as nanodots and nanowires were fabricated with nanolithography techniques. Catalytic selectivity is dominated by several complex factors including the interface between active catalyst phase and oxide support, particle size and surface structure, and selective blocking of surface sites, etc. The advantage of incorporating nanosciences into the studies of catalytic selectivity is the capability of separating these complex factors and studying them one by one in different catalyst systems. The role of oxide–metal interfaces in catalytic reactions was investigated by detection of continuous hot electron flow in catalytic nanodiodes fabricated with shadow mask deposition technique. We found that the generation mechanism of hot electrons detected in Pt/TiO2 nanodiode is closely correlated with the turnover rate under CO oxidation. The correlation suggests the possibility of promoting catalytic selectivity by precisely controlling hot electron flow at the oxide–metal interface. Catalytic activity of 1.7–7.2 nm monodispersed Pt nanoparticles exhibits particle size dependence, demonstrating the enhancement of catalytic selectivity via controlling the size of catalyst. Pt–Au alloys with different Au coverage grown on Pt(111) single crystal surface have different catalytic selectivity for four conversion channels of n-hexane, showing that selective blocking of catalytic sites is an approach to tuning catalytic selectivity. In addition, presence and absence of excess hydrogen lead to different catalytic selectivity for isomerization and dehydrocyclization of n-hexane on Pt(111) single crystal surface, suggesting that modification of reactive intermediates by the presence of coadsorbed hydrogen is one approach to shaping catalytic selectivity. Several challenges such as imaging the mobility of adsorbed molecules during catalytic reactions by high pressure STM and removing polymeric capping agents from metal nanoparticles remain.

A Sum Frequency Generation Vibrational Spectroscopic Study of the Adsorption and Reactions of C6Hydrocarbons at High Pressures on Pt(100)

Sum frequency generation (SFG) vibrational spectroscopy was used to investigate the adsorption geometries and surface reactions of various C6 hydrocarbons (n-hexane, 2-methylpentane, 3-methylpentane, and 1-hexene) on Pt(100). At 300 K and in the presence of excess hydrogen, n-hexane, 3-methylpentane, and 2-methylpentane adsorb molecularly on Pt(100) mostly in “flat-lying” conformations. Upon heating the metal surface to 450 K, the molecules underwent dehydrogenation to form new surface species in “standing-up” conformations, such as hexylidyne and metallacyclic species. 1-Hexene, however, dehydrogenated to form metallocycle Pt3⋮C-(CH2)5-Pt at 300 K in the presence of excess hydrogen and remained unreacted on the surface upon heating the metal surface to 450 K. Dehydrogenation was enhanced in the absence of excess hydrogen in the cases of n-hexane, 2-methylpentane, and 3-methylpentane to form metallocycle Pt3⋮C-(CH2)5-Pt; 2-methyl-1-pentene and 4-methyl-1-pentene; and metallacyclohexane, respectively, at 300 K. These surface species remained unreacted after increasing the surface temperature to 450 K. The mechanisms for catalytic isomerization and dehydrocyclization of n-hexane were discussed on the basis of these results.

The Structures and Reactions of Linear and Cyclic C6Hydrocarbons Adsorbed on the Pt(111) Crystal Surface Studied by Sum Frequency Generation Vibrational Spectroscopy: Pressure, Temperature, and H2Coadsorption Effects

We studied the adsorption structures and reactions of C6 hydrocarbon molecules (cyclohexene, cyclohexane, 1-methylcyclohexene, n-hexane, 2- and 3-methylpentanes, and 1-hexene) on Pt(111) using sum frequency generation (SFG) vibrational spectroscopy. The experiments were performed in the presence and absence of excess hydrogen and as a function of temperature. Upon cyclohexene adsorption on Pt(111) at 1.5 Torr, 1,3- and 1,4-cyclohexadienes and π-allyl C6H9 were observed in the presence of excess hydrogen. Cyclohexane adsorption at 1.5 Torr resulted in the formation of cyclohexyl on the surface in the presence of excess hydrogen but π-allyl C6H9 in absence of excess hydrogen. 1-Methylcyclohexene formed methylcyclohexenyl on the surface only in the presence of excess hydrogen. n-Hexane and 3-methylpentane adsorbed molecularly on Pt(111) at 296 K in the presence of excess hydrogen. 2-Methylpentane and 1-hexene were readily dehydrogenated to form metallacyclobutane and hexylidyne even at 296 K, regardless of the presence of excess hydrogen. n-Hexane was dehydrogenated to form hexylidyne or metallacyclic species at high temperature in the presence of excess hydrogen. Hexylidyne and metallacyclic species were also main surface intermediates in dehydrogenation of 2- and 3-methylpentane. The absence of excess hydrogen induced dehydrocyclization of n-hexane to form π-allyl c-C6H9. On the basis of the SFG results, the mechanism of the n-hexane conversion to benzene is discussed.

Adsorption and reactions of C(6) hydrocarbons at high pressures on Pt(111) single-crystal surfaces studied by sum frequency generation vibrational spectroscopy: mechanisms of isomerization and dehydrocyclization of n-hexane.

The adsorption geometries and surface reactions of various C(6) hydrocarbons (n-hexane, 2-methylpentane, 3-methylpentane, and 1-hexene) adsorbed on Pt(111) were investigated using sum frequency generation (SFG) surface vibrational spectroscopy. The adsorptions and reactions were carried out in 1.5 Torr of C(6) hydrocarbons in the absence and presence of excess hydrogen (15 Torr) and in the temperature range 296-453 K. At 296 K and in the presence of excess hydrogen, n-hexane and 3-methylpentane adsorbed molecularly on Pt(111) mostly in "flat-lying" geometries. Upon heating the sample up to 453 K, the molecules underwent dehydrogenation to form new surface species in "standing-up" geometries, such as hexylidyne and metallacyclic species. However, 2-methylpentane and 1-hexene were dehydrogenated to metallacyclobutane and hexylidyne, respectively, at 296 K in the presence of excess hydrogen. The dehydrogenated species remained unreacted on the surface upon heating the sample up to 453 K. The absence of excess hydrogen enhanced dehydrogenation of n-hexane and 3-methylpentane to form pi-allyl c-C(6)H(9) and metallacyclohexane, respectively, at 296 K. Upon heating to 453 K, the pi-allyl c-C(6)H(9) species underwent irreversible dehydrogenation, while hexylidyne and metallacyclic species remained unreacted. On the basis of these results, the mechanisms for catalytic isomerization and dehydrocyclization of n-hexane, which are the important "reforming" reactions to produce high-octane fuels over platinum, were discussed.

Hydroisomerization, hydrocracking and dehydrocyclization of n-pentane and n-hexane using mono- and bimetallic catalysts promoted with fluorine

The hydroconversion activities and selectivities of 0.35 wt.% Pt/Al 2O 3 promoted by addition of a second metal such as Ir, Rh, Re or U to form adducts (1:1) have been studied for the conversion of n-pentane and n-hexane at different temperatures of 300–500°C, except for Rh catalysts (from 150 to 500°C) in the presence of H 2 atmosphere, 0.049 mol/h. Incorporation of 3.0 wt.% F in the mono- and bimetallic catalysts was also examined. The reaction carried out in a microreactor chromatographic pulse method. The total pressure was 108.5 kPa. The study revealed that three reactions are in competition: hydroisomerization produced isoalkanes, hydrocracking produced methane, ethane and propane and dehydrocyclization produced methylcyclopentane and benzene. The reaction temperature influenced the acidity and selectivities. Over most of the samples hydroisomerization and hydrocracking selectivities changed at ∼400°C. Incorporation of second metal and fluorination improved the activity of Pt/Al 2O 3. The fluorinated PtU/Al 2O 3 showed a significant activity and selectivity. The orders of selectivities of fluorinated bimetallic catalysts are: PtU>PtRe>PtIr>PtRh for hydroisomerization, PtRh>PtU>PtIr>PtRe for hydrocracking and PtRe>PtIr>PtU>PtRh for dehydrocyclization. Rh-containing catalysts are highly selective for hydrogenolysis.

Zirconia/carbon composites as monofunctional catalysts in C 6+ alkane aromatization

High surface area materials composed of microcrystalline zirconia and amorphous carbon have been prepared by a sol–gel method, followed by a high temperature treatment of the resulting xerogel in inert gas. These composites have been characterized and tested in the dehydrocyclization of n-hexane and of n-octane at 550°C. They turned out to be highly selective aromatization catalysts. From n-octane, predominantly ethylbenzene and o-xylene are formed as products, demonstrating a C(1)–C(6) or C(2)–C(7) ring closure of the C 8 chain, respectively. The low rates of side reactions like isomerization or cracking of reactants and products correspond to the low acidity of the new catalysts and show that they are essentially monofunctional. One role of the carbon matrix could consist in the prevention of zirconia sintering, enabling the destruction of acid sites by high treatment temperatures. On the other hand, at present it cannot yet be excluded that the zirconia surface had been modified by carbidization or partial reduction or that the carbon surface of the composites exhibit special properties in contact with the zirconia surface.

Platinum Clusters Supported in Zeolite LTL: Influence of Catalyst Morphology on Performance inn-Hexane Reforming

KLTL zeolite-supported platinum catalysts were synthesized from aqueous tetraammineplatinum(II) nitrate solutions and nonacidic KLTL zeolite crystallites, including some with dimensions as little as 300×500 Å. The zeolite crystallites had various morphologies, some being predominantly disk-shaped particles and some predominantly mosaics of rod-like domains with a range ofc-dimension lengths. The activity and selectivity of each catalyst were evaluated for dehydrocyclization ofn-hexane in the presence of H2to form predominantly benzene at conversions of typically 45–90%. The data presented here provide a detailed characterization of the deactivation of such catalysts in the absence of sulfur. EXAFS data show that the platinum in each catalyst was present in clusters of about 20 atoms each, on average. Electron micrographs show that the platinum clusters were nearly evenly dispersed on the surfaces of the zeolite crystallites, including the intracrystalline and extracrystalline surfaces. The catalytic performance was virtually independent of the zeolite channel length, but activity, selectivity, and resistance to deactivation were found to be correlated with the ratio of the surface area external to the crystallite domains to that within the intracrystalline pores. The catalyst performance is dependent on this ratio (which is related to the zeolite morphology) as follows: in comparison with the others, the catalysts with the relatively low fractions of platinum outside the intracrystalline pores are more active, more selective for benzene formation, and more resistant to deactivation. One well-prepared catalyst, for example, gave greater than 90% selectivity for benzene and no measurable deactivation over 140 h of operation in a flow reactor at 420°C and atmospheric pressure. These data match those characterizing the most selective catalysts reported. Consistent with the interpretation of E. Iglesia and J. E. Baumgartner (in“New Frontiers in Catalysis” (L. Guczi, F. Solymosi, and P. Tetenyi, Eds.), p. 993,Studies in Surface Science and Catalysis,Vol. 75. Elsevier, Amsterdam, 1993), catalyst deactivation is associated with platinum outside the zeolite pores; it is hypothesized that coke formation outside the pores is relatively rapid and that a distinguishing characteristic of the best catalysts is the presence of most of the platinum in the intracrystalline pores, where coke formation may be inhibited by the constraints of the pores.

Acidic property and catalytic behavior of chromium oxide–zirconia catalyst

Chromium oxide–zirconia catalysts were prepared by dry impregnation of powdered Zr(OH) 4 with an aqueous solution of (NH 4) 2CrO 4 followed by calcining in air. Upon the addition of only a small amount of chromium oxide (1 wt.% Cr) to ZrO 2, both the acidity and acid strength of the catalyst increased remarkably, showing the presence of Brönsted and Lewis acid sites on the surface of CrO x /ZrO 2. The catalytic behavior of the sample was investigated through the reaction of n-hexane using a pulse technique. It was found that Cr 6+ species existing on the surface of catalyst was responsible for the formation of strong acid site and the cracking catalytic activity of n-hexane. However, the Cr 6+ species was easily reduced to Cr 3+ species during the catalytic reaction of n-hexane and the reduced Cr 3+ species was active for benzene formation due to the dehydrocyclization of n-hexane. The reduced Cr 3+ species could be reoxidized by treatment with O 2 and subsequently the reoxidized catalyst exhibited catalytic activity for the cracking reaction of n-hexane.

In Situ13C MAS NMR Study ofn-Hexane Conversion on Pt and Pd Supported on Basic Materials: Part I. Identification of the Main Reaction Pathways and Comparison with Microcatalytic Reactor Tests

n-Hexane conversion was studiedin situon Pt and Pd supported on aluminum-stabilized magnesium oxide and Pt on Zeoite KL catalysts (Pt/Mg(Al)O, Pd/Mg(Al)O and Pt/KL) by means of13C MAS NMR spectroscopy.n-Hexane 1-13C was used as a labelled reactant. Forty NMR lines corresponding to 14 different products were resolved and identified. The NMR line assignments were confirmed by adsorption of model compounds. The NMR results were further quantified and compared with continuous flow microreactor tests. Four parallel reaction pathways were identified under flow conditions: isomerization, cracking, dehydrocyclization, and dehydrogenation. Aromatization occurs via two reaction routes: (1)n-hexane dehydrogenation towards hexadienes and hexatrienes, followed by dehydrocyclization; (2) dehydrocyclization ofn-hexane followed by dehydrogenation of a cyclic intermediate. The former reaction pathway is prevented under NMR batch conditions. High pressures induced in the NMR cells at high reaction temperatures (573, 653 K) shift the reaction equilibrium towards hydrogenation. NMR experiments showed that on Pt catalysts aromatization occurs via a cyclohexane intermediate, whereas on Pd it takes place via methylcyclopentane ring enlargement.

Influence of physico-chemical parameters on n-hexane dehydrocyclization over Pt/LTL zeolites

The influence of different physico-chemical parameters such as crystallite size, nature of framework and extra-framework cations of the zeolite LTL and the extent of platinum loading on the catalytic performance of Pt/LTL catalysts in n-hexane dehydrocyclization reaction is reported.

Infrared Spectroscopy of Adsorbed Carbon Monoxide on Platinum/Nonacidic Zeolite Catalysts

Infrared spectroscopy of adsorbed carbon monoxide was investigated on a series of platinum (or palladium) zeolite catalysts plus platinum on silica. The platinum catalysts all showed a strong absorbance band around 2070 ± 20 cm−1 due to linear-bonded CO and a weaker band around 1850 ± 20 cm−1 due to bridge-bonded CO. Four platinum catalysts, Pt/K+ LTL, Pt/K+ MFI, Pt/K+ MTW, and Pt/K+ SiO2, had an additional absorbance near 1970 cm−1. The additional band is not observed for Pt/silica, Pt/La+3 LTL, or platinum supported on H+ LTL or H+ MFI, i.e., neutral and acidic supports. The additional band on the alkalized catalysts is speculated to result from an electrostatic attraction of linear-bonded CO with an alkali cation when Pt is on a nonacidic support. The electrostatic attraction increases the CO adsorption strength and lowers the stretching frequency. The additional linear CO absorption band was also observed on Pd/K+ LTL. The additional band, therefore, is not specific to platinum. Although the presence of this additional infrared band suggests a "new type of metal (catalytic) site," no relation was found between the catalytic activity or selectivity for n-hexane dehydrocyclization and the additional CO absorbance band.

Effect of the reduction temperature on the formation and activity of bimetallic Ni-Tc catalysts

The effect of the temperature on the formation, surface state, and catalytic activity in the reactions of dehydrogenation of cyclohexane and dehydrocyclization of n-hexane by Ni-Tc/γ-Al2O3 mono- and bimetallic catalysts was investigated. TcO2, NiCl2, and metal phases, and at a high temperature (500–700‡C), NiAl2O4 spinel and Ni-Tc clusters, were found on the surface of all of the catalysts. It was shown that the maximum activity is observed in reduction of monometallic catalysts at 500‡C and bimetallic catalysts at 700–800‡C. Synergism appeared in the bimetallic catalysts due to the formation of Ni-Tc clusters.

N-Hexane dehydrocyclization over Pt−Ni supported on alumina catalysts

Pt−Ni/γ-Al2O3 catalysts have been prepared and studied in n-hexane dehydrocyclization. The selectivity for benzene and toluene, a chain lengthening product formation was improved by Ni and correlated with its content.

Mechanism of dehydrocyclization of n-alkanes over platinum—alumina Catalysts

In order to examine whether the metal function or the acid function is “criticalrd for the reforming reaction, the dehydrocyclization of n-hexane, n-heptane and n-octane was carried out over acidic and non acidic platinum—alumina catalysts. It is concluded that the metal function is the “critical” parameter with the C6 and C7 alkanes whereas both the metal and acid could be important with n-octane.

Silylation of porous glass supported platinum catalysts

Effect of silylation with hexamethyldisilazane on adsorption and catalytic properties of porous glass-supported platinum catalysts has been studied. Catalytic activity decreases markedly with an increase in surface coverage by trimethylsilyl groups for all the following reactions examined: hydrogenation of benzene, dehydrogenation of cyclohexane and dehydrocyclization of n-hexane.

Effect of rhenium and chlorine on catalytic activity of L-zeolite Pt-catalysts

Interaction of components of L-zeolite Pt-Re catalyst and their effect on the dehydrocyclization of n-hexane has been examined.

Effect of Germanium and Zirconium on the activity of a platinum catalyst in dehydrocyclization of n-hexane

The addition of zirconium and germanium to an alumino-platinum catalyst (atomic ratio of E/Pt being 1:2) increases its activity and selectivity for dehydrocyclization of n-hexane, apparently due to the part played in the reaction by transition metal clusters containing variable valency platinum.