Dehydrogenation Of N-hexane Research Articles

Cr-MIL-101 derived nano Cr2O3 for highly efficient dehydrogenation of n-hexane

Nano Cr2O3 (n-Cr2O3) was prepared by the thermolysis of the mesoporous Cr-MIL-101, and its catalytic performance for n-hexane dehydrogenation was investigated and compared with Cr2O3 obtained by traditional method. It is found that dehydrogenation of n-hexane on n-Cr2O3 catalyst can produce n-hexenes and benzene efficiently, and the catalytic performance is related to the calcination temperature. The optimal n-hexane conversion can be obtained on n-Cr2O3 calcinated under 600 °C, is 40.6%, and the selectivities to n-hexenes and benzene are 20.1% and 69.3%, respectively. The conversion of n-hexane for n-Cr2O3 catalyst is decreased with calcination temperature increase, while the catalyst stability in dehydrogenation reaction is enhanced. n-Hexane conversion of p-Cr2O3-1 (obtained by precipitation method) and p-Cr2O3-2 (calcinating Cr(NO3)·9H2O directly) catalysts are very low (<7.5%), and their specific activity for n-hexane dehydrogenation are 1.5 and 1.7 g/(m2·h) respectively, lower than that of n-Cr2O3-600 (2.0 g/(m2·h)). The results of BET, XRD, TEM and FT-IR reveal that n-Cr2O3 is the nanoparticles with large specific surface area that more dehydrogenation active sites are exposed, while p-Cr2O3 is the large particles with extremely low surface area that few dehydrogenation active sites are presented. By contrast, industrial Cr2O3/Al2O3 catalyst possesses the highest specific activity of 2.4 g/(m2·h) due to the dispersion effect of Al2O3. Therefore, highly catalytic activity of n-Cr2O3 for n-hexane dehydrogenation is attributed to the unique properties of small particle, large specific surface area and more exposed active sites. This work not only explains the high dehydrogenation activity of nano-Cr2O3 derived by Cr-MIL-101, but also provides guidance for the precise design and synthesis of high-performance CrOx-based catalyst for the dehydrogenation of alkanes.

A site distance dependence of product selectivity in n-hexane dehydrogenation over alumina-supported chromium catalyst

Although CrOx/Al2O3 has been widely applied for light alkanes dehydrogenation, there is insufficient understanding on the dehydrogenation of long-chain alkanes that no solutions to the poor selectivity to mono-olefins and high yield of aromatics. Herein, dehydrogenation of n-hexane was studied on a series of CrOx/γ-Al2O3 catalysts with different densities of Cr sites, and it finds that the dehydrogenation performance is closely related to the distance between Cr sites. With densely populated Cr sites, multiple H atoms of n-hexane are adsorbed on the CrOx/γ-Al2O3 simultaneously (i.e., multi-point adsorption), promoting the formation of n-hexadiene, n-hexatriene and the resultant benzene and/or coke. Therefore, when the Cr loading exceeds the mono-layer coverage (5 wt% Cr loadings), the selectivities to n-hexenes and benzene are kept constant (about 23 wt% and 68 wt%, respectively). While decrease of Cr content in the mono-layer regime leads to a significant decrease in both the distance between Cr sites and the number of adsorbed H atoms, resulting in improved selectivity to n-hexenes. The CrOx@γ-Al2O3 catalyst with 3.6 Cr atoms/nm2 prepared by the equilibrium adsorption method (and the complexation of CrIII and 3,5-dimethylpyridine), exhibits 89 wt% selectivity to n-hexenes (<2 wt% for benzene). This work provides theoretical guidance for the precise design and synthesis of the high-performance CrOx-based catalyst for the dehydrogenation of alkanes, especially for long-chain alkanes.

Pt confined in Sn-ECNU-46 zeolite for efficient alkane dehydrogenation

Highly dispersed and stable Pt-based catalysts play a crucial role in constructing efficient catalytic systems for alkane dehydrogenation. In this study, a novel bimetallic Pt–Sn catalyst confined in extra-large-pore ECNU-46 zeolite (denoted as Pt/Sn-ECNU-46) is prepared by post-treatment. The open-site framework Sn species ((SiO)3Sn–OH) serve as anchors to interact with Pt species, favoring the high dispersion of Pt. On the other hand, the framework Sn species act as the second metal to regulate the geometrical and electronic environment of Pt species, thus suppressing their accumulation. Pt/Sn-ECNU-46 achieves a good performance in propane dehydrogenation reaction with high initial propane conversion (46%) and propylene selectivity (> 99%) as well as regeneration ability. In addition, Pt/Sn-ECNU-46 is also active in the dehydrogenation of n-hexane. This study explores the application of extra-large-pore zeolite as support in constructing metal-confined catalysts for alkane dehydrogenation.

Effect of Residence Time and Carrier Gas on the Dehydrogenation of n-Hexane over Alumina-supported Chromium Catalyst

CrOx/γ-Al2O3 catalyst was prepared by incipient wetness impregnation method, and the influence of residence time and carrier gas on the dehydrogenation of n-hexane were studied. It is found that n-hexane dehydrogenation over CrOx/γ-Al2O3 catalyst with H2 as carrier gas exhibits totally different catalytic behaviors with that of diluting n-hexane with N2. When diluting feed with N2, the selectivity to dehydrogenation product is promoted while the cracking reaction is exhibited with reducing residence time. However, the selectivity to dehydrogenation product is significantly decreased while the isomerization of n-hexane is promoted when using H2 as carrier gas. It is proposed that H2 undergoes dissociation on the dehydrogenation active site, and further facilitates the olefinic dehydrogenation products to form carbocation, which performing isomerization reaction on the acid site on the γ-Al2O3 surface. Therefore, the isomerization of n-hexane is promoted in cost of the selectivity to dehydrogenation product when using H2 as carrier gas.

Effect of chlorine on the performance of Cr-K/γ-Al2O3 catalyst for n-hexane dehydrogenation

Dehydrogenation of n-hexane on Cr-K/γ-Al2O3 catalyst can produce n-hexenes and benzene efficiently, while the catalyst deactivates rapidly. This work discovers that introducing an appropriate amount of Cl (NH4Cl) to the Cr-K/γ-Al2O3 catalyst can significantly improve catalytic activity and enhance stability, which is caused by increased proportion of Cr6+ oxides and number of Lewis acid sites, improved dispersion of CrOx and depressed coke deposition. While excessive Cl further increases the acidity of the catalyst, accelerating the side reactions like cracking and isomerization, meanwhile the inactive Cr-Cl species are formed inevitably. Surprisingly, introducing Cl in the form of KCl can effectively avoid these negative influences owing to the strong interaction between Cl and K. For 13 wt%Cr-2.5 wt%KCl/γ-Al2O3 catalyst, 61.2 wt% of n-hexane conversion, 93.1 wt% of selectivity to dehydrogenation products can be achieved, in which the selectivities to n-hexene and benzene are 20.4 wt% and 70.1 wt%, respectively.

Hexagonal boron nitride for selective oxidative dehydrogenation of n-hexane to olefins

Oxidative dehydrogenation (ODH) of n-hexane to olefins provides an alternative strategy for efficient utilization of n-hexane. Here, we report that hexagonal boron nitride (h-BN) is selective in ODH of n-hexane, achieving 89.6 % selectivity to olefins at n-hexane conversion of 16.3 % and 550 °C. We found that low temperatures and oxygen rich conditions favor the selective hydrogen abstraction from C-H bond over the cleavage of C-C bond, and the presence of gas phase free radical greatly improves the reaction rate of ODH of n-hexane over h-BN. In addition, an anomalous seagull type of relationship exists between the conversion of n-hexane and reaction temperature, which may be related to the re-distribution of the active sites.

Hexagonal Boron Nitride for Selective Oxidative Dehydrogenation of N-Hexane to Olefins

Hexagonal Boron Nitride for Selective Oxidative Dehydrogenation of N-Hexane to Olefins

Hydrodeoxygenation of m-Cresol Over Pt-WOx/C Using H2 Generated In Situ by n-Hexane Dehydrogenation

Hydrodeoxygenation (HDO) of m-cresol using H2 produced by dehydrogenation of n-hexane to hexenes is demonstrated at 673 K over a Pt-WOx/C catalyst at 36 bar. The reaction is possible because deactivation of Pt/C during n-hexane dehydrogenation is greatly suppressed at higher pressures. Flow-reactor measurements of pure n-hexane over Pt/C showed rapid deactivation at 1 bar and 773 K but the catalyst was stable for at least 5 h at 36 bar. While the Pt/C catalyst showed poor activity for HDO of m-cresol, the Pt-WOx/C catalyst was able to utilize the H2 to selectively catalyze m-cresol to toluene, simultaneously pushing the conversion of n-hexane above the equilibrium value that would be achieved in the absence of H2 consumption. To explain the increased stability for alkane dehydrogenation at higher pressures, a microkinetic model of propane dehydrogenation, a surrogate of hexane, was analyzed. The model demonstrates that the coverage of vacancies increases with increasing pressure at the expense of species believed to be coke precursors.

The oxidative aromatization of n-hexane over VMgO catalysts

Vanadium magnesium oxide (VMgO) catalysts with different vanadium loadings were synthesized and tested for catalytic activity in the oxidative dehydrogenation of n-hexane. High surface area catalysts were obtained by the wet impregnation technique. Magnesium oxide (MgO) and magnesium orthovanadate [Mg3(VO4)2] were the only phases observed in each catalyst. All the catalysts were tested at 0.6–7.8 % of n-hexane in air within a temperature range of 350–550 °C, varying the gas hourly space velocities. Catalytic testing conducted below the lower flammability limit (0.6 % n-hexane in air) showed only benzene and carbon oxides in the product stream, whereas for experiments done above the upper flammability limit (7.8 % n-hexane in air), hexenes, in addition to benzene and carbon oxides were observed. Decreasing the feed composition from 7.8 to 5.5 % resulted in higher yields towards benzene and total dehydrogenated products. The effect of variations in the gas hourly space velocity investigated using the 19 wt% vanadium catalyst and a feed concentration of 7.8 % showed that the selectivity to benzene increased slightly with an increase in the gas hourly space velocity.

Introducing Copper as Catalyst for Oxidative Alkane Dehydrogenation

The dehydrogenation of n-hexane and cycloalkanes giving n-hexene and cycloalkenes has been observed in the reaction of such hydrocarbons with hydrogen peroxide, in the presence of copper complexes bearing trispyrazolylborate ligands. This catalytic transformation provides the typical oxidation products (alcohol and ketones) with small amounts of the alkenes, a novel feature in this kind of oxidative processes. Experimental data exclude the participation of hydroxyl radicals derived from Fenton-like reaction mechanisms. DFT studies support a copper-oxo active species, which initiates the reaction by H abstraction. Spin crossover from the triplet to the singlet state, which is required to recover the catalyst, yields the major hydroxylation and minor dehydrogenation products. Further calculations suggested that the superoxo and hydroperoxo species are less reactive than the oxo. A complete mechanistic proposal in agreement with all experimental and computational data is proposed.

On the Mechanism of the Dehydroaromatization of Hexane to Benzene by an Iridium Pincer Catalyst

The developments in the area of transition-metal pincer complexes have opened up new avenues for conversion of saturated hydrocarbons to more useful aromatic compounds under homogeneous reaction conditions. In the backdrop of an interesting series of conversions of unbranched alkanes to benzene, toluene, and xylene (known as the BTX family aromatics) reported by Goldman and co-workers (Nature Chem. 2011, 3, 167), we herein present a comprehensive mechanistic picture obtained by using density functional computations. The reaction involves an iridium-PCP-pincer-catalyzed dehydroaromatization of hexane to benzene (in which PCP=η(3) -C6 H3 (iPrP)2 -1,3) by using tert-butylethylene (TBE) as a sacrificial acceptor. The most energetically preferred pathway for a sequence of dehydrogenations is identified to begin with a terminal CH bond activation of n-hexane leading to the formation of hex-1-ene. Although the initial dehydrogenation of n-hexane was found to be endergonic, the accompanying exoergic hydrogenation of TBE to tert-butylethane (TBA) compensates the energetics to keep the catalytic cycle efficient. Subsequent dehydrogenations provide a hexa-1,3-diene and then a hexa-1,3,5-triene. The pincer bound triene is identified to undergo cyclization to furnish cyclohexadiene. Eventually, dehydrogenation of cyclohexa-1,3-diene offers benzene. In the most preferred pathway, the Gibbs free energy barrier for cyclization leading to the formation of cyclohexa-1,3-diene is found to exhibit the highest barrier (21.7 kcal mol(-1) ).

Catalytic consequences of hydroxyl group location on the kinetics of n-hexane hydroisomerization over acidic zeolites

The measured kinetics of n-C 6H 14 hydroisomerization reactions is consistent with a bifunctional mechanism involving the facile dehydrogenation of n-hexane on the metal catalyst and a kinetically relevant step involving isomerization of n-hexene on zeolitic acidic sites. The measured activation entropy in small 8-MR pockets of MOR (−35 J mol −1 K −1) is similar to that in larger 12-MR channels of MOR ( −37 J mol −1 K −1) and BEA (−33 J mol −1 K −1) but higher than that in medium pore FER (−86 J mol −1 K −1), suggesting that partial confinement of C 6 olefinic reactants results in lower free energy for the isomerization reaction in 8-MR pockets of MOR. The hypothesis that a cyclopropane-like cationic transition state is not completely contained within the 8-MR pockets of MOR is consistent with the observed selectivity to 2-methylpentane and 3-methylpentane in the 8-MR pockets being identical to that measured in larger 12-MR channels of MOR and BEA. The lower activation energy measured in 8-MR pockets compared to larger 12-MR channels of MOR may arise due to greater electrostatic stabilization of the positively charged transition state by framework oxygen atoms located on the pore mouth of the smaller 8-MR pockets of MOR or due to the larger heat of adsorption caused by confinement in smaller 8-MR pockets. The lower activation energy in 8-MR channels and comparable loss in entropy mediated by partial confinement results in the rate per proton in 8-MR pockets being five times larger than the rate in 12-MR channels of MOR. These results provide another conceptual consideration for rigorous and quantitative understanding of local environment effects of zeolite channel size and connectivity on the rate and selectivity of acid-catalyzed reactions.

The Oxidative Dehydrogenation of n-Hexane over a β-NiMoO4 Catalyst

The oxidative dehydrogenation of n-hexane over β-NiMoO4 catalysts was performed in a fixed bed continuous flow reactor. Catalytic testing was done below and above the flammability limits for n-hexane and the effect of reaction conditions was examined to optimize hexene selectivity. The contact times (0.61-2.4 s), n-hexane/oxygen molar ratios and nitrogen dilution (31–60%) were varied. The highest selectivity to total hexenes obtained was 54.7% which was made up of 27.4% 1-hexene, 25.0% trans-2-hexene and 2.3% cis-2-hexene. These selectivities were obtained at a fuel/O2 ratio of 2.2, a contact time of 1.0 s and 43% nitrogen dilution. The oxidative dehydrogenation of n-hexane was investigated over β-NiMoO4. Effects of contact time, dilution and fuel air ratios were investigated. The highest selectivity to the hexenes, mainly 1-hexene, was obtained at a contact time of 1 s at a fuel/air ratio of 2.2 and 43% N2 dilution.

The oxidative dehydrogenation of n-hexane over Ni–Mo–O catalysts

The effect of chemical composition on nickel–molybdenum catalysts in the oxidative dehydrogenation of n-hexane was studied. The catalysts consisted of the following phases: nickel oxide, molybdenum trioxide, nickel molybdate (α-NiMoO 4 and β-NiMoO 4) and a solid solution of nickel in a lattice of nickel molybdate. Characterization of the catalysts was carried out using ICP-OES, BET, XRD, FTIR, SEM, TPR and XPS techniques. The effect of phase composition of the catalysts on their catalytic activity and selectivity to the products obtained in the temperature range 300–500 °C was also studied. The most selective catalyst for the synthesis of hexenes was the pure β-NiMoO 4, which gave 25% selectivity to 1-hexene and 10% selectivity to 2-hexenes and 3-hexenes at 9% conversion. The pure phases NiO and MoO 3, although active in hexane ODH, showed poor selectivity to hexenes, particularly to 1-hexene at comparable conversions suggesting that they are not responsible for the catalytic activity of the NiO–MoO 3 system. The pure β-phase was more selective towards the hexenes than the α-phase with the difference in the molybdenum coordination believed to be the reason for the different activities and selectivities of the two modifications of NiMoO 4. The α-phase with MoO 3 was more active, suggesting that a synergetic effect plays an important role in modifying catalytic activity.

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.

In situ IR investigation of n-hexane isomerization over Pt containing sulfated zirconia

Isomerization of n-hexane over Pt containing sulfated zirconia (Pt/SZ) and SZ was followed by in situ IR measurements. A constant conversion and high selectivity to iso-hexanes were only observed with Pt under hydrogen as carrier gas. In the IR spectra only subtle changes in comparison to the spectrum after activation of Pt/SZ were observed. In the active state of Pt/SZ neither the OH groups nor the sulfates are involved as adsorption sites for n-hexane. Therefore, Pt-proton adducts near sulfate groups and hydride ions connected to Lewis acidic sites are proposed as active sites on Pt/SZ. Over SZ and Pt/SZ using He as carrier gas the isomerization started with an oxidative dehydrogenation of n-hexane followed by a rapid deactivation. The latter reaction is characterized by water formation, by a band near 1670 cm −1 and a broad band in the range of 1540–1580 cm −1 assigned to oxidized carbon species and coke precursors or coke, respectively. The oxidized species are transformed during TOS into coke. Reactivation of the catalysts could only be achieved by oxidation in air.

Study of the role of Zn in aromatization of light alkanes with probe molecules

The role of Zn2+ in ZnZSM-5 catalyst in the process of aromatization of alkanes has been studied with the probe molecules - n-hexane, cyclohexane and 1-hexene. IR measurements showed that the parent HZSM-5 zeolite contains predominantly Bronsted acid sites, while Zn cations in ZnZSM-5 catalyst represent Lewis sites. From the results it follows, that Zn cations in ZnZSM-5 catalyst as Lewis acid sites have strong hydro-dehydrogenation activity and they catalyze the dehydrogenation of n-hexane as well as of cyclic intermediates into the corresponding aromatics. From the results of catalytic tests we conclude that the hydro-dehydrogenation activity of the catalyst plays an essential role in aromatization of light alkanes. The results indicate that for improving the yields of aromatics in aromatization of light alkanes on bifunctional catalysts it is inevitable to increase the participation of the dehydrogenation reactions in the activation of alkane as well as in the formation of the corresponding aromatics from cyclic intermediates.

Silica-supported PtSn alloy doped with Ga, In or, Tl

Abstract Silica-supported trimetallic catalysts containing Pt, Sn and a group 13 metal (PtSnM, M=Ga, In, Tl) were prepared by consecutive impregnation steps from cis-[PtCl2(PPh3)2] and chloride precursors. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected-area electron diffraction (ED) and energy dispersive X-ray analysis (EDX) showed large platelet-like particles of PtSn1−xMx phases. PtSnGa catalyst with a Pt/(Sn+Ga) molar ratio of 1.72 showed a bimodal particle distribution and a Pt phase was identified. Differences in surface structures were also revealed by the performance of catalysts in the dehydrogenation of n-hexane. For PtSnIn and PtSnTl (Pt/(Sn+M) molar ratio of about 1) the dehydrogenation was favoured. In contrast, PtSnGa catalyst yielded hydrogenolysis products. Photoelectron spectra showed the Pt 4f7/2 level at a binding energy of 70.0–71.8 eV in all cases. Moreover, the FT-IR spectra of chemisorbed CO on the PtSnGa showed a slight shift in the ν(CO) toward higher values with respect to the monometallic catalyst, pointing to an electronic effect in accordance with photoelectron spectroscopy.

Aromatization of n-hexane over galloaluminosilicate and gallosilicate

It is shown that proton forms of galloaluminosilicate and gallosilicate exchanged with ammonium nitrate solution have much higher activity for the aromatization of n-hexane than Ga 3+-exchanged H-ZSM-5 or Ga 2O 3-supported H-ZSM-5. This can imply that a large amount of gallium species are uniformly introduced into zeolite crystallite of galloaluminosilicate and gallosilicate, which promote the dehydrogenation of n-hexane to hexene. On the other hand, a proton form of gallosilicate exchanged with hydrochloric acid shows lower activity for the aromatization of n-hexane, but the activity of the proton form of gallosilicate is increased by a small addition of Ga 3+. It is suggested that active gallium species over galloaluminiosilicate and gallosilicate are not gallium species in the framework but those outside the framework.

ChemInform Abstract: Aromatization of n‐Hexane over ZnO/H‐ZSM‐5 Catalysts

The mechanism of transformation of n-hexane into aromatics over ZnO/H-ZSM-5 catalyst has been investigated. The yields of aromatics in the transformation of n-hexane over H-ZSM-5 are enhanced by mechanical mixing of ZnO as well as by ion exchange or impregnation of zinc cation. It is concluded that aromatization of n-hexane over ZnO/H-ZSM-5 is a bifunctional reaction, and that ZnO as well as H-ZSM-5 takes part both in the activation of n-hexane and in the aromatization of lower olefins. By contrasting the conversion of n-hexane with that of 1-hexane, it is found that aromatization of n-hexane over ZnO/H-ZSM-5 involves both the dehydrogenation of n-hexane into hexene and that of the oligomerized products into aromatics. It is proposed that ZnO catalyzes the dehydrogenation of n-hexane into hexene and of the oligomerized products into aromatics.