Hexane Isomers Research Articles

Selective alkane co-inclusion by methylene- and adamantane-bridged macrocycle with biphenyl units

A methylene- and adamantane-bridged macrocycle with biphenyl units showed selective co-inclusion of n-hexane among the hexane isomers through crystallization. In the use of n-pentane or n-heptane, inclusion crystals with alkanes were not formed.

Neutral and cationic methallyl nickel complexes in alkene activation: a combined DFT, ESI-MS and chemometric approach

Herein, we report a comparative study of ethylene activation and 1-hexene isomerization carried out with isomeric neutral and cationic methallyl nickel complexes L1Ni(η3-C3H5) and [L1Ni(η3-C3H5)][B(ArF)4] in the presence of borane co-catalysts.

Peculiarities of palladium-containing MFI-type zeolites as catalysts of isomerization of linear alkanes

Platinum group metals are widely used as a hydrogenating-dehydrogenating component of a number of petroleum refining and petrochemical catalysts, in particular for isomerization of linear alkanes. The main direction in improving the preparation of these catalysts is to reduce their cost by optimizing the metal component amount. However, insufficient attention was paid to the method of introduction of an active metal into the carrier; at the same time, this issue is especially important in case of zeolite catalysts, for which ion exchange can be used in addition to traditional impregnation. Therefore, the purpose of this work was to compare the catalytic efficiencies of Pd-containing MFI zeolites in which metal was introduced by two methods: impregnation from a solution of palladium chloride and ion exchange from tetraamminepalladium(II) chloride in the amount of 0.5 wt.% in terms of pure metal. Study of texture characteristics by nitrogen low-temperature adsorption/desorption technique showed that the specific surface area of samples and the total sorption volume remained practically unchanged, regardless of the procedure of metal component introduction. A significant decrease in average pore size was observed only in case of ion-exchange metal introduction; this indicated the localization of palladium mainly in zeolite pores, which was confirmed by transmission electron microscopy. The stage of transformation from ammonium form to hydrogen one strongly affects the activity of samples, this stage should precede the final stage of metal recovery. The highest yield of hexane isomers of about 46.5 wt.% with the selectivity of 88.7% was observed over a catalyst with Pd introduced by ion-exchange method with the smallest palladium particles (3–7 nm).

Insights into Adsorption of Linear, Monobranched, and Dibranched Alkanes on Pure Silica STW Zeolite as a Promising Material for Their Separation

The adsorption of linear, monobranched and dibranched saturated hydrocarbons in the gasoline range has been studied on pure silica STW zeolite (Si-STW) by using pentane, hexane, and heptane isomers...

Effects of stereoisomeric structure and bond location on the ignition and reaction pathways of hexenes

AbstractThe current work presents new experimental autoignition and speciation data on the two cis‐hexene isomers: cis‐2‐hexene and cis‐3‐hexene. The new data provide insights on the effects of carbon‐carbon double bond location and stereoisomeric structures on ignition delay times and reaction pathways for linear hexene isomers. Experiments were performed using the University of Michigan rapid compression facility to determine ignition delay times from pressure‐time histories. Stoichiometric (ϕ = 1.0) mixtures at dilution levels of inert gas to O2 = 7.5:1 (mole basis) were investigated at an average pressure of 11 atm and temperatures from 809 to 1052 K. Speciation experiments were conducted at T = 900 K for the two cis‐hexene isomers, where fast‐gas sampling and gas chromatography were used to identify and quantify the two cis‐hexene isomers and stable intermediate species. The ignition delay time data showed negligible sensitivity to the location of the carbon‐carbon double bond and the stereoisomeric structure (cis‐trans), and the species data showed no correlation with the stereoisomeric structure, but there was a strong correlation of some of the measured species with the location of the double bond in the hexene isomer. In particular, 2‐hexene showed strong selectivity to propene, acetaldehyde, and 1,3‐butadiene, and 3‐hexene showed selectivity to propanal. Model predictions of ignition delay times were in excellent agreement with the experimental data. There was generally good agreement for the model predictions of the species data for 2‐hexene; however, the mechanism overpredicted some of the small aldehyde (C2‐C4) species for 3‐hexene. Reaction pathway analysis indicates the hexenes are almost exclusively consumed by H‐atom abstraction reactions at the conditions studied (P = 11 atm, T > 900 K), and not by C3‐C4 scission as observed in high‐temperature (>1300 K) hexene ignition studies. Improved estimates for 3‐hexene + OH reactions may improve model predictions for the species measured in this work.

Surface-gel-conversion synthesis of submicron-thick MFI zeolite membranes to expedite shape-selective separation of hexane isomers

Ultrathin zeolite membranes are of paramount importance in accelerating gas transport during membrane separation, and lowering down their membrane thicknesses to submicron scale is deemed to be very challenging. Herein, we develop an advanced approach of surface gel conversion for synthesis of submicron-thick pure silica MFI (silicalite-1) zeolite membranes. Viscous gel is prepared by finely adjusting the precursor composition, enabling its reduced wettability. The unfavorable wetting of the support surface can effectively prevent gel penetration into alumina support voids. Aided by the seeds, the surface gel is directly and fully crystallized into an MFI zeolite membrane with minimal water steam. A membrane with a thickness of 500 nm is successfully acquired and it is free of visible cracks. Additionally, the as-synthesized membranes exhibit rapid and selective separation of hexane isomers by virtue of unprecedentedly high n-hexane permeance of 24.5×10−7 mol m−2 s−1 Pa−1 and impressive separation factors of 13.3-22.6 for n-hexane over its isomers. This developed approach is of practical interest for sustainable synthesis of high-quality zeolite membranes.

Efficient hexane isomers separation in isoreticular bipyrazolate metal-organic frameworks: The role of pore functionalization

Hydrocarbons separation in petrochemical industries is a key, energy-consuming stage in the manufacture of high-quality added-value products—hence the need for more efficient materials and environmentally friendly methodologies to improve this process. In this context, we have studied the effect of metal-organic frameworks (MOFs) pore functionalization in hexane isomers separation, isolating the robust isoreticular zinc(II) bipyrazolates Zn(BPZ), showing no pore decoration, Zn(Me2BPZ), the pores of which are decorated with apolar methyl groups, and Zn(BPZ(NH2)2), the spacers of which possess polar Lewis-basic functions (H2BPZ = 1H,1′H-4,4′-bipyrazole; H2Me2BPZ = 3,3′-dimethyl-1H,1′H-4,4′-bipyrazole; H2BPZ(NH2)2 = 3,5-diamino-1′H,1′H-4,4′-bipyrazole; DMF = dimethylformamide). After characterizing Zn(BPZ(NH2)2) as per its crystal structure and thermal behaviour, and all the three MOFs as per their textural properties, we investigated, from the experimental and computational points of view, the impact of the square one-dimensional channels decoration on the separation of the hexane isomers, demonstrating the relevance of pore constrictions in the resolution of the title alkanes mixture.

Dihydrogen Bond Interaction Induced Separation of Hexane Isomers by Self-Assembled Carborane Metallacycles.

Herein, we describe how to utilize dihydrogen bond interactions to achieve alkane recognition and hexane isomer separation. A series of metallacycles based on carborane backbones are presented herein, revealing interdependent B-Hδ-···Hδ+-C proton-hydride interactions. The metallacycles take advantage of these dihydrogen bond interactions for the separation of hexane isomers. We show that the metallacycle 3a, bearing 1,4-di(4-pyridyl)benzene (DPB), can produce n-hexane with a purity of >99% in a single adsorption-desorption cycle from an equimolar mixture of all five isomers of hexane. The isomers 2-methylpentane and 3-methylpentane can be selectively absorbed by metallacycle 4a, which bears 1,2-di(4-pyridyl)ethylene (DPE). The size of the metallacycle, C-H···π interactions, and particularly B-Hδ-···Hδ+-C interactions are the main forces governing the extent of hexane recognition. This work provides a promising principle for the design of supramolecular coordination complexes (SCCs) for the separation of alkanes.

Separation of hexane isomers by introducing “triangular-like and quadrilateral-like channels” in a bcu-type metal-organic framework

The separation of hexane isomers is of vital importance to produce high quality gasoline in the petrochemical industry. However, the similar vapor pressure and boiling point of hexane isomers bring great difficulties and challenges in the separation process. Sieving effect, which allowing smaller molecules pass through and preventing others, should be a powerful strategy to solve this problem by making good use of porous materials. Therefore, physical separation by metal-organic framework (MOF) materials appears and becomes a burgeoning separation technique in industry. Due to the weak interaction between hexane isomers with absorbents, it puts forward higher requirements for the accurate design of MOF materials with optimal pore system. To address this issue, a novel MOF [Zn9(tba)9(dabco)3]·12DMA·6MeOH (abbreviation: Zn9(tba)9(dabco)3; H2tba = 4-(1H-tetrazol-5-yl)-benzoic acid; dabco = 1,4-diazabicyclo[2.2.2]octane; DMA = N, N-dimethylacetamide) with bcu network has been designed and synthesized by reticular chemistry strategy. Benefiting from the pre-designed topology and suitable linear ligand H2tba and dabco, the structure of Zn9(tba)9(dabco)3 exhibits two types of channels with triangular-like and quadrilateral-like geometry. Zn9(tba)9(dabco)3 with appropriate channel size and shape displays potential selective adsorption capacity of vapor-phase hexane isomers through sieving effect. Moreover, outstanding gas adsorptive separation properties of Zn9(tba)9(dabco)3 could also be speculated by theoretical ideal adsorbed solution theory (IAST), suggesting Zn9(tba)9(dabco)3 can be regarded as a potential adsorbent material for purification natural gas. Breakthrough experiments show that Zn9(tba)9(dabco)3 is capable of discriminating all four hexane isomers at 298 K, and the corresponding research octane number (RON) of the eluted mixture closes to 95, which is higher than the standard for industrially refined hexane blends (about 83). We speculate that sieving effect and diffusion are a synergetic contributory factor in their elution dynamics, which may be ascribed to temperature-dependent interaction between pore aperture and each isomer. This work presents a typical example for design of efficient MOF absorbents by reticular chemistry strategy.

High-Throughput Computational Screening of 12,351 Real Metal–Organic Framework Structures for Separation of Hexane Isomers: A Quest for a Yet Better Adsorbent

Adsorption-based separation of hexane isomers have been investigated widely by researchers across the globe employing variety of adsorbent materials including zeolites and metal–organic frameworks ...

Direct Synthesis of 5A Zeolite From Palygorskite: The Influence of Crystallization Directing Agent on the Separation Performance for Hexane Isomers

AbstractThe efficient separation of hexane isomers from the light naphtha fraction is a significant challenge in the petrochemical industry. 5A zeolite adsorbent is used commercially to sieve alkane isomers. In this study, 5A zeolites were synthesized using a low-cost natural clay mineral precursor, i.e. palygorskite (PAL), with the addition of crystallization directing agent (CDA). By varying the mass ratio of CDA/deionized water, 5A zeolites were obtained as CDA-5%, CDA-7.5%, and CDA-10%. All products were submicron particles with an average particle size of 400–800 nm. A sieving test of CDA-induced 5A zeolites was carried out on hexane adsorbates including n-hexane (nHEX), 2-methylpentane (2MP), and 3-methylpentane (3MP). According to vapor-phase batch adsorption experiments, a significant equilibrium amount (0.149 g/g) of nHEX and only 0.0321 g/g 2MP and 0.0416 g/g 3MP were adsorbed on the 5A zeolite product with CDA-5%. The dynamic adsorption performance of 5A zeolite (CDA-5%) was evaluated by breakthrough curves of binary mixtures of nHEX/2MP and nHEX/3MP. Palygorskite 5A (PAL 5A) zeolite achieved maximum dynamic adsorption capacities of nHEX (0.16 g/g in both cases) at 200°C and 1.2 MPa total pressure. This work provided an economic alternative for the synthesis of 5A zeolites using natural clay minerals instead of chemical raw materials.

Bis(phosphine)hydridorhodacarborane Derivatives of 1,1'-Bis(ortho-carborane) and Their Catalysis of Alkene Isomerization and the Hydrosilylation of Acetophenone.

Deprotonation of [7-(1'-closo-1',2'-C2B10H11)-nido-7,8-C2B9H11]- and reaction with [Rh(PPh3)3Cl] results in isomerization of the metalated cage and the formation of [8-(1'-closo-1',2'-C2B10H11)-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (1). Similarly, deprotonation/metalation of [8'-(7-nido-7,8-C2B9H11)-2'-(p-cymene)-closo-2',1',8'-RuC2B9H10]- and [8'-(7-nido-7,8-C2B9H11)-2'-Cp*-closo-2',1',8'-CoC2B9H10]- affords [8-{8'-2'-(p-cymene)-closo-2',1',8'-RuC2B9H10}-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (2) and [8-(8'-2'-Cp*-closo-2',1',8'-CoC2B9H10)-2-H-2,2-(PPh3)2-closo-2,1,8-RhC2B9H10] (3), respectively, as diastereoisomeric mixtures. The performances of compounds 1-3 as catalysts in the isomerization of 1-hexene and in the hydrosilylation of acetophenone are compared with those of the known single-cage species [3-H-3,3-(PPh3)2-closo-3,1,2-RhC2B9H11] (I) and [2-H-2,2-(PPh3)2-closo-2,1,12-RhC2B9H11] (V), the last two compounds also being the subjects of 103Rh NMR spectroscopic studies, the first such investigations of rhodacarboranes. In alkene isomerization all the 2,1,8- or 2,1,12-RhC2B9 species (1-3, V) outperform the 3,1,2-RhC2B9 compound I, while for hydrosilylation the single-cage compounds I and V are better catalysts than the double-cage species 1-3.

Hexane isomers separation on an isoreticular series of microporous Zr carboxylate metal organic frameworks

The microporous MOF MIL-140B can separate hexane isomers according to the degree of branching, linear >mono-branched >di-branched, with a remarkably high selectivity up to 10 at 343 K. GCMC simulations confirm the origins of the molecular separation.

Preparation and catalytic performance of a novel organometallic CoH/Hβ catalyst forn-hexane isomerization

A novel organometallic catalyst CoH/Hβ was synthesized and directly provided hydrogenation/dehydrogenation effects in the isomerization ofn-hexane.

Reverse shape selectivity of hexane isomer in ligand inserted MOF-74.

Separation of linear, mono-branched, and di-branched isomers is critically important in the petrochemical industry. In this computational study, we demonstrate that the ligand inserted Mg-MOF-74 structure leads to a reverse selectivity effect (i.e. phenomenon that preferentially allows larger species molecules to permeate in a gas mixture) of hexane isomers in the resulting material. Molecular dynamics simulations suggest that strong confinement of the di-branched hydrocarbons in the small pores lead to reverse selectivity. Over a magnitude difference in diffusivity between linear alkanes and their di-branched isomers was observed, clearly showing the steric effects imposed by the pore structure.

Fixed bed dynamics of single and multicomponent adsorption of pentane and hexane isomers in ZIF-8

Single, quinary and septenary fixed bed breakthrough experiments of pentane (nC5/iC5) and hexane (nC6/2MP/3MP/23DMB/22DMB) isomers were performed in shaped zeolitic imidazolate metal organic framework ZIF-8. Adsorption equilibrium isotherms for linear nC5 and nC6 were measured from the single component breakthrough curves covering the temperature range between 373 and 473 K and partial pressures up to 50 kPa. The equimolar septenary (nC5/iC5/nC6/2MP/3MP/23DMB/22DMB) and quinary (iC5/2MP/3MP/23DMB/22DMB) studies were performed at the same temperature range and total hydrocarbon pressure of 25 kPa. In the septenary experiments the following sorption hierarchy was observed: nC6 > nC5 > 2MP > 3MP > iC5 > 23DMB > 22DMB. Results also show that the sorption of the linear nC5 and nC6 is dominated by equilibrium in contrast with the sorption of branched isomers which is kinetically controlled. The quinary experiments with C5/C6 branched paraffins evidences the partially separation by classes of mono- and di-branched isomers. The equilibrium data were modelled by the Sips isotherm and the fitted parameters were used to simulate the breakthrough data through a mathematical model developed in a Matlab code using the method of lines (MOL). In the numerical simulations equal diffusivities constants (Dc/rc2) for linear nC5/nC6 were used, being it 0.002 s−1 at 373 K, which is a value 100 times higher than its respective value for the mono-branched isomers iC5, 2MP and 3MP, and 300 times higher than for the di-branched hexane isomer 22DMB. It is also shown the effect of the residence time of the gas regarding the fixed bed separation of the pentane and hexane isomers, from where it is clear that increasing the residence time improves the separation between the compounds.

Hierarchically pure and M (Cu, Ni)-impregnated ZSM-5 zeolites for the isomerization catalysis of n-hexane and 1-hexene

Conversion of bio-waste into high value catalytic materials is an option in the quest of maintaining sustainable environment. Here, we report an elaborated analysis of the catalytic activity of synthesized hierarchical pure and M (Cu, Ni)-impregnated ZSM-5 zeolite structures from the rice hull ash (RHuA), which is an abundant bio-waste material in most rice producing regions. To achieve the desired characteristics of ZSM-5 framework for metal (i.e., Cu and Ni) impregnation, an organic template-free method synthesis was designed. Further, the catalytic performance of the metal impregnated ZSM-5 in the isomerization of n-hexane and 1-hexene were carried out in a continuous flow stainless steel fixed bed reactor. Three (3) metal impregnation percentages, namely 1, 3 and 5% of Cu/Ni were investigated and compared in respect to the metals. The isomerization of higher alkanes/alkenes is crucial in petroleum industries to increase the octane number of gasoline. Reaction parameters of interest were catalyst amount, reaction temperature, weight hourly space velocity (WHSV), and reaction time were optimized and the obtained results are elaborately discussed. The Ni-ZSM-5 have superior catalytic activity compared Cu-ZSM-5, although the catalytic activity of ZSM-5 zeolite increases with the loading percentage for both Cu and Ni. To explain the metal loading effects on catalytic activity of the ZSM-5, density functional theory calculations were done through adsorption-desorption mechanistic approach. This combined experimental and theoretical investigation strongly validated the reliability of the results.

Adsorption of Alkanes in Zeolites LTA and FAU: Quasi-Equilibrated Thermodesorption Supported by Molecular Simulations

Adsorption of n-alkanes from pentane to decane (C5–C10) in zeolites LTA (ITQ-29 and 5A) and faujasite (FAU, dealuminated high-silica Na-FAU, NaY, and NaX) as well as branched hexane isomers in FAU ...

Ranking of Metal–Organic Frameworks (MOFs) for Separation of Hexane Isomers by Selective Adsorption

The work presented in this article is aimed to assess the performances of 4763 Metal–Organic Frameworks (MOFs) for separation of hexane isomers using computer simulation methods. These MOFs, taken ...

Exploring the Potentials of Metal–Organic Frameworks as Adsorbents and Membranes for Separation of Hexane Isomers

Molecular modelling and computational science tools were employed to select a number of metal–organic frameworks (MOFs) from a pool of 4764 structures to investigate and assess their kinetic and adsorption-based separation performances in separating hexane isomers. The self-diffusivities of all single-component isomers were obtained from molecular dynamics simulations at infinite dilution and at a loading of 4 molecules/unit cell and at several temperatures. The self-diffusivities of the binary mixtures of the hexane isomers were also computed to obtain the kinetic separation metrics. The diffusivities at infinite dilution and at 298 K show a variety of trends as a degree of branching in the chosen MOFs. The linear hexane diffuses faster than other isomers in majority of the MOFs. On the other hand, the MOFs considered here show reverse adsorption selectivity, with the dibranched isomers adsorbing more than the linear one. The diffusivities at infinite dilution, as a function of temperature, of all isomer...