Adsorption Of Cyclohexane Research Articles

Adsorption of cyclohexene and its dehydrogenation intermediates on nAu/Pt(100)( n = 0, 1, 2) surfaces: A DFT study

Abstract Adsorption of cyclohexene and its dehydrogenation intermediates on the nAu/Pt(100) (n = 0, 1, 2 means clean Pt, one monolayer and two layers of Au covered Pt surfaces, respectively.) has been investigated by self-consistent (GGA-PW91) density functional theory combined with periodic slab model. It is found that on the clean platinum, there are two kinds of favorable adsorption sites, i.e., hollow sites and bridge sites, and the adsorption energy at the hollow site is larger than that at the bridge site. However, on the Au/Pt and 2Au/Pt surfaces, there are three kinds of adsorption sites, and the adsorption energies are alike at both the bridge site and the top site. The magnitude order of the adsorption energies is as follows: clean Pt > Au/Pt > 2Au/Pt. The configurations of cyclohexene molecule have been distorted a little during the geometry optimizations. The lengths of C-M (M = Pt or Au, on the top layer of the slab) bonds are closely related to the corresponding adsorption energies.

Photo-catalytic oxidation of cyclohexane over TiO2: a novel interpretation of temperature dependent performance

The rate of cyclohexane photo-catalytic oxidation to cyclohexanone over anatase TiO(2) was studied at temperatures between 23 and 60 °C by in situ ATR-FTIR spectroscopy, and the kinetic parameters were estimated using a microkinetic model. At low temperatures, surface cyclohexanone formation is limited by cyclohexane adsorption due to unfavorable desorption of H(2)O, rather than previously proposed slow desorption of the product cyclohexanone. Up to 50 °C, the activation energy for photocatalytic cyclohexanone formation is zero, while carboxylates are formed with an activation energy of 18.4 ± 3.3 kJ mol(-1). Above 50 °C, significant (thermal) oxidation of cyclohexanone contributes to carboxylate formation. The irreversibly adsorbed carboxylates lead to deactivation of the catalyst, and are most likely the predominant cause of the non-Arrhenius behavior at relatively high reaction temperatures, rather than cyclohexane adsorption limitations. The results imply that elevating the reaction temperature of photocatalytic cyclohexane oxidation reduces selectivity, and is not a means to suppress catalyst deactivation.

Pore-Width-Dependent Preferential Interaction of sp2Carbon Atoms in Cyclohexene with Graphitic Slit Pores by GCMC Simulation

The adsorption of cyclohexene with two sp2and four sp3carbon atoms in graphitic slit pores was studied by performing grand canonical Monte Carlo simulation. The molecular arrangement of the cyclohexene on the graphitic carbon wall depends on the pore width. The distribution peak of the sp2carbon is closer to the pore wall than that of the sp3carbon except for the pore width of 0.7 nm, even though the Lennard-Jones size of the sp2carbon is larger than that of the sp3carbon. Thus, the difference in the interactions of the sp2and sp3carbon atoms of cyclohexene with the carbon pore walls is clearly observed in this study. The preferential interaction of sp2carbon gives rise to a slight tilting of the cyclohexene molecule against the graphitic wall. This is suggestive of a π-π interaction between the sp2carbon in the cyclohexene molecule and graphitic carbon.

Molecular mechanism of hydrocarbons binding to the metal–organic framework

The adsorption and diffusivity of methane, ethane, n-butane, n-hexane and cyclohexane in a metal organic framework (MOF) with the organic linker tetrakis[4-(carboxyphenyl)oxamethyl]methane, the metal salt, Zn2+, and an organic pillar, 4,4′-bipyridin was studied using molecular dynamics simulations. For the n-alkanes, the longer the chain, the lower the free energy of adsorption, which was attributed to a greater number of contacts between the alkane and MOF. Cyclohexane had a slightly higher adsorption free energy than n-hexane. Furthermore, for cyclo- and n-hexane, there were no significant differences in adsorption free energies between systems with low to moderate loadings. The diffusivity of the n-alkanes was found to strongly depend on chain length with slower diffusion for longer chains. Cyclohexane had no effective diffusion, suggesting the selectivity the MOF towards n-hexane over cyclohexane results from kinetics instead of thermodynamics.

Effect of acidity and diffusibility on coke deactivation over nano-sized HZSM-5 zeolites

Four series of nano-sized HZSM-5 zeolite catalysts were prepared with different steaming and HNO3 leaching processes. These samples were studied by XRD, NH3-TPD, FT-IR, n-hexane and cyclohexane adsorption, and fixed-bed aromatization of C4 liquefied petroleum gas (C4 LPG). Results show that, with the steaming temperature increasing, the acidity of catalyst decreased gradually, which was attributed to the removal of Al species from the framework of the zeolite. When steaming is combined with acid leaching, we found that acid leaching-steam catalysts exhibit stronger acidity than steam-acid leaching catalysts. Besides, acid leaching also played an important role in enhancing the diffusibility of catalysts. Catalysts which underwent acid leaching twice exhibit the best microporous diffusibility. Through correlating the acidity and diffusibility of the catalysts with their aromatization performance, it is found that the product distribution of aromatization is mainly determined by catalyst acidity rather than diffusibility. On the other hand, the average coking speed of the catalysts is not only influenced by the acidity of the catalyst, but also by its microporous diffusibility. The improvement of microporous diffusibility remarkably favors suppressing the coking deactivation of nano-sized ZSM-5 zeolite in C4 LPG aromatization.

Methane aromatization in the absence of oxygen over extruded and molded MoO 3/ZSM-5 catalysts: Influences of binder and molding method

The influences of binder and molding method on the catalytic performance of methane aromatization in the absence of O 2 over MoO 3/ZSM-5 catalysts were investigated. SEM, NH 3-TPD, FT-IR of adsorbed pyridine, N 2 adsorption-desorption, cyclohexane adsorption and XPS were employed to characterize the physical and chemical properties of the catalysts. It was found that SiO 2 was a suitable binder for the catalyst due to its appropriate weak acidity. The laminar catalyst comprising of an inert spherical core and a MoO 3/ZSM-5 laminar shell with 0.1-0.2 mm in thickness showed a better catalytic performance than the extruded catalyst. The improved activity of the laminar catalyst could be attributed to the easy carbonization of Mo species and the quick removal of reaction products from the catalyst surface.

Adsorption and Photocatalytic Degradation Kinetics of Gaseous Cyclohexane in an Annular Fluidized Bed Photocatalytic Reactor

The adsorption and photocatalytic degradation kinetics of gaseous cyclohexane using nano-titania agglomerates were investigated in an annular fluidized bed photocatalytic reactor (AFBPR). A series of adsorption and photocatalytic degradation kinetic equations were developed to explore the relationship of adsorption/degradation efficiency and operating variables based on Langmuir adsorption law and photocatalytic elementary reactions. The adsorption equilibrium constant, adsorption active sites, and apparent reaction rate coefficient of cyclohexane were determined by linear regression analysis with variation of gas velocity and relative humidity (RH). It has been demonstrated that the initial concentration, RH, and gas velocity have obviously influenced the adsorption/photocatalytic degradation efficiency and corresponding kinetic parameters. In the adsorption process, the variation of adsorption sites and adsorption efficiency with gas velocity indicated that the adsorption controlling step was related to gas velocity. In the photocatalytic degradation process, the relationship of photocatalytic degradation efficiency and RH indicates that the water molecule played a promotion role in photocatalytic degradation of cyclohexane below a humidity inflection point, while it played an inhibition role in photocatalytic degradation of cyclohexane after this point. In addition, the optimal operating conditions were determined according to the maximum degradation efficiency with respect to RH at 20% and the fluidization number at 1.62.

Adsorption of Harmful Organic Vapors by Flexible Hydrophobic Bis-pyrazolate Based MOFs

Highly porous homoleptic Ni(bpb) and Zn(bpb) materials have been obtained by reaction of nickel(II) and zinc(II) salts with the deprotonated form of the 1,4-(4-bispyrazolyl)benzene ligand (H2bpb). Ab-initio structure solution methods and thermodiffractometry have allowed the determination of their crystal structures, framework flexibility, and thermal stability. The different stereochemical requirements of the Ni(II) and Zn(II) ions induce, in Ni(bpb) and Zn(bpb), rhombic and square channels, respectively, accounting for 57 and 65% of the total cell volume. The two materials feature high adsorption capacities toward small gaseous molecules (N2 and Ar at 77 K, CO2 and CH4 at 273 K), peaking at 22 mmol g−1 of N2 in the case of the zinc(II) derivative, which is reflected by a very large surface area (above 2000 m2 g−1). The flexibility, size, and hydrophobic nature of their channels are adequate also for the incorporation of organic vapors. In this regard, the adsorption of benzene and cyclohexane has been studied under static conditions at 303 K, while that of thiophene has been investigated in dynamic conditions, by measurement, at 298 K, of the breakthrough curves of a flow of CH4/CO2 containing 30 ppm of thiophene. Ni(bpb) and Zn(bpb) are outperforming adsorbents, uptaking up to 0.34 g of thiophene per gram of material. The presence of humidity (60%), which is a major drawback for practical applications of MOFs, does not significantly affect the performance of Ni(bpb) in the removal of thiophene, at variance with Zn(bpb) and HKUST-1, Cu3(btc)2 (btc = benzene-1,3,5-tricarboxylate), which become ineffective in the presence of moisture. Additional XRPD studies have been performed on benzene-loaded Ni(bpb) samples in order to shed some light on the affinity of this material for aromatic guests.

Role of Acidity in the Ethylation of Coking Benzene with Ethanol over Nanosized HZSM-5

Ethylation of coking benzene, the product of coke oven gas, with ethanol over nanosized HZSM-5 zeolites poisoned by 2,6-dimethylpyridine has been investigated in a fixed-bed reactor. Pyridine infrared (Py-IR), amonia temperature programmed desorption (NH3-TPD), thermogravimetry (TG), acidity titration with a Hammett indicator, and cyclohexane and n-hexane adsorption were used to characterize the changes in acidity and porosity of the nanosized HZSM-5 zeolites poisoned by 2,6-dimethylpyridine. Conversion and product distribution depend strongly on the degree of catalyst poisoning. The acidity−activity correlation indicates that acid sites with pKa < −3.0 are responsible for the ethylation of coking benzene and sites with pKa < 2.27 are responsibly for the thiophene conversion. The remaining acid sites, which are not active for the ethylation of coking benzene over the poisoned sample, act as active centers for ethanol dehydration to ethylene. 2,6-Dimethylpyridine not only adsorbs on the strong Bronsted aci...

Adsorption of C6 hydrocarbon rings on mesoporous catalyst supports

The adsorption of cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene and benzene was studied on pristine and shortened multi-wall carbon nanotubes, SBA-15 and a novel high surface area mesoporous carbon (CMH). Data were fitted with the Freundlich adsorption equation and the correlation between the fitted parameters and quantitative structure–activity relationships (QSAR) descriptors of the adsorbates was analyzed. Adsorption on carbon nanotubes is more sensitive to the partial pressure of unsaturated adsorbates, whereas SBA-15 is more sensitive to saturated partners. CMH is a neutral material that appears to be particularly useful for studying catalyst particle efficiency without the influence of the support itself.

Discrimination of the roles of CdSO 4 and ZnSO 4 in liquid phase hydrogenation of benzene to cyclohexene

CdSO 4 and ZnSO 4 as co-modifiers of RuLa/SBA-15 lead to improved catalysts for the partial hydrogenation of benzene to cyclohexene. Based on the experimental results and theoretical calculations, it is shown that CdSO 4 acts as surface modification, suppressing more the adsorption of cyclohexene than that of benzene, while the function of ZnSO 4 is mainly the stabilization of cyclohexene in the liquid phase, accelerating the desorption as well as hindering the re-adsorption of cyclohexene.

Unique surface property of surfactant-assisted mesoporous calcium phosphate

Wormhole-like mesostructured calcium phosphates with different Ca/P molar ratios were prepared using 4-dodecyldiethylenetriamine and N-lauroyl- l-glutamic acid. Structural variations in meso- and micro-scales during calcination of the mesostructured materials were investigated by XRD, TEM and 31P MAS NMR to obtain mesoporous calcium phosphate with high surface area. Mesoporous calcium phosphate (140 m 2 g −1, 0.47 cm 3 g −1 and 7.1 nm) composed of calcium cations and polymorphic P units between ortho- and pyrophosphate species was obtained from the P-rich wormhole-like mesostructured product (Ca/P = 0.95). It is considered that the formation of covalent P−O−P bonds is important for the synthesis of the mesoporous calcium phosphate. A wormhole-like mesostructure of the Ca-rich calcium phosphate (Ca/P = 1.92) collapsed by calcination, because such a polymorphic change did not occur in the calcium phosphate. Adsorption properties of the P-rich mesoporous calcium phosphate were investigated by using water and cyclohexane adsorption measurements, suggesting that the calcium phosphate surfaces were extremely hydrophilic, which has not been observed for surfactant templated mesoporous materials.

Effect of a porous medium on the phase transitions and mobility of cyclohexane molecules

The effect of a porous medium on the phase transitions and molecular mobility of cyclohexane at a liquid content corresponding to a monolayer is studied by pulsed NMR. The times of longitudinal T1 and transverse T2 magnetic relaxation of protons of cyclohexane introduced into granulated porous glasses of the Vycor type with average pore diameters of 4, 11, and 32 nm are measured in the temperature range of 128–293 K. In spite of a relatively low liquid content, two phase transitions are observed for all porous glass samples at temperatures lower than those inherent in pure cyclohexane. At low temperatures, nonfreezing cyclohexane volumes with characteristic times of T2 ∼ 100–200 μs and relative populations of 5–10% remain preserved due to the presence of a small number of micropores commensurable with molecular sizes. The appearance of an additional component with T2 ∼ 200 μs upon temperature elevation to 148 K attests to thawing out of some cyclohexane volumes, which begins long before the crystal-plastic crystal phase transition. The nonexponential character of the transverse magnetization decay of cyclohexane above the temperature of the plastic crystal-liquid phase transition in the porous glass with a pore diameter of 4 nm suggests the existence of barriers for rapid molecular exchange. The obtained experimental results are indicative of the cluster mechanism of cyclohexane adsorption in the studied porous glasses.

Effects of steam and TEOS modification on HZSM-5 zeolite for 2,6-dimethylnaphthalene synthesis by methylation of 2-methylnaphthalene with methanol

Shape-selective methylation of 2-methylnaphthalene (2-MN) over HZSM-5 catalysts modified by steam and tetraethoxysilane (TEOS) treatments was carried out in a fixed-bed reactor. Catalysts were characterized by XRD, NH 3-TPD, BET, FT-IR spectra of pyridine adsorbed, and by adsorption of hexane and cyclohexane. The results show that the TEOS treatment can effectively passivate external active site and narrow pore mouth of catalyst, but it has less effect on the internal surface than steam modification. Hydrothermal treatment can effectively reduce both external and internal acid sites, and adjust channel of catalyst. Steam treatment can effectively increase the ratio of 2,6/2,7-DMN, but the ratio has no increasing trend on TEOS modified catalyst. So adjusting acidity property is more useful to increase the selectivity to 2,6-DMN and the ratio of 2,6/2,7-DMN than narrowing catalyst pore mouth. Tailoring acidic characteristics is more important than spatial control for distinguishing between 2,6-DMN and 2,7-DMN.

Synthesis and Characterization of a Temperature-Responsive Biocompatible Poly(N-vinylcaprolactam) Cryogel: a Step Towards Designing a Novel Cell Scaffold

A poly(N-vinylcaprolactam) (PVCl) cryogel and poly(N-vinylcaprolactam)-co-gelatin interpenetrating cryogel network were synthesized and characterized with respect to physical and biological properties. The PVCl cryogel was synthesized in 5% dimethyl sulfoxide (DMSO) containing aqueous medium and PVCl-co-gelatin interpenetrating cryogel network was synthesized in water as solvent. Both these cryogel networks have good physical morphology as confirmed by scanning electron microscopy. The porosity of these cryogels were characterized by various methods like, adsorption of water and cyclohexane and confirmed by analysis on mercury porosimeter and nitrogen adsorption studies. The porosity of PVCl and PVCl-co-gelatin cryogels was 96% and 98%, respectively, and the permeability of the two types of cryogels was 1.01 × 10−12 m4/Ns and 1.66 × 10−12 m4/Ns, respectively. The effective diffusion coefficients (D eff) of bovine serum albumin (BSA) in PVCl cryogel and PVCl-co-gelatin cryogel were 3.5 × 10−7 cm2/s and 3.4 × 10−7 cm2/s, respectively. These materials were further characterized to demonstrate its interaction with biological system. The blood compatibility studies showed minimal hemolysis (4–6%) caused by these materials and a very low adsorption of BSA (0.001–0.002 mg/g dry scaffold). However, the fetal bovine serum (FBS) adsorption studies demonstrate the protein binding at 37°C. Furthermore, cytotoxicity test and the fibroblast cell adhesion studies showed the potential of these PVCl-based cryogels for suitable biomaterial applications.

Ethylation of coking benzene over nanoscale HZSM-5 zeolites: Effects of hydrothermal treatment, calcination and La2O3 modification

Abstract The objective of this work is to study some post-treatment effects, including hydrothermal treatment, calcination and La2O3 modification, on the catalytic performance of a nanoscale HZSM-5 zeolite for ethylation of coking benzene. The nanoscale HZSM-5 zeolite was treated by hydrothermal treatment, calcination and La2O3 modification in series, and the prepared catalysts were evaluated in a fixed-bed down-flow reactor. The catalyst samples were also characterized by XRD, SEM, NH3-TPD, IR, N2 adsorption–desorption isotherms, cyclohexane adsorption isotherms and TG. The results showed that both the hydrothermal treatment and the calcination led to a drastic decrease in the total amount of acid sites, while the subsequent La2O3 modification resulted in only a slight increase in the number of acid sites. These post-treatments led to a decrease in the Bronsted/Lewis ratio of the nanoscale HZSM-5 zeolites. The hydrothermal treatment followed by the La2O3 modification created new large micropores on the nanoscale HZSM-5 zeolites, which results in the coexistence of micropores and large micropores. The increase in the catalyst lifetime can be attributed to both suppression of carbon deposit formation and partial accommodation of the formed carbon deposit in La-C-HT-HZSM-5 catalyst with lower acidity, lower ratio of B/L and complicated pore structure. The prepared La-C-HT-HZSM-5 catalyst showed good catalytic stability within 1500 h of time on stream in the ethylation of coking benzene containing high sulfur content (375 ppmw) with ethylene under industrial reaction conditions.

Adsorption of cyclohexene on nAu/Pt(1 0 0) ( n = 0, 1, 2): A DFT study

Adsorption of cyclohexene on the nAu/Pt(1 0 0) ( n = 0, clean Pt; n = 1, Pt covered with monolayer of gold (Au/Pt); n = 2, Pt covered with two layers of gold (2Au/Pt), respectively) has been investigated by using self-consistent (GGA-PW91) density functional theory combined with periodic slab model. It is found that on the clean platinum, there are two kinds of favorable adsorption sites, hollow sites and bridge sites, and the adsorption energy in the hollow site is higher than that in the bridge site. Two carbon atoms of the C C moiety are reacted with the same platinum atom through ‘π’ interaction, which forms the chemical bond including the back-donating bond. However, on the Au/Pt and 2Au/Pt surfaces, there are three kinds of adsorption sites, and the adsorption energies are alike in both of the bridge site and the top site. However, the most stable adsorption site is the hollowR45 site on the three surfaces, and the order of the adsorption energies is clean Pt (−1.35 eV) > Au/Pt (−0.63 eV) > 2Au/Pt (−0.59 eV) from the calculations. The configurations of cyclohexene molecule have been distorted a little during the geometry optimizations. The lengths of C–M (M = Pt or Au, in the top layer of the slab) bond are 2.08, 2.77 and 3.07 Å, respectively, which can accord with the difference in adsorption energies. Furthermore, considering the conditions for the formation of the back-donating bond, the d-band centers of clean Pt, Au/Pt and 2Au/Pt are calculated, they are −2.04, −2.73 and −3.04 eV, respectively, which are in line with the order of the adsorption energies.

Energetics of Cyclohexene Adsorption and Reaction on Pt(111) by Low-Temperature Microcalorimetry

The heat of adsorption and sticking probability of cyclohexene on Pt(111) were measured as a function of coverage using single-crystal adsorption calorimetry in the temperature range from 100 to 300 K. At 100 K, cyclohexene adsorbs as intact di-sigma bonded cyclohexene on Pt(111), and the heat of adsorption is well described by a second-order polynomial (130 - 47 theta - 1250 theta(2)) kJ/mol, yielding a standard enthalpy of formation of di-sigma bonded cyclohexene on Pt(111) at low coverages of -135 kJ/mol and a C-Pt sigma bond strength of 205 kJ/mol. At 281 K, cyclohexene dehydrogenates upon adsorption, forming adsorbed 2-cyclohexenyl (c-C6H(9,a)) and adsorbed hydrogen, and the heat of adsorption is well described by another second-order polynomial (174 - 700 theta + 761 theta(2)) kJ/mol. This yields a standard enthalpy of formation of adsorbed 2-cyclohexenyl on Pt(111) at a low coverage of -143 kJ/mol. At coverages below 0.10 ML, the sticking probability of cyclohexene on Pt(111) is close to unity (>0.95), independent of temperature.

Effect of hydrocarbon chain length and cyclization on the adsorption strength of unsaturated hydrocarbons on Pt/3d bimetallic surfaces

Pt-based 3d bimetallic surfaces are known to display higher activities and/or selectivities for various important surface reactions. They exhibit properties that can be entirely different from the parent metal surfaces. Various linear and cyclic unsaturated hydrocarbons were examined over a range of Pt bimetallic surfaces in the present work to help understand the influence of the adsorbate structure and the electronic structure of bimetallic surface on their adsorption properties and reactivity. Density functional theory (DFT) calculations were performed to study the binding of these molecules on the 3d-Pt(1 1 1) surface (over-layer structure) and Pt–3d-Pt (subsurface) structure. The effect of hydrocarbon chain length as well as cyclization was also studied on these surfaces. The DFT results for linear alkenes (1-butene, 1-pentene, 1-hexene) showed that the binding energies of these molecules decreased as the d-band centers moved away from the Fermi level, for 3d as well as 5d surfaces. Cyclohexene adsorption and reactivity were also studied using both theory and experiments. Cyclohexene adsorbed in a boat-type configuration and bonded weakly to the surface as compared to 1-hexene. Benzene adsorption was investigated on the bridge as well as the hollow adsorption sites. Benzene bonded more weakly than 1-hexene and cyclohexene on the subsurface Pt–3d-Pt(1 1 1) bimetallic structures.

Ammonia, cyclohexane, nitrogen and water adsorption capacities of an activated carbon impregnated with increasing amounts of ZnCl 2, and designed to chemisorb gaseous NH 3 from an air stream

The adsorption capacity of ZnCl 2-impregnated activated carbon (AC) for NH 3 is reported in terms of stoichiometric ratio of reaction (NH 3 per ZnCl 2). This ratio depends on the testing conditions used. Compared to the ratio obtained under dry conditions, the ratio is higher under humid conditions or increased NH 3 concentrations. The linear increase of the NH 3 capacity with increasing loading of ZnCl 2 breaks down at about 3.5 mmol ZnCl 2/g AC. This behavior is explained in terms of preferential adsorption of a monolayer of salt followed by aggregation of the impregnant once a monolayer is completed. The effect of increasing the loading of ZnCl 2 on the capacity for gases for which the impregnants are not intended, namely cyclohexane, nitrogen, and water vapor, is also discussed. A break in the linear relationship between water capacity and impregnant loading at about 3.5 mmol ZnCl 2 seems to correspond to a full monolayer coverage of ZnCl 2 on AC. The monolayer of ZnCl 2 is shown to reduce the uptake of water into AC, while the ZnCl 2 aggregates are shown to be hydrophilic.