Adsorption Of Cyclohexane Research Articles

The Effect of Sulphate Doping on NanosizedTiO2andMoOx/TiO2Catalysts in Cyclohexane Photooxidative Dehydrogenation

The effect of sulphate doping of titania in promoting activity and selectivity ofMoOx/TiO2catalysts for the cyclohexane photooxidative dehydrogenation has been investigated in a gas-solid fluidized bed reactor. Sulphate and/or molybdate-modified titania catalysts were prepared by incipient wet impregnation of nanosized (5–10 nm crystallite size) samples. At 60% of titania surface coverage byMoOx, sulphate surface density was obtained up to 19μmol/m2without formation ofMoO3. The catalysts were characterized byN2adsorption-desorption at−196∘C, micro-Raman and UV-visible reflectance spectroscopy, thermogravimetric analysis coupled with mass spectroscopy (TG-MS), and mass titration. Unsulphated and sulphated titania are both active in cyclohexane total oxidation, but sulphate doping of titania has a detrimental effect on the reaction rate. On Mo-based catalysts, polymolybdate species enabled sulphated titania to convert cyclohexane to benzene (99% selectivity) and cyclohexene, reducing at zero the formation ofCO2. Cyclohexane conversion to benzene is almost linearly dependent on sulphate surface density, resulting in enhanced yield to benzene. The enhanced photooxidative dehydrogenation activity and benzene yield by sulphate doping could be attributed to the increase of surface acidity and, as a consequence, of cyclohexane adsorption.

New Insights in Adsorption and Dehydrogenation of Cyclohexene on Pt(111) and Ordered Pt−Sn Surface Alloys: Experiment and Theory

The adsorption of cyclohexene (C6H10) on Pt(111) and two ordered PtnSn/Pt(111) surface alloys has been investigated experimentally using high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and temperature programmed desorption (TPD) as well as theoretically by ab initio density functional theory (DFT) calculations. On Pt(111) and Pt3Sn/Pt(111) a di-σ-bonding of cyclohexene has been found both experimentally and in the DFT calculations. In contrast, on the Pt2Sn/Pt(111) a mixture of weakly bound species (di-σ, π, and physisorbed) has been found. Whereas on Pt(111) a considerable fraction of the cyclohexene dehydrogenates during thermal treatment, the adsorption is completely reversible on the two surface alloys. A detailed theoretical study of the structure and vibrational spectra of the dehydrogenated species has been performed on Pt(111). This allowed the interpretation of the variations in the HREELS spectra when the flash temperature increases by the formation of some dehydrogenated products.

Characterization and catalytic alkylation of hydrothermally dealuminated nanoscale ZSM-5 zeolite catalyst

The acidity and pore structure of nanoscale ZSM-5 zeolite catalysts with both alumina and rare earth as binders and treated by pure steam were studied by various techniques, including X-ray diffraction (XRD), Ammonia Temperature Programmed Desorption (NH 3 -TPD), Fourier-transform infrared spectroscopy (FT-IR) of adsorbed pyridine, modified Hammett indicator, 27 Al MAS NMR spectroscopy, low temperature N 2 adsorption and adsorption of hexane and cyclohexane. The catalytic performance of the catalysts was investigated using alkylation of toluene with methanol and ethylbenzene with ethanol as probe reactions. The results indicated that the nanoscale ZSM-5 catalysts treated at temperatures up to 800 °C maintained good crystallinity. Both the amount of the tetrahedrally coordinated framework aluminum and that of the octahedrally coordinated nonframework aluminum decreased; the average pore diameter increased; and the adsorption capacities for normal hexane and cyclohexane declined on the dealuminated nanoscale ZSM-5 catalyst samples. For the catalyst treated at 700 °C, not only the catalytic activity for the alkylation of toluene with methanol was improved, but was the selectivity of para -xylene, although the total acid amount decreased significantly and strong acid sites almost disappeared. The alkylation activity was very low when the hydrothermal treatment temperature was 800 °C. Correspondingly, the activity of the catalysts for the alkylation of ethylbenzene with ethanol decreased with elevated treatment temperatures.

Adsorption of benzene and cyclohexane on γ-Al2O3 and Pd/γ-Al2O3 at elevated temperature and pressure

Adsorption of benzene and cyclohexane on catalyst supports γ-Al 2 O 3 and catalyst pellets Pd/γ-Al 2 O 3 was studied from 393 to 453 K over an adsorbate relative pressure range of 0.2–1.0. The corresponding adsorption isotherms, which were experimentally measured in the range of the relative pressure p / p 0 larger than 0.2, were considered to be type IV, and can be simulated by a simple adsorption model. The adsorption isotherm of benzene was similar to that of cyclohexane. Compared with the adsorption isotherm of benzene on γ-Al 2 O 3 , a similar one was obtained on Pd/γ-Al 2 O 3 except that the hysteresis loop appeared a little bit earlier. As far as adsorption of benzene on γ-Al 2 O 3 was concerned, only physisorption was involved. But when it came to Pd/γ-Al 2 O 3 , both chemisorption and physisorption occurred, and the chemisorption process facilitated the formation of monolayer, multilayer and capillary condensation in spite of the lower metal loading of the catalyst.

Sensitivity to guest–host force fields in adsorption equilibrium of cyclic hydrocarbons in one-dimensional molecular sieve

We have investigated the effect of different guest–host force fields (united atom, UA; all-atom, AA; and anisotropic united atom, AUA) on the adsorption of cyclic hydrocarbon molecules in AlPO4-5 molecular sieve. Grand canonical Monte Carlo (GCMC) simulations of benzene, xylene and cyclohexane adsorption and positioning are carried out. The results suggest that the force field choice affects considerably the positioning of benzene and cyclohexane molecules while xylenes presented a smaller sensibility. The force fields difference of sensibilities among cyclical molecules seems to be associated with the critical geometric condition identified by Yashonath and Santikary denominated levitation effect. In this condition, the net forces on the guest due to the host is at a minimum, the levitation ratio (γ) is close to unity and we have a weakly bound guest that can be easily moved from its balance position. Unlike xylene, that is only slightly affected by the choice of the force field, benzene and cyclohexane molecules in AlPO4-5 have such dimensions that result in levitation ratio (γ) close to unity. These results point out to the care that should be taken in the choice of the force field when the guest–host size ratio is near the value defined for the levitation effect.

Adsorption, Diffusion and Thermal Desorption Features of Cyclopentane and Cyclohexane in Silicalite‐1

AbstractAdsorption, diffusion and thermal desorption features of cyclopentane and cyclohexane in silicalite‐1 have been investigated using the intelligent gravimetric technique. Both the saturation adsorption loadings and diffusion coefficient of cyclopentane were greater than those of cyclohexane. The diffusivity of cyclopentane was about one order of magnitude greater than that of cyclohexane at the same temperature and initial loading. For cyclopentane, there was only one kind of desorption process at adsorption loadings lower than 4 muc (molecule per unit cell), but two desorption processes appeared at the adsorption loadings higher than 4 muc. While for cyclohexane, one desorption process was found in the whole range of loadings. Both thermal desorption peaks of cyclopentane and cyclohexane moved to higher temperature region with increasing loading.

Adsorption of cyclohexadiene, cyclohexene and cyclohexane on Pt(1 1 1)

The adsorption of 1,3-cyclohexadiene, 1,4-cyclohexadiene, cyclohexene and cyclohexane on Pt(1 1 1) was studied using ab initio density functional theory. For 1,3-cyclohexadiene three adsorption modes were distinguished: bridge 1,2-di-σ/3,4-π, hollow 1,4-di-σ/2,3-π and bridge 1,4-di-σ/2,3-π with adsorption energies of −155, −147 and −75 kJ/mol, respectively. Three stable adsorption modes were also identified for 1,4-cyclohexadiene: bridge quadra-σ, hollow di-σ/π and bridge di-π with adsorption energies of −146 kJ/mol, −142 kJ/mol and −88 kJ/mol, respectively. Cyclohexene was found to adsorb in six modes: 4 di-σ and 2 π-adsorption modes. The preferred configuration was found to be boat di-σ with an adsorption energy of −81 kJ/mol. The three other di-σ adsorption modes have comparable adsorption energies, ranging from −64 to −69 kJ/mol. Molecular strain and C Pt bonding energies are used to elucidate stability trends. Cyclohexane is found to adsorb only at the hollow site whereby the axial hydrogen atoms are positioned over surface Pt-atoms with an adsorption energy of −37 kJ/mol. The calculations correctly predict the weakening of the axial C H bonds and provide a possible explanation for the large shift in the vibrational frequencies.

Time-Resolved Sum-Frequency Generation Spectroscopy of Cyclohexane Adsorbed on Ni(111) under Ultrashort NIR Laser Pulse Irradiation

The adsorption of cyclohexane on Ni(111) was studied by infrared-visible sum-frequency generation (SFG) spectroscopy with and without near-infrared (NIR) pump pulse irradiation. Two adsorption states of cyclohexane were found in the monolayer region, a low-coverage state showing SFG peaks at 2740, 2815, and 2865 cm(-1), and a high-coverage state showing peaks at 2740, 2815, and 2905 cm(-1). Both states coexisted on the saturated Ni(111) surface. The broad peak at 2740 cm(-1) was due to the softened CH stretching mode of the axial CH groups of cyclohexane that point toward the Ni(111) surface. The peaks at 2815 and 2865 (or 2905) cm(-1) were due to the symmetric and asymmetric stretching modes of CH(2) groups, respectively, that were free from the surface. Irradiation with NIR pulses caused a temporary jump in temperature at the Ni(111) surface and enhanced the intensity of the 2905 cm(-1) peak, but weakened the other peaks. This indicates that the temperature jump excited the cyclohexane molecules from the low-coverage state to the high-coverage state. The dynamics of the structural change observed in the adsorbed cyclohexane on NIR irradiation is discussed.

Deactivation of zeolite catalysts for benzene oxidation to phenol

Abstract Coking and deactivating experiments were carried out in a fixed-bed reactor for benzene oxidation to phenol (BTOP). The catalyst used in this study was HZSM-5 (SiO 2 /Al 2 O 3 = 20) treated by high temperature steam at 700 °C. Then the catalysts undergoing different reaction time were analyzed and characterized. The amount of coke was measured by TG. The coke components were detected by infrared. At the same time, the main physics properties of the catalyst with coke deposition were characterized. The pores of the catalyst before and after coking were measured by N 2 adsorption at −196 °C. The amount of hexane and cyclohexane adsorption was measured by weight continuous flow. The acid sites of Bronsted and Lewis were characterized by pyridine adsorption-infrared measurements. Our results show that the deactivation of ZSM-5 in the BTOP reaction system is caused by the coke deposition in the mouth of pores and blocking the pores of the catalyst. It can also be found that the present process agrees with most coking processes over ZSM-5 caused by organic compounds, the Bronsted acid sites are the active sites of the coking.

Characterization and microporosity analysis of mesoporous carbon molecular sieves by nitrogen and organics adsorption

Novel mesoporous carbon molecular sieves have been prepared from SBA-15 materials as templates at different temperatures. This allows the synthesis of hexagonally arranged mesoporous carbons with pore diameters between 3.0 nm (CMK-3-100) and 6.5 nm (CMK-3-150). CMK-1 and CMK-2 have been synthesized using MCM-48 and SBA-1, respectively, as templates for comparison with the above materials. The structural order and textural properties of all the materials were studied by X-ray diffraction (XRD) and nitrogen adsorption. The microporosity analysis of different mesoporous carbons such as CMK-1, CMK-2, CMK-3, CMK-3-130 and CMK-3-150 has been studied by n-heptane and cyclohexane adsorption. It has been found that CMK-2 has a large amount of micropores (upt to 50% of the total pore volume), while the other materials studied possess smaller amounts of micropores. Moreover, it was found that the mechanical stability of CMK-3-100 is high compared to mesoporous SBA-15 silica materials.

Molecular simulations of the adsorption of cycloalkanes in MFI-type silica

A new force field for the simulation of the adsorption of cycloalkanes in nanoporous silica affords a significant improvement over any previously employed force field. The simulated isotherms reproduce the most salient features in the experimental isotherms extremely well. The study of cyclo-pentane, -hexane, and -heptane adsorption in MFI-type silica indicates an inflection for cyclopentane but not for cyclohexane at intermediate pressure. If corroborated by experiments, such an inflection point would afford an excellent calibration point for further force field developments. At low pressures, mixture isotherms of cyclohexane and n-hexane show a temperature dependence on the selectivity in accordance with recent results by J. P. Fox and S. P. Bates, J. Phys. Chem., 2004, 108, 17136. This dependence is caused by a difference in temperature dependence of the Henry coefficient for both molecules. At high pressures entropy effects due to packing always favor the sorption of n-hexane. Furthermore, the influence of the flexibility of the zeolite framework on the adsorption of these rather bulky molecules is investigated. It is found that this influence of the flexibility on the adsorption of cyclohexane is as small as with n-alkanes.

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.

Quantitative Characterization of Hydrophilic−Hydrophobic Properties of MWNTs Surfaces

We report the first direct measurement of surface hydrophilicity−hydrophobicity in multiwall carbon nanotubes (MWNTs) by analyzing the binary solvent adsorption excess isotherms of freshly oxidized as well as thermally annealed MWNT samples. Heat-induced changes in the sample morphology are monitored by XRD and N2 adsorption isotherms, the hydrophilic−hydrophobic surface ratio is calculated from ethanol−cyclohexane adsorption excess isotherms. The surface fractal dimension and the dimension of the capillary condensation are also determined. Results obtained from these independent methods correlate well with each other. The character of the MWNT surface gradually turns from hydrophilic (no treatment) into hydrophobic (12 h in N2 at 1673 K) upon heating. The suggested method appears to be a feasible, reliable, fast, and economic technique of characterizing MWNT surface properties.

Pore Size Engineering and Mechanical Stability of the Cubic Mesoporous Molecular Sieve SBA-1

The influence of HCl/surfactant ratio (nHCl/nS) and synthesis time on the synthesis of the cubic mesoporous molecular sieve SBA-1 were investigated to obtain high-quality materials. The samples prepared at a nHCl/nS ratio of 280 were found to exhibit a higher degree of structural ordering, a higher specific surface area, and a higher specific pore volume as compared to materials prepared at a lower nHCl/nS ratio. The pore diameter of SBA-1 can be tuned from 2.3 to 3.0 nm by simply adjusting the synthesis time between 1 and 72 h. The pore volume increases from 0.7 to 1.03 cm3/g with a concomitant decrease of the surface area from 1430 to 1100 m2/g. Moreover, the stability of the materials toward washing with water was improved by increasing the crystallization time from 1 to 72 h. The mechanical stability of SBA-1 was also investigated using n-heptane and cyclohexane adsorption in addition to nitrogen adsorption. The mechanical stability of SBA-1 is high; by compression of about 217 MPa, the specific pore volume calculated from nitrogen adsorption decreases by 19.7%, whereas the pore volume calculated from the n-heptane and cyclohexane adsorption decreases by 12.5% and 7.5%, respectively. These results together with the large difference between the pore volumes obtained by nitrogen and organics adsorption indicate that the presence of microporosity in the SBA-1 pore walls has to be considered. SBA-1 is mechanically more stable as compared with hexagonal mesoporous materials such as MCM-41 and SBA-15, but exhibits similar mechanical stability as compared to the cubic MCM-48 material.

Characterization and adsorption properties of powders prepared from cactus pulp

Two cactaceous powders, labelled CACMM1 and CACMM2, are shown to contain calcium oxalate and to consist of globular or cubosome particles. The samples were characterized by X-ray diffractometry, spectroscopic, thermic, nitrogen and water adsorption methods and compared with other organic compounds. The pore systems have been analysed from adsorption isotherms, t-plots and pore size distribution curves. The bioadsorbents CACMM1 and CACMM2 exhibited low specific surface area values and presented pores in the mesopore range. Composite adsorption isotherms for the binary mixtures benzene-hexane and benzene-cyclohexane on cactaceous powders have been established and revealed preferential adsorption of hexane and cyclohexane in the respective system. Also the adsorption behavior of dyes from aqueous solution was studied to complete the diversity of adsorbate molecules. Linear adsorption isotherms were obtained up to 6 mM dye equilibrium concentration.

Surface heterogeneity of carbon–silica adsorbents studied on the basis of the complex adsorption investigations

Silica-gel (SG) Si-60 was coated by pyrolytic coke using CVD from methylene chloride. The porous structure of carbon–silica adsorbents (carbosils) was analyzed on the basis of nitrogen adsorption isotherms at 77 K. It has been shown that carbosils are mesoporous adsorbents with complete absence of micropores. Total pore volume and surface area decrease during the carbon deposition on silica surface. Heterogeneity of surface of carbosil samples was estimated by adsorption energy distribution (AEDs) calculated from adsorption isotherms of nitrogen at 77 K and hexane, cyclohexane, benzene and chloroform adsorption isotherms at 473 and 483 K. AEDs were obtained by solving Adsorption Integral Equation with Hill-de Boer kernel function. N2 AEDs show that distribution of silica active surface sites responsible for N2 adsorption at 77 K is not altered during the carbon deposition. However, coke deposition leads to improving of polar–apolar surface composition, resulting in increase of adsorption of apolar (hexane) and decrease of adsorption of polar (chlorophorm) substances. The main changes observed in AEDs are connected with decreasing of silica accessible area at stable carbon structure during CVD of carbon.

The Adsorption of Cyclopropane and Cyclohexane on Cu(111): An Experimental and Theoretical Investigation on the Nature of the CH−Metal Interaction

On contact with transition metals, saturated hydrocarbons reveal pronounced red shifts in the C−H stretching region in their IR spectra (shaded region in picture), which are discussed in connection with C−H activation. Detailed ab initio calculations for cyclopropane and cyclohexane on a Cu surface suggest that this anomaly is caused by an unexpected chemical interaction.

Ni/Pt(111) bimetallic surfaces: unique chemistry at monolayer ni coverage.

We have utilized the dehydrogenation and hydrogenation of cyclohexene as probe reactions to compare the chemical reactivity of Ni overlayers that are grown epitaxially on a Pt(111) surface. The reaction pathways of cyclohexene were investigated using temperature-programmed desorption, high-resolution electron energy loss (HREELS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Our results provide conclusive spectroscopic evidence that the adsorption and subsequent reactions of cyclohexene are unique on the monolayer Ni surface as compared to those on the clean Pt(111) surface or the thick Ni(111) film. HREELS and NEXAFS studies show that cyclohexene is weakly pi-bonded on monolayer Ni/Pt(111) but di-sigma-bonded to Pt(111) and Ni(111). In addition, a new hydrogenation pathway is detected on the monolayer Ni surface at temperatures as low as 245 K. By exposing the monolayer Ni/Pt(111) surface to D2 prior to the adsorption of cyclohexene, the total yield of the normal and deuterated cyclohexanes increases by approximately 5-fold. Furthermore, the reaction pathway for the complete decomposition of cyclohexene to atomic carbon and hydrogen, which has a selectivity of 69% on the thick Ni(111) film, is nearly negligible (<2%) on the monolayer Ni surface. Overall, the unique chemistry of the monolayer Ni/Pt(111) surface can be explained by the weaker interaction between adsorbates and the monolayer Ni film. These results also point out the possibility of manipulating the chemical properties of metals by controlling the overlayer thickness.

Probing the reactivity of C 6-hydrocarbons on Au surfaces: cyclohexane, cyclohexyl and cyclohexene on Au( [formula omitted

Au is often added to reduce the activity of Group-VIII transition metals like Ni, Pd and Pt in dehydrogenation catalysts. In order to improve understanding of the fundamental chemistry of hydrocarbons at Au sites, further studies of stable hydrocarbon molecules and reactive intermediates on Au surfaces are needed. The adsorption and thermal chemistry of cyclohexane (C 6H 12), cyclohexyl groups (C 6H 11) produced by electron-induced dissociation (EID) of cyclohexane, and cyclohexene (C 6H 10) on Au(1 1 1) was studied by using primarily temperature programmed desorption (TPD) and Fourier-Transform Infrared Reflection-Absorption spectroscopy (IRAS). Both cyclohexane and cyclohexene are reversibly adsorbed on Au(1 1 1) surfaces, with desorption peak temperatures of 198 and 213 K from the adsorbed monolayer, respectively, and 143 K from both multilayer films. EID with an incident electron energy of 30 eV on a monolayer film of cyclohexane selectively breaks one C–H bond in weakly bound cyclohexane molecules to form adsorbed C 6H 11 groups on the Au(1 1 1) surface, as established by TPD and IRAS. The Au(1 1 1) surface can readily and selectively dehydrogenate adsorbed cyclohexyl groups to produce cyclohexene at or below 216 K. Cyclohexyl groups also undergo disproportionation reactions at 273 K to produce cyclohexane and cyclohexene. We suggest that surface Au atoms may play a more important, direct role in the hydrogenation/dehydrogenation selectivity found for transition metal–Au alloys than is commonly discussed.

Mechanical Stability of Mesoporous Molecular Sieves SBA-15 and MCM-48 Studied by Adsorption of Benzene, Cyclohexane and n-Heptane

Mechanical Stability of Mesoporous Molecular Sieves SBA-15 and MCM-48 Studied by Adsorption of Benzene, Cyclohexane and n-Heptane