Adsorption Of Neopentane Research Articles

Adsorption of toluene, methylcyclohexane and neopentane on silica MCM-41

Adsorption-desorption isotherms of toluene, methylcyclohexane and neopentane were determined on a silica MCM-41 material of pore diameter ∼3.4 nm over the temperature range 258 K to 308 K (278 K for neopentane). The isosteric enthalpies of adsorption were determined from the isotherms at the various temperatures. It was found that the isotherms of toluene and methylcyclohexane have a similar variation with the temperature, exhibiting hysteresis at 268 K and at lower temperature, while the adsorption of neopentane is reversible at all temperatures. The three organic adsorptives interact differently with the silica surface and the isosteric enthalpies of adsorption indicated that methylcyclohexane has the weakest interaction and toluene the strongest. A slight increase in the adsorption enthalpy at the beginning of the capillary condensation step is observed with methylcyclohexane and neopentane but not with toluene.

In situ X-ray absorption spectroscopy as a unique tool for obtaining information on hydrogen binding sites and electronic structure of supported Pt catalysts: towards an understanding of the compensation relation in alkane hydrogenolysis

L2 and L3 X-ray absorption near edge spectra (XANES) on supported Pt particles, with and without chemisorbed hydrogen, are shown to reflect the type of hydrogen-binding site on the Pt surface. FEFF8 ab initio multiple scattering calculations are used to determine XANES spectral fingerprints for the atop vs threefold H binding sites on Pt. Comparison of the experimental XANES data with the theoretical fingerprints, and further theoretical results, show that the acid/base properties of the support have a profound influence on the hydrogen coverage, and therefore on the mode of hydrogen adsorption on the Pt surface. As the electron richness of the support oxygen atoms increases (i.e., with increasing alkalinity of the support), the H coverage increases and the hydrogen-binding site of the strongly adsorbed hydrogen changes from atop to threefold. This site change is primarily responsible for the observed changes in previously reported kinetic data, which show an increase in negative order (roughly from −1.5 to −2.5) in hydrogen partial pressure for neopentane hydrogenolysis with increasing support alkalinity. This change in negative order directly reflects the greater number of vacant Pt sites that must be available to allow adsorption of the neopentane. A compensation relation is found in the kinetic data of Pt on different supports resulting directly from this change in hydrogen coverage. This implies that the experimentally determined kinetic parameters are apparent values. These apparent values are correlated to the intrinsic kinetic parameters via the thermodynamic properties of the sorption of the reactants, described by the Temkin equation. The TOF of neopentane hydrogenolysis over several catalysts, measured in previous work, decreases with the increasing alkalinity of the support. This can now be directly explained as the result of the change in hydrogen coverage using a Frumkin isotherm, implying that the neopentane adsorption becomes weaker with increased hydrogen coverage. These conclusions, that hydrogen drives the catalysis, are further supported by density functional calculations on small Pt4 clusters, which show that the acid/base properties of the support have a much larger direct influence on PtH bonding than on PtCHn bonding.

Structure of hybrid (organic/inorganic) TiO 2–SiO 2 xerogels as studied by neopentane adsorption at 273 K and Ti K-edge X-ray absorption spectroscopy

TiO 2–SiO 2 hybrid alcogels prepared from two different `organic silicon' precursors, namely Me 2Si(OMe) 2 and (Me 2SiO) 3, were characterized by adsorption of neopentane at 273 K and X-ray absorption spectroscopy (XAS) at the Ti K-edge. Simultaneous fitting of the macro- and mesopore (BET), and micropore (Dubinin–Radushkevitch, DR) neopentane adsorption uptakes at 273 K provides a picture of the near-ambient pore structure of these flexible materials. Both X-ray absorption near edge structures (XANES) and EXAFS indicate that micro-segregation into small TiO x particles takes place during acid hydrolysis in the material prepared from the cyclic (Me 2Si–O) 3 precursor, but a higher Ti dispersion is observed in TiO 2–SiO 2 hybrid glasses prepared from Me 2Si(OMe) 2. Spectral features indicative of pentacoordinated Ti dominate the pre-edge XAS structure of the TiO 2–SiO 2 hybrid glass prepared from Me 2Si(OMe) 2.

Adsorption of saturated hydrocarbons on the Si(111)-7 × 7 surface studied by photoelectron and photon stimulated desorption spectroscopies

Photoelectron spectroscopy (PES) and photon stimulated desorption (PSD) experiments were carried out to follow the thermal chemistry of methane- (CH 4), neopentane- (C 5H 12), and adamantane- (C 10H 16)-dosed-Si(111)-7 × 7 surfaces. Both methane and adamantane adsorb molecularly on the 7 × 7 reconstructed Si surface at temperatures of 30 and 85 K, respectively. In contrast, at low coverages (<7 × 10 15 molecules/cm 2, 85 K; 0.4 ML), a fraction of the neopentane adlayer adsorbs dissociatively; at higher coverages, neopentane adsorption is predominantly molecular. Conversely, the adamantane and neopentane adlayers desorb at temperatures of ∼200 and 115 K, respectively, for heating rates of ∼1 K/min. No desorption temperature was determined for methane, but the methane adlayer was observed to desorb below 100 K. As determined by both PES and PSD, annealed adamantane- and neopentane-dosed surfaces react to form nearly identical surfaces. Si 2p core-level spectra show chemically shifted components of 0.48±0.05, 1.00±0.05, and 1.50±0.05 eV with respect to the bulk component. This demonstrates formation of Si(CH y) x ( x=1-3; y=0-3) type surface species. The H + PSD spectra of the thermally reacted surfaces were measured and show chemical shifts of ∼0.7 eV with respect to bulk Si and two sharp resonances at 100.7 and 101.3 eV. The edge shift and associated structure highlights the chemical specificity of PSD and demonstrates its utility for following surface chemical reactions.

へキサメチルジシラザンで改質したシリカ粉体表面の構造と濡れ性

The wettability and surface structure of precipitated silica surface loaded trimethylsilyl groups were investigated. By measuring water and nitrogen adsorptions, it was found that the non-treated sample had micro pores. The pores of diameter less than 0.8nm were not modified, while the pores of diameter from 0.8 to 0.9nm were modified by trimethylsillyl groups and closed by them. From these results, it was assumed that the former pores would become sites for water vapor as surface silanol groups after modification, and the latter pores would be independent of water adsorption. The character of the bare silica surface disappeared above a surface conversion rate of 33%. These results are deduced from a neopentane adsorption experiment. Furthermore, the water a adsorption capacity near the saturated pressure, as an index of micro-wettability, did not reach theoretical monolayer capacity above the same conversion rate (33%). It was also indicated that the preferential dispersion property as an index of macro-wettability drastically changed at a conversion rate of about 33%. From these results, it was demonstrated that the wettability is closely related to the surface structure interfering with the adsorption of water.

Adsorption of neopentane on HNaY-FAU zeolite studied by IR spectroscopy

Adsorption of neopentane on HNaY-FAU zeolite was studied by IR spectroscopy. At beam temperature neopentane already shows a weak interaction with Brønsted acidic sites. At high temperatures, irreversible consumption of both Brønsted and Lewis sites is observed. According to the results of quantum chemical calculations a proton attack on the central carbon of neopentane seems to be favoured.

Physical adsorption of neopentane on Sterling FT graphite

Physical adsorption of neopentane on Sterling FT graphite

The Effects of Protons on the Hydrogenolysis of Neopentane over Rhodium Catalysts: Rh/Hy, Rh/Nahy, and Rh/SiO2

The hydrogenolysis of neopentane (2,2-dimethylpropane) has been studied over the rhodium catalysts Rh/HY, Rh/NaHY, and Rh/SiO2. The catalytic activity follows the sequence: Rh/HY > Rh/NaHY ≈ Rh/SiO2. For all catalysts, the conversion of neopentane was 100% selective to hydrogenolysis products, methane, and isobutane; they are formed in a ratio of 1 : 1. In an attempt to determine the true activation energies from the apparent activation energies derived from the Arrhenius curves, corrections were made for the preequilibrium between gaseous and physisorbed neopentane. The adsorption of neopentane on the metal-free supports was determined: isosteric heats of adsorption were calculated from these data. For zeolites NaY and HY they are 8.2 and 7.0 kcal/mol, respectively. After correcting for the preequilibrium, the true activation energies are 33.5 and 38.8 kcal/mol for Rh/HY and Rh/NaHY, respectively. The results lend support to the concept that the superior activity of Rh/HY is due to the "electron deficiency" of the rhodium particles.

Ultraviolet–visible spectroscopic investigations on the interaction of neopentane with acidic zeolites

Adsorption of neopentane on faujasite, mordenite and ZSM-5 zeolites has been investigated by UV–VIS spectroscopy. At room temperature, UV–VIS active surface species were detected and assigned to alkenyl-type carbenium ions. Their formation was shown to be influenced by the overall acidity, the ratio of Brønsted to Lewis acidity and the zeolite structure.

Ex-hydroxide magnesium oxide as a model adsorbent for investigation of micropore filling mechanisms

The thermal decomposition of magnesium hydroxide to magnesium oxide has been studied under carefully controlled conditions. Analysis of nitrogen and neopentane adsorption isotherm measurements shows that the micropore structure of the decomposition products is very well defined and consists of slit-shaped pores of width 0.93 ± 0.03 nm from 40% to 90% decomposition. The mean pore width increases, up to ca. 1.8 nm, at higher levels of decomposition. For larger pore sizes two stages of secondary micropore filling by nitrogen (surface coverage followed by enhanced adsorption in the body of the pore) are clearly discernible. The results also show very clearly how the mechanism of micropore filling, and hence the shape of the isotherm, is dependent on the ratio of pore width to molecular diameter (d/σ), and not just on d.

High resolution, quasi-equilibrium sorption studies of molecular sieves

Argon, nitrogen, and neopentane adsorption isotherms from molecular sieves are recorded at 87 K, 77 K, and 273 K, respectively, by a quasi-equilibrium, high resolution gas sorption technique. The molecular sieves used in this study are alkali exchanged zeolite X, AlPO4-11, AlPO4-5, VPI-5, KL, CaA, ZSM-5, and ZSM-11. Little relation is observed between the transition pressure for microporous nitrogen adsorption and pore size. Small changes in the effective pore size resulting from variations in cation size are detected in the transition pressure for argon adsorption. Large shifts in the transition pressure for argon adsorption are found for the 10-, 12-, and 18-membered ring pores of AlPO4-11, AlPO4-5, and VPI-5, respectively. Argon adsorption combined with neopentane adsorption on microporous materials provides additional information regarding transitions in the isotherm that result from dual pore systems and effects that may be due to adsorbate packing. The step in the nitrogen isotherm atP/P0> 0.1 from ZSM-5 is not observed in the nitrogen isotherm from ZSM-11.

Adsorption of neopentane by nonporous carbons and silicas

Adsorption of neopentane by nonporous carbons and silicas

Physical adsorption of simple gases on the (111) face of sulfur; a model for the adsorption by heterogeneous surfaces, including lateral interactions

As revealed by previous ab initio calculations, the surface of the (111) face of rhombic sulfur is characterized by heterogeneous but low adsorption energies. This implies relatively important contributions from lateral interactions in the adsorption mechanism. A theoretical model is presented for this case, also taking into account the random distribution of the sites. One assumes a local Fowler-Guggenheim isotherm and that the adsorption energies have a skew-Gaussian distribution with a non-zero minimum value. The resulting isotherm is tested in a satisfactory way with the data obtained for nitrogen, argon, methane and neopentane adsorption in the submonolayer region.

Molecular statistical calculation of thermodynamic characteristics of neopentane adsorption by zeolite NaX

Molecular statistical calculation of neopentane adsorption by zeolite Na X was carried out using the lattice model of adsorbate. In each cavity of zeolite a system of four centers of adsorption was considered, these centers being disposed on the vertexes of the regular tetrahedron, in respect to the disposition of sites S II. The attraction between the first three adsorbed molecules was taken into account, and it was assumed that the entering of the fourth molecule into the cavity produces the repulsion. The interaction between the molecules adsorbed in adjacent cavities was not taken into consideration. The estimation of the energies of the adsorbate-adsorbent and adsorbate-adsorbate interactions as well as of the frequencies of vibrations of molecules with respect to the zeolite lattice and to each other was made. The approximate expression for the canonical partition function Q n for n molecules in one cavity was calculated. Using the standard methods of statistical thermodynamics the isotherms of adsorption and the dependence of the isosteric heat and entropy of adsorption on the adsorption amount were calculated. The calculated values are in agreement with experimental ones. The differential molar heat capacity of adsorbate and some characteristics of the distribution of adsorbed molecules among the cavities (the variance and the third moment) were also calculated. It is shown that at usual temperatures of adsorption experiments the distribution of molecules among the cavities is essentially nonuniform, but it becomes more homogeneous as the temperature rises. The theoretical calculation of the coefficients of the adsorption isotherm virial equation was also carried out.

Adsorption of neopentane on tungsten and palladium films

The adsorption of neopentane has been studied on tungsten and on palladium films; allied experiments were carried out with methane (W, Pd) and with propane (W). The evolution of hydrogen from each gas/metal system was studied as a function of time and temperature; reaction with gas phase deuterium was used to determine how many of the hydrogen atoms in the surface phase were exchangeable, either rapidly or by slower processes.Rapid dissociative chemisorption, involving the cleavage of an average of 3 or 4 C—H bonds occurred with neopentane or propane on tungsten. Neopentane was chemisorbed on palladium at room temperature but dissociation of methane was not observed below ∼373 K; the extent of the dissociation of both hydrocarbons was smaller than on tungsten. Further breakdown of the surface species, accompanied by evolution of hydrogen, occurred with all systems on raising the temperature. Evidence for C—C bond rupture was obtained from the hydrogenolysis of neopentane at higher temperatures.

Isotherms and heats of adsorption of neopentane and carbon tetra chloride vapor on graphitized carbon black

1. Measurements have been made of the adsorption isotherms and differential heat of adsorption of neopentane and carbon tetrachloride on thermal black graphitized at 3000°. 2. The large sphere-like molecules of these substances are adsorbed in a predominantly nonlocalized way, and interact strongly with one another, which causes a marked increase in the heat of adsorption as the monolayer is being filled and a strongly concave initial portion of the adsorption isotherm. 3. Free rotations or rotational oscillations of these molecules are possible in the adsorbed layer with small amounts of surface filling. 4. A discussion is given of different possible ways of defining the standard state of the adsorbed layer on a homogeneous surface, and values are given for the corresponding thermodynamic quantities of the adsorptive systems studied.