Methane Adsorbs Research Articles

A New, Methane Adsorbent, Porous Coordination Polymer

An astonishing amount of methane is reversibly adsorbed in pores of [{CuSiF6(4,4′-bipyridine)2}n], which exists as a stable 3D microporous network (see picture). The approximate channel sizes of this material is 8×8 Å2 along its c axis and 6×2 Å2 along the a and b axes. At 36 atm, the density of methane adsorbed in micropore volume is 0.21 g mL−1, which is superior to that of any reported zeolites.

Grand Canonical Monte Carlo Simulations of Nonrigid Molecules: Siting and Segregation in Silicalite Zeolite

A method is described for performing grand canonical Monte Carlo simulations of molecules with internal degrees of freedom. The approach is applied to adsorption of methanol and cyclohexane in the zeolite silicalite. Calculated adsorption isotherms and heats of adsorption compare well with experimental data from the literature. In addition, the simulations provide information on preferential siting of molecules in the different pores of silicalite. Binary adsorption results are also predicted for cyclohexane/methane, benzene/methane, and benzene/cyclohexane mixtures. The siting preferences of the single components result in clear segregation effects for these binary systems. Benzene and cyclohexane display preferred siting in the channel intersections, while methane adsorbs preferentially in the channels. In the case of benzene/cyclohexane, the cyclohexane adsorbs in the intersections and the benzene is pushed to the zigzag channels.

1H and 129Xe NMR investigation of the microporous structure of dealuminated H-mordenite probed by methane and xenon

The dynamic behavior of methane and xenon adsorbed in H-mordenite (H-MOR) has been investigated by 1H and 129Xe NMR in a wide range of temperatures (4.2–290 K). Previous results for Na-MOR (Si/Al=7.5) showed that methane adsorbs in both the main channels and the side-pockets of mordenite and that adsorption of methane in the side-pockets is favored relative to adsorption in the main channels. When Na-MOR is converted to its protonated form (H-MOR; Si/Al=8.6), the rotational barrier of methane in the side-pocket increases from 2.0 to 5.0 kJ mol −1, whereas for methane in the main channel, the rotational barrier decreases slightly from 1.0 to 0.53 kJ mol −1. This indicates that methane molecules in the side-pockets interact with acidic Brønsted sites via some sort of “unique” hydrogen bonding. For dealuminated H-MOR (Si/Al=40), the temperature dependence of the 1H spin-lattice relaxation time, T 1, for adsorbed methane indicates that dealumination leads to an enlargement of the pore sizes of both the main channel and side-pocket. This effect is more pronounced for the side-pocket than for the main channel. This observation is consistent with literature results indicating that dealumination opens side-pockets from adjacent channels, thereby forming a secondary pore structure connecting the main channels. 129Xe NMR spectra were also measured for both Na- and H-MOR. Chemical exchange between the two possible locations, i.e. main channel and side-pocket, takes place so that only one broad resonance is observed at room temperature in H-MOR ( E a=18±2 kJ mol −1). The exchange process is slower in Na-MOR ( E a=21±2 kJ mol −1). From this observation and the 1H T 1 results, it is concluded that Na + cations restrict the side-pockets more efficiently than H +, and it is suggested that Brønsted acid sites are located near the opening of the side-pockets.

TAP Study of the Mechanism and Kinetics of the Adsorption and Combustion of Methane on Ni/Al2O3 and NiO/Al2O3

The activation of methane on an industrial Ni/Al2O3 catalyst was studied in a TAP reactor between 723 and 823 K. Methane adsorbs on the reduced Ni catalyst with an activation energy of 54 kJ mol−1. Oxidation of the Ni on the surface of the support up to 13% results in a strong decrease of the methane adsorption rate which is more than proportional to the decrease in the number of reduced Ni sites on the surface. Three parallel pathways are responsible for the adsorption of methane on the oxidized Ni catalyst. Two pathways consist of hydrogen abstraction of methane by oxygen, reversibly chemisorbed on two different active sites. The lifetime of oxygen chemisorbed on the two active sites is limited due to desorption and amounts to respectively 5 and 148 s at 823 K. The rate of adsorption of methane via the third pathway decreases on a time scale of 1000 s. The type of site that is responsible for this pathway cannot be regenerated by oxygen. The combustion of methane on the oxidized catalyst was studied by means of step experiments with methane and isotopically labeled oxygen. Chemisorbed oxygen abstracts hydrogen from gas-phase methane and from adsorbed CHx species. Lattice oxygen is responsible for the carbon-oxygen bond formation. The structure of the Ni/Al2O3 catalyst was studied by means of TPR, XPS, and XRD.

Microbially formed catalysts for enhancement of direct coal liquefaction

The dynamic behavior of methane and xenon adsorbed in H-mordenite (H-MOR) has been investigated by 1H and 129Xe NMR in a wide range of temperatures (4.2–290 K). Previous results for Na-MOR (Si/Al=7.5) showed that methane adsorbs in both the main channels and the side-pockets of mordenite and that adsorption of methane in the side-pockets is favored relative to adsorption in the main channels. When Na-MOR is converted to its protonated form (H-MOR; Si/Al=8.6), the rotational barrier of methane in the side-pocket increases from 2.0 to 5.0 kJ mol−1, whereas for methane in the main channel, the rotational barrier decreases slightly from 1.0 to 0.53 kJ mol−1. This indicates that methane molecules in the side-pockets interact with acidic Brønsted sites via some sort of “unique” hydrogen bonding. For dealuminated H-MOR (Si/Al=40), the temperature dependence of the 1H spin-lattice relaxation time, T1, for adsorbed methane indicates that dealumination leads to an enlargement of the pore sizes of both the main channel and side-pocket. This effect is more pronounced for the side-pocket than for the main channel. This observation is consistent with literature results indicating that dealumination opens side-pockets from adjacent channels, thereby forming a secondary pore structure connecting the main channels. 129Xe NMR spectra were also measured for both Na- and H-MOR. Chemical exchange between the two possible locations, i.e. main channel and side-pocket, takes place so that only one broad resonance is observed at room temperature in H-MOR (Ea=18±2 kJ mol−1). The exchange process is slower in Na-MOR (Ea=21±2 kJ mol−1). From this observation and the 1H T1 results, it is concluded that Na+ cations restrict the side-pockets more efficiently than H+, and it is suggested that Brønsted acid sites are located near the opening of the side-pockets.

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.

Kinetic and mechanistic aspects of the oxidative coupling of methane over a Li/MgO catalyst

The rate of reaction of methane with oxygen in the presence of a Li-doped MgO catalyst has been studied as a function of the partial pressures of CH 4, O 2 and CO 2 in a well-mixed reaction system which is practically gradientless with respect to gas-phase concentrations. It is concluded that the rate determining step involves reaction of methane adsorbed on the catalyst surface with a di-atomic oxygen species. The adsorption of oxygen is relatively weak. Carbon dioxide acts as a poison for the reaction of methane with oxygen, this probably being caused by competitive adsorption on the sites where oxygen (and possibly also methane) adsorbs.

Catalytic oxidation of methane on zeolites containing rhodium, iridium, palladium and platinum

The complete oxidation of methane over zeolites containing palladium, platinum, rhodium and iridium has been studied using a microcalorimetric technique. The results show that isolated ions of these metals are able to adsorb both methane and oxygen but only with platinum do methane and oxygen adsorb competitively. The activation energies for the reaction show that the rate-determining step is the same on each catalyst.