N-butane Sorption Research Articles

Hydrocarbon Sorption in Flexible MOFs-Part II: Understanding Adsorption Kinetics.

The rate of sorption of n-butane on the structurally flexible metal-organic framework [Cu2(H-Me-trz-ia)2], including its complete structural transition between a narrow-pore phase and a large-pore phase, was studied by sorption gravimetry, IR spectroscopy, and powder X-ray diffraction at close to ambient temperature (283, 298, and 313 K). The uptake curves reveal complex interactions of adsorption on the outer surface of MOF particles, structural transition, of which the overall rate depends on several factors, including pressure step, temperature, as well as particle size, and the subsequent diffusion into newly opened pores. With the aid of a kinetic model based on the linear driving force (LDF) approach, both rates of diffusion and structural transition were studied independently of each other. It is shown that temperature and applied pressure steps have a strong effect on the rate of structural transition and thus, the overall velocity of gas uptake. For pressure steps close to the upper boundary of the gate-opening, the rate of structural transition is drastically reduced. This feature enables a fine-tuning of the overall velocity of sorption, which can even turn into anti-Arrhenius behavior.

Sorption of n-butane on sulfated zirconia investigated using a combination of in-situ DRIFT spectroscopy, XPS and TAP

The sorption of n-butane on sulfated zirconia (SZ) was investigated using in-situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS), Photoelectron spectroscopy (XPS) and Temporal Analysis of Products (TAP) measurements. The combination of different spectroscopic techniques like XPS and DRIFTS with transient TAP experiments offer the chance to comprehensively investigate and characterize catalytic sorption sites. Using XPS, DRIFTS, and TAP it was shown that both Brønsted centers (Zr-OH) and the very reactive disulfate groups are involved in the adsorption of n-butane and thus are mainly responsible for the isomerization reaction of n-butane to iso-butane over SZ. Moreover, all methods enable to bridge the pressure gap ranging from UHV to ambient pressure. Especially, adsorption of alkanes with XPS in-situ for different adsorption times has been done for the first time.

The influence of propane and n-butane on the structure and separation performance of cellulose acetate membranes

This work presents the impact of propane and n-butane on the CO2/CH4 separation performance of both cellulose diacetate (CDA) and cellulose triacetate (CTA) membranes by exposing both pristine membranes to either propane (400 kPa) or n-butane (200 kPa) at room temperature (22 ± 2 °C) for 4 weeks. The propane and n-butane sorption isotherms in both membranes were anomalous at 35 °C. X-ray diffraction (XRD) results indicated that the crystalline nature of both polymers was altered by this exposure, although dynamic scanning calorimetry (DSC) did not detect a significant change in the overall crystallinity. Positron Annihilation Lifetime Spectroscopy (PALS) revealed that the average pore size of the CTA polymer and the number of free volume elements of both membranes also increased, even though the sorption uptake was less than 2 wt%. CO2 and CH4 permeabilities at 35 °C were essentially unaffected by the propane or n-butane exposure, indicating that while the crystalline regions of the polymer were affected, plasticization of the glassy amorphous region did not occur. There was a slight decrease in CH4 permeability for the CDA membrane after n-butane exposure, consistent with a slight decline in the CH4 solubility at this feed pressure. The propane and n-butane permeabilities were 0.029 Barrer at 300 kPa and 0.019 Barrer at 125 kPa for the fresh CTA membrane, but these fell significantly after long term exposure to these gases, possibly due to penetrant clustering.

Modelling of methane and n-butane sorption, diffusion and permeation in polydimethylsiloxane using PC-SAFT

Published sorption, diffusion and permeation data for methane and n-butane in polydimethylsiloxane (PDMS) from −20 to 50 °C was simulated using a perturbed chain statistical association theory (PC-SAFT) based model. The use of a temperature-dependent interaction parameter within the PC-SAFT model allowed the pure gas sorption data to be very well represented. The mixed gas sorption results were fully predictable from these pure gas parameters, without the introduction of any additional parameters, and agreed well with the experimental data. The model was also able to model the dilation behavior of PDMS under various gas compositions, making it possible to analyse gas sorption properties using pure gas sorption data only. A diffusion model coupled with the PC-SAFT model was capable of fitting both pure and mixed gas permeation data well by applying an exponential expression to account for such dilation in the diffusivity term. Only two parameters (i.e. infinite dilution mobility coefficient L0 and plasticization factor β) were used and no coupling effect between the two penetrants was needed. The activation energies of L0 were 11.7 and 13.4 kJ mol−1 for methane and n-butane. Moreover, the model was also able to calculate the concentration profiles of the penetrants across the membrane thickness. For n-butane, the mass concentration profile changed from linear to non-linear when the feed pressure increased from 4 to 11 atm for 8 mol% n-butane at 25 °C. Conversely, methane showed a linear concentration profile under both conditions.

Macrochain configuration, stucture of free volume and transport properties of poly(1-trimethylsilyl-1-propyne) and poly(1-trimethylgermyl-1-propyne)

The relationship between poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1-trimethylgermyl-1-propyne) (PTMGP) microstructure, gas permeability and structure of free volume is reported. n-Butane/methane mixed-gas permeation properties of PTMSP and PTMGP membranes with different cis-/trans-composition have been investigated. The n-butane/methane selectivities for mixed gas are by an order higher than the selectivities calculated from pure gas measurements (the mixed-gas n-butane/methane selectivities are 20–40 for PTMSP and 22–35 for PTMGP). Gas permeability and n-butane/methane selectivity essentially differ in polymers with different cis-/trans-composition. Positron annihilation lifetime spectroscopy investigation of PTMSP and PTMGP with different microstructure has determined distinctions in total amount and structure of free volume, i.e. distribution of free volume elements. The correlation between total amount of free volume and gas transport parameters is established: PTMSP and PTMGP with bigger free volume exhibit higher n-butane permeability and mixed-gas n-butane/methane selectivity. Such behavior is discussed in relation to the submolecular structure of polymers with different microstructure and sorption of n-butane in polymers with different free volume.

Gas and vapor sorption and diffusion in poly(ethylene terephthalate)

Equilibrium sorption of n-butane and acetaldehyde in melt-processed poly(ethylene terephthalate) (PET) microtomed flakes is reported. The n-butane sorption isotherm at 35°C is well described by the dual-mode model with the following parameters: kD=0.017cm3 (STP)/(cm3 amorphous polymer cmHg), C′H=1.3cm3 (STP) (cm3 amorphous polymer) and b=0.029cmHg−1. Acetaldehyde isotherms at 35 and 45°C may be described by the Flory–Huggins sorption model, suggesting that penetrant uptake in the non-equilibrium excess volume associated with the glassy polymer made a negligible contribution to the overall sorption level at the conditions of this study. The heat of sorption was essentially equal to the enthalpy of condensation of pure acetaldehyde. At 45°C and acetaldehyde pressures at or above 40.0cmHg, acetaldehyde triggers significant crystallization of PET (up to 37wt%) with increasing concentration. Subsequent sorption experiments at very low penetrant activity reveal solubility coefficients that are markedly higher in the penetrant-crystallized sample than in the initially highly amorphous sample. This result suggests the acetaldehyde-induced formation of microvoids (which act as highly efficient penetrant sorption sites) in the polymer sample. Based on these and literature data, the logarithm of infinite dilution penetrant solubility in amorphous regions of PET was well-correlated with penetrant condensability as characterized by Tc, penetrant critical temperature, or by (Tc/T)2, where T is the experiment temperature. Infinite dilution, amorphous phase penetrant diffusion coefficients in PET decreased according to a power law relation with increasing penetrant critical volume.

Gas permeability and hydrocarbon solubility of poly[1-phenyl-2-[p-(triisopropylsilyl)phenyl]acetylene

The effects of film thickness, physical aging, and methanol conditioning on the solubility and transport properties of glassy poly[1-phenyl-2-[p-(triisopropylsilyl) phenyl]acetylene] are reported at 35 °C. In general, the gas permeability coefficients are very high, and this polymer is more permeable to larger hydrocarbons (e.g., C3H8 and C4H10) than to light gases such as H2. The gas permeability and solubility coefficients are higher in as-cast, unaged films than in as-cast films aged at ambient conditions and increase to a maximum in both unaged and aged as-cast films after methanol conditioning. For example, the oxygen permeability of a 20-μm-thick as-cast film is initially 100 barrer and decreases to 40 barrer after aging for 1 week at ambient conditions. After methanol treatment, the oxygen permeabilities of unaged and aged films increase to 430 and 460 barrer, respectively. Thicker as-cast films have higher gas permeabilities than thinner as-cast films. Propane and n-butane sorption isotherms suggest significant changes in the nonequilibrium excess free volume in these glassy polymer films due to processing history. For example, the nonequilibrium excess free volume estimated from the sorption data is similar for as-cast, unaged samples and methanol-conditioned samples; it is 100% higher in methanol-conditioned films than in aged, as-cast films. The sensitivity of permeability to processing history may be due in large measure to the influence of processing history on nonequilibrium excess free volume and free volume distribution. The propane and n-butane diffusion coefficients are also sensitive to film processing history, presumably because of the dependence of diffusivity on free volume and free volume distribution. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1474–1484, 2000

Pure hydrocarbon sorption properties of poly(1-trimethylsilyl-1-propyne) (PTMSP), poly(1-phenyl-1-propyne) (PPP), and PTMSP/PPP blends

Propane and n-butane sorption in blends of poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1-phenyl-1-propyne) (PPP) have been determined. Solubilities of propane and n-butane increased as the PTMSP content in the blends increased. This result is consistent with the higher free volume of PTMSP-rich blends and the better thermodynamic compatibility between PTMSP and these hydrocarbons. Propane and n-butane sorption isotherms were well described by the dual-mode model for sorption in glassy polymers. PTMSP/PPP blends are strongly phase-separated, heterogeneous materials. A noninteracting domain model developed for sorption in phase-separated glassy polymer blends suggests that sorption in the Henry's law regions (i.e., the equilibrium, dense phase of the blends) is consistent with the model. However, Langmuir capacity parameters in the blends are lower than predicted from the domain model, suggesting that the amount of nonequilibrium excess free volume associated with the Langmuir sites depends on blend composition. © 1996 John Wiley & Sons, Inc.

Location and Conformation of n-Alkanes in Zeolites: An Analysis of Configurational-Bias Monte Carlo Calculations

Results from calculations using a novel Monte Carlo method to simulate the sorption of n-butane to n-decane in various all-silica zeolites are analyzed to obtain information on the location and the conformation of the sorbed molecules. In general, the framework topology determines the conformation of the sorbed molecules. In mordenite, we find that butane is able to adsorb relatively unhindered, whereas longer chains are oriented parallel to the main channel direction and become less kinked with increasing carbon number. In ferrierite, butane molecules are distributed over both the 8- and 10-ring channels, while pentane and longer molecules are restricted to an all-trans conformation in the larger 10-ring channel. Faujasite appears to only slightly perturb the distribution of alkane conformations, compared to those found for gas phase alkanes. In zeolites rho and A, all alkanes are sorbed in a highly coiled conformation inside the α-cages of these structures.

Differential pressure-jump method for routine measurement of sorption kinetics

A sensitive pressure-jump method has been developed for the measurement and characterization of material transport in porous solids. The apparatus is automatically operated and the data evaluation is performed by means of numerical methods based on the Crank-Nicholson procedure. As an example, the sorption ofn-butane and ethene in spherical mesoporous silica gel pellets has been studied. In this way, it is possible to obtain uptake curves and in case of known particle shape to derive values for effective diffusion coefficients. From the pressure and temperature dependence ofDe, information on the transport mechanism by means of Knudsen and surface diffusion can be obtained as well as the tortuosity factor characterizing the pore network.

Adsorption and diffusion of n-butane and 2-butyne in silicalite-I

The sorption and diffusion of n-butane and 2-butyne in silicalite-I zeolite have been measured. Both n-butane and 2-butyne exhibited type I sorption isotherms, but due to its triple bond, the sorbed amount of 2-butyne exceeded that of n-butane by about 20% under the same conditions. The isosteric heats of sorption showed that at lower coverage (< 2 molecules per unit cell), the interaction of the triple bond of 2-butyne with the silanol groups in silicalite-I gave heats nearly 10 kJ/mol higher than did the corresponding heat of sorption of n-butane

Crystal structures of the ammonium and hydrogen forms of zeolite rho

To determine whether or not there is a structural basis for the unusually low n-butane sorption capacity observed for the hydrogen form of zeolite rho (H 12Al 12Si 36O 96), and to study further the surprising elasticity of the zeolite rho aluminosilicate framework (a body-centred-cubic arrangement of α-cages joined through double 8-rings), an X-ray powder diffraction study of the H + form (at 500°C) and of its precursor, the NH 4 + form (desolvated at 350°C and examined at 100°C), has been undertaken. Rietveld refinement of the NH 4—rho structure in the space group l 43m ( a = 14.821 A ̊ ) converged with R f = 0.078 and R wp = 0.105. The α-cages exhibit a tetrahedral distortion which produces oval 8-rings and a concomitant shrinkage of the double 8-ring pore openings. The twelve NH 4 + ions per unit cell are crystallographically equivalent and are located at the centre of each of these oval 8-rings. Each ammonium ion is associated with two framework oxygens at 2.89 Å, and four additional ones at 3.14 Å. The H—rho structure refined in the higher space group Im3m ( a = 14.982 A ̊ ) to R f = 0.058 and R wp = 0.108. In contrast to the NH 4—rho structure, this one displays no distortion of the α-cages and has round, fully open 8-rings. The structure offers no explanation for the low sorption observed.

The sorption and diffusion of n-butane in Linde 5A molecular sieve

Kinetic and equilibrium data are presented for the sorption of n-butane in Linde 5A zeolite crystals and pellets. It is shown that the clay binder which is present in the pellets acts merely as an inert diluent with no significant effect on the sorptive properties. Thus, provided sorbate concentrations are based on the weight of zeolite crystals, there is no significant difference between either the equilibrium sorption isotherms or the diffusivities for pellets and crystals. The equilibrium data are correlated in terms of a simple isotherm equation which is derived from the Dubinin-Polanyi theory. It is shown that the strong positive concentration dependence of the diffusivity is due entirely to the non linearity of the relationship between sorbate activity and concentration and the drag coefficients, calculated from the equation D = ( RT/ k) . (∂ln p/∂ln c), are effectively independent of concentration. A concise semi-empirical correlation of both the equilibrium and diffusivity data is presented which is applicable over a wide range of conditions.

Pore Size Distribution of Porous Aluminum Oxides by Mercury Porosimeter and n-Butane Sorption

Pore Size Distribution of Porous Aluminum Oxides by Mercury Porosimeter and n-Butane Sorption