Investigation of the surface heterogeneity of solids from reversed-flow inverse gas chromatography

In the present work, the novel methodology of the inverse gas chromatographic technique of reversed-flow gas chromatography (RF-GC) was applied to the well-studied catalytic oxidation of carbon monoxide over silica supported Pt, Rh and Pt-Rh alloy catalysts. Adsorption energies, local isotherms, local monolayer capacities, surface diffusion coefficients, lateral interaction energies and energy distribution functions are simultaneously determined in a single experiment. The variation of the determined physicochemical parameters against the nature of the studied catalysts (Pt content) is consistent with the observed catalytic activity. The energy distribution functions, estimated by means of RF-GC, give useful information about the "topography" and the nature of the active sites on the catalyst surface, similar to those of experimental techniques, such as Thermal Desorption Spectroscopy studies of the adsorption of CO on group VIII noble metal surfaces. The experimentally found results explain the superior activity of Pt0.25 + Rh0.75 alloy, in comparison to that of the pure Pt and Rh catalysts.     

Hhdrodenitrogenation catalysis studied by reversed-flow gas chromatography

Summary Many important parameters of surface catalysed reactions can be determined simultaneously, under nonsteady state conditions using Reversed-Flow Gas Chromatography. A simple, slightly modified gas chromatograph is required. The distorted diffusion bands, obtained experimentally for reactant and product(s), can be analysed mathematically, using simple PC programs, to give the pre-exponential factors and the exponential coefficients of a function consisting of the sum of two-four exponential functions of time. From these, and some geometrical and diffusional characteristics of the reaction cell, the values of adsorption, desorption and reaction rate constants, the overall mass transfer coefficients in the gas and in the solid catalyst, and the adsorption equilibrium constant, for both reactant and product(s) can be calculated.The above parameters were determined at various temperatures and over three catalysts for the hydrodenitrogenation of piperidine ton-pentane, an industrially important hydrotreating process. The results obtained can help to understand the mechanism of reactions on solid surfaces and to confirm experimentally theoretical calculations on adsorption and surface reactions.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Physicochemical characterization of dilute n-alcohol/biodiesel mixtures by inverse gas chromatography

Inverse gas chromatography (IGC) has been used to determine the physicochemical parameters that characterize solution thermodynamic interactions in biodiesel-n-alcohol solute systems. Such data is of value to chemical engineers and separation scientists in optimizing separation processes to separate alcoholic solutes at low concentrations in soybean oil methyl ester mixtures (biodiesel). The derived activity and Henry's Law coefficient data can be used to rationalize the interaction of four members of an n-alcoholic homologous series and the soya-based methyl ester solvent in terms of such esters as "green" renewable solvents. Sorption isotherm data confirm linear behavior in most cases between the solute (alcohol) vapor state concentrations and their uptake into the biodiesel phase. Overall, the experimentally determined activity coefficients agree well with those predicted by solution thermodynamic theories as well as correlative chemical engineering equations.     

Ring opening of cyclohexane over ZSM-zeolites followed by reversed-flow gas chromatography

Summary Kinetic parameters for the ring opening of cyclohexane over modified ZSM-5 zeolites can be measured simultaneously under non-steady state conditions using reversed-flow gas chromatography. At relatively low temperatures, the main product detected and measured was propene.The mathematical relations used for the calculation of adsorption-desorption rate constants, surface reaction rate constants and adsorption equilibrium constants for the reactant cyclohexane are different from previously studied catalytic reactions, owing to the different experimental arrangement for feeding the catalytic bed. The diffusion bands obtained experimentally for reactant or product are described by the sum of two or three exponential functions of time, respectively. From the exponential coefficients of time and the pre-exponential factors, all determined by simple PC programs, the above kinetic parameters are calculated.     

Diffusion and adsorption measurements in porous solids by inverse gas chromatography

The new inverse gas chromatography methodology of reversed-flow gas chromatography has been applied to measure diffusion and adsorption in porous solids. The theoretical analysis leads to equations describing the experimental data with very few approximations. From these, the total overall rate constant of transference (k = k(R) + k(2)), its probable error, the diffusion coefficient (D(1)) into porous solids (alpha-alumina and gamma-alumina), and its probable error are calculated by means of a simple PC program. The methodology was applied to pentane, hexane, and heptane diffusing into porous alpha- and gamma-alumina at various temperatures. A comparison of the results is made with those obtained by the Knudsen formula and with those of other researchers.     

Ring opening of cyclohexane over ZSM-zeolites followed by reversed-flow gas chromatography

Summary Kinetic parameters for the ring opening of cyclohexane over modified ZSM-5 zeolites can be measured simultaneously under non-steady state conditions using reversed-flow gas chromatography. At relatively low temperatures, the main product detected and measured was propene. The mathematical relations used for the calculation of adsorption-desorption rate constants, surface reaction rate constants and adsorption equilibrium constants for the reactant cyclohexane are different from previously studied catalytic reactions, owing to the different experimental arrangement for feeding the catalytic bed. The diffusion bands obtained experimentally for reactant or product are described by the sum of two or three exponential functions of time, respectively. From the exponential coefficients of time and the pre-exponential factors, all determined by simple PC programs, the above kinetic parameters are calculated.     

Flux of gases across the air-water interface studied by reversed-flow gas chromatography.

In the present work the reversed-flow gas chromatographic technique was applied for the study of flux of gases across the air-water interface. The model system was vinyl chloride-water, which is of great significance in food and environmental chemistry. Using suitable mathematical analysis, equations were derived by means of which the following physicochemical quantities were calculated: diffusion coefficient of vinyl chloride (VC) into water, partition coefficient of VC between the water (at the interface and the bulk) and the carrier gas nitrogen, overall mass transfer coefficients of VC in the gas (nitrogen) and the liquid (water), gas and liquid film transfer coefficients of VC, gas and liquid phase resistances for the transfer of VC into the water, and finally the thickness of the stagnant film in the liquid phase, according to the two-film theory of Whitman. From the variation of the above parameters with temperature, as well as the volume and the free surface area of the water, useful conclusions concerning the mechanism for the transfer of VC into water were extracted. These are discussed in comparison with the same parameters calculated from empirical equations or determined experimentally by other techniques.     

Kinetic study of the drying step of supported catalysts by reversed-flow gas chromatography

Summary Application of reversed-flow gas chromatography to study the drying step in the preparation of supported catalysts is proposed. It is demonstrated that this technique can be applied to investigate the kinetic law followed, as well as to obtain kinetic parameters of drying (rate constants, activation energies and frequency factors). Amaterial illustrating this application is -Al₂O₃ impregnated with usual organic solvents (n-C₅H₁₂,n-C₆H₁₄,n-C₇H₁₆, CH₃COCH₃ and CH₃CH₂OH). It was found that in all cases above a simple first-order law based onone kind of active site is followed. Moreover, it was observed that the activation energyE _a of the drying process increases almost linearly with the relative molecular mass of the solvent, whereas for the same solventE _a increases with the amount used. The operation of a kinetic compensation effect suggests similar activated complexes of the various solvents used.     

Finite concentration inverse gas chromatography: Diffusion and partition measurements

Inverse gas chromatography (IGC) is a very fast, accurate, and reliable technique to measure diffusion coefficients. This technique however, has been limited to measurements in the infinite dilution region, i.e., in the region of negligible amount of solvent in the polymer. We have extended the scope of inverse gas chromatography to measure diffusion coefficients at finite concentrations of the solvent. This involves doping the carrier gas with a solvent of interest to achieve finite concentrations of solvent in the carrier gas and hence in the polymer. The carrier gas is passed through a saturator maintained at constant temperature to achieve this purpose. Diffusion coefficients for polyvinyl acetate–toluene, and polystyrene–toluene systems were determined at finite concentrations. The results were compared with the traditional gravimetric sorption and piezoelectric sorption measurements reported in the literature. The data are in excellent agreement with the values reported, correlate well with the Vrentas–Duda free volume theory, and can also be predicted from infinitely dilute data using the free volume theory. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1279–1290, 1997     

SEBS triblock copolymer-solvent interaction parameters from inverse gas chromatography measurements

Solubility parameter and Flory-Huggins polymer-solvent interaction parameter of an SEBS (poly(styrene-b-butene/ethylene-b-styrene) triblock copolymer) rubber, were measured by inverse gas chromatography (IGC) technique. The values obtained indicate that cyclohexane, n-pentane and cyclopentane are the most compatible solvents for this kind of polymers. Results were compared with previous data obtained from intrinsic viscosity measurements; also, an estimation using UNIFAC-FV model was performed showing differences less than 35% in terms of infinite dilution activity coefficient of solvents.     

Time distribution of adsorption entropy of gases on heterogeneous surfaces by reversed-flow gas chromatography

The reversed-flow gas chromatography (RF-GC) technique has been applied to measure the adsorption entropy over time, when gaseous pentane is adsorbed on the surface of two solids (gamma-alumina and a silica supported rhodium catalyst) at 393.15 and 413.15K, respectively. Utilizing experimental chromatographic data, this novel methodology also permits the simultaneous measurement of the local adsorption energy, epsilon, local equilibrium adsorbed concentration, c(s)(*), and local adsorption isotherm, theta(p, T, epsilon) in a time resolved way. In contrast with other inverse gas chromatographic methods, which determine the standard entropy at zero surface coverage, the present method operates over a wide range of surface coverage taking into account not only the adsorbate-adsorbent interaction, but also the adsorbate-adsorbate interaction. One of the most interesting observations of the present work is the fact that the interaction of n-pentane is spontaneous on the Rh/SiO(2) catalyst for a very short time interval compared to that on gamma-Al(2)O(3). This can explain the different kinetic behavior of each particular gas-solid system, and it can be attributed to the fact that large amounts of n-C(5)H(12) are present on the active sites of the Rh/SiO(2) catalyst compared to those on gamma-Al(2)O(3), as the local equilibrium adsorbed concentration values, c(s)(*), indicate.     

Solid surface mapping by inverse gas chromatography

Inverse gas chromatography (IGC) at infinite dilution, is a technique for characterising solid surfaces. Current practice is the injection of n-alkane homologous series to obtain the free energy of adsorption of the CH2 group, from which the London component of the solid surface free energy, gamma(d)s, is calculated. A value around 40 mJ/m2 is obtained for poly(ethylene), and 30 mJ/m2 for a clean glass fibre, while the potential surface interactivity of a glass fibre is far greater than that of poly(ethylene). A specific component of the surface, in mJ/m2, should be calculated in order to obtain significant parameters. As applied up to date, when calculating the specific component of the surface energy, the fact that W(sp)a energy values are in a totally different scale than AN or DN values is a major drawback. Consequently, Ka and Kb values obtained are in arbitrary energy units, different from those of the London component measured by injecting the n-alkane series. This paper proposes a method to obtain Ka and Kb values of the surface in the same energetic scale than the London component. The method enables us to correct the traditional London component of a solid, obtaining a new value, where the amount of WaCH2 accounting for Debye interactions with polar sites, is excluded. As a result, an approach to surface mapping is performed in several different substrate materials. We show results obtained on different solid surfaces: poly(ethylene), clean glass fibre, glass beads, chemically modified glass beads and carbon fibre.     

Characterization of polyarylamide fibers by inverse gas chromatography

We determined a group of estrogenic compounds by solid-phase microextraction (SPME) coupled to high-performance liquid chromatography (HPLC) with both ultraviolet (UV) and electrochemical detection (ED). A modified liquid chromatograph was used. Polyacrylate fibers (85 microns) were used to extract the analytes from the aqueous samples. Dynamic and static modes of desorption were compared and the variables affecting both absorption and desorption processes in SPME-HPLC were optimized. Static desorption gave the best recoveries and peak shapes. The performance of the SPME-HPLC-UV-ED method was checked with river water and wastewater. The method enabled estrogenic compounds to be determined at low-microgram l-1 levels in real water samples. Limits of detection were between 0.3 and 1.1 micrograms l-1 using UV detection and between 0.06 and 0.08 microgram l-1 using ED. beta-Estradiol was found in samples from a wastewater treatment plant at concentrations between 1.9 and 2.2 micrograms l-1.     

Characterisation of cement pastes by inverse gas chromatography

Two cement pastes, commonly used in concrete formulations, were characterised by IGC at 35-80 degrees C before and after coating with an epoxy resin and a hardener. The cements are mixtures of hydrates in various proportions, such as calcium silicate hydrate (CaO-SiO2-H2O) and calcium hydroxide Ca(OH)2. Apolar and polar probes were used to determine the dispersive and acid-base characteristics of the cement pastes. These materials have high surface energy as judged from the dispersive contribution to the surface free energy (gamma(s)d) values lying in the 50-70 mJ/m2 range at 60-80 degrees C. Examination of the specific interactions permitted to show that the cement pastes are strongly amphoteric species with a substantial predominant Lewis basicity that is in line with the basic pH of their aqueous suspensions. Following coating with an epoxy resin (DGEBA) and a hardener (triethylene tetramine), the surface energy of the cements decreases substantially with the mass loading of the organic material. The surface thermodynamic properties were also correlated with the surface chemical composition as determined by X-ray photoelectron spectroscopy.     

Glass transitions of polyolefins by inverse gas chromatography

The glass transition temperatures of polypropylene, poly-1-butene, polyisobutylene, and an ethylene-propylene (EP) copolymer have been determined by inverse gas chromatography. The results are consistent with those determined by other procedures. When applied to low-, medium- and high-density polyethylene, values of ?35, ?25, and ?18°C, respectively, were obtained. Because of the fact that the chromatographic technique involves essentially a sensitive detection of free volume in a polymer, it is suggested that these values represent T_g if it is defined as an iso-free volume state of the amorphous phase of the polyethylene.     

Study of the mechanism of the interaction of vinyl chloride with water by reversed-flow gas chromatography.

The investigation of the mechanism for the interaction of vinyl chloride (VC) with liquid foods is of great significance in food science. In the present work the model system VC-water was studied by using the relatively new technique of reversed-flow gas chromatography. Using suitable mathematical analysis the following physicochemical quantities were determined: (a) diffusion coefficients of VC into water, (b) overall mass transfer coefficients of VC in the water, and in the carrier gas nitrogen, (c) partition coefficients of VC between water and nitrogen, and (d) constants of Henry's law for the adsorption of VC by water. From the variation of the above parameters with temperature, and the stirring rate of the water, useful conclusions concerning the mechanism for the VC-water interaction were extracted. The experimental results for the transfer of VC into the bulk water suggest (i) a mechanism consisted of a fast equilibrium step between the VC in the gas phase and in the interface, followed by a slow adsorption of VC into the bulk of water, which is the rate-determining step, when the water is quiescent, and (ii) a mechanism consisted of a slow diffusion of VC from the gas phase to the interface, which is the rate-determining step, followed by a fast equilibrium step between the VC in the interface and in the water bulk, when the water is stirred.     

Examination of polyolefins-organic compounds interactions by inverse gas chromatography

The investigations of interactions between polyolefins and test solutes at temperatures 58–122°C were carried out in the work. The test solutes were intentionally selected as representatives of the most important groups of compounds occurring in technological oils, which may be used as additives in conditions of industrial decomposition of polyolefins in Poland. For this purpose both the Flory-Huggins theory and inverse gas chromatography (IGC) were used. On the basis of retention data the values of both interaction and solubility parameters of analyzed polymers were determined. Solubility parameter δ and interaction parameter χ are related to some heat quantities e.g. excess free energy of mixing. It was observed influence of molecular mass and existence of chain branches on the values of the parameters. The obtained values allowed determination of influence of composition change of typical technological oils on their interactions with polymers and, at the same time, on course of charge preparation in these processes.