Selectivity in adsorptive separations can be enhanced by limiting solute-sorbent interactions to a single or a few specific mechanisms. This work examines the potential of exploiting solute-sorbent hydrogen bonding as a selective adsorption mechanism, for solute adsorption from a nonpolar solvent onto a polycarboxylic ester sorbent. The hydrogen bond is believed to be formed between a proton-donating group on the solute and the carbonyl group on the sorbent. Studies were conducted for three classes of solutes, all of which can hydrogen bond, to determine whether differences in the strengths of adsorption can be exploited for separations. The enthalpies for adsorption from the nonpolar solvent onto the polycarboxylic ester sorbent were determined from calorimetry to be -5.1, -6.4, and -8.2 kcal/mol for the adsorption of N-methylaniline, alcohols, and phenols, respectively.In single-solute-adsorption studies with these solutes, we also observed a strong correlation between the adsorption affinity and the adsorption enthalpy. In studies on the adsorption from mixtures of two solutes, we observed that the solute with the higher adsorption enthalpy was preferentially adsorbed and that the temperature dependency of the separation factor could be related to the difference in the adsorption enthalpies of the two solutes. A simple thermodynamic framework, using data from single-solute studies, was capable of successfully predicting separation factors and the temperature dependence of separation factors.A mass-transfer model for structured packings under supercritical extraction conditions based on a model for structured packings under distillation conditions has been developed. Correlations were used to determine the required physical properties. The model was tested by measuring the height equivalent to a theoretical plate (HETP) of laboratory gauze packing in a column of 3.5-cm internal diameter for the system hexadecane/2-methylnaphthalene/carbon dioxide. Comparison of the measured and calculated HETP as a function of pressure, temperature, composition, and extraction factor showed that the mass-transfer model can be used to predict the mass-transfer efficiency of a laboratory gauze packing with reasonable accuracy. Furthermore, it was demonstrated how the developed model can be used to evaluate the effect of scale-up on the mass-transfer efficiency.

By limiting the number of adsorptive mechanisms, we believe it is possible to develop highly selective sorbents. In this work, our goal was to develop a sorbent that was capable of selectively adsorbing solutes from water through the formation of a hydrogen bond. When water was the solvent, a polycarboxylic ester resin was the sorbent, and a homologous series of phenylalkanols were used as solutes, it appeared that hydrophobic interactions were a predominant mechanism for adsorption. This conclusion is based on the observation that the affinity for adsorption increased with the number of methylene groups of the solute and that the free energy change for adsorption per methylene group was -0.83RT. This value is similar to free energy changes per methylene group observed for various phenomena that result from hydrophobic interactions. In contrast, when hexane was used as the solvent, we observed that the affinity for adsorption of this homologous series of phenylalkanols was independent of the number of methylene groups. Thus it appears that the mechanism of adsorption onto this polycarboxylic ester sorbent varies depending on the solvent. With hexane as the solvent, we believe adsorption results from the formation of a hydrogen bond between the hydroxyl group of the solute and the carbonyl group of the sorbent. To exploit the specificity of the hydrogen bond for the adsorption of solutes from aqueous solutions, we developed a modified sorbent in which the hydrogen bonding site of the sorbent is retained in a nonpolar environment. For this modified sorbent, we filled the pores of the sorbent with hexane. Solutes that adsorb onto this modified sorbent must first partition from the aqueous phase into the hexane pore-phase, and then the solute must adsorb from the hexane pore-phase onto the hydrogen bonding site of the sorbent. Our results demonstrate that adsorption onto this modified sorbent can be quantitatively described by these individual steps. Thus this modified sorbent is capable of adsorbing solutes from aqueous solution through a combination of hydrophobic interactions (Le. the water-hexane partitioning step) and hydrogen bonding (Le. solute adsorption from the hexane pore-phase).

The oxidation of carbon monoxide on the Pt(110)(1×2) surface: The influence of the adlayer composition on the reaction dynamics

No abstract

The reactions of silane, SiH4, disilane, Si2H6, and phosphine, PH3, on single crystalline Si(100) and Si(111) surfaces, and methylsilane, SiH3CH3, on a β-SiC surface have been examined employing supersonic molecular beam scattering. The emphasis here is not on any one experimental result, but rather on the specific experimental approaches adopted and a selected set of results that serve to demonstrate the similarities and differences between these systems and the more extensively studied reactions occurring on transition metal surfaces. All reactions have been examined at substrate temperatures characteristic of steady-state thin film growth. Translational activation is observed to be an efficient means to promote the reactivity of the group IV species: SiH4, Si2H6, and SiH3CH3. In all cases, the reactivity increases exponentially with scaled incident kinetic energy, where the scaling analysis specifically takes into account the microcorrugation of the gas-surface potential in terms of how incident kinetic energy and angle of incidence couple to determine the probability of dissociative chemisorption. The reaction probability of SiH4 is described quantitatively over a wide range of reaction conditions by a recently published model that adapts Rice–Ramsperger–Kassel–Marcus theory to translationally activated dissociative chemisorption. In contrast, PH3, due to its coordinative unsaturation, is found to react almost exclusively via a trapping-mediated precursor dissociation channel. By employing a novel analysis scheme, the dependence of PH3 dissociative chemisorption on the fractional coverage of both P(a) and H(a) has been deduced under conditions where the desorption of H2 and P2 are finite. The experimental techniques described here, and the associated conclusions made in this work, should be of tremendous value in future studies directed at examinations of the gas-surface chemistry involved in epitaxial thin film growth.