RESUMO
During the conversion of natural gas to liquified natural gas, sulfur components are separated by adsorption on zeolites. New zeolite materials may improve this adsorption process. In this paper, the adsorption of hydrogen sulfide is studied on seven faujasite (FAU) zeolites, which differ only in the number of sodium and calcium cations. From a pure NaX zeolite (13X), which contains only sodium cations, the calcium cation content was gradually increased by ion exchange. In a fixed-bed adsorber, cumulative equilibrium loadings of H2S on these zeolites were determined at concentrations between 50 and 2000 ppm at 25 and 85 °C and 1.3 bar (abs). Adsorption isotherms were analyzed considering the influence of cation positioning in the FAU zeolites. The experimental data indicate a superposition of a chemisorptive and a physisorptive mechanism. At a small number of chemisorptive sites, we conclude a dissociation of hydrogen sulfide and covalent bonding of the proton and the hydrogen sulfide ion to the zeolite lattice. The contribution of chemisorption exhibits a very low temperature dependence, which is typical for nearly irreversible reactions with an equilibrium strongly shifted to one side. With an increase in the proportion of Ca2+ cations, only physisorptive adsorption by electrostatic interaction with the cations in the lattice was observed. A large number of physisorptive sites have a lower energetic value. The share of physisorption strongly depends on temperature, which is characteristic of reversible equilibrium reactions.
RESUMO
Since the recent discovery of the template-free synthesis of porous boron nitride, research on the synthesis and application of the material has steadily increased. Nevertheless, the formation mechanism of boron nitride is not yet fully understood. Especially for the complex precursor decomposition of urea-based turbostratic boron nitride (t-BN), a profound understanding is still lacking. Therefore, in this publication, we investigate the influence of different common pre-heating temperatures of 100, 200, 300, and 400 °C on the subsequent properties of t-BN. We show that the structure and porosity of t-BN can be changed by preheating, where a predominantly mesoporous material can be obtained. Within these investigations, the sample BN-300/2 depicts the highest mesopore surface area of 242 m2 g-1 with a low amount of micropores compared to other BNs. By thermal gravimetric analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy, valid details about the formation of intermediates, types of chemical bonds, and the generation of t-BN are delivered. Hence, we conclude that the formation of a mesoporous material arises due to a more complete decomposition of the urea precursor by pre-heating.
RESUMO
The adsorption of elemental mercury (Hg0) on activated carbons modified with 0.2, 0.6, and 1 M HCl is systematically examined. Breakthrough curves are measured, and coupled adsorption and desorption experiments with temperature-programmed desorption (TPD) are performed. The experiments show that impregnation with HCl produces surface-bound chlorine, which significantly increases the capacity of activated carbons for mercury. Physisorptive interactions between elemental mercury and the activated carbon surface dominate on the basic materials. In contrast, on HCl-modified activated carbons, chemisorptive interactions of Hg0 with surface-bound chlorine lead to a complex involving carbon, chlorine, and mercury. Using TPD, two mechanisms could be identified that yield reaction products with different energetic values. By continuously recording Hg0 and Hgtotal concentrations, the formation of Hg0 and Hg x Cl2 during desorption of the complexes from the surface could be studied. It is shown that Hg x Cl2 found in TPD is not present as a solid salt in the pores but is formed by thermal degradation of the mercury chlorine complex on the carbon surface. The mass fraction of Hg measured in TPD which is bound in Hg x Cl2 depends on the Hg loading of the activated carbons, with a maximum mass fraction of 27%. We propose that an important step in the chemisorptive reaction with increasing mercury loading is the conversion of a HgCl2 complex into a Hg2Cl2 complex.
RESUMO
In this work, the influence of water on the adsorption of mercury is systematically investigated on basic and washed activated carbons. Breakthrough curves were measured and temperature-programmed desorption (TPD) experiments were performed with mercury and water. Both physisorptive and chemisorptive interactions are relevant in the adsorption of mercury. The experiments show that the presence of water in the pores promotes chemisorption of mercury on washed activated carbons while there is little influence on chemisorption on basic materials. Washing exposes or forms oxygen functional groups that are chemisorptive sites for mercury. Obviously, effective chemisorption of mercury requires both the presence of water and of oxygen functional groups. As mercury chemisorption is preceded by a physisorptive step, higher physisorptive mercury loading at lower temperature (30 °C) enhances chemisorption though the reaction rate constant is smaller than at higher temperature (100 °C). Sequential adsorption and partial desorption of water at lower temperature changes the surface chemistry without inhibiting mercury physisorption. Here, the highest chemisorption rates were found. The number of desorption peaks in the TPD experiments corresponds to the number of adsorption and desorption mechanisms with different oxygen functional groups in the presence of water. The results of the TPD experiments were simulated using a transport model extended by an approach for chemisorption. The simulation results provide reaction parameters (activation energy, frequency factor, and reaction order) of each mechanism. As in many heterogeneously catalyzed reactions, the activation energy and the frequency factor are independent of mercury loading and increase with increasing temperature.
