Gravimetric adsorption experiment

Figure 1. Schematic arrangement of a simultaneous volumetric and gravimetric adsorption experiment.

Calibration experiments with two nearly equal and well known calibration masses (mj ,m ) prior and after gravimetric adsorption experiments are recommended. This can avoid systematic uncertainties in experimental data due to (slow) shifts of the zero-point-position of the balance. 

To examine the fundamental basis on which experimental definitions depend, consider an adsorption experiment incorporating both volumetric and gravimetric measurements (sec Fig. 1). A measured amount, n, of a specified gas (for simplicity, adsorption of a single gas is considered here), is introduced into the system whose total volume, V, can be varied at con-... 

The detailed adsorption isotherms of N, O, and Ar on mesoporous samples at 77 K were gravimetrically determined with a computer-aided apparatus. Only the adsorption isotherms of MCM41 were measured volumetrically using a commercial equipment (Quantachrome. Autosorb-1). The desorption branch was measured after the measurement of the adsorption branch and it took 20 min-1 h to reach the desorption equilibrium. The samples were preevacuated at 383 K and I mPa for 2 hr prior to the adsorption experiment (the preheating temperature for the volumetric measurement was 423 K). 

In concluding this section we want to emphasize that gas adsorption experiments also can be done at elevated temperatures. Preferently they should be carried out gravimetrically using a highly sensitive microbalance. These so-called thermogravimetric measurements can be used to monitor surface and structural changes of the sorbent material caused by physical or chemical modifications [1.3]. 

To measure binary coadsorption equilibria by tbe volumetric-gravimetric method one proceeds as follows A sorbent sample of 1 g - 3 g and appropriate counterweights, typically lead or silver balls, are placed to the buckets of the microbalance. Then the sorbent is activated by exposing it to helium gas at higher temperatures, i. e. 433 K for activated carbons, 673 K for zeolites and inorganic molecular sieves. After cooling down and evacuation (< 10 Pa) the adsorption chamber is prepared for an adsorption experiment. 

Monte Carlo simulations generate absolute adsorption data, i.e. the actual number of molecules present in the simulated pore space, while adsorption experiments give Gibbs excess properties [3], obtained by eiffier volumetric or gravimetric methods. Therefore, the simulation results must be converted to their excess counterparts before they can be used to analyze experimental data. The excess amount adsorbed (experimental result), Ngx, is given by ... 

Methanol chemisorption experiments have also been carried out at atmospheric pressure in a specially adapted thermal gravimetric analyzer (TGA) microbalance coupled with a PC for temperature and weight monitoring [38]. The system allowed a controlled flow of high purity gases air for pretreatment, helium and a mixture of 2000 ppm methanol in helium for adsorption experiments. 

Experiments Sorption equihbria are measured using apparatuses and methods classified as volumetric, gravimetric, flow-through (frontal analysis), and chromatographic. Apparatuses are discussed by Yang (gen. refs.). Heats of adsorption can be determined from isotherms measured at different temperatures or measured independently by calorimetric methods. 

Pure component loadings for CO2, N2 and O2 on commercial pelleted forms of Linde type 4A, 5A and 13X molecular sieve zeolites were derived from various gravimetric and volumetric measurements. The range of pressures and temperatures over which these measurements were made were at least as broad as those encountered in the breakthrough experiments described here, to permit accurate estimations of heats of adsorption in the manner described by equation (6) above. As mentioned above, the pure component data were correlated to the LRC model, and the CO2 loadings predicted by the multicomponent LRC model compared to actual loadings in the breakthrough runs at bed saturation. 

Several experimental methods are applied to measure adsorption isotherms. The main problem is to determine the amount adsorbed. One method is a gravimetric measurement. In a gravimetric measurement the weight increase as a function of the pressure is determined. The adsorbent, usually in the form of a powder, is placed into a bulb and kept at the desired temperature. The bulb is mounted on a sensitive balance. Before the experiment, the bulb is... 

Gravimetry can also be associated with adsorption manometry, which is a simple and safe way to study co-adsorption of two gases, provided their molar masses are sufficiently different (Keller et al, 1992). The manometric experiment provides a total amount adsorbed n°a = n° + n%, whereas the gravimetric experiment provides the total mass adsorbed m t=m° + m. Since n°Mx = m and n M1 = m" we have two unknowns, n° and n, and two equations. From these we can obtain, for instance ... 

In this study, equilibria and isosteric heat of adsorption for the system of chlorinated hydrocarbons and Y-type zeolite were obtained with gravimetric method and chromatographic method. By comparing an experiment result with a molecular simulation result, the validity of forcefield parameters and zeohte model was exartuned... 

The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method was used to get the initial slope of the isotherms. In the simulation, GCMC method was used to calculate amounts adsorbed for various conditions. When the experiment result and simulation result of chloroform are compared, the simulation for the acid site model was most agreement with chromatographic data and baratron data. The simulation result of tetrachloroethylene with three models corresponded mostly for the non-polar molecule, and above all the acid site model was the closest to the experiment result. Therefore, to get better coincidence between experimental data and simulation, it was found to be necessary to account for aluminum rather than silanol nest. 

IR spectroscopy was mainly used to characterize the sorbed species. The zeolite powder was pressed into self supporting wafers and analyzed in situ during all treatments (i.e., activation, sorption, reaction) by means of transmission absorption IR spectroscopy using a BRUKER IPS 88 FTIR spectrometer (resolution 4 cm" ). For the sorption experiments, an IR cell equipped with IR transparent windows which could be evacuated to pressures below 10" mbar was used [11]. The activated zeolite wafer was contacted with a constant partial pressure (0.001 mbar) of the adsorbate at 308 K until adsorption-desorption equilibrium was reached (which was monitored by time resolved IR spectroscopy). For the coadsorption experiments, the catalysts were equilibrated with 0.001 mbar of both adsorbates admitted in sequentional order. The spectra were normalized for the sample thickness by comparing the intensities of the absorption bands of the adsorbate with the integral intensity of the lattice vibration bands of the zeolite between 2090 and 1740 cm". The surface coverage was quantified by calibration with gravimetric measurements (under conditions identical to the IR spectroscopic experiments). 

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