Adsorption energetics of hydrocarbons on silicalite

The differential adsorption heat curves for hydrocarbons on silicalite feature Langmuir invariance without apparent interaction for heptane, a linear increase with increasing occupancy for pentane, and great complexity with both minima and maxima for benzene. The adsorption heat increment per CH₂ group is 10.0 kJ/mole from the adsorption heat data for ethane, butane, pentane, and heptane, while the free regression term corresponding to the adsorption of 2H or H ₂ is 11.5 kJ/mole. The mean molar entropies of pentane and heptane is less than the entropies of the liquids by -60 J/mole · K, while the state of normal alkanes in silicalite channels is solidlike. The isotherms for the adsorption of hydrocarbons on silicalite are described completely by the volumetricmicropore occupancy theory equations. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2333–2335, October, 1989.     

Analysis of volatile halogenated hydrocarbons on the ppq scale

Summary The use of volatile halogenated hydrocarbons in manufacturing and industry has turned these substances into a severe problem for our environment. Therefore, industries and environmental protection agencies have become very interested in a rapid and trace-scale analysis of these compounds. Investigations have therefore been undertaken on the separation of such components by means of GC-ECD and GC-MS, respectively. The applicability of the analysis system elaborated is demonstrated for actual water samples and biological materials.

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Analyse von flüchtigen halogenierten Kohlenwasserstoffen im ppq-Bereich

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Parts of this work were presented at the 7. Österreichische Chemietage, Vienna, 3–6 Nov. 1987     

Adsorption of hydrocarbons on organo-clays--implications for oil spill remediation

Organo-clays synthesised by the ion exchange of sodium in Wyoming Na-montmorillonite (SWy-2-MMT) with three surfactants: (a) octadecyltrimethylammonium bromide (ODTMA), formula C(21)H(46)NBr; (b) dodecyldimethylammonium bromide (DDDMA), formula C(22)H(48)BrN; and (c) di(hydrogenated tallow)dimethylammonium chloride were tested for hydrocarbon adsorption. Using diesel, hydraulic oil, and engine oil an evaluation was made of the effectiveness of the sorbent materials for a range of hydrocarbon products that are likely to be involved in land-based oil spills. It was found that the hydrocarbon sorption capacity of the organo-clays depended upon the materials and surfactants used in the organo-clay synthesis. Greater adsorption was obtained if the surfactant contained two or more hydrocarbon long chains. Extensive utilisation of chemometrics principally with the aid of MCDM methods, produced models which consistently ranked the organo-clays well above any of the competitors including commercial benchmark materials. Thus, the use of organo-clays for cleaning up oil spills is feasible due to its many desirable properties such as high hydrocarbon sorption and retention capacities, hydrophobicity. The negative effects of the use of organo-clays for oil-spill cleanup are the cost, the biodegradability, and recyclability of the organo-clays.     

Trace analysis of volatile halogenated hydrocarbons in water

Summary Simple sampling and preconcentration technique for the analysis of volatile halogenated hydrocarbons in water is described. For the separation a selective capillary column is used in combination with an ECD. Quantitative data are given.     

Adsorption of hydrocarbons on mesoporous SBA-15 and PHTS materials

Plugged hexagonal templated silica (PHTS) materials are synthesized using a high TEOS/EO(20)PO(70)EO(20) ratio in the SBA-15 synthesis. This generates internal microporous nanocapsules or plugs in part of the channels, which could be inferred from the two-step desorption branch. These materials exhibit a tunable amount of open and plugged pores and a very high micropore volume (up to 0.24 mL/g) and are more stable than the conventional micellar templated structures known so far. In this study the adsorption properties of PHTS are investigated and compared to those of its plug-free analogue SBA-15. For this purpose nitrogen, n-hexane, n-heptane, c-hexane, 3-methylpentane, 1-hexene, and water were adsorbed on SBA-15 and PHTSs with a different ratio of open and plugged mesopores. The adsorption of n-hexane, c-hexane, n-heptane, and 3-methylpentane on SBA-15 and PHTS-A demonstrated that the presence of the plugs had an effect on the uptake of adsorbate in the low relative pressure region, the position of the capillary condensation step, and the total adsorbed amount of adsorbate. The results showed that n-heptane and 3-methylpentane cannot access part of the micropore system of SBA-15 and PHTS-A. Adsorption of c-hexane and n-hexane on PHTS-A indicated that not only the kinetic diameter but also the shape of the molecule is an important factor for being able to be adsorbed into the micropores or past the plugs. Moreover, these two adsorbates were the most efficient in filling up the available pore volume. From the adsorption of n-hexane on PHTSs with a different ratio of open and plugged pores, it was concluded that the size of the plugs differed, which depends on the synthesis conditions. Water adsorption isotherms proved SBA-15 and PHTS-B to be more hydrophobic than PHTS-A. n-Hexane, 1-hexene, and toluene were adsorbed on SBA-15 and the PHTSs to investigate the influence of the polarity of the adsorbate. The isotherms showed higher uptakes for polar adsorbates on more hydrophobic materials and vice versa.     

Adsorption and conversion of various hydrocarbons on monolithic hydrocarbon adsorber

Adsorption and conversion of various hydrocarbons on monolithic hydrocarbon adsorbers were studied using a new experimental model, temperature-programmed adsorption (TPA). In this study, methyl alcohol, acetone, acetaldehyde, 2,2,4-trimethylpentane, n-octane, and toluene were chosen as model hydrocarbons for cold start of a vehicle. The effect of the hydrocarbon components and oxygen concentration on the TPA curve was investigated. Depending on the presence of O(2), the adsorbed and desorbed amounts of the hydrocarbons were decreased, while the conversion efficiency of the hydrocarbons was increased. In the case of hydrocarbons containing oxygen, the thermal decomposition appeared to be in the order methanol, acetaldehyde, and acetone.     

Trends of volatile chlorinated hydrocarbons and trihalomethanes in Antarctica

In order to understand behaviour and environmental fate of manmade chemicals in remote and cold areas, during the XXVII Italian Expedition carried out in Antarctica throughout the austral summer 2011/2012, superficial snow and lake water were sampled along the Ross Sea's South coast and their content of some low-molecular weight volatile halogenated hydrocarbons was evaluated. In consideration of their important role in stratospheric ozone chemistry, some volatile chlorinated hydrocarbons (VCHCs) and trihalomethanes (THMs) were investigated. The analyses were realised with a dedicated system composed by a purge-and-trap injector coupled to a gas chromatograph with a mass spectrometer (PTI-GC-MS) operating in SIM mode. The investigated VCHCs (chloroform; 1,1,1-trichloroethane; tetrachloromethane; trichloroethylene and tetrachloroethylene) were present in all analysed samples, and concentration levels ranged from units to hundreds of ng L^?1 according to considered matrix. For the first time, THMs (bromoform; dibromochloromethane; bromodichloromethane), were measured in Antarctic lake waters and freshly deposited snow; their concentration levels ranged from units to tens of ng L^?1. In order to assess eventual temporal variations, VCHC content in aqueous Antarctic matrices was compared with levels occurring in the past Italian Antarctic expeditions: for some banned substances, a decrease in concentration was observed, probably due to worldwide use restrictions. Finally, current Antarctic and Italian VCHC and THM levels in snow and lake water samples were compared and were found to be quite similar, differing at most by one order of magnitude, corroborating the hypothesis of an accumulation of halogenated compounds in Antarctic aqueous matrices.     

Adsorption of C7 hydrocarbons on biporous SBA-15 mesoporous silica

In our recent studies (Vinh-Thang, H.; Huang, Q.; Eic, M.; Trong-On, D.; Kaliaguine, S. Langmuir 2005, 21, 2051-2057; Vinh-Thang, H.; Huang, Q.; Eic, M.; Trong-On, D.; Kaliaguine, S. Stud. Surf. Sci. Catal. 2005, in press), a series of synthesized SBA-15 materials were characterized using nitrogen adsorption/desorption isotherms at 77 K and SEM images. In the present paper, four of them (MMS-1-RT, MMS-1-60, MMS-1-80, and MMS-5-80) were further investigated with regard to their equilibrium characteristics using n-heptane and toluene as sorbates by the standard gravimetric technique. SBA-15 materials proved to have a broad pore size distribution within the micropore/small-mesopore range in the walls of their main mesoporous channels. The adsorption capacities for toluene were found to be higher than for n-heptane. The isosteric heats of adsorption, estimated by the Clausius-Clapeyron equation, are also higher for toluene compared to n-heptane. They were found to depend on framework microporosity of the relevant SBA-15 samples. The isosteric heats of adsorption for all sorbates decrease with increased loading and approach the heats of evaporation of the respective sorbate. The adsorption capacities of SBA-15 samples are significantly higher than those of silicalite, i.e., the MFI zeolite silica analogue. In contrast to that, the isosteric heats of adsorption in the mesopore channels of SBA-15 were found to be much smaller. This result also suggests that SBA-15 can potentially be a good candidate for separation of C(7) hydrocarbons.     

Solid-phase microextraction-gas chromatographic determination of volatile monoaromatic hydrocarbons in soil

Benzene, toluene, ethylbenzene, three isomers of xylene, and cumene have been isolated and enriched from soil samples by a combination of water extraction at room and elevated temperature and headspace-solid-phase microextraction before their gas chromatographic-mass spectrometric (GC-MS) determination. The conditions used for all stages of sample preparation and chromatographic analysis were optimized. Analytes sampled on a polydimethylsiloxane-coated solid-phase microextraction fiber were thermally desorbed in the split/splitless injector of a gas chromatograph (GC) coupled with a mass spectrometer (MS). The desorption temperature was optimized. The GC separation was performed in a capillary column. Detection limits were found to be of the order of ca. 1 ng g(-1). Relative recoveries of the analytes from soils were found to be highly dependent on soil organic-matter content and on compound identity; they ranged from ca 92 to 96% for sandy soil (extraction at room temperature) and from ca 27 to 55% for peaty soil (extraction at elevated temperature). A few real-world soil samples were analyzed; the individual monoaromatic hydrocarbon content ranged from below detection limits to 6.4 ng g(-1) for benzene and 8.1 for the total of p- + m-xylene.     

The composition of volatile and particulate hydrocarbons in urban air

Summary The hydrocarbon composition of the particle and gas phases in the urban atmosphere has been studied by filtration and parallel adsorption on active charcoal and polyurethane foam (PUF). Gas chromatography (GC) and GC coupled to mass spectrometry (GC-MS) have been used for the analysis of the organic matter extracts obtained with each system. In the case of the particle and PUF samples these extracts were fractionated into aliphatic and aromatic compounds. This approach has allowed to identify up to 247 hydrocarbon molecules showing that C0–C5 alkylbenzenes are the major compounds in the charcoal extracts whereas C14–C23 n-alkanes, C0–C4 alkylnaphthalenes, C0–C4 alkylbiphenyls and C3–C5 alkylbenzenes are those predominantly found in the PUF materials. The particles essentially contain C17–C38 n-alkanes and parent polycyclic aromatic hydrocarbons (PAH). These qualitative differences are paralleled by a progressive concentration decrease from the more to the less volatile hydrocarbons. Thus, the total charcoal extracts average 80 g/m³, the PUF compounds represent 4 /m³ and the particle hydrocarbons 0.7 g/m³. These latter airborne materials are essentially composed of petrogenic residues 0.6 g/m³ (aliphatic fraction) whereas the pyrolytic PAH only involve 0.08 g/m³.     

Adsorption and catalytic conversion of hydrocarbons on nanosized gold particles immobilized on alumina

Gold and nickel particles immobilized on alumina were prepared by the metal vapor synthesis and anionic adsorption methods. The dispersion of metals was determined by X-ray diffraction and transmission electron microscopy. The activity of nanoparticles, tested in model catalytic reactions of CCl₄ addition to multiple bonds and allyl isomerization of allylbenzene, changes in a wide range (from 1 to 3000 (mole of product) (mole of Au)⁻¹ h⁻¹) and is parallel to the chromatographically measured heats of adsorption of the corresponding unsaturated substrates. The heat of adsorption of unsaturated hydrocarbons can serve as a criterion for the efficiency of the gold-containing catalyst in olefin conversion. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2215–2218, October, 2005.     

Adsorption and thermodynamic behavior of uranium on natural zeolite

Adsorptive behavior of natural clinoptilolite-rich zeolite from Balikesir deposites in Turkey was assessed for the removal of uranium from aqueous solutions. The uranium uptake and cation exchange capacities of zeolite were determined. The effect of initial uranium concentrations in solution was studied in detail at the optimum conditions determined before (pH 2.0, contact time: 60 minutes, temperature: 20 °C). The uptake equilibrium is best described by Langmuir adsorption isotherm. Some thermodynamic parameters (ΔH°, ΔS°, ΔG°) of the adsorption system were also determined. Application to fixation of uranium to zeolite was performed. The uptake of uranium complex on zeolite followed Langmuir adsorption isotherm for the initial concentration (25 to 100 μg/ml). Thermodynamic values of ΔG°, ΔS° and ΔH° found show the spontaneous and exothermic nature of the process of uranium ions uptake by natural zeolite. This revised version was published online in August 2006 with corrections to the Cover Date.     

Capture of volatile chlorinated hydrocarbons by aqueous solutions of branched cyclodextrins

The use of aqueous solutions of branched cyclodextrins was examined in order to develop an effective method of capturing toxic volatile chlorinated hydrocarbons such as trichloroethylene and monochlorobenzene. From the experiments in which trichloroethylene diluted with nitrogen gas came into contact with aqueous solutions of branched cyclodextrin mixtures, it was found that absorption could be performed without the formation of inclusion complex solids, which should simplify the whole process of absorption and recovery.     

In-tube extraction for enrichment of volatile organic hydrocarbons from aqueous samples

In-tube extraction (ITEX) is a novel solventless extraction technique in which a headspace syringe with a needle body filled with a sorbent (here: Tenax TA) is used. The analytes are extracted from sample headspace by dynamic extraction. The needle body is surrounded by a separate heater, which is used for thermal desorption of analytes into the injection port of a GC system. We report here for the first time the optimization and evaluation of a fully automated analytical method based on ITEX. As target analytes, 19 common groundwater contaminants such as halogenated volatiles and monoaromatic compounds have been chosen. Method related parameters such as extraction temperature, number of extraction cycles, extraction and desorption volume as well as extraction and desorption flow rates were investigated in detail. The linear dynamic range of the ITEX method ranged over six orders of magnitude between 0.028 microg/L and 1218 microg/L with linear correlation coefficients between 0.990 and 0.998 for the investigated compounds. Method detection limits for monoaromatic compounds were between 28 ng/L (ethylbenzene) and 68 ng/L (1,2,4-trimethylbenzene). For halogenated volatile organic compounds, method detection limits between 48 ng/L (chloroform) and 799 ng/L (dichloromethane) were obtained. The precision of the method with external calibration was between 3.1% (chloroform ethylbenzene) and 7.4% (1,2,3-trimethylbenzene).     

Adsorption of volatile atmospheric pollutants on an heterogeneous surface of surrogate solid aerosols

Adsorption of 23 nonpolar and polar volatile atmospheric organic pollutants on the surface of silica gel, carbon black (Carbopack), and carbon-coated silica gel (Carbosil), materials imitating mineral and carbonaceous aerosols, was studied by inverse gas chromatography at finite concentrations. The results obtained suggest that the surface is nonuniform. The experimental adsorption isotherms were described within the framework of an approach based on the local Langmuir equation and a rectangular surface area distribution over the adsorption energy. It was established how the mean and variance moments of these distributions depend on the polarizability of the sorbates and their acid-base characteristics and how these parameters influence the adsorption characteristics of the system under study.