Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and xylenes on Ca10(PO4)6(OH)2 – CaWO4

A W-containing apatite (W/HAp) catalyst was prepared following a hydrothermal synthesis route and served as a model catalyst. Crystallographic analysis indicated that the resulting material contained hydroxyapatite, Ca_10−3xW_x(PO₄)₆(OH)₂, W-hydroxyapatite, calcium tungstate, CaWO₄, and tricalcium phosphate, Ca₃(PO₄)₂. The catalyst was investigated in liquid phase oxidation of benzyl alcohol and xylenes using hydrogen peroxide as an oxidant. For comparison, commercial calcium phosphate, hydroxyapatite and CaWO₄ were tested in the same reaction. Calcium phosphate and hydroxyapatite appeared as inactive and decomposed hydrogen peroxide non-selectively. A moderate activity but low hydrogen peroxide efficiency was observed for the CaWO₄ phase. In contrast, the W/HAp catalyst showed a reasonable activity and a better hydrogen peroxide efficiency in the oxidation of benzyl alcohol and xylenes. This new W/HAp catalyst showed, after six cycles, losses of the activity below 15% compared to the fresh catalyst with no effect on the selectivity. It is noteworthy that ICP-OES analyses showed no tungsten leaching that is the main advantage of this catalyst.     

Simultaneous HPLC Determination of Urinary Metabolites of Toluene,Xylenes, and Ethylbenzene; and Its Application to Biological Monitoring of Tunisian Workers Exposure

Abstract

A simple, sensitive, and reproducible high performance liquid chromatographic procedure is described for the simultaneous determination of urinary metabolites of several aromatic hydrocarbons: mandelic and phenylglyoxylic acids from ethylbenzene; hippuric acid from toluene, and methylhippuric acids from xylenes. The employed mobile phase and the chromatographic conditions allowed relatively low detection limits (0.1 to 0.2 mg/L). The method was tested on 33 workers in the paint industry and on 33 nonoccupationally exposed Tunisian subjects. A significant difference of metabolite concentrations was found between the two groups. In the exposed group, most of the measured levels were lower than the biological exposure indices.     

吹扫捕集-气相色谱分析海水和沉积物中的苯、甲苯、乙苯及二甲苯

单环芳烃苯、甲苯、乙苯和二甲苯（简称BTEX）是石油的重要组分，也是环境中需要重点监测的致癌污染物。本实验建立了动态顶空（吹扫捕集）和光离子化检测器的气相色谱测量海水、沉积物中痕量BTEX的方法。在120—1200ng／L的浓度范围，苯、甲苯、乙苯、间对二甲苯及邻二甲苯标准溶液的检出限分别为6．4、35．2、15．8、12．3、10．7ng／L，相对标准偏差0．9％-6．1％。样品无需预处理，海水中BTEX回收率为93．50％-98．40％。7个渤海表层海水样品中BTEX的浓度均低于140ng／L；海底沉积物中苯、甲苯、乙苯、间对二甲苯及邻二甲苯浓度分别为169—1243、531—1732、1308—5624、237—1136、510—5194ng／L。测量方法和结果对评价环境污染具有重要意义。     

Liquid Phase Counter-Diffusion Measurements of Aromatics in Silicalite Using the ZLC Method

We have developed a liquid phase Zero Length Column apparatus in order to study the counter-diffusion of aromatic compounds, benzene, toluene, m-, o- and p-xylene, in silicalite crystals. Our results show ZLC desorption curves that are consistent with the model of Brandani and Ruthven (1995). The apparent diffusivities fall in a narrow range of approximately 1 order of magnitude for all the investigated molecules. The results for o-xylene led us to investigate in detail possible mechanisms that can give erroneous results when applying the liquid ZLC technique. We have derived solutions to models for valve leakage and obtained an explanation to the experimental results.     

Solid-phase microextraction and gas chromatography-mass spectrometry for determination of monoaromatic hydrocarbons in blood and urine: Application to people exposed to air pollutants

Summary To assess individual exposure to monoaromatic hydrocarbons (benzene, toluene, ethylbenzene and xylenes-BTEX) in biological fluids, a GC-MS method was developed. Headspace sampling of BTEX was by solidphase microextraction (SPME) with a 75 μm Carboxenpolydimethylsiloxane (PDMS) fiber. Linearity was established for concentrations up to 50 μg L⁻¹. Detection limits, calculated both in human blood and urine, ranged 5–10 ng L⁻¹. Repeatability was in the range 6.5–9.2% for all compounds. The method was applied to the evaluation of the internal dose of BTEX in a group of cyclists running for 2 h within city routes. Benzene and toluene in blood, and toluene and xylenes in urine significantly increased after exercise as compared to prerun values, such changes being consistent with airborne concentrations. The combination of SPME with GC-MS seems to represent an appropriate analytical approach to detect changes in the concentration of monoaromatic hydrocarbons in biological media resulting from exposure to environmental pollution.     

Shape Selectivity by Guest‐Driven Restructuring of a Porous Material

A flexible metal‐organic framework selectively sorbs para‐ (pX) over meta‐xylene (mX) by synergic restructuring around pX coupled with generation of unused void space upon mX loading. The nature of the structural change suggests more generally that flexible structures which are initially mismatched in terms of fit and capacity to the preferred guest are strong candidates for effective molecular separations.     

Separating Xylene Isomers by Commensurate Stacking of p‐Xylene within Channels of MAF‐X8

The development of energy‐efficient processes for selective separation of p‐xylene from mixtures with its isomers is of vital importance in the petrochemical industries. Current industrial practice uses BaX zeolite that has high adsorption selectivity for p‐xylene. Finding para‐selective structures is challenging. With state‐of‐the‐art simulation methodologies we systematically screened a wide variety of zeolites and metal–organic frameworks (MOFs). Our investigations highlight the crucial importance of the channel dimension on the separation. MAF‐X8 is particularly noteworthy because the channel dimensions and geometry allow “commensurate stacking” which we exploit as a separation mechanism at saturation conditions. Due to a significantly improved capacity compared to BaX, the cycle times for p‐xylene with MAF‐X8 are found to be about a factor of 4.5 longer. This is expected to result in significant process improvements.     

Arene Selectivity by a Flexible Coordination Polymer Host

The coordination polymers [Ag₄(O₂CCF₃)₄(phen)₃] ? phen ? arene ( 1? phen ? arene) (phen=phenazine; arene=toluene, p‐xylene or benzene) have been synthesised from the solution phase in a series of arene solvents and crystallographically characterised. By contrast, analogous syntheses from o‐xylene and m‐xylene as the solvent yield the solvent‐free coordination polymer [Ag₄(O₂CCF₃)₄(phen)₂] ( 2 ). Toluene, p‐xylene and benzene have been successfully used in mixed‐arene syntheses to template the formation of coordination polymers 1? phen ? arene, which incorporate o‐ or m‐xylene. The selectivity of 1? phen ? arene for the arene guests was determined, through pairwise competition experiments, to be p‐xylene>toluene≈benzene>o‐xylene>m‐xylene. The largest selectivity coefficient was determined as 14.2 for p‐xylene:m‐xylene and the smallest was 1.0 for toluene:benzene.     

Direct and Oriented Conversion of CO2 into Value-Added Aromatics

The oriented conversion of CO₂ into target high-value chemicals is an effective way to reduce carbon emissions, but still presents a challenge. In this communication, we report the oriented conversion of CO₂ into value-added aromatics, especially para-xylene, in a single pass by combining core–shell structured Zn-doped H-ZSM-5 (Zn-ZSM-5@SiO₂) and a Cr₂O₃ component. Through precise regulation of the acidity of Zn-ZSM-5@SiO₂, high para-xylene selectivity (38.7 % in the total products) at a CO₂ conversion of 22.1 % was achieved. Furthermore, a CO₂-assisted effect in the synthesis of aromatics during the tandem process has been clarified through a control experiment. The CO₂ reactant can act as a hydrogen acceptor to accelerate the dehydrogenation of alkenes, intermediates in the synthesis of aromatics, thereby increasing the driving force towards aromatics in the tandem reaction process.     

Supported C‐Scorpionate Vanadium(IV) Complexes as Reusable Catalysts for Xylene Oxidation

C‐Scorpionate vanadium(IV) [VO_xCl_3−x {κ³‐RC(pz)₃}] [pz=pyrazol‐1‐yl; x= 0, R=SO₃ ( 1 ); x= 1, R=CH₂OH ( 2 ) or CH₂OSO₂Me ( 3 )] complexes supported on functionalized carbon nanotubes (CNTs) are the first V‐scorpionate catalysts used so far for the neat oxidation of o ‐, m ‐ or p ‐xylene, with TBHP (70 % aqueous solution), to the corresponding toluic acids (main products), tolualdehydes and methylbenzyl alcohols. Remarkably, a p ‐toluic acid yield of 43 % (73 % selectivity, TON=1.34×10³) was obtained with 2 @CNT in a simple microwave‐assisted mild oxidation procedure, using a very low catalyst charge (3.2×10⁻² mol ^% vs. substrate). Further, this occurred in the absence of any bromine source, what is significant towards the development of a greener and more sustainable process for oxidation of xylenes. Moreover, reuse of catalysts with preservation of their activity was found for up to six consecutive cycles. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst or type of heating source, on the performance of the above catalytic systems are reported and discussed.