Effects of the Ni–based MOFs structure as a case study on the extraction process

Metal organic frameworks (MOFs) are an attractive class of porous materials, which have exciting features such as high surface area, great potential for post synthetic modification and good thermal stability. Here as a case study, effects of the shape and size of the pores in the nanostructured MOFs on the adsorption and extraction of analytes were investigated. For this purpose, two MOFs based on Ni2 (1, 4–benzene dicarboxylic acid)2 triethylenediamine were synthesized adapting two different methods. Although in both methodologies identical constituents were employed, discriminated crystallographic and morphological structures were produced. The method based on the solvothermal methodology (MOF1) led to the cubic pores, and the one at room temperature (MOF2), gave trigonal and hexagonal pores [1]. The main aim in this study was to investigate whether the MOF cages have any possible influence on the extraction efficiency of some molecules in which their sizes are compatible with the size of the cages. The synthesized nanoscale MOFs were characterized by PXRD, BET, TGA, FESEM and FT–IR. Extraction experiments were performed in a headspace needle trap extraction setup using chlorobenzenes (CBs) and benzene homologs including benzene, toluene, ethylbenzene and xylene (BTEX). After performing the extraction, the entrapped analytes were thermally desorbed and transferred into gas chromatography–mass spectrometry (GC–MS) [2]. To evaluate the effects of pore size on the extraction and adsorption selectivity, CBs and BTEX were selected as analytes and their extraction were examined by the two synthesized MOFs. Results show different patterns for extraction of benzene, toluene and monochlorobenzene, which could be related to the size of trigonal pores in MOF2. These pores apparently, can trap these three molecules based on size similarity between them and trigonal cages. The performance of MOF2 toward CBs and BTEX was favorable. Influencing parameters affecting the extraction process were optimized [3]. Eventually, the developed method was validated by GC–MS. At the optimum conditions, the relative standard deviation (%RSD) values for a double distilled water fortified with the selected CBs at 100 ng L−1 were 5–12% (n = 6) while the limits of detection results were between 2–10 ng L−1. The linear dynamic range was ranged from 6 to 1000 ng L−1 (R2 0.988). Finally the developed method was applied to the analysis of some rivers and tap water samples and the relative recovery values were in the range of 85–96%.