engleski

Influence of postsynthesis treatments on acid site strength of ZSM-5

Introduction Many organic reactions (Friedl-Crafts type, ring transformation, halogenation, condensation, isomerization, etc.) can be catalysed by solid acid catalyst. Among them, zeolites have been often used, due to the known nature of the active (acid) sites. It makes them unique catalysts in comparison with others such as metals, metal oxides, and sulphides. Conventional solid acids, (zeolites such as ZSM-5, USY, beta) of micrometer sized crystal, are model system for research of the relationship among their selectivity, activity, and deactivation in the polyolefins cracking, MTO conversion, methane dehydroaromatization, etc. At the same time, it shows importance of the accessibility of active sites to the larger molecules, due to steric and diffusion limitations. Experimental The ZSM-5 (MFI) was prepared using simple, classical hydrothermal synthesis. starting from the synthesis gel with molar composition: 0.10 Na2O – 0.08 TPABr – 1.00 SiO2 – 0.02 Al2O3 – 20.00 H2O. First, NaOH was dissolved in of distilled water, then Al(OH)3 and TPABr were added to the basic solution. Than precipitated SiO2 was added. Resulting gel was stirred for 1 hour at room temperature and kept in oven at 175 °C for 5 days in teflon-lined stainless steel autoclaves, under static conditions. After synthesis, product was washed, dried at 105°C and calcined in air at 550°C - to eliminate SDA. The H-form was obtained after two cycles of ion exchange with 1M NH4Cl (1 h each) followed by a calcination at 550°C. Desilication of the HZSM-5 was made using 0.2 M NaOH solution for 45 min at 65 °C, with stirring, and again transformed to H-form as described above. Results and discussion PXRD patterns show that all postsynthesis treatments (first desilication with NaOH solution, then wet impregnation (with Ag+, Mg2+, and Fe3+), than reassembly in reactor, and finally calcination) have not significant influence on crystallinity of the samples. At the same time, mesoporosity after dealumina tion increases about 4 times (Fig. 1). In comparison to starting material (HZSM-5) total amount of acid places decrease after desilication from 565.56 μmol/g to 523.83 μmol/g, to 509.77 μmol/g in Mg- ZSM-5, 485.21 μmol/g in Fe-ZSM-5, and surprisnigly increase to 590.64 μmol/g in Ag-ZSM- 5. The ratio between the number of strong NH3 desorption around 400 °C) and weak acid sites (NH3 desorption around 230 °C) significantly decrease in following order: HZSM-5 > desilicated HZSM-5 > Mg-ZSM-5 >. Fe-ZSM-5 > Ag-ZSM-5 (Fig 2), what is also confirmed by vacuum FTIR spectroscopy. The distribution of pores measured by nitrogen adsorption/desorption (BJH) of ZSM-5 crystals before (H-ZSM-5) and after postsynthesis treatment – desilication (parent), wet impregnation (Ag, Fe, Mg) and consecutive thermal treatment of samples were shown in Fig. 1., while NH3-TPD profiles of ZSM-5 samples before and after postsynthesis were shown in Fig. 2. Conclusions Introduction of the various metal cations into hierarchical ZSM-5 does not have influence on samples crystallinity, but significantly affects the ratio between weak and strong acid sites, mainly decreasing the number of strong acid sites, what was measured by NH4-TPD, and confirmed by vacuum FTIR spectroscopy. Acknowledgement: This work was supported by Croatian National Science Fundation, Project no. IP2016-06-2214

zeolite ; ZSM-5 ; acid site strength ; postsynthesis treatments

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