Sinteza in karakterizacija produktov mehanokemijske sinteze imidazolatov in tetrahidridoboratov lahkih kovin

Šibav, Lia (author), Meden, Anton (mentor)

Vodik je eden izmed obetavnih energentov, ki bi se v prihodnosti lahko uporabljali kot gorivo. Ustreznega načina shranjevanja vodika, ki bi bil primeren za množično uporabo tega plina kot gorivo, še ni. V okviru magistrskega dela smo poskušali z mehanokemijsko sintezo pripraviti nove materiale HOF (kovinsko-hidrido-organske mreže), ki bi lahko bili potencialni materiali za shranjevanje vodika. Izvedla sem več mehanokemijskih sintez z različnimi kombinacijami dveh prekurzorjev, in sicer tetrahidridoborata lahkih kovin (Li, Mg in Na) in imidazolata (litijev imidazolat, natrijev imidazolat in litijev benzimidazolat). S posamezno kombinacijo tetrahidridoborata in imidazolata sem izvedla več sintez z različnimi množinskimi razmerji med prekurzorjema brez uporabe topila in z dodatkom topila (acetonitril ali etil acetat). Cilj je bil priti do kristaliničnega produkta in raziskati, katere kombinacije so najobetavnejše za nadaljnje raziskave. Vzorce smo analizirali z rentgensko praškovno difrakcijo, nekatere pa tudi z DSC, TG-MS in temperaturno programirano rentgensko praškovno difrakcijo. Večina kombinacij prekurzorjev se je izkazala za neuspešne in veliko vzorcev po mehanokemijski sintezi je imelo prisotno amorfno fazo. Pri večini vzorcev smo v difraktogramih zaznali še nezreagirane prekurzorje. Ugotovili smo, da je najobetavnejša kombinacija za nadaljnje delo kombinacija litijevega tetrahidridoborata in litijevega benzimidazolata, saj smo v difraktogramih vzorcev z razmerji z večjim deležem litijevega tetrahidridoborata opazili vrhove nove kristalinične faze oziroma faz. Pri množinskem razmerju teh dveh prekurzorjev 3 : 1 sem poskušala optimizirati program mletja in raziskati, kako spreminjanje parametrov sinteze vpliva na rezultat sinteze. Izkazalo se je, da so bolj obetavne manjše hitrosti mletja (650 in 750 rpm) in manj obetavne daljše sinteze (predvsem 12 ciklov). Analize DSC in TG-MS so pokazale, da se pri segrevanju vzorcev zgoraj omenjene kombinacije in razmerja prekurzorjev 3 : 1 iz njih sprošča vodik in po temperaturi 300 °C pirolizno razpadejo. Pri temperaturno programirani analizi XRD treh vzorcev se je izkazalo, da pri temperaturi 500 °C kristalizirata dva litijeva borata.

• Language: Slovenian
• Keywords: mehanokemijska sinteza, kovinsko-hidrido-organske mreže, tetrahidridoborati, imidazolati
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2020
• PID: 20.500.12556/RUL-118579
• COBISS.SI-ID: 27019011
• Publication date in RUL: 27.08.2020

Secondary language

• Language: English
• Title: Synthesis and characterization of mechanochemical synthesis products of light metal imidazolates and tetrahydridoborates

Abstract

Hydrogen is one of the most prominent energy carriers, which could be used as fuel in the future. Currently, there is no suitable storage technology for mass use of hydrogen as fuel. The purpose of this work was preparation of new HOF materials (metal-hydride-organic frameworks), which could potentially fulfill the requirements for large hydrogen storage. A selection of mechanochemical synthesis products was prepared combining two precursors, a light metal tetrahydridoborate (Li, Mg or Na) and an imidazolate (lithium imidazolate, sodium imidazolate and lithium benzimidazolate). Different products were synthesised using different stoichiometric molar ratios between the two precursors with and without solvent (acetonitrile or ethylacetate). The aim was to synthesize a crystalline product and research the most promising combination of precursors for future work. All the samples were analysed with X-ray powder diffraction, some of them were analysed also with DSC, TG-MS and high temperature X-ray powder diffraction. The majority of combinations have been found out to be unsuccessful and a large number of samples contained an amorphous phase after having been synthesized. In the majority of cases, the difractograms of samples after synthesis contained peaks of unused precursors. The combination of lithium tetrahydridoborate and lithium benzimidazolate was found to be the most promising as the difractograms of the samples which were synthesized using a higher proportion of lithium tetrahydridoborate contained peaks of a new crystalline phase or phases. With optimization of ball miling programme for the molar ratio 3:1 of these precursors, we tried to understand how the change of ball miling parametres affected the outcome of the synthesis. It was found that the synthesis is more optimal at lower speeds (650 and 750 rpm) and less optimal at a higher number of cycles (especially 12 cycles). The DSC and TG-MS analyses showed that upon heating, hydrogen evolves and leaves the samples which undergo pyrolysis decomposition at temperatures above 300 °C. Two lithium borates were found to crystallize in three of the samples at 500 °C.

• Keywords: mechanochemical synthesis, metal-hydride-organic frameworks, tetrahydridoborates, imidazolates