| Abstract | Aluminijev sec-butoksid (Asb) je, kako bi se kontrolirala njegova hidroliza, modificiran s
etil-acetoacetatom (Eaa) u različitim omjerima (Eaa : Asb = 0,5; 1; 1,5; 2; 2,5 i 3). Istraživan je utjecaj ovih omjera na solove, gelove i prijelazni aluminijev oksid dobiven termičkom obradom gela. Korištene su metode 1H, 13C i 27Al nuklearne magnetske rezonancije (NMR),
infra-crvene spektroskopije s Fourierovom transformacijom (FTIR), rendgenske difrakcijske analize (XRD), analize veličine čestica (PSA), pretražne elektronske mikroskopije (SEM), simultane diferencijalno termičke i termogravimetrijske (DTA/TGA) analize te
N2 apsorpcijsko/desorpcijske izoterme. U solovima je detektirano pet različitih kelata: monokelirani trimer, Al3(OsBu)8(Eaa), asimetrični bis-kelirani dimer, Al2(OsBu)4(Eaa)2,
tris-kelirani dimer, Al2(OsBu)3(Eaa)3, tetra-kelirani dimer, Al2(OsBu)2(Eaa)4, i tris-kelirani monomer, Al(Eaa)3. Prisutnost ovih kelata potvrđena je dekonvolucijom 1H NMR spektara. Koordinacija aluminija raste s porastom molarnog omjera Eaa/Asb, pa u uzorcima s molarnim omjerom Eaa/Asb od 2,5 i 3 postoji samo heksa-koordinirani aluminij, odnosno tris-kelirani monomer. Oktaedarski Al kompleks dolazi u obliku dva stereoizomera, facijalni (fac) i meridijalni (mer) u omjeru 2 : 3. Tris-kelirani monomer otporan je na hidrolizu te je jedini kelat čija je prisutnost detektirana u hidroliziranim uzorcima. Njegova količina u hidroliziranim uzorcima proporcionalna je početnom omjeru Eaa/Asb te ga skoro nema u uzorku s omjerom Eaa/Asb = 0,5 dok je uzorak s omjerom Eaa/Asb = 3 gotovo čisti Al(Eaa)3. Osim koordinacije 6 koja potječe od Al(Eaa)3, u hidroliziranim uzorcima su zapažena dva slabije definirana okruženja atoma Al. U ovim su uzorcima zapažene dvije različite morfologije, veći komadi gela i fine aglomerirane čestice kelata. Kristalna struktura Al(Eaa)3 određena je rendgenskom difrakcijom u polikristalu. Ovaj kelat kristalizira u monoklinskoj prostornoj grupi P21/n, s parametrima jedinične ćelije a = 16,387(1) Å, b = 19,650(1) Å, c = 6,912(1) Å i
β = 91,91(1)°. Molekulska struktura mu se sastoji od tri O,O-bidentatno povezana
etil-acetoacetatna liganda koja tvore oktaedarsku koordinaciju oko aluminijeva atoma. Molekule su dvjema intermolekulskim vodikovim vezama duž b-osi povezane u lance. Termičkom obradom gela pri 800 °C tijekom 2 sata nastaje čisti, nanokristalinični γ-Al2O3 koji se može primijeniti kao nosač katalizatora. Tekstura i morfologija γ-Al2O3 ovise o inicijalnom omjeru Eaa/Asb pa su tako čestice s manjim Eaa/Asb omjerom kompaktnije dok se čestice s višim Eaa/Asb omjerom čine više aglomerirane. Termička obrada pri višim temperaturama dovodi do transformacije γ-Al2O3 u α-Al2O3. Uzorak s omjerom Eaa/Asb = 0,5 može se ocijeniti kao najprikladniji za pripremu nosača katalizatora. Istraživanje je pokazalo da se različita svojstva γ-Al2O3 dobivenog sol-gel sintezom mogu kontrolirati s kelacijskim agensom. |
| Abstract (english) | Aluminum sec-butoxide (Asb) was modified with ethyl acetoacetate (Eaa) in various ratios (Eaa : Asb = 0,5; 1; 1,5; 2; 2,5 and 3) in order to control its hydrolysis. Sols, gels and transition alumina obtained by gel thermal treatment were investigated with respect to
Eaa/Asb ratio. 1H, 13C, 27Al nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), particle size analysis (PSA), scanning electron microscopy (SEM), simulataneous differential thermal and thermogravimetric (DTA/TGA) analysis and N2 adsorption/desorption isotherms were used. Five different chelates have been detected in sols: mono-chelated trimer, Al3(OsBu)8(Eaa), assymetric bis-chelated dimer, Al2(OsBu)4(Eaa)2, tris-chelated dimer, Al2(OsBu)3(Eaa)3, tetra-chelated dimer, Al2(OsBu)2(Eaa)4 and tris-chelated monomer, Al(Eaa)3.
The deconvolution of 1H NMR spectra confirmed the presence of these chelates.
The coordination of aluminium increases with the increase of molar ratio Eaa/Asb, thus in samples with Eaa/Asb ratio 2,5 and 3 only hexacoordinated aluminium, i.e. tris-chelated monomer, exists. This octahedral Al complex occurs as two stereoisomers, facial (fac) and meridional (mer) in ratio 2 : 3. Tris-chelated monomer is resistant to hydrolisis and it is the only chelate whose presence has been detected in hydrolized samples. Its amount in hydrolized samples is proportional to initial Eaa/Asb ratio and it is almost non-existant in sample with Eaa/Asb = 0.5 while practically whole sample with Eaa/Asb = 3 is Al(Eaa)3. Beside coordination 6 due to Al(Eaa)3, two less defined Al surroundings are noted in hydrolized samples. Two distinct morphologies can be observed in those samples, greater gel chunks and finer agglomerated chelate particles. Crystal structure of Al(Eaa)3 has been determined using powder X-ray diffraction. It crystalizes in monoclinic space group P21/n, with lattice parameters a = 16,387(1) Å, b = 19,650(1) Å, c = 6,912(1) Å and β = 91,91(1) °. The molecular structure comprises of three O,O-bidentatly linked ethylacetoacetate ligands forming octahedral coordination around the aluminium atom. Molecules are connected into chains along the b-axis by two intermolecular hydrogen bonds. Thermal treatment of gel at 800 °C for 2 hours yields pure, nanocrystalline γ-Al2O3 potentially useful as catalyst carrier. Morphology of γ-Al2O3 is also dependent on initial Eaa/Asb ratio, thus particles obtained with lower Eaa/Asb ratio are more compact while particles with greater Eaa/Asb ratio appears agglomerated. Texture is also influenced with Eaa/Asb ratio. Thermal treatment at higher temperatures results with transformation of γ-Al2O3 to α-Al2O3. Sample with Eaa/Asb ratio of 0,5 could be rated as most appropriate for catalyst carrier preparation. Investigation showed that various properties of γ-Al2O3 obtained by sol-gel synthesis can be controlled with chellating agent. |
