Characterisation of Zeolitic Materials with a HEU-Type Structure Modified by Transition Metal Elements: Definition of Acid Sites in Nickel-Loaded Crystals in the Light of Experimental and Quantum-Chemical Results

Abstract

Nickel-loaded HEU-type zeolite crystals have been obtained by well-known synthetic procedures and characterised by X-ray fluorescence (XRF), scanning-electron microscopy/ energy-dispersive spectroscopy (SEM-EDS), FT-IR, diffuse reflectance UV/ Vis spectroscopy (DR(UV/Vis)S) and X-ray photoelectron spectroscopy (XPS) measurements as non-homoionic and non-stoichiometric substances containing exchangeable hydrated Ni2+ ions in the micropores and nickel hydroxide phases supported on the surface. Thermogravimetric analysis/differential gravimetry (TGA/DTG) and differential thermal analysis (DTA) demonstrated that full dehydration below approximately 400 degrees C follows a clearly endothermic process, whereas at higher temperatures the zeolite is amorphised and finally partially recrystallised to Ni(Al,Si) oxides, detected by powder X-ray powder diffraction (XRD). The solid acidity of NiHEU, initially determined by temperature-programmed desorption (TPD) of ammonia to be 8.93 mgg(-1) NH3, is attributed to the weak acid sites (fundamentally Lewis sites) resolved at approximately 183 degrees C, and to the strong acid sites (essentially Brønsted sites) resolved at approximately 461 degrees C in the TPD pattern. A more sophisticated study based on in situ/ex situ FT-IR with in situ/ex situ 27Al MAS NMR and pyridine (Py) as a probe molecule, revealed that the Lewis acid sites can be attributed primarily to Ni2+ ions, whereas the Brønsted ones can probably be associated with the surface-supported nickel hydroxide phases. The spectroscopic measurements in conjunction with powder XRD and 29Si MAS NMR data strongly suggest that distorted Al tetrahedra are formed during the dehydration process and Py chemisorption/complexation (NiHEU-Py), whereas the crystal structure is remarkably well preserved in the rehydrated material (NiHEU-Py/R). The structural, electronic, energetic and spectroscopic properties of all possible nickel(II) aqua and dihydroxy complexes absorbed in the zeolite micropores or supported on the zeolite surface were studied theoretically by density functional theory (DFT). The computed proton affinity, found to be in the range 182.0-210.0 kcalmol(-1), increases with increasing coordination number of the aqua and dihydroxy nickel(II) complexes.

The Use of Heu-Type Zeolitic Tuff in Sustainable Agriculture: Experimental Study on the Decrease of Nitrate Load in Vadose Zone Leachates

“…The sorption and fixation of the different components from their solutions by the micro/nano-pores of HEU-type zeolite, as well as from meso-and macro-pores of the zeolitic tuff, is attributed to absorption (ion exchange), adsorption and surface precipitation processes (Godelitsas et al, 1999(Godelitsas et al, , 2001(Godelitsas et al, , 2003Fu and Wang, 2011;Kantiranis et al, 2011;Malamis and Katsou, 2013). The HEU-type zeolite is characterized by Brønsted acidic and the Lewis basic active sites, that react with the negatively or/and positively charged chemical components, even with molecules in gas condition.…”

“…These chemical processes are related to sorption and fixation physicochemical phenomena of ions and molecules, and are related both to the structural void spaces (micro/nano-pores) and the surface of the HEU-type zeolite crystals, consequently the meso-and macro-pores of the zeolitic tuff. Finally, HEU-type zeolite shows an ability to neutralize the pH of acidic and basic waters, acting either as a proton acceptor or donor, exhibiting thus an amphoteric character (Misaelides et al, 1995;Godelitsas et al, 1999Godelitsas et al, , 2001Godelitsas et al, , 2003Kantiranis et al, 2011).…”

Supramolecular Organization in Confined Nanospaces

“…Mainly because of their impact on catalysis, the location and distribution of extraframework cations in zeolites have been object of many experimental and computational studies. In both hydrophilic and hydrophobic frameworks, water behavior has been intensively explored,, uncovering unexpected supramolecular structures, such as nanohelices (in natrolite), triple helices (in VPI‐5),, cyclic hexamers (in NaY supercages),, or nanoworms (in hydrophobic silicalite, Figure ), along with the more common nanospheres (in zeolite A) and ice‐like nanotubes (in AlPO 4 ‐5 and SSZ‐24) .…”