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

The structural, electronic, energetic and spectroscopic properties of all possible nickel(II) aqua and aqua–dihydroxy complexes (cis- (left) and trans-[Ni(OH2)2(OH)2](T) (right) are depicted) absorbed in the micropores of a nickel-loaded HEU-type zeolite or supported on the zeolite surface were studied theoretically, and the nickel-loaded HEU-type zeolite crystals were characterised by a carefully selected combination of experimental techniques. The acid sites relevant to catalytic activity have also been defined. 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 °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 mg g−1 NH3, is attributed to the weak acid sites (fundamentally Lewis sites) resolved at approximately 183 °C, and to the strong acid sites (essentially Brønsted sites) resolved at approximately 461 °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 kcal mol−1, increases with increasing coordination number of the aqua and dihydroxy nickel(II) complexes. Supporting information (DR(UV/Vis) and FTIR absorption spectra of NaHEU and NiHEU, and in situ FTIR absorption and 27Al MAS NMR spectra of NiHEU-Py and NiHEU-Py. Tables list the computed excitation energies, oscillator strengths, harmonic vibrational frequencies and IR intensities for the high-spin ground states of [Ni(OH2)n]2+ and [Ni(OH2)n−2(OH)2] (n=4–6) complexes) for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2001/f2596_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.