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

Nickel-loaded HEU-type zeolite crystals have been obtained by well-known sy nthetic procedures and characterised by X-ray fluorescence (XRF), scanning- electron microscopy/energy-dispersive spectroscopy (SEM-EDS), FT-IR, diffus e reflectance UV/Vis spectroscopy (DR(UV/Vis)S) and X-ray photoelectron spe ctroscopy (XPS) measurements as non-homoionic and non-stoichiometric substa nces containing exchangeable hydrated Ni2+ ions in the micropores and nicke l hydroxide phases supported on the surface. Thermogravimetric analysis/ di fferential gravimetry (TGA/DTG) and differential thermal analysis (DTA) dem onstrated that full dehydration below approximately 400 degreesC follows a clearly endothermic process, whereas at higher temperatures the zeolite is amorphised and finally partially recrystallised to Ni(Al,Si) oxides, detect ed by powder X-ray powder diffraction (XRD). The solid acidity of NiHEU, in itially 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 degreesC, and to the strong acid sites (essentially Bronsted sites) resolved at approximately 461 degreesC in the TPD pattern. A more sophisticated study based on in situ/ex situ FT- IR with in situ/ex situ Al-27 MAS NMR and pyridine (Py) as a probe molecule , revealed that the Lewis acid sites can be attributed primarily to Ni2+ io ns, whereas the Bronsted ones can probably be associated with the surface-s upported nickel hydroxide phases. The spectroscopic measurements in conjunc tion with powder XRD and Si-29 MAS NMR data strongly suggest that distorted Al tetrahedra are formed during the dehydration process and Py chemisorpti on/complexation (NiHEU-Py), whereas the crystal structure is remarkably wel l preserved in the rehydrated material (NiHEU-Py/R). The structural, electr onic, energetic and spectroscopic properties of all possible nickel(II) aqu a and dihydroxy complexes absorbed in the zeolite micropores or supported o n the zeolite surface were studied theoretically by density functional theo ry (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 a nd dihydroxy nickel(II) complexes.