SAPO-35 Molecular Sieve: Synthesis, Characterization, and Adsorbate Interactions of Cu(II) in CuH−SAPO-35

Abstract

Synthesis of small-pore silicoaluminophosphate SAPO-35 (structurally analogous to the zeolite levyne) using hexamethyleneimine in aqueous and fluoride media is described for the first time. Pure, highly crystalline samples of SAPO-35 were obtained within a short reaction period of 24 h. Various factors affecting crystallinity and phase purity of SAPO-35 were studied. Samples were characterized by X-ray powder diffraction, 27Al, 29Si and 31P solid state MAS NMR spectroscopy, FTIR spectroscopy, thermal analysis (TG and DTA), surface area measurement, electron microprobe analysis and scanning electron microscopy. SAPO-35 crystallizes with highly uniform rhombic crystals about 1 μm across. Two crystallographically distinct tetrahedral framework sites in SAPO-35 are shown from MAS NMR studies. 29Si MAS NMR spectra show that silicon substitutes for framework phosphorus and generates large Bronsted acidity. The location of Cu(II) ions exchanged into H−SAPO-35 and their interaction with deuterated water, ammonia, methanol, and ethylene have been investigated by electron spin resonance (ESR) and electron spin−echo modulation (ESEM) techniques. Simulation parameters of the 31P ESEM spectrum suggest that the hydrated Cu(II) complex, i.e., CuII(H2O)6, is located in the levyne cage. During dehydration some of the Cu(II) ions may migrate into double hexagonal prisms. Room-temperature adsorption of D2O on CuH−SAPO-35 after dehydration at 723 K and subsequent O2 treatment and evacuation at the same temperature (activation) forms a Cu(II)−aquo complex with axially symmetric ESR parameters (g∥ = 2.387 and A∥ = 0.0151 cm-1). This complex is suggested to be CuII(D2O)3 based on 2D ESEM data. Adsorption of ND3 on activated CuH−SAPO-35 produces two CuII(ND3)n complexes with axially symmetric ESR parameters. When CD3OH is adsorbed on CuH−SAPO-35, the resultant Cu(II) complex with ESR parameters g∥ = 2.398 and A∥ = 0.0142 cm-1 contains one methanol molecule coordinating directly with the metal ion based on 2D ESEM measurements. Adsorption of C2D4 on activated CuH−SAPO-35 gives two CuII(C2D4)n complexes based on ESR measurements.