Incorporation of Transition Metal Ions into MeAPO/MeAPSO Molecular Sieves

Photoinduced charge separation in microporous and mesoporous materials

A review of photoinduced charge separation of organic molecules in microporous and mesoporous materials is presented. In particular, the photoionization of N-alkylphenothiazine (PCn), N,N,N′,N′-tetramethylbenzidine (TMB), and porphyrin in microporous materials, such as zeolites, aluminophosphates (AlPOs), silicoaluminophosphates (SAPOs), and mesoporous materials, such as MCM-41, MCM-48, and SBA-15, is discussed.

Synthesis of mesoporous aluminophosphate (AlPO) and investigation of zirconium incorporation into mesoporous AlPOs

Mesoporous aluminophosphate materials with variable amounts of zirconium have been synthesized at room temperature using a nonionic surfactant tri-block copolymer (PEO(20)PPO(70)PEO(20)) as the structure-directing agent. Powder X-ray diffraction of as-synthesized and calcined AlPO and ZrAlPO mesoporous materials shows a single broad peak near 2theta = 2.5 degrees, indicative of the average pore-pore correlation distance. Electron probe microanalysis shows that the ratio of P/Al in the powders is approximately 0.5, far lower than 1.0 for an ideal aluminophosphate framework. XRD, TEM, and N2 adsorption data indicate that the calcined samples consist of wormlike tubular materials having surface areas >350 m2/g and pores in the mesopore range. Electron spin resonance (ESR) studies of the gamma-irradiated and evacuated ZrAlPO samples show signals due to Zr3+ that increase with Zr content in addition to signals due to framework defects (i.e., V centers) and H atoms. The line shape and g values observed for Zr3+ are best explained as arising from a trivalent zirconium ion situated at the framework tetrahedral sites.

Unusual Coordination Behavior of Cr3+ in Microporous Aluminophosphates

A CrAPO-5 molecular sieve has been investigated with X-ray absorption spectroscopy (EXAFS-XANES) as dehydrated material and after loading with water and ammonia to unravel the coordination geometries of Cr3+ in the framework of a microporous crystalline aluminophosphate, more particularly of the AFI-type. A comparison of the XANES data, a preedge analysis with crystal field multiplet calculations and EXAFS data, pointed toward the presence of framework Cr3+ which, on dehydration, takes on a distorted tetrahedral coordination state. Due to the 3d3 configuration of Cr3+, this unusual tetrahedral coordination environment strongly tends to transform into the more stable 6-fold coordination geometry by binding two extraframework water molecules during hydration. In the presence of ammonia, tetrahedral Cr3+ readily transforms into a 5-fold coordination geometry by binding one ammonia molecule. Therefore, depending on the environmental conditions, the Cr3+ ions can occur in a 4-, 5-, or 6-fold coordination. This observation underlines the flexibility of transition metal ions, such as Cr3+, to cope with unusual coordination geometries in inorganic hosts, making them interesting as potential active sites in heterogeneous catalysis.

Isomorphous Substitution of Transition-Metal Ions in the Nanoporous Nickel Phosphate VSB-5

The transition-metal-incorporated nickel phosphate molecular sieves (TMI-VSB-5) have been hydrothermally synthesized at 453 K in weak basic conditions under microwave irradiation. By means of X-ray diffraction, inductively coupled plasma (ICP), ultraviolet-visible (UV-vis) diffuse reflectance, and Mössbauer spectroscopies, successful isomorphous (at least partial) substitution of transition-metal ions in the VSB-5 framework has been verified. Characterization results show that the framework structure of nanoporous VSB-5 can accommodate a substantial level of isomorphous substitution of transition-metal ions up to about 10, 5, and 3 atom % for Fe, Mn, and V, respectively, in both octahedral nickel sites (Mn and Fe) and tetrahedral phosphorus sites (V). The isomorphous substitution including the replacement mechanism was studied by not only the change of unit cell parameters but also spectroscopic analysis. The unit cell parameters of TMI-VSB-5 including a unit cell volume and a-axis length relied on the ionic radii difference between the incorporated ion and the original framework ions such as Ni or P (RTMI - RNi or RTMI - RP).