Abstract
Diffuse Reflectance Infrared Fourier-Transformed (DRIFT) Spectroscopy was used to determine the kinetic behavior of the solid-state migration of a selection of divalent transition metals (Cu+2, Ni+2, Co+2, and Pb+2) in zeolite material. This was completed using a mixture of zeolites comprised of Zeolite Socony Mobil-5 (ZSM-5) and Linde Type A (LTA), also referred to as Zeolite A. Each zeolite contains unique pore shapes, unit cell sizes, and Si/Al ratios. To understand the migration of divalent cation species throughout this zeolite mixture, the diffusion coefficient, which is a function of the concentration of sorbed material over time, was calculated for each exchange. Observation of metal content in ZSM-5 over time using DRIFT spectroscopy required use of a probe molecule pyridine, which was found to add significant influence on a cation’s migration rate. Three sets of experiments were completed. Group 1 experiments involved zeolite mixtures under full pyridine exposure and contained Na-ZSM-5 exchanging separately with either Cu-LTA, Ni-LTA, Co-LTA, or Pb-LTA in the “forward direction.” Forward direction trials involve Na+ beginning an exchange in ZSM-5. The diffusion coefficient for Na+ was calculated using Barrer’s short-term diffusion equation and final results were 6.6 ± 0.5, 4.0 ± 0.3, 3.0 ± 0.2, 2.7 ± 0.4 cm2/hr (*10-5) for exchanges with Cu-LTA, Ni-LTA, Co-LTA, and Pb-LTA respectively. The anisotropic nature of a zeolite’s framework also required experiments performed in the “reverse direction” which places Na+ in LTA at the start of an exchange. Na-LTA was also exchanged separately with Cu-ZSM-5, Ni-ZSM-5, Co-ZSM-5, and Pb-ZSM-5. All of these trials, with the exception of Pb-ZSM-5, experienced minimal to zero diffusion. The diffusion coefficient for Na+ exchanging into Pb-ZSM-5 was 20 ± 5 cm2/hr (*10-5). Diffusion rates in the forward direction followed metal complex stability when divalent metals bonded with pyridine. As stability increases across complexes formed by metal cations and pyridine, the diffusion rate into ZSM-5 was also observed to increase. The metal-pyridine complex interacted very strongly with ZSM-5’s framework, and complex formation was found to strongly influence diffusion rates. Group 2 also performed exchanges in the forward and reverse direction but removed pyridine’s influence on the cation kinetic behavior. Diffusion rates changed significantly and forward exchanges results between Na-ZSM-5 with either Cu-LTA, Ni-LTA, Co-LTA, or Pb-LTA experienced little to no diffusion. Reverse direction exchanges between Na-LTA and Cu-ZSM-5, Ni-ZSM-5, Co-ZSM-5, or Pb-ZSM-5 were observed to experience diffusion. The diffusion coefficients for Na+ exchanging into ZSM-5 were 5 ± 1, 8 ± 1, 6.0 ± 0.5, and 9 ± 1 cm2/hr (*10-5) for Cu-ZSM-5, Ni-ZSM-5, Co-ZSM-5, and Pb-ZSM-5 respectively. Group 2 zeolites were prepared using metal chloride salts and experienced a more acidic environment during exchange. Group 3 trials were also designed to remove pyridine’s influence on migration but in a low acidic environment using acetate salts. Forward direction trials saw minimal to zero diffusion between Na-ZSM-5 and Cu-LTA, Ni-LTA, Co-LTA, or Pb-LTA. Reverse direction trials experienced diffusion between Na-LTA and Cu-ZSM-5, Ni-ZSM-5, Co-ZSM-5, and Pb-ZSM-5. The diffusion coefficients for Na+ exchanging into ZSM-5 were 8 ± 2, 2.1 ± 0.4, 7 ± 1, 8 ± 1 cm2/hr (*10-5) for Cu-ZSM-5, Ni-ZSM-5, Co-ZSM-5, and Pb-ZSM-5 respectively. Diffusion rates in the reverse direction in a low acidic environment and removed from pyridine’s influence were observed to mirror the ranking of water exchange rate constants (k) of the transition metals. By removing pyridine, water molecule interaction with divalent metals becomes the primary mechanism behind cation migration. Cations exchange zeolite active site interactions for water ligand formation and vice versa. The rate at which a cation can exchange between zeolite interactions and water ligands influences its ability to quickly migrate and reach exchange equilibrium. As the water exchange rate constant increased for each divalent metal ion, the diffusion rate was also observed to increase. These results indicate that complex formation with water and pyridine both influence the rate of cation migration. The metal complexation with pyridine having higher stability results in faster diffusion rates than low stability complexes. When pyridine is absent, the migration of metal cations is dependent on exchange rates with water molecules, which is determined by the water exchange rate. As this value increases, so too does the diffusion rate. Lower water exchange rates resulted in slower diffusion coefficients. Understanding both of these mechanisms is extremely important to the overall mechanism in controlling long-range solid state migration of cations in zeolites. This supports the theory that cations migrate through a zeolite by exchanging ligands for an active site with the zeolite framework, and re-exchanging active site interactions for other ligands to further migrate into the zeolite.