Application of fluidized bed film coating for membrane encapsulation of catalysts

Fluidized bed film coating is considered as an efficient method to coat membranes onto catalytic particles producing a micro/mesoporous composite structure. These composite materials described as Membrane Encapsulated Catalysts (MECs) have the potential to increase reaction yield and selectivity by controlling the diffusion of products and reactants to and from the catalytic core. MECs were synthesized by film coating spherical zeolite particles with a nano-alumina suspension in hydroxylpropyl cellulose (HPC) using a conventional fluidized bed (Mini-Glatt). Coated particles were subsequently calcined in a furnace to remove the HPC leaving a porous alumina membrane. The fluidized bed coating process was evaluated by its ability to coat uniformly and defect-free as well as to control the membrane coating thickness. Scanning electron microscopy (SEM), Near Infrared Spectroscopy (NIR), and surface area and pore size distribution analysis (BET) were used to characterize the coated catalyst. Results show that membrane coatings were uniformly applied and free of defects upon optimization of fluidization parameters for the range of membrane thicknesses (6.0 mu m-26.0 mu m) produced in this study. The coating thickness estimated from the experimental techniques was in good agreement with the theoretically predicted thickness demonstrating the precision and controllability of the process. The alumina membrane was also shown to adhere well to the catalyst after removal of HPC and its thickness remained unchanged after calcining. Analysis of the membrane coated zeolite showed both microporous and mesoporous properties which may be particularly advantageous in separation processes. It was shown that NIR spectroscopy may be used as a method to replace image analysis for fast characterization of the membrane thickness. NIR spectra were shown to correlate coating thickness with absorbance at 1184 nm (R-2=.9920). (C) 2011 Elsevier B.V. All rights reserved.