(45c) Zeolitic Coatings Onto Fecralloy Microchannels: Washcoating Vs. in Situ Growing

Introduction: Structured metallic catalysts and reactors offer excellent temperature control in the case of microchannel reactions, different flow patterns with monoliths and foams, and reduced intra particle limitations to diffusion by using thin catalyst coatings. Due to their versatility in pore structure, chemical composition and acidity, zeolites have been widely used in several catalytic processes such as oil refining, fine chemistry, environmental applications etc. The aim of this work is to study the deposition of hydrophobic MFI type zeolite onto Fecralloy microchannels. Two coating methods were compared: washcoating from a stabilized slurry and in situ growing of the zeolite on the metallic surface introduced in the synthesis media . Experimental: Fecralloy monoliths were homemade manufactured and calcined at 900ºC for 22h for generating a rough and non porous alumina whisker layer on the surface. These supports were used for both deposition methods. The dimensions of the channels of the monoliths are in the range of microns (hydraulic diameter 440 microns). Washcoating: Monoliths were washcoated by dipping and withdrawing them in a slurry at constant speed (3 cm/min). Excess slurry was removed by centrifugation. After that, the monoliths were dried at 80ºC for 1h. The process was repeated for 7 times for each monolith and finally calcined at 500ºC for 2 h with heating rate of 1ºC/min. Slurries with different formulation were, prepared with commercial zeolite (Zeolyst CBV 28014), Polivynil alcohol (PVOH), colloidal silica (Ludox AS30) and water. In situ synthesis: Monoliths were introduced in autoclaves together with a 24h hour aged synthesis gel of molar composition 20SiO₂:5TPABr:5NaOH:10000H₂O and placed into an oven. The in situ growing was induced under hydrothermal synthesis at 150ºC for 24h under orbital agitation of 6 rpm. Different surface pre-treatments of the metal were tested: bare oxidized metal and colloidal silica and TPABr pre-coatings. The growing process was repeated for 5 times. Characterization of those micromonoliths was carried out by means of gravimetry, optical microscopy and SEM, XRD, N₂ adsorption. Adherence test was carried out immersing the monoliths in petroleum ether bath and intense sonication for ½ h. Results and discussion: Washcoated monoliths: PVOH acts as a thickener increasing the viscosity of the slurry up to an adequate range of 10-20 cp for washcoating, while colloidal silica acts also as a binder, adhering the zeolite crystals to the metallic surface or to a previously deposited zeolite layer. The higher the amount of silica, the better the adherence. The textural properties of the coating vary with respect to the properties of the pure zeolite sample. The agglomeration of the zeolite crystals produces new mesopores (8 to 10 nm) increasing as the amount of colloidal silica increase. The increase of silica also increases the viscosity of the slurry that produces thicker coatings. Loads of 1.6 to 2.7 mg/cm² were obtained with this coating technique. The adherence in all monoliths was very good with mass losses lower than 8%. Monoliths coated by in situ synthesis: After 5 reactions, the surface of the monoliths was completely covered with a transparent zeolite layer. BET surface area of about 380 m²/g and pure microporous isotherm were obtained. The XRD confirms the obtaining of MFI type zeolite. The zeolite layer is formed by completely interpenetrated crystals forming a continuous layer without other porosity than the zeolite microporosity. This coating technique produces around 1.6-1.7 mg/cm² of zeolite layer after 5 reactions. The result of adherence test was excellent, as no detachment was observed after the ultrasonic treatment. After the first synthesis, a clear promoting effect of the silica or TPABr surface pretreatment was observed with respect to the bare oxidized metal. But this effect disappears for consecutive synthesis. Repeated synthesis produced also other progressive changes. The first one is the increase in the percentage of zeolite growing on the metallic surface with respect to the zeolite nucleating in the solution. The second one is the increase of the overall conversion of the zeolite synthesis in consecutive reactions (40 to 75%). Completely covered Fecralloy micromonoliths were obtained with both coating methods. Surface coating by washcoating seems to be more interesting because is simpler and faster than in situ synthesis. Besides, the use of silica as binder affects the textural properties of the zeolitic coating, generating some inter crystalline mesoporosity that could be interesting for improving material transport properties. Randomly oriented zeolite crystal layer was obtained by washcoating while in situ growing produced more oriented layer. The adherence in all cases was good, being excellent for monoliths coated by synthesis.