Preparation And Applications Of Gradient Porous Zeolite Membrane Materials

With the high-speed development of global economy, increasing emissions of volatileorganic compounds (VOCs) emitted from a large variety of sources scuh as petrochemicals,chemical processes as well as household products produce many undesirable effects onhumans health and environmental safety. Various technologies have been implemented for theremoval of VOCs from air, such as adsorption, catalytic oxidation and thermal combustion etc.Among these methods, adsorption technology and catalytic oxidation using fixed beds havebeen regarded as one of the most promising technologies for the removal of VOCs from airdue to their easy operation, low energy requirement as well as high purification efficiency.However, the drawbacks of poor contacting efficiency, relatively higher mass/heat transfer aswell as bed pressure dorp exist in most of traditional fixed bed adsorbers/reactors due to theuse of pellet shaped or powder adsorbents/catalysts. The design and fabrication of a novelstructured zeolite membrane adsorptional/catalytic material that can effectively enhancemass/heat transfer, contacting efficiency as well as adsorptional/catalytic reaction rate havebeen reconigzed as a cutting-edge research topic. In this paper, the fabrication andcharacterization of gradient porous ZSM-5zeolite membrane adsorptional/catalytic materialwere designed and investigated. The applications of these novel structured materials in theremoval of VOCs in air at low concentrations were also studied. Adsorption dynamics ofVOCs in air at low concentrations in structured fixed bed adsorbers based on these novelstructured materials was measured, and catalytic combustion performances of VOCs in air atlow concentrations over structured zeolite membrane reactor were also investigated.Firstly, the preparation process of paper-like sintered stainless steel fibers (PSSF) supportwas investigated. The paper-like sintered stainless steel fibers (PSSF) support was fabricatedby the wet lay-up papermaking/sintering process using stainless steel fibers (6.5μm diameter,2~3mm length) and cellulose with a weight ratio of2:1. The effects of stainless steel fibersdosage on the bed pressure drop of PSSF were studied, and the experimental results indicatedthat the optimum amount of stainless steel fibers is6g. The micromorphology and structureof stainless steel fibers before and after sintering were observed by using Scanning Electron Microscopy (SEM). The experimental results showed that the junctures of stainless steelfibers in PSSF were completely sintered together to form a three-dimensional networkstructure with large voidage under high temperature condition.Secondly, the preparation process of gradient porous ZSM-5zeolite membranesmaterials was studied. The Tetraethoxysilane (TEOS) and Tetrapropylammoniumhydroxide(TPAOH) were used as silicon source and structure directing agent, respectively. The effectsof various synthesis parameters on the growth of silicalite-1seeds were also investigated. ThePSSF support was treated with a silicalite-1seeds solution to fabricate a thin andhomogeneous seed flim on the surface of stainless steel fibers. The ZSM-5zeolite membraneswere synthesized on the surface of seeded-PSSF by secondary growth method. The secondaysynthesis solution was prepared using the same chemicals as the silicalite-1seed solution, butSodium aluminate (NaAlO2) was added as aluminum source. The effects of various secondarysynthesis parameters on the surface morphology and thickness of ZSM-5zeolite membranewere also studied. The experimental results indicated that the water and Ethanol (EtOH)content in synthesis solution, aging time, crystallization temperature as well as crystallizationtime obviously influenced the growth of silicalite-1seeds. A continuous and dense ZSM-5zeolite membrane was successfully fabricated on the surface of stainless steel fibers by usingsecondary growth method. The BET specific surface area of fabricated gradient porousZSM-5membrane material (ZSM-5/PSSF) was145m2/g, and the thickness of ZSM-5membrane was4μm. Morover, the obtained ZSM-5membrane content in ZSM-5/PSSFmaterial was approximately37wt%. In addition, the aluminum content in secondary synthesissolution significantly influenced the growth of ZSM-5membrane. The experimental resultsalso indicated that the surface morphology and membrane thickness of ZSM-5zeolitemembrane were obviously influenced by the crystallization temperature and time.Thirdly, the preparation process of gradient porous ZSM-5membrane catalysts wasinvestigated. The modified ZSM-5zeolite membrane catalysts were synthesized by theincipient wetness impregnating method using Cobalt nitrate (Co(NO3)2), Manganese nitrate(Mn(NO3)2) and Copper nitrate (Cu(NO3)2) as precursor salts. The effects of molar ratios of transition metal component, total metal loading as well as calcination temperature on thestructure properties and catalytic performances for isopropanol oxidation of modified ZSM-5zeolite membrane catalysts were investigated. Among the series of catalysts, the as-preparedCu-Mn(1:6) mixed oxides modifed ZSM-5membrane catalysts (Cu-Mn(1:6)/ZSM-5/PSSF)and Co-Cu-Mn(1:1:1) mixed oxides modifed ZSM-5membrane catalysts(Co-Cu-Mn(1:1:1)/ZSM-5/PSSF) have exhibited the best catalytic activity for isopropanoloxidation, demonstrating by a lowest T90%of210℃(the temperature at isopropanolconversion approaches90%). Moreover, the gradient porous modified ZSM-5membranecatalysts as high efficiency catalyst for VOCs oxidation can offer a much higher contactingefficiency, excellent catalytic reaction rate as well as reaction stability.Meanwhile, adsorption dynamics of VOCs in air at low concentraions in structured fixedbed adsorber based on the gradient porous ZSM-5zeolite membrane adsorptional materials.The structured fixed bed adsorbers filled with both activated carbon or ZSM-5particles in theinlet of bed and ZSM-5zeolite membrane adsorptional materials (ZSM-5/PSSF) in the outletof bed were designed. Adsorption dynamics of VOCs in air at low concentrations(isopropanol or toluene) in the structured fixed bed adsorbers was performed, comparing withthat of traditional particles fixed beds filled with individual activated carbon or ZSM-5particles. The experimental results indicated that the steepness of breakthrough curves ofVOCs in structured fixed bed adsorber was more upright that that of traditional particles fixedbed adsorber, confirming that the mass transfer was obviously enhanced by addingZSM-5/PSSF adsorptional materials in the outlet of structured fixed bed. Moreover, thebreakthrough times of VOCs in structured fixed bed adsorber decrease significantly asincreasing gas flow rate and inlet concentration of VOCs. The length of unused bed (LUB)theory, Yoon-Nelson as well as Bed Depth Service Time (BDST) models were used toinvestigate the experimental results. The analytical results showed that the bed utilization ofstructured fixed bed adsorber increases10%~18%compared with that of traditional particlesfixed bed. It can be demonstrated that theoretical breakthrough curves predicted using bothYoon-Nelson and BDST models were in good agreement with the experimental results. Meanwhile, catalytic combustion performances of VOCs in single component(isopropanol or toluene) and binary component mixtures (isopropanol and toluene, ethylacetate and toluene) were investigated over the structured zeolite membrane reactor based ongradient porous ZSM-5zeolite membrane catalytic materials. The effects of catalysts bedheight, gas hourly space velocity (GHSV) as well as initial concentrations on the catalyticperformances of structured zeolite membrane reactor for VOCs combustion were alsoinvestigated. The experimental results showed that the best catalysts bed height of structuredzeolite membrane reactor was2cm. Moreover, experimental results also indicated that thecomplete destruction of isopropanol or ethyl acetate in single component over structuredzeolite membrane reactor can be achieved below the temperature of280℃. Results alsoexhibited that the inlet concentration of VOCs and GHSV had a slight effect on the catalyticperformances of structured zeolite membrane reactor for VOCs combustion, demonstating bythat the T50%and T90%of VOCs increase slightly as the initial concentrations and GHSVincreased. The study also indicated that the presence of toluene in VOCs binary mixtures hada minimal inhibition effect on the catalytic conversion of isopropanol or ethyl acetate over thestructured zeolite membrane reactor. It can be concluded that the catalytic activity ofisopropanol or ethyl acetate in binary mixtures was much higher that that of toluene.Finally, the modeling studies of catalytic combustion performances of isopropanol overstructured zeolite membrane reactor were investigated by using Power-rate Law and Mars-vanKrevelen kinetic models. External and internal mass transfer effects were evaluated. Theanalysis results showed that the Mars-van Krevelen kinetic model was more suitable forpredicting the catalytic combustion performances of isopropanol over structured zeolitemembrane reactor than the Power-rate Law kinetic model, and its reaction mechanism isoxidation-reduction. The surface reduction and oxidation reaction activation energies were60.30kJ/mol and57.19kJ/mol, relatively.