Preparation And Properties Of Alumina-supported Inorganic Membranes

Membrane technologies have a wide range of applications in the fields of biomedicine,food,energy and electronic device manufacturing and environment protection.Inorganic membranes exhibit much better thermal and chemical stability than the organic polymer membranes,suitable for applications that require harsh environmental conditions.Porous alumina formed with the phase inversion tape casting method possesses straight open pore structure,ideal for use as a substrate for inorganic membranes.This thesis was devoted to the study of the preparation method and separation properties of molecular sieve membrane on the porous alumina substrate.The hydrophobic modification of the porous alumina and its use in membrane distillation water desalination was also investigated.The hydrophobilzation method developed in the present study was applied to the surface modification of glasses,dense ceramics and metals.In Chapter 1,inorganic ceramic membranes are reviewed in terms of structures,preparation processes and applications.Then,the synthesis methods and potential applications of metal-organic framework(MOF)materials are introduced.The hydrophobic modification of inorganic materials and the applications in membrane distillation water desalination are described.Finally,the scope and content of this thesis are presented.In Chapter 2,the synthesis and properties of an ultrathin ZIF-8 membrane supported on a porous alumina are investigated.The porous alumina substrate with straight open pores was prepared by the phase inversion tape casting/sintering method,and modified with polydopamine(PDA).Then,the hybrid ZIF-8/CNT seed solution was deposited on the alumina substrate,and an ultra-thin ZIF-8 film was grown on it via a spatially confined contra-diffusion process.The as-synthesized ZIF-8 film had uniformed thickness of?400 nm,and adhered tightly with the substrate.No cracks were found in the film from the SEM image.The ZIF-8 film was grown well on the alumina substrate as revealed by XPS analysis.The permeation measurements showed that the H2/CO2 permeability ratio reached 12.9,higher than those given in the literature.At a raised temperature of 150°C,the permeability ratio remained unchanged during a period of 90 hours,indicating that the membrane possessed satisfactory thermal stability.In Chapter 3,the seed-free synthesis of the ZIF-8 film on the porous alumina is presented.The porous alumina support was immersed in a dopamine solution with catecholamine moieties,so as to enhance the adherence with the ZIF-8.A uniform and crack-free ZIF-8 film of thickness?700nm was prepared by raising the temperature to 50°C and with an optimized reaction time of 12 hours.The as-synthesized ZIF-8 membrane showed a H2/CO2 permeability ratio of 21,higher than the one for the membrane prepared with the seed described in the previous chapter.The membrane also showed a high H2/N2 permeability ratio of 13.In Chapter 4,the super-hydrophobic surface modification and self-cleaning properties of porous alumina is presented.Artificial super-hydrophobic surfaces demonstrate self-cleaning and anti-icing function,promising for a variety of applications.The application of artificial super-hydrophobic surfaces is hindered by their poor durability.This problem is even severer for the surfaces created by applying the organic coating on inorganic substrates.The present study reports for the first time the all-inorganic super-hydrophobic surface.The ?-Al2O3 layer was grown on the alumina substrate by using the hydrothermal method.The SEM showed that the flower-like layer of thickness 53 ?m was composed of nano-needles of length 300-400 nn and width 40-50 nm.A polydimethylsiloxane(PDMS)film was applied onto an alumina substrate by the dip-coating method,and then pyrolyzed at 400 °C in a non-oxidizing atmosphere.The TEM and EDS mapping showed that the pyrolysis treatment converted the PDMS film into a SixCyOz ceramic coating which adhered tightly to the underlying ?-Al2O3 nano-needles.The FTIR analysis revealed that the polymer-derived ceramic coating contained low surface-energy methyl groups surviving the pyrolysis.A water contact angle(CA)of 1700?and a sliding angle(SA)of 5° were obtained,indicating that as-modified alumina was super-hydrophobic.The super-hydrophobicity is attributed to the presence of low surface-energy methyl groups in the polymer-derived ceramic coating and the high roughness of the alumina substrate.After immersion in strong acidic solution(pH=2),alkaline solution(pH=12)and hot water for 48 hours,the alumina retained the super-hydrophobicity.The super-hydrophobic alumina exhibited desired water and dirty repelling properties.In Chapter 5,the membrane distillation water desalination with the super-hydrophobic porous alumina is described.The porous alumina was modified with PDMS-derived hydrophobic ceramic coating as described in the previous chapter.The bare alumina exhibited a nitrogen permeability of 1.89×105 Lm-2h-1bar-1.The permeability decreased to 1.77×105 Lm-2h-1bar-1 after modification.The bare alumina allowed liquid water to permeate at a rate of 4.4×104Lm-2h-1hbar-1,while the modified alumina was water repelling.The membrane distillation was conducted by exposing one side of the PDMS-modified alumina to a hot NaCl solution(70 ?)and sweeping the other side with dry nitrogen to carry away the permeated water.The water permeation flux was measured to be 2.65,2.21,2.04 and 1.85 Lm-2h-1 for NaCl concentrations of 2,4,8 and 12 wt.%,respectively,and the salt rejection was 99.9%.As the testing time increased,the surface of the membrane became covered with sodium chloride crystals,blocking the pores of the membrane and thus leading to a decrease in the water flux.After changing to a fresh hot solution and cleaning the membrane,the water flux was recovered.In Chapter 6,the surface hydrophobic modification method explored for porous alumina is applied to glasses,dense ceramics and metals.These materials were coated with a PDMS film followed by heat treatment at elevated temperature of 450°C in a non-oxidizing atmosphere.As a result,the PDMS film was converted into a SixCyOz ceramic coating.The as-modified ground glass and(dense)alumina showed super-hydrophobicity with water contact angles of 151°and 163°,respectively.A copper-zinc alloy(Cu64Zn36)treated with the same method demonstrated a contact angle of 137°.The hydropobilzation method developed in this study is effective,simple and low cost,promising for wide range of applications.In Chapter 7,the summary of this thesis is presented,suggestions and prospects for the future research work are also proposed.