Study On Pore Structure And Surface Properties Of Diatomite And Composite Material And Humidity Control Performance

Relative humidity is one of significant factors to evaluate comfort of interior air environment.Both damp and dry environment have adverse influence on human health and livelihood.Air-conditioning is the most widely used humidity control method presently,which consumed a large amount of energy and unsatisfied the demand of energy-saving and environment protection.As a kind of novel energy-saving and environment protection material,humidity control material can regulate moisture content of interior air automaticly according to the vary of relative humidity with moisture adsorption/desorption ability.Due to light weight,high porosity,excellent absorption capacity,chemical inertness,low-price and large reserves,diatomite is ideal raw material for humidity control.During the past decades,diatomite has attracted large attention of researchers from all around the world as humidity control material,especially the development of diatomite interior wall material and diatom ozoo.However,until now,few attentions have focused on the pore structure and surface properties of diatomite.Meanwhile,few researches about utilizing the structure-activity relationship of pore structure and surface properties of diatomite to control the preparation of diatomite humidity control materials were found.Mixture and recipe are the two main technologies for industry of diatom ozoo,which limited the improvement of humidity control performance of diatomite.Thus,it is vitally important to analyze the pore structure and surface properties of diatomite and study the structure-activity relationship among humidity contril performance,pore structure and surface properties.And it is meaningful to optimize the pore structure and surface properties of diatomite and prepare novel efficient diatomite humidity control material.Based on the research status and practical demand,this paper has analyzed the pore structure and surface properties of diatomite using Linjiang diatomite as raw material,and the structure-activity relationship among humidity control performance,pore structure and surface properties was analyzed.There mining processing methods which are calcination,alkali dissolution and grinding were used to regulate the pore structure and surface properties of diatomite,and the influence of calcination temperature,alkali dosage and grinding time were analyzed.The structure-activity relationship among humidity control performance,pore structure and surface properties was obtained.Meanwhile,three kinds of novel diatomite humidity control materials(diatomite/ground calcium carbonate humidity control material,diatomite/silica humidity control material and diatomite/hydroxyl alumina humidity control material)were prepared,and preparation meschiem,pore structure,surface properties and humidity control performance of composite materials were analyzed.The humidty control meschiem was diacussed combined with the analysis of dynamic and thermodynamic analysis.The main work of this paper and the main achievements are as follows:(1)The adjusting of pore structure and the change of pore structure and surface properties of diatomiteThe raw diatomite sample was obtained from Linjiang area.The calcination,alkali dissolution and mechanical grinding of the diatomite were carried out at different calcination temperature,alkali dosage and grinding time respectively.The particle size distribution,specific surface area,pore volume,pore size distribution,pore surface roughness and pore structure inhomogeneity,surface morphology,surface structure,surface hydroxyl groups,surface morphology of diatomite treated with different pore structure adjustment methods were studied by means of X-ray diffraction(XRD),particle size analysis,low temperature nitrogen adsorption,mercury intrusion(MIP),fractal theory,scanning electron microscopy(SEM),transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FTIR),Boehm titration and thermal analysis(TG-DSC).The specific surface area and mesoporous volume of diatomite were analyzed by BET theory.The t-plot method based on de Boer model was used to study the specific surface area and volume of the micropore in diatomite.The change of diatomite's specific surface area and the mesoporous volume with pore size was analyzed by BJH model and DFT model using the adsorption isotherm curve.Based on Washburn equation,the MIP was used to analyze the volume and distribution of the macropores.Based on the principle of nitrogen adsorption,the data of adsorption isotherm was fitted by FHH fractal model to analyze the fractal dimension of surface and the mesopores and evaluate the pore surface roughness and the mesoporous structure inhomogeneity of diatomite.From the result of the mercury intrusion test,the Menger sponge model and the thermodynamic model were used to analyze the fractal dimension of the macropores of the diatomite disposed by calcination,alkali dissolution and mechanical grinding so that the inhomogeneity of the macropores was quantitatively characterized.The results showed that the porous structure of diatomite was a three-dimensional pore structure composed of the mesopores and macropores.The diatomite contained almost no microporous structure.Alkaline surface etching could produce a certain amount of micropores.There was a certain amount of blind holes in the internal structure,which could be opened by mechanical grinding.The proportion of macropores in diatomite was larger than that of mesopores.The mesopores of diatomite were greatly affected by calcination.Alkali solution could control the mesopores and macropores at the same time.The macropores were easily worn by mechanical grinding.Diatomite and its composite humidity control materials have typical fractal characteristics.The surface fractal dimension and volume fractal dimension could be used to characterize the surface roughness and pore structure inhomogeneity of diatomite and its composites.FHH model and thermodynamic model could be used to analyze the fractal dimension of mesoporous and macroporous structure of diatomite.The thermodynamic relationship model is more reliable than the Menger sponge model on the analysis of macroporous fractal dimension.With the increase of calcination temperature,the specific surface area and mesoporous volume of diatomite decrease,and the proportion of macropores increase.The surface fractal dimension of diatomite decreased with the increase of calcination temperature and the surface roughness decreased and the surface tended to be smooth.When the calcination temperature was lower than 800?,the fractal dimension of the mesoporous volume and the heterogeneity of mesoporous structure decreased with the increase of calcination temperature.The deformation of mesoporous structure and the mesoporous volume fractal dimension increased.The mesoporous structure becomes more complicated when the calcination temperature exceeded 800?.When the calcination temperature was higher than 600 ?,the fractal dimension of macroporous pores decreased and the roughness and inhomogeneity of macroporous surface of diatomite under high temperature calcination were reduced.The hydroxyl groups on the surface of diatomite were condensed,the number of hydroxyl groups on the surface less,the surface active sites decreasing,and the surface hydroxyl groups reached the minimum value of 2.117/nm2 at 800?.With the increase of the amount of alkali,the specific surface area and mesoporous volume decreased first and then increased,which is as same as the fractal dimension of the surface,mesoporous volume and the macroporous pores.The surface roughness and inhomogeneity decreased first and then increased.The surface hydroxyl content of diatomite increased first and then decreased,which was negatively correlated with the change of specific surface area of diatomite.With the increase of grinding time,the specific surface area and the mesoporous volume increased and the fractal dimension of the diatomite surface and the fractal dimension of the mesoporous volume decreased first and then increased and then decreased,which was related to a certain degree of damage to the initial mesoporous structure in the grinding process and the pores of the mesoporous opening and particle agglomeration caused by grinding over time.The fractal dimension of the macroporous pores gradually increased.The mechanical grinding increased the roughness and inhomogeneity of the macroporous surface of diatomite.The surface hydroxyl density of diatomite increased first and then decreased.(2)Effect of pore structure adjustment on diatomite humidity control performanceThe adsorption and desorption experiments of 72 hours under different temperature and reltive humidity conditions were applied to the sample of diatomite managed by calcination,alkali dissolution and grinding.The results showed that the humidity control performance of diatomite under different temperature increased with the increase of the ambient temperature and relative humidity(or relative humidity difference).The releasing moisture content of diatomite was positively correlated with its moisture content of absorption under different temperature and humidity conditions.The moisture absorption performance of diatomite determined the strength of its moisture desorption performance.The humidity control performance of diatomite decreased with the increase of calcination temperature.When the calcination temperature exceeded 1000?,the humidity control performance of diatomite decreased significantly.The humidity control performance of diatomite decreased first and then increased with the increase of alkali dosage.The humidity control performance of diatomite decreased first,then increased and then decreased with the increase of grinding time.Under the experimental condition of this research,the humidity control performance of diatomite was the highest when the grinding time was 90 minutes.(3)The structure-activity relationship of pore structure,surface properties and humidity control performance of diatomite.The pore structure and surface properties of diatomite had a significant effect on the humidity control performance of diatomite.The results of analysis showed that The specific surface area of diatomite,the ratio of mesoporous volume,the proportion of macroporous volume,the surface roughness,the inhomogeneity of pore structure and the number of surface hydroxyl groups were important factors to determine the humidity control performance of diatomite.The humidity control performance of diatomite was positively correlated with the specific surface area,the mesoporous volume,the surface roughness,the inhomogeneity of the pore structure and the number of surface hydroxyl groups,and negatively correlated with the proportion of large pore volume.The influence of mesoporous on the humidity control performance of diatomite was greater than that of the macropores and the effect of mesoporous surface area is was greater than that of the macropores.The mesoporous structure could be occurred the phenomenon of capillary condensation in the relatively lower external vapor pressure than that of the macropores,which significantly increased diatomite moisture capacity.The macroporous structure mainly acted as a channel for the diffusion of water vapor molecules during the process of humidity control and acted as a space for the adsorption and storage of water vapor molecules.On the surface of diatomite pores,there were a large number of hydroxyl groups with good hydrophilic properties,which could improve the amount of diatomite moisture by forming hydrogen bond with water vapor molecules.The number and distribution of hydroxyl groups were influenced by the pore specific surface area and pore surface roughness and the heterogeneity of pore structure distribution of the diatomite and its composite humidity control materials,which then changed the humidity control performance of diatomite as well as its composites.(4)Preparation and humidity control performance of diatomite humidity control materials.Diatomite/ground calcium carbonate humidity control material(DE/GCC)was prepared by uniform mixture and high temperature calcination using diatomite and ground calcium carbonate as raw materials.Diatomite/silica humidity control material(DE/S)was prepared visa partial alkali dissolution and acid sediment using diatomite,sodium hydroxide and sulfuric acid as raw materials.Diatomite/hydroxyl alumina humidity control material(DE/A)was prepared visa hydrolysis precipitation reaction and hydrothermal molding using diatomite,sodium hydroxide and aluminum chloride as raw materials.The preparation meschian,pore structure and surface properties of composite materials were characterized by XRD,ICP,particle size distribution,low temperature nitrogen gas adsorption,MIP,factral theory,SEM,EDS,TEM,FTIR,Boehm titration,TG-DSC and contact angle test.The humidity control performances of composite materials were conducted under different temperature and relative humidity condition.And the reuseability of composites was also analyzed.The results showed as follow:Ground calcium carbonate decomposes at 750? and develops calcium oxide.Then some silicon converts to calcium silicate.Some calcium oxide converts to calcium hydroxide due to reaction with water vapour as exposing in the air after calcination.The surface hydroxyl density and acid sites increase,and van der Weals forces increase.While the specific surface area,mesoporous volume,surface roughness and pore structure inhomogeneity decreased compared with DE.The moisture adsorption performance of DE/GCC improved while the moisture desorption content declined.DE/GCC doesnot have good reuseability.And the pore structure of DE/GCC needs to be improved further.The preparation mechanism of DE/S was as follow: part of diatom reacts as silicon source to prepare the sodium silicate,which is precursor of precipitate silica,silica was coated on the surface and pore structure of DE afer acid sediment and aging,and the rest diatom works as skeleton of composite material.The specific surface area,mesoporous volume,surface roughness,pore structure inhomogeneity,surface hydroxyl and surface energy of DE/S improved apparently,and the macroporous volume declined.DE/S showed excellent performances as humidity control material with 72 h constant temperature(30?)moisture adsorption content 32.042% and moisture desorption content 24.944% at 98%RH and 33%RH,respectively.The humidity control ability of DE/S improves 4 to 7 times compared with that of DE.Meanwhile,DE/S showed good reusability during three moisture adsorption and desorption cycles of 216 h,the moisture desorption contents of DE/S are over 75% of moisture adsoption contents.The preparation mechanism of DE/A was as follow: the pore structure of DE works as microreactor and carrier of hydroxyl alumina,the crystal and morphology of hydroxyl alumina is controlled by hydrothermal.Speculate hydroxyl alumina coated on the pore surface of DE homogeneously.Hydroxyl alumina has larger specific surface area and abundant mesoporous volume,which improved the mesoporous inhomogeneity.Abundant silicon hydroxyl and aluminium hydroxyl in DE/A increased the surface active sites and surface energy.DE/A showed excellent performances as humidity control material with 72 h constant temperature(30?)moisture adsorption content 29.127% and moisture desorption content 23.090% at 98%RH and 33%RH,respectively.The humidity control ability of DE/A improved 4 to 7 times compared with that of DE.Meanwhile,DE/A shows good reusability during three moisture adsorption and desorption cycles of 216 h,the moisture desorption contents of DE/A are over 75% of moisture adsoption contents.(5)Dynamic and thermodynamic analysis of diatomite and composites materials and their humidity control mechanism.Moisture adsorption/desorption dynamic of DE,DE/S and DE/A under different temperature and relative humidity condition were conducted with pseudo-second-order kinetics and intraparticle diffusion models.Thermodynamic parameters of moisture adsorption of DE,DE/S and DE/A were analyzed using thermodynamic principle of porous media.Isothermal moisture adsorption(or desorption)heat of diatomite and composites were calculated by Clausius-Claperon equation.The results showed that moisture adsorption/desorption of DE,DE/S and DE/A follow pseudo-second-order kinetics model.Intraparticle diffusion model is suitable for moisture adsorption dynamic,especially in high relative humidity.Pseudo-second-order kinetics model can reflect the moisture adsorption/desorption mechanism of diatomite and composite.Isothermal moisture adsorption of DE,DE/S and DE/A follow S type physisorption.And the isothermal moisture adsorption heat is lager than 0.Moisture adsorption is endothermic,and diatomite and composite have better adsorption performance under lower temperature.Van der Waals force and hydrogen bond are the main forces.