Study On Iron Molybdate Catalytic Prepared By Mechanochemical Method And Their Catalytic Performance For Partial Oxidation Of Methanol To Formaldehyde

Formaldehyde is an important raw material for fine chemicals,which is widely used to produce polyformaldehyde,resin,1,4-butanediol,urotropine,etc.,and its demand is increasing year by year.Iron molybdenum（FeMo）catalyst is an industrial catalyst for methanol oxidation to formaldehyde.The preparation process of iron-molybdenum catalyst by co-precipitation method is complex and produces large amounts of wastewater.The selectivity of iron-molybdenum catalyst needs to be further improved,and Mo O₃is easy to sublimation.Mechanochemical method is a simple and environmentally friendly green preparation method.In this paper,the synthesis of FeMo catalyst prepared by mechanochemical method and the structure-activity relationship of FeMo catalyzed methanol to formaldehyde were studied.The formation mechanism of Fe₂（Mo O₄）₃and Mo O₃crystal phases were explored.The influence of reaction conditions and raw materials on the physical and chemical properties of FeMo catalyst was investigated,and the formation mechanism of FeMo catalyst prepared by mechanochemical method was preliminarily revealed.Finally,FeMo catalyst was modified by metal oxides.This paper provides a new way for green preparation and application performance improvement of novel and efficient FeMo catalysts.The main contents are as follows:（1）To clarify the synthesis mechanism of FeMo catalyst prepared by mechanochemical method.The effects of different calcination temperatures and Mo/Fe molar ratios on the formation mechanism of FeMo catalysts prepared by mechanochemical method have been investigated using in-situ XRD,SEM,TG and XPS,et al.In the preparation of FeMo catalyst by mechanochemical method,it is impossible to synthesize FeMo catalyst without calcination.The calcination condition is a crucial step to adjust the proportion of Mo O₃and Fe₂（Mo O₄）₃phase.When the Mo/Fe molar ratio was 1.5,Mo O₃and Fe₂（Mo O₄）₃phases began to form at 180℃,and the Fe₂（Mo O₄）₃phase was stable at 600℃,and the aggregation increased with the increase of temperature.When the temperature higher than 500℃,the sublimation phenomenon of Mo O₃will also be accelerated.When the Mo/Fe molar ratio is 1.0,it is mainly composed of Fe₂（Mo O₄）₃phase.When the Mo/Fe molar ratio is more than 1.5,Mo O₃and Fe₂（Mo O₄）₃phases can occur simultaneously.When the molar ratio of Mo/Fe was2.6,the calcination temperature was 500°C,and the catalytic reaction is 4 h,the optimal catalytic performance is obtained,the conversion rate of methanol is 100%,and the selectivity of formaldehyde is 92.27%.（2）Effect of milling conditions on physicochemical properties of FeMo catalysts by mechanochemical method was investigated.The effects of ball number ratio,quality ratio of balls to materials and milling time on the physicochemical properties of FeMo catalysts prepared by mechanochemical method were employed by XRD,SEM,Raman and XPS.The results showed that the catalyst prepared by the all small balls（diameter=6 mm）had the highest formaldehyde yield（95.2%）.The influence of quality ratio of balls to materials ratio on the physical and chemical properties of FeMo catalyst was investigated.Except that the Mo O₃/Fe₂（Mo O₄）₃phase content of the sample with quality ratio of balls to materials ratio of 2 was the lowest,the Mo O₃/Fe₂（Mo O₄）₃phase content of the other three samples increased with the increase of quality ratio of balls to materials ratio.When the milling time was 10 min,only part of the Fe₂（Mo O₄）₃phase characteristic peaks appeared.The catalysts prepared in the range of 30～120 min showed typical FeMo catalyst diffraction peaks.The extension of milling time had no obvious effect on the crystal structure of FeMo catalyst.When the milling time was 120min,the catalyst had the highest formaldehyde selectivity（97.56%）,which could be due to the increasing content of Fe₂（Mo O₄）₃and the improved uniformity of particles with the increasing ball grinding time.（3）Effect of raw material type on physicochemical properties of FeMo catalyst by mechanochemical method was investigated.The effects of Fe（NO₃）₃·9H₂O,Fe₂（SO₄）₃and Fe Cl₃·6H₂O as iron sources on the physicochemical properties of FeMo catalysts prepared by mechanochemical method were investigated by XRD,SEM,Raman and XPS.FeMo catalyst prepared by mechanochemical method using Fe（NO₃）₃·9H₂O as iron source has the highest methanol conversion and formaldehyde yield.The formaldehyde yield of the catalyst prepared with Fe₂（SO₄）₃as the iron source was lower than that prepared with Fe（NO₃）₃·9H₂O as the iron source,mainly because the active phase Fe₂（Mo O₄）₃of the former was reduced.It was difficult to adjust the phase composition by adjusting the Mo/Fe molar ratio when Fe₂（SO₄）₃was used as the iron source.Combined with mechanochemical method and co-precipitation method,it can be seen that when Fe Cl₃·6H₂O and Fe₂（SO₄）₃are used as iron sources to prepare catalysts,solvent water is the key factor for the formation of Mo O₃and Fe₂（Mo O₄）₃phases by mechanochemical method.The impurity element Cl in the catalyst has a certain influence on the catalytic performance of the catalyst.（4）Effect of metal oxide doping on the structure and properties of FeMo catalysts by mechanochemical method was investigated.Bi（NO₃）₃·5H₂O,Co（NO₃）₂·6H₂O,Ni（NO₃）₂·6H₂O,Al（NO₃）₃·9H₂O and kaolin were added to modify FeMo catalysts in preparation of catalysts.The crystal particle size of the catalyst modified by metal oxides was smaller than that of the unmodified catalyst.The crystal distribution of the catalyst modified by Ni,Al,and kaolin was more irregular.Raman analysis showed that the content of Fe₂（Mo O₄）₃phase in the catalyst modified by Al and Co oxides increased,which improved the selectivity of formaldehyde.The effects of different Co additions on FeMo catalysts were discussed.The Co/Mo molar ratio was favored the formation of micro-cluster structure in the range of 0.01～0.05,and the micro-cluster gradually became dense phase in the range of 0.07～0.11.The Co oxides were uniformly distributed in the FeMo catalyst in amorphous form.In addition,according to the chemical state analysis,the formation of Co O can reduce the sublimation rate of Mo O₃,thereby improving the selectivity of formaldehyde.When the Co/Mo molar ratio was0.05,the formaldehyde selectivity was the highest（97.37%）.