Selective Oxidation Of Propane To Acrolein Over Mo-based Multi-component Composite Metal Oxide Catalyst

Propane is a cheap chemical material with abundant reserve in the world. Acquisition of oxy-compounds with high additive value as acrolein from propane always attracts extensive attention in both academe and industry field,because the study of one-step oxidation of propane to partial oxidation products has much significance both in theory and application.An effective catalyst has not been found for the reaction up to date,and there is no consummate theory of the reaction mechanism and of the relation between the structure of catalyst and the catalytic performance.More work on developing catalysts for the direct selective oxidation of propane to acrolein is needed.The recent related research mainly focuses on the multi-component MMO catalysts which are based on the Mo-V system.The best yield of acrolein(21.9%)was reported on the Mo-V-Te-P system.On account of the strong oxidation of V,these catalysts cannot reach the request of industry.Therefore finding some elements to substitute V in order to acquire a catalyst with the weaker oxidizability can be a way to settle the problem.In chapter two,the tetra-component catalysts Mo-Mn-Fe-M-O(M = Co,Bi), Mo-Cr-Te-Bi-O were prepared and tested for the synthesis of acrolein from propane. The addition of elements Co and Bi as the forth component to the ternary system Mo-Mn-Fe-O generally caused the decrease of the conversion of propane.The XRD results indicated that the addition of elements Co and Bi caused the transition of the active phaseα-MnMoO₄ to the phase MnMoO₄,which might be the reason of the restraint of the propane active.The addition of elements Bi as the forth component to the ternary system Mo-Cr-Te-O caused the decrease of the conversion of propane. The addition of Bi with the low content enhanced the selectivity of acrolein,but depressed with high content as a result of the augment of propane cracking reaction.In chapter three the multi-component catalyst Mo-Cr-Te-Co-P-O and Mo-Cr-Te-Co-P-K(Cs)-O were prepared and tested for the synthesis of acrolein from propane.The content of Cr,Te and Co component and the reaction temperature affected the influence of element P on the catalyst.The addition of P caused the increase of the conversion of propane evidently at the low reaction temperature.The prime production was acrylic acid.At the high reaction temperature a little addition of P made the aggrandizement of conversion of propane and the selectivity of acrolein with the low content of Te and Co,but the selectivity of acrylic acid with the high content of Co.The high content of P caused the great increase of conversion of propane and selectivity of acrolein with the high content of Cr and Te,and the highest yield of acrolein(34.71%)was obtained for now.Addition of P to Mo-Cr-Te-Co systems forming the P³⁺/P⁵⁺pairs enhanced the oxygen insertion velocity,which did not only encourage the formation of ACR but also oxidation of ACR to AA.Addition of P to Mo-Cr-Te-Co systems also advanced the ratio of the Mo⁵⁺in the catalysts to improve the redox ability.For the MoCr_xTe_yCo_zP_nO_m catalyst system the higher content of the chromium component could make the selectivity of acrolein increase, but the conversion of propane decreased for the same reason.The addition of elements potassium(or cesium)as the sixth component to some penta-component Mo-Cr-Te-Co-P-O system generally caused the decrease of the conversion of propane and the selectivity of acrolein.It was noted that propylene is tested as a main byproduct on the catalyst system including K(Cs)-component unlike the penta-component catalyst system.The result of XRD,H₂-TPR and XPS pointed out that the Mo⁶⁺/Mo⁵⁺pairs build the framework and make a function of charge neutrality.And the Co³⁺=O terminal group played an important role in the propane activation.The Co³⁺/Co²⁺pairs helped the transfer of oxygen species for both deep and selective oxidation.In chapter four the effects on different preparation conditions of the catalysts such as the acidity in the process of precipitation,calcinations temperature or time and on the different reaction conditions including the GHSV and the scale of propane and oxygen were measured and ascertained.In mass the preparation conditions made greater influence on the conversion of propane than the selectivity of acrolein for the catalysts.For the MoCr_0.0429Te_0.20Co_0.05P_0.20O_n catalyst,the best conversion of propane and the yield of acrolein were obtained on the occasion that the pH of the precursor solution is 3.The influence of acidity in the process of precipitation is less for the MoCr_0.0429Te₍0.15_Co_0.20P_0.15O_n catalyst.For the MoCr_0.0429Te_0.20Co_0.05P_0.20O_n catalyst,the long calcinations time was propitious to the yield of acrolein relative to the high calcinations temperature,but with the low calcinations temperature the short calcinations time did a favor for the yield of acrolein.The causation might be the change of the active phase of the catalyst.High GHSV was propitious to gain good selectivity of acrolein,but makes against the conversion of propane.In conclusion,we picked out the GHSV 750 h^-1and the ratio of C₃H₈:O₂=1:1.6 as the best condition in the reaction of elective oxidation from propane to acrolein for the MoCr_0.0429Te_0.20Co_0.05P_0.20O_n catalyst.The conclusions were made in the chapter five.In a word the catalyst MoCr_0.0429Te_0.20Co_0.05P_0.20O_n was picked out with the best yield of acrolein(34.71%)up to date,and some discussion was made on the relationship between structures and the performance of the catalysts which lay a solid foundation in order to gain the greater development of catalysts for the direct selective oxidation of propane to acrolein.