Study On Catalysts For Selective Oxidation Of Isobutane To Methacrolein And Methacrylic Acid

Selective conversion of cheapness and abundance light alkanes to thehigher valued oxygenated products is an important process in the chemicalindustry in recent years. Process for selective oxidation of n-butane tomaleic anhydride has been industrialized. Methacrylic acid (MAA) is animportant resin monomer, and is principally used for the synthesis ofpolymethyl methacrylate (PMMA). MAA is currently synthesized via theacetone-cyanohydrin (ACH) route, which use the toxic hydrogen cyanideand co-produces a massive amount of spent acid NH4HSO4. On the otherhand, oxidation of isobutene to MAA via methylacrolein (MAL), andmethylation of propylene (or ethylene) are developed. However, theseprocesses exist many disadvantages such as high cost of raw materials andmultistep reactions et al. Recently, interesting is increased in the one-stepselective oxidation of isobutane into MAA and MAL due to its simplicity,inexpensive raw materials and negligible environmental impact.Several catalytic systems have been reported for the direct oxidation ofisobutane. Among them, PMo heteropoly compounds (HPC) withKeggin-type structure exhibited relative high catalytic performance.Recently, multi-metal oxides have been reported to have catalytic activityfor the title reaction. It should be pointed out that compared to ethane andpropane, isobutane is more chemical active. This may be the main reasonfor the lower selective on metal oxide catalysts. So, we can increase thecatalytic performance of metal oxide catalysts for the oxidation of isobutaneby changing consist of catalysts, valent state of element and preparationmethods. It has been reported that MoVTeNbOx metal oxides prepared byhydrothermal method exhibited good catalytic performance on the oxidationof ethane, propane and ammoxidation of propane. The crystalline phase ofthe catalysts is simple and uniform. However, further work is still requiredin order to study the catalytic performance of various catalyst systems, theproperties of active sites, the regularity of selective oxidation of isobutaneand the catalytic mechanism of the reaction.In this work, various HPC and multi-ingredient metal oxides wereprepared, and the catalytic performance was investigated for the purpose tofind some novel effective catalysts for the title reaction and to establishrelationship between catalytic performance and chemical-physical featuresof the catalysts. The main experimental results and conclusions are asfollows:1, Multi-ingredient P-Mo HPC precursors are prepared by co-precipitationmethod, and the calcined products are P1.13Mo12VaCubSbcAsdK1.5Oz. TheXRD and FTIR results show that the catalysts have Keggin-type structureand the structure is stability after reaction. The TG-DTA results show thatthe HPC catalysts have relative high thermal-stability (723 K). The resultsof ESR indicate that: in the course of reaction, redox reaction of thetransition elements is occurred in P-Mo HPC catalysts, and Cu2+ is mainlyresponsible for the oxygen transfer. The introduction of As, V, Cu, Sb canincrease both the activity of the reaction and the selectivity of MAL andMAA. Good catalytic performance ( isobutane conversion = 10.8%,selectivity to MAL + MAA = 67.1 %, yield of MAL + MAA = 7.2%) for theoxidation of isobutane can be obtained on the Multi-ingredient P-Mo HPCcatalyst when reaction conditions are: reaction temperature = 633 K,isobutane/O2 = 2.5/1 (mol), GHSV = 3600 h-1. The structure of the catalystand the catalytic performance are the same on the catalyst prepared byreflux method and co-precipitation method.2, Effects on the properties and catalytic performance by addition oftransitional metal cation and alkali metal ion using HPC as precursor areinvestigated. The results indicated that the addition of VO2+ could increasethe activity, selectivity and yield of MAA and MAL. The addition of Cu2+can obviously increase the selectivity of MAA that can be mainly due to theeasily re-oxidated of Cu+, thus accelerated the cycle of oxidation-reductionreaction. The addition of suitable amount of Cs+ is of benefit to the catalyticperformance of the catalysts.3, Supported molybdophosphates catalysts are synthesized by two-stepimpregnation method. The supports included silica gel, expanding poresilica gel, MCM-41 and SBA-15. The effects of structure of supports andpreparation conditions on the catalytic performance are studied. The resultsshowed that higher surface area of catalysts is favorable to the conversionof isobutane, and larger pore diameter is in favor of the formation of mainproducts. It is obviously that the impregnated conditions (for example, thesolvent, the impregnation order of raw materials, loading amount et al.)have effect on the catalytic performance. We can conclude that the catalystsprepared by using water as solution, first impregnate heteropoly acid, thenimpregnate alkali salt exhibit high catalytic performance. However, thecatalytic performance on the catalysts prepared by impregnation method islower than the bulk phase catalysts. More work is required to study theselect of supports and the structure properties of supported HPC.4, Multi-ingredient metal oxides (MoVTeNbOx) are prepared byhydrothermal method. The elementary composition, oxidation state, thecrystalline structure and properties of the catalysts are characterized by BET,ICP, SEM, XRD, FTIR, Raman, XPS and TPR methods. The catalyticperformance of oxidation of isobutane is also investigated. The resultsshowed that the catalysts have simple crystalline phase and have stability ofstructure and compose during calcinations and reaction. The catalyticperformance is various with different crystalline phase of the catalysts. Theintroduction of suitable amount of Te, V, Nb can increase the activity andthe selectivity to main products. MoVTeNbOx oxides exhibit the highestcatalytic performance. According to the results of characterization, theco-cooperation of MoO3 phase, M1 and M2 can be responsible for the highcatalytic performance of the catalysts.