Selective Oxidation Of Isobutane Over Mixed Metal Oxide Catalysts

In recent years, methacrylic acid surges in demand in the world. It is mainly used for the preparation of methyl methacrylate and its derivatives. At present, the methyl methacrylate production technologies include acetone-cyanohydrin route, isobutene selective oxidation route and ethylene route. From the 1980s, people began to research the one-step synthesis of methacrylic acid from isobutane. Although MAA yield is currently lower by the method and it can not be applied for industrial production, it will attract increasing attention in the long term, because this method does not require an intermediate product separation and the process is relatively simple, besides, it does less pollution to the environment and produces no inorganic by-products. There are abundant C4 resources in our country, with the exception of some raw materials for the chemical industry, is basically used as fuel. Therefore, isobutane exploitation is necessary and urgent. Because heteropoly acid catalysts have good crystalline structure, strong redox capacity, high catalytic activity and selectivity in the selective oxidation of alkanes, they received wide attention in the past decades. However, heteropoly acid and its derivatives have a common shortcoming, which is their poor thermal stability. They are prone to decompose in the catalytic reaction conditions, therefore reducing their catalytic activity and selectivity.In very recent years, research on complex metal oxide catalysts is progressing very rapidly. Mo-V-O complex oxide catalysts are widely used for selective oxidation of light alkanes and ammonia oxidation reaction. Back in the 1970s, Mo-V-Nb catalysts began to be used in the oxidation of ethane to ethylene and acetic acid. By the 1990s, MoVTe(Sb)NbOx catalysts were reported for oxidation and ammonia oxidation of propane. In addition, MoVTe(Sb)NbOx catalysts also showed good catalytic performance in the catalytic oxidation of propane to acrylic acid. Recently, we reported that Mo-V-Te-based oxides have high catalytic activity and selectivity in selective oxidation of isobutane, the methacrolein selectivity reaches as much as 37% over Mo-V-Te-Sb-O catalysts. Moreover, we demonstrate the exfoliation of the layered vanadium (IV) orthophosphate VOHPO4·0.5H2O as well as the organization of this exfoliated solid into a mesostructured lamellar phase having the same V–P–O connectivity as in VOHPO4·0.5H2O using a long chain alkyl amine surfactant such as dodecylamine as the structure directing agent. VPO catalysts can be obtained by calcining the VOHPO4·0.5H2O precursors. And these catalysts exhibit better catalytic performance in the isobutane selective oxidation reaction than no addition of dodecyl amine. It is demonstrated that the Mo-based and V-based catalysts are economic, environmental protection and high activation as the new-type catalyst for isobutane oxidation.The main research work is as follows:1. We systematically studied the isobutane and isobutene selective oxidation reaction over Te-Mo-O catalysts. We also amply study the influence of the reaction conditions (including temperature, reaction space velocity, water content and alkane/oxygenl ratio) on the catalytic performance. The experimental results show that: (1) Under the reaction conditions (400 oC, isobutane:O2:N2:H2O =1:2:2:2 (mol); GHSV=3000h-1), when Te is added at Te:Mo = 1:3, the catalyst exhibits best catalytic performance in the selection of isobutane, in which the yield to MAL+MAA reaches 10.1% and the selectivity to MAL+MAA is 46%. (2) Under the reaction conditions (440 oC, isobutene:O2:N2:H2O = 1:13:32:4 (mol), GHSV=10200h-1), when Te is added at Te:Mo = 1:4, the catalyst exhibits best catalytic performance in the selection of isobutene, in which isobutene conversion reaches as high as 77.5%, the selectivity to MAL and MAA is 73% and 9% respectively.2. We have synthesized Mo-V-Te-O catalysts by evaporation of the solution and researched their catalytic performance in the selective oxidation of isobutane and isobutylene. Their physicochemical properties were investigated by XRD, FTIR, BET, TPR and XPS. The catalytic results show that V-content catalysts exhibit higher catalytic activity and lower selectivity. This might be related to higher oxidation capacity and stronger surface acidity of V atoms than Mo atoms. If the surface acidity is very strong, the desorption of methacrolein product will become more difficult from the catalyst surface, leading to low selectivity to MAL and high selectivity to complete oxidation products COx.3. On the basis of the Mo-V-Te-O catalyst system, we added non-metallic atoms (e.g. P), weak oxidation capability metal atoms (e.g. Sb) and strong oxidation capability and oxygen reserves metal atom (e.g. Ce) into the Mo-V-Te system for the preparation of a series of MoV0.3Te0.23MxOn catalyst. Combining XRD, FTIR, BET, TGA, TPR and XPS characterization and catalytic performance, we systematically studied the effect of P, Sb and Ce atoms in the selective oxidation of isobutane and isobutylene. The results showed: (1) TPR results showed that the redox capability of the catalysts has been improved due to the addition of P atoms, and synergy effect of the components has been greatly strengthened. Since there is an interaction between P and Mo, Mo and Te, a certain amount of Mo5+ is formed. And the interaction between Mo5+ and Mo6+ improves the catalytic performance. When the composition of the catalyst is MoV0.3Te0.23P0.3Ox, it achieves the best catalytic performance. (2) Adding Sb to Mo-V-Te-O system can suitably adjusting the concentration of Mo5+: Mo6++Sb4+ ? Sb5+ Mo5+. Meanwhile, TPR results showed that the redox capability of the catalysts has been improved due to the addition of Sb atoms; synergy effect of the components has been greatly strengthened. Isobutane selective oxidation results show that when Sb is added at Sb/Mo=0.5 (MoV0.3Te0.23Sb0.5), the selectivity and yield to purpose product MAL is the best, respectively 39% and 7.8%, which is to our knowledge the highest ever reported for the direct oxidation of isobutane. (3) by adding Ce to Mo-V-Te-O system will not only improve the oxygen storage capacity and electronic transfer capacity to the system, but also can be formed a redox cycle in VI the system: Ce3++Mo6+ ? Mo5+ +Ce4+. Under the reaction conditions (420 oC, isobutane:O2:N2:H2O =1:2.5:2:2 (mol), GHSV=3600h-1), over the MoV0.3Te0.23Ce0.2 catalyst, isobutane conversion can be reached 20.2%, and the selectivity to MAL and MAA can be reached 33% and 20%, respectively.4. We have studied the use of dodecylamine to form mesostructured lamellar vanadium phosphate, and these catalyst precursors were calcined at 550 oC to gain the VPO catalysts. Combined with catalyst characterization, we investigated their catalytic performance in the selective oxidation of isobutane and isobutene. The catalytic results show that the proper addition of dodecylamine to catalyst precursor can form reducedβ-VOPO4 phase and modify the catalytic properties of vanadium phosphate in the isobutane selective oxidation reaction, therefore, the selectivity to MAL and MAA can be largely improved. However, the isobutene selective oxidation reaction shows exactly the opposite results.