Green Oxidation Of Hexadecane Catalyzed By The Noble-Metal Ru-Supported Polyoxometalates

Derivatives of alkane as important chemical raw materials can be obtained by theoxidation of alkanes, but alkanes contain inactive C-H bond, so a series of studies havebeen developed in order to do affect the oxidation of inactivated C-H bond, such assacrificial oxidant, using organic solvents, adding additives, and the fierce conditions （hightemperature or high pressure）. However there are some problems brought along with thedesired products, vast amounts of waste are generated. On one hand, the separation ofwaste and product is required; on the other hand, the waste is needed to be dealt with, thesecomplexes post-processing are bring a higher operating costs, especially increasing theenvironmental pollution, therefore it is imperative to establish green chemical processes. Itcan be seen from the recent studies that oxygen or air is using as oxidant instead of thetraditional for reducing pollution and cost, but the oxidation of alkane becomes moredifficult. Therefore, many opportunities and challenges for the chemists are present for thedevelopment of the new and efficient green catalysts in the oxidation of alkanes.The noble metal ruthenium has the4d⁷5s¹electronic structure, so in the periodic tableit presents the widest oxidation state changes （from-2to+8）. In addition, the studiesindicate that the ruthenium-containing polyoxometalates can anchor or transport oxygenduring the catalytic process, leading to their unique redox properties and excellentcatalytic activities in the oxidation reactions, so the exploitation of noble metal rutheniumcontaining polyoxometalates as catalysts has received consideration attention. Especially,the noble metal ruthenium containing POMs have been the important candidate catalystsfor green oxidation of alkanes.According to the current research on oxidation of alkanes, the development of greenoxidation catalysts for the long chain alkane of liquid hexadecane has just been started. Sofar, there have only three articles published, for example Kortz et al anchored the Cu containing polyoxometalate [Cu₂₀Cl（OH）₂₄（H₂O）₁₂(P₈W₄₈O₁₈₄)]^25-on the SBA-15toprepare the solid catalyst and realized the multi-phase green oxidation of hexadecane.These words show the polyoxometalate can be the ideal catalysts for green oxidation ofhexadecane. However, based on the current studies, the all catalysts are the singletransition metal substituted polyoxometalate. Obviously, this system is still in its infancy,the development of mixed transition metal and ruthenium containing POMs as thecatalysts for green oxidation of hexadecane has never been reported in the world.Based on the above introdruction, we have performed the following three works:Firstly, We have designed and synthesized the noble metal ruthenium supportedpolyoxometalate Na₆[{Ru（C₆H₆）}₂W₈O₂₈（OH）₂]·16H₂O （W₈Ru₂）, and then usedmesoporous molecular sieve SBA-15as the catalyst supporter to immobilize W₈Ru₂onthe surface of SBA-15to prepare the solid-phase catalyst. The catalytic activity of thesolid catalyst for heterogeneous green oxidation of hexadecane was investigated includingthe influence of different W₈Ru₂loading amount on the catalytic activity. The results showW₈Ru₂and its soild catalyst SBA-15-Apts-W₈Ru₂exhibited the higher catalytic activitiestowards the higher conversions of hexadecane （more than40%）. Meanwhile we find outthe different W₈Ru₂loading amount shows a greater impact on the activity of thecatalysts.Secondly, we have designed and synthesized a series of sandwich-type mixedtransition metal ruthenium supported polyoxometalates [{B-α-AsW₉O₃₃（OH）}{B-β-AsW₈O₃₀（OH）}{M₄（OH）₂（H₂O）2}{（RuC₆H₆）₃}]^6-（AsWRuM）（M=Ni^Ⅱ, Zn^Ⅱ, Cu^Ⅱ, MnⅡ,Co^Ⅱ） which possess the same structure but with different sandwich atoms. Thesecompounds were used as catalysts for oxidation of hexadecane under green catalyticconditions. The results indicate AsWRuM exhibit the synersistic effect of Ru andtransition metal ions to enhance catalytic activities and the order of the relative catalyticactivities: AsWRuCu<AsWRuMn<AsWRuNi<AsWRuZn<AsWRuCo.Thirdly, in order to investigate the impact of the central atoms on the catalyticactivities of the noble metal ruthenium supported POMs, we selected two compounds withthe same structure but different central atoms, such as [（RuC₆H₆）XW₉O₃₄]^7-（MWRu）（X=As, P） as catalysts for the green oxidation of hexadecane. At same time, we also studiedthe influence of amount of catalysts, reaction times, and reaction temperatures on the catalytic activities of the soild catalyst SBA-15-Apts-MWRu （M=As, P） to detect therelationship of the centrol atoms with the catalytic activity for such ruthenium containingpolyoxometalates. The results show the compound with the P as the centrol atom exhibitsmuch higher activity than the As, meanwhile the conversions of hexadecane obtained bythe soild catalysts SBA-15-Apts-PWRu（2.23%）and SBA-15-Apts-AsWRu（2.15%）are both higer than the before anchored by20%.In conclusion, we have used a series of noble metal ruthenium supportedpolyoxometalates as catalysts under the system of green oxidation of hexadecane toinvestigate the influence of reaction time, amount of catalyst, reaction temperature,polyoxometalate loading amount for the soild catalysts on the catalytic reactions and theirreusability. Importantly, based on the comparative analyses of the catalytic results, wehave found out that the sandwich atoms and centrol atoms have the major impact on itscatalytic activities for this system, providing a widespread basis in such oxidation ofalkanes.