Fine Control of Active Sites of Metal Hydrogenation Catalysts and Their Performance for Catalytic Transformation of Biomass Platform Molecules

Author:Gao Zhi

Supervisor:li feng


Degree Year:2018





With the rapid depletion of non-renewable fossil resources and resulting increasing environment problems,the exploration of suitable and renewable resources to replace fossil resources attract more and more attention.Biomass serving as excellent renewable resources can be converted to a variety of important platform molecules.Furthermore,these platform compounds can be further transformed to highly value-added chemicals by green catalytic hydrogenation processes.For hydrogenation of 5-Hydroxymethylfurfural(HMF),the products can be divided to two categories.One is the promising biofuel with high energy densities and octane numbers,including 2,5-dimethylfuran(DMF)and 2,5-dimethyltetrahydrofuran(DMTHF).Another category is the diol precursor of polymer production,such as 2,5-dihydroxymethylfuran(DHMF)and 2,5-dihydroxymethy ltetrahy drofuran(DHMTHF).Thus,biomass is considered to be an ideal substitute of fossil resources.In present,the hydrogenation of biomass-derived platform molecules has been widely investigated and some progresses have been achieved.But,some drawbacks,such as low activity,poor stability and the use of high H2 pressure,limit the scale-up applications.Therefore,the design and synthesis of highly efficient catalysts for hydrogenation of biomass-derived platform molecules is a very meaning work.Additionally,the hydrogenation of biomass-derived platform molecules is generally performed under high H2 pressure.Using H2 as hydrogen donor exist the drawbacks of security concerms,low utilization,difficult transportation and high cost.Compared with formic acid,noncorrosive alcohols can be more suitable hydrogen donors for catalytic hydrogenation of biomass-derived platform molecules.Moreover,the dehydrogenation products of alcohols can be easily separated from reaction system.For the hydrogenation of biomass-derived platform molecules,the selectivity of target products is low.The selectivity of desired products usually can be enhanced using alcohols instead of HZ as hydrogen donors,which is attributed to the gentle dehydrogenation process of alcohols.However,the activity of hydrogenation reactions in biomass field is poor under alcohols as hydrogen donors.Thus,the fabrication of highly efficient catalysts for catalytic transfer hydrogenation of biomass-derived platforms molecules using alcohols as hydrogenation sources is a very meaning work.Hydrotalcite,also known as layered double hydroxides(LDHs),has the advantages of controllable layer composition and interlayer anion.The facile maneuverability of LDHs provides the new ideas for the rational design of future catalysts,where different active compositions can be introduced for typical catalytic reactions.Through the post process of LDHs,the active sites of catalysts can be finely adjusted and thus the highly active and selective multi-functional metal base catalyst can be prepared.In this work,metal base catalysts(Ru-based and Cu-based)with different microstructures were synthesized by one-pot embedding method or LDH precursor method and used for catalytic hydrogenation of biomass-derived platforms molecules.The structure of catalysts and catalytic performance were investigated systematically.The plausible reaction mechanism was proposed and the main work is as follows:1.The fabrication of dandelion-like cobalt oxide microsphere-supported RuCo bimetallic catalysts and their performance in selective hydrogenolysis of 5-hydroxymethylfurfuralA simple and effective one-pot embedding method was developed for the fabrication of RuCo bimetallic NPs inside dandelion-like CoOx microspheres structure with a large number of surface defects(i.e.Co2+species and oxygen vacancies)simultaneously constructed.As-formed RuCo/CoOx bimetallic catalyst exhibited excellent catalytic performance for the hydrogenolysis of HMF to produce DMF(96.5%yield after 2 h)even when the HMF/Ru molar ratio was as high as 252.7.In particular,RuCo/CoOx also showed a high TOFHMF(420 h-1)and TOFC-O(437 h-1),as well as low apparent activation energy(55 kJ/mol).Compared with Ru/Co3O4 and Ru/Co/CoOx catalysts prepared using NaBH4 reduction agent,RuCo/CoOx exhibits higher stability in the hydrogenolysis of HMF due to the strong interaction between RuCo bimetallic nanoparticles and CoOx support.The high catalytic performance of RuCo/CoOx in the HMF hydrogenolysis can be related to the following several factors:1)RuCo bimetallic NPs can rapidly dissociate the H2 to form active hydrogen species;2)surface defects(i.e.,oxygen vacancies,Co2+ species)can absorb and then activate C=0 and C-0 in HMF 3)The significantly enhanced hydrogen spillover effect in the present RuCo/CoOx can contribute to the improved catalytic performance to some extent,iiii)The specific 3D dandelion-like architecture of RuCo/CoOx with suitable pore size,high specific surface area and metal dispersion is good for the access of spillover hydrogen and close contact of HMF with surface active sites.As a result,the enhanced hydrogenolysis of HMF to DMF is attributed to the Ru-Co synergistic effect in RuCo NPs,favorable surface defect in proximal CoOx sites and the enhanced hydrogen spillover effect,as well as more active sites and reaction centers provided by distinct dandelion-like structure.2.The fabrication of surface defects-promoted Ru-based catalysts and their performance in transfer hydrogenation of furfuralHighly dispersed Ru-based catalysts with abundant surface defects(Co-Ru/C)were fabricated by a two-step hybridization-self-reduction route.Compared with other catalysts(Mg-Ru/C,Zn-Ru/C and Ni-Ru/C)prepared using same procedures,Co-Ru/C shows higher catalytic performance.The completed conversion of furfural and 100%selectivity of furfuryl alcohol in Co-Ru/C catalyst were obtained at 150 ℃ for 12 h under N2 atmosphere.Further,the TOF value can reach as high as 173.8 h-1 and the apparent activation energy is lowest compare to other three samples.Based on a series of characterization,we found that Co-Ru/C catalyst has more surface defects(i.e.,oxygen vacancies,Co2+ species)and Ruσ+ species,which can promote the absorption and sequent activation of furfural.Moreover,Ru0 species is benefit for the dehydrogenation of 2-propanol and subsequent transfer of active hydrogen.Three key role(Ruσ+、Ru0 and surface defects)can synthetically govern catalytic transfer hydrogenation of furfural.Furthermore,Co-Ru/C catalyst shows excellent stability in transfer hydrogenation of furfural.Noticeably,Co-Ru/C also shows good activity in the transfer hydrogenation of a series of biomass-derived compounds that contain carbonyl groups.3.The fabrication of double-active sites promoted Ru-based catalyst and their performance in transfer hydrogenation of ethyl levulinateDouble-active sites promoted Ru-based catalyst(Ru/ZnAlZr-LDH)was prepared through a separate nueleation and aging steps method.As-prepared Ru/ZnAlZr-LDH catalyst exhibited high catalytic activity in the transfer hydrogenation of ethyl levulinate(EL)to produce γ-valerolactone(GVL).The yield of GVL can reach 98%at 200 ℃ for 10 min under N2 pressure using 2-propanol as hydrogen donor and simultaneously,the production rate of GVL is as high as 1250μmol/g/min,which much higher than the value reported previously.Based on a series of catalytic tests and characterizations,we found that the introduction of Zr and Ru species can sharply facilitate the dehydrogenation of 2-propanol and subsequent improvement of GVL yield.Based on XPS tests,we found that Ru species was existed in electron-rich state due to the strong interaction between Ru species and ZnAlZr-LDH support.The electron-rich Ru3+ species and a large amount of surface hydroxyl groups can synergistically derive the transfer hydrogenation of EL by metal hydride route and MPV route,respectively.Importantly,Ru/ZnAlZr-LDH also can highly catalyze a series of biomass-derived carbonyl compounds.Thus,the Ru/ZnAIZr-LDH possesses promising potential for industrial application.4.The fabrication of nitrogen-doped carbon-decorated Cu-based catalysts and and their performance in transfer hydrogenolysis of 5-hydroxymethy lfurfuralA high-performance Cu-based catalyst(NC-Cu/MgAlO)was developed by a facile thermal decomposition of CuMgAl-LDH/melamine hybrid precursor and employed for the transfer hydrogenolysis of HMF using cyclohexanol as hydrogen donor.The high yield of DMF or DMTHF can be convertibly achieved by modulating the reaction time.A series of characterization results reveal that the doping of N species can not only modulate the electronic state but also increase the amount of surface basic sites.The Cu0,Cu+ and lewis basic sites of NC-Cu/MgAlO catalyst synergistically promote the dehydrogenation of cyclohexanol and subsequent hydrogenation of HMF.Additionally,the catalytic activity of NC-Cu/MgAlO catalyst exhibits negligible loss after five consecutive cycles,demonstrating the strong ability in transfer hydrogenolysis of HMF.