| The non-renewable nature of fossil fuels and global environmental changes urge the society to develop renewable energy technologies to reduce the dependence of human society on fossil fuels.Because of its abundance,it is believed that lignocellulosic biomass could be a viable alternative to petroleum-derived liquid fuels.Cellulose and hemicellulose,the main components of lignocellulosic biomass,can be catalytically converted via a multitude of reactions such as hydrolysis and dehydration to produce furanic compounds,which can be further upgraded into platform molecules such as levulinic acid.The production of yvalerolactone(GVL),a value-added product with versatile applications,from levulinic acid and its esters is a widely explored route in the current settings of biorefinery.Given the issues associated with the hydrogenation strategies based on hydrogen gas(e.g.,over-hydrogenation,process safety and high cost of precious metal catalysts),it is essential to develop efficient catalysts that can achieve the hydrogenation-cyclization of levulinic acid and its esters into GVL at high rates and selectivities under mild conditions.In this thesis,we chose to investigate a catalytic transfer hydrogenation strategy based on the well-known Meerwein-Ponndorf-Verley(MPV)reduction chemistry over several representative Zr-based heterogeneous catalysts,which contain Lewis acidic Zr centers embedded within different coordination environments and even under the confinement of molecularly sized voids.Ethyl levulinate(EL)was used as a model compound and 2-butanol served as the hydrogen donor.Through a combination of rigorous kinetic analysis,ex situ and in situ characterizations and isotopic labeling studies,we arrived at a deeper understanding of the site-specific activities of these catalysts and the fundamental origins of their different reactivities.Specifically,we synthesized four types of Zr catalysts and used a battery of techniques including temperature-programmed desorption(NH3-and CO2-TPD),X-ray photoelectron spectroscopy(XPS)and in situ infrared spectroscopy(IR)with probe molecules to characterize the acid-base properties and the electronic states of Zr in these catalysts:(1)A metal-organic framework(MOF)catalyst,MOF-808,was synthesized by solvothermal method,and the effects of the amount of formic acid regulator,activation temperature,the organic linker,and the Zr precursor on the catalytic activity were investigated;(2)A Zr-BEA zeolite catalyst was synthesized by hydrothermal method,and the effect of crystallization time on the catalytic activity was investigated;(3)An organic-inorganic hybrid(Zr-phytic acid,abbreviated as Zr-PhyA)catalyst was synthesized by solvothermal method,which was compared with MOF-808 to furnish insights into the effects of pore confinement and coordination spheres(organic ligands)on the catalytic activity;(4)A ZrO2/SiO2 catalyst was prepared by incipient wetness impregnation and when compared with Zr-BEA,allowed discussing the catalytic consequences of pore confinement in a purely inorganic coordination environment.After excluding transport limitations and the influence of non-chemical and non-catalytic artifacts,the catalytic activities were measured and compared first on a mass basis(massspecific rates).To facilitate a comparison of activity on a per active site basis,and considering that not necessarily all Zr atoms in the catalysts contribute to the MPV reduction catalysis,methylphosphonic acid was employed as a titrant that selectively binds to Lewis acid sites to gain a quantitative metric of active site density under reaction conditions.Using this in situ titration approach,we not only determined the active site concentrations in the individual catalysts,but also obtained the TOFs(rates normalized to the number of active sites)of each catalyst,which represents the first example of this kind in MPV reactions catalyzed by heterogeneous catalysts.It was found that MOF-808 contained the highest concentration of active sites(2.6±0.2 mmol g-1)among all studied catalysts,followed by Zr-PhyA(1.7±0.1 mmol g-1)and Zr-BEA(～0.11 mmol g-1),while the active sites on ZrO2/SiO2 only accounted for 10%of the total amount of Zr in the sample.Under prevalent conditions,MOF-808 exhibited the highest mass-specific activity,though Zr-BEA was undoubtedly the best catalyst in terms of TOF at relatively high[EL],reaching a TOF as high as 2 s-1 at 453 K and 0.88 M[EL].In contrast to the seemingly promising performance in the literature,Zr-PhyA turned out to be a rather poor catalyst for MPV reduction of levulinic acid esters in terms of TOF despite a relatively high density of active sites.Kinetic experiments showed that MOF-808 and Zr-BEA had comparably high TOFs that exceeded those of the other two catalysts by one to two orders of magnitude,but at higher[EL]concentrations,MOF-808 transitioned into a zero-order kinetic regime whereas TOFs of other catalysts remained nearly first-order with respect to[EL].Replacing all hydrogen atoms in the alcohol solvent with deuterium atoms led to a decrease in the measured rates,reflecting kinetic isotope effects of 1.2-2.0 that suggest hydride shift as the rate-controlling elementary step.A plausible mechanistic sequence comprised of quasi-equilibrate EL adsorption and alcohol deprotonation and the rate-controlling hydride shift between the co-adsorbed 2-alkoxide and EL was put forward,which allowed us to derive a rate equation containing kinetic and thermodynamic constants associated with the individual elementary steps.Apparent zeroorder rate constants and EL adsorption equilibrium constants determined by regression of the measured rates to the rate equation span a two-order-of-magnitude range across the studied Zr catalysts;the van’t Hoff and Eyring analyses of these regressed kinetic and thermodynamic constants enabled quantifying the enthalpies/entropies of EL adsorption and of activation.Because of the vastly different kinetic dependences on[EL]concentrations over different catalysts,no correlation could be identified between the TOF and acid-base or electronic properties.Taken together,the results inform that constructing molecular-sized confinement around the Zr active sites is a more effective measure than tuning the local coordination environments to enhance the catalytic activity for MPV reactions. |
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