Synthesis Of Palladium Nanoparticles/UiO-66 Composites And Investigation For Catalyzing Transfer Hydrogenation Properties

With the demand for industrial development and scientific and technological progress,more and more important reactions have been industrially applied,among which catalytic hydrogenation reaction,as an integral part of various industrial catalytic reactions,has been a hot direction of multiphase catalytic research.Conventional catalytic hydrogenation mainly uses Pt,Ni and other metals as catalysts for hydrogenation using high-pressure gaseous H₂.However,this process is plagued by high energy consumption,non-selective hydrogenation,low yield,complicated subsequent separation process,and high equipment and maintenance investment.Therefore,the development of green,efficient and mild highly selective catalytic hydrogenation technology to replace the conventional high-pressure gaseous H₂ hydrogenation has become a hot research topic.Catalytic transfer hydrogenation（CTH）with loaded Pd nanoparticles（NPs）as the catalyst is an emerging class of catalytic reduction hydrogenation systems that has been developed recently and has unique advantages in terms of reaction conditions,product selectivity,and subsequent separation process.Unfortunately,due to their high surface energy,Pd NPs are prone to agglomeration during preparation and reaction processes,resulting in the absence of active sites,which,in turn,can affect their catalytic activity.Therefore,the development of highly stable,highly dispersed,ultra-fine particle size Pd NPs-based high-efficiency catalysts for efficient CTH is an urgent challenge in this field.Metal-Organic Frameworks（MOFs）,also known as Porous Coordination Polymer（PCP）,are emerging crystalline porous framework materials formed by the self-assembly of organic bridging ligands and metal ions/clusters through coordination interactions.Ui O-66,an important representative Zr carboxylic acids MOF,has shown great potential for multiphase catalysis due to its easily modified organic ligands and outstanding stability in aqueous or organic phases.These excellent properties ensure Ui O-66 an ideal carrier to load metal nanoparticles（MNPs）,and MNPs/Ui O-66 has been envisioned as one of the most promising MOFs composites for industrial catalytic applications.In this thesis,we designed a new process for preparing highly stable,dispersed,and ultra-fine Pd nanoparticles by leveraging the structural and property characteristics of Ui O-66.Our goal is to create efficient Pd nanoparticle/Ui O-66 composites that can be used for stable applications in CTH reactions.And to reveal the correlation between the structure and catalytic performance of such composites.The specific research contents and main results of this thesis are as follows:1.Pd/Ui O-66 composites were prepared by a simple one-step impregnation-reduction method using CO（generated by the thermal decomposition of DMF under acidic conditions）as the reducing agent and Pd Cl₂ as the precursor.The process effectively preserved the pore structure and specific surface area of Ui O-66,and the highly porous structure of Ui O-66 effectively limited the growth of Pd NPs to obtain highly stable,dispersed,and ultrafine particle size Pd NPs（1.77±0.35 nm）.The dehalogenation hydrogenation reaction of halogenated aromatic compounds was achieved by the CTH hydrogenation process under mild condition.The Pd/Ui O-66 CTH dehalogenation of4-chlorophenol at 35 ^oC with water as the solvent and ammonium formate as the hydrogen donor achieved 99.5%conversion of 4-chlorophenol with 99.9%selectivity of phenol after5 h reaction.These results were better than other types of palladium-based catalysts under the same reaction conditions,and the condition of this reaction system was milder compared with the reported dehalogenation processes.Moreover,Pd/Ui O-66 showed better applicability and cycling stability,and no significant agglomeration and desorption of Pd were observed up to after five cycles.2.The functionalized Ui O-66-SO₃H was successfully prepared owing to easily modified organic ligands H₂BDC.Ui O-66 itself has strong Lewis acidic sites,and the introduction of the-SO₃H group adds Br?nsted acidic sites to Ui O-66.Bifunctional Pd/Ui O-66-SO₃H-40 with Lewis acidity,Br?nsted acidity,and hydrogenation activity were prepared by the previously developed simple one-step impregnation-reduction method.It was found that Pd/Ui O-66-SO₃H-40 achieved the reaction of methyl levulinate for the preparation ofγ-enterolactone under relatively mild reaction condition by CTH process,the conversion of methyl levulinate reached 95.3%and the yield of enterolactone reached91.5%.Moreover,the Pd/Ui O-66-SO₃H-40 exhibited good applicability and stability,and no significant deactivation was observed after 5 cycles.An investigation into the catalytic mechanism revealed that Ui O-66 exhibits Lewis acidity,which can activate the conversion of methyl levulinate to methyl 4-hydroxy pentanoate.However,Ui O-66 alone shows relatively low conversion rates.In contrast,the addition of Pd nanoparticles significantly enhances its catalytic activity.Moreover,the introduction of-SO₃H groups provides Br?nsted acidic sites,enabling efficient conversion of methyl 4-hydroxy pentanoate to high pentane-selective ester.The synergistic effect of Lewis acid,Br?nsted acid,and Pd NPs are the reason for the excellent reactivity and selectivity of the catalyst.3.A"pre-coordination"process was designed to regulate the growth and distribution of Ui O-66 on the GO surface by taking advantage of the excellent adsorption properties of GO on metal ions.The Ui O-66-GO-40 composite was prepared,and the Ui O-66 was densely and uniformly grown on the GO surface,and the Ui O-66 particle size（154±15nm）was effectively controlled.The Pd/Ui O-66-r GO-40 catalyst was obtained by a one-step impregnation-reduction method as mentioned before.It was found that Pd/Ui O-66-r GO-40 catalyzed the hydrogen transfer reaction ofα-methyl styrene（AMS）to isopropylbenzene with a TOF value of 692.0 h^-1 at room temperature using ammonium formate as the hydrogen donor and water as the reaction solvent,the catalytic efficiency of Pd/Ui O-66-r GO-40 was superior when compared with other similar catalysts.Pd/Ui O-66-r GO-40 showed excellent applicability and cycling stability,and no significant loss of activity was observed after 5 cycles.4.Using Ui O-66-GO-40 as a sacrificial template,the Zr O₂/C/r GO porous carbon material was obtained by high-temperature pyrolysis of Ui O-66-GO-40,and the Pd Au/Zr O₂/C/r GO obtained after one-step impregnation-reduction method loaded with Pd NPs and Au NPs could CTH of nitroaromatics to obtain the corresponding arylamines.Results showed that Pd Au/Zr O₂/C/r GO could catalyze CTH of various nitroaromatics to the corresponding arylamines at room temperature,with ammonium formate as the hydrogen donor and water as solvent,and the conversion of nitroaromatics exceeded 90%and the selectivity of the target products was as high as 99%.The experimental results show that the Zr O₂ formed by the pyrolysis of Ui O-66-GO-40 composites not only generated Lewis acidic sites,but also played a good domain-limiting effect on Pd NPs and Au NPs,and Au NPs effectively improved the electronic structure of Pd NPs,contributing to the high catalytic activity of Pd Au/Zr O₂/C/r GO.