Study Of Parahydrogen-induced Hyperpolarization On Metal-loaded Catalysts

Nuclear magnetic resonance(NMR)is a powerful analytical technique,which is widely used in many fields of physics,chemistry,biology,and medicine,and is of great importance.Compared with other characterization techniques,the further application and development of NMR are constrained by the low sensitivity.The hyperpolarization technique can directly change the Boltzmann distribution of atomic nuclei and obtain 2-4 orders of magnitude enhancement of the NMR signal,thus achieving the purpose of improving the sensitivity of NMR.Among them,the Parahydrogen-Induced Polarization(PHIP)technique can achieve strong polarization signal enhancement by simple and convenient experimental operation,with the advantages such as low cost,fast polarization rate and obvious enhancement,thus gaining wide attention.Heterogeneous catalysis Parahydrogen-Induced Hyperpolarization(HET-PHIP)has been widely studied by combining PHIP technique with heterogeneous catalysis hydrogenation reaction system,which has the advantages of easy separation and purification of catalyst,high activity and high stability.Meanwhile,the HET-PHIP technique is able to selectively observe the pairwise addition process of hydrogen molecules in heterogeneous catalysis systems,and can be applied as a unique mechanism research tool in hydrogenation reaction mechanism studies.However,the main drawback of this technique is the low efficiency of polarization generation.How to improve the polarization efficiency of the HET-PHIP technique is an urgent issue to be solved.More and more studies indicate that the catalyst structure has a strong influence on the polarization efficiency of HET-PHIP technique,but the exact mechanism of the influence still remains unknown,which requires a deeper understanding and recognizing of the relationship between catalyst structure and polarization generation.In this thesis,I have investigated the hydrogenation reaction catalyzed by bimetallic nanoparticles using HET-PHIP technique.The systematic regulation of the structure and properties of bimetallic catalysts was achieved by changing the ratio of the two metals and the metal loading methods.The reaction of unsaturated hydrocarbon hydrogenation catalyzed by bimetallic catalysts was thoroughly investigated by using the PASADENA and ALTADENA methods of PHIP,and the reaction mechanism and polarization mechanism were discussed.The main aspects are as follows:(1)The mechanism of selective hydrogenation of 1,3-butadiene over Pd-Sn bimetallic catalysts was investigated using the HET-PHIP technique.A series of bimetallic catalysts with different Pd/Sn ratios,Pd1-Snx/Al2O3(x=0.5,1.0,1.5,2.0),were synthesized using a co-impregnation method.The differentiation of the product 2-butene generation pathway was achieved by the HET-PHIP technique,which was used to qualitatively analyze the hydrogenation pathway and isomerization pathway of the reaction.It was found that the decrease in the Pd/Sn ratio of the catalyst led to a decrease in the reaction activity of 1,3-butadiene and an increase in the selectivity of the product butene,and also inhibited the isomerization process between the hydrogenation products 1-butene and 2-butene.This is due to the change in the Pd/Sn ratio in the bimetallic catalyst which changes the morphology effect and electronic effect of the Pd active centers on the surface:with the increase in the Sn component content,on the one hand,the exposed Pd component on the surface decreases and the activity decreases;on the other hand,the electron density on Pd is increased and the produced butene becomes more easily desorbed,reducing the over-hydrogenation and isomerization.(2)HET-PHIP studies on Pd-Co metal-loaded Silicalite-1(S-1)zeolites.The impregnated monometallic catalyst Pd/S-1-im was synthesized by isovolumic impregnation method,the encapsulated monometallic catalysts Pd@S-1 and bimetallic catalysts Pd1-Cox@S-1(x=3,6,9)were synthesized in situ by the one-pot hydrothermal method.The reactivity and PHIP polarization efficiency of the different catalysts in hydrogenation reaction of unsaturated low carbon hydrocarbons were compared and analyzed,and it was found that the catalytic reactivity and polarization efficiency of Pd@S-1 were weaker than those of Pd/S-1-im,while the catalytic reactivity and polarization efficiency of the bimetallic catalyst Pd1-Cox@S-1 were better than those of Pd@S-1.It was also found that part of Pd particles distributed inside the zeolites is wrapped by the zeolite structure,resulting in a smaller total area of Pd active centers that can be exposed to the reaction gas and weaker reaction activity;the lower polarization efficiency was due to the accelerated relaxation of the polarized molecules inside the zeolite micropores,which results in a larger loss of polarization signal and leads to a lower observed signal enhancement amplification.The presence of the second metal Co in the bimetallic catalyst helps to improve the dispersion of Pd in the zeolite,which increases the accessibility of Pd metal particles to the reacting molecules and improves the reaction activity,and also helps to limit the splitting and overflow of the parahydrogen molecules on the active centers,thus improving the pairwise addition selectivity and the PHIP polarization efficiency.