| Glucose,as a biomass resource with large reserves and diverse ways to acquire,has been attracting attention for its transformation and utilization.There are many ways to transform glucose,which include glucose isomerization,conversion to 5-hydroxymethylfurfural(5-HMF),and oxidation to gluconic acid.Among them,the conversion of glucose to gluconic acid is one of the most direct and efficient ways.Gluconic acid is a widely used chemical,and there is a huge demand in chemical,pharmaceutical,food industries.At present,production of gluconic acid on the market is mainly carried out by the biological fermentation method,which has the disadvantages of long production cycle,low selectivity and relatively harsh reaction conditions.Therefore,it is necessary to develop chemical catalysis methods,which is more technical and economical.To realize the chemical production of gluconic acid,the main scientific problem lies in the development of catalyst materials with high efficiency and excellent chemical properties.At the beginning of the research,catalysts with traditional precious metal components such as Pt and Pd were developed to catalyze the oxidation reaction of glucose,but the product’s poor selectivity and serious loss of active components could not be effectively solved until the "emerging" nano-Au have been found to have excellent catalytic activity,and they have been extensively used as catalysts for the oxidation of glucose to gluconic acid.Compared with metals such as Pt and Pd,nano-Au has higher low-temperature oxidation properties,higher product selectivity,and is less prone to deactivation.In recent years,most of the related research has been devoted to the improvement of the properties of Au catalyst materials,but little information has been provided on the catalytic effect of Au catalysts and the mechanism of the reaction.However,it has been well accepted that Au’s catalytic mechanism is of great advantage in developing the excellent materials,and realizing the industrialization of this reaction ahead of schedule.In this dissertation,the conversion process of glucose on Au-molecular sieve catalysts was studied by solid-state NMR and other spectroscopic techniques.In particular,the catalytic behavior of Au-molecular sieve catalysts and the mechanism of glucose oxidation were revealed.The main results are as follows:(1)Au-Al/SBA-15 molecular sieve catalysts were prepared by impregnation reduction method.A1 species and Au species on the catalyst were characterized.It was found that the catalytic activity and selectivity were affected by both the Au content and the Al content of the modified support.Through UV-Vis and TEM characterization analysis,it was found that using the Al-modified carrier can prepare nano-Au particles with smaller particle size,thereby improving the reactivity.When the A1 content is too high,the selectivity of gluconic acid decreases sharply.As revealed by solid-state NMR,more non-framework A1 species are produced on the carrier to catalyze the side reaction of glucose isomerization,resulting in a decrease in selectivity.Finally,it was found that either of the single mental catalysts have the activity,illustrating that the Au-Al/SBA-15 can catalyze the glucose oxidation because of the cooperation between the Au and Al.(2)Through in-situ NMR technology,the prepared Au-Al/SBA-15 molecular sieve catalyst was used to perform in-situ glucose oxidation in a special rotor,and the reaction mechanism was explored.It was found that the oxidation of glucose to gluconic acid proceeds through an intermediate with a structure similar to gluconolactone.The lye added to the reaction helps to strip and desorb the strongly adsorbed product gluconate from the surface of the catalyst,re-exposing the active site and keeping the reaction ongoing. |
