As shown in Figure , the c-nanotube structure of the catalyst is clearly visible, with a small amount of graphene spherical shell encapsulated catalyst particles, while the carbon nanotube structure is almost completely lost in the cycled catalyst (Fig. ), replaced by graphene spherical shell encapsulated metal The catalysts were replaced by metal nanoparticles encapsulated by graphene spherical shells, but still uniformly distributed. It is assumed that the change in catalyst activity is due to the change in catalyst structure. Further high resolution (HR) TEM analysis showed that the nanospheres consisted of metallic nanoparticles (NPs) that were completely coated with a graphene shell (Fig. ). The d-spacing of the Ni/Co@NDC metal nanoparticles is 0.20 nm, corresponding to the (111) plane of the nickel alloy (Fig. ). Statistical analysis by high resolution transmission electron microscopy (HR-TEM) showed that the graphene shells on the Ni/Co nanoparticles are very thin, with >90% of the C-shell consisting of a few layers. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images with subÅ resolution further confirm that homogeneous metal nanoparticles have been formed (Fig. ), and the corresponding energy dispersive X-ray (EDX) maps indicate that N and O atoms are uniformly distributed over all NPs (Fig. ). X-ray diffraction (XRD) was used to investigate the electronic and structural properties of Ni/Co@NDC. The XRD spectrum (Figure ) shows a graphitic carbon shell C(002) between 20 and 30, which confirms the formation of a thin graphene shell. However, the characteristic peaks of Co metal were not found in the XRD spectrum, presumably creating an alloy structure with Co. The XRD peak position of the catalyst did not change after cycling, but the peak signal was slightly weakened, which may be related to the slight metal loss from the catalyst after cycling. These results show that the homogeneous Ni/Co nano-alloys are fully encapsulated in the thin graphene shells. It can be seen that the metal is evenly distributed in the catalyst particles and that each catalyst particle is uniformly encapsulated by the C shell. We first tested the effect of the catalyst on the hydrogenation of vanillin in a methanol solvent (Table , Entry 1) in batch reactor and found that although the selectivity for VA was high, the reaction produced DMP by-products, presumably due to the involvement of methanol. We then reran the reaction in a methanol solvent at a reduced reaction temperature (Table , Entry 2) and chose two other solvents (Table , Entry 3, 4) that did not contain active hydroxyl groups to test if the by-product formation could be reduced and the reaction would stay in the conversion to VA, but apparently the results were not as expected at low temperatures. We proceeded to restore the reaction temperature and chose ethanol, a solvent less polar than methanol but more soluble in hydrogen , and achieved a VA selectivity of 95.11 without any by-products (Table , Entry 5). We then transferred the reaction system to a continuous stream and used the excellent mass and heat transfer properties of the continuous stream for subsequent screening of the conditions. As shown in Figure , the conversion reached over 99.99% with increasing temperature. Interestingly, at a reaction temperature of 100 °C, the selectivity of VA reached 96.06%, while a further increase of 20 °C resulted in the conversion of all VL to MMP. In the pressure screening process (Fig. b), we found that at a hydrogen pressure of 3 bar, VA could still achieve more than 99.99% MMP selectivity, which is a very mild reaction condition in our known previous studies. However, we found that the reaction required a certain reaction time. As shown in Figure , during the flow rate screening process, not only did the conversion decrease significantly with increasing flow rate, but also the selectivity of both VA and MMP decreased. The decrease in MMP selectivity with reaction time and the peak-shaped change in VA selectivity are due to the fact that during the hydrogenation of VL, VA is first hydrogenated to produce VA and then VA continues to be hydrogenated to produce MMP, so the shorter reaction time is more favourable to the production of VA. After a long screening run, the optimized conditions were used for a 15 h continuous experiment without catalyst replacement (Fig. ). The experimental results show that the catalyst has a certain degree of deactivation. The ICP results show that the Co content of the original NOM@NDC catalyst is 44.38%, the Ni content is 20.36%, and the ratio between metals is 2.17. The Co content of the recycled catalyst is 35.48 %, the Ni content was 16.27%, and the metal ratio was 2.18. It indicated that the catalyst after cycling had a slight loss of metal in the same proportion. Since the catalyst metal was not seriously lost, we suspected that the deactivation phenomenon was related to the change of the catalyst structure. First, the substrate is adsorbed onto the catalyst active material and the C-O bond is activated and broken with the help of Ni/Co-H substances. Then it is hydrogenated to hydroxyl groups to produce vanillin (VA). VA can be hydrogenated in one step to produce MMP, or it can be reacted with ethanol to produce a DMP intermediate, which removes the CH3OH added in the previous step and is hydrogenated again to produce MMP. if the reaction conditions are too mild or the reaction time is not sufficient, the reaction will stay in the DMP step and produce by-products.