Herein, we would like to report this idealist hydrogenation of 4-nitrostyrene system using thin graphene layer encapsulated Ni@graphene and Pd@graphene catalyst in both batch reactor and industrially applicable flow reactor. Graphene, which has been recognized as one of the hardest, thinnest, thermal and electrical conductivity materials, especially for its recent discovered superconductivity and correlated insulating phases in the magic-angle twisted bilayer, three-layer graphene, was successfully prepared via situ facile synthesis method with a thin layer as the "bulletproof vests" for active metal species in this work. The prepared Ni@graphene and Pd@graphene catalysts having the graphene "bulletproof vests" which enhance the anti-interference ability and stability of the active metal components, showed excellent activity (99%) and selectivity (99%) in the chemo selective hydrogenation of the 4-nitrostyrene system. Moreover, by optimizing the reaction conditions, three types of desired products 4-VA , 4-EA, and 4-ENB could be obtained with over 99% yield respectively, which was not mentioned during the previous report. In addition, the highly chemo-selective hydrogenation of 4-nitrostyrene can be carried out in the industrially applicable flow reactor, and catalysts can be easily separated from the product because of its magnetic properties or flow reactor systems and showed excellent stability during the reaction. The Ni@graphene and Pd@graphene catalysts were synthesized with the modified facile synthesis method (Scheme S1). All these prepared catalysts were numbers as M1/M2@(N)C-a-b-c-d, where M1= Ni, Pd; M2=NiO, PdO; a is pyrolysis temperature; b is oxidation temperature; c is oxidation time, and d is solvent. Figure showed some representative scanning electron microscopy (SEM) images and high-angle annular dark-field-STEM (HAADF-STEM) images of oxidized and non-oxidized samples of graphene encapsulated Ni@C catalysts, and nitrogen-doped graphene encapsulated Pd@NC catalyst. The other catalysts' corresponding images are presented in Supplementary Information. As illustrated in Figure and 1b, the oxidized catalyst Ni@C-700-200-1h-H2O has the obvious pores structure on the catalyst surface. Also, the solvent and pyrolysis temperature can affect the catalyst's surface significantly . The Ni@C catalysts prepared with H2O have bigger catalyst particles compared with the catalysts prepared with EtOH (Figure ). According to the SEM-EDS element analysis, the oxidized Ni@C catalysts have a higher metal loading of Ni, lower loading of C, and O, compared with the un-oxidized catalysts (Table ). According to the HR-TEM analysis, the prepared Ni@C and N-doping Pd@NC-2 catalysts consisted of metal nanoparticles that were encapsulated by less than 5 graphene layers (Figure , Figure ), and >90% of metal species were encapsulated by a few graphene layers . As shown in the corresponding energy-dispersive X-ray (EDX) images, the C, N, and O atoms were dispersed homogeneously over all the Pd NPs (Figure ), and this confirmed that the Pd@NC-2 catalyst has the alloy structure. The C, O, and Ni atoms were also distributed homogeneously over the prepared Ni@C catalysts (Figure ). The oxidized catalyst Ni@C-700-200-1h-H2O had a smaller pore size of 5.6 nm and a similar surface area of 160 m 2 g -1 compared with the un-oxidized catalyst Ni@C-700-H2O (6.5 nm, 162 m 2 g -1 ) (Table ). When the pyrolysis temperature increased from 600 °C to 700 °C, the pore size and surface area of the Ni@C-600-H2O and Ni@C-700-H2O catalysts both increased from 3.8 nm to 6.5 nm, 132 m 2 g -1 to 162 m 2 g -1 respectively (Table ). Based on the X-ray photoelectron spectroscopy (XPS) (Figure -f) analysis, both metallic Ni 0 and Ni 2+ species were presented on the catalyst's surface (Figure ). Also, for the N-doping catalyst Pd@NC-2, the metallic Pd 0 and Pd 2+ species were observed. All of these observations indicated that the active metal species were easily oxidized during the preparation. The well-defined graphene shell encapsulated structure characteristics of the catalysts were confirmed by X-ray diffraction (XRD) measurement, as shown in Figure . All the prepared catalysts showed graphitic carbon shell C (002) between 20 and 30. Apart from the graphene shell C, Ni alloy and NiO species were also present in the Ni@C catalysts (Figure ). This indicated that the active metal Ni species of the Ni@C catalysts prepared from the non-oxidation step were also oxidized by air after the reduction process during storage. However, the observed increasing intensity peaks of NiO in the catalysts confirmed that more active metal species were oxidized during the oxidation process, which is consistent with the TEM statistical analysis (Table ) and XPS analysis. As mentioned previously, the oxidation step significantly affects the catalyst physical properties, here, it is clear see that the catalysts E, F, J, H prepared with the oxidation step had higher conversion than the corresponding catalysts A, B, C, D without the oxidation step. The solvent used for preparing the catalysts also affects the catalytic activity and product selectivity activity. The catalysts A, B, E, F prepared from H2O generally had higher reactivity and 4-VA selectivity than the corresponding catalysts C, D, J, H which were prepared from EtOH. When the pyrolysis temperature increased from 600 ºC to 700 ºC for the catalysts in pairs of and and and and, it is obvious to see the trend that the catalytic activity decreased respectively, no matter for the un-oxidized catalysts and oxidized catalysts. But for the 4-VA selectivity, all the un-oxidized catalysts in pairs of A and B, C and D, did not give any changes with all 99% of selectivity. For the oxidized catalysts in pairs of E and F, J and H, the product 4-VA selectivity increased when the pyrolysis temperature increased from 600 ºC to 700 ºC . Among all the prepared thin graphene layer encapsulated Ni-based catalysts, the catalyst F Ni/NiO@-700-200-1-H2O showed the highest reactivity of 99%, highest and excellent selectivity of 4-VA without observing 4-EA and 4-ENB. The other commercially available noble metal catalysts Ru/C, Pd/C, and Pt/C, which were usually applied in hydrogenation reaction due to their excellent hydrogenation ability, could also give excellent activity and selectivity of the other two products 4-EA and 4-ENB. Interestingly, the Ru/C gave the 4-ENB with 94% selectivity, while the Pd/C and Pt/C catalysts gave the fully over-hydrogenated product 4-EA with excellent selectivity. Here, it is a great need to mention some general conclusions. First, the prepared Ni catalyst has unique hydrogenation properties that are different from other noble metal catalysts Ru/C, Pd/C, Pt/C. Generally speaking, the Ni-based catalyst has been reported previously with similar hydrogenation properties like Pd, Pt, and Ru , however, in this chemo-selective hydrogenation 4-nitrostyrene that containing competing hydrogenation sensitive functional groups -NO2 and terminal olefin C=C double bond, they showed different hydrogenation properties. In the non-polar, aprotic solvent toluene, the Ni@C catalyst preferred to hydrogenate the -NO2 functional group first without touching the usually more active terminal olefin C=C double bond, however, the Ru/C catalyst, instead, prefer first to hydrogenate the C=C functional group. The highly active Pd/C and Pt/C are active and no-selective to both -NO2 and C=C functional groups and can hydrogenate C=C and NO2 functional groups at the same time, giving 4-EA the only product. Second, the solvent, pyrolysis temperature, and the oxidation step are crucial factors when preparing the graphene encapsulated Ni@C catalyst, which resulting in the dramatic difference in the chemo-selective hydrogenation of 4-nitrostyrene. Ni/NiO@-700-200-1-H2O, 0.5 mmol 4-nitrostyrene, 6 mL cyclohexane, 15 bar H2, different reaction time, 90 °C. Conversion and selectivity were determined by GC using 1, 3, 5-trimethoxybenzenen as an internal standard. The types of products were confirmed by GCMS and 1 H NMR Based on the above initial catalysts' screening investigation, we further test reaction factors such as solvent, reaction temperature, hydrogen pressure, and reaction time using the optimized catalyst F. As shown in Figure , the solvent had a great effect on the hydrogenation of 4-nitrostyrene under the optimized reaction conditions (20 bar H2, 1 h, 120 °C). By changing from the solvent from no-polar, aprotic solvent toluene to the polar and protic solvent EtOH, the catalytic reactivity and product selectivity changed dramatically. In the no-polar solvent toluene, petroleum ether, and cyclohexane, the selectivity of product 4-VA is higher than product 4-EA. While in the polar, protic solvent acetonitrile and EtOH, higher selectivity of 4-EA than 4-VA could be obtained. And using the graphene encapsulated no-noble Ni-based catalyst, we can both get the challenging product 4-VA with excellent selectivity of 99% and fully hydrogenated product 4-EA with super selectivity (>99%) through changing the solvent simply, which could be a promising application in industry. Here, for the Ni@C catalyst, in the polar, protic solvent EtOH, the hydrogenation rate of the C=C double bond was increased dramatically compared with the non-polar, non-protic solvent cyclohexane, and the NO2 and C=C double bond functional groups can be both hydrogenated completely. We also checked the reaction temperature, which is an important factor and costs a lot of energy during the industrial process. As illustrated in Figure , the hydrogenation conversion of 4-nitrostyrene increased significantly from 83% to 99% when the reaction temperature increased from 80 °C to 90 °C, and then kept the full conversion at the reaction temperature of 100 °C, 110 °C, and 140 °C. The selectivity of 4-VA decreased slowly from 99% to 78% at 140 °C, and 4-EA selectivity increased from 1% to 22% correspondingly, that is because some of 4-VA were further hydrogenated to 4-EA. This also indicated that, in the non-polar, aprotic solvent media cyclohexane, the hydrogenation rate of NO2 is fast than the double bond C=C. The H2 pressure, as another crucial factor that generally affects hydrogenation reaction significantly, was also evaluated in the chemo-selective hydrogenation of 4-nitrostyrene under 1 h, 90 °C conditions with varying different H2 pressures. As shown in Figure , the conversion of 4-nitrostyrene increased in accordance with the H2 pressure increasing from 5 bar to 10 bar, 15 bar, 20 bar, and 25 bar gradually. And the selectivity of 4-VA gradually decreased from 100% to 85%, since some of the 4-VA product were over-hydrogenated to 4-EA, which can be seen from the increasing yield of 4-EA. Here, it is interesting to see the phenomenon that the simultaneous hydrogenation of C=C double bond and NO2 could also occur under the highest H2 pressure. Since not only the generally first step hydrogenated product 4-VA was observed in high yield, but also the over-hydrogenated product 4-EA could be obtained in high yield, and the other un-normally hydrogenated product 4-ENB could also be determined during the reaction. This indicated that, during the highest H2 pressure, the chemo-selective properties of the Ni@C began to non-distinguish the C=C double bond and the NO2 functional group. Under this reaction condition, it can exceed the transition state energy barrier of the reaction pathways that produce three different products, therefore, three reaction routes can be followed to produce all three reaction products 4-VA , 4-EA, and During the preparation of Ni@C catalysts, we also prepared the nitrogen-doped graphene encapsulated Pd@C with a similar modified synthesis method. Changing the Pd catalyst properties with nitrogen doping could affect its hydrogenation ability significantly . As shown in the previously catalyst screening test Figure , the commercially available Pd/C gave the fully hydrogenated product 4-EA. Herein, surprisingly, using the nitrogen-doped Pd@NC-2, excellent selectivity of 4-ENB was observed, which was not reported previously using the strong hydrogenation ability catalyst Pd/C and Pt/C (Figure ). As shown in Figure , an excellent yield of up to 94% 4-ENB could be obtained only when using the Ru/C catalyst. Here, as illustrated in Figure , the Pd@NC-2 catalyst was tested in the chemo-selective hydrogenation of 4-nitrostyrene in cyclohexane and MeOH. When the reaction was carried out in cyclohexane, excellent reactivity of 98% conversion and super selectivity of 4-ENB up to 100% were obtained under the very mild reaction conditions (r.t., 5 bar H2, 1 hour). And during the other modified reaction conditions, the yield of the other two products 4-VA and 4-EA was lower than 2%. It could conclude that in this reaction condition, unlike the above-mentioned graphene encapsulated Ni@C catalyst and other commercially available catalysts Pd/C and Pd/C, the Pd@NC-2 catalyzed hydrogenation rate of double bond C=C is faster than the hydrogenation rate of NO2 functional group. In the protic solvent MeOH, when the H2 pressure decreased from 5 bar to 1 bar, the product selectivity of 4-ENB increased accordingly, which is a similar trend observed in cyclohexane. However, a lower conversion of 14% and only 4-EA could be obtained when the H2 pressure increased to 5 bar in solvent MeOH. The catalyst's reusability and stability are important factors when concerning its application in industry. Both of the prepared graphene thin layers encapsulated Ni@C and Pd@NC catalysts showed excellent stability and reusability. Furthermore, it is also interesting to mention that the catalysts could be used in the advanced flow reactor and produce the three products 4-VA , 4-EA, and 4-ENB with super selectivity by simply varying the reaction conditions (Figure ). This opens a promising application in industrial using heterogeneous, sustainable, low-cost catalysts.