## 1 Introduction  
Aniline, as a vital raw material and key intermediate for many chemical products [1], is an important organic compound with a wide range of applications in the chemical industry, pharmaceuticals, and dyestuffs [2]. Furthermore, aniline serves as a raw material for the production of polyurethane raw material diphenylmethane diisocyanate (MDI), having a great market potential [3, 4]. At present, the production methods of aniline mainly consist of nitrobenzene iron powder reduction [5], phenol amination [6], and nitrobenzene catalytic hydrogenation [7]. Among them, nitrobenzene catalytic hydrogenation has advantages of low pollution and high product quality, accounting for 85% of the total production capacity of aniline [8]. The chemical reaction of nitrobenzene catalytic hydrogenation reduction to aniline is [9]:  
\[\text{C}_{6}\text{H}_{5}\text{NO}_{2}+3\text{H}_{2}\rightarrow\text{C}_{6} \text{H}_{5}\text{NH}_{2}+2\text{H}_{2}\text{O} \tag{1}\]  
The procedure of nitrobenzene catalytic hydrogenation mainly consists of gas-phase catalytic hydrogenation [10] and liquid-phase catalytic hydrogenation [11]. Gas-phase catalytic hydrogenation is widely used due to its high conversion, whereas it is greatly hindered by the stringent operating conditions (high temperature and pressure) and unavoidable by-products (nitro and azo compounds) [12]. On the other hand, liquid-phasecatalytic hydrogenation exerts better catalytic effects, including fewer by-products, high selectivity, low hazard factor, easy separation of products, and low energy consumption [13; 14], and has gradually become the mainstream technology for aniline production. In liquid-phase catalytic hydrogenation, the mixing and mass transfer between the gas and liquid phases is one of the most important factors affecting the hydrogenation efficiency and product conversion [15; 16; 17]. At present, most of the catalysts used for nitrobenzene liquid-phase catalytic hydrogenation reaction are granular catalysts, in which the reactor bed pressure drop is large and the mass transfer efficiency between the gas and liquid phases is low, leading to limitations in the improvement of aniline production. In recent years, the application of structured catalysts in multiphase catalysis has attracted the attention of numerous scientists [18]. Structured catalysts can significantly improve the mass and heat transfer performance of the catalyst bed, optimize the structure of the flow field, reduce the pressure drop, and lower the reaction hazard factor [19]. To be more specific, 3D-printed catalytic static mixers (csm) [20] had great achievements in various hydrogenation reactions [21; 22]. In addition, researchers [23; 24; 25] also prepared aluminum oxide powder coating catalysts to reduce pressure drop and strengthen stability. However, most of the 3D-printed catalytic static mixers are limited in size and are not suitable for mass production applications [26]. Meanwhile, aluminum oxide powder coating can easily fall off of the aluminum oxide powder coating catalyst substrate, especially in reactions including liquid phases [27]. Therefore, such dilemmas require us to develop a catalyst with stronger bonding to the substrate and greater production capacity. In this work, structured mesh-type \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al supports were prepared by anodic oxidation technology and then structures of various pore sizes were adjusted by acid pore-widening treatments. After loading, the Pd/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al catalysts were applied to nitrobenzene liquid-phase catalytic hydrogenation reaction in a fixed-bed reactor. Firstly, the various pore physical properties of structured mesh-type \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)/Al supports were investigated by SEM and BET, and the differences in pore structure of a series of acid pore-widening supports were further explored.