## 1 Introduction
We are familiar with porous materials such as metal-organic frameworks (MOFs), porous organic polymers (POPs), activated carbon, silica, clay, and so on [21]. They have been used in a wide range of fields, nowadays, such as porous silicon for the manufacture of photoelectric components, porous membranes, and porous adsorbents for separation, thermal, and sound insulation materials, biomedical materials, and so on [22]. Porous materials can not only improve the utilization of the volume of material based on the stability and screening characteristics of the pores but also provide more space for the contact between active sites and reaction substrates, which greatly promotes the performance of catalysts [23]. When metal components are the important active sites, a supported porous catalyst can ensure high efficiency, but there are also side reactions, low selectivity, and metal leaching  
Figure 1: Partial transformation route of HMF [7]. into the product [24]. Hence, as one of the hot topics in catalysis research, nonmetallic porous carbon materials have the advantages of low density, no metal leakage pollution, good structural ductility, and strong acid and alkali resistance [25]. They are often used as a kind of catalytic material with great potential for energy storage and conversion [26] and can realize efficient conversion through the functionalization and loading of active components. They have excellent application in catalytic biomass valorization and environmental remediation [27]. However, since porous carbon materials are usually prepared by high-temperature pyrolysis, it is relatively difficult to predict and control their structural features [28]. In addition, their energy consumption and equipment cost are relatively high. Therefore, the development of high-performance catalysts with well-defined synthetic structures, low cost, and environmentally friendly features is of paramount importance, especially for the catalytic field of the preparation and conversion of HMF. In the pursuit of the desired characteristics mentioned above, researchers are starting to pay more attention to the gradual update of catalyst structures from an inorganic skeleton to an organic skeleton [29]. At present, POPs are widely used in the field of catalysis due to their excellent physical and chemical properties, good thermal stability, large specific surface area, controllable pore size, and designable chemical structure [30]. Specifically, in the fields of photocatalysis [31; 32] and photoelectrocatalysis [33], they have shown good performance. Compared to other inorganic porous materials, the advantages of POPs is that, first of all, they are mainly composed of light elements--C, H, O, N, and B--and have low skeletal density [34]. Secondly, they have rich organic synthesis strategies and various types of monomers, and they are connected by high-energy covalent bonds, with inherent porosity and high physical and chemical stability. Therefore, they can effectively avoid the above-mentioned catalyst recovery and product separation problems, and in the design of the catalyst structure, compared to the popular carbon materials, more accurate control can be achieved [35]. They can even achieve high-efficiency metal-free catalytic conversion by virtue of their special structure. Depending on the framework features and synthesis method, POPs mainly include covalent organic frameworks (COFs) [36], porous aromatic frameworks (PAFs) [37], hyper-cross-linked polymers (HCPs) [38], conjugated microporous polymers (CMPs) [39], and other types. They not only have many applications in catalysis but also certain advantages and effects on gas storage, molecular separation, drug delivery, and sensing [40; 41]. Likewise, the heterogeneous catalytic system for the preparation of HMF from biomass resources, and the subsequent conversion of HMF discussed in this paper have also shown good performance and have provided important knowledge for subsequent research.