How does the concentration of HMF influence its reactivity in catalytic or polymerization reactions?

In catalysis or polymerization reactions, HMF concentration directly affects the number of effective reaction molecules per unit volume. At higher concentrations, the collision frequency between molecules increases, which speeds up the reaction rate. In multi-step reaction pathways, this concentration effect may also promote the progress of some rate-limiting steps, thereby improving the overall conversion efficiency. However, above the critical concentration, the system may enter the reaction diffusion control region, which in turn inhibits the reaction activity.

HMF is a highly reactive multifunctional compound that is prone to crosslinking and condensation reactions under catalytic conditions. The higher the concentration, the greater the possibility of side reactions, such as the self-condensation reaction between carbonyl and hydroxyl groups, which will generate macromolecular byproducts and deposit on the catalyst surface, causing problems such as pore blockage and metal center passivation, which in turn leads to reduced catalyst activity, accelerated selectivity transfer or deactivation rate.

In the preparation of HMF-based functional polymers (such as bio-based phenolic resins and polyesters), concentration control is crucial. High concentration of HMF is conducive to increasing the probability of cross-linking reaction, thereby obtaining higher mechanical strength and thermal stability, but it will also increase the gel risk of the system, reduce processability and fluidity, and bring challenges to the control of polymerization rate and terminal groups.

The increase of HMF concentration will increase the total heat load of the system. If the temperature is not properly controlled, it is easy to induce the formation of by-products such as furfural derivatives and polymerized tar in strong exothermic reactions such as catalytic oxidation or dehydration. These by-products will reduce product purity, increase separation difficulty, and cause corrosion or blockage risks to equipment.

High-concentration HMF solution often has a high viscosity, which will significantly reduce the diffusion rate of reactants in the liquid phase, reduce the macroscopic mixing and microscopic mass transfer efficiency in the reactor, cause local uneven reaction, and even cause side reactions to occur in certain hot spots. This places higher requirements on the design of continuous reactors and microchannel equipment, which usually need to be optimized through diluent or fluid dynamic design.

The increase in HMF concentration will induce more frequent condensation, etherification, esterification and other side reactions between its hydroxymethyl and aldehyde groups, resulting in impurities with complex structures and difficult to separate. These impurities not only affect the yield of the target product, but also interfere with the selectivity of the analytical method, increasing the cost and complexity of separation and purification.

High-concentration HMF is prone to cause a sudden rise in the temperature of the reaction system in highly exothermic reactions such as catalytic oxidation, bringing the risk of thermal runaway of the system. It is necessary to accurately adjust the reaction heat flow distribution through intermittent feeding, dynamic temperature control, multi-point monitoring and other means to ensure equipment safety and process stability.