Thread Content
This post was last edited by Maimai on 2025-11-3 at 10:53: https://pic1.zhimg.com/v2-71e9f5a4a4c32354e0f4f5ef96becd90_b.jpg Solid acid catalysis is one of the most important tools for practicing green chemistry; it is widely used in reactions such as esterification, etherification, and Friedel-Crafts alkylation. Acid strength and acid amount are often regarded as the key properties of a catalyst, as they generally determine the way in which the catalyst activates the substrate as well as the rate of the catalytic reaction. Measurement of acid strength: The acidity of solid acid catalysts is commonly characterized using spectroscopic methods; for example, ammonia temperature-programmed desorption (NH3-TPD) and pyridine infrared (Py-IR) spectroscopy can reveal the number and type of acid sites. Such tests are usually carried out under vacuum or gas phase conditions, allowing for the scientific measurement of the inherent acidity of the material itself. The H₀ acid function quantifies the acidity of a solution by measuring the degree of protonation of neutral basic indicators in acidic solutions; it is also used to estimate the acidity strength of acidic sites on solid surfaces. (The lower the value, the stronger the acidity.) https://pic2.zhimg.com/v2-0075501ca37f341d8587c08b3d827e1d_1440w.jpg (Acidity values of some common solid acid catalysts – Hammett indicator method) The complexity of liquid-phase reaction systems: In liquid-phase reaction systems, the situation is often much more complex than in testing environments. Many of the key reactions in fine chemical manufacturing take place in liquid-phase systems, where solvents, reactants, and trace amounts of water molecules can \"coat\" or \"neutralize\" acid sites, thereby affecting their catalytic capacity. The properties of the solid acid obtained through testing do not indicate its capability in catalyzing reactions. Selection of solid catalysts in liquid-phase systems 1. Optimize the catalyst to match the liquid-phase reaction system. Catalyst – Zirconium sulfate SO4²⁻/ZrO2. Zirconium sulfate SO4²⁻/ZrO2 is a typical super-strong solid acid; theoretically, its acidity can compare to that of 100% sulfuric acid. In a dry gas phase environment, it can catalyze high-energy reactions such as isomerization and alkylation. In liquid-phase reaction systems such as **autocoupling, esterification, aldol condensation, etc., the catalytic activity often decreases significantly when SO₄²⁻/ZrO₂ is present. Optimization of liquid-phase conditions: By introducing TiO₂, a sulfated composite catalyst (SO4²⁻/ZrO2-TiO2) was prepared to enhance the lattice bonding force and make the catalyst structure more stable. Composite substrates can enhance the anchoring capacity of sulfate groups and their resistance to hydrolysis. In the esterification reaction of nitrogen-containing substrates, the lifetime of the SO4²⁻/ZrO2-TiO2 catalyst is more than twice that of zirconium sulfate. https://pica.zhimg.com/v2-eca6c5333a57b81c8cbe0b30abb00a78_1440w.jpg 2. Based on the liquid-phase system, select an appropriate catalyst and solvent. The sulfonic acid resin SDA-AC-D600 has limited acidity in dry or non-polar solvents; however, in aqueous systems, its -SO3H groups can dissociate fully, resulting in a significant improvement in its catalytic performance. The working acidity of a catalyst is closely related to its solvent environment. In non-polar solvents, proton migration is restricted, making it difficult for reactants to utilize the acid sites. In aldol condensation or esterification reactions, the reaction rate of SDA-AC-600 in water/alcohol co-solvent systems is more than 3 times higher than that in non-polar solvents. 3. Depending on the acid strength required for the reaction, an appropriate catalyst should be selected for the esterification of short-chain fatty acids; in acid-catalyzed esterification, it is the amount of acid that plays a key role, rather than its strength. Sulfonic acid resins can efficiently catalyze reactions in the system, without causing side reactions such as dehydration and etherification due to excessive acidity. For the esterification of long-chain or hydrophobic reactants, the acid site requires both a strong acid and sufficient contact with the reactant. For reactants with longer carbon chains and increased hydrophobicity, both carbonyl protonation and intermediate stabilization require higher acid strength. Meanwhile, the diffusion limitation of long-chain reactants and the accessibility of acid sites become decisive factors. By adjusting the silica-alumina ratio and employing special molding processes, molecular sieves can meet both the requirements for acid strength and acid quantity.