Acid catalysts are substances that increase the rate of a chemical reaction by donating protons (H⁺ ions) to reactants. They are commonly used in various chemical processes, including esterification and alkylation. Acid catalysts can be either strong or weak, depending on their ability to donate protons, and they help facilitate reactions without being consumed in the process. These catalysts can be classified into two main types: Brønsted acids, which donate protons, and Lewis acids, which accept electron pairs. Acid catalysts play a crucial role in industrial applications, such as the production of petrochemicals and biomass conversion, enhancing efficiency and selectivity in chemical reactions.