Abstract
Catalysis is central to green chemistry because it can increase selectivity, reduce activation-energy requirements, minimize stoichiometric waste, and enable transformations under milder conditions. This chapter reviews the green-chemistry roles of heterogeneous and homogeneous catalysis, biocatalysis, organocatalysis, photocatalysis, and catalytic processing in flow. Turnover number, turnover frequency, selectivity, catalyst loading, recovery, metal contamination, and solvent compatibility are used to compare catalytic strategies. Representative applications illustrate how catalytic oxidation, reduction, carbon-carbon bond formation, asymmetric synthesis, enzymatic transformations, and photocatalytic processes can replace more waste-intensive stoichiometric routes. The discussion also emphasizes trade-offs: high activity does not by itself guarantee sustainability when catalyst synthesis, scarce metals, separation, energy inputs, or solvent requirements create significant burdens. Effective catalyst selection therefore requires integrated assessment of reaction performance, recoverability, life-cycle impacts, safety, and scale-up. Catalysis emerges as an enabling technology for translating green-chemistry principles into practical process design.