A simple heat treatment can convert a common metal-organic framework (MOF) to an active catalyst for ethylene dimerization. The resulting solid catalyst could potentially replace the expensive solution-phase catalyst currently used for making the monomer 1-butene, a common ingredient in polybutylene and other polymers (Nat. Commun. 2019, DOI: 10.1038/s41467-019-10013-6). Most large-scale catalytic processes rely on solid catalysts. Compared with solution-phase catalysts, which are often organometallic compounds, solid catalysts do not require solvents and cocatalysts or activators, and they are generally easier and less expensive to separate from the product and reuse. But unlike their liquid-phase counterparts, solid catalysts lack uniform active sites and are therefore prone to making unwanted side products. MOFs could offer the best of both worlds: they are porous solids with molecular order. But making them catalytically active remains challenging. Through a series of tests on a ruthenium-based MOF known as HKUST-1, Iker Agirrezabal-Telleria of the University of the Basque Country, Manuel A. Ortuño of the Institute of Chemical Research of Catalonia, and coworkers found an easy solution. When they heated the MOF gently in the presence of nitrogen and hydrogen, it formed uniform, catalytically active Ru-H sites. In the absence of cocatalysts and solvents, the activated MOF converts ethylene to 1-butene with greater than 99% selectivity and remains stable for more than 120 h.