The catalytic properties of the metal-organic framework compound Cu₃(BTC)₂(H₂O)₃ ·xH₂O (BTC=benzene 1,3,5-tricarboxylate) were explored. Cu₂O-free powder samples of Cu₃ (BTC)₂(H₂O)₃·xH₂O were obtained using an improved synthesis at 393 K under hydrothermal conditions. The microporous material has a high specific pore volume of 0.41 cm³g^-1 and a pore diameter of 10.7 Å (Horvath-Kawazoe). Removal of the three copper-bound water molecules allows to access the Lewis acid copper sites. The exchange of coordinated water by substrates or solvent molecules is recognized from the color change of the compound. For chemisorbed benzaldehyde, the IR stretching frequency ν(C=O) is decreased from 1702 to 1687 cm^-1. The chemisorption results in an activation of benzaldehyde for the liquid phase cyanosilylation with a reasonable yield of 50-60% after 72 h (313 K) and a high selectivity. Filtration experiments demonstrate that the reaction mechanism is heterogeneous. Coordinating solvents such as THF completely block the Lewis acid sites of the catalyst. Solvents such as CH₂Cl₂ or higher reaction temperatures (353 K) cause decomposition of the catalyst.