Various alkylcarboxylate-intercalated layered yttrium hydroxides (C_n−1H_2n−1COO⁻/Y-layered rare-earth hydroxide (LRH), n = 1–10) were synthesised by a simple anion exchange method using Cl⁻/Y-LRH as a parent material. The anion exchange from Cl⁻ into CH₃COO⁻ proceeded rapidly, as confirmed by the time-resolved synchrotron-radiation X-ray diffraction (t-SXRD) analysis. The interlayer distance of C_n−1H_2n−1COO⁻/Y-LRH could be controlled precisely, and the basal spacings calculated from XRD were linearly proportional to the length of alkyl groups. In water solvent, the basal spacing of the CH₃COO⁻/Y-LRH catalyst based on the (00l) plane increased from 1.00 nm under dry conditions to 1.35 nm, and this lifted-up layered structure was generated immediately. By use of the CH₃COO⁻/Y-LRH catalyst, a wide range of substrates were converted into the corresponding α,β-unsaturated nitriles in water via Knoevenagel condensation. Our synthesised heterogeneous catalyst acted as a Brønsted base for this organic transformation, and it was reusable without any loss of its catalytic activity and selectivity.