General procedure for C(sp 3 )-H alkylation with inline TBADT recovery. A 50 mL volumetric flask was charged with olefin (0.1 M, 5 mmol, 1 equiv.), H-donors (1 M, 50 mmol, 10 equiv.) and tetrabutylammonium decatungstate (TBADT) (1 mM, 50 μmol, 1 mol%). Next, CH3CN was added to acquire a total volume of 50 mL. The prepared solution was transferred to the holding tank and delivered to a Vapourtec UV-150 Reactor (PFA tubing, 3.06 mL, 750 μm inner diameter) via HPLC pump. The outlet solution of photoreactor directly injected into the two-stage flow cells. Samples from permeate and retentate solutions were collected and analyzed until reaching the equilibrium of the membrane. Another solution with olefin and H-donors in acetonitrile was prepared, which was delivered to the holding tank by the peristaltic pump. Meanwhile, the retentate feed was put into the holding tank, mixing with the fresh starting material solution. The samples were collected from two permeate feeds and analyzed with 1 H NMR. After certain experimental time, a pure acetonitrile was used to push out the solution left inside the system. A total turnover number was calculated by combining solutions in permeate feeds and remaining in the system. A detailed description of the experimental setup and protocols is available in Section 3.2 of the Supplementary Information.