Résumé
A new material, consisting of alginate-polyethyleneimine (AP) foam, with high percolating properties has been designed for palladium recovery in fixed-bed reactor. The foam, having high affinity for Pd(II), can be also used for manufacturing heterogeneous hydrogenation catalyst. SEM-EDX and TEM analyses were performed to determine the structure of the foams and the distribution of Pd nanoparticles (after metal reduction). Metal-sorbent interactions and oxidation state of Pd are characterized by XPS. Pd(0)-bearing foams are investigated for the hydrogenation of 3-nitrophenol (3-NP) using HCOOH as the hydrogen donor. The maximum sorption capacity of Pd(II) by AP foams reaches up to 224 mg g(-1). The water flux (under water-depth pressure of 6 mbar) reaches 24.8 mL cm(-2) min(-1) (superficial flow velocity: 14.9 m h(-1)). The foams are remarkably stable: The mass loss under strong shaking for 2 days does not exceed 3%. For catalytic application, Pd loading conditions were optimized (flow rate, metal concentration) to reach 101 mg Pd g(-1) (97% metal removal). Under these conditions, Pd overloading and nanoparticles aggregation can be minimized. The catalytic hydrogenation of 3-nitrophenol (using formic acid as the hydrogen donor) was optimum for a HCOOH/3-NP molar ratio close to 160 (pH between 3 and 4). High flow rates minimize diffusion effects; the apparent rate constant (for pseudo-first-order rate equation) reaches 9.7 x 10(-3) s(-1). The reuse of the foams over 30 cycles shows the long-term stability of catalytic activity. The test on continuous (one-pass) mode shows a progressive poisoning of the catalyst. However, a simple washing with water is sufficient for recovering catalytic activity.