Abstract
The ordered mesoporous silica supported Pd NPs, denoted as Pd NPs@[KIT-6] 2 and Pd NPs@[KIT-6]-PEG-imid 6, were synthesized by a deposition precipitation method using the [KIT-6] 1 or [KIT-6]-PEG-imid 5, respectively, and Na2PdCl4 followed by in situ reduction by NaBH4. The obtained supported Pd NPs 2 and 6 were fully characterized by XPS, XRD, TEM, SEM, FT-IR and solid-state Si-29 and C-13 NMR spectroscopy. The effect of several parameters such as catalyst loading, temperature, NaOH and NaBH4 concentration were examined to obtain the optimum reaction conditions. The supported Pd NPs 2 and 6 catalyze the hydrolytic production of H-2 from NaBH4 with notable efficiencies and the catalyst stabilized by PEG and imidazolium ionic liquid based-[KIT-6] 6 is more active than its [KIT-6] counterpart 2. A TOF of 176 mole(H2).mol(cat)(-1).min(-1) for 6 was achieved when the reaction was performed with a low catalyst loading of 0.1 mol%. Reaction kinetics studies showed that the hydrolysis is first order in catalyst and NaBH4 with apparent E-a of 35.0 and 35.7 kJ mol(-1) for 2 and 6, respectively. The efficiency of catalyst 6 for the hydrolysis of NaBH4 was investigated in H2O and D2O and indicated that the reaction is considerably faster in H2O than in D2O with primary KIE k(H)/k(D) = 2.36. In addition, the PdNPs based [KIT-6] mesoporous silica 6 retains 71.6% of its initial efficiency after five cycles.