•Octahedral aluminoborate: alternative candidate to prepare redox molecular sieves.•High substitution upper-limit and strong accommodation capacity for some transition metals.•Sec-alcohols oxidized to ketones or other over-oxidative products.•Cr3+ functioned as catalytic center to modulate the formation of free radicals.•A mechanism proposed to demonstrate the probable reaction pathway.Octahedral-based redox molecular sieves M-PKU-1 (MCr, Fe) were synthesized by isomorphous metal-substitution and used as catalysts for catalytic dehydrogenation of sec-alcohols using H2O2 as oxidant. Various characterizations, such as X-ray diffraction Rietveld refinement and XPS spectrum, confirmed that transition metals were embedded successfully inside the PKU-1 framework with a high level (about ∼50 atom %) and presented in the valence state of +3. Molecular probe analyzes suggested that Cr sites catalyzed the quick formation of active OH radicals, however strongly suppressed the generation of superoxide O2- ions. Under the performed reaction conditions, 10%Cr-PKU-1 exhibited excellent catalytic performances (>99 % selectivity) and kept favorable recyclable stability. A hypothetical mechanism was proposed, which involved a Cr3+-Cr2+-Cr3+ circle when the oxidative dehydrogenation reaction happened. Furthermore, qualitative and quantitative analyzes were performed to illustrate the stepwise by-products generated in the probable pathway due to the over-oxidization, but the selectivity to the two proposed pathways seemed to be in equal portions and didn’t have any obvious preference. Obviously, our preliminary results still merit further exploration; we believe however, they would provide helpful information to better understand the structure-activity relationship and the key function of Cr-PKU-1 in the catalytic activation of H2O2.Download high-res image (97KB)Download full-size image

•Octahedron-based framework aluminoborates provide a powerful alternative strategy to prepare redox molecular sieves with different transition metals.•Cr-PKU-1 is a dual-centered solid acid catalyst and shows higher activity than mono-centered solid catalyst in the heterogeneously-catalyzed Strecker reaction.•Site-isolated Lewis or Brönsted acid centers independently activate the Strecker reaction and undergo two-type parallel reaction pathways.Cr-PKU-1, an octahedron-based redox molecular sieve, was found to be an efficient, environmentally benign and easily recoverable catalyst for the heterogeneously-catalyzed Strecker reaction under the mild conditions. This material was evidenced to be a dual-centered solid acid with a large density of Brönsted acid (borate hydroxyl groups, B-OH) and Lewis acid sites (Cr3+), and therefore showed much higher catalytic activity than mono-central solid acid in the formation of α-aminonitriles. Although commonly considered as Lewis acid centers, electron-deficient boron atoms (tri-coordinated BO3) in Cr-PKU-1 failed to exhibit the desired catalytic activity in the present investigations probably due to steric hindrance against the large organic substrates. A plausible mechanism about two-centered catalysis in Strecker reaction were proposed based on the comprehensive and sufficient analysis. This work illustrates for the first time the possible application in the Strecker reaction using the dual-centered redox molecular sieve, and highlights their potential ability to address the various functions involved in the reaction mechanism.Download high-res image (177KB)Download full-size image

An octahedron-based gallium borate (Ga-PKU-1) with an open framework was prepared by the boric acid flux method and studied by XRD, SEM, FT-IR, TGA-DSC, BET and NH3-TPD. The borate hydroxyl groups in the 18-ring membered channels functioned as Brönsted acid centers and were proved to play an important role in the dehydration of isopropanol to propylene with high selectivity. However, the other two gallium borates, Ga4B2O9 and GaBO3, with similar elemental compositions showed completely different acidities and catalytic performance for isopropanol decomposition. Our results indicated that such a great difference in selectivity most probably originated from the changes in the coordination microenvironment from GaO6 to GaO5. The relationship between the structure and activity was discussed in detail, and an intrinsic mechanism for the selectivity preference was proposed according to the obtained results.

An octahedron-based gallium borate (Ga-PKU-1) with an open framework was prepared by the boric acid flux method and studied by XRD, SEM, FT-IR, TGA-DSC, BET and NH3-TPD. The borate hydroxyl groups in the 18-ring membered channels functioned as Brönsted acid centers and were proved to play an important role in the dehydration of isopropanol to propylene with high selectivity. However, the other two gallium borates, Ga4B2O9 and GaBO3, with similar elemental compositions showed completely different acidities and catalytic performance for isopropanol decomposition. Our results indicated that such a great difference in selectivity most probably originated from the changes in the coordination microenvironment from GaO6 to GaO5. The relationship between the structure and activity was discussed in detail, and an intrinsic mechanism for the selectivity preference was proposed according to the obtained results.