Revealing Microstructural Evolution and Boron Depletion-Driven Acid Site Formation in Cr³⁺-Doped Aluminoborate for Xylose-To-Lactic Acid Catalysis

Ionic doping is widely used to regulate surface acidity in catalytic materials, yet the microscopic mechanisms remain difficult to observe. Clarifying them is essential for acid catalysis design. Prior work showed that Cr³⁺ doping into the AlO₆-based open-framework aluminoborate PKU-1 alters acidity. Here, we probe the microstructural changes and formation of new acid sites induced by high-level Cr³⁺ doping. PXRD and HRTEM show that up to 40% Cr³⁺ is homogeneously incorporated into the PKU-1 framework. N₂ adsorption-desorption, NH₃-TPD, Py-FTIR, XPS, and ICP-OES indicate that Cr³⁺ doping increases surface area, acid-site density, and the Lewis/Brønsted ratio, while decreasing boron content, with B/(Al + Cr) dropping from 2.23 to 1.86. We propose that a high Cr³⁺ content promotes partial cleavage of edge-sharing CrO₆-AlO₆ linkages and concurrent removal of BO₃/B₂O₅ species, causing boron depletion and channel opening. The resulting oxygen vacancies coordinate water to form Brønsted sites or expose Cr³⁺ as a Lewis site. These microstructural changes correlate directly with enhanced acidity and catalytic behavior. As a demonstration, Cr-PKU-1 exhibits outstanding xylose-to-lactic-acid conversion, achieving 99.9% xylose conversion and a lactic-acid yield of 57.8%, over three times that of undoped PKU-1. This work offers new insight into cation-doping-induced acidity modulation via local structural reorganization.