The presence of boron significantly promoted propylene oxide (PO) formation over chlorine-free K+-modified FeOx/SBA-15 catalysts with K/Fe ratios >= 2.5. Boron also improved the catalytic performances of Rb+- and Cs+-modified FeOx/SBA-15 catalysts, whereas it did not play significant roles over Li+- and Na+-modified catalysts. Boron-promoted K+-FeOx/SBA-15 exhibited the best catalytic performance for PO formation. Potassium ions could enhance the dispersion of FeOx clusters, forming active iron sites. The addition of K+ at a higher content destroyed the ordered mesoporous structure of SBA-15, likely causing the reaggregation of iron species. The modification by boron could keep the ordered mesoporous structure and the high dispersion of iron species, which were beneficial to PO formation. The addition of boron with a proper content also suppressed further conversion of PO by diminishing the acidity and basicity of the catalyst. We propose that the interactions among boron, potassium ions, and iron species play pivotal roles in enhancing PO formation. (C) 2008 Elsevier Inc. All rights reserved.