Proton enriched high-surface-area cesium salt of phosphotungstic heteropolyacid with enhanced catalytic activity fabricated by nanocasting strategy

The elimination by HF of the silica matrix from the composites obtained by the two-step reaction deposition of Cs_xH_3-xPW₁₂O₄₀ (CsHPW) salt nanocrystals with a Cs/W₁₂ ratio equal 2.5 on SBA-15 yields materials with substantially lower Cs/W₁₂ ratios of 1.7-2.0. The value of the Cs/W₁₂ ratio in the nanocasts is determined by the Cs-precursor (Cs n-propoxide or Cs-acetate) used at the first stage of materials preparation. The surface area of the CsHPW nanocasts is 41-45 times higher than their co-precipitated analogs at the same Cs/W₁₂ ratios. We report here that implementation of the nanocasting preparation technique yields for the first time a bulk CsHPW material that combines a high concentration of acid sites (Cs/W₁₂ = 1.7-2.0) with a high surface area of 41-93 m² g^-1. Co-precipitated analogues at the same Cs/W₁₂ ratios are nonporous and exhibit a surface area smaller than 5 m² g^-1. Our nanocasted CsHPW materials are stable against leaching and colloidization in polar solvents, and their catalytic performance exceeded that of bulk Cs_2.5H_0.5PW₁₂O₄₀, known as the most active among the acidic HPW salts. The catalytic activity of CsHPW nanocasts in MTBE synthesis and in the isopropanol dehydration reactions is shown to be higher by a factor of 2-3 than that of the standard Cs_2.5H_0.5PW₁₂O₄₀ material.