Mason Alexander H, Motta Alessandro, Kratish Yosi, Marks Tobin J
Department of Chemistry, Northwestern University, Evanston, IL 60208 3113.
Trienens Institute for Sustainability and Energy, Northwestern University, Evanston, IL 60208 3113.
Proc Natl Acad Sci U S A. 2024 Jul 23;121(30):e2406133121. doi: 10.1073/pnas.2406133121. Epub 2024 Jul 15.
A kinetic/mechanistic investigation of gaseous propane hydrogenolysis over the single-site heterogeneous polyolefin depolymerization catalysts AlS/ZrNp and AlS/HfNp (AlS = sulfated alumina, Np = neopentyl), is use to probe intrinsic catalyst properties without the complexities introduced by time- and viscosity-dependent polymer medium effects. In a polymer-free automated plug-flow catalytic reactor, propane hydrogenolysis turnover frequencies approach 3,000 h at 150 °C. Both catalysts exhibit approximately linear relationships between rate and [H] at substoichiometric [H] with rate law orders of 0.66 ± 0.09 and 0.48 ± 0.07 for Hf and Zr, respectively; at higher [H], the rates approach zero-order in [H]. Reaction orders in [CH] and [catalyst] are essentially zero-order under all conditions, with the former implying rapid, irreversible alkane binding/activation. This rate law, activation parameter, and DFT energy span analysis support a scenario in which [H] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular β-alkyl elimination. The Zr and Hf catalyst activation parameters, ΔH = 16.8 ± 0.2 kcal mol and 18.2 ± 0.6 kcal mol, respectively, track the relative turnover frequencies, while ΔS = -19.1 ± 0.8 and -16.7 ± 1.4 cal mol K, respectively, imply highly organized transition states. These catalysts maintain activity up to 200 °C, while time-on-stream data indicate multiday activities with an extrapolated turnover number ~92,000 at 150 °C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.
对单中心多相聚烯烃解聚催化剂AlS/ZrNp和AlS/HfNp（AlS = 硫酸化氧化铝，Np = 新戊基）上气态丙烷氢解反应进行动力学/机理研究，以探究催化剂的本征性质，避免因时间和粘度依赖的聚合物介质效应带来的复杂性。在无聚合物的自动活塞流催化反应器中，丙烷氢解反应的周转频率在150℃时接近3000 h⁻¹。两种催化剂在亚化学计量的[H]下，速率与[H]之间均呈现近似线性关系，对于Hf和Zr，速率定律的反应级数分别为0.66±0.09和0.48±0.07；在较高的[H]下，速率接近[H]的零级反应。在所有条件下，[CH]和[催化剂]的反应级数基本为零级，前者意味着烷烃快速、不可逆的结合/活化。该速率定律、活化参数和DFT能量跨度分析支持了这样一种情况，即[H]在两个合理且相互竞争的速率决定过渡态之一中起关键作用——双分子金属 - 烷基键氢解与单分子β - 烷基消除。Zr和Hf催化剂的活化参数分别为ΔH = 16.8±0.2 kcal mol⁻¹和18.2±0.6 kcal mol⁻¹，与相对周转频率相符，而ΔS分别为 - 19.1±0.8和 - 16.7±1.4 cal mol⁻¹ K⁻¹，意味着过渡态高度有序。这些催化剂在高达200℃时仍保持活性，而在线时间数据表明，Zr催化剂在150℃下具有多天的活性，外推的周转数约为92,000。这种方法对于解聚催化剂的发现和工艺优化具有吸引力。
