Parahydrogen enhanced NMR reveals correlations in selective hydrogenation of triple bonds over supported Pt catalyst

Abstract

Parahydrogen induced polarization using heterogeneous catalysis can produce impurity-free hyperpolarized gases and liquids, but the comparatively low signal enhancements and limited scope of substrates that can be polarized pose significant challenges to this approach. This study explores the surface processes affecting the disposition of the bilinear spin order derived from parahydrogen in the hydrogenation of propyne over TiO2-supported Pt nanoparticles. The hyperpolarized adducts formed at low magnetic field are adiabatically transported to high field for analysis by proton NMR spectroscopy at 400 MHz. For the first time, the stereoselectivity of pairwise addition to propyne is measured as a function of reaction conditions. The correlation between partial reduction selectivity and stereoselectivity of pairwise addition is revealed. The systematic trends are rationalized in terms of a hybrid mechanism incorporating non-traditional concerted addition steps and well-established reversible step-wise addition involving the formation of a surface bound 2-propyl intermediate.

Selective Single‐Site Pd−In Hydrogenation Catalyst for Production of Enhanced Magnetic Resonance Signals using Parahydrogen

“…The scope of heterogeneous catalysts which are suitable for PHIP increased significantly in the past 10 years. For instance, PHIP was successfully observed over various metal catalysts such as Rh, Pt, Pd, Ir, Au, Cu, bimetallic systems Pt−Sn, Pd−Au, Pd−M (M=Sn, Zn, Ag, Mn, Pb, Au) supported on support materials such as SiO 2 , ZrO 2 , Al 2 O 3 , TiO 2 , Al−Si fiberglass, chitosan, carbon nanotubes . Moreover, the PHIP effect was demonstrated for bulk Pt metal and several metal oxides (PtO 2 , PdO and CeO 2 ).…”

Parahydrogen‐Based Hyperpolarization for Biomedicine

“…HET-PHIP effects were also successfully observed using heterogeneous catalysts comprising Pt and Pd nanoparticles supported on γ-Al 2 O 3 , [79] and later for many other supported metals (e.g., Rh, Ir, Co, Ni, Au, Cu) catalysts in gas, liquid and aqueous phase hydrogenations. [11–12, 25, 27, 78f, 78g, 80] Variation of the type of metal, metal particle size [80a, 80d, 81] and shape, [80e, 80h] type of support [80a, 80d, 80g, 80i] and reaction conditions were all shown to influence the magnitude of the PHIP effect. Polarization transfer from 1 H to heteronuclei ( 13 C, 15 N) during heterogeneous hydrogenation [9b, 62a, 62b] helps to increase the hyperpolarization lifetime significantly, and is particularly important for potential biomedical applications along with the use of aqueous media.…”

Semihydrogenation of Propyne over Cerium Oxide Nanorods, Nanocubes, and Nano‐Octahedra: Facet‐Dependent Parahydrogen‐Induced Polarization

“…PHIP is a simple and robust method offering high polarization rates using relatively inexpensive instrumentation. The ease of separation of the hyperpolarized product from the polarizing agent is a key advantage of PHIP by heterogeneous catalysis . However, hyperpolarization levels afforded by supported metals and metal‐oxide catalysts have up to now been limited to only a few percent owing to the low pairwise selectivity of hydrogenation on metal surfaces.…”

“…The ALTADENA signal enhancement of PE was calculated from the ratio of the signal integrals for the ALTADENA and thermally polarized PE CH group. The hyperpolarized signals were corrected for relaxation losses so that they could be used to determine the pairwise selectivity . The calculation also accounted for the sharing of the scalar spin order with ancillary spins (not originating as p‐H 2 ).…”