NMR imaging of low pressure, gas‐phase transport in packed beds using hyperpolarized xenon‐129

Abstract

Gas‐phase magnetic resonance imaging (MRI) has been used to investigate heterogeneity in mass transport in a packed bed of commercial, alumina, catalyst supports. Hyperpolarized 129Xe MRI enables study of transient diffusion for microscopic porous systems using xenon chemical shift to selectively image gas within the pores, and, thence, permits study of low‐density, gas‐phase mass‐transport, such that diffusion can be studied in the Knudsen regime, and not just the molecular regime, which is the limitation with other current techniques. Knudsen‐regime diffusion is common in many industrial, catalytic processes. Significantly, larger spatial variability in mass transport rates across the packed bed was found compared to techniques using only molecular diffusion. It has thus been found that that these heterogeneities arise over length‐scales much larger than ∼100 µm. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4013–4019, 2015

Robust In Situ Magnetic Resonance Imaging of Heterogeneous Catalytic Hydrogenation with and without Hyperpolarization

“…The necessity to use noble gases in SEOP narrows the scope of possible applications, in particular catalytic applications, but can provide information about gas mass transfer. [28] Other hyperpolarization methods are dissolution-DNP, a powerful tool to hyperpolarize small molecules in liquid solutions, [29,30] and DNP surface-enhanced NMR spectroscopy (DNP-SENS), which was used to study metalorganic frameworks and mesoporous materials. [31] However, DNP techniques require rather complex and very costly equipment [30] making studies exploiting these methods for polarization of gases limited to the preparation of small amounts of polarized propane, butane and ethylene using HYPSOs (Hyper Polarizing Solids).…”

“…are at the cornerstone of chemical and petrochemical industry [1] and, to make these processes more sustainable, control over reaction selectivity, conversion rates, mass and heat transport inside the working reactor (that is under operando conditions) is required. [28] Other hyperpolarization methods are dissolution-DNP, a powerful tool to hyperpolarize small molecules in liquid solutions, [29,30] and DNP surface-enhanced NMR spectroscopy (DNP-SENS), which was used to study metalorganic frameworks and mesoporous materials. [8][9][10] MRI of reactors utilizing gases are not as developed as studies of liquids because the spin density in the gas phase is ca.…”

“…[12] Overall, to date, there are only a handful of MRI studies of the gas phase reactions with regular non-hyperpolarized gases. [28] Other hyperpolarization methods are dissolution-DNP, a powerful tool to hyperpolarize small molecules in liquid solutions, [29,30] and DNP surface-enhanced NMR spectroscopy (DNP-SENS), which was used to study metalorganic frameworks and mesoporous materials. SEOP relies on the polarization of noble gases, e. g., 129 Xe, [25] 3 He, [26] 83 Kr, [27] which are unreactive and very expensive.…”

NMR Hyperpolarization Techniques of Gases

“…[59] The advent of continuous-flow production of HP 129 Xe [45a] was soon applied to greatly facilitate studies of materials surfaces, [60] including under conditions of magic angle spinning. [61] Since that work, HP xenon has been used to study diffusion in confined spaces or porous media [62] , [63] , [64] ; image such systems as a function of gas flow [65] or 129 Xe chemical shift [66] ; or spectroscopically probe single-crystal surfaces [67] , liquid crystals, [68] or combustion processes. [69] However, the greatest body of materials-related work has concerned the effort to probe void spaces and surfaces in microporous or nanoporous materials with HP 129 Xe, thereby providing information about pore size, pore shape, and gas dynamics in: nanochanneled organic, organometallic, and peptide-based molecular materials [70] (including in macroscopically oriented single crystals [71] ); multi-walled carbon nanotubes [72] ; gas hydrate clathrates [73] ; porous polymeric materials and aerogels [74] ; metal-organic frameworks [75] ; calixarene-based materials and nanoparticles [76] ; organo-clays [77] ; mesoporous silicas [78] ; and zeolites and related materials [79] — efforts that have been aided by computational studies of xenon in confined spaces (e.g., Refs.…”

Spatially resolved NMR spectroscopy of heterogeneous gas phase hydrogenation of 1,3-butadiene with parahydrogen

“…12,[15][16][17] A major improvement in gas imaging can be obtained by exploiting hyperpolarized gases which can provide an up to several orders of magnitude increase in signal intensity and hence in the signal-to-noise ratio (SNR) in NMR experiments. The hyperpolarization methods include, for instance, spinexchange optical pumping (SEOP), [18][19][20][21] dynamic nuclear polarization (DNP), 22,23 parahydrogen-induced polarization (PHIP) [24][25][26][27] and signal amplification by reversible exchange (SABRE). [28][29][30] All these techniques are powerful, yet their applicability in catalysis research is limited.…”