Quantitative In Situ Monitoring of Parahydrogen Fraction Using Raman Spectroscopy (original) (raw)
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Use of Raman Spectroscopy to Characterize Hydrogenation Reactions
Organic Process Research & Development, 2006
Raman spectroscopy was used to characterize hydrogenation reactions involving single-step and two-step processes. The Raman technique was shown to be well-suited for endpoint determination as well as process optimization. In this investigation, hydrogenation of cyclohexene to produce cyclohexane was used as a model system. Conditions were varied to determine the effect of catalyst loading, solvent ratios, and reactant concentrations. Four catalysts were evaluated. The kinetic profiles of each reaction process were determined for each of the catalysts. In one case, a side reaction leading to an intermediate was observed for the hydrogenation reaction when run under hydrogen-starved conditions. After these cyclohexene hydrogenations were characterized, Raman spectroscopy was applied to the conversion of carvone to tetrahydrocarvone and the hydrogenation of 2-(4-hydroxyphenyl) propionate. Raman was used to characterize the kinetics of these reactions and was also used to prove that two-step hydrogenation mechanisms occurred in each. Raman was shown to be useful for process understanding, process optimization, process monitoring, and endpoint determination. Accomplishment of these goals leads to better process controls upon transfer of the procedure to a process environment. This ultimately leads, in turn, to the mitigation of risk of making out-of-specification product in manufacturing.
Detection of Molecular Hydrogen by Stimulated Raman Emission
Applied Spectroscopy, 1994
FiG. 4. Analytical curves for melamine and melamine cyanurate in nylon. Lines are for best fit obtained by linear regression. Circles show experimental points for melamine; squares, for melamine cyanurate. CONCLUSIONS FT-Raman spectroscopy is readily applied to the determination of melamine and melamine cyanurate in nylon. Results reported above are for powdered material. This form was used only because it was the only form in which we could prepare samples with known content of additives in our laboratory setting. We have, however, run spectra of equal quality on flake and other forms of nylon resin. These results suggest that the method described here can be used to determine the concentration of additives in the feedstock or finished product of nylon spinning operations without sample pretreatment. It should also be possible to make measurements on the melted resin, before or after extrusion, but blackbody emission from the sample in this case may somewhat degrade the signal-to-noise ratio of the measurement. This complication is, of course, one of the disadvantages of using 1064-nm excitation. Even though the major spectral features of the two analytes show substantial overlap, the least-squares fitting operation permits quantitation of both components with the use of spectral features of the nylon matrix as an internal standard. The limit of detection is less than 1% by weight for both analytes. Other additives could be determined in the same way?
Production and characterization of para-hydrogen gas for matrix isolation infrared spectroscopy
Normal hydrogen (n-H 2) has 3:1 ortho/para ratio and the production of enriched para-hydrogen (p-H 2) from normal hydrogen is useful for many applications including matrix isolation experiments. In this paper, we describe the design, development and fabrication of the ortho-para converter that is capable of producing enriched p-H 2. The p-H 2 thus produced was probed using infrared and Raman techniques. Using infrared measurement, the thickness and the purity of the p-H 2 matrix were determined. The purity of p-H 2 was determined to be >99%. Matrix isolation infrared spectra of trimethylphosphate (TMP) and acetylene (C 2 H 2) were studied in p-H 2 and n-H 2 matrices and the results were compared with the conventional inert matrices.
A pulsed injection parahydrogen generator and techniques for quantifying enrichment
Journal of Magnetic Resonance, 2012
A device is presented for efficiently enriching parahydrogen by pulsed injection of ambient hydrogen gas. Hydrogen input to the generator is pulsed at high pressure to a catalyst chamber making thermal contact with the cold head of a closed cycle cryostat maintained between 15 and 20 K. The system enables fast production (0.9 standard liters per minute) and allows for a wide range of production targets. Production rates can be systematically adjusted by varying the actuation sequence of high-pressure solenoid valves, which are controlled via an open source microcontroller to sample all combinations between fast and thorough enrichment by varying duration of hydrogen contact in the catalyst chamber. The entire enrichment cycle from optimization to quantification and storage kinetics are also described. Conversion of the para spinisomer to orthohydrogen in borosilicate tubes was measured at 8 minute intervals over a period of 64 hours with a 12 Tesla NMR spectrometer. These relaxation curves were then used to extract initial enrichment by exploiting the known equilibrium (relaxed) distribution of spin isomers with linear least squares fitting to a single exponential decay curve with an estimated error less than or equal to 1 %. This procedure is time-consuming, but requires only one sample pressurized to atmosphere. Given that tedious matching to external references are unnecessary with this procedure, we find it to be useful for periodic inspection of generator performance. The equipment and procedures offer a variation in generator design that eliminate the need to meter flow while enabling access to increased rates of production. These tools for enriching and quantifying parahydrogen have been in steady use for 3 years and should be helpful as a template or as reference material for building and operating a parahydrogen production facility.
Process Raman gas analysis in ammonia production and refining
2017
On-line process measurement of the composition of gas streams in refining, fertilizer, and other manufacturing industries is essential for the optimal operation of different process units within these facilities. Process analyzers based on gas chromatography, mass spectrometry, and electrochemical technologies are commonly used in these facilities. However, process conditions for certain streams present major challenges for these traditional technologies. Techniques based on optical spectroscopy, including near-infrared (NIR), infrared (dispersive and Fourier transform), and Raman spectroscopy, can provide analysis solutions for these challenging stream conditions. Raman spectroscopy is particularly useful for streams containing homonuclear diatomic gases, such as H2 and N2. These gases are key components in many chemical processes involving the creation and use of syngas (H2, CO, and CO2), such as the manufacturing of ammonia and methanol. Hydrogen is also an essential feedstock fo...
Sensors
Highly accurate, quantitative analyses of mixtures of hydrogen isotopologues—both the stable species, H2, D2, and HD, and the radioactive species, T2, HT, and DT—are of great importance in fields as diverse as deuterium–tritium fusion, neutrino mass measurements using tritium β-decay, or for photonuclear experiments in which hydrogen–deuterium targets are used. In this publication we describe a production, handling, and analysis facility capable of fabricating well-defined gas samples, which may contain any of the stable and radioactive hydrogen isotopologues, with sub-percent accuracy for the relative species concentrations. The production is based on precise manometric gas mixing of H2, D2, and T2. The heteronuclear isotopologues HD, HT, and DT are generated via controlled, in-line catalytic reaction or by b-induced self-equilibration, respectively. The analysis was carried out using an in-line intensity- and wavelength-calibrated Raman spectroscopy system. This allows for continu...
Raman gain measurement in solid parahydrogen
Optics Letters, 2000
We report a steady-state Raman gain measurement of the Q 1 ͑0͒ transition ͑v 1 √ 0, J 0 √ 0͒ in solid parahydrogen. We carry out measurements by pumping with a continuous-wave frequency-doubled YAG laser at 532 nm and observing the direct amplif ication of a probe-laser beam for the f irst Stokes transition at 683 nm. A large single-pass amplif ication coeff icient of 2.3 6 0.2 is obtained at a pump intensity of 46 kW͞cm 2 , with an interaction length of 1 cm, giving a steady-state Raman gain coeff icient of 18 6 3 cm͞MW.
Application of parahydrogen for mechanistic investigations of heterogeneous catalytic processes
Russian Chemical Bulletin, 2017
Parahydrogen induced polarization technique (PHIP), based on the pairwise addition of molecular hydrogen to a substrate, was successfully applied to obtain novel information on the mechanisms of heterogeneous catalytic hydrogenation, hydrodesulfurization, and oligo merization processes. In particular, the PHIP effects were observed upon hydrogenation with parahydrogen catalyzed by the immobilized neutral complexes of rhodium and iridium, which confirms the similarity in the mechanisms of homogeneous and heterogeneous hydrogenation for such systems. In the study of acetylene oligomerization, a significant NMR signal enhance ment was revealed for a number of C 4 oligomers, with the enhancement levels by far exceeding that observed in hydrogenation of carbon carbon triple bonds. The mechanistic features of heterogeneous hydrogenation of a number of six membered cyclic hydrocarbons over support ed metal catalysts were investigated, and their hydrogenation scheme based on the pairwise addition of molecular hydrogen was proposed. Furthermore, the PHIP technique revealed that heterogeneous hydrodesulfurization of thiophene mainly proceeds via hydrogenation followed by a C-S bond cleavage. A significant enhancement of sensitivity in combination with charac teristic line shapes of NMR signals make the PHIP method a unique and highly informative tool for the investigation of heterogeneous catalytic processes.
2011
An optical cell is described for high-throughput backscattering Raman spectroscopic measurements of hydrogen storagematerials at pressures up to 10 MPa and temperatures up to 823 K. High throughput is obtained by employing a 60 mm diameter × 9 mm thick sapphire window, with a corresponding 50 mm diameter unobstructed optical aperture. To reproducibly seal this relatively large window to the cell body at elevated temperatures and pressures, a gold o-ring is employed. The sample holder-to-window distance is adjustable, making this cell design compatible with optical measurement systems incorporating lenses of significantly different focal lengths, e.g., microscope objectives and single element lenses. For combinatorial investigations, up to 19 individual powder samples can be loaded into the optical cell at one time. This cell design is also compatible with thin-film samples. To demonstrate the capabilities of the cell,in situ measurements of the Ca(BH4)2 and nano-LiBH4–LiNH2–MgH2hydr...
Journal of Raman Spectroscopy, 2020
Wavenumber and intensity calibration of a Raman spectrometer is performed with the use of pure rotational Raman bands (Δν = 0, ΔJ = ± 2) of H 2 , HD, D 2 , and vibration-rotation Raman bands (Δν = 1, ΔJ = ± 2) of O 2 as primary standards. Wavenumber calibration is based on reference transition wavenumbers available from accurate theoretical and experimental results. Intensity calibration is based on ratios of accurate theoretical Raman intensities for transitions from common rotational states to eliminate temperature effects. Polarization dependence is corrected to ensure that all of these bands have the correct depolarization ratio, ρ = 0.75. The calibrated Raman spectrometer is used to measure standard Raman spectra of carbon tetrachloride, benzene, cyclohexane, toluene, and benzonitrile, for which the relative Raman intensities and depolarization ratios are determined with carefully estimated uncertainties. Vibrational frequencies of indene used for routine wavenumber calibration are updated.