Pd-based bimetallic catalysts for HET-PHIP

Burueva, Dudari B.; Stakheev, Aleksandr Y.; Koptyug, Igor V.

doi:https://doi.org/10.5194/mr-2021-5

Preprints

https://doi.org/10.5194/mr-2021-5

Preprints

19 Jan 2021

Review status: this preprint is currently under review for the journal MR.

Pd-based bimetallic catalysts for HET-PHIP

Dudari B. Burueva^1,2, Aleksandr Y. Stakheev³, and Igor V. Koptyug^1,2 Dudari B. Burueva et al.

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¹Laboratory of Magnetic Resonance Microimaging, International Tomography Center, SB RAS, Novosibirsk, 630090, Russia
²Novosibirsk State University, Novosibirsk, 630090, Russia
³N.D. Zelinsky Institute of Organic Chemistry, RAS, Moscow, 119991, Russia

Received: 14 Jan 2021 – Accepted for review: 17 Jan 2021 – Discussion started: 19 Jan 2021

Abstract. Production of hyperpolarized catalyst-free gases and liquids by heterogeneous hydrogenation with parahydrogen (HET-PHIP) can be useful for various technical as well as biomedical applications, including in vivo studies, investigations of mechanisms of industrially important catalytic processes, enrichment of nuclear spin isomers of polyatomic gases, and more. In this regard, the wide systematic search for heterogeneous catalysts effective in pairwise H₂ addition required for the observation of PHIP effects is crucial. Here in this work we demonstrate the competitive advantage of Pd-based bimetallic catalysts for HET-PHIP. The dilution of catalytically active Pd with less active Ag or In atoms provides the formation of atomically dispersed Pd₁ sites on the surface of Pd-based bimetallic catalysts, which are significantly more selective toward pairwise H₂ addition compared to the monometallic Pd. Furthermore, the choice of the dilution metal (Ag or In) has a pronounced effect on the efficiency of bimetallic catalysts in HET-PHIP, as revealed by comparing Pd-Ag and Pd-In bimetallic catalysts.

Dudari B. Burueva et al.

Status: open (until 02 Mar 2021)

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RC1:
'Comment on mr-2021-5', Anonymous Referee #1, 01 Feb 2021 reply

Please see pdf.

Reply

Citation: https://doi.org/10.5194/mr-2021-5-RC1
- AC1:
  'Reply on RC1', Igor Koptyug, 18 Feb 2021 reply
  
  We thank Anonymous Referee # 1 for his/her comments and suggestions.
  The detailed response can be found in the attached file.
  
  Reply
  
  Citation: https://doi.org/10.5194/mr-2021-5-AC1
  - RC2: 'Reply on AC1', Anonymous Referee #1, 18 Feb 2021 reply
    
    The authors, have considered all my comments in a sufficient way. I have no further comments and suggest the mansucript for publication in "Magnetic Resonance"
    
    Reply
    
    Citation: https://doi.org/10.5194/mr-2021-5-RC2
RC3:
'Comment on mr-2021-5', Anonymous Referee #2, 23 Feb 2021 reply
The authors present PHIP experiments in the gas phase of propyne employing heterogeneous bimetallic catalysts. The study provides useful information for improving HET-PHIP experiments but should be revised with respect to the following specific comments:

Line 97: “Pd-Ag/Al2O3 catalyst sample contained 2 wt.% of Pd and 6 wt.% of Ag; Pd-In/Al2O3 catalyst contained 2 wt.% of Pd and 2 wt.% of In”. This result in different dilutions of Pd in the other (less catalytically active) metal. Therefore, the amount of catalytically active Pd1 sites should be different in the two bimetallic catalysts? How does this impact on the comparability of the results for the Pd-Ag and the Pd-In catalyst?

Very few experimental details are presented in the entire manuscript. Reviewer 1 already asked for more details on the synthesis and analysis of the bimetallic catalysts, which was sufficiently answered by the authors in the revised version of the manuscript. This reviewer is more concerned with the understanding of the NMR experiments and the reaction control of the hydrogenation experiments, which are both not sufficiently described in the experimental section.

Questions on hydrogenation experiments:

In line 110 the authors mention that “propyne was premixed with p-H2 in the molar ratio of 1:4”. What was the overall pressure of the gas mixture before the reaction?

The authors performed the hydrogenation experiments at different temperatures, showing how temperature affects the conversion and selectivity of the reaction. It would be also interesting to know how different pressures would affect the reaction. Did the authors ever tried to perform the hydrogenation with different pressures and can give a hint in which way the hydrogenation is affected by different pressures?

In table 1 the authors provide conversion rates and selectivity values for different flow rates of the gas mixture and state in line 124-125 that “Slightly higher selectivity values at high gas mixture flow rates is explained by the lower catalyst contact times”. However, any information on the design of the tubular reactor for the hydrogenation reaction is missing (i.e. inner diameter and length of the reactor, length of fixed bed containing solid catalyst) that would allow the reader to estimate contact times of the gas stream with the catalyst bed from the provided flow rates.

Questions on NMR experiments and relaxation issues

Line 111: Were the NMR experiments conducted as continuous flow experiments (i.e. on the flowing gas) (if so, please provide the mean velocity of the flowing sample) or in a stopped flow fashion? Was the used probehead a standard probehead for 5mm NMR tubes? How does the sample container in the NMR probehead looked like? Was it just a tube passed through the NMR coil (if so, please provide the inner diameter of the tube) or a flow cell with a more sophisticated geometry (if so, please provide a description of the geometry)? These points are important for a better understanding of the NMR experiments, as all points have an impact on the SNR of the spectra (i.e. filling factor, outflow effects and line broadening in continuous flow NMR).

The distance between the tubular reactor and the RF coil of the NMR spectrometer should also be mentioned in the experimental section or even better the transport time (for the two different flow rates) of the hyperpolarized gas from the reactor to the detection site. These times together with the T1 relaxation time of propene (which should also be provided) would help the reader to estimate how severe hyperpolarization loss due to T1 relaxation was. This could also explain the very different SE values for the experiments conducted with 1.3 and 3.8 ml/s flow rates. The authors started a discussion on hyperpolarization losses due to T1 relaxation in chapter 3.3 (line 194-211), but do not provide enough information to understand if the hyperpolarization losses due to T1 relaxation were dramatic (e.g. on the order of 60% of the initial signal enhancement) or minor (e.g. on the order of 10% of the initial signal enhancement) .

Technical comments: Line 199: please correct the wrong spelling of “experimental”

Line 265: add “in” before “order”

Reply
Citation: https://doi.org/10.5194/mr-2021-5-RC3

Dudari B. Burueva et al.

Data sets

Pd-based bimetallic catalysts in hydrogenation with parahydrogen Burueva, D. B., Stakheev, A. Y., and Koptyug, I. V. https://doi.org/10.5281/zenodo.4436159

Dudari B. Burueva et al.

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Short summary

Catalytic hydrogenation of unsaturated molecular precursors with parahydrogen provides a major signal enhancement in NMR, which is vital for many technical as well as biomedical applications. We show that dilution of catalytically active Pd with a less active metal in heterogeneous Pd-based catalysts provides significantly larger enhancements, and that the judicious choice of the dilution metal can further improve the performance, as revealed by comparing Pd-Ag and Pd-In bimetallic catalysts.


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