Thursday, January 23, 2014

NMR thermometry

The paper for this week was published back in late October in Nature.  I'm just getting to it now though,  because a) I'm always a bit behind and b) the title "Thermal maps of gases in heterogeneous reactions" does not alert the reader that the content of this paper is highly relevant to anyone who wishes to be fluent in contemporary NMR research.  The full citation is

"Thermal maps of gases in heterogeneous reactions"


Jarenwattananon, Gloggler, Otto, Melkonian, Morris, Burt, Yaghi and Bouchard

Nature 2013 502, 537

This paper describes a novel NMR thermometry technique to measure gas temperatures at gas-solid interfaces in heterogeneous catalysts.  For engineers who optimize reactor design, temperature gradients on catalyst beds contain vital information on reaction energetics (so I am told).  The key insight of the authors is that in the presence of a weak magnetic field gradient (<< 1 G cm^-1) motional averaging will result in an inverse relationship between line width of a given peak and temperature of the system.  Figure 1a provides an excellent pictorial description of this technique.

To demonstrate the feasibility of NMR thermometry, the authors design a simple calibration experiment using a 10 mm NMR tube with catalyst-loaded glass wool (more on their catalysts later) and an analyte of propylene gas flowing at 15 cc min^-1, 40 PSI.  A 1D 1H spin echo spectrum is recorded with gradient applied during acquisition on a 400 MHz NMR (with microimaging capabilities) and the probe temperature set from 303 to 413 K in 10 K steps.  A sum of Lorentzians is fit to the olefin region of the propylene spectrum to get line widths.  Figure 1b shows linear fit of a plot of the change in linewidth relative to 303 K (deltaF) versus temperature (T) for three different gradient strength.

In the absence of a gradient, the slope of deltaF versus T is negative but small.  For a gradient strength of 0.1 G cm^-1, the slope is -0.13 Hz K^-1.  A stronger gradient results in a larger slope.

The authors settle on 0.005 G cm^-1 for gradient strength for their NMR thermometry setup.  They measure deltaF vs. T for two catalysts: Pt nanoparticles (PtNP) and Pd metal-organic frameworks (Pd-MOF).  Least squares fit results in a slope and intercept of -0.16 Hz K^-1 and 151 Hz and -0.10 Hz K^-1 and 119 Hz for PtNP and Pd-MOF, respectively.  Now the authors have the tools in place to measure T by NMR.  Simply measure the line width (of propylene olefin peaks) in 1D 1H spectrum with a gradient of 0.05 G cm^-1 during acquisition and use the appropriate linear equation to calculate temperature.

Having convincing demonstrated feasibility, the authors move to an application: thermal maps of lab-scale demo reactors with each catalysts.  In this experiment, propylene and hydrogen gas (actually para-hydrogen) are flowed through the reactor at 15 cc min^-1, 40 PSI and catalysis (hydrogenation) takes place.  The microimaging experiment (which takes ~30 min) divides the reactor into voxels and in each voxel the spin echo/gradient experiment is performed.  Line widths and, subsequently, temperature is determined for each 0.73 x 0.73 mm pixel in the reactor.  Figure 3 shows false color image of axial, coronal and sagittal views of the reactors.

Significant variations in the temperature of the catalyst bed is observed.  There are spots with T > 600 K and other spots with T < 400 K!  To validate these results, the authors carefully place a fiber optic temperature sensor at 3 spots in the demo reactor (you can see where in the coronal view of PtNP and sagittal view of Pd-MOF reactors).  The sensor agrees well with NMR thermometry-derived T.  The error is at most 4%.

Finally, the authors demonstrate NMR thermometry on a 1 mm microreactor with catalyst supported on silica gel.  Para-hydrogen preheat to 418 K is flowed into the reactor.  I assume that there is also propylene.  I have no clue what the flow and pressure is because the authors do not go out of their way to bore us with any details.  The temperature calibration experiment results in a slope of -0.2 Hz K^-1.  No mention of the intercept.  Figure 4 show a false color image thermal map from the microimaging experiment.

Once again, NMR thermometry reveals significant temperature variations in the reactor from 30 K higher than gas T at the inlet to 30 K lower at the outlet.

This is an amazing paper!  It is easy to understand why it was published in Nature.  The thermal maps in Fig. 2 and 4 are incredible.  I am no expert, but  don't see any other way to measure these types of temperature gradients on this scale other than NMR thermometry.  In Fig. 4 for instance, there is a gradient of ~60 degrees over a space of ~2.5 mm (which is less than 1/10 of 1 inch).


I'm going to make this a two-part blog and return tomorrow and sharpen my critique up a little bit.  See you then.


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