Science - USA (2021-07-16)

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INSIGHTS | PERSPECTIVES

278 16 JULY 2021 • VOL 373 ISSUE 6552 sciencemag.org SCIENCE

PIEZOELECTRICS

Autonomous


biocompatible


piezoelectrics


By Shlomo Berger

M

uscles provide mechanical forces
needed for the dynamic activity
of a human body. The mechanical
forces applied on various human
organs lead to different types of
physical movements (e.g., contrac-
tion and extension, rotation, and bending).
Injuries and diseases can disable muscle
functionality, which may lead to the pro-
nounced deterioration of a human activity
or even cause death. Surgical correction
is not always possible or does not always
provide optimal results. Implanted artifi-
cial biocompatible devices can replace de-
fective muscles. These should operate lo-
cally at the defective site by monitoring the
need for a specific physical movement and
then applying the correct mechanical force
to obtain it. These devices should also be
self-energized using the body’s energy re-
sources and programmed for optimal oper-
ation. On page 337 of this issue, Yang et al.
( 1 ) present a new approach for fabricating
thin piezoelectric biocompatible thin films
that actuate physical movements, demon-
strated on mice muscles, under applied
electric field.
Biocompatible piezoelectric materi-
als can provide sensing and actuating of
physical movements of organs inside a hu-
man body ( 2 ). They possess a piezoelectric
property ( 3 ) that interconverts electrical
and mechanical energy. The piezoelectric
response of materials results from a revers-
ible change in the length of neighboring
ionic bonds that create electric dipoles in
response to applied mechanical forces or
electric fields. Piezoelectric materials exist
naturally in a human body ( 4 ), such as in
bones and muscles (5, 6), and contribute

Faculty of Materials Science and Engineering, Technion–
Israel Institute of Technology (IIT), Haifa 32000, Israel.
Email: berger@technion.ac.il

the transport of molecules in the vicinity of
the active site ( 6 ).
Zeolites are a diverse family of robust,
microporous aluminosilicate materials that
are widely used as catalysts in hydrocarbon
processing on an industrial scale. The ion-
exchanged metal sites in iron- and copper-
containing zeolites resemble key structural
components of the active sites in enzymes
( 2 ). Snyder et al. installed the same active
iron sites in the pores of two zeolites. Both
have similar silicon-to-aluminum ratios,
iron loadings, and cage diameters, and
might be expected to have nearly identical
activities toward methane. However, the
zeolite structures differ in the accessibility
of their iron sites. Beta zeolite (*BEA) has
a system of interconnected pores composed
of “large” 12-rings (Si 12 O 12 ), and no smaller
constrictions ( 7 ). Although chabazite (CHA)
cages have the same diameter, entry into
the cages must occur through smaller
8-ring apertures (Si 8 O 8 ) ( 7 ).
Methane hydroxylation in these zeolites
was studied as a sequence of stoichiometric
reactions at room temperature. First, the re-
duced Fe(II) sites were activated by the oxi-
dant, N 2 O, which installs reactive a-O atoms
(see the figure). When one of the activated
Fe(IV)O sites abstracts a hydrogen atom from
methane, a methyl radical and a hydroxy site,
Fe(III)OH, are simultaneously created in the
same zeolite cage. If rebound of the methyl
radical to the colocated Fe(III)OH site ensues,
the Fe(II) site is regenerated and can perform
the reaction sequence again. If the methyl
radicals diffuse away, they can be trapped by
neighboring ferryl sites, which are converted
to inert methoxy sites, Fe(III)OCH 3. Although
a C–O bond is formed in both types of reac-
tion, radical trapping at a ferryl site is un-
productive because neither of the resulting
Fe(III) sites (hydroxy or methoxy) can be re-
oxidized by N 2 O under mild conditions. The
ratio of radical escape relative to rebound is
very high in *BEA.
By contrast, diffusion of the methyl radical
in CHA zeolite is restricted by virtue of the
small size of the window through which the
radical must escape from the cage where it
is generated. This cage effect is reminiscent
of radical confinement in enzymes. Using
Mössbauer and resonance Raman spectros-
copies, Snyder et al. show that the yield of
Fe(II) (resulting from radical capture in the
same zeolite cage) under single-turnover
conditions is ~40% for CHA, whereas it is
near zero in *BEA. In CHA, the methanol
product migrates spontaneously from the
Fe(II) sites to Brønsted acid sites in the zeo-
lite pores, allowing a second reaction cycle
to take place. When the isotopic identity of
the methane was switched, from^13 CH 4 in the
initial cycle to^12 CH 4 in a subsequent cycle,

the isotopic composition of the methanol
changed, which demonstrates that Fe(II)
sites in CHA can be reactivated by N 2 O to
produce a second equivalent of methanol.
Efficient processes for converting meth-
ane to an energy-dense liquid hydrocarbon
such as methanol are important not only
for making greater use of abundant natu-
ral gas resources but also to reduce the
need to flare stranded natural gas. Rather
than generating the greenhouse gas CO 2
unproductively, conversion to methanol
would allow transport in a cost-effective
way to population centers where its en-
ergy, chemical value, or both, could be
extracted. However, using the strategy of
Snyder et al. to this effect will require the
process to become much more efficient.
For example, combining the oxidant with
methane in the zeolite should allow the re-
action sequence to proceed in a single step.
However, this approach presents a selectiv-
ity challenge. Because the relative difficulty
of activating a C–H bond in methane ver-
sus methanol is roughly constant, there is
a universal, catalyst-independent trade-off
between conversion and selectivity ( 8 ). It is
not yet clear how to achieve rapid diffusion
of methanol away from the active sites,
preventing its further oxidation, while si-
multaneously confining methyl radicals
near the active sites to form methanol and
regenerate Fe(II).
A second challenge is to replace the N 2 O
oxidant by a less expensive oxidant such as
O 2. Colocating two iron sites in a ferrierite
zeolite was recently shown to facilitate O 2
splitting ( 9 ). However, this geometry will
enhance the undesired ferryl trapping of
methyl radicals that leads to Fe(III)OH/
Fe(III)OCH 3 sites. The soluble methane
monooxygenase enzyme achieves methane
oxidation at diiron active sites linked by
bridging oxygens, Fe(IV) 2 (m-O) 2 , but it also
produces ferric sites that require an external
reductant for reactivation. A useful process
for converting stranded methane will need
to overcome both of these challenges. j

REFERENCES AND NOTES


  1. N. J. Gunsalus et al., Chem. Rev. 117 , 8521 (2017).

  2. K. T. Dinh et al., ACS Catal. 8 , 8306 (2018).

  3. R. M. Bullock et al., Science 369 , eabc3183 (2020).

  4. B. E. R. Snyder et al., Science 373 , 327 (2021).

  5. X. Huang, J. T. Groves, J. Biol. Inorg. Chem. 22 , 1 85 ( 2017 ).

  6. R. Breslow, Acc. Chem. Res. 28 , 146 (1995).

  7. C. Baerlocher, W. M. Meier, D. H. Olson, Atlas of Zeolite
    Framework Types (Elsevier, ed. 6, 2007).

  8. A. A. Latimer, A. Kakekhani, A. R. Kulkarni, J. K. Nørskov,
    ACS Catal. 8 , 6894 (2018).

  9. E. Tabor et al., S c i. A d v. 6 , eaaz9776 (2020).


ACKNOWLEDGMENTS
S.L.S. acknowledges the US Department of Energy, Office
of Science, Division of Basic Energy Sciences, under the
Catalysis Science Initiative (DE-FG-02-03ER15467) for
financial support.

10.1126/science.abj4734

Implants that work


with muscles should


monitor, correct,


and be self-energized


0716Perspectives.indd 278 7/9/21 5:20 PM

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