Photochemistry of ketones adsorbed on size/shape selective zeolites. A supramolecular approach to persistent carbon centered radicals

Article abstract of DOI:10.1039/a708691a

²H NMR, EPR, computational and product analyses of the photolysis of 2,4-diphenylpentan-3-one (DPP) adsorbed on MFI size/shape selective zeolites are consistent with supramolecular structural changes as a function of surface coverage that provi

Full text of DOI:10.1039/a708691a

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Photochemistry of ketones adsorbed on size/shape selective zeolites. A
supramolecular approach to persistent carbon centered radicals
a
a
a
a
b
c
Nicholas J. Turro,* † Ann McDermott, Xuegong Lei, Wei Li, Lloyd Abrams, M. Francesca Ottaviani,
d
e
e
e
Hege Støgård Beard, Kendall N. Houk, Brett R. Beno and Patrick S. Lee
a
Chemistry Department, Columbia University, New York, NY 10027, USA
b
E. I. duPont de Nemours, Central Research Department, Experimental Station, Wilmington, DE 19880, USA
University of Florence, Department of Physical Chemistry, Via Gino Capponi, 9, 50121 Florence, Italy
Chemistry Department, University of Oslo, Oslo, Norway
c
d
e
Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA
2H NMR, EPR, computational and product analyses of the
after rapid loss of CO, a-methylbenzyl radicals (MB ) [eqn.
(1)], which undergo random radical combination to produce
2,3-diphenylbutane (DPB) (ca. 95%) and disproportionates to
styrene (S) and ethylbenzene (EB) (ca. 5%). The rates of the
radical–radical reactions in eqns. (2) and (3) are close to
diffusion controlled5 and, in the absence of radical scavengers,
·
photolysis of 2,4-diphenylpentan-3-one (DPP) adsorbed on
MFI size/shape selective zeolites are consistent with supra-
molecular structural changes as a function of surface
coverage that provide a novel method for the generation of
persistent diffusing organic free radicals.
,8
·
determine the lifetime of the ‘molecular’ radicals MB .
The MFI topology zeolites (e.g. silicalite and ZSM-5) are of
technical significance in both size/shape selective catalysis and
in molecular sieve separations.1 The technically attractive
properties of MFI zeolites originate from supramolecular,
structural (substrates as guests, zeolite crystal as host) and
dynamic (shape/size dependent supramolecular molecular dif-
fusion) characteristics, i.e. substrates possess ‘effective’ molec-
ular diameters comparable to the size/shape of the holes on the
external surface of the zeolite crystal and/or the channels and
Results of product analysis and EPR measurements as a
function of coverage [DPP/(Na)ZSM-5 (w/w), Si/Al = 80,
average particle size ca. 0.1 m] showed that (i) the dominant
products depend on the coverage, with disproportionation to
form S and EB being favored by low coverage ( < 0.3%) and
combination to form DPB being increasingly favored as the
9
coverage increases, and (ii) the maximum EPR intensity of the
·
signal of the persistent MB radicals is insensitive to loading.
For example, the ratio of disproportionation to combination of
2
·
intersections on the internal surface. The siting and diffusion of
MB radicals in solution is typically of the order of 0.1; at 0.3%
a substrate are vital in determining the product selectivity in
catalysis and the resolution efficiency in separations.3
Understanding the role of diffusion and siting requires the
uncoupling of both supramolecular, structural and dynamic
features from the effects of activation at active sites and
rearrangements of reactive intermediates. The effects of
activation are pronounced at high temperatures with zeolites
possessing strongly acidic sites (e.g. HZSM-5).
loading of DPP the ratio is 3.1.
2
The values of the half-widths (DH1/2) of the H NMR spectra
2
of C
D
6 5
CHMeCOCHMeC
D
6 5
([ H10]-DPP) as a function of
1
0
coverage are shown in Fig. 1. These provide information
concerning both the supramolecular structure and dynamics of
2
the [ H10]-DPP/ZSM-5 system as a function of coverage: (i) a
‘sharp’ signal ( < 1 kHz) indicates rapid, isotropic motion and
the lack of constraints of the molecular structure of [ H10]-DPP
2
4
We report a supramolecular photochemical investigation at
by the host ZSM-5 structure, i.e. the system is essentially
molecular (very weak intermolecular bonds between guest and
host); (ii) a ‘relatively broad’ signal (ca. 40 kHz) indicates an
anisotropic and partial constraint of a weakly bonded supramo-
lecular structure of a DPP/ZSM-5 system; (iii) a ‘limiting
broad’ signal (ca. 130 kHz) indicates relatively strong inter-
molecular bonds which result in inhibition of motion of a
room temperature in which the activation and diffusional/siting
effects of zeolite catalysis are uncoupled. Activation of the
substrate towards reaction is provided by photochemical
methods which (i) excite adsorbed substrates whose sitings and
2
reaction products are directly determined by H NMR analysis
and (ii) produce persistent5 adsorbed carbon-centred radical
intermediates whose structure and mobility are directly deter-
mined by EPR spectroscopy. Computational methods are
employed for (i) analysis of the plausibility of the siting and
diffusional processes on the external and internal surfaces, (ii)
rationalization of the variation of product distribution as a
function of supramolecular structure, and (iii) simulation of the
EPR spectra of the persistent supramolecular radical species
formed upon photoexcitation of the adsorbed substrates.
In the molecular (solution) photochemistry [eqns. (1)–(3)] of
,6
2
supramolecular [ H10]-DPP/ZSM-5 system. If the ketone were
located on the flat part of the external surface, its motion would
1
⁄
2
be essentially isotropic, and DH would be expected to be of the
(
a)
∆H1/2
(b)
(v)
0.3 kHz
(
iv)
0.6 kHz
2.8 kHz
(
iii)
(ii)
(i)
1
30 kHz
2,4-diphenylpentan-3-one (DPP) a-cleavage from T
1
produces,
(
ii)
14 kHz
38 kHz
hn
hn
CO
•
PhCHMeCOCHMePh
DPP
PhCHMe
(1)
(2)
(
i)
–
•
MB
(
a)
b)
PhCHMeCHMePh
DPB
200
0
kHz
–200
200
0
–200
•
kHz
PhCHMe
(
•
2
2
MB
Fig. 1 (a) H NMR spectra of ([ H10]-DPP) at (i) 0.3, (ii) 1, (iii), 2, (iv) 5
and (v) 10 wt% loading on zeolite. (b) Result of 1% loading sample (i)
before and (ii) after photolysis.
PhCH=CH2 + PhEt
EB
(3)
S
Chem. Commun., 1998
697

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order of 1 kHz. If the ketone were located on the inside of the
would be of the order of 130 kHz. The observed
for this selectivity may be either a supramolecular ‘steric’ or
‘dynamic’ effect. The steric effect would result from the high
energy required for a C–C bond to form DPB at an intersection,
as indicated by computation, and resulting from the supramol-
ecular steric effects associated with the compression of a DPB
molecule into the limited space available in the vicinity of the
intersections and channels. The less sterically demanding
radical–radical disproportionation in the vicinity of the inter-
sections becomes the default supramolecularly-allowed radi-
cal–radical reaction, but even this reaction is still remarkably
zeolite DH
1
⁄
2
value of 40 kHz is consistent with the ketone being moderately
constrained in the holes on the external surface.
2
Before photolysis (Fig. 1), the H NMR spectrum of the DPP/
ZSM-5 system at low loading ( < 1% loading) exhibits a half
width of ca. 40 kHz, characteristic of a moderately constrained
2
[
H10]-DPP molecule. Thus, we conclude that the DPP
molecules at low coverage are adsorbed in the holes at the
interface of the external and internal surfaces of the ZSM-5
2
·
crystal. Photolysis of [ H10]-DPP at room temperature results in
slow and allows the supramolecular MB /ZSM-5 radicals to
2
a change in the H NMR spectrum and the appearance of a new
become persistent for hours at room temperature. A dynamic
mechanism, termed a diffusional ‘maze’ effect, could also cause
the persistence of otherwise reactive radicals. In the maze
effect, the molecular traffic patterns of the radicals lead to rate
limiting infrequent encounters in the vicinity of the intersection.
In the extreme form of the maze effect an encounter leads to a
‘diffusion controlled’ reaction.
broad feature (ca. 130 kHz) which grows as the extent of
photolysis increases (Fig. 2).
For high coverage before photolysis the spectrum shows
mainly a sharp peak (DH1/2 < 1 kHz), with the broad peak
2
being buried in the base line due to the [ H10]-DPP molecules in
2
the holes. We conclude that photolysis of [ H10]-DPP at high
coverage does not lead to a significant increase of the broad
The 2H NMR, EPR, computational and photochemical
product analyses are all supported by the same supramolecular
structural and dynamic interpretation of the results at high and
low coverages. The supramolecular photochemistry of DPP
adsorbed on ZSM-5 molecular sieve zeolites depends dramat-
ically on the composition of the DPP/ZSM-5 system, because
the supramolecular constitutional structure (connectivity rela-
tionship of the guest and host structures) depends on the
system’s composition. At low loading ( < 0.3%) DPP is mainly
adsorbed in the holes on the external surfaces that provide
access to the internal surface. At intermediate loadings (ca. 1%),
when the limited amount of holes is plugged with DPP
molecules, as the coverage increases, the external framework
surface between the holes becomes covered with DPP mole-
cules until a monolayer is formed. At ‘high’ loadings ( > 1%),
both the holes and the framework’s external surface are covered
(a monolayer is formed) so that, as the coverage increases,
multilayers or two dimensional films of DPP are formed on the
external surface.
2
peak because most of the photolyzed [ H10]-DPP molecules
occur in multilayers at high coverage, and the radicals produced
in these fluid multilayers are very mobile and undergo random
radical–radical combination [eqn. (2)]. This view is supported
by the fact that the major product of photolysis at high coverage
is that expected from ‘molecular’ or solution conditions, i.e.
DPB (ca. 95%); most of the absorbed light excites the large
excess of ketones in the multilayers rather than the relatively
smaller number adsorbed in the holes at the interface.
The steady state photolysis of DPP adsorbed on ZSM-5
produced intense, long lived (many hours) EPR signals at all
12
coverages studied. The observed spectrum fits a simulation of
·
a powder spectrum of MB as expected from the restricted
·
mobility of MB adsorption on the internal surface. Computa-
tional analysis indicates that the lowest energy supramolecular
·
structure of MB has the Ph group placed in an intersection
between the channels, and the alkyl radical moiety placed in a
channel between the intersections. This supramolecular geome-
try possesses a substantial barrier to motion, but allows the
achievement of a roughly planar structure at the radical center.
The authors thank Dr Paul Krusic (DuPont), Professors Hans
Fischer and Henning Paul for enlightening discussions, and the
NSF for financial support. X. G. L. and W. L. thank the
Kanagawa Academy of Science and Technology for funding.
The hyperfine coupling constants computed and determined
experimentally,1
1,12
taking into account anisotropies due to
restricted motion, are sufficient for the simulation. Finally,
computations were made of the external surface area available
for adsorption of DPP in order to estimate the relationship
between the macroscopic composition (loading w/w) and the
surface coverage. The measured external surface area of the
Notes and References
† E-mail: turro@chem.columbia.edu
1
2
See, for example, R. M. Dessau, Selective Sorption Properties of
Zeolties, ACS Symp. Ser., 1980, 135, 123.
V. R. Choudhary, V. S. Nayak and T. V. Chaudhary, Ind. Eng. Chem.
Res., 1997, 36, 1812.
9
2
21
ZSM-5 sample is of the order of 16 m g , which, when
compared to the computations, implies that a monolayer of DPP
will be formed when the coverage is ca. 0.4%, the coverage at
which the experimentally observed salient effects on products
and spectroscopic properties begin to change. As the coverage
increases to values of ca. 1% and greater, the results become
characteristic of a molecular, two dimensional film rather than
of an adsorbed layer, because the bulk of the ketone molecules
are in the multilayer and not adsorbed on the zeolite external
surface. As the coverage increases, the lifetime of the radicals
decreases and the ratio of disproportionation to combination
decreases, i.e. the product distribution becomes more like that in
homogeneous solution. These results are also consistent with
the formation of multilayers of ketones on the external surface,
so that ketone molecules finds themselves increasingly in a
3 P. B. Weisz, Pure Appl. Chem., 1980, 52, 2091; P. B. Weisz, Ind. Eng.
Chem. Fundam., 1986, 25, 53.
4
For a review of the photochemistry of organic molecules adsorbed on
zeolites, see N. J. Turro, Pure Appl. Chem., 1986, 58, 1219.
5
(a) D. Griller and K. U. Ingold, Acc. Chem. Res., 1976, 9, 13; (b)
G. D. Mendenhall and K. U. Ingold, J. Am. Chem. Soc., 1973, 95,
3
422.
6
Self reaction via combination or disproportionation determines the
persistence of radicals inert to rearrangement and reaction with the
environment.
7 N. D. Ghatlia and N. J. Turro, J. Photochem. Photobiol. A: Chem., 1991,
57, 7; B. H. Baretz and N. J. Turro, J. Am. Chem. Soc., 1983, 105,
1
309.
8
9
H. Fischer, J. Am. Chem. Soc., 1986, 108, 3925 and references
therein.
‘
two-dimensional’ liquid as the loading increases, and the
The surface area of the ZSM-5 samples was determined to be ca. 15
results tend toward those for homogeneous solutions.
2
21
m g by mercury porosimetry.
6,7
·
The persistence, or lifetime, of MB is limited by either (i)
2
1
0 For a discussion of the relationship between motion and line shape or H
NMR spectroscopy, see R. J. Wittebort, E. T. Olejniczak and
R. G. Griffin, J. Chem. Phys., 1987, 86, 5411.
·
the diffusion of two MB radicals into the vicinity of an
intersection, or (ii) disproportionation, the lowest energy
supramolecular reaction of the system within the intersection.
Thus, as a consequence of their supramolecular structure, two
11 R. Batra, B. Giese, B. M. Spichty, G. Gescheidt and K. N. Houk, J. Phys.
Chem., 1996, 100, 1837.
12 M. S. Conradi, H. Zeldes and R. Livingston, J. Phys. Chem., 1979, 83,
·
encountering MB radicals to not undergo the ‘molecularly
6
33.
favored’ radical–radical combination, but instead undergo
disproportionation to S and EB [eqn. (3)]. The structural basis
Received in Corvallis, OR, USA, 1st December 1997; 7/08691A
698
Chem. Commun., 1998