Nitrate-Mediated Alcohol Oxidation on Cadmium Sulfide Photocatalysts

Article abstract of DOI:10.1021/acscatal.9b01051

Developing photocatalysts capable of organic oxidations enables the generation of value-added products from biomass feedstocks through visible light irradiation. Through a series of nonaqueous photocatalytic experiments, we have uncovered that CdS nanowir

Full text of DOI:10.1021/acscatal.9b01051

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Article
Nitrate Mediated Alcohol Oxidation on Cadmium Sulfide Photocatalysts
John DiMeglio, Andrew G Breuhaus-Alvarez, Siqi Li, and Bart M Bartlett
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01051 • Publication Date (Web): 08 May 2019
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ACS Catalysis
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Nitrate Mediated Alcohol Oxidation on Cadmium Sulfide
Photocatalysts
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John L. DiMeglio, Andrew G. Breuhaus-Alvarez, Siqi Li, and Bart M. Bartlett*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United
States
KEYWORDS: Semiconductor photocatalysis, alcohol oxidation, redox mediator, cadmium sulfide, nitrate radical
ABSTRACT: Developing photocatalysts capable of organic oxidations enables the generation of value added products from
biomass feedstocks through visible light irradiation. Through a series of non-aqueous photocatalytic experiments, we have
uncovered that CdS nanowires catalyze benzyl alcohol (BnOH) oxidation and 5-hydroxymethylfufural (HMF) oxidation. The
rate can be improved by introducing nitrate salts that act as a redox mediator in solution. Specifically, nitrate salts of lithium,
magnesium, calcium, and manganese promote the selective photooxidation of BnOH to benzaldehyde on CdS in 70–100 %
–
1
–1
yields at rates up to 13.6 mM•h , compared to 8 % yield at 3 mM•h in the absence of a nitrate mediator. Kinetic analysis
reveals that in the absence of nitrate salts, the reaction is first order with respect to BnOH, while in the presence of nitrate,
the reaction is half order in BnOH. This rate law disparity, along with radical trapping and kinetic isotope experiments suggest
that nitrate-mediated alcohol oxidations proceed through a mechanism involving the catalytic generation of a nitrate radical,
•
NO
3
. The generation of this radical also enables the selective photooxidation of HMF to 2,5-diformylfuran at a rate of 2.6
–
1
mM•h using CdS nanowires.
precursors: 2,5-diformylfuran and 2,5-furandicarboxylic
acid.
INTRODUCTION
5
Biomass upgrading represents a unique opportunity for
the renewable generation of valuable chemicals and fuels
from abundant cellulosic waster mater. Feedstocks for
Given the high value of oxidized HMF products, significant
efforts have been directed at preparing efficient and
selective organic oxidation catalysts. Historically, some of
the best catalysts include enzyme-based biocatalysts,
homogeneous manganese and cobalt salts, as well as
1
6,7
these processes include residues separated from the
processing of agricultural products such as rice or
8
9
–
1 2
sugarcane, which total over 300 megatonnes•y . While
some of this matter is burned as fuel in processing facilities,
this is a low-value use of biomass waste. In particular, these
lignocellulose materials contain a variety of aromatic
species as well as nitrogen and oxygen functional groups
that can be used for the generation of value-added platform
carbon or aluminum supported Au, Pt, Pd, and Ru thermal
6,7,10
catalysts.
While these catalyst systems display high
yield and selectivity for oxidized HMF products, many
require high oxygen pressure, temperature, or the addition
of a precious metal co-catalyst. In addition to traditional
thermal catalysis, photo(electro)catalysts composed of
3
chemicals. One pathway to isolate a chemical precursor
metal oxides or sulfides can also deliver oxidized organic
from cellulose involves a series of hydrolysis and
dehydration reactions to yield 5-hydroxymethylfurfural
11–13
products.
Photocatalysis enables oxidative chemistry
under ambient conditions while using renewable solar
energy inputs. However, compared to the wealth of reports
on thermal HMF oxidations, there are fewer examples of
HMF photo(electro)chemical oxidations. Instead, most
reports apply benzyl alcohol (Figure 1, BnOH) as a model
4
(HMF). As shown in Figure 1, HMF contains significant
chemical
1
4–20
compound for the alcohol functionality in HMF.
fastest alcohol oxidation rates occur on CdS/graphite/TiO
The
2
ternary platforms and cyanamide functionalized graphitic
carbon nitride materials interfaced with molecular
electrocatalysis such as cobalt thioporphyrazine or nickel
1
4,21,22
Figure 1. Structure of the bio-derived HMF (left) and the model
bis(diphosphine) catalysis.
substrate used in this study, BnOH (right).
While interfacing electrocatalysts can significantly
improve the efficiency and activity of organic
photooxidations, this approach is limited by the cost and
preparation of the co-catalyst or the synthetic complexity of
complexity owing to its aldehyde, alcohol, and furan
functionalities. Through these functional groups, HMF can
be oxidized to a variety of species, including the polymeric
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a multi-component material. As an alternative to surface
Page 2 of 25
O
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23–25
catalyst
photochemical
incorporation,
many
electrochemical,
h460 nm
OH
H
(1)
2
6–28
+
O2
+
H2O2
and photoelectrochemical organic
CdS, MeCN
oxidation reactions rely on N-oxyl radical mediators such as
2,2,6,6,-tetramethylpiperidinyloxyl (TEMPO) to improve
their catalytic activity. Mediators like TEMPO function by
undergoing facile oxidation at a particle or electrode surface
followed by homogeneous charge transfer with the
substrate. This homogeneous charge transfer has unique
reactivity compared to direct heterogenous charge transfer,
enabling the oxidation of various inactivated
After two days of irradiation in a BnOH solution without
any mediator, the CdS nanowires produced a trace quantity
of benzaldehyde in 7 ± 0.5 % yield. Upon adding 50 mM
lithium nitrate, the benzaldehyde yield increased by an
order of magnitude to 70 ± 10 %. Kinetic analysis performed
during the first eight hours of the reaction show that initial
rate for benzaldehyde production increases from 3 to 10
mM•h when lithium nitrate is added. Gas chromatography
reveals that benzaldehyde is the only product detected,
giving rise to >99 % selectivity (Figure S1). Control
reactions performed without CdS, in the dark, or under
nitrogen display significantly suppressed benzaldehyde
yields, highlighting that this is a photocatalytic process that
requires CdS, lithium nitrate, oxygen, and light (Figure S2).
–
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9,30
compounds.
ZnO and Fe
oxidation rate increases in the presence of N-oxyl
In particular, reactions employing TiO
2
,
2
O have demonstrated significant alcohol
3
26–28
mediators.
Alternatively, Ni-modified CdSe quantum
dots may also mediate alcohol oxidations through their
3
1
thiol-based capping ligands. While the catalytic properties
of N-oxyl mediators have been well established, there are a
variety of redox mediators including simple organic
To elucidate further the importance of nitrate in the
reaction, we conducted a control experiment with LiPF
6
compounds and commercially available salts that have not
yet been fully investigated.32–34
(nitrate-free). After two days of photocatalysis using this
+
alternate Li salt, the benzaldehyde yield drops to 18 ± 7 %,
One particularly promising mediator class that may have
applications in photocatalysis are nitrate salts. Previous
electrochemical work has demonstrated nitrate-mediated
–
demonstrating that NO
3
is critical to the observed catalysis.
Based on these control experiments and established nitrate
radical chemistry (vide supra), we hypothesize that CdS
3
5–37
alcohol oxidations on carbon and platinum electrodes.
In addition to electrochemical mediation, the nitrate
–
oxidizes NO
3
3
to the NO • nitrate radical, which then
mediates BnOH oxidation to benzaldehyde.
–
ion/nitrate radical couple (NO
atmospheric volatile organic compound oxidations.
3 3
/NO •) is involved in
3
8,39
While the low photocatalytic BnOH activity of LiPF6
This
solutions supports that nitrate is required for the observed
reactivity towards organic compounds has also been
applied to solution oxidations, where nitrate radical
+
catalysis, we next sought to determine if Li is required for
the observed reactivity. The nitrate salts of calcium,
magnesium, and manganese are at least partially soluble in
acetonitrile solvent at 50 mM. Figure 2 (red bars) shows
generation occurs by laser flash photolysis or radiolysis of
40–42
dissolved nitrate salts.
In addition to these generation
methods, photochemical nitrate radical formation is
43
possible by TiO
2
in aqueous nitrate solutions. Despite
these promising characteristics, alcohol oxidation mediated
4
4
2
by nitrate has been limited to gas phase TiO applications
4
5
and homogenous oxidations on organic dyes. However,
these acridinium-based light absorbers are challenged by
their inherent reactivity towards nitrate radicals.
To test the ability of nitrate-based mediators to promote
primary alcohol oxidation reactions, we first employed
4
6
BnOH as a bio feedstock model substrate. CdS nanowires
act as the photocatalyst, and have the advantages to its
excellent photophysical properties and established
4
7–49
reactivity with organic substrates.
We carried out
reactions in acetonitrile to minimize aqueous hydroxy
5
0
radical formation and to maximize substrate solubility.
Herein, we show that unfunctionalized CdS nanowires are
capable of yielding benzaldehyde at maximum rate of 13.6
–1
mM•h from 2 mL solutions containing 0.1 mmol of
Mn(NO and 1 mg CdS illuminated with 170 W 465 nm
Figure 2. Average benzaldehyde yield (red) from a two day
irradiation of CdS suspensions in acetonitrile with the listed
meditator at 50 mM, and the electrical conductivity of the
solution (black). Error bars represent the standard deviation
among 3 trials.
3 2
)
blue LED light. Moreover, we validate using BnOH as a
model by also demonstrating that HMF is selectively
oxidized to 2,5-diformylfuran (DFF) under the same
–
1
conditions at a rate of 2.6 mM•h .
that all of these metal cations catalyze BnOH oxidation to
RESULTS
benzaldehyde in 70-100
tetrabutylammonium cation (Bu
yield, only 10 ± 7 %, nearly that that of the unmediated
reaction. The low Bu NNO reactivity may be rationalized
by measuring solution conductivity, which directly relates
%
yield.
However, the
+
4
N ) displays a much lower
To test the catalytic activity of nitrate-based redox
mediators for organic oxidations on semiconductors, we
performed BnOH photocatalysis as shown in Equation 1.
4
3
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ACS Catalysis
species, enabling BnOH oxidation. Moreover, having either
to ion activity. Solution conductivity measurements reveal
that lithium, calcium, magnesium and manganese nitrate
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4 6
Bu N or PF in solution does not inhibit the reaction.
+
–
show conductivity values ≤ 1.2 mS/cm, while both the
Bu NNO and LiPF solutions display conductivities of ~5
4 3 6
mS/cm (Figure 2, black bars).
To probe the nature of nitrate-enhanced BnOH oxidation
further, we analyzed the initial rate during the first eight
In addition to conductivity measurements, solution FT-IR
–
1
analysis in Figure 3 shows an ~10 cm bathochromic
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Figure 4. Concentration of BnOH over time, upon CdS
photocatalysis of LiPF (black), Bu
6
4 3 6
NNO (red), LiPF (green),
or a Bu NNO and LiPF mixture (blue) in acetonitrile solutions.
4
3
6
hours of the reaction using gas chromatography to quantify
BnOH consumption (Figure S4). Figure 5a shows that
Figure 3. FTIR (a) and UV-Vis (b) spectra of LiNO
Bu NNO in MeCN.
3
and
4
3
shift of the LiNO
3
N-O frequency compared to that of
Bu NNO . Furthermore, UV-Vis spectra acquired of the
4
3
same solutions (Figure 3b) display a hypsochromic shift in
the electronic absorption, as has been previously
Figure 5. Kinetic plots for illumined CdS samples in solutions
with various BnOH concentrations for mediator-free (a) and
demonstrated with Ca(NO
)
3 2
.51
LiNO
varying LiNO
3
containing solutions (b); as well as solutions with
(c) or CdS loading (d). The highest concentration
3
Figure 4 shows that individually, LiPF
6
4 3
and Bu NNO are
were omitted from analysis due to surface site saturation.
inert toward photocatalytic BnOH oxidation (black and red
profiles, respectively). However the photocatalytic reaction
without any mediator, the reaction is first order in BnOH
of a solution containing both LiPF
catalytic profile closely resembling that observed for LiNO
blue and green profiles, respectively). This experiment is
repeated in Figure S3. This recovery of catalysis supports
that metathesis in solution generates the active LiNO
6 4 3
and Bu NNO yields a
However, with LiNO
in BnOH (Figure 5b). In addition, the reaction is first order
in LiNO (Figure 5c) and zero order in CdS near our 10 mg
3
in solution, the reaction is half order
3
(
3
loading (Figure 5d). The zero-order photocatalyst
dependence likely arises because of light scattering (Figure
3
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ACS Catalysis
occur in this system (Figure S16), in contrast to TiO
Page 4 of 25
2
BnOH
S5). In accord with the half-order dependence on [BnOH],
the slope of the plot of [BnOH] vs. t starting from 250 mM
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photocatalysis.
12
BnOH is linear over the first 10 h of the reaction and
provides an initial rate constant ~ 0.6 mM h (Figure S6).
Nitrate-mediated CdS photocatalysis also oxidizes HMF, a
true bio-derived substrate, as illustrated in equation 3.
1
/2 –1
While the catalytic behavior we observe with CdS and
–
NO
3
is indicative of NO
3
• formation, we do not directly
observe any adsorbed nitrate species by ex-situ Raman
spectroscopy (Figure S7), infrared spectroscopy (Figure
S8), X-ray photoelectron spectroscopy (Figure S9), or in situ
electron paramagnetic resonance spectroscopy (Figure
3
S10). We surmise that any NO • that is formed is very
0
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reactive, and therefore short lived. While Raman analysis
displays no adsorbed nitrate signals, a small change in the
Raman overtone intensity, attributed to CdS surface trap
52
state formation was observed. We propose that these trap
states are a result of CdS corrosion, which has been
previously
observed
during
non-aqueous
CdS
53
photocatalysis. Emerging infrared stretching frequencies
are also consistent with the formation of a sulfate species
(Figure S8). Ultimately, scanning electron microscopy
imaging shows a significant morphological change in the
CdS nanowires after the photocatalytic reaction with BnOH
(Figure S11), corroborating their corrosion. In addition to
this loss of morphology, inductively coupled plasma mass
spectrometry carried out on solution after the reaction
detects leached cadmium at 106 ± 5 ppm. X-ray
photoelectron spectroscopy (XPS) also confirms surface
corrosion by the appearance of 48 ± 2 % sulfate (Figure
S12). Further XPS experiments reveal detectable lithium on
the particle surface in the form of lithium sulfate, which
Figure
photocatalysis of a mediator free solution (black), a mediator
free solution with TME (blue), a LiNO solution with TME
(green), and a LiNO solution (red).
6
Concentration of BnOH over time, upon
3
3
+
2+
would result from Li transmetalation of Cd . The presence
of crystalline lithium sulfate was further confirmed through
powder X-ray diffraction (PXRD) on the CdS nanowire
photocatalysts after the reaction (Figure S13).
Therefore, in order to further ascertain the function of
nitrate in this system, we performed photocatalytic BnOH
oxidation in the presence of a nitrate radical trap,
54,55
tetramethylethylene (TME).
reaction
Equation 2 shows one
O
NO3
+
+
NO2
(2)
3
between NO • and TME, olefin epoxidation that liberates
NO . Figure 6 shows that when TME is added to a nitrate-
2
free BnOH solution, the photocatalytic reaction rate is
unaffected (blue trace vs black trace; Figure S14). However,
when TME is introduced into LiNO
3
containing solution, the
Figure 7. Concentration of BnOH over time during CdS
photocatalysis with LiNO in acetonitrile when using
catalytic activity significantly decreases (red vs. green
traces), and the reactivity reverts to that of unmediated
reaction (black trace).
3
2
protonated (black) and deuterated (α,α-d ) benzyl alcohol
(red).
To elucidate the key steps in which this nitrate radical
facilitates BnOH conversion, we investigated the kinetic
behavior of a deuterated BnOH substrate. Upon CdS
photocatalysis of a LiNO
3
solution containing benzyl
alcohol-,-d , Figure 7 shows a negligible kinetic isotope
2
effect (KIE) of 1.2 ± 0.3 for deuterated benzaldehyde
production (Figure S15), indicating that -deprotonation is
3 2
Figure 8 shows that in the presence of Mn(NO ) ,
quantitative conversion to 2,5-diformyl-furnan (DFF)
not
rate-determining.
Additionally,
CdS
BnOH
yields
–
photocatalysis performed in the presence of 8O
1
occurs between 24 and 48 h with an initial rate of 2.6 mM•h
2
1
1
8
. When the nitrate mediator was removed from the
benzaldehyde with no O incorporation, suggesting that
oxygen atom transfer from reduced oxygen species does not
solution, no HMF oxidation was observed (Figure S17).
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DISCUSSION
ACS Catalysis
alcohols are a far more lucrative substrate since they are
bioavailable from agricultural feedstocks, and their
oxidized products have value In particular, significant
efforts are aimed at cellulose-derived HMF oxidation to
yield products including 2,5-furandicarboxylic acid and 2,5-
diformylfuran (DFF), as valuable precursors for furan-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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2
2
3
3
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4
4
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4
4
4
5
5
5
5
5
5
5
5
5
5
6
Expanding selective catalysts for organic oxidations to
include heterogeneous photocatalysts is challenging
because of the low activity of unfunctionalized light
absorbers. Consequently, most preliminary photocatalytic
transfor-
.
59
60
61
based polyesters and furanic poly Schiff bases.
Table 1 compares the BnOH and HMF activity of our
nitrate mediated CdS platform with representative
examples of other heterogeneous photocatalysts. These
materials highlight the various classifications of
heterogenous photocatalytic materials active for alcohol
oxidation, including: unfunctionalized metal oxides,
multijunction catalysts, photocatalysts modified with
precious metal or homogeneous catalysts, and reactions
performed with redox mediators. In these studies, BnOH is
a biomass model compound and displays high selectivity for
aldehyde formation over carboxylic acid products. While
catalyst and substrate loading are not standardized, the
reaction rate typically increases with the use of complex
photocatalyst design or with metal co-catalysts. Generally,
the highest reported rates for alcohol oxidation are
0
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3
4
5
6
7
8
9
0
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2
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4
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5
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0
62–64
observed
with
multi-junction
CdS,
cyanamide
functionalized graphitic carbon nitride materials interfaced
with molecular electrocatalysis such as cobalt
2
2,65
thioporphyrazine or nickel bis(diphosphine) catalysis,
3
Figure 8. Concentration of HMF over time using Mn(NO) as a
mediator. Data for four separate trials are presented.
and dye-sensitized ZnO photochemistry performed in the
presence of the N-oxyl redox mediator, TEMPO.
mations target amine-based substrates due to their
Compared to these previously reported examples, we find
that our optimized conditions comprising 2 mL acetonitrile
containing 50 mM Mn(NO ) with 1 mg of suspended CdS
3 2
56
electron-rich functionality, which leads to facile oxidation.
57
11
58
47–49,53
Materials such as BiVO
4
,
CuWO
4
,
TiO
2
and CdS
establish that semiconducting photocatalysts can
selectively deliver imines from amines. While amines serve
as an excellent test reaction for evaluating photocatalysts,
nanowires with 250 mM BnOH substrate yields
–
benzaldehyde exclusively with an initial rate of 13.6 mM•h
,
1
which is competitive with some of the most active systems
Table 1. Comparison of various heterogeneous alcohol oxidation photocatalysis. Rates were calculated from the kinetic data
presented in the listed references
Photocatalyst
Solvent
Water
Substrate
Me-BnOH
BnOH
HMF
Product
Me-BnAl
BnAl
Selectivity / %
Rate / mM•h–1
15
TiO
2
60
>99
65
0.21
3.3
0.16
7.1
16
2
1
7
HNb
3
O
8
BTF
P-Zn
X
2
Cd1–xS14
Water
DFF
2
0
In
S
3–X
BTF
BnOH
BnOH
BnOH
FFA
BnAl
>99
90
62
CdS/Graphite/TiO
2
BTF
BnAl
64
WO
3
/TiO
2
Water
BnAl
56
Ni/CdS63
Water
Furfural
BnAl
>99
>99
97
2
1
6
Au/CeO
CoPz, g-C
Ni-P, g-C
2
Water
BnOH
HMF
0.8
1.6
6.6
3
2
2
3
N
4
Water
FDCA
MeO-BnAl
BnAl
19
3
N
4
Water
MeO-BnOH
BnOH
BnOH
BnOH
BnOH
HMF
>99
98
Ni / MPA-CdSe31
DS-ZnO27
Water
Water / TEMPO / Ag+
BnAl
>99
98
27
4
2
6
DS-TiO
CdS
2
BTF / TEMPO
BnAl
MeCN / Mn(NO
MeCN / Mn(NO
3
)
)
2
2
BnAl
>99
>99
13.6
2.6
CdS
3
DFF
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Page 6 of 25
Abbreviations: BnOH = Benzyl Alcohol, BnAl = Benzaldehyde, HMF= 5-hydroxymethylfurfural, DFF = 2,5-diformylfuran, FFA
= furfuryl alcohol, MeO-BnOH = 4-methoxy benzyl alcohol, MeO-BnAl = 4-methoxybenzaldehyde, Me-BnOH = 4-methyl benzyl
alcohol, Me-BnAl = 4-methyl benzaldehyde, BTF = trifluorotoluene, MeCN = acetonitrile, CoPz = cobalt thioporphyrazine, Ni-
P = nickel bis(diphosphine) DuBois catalyst, MPA-CdSe = mercaptopropionic acid-capped CdSe quantum dots, DS-ZnO and
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2 2
DS-TiO = dye sensitized ZnO and TiO , respectively.
reported to date, without the use of metal co-catalysts or
multi-phase photocatalysts. We note that optimization
involves improving the photocatalyst dispersion using a 10-
minute sonication (Figure S18), an enhancement that has
been previously reported for carbon nitride. While the
rate of BnOH oxidation in this system is very competitive,
corrosion of the CdS material as shown through SEM
imaging, XPS-, PXRD-, and ICP-MS analysis (vide supra)
limits the recyclability of the photocatalyst material. The
photocatalytic activity of the CdS nanowires continuously
drops over the five days that it shows activity (Figure S19).
3
as NO • radical oxidizes benzyl alcohol. This reaction order
and proposed mechanism are similar to what is proposed
for electrochemical BnOH oxidation mediated by
2
3
(bpy)Cu/TEMPO, where the catalytic current is half order
in BnOH substrate. These results agree with established
electrokinetic models of homogeneous systems limited by
2
1
0
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2
3
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2
3
4
5
6
7
8
9
0
,
68
chemical steps involving the catalyst. and also match
what has been observed experimentally with adsorbed
–
69
CrO4 reduction on TiO2.
The fractional reaction order suggests that BnOH
oxidation on our LiNO /CdS system proceeds through a
3
+
2+
2+
2+
While nitrate salts of Li , Mg , Ca , and Mn catalyze
alcohol oxidation, Bu NNO has no effect on the reaction
different mechanism than on CdS, we next sought to obtain
evidence for the proposed nitrate radical intermediate.
Previous reports indicate that TME is highly sensitive to the
presence of nitrate radical and will quickly undergo
oxidation to a variety of oxygenated products, including
4
3
rate (Figure 2). We rationalize this reactivity by considering
the solution conductivity and hard soft acid- base concepts.
In acetonitrile, hard metal ions tend to bind tightly to the
hard nitrate ion, leading to generally low solubility. The soft
5
4,55
epoxides as shown in equation 2.
The kinetic
+
–
Bu
Bu
4
N cation and soft PF
6
anion lead to high solubility for
experiments in Figure 6 reveal that TME inhibits the
photocatalytic activity of nitrate-containing solutions,
suggesting that any generated nitrate radical preferentially
reacts with TME over BnOH. Importantly, TME has no effect
on the native, nitrate-free CdS BnOH activity, highlighting
that TME does not compete with BnOH for surface sites.
Based on the established reactivity of alkenes as traps for
nitrate radical and the suppression of nitrate catalysis in the
presence of TME, we propose that catalytic nitrate oxidation
to nitrate radical is an elementary step in catalytic 1° alcohol
oxidation.
4
NNO and LiPF . We find that catalysis results only from
3
6
the partially associated ionic solids. Moreover, in situ
formation of LiNO from the metathesis of the soluble salts
6
Bu NNO and LiPF gives rise to nearly the same reactivity
3
4
3
and suppressed conductivity (Table S1) observed in the
pristine LiNO solution. That is, the other cations do not
3
inhibit substrate binding or subsequent oxidation.
FT-IR and UV-Vis spectroscopy of Li+ and Bu
N+
4
acetonitrile solutions corroborated an associated metal
cation-nitrate interaction. Figure 3a shows a bathochromic
shift in the N-O stretching vibration in LiNO
3
compared to
In addition to nitrate oxidation to nitrate radical, there
are a diverse set of electrochemical pathways in which
nitrate may serve as an oxidizing agent through the
formation of either nitrous oxide, nitric acid, or nitrogen
dioxide (Figure S20). While stoichiometric consumption of
nitrate is unlikely as catalysis yields benzaldehyde
concentrations higher than the lithium nitrate
concentration, we sought to quantify nitrate consumption
through gravimetric analysis. An excess of sodium
perchlorate was added to a series of solutions isolated after
photocatalysis, yielding a white precipitate shown to be
sodium nitrate by PXRD analysis (Figure S21). Based on the
isolated solid, we calculated a nitrate recovery of 90 ± 10 %,
supporting that nitrate is not consumed in this reaction.
While gravimetric analysis supports the catalytic nature of
nitrate under aerobic conditions, we found a significant
drop in solution nitrate concentration in those reactions
2
+
Bu
and NO
4
NNO
3
–
, suggesting a stronger interaction between Li
in acetonitrile. Similar IR spectra shifts have been
3
reported for the effect of water coordination on differently
–
66
hydrated nitrate ions, NO
evidence of a strong Li-NO
cations also result in a hypsochromic shift the NO
absorption profile as shown in Figure 3b. Previous UV-Vis
spectra reported for Ca(NO shows that a stronger cation-
3
(H
3
2
O)1–6
.
In addition to FT-IR
+
interaction, we find that Li
–
3
UV-Vis
3 2
)
nitrate interaction blue shifts the absorption due to the
reduced symmetry and conjugation of a metal-nitrate
species compared to free or more weakly coordinated
5
1
ntirate. The exact nature of this interaction and its
importance to the photocatalytic oxidation of BnOH on CdS
is unclear. We hypothesize that the relatively stronger
4 3
metal-nitrate interaction (compared to Bu NNO ) may
facilitate nitrate radical desorption from CdS required to
mediate homogeneous electron transfer.
carried out under N . This nitrate consumption suggests
2
that the small PhCHO yields detected under N are due to
The reaction order with respect to BnOH substrate differs
whether or not metal nitrates are present, suggesting that
BnOH oxidation coupled with stoichiometric nitrate
oxidation. Investigations into the nature of anaerobic BnOH
mediation is a subject of ongoing study in our laboratory.
From the kinetic experiments outlined above, we propose
the mechanism shown in Error! Reference source not
found.. Principally,
the reactions proceed by different mechanisms. Without
–
the NO
3
mediator, the reaction is first-order in BnOH, but
present, we observe a half-order dependence in
with LiNO
3
BnOH (Figure 5). Without nitrate, the CdS activity is likely
limited by heterogeneous charge transfer to BnOH as is
67
commonly proposed in photocatalytic reactions. We
propose that the half-order dependence with nitrate arises
ACS Paragon Plus Environment

Page 7 of 25
ACS Catalysis
interaction. These results present the application of simple
1
2
3
4
5
6
nitrate salts to mediate oxidations relevant to biomass
upgrading, and present new opportunities and challenges
for continued studies of mediated photocatalytic systems.
EXPERIMENTAL
Materials and Methods. All solvents were purchased
from Sigma Aldrich and used without further purification.
All alcohols were purchased from Fisher scientific, the
mediator salts and deuterated benzyl alcohol were
purchased from Sigma Aldrich. Nylon Syringe filters (25
mm, 0.2 mm membrane) were purchased from VWR. To
Scheme 1. Proposed mechanism for LiNO
photocatalytic oxidation of benzyl alcohol.
3
mediated
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
photocatalytic oxidation of nitrate to a nitrate radical
species occurs. Cations such a lithium participate either in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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0
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3
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5
6
7
8
9
0
nitrate
absorption/desorption
or
intermediate
3 3
achieve 50 mM LiNO concentrations mixtures of LiNO ,
stabilization. In accordance with previous mediated
mechanisms we suggest that the nitrate radical desorbs
from the surface and interacts with BnOH in a rate
MeCN and BnOH were prepared and then sonicated under
heat (~50 ℃ ) for 20 minutes to give a clear, colorless
solution.
3
2–34
determining electron transfer process.
The nitrate
radical intermediate likely reacts with BnOH in an
electrochemical process without coupled α-proton
abstraction, as no significant KIE (1.2 ± 0.3) was observed
Preparation of CdS Nanowires. CdS Nanowires (NWs)
were prepared by adopting
a previously reported
71
hydrothermal synthesis. Cadmium nitrate tetrahydrate
(751 mg, 2.4 mmol), thiourea (556 mg, 7.3 mol), and
ethylenediamine (13 mL) were mixed in an 18 mL PTFE
lined autoclave. The vessel was sealed and warmed to 160
˚C at a of 10 ˚C/min ramp rate. After two days at 160 ˚C the
reaction vessel was cooled naturally to room temperature
to give a yellow powder suspended in an orange solution.
This suspension was centrifuged at 4000 rpm for 5 minutes
to isolate a bright yellow powder. This powder was washed
four times with Millipore (MQ) water (4 × 15 mL), and then
dried overnight in a vacuum oven at 50 ˚C.
(
Figure 7). Further studies are required to elucidate the
mechanistic steps following nitrate-meditated substrate
18
oxidation. However, given that isotopically labeled
does not support oxygen-atom transfer from reduced
oxygen in this reaction (Figure S16), it is unlikely that
O
2
previously proposed mechanisms for non-aqueous TiO
2
1
2, 70
photocatalysis are operative.
We surmise that
superoxide radical generated from photogenerated
electrons serves as a base and another nitrate radical
mediates the second electron transfer necessary for
benzaldehyde formation.
Characterization Details. Scanning electron microscopy
(SEM) images were obtained from a Zeiss LEO 1455VP
tungsten filament SEM at an accelerating voltage of 3 kV and
working distance of 5 mm. Powder X-ray diffraction (PXRD)
Through these studies we have established that nitrate
salts dissolved in acetonitrile are capable of mediating
BnOH oxidation. To test the ability of other common
mediators for CdS promoted BnOH oxidation we performed
photocatalysis in solutions containing triarylamines,
polycyclic aromatic hydrocarbons, perchlorate and halide
data was collected on
a
Panalytical Empyrean
diffractometer at a power of 1.8 kW (45 kV, 40 mA) with Cu
K흰 (흺 = 1.5418 nm) radiation. The diffractometer was a
3
2–34
salts (Figure S22).
After two days of illumination, we
X’Celerator Scientific, a position-sensitive 1D detector and
HD
found that of the tested mediators, only lithium salts of
bromine and chlorine displayed improved benzaldehyde
conversions of 60 ± 10 % and 62 ± 9 %, respectively. While
these halide mediators promote BnOH oxidation, PXRD
carried out after the reaction revealed significant CdS
corrosion in lithium halide solutions compared to nitrate
(Figure S13).
was equipped with a Bragg-Brentano
X-ray optic
delivering only K흰 radiation. Patterns were collected with a
sampling step of 0.017 ° and a scan rate of 6 °/min while
spinning at a rate of 0.25 Hz. X-ray photoelectron
spectroscopy (XPS) was conducted using a Kratos Axis Ultra
X-ray Photoelectron Spectrometer. The X-rays used were
monochromatic Al K흰 X-rays (1486.7 eV). All data collected
–
9
while the analysis chamber pressure was ~ 1 × 10 torr.
The charging effects of the semiconducting substrates were
compensated using an electron flood gun. High-resolution
spectra were collected at a pass energy of 20 and a step size
of 0.1 eV. XPS spectra interpretation was performed using
Casa XPS with a Shirley-type baseline to calculate peak
areas. Binding energies were calibrated by positioning the
adventitious carbon signal at 248.8 eV. Raman spectra were
collected on a Renishaw inVia microscope equipped with a
CONCLUSIONS
Photocatalytic BnOH oxidation on CdS particles is
significantly enhanced in the presence of either lithium,
magnesium, manganese, or calcium nitrate salts. These
reactions yield benzaldehyde selectivity at a rate of
–
1
1
3.6 mM•h . Through a series of kinetic and trapping
experiments we propose that benzaldehyde is produced
through a mechanism involving the formation of a nitrate
radical which oxidizes the alcohol substrate rather than
direct oxidation by photogenerated holes. In addition to
promoting BnOH oxidation, this nitrate radical mediator
5
32 nm laser. Fourier transform- infrared spectroscopy was
performed on a Thermo-Nicolet IS-50 spectrometer
equipped with an ATR accessory. UV-VIS spectra were
acquired on a Cary 5000 UV-Vis NIR Spectrophotometer.
Conductivity measurements were obtained using a Mettler-
Toledo FEPO30 conductivity meter equipped with an InLab
can direct the course of HMF oxidation to selectively deliver
–1
2
,5-diformylfuran at a rate of 2.6 mM•h . Through the
course of this study, we have determined that
tetrabutylammonium cations are not compatible with
nitrate-based catalysis, likely due to a weaker nitrate
7
10 probe at room temperature.
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Page 8 of 25
Leached Cadmium Detection. Cadmium concentration reaction mixture and combining with a chlorobenzene
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
in the post photocatalysis solution was determined via
inductively coupled plasma mass spectrometry (ICP-MS)
using a Perkin-Elmer Nexion 2000 ICP-MS. The post
photocatalysis suspension were filtered through a syringe
filter, and rinsed with acetonitrile three times (3 x 1 mL).
Volatile organics were then removed by rotary evaporation
to yield a viscous oil. This oil was dissolved in Milli-Q water
and filtered through a new syringe filter. Samples were
referenced to an external bismuth standard (20 ppb) and
concentrations of cadmium were determined via a series of
standard cadmium solution prepared from a 1000 ppm Cd
reference solution (Sigma Aldrich).
standard (50 휇L of a 21.6 mM standard) and acetonitrile
(940 휇L). Before aliquot collection, the solution mass was
measured, and any evaporation was corrected by adding an
appropriate amount of acetonitrile. Before returning the
samples to the reaction vessel, the oxygen balloon was
refilled with fresh oxygen. The samples were analyzed on
either a Shimadzu QP-2010 GC/MS or Thermo Fisher Trace
13-10 gas chromatograph equipped with a capillary column
and FID detector. During analysis the column oven was
warmed from 40 to 290 ℃ at a 10 ℃ /min ramp rate.
Quantification was enabled by the integration ratio of the
analyte and chlorobenzene signals in relation to an obtained
calibration curve.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In situ EPR Spectroscopy. In situ EPR spectroscopy was
performed on a Bruker EMX electron spin resonance
spectrometer at room temperature. Spectra were recorded
both in the dark and after 20 minutes of illumination. The
resonator cavity used was a 4102-ST cavity. Illumination
was provided by a Newport-Oriel 150 W Xe arc lamp to give
Gravimetric
Nitrate
Determination.
Nitrate
concentrations were determined by addition of excess
sodium perchlorate after the photocatalytic reaction. The
CdS photocatalyst as separated from the reaction mixture
by centrifuging the reaction mixture at 4000 rpm for 5
minutes. The reaction solution was transferred to a tared
vial and the CdS was washed with MeCN (3x 2 mL). These
washes were combined and a solution of sodium
perchlorate (1 M, 4 mL) was added to produce a white
precipitate. This suspension was left overnight in a
refrigerator. The white solid was then isolated by
centrifuging and washed with MeCN (3 x 2 mL). The
resulting powder was dried overnight in a vacuum oven at
50 ˚C and was confirmed to be sodium nitrate by PXRD
analysis.
–
2
a power of ~200 mW cm as measured by an optical power
meter (Newport 1918-R) equipped with a thermopile
detector (Newport 818P-015-19). The microwave
frequency was set to 9.822 GHz with a power of 20.51 mW.
The data was collected with the receiver set to a modulation
amplitude of 1 G and a modulation frequency of 100 kHz.
The data was converted to g-factor using the following
conversion:
(
ℎ × 휈)
푔 =
(
휇퐵 × 퐵0
)
where h = planks constant, 휐 = microwave frequency, 휇
B
is
ASSOCIATED CONTENT
the Bohr magneton, and B
strength.
0
is the applied magnetic field
Photochemistry. CdS (1-40 mg) was suspended in
acetonitrile (2 mL) in a borosilicate 15 x 44mm OD 4 mL vial
(Kimble) equipped with a stir bar. Unless otherwise noted,
all experiments was performed at 250 mM substrate and 50
mM mediator. The vials were capped with a septum
AUTHOR INFORMATION
Corresponding Author
*
(B.M.B) * E-mail: bartmb@umich.edu
Author Contributions
(
Chemglass 14/20-14/35) and kept under an oxygen
The manuscript was written through contributions of all
authors. / All authors have given approval to the final version
of the manuscript.
atmosphere using a balloon. Prior to illumination, we found
that 10 minutes of sonication significantly improves the
dispersion and performance of the photocatalyst material.
The reaction mixture was placed in a custom-built
aluminum LED photoreactor (Figure S23). This
Supporting Information
Additional kinetic data, Raman, FTIR, EPR, XPS, FTIR, mass
spectra, solution conductivity measurements, images of CdS
dispersion, gravimetric analysis results, and images of a
custom-built LED photoreactor. The Supporting Information is
available free of charge on the ACS Publications website.
photoreactor is equipped with five royal blue LEDs (휆max
=
4
60 nm) with a power rating of 1.03 W (Mouser Electronic,
Part: 997-LXML-PR02-A900). These LEDs were reflow
soldered onto a SinkPAD star board (Adura LED Solutions,
Part: 1903), aligned in series and powered using a 12 W
700mA constant current LED driver (semperlite, Part: APC-
12-700). While in use, the photoreactor was cooled using a
fan to an average temperature of 29.9 ± 0.5 ˚C. The average
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences, Catalysis Science
Program, under Award # DE-SC0006587. For the XPS
instrument, the authors acknowledge the financial support of
the University of Michigan College of Engineering and NSF
grant #DMR-0420785 and technical support from the Michigan
Center for Materials Characterization. In particular, we thank
Drs. Kai Sun, Haiping Sun, and Bobby Kerns for assistance.
AGBA thanks the University of Michigan Rackham Graduate
School for funding through a Rackham Merit Fellowship.
–
2
light intensity was measured to be 170 ± 10 mW cm . For
1
8
experiments performed under
2 3
O , a LiNO and BnOH
solution in MeCN was prepared and degassed through six
cycles of freeze-pump-thaw. This degassed solution was
brought into a N glovebox where 2 mL was dispensed into
2
a vial loaded with 10 mg CdS. This vial was sealed with a
septum, brought out of the glovebox and a balloon
18
containing
2
O (97 atom %, Aldrich) was affixed on top.
Substrate and product concentrations were measured using
GC-FID by periodically removing an aliquot (10 휇L) from the
ACS Paragon Plus Environment

Page 9 of 25
ABBREVIATIONS
ACS Catalysis
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
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3
4
4
4
4
4
4
4
4
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4
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5
5
5
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5
5
6
BnOH, Benzyl alcohol; BnAl, Benzaldehyde, HMF, 5-
hydroxymethylfurfural; DFF, 2,5-diformylfuran; TME,
tetramethylethylene, FFA = furfuryl alcohol, MeO-BnOH = 4-
methoxy benzyl alcohol, MeO-BnAl = 4-methoxybenzaldehyde,
Me-BnOH = 4-methyl benzyl alcohol, Me-BnAl = 4-methyl
benzaldehyde, BTF = trifluorotoluene, MeCN = acetonitrile,
2
Au/CeO Exhibiting Strong Surface Plasmon Resonance Effective
for Selective or Chemoselective Oxidation of Alcohols to Aldehydes
or Ketones in Aqueous Suspensions under Irradiation by Green
Light. J. Am. Chem. Soc. 2012, 134, 14526–14533.
1
7) Liang, S.; Wen, L.; Lin, S.; Bi, J.; Feng, P.; Fu, X.; Wu, L.
Monolayer HNb for Selective Photocatalytic Oxidation of
3 8
O
CoPz
=
cobalt thioporphyrazine , Ni-P
=
nickel
Benzylic Alcohols with Visible Light Response. Angew. Chemie. Int.
Ed. 2014, 53, 2951–2955.
bis(diphosphine)
1
8) Qamar, M.; Elsayed, R. B.; Alhooshani, K. R.; Ahmed, M. I.;
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ACS Paragon Plus Environment

ACS Catalysis
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ACS Paragon Plus Environment

Page 21 of 25
ACS Catalysis
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ACS Paragon Plus Environment

ACS Catalysis
Page 22 of 25
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ACS Paragon Plus Environment

Page 23 of 25
ACS Catalysis
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ACS Paragon Plus Environment

ACS Catalysis
Page 24 of 25
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ACS Paragon Plus Environment

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ACS Catalysis
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ACS Paragon Plus Environment