Quantitative Determination of Pt- Catalyzed d -Glucose Oxidation Products Using 2D NMR

Article abstract of DOI:10.1021/acscatal.8b03838

Quantitative correlative ¹H-¹³C NMR has long been discussed as a potential method for quantifying the components of complex reaction mixtures. Here, we show that quantitative HMBC NMR can be applied to understand the complexity of the catalytic oxidation of glucose to glucaric acid, which is a promising bio-derived precursor to adipic acid, under aqueous aerobic conditions. It is shown through 2D NMR analysis that the product streams of this increasingly studied reaction contain lactone and dilactone derivatives of acid products, including glucaric acid, which are not observable/quantifiable using traditional chromatographic techniques. At 98% glucose conversion, total C₆ lactone yield reaches 44%. Furthermore, a study of catalyst stability shows that all Pt catalysts undergo product-mediated chemical leaching. Through catalyst development studies, it is shown that sequestration of leached Pt can be achieved through use of carbon supports.

Full text of DOI:10.1021/acscatal.8b03838

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Article
Quantitative determination of Pt- catalyzed D-
glucose oxidation products using 2D NMR
Robert David Armstrong, Jun Hirayama, David W Knight, and Graham J. Hutchings
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03838 • Publication Date (Web): 30 Nov 2018
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ACS Catalysis
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Quantitative determination of Pt- catalyzed D-glucose oxidation
products using 2D NMR
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R. D. Armstrong*†, J. Hirayama†‡, D. W. Knight†, G. J. Hutchings*†.
† Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Park Place, Cardiff, UK, CF10 3AT
‡ Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo 001-0021, Japan
ABSTRACT: Quantitative correlative 1H – 13C NMR has long been discussed as a potential method for quantifying the
components of complex reaction mixtures. Here we show that quantitative HMBC NMR can be applied to understand
the complexity of the catalytic oxidation of glucose to glucaric acid, which is a promising bio- derived precursor to
adipic acid, under aqueous aerobic conditions. It is shown through 2D NMR analysis that the product streams of this
increasingly studied reaction contain lactone and dilactone derivatives of acid products, including glucaric acid,
which are not observable/ quantifiable using traditional chromatographic techniques. At 98 % glucose conversion,
total C6 lactone yield reaches 44 %. Furthermore, a study of catalyst stability shows all Pt catalysts to undergo product-
mediated chemical leaching. Through catalyst development studies, it is shown that sequestration of leached Pt can
be achieved through use of carbon supports.
KEYWORDS Partial Oxidation, Quantitative 2D NMR, Glucose, Glucaric Acid, Gluconic Acid, Carbohydrates.
6, 27, 32, 35
categories; (i) pH controlled
and (ii) non pH con-
1. Introduction
trolled.21, 29-31, 33 Dirkx et al.,27 reported that strict control of pH
at 9-10 enhances the activity of Pt catalysts, which was at-
tributed to more rapid desorption of free- acid products from
the active site. Meanwhile Rennovia Inc. recently claimed glu-
caric acid yields of 66 % in the absence of pH control, over a
range of monometallic Pt and bimetallic AuPt catalysts.20, 29, 30
To date however, studies have failed to address either spectro-
scopic analysis of product streams or the long term stability of
catalysts operating under autogeneous pH. The latter is par-
ticularly pertinent as products of glucose oxidation; gluconic,
glucaric and tartaric acids, are known metal- sequestering
agents.27, 36 Indeed, Karski et al., reported leaching of M (M=
Bi, Tl, Sn and Co) and low levels of Pd when bimetallic Pd-M/
carbon and silica catalysts were used to catalyze the oxidation
of glucose to gluconic acid.37 This was attributed to the strong
chelating properties of gluconic acid.37
Combined environmental, economic and political pressures
are incentivising the drop-in integration of bio-derived, sus-
tainable feedstocks within the chemical industry. As a highly
abundant C6 monosaccharide, the selective oxidation of glu-
cose has received significant attention in recent years. In par-
ticular, research has focussed upon the selective oxidation of
glucose to gluconic acid, which is used in the food, pharma-
ceutical and paper industries. A number of heterogeneous,
precious metal catalysts have been reported to catalyze this
reaction using molecular oxygen as oxidant. These often com-
prise Pt, Pd and Au,1-6 and might also contain promoters such
as Bi.2, 7-9 Less extensively studied is the selective oxidation of
glucose to glucaric acid, an aldaric acid which was recently
recognised by the US Department of Energy as a key bio- de-
rived platform molecule.10
Glucaric acid is produced industrially through oxidation of
glucose with nitric acid, a process which dates back to 1888
and affords ca. 40% yield of the mono-potassium salt.11-13 By-
products for this reaction include oxalic, tartaric and 5-ke-
togluconic acid.12 With potential applications in food, phar-
maceutical and polymer industries, global demand for glu-
caric acid is predicted to increase through the 21st century.14-21
Processes for the enzymatic, electrocatalytic and heterogene-
ously catalysed synthesis of glucaric acid from glucose have
also been reported.19, 20, 22-33 The TEMPO- promoted oxidation
of glucose to glucaric acid (> 90% yield), with NaBr, bleach
and strict pH control has also been reported,34 however, these
reactions generate toxic by-products. Of the many approaches
taken, the heterogeneously catalyzed systems utilizing molec-
ular oxygen present the most atom efficient, inherently green
approach to glucaric acid production. Studies into heteroge-
neously catalyzed oxidation processes typically fall into two
Scheme 1. Oxidative conversion of glucose to glucaric
acid
Chromatographic techniques are favoured for separation and
quantification of the products of glucose oxidation and C6
products are accepted to form according to the pathways
shown in Scheme 1.19-21, 27, 29-33, 35, 38, 39 Glucaric acid is generally
accepted to be the terminal C6 oxidation product, with lower
molecular weight carboxylic acids formed through undesira-
ble C-C scission pathways. However, juxtaposed with high re-
ported mass balances in catalysis studies, are equilibria known
to establish between D-glucaric acid and its mono and di- lac-
tone derivatives; D-glucaro 1,4 lactone, D-glucaro 6,3 lactone
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Page 2 of 12
and D-glucaro 1,4: 6,3 dilactone under acidic conditions
tion products, substrate conversion and for identifying prod-
(Scheme 2).40 Whilst never reported as quantified products of
glucose oxidation, Boussie et al., implied that these derivatives
might form in situ under their reaction conditions.20 Mean-
while, reported short chain acid by-products of Pt- catalysed
aerobic glucose oxidation include tartronic acid, tartaric acid,
oxalic acid, formic acid, glycolic acid and glyceric acid.
ucts based on retention times. Indeed, these techniques are
favored for such processes, owing to the availability of a wide
range of stationary phases for the separation of low molecular
weight organic acids. Our previous studies into the prepara-
tion of crystalline D-glucaric acid showed its rapid lactoniza-
tion to occur at mild temperatures (< 50 oC), despite being in
an aqueous environment.43 Indeed, the pathways shown in
Scheme 2 have previously been studied using 1H NMR.40 How-
ever, whilst alluded to within the patent literature,30, 38, 44 no
study has as yet reported formation of lactone or dilactone de-
rivatives of D-glucaric acid under catalytic conditions, let
alone sought to quantify these potential reaction products.
Similarly, often overlooked are D-glucuronic and L-guluronic
acids, both glucose- derived uronic acids which have been re-
ported to undergo oxidation to yield glucaric acid.27, 45 In line
with previous studies these molecules and other potential C6
products of D-glucose oxidation were analyzed via HPLC us-
ing an acid- specific stationary phase (Agilent, Metacarb
67H)(ESI Fig. 1). Despite method optimization, significant
overlapping of peaks was observed in both RID and DAD (210
nm) signals. Indeed, it was apparent that separation of D-glu-
curono, L-gulurono, D-glucono and D-glucaro- products was
not feasible, as shown in Fig. S1.
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Scheme 2. Equilibration of D-glucaric acid with its
mono and dilactone derivatives
Of the analytical techniques available, nuclear magnetic reso-
nance (NMR) is amongst the most versatile, with the potential
for simultaneous quantification of multiple small organic mol-
ecule components within a complex mixture.41, 42 Two dimen-
sional NMR, in particular heteronuclear multiple- bond corre-
lation (HMBC) spectroscopy, exemplifies this due to its ability
to separate overlapping 1H resonances. However, a key draw-
back with 2D techniques is that the intensity of a 2D NMR
correlation is molecule and environment dependant, leading
to difficulty in determining absolute concentrations. Whilst it
has been shown that this can be mitigated to some degree
through addition of a standard,42 the correlations representa-
tive of the standard molecule also suffer environmental ef-
fects, leading to non-linear calibrations.42
To determine which C6 products form under glucose oxida-
tion conditions, 5 wt. % Pt on TiO2 (P25) was prepared by wet
impregnation (5 wt. % Pt/TiO2IMP Red 400). This catalyst was as-
sessed in D2O under the following reaction conditions; 40 mg
o
catalyst, 1000 rpm, P(O2) = 20 bar, 80 C, 24 h, [D-glucose] =
0.554 M, 5.54 mmol (D-glucose/ Pt = 540 : 1 mol : mol). The 1H
NMR spectrum of aqueous products (ESI Fig. 2) showed mul-
tiple overlapping resonances within the δ = 3.0 ppm to δ = 5.6
ppm region, with a strong solvent resonance centered at δ =
4.79 ppm attributable to H2O. Objective assignment of 1H res-
onances in ESI Fig. 2 was not possible, therefore 2D HMBC
NMR was employed, with an aim to separate 1H environments
along the 13C axis. The 13C-1H HMBC spectra are shown in Fig.
S3, with correlations assigned through analysis of commercial
standards where possible (alternately, synthesized standards)
and chemical structures are shown in Table S1, with 13C-1H as-
signments shown in Table S2. An expanded region of this
spectrum is shown in Fig. 1, with spectral features assigned
through false coloring based on analysis of standards. It is
clear from Fig. 1 that Pt- catalyzed aerobic oxidation of glucose
under autogeneous pH affords a far more complex mixture of
C6 oxidation products than shown in Scheme 1. Indeed, 13C- 1H
correlations consistent with three uronic acids; D-glucuronic
acid, D-gluconic acid and L-guluronic acid are observed. Lac-
tone derivatives of these; D-glucurono 6,3- lactone, D-glucono
1,4- lactone and L- gulurono 6,3- lactone were also observed,
with integrated peak areas suggesting that these form in sig-
nificant concentrations. D-glucaro 1,4 lactone and D-glucaro
6,3 lactone are also confirmed, for the first time, to be prod-
ucts of D-glucose oxidation under aqueous conditions. Poten-
tial pathways to formation of these C6 products are shown in
Scheme 3. 5-Keto D-gluconic acid and the hemiacetal forms of
D-fructose; α-D fructofuranose / α-D fructopyranose however,
were not observed as reaction products. No dialdehyde prod-
ucts were observed. Meanwhile, studies on commercial stand-
ards of D-glucono-1,5 lactone (10) showed it to undergo spon-
taneous conversion (STP) to D-gluconic acid (8). This was
characterised by a shift in δ 13C from 173.8 ppm to 175.9 ppm
In the current study, aerobic oxidation of glucose over sup-
ported Pt catalysts is studied under autogenous pH. Both C6
products and glucose conversion are quantified using quanti-
tative 13C-1H HMBC NMR. It is shown that the mono and di-
lactone derivatives of D-glucaric acid featured in Scheme 2
form in situ and at appreciable selectivities, as do D-glucu-
ronic and L-guluronic acids, the derived 5- membered lac-
tones thereof and also carbon dioxide. Elemental analyses of
product streams show that heterogeneous Pt- catalysts un-
dergo product- mediated chemical leaching. This non- transi-
ent leaching increases with glucose conversion, with leaching
levels of up to 24 % of total supported Pt observed. It is shown
that by supporting Pt on a carbon support, or addition of car-
bon, leached Pt can be effectively sequestered to yield a prod-
uct stream which is effectively platinum- free. At 98 % D-glu-
cose conversion, a catalyst comprising 5 wt.% Pt/C prepared
by wet impregnation afforded 59 % yield of D-glucaric acid
and its lactone derivatives, mass balance of 93 % and low Pt-
concentration of 1.4 ppm.
2. Results and discussion
2.1 Product identification by NMR. To date, all studies re-
lating to catalytic D-glucose oxidation have utilized chroma-
tographic (HPLC/ IC) techniques for quantification of reac-
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and decreased intensity of correlations at δ 13C = 81.6 ppm.
Thermodynamic instability of 6- membered D-glucono 1,5-
lactone (10) relative to 5- membered D-glucono 1,4- lactone
(9) is consistent with previous studies.46
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Figure 1. Expanded region of a false- colored 13C-1H HMBC-NMR spectrum of the product stream from D-glucose oxidation cata-
IMP Red 400
lyzed by 5 wt. % Pt/TiO2
with O2. The structures of C6 gluconic, glucuronic, guluronic and glucaric acids and their
1
derivative products are shown. F1 axis is the 13C spectrum, F2 is the H spectrum. ( Unassigned). *D-glucose and D-glucuronic
acid present in solution as α/β cyclic anomers. Conditions; 5 % Pt/TiO2IMP Red 400 (40 mg), [D-glucose] = 0.554 M (5.54 mmol in
D2O), 80 oC, 1000 rpm, P(O2) = 20 bar, 24 h.
Recently, Lee et al., identified dehydrogenation of D-gluconic
To confirm the C6 pathways proposed in Scheme 3, the three
acid as a potential rate limiting step in formation of D-glucaric
uronic acid products identified in Fig.1; D-glucuronic acid (4),
acid from glucose, though reported no formation of L-gulu-
D-gluconic acid (8) and L-guluronic acid (11) were treated un-
ronic acid or any derivatives.33 Indeed, Scheme 1 and Eq. 2 are
IMP Red
der oxidation conditions in the presence of 5% Pt/TiO2
representative of C6 pathways reported in the vast majority of
D-glucose oxidation studies, with K4 >> K5 widely reported.27,
35, 39 In addition to limitation arising from K1, the observed for-
mation of thermodynamically stable γ-lactones; D-glucono
1,4- lactone and L-gulurono 6,3 lactone, would also slow the
rate of D-glucaric acid formation. Experimentally determined
400. Conversion of the target aldaric acid; D-glucaric acid (13)
was also assessed under the same conditions and HMBC NMR
spectra of product streams are shown in Table S3, Entries 1-4.
These studies confirmed that the three uronic acids undergo
oxidation to yield D-glucaric acid (13). An equilibrium with D-
glucaro 1,4 lactone (14) and D-glucaro 6,3 lactone (15) is then
established. Dehydrogenation of D-gluconic acid (8) was ob-
served, yielding L-guluronic acid (11) and L-gulurono 6,3- lac-
tone (12)(Table S3, Entry 1), with D-glucono- 1,4 lactone (9)
also observed. This was found to be consistent with previous
work by Dirkx et al., who reported that for Eq. (i), K1 << K2.27
initial rates of D-gluconic acid (8) and D-glucono 1,4 lactone
IMP Red 400
(9) conversion over 5% Pt/TiO2
are shown in Table
S4. The equilibrium between acid and γ- lactone dominates
under reaction conditions, with comparable conversion rates
observed for both substrates under aerobic conditions. Con-
version of D-gluconic acid (8) under anaerobic conditions
(Table S4, Entry 3) afforded D-glucono 1,4 lactone (9) and L-
guluronic acid (11) only, suggesting that the transformation of
D-gluconic acid (8) to L-guluronic acid (11) shown in Equation
1 proceeds via non- oxidative catalytic dehydrogenation with
respect to PO2. Given the literature precedent for adding Cu
풌ퟏ
풌ퟐ
퐃 − 퐠퐥퐮퐜퐨퐧퐢퐜 퐚퐜퐢퐝 → 퐋 − 퐠퐮퐥퐮퐫퐨퐧퐢퐜 퐚퐜퐢퐝
→
푫 − 퐠퐥퐮퐜퐚퐫퐢퐜 퐚퐜퐢퐝
(i)
풌ퟒ
풌ퟓ
퐃 − 퐠퐥퐮퐜퐨퐬퐞 → 퐃 − 퐠퐥퐮퐜퐨퐧퐢퐜 퐚퐜퐢퐝
→
푫 − 퐠퐥퐮퐜퐚퐫퐢퐜 퐚퐜퐢퐝
(ii)
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as a promotor in Pt catalysts for non- oxidative dehydrogena-
Based on NMR and product conversion studies, the C6 product
pathways in operation under aerobic D-glucose oxidation con-
ditions is now reported for the first time in Scheme 3.
tion,47, 48 this correlates well with the findings of Jin et al., who
reported enhanced D-glucaric acid productivities from glu-
cose for Pt-Cu/TiO2 catalysts, relative to Pt/TiO2 analogues.35
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Scheme 3. Proposed reaction network for formation of C6 derivatives in catalytic aerobic D-glucose oxidation
solutions. In this way those correlations were identified which
were least susceptible to drift from their linear calibrations
2.2 Quantification of reaction products using HMBC
NMR. Application of NMR techniques as quantitative tools in
when present in complex product streams. In addition to sen-
catalytic processes is not a novel concept. Indeed, quantitative
sitivity analyses, only distinct, non- overlapping peaks were
1H NMR has previously been employed in a number of stud-
considered for quantitative analyses. An example of route (i)
ies.49-54 2D NMR techniques, however, have not been broadly
depicting calibration of D-glucaric acid (13) is shown in Fig. S4
adopted due to the limitations previously described. To re-
move one of these limitations, specifically, interaction of reac-
tion products with the NMR standard itself, a melt- sealed
glass ampule containing 1 wt.% TMS/CDCl3 was used as inter-
nal standard in calibrations and product analyses. Two differ-
ent methods were then used in calibrating for glucose oxida-
and Tables S5 – S6, whilst calibration of D-glucaro 6,3 lactone
(15) via route (ii) is shown in Figs. S5-S6 and Tables S7- S9. A
truly quantitative, standardised first order NMR method for
determining the concentration D-glucose (as α- and β- D-glu-
copyranose) in solution was employed (Fig. S7), with valida-
tion of rFs shown in Fig. S8. A representative sensitivity anal-
tion reaction products using HMBC NMR. These were; (i) lin-
ysis is shown in Table S6. Such an analysis was carried out for
ear calibrations using commercially available standards and
all derivatives shown in Table S1, with the exception of D-glu-
(ii) calibration through equilibrium studies. Approach (ii) was
adopted in calibration of molecules which could be neither
purchased nor isolated in pure, quantifiable crystalline form,
but rather exist as equilibrium mixtures. These include; (a) α-
D- glucopyranose (2) ⇌ β-D- glucopyranose (3) the aqueous
forms of D-glucose (1), (b) α-D- glucuronic acid (5) ⇌ β -D-
glucuronic acid (7) and (c) D-glucaro 6,3 lactone (15), which
was cross- calibrated via the equilibria shown in Scheme 2 us-
caro 6,3 lactone and α/ β -D- glucuronic acid – for which pure
standards were unavailable. For these, non-overlapping corre-
lations (where R2 > 0.99) were used in quantification. The 1H-
13C correlations used in quantifying unreacted glucose and ox-
idation products are shown in Table S10. 13C-1H HSQC NMR
was also considered for use in product stream analysis due to
its being a more sensitive technique and less susceptible to
variation in 13C-1H couplings. However, observed 13C-1H corre-
ing rFs derived for D-glucaric acid (13), D-glucaro 1,4 lactone
lations were too intense for objective TMS- normalised quan-
titative analysis of product streams without product stream
dilution (Fig. S9), which could shift the position of the dy-
namic equilibria shown in Scheme 3.
(14) and D-glucaro 1,4: 6,3 dilactone (16). For a range of con-
1
centrations (in D2O), each characteristic H-13C correlation
was normalised to the TMS standard at δ 1H = 0 ppm, δ 13C = 0
ppm to afford a response factor (rF). Sensitivity Analyses were
then carried out for the rFs of each 1H-13C correlation within a
specific HMBC spectrum of a product using model reaction
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Table 1 The aqueous aerobic oxidation of glucose over various supported 5 wt. % Pt catalysts; catalytic activity and stability
Product Selectivities / % [b]
[M]
Cbal
2
Entry
Cat
χ / % [a]
ppm
5K-
GLU
GLUU
Other
/ %
GLU
GLU1,4
GLA
GL1,4
GL6,3
GL1,4:6,3
GLUU6,3
GUL
GUL6,3
CO2
[e]
[c]
[d]
3
4
5
6
7
8
9
1
2
None
TiO2
0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
101
99
101
101
100
95
-
-
0
-
-
-
-
-
-
-
-
-
-
-
3
SiO2
0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
4
C (XC72R)
0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
IWI
5
Pt/TiO2
42.2
55.5
49.0
26.0
76.9
16.2
44.3
57.8
56.9
14.8
22.7
17.9
19.3
2.9
34.3
32.2
24.8
25.5
11.4
18.6
16.6
16.4
1.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
8.5
5.7
6.7
4.5
11.5
0.0
4.6
6.5
5.9
1.7
2.0
1.7
1.6
3.0
2.9
1.2
2.1
1.8
9.1
7.5
6.0
5.2
8.0
1.0
5.4
5.8
6.6
0.6
0.5
0.4
0.8
1.0
0.0
0.2
0.7
0.4
11.1
4.0
6.1
10.4
11.6
6.7
3.1
4.4
4.0
10.6
5.1
7.6
11.6
17.0
5.2
4.1
5.2
4.2
15.6
13.8
14.2
14.4
15.8
9.2
11.0
12.1
13.6
11.0
12.5
13.2
13.5
10.3
9.7
8.3
10.8
10.2
5.2
6.0
5.7
1.7
13.5
3.6
2.3
3.9
4.6
0.4
1.6
3.9
0.6
3.9
1.2
0.4
0.8
1.2
6.8
7.0
7.9
2.0
12.6
1.1
CVI
6
7
Pt/TiO2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
IMP
Pt/TiO2
97
IWI
8
Pt/SiO2
102
74
CVI
9
10
11
12
13
Pt/SiO2
IMP
Pt/SiO2
26.1
27.2
22.8
22.0
96
102
102
95
Pt /CIMP
Pt /CIWI
Pt /CCVI
1.0
1.0
1.0
GLU (D-gluconic acid), GLU1,4 (D-glucono 1,4- lactone), 5K-GLU (5-keto D-gluconic acid), GLA (D-glucaric acid), GL1,4 (D-glucaro 1,4 lactone), GL6,3 (D-glucaro 6,3 lactone), GL1,4:6,3 (D-glucaro
1,4:6,3 dilactone), GLUU (D-glucuronic acid), GLUU6,3(D-glucurono 6,3- lactone), GUL (L-guluronic acid), GUL6,3 (L-gulurono 6,3-lactone)
Conditions; 40 mg catalyst, [D-glucose] = 0.554 M (5.54 mmol in D2O), 80 oC, 1000 rpm, P(20 % O2/ N2) = 25 bar, 24 h, Glucose : Pt (mol: mol) = nominal 540 : 1 where applicable.
[a] Glucose conversion quantified from disappearance of glucose (α & β glucopyranose) using HMBC NMR. [b] Based on observed reaction products. [c] Quantified as α- and β- D-
glucuronic acid. [d] Other (tartaric acid, tartronic acid, glycolic acid, glyceric acid, oxalic acid, acetic acid) [e] Determined by MPAES (note: where applicable, 100 % Pt leaching
would afford a [Pt] of 200 ppm).
To further validate the applicability of quantitative 13C-1H
HMBC NMR in these systems, 5% Au/TiO2
2.3 Assessment of supported Pt catalysts in the aerobic
IMP Red 400
was pre-
oxidation of D-glucose. Blank reactions were carried out in
the absence of a catalyst (Table 1, Entry 1) and also in the pres-
ence of unmodified support materials P25 TiO2, SiO2 and car-
bon (Table 1, Entries 2, 3 and 4 respectively). No glucose con-
version was observed in the presence of these unmodified sup-
port materials. For these experiments, the carbon mass bal-
ance at t = 24 h was 100 % ± 1 %, with glucose quantified post-
reaction (as α & β glucopyranose) using HMBC NMR. Direct
quantification of glucose conversion in this way is a key dis-
tinction between the reported quantitative NMR technique
and chromatographic methodologies, which suffer from co-
elution of products and substrate. Consistent with previous
reports on the oxidation of D-glucose to D-glucaric acid, a
range of catalysts containing a nominal 5 wt.% loading of Pt
supported on SiO2 (Divasil 635), TiO2 (Degussa, P25) and car-
bon (Vulcan, XC72R) were prepared via IMP, IWI and CVI.
This being a representative sample of preparation methodolo-
gies, support materials and Pt- precursors. To allow for com-
parison of catalysts, assessments were carried out at < 100 %
pared and assessed for non pH- controlled D-Glucose oxida-
tion, a reaction which is broadly reported to yield D-gluconic
acid (8) as the terminal C6 product. Two C6 products were
observed in HMBC NMR spectra of reaction solutions across
20 h on line; D-gluconic acid (8) and D-glucono 1,4 lactone
(9). As was shown in Fig. S1, these products are separable via
HPLC. Comparative quantitative analyses of product streams
at 2- 20 h on line via HMBC NMR and HPLC are shown in Fig.
S10. Ring- opening of the lactone was observed during calibra-
tion and is attributed to the polar mobile and strongly cationic
stationary phase within the HPLC column. This was charac-
terised by a consistently lower lactone: acid ratio observed in
HPLC relative to HMBC NMR data (Figs. S10a and 10b respec-
tively). Nonetheless, the Σ yield of D-gluconic acid derivatives
(D-gluconic acid + D-glucono 1,4 lactone) was consistent be-
tween analytical techniques (Fig. S10c).
To discount any potential effects from the use of D2O as sol-
vent, 5% Pt/TiO2 IMP Red 400 was also assessed in deionised H2O.
A comparison of HMBC NMR spectra for reactions carried out
in either D2O or H2O can be found in Fig. S11. Whilst the same
distribution of products formed as in Fig. 1, spectral resolution
is significantly obscured by the presence of a strong solvent
o
conversion; [D-glucose] = 0.554 M, 24 h, 80 C, P(20 % O2 /
N2) = 25 bar. Through quantitative HMBC analysis of product
streams shown in Table 1, 6 additional C6 products of Pt- cat-
alyzed D-glucose oxidation are now reported for the first time
(Table 1, Entries 5-13). These are; D-glucono 1,4 lactone
(GLU1,4)(9), D-glucaro 1,4 lactone (GL1,4)(14), D-glucaro 6,3
lactone (GL6,3)(15), D-glucaro 1,4:6,3 dilactone (GL1,4:6,3)(16),
D-glucurono 6,3 lactone (GLUU6,3)(6) and L-gulurono 6,3 lac-
tone (GUL6,3)(12). Total oxidation to CO2 was also assessed,
with yields of up to 2.0 % determined (Table 1, Entry 7). In the
presence of Pt- catalysts, low glucose conversion correlated
with high selectivity towards D-gluconic acid and its deriva-
tives (Table 1, Entry 10). With increasing conversion, selectiv-
ity towards derivatives of D-glucaric (13,14,15,16) and L-gulu-
ronic acid (11,12) increases. At near isoconversion, comparable
product selectivities were observed for Pt/TiO2CVI, 5% Pt/CCVI
1
resonance centred at ca. δ H = 4.9 ppm (± 0.25 ppm), with
certain characteristic 1H - 13C interactions effectively obscured.
These are indicated in Fig. S11a. Analysis of the product
streams from Fig. S11 using 2H NMR indicated that no deuter-
ium exchange occurred during catalytic assessments (Fig. S12).
All reactions and calibrations were therefore carried out in
D2O to allow for accurate quantification of products. In addi-
tion and for the first time, CO2 is quantified as a product of Pt-
catalyzed D-glucose oxidation, as determined through GC-
FID analysis of gaseous product streams.
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 12
and 5% Pt/CIWI (Table 1, Entries 6, 12 and 13). Despite the sig-
conditions in a trickle flow regime (Fig. S15a). Pt- leaching was
1
2
3
4
5
6
7
8
nificant disparity in BET surface areas of the TiO2 and carbon
supports (48 and 226 m2g-1 respectively). Indeed, the physico-
chemical property which best correlated with observed D-glu-
cose conversion rates was Pt- surface area, as determined by
CO pulse titration (Fig. S13a). ICP-MS analysis of the product
streams also showed leaching of Pt into solution. This was de-
termined to be product- mediated, with leaching observed
only when the reactor was charged with both the O2 and D-
glucose (Table S11). At isoconversion, lower [Pt] concentra-
tions were observed for Entries 12 and 13 than Entry 7 and this
can be attributed to the ability of carbon to effectively se-
quester leached Pt from aqueous solution as confirmed by
studies shown in Fig. S14. To determine whether the observed
product- mediated leaching of Pt was a transient phenome-
non, 5% Pt/TiO2 IMP Red 400 was assessed under continuous flow
observed to continue at steady state over 40 h on stream, in
line with steady state glucose conversion. Consistent with
batch reaction studies in Table 1, it is shown in Fig. S15b that
through addition of carbon extrudates (Norit ROW Supra,
Sigma Aldrich) after the Pt-catalyst bed, leached Pt can be ef-
fectively sequestered from the product stream onto the car-
bon. It is perhaps serendipitous therefore, that Rennovia Inc.
recently patented the synthetic procedure for a series of
shaped porous carbons.55-57 Further still, that the examples in
said patents featured use of the synthesized carbons as sup-
ports for Pt- based glucose oxidation catalysts.55-57 To further
study the complex reaction pathways in operation in Pt- cata-
lyzed glucose oxidation, a time on line study of the catalytic
performance of 5% Pt/TiO2IMP Red 400 was carried out and data
is shown in Fig. 2.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(c)
(a)
(b)
20
30
40
50
60
70
20
30
40
50
60
70
20
30
40
50
60
70
50
45
40
35
30
25
20
15
10
5
50
45
40
35
30
25
20
15
10
5
10
9
8
7
6
5
4
3
2
1
0
D-Glucaric Acid
D-Glucaro 1,4 lactone
D-Glucaro 6,3 lactone
D-Glucuronic Acid
D-Gluconic Acid
L-Guluronic Acid
D-Glucurono 6,3 lactone
D-Glucono 1,4 lactone
5- keto D- gluconic acid
L-Gulurono 6,3 lactone
D-Glucaro 1,4: 6,3 dilactone
0
0
20
20
30
40
50
60
60
70
70
20
20
30
30
40
50
60
60
70
70
20
20
30
30
40
50
60
60
70
70
XGlucose / %
XGlucose / %
XGlucose / %
(f)
(d)
(e)
40
50
40
50
30
40
50
17.5
5
4
3
2
1
0
Glucuronates
 Gluconates
 Guluronates
 Glucarates
Tartaric Acid
Tartronic Acid
Oxalic Acid
Glycolic Acid
Glyceric Acid
Acetic Acid
CO2
100
80
60
40
20
0
70
15.0
12.5
10.0
7.5
 < C6 products
60
50
40
30
20
10
0
Carbon Balance
[Pt] Leached
5.0
2.5
0.0
20
30
40
50
60
70
20
30
40
50
60
70
20
30
40
50
60
70
XGlucose / %
XGlucose / %
XGlucose / %
IMP Red 400
Figure 2. The temporal evolution of products in aerobic D-glucose oxidation catalyzed by 5% Pt/TiO2
showing glucose
conversion (XGlucose), selectivities towards (a) uronic acid products, (b) lactone- derivatives of uronic acid products, (c) glucarate
products and (d) < C6 products. Also showing (e) Σ selectivities towards product groups and (f) carbon mass balances + Pt leaching.
Conditions; 5 % Pt/TiO2IMP Red 400 (40 mg), [D-glucose] = 0.554 M (5.54 mmol in D2O), 80 oC, 1000 rpm, P(20% O2/ N2) = 25 bar.
ACS Paragon Plus Environment

Page 7 of 12
ACS Catalysis
Table 2 A parametric study of 5% Pt/TiO2IMP Red 400 and 5% Pt/CIMP Red 400 catalyzed D-glucose oxidation
1
2
3
Observed Product Yields / C-mol % [b]
[Pt]
Conditions;
% O2/ Temp oC/ mg cat
Cbal
/ %
χ / %
Entry
ppm
[a]
GLUU
Other
GLU
GLU1,4
GLA
GL1,4
GL6,3
GL1,4:6,3
GLUU6,3
GUL
GUL6,3
CO2
[e]
[c]
[d]
4
5
6
7
8
9
10
1
20% O2/ 60 oC/ 40 mg
20% O2/ 80 oC/ 40 mg
20% O2/ 100 oC/ 40 mg
100% O2/ 60 oC/ 40 mg
100% O2/ 80 oC/ 40 mg
100% O2/ 100 oC/ 40 mg
100% O2/ 80 oC/ 80 mg
20% O2/ 60 oC/ 40 mg
20% O2/ 80 oC/ 40 mg
20% O2/ 100 oC/ 40 mg
100% O2/ 60 oC/ 40 mg
100% O2/ 80 oC/ 40 mg
100% O2/ 100 oC/ 40 mg
100% O2/ 80 oC/ 80 mg
100% O2 / 100 oC/ 80 mg
34.6
49.0
90.0
43.1
13.2
8.3
7.0
7.7
1.3
3.1
0.2
0.8
0.3
0.4
1.6
2.0
0.8
0.3
0.6
1.3
1.0
2.8
0.9
1.9
4.1
5.3
2.3
1.2
0.0
0.2
0.8
0.
1.5
2.8
1.3
1.2
3.5
4.6
1.4
5.0
5.3
2.1
3.7
6.6
5.5
5.0
7.1
2.0
6.1
7.5
2.5
8.2
4.2
3.3
1.3
0.4
2.6
4.2
3.4
3.1
0.1
1.8
2.2
0.3
0.3
2.6
0.8
0.1
0.2
1.1
97
97
43
1.8
7.9
34.8
3.5
2
3
1.2
3.1
1.2
4
15.7
12.4
3.0
9.7
10.2
2.9
4.4
11.3
12.6
10.5
14.3
16.6
7.2
2.0
3.8
5.1
2.4
3.3
2.3
1.0
1.0
1.4
2.1
102
100
55
5
59.5
89.8
90.6
41.6
44.3
69.8
44.4
57.9
90.7
82.7
99.5
0.5
0.6
0.2
0.1
11.3
48.8
27.4
0.4
1.0
6
4.5
9.9
4.7
5.1
7.1
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7
8
5.2
3.4
1.4
2.2
5.2
1.0
2.3
3.3
9.2
4.2
2.9
0.7
1.1
43
18.4
14.9
9.9
20.1
18.5
6.8
17.8
3.2
0.9
1.9
3.9
0.7
1.7
100
102
85
9
2.5
5.0
0.5
2.5
2.4
8.2
3.4
0.1
3.8
6.4
1.6
4.6
4.3
7.1
10
11
0.9
0.1
5.6
3.4
6.0
2.1
2.9
0.4
1.7
11.6
0.1
0.1
0.6
1.6
0.7
2.7
0.1
0.1
0.3
1.1
98
99
45
12
13
14
15
0.6
1.0
0.4
0.8
0.9
1.9
1.1
1.5
4.8
3.0
5.4
10.0
2.3
15.8
3.6
3.2
1.2
2.3
0.7
10.0
2.0
0.5
2.8
98
31
2.4
12.4
100% O2/ 100 oC/ 80 mg
+ 80 mg XC72R[f]
16
17
98.2
98.4
4.1
4.1
4.1
4.1
16.7
23.8
6.2
7.5
14.9
23.0
2.6
6.0
0.5
0.6
1.0
4.0
3.7
3.6
3.3
8.4
11.5
2.9
2.8
71
5.2
1.4
100% O2/ 100 oC/ 80 mg+
160 mg XC72R[f]
0.7
93
GLU (D-gluconic acid), GLU1,4 (D-glucono 1,4- lactone), GLA (D-glucaric acid), GL1,4 (D-glucaro 1,4 lactone), GL6,3 (D-glucaro 6,3 lactone), GL1,4:6,3 (D-glucaro 1,4:6,3 dilactone), GLUU (D-
glucuronic acid), GLUU6,3(D-glucurono 6,3- lactone), GUL (L-guluronic acid), GUL6,3 (L-gulurono 6,3-lactone)
General conditions; [D-glucose] = 0.554 M (5.54 mmol in D2O), 1000 rpm, Ptotal (X % O2/ N2) = 25 bar, 24 h. Entries 1-7; 5% Pt/TiO2IMP Red 400, Entries 8-17; 5% Pt/CIMP Red 400
.
[a] Glucose conversion quantified from disappearance of glucose (α & β glucopyranose) using HMBC NMR. [b] As CMB deviated from 100 % under certain conditions, data
presented as yield in C-mol%, [c] Quantified as α- and β- D-glucuronic acid. [d] Other (tartaric acid, tartronic acid, glycolic acid, glyceric acid, oxalic acid, acetic acid), [e] Deter-
mined by MPAES (note: where 40 and 80 mg catalyst were used, 100 % Pt leaching would afford a [Pt] of 200 and 400 ppm respectively). [f] Reactor charged with Carbon
XC72R at t=0.
products (tartaric, tartronic, oxalic, glycolic, glyceric and ace-
tic acids in addition to CO2) reached 12.8 % at 40 h on line (Fig.
2e). Selectivity towards the total oxidation product; CO2
D-Gluconic acid (8) and D-glucono 1,4 lactone (9) are appar-
ent primary products (Fig. 2a), with their total selectivity de-
creasing from 70.7 % at 2 h on line (χ = 19 %) to 24.8 % at 40 h
reached 4.1 % at χ = 65 % (1.96 % yield) based on observed
(χ = 65 %). Selectivity to L-guluronic acid (11) and the derived
products. A decrease in specific activity was observed with in-
lactone L-gulurono 6,3 (12) increases, consistent with their
forming through dehydrogenation of D-gluconic acid (8). Se-
lectivity towards D-glucuronic acids (4) and D-glucurono 6,3
lactone (6) also increase with increasing time on line, which
suggests that this competing reaction pathway (Scheme 3) be-
comes more favorable at low pH. This is a minor pathway
however, with formation of D-gluconic acid favored. This is
consistent with more facile oxidation of D-glucose (1) at the
terminal RCHO functional group than at the RCH2OH. Selec-
tivity towards D-glucaric acid and its derivatives also increases
with increasing glucose conversion, with comparable selectiv-
ity towards D-glucaric acid (13) and D-glucaro 6,3 lactone (15)
observed across 40 h on line (Fig. 2c). Given the co-elution of
products shown in Fig. S1, it should be noted that had product
streams from Fig. 2 been analyzed by HPLC, it would not have
been possible to distinguish between key major products, for
example D-gluconic acid (8) and D-glucaro 6,3 lactone (15).
Indeed, whilst lactone products have never been reported in
catalytic D-glucose oxidation literature, C6 lactones accounted
for 45 ± 2.0 % of all products represented across the 40 h time
period in Fig.2. Consistent with Scheme 3, D-glucaric acid and
its derivatives are the terminal C6 oxidation products, and C-
C scission yields increasing selectivity towards < C6 products
(Fig. 2d and Fig. 2c). Indeed, total selectivity towards < C6
creasing time on line (Fig. S13b) suggesting either competitive
adsorption of reaction products or the onset of mass transport
limitation at high degrees of either D-glucose or O2 conver-
sion. Indeed, using the CO chemisorption- determined Pt-sur-
face site density (4.43 m2Pt gcat-1), the TOF of 5% Pt/TiO2
IMP Red
400 at 2 h on-line was calculated as 144 molGlucose converted molSur-
-1
h . This decreased to 25 molGlucose converted molSurface Pt
h , averaged at t= 39 h. Up to 24 h on-line (χ = 49.0 %), the
average observed carbon mass balance (CMB) was 98 % ± 2,
but this fell to 83 % at 40 h on-line (χ = 65 %)(Fig. 2f). This
face Pt sites
-1
sites
decrease in CMB might be attributed to formation of insolu-
ble humins, a process which is favorable under acidic condi-
tions and elevated temperatures.58 To confirm this, 5%
IMP Red 400
Pt/TiO2
was assessed across a temperature range of
60 – 100 oC (Table 2, Entries 1-3).
Glucose conversion, measured at 24 h online, increased from
34.6 % at a reaction temperature of 60 oC to 49.0 % at 80 oC.
Consistent with conversion selectivity relationships shown in
Fig. 2(a-d), this increase in conversion was coupled with de-
creased selectivity towards primary products; D-gluconic
acid/ D-glucono 1,4 lactone, with increased selectivity towards
D-glucuronic acid, L-guluronic acid, D-glucaric acid, lactone
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 12
derivatives thereof and < C6 products. At a reaction tempera-
1H-13C through- bond couplings. The role of leached Pt in ef-
fecting low CMBs was confirmed through analysis of com-
bined data from Tables 1 & 2 and Figure 2.
ture of 100 oC, (25 bar of 20 % O2/ N2) the system became an-
aerobic (χ glucose = 90.0 %, 4.98 mmol glucose converted)
with a 1: 1 stoichiometry of converted glucose: O2 in the charge
gas (5.05 mmol O2) of 1: 1. The apparent CMB fell to 43 % under
these conditions (Table 2, Entry 3). In the absence of O2, for-
mation of L-gulurono products (GUL/ GUL6,3) continued. In-
deed, L-guluronic acid and its lactone derivative accounted for
40 % of observed products (Table 2, Entry 3). This is consistent
with previous observations shown in Table S4, which further
indicates that conversion of D-gluconic acid to D-glucaric acid
proceeds via L-guluronic acid and, furthermore, that this pro-
ceeds via dehydrogenation of D-gluconic acid with zero order
dependence with respect to PO2. In the absence of O2, oxida-
tion of L-guluronic acid to yield D-glucaric (13) was prevented,
with the total yield of D-glucaric acid and lactone derivatives
1
2
3
4
5
6
7
8
XGlucose / %
0
20
40
60
80
100
100
80
60
40
20
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5% Pt/TiO2
5% Pt/SiO2
5% Pt/C
(a)
o
thereof, falling to 3.2 % (c.f 6.9 % at 80 C Table 2, Entries 2
0
and 3). This represents a potential chemocatalytic route to-
wards the selective synthesis of L-guluronic acid, the mono-
mer which affords rigidity in alginate biopolymers which have
proven biomedical applications.59-61 L-Guluronic acid is cur-
rently produced commercially by hydrolysis of alginic acid,
yielding the monomers; L-guluronic acid (11) and D-mannu-
ronic acid.
50
40
30
20
10
Analysis of the liquid and gaseous products streams of reac-
tions represented in Table 2, Entries 1-7 with low apparent
CMBs showed no unidentified by-products. To determine the
fate of the missing carbon, the solid residue for reactions in
Table 2 Entries 1-3, representing a CMB range of 97 – 43 %,
were recovered by filtration, washed (100 ml H2O) and dried
(b)
0
100
80
60
40
20
0
o
o
(16 h, 110 C) prior to thermogravimetric analysis (TGA)(5 C
min-1, 20 ml min-1 air, 800 oC). Despite showing markedly dif-
ferent CMBs, no significant difference in mass loss was ob-
served, suggesting that insoluble humin products or other car-
bonaceous species did not form under reaction conditions
(Fig. S16). Spectroscopic, chromatographic and thermogravi-
metric analyses therefore offered no explanation as to the fate
(c)
of the -57 % CMB deficit in Table 2, Entry 3. In a parametric
0
10
20
30
40
50
60
IMP Red 400
study of 5% Pt/TiO2
catalyzed D-glucose oxidation
[Pt] / ppm
(Table 2, Entries 1-7) isoconversion of 90.1 % ± 0.4 was ob-
served under three different reaction conditions (Table 2, En-
tries 3, 6 and 7). Under these conditions, the observed CMB
was < 50 % and significant Pt- leaching was observed, exceed-
ing 24 % of total supported Pt (Table 2, Entry 6).
Figure 3. The interdependency between χGlucose, observed CMB
and [Pt] showing data from Tables 1 & 2 and Figure 2 - repre-
senting  5% Pt/C,  5% Pt/SiO2 and  5% Pt/TiO2 catalysts
prepared by CVI, IMP and IWI. ( 5% Pt/CIMP Red 400 from Ta-
ble 2, Entries 15-17)
It is well known that the intensity of 1H NMR resonance peaks
is directly proportional to the population of 1H. Indeed, that is
the basis of quantitative 1H NMR techniques. The intensity of
HMBC NMR correlation peaks however, are determined by
through-bond coupling of two different spin active nuclei; 13C
– 1H. At 100 oC, P(O2) = 5 bar, a significant concentration of Pt
was leached into solution (34.8 ppm, Table 2, Entry 3) This
equated to leaching of ca. 17 % of supported Pt. Low CMB re-
actions in Table 2 also correlated well with formation of or-
ange product solutions which is consistent with observations
by Dirkx et al., Sugar acids are effective chelating agents, in-
deed this an established commercial application for D-glu-
caric acid 27, 36, 62 and a general transfer of electron density oc-
curs when an organic acid chelates a cationic metal center.
This might be expected to effect changes in absolute areas of
For all Pt- catalysts, increasing D-glucose conversion corre-
lated well with decreasing CMB (Fig. 3a) and increasing Pt-
leaching (Fig. 3b). When the [Pt] remaining in solution post-
reaction exceeded 10 ppm, a rapid decrease in CMB was ob-
served (Fig. 3c). In general, 5% Pt/C catalysts showed lower
[Pt] at isoconversion than did SiO2 - and TiO2 – supported an-
alogues (Fig.3b), which is consistent with the ability of carbon
to sequester leached Pt from solution, as shown in Fig. S14. To
confirm the role of leached metal species in effecting de-
creased CMBs, 5% Pt/CIMP Red 400 was assessed under high con-
version conditions with an increasing mass of XC72R (0 – 160
mg) added at t=0 to promote sequestration of metals from so-
lution (Table 2, Entries 15-17). Isoconversion of glucose was
observed across these conditions (98.7 ± 0.7 %). Addition of
160 mg carbon led to decreased [Pt] (12.4 – 1.4 ppm) and in-
creased observed CMB (31 – 93 %) (Table 2, Entries 15 and 17).
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The total yield of C6 lactones in Table 2, Entry 17 was 43.6 %.
uronic acid products form; D-glucuronic acid (4) and D-glu-
1
2
3
4
5
6
7
8
HMBC NMR quantification of D-glucaric acid and D-tartaric
acid, both well-known chelating agents, suffered significantly
in the presence of leached cations. Indeed, the observed D-
glucaro (GLA, GL1,4, GL6,3 and GL1,4:6,3)/ D-tartaric acid yields
increased from 8.8 % / 2.3 % to 60.3 % / 8.4 % when 160 mg
carbon XC72 R was added to the reactor. The average mass-
normalized D-glucaro (GLA, GL1,4, GL6,3, GL1,4:6,3) productivity
across 24 h on-line for data in Table 2 Entry 17 was 1.71 mol D-
glucarates kgcat-1 h-1. This compares favorably with the optimal D-
glucaric acid synthesis rate previously reported by Boussie et
conic acid (8). These both establish equilibria with their 6,3
(6) and 1,4 (9) lactones respectively. Whilst D-glucuronic acid
and its lactone undergo direct oxidation to yield D-glucaric
acid (13), D-gluconic acid and its derivative lactone are shown
to undergo dehydrogenation to yield L-guluronic acid (11),
which rapidly dehydrates to form L-gulurono 6,3 lactone (12).
Oxidation of L-guluronic acid and its lactone leads to the de-
sired product of Pt-catalyzed D-glucose oxidation; D-glucaric
acid (13). Equilibration of D-glucaric acid to D-glucaro 1,4 lac-
tone (14), D-glucaro 6,3 lactone (15) and D-glucaro 1,4: 6,3 di-
lactone (16) is then observable and quantifiable via HMBC
NMR, with D-glucaro 6,3 lactone yields often equaling those
of D-glucaric acid itself. Crucially, observation of D-glucaro
lactones is not possible using chromatographic techniques
used in all previous studies of this reaction, indeed we attrib-
ute the low D-glucaric acid yields reported in many previous
studies, to co-elution of D-glucaro lactones with D-gluconic
acid, which is normally reported to be both primary and major
reaction product. Extensive product- mediated leaching of the
active phase – Pt, is reported and attributed to chelation of
oxidized Pt-species by carboxylic acid products. A strong che-
lating effect was confirmed, with the 13C-1H correlation peaks
of D-glucaric acid shown to be significantly quenched by [Pt]
of as low as 10 ppm. This was shown to impact significantly
upon product quantification, with carbon mass balances of <
50 % observed. Catalyst design studies showed D-glucose con-
version to increase proportional to Pt- surface area. Through
supporting of Pt onto carbon black (XC72R) via a range of
preparation techniques, catalysts can be synthesized which af-
ford appreciable specific activities and product streams con-
taining a low [Pt]. Rather than being due to enhanced stability
towards leaching, the latter is attributed to carbon’s affinity
for adsorbing leached Pt in situ. Preliminary parametric opti-
mization of reaction conditions employed in assessing 5%
Pt/C prepared by IMP led to high D-glucose conversion (98.4
%), relatively low [Pt] in solution (1.4 ppm) and consequently
high CMB (93 %). Under said condition, the total yield of pre-
viously unreported C6 lactones was 43.6 %, with 35.5 % yield
being D-glucaro lactone and dilactone products. This work
therefore lays the foundation for future studies into catalytic
biomass- valorization; detailing a new approach to analysis of
complex reaction mixtures and application thereof to provide
vital insight as to the true pathways in operation during the
Pt-catalyzed oxidation of D-glucose to D-glucaric acid. This
should inform future kinetic studies of the competing path-
ways in operation during catalytic D-glucose oxidation as well
as catalyst design/ screening studies.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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31
32
33
34
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60
-1
al. for a 4 % Pt/SiO2 catalyst (1.42 mol D-glucaric acid kgcat h-1)(8
o
h, 90 C, PO2 = 5 bar, Pt: Glucose = 1: 84 mol: mol). Dirkx et
al., previously reported increased rates of D-glucaric acid for-
mation over Pt-catalysts when the reaction pH was main-
tained at pH 9-10 through autotitration of NaOHaq.27, 39 This
was attributed to more efficient desorption of acid products
27, 39
from the catalyst surface under basic conditions.
Such
decreased contact times could decrease product- mediated
chemical leaching and therefore [Pt]. Unfortunately, addition
of NaOH (1 eq.) at t=0 caused a significant decrease in the res-
olution of HMBC NMR spectra (Fig. S17). As a result, it was
not possible to quantify either D-glucose conversion or for-
mation of products under such pH- controlled conditions.
It is clear from Figs. 3a and 3b that addition of increasing
masses of carbon (XC72R) significantly decreased observed
[Pt] and consequently led to increased CMB. To avoid intro-
duction of mass transport limitation effects, the carbon addi-
tive charge was limited to ≤ 160 mg in the 10 ml reaction. Ex-
trapolation of data in Table 2, Entries 15-17 suggests that fur-
ther addition of carbon would yield a metal-free product
stream and closed CMB. Based on trends in closed batch
(Fig.3b) and trickle flow regimes (Fig. S15) it is therefore clear
that operation in continuous flow would be favorable on scale-
up. Use of a Pt/C catalyst with downstream high purity carbon
beds, might be expected to afford product streams rich in D-
glucaric acid/ its lactone derivatives, with leached metal se-
questered and thereby recoverable. Development of such a
process will be addressed in a future study.
3. Conclusions
For the first time, quantitative 2D HMBC NMR has been ap-
plied in both determining and quantifying the products of
chemocatalytic reactions. Linear, 1st order calibrations, use of
a self-contained TMS standard and sensitivity analyses re-
move degrees of uncertainty encountered in previous at-
tempts to employ 2D NMR as a quantitative tool for analysis
of complex mixtures. The developed analytical technique was
then employed in identifying and quantifying the myriad C6
products; both carboxylic acids and lactones, which form
spontaneously during non-pH controlled aerobic oxidation of
D-glucose over heterogeneous Pt catalysts. For the first time,
CO2 has been quantified as a product of this oft studied reac-
tion, with not- insignificant yields of up to 2.9 % observed. The
reaction pathway is more complex than has previously been
reported, with C6 5-membered lactone products accounting
for ca. 45 % of all products observed over 40 h on line. All lac-
tonizations proceed with retention of stereochemistry at the
secondary alcohol center, that is, by activation of the carbox-
ylic acid, followed by nucleophilic attack by said alcohol (hy-
droxyl) and loss of a hydroxyl from the acid. Two primary
AUTHOR INFORMATION
Corresponding Author
* To whom correspondence should be addressed. Dr. R. D.
Armstrong (ArmstrongR4@Cardiff.ac.uk), Prof. G. J. Hutch-
ings (Hutch@Cardiff.ac.uk)
All authors have given approval to the final version of the
manuscript.
ORCID
Armstrong. R. D. (0000-0001-9075-3492)
Hutchings. G. J. (0000-0001-8885-1560)
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Page 10 of 12
glucose oxidation using air with supported gold catalysts.
Green Chemistry 2014, 16, 3132-3141.
6. Solmi, S.; Morreale, C.; Ospitali, F.; Agnoli, S.;
Cavani, F. Oxidation of d-Glucose to Glucaric Acid Using
Au/C Catalysts. ChemCatChem 2017, 9, 2797-2806.
Funding Sources
1
2
3
4
5
6
7
8
This work was supported by the Engineering and Physical
Sciences Research Council) EPSRC, UK (EP/K014749).
Notes
The authors declare no competing financial interest.
7.
Besson, M.; Flèche, G.; Fuertes, P.; Gallezot, P.;
Lahmer, F. Oxidation of glucose and gluconate on Pt, Pt Bi, and
Pt Au catalysts. Recueil des Travaux Chimiques des Pays-Bas
1996, 115, 217-221.
ASSOCIATED CONTENT
Supporting Information. Detailed NMR spectral assign-
ments, details pertaining to development of the quantitative
HMBC protocol and data for product conversion reactions.
Also includes plots showing specific catalyst activity, Pt-leach-
ing/ sequestration studies and thermogravimetric analyses of
catalysts prior to and following catalytic testing. This material
is available free of charge via the Internet at
http://pubs.acs.org.
8.
Besson, M.; Lahmer, F.; Gallezot, P.; Fuertes, P.;
9
Fleche, G. Catalytic oxidation of glucose on bismuth-promoted
palladium catalysts. J. Catal. 1995, 152, 116-21.
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Werpy, T.; Peterson, G. Top Value Added Chemicals
ACKNOWLEDGMENT
From Biomass, Volume 1—Results of Screening for Potential
Candidates from Sugars and Synthesis Gas; Energy Efficiency
This work was supported by the Engineering and Physical Sci-
ences Research Council EPSRC, UK (EP/K014749). Infor-
mation on the data underpinning the results presented here,
including how to access them, can be found in the Cardiff Uni-
versity data catalogue at DOI
and
http://www.eere.energy.gov/biomass/pdfs/35523.pdf, 2004.
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