Direct speciation methods to quantify catalytically active species of AlCl3 in glucose isomerization

Article abstract of DOI:10.1039/c8ra03088j

While homogeneous metal halides have been shown to catalyze glucose to fructose isomerization, direct experimental evidence in support of the catalytically active species remains elusive. Here, we integrate direct speciation methods with kinetics to provide strong evidence for the active species of AlCl₃ in glucose-fructose isomerization in water. We investigate the effect of Lewis (AlCl₃) and Br?nsted (HCl) acids on aluminum hydrolysis and glucose conversion. We demonstrate the interplay between the acids using the Optimum Logic Inc. speciation model (OLI software). We measure aqueous aluminum species and protons through in situ and ex situ²⁷Al quantitative nuclear magnetic resonance (qNMR) and pH measurements, respectively, and quantify aluminum nanoparticles through a combination of inductively coupled plasma-mass spectrometry (ICP-MS), dynamic light scattering (DLS), and ultrafiltration. Direct speciation measurements correlated with the glucose isomerization rate indicate that the hydrolyzed Al(iii) complex [Al(H₂O)₄(OH)₂]¹⁺ is the active species in glucose isomerization.

Full text of DOI:10.1039/c8ra03088j

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Direct speciation methods to quantify catalytically
active species of AlCl in glucose isomerization†
3
Cite this: RSC Adv., 2018, 8, 17101
Angela M. Norton, Hannah Nguyen, Nicholas L. Xiao and Dionisios G. Vlachos
*
While homogeneous metal halides have been shown to catalyze glucose to fructose isomerization, direct
experimental evidence in support of the catalytically active species remains elusive. Here, we integrate
direct speciation methods with kinetics to provide strong evidence for the active species of AlCl
3
in
glucose–fructose isomerization in water. We investigate the effect of Lewis (AlCl ) and Brønsted (HCl)
3
acids on aluminum hydrolysis and glucose conversion. We demonstrate the interplay between the acids
using the Optimum Logic Inc. speciation model (OLI software). We measure aqueous aluminum species
27
and protons through in situ and ex situ Al quantitative nuclear magnetic resonance (qNMR) and pH
measurements, respectively, and quantify aluminum nanoparticles through a combination of inductively
coupled plasma-mass spectrometry (ICP-MS), dynamic light scattering (DLS), and ultrafiltration. Direct
speciation measurements correlated with the glucose isomerization rate indicate that the hydrolyzed
Received 11th April 2018
Accepted 25th April 2018
DOI: 10.1039/c8ra03088j
1+
rsc.li/rsc-advances
Al(III) complex [Al(H
O)
2 4
2
(OH) ]
is the active species in glucose isomerization.
3,9–12
Metal halides are related homogeneous chemo-catalysts.
Introduction
Hu and co-workers compared the catalytic activity of AlCl3,
, and SnCl and found that CrCl exhibits the highest
dence on fossil fuels has prompted considerable research in the initial turnover frequency (TOF), while AlCl is the most selec-
The need to reduce greenhouse gas emissions and our depen- CrCl
3
4
3
3
9
production of fuels and chemicals from lignocellulosic tive catalyst to fructose. Mechanistic studies of glucose isom-
1
biomass. Biomass is a promising renewable feedstock due to erization with AlCl
3
and CrCl
3
show that the isomerization
its abundance and its ability to capture CO from the atmo- proceeds through a 1,2 hydride transfer, and this step is rate-
2
2
3
sphere via photosynthesis. Among the top biomass-derived limiting according to kinetic-isotope effect measurements.
chemicals, 5-(hydroxymethyl)furfural (HMF) has gained signif- Based on theoretical investigations, the catalytic species
icant interest as a platform chemical. HMF synthesis results responsible for the isomerization likely exists as a bifunctional
13
2
from the hydrolysis of cellulosic biomass to glucose, glucose Lewis acidic/Brønsted base site, in which the Brønsted base
1
isomerization to fructose, and fructose dehydration to HMF.
(–OH) facilitates an initial proton transfer, activating the
When the isomerization and dehydration steps occur in a single subsequent Lewis acid (metal center, M) catalyzed C2 / C1
10
pot, dehydration drives the equilibrium-limited isomerization hydride shi (Scheme 1). Direct experimental evidence in
3
resulting in higher yields. This then requires an isomerization support of the catalytic species has though been lacking. This
catalyst that is compatible with the Brønsted acid catalyst and lack of understanding stems from a number of metal hydrolysis
high temperatures associated with the dehydration reaction. steps and self-condensation leading to oligomeric species and
1
4–16
Currently, industrial applications use immobilized D-xylose potentially metal nanoparticles.
ketoisomerase to catalyze the isomerization; however, these
4,5
Different approaches have emerged to identify the catalyti-
catalysts are expensive, operate under narrow conditions, and cally active species. Our group combined kinetic rate measure-
6
require highly pure glucose. As a result, research has turned to ments with
a speciation thermodynamic model, called
the development of chemo-catalytic processes to carry out Optimum Logic Inc. Systems' Stream Analyzer soware (OLI
2
+
2 5 3
glucose isomerization, starting with the pioneering work of soware, 2017), to suggest [Cr(H O) (OH)] is active in CrCl -
17
Davis and co-workers on Sn-BEA zeolite that enable HMF catalyzed glucose isomerization. Hu and co-workers per-
7,8
production in a single pot.
formed tandem electrospray ionization mass spectrometry (ESI-
1
+
MS/MS) to propose, due to its abundance, [Al(H O) (OH) ] is
2
4
2
9,18
the active species when AlCl
3
is used as a catalyst. Given the
difference in the active species between CrCl and AlCl , the fact
3 3
Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular
Engineering, University of Delaware, 221 Academy St., Newark, DE 19716, USA. that ESI-MS/MS may alter speciation during the measurement,
19
E-mail: vlachos@udel.edu
and that observed species are oen spectators rather than the
RSC Adv., 2018, 8, 17101–17109 | 17101
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra03088j
1
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Scheme 1 Proposed mechanism of glucose to fructose isomerization in water. “M” represents a metal center [e.g., Cr, Al, and Sn (in Sn BEA)].
Redrawn from ref. 3.
active ones, the active aluminum species still remain(s) catalyst solution was stirred and heated for a specied time. The
unknown.
pH sensor was then inserted into the vessel and pH was ob-
In this work, we integrate kinetics with more direct specia- tained in situ. Heated samples were then cooled to room
tion measurements for the rst time to provide evidence for the temperature and pH was obtained ex situ. The sensor was
active species of AlCl . Our judicious choice of AlCl is based on conrmed to measure the pH of buffer solutions (Fisher
3
3
the fact that it is a selective catalyst in glucose isomerization Scientic) within their specied temperature range (298 to 373
2
0–24
and an excellent salt to be followed spectroscopically.
investigate the interplay between Lewis (AlCl ) and Brønsted
HCl) acids using the OLI soware, while simultaneously 27
We K, see Fig. S1†).
3
(
Al qNMR spectroscopy
developing an experimental protocol to quantify the various
2
7
2
7
Al qNMR was carried out on an Avance III 400 MHz NMR
aluminum species. We use Al quantitative nuclear magnetic
resonance (qNMR) and pH to measure the aqueous aluminum
species and protons, respectively, and combine inductively
coupled plasma-mass spectrometry (ICP-MS), dynamic light
scattering (DLS), and ultraltration to measure the aluminum
nanoparticles. Finally, we correlate the glucose isomerization
rate with the aluminum species' concentrations to propose an
active species.
spectrometer (Bruker). Spectra were measured at 104.27 MHz
pulse width: 12.50 ms, acquisition time: 0.1966 s, scans: 64,
solvent: 90% H O, 10% D O) and processed using Mestrelab
(
2
2
Research soware (mNOVA). The samples were prepared in
quartz NMR tubes (NewEra), containing 0.5 mL of reaction
mixture and were studied at 303 or 363 K. For quantication
purposes, an external standard of Al(H
5
3
+
2
O)
6
was prepared with
mM AlCl and 100 mM HCl (Fig. S2†).
3
Methods
Speciation model
Ultraltration
Ultraltration was performed using Vivaspin 500 concentrators
(Sartorius) with a 10 000 molecular weight cut-off (MWCO). The
concentrators were lled with 500 mL of either freshly prepared
or heat-treated catalyst solutions and placed into the centrifuge
AlCl3 speciation was predicted using the mixed-solvent elec-
25,26
trolyte OLI soware.
The model predicts speciation by
solving chemical equilibria of multicomponent systems and
combining standard-state thermochemical properties of solu-
tion species with an expression for the excess Gibbs free energy.
The standard-state properties are calculated using the Helge-
son–Kirkham–Flowers (HKF) equation of state for aqueous ions
and electrolytes at innite dilution, while the excess Gibbs free
energy model incorporates short, middle, and long-range elec-
trostatic interactions. Further details regarding these calcula-
(Eppendorf Centrifuge 5424) for 15 minutes at a spin speed of
15 000g. Samples were then recovered from the bottom of the
concentrate pocket with a pipette.
ICP-MS
ICP-MS measurements on freshly prepared and heat-treated
catalyst solutions were carried out using an Agilent 7500cx
Series instrument (Wilmington, DE). Samples underwent
ultraltration, followed by acid treatment prior to ICP-MS
analysis. The aluminum standards (Agilent) were prepared in
8
tions can be found in our previous work. The OLI model has
been parameterized at conditions that differ from those typi-
cally encountered in isomerization chemistry and thus, its
accuracy is unknown. We address this point in this paper by
more direct measurements of aluminum speciation.
3
HNO (70 w/w%, Sigma Aldrich) and diluted for the ICP-MS
calibration.
pH measurements
DLS
pH measurements were obtained using the 3300 High
Temperature (HT) PERpH-X sensor (Rosemount) and the 56 DLS experiments were conducted on a Brookhaven ZETAPALS
Advanced Dual-Input analyzer (Rosemount). typical instrument. DLS measurements were obtained both before and
A
measurement was conducted in a 100 mL thick-walled glass aer heating the catalyst solutions. The autocorrelation func-
vessel (Fisher Scientic) containing catalyst solution. The tions were analyzed using the Multimodal Size Distribution
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algorithm of the accompanying soware, where the viscosity of
the solutions was assumed to equal that of water.
Catalytic measurements
All chemicals were purchased from Sigma Aldrich and used as
received. The reactions were conducted in 10 mL glass vials
(
Sigma Aldrich) heated in an aluminum reactor block with
controlled stirring (Fisher Scientic). Catalyst solutions of
AlCl $6H O (Sigma Aldrich) and HCl (37 w/w%, Sigma Aldrich)
3
2
were stirred and preheated at reaction temperature for 24 h.
Kinetic experiments were then carried out in sealed reactor vials
containing the preheated catalyst solution (2 mL) and glucose.
The Al-to-reactant molar ratio was kept at 9 : 100, which corre-
sponds to 5 mM AlCl and 1 wt% glucose solution. The HCl
3
concentration varied from 3 to 44 mM when used with AlCl
except for the external standard mentioned above). At different
time points, the vials were removed from the oil bath, quenched
3
(
Fig. 1 Distribution of AlCl
temperature (413 K), calculated using the OLI software.
3 3
speciation (5 mM AlCl ) at reaction
in ice to stop the reaction, and ltered with a 0.2 mm lter
(
Fisher Scientic).
Quantication of the liquid products was achieved using
the formation of the hydrolyzed aluminum species and
3
+
3+
increasing the concentration of Al . The dominance of Al
species at high acid concentrations has been reported before.
high-performance liquid chromatography (HPLC, Waters Alli-
ance Instruments e2695), equipped with a refractive index (RI)
detector and a photodiode array (PDA) detector. Sugars were
separated using a Biorad HPX87C column at 348 K with HPLC-
grade water owing at 0.5 mL min as the mobile phase. Acid
byproducts and HMF were separated using a Biorad HPX87H
column heated to 323 K with 5 mM sulfuric acid owing at 0.5
3,15
In addition, the prediction of solid boehmite is consistent with
Hem et al.'s ndings, which demonstrated that heating aqueous
aluminum results in the formation of boehmite, whose struc-
ꢀ
1
28
ture was conrmed by X-ray diffraction (XRD). To our knowl-
edge, the concentrations of the hydrolyzed aluminum species
have not been quantied; however, Mesmer et al. determined
the hydrolyzed species are most signicant at relatively low
ꢀ
1
mL min as the mobile phase.
29
concentrations of aluminum (e.g., #10 mM), as considered
here. The model predicts the hydrolyzed species' concentra-
tions increase with an increase in solution temperature, which
is expected given that aluminum hydrolysis is endothermic
Results and discussion
Model-predicted AlCl speciation in aqueous media
3
In aqueous media, AlCl3 dissociates to form metal cations.
These cations are solvated by water, forming complexes, such as
27
(
Fig. S3†). Qualitatively, the results remain similar when
3
+
temperature changes.
2 6
[Al(H O) ] , which can be further hydrolyzed, as shown in eqn
15,16,27
(
1).
3
Equilibration of AlCl –HCl catalyst solutions
3
+
(3ꢀx)+
3
+ xH O
+
[
Al(H
2
O)
6
]
+ xH
2
O 4 [Al(H
2
O)6ꢀx(OH)
x
]
Next, we investigated the time necessary for the AlCl –HCl
(1)
3
catalyst solutions to reach equilibrium. Equilibration allows
30
The main species predicted using the OLI soware vs. HCl formation of the various aluminum species and thus proper
3
+
2 6
concentration include the hexa-aqua species, [Al(H O) ] , the comparison between the experimentally measured and pre-
26
stable, crystalline solid form of aluminum hydroxide, called dicted speciation, and ensures quasi-equilibrated speciation
boehmite, AlO(OH), and the mono- and di-hydroxy species, during short reaction times to enable kinetic analysis (which
2
+
1+
[Al(H
2 5 2 4 2
O) (OH)] and [Al(H O) (OH) ] , respectively (Fig. 1 and otherwise can be inuenced by varying speciation during
3
+
2+
Scheme 2). The cations are represented as Al , AlOH , and kinetics measurements). Different approaches have been
Al(OH)2 (for simplicity).
1
+
employed in prior work to determine the time necessary for
3
+
With increasing HCl concentration, the Al concentration equilibrium. Frink et al. measured the pH of aluminum salt
increases monotonically, and that of AlO(OH) decreases solutions as a function of time and temperature (298 K and 313
2
+
30
monotonically, whereas the concentrations of Al(OH) and K) upon cooling (ex situ). Hem et al. performed measurements
1
+
Al(OH)
2
exhibit volcano-like curves with peaks at intermediate of the pH together with quantication of polymeric and solid
HCl concentrations. At low HCl concentrations, the concentra- aluminum. Since OLI predictions indicate the formation of
28
2
+
1+
tions of Al(OH) and Al(OH)
2
increase as the pH drops at the solid aluminum, we measure pH and solids. The catalyst solu-
expense of the solid. The distribution of aluminum species at tions were preheated at reaction temperature for 24 h, and the
high HCl concentrations can be rationalized by Le Chatelier's pH was obtained in situ and ex situ; not a noticeable difference
+
principle, where an increase in H concentration shis the between in situ and ex situ pH measurements was found (see
thermodynamic equilibrium to the le in eqn (1), suppressing Fig. 2 at 363 K and Fig. S4† for measurements at 303 K and 413
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3
Scheme 2 Al(III) ions generated from the dissolution of AlCl in aqueous media.
K). The irreversible pH behavior observed upon cooling is most non-destructive, quantitative, and uses a low-energy electro-
likely due to the formation of stable solid aluminum species. magnetic wave, causing an extremely small energy perturbation
3
2–34
Thereaer, we present only ex situ pH data. The solid was to the system.
In addition, the quantitative accuracy of NMR
32–34
quantied through ultraltration followed by ICP-MS analysis analysis has reached three signicant gures. Following
of the permeate. The difference between the initial amount of a similar approach to that of Maki et al., we measured the
aluminum, prior to preheating, and the concentration of concentration of Al by comparing the qNMR peak areas of
aluminum in the permeate was taken as the amount of solid. experimental samples to an external standard, which consisted
Fig. 3a shows the ex situ measured pH and Fig. 3b shows the of 5 mM AlCl and 100 mM HCl. The concentration of HCl in the
32
3
+
3
3
+
amount of solid as a function of time. The pH initially drops standard is high to ensure all aluminum is in the form of Al ,
during the rst 1 h and then remains approximately unchanged according to Fig. 1. Similar to the pH measurements, it did not
3
+
aer 8 h, whereas solid formation increases with time and make a difference if the Al species was quantied in situ
eventually remains constant aer 24 h. Fig. 3 indicates multiple (Fig. S7 and Table S1†) or ex situ (Fig. 5 and Table S2†); both
time scales whereby solids form slowly compared to equilibra- measurements were in reasonable agreement with the specia-
tion of small species. Hsu proposed the solid results from an tion model (Fig. 6a). We hypothesize the unaccounted
14
intermediate polymeric species, which forms within the rst aluminum exists primarily as suspended solids, which are not
15
hour of heating, and, in combination with the formation of the detect by liquid-phase qNMR.
hydrolyzed species, results in the formation of H , which
+
decreases the pH. As time increases, the polymeric species
Solid aluminum
undergo nucleation to form stable solid species, such as
14
boehmite. This nucleation and/or subsequent growth is slow, We next quantied the solid aluminum species. Techniques
given the number of particles continue to increase for 24 h. DLS such as dialysis and ultraltration are commonly used to
measurements further indicate the formation of nanoparticles separate ne colloidal mineral aluminum and polymeric
15
and provide the average particle growth as a function of time aluminum species from soluble aluminum. Therefore, we
Fig. S5†). The average particle diameter remains constant aer
4 h of heating, which is consistent with the time taken for
(
2
observable solid formation using ultraltration/ICP-MS. We
observed similar results for samples preheated at 363 K
(Fig. S5†). For these samples, solids were present aer being
heated for 24 h or longer, and their size and amount remained
constant thereaer whereas the pH equilibrated faster
(Fig. S6†). Therefore, we have decided to preheat our samples
for 24 h at reaction temperature for equilibration of aluminum
species prior to conducting kinetic experiments.
Direct measurements of AlCl speciation
3
Fig. 4 outlines the methodology to quantify each species. qNMR
measurements were performed in situ at 363 K and ex situ at 413
K due to instrument limitations.
3
+
Fig. 2 pH measured ex situ (closed circles) or in situ (open circles)
compared to pH calculated from OLI software (line) at 363 K. Samples
Hexa-aqua aluminum (Al )
3
+
27
We rst quantied the hexa-aqua monomer, Al , using Al
contained 5 mM AlCl
qNMR, at a chemical shi of ꢁ0 ppm. qNMR spectroscopy is interval.
3
. Error bars correspond to 95% confidence
31
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3
Fig. 3 Determination of equilibrium from (a) pH, measured ex situ, and (b) amount of solid formed with time. Samples contained 5 mM AlCl and
were cooled from 413 K to 303 K.
combined ultraltration with DLS and ICP-MS measurements diameter of 5 nm, signicantly less than that detected by DLS
to quantify solid aluminum species in our catalyst solutions. (Fig. S8†). The samples then underwent heating and ultral-
DLS measurements detected the presence of aluminum parti- tration. ICP-MS determined the concentration of aluminum in
cles and provided the average particle diameter. This allowed the permeate. Our experimentally inferred data are consistent
for selection of an ultraltration membrane with a pore with the calculated amount of solid aluminum (Fig. 6b).
3
Fig. 4 Methodology to quantify aluminum speciation at specified concentrations of AlCl and HCl, temperature, and heating time.
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27
3+
Fig. 5
03 K.
Al qNMR spectra for Al . Experimental conditions: preheat 413 K for 24 h, 5 mM AlCl
3
. qNMR measurements obtained upon cooling to
3
2
+
1+
Hydrolyzed monomers (AlOH and Al(OH)2
)
the main product observed (Fig. S9 and S10†). Fig. 7 shows these
rates as a function of HCl concentration.
The low concentration and rapid proton exchange associated
with the hydrolyzed species make them difficult to observe
experimentally. Therefore, we deduced their concentration
through the fundamental equilibrium relation (Tables S3 and
S4†). This required the Al and H concentrations and the acid
dissociation constants (KA,1 and KA,2). The Al and H concen-
trations were obtained by qNMR and pH measurements,
respectively. The acid dissociation constants were obtained
from the OLI soware (Tables S3 and S4†). The experimentally
estimated mono- and di-hydrolyzed concentrations exhibit the
same volcano-like behavior predicted by the OLI soware as
a function of HCl concentration, as shown in Fig. 6c and d,
respectively.
Interestingly, with increasing HCl, the glucose conversion
rate exhibits a trend that is qualitatively consistent with the
27
2
+
1+
Al(OH) and Al(OH)2 species' concentrations, shown in Fig. 1
(see Fig. S3† for 363 K data). The slow glucose consumption at
low HCl concentrations (before the maximum rate) is congruent
with the formation of solid aluminum species, indicating the
solid likely hinders glucose conversion. Tang et al. observed,
3
+
+
3
+
+
when using solid Al(OH)
effectively catalyze the reaction, and resulted in low glucose
conversion (X ¼ 7.8%) and fructose yield (Y ¼ 6.5%)
3 3
as a catalyst, that Al(OH) did not
Fru
compared to AlCl (X ¼ 31.8% and Y ¼ 26.3%) at the same
3
Fru
3+
9
reaction conditions. Similarly, the Al species is not likely
catalytically active given the reaction rate continues to decrease
3
+
as the Al species concentration increases. The correlation
between the estimated glucose consumption rate and the
hydrolyzed species strongly indicates that one or both of these
Glucose conversion and catalytically active species in
equilibrated catalyst solution
(
3ꢀx)+
Al(OH)
x
species may be catalytically active for glucose
A series of kinetic experiments with glucose added to preheated
catalyst solutions containing AlCl and HCl were performed.
The initial rates of glucose isomerization were obtained by
tting the glucose concentration proles at low conversions
26
isomerization. Since the speciation model was developed at
conditions different from typical sugar experiments, we used
our direct speciation measurements to elucidate the aluminum
species.
3

(
#15%) with a rst-order rate expression, where fructose was
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Fig. 6 Aluminum speciation calculated by OLI software at 363 K (red dash lines) and 413 K (blue dotted lines) compared to experimentally
3+
inferred species (data represented by red and blue dots). (a) Al (measured in situ at 363 K and ex situ upon cooling from 413 K to 303 K), (b) solid
2+
1+
Al (measured ex situ), (c) Al(OH) , and (d) Al(OH)
2 3
. Experimental samples contained 5 mM AlCl and were preheated at specified temperature
for 24 h.
3
Fig. 7 Effect of HCl on glucose isomerization rate in AlCl solutions (5 mM) that have been equilibrated at (a) 413 K and (b) 363 K for 24 h prior to
kinetic study. Reaction conditions: glucose 1 wt%, Al to glucose molar ratio 9 : 100.
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Fig. 8 Glucose conversion as a function of measured Al species' concentrations, normalized to the maximum observed species' concentration
see Table S3† for observed species' concentrations). Catalyst solutions were preheated at 413 K for 24 h prior to kinetic study. Reaction
(
conditions: glucose 1 wt%, Al to glucose molar ratio of 9 : 100, 413 K.
1
+
At 413 K, the linear scaling observed between the glucose that the hydrolyzed Al(III) complex [Al(H
2
O)
4
(OH)
2
]
is the
1
+
isomerization rate and the di-hydrolyzed Al(OH)
2
concentra- active species in glucose isomerization. This nding supports
is the catalytically the hypothesis of Hu and co-workers, who observed [Al(H -
1
+
tion (Fig. 8) strongly indicates Al(OH)
active species. At 363 K, the low reaction rate, extremely small O) (OH) ]
2
2
1
+
9
using ESI-MS/MS. Furthermore, our ndings
4
2
values of the rate constants and low concentrations of the support the idea that the catalytic species responsible for the
aluminum species (see Table S4 and Fig. S11†) render our data isomerization exists as a bifunctional Lewis acidic/Brønsted
3
inconclusive regarding the active species.
base site. The approach developed here can be applied to study
the speciation of other metal halides. For metals not observed
by qNMR, such as CrCl
as ultraviolet visible light spectrometry (UV-Vis), could possibly
3
, other spectroscopic techniques, such
Conclusions
We have coupled kinetic studies with direct speciation be employed to study the dominant hexa-aqua species.
measurements to elucidate the active species of AlCl in glucose
3
to fructose isomerization. Experiments were designed based on
insights obtained from modeling the aluminum hydrolysis
Conflicts of interest
using the OLI soware. We established an experimental There are no conicts to declare.
protocol to quantify the various aluminum species that employs
2
7
Al qNMR for the hexa-aqua aluminum monomer, ultraltra-
Acknowledgements
tion and ICP-MS for the solid, and pH measurements combined
with equilibrium relations for the mono- and di-hydroxy We acknowledge support from the National Science Foundation
aluminum species. We found that speciation reaches equilib- under Award No. 1434456. The authors would like to thank
rium aer heating the catalyst solutions for long times. Linear Caroline Golt (UD) for performing the ICP-MS measurements,
scaling between the glucose isomerization rate and the specia- Dr Shi Bai (UD) and Dr Elizabeth McCord (UD) for their assis-
tion measurements at sufficiently high temperatures indicates tance with the qNMR method, and John Ruano-Salguero (UD),
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Prof. Raul Lobo (UD), and Dr Basudeb Saha (UD) for useful 16 C. F. Baes, The Hydrolysis of Cations, Krieger Publishing
discussions.
Company, Malabar, FL, 1976.
1
7 V. Choudhary, S. H. Mushrif, C. Ho, A. Anderko,
V. Nikolakis, N. S. Marinkovic, A. I. Frenkel, S. I. Sandler
and D. G. Vlachos, J. Am. Chem. Soc., 2013, 135, 3997–4006.
References
1
R. J. van Putten, J. C. van der Waal, E. de Jong, 18 J. Q. Tang, L. F. Zhu, X. Fu, J. H. Dai, X. W. Guo and C. W. Hu,
C. B. Rasrendra, H. J. Heeres and J. G. de Vries, Chem. Rev.,
ACS Catal., 2017, 7, 256–266.
013, 113, 1499–1597.
19 I. I. Stewart, Spectrochim. Acta, Part B, 1999, 54, 1649–1695.
A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411– 20 P. M. Bertsch, R. I. Barnhisel, G. W. Thomas, W. J. Layton
502.
and S. L. Smith, Anal. Chem., 1986, 58, 2583–2585.
V. Choudhary, A. B. Pinar, R. F. Lobo, D. G. Vlachos and 21 M. A. Curtin, L. R. Ingalls, A. Campbell and M. James-
S. I. Sandler, ChemSusChem, 2013, 6, 2369–2376.
Pederson, J. Chem. Educ., 2008, 85, 291–293.
T. D. Fenn, D. Ringe and G. A. Petsko, Biochemistry, 2004, 43, 22 B. C. Faust, W. B. Labiosa, K. H. Dai, J. S. Macfall,
2
2
3
4
5
6
7
8
9
2
6
464–6474.
B. A. Browne, A. A. Ribeiro and D. D. Righter, Geochim.
Cosmochim. Acta, 1995, 59, 2651–2661.
23 K. Shafran, O. Deschaume and C. C. Perry, Adv. Eng. Mater.,
2004, 6, 836–839.
24 A. Singhal and K. D. Keefer, J. Mater. Res., 1994, 9, 1973–
1983.
25 P. M. Wang and A. Anderko, Fluid Phase Equilib., 2001, 186,
103–122.
R. DiCosimo, J. McAuliffe, A. J. Poulose and G. Bohlmann,
Chem. Soc. Rev., 2013, 42, 6437–6474.
S. H. Bhosale, M. B. Rao and V. V. Deshpande, Microbiol.
Rev., 1996, 60, 280–300.
M. Moliner, Y. Roman-Leshkov and M. E. Davis, Proc. Natl.
Acad. Sci. U. S. A., 2010, 107, 6164–6168.
E. Nikolla, Y. Roman-Leshkov, M. Moliner and M. E. Davis,
ACS Catal., 2011, 1, 408–410.
26 P. M. Wang, A. Anderko and R. D. Young, Fluid Phase
Equilib., 2002, 203, 141–176.
J. Q. Tang, X. W. Guo, L. F. Zhu and C. W. Hu, ACS Catal.,
2015, 5, 5097–5103.
27 C. F. Baes and R. E. Mesmer, Am. J. Sci., 1981, 281, 935–962.
1
1
1
1
0 T. D. Swi, H. Nguyen, A. Anderko, V. Nikolakis and 28 J. D. Hem, Adv. Chem. Ser., 1968, 98–114.
D. G. Vlachos, Green Chem., 2015, 17, 4725–4735.
1 Y. J. Pagan-Torres, T. F. Wang, J. M. R. Gallo, B. H. Shanks
and J. A. Dumesic, ACS Catal., 2012, 2, 930–934.
29 R. E. Mesmer and C. F. Baes, Inorg. Chem., 1971, 10, 2290–
2296.
30 C. R. Frink and M. Peech, Inorg. Chem., 1963, 2, 473–478.
2 V. Choudhary, S. I. Sandler and D. G. Vlachos, ACS Catal., 31 J. W. Akitt, Multinuclear NMR, Springer, US, New York, 1987.
2012, 2, 2022–2028.
32 H. Maki, G. Sakata and M. Mizuhata, Analyst, 2017, 142,
3 R. Bermejo-Deval, R. S. Assary, E. Nikolla, M. Moliner,
1790–1799.
Y. Roman-Leshkov, S. J. Hwang, A. Palsdottir, 33 F. Malz and H. Jancke, J. Pharm. Biomed. Anal., 2005, 38, 813–
D. Silverman, R. F. Lobo, L. A. Curtiss and M. E. Davis,
Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 9727–9732.
4 P. H. Hsu, Clays Clay Miner., 1988, 36, 25–30.
5 G. Sposito, The Environmental Chemistry of Aluminum, CRC
Press Inc., Boca Raton, FL, 1989.
823.
34 G. F. Pauli, B. U. Jaki and D. C. Lankin, J. Nat. Prod., 2005, 68,
133–149.
1
1
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