Enhanced Acidity and Activity of Aluminum/Gallium-Based Ionic Liquids Resulting from Dynamic Anionic Speciation

Article abstract of DOI:10.1021/acscatal.9b03132

To explore if the catalytic activity of single-metal species in halometallate catalysts can be enhanced with mixed-metal species, aluminum and gallium were simultaneously introduced to prepare a mixed-metal ionic liquid (IL) through a one-pot reaction of [HN₂₂₂]Cl, AlCl₃, and GaCl₃ in various amounts. The Lewis acidity in Friedel-Crafts acylation and alkylation reactions of these IL compositions were evaluated and all mixed-metal [HN₂₂₂][xAlCl₃ + (2 - x)GaCl₃]Cl ILs exhibited higher catalytic activity than AlCl₃, GaCl₃, [HN₂₂₂][Al₂Cl₇], or [HN₂₂₂][Ga₂Cl₇]. The broadening in ²⁷Al nuclear magnetic resonance (NMR) of these ILs indicates the presence of dynamic anionic species, whereas matrix-assisted laser-desorption ionization time-of-flight (MALDI-TOF) and acetonitrile-probed Fourier transform infrared (FT-IR) analyses correlates the structure-activity relationship, for instance, identifying the presence of mixed-metal-containing [Cl₂GaOClAlClOGaCl₂]^- species in the better-performing ILs.

Full text of DOI:10.1021/acscatal.9b03132

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Letter
Enhanced Acidity and Activity of Aluminum/Gallium Based
Ionic Liquids Resulting from Dynamic Anionic Speciation
Kai Li, Hemant Choudhary, Manish Kumar Mishra, and Robin D Rogers
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03132 • Publication Date (Web): 26 Sep 2019
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ACS Catalysis
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Enhanced Acidity and Activity of Aluminum/Gallium Based Ionic
Liquids Resulting from Dynamic Anionic Speciation
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Kai Li,a,‖ Hemant Choudhary,a,ξ Manish Kumar Mishraa,§ and Robin D. Rogersa,b,*
a College of Arts & Sciences, The University of Alabama, Tuscaloosa, Alabama 35487, United States
b 525 Solutions, Inc., P.O. Box 2206, Tuscaloosa, Alabama 35403, United States
ABSTRACT: To explore if the catalytic activity of single metal species in halometallate catalysts can be enhanced with mixed
metal species, aluminum and gallium were simultaneously introduced to prepare a mixed metal ionic liquids (IL) through a
one-pot reaction of [HN222]Cl, AlCl3, and GaCl3 in various amounts. The Lewis acidity in Friedel-Crafts acylation and alkylation
reactions of these IL compositions were evaluated and all mixed metal [HN222][xAlCl3+(2-x)GaCl3]Cl ILs exhibited higher
catalytic activity than AlCl3, GaCl3, [HN222][Al2Cl7], or [HN222][Ga2Cl7]. The broadening in 27Al NMR of these ILs indicates the
presence of dynamic anionic species, whereas MALDI-TOF and acetonitrile-probed FT-IR analyses correlates the structure-
activity relationship, for instance, identifying the presence of mixed metal containing [Cl2GaOClAlClOGaCl2]- species in the
better performing ILs.
KEYWORDS. Double salt ionic liquids, Friedel-Crafts acylation/alkylation, Lewis acid catalyst, chloroaluminate,
chlorogallate
Ionic liquids (ILs, salts with melting points below 100
been used as a catalyst for industrial isobutane alkylation.14
Very recently, we have also reported superior catalytic
°C1), have aroused extensive attention in different areas,
such as biomass processing,2,
active pharmaceutical
performances
of
an
Al-Zn
IL
system
3
ingredients,4 process development,5 materials sciences,6
etc., due to their easily accessible and tunable properties.1, 7
The ease of combining different cations and anions in IL
structures to tune their catalytic properties has also been a
promising and active area in the search for alternatives to
traditional catalysts. For instance, halometallate ILs (a
subclass of ILs possessing metal halide(s) in/as anion8)
have been utilized as reaction media or active acidic
catalysts in many different reactions.8a, 9
([HN222]2x[(1−x)AlCl3+xZnCl4]) in Beckman rearrangement
reactions when compared to the individual single metal
catalysts.15 The results suggested that this approach could
be a unique methodology for the design of highly pertinent
catalysts. Motivated by these results and with an urge to
develop highly active acidic catalyst, we sought to
strategically design new multiple metal center containing IL
catalytic systems, which could be prepared in
straightforward one-pot reactions.
Chauvin’s pioneering work with chloroaluminates for
catalytic olefin dimerization and metathesis was awarded
the 2005 Noble Prize in Chemistry.10 Since then, these ILs
have been exploited to replace traditional toxic and
corrosive Lewis acids e.g., aluminum chloride (AlCl3) or
boron trifluoride (BF3), and Brønsted acids, such as
hydrofluoric acid (HF) or concentrated sulfuric acid
(H2SO4), etc., as catalysts in industrially relevant chemical
transformations,11 such as Friedel-Crafts reactions,
esterifications, and oxidations, among others. In fact,
Chevron recently announced a scaled-up facility for liquid-
phase alkylation using chloroaluminate ILs replacing the
toxic HF.12
Both AlCl3 and gallium chloride (GaCl3) are established
Lewis acids, belonging to the same group and having similar
coordination
environments.
Aluminum
is
more
electronegative than Ga indicating more electrostatic
attraction, while Ga has a larger size that can help
electron/charge delocalization. Both chloroaluminate and
chlorogallate ILs have been separately reported to be active
16
in acid-catalyzed reactions.7, We hypothesized that ILs
containing both Al3+ and Ga3+ would possess an ideal Lewis
acidity leading to synergistic activity. Interestingly, despite
the ease of making mixed-metal ILs of this type, the
combination of both Al and Ga into one IL has not yet been
reported.
The field of ILs has experienced extensive growth and
development in the past few years, and recently attention is
starting to focus on more complex liquids containing more
than two types of ions, variously named binary mixtures,
high ionicity ionic liquids, and double salt ionic liquids
(DSILs).13 Compared to single metal halometallate ILs, these
multiple metal center containing ILs can allow fine tuning of
the physical and chemical properties of the final IL
composition. ILs based on chloroaluminates and CuCl have
In our work, Al/Ga-based ILs were easily synthesized
in a one-pot approach by direct mixing, under an inert (Ar)
atmosphere and without additional solvents. Solid
triethylammonium chloride ([HN222]Cl), AlCl3, and GaCl3
were mixed in a 10 mL vial where the molar ratio of total
metal chlorides to ammonium salt was 2:1. The Al:Ga molar
ratio was varied to give systems of the following formulas
[HN222][xAlCl3+(2-x)GaCl3]Cl, where x = 2.0, 1.5, 1.33, 1.0,
0.67, 0.5, or 0 (ESI, Scheme 1). Mixing of the solid reactants
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led to an exothermic reaction immediately yielding
transparent liquids in all molar ratios studied. These
systems were then sealed and heated at 50 oC for 24 h.
CH3CN molecules in a distorted octahedral geometry
(Figure 2). The four equatorial acetonitrile ligands in the
cation are in the plane perpendicular to A1-C1(1) bond. This
bond slightly bends away from the equatorial acetonitrile
ligands plane with a mean C1-A1-N angle of 94.6° due to the
consequence of steric effects generated by Cl. The [GaCl4]-
anions have approximate tetrahedral geometry.
1
2
3
4
5
6
+ x AlCl3 + (2-x) GaCl3
[HN222][xAlCl3+(2-x)GaCl3]Cl
H
N
Cl
7
8
9
Scheme 1. One pot synthesis of [HN222][xAlCl3+(2-
x)GaCl3]Cl liquids from solid precursors.
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The Lewis acidity of the [HN222][xAlCl3+(2-x)GaCl3]Cl,
ILs was investigated by FT-IR spectroscopy with
acetonitrile (CH3CN) as probe,17 and compared with AlCl3
and GaCl3. The ILs, AlCl3, and GaCl3 were treated with CH3CN
in 1:1 molar ratio and the FT-IR spectra were recorded
(Figure 1). Pure CH3CN has two characteristic bands at
2292 and 2252 cm-1, originating from its C≡N stretching
vibrations. For AlCl3, GaCl3, and [HN222][xAlCl3+(2-
x)GaCl3]Cl, two new bands at ca. 2320 and 2300 cm-1
appeared after mixing with acetonitrile, confirming
coordination of CH3CN and allowing an evaluation of the
overall Lewis acidity of the metal centers.
Figure 2. Probability ellipsoid (50%) plot of the formula
unit of [Al(CH3CN)5Cl][GaCl4]2•CH3CN.
Overall, these results confirmed that the ILs with
double metal centers had higher Lewis acidity compared
with the Lewis acid precursors, confirming our hypothesis.
We next tested their catalytic performance in the classic
Lewis acid catalyzed Friedel-Crafts acylation reaction of
1,3-dimethoxybenzene (DMB) and acetyl chloride. Table 1
summarizes the results for acylation of DMB with 10 mol%
[HN222][xAlCl3+(2-x)GaCl3]Cl, AlCl3, and GaCl3 in benzene at
30 °C for 3 h. The only product observed under our
employed reaction conditions was the monoacylated
product 2',4'-dimethoxyacetophenone (DMAP). Among all
of the acid catalysts, [HN222][1.33AlCl3+0.67GaCl3]Cl
resulted in the highest yields of DMAP (25.4%; Table 1,
entry 6).
Interestingly, the well-established state-of-the-art
Lewis acidic catalysts, AlCl3 and GaCl3, afforded DMAP in
only 7.3% and 3.6% yield, respectively, under identical
conditions (Table 1, entries 3-4). The all Al-based IL,
[HN222][Al2Cl7], could not convert DMB efficiently and
offered only 3.7% DMAP yield under the employed reaction
conditions, even lower than AlCl3 (Table 1, entry 5). The all
Ga-based IL, [HN222][Ga2Cl7], on the other hand, had a
decent DMB conversion to form 16.6% DMAP (Table 1,
entry 11). Nevertheless, all of the Al/Ga ILs could convert
48-70% DMB to afford DMAP yields of 13-25% (Table 1,
entries 6-10).
Figure 1. FT-IR spectra of the ILs and starting materials
after treatment with acetonitrile.
In these spectra, the higher the wavenumber of the
coordinated CH3CN bands, the stronger the coordination
and the higher the Lewis acidity. Compared to AlCl3 and
GaCl3, all [HN222][xAlCl3+(2-x)GaCl3]Cl ILs exhibited higher
CH3CN band wavenumbers (around 2330 cm-1; Figure 1)
indicating each IL was more Lewis acidic than either pure
metal chloride. Of the ILs, [HN222][1.33AlCl3+0.67GaCl3]Cl,
was found to be the most acidic (υ = 2335 cm-1).
The catalytic activity of the mixed metal ILs was also
evaluated for the alkylation of benzyl chloride (BzCl) using
benzene under inert atmosphere at 30 °C for 15 min (Table
1). Diphenylmethane (DPM) was the only product detected.
The trend in catalytic activities was similar to that observed
Interestingly, during this study crystals were formed
overnight from the 1:1 solution of [HN222][xAlCl3+(2-
x)GaCl3]Cl and acetonitrile. Single crystal X-ray diffraction
of these crystals revealed them to be the previously
for
the
acylation
reaction,
with
[HN222][1.33AlCl3+0.67GaCl3]Cl exhibiting the highest DPM
yield (72%, Table 1, entry 7). DPM yields with AlCl3 and
GaCl3 under identical conditions were 63.6% an 64.1%,
respectively (Table 1, entries 3-4). The all Al- or Ga-based
ILs afforded only 65.5% and 57.8% DPM, respectively
reported
[Al(CH3CN)5Cl][AlCl4]2•CH3CN18
and
[Al(CH3CN)5Cl][GaCl4]2•CH3CN (Figure 2). The crystal
structure of the latter is isostructural with the former,18
with one [Al(CH3CN)5Cl]2+ cation and two [GaCl4]- anions. In
the cation, the Al atom is bonded to one Cl atom and five
(Table 1, entries
5 and 11). The [HN222][xAlCl3+(2-
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ACS Catalysis
x)GaCl3]Cl ILs have better or similar catalytic performance
considering the yields, compared with the well-established
Lewis acids catalyst, e.g., AlCl3, GaCl3, [HN222][Al2Cl7], and
[HN222][Ga2Cl7].
[HN222][Al2Cl7]. 71Ga NMR was attempted to characterize the
ILs, however no signal was obtained, which is consistent
with the reported absence of a 71Ga signal in studies where
polynuclear anions containing Ga were studied.21
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5
6
7
8
9
Table 1. Acylation and alkylation reactions studied
MeO
OMe
MeO
OMe
O
C
10 mol% Catalyst
30 oC, 3 h
+
C
O
CH3
H3C
Cl
DMB
DMAP
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10 mol% Catalyst
30 oC, 15 min
Cl
+
DPM
BzCl
DMAPa
DPMb
Entry Catalyst
Yield Sel. Yield Sel.
(%)
0
(%)
0
(%)
0
(%)
0
Figure 3. 27Al NMR spectra of [HN222][xAlCl3+(2-x)GaCl3]Cl
ILs at 300 K, DMSO-d6 as external lock solvent in a capillary.
1
2
3
4
5
None
[HN222]Cl
0
0
0
0
AlCl3
GaCl3
To
further
understand
the
speciation
in
7.3
3.6
3.7
34.1 63.6 63.6
32.0 64.1 64.1
12.3 65.5 65.5
[HN222][xAlCl3+(2-x)GaCl3]Cl, these ILs were analyzed with
matrix-assisted laser-desorption ionization time-of-flight
(MALDI-TOF). All of the ILs have the expected peak at m/z
102 in the positive-ion spectra (Figure S4), indicating the
existence of the [HN222]+ cation. The negative-ion spectrum
of [HN222][Al2Cl7] (Figure 4) has two signals at m/z 168 and
302 that can be assigned to [AlCl4]- and [Al2Cl7]-. This is in
agreement with the broad signal assigned to [AlnCl3n+1]- in
the 27Al NMR.22 For [HN222][Ga2Cl7], GaCl3 and [GaCl4]- were
observed, confirming the dynamic equilibrium between
GaCl3, [GaCl4]-, and [Ga2Cl7]-.21
[HN222][Al2Cl7]
[HN222][1.5AlCl3
+0.5GaCl3]Cl
6
7
8
9
18.7
25.4
20.3
13.7
36.0 69.3 69.3
20.9 72.0 72.0
31.6 71.7 71.7
28.5 68.0 68.0
[HN222][1.33AlCl3
+ 0.67GaCl3]Cl
[HN222][1.0AlCl3
+1.0GaCl3]Cl
[HN222][0.67AlCl3
+1.33GaCl3]Cl
[HN222][0.5AlCl3
+1.5GaCl3]Cl
10
11
19.1
16.6
31.1 62.5 62.5
33.7 57.8 57.8
[HN222][Ga2Cl7]
aDMB: Acetyl Chloride: Catalyst = 1:1:0.1 (molar ratio),
o
b
benzene (1 mL), 30 C, 3 h. BzCl: Catalyst = 1:0.1 (molar
o
ratio), benzene (5 mL), 30 C, 15 min. Product yield and
selectivity were determined by GC-MS using naphthalene as
an internal standard.
Having shown [HN222][1.33AlCl3+0.67GaCl3]Cl to be the
best catalyst, we sought to understand what species was
responsible for the synergistic catalytic activity. 1H, 13C, 27Al,
and 71Ga NMR spectra were recorded for all of the ILs
Figure 4. Negative-ion MALDI-TOF mass spectra of (a)
[HN222][Al2Cl7], (b) [HN222][1.33AlCl3+0.67GaCl3]Cl, and (c)
[HN222][Ga2Cl7].
studied (see Figures 3, S1, S2). 1H and 13C NMR confirmed
+
the presence of [HN222
]
rather than any N-coordinated
species. 27Al NMR (Figure 3) showed a broad peak centered
at 106 ppm for[HN222][Al2Cl7] which was assigned to the
[AlnCl3n+1]- anion in dynamic equilibrium based on previous
reports (Figure S3).19 The spectrum for Ga containing ILs,
exhibited a clear hump at 98 ppm and also the broader peak
for [AlnCl3n+1]-. This suggests that the chemical environment
of Al in the anion is not the same in these ILs compared with
[HN222][Al2Cl7]. The peak shapes of these ILs are also
different, which is possibly due to the dynamic equilibrium
of the anion species and their different concentrations20 We
believe the presence of Ga modifies the chemical
environment leading to stronger acidic species, which could
explain the higher activity of these ILs compared with
When [HN222][1.33AlCl3+0.67GaCl3]Cl was studied by
MALDI-TOF, a [GaCl4]- peak at m/z 211 was observed,
however, a peak corresponding to [AlCl4]- was not found.
Instead, a new peak at m/z 415 appeared suggesting the
presence of an anion containing both Al and Ga. This peak
was also observed for [HN222][1.0AlCl3+1.0GaCl3]Cl and
[HN222][0.67AlCl3+1.33GaCl3]Cl (Figure S5) and suggests
the presence of a complex polynuclear species consisting of
Al, Ga, O, and Cl atoms based on the mass and charge, which
could possibly be [Cl2GaOClAlClOGaCl2]- (Figure S6), where
the oxygen and chloride atoms are shared by both Al and Ga.
The presence of oxygen is likely due to the highly
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