Orderly Layered Zr-Benzylphosphonate Nanohybrids for Efficient Acid–Base-Mediated Bifunctional/Cascade Catalysis

Article abstract of DOI:10.1002/cssc.201601570

The development of functional metal–organic materials that are robust and active for bifunctional/cascade catalysis is of great significance. Herein, a series of mesoporous and orderly layered nanohybrids were synthesized for the first time through simple and template-free assembly of ortho-, meta-, or para-xylylenediphosphonates (o-, p-, or m-PhP) containing zirconium. It was found that m-PhPZr nanoparticles (20–50 nm) with mesopores centered at 7.9 nm and high Lewis acid–base site ratio (1:0.7) showed excellent performance under mild conditions (as low as 82 °C) in transfer hydrogenation of carbonyl compounds, including bioaldehydes and alcohols, with near quantitative yields and little Zr leaching. Isotopic labeling studies indicated the occurrence of direct hydrogen transfer rather than metal hydride route by bifunctional catalysis. Lewis acidic (Zr) and basic (PO₃) centers of the heterogeneous catalyst were further revealed to play a synergistic role in one-pot cascade transformations, for example, of ethyl levulinate to γ-valerolactone and glucose to 5-hydroxymethylfurfural.

Full text of DOI:10.1002/cssc.201601570

Accepted Article
Title: Orderly layered Zr-benzylphosphonate nanohybrids for efficient
acid/base-mediated bifunctional/cascade catalysis
Authors: Hu Li, zhen fang, Jian He, and Song Yang
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10.1002/cssc.201601570
ChemSusChem
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Orderly layered Zr-benzylphosphonate nanohybrids for efficient
acid/base-mediated bifunctional/cascade catalysis
Hu Li,[a,b] Zhen Fang,*[a] Jian He,[b] Song Yang[b]
hydrogenation and dehydration processes are often used for the
efficient removal of oxygen to increase C and H contents for the
production of value-added chemicals and biofuels [10]. However,
noble metals (e.g., Au, Pd, and Ru), high molecular hydrogen
pressure, and mordant acids are generally required to achieve
the desirable selectivity of biomass-derived products [11]. In
contrast, catalytic transfer hydrogenation (CTH) using liquid
organic hydrogen donors (e.g., alcohols and formic acid) can
enhance the degree of control in selective hydrogenation,
alleviate the safety concern of handling flammable H2, and
reduce the cost and complexity of reaction units [12]. Besides
noble and transition metal nanoparticles [13], cheap Zr-based
catalysts (e.g., ZrO2, ZrO(OH)2 and ZrFeOx) are active for CTH
Abstract: The development of functional metal-organic materials
being robust and active for bifunctional/cascade catalysis is of great
significance. Herein, a series of mesoporous and orderly layered
nanohybrids were synthesized for the first time via simple and
template-free
assembly
of
ortho-,
meta-,
or
para-
xylylenediphosphonates (o-, p-, or m-PhP) with zirconium. It was
found that m-PhPZr nanoparticles (20-50 nm) with mesopores
centered at 7.9 nm and high Lewis acid/base site ratio (1:0.7)
showed prominent performance under mild conditions (as low as 82
ºC) in transfer hydrogenation of carbonyl compounds including bio-
aldehydes to alcohols with near quantative yields and little Zr
leaching. Isotopic labeling study indicated the occurrence of direct
hydrogen transfer rather than metal hydride route via bifunctional
catalysis. Lewis acidic (Zr) and basic (PO3) centers of the
heterogeneous catalyst were further revealed to play a synergistic
role in one-pot cascade transformations, e.g., of ethyl levulinate to γ-
valerolactone and glucose to 5-hydroxymethylfurfural.
of
carbonyl
compounds
(e.g.,
levulinates,
5-
[14]
hydroxymethylfurfural (HMF) and furfural)
,
but, some
drawbacks including harsh reaction conditions and deactivation
of them are always encountered. Hence, there is still a high
demand to develop stable and effective Zr-containing materials
for CTH reactions. To our knowledge, rare studies have been
made on the structural optimization and use of zirconium
phosphonate-based UMOFs for biomass valorization via
bifunctional catalysis [14f,14g]. Herein, a series of novel zirconium
benzylphosphonate nanohybrids were prepared by simply
assembling ortho-, para-, and meta-xylylenediphosphonic acids
with ZrCl4 (denoted as o-, p-, and m-PhPZr) for CTH and
selected cascade reactions in high efficiency.
Carboxylate-based metal-organic frameworks (MOFs), a class of
porous coordination polymers, have been demonstrated to
exhibit a wide range of potential applications such as gas
storage, adsorption and separation, drug delivery, and
luminescence [1]. As a promising candidate to conventional
heterogeneous catalysts, crystalline MOFs with flexible organic
ligands, functional organic sites, embedded metal nanoparticles,
and coordinatively unsaturated metal sites are active for solar
fuel production and organic synthesis via photocatalysis [2]. In
chemocatalytic reactions, the microporous cavities of MOFs
typically offer substrate-size selectivity [3], which on the other
hand may hinder the access of bulk molecules (e.g., highly
branched- and long-chain organics) to the active sites of MOFs,
thus possibly lowering the reaction rate and performance. To
alleviate the mass diffusion and transfer obstacle of large
molecules, microwave- and surfactant-assisted strategies are
thus adopted to synthesize MOFs with mesoscale domains of 2-
50 nm [4]. However, the cost and stability of the resulting
mesoporous materials are rarely taken into account [5]. Unlike
typical MOFs, trivalent/tetravalent metal phosphonate-based
unconventional MOFs (UMOFs) are rarely crystalline, but tend to
The prepared o-, p-, and m-PhPZr hybrids were initially
characterized by transmission electron microscopy (TEM). In Fig.
1A-C, all the samples with disordered particle dimension are in
nano scale (< 100 nm), which can be ascribed to the amorphous
structure of zirconium phosphonates illustrated by X-ray
diffraction (XRD, Fig. S1). Notably, m-PhPZr shows highly
ordered interlayers, while a larger amount of platelike particles
are bonded together in the cases of o- and p-PhPZr, thus
exhibiting no regular layer distribution (Fig. S2). Inductively
coupled plasma-optical emission spectrometer (ICP-OES)
analysis shows an approximately 1:2 of Zr/P molar ratio for o-, p-,
and m-PhPZr nanohybrids (Table S1), and the possible
connectivity
patterns
between
Zr4+
and
different
benzylphosphonates are provided in Fig. S3. The broad and
strong absorptions ranging from 900 to 1200 cm-1 in Fourier
transform infrared (FT-IR) spectra of o-, p-, and m-PhPZr
nanohybrids (Fig. S4) are assigned to P−O stretching vibrations
precipitate as irregularly layered hybrids and are highly insoluble
[6]
as well as stable even in acidic system
. Owing to
phosphonates capable of segregating hydrophobic moieties
from hydrophilic groups [7], the surfactant-free self-assembly of
homogeneous metal and R–PO3H compositions render the
possibility of creating mesoporous hybrid materials with robust
[15], from the characteristic of α-type ZrP materials [16]
.
structure [8]
.
Lignocellulosic biomass, the most abundant organic carbon
source in the nature, has been deemed as a prospective
[9]
alternative to fossil fuel-based industry
.
Catalytic
Fig. 1. TEM images of (A) o-PhPZr, (B) p-PhPZr, and (C) m-PhPZr
[a]
[b]
Dr. H. Li, Prof. Z. Fang
Biomass Group, College of Engineering, Nanjing Agricultural
University, 40 Dianjiangtai Road, Nanjing, Jiangsu 210031, China.
E-mail: zhen.fang@mail.mcgill.ca, zhenfang@njau.edu.cn. URL:
http://biomass-group.njau.edu.cn/
Thermogravimetric
(TG)
analysis
demonstrates
the
thermostability of m-PhPZr (up to ~300 ºC) is superior to that of
o- and p-PhPZr nanohybrids (Fig. 2A). Despite the presence of
micropores in these nanohybrids (Fig. 2B), the average pore
diameter of m-PhPZr is centered at 7.9 nm (Fig. 2C), showing
more regular pore-size distribution to others (Fig. S5). Moreover,
a good spatial correspondence of Zr, P and O elemental maps in
Dr. H. Li, J. He, Prof. S. Yang
State-Local Joint Engineering Laboratory for Comprehensive
Utilization of Biomass, State Key Laboratory Breeding Base of
Green Pesticide and Agricultural Bioengineering (Ministry of
Education), Center for R&D of Fine Chemicals, Guizhou University,
Guiyang 550025, China
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10.1002/cssc.201601570
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m-PhPZr is observed by scanning transmission electron
microscope and high-angle annular dark-field (STEM-HAADF)
(Fig. 2D-G), indicating the even connectivity and dispersion of –
frequency (TOF; 8.6 h-1). The promotional and synergistic effect
of Lewis acid-base sites on increasing GVL selectivity and EL
conversion are clearly confirmed by the active sites-poisoned
experiments (entries 5-6), wherein, GVL yield is significantly
decreased by poisoning either Lewis acid or base sites.
2–
PO3 and Zr4+ moieties. The presence of O2− and Zr species in
the bonding mode of −P−O−Zr− was most likely to be
responsible for forming basic and acidic sites of the hybrids,
respectively[14f]. For comparison, other Zr-containing hybrids
were also prepared with similar structural and textural properties
(Figs. S1, S4-S9, Tables S2-S3). It should be noted that the
difference in basicity and acidity of these Zr-based catalysts
(Table S3), determined separately by CO2-TPD (temperature
programmed desorption, Fig. S8) and pyridine adsorbed FT-IR
(Fig. S9), could be ascribed to the discrepant types of acid
radicals (i.e., phosphonate and carboxylate)[14f] and their
different substitution positions (i.e., ortho-, meta-, and para) in
the aromatic ring[17] that possibly affected the binding strength of
the acid radical and Zr4+.
Catalytic transformation of levulinic acid and its esters to γ-
valerolactone (GVL) is one of the important reactions for
producing biofuel additives from biomass [18], which generally
involves cascade reduction of carbonyl groups and
intramolecular transesterification (i.e., cyclization). Hereby, CTH
of ethyl levulinate (EL) to GVL in isopropanol (i-PrOH) was thus
selected to study the performance of o-, p-, and m-PhPZr
nanohybrids (Table 1). Pristine ZrO2 gave a high EL conversion
of 97%, but a moderate GVL yield of 76% with isopropyl
levulinate (IPL) as the major byproduct (entry 1), which may be
ascribed to the occurrence of intermolecular transesterification
facilitated by more base sites (Table S3, Fig. S8). In contrast, o-,
p-, and m-PhPZr hybrids having relatively low molar ratios of
base/acid sites and moderate Lewis acidity (Table S3, Fig. S9)
were likely to impede the formation of IPL (1-6% yields),
showing improved GVL yields of 89-98% (entries 2-4). In
addition to the superior GVL yield, m-PhPZr also exhibited the
highest GVL formation rate (38.9 μmol g-1 min-1) and turnover
Fig. 2. (A) TG curves and (B) N2 adsorption-desorption isotherms for o-, p-,
and m-PhPZr hybrids, as well as (C) pore-size distribution, (D) STEM-HAADF
image, and (E) Zr, (F) P & (G) O elemental mappings for m-PhPZr nanohybrid
Table 1. Catalytic transfer hydrogenation-cyclization of EL to GVL with Zr-containing catalysts
Temp.
(ºC)
Time
(h)
Catal./EL mass
ratio
Base/acid sites
ratio[a]
GVL Yield
[%][b]
EL Conv.
[%][b]
IPL Yield
[%][b]
GVL FR (μmol g-1
TOF
Entry
Catal.
min-1)[c]
[h-1][d]
1
2
3
ZrO2
160
160
160
160
160
160
160
160
160
150
150
82
6
6
6
6
6
6
6
6
6
4
6
18
18
1:2.1
1:2.1
1:2.1
1:2.1
1:2.1
1:2.1
1:2.1
1:2.1
1:2.1
1:0.7
1:0.7
1:0.6
1:0.6
2.6:1
0.9:1
0.8:1
0.7:1
--
76
89
91
98
48
37
72
79
86
94
92
4
97
95
98
100
65
40
93
94
96
20
6
5
1
15
2
19
13
8
--
--
30.2
35.3
36.1
38.9
19.0
14.7
28.6
31.3
34.1
19.6
12.8
0.2
1.9
6.3
6.1
8.6
--
o-PhPZr
p-PhPZr
m-PhPZr
m-PhPZr
m-PhPZr
UiO-66(Zr)
m-PhOZr-H
m-PhPZr-H
Zr-HBA
4
5[e]
6[f]
7
--
--
3.9:1
2.8:1
1.3:1
--
--
--
3.4
3.8
6.7
3.5
2.4
0.3
1.2
8
9
10[g]
11[g]
12[h]
13
100
100
6
Zr-Beta
Zr-Beta
m-PhPZr
--
12
82
0.7:1
85
100
3.3
[a] The contents of acid and base sites were determined by pyridine-adsorbed FT-IR spectra and CO2-TPD (temperature programmed desorption), respectively. [b]
EL conversion (Conv.), and GVL & IPL (isopropyl levulinate) yields were determined by gas chromatography (GC); Selec.: Selectivity. [c] GVL FR (formation rate)
= (mole of GVL) / (catalyst amount × time). [d] TOF (turnover frequency) is defined as (mole of GVL) / (mole of catalyst acid-base sites × time) at EL conversion of
~15%. [e] Pretreatment with 100 mg pyridine to cover Lewis acid sites of m-PhPZr [14a] [f] Titration with 100 mg benzoic acid to poison Lewis base sites of m-PhPZr
.
[14a]
.
[g] The data were obtained from ref. [19]. [h] The data were obtained from ref. [20].
an even lower base/acid site ratio of 1.3:1. The enhanced GVL
yield (86%) from EL catalyzed by m-PhPZr-H (entry 9) revealed
the more predominant role of Lewis acid sites than base ones,
despite the concerted effect of acid-base couple sites had been
evidently demonstrated for this reaction. On the other hand, the
method used to prepare zirconium phosphonate-based UMOFs
(i.e., o-, p-, and m-PhPZr) in the present study without a long
period of hydrothermal treatment process was able to facilitate
the formation of Lewis acid sites (Table S3, Figs. S8-S9), thus
preferably promoting CTH of EL to GVL. Furthermore, m-PhPZr
was also manifested to have unprecedented performance to
other previously reported Zr-based catalysts even under open
refluxing conditions (entries 10-13), demonstrating that m-PhPZr
was more optimal for producing GVL from EL via the CTH
process due to its superior ratio and content of base/acid sites.
To investigate the universality of the concerted role of acid-base
sites on converting EL to GVL, UiO-66(Zr), a typical carboxylate-
based metal-organic framework being synthesized from
terephthalic acid and ZrCl4 under hydrothermal treatment, was
also used as catalyst but only afforded a GVL yield of 72%
(entry 7). The relatively higher content of base sites (0.31 mol g-
1) but lower Lewis acidity (0.08 mol g-1; Table S3) in UiO-66(Zr)
was more prone to cause IPL (19% yield) being generated from
EL. As a counterpart of UiO-66(Zr), m-PhOZr-H prepared from
isophthalic acid and ZrCl4 with the same method displayed a
reductive molar ratio of base/acid sites (2.8:1 vs 3.9:1), but still
could not effectively restrain the generation of IPL (entry 8). By
using the identical synthetic approach, the resulting m-PhPZr-H
hybrid was found to have a comparable content of Lewis acid-
base sites to m-PhOZr-H (0.36 vs 0.38 mol g-1; Table S3) while
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As discussed above, Lewis acid-base sites generated from
zirconium and phosphate (P-O-Zr) or carboxylate (C-O-Zr)
moieties were crucial for efficient CTH. Therefore, the local
environment of Zr and O species was further examined by X-ray
photoelectron spectroscopy (XPS; Fig. 3). Fig. 3A shows that
the highest Zr 3d binding energy of m-PhPZr corresponds to the
largest positive charge of Zr atoms, directly leading to the
strongest Lewis acidity of Zr centers [21]. Moreover, the binding
energy of O 1s assigned to P-O-Zr interactions coexistent with
P=O species in m-PhPZr was lower than that of C-O-Zr
prolonging reaction duration, even though a much lower
catalyst/EL mass ratio of 1:6.3 was utilized (Fig. S10 D). These
results are well consistent with the good activity of m-PhPZr in
the open sytem for CTH of EL at the boiling point of i-PrOH (82
ºC) after 18 h (Table 1, entry 13).
To examine the catalytic behavior of m-PhPZr in CTH of EL to
GVL, the catalyst was filtered out from the reaction mixture after
2 h at 160 ºC (Fig. S13). No obvious reaction was observed to
occur for another 2-4 h, which clearly verified the heterogeneous
catalysis of m-PhPZr. ICP-OES analysis shows an extremely low
concentration of Zr (~0.2 ppm) leached into the reaction solution,
indicating the robust structure of active species in m-PhPZr.
Moreover, m-PhPZr was cycled for five times with no evident
decline in activity (Fig. S14). The recovered m-PhPZr catalyst
after five cycles was further characterized by NH3-TPD, XRD,
pyridine-adsorbed FT-IR, and N2 adsorption-desorption (Fig.
S15). It was revealed that the acid-base properties and textural
structures between the fresh and used m-PhPZr catalysts
remained almost unchanged.
interactions in m-PhOZr-H (Fig. 3B), illustrating
a higher
negative charge on oxygen and more enhanced basicity of the
m-PhPZr catalyst. The superior strength of Lewis acid sites
(Zr4+) in m-PhPZr was beneficial to activate carbonyl groups of
EL while a relatively higher basicity (O2-) was favorable for
dissociating the hydroxy groups in i-PrOH (Scheme S1), both of
which contributed to increase the selectivity and reaction rate
towards GVL. However, too much higher basicity (e.g., ZrO2)
resulted in the rapid formation of IPL via intermolecular
[22]
transesterification
.
Further,
two
biomass-based
transformations promoted by acid-base couple sites including
[23]
cascade isomerization-dehydration of glucose to HMF
and
simultaneous esterification-transesterification of acidic seed oil
to biodiesel [24] were conducted (Tables S4-S5). As expected, m-
PhPZr exhibited better performance than m-PhOZr-H and ZrO2
for both reactions, which confirmed the better compatibility and
more concerted catalytic effect of acid-base sites in m-PhPZr
during reactions, compared to m-PhOZr-H and ZrO2.
Fig. 4. Reaction kinetics study on CTH of EL to GVL in isotopic i-PrOH-d8
using 1H NMR (Reaction conditions: EL 2 mmol, m-PhPZr 0.14 g, i-PrOH-d8
10 mL, and 160 ºC)
Mechanistic insight into CTH of EL to GVL was performed with
ex situ Nuclear Magnetic Resonance (NMR) studies in i-PrOH-d8
(completely deuterated isopropanol) in the presence of m-PhPZr
by varying reaction time (1-6 h), and the obtained 1H NMR
spectra of the reaction mixture are provided in Fig. 4. For clear
comparison, the NMR spectra of sole i-PrOH-d8, as well as
normal EL, GVL and ethanol (EtOH) in i-PrOH-d8 are also
supplied. The protons belonging to 1a, 4a and 5a of EL
gradually disappeared with increasing addition of deuterium (D)
from isotopic i-PrOH-d8 to its C=O group, which can be
straightforwardly detected from the NMR spectra of the mixture
after reacting for 1-6 h. A slight chemical shift migration of 2a
and 3a protons in EL to 2b’ and 3b’ of in situ formed GVL, and
even to 2b and 3b of the normal GVL in i-PrOH-d8 can be
observed, respectively, which may be resulted from the
presence of deuterium element in the in situ produced GVL
despite of their similar proton environments. Compared to the
normal GVL, the absence of 4a’ proton in the partially
deuterium-exchanged GVL throughout the reaction, and the
gradually increased proton intensity of EtOH at around 5.7 ppm
(Fig. 4) formed by intramolecular transesterification in reaction
system prove the proceeding of CTH reaction. Nevertheless, it is
still ambiguous to know whether the reaction occurs through
direct hydrogen transfer or metal hydride route due to the lack of
selectivity towards D addition [25]. To clarify these two pathways,
i-PrOH-d1 [i.e., CH3CD(OH)CH3] instead of i-PrOH-d8 was used
as hydrogen-donor and solvent for CTH of EL. Accordingly, GVL
should be produced in a molecular weight of either MS+1 with
100% yield via direct hydrogen transfer, or 50% MS+0 and 50%
Fig. 3. XPS spectra of (A) Zr 3d and (B) O 1s in Zr-containing catalysts
Reaction time and catalyst dosage were also found to
significantly affect the performance of m-PhPZr in CTH of EL to
GVL (Fig. S10). With the increase of catalyst/EL mass ratio from
1:2.1 to 1:1, a comparable GVL yield of 97% at almost complete
EL conversion was obtained in a shorter time of 4 h at 160 ºC
(Fig. S10 A-B). However, more IPL was generated in the early
stage of the reaction when less m-PhPZr was used (Fig. S10 C-
D), as illustrated by GC-MS spectra in Fig. S11. Likewise, a
lower reaction temperature (100-140 ºC) was favorable for
forming IPL (up to 18%) at a relatively decreased EL conversion
of 43-87% (Fig. S12). To our delight, the in situ generated IPL
along with unreacted EL could be further converted to GVL by
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10.1002/cssc.201601570
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MS+1 by the way of metal hydride (Scheme S2). Expectedly,
GC-MS analysis shows the molecular ion peak of the generated
GVL only containing an additional 1 amu (m/z = 101), which
explicitly substantiates the happening of direct hydrogen transfer
over m-PhPZr (Fig. S16).
We wish to thank the financial supports from Nanjing Agricultural
University (68Q-0603), Postdoctoral Science Foundation of
China (2016M600422), and Jiangsu Postdoctoral Research
Funding Plan (1601029A).
Keywords: nanoparticles • heterogeneous catalysis • cascade
reaction • mesoporous material • biomass conversion
Delighted by the pronounced catalytic performance of m-PhPZr
in CTH of EL to GVL, the substrate scope was further extended
to other carbonyl compounds including aliphatic and aromatic
ketones, and biomass-derived aldehydes (Table 2). All
examined ketones and aldehydes can be efficiently catalyzed by
m-PhPZr to produce corresponding alcohols in high yields.
Notably, harsh reaction conditions are required for CTH of
ketones (entries 1-2), due to the steric hindrance and electron
donating nature of the alkyl groups in these molecules. In
contrast, biomass derivatives like citral, cinnamaldehyde, furfural,
5-methylfurfural, HMF, and veratraldehyde give near quantitative
yields of corresponding alcohols in the presence of m-PhPZr
under mild conditions (entries 3-8). These results indicate the
great potential applications of m-PhPZr for the CTH reactions in
both organic synthesis and biomass transformation.
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Temp
(ºC )
Time
(h)
Conv
(%)[b]
Yield
(%)[b]
Reactant
Product
Entry
1
160
8
97
96
2
3
160
6
99
99
[10]
120
6
96
94
4
5
6
7
120
120
120
120
6
2
2
2
98
98
99
98
93
>99
>99
>99
[11]
[12]
8
120
2
98
97
[13]
[14]
[a] Reaction conditions: 2 mmol substrate, 0.14 g m-PhPZr, and 10 mL i-PrOH.
[b] Conversion (Conv) and yield were quantified using GC.
In brief, a facile and effective approach has been developed to
prepare a series of orderly layered and mesoporous zirconium
phosphonate by simply reacting ZrCl4 with different
xylylenediphosphonic acids. The resulting m-PhPZr nanohybrid
shows prominent performance for CTH of various carbonyl
compounds to corresponding alcohols, particularly for cascade
CTH-cyclization of EL to GVL under mild conditions. Deuterium
labeling experiments affirm the CTH reaction proceeding
through direct intermolecular hydrogen transfer, other than metal
hydride route. Comprehensive studies indicate that both Lewis
acidic (Zr4+) and basic (PO32-) moieties contribute significantly to
the excellent catalytic activity of m-PhPZr. The robust and
heterogeneous nanohybrid shows great potential for biomass
valorization and organic transformations mediated by acid-base
couple sites, and the simple method can be used to synthesize
other organic-inorganic functional hybrids.
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[22]
[23]
[24]
[25]
Acknowledgements
This article is protected by copyright. All rights reserved.

10.1002/cssc.201601570
ChemSusChem
COMMUNICATION
COMMUNICATION
Two is better than one: Acid-base
bifunctional Zr-benzylphosphonate
nanohybrids, a class of
H. Li, Z. Fang,* J. He, S. Yang*
Page No. – Page No.
unconventional metal-organic
Orderly layered Zr-
frameworks (UMOFs), are simply
prepared to have unique properties
(see image) and show predominant
performance and stability in synthesis
of valuable chemicals and biofules via
bifunctional/cascade catalysis.
benzylphosphonate nanohybrids for
efficient acid/base-mediated
bifunctional/cascade catalysis
This article is protected by copyright. All rights reserved.