Soluble/MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids

Alcohols to Benzoic Acids

Article

Soluble/MOF-Supported Palladium Single Atoms Catalyze the

Ligand‑, Additive‑, and Solvent-Free Aerobic Oxidation of Benzyl

∇

Estefan í a Tiburcio, Rossella Greco, Marta Mon, Jordi Ballesteros-Soberanas, Jesus

Miguel L ó pez-Haro, Juan Carlos Hernandez-Garrido, Judit Oliver-Meseguer, Carlo Marini,

Mercedes Boronat, Donatella Armentano,* Antonio Leyva-P é rez,* and Emilio Pardo*

Ferrando-Soria,

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sı Supporting Information

ABSTRACT: Metal single-atom catalysts (SACs) promise great rewards in terms of

metal atom eﬃciency. However, the requirement of particular conditions and

supports for their synthesis, together with the need of solvents and additives for

catalytic implementation, often precludes their use under industrially viable

conditions. Here, we show that palladium single atoms are spontaneously formed

after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze

their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents.

With this result in hand, the gram-scale preparation and stabilization of Pd SACs

within the functional channels of a novel methyl-cysteine-based metal−organic

framework (MOF) was accomplished, to give a robust and crystalline solid catalyst

fully characterized with the help of single-crystal X-ray diﬀraction (SCXRD). These

results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid

catalysts for potential industrial implementation.

INTRODUCTION

■

Single-atom catalysts (SACs) attract great interest due to their

unique catalytic properties in diﬀerent reactions of capital

importance. However, various limitations linked with their

real applications, such as the diﬃculties related to their gram-

scale preparation, their challenging characterization, and the

need to use protective ligands to stabilize these SACs,

−4

preventing their agglomeration, still need to be overcome.

In this context, the formation, stabilization, and catalytic action

of soluble SACs in the neat reactant is still a quite unexplored

ﬁeld, since metals, even in very low amounts, tend to

−7

agglomerate in organic solutions after reduction.

However,

this may not be the case for molecules able to concomitantly

reduce, stabilize and be activated by the in situ formed SACs.

Figure 1. Stepwise and direct catalytic oxidation of benzyl alcohols to

benzoic acids (top) and the catalytic systems used here (bottom).

Benzyl alcohols are fundamental starting materials in organic

synthesis such as in the formation of benzaldehydes and

,10

benzoic acids after oxidation.

Figure 1 shows that the

dehydrogenation of benzyl alcohols occurs under a variety of

11−16

metal catalysts and reaction conditions

and that the

radical oxidation of benzaldehydes occurs spontaneously under

air for substrates with >98% purity; otherwise, just a >2% of

remaining benzyl alcohol acts as a very good quencher of

radical oxygen species (ROS). With these results in mind, it

is not surprising that in contrast to the stepwise process the

direct aerobic oxidation of benzyl alcohols to benzoic acids is a

challenging reaction that requires of harsh oxidation agents or

Received: November 26, 2020

Published: February 4, 2021

2021 American Chemical Society

Journal of the American Chemical Society

J. Am. Chem. Soc. 2021, 143, 2581−2592

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Figure 2. (a) Initial turnover frequency (TOF ) for the aerobic oxidation of neat benzyl alcohol 1a with increasing amounts of Pd(OAc) at 150 °C

under 4 bar of O . (b) Representative time−yield kinetic plot of the reaction for 0.05 mol % Pd(OAc) . (c) Two diﬀerent AC-HAADF-STEM

images of the Pd species in solution during reaction, after being trapped in active charcoal. Some Pd SACs are marked with yellow circles. (d) X-ray

absorption near-edge structure (XANES, top) and extended X-ray absorption ﬁne structure (EXAFS, bottom) spectra of the solution (red lines),

compared to Pd foil (blue lines).

organometallic complex catalysts, additives, and solvents;

conversions, when >50% of 1a still remains in solution. These

results conﬁrm that the Pd catalyst formed under these

conditions is able to override the poisoning of 1a under

aerobic conditions. An acceptorless dehydrogenation pathway

can be ruled out since an open vial reaction gave very

otherwise, diﬀerent undesired reactions such as ether and

18−28

ester formation occurs.

Benzyl alcohol is an industrial reagent for the reduction of

noble metal salts (e.g., Pd, Au, etc.) to nanoparticles, with the

concomitant formation of benzaldehyde. Thus, we envi-

sioned that perhaps the dissolution of a metal salt into tiny

amounts of neat benzyl alcohols could generate a viable, self-

stabilized redox single metal atom for the catalytic oxidation of

benzyl alcohols to benzoic acid under air. This highly reactive

single atom would, in principle, feature the empty coordinating

sites required for the diﬀerent chemical events during reaction,

conversion of 1a, thus conﬁrming the need of O to facilitate

the one-pot oxidation.

Figure 2c shows aberration-corrected high-angle annular

dark ﬁeld scanning-transmission electron microscopy (AC-

HAADF-STEM) measurements of the metal species in

solution during reaction, trapped in situ with active charcoal.

Since AC HAADF-STEM imaging is proportional in good

19,20

including dehydrogenation and oxygen activation,

while

approximation to the squared atomic number, Z , Pd species

most likely circumventing severe benzyl alcohol poisoning.

From a material synthesis point of view, the SAC could be

considered an arrested state of the metal during the reduction/

aggregation process into the benzyl alcohol, which keeps the

metal seeds alive and catalytically active for the solvent-,

ligand-, and additive-free oxidation reaction, beyond which

can be reliably identiﬁed as the brightest contrasts in the image

(

some of them have been marked with orange circles). While

Pd single atoms are the main species present at <0.1 Pd mol %,

only clusters and eventually NPs were found at >0.1 Pd mol

(

see Figures S3 and S4). Figure 2d shows X-ray absorption

near edge structure (XANES) and extended X-ray absorption

ﬁne structure (EXAFS) measurements of the solution, which

conﬁrm the reduction of Pd and the generation of very small

agglomerates, with an average number of ∼6 Pd−Pd bonds

30−33

some other metal species could be present.

RESULTS AND DISCUSSION

■

(

see also Table S2), much lower than in Pd foil (12 Pd−Pd

bonds). Ultraviolet−visible (UV−vis) spectrophotometric

titrations with PPh conﬁrm the progressive disappearance of

Figure 2a shows the catalytic results for the aerobic oxidation

of neat benzyl alcohol 1a to benzoic acid 2a with a 0.03−0.3

mol % of dissolved Pd(OAc) , and it can be observed that a

Pd in solution during reaction (Figure S5 top). These results

strongly support that partially reduced Pd1 species could be

the catalytic active species for the direct aerobic oxidation of 1a

to 2a.

high initial turnover frequency (TOF ) for 2a is achieved with

0.1 Pd mol % but not with higher amounts of Pd, with yields

of 2a around 50−80% after 4 h of reaction time (Figure S1).

Similar results were observed with other Pd sources, including

K PdCl , Pd (dba) , and Pd(acac) , but not with Pd

The very low catalytic activity found with intermediate Pd

complexes having a stronger ligand such as a phosphine, i.e.,

Pd(PPh ) or Pd(PPh ) Cl (Table S1 and Figure S2). Figure

amounts (0.1−0.25 mol %) is consistent with the formation of

subnanometric Pd clusters, catalytically inactive in this case.

3 2

b shows that remarkably 2a starts to form at intermediate

In order to check this hypothesis, subnanometric Pd clusters in

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Figure 3. PS approach showing the structures of 3−5 determined by single-crystal X-ray diﬀraction which consists of two consecutive processes:

ﬁrst, the insertion of [Pd(NH ) ] cations within the channels of 3 (a, b) to give 4 (c, d) and, second, the reduction of Pd cations to form the

Pd single atoms in 5 (e, f). Copper and strontium atoms from the network are represented by cyan and purple spheres, respectively, whereas

organic ligands are depicted as gray sticks. Yellow and blue spheres represent S and Pd atoms (gray spheres in e represent Pd atoms). Dotted lines

represent the Pd···S interactions.

solution were independently prepared by two reported

showed that the catalytic activity comes from species in

solution (Figure S7). These results strongly support that the

oxidation of 1a to 2a in the neat reagent is catalyzed by soluble

methods, i.e., endogenous reduction in aqueous N,N-

dimethylformamide and supporting solvent as well as

reduction and leaching from ethylene vinyl alcohol polymer

Pd1 species, regardless the amount of Pd employed.

1−4

(

EVOH), and tested as catalyst for the oxidation of 1a under

SACs are, by deﬁnition, supported metal species.

Thus,

the same conditions than Pd salts and complexes. The results

Figure S5, bottom) show that these clusters are inactive as

catalysts for the oxidation of 1a to 2a, which strongly supports

once we established the catalytic activity of in situ prepared Pd₁

SACs for the one-pot oxidation of 1a to 2a, it is of interest to

ﬁnd a solid able to generate and stabilize such Pd species.

that Pd is the main catalytic active species for the oxidation

However, this is a quite diﬃcult task. The appropriate solid has

to be able to preserve the required electronic and structural

chemical nature of SACs, preventing their leaching out under

reaction conditions and diﬀusing onto the solid to aggregate

and, at the same time, enable a clear-cut characterization of the

reaction. It is reasonable to think that the combination of a

mild reductant agent (benzyl alcohol), which can at the same

time act as a stabilizer, could have the same eﬀect than a

support/strong reductant system for the preferential formation

−3

of Pd species.

In any case, the Pd clusters may be unable

supported Pd site and surroundings. In this context, metal−

36−41

not only to catalyze the redox reaction but also to dislodge Pd

single atoms, according to the canonical Ostwald ripening

mechanism. In accordance with this hypothesis, the formation

of 2a starts to be observed again at Pd concentrations where

NPs are formed (>0.3 mol %). This is in good agreement with

the reported catalytic activity of some metal NPs for this

organic frameworks

(MOFs) are one of the most suitable

platforms to overcome these diﬃculties. MOFs chemistry have

reached high microporosity control, ﬁne-tuning of the

functionalities decorating their channels, and in-depth

characterization of the ﬁnal hosted metal species using

42−48

single-crystal X-ray diﬀraction (SC-XRD).

As a direct

3−28

reaction,

as well as with the ability of metal NPs to

consequence, MOFs have experienced rapid growth over the

few last decades in catalysis, by means of their constituting

building blocks, both organic linkers and open metal sites,

dissociate O₂and dislodge single atoms in solution.

Commercially available samples of Pd/C with diﬀerent Pd

loadings (1−10 wt %) and particle size (5−50 nm average

diameter) were tested as catalyst for the reaction and only the

sample with highest loading and biggest NP size was active for

the formation of 2a (Figure S6). A quenching test with

triphenylphosphine, under the indicated reaction conditions,

49−57

and/or catalytically active guest species within their pores.

Herein, we report a novel three-dimensional (3D) MOF,

derived from the amino acid S-methyl-L-cysteine, with formula

{Cu Sr[(S,S)-Mecysmox] (OH) (H O)}·15H O (3) (Mecys-

mox = bis[S-methylcysteine]oxalyl diamide) (Figure 3a),

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featuring pores densely decorated with dimethyl thioether

groups, which allow the sequential formation and stabilization

of Pd SACs within their functional channels (Figure 3). In

order to do so, a two-step postsynthetic (PS) strategy has

been applied, leading to the formation of two novel adsorbates

with formulas [Pd (H O)(NH ) ] Cl @{Sr Cu [(S,S)-

6 0.5

Mecysmox] (OH) CH OH)}}·12H O (4) (Figure 3c) and

(

Pd ) ([Pd (H O)(NH ) ]Cl ) @{Sr Cu [(S,S)-Mecys-

1 0.5 2 3 3 2 0.5 6

mox] (OH) CH OH)}}·13H O (5), respectively, (Figures 1

bottom and 3e, Table S3 ). Figure 3 serves a dual purpose:

shows the crystal structures of 3−5 and illustrates the SACs

formation route. First, this consists of the insertion of

[

Pd(NH ) ]Cl cations in the starting MOF (3) to form the

3 4 2

Pd -containing MOF (4) and the concomitant in situ

reduction of half of the Pd cations to form the mixed

valence Pd /Pd hybrid compound (5) (see the Experimental

Section). Notably, the sulfur-containing groups play a dual key

role in this PS approach. They retain the Pd²cations in

speciﬁc positions after the insertion process, which allows their

homogeneous distribution along the channels and prevents

their agglomeration and forming of NCs or NPs during the

reduction process. Besides, the crystal structure of each phase

could be unveiled by SC-XRD given the high crystallinity and

8−64

robustness

of the pristine MOF (Figure 3 and Table S4).

Compounds 3−5 are isomorphous, crystallizing in the chiral

P6 space group, and exhibit a chiral 3D strontium(II)−

copper(II) network, featuring hexagonal channels where the

dimethyl thioether chains from methylcysteine residues are

conﬁned. These functional arms exploit their intrinsic ﬂexibility

adopting diﬀerent most stable conformations depending on the

nature of target, i.e., solvent molecules in 3 (Figures S8 −S10 ),

Figure 4. Perspective views of one single channel of 5 along the c- (a)

and b-axes (b). Copper and strontium atoms from the network are

represented by cyan and purple polyhedra, respectively, whereas

organic ligands are depicted as gray sticks. Yellow and blue spheres

represent S and Pd atoms. Dual color sticks represent the Pd···S

interactions.

Pd in 4 (Figures 3c,d and S11) or both Pd cations and Pd

SACs in 5 (Figures 3e,f, 4, and S14). The Pd −S bond

distances [1.96(4) and 2.17(2) Å (4)] (Figures 3d and S13a)

are close enough to those observed previously, and similar to

the Pd −S bond distance observed in 5 2.16(2) Å] (Figures 3f

SC-XRD data also allows to suggest certain parameters of

the Pd₁formation mechanism. First, the bridging water

molecule in [Pd (H O)(NH ) ] units (4) (Figures 3d and

and S13b ). In 3−5 the thioether chains from the Mecysmox

ligand show as basic conformation one of the two distinct

moieties in a distended conformation toward the center of the

pores, and the other one regularly bent, with the terminal

methyl groups pointing toward the smaller interstitial voids

residing along the a-axis (Figures S10 and S14). Both

conformations allow amino acid arms to eﬃciently target

3 6

S13a ) might play a crucial role during the SACs formation

process (5), acting synergistically with the ﬂexible dimethyl

thioether chains from methylcysteine residues stabilizing Pd

metal ions in 4. The reduction process of Pd −S units located

inside the most accessible pores breaks water bonds from one

side, which meanwhile generates the Pd −S ones in 5. This

leaves the water molecule still coordinated and stabilizing the

48,66−68

Pd ions by S binding sites,

as conﬁrmed by the crystal

structure of 4. However, only the Pd ions residing in the

most accessible pores [50% of total Pd ions] could be

chemically reduced to Pd , as con ﬁ rmed by XPS spectrum of 5

unreduced mononuclear Pd complexes residing in hindered

interstitial voids (Figures 3f and S13b ). Second, the length of

the amino acid residue also seems to play a key role in the

nuclearity of the metal species formed. Thus, in the present

(

vide infra) (Figures 4b and S11 −S14). This situation was

observed previously. In 5, Pd SACs are ﬁxed to sulfur atoms

in the larger hexagonal pores (Figure 4a and S12), whereas

MOF, where Pd cations are connected to shorter dimethyl

Pd ions are still stabilized by sulfur atoms of dimethyl

thioether located in interstitial voids (Figure S14). The water

thioether chains within the channels, Pd SACs are formed

during the reduction process. In turn, in a previously reported

work with an isoreticular MOF prepared from the amino acid

molecule acting as bridge between two-coordinated Pd ions

6,67

in 4, still remains coordinated in 5 but as a terminal ligand, to

L-methionine,

the larger length of the ethylmethyl

only one of Pd metal ions, as a consequence of the breaking

thioether chains decorating the channels allows a closer

linkage after Pd SAC formation [Pd −Ow 2.00(6) and 3.03(7)

approach of the metal species, and dinuclear Pt nanoclusters

could be obtained.

Å in 4 and 1.99(2) Å in 5] (Figure S13) (vide infra). Pd

weakly also interacts with oxamate moieties with Pt ···O

distances of 2.84(1) and 2.88(1) Å (see also Figures S12

and S13b). As far as we know, no examples of crystallo-

graphically precise Pd₁SACs have been reported so far.

Nevertheless, this Pd···O distance is close enough to that

The virtual diameter of the channels only slightly decreases

from ca. 0.9 nm in the precursor material 3 to ca. 0.7 nm in 4

and 5 (Figures S9, S11, and S12). This is in total agreement

with adsorption measurements. Thus, the permanent porosity

of the samples, particularly important in the case of 4 and 5 for

catalytic applications, was veriﬁed by measuring their N₂

previously reported for Pd nanoclusters (2.9 Å).

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compared to Pd foil. The results conﬁrm the partial reduction

Article

adsorption isotherms at 77 K. They conﬁrm the permanent

porosity for 3−5 (Figure S15), which is slightly lower for 4 and

Pd , can be observed, which supports the partial reduction of

Pd. However, two clear broad signals centered at 2114 and

−

, as expected from the decrease of their accessible void due to

2012 cm , attributable to unreduced Pd and highly

dispersed, partially reduced Pd atoms (δ = 0−1),

respectively, can be clearly seen, together with the increasing

δ+

the presence of the Pd guests within the channels (calculated

Brunauer−Emmett−Teller (BET) surface areas for 3, 4, and

−1

are 719, 548, and 572 m /g, respectively).

sharp signal of free CO (2137 cm ) at high CO doses. These

−

Besides the structural characterization, the chemical

peaks are accompanied by a very broad signal at 1820 cm ,

identities of 3−5 were also established by elemental analyses

which can be assigned to Pd(0) nanoparticles.

(

C, H, S, and N), inductively coupled plasm−mass

Figure 6 shows the EXAFS and XANES spectra of MOF 5,

spectrometry (ICP−MS), powder X−ray diﬀraction (PXRD),

electronic microscopy, X-ray photoelectron spectroscopy

XPS) and thermogravimetric (TGA) analyses (see the

Supporting Information). Figure S16 shows the experimental

powder X-ray diﬀraction (PXRD) patterns of 3−5 . They are

identical to the theoretical ones (bold lines in Figure S16),

which conﬁrms that the bulk samples are pure and

homogeneous. The solvent contents of 3 −5 were, however,

deﬁnitively established by TGA (see Figure S17). The XPS

spectra of 4 and 5 are shown in Figure S18 . The Pd 3d line of 2

is only one doublet with a binding energy (BE) of the Pd 3d

peak of 337.8 eV, typical of Pd²⁺cations (Figure S18a ) which

is close enough to other reported values. In turn, Figure S18b

clearly shows, apart from the same Pd 3d₅_/₂doublet with a BE

of 337.7 eV, an additional peak at 335.8 eV, attributed to

reduced Pd SACs, with a 1:1 ratio respect to Pd . This

feature indicates that only 50% of Pd , those occupying

accessible positions, are reduced when in contact with reducing

agent, whereas those Pd²cations situated in inaccessible

sheltered interstitial positions (see structural description)

remain in their original oxidation state. These values are

close enough to those observed for other Pd / Pd species.

Therefore, they suggest that the thioether groups do not alter

signiﬁcantly either the native electronics nor the open-shell

structure of the Pd site, which is ready to catalyze the aerobic

oxidation of 1 to 2.

In order to further conﬁrm the presence of partially reduced

Pd SACs within 5, Fourier transform infrared under CO

(

FTIR−CO), XANES, and EXAFS spectroscopic measure-

ments and computational calculations based on the density

functional theory (DFT) were carried out. Figure 5 shows the

low-temperature (−196 °C) FTIR-CO results of 5, where

Figure 6. XANES (top) and EXAFS (bottom) spectra of fresh MOF

5 (red lines), compared to Pd foil (blue lines).

−

there are no signals above 2150 cm , corresponding to bare

of Pd, as occurred for the Pd catalyst in solution (see Figure 2d

above and Figure S19 for comparison and ﬁtting). It can be

observed in both cases, i.e., in solution and in MOF 5, that the

ﬁrst oscillations beyond the edge are ﬂattened respect to the

foil due to quantum size eﬀects of the single atoms, also

indicating a large fraction of low coordination Pd atoms, more

intensiﬁed in the case of MOF 5. No Pd−Pd bond signals can

be detected for the latter, but an average of 3 Pd −S bonds are

the Supporting Information, with references), in nice agree-

ment with the SCXRD structure. Combined, these results

strongly support the single-atom nature of Pd within MOF 5.

In order to further support the stability of the Pd SACs in

MOF 5, DFT calculations were performed. Figure 7 shows the

periodic DFT calculations through geometry optimization of

Pd(0) in three di ﬀ erent environments within the MOF (see

also Figure S20 and Table S5), which support the crystallo-

graphic characterization of 5. Pd(0) is always linearly

coordinated to two ligands (O or S) with optimized Pd−O

and Pd−S distances between 2.1 and 2.3 Å, as measured in the

Figure 5. Low-temperature Fourier transform infrared spectrum,

under CO (FTIR-CO), of fresh MOF 5. Peak assignments: free CO

−

−1

δ+

−1

(

2137 cm ), Pd (2114 cm ), Pd (2012 cm , δ = 0−1), and Pd

−1

NPs (1820 cm ).

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Figure 7. DFT-optimized structures of Pd(0) and PdCl within the

MOF, for three di ﬀ erent coordination environments (Pd0−A−C, see

also Figure S20 and Table S5 ). Pd, Cu, Sr, S, N, O, Cl, C, and H

atoms are depicted as cyan, orange, pink, yellow, blue, red, green, gray,

and white balls, respectively.

X-ray absorption spectroscopy (XAS) techniques (compare

with Table S2 ). The calculated atomic charge on Pd in PdCl₂

within the MOF is nearly the same as in the gas phase

calculated at the same level of theory (0.678 e), while the net

atomic charges on two Pd(0) models are just slightly positive

and slightly negative in the third one (see Table S5). With the

optimized structure of MOF 5 in hand, the interaction of the

Pd SACs with CO was simulated by DFT calculations, in order

to compare with the experimental results in Figure 5. The

results (Table S6 and Figure S21) show that CO interacts

Figure 8. (a) Reusability of Pd⁰SACs-MOF (5); reaction

conditions: 1.96 mmol substrate, 0.1% mol Pd SACs-MOF, 4 atm

O , 150 °C, 450 rpm, 15 h; GC yields. (b) Plausible reaction

mechanism for Pd in solution. (c) Reaction scope with Pd(OAc)₂

(

0.3 mol %). Reaction conditions: 1.96 mmol substrate, 0.3% mol

Pd(OAc) , 4 atm O , 150 °C, 450 rpm, 4 h. GC yields. *15 h. 0.1%

mol Pd SACs-MOF, 24 h.

microscopy experiments were carried out after the catalytic

experiments, where no SC-XRD measurements could be

carried out due to the loss of the crystallinity of the material.

Figures 9 and S22 show representative AC-HAADF-STEM

images of 5. Highly dispersed Pd species are clearly observed.

In particular, they show a 0.135 nm average diameter, which is

strongly with all models of Pd0 but not with PdCl , with

calculated interaction energies between −28 and −40 kcal·

−

mol and with optimized Pd−CO distances around 1.8 Å. In

particular, the interaction of Pd with CO in the Pd0−A system

is so strong that the Pd−O bond is broken. The calculated

−

ν(CO) frequency in this system, 1997 cm , is similar to that

obtained for CO adsorbed on top of a corner Pd atom in a

−

small Pd cluster used as reference (1999 cm ) and nicely ﬁts

−

with the observed experimental peak at 2012 cm (see

above). In the other two models (Pd0−B−C), where the

Pd(0) atom remains attached to two MOF ligands after CO

coordination, the calculated ν(CO) frequency is slightly shifted

−

to 1930−1945 cm , in any case still assignable to the

experimental peak observed.

The catalytic results for the aerobic oxidation of benzyl

alcohol 1a with MOFs 3−5 show that only Pd SACs-MOF

(

5) catalyzes the oxidation with good eﬃciency (28% of

benzaldehyde and 43% of benzoic acid 2a). In contrast, 4

barely catalyzes the reaction, and 3 is completely inactive. The

inactivity of 4 can be explained by the need of using a reducing

agent stronger than 1a to obtain the catalytically active Pd₁

species within the MOF, which then show a catalytic activity

comparable to the Pd(0) complex Pd (dba) (see Table S1).

Figure 8a shows that MOF 5 is recyclable, without minimal

depletion of the catalytic activity after 3 reuses. In order to

verify the integrity of the SACs in MOF 5, electronic

Figure 9. AC-HAADF-STEM image of reused MOF 5 showing the

presence of Pd SACs.

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in a good agreement with isolated atomic species. Only very

scarce, small agglomerations could be observed in some areas,

with diameters <0.5 nm, thus conﬁrming that the Pd atoms do

not aggregate into large NCs. In the same vein, PXRD pattern

of 5, recovered after catalysis (Figure S23a ), conﬁrm that the

material remains crystalline and that no characteristic XRD

peaks of Pd NPs or oxides are observed, further conﬁrming the

from that of soluble Pd (compare with Figure S25). The lack

of inﬂuence of O for the solid catalyst can be explained by

diﬀusion limitations; thus, the dehydrogenation step is slower

than that in solution. Indeed, the experimental activation

−

energy for the latter (7.7 kcal·mol ), also on the basis of initial

rates, is much higher than that for the former (35.2 kcal·

−

mol ), showcasing the more diﬃcult access to the catalytic

sites in the solid. The inhibiting eﬀect with the strong ligand

integrity of Pd SACs. Moreover, the XPS spectra of 5 after

catalytic experiments (5′) is very similar to that of the starting

PPh also occurs (Table S1).

material (Figure S23b), conﬁrming that a 1:1 ratio for Pd and

The fact that di ﬀerent Pd sources work well as catalysts in

solution (Table S1) suggests that the dynamic system drives to

a common catalytically active reduced Pd species in variable

amounts, while in contrast the reduced Pd species are directly

obtained within the MOF 5 during the reduction treatment

and not during reaction, since MOF 4 does not work well.

These results illustrate the stability conferred by the MOF

structure to the conﬁned Pd single atoms, at expense of losing

substrate availability. Nevertheless, Figure 8c shows the

Pd remains after catalysis. In accordance with all the

characterization made to the used MOF 5 sample, leaching

tests after ﬁltration in hot of the catalyst (Figure S24) reveals

that no reaction occurs after ﬁltration of the solid catalyst,

neither for the benzaldehyde intermediate nor product 2a,

which disproves the presence of catalytically active Pd species

in solution from MOF 5.

The potential cocatalysis by the Cu atoms in the MOF was

discarded on the basis of comparative experiments (Table S7),

reaction scope for the Pd catalyst in neat benzyl alcohols

since Cu(OAc) merely does not catalyze the reaction (0.6% of

1a−i, which provides a limited number of benzoic acids 2a−i

a and 2.8% of benzaldehyde under optimized reaction

in moderate yields, fairly comparable to most of the catalytic

conditions), while a Cu-MOF treated under reduction

metal systems previously reported. Besides, the calculated

conditions (NaBH in methanol) and not having any Pd

turnover frequency for product 2a under optimized conditions

−

showed a similar catalytic activity than Cu(OAc) (0.8% of 2a

is 7.95 min , which is a 50-fold increase with respect to any

other catalytic system previously reported for this reaction

(Table S8).

and 5.0% of benzaldehyde). These results disprove Cu, on its

own, as a catalyst of the reaction, and when Cu(OAc) was put

together with Pd(OAc) , the yield of benzoic acid 2a was lower

than that with Pd alone. These results together conﬁrm that Pd

is the only metal catalyst for the reaction here. Overall, these

results nicely ﬁt the observations during the reactions in

CONCLUSIONS

■

In summary, we report, in the ﬁrst part of the manuscript, the

in situ formation of Pd in neat benzyl alcohols which are able

to catalyze the aerobic oxidation to benzoic acids. Then, we

present the gram-scale preparation of well-deﬁned Pd SACs in

a methyl-cysteine-based MOF, homogeneously distributed and

stabilized along the functional channels. Synchrotron SC-XRD

allows, for the ﬁrst time, to clearly visualize the Pd SACs and

solution and strongly support the idea that ligand-free Pd are

the catalytically active species during the one-pot oxidation of

benzyl alcohols to benzoic acids under additive- and solvent-

free conditions.

Kinetic experiments evidence that the rate equation for Pd

−

in solution is v = k [Pd][O ][1a] (Figure S25); this

exp

surroundings. The nature of Pd in both solution and MOF is

equation rate is similar if one starts from benzaldehyde rather

than benzyl alcohol 1a (Figure S26 ) and that the kinetic

further supported by microscopic and XAS techniques, in

addition to DFT calculations for the solid. The latter enable us

to support SC-XRD results unveiling the main interactions

between palladium atoms and the network, as well as to infer a

plausible formation mechanism of Pd SACs. The present

results show a straightforward manner to obtain, on a

isotopic eﬀect (KIE) is 2.6(7) when 1a-d is used as the neat

substrate. The inverse reaction order for 1a, obtained by

dilution experiments with n-hexadecane, is in accordance with

the expected tendency of 1a to poison the oxidation catalyst.

The good linearity of the reaction rate with O pressure

multigram scale, well-deﬁned ligand-free Pd SACs, which

supports the lack of diﬀusion eﬀects and suﬃcient solubility in

can be eﬀectively used in catalysis under industrially viable

reaction conditions without additional reagents. This further

demonstrates the great versatility of MOFs and represents a

step closer to the real application of MOF-based materials in

catalysis.

the neat reactant. Trapping of benzaldehyde as an acetal with

a diol, in situ, completely stops the formation of benzoic acid

a, and starting the reaction from the corresponding ester does

not give any product 2a, while dibenzyl ether does give 2a

Figures S27 and S28). Moreover, the addition of the oxygen

radical inhibitor DABCO to the reaction mixture stops the

formation of 2a , but it did not aﬀect the formation of

benzaldehyde (Figure S29). These kinetic, isotopic, and

reactivity results together strongly support the reaction

mechanism proposed in Figure 8b for Pd in solution, where

the rate-determining step of the reaction is the oxidation of

benzaldehyde to 2a. This explains the catalytic activity of Pd₁

in the neat reactant, since the dehydrogenation of 1a proceeds

extremely well, and no base or additional stabilizing are

needed. However, given that this is an early step in the reactive

sequence, the addition of catalytic amounts of NaOAc did

EXPERIMENTAL SECTION

■

Preparation of {Cu Sr[(S,S)-Mecysmox] (OH) (H O)}·

15H O (3). (Me N) {Cu [(S,S)-methox](OH) }·4H O

(4.32 g, 6.0 mmol) was dissolved in 50 mL of water. Then,

another aqueous solution (10 mL) containing Sr(NO ) (0.42

g, 2.0 mmol) was added dropwise under stirring. After further

stirring for 10 h, at room temperature, a green polycrystalline

powder was obtained and collected via ﬁltration and dried with

ethanol, acetone and diethyl ether. Yield: 2.91 g, 83%. Anal.

calcd for C Cu SrH S N O (1752.2): C, 20.56; H, 4.03; S,

18−28

increase the reaction rate (Figure S30).

In contrast, the

10.98; N, 4.80%. Found: C, 20.51; H, 4.00; S, 10.99; N, 4.83%.

−

rate equation obtained with MOF 5 as a catalyst on the basis of

initial rates (Figure S31) is v = k′ [5][1a], which diﬀers

IR (KBr): ν = 1605 cm (CO). Well-shaped hexagonal

prisms of 1 suitable for X-ray structural analysis could be

exp

587

J. Am. Chem. Soc. 2021, 143, 2581−2592

Journal of the American Chemical Society

Article

obtained by slow diﬀusion in an H-shaped tube of H O/DMF

to a manometer, where O can be introduced at the desired

pressure through a cannula. After the reaction time, the

mixture was analyzed as above.

The following procedure was followed to get the active

catalytic species trapped in situ after 60 min of reaction, at 150

(

1:9) solutions containing stoichiometric amounts of

Me N) {Cu [(S,S)-Mecysmox](OH) }·5H O (0.13 g, 0.18

mmol) in one arm and Sr(NO ) (0.012 g, 0.06 mmol) in the

other. They were isolated by ﬁltration on paper and air-dried.

Preparation of [Pd (H O)(NH ) ] Cl @{Sr Cu [(S,S)-

°C and 4 bar O . Active charcoal was added after

3 6 0.5

Mecysmox] (OH) (CH OH)}·12H O (4). Well-formed hex-

depressurization of the reaction, while the reaction mixture

was being stirred at reaction temperature. While still hot, the

reactor was set aside, and 2 mL of methanol was added to

reactor. The mixture was kept in stirring for 10 min. Afterward,

the whole mixture was transferred to a 2 mL vial, which was

then centrifuged and washed 3 times with 2 mL of fresh

methanol each time. After this procedure, the samples were

dried at 70 °C under vacuum overnight and then analyzed by

HR-TEM. The amount of charcoal used to trap the species

(5−30 mg) was calculated in order to obtain a sample with

approximately 2−3 wt % of Pd.

agonal green prisms of 2, which were suitable for X-ray

diﬀraction, were obtained by soaking crystals of 1 (ca. 25 mg,

.015 mmol) in a H O/CH OH (1:1) solution of [Pd-

2 3

(

NH ) ]Cl (0.015 mmol) for 6 h. The process was repeated

3 4 2

ﬁve times to ensure the maximum loading of [Pd(NH ) ]Cl .

Crystals were washed with a H O/CH OH (1:1) solution

several times, isolated by ﬁltration on paper and air-dried. Anal.

calcd for C Cl Cu SrH PdS N O (1949.6): C, 19.10; H,

33.5

.93; S, 9.87; N, 6.47%. Found: C, 19.07; H, 3.89; S, 9.91; N,

.45%. IR (KBr): ν = 1603 cm−1 (CO).

A multigram-scale procedure was also developed by using

General Procedure for Ultraviolet−Visible (UV−Vis)

Spectrophotometric Titrations. In a 10 mL glass vial

equipped with a stirring bar, the corresponding benzyl alcohol

(1.96 mmol) was charged with 0.006 mmol of palladium

acetate. The vial was closed with a septum equipped with a

manometer and charged with 4 bar of oxygen. Then, it was

placed in a preheated metal heating plate at 150 °C and stirred

at 450 rpm for the indicated time. Afterward the mixture was

quenched with 4 mL of a 0.02 M triphenylphosphine solution

in CH Cl and analyzed by UV−vis spectrophotometry in

the same synthetic procedure but using a higher amount of a

polycrystalline sample of 1 (2 g, 1.15 mmol), which were

suspended a H O/CH OH (1:1) solution of [Pd(NH ) ]Cl

3 4

(

1.1 mmol) for 1 h under a mild stirring. The process was

repeated 5 times. Finally, the product was collected by

ﬁltration, washed with a H O/CH OH (1:1) solution and air-

dried. Anal. calcd for C Cl Cu SrH PdS N O (1949.6):

33.5

C, 19.10; H, 3.93; S, 9.87; N, 6.47%. Found: C, 19.02; H, 3.87;

−

S, 9.91; N, 6.47%. IR (KBr): ν = 1602 cm (CO).

(

Pd ) ([Pd (H O)(NH ) ]Cl ) @{Sr Cu [(S,S)-Mecys-

quartz cuvettes with an optical path of 10 × 10 mm .

3 3

2 0.5

mox] (OH) (CH OH)}·13H O (5). The same procedure was

X-ray Crystallographic Details. Diﬀraction data for 3

were collected on a Bruker-Nonius X8APEXII CCD area

detector diﬀractometer using graphite-monochromated Mo Kα

radiation (λ = 0.71073 Å), whereas data for 4 and 5 were

collected using synchrotron radiation at I19 beamline of the

Diamond Light Source at λ = 0.6889 Å. Crystal data for 3−5:

applied, with the same successful results to both crystals (ca. 25

mg) and a powder polycrystalline sample of 2 (ca. 2 g). They

were suspended in H O/CH CH OH (1:1) solutions to which

NaBH , divided in 15 fractions (0.4 mmol of NaBH per mmol

of MOF each), were added progressively in the space of 72 h.

Each fraction was allowed to react for 1.5 h. After this period,

samples were gently washed with a H O/CH OH solution and

hexagonal, space group P6 , T = 100(2), Z = 2. 3:

C Cu H N O S Sr, a = 18.057(4) Å, c = 12.800(3) Å, V

36 6

ﬁltered on paper. Anal. calcd for C ClCu SrH PdS N O

= 3614.6(17) Å ; 4: C Cu H N O S SrPdCl , a =

73.5

7.5 34.5

31 6 76 9 33.5 6 2

(

1906.6): C, 19.53; H, 3.89; S, 10.01; N, 5.51%. Found: C,

9.48; H, 3.87; S, 10.03; N, 5.49%. IR (KBr): ν = 1601 cm

17.86780(10) Å, c = 12.80840(10) Å, V = 3541.34(5) Å ; 5:

C Cu H N O S SrPdCl, a = 17.8206(2) Å, c =

−

73.5 7.5 34.5 6

(

CO).

12.7821(3) Å, V = 3515.42(11) Å . Further details can be

found in the Supporting Information. CCDC 1995182,

1995183, and 1995184 for 3, 4, and 5, respectively, contain

the supplementary crystallographic data for this paper. These

data can be obtained free of charge via www.ccdc.cam.ac.uk/

conts/retrieving.html or from the Cambridge Crystallographic

Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax:

(+44)1223−336−033; or deposit@ccdc.cam.ac.uk.

Catalysis Details. All reactions were performed under

aerobic solvent-free conditions. Palladium acetate (Sigma-

Aldrich, >99.8% purity) was weighed (0.13−1.3 mg, which

corresponds to 0.03 to 0.30%mol, respectively) in a double-

walled 5 mL reactor equipped with a needle connected to a

manometer. Then, the corresponding benzyl alcohol (0.2 mL,

Sigma-Aldrich, > 98%) was added, and after setting an

atmosphere of 4 bar O , the reactor was placed at 150 °C at a

X-ray Powder Diﬀraction Measurements. Polycrystal-

line samples of 3, 4, and 5 were introduced into 0.5 mm

borosilicate capillaries prior to being mounted and aligned on a

Empyrean PANalytical powder diﬀractometer, using Cu Kα

radiation (λ = 1.54056 Å). For each sample, ﬁve repeated

measurements were collected at room temperature (2θ = 2−

60°) and merged in a single diﬀractogram. A polycrystalline

sample of 5 was also measured after catalysis following the

same procedure.

X-ray Photoelectron Spectroscopy (XPS) Measure-

ments. Samples 4, 5, and 5′ (after catalysis) were prepared by

sticking, without sieving, the MOF onto a molybdenum plate

with cellophane tape, followed by air drying. Measurements

were performed on a K-Alpha X-ray Photoelectron Spec-

trometer (XPS) System using a monochromatic Al K(alpha)

source (1486.6 eV). As an internal reference for the peak

stirring rate of 450 rpm for the required reaction time. Aliquots

were taken periodically to follow the course of the reaction by

GC and by GC-MS, after adding mesitylene (3 μL) as an

external standard. Supported Pd nanoparticles (Sigma-Aldrich,

8%) of diﬀerent loadings 10, 5, and 1% in weight (3.1, 6.1,

and 30.7 mg, respectively) and MOF 5 (1% in weight, 30.7

mg) were used as catalysts for the same purpose.

General procedure for the oxidation of 1a with palladium

catalysts: In a 10 mL glass vial equipped with a stirring bar, the

corresponding benzyl alcohol (1.96 mmol) was charged with

the diﬀerent amounts of the palladium catalyst. The vial was

closed with a septum, an oxygen balloon was connected, and

the mixture was placed in a preheated metal heating plate at

50 °C and stirred at 450 rpm for the indicated time.

Alternatively, we used a lab-made double-walled vial connected

588

J. Am. Chem. Soc. 2021, 143, 2581−2592

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.0c12367.

Article

positions in the XPS spectra, the C 1s peak has been set at

Jesus Ferrando-Soria − Instituto de Ciencia Molecular

(ICMol), Universidad de Valencia, 46980 Paterna, Valencia,

Spain

84.8 eV.

FTIR Spectroscopy of Adsorbed CO. Fourier transform

infrared (FTIR) using CO as a probe molecule was used to

evaluate electronic properties of MOF 5. Spectra were

recorded once complete coverage of CO at the speci ﬁ ed CO

partial pressure was achieved. See the Supporting Information

for details.

Computational Methods. Periodic DFT calculations

were performed with the Vienna Ab-initio Simulation Package

Miguel López-Haro − Departamento de Ciencia de los

Materiales e Ingeniería Metalúrgica y Química Inorgánica,

Facultad de Ciencias, Universidad de Cádiz, Campus

Universitario de Puerto Real, 11510 Puerto Real, Cádiz,

Spain; Instituto Universitario de Investigación en Microscopía

Electrónica y Materiales (IMEYMAT), Facultad de Ciencias,

Universidad de Cádiz, Campus Universitario de Puerto Real,

11510 Puerto Real, Cádiz, Spain; orcid.org/0000-0003-

2560-8015

(

VASP) code, using the Perdew−Burke −Ernzerhof (PBE)

exchange-correlation functional. See Supporting Information

for details.

Juan Carlos Hernandez-Garrido − Departamento de Ciencia

XAS Techniques. XANES and EXAFS measurements of

Pd foil, Pd acetate, and MOF 5 were carried out on CLAESS

beamline at ALBA Synchrotron Light Source, Barcelona, Spain.

See the Supporting Information for details.

de los Materiales e Ingeniería Metalúrgica y Química

Inorgánica, Facultad de Ciencias, Universidad de Cádiz,

Campus Universitario de Puerto Real, 11510 Puerto Real,

Cádiz, Spain; Instituto Universitario de Investigación en

Microscopía Electrónica y Materiales (IMEYMAT), Facultad

de Ciencias, Universidad de Cádiz, Campus Universitario de

Puerto Real, 11510 Puerto Real, Cádiz, Spain; orcid.org/

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

000-0001-8499-0395

Judit Oliver-Meseguer − Instituto de Tecnología Química

UPV−CSIC), Universitat Politècnica de València−Consejo

(

Physical techniques. Crystallographic and catalytic

details. Tables S1−S8. Figures S1−S31. (PDF)

Superior de Investigaciones Cientíﬁcas, 46022 Valencia,

Spain

Carlo Marini − CELLS−ALBA Synchrotron, E-08290

Accession Codes

Barcelona, Spain

CCDC 1995182 −1995184 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Mercedes Boronat − Instituto de Tecnología Química

(

UPV−CSIC), Universitat Politècnica de València−Consejo

Superior de Investigaciones Cientí ﬁ cas, 46022 Valencia,

Spain; orcid.org/0000-0002-6211-5888

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c12367

AUTHOR INFORMATION

Corresponding Authors

■

Author Contributions

E.T. and R.G. contributed equally to this work.

∇

Donatella Armentano − Dipartimento di Chimica e

Tecnologie Chimiche (CTC), Università della Calabria,

Rende 87036, Cosenza, Italy; orcid.org/0000-0002-

Notes

The authors declare no competing ﬁnancial interest.

502-8074 ; Email: donatella.armentano@unical.it

Antonio Leyva-Pérez − Instituto de Tecnología Química

UPV−CSIC), Universitat Politècnica de València−Consejo

ACKNOWLEDGMENTS

■

(

This work was supported by the Ministero dell’Istruzione,

Superior de Investigaciones Cientí ﬁ cas, 46022 Valencia,

Spain; orcid.org/0000-0003-1063-5811;

Email: anleyva@itq.upv.es

Emilio Pardo − Instituto de Ciencia Molecular (ICMol),

Universidad de Valencia, 46980 Paterna, Valencia, Spain;

orcid.org/0000-0002-1394-2553; Email: emilio.pardo@

uv.es

dell’Universita

e della Ricerca (Italy) and the MINECO

(

Spain) (Projects PID2019−104778GB−I00, CTQ 2017−

6735−P, RTC−2017−6331−5, Severo Ochoa program

SEV−2016−0683 and Excellence Unit “Maria de Maeztu”

CEX2019−000919−M). E.T. and M.M. thank MINECO and

ITQ for the concession of a contract. D.A. acknowledges the

ﬁnancial support of the Fondazione CARIPLO/“Economia

Circolare: ricerca per un futuro sostenibile” 2019, Project

code: 2019−2090, MOCA and Diamond Light Source for

awarded beamtime and provision of synchrotron radiation

facilities and thanks Dr. Sarah Barnett and David Allan for their

assistance at I19 beamline (Proposal No. MT18768-1).

Thanks are also extended to the “2019 Post-doctoral Junior

Leader-Retaining Fellowship, la Caixa Foundation

(ID100010434 and fellowship code LCF/BQ/PR19/

11700011” (J.F.-S.) and “La Caixa” scholarship (ID

100010434) LCF/BQ/DI19/11730029 (J.B.-S). E.P. acknowl-

edges the ﬁnancial support of the European Research Council

under the European Union’s Horizon 2020 research and

innovation programme/ERC Grant Agreement No 814804,

MOF reactors. J.O.-M. acknowledges the Juan de la Cierva

Authors

Estefanía Tiburcio − Instituto de Ciencia Molecular (ICMol),

Universidad de Valencia, 46980 Paterna, Valencia, Spain

Rossella Greco − Instituto de Tecnología Química

(

UPV−CSIC), Universitat Politècnica de València−Consejo

Superior de Investigaciones Cientíﬁcas, 46022 Valencia,

Spain

Marta Mon − Instituto de Tecnología Química (UPV−CSIC),

Universitat Politècnica de València−Consejo Superior de

Investigaciones Cientíﬁcas, 46022 Valencia, Spain

Jordi Ballesteros-Soberanas − Instituto de Tecnología

Química (UPV−CSIC), Universitat Politècnica de

València−Consejo Superior de Investigaciones Cientíﬁcas,

6022 Valencia, Spain

589

J. Am. Chem. Soc. 2021, 143, 2581−2592

Journal of the American Chemical Society

F.; Wang, Q.; Liu, Z.; Zhang, B.; Yu, H. Competitive Adsorption on

Article

program for the concession of a contract (IJC2018-036514-I).

We gratefully acknowledge to ALBA synchrotron for allocating

beamtime and CLÆSS beamline staﬀ for their technical

support during our experiment. The computations were

performed on the Tirant III cluster of the Servei d’Informatica

of the University of Valencia.

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