Efficient Conversion of Biomass-Derived Levulinic Acid to γ-Valerolactone over Polyoxometalate@Zr-Based Metal-Organic Frameworks: The Synergistic Effect of Bro?nsted and Lewis Acidic Sites

Jie Li, Shuaiheng Zhao, Zhen Li, Dan Liu, Yingnan Chi,* and Changwen Hu *

Article

Eﬃcient Conversion of Biomass-Derived Levulinic Acid to

γ‑Valerolactone over Polyoxometalate@Zr-Based Metal−Organic

Frameworks: The Synergistic Eﬀect of Bro

nsted and Lewis Acidic

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sı Supporting Information

ABSTRACT: Catalytic transformation of levulinic acid (LA) to γ-

valerolactone (γ-GVL) is an important route for biomass

upgradation. Because both Bro̷nsted and Lewis acidic sites are

required in the cascade reaction, herein we fabricate a series of

H PW O @Zr-based metal−organic framework (HPW@MOF-

12 40

08) by a facile impregnation method. The synthesized HPW@

MOF-808 is active for the conversion of LA to γ-GVL using

isopropanol as a hydrogen donor. Interestingly, with the increase in

the HPW loading amount, the yield of γ-GVL increases ﬁrst and

then decreases, and 14%-HPW@MOF-808 gave the highest γ-

GVL yield (86%). The excellent catalytic performance was ascribed to the synergistic eﬀect between the accessible Lewis acidic Zr

sites in MOF-808 and Bronsted acidic HPW sites. Based on the experimental results, a plausible reaction mechanism was proposed:

the Zr sites catalyze the transfer hydrogenation of carbonyl groups and the HPW clusters promote the esteriﬁcation of LA with

isopropanol and lactonization to aﬀord γ-GVL. Moreover, HPW@MOF-808 is resistant to leaching and can be reused for ﬁve cycles

without signiﬁcant loss of its catalytic activity.

Lewis-basic O²sites, γ -GVL is synthesized under relatively

mild conditions (Table S1 in the Supporting Information).

Metal−organic frameworks (MOFs) are a class of crystalline

porous materials constructed by metal nodes/clusters and

−

INTRODUCTION

■

Biomass as a renewable and abundant natural source provides

an attractive alternative to fossil-based feedstock. Levulinic acid

LA) is one of the most important biomass-derived platform

(

7,28

organic ligands.

The large surface area, tunable pore size,

molecules, which is generally obtained by the hydrolysis of

and permeable channels make MOFs promising heterogeneous

catalysts or/and excellent catalyst supports.

reported MOFs, Zr-based MOFs (Zr-MOFs) have attracted

continuous attention, owing to their high chemical/thermal

stability and Lewis acidity of Zr sites.

Zr-based MOFs including MOF-808, UiO-66, and MOF-801

were reported to be eﬀective for the synthesis of γ-GVL using

isopropanol as a hydrogen donor. After this, sulfonic acid-

functionalized UiO-66 was prepared, and the combination of

lignocellulose. Recently, the catalytic conversion of LA to

30−32

Among the

value-added chemicals, such as levulinate esters,

γ-

7,8

valerolactone (γ-GVL), and 1,4-pentanediol, has attracted

considerable attention. Among the downstream products, γ-

GVL is a versatile molecule, which can be widely used as a

food additive, fuel additive, organic solvent, and precursor for

33,34

Recently, several

−15

the production of valuable chemicals and liquid biofuels.

Up to now, several approaches based on using diﬀerent

hydrogen sources have been developed for the synthesis of γ-

GVL from LA. Although a high γ-GVL yield was achieved by

Lewis acidic Zr sites and Bro̷nsted acidic −SO H groups

results in the improvement of the γ-GVL yield. However, the

good catalytic activity of the abovementioned Zr-MOFs

signiﬁcantly relies on using levulinate ester as the substrate.

using H or HCOOH as the hydrogen source, the use of noble

6,17

18,19

catalysts (e.g., Ru,

Pd,

and Au ) and harsh reaction

conditions (high temperature and high H pressure) limit their

industrial applications. In contrast, the catalytic transfer

hydrogenation in the presence of alcohols as hydrogen donors

provides an energy-saving and cost-eﬀective pathway. Previous

investigations indicate that Zr-based materials including

Received: January 20, 2021

1,22

25,26

ZrO₂,

ZrO(OH)₂, Zr-beta, and Zr-complexes

are

active for the hydrogenation of LA or levulinate esters, and due

to the cooperative eﬀect between Lewis-acidic Zr⁴sites and

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Catalytic Conversion of LA to γ-GVL Using Alcohols as Hydrogen Donors

When LA was used, only a very limited γ-GVL yield was

obtained because the carboxylic groups of LA have strong

binding ability to the active sites of Zr-MOFs. Compared with

levulinate esters, using LA as the starting material is more cost-

eﬀective and atom-economic. As shown in Scheme 1, the

cascade transformation of LA to γ-GVL is composed of

commercially available reagents for the catalytic test were obtained

from Sinopharm Chemical Reagent Co., Ltd. All the chemicals

purchased in this work were used without further puriﬁcation.

Powder X-ray diﬀraction (PXRD) data were recorded on a Rigaku

MiniFlex 600 diﬀractometer with a Cu-Kα X-ray radiation source (λ =

.54056 Å). Fourier transform infrared (FT-IR) spectra were

collected on a Bruker FT-IR spectrophotometer in the range of

37,38

esteriﬁcation, hydrogenation, and lactonization.

As the

−1

2800−400 cm using KBr pellets. The Zr and W elements were

transfer hydrogenation of the carbonyl group is driven by

Lewis acid, while esteriﬁcation and lactonization are accel-

determined by using a Thermo iCAP Q mass spectrometer. X-ray

photoelectron spectroscopy (XPS) was performed by a Thermo

Scientiﬁc Kα XPS spectrometer equipped with a monochromatic Al

Kα X-ray source (1486.6 eV), and the binding energy was calibrated

via C1s (284.8 eV). The morphologies of MOF-808 and HPW@

MOF-808 were observed on a HITACHI BCPCAS4800 ﬁeld-

emission scanning electron microscope with an accelerating voltage

of 15 kV. The Brunauer−Emmett−Teller (BET) speciﬁc surface areas

were analyzed at 77 K by a Belsorp-max surface area detecting

instrument. Pore size distribution data were collected on the basis of

erated by Bro

Bronsted acidic sites is highly desired in the synthesis of γ-

GVL.

̷nsted acid, a catalyst with both Lewis and

Polyoxometalates (POMs) are a type of anionic metal-oxo

clusters composed of d-block transition metal ions (Mo, W, V,

9,40

and Nb) and oxo ligands.

Bronsted acidity, high proton mobility, and good thermal

stability but also are less corrosive compared to traditional

inorganic acids (e.g., H SO , HNO , and HCl). Moreover,

POMs not only have strong

nonlinear density functional theory (NLDFT). The acidic sites of

MOF-808 and 14%-HPW@MOF-808 were tested by temperature-

during the catalytic process, the polyanions can stabilize

programmed desorption of ammonia (NH -TPD). The samples were

1,42

organic cation intermediates.

Therefore, POMs have been

pretreated at 150 °C for 1 h in ﬂowing Ar, cooled to 50 °C, saturated

,43

widely used to catalyze esteriﬁcation,

transesteriﬁcation,

with NH for 30 min, and then purged for 60 min in Ar. After this, the

5,46

48,49

−1

hydrolysis,

hydration, and Friedel-Crafts reactions.

samples were heated to 150 °C at a rate of 10 °C min , and the

^d^e^s^o^r^b^e^d^N^H3 was recorded by a TCD detector. UV−Vis

spectroscopy was recorded on UV-2600. Thermogravimetric analyses

Nevertheless, high solubility of POMs in polar solvents and a

low surface area restrict their application as heterogeneous

catalysts. Immobilization of POMs on suitable supports has

been proven to be an eﬀective method to recycle POMs.

Among the reported solid supports including mesoporous

(

TGA) were conducted on a Shimadzu DTG-60 thermal analyzer

−

under a N₂atmosphere at a heating rate of 10 °C min . Gas

chromatography−mass spectrometry (GC−MS) was performed in EI

mode on an Agilent 7890A GC/5975C MS. The conversion and yield

were monitored on a Shimadzu GC-2014C instrument using a gas

chromatograph (GC) with a ﬂame ionization detector (FID).

Synthesis of MOF-808. MOF-808 was synthesized by a modiﬁed

52,53

silica, carbon, ﬁbers, and layered double hydroxides,

0,31

MOFs

are promising candidates.

Herein, Keggin-type H PW O (HPW) clusters were

12 40

successfully incorporated into the cavities of MOF-808 by a

simple impregnation method and the synthesized HPW@

MOF-808 composite was assessed as a heterogeneous catalyst

for the production of γ-GVL from LA using isopropanol as a

hydrogen donor. The integration of MOF-808 with HPW was

based on the following considerations. (i) MOF-808 was

shown to be more active than other Zr-MOFs for the catalytic

literature method. ZrOCl ·8H O (1.67 mmol, 538.9 mg), H BTC

(0.55 mmol, 116.7 mg), DMF (25 mL), and formic acid (25 mL)

were mixed and stirred for 5 min under ambient conditions. Then, the

mixture was transferred to a 100 mL Teﬂon-lined pressure autoclave

and heated in an oven at 130 °C for 3 days. After cooling to room

temperature, the white precipitate was isolated by centrifugation and

washed three times with DMF. The obtained precipitate was

immersed in DMF, water, and acetone for 3 days each, respectively,

and all the solvents were changed three times per day. After this, the

residual solvent was evaporated at room temperature, and the

precipitate was further activated in a vacuum oven at 150 °C for 1 day.

Synthesis of HPW@MOF-808. H PW O (15, 30, 45, or 60 mg)

transfer hydrogenation, but its Bro

requirement. (ii) HPW as a strong Bro

nsted acidity is far from the

nsted acid can

eﬀectively facilitate esteriﬁcation of LA with isopropanol and

the following lactonization reaction. (iii) MOF-808 is highly

stable even in concentrated HCl and its cavities are large

12 40

was dissolved in 10 mL distilled water in a 25 mL breaker and to this

the activated MOF-808 (100 mg) was added followed by mechanical

vibration for 12 h. Then, the solid product was isolated by

centrifugation, washed with plenty of water, and dried at 130 °C

under vacuum overnight.

Catalytic Test. In a typical reaction, LA (1 mmol), catalysts (50

mg), naphthalene (internal standard, 0.5 mmol), and isopropanol (5

mL) were mixed and transferred to a 23 mL Teﬂon-lined pressure

autoclave. The reaction vessel was heated to 160 °C for 6 h with

magnetic stirring and then cooled with running water. The obtained

liquid products were monitored by GC and qualitatively analyzed by

GC−MS and authorized chemicals. The catalytic data presented in

the time proﬁle for the conversion of LA to γ-GVL over 14% HPW@

MOF-808 are obtained from six parallel catalytic experiments. In the

recycle test, the catalyst was separated after the reaction via

centrifugation, washed with dichloromethane and dried under vacuum

at 85 °C for 12 h, and used for the next cycle. After three cycles, the

54,55

enough to encapsulate HPW.

As a result, a high yield of γ-

GVL was achieved by HPW@MOF-808 under relatively mild

conditions and the synergistic eﬀect between Lewis acidic sites

of MOF-808 and Bro̷nsted acidic sites of HPW plays a key role

in the valorization of LA.

EXPERIMENTAL SECTION

■

Materials and Methods. ZrOCl ·8H O and 1,3,5-benzenetricar-

boxylic acid (H BTC) were obtained from Aladdin Industrial

Corporation. Formic acid (purity ≥99%) was obtained from J&K

Scientiﬁc Co. Ltd. N,N-Dimethylformamide (DMF) was obtained

from Meryer Chemical Technology Co., Ltd. HPW and γ-GVL

(

≥

purity ≥98%) were obtained from Energy Chemical. LA (purity

97%) and naphthalene were obtained from Tokyo Chemical

Industry, Ltd. Acetone, isopropanol, dichloromethane, and other

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 2. Synthesis of HPW@MOF-808 by a Direct Impregnation Method

Figure 1. (a) PXRD patterns and (b) FT-IR spectra of HPW, MOF-808, and x%-HPW@MOF-808 (x = 9, 14, 20, and 27%). (c) W 4f and (d) P

p XPS spectra of 9%-HPW@MOF-808 (top) and HPW (bottom).

collected catalyst was activated at 130 °C for 12 h to remove the

experiments displayed that the HPW loading amount in

MOF-808 can be readily regulated by the amount of used

HPW. As a result, four HPW@MOF-808 composites with

HPW loading amounts of 9, 14, 20, and 27%, were synthesized,

respectively, which provide a good platform to investigate the

impact of the HPW loading amount on the catalytic

conversion of LA to γ-GVL. The obtained catalysts were

denoted as x%-HPW@MOF-808 and the HPW weight percent

residual solvent molecules.

RESULTS AND DISCUSSION

■

Synthesis and Characterization of HPW@MOF-808. MOF-

08 was prepared under solvothermal conditions according to

a modiﬁed literature method. The 3D open framework of

MOF-808 is formed by the interconnection of {Zr (μ -O) (μ -

(

x%) in the composite was determined by inductively coupled

OH) (HCOO) } clusters with 1,3,5-benzene-tricarboxylate

plasma-mass spectrometry (ICP-MS) data (see Table S2).

To check the phase purity of HPW@MOF-808 composites,

the PXRD patterns of MOF-808, HPW, and HPW@MOF-808

were recorded and compared (Figure 1a). The PXRD patterns

of HPW@MOF-808 are basically identical to the synthesized

and simulated ones of MOF-808, demonstrating that the

framework of MOF-808 is maintained after the incorporation

of HPW. In addition, there are no diﬀraction peaks of Keggin-

type HPW in all composites, suggesting the well-dispersion of

linkers. There are two kinds of cages in MOF-808: a

tetrahedral cage with a size of approximately 4.8 Å and an

adamantane-shaped cage with an internal pore diameter of

approximately 18.4 Å. Considering the size of the Keggin-type

cluster (ca. 12 Å), HPW might be incorporated in the large

adamantane-shaped cages. Due to the excellent stability of

MOF-808 under acidic conditions and the existence of suitable

cages, we combine MOF-808 with HPW via a direct

impregnation method, as shown in Scheme 2. Control

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 2. (a) SEM image of MOF-808. (b) SEM image and (c) EDS mapping of 9%-HPW@MOF-808.

HPW clusters in the cavities of MOF-808. It is found that the

crystallinity of HPW@MOF-808 decreases with the increase in

the HPW loading amount, which was also observed in the

previous studies. The FT-IR spectra of HPW, HPW@MOF-

08, and MOF-808 are illustrated in Figure 1b. The

−

characteristic peaks of the HPW precursor are at 1081 cm

−

for P−O (a: tetrahedral O atoms), 986 cm for WO (d:

−

terminal O atoms), 891 cm for W−O −W (b: edge-shared

O atoms), and 804 cm⁻for W−O −W (c: corner-shared O

atoms), respectively. The characteristic bands in the pristine

MOF-808 at 1615, 1448, and 1383 cm⁻correspond to the

vibrations of the carboxylate group, and the peaks at 1574,

111, 942, 760, and 715 cm⁻are assigned to the vibrations of

Figure 3. N sorption isotherms of MOF-808 and x%-HPW@MOF-

the ligands. In comparison with MOF-808 and HPW, the

HPW@MOF-808 composites clearly exhibit all characteristic

peaks of both the components, which demonstrates successful

incorporation of HPW into MOF-808. Moreover, compared

with the HPW precursor, the vibrations of P−O and WO

08 (x = 9, 14, 20, and 27%. Solid graphics: N absorption; hollow

graphics: N desorption).

comparable to the reported values. With the increase in the

HPW loading amount, the BET surface areas of the composites

−1

in HPW@MOF-808 undergo a distinct red shift (above 20

gradually decrease to 628 m g in 9%-HPW@MOF-808, 414

−

−1

cm ), implying the strong interactions between polyanions

m g in 14%-HPW@MOF-808, 282 m g in 20%-HPW@

59−61

−1

and MOF-808. According to previous investigations,

MOF-808, and 155 m

in 27%-HPW@MOF-808,

speculate that the formate ligands on the Zr (μ -O) (μ -

respectively. Similarly, the pore volumes of HPW@MOF-808

−1

OH) (HCOO) clusters might be partially replaced by −OH

also decrease from 0.61 to 0.34−0.13 cm g . The decrease in

the speciﬁc surface area and pore volume (Table 1) shows that

ligands, and the protonated −OH interacts with HPW

polyanions by electrostatic interactions. The strong interaction

between HPW and MOF-808 is also proven by the leaching

test. When HPW@MOF-808 was soaked in deionized water

for 4 h, no characteristic absorption peak of HPW was detected

by UV−Vis spectroscopy (Figure S1).

To conﬁrm the elemental compositions and their oxidation

states, the XPS spectra of 9%-HPW@MOF-808 as a

representative and HPW were performed and compared. As

illustrated in Figure 1c,d, in the HPW precursor the peaks at

Table 1. Physicochemical Properties of the Synthesized

Catalysts

catalyst

MOF-808

9%-HPW@MOF-808

14%-HPW@MOF-808

20%-HPW@MOF-808

27%-HPW@MOF-808

^SBET (m g⁻¹)^Vtotal (cm g⁻¹) pore size (Å)

1228

628

414

282

155

0.61

0.34

0.25

0.19

0.13

19.9

21.5

23.7

26.7

32.5

6.26, 38.40, and 134.66 eV are assigned to W(VI) 4f₇_/₂,

W(VI) 4f₅_/₂, and P 2p, respectively. Notably, after combination

with MOF-808, the W 4f₇_/₂, W 4f₅_/₂, and P 2p peaks of HPW

shift to lower binding energies, appearing at 35.72, 37.85, and

Measured by using the N -sorption technique. Average pore size.

34.06 eV, respectively. The decrease in binding energy (0.54

HPW is successfully immobilized in the cages of MOF-808. In

addition, the pore size distribution of MOF-808 and HPW@

MOF-808 was investigated (Figure S4 ). The pore diameter in

MOF-808 is concentrated at 21.8 Å, which is larger than that

reported in the literature (18.4 Å) probably owing to the

formation of more defect sites in the synthesis. After the

incorporation of HPW, the pore size distribution becomes

narrow and the pores of 7 and 14 Å appear (Figure S4 ).

Catalytic Conversion of LA to γ-GVL by HPW@MOF-808.

The catalytic performance of HPW@MOF-808 composites

was evaluated in the conversion of biomass-derived LA to γ-

GVL using isopropanol as a hydrogen donor. These results are

summarized in Table 2. MOF-808 as a catalyst converts 54% of

LA at 160 °C in 6 h and the γ-GVL yield is 32% (Table 2,

entry 1). In this case, the esteriﬁcation product, isopropyl

levulinate (iPL), is the main byproduct and a small amount of

4-hydroxyvaleric isopropanyl ester (4-HVIE) and 4-hydrox-

eV for W 4f₇_/₂, 0.55 eV for W 4f₅_/₂, and 0.6 eV for P 2p) might

be related to the electrostatic interaction between MOF-808

and HPW. In contrast, no obvious shift was found in the Zr 3d

region (Figure S2c ). The ﬁeld-emission scanning electron

microscopy (FE-SEM) images show that HPW@MOF-808

has the typical octahedral morphology with an average crystal

size of 200−500 nm (Figure 2b and Figure S3), and compared

with the SEM image of MOF-808 (Figure 2a), no distinct

morphology change is observed. Additionally, the EDS

mapping measurements (Figure 2c) conﬁrm the existence of

Zr, O, P, and W elements in the composite and the

homogeneous distribution of HPW in MOF-808, which is

consistent with the result of PXRD.

N sorption isotherms of MOF-808 and HPW@MOF-808

are shown in Figure 3. The pristine MOF-808 with a type II

isotherm has a large surface area of 1228 m g , which is

−1

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 2. Conversion of LA to γ-GVL by Using Diﬀerent Catalysts

yield (%)

4-HVIE

entry

catalyst

conv. (%)

sel. (%)

γ-GVL

iPL

4-HVA

MOF-808

>99

2.7

trace

4.5

3.4

9.8

6.3

13.2

13.4

14.7

2.4

2.6

7.2

5.0

3.5

trace

9%-HPW@MOF-808

14%-HPW@MOF-808

20%-HPW@MOF-808

27%-HPW@MOF-808

none

trace

H PW O

12 40

Reaction conditions: LA (1 mmol), catalyst (50 mg), isopropanol (5 mL), T = 160 °C, t = 6 h, naphthalene as the internal standard. Conversion

conv.) and γ -GVL yield were monitored by GC. sel. = Selectivity. MOF-808 = 43 mg. H PW O = 7 mg.

3 12 40

Figure 4. (a) Eﬀects of the reaction temperature, (b) type of alcohols, and (c) catalyst amount on the catalytic conversion of LA to γ-GVL over

4%-HPW@MOF-808. (d) Time proﬁle for the conversion of LA to γ-GVL over 14% HPW@MOF-808. Reaction conditions: LA (1 mmol),

catalyst (50 mg), solvent (5 mL), T = 160 °C, t = 6 h, and naphthalene as the internal standard.

yvaleric acid (4-HVA) was observed. In contrast, a negligible

amount of γ-GVL is detected in the absence of the catalyst,

although 22% of LA was consumed with an iPL yield of 14.7%

XPS spectra of the used MOF-808 were measured (Figure

S6b). Compared with the fresh MOF-808 (182.71 and 185.08

eV), the Zr 3d peaks of the used MOF-808 shift to lower

binding energies (182.35 and 184.71 eV), which further

supports the coordination of LA with Zr centers (Figure S6c).

(

Table 2, entry 6). The observed activity of MOF-808 is

2−

mainly ascribed to the well-distributed Zr -O sites, which

can catalyze the transfer hydrogenation, evidenced by the

formation of hydrogenation products (4-HVIE and 4-HVA).

To attain a higher γ-GVL yield, HPW as a strong Bro̷nsted

acid was introduced into MOF-808. As shown in Table 2, entry

2, both the conversion of LA and the yield of γ-GVL are

improved slightly by using 9%-HPW@MOF-808. Notably,

when the loading amount of HPW reaches 14 wt %, the

composite gives nearly complete conversion of LA and 86% γ-

GVL yield (Table 2, entry 3). Although 14%-HPW@MOF-808

As the Bro̷nsted acidity in MOF-808 is too limited to

eﬀectively promote esteriﬁcation of LA with isopropanol, the

unreacted LA binds to metal sites of MOF-808 leading to the

decrease of catalytic activity. The FT-IR spectrum of the used

−

MOF-808 shows two new peaks at 1704 and 1168 cm

−1

compared to that of the fresh one (Figure S6a ), which are

has a lower speciﬁc surface area (414 m g ) relative to that of

−1

ascribed to the stretching vibration of CO and C−O bonds

MOF-808 (1228 m g ) or 9%-HPW@MOF-808 (628 m

−1

of LA, respectively. Moreover, the two peaks exhibit a slight

g ), its catalytic activity is signiﬁcantly higher than the two

catalysts. Moreover, the intermediate 4-HVIE observed in

MOF-808 and 9%- HPW@MOF-808 nearly disappeared in

−

red shift (ca. 10 cm ) compared to the free LA, indicating the

coordination of LA with Zr sites in MOF-808. In addition, the

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

4%-HPW@MOF-808. To understand the role of POMs in

Article

the products suggests that the esteriﬁcation reaction is faster

than the following transfer hydrogenation and the latter is the

rate-limiting step in the whole process, which is consistent with

the catalytic reaction, a control using HPW as a homogeneous

catalyst was performed. HPW alone produces negligible γ-GVL

but 96% of LA was converted to iPL (Table 2, entry 7). When

HPW was replaced by H SiW O (HSiW) with relatively

23,36

previous reports.

Importantly, only a trace amount of 4-

HVIE was detected, indicating that the immobilized HPW can

eﬀectively catalyze the intramolecular dealcoholization of 4-

HVIE to γ-GVL. Moreover, the detection of 4-HVIE provides

a direct evidence for the transfer hydrogenation of iPL. Since

LA has a lower reactivity than iPL in the transfer hydro-

genation, only a small amount of 4-HVA was produced in the

ﬁrst 4 h. After this, the amount of 4-HVA gradually increases,

which might be generated by the direct transfer hydrogenation

of LA, the hydrolysis of 4-hydroxyvaleric isopropanyl ester (4-

HVIE), or the ring opening of γ-GVL.

12 40

weaker acidity, HSiW@MOF-808 exhibited a reduced catalytic

activity (conversion: 67%, yield: 49%). According to the above

results, we think that the excellent activity of 14%-HPW@

MOF-808 is mainly attributed to enough Bro̷nsted acidic sites

oﬀered by HPW, which can greatly accelerate the esteriﬁcation

of LA and the lactonization of 4-HVIE to produce γ-GVL.

Therefore, the acidic sites of 14%-HPW@MOF-808 and

MOF-808 are examined by NH -TPD (Figure S7 ), and the

results suggest that 14%-HPW@MOF-808 has higher acidity

than MOF-808. Nevertheless, the further increase in the HPW

loading amount leads to the decrease of LA conversion and γ-

GVL yield (Table 2, entries 4 and 5). This decreased activity

might be explained by the following two points. On one side,

when excessive HPW clusters occupy the cages of MOF-808,

the accessible Lewis acidic sites (Zr-oxide cluster) correspond-

ingly decrease and thereby the catalytic transfer hydrogenation

is inhibited. On the other side, the surface areas of 20%-

On the basis of the experimental results and the previous

25,26,35−38,62−64

reports,

we proposed a possible mechanism for

the production of γ-GVL by HPW@MOF-808 (Scheme 3).

Scheme 3. Possible Mechanism for the Conversion of LA to

γ-GVL by HPW@MOF-808

−1

HPW@MOF-808 (282 m g ) and 27%-HPW@MOF-808

−1

(

155 m g ) signiﬁcantly decrease and accordingly less

catalytic active sites are exposed. The catalytic activity order of

4%-HPW@MOF-808 > 20%-HPW@MOF-808 ≈ 9%-

HPW@MOF-808 > 27%-HPW@MOF-808 > MOF-808

demonstrates that accessible balanced Bronsted and Lewis

acidic sites are a key factor to inﬂuence the conversion of LA to

γ-GVL. Importantly, there appears to be synergistic eﬀect

between MOF-808 and HPW in HPW@MOF-808.

Due to the good catalytic performance, 14%-HPW@MOF-

08 was used in the following experiments. Usually, the

conversion of LA to γ-GVL can be inﬂuenced by many factors,

such as temperature, solvent, time, and catalytic dosages. As

shown in Figure 4a, the temperature dependence was observed

in the reaction. Both the conversion of LA and the selectivity

of γ-GVL increase with the reaction temperature in the range

of 120−180 °C. Although a higher yield of γ-GVL (91%) was

obtained at 180 °C, compared with the yield (86%) at 160 °C

no substantial enhancement was obtained. From the energy-

saving point of view, the temperature of 160 °C was selected.

In this reaction, alcohols were used not only as hydrogen

donors but also as solvents, and so the impact of diﬀerent

alcohols on the cascade reaction was investigated. As shown in

Figure 4b, the best performance (conversion: >99%,

selectivity: 87%) is achieved by using isopropanol (2-PrOH).

In the presence of tert-butanol (t-BuOH), no γ-GVL is

produced due to the lack of β-H. Moreover, the selectivity of

secondary alcohols for γ-GVL is better than that of primary

alcohols because it is relatively easy to eliminate the β-hydride

First, the carbonyl oxygen of LA is protonated by HPW and

the formed carbocation intermediate is stabilized by poly-

1,42

anions,

and the subsequent nucleophilic attack of

isopropanol gives rise to iPL. The catalytic eﬃciency of

HPW in the esteriﬁcation has been proven in the

homogeneous system, where 96% of LA was converted to

iPL in 4 h. Then, the generated iPL and isopropanol interact

2−

with the Zr -O Lewis acid−base sites in MOF-808 to form a

six-membered ring transition state. Next, the β-H atom of

isopropanol attacks the carbonyl group of iPL to produce 4-

HVIE. Finally, the generated 4-HVIE is rapidly converted to γ-

GVL through Bro

the synergistic eﬀect of Bro

composite plays an important role in the highly eﬃcient

transformation of LA to γ-GVL.

nsted acid-driven lactonization. Therefore,

nsted and Lewis acidic sites in the

The heterogeneous nature of HPW@MOF-808 was

examined by the leaching test and incubation experiment.

When the reaction was conducted for 2 h with a conversion of

about 43%, the 14%-HPW@MOF-808 catalyst was ﬁltered oﬀ

and the resulting solution continued to react for another 4 h

under identical conditions. Considering the 22% conversion of

LA in the blank test, the observed slight increase in conversion

is reasonable (Figure 5a). In addition, the catalyst was

incubated in isopropanol at 160 °C for 6 h and removed by

centrifugation. Then, LA was added and the solution was

allowed to react at 160 °C for another 6 h. The yields of γ-

GVL and iPL is basically identical to that of the blank test,

suggesting that no HPW is leaching in the reaction. The above

result also demonstrates that MOF-808 is not only an active

catalyst for the transfer hydrogenation but also a good support

from the alkoxide intermediate in secondary alcohols. In

addition, the control experiments indicate that 50 mg is an

optimal catalyst dosage (Figure 4c).

To further understand the catalytic process, the reaction was

monitored in the time range of 0−12 h over 14%-HPW@

MOF-808 under the optimized conditions (Figure 4d). The

maximum yield of γ-GVL was obtained in 6 h and after that the

γ-GVL yield was basically unchanged. In addition, iPL, 4-HVA,

and 4-HVIE (the hydrogenation product of iPL) as byproducts

were detected by GC and determined by GC−MS (Figure S8).

As shown in Figure 4d, the yield of iPL increases ﬁrst and then

decreases and the maximum appears in 4 h. The time proﬁle of

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

Figure 5. (a) Leaching test for the conversion of LA to γ-GVL over 14%-HPW@MOF-808. (b) Recycling test for the conversion of LA to γ-GVL

over 14%-HPW@MOF-808. (c) FT-IR spectra and (d) PXRD patterns of 14%-HPW@MOF-808 before and after ﬁve cycles. Reaction conditions:

mmol LA, 50 mg catalyst, 5 mL isopropanol, T = 160 °C, t = 6 h, and naphthalene as the internal standard.

to immobilize HPW. Moreover, we evaluate the recyclability

and stability of the 14%-HPW@MOF-808 catalyst under the

optimized conditions. After the ﬁrst cycle, the catalyst was

separated from the reaction solution and washed with

dichloromethane. After drying at 85 °C, it was used for the

next run. As shown in Figure 5b, a decrease of the conversion

and γ-GVL yield was observed for the ﬁrst three cycles. We

speculate that the decrease of catalytic activity is possibly

related to the absorption of solvent molecules in MOF-808

leaching and can be reused for ﬁve cycles without signiﬁcant

loss of catalytic activity. The present work provides some

insights into the rational design of heterogeneous catalysts for

biomass upgrading.

ASSOCIATED CONTENT

sı Supporting Information

■

https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00185.

(Figure S9). Therefore, the reused catalyst was activated at 130

C under vacuum for 12 h. After activation, the activity of 14%-

Summary of the catalytic synthesis of γ-GVL from LA,

elemental analysis, UV−Vis spectra, XPS spectra, SEM

images, pore size distribution, TGA, FT-IR spectra,

HPW@MOF-808 was basically recovered. The comparison of

the FT-IR spectra and PXRD patterns before and after the

recycling test conﬁrms that the structure of HPW@MOF-808

remains stable under the turnover conditions (Figure 5c,d).

Moreover, the SEM and EDS elemental mapping measure-

ments (Figure S10) show that the morphology of 14%-HPW@

MOF-808 is basically maintained after ﬁve cycles and the POM

clusters are still highly dispersed in MOF-808.

NH -TPD, GC−MS, and EDS mapping (PDF)

AUTHOR INFORMATION

■

Corresponding Authors

Yingnan Chi − Key Laboratory of Cluster Science Ministry of

Education, Beijing Key Laboratory of Photoelectronic/

Electrophotonic Conversion Materials, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, P.R. China; orcid.org/0000-0003-3983-

CONCLUSIONS

■

In summary, given that both Bro

are required for the cascade synthesis of γ-GVL from biomass-

nsted and Lewis acidic sites

997; Email: chiyingnan7887@bit.edu.cn

derived LA, a strong Bro̷nsted acid, HPW, was incorporated

Changwen Hu − Key Laboratory of Cluster Science Ministry

of Education, Beijing Key Laboratory of Photoelectronic/

Electrophotonic Conversion Materials, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, P.R. China; Email: cwhu@bit.edu.cn

into MOF-808 by a simple impregnation method and the

synthesized HPW@MOF-808 composites were characterized

by routine techniques. HPW@MOF-808 was tested as the

heterogeneous catalyst for γ-GVL production using isopropa-

nol as a hydrogen donor. Interestingly, 14%-HPW@MOF-808

exhibits a better catalytic performance than either of the

individual components alone or the composites with other

Authors

Jie Li − Key Laboratory of Cluster Science Ministry of

Education, Beijing Key Laboratory of Photoelectronic/

Electrophotonic Conversion Materials, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, P.R. China

HPW loading amounts, showing that the synergistic eﬀect

between the accessible Lewis acidic Zr⁴sites and Bro

nsted

acidic HPW sites plays an important role in the improvement

of the γ-GVL yield. Moreover, HPW@MOF-808 is resistant to

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

H.; Halligudi, S. B.; Hwang, J. S.; Chang, J. S. Direct Hydrocyclization

Article

Shuaiheng Zhao − Key Laboratory of Cluster Science Ministry

of Education, Beijing Key Laboratory of Photoelectronic/

Electrophotonic Conversion Materials, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, P.R. China

Zhen Li − Key Laboratory of Cluster Science Ministry of

Education, Beijing Key Laboratory of Photoelectronic/

Electrophotonic Conversion Materials, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, P.R. China

Dan Liu − Key Laboratory of Cluster Science Ministry of

Education, Beijing Key Laboratory of Photoelectronic/

Electrophotonic Conversion Materials, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, P.R. China

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Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.1c00185

Author Contributions

Y.N.C. and C.W.H. conceived and directed the research. J.L.

prepared the catalysts and performed the catalytic experiments.

S.H.Z. gave some advice about the preparation of catalysts.

(17) Deng, L.; Li, J.; Lai, D. M.; Fu, Y.; Guo, Q. X. Catalytic

Z.L. helped to perform the N sorption experiment.

Conversion of Biomass-Derived Carbohydrates into γ -Valerolactone

Notes

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The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS

This work was ﬁnancially supported by the National Natural

Science Foundation of China (21671019, 21871026,

1771020, and 21971010).

■

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