Cascade reaction engineering on zirconia-supported mesoporous MFI zeolites with tunable Lewis-Br?nsted acid sites: a case of the one-pot conversion of furfural to γ-valerolactone

Article abstract of DOI:10.1039/d0ra06915a

Catalytic cascade reactions are strongly desired as a potential means of combining multistep reactions into a single catalytic reactor. Appropriate catalysts composed of multi-reactive sites to catalyze cascade reactions in a sequential fashion are central to such efforts. Here, we demonstrate a bifunctional zeolite catalyst with close proximity of Br?nsted and Lewis acid sites through the synthesis of a mesoporous ZrO₂[Al]MFI nanosponge (NS). The unique mesopores of the MFI-NS allow the confinement of zirconium oxide clusters (Lewis acid sites, LA) within the few-unit-cell-thin MFI aluminosilicate zeolite wall (Br?nsted acid sites, BA). Such a structure is clearly distinct from the conventional MFI zeolite, where the agglomeration of zirconium oxide clusters onto the external surface area within the crystal bulk is not possible, resulting in segregated BA and LA sites on the internal and external zeolite, respectively. By bringing the BA and LA within ZrO₂[Al]MFI-NS 30, we uncovered a more efficient catalytic route for the conversion of furfural (100% within 2 h) to γ-valerolactone (GVL) (83%). This route is only evident when the long molecular diffusion path, in the most extreme case of physically mixed ZrO₂-(LA) and Al-zeolites (BA) (45% of GVL yield), is eliminated. Unlike the bifunctional ZrO₂-Al-beta (GVL yield of 75%), where the BA concentration is greatly compromised at the expense of LA formation, we also show that the ZrO₂[Al]MFI-NS is able to maintain a high density and good stability of both types of acids.

Full text of DOI:10.1039/d0ra06915a

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PAPER
Cascade reaction engineering on zirconia-
supported mesoporous MFI zeolites with tunable
Lewis–Brønsted acid sites: a case of the one-pot
conversion of furfural to g-valerolactone†
b
Cite this: RSC Adv., 2020, 10, 35318
Kyung Duk Kim,a Jaeheon Kim,a Wey Yang Teoh,
Jeong-Chul Kim,a
c
ad
*
Jun Huang
and Ryong Ryoo
Catalytic cascade reactions are strongly desired as a potential means of combining multistep reactions into
a single catalytic reactor. Appropriate catalysts composed of multi-reactive sites to catalyze cascade
reactions in a sequential fashion are central to such efforts. Here, we demonstrate a bifunctional zeolite
catalyst with close proximity of Brønsted and Lewis acid sites through the synthesis of a mesoporous
ZrO2[Al]MFI nanosponge (NS). The unique mesopores of the MFI-NS allow the confinement of zirconium
oxide clusters (Lewis acid sites, LA) within the few-unit-cell-thin MFI aluminosilicate zeolite wall
(Brønsted acid sites, BA). Such a structure is clearly distinct from the conventional MFI zeolite, where the
agglomeration of zirconium oxide clusters onto the external surface area within the crystal bulk is not
possible, resulting in segregated BA and LA sites on the internal and external zeolite, respectively. By
bringing the BA and LA within ZrO2[Al]MFI-NS 30, we uncovered a more efficient catalytic route for the
conversion of furfural (100% within 2 h) to g-valerolactone (GVL) (83%). This route is only evident when
the long molecular diffusion path, in the most extreme case of physically mixed ZrO2-(LA) and Al-
zeolites (BA) (45% of GVL yield), is eliminated. Unlike the bifunctional ZrO2–Al-beta (GVL yield of 75%),
where the BA concentration is greatly compromised at the expense of LA formation, we also show that
the ZrO2[Al]MFI-NS is able to maintain a high density and good stability of both types of acids.
Received 11th August 2020
Accepted 14th September 2020
DOI: 10.1039/d0ra06915a
rsc.li/rsc-advances
catalysts initially converts furfural into levulinic acid, followed
by the reductive hydrogenation of levulinic acid to GVL over
precious-metal catalysts such as Pt and Ru.5 In spite of the
1. Introduction
The exploitation of efficient catalytic processes capable of con-
verting renewable cellulosic biomass, especially lignocellulose,
to versatile platform chemicals is of importance for the
production of sustainable chemicals and fuels, conventionally
obtained from crude oil.1 Biomass-derived g-valerolactone
(GVL) has been identied as a versatile platform molecule due
to its attractive physicochemical properties (low toxicity and
biodegradability) and unique features that allow it to be trans-
formed into a wide collection of bio-products, ranging from
green fuels to bio-polymers.2–4 Conventionally, the production
of GVL from furfural is accomplished through multistep pro-
cessing, where the combination of Brønsted and Lewis acidic
burgeoning interest, the requirements of high-pressure
hydrogen and the costly noble-metal catalysts required during
the reductive hydrogenation step have been shown to impact
the economic feasibility of this process negatively.6,7 Hence, the
catalytic transfer hydrogenation (CTH) route using the Meer-
wein–Ponndorf–Verley (MPV) reaction is oen used as an
alternative to reductive hydrogenation, in which the Lewis acids
can serve as an efficient catalyst with the aid of secondary
alcohols (e.g. 2-propanol, 2-butanol, and cyclohexanol) as an H-
donor.
Zeolites containing isomorphously substituted-heteroatoms
such as Sn and Zr, are oen used as a catalyst for the CTH
reaction due to the high intrinsic Lewis acid (LA) activity of
single-atomic Sn or Zr sites.8,9 Most conventionally, these
heteroatom-containing zeolites can be obtained by a hydro-
thermal synthesis route. However, there remain a number of
fundamental restrictions though these have been gradually
overcome in recent years. For example, it is well-known that this
route requires a long synthesis time (nearly one month) due to
the extremely slow rate of heteroatom incorporation8,10 and
aCenter for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS),
Daejeon 34141, Korea. E-mail: rryoo@kaist.ac.kr
bSchool of Chemical Engineering, The University of New South Wales, Sydney, NSW
2052, Australia
cLaboratory for Catalysis Engineering, School of Chemical and Biomolecular
Engineering, Sydney Nano Institute, The University of Sydney, NSW 2006, Australia
dDepartment of Chemistry, KAIST, Daejeon, 34141, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06915a
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necessitates the use of a toxic chemical, e.g., uoride as capable of rapid molecular diffusion under various catalytic
mineralizing agent to incorporated heteroatoms effectively in reaction conditions.22–24,27,28
the framework position.8 These problems can be partially
In this work, we demonstrate this concept via the one-pot
overcome through the advancement of a seeding-mediated conversion of furfural to GVL. BA–LA tunable ZrO2[Al]MFI-NS
synthesis route, nevertheless, this approach is oen far from zeolites, whilst mitigating the various limitations of conven-
optimum. To circumvent these limitations, Sn- or Zr-containing tional zeolites, exhibit the efficient one-pot conversion of
zeolites were oen obtained by a post-synthetic route consisting furfural with a high GVL yield (83%) without the need for
of demetallation and a subsequent remetallation step.11,12 In hydrogen pressure and precious metals. In fact, the high
this route, aluminosilicate or borosilicate zeolites initially stability of the ZrO2[Al]MFI-NS is further demonstrated through
synthesized and then treated with an acid solution to remove a number of recycling reactions with a minimal decrease in the
the framework Al- or B-species. A subsequent incorporation step performance outcome.
is conducted on the defect sites of the framework to generate
isolated tetrahedral single Sn or Zr sites. In this way, a several
Lewis acid zeolites have been reported to exhibit excellent
catalytic activity for the CTH reaction.12–14 In addition, bifunc-
2. Experimental
2.1 Catalyst preparation
tional zeolites could be prepared by adjusting the degrees of Al
Detailed processes for the synthesis of pristine catalysts such as
removal (BA sites) and heteroatom incorporation (LA
mesoporous MFI-NS, bulk-MFI, and the Beta catalysts used here
are provided in the ESI.† ZrO2-containing MFI zeolites were
synthesized via impregnation as reported in previous work.29 To
prepare all of the zeolites (ZrO2-[Al]MFI-NS, ZrO2 bulk-MFI and
ZrO2-Al-beta), 0.032 mg of zirconium(IV) oxynitrate hydrate was
sites).11,12,15–17 Controlled densities of BA and LA sites within
close proximity in bifunctional zeolites are spatially segregated
such that they are aligned with the residence time of each
reaction step, hence increasing GVL productivity.11,12,15–18
Nevertheless, this process of controlled partial dealumination
dissolved into 5 ml of deionized water. 0.2 mg of zeolite was
to obtain the optimum Si/Al ratio is highly sensitive, rendering
added to the aqueous solution. The mixture was stirred at 80 ꢀC
the tuning of the BA–LA ratio in the product catalyst rather
for 2 h until the excess water was removed. Subsequently, the
cumbersome with demetallation possibly leading to certain
zeolite powder dried at 80 ꢀC for 6 h in an oven and calcined at
problems associated with the crystallinity of zeolites.
Another notable way to design a bifunctional BA–LA catalyst
is to impregnate LA-components into aluminosilicate zeolites.
It is well-known that some metal oxides such as ZrO2, SnO2, and
ꢀ
550 C for 6 h.
2.2 Characterization of materials
TiO2, can function as solid LA-catalysts when they form Powder X-ray diffraction (XRD) patterns of zeolite materials were
a nanoparticle-like morphology.19–21 Therefore, it is simple to collected at two angles between 5ꢀ and 70ꢀ using a Rigaku Mul-
propose that if the tiny nanoparticles of metal oxides are sup- tiex diffractometer equipped with Cu-Ka radiation (l ¼ 0.1541
ported inside the zeolite micropore channels, the resulting nm) at 40 kV and 30 mA. Argon adsorption–desorption isotherms
catalysts could exhibit high activity as a bifunctional BA–LA were obtained using an ASAP 2020 at 87 K. Prior to the adsorption
catalyst for the furfural to GVL. Unfortunately, the deposition of measurements, all samples were degassed at 573 K for 6 h. The
ZrO2 nanoparticles onto the zeolite can in principle fulll the pore-size distributions were derived from the adsorption branch
requirements of LA sites; however, this has remained limited to of the argon isotherms using non-linear density functional theory
the external surface of the zeolites as stability is not straight- (NLDFT), with the assumption of cylindrical pores. The specic
forward in the microporous channels of zeolites. The micro- surface area was calculated from the adsorption branch in the P/
porous exclusion of metal oxide nanoparticles is a common P0 range of 0.05–0.2 using Brunauer–Emmett–Teller (BET) equa-
feature in which the high surface energy from small nano- tions. High-resolution scanning electron microscopy (SEM)
particles confers a large amount of mobility to migrate to images were taken with a FEI VERIOS 460 instrument operating at
external surfaces of the crystal zeolite.22–24 Because microporous 1 kV in beam-deceleration mode. Transmission electron micros-
zeolites have a very low external surface area, small nano- copy (TEM) images were obtained with a Tecnai microscope
particles are more susceptible to agglomeration, inevitably operating at 300 kV (G2F30). Before these measurements, the
limiting exibility during the engineering of in-pore reactions. power sample was suspended in acetone by ultra-sonication. A
To circumvent the difficulty of ZrO2 deposition within the few droplets of the suspended solution were placed on a Cu
narrow and tortuous zeolite micropores, here we employ the use microgrid and dried under ambient conditions. The silicon,
of mesoporous MFI nanosponge (NS) consisting of MFI zeolite aluminum and zirconia loadings of the zeolite materials were
with mesoporous channels.22,23,25,26 Silt-shaped mesoporous determined by a Thermo IRIS Intrepid II XSP atomic emission
channels, bound by a few unit cells-thin MFI aluminosilicate spectrometer (ICP-AES). The solid 27Al MAS NMR spectra were
zeolitic walls (BA-rich sites) with highly uniform diameters can recorded using a Bruker Avance III 400WB spectrometer at room
strictly support ZrO2 nanoparticles with the connement temperature operated at a B0 eld of 9.4 T with Larmor frequen-
contact effect of meso-micro pore walls, resulting in a high cies of 104.3 MHz. A sample spinning rate of 10 kHz using 4 mm
dispersion of ZrO2 clusters (LA-rich sites) and thereby allowing rotors was used in all of the experiments.
very close proximity of BA–LA sites within cascade reaction
The concentrations of the acid sites were measured using
distances. In addition, mesoporous MFI-NS have is reportedly Fourier transform infrared (FT-IR) aer adsorbing pyridine as
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a probe molecule. Pyridine (Py) with a kinetic diameter of about equations that describe the rst-order reaction kinetics of each
0.5 nm was used to detect the total number of Brønsted and step in the cascade reactions:
Lewis acid sites existing on the internal pore walls and external
d½FALꢃ
¼ ꢂðk3 þ k1Þ½FALꢃ þ k2½FDAꢃ
(1)
(2)
(3)
(4)
(5)
(6)
surfaces of the zeolite crystals. 2,6-di-tert-butyl-pyridine (DTBP)
was used to measure the Brønsted acid sites located on the
external surfaces of the samples due to its larger kinetic diam-
dt
d½FDAꢃ
¼ k1½FALꢃ ꢂ k2½FDAꢃ
˚
˚
eter (0.8 nm) than the 10-ring channels (5.1 ꢁ 5.5 A, 5.3 ꢁ 5.5 A)
of MFI zeolites. FT-IR spectra were acquired using a JASCO
FTIR-6100, following a method reported in the literature.30 Prior
to the measurement, a self-supported zeolite wafer (ca. 10 mg
cmꢂ2) was placed in a lab-made IR cell and evacuated at 400 ꢀC
for 4 h. Then, adsorption of the probe molecules (Py and DTBP)
dt
d½FAꢃ
dt
¼ k3½FALꢃ ꢂ ðk4 þ k5Þ½FAꢃ
d½ALꢃ
¼ k4½FAꢃ ꢂ k6½ALꢃ
dt
ꢀ
was carried out at 150 C for 30 min at an equilibrium probe
vapor pressure for 30 min. Desorption proceeded at the same
temperature for 1 h, followed by the collection of the spectra at
room temperature. All IR data were recorded with a resolution
of 2 cmꢂ1 by collecting 40 scans for a single measurement. For
quantication of the total Brønsted and Lewis acid sites, the
molar extinction coefficient [3(B) ¼ 1.67 cm mmolꢂ1 and 3(L) ¼
2.22 cm mmolꢂ1] of the pyridine IR band at 1545 cmꢂ1 and at v ¼
1454 cmꢂ1 was used. To calculate the concentration of external
Brønsted acid sites, extinction coefficients for pyridine at v ¼
1616 cmꢂ1 were also used.
d½IPLꢃ
dt
¼ k5½FAꢃ þ k6½ALꢃ ꢂ k7½IPLꢃ
d½GVLꢃ
¼ k7½IPLꢃ
dt
where FAL, FDA, FA, AL, IPL and GVL are the abbreviations for
furfural, furfural diisopropyl acetal, furfural alcohol, b-angelica
lactone, isopropyl levulinate, and g-valerolactone, respectively.
Matlab code was used to perform the numerical integration and
optimization. The global error, taken as the square of the
difference between the estimated and observed values, was
minimized to determine the best t reaction rate constants. The
2.3 Catalytic reaction
global error is typically less than 3 ꢁ 10ꢂ5
.
The catalytic conversion of furfural was performed both in a thick-
walled glass reactor and in 150 ml stainless reactor continuing
a magnetic stir bar with thermal couple monitoring. Typically,
prior to the reaction, all zeolites were activated under a dry
nitrogen ow (30 ml minꢂ1) at 400 ꢀC for two hours. Aer cooling
the catalysts under a ow of N2, the zeolite was transferred into the
reactor. Subsequently, 0.04 M furfural at a 1 : 50 molar ratio of
furfural to a 2-propanol solution (2-propanol as the solvent and
hydrogen donor) was added to the rector. The mass ratio of the
catalyst to furfural was 1 : 2.5. The reactor was heated to 170 ꢀC in
a temperature-controlled oil bath on a magnetic stir plate, and the
product mixtures were collected at various times. The products
were separated by ltration and analyzed by a gas chromatograph
(youngling) equipped with a capillary column (Rtx-5 column, 30 m
ꢁ 0.25 mm ꢁ 3 mm) and a ame ionization detector (FID). The
products were identied from known standards and by gas
chromatography-mass spectrometry (Agilent, Rtx-5 column, 30 m
ꢁ 0.25 mm ꢁ 3 mm). The products were quantied based on the
retention times and response factors of the internal standard
chemicals (n-decane). In the catalyst recycling experiments (fourth
cycles), the catalyst was separated from the reaction mixture using
a centrifuge and washed with 2-propanol three times to remove
any residuals and organics, followed by drying at 100 ꢀC in an oven
overnight. The cascade furfural conversion experiments were then
carried out using the recovered catalyst with the same procedure
described above.
3. Results and discussion
3.1 Physiochemical properties of the investigated catalysts
In the present work, two different types of zeolites were
synthesized and compared to each other as a catalyst for the
efficient one-pot conversion of furfural to GVL. The rst sample
was an MFI zeolite exhibiting the morphology of a nanosponge
(NS) which was synthesized using dual structure-directing
multiammonium surfactants.25,26 As shown in Fig. S1,† the
MFI zeolite-like nanosponge samples showed the presence of
ultra-thin nanocrystals (2.5 nm), as corroborated by the broad-
ened MFI peaks measured using XRD, corresponding to
a single-unit-cell size along the (010) direction with irregular
interconnection. The zeolite nanosheets were rigidly self-
supported in a highly mesoporous structure. The adsorption–
desorption isotherm is the characteristic type IV, exhibiting
a rapid increase in the low-pressure region of P/P0 < 0.1 and
a mid-pressure jump with hysteresis from P/P0 0.4 to 0.7 of
relative pressure (Fig. S1†). The former is involved in micropore
lling in the zeolite framework, while the later reects the
capillary condensation of gas in the intercrystalline mesopores
between zeolite nanosheets, which is a distinguishing feature of
disordered mesoporous materials.31 [Al]MFI-NS samples with
different Si/Al ratios e.g. 30, 60, and 100, used to investigate the
effect of the density of the Brønsted acidity on the furfural
conversion to GVL, were synthesized. These nanosponge
2.4 Kinetic modeling
Kinetics modelling was carried out by tting the time–concen- samples are denoted as ‘[Al]MFI-NS n (n is the number denoting
tration of the furfural reactant, GVL product and other inter- the Si/Al ratio)’. Another sample was MFI zeolite with a bulk
mediates, as described by the following set of ordinary different crystalline morphology synthesized by a conventional method
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using tetrapropylammonium hydroxide as a SDA.22 The MFI readily reects the hierarchical construct of the ZrO2-[Al]MFI-NS
framework structure was conrmed by XRD and the diameters 30 sample (Fig. 1d). Fig. 1e shows the pore size distribution for
of the particles ranged from 500 nm to 1 um according to STEM the microporous channels with diameters of ca. 0.55 nm and
(Fig. S3†), which are denoted as ‘bulk-MFI 30 (where the the mesoporous channels with diameters of ca. 6 nm (Fig. 1e,
number denotes the Si/Al ratio)’.
insert). Unlike ZrO2-[Al]MFI-NS 30, as shown in the STEM image
In an ideal BA–LA zeolite catalyst, the availability of LA-rich in Fig. 1a, the deposited ZrO2 nanoparticles (dTEM ꢄ 16 nm,
sites (i.e., through the deposition of LA-mediated metals, in Fig. S5†) of the bulk-MFI 30 zeolite are composed of relatively
our study, ZrO2) should lie within the molecular distances of the large aggregates measuring up to a few hundreds of nanometers
active BA sites (i.e., the MFI zeolite framework) in such a way and are located on the external surfaces (Fig. 1a). As anticipated,
that intermediates are exposed to the appropriate active sites, as only the microporous structure (0.55 nm) was conrmed as
required at every step of cascade reactions. In order to provide shown in Fig. 1e.
LA-rich sites on the zeolites, 5 wt% zirconium oxide (ZrO2) was
Table 1 shows the total pore volume, micropore volume and
deposited onto [Al]MFI-NS samples with different ratios of Si/Al, the BET surface area and external surface area of the synthe-
denoted as ‘ZrO2-[Al]MFI-NS n’. For comparison, bulk-MFI 30 sized zeolites. The result indicated that mesoporous MFI-NS
was supported with 5 wt% in the same manner. This is desig- showed a higher BET and external surface area compared to
nated as ‘ZrO2-bulk MFI 30’.
the bulk-MFI 30 due to the mesoporous structure. Aer the
Fig. 1 shows a comparison of the physicochemical properties deposition of ZrO2, the pore volumes and surfaces areas of both
of both prepared zeolites. The highly mesoporous ZrO2[Al]MFI- zeolites were slightly decreased, indicating the ZrO2 nano-
NS 30 exhibited mesoporous channels which are bound by thin particles could be adhered into the porous structure.
crystalline [Al]MFI walls and which allow the homogenous
The deposition of ZrO2 nanoparticles (LA-rich sites) within
dispersion of ZrO2 clusters (bright spots, Fig. 1b). The compa- the porous channels (BA-rich sites) on the zeolite provides
rable diffraction intensities are exhibited by XRD, ruling out the spatial heterogeneity of the acid sites, as required by the
collapse of the zeolite framework up on modication with ZrO2- reacting molecules when diffusing along the porous channel.
metal and no diffraction peaks corresponding to a bulk ZrO2 However, only a handful of metals, e.g., Pt with sufficiently high
phase or other crystalline impurities could be detected due to atomic binding energy can exist as clusters small enough to be
the small size of the ZrO2 (dTEM ¼ 2.0 nm, Fig. S5†) (Fig. 1c), conned within the narrow width of the 10 MR microporous
32
˚
˚
consistent with the STEM observation. The type IV isotherm channels (5.1 ꢁ 5.5 A, 5.3 ꢁ 5.5 A) of the MFI topology; thus,
Fig. 1 STEM images of (a) ZrO2-bulk-MFI 30, (b) ZrO2-[Al]MFI-NS 30, and the corresponding (c) XRD patterns, (d) Ar adsorption–desorption
isotherms, and (e) pore size distributions.
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Table 1 Physicochemical properties of the various zeolite catalysts
used in the current study
the highly porous nature of a mesoporous material with a high
external surface (339 m2 gꢂ1, Table 1), which assisted with the
dispersion of the ZrO2 into tiny separated nanoparticles due to
a connement effect between the two adjacent mesopore walls,
preventing the further agglomeration of ZrO2 into large
particles.22,24
Pore volume (cm3 gꢂ1
)
Surface area (m2 gꢂ1
)
a
a
b
c
Catalyst
Vtot
Vmicro
SBET
Sext
[Si]MFI-NS
0.67
0.69
0.64
0.69
0.19
0.56
0.59
0.56
0.09
0.09
0.1
582
577
589
590
379
518
510
516
516
339
375
382
365
371
81
346
339
326
310
72
[Al]MFI-NS 30
[Al]MFI-NS 60
[Al]MFI-NS 100
Bulk-MFI 30
ZrO2-[Si]MFI-NS
ZrO2-[Al]MFI-NS 30
ZrO2-[Al]MFI-NS 60
3.2 Acidic properties of the prepared catalysts
0.1
0.13
0.058
0.076
0.086
0.09
0.12
The aforementioned investigations have shown the dispersion
of the ZrO2 nanoparticles on the [Al]MFI-NS and bulk-MFI 30
samples, inuencing their BA- and LA-properties. Thus, the
distributions of BA and LA sites were investigated by FTIR
spectroscopy of the adsorbed pyridine.33 The characteristic
vibration of coordinately bonded pyridine at LA (1450–
1455 cmꢂ1) was observed. The band at 1555–1560 cmꢂ1 is
assigned to pyridine adsorbed at BA, while that at 1490 cmꢂ1
results from for pyridine adsorbed at either BA and LA sites.
As shown in Fig. 2b and c, the deposition of ZrO2 nano-
particles onto ZrO2-[Al]MFI-NS 30 and 100 samples signicantly
increased the LA content (from 0.095 to 0.222 mmol gꢂ1 and
0.021 to 0.102 mmol gꢂ1, respectively), as reected by the
increased pyridine-FTIR absorption band centered at
1453 cmꢂ1. Although a signicant amount of LA pre-existed as it
originated from the octahedral-coordinated framework Al
species, as conrmed by 27Al MAS NMR (Fig. S6†), the amount
of LA was increased by more than a factor of 3 upon the
ZrO2-[Al]MFI-NS 100 0.64
ZrO2 bulk-MFI 30
0.14
a
Vtot and Vmicro are the total pore volume obtained at P/P0 ¼ 0.95. b SBET
is the BET surface area obtained from Ar desorption in relative pressure
range (P/P0) of 0.05–0.20. Sext is the external surface area evaluated
c
from the t-plot method.
the ZrO2 nanoparticles on the zeolite tend to migrate to the
external surface area, as mentioned in the Introduction.
Because the deposition of ZrO2 nanoparticles onto the bulk-
MFI-30 sample was limited to their exterior surface (72 m2
gꢂ1, Table 1), these nanoparticles formed large aggregates of
ZrO2 nanoparticles. In contrast, the ZrO2-[Al]MFI-NS exhibited
Fig. 2 Pyridine-FTIR spectra of (a) ZrO2-bulk-MFI 30, (b) ZrO2-bulk-MFI-NS 30, and (c) ZrO2-bulk-MFI-NS 100 before and after deposition with
ZrO2, as well as (d) the physical mixture of ZrO2-[Si]MFI-NS and [Al]MFI-NS 30 and its individual constituents. The quantified density levels of (a)
BA and (b) LA sites for the different catalysts before (open symbols) and after ZrO2 deposition (solid symbols) are also shown. The symbols are as
follows: MFI-NS (square), bulk MFI (diamond), and the physical mixture (triangle). The linear lines connecting the data from the MFI-NS samples
serve as a guide for the eyes.
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deposition of ZrO2 nanoparticles. This is accompanied by increased number of LA sites despite the similar Al/Zr atom
a slight increase in the BA content (1543 cmꢂ1) (from 0.078 to ratios (Table 2). While the resultant surface-segregated ZrO2-
0.102 mmol gꢂ1, 0.052 to 0.06 mmol gꢂ1, respectively) (Fig. 2b bulk MFI 30 catalyst contains both BA and LA sites, it is perhaps
and c and Table 2). This effect can be traced to the partial more suited to short (e.g., two or three steps) cascade reactions,
hydrolysis of the Zr–O–Si bridge to form Zr(OSi)3OH, which has where the reactants would react with the LA prior to entering
both LA and weak BA properties, as well as that of the surface and/or upon exiting the BA-rich zeolite micropores. Moreover,
Zr–OH.34 By tuning the Si/Al ratio of the [Al]MFI-NS, we further as will be shown the later, the overall reaction kinetics can be
demonstrate the effect on the distribution of the acid sites. As limited by the sluggish diffusion through the bulk MFI crystals.
shown in Fig. 2e and f, the amounts and surface density levels of
We carried out additional acidity investigations using phys-
BA and LA can be systematically reduced by up to half when ically mixed samples containing ZrO2-[Si]MFI-NS (as meso-
increasing the Si/Al ratio from 30 to 60 and to 100. The porous LA-rich zeolite) and [Al]MFI-NS 30 (as mesoporous BA-
description of the BA distribution in ZrO2 MFI-NS zeolites is rich zeolite) (Fig. 2e). While both acidity levels of the catalysts
completed by considering the external surface acidity (meso- compensated for the lack of acidity of these samples, they
porous MFI wall) with BA density levels of 0.067, 0.057, and showed lower acidity levels in both cases compared to the
0.036 mmol gꢂ1, as well as the corresponding levels of 0.035, counterpart catalyst (ZrO2-MFI-NS 30).
0.034, and 0.024 mmol gꢂ1of the internal surface area (micro-
porous MFI wall) according to the Si/Al ratio (Fig. S7† and Table
2). The nearly doubled acid content on the mesoporous surface
compared to the microporous wall is commensurate with the
corresponding measured surface areas. This provides impor-
tant insight in that the mesoporous channels not only enhance
the diffusion of the molecules but the immediate mesoporous
surface actually exposes more acid sites than the microporous
channels.
3.3 Catalytic properties
3.3.1 Preliminary investigation of the catalytic perfor-
mance. To showcase the efficacy of the hierarchical BA–LA
zeolite catalysts, they were assessed for the one-pot cascade
conversion of furfural to GVL in an isopropanol solvent. Iso-
propanol, besides serving as the reaction medium, also func-
tions as a sacricial hydrogen donor for the catalytic transfer
hydrogenation via the Meerwein–Ponndorf–Verley (MPV)
reduction.35,36 We conrm that with limited LA sites, i.e., [Al]
In the absence of mesoporosity, as in the ZrO2-bulk-MFI 30
case here, the resultant aggregated ZrO2 nanoparticles demon-
strated a lower increase in the LA content (0.119 mmol gꢂ1
)
MFI-NS without ZrO2 deposition, furfural conversion was
limited to less than 50%, with a GVL yield of less than 4%, most
likely attributed to the inherent LA sites from the octahedral-
coordinated Al according to 27Al MAS NMR (Table 3). Like-
wise, for the [Si]MFI-NS, which lacks both BA and LA, furfural
conversion was limited to 2% with a zero yield of GVL (Table 3).
While the abundant LA sites of ZrO2[Si]MFI-NS showed limited
overall cascade furfural conversion to GVL, conrming the
greater accumulation of FA (65.7%) with a zero GVL yield, the
bifunctional ZrO2[Al]MFI-NS zeolites regained efficient
production of GVL (82.8, 81.6, and 78.3%), according to the Si/Al
ratio (Table 3), via the cascade conversion of furfural to GVL as
will be shown in the next Section. We conrmed that scaling-up
(Fig. 2b and f) in spite of the similar Al/Zr atom ratio compared
to the ZrO2-[Al]MFI-NS (Table 2). Nevertheless, the rich BA sites
within the framework of the bulk MFI were unperturbed,
explaining the highest BA amount among all samples assessed
(0.132 mmol gꢂ1) (Fig. 2a and Table 2). The level of nearly two
orders of magnitude of LA sites on the ZrO2-MFI-NS zeolites
compared to the ZrO2 bulk MFI 30 is attributed to the small
ZrO2 particle size and the greater dispersion on the meso-
porous. This indicates that the mesoporous structure provides
strong stability of ZrO2 nanoparticles of a very small size and
prevents the surface segregation of ZrO2, giving rise to an
Table 2 Compositions (in Si/Al and Si/Zr atomic ratios), and density levels of LA and BA sites as well as the ratio of BA-to-LA as determined by
pyridine-FTIR
Si/Ala
Si/Zra
LAtot (mmol gꢂ1
)
BAtot (mmol gꢂ1
)
BAext (mmol gꢂ1
)
BAintd (mmol gꢂ1
)
b
b
c
Zeolite
ZrO2-[Si]MFI-NS
ZrO2-[Al]MFI-NS 30
ZrO2-[Al]MFI-NS 60
ZrO2-[Al]MFI-NS 100
ZrO2-[Si]MFI-NS + [Al]MFI-NS 30
ZrO2ebulkeMFI 30
[Si]MFI-NS
n.d.d
31
58.6
101
36
47.6
n.d.d
32
16.8
15.8
16.2
15.1
16.8
16.6
n.d.e
n.d.e
n.d.e
n.d.e
n.d.e
0.037
0.222
0.15
0.006
0.102
0.091
0.06
n.d.d
0.067
0.057
0.036
n.d.e
n.d.d
0.035
0.034
0.024
n.d.e
0.102
0.163
0.03
0.09
0.132
n.d.e
0.095
0.082
0.052
0.126
0.028
0.104
n.d.e
0.078
0.031
0.021
0.086
n.d.e
n.d.e
[Al]eFI-Ne 30
0.063
0.051
0.033
0.027
0.032
0.031
0.019
0.099
[Al]MFI-NS 60
[Al]MFI-NS 100
Bulk MFI
61
110
32
a
Si/metal molar ratio determined by ICP-AES analysis. b The number of LAS and BA sites were determined by FT-IR with pyridine adsorption. c The
number of external BA sites were determined by FT-IR with DTBP. d The number of internal BA sites were calculated by BAtot ꢂ BAext
.
Not detected.
e
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Table 3 Furfural conversion and product yields of the different catalystsa
Product yieldb (mol%)
Catalyst
Furfural conversion (%)
GVL
FA
AL
IPL
0
17.2
18.4
21.7
10.7
0
FDA
7.2
0
0
ZrO2-[Si]MFI-NS
100
100
100
100
68
48
2
41
49
0
65.7
0
0
0
0
0.5
1.5
0.1
0.2
0.2
27.2
0
0
ZrO2-[Al]MFI-NS 30
ZrO2-[Al]MFI-NS 60
ZrO2-[Al]MFI-NS 100
ZrO2-[Si]MFI-NS + [Al]MFI-NS 30
ZrO2-bulk-MFI 30
[Si]MFI-NS
[Al]MFI-NS 30
[Al]MFI-NS 60
[Al]MFI-NS 100
82.8
81.6
78.3
45.1
0
0
0
6.8
26.4
0
13.7
20.7
15.2
5.7
21.5
0.5
14.7
14.9
10.6
0
0
3.4
3.2
1.2
9.2
9.6
11.6
39
a
Reaction condition: 170 ꢀC, 36 h; 0.04 M furfural in a 1 : 50 molar ratio of furfural to 2-propanol solution (2-propanol as solvent and hydrogen
b
donor), the mass ratio of the catalyst to furfural was 1 : 2.5. g-Valerolactone (GVL); furfural alcohol (FA); b-angelica lactone (AL); 2-propyl
levulinate (IPL); furfural diisopropyl acetal (FDA).
of the reaction by a factor of 20 (100 ml reaction, 1 g catalyst) constants (ki) (right) based on the cascade mechanism
yield similar conversion and selectivity as that carried out at the described in Scheme 1. As shown in Fig. 3d–f, all of the meso-
current out at the current small scale (Table S1†).
porous ZrO2-[Al]MFI-NSs recorded complete conversions within
3.3.2 Cascade conversion of furfural to GVL and kinetic 1 h, attributed to the simultaneous production of FA and FDA.
modelling. On the basis of our own experimental observations Although both intermediates were the primary reaction prod-
and previous studies,36 the suggested catalytic cycles for each ucts, the former was instantly converted to a subsequent
step for the one-pot conversion of furfural to GVL is described in intermediate, i.e., AL or IPL, while the latter reached
Scheme 1 where furfural diisopropyl acetal (FDA) was detected a maximum concentration before the concentration gradually
here for the rst time, existing in equilibrium with furfural (K ¼ decreased with time upon the reversible reaction to furfural.
k1/k2). FDA was synthesized from furfural via acetalization via With the abundance of LA sites in the mesoporous ZrO2-MFI-NS
two 2-propanol molecules. As reported previously,37–40 the ace- zeolites, FDA showed a more rapid decrease compared to the
talization is normally carried out with strong Brønsted mineral other zeolites, attributed to the rapid furfural conversion to FA
acids (e.g. HCl or H2SO4) or solid Brønsted acids (e.g., Amberlyst via the rst MPV reduction. This is accordance with the
70 or Naon-H). We veried the observation using Naon-H, constant rates of FDA to Furfural (k2) in ZrO2-MFI-NS zeolites (k2
which is a phase-pure BA, showing the exclusive production of ¼ 0.03, 0.06 and 0.1 hꢂ1 for ZrO2-[Al]MFI-NS at 30, 60 and 100,
FDA (Table S2†).The conversion of furfural to furfural alcohol respectively) compared to ZrO2-bulk-MFI (k2 ¼ 1.2 hꢂ1). More-
(FA) (k3) by MPV reduction involves a six-membered transition over, the rate constants of the furfural conversion to FA were
state on the catalyst surface, where the 2-propanol and furfural increased by two orders of magnitude (k3 ¼ 2.9, 2.0 and 1.8 hꢂ1
are coordinated to the same LA-catalyzed Zr center.18 The FA is for ZrO2-[Al]MFI-NS 30, 60 and 100, respectively) compared to
subsequently etheried to furfural alcohol ether (FE). From the limited LA-sites in the micropore ZrO2-bulk-MFI (k3 ¼ 0.02
here, FE can undergo two pathways: (1) it can be converted to hꢂ1). This nding arose due to the limited number LA-rich sites
angelica lactone (AL) (k5) by the addition of water and followed as a result of the poorly dispersed ZrO2 nanoparticles in the
by ring-opening transesterication with 2-propanol to produce ZrO2-Bulk-MFI 30 sample. The produced FA can be converted to
the isopropyl levulinate (IPL) (k6), or (2) it can be directly formed IPL via two different pathways. FA can be converted to AL by the
to the IPL (k4) via the hydrolytic cleavage of the furan ring elimination of 2-propanol followed by an addtion of water to the
opening of FE.41 As a nal step in the cascade reactions, the IPL IPL, or via the direct transformation of the IPL via the hydrolytic
is reduced to GVL (k7) via the LA-catalyzed MPV reaction.35,41–43 cleavage of the furan ring opening in FA.41 Interestingly, the
Strictly speaking, the latter step involves the hydrogenation of constant rate of FA to AL (k4) appears to be the overwhelming
IPL to isopropyl 4-hydroxypentanoate (4-HP), which rapidly pathway compared to that of FA to IPL (k5) over all samples,
undergoes lactonization to form GVL. However, due to the rapid which was previously found to be correlated with the hydro-
lactonization step, 4-HP was rarely detected41,42,44 and for philicity and high BA density of the zeolite.41,45 While reported
simplicity, the conversion of IPL to GVL is represented here as bifunctional zeolites have showed that FA conversion to IPL was
a single step. As can be seen above, the cascade mechanism the a rate-limiting step for the overall cascade due to the lack of
requires different proles of acid sites at every step, rendering it BA sites,35,42,45 the abundant BA sites as active sites in ZrO2-[Al]
an ideal test reaction for the engineering of BA–LA zeolite MFI-NS zeolites are unassociated with this bottleneck here. As
catalysts.
the BA content in the ZrO2-[Al]MFI-NS increases (by decreasing
Fig. 3 reveals the kinetics of the furfural conversion and the Si/Al ratio from 100 to 30), the overall kinetics is improved,
product formation (le) and the pseudo-rst-order rate especially during the acid-catalyzed slow step of AL to IPL (k6 ¼
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that during the conversion of AL to IPL is observed for the nal
and second MPV reduction of IPL to GVL (k7 ¼ 0.12, 0.11 and
0.08 hꢂ1 for ZrO2-[Al]MFI-NS 30, 60 and 100, respectively). While
Step 6 is the slow step among the MFI-catalyzed cascades (BA
sites), Step 7 is the slow-step on the LA-rich ZrO2 surface. In
cascade reactions where there could be multiple active sites
catalyzing each of the sequential reaction steps, here we iden-
tied two slow steps that require the optimization of the acid
distributions on both the MFI framework and the ZrO2 to obtain
a high GVL production rate.
As shown Fig. 3a, despite recording a furfural conversion rate
of less than 50%, the microporous ZrO2-bulk-MFI produces
predominantly of FDA and AL with a trace amount of FA. Again,
the slow furfural conversion is related to the limited number of
LA-rich sites as a result of the poorly dispersed ZrO2 nano-
particles. While the produced FA is quickly converted to AL, the
slow diffusion within the micropores of the bulk MFI crystal for
the subsequent conversion to IPL also contributes to the overall
sluggish reaction. Possibly in an inter-related fashion, the
catalyst only managed a single-pass reaction through the
surface-segregated bifunctional catalyst, i.e., Step 3 on the LA
sites on the catalyst surface, followed by Step 4 on the BA sites
through the bulk zeolite. To exemplify the superiority of BA–LA
within a single hierarchical zeolite particle, we carried out
another reaction using a physically mixed sample containing
ZrO2-[Si]MFI-NS (as a mesoporous LA-rich zeolite) and [Al]MFI-
NS 30 (as a mesoporous BA-rich zeolite). As shown in Fig. 3b,
a sluggish reaction was found resulting in a furfural conversion
of 68%. The low furfural conversion rate is related to the slug-
gish interparticle diffusion of FDA from the BA-rich zeolite to
the LA-rich zeolite. The lower k3 by more than ve-fold compare
to that of ZrO2-[Al]MFI-NS 30 resulted in low furfural conver-
sion, an outcome related to the sluggish interparticle diffusion
of FDA. Up to this juncture, it is sufficient to say that the careful
tuning of the BA–LA sites enhanced the cascade reactions
within a single catalyst particle, while the high dispersion of
ZrO2 nanoparticles in the mesoporous channels mitigates the
overall slow molecular diffusion through the zeolites as well.
On the other hand, bifunctional Zr-Al-beta is known in the
literature as a catalyst showing among the best performance
capabilities for the conversion of furfural to GVL when Zr-
species are introduced aer
a
partial dealumination
process.36,45–47 This sample is denoted as ‘ZrO2-Al-beta’
(Fig. S4†). We investigated its catalytic performance for the
conversion of furfural to GVL as well. A GVL yield of 74.8% was
recorded when using the ZrO2-Al-beta catalyst (Fig. S8† (le)).
With both the microporous (primary) and macroporous
(secondary) dimensions of the ZrO2-Al-beta catalyst (Fig. S4d
and e†) larger than those of the ZrO2-[Al]MFI-NS catalysts,
diffusion limitation is not expected to be an issue. Rather, the
lower GVL yield in the former case is commensurate with the
sluggish rate-limiting MPV reduction of IPL to GVL (k7 ¼ 0.06
hꢂ1) (Fig. S8† (right)). This is true despite the relatively efficient
conversion of AL to IPL (k6 ¼ 0.3 hꢂ1) likely due to the faster
diffusion of molecules through the inltrating porous channels.
Importantly, the ZrO2-Al-beta result consolidates the discussion
Scheme 1 Proposed catalytic cycles of the cascade reactions for the
conversion of furfural to GVL, as adapted from earlier work.36 (a)
Initiation reaction in which furfural was converted into furfural diiso-
propyl acetal (FDA) (k1, k2), (b) furfural to furfural alcohol (FA) (k3) via
MPV reduction, (c) furfural alcohol ether (FE) to the isopropyl levulinate
(IPL) (k4) by direct ring-opening reaction (d) furfural alcohol ether (FE)
to angelica lactone (AL) (k5) by the addition of water, (e) AL to IPL (k6) by
ring-opening transesterification, (f) IPL to GVL (k7) via the LA-catalyzed
MPV reaction (BA: Brønsted acid, LA: Lewis acid).
0.10 to 0.16 hꢂ1, respectively, k6). The relatively slow hydration
kinetics in Step 6 is most likely limited by the non-aqueous
solvent used in the current work. A slow step comparable to
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Fig. 3 Kinetic plots of furfural, product GVL and other intermediates over zeolites (left) and the estimated pseudo-first order rate constants based
on the mechanism in Scheme 1 (right) for (a) ZrO2-bulk MFI 30, (b) the physical mixture of ZrO2-[Si]MFI NS + [Al]MFI-NS 30, and ZrO2-MFI-NS
with Si/Al at ratios of (c) 0, (d) 30, (e) 60, and (f) 100. Reaction condition: 170 ꢀC, 36 h; 0.04 M furfural in a 1 : 50 molar ratio of furfural to 2-
propanol solution (2-propanol as the solvent and hydrogen donor), The mass ratio of the catalyst to furfural was 1 : 2.5 under magnetic stirring.
furfural and still higher GVL yields from 76 to 82% during the
fourth recycling. The high recyclability of ZrO2[Al]MFI-NS 30 is
a result of impressive chemical and thermal stability of the
catalyst, as reected by the well-maintained MFI crystallinity,
micro/mesoporous structure, and ZrO2 dispersion, as well as
the absence of leaching, even aer 4 consecutive reaction runs
(Fig. S9†).
4. Conclusion
The work demonstrates the design of mesoporous MFI-NS
zeolite catalysts that can be utilized in cascade reactions, as
exemplied in the current study in a demonstration of the one-
pot conversion of furfural to GVL. The advantage of the highly
inltrated mesoporous channels enabled the deposition of
Fig. 4 Recycling experiments of ZrO2[Al]MFI-NS 30 for the one-pot
ultrane ZrO2 nanoparticles (>2 nm) as LA-rich sites throughout
the MFI crystal, which otherwise contains predominantly BA
and some lesser number of LA sites. These intrinsic MFI acid
sites are accessible on the internal surface of the mesoporous
channels, as well as the microporous channels contained within
the few-unit-cell-thin MFI wall. Besides enhancing the molec-
ular diffusion, the mesoporous channels are an important
source of acid sites given that they contain twice as many acid
sites as the short microporous channels within the thin MFI.
The heterogeneous distribution of LA-rich sites (from ZrO2) and
that of mixed BA–LA sites (from the MFI framework) inuence
the occurrence of the different acid sites experienced by the
furfural conversion to GVL. Reaction condition: 170 ꢀC, 36 h; 0.04 M
furfural at a 1 : 50 molar ratio of furfural to a 2-propanol solution (2-
propanol as a solvent and hydrogen donor), The mass ratio of the
catalyst to furfural was 1 : 2.5 under magnetic stirring.
above regarding the optimization of both k6 and k7 to achieve
the highest GVL yield.
3.3.3 Reusability. A reusability test of ZrO2[Al]MFI-NS 30,
which is the best performing catalyst for GVL production, was
carried out for four times under identical conditions. As shown
in Fig. 4, ZrO2 [Al]MFI-NS 30 maintained 100% conversion of
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molecules as the cascade reactions proceed, hence affecting 10 M. Renz, et al., Selective and shape-selective Baeyer-Villiger
their kinetics and selectivity. The two slow steps are the
conversion of AL-to-IPL and that of IPL-to-GVL, requiring the
simultaneous optimization of both MFI and ZrO2, respectively.
oxidations of aromatic aldehydes and cyclic ketones with
Sn-Beta zeolites and H2O2, Chem.–Eur. J., 2002, 8(20),
4708–4717.
A high yield of 82.8% GVL was achieved using the optimum 11 B. Tang, et al., Improved Postsynthesis Strategy to Sn-Beta
ZrO2-[Al]MFI-NS 30, an outcome higher than that with the
benchmark ZrO2-Al-beta catalyst (74.8%). We envision that the
Zeolites as Lewis Acid Catalysts for the Ring-Opening
Hydration of Epoxides, ACS Catal., 2014, 4(8), 2801–2810.
design concept of the hierarchical BA–LA zeolite shown here 12 P. Li, et al., Postsynthesis and Selective Oxidation Properties
can be extended to a range of other cascade acid-base types of
reactions. In the same manner, innovative design of other well-
of Nanosized Sn-Beta Zeolite, J. Phys. Chem. C, 2011, 115(9),
3663–3670.
ordered, hierarchical BA–LA zeolites especially using econom- 13 Z. H. Kang, H. O. Liu and X. F. Zhang, Preparation and
ical SDA would pave the way for their wider industrial
implementations.
Characterization of Sn-beta Zeolites by a Two-Step
Postsynthesis Method and Their Catalytic Performance for
Baeyer-Villiger Oxidation of Cyclohexanone, Chin. J. Catal.,
2012, 33(5), 898–904.
Conflicts of interest
14 J. Wang, et al., Post-synthesized zirconium-containing Beta
zeolite in Meerwein-Ponndorf-Verley reduction: Pros and
cons, Appl. Catal., A, 2015, 493, 112–120.
There are no conicts to declare.
15 J. Dijkmans, et al., Cooperative Catalysis for Multistep
Biomass Conversion with Sn/Al Beta Zeolite, ACS Catal.,
2015, 5(2), 928–940.
16 B. Tang, et al., Mesoporous Zr-Beta zeolites prepared by
a post-synthetic strategy as a robust Lewis acid catalyst for
the ring-opening aminolysis of epoxides, Green Chem.,
2015, 17(3), 1744–1755.
Acknowledgements
This work was supported by IBS-R004-D1.
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