Zeolite-Tailored Active Site Proximity for the Efficient Production of Pentanoic Biofuels

Article abstract of DOI:10.1002/anie.202108170

Biofuel production can alleviate reliance on fossil resources and thus carbon dioxide emission. Hydrodeoxygenation (HDO) refers collectively to a series of important biorefinery processes to produce biofuels. Here, well-dispersed and ultra-small Ru metal nanoclusters (ca. 1 nm), confined within the micropores of zeolite Y, provide the required active site intimacy, which significantly boosts the chemoselectivity towards the production of pentanoic biofuels in the direct, one-pot HDO of neat ethyl levulinate. Crucial for improving catalyst stability is the addition of La, which upholds the confined proximity by preventing zeolite lattice deconstruction during catalysis. We have established and extended an understanding of the “intimacy criterion” in catalytic biomass valorization. These findings bring new understanding of HDO reactions over confined proximity sites, leading to potential application for pentanoic biofuels in biomass conversion.

Full text of DOI:10.1002/anie.202108170

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Zeolite-Tailored Active Site Proximity for the Efficient Production
of Pentanoic Biofuels
Authors: Jiang He, Zhijie Wu, Qingqing Gu, Yuanshuai Liu, Shenqi
Chu, Shaohua Chen, Yafeng Zhang, Bing Yang, Tiehong
Chen, Aiqin Wang, Bert M. Weckhuysen, Tao Zhang, and
Wenhao Luo
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202108170
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10.1002/anie.202108170
Angewandte Chemie International Edition
RESEARCH ARTICLE
Zeolite-Tailored Active Site Proximity for the Efficient Production
of Pentanoic Biofuels
＃
＃
＃
Jiang He[a], [b], , Zhijie Wu[c], , Qingqing Gu[d], , Yuanshuai Liu[e], Shengqi Chu[f], Shaohua Chen[g],
Yafeng Zhang[d], Bing Yang[a],[d], Tiehong Chen[g], Aiqin Wang[a], Bert M. Weckhuysen*[e], Tao Zhang*[a]
and Wenhao Luo*[a]
[a]
J. He, Dr. B. Yang, Prof. A. Wang, Prof. T. Zhang, Dr. W. Luo
CAS Key Laboratory of Science and Technology on Applied Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian, 116023, P. R. China
*E-mail: taozhang@dicp.ac.cn; w.luo@dicp.ac.cn
J. He
University of Chinese Academy of Science
19A Yuquan Road, Shijingshan District, Beijing, 100049, P. R. China
Prof. Z. Wu.
[b]
[c]
State Key Laboratory of Heavy Oil Processing and Key Laboratory of Catalysis of CNPC
China University of Petroleum
18 Fuxue Road, ChangPing, Beijing, 102249, P. R. China
Dr. Q. Gu, Dr. Y. Zhang, Dr. B. Yang
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian, 116023, P. R. China
Dr. Y. Liu, Prof. B. M. Weckhuysen
[d]
[e]
Inorganic Chemistry and Catalysis group
Debye Institute for Nanomaterials Science, Utrecht University
Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
*E-mail: B.M.Weckhuysen@uu.nl
[f]
[g]
#
Dr. S. Chu
Beijing Synchrotron Radiation Facility
Institute of High Energy Physics, Chinese Academy of Sciences
19B Yuquan Road, Shijingshan District, Beijing, 100049, P. R. China
Dr. S. Chen, Prof. T. Chen
Key Laboratory of Advanced Energy Materials Chemistry
Institute of New Catalytic Materials Science, Nankai University
38 Tongyang Road, Tianjin, 300350, P. R. China
These authors contributed equally to this work
Abstract: Biofuel production can be a viable option for alleviating
current heavy reliance on fossil resources and the related carbon
dioxide emission issue. Hydrodeoxygenation refers collectively to a
series of important biorefinery processes to produce biofuels. Here,
we show that well-dispersed and ultra-small Ru metal nanoclusters (~
1 nm), confined within the micropores of zeolite Y, provide the
required active site intimacy, which significantly boosts the
chemoselectivity towards the production of pentanoic biofuels in the
direct, one-pot hydrodeoxygenation (HDO) of neat ethyl levulinate.
Crucial for improving catalyst stability is the addition of La, which
upholds the confined proximity by preventing zeolite lattice
deconstruction during catalysis. Furthermore, we have established
and extended an understanding of the ‘intimacy criterion’ in catalytic
biomass valorization. These findings bring fundamental new
understanding of HDO reactions over confined proximity sites, and
may facilitate a new application potential for the practical production
of pentanoic biofuels in biomass conversion.
alternative feedstocks and necessity of driving the essential
transition to a more renewables-based, sustainable society[1].
Non-edible biomass is one prime, renewable and abundant
carbon-based alternative, which can serve as a viable substitute
for the sustainable production of fuels, chemicals and materials.
The exploration and development of biomass for sustainable
energy production, including the manufacturing of biofuels and
bio-based additives, such as lubricants, can be a promising option
for the energy sector[2]. Additionally, there is a clear tendency
towards increasing utilization of renewables for the transportation
sector for alleviating our reliance on fossil resources, further
spurred by the rising concerns about climate change[3].
The current implemented option is using bio-ethanol as a
blend with gasoline, which is widely applied in China and several
European countries (i.e., E10, referring to a 10% blend of
ethanol)[4]. However, limitations referring to a lower energy
density and inferior compatibility, compared to that of traditional
gasoline, lead to an ongoing debate about developing alternatives
for bio-ethanol. To this end, alternative molecules, such as
pentanoic esters, produced from lignocellulose-derived levulinic
acid (LA) have been identified as a promising class of biofuels
(additives) by Shell laboratories[5]. An example is ethyl pentanoate
(EP) at a 15 vol% blend with fossil-derived gasoline, representing
superior fuel properties and excellent compatibility with existing
combustion engines, as evidenced by a successful road trial of
250 000 km. The reported production of EP from
Introduction
The finite reserve of non-renewable fossil resources, such as coal,
natural gas and crude oil, and the growing global energy demand,
combined with increasingly stringent legislation and mandates on
CO2 emissions, have prompted the wide-ranging search for
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Scheme 1. Reaction scheme for the production of pentanoic biofuels starting from ethyl levulinate (route 1: the proposed synthesis route described in this work)
and levulinic acid (route 2: the previously reported synthesis process).
lignocellulose-derived downstream platform molecule of LA,
consists of the hydrogenation of LA to γ-valerolactone (GVL) and
subsequently into pentanoic acid (PA), and finally esterification to
EP (Scheme 1). Metal/zeolite bifunctional catalysts with
considerable activity and selectivity for LA hydrodeoxygenation
(HDO) has been developed for the efficient production of
pentanoic biofuels[6]. However, the developed catalysts still
encompass several challenges, with respect to 1) the lack of
precision synthesis methods enabling the active site control at the
nanometric level; 2) unpractical dilute substrate solutions; and 3)
underestimated aspects on catalyst stability as the highly oxygen-
functionalized, polar and even corrosive reaction medium under
applied elevated temperatures often results in a poor durability of
the solid catalysts studied. Therefore, there is clear room in
developing more stable and well-defined bifunctional catalysts via
rational design approaches, operating under more demanding
experimental conditions.
In this work, we propose a novel strategy for the direct
production of pentanoic biofuels starting from neat ethyl levulinate
EL), instead of the usually starting molecule LA, using zeolite-
supported bifunctional catalysts with confined active site proximity.
Compared to the previously reported route based on LA (Route 2
in Scheme 1), the proposed route based on EL (Route 1 in
Scheme 1) shows significant advantages: 1) EL can be efficiently
produced as the downstream platform molecule via the
alcoholysis of lignocellulose materials, wherein humin formation
via undesired condensation reactions can be more efficiently
suppressed compared to the route based on LA; and 2) alleviating
the severeness of the reaction medium for catalysts by using a
mild ester instead of protic acid as substrate, and thus more
feasible for applying higher substrate concentrations or even neat
substrate solutions. We present a simple and efficient synthesis
method to directly generate highly dispersed ultra-small Ru
particles confined in the micropores of zeolite Y. By a combination
of state-of-the-art electron microscopy-based techniques and
experimental approaches, we found that zeolite-tailored confined
proximity between metal and acid sites is pivotal for catalytic
performance in the direct HDO of EL into pentanoic biofuels,
enabling a remarkable increase in activity and selectivity to
pentanoic biofuels. Furthermore, we show that the presence of La
in zeolite Y can significantly improve catalyst stability and
suppresses zeolite dealumination and desilication during
catalysis. Besides, an enhanced understanding of the improved
HDO performance of intimacy functional sites behind this reaction,
provided by this study, will not only be a step forward in the
efficient production of pentanoic biofuels, but it will also aid in
understanding similar hydrogenation reactions in tandem
catalysis, which may bring a great and multidisciplinary impact in
fundamental aspects.
Results and Discussion
Catalytic performance and structural characterization
For the economical production of pentanoic biofuels (i.e., EP and
PA), it is desirable to study the hydrodeoxygenation (HDO) of EL
at high substrate concentrations. We have performed the HDO of
EL under batch conditions with neat EL as substrate at 220 °C
and 40 bar H2. Previous reports have revealed that Ru-based
catalyst materials appear as the catalyst of choice for the
hydrogenation of LA, owing to the excellent activity and
selectivity[7]. A series of state-of-the-art Ru-based catalysts,
prepared by a wet impregnation method (see Methods in
Supplementary Information and Fig. S1), have been rationally
screened (Fig. 1a). The catalytic screening results demonstrate
that controllably depositing Ru nanoclusters inside the 12 MR
channels (FAU and BEA) is beneficial for both activity and
selectivity. Especially, the level of activity afforded by the Ru/H-Y
and Ru/La-Y catalysts has so far proven to be unique for the one-
pot conversion of EL into EP/PA, as a quantitative conversion of
EL and a combined EP/PA yield of ≥ 94% observed for both
catalysts after 10 h. The other Ru-based catalysts, such as Ru
nanoclusters inside the 10 MR channels (MFI), Ru nanoclusters
with less acid sites (Ru/Na-Y and Ru/C), the intimate mixture of
Ru nanoclusters and zeolites (Ru/C combined with different
zeolites) or other Ru nanoparticles with inferior activity or
negligible selectivity towards EP/PA for the HDO of EL, of the
confinement of Ru nanoparticles inside zeolite Y for the promotion
of catalysis. Furthermore, EP/PA productivity, expressed as the
number of EP/PA molecules produced per gram of metal per time,
reflects the turnover rate of deep hydrogenation products. Based
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on the time profiles (Fig. 1b-c and Fig. S2), the EP/PA
productivities are determined to be 11.514 molgRu-1h-1 for the
Ru/H-Y and 5.965 molgRu-1h-1 for the Ru/La-Y respectively, which
are both much higher than the current highest values obtained for
the 1 wt% Rh/H-β and 1 wt% Ru/H-β (Table S1), reported by the
Shell laboratories for the one-pot HDO of EL to EP/PA at a higher
reaction temperature of 250 °C and reproduced in this work[8].
Encouraged by the promising screening results, the time-on-line
concentration profiles of substrate and products for the Ru/H-Y
and Ru/La-Y are further performed and shown in Fig. 1b-c. For
both catalysts, GVL, PA and EP are detected as the major
products, with no pentanediol and methyl-tetrahydrofuran
observed. Notably, both the Ru/H-Y and Ru/La-Y can afford a
combined EP/PA yield of 37% and 22% even at 5 min under
reaction conditions, which is significantly higher than those for
previous reported catalysts (EP/PA yield <1%)[5, 7c]. Trace amount
of 4-hydroxypentanoic acid ester (4-HPE) is also detected at low
EL conversion level by GC-MS (Fig. S3). The significant
enhancement in EP/PA yields at initial stage might indicate an
alternative reaction pathway to EP via an intermediate of 4-HPE.
At 150 min, both catalysts show a full conversion of EL, with EP
and PA as the major products. The Ru/H-Y shows a combined
EP/PA yield of 92% and minor amounts of GVL (3%), whereas
the Ru/La-Y affords a maximum EP/PA yield of 96% and 3% GVL.
After 150 min, a progressive increase in the selectivity towards
PA is observed for both catalysts over time, as a result of the
consecutive hydrolysis of EP to PA by acid catalysis. Notably, a
total EP/PA yield of 94% and 95% could be maintained for the
Ru/H-Y and for the Ru/La-Y respectively, even extended to a
reaction time of 10 h. Such excellent yields of EP/PA (≥94%) and
high EP productivities, to the best of our knowledge, are first
reported for the direct HDO of neat EL under relatively mild
conditions (220 °C and 40 bar H2, under which (Table S1) GVL
was normally observed as the dominant product for the
bifunctional catalysts in the HDO of EL).
The stability of both Ru/H-Y and Ru/La-Y is examined by
performing consecutive runs in the direct HDO of EL into EP/PA
(Fig. 1d and 1e). For the Ru/H-Y, the high EP/PA selectivity and
yield are only sustained upon three consecutive 10 h runs. A clear
deactivation was observed in the fourth run, with a decrease in
the EL conversion from an initial 100% to 77%, and a concomitant
drop in EP/PA yields from initial 91% to 9%. Even after a
regeneration step involving coke burning-off at 450 °C under a
dilute hydrogen flow[7c], the catalytic performance of Ru/H-Y could
only get partially recovered in the sixth run, with the EL conversion
of 92% and the EP/PA yield of only 50%, indicating irreversible
deactivation of the Ru/H-Y during reactions. Similar deactivation
of the 1wt% Ru/H-ZSM-5 during recycling of LA hydrogenation
was also reported, primarily caused by the dealumination of the
zeolite support[9]. For the Ru/La-Y, marginal drop in activity and
selectivity were observed even upon seven consecutive runs. A
full EL conversion and an EP/PA yield of > 90% could be still
sustained even in the seventh run, indicating an excellent stability
of the Ru/La-Y. The stability tests of both catalysts were also
performed at the sub-complete conversion levels (Fig. S4). Again,
the Ru/H-Y showed an obvious, continuous deactivation, while
the Ru/La-Y showed no apparent decrease in both EL conversion
and EP/PA yield during reusability tests. The results verify the
superior stability of the Ru/La-Y in the HDO of EL.
Figure 1. Catalytic performance of the different bifunctional catalysts under
study. (a) Catalytic hydrodeoxygenation of neat EL over different catalysts after
a reaction time of 10 h. (b, c) Time profiles of the catalytic hydrodeoxygenation
of neat EL over Ru/H-Y and Ru/La-Y, from which the EP/PA productivities of
both catalysts were determined at the ~100% EL conversion levels at the
reaction time of 30 min. (d, e) Reusability tests of Ru/H-Y and Ru/La-Y,
respectively, the column behind dash line was performed over the regenerated
Ru/H-Y catalyst. The sixth run in (d), noted as the column behind dash line was
performed over the regenerated Ru/H-Y catalyst. Reaction conditions: 220ꢀ°C,
40 bar H2, 1wt% Ru loading with different supports and a mechanical stirring
speed of 1000 rpm. EL: ethyl levulinate; LA: levulinic acid; GVL: γ-
valerolactone; EP: ethyl pentanoate; PA: pentanoic acid. The level of activity
exhibited by zeolite Y confined Ru nanoparticles has shown to be unique.
optical emission spectrometer (ICP-OES) and aberration-
corrected scanning transmission electron microscopy (AC-STEM).
The Ru content was determined to be 1 wt% for both catalysts
(Table S2). The high-angle annular dark-field (HAADF) images of
the Ru/H-Y and Ru/La-Y samples reveal a high dispersion of ultra-
small Ru nanoparticles (around 1.5 nm) located within the zeolite
crystals (Fig. 2a-d), although they slightly exceed the size of
micropores (roughly 1 nm), as reported previously[10]. H2
chemisorption results of both Ru/H-Y and Ru/La-Y further
corroborate the high dispersion of Ru particles (D ≥ 72%), with an
average Ru particle size of about 1.5 nm for both samples (Table
S3). Additionally, close inspection of X-ray diffraction (XRD)
patterns of both Ru-containing zeolites (Fig. S5) shows a shift of
peaks to a lower 2θ angle after the deposition of Ru, indicating a
slight lattice expansion by doping Ru particles inside the cavities
of the zeolite. This is also in good accordance with the observed
decrease in micropores volume of the zeolite supports after Ru
deposition (Table S2). To further discriminate the La and Ru
species in the HAADF images, Energy dispersive X-ray
spectroscopy (EDX) has been applied for the Ru/La-Y, which
confirms an atomic dispersion of La species and highly dispersed
Ru clusters in the zeolite Y (Fig. 2e-g and Fig. S6g-h).
The composition and nanostructure of both Ru/H-Y and
Ru/La-Y, were first characterized by inductively coupled plasma
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Figure 2. Structural characterization of the bifunctional catalyst materials under study. Representative aberration-corrected high-angle annular dark-field scanning
transmission electron microscopy (AC-HAADF-STEM) images of (a, b) the Ru/H-Y and (c, d) the Ru/La-Y showing the existence of Ru nanoparticles confined in
zeolite Y, corresponding particle size distribution derived from measurements of over 200 particles; (e, f) Atomic resolution of AC-HAADF-STEM images of the
Ru/La-Y and (g) EDX spectral imaging of the Ru/La-Y and corresponding elemental maps: Ru (pink), La (red), Si (green), Al (yellow), and O (blue), showing that
Ru and La species are indeed highly dispersed in zeolite Y.
Complementing the direct and local AC-HAADF-STEM
observation, the chemical states and local coordination
environment of the zeolite Y supported bifunctional catalysts were
characterized by X-ray absorption spectroscopy (XAS). XAS of
Ru/H-Y and Ru/La-Y were measured after in situ reduction under
H2. The X-ray absorption near-edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS) results are
shown in Fig. 3a-b, with the fitting results of EXAFS in Fig. S7 and
Table S4. Both Ru/H-Y and Ru/La-Y share a comparable
chemical valence and coordination of Ru. A slightly higher
valence state of Ru species is shown in the Ru/H-Y (Run+) than
eV[11], was shown for the Ru/H-Y, than that at 280.6 eV for the
Ru/La-Y. A larger number of Ru-O and a much lower Ru-Ru
coordination numbers (CN) with both Ru/H-Y (CNRu–O =3.7±0.4,
CNRu–Ru=6.0±0.6) and Ru/La-Y (CNRu–O =2.8±0.3, CNRu–Ru =7.3
±0.6) samples, as compared to the standard Ru metal foil (CNRu–
=0, CNRu–Ru = 12), reveal the formation of ultrasmall Ru
O
nanoparticles for both catalysts. Similar values of CNRu–O and
CNRu–Ru have been reported for the Ru/TiO2 and Ru-Pd/TiO2,
attributed to the formation of a significant fraction of Ru sub-
nanometric clusters[12]. Therefore, the XAS results confirm the
representative structure of a substantial fraction of Ru ultrasmall
that (Ruδ+) in the Ru/La-Y (n>δ≈0), between Ru0 and Ru4+ (Fig.3a). nanoparticles or even clusters in the zeolite Y for the entirety of
This is consistent with the X-ray photoelectron spectroscopy
results (Fig. S8) that a higher binding energy at 281.0 eV,
compared to the binding energy of metallic Ru0 at about 280.6
the both bifunctional catalysts.
The Fourier transform-infrared (FT-IR) spectra after CO
adsorption for Ru/H-Y and Ru/La-Y catalysts were conducted at
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embedded catalysts with an ion beam produces thin slices
exposing the inner parts of catalyst granules, and thus provides
information of spatial arrangements of metal particles inside the
shaped catalyst bodies (Fig. 4a). Fig. 4b-c show the inside section
of Ru/H-Y and Ru/La-Y. Those sections of both samples clearly
show a significant fraction of Ru nanoparticles located inside the
zeolite crystals, with an average particle size of around 1.5 nm for
both samples. This is in an excellent agreement with the AC-
HAADF-STEM and H2 chemisorption results. Those indicate that
substantial Ru nanoparticles are present inside the cavities of
zeolite Y. The sizes of Ru nanoparticles inside the zeolite being
slightly larger than the micropores cavities, could be ascribed to
the collapse of neighboring cavities during the growth of Ru
particles, in a good accordance with previous observation[10a]
.
XRD results further confirm such changes in the local cavities, as
evidenced by the expanding of the zeolite unit cell and a decrease
in zeolite crystallinity after metal deposition (Fig. S5), probably
indicating the Ru particles’ growth in a cavity-passing mode[17]
.
Figure 3. Local environment and electronic feature of Ru nanoparticles. (a)
Normalized X-ray absorption near edge structure (XANES) of Ru k-edge for Ru
foil, RuO2, Ru/H-Y and Ru/La-Y; (b) Fourier transforms of the k2-weighted
extended X-ray absorption fine structure (EXAFS) of Ru K-edge for the Ru foil,
Ru/H-Y and Ru/La-Y; (c, d) Low-temperature FT-IR spectra after CO adsorption
on the Ru/H-Y and Ru/La-Y at -189 °C, respectively.
To further quantify the fraction of such Ru nanoclusters
constraint inside the zeolite cavities, we further apply
hydrogenation tests with two probing molecules with different
size: toluene (kinetic diameter: 0.55 nm, accessible into the
cavities of FAU) and 1,3,5-triisopropylbenzene (TIPB, kinetic
diameter: 0.85 nm, inaccessible into the cavities of FAU) (Fig. 4d
and Table S3). The portion of Ru nanoclusters inside the cavities
of the zeolite is thus determined to be 78% for the Ru/H-Y and
85% for the Ru/La-Y separately, confirming the validation of most
Ru nanoclusters (~1 nm) confined in the micropores of zeolite Y.
Consequently, such a significant fraction of Ru nanoclusters
-189 °C upon a stepwise increase of CO pressure from sub-mbar
to 1 mbar, as shown in Fig. 3c and 3d. For the Ru/H-Y, a feature
initially at 2178 cm-1 and a red-shift of 10 cm-1 are visualized as a
function of CO pressure in the sub-mbar range, attributed to CO
adsorbed on the protons of H-Y[13]. Notably, a feature at 2128 cm-
1 develops at a CO partial pressure of above 0.8 mbar, assigned
to the multi-coordinated Run+-CO according to previous reports[12]
.
A weak adsorption of Run+-CO was indicated in the desorption
profile (Fig. S9). Interestingly, no observed Ru0-CO signal
suggests that the Ru particles mainly exhibit a positive charge of
Run+ in the Ru/H-Y. The modulation of the electronic structure of
the small Ru nanoparticles confined inside the zeolite is induced
by the interaction between the metal species and the zeolite
framework. For the Ru/La-Y, a feature initially at 2177 cm-1 and a
red-shift of 2 cm-1 are observed as the adsorbate surface
coverage increased, attributed to CO adsorbed on the cations in
La-Y[14]. Notably, a feature at 2040 cm-1 develops at a relatively
low CO pressure of 0.2 mbar, attributed to the linear Ruδ+-CO
species[15]. Upon applying high vacuum and heating, the Ruδ+-CO
remains up to 100 °C, indicating metallic feature of Ruδ+ with a
strong CO adsorption (Fig. S9). A weak feature at 2128 cm-1
assigned to the multi-coordinated Run+-CO is also shown at PCO
≈ 1 mbar. The La modification enriches the Ru nanoparticles in
electrons and reduces its effective charge, as also confirmed by
XPS and XANES results. This corroborates a degenerated
interaction between metal and the zeolite framework after the
addition of La cations in zeolite Y, probably caused by the strong
electronic interaction between Ru nanoparticles and La species,
which is also supported by H2-TPR (Fig. S10). Similar increase in
electron charge on Pt clusters via the addition of metal cations
has been noted by Visser et al. for their Pt/Y catalyst system[16]
.
Figure 4. Validation of zeolite-tailored proximity between metal sites and acid
sites. (a) Schematic diagram of the milling process of resin-embedded
catalyst particles with a precision ion polishing system. (b, c) TEM images
of cross sections of Ru/H-Y and Ru/La-Y respectively. (d) Schematic diagram
of hydrogenation tests with two probing molecules with different size: toluene
(kinetic diameter: 0.55 nm) and 1,3,5-triisopropylbenzene (TIPB, kinetic
diameter: 0.85 nm). This reaction process is used to quantify the fraction of
Ru nanoclusters constraint inside the zeolite cavities
Validation of zeolite-tailored confined proximity
To clarify the spatial distribution of Ru nanoparticles in FAU
zeolites, precision ion polishing system (PIPS) was applied for the
preparation of cross-sectional TEM samples. Milling the resin-
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encapsulated inside the zeolite cavity provides a sub-nanometric
proximity between Ru particles and acid sites within the confined
space provided by the FAU zeolite, which is essential to offer
additional benefits in activity for the direct conversion of EL to
pentanoic biofuels. Although ‘the-closer-the-better’ with respect to
the location of the acid and metal sites, generally recognized for
improving activity of the bifunctional catalyst in gas-phase
reactions, is challenged for the industrially highly important
hydrocracking reaction[10b], this notion is still advocated for
tandem catalysis in the liquid-phase biomass conversion process,
as studied in this work.
significant EP/PA yields at the initial stage over the zeolite Y
supported Ru particles with confined proximity. In the case of the
Ru-based bifunctional catalysts with non-confined proximity (Fig.
5b and Fig. S11), the yields of EP/PA are generally much lower
than that for bifunctional catalysts with confined proximity. The
reactive 4-HPE intermediate either cannot be stabilized on the
external surface of the zeolite owing to the lack of confinement
environment, or quickly hydrolyzed to LA inside the cavity of
zeolites and subsequently hydrogenated to GVL when diffusing to
the metal sites located on the external surface. Such structure
normally results in similarity in product selectivity, with GVL as the
dominate product. Accordingly, the FAU zeolite-tailored confined
proximity shows an enhanced selectivity towards the production
of EP/PA compared to that of the Ru nanoparticles combined with
open structure zeolites (Ru/C, Ru/SiO2 or Ru/Al2O3 mixed with H-
Y or La-Y in Fig. S11 and Table S1), indicating that the confined
intimacy sites are the required essential sites for the efficient
production of EP/PA in the HDO of EL.
Insight into the intimacy effect during biomass catalysis
Our experimental results of the HDO of EL demonstrated that the
confined proximity in the zeolite Y supported bifunctional catalysts
(Ru/H-Y and Ru/La-Y) has significantly promoted the catalytic
activity and selectivity (Fig. 1a). Specifically, when Ru is located
inside the FAU zeolite, EL can be quickly hydrogenated to 4-HPE,
as evidenced by a significant yield of EP even at an early stage
of reaction time (5 min in Fig.1b, c). Notably, EP/PA are also
observed as the dominant products with the Ru/La-Y at a low EL
conversion level (with a less amount of catalyst after 30 min in Fig.
S11). Additionally, 4-HPE, has been detected in GC-MS (Fig. S3).
Those all point to the occurrence of an alternative pathway of EL-
4-HPE-EP. Similar reactive analogue of 4-hydroxypentanoic acid
(4-HPA) has been proposed for both thermostatically and
electrochemically hydrogenation of LA to PA[7c, 18]. The 4-HPE
intermediate might be stabilized by an acid site in zeolite cavities
via the confinement and nest effects[19], and subsequently directly
hydrogenated to PE by the adjacent metal site within sub-
nanoscale proximity, rather than it being self-ring-closed to GVL
on the same acid site. Thus, the rate determining step of GVL
ring-opening can be efficiently suppressed. Even with the
formation of GVL, GVL ring-opening step can be still facilitated
when the metal and acid sites are in a close proximity. Therefore,
EP/PA can be produced fast and efficiently even at an early stage
of the reaction (Fig. 5a), as consistent with the observed
The role of La species on Ru dispersion and catalyst stability
1
The solid-state H MAS NMR spectra of the H-Y, La-Y, Ru/H-Y
and Ru/La-Y materials after dehydration are depicted in Fig. 6a.
Different zeolitic hydroxyl species can be discriminated and
quantitatively established for all the samples. Four major signals
can be discriminated and quantified, assigned to the hydroxyl
groups located in the FAU sodalite cages (5.0 ppm) and
supercages (3.9 ppm), extra-framework aluminol (2.5 ppm) and
non-acidic silanol groups (1.9 ppm) respectively, with two
additional minor signals attributed to NH4+ species (7.0 ppm) and
hydroxyl groups of metal cation (1 ppm i.e. Na+ or Ru)[20]. By
1
comparing the La-Y to the H-Y, we find a clear difference in H
MAS NMR signal patterns. The addition of La into zeolite Y,
mainly results in an increase of hydroxyl species in sodalite cages,
together with a decrease of the extra-framework Al-OH species.
Combining with the atomically dispersed La species in the La-Y
determined by AC-HAADF-STEM (Fig. 2f-g and Fig. S6g-h), the
increase of hydroxyl species in sodalite cages can be attributed
to
a significant amount of hydroxyl species as charge
compensators of La cations, located inside the sodalite cages of
the zeolite Y. This is in line with the nature and location of the La
species in FAU-type zeolites, as La hydroxylated species majorly
in the cavities of FAU topology[21]. After Ru deposition, a decrease
of the hydroxyl species in both supercages and sodalite cages
was observed for both Ru/H-Y and Ru/La-Y, compared to the
parent H-Y and La-Y. This further validates the formation of sub-
nanoscale intimacy confined in the cavities of zeolite Y. Notably,
the AC-HAADF-STEM of the Ru/La-Y shows that highly dispersed
La species closely surrounds the Ru nanoparticles, indicating that
La hydroxylated species might provide coordinative sites for Ru
species. Combined with the strong interaction between Ru and La
species indicated by H2-TPR (Fig. S10), the obtained results of
probing tests and H2 chemisorption (Table S3) consistently show
the La modification further improves the dispersion of Ru species
into the cavities of the zeolite Y. It is acknowledged that the rare
earth metal modification can be an efficient approach for
improving the zeolite stability for gas-phase reactions[22]. Recently,
especially for biomass related liquid phase reactions,
dealumination of zeolite materials in liquid phase has been
recognized as a major issue for catalyst deactivation. To confirm
Figure 5. Insight into the proximity effect of the bifunctional catalysts for the
hydrodeoxygenation (HDO) of ethyl levulinate (EL). Reaction pathway of
selective conversion of EL over bifunctional catalysts (a) with confined
proximity. (b) with non-confined proximity.
6
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10.1002/anie.202108170
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RESEARCH ARTICLE
Figure 6. The role of La species on Ru dispersion and catalyst stability. (a) 1H MAS NMR spectra of H-Y, Ru/H-Y, La-Y and Ru/La-Y. Relative amounts of each
hydroxyl species normalized to the hydroxyl content in the parent zeolite sample; (b, c) 27Al and 29Si MAS NMR spectra of fresh and spent Ru/H-Y and Ru/La-Y
catalysts (after five consecutive runs), respectively; The solid and dash lines in the NMR spectra represent the measured signals and cumulative peak fitting curve
respectively
the validity of
a
stabilizing effect of La species on EL
deactivation was also reported for the Ru/H-beta and Ru/H-ZSM-
5 for the HDO of LA upon multiple reuse under such liquid phase
conditions[7c, 25]. Additionally, support desilication has also
occurred for the Ru/H-Y during multiple reuses, whilst no Si
leaching observed for the Ru/La-Y, as evidenced by ICP analysis
(Table S6). Notably, La cations indeed stabilize the FAU zeolite
structure and efficiently suppress the deconstruction of FAU
zeolites during catalyst preparation and catalysis[26]. Compared to
the fresh Ru/La-Y, the spent Ru/La-Y after five consecutive runs,
shows only trace amount (3%) of transformation of FAL (AlIV) to
EFAL (AlV), indicating no apparent dealumination of the La-Y. This
is also supported by the limit change of Si/Al ratio from 2.8 to 2.9
even after five consecutive runs, shown by 29Si MAS NMR (Fig.
6c). The presence of La cations in zeolite Y indeed efficiently
prevents the dealumination in the liquid phase under reaction
conditions, which correlates well with no apparent drop in catalyst
reactivity even after consecutive runs. Recently, Louwen et al.
reported that the presence of La species in the zeolite Y indeed
resulted in a higher energy barrier of dealumination, supporting a
stabilizing effect of La species on zeolite structure[27]. Furthermore,
no apparent leaching of Al and Si species detected by ICP
corroborates the stabilizing effect of La species (Table S6).
Additionally, the presence of La cations in the Ru/La-Y,
significantly alleviates the coke formation during catalysis, as
evidenced in thermal gravimetric analysis (Fig. S14). Therefore,
the excellent stability of the Ru/La-Y for the direct, one-pot
conversion of EL to EP/PA can be primarily attributed to the
retained structure of the confined proximity by prevention of
hydrodeoxygenation, 27Al and 29Si MAS NMR were performed for
the fresh and spent Ru/H-Y and Ru/La-Y catalysts. The 27Al MAS
NMR spectra (Fig. 6b) of all the catalysts show three resonances
in the four-coordinated (AlIV), five-coordinated (AlV) and six-
coordinated (AlVI) aluminum region. The AlIV signal at ~57 ppm
and the AlVI signal at ~0 ppm represents tetrahedral framework
(FAL) and octahedral extra-framework aluminum (EFAL) species
respectively[23]. Trace amount of the AlV signal with a broad
resonance at around 30 ppm, assigned to EFAL[24], is also
detected for all the catalysts. For the fresh catalysts, the Ru/H-Y
shows a FAL of 67% and an EFAL of 32%, while the Ru/La-Y
depicts a FAL of 89% and an EFAL of 10%. The more prevalence
of tetrahedral FAL in the Ru/La-Y, in line with the dominant
Brønsted acid sites (BAS) probed by NH3-TPD and FT-IR
spectroscopy of adsorbed pyridine results (Table S5 and Fig.
S12-13), indicates that La modification can efficiently suppress
the generation of EFAL species during the preparation processes
(i.e. calcination and reduction), which is in a good agreement with
1H MAS NMR. After five consecutive runs, the spent Ru/H-Y
clearly shows a significant decrease in FAL (AlIV) and a
simultaneous increase in EFAL (both AlV and AlVI). The severe
dealumination of FAL observed for the spent Ru/H-Y, indicates a
severe loss of BAS by 45%, pointing to a severe dealumination of
H-Y during the multiple reuse tests, as also evidenced Al leaching
by ICP analysis (Table S6). The 29Si MAS NMR results confirm
the validation of a severe dealumination occurred for the Ru/H-Y,
with an increase of Si/Al ratio from 3.6 to 4.4 during the multiple
reuses. Support dealumination accounted for the catalyst
7
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RESEARCH ARTICLE
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The National Key Projects for Fundamental Research and
Development of China (2018YFB1501602), National Natural
Science Foundation of China (21721004, 21703238 and
22078316) are acknowledged for financial support. Z.W. and S.C.
acknowledges the support of Science Foundation of China
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Keywords: bifunctional catalysis
• zeolite • proximity •
hydrodeoxygenation • biomass conversion
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8
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Entry for the Table of Contents
Zeolite-tailored active site proximity, with well-dispersed and ultra-small Ru nanoclusters (~1 nm) confined within the internal cavities of zeolite Y, significantly
boosts the catalytic activity and selectivity for the direct HDO of neat ethyl levulinate (EL) into pentanoic biofuels. Furthermore, La modification for zeolite Y could
improve the metal dispersion and the stability of zeolite during catalysis.
9
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