Recent Progress of Hydrogenation and Hydrogenolysis Catalysts Derived from Layered Double Hydroxides

Abstract

Layered double hydroxides (LDHs), also known as hydrotalcite−like compounds, are widely used in many fields due to their unique structural advantages. Based on LDHs, a wide range of metal catalysts could be synthesized with high metal dispersion, tunable acid−base properties, facile but flexible preparation methods, strong metal−support interaction, and thermal stability. Owing to these outstanding advantages, LDH−derived materials manifest great potential as catalysts, particularly in hydrogenation and hydrogenolysis reactions. More than 200 papers published in the past five years in this field clearly indicated the rapid development of these materials. In this respect, it is imperative and essential to provide a timely review to summarize the current progress and motivate greater research effort on hydrogenation and hydrogenolysis catalysts derived from LDHs. In this review, the applications of LDH−derived materials as heterogeneous catalysts in various hydrogenation and hydrogenolysis reactions were comprehensively discussed. Hydrogenation of unsaturated chemical bonds, hydrodeoxygenation of oxygenated compounds, hydrogenolysis of carbon–carbon bonds and hydrogenation of nitrites and nitriles were described. This review demonstrates the extraordinary potentials of LDH−derived catalysts in hydrogenation and hydrogenolysis reactions, and it is undoubted that LDH−derived catalysts will play an even more significant role in the foreseeable future.

Full text

Catalysts 2022, 12, 1484. https://doi.org/10.3390/catal12111484 www.mdpi.com/journal/catalysts
Review
Recent Progress of Hydrogenation and Hydrogenolysis
Catalysts Derived from Layered Double Hydroxides
Zhihui Wang
1
, Wei Zhang
1
,
Cuiqing Li
2,
* and Chen Zhang
2,
*
1
School of New Materials and Chemical Engineering, Beijing Institute of Petrochemical Technology,
Beijing 102617, China
2
Beijing Key Laboratory of Enze Biomass Fine Chemicals, Beijing Institute of Petrochemical Technology,
Beijing 102617, China
* Correspondence: licuiqing@bipt.edu.cn (C.L.); zhangc@bipt.edu.cn (C.Z.)
Abstract: Layered double hydroxides (LDHs), also known as hydrotalcite−like compounds, are
widely used in many fields due to their unique structural advantages. Based on LDHs, a wide range
of metal catalysts could be synthesized with high metal dispersion, tunable acid−base properties,
facile but flexible preparation methods, strong metal−support interaction, and thermal stability.
Owing to these outstanding advantages, LDH−derived materials manifest great potential as cata-
lysts, particularly in hydrogenation and hydrogenolysis reactions. More than 200 papers published
in the past five years in this field clearly indicated the rapid development of these materials. In this
respect, it is imperative and essential to provide a timely review to summarize the current progress
and motivate greater research effort on hydrogenation and hydrogenolysis catalysts derived from
LDHs. In this review, the applications of LDH−derived materials as heterogeneous catalysts in var-
ious hydrogenation and hydrogenolysis reactions were comprehensively discussed. Hydrogenation
of unsaturated chemical bonds, hydrodeoxygenation of oxygenated compounds, hydrogenolysis of
carbon–carbon bonds and hydrogenation of nitrites and nitriles were described. This review
demonstrates the extraordinary potentials of LDH−derived catalysts in hydrogenation and hydro-
genolysis reactions, and it is undoubted that LDH−derived catalysts will play an even more signif-
icant role in the foreseeable future.
Keywords: layered double hydroxides; hydrotalcite; hydrogenation; hydrogenolysis; catalysis
1. Introduction
Hydrogenation is one the most fundamental processes in the chemical industry with
diverse applications. In hydrogenation, atomic hydrogen is added to unsaturated chemi-
cal bonds in the presence of homogeneous or heterogeneous catalysts. Hydrogenolysis is
another important type of reaction where a carbon–carbon bond or carbon–heteroatom
bond is cleaved by hydrogen. In the past century, hydrogenation and hydrogenolysis are
particularly important in the petrochemical industry to saturate alkenes or aromatics, re-
move sulfur or nitrogen atoms, increase fuel stability and decrease toxicity [1]. Recently,
hydrogenation and hydrogenolysis attracted extensive attention for new objectives. Bio-
mass utilization [2,3] and CO
2
hydrogenation [4,5], which are crucial processes for a sus-
tainable human society, also heavily rely on the technological development of hydrogena-
tion and hydrogenolysis. Using renewable H
2
produced from eco−friendly processes [6,7],
hydrogenation and hydrogenolysis in biomass upgrade and CO
2
hydrogenation could
greatly reduce the dependency on fossil fuels. In most industrial cases, hydrogenation or
hydrogenolysis are catalyzed by supported metal catalysts based on transition metals
such as Ni, Cu, Pd, or Pt [2,8,9].
Layered double hydroxides (LDHs) are layered inorganic materials consisting of pos-
itively charged brucite Mg(OH)
2
−like layers and interlayer charge−compensating anions,
Citation: Wang, Z.; Zhang, W.; Li,
C.; Zhang, C. Recent Progress of
Hydrogenation and Hydrogenolysis
Catalysts Derived from Layered
Double Hydroxides. Catalysts 2022,
12, 1484. https://doi.org/
10.3390/catal12111484
Academic Editor: Vladimir Sobolev
Received: 21 October 2022
Accepted: 17 November 2022
Published: 20 November 2022
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
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Copyright: © 2022 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://cre-
ativecommons.org/licenses/by/4.0/).

Catalysts 2022, 12, 1484 2 of 32
as shown in Figure 1 [10]. A typical type of LDHs is hydrotalcite (HT), a natural mineral
with chemical composition of Mg
6
Al
2
(OH)
16
CO
3
·4H
2
O [11]. LDHs and natural hy-
drotalcite share similar structure, in which metal cations can be replaced by divalent and
trivalent metal ions with similar atomic radii in the same crystal, and interlayer anions
can be replaced by other intercalating anions. LDHs, also called hydrotalcite−like com-
pounds, could be expressed with the general formula [M
2+(1−x)
·M
3+x
(OH)
2
]
x+
·[(A
n−
)
x/n
·zH
2
O]
x−
[12]. Common M
2+
ions include Mg
2+
, Cu
2+
, Zn
2+
and Ce
2+
, while common M
3+
include Al
3+
,
Fe
3+
, and Cr
3+
. These cations can be atomically and uniformly dispersed in the hydrotalcite
layer. The formed cation layers lack interlayer interaction and have a negative net charge,
which must be balanced by the inserted anions, and water molecules might also enter the
interlayer region [13]. Interlayer anions are exchangeable, such as CO
32−
, NO
3−
, SO
42−
, Cl
−
,
F
−
, acetate and salicylate. The x value in the general formula represents the molar ratio of
M
2+
/(M
2+
+
M
3+
), and the range of its value has great influence on the composition and
structure of hydrotalcite materials [10]. Both the magnitude of the charge density on the
laminate and the extent to which the homocrystalline substitution of metal cations occurs
depend on the x value, and a genuine LDH phase is usually found only at 0.2 < x < 0.4
[14,15].
The unique structure, tunability of the layer and interlayer element composition, de-
lamination property, structural topological transformations, and the confinement effect of
LDHs provide LDH−derived materials great application potentials in many fields [14,16].
In heterogeneous catalysis, LDH can be applied as catalyst itself, catalyst support or cata-
lyst precursor in many reactions such as C
−
C coupling, N−arylation, oxidation, hydro-
genation, hydrogenolysis, etc. Although several review articles on catalytic applications
of LDHs have been published [12,17–21], reviews focusing on hydrogenation and hydro-
genolysis are still scarce to the best of the authors’ knowledge. Furthermore, in the past
few years, a large number of research articles on hydrogenation and hydrogenolysis over
LDH−derived catalysts were published, demonstrating the significant development in the
field. Therefore, we believe it is timely to provide a comprehensive and in−depth sum-
mary on the latest progress of LDH−derived catalysts for hydrogenation and hydrogen-
olysis. This review will focus on the applications of LDH−derived materials as hydrogena-
tion and hydrogenolysis catalysts, highlighting physiochemical properties, structural
properties and reactivity advantages of LDH−derived catalysts. Thermocatalytic pro-
cesses catalyzed by heterogeneous catalysts will be the emphasis of this review, while
electrocatalytic or photocatalytic research will not be discussed due to their distinct nature
in mechanisms.
Figure 1. A Schematic illustration of LDH structure.

Catalysts 2022, 12, 1484 3 of 32
2. Type of LDH−Derived Catalysts for Hydrogenation and Hydrogenolysis
Generally, three types of LDH−derived catalysts are prepared and investigated:
LDH−supported catalysts, LDH−derived mixed metal oxides (MMOs), and LDH−derived
intermetallic compounds (IMCs). Table 1 shows several examples of different types of
LDH−derived catalysts and their reported physiochemical properties. It could be noticed
in the table that high specific surface area and small particle size could be readily achieved
with catalysts from LDHs. This section will describe in detail the general advantages and
synthesis protocols of these three types of LDH−derived materials.
Table 1. Examples of LDH−derived catalysts prepared with different methods.
Catalyst Preparation Method
Surface Area
(m2/g)
Particle Size
(nm) Other properties Ref.
PdAg/ZnTi−LDH Coprecipitation and photo-
chemical reduction 131 5 Pore volume 2.63 cm3·g−1; mean
pore size 80.3 nm [22]
Pt/CoAl−MMO Coprecipitation and reduc-
tion–deposition 87 4–5
Pore volume 0.29 cm3·g−1; mean
pore diameter 3.83 nm [23]
CuMgAl−MMO Urea hydrolysis 213 − Pore volume 0.95 cm3·g−1 [24]
CuCoAl−MMO Coprecipitation 92.9 3.1 Pore volume 0.69 cm3·g−1 [25]
NiMoAl−MMO Coprecipitation and ion ex-
change 93 4.6
Pore volume 0.13 cm3·g−1; total
acid sites 2.077 mmol·g−1 [26]
NiIn−IMC Co−precipitation 126.4 5.8 − [27]
NiMo−IMC In situ co−reduction 189.2 18.6 − [28]
2.1. LDH−Supported Catalysts
Because heterogeneous catalysts for hydrogenation and hydrogenolysis usually re-
quire the participation of metallic sites, pristine LDHs could not be used directly as hy-
drogenation and hydrogenolysis catalysts. Nevertheless, LDHs act as perfect support ma-
terials due to intrinsic structural superiorities. High specific surface area, hierarchical pore
structure, abundant surface defects, strong metal−support interaction, surface acid−base
sites with tunable strength or concentration, or inherent confinement effects have been
reported for LDHs [14,20].
Major synthesis methods of LDHs include coprecipitation [29], urea hydrolysis, hy-
drothermal synthesis [30] and the sol−gel method [15]. Different synthesis methods di-
rectly affect the physical and chemical properties of the obtained LDH materials [31–33].
The most facile and common method is coprecipitation. By coprecipitation, crystallinity,
particle size distribution, and stability of LDHs can be precisely controlled by adjusting
the synthesis parameters such as pH, temperature, aging time, mixing rate, cation ratio
and solution concentration [31,32]. Coprecipitation can be combined with the anion ex-
change method to modulate the structure and function of LDH−based materials by insert-
ing ions or molecules between layers while keeping the basic structure unchanged, mod-
ifying the physiochemical properties or the surface properties of the material.
2.2. LDH−Derived Mixed Metal Oxide Catalysts
Mixed metal oxides (MMOs), which could be synthesized by the heat treatment of
LDHs, showed high catalytic performance and received broad attention in hydrogenation
and hydrogenolysis. LDH−derived MMOs inherit LDH characteristics such as controlled
composition, high specific surface area, and uniform morphology. In addition, by con-
verting metal hydroxides into metal oxides, highly dispersed metal oxides particles were
generated, which could act as acid or base sites. The nature, intensity and concentration
of these acid/base sites on MMOs could be tuned by controlling the type and molar ratio
of metal cations in the precursors, preparation method, calcination temperature and type
of interlayer anions [20,34,35]. The high surface area and tunable acid/base property, as

Catalysts 2022, 12, 1484 4 of 32
well as other advantages, make LDH−derived MMOs an ideal catalyst support material
compared to the conventional metal oxide support. MMOs could also be used inde-
pendently as hydrogenation/hydrogenolysis catalysts. For MMOs with hydrogena-
tion−active metals such as Ni or Cu, the reduction of MMOs could generate highly dis-
persed metallic sites in situ which are catalytically active for hydrogenation/hydrogenol-
ysis. Compared with supported metal catalysts prepared by impregnation, reduced
MMOs as hydrogenation/hydrogenolysis catalysts are often more reactive, due to the high
metal dispersion, abundant acid/base sites and strong metal–support interaction, as
shown later in this review.
The calcination of LDHs to generate MMOs is a topological transformation process.
During thermal transformation three temperature domains of LDH structure changes can
be observed. The first structural change at around 190 °C corresponds to the dehydration
of loosely bonded interlayer water, accompanied by the decrease in the layer spacing,
while the layered structure is retained [36]. The second temperature domain in the range
of 200–400 °C indicated dehydroxylation and the collapse of the layered structure of
LDHs, in situ forming highly dispersed metal oxides [37]. The third domain in the range
of 500–1000 °C is ascribed to the formation of sintered metal particles and spinel phases
[36,38]. The structure and function of MMOs can be controlled by changing the topological
transformation parameters such as heating rate, calcination temperature and calcination
atmosphere of LDH precursors during the thermal treatment, leading to the modification
of the catalytic performance of MMOs [10,17,34,39,40].
2.3. Intermetallic Compound Catalysts
Intermetallic compounds (IMCs), also called ordered alloy, is a special type of alloy
composed of two (or more) metal elements with specific crystal structure and atomic com-
position. IMCs can be expressed as AxBy, where x and y are small integers, usually 1, 2 and
3 [41,42]. Compared with conventional alloys, IMCs exhibit completely or partially or-
dered surface structure. Figure 2 showed the structure of NiSb and NiBi IMCs catalysts
prepared by Wei’s group [43]. It could be noticed that high metal dispersion and uniform
alloy composition were obtained with IMCs, which were important for catalytic reactivity.
Due to the unique electronic and geometric properties, IMCs promote the activated ad-
sorption of substrate molecules with specific patterns or configurations, thus achieving
improved catalyst activity, selectivity and stability [41,44,45].
Figure 2. (B1) SEM, (B2) TEM and particle size distribution, (B3) elemental mapping, and (B4) lattice
spacing images of NiSb IMC; (C1) SEM, (C2) TEM and particle size distribution, (C3) elemental
mapping, and (C4) lattice spacing images of NiBi IMC [43]. (Copyright 2020, Elsevier).
The preparation of IMCs can be categorized into three methods, controlled colloid
synthesis, inorganic capsule synthesis, and layered double hydroxide synthesis [42]. The

Catalysts 2022, 12, 1484 5 of 32
synthetic method based on LDHs has particular advantages. The composition flexibility
of LDH precursors enable the possibility of synthesizing a variety of IMC catalysts [45,46].
Metal elements of LDH layers have a high degree of dispersion at the atomic level, which
is conducive to the formation of metal catalysts with high metal dispersion after the calci-
nation−reduction process. The preparation of IMC nanocrystals using LDH precursors can
be achieved by two methods, endogenous method or exogenous method. For the endog-
enous method, two metal components of IMCs (e.g., Ni, Co, Cu, Ga) are introduced sim-
ultaneously into the host layer of LDHs, followed by the calcination and reduction of LDH
precursors, affording IMC catalysts. For noble metal elements (Au, Rh, Pd, Ir, Pt, etc.) or
heavy metal elements (Sb, Sn, Bi, Pb, etc.) which are difficult or impossible to introduce
into the LDH layer, the exogenous method can be adopted, in which noble metal or heavy
metal salts are mixed with LDH precursors by impregnation, followed by calcination and
reduction to obtain IMC catalysts.
3. Hydrogenation of Carbon−Oxygen Unsaturated Bonds
3.1. Hydrogenation of Ketones and Aldehydes
The hydrogenation of carbonyl groups in ketones and aldehydes to form hydroxy
groups is an important process particularly in synthetic chemistry. This section will re-
view research work on ketone or aldehyde hydrogenation catalyzed by LDH−derived cat-
alysts. The selective hydrogenation of α−β unsaturated aldehydes including crotonalde-
hyde, citral and cinnamaldehyde will also be discussed. The hydrogenation of furfural, a
highly important furan aldehyde derived from hemicellulose, will be reviewed in the next
section separately.
For hydrogenation over heterogeneous catalysts, metal dispersion is crucial because
high dispersion would naturally mean more active sites for reactions to take place.
Non−precious metal catalysts such as Ni, Cu and Co could be synthesized by the calcina-
tion of corresponding LDH precursors prepared by coprecipitation. Because M2+ and M3+
ions are uniformly distributed in the brucite layers, the calcination of LDHs could in situ
generate highly dispersed metal particles and more catalytic active sites for hydrogena-
tion. For instance, Dragoi’s group have shown that a reduced CuMgAl catalyst synthe-
sized from LDH precursors exhibited particle sizes of 2.6 to 6.5 nm and high activity in
cinnamaldehyde hydrogenation to cinnamyl alcohol [40]. Another study using CuM-
gAl−MMO of 1.4 to 2.4 nm metal particle size for benzyl aldehyde hydrogenation, ob-
tained 85–93% selectivity of benzyl alcohol [47].
As shown in Table 2, precious metals are particularly effective in selective hydro-
genation of unsaturated aldehydes. For precious metal catalysts such as Pt, Pd or Ru, us-
ing LDHs or LDH−derived MMOs as support could also generate catalysts with high pre-
cious metal dispersion. Many examples were found in the literature reporting metal par-
ticle sizes less than 5 nm using the conventional impregnation method on LDHs/MMOs
owing to the abundant surface basicity to anchor metal species [48], such as Ru/MgAl−HT
[49], Pd/MgAl−MMO [50] and Au/MgAl−MMO [51] for aldehyde/ketone hydrogenation.
Notably, due to the special interlayer galleries of LDHs, an alternative way to load metals
to the support other than impregnation is by intercalating metal−containing anions in
LDHs by the ion exchange method. With this method, researchers were able to obtain a
Pt/MgAl−HT catalyst with an average diameter of Pt nanoparticle being only 1.9 nm using
PtCl62− [52]. Generally, as the catalyst particle size decreases, the number of low coordina-
tion sites such as the edges and corners of the metal increases, providing more active sites
for the reaction and thus enhancing the catalytic activity of the catalyst [23,27,53].

Catalysts 2022, 12, 1484 6 of 32
Table 2. Catalytic performances of LDH−derived catalysts for hydrogenation of ketones and alde-
hydes.
Catalyst Substrate Reaction Conditions Conversion (%) Product Selectivity (%) Ref.
Pt/ZnSnAl/C 2−Pentenal 80 °C; 3.0 MPa H2 28.5 2−Pentenol, 92.0 [54]
Cu/MgAl−HT Benzaldehyde 250 °C 68 Benzyl alcohol, 93 [47]
Ru/MgAl−HT Benzaldehyde 100 °C; 3.5 MPa H2 >63 Cyclohexanemethanol, 91.9 [49]
Au/Mg2AlO Crotonaldehyde 120 °C; 0.93 MPa H2 23.6 Crotyl alcohol, 62 [51]
Pt/MgAl−LDH Cinnamaldehyde 80 °C; 2 MPa H2 92.6 Cinnamyl alcohol, 75.5 [52]
Pt/CoAl−MMO Cinnamaldehyde 70 °C; 2 MPa H2 99.7 Cinnamyl alcohol, 72.5 [23]
Au/ZnAl Cinnamaldehyde 130 °C; 1.5 MPa H2 100 Cinnamyl alcohol, 95.7 [53]
Pt/MgAl−LDH Cinnamaldehyde 60 °C; 1 MPa H2 79.7 Cinnamyl alcohol, 85.4 [55]
PtGa/MgAlGa Cinnamaldehyde 70 °C; 3 MPa H2 52.8 Cinnamyl alcohol, 70.7 [56]
Ir/MgAlFe Cinnamaldehyde 60 °C; 3 MPa H2 94.4 Cinnamyl alcohol, 79.1 [57]
Pt/CoAl−LDH Cinnamaldehyde 70 °C; 3 MPa H2 94.3 Cinnamyl alcohol, 91.9 [58]
CoGa−IMC Cinnamaldehyde 100 °C; 2 MPa H2 100 Cinnamyl alcohol, 96 [59]
NiZnAl/C Citral 140 °C; 1 MPa H2 100 Citronellol, 92.3 [60]
CoSn−IMC Citral 160 °C; 4.0 MPa H2 100 Citronellol, 67.6 [61]
CuZnAl−MMO Citral 80 °C; 1.0 MPa H2 99.8 Allylic alcohol, 75.1 [62]
NiBi−IMC Unsaturated aldehydes 100 °C; 2 MPa H2 >90 Unsaturated alcohol, >93.2 [43]
NiIn−IMC Unsaturated aldehydes 120–145 °C; 3 MPa H2 >56 Unsaturated alcohol, >44 [27]
Pt/MgCoAl Unsaturated aldehydes 80 °C; 2 MPa H2 >87 Unsaturated alcohol, >80 [63]
Because metal particle size plays an important role in the reactivity of the catalyst,
particularly for the selective hydrogenation of α−β unsaturated aldehydes, several meth-
ods were developed for tuning particle size in LDH−related catalysts. Xiang et al. pre-
pared HT−supported Pt nanocrystal catalysts for the selective hydrogenation of cinnamal-
dehyde [55]. It is noticed in this research that the selectivity of cinnamyl alcohol was
closely related to Pt particle size, while the size of the Pt nanocrystals could be finely tuned
by controlling the amount of surfactant added. Another study provides an example of
using different alcohols including ethylene glycol, methanol and ethanol for the reduction
of Pt on MgAl−LDHs, generating different sizes of reduced Pt particles with distinct reac-
tivity in cinnamaldehyde hydrogenation [64].
For LDH−derived catalysts, the fact that active metal particles were incorporated in
the LDH or MMO matrix indicates strong metal−support interaction (SMSI) [65]. SMSI is
a specific interaction effect between the support and the metal nanoparticles (NP). This
interaction may lead to metal–metal bonding or the formation of intermetallic compounds
in the catalyst [66], enhancing the stability of the metal particles in the catalyst. In addition,
the topological transformation from LDHs to MMOs can be exploited to create SMSI by
stabilizing the active metal particles in the oxide matrix, to prevent leaching and agglom-
eration [67–69]. Wang et al. reported a slight increase in particle size from 1.2 nm to 1.6
nm for the Pt catalyst supported on ZnSnAl−MMOs after 12 h of 2−pentenal hydrogena-
tion reaction [54]. Metal–support interaction could also electronically and geometrically
affect metal atoms, shifting the product distribution in selective hydrogenation [56,57,67].
For instance, the addition of ZnO into NiZnAl−MMOs could deactivate C=C adsorption
on Ni sites, inducing increased citronellol selectivity in citral hydrogenation [60]. Oxygen
vacancies, which play important roles in carbonyl hydrogenation, could also be readily
generated by SMSI. Miao et al. reported the formation of oxygen vacancies at the metal–
support interface of Pd/MgAl−LDHs [70] and Pt/CoAl−LDHs [58]. It is found that these
oxygen vacancies are activation sites for C=O bonds, and significantly enhance the selec-
tivity toward C=O hydrogenation over C=C bond hydrogenation. Although SMSI is im-
portant for catalysts, its actual mechanism on product selectivity is still complex and
case−dependent. It is proposed in a study that SMSI could enhance the C=C bond adsorp-
tion on Pd sites on MgAl−LDHs, leading to an increased selectivity toward citronellal in
citral hydrogenation [71], while another research on cinnamaldehyde hydrogenation over

Catalysts 2022, 12, 1484 7 of 32
Pt/CoMgAl suggested that SMSI strengthened the carbonyl adsorption and inhibited C=C
hydrogenation [63].
Wei’s groups synthesized a series of intermetallic compound (IMC) catalysts using
LDH precursors, including CoGa, CoIn, NiBi, CoSn and NiIn, for selective hydrogenation
of unsaturated aldehydes [43,59,61]. It could be concluded from their research that by
forming IMC catalysts, the electron density of active metal (Ni or Co) was modified due
to electron transfer, leading to a change in adsorption conformation or adsorption strength
which eventually affected product selectivity. The promising results from IMCs, as well
as the convenient preparation method using LDH precursors, suggest a promising pro-
spect of IMCs in heterogeneous catalysis.
For α−β unsaturated aldehydes, total hydrogenation of both C=C bonds and C=O
bonds will also decrease yields toward desired products, and in most cases selective hy-
drogenation is pursued rather than total hydrogenation. The tunable acid–base properties
of LDHs or MMOs play an important role in product distribution of selective hydrogena-
tion of α−β unsaturated aldehydes [72,73]. It is shown that CuZnAl−MMO catalysts with
the most Lewis acid sites which adsorbed citral molecules via the carbonyl group showed
the highest selectivity toward allylic alcohol products such as geraniol and nerol [62]. An-
other study on Pt/MgAl−MMO also suggested that catalysts with stronger acidity were
more prone to unsaturated alcohol production [74]. It is deduced that Lewis acid sites,
such as metal cations, are adsorption and activation sites for C=O bond, and thus the se-
lectivity of C=O bond hydrogenation products is closely associated with Lewis acidity of
catalysts. LDH−derived catalysts often exhibit a close vicinity to Lewis acid sites for car-
bonyl adsorption and metal sites for hydrogenation, inducing synergistic effects for effi-
cient hydrogenation of carbonyl bond in aldehyde or ketone hydrogenation [75].
3.2. Hydrogenation of Furfural
Furfural (FAL) is an important biomass platform chemical bridging biomass feed-
stock and bio−derived chemicals with an annual production exceeding 652 kilotons [76].
FAL could be converted from lignocellulose and their chemical versatility implies plenti-
ful potential applications in the production of renewable C5 chemicals [77]. C=C double
bond and C=O double bond in furfural molecules require selective hydrogenation capa-
bility from catalysts to yield desired products. The most prevalent product derived from
furfural is furfuryl alcohol (FOL), as more than 65% of FAL produced worldwide was
converted into FOL [78]. The hydrogenation of FAL to yield furfuryl alcohol (FOL) could
be achieved by various metals [77]. Non−precious metal Cu− and Ni−based catalysts pre-
pared from LDH precursors were studied for carbonyl group hydrogenation to produce
furfuryl alcohol, such as CuAl [79,80], CuCr [81,82], NiAl [83,84], NiSn [85], and CuNiAl
[86,87]. Advantages of preparing Cu or Ni catalysts from LDH precursors are the small
metal particle size and high stability, due to the uniform atom distribution in LDH pre-
cursors. Yang et al. reported the NiSn intermetallic compound catalyst prepared from
LDH precursors exhibited a monometallic Ni particle with a size of around 10 nm. The
high dispersion and the electronic modification effect of Sn resulted in a 99% selectivity
toward FOL from FAL at 100 °C and 2 MPa, and a high stability after more than 30 h [85].
Considering that the C=O bond from the furfural molecule could interact with the
basic site on the catalyst surface, metal catalysts with surface basicity were synthesized,
with Mg being the most commonly used metal due to the facile preparation from
MgAl−hydrotalcite. NiMgAl−MMOs synthesized by calcining hydrotalcite precursors
displayed high reactivity for the hydrogenation of FAL to FOL, with the synergistic effect
between metallic Ni species to dissociate H2 and surface basic sites generated by Mg in-
troduction to activate furfural via carbonyl groups [88]. Villaverde et al. prepared CuM-
gAl catalysts derived from HT−like phases possessed high copper dispersion as well as
strong interactions between metallic copper and magnesium–aluminum support, result-
ing in better activity, selectivity and stability than impregnated Cu catalysts [89,90]. Since

Catalysts 2022, 12, 1484 8 of 32
CuNiAl, NiMgAl, and CuMgAl all showed outstanding reactivity in furfural hydrogena-
tion, it is reasonable that CuNiMgAl would also display desirable catalytic properties
[91,92], and it is demonstrated that the uniform distribution of highly dispersed CuNi
particles and surface basicity were crucial for the enhanced catalytic performances [93].
3.3. Hydrogenation of Levulinic Acid
Levulinic acid (LA) is an important biomass platform chemical which can be pro-
duced by the acid−catalyzed hydrolysis of lignocellulose [94]. One of the most important
downstream chemicals from LA is gamma−valerolactone (GVL), a stable and nontoxic
lactone that has broad applications and could be further converted into valuable com-
pounds [95]. For example, GVL could be transformed into 1,4−pentanediol (1,4−PDO) by
a hydrogenative ring−opening reaction, and 1,4−PDO could go through an intermolecular
dehydration to form 2−methyltetrahydrofuran (2MTHF) [26,96]. Two mechanisms for LA
hydrogenation into GVL were proposed based on literature research. In the first mecha-
nism [97], LA was initially hydrogenated to form 4−hydroxypentanoic acid on metallic
sites, followed by the lactonization to yield GVL at acid or basic sites. The second mecha-
nism [98] started with an isomerization of LA to the enol form, followed by the lactoniza-
tion to produce angelica lactone, and ends with hydrogenation to produce GVL. Some
studies stated that the hydrogenation was the rate−determining step, while the lactoniza-
tion reaction proceeded rapidly [99]. High hydrogen pressure (0.5–3 MPa) is usually nec-
essary for high conversion of LA, so that the sufficient amount of dissolved hydrogen in
solvent enables LA hydrogenation [100]. Many catalysts based on LDHs or MMOs were
reported to be efficient for converting LA to GVL with high yields. Based on the published
research, it seems that MMO catalysts based on Cu, Ni or Co showed comparably high
selectivity toward GVL from LA. For example, CuAl−MMO [82], NiAl−MMO [101,102]
and CoAl−MMO [103] catalysts were synthesized by different research groups and all
showed comparably high selectivity (>85%) toward GVL. Analogously, CoMoAl [104],
NiMgAl [101] and CuMgAl [105] catalysts synthesized by calcination of LDH precursors
also showed dominant GVL production. Noble metal supported on LDH or MMO mate-
rials were also efficient for LA hydrogenation. Notably, MMOs−supported noble metal
catalysts with a much lower reaction temperature (40–80 °C) were reported recently, com-
pared to a reaction temperature of 140–260 °C used by most non−noble metal catalysts. Pt
supported on MgAl−MMO demonstrated a nearly complete LA conversion into GVL at
40 °C and 50 bar [106], while Ru supported on MgAl−MMO or MgLa−MMO also showed
a GVL selectivity higher than 99% at 80 °C and 5 bar [107].
Although high selectivity toward GVL from LA was achieved by many catalysts, a
major concern in LA hydrogenation catalysts that should be dealt with is catalyst deacti-
vation. Several mechanisms were proposed, including particle sintering, coke deposition,
or metal leaching [108,109]. Because LA conversion into GVL produces water in lactoni-
zation, it is probable that MgO and Al2O3 may undergo phase transition in the hydrother-
mal environment, with brucite MgO or γ−Al2O3 transformed into periclase or boehmite,
respectively, leading to undesired changes in pore structure and surface property [110–
112]. Deactivation of LDH or MMO catalysts in LA hydrogenation was also observed and
studied for its mechanisms. Hu’s research reported Ni leaching in NiMgAl−MMO cata-
lysts in LA hydrogenation, mainly due to the detrimental effect of water to Mg or Al lead-
ing to structure collapse [101]. Hussain et al. observed significant decreases of LA conver-
sion in a time−on−stream study of MgAl−MMO. After characterization on spent catalysts,
it is confirmed that in addition to the coke deposition on catalyst surface, the MgAl−MMO
catalyst adsorbed water to regain the layered structure due to the memory effect, though
this deactivation was largely reversible by calcination at 450 °C in air [113]. Considering
the structure destruction during the reaction, metal particle sintering becomes inevitable,
as observed with Cu/MgAl−MMO catalysts that Cu particles sized increased from 2 to 5.3

Catalysts 2022, 12, 1484 9 of 32
nm after LA hydrogenation at 260 °C [114]. Therefore, the stability of LDH or MMO cata-
lysts for LA hydrogenation is an unavoidable issue before the commercialization of the
catalytic process.
3.4. Catalytic Hydrogenation of Monosaccharides
The conversion of biomass feedstocks to fuels or chemicals is an important approach
to generate renewable energy resources. Lignocellulose depolymerization or fermentation
could produce monosaccharides or sugar alcohols, which could be further transformed to
various types of chemicals [115]. For example, cellulose or hemicellulose could be hydro-
lyzed into C5 and C6 sugar alcohols such as glucose, xylose, and arabinose. These sugar
alcohols were listed by the US Department of Energy among the top 12 sugar−derived
building block chemicals, which are stable and versatile to upgrade to commodity chem-
icals [116]. Yamaguchi et al. prepared hydrotalcite supported nickel phosphide with high
activity and stability for the hydrogenation of glucose [117], xylose [118], and maltose
[119] under mild condition. The authors indicated that the hydrotalcite support activated
the carbonyl group of the saccharide molecules and donated electrons to Ni, while molec-
ular hydrogen was activated by unsaturated Ni sites from Ni2P. Another study on sugar
hydrogenation using a physical mixture of PtSn/Al2O3 and MgAl−MMO proposed that
the addition of MgAl−MMO could provide an alkaline environment in the reaction system
to facilitate ring opening reactions of sugar molecules [120], which is a key step for car-
bonyl hydrogenation [121]. While supported LDH catalysts showed synergistic effects be-
tween support and active metal, LDH−derived MMOs were also active in sugar hydro-
genation. Wu et al. have shown that reduced CuNiAl and CuNiAlM (M = Mg, Co, Cr and
Fe) catalysts were effective for the hydrogenation of glucose and fructose into sorbitol and
mannitol [122,123]. Pérez−Ramírez et al. also reported HT−derived CuNiAl catalysts
could catalyze transfer hydrogenation of these sugars to the corresponding polyols [124].
In both research, characterization results indicated that metallic Cu and Ni were formed
in the reduced catalysts, which could act as active sites for hydrogenation.
4. Hydrogenation of Carbon−Carbon Unsaturated Bonds
4.1. Partial Hydrogenation of Alkynes
The production of ethylene in oil refinery is often accompanied with acetylene as the
byproduct. Because acetylene could poison Ziegler–Natta catalysts and significantly de-
teriorate the quality of polyethylene, a partial hydrogenation process is needed to elimi-
nate ethylene and not overhydrogenate ethylene to ethane [125]. Acetylene hydrogenation
is considered to be a structure−sensitive reaction in which catalyst composition plays a
vital role in product selectivity [126]. Ma et al. obtained a Pd/MgAl−LDHs/Al2O3 catalyst
by in situ synthesis of Pd/MgAl−LDHs on spherical Al2O3 surface, and another
Pd/MgO−Al2O3 catalyst by calcination and reduction of synthesized
Pd/MgAl−LDHs/Al2O3 [127]. The results indicated that both catalysts had larger surface
area, lower surface acidity, uniform Pd particle size, and strong metal−support interac-
tion, leading to higher catalytic activity and selectivity than impregnated Pd/Al2O3 [128].
The high dispersion of Pd on the HT surface is presumably originated from the acidic sites
on the support surface which weakened the electron density of Pd, favored the formation
of low−coordinated Pd sites and enhanced dispersion [129,130]. Because Ag could effec-
tively inhibit the formation of coke and improve the activity and stability of the catalyst
[125], PdAg bimetallic catalysts supported on LDH−derived MMOs were also studied for
acetylene hydrogenation. Research on PdAg supported on LDH−derived ZnO−Al2O3
found that compared with Al2O3, ZnO−Al2O3−MMO could significantly inhibit the over-
hydrogenation of acetylene and decrease oligomer formation [131]. Research on PdAg
supported by NiTi−MMOs for acetylene hydrogenation found that a large number of Ti3+
defective sites existed on the NiTi support, acting as active centers to activate hydrogen
and increasing the electron density Pd, thus promoting the desorption of ethylene and

Catalysts 2022, 12, 1484 10 of 32
improving reaction selectivity [132]. When switching NiTi to MgTi as the PdAg support,
the electronic effect between Ti3+ and Pd was still present, combined with the acid–base
property of the MgTi support, contributing to nearly 100% conversion and 83.8% selectiv-
ity to ethylene at 70 °C [133].
In addition to being used as a catalyst support, LDH materials after calcination and
reduction can also be used directly as acetylene hydrogenation catalysts. Rives et al.
[134,135] prepared a series of NiZnAlCr hydrotalcite−like catalysts for the selective hy-
drogenation of acetylene. The introduction of Cr inhibited the formation of coke, while
the addition of Zn enhanced the metal–support interaction and improved the dispersion
of Ni. Liu et al. [136] prepared CuNiMgAl nanoalloy catalysts for acetylene partial hydro-
genation using LDHs as the precursor. High metal dispersion and structural homogeneity
of NiCu nanocrystals were observed, and this LDH−derived CuNiMgAl showed better
selectivity, anti−coking ability and stability than the CuNi/MgAl−HT catalysts prepared
by the impregnation method. The addition of iron in Cu−based MMOs could further im-
prove selectivity and reduce oligomerization, because iron served as a structural promoter
to disperse active metals [137]. Moreover, in another research investigating
Cu/FeyMgOx−type catalysts derived from LDHs, iron is proposed to facilitate the for-
mation of bifunctional interfacial active site Cuδ−Fe0.16δ+MgOx, which is effective in activat-
ing acetylene and hydrogen and promoting the desorption of ethylene [138].
Similar to acetylene partial hydrogenation, the partial hydrogenation of phenylacety-
lene to styrene for the elimination of phenylacetylene is an important pretreatment step
in styrene polymerization process. Several LDH−based catalysts have shown excellent
performance in this reaction. Pd/HT catalysts were synthesized for the phenylacetylene
hydrogenation, and it is proposed that the layered structure of HT may impose steric re-
striction on reactants, making Pd/HT more stereoselective in the product than the conven-
tional supported Pd catalysts [139]. Duan’s group presented a novel series of nickel phos-
phide catalysts from LDH precursors with high selectivity toward styrene. It is claimed
that phosphorus incorporation increased the Ni–Ni bond length and decreased Ni elec-
tron density, leading to the desorption of styrene so that further hydrogenation was
avoided [140].
4.2. Hydrogenation of Aromatic Ring
Aromatic compounds in transportation fuels are responsible for reduced cetane
number and increased particulate emission, as well as potential health hazard accompa-
nied by the emission [141–143]. Strict legislative restrictions on aromatic content all over
the world encourage research efforts on aromatic rings hydrogenation as an effective strat-
egy to eliminate aromatic compounds in fuels [144]. The urgent need for lignin upgrade
in recent years also inspires research on efficient aromatic hydrogenation [145]. Bai et al.
investigated the hydrogenation of phenol over a series of Ni−based MMO catalysts and
over 90% selectivity of cyclohexanol was obtained at 110–150 °C [146–148]. It is pointed
out that the strong metal–support interaction of MMO catalysts induced high metal dis-
persion and prevented agglomeration or sintering. Strong metal–support interaction was
also important in aromatic hydrogenation over Pd supported by MMOs [149]. Highly dis-
persed Pd particles were formed on CoCeAl−MMO surface with a particle size smaller
than 4 nm, providing plentiful active sites for H2 dissociative adsorption and ring hydro-
genation. The addition of Ce generated abundant oxygen vacancies, which activated phe-
nol by forming phenoxy and accelerated the hydrogenation reaction. Reaction tempera-
ture and pressure also significantly affected hydrogenation activity, as relatively high
temperature and high pressure were more preferable for benzyl ring hydrogenation prod-
ucts [150,151].

Catalysts 2022, 12, 1484 11 of 32
5. Hydrogenolysis of Oxygenated Compounds
5.1. Hydrogenolysis of Esters
Ester hydrogenation and hydrogenolysis are important reactions with particular in-
terest in biomass upgrade. Esters, including fatty acid esters, lactones, and levulinate es-
ters, could be either directly extracted or chemically converted from biomass feedstock.
Because esters themselves have limited uses in the chemical industry, hydrodeoxygena-
tion or hydrogenolysis is usually necessary to convert esters into valuable commodities
such as alcohols or alkanes. Triacylglycerols from non−edible plant oils or animal fats
could be converted into fatty acid esters by transesterification reactions. Due to their high
oxygen content and low heating value as fuels [152], researchers attempted to upgrade
fatty acid esters to diesel−range hydrocarbon (C15−C18) by hydrodeoxygenation. Cao et al.
utilized LDH−derived NiCuAl catalysts for the hydrogenolysis of soybean and waste
cooking oils, and more than 80% yield for diesel−range hydrocarbons was obtained at 260
°C and 3 MPa of H2 [153]. Lewis acid sites (Al3+) was thought to be active sites for decar-
bonylation by interacting with the oxygen atom and cleaving acyl C−O bond. Another
study [154] using NiGaMgAl to catalyze the hydrodeoxygenation of methyl laurate
(C11H23COOCH3) also present a 99% yield toward C11+C12 hydrocarbons at 400 °C and 3
MPa. The promotional effect of Ga to Ni suppressed the C−C bond hydrogenolysis, ac-
counting for the high hydrocarbon yield.
Converting esters to diols which could be used as monomers for polyester materials
is another attractive tactic with practical needs. Ethylene glycol, an important monomer
in the polymer industry, could be produced by an indirect pathway from syngas via di-
methyl oxalate. Wei’s group prepared CuZrMgAl−MMO catalysts for dimethyl oxalate
hydrogenation and obtained 99.5% yield of ethylene glycol under 180 °C and 2 MPa [155].
According to in situ characterization techniques, the authors attributed the high reactivity
to the Cu−O−Zr metal−support interfacial sites, where carbon–oxygen bonds were acti-
vated at Zr−related oxygen vacancies and hydrogenated at adjacent Cu sites. Li’s group
demonstrated that 1,4−pentanediol could be produced from bio−derived ethyl levulinate
by hydrodeoxygenation over CuCoAl−MMO catalysts, where Lewis acid sites such as
electrophile Cu+ or electron−deficient CoOx were considered to be crucial for carbonyl ad-
sorption and activation [25].
5.2. Hydrodeoxygenation of Lignin Derivatives
Lignin is a cross−linked three−dimensional amorphous polymer composed of aro-
matic units, accounting for 10–35 wt.% in lignocellulosic material [145,156]. The rapid de-
velopment of the biomass industry cogenerated more than 100 million tons of lignin an-
nually [157]. Various aromatic chemicals could be produced by lignin valorization, yet
only a small portion of lignin was actually utilized for chemical production. Lignin depol-
ymerization is a complex and challenging task, with the pivotal issue being the cleavage
of C−C bond and C−O bond. Hydrogenolysis is an effective way to break C−O linkage,
and various LDH−derived catalysts were demonstrated to be highly reactive for lignin
hydrogenolysis due to its high metal dispersion and adjustable acid–base properties.
Flower−like Ni2P−Al2O3 catalysts were obtained by reduction of red phosphorus with
NiAl−LDH precursors at 500 °C by Li’s group [158]. Ni2P−Al2O3 showed high alkane se-
lectivity in hydrogenolysis of poplar lignin oil. The high reactivity was attributed to the
high exposure of cantilevered conical Ni sites with strong C−O bond break capability and
plentiful acid sites to activate the substrate and H2, as shown in Figure 3. Other research
stated that acid sites, especially Lewis acid sites, facilitated deoxygenation of lignin deriv-
atives by adsorbing the electron−rich phenyl ring or oxygen atoms, promoting ring hy-
drogenation or deoxygenation of methoxy or carbonyl [159,160]. Oxygen vacancies are
also important for constructing high reactivity LDH−derived catalysts for lignin hydro-
genolysis. Reducible oxide such as CoOx [161] or CeOx [162] in MMO catalysts could form

Catalysts 2022, 12, 1484 12 of 32
surface oxygen vacancies to adsorb or activate oxygen−containing functional groups,
greatly benefiting lignin deoxygenation reaction.
Figure 3. Reaction mechanism of guaiacol hydrodeoxygenation over NiP catalysts [158]. (Copyright
2022, American Chemical Society).
Among the common linkages that exist in lignin, the most recurring linkage is the
β−O−4 linkage, constituting 50% of all linkages [145]. Therefore, model compounds with
β−O−4 linkages were often synthesized to investigate catalyst reactivity in lignin depoly-
merization [163–165]. Wang et al. prepared a NiMgAl−C composite catalyst using ligno-
sulfonate as the carbon precursor to impose electronic modifications to Ni [164]. The syn-
thesized catalysts showed promising results in terms of conversion and selectivity in hy-
drogenolysis of model compounds and real lignin. The lamellar structure of LDHs could
also be utilized to improve catalytic performance. Beckham and colleagues presented an
inspiring study, where HT−derived catalysts with intercalated nitrates produced phenol
monomers 2–3 times more than catalysts without nitrates in hydrogenolysis of model
compounds [166]. It is proposed that intercalated nitrates not only increased accessibility
to basic active sites but also directly participated in the hydrogenolysis process, as the
activity could be recovered after the nitrate reservoir was depleted and replenished.
5.3. Hydrodeoxygenation of Furfural
C5 polyols are important types of chemicals with huge demands in the polymer in-
dustry. Such polyols are traditionally produced from petroleum resources. Recently, re-
searchers have developed the catalytic transformation of furfural to various polyols via
hydrodeoxygenation. For furfural conversion to pentanediols (PDO), although early in
1930s Adkins and Connor presented Cu−Cr catalysts to convert furfuryl alcohol to pen-
tanediols [167], MMO catalysts for catalytic conversion of furfurals to pentanediols were
extensively studied in the last decade. Initially, Pt supported on CoAl−MMO [168] or
MgAl−HT [169] synthesized by two separate research groups showed high reactivity in
furfural hydrogenolysis toward 1,5−pentanediol and 1,2−pentanediol, respectively, at
110–140 °C and 1.5–3 MPa. The role of these supports is related to the basicity which might
change adsorption preference leading to ring−opening reactions. Subsequently, Huang’s
group noticed that CuMgAl−MMO [170] or CuAl−MMO [171] alone could also catalyze

Catalysts 2022, 12, 1484 13 of 32
the formation of pentanediols at a relatively high pressure of 6 MPa. The distinct differ-
ence in reactivity and selectivity between conventional impregnated Cu/Al2O3 catalyst
and coprecipitated CuAl−MMO catalyst demonstrated the advantage of the much smaller
Cu particle size of the MMO catalyst (1.9 nm) compared to the impregnated catalyst (16.7
nm) [171]. Another advantage of MMO catalysts is the partially reduced metal species on
the surface, as several studies presented positive correlations between partially reduced
metal species and pentanediol selectivity [24,172,173]. Based on published literature, the
mechanism of furfural conversion to pentanediols could be summarized, as shown in Fig-
ure 4. Furfural is first hydrogenated to produce furfuryl alcohol, and then furfuryl alcohol
will go through C−O bond scission to afford 1,2−pentanediol or 1,5−pentanediol depend-
ing on the catalyst property. Acid–base active sites [24,174–177] or partially reduced metal
species [172,173] are proposed to influence the adsorption configuration of furfuryl alco-
hol, resulting in the activation of different C−O bond and different pentanediol products.
Figure 4. Reaction mechanism of furfural hydrogenolysis over CuMgAl−MMO catalysts [170]. (Cop-
yright 2022, Royal Society of Chemistry).
5.4. 5−Hydroxymethylfurfural Hydrodeoxygenation
Cellulose could be transformed into 5−hydroxymethylfurfural (HMF), a platform
chemical with various downstream products [77]. Many important compounds can be
produced from HMF. For example, 2,5−dimethylfuran (DMF) by HMF hydrodeoxygena-
tion could be used as fuel additive due to its high−octane number and energy density
[178,179]. HMF could also be converted to 2,5−furandimethanol (FDM) [180,181] via hy-
drogenation of carbonyl group, to 2,5−tetrahydrofurandimethanol (THFDM) [180,182] via
furan ring hydrogenation of FDM, or to 2,5−dimethyltetrahydrofuran (DMTHF) [183–185]
via furan ring hydrogenation of DMF. FDM and THFDM could be used as building blocks
in polymer synthesis, and DMTHF is a potential fuel additive [77]. As shown in Table 3,
the selectivity toward DMF, FDM, or DMTHF could be controlled by choosing adequate
reaction conditions or metal catalysts [180,184].

Catalysts 2022, 12, 1484 14 of 32
Table 3. Catalytic performances of LDH−derived catalysts for 5−hydroxymethylfurfural hydrode-
oxygenation 1.
Catalyst Reaction Condition Conversion (%) Major Product Selectivity (%) Ref.
NiAl−MMO 80 °C; 2 MPa H2 96.0 THFDM, 74 [180]
Ru/MgAl−HT 220 °C; 1 MPa H2 100.0 DMF, 58 [181]
NiAl−MMO 180 °C; 1.2 MPa H2 100.0 DMTHF, 97.4 [182]
Ni−Cu/HT 90 °C; 1 MPa H2 99 DMF, 67 [185]
CoZnAl−MMO 130 °C; 0.7 MPa H2 >99.9 DMF, 74.2 [186]
CuCoNiAl−MMO 180 °C; 1 MPa H2, 99.8 DMF, 95.3 [187]
CuZnAl−MMO 180 °C; 1.2 MPa H2 100 DMF, 90.1 [188]
Co−N−C/NiAl−MMO 170 °C; 1.5 MPa H2 99.9 DMF, 100 [189]
NiZnAl 100 °C; 1.5 MPa H2 100 FDM, 98.2 [190]
Cu@Co/CoAlOx 180 °C; 1.2 MPa H2 100 DHTMF
,
83.6 [191]
CuCoCe−MMO 210 °C; 1.5 MPa H2 100 DMF, 96.5 [192]
NiCoAl−MMO 120 °C; 4 MPa H2 100 1,2,6−HTO, 64.5 [193]
Cu1.5Mg1.5Al 150 °C, 6 MPa H2 100 1,2−HDO, 40 [194]
1. THFDM, 2,5−tetrahydrofurandimethanol; FDM, 2,5−furandimethanol; DMF, 2,5−dimethylfuran;
DMTHF, 2,5−dimethyltetrahydrofuran; 1,2,6−HTO, 1,2,6−hexanetriol; 1,2−HDO, 1,2−hexanediol.
The transformation of HMF to afford DMF could be accomplished by various metal
catalysts. CuAl− [80], NiAl− [180,182], and CuNiAl−MMO [195] catalysts with high metal
dispersion showed excellent performance for the conversion of HMF to DMF, with DMF
yields higher than 90%. In contrast, CoAl showed relatively lower DMF selectivity
[186,187], likely due to its lower hydrogenation reactivity [196]. The conversion of HMF
into DMF requires multifunctional catalysts with dual active sites catalyzing C=O hydro-
genation and C−O hydrogenolysis. Wang et al. [188]. studied the HMF hydrodeoxygena-
tion over CuZnAl−MMO catalysts, claiming that Cu+ species acted as C−O cleavage sites
and Cu0 species acted as C=O adsorption sites. The synergy between Cu+ and Cu0 species
promoted the selective transformation of multi−functional groups of HMF molecules, thus
improving catalyst reactivity and selectivity. However, these active sites might be suscep-
tible to deactivation caused by changes in the metal chemical state [189]. This issue could
be tackled by the incorporation of Zn generating strong Cu−O−Zn interaction by charge
compensation, stabilizing active Cu+ species, influencing adsorption configuration and
preventing Cu agglomeration [186,190]. In another study on LDH−derived CuCoAl cata-
lysts, Wang et al. pointed out that interfacial sites played key roles in the formation of
different reaction intermediates in the catalytic hydrodeoxygenation of HMF [191]. C=O
bond in the reactant was hydrogenated by the oxygen vacancies from CoOx site to form
hydroxy groups, while C=C bond was hydrogenated by Cu−Co interface positions. A sim-
ilar interfacial effect is observed in reduced CuCoCe [192] and CoMgFe [197] catalysts as
well, where oxygen vacancies formed on CuCeOx or CoFeOx interface activated the C−O
bond and promoted C−O cleavage.
The hydrogenolysis of HMF to hexanediols or hexanetriols over LDH−derived cata-
lysts were also probed by researchers. NiCoAl−MMO [193] and Pt supported on
MgAl−MMO [198] were effective in converting HMF to 1,2,6−hexanetriol at 120–160 °C
and 3–4 MPa. A 64.5% yield was obtained with a 0.5Ni2.5CoAl catalyst at 120 °C and 4
MPa. 1,2−Hexanediol could be produced over CuMgAl−MMO catalysts at 150 °C and 4–
6 MPa [194]. The structure–activity relation could be inferred by two correlations reported
in Hu’s research [194], as shown in Figure 5: a positive correlation between basic sites and
catalyst turnover frequency (TOF), and a negative correlation between Cu particle size
and catalyst TOF. These observations hinted at the crucial role of Cu particle size and
basicity, which controlled the catalysts reactivity and selectivity in HMF hydrogenolysis,
analogous to furfural hydrogenolysis. Conceivably, HMF or its hydrodeoxygenated prod-
uct 5−methyl furfuryl alcohol were adsorbed on active sites in a tilted mode with C−O

Catalysts 2022, 12, 1484 15 of 32
bonds interacting with active sites, leading to the C−O bond cleavage to form polyols
[193,199].
Figure 5. Influence of the molar ratio of Cu/Mg on the (a) distribution of basic sites and (b) Cu
particle size. (c) Correlation between the Cu particle size and TOF value and (d) basic sites and TOF
value [194]. (Copyright 2022, American Chemical Society).
6. CO2 Hydrogenation
The chemical fixation of CO2 into value−added products is a pivotal process in CO2
utilization to reduce CO2 emission, decrease fossil fuel usage, and alleviate the greenhouse
effect. Numerous research on catalytic conversion of CO2 were conducted in recent years,
and exciting progress was made for the hydrogenation of CO2 into various chemicals in-
cluding methane, methanol, carbonates, carboxylic acid, etc. [200,201]. Compared to reg-
ular hydrogenation catalysts, unique advantages in adsorption capacity of LDH−derived
catalysts for CO2 hydrogenation are recognized to further promote catalyst performance
through adsorption enhancement [202]. Because a number of reviews were published in
the past few years regarding the broad CO2 hydrogenation field [9,201,203,204] or specific
CO2 hydrogenation reaction [205], the goal of this review is to analyze the applications of
LDH or MMO catalysts in CO2 hydrogenation, focusing on CO2 thermocatalytic conver-
sion into methanol and methane.
6.1. CO2 Conversion to Methanol
The CO2 hydrogenation to methanol is a promising strategy for CO2 utilization, as
methanol is a key feedstock that can be industrially converted to a wide range of chemicals
such as light olefins and gasoline [206,207]. For example, the methanol−to−olefin (MTO)
process by Dalian Institute of Chemical Physics, China, has been commercialized with an
annual production of 7.16 Mt per year by the end of 2018 [208]. Methanol could be pro-
duced from syngas (CO2/CO/H2) using an industrial Cu/ZnO/Al2O3 catalysts synthesized
from zincian malachite, (Cu,Zn)2(OH)2CO3, by the coprecipitation of metal nitrates (Cu,
Zn and Al) [209]. It has been conclusively demonstrated that the superior reactivity of
Cu/ZnO/Al2O3 stemmed from the structure defects of the Cu surface and the promotional
effect of Zn to Cu [210]. Owing to the advantages of the MMO catalysts mentioned above,

Catalysts 2022, 12, 1484 16 of 32
research efforts were devoted to preparing CuZnAl−based catalysts from LDH precursors
aiming to obtain catalysts more reactive than the industrial Cu/ZnO/Al2O3 catalyst. Beh-
rens and coworkers presented CuZnAl−MMO catalysts through the coprecipitation of ni-
trate solution at 25 °C and a constant pH value of 8 ± 0.7. The synthesized CuZnAl was
intrinsically more active in CO2 hydrogenation than the industrial CuZnAl catalyst when
normalized by Cu surface area, due to the highly dispersed Cu particles (7 nm) and strong
interfacial interaction between embedded Cu particles and ZnAl2O4 matrix [211].
To further improve the catalyst reactivity of CuZnAl catalysts, the addition of fourth
metal elements was explored in CO2 hydrogenation to methanol. A wide range of metals
have been added to CuZnAl by coprecipitation of metal precursors, with different metals
exhibiting different modification effects. The addition of zirconium to CuZnAl could in-
crease the basicity, strengthen the adsorption of CO2 and promote the formation of meth-
anol [212,213]. Similar effects were observed for Mn, La, Ce and Y, as a series of CuZnAlX
(X = Mn, La, Ce, Zr and Y) catalysts were synthesized in a study from Sun’s group, and
modified CuZnAlX catalysts showed a higher portion of strong basic sites, CO2 conver-
sion and methanol selectivity than the unmodified CuZnAl [214]. Yttrium modified CuZ-
nAl possessed a higher surface area and more dispersed Cu particles, resulting in a higher
methanol yield per gram of catalysts than the unmodified CuZnAl [215]. The addition of
indium to CuZnAl increased the methanol selectivity by inhibiting CO formation, yet de-
creased TOF of methanol formation [216].
Modifications on synthesis procedures were also attempted to induce structural
transformation of the CuZnAl−based MMO catalysts. Different alumina sources [217,218],
Zn precursors [219,220], phase of precipitated precursors [221], atomic ratios [222–224],
sequence of precursor addition [225,226], calcination temperature [227], precipitation pH
[228] and precipitation agents [229] in synthesis procedures of MMO catalysts commonly
resulted in diverse catalytic activity, selectivity or stability for CO2 hydrogenation. These
phenomena were often attributed to the difference in acid–base properties, exposed Cu
surface area, particle size, phase composition, etc. However, due to the structural com-
plexity of active sites in methanol synthesis [210,230], it is not easy to acquire a direct
correlation between catalyst physiochemical properties and catalytic performance. More
in−depth research on catalyst structure, especially in situ characterizations, is essential to
unveil the structure–activity relationship for CuZnAl catalysts.
For the industrial Cu/ZnO/Al2O3 catalyst, the role of alumina is considered to be a
structural promoter that prevents Cu particles from aggregation [231]. Kühl et al. con-
ducted experiments to partially replace Al with trivalent cations Cr and Ga [232]. The
substitution of Al by Cr reduced the interaction between Cu and oxide matrix, causing Cu
particle growth during CO2 hydrogenation. In contrast, the substitution of Al by Ga turns
out to be rather effective in improving catalyst activity, and this improvement was as-
cribed to the fact that Ga addition could stabilize Cu phase and benefit the catalytic activ-
ity of Cu particles. Other research presented consistent conclusions. CuZnGa−MMO cat-
alysts showed higher Cu surface area and dispersion than CuZnAl using the aqueous
miscible organic solvent treatment (AMOST) method, which involves an additional step
of treating LDHs precipitates with an organic solvent [233,234].
6.2. CO2 Conversion to Methane
The conversion of CO2 to methane, namely CO2 methanation, is also known as the
Sabatier reaction first reported in 1897. In the current age of pursuing carbon neutrality,
Sabatier reaction is experiencing its renaissance, as CO2 methanation becomes a conven-
ient strategy to convert CO2 to a widely used energy resource in a sustainable manner.
Two types of reaction mechanisms for CO2 methanation were proposed. The first mecha-
nism presumes the direct methanation of CO2 to methane via formate, carbonate or bicar-
bonate, while the second mechanism requires the conversion of CO2 to CO before CO is
converted to CH4 [235–237]. The actual mechanism may vary depending on the catalyst

Catalysts 2022, 12, 1484 17 of 32
type, reaction temperature, etc. As CuZn−based catalysts constitute the majority of cata-
lysts for methanol synthesis, Ni−based catalysts represent the most important category of
CO2 methanation catalysts [236]. Another important factor for the catalyst of methane for-
mation is the amount and strength of basic sites [238,239]. In this respect, MMO catalysts
derived from MgAl−LDHs were studied comprehensively due to its advantageous capa-
bility of combining tunable basic sites with dispersed metal particles.
The strong dependence of CO2 conversion to catalyst basicity, especially medium
strength basic sites, was reported by many researchers, as indicated by the positive corre-
lation between medium strength basic sites and CO2 conversion or TOF [240–242]. Basic
sites, such as OH− and O2−, could adsorb and activate CO2 to form carbonate or bicarbonate
species [243,244]. It is noticed that increased basicity was related to the increased amount
of bicarbonate−like species for NiMgAl−MMO catalysts, while bicarbonate is the main
CO2 adsorption product and an important intermediate for methane formation at low
temperatures [245]. It is also speculated that higher basicity promotes the hydrogenation
rate of surface−adsorbed carbonates [246]. Moreover, medium strength basic sites pro-
mote monodentate carbonates formation, as monodentate is more readily hydrogenated
to form CH4 than bidentate carbonates [238,245]. The introduction of certain metal pro-
moters is a facile way to manipulate MMO catalyst basicity. Metals including V [247], Y
[240], Fe [246,248], Mn [242], Zr [249], Ce [250], La [251–253] and Cu [241] have been
demonstrated to result in increased concentration of basic sites. Apart from changes in
basicity, metal promoters could also adjust the metal–support interaction and increase Ni
dispersion. Ho and colleagues [253] claimed that CO2 interacted more strongly with
NiLaAl than NiAl due to higher basicity and more dispersed Ni particles, leading to a
much higher low−temperature activity in CO2 methanation of NiLaAl. In situ characteri-
zation studies suggested that both dissociative activation (forming CO) and associative
activation (forming bicarbonate and carbonate) of CO2 were simultaneously observed for
NiLaAl, while for NiAl, CO2 was successively activated by the dissociative pathway and
then associative.
Oxygen vacancies are also proposed as a crucial factor for CO2 methanation, as
shown in Figure 6 [250]. Oxygen vacancies could act as Lewis acid sites to interact with
electron−rich oxygen atoms from CO2 for CO2 chemisorption and activation [254]. DFT
calculations confirmed that oxygen vacancies on simulated the NiCeAl surface contrib-
uted to the lower CO2 adsorption energy than the NiAl surface [255]. Oxygen vacancies
also facilitate the formation of active oxygen species, which could react with CO2 to pro-
duce monodentate or bidentate carbonates, accelerating the methanation reaction
[249,250]. He et al. confirmed that NiZrAl ternary MMO catalysts showed more oxygen
vacancies and basic sites than NiAl binary counterpart.
Figure 6. The role of oxygen vacancies in CO2 methanation via (a) formate pathway and (b) CO
pathway [250]. (Copyright 2022, Elsevier).

Catalysts 2022, 12, 1484 18 of 32
7. Hydrogenation in C−C Coupling Processes
In recent years, the demand for bio−derived fuels or chemicals stimulates extensive
interests in converting biomass feedstock into various types of renewable fuels. Regarding
the fact that most biomass platform chemicals consist of oxygen−containing functional
groups such as hydroxy or carbonyl groups, C−C coupling or C−C bond formation via
aldol condensation, dehydration/hydrodeoxygenation, and hydrogenation are effective
strategies to produce downstream chemicals, especially in the synthesis of long chain hy-
drocarbons such as jet fuel in the range of C8−C16. Regarding the advantages of LDH−de-
rived MMOs simultaneously possessing metallic, acidic and basic sites, they become ideal
candidates either as catalyst supports or as the catalyst themselves to achieve the cascade
hydroconversion of C−C coupling for biomass upgrade. In some cases [256–262], C−C cou-
pling to produce chemicals was accomplished by a two−stage process, with the first stage
aimed at aldol condensation and dehydration using HT or MMO catalysts, and the second
stage aimed at hydrogenation using non−HT and non−MMO catalysts. These studies,
which did not use HT or MMO as hydrogenation catalysts, will not be discussed in this
review. The following section will examine literature work on HT− or MMO−catalyzed
hydrogenation in the C−C coupling processes.
Acetone, an inexpensive byproduct in lignocellulose fermentation, could undergo al-
dol condensation with itself or other chemicals to produce products with longer chains.
The transformation of acetone to methyl isobutyl ketone (MIBK), an important chemical
with extensive applications, is a typical example of the multifunctionality of LDH−derived
materials. MIBK could be converted from acetone via three consecutive steps: aldol con-
densation catalyzed by bases, dehydration catalyzed by acids, and hydrogenation cata-
lyzed by metals [263]. Regarding the advantages of LDH−derived MMOs, they are cata-
lytically active as supports (Pd on ZrCr [263], MgAl [264,265] and CoAl [266]) or catalysts
themselves (NiMgAl [267] and CuAl [268]) to achieve the one−pot conversion of acetone
to MIBK. Because the sequence of the three steps are crucial for MIBK production from
acetone, the concentration and the strength of acidic, basic, and metallic sites are expected
to be balanced in order to minimize side products [264]. In order to obtain the optimal
catalysts, catalysts with different concentrations of acid or base sites were prepared and
tested. According to experimental results, it is generally believed that excessively high
acidity or basicity would lead to over−condensation products and reduce MIBK yields
[264,267].
Cyclopentanone (CPO, C5H8O), a bio−derived chemical produced by aqueous phase
furfural rearrangement, could be transformed to jet−fuel range cycloalkanes by C−C cou-
pling reactions. Cai and coworkers synthesized a bifunctional Ni/MgAl/active carbon cat-
alyst using a MgAl−HT precursor in an integrated C−C coupling/hydrogenation process
for the conversion of CPO with over 80% yield toward C10 or C15 alkanes [269,270]. The
authors ascribed the remarkable performance to the enhanced strength of the basic sites
of MgAl−MMO, promoting trimerization of CPO more effectively than conventional ox-
ide support such as MgO, Al2O3, NaY or TiO2, as well as the strong hydrodeoxygenation
activity of Ni than Fe, Co or Cu. Another research combining MgAl−MMO with Pd in
CPO trimerization also obtained high trimer yield [271]. In ad dition t o CPO, C−C coupling
of other ketones were also attempted to produce elongated alkanes using LDH or MMO
catalysts. Sheng et al. performed MIBK self−condensation over Pd−modified MgAl−HT to
produce dodecanol, with MgAl−HT support acting as a base catalyst for self−aldol con-
densation [272].
Alcohols could also be used for C−C coupling after dehydrogenation to form alde-
hydes or ketones, followed by aldol condensation, dehydration and hydrogenation. For
these processes, metallic sites on MMO catalysts could catalyze the dehydrogenation of
the initial alcohol and the hydrogenation of the unsaturated ketones, while acid–base sites
catalyze condensation and dehydration reaction. A few alcohols were studied for their
C−C coupling applications, including 2−hexanol [273], propanol [274], octanol [275] and

Catalysts 2022, 12, 1484 19 of 32
ethanol [276,277]. It is commonly believed in these studies that numbers of metal, acid,
and base sites should be delicately designed to prevent side reactions and by−products.
8. Hydrogenation of Nitrites and Nitriles
8.1. Hydrogenation of Organic Nitrites
The selective hydrogenation of aromatic nitro compounds to produce anilines is im-
portant in synthetic chemistry. The major challenges in this process are to selectively re-
duce nitro groups and not affect other functional groups. Various types of noble or
non−noble metal catalysts were developed to achieve this goal [278]. For catalysts with
noble metals such as Pd or Pt, key factors that determine their catalytic performance are
metal dispersion and the resistance to particle agglomeration. LDHs or MMOs, which ex-
hibit hierarchical structure able to immobilize active species, stand out as desirable cata-
lyst support. Wang et al. showed that Pd supported on MgAl−LDHs demonstrated higher
catalytic activity and selectivity than Pd/SiO2 or Pd/C in substituted nitrobenzene hydro-
genation, likely due to the confining effect of metallic Pd in layers of LDHs [279]. Similar
conclusions were reached in another study studying Pd supported on MgAl−MMOs,
MgO and γ−Al2O3 in nitrobenzene hydrogenation [48]. Results showed that Pd/MMOs
were superior to the latter two in terms of turnover frequency and stability, and authors
ascribed the superiority to the high dispersion of Pd particles on MMOs and the anchoring
effect of MMOs to prevent Pd aggregation. Non−noble MMO catalysts such as CuZnAl
also showed high activity in the hydrogenation of nitro groups [280]. The activity of CuZ-
nAl was attributed to the Cu−ZnOx active sites, similar to the one used in CO2 conversion,
as mentioned above.
A more challenging case in aromatic nitro compound hydrogenation is the selective
hydrogenation of nitro groups when other readily reducible groups are presented, as in
the case of nitrostyrene hydrogenation to yield vinylaniline. Corma and Serna have re-
ported a breakthrough by using Au/TiO2 and Au/Fe2O3 to hydrogenate 3−nitrostyrene to
selectively produce 3−vinylaniline [281]. Recently, a series of gold catalysts supported on
hydrotalcites also showed promising activity. Zhang’s group discovered that thiolated
Au25 nanoclusters supported on calcined ZnAl−HT showed complete 3−nitrostyrene con-
version in a wide temperature window from 90 to 135 °C with 3−vinylaniline selectivity
higher than 98%. The high selectivity was attributed to the ZnAl−HT support adsorbing
nitro groups rather than vinyl groups [282]. Follow−up research comparing MgAl−,
ZnAl−, and NiAl−HT as catalyst support observed that the amount of basic sites were in
the order of MgAl > NiAl > ZnAl, while the 3−vinylaniline yield followed the order of
MgAl < NiAl < ZnAl, suggesting that medium to weak basicity of the HT surface might
benefit the reactivity [283]. Because nitro groups were generally believed to readily adsorb
on basic surface, the role of calcined HT is speculated to be adsorption sites for nitro
groups, in synergy with the gold particles which activate hydrogen. Nonetheless, the
choice of HT support used for nitrostyrene hydrogenation catalysts still request special
attention, as another research indicated that the selectivity toward vinyl hydrogenation
could be enhanced by changing HT composition [284].
8.2. Hydrogenation of Nitriles
Hydrogenation of nitrile is an important industrial route for amines production. To
inhibit byproducts such as secondary or tertiary imines, bases such as alkali bases or am-
monia are commonly added along with metal catalysts [285]. The inherent basicity of
MgAl−MMO materials makes them potential candidates for the hydrogenation of nitriles
to produce amines. Tichit and coworkers prepared a series of Ni−containing catalysts
based on LDH precursors for catalytic hydrogenation of nitriles. They discovered that the
introduction of Mg in LDH precursors was effective in decreasing the surface acidity of
the catalyst, decreasing the adsorption strength of primary amines, and thus preventing
further coupling reaction between amines and imines. [286,287]. Cao et al. formulated a

Catalysts 2022, 12, 1484 20 of 32
core–shell catalyst by coating nickel−based nanocomposites with LDHs or layered double
oxides (LDOs), to catalyze the selective hydrogenation of benzonitrile to N−benzylaniline
or benzylmethylamine. It was found that the structural and acid−base properties of the
LDH/LDO−coated nanocomposites could be switched by calcination or hydration accord-
ing to the memory effect of LDHs [288]. Their following work on LDH−derived NiM-
gAl−MMO catalysts for the hydrogenation of benzonitrile also showed that the presence
of strong metal–support interactions in NiMgAl catalysts could effectively inhibit the
leaching or aggregation of Ni nanoparticles, accounting for the excellent stability of NiM-
gAl−MMO than the impregnated Ni/Mg0.75Al0.25 catalysts [289]. The strong metal–support
interaction was also observed in another study, in which a highly stable and active Co2P
supported on HT was prepared and studied for the ammonia−free selective hydrogena-
tion of various nitriles to corresponding primary amines [290]. Compared to Al2O3, SiO2,
or carbon support, HT support stabilized Co species and prevented their oxidation, lead-
ing to enhanced reactivity and stability.
9. Future and Prospect of LDH−Derived Catalysts for Hydrogenation and Hydrogen-
olysis
Hydrogenation and hydrogenolysis are important reactions particularly in the pet-
rochemical industry, biomass upgrade and CO2 conversion. In the past decade, research-
ers have carried out extensive research on the application of LDH−derived catalysts in
hydrogenation and hydrogenolysis. As reported in numerous studies mentioned above,
superior reactivity of LDHs and their derived catalysts undoubtedly demonstrates out-
standing advantages over conventional catalysts in the following aspects:
(1) Various facile preparation methods, including co−precipitation, hydrothermal meth-
ods, ion exchange, urea hydrolysis, etc., could be used to prepare LDH−derived cat-
alysts. These preparation methods are generally mature and well−established, facili-
tating the wide application and scale−up production of LDH materials.
(2) The appropriate amount and strength of acidity/basicity are crucial for the design of
multifunctional catalysts, and these properties could be achieved with LDH−derived
materials. LDHs and MMOs possess high concentrations of acid/base sites, which are
key sites for adsorption and reaction. For example, as discussed in this review, acidic
sites could catalyze deoxygenation reactions, while basic sites could interact with
carbonyl groups or catalyze aldol condensation. More importantly, the acid–base
properties of LDHs and their derivative materials can be finely modulated by con-
trolling the main layer element composition, interlayer ion species of the LDH pre-
cursor, and other synthesis parameters.
(3) Metal particle size is crucial for the reactivity and selectivity of metal catalysts. In
hydrogenation or hydrogenolysis reactions, metal catalysts with high metal disper-
sion will provide more active sites for dissociative activation of hydrogen and surface
reaction to take place, resulting in increased reaction rates and decreased catalyst us-
ages. For LDH−supported metal catalysts, hydroxy groups and interlayer galleries
could promote the dispersion of metal particles. For MMO or IMC catalysts, because
metal cations are uniformly distributed in the atomic level within LDH layers, the
calcination of LDH precursors would generate MMOs with highly dispersed metal
atoms. Therefore, LDH−derived materials become ideal catalysts for hydrogenation
or hydrogenolysis due to their capability of generating small metal particles.
(4) For MMO or IMC catalysts, when LDHs are calcined at high temperatures, active
metal atoms are immobilized in the metal oxide matrix. This interaction between ac-
tive metals and oxide matrix results in the formation of strong metal–support inter-
actions (SMSI). SMSI is important not only in effectively preventing particle aggre-
gation or sintering of the active metal during reaction but also modifying the elec-
tronic properties and catalytic reactivity of active metals.

Catalysts 2022, 12, 1484 21 of 32
Although much progress was made in the field of hydrogenation or hydrogenolysis
catalysis over LDH materials, several challenges remained as follows:
(1) The synthesis procedures of LDHs were studied thoroughly, but for LDH−derived
catalysts more optimization and mechanism research on the preparation methods are
still necessary. For example, for MMO catalysts, the relationship between prepara-
tion parameters (e.g., metal precursors, precipitation pH, crystallization time, calci-
nation temperature, reduction temperature, etc.) and physiochemical properties is
still vague or case−dependent.
(2) The structure of LDH−derived catalysts is also currently unclear, which deserves
more characterization efforts or theoretical predictions. The surface composition of
LDH−derived catalysts, the electronic and geometric interactions between metal na-
noparticles and neighboring components, and the origin of acidity/basicity of
LDH−derived materials, are largely unknown, leading to difficulties in studying
structure–reactivity relationships and catalyst design.
(3) LDHs show a unique “memory effect” and good reversible topological conversion
properties after heat treatment over a wide range of temperatures. Accordingly, how
LDH−derived catalysts go through structure transformation during the reaction pro-
cess also needs in−depth research. For biomass−related hydrogenation or hydrogen-
olysis research, this issue is important because water is often present either as reac-
tant or product, which might lead to topological transformation of LDH derivates.
More research on structure transformation of LDH−derived catalysts before, during,
or after reaction will be valuable for a broader application of LDH−derived catalysts.
10. Conclusions
In this review, the applications of LDH−derived materials in the field of catalytic hy-
drogenation and hydrogenolysis are comprehensively summarized. LDH−derived cata-
lysts in hydrogenation and hydrogenolysis could be categorized into three types:
LDH−supported catalysts, mixed metal oxides, and intermetallic compounds, while dif-
ferent types of catalysts exhibit different structure and physiochemical properties. The
tunability in composition and property of these LDH−derived materials makes them ver-
satile in many applications. As demonstrated by numerous examples mentioned above,
LDH−derived catalysts showed superior reactivity owing to their unique advantages.
Therefore, based on the current research progress, it could be envisaged that research on
hydrogenation or hydrogenolysis over LDH−derived catalysts will keep on increasing
rapidly in the future, and industrial applications of LDH−derived catalysts will also be
expanding due to their desirable characteristics.
Author Contributions: Z.W.: Investigation, Writing—Original Draft, W.Z.: Validation, Supervision.
C.L.: Writing—Review and Editing. C.Z.: Conceptualization, Writing—Review and Editing, Fund-
ing acquisition. All authors have read and agreed to the published version of the manuscript.
Funding: The authors would like to acknowledge funding support provided by National Natural
Science Foundation of China (No. 21905027) and Beijing Education Committee Science and Tech-
nology Project (No. KM202010017007).
Conflicts of Interest: The authors declare no conflict of interest.
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