Methanol as a clean and efficient H-transfer reactant for carbonyl reduction: Scope, limitations, and reaction mechanism

Article abstract of DOI:10.1016/j.jcat.2014.06.023

The previously unexplored use of methanol as a H-transfer agent for the Meerwein-Ponndorf-Verley reduction of aromatic aldehydes and aryl ketones is described. Furfural, 5-hydroxymethylfurfural, benzaldehyde, and acetophenone were selectively reduced to t

Full text of DOI:10.1016/j.jcat.2014.06.023

Journal of Catalysis 317 (2014) 206–219
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Methanol as a clean and efficient H-transfer reactant for carbonyl
reduction: Scope, limitations, and reaction mechanism
Thomas Pasini a,b, Alice Lolli a,b, Stefania Albonetti a,b, Fabrizio Cavani a,b, , Massimo Mella c,
⇑
⇑
a
Dipartimento di Chimica Industriale ‘‘Toso Montanari’’, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
b
Consorzio INSTM, Unità di Ricerca di Bologna, Florence, Italy
c
Dipartimento di Scienze ed Alta Tecnologia, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The previously unexplored use of methanol as a H-transfer agent for the Meerwein–Ponndorf–Verley
reduction of aromatic aldehydes and aryl ketones is described. Furfural, 5-hydroxymethylfurfural, benz-
aldehyde, and acetophenone were selectively reduced to the corresponding alcohols in mild conditions.
The reaction mechanism was elucidated by means of reactivity tests and DFT calculations. It was found to
include the highly efficient H-transfer with the formation of formaldehyde, which further reacted with
excess methanol to generate the adsorbed hemiacetal. In turn, the latter reduced carbonyl, with the
Received 11 March 2014
Revised 25 May 2014
Accepted 19 June 2014
Keywords:
Hydrogen transfer
Meerwein–Ponndorf–Verley reduction
MgO
4 2
formation of methylformate, which further decomposed into CO, CH , and CO . Compared to the alcohols
typically used for carbonyl reductions, methanol showed the advantage of producing gaseous
components as the only co-products, which are easily separated from the reaction medium. In the case
of furfural, a 100% yield to furfuryl alcohol was obtained, using the high-surface area MgO as the easily
recoverable and reusable catalyst.
5
-Hydroxymethylfurfural
Furfural
Methanol
Ó 2014 Elsevier Inc. All rights reserved.
1
. Introduction
The reduction of carbonyl groups using alcohols as hydrogen
still being sought with the aim of developing sustainable hydroge-
nation protocols.
Within this framework, several different solid catalysts have
been reported in the literature, mostly based on alkali and alkaline
earth oxides, as well as zeolites or mesoporous materials, sometimes
incorporating metal ions acting as Lewis acid sites. More specifically,
examples include the following: (a) MgO, either doped or as is, and
Mg/M mixed oxides, used for the reduction of substrates such as
citral [16,17], cyclohexanone [18–20], acrolein [21], acetophenone
[22,23], hexenone [24–26], mesityl oxide [27–30], acetone [31],
benzaldehyde [32,33], crotonaldehyde [34], furfural [35,36] and, in
general, various aliphatic aldehydes and ketones [37–43] or
aralkylketones [44,45], in most cases using isopropanol as the H-
transfer reactant, with a few exceptions in which ethanol [31,33],
sources, i.e. the Meerwein–Ponndorf–Verley (MPV) reaction, offers
an alternative approach to work under hydrogen pressure with
supported precious metal catalysts [1–8]. Using appropriate
conditions, the reaction can be highly chemoselective toward
carbonyl groups. The MPV is usually carried out through homoge-
neous catalysis by using Lewis acids such as Al, B, or Zr alkoxides
(
isopropoxide); other catalysts are based on precious metals such
as Pd, Rh, Ru, and Au. However, the MPV reaction usually requires
a large amount of alkoxide in order to obtain an acceptable yield,
and often, the procedure used for catalyst recovery ends up leading
to non-reusable compounds. Alternative homogeneous catalysts
have been sought, based on either non-precious metals, such
as Fe [9–12], or alkali metal ions, such as Li alkoxides [13,14] or
KOH [15]. Nevertheless, alternative catalysts, based on heteroge-
neous, inexpensive, easily available, and non-toxic materials, are
4 2
methanol [22], or other C alcohols [32] were used; (b) ZrO and
hydrous zirconia, either doped or as is [46–53], anchored/grafted
Zr over supports [54–57], and Zr-beta [58,59], for the reduction of
the same substrates as for MgO, again using isopropanol; (c) zeolites
such as H-beta [60–63] and alumina [64] using isopropanol, ethanol,
2
or cyclopentanol; (d) CuO- [65–69] and MnO -based [70] catalysts,
with cyclohexanol, 1,4-butandiol, or ethanol; and (e) Ti or Sn
incorporated in MCM or beta zeolite [71–74], with isopropanol or
⇑
Corresponding authors. Addresses: Dipartimento di Chimica Industriale ‘‘Toso
Montanari’’, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
F. Cavani), Dipartimento di Scienze ed Alta Tecnologia, Università degli Studi
(
2
-butanol as the reducing agents.
dell’Insubria, Via Valleggio 11, 22100 Como, Italy (M. Mella).
E-mail addresses: fabrizio.cavani@unibo.it (F. Cavani), massimo.mella@
uninsubria.it (M. Mella).
Among the various carbonyl-bearing substrates, furfural (FAL)
and 5-hydroxymethylfurfural (HMF) are important renewable
http://dx.doi.org/10.1016/j.jcat.2014.06.023
0
021-9517/Ó 2014 Elsevier Inc. All rights reserved.

T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
207
building blocks, as key precursors for the production of biofuels
and chemicals [75–79]. The upgrading of these molecules includes,
among others, the hydrogenation of carbonyl groups to the
corresponding aromatic alcohols, furfuryl alcohol (FFA), and
catalysis in organic synthesis, for example alcohols, resulted in a
substantial decrease in activity. For example, FAL was totally
reduced to 2-methylfuran by contact with 1,4-butanediol, using a
Cu–Zn–Al catalyst at 225 °C, but the reaction needed the presence
2
,5-bis(hydroxymethyl)furan (BHMF), respectively (Scheme 1).
FFA is used as a modifier for phenolic and urea resins, as a non-
2
of a large excess of H [69,106]. Nagaraja et al. [68] showed that
Cu–MgO catalysts made possible the reduction of FAL to FFA with
reactive solvent in the production of epoxy resins, as a chemical
intermediate for producing lubricants, and for the synthesis of
lysine, vitamin C, and tetrahydrofurfuryl alcohol. It is industrially
produced through the hydrogenation of FAL, which is carried out
in either the vapor or the liquid phase. The industrial catalyst is
made of a mixed Cu–Cr oxide [80,81]; in general, Cu-based cata-
lysts catalyze the gas-phase hydrogenation of FAL to FFA with good
selectivity [65,82–86]. Indeed, hydrogenation may lead to several
compounds, because of the hydrogenolysis of the CAO bond,
decarbonylation, hydrogenation, and furan ring opening; therefore,
alternative, Cr-free, highly-selective catalysts that can operate at
mild conditions have been sought. Alternative catalysts studied
include the following: (a) systems based on Ni or Co-Raney, and
Ni or Co alloys with Cu, Fe or Ce, for liquid-phase hydrogenation,
which may reach 98% selectivity to FFA [87,88]; Ni–B and Co–B
amorphous alloys [89–92] are some examples. On the other hand,
undoped silica-supported Ni mainly catalyzes the formation of
ring-opening products (butanal, butanol, and butane); (b) sup-
ported Pt and Pt/Sn-based systems, also containing various pro-
moters [93,94]; (c) supported Ru [95] and Ru(II) bis(diimine)
homogeneous complexes [96]; and (d) supported Ir [97]. Con-
versely, silica-supported monometallic Pd and bimetallic Pd–Cu
mainly catalyze the formation of the decarbonylation product,
cyclohexanol as the HT agent in the 200–300 °C T range, without
needing the H input, but with a yield of FFA no higher than 60%.
2
Recently, a 99% yield of FFA was achieved using isopropanol as
the HT alcohol, with a Ru carbene complex as the catalyst, KOtBu,
KOH, and THF as the solvent, at 60 °C and 24 h reaction time [107].
The catalytic conversion of HMF to dimethylfuran, dimethyltetra-
hydrofuran, and 2-hexanol was obtained over Cu-doped metal
oxides in supercritical methanol [108].
Here, we report on the reduction of FAL and HMF to the
corresponding unsaturated alcohols, FFA and BHMF, using a simple
procedure which enables selective HT from methanol to reactants,
with MgO as the HT catalyst. The reaction is carried out in a liquid
monophasic system, in which methanol also acts as a fully recycla-
ble solvent, since the only co-products obtained in methanol trans-
2 4
formation are gaseous compounds, i.e. CO, CO , and CH . Moreover,
we extended the procedure to other aldehydes and ketones, in
order to demonstrate the general soundness of the approach used.
The further aim of the present work was to assess the reaction
mechanism, through the validation of the reactivity experiments
by means of DFT calculations. This allows the identification of
validity boundary conditions and limitations of the method used
for carbonyl reduction.
2 3
furan [98]. Recently, Hermans reported that Fe O -supported Cu
and Ni catalysts permit over 70% selectivity to FFA at moderate
FAL conversion, with isopropanol as the H-transfer reagent;
2. Materials and methods
however, Pd/Fe
2
O
3
exhibited extraordinary activity in the
2.1. Catalyst preparation and reactivity experiments
further hydrogenolysis to methylfuran, to the ring-hydrogenated
compound, and to furan, with a combined yield of 62% under
continuous flow conditions. A similar performance was shown
with HMF [99].
MgO was prepared by means of thermal decomposition of
brucite. Brucite was synthesized following the conventional
co-precipitation method reported in the literature [109,110]. A
The reduction of HMF to BHMF (or to the saturated molecule,
,5-bis(hydroxymethyl)tetrahydrofuran, DHMTHF) has been
Mg(NO
3
)
2
ꢀ6H
2
O (Sigma–Aldrich) aqueous solution was added
2
dropwise into a solution containing 1 M NaOH (Sigma–Aldrich).
While the brucite was precipitating, the slurry was maintained at
55 °C and pH 10.5. At the end of the precipitation, an 1 h aging
treatment was carried out in order to increase the crystallinity of
the formed phase. The obtained solid was then filtered and washed
with 2 L water per gram of solid. Lastly, brucite was dried at 70 °C
in static air overnight. The precursor obtained was calcined in air at
450 °C for 5 h. Both phases of the precursor and magnesium oxide
were characterized by means of X-ray diffraction, using a Bragg/
Brentano X’pertPro PANalytical diffractometer (5–80° 2h, with
studied previously in the literature by a number of authors (see
the recent review on the reduction of FAL and HMF [100]). For
example, catalysts based on Ni, Cu, Pt, Pd, or Ru in a neutral solu-
tion have made it possible to obtain aromatic compounds, whereas
when used in acidic solution, the main products were ring-opened
compounds. Ni–Pd bimetallic catalysts gave primarily DHMTHF
selectivity 96%) [101,102].
As for the catalytic HT aimed at the reduction of FAL and HMF,
the reduction of the latter into BHMF has recently been achieved
with an excellent yield using formic acid (which is used as a source
(
acquisitions of 10 s every 2h 0.1°). The surface area of MgO
2
of H
2
) [103,104] and various catalysts based on Ir or Ru complexes.
(125 m /g) was measured using
a
Fisons Sorpty 1750 CE
This reaction was carried out at mild conditions, at 40 °C, in THF
solvent; the presence of a base greatly accelerated the reaction rate
instrument (single point BET method). The analysis was conducted
after dry-treating the sample at 120 °C.
[
105]. Conversely, hydrogen donors traditionally used for HT
The following reagents and products were used for reactivity
experiments: furfural (FAL) (Sigma–Aldrich), 5-hydroxymethyl-
furfural (HMF) (Sigma–Aldrich), furfuryl alcohol (FFA) (Sigma–
Aldrich), and 2,5-bis(hydroxymethyl)furan (BHMF) (Toronto
Research Chemicals). The hydrogenation of FAL and HMF was car-
ried out using a Parr Instrument 4561 autoclave reactor (300 mL
capacity). The reaction was carried out in methanol, using the
appropriate amount of catalyst. If not otherwise indicated, each
test was conducted for 3 h at 160 °C, with the following amounts
of reagents: 50 mL methanol; 1.21 mmol FAL or HMF; 0.5 g MgO;
1
bar of nitrogen. After loading the methanol, reactant, and
catalyst, the autoclave reactor was purged 3 times with N
20 bar) and then pressurized at 1 bar (N ). The temperature was
increased up to 160 °C and the reaction mixture was stirred at
2
(
2
Scheme 1. Hydrogenation of FAL and HMF to FFA and BHMF, respectively.

2
08
T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
4
00 rpm for the time needed. At the end of the reaction, the reac-
substituent effects are found with respect to the vibrational modes
entailed in these reactions.
tion mixture was cooled in an ice bath and the MgO was separated
by filtration.
Preliminary experiments were carried out in the aim of finding
the lowest stirring rate to apply in order to avoid mass transfer
limitations. Results of experiments are reported in Fig. S1, plotting
yield to FFA (which corresponds to FAL conversion, being FFA
selectivity equal to 100%) in function of the stirring rate, at
3
. Results and discussion
3.1. Furfural and 5-hydroxymethylfurfural reduction with methanol as
HT agent
1
4
30 °C. From these results, we decided to use a stirring rate of
00 rpm for catalytic experiments.
Liquid products were analyzed by means of HPLC (Agilent
Fig. 1 reports the FAL conversion and selectivity to FFA for fixed
reaction conditions (3 h reaction time, 160 °C), as a function of the
catalyst amount. It is shown that under the conditions chosen,
00% yield to FFA was obtained using 1.0 g MgO; the C balance,
as well as a careful analysis of the reaction mixture by means of
both HPLC and NMR, confirmed the total yield to FFA. However,
a lower catalyst amount led to a partial FAL conversion only, but
still with 100% selectivity. It is worth noting that the methanol
was both HT reagent and solvent, which may explain the complete
FAL conversion experimentally observed.
Table 1 summarizes the results obtained after changing the
reaction temperature. The table also shows the values of TON
and TOF, calculated by assuming that all the surface basic sites,
Technologies 1260 Infinity), using a 50 ꢁ 4.6 mm C-18 core–shell
1
column using an 80% solution of 0.01 M H PO and 20% acetonitrile
3
4
as the mobile phase. The gas phase was collected and analyzed
with an Autosystem XL (Perkin Elmer) GC equipped with a
3
0 mm ꢁ 0.32 mm Elite Plot Q capillary column attached to a
methanizer assembly flame ionization detector (FID). Compounds
were identified by calibration using reference commercial samples,
and by means of NMR. NMR analyses were performed using a
4
00 MHz NMR instrument.
2.2. Modeling approach
2
previously determined by means of CO -TPD [109,110], contribute
to the reaction; this assumption clearly leads to an underestimation
of the TON and TOF values.
Gas-phase electronic structure calculations were carried out
using the Gaussian09 [111] suites of codes and the B3LYP
112,113] density functional theory (DFT), including all the
electrons in the calculations. The basis set employed was
-31++G(d,p). The presence of methanol as the solvent was
modeled using the PCM model with the appropriate dielectric
constant, as necessary.
MgO nano-crystals were modeled using a cluster approach that
was previously benchmarked for structure, energetics, and spectral
properties [114,115], by evaluating possible size effects on adsorp-
tion energies and structures by increasing the number of atom
clusters while maintaining the overall species neutrality. The
geometrical parameters of the MgO (cubic, dMgO = 2.1084 Å) clus-
ters were kept frozen in all calculations, as one would expect minor
effects for them [116] as well as for energetic quantities. Geome-
tries for the adsorbed species and reaction-relevant structures
With regard to the effect of temperature, when using less mild
reaction conditions (entries 5–7 in Table 1), it was possible to
decrease both reaction time and catalyst amount, while obtaining
a significant FFA yield, still with 100% selectivity. On the other
hand, when a lower temperature was used (entries 1 and 2 in
Table 1), the selectivity to FFA was not total; in fact, in addition
to a 55% yield to FFA, a 2% yield to the acetal formed by reaction
between FAL and methanol was also obtained at 130 °C. At
[
6
1
00 °C, the acetal was the only product, obtained with a 15% yield
in 3 h reaction time; at 160 °C and above, the acetal did not form at
all. Therefore, under our conditions, methanol reacts with the
carbonyl moiety at low temperatures, whereas a temperature
higher than 130 °C is needed to activate methanol for the HT
reaction. However, it is worth noting that the formation of the
acetal might derive from acid impurities present in FAL.
TOF showed an exponential increase when the temperature was
raised. In general, TON and TOF values were low, mainly because of
both the high catalyst amount and the low FAL concentration used.
On the other hand, under the assumptions made, the calculated
turnover values demonstrate that the reaction was catalytic in nat-
ure. At 230 °C, 86% FAL conversion (corresponding to a TOF value
(
i.e. aldehydes, methanol, hemiacetals, transition states, TSs) were
fully optimized and the stationary points found were characterized
by means of frequency calculations. Putative structures for energy
minima were built using literature data [117,118] followed by a
complete geometrical relaxation keeping the MgO cluster con-
strained. On the other hand, TS geometries were often built using
relaxed scans along the reaction coordinates (e.g. CAH distances);
the latter approach helped in locating sensible initial structures
with small forces as well as the correct local curvatures of the
potential energy surface (PES). In order to propose possible reac-
tive H-transfer pathways, we exploited both the formal similarity
between the b-hydride transfer (bHT) termination reaction typical
of olefin polymerization with the reduction of aldehydes (furfural
and formaldehyde) and previous studies on the MPV reaction in
homogeneous [117,119,120] and heterogeneous [121] catalysis.
The only deviation from the procedures just discussed is the
optimization of hemiacetal, which was spontaneously obtained
while attempting to minimize the structure of a system made of
both formaldehyde adsorbed on the corner Mg (Mg3C) of MgO
and a methanol molecule adsorbed onto a vicinal-face Mg (Mg5C).
As the transferring atom is light, harmonic frequency calcula-
tions may be needed in order to account for possible differences
in vibrational zero point energy (ZPE) properties between
minimum and TS structures. In this respect, such correction is
found to lower the electronic energy barrier by roughly 1 kcal/
mol for all species due to the similar structure of carbonyl and
alcoholic parts of reactants, TSs, and products. Therefore, no strong
Fig. 1. FAL conversion (j) and FFA selectivity (h) as a function of catalyst amount
in the reaction of FAL reduction over MgO in methanol. Reaction conditions: FAL
1.21 mmol, methanol 50 mL, reaction time 3 h, temperature 160 °C.

T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
209
Table 1
Effect of the reaction temperature on catalytic performances of MgO catalyst in FAL reduction with methanol.
Entry
Temperature (°C)
Time (h)
FAL (mmol)
MgO (g)
FFA yield, FAL conv. (%)
TONa
TOFb (hꢂ1
)
1
2
3
4
5
6
7
100
130
150
160
210
230
230
3
3
3
3
0.75
0.5
1
1.21
1.21
1.21
1.21
1.21
2.42
2.42
0.5
0.5
0.5
0.5
0.25
0.1
0.1
0, 15
0
0
55, 57
90, 90
97, 97
51, 51
43, 43
86, 86
3.7
6.0
6.5
6.9
28.9
57.8
1.2
2.0
2.2
9.1
57.8
57.8
All experiments were conducted using 50 mL methanol.
a
TON expressed as mol FFA produced per basic site; the number of basic sites is assumed to coincide with the number of moles of CO
experiments [109,110].
2 2
adsorbed during CO -TPD
b
TOF = TON/reaction time.
ꢂ1
close to 58 h ) was achieved after 1 h reaction time, still with
1
00% selectivity to FAA (entry 7 in Table 1).
The absence of condensation compounds might be due to the
low concentration of FAL in methanol; however, experiments
carried out at higher substrate concentrations (see below) still
led to a fairly significant yield to FFA with 100% selectivity, with
an acceptable TON value.
Fig. S2 shows FAA yield (coincident with FAL conversion, since
at conditions used, selectivity was always 100%) and TOF values
plotted in function of reaction time, at 160 °C; the test carried
out at 3 h reaction time corresponds to entry 4 in Table 1. An
increase in reaction time led to an increase of FFA yield (with an
obvious analogous increase of TON value), whereas the TOF
declined.
The effect of FAL concentration on FFA yield (once again, yields
were coincident with FAL conversion, because of the 100% selectiv-
ity achieved) is shown in Fig. 2, for experiments carried out at
Fig. 3. HMF conversion (j) and BHMF selectivity (h) as a function of catalyst
amount in the reaction of HMF reduction over MgO in methanol. Reaction
conditions: HMF 1.21 mmol, methanol 50 mL, reaction time 3 h, temperature
1
60 °C. It is shown that the reaction rate was positively affected
160 °C.
by FAL initial concentration, even though a decline of FFA yield
indicates a surface saturation effect.
Fig. 3 shows the results obtained in HMF reduction (HMF
conversion and selectivity of BHMF), in conditions similar to those
used for FAL reduction. In this case, also, total selectivity to the
corresponding alcohol was obtained at every level of reactant
conversion; the NMR analysis of the reaction mixture confirmed
the formation of BHMF as the only product (Figs. S3–S5). It is
worthy of note that similar reactant conversion (and alcohol yield)
values were obtained from either HMF or FAL, when the same
values of substrate/catalyst (mol/wt) ratio were used. This
indicates that the reaction rate is not affected by the type of
Scheme 2. Summary of main reactions occurring to methanol over MgO.
substrate and that the rate-determining step of the process
involves the activation of the HT agent; experiments aimed at
determining the amount of active hydrogen species generated by
methanol transformation will confirm this hypothesis (see below).
With respect to the amount of active hydrogen species involved
in the HT reaction, we noted that the only products of methanol
2 4
transformation were CO, CO , and CH , while we did not detect
the formation of partially dehydrogenated or dimerization com-
pounds, such as formaldehyde, methylformate, formic acid, the
hemiacetal or the acetal, dimethoxymethane. We did not even
detect the formation of molecular hydrogen. This strongly suggests
that the HT, from methanol to the substrate, generates formalde-
hyde, which either also acts as a HT reagent (with the ultimate
co-production of CO), or dimerizes to yield methylformate, which,
4 2
however, is rapidly decomposed into CH and CO . As for this,
Scheme 2 gives a formal set of reactions that would account, at
least in principle, for both the reduction capability of methanol
and gaseous by-products (vide infra for a more detailed analysis
based on modeling results) and would also allow for hydrogen
mass balance calculation. As shown in Fig. 4, which reports the
Fig. 2. FFA yield (j), TON (N) and TOF (ꢀ) values as a function of FAL initial amount
in the reaction of FAL reduction over MgO in methanol. Reaction conditions: MgO
0
.5 g, methanol 50 mL, reaction time 3 h, temperature 160 °C. TON expressed as mol
FFA produced per basic site; the number of basic sites is assumed to coincide with
the number of moles of CO
TOF = TON/reaction time.
2 2
adsorbed during CO -TPD experiments [109,110].
4 2
number of moles of CO, CH , and CO formed based on the time

2
10
T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
sure and with temperatures in the range of 130–160 °C, without
the need for H input.
2
The comparison between MgO catalyst maximum productivity
in FAL reduction by means of selective HT (entries 6 and 7 in
Table 1) and the best results reported in the literature obtained
by means of liquid-phase FAL hydrogenation with H (Fig. S6) indi-
2
cates that the best productivity obtained under MgO-catalyzed HT
conditions was the same order of magnitude as those reported for
2
supported noble metal catalysis, under H pressure.
We also carried out experiments recovering and recycling the
used MgO catalyst (Fig. S7); we found that the MgO catalyst can
be recovered by filtration and recycled without a significant loss
of activity, but a thermal treatment at 450 °C for 5 h in air is
required to regenerate the partially deactivated catalyst.
Fig. 4. FFA yield (j) and number of moles of CO (}), CO
2
(s) and CH
4
(4) generated
Concerning the use of methanol as a HT agent for a MPV-type
reduction, very few examples are reported in the literature. Ara-
as a function of reaction time in the reaction of FAL reduction over MgO in
methanol. Reaction conditions: FAL 1.21 mmol, MgO 0.5 g, methanol 50 mL,
temperature 150 °C.
´
mendıa et al. [22] reported on the reduction of acetophenone with
MgO and Mg phosphate catalysts using methanol as the reagent. In
previous papers, we reported that in basic-catalyzed vapor-phase
methylation of phenol with methanol, at temperatures over
3
00 °C, the dehydrogenation of methanol led to the in situ genera-
tion of formaldehyde, which reacted with the activated phenolic
compound to produce a hydroxymethylphenol compound. The lat-
ter compound was either dehydrogenated into the corresponding
aldehydic compound and then reduced by methanol to yield the
methyl group, or directly reduced by the formaldehyde generated
in situ [122–127]. The ultimate product of the reaction was the cor-
responding ring-methylated phenol (o-cresol).
Other examples are reported in which, over MgO-based cata-
lysts also with acid features, the in situ dehydrogenation of meth-
anol generates formaldehyde, which then reacts with activated
CAH-containing compounds (nitriles, ketones, ethers) to form
a,b-unsaturated compounds via aldol condensation and dehydra-
tion [128,129]; in these cases, however, methanol was not reported
to act as a H-transfer reagent.
Fig. 5. Number of moles of FFA produced (j) and theoretical number of H
generated by methanol dehydrogenation (}) (calculated taking into account the
number of moles of CO and CO generated during reaction, see text for explanation),
as a function of time in the reaction of FAL reduction over MgO in methanol.
Reaction conditions: FAL 1.21 mmol, MgO 0.5 g, methanol 50 mL, temperature
2
moles
The present work results demonstrate that it is possible to
reduce FAL and HMF to FFA and BHMF, respectively, using mild
reaction conditions, an MgO catalyst, and methanol as a HT
reagent, achieving 100% yield to the alcohol within a few hours’
reaction time, at a temperature of 160 °C, and under autogenous
pressure. Methanol also acts as a solvent. The co-products of
methanol transformation are exclusively light molecules which
are transferred into the gas phase under reactor depressurization
and, in the end, easily separated from the liquid mixture.
2
1
50 °C.
4 2
in FAL reduction, the number of CH and CO moles generated was
identical, thus supporting the hypothesis of a rapid methylformate
decomposition; both were approximately half the number of CO
moles.
When the number of H
2
molecules theoretically used for the
3.2. Other carbonyl substrates
reduction of the substrate, produced during methanol transforma-
tion into formaldehyde and then into CO – this value was calcu-
Table 2 shows the results obtained with benzaldehyde and
acetophenone as substrates (1.21 mmol), using methanol (50 mL)
as the HT reagent and MgO as the catalyst. It is shown that other
aromatic aldehydes could be reduced with methanol, although
under more demanding reaction conditions than for FAL and
HMF. In fact, either longer reaction times (for benzaldehyde) or
the combination of longer times and higher temperature (for ace-
tophenone) was required in order to achieve good conversion to
the corresponding alcohol. At 160 °C, for instance, acetophenone
conversion was very low. With both substrates, the selectivity
was not total, because of the formation of the acetal in the case
of benzaldehyde and of 1-phenylpropanone in the case of aceto-
lated from the sum of the number of CO and CO
determined by analysis of the gas phase, both multiplied by 2
see Scheme 2) – were plotted in function of reaction time, we
obtained the plot shown in Fig. 5; the figure also reports the num-
ber of FFA moles generated. Since neither H nor formaldehyde, the
2
moles as
(
2
hemiacetal or methylformate was found either in gas or in liquid
phase, this implies that all the hydrogen atoms in converted meth-
anol (with the exception of those finally contained in methane)
were used for the reduction of the carbonyl moiety in FAL. There-
fore, the two curves reported in Fig. 5 should be coincident, which
in fact was the case. The overall selectivity to the hydrogen incor-
porated in the substrate, calculated with respect to the converted
methanol, was about 45%, because of the unproductive formation
´
phenone. As also reported by Aramendıa et al. [22], the formation
of 1-phenylpropanone occurs by aldol condensation between the
formaldehyde generated in situ and the ketone, followed by dehy-
dration and hydrogenation. Therefore, it is apparent that the use of
methanol as a selective H-transfer agent is possible only with
substrates, which do not bear activated CAH bonds.
of CH
4
. Therefore, our method made possible the reduction (with
1
00% yield with respect to the substrate and 45% selectivity with
respect to hydrogen atoms in converted methanol) under mild
reaction conditions, i.e. in the liquid phase under autogenous pres-

T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
211
Table 2
the reaction, we opted for directing our modeling effort toward
low-coordination defective sites such as a tri-coordinated vertex
Mg (Mg3C). On these, the reaction can take place via direct atomic
transfer without imposing strong geometrical constraints on the
various species, as suggested for the mechanism of the MPV reac-
tion with homogeneous [4,121,133] and heterogeneous [134] cat-
alysts (a hydride transfer from the OH-bearing carbon in the
alcohol to the carbonyl one). There are also strong similarities with
the b-hydride transfer (bHT) termination reaction of Ziegler–Natta
olefin polymerization [135–139]. In both cases, six-center TSs are
invoked, with one of the centers being a cationic Lewis acid.
Despite such structural similarities, however, it is stressed that
the assistance of transition metal d orbitals in stabilizing the par-
tial negative charge [135] on the transferring H atoms is not avail-
able in the six-center transition state for MPV when the Mg cation
is involved. Thus, the energetic cost required for inducing the bond
restructuring should be expected to be higher in the latter case.
This observation provides additional support to the idea that the
Mg3C site may play a key role as a reactive defect, as it is expected
to provide regions of higher electrostatic potential than in
the vicinity of edge (Mg4C) or terrace (Mg5C) sites [122–
Reactivity of other aldehydes for reduction with methanol.
Substrate,
conversion (%)
Product, yield
(%)
T (°C), t
(h), MgO
amount
By-product
(
g)
160, 3,
.25
160, 6,
.50
1-Phenylethanol, 38 210, 6,
.50
Benzaldehyde, 12
Benzaldehyde, 90
Acetophenone, 52
Benzyl alcohol, 12
Benzyl alcohol, 52
None
0
Benzaldehyde acetal
1-Phenylpropanone
0
0
We also tested the reactivity of crotonaldehyde and butyralde-
hyde; however, in this case, not only the formation of dimerization
and aldol condensation compounds was observed, but also the
conversion was very scarce.
3.3. The mechanism of HT with methanol
The reaction mechanism for the homogenous Al-alkoxy MPV
1
26,135,140]. In fact, Mg3C is known to be the most stabilizing
reaction involves a cyclic six-membered transition state in which
both the reducing alcohol and the carbonyl compound are coordi-
nated to the same metal center. The reaction proceeds by a hydride
transfer from the alcohol, bound to the metal center as an alkoxide,
to the carbonyl (Scheme 3, left). On strong Lewis basic sites M@O,
the reducing alcohol adsorbs with the oxygen and hydrogen to the
metal ion and the oxo-ion site, respectively. The carbonyl group
coordinates by hydrogen-bonding to this surface hydroxyl group
rather than to the metal; the reaction proceeds again via a cyclic
seven-membered transition state (Scheme 3, right). As an
exception, a single electron transfer pathway was observed for
alkali-catalyzed MPV reductions [130]; metal hydrides are only
formed during hydrogenations catalyzed by transition metals such
as ruthenium [131].
adsorption location for many adsorbates [125].
To test this idea, we have verified that this is also the case for
formaldehyde. In fact, our results suggest a decrease of roughly
2
0 kcal/mol for the CH
to the Mg3C site on the largest of our MgO models (Mg13
Table 3). The structure of the latter and of Mg10 10, the two most
2
O adsorption energy on Mg4C compared
O13, see
O
frequently used MgO models in this work, is shown in Fig. 6. The
latter have been previously used to study molecular adsorption
energies and have shown to provide a sufficiently accurate descrip-
tion even in cases when dissociative chemisorption takes place
[
124–126,140].
Note that, at least in principle, H
2
could be formed (see
Scheme 2) and spontaneously dissociated over the MgO surface
as already discussed [115]. However, the temperature in the reac-
tor is sufficiently high to force H desorption [115], so that the
2
direct hydride transfer appears as to be the most sensible pathway
to explore. To test the generality of the identified pathway, we
opted to investigate the energy profile for the hydride transfer to
all the mentioned carbonyl-containing compounds and also to
formaldehyde. The results for the latter species should provide
Analogous mechanisms involving a six-membered cyclic transi-
tion state are also suggested for heterogeneous catalysis, but reac-
tion may also occur on Brønsted acid sites, Lewis acid sites, and
basic sites [5,20,46,48,63,71,72]. The cooperation of acid–base
pairs is believed to contribute in the activation of the two reactants
[
16,18,28,29,31,33,34,41]; in this case, the HT is thought to take
place again via a concerted process involving a six-center transi-
tion state, with the alcohol and carbonyl compound adsorbed on
a surface acid–base pair. If the HT takes place on a flat surface,
the rate-determining step might be the interaction of the alcohol
with the acid–base site and thus related to the catalyst basicity.
However, methanol is known to dissociate on less coordinated
sites [132] (vide infra), a finding suggesting that the HT may be
the kinetic bottleneck in these alternative situations. However, it
seems that these ideas should be taken as sensible indications
rather that proven facts in view of the lack, for instance, of any
quantitative support from theoretical modeling.
the data for
a quantitative comparison among differently
substituted aldehydes.
Table 3 lists adsorption energies for a few of the species
involved in MPV reactions – namely methanol, formaldehyde,
and furfural – on the Mg3C site for the two aforementioned cluster
sizes; the structures of the minima obtained are shown in Fig. 6.
Note that one strong reason for using such a large MgO model is
based both on the size of the substituted aldehydes and on the
need to limit effects due to edge or vertex polarization that may
introduce artefacts when studying reactivity.
From the structures shown in Fig. 6, it can be seen that the
adsorbed methanol dissociates when transferring the OH acid pro-
ton onto vicinal O4C oxygen in the MgO cluster as previously sug-
Bearing in mind what is suggested in the literature as well as
the fact that the experiments in this work did not reveal any
molecular hydrogen in the gaseous products developed during
H
R
Table 3
H
Electronic adsorption energies (Ead, kcal/mol) for methanol, formaldehyde and
H
R
C
O
H
furfural on the vertex of the MgO clusters used to model the defective Mg3C site at
the B3LYP/6-31++G(d,p) level. Methanol dissociates while transferring the hydroxyl
proton to a vicinal O4C site. The value in parentheses is for formaldehyde on the
Mg4C site.
H C
C
2
O-
O
CH
2
H+
O
O-
M
Al
+HL
L'
Ead (kcal/mol)
Methanol
Formaldehyde
Furfural
O
Cubic Mg10
Cubic Mg13
O
O
10
13
40.4
38.2
27.0
23.7 (3.4)
33.5
28.5
Scheme 3. Sketch of the transition state for H-transfer as proposed in the literature.

2
12
T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
Fig. 6. MgO cluster models used in this work (d(MgO) = 2.168 Å). The structure of Mg10
furfural (e) on the largest cluster (Mg13 13) tested in this study are also shown here.
O10 (a) the adsorbed methanol (b), formaldehyde (c, on MgC4; d, on MgC3), and
O
Table 4
gested [117], thus confirming its basicity. The formation of a
stabilizing hydrogen bond between the transferred proton and
the just-formed methoxide is worthy of note, as well as the fact
à
TS energy barrier (E , kcal/mol) for the hydride transfer between methanol and the
reactive carbonylic species; reactants coordinated on the Mg3C vertex of the Mg10O10
cluster modeling site; B3LYP/6-31++G(d,p) level. The last column gives the energy
released by adsorbing a second molecule (either methanol or a carbonyl compound)
on the Mg3C site already occupied by a dissociated methanol molecule.
2
that CH O and FAL are found with the aldehydic H atom pointing
toward the O4C site. The latter finding suggests the presence of
both a partial positive charge on the H atoms and a quadrupolar
charge distribution on the aldehydic oxygen. The acidic dissociation
of methanol ought to be considered as preparatory for the possible
hydride shift, as the negatively charged oxygen would represent
the source of electrons for the post-transfer formation of the
carbonylic double bond.
à
Electronic E
Second molecule Ead
Methanol/methanol
Methanol/formaldehyde
Methanol/furfural
Methanol/benzaldehyde
Methanol/acetophenone
No reaction
14.0
27.5
25.3
27.8
17.9
10.0
13.5
13.4
13.3
From the quantitative standpoint, methanol adsorption is more
exothermic than that of the two aldehydes, a finding explained by
its weak proton-donating ability. Additionally, we found that FAL
as it is necessary for the latter to bind on the same acid center of
the dissociate methanol to react. This quantitative observation also
explains the increase in TON seen as the concentration of FAL
(Table 1) in the reaction mixture is increased, as it pushes the equi-
librium toward the formation of doubly-occupied methanol/FAL
Mg3C sites fostering the reactivity of the aldehyde.
releases more energy upon adsorption than CH
the electron donating capability of the furanyl ring. Also, it is
possible to see the weak dependency (
ad ꢃ 2 kcal/mol) on the
cluster size observed for the adsorption energy of methanol, and
the slightly more marked decrease ( ad 3–5 kcal/mol) for the
2
O, likely due to
D
E
D
E
aldehydes; the latter is probably due to a weak termination effect
influencing the Mg3C and O4C charge distributions. Despite this
effect, the Mg10O10 model seems to provide reasonably accurate
Turning to the reaction barriers, the energetic effect of the sub-
stitution on the carbonyl moiety is very clear. The net influence of
both aromatic and aliphatic substituents causes the deactivation of
the carbonyl group with respect to hydride transfer: a finding
mainly connected to the reduction of the partially positive charge
on carbonyl carbon due to the electron donor character of the
substituents. Nevertheless, the barrier heights remain sufficiently
low to support the idea that the related TSs are sensible stationary
points along the reaction pathways. In fact, when comparing the
energy barriers in Table 4 with the MPV and bHT ones in the
results at a reduced computational cost; thus we opted to use
the latter as a testing ground for our mechanistic study.
à
Table 4 shows the TS energy barrier (E ) for the hydride shift to
the species found to be reactive, and Ead for a molecule adsorbing
onto a Mg3C site already bearing a dissociated methanol molecule.
The stationary points found during the optimization process of the
reactants and TS geometries are shown in Fig. 7.
In commenting the results in Table 4, we begin by noticing that
there is a clear reduction of Ead for a second molecule adsorbing
onto a Mg3C site already bearing a methanol molecule if compared
with the results in Table 3. Nevertheless, adsorption of the second
molecules on Mg3C still remains energetically advantageous for
the system by roughly 17.7 kcal/mol for methanol, 10 and
2
literature, we can see that the barrier for the CH O reduction (i)
is similar to the one for bHT when the latter is a very efficient chain
termination process [138], (ii) is 2 kcal/mol lower than for MPV
catalyzed by Sn or Zr [72] in zeolites, and (iii) is more than 6 kcal/
lower than in the case of Al bi-naphtolate [121] (20.4 kcal/mol) or
bis-cyclopentadienyl Ti(IV) (26.3 kcal/mol) complexes, which are
known to be active with respect to bHT. The latter species
appears to induce a TS barrier similar to the one found for aromatic
species in Table 4, thus suggesting that TSs obtained in this study
1
2
3.5 kcal/mol for CH O and the remaining species. Similar values
for the adsorption of methanol and carbonyl-bearing compounds
are indeed a prerequisite for the hydride transfer to take place,

T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
213
Fig. 7. Reactant and TS structures for species (b and c, methanol/formaldehyde; d and e, methanol/FAL; f and g, methanol/benzophenone; h and i, methanol/benzaldehyde)
involved in the hydrogen transfer reaction between methanol and the tested carbonylic species. Also, panel (a) shows two methanol molecules co-adsorbed on the same
Mg3C site.
are likely to represent the kinetic bottleneck of the MPV process
over MgO.
supra). These features appear to be present even when Ti [138]
and Zr are involved in the reaction instead of Al [135] or Sn
[134]. Unlike many cases studied so far, we must also point out
that our TS geometries are strongly unsymmetrical (e.g. in terms
of the CAH distance) due to substantially different local molecular
environments. In particular, the hydride-receiving carbon distance
is ꢃ0.4 Å shorter than the other one, thus indicating a late TS and
the need for injecting energy into the methanol CAH vibration to
facilitate the reaction. It is also evident from Fig. 7 that the devel-
oping formaldehyde has increased its OAMg3C distance thanks to
an incipient hydrogen bond formed with the proton sitting on O4C.
The latter interaction is, in fact, sufficiently strong to make possible
As for the TS structures obtained, it is worth pointing out that
reactants remain close to the C
3
axis passing through the Mg3C site
of the Mg10 10 cluster and away from its lower edges: a situation
O
that should reduce any bias due to size-related polarization effects
and thus support the validity of our initial choice for the cluster
size. On comparing TS geometries and barriers found for the
methanol-based reduction processes and a few previously studied
bHT [135,138,139] and MPV hydride transfers [121,134], it can be
seen that the TS structure of the MPV reduction conforms to the
B-type TS discussed by Talarico and Budzelaar [138] for bHT in
homo- and hetero-olefins. This is characterized by a longer dis-
tance between metal and hydride than between the metal and
the C atoms involved in the transfer; this finding supports the lack
of hydride–metal interaction mediated by metal orbitals (vide
2
the migration of the forming CH O away from the Mg3C site, as
evidenced by the post-TS reaction pathway in all cases.
Having shed some light on a few mechanistic aspects of the
reduction of aromatic carbonyls by methanol, we now focus on

2
14
T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
clarifying the fate of the produced CH
2
O, with an eye on the gas-
The possible formation of methyl formate gives us the chance to
explain the presence of gaseous products. Indeed, the thermal
chemistry of methyl formate, either on its own [141,142] or on
basic oxides such as MgO [143], is known to produce both metha-
eous by-products methane, carbon dioxide, and carbon monoxide.
In this, we are helped by the spontaneous (and serendipitous) for-
mation of a hemiacetal molecule by adding a methanol molecule to
formaldehyde. This was obtained while trying to optimize a system
2
nol and CO, or CO and methane via monomolecular rearrange-
with methanol adsorbed onto a Mg4C site in Mg13
O
13 and CH
2
O
ments. It is worth noting that gas-phase processes require rather
high temperatures to occur [141,142], while the decarbonylation
of ester is already active at 100 °C [143] in the presence of
activated MgO. Thus, it is clear that the oxide surface plays a key
role in increasing the decomposition rate by catalyzing the other-
wise kinetically demanding step. According to the gas-phase TSs
previously adsorbed onto Mg3C. At first, the minimization led
the alcohol molecule to transfer its OH proton onto a vicinal O4C
site, with the negatively charged methanolate subsequently
binding to the carbonylic carbon. The structure of the methoxy
methanol (hemiacetal) adduct is shown in Fig. 8; note that the
hemiacetal is 12.1 kcal/mol below the parent species with
2
for both decarbonylation and CO elimination of methyl formate,
methanol and CH
finding contributes to some extent to explaining the formation of
acetal compounds reported above for some species.
As the hemiacetal is a fairly reactive species, its formation may,
at least in principle, open a few reactive channels. Perhaps more
interesting for our work, the hemiacetal may subsequently react
2
O co-adsorbed onto the Mg3C site. Also, this
the four-center TS for the latter process should be more affected
by the presence of an acid site such as Mg3C due to the very long
distance between the O atom in the ester bond and methyl carbon,
a structure that requires stabilization of a partially negative charge
present on the detaching oxygen [141,142]. Thus MgO should be
2
expected to lower the TS for CO more than for decarbonylation,
with another aldehyde (whether CH
2
O or one of the aromatic spe-
which does not have a polar TS. Indirect support to these theories
comes from the de-carboxylation of carboxylic acids, which
appears to be active already around 150 °C [144]. Conversely, a
more direct support comes from TS barriers in the presence of
methanol, a polar molecule that is in clear excess in the reaction
environment, obtained using the model PCM. In fact, the computed
cies is inconsequential, vide infra), reducing the latter via hydride
transfer. If the transfer occurred from the di-oxo methylene, it
would produce methyl formate (Scheme 4, top); alternatively, if
it occurred from the methyl group, it would generate a methanal
dimer that should decompose quickly (Scheme 4, bottom).
The TS leading from the hemiacetal to the methyl formate via
hydride transfer was located (see Fig. 8), and it required surmount-
ing a ZPE-uncorrected barrier of 13.6 kcal/mol (11.8 kcal/mol if
ZPE-corrected). Note that this value is slightly lower than the one
for the hydride transfer from the methoxy species coordinated to
Mg3C, in agreement with the presence of two partially negative
O atoms capable of donating electrons to the incipient electron-
poor carbon. Clearly, one would expect a higher barrier for trans-
ferring the hydride to aromatic species than for formaldehyde;
whether the latter reaction is active or not, it is of limited practical
consequence for the overall process, as the formaldehyde con-
sumed in the former transformation would come from the oxida-
tion of methanol. The hydride transfer from the methyl group to
formaldehyde was also studied and found to require at least
2
TS barrier for CO loss decreases from 77.8 to 72.8 kcal/mol while
going from the gas phase to the model solvent. Conversely, the
same quantity slightly increased from 66.1 to 69.7 kcal/mol for
decarbonylation, in agreement with what has just been discussed.
2
Clearly, similar barrier heights would make CO loss a competitive
process with respect to CO loss. We also located TSs for two
decomposition channels in the presence of MgO (see Fig. 9), and
a similar effect was indeed seen. However, the two barriers (65.2
2
and 83.0 kcal/mol for CO and CO loss, respectively) still seem
too high to lead to gaseous products in reaction conditions. As a
possible reason for these findings, we suggest that oxygen vacan-
cies may play a role, as previously found for the decomposition
of formic acid on MgO [145]. Given the previous experimental
evidences of methyl formate decomposition on metal oxides which
supported our interpretation of the MPV results, we feel that
testing whether or not this is the case would be best left for a more
focused study including details on how the decomposition depends
on the defective nature of MgO.
3
8 kcal/mol of energy; this result indicates that such a process is
almost irrelevant in this context. A similar conclusion was reached
with respect to the possible decomposition via SN2-type intramo-
lecular hydride transfer from the methyl group onto the di-oxo
2
methylene producing CH O through four-atom TS, which we were
not able to locate despite several attempts. Finally, it is also worth
mentioning that the possible dimerization of formaldehyde
directly producing methyl formate should be expected with a very
high barrier (49.3 kcal/mol in gas phase [141], vide infra, for solvent
and surface effects).
3.3.1. Comparison of reactivity between aromatic and aliphatic
aldehydes
As indicated in the section describing the results for other
aldehydes, aliphatic species were scarcely reactive with respect
to accepting a HT from methanol. The latter is expected to be a
Fig. 8. Hemiacetal formed by the spontaneous reaction between methanol placed on the Mg4C site and formaldehyde adsorbed on the corner Mg (a); hemiacetal and
formaldehyde co-adsorbed on Mg3C (b); TS for the hydrogen transfer from the di-oxo methylene part of the hemiacetal to the carbonyl carbon in formaldehyde (c).

T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
215
H C
O
H
3
H C
O
3
H
H
C
O
M
O
C
+H
O
O-
M
O
+H
H
H
C
CH3
O
O-
O
O
O+
CH2
H C
CH2
2
CH2
O
2
H C
H CO
2
+H
O-
H
+H
O-
+H
H
C
CH3
O
M
O
O
O-
CH
3
O-
O
M
H
O
M
O
O
Scheme 4. The H-transfer from hemiacetal to formaldehyde leading either to methylformate and methanol (top), or to a dimer which quickly decomposes to formaldehyde
(
bottom).
2
Fig. 9. Methyl formate adsorbed onto the corner Mg and produced by HT from hemiacetal to formaldehyde (a); CO loss TS onto Mg3C (b); CO -loss TS onto Mg3C (c).
Table 5
weaker reducing agent than primary or secondary alcohols (e.g.
isopropanol) due to the stabilization permitted by aliphatic chains
to the forming carbonyl group; thus, the reason for this difference
may be related to the thermodynamics of processes. As a first step
to investigate possible reasons for the experimental results, TS energy
barriers for the reduction of crotonaldehyde and butyraldehyde with
Gas-phase and surface-adsorbed electronic
between methanol and carbonylic species tested in this work; reactants and products
are considered to be coordinated on the most stable site of the Mg10
(usually the corner); B3LYP/6-31++G(d,p) level.
DE (kcal/mol) for the hydride transfer
O10 cluster
Carbonyl species
Gas-phase
D
E
Surface DE
Furfural
8.7
9.4
11.5
9.6
7.3
6.4
11.2
16.9
11.7
methanol on Mg10O10 were calculated, and these were roughly 23
Benzaldehyde
Acetophenone
Crotonaldehyde
Butyraldehyde
and 25 kcal/mol, i.e. slightly lower than the aromatic species in
Table 4. With TS and reactant geometries being similar to those
found for aromatic species, this finding clearly indicates that a
kinetic hurdle should not be expected to be present for aliphatic
aldehydes and that these experimental results may either be due
to alternative reactivity or related to energy issues.
5.9
To verify whether or not the latter case may provide sound evi-
in the direction of reducing the energetic request for the HT; the
opposite is true for aliphatic species (increasing by 6–7 kcal/mol),
whose reduction becomes more energetically demanding than for
aromatic species.
In this case, the different behavior is explained by the structure
of the adsorbed products shown in Fig. 10; there, it can be seen
dence, we computed the energy change
DE for the reduction of all
the tested carbonyl species with methanol in the gas phase, and
assuming that product and reactants are adsorbed on the MgO
surface. Data are shown in Table 5. As can be seen, all reactions
are energetic (by 6–17 kcal/mol), both in the gas phase and on
the MgO surface. Furthermore, it should be noted that – in the
that the electron cloud of the
p system in aromatic rings interacts
gas phase – the lowest
D
E is the one for the non-conjugated alde-
directly with the dissociated proton bearing a partial positive
charge. Such interaction reduces, for example, the energy of the
adsorbed benzyl alcohol by roughly 9 kcal/mol, compared to a con-
former where the ring points away from the proton. In conclusion,
it seems that the lower activity with respect to reduction by HT of
aliphatic aldehydes may be explained by the thermodynamics of
hyde butyraldehyde, thus suggesting that breaking the conjugation
of carbonyl with a double bond or an aromatic group destabilizes
the aldehyde. However, the effect related to adsorption for
aromatic species is clearly substantial (
DE is lowered by up to
3
kcal/mol, the smallest change being for acetophenone) and works

2
16
T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
the process; nevertheless, alternative reaction channels might also
provide some contribution to a different behavior.
equilibrium involved in forming acetal and hemiacetal may be in
favor of the reactants and fairly sensitive to thermal effects, so that
the hemiacetal from FAL should be present in low concentrations,
if any. To verify whether this may be the case, we computed the
change in gas-phase energy upon forming the methoxy hemiacetal
for both formaldehyde and FAL, obtaining respectively ꢂ14.6 and
ꢂ2.8 kcal/mol. These results clearly indicate a net disparity in reac-
tion energy and that – even at the lowest temperature explored in
our experiments – entropic effects should tilt the balance in favor
of free FAL, due to the net decrease in the number of molecules
upon hemiacetal formation. Conversely, the stronger energy
decrease estimated for formaldehyde would be expected to bias
the equilibrium toward the formation of methoxy methanol, a
process facilitated by the presence in MgO of Mg acting as a Lewis
acid (vide supra) even at low formaldehyde concentrations. Note that
our results are in line with thermodynamic data reported in the
literature [146], which indicate a less negative (by 2–3 kcal/mol)
acetal formation enthalpy for aromatic species than for aliphatic
ones, probably due to the stabilization effects induced by the con-
jugation with aromatic substituents, and a very low equilibrium
In conclusion, Fig. 11 summarizes the reaction pathway, and
Scheme 5 shows a simplified picture of the sequential steps in
the reduction of FAL into FAA. The reaction energy profile assumes
as energy zero a doubly-ligated Mg3C bearing two methanol mol-
ecules, the latter being present in much higher abundance than
other chemicals, plus the two molecules involved in the redox pro-
cess via HT. While the first step describes the energetic change
upon methanol substitution by FAL on Mg3C, the second describes
the HT through a TS state 27.5 kcal/mol higher than the co-
adsorbed reactants. Note the substantial decrease in energy upon
substituting the produced formaldehyde with methanol in the
third step, as well as the energetic cost of desorbing the formed
FFA due to the previously discussed effect of the aromatic ring.
In conclusion, it can be seen that the HT is slightly endothermic
due the cost of desorbing redox products, supporting the experi-
mental evidence indicating that the somewhat limited require-
ment for the reaction temperature derives from the HT step.
Scheme 5 explicitly indicates the possible formation of methyl
formate via HT transfer from methoxy methanol (the hemiacetal)
and its two decomposition channels. In this respect, one might
wonder if a similar channel is also open for FAL, which would lead
to the formation of both the methyl ester of furoic acid and,
perhaps, the relevant decomposition products also. As for this,
the experimental results discussed for FAL suggested that the
ꢂ3
constant (7.4 ꢁ 10 ). Thus FAL hemiacetal should not be expected
to play any substantial role in the reaction environment.
To conclude our discussion on the reaction processes involved
in FAL reduction, it may be useful to mention that there are other
alternatives that might explain the gaseous by-products, in
addition to the decomposition channels discussed above. These
Fig. 10. Product alcohols adsorbed on the corner Mg: (a) butanol; (b) crotylic alcohol; (c) benzylic alcohol; (d) furfuryl alcohol; (e) 1-phenyl ethanol. Note the orientation of
the dissociated proton with respect to aromatic rings.

T. Pasini et al. / Journal of Catalysis 317 (2014) 206–219
217
Fig. 11. Reaction energy profile for FAL reduction by methanol on MgO. Note that energy zero has been chosen in order to have reactants in their most stable form (e.g. FAL in
solution and two coordinated Mg3C sites).
low as the one found for methoxy methanol, due to the absence
of positive charge-stabilizing groups such as the coordinated
ꢂ
O
in methoxy methanolate, OH, or the methoxy group. In
conclusion, we feel the methyl formate route is the most likely
pathway along which gaseous products are formed.
4
. Conclusions
The liquid-phase reduction of furfural and of 5-hydroxymeth-
ylfurfural, catalyzed by high-surface-area MgO, was carried out
with 100% yield into furfuryl alcohol and bis(2,5-dihydroxy-
methyl)furan, respectively, using methanol as both the H-trans-
fer reagent and the reaction solvent, under mild conditions
(
T 160 °C), with autogenous pressure, within a few hours’ reac-
tion time. The reaction co-products were exclusively gaseous
compounds – CO, CO , and CH – which were easily removed
2
4
during reactor depressurization at room temperature. The use
of methanol, as an unconventional H-transfer reagent, proved
to be possible under generally mild reaction conditions, but only
with aromatic aldehydes and ketones, because of thermody-
namic constraints which limit the reaction in the case of
Scheme 5. The simplified sequence of reactions occurring at the adsorbed state
between FAL and methanol.
are the Cannizzaro-type disproportionation of formaldehyde lead-
ing to formic acid and methanol, and a direct HT from formalde-
hyde to FAL producing FFA and CO. In the first process, which
aliphatic substrates, and with substrates not bearing a-activated
H atoms. The reaction mechanism was investigated in detail, and
proved to consist of a rate-determining step of H-transfer from
methanol to carbonyl through a six-center TS involving the alde-
hyde and alcohol coordinated onto a Mg3C site. Other significant
steps in the mechanism are represented by the adsorption of FAL
onto Mg3C and the desorption of FAA from it, both presenting a
positive energy requirement. With our modeling, we were also
able to investigate gaseous by-product formation, which led us
to suggest methyl formate decomposition as the most likely
pathway. The ester is thought to form via HT transfer by a
hemiacetal formed from formaldehyde and methanol to another
aldehyde.
would allow formic acid decomposition into CO and H
oxygen from a water molecule is required. This makes this process
unlikely, as H O is not among the reactants introduced into the
2
O [145],
2
reactor. Moreover, it may be expected that the mechanism for
the Cannizzaro reaction would follow closely what was proposed
for the methyl formate formation (i.e. via HT) [147] with the
substitution of methanol by water, so that the latter species should
be favored due to the large methanol excess. As for the second
possibility, it seems unlikely that the process transferring
hydrogen from formaldehyde to FAL could have a TS barrier as

2
18
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