New insights for the valorisation of glycerol over MgO catalysts in the gas-phase

Article abstract of DOI:10.1039/c8cy02214c

Aqueous glycerol solutions of up to 50 wt% were reacted over magnesium oxide catalysts at temperatures greater than 300 °C, the reactivity of which was compared to catalyst-free reactions. Under catalyst-free conditions, modest levels of dehydration to hydroxyacetone were observed at temperatures >400 °C in a steel reactor tube and >320 °C over silicon carbide. For reactions over MgO, the product distribution becomes more diverse, resulting in the formation of methanol, acetaldehyde, ethylene glycol, 1,2-propanediol and acetic acid. The methanol space-time-yield over MgO catalyst samples (0.5 g) was found to be highest at 400 °C (205 g h^-1 kg_cat^-1) with a 50 wt% solution of glycerol, or with a glycerol concentration of 10 wt%; 255 g h^-1 kg_cat^-1 over 0.1 g of catalyst. Despite the high glycerol conversion achieved, the MgO catalyst was found to be stable over 48 h, following a modest decrease in glycerol conversion during the initial 2 h of reaction. Post-reaction characterisation revealed that the level of coking at high glycerol conversions (>90%) was ≥120 mg g_cat^-1. The carbon mass balance determined by GC analysis for a typical reaction was 75% and so the carbon lost from catalyst coking only represents a modest quantity of the missing carbon; typically <10%. MgO was also found to promote the formation of high molecular weight products via condensation reactions, which were responsible for the remainder of the missing carbon; ca. 15%. Therefore, the total organic content of the post-reaction mixture and coke was calculated to be 94% of the starting solution. We conclude that the catalyst surface directs the formation of methanol, however, the results indicate that the reaction conditions are crucial to obtain optimum yields.

Full text of DOI:10.1039/c8cy02214c

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New insights for the valorisation of glycerol over
MgO catalysts in the gas-phase†
Cite this: DOI: 10.1039/c8cy02214c
Louise R. Smith, Paul J. Smith, Karl S. Mugford, Mark Douthwaite,
*
Nicholas F. Dummer,
David J. Willock, Mark Howard, David W. Knight,
*
Stuart H. Taylor and Graham J. Hutchings
Aqueous glycerol solutions of up to 50 wt% were reacted over magnesium oxide catalysts at temperatures
greater than 300 °C, the reactivity of which was compared to catalyst-free reactions. Under catalyst-free
conditions, modest levels of dehydration to hydroxyacetone were observed at temperatures >400 °C in a
steel reactor tube and >320 °C over silicon carbide. For reactions over MgO, the product distribution be-
comes more diverse, resulting in the formation of methanol, acetaldehyde, ethylene glycol, 1,2-
propanediol and acetic acid. The methanol space–time–yield over MgO catalyst samples (0.5 g) was found
to be highest at 400 °C (205 g h−1 kgcat−1) with a 50 wt% solution of glycerol, or with a glycerol concentra-
tion of 10 wt%; 255 g h−1 kgcat over 0.1 g of catalyst. Despite the high glycerol conversion achieved, the
−1
MgO catalyst was found to be stable over 48 h, following a modest decrease in glycerol conversion during
the initial 2 h of reaction. Post-reaction characterisation revealed that the level of coking at high glycerol
conversions (>90%) was ≥120 mg gcat−1. The carbon mass balance determined by GC analysis for a typical
reaction was 75% and so the carbon lost from catalyst coking only represents a modest quantity of the
missing carbon; typically <10%. MgO was also found to promote the formation of high molecular weight
products via condensation reactions, which were responsible for the remainder of the missing carbon; ca.
15%. Therefore, the total organic content of the post-reaction mixture and coke was calculated to be 94%
of the starting solution. We conclude that the catalyst surface directs the formation of methanol, however,
the results indicate that the reaction conditions are crucial to obtain optimum yields.
Received 26th October 2018,
Accepted 7th February 2019
DOI: 10.1039/c8cy02214c
rsc.li/catalysis
land use change effects. This has led to uncertain regulatory
support and limited investment, as well as increased use of
1. Introduction
The growing concerns regarding the contribution of CO2
emissions to climate change and global warming have en-
couraged researchers to invest in the discovery of green and
sustainable routes for the production of liquid fuels.1,2 The
contribution of ‘renewable’ liquid fuels to final energy de-
mand in the transport sector during 2015 comprised ethanol
at 1.6% and biodiesel at 0.8%, with all other liquid biofuels
contributing 0.4%.3 Biodiesel is manufactured via the trans-
esterification of natural triglyceride oils with methanol, giving
approximately 90 wt% yield of fatty acid methyl esters (biodie-
sel) and a 10 wt% crude glycerol by-product.4,5 Future growth
in renewable fuel production based on virgin oils, such as
palm, soy or rapeseed, is challenged by concerns over sustain-
ability, actual carbon footprint, competition for land, and
recycled oils and fats. There is therefore growing interest in
alternative sources of triglyceride oils, for example from oil
crops with potentially improved sustainability (e.g. camelina,
jatropha), or from microalgae, despite the very considerable
cost challenges.6,7 Transesterification to biodiesel (fatty acid
methyl esters) remains one of two main options for conver-
sion of these potentially more sustainable triglyceride feed-
stocks to finished transport fuel, with hydrogenation/iso-
merisation to hydrocarbon fuels being the other. Adding
value to the crude glycerol by-product from triglyceride based
biodiesel manufacture is one way of improving the overall
economics of such routes, and therefore their future
potential.
The valorisation of glycerol to value-added chemicals is
not a novel concept. Numerous reviews have been published
highlighting the diverse range of compounds into which glyc-
erol can be upgraded.8–10 Such processes include the selective
oxidation and reduction of glycerol,11–13 esterification,14,15
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building,
Park Place, Cardiff, CF10 3AT, UK. E-mail: MugfordK1@cardiff.ac.uk,
dummernf@cardiff.ac.uk, hutch@cardiff.ac.uk
etherification,16,17
cyclisation,18
dehydration,19–22
and
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cy02214c
reforming.23,24 These remain interesting options where scale
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and processing costs are consistent with the relevant
chemicals markets can be achieved. An alternative approach
could be to convert the crude glycerol by-product into
blendable liquid fuel components, which may include metha-
nol either for direct gasoline blending or for a feedstock into
transesterification. It is important to emphasise that crude
glycerol would be required as a feedstock to make such a pro-
cess economical.
There are numerous examples in the literature of catalytic
systems which utilize hydrogen as a means of transforming
glycerol into lower carbon-containing alcohols by reductive
routes.25,26 Whilst these offer potential, the requirement for
co-fed hydrogen will inevitably impact process costs and im-
part an energy/environmental penalty. The surface properties
of the catalyst predominantly dictate the reaction selectivity,
for example, Brønsted acidic surface sites typically favour de-
hydration routes to form hydroxyacetone and acro-
lein.20,21,27,28 However, the absence of a gaseous oxidant or
reductant ultimately reduces experimental control, leading to
a large variety of products.
As discussed previously, many different strategies can be
invoked to valorise glycerol.8–10 However, establishing which
transformations are economically viable is challenging, but
can be assessed by the careful implementation of life cycle
analysis methodologies.29–31 Unfortunately, the most eco-
nomic approach for the up-grading of glycerol is often vari-
able and driven by the market value of the reaction products.
We therefore consider it vitally important that research and
development is under taken to offer as many options for the
valorisation of glycerol, so that technologies can be
implemented swiftly when deviations in the market occur.
The primary aim of this work is to develop an efficient
method of producing methanol from glycerol and over a sim-
ple metal oxide catalyst. Methanol is used as a chemical re-
agent in the production of bio-diesel; a process in which glyc-
erol is an un-desirable by-product of. One could therefore
envisage how bio-diesel companies could reduce feedstock
costs by generating methanol in-house, should they wish to
invest and adopt such a technology. This work expands on
our previous research in this area, accounting for any
changes in the chemistry taking place which arise from the
higher glycerol feed concentration and reaction temperatures
used previously.
We have previously shown, that under specific reaction
conditions, it is possible to achieve a reasonably high selec-
tivity to methanol with dilute aqueous solutions of glycerol
over MgO, CeO2 and other metal oxides without the need for
additional gaseous hydrogen.32 In our previous study, prelim-
inary results also revealed that higher glycerol concentrations
in the feed resulted in a significant increase in the quantity
of side reactions. This was common to both pure and crude
glycerol solutions with a feed concentration of 50 wt% of
glycerol. A reaction mechanism was subsequently proposed
to rationalise the formation of the major products observed.
In the present study, we have established that in addition to
the reactions proposed previously, numerous other side reac-
tions can also occur on this MgO catalyst, particularly when
high concentrations of glycerol are present in the feed. We
have focused on the influence of reaction temperature and
partial pressures of water and glycerol on the product distri-
bution. Additionally, analysis protocols suitable for the analy-
sis of the complex product mixture and a total carbon con-
tent of a typical reaction are reported in an attempt to close
the carbon balance. We consider that this increased under-
standing of the parameters that promote the competing path-
ways, provides us with solid foundation to begin further opti-
misation of the system.
2. Experimental section
2.1. Materials
MagnesiumIJII) nitrate hexahydrate (≥99.0%), magnesiumIJII)
hydroxide (≥99.0%), sodium carbonate (anhydrous, ≥99.0%),
and glycerol (≥99.5%) were all purchased from Sigma-Al-
drich. Argon gas was supplied by BOC. All purchased mate-
rials were used as received. Deionised water was provided in-
house. Silicon carbide (98%, Alfa Aesar, 40–50 mesh size) was
washed with deionised water and dried prior to use at 110 °C
for 24 h.
2.2. Catalyst preparation
MgO catalysts were prepared using a reflux technique we
reported previously.32 The as-received metal hydroxide was
calcined to 450 °C for 24 h (10 °C min−1, static air). The
resulting metal oxide was refluxed in deionised water (15 ml
g−1) for 3 h to form a slurry and then dried at 110 °C for 24
hours. The materials were then calcined (600 °C at 10 °C
min−1 and held for 3 h in flowing nitrogen) to obtain the fi-
nal oxide catalyst.
2.3. Catalyst testing
Catalytic reactions were performed using a gas-phase plug
flow micro-reactor. Typically, aqueous glycerol solutions (50
wt%) were introduced into a preheater and vaporiser (305 °C)
using an HPLC pump at a flow rate of 0.016 mL min−1. The
glycerol feed was swept through the reactor using argon as
carrier gas (50 mL min−1). All lines were heated to 300 °C
pre-catalyst bed and 100 °C post-catalyst bed to prevent any
condensation taking place. Catalysts were pelleted, crushed
and sieved to a uniform particle size (250–425 μm) prior to
testing. Typically, 0.5 g of catalyst was combined with silicon
carbide to a uniform volume (1 mL) and packed into an
8 mm inner diameter stainless steel tube between two plugs
of quartz wool. The resultant mass velocities and space veloc-
−1
ities studied were between 6000–29 850 LAr h−1 kgcat and
−1
4615–20 000 LAr h−1 Lcat respectively. Reactions were carried
out between 320–480 1 °C in an oven controlled by a PID
temperature controller with a thermocouple placed in the cat-
alyst bed. Liquid reaction products were collected using an
ice cold stainless steel trap. A gas bag was attached at the exit
line to collect the gaseous products.
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Liquid reaction products were analysed offline using a
Varian CP 3800 gas chromatograph (GC1) equipped with a
capillary column (ZB-Wax plus, 30 m × 0.53 mm × 1 μm).
Cyclohexanol was used as an external standard. Carbon
based gas reaction products were analysed offline using a
Varian 450-GC gas chromatograph (GC2) equipped with a
capillary column (CP-Sil5CB, 50 m × 0.32 mm × 5 μm). H2
and O2 were analysed using a Varian CP3380 gas chromato-
graph (GC3) equipped with a Porapak Q column. A full
product list and retention times according to GC used is
displayed in Table S1.† Some additional qualitative analysis
of the post reaction effluent was conducted by liquid
chromatography-mass spectrometry (LCMS). This was
conducted on a Bruker Amazon SL ion trap mass spectro-
meter which was operated in positive electrospray ion mode
and coupled to a Thermo Ultimate HPLC system. The HPLC
was equipped with a C-18 column (maintained at 40 °C)
and utilized a stepped elution consisting of 0.1% formic
acid in H2O (A) and 0.1% formic acid in acetonitrile. 10 μL
of sample was injected and the gradient elution was
performed as illustrated in Table 1.
SG, multiplied by conversion CGLY, multiplied by the carbon
balance xCb, excluding coke (eqn (3)).


S
C

G 




GLY 
(3)
Y % 
 xCb exc. coke %


 


100
The carbon balance xCb was calculated (eqn (4)) by divid-
ing the sum of the carbon moles of products xCp, coke xCcoke
and unreacted glycerol gmo by the carbon moles of glycerol
injected in the reactor gmi
.
x
Cp  xCcoke  g



mo 
xCb % 
100
(4)




gmi
Carbon deposition (coke) on the catalyst was calculated by
dividing the mass loss as analysed by TGA of the used cata-
lyst mLOST, by the carbon moles of glycerol feed over the cata-
lyst gmi (eqn (5)).
2.4. Calculations






mLOST
gmi
Coke % 
100
(5)


Eqn (1) was used to calculate the glycerol conversion (CGLY
based on the molar difference between the carbon moles of
)
glycerol fed into the reactor, gmi, and that detected at the out-
The methanol space–time–yield STYMEOH, was calculated
(eqn (6)) from the mass of methanol mMEOH, produced per h
(reaction time Rt), per mass of catalyst (mcat, kg).
let, gmo
:






gmi  gmo
CGLY % 
100
(1)


gmi


mMEOH
g
 
(6)
STYMEOH





The product selectivity (SpIJx), carbon mol%) for any prod-
uct, x, was calculated from the moles of carbon recovered of
x (xCm) divided by the total moles of carbon in all products,
yCm (eqn (2)).
Rt h m kg
  cat 



2.5. Material characterisation
Powder X-ray diffraction (XRD) analysis was performed using
a PANalytical X'pert Pro diffractometer with a copper X-ray
source operating at 40 keV and 40 mA, and Kα1 X-rays were
selected using a Ge (111) single crystal monochromator. Pat-
terns were recorded over the 2θ angular range 10–80° using a
step size of 0.016°.






xCm
y
Sp
x
 
% 

100
(2)
 
Cm


y

Functional group yield (Y, carbon mol%) data were calcu-
lated by the sum of the selectivities of that functional group
Carbon deposition on catalysts post-reaction was deter-
mined using thermal gravimetric analysis (TGA) and differen-
tial thermal analysis (DTA) and was performed using a
Setaram Labsys 1600 instrument. Samples (20–50 mg) were
loaded into alumina crucibles and heated to 800 °C (5 °C
min−1) in a flow of synthetic air (50 mL min−1). For all speci-
fied TGA runs, blank runs were subtracted from the relevant
data to remove buoyancy effects.
Brunauer Emmett Teller (BET) surface area analysis was
performed using a Micromeritics Gemini 2360 surface
analyser. A five point analysis was performed using N2 as the
adsorbate gas at −196 °C. Samples were degassed for 60 min
at 105 °C prior to analysis.
Table 1 The makeup of the mobile phase for the gradient elution
Time (min)
A (%)
B (%)
0.0
1.0
15.0
17.0
18.0
20.0
98
98
2
2
98
98
2
2
98
98
2
2
A
=
0.1% formic acid in H2O and
B
=
0.1% formic acid in
acetonitrile.
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3. Results and discussion
3.1. Catalyst-free reactions
products formed from reactions at high temperatures, as
comparable product distributions are observed in the reac-
tions over metal oxide catalysts.35
Vapour-phase reactions of dilute glycerol over metal oxide
catalysts have been reported by other workers from 250 to
700 °C.27,33 Catalyst free reactions were reported by
Hernandez et al. indicating that significant quantities of glyc-
erol can react in the absence of any catalyst at temperatures
above 400 °C.34 However, the corresponding product distribu-
tions of these experiments were not discussed. In the present
study, catalyst-free reactions were carried out over a tempera-
ture range of 320–480 °C. Reactions were conducted with a
50 wt% glycerol feed (0.016 mL h−1) and an Ar flow rate of 50
mL min−1 with both an empty stainless-steel tube and with a
plug of silicon carbide (1 mL) in place. The glycerol conver-
sion and the distribution of recovered products are displayed
in Table 2. There is no reaction at 320 °C in the blank tube,
however, at 400 and 480 °C glycerol conversions were calcu-
lated to be 6 and 11% respectively. The major products at
both these reaction temperatures are allyl alcohol with a car-
bon mole selectivity (hereafter selectivity) of ca. 17% and
hydroxyacetone with a selectivity of ca. 30% (see Table S2a†
for full product selectivity). These both contribute to a respec-
tive yield of 2.5% and 3.1% to alcohols and ketones at 480 °C
(Table 2; entry 3). Methanol, acetaldehyde, ethylene glycol,
acetic and propionic acids, and 1,2-propanediol are present
in the recovered reaction mixture (Table S2a†). The relatively
high concentration of hydroxyacetone observed at both tem-
peratures suggests that dehydration of glycerol to acrolein is
unlikely to be the principle pathway. We consider that
hydroxyacetone forms through the thermal dehydration of
glycerol, and the modestly reductive atmosphere at the reac-
tion temperatures used maintains the carbonyl functionality.
Unidentified products are defined as the collected,
unidentified signals (GC-FID peak areas) from the liquid
product mixture, analysed in GC1. The carbon, oxygen and
hydrogen mass balances are high and remain over 95% at all
temperatures. Furthermore, the implication of these results
is that glycerol can undergo both C–C and C–O scission in
the gas phase, potentially via a radical mechanism initiated
by the heated surfaces within the reactor tube. As such, it is
important to consider this contribution when assessing the
To investigate the conversion of glycerol under catalyst-
free conditions further, silicon carbide (40–50 mesh) was
used to assess the effect of increased contact with a hot sur-
face (Table 2; entries 4–6). Additionally, the residence time in
the heated zone of the reactor has decreased with the associ-
ated reduction in volume within the reactor. The residence
time was calculated to be 1.2 s across the 1 mL bed-length in
the empty tube, this was reduced to 0.59 s with the presence
of silicon carbide. Silicon carbide is used as a diluent in the
catalyst bed and therefore should ideally have no impact on
the chemistry taking place. At a lower reaction temperature
(320 °C) over silicon carbide, the glycerol conversion is less
than 2%; however, at 480 °C the conversion increases to ca.
18%. The modest glycerol conversion suggests that the SiC is
not necessarily inert; however, we consider that the enhanced
contact between the gaseous reactants in the heated zone of
the reactor over the SiC bed greatly contributes to the in-
creased glycerol conversion, which can be said to be ther-
mally initiated, particularly at 480 °C. In the empty reactor-
tube the majority of the gas stream will have no contact with
a hot surface due to the increased available volume. The
main products over the temperature range studied (320–480
°C) were hydroxyacetone and allyl alcohol (Table S2b†), which
is comparable to the product distribution observed with only
the empty reactor tube (Table S2a†). In addition, the selectiv-
ity (Table S2b†) to hydroxyacetone was comparable to the re-
action selectivity observed in an empty tube: 37% at 360 °C,
which decreased to 23% at 480 °C. Interestingly, with the SiC
present, the selectivity to allyl alcohol was found to be higher
when compared to the reaction in the empty reactor: 35% at
360 °C, which decreased to 22% at 480 °C. Potentially, this
was due to a decreased selectivity to unknown products over
SiC and can be attributed to the increased contact with SiC,
resulting in a greater proportion of substrate and/or interme-
diate activation on a hot surface in the reactor.
At the higher reaction temperatures, significant quantities
of acetaldehyde, acrolein, 1,2-propanediol and ethylene glycol
were detected. As noted above, the implication of these re-
sults further complicates the origin of products formed over
Table 2 Glycerol conversion and product distribution for reactions under catalyst-free conditions
a
Mass balanceb (%)
Yieldc (%)
Alc.
Condition
(°C)
CGLY
(%)
Entry
C
H
O
Diols
Ald.
Ket.
Ac.
COx
Unk.
1
2
3
4
5
6
7
320
400
480
320 w/SiC
360 w/SiC
400 w/SiC
480 w/SiC
0.0
6.1
11.0
1.6
4.4
11.9
17.9
102
95
98
99
97
95
95
102
95
98
98
97
93
93
102
95
98
99
97
92
90
—
—
—
—
—
—
—
1.1
2.5
0.7
1.6
3.8
5.0
0.5
1.4
0.0
0.2
0.7
1.9
0.4
1.1
0.0
0.2
0.9
3.5
1.8
3.1
0.9
1.6
3.9
4.2
0.1
0.5
0.0
0.2
0.5
0.3
0.2
0.3
0.0
0.2
0.4
0.4
1.3
2.1
0.0
0.2
1.0
1.7
a
b
c
Glycerol conversion. Carbon mass balance ( 3%) of products detected in GC1 and GC2. Yield of products detected in GC1 and GC2; Alc.,
alcohols; Ald., aldehydes; Ket., ketones; Ac., acids; Unk., unknowns (full product list in Table S2). Reaction conditions; 50 wt% glycerol (0.016
mL min−1), 50 mL min−1 Ar, 3 hours.
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metal oxides and suggests that a high surface area catalyst
promotes the conversion of glycerol and the reaction interme-
diates. The formation of allyl alcohol as a major product un-
der these conditions suggests that glycerol reacts via dehydra-
tion at the primary positions (C1 or C3) to produce
hydroxyacetone or via the C2 position to form acrolein,22
however, hydrogen is required to complete these reactions to
allyl alcohol. Therefore, as hydrogen is not added to the feed-
stream an alternative pathway can be proposed to form allyl
alcohol according to a radical mechanism as described in
Fig. 1. Given that significantly lower selectivity to acrolein
and allyl alcohol is observed in reactions over MgO, it is
likely that MgO promotes the activation of a primary alcohol
group and a radical reaction to allyl alcohol is inhibited.
is consumed through various reductive reactions.36,37 The
presence of CO2 in the product distribution is proposed to be
a result of organic acid decarboxylation,38,39 but is also likely
to be produced via the water gas shift reaction, which has
been reported over MgO at temperatures above 300 °C.40
The product distribution varied with temperature, with
the main groupings of product distributions summarised in
Fig. 2. At the lower reaction temperatures, the major product
formed was hydroxyacetone (reaction selectivity equal to
26%), which decreased with increasing temperature and con-
version, suggesting this is an intermediate in the reaction.
The diol yield, which is a combined sum of the yields to 1,2-
propanediol, 1,3-propanediol and ethylene glycol, also de-
creased with increasing temperature and conversion,
suggesting that these too are further converted.
Similarly to the catalyst free reactions, the aldehyde selec-
tivity increased steadily with increasing temperature over
MgO, largely attributed to the significant quantities of acetal-
dehyde produced (17% selectivity at 440 °C), with smaller
amounts of propionaldehyde and acrolein present. In con-
trast to the aldehyde selectivity, the alcohol selectivity
reached a maximum at 400 °C, where the methanol selectiv-
ity was 28%, corresponding to a methanol space time yield of
205 g h−1 kgcat−1. Due to the increasing selectivity towards un-
desirable by-products such as acetaldehyde and COx at tem-
peratures exceeding 400 °C, and methanol selectivity being at
a maximum at this temperature; higher temperatures were
not explored. Increasing or decreasing the flow rate of the
carrier gas and hence varying the glycerol contact time with
the catalyst results in comparable product distributions. That
is, with a higher contact time the effect is comparable to
higher reaction temperatures and vice versa. Standard reac-
tion conditions utilize a GHSV of 4615 h−1 and were com-
pared to the reaction data obtained with GHSVs of 2300 h−1
and 6920 h−1. The conversion of glycerol and product distri-
bution observed in these experiments were influenced by the
flow-rate change. For example; a GHSV of 6920 h−1 resulted
in a lower glycerol conversion and greater reaction selectivity
to intermediate products such as hydroxyacetone and 1,2-
propanediol and a reduction in selectivity to terminal reac-
tion products such as acetaldehyde and methanol (Table
S4†). In contrast, with a GHSV of 2300 h−1 a higher glycerol
conversion was observed and a greater selectivity to terminal
products was observed at the expense of the intermediates.
The above experiments and enhanced analytical methods
used in this work has led to a greater understanding of the
chemistry taking place in this reaction. Based on our experi-
mental observations, a reaction map has been developed
3.2. Reactions over MgO
3.2.1. Influence of reaction temperature. The effect of the
reaction temperature on the conversion of glycerol and prod-
uct distribution over MgO is shown in Table 3. At 360 °C, the
glycerol conversion was 74%, with full conversion of glycerol
observed at temperatures above 440 °C. Over this tempera-
ture range, the carbon mass balance was calculated to be be-
tween 73 and 77% and decreased with an increasing reaction
temperature. The main liquid phase products detected with
GC1, and their respective selectivities at 400 °C, were acetal-
dehyde (13.3%), methanol (27.9%), hydroxyacetone (17.9%)
and ethylene glycol (5.8%), with smaller quantities of
propionaldehyde, acrolein, 2,3-butanedione, acetic acid and
1,2-propanediol. We have discussed the potential routes to
these main products previously.32 The yield of the product
functional groups is illustrated in Table 3 (full product selec-
tivities can be found in Table S3†); numerous other liquid
phase products were detected and assigned according to the
retention times of commercial samples, although the selectiv-
ities to these other compounds were by comparison very low.
These include acetic acid, propionic acid, 2-butanol, 3-ethoxy-
1-propanol and propanol. Potential routes to these products
are illustrated in the ESI† (Fig. S1). These are typically formed
from reaction intermediates such as hydroxyacetone or alde-
hydic radicals and require a reductive atmosphere. Interest-
ingly, the selectivity to allyl alcohol is low at (<2.5%) in the
presence of MgO over all the reaction temperatures studied,
which is notably different to the selectivity observed over SiC
(Table 2). Additionally, gas phase products in the form of
COx and CH4 were also detected, no higher hydrocarbons
were detected. CO was ascribed to the decomposition of
methanol over MgO, yielding CO and H2, the latter of which
Fig. 1 Potential reaction scheme for the formation of allyl alcohol via a radical mechanism, which initially dehydroxylates at the C2 position of
glycerol then at the C3 position.
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Table 3 Glycerol conversion and product distribution over MgO at different temperatures
Mass balanceb (%)
Yieldc (%)
MeOH
S.T.Y.
Carbon
a
Condition
CGLY
(g1 h−1
deposition
−1
Entry
(°C)
(%)
C
H
O
Alc.
Diols
Ald.
Ket.
Ac.
COx
Unk.
kgcat
)
(mg g−1
)
1
2
3
360
400
440
74
90
100
74 (77)
73 (77)
68 (73)
72
71
61
67
61
55
14.4
21.2
21.3
8.0
5.2
2.7
6.1
11.0
15.9
15.4
13.6
13.1
1.2
1.7
1.8
3.9
6.6
7.8
5.9
6.2
5.4
131
205
204
81
122
125
a
b
Glycerol conversion. Carbon mass balance ( 3%) of products detected in GC1 and GC2, values in parenthesis include coke deposited on
catalyst. c Yield of products detected in GC1 and GC2; Alc., alcohols; Ald., aldehydes; Ket., ketones; Ac., acids; Unk., unknowns (full product list
in Table S3). Reaction conditions; 50 wt% glycerol (0.016 mL min−1), 0.5 g MgO, 50 mL min−1 Ar, 3 hours.
Fig. 2 Collected product selectivities expressed as carbon mole selectivity following reaction at 360 °C (black), 400 °C (grey) and 440 °C (white)
over MgO. Reaction conditions; 50 wt% glycerol (0.016 mL min−1), 0.5 g MgO, 50 mL min−1 Ar, 3 hours.
(Fig. 3), comprising of the major products observed in these
reactions, and proposed routes to their formation.
pressure did not vary as greatly from 0.29 to 0.18 mbar with
the respective 10 to 50 wt% solutions. An additional reaction
was also performed with 10 wt% glycerol in the feed and a re-
duced amount of catalyst in order to examine the effect of
glycerol to catalyst ratio.
3.2.2. The influence of glycerol concentration in the feed-
stream. Thus far, all reactions were performed with a 50 wt%
glycerol solution. Commonly, catalytic conversions of glycerol
are performed with more dilute feedstock solutions and vary-
ing flows of carrier gas, making direct comparisons of space–
time yields difficult.19,41 Furthermore, the use of more dilute
aqueous feedstocks for gas phase conversions increases the
energy demand of the system as increasing quantities of wa-
ter require vaporisation.42 Whilst effective conversions of
concentrated glycerol solutions are highly desirable, the high
functionality of the molecule often leads to unwanted side-
reactions and decreased product selectivity43 and as such, the
effect of the glycerol concentration was investigated. Reac-
tions were performed at 400 °C with feedstocks varying be-
tween 10 and 50 wt% glycerol, whilst maintaining the same
catalyst mass and volume; the partial pressure of glycerol in-
creased from 0.026 mbar to 0.14 mbar. The water partial
The glycerol conversion and the yield of the different
product groups are displayed in Table 4, where the glycerol
to catalyst ratio was varied. Only traces of unconverted glyc-
erol were detected for a 40 wt% feedstock, with complete con-
version of glycerol observed with the more dilute feedstocks.
Whilst the carbon balance was 77% with a 50 wt% feedstock,
a reduction in the glycerol concentration resulted in im-
proved carbon balances. This was to be anticipated, since the
formation of high molecular weight products, which are not
quantified and therefore not counted in the carbon balance,
has been shown to increase with more concentrated glycerol
feed-stocks.44,45 Furthermore, the presence of water has been
reported to prevent condensation reactions,45 and the reduc-
tion in the partial pressure of glycerol and products resulting
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Fig. 3 Proposed reaction network for the catalytic transformation of glycerol into a range of different products over MgO under N2. Red arrows
correspond to dominant reaction pathways over MgO. The green arrow corresponds to a dominant pathway occurring in the absence of MgO. 1.
Glycerol; 2. 3-hydroxypropenal; 3. 1,3-propanediol; 4. 1-propanol; 5. hydroxyacetone; 6. propanoic acid; 7. 1,2-propanediol; 8. 2-propanol; 9. ace-
tone; 10. acetaldehyde; 11. ethanol; 12. 2,3-butanedione; 13. 2,3-butandiol; 14. 2-butanol; 15. glycolaldehyde; 16. ethylene glycol; 17. ethenone;
18. acetic acid; 19. allyl alcohol; 20. 1-propanal; 21. 1-hydroxyl-2-butanone; 22. acrolein; 23. 3-alkoxypropanal; 24. 3-alkoxy propanol.
Table 4 Glycerol conversion and product distribution over MgO with differing feedstock concentration at 400 °C
Cat :
gly
ratio
(g/g)
Mass balanceb (%)
Yieldc (%)
MeOH
S.T.Y.
Carbon
a
−1
Glycerol
weight%
CGLY
(%)
(g h
kgcat
deposition
−1
Entry
C
H
O
Alc.
Diols
Ald.
Ket.
Ac.
COx
Unk.
)
(mg g−1
)
1
2
3
4
5
10
10
20
40
50
1.8
0.3d
0.8
0.4
0.3
100
96
100
99
83 (84)
83 (98)
80 (88)
78 (81)
73 (77)
80
84
76
74
71
68
67
67
62
61
36.5
31.2
31.1
27.2
21.4
0.5
6.9
1.2
2.9
5.2
20.0
13.9
19.3
17.2
11.0
8.2
16.3
9.3
14.1
13.6
0.9
1.0
1.8
1.6
1.7
8.4
2.7
8.6
6.0
5.1
8.4
7.8
8.5
7.4
6.2
61
55
75
80
87
122
255
105
219
205
90
a
b
Glycerol conversion. Carbon mass balance ( 3%) of products detected in GC1 and GC2, values in parenthesis include coke deposited on
catalyst. c Yield of products detected in GC1 and GC2; Alc., alcohols; Ald., aldehydes; Ket., ketones; Ac., acids; Unk., unknowns (full product list
in Table S4). d 100 mg catalyst used. Reaction conditions; 400 °C, glycerol/water flow 0.016 mL min−1, 0.5 g MgO, 50 mL min−1 Ar, 3 hours.
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from more dilute feedstocks could reduce the formation of
high molecular weight products. Similar products were ob-
served in each of the reactions; however, the relative yield of
the products (Table 4) and product selectivities (Table S5†)
did vary. An increase in the catalyst to glycerol ratio did result
in an increase in selectivity to aldehydes and alcohols (Fig. 4)
particularly acetaldehyde and methanol (Table S5†). The se-
lectivity towards two of the major reaction intermediates,
hydroxyacetone and ethylene glycol,35 reduced from 17.9%
and 3.6% to 3.6% and 0.2% respectively, upon increasing the
ratio of catalyst to glycerol from 0.3 to 1.8 (Table S5†). Addi-
tionally, a higher proportion of catalyst (cat : gly 0.8–1.8)
resulted in decreased propanediol selectivity (both 1,2- and
1,3-propanediol), with almost complete diol conversion or
non-formation with the highest ratio of catalyst to glycerol.
These observations are comparable to findings by Montassier
et al. where it was reported that ethylene glycol could be
readily converted to C1 products after the complete glycerol
conversion was achieved.46
The increased conversion of reaction intermediates at in-
creased catalyst to glycerol ratios resulted in a decrease in
the ketone and diol selectivities and corresponded to an in-
crease in the selectivity to alcohols, aldehydes and COx
(Table 4). The methanol selectivity steadily increased with in-
creasing proportions of catalyst, reaching a maximum of
34.9% with a 10 wt% glycerol feedstock and 500 mg of MgO
(Table S5†); in addition to methanol, the selectivity towards
other mono-alcohols, namely ethanol, 1-propanol and 3-eth-
oxy propanol, increased with decreasing quantities of glycerol
to catalyst. The increased aldehyde yield can be attributed to
increasing selectivity to acetaldehyde and propionaldehyde.
Where the catalyst mass was kept constant (entries 1 and 3–5
in Table 4), the methanol space time yield reached a maxi-
mum using a 40 wt% glycerol feed (219 g h−1 kgcat−1); higher
methanol yields were achieved from 40 wt% glycerol com-
pared with 50 wt%, due to the increased glycerol conversion
and improved carbon balance. Since complete glycerol con-
version was achieved for both 20 and 10 wt% feedstocks, the
amount of catalyst present was reduced proportionally. Com-
paring entries 2 and 4, where the catalyst to glycerol ratio
was kept constant, an improved methanol space time yield
(255 g h−1 kgcat−1) was achieved from a more dilute feedstock
and reduced amount of catalyst. This was attributed to the
improved carbon balance and slightly higher methanol selec-
tivity and glycerol conversion. Interestingly, the product selec-
tivities were largely comparable when the glycerol to catalyst
ratio was kept the same (Table S5†). For example the selectiv-
ity to hydroxyacetone, considered an intermediate, was found
to be highest when the GLY : cat was 0.3, suggesting that con-
tact time or the glycerol partial pressure to catalyst surface
area ratio are important variables in this reaction to achieve
a high methanol selectivity. However, more work is required
in order to accurately deconvolute the influence of the con-
tact time and the glycerol partial pressure on the yield of
methanol observed.
3.2.3. Investigating catalyst stability. The longer term sta-
bility of the catalyst was investigated with a more concen-
trated reactant feedstock of 50 wt% glycerol, as more signifi-
cant catalyst deactivation can be expected with a higher
carbon feed-content. For all reactions, including the longer
study, the product mixture was diverted to a gas bubbler for
a period of 2 hours 15 minutes after the glycerol flow was
Fig. 4 Collected product selectivities expressed as carbon mole selectivity following reaction with 10 wt% (black; catalyst to glycerol {g/g} ratio of
1.8), 20 wt% (dark grey, 0.8) and 40 wt% (light grey, 0.4) and 50 wt% (white, 0.3) over MgO. Reaction conditions; 400 °C, glycerol/water flow 0.016
mL min−1, 0.5 g MgO, 50 mL min−1 Ar, 3 hours.
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initiated. Analysis of this mixture showed an extremely poor
carbon balance and gave a different product distribution to
that observed over the remainder of the reaction. After this 2
hour 15 minute period was over, the reaction mixture was
collected and analysed periodically, and the corresponding
time-on-line data are shown in Fig. 5. Glycerol conversion
dropped by approximately 10% between hours 2 and 4, after
which it remained stable at ca. 87%. Similarly, the carbon
balance showed an increase during the initial 4 hours, before
decreasing slightly to stabilise at 65%. The stability of the
catalyst is surprising given the poor carbon balance,
suggesting that the active sites on the catalyst are not suffi-
ciently blocked by carbon deposition so as to lead to catalyst
deactivation.
The methanol selectivity and space time yield aligned with
the trends observed for the glycerol conversion, indicating a
period of 4 hours is required for the system to approach
steady-state, after which, both conversion levels and product
selectivities are quite stable (full product list in Table S6†).
This provides further evidence to suggest that only modest
catalyst deactivation occurs over the duration of our experi-
ments, despite the low carbon balances observed throughout
this period. This is further evidenced by the XRD patterns of
the pelleted fresh and used catalyst, displayed in Fig. 6. The
Scherrer equation was used to estimate the crystallite size of
each material from the (200) reflection; there was no indica-
tion of MgO sintering (Table S7†). Large losses in carbon
were also observed under similar reaction conditions by
Batiot-Dupeyrat and co-workers when studying the conversion
of glycerol over basic lanthanum-based materials.34,47
served during many of the reactions conducted, additional at-
tempts were made to identify the pathway(s) by which this
carbon was lost. One possible explanation for the observed
loss in carbon was the coking of organic matter onto the sur-
face of the catalyst during the reaction. In order to determine
whether this was a contributing factor to the low carbon bal-
ances observed, TGA was carried out on all of the used cata-
lysts from this study. The traces for these experiments are
displayed in Fig. 7. The samples were initially heated to 110
°C and held at this temperature for 30 minutes, following
which the temperature was increased to 800 °C at a rate of 20
°C min−1. For all of the used catalysts, a significant mass loss
is observed at approximately 400 °C, which can be attributed
to the combustion of organic material from the surface of the
catalyst. Fig. 7c illustrates the exothermic release of adsorbed
species with the positive values observed in the TGA-DTA
heat flow measurements. These mass losses were subse-
quently used to estimate the quantity of retained carbon after
each reaction expressed as mg of carbon per gram of catalyst.
The results of these estimations, which are displayed in an
additional column in Tables 3 and 4, indicate that only a
small proportion of the missing carbon is attributable to cok-
ing on the catalyst. There appears to be a relationship be-
tween the quantity of carbon deposited on the catalyst and
the reaction conditions used. In general, increasing the con-
centration of glycerol in the feed, and increasing the reaction
temperature, typically leads to an increase in the quantity of
carbon deposited on the catalyst.
From the time-online experiment in Fig. 5, it is evident
that the carbon mass balance is lower at the initial stages of
the reaction. To investigate this further, an additional TGA
experiment was conducted on the MgO catalyst, retrieved
3.2.4. Investigating the carbon mass balance. Given that
carbon mass balances of less than 80% were commonly ob-
Fig. 5 Glycerol conversion (■), methanol selectivity (●), carbon mass balance (▲) and methanol space time yield (▽) over MgO catalyst as a
function of reaction time. Reaction conditions; 400 °C, glycerol flow (0.016 mL min−1), 0.5 g MgO, 50 mL min−1 Ar.
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Fig. 6 X-Ray diffraction patterns for (A) the fresh, pelleted MgO catalyst and (B) a used pelleted MgO catalyst after reaction with 50 wt% glycerol
in H2O for 3 h at 400 °C. Residual SiC from the catalyst bed denoted with (■).
Fig. 7 Thermal gravimetric analysis of post reaction samples of MgO; investigating the influence of reaction temperature (a) and glycerol to
catalyst ratio (b). The values next to each line refer to the reaction temperature in (a) and the values in (b) refer to glycerol to catalyst ratio. Mass
loss and heat flow of a post-reaction MgO sample following a 48 h reaction (solid line = mass loss, dashed line = heat flow) highlighting removal
of carbon deposition (c).
after 135 minutes of reaction (Fig. S2†), prior to the onset of
a steady glycerol flow. The quantity of coke in this period
(80 mg g−1) evidences a non-proportionate correlation be-
tween the quantity of coke deposited and reaction time; more
coke is deposited during the initial stages of the reaction. As
such, it is likely that a proportion of the active sites responsi-
ble for the coking and/or production of unknown products
are blocked during the initial stages of the reaction and the
coke deposited during this initial period assists with the
stabilisation of the catalyst surface.
XRD was conducted on a pre- and post-reaction catalyst to
ensure that the observed mass losses could not be attributed
to a phase change from MgO to MgIJOH)2. It is known that
MgO readily hydrates to MgIJOH)2 when exposed to liquid
phase water even at room temperature.48 The corresponding
XRD is displayed in Fig. 7, and indicates that the post-
reaction catalyst from the experiment in Table 4, entry 5 is
MgO and confirms that any mass losses observed in the TGA
experiments can be attributed to deposition of carbon on the
surface of the catalysts during the reaction. Accordingly, the
mass lost at 400 °C (Fig. 7) appears to have a positive correla-
tion with the concentration of glycerol in the feed confirming
the source of coke is based on the reaction concentration.
The formation of large organic species via intermolecular
condensation reactions could also be responsible for some of
the missing carbon in the system. The formation of these
species typically have reaction orders ≥1.5 with respect to the
substrate in the presence of hydrophilic reaction solvents49
and as such, the rate of their formation would likely be de-
pendent on the concentration of reactive substrate/intermedi-
ates in the feed. Given that larger quantities of carbon do ap-
pear to be lost as the partial pressure of glycerol is increased,
it is therefore feasible to suggest that condensation reactions
of this nature do occur in the present system. Previous
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esterification and dimerization reactions have been reported
to readily occur in the gas phase with glycerol at comparable
reaction temperatures.50 LC-MS was subsequently utilized in
an attempt to derive some qualitative evidence for the pres-
ence of these larger compounds in a post reaction solution. A
reaction was run for 6 h, analysed by GC-FID and subse-
quently submitted for LC-MS. The calculated carbon mass
balance for this sample, based on products observed by GC
in the liquid and gas phase, was 68%. The corresponding LC-
MS chromatogram is displayed in Fig. S3.† Despite only
screening for products and fragments between 100 and 1000
m/z, the chromatogram appears to be extremely complex,
which ultimately made compound identification extremely
challenging. This does however confirm the presence of nu-
merous large compounds in the post reaction sample. CHN
analysis was subsequently carried out on the post reaction so-
lution to determine the total carbon content. By combining
tion is accompanied by an increase in the concentration of
undesirable aldehydes. Preventing the formation of aldehydes
such as acetaldehyde is crucial as these products could be re-
sponsible for the formation of high molecular weight prod-
ucts via condensation reactions over basic sites. As these
products are not routinely detected in the GC-FID setup used
for reaction analysis, the mass balance calculations do not
take these in to account. Despite the relatively high reaction
temperatures used, catalyst fouling by carbon deposition on
the catalyst was found to be modest and does not appear to
be detrimental to the catalyst activity over 48 h. The low mass
balance of the reaction at ca. 75% is in part due to the forma-
tion of high molecular weight products, the presence of which
in the post reaction effluent was confirmed by LC-MS and TOC
analysis. Optimisation of the catalyst surface in order to reduce
aldehyde formation should be a priority going forward.
the contributions of carbon from coke on the surface of the Conflicts of interest
catalyst, COx and CHN analysis, a total carbon balance of
The authors declare no competing financial interest.
94.0% was obtained (Table S8†). We consider that the miss-
ing carbon (6.0%) is lost through reactor fouling. This further
supports the theory that a significant quantity of high molec-
ular weight compounds are present in the post reaction solu-
tion, which are not observed by GC-FID analysis. MgO is a ba-
Acknowledgements
We would like to thank the EPSRC for funding this work
(Grant reference codes: EP/P033695/1 and EP/L027240/1). The
authors would also like to thank Exeter Analytical UK Ltd. for
the Total Organic Content analysis. We would also like to ac-
knowledge Thomas Williams for assistance with the operation
of and processing of the LC-MS data. Information on the data
underpinning the results presented here can be found in the
Cardiff University data catalogue at [http://doi.org/10.17035/
d.2018.0049493818].
sic metal oxide and has been widely studied as
a
heterogeneous catalyst for base-catalysed reactions.51,52 As
such, it seems appropriate to consider whether this property
could promote some of the undesirable side reactions, which
are evidently occurring. It is known that aldol condensation
reactions can readily occur in the gas phase over solid basic
oxides53–55 and over MgO in particular, these condensation
reactions are typically attributed to the population of basic
sites.56 We can conclude that a large quantity of the missing
carbon in these reactions is likely attributed to the formation
of larger organic species via such intermolecular condensa-
tion and/or esterification reactions. These undesirable side
reactions appear to be promoted by increasing the reaction
temperature and glycerol partial pressure in the feed. There-
fore, the challenge from a catalyst design and engineering ap-
proach would be to establish a means of reducing these com-
petitive pathways, without influencing the formation of any
of the high value products.
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