Continuous-Flow O-Alkylation of Biobased Derivatives with Dialkyl Carbonates in the Presence of Magnesium–Aluminium Hydrotalcites as Catalyst Precursors

Article abstract of DOI:10.1002/cssc.201601765

The base-catalysed reactions of OH-bearing biobased derivatives (BBDs) including glycerol formal, solketal, glycerol carbonate, furfuryl alcohol and tetrahydrofurfuryl alcohol with non-toxic dialkyl carbonates (dimethyl and diethyl carbonate) were explored under continuous-flow (CF) conditions in the presence of three Na-exchanged Y- and X-faujasites (FAUs) and four Mg–Al hydrotalcites (HTs). Compared to previous etherification protocols mediated by dialkyl carbonates, the reported procedure offers substantial improvements not only in terms of (chemo)selectivity but also for the recyclability of the catalysts, workup, ease of product purification and, importantly, process intensification. Characterisation studies proved that both HT30 and KW2000 hydrotalcites acted as catalyst precursors: during the thermal activation pre-treatments, the typical lamellar structure of the hydrotalcite was broken down gradually into a MgO-like phase (periclase) or rather a magnesia–alumina solid solution, which was the genuine catalytic phase.

Full text of DOI:10.1002/cssc.201601765

Accepted Article
Title: Continuous-Flow O-alkylation of Bio-Based Derivatives with
Dialkyl Carbonates in the Presence of Mg-Al hydrotalcites as
Catalyst Precursors
Authors: Maurizio Selva, Alvise Perosa, Pietro Riello, Thomas
Maschmeyer, and Lisa Cattelan
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201601765
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10.1002/cssc.201601765
ChemSusChem
FULL PAPER
Continuous-Flow O-alkylation of Bio-Based Derivatives with
Dialkyl Carbonates in the Presence of Mg–Al hydrotalcites as
Catalyst Precursors
Lisa Cattelan,[a], [b] Alvise Perosa,[a] Piero Riello,[a] Thomas Maschmeyer,[b] and Maurizio Selva*[a]
[3]
Abstract: The base-catalysed reaction of OH-bearing bio-based
derivatives (BBDs) including glycerol formal, solketal, glycerol
carbonate, furfuryl alcohol and tetrahydrofurfuryl alcohol with non-
toxic dialkyl carbonates (dimethyl and diethyl carbonate) was
explored under continuous-flow (CF) conditions in the presence of
three Na-exchanged Y- and X-faujasites (FAUs) and four Mg–Al
hydrotalcites (HTs). Compared to previous etherification protocols
mediated by dialkyl carbonates, the reported procedure brings about
substantial improvements not only in terms of (chemo)-selectivity,
but also for the recyclability of catalysts, work-up, ease of product
purification, and importantly, process intensification. Characterisation
studies proved that both HT30 and KW2000 hydrotalcites acted as
catalyst precursors: already during the thermal activation pre-
treatments, the typical lamellar structure of hydrotalcites was
gradually broken down into a MgO-like phase (periclase) or rather a
magnesia–alumina solid solution which was the genuine catalytic
phase.
protocols mediated by dialkyl carbonates (ROCO2R, DAlCs
with the chemical valorisation of bio-based platform molecules.
Necessarily such an activity requires multidisciplinary
)
a
approach, combining aspects of organic and physical chemistry
as well as chemical engineering and materials science.
Within this context, our attention has been focused on five model
OH-bearing BBDs (Scheme 1).
Scheme 1. Model OH-bearing BBDs.
They include solketal (1a), glycerol formal (2a/2a’), and glycerol
carbonate (3a): these compounds are among the most
celebrated derivatives of glycerol and their applications span
across multiple sectors: from fine chemicals, cosmetics and
pharmaceuticals to bio- fuels and lubricants, bio-based solvents
and polymers.[4] The availability of glycerol is mostly fueled by
plant-oil-based biodiesel manufacture generating large amounts
of glycerol as a co-product.[5] The other two compounds of
Scheme 1, specifically furfuryl alcohol (4a) and its hydrogenated
homologue tetrahydrofurfuryl alcohol (5a), are derived from
furfural which in turn, is generated prior by the dehydration of
sugar components (glucose and xylose) that constitute a large
portion of lignocellulosic biomass.[6] Both 4a and 5a find uses as
modifiers and templates for polymers, as fibers and
nanocomposites,[7] as well as sources of polyols.[8]
Introduction
In the past 15 years, an enormous effort has been devoted to
the identification of the most promising biomass-derived
compounds. Among the available analyses, an extensive work
carried out by the US Department of Energy in 2004 and its
revisitation in 2010 still represent cornerstones in this field. For
the first time, they offered rational criteria for the selection of the
so-called “top 10” platform chemicals, namely a small group of
bio-based derivatives (BBDs) that could be utilized as building
blocks for higher-value products and materials.[1] Although this
approach has been refined further over the years,[2] current ”top
ten” lists of biomass-derived platform compounds still include
most of the originally identified compounds, particularly mono-
and di-carboxylic functionalised acids, 3-hydroxybutyrolactone, a
bunch of bio-hydrocarbons derived from isoprene, glycerol and
derivatives, and few other sugars such as sorbitol and xylitol.
These considerations have inspired us to try and integrate green
In addition the presence of
a
OH-capped tether
(hydroxymethylene group) enables an avenue for the
derivatisation of all five compounds (1a-5a): of particular note,
esterification and alkylation protocols allow to expand the
potential of OH-BBDs through the synthesis of substantially
value-added products, including intermediates, solvents and
biologically active molecules as glycerol ethers, esters and
carbonates,[9] and additives for biodiesel blends based on
furanyl ethers derivatives.[6a]
The combined effect of a high reaction temperature (200-
220 °C) and a base catalyst (K2CO3) allowed an almost
exclusive O-alkylation of GAs. The reaction showed excellent
features from both synthetic and environmental standpoints,
since it not only proceeded with etherification selectivities and
yields as high as 99% and >80%, respectively, but it also
coupled the use of renewable and/or non-toxic reactants as
glycerol acetals and DAlCs in a catalytic process with no by-
products other than CO2 (which did not present a disposal
problem) and alcohols that were recyclable for the synthesis of
dialkyl carbonates. Only trace amounts of by-products from
competitive transesterification processes were detected.
[a]
[b]
L. Cattelan, Prof. A. Perosa, Prof. P. Riello, Prof. M. Selva
Department of Molecular Sciences and Nanosystems
Università Ca’ Foscari Venezia
Via Torino, 155 – Venezia Mestre (Italy)
E-mail: selva@unive.it
L. Cattelan, Prof. T. Maschmeyer
Laboratory of Advanced Catalysis for Sustainability, School of
Chemistry F11,
University of Sydney
Sydney, 2006, Australia
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201601765
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However, under the explored batch conditions, the sequence
>99% selectivity in the presence of calcined HT Pural® MG30
suffered from two major drawbacks: i) slow alkylation kinetics, i.e. (cHT30; Mg/Al= 0.5) as a catalyst.
reactions could require up to 80 hours for completion; ii)
extensive competitive decarboxylations of dialkyl carbonates
with formation of the corresponding dialkyl ethers,[11] (dashed
path, Scheme 2).
The CF-protocol can be extended to the reaction of substrates of
Scheme 1 with both dimethyl and diethyl carbonate: quantitative
conversion and O-alkylation selectivity comparable to those of
solketal, are achieved over calcined HTs catalysts, except for
the case of furfuryl alcohol (4a) for which only the
transesterification derivative (furan-2-ylmethyl methyl carbonate)
can be obtained. Overall, the CF-procedure not only exemplifies
an original approach for the upgrading of OH-bearing biobased
substrates, but it can also overcome the safety/scale-up
limitations of batchwise reactions since it operates at
atmospheric pressure and with a productivity [2 g/(gcat·h)] on
ether derivatives which is up to 200-fold higher than that of batch
methods.
Scheme 2. The etherification of glycerol acetals with dialkyl carbonates in the
presence of K2CO3 catalyst (top). The dashed path shows a concurrent
decarboxylation reaction of dialkyl carbonates.
Results and discussion
This necessitates the use of a high excess of alkylating agents,
and importantly, it involved the co-generation of an autogenous
pressure of ≥ 60 bar, both these aspects hindered the scale-up
of the process. However, the benefits and disadvantages
stimulated us to devise a new broad-based protocol aimed at
expanding the alkylating capabilities of dialkyl carbonates to
different OH-bearing BBDs, and at the same time, minimising
the limitations described above. The implementation of a
continuous-flow (CF) procedure was an attractive option with
which to reach the conversion target, since it offered the best
possible control of the reaction parameters (T, p, and reactants
molar ratio) to improve the process kinetics, productivity and
safety. Under CF-conditions, however, alkaline carbonates
needed anion activators (polyethylene glycols) and/or
specifically shaped reactors (e.g. continuous-stirred-tank-
reactors, CSTR) in order to perform as catalysts,[12] since not
only they (alkaline carbonates) were too weak bases, but they
were also partially soluble in both reactant DAlCs and in the co-
produced alcohols (Scheme 2).[13] Alternative solid systems had
to be considered. Candidates were chosen from the families of
faujasite (FAU) and Mg-Al hydrotalcite (HT) solids. Both Y- and
X-type FAU have been extensively reported by us as catalysts
for DAlCs-promoted N, O, and S-alkylations of a large variety of
nucleophiles, including anilines, benzyl alcohols, aminophenols,
aminobenzyl alcohols, mercaptophenols, mercaptobenzoic acids,
hydroxybenzoic acids, and functionalised phenols of the lignin
scaffold.[14] Yet HTs have been investigated almost exclusively
as catalysts for transesterifications with dialkyl carbonates,[15]
while only few reports described the methylation of some
phenols and benzyl alcohol with dimethyl carbonate over Mg-Al
hydrotalcites as such or over modified HT systems.[16] In fact, the
role of the catalyst is still being debated.
The apparatus used for continuous-flow (CF) reactions was
comprised of an HPLC pump, an oven containing a tubular
reactor filled with the catalyst, a Rheodyne sampling valve, and
a back-pressure regulator (details are reported in SI).
Dialkyl carbonates including dimethyl- and diethyl-carbonate,
and compounds 1a-5a were commercially available ACS grade
compounds.
Catalysts. Three alkali metal exchanged X- and Y-type faujasites
(FAUs) including two commercially available NaX and NaY
solids, and a CsY zeolite were used. The latter was prepared as
described in the experimental section.
Four hydrotalcites (HTs) were also used. Three of them were
labelled as HT30, HT63 and HT70: these were from Sasol Italy.
A fourth sample was labelled as KW2000 and it was sourced
from Kyowa Kagaku Kogyo Company Limited. Some features of
these HT solids are summarized in Table 1.
Table 1. Hydrotalcites used in this work
Post-
MgO:
Al2O3
(%)
Surface
area
Label of
sample
Synthesis
residues
(%)
Entry
Source
(m²/g) b
Pural® MG30,
Sasol
1
2
3
HT30
HT63
HT70
30:70 a
63:37 a
70:30 a
250
230
180
Na:
1.5 10-3 c
Pural® MG63,
Sasol
K:
1.7 10-3 c
Pural® MG70,
Sasol
Kyowa
Kagaku Kogyo
Co. Ltd
Na: 2.5 10-2
64:36 a
d
4
KW2000
190
(65:35)d
K: 1.1 10-2 d
The present work demonstrates that the use of HTs allows for
the setup of a particularly robust catalytic CF-method for the
etherification of OH-bearing BBDs with DAlCs. For example, at
210 °C and ambient pressure, the model reaction of solketal (1a)
with dimethyl carbonate shows the formation of the
corresponding methyl ether with quantitative conversion and
b,c
a % Wt ratio specified by the Supplier.
dDetermined by ICP measures
Data specified by the Supplier.
In the case of KW2000, ICP analysis were used to determine
post-synthesis residues of Na and K (see experimental).
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MgO as such and a 30:70 physical mixture of MgO (Aldrich,
particle size: 325 mesh; surface area: 114 m2/g; average pore
diameter: 9.4 nm) and basic γ-Al2O3 (Macherey-Nagel, pH 9,5 ±
0,3; particle size: 50-200 µm; surface area: 130 m2/g) were also
used as additional catalysts for two comparative tests.
The reaction of solketal (1a) with DMC. This reaction was
chosen as a model to investigate the activity of different
catalysts and the effect of three major reaction parameters, i.e.
temperature, pressure, time, and reactants molar ratio.
followed by methylation and transesterification of the
corresponding (linear) derivatives.
At 250-275 °C and 10 bar, FAUs proved unsuitable catalysts for
the reaction investigated. At best, the conversion of solketal was
moderate (60-70%). The more basic NaX and CsY systems
were slightly more active than NaY, but they favoured the
transesterification product 1c with a selectivity of 60-70%.[19]
NaY improved the formation of the O-methyl derivative 1b (up to
40%), but it also induced side-reactions leading to a sizeable
amount of unidentified by-products. In all cases, conversion and
selectivity were steady after 180 min and they did not
substantially vary when CF-tests were prolonged up to 18 hours
(SI section: Figures S2a-b, and Table S1).
This behaviour neatly contrasted our previously reported results
for the reaction of benzyl-type alcohols with DMC during which
the corresponding O-methyl ethers were achieved in
substantially quantitative yields and selectivity in the presence of
NaX and NaY catalysts.[20] Unlike aromatic or benzyl
substrates,[14,21,22] the cycloaliphatic structure of solketal was
plausibly more weakly coordinated by the FAU surface, thereby
altering the reaction outcome. According to the HSAB principle,
other Authors have also proposed that hard nucleophiles such
as aliphatic alcohols prefer the hard electrophilic site of DMC (i.e.
the carboxyl carbon), rather than the softer methyl group.[23]
The scenario completely changed in the presence of HTs.
Particularly, KW2000 allowed to improve the conversion and,
even more remarkably, the methylation selectivity. Reactions
were carried out under the same conditions of Figure 2b (275 °C,
10 bar, W=5; F=0.1 mL/min), but they were followed for a longer
time (18-20h). Figures 1a-b summarise the performance of HT
catalysts with emphasis on KW2000.
CF-tests over different catalysts. The performance of FAUs and
HTs catalysts were compared using the above described CF-
apparatus. Temperature and pressure for initial tests were
selected according to our previous results obtained for both
batch alkylation and CF-transesterification reactions of glycerol
acetals with dialkyl carbonates:[10,11] in particular, experiments
were carried out at T and p in the range of 200-275 °C, and 5-50
bar, respectively.
Each of the investigated catalyst was used to fill the inner
volume of the CF-reactor as uniformly and completely as
possible.[17] Therefore, the amounts of solid samples were
adjusted according to their apparent density: NaY, NaX, CsY,
KW2000, HT30, HT63, and HT70 were used as such in slightly
different quantities of 0.68, 0.54, 0.86, 0.52, 0.73, 0.51, and 0.85
g, respectively. Each catalyst was dehydrated under vacuum
(70 °C, 18 mm, overnight) before use. In all tests, the same 1.83
M solution of solketal in DMC [DMC:solketal molar ratio (W)=5]
was fed to the reactor at a total volumetric flow rate (F) of 0.1 mL
min−1 corresponding to a contact time of about 10 minutes. The
excess DMC served both as a reagent and a carrier/solvent.
During the CF-experiments, periodic GC/MS analyses of the
mixtures collected at the reactor outlet, allowed to evaluate both
the reaction conversion (measured with respect to solketal as
the limiting reagent) and the products distribution. Each test was
duplicated to check for reproducibility; in each reaction, fresh
samples of the same batch of catalysts were used.[18]
All the hydrotalcites investigated were, on average, better
catalysts than FAUs, but KW2000 offered a performance far
superior than other HTs (HT30, HT63, and HT70) (Figure 1a).
The KW2000-catalyzed reaction proceeded with a very good
conversion and O-methylation selectivity of 99% and 89%,
respectively (dashed green line and blue bar); particularly, the
ring of the reactant acetal was substantially preserved, avoiding
further transformations, and only traces of the transesterification
product 1c were observed. The second best hydrotalcite, i.e.
Several reactions took place under the conditions explored, i.e.
the double electrophilic reactivity of dimethyl carbonate
accounted for the formation of O-methyl and transesterification
derivatives 1b and 1c, respectively (Scheme 3: paths i and ii).
HT30, still allowed
a
substantially quantitative process
(conversion 96%), although the methyl derivative 1b did not
exceed 60% of the total of observed products (second group of
bars, from left).
The reaction catalysed by KW2000 and HT30 were explored in
greater depth by analysing samples of the mixture collected at
the CF-reactor every 60 minutes for up to 20 hours (see SI
section, Figures S3a-b). In the presence of KW2000, a steady
and almost complete conversion was obtained after just one
hour, while the O-methylation selectivity reached a maximum of
95% in 4 h and it only slight decreased to 87-89% at the end of
the test (18-20 h; Figure S3a). The HT-30 catalysed process
showed that in the first 4 hours, the conversion was below 80%
and the transesterification compound 1c was the dominant
product (Figure S3b); while, after 18-20 h, the residual solketal
was in trace amounts (2-5%) and a steady formation of both 1b
and unidentified products was observed (60% and 40%,
Scheme 3. Major products of the CF-catalytic reaction of solketal with DMC.
Subsequently, compound 1c plausibly underwent a further
transesterification with solketal and/or a dismutation reaction
that afforded the symmetric carbonate product 1d (Scheme 3:
paths iii and iv). The structures of 1b, 1c, and 1d were assigned
by GC/MS and NMR analyses, and by comparison to authentic
samples previously synthesized by us. Moreover, some
unidentified side-products were also observed. These
compounds are referred to as “others” and their GC/MS data are
consistent with a ring-opening reaction of the acetal ring
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10.1002/cssc.201601765
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respectively). Albeit with a different products distribution, both
the reactions displayed an initial (induction) period during which
mixtures of products formed. Thereafter, conversion and
selectivity stabilized: the almost exclusive formation of ether 1b
was noticed in the case of KW2000. Based on this catalyst,
three additional short experiments (4 hours each) were carried
out to examine the effect of the temperature in the range of 200-
275 °C. Other conditions were kept unaltered (10 bar, W=5;
F=0.1 mL/min). (Figure 1b).
In the presence of KW2000, further tests were also devised to
investigate on the effects of the pressure and the DMC:solketal
molar ratio (W) (see SI section, Figures S4a-b). At 275 °C (W=5;
F=0.1 mL/min, 4 h), if the pressure was either decreased or
increased to 5 and 50 bar, respectively, the conversion slightly
diminished (from 99% to 95%), but the methylation selectivity
dropped from 95% to 75-80% because of the onset of
transesterification and unidentified side-reactions. Pressure
plausibly acted on the partition of reactants between the liquid
and the vapour phases and consequently, on the contact of
solketal and DMC with the catalyst surface. A similar behaviour
was already noticed by us during the investigation of thermal
(catalyst-free) transesterification of both glycerol acetals and
glycerol with DMC.[24] Yet, at 275 °C and 10 bar (F=0.1 mL/min,
4 h), if W was decreased from 5 to 1.1, both the conversion and
the O-methylation selectivity fell to 58% and 62%, respectively,
with a sizeable formation of products 1c, 1d and “others”.
Two effects might plausibly account for this result at a constant
F: i) the higher the DMC amount (W=5), the higher the rate (and
conversion) of DMC-mediated reactions. Under these conditions,
even though (reversible) transesterifications took place, the
corresponding products (1c and 1d) were observed in only
minor amounts, because these compounds were consumed by
the onset of the parallel irreversible O-alkylation reaction which
proceeded almost to completion (Scheme 3 and Figures 1a and
S3-4). However, if W<5, particularly at W=1.1, less energy
demanding reaction pathways, i.e. transesterification processes,
became more evident and so, the quantities of derivatives 1c
and 1d increased.
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
(a)
275 °C
10 bar
18 h
KW2000
HT30
HT63
HT70
Hydrotalcite catalysts
110
100
90
80
70
60
50
40
30
20
10
0
110
100
90
80
70
60
50
40
30
20
10
0
(b)
KW2000
10 bar
4 h
ii) a low DMC:solketal molar ratio (W=1.1-2) not only disfavored
the solvation of the acetal by DMC and the mutual interactions
between reactants, but it also facilitated the occurrence of
intramolecular side-processes including, for example, the ring
opening of solketal (“others” as by-products) adsorbed over the
catalyst surface.
CF-tests with calcined HTs. When heated at 200 °C,
hydrotalcites release water, but in the proximity of 300 °C (near
the temperature used in Figure 1) and above, a collapse of the
typical layered structure of HTs occurs with formation of Mg/Al
mixed oxides.[25] These facts prompted us to explore the
catalytic activity of calcined hydrotalcites (c-HTs) in the reaction
of solketal with DMC. c-HTs were prepared by heating Pural®
(HT30, HT63, HT70) and KW2000 solids at 450 °C in a dried air-
flow for 16 hours.[26] c-HTs showed quite similar apparent
densities and were used in the same amount (0.5 g each) for
catalytic tests. In all cases, a 1.83 M solution of solketal in DMC
[DMC:solketal molar ratio (W)=5] was fed to the reactor at a total
volumetric flow rate (F) of 0.1 mL min−1. Experiments proved that
calcined hydrotalcites, particularly c-HT30, dramatically
improved the reaction outcome: not only both the solketal
conversion and the O-methylation selectivity increased, but also
the c-HT30 catalyst was active at temperature and pressure
remarkably lower than those previously used. With other
conditions unaltered (W=5, F=0.1 mL/min), reactions could be
run at temperatures and pressures as low as only 210 °C and 1
bar.
200
250
275
Temperature (°C)
Figure 1. Trends of conversion and products distribution for the CF-reaction of
solketal with DMC: a) comparison of different HT catalysts at 275 °C and 10
bar; b) effect of the temperature during tests catalysed by KW2000 at different
temperatures. Coloured bars (from left to right) show the selectivity towards
the O-methyl product (1b, ●), the transesterification and/or dismutation
derivatives (1c, ●, and 1d, ●, respectively), and unidentified compounds
(others, ●). The dashed line (from right to left) shows the trend of solketal
conversion. DMC and solketal were used in a 5:1 molar ratio (W), respectively,
and the total flow rate was of 0.1 mL/min.
At 200 °C, the transesterification and/or disproportion reactions
were the major transformations also on KW2000 (green and
ochre bars). However, the O-methylation process was triggered
by an increase of the temperature, and it was progressively
favored over 250 °C.
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Figure 2 details the performance of c-HT30 during 20 hours of
time-on-stream, while Table 2 compares the activity and the O-
methylation selectivity of different c-HTs. Each test was
duplicated to check for reproducibility (see note 18).
The outcome of the reaction catalyzed by c-HT30 exemplified
one of the best ever reported O-methylation reaction of alcohol-
like functions mediated by DMC: solketal was quantitatively
transformed in the methyl ether 1b, and a steady efficiency was
ensured for up to at least 20 hours. The comparison of Figures 2
to Figures 1 and S2a-b also indicated that not only the
calcination of HTs, but even the thermal treatment during high-
temperature (275 °C) reactions considerably modified the
catalytic properties of both HT30 and KW2000.
The ability of calcined hydrotalcites to promote the investigated
reaction was contrasted with other CF-experiments in which c-
HTs were replaced by either MgO (0.85 g of a mixture of MgO in
60% wt with ground glass Raschig rings) or a physical mixture of
MgO and basic -Al2O3 in 30:70 molar ratio, respectively (0.85 g)
which mimicked the formal composition of HT30. Both these
solids were calcined at 450 °C for 6 hours before use. Under the
conditions described above (275 °C, 10 bar, W=5, F=0.01
mL/min), the two catalysts gave modest results: the best one
was MgO by which the reaction proceeded with conversion and
O-methyl selectivity of only 80% and 48%, respectively.
Notwithstanding MgO and -Al2O3 were recently reported to
catalyse both O- and N-alkylations mediated by DMC,[27,28] these
solids were obviously inadequate for the reaction investigated.
The study so far demonstrated that KW2000 and HT30 were the
most promising catalysts. To continue exploring the potential of
such materials, in particular of the calcined system c-HT30, the
investigation was focused on the scope and limitations of the
CF-etherification of other OH-bearing bio-based derivatives with
both dimethyl and diethyl carbonate.
104
102
100
98
104
102
100
98
c-HT30
210 °C
1 bar
96
96
94
94
92
92
90
90
88
88
Scope of the reaction: different reactants, mass balance, and
productivity. Substrates of Scheme 1 including glycerol formal
(2a+2a’), glycerol carbonate (3a), furfuryl- and tetrahydrofurfuryl-
alcohols (4a and 5a) were set to react with DMC or DEC under
CF-conditions. Accordingly, a homogeneous solution of dialkyl
carbonate and the substrate was delivered for 6 hours to the CF-
reactor filled with c-HT30 (0.5 g) as a catalyst. The total flow rate
was of 0.1 mL/min. All CF-tests were repeated twice to check for
reproducibility.
Results are shown in Table 3 which reports reaction conditions,
isolated yields (Y) and productivity (P: grams of products
obtained in one hour per g of catalyst) for each of the studied
processes, and Scheme 4 which summarizes the reactions and
the structure of the products. For the sake of completeness,
Table 3 also includes the above described results of the reaction
of solketal with DMC.
86
86
30
20
10
0
30
20
10
0
1
2
3
4
18 19 20
Time (h)
Figure 2. CF-reaction of solketal with DMC in the presence of c-HT30 catalyst
at 210 °C and ambient pressure. Trends of O-methylation selectivity and
conversion of solketal with time (blue bars and dashed green profile). Other
conditions: W=5, F=0.1 mL/min.
At the same reaction temperature (210 °C) used for c-HT30,
other c-HT catalysts allowed conversion and O-methylation
selectivity not exceeding 88% and 27%, respectively, even when
operating at 10 bar (Table 2: compare c-HT70, c-KW2000, and
c-HT63 in entries 2-4, columns 4-7). Minor improvements were
achieved at 225 °C (entries 2-4, columns 8-11). Only at 275 °C
(10 bar, W=5, F=0.01 mL/min), did the reaction proceed with
complete conversion of solketal to the methyl derivative 1b over
all c-HTs catalysts.
Reactions could be carried out at atmospheric pressure, but the
operating temperature and the reactants molar ratio must be
optimised case-by-case in the range of 210-275 °C and 5-20,
respectively: except for compound 4a, a quantitative conversion
was reached for all the other substrates with O-alkylation
selectivity of 65-99%.
Table 2. Comparison of c-HTs catalysts for the reaction of solketal with DMC
Reaction Temperature
210 °C
225 °C
Sel.(%)b
Sel.(%)b
p/t
(bar/h)
Conv.’n
Conv.’n
Entry
Catalyst
(%)a
(%)a
1b
99
27
15
2
1c
-
Others
1b
99
40
20
8
1c
-
others
1
2
3
4
c-HT30
99
88
85
78
-
99
95
93
87
-
c-HT70
49
61
68
24
24
30
42
68
61
18
12
31
10/18
c-KW2000
c-HT63
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a,b
Conversion of solketal and selectivity towards the O-methyl derivative 1b, the transesterification compound 1c, and other products (including the
disproportionation derivative 1d and unidentified by-products), respectively. Other conditions: W=5, F=0.1 mL/min.
Table 3. The CF-reactions of substrates 1a-5a with both DMC and DEC in the presence of c-HT30 catalyst.a
Product
Dialkyl
carbonate
Wb
ratio
T
(°C)
Conv.’n
Sel.
(%)d
Entry
1
Substrate
(%)c
Y e
(%)
P f
Structure
[gprod/(gcat h)]
1a
DMC
DMC
5
5
210
220
99
99
99
99
92
81
2.64
1.92
2
3
4
2a/2a’
3a
DMC
DMC
20
10
210
150
98
88
65
91
55
80
0.42
1.26
4a
5
6
5a
1a
DMC
DEC
5
5
260
250
99
99
92
99
82
92
1.94
2.04
7
2a/2a’
DEC
5
275
99
99
84
1.62
a
b
Reactions were carried out for 6 hours (20 hours only for 1a, entry 1) at ambient pressure and at a total flow rate of 0.1 mL/min. Dialkyl carbonate:substrate
molar ratio. c Conversion of the substrate (determined by GC). d Selectivity towards the shown product (determined by GC). e Isolated yield was evaluated by the
work-up of a mixture collected at the reactor outlet for 6 hours (15 hours for 1a, entry 1). f Reaction productivity was calculated on isolated yields.
and 2e/2e’ were obtained at 220 and 275 °C, respectively, while
compounds 1b and 1e formed at the lower temperatures of 210
and 250 °C (compare entries 1-2 and 6-7). A similar difference
was observed also in the transesterification of glycerol acetals
with dialkylcarbonates (DAlCs).[24] The higher density of glycerol
formal (1.21 g mL−1) with respect to solketal (1.07 g mL−1) might
play a role. ii) The synthesis of ethyl ethers required a
considerably higher temperature (250-275 °C) than that of
methyl ethers (210-220 °C; compare entries 1 and 6, 2 and 7).
This result was in line with the trend noticed in a number of
Scheme 4. The etherification of OH-bearing BBDs 1a-3a and 5a (top). The
transesterification of furfuryl alcohol with DMC (bottom).
processes mediated by DAlCs, including transesterifications,
decarboxylations, etherifications, and alkylations.[11] Steric
reasons most plausibly account for the lower electrophilic
reactivity of DEC compared to DMC.
GC/MS (details are in the SI section).
All products were isolated and characterised by NMR and
Products were recovered in good-to-excellent isolated yields
(81-92%); though, the more volatile derivatives of glycerol formal
gave slightly poorer results because of some technical
Solketal and glycerol formal (1a and 2a/2a’) could be
quantitatively converted into the corresponding methyl and ethyl
ethers (1b and 2b/2b’) by using DMC and DEC as O-alkylating
agents. Two facts emerged: i) regardless of the dialkyl
carbonate, the etherification of glycerol formal was more energy
demanding than that of solketal: methyl and ethyl ethers 2b/2b’
difficulties in the separation from DMC or DEC (2b/2b’: 81%;
2e/2e’: 84%; entries 2 and 7).[29] Isomeric ethers 2b/2b′ and
2e/2e′ were obtained in the same (3:2) relative ratio of the
starting acetals 2a and 2a’.
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The reaction productivity – calculated on the isolated yield –
allowed further remarkable considerations: if compared to our
previously reported batch-wise (autoclave) method[10] which
allowed P values of 0.01-0.02 gprod/(gcat h) for the synthesis of
compounds 1b and 2b/2b’, the CF-procedure boosted the
productivity by a factor of 100-200 [up to  2 gprod/(gcat h); entries
1 and 2], thereby substantiating the synthetic potential of such a
protocol. The overall mass balance for the alkylation of glycerol
acetals was also validated by NMR of crude mixtures collected
at the reactor outlet and gravimetric analyses of the catalytic bed
carried out before and after CF-tests: both such checks
indicated that neither heavy products nor noticeable coke
formation (on the catalyst) occurred even after prolonged
experiments (up to 20 hours, entry 1).
A similar outcome was also observed for the CF-reaction of
tetrahydrofurfuryl alcohol (5a) with DMC, in which we obtained
the corresponding O-methyl ether (5b) with 92% and 80%
selectivity and isolated yield, respectively (entry 5). Though, a
higher operating temperature of 250 °C was necessary.
The control of the chemoselectivity was significantly more
difficult for the reactions of glycerol carbonate (3a) and furfuryl
alcohol (4a).
methyl chloroformate;[33] the process described here was the first
successful example of the same preparation using DMC as a
nontoxic reagent.
Experiments of Table 4 proved that c-HT30 was not only an
efficient catalyst, but it could also be recycled without loss of
performance. As an example, once the CF-alkylation of solketal
was run for 20 hours (entry 1), a simple cleaning cycle of the
catalytic bed with methanol (50 mL at 1 mL/min, 50 °C,
atmospheric pressure) allowed to reset the system to its initial
conditions, and to restart the same or a new reaction (e.g. the
alkylation of glycerol formal). No differences of activity were
appreciated when a used c-HT30 catalyst was compared to a
freshly calcined HT30 sample, thereby confirming the
robustness of the CF-procedure. This was further substantiated
by the very low metal leaching from the catalyst: after the test of
entry 1 (Table 4), ICP measures demonstrated that the Al and
Mg concentrations in the stream recovered at the outlet of the
reactor were 40 and 85 ppb, respectively (see experimental and
SI sections for details). The estimated mass loss of the catalytic
bed corresponded to 38 µg per 20 working hours. Very low level
of metal leaching was observed also for others reactions
catalyzed by HT-derived Mg/Al mixed oxides: two examples are
the transesterifications of natural fats and liquid-phase Michael
additions.[34]
Glycerol carbonate was sensitive to the competitive reactions of
transesterification[24b] and decarboxylation [30, 31] producing higher
carbonate homologues and glycidol, respectively. In this case, a
O-methylation selectivity of 65% was achieved at 210 °C in the
presence of a large excess of the alkylating agent (entry 3 and
Scheme 5).
It was also estimated that, after vacuum distillation of mixtures
collected at the reactor outlet, up to 80% of the unreacted DMC
(and its azeotrope with MeOH,[35]) and DEC could be recovered
and recycled with minimal generation of wastes.
Overall, the CF-procedure proved versatile and suitable to
different reactants, both carbonates and alcohols. In particular,
c-HT30 provided a rational long term stability and selectivity
(with no appreciable leaching or poisoning phenomena) and a
productivity which could be orders of magnitude higher than that
achieved by batch alkylation methods. To the best of our
knowledge, a result comparable to that of c-HT30 was reported
only in the continuous-flow O-methylation of primary alcohols
with DMC that was catalyzed by -Al2O3.[28] However, this
protocol could hardly be extended: sec-alcohols gave substantial
side reactions of eliminations to alkenes promoted by the acidity
of the catalyst;[36] moreover, even at 150 °C, DMC underwent an
extensive decarboxylation reaction to dimethyl ether.[37] -Al2O3
Scheme 5. The CF-reaction of glycerol carbonate with DMC in the presence
of c-HT30 (the structure of 3c was assigned by GC/MS; others were
unidentified by-products).
Due to dilution/solvation effects, these conditions contribute to
minimise the contact between glycerol carbonate and the
catalytic surface, thereby limiting undesired decarboxylation
reactions. Of note, the methylation productivity (0.42) of the
process was two-fold higher than the best previously reported
value (0.21) obtained in a batch reaction of glycerol carbonate
with DMC catalysed by Al2O3.[32]
would have posed
a chemoselectivity concern also for
functionalised alcohols used in this study: glycerol acetals
(solketal and glycerol formal) and furfuryl alcohol are extremely
sensitive to ring aperture and polymerisation reactions catalysed
by acids, while, glycerol carbonate would undergo a rapid
release of CO2.
It should be noted that the decarboxylation of dialkyl carbonates
may be catalyzed by FAUs,[11] or by hydrotalcites as such.[38]
However, under the CF-conditions here explored, we observed
that calcined HTs were far less efficient for the same process.
This was corroborated by an additional test in which solketal
methyl carbonate (1c) was set to react at 210 °C and ambient
pressure over a catalytic bed of c-HT30. At a conversion of 75%,
compound 1c underwent a predominant disproportion reaction
towards bis((2,2-dimethyl-1,3-dioxolan-4-yl)methyl) carbonate
Furfuryl alcohol (4a) proved too reactive to allow the formation of
the corresponding alkyl ether. At T≥180 °C, the reaction of 4a
with DMC gave polymeric by-products, which blocked the
catalytic bed rapidly. However, at a lower temperature (150 °C),
a highly selective transesterification occurred, providing furan-2-
ylmethyl methyl carbonate (4b) in 80% isolated yield (entry 4).
This equilibrium reaction was also favored by a large DMC
excess (10 molar equivs. with respect to 4a). Of note, previously
reported syntheses of 4b were only ever based on the reaction
of furfuryl alcohol with a harmful phosgene derivative such as
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(1d, 80%), rather than the decarboxylation to derivative 1b
(10%) (Scheme 6).
of MgO was smaller (4.190±0.001 Å) than that of pure MgO
(4.2112 Å), thereby indicating the formation of a solid solution
(s.s.) of alumina in magnesia where Al occupied 15% of cationic
sites.[42] By assuming that the s.s. dissolved the Al2O3 phase, the
Rietveld quantitative analysis of Figure 4 allowed to estimate
proportions of Al2O3 (39 wt%) and MgO (61 wt%) very close to
the nominal composition of the sample. This indirectly supported
the presence of almost pure Al2O3 as the amorphous phase.
Finally, the s.s. was totally absent in the fresh sample of
KW2000 (Figure 3b) in which periclase showed a unit cell
parameter of 4.213±0.002 Å.
Scheme 6. CF-reaction of 1c over c-HT30 at 210 °C and ambient pressure.
Cyclohexane was used as a solvent (5 molar equivs. with respect to 1c). Total
flow rate: 0.1 mL/min.
The XRD patterns of c-HT63 and c-HT70 solids were very
similar to that of c-KW2000 (see Figure S32, SI section).
By contrast, the diffraction pattern of c-HT30 - containing a
higher fraction of Al – was totally different (Figure 5). The
Rietveld analyses carried out as mentioned above, allowed to
identify the presence of a s.s. MgO/Al2O3, the spinel Al2MgO4,
and k-Al2O3, and to estimate that the corresponding amounts of
these three phases were 15, 63, and 22 wt%, respectively.
Although the obtained fit was not as good as the previous one
(cfr Rwp indexes of Figures 4 and 5), the calculated proportions
of Al2O3 (69 wt%) and MgO (31 wt%) were very close to those of
the nominal composition of the sample.
BET results were in substantial agreement with the trend of
surface area that was usually observed for fresh and calcined
HTs:[25a, 45] notwithstanding fresh samples of KW2000 and HT30
were not pure hydrotalcite phases, an increase of SBET was
noticed once both solids were calcined at 450 °C (Table S2: 108
and 199 m2/g for f- and c-KW2000, and 136 and 247 m2/g for f-
and c-HT30, respectively).
Product 1d was isolated in a 58% yield and it was fully
characterised by GC/MS and NMR analyses (see SI section).
Albeit indirectly, this test confirmed that c-HT30 could also
improve the efficiency of the alkylation reactions desired here.
Characterization and role of the catalyst. The performance of the
investigated catalysts was consistent with structural
modifications induced by calcination and, potentially during the
reactions itself. The two most active systems, KW2000 and
HT30, were therefore subjected to a more in-depth analysis and
characterization.
A total of four samples were examined for XRD analyses: two of
them (f-KW2000 and f-HT30) were fresh solids never used for
catalytic tests; the other two specimens (c-KW2000 and c-HT30)
were calcined catalysts used for reactions carried out under the
conditions of Table 2 (For details, see Table S2 in the SI). The
XRD patterns are shown in Figures 3 for fresh materials, and
Figures 4 and 5 for calcined solids, respectively.
Fresh solids were constituted by at least two phases in which
the hydrotalcite structure (ICSD 81963) was present along with
large fractions of: i) Periclase (MgO: ICSD 9863; 60±1 wt%) in f-
KW2000; ii) Boehmite (aluminum oxide-hydroxide: ICSD 36340;
53±1 wt%) in f-HT30. The quantitative relationships between the
constituent phases were obtained by the Rietveld analysis
shown in Figures 3c-d. For comparison, XRD patterns were
acquired also for fresh solid samples of HT63 and HT70: these
materials were single phase hydrotalcites (for further details, see
Figures S31, SI section).
Overall, the comparative bulk characterisation indicated that
fresh solids, both f-KW2000 and f-HT30, acted as precursors of
active catalysts for the O-alkylation reactions investigated here.
Such active phases were composed by a solid solution of Mg/Al
oxides along with amorphous alumina, and they formed during
thermal treatments of the hydrotalcites investigated.[27,39,40] This
matched the behaviour described in the literature which is
summarized in Scheme 7.[25]
The XRD pattern of c-KW2000 showed the presence of cubic
MgO (periclase) along with an extremely dispersed or
amorphous phase responsible for a broad peak detected at
2θ=35° (Figure 4). The pattern was consistent with typical XRD
of c-HT systems characterized by an Al/(Al+Mg) molar fraction
up to about 50% (≈55wt% of Al2O3).[39] [40] A Rietveld analysis
was then carried out by considering that: i) in addition to MgO,
both Al2O3 and the spinel Al2MgO4 could be the other most
probable components of the c-KW2000 system;[41] ii) if the
thermal degradation of a hydrotalcite occurred at 300-500°C,
metastable phases of MgO and finely dispersed Al2O3 or solid
Scheme 7. The decomposition of Mg–Al hydrotalcite.
Above 250 °C, HTs decompose because of the release of water
and extensive dehydroxylation and decarbonation reactions of
2-
the OH- and CO3 intralayer anions. However, the produced
phases (MgAl2O4, Al2O3 or Al-doped MgO) are often as small
nuclei hardly distinguishable from each other,[43,44] thereby
making not only the outcome of thermal process, but also the
precise nature of mixed oxides still not completely resolved.
[42]
solutions of Al2MgO4/Al2O3 and/or MgO/Al2O3 could form.
Among aluminas, k-Al2O3 (ICSD 94485 ) gave the best fit of the
XRD pattern (Figure 4). It was also noticed that the refined cell
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These considerations allowed us to formulate a hypothesis to
explain why in the investigated O-alkylation reactions, KW2000
could be used as such (Figure 1: f-KW2000), while HT30 was
active only upon calcination (Figure 2 and Table 3). Since the
two solids showed quite different chemical compositions in
which the layered HT structure coexisted with large proportions
of Periclase in f-KW2000 or Boehmite in f-HT30 (Figures 3a and
3b, respectively), it was plausible that the breakdown of Scheme
7 took place under different conditions: in the case of f-KW2000,
the process could be completed during the CF-alkylation
reactions with heating at 275 °C, while the collapse of f-HT30
required a more energy demanding transformation which was
achieved only during a high temperature calcination at 450 °C.
d
f-KW200
fit
f-HT30
fit
c
bkg
bkg
Hydrotalcite
Periclase
Hydroalcite
Bohemite
10
20
30
40
50
60
70
20
40
60
80
100
2
2
Figure 3. f-HT30 and f-KW2000 samples. Left: XRD patterns. Right: Rietveld fit.
1200
MgO-Al2O3 s.s.
c-HT30
fit
bkg
MgO
Al2MgO4
500
400
300
200
100
0
1000
c-KW2000
MgO/Al2O3 s.s.
800
k-Al2O3
k-Al2O3
fit
bkg
600
k-Al2O3
400
200
0
20
40
60
2
80
100
20
30
40
50
60
70
80
90
100
2
Figure 4. XRD pattern and Rietveld fit of c-KW2000 (Rwp=9.5%).
Figure 5. XRD pattern and Rietveld fit of c-HT30 (Rwp=11.6%).
Another question was the different performance of the
investigated c-HT systems. Mg/Al mixed oxides obtained by
calcination of HTs, are often defined as amphoteric solids.[27,40,46]
This dual (acid/base) activity has been invoked to account for
both the nucleophilic and electrophilic activation of organic
substrates promoted by c-HTs in several model reactions
including transfer hydrogenations and aldol condensations of
ketones,[27,47] transesterification and carbonylation processes,[48]
as well as eliminations and condensations of alcohols.[43,49]
The basicity of c-HT systems has been the subject of many
fundamental investigations that generally agreed to define basic
sites of different strength including OH groups (weak), Mg–O or
Al–O pairs (medium), and low-coordinated O2- anions (strong).
[25,39,50] In this respect, the density of total basic sites determined
by CO2-TPD profiles of HT30 and HT70 solids after calcination
at 450 °C, was of 1.35 and 3.00 mol/m2, respectively.[51] This
finding was consistent with the Mg/Al ratio of the samples: the
higher the Al content (the more electronegative metal cation),
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the higher the nucleation of Al-rich phases, and the lower the
average basicity of the solid.[27,40,47] In particular, the segregation
of a large amount of a spinel phase observed by XRD analysis
of c-HT30 (Al2MgO4: 63 wt %; Figure 5) probably determined a
partial coverage of the sites at the surface of MgO, thereby
decreasing the basicity with respect to c-HT70 solid (the latter
was mostly composed of a solid solution of alumina in
magnesia: Figure S32). An extensive analysis of these aspects
was carried out by Cavani et al.[25,50,52] that concluded how the
Al/Mg ratio of c-HTs induced a variability of surface properties,
but in the range 2.0 < Mg/Al < 3.5, all materials possessed basic
sites mostly of medium strength, while interestingly, Lewis acid
sites were manifest at low Mg/Al ratios in the form of
coordinatively unsaturated Al3+ species. Moreover, c-HTs were
less basic than MgO. The same Authors then proposed that in
the presence of c-HTs of Mg/Al=2 as catalysts, a cooperative
mechanism via a synergetic effect of basic and Lewis acidic
sites could explain the selective O-methylation of phenol with
methanol in the gas phase.[52]
A similar formulation appeared consistent with the behavior of c-
HTs described by Figure 2 and Tables 2-3. Results were
obviously not due to a purely basic mechanism since the most
performant catalyst c-HT30 was less basic than c-HT70 and by
far, than pure MgO. On the other hand, among the tested c-HTs,
c-HT30 had not only a different bulk structure (Figure 5), but
also the lower Mg/Al ratio which plausibly favored the
occurrence of Lewis acidic sites. c-HT30 apparently displayed
the best compromise of acid/base properties to hypothesize the
mechanism of Scheme 8.
product distribution was also steered by the temperature. In-line
with previously reported results,[3,10-11,15,33] alkylation and
transesterification processes involving DAlCs are discriminated
between based on their different energy demands: methylations
occur only at relatively high temperatures usually, above 150 °C.
A different situation was manifest for the four fresh HT solids
investigated here, of which f-KW2000 was the best catalytic
system (Figure 1). As mentioned above, the composition of f-
KW2000 containing a large excess of periclase (Figure 3b) could
facilitate the decomposition of the solid towards a catalytically
active phase (s.s. MgO/Al2O3) even at the temperature of 275 °C
used for CF-tests. KW2000 however, showed remarkable
similarities to HT63 and HT70 samples for the comparable Mg/Al
ratios (Table 1), and the bulk structure obtained by calcination
(cfr Figure 4 and S32). This was further substantiated by the
same catalytic performance of the three calcined solids (c-
KW2000, c-HT63 and c-HT70) in the O-methylation of solketal of
Table 2. Therefore, although f-KW2000 could be converted into
a catalyst at 275 °C (and 10 bar), the corresponding active
phase of Mg/Al oxides had to be more basic and consequently,
less efficient than c-HT30.
The effect of post-synthetic alkaline impurities on the catalytic
activity of c-HTs was finally considered. Although the doping
with alkali metal carbonates and hydroxides was known to
increase the basicity and the activity of HT-systems, the
necessary quantity of alkaline dopants must corresponded to an
amount of alkali metals (Na, K) of 0.5-10 wt%.[53] These values
were at least 250- and 20-fold higher than those of HT30 and
KW2000, respectively (Table 1). Reasonably, alkalinity based on
Na- or K-residual salts in the investigated catalysts had a
negligible, if any, role in the catalysis of O-alkylation reactions by
DAlCs.
Conclusions
The investigation integrates an archetypical green reaction
involving nontoxic and renewable reactants with an efficient
continuous flow procedure for the synthesis of alkyl ethers of a
class of biobased derivatives. Calcined hydrotalcites, particularly
c-HT30, have proved effective to highly chemoselective
transformations: functionalised bio-based alcohols and dialkyl
carbonates are activated towards O-alkylations with respect to
the competitive transesterifications and other side-reactions
including the ring opening of acetal functions, decarboxylations
of dialkyl carbonates, and oligo-/poly-merisations. An O-
alkylation selectivity as high as 99% at complete conversion is
achieved. To the best of our knowledge, this result have no
precedents in the class of biomass derived compounds
investigated.
The analysis of the bulk structure of fresh and calcined
hydrotalcites here investigated is consistent with the formation of
catalytically active phases constituted by Al/Mg mixed oxides.
These are originated either upon calcination or simply heating to
reaction temperature and their composition varies between
different HTs. The activity of the more performant catalyst (c-
HT30) is compatible with both a lower density of basic sites and,
at the same time, a more pronounced acid character than other
solids of the family.
Scheme 8. Proposed mechanism for the reaction of OH-bearing BBDs (ROH)
and DMC at the HT surface.
Both nucleophilic and electrophilic partners were activated by
the catalyst. After the adsorption of an OH-bearing BBD (ROH:
Scheme 1) on Brønsted basic sites (e.g. Mg–O pairs), a
deprotonation reaction followed with the formation of the
alkoxide intermediate ROads. Whereas the dialkyl carbonate
(DMC in Scheme 8) was activated by Lewis acid sites (Al3+
cations) in the HT framework. An electrophilic activation of
DAlCs by metal cations was already noticed by us in FAU
catalysts.[22] Then, ROads underwent a BAl2-type reaction on
adsorbed DMC to produce the desired methyl ether (ROMe) and
methyl hydrogen carbonate (MeOCO2H). The latter was an
unstable derivative, which spontaneously decomposed into
MeOH and CO2. ROads could also attack the carboxyl carbon of
DMC to form the corresponding transesterification product
(ROCO2Me; BAc2 mechanism, see Scheme 3). However, the
transesterification as an equilibrium process was forced to
backtrack by the irreversible O-methylation reaction. The
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10.1002/cssc.201601765
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physisorption isotherm. From the data, the Brunauer–Emmett–Teller
(BET) equation was used to calculate the specific surface area
Overall, the CF-protocol provides multiple advantages of which
the most relevant ones include the high process safety, the
recycle of solvents/unconverted reagents and reuse of catalysts,
and the simplification of downstream operations for the isolation
and purification of products with a benefit on productivity
(greater by orders of magnitude) with respect to batch methods.
X-ray powder diffraction (XRPD) analyses were recorded with a Philips
X'Pert powder diffractometer (Bragg–Brentano parafocusing geometry).
A nickel-filtered Cu Ka1 radiation (l ¼ 0.15406 nm) and a voltage of 40
kV were employed. XRD patterns were carried out for all fresh (as-
received) hydrotalcites and the corresponding calcined solids.
Experimental Section
Inductively coupled plasma (ICP) analysis were performed on a Perkin
Elmer Nexion 300XX Inductively Coupled Plasma Mass Spectrometer
(ICP-MS).
General
Solketal (1a), Glycerol formal (GlyF, 2a+2a’), tetrahydrofurfuryl alcohol
(4a), furfuryl alcohol (5a), dimethyl carbonate (DMC), diethyl carbonate
(DEC) and MgO were ACS grade from Aldrich. If not otherwise specified,
they were employed without further purification. Glycerol carbonate (3a)
was from JEFFSOL® and used as received. Solketal methylcarbonate
(1c) was prepared via the transesterification reaction of Solketal with
DMC, by adjusting a method developed by us.[10]
CF-Apparatus
The apparatus used for the investigation was assembled in house
according to the chart reported in Figure S1. An HPLC pump was used to
deliver liquid reactants to a stainless steel tubular reactor (L=12 cm,
Ø=1/4", 1.16 cm3 inner volume) containing the catalyst whose amount
was chosen based on the apparent density of both hydrotalcites and
faujasites. Typical catalyst loadings were in the range of 0.5-0.85 g (see
Figures 1-2 and Tables 2 and 3). The reactor was placed in the upright
position in a thermostated oven, and heated at the desired temperature.
A Swagelok back pressure regulator placed at the outlet of the reactor,
was used to keep the pressure constant over the whole system
throughout the reaction. When experiments were carried at ambient
pressure, the BPR was bypassed. A Rheodyne Model 7725i injector,
equipped with a 10 μL sample loop, was placed before the BPR and
used for sampling.
GC-MS (EI, 70 eV) analyses were run using a HP5-MS capillary column
(L=30 m, Ø=0.32 mm, film=0.25 μm) and CG/FID analyses were run
using an Elite-624 capillary column (L=30 m, Ø=0.32 mm, film=1.8 μm).
1H NMR were recorded at 400 or 300 MHz, 13C NMR spectra at 100 MHz,
and chemical shifts were reported in δ values downfield from TMS; CDCl3
was used as the solvent.
Catalysts
Faujasites of formula Mz[(AlO2)z(SiO2)w]mH2O (M=Na, Cs; X- and Y-type:
z=86 and 56, w=104 and 136, m=264 and 250, respectively) included
NaX and NaY faujasites from Aldrich and a CsY zeolite synthesised by a
conventional ionic exchange reaction of NaY with aq. CsCl.[18] The
percentage of ionic exchange (Na+Cs+) was 58%: it was evaluated
through atomic emission according to a procedure already reported by
us.[54]
SAFETY WARNING. Operators of high pressure equipment should take
proper precautions to minimise the risk of personal injury.[55] The
individual components that we describe work well, but they are not
necessarily the only equipment of this type available.
General procedure for the CF-reactions of 1a, 2a+2a’, 3a, 4a, 5a with
DMC
Before their use, they were dehydrated under vacuum (70 °C, 18 mm,
overnight).
A typical reaction was carried by using the CF-apparatus described
above through the following procedure. The oven was set at
a
temperature of 150 °C and N2 was flushed for one hour throughout the
system. Then, the reacting mixture (Dialkyl carbonate and the bio-based
alcohol) was allowed to flow for 10 minutes at 1 mL/min, while the BPR
and the oven were set to the operating pressure and temperature (5-60
bar, and 150-300 °C, respectively). Once T and p were stabilised, the
reactants flow was adjusted at the desired rate (0.07-0.2 mL/min). The
reaction mixture was collected through a Rheodyne® valve (7725i fitted
with a 10 μL sample loop) at time intervals of about 30 minutes, diluted
with 1.5 mL of diethyl ether and analysed by GC/FID or GC/MS.
Hydrotalcites (HTs) were aluminium magnesium hydroxyl carbonate
hydrates of formula Mg2xAl2(OH)4x+4(CO3)·nH2O (x = 0.5-2.3). Four
commercial HTs were considered (their Mg/Al molar ratio is given
between parentheses): KW2000 (1.8); PURAL® MG 30 (0.5), MG 63
(1.9) and MG 70 (2.3). KW2000 was from Kyowa Chemical Industry Co.,
Ltd., while PURAL® MG solids were from CONDEA/Sasol Germany
GmbH, Inorganic Specialty Chemicals. According to the manufacturer
specifications,
these
materials
are
aluminium
magnesium
hydroxycarbonate hydrates Mg2xAl2(OH)4x+4(CO3)·nH2O with x varying
between 0.5 and 2.3. Carbonate content is about 10 wt.% and the
maximum loss on ignition (3 h, 1000 °C) is in the 40–45% range. All HTs
were used both as-received and after calcination. If used as received,
solids were dehydrated under vacuum (70 °C, 18 mm, overnight) before
being loaded in the CF-reactor. Otherwise, the calcination treatment was
carried out according to a procedure already reported for HTs:[21] solid
samples (5 g) were heated in a quartz reactor (in the upright position)
under a flow of dry air at 450 °C for 16 h. The heating rate was 30 °C/min.
Change of reaction conditions. The oven and the BPR were set to the
new desired values of T and p, respectively. Under these conditions, the
reactants mixture was flushed for 10 minutes at 1 mL/min. Then, the flow
was adjusted at the chosen rate and a new experiment started.
System cleaning and restart. At the end of each experiment, the oven
and the BPR were set to 50 °C and atmospheric pressure, respectively,
while a cleaning solution of methanol (50 mL at 1 mL/min) was allowed to
flow through the system. The pump was then stopped and the oven was
allowed to cool at rt. The CF-reactor was then disassembled and the
catalytic bed could be replaced by a fresh one.
Catalyst characterisation
Adsorption–desorption isotherms of nitrogen were obtained at the liquid
nitrogen temperature using a Micromeritics ASAP 2010 system. Each
sample has been degassed at 130 °C overnight before measuring the N2
Reaction of Solketal (1a) and DMC with alkali metal exchanged
faujasites (FAUs) as catalysts
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The above described procedure was used to tests Faujasites as catalysts.
Before use, each zeolite was dehydrated under vacuum (70 °C, 18 mm,
overnight): 0.68, 0.54, and 0.86 g of NaY, NaX, and CsY, respectively,
were charged in the CF-reactor. In all tests, a 1.83 M solution of solketal
in DMC [DMC:solketal molar ratio (W)=5] was fed to the reactor at a total
volumetric flow rate (F) of 0.1 mL min−1. CF-reactions were performed
under a constant pressure of 10 bar, while T was set at 250 and 275 °C,
respectively. Experiments were followed for 3 hours. Two additional runs
with both NaY and NaX catalysts were carried out for up to 18 hours.
Acknowledgements
MIUR (Italian Ministry of University and Research is
acknowledged for funding the PhD program of LC through
“Borse aggiuntive di dottorato, fondo giovani”. Dr. Roberto
Clerici from Sasol Italy SpA is gratefully acknowledged for his
generous supply of Pural® hydrotalcites and the discussion of
the properties of these solids. Dr. Alexander Malyschew from
Sasol Germany GmbH is also acknowledged. Finally, Mr. Tom
Savage (University of Sydney) is acknowledged for his support
with ICP analyses.
Reaction of Solketal (1a) and DMC with hydrotalcites
As-received hydrotalcites. HTs were dehydrated under vacuum (70 °C,
18 mm, overnight): 0.52, 0.73, 0.51, and 0.85 g of KW2000, HT30, HT63,
and HT70, respectively, were used for CF-tests. A 1.83 M solution of
solketal in DMC [DMC:solketal molar ratio (W)=5] was fed to the reactor
at a total volumetric flow rate (F) of 0.1 mL min−1. At 275 °C, reactions
were performed under a constant pressure of 10 bar, and they were
followed for 18 hours (Figure 3a). In the case of KW2000, additional
experiments were carried out under the following conditions: i) T= 200
and 250 °C, p= 10 bar, W=5, F=0.1 mL/min; ii) T=275 °C, p= 5 and 50
bar, W=5, F=0.1 mL/min; ii) T=275 °C, p= 10 bar, W= 1.1, 2, 3.5, F=0.1
mL/min.
Keywords: O-alkylation • bio-based derivatives • dialkyl
carbonates • continuous flow • hydrotalcites
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A sustainable route for the upgrading
of bio-based derivatives is
developed through a selective,
continuous-flow, and catalytic O-
alkylation reaction. Dialkyl
Lisa Cattelan, Alvise Perosa, Piero
Riello, Thomas Maschmeyer, and
Maurizio Selva*
Page No. – Page No.
Continuous-Flow O-alkylation of
Bio-Based Derivatives with Dialkyl
Carbonates in the Presence of Mg–
Al hydrotalcites as Catalyst
Precursors
carbonates are used as green
alkylating reagents in the presence
of hydrotalcites acting as precursors
of a catalytic phase comprised of a
magnesia-alumina solid solution.
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