B-TUD-1: A versatile mesoporous catalyst

Article abstract of DOI:10.1039/c3ra44406f

Novel amorphous mesoporous borosilicate, B-TUD-1, was prepared to test its performance for different sustainable reactions. The structure of the material, the effective incorporation of boron into the framework as well as the nature of incorporated boron were verified by N₂-sorption, XRD, ICP-OES, TEM, NH₃-desorption, MAS NMR and FTIR. The potential of these materials as catalysts was tested in two reactions under different conditions. They showed an apparently good activity and recycling potential for HMF (5-hydroxymethyl- furfural) synthesis. However leaching of boron occurred during this aqueous reaction. We could demonstrate that the leached boron was actually the catalyst of the reaction. In a second reaction, this time in organic solvent, the B-TUD-1 materials were tested for the Prins cyclisation of citronellal. A very promising activity was obtained. The catalyst could be recycled and no boron leached, as demonstrated by hot filtration experiments. The Royal Society of Chemistry 2013.

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B-TUD-1: A versatile mesoporous catalyst
DOI: 10.1039/C3RA44406F
5
Adeline Ranoux, Kristina Djanashvili, Isabel W. C. E. Arends and Ulf Hanefeld*
Page – Page
Introducing boron into siliceous mesoporous TUD-1gave environmentally benign and catalytically highly active materials,
containing dynamic boron in the framework. The performance and the recycling of these new materials was tested in
different reactions. They were demonstrated to be promising and versatile acidic catalysts under non-aqueous conditions.
10
B-TUD-1
15

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B-TUD-1: A versatile mesoporous catalyst†
Adeline Ranoux, Kristina Djanashvili, Isabel W. C. E. Arends and Ulf Hanefeld*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
Novel amorphous mesoporous borosilicate, B-TUD-1, was prepared to test its performance for different sustainable reactions. The
structure of the material, the effective incorporation of boron into the framework as well as the nature of incorporated boron were
verified by N2-sorption, XRD, ICP-OES, TEM, NH3-desorption, MAS-NMR and FTIR. The potential of these materials as catalysts was
tested in two reactions under different conditions. They showed an apparently good activity and recycling potential for HMF (5-
hydroxymethyl-furfural) synthesis. However leaching of boron occurred during this aqueous reaction. We could demonstrate that the
10 leached boron was actually the catalyst of the reaction. In a second reaction, this time in organic solvent, the B-TUD-1 materials were
tested for the Prins cyclisation of citronellal. A very promising activity was obtained. The catalyst could be recycled and no boron
leached, as demonstrated by hot filtration experiments.
imparts activity to mesoporous materials. In this paper a novel
50 approach towards the synthesis of heterogeneous mesoporous B-
Introduction
catalyst is presented, B-TUD-1. Combining the advantageous
15 Heterogeneous catalysts were widely studied during the last
properties of the TUD-1 materials (notably their 3D structure
years, in particular for the development of sustainable processes.
suppressing diffusion limitation of the substrates) and excellent
Indeed, while representing powerful catalysts, they have the
catalysis efficiency of the boron derivatives, the obtained B-
additional advantage that they are easily recyclable and thus lead
55 TUD-1 catalysts should present new versatile catalytic properties.
to significant reductions of waste. In this context, TUD-1
We present here not only the preparation of the catalyst and its
20 materials were developed.
full characterization, and additionally performance tests for two
The special properties of mesoporous TUD-1, such as its
important, sustainable reactions, the HMF synthesis, and, the
three dimensional pore structure with a good accessibility for
Prins cyclisation of citronellal.
substrates and products, tuneable pore properties,1, 2 and a cost-
effective one-pot, surfactant-free synthesis by the sol-gel method,
60 Results and Discussion
25 have made it a versatile heterogeneous catalyst for many
interesting synthesis applications.3 TUD-1 has been successfully
Preparation of the material
functionalized with numerous incorporated metals (Al, Ti, Co,
The M-TUD-1 materials (Al and B) were prepared by the sol-gel
Fe, V, Cr, Mo, Hf, Ga, Ni, Mn, Zr and Cu) aiming at a variety of
method using triethanolamine as a complexing agent following
catalytic properties.4-29 Surprisingly, the incorporation of B, in the
the protocols described earlier.9 The M was included in the form
30 same group as Al, was never investigated.
65 of the isopropoxide M(iPrO)3.
Boron derivatives have proven to be efficient catalysts for a
The use of triethanolamine permits to avoid the use of any
large range of reactions (reduction, Diels-Alder, aldol
surfactants or liquid crystal templates. Complexing with the metal
31
reactions).30,
Boric acid itself is a good Lewis acid for
triethanolamine leads to the formation of the mesopores (Fig. 1).
52
esterifications,32 amidifications33 and dehydrations of polyols.34-36
Both TUD-1 materials and boric acid have been demonstrated
as excellent catalysts for sustainable reactions, conversion of
renewable materials and synthesis of platform chemicals (for
example, boric acid catalysed synthesis of HMF37 or Hf-TUD-1
catalysed conversion of glycerol to solketal.14
Initially the active metal is complexed to form an atrane.51,
35
70 Then these atranes complex together with free triethanolamine
creating the framework of the material. During gelation and also
after calcination and consequent removal of organic compounds,
the metal is incorporated into the framework of the material as
oxide species.3, 4 This is also known as the atrane route.53 The
75 formation of the boratranes has been demonstrated as to be a set
of complex equilibria between the different coordination forms.
Several stable boratranes with three six-membered rings where
the boron has a tetrahedral structure could be characterised.54-56
Another interesting point is that by the use of a similar method
80 introducing surfactants, ordered mesoporous materials could be
obtained (for example SBA-1557) in contrast to the amorphous,
mesoporous TUD-1 materials.
40
Previously other boron-incorporated zeolites and mesoporous
materials (Beta- and ZSM-5 zeolites, SBA-15, MCM-41)38-49
have been reported. Key examples are the B-MCM-41-catalysed
Beckmann rearrangement of cyclohexanone oxime44 permitting
the efficient synthesis of caprolactam, and, the B-MCM-41-
45 catalysed three component Strecker reaction also leading to the
formation of α-aminonitriles,50 intermediates for the synthesis of
α-aminoacids or N-containing heterocycles. Boron is herewith a
promising, cheap and environmentally benign element that
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Table 1 Physico-chemical characterization of M-TUD-1 with different Al
or B loadings.
Different ratios of M were introduced, Si/Al 25 (Al-TUD-1 25),
and Si/B 50, 25 and 10 (B-TUD-1 50, 25 and 10 resp.). A
hydrothermal treatment of 6 h at 180˚C was applied that has
defined the size of the pores and the mesoporous structure.1
Subsequent calcination at 600 ˚C for 10 hours led to the final
Catalyst
Al-TUD-1
25
B-TUD-1
Si/X gel
50
25
10
Si/X product
SBET (m2/g)
26[a]
54[a]
428
8.5
26[a]
532
5.9
11[a]
637
5.2
5
850
material.
Pore diameter Dpore (nm)
Total volume pore Vpore (mL/g)
Micropore volume Vmicro (mL/g)
3.6
B
A)
B)
0.7539
n.d.[b]
0.883 0.784 0.831
O
O
O
O
B
N
O
0.0075 0.035 0.027
Si
N
O
[a] From ICP-OES measurements; [b]not determined because not measurable.
Fig. 1 Atranes complexes, triethanolamine as A) metal complexing agent
and B) templating agent.
10 Characterization: a classical mesoporous catalyst
The ratio of incorporated boron was verified by elemental
analysis by inductively coupled plasma atomic emission
spectroscopy (ICP-OES). The mesoporous character of the
materials was confirmed by X-ray diffraction (XRD), N2 sorption
15 and high-resolution transmission electron micrographs (TEM).
The acid sites were characterised by temperature-programmed
desorption of NH3. Finally, the incorporation of boron was
verified by XRD at small angles and by MAS NMR. The latter
permitted also to characterise the nature of the boron species
20 incorporated into the mesoporous framework.
55
Fig. 2 N2 adsorption and desorption isotherms at 77K.
The X-ray diffractograms of the materials (Fig. 3) show a broad
intense peak at low angles around 0.5-1 2θ˚ in line with the
mesostructured character of these materials. No correlation could
Results of the elemental analysis (ICP-OES) and the porosity
measurements are given in Table 1. The elemental analysis 60 be established between the peak intensity and the Si/B ratio.
reveals Si/B ratios of the calcined catalysts very close to the
actual added amount. ≥95% of the added boron compounds
25 during the preparation of the gel was incorporated. This
demonstrates the good predictability of the preparation method,
already observed with other M-TUD-1 materials.8, 58
According to Jansen et al.,1, 2 the pore size of TUD-1 material
is tuneable and can be controlled by the time of the hydrothermal
30 treatment. After a treatment of 6h, the expected average pore size
was between 6-8 nm. The measured pore diameters correspond to
The complete XRD spectra of B-TUD-1 10, 25 and 50, as well
as that of Al-TUD-1 25 for comparison, were recorded. No
crystalline structure was observed in any case. A spectrum of
boric acid was measured as a reference (multiple peaks,
65 characteristic of this crystalline product). None of the
characteristic signals of boric acid could be detected in the B-
TUD-1 spectra, not even at a Si/B ratio of 10. This is quite
remarkable compared to other M-TUD-1 materials. It indicates a
complete framework incorporation even at the highest metal
70 loading, where most of the M-TUD-1’s contain a certain
percentage of metal oxides visible on TEM pictures.3, 8 Complete
Boron incorporation at high loading has also been reported for
other boron-containing zeolites38,45 or mesoporous silicates. 43,44
The TEM pictures (Fig. 4) show “worm/sponge-like”
75 structures typical for amorphous material and TUD-1 samples.1,3
This confirms the mesoporous structure of the material.
Furthermore, no crystalline boron particles could be observed
even at higher loading confirming the incorporation of the boron
species. Therefore, the material appeared completely amorphous,
80 as no ordered pores or channels were visible, and as no boron (or
aluminium) oxide could be detected (Fig. 3 and 4).
the theoretical values and are slightly larger than those for
classical M-TUD-1 (average around 4.0 nm)3 and close to other
boron containing mesoporous silicates (for example, B-MCM-41,
35 average around 4.5 nm, B-SBA-15, average around 6.5 nm).43, 46,
47
The surface area of B-TUD-1 increases upon increasing of the
metal loading. At the same time, the average pore diameter
decreases in line with the results for other M-TUD-1 materials3
and the boron substituted mesoporous SBA-15.43
40
The N2 adsorption and desorption isotherms of the B-TUD-1
50 and 25 (Fig. 2) reveal a typical “type IV” hysteresis loop, with
a large uptake of nitrogen at relative pressures between 0.5 and
0.9 due to capillary condensation in the mesopores. A plateau at
relative pressures of above 0.9 p/p0 indicates the absence of large
A temperature-programmed desorption (TPD) profile (from
100 to 500 ˚C) of ammonia for different B- and Al-TUD-1
samples (Fig. 5) was measured. For all the boron-incorporated
85 materials two peaks were observed, an intense peak at 150˚C
corresponding to weak acids, and a broad peak with low intensity
at around 400-500 ˚C, corresponding to strong acid sites (Table
2). The medium acid sites (temperature around 250-300 ˚C) are
hardly present. The intensity of the peaks (especially for the weak
90
45 mesopores or macropores. The B-TUD-1 10 isotherm (Fig. 2)
differs slightly by the fact that it shows a lower uptake in the first
phase, possibly related to a lower mesoporosity. The material has
the lowest pore diameter and micropore volume (Table 1). This
might result from the intensified interaction of the B atoms with
50 the silica structure as it has been observed for the Zr-TUD-1
material.58
2
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Fig. 3 Powder XRD diffraction patterns of calcined B-TUD-1 A) at small
angles (0.5 to 10 deg.); B) at higher angles (4 to 90 deg.).
acid sites) increases with the B loading (i.e. decreasing Si/B ratio)
as well as the total acidity. In the case of Al-TUD-1 25, we could
observe one broad band from 100 to 350 ˚C corresponding to
weak and medium acids, more classically observed with M-TUD-
1 catalysts.11
5
Table 2 Acidity derived from temperature-programmed desorption (TPD)
10 of ammonia for Al- and B-TUD-1 samples with different Si/M ratios.
Catalyst
Total acidity
(mmol NH3/g)
Temp
(°C)
161
Quantity
ꢀmol/NH3/g)
273.3
B-TUD-1 Si/B 10/1
B-TUD-1 Si/B 25/1
295
459
0.86
152.4
0.428
0.195
152
473
71.7
115.8
35.9
B-TUD-1 Si/B 50/1
Al-TUD-1 Si/Al 25/1
0.147
0.372
148
107.9
179 (max) 372
The solid-state 11B MAS-NMR (Fig. 6) showed three main peaks
for the B-TUD-1 samples, corresponding to the earlier results for
the boron containing mesoporous silicates as MCM-4142 or SBA-
15 1543 (and to the results obtained for many zeolites39, 40, 49). The
first peak at around -22.9 ppm, sharp and narrow, corresponds to
the 4-coordinated tetrahedral boron (mainly, 2-coordinated also
possible,43 symmetric form). The two other broader bands from -
11.5 ppm to -2.5 ppm correspond to different trigonal forms of
20 boron species with different degrees of coordination to silica (in
line with literature39, 40, 42, 49), namely 3-coordinated (11.5 ppm),
and trigonal B with a lower coordination number (structures in
Fig. 7). With a decrease in boron loading, i.e. higher Si/B ratio,
relatively more tetrahedral boron is observed. It is however
25 important to notice the dynamic nature of these incorporated
boron species. Diverse equilibria are established between the
different species, especially between tetrahedral and trigonal
30
Fig. 4 TEM pictures of B-TUD-1 10 revealing the mesostructure of the
catalysts (scale in nm) A) 10 nm, B) 20 nm, C) 50 nm.
forms, as function of the temperature and media (hydratation,
presence of cations),45, 49 explaining the indistinguishable broad
35 peaks observed in the NMR spectra. For this reason, the nature of
the acid sites is also dynamic. Due to this dynamic equilibrium
that is also influenced by water, it is not meaningful to determine
the ratio of Lewis to Brønsted acid sites as this is dynamic too.
Furthermore, the 27Al MAS-NMR spectrum of the Al-TUD-1
40 material was also measured (cf ESI; Fig. S1). Two main peaks
were observed at 0 and 55 ppm, corresponding to the octa- and
tetrahedral aluminium resp., as described in the literature.59-61
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The intensity of these two peaks, related to the presence of B,
15 decreases in line with the B loading. For comparison, these peaks
are absent in the spectra of Al-TUD-1. The absorption bands at
460, 810 and 1070 with a shoulder at 1200 cm-1, present in all
the B- and Al-TUD-1, can be assigned to the symmetric and
asymmetric stretching of Si-O-Si vibrations of tetrahedral SiO2
20 units (Fig. 8). Based on these results, we can postulate that B-
TUD-1 materials with fully framework incorporated boron were
prepared. They contain trigonal boron representing Lewis acidity
as well as tetrahedral boron representing Brønsted acidity (Fig. 6
and 7) in a dynamic equilibrium (Fig. 8).
25 Performance test in aqueous or biphasic media: HMF
synthesis.
Fig. 5 Temperature-programmed desorption (TPD) profile of ammonia
for various Al- and B-TUD-1 samples.
HMF, 5-hydroxymethylfurfural, is a biobased platform chemical,
obtained from sugars, especially fructose, by acid-catalysed
dehydration (Scheme 1).62 However, despite the many studies
30 that have been reported, no real efficient, sustainable and scalable
route has been proposed yet.62
O
HO
OH
Fru
O
O
Acid catalyst
- 3 H2O
HO
HO
H
OH
HMF
OH
Scheme 1. Acid-catalysed dehydration of Fructose (Fru) leading to the
formation of HMF.
35 Many Lewis acids,63, 64 zeolites and mesoporous materials15, 22, 65-
70
were used as catalysts for this reaction. Because of the
numerous advantages (recyclability, easy separation of the
product, no corrosion of equipment), heterogeneous catalysts in
general have been the main focus.
40 As Al-TUD-160 and boric acid37,
5
Fig. 6 11B MAS NMR of B-TUD-1 catalysts (25, 10) referenced to 0.1M
aq. B(OH)3 (most of the cited articles use BF3(OEt)2 as reference leading
to a shift of the signals of 19.8 ppm).
71
were already successfully
tested for HMF synthesis in biphasic systems or ionic liquids, B-
TUD-1 materials appear to be promising heterogeneous, thus
potentially recyclable, catalysts for HMF production. Indeed,
highly stable, TUD-1 materials have been shown to be easily
45 recovered and recycled after reaction by a calcination similar to
the one used during the preparation.3
The presence of tetrahedral boron species in B-TUD-1 materials
was confirmed by FTIR as a shoulder at 920-930 cm-1 (mainly
10 visible for B-TUD-1 10, Fig. 8). Another peak characteristic for
the presence of trigonal boron can be identified at 1395 cm-1.43, 47
Fig. 7 Possible tetrahedral and trigonal structures of boron incorporated in TUD-1 framework.
4
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40 point, a recycling experiment of the aqueous phase of the reaction
was carried out (Fig. 12). The catalyst was filtered off after one
hour and the reaction was continued with the remaining aqueous
phase for an extra hour. In parallel, a reference sample was
running, where the catalyst was not removed.
45 The reaction was shown to continue despite the removal of B-
TUD-1. Both conversion and yield continued to increase.
Moreover, very similar results were obtained with the solid
catalyst or after removal of the catalyst (Fig. 12). Our conclusion
is that the released boric acid catalyses the HMF formation.
50 Finally, a last experiment (conditions biphasic, 190 °C, 40 min),
conducted with a small concentration of boric acid (4 wt%) led to
similar results (89% conversion, 59% HMF yield) as the ones
observed with the solid catalysts (91% conversion, 60% HMF
yield), thus confirming this assertion.
Fig. 8 FTIR spectra of B-TUD-1 50, 25, 10 and Al-TUD-1 25.
55
The synthesis of HMF in aqueous media is a challenging
reaction. The B-TUD-1 catalysts showed very good activity for
this reaction, especially under biphasic conditions with toluene.
In Figure 9, experimental results are presented. With the B-TUD-
1 catalysts under purely aqueous conditions, the conversion and
yield appeared limited (same results after at 150 ˚C for 90 min or
4h, ≤ 20% yield); whereas in biphasic media, at 150 ˚C for 4h, 65
5
10 to 70% conversion of fructose was observed with a yield between
40 to 45%. Interestingly, these results were similar for all Si/B
ratios and very close to what can be obtained under similar
conditions with Al-TUD-1.60
In the same way, consecutive recycling experiments were
15 carried out in a purely aqueous phase (Fig. 11A). However, the
liquid 11B NMR analysis of the reaction media revealed the
presence of a broad peak at 0 ppm, corresponding to the presence
of boric acid. Furthermore, the elemental analysis of the recycled
catalysts after one, two or three reactions revealed an increasing
20 Si/B ratio (from 25 to 85 after 3 reactions, Fig. 11B), thus a
decreasing B content on the mesoporous material. The B is
probably slowly released during the reaction. Furthermore, a
porosity analysis of the used catalysts after calcination showed a
clear change in their mesoporous structure (Table 3). Notably a
25 net decrease of the mesoporous surface and pore volume could be
observed. The structure is probably affected by the release of B
and the hydrolysis of the Si-O-B bonds. Finally, a 11B MAS
NMR of the used catalysts (cf ESI; Fig. S2) showed a net
decrease of the peaks corresponding to tetrahedral 4-coordinated
30 B (sharp peak at -22.9 ppm), but also to the trigonal 3-
coordinated B (broad peak at -11.5 ppm). We observed thus
Fig. 9 B or Al-TUD-1 catalyzed HMF synthesis in A) aqueous or B)
biphasic conditions (at 150 ˚C, 30 wt% Fructose, 10 wt% M-TUD-1,
Toluene/Water 3/1).
60
under acidic conditions, in line with literature descriptions in the
case of different boron containing zeolites,45,
49
both the
conversion of tetrahedral to trigonal boron, as well as the
35 progressive cleavage of the Si-O-B bridges, leading to a partial
deboronation.
Fig. 10 Results of the recycling experiments of B-TUD-1 25 in biphasic
conditions (190 ˚C, 40 min, 30 wt% Fructose, 10 wt% M-TUD-1,
Toluene/water 3/1), conversion of fructose (%), yield of HMF, selectivity
to HMF (%)
This observed leaching, questions the character of the
catalysis, heterogeneous ensured by the solid catalyst, or
homogeneous ensured by the released boric acid. To clarify this
65
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Fig. 12 Results of the recycling experiment of the aqueous phase of the
reaction medium (175 ˚C, 1h, +1h after filtration or +1h without filtration
30 for the reference, 30 wt% Fructose, 10 wt% B-TUD-1 25), conversion of
fructose (%), yield of HMF (%).
This intramolecular cyclisation of citronellal,
a renewable
Fig. 11 A) Results of the recycling experiments in aqueous phase (A, 1
product, is an essential step in the Takasago synthesis of
menthol.77 Industrially, the reaction is catalysed by stoichiometric
35 amounts of solid ZnBr2 in benzene.78 Recently, the
implementation of alternative processes using heterogeneous
catalysts has attracted considerable attention.11, 58, 79-89 The Prins
reaction was carried out with the different B-TUD-1 (50, 25 and
10) catalysts as well as with Al-TUD-1 25 as a reference (Scheme
40 3).
reaction cycle; B, 2 reaction cycles/1 recycling step; C, 3 reaction
cycles/2 recycling steps; 160 ˚C; 1h, 30 wt% Fructose, 10 wt% B-TUD-1
25), conversion of fructose (%), yield of HMF; B) Ratio Si/B measured
by elemental analysis after one (1A), two (2B), or three (3C) reactions.
5
Table 3 Porosity measurements of the used B-TUD-1 catalysts after the
HMF synthesis or Prins reaction.
Treatment
HMF
25/1
Prins
The obtained activity of the B-TUD-1 catalysts was quite
high (Fig. 13) and was increasing with the amount of
incorporated boron (thus with a decreasing Si/B ratio). It should
be noticed that the experiments were carried out with industrial
45 grade citronellal containing around 5% isopulegol. As it has
previously been shown, the presence of isopulegol partially
inhibits the conversion of citronellal. Higher activities can be
expected starting from optically pure citronellal.58
Si/B (initiaI)
50/1
10/1
25/1
10/1
SBET (m2/g)
375
8.8
434
5.5
503
5.5
530
5.1
599
5.0
Pore diameter Dpore (nm)
Total volume pore Vpore
(mL/g)
Micropore volume Vmicro
(mL/g)
0.825
n.d[a]
0.599
0.137
0.696
0.003
0.675
0.030
0.748
0.028
[a]not determined because not measurable.
10
We could observe full conversion after one hour for the B-
50 TUD-1 10. This result is very close to the conversion that we
obtained via Al-TUD-1 (full conversion after approximately 50
minutes). Full conversion was reached after four hours for B-
TUD-1 25, 80 % conversion was obtained after 6h with B-TUD-1
50. Despite an apparent lower activity of the latter two catalysts,
55 the comparison of their turn over frequencies (TOF, Fig. 13B)
showed their activity per active site to be actually higher than for
B-TUD-1 10.
This result with B-TUD-1 can be compared to the stability of Al-
TUD-1 under similar conditions. Lima et al.60 showed a good
recyclability of Al-TUD-1 under similar reaction conditions.
Moreover, our measurement of the 27Al MAS NMR after reaction
15 showed identical spectra to the original calcined sample. This
difference in stability can be explained by a smaller size of the
boron and its greater dynamic reactivity in the silica framework.
Performance test in organic media: Prins cyclisation
The catalyst performance was also tested in the Prins reaction72-74
20 (Scheme 2), an important C-C bond forming reaction catalysed
by acids.75, 76
OH
OH
M-TUD-1
Isopulegol (I)
Nei-Isopulegol (NI)
O
Toluene
80 °C
Citronellal
Acid catalyst
OH
OH
O
OH
Iso-Isopulegol (II)
Neo-Iso-Isopulegol (NII)
Citronellal
Isopulegol
60
Scheme 3 Cyclisation of citronellal to isopulegol and its isomers
catalysed by M-TUD-1 material.
25
Scheme 2 Intramolecular Prins cyclisation of citronellal.
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DOI: 10.1039/C3RA44406F
These results are similar to the results with Zr-TUD-1 100. We
can also compare this performance to other examples of zeolite
catalysts for this reaction. Similar performance was observed
under the exact same conditions with Al-MSU-5083 (88.6 %
conversion after 30 minutes, and 96.7 % after 60 minutes),
whereas Zr-incorporated MOF89 only presented a full conversion
after a minimum of 4h at 100˚C for a citronellal/Zr ratio of 10 (in
our conditions, with B-TUD-1 10, citronellal/B ratio of 9, with B-
TUD-1 25, citronellal/B ratio of 21). Our catalyst thus presents an
5
10 interesting activity for the Prins cyclisation of citronellal. The
product selectivity to isopulegol (all diastereoisomers) for all the
catalysts appeared excellent (≥95%, Table 4). The observed
diastereoselectivity to isopulegol (vs its other diastereoisomers) is
around 63/37 for all B-containing TUD-1 catalysts (Table 4).
Fig. 14 Hot filtration test on B-TUD-1 10 during the Prins cyclisation of
15 This result is close to the measured diastereoselectivity of Al- 45 citronellal; catalyst filtered after 20 minutes; 80 °C.
TUD-1 25 (66/34) and to the values described in literature for Zr-
TUD-1 (average 65/35).58 The reduced pore diameter of B-TUD-
1 10 does not influence this ratio.58
Recycling steps were realised with the catalysts B-TUD-1 10 and
25, with or without calcination before the recycling reaction (Fig.
15). Very similar results were obtained with the calcined recycled
50 catalysts, whereas we could observe a net decrease of the activity
of the non-calcined samples, probably partially inhibited by the
product remaining on the catalyst as described earlier.58
20
Fig. 13 A) Evolution of the yield of isopulegol formed during the Prins
cyclisation catalysed by various B- and Al-TUD-1 catalyst; B) Evolution
of the TOF of the catalysts during the reaction (in mol converted product
per mol B or Al per min).
55
Fig. 15 Results of the recycling experiments with or without intermediate
25 Table 4. Conversion, selectivity and diastereoselectivity for M-TUD-1
calcination of the B-TUD-1 25 and 10 catalysts, yield of isopulegol (%).
catalysts in the Prins cyclisation of citronellal.
Conclusions
Catalyst
Time
Conversion
Selectivity Diastereoselectivity
(%)
98
(%)
98
I/MI/II/NII (%)
We successfully obtained and characterized a new, easy to
60 prepare, three-dimensional and mesoporous acidic B-TUD-1
catalyst. We verified the mesoporous and amorphous character of
the material as well as the incorporation of the boron into the
framework even at low Si/B ratio. We also characterized the
mixed acidity of the material (Brønsted and Lewis acid sites) as
65 well as the different possible structures of the incorporated boron
(trigonal and tetrahedral) and their dynamic equilibrium.
Catalytic tests showed the efficiency of this material for the
catalysis of different sustainable reactions. However, in the case
of HMF synthesis, the acidic aqueous reaction conditions caused
70 boron leakage and a change of the structure of the material was
observed. On the contrary, B-TUD-1 demonstrated high
reactivity as well as reasonable recyclability for the Prins
cyclisation of citronellal in toluene. B-TUD-1 is there with a
promising heterogeneous acid that catalyses reactions under non-
75 aqueous conditions.
Al-TUD-1 25
B-TUD-1 10
B-TUD-1 25
B-TUD-1 50
60
66.3/22.6/7.05/0.05
60
97
96
81
98
95
98
63.2/30.1/5.1/1.6
63.2/30.1/4.8/1.8
62.7/30.2/5.1/2.0
240
360
A liquid 11B NMR of the reaction medium after filtration of the
30 catalyst did not show the presence of boron compounds,
indicating the absence of leaching. Furthermore a porosity
measurement of the catalysts after reaction and calcination
showed very similar results (Table 3) to the initials catalysts
(Table 1). Finally, a hot filtration test was carried out. After 20
35 min, the reaction medium is sampled, immediately filtered, and
left again at 80˚C. This operation is performed on a hot sample to
prevent any possible precipitation of diluted compounds.90 After
the removal of the solid catalyst, the reaction clearly stopped
(Fig. 14), proving the heterogeneous nature of the catalytic
40 activity.
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weighed, and transported into a thoroughly rinsed teflon vessel.
All samples are dissolved in an aqua regia acid mixture
containing 1.2 ml concentrated hydrochloric acid (36% v/v), 0.4
ml concentrated nitric acid (69% v/v) plus 0.4 ml concentrated
60 hydrofluoric acid (40% v/v).
Experimental Section
Materials and Methods
All chemicals were purchased from Aldrich, Janssen or Acros.
For the catalysis experiments, the anhydrous solvents and solids
were used as received, all other liquids were dried and distilled
prior use. M-TUD-1 catalysts were activated in the presence of
air at up to 600˚C at a ramp of 1 ˚C.min-1 and heated at 600 ˚C for
10 h. The experiments were performed in dried glassware and
under N2 atmosphere.
Temperature-programmed desorption (TPD) of ammonia was
carried out on a Micromeritics TPR/TPD 2920 equipped with a
thermal conductivity detector (TCD). The sample (30 mg) was
pre-treated at 600˚C to remove volatile components. Prior to the
65 TPD measurements the samples were degased at 250 ˚C (after
ramping from room temperature at a ramp rate of 10˚C.min-1) for
1h then saturated with ammonia gas at 100˚C for 30 min at a flow
of 20 cm3.min-1. After a purge of ammonia by helium for 30 min
at a flow of 10 cm3.min-1 at 100˚C, desorption of NH3 was
70 monitored in the range between 100 and 600˚C at a ramp rate of
10˚C.min-1.
5
10 Catalyst preparation
General protocol: Dry isopropanol and ethanol were added to a
plastic flask (washed with acetone and placed 30 min in an oven
at 60 °C) and the flask was closed. M(iPrO)3 (aluminium or boron
isopropoxide) was ground (in case of Al; B is liquid) before
15 addition to the isopropanol/ethanol mix. Then TEOS (tetraethyl
orthosilicate) was added. This mixture was stirred 20 min,
resulting in a milky solution. TEA (triethanolamine) was weighed
and diluted with water. The TEA/water mix was added dropwise
over a period of 25 min, whilst stirring, then the mixture was
20 stirred an additional 20 min. This resulted in a milky/glassy white
gel. TEAOH (tetraethylammonium hydroxide solution 35%) was
then added dropwise over a period of 30 min. This yielded a gel
with a molar ratio of SiO2/xM(iPrO)3/(0.3-0.5)TEAOH/(0.7-
1.1)TEA/(10-20)H2O. After 30-45 min extra stirring, the gel
25 formed was transferred to a porcelain bowl and was allowed to
dry at room temperature overnight. The porcelain bowl was
placed in an oven at 98 °C for 24h (drying step). It turned into a
yellow, transparent solid. This solid was ground, then transferred
in Teflon lined steel autoclaves, and placed in an oven at 180 °C
30 for 6h (hydrothermal treatment). After this hydrothermal
treatment, the material was transferred again into the porcelain
bowl and placed in the calcination oven. The following program
was applied (calcination step): heating at 1 ˚C.min-1 until 600 °C,
hold at this temperature for 10 h, then cooled down to 50°C at a
35 rate of 1 ˚C.min-1. The M-TUD-1 was recovered as a white solid.
The FT-IR spectra were measured on a Perkin Elmer Spectrum
ONE instrument. A KBr wafer was prepared containing a
particular amount of the calcined sample that allowed for a
75 transmission of minimally 50% (< 1 mg). The spectrum was
taken over a range of 450-4000 cm-1 with a resolution of 1 cm-1.
Transmission Electron microscopy was performed by using a
Philips CM30T electron microscope with a LaB6 filament as
source of electrons operated at 300 kV.
80
The MAS NMR was recorded on a Bruker Avance-400
Spectrometer with a 4mm zirconium rotor sample holder spinning
at 10 kHz. The 11B/27Al MAS NMR was measured at a frequency
of 128.31/104.20 MHz with a spectral width of 25/100 kHz,
acquisition time of 0.05s/0.08s, and acquisition delay 1s, the total
85 number of scans was 2000/4000. Liquid 11B NMR spectra were
recorded with a Bruker Avance 400 spectrometer with chemical
shifts (δ) reported in ppm downfield with B(OH)3 0.1 M in D2O.
HMF synthesis
General protocol: 600 mg of fructose and 200 mg of M-TUD-1
90 were weighed and filled in an Ace® pressure 15 mL-glass tube.
The solution was completed to 2 mL by adding demi-water, then
6 mL of toluene was added in the case of the biphasic
experiments. The solution was placed in an oil bath at the desired
temperature. After the defined time of reaction, the tube was
95 removed from the oil bath and allowed to cool down to room
temperature (in 5 min approx.). In the case of biphasic reactions,
the two phases were allowed to separate for a few minutes. 100
µL of the organic phase was then sampled, derivatised and
analyzed by GC (replicated). The aqueous phase was diluted 5
100 times by adding 8 mL 0.005 M H2SO4. 2 mL of this diluted
solution was transferred into an Eppendorf tube, centrifuged and
the supernatant analyzed by HPLC.
Catalyst characterization
The volumetric nitrogen adsorption was realised on
a
Quantachrome Autosorb-6B at 77 K. Prior to the physisorption
experiments, samples were degassed at 250˚C for 16h.
40
Powder XRD patterns were obtained on a Philips PW 1840
diffractometer equipped with a graphite monochromator using
CuKα radiation. The powder XRD patterns at small angles were
obtained with a Bruker D8 Discover X-ray diffractometer
equipped with the 2-dimensional Hi-Star Area Detector and Cross
45 Coupled Göbel Mirrors. The measurements were performed in
transmission mode at room temperature using monochromatic
CuKα1 radiation and a sample to detector face of 30 cm.
Recycling step (recycled catalyst): Just after reaction and cooling
(cf protocol above), the Ace® tubes were centrifuged 10 min at
105 1800 rpm. The recovered pellets (humins and catalyst) were
placed in an oven and calcined at 600˚C for 6h, then used in a
recycling step (same protocol followed).
The elemental analysis of the materials was carried out using
an ICP-OES 4300DV plasma emission spectrometer (Perkin
50 Elmer, United States), enabling simultaneous recording of the full
emission spectrum of the sample in the range from 166.25 to 847
nm with the help of a Charge Injection Device (CID). The
spectrometer was equipped with a cyclonic spray chamber with
concentric nebulizer. A double system of observation of plasma
55 was used (axial and radial). Approximately 10-40 mg sample was
Recycling step (recycled solution): Just after reaction and cooling
(cf protocol above), the Ace® tubes were centrifuged 10 min at
110 1800 rpm. The supernatant was placed in the same conditions as a
classical reaction.
HMF synthesis: HPLC analysis (aqueous phase): Samples were
diluted by a factor 10 or 20 with an aqueous solution of H2SO4 at
8
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0.005 M. 10 µL of the diluted samples were measured by HPLC.
The used eluent was an aqueous solution of H2SO4 at 0.005 M.
The method lasted 35 min with a flow rate of 0.8 mL.min-1. Both
BeBasic and Dr Harald Ruijssenaars (Bird Engineering) for
fruitful and valuable discussions about this work. The authors
would like to acknowledge the TU Delft colleagues for their help
UV (for HMF and Furaldehyde, Fal) and RI (for HMF, Fructose, 60 in the different analyses: Willy Rook (physisorption), Dr. Olav
5
Levulinic acid, LA, Formic acid, FA, and Fal) detections were
used. Retention times: Fructose: 7.3 min, FA: 10.3 min, LA: 11.5
min, HMF: 21.7 min, Fal: 32.6 min.
Steinebach (ICP-OES), Dr. Patricia Kooyman (TEM), Ben
Norder (XRD), Remco van Oosten (GC), Maarten Gorseling
(HPLC, FTIR).
HMF synthesis: Derivatisation and GC analysis (organic phase):
100 µL of the toluene phase was sampled. 50 µL of the IS
Notes and references
10 solution (hydroquinone, 20 mg in 10 mL, 2g/L) was added. 150
µL of BSTFA (N,O-bis-trimethylsilyltrifluoroacetamide) reagent
was finally added. After agitation (5 min) the mixture was
measured in GC. The method lasted 15 min. GC method (Varian
FactorFour VF-1ms column): Carrier gas: He; Detector
15 temperature: 330 °C; Injector temperature: 300 °C; Temp.
gradient: beginning at 100°C hold for 4.2 min, then increase of 15
°C.min-1 to 140 °C, of 20 °C.min-1 to 240 °C and finally of 40
°C.min-1 to 325 °C (hold 1 min); Column flow: 1.08 mL.min-1
(linear velocity 42.2 cm.sec-1); Split ratio: 20, Retention times:
20 HMF: 6.0 min, IS: 7.3 min.
65 aAdeline Ranoux, Kristina Djanashvili, Isabel W. C. E. Arends, Ulf Hanefeld,
Technische Universiteit Delft, Department of Biotechnology, Gebouw voor
Scheikunde, Julianalaan 136, 2628 BL Delft, The Netherlands; Fax: +31 (0) 15
2781415; E-mail: u.hanefeld@tudelft.nl
70 † Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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