Synthesis of long alkyl chain ethers through direct etherification of biomass-based alcohols with 1-octene over heterogeneous acid catalysts

Article abstract of DOI:10.1016/j.jcat.2009.09.023

Heterogeneous etherification of various biomass-based alcohols with 1-octene was investigated as a direct route for the synthesis of long alkyl chain ethers. Several acid catalyst materials including Amberlyst resins and various zeolites were screened as etherification catalysts in a solventless system. It was found that H-Beta zeolites are the most selective catalysts for the etherification of biomass-based alcohols with 1-octene to the corresponding mono-ethers. With H-Beta the conversion of neat glycerol was around 15-20% and increased to 54-89% for glycols such as ethylene glycol and 1,2-propylene glycol, with high selectivities to mono- and di-octyl ethers of 85-97%. Other linear alkenes like 1-dodecene and 1-hexadecene were successfully employed in the direct etherification of glycols as well. Crude glycerol was also etherified, albeit with low conversions. The influence of several reaction parameters on the etherification activity of H-Beta has been investigated together with catalyst recovery and re-use.

Full text of DOI:10.1016/j.jcat.2009.09.023

Journal of Catalysis 268 (2009) 251–259
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Synthesis of long alkyl chain ethers through direct etherification
of biomass-based alcohols with 1-octene over heterogeneous acid catalysts
Agnieszka M. Ruppert a,1, Andrei N. Parvulescu a, Maria Arias a, Peter J.C. Hausoul a,b
,
Pieter C.A. Bruijnincx a, Robertus J.M. Klein Gebbink b, Bert M. Weckhuysen a,
*
a Inorganic Chemistry and Catalysis group, Debye Institute for Nanomaterials Science, Utrecht University, Sorbonnelaan 16, 3508 TB Utrecht, The Netherlands
b Chemical Biology & Organic Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 May 2009
Revised 11 September 2009
Accepted 26 September 2009
Available online 28 October 2009
Heterogeneous etherification of various biomass-based alcohols with 1-octene was investigated as a
direct route for the synthesis of long alkyl chain ethers. Several acid catalyst materials including Amber-
lyst resins and various zeolites were screened as etherification catalysts in a solventless system. It was
found that H-Beta zeolites are the most selective catalysts for the etherification of biomass-based alco-
hols with 1-octene to the corresponding mono-ethers. With H-Beta the conversion of neat glycerol was
around 15–20% and increased to 54–89% for glycols such as ethylene glycol and 1,2-propylene glycol,
with high selectivities to mono- and di-octyl ethers of 85–97%. Other linear alkenes like 1-dodecene
and 1-hexadecene were successfully employed in the direct etherification of glycols as well. Crude glyc-
erol was also etherified, albeit with low conversions. The influence of several reaction parameters on the
etherification activity of H-Beta has been investigated together with catalyst recovery and re-use.
Ó 2009 Elsevier Inc. All rights reserved.
Keywords:
Etherification
Zeolites
Biomass
Glycerol
Glycols
Octyl-ethers
Alkenes
1. Introduction
Etherification of glycerol or bio-based glycols represents an
important application, as it will directly afford compounds that
In recent years, a growing interest in the use of biomass as raw
material for the chemical industry has developed [1,2]. Biomass is
the primary renewable source of carbon besides carbon dioxide
and can therefore be a sustainable alternative to the depleting fos-
sil resources for the production of both transport fuels and chem-
icals [3]. In this way, new platform molecules can be obtained from
biomass and further used as starting materials for bulk and fine
chemicals [4,5]. In contrast to fossil-derived feedstock, some of
these molecules are highly oxygenated compounds, such as poly-
ols. An important example of the latter is glycerol. The increase
in biodiesel production and development of the vegetable oil
industry results in the production of glycerol in large quantities
as a by-product. For example, about one kilogram of glycerol is ob-
tained for each ten kilograms of biodiesel produced. In order to
have a sustainable biodiesel production, new processes for the con-
version of glycerol to value added products are absolutely neces-
sary [2]. Besides glycerol other polyols, such as glycols, also
represent important starting materials for various chemical syn-
theses and they are readily available from biomass sources via fer-
mentation, thermochemical or catalytic processes [5].
can be used as either fuel additives [6], intermediates in the phar-
maceutical industry, agrochemicals [7] or as non-ionic surfactants
[2,8]. More specifically, the direct etherification with long linear al-
kenes like 1-octene will produce C8-chain mono and di-ethers
which have valuable properties and potential large scale applica-
tions [9–13]. Indeed, C8 mono-ethers of glycerol or ethylene glycol
in particular were found to present remarkable antimicrobial and
antiseptic properties [13,10] and C8 mono-ethers of glycerol and
glycols have in general found wide application as non-ionic surfac-
tants or solvents [13,9,12,14].
Long alkyl ethers of glycerol are classically obtained indirectly
using several synthetic steps or by employing environmentally
hazardous compounds, using strong mineral acids or alkaline bases
[15] as catalysts and alkyl chlorides as etherification agents. Corre-
sponding alkyl ethers of ethylene glycol and propylene glycol are
generally obtained using oil-based ethylene oxide or propylene
oxide as starting materials [9]. These routes are not only time-con-
suming, but produce stoichiometric amounts of waste as well.
Products with similar properties have been recently obtained by
telomerization of 1,3-butadiene with glycerol and glycols, using
homogeneous Pd/phosphine catalysts, but an extra hydrogenation
step is needed in this case to obtain the saturated C8-ethers
[16,17]. A direct one-step etherification process of glycols and
glycerol with 1-octene catalyzed by a heterogeneous system is
* Corresponding author. Fax: +31 30 251 1027.
E-mail address: b.m.weckhuysen@uu.nl (B.M. Weckhuysen).
On leave from Institute of General and Ecological Chemistry, Technical University
1
of Lodz, ul. Zeromskiego 116, 90-924, Lodz, Poland.
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.09.023

252
A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
therefore desired as it will represent a greener and more straight-
forward route to these valuable compounds.
in the NHþ4 form were calcined before reaction and transformed
into the H+ form. The following calcination program was used:
heating from 25 °C to 500 °C with a rate of 1 °C/min and keeping
the temperature at 500 °C for 6 h. From here on, zeolites will be de-
noted as e.g. H-Beta (37.5) instead of H-Beta (Si/Al = 37.5).
Until now most of the reports concerning heterogeneous ether-
ification of such alcohols with alkenes involve the use of isobutene
or butene [18,20], or cyclic alkenes [21] using acid catalysts such as
ion-exchange resins, zeolites (i.e. H-Beta and HY) or mesoporous
sulfonated silica. Recently, Corma and co-worker [19] reported
the direct synthesis of the 1-octyl ether of methanol from the di-
rect reaction of methanol with 1-octene in the presence of a homo-
geneous AuCl3/CuCl catalyst system. Knifton [12] reported the
2.2. Catalyst characterization
X-ray powder diffraction (XRD) was performed using a Bruker-
AXS D8 Advance powder X-ray diffractometer, equipped with
automatic divergence slit, Våntec-1 detector and Cobalt Ka1,2
etherification of ethylene glycol and 1,2-propylene glycol with
a-
olefins over homogeneous heteropolyacid catalysts and a hetero-
geneous montmorillonite clay material. Both the homogeneous
and heterogeneous acid catalysts presented high etherification
activity for ethylene glycol, but needed to use an organic solvent
and a continuous flow system and very low activities were re-
ported for 1,2-propylene glycol. Surprisingly, there is no detailed
investigation of the direct etherification of glycerol with long linear
alkenes, although one report indicates H-Beta as an active catalyst
for the etherification of glycerol with 1-dodecene [22].
Given the little attention hitherto paid to direct etherification
and its large potential for biomass-based alcohol valorization, we
have decided to investigate the direct synthesis of C8-chain mono-
and di-ethers of glycerol and a number of renewable glycols from
the reaction with 1-octene (Scheme 1). This complementary route
would provide an attractive alternative to our research effort on
glycerol, polyols and sugars telomerization [16]. The reactions
were performed in a solventless system in the liquid phase and
various commercial solid acids such as zeolites and ion-exchange
resins were tested as heterogeneous etherification catalysts.
(k = 1.79026 Å) source. The specific surface areas and pore volume
were determined by N2 sorption measurements using a Micromer-
itics ASAP 2400 instrument. Surface areas were calculated using
the BET model.
2.3. Catalytic experiments and analytical methods
In a typical etherification reaction 0.12 mol of glycerol, 0.24 mol
of 1-octene and 1 g of catalyst were loaded in a 100 mL stainless
steel Parr autoclave. The autoclave was flushed with Ar three times
and then pressurized with an additional 10 bar of Ar. The autoclave
was heated to 120 °C or 140 °C under continuous mechanical stir-
ring (750 rpm). The starting point of the reaction was taken when
the autoclave reached the reaction temperature. When the reaction
was considered finished, the autoclave was cooled to 40 °C and the
reaction mixture was dissolved in a known amount of ethanol. The
catalyst was separated by filtration, and a sample of the solution
was taken for further analysis. The conversions were calculated
using a GC 2010 system from Shimadzu with a CP-WAX 57CB col-
umn (25 m ꢀ 0.2 mm ꢀ 0.2
done by a GC–MS from Schimadzu with a Supelcowax 10 column
(30 m ꢀ 0.2 mm ꢀ 0.2 m) and a HPLC-ESI-MS from Schimadzu
with a CP-WAX 57CB column (25 m ꢀ 0.2 mm ꢀ 0.2 m). Since
lm). Identification of the products was
2. Experimental section
l
l
2.1. Chemicals and catalysts
the main product of glycerol etherification (mono octyl-ether (3-
(2-octyloxy)propane-1,2-diol) (C8Glyc)) is not commercially avail-
able, it was isolated and purified by column chromatography. The
purity was checked by 13C NMR, 1H NMR, and GC–MS. Calibration
was done using the pure compound and the correction factor of the
mono-ether was considered to be the same for the di-ether and
other by-products. The same procedure was used for the other
tested alcohols.
Glycerol (>99%), 1,2-propylene glycol (>99%) and 1-octene
(99+%) were purchased from Acros, while 1,3-propylene glycol
(>99%) and ethylene glycol (>99%) were purchased from Fluka.
Crude glycerol (15 wt.% water) was received as a gift from DOW
Chemical. Amberlyst-70 was a gift from Sabic. The wet ion-ex-
change resin was washed with methanol and dried for 6 h at
60 °C before reaction. Zeolite NHþ4 -Beta (Si/Al = 12.5) CP814E, H-
Beta (Si/Al = 37.5) CP811E, NHþ4 -ZSM-5 (Si/Al = 15) CBV3024E, H-
Y (Si/Al = 40) CBV 910 were purchased from Zeolyst and NHþ4 -
USY (Si/Al = 25) was a gift from Albermarle. The zeolites that were
3. Results and discussion
3.1. Catalyst screening
OR
catalyst
As part of our recent efforts aimed at catalytic glycerol valoriza-
tion, the etherification of glycerol with 1-octene was chosen as a
test reaction. The aim was to develop an alternative route for the
synthesis of long alkyl chain ethers complementary to our previous
telomerization studies [16]. Solid acid catalysts, such as zeolites
and ion-exchange resins, were previously reported as active cata-
lysts for the etherification of glycerol with isobutene [18,20].
Therefore, these solid acids were screened in the direct etherifica-
tion of glycerol with 1-octene and the catalytic activities were
benchmarked against a homogeneous acid catalyst, para-toluene
sulfonic acid (pTSA). The etherification reactions were performed
at 120 °C or above since no significant conversion to octyl-ethers
was observed at lower temperatures for any of the tested catalysts.
The reaction time was fixed at 10 h. The conversions and yields
were calculated based on glycerol.
+ R-OH
R-OH:
OH
OH
HO
OH
HO
HO
1,2PG
OH
1,3PG
Glyc
OH
OH
OH
HO
HO
OH
1,3BG
EG
1,2BG
HO
HO
OH
1-hexanol
1,5PENG
Etherification of glycerol with 1-octene over the acid catalyst
can result in the formation of a mixture of mono-, di- and tri-ethers
according to Scheme 2 and, additionally, in the formation of some
Scheme 1. Etherification of alcohols with 1-octene.

A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
253
OH
O
O
OH
O
+ linear isomers
+ linear isomers
+
OH
OH
acid catalyst
HO
OH
+
+
O
O
O
+ linear isomers
Scheme 2. Etherification of glycerol with 1-octene over acid catalysts. Tri-octyl ether of glycerol was not observed as a reaction product.
unwanted by-products (vide infra). The product distribution was
found to depend strongly on the type of acid catalyst employed.
The only observed octyl-ethers of glycerol were the mono-ether
C8Glyc and di-ether C16Glyc, however. The tri-octyl ether of glyc-
erol was not formed, most probably because of steric constrains.
Since at temperatures above 120 °C 1-octene is in equilibrium be-
tween the liquid and the gas phase, glycerol is essentially found in
excess in the liquid phase and the reaction will proceed according
to the mechanism proposed by Françoise and Thyrion [23] for the
etherification of ethanol with isobutene [24]. The alcohol is proton-
ated first and further reaction with the alkene regioselectively
gives the C8Glyc and C16Glyc as branched isomers (>95%) with
only small traces of linear and mixed linear-branched ethers
(<5%) (Scheme 2).
The results of the catalyst screening are presented in Table 1. In
the absence of a catalyst (Table 1, entry 1) no conversion was ob-
served irrespective of the reaction conditions. Since water is
known [20a] to decrease the activity of etherification catalysts,
all catalysts were dried and the reaction mixture was flushed with
Ar. The catalytic etherification activities of Amberlyst-70 and pTSA
were found to be similar (Table 1, entries 2–7). With both catalysts
selectivities below 20% to the glycerol octyl-ethers were obtained.
C8Glyc was formed with low selectivity and only traces of C16Glyc
could be detected with both catalysts, as unwanted by-products
made up most of the product. The selectivity for C8Glyc was 12%
over Amberlyst-70 at 120 °C, and remained the same with an in-
crease of temperature to 140 °C (entries 2 and 3). The increase in
temperature was found to affect only the conversion, which in-
creased from 12% to 38% for Amberlyst-70 (Table 1, entries 2 and
3). As a homogeneous counterpart of Amberlyst-70, the use of pTSA
resulted in higher conversions of glycerol and also in a higher
selectivity to C8Glyc (Table 1, entries 5–7). It should be noted that
1 g of pTSA contains 5.81 ꢀ 10ꢁ3 moles of sulfonic acid groups,
whereas 1 g of Amberlyst-70 contains 2.55 ꢀ 10ꢁ3 moles of acid
groups. Therefore, using the same weight loading of catalyst, al-
ready at 120 °C the glycerol conversion was 22% with pTSA (com-
pared to 12% for Amberlyst-70) and at 140 °C conversion reached
57% (Table 1, entries 2–6). Nonetheless, the homogeneous acid
was still more active than Amberlyst-70 even when an equivalent
concentration of acid species was used (Table 1, entries 3 and 7). As
in the case of Amberlyst-70, by-products were predominantly
formed, with selectivities higher than 70%.
In contrast to the results obtained with either Amberlyst-70 or
pTSA, C8Glyc and C16Glyc were obtained as the main products in
the etherification of glycerol with 1-octene when zeolites were
employed as heterogeneous catalysts (Table 1, entries 8–18). H-
Beta zeolites gave the best etherification activities, with excellent
selectivities to the octyl-ethers of glycerol, particularly to the de-
sired C8Glyc. Two H-Beta zeolites with a different Si/Al ratio were
tested in the etherification of glycerol to assess their influence on
conversion and product distribution (Table 1, entries 8–15). H-Beta
(12.5) gave the highest conversion of glycerol at 140 °C, namely
Table 1
Screening of acid catalysts for etherification of glycerol with 1-octene.
Entry
1
Catalyst
T (°C)
Conversiona (%)
0
SelectivityC8Glyc (%)
–
SelectivityC16Glyc (%)
–
Selectivityother (%)
–
Blank
140
2
3
4
Amberlyst-70
120
140
140b
12
38
17
12
11
9
0
1
1
88
88
90
5
6
7
pTSA
120
140
140c
22
57
45
20
19
19
3
2
1
77
79
80
8
9
10
11
12
H-Beta (37.5)
120
140
160
140b
140d
10
14
14
13
17
72
79
54
81
78
16
12
8
12
15
12
9
38
7
7
13
14
15
H-Beta (12.5)
140
15
12
19
80
84
81
14
13
14
6
3
5
140b
140d
16
17
18
H-Y (40)
140
140
140
12
10
1
47
75
83
12
12
5
41
13
12
USY (27.5)
H-ZSM-5 (15)
Standard etherification conditions: 1 g catalyst, 10 h reaction, 10 bar Ar, 1-octene:glycerol 2:1.
a
Conversion of glycerol.
5 h.
0.5 g of pTSA.
Molar ratio 3:1.
b
c
d

254
A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
15% as well as the highest selectivity for the octyl-ethers (94%).
C8Glyc was obtained as the main product with a selectivity of
80% (Table 1, entry 13). A slightly lower conversion of 14% and
selectivity of C8Glyc of 79% was obtained at 140 °C with a zeolite
with a higher Si/Al ratio, H-Beta (37.5) (Table 1, entry 9). H-USY
(27.5) and H-ZSM-5 (15) (Table 1, entries 17 and 18) were less ac-
tive in the etherification of glycerol with 1-octene, yet the selectiv-
ity to octyl-ethers remained high, above 90%. For H-Y (40) a
conversion of glycerol of 12% was reached, but compared to the
other zeolites the selectivity to C8Glyc dropped to 47%, while the
selectivity to C16Glyc remained around 12% (Table 1, entry 16).
The screening of the different zeolites clearly indicated that H-Beta
zeolites are the most promising etherification catalysts for this par-
ticular reaction.
and may form ethers with non-ionic surfactant properties [12] (Ta-
ble 2).
For all substrates higher yields of ethers were obtained com-
pared to glycerol. The conversions and selectivities to the octyl-
ethers were influenced by both the alcohol and the catalyst that
was used.
In fact, the change of substrate increased the etherification
activity considerably for the H-Beta zeolites, while maintaining
the high selectivity for the octyl-ethers (Table 2, entries 6–18).
Conversions between 40% and 90% were obtained using the two
H-Beta zeolites depending on the alcohol. These conversions are
considerably higher than those obtained for glycerol for which
maximum conversion under the same conditions was 16% (Table
2, entry 6).
The optimal reaction temperature for the etherification activity
of H-Beta zeolites was found to be 140 °C. For H-Beta (37.5), a low-
er temperature gave a lower conversion, whereas at 160 °C the
selectivity to by-products increased rapidly from 13% to 38% (Table
1, entries 8–10). A higher reaction temperature did not increase the
amount of di-ethers which is in contrast with the results obtained
for Beta zeolites in the etherification of glycerol with isobutene
[20a]. Inherent differences in reactivity between 1-octene and iso-
butene and the size of the product may prevent an increase in the
amount of C16Glyc in this case. Decreasing the reaction time from
10 h to 5 h had very little influence on the conversion of glycerol
with H-Beta (37.5), i.e. 13% instead of 14% (Table 1, entry 11). For
H-Beta (12.5) the conversion of glycerol dropped to 12% from
15% but the selectivity to C8Glyc slightly increased to 84% (Table
1, entry 14). Increasing the alkene to glycerol molar ratio to 3:1 im-
proved the etherification activity of both H-Beta zeolites (Table 1
entries 12 and 15). The conversion increased from 14% to 17% for
H-Beta (37.5) where it reached 19% for H-Beta (12.5). In both cases
the selectivity for C8Glyc remained around 80%.
Similar conversions of 70% were obtained for both H-Beta zeo-
lites in the etherification of ethylene glycol (EG) (Table 2, entries 7
and 12). H-Beta (37.5) gave a higher selectivity for the di-ether
(21%), where H-Beta (12.5) presented an excellent selectivity
(81%) for the mono-ether. 1,2-Propylene glycol (1,2PG) also proved
a very good substrate for the etherification with 1-octene with
both H-Beta zeolites. With H-Beta (12.5) a 76% conversion of
1,2PG was obtained after 5 h of reaction and the selectivity to
C8-ether was 82%. For H-Beta (37.5) the conversion was 87%, but
the selectivity to C8-ether dropped to 70% (Table 2, entries 8 and
13). For 1,2-butylene glycol (1,2BG) a lower conversion of the alco-
hol (59%) was obtained with H-Beta (12.5), but the selectivity for
the C8-ether increased to 90%. In contrast, H-Beta (37.5) presented
the highest conversion when 1,2BG was used as a substrate, i.e.
89% after just 5 h of reaction. The selectivity for the C8-ether was
still 76% in this case (Table 2, entry 15). 1,3-Propylene glycol
(1,3PG) turned out to be a more difficult substrate for both H-Beta
zeolites, as conversions of 52% and 46% were obtained for H-Beta
(12.5) and H-Beta (37.5), respectively (Table 2, entries 9 and 14).
Other glycols, 1,3-butylene glycol (1,3BG), for example, could also
be successfully etherified with H-Beta (37.5) (Table 2, entry 16).
Larger alcohols, like 1,5-pentylene glycol (1,5PENG) and 1-hexanol,
were also etherified with 1-octene with good results (Table 2,
entries 17 and 18). The conversion of 1,5-pentene glycol was 71%
over H-Beta (37.5) again with a very high selectivity for the
C8-ether of 92% (Table 2, entry 17); 1-hexanol selectivity for the
C8-ether was 91%.
3.2. Substrate scope and product selectivity
The initial catalyst screening for glycerol etherification activity
with 1-octene indicated that H-Beta zeolites presented a high
selectivity to the octyl-ethers, whereas Amberlyst-70 and pTSA
showed higher conversions of the alcohol but a very low selectivity
to the desired products. To follow up on this encouraging lead, we
intended to explore the substrate scope of the system. Indeed, be-
sides glycerol, other alcohols are also easily available from biomass
Although the selectivity to C8-ethers was somewhat higher for
the glycols than for glycerol with Amberlyst-70 and pTSA, the by-
Table 2
Substrate screening in etherification with 1-octene.
Entry
Catalyst
pTSA
Substrate
Conversion (%)
Selectivityc8-ether (%)
Selectivityc16-ether (%)
Selectivityother (%)
1
2
Glycerol
1,2PG
40
80
10
42
2
8
88
50
3
4
5
Amberlyst
Glycerol
EG
1,2PG
35
68
85
5
33
26
1
1
7
93
66
67
6
7
8
9
H-Beta (12.5)
Glycerol
EG
1,2PG
1,3PG
1,2BG
16
70
76
51
59
83
81
82
70
90
13
17
6
11
3
3
2
9
19
7
10
11
12
13
14
15
16
17
18
H-Beta (37.5)
Glycerol
EG
1,2PG
1,3PG
1,2BG
1,3BG
1,5PENG
1-Hexanol
14
71
87
46
89
54
71
65
78
74
70
78
76
71
92
91
14
21
17
12
13
14
5
8
4
13
10
12
15
3
–
9
Reaction conditions: 1 g catalyst, 0.12 mol substrate, 0.36 mol 1-octene, 140 °C, 5 h, 10 bar Ar.

A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
255
products were still formed predominantly (Table 2, entries 1–5).
For 1,2-propylene glycol (1,2PG) a conversion of 80% was obtained
with pTSA with a selectivity of just 42% for the mono-ether. As pre-
viously observed, pTSA was generally more selective than Amber-
lyst-70. The conversion of 1,2PG was 85% with Amberlyst-70, but
the selectivity for the C8-ether just 26%. Like in the case of glycerol,
with both pTSA and Amberlyst-70 the di-ether was formed with
very low selectivities (Table 2, entries 1–5).
The size of the alcohol and the hydrophilic/hydrophobic proper-
ties clearly influence the etherification activity of the heteroge-
neous catalysts. Glycols are more hydrophobic than glycerol [25]
and in the absence of a solvent they will mix better with the al-
kene. The hydrophobic character increases in the series glyc-
erol < ethylene glycol < 1,2-propylene glycol < 1,2-butylene glycol.
The activity of the etherification catalysts is following a similar
trend. For pTSA and Amberlyst-70 the conversion increased from
glycerol towards 1,2PG. The same trend was observed for H-Beta
zeolites with few exceptions. With 1,5PENG and 1-hexanol smaller
conversions were obtained even though these alcohols are the
most hydrophobic ones. The size of the alcohol and of the products
may be responsible for this exception.
In addition to the physicochemical properties of the alcohol, the
catalyst properties are also very important. For instance, pTSA and
Amberlyst-70 are more hydrophilic than H-Beta zeolites. This dif-
ference is reflected in the product distribution over these materials
and especially by the high amounts of by-products, which were ob-
tained for pTSA and Amberlyst-70. Particularly for glycerol the
selectivity for these by-products can go up to 90%. A closer look
at the type of by-products (Scheme 3) shows that most of them
are formed by glycerol dehydration, condensation or etherification
of the polyglycerols. Moreover, the interaction with glycerol is en-
hanced by the highly hydrophilic character of Amberlyst-70 [26]
making the adsorption of 1-octene on the catalyst more difficult.
Indeed, Amberlyst catalysts are highly hydrophilic materials that
are known to concentrate polar compounds such as alcohols in
their structure [24,27]. Polyglycerol ethers were also found in the
complex mixture of products in the reaction catalyzed by pTSA
and Amberlyst-70. They were found to be formed from the ether-
ification of polyglycerols and not from successive reactions of
C8Glyc as indicated by the results obtained after 5 h of reaction
with Amberlyst-70. The polyglycerols consist mostly of tetra- or
penta-glycerol units and their formation was favored by the mac-
roporous structure of Amberlyst-70. The same types of by-prod-
ucts were obtained for the other glycols with Amberlyst-70 and
pTSA, albeit to a lesser extent (Table 2, entries 1–5).
Only small amounts of by-products were obtained for H-Beta
zeolites (Tables 1 and 2). Dehydration products and traces of poly-
glycerol derivatives were obtained in the etherification of glycerol
with H-Beta zeolites with selectivities below 10% (Table 2, entries
6–11). The C8Glyc was predominantly formed with selectivities
around 80% and only a small amount of bulky polyglycerols of
low oligomerization degree (two to maximum three glycerol
units). The uniform microporous structure of H-Beta, with pore
dimensions of around 7.5 Å, is thus likely inducing a shape selec-
tivity effect. In addition to the shape selectivity effect, the selectiv-
ity is influenced by the fact that due to its high Si/Al ratio H-Beta is
more hydrophobic than Amberlyst-70. Furthermore, H-Beta also
has a higher surface area. Indeed, the nitrogen sorption measure-
ments indicated that the tested H-Beta had surface areas higher
than 500 m2/g whereas for Amberlyst-70 this amounted to just
36 m2/g. Both the hydrophobic properties and the high surface area
allow a better adsorption of 1-octene over H-Beta zeolites com-
pared with Amberlyst-70. The low selectivity for the by-products
of H-Beta zeolites was kept for the other alcohols as well. For gly-
cols the main by-products were found to be dehydration products,
cyclic products and low oligomers of the alcohols. The different Si/
Al ratio influences the etherification activity of the two zeolites.
With H-Beta (12.5) the etherification activity with hydrophilic sub-
strates, like glycerol or 1,3-propylene glycol was higher compared
to the one of H-Beta (37.5) for the same substrates. Also the selec-
tivity for the C8-ether of H-Beta (12.5) was always higher com-
pared to H-Beta (37.5). As a result of its higher Si/Al ratio, H-Beta
(37.5) has both a higher acid strength and is more hydrophobic.
As a result, higher conversions are obtained with more hydropho-
bic alcohols like 1,2PD and 1,2BD (table 2, entries 13 and 15). The
C16-ethers were also formed in higher amounts since the C8-ether
can more easily interact with the more hydrophobic H-Beta (37.5)
and be further transformed. This indicates that an optimum com-
bination between the properties of the catalyst and substrate must
be achieved in order to have a maximum yield of desired product.
3.3. Influence of the reaction parameters on the etherification activity
of H-Beta zeolites
Since the reactions were carried out in the absence of a solvent
it was essential to exclude any influence of external mass transfer
O
O
O
O
O
O
O
O
O
O
O
OH
O
O
O
O
HO
OH
OH
OH
OH
O
HO
O
OH
O
OH
O
O
O
n
n
O
HO
dehydration products
cyclic products
OH
OH OH
HO
O
O
OH
HO
n
OH
OH
OH
OH
OH
HO
O
O
O
n
polyglycerols
octanols
n = 0, 1, 2, ...
Scheme 3. Observed by-products of glycerol etherification with 1-octene.

256
A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
limitation on the performance of the heterogeneous catalysts. The
influence of stirring speed and the amount of catalyst on the
etherification activity was therefore carefully analyzed. An extra
pressure of inert gas (10 bar Ar) was used in order to maintain 1-
octene in the liquid phase during the whole reaction.
3.4. Catalyst recovery and re-use
The reduced etherification activity of the H-Beta catalysts in the
etherification of glycerol with 1-octene is most probably caused by
deactivation of the catalyst due to the formation of some coke-like
compounds. After the reaction a change in color of the H-Beta cat-
alysts was observed. The brown-yellow color is more pronounced
in the case of glycerol compared with the other alcohols and with
an increase in the Si/Al ratio. Most probably this change of color is
caused by the oligomerization of the alkene on the acid catalyst
and irreversible adsorption of these products. Similarly, oligomer-
ization of isobutene was previously observed in the etherification
of glycerol even at reaction temperatures of 90 °C [20a]. Recent re-
ports also suggest that glycerol and its polyglycerols may irrevers-
ibly adsorb on the zeolite [28]. Irreversible adsorption of glycerol
was found to be less than 2% in our studies, but the complex mix-
ture of polyglycerols and etherified polyglycerols (Scheme 3) may
irreversibly adsorb and deactivate the solid catalyst. Further spec-
troscopic work is currently ongoing to identify the species that are
causing this deactivation and to elucidate how the process occurs.
The deactivation of the H-Beta zeolites was followed in time for
four alcohols (Fig. 1). The strongest deactivation was found for
glycerol where after 15 h of reaction conversion reached a maxi-
mum of 22% (Fig. 1a). From 5 h to 15 h the conversion increases
with 6% and the selectivity to C8Glyc slightly decreased from
83% to 81%. For the glycols tested deactivation of the H-Beta cata-
lyst is less pronounced (Fig. 1b–d), as conversions over 72% are
being easily obtained. A maximum in conversion different for each
of the alcohols was obtained after 10 h of reaction. For example,
the conversion reached 90% after 10 h for 1,2PG (Fig. 1c) and just
72% for EG. In time the selectivity for C8-ether is slightly decreas-
ing and an increase of the C16-ether selectivity and by-products
was observed.
The most sensible substrate to study the possible influence of
external mass transfer limitations is certainly glycerol as it is the
most viscous and hydrophilic of the tested alcohols. Above
500 rpm no influence of stirring rate on conversion of glycerol
was observed (Table 3, entries 1 and 2). Using a lower stirring rate
like 200 rpm resulted in both a lower conversion and selectivity for
the C8Glyc (Table 2, entry 3). A stirring rate of 750 rpm was there-
fore subsequently employed in all the etherification reactions. The
influence of the amount of catalyst was also checked. A loading 1 g
of zeolite proved to be the optimum amount in order to have a
good combination of conversion and selectivity for etherification
of the various alcohols (Table 3, entries 1, 4–9). For both glycerol
and ethylene glycol the conversion increased linearly with the
amount of catalyst which suggests that the reaction is not influ-
enced by external mass transfer limitations.
The effect of the 1-octene to glycerol molar ratio was already
pointed out to influence the etherification activity of H-Beta zeo-
lites (see Table 1). Since no solvent is used it is necessary to adjust
the ratio between the two reactants to optimize the access to the
active sites of the catalyst and desorption of the reaction products.
The influence of the 1-octene:alcohol molar ratio was exemplified
in the etherification of glycerol and 1,2-propylene glycol with 1-oc-
tene over H-Beta (12.5). A 1:1 molar ratio led to a lower conversion
for both alcohols (Table 4, entries 1 and 5). An increase of the 1-
octene:alcohol molar ratio to 3:1, resulted in higher conversions
for both substrates. A 6% and 20% increase of conversion was ob-
tained for glycerol and 1,2PG, respectively (Table 4, entries 1–3
and 5–7). Besides conversion, also the selectivity for the C8-in-
creases for both alcohols (>80%) with an increase of the 1-
octene:alcohol molar ratio to 3:1. Further increase of the 1-octene
amount gave rise to a decrease in the conversion of the two
alcohols.
In order to investigate if the deactivation of the zeolite was
reversible, the catalyst was recovered and re-used. The recovered
H-Beta (37.5) presented a lower etherification activity for glycerol
in the second run, with a 70% loss of the initial activity (Table 5,
Table 3
Stirring speed and catalyst weight influence on the etherification activity of H-Beta (37.5).
Entry
Substrate
Glycerol
Weight catalyst (g)
Stirring rate (rpm)
Conversion (%)
SelectivityC8-ether (%)
SelectivityC16-ether (%)
Selectivityother (%)
1
2
3
4
5
1
1
1
0.5
1.6
750
570
200
750
750
12
12
6
8
15
84
83
75
81
79
13
13
8
14
13
3
4
17
5
9
7
8
9
EG
0.5
1
2
750
750
750
34
61
72
92
80
64
6
18
31
2
2
5
Reaction conditions: 5 h, 1-octene:substrate 2:1, 140 °C, EG – ethylene glycol.
Table 4
Molar ratio 1-octene:alcohol versus the etherification activity of H-Beta (12.5).
Entry
Catalyst
Glycerol
1-Octene:alcohola
Conversion (%)
SelectivityC8-ether (%)
SelectivityC16-ether (%)
Selectivityother (%)
1
2
3
4
1:1
2:1
3:1
5:1
10
12
16
11
71
84
83
82
12
13
13
16
17
3
3
2
5
6
7
8
1,2PG
1:1
2:1
3:1
5:1
57
62
76
61
80
86
82
86
9
7
6
4
11
7
9
10
Reaction conditions: 1 g catalyst, 5 h, 10 bar Ar, 140 °C.
a
Molar ratio 1-octene:alcohol.

A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
257
yield of C8Glyc and C16Glyc were found to remain the same (Table
5). XRD measurements indicated that neither the calcination nor
the etherification reaction did affect the structure of the zeolite.
The re-use of H-Beta (12.5) was also attempted for 1,2PG. As was
previously observed from the conversion-time profile for the
etherification of 1,2PG, the deactivation of the H-Beta (12.5) is
much less pronounced compared to glycerol. The recovery and
re-use of the zeolite confirmed this observation. After a second
reaction cycle the conversion of 1,2PG was 57% with 90% selectivity
for the C8-ether. This corresponds to a drop of 25% of the initial
conversion. In a third reaction cycle conversion dropped further
to 40%, which nonetheless still represents more than 50% of the ini-
tial activity (Table 5, entries 5–7). The type of the alcohol thus
influences the extent of deactivation of the H-Beta zeolite and this
process is less pronounced for glycols.
a
100
80
60
40
20
0
0
2
4
6
8
10
12
14
16
Time (h)
100
80
60
40
20
0
b
3.5. Control of the alkyl chain length
Since our goal is to synthesize compounds with possible non-io-
nic surfactant applications it would be interesting if ethers with
various lengths of the alkyl chain may be obtained. It is known that
ethers with chain lengths of C10 or C16 have already found many
applications as non-ionic surfactants [9]. Therefore, in addition to
1-octene, alkenes with longer alkyl chain such as 1-dodecene and
1-hexadecene were also tested in the etherification of 1,2PG over
H-Beta (12.5) (Table 6). Good conversions after 5 h of reaction
and selectivities to the mono-ether over 90% were obtained with
1-dodecene and 1-hexadecene (Table 6). The conversion was 71%
when 1,2PG was etherified with 1-dodecene and dropped to 46%
for 1-hexadecene. With a shorter alkene like 1-hexene the selectiv-
ity for the mono-ether was just 55% and the di-ether was obtained
with 40% selectivity. On the contrary, with 1-hexadecene no di-
ether was obtained at all. The selectivity for the mono-ether was
therefore found to increase with the chain length of the alkene
(Table 6). This result may be correlated with the literature data
for etherification of glycerol with isobutene where very high selec-
tivities for the di-ethers are normally obtained with H-Beta zeolites
[20]. Also the properties of the H-Beta (12.5) may influence this
selectivity since it was already shown that this zeolite presented
a higher selectivity for the mono-ether due to its hydrophilic/
hydrophobic properties. With increasing the chain length the
interaction of the mono-ether with the zeolite may therefore de-
crease and higher selectivities for the mono-ether are obtained.
0
2
4
6
8
10
12
Time (h)
c
100
80
60
40
20
0
0
2
4
6
8
10
12
Time (h)
100
80
60
40
20
0
d
3.6. Etherification of crude glycerol
Crude glycerol coming from a caustic catalyzed methanolysis
plant was also tested in the etherification with 1-octene over H-
Beta zeolites (Table 7). When crude glycerol was used as such (Ta-
ble 6, entry 2) no catalytic activity was observed. Some etherifica-
tion activity, i.e. 3% conversion, was obtained by partially drying
the crude glycerol prior reaction (Table 6, entry 3). This aqueous
(15 wt.%) alkaline solution of glycerol affects both the acidity of
the zeolite and its structure upon heating [28]. We observed that
the presence of the alkali metal in crude glycerol (as alkali salts,
soaps, unreacted base), is detrimental for the catalytic activity of
H-Beta. That the poisoning effect is due to the alkali metal residues
rather than the large amount of water in crude glycerol is further
illustrated by the results obtained from the etherification of a
15 wt.% water solution of neat glycerol. Indeed, the etherification
activity of the H-Beta (12.5) was not influenced by the presence
of water (Table 6, entries 1 and 4) and the selectivity to C8Gly re-
mained around 84%. In order to pinpoint the poisoning effect of the
alkali ions on the zeolite etherification activity further tests were
performed in which the alkali metal content was controlled. Alkali
metals may be found in the crude glycerol as salts of the fatty acids,
0
2
4
6
8
10
12
Time (h)
Fig. 1. Time vs. etherification of various substrates with 1-octene over H-Beta
(12.5) at 140 °C, 10 bar Ar, 1-octene:alcohol 3:1, (a) glycerol; (b) ethylene glycol; (c)
1,2-propylene glycol; (d) 1,2-butylene glycol; j – conversion (%), N – selectivity of
C8-ether (%); d – selectivity of C16-ether (%); ꢀ – selectivity to other products. The
lines are intended as visual aid only.
entries 1 and 3). However, calcination of the zeolite could restore
the initial etherification activity completely. In this way, H-Beta
(37.5) was successfully recovered and re-used for 3 reaction cycles
in the etherification of glycerol with 1-octene. Both conversion and

258
A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
Table 5
Re-use of H-Beta zeolites in the etherification of glycerol with 1-octene.
Substrate
Glycerol
Run
Conversion (%)
SelectivityC8-ether (%)
SelectivityC16-ether (%)
Selectivityother (%)
1
2
2
3
Fresh
14
13
4
78
80
75
79
14
13
11
12
8
7
14
9
Calcined
Uncalcined
Calcined
14
1,2-PG
1
2
3
Fresh
Uncalcined
Uncalcined
76
57
40
82
90
91
6
3
3
9
7
6
Reaction conditions: 1 g of catalyst, 1-octene:glycerol 3:1, 5 h, 10 bar Ar, 140 °C.
Table 6
Alkene screening in etherification of 1,2PG over H-Beta (12.5).
Alkene
Conversion (%)
Selectivitymono (%)
Selectivitydi (%)
Selectivityother (%)
1-Hexene
1-Octene
1-Dodecene
1-Hexadecene
56
76
71
46
55
82
93
90
40
6
3
5
9
3
10
0
Reaction conditions: 1 g catalyst, 0.12 mol substrate, 0.36 mol alkene, 140 °C, 5 h, 10 bar Ar.
Table 7
Crude glycerol etherification with 1-octene over H-Beta (12.5).
Entry
Substrate
Conversion (%)
SelectivityC8Glyc (%)
SelectivityC16Glyc (%)
Selectivityother (%)
1
2
3
4
5
6
Glycerol
Glycerola
Glycerolb
Glycerolc
Glycerold
Glycerole
12
0
3
13
0
5
84
–
90
83
–
13
–
3
13
–
3
3
–
7
4
–
88
9
Reaction conditions: 1 g catalyst, 1-octene:glycerol 2:1, 5 h, 10 bar Ar, 140 °C.
a
Crude glycerol.
Partially dried crude glycerol.
15 wt.% aqueous glycerol solution.
86 wt.% glycerol, 10 wt.% water, 2 wt.% sodium oleate, 2 wt.% NaCl.
b
c
d
e
86 wt.% glycerol, 10 wt.% water, 2 wt.% hexanoic acid, 2 wt.% NaCl.
inorganic salts or residual catalyst. Therefore, solutions of glycerol
containing water and sodium salts were prepared (Table 6, entries
5 and 6). When a solution of glycerol containing 2 wt.% of sodium
oleate and 2 wt.% of sodium chloride was used no transformation
of the glycerol was observed. By replacing the sodium oleate with
hexanoic acid, some etherification activity was restored and in
addition to the octyl ethers some glycerol esters of hexanoic acid
were formed. Therefore, the use of crude glycerol resulting from
a heterogeneous biodiesel synthesis process is expected to avoid
such deactivation problems.
ity to the octyl-ethers were obtained in the etherification of
glycerol with 1-octene over H-Beta zeolites, more particularly with
H-Beta (12.5). Switching to other biomass-based alcohols such as
glycols resulted in a strong increase in the etherification activity
of the H-Beta zeolites, while maintaining the very high selectivity
toward the desired octyl-ethers. Conversions up to 89% were ob-
tained for 1,2PD and 1,2BD with H-Beta zeolites and selectivities
for the octyl-ethers over 90%. Using alkenes with longer alkyl
chains like 1-dodecene and 1-hexadecene also proved possible
with the H-Beta zeolites. Crude glycerol was also etherified, albeit
with low conversions and the alkaline nature of the substrate was
demonstrated to be responsible for this low activity. The H-Beta
catalysts were recovered and re-used for three reaction cycles in
the etherification of both glycerol and a glycol substrate. These re-
sults show the potential of using heterogeneous acid catalysts for
the green synthesis of valuable long alkyl mono- or di-ethers of var-
ious bio-based alcohols.
4. Conclusions
The direct etherification of glycerol and glycols with 1-octene in
a solventless system is possible by using heterogeneous acid cata-
lysts. The type of the solid acid used strongly influences the activity
and selectivity of the etherification process. Several factors were
found to influence the activity and selectivity of the acid catalysts
in the etherification of glycerol with 1-octene. Foremost, hydrophi-
licity and pore structure of the catalyst turned out to be the critical
parameters. Optimization of other parameters, like reaction time,
stoichiometry of reactants, reaction temperature, and the addition
of an inert gas resulted in an improved etherification activity of the
catalyst as well. H-Beta zeolites proved to be the most selective
etherification catalysts, whereas for Amberlyst-70 mainly by-prod-
ucts were formed. Maximum conversions of 19% with 93% selectiv-
Acknowledgment
The authors would like to thank the ACTS-ASPECT program for
financial support.
References
[1] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411.
[2] A. Behr, A. Westfechtel, J. Pérez Gomes, Chem. Eng. Technol. 31 (2008) 700.

A.M. Ruppert et al. / Journal of Catalysis 268 (2009) 251–259
259
[3] (a) M.A.R. Meier, J.O. Metzger, U.S. Schubert, Chem. Soc. Rev. 36 (2007) 1788;
(b) M. Stöcker, Angew. Chem. Int. Ed. 47 (2008) 2.
[4] E. Taarning, I.S. Nielsen, K. Egeblad, R. Madsen, C.H. Christensen,
ChemSusChem 1 (2008) 75.
(c) R. Palkovits, A.N. Parvulescu, P.J.C. Hausoul, C.A. Kruithof, R.J.M. Klein
Gebbink, B.M. Weckhuysen, Green Chem. 11 (2009) 1155;
(d) P.J.C. Hausoul, P.C.A. Bruijnincx, R.J.M. Klein Gebbink, B.M. Weckhuysen,
ChemSusChem 2 (2009) 855.
[5] (a) P. Gallezot, ChemSusChem 1 (2008) 734;
[17] A. Behr, M. Becker, T. Beckmann, L. Johnen, J. Leschinski, S. Reyer, Angew.
Chem. Int. Ed. 48 (2009) 3598.
[18] J.A. Melero, G. Vicente, G. Morales, M. Paniagua, J.M. Moreno, R. Roldán, A.
Ezguerro, G. Pérez, Appl. Catal. A 346 (2008) 44.
[19] X. Zhang, A. Corma, Chem. Commun. (2007) 3080.
[20] (a) C. Klepácˇová, D. Mravec, M. Bajus, Appl. Catal. A 294 (2005) 141;
(b) J. van Haveren, E.L. Scott, J. Sanders, Biofuels. Bioprod. Bioref. 2 (2007)
41.
[6] F. Jérôme, Y. Pouilloux, J. Barrault, ChemSusChem 1 (2008) 586.
[7] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C. Della Pina, Angew. Chem. Int.
Ed. 46 (2007) 4434.
ˇ
[8] (a) J.M. Clacens, Y. Pouilloux, J. Barrault, Appl. Catal. A 227 (2002) 181;
(b) A. Piasecki, A. Sokolowski, B. Burczyk, U. Kotlewska, J. Am. Oil Chem. Soc.
74 (1997) 33;
(b) C. Klepácová, D. Mravec, A. Kaszonyi, M. Bajus, Appl. Catal. A 328 (2007) 1;
(c) R.S. Karinen, A.O.O. Krause, Appl. Catal. A 306 (2006) 128;
(d) A. Behr, L. Obendorf, Chem. Eng. Technol. 73 (2001) 1463;
(c) A. Corma, S. Iborra, S. Miquel, J. Primo, J. Catal. 161 (1996) 713.
[9] E. Smulders (Ed.), Laundry Detergents, Wiley-VCH, 2002, p. 52.
[10] K. Nishizawa, H. Nakayama, T. Ukune, Y. Yamoto, JP Patent 51076423, 1976.
[11] C.L. Edwards, WO2005019138, 2005.
(e) A. Behr, J. Eilting, K. Irawadi, J. Leschinki, F. Lindner, Green Chem. 10 (2008)
13.
[21] Y. Gu, A. Azzouzi, Y. Pouilloux, F. Jérôme, J. Barrault, Green Chem. 10 (2008)
164.
[12] J.F. Knifton, Appl. Catal. A 130 (1995) 79.
[22] J. Stephan, M. Röper, T. Heidemann, M. Triller, J. Tropsch, US Patent 80176782,
2008.
[23] O. Françoise, F.C. Thyrion, Chem. Eng. Process. 30 (1991) 141.
[24] J.H. Clark, G.L. Monks, D.J. Nightingale, P.M. Price, J.F. White, J. Catal. 193 (2000)
348.
[25] C. Reichardt, Chem. Rev. 1 (1999) 2319.
[26] R. Bringue, J. Tejero, M. Iborra, J.F. Izquierdo, C. Fité, F. Cunill, Top. Catal. 45
(2007) 181.
[27] (a) F. Ancilloti, M.M. Mauri, E. Pescarollo, J. Catal. 46 (1977) 49;
(b) A. Gicquel, B. Torck, J. Catal. 83 (1983) 9.
[28] Y.K. Krisnandi, R. Eckelt, M. Scheider, A. Martin, M. Richter, ChemSusChem 1
(2008) 835.
[13] K.H. Diehl, H. Eggensperger, US Patent 5591442, 1997.
[14] S. Queste, P. Bauduin, D. Touraud, W. Kunz, J.-M. Aubry, Green Chem. 8 (2006)
822.
[15] (a) E. Baer, H.L. Fisher, J. Biol. Chem. 140 (1941) 397;
(b) N. Bagget, in: D. Barton, W.D. Ollis (series Eds.), J.F. Stoddart (Ed.),
Comprehensive Organic Synthesis, vol. 1, Pergamon, Oxford, 1979, p. 799;
(c) K. Miura, M. Tomita, Y. Yamada, A. Hosomi, J. Org. Chem. 72 (2007) 787.
[16] (a) R. Palkovits, I. Nieddu, C.A. Kruithof, R.J.M. Klein Gebbink, B.M.
Weckhuysen, Chem. Eur. J. 14 (2008) 8995;
(b) R. Palkovits, I. Nieddu, R.J.M. Klein Gebbink, B.M. Weckhuysen,
ChemSusChem 1 (2008) 193;