Dehydration of sorbitol to isosorbide over H-beta zeolites with high Si/Al ratios

Article abstract of DOI:10.1039/c5gc00319a

Conversion of sorbitol to isosorbide by heterogeneous catalysts is a challenge in biorefinery. Herein, H-beta zeolites with specific Si/Al ratios uniquely give isosorbide in up to 76% yield under mild conditions. Mechanistic study has suggested that acid sites on hydrophobic internal surface are active for this reaction.

Full text of DOI:10.1039/c5gc00319a

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Dehydration of sorbitol to isosorbide over H-beta
zeolites with high Si/Al ratios†
Cite this: DOI: 10.1039/c5gc00319a
Hirokazu Kobayashi,a Haruka Yokoyama,a,b Bo Feng‡a and Atsushi Fukuoka*a
Received 10th February 2015,
Accepted 9th March 2015
DOI: 10.1039/c5gc00319a
www.rsc.org/greenchem
Conversion of sorbitol to isosorbide by heterogeneous catalysts is plasticisers for the production of flexible polyvinyl chloride.
a challenge in biorefinery. Herein, H-beta zeolites with specific Isosorbide nitrates are medicines for angina pectoris, and iso-
Si/Al ratios uniquely give isosorbide in up to 76% yield under mild sorbide itself is used for treating glaucoma, brain hypertension
conditions. Mechanistic study has suggested that acid sites on and Ménière’s disease.
hydrophobic internal surface are active for this reaction.
The conversion of sorbitol to isosorbide (Scheme 1) has
required liquid sulphuric acid as a catalyst in industry, which
provides good yields of isosorbide (70–77%) at ca. 400 K
within a few hours in batch reactors.7,8 Major obstacles of this
system are difficult separation of isosorbide from the reaction
mixture and discharge of a large amount of sulphuric acid
pitch.9 Thus, solid acid catalysts such as zeolites,8,10 mixed
oxides,11 phosphated or sulphated oxides,10d,12 sulphonic
resins13 and Ru–Cu bimetals14 have been investigated to
replace the homogeneous acid. Additionally, hot compressed
water was applied to the synthesis of isosorbide.15 Among
them, zeolites would be prospective choices for the
dehydration of sorbitol, as they are composed of ubiquitous
elements (Si, Al, O) with thermal stability and tunable pro-
perties. However, zeolites tested in previous reports have
shown low activities and required severe reaction conditions.
For example, an H-beta with a Si/Al ratio of 12.5, denoted
Hβ(12.5), gave only a 38% yield of isosorbide in the de-
hydration reaction at 423 K for 12 h.8 HZSM-5(40) produced
isosorbide in 59% yield at 533 K over 14 h.10b Thus, it is
necessary to explore zeolite catalysts working under mild con-
ditions similar to those for sulphuric acid. Herein, we report
that raising Si/Al ratio of Hβ to 75 drastically improves the
activity, giving isosorbide in 76% yield at 400 K within 2 h.
Dehydration of sorbitol was conducted at 400 K for 2 h in
the presence of various zeolites with similar Si/Al ratios
(Table 1) in a Pyrex flask (Fig. S1, ESI†). Hβ(50) produced iso-
sorbide in 72% yield with >99% conversion of sorbitol
(entry 2), which was in contrast to the low activity of Hβ(12.5)
reported previously (38% yield even at 423 K for 12 h).8 An
intermediate for the formation of isosorbide, 1,4-sorbitan, was
yielded in 4.5%. Other identified products were 2,5-mannitan
(3.1%) and 2,5-iditan (Scheme S1, ESI†). 2,5-Iditan and a
monoanhydrohexitol (AH) other than 3,6-sorbitan were over-
lapped in our HPLC analysis (Fig. S2, ESI†), and their total
Catalytic transformation of cellulosic biomass to chemicals is
a crucial technology in the biorefinery for pursuing sustain-
ability.1 The hydrolytic hydrogenation of cellulose gives sorbi-
tol in up to 90% yield,2 and cellulose in real biomass has also
been successfully converted to sorbitol.3 Sorbitol is one of
the top-ten platform chemicals in biorefinery proposed by US
Department of Energy,4 and the most promising derivative of
sorbitol is isosorbide (Scheme 1).1d,5 Isosorbide polycarbonate,
commercialised as DURABIO® and PLANEXT®, is an engineer-
ing plastic with superior characteristics of both polymethyl
methacrylate and bisphenol-A polycarbonate. Incorporation of
isosorbide units into polyethylene terephthalate is a solution
for high-temperature applications such as hot-beverage con-
tainers. Dimethyl isosorbide is a low-toxic and high-boiling
solvent (b.p. 509 K).6 Besides, isosorbide diesters are
Scheme 1 Dehydration of sorbitol to isosorbide.
aCatalysis Research Centre, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo,
Hokkaido 001-0021, Japan. E-mail: fukuoka@cat.hokudai.ac.jp
bDepartment of Chemistry, Faculty of Science, Hokkaido University, Kita 10 Nishi 8,
Kita-ku, Sapporo, Hokkaido 060-0810, Japan
†Electronic supplementary information (ESI) available: Experimental, additional
discussion. See DOI: 10.1039/c5gc00319a
‡Current address: Debye Institute for Nanomaterials Science, Utrecht University,
Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.
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Table 1 Dehydration of sorbitol by zeolite catalystsa
Yield of product/%
Yield of by-product/%
2,5-Mannitan
Reaction
time/h
Entry
Catalyst
Conv./%
Isosorbide
1,4-Sorbitan
2,5-Iditan and AHb
Othersc
1
2
3
4
5
6
7
8
9
None
Hβ(50)
2
2
2
2
2
2
1
1
1
<1
>99
97
51
18
>99
94
96
0.0
72
28
27
2.3
76
53
57
56
0.0
4.5
29
7.1
11
4.6
24
0.0
3.1
10
1.0
0.9
2.9
2.7
2.8
2.7
0.0
9.2
20
0.9
2.6
5.9
6.5
6.9
6.8
<1
11
10
15
1
10
8
HUSY(40)
HZSM-5(45)
HMOR(45)
Hβ(75)
Hβ(75)
Hβ(75)d
Hβ(75)e
18
22
11
9
96
a Sorbitol 182 mg (1.00 mmol), catalyst 50 mg, 400 K. b AH: a monoanhydrohexitol other than 3,6-sorbitan. c (Conversion) − (total yield of shown
products). d Modified with triphenylsilane. e 2,4,6-Tri-tert-butylpyridine (5 mg) was added.
yield was 9.2%. Hβ(50) became brownish after the reaction due [16 μmol in 50 mg Hβ(75), determined by NH3-temperature
to slight coking, but it can be regenerated by calcination (see programmed desorption; Fig. S3, ESI†]. In addition, HUSY and
below). HUSY(40) provided a high conversion of 97% but the HZSM-5 also provided volcano-type dependence (Fig. S4, ESI†).
yield of isosorbide was as low as 28%, which was due to the We assume that both acid site and hydrophobicity are necess-
formation of large amounts of by-products (40%; entry 3). ary for good catalytic performance. In general, increase of Si/Al
HZSM-5(45) and HMOR(45) were also less active than Hβ(50) improves hydrophobicity, which thermodynamically enhances
(isosorbide yield: 27% and 2.3%, respectively; entries 4 and 5). the removal of water molecules formed in the dehydration of
Hβ was uniquely active and selective for the formation of iso- sorbitol. It is known that hydrophobic zeolites can function as
sorbide among the zeolite catalysts tested. The good activity of acid catalysts even in aqueous phase.17 At the same time,
Hβ may be due to the twelve-membered-ring and three-dimen- raising the Si/Al ratio reduces the number of acid site, which is
sional porous structure with no excess space (supercage) roughly equal to the content of Al in these cases.18 In contrast
causing side-reactions, whereas acid strength16 is a minor to acid amount, acid strength is a minor factor, since this para-
factor in this case. The detailed discussion is available in ESI.† meter is independent of Si/Al ratio for high-silica zeolites as
The use of Hβ(50) instead of Hβ(12.5)8 remarkably improved used in this study.18 Effect of porosity is also minor in
the reaction performance as described above, which prompted this comparison, as the Hβ zeolites have similar micropore
us to study the effect of Si/Al ratio on dehydration activity of volumes (0.25–0.29 cm3 g−1
) and specific surface areas
Hβ. Fig. 1 represents the correlation between the Si/Al ratios of (590–670 m2 g−1) except for Hβ(12.5) (Table S1, Fig. S5a,b,
Hβ and the results of the dehydration of sorbitol at 400 K for ESI†). In addition, external surface area has no influence on
1 h. Yield of isosorbide and conversion of sorbitol were maxi- the activity (Table S1, Fig. S5c, ESI†), because internal acids
mised at Si/Al = 75, and both decreasing and increasing the work for the reaction as described below. Hence, the volcano-
Si/Al ratio declined the activity. It is noteworthy that Hβ(75) type curve can be depicted for the catalytic activity against
provided the highest yield of isosorbide (76%) at a longer reac- Si/Al ratio, and the best catalyst, Hβ(75), has been used for the
tion time of 2 h (Table 1, entry 6). Turnover number for the further study.
dehydration was 100, based on the quantity of acid sites
Active sites of Hβ(75) on external or internal surfaces were
estimated by selectively blocking external ones. First, external
surface of Hβ(75) was covered with triphenylsilane;19 however,
pristine and the modified Hβ(75) provided similar catalytic
activity in the dehydration of sorbitol for 1 h (isosorbide yield:
53% and 57%, respectively; Table 1, entries 7 and 8). Second,
we conducted the dehydration of sorbitol over Hβ(75) in the
presence of 2,4,6-tri-tert-butylpyridine in order to poison exter-
nal acid sites,20 but a similar isosorbide yield was obtained
(56%, entry 9). Hence, the activity is not ascribed to external
acid sites but internal ones. Besides, the apparent activation
energy determined by an initial rate method was 89 kJ mol−1
for Hβ(75) (Fig. S6, ESI†), which was not in the range of
diffusion21 but of chemical reactions (dehydration). Since pore
size of *BEA (6.6 × 6.7 Å) is larger than the cross-sections of
sorbitol (5.7 × 5.9 Å) and isosorbide (5.9 × 6.2 Å) (Fig. S7, ESI†),
quick diffusion of these molecules are possible in the pores.
Fig. 1 Effect of Si/Al ratio of Hβ on the dehydration of sorbitol at 400 K
for 1 h.
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internal surface of Hβ with hydrophobic natures are active for
this reaction.
Acknowledgements
This work was supported by a Grant-in-Aid for Young Scientists
(26709060) from Japan Society for the Promotion of Science
(JSPS).
Fig. 2 Reuse experiments of Hβ(75) in the dehydration of sorbitol at
400 K for 2 h.
Notes and references
Davis et al. also have revealed that the rate-determining step of
a sugar conversion (isomerisation of glucose) over β zeolite is a
chemical reaction (hydride shift).22 Accordingly, it is con-
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which is a cause of the obvious dependence of catalytic activity
on pore structures of zeolites. This fact also supports the
importance of hydrophobicity; water molecules produced by
the reaction can be strongly adsorbed in pores due to the
small diameter, and therefore we tentatively propose that water
needs to be destabilized by hydrophobic nature for the
desorption.
Finally, reuse experiments of Hβ(75) were conducted to
evaluate the durability in the dehydration of sorbitol at 400 K
for 2 h, as fresh Hβ(75) required 2 h for completing the reac-
tion (Fig. S8, ESI†). The first reaction gave an isosorbide yield
of 76%, but the used catalyst afforded a decreased yield (67%)
of isosorbide. Coking (carbon 9.2 wt%) was found in the used
catalyst, and it decreased Brunauer–Emmett–Teller (BET)
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g−1, respectively. Al content was maintained after the calcina-
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(Fig. 2). The catalytic activity is largely recovered by the
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weakened (Fig. S9, ESI†), partial degradation of the crystals
probably reduces the catalytic activity. This assumption agrees
well with a fact that the good catalytic activity is provided
by *BEA structure.
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