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Table 1. Conversion (X) and selectivity of trioses toward methyl lactate
(S1), pyruvaldehyde dimethyl acetal (S2), and lactic acid (S) in methanol
and water, respectively.[a]
Entry Catalyst
Si/B Si/M Substrate[b]
Methanol
Water
X [%] S1/S2 [%]
X [%] S [%]
1
2
3
4
5
6
B-MWW
Ti-MWW
Sn-MWW-a 120 54 DHA
Sn-MWW-a 120 54 GLA
Sn-MWW-b 190 52 DHA
Sn-MWW-b 190 52 GLA
11
–
DHA
72
92
>99
>99
0/24
90
9
35 180 DHA
26/17
99/<1
99/<1
50 20
93 84
93 84
>99 >99/trace >99 96
>99 >99/trace >99 96
[a] Reaction conditions: 2.5 mL of the solvent, 40 mg of the catalyst,
58 mg of the substrate. Reaction time: 393 K, 24 h in methanol and
383 K, 6 h in water. [b] DHA=dihydroxyacetone; GLA=glyceraldehyde.
Figure 1. a) XRD patterns of as-synthesized B-MWW (trace 1), deboronated
and calcined MWW (trace 2), as-synthesized Sn-MWW-b (trace 3), and cal-
cined Sn-MWW-b (trace 4) zeolites; b) UV/Vis spectrum of Sn-MWW-b;
c) in situ FTIR spectra of Sn-MWW-b after pyridine adsorption and evacua-
tion at 1008C (trace 1), 2008C (trace 2), and 3008C (trace 3). Absorption
band characteristics for specifically adsorbed pyridine are denoted by H for
hydrogen-bonded pyridine, B for adsorption on Brønsted sites, and WL and
SL for weak and strong Lewis acid sites, respectively.
Sn-modified MWW zeolite is also the most efficient catalyst for
this reaction. Upon decreasing the B content of MWW zeolite,
the conversion of DHA and the selectivity of lactic acid in-
creased from 93% and 85% (Table 1, entry 3) to approximately
100% and 96% (Table 1, entry 5), respectively. This result indi-
cates the importance of removing the strong Brønsted acidity
to avoid undesired side reactions (Table 1, entry 1). With glycer-
aldehyde (GLA) as the substrate, similar trends were observed,
which points to the high GLA isomerization activity of
Sn-MWW (Table 1, entries 4 and 6).
ditions in the presence of hexamethyleneimine (HMI). The
lamellar structure was slightly recovered during this modifica-
tion step (Figure 1a, trace 3). The desired 3D Sn-MWW zeolite
was finally obtained through calcination (Figure 1a, trace 4).
The XRD reflection at approximately 68 was shifted slightly to
a lower angle compared to the parent zeolite, which indicates
Sn incorporation (inset of Figure 1a). The UV/Vis diffuse reflec-
tance spectrum of Sn-MWW zeolite shows an intense band
around 200 nm, which arises from Sn in tetrahedral coordina-
tion (Figure 1b).[24,45–47] This coordination of Sn is usually con-
sidered to give rise to Lewis acidity.[17,24,47]
The time course of the conversion of DHA at 808C in metha-
nol was also followed (Figure 2a). In the presence of the
Sn-modified MWW, a complete conversion of DHA to methyl
lactate is observed after 4 h of the reaction. To obtain the
same result with Sn-BEA, a reaction time of 24 h was re-
quired.[17] Similar yields required a reaction time of 6 h at 908C
for Sn-modified MCM-41.[24] The better performance in the
present study is ascribed to the efficient postsynthesis
method, which allows for a higher Sn content. An experiment
at twice the DHA concentration also led to complete conver-
sion, although a longer reaction time was required (Figure S2).
We also tested the recyclability of Sn-MWW. The two
Sn-MWW catalysts were found to retain their activity after
three runs on performing the reaction in methanol (Figure 2b).
Similar results have been obtained previously for DHA conver-
sion in methanol with Sn-BEA or bifunctional Sn-MCM-41 cata-
lysts.[17,24] However, on performing this reaction in water, large
amounts of carbonaceous deposits are typically deposited on
the catalyst surface requiring regeneration by calcination.[17,24]
It is difficult to selectively remove the carbonaceous deposits
from the bifunctional Sn-MCM-41 silica because this catalyst
also contains a graphite-like carbon that is the source of mild
Brønsted acidity.[24] As in principle calcination can be used effi-
ciently to regenerate Sn-BEA, we compared its recyclability
with that of Sn-MWW for the conversion of DHA in water (Fig-
ure 2c). Sn-BEA was prepared according to the conventional
method.[17] It shows activity and selectivity similar to those of
lactic acid, as reported previously (Figure S3).[17] Upon repeated
use in aqueous DHA conversion, the catalytic performance of
Sn-BEA decreases. In contrast, the activity and selectivity of
Sn-MWW remains nearly unaffected, which demonstrate the
The Lewis acidity of Sn-MWW zeolite was then studied by
using FTIR spectroscopy of pyridine adsorption. The IR spectra
of pyridine adsorbed on Sn-MWW zeolite after evacuation at
different temperatures are shown in Figure 1c. The bands at
1445 and 1596 cmÀ1, which are due to weakly adsorbed (hy-
drogen-bonded) pyridine, already disappear after evacuation
at 2008C. The bands characteristic to pyridine adsorbed on
Lewis acid sites (1455 and 1615 cmÀ1) were observed in the IR
spectra even after evacuation at 3008C, which demonstrates
their high strength.[18,24]
The direct conversion of dihydroxyacetone (DHA) in metha-
nol was performed by using various modifications of MWW
zeolite. B-MWW, a typical Brønsted acid catalyst, demonstrates
a high DHA conversion albeit with a low selectivity toward the
desired product (Table 1, entry 1). This is in line with previous
reports, which suggests that the low selectivity toward alkyl
lactate can be attributed to the competitive acetalization of
pyruvic aldehyde in the presence of strong Brønsted acidi-
ty.[24,48] Upon exchange of trivalent lattice boron sites by tetra-
valent Sn or Ti atoms, pronounced Lewis acidity is incorporat-
ed into the zeolite. Thus, the selectivity toward methyl lactate
increases significantly. Sn-modified MWW zeolite catalyzes the
complete conversion of DHA with a high selectivity toward
methyl lactate (Table 1, entries 3 and 5). The performance of
the catalyst was also assessed in water. In this case, lactic acid
is the desired product. It is apparent from Table 1 that
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Guo, Qiang
