C. Liu, et al.
MolecularCatalysis485(2020)110514
Table 3
The hydrogenolysis of glucose on the CuB/Al2O3 catalysts.a
Catalyst
WHSV (h−1
)
Conv. (%)
Selectivity (on a carbon basis, %)
TC (%)
Sorbitol
Erythritol
EG
Glycerol
1,2-PDO
Ethanol
Cu/Al2O3
0.36
0.48
0.60
0.36
0.48
0.60
0.36
0.48
0.60
0.36
0.48
0.60
0.36
0.48
0.60
0.36
0.48
0.60
91.6
87.9
87.2
96.6
94.6
92.6
88.7
87.4
84.6
96.6
93.9
92.2
96.6
94.7
93.0
96.6
94.3
92.0
18.0
22.7
38.5
11.0
14.6
17.9
35.7
39.9
44.1
15.9
19.8
22.5
16.7
18.4
20.7
17.8
19.5
21.9
4.8
5.4
2.3
4.3
4.6
5.1
1.0
1.1
0.4
4.5
5.1
5.4
4.8
5.1
5.2
4.5
5.3
5.5
13.0
12.6
10.4
14.3
13.4
13.0
0.9
15.2
16.0
17.6
13.9
14.8
15.8
10.2
11.4
15.5
17.6
17.9
17.9
18.3
17.5
17.7
18.4
17.8
17.6
43.4
38.9
27.8
49.5
47.1
43.5
36.3
33.8
27.1
44.9
41.4
38.3
43.9
41.2
38.1
42.6
39.5
36.5
2.2
0.0
0.0
2.4
2.0
1.7
0.1
0.1
0.1
2.1
1.8
1.6
2.2
2.0
1.5
2.0
1.9
1.6
3.4
4.4
3.4
4.6
3.5
3.0
15.8
12.5
11.3
0.1
0.6
1.2
0.1
1.4
3.3
0.1
1.5
4.1
96.9
96.1
97.0
95.6
96.7
97.2
86.0
89.1
90.4
99.9
99.4
98.9
99.9
98.7
96.9
99.9
98.6
96.2
1CuB/Al2O3
1CuB/Al2O3
3CuB/Al2O3
5CuB/Al2O3
7CuB/Al2O3
1.2
1.5
14.9
13.4
13.1
14.0
14.4
13.5
14.6
14.5
12.8
a
b
c
Reaction condition: 5 wt% glucose aqueous solution, 4 MPa H2, 180 °C.
Others: humins.
Reduced at 300 °C.
3.7. Correlation between the properties and performance of CuB/Al2O3
that the high-temperature reduction changed the ratio of Cu/Cuδ+ and
caused the aggregation of Cu species, which resulted in the lower hy-
drogenolysis activity of the catalyst. Considering the tandem reaction
process, it suggested that 1,2-PDO selectivity was determined by both
the acidic and metal sites. An interesting found was that 1,2-PDO se-
lectivity displayed nearly the same trend with the Cuδ+ percent on the
CuB/Al2O3 catalysts. It seems that the proportion of Cuδ+/Cu0 plays
great roles on the selective hydrogenolysis of glucose. The 1CuB/Al2O3
catalyst after reaction was also characterized by XPS and XAES to detect
the ratio of Cu cation to Cu metal (Fig. 4 and Table 2). The Cu 2p XPS
spectra of the used 1CuB/Al2O3 catalyst displayed the similar results to
the reduced 1CuB/Al2O3 catalyst, which indicates the presence of both
Cu0 and Cuδ+ species. Furthermore, the XAES results turned out that
the percentage of Cuδ+ was 77%, which is much higher than that on the
reduced 1CuB/Al2O3 catalyst. We speculate that the small Cu nano-
particles on the 1CuB/Al2O3 catalyst are prone to be oxidized in air.
It has been reported that the metal cations such as Snx+ and Wx+ on
the solid catalysts could act as Lewis acid in the hydrogenolysis of
saccharides, which could catalyze the isomerization between glucose
and fructose [29,42]. Furthermore, many researchers also pointed out
that the Cuδ+ species on the reduced copper-based catalysts could
function as Lewis acid sites [29,43,44]. It can be concluded that the
electronic deficient Cuδ+ species could provide Lewis acid sites, which
B2O3 has been widely used as an additive for various catalysts due to
its structure and electronic effects [33,34]. Based on the N2O-TPD and
H2-TPR results, the close contact between B2O3 and copper not only
promotes the dispersion of copper but also has a coverage effect on
copper species, resulting in the decrease of copper surface area with the
increase of B2O3 contents. The coverage and dispersion effects have
been widely reported in the case of WOx [29], B2O3 [32], MoOx [41],
etc. For example, Zhu et al. have prepared the CuB/SiO2 catalysts. They
suggested that the doping of B2O3 could inhibit the agglomeration of
copper species during the heating treatment and facilitate the formation
of small CuO nanoparticles. As revealed by H2-TPR, CO-DRIFTS, and Cu
LMM XAES results, the close contact between B2O3 and copper resulted
in a strong electron interaction, which inhibited the reduction of CuO
and decreased the ratio of Cu0/Cuδ+ on the reduced catalysts. Ad-
ditionally, the doping of B2O3 introduced more acidity sites onto the
catalysts.
The hydrogenolysis of glucose to 1,2-PDO undergoes some tandem
reactions, as shown in Scheme 1 [29]: (1) the isomerization of glucose
and glyceraldehyde through retro aldol condensation reaction, which is
supposed to be conducted on metal sites; (3) The CeO bond cleavage of
dihydroxyacetone and glyceraldehyde through dehydration reaction;
(4) The hydrogenation of the intermediates.
As previously suggested, sorbitol is obtained by the hydrogenation
of glucose, which acts as the competitive reaction with the retro aldol
condensation reaction [29]. Moreover, the selectivity of 1,2-PDO, the
percent of Cuδ+
(
X
Cu
+
), the surface area of Cu (SCu), and the amount of
acid sites on different CuB/Al2O3 catalysts were shown in Fig. 7. The
results demonstrated that, with the increasing B2O3 content, the acid
amounts increased while the copper surface area decreased. Accord-
ingly, the optimal 1,2-PDO selectivity was achieved on the 1CuB/Al2O3
catalyst. The hydrogenolysis of glucose has been performed on the
1CuB/Al2O3 catalyst reduced at 300 °C. The results turned out that the
conversion of glucose was lower than that on the 1CuB/Al2O3 catalyst
reduced at 250 °C. Furthermore, the reduction temperature of 1CuB/
Al2O3 also affected the selectivity of products. For example, the se-
lectivity of sorbitol was higher than that on 1CuB/Al2O3 reduced at
250 °C. Correspondingly, the selectivity of 1,2-propanediol decreased
when 1CuB/Al2O3 was reduced at higher temperature. We speculated
Fig. 8. Stability of 1CuB/Al2O3 in glucose hydrogenolysis.
6