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W. Yu et al. / Catalysis Communications 13 (2011) 35–39
3.2.2. Effects of acidity of Al-SBA-15 on the OHE of FAL and HAc
To examine the effects of acidity of supports on reaction activity,
reactions were studied over 5%Pd/Al-SBA-15(X) with different Si/Al
ratios and the catalytic results are also shown in Table 3. The
correlation of Table 2 and Table 3 tells us the structure–activity
relationship of the catalysts. It is illustrated that, with increasing Si/Al
of Al-SBA-15 (decrease in acidity of the supports), X(FAL) (conversion
of furfural) decreases but S(D) increases. As a result, Y(D) increases at
first and then drops. From the results in Tables 2 and 3 (entries 2–6)
and Fig. 5, it can be inferred that the acidity of Al-SBA-15(X) shows
significant effect on the Y(D). To be specific, too much strong acid sites
(582.5 μmol/g) only favor the formation of byproducts, resulting in
lower S(D) (shown in Table 3, entry 2) .Moderate amount of weak
acid sites (142.6 μmol/g) is favorable to the esterification of FOL and
HAc (Table 3, entry 4). But too little amount of acid sites (86.6 and
48.3 μmol/g) does not facilitate the esterification of FOL and HAc
(Table 3, entries 5 and 6), thus resulting in lower Y(D). In summary,
for OHE of FAL and HAc over Pd/Al-SBA-15, the acidity of the support
must be medium to obtain high yield of desired products.
Al-SBA-15(22)
Al2(SiO3)3
Al-SBA-15(300)
Al-SBA-15(500)
SBA-15
100
200
300
400
T / oC
500
600
700
Fig. 5. NH3–TPD profiles of Al-SBA-15(X=22, 300, 500), SBA-15 and Al2(SiO3)3.
3.2.3. Comparison of Al-SBA-15 and Al2(SiO3)3
By comparison of entries 1, 4, 7 and 8 (from Table 3), it is obvious
that Al-SBA-15 is much better than Al2(SiO3)3 for the OHE of FAL and
HAc in terms of X(FAL) and Y(D). To explain this phenomenon, BET
surface area (Table 1) as well as acidity (Table 2) of Al2(SiO3)3 and Al-
SBA-15 was measured and compared. Firstly, compared to the lower
surface area of Al2(SiO3)3 (~150 m2/g), Al-SBA-15 has a larger specific
surface area (~750 m2/g.), which is in favor of dispersion of palladium
(11.0 nm vs. 15.6 nm). Secondly, the strength and amount of acid sites
of Al2(SiO3)3 is higher than that of Al-SBA-15 (except for Al-SBA-15
(22)), which facilitates the formation of byproducts. For 5%Pd/Al-SBA-
15(22), the acidity of the support is not beneficial to the OHE of FAL
and HAc. But compared to 5%Pd/Al2(SiO3)3, higher specific surface
area thus higher Pd dispersion (Table 1) compensate for the adverse
factor of high acidity.
acetate) and Y(D) (yield to desired products) over 5%Pd/Al2(SiO3)3
are higher than those over Pd/C+Al2(SiO3)3[22]. In the same way, by
comparison of entries 1 and 4, both S(D) and Y(D) over 5%Pd/Al-SBA-
15(300) are higher than those over 5%Pd/C+Al-SBA-15(300). These
catalytic performances may be attributed to a synergetic effect
between metal sites and acid sites over the composite bifunctional
catalysts. It is well known that for cooperative catalysis the functional
groups must be close enough to each other. The physical mixture of 5%
Pd/C and Al-SBA-15(300) exhibited poorer catalytic performances
than the composite bifunctional catalyst 5%Pd/Al-SBA-15(300), which
demonstrates that the acid groups and the metallic sites must be in
close proximity to each other. From our previous study we know that
the principal loss in yield of furfuryl acetate (FA) is due to a side
reaction of oligomerization of furan compounds [22]. Therefore, for
composite bifunctional catalysts, after FAL being hydrogenated over
metal sites to furfuryl alcohol (FOL), the in-situ produced FOL
immediately reacts with HAc to form FA over the acid sites, which
may prevent the FOL from undergoing polycondensation reaction to
some extent.
In light of the fact that this study is in its infancy and far from being
ready for industrialization, the stability of the adopted catalysts is
examined just preliminarily. The representative result is listed in
Table S1. The catalyst of 5%Pd/Al-SBA-15(300) shows good stability in
the first three repeated uses. In the fourth and fifth times, both activity
and selectivity drop. Detailed mechanisms of deactivation and
methods for regeneration need further investigation, which is not
the focus of this study.
4. Conclusions
The study leads us to the conclusions as follows. (1) As supports of
bifunctional catalysts, Al-SBA-15 is much better than Al2(SiO3)3 for
the OHE of FAL and HAc in terms of Y(D). (2) Al-SBA-15 only with
medium acidity favors the esterification of FOL and HAc, thus benefits
the OHE of FAL and HAc. (3) Compared to mixed bifunctional
catalysts, there is a synergistic effect for the OHE reaction over
composite bifunctional catalysts of 5%Pd/Al-SBA-15 or 5%Pd/Al2
(SiO3)3. (4) The OHE reaction of FAL and HAc is viable over composite
bifunctional 5%Pd/Al-SBA-15 or mixed bifunctional 5%Pd/C+Al-SBA-
15, both of which possess hydrogenation and esterification properties.
In view of the greater accessibility of large molecules to acid sites in
Table 3
Catalytic performances for OHE reaction of FAL and HAca.
Entry
Catalyst
X(FAL)/%
S(FOL)/%
S(FA)/%
S(BP)/%
Y(D)/%
S(D)/%
1
2
3
4
5%Pd/C+Al-SBA-15(300)
5%Pd/Al-SBA-15(22)
5%Pd/Al-SBA-15(100)
5%Pd/Al-SBA-15(300)
5%Pd/Al-SBA-15(500)
5%Pd/SBA-15
70.7
73.2
71.9
70.3
48.6
35.2
69.4
56.9
43.1
49.3
56.9
61.8
79.6
92.2
19.7
52.8
16.0
15.8
16.5
18.2
9.6
3.3
9.1
13.6
40.9
34.9
26.6
20.0
10.8
4.5
41.8
47.7
52.8
56.2
43.4
33.6
20.0
37.8
59.1
65.1
73.4
80.0
89.2
95.5
28.8
66.4
5
6
7b
8b
5%Pd/C+Al2(SiO3)3
5%Pd/Al2(SiO3)3
71.2
33.6
a
Reaction conditions: TR =150 °C, PH2 =2.0 MPa, 800 rpm, tR =4 h, 9.60 g FAL+6.00 g HAc+10.0 mL Toluene. X(FAL) — conversion of furfural, S(FOL) — selectivity to desired
product of furfuryl alcohol (the hydrogenation product), S(FA) — selectivity to desired product of furfuryl acetate (the esterification product), S(D)=S(FOL)+S(FA), S(BP) —
selectivity to byproducts, Y(D) — yield to desired products.
b
Data from reference [22].