Full Papers
It is well-known that the hydrogenolysis reaction of GVL into
1,4-pentanediol (PDO) requires very high H2 pressure;[10,17]
therefore, hydrogen availability arises as a key parameter in
this reaction. The reversible 2-PrOH$acetone+H2 reaction can
continuously produce hydrogen on the active sites of the cata-
lyst, dramatically increasing the hydrogen availability and,
hence, the GVL ring-opening reaction rate. Despite the fact
that the activity tests were carried out under high H2 pressure,
2-PrOH dehydrogenation was confirmed by the significant
amounts of acetone detected in the reaction products (up to
1.5 molLÀ1). However, when the experiment was carried out
with LA in 2-PrOH without a catalyst present, only trace
amounts of acetone were detected, pointing to the need of
a catalyst for the 2-PrOH dehydrogenation to occur. Moreover,
the formation of AL from GVL was negligible under these reac-
tion conditions as no AL was detected in the reaction medium.
It can be speculated that this higher hydrogen availability min-
imizes dehydrogenation reactions. On the other hand, signifi-
cant yields of side products were obtained [14.2% of 2-BuOH,
9.3% of CH4 and 4.1% of 2-pentanol (2-PeOH)].
of the GVL converted (8% yield). However, a 9.6% VA yield
was detected as the main side product. This is, to the best of
our knowledge, the highest MTHF yield achieved using non-
noble metal catalysts and green solvents.
The superior performance of the bimetallic catalyst with
a 23:12 Ni/Cu ratio (23Ni-12Cu/Al2O3) compared to 35Ni/Al2O3
was confirmed by lower reaction temperature tests (Table 1,
entries 11 and 12). At 2308C, 23Ni-12Cu/Al2O3 afforded a MTHF
yield of 44.5% whereas the yield of MTHF was only 29.9% for
35Ni/Al2O3. These results suggest a synergetic effect between
Ni and Cu, Ni providing high activity to convert the intermedi-
ate GVL and Cu improving the selectivity towards MTHF.
As discussed above, MTHF can further react to yield side
products; therefore, the stability of the MTHF produced in the
reaction medium is important to determine the suitability of
the catalyst. MTHF stability tests were performed under similar
reaction conditions as outlined above using 5 wt% MTHF in 2-
PrOH and Ni-Cu/Al2O3 catalysts. The results are shown in
Table 2.
According to Scheme 1 and the observed reaction products,
the reaction seems to proceed by the hydrogenation of GVL to
PDO and this compound undergoes (i) dehydration–cyclization
to produce MTHF, (ii) dehydrogenation and decarbonylation to
yield 2-BuOH and CO or CH4, and (iii) hydrogenolysis of the ter-
minal ÀOH group to end in 2-PeOH. In addition to this, MTHF
can further react to yield 2-PeOH. Ni is well known for its high
CÀC cleavage activity,[27] which seems to be the reason for the
high yield of 2-BuOH. Looking for catalysts with similar activity
and enhanced selectivity towards MTHF, a series of Ni-Cu/Al2O3
catalysts was prepared and tested.
Table 2. Results from MTHF stability tests.[a]
Catalyst
[/Al2O3]
T
MTHF conv. Yield [%]
CB
[8C] [%]
2-BuOH 2-PeOH 1-PeOH [%]
35Ni
35Cu
23Ni-12Cu
23Ni-12Cu
250 55.3
250 12.1
250 20.7
230 12.0
5.0
0.0
0.7
0.7
23.0
0.0
5.1
0.0
8.1
15.2
3.8
104.4
96.0
100.3
94.8
2.3
[a] Reaction conditions: reaction T 2508C, 5 wt% MTHF in 2-PrOH, MTHF/
cat. ratio 10 ggÀ1, 70 bar H2, 5 h.
Table 1 (entries 4–9) shows MTHF and GVL yields and the
carbon balance (CB) for the six pre-reduced Ni-Cu/Al2O3 cata-
lysts (with different Ni/Cu atomic ratios) using 2-PrOH as the
reaction solvent. The results of the tests carried out at 2508C
correlated the increase in the Ni/Cu ratio from 0:35 to 23:12
with a decrease in the GVL yield from 64.5% to 13.3%. A de-
crease in the GVL conversion was observed when the Ni/Cu
ratio was increased from 23:12 to 30:5; however, the monome-
tallic 35Ni/Al2O3 catalyst converted almost all the GVL (only
a 3.6% GVL yield was detected). These results clearly indicate
that Ni is more active than Cu to convert the highly stable
GVL.
These tests confirmed that 35Ni/Al2O3 is the most active and
35Cu/Al2O3 is the least active for MTHF degradation. The 23Ni-
12Cu/Al2O3 catalyst, which was nearly as active to convert GVL
as 35Ni/Al2O3, showed less than half of the MTHF conversion
shown by 35Ni/Al2O3. Based on these results, it is concluded
that the highest MTHF yield obtained using 23Ni-12Cu/Al2O3
(see Table 1 entry 6) is the result of an optimal combination of
the high Ni activity for GVL conversion and the high Cu selec-
tivity towards MTHF.
The product distribution related to the metal active sites is
of particular interest; the 35Cu/Al2O3 catalyst selectively
opened the MTHF cycle from the substituted side, whereas
35Ni/Al2O3 preferably opened the cycle from the less impeded
side, in agreement with previous works on this mechanism.[28]
These data help to clarify the mechanism of the side reactions
discussed above by suggesting that the main route for the
production of 2-PeOH is MTHF degradation whereas 2-BuOH is
mainly obtained from GVL and/or PDO. The 23Ni-12Cu/Al2O3
catalyst, which exhibited intermediate activity for the conver-
sion of MTHF, also showed an intermediate product distribu-
tion closer to that of 35Cu/Al2O3. The activity for MTHF conver-
sion of 23Ni-12Cu/Al2O3 was reduced by half at 2308C (Table 2,
entry 4), matching the conversion showen by 35Cu/Al2O3 at
2508C, but similarly favoring both ring-opening options.
The MTHF yield profile, on the other hand, showed a typical
volcano shape. MTHF yield slowly increased with the Ni/Cu
ratio, for ratios lower than 17:17 (average MTHF yields around
33% for these three catalysts). A maximum 56% MTHF yield
was achieved for the 23:12 Ni/Cu ratio; higher Ni contents re-
sulted in lower yields with a plateau around 44%. Interestingly,
the yields of other products [2-BuOH, 1- and 2-PeOH, and vale-
ric acid (VA)] also increased with the Ni/Cu ratio. Therefore,
high Cu proportions seem to prevent side reactions leading to
these products (see Scheme 1). To prove that the presence of
Cu is beneficial to improve the selectivity towards MTHF,
a 24 h test was performed at 2508C using the monometallic
Cu/Al2O3 catalyst with 35 wt% Cu loading (35Cu/Al2O3). The re-
sults (Table 1, entry 10) showed a 75% MTHF yield with most
ChemSusChem 2015, 8, 3483 – 3488
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