beginning (1–2 h), 5 was the major product, but it started to
decrease gradually with the formation of 6. Therefore, it could
be suggested that a longer reaction time is necessary for the
formation of linear alkane under the studied reaction conditions
(temp. = 80 ◦C; PH = 4.0MPaand PCO = 14MPa)usingPd/Al-
Experimental
Aldol condensation
Aldol condensation between 5-hydroxymethyl furfural (HMF)
and acetone was carried out at 25–40 ◦C without using any solid
catalysts. Different ratios of acetone and HMF (mole ratio varies
from 10 : 1 to 1 : 1) were placed in a bottle with the addition of
0.3 ml of 0.1 M NaOH under constant stirring for 4–6 h. A
bright yellow intermediate was formed and extracted with 2 ml
of CHCl3 to remove the water (from NaOH solution). Finally,
after the purification by chromatographic column separation
and evaporation of the solvent, the product was separated and
subjected to 13C and 1H NMR analysis to confirm the formation
of desired structure. Both of the spectra show all the desired 9
peaks to ensure the formation of the intermediate (1; Scheme 1a).
The product is stable and can be stored for a long period of
time.
2
2
MCM-41 catalyst. Interestingly, from the course of the reaction
it was clear that the hydrogenation of 4 to 5 was very fast and
reached highest selectivity of 5 within 1 h. On the contrary,
the formation of 6 via dehydration/hydrogenation was slow.
This process started from 2 h of reaction time and attained
the highest selectivity during 15–20 h in the present reaction
conditions.
As the ring-opening reaction of furan was facilitated by
the combination of acid and metal catalysts, Al has been
incorporated into the Si support to increase the acidity, because
the acidity of the support is directly related to the framework
Al.11,12
As mentioned before, in comparison with the other catalysts,
highest activity was achieved from the Pd/Al-MCM-41 and
it might be associated with the highly dispersed Pd particles12
as determined by the transmission electron microscopic image
(TEM) of the catalyst (ESI, Fig. S3†) or support. However, un-
derstanding of the heterogeneous catalyzed reaction depending
on the nature of the active site, particle size and support in scCO2
is a difficult and complicated issue.
After optimization, the same reaction conditions were re-
peated on 1 (aldol condensation product of HMF and ace-
tone). As a result, the successful formation of the product
3 (Scheme 1a) with >99% selectivity (Table 1; entry 8) was
evident. Regarding the parameters affecting selectivity, the
hydrogenation and dehydration/hydrogenation of 1 follows a
similar trend to that of 4 in scCO2 under similar reaction
conditions.
It is well known that catalyzed ring opening of a heterocyclic
compound and the removal of the heteroatom, such as sulfur,
is an important part of the petroleum industry. To achieve the
ring-opening product and the removal of heteroatom, the same
methodology was further extended to the nitrogen and sulfur-
containing compounds such as imidazole and thiophene (ESI,
Table S1†), respectively. Unfortunately, ambiguous results were
obtained by this method. Depending on the catalysts, imidazole
shows 70 to 95% conversion, but the detection of the ring-
opening product was greatly hampered by its volatile nature. For
thiophene, the catalyst was deactivated easily because of sulfur
and no reaction took place. The deactivation of the catalyst was
again confirmed by inactivity during the ring opening of 4 in the
presence of thiophene (ESI, Table S1; last entry†). Therefore,
hydrogenation of these compounds requires modification of
the reaction conditions as well as improvement in the product
collection and identification, which might be a topic of future
study.
In conclusion, we have developed a ring-opening method
for the product of the aldol condensation between HMF and
acetone, under very mild reaction conditions. This process has
the potential to avoid the undesired coke formation as the
catalyst exhibited long-term stability under the studied reaction
conditions. Furthermore, we expect that it is possible to improve
the catalyst efficiency to achieve high selectivity of the desired
compound within a short reaction time.
Ring opening by dehydration/hydrogenation
The synthesized pure intermediate was hydrogenated in scCO2
in the presence of Pd catalyst at 80 ◦C. 0.2 g of the catalyst
along with 0.5–1 g of the compound was introduced into a
stainless steel batch reactor. Then, the reactor was sealed and
flashed two times with 2 MPa of CO2 to remove air. After
flushing, the reactor was placed into the oven with a fan heater to
maintain the constant temperature. Hydrogen was first loaded
into the reactor followed by the liquid CO2 using a high pressure
liquid pump (JASCO), and compressed to the desired pressure.
The reaction mixture was stirred continuously with the help
of a Teflon-coated magnetic bar during the reaction. After
the reaction, the reactor was cooled in ice and depressurized
very carefully. The liquid mixture was identified by 13C and 1H
NMR and analyzed quantitatively. The path of the reaction was
followed by analysis of the product by NMR (Fig. 2). Due to the
limitations of the experimental facility to scale up the process
to the industrial level, we performed the reaction in the small
scale.
Notes and references
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3 (a) F. Ma and M. Hanna, Bioresour. Technol., 1999, 70, 1; The
Biodiesel Handbook, ed. G. Knothe, J. Van Gerpen and J. Krahl,
AOCS Press, Urbana, IL, USA, 2005; (b) M. Verziu, B. Cojacaru,
J. C. Hu, R. Richard, C. Ciuculescu, P. Filip and V. I. Pervulescu,
Green Chem., 2008, 10, 373; (c) G. W. Huber, P. O’Connor and A.
Corma, Appl. Catal., A, 2007, 329, 120.
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5 T. A. Werpy, J. G. Frye, A. H. Zacher and D. Miller, US Pat.
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8 (a) R. D. Cortright, R. R. Davada and J. A. Dumesic, Nature, 2002,
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