CHEMSUSCHEM
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be an attractive source of moderate basicity. The resulting cat-
alysts combine high activity, selectivity, and stability in reac-
tions of relevance to the upgrading of bio-oil, in which they
outperform traditional basic zeolites and strong bases such as
MgO and rely only on abundant and accessible raw materials.
The catalysts introduced in this work represent a cost-efficient
and environmentally benign solution to reduce hydrogen con-
sumption in the cascade upgrading of bio-oil, and thereby ac-
celerate the industrial realization of sustainable fuels.
Experimental Section
Catalyst preparation
Parent high-silica zeolites were obtained in the protonic form from
Tosoh Corporation: HSZ-390HUA with molar Si/Al=385 (USY-P),
HSZ-890HOA with molar Si/Al=1060 (MFI-P), HSZ-980HOA with
molar Si/Al=220 (BEA-P). Zeolite X with molar Si/Al=1.4 (X) was
supplied by Sigma–Aldrich in the Na form. Silicon dioxide nano-
powder 10–20 nm (SiO2) was purchased from Sigma–Aldrich, and
MgO samples (magnesium oxide 99.5% mesh 375, MgO; magnesi-
um oxide nanopowder, MgO nano) were obtained from Strem
Chemicals. The properties of the as-received catalysts are summar-
ized in Table 2. Activation treatments of high-silica zeolites were
performed in 50 cm3 glass reactors at RT. In a typical experiment,
the reaction solution was prepared by dissolving the desired
amount of base (NaOH, Na2CO3, or NH4OH, 10À4–1m) and TPABr
(0–0.2m) in water (30 mL). Then, the zeolite (1 g) was treated
under vigorous stirring (1–10 min), collected by filtration and
washed thoroughly with deionized water. Acid treatments were
conducted in aqueous 0.1m HCl. Ion-exchange procedures with
NH4NO3, NaCl, and CsCl were performed by subjecting the zeolite
to three subsequent treatments in aqueous solutions of the salt
(0.1m, 8 h, 298 K, 10 gzeolite LÀ1). Hierarchical MFI zeolite was ob-
tained by treating the zeolite (33 gLÀ1, 0.5 h, 338 K) in a solution of
NaOH (0.2m) and TPABr (0.2m). To remove organic species ad-
sorbed during the treatment, all catalysts were calcined in static air
(823 K, 5 h, 5 KminÀ1). Sample names are summarized in Table 1.
Figure 13. Product yields in the ketonization of propionic acid at 673 K over
USY zeolites and traditional base catalysts after 2 h on stream.
the acid sites are deactivated in the alkaline environment, and
basic siloxy sites are formed, only the anhydride and ketoniza-
tion products are observed, and the selectivity to the latter
product is increased significantly. A similar increase in the 3-
pentanone selectivity was reported for MFI zeolites modified
by treatment with 3.7m NH3 +0.7m NH4NO3.[25] The interplay
between the strength of basic sites and the level of the forma-
tion of this compound is also reflected if traditional basic ma-
terials are considered. The acetal intermediate was converted
to 3-pentanone to a high extent over Cs-X zeolite. With the
MgO catalyst that possesses an even stronger basicity, pro-
pionic anhydride was not detected anymore. These observa-
tions suggest that the basic sites generated in the USY zeolite
have a moderate strength.
Besides their outstanding performance in the aldol conden-
sation of aldehydes, high-silica zeolites also catalyze acetaliza-
tion and ketonization reactions of carboxylic acids efficiently.
These findings underline the advantage of an intermediate cat-
alytic deoxygenation of bio-oil, in which a pronounced reduc-
tion of the oxygen content of pyrolysis vapors can be expect-
ed through the combination of multiple catalytic pathways.
Besides the improved stability, decreased corrosivity, and in-
creased heating value of the thereby obtained bio-oil conden-
sate, catalytic upgrading through CÀC bond formation reac-
tions also has the unique property of increasing the contents
of higher hydrocarbons by the assembly of short-chain mole-
cules. Although it poses challenging tasks in terms of catalyst
design, we expect intermediate catalytic deoxygenation to
play a key role in the production of second-generation bio-
fuels.
Characterization
Porous properties were determined by N2 sorption at 77 K on
dried samples (673 K, 3 h, 10À5 bar) by using a Micromeritics TriStar
II instrument. The total surface area (SBET) was calculated using the
BET model, and the t-plot method was used to determine the mi-
cropore volume (Vmicro) and mesopore surface area (Smeso). XRD pat-
terns were measured by using a PANalytical X’Pert PRO-MPD dif-
fractometer using Ni-filtered CuKa radiation (l=0.1541 nm). Data
were recorded in the 2q range of 5–708 with an angular step size
of 0.058 and a counting time of 8 s per step. The sodium content
in the solids was determined by AAS by using a Varian SpectrAA
220 FS instrument. 23Na MAS NMR spectra were recorded at a spin-
ning speed of 10 kHz by using a Bruker Avance 700 spectrometer
equipped with a 4 mm probe head and 4 mm ZrO2 rotors at
185.2 MHz. Spectra were obtained using 4096–8192 accumulations
with a pulse length of 1 ms, a recycle delay of 0.25 s, and 0.1m
NaCl in D2O as a reference. Before the measurement, samples were
evacuated at 573 K for 4 h. Thermogravimetric analysis (TGA) of
the used catalysts was performed by using a Mettler-Toledo TGA/
DSC 1 instrument by heating the solid (20 mg, 298–1173 K,
10 KminÀ1) in flowing air (60 cm3 minÀ1). The TPD of propanal was
performed by using a Micromeritics Autochem II analyzer. The cat-
Conclusions
We demonstrate the generation of siloxy sites in silica-rich ma-
terials through alkaline activation treatments, which prove to
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