A. Ausavasukhi et al. / Journal of Catalysis 268 (2009) 68–78
77
þ
ꢂ
þ
ꢂ
GaðH
2
Þ Z þ D
2
¢ ðDÞðGaðH
2
ÞðDÞÞ Z
ð4Þ
ð5Þ
(Table 3), a straight line results, which supports the concept that
the direct decarbonylation of benzaldehyde to form benzene is cat-
alyzed by strong Brønsted site.
þ
ꢂ
þ
ꢂ
ðDÞðGaðH
2
ÞðDÞÞ Z ! GaðHDÞ Z þ HD
An interesting difference in the response to addition of water to
the feed was observed between the unpromoted and the Ga-pro-
moted HZSM-5. While the activity of the former decreased with
water added to the feed, the activity of the latter increased. As
mentioned above, the decrease in activity is simply due to the
adsorption competition, while the increase parallels the increase
in the density of Brønsted acid sites observed by IPA-TPD (Table
The D
2
molecule can dissociate into two D atoms; one interacts with
ꢂ
þ
the Z GaH2 site, the other is bonded to an O atom in the zeolite
framework. In the case of benzaldehyde conversion in the presence
of gas-phase H
ated H
forming toluene.
Benzene is formed primarily by benzaldehyde decarbonylation
over the Brønsted acid sites. It can also be produced via toluene
hydrodealkylation, with the formation of methane in parallel.
The main reaction paths occurring on the catalysts investigated
can be summarized in the following scheme.
þ
2
, the GaH2 species can possibly transfer the dissoci-
2
to the benzaldehyde and consecutively hydrogenolyze it
3). This increase can be interpreted in terms of the formation of
GaO(OH) species according to the following steps, previously pro-
posed [53–55]
þ
O ! GaOþ þ H
Ga þ H
2
2
ð6Þ
ð7Þ
þ
þ
GaO þ H
2
O ! GaOðOHÞ þ H
O
Some of us [41] have recently proposed that the GaO(OH) spe-
cies can be incorporated into the zeolites framework by reaction
with available silanol defects, forming additional Brønsted acid
sites („Ga–OH–Si„), according to
Reduced Ga species
H
H2
GaOðOHÞ þ 4ðBSi—OHÞ ! GaHðOSiBÞ þ 2H
2
O
ð8Þ
4
2
Under H , the dominant hydrogenation/hydrogenolysis activity of
Ga/HZSM-5 decreased while decarbonylation increased when water
was added. Based on the concepts discussed above, the interaction
of water with the Ga cationic species not only forms additional
Brønsted sites, which accounts for the enhanced decarbonylation,
H2
- CH4
-
CO
þ
but also reduces the number of GaH2 species, active for hydrogena-
tion/hydrogenolysis. Interestingly, the hydrogenation/hydrogenoly-
sis activity can be recovered when water is removed from the feed,
which indicates that Eqs. (7) and (6) are reversible.
Strong Brønsted acid sites
5
. Conclusions
The benzaldehyde conversion over Ga/HZSM-5 yields benzene
and toluene as main products. Benzene is produced mainly by di-
rect decarbonylation over Brønsted acid sites. Toluene is only pro-
Over non-reduced 3Ga/HZSM-5 catalyst and in the absence of
2
, only benzene is obtained. The addition of Ga does not seem
duced in the presence of H
GaH2 species. These species are only generated under H
ene is formed in steady state in the absence of H
2
by hydrogenation/hydrogenolysis over
. No tolu-
. The presence of
H
þ
2
to have any effect on this reaction, other than reducing the density
of strong acid sites. In fact, the rate of benzene formation correlates
very well with the number of strong acid sites. As shown in Fig. 11,
when the rate of benzene formation is plotted as a function of the
density of strong Brønsted acid sites, as measured by IPA-TPD
2
water causes an increase in the benzene/toluene ratio in the prod-
ucts from Ga-modified HZSM-5.
Acknowledgments
300
This work was financially supported by the Oklahoma Secretary
of Energy and the Oklahoma Bioenergy Center. One of the authors
(
A. Ausavasukhi) is grateful for a financial support from the Thai-
land Research Fund through the Royal Golden Jubilee Ph.D. Pro-
gram (Grant No. PHD/0213/2548).
200
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