A. Ausavasukhi et al. / Journal of Catalysis 290 (2012) 90–100
99
IPA-TPD (see Figs. 1 and 2) suggest that reduction in octahedrally
coordinated Ga2O3 (Ga3+ ions) leads to the formation of reduced
univalent Ga+ ions, as previously reported [20,28,46,47], which re-
place acidic protons.
– The main products observed from m-cresol are toluene, ben-
zene, and xylene. In addition, phenol, oxygenated, and bicyclic
compounds are also formed. The presence of these products
suggests that the formation of a ‘‘surface pool’’ of oxygenated
intermediates may play a role in the reaction pathway.
– The pulse-mode experiments show that m-cresol is readily
trapped inside the zeolite. These trapped species can undergo
several reaction paths, as follows:
Ga2O3 þ 2H2 ! Ga2O þ 2H2O
ð1Þ
Ga2O þ 2ZꢂHþ ! 2ZꢂGaþ þ H2O
ð2Þ
where Zꢂ represents the negative framework charge of the zeo-
lite.Alternatively, these Ga+ ions can chemisorb molecular H2 result-
ing in the formation of gallium dihydrides (GaH2+), which have also
been proposed as active sites that promote hydrogenolysis
[34,46,48,49].
a. Condensation to (deactivating) heavier products.
b. Decomposition to phenol and aromatics.
c. Hydrogenolysis of trapped products as well as of some of
the larger molecular aggregates that have started to
condense.
ZꢂGaþ þ H2 ! ZꢂGaHþ
ð3Þ
2
The last path, (c), is greatly accelerated in the presence of Ga
and H2 in the gas phase. Under these conditions, path (a) is much
less predominant, so deactivation is slow.
Path (b) occurs even in the absence of H2, but in that case, path
(a) dominates, leading to a fast deactivation. Reactions in He or
those in H2, but without Ga (in the gas phase or the surface), show
severe deactivation.
The existence of the GaHþ2 on this type of catalysts has been previ-
ously proposed, on the basis of IR absorption bands observed at
ꢁ2040 cmꢂ1 [30]. Since hydride transfer plays an important role
in the formation of light aromatics from phenolics [50,51], we be-
lieve that H2 not only keeps Ga species in its reduced form but
+
can also stabilize GaH2 species, which can promote hydride trans-
fer to the CAC and CAO bond of the condensation product, leading
to the formation of lighter aromatics, as discussed above.
– The continuous-flow-mode experiments show that without H2
in the feed, the hydrogenolysis activity is rapidly depleted.
However, the BrØnsted sites of the zeolite are still active for
generating the surface pool of condensation products as well
as for the decomposition of these deposits, but not for deoxy-
genation, so mainly oxygenated compounds are produced. Also,
it is observed that the lack of hydrogenolysis activity in the
absence of H2 leads to a rapid catalyst deactivation.
– By contrast, the presence of H2 in the feed greatly enhances the
deoxygenation activity, particularly on the Ga/HBEA catalysts,
being significantly lower on Ga/HMFI and almost negligible on
Ga/SiO2. While the former is due to steric constraints of the pore
structure, the latter is due to the inability of the support to sta-
bilize well-dispersed Ga species, which under H2 are the active
sites for hydrogenolysis.
In the case of the 3Ga/silica catalyst, it is clear that the inability
of silica to stabilize well-dispersed Ga species and promote the for-
mation of gallium dihydrides is responsible for the lack of any
hydrodeoxygenation activity (Table 4). Another interesting trend
was observed on this catalyst. While the selectivity to phenol
was relatively high, there was practically no formation of heavy
oxygenated compounds, which appeared in significant amounts
on the zeolite catalysts. This difference shows that the formation
of the surface pool of condensed intermediates requires the pres-
ence of a high density of acid sites as found in zeolites.
The different reactivity exhibited by the Ga-modified HMFI and
HBEA zeolites is an interesting matter to discuss. For example,
while both 3Ga/HMFI and 3Ga/HBEA stabilize the same type of
Ga species and exhibit similar TPR profiles (Fig. 1), the overall m-
cresol conversion is significantly lower over 3Ga/HMFI than over
3Ga/HBEA. Then, the difference may have a shape-selectivity ori-
gin. First, the diffusion of m-cresol into the restricted pores of
3Ga/HMFI may be much more constricted than in the pores of
3Ga/HBEA. Second, if as proposed above, the conversion of m-cre-
sol involves bimolecular interactions, this reaction path will be
hindered inside the structure of 3Ga/HMFI. Therefore, the lower,
but still significant activity of 3Ga/HMFI must derive from one of
the following possibilities: (i) pore mouth sites or (ii) isomerization
of m-cresol to a less restricted isomer that can readily diffuse into
the pores.
Acknowledgments
This research was supported by NSF EPSCoR (Grant 0814361)
and DoE EPSCOR (Grant DE-SC0004600). One of the authors
(A.A.) is grateful for a financial support from the Thailand Research
Fund through the Royal Golden Jubilee Ph.D. Program (Grant No.
PHD/0213/2548).
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