S. Liu et al. / Catalysis Today 269 (2016) 122–131
125
involved in the production of gasoline-ranged products from sor-
3.2. Effect of reaction temperature
bitol. Therefore, all the noble-metals modified Ir-ReOx/SiO2 should
have bi-functional roles of the activities of APR and hydrogenolysis.
On the one hand, Dumesic et al. have found that the overall cat-
alytic performance for ethylene glycol reforming decreased in the
following order for silica-supported metals: Pt > Ru > Rh ∼ Pd > Ir
Table 2 shows the effect of reaction temperature on conversion
of sorbitol over Pt(3)-Ir-ReOx/SiO2 catalyst. The sum yield of CO2
and gasoline-ranged products increased as the reaction tempera-
ture was increased from 443 to 463 K. The amount of remaining
H2 was not so changed by the temperature change. This behavior
indicated that higher temperature improved the APR activity effi-
ciently to give larger amount of in situ hydrogen which was used in
the C O hydrogenolysis reactions. The hydrogenolysis activity of
Ir-ReOx/SiO2 catalyst also increases with the increase in the reac-
tion temperature [41]. In addition, the addition of Pt improved the
hydrogenolysis activity of Ir-ReOx/SiO2 in pressurized hydrogen
[
56]. Silica-supported Pt and Pd catalysts exhibited relatively high
selectivities for production of H , while silica-supported Rh and
2
Ru catalysts showed a low selectivity for production of H . This
2
trend could be related to our results that the noble metals modified
Ir-ReOx/SiO2 catalysts except Ru-modified Ir-ReOx/SiO2 showed.
On the other hand, in our previous work, the activity of silica-
supported metals in hydrogenolysis of glycerol decrease in the
order: Ru > Rh > Pt, Pd while ReOx modified silica-supported metals
with the order: Rh-ReOx > Ru-ReOx > Pt-ReOx > Pd-ReOx. In addi-
(Table S2). The yield of CO decreased slightly with further increase
2
of the reaction temperature (Entry 4). That could be attributed
to faster growth rate of C O dissociation than that of C C cleav-
age with the increasing temperature, and more amount of sorbitol
was consumed by C O dissociation. The highest yield of gasoline-
ranged products reached 42% when the reaction temperature was
at the range of 453–463 K (Entry 2 and 3). However the yield of
C5–C6 alkanes at 463 K was higher than that at 453 K.
tion, Ru, Rh and Pd modified Ir-ReOx/SiO showed that Ru improved
2
the hydrogenolysis activity of Ir-ReOx/SiO while Rh and Pd showed
2
negative effect, especially Pd [57,58]. The addition of Pt also
improved the hydorgenolysis performance of Ir-ReOx/SiO2 which
was shown below. This could be related to the yields of gasoline-
ranged products over Pt, Rh and Pd modified catalyst.
To obtain high yield of gasoline-ranged products, controlling the
relative rates of C O and C C cleavage is a key. Activity in C
C
3.3. Effect of HZSM-5 addition
cleavage is important to obtain enough H2 via APR, whereas much
higher rate of C C cleavage than C O cleavage decreases the yield
of products with larger carbon number, which are suitable for gaso-
line. Therefore, the effect of Pt loading amount of Pt-Ir-ReOx/SiO2
on the performance was investigated (Table 1, entries 5–9). The
activity of APR, in terms of CO2 yield, increased as the Pt loading
amount increased. The sum yield of all the gasoline-ranged prod-
ucts increased when the loading amount increased to 3 wt%. When
Pt loading amount was further increased to 5 wt%, the sum yield of
the gasoline-ranged products decreased slightly. Therefore, Pt(3)-
Ir-ReOx/SiO2 was chosen as the standard catalyst and the molar
ratio of Pt:Ir:Re was 1:1.3:2.6 on this catalyst. Another important
Although over 40% yield of gasoline-ranged products can be pro-
duced over Pt(3)-Ir-ReOx/SiO2 catalyst, of them, over 65% of the
products are oxygen-containing organic compounds which may
not meet the gasoline requirement. Therefore further hydrodeoxy-
genation of products to alkanes would be preferred. To obtain
high yield of alkanes, increasing the rate of C O hydrogenoly-
sis of Pt(3)-Ir-ReOx/SiO2 catalyst may be the key. It was reported
that the addition of solid acids such as HZSM-5 can enhance
the C O hydrogenolysis activity of Ir-ReOx/SiO2 catalyst [44].
The combination of Ir-ReOx/SiO2 catalyst and HZSM-5 showed
excellent performance in production of alkanes from cellulose
or sugar alcohols with external hydrogen [38,39]. When HZSM-5
was used as co-catalyst in conversion of sorbitol without hydro-
gen, the total yield of gasoline-ranged products decreased slightly
(Table 3). However, the distribution of products changed signif-
icantly. The yield of C5–C6 alkanes much increased while the
oxygen-containing organic compounds decreased (Entry 2), indi-
cating that HZSM-5 also improved the C O cleavage performance
of Pt(3)-Ir-ReOx/SiO2 catalyst. The yield of H2 was much lower
than that without HZSM-5, indicating that almost all of the hydro-
gen was consumed by hydrogenolysis reaction. At longer reaction
time, the yield of C5–C6 alkanes was slightly increased (30%) and
the amount of them accounts for about 75% of the total gasoline-
ranged products (Entry 3). The yield of CO2 was almost the same
as that without HZSM-5 indicating that HZSM-5 did not affect the
APR performance of Pt(3)-Ir-ReOx/SiO2 catalyst. Almost no mono-
alcohols were detected with the addition of HZSM-5 and the yield
of ketones decreased gradually with the reaction time. The results
suggest that the mechanism of hydrogenolysis of mono-alcohols
and ketones proceeded by dehydration + hydrogenation and hydro-
genation + dehydration + hydrogenation, respectively (Scheme 1).
In particular, it has been reported that Ir-ReOx/SiO2 showed high
activity in the hydrogenation of C O bond [47]. The production
of C5–C6 alkanes especially hexanes was limited by the hydro-
genation of ketones. The addition of HZSM-5 not only enhanced
the dehydration of mono-alcohols, but also improved the ketones
hydrogenation performance of Pt(3)-Ir-ReOx/SiO2 catalyst.
point is that Pt(3)/SiO , Pt(3)-ReOx/SiO and Ir-ReOx/SiO2 showed
2
2
very low APR performance. Although the Re addition to Pt/SiO2
enhanced the APR activity, the additive effect was not so significant.
This can be because the reaction temperature (453 K) was signifi-
cantly lower than that in the literature for APR over Pt- and Pt-Re
catalysts (∼500 K) and the different support materials [10,21–24].
The yield of gasoline-ranged products over the physical mixture of
Pt(3)/SiO2 and Ir-ReOx/SiO2 catalysts was much lower than those
over Pt(3)-Ir-ReOx/SiO2 catalyst. This indicated the strong syner-
getic effect of Pt and Ir-ReOx. The product yields and distributions
were almost unchanged when the reaction time was prolonged
(
Entry 12). The conversion of sorbitol under pressurized H2 was
also conducted (Table S2). The main products were hexanols which
was very different from that under Ar. The yield of hexanols over
Pt(3)-Ir-ReOx/SiO2 was higher than that over Ir-ReOx/SiO , which
2
indicated that the addition of Pt improved the hydrogenolysis activ-
ity of Ir-ReOx/SiO2 in pressurized hydrogen.
When the reaction was conducted without sorbitol, almost no
n-decane was converted. As a result, it is verified that the organic
solvent was inert under the reaction condition. When the reac-
tion was conducted without n-decane, the yield of gasoline-ranged
products was much lower than that with n-decane. This can be
attributed to the fact that the organic solvent (n-decane) not only
capture the products with low-boiling point for analysis, but also
suppress the consecutive reaction of intermediates with high sol-
ubility to alkanes. The product distribution in different phases is
shown in Table S1. In fact, most of ketones were present in the
organic phase. Lowering the reactive ketones or aldehydes in the
aqueous phase containing catalysts can suppress the polymer-
ization, which starts with bi-molecular reaction, to undesirable
by-products (loss of carbon balance).
3.4. Catalyst stability
Table 4 lists the results of the reuse test of Pt(3)-Ir-ReOx/SiO2.
The yield of gasoline-ranged products was decreased gradually