ters was confirmed by powder X-ray diffraction analysis (Sup-
porting Information, Figure S2). It seems that the faceted Ni
pear-shape implies distinguished catalytic behavior compared
to reference polycrystalline Ni catalysts;[12] consider, for exam-
ple, the orders-of-magnitude lower sorbitol yields reported on
conventional Ni/Al2O3 and Ni/AC catalysts. It should be stressed
that simple impregnation of carbon nanotubes with Ni, which
consists of small Ni particles at the outer surface of the tubes in-
stead of larger reshaped Ni particles attached at the tips, affords
poor yields of less than 1% sorbitol (an observation recently rec-
ognized by Deng and co-workers).[5] Although details of the
direct correlation between surface structure and product distri-
bution are still under intense study, we suggest that the bond-
breaking selectivity is controlled by controlling the shape of the
Ni particles during the carbon nanofiber growth process.
Experimental Section
Nickel–carbon nanofiber catalysts were prepared by chemical
vapor deposition (CVD) of methane over Ni/g-Al2O3. Detailed syn-
thesis procedures as well as various characterization methods are
provided as Supporting Information. In a typical reaction, cellulose
(Sigma–Aldrich; microcrystalline Avicel PH-101, 1 g), Ni/CNF (0.5 g),
and water (50 mL) were loaded in a stainless steel autoclave (Parr
Instruments Co., 100 mL). The reaction mixture was stirred at a
rate of 700 rpm, pressurized with H2 to 4 MPa at room tempera-
ture, and subsequently heated at 503 K for 4 h. After the reaction,
the product mixture was centrifuged, filtered, and analyzed by
HPLC [Agilent 1200 Series, RI detector, Varian Metacarb 67 C
column (300ꢁ6.5 mm), mobile phase: water]. The product yield
was calculated as follows: yield (%)=(weight of polyol)/(weight of
cellulose charged in reactor). The conversion of cellulose was de-
termined by total organic carbon (TOC) analysis of the liquid
phase, as reported earlier.[4b]
An intriguing way to improve the yield of sugar alcohols
relies on mild pretreatment of cellulose using mechanical ball-
milling, which is expected to increase the amorphous fraction
(for SEM images, XRD patterns, CP/MAS 13C NMR, and IR spec-
tra of the feedstock before and after pretreatment, see the
Supporting Information).[13] When ball-milled cellulose was ex-
ploited as substrate, an overall unprecedented yield of 70%
sugar alcohols was achieved with Ni/CNF (entry 10). The ratio-
nale behind this improved result is found in the mechanical
disruption of microcrystalline cellulose by breaking hydrogen
bonds, which translates into a better accessibility of the b-gly-
cosidic linkages, higher reaction rates, and hence to higher
yields of polyols at lower temperatures (463 vs. 503 K). More-
over, similar catalytic results are observed when starting from a
five times more concentrated cellulose feed (viz. 10 wt%,
entry 11). With this knowledge, we anticipate that the Ni/CNF
catalysts might also be well-suited in biorefineries for the bi-
functional conversion of cello-oligomers, formed for example
during the selective depolymerization of cellulose or wood,
using acidic resins in ionic liquids.[14]
Acknowledgements
S.V.d.V. and M.D. acknowledge FWO–Vlaanderen, and J.G. thanks
IWT for a doctoral fellowship. This work was performed within
the framework of IAP, IDECAT, and CECAT projects. We are grate-
ful to Kristof Houthoofd for 13C NMR spectroscopy, and to Chris-
toff Van Moorleghem for TOC measurements.
Keywords: carbohydrates
catalysis · nanostructures · nickel
·
cellulose
·
heterogeneous
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