Robust heterogeneous nickel catalysts with tailored porosity for the selective hydrogenolysis of aryl ethers

Article abstract of DOI:10.1002/cctc.201300763

SiC materials with tailored porosity and integrated Ni NPs (Ni@SiC) were synthesized via microphase separation of polycarbosilane-block-polyethylene followed by its pyrolysis. Changing the length of organic block allowed the synthesis of micro, meso and hierarchical Ni@SiC materials which were characterized by PXRD, TEM, TGA, and nitrogen physisorption. Selective hydrogenolysis of aryl ethers mimicking the most abundant linkages of lignin was achieved in water avoiding the possible hydrogenation of aromatic rings. Copyright

Full text of DOI:10.1002/cctc.201300763

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DOI: 10.1002/cctc.201300763
Robust Heterogeneous Nickel Catalysts with Tailored
Porosity for the Selective Hydrogenolysis of Aryl Ethers
Muhammad Zaheer,[a] Justus Hermannsdçrfer,[a] Winfried P. Kretschmer,[a] Gꢀnter Motz,[b] and
Rhett Kempe*[a]
Abstract: SiC materials with tailored porosity and integrated Ni
NPs (Ni@SiC) were synthesized via microphase separation of
polycarbosilane-block-polyethylene followed by its pyrolysis.
Changing the length of organic block allowed the synthesis of
micro, meso and hierarchical Ni@SiC materials which were
characterized by PXRD, TEM, TGA, and nitrogen physisorption.
Selective hydrogenolysis of aryl ethers mimicking the most
abundant linkages of lignin was achieved in water avoiding
the possible hydrogenation of aromatic rings.
the cleavage of ethers in water.[12–14] The substantial reduction
of arenes was also observed. Reports on the reusability of the
hitherto applied catalysts are rare. Thus, there is a need for re-
usable catalysts offering hydrothermal stability for the selective
cleavage of CArÀO bonds.
In the past few years, others and ourselves have developed
late transition metal-containing polymer-derived SiCN materials
(M@SiCN) as robust heterogeneous catalysts.[15] The Wiesner
group fabricated Pt@SiCN materials which, despite the struc-
tural control over multiple length scales, showed a rather low
surface area.[16] By the controlled pyrolysis of Ni modified poly-
silazane, we managed to get microporous high surface area
materials. They were found to be selective and active catalysts
for the semi-hydrogenation of acetylenes.[17] Unfortunately, the
SiÀN bonds of the materials pyrolysed at 6008C were under
harsh conditions. SiC materials do not contain such bonds and
should offer additional hydrothermal stability. Commercially
available silicon carbide possesses low surface area, which
limits the effective transport of substances through it for effi-
cient catalysis.[18] Self-assembly of block copolymers[19] has
been applied to the fabrication of mesoporous SiC materials
by Kim and co-workers and allows fine tuning of pore mor-
phology.[20] Porous iron containing SiC materials have been fab-
ricated by the Manners group through the self-assembly of
metal-containing block copolymers.[21]
The sustainable production of fine chemicals and fuels from re-
newable resources is a burgeoning and challenging research
area.[1] Lignocellulose biomass (cellulose, hemi-cellulose and
lignin) is a key resource whose availability is plentiful and its
utilisation will allay the fears regarding competition with food
supplies.[1,2] Although cellulose can be depolymerised in many
ways,[3] selective cleavage of aryl ether (CArÀO) and especially
diaryl ether substructures of lignin is challenging.[4] Lignin con-
stitutes 15–30% of woody biomass[5] with an energy content
of up to 40% of this biomass[6] and its selective depolymerisa-
tion (through hydrogenolysis) is crucial for the generation of
fuel and chemicals from renewable resources. Selective cleav-
age of the CArÀO bond of lignin model compounds in organic
solvents has been reported by the use of molecular catalysts
(V,[7] Ru[8] and Ni[9] complexes) and heterogeneous catalysts
(Ni[10] and Zn/Pd[11]). The molecular catalysts offer better che-
moselectivity in the selective cleavage of CArÀO bonds and are
operative under mild conditions (80–1358C). Unfortunately,
they are mostly sensitive to high concentrations of water, the
removal of which from the crude biomass is challenging and
uneconomical. Heterogeneous catalysts, on the other hand,
are not very selective and require higher reaction temperatures
or hydrogen pressures, leading to the reduction of arenes and
subsequent hydrogen loss. The Lercher group has reported
some elegant work on the use of heterogeneous catalysts for
Herein, we report on the synthesis of porous SiC composites
with integrated nickel nanoparticles (Ni NPs) starting from or-
ganic–inorganic block copolymers. The synthesis, cross-linking
and microphase separation of the block copolymers is ach-
ieved in a concerted process[22] followed by controlled pyrolysis
that leads to porous Ni@SiC composite materials. The porosity
of the materials can be tailored from micro- to meso-scale in-
cluding hierarchical (micro–meso) pore structures by changing
the molecular weight of the organic block. In particular, the
synthesis of mesoporous Ni@SiC materials with an average
pore diameter of 4–5 nm is achieved, which is challenging
owing to high Laplace pressure leading to pore collapse.[22]
Phenethoxybenzene (PEB) and diphenylether (DPE), which
mimic the most frequent linkages of lignin [b-O-4 (45–62%)
and 4-O-5 (4–9%), see Figure 1], are used as model com-
pounds for the chemoselective cleavage of CArÀO bond. The
Ni@SiC catalysts are active under mild reaction conditions (90–
1208C, 6 bar hydrogen; 1 bar=100 kPa) in water providing
high selectivity for hydrogenolysis of the CArÀO bond and
reusability.
[a] M. Zaheer, J. Hermannsdçrfer, Dr. W. P. Kretschmer, Prof. Dr. R. Kempe
Lehrstuhl Anorganische Chemie II
Universitꢀt Bayreuth
95440 Bayreuth (Germany)
E-mail: kempe@uni-bayreuth.de
[b] Dr. G. Motz
Lehrstuhl Keramische Werkstoffe
Universitꢀt Bayreuth
95447 Bayreuth (Germany)
The synthesis of Ni@SiC materials is summarised in
Scheme 1. An organic–inorganic block copolymer is formed by
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300763.
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after pyrolysis contain 4.3 wt%
of nickel (Si/Ni atomic ratio
ꢀ20:1, Table S2). The molecular
weight of the organic block (PE)
affects the type of porosity gen-
erated in the Ni@SiC catalyst.
The use of PE with a molecular
weight of 1740 gmolÀ1 provides
a mesoporous Ni@SiC material
after self-assembly and pyrolysis.
The material, as investigated by
TEM, shows Ni NPs with an aver-
age size of 3.3 nm (Figure 2c).
The nitrogen adsorption–desorp-
tion isotherm as presented in
Figure 2e is typical for a mesopo-
rous material. The closure of the
desorption branch at a relative
pressure of ꢀ0.4 owes to capilla-
ry condensation, a characteristic
process known for mesopores.
To a BET surface area (SBET) of
450 m2 gÀ1, only 38 m2 gÀ1 is con-
tributed by micropores, as calcu-
lated by t-plot method (Table S3,
entry 1). The size of the pores is
Figure 1. A fragment of hardwood lignin.
calculated by non-localised DFT
(adsorption branch) and is in the
2–8 nm size range (Figure 2 f).
The presence of Ni NPs (cubic nickel, ICDD PDF number: 00-
001-1260) was confirmed by powder X-ray diffraction (PXRD,
see Figure 2d). If the molecular weight of the organic block
(PE) is decreased to 326 gmolÀ1, microporous materials (SBET
=
552 m2 gÀ1) are obtained. The material shows a typical type-I
adsorption isotherm (Figure 3c) with an average pore width of
1.3 nm (Figure S6). The specific surface area shows a small con-
tribution (69 m2 gÀ1) from mesopores (Table S3, entry 2). The Ni
NPs are smaller in size (mean diameter=2.4 nm, Figure 3b)
than the mesoporous materials and are uniformly distributed
over the material (Figure 3a). The PXRD pattern of the material
(Figure S7a) confirms the presence of Ni NPs. At this stage, it
seems that porosity of the Ni@SiC materials can be tailored by
varying the molecular weight of PE. Lower molecular weight
PE (326 gmolÀ1) provides microporous materials, whereas mes-
oporous materials can be obtained by higher molecular weight
PE (1740 gmolÀ1). We thus expected to obtain a hierarchically
structured material (micro- and mesopores) by the use of PE
with intermediate chain lengths. If PE with a molecular weight
of 550 gmolÀ1 was used, composites with hierarchical porosity
(SiC-hier) were obtained. The formation of amorphous SiC was
confirmed by FTIR and solid state 29Si NMR analysis of the ma-
terials (Figure S8). The latter shows broad peaks at À16 and
À65 ppm assigned to SiC4 and CSiO3 environments, respective-
ly.[26] A slight amount of oxygen in the materials comes from
the hydroxy-terminated PE. As shown in Figure 3d, these ma-
terials contain nickel nanoparticles with an average size of
2.7 nm. High uptake of nitrogen both at lower and higher rela-
Scheme 1. The synthesis of porous Ni@SiC materials through the self-assem-
bly of PCS-block-PE. The block copolymer is synthesised through Ni com-
plex-catalysed dehydrocoupling of PCS with PE.
the reaction of the commercially available polycarbosilane
(PCS) SMP-10 (Starfire Systems) with hydroxy-terminated poly-
ethylene (PE) through nickel-catalysed dehydrocoupling of
SiÀH and OÀH functions.[23] As both blocks are immiscible and
covalently linked, the microphases separate into microdo-
mains, providing a nanostructured material. The PE block used
is inexpensive and can be produced efficiently by using molec-
ular catalysts.[24] The crystallisation behaviour of strictly linear
PE allows good morphology control even in combination with
rather ill-defined inorganic blocks.[25] The added nickel complex
([Ni(Ap)2]2 where Ap-H=4-methyl-2-((trimethylsilyl)amino)pyri-
dine, see Figure S1 in the Supporting Information for its struc-
ture) also catalyses the cross-linking of the PCS block, for ex-
ample, through homocoupling of the SiÀH bonds. The organic
block is burnt out during pyrolysis (7008C in Ar), whereas NiII
species are reduced to elemental nickel. Thus, porous SiC ma-
terials with integrated Ni NPs are generated. The loading of
the nickel complex is adjusted such that the final materials
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Figure 3. a) TEM micrograph with a high resolution inset, b) Ni particle size
distribution and c) nitrogen adsorption isotherm of the SiC-micro material.
d) TEM micrograph with Ni particle size count and high resolution insets,
e) nitrogen adsorption isotherm and f) pore size distribution calculated by
using a non-localised DFT model of the SiC-hier material.
Figure 2. a,b) TEM micrographs of mesoporous Ni@SiC (SiC-meso) and c) the
corresponding Ni particle size distribution. d) PXRD pattern with nickel re-
flexes and asterisk denoting graphitic phases. The carbon phase forms as
a result of considerable polyethylene cracking during pyrolysis. e) Nitrogen
adsorption–desorption isotherm and f) the corresponding pore size distribu-
tion, calculated by using a non-localised DFT model with silica kernels of cy-
lindrical pore geometry.
a TBAB (tetra-n-butylammonium bromide) could be advanta-
geous. With the addition of 0.3 equivalents of TBAB, hydroge-
nolysis of benzyl phenyl ether was achieved even at 908C.
tive pressure values (Figure 3e) is indicative of the existence of
both micro- and mesopores. The non-localised DFT model ap-
plied to the nitrogen adsorption isotherm (Figure 3 f) confirms
the existence of micro- and mesopores. Contributions to the
total surface area of SiC-hier (527 m2 gÀ1) came partly from the
micropores (326 m2 gÀ1) and partly from the mesopores
(202 m2 gÀ1), as tabulated in Table S3. The pores were generat-
ed because of gaseous emissions (mostly cracked hydrocar-
bons) from the organic block during pyrolysis (see Figure S9
for thermogravimetric analyses of all materials). Mass losses
(300–5008C) are in accordance with the loss of the PE block
during pyrolysis.
Table 1. Hydrogenolysis of benzyl phenyl ether with Ni@SiC.[a]
Entry Catalyst
T
KOtBu
TBAB
[equiv.]
Conv. [%]
(TON)[c]
Sel.[d]
[mol%]
[8C] [equiv.]
1
2
3
4
SiC-
110
110
110
90
–
–
–
1
–
99 (16)
75 (12)
26 (4)
81
94
98
99
hier[b]
SiC-
–
micro[b]
SiC-
meso[b]
SiC-
–
The selective hydrogenolysis of lignin model compounds
was studied in water under 6 bar hydrogen pressure. Compari-
son of the activities of all three types of Ni@SiC materials was
tested in the hydrogenolysis of benzyl phenyl ether at 1108C
for 20 h (Table 1). SiC-hier was found to be the most active cat-
alyst providing 99% conversion (entry 1). In case of micropo-
rous SiC-micro, a conversion of 75% was observed (entry 2)
under the same conditions. Mesoporous Ni@SiC (SiC-meso)
showed only 26% conversion (entry 3). The selectivity towards
aromatic products (phenol and toluene) was high in all experi-
ments. As aryl ethers are insoluble in water, the addition of
0.3
90 (38)
micro
SiC-hier
SiC-
5
6
90
90
1
1
0.3
0.3
99 (42)
64 (27)
99
99
meso
[a] Conditions: 0.5 mmol of ether was mixed with 15 mg of catalyst
(1.17ꢂ10À5 mol Ni) in 2 mL H2O at a H2 pressure of 6 bar and stirred
(1000 rpm) at a specified temperature for 20 h. [b] 40 mg catalyst (3.13ꢂ
10À5 mol Ni) and a H2 pressure of 10 bar was used. [c] Turnover numbers
were calculated based on the total amount of Ni in the materials. [d] Se-
lectivity to phenol.
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Again, SiC-hier showed the highest activity (entry 5) compared
with SiC-micro (90%, entry 4) and SiC-meso (64%, entry 6). The
higher activity of SiC-hier could have resulted from the better
accessibility of catalytically active nickel NPs in the hierarchical-
ly porous material. At this temperature, no further hydrogena-
tion of phenol to cyclohexanol was observed, providing
phenol in 99% selectivity.
ether, after which it remained almost constant (see Fig-
ure S11a). Keeping the amount of TBAB at 0.3 equivalents, in-
creasing the concentration of NaOtBu base increased the over-
all conversion up to 1 equivalent of base relative to the ether.
With more than 1 equivalent of base, the conversion decreased
(see Figure S11b). These optimised conditions were also ap-
plied to selectively cleave DPE, the model compound for the
4-O-5 linkage (Figure 1). 50% of DPE was converted into
phenol and benzene by using SiC-hier (entry 10). The phenol
selectivity was nearly quantitative. By doubling the catalyst
loading and increasing the amounts of base and phase-transfer
catalyst, an almost quantitative conversion of DPE was ach-
ieved within 20 h (entry 11). The selectivity towards phenol
and benzene was again as high as 97%. These results showed
the hierarchical Ni@SiC materials to be highly selective towards
hydrogenolysis of the CÀO bond of DPE without hydrogena-
tion of the aromatic ring(s). In comparison to previous works,
we have achieved the selective cleavage of CÀO bonds in
water (avoiding the use of organic solvents) and with a lower
nickel content (4.3 wt%). The recycling of the catalyst SiC-hier
was tested with almost 50% conversion at 1208C under 6 bar
of hydrogen. No prominent loss in the activity was observed
after up to five consecutive catalytic runs (see Figure S12).
In summary, we have synthesised robust porous SiC materi-
als with integrated Ni nanoparticles. Porosity was achieved
through the self-assembly of organic–inorganic block copoly-
mers. Depending upon the molecular weight of the organic
block (inexpensive polyethylene), the fabrication of micro-,
meso- and hierarchically porous Ni@SiC catalysts was possible.
All catalysts were found to be active and highly selective in
ether hydrogenolysis. The hierarchically structured Ni@SiC ma-
terial was the most active catalyst. It was reusable and highly
selective towards the hydrogenolysis of diphenylether, a chal-
lenging lignin model compound.
SiC-hier, owing to its higher activity than the other two cata-
lysts (SiC-micro and SiC-meso), was chosen for the selective
cleavage of the b-O-4 linkage to phenol and ethylbenzene
(Table 2). No conversion was observed at 1208C in the absence
Table 2. Hydrogenolysis of phenethoxybenzene (entries 1–9) and DPE
(entries 10-11) with Ni@SiC.[a]
Entry Catalyst Base [equiv.] TBAB [equiv.] Conv. [%] (TON) Sel.[b] [%]
1
2
3
4
5
6
7
8
SiC-hier
SiC-hier
SiC-hier
SiC-hier
SiC-hier
SiC-hier
SiC-hier
SiC-micro NaOH (1)
SiC-meso NaOH (1)
SiC-hier
SiC-hier[c] KOtBu (2.5) 1.0
–
–
–
0.3
–
0.3
0.3
0
0
27
93
71
89
99 (42)
78 (33)
25 (11)
50
0
0
NaOH (1)
NaOH (1)
K2CO3 (1)
NaOtBu (1) 0.3
KOtBu (1)
99
92
84
94
96
95
99
98
97
0.3
0.3
0.3
0.3
9
10
11
KOtBu (1)
96 (14)
[a] Conditions: 0.5 mmol of ether was mixed with 15 mg of catalyst
(1.17ꢂ10À5 mol Ni) in 2 mL H2O at a H2 pressure of 6 bar and then stirred
(1000 rpm) at 1208C for 20 h. [b] Selectivity to phenol (ethylbenzene/ben-
zene selectivity >99% in all cases). Turnover numbers were calculated
based on the total amount of Ni in the materials. [c] 30 mg Catalyst was
used (3.42ꢂ10À5 mol Ni).
Experimental Section
The starting materials 4-methyl-2-((trimethylsilyl)amino)pyridine
(Ap-H),[27] Ni complex [Ni(Ap)2]2 and phenethoxybenzene[13a] were
synthesised by following reported methods.
[28]
of base and TBAB (entry 1) or in the presence of TBAB alone
(entry 2). A minor conversion (27%) was achieved by the addi-
tion of a base (entry 3), which increased greatly (93%) upon
addition of both base and TBAB (entry 4). Addition of TBAB
would have improved the solubility of aryl ethers in water,
whereas the base promoted the cleavage of CÀO bonds, as
previously documented.[10b] Whereas lower conversion and se-
lectivity was obtained by using K2CO3 as a base (entry 5),
NaOtBu and KOtBu provided good yields and selectivities (en-
tries 6–7).
SiC-micro and SiC-hier Ni@SiC materials were fabricated by the re-
action of PE-326 and PE-550 with PCS in THF, respectively. In a vial
placed in a Schlenk tube, PE (1 g) was dissolved in THF (8 mL), fol-
lowed by the addition of PCS (1 g) and Ni complex [Ni(Ap)2]2
(0.400 g, 4.79ꢂ10À4 mol, Ni/Si=1:20). The solution was annealed at
808C for 24 h, during which the solvent came out of the vial into
the Schlenk tube. Finally, the solid was cooled slowly to RT.
For the synthesis of SiC-meso materials, PCS SMP-10 (1 g) and PE-
1739 (1 g) were dissolved in cumene (10 mL) at 1508C. Afterwards,
a solution of the Ni complex [Ni(Ap)2]2 (0.400 g, 4.91ꢂ10À4 mol) in
cumene (7 mL) was added. The mixture was cooled to 1408C and
annealed at this temperature for 24 h. Lastly, the obtained solid
was slowly cooled to RT and traces of the solvent were removed
under vacuum.
In comparison to SiC-hier, the other two materials (SiC-micro
and SiC-meso) had lower activities (entries 8–9). A minor con-
version of phenol to cyclohexanol (hydrogenation product of
phenol) was observed in all cases, indicating a high chemose-
lectivity of hydrogenolysis over hydrogenation provided by the
catalysts. The effect of the amount of TBAB and base was also
investigated. On increasing the amount of TBAB, conversion in-
creased with up to of 0.3 equivalents of TBAB relative to the
The catalytic selective hydrogenolysis of lignin model compounds
was done in a Parr autoclave under a H2 pressure of 6–10 bar. A
glass tube was charged with a magnetic stirrer bar and milled cata-
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gratitude to HEC Pakistan and DAAD for the fellowship.
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Received: September 11, 2013
Published online on October 21, 2013
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