Template-derived carbon: An unexpected promoter for the creation of strong basicity on mesoporous silica

Article abstract of DOI:10.1039/c4cc04742g

Template-derived carbon is demonstrated to effectively promote the creation of strong basicity on mesoporous silica, for the first time. New materials owning ordered mesoporous structure, strong basicity, and excellent catalytic activity are thus successfully constructed at low temperatures, which are impossible to achieve using conventional methods.

Full text of DOI:10.1039/c4cc04742g

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Template-derived carbon: an unexpected
promoter for the creation of strong basicity
on mesoporous silica†
Cite this: Chem. Commun., 2014,
0, 11192
5
Received 23rd June 2014,
Accepted 29th July 2014
Lin-Bing Sun,* Xiao-Yan Liu, Ai-Guo Li, Xiao-Dan Liu and Xiao-Qin Liu
DOI: 10.1039/c4cc04742g
www.rsc.org/chemcomm
Template-derived carbon is demonstrated to effectively promote as support, quite high temperatures are obligatory for such a
the creation of strong basicity on mesoporous silica, for the first well-known method. For instance, the decomposition of the
time. New materials owning ordered mesoporous structure, strong precursor KNO to the basic species K O on silica only takes
3
2
6
basicity, and excellent catalytic activity are thus successfully con- place at temperatures higher than 600 1C. It is no doubt that
structed at low temperatures, which are impossible to achieve using the energy consumption in the synthetic process is quite high.
conventional methods.
Furthermore, the reaction of strongly basic species with silica
supports is almost unavoidable at such high temperatures.
For the development of sustainable chemistry and engineering, Accordingly, the resultant materials only show quite weak
increasing attention has been paid to the replacement of basicity and the mesoporous structure of supports is severely
traditional homogeneous catalytic processes with heterogeneous destroyed. In spite of great efforts, the generation of strong
ones. Amongst the heterogeneous catalysts, mesoporous solid basicity on mesoporous silicas remains an open question.
strong bases are highly fascinating for applications in green and Aiming to create strong basicity on mesoporous silicas with
economic catalytic processes, because of some obvious merits the preservation of ordered mesostructure, it is extremely
such as no corrosion, easy separation, and negligible waste desirable to develop a facile approach to convert precursors
1
production. Their large pore diameters favor mass transfer to a basic species at low temperatures.
and avoid deactivation resulting from coke formation that
Herein, we report an effective strategy to generate strong
usually occurs in microporous catalysts. So far many efforts have basicity on the mesoporous silica SBA-15 by using the particular
been dedicated to fabricate mesoporous materials with strong function of carbon derived from the template. Conventionally,
2
basicity. Due to their low cost and high synthetic controllability, as-prepared mesoporous silica was calcined in an oxygen-
3
mesoporous silicas are considered the ideal choice of support.
containing atmosphere to remove the template completely
It is therefore of great interest to generate strong basicity on before the introduction of precursors (Scheme 1). Instead of
mesoporous silicas ever since their discovery. calcination, the treatment of as-prepared SBA-15 in a nitrogen
Nitrogen incorporation and amino groups grafting offer atmosphere was adopted, leading to the formation of a carbon
interesting approaches to create basic sites on mesoporous layer coated on pores. The reducibility of the carbon layer
4
silicas. However, the basicity of these solid bases is relatively
weak due to the inherent properties of basic sites. In order to
improve base strength, the introduction of strongly basic metal
oxides (such as alkali metal oxides) was tried, which has been
widely used for the generation of strong basicity on a variety of
5
supports. The preparation includes two main steps, namely
(
i) loading of base precursors (for example KNO
tion to decompose precursors to corresponding metal oxides
for example K O). It should be stated that, when silica is used
3
) and (ii) calcina-
(
2
State Key Laboratory of Materials-Oriented Chemical Engineering,
College of Chemistry and Chemical Engineering, Nanjing Tech University,
Nanjing 210009, China. E-mail: lbsun@njtech.edu.cn
3
Scheme 1 Low-temperature conversion of LiNO and creation of strongly
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc04742g
basic sites on mesoporous silica promoted by template-derived carbon.
11192 | Chem. Commun., 2014, 50, 11192--11195
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
3
makes the conversion of the precursor LiNO take place at
temperatures of around 400 1C, which is much lower than that
of conventional thermally induced decomposition of LiNO3
(ca. 600 1C). As a result, strong basicity was successfully created
on mesoporous silica SBA-15 at low temperatures, and the
ordered mesostructure is well preserved, which is impossible
to achieve by means of conventional methods. Of course, this
new strategy can save a lot of energy since the preparation
temperature is greatly decreased. Our results also demonstrate
that the obtained materials are highly active in heterogeneous
transesterification reactions, and the activity is apparently
superior to the catalysts prepared using conventional methods
and typical strongly basic catalysts.
As-prepared SBA-15 was treated under a nitrogen atmo-
sphere at different temperatures, leading to the formation of
carbon-coated samples which were denoted as T S, where x is
x
the treatment temperature (300–900 1C). The carbon content
decreases with the treatment temperature and is 6.7, 4.0, 3.3,
and 1.8 wt% for T300S, T500S, T700S, and T900S, respectively
Fig. 1 TG curves and MS signals of gaseous products from the decom-
3
position of LiNO supported on (A) SBA-15 and (B) T500S.
(Table S1, ESI†). The introduction of carbon does not worsen
the structure of the samples. The preservation of ordered
mesostructure can be proven by three well-resolved diffraction
In the case of pristine SBA-15, the decomposition of LiNO
place predominantly at 580 1C. Surprisingly, on the support T500
containing carbon, most LiNO can be converted at the tempera-
ture as low as 350 1C. The decomposition temperature generally
increases with the decrease of carbon content in the support. For
3
takes
lines observed in low-angle XRD patterns (Fig. S1, ESI†) and N
2
S
adsorption–desorption isotherms of type IV with H1 hysteresis
loops (Fig. S2, ESI†). For the sample treated at 300 1C, a small
amount of template remains as can be seen from IR bands in the
3
À1
range of 2860–3000 cm (Fig. S3, ESI†). When the treatment
example, the majority of LiNO is converted at about 350 1C on
3
temperature is higher than 500 1C, almost all of the template
could be decomposed and/or converted to carbon.
T
300S with a carbon content of 6.7 wt%, while 500 1C on T900
with a carbon content of 1.8 wt%. Anyway, these temperatures are
lower as compared with the decomposition of LiNO on pristine
S
3
The base precursor LiNO was loaded onto the supports TxS
3
by impregnation and activation at 400 1C, leading to the forma-
tion of Li400TxS (where x is the carbonization temperature of
SBA-15 without carbon. The TG results, along with the IR and
XRD data, evidently demonstrate that template-derived carbon
template and varies from 300 to 900). Similarly, LiNO
3
was loaded
acts as an efficient promoter for the conversion of LiNO at very
3
onto pristine SBA-15 and activated at 400 and 600 1C, producing
low temperatures.
Li400S and Li600S, respectively. All LiNO -loaded samples show an
3
À1
The structure of the materials after loading of lithium was
characterized using various methods. The diffraction lines corre-
sponding to a 2D hexagonal pore regularity can be well recognized
in low-angle XRD patterns of carbon-containing samples (Fig. 2
and Fig. S9, ESI†). Despite the absence of carbon in Li400S, the
diffraction lines can be identified, which is due to the majority
apparent vibration band at 1386 cm originated from nitrate
3b
(
Fig. S4, ESI†). Activation at 400 1C only slightly weakens the
band of nitrate on pristine SBA-15. Increasing the temperature to
00 1C results in the disappearance of this band, whereas the
mesoporous structure was entirely destroyed as discussed later.
The introduction of carbon can promote the conversion of LiNO
obviously. The band of nitrate is hard to identify on Li400
6
3
of LiNO not being decomposed on pristine SBA-15 at 400 1C.
3
T500S,
indicating the complete conversion of LiNO at 400 1C in the
3
presence of carbon. However, the band of nitrate is still apparent
on Li400T900S; this is caused by the low carbon content in the
support prepared at the high carbonization temperature of
900 1C. There are no detectable diffraction lines on wide-angle
XRD patterns of LiNO -modifed samples before and after activa-
3
tion at 400 1C (Fig. S5, ESI†). That means that lithium species can
be well dispersed on SBA-15. Nevertheless, on the sample acti-
vated at 600 1C (Li600S), diffraction lines ascribed to Li Si O
2
2 5
(JCPDS No. 72-0102) become visible. This gives evidence of the
formation of a weakly basic compound due to reaction of the
lithium species and siliceous support at high temperatures. TG
and MS techniques provide further information on the conver-
Fig. 2 (A) Low-angle XRD patterns and (B) N
2
adsorption–desorption
sion of LiNO on different supports (Fig. 1 and Fig. S6–S8, ESI†). isotherms of Li400T500S, Li400S, and Li600S samples.
3
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 11192--11195 | 11193

View Article Online
ChemComm
Communication
By raising the temperature to 600 1C, the decomposition of
LiNO can proceed; however, all of the diffraction lines vanish,
3
reflecting the serious destruction of mesostructure. The N2
adsorption–desorption isotherms of carbon-containing materials
are of type IV with H1 hysteresis loops (Fig. 2 and Fig. S10 and S11,
ESI†), indicating the cylindrical mesopores of supports are well
2
maintained. The adsorption amount of N on Li600S is neglectable,
2
À1
corresponding to a quite small surface area of 17 m g (Table S2,
ESI†). TEM provides another useful approach to characterize the
periodic ordering of mesoporous structure. The long-range
channel ordering is observable for carbon-containing materials
Fig. 4 The yield of DMC under the catalysis of Li400T500S, Li400S, and Li600S.
(Fig. S12, ESI†). In terms of these results, it is safe to say that
the ordered mesoporous structure is well preserved in final and has been considered as carbonylating and methylating
8
materials, which can be attributed to the low-temperature agents as well as a promising fuel additive. Conventionally,
conversion of LiNO
3
induced by template-derived carbon.
DMC is synthesized in the presence of homogeneous strong
The basic properties of different materials were first deter- bases; recently, growing attention has been given to the develop-
mined by the amount of basic sites. For Li400S and Li600S, the ment of heterogeneous catalysts. Under the catalysis of material
À1
amount of basic sites is only 0.69 and 0.71 mmol g , respec- without lithium (T500S), no DMC is produced at all even at 4 h
tively, owing to the incomplete decomposition of LiNO and/or (Fig. S14, ESI†). On the materials without carbon, that is Li400
S
3
the reaction of lithium species with siliceous support (Fig. S13, and Li600S, the reactions can occur to some extent and the yield of
ESI†). It is worth noting that, for the material Li400T300S contain- DMC is 12.9% and 10.2%, respectively (Fig. 4). It is worthy of note
À1
ing carbon, the amount of basic sites increases to 2.04 mmol g
.
that the yield of DMC reaches as high as 43.4% on Li400T500S,
À1
The amount of basic sites reaches as high as 2.42 mmolg on which is obviously higher than that on Li400S and Li600S in spite
Li400
T
500S. With the further increase of carbonization tempera- of an identical lithium content. The recyclability of the catalyst
ture, the amount of basic sites diminishes, which is consistent Li400 500S was also examined. After three cycles, the yield of DMC
with the decomposition quantity of LiNO as described above. (41.2%) is slightly lower as compared with that on the fresh
T
3
CO -TPD was further employed to characterize the basic proper- catalyst (43.4%). That means, the present catalyst is able to retain
2
ties of lithium-containing materials. The peaks of CO desorption most of the activity during recycling. To further understand the
2
can be generally divided into two parts at around 200 and 500 1C, catalytic performance, some well-known basic catalyst were
which are tentatively ascribed to weakly and strongly basic sites, employed for comparison. In the presence of typical basic catalyst
respectively (Fig. 3). Both materials without carbon (namely Li400
and Li600S) display a major peak originated from weakly basic typical support (i.e. Al
sites, together with a negligible peak derived from strongly basic bases, which is different with SiO
sites. It is noticeable that the material Li400 500S containing basic sites with support can be avioded. The obtained material
carbon gives rise to an intense peak attributed to strong basicity Li O/Al O gives a DMC yield of 18.8%. A solid superbase CaO/
S
MgO, the yield of DMC is only 7.6%. LiNO
) for the preparation of solid strong
and the reaction of strongly
3
was loaded on a
2 3
O
2
T
2
2 3
besides the one caused by weak basicity. Furthermore, the ZrO was also used as the catalyst, and the yield of DMC is 28.2%.
2
temperature for CO desorption continues up to 720 1C, which It is therefore clear that the present materials show excellent
2
7
is comparable to, if not higher than, some solid superbases.
catalytic performance in the transesterification reaction, which is
Bases on these results, it is conclusive that the template-derived obviously superior to various well-known basic catalysts.
carbon effectively promotes the creation of basic sites with regard
to both amount and strength.
To examine the pathway of LiNO
products formed in the activation process were analyzed by
3
conversion, gaseous
Our materials were also used to catalyze the synthesis of MS. Four signals of m/z = 30, 32, 44, and 46 were detected; they
dimethyl carbonate (DMC) via the transesterification of ethylene are originated from NO, O , CO , and NO , respectively (Fig. 1,
2
2
2
carbonate and methanol. DMC is a versatile green chemical, and Fig. S6–S8, ESI†). The signal of m/z = 15 originated from the
decomposition of copolymer template was also observed in the
gaseous products from the decomposition of LiNO
3
supported on
9
T300S, while absent on the supports carbonized at higher tem-
peratures. This indicates that temperatures higher than 500 1C is
more appropriate aiming to fully carbonize the copolymer tem-
plate. On pristine SBA-15, the main gaseous products from LiNO
3
decomposition are NO and O . The existence of carbon in supports
2
leads to a quite different manner for the conversion of LiNO ; NO
3
and CO
conversion of LiNO
carbon-containing SBA-15 could be depicted respectively through
eqn (1) and (2). The special pathway for LiNO conversion induced
2
become predominant gaseous products. Hence, the
3
to basic species on pristine SBA-15 and
Fig. 3 CO
2
-TPD profiles of Li400T500S, Li400S, and Li600S samples.
3
11194 | Chem. Commun., 2014, 50, 11192--11195
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
by template-derived carbon are believed to account for the make the obtained materials highly promising in applications
much lower conversion temperature.
LiNO - Li O + 2NO (m) + 1.5O
+ 1.5C - Li O + 2NO (m) + 1.5CO
as heterogeneous basic catalysts for various reactions.
This work was supported by the Distinguished Youth Foun-
dation of Jiangsu Province (BK20130045), the Fok Ying-Tong
Education Foundation (141069), and 863 Program (2013AA032003).
2
3
2
2
(m)
(m)
(1)
(2)
2
LiNO
3
2
2
Notes and references
In theory, 2.6 wt% of carbon is required for the conversion of
all supported LiNO (20 wt%) according to eqn (2). TG analysis 1 (a) G. Busca, Chem. Rev., 2010, 110, 2217; (b) X.-Y. Liu, L.-B. Sun,
3
F. Lu, X.-D. Liu and X.-Q. Liu, Chem. Commun., 2013, 49, 8087;
also indicates that the support T500S itself gives a weight loss of
(
c) G. Wu, S. Jiang, L. Li and N. Guan, Appl. Catal., A, 2011, 391, 225.
1
3
.1 wt% during heating. Hence, a minimum carbon content of
.7 wt% is obligatory, which consists of 2.6 wt% of carbon that
2
(a) Z. Y. Wu, Q. Jiang, Y. M. Wang, H. J. Wang, L. B. Sun, L. Y. Shi,
J. H. Xu, Y. Wang, Y. Chun and J. H. Zhu, Chem. Mater., 2006,
18, 4600; (b) L. B. Sun, Y. Chun, F. N. Gu, M. B. Yue, Q. Yu, Y. Wang
reacted with LiNO
process. It is therefore easy to understand that LiNO
completely converted on T500S with a carbon content of 4.0 wt%.
This can also explain that only part of LiNO is converted on T700
3
and 1.1 wt% of carbon that consumed in the
and J. H. Zhu, Mater. Lett., 2007, 61, 2130; (c) L. B. Sun, J. H. Kou,
Y. Chun, J. Yang, F. N. Gu, Y. Wang, J. H. Zhu and Z. G. Zou, Inorg.
Chem., 2008, 47, 4199.
3
can be
3
4
(a) T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature,
3
S
1
999, 402, 867; (b) X.-Y. Liu, L.-B. Sun, X.-D. Liu, A.-G. Li, F. Lu and
and T900S, since their carbon contents are lower than 3.7 wt%.
It is noticeable that the carbon content of T300S (6.7 wt%) is
X.-Q. Liu, ACS Appl. Mater. Interfaces, 2013, 5, 9823; (c) Y. Deng, J. Wei,
Z. Sun and D. Zhao, Chem. Soc. Rev., 2013, 42, 4054.
(a) S. Y. Chen, C. Y. Huang, T. Yokoi, C. Y. Tang, S. J. Huang, J. J. Lee,
J. C. C. Chan, T. Tatsumi and S. Cheng, J. Mater. Chem., 2012,
higher than 3.7 wt%, while supported LiNO cannot be comple-
3
tely converted either. The carbonization of template P123 is still
22, 2233; (b) Y. D. Xia and R. Mokaya, Angew. Chem., Int. Ed., 2003,
in its infancy at 300 1C, and some preliminary carbonization
products, which are unfavourable to LiNO conversion, may
3
42, 2639; (c) K. Sugino, N. Oya, N. Yoshie and M. Ogura, J. Am. Chem.
Soc., 2011, 133, 20030.
(a) R. Sundararaman and C. S. Song, Appl. Catal., B, 2014, 148, 80;
5
contribute to the carbon content. It is therefore conclusive that
template-derived carbon can efficiently promote the conversion
of LiNO at low temperatures. Moreover, the conversion of LiNO
(
b) L.-B. Sun, Y.-H. Li, Y. Yin, X.-L. Wu and X.-Q. Liu, Curr. Org. Chem.,
2013, 17, 2249; (c) L. B. Sun, J. Yang, J. H. Kou, F. N. Gu, Y. Chun,
Y. Wang, J. H. Zhu and Z. G. Zou, Angew. Chem., Int. Ed., 2008,
3
3
47, 3418.
is well related to basicity and subsequently, catalytic performance
of resultant materials.
6
7
8
L. B. Sun, F. N. Gu, Y. Chun, J. Yang, Y. Wang and J. H. Zhu, J. Phys.
Chem. C, 2008, 112, 4978.
T.-T. Li, L.-B. Sun, L. Gong, X.-Y. Liu and X.-Q. Liu, J. Mol. Catal. A:
Chem., 2012, 352, 38.
(a) M. Honda, A. Suzuki, B. Noorjahan, K.-i. Fujimoto, K. Suzuki and
K. Tomishige, Chem. Commun., 2009, 4596; (b) J. Xu, H.-T. Wu,
C.-M. Ma, B. Xue, Y.-X. Li and Y. Cao, Appl. Catal., A, 2013,
In summary, we develop a strategy to convert base precursors
by use of the reducibility of carbon, which originates from
template that is usually removed as a useless thing after the
formation of mesopores. This strategy can save lots of energy
due to the decreased preparation temperature. The ordered
mesostructure, strong basicity, and excellent catalytic activity
464–465, 357.
9
F. B ´e rub ´e and S. Kaliaguine, Microporous Mesoporous Mater., 2008,
115, 469.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 11192--11195 | 11195