Realizing both selective adsorption and efficient regeneration using adsorbents with photo-regulated molecular gates

Article abstract of DOI:10.1039/c5cc10634f

A new generation of smart adsorbents was designed by grafting photo-responsive molecules onto the pore entrances of mesoporous silica. These molecules act as the gates of the mesopores, which are reversibly closed/opened upon light irradiation. Our smart adsorbents thus realize both selective adsorption and efficient desorption, which is highly expected for adsorption but impossible for traditional adsorbents with fixed pore entrances.

Full text of DOI:10.1039/c5cc10634f

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Realizing both selective adsorption and efficient
regeneration using adsorbents with photo-
regulated molecular gates†
Cite this:DOI: 10.1039/c5cc10634f
Received 29th December 2015,
Accepted 3rd February 2016
Jing Zhu, Jia-Jia Ding, Xiao-Qin Liu,* Peng Tan and Lin-Bing Sun*
DOI: 10.1039/c5cc10634f
www.rsc.org/chemcomm
A new generation of smart adsorbents was designed by grafting
photo-responsive molecules onto the pore entrances of mesoporous
silica. These molecules act as the gates of the mesopores, which are
reversibly closed/opened upon light irradiation. Our smart adsorbents
thus realize both selective adsorption and efficient desorption, which
is highly expected for adsorption but impossible for traditional
adsorbents with fixed pore entrances.
Separation is one of the most important unit operations in the
chemical industry. Being the opposite process of mixing, which
is favored by the second law of thermodynamics, separation is
normally not a spontaneous procedure. Consequently, conven-
tional separation techniques are often somewhat expensive and
energy-intensive. At present, the dominating separation method
is distillation, which consumes more than 50% of the total
production energy.1 Adsorption is well-positioned to play a key
role in the development of many future energy and environ-
mental technologies, because of its low equipment investment,
easy operation, and high energy efficiency.2 The design and
fabrication of efficient adsorbents is crucial for adsorption
processes.3 Micropores and mesopores are two predominant
pore systems for traditional adsorbents.4 Microporous materials
with pore diameters of o2 nm (e.g. zeolites) display excellent
performance in selective adsorption with a high steric effect
(Fig. 1A).5 In this case, only small and properly shaped molecules
can diffuse into the channels, whereas other molecules are
totally excluded. However, the regeneration process of micro-
Fig. 1 (A) Scheme of conventional microporous adsorbents with high adsorp-
tion selectivity but low desorption efficiency, (B) scheme of conventional
mesoporous adsorbents with high desorption efficiency but low adsorption
selectivity, (C) smart adsorbents with photo-regulated molecular gates for
selective adsorption and efficient regeneration, and (D) the schematic structure
of molecular gates when closed and open caused by photo-dimerization
(>310 nm light irradiation) and cleavage (254 nm light irradiation) of a coumarin
derivative.
porous adsorbents is relatively slow and difficult.6 In the case of pore entrances to achieve selectivity in the adsorption process,
mesoporous materials (with pore diameters of 2–50 nm), their while show enlarged pore entrances to facilitate desorption in
large pore entrances allow efficient desorption but at the expense the regeneration process. Unfortunately, traditional adsorbents
of adsorption selectivity (Fig. 1B). From the viewpoint of practical with fixed pore entrances are unlikely to realize both of the
applications, an ideal adsorbent is expected to possess small requirements. Despite great challenges, it is extremely desirable
to develop a new type of adsorbent possessing pore entrances
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
that are adjustable during adsorption/desorption.
Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009,
Inspired by sophisticated biological systems, the implementa-
China. E-mail: liuxq@njtech.edu.cn, lbsun@njtech.edu.cn; Fax: +86-25-83587191;
tion of switchable molecules in artificial systems would lead to a
Tel: +86-25-83587177
variety of advanced stimuli-responsive properties.7 The regulation
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cc10634f
of the behavior of switchable molecules upon external stimuli
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(such as light, temperature, and pH) has attracted great attention,
not only in relation to biological supramolecular machines but
also for the development of materials science.8 In comparison
with other types of external stimuli, light has several additional
advantages.9 It can be used to control the behavior of switchable
molecules remotely, and can be delivered to a precise location,
and applied/removed instantaneously. Consequently, light has
been investigated extensively as a type of external stimulus,
which results in a mass of photo-controlled materials for various
applications.10
Here we report the fabrication of a new generation of
adsorbents by grafting a coumarin derivative 7-[(3-triethoxysilyl)-
propoxy]coumarin (TPC) onto the pore entrances of mesoporous
silica. The TPC molecules act as photo-regulated molecular gates
for the mesopores, which are reversibly switched on and off
upon UV irradiation (Fig. 1C). Upon irradiation using light with
wavelengths >310 nm, intermolecular dimerization takes place
and produces cyclobutane coumarin dimers in anti-head-to-head
configuration (Fig. 1D),11 which obstructs part of the pore
entrances. The sizes of the pore entrances are thus reduced,
through which the adsorbates with smaller diameter can easily
Fig. 2 (A) Low-angle XRD patterns, (B) IR spectra, (C) N2 adsorption–
desorption isotherms, and (D) pore size distributions of MCM-41 before
and after introduction of the molecular gates.
pass but the larger ones are blocked, achieving the selective (XRD) patterns (Fig. 2A), all of the samples show an intense
adsorption of two molecules with different diameters. Further diffraction line accompanied by two weak ones, which can be
irradiation with 254 nm UV light causes photo-cleavage of indexed as the (100), (110), and (200) reflections corresponding
dicoumarins, which makes the molecular gates open, and thus to the 2D hexagonal pore regularity of the p6mm space group.13
the desorption of adsorbates is facilitated. Accordingly, the This indicates that the ordered mesostructure is well preserved
smart adsorbent exhibits the ability to undergo both selective after organic functionalization and template removal. The
adsorption and efficient desorption, which is highly expected for wide-angle XRD patterns of MCM-41 before and after introduc-
adsorptive separation but impossible to realize using traditional tion of the molecular gates present a single broad diffraction
adsorbents with fixed pore entrances.
peak centered at 231 attributed to amorphous silica walls
The gate molecule TPC was first synthesized (Fig. S1, ESI†) (Fig. S5, ESI†). No new diffraction peaks emerge after the
and characterized using 1H nuclear magnetic resonance (NMR) introduction of TPC, suggesting the lack of long-range order
spectra (Fig. S2 and S3, ESI†). The molecular length of TPC, in the arrangement of coumarin molecules. All samples exhibit
attached to silanol groups on mesoporous silica MCM-41 is an N2 adsorption–desorption isotherm of type IV and a narrow
estimated to be 1.30 nm. The photo-dimerization yields cyclo- pore size distribution (Fig. 2C and D), indicating that the
butane coumarin dimers in an anti head-to-head configuration cylindrical mesopores of MCM-41 are well maintained. More-
with a length comparable to the pore size of MCM-41 (2.67 nm). over, the samples before and after irradiation with UV light
To graft as many coumarin molecules as possible onto the pore show identical adsorption behavior for N2 (Fig. S6, ESI†),
entrances, two factors were considered. On the one hand, suggesting that light irradiation does not damage the meso-
as-prepared mesoporous silica (whose pore channels are occluded structure and that N2 adsorption cannot probe the change of
with the template) was used for the grafting of coumarin. On the the pore entrances. Transmission electron microscopy (TEM) is
other hand, a short grafting time (15 min) was adopted to hinder another useful technique to characterize the long-range chan-
the diffusion of coumarin molecules into the pore channels nel ordering of mesoporous materials. The images show that
and the elimination of the template in the reaction solvent. the periodic ordering of the mesostructure is well maintained
Both considerations can ensure that the coumarin molecules (Fig. S7, ESI†). On the basis of the aforementioned results, it is
attach preferentially to the pore entrances rather than the clear that the obtained adsorbents show an ordered meso-
inside walls (Fig. S4, ESI†).12 Then an acidic alcohol solution structure that is comparable to pristine mesoporous silica.
was used to extract the template in the pore channels. The
The successful introduction of the coumarin substituents
resultant materials were denoted by CM-1, CM-2, CM-3, and into mesoporous silica was demonstrated using different char-
CM-4, representing the grafted amounts of 0.3, 1.0, 2.1, and 5.6 acterization techniques. In the infrared (IR) spectra (Fig. 2B),
wt%, respectively (Table S1, ESI†). To confirm the point above, the band at 1700 cmꢀ1 corresponds to the stretching vibration
template-free MCM-41 was employed for grafting. The amount of carbonyl groups, and the bands at 1557 and 1508 cmꢀ1
of coumarin is identical to the sample CM-2, and the obtained derive from the ring CQC vibration of the conjugated coumarin
reference sample is denoted by CM0-2.
ring.14 These results indicate the successful grafting of the
Structural characterization of the samples was performed coumarin substituents onto MCM-41. With increasing amount
using various techniques. In the low-angle X-ray diffraction of coumarin substituent (from CM-1 to CM-4), the characteristic
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388 nm (Fig. 3B).17 Upon irradiation of UV light (>310 nm), the
formation of coumarin dimers led to a decrease in fluorophore
CQC, then the fluorescence intensity weakened gradually. In
addition, the color of the sample changed from white to light
yellow due to the dimerization of the coumarin monomers
(Fig. 3A, inset). These results are consistent with the UV-vis
spectra, thus confirming that the photo-responsive property of
the coumarin molecules is well retained even after immobiliza-
tion on mesoporous silica. Because the coumarin molecules are
located on the pore entrances, they might act as molecular
gates and might be switched on and off via light irradiation.
The photo-responsive property endows the adsorbents with
interesting adsorption performances as shown below.
Fig. 3 (A) UV-vis spectra and (B) fluorescence emission spectra of the
smart adsorbent CM-2 with different irradiation times. Insets in (A) are
typical photographs for CM-2 before and after irradiation.
absorption bands become stronger, which is in line with the
Two dyes with different diameters (Fig. S12, ESI†), namely
TG analysis (Fig. S8, ESI†) and UV-vis spectra (Fig. S9, ESI†). methylene blue (MB, 1.36 nm) and Coomassie brilliant blue
In addition, no peaks related to the template cetyltrimethyl- (CBB, 1.91 nm), were utilized to probe the adsorption perfor-
ammonium bromide (CTAB) are observed, suggesting the efficient mance. The adsorption behavior of the support MCM-41 was
removal of the template via extraction (Fig. S10, ESI†).15 On the first examined. The adsorption amount of the small molecule
basis of the above-mentioned results, it is obvious that the gate MB or the large molecule CBB does not change before/after UV
molecules (TPC) are successfully introduced to the entrances irradiation (Fig. S13A and S14A, ESI†). In term of the equili-
of MCM-41.
brium adsorption capacity of MCM-41, the selectivity of MB
Photo-dimerization of coumarin-modified MCM-41 was first (small molecule) over CBB (large molecule) is calculated to
examined using UV-vis spectroscopy. Coumarin and its deriva- be 2.1. Unlike pristine MCM-41, the coumarin-containing samples
tives show two characteristic absorption bands in their UV-vis (CM-1, CM-2, CM-3, and CM-4) exhibit interesting adsorption
spectra (Fig. 3A). Between 310 and 340 nm, an n - p* like behavior upon light irradiation. Taking CM-2 as an example,
transition is observed, related to the carbonyl function. At the adsorption amount of the small molecule MB is not much
higher energies there is a p - p* transition between 250 and affected upon irradiation with UV light (Fig. 4A). However, after
300 nm corresponding to the conjugated p system.16 UV light UV irradiation, the adsorption amount of the large molecule
irradiation (>310 nm) results in a decrease in the absorption CBB presents a sharp decline of 75.9% (from 10.1 to 2.4 mg gꢀ1
,
between 310 and 340 nm as well as an increase in the absorp- Fig. 4B). A great difference in color is also observable for the
tion between 250 and 300 nm. This is due to the formation of a solutions after adsorption by the adsorbent before and after
cyclobutane ring via the dimerization of the coumarin monomers, irradiation (Fig. 4B, inset). This results in the high selectivity of
and the length of the conjugated p system is reduced. After
irradiation for 240 min, the absorption between 310 and
340 nm almost disappears, while the absorption between 250
and 300 nm becomes quite sharp. The UV-vis spectra (Fig. S11,
ESI†) show that in the case of CM-2, the absorption at 320 nm
almost disappeared after UV irradiation, indicating the efficient
dimerization of coumarin. However, the decrease in absorption
at 320 nm is much smaller for CM0-2 under the same condi-
tions. It is known that for the photo-dimerization of coumarin,
two molecules must be located closely enough to combine their
coumarin parts.12 By use of the short-time grafting method with
template occluded MCM-41, the coumarin molecules are likely
to be densely grafted on the pore entrances, which enables
efficient photo-dimerization. On the contrary, in the case of
CM0-2, the relatively dispersed arrangement of the coumarin
molecules on the pore channels make the photo-dimerization
quite difficult, which is responsible for the small decrease in
absorption at 320 nm. In addition to the UV-vis spectra,
fluorescence emission spectra were also employed to monitor
the photo-dimerization of the coumarin-modified samples.
Coumarin derivatives with fluorophores (e.g. benzene ring,
CQC, CQO, and lactone structure) have strong fluorescence.
Fig. 4 Dynamic adsorption curves of (A) MB and (B) CBB on the smart
adsorbent CM-2 before and after irradiation. (C) Change in the adsorption
amount of the different adsorbents on MB and CBB. (D) Desorption
curves of CBB on the smart adsorbent CM-2 before and after irradiation.
Upon excitation at 330 nm, the fluorescence emission spectra
of coumarin monomers exhibit strong fluorescence emission at
Insets in (B) are the photographs for the initial CBB solution and the
solution after adsorption by CM-2 with or without irradiation.
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MB over CBB on CM-2 after irradiation (11.9), which is much stimuli-responsive groups (e.g. thermo, magnet, and pH)
higher than that before irradiation (2.2). The effect of the TPC as molecular gates, aiming to develop adsorptive separation
content on the adsorption performance was also explored processes with high efficiency and low energy consumption.
(as shown in Fig. S13 and S14, ESI†). The adsorbent CM-2
This work was supported by the Distinguished Youth Founda-
shows the largest difference in the adsorption amount of CBB tion of the Jiangsu Province (BK20130045), the National Natural
(Fig. 4C). Further increasing the content of TPC, the change Science Foundation of China (21576137), the Fok Ying-Tong
amount of CBB can hardly increase after photo-irradiation. In Education Foundation (141069), the National Basic Research
this regard, it is possible that excess coumarin monomers are Program of China (973 Program, 2013CB733504), and the Project
randomly grafted onto the pore entrances or the outer surface of Priority Academic Program Development of Jiangsu Higher
of mesoporous silica. Photo-dimerization of these monomers Education Institutions.
might not affect the size of the pore entrances. Hence, the
suitable content of coumarin for photo-regulated selective
adsorption is about 1.0 wt% (i.e. the sample CM-2).
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