Increasing the H+ exchange capacity of porous titanium phosphonate materials by protecting defective P-OH groups

Article abstract of DOI:10.1039/c1cc11583a

Hierarchically macro-/mesoporous titanium phosphonates with enlarged H ⁺ exchange capacity were synthesized in the presence of a series of alkyl amines that acted as protective groups for the defective P-OH, which were used as promising solid acid catalysts to replace conventional liquid acid catalysts and acidic resins in some acid-catalytic reactions.

Full text of DOI:10.1039/c1cc11583a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 6015–6017
www.rsc.org/chemcomm
COMMUNICATION
Increasing the H+ exchange capacity of porous titanium phosphonate
materials by protecting defective P–OH groupsw
Tian-Yi Ma, Lei Liu, Qing-Fang Deng, Xiu-Zhen Lin and Zhong-Yong Yuan*
Received 18th March 2011, Accepted 7th April 2011
DOI: 10.1039/c1cc11583a
Hierarchically macro-/mesoporous titanium phosphonates with
enlarged H+ exchange capacity were synthesized in the presence
of a series of alkyl amines that acted as protective groups for the
defective P–OH, which were used as promising solid acid
catalysts to replace conventional liquid acid catalysts and acidic
resins in some acid-catalytic reactions.
materials to be outstanding acid catalysts.9 In this contri-
bution, hierarchically macro-/mesoporous titanium phospho-
nate materials were synthesized by a one-pot template-free
method, constructed from diethylenetriamine penta(methylene
phosphonic acid) (DTPMPA, Scheme S2, ESIw). The
preparation was carried out in the presence of a series of alkyl
amines with different concentrations, acting as protective
groups during the condensation process to increase the
defective P–OH content (Scheme 1). The designed porous
titanium phosphonates with large H+ exchange capacity were
further used as acid catalysts in the synthesis of methyl-2,3-o-
isopropylidene-b-D-ribofuranoside, the key intermediate for the
preparation of several important nucleosides, which
exhibit a variety of biological functions.10
Mesoporous metal phosphate and phosphonate materials
have become new potential candidates for ion exchange and
acid catalysis due to their advantages of accessible ion-
exchange capacities,1,2 well-defined pore structures with large
specific surface areas and some hierarchical micro-architectures,3
and high thermal stability,4 which are superior to the traditional
polymer ion-exchange resins. However, the H+ exchange
capacity of metal phosphates and phosphonates was usually
limited, because the H+ exchange of these materials is due to
the defective P–OH groups in the resultant solids,5 while the
condensation between P–OH and metal ions during the prepara-
tion process often results in the extensive formation of P–O–Me
(Me = Ti, Zr, V, Al, etc.) bonding mode instead of defective
P–OH.6 Thus it is urgent from a technical viewpoint to develop
an efficient way to increase the defective P–OH concentration in
metal phosphates and phosphonates for the improvement of the
H+ exchange capacity, namely, the acid content.
The condensation between DTPMPA and tetrabutyl
titanate was performed in the presence of n-propylamine,
n-butylamine and n-pentylamine with different concentrations
(ESIw), and the obtained porous titanium phosphonates were
denoted as TiPPh-amine. In comparison, the sample synthe-
sized in the absence of alkyl amine was denoted as TiPPh-non.
As shown in Scheme 1, the reversible reaction between alkyl
amines and P–OH groups could greatly increase the defective
P–OH in the resultant solids, which could be proved when
appropriate amounts of n-butylamine (amine/DTPMPA
molar ratio of 0.3) were added into the reaction system with
increase of P/Ti molar ratios (Fig. 1, left). In the absence of
amines, although the P/Ti molar ratio in the obtained solids
increased by increasing the added P/Ti ratio in raw materials,
due to the limit of coordination ability of Ti4+ ions with
phosphonic acids,4 the P/Ti ratio reached a plateau of
1.35–1.51 when the added P/Ti ratio was larger than 1.75.
Metal phosphonates stand for an important family of
organic–inorganic hybrid materials, which have recently
attracted much attention because of the combination of
properties due to both inorganic and organic moieties.7 The
integration of bridging organic groups into the inorganic
component framework could not only adjust the hydrophilic/
hydrophobic nature of the pore surface but also introduce
functional sites into the hybrid network.8 Therefore, the
host–guest selectivity between the organophosphonate groups
and the substrates, which hardly exists in purely inorganic
metal phosphate materials, and the inherent acidity
from P–OH defects have determined the metal phosphonate
Institute of New Catalytic Materials Science, Key Laboratory of
Advanced Energy Materials Chemistry (Ministry of Education),
College of Chemistry, Nankai University, Tianjin 300071, China.
E-mail: zyyuan@nankai.edu.cn; Fax: +86 22 23509610;
Tel: +86 22 23509610
w Electronic supplementary information (ESI) available: Experimental
details, N2 sorption, XRD, FT-IR spectra, Hammett acidities and
recycle profiles of the catalysts. See DOI: 10.1039/c1cc11583a
Scheme 1 Alkyl amine-assisted preparation of titanium phosphonates.
Chem. Commun., 2011, 47, 6015–6017 6015
c
This journal is The Royal Society of Chemistry 2011

View Article Online
Fig. 1 P/Ti molar ratios in the obtained solids and H+ exchange
capacities versus added P/Ti ratios in the raw materials (n-butylamine/
DTPMPA molar ratio of 0.3) (left). H+ exchange capacities of titanium
phosphonates synthesized in the presence of various alkyl amines with
different concentrations (added P/Ti molar ratio of 2.5) (right).
Fig. 2 FT-IR (left) and MAS NMR (right) spectra of (a) titanium
phosphonates synthesized without amines, and titanium phosphonates
synthesized with n-butylamines (b) before and (c) after HCl extraction.
bands at 1462 and 1433 cmÀ1 are assigned as the C–H
bending in –CH2– groups and the P–C stretching vibrations,
respectively. The small bands at 1323 cmÀ1 could be attributed
to C–N stretching.4 No bands at around 1380 or 1137 cmÀ1
were found for phosphoryl (PQO) frequency, indicating the full
condensation between phosphoryl and Ti atoms.11 In addition,
several bands around 2900–3000 cmÀ1 are assigned to the C–H
stretching modes. The band at 970 cmÀ1 assigned to P–OH
stretching vibrations11,12 was not observed in TiPPh-non
(Fig. 2a), showing the low content of defective P–OH groups
in TiPPh-non, which was enveloped in the strong band at
1050 cmÀ1 (P–OÁ Á ÁTi). A shoulder peak is observed at around
970 cmÀ1 for the as-synthesized TiPPh-butylamine sample
before HCl extraction (Fig. 2b), but transformed into a
newborn peak after HCl extraction (Fig. 2c), indicating a large
amount of P–OH defects.11,12 Noticeably, the increase of C–H
stretching strength at 2900–3000 cmÀ1 and C–N stretching
strength at 1323 cmÀ1, and the existence of N–H bending
vibrations in NH2– groups at 845 cmÀ1 (Fig. 2b) also confirmed
the participation of alkyl amines.13 Correspondingly, the 31P
MAS NMR spectrum of TiPPh-non shows one broad signal
around 21.5 ppm (Fig. 2a), which is in the area characteristic of
phosphonates,6,8 while the spectrum of TiPPh-butylamine
(Fig. 2c) shows two resonance signals of the phosphorus nuclei
at 21.3 and 29.2 ppm, which could be assigned to P atoms in
–RPO3Ti3 and in the mixture of –RPO3H2Ti and –RPO3HTi2,
respectively.
In the presence of amines, the P/Ti ratio of obtained solids also
exhibited a sharp initial rise, and finally reached a plateau of
1.59–1.80, which was higher than that without amines added.
Correspondingly, a similar tendency could be observed for H+
exchange capacity of the synthesized materials. The highest
H+ exchange capacities could be confirmed as 2.44–2.79 and
5.51–5.80 mmol gÀ1 for the samples synthesized without and
with n-butylamine assistance, respectively, which demon-
strated that the presence of alkyl amines in the reaction system
could efficiently increase the P/Ti molar ratio, more impor-
tantly the H+ exchange capacity, of resultant materials. This
could be explained by the fact that the alkyl amines first
partially occupied P–OH sites by acid–base reactions, followed
by the condensation between the added alkoxides and residual
P–OH and PQO groups (Scheme 1). And the extraction with
HCl would finally release P–OH defects of the resultant solids,
leading to the high H+ exchange capacity and acid content.
However, the enhancement effect was hardly detected when
NH3ÁH2O was used instead of alkyl amines, probably because
NH3 was too volatile to form relatively stable bonding with
P–OH in the competitive reaction with alkoxides. The
observed H+ exchange capacities of 5.51–5.80 mmol gÀ1 in
resultant samples are much larger than those in the previously
reported titanium phosphates5 (1.7–3.4 mmol gÀ1
) and
titanium phosphonates2 (3.93 mmol gÀ1). Furthermore, when
the added P/Ti molar ratio was fixed at 2.5, which was within
the plateau range mentioned above, and various amines were
employed into the system with different concentrations, it
could be seen in Fig. 1 (right) that the H+ exchange capacities
increased when the amine concentration (amino/DTPMPA
molar ratio) is in the range from 0.05 to 0.25, and kept stable
from 0.25 to 0.35. The enhancement effect provided by
n-butylamine and n-pentylamine was approximate but greater
than that by n-propylamine. Thus titanium phosphonate
material synthesized with an added P/Ti ratio of 2.5 and
n-butylamine/DTPMPA ratio of 0.3 (TiPPh-butylamine) was
chosen here for the following discussions.
The morphology, pore structure and texture properties of
the synthesized TiPPh-amine catalyst were then characterized.
SEM images revealed a channel-like macroporous structure
with a uniform pore diameter distribution of 500–1000 nm
(Fig. 3a). The macrochannels are mainly of one-dimensional
orientation, parallel to each other, perforative through almost
the entire particle (Fig. 3a, inset). It is revealed by the TEM
observation (Fig. 3b) that the walls of the macroporous network
are composed of accessible wormhole-like mesopores. The N2
sorption analysis also indicated the hierarchical porosity with a
surface area of 269 m2 gÀ1, a pore volume of 0.22 cm3 gÀ1 and a
pore size of around 5.1 nm (Fig. S1a, ESIw).
FT-IR and MAS NMR measurements were employed to
further confirm the functions of alkyl amine assistance. The
FT-IR spectra of TiPPh-butylamine and TiPPh-non are shown
in Fig. 2. The strong broad band at 3400 cmÀ1 and the sharp
band at 1642 cmÀ1 correspond to the surface-adsorbed water
and hydroxyl groups. The strong band at 1050 cmÀ1 is due to
phosphonate P–OÁ Á ÁTi stretching vibrations.4,6 The overlapped
The large H+ exchange capacity of titanium phosphonates
synthesized with amines assistance could make these materials
act as practical acid catalysts potentially useful in many
reactions. The condensation reaction of D-ribose with acetone
and methanol to produce methyl-2,3-o-isopropylidene-b-D-
ribofuranoside was chosen herein, which was catalyzed by
c
6016 Chem. Commun., 2011, 47, 6015–6017
This journal is The Royal Society of Chemistry 2011

View Article Online
noticeable that with both catalysts having a similar pore
structure, acid content and strength, the organic–inorganic
hybrid TiPPh-butylamine catalyst has a higher yield (+5.9%)
than the purely inorganic TiP-butylamine catalyst, which
might be explained by the strong affinity between the organic
motifs in the hybrid network of titanium phosphonates and
the substrates.2,15 The conc. HCl catalyst showed the yield of
26.2% after refluxing for 3 h; but by extending the refluxing
time to 18 h, the yield while using the HCl catalyst could be
increased to 50%,14 similar to the yield while using the
TiPPh-butylamine catalyst after refluxing for 3 h (48.7%),
which indicated the high reaction rate by using phosphonate-
based catalysts. After 10-time reuse, the product yield was
hardly decreased (Fig. S2, ESIw). Thus the TiPPh-butylamine
material was proved to be an efficient acid catalyst with
desirable stability and good recyclability, which could replace
the conventional acid catalysts like conc. HCl and acidic
resins in this reaction shorten the reaction time and over-
come disadvantages of homogeneous catalysts such as
being difficult to separate and recover and harmful to the
environment.
Fig. 3 SEM (a) and TEM (b) images of the TiPPh-butylamine
material.
some homogenous catalysts such as H2SO4 and HCl
(Scheme S1, ESIw).14 Porous titanium phosphate (TiP-butylamine)
was also synthesized with the use of phosphoric acid instead of
DTPMPA in the presence of n-butylamine. TiPPh-butylamine,
TiP-butylamine, TiPPh-non, previously reported ordered
mesoporous titanium phosphonates constructed by ethylene-
diamine tetra(methylene phosphonic acid) (PMTP-1),4
commercial acidic resin NKC-9 and concentrated HCl
(conc. HCl) were all tested for a 3 h reaction (Table 1). A
scarce product was obtained when catalyzed by TiPPh-non
and PMTP-1 due to their low acid content (2.54 and
2.93 mmol gÀ1) and acid strength (H0 4 À8.20, Table S1,
ESIw). However, with the assistance of alkyl amines, both the
acid contents of TiPPh-butylamine (5.76 mmol gÀ1) and
TiP-butylamine (5.71 mmol gÀ1), and the acid strength
(H0 o À8.20) were increased, leading to high yields of 48.7
and 42.8% for methyl-2,3-o-isopropylidene-b-D-ribofurano-
side, respectively. The yields were even higher than that of
NKC-9 (33.0%) with the acid content of 4.33 mmol gÀ1, which
could be attributed to the low surface area of NKC-9
(12 m2 gÀ1). Thus under the present experimental conditions,
the catalytic ability of these solid acids was not only related to
the H+ exchange capacity (acid content) but also to the acid
strength. For the solid catalysts with H0 4 À8.20 (TiPPh-non,
PMTP-1), the condensation reaction shown in Scheme S1
(ESIw) could hardly proceed; for the solid catalysts with
H0 o À8.20 (TiPPh-butylamine, TiP-butylamine, NKC-9),
the condensation reaction could proceed and the yields were
further determined by the acid contents and pore structure
(surface area, pore volume, etc.) of the materials. It is
This strategy is supposed to open a new area in the
development of outstanding phosphonate-based acid catalysts
and ion exchangers by the defect protection method. Only
small amounts of alkylamines were necessary instead of other
expensive additives, and the facile one-pot preparation process
also makes the present acid catalyst practical in some reactions
in the chemical industry. Moreover, because the P–OH groups
serve as carriers of protons, these materials may also find use
as an electrolyte for fuel cells.
This work was supported by the National Natural Science
Foundation of China (20973096 and 21073099), the National
Basic Research Program of China (2009CB623502), and the
Ministry-of-Education Program for Innovative Research
Team in University (IRT0927).
Notes and references
1 D. J. Jones, G. Aptel, M. Brandhorst, M. Jacquin, J. Jime
Jimenez, A. Jimenez-Lopez, P. Maireles-Torres, I. Piwonski,
E. Rodrıgues-Castellon, J. Zajac and J. Roziere, J. Mater. Chem.,
2000, 10, 1957.
´
nez-
´
´
´
´
´
2 T. Y. Ma and Z. Y. Yuan, Chem. Commun., 2010, 46, 2325.
3 T. Z. Ren, Z. Y. Yuan, A. Azioune, J. J. Pireaux and B. L. Su,
Langmuir, 2006, 22, 3886.
4 T. Y. Ma, X. Z. Lin and Z. Y. Yuan, J. Mater. Chem., 2010, 20,
7406.
5 A. Bhaumik and S. Inagaki, J. Am. Chem. Soc., 2001, 123, 691.
6 T. Kimura, Chem. Mater., 2003, 15, 3742.
7 C. Sanchez, G. J. de A. A. Soler-Illia, F. Ribot, T. Lalot,
C. R. Mayer and V. Cabuil, Chem. Mater., 2001, 13, 3061.
8 T. Kimura, K. Kato and Y. Yamauchi, Chem. Commun., 2009,
4938.
9 P. H. Mutin, G. Guerrero and A. Vioux, J. Mater. Chem., 2005,
15, 3761.
10 P. Sairam, R. Puranik, B. S. Rao, P. V. Swamy and S. Chandra,
Carbohydr. Res., 2003, 338, 303.
11 M. Vasylyev, E. J. Wachtel, R. Popovitz-Biro and R. Neumann,
Chem.–Eur. J., 2006, 12, 3507.
12 G. Guerrero, P. H. Mutin and A. Vioux, Chem. Mater., 2000, 12,
1268.
13 R. Guillet-Nicolas, L. Marcoux and F. Kleitz, New J. Chem., 2010,
34, 355.
14 A. G. M. Barrett and S. A. Lebold, J. Org. Chem., 1990, 55, 3853.
15 A. P. Wight and M. E. Davis, Chem. Rev., 2002, 102, 3589.
Table 1 Comparison of different acid catalysts
SBETa/ Vpore
/
Dporec/ H+ exchange
capacity/mmol gÀ1 (%)
Yield
b
Sample
m2 gÀ1 cm3 gÀ1 nm
TiPPh-butylamine 269 0.22
5.1
5.1
5.2
2.8
—
5.76
2.54
5.71
2.93
4.33
—
48.7
1.3
42.8
2.2
33.0
26.2
TiPPh-non
TiP-butylamine
PMTP-1d
242 0.21
281 0.23
1066 0.83
12 0.02
NKC-9e
Conc. HCl
—
—
—
a
BET surface area calculated from the linear part of the 10-point BET
b
plot. Single point total pore volume of pores at P/P0 = 0.97.
c
Estimated using the adsorption branch of the isotherm by the BJH
d
method. Periodic mesoporous titanium phosphonate materials
e
(see ref. 4). The commercial acidic resin.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6015–6017 6017