Effect of polyvinylpyrrolidone on mesoporous silica morphology and esterification of lauric acid with 1-butanol catalyzed by immobilized enzyme

Article abstract of DOI:10.1016/j.jssc.2014.03.004

Mesoporous silica materials with a range of morphology evolution, i.e., from curved rod-shaped mesoporous silica to straight rod-shaped mesoporous silica, were successfully prepared using polyvinylpyrrolidone (PVP) and triblock copolymer as dual template. The effects of PVP molecular weight and concentration on mesoporous silica structure parameters were studied. Results showed that surface area and pore volume continuously decreased with increased PVP molecular weight. Mesoporous silica prepared with PVP K30 also possessed larger pore diameter, interplanar spacing (d₁₀₀), and cell parameter (a₀) than that prepared with PVP K15 and PVP K90. In addition, with increased PVP concentration, d₁₀₀ and a₀ continuously decreased. The mechanism of morphology evolution caused by the change in PVP concentration was investigated. The conversion rate of lauric acid with 1-butanol catalyzed by immobilized Porcine pancreatic lipase (PPL) was also evaluated. Results showed that PPL immobilized on amino-functionalized straight rod-shaped mesoporous silica maintained 50% of its esterification conversion rate even after five cycles of use with a maximum conversion rate was about 90.15%.

Full text of DOI:10.1016/j.jssc.2014.03.004

Journal of Solid State Chemistry 213 (2014) 210–217
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Effect of polyvinylpyrrolidone on mesoporous silica morphology
and esterification of lauric acid with 1-butanol catalyzed
by immobilized enzyme
n
Jinyu Zhang, Guowei Zhou , Bin Jiang, Minnan Zhao, Yan Zhang
Key Laboratory of Fine Chemicals in Universities of Shandong, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology,
Jinan 250353, Shandong, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Mesoporous silica materials with a range of morphology evolution, i.e., from curved rod-shaped
mesoporous silica to straight rod-shaped mesoporous silica, were successfully prepared using poly-
vinylpyrrolidone (PVP) and triblock copolymer as dual template. The effects of PVP molecular weight and
concentration on mesoporous silica structure parameters were studied. Results showed that surface area
and pore volume continuously decreased with increased PVP molecular weight. Mesoporous silica
prepared with PVP K30 also possessed larger pore diameter, interplanar spacing (d100), and cell
parameter (a0) than that prepared with PVP K15 and PVP K90. In addition, with increased PVP
concentration, d100 and a0 continuously decreased. The mechanism of morphology evolution caused
by the change in PVP concentration was investigated. The conversion rate of lauric acid with 1-butanol
catalyzed by immobilized Porcine pancreatic lipase (PPL) was also evaluated. Results showed that PPL
immobilized on amino-functionalized straight rod-shaped mesoporous silica maintained 50% of its
esterification conversion rate even after five cycles of use with a maximum conversion rate was about
90.15%.
Received 19 November 2013
Received in revised form
26 February 2014
Accepted 2 March 2014
Available online 12 March 2014
Keywords:
Morphology evolution
Polyvinylpyrrolidone
Curved mesoporous silica
Immobilized lipase
Esterification
& 2014 Elsevier Inc. All rights reserved.
1. Introduction
hierarchical compounds [13–15], membranes [16], fibers [17–21],
and core–shell mesoporous nanoparticles [22–24]. Many studies
Organic acid esters are widely used as chemical intermediates
for the synthesis of biodiesel [1,2], textile, cosmetic, flavor, and
fragrant compounds because of their great practical applications in
various industries. Generally, esterification reactions of carboxylic
acids with alcohols are commercially catalyzed using heteroge-
neous [1–4] and homogeneous catalysts [5,6]. Immobilized
enzymes, as heterogeneous catalysts, currently attract consider-
able attention because of their considerable physiological and
industrial significance [3,4,7]. These enzymes have many advan-
tages, such as high selectivity and specificity for substrate, high
stability, reusability, and applicability to extreme conditions [8].
However, the stability and reusability of immobilized enzymes are
closely linked with a suitable support.
have reported mesoporous silica materials with the morphology of
hollow spheres [23,24], spheres [25], and fibers by PVP-assisted
reaction. However, only a few works have described mesoporous
silica rod prepared using PVP as template. Zhu et al. [24] success-
fully synthesized hollow mesoporous silica spheres with uniform
size and morphology, where PVP contributes to the formation of a
hollow structure. Rod-shaped mesoporous silica with open-
framework structure have received much attention in the past
decade as potential materials for drug delivery systems, catalyst
supports, and sensors because of their ordered mesochannels, high
BET surface area, large pore volume, uniform porosity, stable
aqueous dispersion, and excellent biocompatibility [26,27]. Huang
et al. [28] proved that rod-shaped mesoporous silica nanoparticles
are superior to their spherical counterparts in drug delivery
applications. Rod-shaped mesoporous silica are advantageous over
spherical ones. For example, rod-shaped mesoporous silica with
short pore channels possess remarkably improved immobilization
abilities for enzymes because of the doubled open mouth at the rod
terminal, short diffusion paths, fast adsorption, and ease in mass
transfer. Butyl laurate is mainly used to prepare flavors and
fragrances because of its fruit- or peanut-like aroma. Brahmkhatri
et al. [29] synthesized butyl laurate catalyzed by H3PW12O40
Ordered mesoporous silica has been proposed as a suitable
support for enzyme immobilization [9] and is synthesized with a
surfactant as template using supramolecular-template methods.
Polyvinylpyrrolidone (PVP) is a common non-ionic surfactant that
can be used to form metal–inorganic composite particles [10–12],
n
Corresponding author. Tel.: þ86 531 89631696.
E-mail addresses: guoweizhou@hotmail.com, gwzhou@qlu.edu.cn (G. Zhou).
http://dx.doi.org/10.1016/j.jssc.2014.03.004
0022-4596/& 2014 Elsevier Inc. All rights reserved.

J. Zhang et al. / Journal of Solid State Chemistry 213 (2014) 210–217
211
supported on MCM-41. Barros et al. [30] also studied the esterfica-
tion of lauric acid with butanol over mesoporous materials. Results
showed that ZnO/SBA-15 and MgO/SBA-15 led to high esterifica-
tion yields at ambient pressure.
and NH-SRMS-S2-PPL. The amount (P, mg gꢀ1) of PPL immobiliza-
tion and activity (Ea, U gꢀ1) of immobilized PPL was obtained.
2.4. Esterification procedure
In this study, mesoporous silica materials including curved rod-
shaped mesoporous silica (CRMS) and straight rod-shaped meso-
porous silica (SRMS) were successfully synthesized using PVP and
P123 as co-template by varying different reaction parameters such
as the mole ratio of PVP to P123 and the molecular weight of PVP.
We investigated the characteristics of mesoporous silica size,
morphology, and structure, which significantly influenced its
physical and chemical properties, as well as its applications in
enzyme immobilization. Esterification of lauric acid with 1-
butanol catalyzed by immobilized Porcine pancreatic lipase (PPL)
(SRMS-S2-PPL and NH-SRMS-S2-PPL) was then investigated.
Ester synthesis in n-hexane (10 mL) and 3 mL of phosphate
buffer solution (pH 7.0) was carried out in a 100 mL three-necked
flask using lauric acid (0.2 mol Lꢀ1
, 0.4 g) and 1-butanol
(0.4 mol Lꢀ1, 0.296 g) as substrates. SRMS-S2-PPL or NH-SRMS-
S2-PPL (0.06 g) was also added to the mixture. Esterification was
then carried out at 313 K with constant stirring using a magnetic
stirrer.
Results of the reaction were determined by measuring the lauric
acid conversion rate (α, %) according to reference reports [34].
2.5. Characterization
2. Experimental
Small-angle X-ray powder diffraction (SAXRD) patterns were
recorded in the 2 range 0.51–31 at a step size of 0.021 with a
Bruker D8 advance diffractometer, using CuK radiation (30 kV,
30 mA,
¼0.1541 nm). Field emission scanning electron micro-
θ
2.1. Materials
α
λ
scopy (FESEM) micrographs were observed by a SUPRATM 55
microscope operating at an accelerating voltage of 5 kV. Transmis-
sion electron microscopy (TEM) micrographs were obtained on a
JEM-1400 microscope with an acceleration voltage of 120 kV.
The dry-weight-based C, H, and N contents were determined
using a Vario ELIII elemental analyzer with the oxygen content
calculated by the mass difference. N2 adsorption–desorption
apparatus (Micromeritics, TriStar 3020) was used to determine
surface areas, pore volumes, and pore size distributions.
The surface areas were calculated by the Brunauer–Emmett–Teller
(BET) method. The pore diameters were estimated from the
desorption branches of the isotherms based on the Barrett–Joy-
ner–Halenda (BJH) model. Small-angle X-ray scattering (SAXS)
experiments were performed using a SAXSess mc2 system (Anton
Poly(ethylene glycol)-block-poly(propyleneglycol)-block-poly
(ethylene glycol) (P123), PPL, (3-aminopropyl)triethoxysilane
(APTES) (98%), bovine serum albumin (BSA), Coomassie brilliant
blue G-250, and triacetin (C9H14O6, 99%) were purchased from
Sigma-Aldrich Co. Polyvinylpyrrolidone (PVP (K15, K30, and K90)),
tetraethoxysilane (TEOS), and lauric acid were used as received
from Sinopharm Chemical Reagent Co., Ltd. Other chemicals were
of analytical grade and were all obtained from Tianjin Chemical
Agent Co. (China).
2.2. Synthesis of mesoporous silica materials and
amino-functionalization
CRMS and SRMS were prepared by tuning the molar ratio of the
two structure-directing agents to obtain ordered hexagonal-like
mesoporous channels and rod-shaped silica. To obtain an appro-
priate concentration ratio, the concentration of PVP (K30) was
varied while fixing the amount of P123. The molar ratio of
TEOS:P123:PVP:HCl:H2O was 1:0.01671:(0, 0.0116, 0.058, 0.116,
0.174):5.814:190.0, and the samples were designated as CRMS-S0,
CRMS-S1, SRMS-S2, SRMS-S3, and CRMS-S4. Samples synthesized at
a molar ratio of 1:0.116 (P123:PVP) were designated as SRMS-S5
(PVP K15) and CRMS-S6 (PVP K90). About 1.0 g of P123 was placed
at the bottom of a beaker and mixed with 30.0 g of aqueous HCl
solution (2.0 mol Lꢀ1) and 7.5 g of deionized water. After initial
stirring for 2 h, the solution became clear. Then, 2.15 g of TEOS was
added at a stirring rate of 500 rpm throughout the reaction period.
Certain amounts of PVP were then sequentially added to the
solution, which was stirred at 308 K for 24 h. The solution along
with the produced precipitate was transferred to an autoclave and
heated at 373 K for another 24 h. The product was filtered, washed
three times with deionized water, and calcined at 823 K in flowing
air for 6 h to remove surfactant. The functional group used to
functionalize CRMS and SRMS came from APTES, and the functio-
nalization procedure was performed by postsynthesis-grafting
method [31–33]. The obtained samples were designated as
NH-CRMS or NH-SRMS (NH-CRMS-S0, NH-CRMS-S1, NH-SRMS-S2,
NH-SRMS-S3, NH-CRMS-S4, NH-SRMS-S5, and NH-CRMS-S6).
Paar) with a CuK
α
radiation (0.1542 nm) operating at 40 kV and
50 mA. The sample-to-detector distance was 0.26 m, and the test
temperature was 308 K. Scattering curves of P123/PVP/SiO2 sols
with different PVP concentrations were recorded for q ranging
from 0.2 nmꢀ1 to 1.5 nmꢀ1
.
3. Results and discussion
3.1. The structural characteristics
3.1.1. SAXRD analysis
Fig. 1a shows the SAXRD patterns of samples synthesized with
different P123:PVP molar ratios. All samples had three well-
resolved Bragg diffraction peaks unambiguously indexed as the
(1 0 0), (1 1 0), and (2 0 0) reflection of a hexagonal symmetry
structure (p6mm) [35,36]. The interplanar spacings (d100) values
and cell parameter (a0¼2d/31/2) are summarized in Table 1. With
increased PVP concentration, diffraction peak position moved to a
higher angle side and relative intensities increased. However, the
relatively wide and similar peak of the sample synthesized
with P123 as a template was located at the highest angle,
indicating relatively poor structural ordering. In addition, the
structural ordering of samples templated by mixed P123/PVP
surfactants increased, and a0 and d100 continuously decreased
with increased PVP. The SAXRD patterns of amino-functionalized
samples are shown in Fig. 1b. All samples had three peaks
and structural ordering, and a0 and d100 decreased with increased
PVP, indicating that the hexagonal symmetry structure did not change
after functionalization. The structural ordering of amino-functionalized
samples was reduced to a certain extent. Compared with CRMS-S0,
2.3. Immobilization of PPL and activity assays
PPL immobilization onto SRMS-S2 or NH-SRMS-S2 and activity
assay were carried out according to our previous work [8]. Final
samples after PPL immobilization were designated as SRMS-S2-PPL

212
J. Zhang et al. / Journal of Solid State Chemistry 213 (2014) 210–217
NH-CRMS-S0 had larger d100 and a0, and other samples exhibited the
opposite. In addition, compared with PVP (K15, 10000) and PVP (K90,
360000), SRMS-S3 synthesized with PVP (K30, 40000) had narrower
peaks (Fig. S1, Supporting information), indicating better structural
ordering. SRMS-S2 also had narrower peaks than CRMS-S1, SRMS-S3,
and CRMS-S4. The peaks of NH-SRMS-S2 were also narrower than
other functionalized samples. Thus, a molar ratio of 1:0.058 and PVP
(K30) were the best conditions.
3.1.2. TEM images
CRMS and SRMS exhibited arrays of long-range mesoporous
channels with an average pore diameter of around 5 nm to 7 nm
(Fig. 2), consistent with nitrogen adsorption–desorption measure-
ments. The morphology of CRMS-S1, CRMS-S4, and CRMS-S6
(Fig. S2d, Supporting information) had a certain curvature, whereas
SRMS-S2, SRMS-S3, and SRMS-S5 (Fig. S2c, Supporting information)
presented straight characteristics. The morphology of CRMS-S0
showed slightly curved rod-shaped silica with an irregular shape, as
shown in Fig. S2a and b (Supporting information).
3.1.3. SEM images
To determine the relationship of morphology evolution with
PVP concentration, CRMS and SRMS were further studied by
FESEM. Results showed that CRMS-S1 (Fig. 3a) and CRMS-S4
(Fig. 3d) were slightly curved rod-shaped mesoporous silica,
whereas SRMS-S2 (Fig. 3b) and SRMS-S3 (Fig. 3c) were straight
rod-shaped mesoporous silica. When the P123/PVP molar ratio
was 1:0.0116 or 1:0.174, CRMS-S1 and CRMS-S4 had an average
length of 1 m, but the widths were 200 and 300 nm, respectively.
μ
As shown in Fig. 3a and d, slightly curved rod-shaped mesoporous
silica appeared to aggregate to some extent. When the P123/PVP
molar ratio was 1:0.058 or 1:0.116, the length decreased to 800 nm
with increased width to around 500 nm. Compared with SRMS-S3,
SRMS-S2 was uniform without aggregation. Moreover, PVP con-
centration influenced the morphology of CRMS or SRMS; hence,
SRMS-S2 was the best sample.
3.1.4. Element analysis
A sample of SRMS-S2 is correctly shown in Table 2 as a material
with a H content of 1.424% without C and N. This result indicated
that the template agent P123 and PVP were completely removed
during calcination. The element content of C, N, and H in
NH-SRMS-S2 increased to 1.977%, 7.162%, and 2.297%, respectively,
because of subsequent functionalization. A comparison of SRMS-
S2-PPL and NH-SRMS-S2-PPL revealed that the contents of C, N,
and H increased from 1.212%, 5.618%, and 2.287% to 2.805%,
9.808%, and 2.612%, respectively, which can be attributed to the
appearance of PPL. These results confirmed that the aminopropyl
group and PPL were located on RMS and NH-RMS.
Fig. 1. SAXRD patterns of (a) CRMS or SRMS and (b) NH-CRMS or NH-SRMS:
(i) CRMS-S0, (ii) CRMS-S1, (iii) SRMS-S2, (iv) SRMS-S3, and (v) CRMS-S4.
Table 1
Prepared samples and their characterization results.
Da (nm)
SBET (m2 gꢀ1
)
Va (cm3 gꢀ1
)
Tp (nm)
a
b
Sample
P123:PVP (molar ratio)
d100 (nm)
a0 (nm)
2θ (1)
CRMS-S1
SRMS-S2
SRMS-S3
CRMS-S4
NH-CRMS-S1
NH-SRMS-S2
NH-SRMS-S3
NH-CRMS-S4
SRMS-S2-PPL
NH-SRMS-S2-PPL
1:0.0116
1:0.058
1:0.116
1:0.174
1:0.0116
1:0.058
1:0.116
1:0.174
1:0.058
1:0.058
9.90
9.53
9.10
9.07
9.69
9.64
9.00
8.59
11.43
11.00
10.51
10.48
11.19
11.13
10.39
9.92
0.892
0.926
0.970
0.973
0.911
0.916
0.981
1.027
5.97
5.40
5.32
5.34
5.94
5.32
5.28
5.30
5.32
5.02
637
615
615
620
252
351
277
351
286
198
0.80
0.73
0.73
0.73
0.42
0.56
0.44
0.55
0.49
0.32
5.46
5.60
5.19
5.14
5.25
5.47
5.11
4.47
a
D, SBET, and V stand for BET pore diameter, surface area, and pore volume, respectively.
pore wall thickness as Tp¼a0ꢀD.
b

J. Zhang et al. / Journal of Solid State Chemistry 213 (2014) 210–217
213
Fig. 2. TEM images of (a) CRMS-S1, (b) SRMS-S2, (c) SRMS-S3, and (d) CRMS-S4.
3.1.5. N2 adsorption–desorption isotherms
Consequently, most aminopropyl groups were located on the surface
of CRMS and SRMS.
The N2 adsorption–desorption isotherms and the correspond-
ing BJH pore size distribution curves of the samples are shown in
Fig. 4. All samples exhibited type IV isotherms with H1-type
hysteretic loops within a relative pressure (P/P0) range of 0.50 to
0.80, which were related to a highly ordered mesoporous structure
and cylindrical channels [37,38]. The mean value of pore diameter (D),
surface area (SBET), wall thickness (Tp), and pore volume (V) were
evaluated in Table 1. D and SBET initially decreased and then increased
with increased PVP concentration, whereas Tp initially increased and
then decreased. With increased PVP molecular weight, D and Tp had
the same trend as the change in concentration, but SBET and V
decreased (Table S1, Supporting information). This result indicated
that PVP played an important role in the formation of CRMS and
SRMS, and that PVP molecular weight and concentration were
important influencing factors. Textural parameters did not present a
linear change with increased PVP concentration, which can be
predicted by SEM. Notably, pore size distribution showed a narrow
pore distribution (Fig. 4b). The pore size distribution of SRMS-S3 had a
narrower range than those of SRMS-S5 and CRMS-S6 (Fig. S3,
Supporting information), which meant higher structural ordering.
The sample of SRMS-S2 displayed narrower pore size distribution
than CRMS-S1, SRMS-S3, and CRMS-S4, consistent with SAXRD results.
Compared with CRMS and SRMS, the SBET, Tp, and V of NH-CRMS
and NH-SRMS decreased, whereas the pore size irregularly varied.
3.2. SAXS analysis of P123/PVP/SiO2 sols with different PVP
concentrations
Xu et al. [39] studied the effect of PVP on different micro-
structures of spherical PVP/SiO2 sol particles by SAXS. They found
that the PVP concentration has an important function in the SiO2
sol process and the growth of SiO2 clusters. To distinguish the
formation of different P123–PVP structures, P123/PVP/SiO2 sols
formed using different P123–PVP mole ratios were tested to obtain
the scattering curves (Fig. 5). The scattering vector amplitude q is
defined as q¼4
π
sin
θ
/
λ
, where
θ
is the scattering angle, and
λ is
/q) and
the wavelength of the incident X-ray. The d spacing (d¼2
π
a0 (a0¼2d/31/2) can be obtained from q. The SAXS profiles reveal
that all samples have a strong scattering peak centered at
qE0.56 nmꢀ1 and correspond to the (10) reflection from the
hexagonal sols with different peak intensities and widths [40].
Based on calculations, d¼11.22 nm and a0¼12.96 nm, which were
all greater than d and a0 from XRD analysis because of structural
shrinkage following the removal of P123/PVP template agents. The
results suggest that P123, TEOS, and PVP interact with each other
in TEOS hydrolysis to form hexagonal P123/PVP/SiO2 sols, and PVP

214
J. Zhang et al. / Journal of Solid State Chemistry 213 (2014) 210–217
Fig. 3. FESEM images of (a) CRMS-S1, (b) SRMS-S2, (c) SRMS-S3, and (d) CRMS-S4.
Table 2
molecules in the P123/TEOS hydrolysis solution encloses a series of
P123 spherical micelles with PVP molecular chains through the
hydrogen bonding of silanol groups on the particle surface with the
electronegative inner amide of PVP side chain [39]. The amount of PVP
was relatively low, and the PVP molecules were separately surrounded
by the mesophase, which were not sufficient to form a continuous
layer and restrain the growth of the mesophase radially. Curved and
rod-shaped P123/PVP/SiO2 complex sols with 2D hexagonal meso-
phase surrounded by PVP molecule chains were formed [43,44].
Slightly curved and rod-shaped silica with smaller D and larger Tp
were obtained because of the small restrictive power of PVP molecular
chains. Therefore, curved CRMS-S1 with a high aspect ratio was
formed because of the main structure-directing agent of P123, such
as CRMS-S0 prepared only by P123. At higher PVP concentrations,
more PVP molecule chains were adsorbed on P123 rod-shaped
micelles and hexagonal aggregation, forming a continuous layer,
similar to a colloid, to inhibit the growth of P123/PVP/SiO2 sol in all
directions [45,46]. Thus, the dimension of mesoporous silica increased
radially and decreased horizontally because of the large restrictive
power of PVP molecular chains, which resulted in straight rod-shaped
silica CRMS-S2 and CRMS-S3 with low aspect ratios [47]. Further
increase in the PVP concentration dissolved more PVP into the P123/
TEOS hydrolysis solution for the adsorption on P123 rod-shaped
micelles [48]. The excess PVP self-assembled with P123 into com-
pound spherical micelles or with P123/TEOS hydrolysis solution into
curved rod-shaped micelles and hexagonal compound aggregation of
P123/PVP/SiO2 sol because of the affinity between the hydrophilic
groups [48]. This phenomenon yielded curved CRMS-S4 with high
Elemental contents of (a) SRMS-S2, (b) NH-SRMS-S2, (c) SRMS-S2-PPL, and
(d) NH-SRMS-S2-PPL.
Sample
Element
N (wt%)
C (wt%)
H (wt%)
SRMS-S2
0
0
1.424
2.297
2.287
2.612
NH-SRMS-S2
SRMS-S2-PPL
NH-SRMS-S2-PPL
1.977
1.212
2.805
7.162
5.618
9.808
concentration is important in the structural ordering and para-
meters [39].
3.3. Mechanism for the formation of CRMS and SRMS
The possible mechanism of such structural evolution may be
related to the varied PVP concentration (Fig. 6). PVP is an amphiphilic
molecule with a polyvinyl backbone and pyrrolidone group as hydro-
phobic and hydrophilic groups, respectively. This molecule is a long
chain polymer with a random coil structure that has strong selective
adsorption [41,42] and reduces the hydrolysis and condensation rates
of TEOS [39]. The SAXS results revealed that P123, TEOS, and PVP
interact with each other in TEOS hydrolysis to form hexagonal P123/
PVP/SiO2 sols. To describe the morphological evolution of the samples,
TEOS was omitted in the initial part of the mechanism. The “necklace
model” [14] states that the dissolution of a small number of PVP

J. Zhang et al. / Journal of Solid State Chemistry 213 (2014) 210–217
215
Fig. 4. N2 adsorption–desorption isotherms (a) and (c) and corresponding BJH pore size distribution curves (b, d) of CRMS, SRMS, NH-CRMS, and NH-SRMS.
3.4. Loading amounts and activities of immobilized PPL
Table 1 shows that NH-SRMS-S2-PPL had smaller SBET, D, and V
than SRMS-S2-PPL. In addition, SRMS-S2-PPL had smaller SBET, D,
and V than SRMS-S2. Most PPLs were located in the pore channel
of RMS, CRMS, and SRMS. PPL also had minimal effect on the
structure of well-ordered hexagonal arrangements (Fig. S4, Sup-
porting information). The effect of temperature and medium pH
on the activity of SRMS-S2-PPL and NH-SRMS-S2-PPL were inves-
tigated from 293 K to 333 K (Fig. S5, Supporting information) and
5.0 to 9.0 (Fig. S6, Supporting information), respectively. Notably,
the optimum conditions of SRMS-S2-PPL were 308 K and pH 7.0,
whereas those of NH-SRMS-S2-PPL were about 313 K and pH 8.0.
SRMS-S2 possessed good loading amounts of 45.2 mg gꢀ1
,
whereas that of NH-SRMS-S2 was 77.64 mg gꢀ1. The activity of
NH-SRMS-S2-PPL (2337.71 U gꢀ1) was larger than that of SRMS-
S2-PPL (1493.36 U gꢀ1
)
because of amino-functionalization.
Enzyme molecules approached mesoporous materials through
hydrogen bonding, which occurred between amino and carboxylic
groups on the enzyme molecule, and surface silanols. With the
incorporation of organosilanes, surface hydrophobicity increased,
resulting in better enzyme immobilization within mesoporous
structures because lipase had better affinity to hydrophobic
supports.
Fig. 5. SAXS curves of P123/PVP/SiO2 sols formed with different P123:PVP mole
ratios: (i) 1:0.0116, (ii) 1:0.058, (iii) 1:0.116, and (iv) 1:0.174.
aspect ratio. P123 was the main structure-directing agent, and the
curvature phenomenon recurred. Compared with CRMS-S1, the length
of CRMS-S4 was uneven, and the amount of irregular rod-shaped
particles increased. After TEOS whole hydrolysis and condensation, as
well as calcination of the as-synthesized products, PVP and P123 were
removed without destroying the mesoporous SiO2 2D hexagonal
arrangement.
3.5. Esterification of lauric acid with n-butanol by immobilized PPL
catalysis
To overcome the equilibrium limitation, esterification of lauric
acid with n-butanol catalyzed by SRMS-S2-PPL or NH-SRMS-S2-PPL

216
J. Zhang et al. / Journal of Solid State Chemistry 213 (2014) 210–217
Fig. 6. The mechanism of morphology evolution between CRMS and SRMS.
maximum yields. To study the effect of the amount of the catalyst,
the reaction was carried out using different amounts of NH-SRMS-
S2-PPL from 20 mg to 100 mg under otherwise similar conditions.
The lauric acid conversion rate is reported in Fig. 7. The lauric acid
conversion rate initially increased with increased amount of NH-
SRMS-S2-PPL from 20 mg to 60 mg. The maximum conversion rate
of 90.15% was obtained with further increased amount of NH-SRMS-
S2-PPL; hence, 60 mg was deemed to be the optimum amount of
catalyst. In the reaction involving SRMS-S2-PPL and NH-SRMS-S2-
PPL, the maximum conversion rate of lauric acid was 76.85% and
90.15%. In addition, SRMS-S2-PPL or NH-SRMS-S2-PPL reusability
studies were carried out to determine the stability of immobilized
PPL during the reaction. The catalyst was separated from the
reaction mixture by simple filtration, washed with a solvent, dried
at room temperature, and reused as such. Fig. 8 shows that NH-
SRMS-S2-PPL maintained 50% of its esterification conversion rate
even after five cycles of use, whereas SRMS-S2-PPL maintained
only 30%.
Fig. 7. Effect of NH-SRMS-S2-PPL amount on the lauric acid conversion rate.
4. Conclusions
Mesoporous silica materials with a 2D mesostructure (p6mm)
were successfully prepared with P123 and PVP as templates and
used for the immobilization of PPL. Changes in PVP concentration
and molecular weight caused mesoporous silica materials to vary
between CRMS and SRMS with changes in length, width, and
structure parameters. With increased PVP molecular weight, SBET
and V continuously decreased, and SRMS-S3 possessed larger D,
d100, and a0, as well as better structural ordering compared with
SRMS-S5 and CRMS-S6. Analysis of the effect of PVP concentration
on the synthesis of mesoporous silica showed that SRMS-S2 was
uniform without aggregation and had better structure ordering
than CRMS-S1, SRMS-S3, and CRMS-S4. NH-SRMS-S2 also exhib-
ited a higher loading amount than SRMS-S2. SRMS-S2-PPL and
NH-SRMS-S2-PPL were able to promote the cycles of use and
esterification of lauric acid with n-butanol, resulting in good
conversion rates of 90.15% and 76.85%, respectively.
Acknowledgments
Fig. 8. Effect of recycle numbers on the lauric acid conversion rate.
This work was supported by the National Natural Science
Foundation of China (Grant No. 20976100, 51372124), the Natural
Science Foundation of Shandong Province (Grant No. ZR2010BM013,
ZR2011BQ009), and the Program for Scientific Research Innovation
Team in Colleges and Universities of Shandong Province.
was carried out using excess n-butanol. The esterification was
investigated by varying different reaction parameters such as the
amount of catalyst and reaction time to optimize the conditions for

J. Zhang et al. / Journal of Solid State Chemistry 213 (2014) 210–217
217
Appendix A. Supporting information
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tionꢀdesorption isotherms, corresponding BJH pore size distribu-
tion curves, and structure parameters on different samples and
more detailed discussions of influence factors about immobilized
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