chapter 3 investigation of catalytic activity of...

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Investigation of Catalytic Activity of Mesoporous Silica

Chapter 3

INVESTIGATION OF CATALYTIC ACTIVITY OF MESOPOROUS

SILICA

3.1. Introduction

Natural and synthetic polymers have been used as matrices for various biomedical

applications such as drug delivery, gene delivery, tissue engineering, etc [1–5].

Commonly used synthetic polymers such as polyethylene (PE), polypropylene (PP) and

polystyrene (PS) have limited biomedical applications due to their non–biodegradability

and immunogenicity. Hence, biodegradable polymers such as polyesters,

polyphosphazenes, polyanhydridres, etc. are preferred for use in biological systems as

drug delivery vehicles, wound closure dressings, prosthetic implants and three–

dimensional porous scaffolds for tissue engineering applications [6–12]. Biodegradable

polyesters such as poly(lactic acid), poly(glycolic acid), poly(ε–caprolactone),

poly(hydroxy butyrate) and their copolymers have received considerable attention for

biomedical applications due to their biocompatibility [13–17]. These polymers undergo

degradation in vivo via hydrolytic, enzymatic or oxidative hydrolysis. It is essential that

the products of degradation are not toxic and can either be taken up the cells (resorbable)

or removed from the body (eliminable) by natural metabolic pathways. Apart from

biocompatibility, the polymer should also be easy to process and should possess the right

amount of hydrophilicity to favour cell adhesion [18].

Though many types of polyesters have been synthesized over the years, there have been

limited attempts to develop polyesters that can find applications in vivo. One of the

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major challenges in aliphatic polyester synthesis has been the separation of reaction

products from the liquid phase. Tremendous efforts have been directed to synthesize an

ideal catalytic system, which combines the advantages of both homogeneous and

heterogeneous catalysts. Although solid supports such as zeolites, alumina and metals

have been used as heterogeneous catalysts, they are usually accompanied with a

significant loss in activity and selectivity [17–23]. Conventional catalysts for polyester

synthesis include tetrabutyl titanate, H2SO4, HNO3, SnCl2.2H2O and organometallic

compounds which are highly corrosive and hazardous to the environment and our body

[24–28].

Materials with high surface area are invaluable for catalytic applications as they provide

numerous active sites for the reaction. Further, the use of a high surface area

heterogeneous catalyst enables higher conversion as well as makes it convenient to

separate the product with high purity unlike conventional homogeneous catalysts.

Mesoporous materials belong to a class of high surface area nanostructures that have

elicited immense interest for their highly efficient catalytic properties and have been used

for catalyzing a wide range of reactions from Freidel–Crafts acylation to Knoevenagal

reactions [29–36]. Over the past decade, many types of mesoporous materials have been

developed which include mesoporous carbon, mesoporous titania, mesoporous silica,

mesoporous alumina, mesoporous zinc oxide etc. [37–41]. Among these, mesoporous

silica with different pore dimensions, morphology and surface area have been developed

and extensively used as a heterogeneous catalyst for a wide range of reactions. Table 3.1

summarizes the various reactions that have been efficiently catalysed using mesoporous

silica when compared with conventional catalysts.

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Table 3.1: Mesoporous silica catalyzed reactions Mesoporous material Reaction Reference

i. Poly(4–methyl vinylpyridinium hydroxide) / SBA–15 Composite

ii. PVAm / SBA–15

Knoevenagel Reaction

[42] [43]

Aminopropyl–functionalized SBA–15 Claisen–Schmidt condensation [44]

i. SBA–15 ii. Primary amine and sulfonic acid

functionalized SBA–15 Aldol condensation [45]

[46]

SBA–15 supported silicotungstic acid Baeyer–Villiger Oxidation [47]

Sn–SBA–15 Baeyer–Villiger oxidation and Meerwin–Pondorf–

Verly reduction [48]

Ionic liquid–functionalized SBA–15

i. Cycloaddition of carbon dioxide (CO2) with epoxide

ii. Aza–Michael addition iii. Biginelli reaction of

aldehyde, ethyl acetoacetate and urea

[49]

SBA–15–supported ionic liquid–Pd(OAc)2 Heck reaction [50]

Amino–functionalized SBA–15 Henry reaction [51] Au nanoparticles supported on periodic

mesoporous organosilica (PMO) Ullmann reaction [52]

Noble metal nanoparticles stabilized in SBA–15

Suzuki–Miyaura coupling reaction [53]

Vanadium oxide catalyst supported on MCM–41, MCM–48, silica

Hydroxylation of benzene to phenol [54]

SBA–15 FischerTropsch synthesis [55]

Sulfonic Acid Functionalized SBA–15 Hantzsch four component condensation reaction [56]

SBA–15

• Michael addition • Wohl–Ziegler reaction • Acylation • Heck reaction • Tishchenko reaction • Diels–Alder reaction • Reformatsky • Luche reaction • Oxidative coupling • Reaction, synthesis of

chalcones • Synthesis of

[57]

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dihydropyrimidinones

Vanadyl (IV) Acetylacetonate Anchored SBA–15 Catalyst

Oxidative Cleavage of C=C Bond of Styrene and Its

Derivatives [58]

Ru/SBA–15

Hydrogenation of benzene to cyclohexene [59]

MCM –41, 48, SBA–15 FischerTropsch synthesis [60] V–HMS, V–MCM –41 and V–SBA –15

Oxidative dehydrogenation

(ODH) of ethane . [61]

Co–SBA–15 Aerial oxidation of ethylbenzene [62]

Primary and quaternary amine, sulfonic acid, and amino acid,

especially l–proline, functionalized SBA–15 & 16

• Knoevenagel and Henry reactions

• Claisen–Schmidt condensation

• Diethyl malonate addition reactions

[63]

The immense popularity of the mesoporous silica stems from its ease of synthesis,

tunable properties, excellent hydrothermal stability and inertness that enables it to

catalyse a broad range of reactions. It is evident that mesoporous silica with ordered

mesopores between 2–50 nm, high surface area, large pore volume and functionalities

[64], may also serve as a potential catalyst for polycondensation reactions. Recently, a

type of mesoporous silica, MCM–41, has been employed as a highly ordered catalyst

support to prepare dendrimers, a class of hyper–branched polymers [65]. Similarly, we

hypothesize that SBA–15, a class of mesoporous silica, which has larger pore size and

higher thermal stability may serve as a better catalyst for polymerization reactions [66].

Therefore, the aim of the current study was to synthesize for the first time three

polyesters namely poly(butylene succinate) (PBSu), poly(butylene sebacate) (PBSe) and

poly(butylene pimelate) (PBPi) using the mesoporous catalyst, SBA–15 with different

surface area and compare the products obtained using a homogeneous catalyst,

SnCl2.2H2O.

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The choice of the polymers was based on the fact that these are not only biodegradable

due to the ester links present, but the products of degradation can be either resorbed by

the body or eliminated without producing any toxic effects. Poly(butylene pimelate) has

been chosen for synthesis since it can degrade by hydrolysis and the degradation

products, namely 1, 4–butane diol and pimelic acid are non–toxic to the biological

system. The body eliminates 1, 4–butane diol after conversion to γ–hydroxy butyrate and

pimelic acid is a precursor of biotin (vitamin B7) [67]. L–lysine, an essential amino acid

that plays an important role in calcium absorption and suppresses viral infections [68].

The degradation products of poly(butylene succinate) are 1, 4–butane diol and succinic

acid. While 1,4–butane diol can be easily eliminated from the body, succinic acid is an

intermediate in the tricarboxylic acid (TCA) cycle, which can be converted to fumarate

by succinate dehydrogenase [69].

An interesting prospect for this polymer in drug delivery applications stems from the fact

that the brain expresses high levels of succinate dehydrogenase thereby enabling faster

and efficient degradation of this polymer. Hence, this polymer may be further explored

for brain targeted drug delivery applications. Poly(butylene sebacate) on degradation

produces 1,4–butane diol that can be easily eliminated and sebacic acid, which can be

metabolized to acetyl CoA and succinyl CoA [70].

The synthetic route and catalyst employed influences the molecular weight and purity of

the product, which is vital in determining the nature and magnitude of biological

response towards the polymer. Among the polyesters chosen, reports on the synthesis of

PBSu using direct polycondensation of dicarboxylic acid with diol in the presence of

hafnium (IV) or zirconium (IV) salts and reusable rare–earth triflates are available [36,

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37]. Reports on enzymatic synthesis of poly(butylene succinate) and poly(butylene

sebacate) are also available in literature [71, 72]. However, the process is time

consuming and the use of lipase enzyme limited the reusability and recovery of the

catalyst [73]. Poly(butylene succinate) has also been formed in the presence of other

homogeneous catalysts like titanium tetraisopropoxide and SnCl2.2H2O [74, 75]. The

synthesis of poly(butylene pimelate) has not been reported elsewhere. However, apart

from a report on the polymerization of grafted aniline using mesoporous SBA–15, no

reports are available on the catalytic efficiency of mesoporous silica on aliphatic

polyester synthesis including the above mentioned three polymers, thus making this

attempt the first of its kind [76].

3.2. Materials & Methods

3.2.1. Materials

Chloroform, methanol, tin (II) chloride, succinic acid, pimelic acid, sebacic acid and 1,4–

butane diol were purchased from Merck, India. All chemicals were of GR grade and

were used as such without any further purification.

3.2.2. Synthesis of polyesters

A typical synthesis of the aliphatic polyesters was carried out as follows. The chosen

polyester was synthesized by taking 5.1 mM of 1,4–butanediol and 5 mM of the

dicarboxylic acid (succinic acid or pimelic acid or sebacic acid) in a round–bottom flask.

The mixture was refluxed at 140°C with the catalyst (SnCl2.2H2O or different

mesoporous silica – SW, SEG, SG) for 24 h with continuous stirring in a nitrogen

atmosphere. The Dean–Stark apparatus was used to remove water from the reaction

mixture. After cooling to room temperature, the polymer obtained was dissolved in

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chloroform and filtered to separate the catalyst. The polymer was then reprecipitated in

excess methanol, filtered, dried under vacuum and characterized. The general reaction

scheme for formation of the polymer is shown below.

Scheme 3.1: Reaction scheme for the synthesis of polyester

3.2.3. Characterization of polymers

Fourier transform infrared spectroscopy (FT–IR) (Spectrum 100, Perkin Elmer, USA)

was used to analyze the polymer samples. FT–IR analysis was performed between 4000

and 450 cm–1 at a resolution of 4 cm–1 averaging 10 scans. The moisture–free polymer

sample was pelletized with KBr (IR grade, Merck) and analysed.

X–ray diffraction was used to characterize the crystallinity of polymer samples. The

samples were finely ground using a mortar and pestle and the fine powder was mounted

on a polymer slide and analyzed using an X–ray diffractometer (D8 Focus, Bruker,

Germany). The analysis was carried out from 10° to 60° in steps of 0.01° and at the scan

rate of 0.5° per step. Cu–Kα radiations were used to irradiate the sample.

Proton nuclear magnetic resonance spectroscopy was used to ascertain the structure of

the synthesized polymers. The analysis was done using CDCl3 as solvent and TMS as

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internal reference and the spectrum was recorded in a 300 MHz NMR spectrometer

(Bruker, Germany).

The hydrophobicity of the polymer films were determined using contact angle

measurements. The contact angle measurements were made between a water droplet and

the polyester film using Goniometer (Ramehart, USA). Initially, a polymer film was

prepared by applying a pressure of 100 kg/cm3 following which it was placed on a

sample holder. A water droplet was placed carefully on the prepared film and viewed

through a microscope. The contact angle was measured at constant time intervals and the

images were captured using a CCD camera.

Molecular weights of the synthesized polyesters were analyzed by gel permeation

chromatography (1200 Series, Agilent GPC, USA) in tetrahydrofuran (HPLC grade,

Merck) solvent with the sample concentration of 1 wt%. The sample (20 μL) was

injected into the column at a flow rate of 1 mL/min using an isocratic pump. The

molecular weight and molecular weight distribution of polymer samples were calculated

from a reference calibration curve obtained using polystyrene standards.

The phase transitions of the polymer samples were analyzed using differential scanning

calorimetry (DSC Q20, TA instruments, USA). Nitrogen was used as a purge gas with a

flow rate of about 20 cm3/minute. Both temperature and heat flux were calibrated

initially and about 20 mg of the samples were analyzed in the central region of the

injection parts. All the experiments were performed at a ramp rate of 10°C/minute,

starting from room temperature. The first run was eliminated to avoid interference from

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any moisture present and the third run was recorded to reveal the melting behavior of

synthesized polyesters.

3.2.4. Formation of poly(butylene succinate), poly(butylene pimelate) and poly(butylene

sebacate) microspheres

Polyester microspheres were prepared by the single emulsion technique. 0.2 g of

synthesized polymer was dissolved in 5 mL of chloroform and this was added drop wise

to 2% w/v poly(vinyl alcohol) (Sigma–Aldrich Chemicals, USA) under constant stirring

at 500 rpm. After 4 h, the microspheres formed were filtered and air–dried.

Figure 3.1: Polymer microspheres preparation through single emulsion technique

The morphology of the prepared polymer microspheres was studied using a cold field

emission scanning electron microscope (FE–SEM, JSM 6701F, JEOL, Japan). Initially

the samples were sputter coated with a thin layer of platinum. The applied voltage was

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maintained at 3 kV with a working distance of 6 mm and the images were recorded using

secondary electron detector.

3.3. Results & Discussion

3.3.1. Influence of catalyst on the polymer yield and molecular weight

Table 3.2 shows the properties of the polymers obtained using different mesoporous

materials (SW, SEG & SG) as catalyst compared to conventional catalyst such as tin

chloride (SnCl2.2H2O).

Table 3.2: Influence of conventional catalyst (SnCl2.2H2O) and mesoporous catalyst (SW) on the yield of PBSu, PBPi & PBSe.

Synthesized polyester

Physical parameters

Catalyst SnCl2.2H2O SW SG SEG

PBSu

% yield 54 90 61 42 ηint 0.05 0.38 0.25 0.08

Molecular weight (g/mol)

Mn 5466 10041 9162 398

Mw 5969 21954 10539 722

Mw/Mn 1.1 2.2 1.2 1.8

PBPi

% yield 6 25 23 10 ηint 0.08 0.55 0.32 0.15


Mn 1098 1899 1249 776

Mw 1112 5955 1326 1602

Mw / Mn 1.1 3.1 1.1 2.1

PBSe

% yield 25 40 32 15 ηint 0.06 0.65 0.35 0.2


Mn 1317 8809 7974 841

Mw 1340 10857 9727 1269

Mw / Mn 1.0 1.2 1.2 1.5 PBSu: Poly(butylene succinate); PBPi: Poly(butylene pimelate) &

PBSe: Poly(butylene sebacate).

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It was observed that among the four catalysts used, SW showed the maximum catalytic

efficiency for the synthesis of the three polymers. Also, the molecular weight of the

synthesized polymers obtained (Mn and Mw) and the corresponding intrinsic viscosity

(ηint) was highest for SW followed by SG. The catalytic efficiency of SW and SG are

higher than the conventional catalyst (SnCl2.2H2O). This difference may be attributed to

the advantages provided by a heterogeneous catalyst with high surface area that offers

greater number of active sites for the reaction to occur. A homogeneous catalyst such as

tin chloride does not possess these characteristics and hence the observed difference in

catalytic efficiency. Interestingly, the catalytic efficiency of the mesoporous silica

synthesized using ethylene glycol as solvent (SEG) was found to be much lower when

compared to the other two mesoporous catalysts. The order for catalytic efficiency for

the three mesoporous catalysts was found to be SW > SG > SEG. This difference in

catalytic efficiencies may not be attributed to an increase in surface area, as SEG has the

maximum surface area (Chapter 2) and however, least catalytic activity. Hence, some

other parameter seems to have an implementing role in catalytic activity of the

mesoporous samples. The transmission electron micrographs of the three mesoporous

samples reveal that both SW and SG have ordered hexagonal pores while SEG has

distorted hexagonal pores (Figure 2.6). It is therefore most likely that the pore

morphology and order have a more pronounced effect on catalysis than surface area. The

relatively high molecular weights of the polymers obtained using SW can also be

attributed to the presence of continuous tubular pores, which is absent in SEG. Similarly,

SG also has more number of ordered tubular pores and pore morphology resulting in

higher yield and molecular weight of the polymers than SEG (Figures 2.5 & 2.6).

However, the yield of the polymers and their molecular weight obtained using SG are

lower than those observed with SW. Therefore, neither surface area nor pore order might

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be the sole determinants for this difference. The 29Si NMR reveals that the SG sample

has lesser number of free –OH when compared with SW (Figure 2.7). This indicates the

importance of surface functionalities in polycondensation catalysts (Scheme 3.2).

O

O

O

Si OH (R)nHO OH (R)mC CHO OH

O O+ +

O

O

O

SiOH

(R)nO O(R)mC C

HOOH

O O

O

O

O

SiOH

(R)nOO

(R)mC C OHO O

H

O

O

O

SiOH

(R)nO

O

(R)mC C

O O

(R)n

O

O(R)mC C O

O O

n-1

O

O

O

SiOH

(R)nO O(R)mC C O

O O

n

+

-H2O

-(n-2)H2O

SW Polyester Scheme 3.2: Mechanism of polyester formation with heterogeneous mesoporous silica

catalyst

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Thus our results demonstrate that apart from surface area, pore order and surface

functionalities have a major role in imparting a high degree of catalytic efficiency to

mesoporous catalysts for polycondensation reactions. Based on the results obtained,

polymer samples obtained from SW were characterized further and compared with those

obtained using conventional homogeneous catalyst, SnCl2.2H2O.

3.3.2. Characterization of polymers

The FTIR spectra of PBSu, PBPi and PBSe samples synthesized using different catalysts

are shown in Figure 3.2, 3.3 & 3.4 respectively. The presence of absorption bands at

2800, 1740, 1480, 1100 and 780 cm–1 indicate the formation of polyester. The band at

2800 cm–1 represents –CH2– stretching, 1740 and 1480 cm–1 indicate the –C=O

stretching and –CH2– bending respectively, while the bands at 1100 and 780 cm–1

represent the –O–C–C– stretching and –CH2– rocking respectively. Most of these bands

are found in all samples, but the band at 780 cm–1 is very weak in the case of PBSu and is

absent in the other two polymer samples obtained using SnCl2.2H2O. This band is

usually associated with molecules having multiple –CH2– groups. It is likely that the

sample obtained with SnCl2.2H2O does not have a high molecular weight or is only an

oligomer.

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4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

%T

Figure 3.2: FT–IR spectra of poly(butylene succinate) synthesized with A)

SnCl2.2H2O; B) SW; C) SEG & D) SG catalyst

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

%T

Figure 3.3: FT–IR spectra of poly(butylene pimelate) synthesized with A)


A

B

C

D

A

B

C

D

80


4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

%T

Figure 3.4: FT–IR spectra of poly(butylene sebacate) synthesized with A)


Figure 3.5 shows the powder XRD of polyesters namely poly(butylene succinate),

poly(butylene pimelate) and poly(butylene sebacate) synthesized using SW and

compared with conventional homogeneous catalyst, SnCl2.2H2O. The XRD pattern of

the PBSe polyester synthesized in the presence of SnCl2.2H2O exhibits a semi–crystalline

nature which may be attributed to its low molecular weight (Figure 3.5E), while all the

polyesters synthesized using mesoporous SW show predominantly an amorphous nature

which can be attributed to the formation of high molecular weight polyesters.

A

B

C

D

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10 20 30 40 50

F

E

2 theta (deg.)

D

C

B

A

Figure 3.5: XRD of poly(butylene succinate) (A & B), poly(butylene pimelate) (C

& D) and poly(butylene sebacate) (E & F) synthesized with SnCl2.2H2O (A, C & E) and SW (B, D & F) as catalyst

The 1H–NMR spectra of the PBSu, PBPi and PBSe samples obtained using homogeneous

and mesoporous SW catalysts are shown in Figure 3.6, 3.7 & 3.8 respectively. The

structure of PBSu indicates that it possesses three sets of magnetic equivalent protons

denoted as a, b and c in the inset of Figure 3.6. The proton NMR of PBSu shows three

signals at δ 1.62, δ 2.85 and δ 4.13 corresponding to A, B and C set of equivalent protons

respectively (Figure 3.6). The NMR shows no difference in the products obtained using

SnCl2.H2O and mesoporous SW catalysts.

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5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0ppm

SW B

O

OO

OO

n

A

AB

B

C

C

C

SnCl2.2H2O

A

Figure 3.6: 1H–NMR of PBSu synthesized with SnCl2.2H2O & SW as catalyst

In the case of PBPi, five sets of magnetically equivalent protons are present which are

denoted as A, B, C, D and E (Inset of Figure 3.7). It is seen that the samples obtained

using 0.1 g of SW show peaks at δ 1.29, δ 1.57, δ 1.65, δ 2.25 and δ 4.08 corresponding

to the protons denoted by A, B, C, D and E respectively. However, only the peak at δ

1.57 is prominent in the samples obtained using tin chloride catalyst, implying

incomplete polymerization and this is in agreement with the FTIR data (Figure 3.3).

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4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

O

nA E

B

D D

O

O

O

O B

C C E

SW

A

DE C

B

SnCl2.2H2O

ppm Figure 3.7: 1H–NMR of PBPi synthesized with SnCl2.2H2O & SW as catalyst

The structure of PBSe indicates that it has five sets of magnetically equivalent protons

denoted as A, B, C, D and E (inset of Figure 3.8). Figure 3.8 shows that the polymer

sample obtained using SW exhibits five signals at δ 1.3, δ 1.6, δ 1.7, δ 2.3, & δ 4.1

corresponding to A, B, C, D and E respectively. The polyester synthesized with tin

chloride shows an additional low intensity signal at δ 3.7, which indicates the presence of

small amounts of –OH, most likely from butane diol. This indicates that there is

incomplete polymerization with this catalyst.

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4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

O OH

n

A

A

B

D

DO

OBC

C

E

O

O

E

A

A F

SW

SnCl2.2H2O

D

F

EC

B

A

ppm Figure 3.8: 1H–NMR of PBSe synthesized with SnCl2.2H2O & SW as catalyst

The hydrophobicity of the polyesters was determined using a goniometer and figure 3.9

shows the contact angles of PBSu, PBPi and PBSe at various time intervals.

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Figure 3.9: Contact angle measurement of polyesters synthesized with SW as catalyst

The absolute contact angles for PBSu, PBPi and PBSe were calculated to be 60°, 68° and

72° respectively. The higher contact angle represents a greater hydrophobicity and as

expected the poly(butylene sebacate) polymer exhibited the maximum hydrophobicity

when compared with the other two polyesters. Thus, greater number of methylene

groups and a corresponding increase in the alkyl chain length confers greater

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hydrophobic character to the polymers as is evident from the contact angle data for the

three polymers which increase in the order PBSu < PBPI < PBSe. It was also observed

that over a period of nine minutes, the contact angle decreased from 60˚ to 10˚ for PBSu,

68˚ to 8˚ for PBPi and 72˚ to 5˚ for PBSe. This may be attributed to the pores present in

the surface of the polymer that permit diffusion of water droplet. The contact angle of a

polymer film has a key role in determining the degradation rate of the polymer.

Thermal analysis (DSC) data of synthesized polyesters with tin chloride and mesoporous

silica (SW) catalysts are shown in Figure 3.10. The melting point of the PBSu

synthesized using tin chloride and mesoporous silica (SW) is 97.5˚C and 96˚C

respectively. It is observed that when the SnCl2.2H2O was used as catalyst, a sharp

melting peak with higher heat flow is obtained (Figure 3.10 A). In contrast, the polymer

obtained using mesoporous silica exhibits a broad melting pattern indicating formation of

a higher molecular weight product with greater amorphous character. This is in

concurrence with the molecular weight data and spectroscopic analysis of the polymer

samples.

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70 75 80 85 90 95 100

SW

SnCl2.2H2O

AEn

do

Temperature (oC)

2

45 50 55 60 65 70

Endo

Temperature (oC)

SW

SnCl2.2H2O

B

50 55 60 65 70

Endo

Temperature (°C)

CSW

SnCl2.2H2O

Figure 3.10: DSC thermograms of polyesters [A] PBSu; [B] PBPi; and [C] PBSe

synthesized with SnCl2.2H2O & SW as catalyst

In the case of PBPi (Figure 3.10 B), it is observed that when SnCl2.2H2O was used as

catalyst, a sharp melting peak with higher heat flow is obtained at 54.1°C, while the

polymer formed using SW, a broad melting peak between 58.6°C and 59.3°C is observed

which may be attributed to the formation of a higher molecular weight polymer with

greater amorphous character due to better polymerization. A similar observation can be

made for PBSe, where the polymer formed using SW shows a broader signal around

60°C while a sharper signal is obtained around the same temperature for the polymer

formed using tin chloride (Figure 3.10 C).

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3.3.3. Fabrication of polyester microspheres

The molecular weight and hydrophobicity of a polymer are responsible for formation for

micro– and nanospheres that can find applications as drug carriers. As the polyesters

formed using SW show higher molecular weight when compared to those formed using

other catalysts, an attempt was made to check the feasibility of transforming them into

polymer microspheres using single emulsion technique. Figure 3.11 shows the scanning

electron micrographs of the polymer microspheres formed using the three different

polyesters synthesized using SW. The conditions for microsphere formation were

identical in all three cases.

A B C

Figure 3.11: Scanning electron micrographs of microspheres formed from [A] PBSu; [B]

PBPi and [C] PBSe

Among the three polymers, PBSu formed smoother microspheres while the roughness

increased for PBSe. In the case of PBPi, macroporous spheres were obtained. This

difference in the surface morphology of the spheres can be attributed to the difference in

molecular weight of the polymers. The PBSu polymers had the largest molecular weight

among the three polymers followed by PBSe while PBPi had the least molecular weight.

Therefore, the viscosity and rate of solvent evaporation from the surface of the

microspheres will vary resulting in the observed differences in surface morphology.

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These results indicate the significance of polymer molecular weight for microsphere

formation. Very low molecular weight polymers are difficult to be transformed in to

microspheres, which will in turn limit the applicability of the polymer in the biomedical

field.

3.3.4. Characterization of mesoporous silica after polymerization

The structural and textural properties of SW were analyzed post catalysis to ascertain any

alterations in the pore order and morphology. Figure 3.12 shows the FT–IR and nitrogen

adsorption–desorption isotherm of the SW post catalysis followed by 6 hours of

calcination at 550°C. The FT–IR shows no shift in the bands for mesoporous silica

before and after catalysis. Similarly, the BET surface area and BJH analysis of pore size

of the mesoporous silica are 620 m2/g and 8 nm respectively, which confirms that there is

no alteration in the structure and texture of mesoporous SBA–15 silica due to the

catalysis reaction conditions.

B

4000 3500 3000 2500 2000 1500 1000 500

2

Wave number (cm-1)

1

%T

A

Figure 3.12: [A] FT–IR spectra and [B] nitrogen adsorption–desorption isotherm of calcined heterogeneous SW, which was used already as a catalyst for polymerization

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3.4. Conclusions

The mesoporous SBA–15 silica has been successfully demonstrated to act as a catalyst

for polycondensation reactions in this study. An interesting aspect that comes to the fore

from this study is that high surface area alone is not the sole contributor for effective

catalysis, but high pore order and surface functionalities also play a key role in improving

the catalytic efficiency towards polycondensation reactions. The mesoporous SBA–15

synthesized using water solvent integrates all these properties and hence was found to

result in relatively higher molecular weight polymers with better yield when compared

with the other mesoporous counterparts used for the study. The work also resulted in the

synthesis and characterization of poly(butylene succinate), poly(butylene pimelate) and

poly(butylene sebacate) of which the poly(butylene pimelate) is reported for the first

time. These polymers were successfully fabricated into microspheres which may be

explored in future for potential biomedical applications especially in the field of drug

delivery.

3.5. References

1. Ward MA, Theoni KG. Thermoresponsive Polymers for Biomedical Applications. Polymers 2011; 3: 1215–42.

2. Rajesh V, Dhirendra SK. Nanofibers and their applications in tissue engineering.

Int J Nanomedicine 2006; 1: 15–30. 3. Lakshmi SN, CatoTL. Polymers as Biomaterials for Tissue Engineering and

Controlled Drug Delivery. Adv Biochem Engin/Biotechnol 2006; 102: 47–90. 4. Huayu T, Zhaohui T, Xiuli Z, et al. Biodegradable synthetic polymers: Preparation,

functionalization and biomedical application. Prog Polym Sci 2012; 37: 237–80. 5. Naim JO, Van Oss CJ. The effect of hydrophilicity-hydrophobicity and solubility

on the immunogenicity of some natural and synthetic polymers. Immunol Invest 1992; 21:649–62.

91


6. Breitenbach A, Li YX, Kissel T. Branched biodegradable polyesters for parenteral drug delivery systems. J Control Release 2000; 64: 167–78.

7. Lakshmi S, Katti DS, Laurencin CT. Recent developments with Biogradable

Polymers Biodegradable polyphosphazenes for drug delivery applications. Adv Drug Deliver Rev 2003; 55: 467–82.

8. Laurencin CT, Gerhart T, Witschger P, et al. Bioerodible polyanhydrides for

antibiotic drug delivery: in vivo osteomyelitis treatment in a rat model system. J Orthop Res 1993; 11: 256–62.

9. Payam Z, Iraj R, Seyed, et al. A review on wound dressings with an emphasis on

electrospun nanofibrous polymeric bandages. Polym Adv Technol 2010; 21: 77–95. 10. De Groot JH, De Vrijer R, Pennings AJ, et al. Use of porous poly-urethanes for

meniscal reconstruction and meniscal prosthses. Biomaterials 1996; 17: 163–73. 11. Kopinkea FD, Remmlera M, Mackenziea K, et al. Thermal decomposition of

biodegradable polyesters—II. Poly(lactic acid). Polym Degrad Stabil 1996; 53: 329–42.

12. Athanasiou KA, Niederauer GG, Agrawal CM. Sterilization, toxicity,

biocompatibility and clinical applications of polylactic acid/polyglycolic acid copolymers. Biomaterials. 1996; 17: 93–102.

13. Serranoa MC, Pagania R, Vallet-Regı́b M, et al. In vitro biocompatibility

assessment of poly(ε-caprolactone) films using L929 mouse fibroblasts. Biomaterials 2004; 25: 5603–11.

14. Luo L, Wei X, Chen GQ. Physical Properties and Biocompatibility of Poly(3-

hydroxybutyrate-co-3-hydroxyhexanoate) Blended with Poly(3-hydroxybutyrate-co-4-hydroxybutyrate). J Biomat Sci 2009; 20: 1537–53.

15. Ahmed T, Marçal H, Lawless M, et al. Polyhydroxybutyrate and its copolymer

with polyhydroxyvalerate as biomaterials: influence on progression of stem cell cycle. Biomacromolecules 2010; 11: 2707–15.

16. Lydon MJ, Minett TW, Tighe BJ. Cellular interactions with synthetic polymer

surfaces in culture. Biomaterials 1985; 6: 396–402. 17. Simone CM, Maria de Fatima VM. Polyethylene synthesis using zeolite as support

for methallocene catalyst. Eur polym J 2001; 37: 2123–30. 18. Hegedüs A, Hell Z, Vargadi T, et al. A new, simple synthesis of 1,2-

dihydroquinolines via cyclocondensation using zeolite catalyst. Catal Lett 2007; 17: 99–101.

19. Corma A, Nemeth LT, Renz M, et al. Sn-zeolite beta as a heterogeneous

chemoselective catalyst for Baeyer–Villiger oxidations. Nature 2001; 412: 423–5.

92


20. Lesainta C, Glomma WR, Borgb O, et al. Synthesis and characterization of

mesoporous alumina with large pore size and their performance in Fischer–Tropsch synthesis. App Catal A-Gen 2008; 351: 131–5.

21. Rosa CD, Auriemma F, Spera C, et al. Crystallization properties of elastomeric

polypropylene from alumina-supported tetraalkyl zirconium catalysts. Polymer 2004; 45: 5875–88.

22. Allenger VM, McLean DD, Ternan M. Simultaneous polymerization and

oligomerization of acetylene on alumina and fluoridated alumina catalysts. J Catal 1991; 131: 305–18.

23. Lee IS, Kwak YW, Lee HD, et al. Synthesis of conjugated spirocyclic polymers:

Cyclopolymerization of 1,1-dipropargyl-1-silacycloalkanes by transition metal catalysts. J Ind Eng Chem 2008; 14: 720–5.

24. Wei Z, Liu L, Qi M. Synthesis and characterization of homo- and co-polymers of

(R,S)-β-butyrolactone and γ-butyrolactone or β-valerolactone initiated with cyclic tin alkoxide. React Funct Polym 2006; 66: 1411–9.

25. Zeng JB, Li YD, Zhu QY, et al. A novel biodegradable multiblock poly(ester

urethane) containing poly(l-lactic acid) and poly(butylene succinate) blocks. Polymer 2009; 50: 1178–86.

26. Ishihara K, Nakayama M, Ohara S, et al. Direct ester condensation from a 1:1

mixture of carboxylic acids and alcohols catalyzed by hafnium(IV) or zirconium(IV) salts. Tetrahedron 2002; 58: 8179–88.

27. Nagai A, Sato D, Ishikawa J, et al. A Facile Synthesis of N-Carboxyanhydrides and

Poly(α-amino acid) Using Di-tert-butyltricarbonate. Macromolecules 2004; 37: 2332–4.

28. Takasu A, Iio Y, Oishi Y, et al. Environmentally Benign Polyester Synthesis by

Room Temperature Direct Polycondensation of Dicarboxylic Acid and Diol. Macromolecules 2005; 38: 1048–50.

29. Corma A, Kumar D. Possibilities of mesoporous materials in catalysis. Stud Surf

Sci Catal 1998; 117: 201–22. 30. Chiu JJ, Pine DJ, Bishop ST, et al. Friedel–Crafts alkylation properties of

aluminosilica SBA-15 meso/macroporous monoliths and mesoporous powders. J Catal 2004; 221: 400–12.

31. Corma A. Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon

Reactions. Chem Rev 1995; 95: 559–614. 32. Tanabe K, Hölderich WF. Industrial application of solid acid–base catalysts. Appl

Catal A 1999; 181: 399–434.

93


33. Bellussi G, Pazzuconi G, Perego C, et al. Liquid-Phase Alkylation of Benzene with

Light Olefins Catalyzed by β-Zeolites. J Catal 1995; 157: 227–34. 34. Perego C, Amarilli S, Carati A, et al. Mesoporous silica-aluminas as catalysts for

the alkylation of aromatic hydrocarbons with olefins. Micropor Mesopor Mater 1999; 27: 345–54.

35. Zhao W, Salame P, Launay F, et al. Sulfonic acid functionalised SBA-15 as

catalysts for Beckmann rearrangement and esterification reaction. J Porous Mater 2008; 15: 139–43.

36. Kim KS, Song JH, Kim JH, et al. Preparation of guanidine bases immobilized on

SBA-15 mesoporous material and their catalytic activity in knoevenagel condensation. Stud Surf Sci Catal 2003; 146: 505–8.

37. Liang C, Li Z, Dai S. Mesoporous carbon materials: synthesis and modification.

Angew Chem Int Ed Engl. 2008; 47: 3696–717. 38. Jin-Yu Z, Jie-Bin P, Kun-Yuan Q, et al. Synthesis and characterisation of

mesoporous tiatania and silica-titania materials by urea template sol-gel reactions. Micropor mesopor mater. 2001; 49: 189–95.

39. Qiao ZA, Zhang L, Guo M, et al. Synthesis of Mesoporous Silica Nanoparticles via

Controlled Hydrolysis and Condensation of Silicon Alkoxide. Chem Mater 2009; 21: 3823–9.

40. Kima P, Kim Y, Kim C, et al. Synthesis and characterization of mesoporous

alumina as a catalyst support for hydrodechlorination of 1,2-dichloropropane: �effect of catalyst preparation method. Catal Lett 2003; 89: 185–92.

41. Chandra D, Mridha S, Basak D, et al. Template directed synthesis of mesoporous

ZnO having high porosity and enhanced optoelectronic properties. Chem Commun 2009; 2384–6.

42. Kalbasi RJ, Kolahdoozan M, Massah A, et al. Synthesis, Characterization and

Application of Poly(4-Methyl Vinylpyridinium Hydroxide)/SBA-15 Composite as a Highly Active Heterogeneous Basic Catalyst for the Knoevenagel Reaction. Bull Korean Chem Soc 2010; 31: 2618–26.

43. Kalbasi RJ, Kolahdoozan M, Rezaei M. Synthesis and characterization of

PVAm/SBA-15 as a novel organic–inorganic hybrid basic catalyst (Knoevenagel Reaction). Mater Chem Phys 2011; 125: 784–90.

44. Wang X, Cheng S. Solvent- free synthesis of flavanones over aminopropyl-

functionalized SBA-15. Catal Commun 2006; 7: 689–95.

94


45. Zeidan RK, Davis ME. The effect of acid–base pairing on catalysis: An efficient acid–base functionalized catalyst for aldol condensation. J Catal 2007; 247: 379–82.

46. Zeidan RK, Hwang SJ, Davis ME. Multifunctional Heterogeneous Catalysts: SBA-

15-Containing Primary Amines and Sulfonic Acids. Angew Chem Int Ed 2006; 45: 6332–5.

47. Zhiwang Y, Zhenhong MA, Lengyuan NIU, et al. Baeyer-Villiger Oxidation of

Cyclohexanone Catalyzed by SBA-15 Supported Silicotungstic Acid. Chinese Journal of Catalysis 2011; 3: 463–7.

48. Sasidharana M, Kiyozumia Y, Malb NK, et al. Incorporation of tin in different

types of pores in SBA-15: Synthesis, characterization and catalytic activity. Micropor Mesopor Mater 2009; 126: 234–44.

49. Xu LW, Yang MS, Jiang JX, et al. Ionic liquid-functionalized SBA-15 mesoporous material: efficient heterogeneous catalyst in versatile organic reactions. CEJC 2007; 5: 1073–83.

50. Jung JY, Taher A, Kim HJ, et al. Heck reaction catalyzed by mesoporous SBA-15-

supported ionic liquid–Pd(OAc)2. Synlett 2009; 1: 39–42. 51. Sujandi, Prasetyanto EA, Park SE. Synthesis of short-channeled amino-

functionalized SBA-15 and its beneficial applications in base-catalyzed reactions. App Catal A-Gen 2008; 350: 244–51.

52. Karimi B, Esfahani FK. Unexpected golden Ullmann reaction catalyzed by Au

nanoparticles supported on periodic mesoporous organosilica (PMO). Chem Commun 2011; 47: 10452–4.

53. Zheng Z, Li H, Liu T, et al. Monodisperse noble metal nanoparticles stabilized in

SBA-15: Synthesis, characterization and application in microwave-assisted Suzuki–Miyaura coupling reaction. J Catal 2010; 270: 268–74.

54. Lemke K, Ehrich H, Lohse U, et al. Selective hydroxylation of benzene to phenol

over supported vanadium oxide catalysts. App Catal A Gen 2003; 243: 41–51. 55. Wang Y, Noguchi M, Takahashi Y, et al. Synthesis of SBA-15 with different pore

sizes and the utilization as supports of high loading of cobalt catalysts. Catal Today 2001; 68: 3–9.

56. Mohammadi ZGH, Badiei AR, Khaniania Y, et al. One Pot Synthesis of

Polyhydroquinolines Catalyzed by Sulfonic Acid Functionalized SBA-15 as a New Nanoporous Acid Catalyst under Solvent Free Conditions. Iran J Chem Chem Eng 2010; 29: 1–10.

57. Marvaniya HM, Modi KN, Sen DJ. Greener Reactions Under Solvent Free

Conditions. Int J Drug Dev & Res 2011; 3: 42–51.

95


58. Anand N, Reddy KHP, Rao KSR, et al. Oxidative Cleavage of C=C Bond of Styrene and Its Derivatives with H2O2 using Vanadyl (IV) Acetylacetonate Anchored SBA-15 Catalyst. Catal Lett 2011; 41: 1355–63.

59. Liu JL, Zhu LJ, Pei Y, et al. Ce- promoted Ru/SBA-15 catalysts prepared by a

‘‘two solvents’’ impregnation method for selective hydrogenation of benzene to cyclohexene. Appl Catal A-Gen 2009; 353: 282–7.

60. Martınez A, Prieto G. The Application of Zeolites and Periodic Mesoporous Silicas

in the Catalytic Conversion of Synthesis Gas. Top Catal 2009; 52: 75–90. 61. Capek L, Adam J, Grygar T, et al. Oxidative dehydrogenation of ethane over

vanadium supported on mesoporous materials of M41S family. Appl Catal A 2008; 342: 99–106.

62. Sujandi, Prasetyanto EA, Han DS, et al. Immobilization of Co(III) using tethered

cyclam ligand on SBA-15 mesoporous silica for aerial oxidation of ethylbenzene. Catal Today 2009; 141: 374–7.

63. Park SE, Prasetyanto EA. Organocatalytic Application of Direct Organo-

Functionalized Mesoporous Catalysts Prepared by Microwave. Top Catal 2009; 52: 91–100

64. Selvam P, Krishna NV, Viswanathan B. Architecting mesoporous AlSBA-15: An

overview on the synthetic strategy. J Indian I Sci 2010; 90: 271–85. 65. Bhagiyalakshmi M, Park SD, Cha WS, et al. Development of TREN dendrimers

over mesoporous SBA-15 for CO2 adsorption. Appl Surf Sci 2010; 256: 6660–6. 66. Qingyi L, Feng G, Sridhar K, et al. Ordered SBA-15 Nanorod Arrays Inside a

Porous Alumina Membrane. J Am Chem Soc 2004; 126: 8650–1. 67. Snead OC, Furner R, Liu CC. In vivo conversion of γ-aminobutyric acid and 1,4-

butanediol to γ-hydroxybutyric acid in rat brain: Studies using stable isotopes. Biochem Pharmacol 1989; 38: 4375–80.

68. Griffith RS, Norins AL, Kagan C. A Multicentered Study of Lysine Therapy in

Herpes simplex Infection. Dermatologica 1978; 156: 257–67. 69. King A, Selak MA, Gottlieb E. Succinate dehydrogenase and fumarate hydratase:

linking mitochondrial dysfunction and cancer. Oncogene 2006; 25: 4675–82. 70. Halarnkar PP, Blomquist GJ. Mini-review Comparative aspects of propionate

metabolism. Comp Biochem Phys B 1989; 92: 227–31. 71. Sugihara S, Toshima K, Matsumura S. New Strategy for Enzymatic Synthesis of

High-Molecular-Weight Poly(butylene succinate) via Cyclic Oligomers. Macromol Rapid Comm 2006; 27: 203–7.

96


72. Linko YY, Wanga ZL, Seppälä J. Lipase-catalyzed synthesis of poly(1,4-butyl sebacate) from sebacic acid or its derivatives with 1,4-butanediol. J Biotechnol 1995; 40: 133–8.

73. Panova AA, Kaplan DL. Mechanistic limitations in the synthesis of polyesters by

lipase-catalyzed ring-opening polymerization. Biotechnol Bioeng 2003; 84: 103–13.

74. Velmathi S, Nagahata R, Sugiyama J, et al. A Rapid Eco-Friendly Synthesis of

Poly (butylene succinate) by a Direct Polyesterification under Microwave Irradiation. Macromol Rapid Comm 2005; 26: 1163–7.

75. Chen CH, Yang CS, Chen M, et al. Synthesis and characterization of novel poly

(butylenes succinate-co-2-methyl-1,3-propylene succinate)s. EXPRESS Polymer Letters 2011; 5: 284–94.

76. Sasidharan M, Mal NK, Bhaumik A. In-situ polymerization of grafted aniline in the

channels of mesoporous silica SBA-15. J Mater Chem 2007; 17: 278–83.

chapter 3 investigation of catalytic activity of...

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