development of sulfonic‐acid‐functionalized mesoporous

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Title: Development in Sulfonic Acid-Functionalized Mesoporous Materials: Synthesisand Catalytic Applications

Authors: Esmail Doustkhah, Ph.D; Jianjian Lin; Sadegh Rostamnia, Ph.D; Christo-phe Len, Ph.D; Rafael Luque, PhD; Xiliang Luo; Yoshio Bando, PhD; KevinC.-W. Wu, Ph.D; Jeonghun Kim, PhD; Yusuke Yamauchi; Yusuke Ide, Ph.D

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1

Development in Sulfonic Acid-Functionalized Mesoporous Materials: Synthesis and

Catalytic Applications

Esmail Doustkhah,1 Jianjian Lin,

2 Sadegh Rostamnia,

3* Christophe Len,

4 Rafael Luque,

4,5* Xiliang Luo,

2

Yoshio Bando,1,6

Kevin C.-W. Wu,7 Jeonghun Kim,

8 Yusuke Yamauchi

2,8,9* and Yusuke Ide

1*

1 International Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials

Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

2 College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,

Qingdao 266042, China

3 Organic and Nano Group (ONG), Department of Chemistry, Faculty of Science, University of

Maragheh, P.O. Box. 55181-83111, Maragheh, Iran

4 Sorbonne Universités, Université de Technologie de Compiègne (UTC), EA 4297 UTC-ESCOM, CS

60319, 60203 Compiègne Cedex, France

5 Departamento de Quimica Organica, Universidad de Cordoba, Edif. Marie Curie, Ctra Nnal IV-A, Km

396, 14014 Cordoba, Spain

6 Australian Institute for Innovative Materials (AIIM), University of Wollongong, Squires Way, North

Wollongong, NSW 2500, Australia

7 Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei

10617, Taiwan

8 School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology

(AIBN), The University of Queensland, Brisbane, QLD 4072, Australia

9 Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero,

Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea

Keywords: mesoporous materials; catalysts; sulfonic acid functionalization


2

Abstract

Sulfonic acid-based mesostructures (SAMs) have been developed in recent years and have important

catalytic applications. The primary applications of these materials are in various organic synthesis reactions

such as multicomponent reactions, carbon-carbon bond couplings, protection reactions, and Fries and

Beckman rearrangements. This review aims to provide an overview of the recent developments in the field

of SAMs with a particular emphasis on the reaction scope and advantages of heterogeneous solid acid

catalysts.

Content

Abstract

1. Introduction

2. Precursors for sulfonation

3. Development of new mesostructures for the fabrication of sulfonic acid-based mesostructures (SAMs)

3.1. Carbon mesoporous sulfonic acids (CM-n-SO3H)

3.2. Sulfonated ordered mesoporous polymers (OMP-SO3H)

3.3. Sulfonated mesoporous composites

3.3.1. Sulfonated polymer-silica (SPS) mesocomposites

3.3.2. Sulfonated carbon-silica (SCS) mesocomposites

3.4. Sulfonated periodic mesoporous organosilicas (PMO-SO3H)

4. Conclusions

5. Abbreviations

6. References


3

1. Introduction

In recent years, demands for the design and fabrication of new mesoporous catalysts with superior features

such as being recyclable, having a unique molecular architectures and being atom economical to adhere to

the tenants of green chemistry are increasing. Heterogeneous solid acid catalysts[1]

play an important role in

the development of greener catalytic protocols due to their recoverability, reusability and stability in

chemical processes. Among these catalysts, sulfonic acid-based mesostructures (SAMs) are a class of hybrid

organic-inorganic nanoporous materials that are attracting increasing attention from researchers due to their

aforementioned advantages.[2]

Sulfonic acid-functionalized mesoporous materials are superior to other

corresponding solid acid catalysts because they can provide a large number of reaction sites and realize the

size selectivity.[3]

In addition, these materials can be co-functionalized with other functional groups to

increase their efficiency by balancing their hydrophobicity, acidity, and basicity.[4]

Our ongoing research focus is the development of catalytic applications of mesoporous materials.[5]

This contribution seeks to review the recent advancements in the catalytic applications of SAMs with

diverse structures including silicates, polymers, hybrid polymer-silicates, organosilicates, and carbon-

containing compounds in a comprehensive manner (Scheme 1).

The preliminary reports on SAMs in 1998 were based on silica frameworks.[6]

The earliest versions of

SAMs were prepared by two general routes: 1) post-functionalization of mesoporous silica with 3-

mercaptopropyltrimethoxysilane (MPTMS) and 2) cocondensation of MPTMS and a silica source (e.g.,

tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS)). The final key step in the production of

SAMs was the oxidation of the thiol groups to sulfonic acids using oxidants such as H2O2. In this regard,

many advances,[6-7]

including enhancing the MPTMS loading capacity using a coating method,[6c]

using

cocondensation with TMOS instead of TEOS,[6a]

replacing calcination with extraction,[7g]

and

cofunctionalizing MPTMS with octyl substituents to enhance the catalytic activity by increasing the acidic

strength and hydrophobicity,[8]

have been made. Importantly, a number of the prepared SAMs are primarily

employed in biomass conversion.[6, 7g, 9]

In 2006, Melero and coworkers[10]

extensively reviewed and

discussed all types of sulfonating precursors and their catalytic applications. However, their review was


4

limited to the SASMs that had been reported to that date. Herein, recent advances in all areas of sulfonic

acid-based mesoporous materials will be discussed in detail.

Catalyzing the synthesis of 2,2-bis(5-methylfuryl)propane via the condensation of acetone with 2-

methylfuran,[6b]

the esterification of D-sorbitol with lauric acid,[11]

the synthesis monolaurin through the

direct esterification of glycerol with lauric acid,[9]

the three-component syntheses of 3,4-

dihydropyrimidinones through Biginelli reactions,[12]

Fries and Beckmann rearrangements,[13]

the syntheses

of polyhydroquinoline derivatives,[5c]

the synthesis of β-amino carbonyls via Mannich reactions,[14]

the

synthesis of xanthenes and bis(indolyl)methanes,[15]

syntheses of benzoxazole derivatives,[16]

the synthesis

of 4-phenyl-1,3-dioxane,[17]

the synthesis of chromenes from chromanols,[18]

the esterification of salicylic

acid with dimethyl carbonate,[19]

the multicomponent synthesis of spiro[indole-tetrahydropyrano(2,3-

d)pyrimidine] derivatives,[20]

etc.[21]

illustrate the versatility of these materials in catalytic applications.

Scheme 1. Overall strategies for the sulfonation of mesoporous materials.


5

2. Precursors for Sulfonation

Several sulfonic acid precursors (SAPs) have been reported for the sulfonation of mesoporous materials.

Sulfonation with concentrated sulfuric acid is the most common method. Among the organosiloxane-based

SAPs, MPTMS is generally employed to link silica with siloxane moieties. However, these methods are less

than ideal and can not necessarily be used in the sulfonation of all types of mesoporous compounds.

Consequently, several types of SAPs have been developed to modify mesoporous materials to control their

acidity, leaching, hydrophobicity and other parameters. For example, increases in hydrophobicity, and the

concomitant improvements in catalyst deactivation by water and the mass transfer of hydrophobic

compounds, could be achieved in a facile manner by replacing the propylsulfonic acid moiety in MPTMS

with phenylsulfonic acid. Some of the most commonly used SAPs for the sulfonation of mesoporous

materials are summarized in Table 1.

Recently, supported N-propylsulfamic acids have attracted significant attention in the field of

catalysis.[22]

Sulfamic acid-based catalysts can be regarded as strong acids that are zwitterionic in the

absence of water.[23]

Moreover, such catalysts are easily separable from reaction mixtures and can be

recycled a number of times when supported on the surface of mesopores. In neutral or alkaline solutions,

sulfamic acid derivatives can be boiled without appreciable hydrolysis; however, they slowly hydrolyze

under aqueous conditions.[23a]

Hajjami and coworkers[22d]

prepared MCM-41-N-propylsulfamic acid in a

one-pot multicomponent synthesis from 1-amidoalkyl-2-naphthols and studied its catalytic activity.

The functionalization of fluoro-based sulfonic acid precursors (F-SAP) inside the mesopores was first

reported by Harmer and coworkers.[24]

These hybrid mesostructures are strongly acidic due to the presence

of electronegative fluorine atoms. However, the preparation and stability of these materials are major

drawbacks as they often undergo leaching, which causes deactivation.[25]

1,2,2-Trifluoro-2-hydroxy-1-

trifluoromethylethane sulfonic acid sultones, F-SAPs, can be directly anchored to silica surfaces by a direct

synthetic strategy.[26]

Harmer et al.[24]

disclosed another strategy for the preparation of F-SAPs through a

platinum-catalyzed hydrosilylation procedure (Scheme 2), and the F-SAP can then be anchored to the silica

surface by a cocondensation reaction or grafting.[27]


6

Scheme 2. Synthesis of F-SAP. [24]

Phenylsulfonic acid siloxane-based precursors (Ph-SAP- Table 1, entries 6, 7, and 11) are advanced

types of SAPs that have improved acidic properties relative to the analogous commercial versions such as

Amberlyst-15.[31i]

Lindlar et al.[36]

reported the synthesis of MCM-41-Ph-SO3H (using Ph-SAP in Table 1,

entry 6) with enlarged pore diameters up to 60 Å using swelling agents. The phenyl groups were first grafted

onto the silica surface and subsequently sulfonated with chlorosulfonic acid. However, in this step, free

silanol groups can also undergo sulfonation. To avoid this phenomenon, they incorporated capping agents

(such as trimethoxymethylsilane) that can be grafted after the functionalization of the phenyl precursor and

will protect the free silanols from sulfonation with chlorosulfonic acid. For the sulfonation of mesoporous

silica materials, grafting is the most commonly selected method due to its advantages over cocondensation

functionalization (in which the structure detection agent (SDA) would need to be removed by extraction).

However, the Launay and Gédéona group found that a low temperature and gentle calcination at 200 °C can

completely remove the P123 from the extracted sample. Therefore, the P123 surfactant in SBA-15-Ph-

SO3H, which is prepared by a cocondensation method, can be removed by calcination.[31a]


7

Table 1. List of most popular precursors for the sulfonation of mesoporous materials.

Entry Structural formula Properties and applications Reference

1

-A famous family of SAPs that are generated

by the oxidation of MPTMS

[6a, 7d]

2

O

Si

O

OEt

Et

Et

OSO3H

F F F F

F F F F

-Prepared in one step prior to functionalization

and attached to a silica surface through the

siloxane moiety

-Strongly acidic and highly hydrophobic

-Expensive to produce

[27]

3

-Can be directly functionalized onto a silica

surface

-Strongly acidic due to the fluorine atoms

-Higher leaching and more expensive

[26, 28]

4

-Attaches to a silica surface via the siloxane

moiety

-By adding a nucleophile, a wide variety of

bifunctionalized SAMs can be produced

[29]

5

-Can directly attach to a silica surface or

undergo ring opening in the presence of an

amine

[30]

6

-Can be prepared in two steps:

1) functionalization of PTS onto a silica

surface;

2) sulfonation of a phenyl group by H2SO4

-Their acidity is similar to that of commercial

sulfonated Amberlyst-15 resins

[25, 31]

7

Si

O RO

R

OR

S

O

O

Cl

-It can easily hydrolyze to give other SAP and

can be used to tailor surfaces through

nucleophile attack of -SO2Cl moieties. These

kinds of precursors can decrease the pore size

and surface area

[32]

8

-Is similar to entry 6’s precursor except with

more flexibility and a longer length

-These two SALs are more water tolerant than

the SAL shown in entry 1

[31j]

9

-Can be embedded into a silica wall and then

oxidize to -SO3H to give a uniform distribution

of sulfonyl groups

[33]

10

-Can be prepared from chlorosulfonic acid and

ATPS.

-Converts to a zwitterion in water and slowly

hydrolyzes to the amine and H2SO4

[22-23]

11

-Generally used for CMs and functionalized

through displacement of the diazonium[34]

in

the presence of H3PO2

[35]


8

3. Development of new mesostructures for the fabrication of sulfonic acid-based mesostructures

(SAMs)

3.1. Carbon mesoporous sulfonic acids (CM-n-SO3H)

Mesoporous carbons (MCs) were developed recently and are synthesized from hard or soft-templating

carbonization.[37]

Owing to their high stability and potential for undergoing sulfonation, MCs have emerged

as the next generation of sulfonic acid-based carbon mesopores (SACMs), and they are stable at high

temperatures.[35a, 38]

Wu et al.[38a]

pioneered a sulfonation of the controllable carbonized CMK-3 via a vapor-

phase method in a closed autoclave with fuming H2SO4 and subsequent treatment with pure SO3 gas. This

group also studied the influence of carbonization on the functionalization of CMK-3 at various temperatures

from 673-1173 K. At 1173 K, a highly ordered mesoporous carbon material could be produced (Scheme 3).

However, the balance between the structural order and the content of polycyclic aromatic carbons

deteriorated when the carbonization temperature was 873 K. The presence of polycyclic aromatic rings was

found to be essential for obtaining a highly functionalized -SO3H-containing material. According to the

XRD patterns, the intensity of the main diff raction lines slightly decreased when using vapor transfer

sulfonation, which indicated that the ordered 2D hexagonal p6mm structure of CMK-3(873)-SO3H was

preserved during sulfonation. This material was then tested as a catalyst for the Beckman rearrangement of

cyclohexanone oxime and the condensation of bulky aromatic aldehydes.


9

Scheme 3. Functionalization of CMK-3 with fumed H2SO4.[38a]

Following that study, Janaun and Ellis[39]

extensively explored the effects of sulfonation on the structure

of MC and its corresponding effects on the catalytic activity of CMK-3-SO3H. They also attempted the

sulfonation of the material before and after the removal of the silica template. Their research showed that the

sulfonation of CMK-3 after removal of the template can result in remarkable damage to the internal pores,

whereas sulfonation before removing the silica template did not impact the mesoscopic structure. The

authors attributed these findings to silica serving as a barrier, which prevented sulfuric acid from penetrating

into the internal surface. TEM images (Figure 1) indicated that two different structures were obtained from

the sulfonation of MC before and after silica removal (after silica removal, sulfonation caused the total

destruction of the pore walls). The carbonization temperature was 400 °C, and the proposed system was

successfully tested in biodiesel production.


10

Figure 1. CMK-3-SO3H TEM images of the two different structures obtained from sulfonation of MC

before (a) and after (b) removal of the silica template (Reproduced from ref. [39]

).

Later, Sels and Jacobs[38b]

further optimized the sulfonation of CMK-3-SO3H. Instead of fuming H2SO4

or SO3 gas, concentrated sulfuric acid at a higher temperature was employed in the sulfonation of the

carbonized mesoporous carbon inside SBA-15, and then the SBA-15 (hard template) was removed using

HF. Zhang and coworkers[40]

hydrolyzed cellulose into glucose with the highest yield to that point using

CMK-3-SO3H at a high temperature. The stability of CMK-3-SO3H at high temperature (250 ºC) was key to

achieving the high yields (94% cellulose conversion with a glucose yield of 74.5%).

An alternative approach was also developed for the preparation of sulfonated CMK-5 (CMK-5-

SO3H)[41]

with the reduction of the diazonium salts, bearing sulfonic acid functional group, with

hypophosphorous acid, and the final material was employed as a catalyst in the production of bisphenols

(Scheme 4). The catalytic performance was compared with those of two other SASMs (e.g., SBA-Pr-SO3H

and Et-PMO-SO3H), and the fully sulfonated CMK-5 provided the best results. The catalytic activity of

sulfuric acid was also tested, but it showed worse results in terms of yield versus time. However, CMK-5-

SO3H was a greener, more recoverable and more reusable solid acid catalyst than deleterious and corrosive

sulfuric acid.


11

Scheme 4. Sulfonation of CMK-5 with benzene para-diazonium sulfonic acid (Reproduced from ref. [41]

).

Another type of sulfonated ordered mesoporous carbon (OMC-SO3H), a member of the SACM family,

was also prepared, and various parameters of the preparation procedure were studied. These parameters

include the effect of aging temperature on the mesoscopic structure at three different points [(OMC-n-SO3H)

n = 100, 130, and 150 ºC], and the synthesis was carried out by nanocasting silica SBA-15 and using

furfuryl alcohol as the carbon source.[41b]

Aging at 150 ºC afforded materials with a higher pore diameter

and therefore a larger pore diameter than the corresponding OMC. As a result, sulfonation decreased the

pore diameters of these materials. Compared to SBA-Ph-SO3H, OMC-SO3Hs was more acidic and therefore

showed higher catalytic activity. Accordingly, OMC-150-SO3H provided a higher conversion of oleic acid

to the corresponding ester with ethanol, and OMC-130-SO3H and OMC-100-SO3H resulted in similar

conversions. SBA-Ph-SO3H provided the lowest conversion among the OMC-SO3Hs tested, but

interestingly, it was more efficient than commercial Nafion (Figure 2).


12

Figure 2. Conversion of oleic acid to the ester with ethanol over different catalysts over 10 h (Reproduced from ref. [41b]

).

Li and Liang[42]

anchored phenylsulfonic acid groups onto the surface of OMC through an in situ radical

polymerization of sulfanilic acid and isoamyl nitrite under ambient conditions. By this method, they reduced

the number of synthetic steps to one and subsequently utilized OMC-SO3H as a support for palladium

nanoparticles via PdCl2 reduction using NaBH4 as the reducing agent. The synthesized nanocatalyst was

successfully employed in the electrooxidation of formic acid (cyclic voltammetry and chronoamperometry).

The catalytic activity of CMK-5-SO3H was also investigated in a range of organic reactions,[35b, 43]

such

as the silylation of alcohols and phenols with hexamethyldisilazane at room temperature in dichloromethane,

which was achieved in high yields, and the synthesis of pyrimidine derivatives.[43a]

The latter reaction was

performed in ethanol at room temperature. The catalyst could be recycled several times with no significant

loss in its activity, and excellent yields of the derivatives could be generated in the first runs with short

reaction times.[43b]

In a separate work, CMK-5-SO3H was also employed as a catalyst for the solvent-free

one-pot synthesis of coumarins through a Pechmann condensation (Scheme 5). Coumarins could be

synthesized from phenols and ethyl acetoacetate at moderate to higher temperatures (typically 130 ºC), and

CMK-5-SO3H was highly stable under the investigated conditions.[35b]


13

Scheme 5. Synthesis of CMK-5-SO3H and its catalytic applications in organic reactions.

[35b, 43]

Corma and coworkers[35a]

recently designed a new and interesting sulfonic acid-based mesoporous

carbide derived from ordered silicon carbide with an extraordinary high specific surface area (up to 2800 m2

g−1

). This material was designed via the selective extraction of silicon from ordered mesoporous silicon

carbide and subsequent functionalization with sulfonic groups, and it is an excellent solid acid catalyst for

the esterification of stearic acid to ethyl stearate (Figure 3).

Figure 3. Synthesis of SiC-derived MC-SO3H by a hard-templating approach (Reproduced from ref. [35a]

).

In addition to the nanocasting method, self-assembly via soft templating was recently developed, and it

has surpassed nanocasting and hard templating in the preparation of OMCs (Figure 4). This method was

pioneered by Dai et al.,[44]

Nishiyama et al.,[45]

and Zhao et al.,[46]

and it has advantageous over nanocasting

due to the reduced preparation costs, the possibility for large scale production, the lower number of synthetic

steps, the narrower pore size distribution and the shorter synthesis time.


14

Figure 4. Synthesis of OMPs with various structures (Reproduced from ref. [46a]

).

In this way, OMC-SO3H could be directly prepared by the self-assembly of resorcinol/formaldehyde

(RF) under aqueous conditions using F127 as an SDA.[47]

The impact of various factors on the resulting

properties of the OMC-SO3H were studied. The obtained organic mesoporous polymer (OMP) was

carbonized at high temperature (> 400 °C) under an inert atmosphere to produce OMC. The acid (HCl)

concentration and molar ratio of resol and F127 were found to have significant effects on the mesoporous

structure during the polymerization process. According to their report, a highly structured OMC could be

manufactured when the acid concentration was 0.6-2.0 M and the mass ratio of resol to F127 was 3.5-4.0.

Sulfonation could be achieved using concentrated H2SO4 at moderate temperatures, and the temperature

must be carefully controlled as OMCs are susceptible to H2SO4 degradation at high temperatures (i.e.,

H2SO4 damages the pore walls of OMC and converts the material to amorphous carbon). Gou et al. [47]

studied the carbonization step at different temperatures (400, 600, 700, and 850 °C) and demonstrated that

the optimum temperature for obtaining a highly ordered OMC is 400 °C. Interestingly, the sulfonation step

(carried out at 200 °C) indicated that calcination of the OMC at 850 °C resulted in the optimum

carbonaceous material with retention of the porosity (Figure 5). Hara et al.[48]

found that after carbonization,

OMC is converted to amorphous carbon in the sulfonation step. In addition, graphene sheets with high

densities of -SO3H groups were also found.


15

Figure 5. TEM images of samples: (a and b) C600-3.5–2.0 M; (c) C600-4.0–2.0 M; (d) C600-4.5–2.0 M; (e) C600-

5.0–2.0 M (Reproduced from ref. [47]

).

Dong and coworkers[38c]

prepared OMC-SO3H in a three-step process using F127 as the SDA and FA as

the carbon source (Figure 6). The OMP obtained from the first step was carbonized at various temperatures,

and carbonization at 400 °C was found to afford a catalyst with higher acidity and catalytic activity towards

the esterification of oleic acid with methanol.

Figure 6. Synthesis of MC-SO3H by a soft-templating approach (Reproduced from ref.

[38c]).

Hierarchical monolithic carbons with mesoporous arrays can also be prepared via a soft-templating

approach.[49]

These materials can be rapidly prepared on a remarkable scale, and they have promise for

applications in catalysis[50]

and separation and storage.[51]

Mukai and Oginio[52]

prepared a monolithic carbon

from RF copolymerization and carbonization at 400-800 °C for 4 h followed by sulfonation using

concentrated sulfuric acid at 80 °C for 10 h. This material was found to have a mesoporous structure as


16

proven by the N2 adsorption/desorption isotherm (Figure 7). The designed sulfonated monolithic carbon

was successfully tested as catalyst in the liquid-phase esterification of acetic acid with ethanol at 60 °C in a

flow reaction system over 50 h of operation.

Figure 7. (a) SEM images for characterizing the CMHC. The inset shows a photograph of CMHC. Nitrogen

adsorption data for the characterization of CMHC: (b) Adsorption (●) and desorption (○) isotherms, (c) mesopore size

distribution calculated by applying the Dollimore-Heal equation to the adsorption isotherm (Reproduced from ref. [52]

).

3.2. Sulfonated ordered mesoporous polymers (OMP-SO3H)

OMPs themselves can be independently designed as SAM catalysts for a number of acid-catalyzed

reactions.[53]

These polymers have a variety of structures that can bear various functionalities. Rapid

syntheses and facile modification/functionalization are relevant advantages of OMPs. These OMPs can also

be modified to have SO3H groups inside the pore walls. However, there are few reports of sulfonated OMPs

(OMP-SO3H). The porosity of OMPs is generated through a) the steric orientation and geometric structures

of polymers (template-free method)[54]

or b) the addition of a template during polymerization to replicate the

template with a tunable and mesoscopic structure (templating method).[55]

There are also other types of

organic porous polymers featuring 2D or 3D networks with different pore sizes (metal-organic frameworks

and microporous polymers).

Ryoo and Choi[56]

synthesized an OMP-SO3H through a free-radical polymerization method starting

from crosslinkable olefinic monomers within the interstices of the porous templates in the presence of

mesoporous silica KIT-6 as hard template for replication. The polymerization reaction proceeded under

heating at 150 °C, and then the sulfonation step was conducted by heating the material with sulfuric acid to


17

90 °C (Figure 8). The etching step to remove the template was achieved by using hydrofluoric acid. The

OMP structures collapse when the pore diameters of the parent mesoporous silica are larger than the

distance between the crosslinking points. Under the optimized replication conditions, the replica exhibited a

very narrow pore size distribution and a high specific surface area, which is based on the structural standards

of the parent silica template. This OMP-SO3H, with a surface area of 300 cm3.g

-1, was tested as a catalyst for

the esterification of hexanoic acid with benzyl alcohol, and its catalytic activity was superior to that of

commercially available Amberlyst-15, while its active acidic sites were weaker than those of Amberlyst-

15.[56]

Figure 8. (a) Synthesis of OMP-SO3H through poly-DVB using TMS-coated KIT-6 as the template. TEM images of

(b) TMS-coated KIT-6 and (c) poly-DVB materials (Reproduced from ref. [56]

).

Resol-based OMPs can be synthesized in the presence of hexamethyl tetraamine (HMTA) (OMR-

[HMTA]) as a crosslinking agent and F127 as the SDA.[57]

HMTA inside the fabricated OMP can also be

post-modified by 1,3-propansulfone through its free amine groups (Scheme 6), which finally leads to the

formation of a zwitterion on the surface (quaternary amine cation and sulfonate anion). This zwitterionic

mesoporous material can be converted in the subsequent step to an acidic SAM by adding CF3COOH, a

strong acid. This route led to an ionic liquid anchored to the pore walls of OMR-[HMTA]. This catalyst was

tested in the esterification of acetic acid with cyclohexanol, the hydration of propylene oxide, the Pechmann

reaction of resorcinol with ethyl acetoacetate and the transesterification of tripalmitin with methanol, and it


18

showed remarkably superior activities to those of Amberlyst 15, sulfonic group-functionalized ordered

mesoporous silicas, and acidic zeolites and comparable activities to those of sulfuric acid.[57]

Scheme 6. Synthesis of OMP-SO3H/CF3SO3 based on HPTA (Reproduced from ref.

[57]).

3.3. Sulfonated mesoporous composites

3.3.1. Sulfonated polymer-silica (SPS) mesocomposites

One of the methods for fabricating SPS mesocomposites is the ‘dissolution and entrapment’ method.[58]

In

this method, soluble polymer with a low degree of polymerization was added to the hydrolysis of the silica

source (e.g., TEOS or TMOS) in the presence of an SDA. During the hydrolysis and formation of the silica

network, polymer chains can be trapped within the wall of the mesoporous silica, which ultimately generates

an SPS mesocomposite. In this manner, Wan et al.[59]

incorporated MPTMS as an SAP to generate a

sulfonated SPS composite using dissolution and entrapment method. Thus, TEOS and phenolic resin with a

low degree of polymerization were employed as precursors, MPTMS was used for further conversion to -

SO3H groups, and F127 was used as the SDA. Interestingly, the authors did not use an organosiloxane

coupling agent to couple the organic polymer and the SiO2, and instead, they take advantage of the coupling

of Si to the -OH of resol during the final steps of the polymerization and TEOS hydrolysis. The polymer was

hydrophobic, which enhanced its catalytic activity and decreased catalyst poisoning due to water. The

catalyst was also successfully tested in the acetalization of ketones and in the condensation of acetone and

phenol to yield bisphenol (Figure 9).[59]


19

Figure 9. Acetalization of carbonyl compounds catalyzed by silica-organic polymer based SAM (Reproduced from

ref. [59]

).

New hybrid hollow nanospheres containing silica and sulfonated polystyrene (PS-SO3H/PMA-SiO2) were

designed and synthesized by Yang and coworkers, and the nanospheres aligned uniformly in the mesoporous

channel of a silica shell.[60]

The template components in this synthesis were polystyrene nanospheres and

cetyltrimethylammonium bromide (CTAB). TEOS and Si-PMA were the precursors for the synthesis of the

SPS mesocomposite. Finally, after the synthesis of the hybrid hollow nanospheres, the template, CTAB, was

extracted using EtOH/HCl. The obtained polymer-silica composite (PS/PMA-SiO2) then underwent

sulfonation of the polystyrene moieties using chlorosulfonic acid (which has an acid exchange capacity in

the range 0.8 to 2.0 mmol g−1

) to produce the SPS mesocomposite (PS-SO3H/PMA-SiO2). This catalyst was

tested in the esterification of lauric acid with EtOH. To increase the surface hydrophobicity and

consequently increase the catalytic activity, octyl organosilane chains were added in a post-modification step

(Figure 10).[60]

For comparison, corresponding free silica HN moieties were sulfonated by the same method and compared

with PS-SO3H/PMA-SiO2 HNs in the esterification of lauric acid with ethanol. Additionally, this new

catalyst was compared with the commercial Amberlyst®-15 catalyst. This new hybrid SPS showed higher

catalytic activity than the other two catalysts, and the turnover frequency (TOF) of the optimized hybrid SPS

was almost identical to that of concentrated sulfuric acid, which shows that the catalyst’s activity is similar

to that of its homogeneous analog. The high activity of the hybrid HN moieties was attributed to the uniform

distribution of sulfonic acid functionalities on the surface of the PS-SO3H nanospheres inside the


20

mesoporous silica shell, the penetrable mesochannels, and the suitable surface hydrophobicity, which

increases the mass transfer. Furthermore, the recyclability of the hybrid HN moieties could be greatly

enhanced by octyl group substitution, which may prevent the leaching of PS-SO3H during the catalytic

process. For the synthesis of PS-SO3H/PMAn–SiO2-SO3H, different ratios (n) of PMA-Si to PS (n = 2.5,

3.3, and 5) were used to obtain different hybrid SPS HNs and to determine the effect of the ratio on the

catalytic activity. Among the prepared materials, PS-SO3H/PMA2.5–SiO2-SO3H, which underwent octyl

chain post-modification, was more active than non-octylated derivatives, samples prepared with other ratios

and free sulfonated SiO2 HNs.[60]

Figure 10. (a) Synthesis of PS-SO3H/PMAn–SiO2-SO3H, and TEM images of (b) PS-SO3H/SiO2 and (c) octylated PS-

SO3H/PMA2.5–SiO2-SO3H (reproduced from ref. [60]

).

Polymerization of organic monomers inside of mesoporous silica materials has permitted reach into

various functionalized polymer-silica composite materials with well-defined mesoporosities. Some ordered

mesoporous silica materials contain disordered micropores within their walls. The structure and pore size

range of the (micro)pores can play a crucial role in the polymerization mechanism inside the mesoporous

silica materials and subsequent impact the stability of the polymer/silica mesocomposites. Ryoo and

coworkers[61]

synthesized two different SBA-15 silica mesoporous materials containing different pore sizes.

The pore size could be fine-tuned to 2-4 nm when the molar ratio of SiO2/P123 was 45, while a further

increase in this ratio to 75 essentially rendered porous materials with pore sizes > 2 nm. Based on XRD

analyses, their prepared materials exhibited better compatibility with SBA-15 in r = 45. The location of the


21

polymers was systematically controlled by adjusting the micropores of the silica framework and the

polymerization conditions (Figure 11). They used various vinyl monomers, such as styrene, chloromethyl

styrene, 2-hydroxyethyl methacrylate, and methacrylic acid, for the polymerization inside the mesoporous

silica. Among these monomers, styrene and divinylbenzene with AIBN inside the SBA-15 provided the

optimum results, and subsequent sulfonation was conducted using concentrated sulfonic acid. This

sulfonated mesocomposite was then tested as a catalyst in the esterification of benzyl alcohol with hexanoic

acid in toluene at 75 °C.[61]

Figure 11. (a) Controlled polymerization of organic polymers inside the micropores. (b) XRD patterns of SBA-15 and

polymerization-controlled mesoporous silica SBA-15 (PCMS/SBA-15). (c) BET analyses of SBA-15 and

PCMS/SBA-15 (Reproduce from ref. [61]

).

A similar strategy was also devised to synthesize a sulfonated mesocomposite polymer/silica while

maintaining the main structure of the mesopores and the porosity.[62]

To generate such materials, they used

divinylbenzene (DVB) and sodium p-styrene sulfonate as the monomers for the copolymerization on the

surface of mesoporous silica under solvothermal conditions (Figure 12). This polymer made the surfaces of

the pores hydrophobic (as determined by contact angle analysis) during sulfonation. This hydrophobicity

significantly reduced catalyst poisoning by water. The mesocomposite exhibited high stability (372 °C)


22

according to TG analysis and an excelling catalytic performance in the esterification of acetic acid with

cyclohexanol, and 1-butanol and the condensation of benzaldehyde with ethylene glycol making this

material remarkably more active than Amberlyst 15, SBA-15-Pr-SO3H, and homogeneous H2SO4.[62]

Figure 12. Copolymerization of divinylbenzene (DVB) and sodium p-styrene sulfonate on the surface of mesoporous

silica to produce a hydrophobic organic polymer-silica composite-based SAM (Reproduced from ref. [62]

).

3.3.2. Sulfonated carbon-silica (SCS) mesocomposites

A facile template carbonization strategy to synthesize ordered large-pore mesoporous silica

microspheres with sulfonated carbon nanoparticles trapped inside accessible mesopores through a solvent-

evaporation-induced aggregating assembly (EIAA) approach is an interesting route towards the design of

advanced carbon-silica-based composites of SAMs. In this approach, amphiphilic poly(ethylene oxide)-b-

polystyrene (PEO-b-PS) and TEOS were used as the template and silica source, respectively. However, the

template was also used as the carbon source for the mesoporous structure. The synthesis of carbon/silica

mesocomposites requires several consecutive steps including a) hydrolysis and condensation of TEOS with

silica in the presence of PEO-b-PS, b) hydrothermal aging of the material obtained in the first step, c) in situ

carbonization of PEO-b-PS to generate carbon nanoparticles within the mesoporous structure, and finally, d)

sulfonation of the obtained carbon-silica mesocomposite to produce a new SAM. PEO-b-PS molecules are

employed not only as a template for the creation of uniform mesopores but also for the production of carbon

nanoparticles in the mesopores. This protocol provides an alternative approach that avoids the use of organic

templates by introducing other external carbon sources. This unique mesocomposite structure with a


23

microsphere shape and abundant SO3H groups exhibited excellent catalytic activity in the condensation of

benzaldehyde with ethylene glycol (93% conversion) and good reusability (Figure 13).[63]

Figure 13. Catalysis by template-carbonized SASM (Reproduced from ref. [63]

).

Viswanadham and coworkers[64]

prepared a new sulfonated silica/carbon mesocomposite through

template carbonization in which glucose was selected as both the template and carbon source. Sulfuric acid

was added to the glucose/TEOS reaction mixture to hydrolyze and sulfonate the carbonaceous part, which

was subsequently converted to the SCS mesostructure via carbonization under a N2 atmosphere (Scheme 7).

Although the sulfonated mesocomposite possesses a less ordered mesopore structure, it exhibited efficient

catalytic activity in the butylation of phenol. The hydrophilicity of the sulfonic acid linked to hydrophobic

carbon can lead to strong interactions with the hydrophilic surface of the silica. Another interesting feature

of this work was the surfactant-free synthesis and the ready availability of glucose in the synthesis of the

mesocomposite.

Scheme 7. Use of glucose as both the template and carbon source for the production of sulfonated silica/carbon

mesocomposites (Reproduced from ref. [64]

).


24

Magnetic separation can also offer additional advantages for certain types of materials/catalysts in terms

of recovery/separation from the reaction mixture after the reaction. In this regard, an advanced, new

sulfonated magnetic spherical silica/carbon mesoporous composite was synthesized by a simple route[65]

as

indicated in Figure 14. First, the silica layer encapsulated the magnetic core (Fe3O4), and that layer was

covered by another carbon layer created via carbonization of the absorbed glucose in the material as the

carbon source. After the successful modification with -SO3H, a solid acid composite carbonized at a low

temperature (400 °C) exhibited the highest acidity (1.98 mmol H+). The high surface area, large pore volume

and high acidity gave this solid acid material excellent catalytic activity in the transesterification of soybean

oil with methanol.[65]

Figure 14. (a) Synthesis of Fe3O4@SiO2@PCS. TEM images of the core/shell structure of (b) Fe3O4@SiO2 and (c)

Fe3O4@SiO2@C (Reproduced from ref. [65]

).

Sugar-derived amorphous carbonaceous materials bearing -SO3H groups cannot catalyze water-

sensitive reactions such as the dimerization of α-methylstyrene because of their small surface areas (Figure

15). Furthermore, these solid acid catalysts also suffer from significant leaching and deactivation that has

not yet been addressed. To solve these issues, a composite of amorphous sulfonated sugar-derived carbon

within mesoporous silica was designed and was reported to exhibit remarkable catalytic activity in the

dimerization of α-methylstyrene. Under the optimized conditions, the selectivity for unsaturated dimers in

the presence of catalyst exceeded 98%. The authors also demonstrated that SO3H groups occupy a large


25

portion of the surface area of the amorphous carbon inside the pores thus preventing the production of side

products i.e., intramolecular Friedel-Crafts alkylation, leading to high catalytic activity and selectivity.[38d]

Figure 15. Composite of amorphous sulfonated carbon inside of mesoporous silica (Reproduced from ref.

[38d]).

3.4. Sulfonated periodic mesoporous organosilicas (PMO-SO3H)

In the development of SAMs as catalysts, many studies have been conducted on their hydrophobicity as this

parameter plays a crucial role in the catalytic activity of solid acid catalysts especially when the reactants are

hydrophobic (i.e., in fatty acid transesterification reactions).[66]

Traditional SASMs could be transformed

into compatible hydrophobic SASMs through the incorporation of a hydrophobic SAP or surface

modification with alkyl or phenyl chains.[4b, 67]

However, these methods have some drawbacks, including

decreasing the pore size and consequently causing molecular transfer limitations in the cases of bulky

compounds, which can be detrimental for the performance of such materials.[4b]

Periodic mesoporous organosilicas can be considered next-generation SASMs with sufficient

hydrophobicity and crystal-like pore walls, and they were first reported by Inagaki and coworkers.[68]

The

first sulfonated benzene-bridged PMO (Ph-PMO-SO3H) was synthesized via the condensation of 1,4-

bis(triethoxysilyl)benzene (BTEB) with an SDA. The obtained white solid was then sulfonated with 25%

SO3/H2SO4 solution. Then, the same group was simultaneously condensed with both BTEB and MPTMS

under basic conditions in the presence of an SDA to fabricate a Ph-PMO with sulfonic acid groups.[69]

The

oxidation of the -SH groups, which are produced in the first step, to -SO3H groups on the surface of the Ph-

PMO can be achieved by treatment with concentrated HNO3 at room temperature for 24 h.


26

Fukuoka and coworkers[70]

hydrolyzed starch in water using two water-tolerant PMO-SO3H materials,

ethylene-bridged (Et-PMO) and phenylene-bridged compounds, and this was the first report of using this

type of catalyst in an aqueous reaction of this kind.[70]

Jerome and coworkers[71]

reported the catalytic

application of Ph-PMO-SO3H in two different reactions including the synthesis of bis(indolyl)methanes

from aromatic aldehydes and indole and the mono-etherification of glycerin with 1-phenylpropan-1-ol under

aqueous conditions.

Cho and coworkers studied the possibility of directly synthesizing 2D hexagonal (p6mm) Ph-PMO-

SO3H mesostructures via the condensation of BTEB and MPTMS in the presence of H2O2 using P123 under

dilute acidic conditions.[72]

Sulfonic acid groups (-SO3H) were successfully generated in situ by the

oxidation of -Pr-SH using H2O2 as the oxidant during the synthesis of the sol-gel. The SEM image showed

that Ph-PMO-SO3H existed as spheres with diameters of 2-5 μm (Figure 16).

Figure 16. (a) TEM and (b) SEM images of 2D hexagonal (p6mm) Ph-PMO-SO3H (Reproduce from ref.

[72]).

The proposed approach was further expanded to the design of Ph-PMO-SO3H mesostructures with 3D

cubic (Pm3n symmetry) structures using a highly acidic medium and CTAB as the SDA (Figure 17a and

b).[73]

In this work, BTEB was condensed with the optimum amount of MPTMS. Ordered and uniform

mesopores could be obtained by using up to 25 mol% of MPTMS in the initial reaction mixture. The

chemical structures of the precursor and the SDA had dramatic effects on the mesostructure. Similarly,

another 3D (Pm3n) cubic Ph-PMO-SO3H was prepared by using a novel allylorganosiloxane precursor, 1,4-

bis(triallylsilyl)phenylene, and cetyltrimethylammonium chloride (C16TMACl) as the SDA in an acidic


27

medium.[74]

This PMO was particularly efficient in Friedel-Crafts acylation reactions and in controlling the

atmospheric emission of volatile organic compounds that are responsible for ground-level ozone, air toxicity

and smog (Figure 17c-f). Sulfonation of the phenylene bridges was performed using concentrated sulfuric

acid.

Figure 17. (a) TEM and (b) SEM images of the Ph-PMO-SO3H mesostructure with a 3D cubic (Pm3n symmetry)

structure. (c) TEM and (d-e) SEM images of 3D (Pm3n) cubic Ph-PMO-SO3H prepared using a novel

allylorganosilane precursor, 1,4-bis(triallylsilyl)phenylene, and C16TMACl. (f) Friedel-Crafts acylation over the 3D

cubic phenylene-bridged mesoporous silica (Reproduced from ref. [73-74]

).

Ph-PMOs have good thermal stability especially in the presence of air because phenyl groups are easily

oxidized, and therefore, the structure of their sulfonated derivatives are maintained at higher temperatures.

In addition, the stability of Ph-PMOs under N2 is remarkably higher than their stability under air. The

thermal stability of Ph-PMO has been extensively studied by TGA.[75]

Accordingly, decomposition of the

phenyl groups is observed from 500-700 °C. Ph-PMO sulfonated with chlorosulfonic acid was further tested

in the esterification of acetic acid with ethanol. The authors claimed that the Ph-PMO synthesized via the

self-assembly of Brij76 and BTEB should be calcined under a N2 atmosphere, and structural changes were

observed with increasing calcination temperatures.[75]

Additionally, the use of acidic SBA-15-like PMOs, in which the location of the acid sites can be

controlled, in acid-catalyzed reactions in hot water (i.e., the hydrolysis of cellobiose) has also been

proposed.[76]

The hybrid silica was prepared by the condensation of BTEB. The material was sulfonated


28

using chlorosulfonic acid or 3-mercaptopropyltrimethoxysilane and further oxidized with H2O2 to afford

materials containing Bronsted acid sites, and were fully characterized. The acidic features of the materials

were characterized by calorimetry of the adsorption of ammonia. Furthermore, the catalytic potentials of the

materials were tested in the gas-phase dehydration of isopropanol as a model reaction and compared with the

reference acidic sulfonated resin, Amberlyst 15. The introduction of -Pr-SO3H by the oxidation of -PrSO3H

groups using H2O2 did not change the mesoscopic structure, while sulfonation with chlorosulfonic did

change the mesoscopic structure. However, the results of the ammonia adsorption calorimetry indicated the

heterogeneity of this solid acid, confirming that there are distinct sulfonation sites. This conclusion was also

supported by XPS analysis.

In the gas-phase dehydration of isopropanol, the solids sulfonated with chlorosulfonic acid exhibited a

catalytic activity equivalent to that of Amberlyst 15, but they were less stable due to leaching of the sulfur

species. SBA-15-like PMOs obtained by H2O2 oxidation of the -SH groups were less acidic and showed

lower catalytic activity in the gas-phase dehydration of isopropanol. However, no significant sulfur leaching

was observed for this catalyst. The catalytic activities of these materials were also tested in a biomass

valorization such as in cellobiose hydrolysis in hot water. The solids were active at 150 °C; however, a

remarkable amount of sulfur leaching was observed, and therefore, the reaction proceeded mainly

homogeneously, especially with the chlorosulfonic acid-modified material. The author found that a

pretreatment step including the hot washing of catalysts containing Pr-SO3H moieties lead to a decrease in

their activities in hydrolysis reactions and increases in their stability and recyclability (Scheme 8).[76]


29

Scheme 8. Synthesis of double sulfonated phenylene-bridged PMO-SO3H. [76]

Bion and coworkers also studied four different types of SASMs and monitored the catalytic activities in

the aqueous synthesis of bis(indolyl)methanes. They extensively studied the effect of hydrophobicity on the

catalyst activities and found that an increase in hydrophobicity enhanced the catalytic efficiency.[77]

After the calculation of the TOFs, they demonstrated that among all the samples, Ph-PMO-Pr-SO3H

exhibited the highest catalytic activity, TOF, yield, and H+ exchange capacity. Embedding the phenyl groups

within the pore walls can significantly increase the hydrophobicity relative to the corresponding SBA-15-

PrSO3H. This result was obtained based on the adsorption behavior of toluene/water over both SASMs. The

TOF was highly dependent on the -SO3H loadings. Therefore, increasing the -SO3H loading improved the

catalytic activity of the systems. The authors claimed that silylation of the silanol groups increased the

hydrophobicity and consequently improved the catalytic efficiency (Scheme 9).[74]


30

Scheme 9. Comparing the catalytic activities of four different types of sulfonated mesoporous materials.[74]

Ferreira and Jérôme[78]

extensively studied the effects of various parameters on the catalytic activity of

Ph-PMO-SO3H including the effects of the density and the location of -SO3H groups and the hydrophobicity

on the conversion of fructose to 5-hydroxymethylfurfural (HMF). The series of synthesized catalysts were

contained phenylene and biphenylene bridges. In addition, the sulfonation was achieved by two different

methods: the first is the sulfonation of the benzene rings with sulfuric acid and the second is via

condensation with MPTMS and subsequent oxidation to sulfonic acid. Among the various loadings of

sulfonic acid sites, 0.36 mmol g-1

was reported to be the optimum loading for sulfonated PMOs. In addition,

of the two sulfonation methods, propylsulfonic acid linkers resulted in catalysts with higher activity in the

conversion of fructose to HMF. A comparison between BTEB and 4,4′-bis-(triethoxysilyl)biphenyl

(BTEBP) bridges indicated that the phenylene bridge provided superior activity compared to the

biphenylene bridge (Figure 18).


31

Figure 18. Using Ph-PMO-SO3H for HMF production (Reproduce from ref. [78]

)

Yang and coworkers[79]

also prepared a new Ph-PMO-SO3H bearing aliphatic chains between the Si and

benzene. 1,4-Bis(trimethoxysilylethyl)benzene (BTSEB) was employed as an organic moiety in the PMOs

and was condensed with TMOS. The authors claimed obtain highly ordered PMOs was difficult due to the

long chain and flexible organic group in BTSEB when it was utilized in the absence of TMOS. In the second

step, the prepared Ph-PMO was sulfonated with chlorosulfonic acid, and the resulting catalyst was

successfully employed in the esterification of hexanoic acid with ethanol (Scheme 10). In the synthesis of

the catalyst, two molar percentages of BTSEB in the TMOS/BTSEB mixture were 30 and 70 mol%. The

mesostructure generated with 70% BTSEB was relatively collapsed, while in the material prepared with

30% BTSEB, the mesostructure was retained. In most cases, increasing the molar ratio of the organic bridge

leads to the deterioration of the mesostructure. However, in this work, under similar conditions, 30 mol% of

BTSEB as the bridge resulted in a lower catalytic activity than that of the material prepared with 70 mol%

BTSEB. This observation can be attributed to the high concentration of sulfonate groups in the 70 mol%

material via the phenylene bridges on the surface.[79]


32

Scheme 10. Sulfonation of the aliphatic chain-containing Ph-PMO for esterification reactions.[79]

Kaliaguine[80]

synthesized a series of ethylene-bridged PMOs with different amounts of sulfonic acid

via the condensation of MPTMS with bis(trimethoxysilyl)ethane (BTME) under basic conditions in the

presence of cetyltrimethylammonium chloride (CTAC) as the SDA. The obtained SASM had an ordered

Pm3n cubic-like mesostructure with a high surface area (up to 950 m2/g) and narrow pore size distribution

(up to 3.50 nm). The order of the structural was retained during oxidation. This group also examined the

effects of the amount of MPTMS (15, 25, 50 mol% of MPTMS in PMO) on the structural properties,

morphology, and thermal behavior of the material. With 15 mol% of MPTMS, the morphology, XRD

pattern, and BET results were similar to those of the parent mesopore structure, while with 50 mol% of

MPTMS, a significant deformation of the morphology and a decrease in the intensity of the peak at 2θ ~ 2°

was observed. This observation indicates that by increasing the amount of MPTMS to 50 mol%, a

significant loss of mesostructural ordering occurred. In the case of 50 mol%, other diffraction peaks also

disappeared, probably due to the decreased mesostructural ordering by the functionalization.

As mentioned discussed in the context of traditional SASMs, researchers tend to improve the catalytic

efficiency of SASMs through the incorporation of additional group(s), making them bifunctionalized

materials. For instance, -SO3H is widely used in the presence of amine-type functionalities to improve the

catalytic behavior. Shylesh and Thiel’s groups[81]

introduced amine groups via post-modification of BTEB-

based PMO-SO3H to obtain bifunctionalized PMO-based systems. To achieve bifunctionalization, they first

condensed BTEB with Boc-protected APTS and then sulfonated the intermediate with chlorosulfonic acid.


33

Finally, the protecting group was removed from the catalyst before its utilization in nitroaldol condensations.

The results showed a quantitative product conversion with 97.5% yield. The authors also tested the catalytic

activity of N-protected PMO-SO3H/-NHBoc in this a nitroaldol condensation, and they observed no

conversion. However, the protected system exhibited remarkable activity in the conversion of

dimethoxyphenylmethane to benzaldehyde and provided the target product in quantitative yield. This

catalytic system was also compared with monofunctionalized mesoporous materials (SBA-NH2 and PMO-

SO3H), and the results indicated PMO-SO3H/-NH2 was more efficient in this reaction. The protons from the

-SO3H groups in these mesostructured materials can be exchanged with positively charged materials. This

method is an alternative and useful strategy for immobilizing anionic and even bulkier catalytically active

species.[82]

On the other hand, sulfonate groups can act as a ligands to immobilize transition metals. Zhang

and Li[83]

used Ph-PMO-SO3H to electrostatically immobilize Sc(OTf), a catalytically active transition

metal, through chelation with the -SO3H moieties (Scheme 12) to generate an appropriate hybrid catalyst for

Mukaiyama-aldol reactions. The superior catalytic activity of this material was correlated to the

mesoporosity and hydrophobicity of the pore walls due to the presence of embedded phenyl moieties.

Scheme 11. Synthesis of a bifunctionalized acid-based PMO to catalyze nitroaldol reactions.[81]


34

Scheme 12. Supporting Sc(OTf)2 on sulfonated Ph-PMO.[83]


35

Inagaki and coworkers[84]

synthesized the first biphenylene-bridged PMO (biPh-PMO-SO3H) in 2002,

and then they separately functionalized biPh-PMO-SO3H and Ph-PMO-SO3H using a new approach.[85]

Their methodology involved a post-functionalization with two organosilane epoxides and subsequent

treatment with sulfite to produce a new series of PMO-SO3H derivatives. The catalytic activities of the

newly synthesized PMO-SO3H materials were investigated in the esterification of acetic acid with ethanol,

the acylation of benzyl alcohol with acetic acid and the condensation of phenol with acetone (Scheme

13).[85]

When comparing the catalytic activities of these four catalysts with those of previously reported

catalysts, including MCM-SO3H and Ph-PMO, which were both sulfonated by conc. sulfuric acid and

MPTMS through both grafting and condensation approaches. Indeed, testing the catalysts in the

esterification of acetic acid with ethanol indicated that the new biphenylene-bridged PMOs, including BiPh-

PMO-CySO3H and Ph-PMO-GlySO3H, are the most active catalysts among those tested for this reaction.

For instance, these new phenylene-based PMOs were more active catalysts than those that were sulfonated

by conc. sulfuric acid. These observations indicate that the new SAPs are more efficient than traditional

SAPs such as MPTMS and conc. sulfuric acid.[85]


36

Scheme 13. Synthesis of sulfonated phenylene- and biphenylene-bridged PMOs via epoxide

functionalization and ring opening.[85]

Imidazolium ionic liquid-PMO (IL-PMO) is a type of mesostructure in which the imidazolium salt is

embedded inside the PMO framework, and they were first designed by Karimi and coworkers.[86]

Elhamifar

and Karimi[87]

modified imidazolium-based IL-PMOs by grafting MPTMS onto their surface and then

oxidizing them to generate -Pr-SO3H groups on the modified surface. The catalytic activities of these

compounds were then tested in the esterification of various alcohols and acids at room temperature, and

good to high yields were observed. The TEM images indicated that this modification did not significantly

influence the mesoscopic structure of the IL-PMO (Scheme 14). In another work, the same group

synthesized this material by the condensation of TMOS, MPTMS, and the IL bridge in the presence of P123

as a surfactant. The sulfonic acid groups present in the ionic framework of the imidazolium chloride were

highly active. This catalyst had higher recyclability and shorter reaction times than the corresponding

SASMs in Biginelli reactions for the synthesis of dihydropyrimidinones (Scheme 14). Condensation with

TMOS did not negatively impact the hexagonal array of the mesoporous organosilica.[88]

Scheme 14. Imidazolium-bridged PMO sulfonated by MPTMS as a catalyst for Biginelli and esterification reactions

(Reproduce from ref. [87]

).


37

Additionally, a new and straightforward post-synthetic method involving the use of mercaptol/H2O2 or

concentrated H2SO4 for the sulfonation of the ethylene groups of ethylene-PMO to generate a new type of

PMO-SO3H was recently developed.[93]

Synthesized hexagonal mesoporous material with ethenylene-silica

pore walls was obtained by with different pore size distributions using different SDAs (P123, Brij76, and

Brij56). For the sulfonation, the ethylene groups were first converted to epoxides by epoxidation at 5 °C.

The resulting epoxides sulfonated to generate HME-SO3H, a β-hydroxysulfonic acid, in the presence of

bisulfite ions at 65 °C. The catalytic activity of HME-SO3H was tested in the esterification of acetic acid

with ethanol at 70 °C for 24 h. The epoxidation step was hypothesized to play a crucial role in determining

the yield of -SO3H groups attached to the silica surface. Therefore, they optimized temperature, oxidant, pH,

water content and reaction time in the epoxidation reaction (Scheme 15).

Scheme 15. Sulfonation of vinylene-bridged PMO by epoxidation followed by nucleophilic attack of the ring by

sulfite.[93]

Kondo and coworkers[94]

also prepared a new type of PMO-SO3H by the Diels-Alder reaction of the

ethenylene groups on the framework of hybrid mesoporous ethenylene-silica (HME) with pendant

phenylene groups followed by sulfonation with concentrated H2SO4. The catalytic activity of this hybrid

mesostructure was investigated in three types of reactions, including the esterification of acetic acid and

Beckmann and pinacol-pinacolone rearrangements. The activity of Ph-SO3H HME was compared with those

of various other catalysts such as H-Beta, H-ZSM5, H3PW12O40, Amberlyst 15, p-TsOH, and Nafion-H

(NR50), and PMO showed superior selectivities and conversions compared to the other catalysts in the


38

selected chemistries. The replacement of anthracene with the diene surrogate in the Diels-Alder reaction

with HME (Scheme 16) also led to a new Ant-HME-SO3H material,[95]

which was also successfully

employed in the esterification of acetic acid with ethanol.

Scheme 16. Sulfonation of post-synthesized vinylene bridged PMO by using Diels-Alder reaction.[95]


39

a simple approach for the preparation of a new class of bifunctionalized PMO-SO3H materials featuring

partly unoxidized thiol groups was also recently disclosed.[96]

Au nanoparticles could be synthesized within

the pores of PMO which contains -SH functionalized groups and in situ Au3+

reduction inside the pores

under acidic conditions. Au nanoparticles with a uniform, narrow size distribution of approximately 1-2 nm,

which is of great significance for catalytic reactions, could indeed be produced within Et-PMO-SH/SO3H

materials (Scheme 17). This group investigated the synthesized catalysts in a number of reactions including

alkyne hydration, intramolecular hydroamination, styrene oxidation and three-component coupling

reactions. The amphiphilicity of the Et-PMO-SO3H/-SH/Au nanostructures enabled the organic reactions to

be performed efficiently in pure water with no organic cosolvent. The catalyst recyclability highlighted the

possibility of reusing the catalysts at least 10 times without appreciable loss of catalytic efficiency in the

model reactions.[96]

Scheme 15. Bifunctional thiol-sulfonic acid-based Et-PMO that can catalyze multiple organic reactions.[96]

An uniform distribution of SO3H groups is crucial for certain applications e.g., in PEM fuel cells.[89]

One of

the most interesting works in the field of PMO-SO3H materials is the design of uniformly distributed PMO-


40

SO3H through bridged sulfides followed by oxidation to the sulfonic groups.[33, 89-90]

Additionally, the

location of the -SO3H groups has a substantial impact on the catalytic activity of the SASMs.[91]

In this

method, generating the appropriate number of bridged sulfides is essential for obtaining a nanoporous

structure.[33a]

Romero-Salguero and coworkers[33a]

designed two types of SASMs with highly distributed

tetrasulfide-bridged linkers that were synthesized by the condensation of

bis[3-(triethoxysilyl)propyl]tetrasulfide with TEOS or BTEB under acidic conditions in the presence of

Brij76 as the SDA. Hydrogen peroxide was then selected to convert the tetrasulfides to sulfonic acid groups.

In this regard, sulfonic acids were uniformly distributed over the framework. The authors claimed that

increasing the S content in the syntheses of these SASMs caused significant erosion of the mesoscopic

structure. Therefore, with a lower tetrasulfide content, a more ordered mesoscopic structure was obtained

(Scheme 18).

The optimized derivative of this SASM was employed in the esterification of acetic acid with ethanol, and

the results indicated that the activities of PMO-SO3H materials and Amberlyst-15 were comparable.


41

Scheme 18. Using tetrasulfide- and disulfide-bridged PMO for sulfonation reactions.[33a]

Scheme 19. Synthesis of bifunctionalized disulfide-bridged sulfonated PMOs with amine functionalities.[33b, 90]

Mehdi and coworkers[33b, 90]

also synthesized a bifunctionalized PMO containing disulfide groups within

the framework and propyl amine moieties on the surface. The sulfides were then converted into -SO3H

groups through one sequential reduction and oxidation processed using NaBH4 and H2O2, respectively

(Scheme 19). Similar mesoporous organosilica materials with disulfide bridges, bis[3-

(triethoxysilyl)propyl]disulfide (BTPDS), were also prepared and chemically oxidized to generate sulfonic

acid groups.[92]

The ratio of BTPDS to TMOS was found to be crucial to the mesoscopic structure of the

mesoporous organosilica. For example, by increasing the molar ratio of BTPDS, the mesostructure shifts

from a 2D hexagonal structure to a cellular foam-like structure. The structural characterization after the

oxidation showed that a remarkable change occurred in the mesostructure especially when the molar ratio of


42

disulfide was high (Figure 19). These materials were investigated it in the esterification of aliphatic acids

with ethanol.

Figure 19. Effect of the TMOS:BTPDS ratio on the porous structure of PMO. Molar ratios of (a) 10; (b) 10-SO3H; (c)

20; (d) 20-SO3H; (e) 30; (f) 30-SO3H; (g) 40; and (h) 40-SO3H (Reproduce from ref. [92]

).


43

4. Conclusions

Several SAMs that can be sulfonated by various approaches are discussed and reviewed in this paper. Due to

presence of sulfonic acid functionalities, these SAMs are promising candidates for solid Bronsted acid

catalysts for organic transformations. Therefore, several SAPs and their applications, features and properties

were discussed in terms of their use in the further modification of mesoporous materials. In addition, the

catalytic power and applications of these SAMs, which are reported for a variety of catalyzed reaction, were

addressed. Sulfonation can provide a rapid method of manufacturing catalytically effective mesopores.

Moreover, sulfonic acid groups on the surface of mesopores can act as active and stable Bronsted acid sites

to catalyze a wide range of organic reactions. Additionally, the newly developed SAMs that were mentioned

in this article may have more opportunities for further investigation.


44

5. Abbreviation

ATPS aminopropyltrialkoxysilane

TOF turnover frequency

PMO periodic mesoporous organosilica

SDA structure directing agent

BTEB 1,4-bis(triethoxysilyl)benzene

BTEBP 4,4′-bis-(triethoxysilyl)biphenyl

PEG polyethylene glycol

IL ionic liquid

HMF 5-hydroxymethylfurfural

TEOS tetraethylorthosilicate

TMOS tetramethylorthosilicate

MPTMS 3-mercaptopropyltrimethoxysilane

SASM sulfonic acid-based silica mesostructure

SACM sulfonic acid-based carbon mesostructure

SAM sulfonic acid-based mesostructure

SAP sulfonic acid-based precursor

HME hexagonal mesoporous ethenylene-silica

AIBN azobisisobutyronitrile

CTAB cetyltrimethylammonium bromide

Boc tert-butyloxycarbonyl

BTSEB 1,4-bis(trimethoxysilylethyl)benzene

BTME bis(trimethoxysilyl)ethane

CTAC cetyltrimethylammonium chloride

SCS sulfonated carbon-silica

SAL sulfonic acid linker

Si-PMA alkoxysilylated poly(methyl acrylate)


45

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