ZnAlMCM-41: a very ecofriendly and reusable solid acid catalyst for the highly selective synthesis of 1,3-dioxanes by the Prins cyclization of olefins

Article abstract of DOI:10.1039/d0dt04158k

The Prins cyclization of styrene (SE) with paraformaldehyde (PFCHO) was conducted with mesoporous ZnAlMCM-41 catalysts for the synthesis of 4-phenyl-1,3-dioxane (4-PDO) using a liquid phase heterogeneous catalytic method. For a comparison study, the Prins cyclization reaction was also conducted over different nanoporous catalysts,e.g.mesoporous solid acid catalysts, AlMCM-41(21) and ZnMCM-41(21), and microporous catalysts, USY, Hβ, HZSM-5, and H-mordenite. The recyclable mesoporous ZnAlMCM-41 catalysts were reused in this reaction to evaluate their catalytic stabilities. Since ZnAlMCM-41(75) has higher catalytic activity than other solid acid catalysts, washed ZnAlMCM-41(75)/W-ZnAlMCM-41(75) was prepared using an efficient chemical treatment method and used with various reaction parameters to find an optimal parameter for the highly selective synthesis of 4-PDO. W-ZnAlMCM-41(75) was also used in the Prins cyclization of olefins with PFCHO and formalin (FN, 37% aqueous solution of formaldehyde (FCHO)) under different reaction conditions to obtain 1,3-dioxanes, which are widely used as solvents or intermediates in organic synthesis. Based on the nature of catalysts used under different reaction conditions, a reasonable plausible reaction mechanism for the Prins cyclization of SE with PFCHO is proposed. Notably, it can be seen from the catalytic results of all catalysts that the W-ZnAlMCM-41(75) catalyst has higher 4-PDO selectivity with exceptional catalytic activity than other microporous and mesoporous catalysts.

Full text of DOI:10.1039/d0dt04158k

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ZnAlMCM-41: a very ecofriendly and reusable solid
acid catalyst for the highly selective synthesis of
1,3-dioxanes by the Prins cyclization of olefins†
Cite this: Dalton Trans., 2021, 50,
1672
Manickam Selvaraj, *a Mohammed A. Assiri,a Hari Singh,b
e
Jimmy Nelson Appaturi, c Ch. Subrahmanyam d and Chang-Sik Ha
The Prins cyclization of styrene (SE) with paraformaldehyde (PFCHO) was conducted with mesoporous
ZnAlMCM-41 catalysts for the synthesis of 4-phenyl-1,3-dioxane (4-PDO) using a liquid phase hetero-
geneous catalytic method. For a comparison study, the Prins cyclization reaction was also conducted
over different nanoporous catalysts, e.g. mesoporous solid acid catalysts, AlMCM-41(21) and ZnMCM-41
(21), and microporous catalysts, USY, Hβ, HZSM-5, and H-mordenite. The recyclable mesoporous
ZnAlMCM-41 catalysts were reused in this reaction to evaluate their catalytic stabilities. Since
ZnAlMCM-41(75) has higher catalytic activity than other solid acid catalysts, washed ZnAlMCM-41(75)/
W-ZnAlMCM-41(75) was prepared using an efficient chemical treatment method and used with various
reaction parameters to find an optimal parameter for the highly selective synthesis of 4-PDO.
W-ZnAlMCM-41(75) was also used in the Prins cyclization of olefins with PFCHO and formalin (FN, 37%
aqueous solution of formaldehyde (FCHO)) under different reaction conditions to obtain 1,3-dioxanes,
which are widely used as solvents or intermediates in organic synthesis. Based on the nature of catalysts
used under different reaction conditions, a reasonable plausible reaction mechanism for the Prins cycliza-
tion of SE with PFCHO is proposed. Notably, it can be seen from the catalytic results of all catalysts that
the W-ZnAlMCM-41(75) catalyst has higher 4-PDO selectivity with exceptional catalytic activity than other
microporous and mesoporous catalysts.
Received 7th December 2020,
Accepted 24th December 2020
DOI: 10.1039/d0dt04158k
rsc.li/dalton
Brønsted acids and Lewis acids, e.g., phosphoric acid (H3PO4),
acetic acid (CH3COOH), boron trifluoride (BF3) and stannic
1. Introduction
The addition of aldehydes and ketones to alkenes in the pres- chloride (SnCl4), were used as catalysts.3–5 The mineral
ence of Brønsted acids is usually called the Prins reaction, Brønsted and Lewis acid catalysts used in homogeneous cata-
which is a straightforward catalyst method for the synthesis of lytic systems can easily damage the reactors because they
1,3-dioxanes using a variety of olefins with FCHO in the pres- cause high corrosion and excessive toxic wastes. The recovery
ence of sulfuric acid (H2SO4), which is called a mineral of identical catalysts takes a long time after the reaction is
Brønsted acid catalyst, under a homogeneous medium, and in complete, most undesirable side products are likely formed by
acid-catalyzed condensation reactions, water and glacial acetic the use of large quantities of raw chemicals, and many steps
acid were used as solvents.1,2 Citronellal to isopulegol is one are needed to recover them from the main product. A major
of the Prins cyclization reactions.2 In this reaction, many drawback is that homogeneous catalysts cannot be used as
recyclable catalysts in the chemical industry and are con-
sidered as non-ecofriendly.
aDepartment of Chemistry, Faculty of Science, King Khalid University, Abha 61413,
In order to overcome the above problems, the hetero-
Saudi Arabia. E-mail: mselvaraj@kku.edu.sa; Fax: +966 17 241 7637
geneous catalytic chemistry mainly involves the development
of a highly efficient and versatile catalyst system for producing
1,3-dioxanes through Prins cyclization reactions. Moreover, the
heterogeneous catalytic system produces the main product
with high selectivity, reduces the trace number of byproducts
and replaces hazardous and stoichiometric reagents, in which
the ecofriendly catalytic materials are easily recovered from the
final reaction mixture and used as recyclable catalysts. Recent
bDepartment of Chemistry, School of Basic and Applied Sciences, RIMT University,
Mandi Gobindgarh, Punjab, 147301, India
cSchool of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
dDepartment of Chemistry, Indian Institute of Technology Hyderabad, Kandi,
Sangareddy, Telangana, India
eDepartment of Polymer Science and Engineering, Pusan National University,
Busan 609-735, Republic of Korea
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0dt04158k
1672 | Dalton Trans., 2021, 50, 1672–1682
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research has focused on replacing traditional homogeneous
Prins cyclization of SE with PFCHO was conducted in a
catalysts with green heterogeneous catalysts. For the Prins liquid phase reaction using different nanoporous catalysts
cyclization reaction of olefins, several heterogeneous solid acid under vigorous stirring in a thermostated quartz-vessel reactor.
catalysts have been designed and developed, e.g., zeolites, In a typical experimental procedure, 100 mg of ZnAlMCM-41
organic heteropolysalts, sulfonic group functionalized SBA-15 (75) was placed in the reactor with the required amounts of SE
MoO3/SiO2 and sulfated zirconia.6–11 Using the above catalysts, (20 mmol), PFCHO (20 mmol) and 1,2-dichloroethane solvent
1,3-dioxane derivatives synthesized by the Prins cyclization of (5 mL). The reactor fitted with a condenser was slowly heated
olefins with PFCHO/FN are used in industry as solvents, plasti- to 353 K with continuous stirring. After reaching 353 K, the
cizers, curing agents and pigment dispersants, and as inter- reaction mixture was continuously refluxed for 6 h under
mediates in the preparation of pharmaceuticals in medicinal atmospheric pressure. After completion of the reaction, the
organic synthetic chemistry.6–13
products were collected through the recovery of ZnAlMCM-41
Corporate pressure to reduce the amount of toxic waste (75). The products cooled to room temperature were centri-
arising from environmental protection laws and chemical pro- fuged, analyzed with authentic samples using a gas chromato-
cesses has fostered the development of innovative and environ- graphy system (GC, Shimadzu-2041, FID detector) equipped
mentally friendly chemical technologies. For application in with an RTX-5 column (0.25 mm I.D. and 30 m length). All the
fine chemical industries, suitable solid acid catalysts are in products were further confirmed using a combined gas chrom-
high demand, synthesized with a large number of catalytically atography-mass spectrometry system (GC-MS; Hewlett G1800A).
active sites, and used under mild reaction conditions without
To find the best catalyst, various nanoporous catalysts were
the use of toxic materials. For the green catalytic process by used for the Prins cyclization of SE with PFCHO under similar
reducing environmental pollution, Selvaraj’s research group, reaction conditions. The studies of recyclable and washed cata-
for the first time, developed hexagonally mesostructured lysts were performed. The Prins cyclization of SE with PFCHO
ZnAlMCM-41 catalysts with good physiochemical properties.14 was also performed with different solvents to identify the best
ZnAlMCM-41catalysts have been widely used for the selective solvent. Various reaction parameters, i.e. the temperature, time
synthesis of nopol through the Prins condensation reaction.15 and stoichiometric ratio of the reactants (SE to PFCHO ratios),
To the best of our knowledge, ZnAlMCM-41 catalysts have not were studied to determine the optimal reaction conditions. For
been used for the Prins cyclization of olefins with PFCHO/FN, the production of 4-PDO, FN was also used instead of PFCHO.
and their catalytic results have not been clearly reported in the The Prins cyclization of olefins with PFCHO was also con-
open literature. Our present investigation brings out a unique, ducted with different reaction times and temperatures under
eco-friendly and simple method for the selective synthesis of similar reaction conditions. The organic products were
4-PDO and different 1,3 dioxanes using low cost reactants extracted after completion of the reaction, cooled to room
under a liquid phase catalytic system while true heterogeneous temperature, and analysed with authentic samples by GC and
catalysts, ZnAlMCM-41, have been recently used in this catalyst further confirmed by GC-MS. The olefin conversion, product
system. The catalytic results obtained from PFCHO and FN yield and selectivity were calculated using the standard for-
using ZnAlMCM-41 catalysts were thoroughly discussed to find mulas. The NMR (1H and 13C) data for the selected 1,3-dioxane
the best one. The catalytic results obtained over ZnAlMCM-41 products are specified in the ESI.†
catalysts, for the selective synthesis of 1,3-dioxanes, have also
been correlated and compared with those of other nanoporous
solid catalysts, such as AlMCM-41(21), ZnMCM-41(21), USY,
Hβ, HZSM-5 and H-mordenite.
3. Results and discussion
3.1. Selective synthesis of 4-PDO
The Prins cyclization of SE with PFCHO was carried out over
different nanoporous solid catalysts with a corresponding ratio
2. Experimental
of 1 : 2 at 353 K for 6 h, as noted in Table 1. In this cyclization
According to previously published procedures,14,16,17 various reaction, 4-PDO formed as a major product by cycloaddition of
mesoporous catalysts ((ZnAlMCM-41, nSi/(nZn + nAl) = 75, 151, the primary product intermediate, 3-phenylpropanol-like
228, 304 and 380), (AlMCM-41, nSi/nAl = 21), (ZnMCM-41, nSi/ cation, with another FCHO molecule being derived from
nZn = 21), and MCM-41) were synthesized using a direct hydro- PFCHO, as illustrated in Scheme 1.
thermal method and characterized. The discussion of physico-
The sulfonic acid functionalized SBA-15 (SBA-15-SO3H) pro-
chemical characterization and the details of the synthesis pro- duced 100% 4-PDO selectivity when the Prins cyclization was
cedures of mesoporous catalysts are given in the ESI.† conducted with SE and FCHO at a molar ratio of 1 : 2 with di-
Microporous catalysts, Hβ (nSi/nAl = 20, Strem), HY (nSi/nAl
2.9, PQ), H-mordenite (nSi/nAl = 20, PQ), and HZSM-5 (nSi/nAl
=
=
chloromethane as a solvent in refluxing temperature for 35 h
in a liquid phase reaction but catalyzed to 52% 4-PDO selecti-
30, PQ), were obtained as commercial sources. Prior to the use vity only when water was used as a solvent under similar reac-
of catalytic materials in this catalytic reaction, they were cal- tion conditions.7 The catalytic results show that SBA-15-SO3H
cined at 473 K in air for 6 h to completely remove moisture has large numbers of free silanol groups, which reduce hydro-
from their inner pore surfaces.
phobicity and cause the migration of acid sites, indicating that
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Table 1 Prins cyclization of SE with PFCHO and FN over various types of catalystsa
Catalysts (100 mg)
Pore size (Å)
Conversion of SE (%)
Selectivity of 4-PDO (%)
Yield of 4-PDO (%)
ZnAlMCM-41(75)
ZnAlMCM-41(151)
ZnAlMCM-41(228)
ZnAlMCM-41(304)
ZnAlMCM-41(380)
HY (2.6)
Hβ (20)
H-Mordenite (20)
HZSM-5(15)
AlMCM-41(21)
ZnMCM-41(21)
MCM-41
Without catalyst
ZnAlMCM-41(75)b
ZnAlMCM-41(75)c
ZnAlMCM-41(75)d
ZnAlMCM-41(75)e
22.8
25.6
27.0
29.3
32.2
7.4
7.6 × 5.4
7.1
5.4
27.5
27.3
29.3
—
22.8
22.8
22.8
22.8
100
91
83
75
67
60
64
59
57
85
40
11
9
100
99
97
93
90
49
79
43
20
95
81
38
33
100
100
100
100
100
90
80
70
60
29
51
25
11
81
32
4
3
99
100
97
100
99
100
97
100
a Reaction conditions: 10 mmol of SE, 20 mmol of PFCHO, 5 ml of 1,2-dichloroethane, temperature = 353 K, time = 6 h. b 30 mmol of FN (37%
aqueous solution of FCHO) instead of 20 mmol of PFCHO. c The reaction was carried out with PFCHO at 413 K in an autoclave under autogenous
pressure for 3 h. d 30 mmol of FN without solvent. e The reaction was carried out using 30 mmol of FN at 413 K in an autoclave under autogenous
pressure for 3 h.
Scheme 1 Plausible reaction mechanism for the Prins cyclization of SE with PFCHO over ZnAlMCM-41 catalysts.
FCHO is an inappropriate source for the Prins cyclization of when 1,2-dichlorethane was used as a solvent. Although the SZ
SE over SBA-15-SO3H. The sulfated zirconia (SZ) catalyst was catalyst produces a better 4-PDO selectivity, the characteristic
used in the Prins cyclization with SE and PFCHO at a molar results show that the surface area of SZ is fourteen times less
ratio of 1 : 3 at 353 K for 7 h and gave 92% 4-PDO selectivity than that of the ZnAlMCM-41(75) catalyst whereas the SZ cata-
1674 | Dalton Trans., 2021, 50, 1672–1682
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lyst has different pore sizes, which enhance the production of than other porous catalysts, as depicted in Table 1, because its
polymerized products with 8% selectivity.11 The Wells-Dawson mild acid sites are more suited to this catalytic system than
type heteropolyacid (H6P2W18O62·24H2O) catalyst yielded 68% other catalysts, as revealed elsewhere.8 ZnMCM-41(21) pro-
selectivity with 4.8% SE conversion at 298 K when the Prins duces a lower yield of 4-PDO with 81% selectivity than other
cyclization of SE was conducted with FCHO using water as a mesoporous catalysts but increases the selectivities of the
solvent.9 This result indicates that the heteropolyacid catalyst byproducts, e.g. cinnamyl alcohol (15%) and 3-phenylpropio-
has high hydrophilicity with a low surface area, and even it naldehyde (4%, 3-PPCHO). This catalytic result shows that the
has more strong acid sites compared to solid acid catalysts acid sites of ZnMCM-41(21) could not favor the Prins cycliza-
while water molecules inhibit its acid sites during the pro- tion reaction, as revealed in the literature.19 Based on the lit-
duction of 4-PDO. Although the triflic acid (TfOH) catalyst erature results, the results of FT-IR-pyridine and ammonia-
gives a 92% yield of 4-PDO at 353 K for 10 h during the Prins TPD indicate that the acid sites of all microporous catalysts
cyclization of SE with FCHO, it is a non-ecofriendly catalyst are almost identical, except for ZSM-5, as confirmed else-
due to the highly corrosive and eye irritant liquid and it where.20,21 Even though zeolite Hβ(20) with pore sizes of 7.6
cannot be easily reused.18 To overcome these disadvantages, and 5.4 Å has fewer acid sites than HY(2.6) due to a lower
mesoporous ZnAlMCM-41 catalysts were used in Prins cycliza- aluminum content, it produces a higher yield of 4-PDO with
tion, and they showed excellent 4-PDO selectivity. This is due 64% selectivity than HY(2.6). This catalytic result implies that
to the Zn ions incorporated into mesoporous AlMCM-41 cata- the pore size (7.4 Å) of HY(2.6) could not enhance in the pro-
lysts, which are very favorable for cyclization, condensation, duction of 4-PDO but it rather increases the selectivity of cin-
hydrogenation and dehydrogenation, as reported elsewhere.16 namyl alcohol (40%), as shown in Table 1. These catalytic
The yield of 4-PDO, as shown in Table 1, increases in the order results clearly demonstrate that both catalysts (Hβ and HY)
of ZnAlMCM-41(75) > ZnAlMCM-41(151) > AlMCM-41(21) ≈ cannot generate high yields of 4-PDO in their inner pores due
ZnAlMCM-41(228) > ZnAlMCM-41(304) > ZnAlMCM-41(380) > to diffusion limit constraints for 4-PDO; furthermore, large
Hβ(20) > ZnMCM-41(21) > HY(2.6) > H-mordenite(20) > numbers of reactants and large organic molecules cannot nor-
HZSM-5(15) > MCM-41. Mesoporous ZnAlMCM-41(75) has mally enter into their inner pores as commonly reported else-
higher SE conversion (100%) and 4-PDO selectivity (100%) as where.21 In contrast, H-mordenite(20) and HZSM-5(15) are less
well as 100% yield of 4-PDO compared to other mesoporous active in the reaction because aryl-1,3-dioxanes are too large
and microporous catalysts (Table 1) and shows excellent cata- and cannot enter into their pores (ZSM-5 (5.4 Å) and
lytic activity that enhances the 4-PDO selectivity because it H-mordenite (7.1 Å)). Hence, 4-PDO forms only on the outer
does not have any diffusional constraints. While the results of surfaces of ZSM-5 and H-mordenite, as referred to in the litera-
the catalytic activity prove that ZnAlMCM-41(75) has a larger ture.8 Although this reaction is catalyzed by moderate active
pore size than the molecular size of 4-PDO as reported else- sites, H-mordenite(20) produces a higher yield of 4-PDO with
where.16 Due to an increase in the amounts of Zn and Al ions, 43% selectivity than HZSM-5(15). These catalytic results show
as shown in Tables 1S and 2S†, ZnAlMCM-41(75) has higher that the high-acid sites and small pore-sized zeolites of ZSM-5
Brønsted acid sites with a less pore size compared to other do not require the Prins cyclization reaction, as mentioned in
ZnAlMCM-41 catalysts while its acidity enhances the 4-PDO Table 1. Overall, this is corroborated from all the above cata-
selectivity compared to other ZnAlMCM-41 catalysts. lytic results, as all the mesoporous catalysts, viz. ZnAlMCM-41
Additionally, within
6 h, the ZnAlMCM-41(75) catalyst and AlMCM-41(21), with the exception of ZnMCM-41(21),
improves the selectivity of 4-PDO compared to other catalysts produce higher yields of 4-PDO with exceptional selectivity
e.g. SZ and TfOH, as observed in the literature.11,18 In this cata- compared to all the microporous catalysts, which are not suit-
lytic reaction, the selectivity (100%) of 4-PDO remains constant able for this reaction to achieve a higher yield of 4-PDO.
when FN is used instead of PFCHO/when the catalytic reaction Furthermore, ZnAlMCM-41(75) proves to be an excellent cata-
is carried out with PFCHO at 413 K for 3 h under autogenous lyst for the highly selective synthesis of 4-PDO. In conclusion,
pressure/when FN is used without solvent/when the Prins cycli- it confirms that the highly ordered mesopore with large
zation of SE is conducted with FN at 413 K for 3 h under auto- amounts of Zn and Al loaded into MCM-41 plays an important
genous pressure. All catalytic studies have shown that a large catalytic role in the obtainment of high 4-PDO selectivity.
number of Brønsted acid sites in ZnAlMCM-41(75) can be pro-
3.2. Recyclability
moted by the introduction of large amounts of Zn ions onto
the surface of mesoporous AlMCM-41, which has been con- Of the mesoporous catalysts used in the Prins cyclization reac-
firmed by FTIR-pyridine-acidity studies.14 In addition, we tion, ZnAlMCM-41(75) and ZnAlMCM-41(380) produce the
found from the acidity studies of FTIR-pyridine and TPD-pyri- highest and lowest 4-PDO selectivities, respectively. After the
dine that the total acidity, and Brønsted and Lewis acid sites investigation of the above catalytic results, the reusabilities of
increase with an increase in the amount of Zn-ions incorpor- the mesoporous catalysts, ZnAlMCM-41(75) and ZnAlMCM-41
ated on the surface of AlMCM-41 as reported in Table 2S,† (380), were mainly evaluated to investigate their catalytic stabi-
while ZnAlMCM-41(75) has high hydrophobicity, which is con- lities under the reaction conditions specified in Table 2.
firmed by the study of its hydrothermal stability.14 The Hβ(20) Initially, the catalysts used for this reaction suffered from loss
catalyst produces a higher yield of 4-PDO with better selectivity of catalytic activity and cannot be directly reused because trace
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Table 2 Recycling of ZnAlMCM-41(75) and ZnAlMCM-41(380) for Prins cyclization of SE with PFCHOa
Catalysts (100 mg)
Pore size (Å)
Run
Conversion of SE (%)
Selectivity of 4-PDO (%)
Yield of 4-PDO (%)
ZnAlMCM-41(75)b
ZnAlMCM-41(75)c
ZnAlMCM-41(75)c
ZnAlMCM-41(75)c
ZnAlMCM-41(75)c
W-ZnAlMCM-41(75)d
ZnAlMCM-41(380)c
ZnAlMCM-41(380)c
ZnAlMCM-41(380)c
ZnAlMCM-41(380)c
22.7
22.8
22.9
22.9
22.9
22.9
31.5
31.5
31.5
31.5
—
1
2
3
4
—
1
2
3
4
90
99
100
100
100
100
65
66
67
67
92
83
99
100
100
100
100
57
59
60
60
100
100
100
100
100
88
89
90
90
a Reaction conditions: 10 mmol of SE, 20 mmol of PFCHO, 5 ml of 1,2-dichloroethane, temperature = 353 K, time = 6 h. b The used catalyst was
directly used (no chemical treatment/calcinations). c The reused catalysts were obtained, as the used catalysts were chemically treated with
acetone several times, dried at 393 K overnight and calcined at 773 K for 6 h in air to remove the unwanted organics completely. d The catalyst
was washed with ammonium chloride solution for removal of non-framework zinc and aluminium ions on the active surface.
amounts of unreacted organic coke accumulated on the reaction is conducted without a catalyst (Table 1). In con-
surface of the catalysts. Therefore, the catalysts were regener- clusion, the major catalytically active sites are created only by
ated by chemical treatments and calcinations before use. An both the metal ions Al and Zn incorporated/substituted on the
experimental procedure for a clear observation is illustrated. surface of silica walls.
To remove the unwanted organics completely, the reused cata-
lyst was chemically treated with acetone several times, dried at
3.3. Effect of solvents
393 K overnight and calcined at 773 K for 6 h in air. Under the Since W-ZnAlMCM-41(75) gives excellent SE conversion and
same experimental procedure, all recyclable catalysts were yield as well as selectivity of 4-PDO among the other nanopor-
recovered before each successive run. The conversion of SE ous catalysts, it is used to find the best solvent for the highly
and selectivity, and the yield of 4-PDO slightly decrease in the selective synthesis of 4-PDO through Prins cyclization under
first two runs. These results indicate that trace amounts of the reaction conditions specified in Table 3. The Prins cycliza-
non-framework octahedral aluminum and zinc oxides accumu- tion of SE with PFCHO was conducted with non-polar, polar
lated on the inner pore surface of ZnAlMCM-41(75) screen aprotic and polar protic solvents. Initially, the different types
some active sites and can be completely removed using the of solvents with nearly identical dielectric constants (DCs)
above procedure. After two runs, the catalysts in cycles without used in this reaction show different yields for 4-PDO and SE
losing their activities on the surface of the mesoporous silica conversions as mentioned in Table 3. For example, it is inter-
matrix remain unchanged with the conversion of SE and esting to note that the yield of 4-PDO is different in solvents
selectivity as well as the yield of 4-PDO. This result is corrobo- with identical DCs, e.g., toluene (DC 2.38), cyclohexane (DC
rated by the results of ICP-AES obtained before/after the reuse 2.02), triethylamine (DC 2.42) and 1,4-dioxane (2.25), as
of ZnAlMCM-41(75), as depicted in Table 1S.† The catalytic shown in Table 3. These catalytic results demonstrate that DC
results of each recycle show that ZnAlMCM-41(380) predicts is not an important factor in evaluating the catalytic activity
nearly identical catalytic activity given in Table 2 because low of ZnAlMCM-41(75) by the Prins cyclization of SE with
active species are homogenously incorporated in the inner PFCHO.
silica pore walls. This process is a truly heterogeneous catalytic
The above results decide further to study the Acceptor
process.21 To ensure the catalytic stability with the results of number (AN) and Donor number (DN) of solvents used in the
the recycling experiments, ZnAlMCM-41(75) was simply Prins cyclization reaction when PFCHO is used as the FCHO
washed three times with 1.0 M NH4Cl aqueous solution source, as noted in Table 3. Solvent properties such as DN and
(10 mg of ZnAlMCM-41/20 mL of 1.0 M NH4Cl solution) at AN clearly illustrate the role of solvents in this catalytic reac-
room temperature for 1 h, filtered, washed with copious tion. Gutman first reported the properties of DN and AN of
amounts of deionized water, and air-dried at 373 K for 1 h. different solvents by investigating their acidic and basic
The treated catalyst is denoted as W-ZnAlMCM-41(75), which nature.22 Furthermore, many chemists working in academia
was used in the preparation of 4-PDO. The yield of 4-PDO for and industry have adopted the Gutman concepts to evaluate
W-ZnAlMCM-41(75) is similar to that of the third cycle the impacts of solvents on this reaction using a liquid phase
obtained for recyclable ZnAlMCM-41(75). This catalytic state- reaction method.23,24 As shown in Table 3, the yield of 4-PDO
ment confirms the catalytic stability because the acid sites are increases when the AN of the solvents is higher than the DN
almost unchanged from the surface of the two catalysts. The but it decreases when the DN of the solvents is higher than
pure mesoporous silica, MCM-41 was used in this reaction but the AN. However, dichloromethane, which is a volatile and
gave the lowest SE conversion (11%) with the trace yield of polar aprotic solvent and has a better AN, produces 29% yield
4-PDO (Table 1). The conversion of SE is much less when the of 4-PDO with 45% SE conversion only because it could not
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Table 3 Prins cyclization of SE with PFCHO and FN using different solvents over W-ZnAlMCM-41(75)a
Boiling
Dipole
Dielectric
Acceptor
Donor number Conversion Selectivity of Yield of
Catalysts
point (°C) moment (D) constant (DC) number (AN) (DN)
of SE (%)
4-PDO (%)
4-PDO (%)
Dichloromethane
Chloroform
1,2-Dichloroethane
Toluene
Cyclohexane
Tetrahydrofuran
Dimethylformamide 153
Dimethylactamide
Acetonitrile
Nitrobenzene
Triethylamine
Isopropylalcohol
1,4-Dioxane
39.6
61.2
55
110.6
80.75
66
1.55
1.15
1.86
0.43
0
1.73
3.86
3.72
3.45
4.02
0.87
1.66
0.45
1.82
—
8.93
4.81
10.36
2.38
2.02
7.58
36.7
37.8
37.5
34.82
2.42
17.9
2.25
80.1
—
20.4
23.1
16.7
8.2
0
8.0
16.0
13.6
18.9
14.8
1.4
33.5
10.8
54.8
—
10.8
20.4
54.8
—
1
4
0
0
45
75
100
85
15
14
16
18
23
61
10
39
96
92
30
98
100
97
91
65
83
100
88
65
53
58
49
69
72
28
52
98
100
47
100
98
100
100
29
62
100
75
10
7
9
9
16
44
3
20
94
92
14
98
98
97
91
0
20
26.6
27.8
14.1
4.4
61
21.1
0.38
18
—
165
82
210
89
82.5
101
100
—
101
39.6
100
—
Water
Without solvent
1,4-Dioxaneb
Dichloromethaneb
Waterb
0.45
1.55
1.82
—
2.25
8.93
80.1
—
0.38
1
18
—
Without solventb
a Reaction conditions: 100 mg of W-ZnAlMCM-41(75), 10 mmol of SE, 20 mmol of PFCHO, 5 ml of solvent, temperature = 353 K, time = 6 h.
b 30 mmol of FN instead of 20 mmol of PFCHO.
achieve the complete depolymerization of PFCHO in the for- polar) and water (polar protic), as specified in Table 3.
mation of FCHO. Nitrogen containing solvents are unable to Dichloromethane plays a unique role in obtaining 98% yield
produce a better yield of 4-PDO because their DNs are higher of 4-PDO with 100% SE conversion. The other non-polar
than ANs, except nitrobenzene, which produces 44% yield of solvent, 1,4-dioxane, yields almost the same catalytic activity as
4-PDO because it has a higher AN compared to the DN. It is mentioned in Table 3. High catalytic activity (91% SE conver-
noteworthy from the above catalytic results that solvents with sion and 100% 4-PDO selectivity) is observed when the reac-
high ANs and low DNs show higher selectivity of 4-PDO. For tion is carried out in the absence of a solvent. The catalytic
clear information, water has a higher AN compared to the DN results indicate that when FN is used instead of PFCHO, an
and shows higher selectivity of 4-PDO. This result indicates organic solvent is not required to produce high yields of
that the hydrophobicity of ZnAlMCM-41(75) leads to higher 4-PDO because the water content of FN acts as a solvent. For
catalytic activity as confirmed elsewhere.14 The complete depo- additional information, the catalytic reaction when using
lymerization of PFCHO to form FCHO over ZnAlMCM-41(75) water as a solvent provides 100% 4-PDO selectivity with 97%
occurs when a solvent with a high AN is used in this reaction. SE conversion. In this reaction, ZnAlMCM-41(75) possesses
While the solvent represents the Lewis acidic property, it higher catalytic activity than other solid acid catalysts.7,9 The
enables the formation of FCHO from PFCHO without the results of the above catalyst demonstrate that catalytically
impacts of catalytically active sites. The nitrogen-containing active sites cannot be excreted from the surface of
solvents (Table 3) with high DNs make poison-treatment on W-ZnAlMCM-41(75) because it has high hydrophobicity with
the acid sites of ZnAlMCM-41(75) and reduce the active sites, improved hydrothermal stability.14 Moreover, FN and water are
while the solvents reduce the formation of FCHO from PFCHO the most attractive substrate and solvent, respectively, and are
and the yield of 4-PDO because they are Lewis basic in nature, used in fine chemical industries to produce high quantities of
which tends to coordinate with Lewis acceptors with various 4-PDO with high yields, as reported elsewhere.7 Overall, water
interactions.25 Isopropyl alcohol is one of the protic solvents, is a green solvent, inexpensive and readily available for the
has a higher DN compared to non-nitrogen containing sol- highly selective synthesis of 4-PDO by the Prins cyclization of
vents and could not help to improve the catalytic activity. The SE with FN being used as the FCHO source only while this
catalytic reaction conducted without a solvent produces 14% type of synthesis procedure is considered as a green hetero-
yield of 4-PDO. Overall, to produce 100% yield of 4-PDO with geneous catalytic method.
100% SE conversion, 1,2-dichloroethane, which has a high AN
and zero DN, is a good solvent compared to other solvents for
3.4. Effect of reaction parameters
the Prins cyclization of SE with PFCHO.
The Prins cyclization of SE with PFCHO was performed with
To evaluate the solvent effect for the highly selective syn- different reaction temperatures using W-ZnAlMCM-41(75)
thesis of 4-PDO, the FN was used as an FCHO source for the under the reaction conditions specified in Fig. 1. As the temp-
Prins cyclization of SE over W-ZnAlMCM-41(75) with different erature increases from 293 to 353 K, the SE conversion with
solvents, viz. 1,4-dioxane (non-polar), dichloromethane (non- selectivity and the yield of 4-PDO reach 100% within 6 hours.
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Fig. 2 Effect of reaction time over W-ZnAlMCM-41(75). Reaction con-
ditions: 100 mg of catalyst, 10 mmol of SE, 20 mmol of PFCHO, 5 ml of
1,2-dichloroethane, temperature = 353 K, SE to PFCHO mole ratio =
1 : 2.
Fig. 1 Effect of reaction temperature over W-ZnAlMCM-41(75).
Reaction conditions: 100 mg of catalyst, 10 mmol of SE, 20 mmol of
PFCHO, 5 ml of 1,2-dichloroethane, time = 6 h, SE to PFCHO mole ratio
= 1 : 2.
W-ZnAlMCM-41(75) was used in the Prins cyclization reac-
tion with various molar ratios of SE to PFCHO under the reac-
tion conditions specified in Fig. 3 to find an appropriate molar
ratio of SE to PFCHO. When the concentration of PFCHO is
increased only in the molar ratios from 1 : 1 to 1 : 4, the SE con-
version with selectivity as well as yield of 4-PDO reaches 100%
at a molar ratio of 1 : 2, slightly decreases at a molar ratio of
1 : 3 and is lower at a molar ratio of 1 : 4 compared to a molar
ratio of 1 : 3. Moreover, the conversion and selectivity as well as
yield decrease in other molar ratios of 2 : 1 and 2 : 5. The for-
mation of 3-PPCHO with 5–20% selectivity is observed at other
ratios, except for a molar ratio of 1 : 2. In the literature, the SZ
solid catalyst shows higher selectivity of 4-PDO at a molar ratio
of 1 : 3 but in this study, W-ZnAlMCM-41 produces 100% cata-
lytic activity in a molar ratio of 1: 2. The above catalytic results
This is because the depolymerization of PFCHO is very fast in
the formation of FCHO; while a complete SE conversion is
observed within 6 hours. The main product, 4-PDO, selectivity
gradually decreases to ∼85% but the SE conversion remains
constant when the reaction is carried out at above 353 K
because the formation of byproducts could be observed (not
shown in Fig. 1). From the literature, it is noteworthy that
W-ZnAlMCM-41(75) has 100% catalytic activity at 353 K than
other solid acid catalysts.9,11 The aforementioned catalytic
results demonstrate that the Prins cyclization reaction con-
tinues to exhibit better conversion and selectivity at 353 K than
other reaction temperatures.
To find the best selective synthesis of 4-PDO, the Prins cycli-
zation of SE with PFCHO is carried out at different reaction
times over W-ZnAlMCM-41(75) under the reaction conditions
shown in Fig. 2. An increase in conversion from 55 to 100%
and 4-PDO selectivity from 63 to 100% as well as yield from 35
to 100% is notably observed when the reaction temperature is
increased from 2 to 6 h. The catalytic activity (SE conversion,
selectivity and yield of 4-PDO) remains constant when the reac-
tion temperature is further increased from 6 to 7 h. The selecti-
vity of the byproducts, viz. 3-PPCHO and cinnamyl alcohol,
increases when the time is reduced from 5 to 2 h. Moreover,
3-PPCHO selectivity is higher than cinnamyl alcohol selecti-
vity. The catalytic reports show that the generation of FCHO
from PFCHO is less at lower reaction times (<5 h) and there-
fore, the formation of byproducts is predominantly observed at
a lower time. The development of the byproducts reduces with
a higher reaction time, which is more favorable for the devel-
opment of 4-PDO selectivity, which is in agreement with that
reported elsewhere.9,11 This information demonstrates that
W-ZnAlMCM-41(75) is more active than the SZ catalyst at a
lower reaction time.11 From all the above catalytic results, it is
worth mentioning that the best time to obtain excellent selecti-
vity with 100% SE conversion is 6 h.
Fig. 3 Effect on the ratios of SE to PFCHO over W-ZnAlMCM-41(75).
Reaction conditions: 100 mg of catalyst, 5 ml of 1,2-dichloroethane,
temperature = 353 K, time = 6 h.
1678 | Dalton Trans., 2021, 50, 1672–1682
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show that a molar ratio of 1 : 2 is the best ratio to achieve stituted at the α-position of the double bond in SE
100% catalytic activity among the other ratios.
(α-methylstyrene) produces higher reactivity compared to that
substituted at the β-position (β-methylstyrene) due to the
inductive effect of the methyl group. Thus, the high reaction
3.5. Selective synthesis of 1,3-dioxanes
Since the W-ZnAlMCM-41(75) catalyst is the best catalyst time needs to complete the Prins cyclization of
among other nanoporous catalysts for the Prins cyclization of β-methylstyrene with PFCHO in order to get the corresponding
SE, it was used in the Prins cyclization of different olefins with 1,3-dioxane in 74% yield with 83% conversion (entry 7). For
PFCHO. In this catalytic system, the ten SE derivatives and further understanding, the Prins cyclization of β, β′-dimethyl-
three aliphatic linear olefins were used as olefin sources and styrene with PFCHO proceeds sluggishly at a higher reaction
were allowed to react smoothly with PFCHO under the reaction temperature and time and produces the corresponding 1,3-
conditions specified in Tables 3 and 4 to obtain the corres- dioxane with 69% yield (entry 8). The catalytic results show
ponding 1,3-dioxanes in excellent yields with high conversions that the two-methyl groups at the β-position of the double
of olefins. In all cases, the Prins cyclization of olefins with bond cause higher steric hindrance than one-methyl group.
PFCHO was systemically performed with different reaction Indene has less reactivity at a lower reaction time and pro-
temperatures and times to obtain higher selectivity of 1,3- duces the corresponding ketone with 96% yield at a high reac-
dioxanes.
tion time (12 h) at 353 K (entry 9). Naphthyl olefin,
The Prins cyclization of 4-methylstyrene with PFCHO using β-vinylnaphthalene, gives an inferior yield of the corres-
W-ZnAlMCM-41(75) was carried out with different tempera- ponding 1,3-dioxane when it is allowed to react with PFCHO at
tures and times (entry 1). In this reaction, the formation of 4- a lower reaction temperature and time (entry 10), as shown in
(p-tolyl)-1,3-dioxane with 99% yield is observed at 333 K with Table 4. However, this catalytic system produces better yield at
4 h. The selectivity of 4-(p-tolyl)-1,3-dioxane with the conver- a high reaction temperature and time (363 K and 12 h) com-
sion of 4-methylstyrene (100%) is higher at 333 K for 4 h than pared to other catalytic systems, in which different catalysts
that at other temperatures and times. An almost identical cata- are used as reported in the literature.28,29 As reported in the
lytic activity is observed for the Prins cyclization performed literature,9,11 the long chain aliphatic linear olefins are con-
using 4-tert-butylstyrene and 4-methoxystyrene to give the 1,3- sidered to be inactive in many catalytic systems (Table 4,
dioxanes shown in Table 3 (entries 2 and 4). However, 100% entries 11–13) but react well in this catalytic system with
1,3-dioxane selectivity is achieved at 333 K for 3 h because at a W-ZnAlMCM-41(75) under the reaction conditions given in
low reaction time the bulk electron donating group (t-butyl) Table 4. The olefins increase the reactivity at a high reaction
substituted at the p-position of SE reacts faster with PFCHO time and temperature to get the corresponding 1,3-dioxanes in
compared to other electron donating groups substituted at the yields of 81–93% with 85–95% olefin conversion. The catalytic
p-position of SEs (–CH3 and –OCH3), including SE.26 From the activity almost remains constant when FN is used as an FCHO
above catalytic results, it is clearly established that the electron source in the Prins cyclization of different olefins under the
donating groups (–CH3 and –OCH3) substituted at the p-posi- reaction conditions specified in Table 4. The same catalytic
tion of SE enhance the smooth reaction with PFCHO at a lower effects are shown in the 4th run of recycling. Overall, in the
reaction temperature and time. Because the electron-donating Prins cyclization of various olefins, W-ZnAlMCM-41(75) has
groups substituted at the p-position of SE have negative superior catalytic activity compared to other catalysts.7,11
Hammett constants [σp (Me) = −0.14 and σp (OMe) = −0.28]
3.6. Mechanism of Prins cyclization of SE with PFCHO
and enable the conduction of the reactions at lower tempera-
ture as mentioned elsewhere.27 The electron withdrawing Scheme 1 shows a plausible mechanism of the Prins cycliza-
group (–Cl) substituted at the p-position of SE has less reactivity tion of SE with PFCHO. Initially, monomeric FCHO formed by
with PFCHO at a lower reaction temperature and time because the decomposition of PFCHO reacted with ZnAlMCM-41 at an
it has positive Hammett constant [σp (Cl) = 0.22], as reported in appropriate reaction temperature of 353 K due to an inter-
the literature.27 Therefore, in order to increase the catalytic action of PFCHO with the acid sites of the surface of the cata-
activity of this reaction, a high reaction temperature and time lyst. An appropriate reaction temperature is confirmed by the
are highly required. For example, 4-chlorostyrene with PFCHO studies of different temperatures, as shown in Fig. 1. The acid
gives the corresponding 1,3-dioxane in 83% yield with 95% con- sites of ZnAlMCM-41 can be found by the studies of FTIR and
version of 4-chlorostyrene at 353 K for 6 h (entry 3).
TPD-pyridine studies and their results can be described in
The electron donating group (–CH3) substituted at the Table 2S and Fig. 4S.† According to the mechanism of the
α-position of the double bond in SE (α-methylstyrene) Prins cyclization reaction using solid acid catalysts reported
increases the reactivity and has an outward promoting effect to elsewhere,2,8,11 ZnAlMCM-41 leads to the protonation of the
get the corresponding 1,3-dioxane with 98% selectivity (entry CvO bond in FCHO and produces a carbocation (HC+(H)OH)
5). Furthermore, the electron withdrawing (−Cl) and donating on the surface of the catalyst. Then the cation formed on the
(–CH3) groups substituted correspondingly at the p-position of surface of the catalyst strikes the double bond of SE to afford a
the ring and the α-position of the double bond in SE (p-chloro- 3-phenylpropanol-like cation as an intermediate. The carbo-
α-methylstyrene) boost similar reactivity and catalytic activity cation intermediate undergoes cycloaddition of the second
of α-methylstyrene (entry 6). Moreover, the methyl group sub- FCHO molecule at 353 K for 6 h to give the desired product,
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Table 4 Prins cyclization of different olefins with PFCHO and FN over W-ZnAlMCM-41(75) and recyclable ZnAlMCM-41(75)a
Different
olefins
Conversion of
olefins (%)
Selectivity of
1,3-dioxanes (%)
Yield of
1,3-dioxanes (%)
S. No.
1
Product
T (K)
Time (h)
313
313
333
353
333b
333c
313
313
333
353
333b
333c
323
323
353
353
353b
353c
313
313
333
353
333b
333c
323
323
353
353
353b
353c
323
323
353
353
353b
353c
323
323
353
353
353b
353c
323
323
363
363
363b
363c
323
323
353
353
353b
353c
323
323
363
363
363b
363c
323
323
363
363
363b
363c
4
51
65
99
91
98
99
47
55
98
93
99
99
35
42
39
87
85
87
56
77
97
91
98
97
31
45
35
93
94
93
35
51
43
95
90
96
48
63
55
83
71
84
30
42
38
71
68
71
31
48
55
97
99
98
34
45
39
71
68
71
25
52
37
95
89
96
69
72
100
95
100
100
56
65
100
97
100
100
41
53
41
95
90
95
60
86
100
96
100
100
43
57
41
98
98
99
41
58
61
100
100
100
51
67
77
89
85
89
37
51
47
98
91
98
42
76
83
99
98
99
41
51
44
95
91
96
35
67
48
98
95
98
35
47
99
86
98
99
26
36
98
90
99
99
14
22
16
83
76
83
34
66
97
87
98
97
13
26
14
91
92
92
14
29
26
95
90
96
24
42
42
74
60
75
11
21
18
69
62
69
13
36
46
96
97
97
14
23
17
67
62
68
9
6
4
4
4
4
3
6
3
3
3
3
3
6
3
6
6
6
4
6
4
4
4
4
3
6
3
6
6
6
3
6
3
2
3
4
5
6
6
6
6
6
7
15
6
15
15
15
6
15
6
15
15
15
6
12
6
12
12
12
6
12
6
12
12
12
6
8
9
10
11
12
6
12
12
12
35
18
93
84
94
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Table 4 (Contd.)
Different
olefins
Conversion of
olefins (%)
Selectivity of
1,3-dioxanes (%)
Yield of
1,3-dioxanes (%)
S. No.
12
Product
T (K)
Time (h)
323
323
363
363
363b
363c
323
323
363
363
363b
363c
6
12
6
12
12
12
6
12
6
20
42
29
93
88
94
15
31
27
85
78
86
35
59
37
98
95
98
29
45
37
95
90
96
7
25
11
91
84
92
4
14
10
81
70
82
13
12
12
12
a Reaction conditions: 100 mg of W-ZnALMCM-41(75)/recyclable ZnAlMCM-41(75), 10 mmol of olefin, 20 mmol of PFCHO, 5 ml of 1,2-dichlor-
oethane. b 30 mmol of FN instead of 20 mmol of PFCHO. c The catalytic results received in the 4th run.
4-PDO with the regeneration of the ZnAlMCM-41 catalyst. the Prins cyclization of different olefins with PFCHO/FN as the
When the formation of FCHO from the PFCHO is less at the FCHO sources. The above catalytic studies prove that the
lower reaction times and unsuitable ratios of SE to PFCHO, as W-ZnAlMCM-41(75) catalyst has higher catalytic activity than
shown in Fig. 2 and 3, the intermediate carbocation creates other nanoporous catalysts, produces 100% yield of 4-PDO and
cinnamyl alcohol and 3-PPCHO, as the byproducts, by releas- 67–100% yield of other 1,3-dioxanes, and is a highly active and
ing a proton and transformation of the proton, respectively. A very ecofriendly heterogeneous catalyst for the selective syn-
similar reaction mechanism can be followed for the Prins cycli- thesis of 1,3-dioxanes.
zation of different olefins with PFCHO/FN. The hydrophobi-
city, hydrothermal stability and active sites of ZnAlMCM-41(75)
are key factors in the production of 1,3-dioxanes with excellent
yields.
Conflicts of interest
There are no conflicts to declare.
4. Conclusions
Acknowledgements
Many
ZnAlMCM-41, have been successfully used in the Prins cycliza-
tion of SE with PFCHO in the production of 4-PDO. Scientific Research at King Khalid University for funding this
nanoporous
catalysts,
including
mesoporous
The authors extend their appreciation to the Deanship of
work through a General Research Project under grant number
G.R.P. 317/1442.
Recyclability studies have been performed to find the stability
of the catalyst. A chemical treatment method is successfully
used to remove trace amounts of non-framework zinc and
aluminum ions on the surface of ZnAlMCM-41(75), and then
the treated catalyst is called W-ZnAlMCM-41, which is success-
fully used in this reaction to further evaluate the catalytic stabi-
lity. The catalytic results show that both the catalysts, viz.
W-ZnAMCM-41(75) and recyclable ZnAlMCM-41(75), have
almost similar catalytic activity, which remains constant after
the 4th run and confirms that the loss of active sites on the
surface of the catalyst could not be observed. More than thir-
teen solvents have been tested to find the best solvent. Even
though 1,2-dicloroethane is the best solvent among the other
solvents for the highly selective synthesis of 4-PDO, it is vola-
tile, toxic and not considered as a green solvent. Actually, FN
is a good source of FCHO and has been successfully used
instead of PFCHO with water as a solvent to get a higher yield
of 4-PDO. This is highly considered as a very ecofriendly and
heterogeneous catalytic process. Effects of various reaction
parameters are systematically investigated to find the best
optimal conditions. W-ZnAlMCM-41(75) is successfully used in
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