Dealuminated BEA zeolite for selective synthesis of five-membered cyclic acetal from glycerol under ambient conditions

Article abstract of DOI:10.1039/c6ra01437b

BEA zeolite was treated with phenoldisulfonic acid (PDSA) of different concentrations from 0.1 M to 1.0 M to obtain dealuminated zeolites. The treated BEA samples were characterized by XRD, ²⁷Al-MAS NMR, SEM-EDX, TPD NH₃, pyridine-FTIR and BET measurements. Modified zeolites showed an increase in surface area and pore volume and a decrease in acid amount with an increase in PDSA concentration. An untreated BEA sample catalyzed the reaction between glycerol and acetone to form glycerol acteals to yield two products, five-membered dioxalane and six-membered dioxane. Under optimized reaction conditions, the untreated BEA sample showed 83% dioxalane and 17% dioxane while the modified samples showed an increase in dioxalane selectivity with an increase in the acid concentration used for the treatment. The 1 M PDSA treated sample exhibited 100% selectivity for dioxalane. This is attributed to an increase in pore volume and decrease in acid amount of the modified BEA samples with an increase in PDSA concentration. The combined effect of pore volume and acidity in determining dioxalane selectivity is discussed.

Full text of DOI:10.1039/c6ra01437b

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Page 1 of 22
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Dealuminated BEA Zeolite for selective synthesis of five-membered cyclic acetal
from glycerol under ambient conditions
Venkatesha*, Y.S. Bhat, B.S. Jai Prakash
Chemistry Research Centre, Bangalore Institute of Technology, K.R. Road,
Bangalore 560 004, India
Eꢀmailꢀvenkatesha.312@gmail.com,
Tel: +91 9901003120
Abstract: BEA zeolite was treated with phenoldisulfonic acid (PDSA) of different
concentrations from 0.1 M to 1.0 M to obtain dealuminated zeolites. Treated BEA samples were
characterized by XRD, 27AlꢀMASNMR, SEMꢀEDX, TPD NH3, pyridineꢀFTIR and BET
measurements. Modified zeolites showed increase in surface area and pore volume and a
decrease in acid amount with increase in PDSA concentration. Untreated BEA sample catalyzed
reaction between glycerol and acetone to form glycerol acteals to yield two products, fiveꢀ
membered dioxalane and sixꢀmembered dioxane. Under optimized reaction conditions, untreated
BEA sample showed 83% dioxalane and 17% dioxane while the modified samples showed an
increase in dioxalane selectivity with increase in acid concentration used for treatment. 1MPDSA
treated sample exhibited 100% selectivity for dioxalane. This is attributed to increase in pore
volume and decrease in acid amount of modified BEA samples with increase in PDSA
concentration. Combined effect of pore volume and acidity in determining dioxalane selectivity
is discussed.
Keywords: BEA zeolite, dealumination, phenoldisulfonic acid, dioxalane.
1. Introduction
The condensation of glycerol with aldehydes or ketones gives mixture of cyclic acetal products,
1,3 dioxalane, a 5ꢀmembered solketal and 1,3 dioxane, a 6ꢀmembered solketal with an excess of
one product or approximately in equimolar amounts1,2. This phenomenon seems to be related to
the nearly identical reactivity of the primary and secondary hydroxyl group in glycerol and to
similar tendencies for the formation of the 5ꢀ and 6ꢀmembered ketals. Only by changing their
physical properties by converting them into their derivatives, an isolation of one or both of the
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acetals on a large scale is feasible3. However, this procedure is disadvantageous because of
expensive purification steps which result in considerable loss of the target product3ꢀ5.
Selective protection of carbonyl groups by glycerol during the manipulation of multifunctional
organic molecules6 is employed in production of fragrances, food and beverage additives,
pharmaceuticals, detergents, lacquers and also as fuel additives by blending with petrol to reduce
the cost and enhance efficiency by controlling the knocking6ꢀ10
.
The general method for acetal synthesis involves reaction of carbonyl compounds with an
alcohol or an orthoester in the presence of protic or Lewis acid catalysts. Protic acids such as
pTSA, HCl, H3PO4, H2SO4 11 and Lewis acids such as AlCl3, FeCl3, ZnCl2, SnCl2 12,13 are largely
used in the homogeneous medium for acetal synthesis. These acids, however, have limitations
which include tedious workꢀup procedure and production of undesired wastes in addition to their
costs. Heterogeneous acid catalysts have obvious advantages over homogeneous acids such as
their easy separation, product selectivity, solventless conditions and reusability13,14. There are
several reports available on the synthesis of glycerol acetals by different carbonyl compounds
like formaldehyde, cinnamaldehyde, benzaldehyde, furfural, acetone, benzophenone,
cycloheaxanone1,7,9,14ꢀ16 using various catalysts such as, Amberlyst, MCM, NafionꢀH1, alumina17,
montmorillonite18,19, zeolites9,20ꢀ22, mesoporous aluminosilicates23,24, ion exchange resins12,25
[Cp*IrCl2]2, Lu(OTf)326 and supported silica27ꢀ29
,
.
Modified zeolites have been used as heterogeneous acid catalysts for organic synthesis. Post
synthesis modification of zeolites is commonly practiced to vary their catalytic activity.
Dealumination of zeolites is an effective post synthesis method of varying the Si to Al ratio
(SAR) and acidity. Mineral acids such as HCl, HNO3, H2SO430,31 and organic acids like pTSA32,
citric acid, oxalic acid, acetic acid and dicarboxylic acid33ꢀ36 have been used as dealuminating
agents for zeolite samples. When treated with mineral acids, dealumination is caused by the acid
hydrolysis of AlꢀOꢀSi bonds whereas organic acids essentially remove Al by complexation32ꢀ36
.
In both cases, post modification results in a zeolite having steady catalytic performance for a
long period in acid catalyzed reactions such as alkylation, acylation, esterification and hexane
cracking reactions32ꢀ36
.
In the recent years, microwave heating is used in the post synthesis modification like
dealumination of zeolite. Microwave heating brings about the dealumination considerably faster.
Apart from saving energy, it also results in more pronounced effect on the surface and acidic
2

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properties compared to conventional heating. Thus, use of microwave in the zeolite modification
is a valuable green chemistry process32, 37, 38
.
In the present study, dealumination of BEA zeolite sample was brought about by treatment with
organic acids such as phenoldisulfonic acid (PDSA), methanesulfonic acid (MSA) and paraꢀ
toluenesulfonic acid (pTSA). The objective was to investigate the catalytic activity of the
dealuminated samples for the reaction between glycerol and acetone to form glycerol acetals.
Selectivity aspect of five and six membered cyclic acetals formed as a result of dealumination is
also examined.
2. Experimental
2.1 Materials and methods:
The BEA zeolite sample having Si to Al ratio (SAR) 25 was obtained from Süd chemie,
Vadodara, Gujarat, India. Commercial zeolite obtained is in Na+ form, which was further
converted into H+ form by ion exchanging with 1M NH4NO3 solution followed by calcination at
540 ˚C. MSA and pTSA were obtained from SD fine chemicals, PDSA was procured from Loba
chemicals, glycerol was supplied by Merck chemicals and acetone was received from CDH
chemicals. Glycerol formal [mixture of dioxalane (64%) and dioxane (36%)] and dioxalane were
procured from Sigma Aldrich.
2.2 Catalyst preparation:
The modification of zeolite samples by organic acid treatment was performed under microwave
irradiation. Milestone, “STARTꢀS” microwave lab station for synthesis, Italy has been used.
This has a facility to control reaction temperature with the aid of infrared sensor by regulating
microwave power output has been used. Variable power up to 1200 W was applied by
microprocessorꢀcontrolled singleꢀmagnetron system.
Acid treatment of zeolites was performed by following the procedure reported elsewhere32.
PDSA solution of different concentrations (0.1M, 0.25M, 0.5M and 1.0M) were prepared using
deionized water in 100 cm3 volumetric flasks. BEA zeolite samples (10 g each, Hꢀform) was
separately dispersed with continuous stirring in PDSA solutions of different concentrations. The
suspensions were subjected to refluxing under microwave irradiation for 30 min initially by
o
applying 800 W of energy to reach 110 C. Then the same temperature was maintained for 29
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min by applying the energy in the range of 200ꢀ450 W. After that the mixture was cooled to
ambient temperature, centrifuged and repeatedly washed to remove the excess of acid from the
o
sample. The centrifuged solid sample was dried at 120 C, finely ground and subjected to
calcination at 540 oC for 6 hours. Similarly, BEA was treated with 1M each of pTSA and MSA.
The samples were designated as 0.1PDSABEA, 0.25PDSABEA, 0.5PDSABEA, 1.0PDSABEA,
1.0pTSABEA and 1.0MSABEA.
2.3 Characterization: Different acid treated zeolite samples were characterized using various
techniques such as pyridine adsorbed by FTIR, NH3ꢀTPD, 27Al MASNMR, powder XRD, SEMꢀ
EDX and BET analyses.
Acidity of samples was recorded before and after organic acid treatment of the zeolite. All the
o
samples were activated by degassing at 150 C for 2 hours, then cooled under vacuum and
o
saturated with liquid pyridine for 1 hour. Samples were again heated to 110 C to remove
physisorbed pyridine. Spectra of the samples were recorded by KBr pellet technique using
Shimadzu, IRAffinityꢀ1 instrument with 4 cmꢀ1 resolution of 40 scans in the wavenumber range
1700ꢀ1400 cmꢀ1.
Surface area and pore characteristics of the parent and modified zeolite samples were
characterized under liquid nitrogen temperature using Quanta Chrome Novaꢀ1000 surface
analyzer instrument. BET and de Boer tꢀplot methods were used to determine surface area and
pore volume.
Structural integrity of the catalyst samples was checked by powder XRD. The data were
recorded by step scanning at 2θ = 0.020˚ per second from 3˚ to 80˚ on PANalytical X’Pert PRO
MPD Xꢀray diffractometer with graphite monochromatized Cu Kα radiation (λ = 0.15406 nm).
27Al MAS NMR spectra of untreated and dealuminated BEA zeolites were recorded using Joelꢀ
ECX instrument having 8mm rotor of speed 10k.
Morphology and Si/Al ratio of BEA and acid treated zeolite samples were recorded using ZEISS
ULTRAꢀ55 Field Emission Scanning Electron Microscope (FESEM) having Energy Dispersive
xꢀray Analysis (EDXA) under high vacuum mode.
Strength of the zeolite acid sites was evaluated by NH3ꢀTPD method. Typically it involved
around 0.1g of sample, which was placed in a Uꢀshaped quartz sample tube. Prior to the TPD
o
measurement, the catalyst was pretreated with He at 120 C for 2 h. A mixture of NH3 in He
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o
(10%) was passed at the rate of 30 cm3/min at 150 C for 30 min. The sample was then flushed
with He (30 cm3/min) at 150 oC for 1 h. The TPD measurements were carried out in the range of
100ꢀ600 oC at a heating rate of 10 oC/min. Ammonia concentration in the effluent was measured
with a goldꢀplated filament thermal conductivity detector.
Molecular dimensions of the reactant and product molecules were calculated using Chem Bio 3D
(version 7) and Gaussian software theoretically.
2.4 Catalytic activity tests
Ten mmol each of glycerol and acetone were mixed with 0.5 g catalyst sample in 50 ml reaction
vessel and stirred on a magnetic stirrer for 30 min. Reaction temperature was maintained using a
thermostat. After the reaction, the mixture was extracted by stirring with 4 ml of nꢀpentanol for 5
minutes and filtered to remove the catalyst. The reaction mixture was analyzed using Chemito
GCꢀ1000 gas chromatograph with TRꢀWax capillary column and flame ionization detector. The
cyclic acetal products [5ꢀmembered cyclic ketal (2,2ꢀdimethylꢀ1,3ꢀdioxolanꢀ4ꢀyl) methanol
simply called dioxalane and 6ꢀmembered cyclic ketal (2,2ꢀdimethylꢀ1,3ꢀdioxanꢀ5ꢀol)] simply
called dioxane were analyzed using nꢀheptane as internal standard. The products were also
confirmed by GCꢀMS analysis. The conversion and selectivity of products were quantified with
respect to standard samples of dioxalane, glycerol formal and glycerol. Reaction occurring is
shown in scheme 1.
OH
O
O
catalyst
O
O
OH
+
H2O
O
+
+
30 °C, 30 min
OH
OH
OH
2,2-dimethyl-1,3-dioxan-5-ol [dioxane]
Acetone
Glycerol
(2,2-dimethyl-1,3-dioxolan-4-yl)
methanol [dioxalane]
Scheme 1: Reaction carried out using dealuminated BEA zeolite samples
3. Results and discussion:
Table 1 gives the surface characteristics and acidity of zeolite samples before and after
modification with organic sulfonic acids. BEA zeolite having SAR 25 was used for the
experiments. Before modification, it possessed a measured surface area of 538 m2/g. Surface
area and pore volume of BEA zeolite, before and after modification, are also presented in table 1.
Three different organic sulfonic acids such as pTSA, MSA and PDSA were used for
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modification. All the three organic acids caused removal of structural Al accompanied by
increase in surface area and pore volume; the changes observed was in the sequence PDSA >
MSA > pTSA.
Table 1 Surface characteristic and acidity of acid treated and other zeolite samples
Catalyst
samples
Si/Al
ratio
Surface
area,
Pore
Average Pore Pyridine FTꢀIR
volume,
diameter
(Å)
acidity
mmol/g
S
BET (m2/g)
V
(cm3/g)
B
L
BEA
25
48
69
96
538
579
628
674
0.307
0.364
0.388
0.412
31.04
30.9
30.01
0.644
0.626
0.602
0.531
0.394
0.372
0.366
0.347
1.0pTSABEA
1.0MSABEA
1.0PDSABEA
29.24
Standard deviation values: Si/Alꢀ±1%, SBETꢀ± 2.1%, Vꢀ±0.35%, Average pore diameterꢀ±1.2%, Pyridine
FTꢀIR acidityꢀ±0.42%
Si/Al ratio was calculated using SEMꢀEDX analysis
3.1 Acidity: Acidity in zeolite is mainly due to isomorphous substitution of aluminum,
generating a negative charge on the framework. This charge is compensated by H+ ions in Hꢀ
form of BEA and is responsible for Bronsted acidity. Aluminum present on the framework edges
is the Lewis acid site. Organic sulfonic acid treatment of the zeolite removes aluminum
from the framework and also from edges of the framework. This causes decrease in both
Bronsted and Lewis acidity. The changes in the Bronsted (B) and Lewis (L) acidities of
different organic sulfonic acid treated BEA samples were measured by the pyridine adsorption
using FTIR method reported elsewhere39, 40. The results are given in table 1 and the pyridineꢀ
FTIR patterns are shown in Fig 1.
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7
6
5
%T
4
3
2
1.0PDSA BEA
1.0MSA BEA
1.0pꢀTSA BEA
BEA
1600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400
Wavenumber (cmꢀ1)
Fig 1 pyridine adsorbed FTꢀIR spectra of the catalyst samples
Both the methods of acidity measurement showed a decrease in the acid amounts of BEA on
treatment with organic acids. The decrease in the Bronsted and Lewis acid amounts due to
dealumination was found to be in the order PDSA>MSA>pTSA. Different organic acids have
the ability to remove structural (tetrahedral) Al to different extents thereby altering the Si/Al
ratio of the treated BEA zeolite. PDSA is able to remove more Al than the other two acids. This
27
is confirmed by decrease in the intensity of the band at 60 ppm in the Al MAS NMR spectra
corresponding to the tetrahedral aluminum present in the zeolite framework ((Fig 2). From the
27Al MAS NMR spectra it can be seen that the decrease in the Al content of the organic acid
treated zeolite follows the order PDSA>MSA> pTSA. Consequently, among the organic acid
treated zeolites, PDSA treated zeolite shows the lowest acid amounts and pTSA treated zeolite
shows the highest acid amount. Al removal from the zeolite structure is expected to cause
enhancement in surface area and pore volume. This is supported by the fact that the increase in
surface area, pore volume and Si/Al ratio for the treated zeolites is in the order pTSA<
MSA<PDSA (table 1). Also there is no change in structural integrity of the acid treated BEA
samples, as evident from the XRD patterns (Fig 3). Morphology of zeolite samples, as shown by
FESEM images, does not show any major change upon organic acid treatment (Fig 4).
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3.0
2.5
2.0
1.5
1.0
0.5
0.0
a
b
c
d
ꢀ100 ꢀ80 ꢀ60 ꢀ40 ꢀ20
0
20 40 60 80 100 120 140
δ (ppm)
Fig 2 27AlꢀMAS NMR plots of (a) BEA (b) 1.0pTSABEA (c) 1.0MSABEA and (d)
1.0PDSABEA
1.0PDSA BEA
1.0MSA BEA
1.0pꢀTSA BEA
BEA
10
20
30
40
50
60
70
80
2θ (degree)
Fig 3 XRD pattern of BEA and acid treated BEA samples
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Fig.4 FESEM images of (a) BEA (b) 1.0pTSABEA (c) 1.0MSABEA and (d) 1.0PDSABEA
samples
3.2 Reaction parameter studies with untreated BEA
The effect of different reaction parameters such as temperature, catalyst amount, mole ratio and
reaction time on conversion and selectivity was carried out in the presence of untreated BEA
catalyst. Each experiment was performed three times and the mean of the obtained values was
considered for the calculation of standard deviation. Internal and external diffusion effects were
calculated under similar experimental conditions using Mear criteria (3.54×10ꢀ7) for external
diffusion and WeiszꢀPrater (2.984×10ꢀ7) and KorosꢀNowak criteria for internal diffusion. Both
the diffusion effects are found to be negligible. The results of variation of reaction parameters on
catalytic activity are discussed in the ensuing sections.
3.2.1 Effect of catalyst amount:
There is a significant effect of catalyst amount on the glycerol conversion and the product
selectivity. In the absence of catalyst, reaction does not proceed and in the presence of the
catalyst reaction proceeds slowly with gradual increase in conversion of glycerol with the
amount of the untreated BEA catalyst (Fig 5). At lower catalyst amounts up to 0.5 g there is a
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substantial increase in conversion which gets stabilized beyond 0.5 g. Fig 5 also shows the effect
of catalyst amount on the selectivity of the two products. The selectivity of fiveꢀmembered
dioxalane increased accompanied by decrease in the sixꢀmembered dioxane. At 0.1 g of catalyst,
the selectivity of dioxalane, 72% and dioxane, 28% was observed. Further increase in catalyst
amount beyond 0.5 g, the selectivity distribution was found to be 94% dioxalane and 6%
dioxane.
100
90
80
70
Glycerol conversion
Dioxalane selectivity
Dioxane selectivity
60
50
40
30
20
10
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Catalyst amount (g)
Fig 5 Effect of untreated BEA catalyst amount on acetalization of glycerol with acetone and
product selectivity Reaction conditions: mole ratio 1:1 of glycerol to acetone, temperature; 30
oC, reaction time; 30 min.
3.2.2 Effect of reaction time:
Effect of reaction time was studied in the range of 5 to 100 min (Fig 6). As the reaction time
increases from 5 to 30 min, glycerol conversion also increased and reached a maximum around
40 min. Beyond 40 min, a slight decrease in conversion was observed, but the selectivity
distribution of the two products remained almost the same. The decrease in the conversion of
glycerol is explained due to hydrolysis of cyclic acetals41ꢀ43. In the present study, it is noticed
that, when the glycerol conversion reaches a point (around 78%), the hydrolysis of cyclic acetals
becomes significant.
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100
90
80
70
60
50
40
30
20
10
0
Glycerol conversion
Dioxalane selectivit
Dioxane selectivity
y
0
10
20
30
40
50
60
70
80
90 100 110
Reaction time (min)
Fig 6 Effect of reaction time on acetalization of Glycerol with acetone in presence of BEA as
catalyst
o
Reaction conditions: mole ratio 1:1 glycerol to acetone, temperature, 30 C, catalyst amount;
0.5g.
3.2.3 Effect of temperature:
100
90
80
70
60
50
Glycerol conversion
Dioxalane selectivity
Dioxane selectivity
40
30
20
10
0
25
30
35
40
45
50
55
60
65
70
Temperature (0C)
Fig 7 Effect of reaction temperature on acetalization of Glycerol with acetone
Reaction conditions: mole ratio 1:1 of glycerol to acetone, reaction time; 30 min, catalyst
amount; 0.5 g.
Fig 7 shows effect of temperature variation on the acetalization of glycerol with acetone. Effect
was studied from ambient temperature (30 oC) to 65 oC. At the ambient temperature, conversion
was 61% with selectivity 83% for dioxalane and 17% for dioxane. Increase in temperature
caused an increase in conversion and reached a maximum at 50 oC, with 96% dioxalane and 4%
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o
dioxane. Beyond 50 C, conversion slightly decreased possibly because of the hydrolysis of the
cyclic acetals formed but again there was no change in product selectivity.
3.3 Comparison with dealuminated BEA samples:
Under similar reaction conditions catalytic reactions using organic sulfonic acid treated BEA
samples were carried out to study their behavior in catalytic activity and selectivity in
acetalization of glycerol with acetone. Untreated BEA sample showed 76% conversion with
selectivity 83% for dioxalane and 17% for dioxane. Conversion and selectivity of products were
found to be higher in modified BEAs (Table 2). 1.0MSABEA and 1.0pTSABEA showed almost
the same conversion and selectivity. 1.0PDSABEA however showed 100% selectivity for the
dioxalane with no formation of dioxane. This is perhaps due to the rearrangement of dioxane to
dioxalane over 1.0PDSABEA.
Catalytic acetalisation reaction mainly occurs on the internal surface (pores) of the zeolite.
Molecular dimensions of the dioxane and dioxalane computed theoretically are given below (Fig
8). It can be seen that dimensions of both reactant and product molecules are smaller than
internal pore dimensions. Further, most of the active sites are located within the pores.
Fig 8 Molecular dimension and 3D structures of reactant and product molecules
Generally glycerol acetalization reaction is a Bronsted acid catalyzed reaction but also can take
place to a lesser extent in the presence of a Lewis acid. The rearrangement of dioxane to
dioxalane however, can take place only in the presence of Bronsted acid sites. Accordingly,
selectivity is only dependent on the Bronsted acid sites.
Strength of the acid sites was studied by using NH3ꢀTPD method. Desorption of ammonia at
different temperatures could be seen in the TPD profile (Fig 9). It indicates desorption of
o
ammonia at lower temperature range (T1 = 100ꢀ300 C) corresponding to weak acid sites and
desorption at higher temperature (T2 = 325ꢀ550 oC) corresponding to the stronger acid sites. The
amounts of NH3 desorbed are given in table 2.
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On treatment with organic acids, BEA zeolite showed changes in strength of the acid sitesꢀ a
decrease in weaker acid sites and a moderate increase in stronger acid sites (Fig 9). Increase in
strength of the acid sites enhanced the selectivity of dioxalane from dioxane with insignificant
change in conversion of glycerol in the acetalization reaction.
Table.2 comparison of conversion of glycerol and dioxalane selectivity in glycerol acetalization
with acetone using BEA samples dealuminated with different acids
Catalyst
samples
%
%
%
Amount of NH3ꢀ Total
Conversion selectivity selectivity desorbed (mmol)
of glycerol
acidity
(mmol)
of
dioxalane
83
of
dioxane
17
T1
T2
BEA
76
82
84
80
0.64
0.45
0.29
0.19
0.47
0.51
0.58
0.49
1.11
0.96
0.87
0.68
1.0pTSABEA
1.0MSABEA
1.0PDSABEA
90
93
100
10
07
00
Reaction conditions: time; 30 min, temperature; 30 oC, catalyst amount; 0.5 g, mole ratio 1:1 of
glycerol and acetone. T1= temperature range 100ꢀ300 oC, T2 = temperature range 325ꢀ550 oC
Standard deviation values: Glycerol conversionꢀ±1.6%, dioxalane selectivityꢀ± 2.1%, dioxaneꢀ±1.35%,
Amount of NH3 desorbed ꢀ±0.52%,
120
BEA
100
1.0pTSABEA
80
60
1.0MSABEA
40
1.0PDSABEA
20
0
100 150 200 250 300 350 400 450 500 550 600
Temperature (oC)
Fig 9 NH3ꢀ TPD profiles of HBEA and acid treated BEA zeolite samples.
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3.4 Selective dioxalane formation:
Catalytic activity with 1.0PDSABEA was carried out by varying reaction conditions such as
temperature, catalyst amount and reaction time. The results obtained are shown in table 3Aꢀ3C.
Though the glycerol conversion varied under different conditions, 1.0PDSABEA sample gave
dioxalane as the single product. This shows that only catalyst characteristics are responsible for
the selectivity and not the reaction conditions. Further studies were made with the catalyst
samples dealuminated with 0.1 M to 0.5 M PDSA. This was to investigate the changes in acid
amounts and pore volumes that arise upon acid treatment and their relation with dioxalane
selectivity.
Table.3A Effect of catalyst amount on the conversion of glycerol and product selectivity with
1.0PDSABEA catalyst
Catalyst
% conversion Selectivity of Selectivity
amount (g) of glycerol
dioxalane
of dioxane
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
35
48
62
76
81
82
84
85
85
86
100
00
Reaction conditions: Time; 30 min, temperature; 30 oC, mole ratio 1:1 glycerol: acetone.
Standard deviation values: Glycerol conversionꢀ±1.8%, dioxalane selectivityꢀ±0.15%, dioxaneꢀ±0.2%,
Table.3B Effect of reaction temperature on the conversion of glycerol and product selectivity
with 1.0PDSABEA catalyst
Temperature % conversion Selectivity of Selectivity
(oC)
of glycerol
dioxalane
of dioxane
30
35
40
45
50
55
60
70
80
56
66
71
76
80
82
84
85
85
100
00
Reaction conditions: Time; 30 min, catalyst; 0.5 g, mole ratio 1:1 glycerol: acetone.
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Standard deviation values: Glycerol conversionꢀ±1.48%, dioxalane selectivityꢀ±0.09%, dioxaneꢀ±0.17%,
Table.3C Effect of reaction time on the conversion of glycerol and product selectivity with
1.0PDSABEA catalyst
Reaction
% conversion Selectivity of Selectivity
time (min) of glycerol
dioxalane
of dioxane
5
10
20
30
40
50
60
90
120
39
48
62
76
80
78
80
82
84
100
00
Reaction conditions: Catalyst; 0.5 g, temperature; 30 oC, mole ratio 1:1 glycerol: acetone.
Standard deviation values: Glycerol conversionꢀ±1.84%, dioxalane selectivityꢀ±0.16%, dioxaneꢀ±0.20%.
3.5 Reusability of the samples
The reusability of the catalyst samples was checked for four cycles. The results (table 4) are
similar to fresh catalyst samples indicating negligible changes in catalyst characteristics after use
in the reaction. Further catalyst samples were recovered after the reaction and subjected to
pyridine FTꢀIR, XRD and BET analysis. These results do not show any significant change in
catalyst characteristics before and after use in the reaction indicating there is no deactivation of
zeolite sample.
Table 4 Recyclability of 1.0PDSABEA catalyst and their surface and acidity characteristics after
the reaction
% Conversion
of glycerol
% selectivity
of dioxalane
% selectivity
of dioxane
Cycles
Surface Pore
Pyridine
area
volume FTꢀIR
(m2/g)
(cm3/g) acidity
(mmol/g)
1st
673
671
672
670
0.410
0.409
0.411
0.410
0.875
0.869
0.872
0.870
80
78
79
77
100
100
100
100
00
00
00
00
2nd
3rd
4th
Reaction conditions: time; 30 min, temperature; 30 oC, catalyst amount; 0.5 g, mole ratio 1:1 of
glycerol and acetone.
Standard deviation values: Glycerol conversionꢀ±2%, dioxalane selectivityꢀ±0.2%, dioxaneꢀ±0.25%,
SBETꢀ± 2.4%, Vꢀ±0.28%, Pyridine FTꢀIR acidityꢀ±0.38%,
The possible reaction mechanism for dioxalane formation is shown in scheme 2. The mechanism
of formation of glycerol acetals involves protonation by Bronsted acid site or Lewis acid
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coordination to carbonyl oxygen of acetone generating positive charge on the carbonyl carbon
(scheme 2). The carbocation thus formed gets attacked readily by nucleophilic hydroxyl group of
glycerol giving hemi acetal. This is further attacked by another hydroxyl group resulting in
cyclic acetal with the elimination of a molecule of water. When 1,2 hydroxy groups are involved
dioxalane [5ꢀmembered] is formed and dioxane [6ꢀmembered] is formed when 1,3 hydroxy
groups are involved. Further, in the presence of Bronsted or Lewis acid sites, dioxane gets
rearranged to dioxalane.
O
O
O
-
Al
O
O
+
Si
O
H
O
O
H
H
O+
O
H
H
H
O+
O
O
H
O
O
O+
+
O+
HO
O
C
H
HO
O
HO
OH
HO
O
Si
HO
O
HO
O
OH
-
O
Al
HO
O
O
O
H
H
O+
+
O
O
O
C
Al-
O+
O
OH
O
H
+
O
O
OH
Si
C
+
H
O
HO
O
O
O
O
OH
OH
OH
OH
OH
Scheme 2 Possible reaction mechanism for dioxalane formation
3.5 Role of pore volume and acidity in dioxalane selectivity:
The selectivity for dioxalane and dioxane in presence of untreated BEA and selectivity for those
treated with different concentrations of PDSA are shown in Table 5. It reveals that the
selectivity for the dioxalane increased with the increase in PDSA concentration from 0.1 M to
1.0 M. The selectivity for dioxalane with 0.1PDSABEA was the same as that of untreated BEA
catalyst (83%) but the selectivity increased to 100% with 1.0PDSABEA. Since this increase of
17% has significance with respect to mechanism, its relation with changes in various parameters
observed in the catalyst samples upon acid treatment was studied.
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Table 5 Comparison of glycerol conversion and product selectivity for BEA zeolite treated with
different concentrations of PDSA
Catalyst
samples
Conversi
on of
glycerol
(%)
%
%
Si/Al Acidity
Pore
volume
Surface
area
selectivity selectivity ratio
(Pyrꢀ
of
of dioxane
FTIR) (V) cm3/g (SBET
)
m2/g
dioxalane
(A)
mmol/g
0.1PDSABEA
0.25PDSABEA
0.5PDSABEA
1.0PDSAꢀBEA
R
72
75
76
80
ꢀ
83
86
95
100
ꢀ
17
14
05
00
ꢀ
48
63
84
96
ꢀ
0.992
0.953
0.928
0.878
ꢀ0.963
0.345
0.364
0.392
0.412
0.994
556
589
635
674
0.993
o
Reaction conditions: time, 30 min; temperature, 30 C; catalyst amount, 0.5 g; glycerol and
acetone mole ratio, 1:1.
Standard deviation values: Glycerol conversionꢀ±1.6%, dioxalane selectivityꢀ± 2.1%, dioxaneꢀ±1.35%,
Si/Alꢀ±1%.
It could be seen from Table 5 that the catalyst properties that change on increase in the acid
concentration used for treatment are surface area, pore volume and acidity. Increase in pore
volume is evidently due to the removal of structural Al causing decrease in acid amount (table
5). Acidity (A) was calculated by summing up the Bronsted and Lewis acidities by the pyridineꢀ
FTIR method. In order to know whether the change in pore volume and acidity are responsible
for the increase in dioxalane selectivity, these characteristics were correlated with the increase in
selectivity. The correlation coefficients are given in table 5. The correlation coefficient (r) values
show good correlation for the dioxalane selectivity with all the parameters. Surface area and pore
volume showed very nearly the same r value. This is expected because these two parameters are
caused due to dealumination and therefore there exists a good proportionality between surface
area and pore volume. The acidity showed a decreasing trend with a simultaneous increase in
pore volume. Since both pore volume and acidity appear to be responsible, some combination of
both have to be considered in explaining the rearrangement of dioxane leading to 100 %
dioxalane selectivity.
3.6 Volume space acidity (VSA):
Increase in pore volume (by about 25%) on organic acid treatment is caused by the removal of
framework Al thereby diminishing the acid amount (by about 14 %) in the enhanced pore
volume. A combination of both these factors, that is, acidity and pore volume, apparently
determines the rearrangement of dioxane. Extent of rearrangement is dependent on the
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progressive removal of Al which causes decrease in acid amounts and increase in pore volume.
The combination of both these factors is expressed as volume space acidity factor (VSA). VSA,
described as the product of pore volume (V) and inverse acidity (Aꢀ1), provides a measure of
volume space for each acid site available for the dioxane molecule to orient and undergo
rearrangement.
In the present work, the rearrangement of the dioxane to dioxalane is facilitated by the pore
volume to approach the acid sites in such a way that the molecule readily orients to rearrange
itself. With the increase in acid concentration used for treatment, the pore volume increases with
proportional decrease in the acid amounts (or increase in VSA) till it reaches an optimum value
to achieve 100 % dioxalane selectivity with 1.0PDSABEA catalyst (Fig 10). Correlation
coefficient (r) between VSA and the dioxalane selectivity for catalysts treated with different acid
concentrations also showed a high correlation (r = 0. 989). Thus, combination of increase in
pore volume and decrease in acid amounts favoring the rearrangement of the dioxane to
dioxalane is probable.
100
95
90
85
80
75
0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50
Volume Space Acidity (VSA)
Fig 10 Representation of relation between VSA and selectivity of dioxalane
o
Reaction conditions: time, 30 min; temperature, 30 C; catalyst amount, 0.5 g; glycerol and
acetone mole ratio, 1:1
3.7 Acidity, hydrophobicity and dioxalane selectivity:
Increase in acid amounts is known to cause the hydrolysis of dioxane/dioxalane favoring the
reverse reaction giving back the reactant molecules20,26,29,41ꢀ45. On the contrary, decrease in
framework Al brings down the acidity and consequently hydrolysis of the dioxane/dioxalane is
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reduced. Furthermore, Al removal causes increased hydrophobicity in the zeolite channels.
Increased hydrophobicity prevents the dioxane molecules to hydrolyze back into the reactants
thereby favoring the rearrangement of the dioxane molecules to dioxalane. This results in 100%
selectivity for dioxalane in 1.0PDSABEA. High correlation factor observed between VSA and
dioxalane selectivity is not surprising. On the other hand VSA correlation of acid amount of
higher strength (as measured by the ammonia desorbed at higher temperature T2) with dioxalane
selectivity showed a poor correlation (r = 0.846). It appears that variation of acid strength as
measured by TPDꢀNH3 has no influence on the dioxalane selectivity. And also VSA of average
pore diameter when correlated with the selectivity of dioxalane showed r value 0.953.
4. Conclusion:
Removal of Al from the framework on treatment with PDSA modifies the characteristics of BEA
zeolite. Increase in PDSA concentration for treatment results in dealuminated zeolite with
gradual increase in surface area and pore volume but decrease in acid amounts. Untreated BEA
zeolite catalyzes the reaction between glycerol and acetone to form sixꢀmembered dioxane and
five membered dioxalane, the maximum selectivity of the latter being 83%. BEA sample treated
with 1MPDSA, however, is found to exhibit complete rearrangement of sixꢀmembered dioxane
into the fiveꢀmembered dioxalane. Augmented selectivity for the five membered species is
attributed to the increase in pore volume and decreased acid amount. A factor called volume
space acidity (VSA) described as the product of pore volume (V) and inverse acidity (Aꢀ1),
provides a measure of volume space available for molecular rearrangement on each acid site.
VSA correlates well with the dioxalane selectivity exhibited by the modified BEA zeolite.
Acknowledgements:
The authors wish to thank the Rajya Vokkaligara sangha and Principal Bangalore Institute of
Technology for the facilities provided. The authors also extend their thanks to Dr. Arun Basrur,
General Manager (R&D) Sud Chemie, India, for providing zeolite samples, L.V. Narendra
NMRC IISc, Bengaluru for MAS NMR, Mr. Shanmugam IIT madras for helping in molecular
dimension calculation and PPISR for XRD and Mr. Hareesh.N JU for NH3ꢀTPD measurements.
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Graphical Abstract:
O
H
Unmodified BEA
Modified BEA
O
H
O
+
O
H
Organic sulfonic acid
MW irradiation
O
O
O
O
H
O
2
O
+
O
+
+
H
O
2
O H
O H
HO