Pore-engineered silica-alumina: Texture, acidity, and activity for conversion of longifolene to isolongifolene

Article abstract of DOI:10.1007/s00706-011-0666-4

Pore-engineered silica-alumina was synthesized for the conversion of longifolene to isolongifolene, and the effects of texture and surface properties on the activity were examined. The acidity and texture of the modified silica-alumina play a vital role in governing the catalytic isomerization of longifolene to isolongifolene. A conversion of 97% of longifolene with 95% selectivity has been achieved. Springer-Verlag 2011.

Full text of DOI:10.1007/s00706-011-0666-4

Monatsh Chem (2012) 143:747–751
DOI 10.1007/s00706-011-0666-4
ORIGINAL PAPER
Pore-engineered silica–alumina: texture, acidity, and activity
for conversion of longifolene to isolongifolene
•
Jyoti Patial Bashir Ahmad Dar Parveen Sharma
•
•
•
Kusunuru Anil Kumar P. R. Sharma Shyam Kr Ray
•
•
•
Debaraj Mukharjee Baldev Singh
Received: 23 June 2011 / Accepted: 25 September 2011 / Published online: 20 October 2011
Ó Springer-Verlag 2011
Abstract Pore-engineered silica–alumina was synthe-
sized for the conversion of longifolene to isolongifolene,
and the effects of texture and surface properties on the
activity were examined. The acidity and texture of the
modified silica–alumina play a vital role in governing the
catalytic isomerization of longifolene to isolongifolene. A
conversion of 97% of longifolene with 95% selectivity has
been achieved.
activity. Tetrahedral Al atoms in the silica network are
assumed to be highly active components in aluminosili-
cates. Aluminum has only three valence electrons, whereas
silicon has four. Substitution of silicon by aluminum in the
tetrahedron therefore creates a vacancy which is filled by
accommodating an electron and a negative charge is thus
obtained. Compensation of the resulting negative charge by
a proton gives rise to a Brønsted acid site [10]. Lewis acid
sites usually predominate on dehydrated silica–alumina
although adsorbed water leads to the conversion of Lewis
to Brønsted acid sites [11, 12].
Keywords Silica–alumina ꢀ Longifolene ꢀ
Isolongifolene ꢀ Acidity ꢀ Surface area distribution ꢀ
Pore size distribution
In the present work, pore-regulated silica–alumina was
synthesized by a stepwise precipitation technique using
pore-regulating agents (PRAs) such as tetraalkylammo-
nium salts (methyl, ethyl, propyl, and butyl) and the texture
and surface properties were determined. An attempt was
made to correlate these properties with activity in the
transformation of longifolene to isolongifolene through
acid-catalyzed isomerization reaction (Scheme 1).
Introduction
Aluminosilicate catalysts find application in various types
of reactions like cracking, acylation, alkylation, dealkyla-
tion, cyclodehydrogenation, isomerization, and other acid-
catalyzed reactions [1–9]. Proper choice of the surface
characteristics of a catalyst can increase activity, selectiv-
ity, and catalyst life. In aluminosilicate catalysts, acid sites
generated by substitution are responsible for the catalytic
Longifolene (C15H24, decahydro-4,8,8-trimethyl-9-
methylene-1,4-methanoazulene), a tricyclic sesquiterpene
hydrocarbon, is a commercially important chemical whose
oxygenated derivatives (e.g., alcohols, ketones, oxides,
aldehydes) have been extensively used in the perfumery
and pharmaceutical industries because of their woody and
floral notes [13, 14]. Several investigations were reported
on the conversion of longifolene to isolongifolene using
acids like sulfuric acid/acetic acid [15], BF3ꢀOEt2 [16, 17],
bromoacetic acid [18], camphenecarboxylic acid [19], and
others [20, 21]. The disadvantages inherent in the use of
acids are multiple steps, long reaction times, and above all
the existing processes are not eco-friendly [15]. Moreover
other rearranged products such as tetralin and octal are also
formed as side products [22]. The catalyst prepared in
the current paper has better activity, selectivity, and
J. Patial ꢀ B. A. Dar ꢀ P. Sharma ꢀ K. A. Kumar ꢀ
D. Mukharjee ꢀ B. Singh (&)
Catalysis Discipline, Indian Institute of Integrative Medicine
(CSIR), Canal Road, Jammu 180001, India
e-mail: drbaldevsingh@yahoo.co.in
P. R. Sharma
Cancer Pharmacology Division, Indian Institute of Integrative
Medicine (CSIR), Canal Road, Jammu 180001, India
S. K. Ray
CIMFR, Dhanbad, Jharkhand, India
123

748
J. Patial et al.
Scheme 1
recyclability as compared to conventional catalysts [23,
24]. In our earlier work [25] an attempt was made to cor-
relate the activity of the catalysts with total acidity,
number, and strength of the acids for the conversion of
longifolene to isolongifolene with 70–80% conversion. In
the present work we studied the role of acidity and mor-
phology of the modified silica–alumina for catalytic
isomerization of longifolene to isolongifolene, and 97%
conversion of longifolene with 95% selectivity has been
achieved.
Fig. 1 SEM of SiO2–Al2O3 (without PRA)
Results and discussion
aggregate is about 350–400 nm and particles are more or
less spherical in shape, whereas in the sample prepared
with tetraethylammonium (TEA) compound the size of
aggregate as shown in Fig. 3 is 400–550 nm.
Compositions of the catalysts and their specific surface
area, pore volume, and pore size distribution are summa-
rized in Table 1. The morphological aspects of the samples
are shown in Figs. 1, 2, 3, and 4. The results show that
surface area, pore size, and morphological properties of
silica–alumina are greatly influenced by various factors
like method of preparation, pH, silica to alumina ratio,
synergism, ageing, etc. It is believed that the pore-regu-
lating effect of PRAs is based on the volume occupied at
the molecular level in the gel before removal of compo-
nents by combustion or evaporation [26]. The
tetraalkylammonium cations decompose after combustion
or evaporation releasing alkaline material [27].
Table 1 reveals that the maximum surface area for
samples prepared without PRA is 278.5 m2 g-1. On treat-
ment with PRA the surface area is sharply reduced because
the carbon number in the alkyl group influences the surface
area as well as texture properties of the silica–alumina
catalyst. It is interesting to observe that the surface area of
samples prepared from TMA to tetrapropylammonium
(TPA) gradually increases with the increase of carbon
number in the alkyl group but suddenly declines to
102.2 m2 g-1 for the sample prepared with tetrabutylam-
monium (TBA) compounds, which is not in accordance
with the finding made by Manton and Davidtz [28]. This
may be due to differences in preparation methods, the
source materials, pH, and product concentration, which
affect the surface area to some extent [29]. Table 1 also
Figure 1 shows that the silica–alumina aggregate pre-
pared without PRA has a pore size distribution of
700–750 nm and the aggregates are not spherical in shape.
Figure 2 shows that when the sample is prepared with
tetramethylammonium (TMA) compound, the size of
Table 1 Surface area, pore volume, and pore size distribution of different catalysts
Catalyst
Surface
area/m2 g-1
Pore
volume/cm3 g-1
Pore size distribution/nm
0.4–6
6–10
10–20
20–30
30–40
40–50
[50
SiO2–Al2O3 without PRA, stepwise
SiO2–Al2O3 with TMA
SiO2–Al2O3 with TEA
278.5
126.5
129.5
179.1
102.2
1.0301
0.9012
1.4878
1.2905
1.4302
73.2
31.7
9.5
12.8
8.7
3.3
7.6
3.8
6.9
12.4
7.0
2.6
10.9
7.6
0.9
6.9
3.6
8.1
6.1
1.0
7.0
2.6
22.4
62.8
33.1
58.4
6.2
SiO2–Al2O3 with TPA
9.1
14.8
2.8
11.4
2.8
15.9
13.6
SiO2–Al2O3 with TBA
12.5
TMA tetramethylammonium, TEA tetraethylammonium, TPA tetrapropylammonium, TBA tetrabutylammonium
123

Pore-engineered silica–alumina
749
Fig. 4 SEM of SiO2–Al2O3 with TBA as PRA
programmed desorption (TPD) results show that without
PRA, the total acidity of the catalyst m(H?) is
1.10 mmol g-1. The activity in terms of selectivity of
isolongifolene is 50%, whereas conversion of longifolene
is 48%. Modified with TMA compound produces a total
pore volume of 0.9012 cm3 g-1 and 31.7% of pores are of
diameter 0.4–6 nm. NH3 TPD shows that total acidity after
modification increases to 1.18 mmol g-1, comprising weak
Fig. 2 SEM of SiO2–Al2O3 with TMA as PRA
acidity 1.15 mmol g-1, moderate acidity 0.01 mmol g-1
,
and strong acidity 0.02 mmol g-1. Selectivity of isol-
ongifolene sharply increased to 82% and total conversion
of longifolene is 84%. When Si–Al is modified with TEA,
TPA, and TBA compounds, the acid strength and number
of acidic sites subsequently change. Total acidity with TEA
compound is 1.14 mmol g-1 comprising 98.38, 0.81, and
0.81% of weak, moderate, and strong acid sites, respec-
tively, and total pore volume observed is 1.4878 cm3 g-1
where 62.8% of pores are of diameter greater than 50 nm
and selectivity of isolongifolene enhances very appreciably
to 95% with 97% conversion. The enhanced selectivity and
conversion are probably due to the acidity and distribution
of pores. When modified with TPA compound, total acidity
increases to 1.35 meq H? g-1, where weak acid concen-
tration is 92.71%, moderate 3.68%, and strong 3.61%
(Table 2). Total pore volume slightly decreases to
1.2905 cm3 g-1 and 33.1% of pores are of diameter above
50 nm. Selectivity of isolongifolene slightly decreases to
81.5% and conversion of longifolene is 90%.
Fig. 3 SEM of SiO2–Al2O3 with TEA as PRA
shows that the pore volume of SiO2–Al2O3 prepared
without any PRA treatment is 1.0301 cm2 g-1 and 73.2%
of pores are observed to be of diameter between 0.4 and
6 nm.
Table 2 shows that total acidity and acid distribution
undergoe appreciable change and exhibit different
strengths of acidity. It may be assumed that the PRA
affects the acidic properties, pore structure, and surface
When modified with TBA compound as the PRA,
noticeable changes are observed in the number of acidic sites
and strength of acidity. Total acidity is 1.58 mmol g-1 and
concentrations of weak and moderate acid sites are 95.56 and
4.44%, respectively. Selectivity to isolongifolene sharply
declines to 58% and conversion of longifolene decreases to
`
area of silica–alumina gels because of the volume vis-a-vis
the charge density of the respective PRA. NH3 temperature
123

750
J. Patial et al.
Table 2 Effect of total acidity and acid concentration on conversion and selectivity (GC yields)
Catalyst
Weak acidity/ Moderate acidity/ Strong acidity/ Total acidity/ Activity
mmol g-1
mmol g-1
mmol g-1
mmol g-1
Conversion/% Selectivity/%
SiO2–Al2O3 without PRA, stepwise 1.10 (100.00)
0.00
0.00
1.10
1.18
1.14
1.35
1.58
48
50
82
95
90
58
SiO2–Al2O3 with TMA
SiO2–Al2O3 with TEA
SiO2–Al2O3 with TPA
SiO2–Al2O3 with TBA
1.15 (97.46%) 0.01 (0.90%)
1.12 (98.38%) 0.01 (0.81%)
1.26 (93.33%) 0.05 (3.71%)
1.51 (95.56%) 0.07 (4.44%)
0.02 (1.64%)
0.01 (0.81%)
0.049 (2.9%)
0.00 (0%)
84
97
81.5
75
75%. From Table 1 it is observed that the diameter of pores
and pore volume are very near to those of Si–Al modified
with TEA compound. The change in activity is probably due
to a change in number and strength of acidity as well as the
texture properties of the catalyst as observed from both
`
Tables 1 and 2. So it may be assumed that volume vis-a-vis
the charge density of the respective PRA highly influences
the activity of the catalyst.
Catalyst recyclability
The catalyst recyclability was tested by conducting the
reactions using the Si–Al modified with TEA compound as
the PRA under optimized reaction conditions and the
samples for analysis were collected after each hour. It is
interesting to note that there is no significant loss in cata-
lytic activity up to 2 h. The spent catalyst was regenerated
after each 2 h and reused. The catalytic activity was almost
similar to that of the fresh catalyst. The results obtained are
shown in Fig. 5.
Fig. 5 Recyclability of the catalyst
(99%), and tetrabutylammonium bromide (98%) were pur-
chased from Loba Chemie L.R. Tetrapropylammonium
bromide (99%) was obtained from Merck-Schuchardt, sulfu-
ric acid (7%) from BDH L.R, and hydrochloric acid (min.
35%) from Glaxo A.R.
Thus itis evident from the above studies that the size of the
PRA cations affects the texture and catalytic properties of
silica–alumina, particularly total acidity, and generates dif-
ferent acid sites. Silica–alumina prepared with TEA
compounds has the maximum selectivity for the production
of isolongifolene through isomerization of longifolene. This
may be ascribed to the characteristic morphology as well as
the required total acidity and acid distribution. Thus, for an
active, selective, and stable catalyst based on modified
silica–alumina, compromising between distributions, con-
centration, and total acid strength with characteristic
morphology of the catalyst is essential.
Preparation of silica–alumina and modified
silica–alumina
Silica gel was prepared by adding H2SO4 to a hot solution
of Na2SiO3ꢀ5H2O with vigorous stirring until the pH
reached 7. Alumina gel was prepared by adding NH4OH
(1:1) to Al2(SO4)3ꢀ16H2O solution with vigorous stirring
until pH reached 7.5–8.5. Both gels were aged for 1 h and
then filtered and washed to free them from sulfate ions.
Pore-regulated silica–alumina was synthesized by dissolv-
ing silica gel and alumina gel as prepared and dissolved
separately in measured amounts of aqueous solution of the
PRA and concentrated solution of sodium hydroxide. The
two solutions were mixed thoroughly and a gel of silica–
alumina was prepared at pH 9. The gel was then aged for
24 h, washed, and base exchanged with 2% ammonium
Experimental
Materials
Sodium silicate (99.95%), aluminum sulfate hexadecahydrate
(98%), sodium hydroxide (98%), tetramethylammonium
hydroxide (25% aq. solution), tetraethylammonium bromide
123

Pore-engineered silica–alumina
751
nitrate solution. The mass was dried in air in an oven at
The surface area of the catalyst samples was measured
by adsorption of nitrogen with help of a Brunauer–
Emmett–Teller (BET) apparatus at liquid N2 temperature.
Pore size distributions of catalysts were determined by
using an AMNICO slow mercury porosimeter. The pre-
pared samples were screened in a JEOL-100 CXII electron
microscope with ASID at 40 kV and images are presented
in Figs. 1, 2, 3, and 4.
110 °C and finally calcined at 450 °C for 3–4 h.
Experimental set-up
The experiments for vapor-phase reactions were performed
in a fixed-bed continuous flow reactor. The arrangement for
testing the catalytic activity is described elsewhere [30]
except that, instead of top feeding, bottom feeding was
used. The vapors of longifolene and nitrogen were intro-
duced from separate inlets. All the experiments were
carried out between 160 and 200 °C.
Acknowledgments The authors wish to express their sincere grat-
itude to the Director IIIM, Jammu (CSIR) for his keen interest and
permission to publish the paper.
GC analysis
References
1. Anand B, Halgeri (2003) Bull Catal Soc India 2:184
2. Hara MO (1965) US Patent 3,216,922
3. Padro CL, Apesteguı CR (2005) Catal Today 107–108:258
4. Hilfman L (1974) US Patent 3,825,504
5. Hilfman L (1974) US Patent No. 4061563
6. Massoth FE, Murali DG, Shabtai J (1984) J Catal 85:53
7. Murrell LL, Dispenziere NC, Kim KS (1988) Catal Lett 2:263
8. Thomas JM, Klinowski J, Wright PA, Roy R (1983) Angew
Chem Int Ed 22:614
9. Mediavilla M, Melo L, Yraida D, Joaquı0n L, Alberto A, Roger S
(2007) Microporous Mesoporous Mater 102:86
10. Dilip GP, Rob van Veen JA, Marcello SR, Emiel JMH (2010)
Chem Commun 46:3466
11. Cant NW, Little LH (1966) Nature 211:69
12. Dar BA, Mohsin M, Basit A, Mazahar F (2011) J Saudi Chem
Soc. doi:10.1016/j.jscs.2011.03.004
13. Pybus DH (1999) In: Sell CS (ed) The chemistry of fragrances.
RSC, Cambridge
14. Panten J, Bertram HJ, Surburg H (2004) Chem Biodivers 1:1936
15. Ziess HH, Arakawa M (1954) J Am Chem Soc 76:1653
16. Beyler RE, Ourisson G (1965) J Org Chem 30:2838
17. Ranganathan R, Nayak UR, Santhanakrishnan TS, Dev Sukh
(1970) Tetrahedron 26:621
GC analysis with respect to isolongifolene conversion was
carried out on a Nucon 5765 gas chromatograph equipped
with a flame ionization detector (FID) and AIMIL chro-
matography data processor. The separation was achieved
using a FFAP, SE-30 fused-silica capillary column
(20 m 9 0.25 mm i.d., 0.25 lm film thickness); column
temperature 90 °C (2 min) to 220 °C at 4 °C/min; injection
temperature 240 °C; detector temperature 260 °C; using
split mode, and helium as carrier gas with column flow rate
of 1.05 cm3/min (100 kPa). Retention indices (RI) of the
sample components and authentic compounds were deter-
mined on the basis of homologous n-alkane hydrocarbons
(n-nonane to n-nonadecane) under the same conditions.
The quantitative analysis was determined by peak area
normalization, and the response factor for each component
was considered to be equal to 1.
Catalyst characterization
18. Kula J, Masarwesh A (1998) Flavour Fragrance J 13:277
19. Sobti RR, Sukh D (1970) Tetrahedron 26:649
20. Roy A, Paul T, Mukherjee D (2005) Arkivoc (xi):218
21. Wu F, Xu Z, Wang Z, Shi Y, Li L, Zhang Z (2010) Chem Eng J
158:426
TPD experiments were performed after NH3 adsorption on
sample by a Chemisorbs 27820 (Micrometrics, USA)
instrument. A 0.02- to 0.03-g sample was loaded in a quartz
microreactor. Sample was dried in a flow of ultra pure
argon (20 cm3/min) for 2 h at 200 °C then was saturated
with NH3 by flowing 10% NH3 in helium at room tem-
perature and normal pressure. The saturation of the catalyst
with NH3 was confirmed by the stability of the thermal
conductivity detector (TCD) baseline. The sample tem-
perature was then ramped to 550 °C at a linear heating rate
at 5 °C/min in flowing helium (20 cm3/min). Desorption of
NH3 was measured with a TCD. Data were analyzed with
the help of the Chemi software.
22. Sukh D (1981) Acc Chem Res 14:82
23. Breysse M, Portefaix JL, Vrinat M (1991) Catal Today 10:489
24. Luck F (1991) Bull Soc Chim Belg 100:781
25. Singh B, Patial J, Sharma P, Agarwal SG, Qazi GN, Maity S
(2007) J Mol Catal A Chem 266:215
26. Snel R (1984) Appl Catal 12:347
27. Whyte TE, Wu EL, Kerr GT, Venuto PB (1971) J Catal 20:88
28. Manton MRS, Davidtz JC (1979) J Catal 60:156
29. Bandopadhyay AK, Das J, Roy SK (1990) J Catal 124:241
30. Singh B, Roy SK, Sharma KP, Goswami TK (1998) J Chem
Technol Biotechnol 71:246
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