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Acidity and porosity modulation of MWW type zeolites for Nopol production by Prins condensation
Catalysis Communications 12 (2011) 1131–1135
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Acidity and porosity modulation of MWW type zeolites for Nopol production by Prins condensation Jie Wang a, Stephan Jaenicke b, Gaik Khuan Chuah b,⁎, Weiming Hua a, Yinghong Yue a,⁎⁎, Zi Gao a a b
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, 200433 Shanghai, PR China Department of Chemistry, National University of Singapore, 3 Science Drive 3, Kent Ridge, 117543 Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 17 February 2011 Received in revised form 21 March 2011 Accepted 22 March 2011 Available online 27 March 2011 Keywords: Prins condensation Nopol MWW zeolites Exposed acid sites Lewis acid sites
a b s t r a c t Prins condensation of β-pinene with paraformaldehyde was carried out over MCM-22, delaminated ITQ-2 and silica pillared MCM-36. The mesopore-containing MCM-36 and ITQ-2 catalysts exhibit higher conversion of βpinene due to more exposed acid sites. Lewis acid sites are responsible for Prins condensation while Brønsted acid sites favor the isomerization of pinene. The Brønsted acid sites can be removed mostly by ion-exchanging the zeolites with sodium cations. Thus, NaMWW zeolites had a higher selectivity towards Nopol. Of these, NaITQ-2 showed the highest activity and selectivity, and is a stable and reusable catalyst for production of Nopol. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Biomass carbohydrates, which are the most abundant renewable resources, are currently viewed as a feedstock for the green chemistry of the future. Among them, pinene is a worldwide available and inexpensive terpene, which is the major component of essential oil in pine trees [1]. Nopol, the condensation product of β-pinene and paraformaldehyde, is generally used in the agrochemical industry to produce pesticides and also in manufacturing household products such as soaps, detergents and polishes. However, the industrial methods for Nopol synthesis are high energy-consuming and form side products like monocyclic isomers along with Nopol [2]. In accordance with the concept of green chemistry, heterogeneous catalysts were widely explored in the past decades and also utilized in this reaction. Besides acid clays [3,4], mesoporous materials were studied as catalysts or catalyst supports for Nopol synthesis [5]. The most widely explored ones are mesoporous silicas, such as MCM-41 and SBA-15 [6]. Zinc, aluminum and tin were added to MCM-41 [4,6–10] and SBA-15 [11,12] to introduce Lewis acidic sites. Nevertheless, doping other metals into mesoporous silica is complicated and due to the amorphous nature,
⁎ Corresponding author. Tel.: + 65 65162839; fax: + 65 67791691. ⁎⁎Corresponding author. Tel.: + 86 21 65642409; fax: + 86 21 65641740. E-mail addresses: [email protected] (G.K. Chuah), [email protected] (Y. Yue). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.03.034
metal-doping mesoporous material may not be stable enough for industrial purpose. Zeolites, crystallized microporous aluminosilicas, are widely used in petrochemistry and fine chemical production due to their high hydrothermal stability and strong solid acidity. However, the pore size of conventional zeolites is too small so that the diffusion constraint of bulky molecules is severe. Moreover, for Nopol synthesis, the acidity of zeolites is too strong so that many mono-isomers of β-pinene are formed instead of Nopol [8]. The existence of Brønsted acid sites is also favorable to form monoisomers through pinene isomerization rather than Nopol through condensation. Therefore, the acid sites on zeolites should be modulated by ion-exchange with NH+ 4 [13] or alkali cations such as Na+[14,15]. MCM-22 zeolite has a unique pore structure and acid property [16– 18]. The half pockets on its external surface contribute most to the acidcatalyzed reactions involving bulky molecules. Furthermore, lamellar MWW zeolites can be swollen, exfoliated and pillared under suitable conditions. For instance, ITQ-2 [19] and MCM-36 [20] are two typical derivatives of the MCM-22 precursor. Both of these two materials have enhanced surface area and accessibility of the acidic sites. In this work, catalysts derived from MWW-type zeolites, MCM-22, delaminated ITQ-2 and silica pillared MCM-36 were synthesized and employed for Prins condensation of β-pinene with paraformaldehyde. Since the acid nature and acid strength are essential to the formation of Nopol, modulating the acidity of MWW-type zeolites by ion-exchanging with Na+ was
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J. Wang et al. / Catalysis Communications 12 (2011) 1131–1135
designed. The influence of temperature and the reusability of the catalyst were also studied.
X5
2. Experimental
(310) (220)
(101) (100)(102)
(d)
X5
2.1. Catalyst preparation 2.1.1. Synthesis of MCM-22, ITQ-2 and MCM-36 An MCM-22 precursor with a Si/Al = 20 gel ratio was synthesized in a similar manner to reported literature [21], except that a static and relatively long aging process was used [22]. MCM-22 was obtained directly from calcination of the MCM-22 precursor. ITQ-2 and MCM-36 were prepared according to methods reported in literature [19,20,22].
(c)
X5
(b) X5
(a) 2.1.2. Modulating the acidity on MWW zeolites The proton form of MWW zeolites was obtained by ion-exchanging the as-synthesized zeolites three times with 1 M NH4NO3 at 80 °C for 4 h. After that, the catalysts were calcined at 500 °C for 3 h. The samples were denoted as HMCM-22, HITQ-2 and HMCM-36, respectively. Ionexchange with sodium ions was carried out with 1 M NaCl under similar conditions. The catalysts were washed and dried overnight at 100 °C. The sodium-exchanged samples were denoted as NaMCM-22, NaITQ-2 and NaMCM-36.
2
4
5
10
15
20
35
40
effective carbon numbers (ECN). The products were confirmed by GCMS (Shimadzu-QP5000, DB5MS column).
3. Results and discussion
The powder XRD patterns were recorded on a Bruker D8 ENDEAVOR diffractometer (Cu Kα radiation, 40 kV, 40 mA). The bulk Si/Al ratio was determined using a Philips PW2404 XRF elemental analysis spectrometer. The N2 adsorption/desorption isotherms were measured with a Tristar 3000 porosimeter at liquid N2 temperature. Ammonia temperature programmed desorption (NH3-TPD) of the samples was carried out in a flow-type fixed-bed reactor at ambient pressure. The catalysts were pretreated in helium at 550 °C for 2 h. The NH3 adsorption temperature was 120 °C. Flushing with helium for 2 h was carried out prior to NH3 desorption. For NH3 desorption, the temperature was raised at a rate of 10 °C/min. The NH3 desorbed was detected by a thermal conductivity detector and collected in liquid N2 traps. The amounts trapped in the region of 100–350 °C and 350– 600 °C were quantified again after the desorption process. FT-IR spectra of pyridine absorption were recorded on a Bio-Rad Excalibur FT-IR spectrometer with a resolution of 4 cm− 1. The sample disk (10–20 mg) was pretreated in a glass cell at 300 °C for 2 h under a vacuum of 100 Pa and then cooled to room temperature. After the background spectrum was recorded, pyridine was introduced into the cell for 15 min. The cell was re-evacuated at room temperature for 1 h before measuring the IR spectrum. Further measurements were made after the sample had been evacuated at 100 °C and 200 °C for 1 h, respectively. Quantitative determination of Brønsted and Lewis acid sites was derived from the integrated areas of the IR bands at ca. 1540 and 1450 cm− 1, respectively. The integrated molar extinction coefficients were obtained from previous work of Emeis [23].
3.1. Textural properties of the catalysts XRD patterns of the MCM-22 precursor, calcined MCM-22, ITQ-2 and MCM-36 have similar peak positions for the strongest reflections of (100) and (310) at 2θ=7.2° and 25.9°, respectively [24] (Fig. 1). However, a broad peak occurs at 2θ in the region 7.5–11° and covers the strongest h0l reflections such as (101) and (102) for ITQ-2 and MCM-36. It indicates that ITQ-2 and MCM-36 possess wide a–b planes with large coherent domains, while the parameter along the c-axis is different. The N2 adsorption/desorption isotherms showed that the porosity of these three samples was different (Fig. 2). MCM-22 has a typical isotherm for zeolites (type I according to IUPAC), whereas ITQ-2 and MCM-36 have isotherms of type IV with H3 hysteresis loop, showing that mesopores were generated. The composition and textural properties of the catalysts are listed in Table 1. For MCM-22 and ITQ-2, the bulk Si/Al ratio was slightly lower than the initial gel. For MCM-36, the Si/Al ratio was 31, much
500
Volume adsorbed/desorbed (cm3/g)
The Prins condensation reaction for the synthesis of Nopol was carried out in a two-necked flask equipped with a condenser. Typically, 50 mg catalyst, 1 mmol β-pinene, 2 mmol paraformaldehyde and 5 ml toluene were added into the flask with magnetic stirring. The reaction was carried out between 70 and 90 °C. The catalysts were dried in 80 °C overnight before use. The products were analyzed by GC (Agilent GC6890 fitted with a HP-5 capillary column). Besides the condensation product Nopol, by-products of mono-isomers of β-pinene such as αpinene, camphene, limonene, terpinolene and terpinene were formed. The conversion and selectivity were calculated from the GC results using
30
Fig. 1. XRD patterns of (a) MCM-22, (b) MCM-22 precursor, (c) ITQ-2 and (d) MCM-36.
2.2. Characterization
2.3. Prins condensation reaction
25
2 Theta
(c)
450 400
(b)
350 300
(a)
250 200 150 100 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 2. Nitrogen absorption/desorption isotherms of (a) MCM-22, (b) MCM-36 and (c) ITQ-2.
J. Wang et al. / Catalysis Communications 12 (2011) 1131–1135
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(f)
Table 1 Composition and textural properties of the catalysts. Catalyst
Si/Ala
Stotalb (m2/g)
Sexternalc (m2/g)
Vtotald (cm3/g)
Vmicroc (cm3/g)
(e)
HMCM-22 HITQ-2 HMCM-36
19 19 31
524 676 558
127 625 210
0.38 0.69 0.59
0.21 0.03 0.18
(d)
a b c d
(c)
Determined by XRF. Surface area was calculated by BET method. Determined by t-plot method. Calculated by BJH method.
(b) (a)
higher than the MCM-22 precursor. The reason is that the pillaring material is pure silica so that the bulk silica content is increased.
3.2.1. NH3-TPD NH3-TPD was utilized to determine the total acidity on the catalysts (Table 2). On HITQ-2 and HMCM-36 samples, the amount of strong acid sites as well as the total acid sites decreased in comparison with HMCM-22. Furthermore, the acid strength decreased along with the swelling and pillaring process so that the peak temperatures were also decreased. After ion-exchange with NaCl, the acid amount of MWW zeolites was significantly lower than that of the protonated forms since the Brønsted acid sites was removed. Brønsted acidity was significantly decreased while Lewis acidity decreased by a lesser amount. 3.2.2. FT-IR spectra of pyridine adsorption The FT-IR of pyridine adsorption was used to study the nature of the acid sites. Fig. 3 shows the normalized intensity (by weight of sample) of the absorption bands after evacuation at 100 °C for 1 h. For HMCM-22 and HITQ-2 samples, absorption bands around 1540 cm− 1 for Brønsted acid sites and 1450 cm− 1 for Lewis acid sites were observed, showing that both Lewis and Brønsted acid sites are present. For HMCM-36 sample, the Brønsted/Lewis (B/L) ratio decreased due to a substantial decrease in the Brønsted acid sites (Table 3). HMCM-22 has the most acid sites and the highest B/L ratio. After swelling and exfoliation, the acidity of HITQ-2 was reduced along with a lower B/L ratio. As the swelling process occurred under basic conditions, dealumination could have occurred forming more Lewis acid sites on ITQ-2. For the pillared MCM-36, as the pillaring process involved mainly the external hydroxyl groups to form T―O―T moieties, its acidity was further reduced. After ion-exchange with NaCl, the Brønsted acid sites almost vanished but the Lewis acid sites were still present although their intensity was smaller. The decrease of Brønsted acid sites by ionexchange agrees with reported literature where a Na+ exchange rate of over 90% results in the disappearance of Brønsted acid sites [15].
Table 2 NH3-TPD data of the catalysts.
HMCM-22 HITQ-2 HMCM-36 NaMCM-22 NaITQ-2 NaMCM-36
1600
1500
1400
Wavenumber (cm-1) Fig. 3. Pyridine absorption spectra recorded after evacuation at 100 °C for (a) HMCM22, (b) HITQ-2, (c) HMCM-36, (d) NaMCM-22, (e) NaITQ-2 and (f) NaMCM-36.
3.2. Acidity measurement
Catalysts
1700
Furthermore, after ion-exchange, the acid density was sharply reduced for all the samples. The total acid sites remaining for NaMCM-22, NaITQ-2 and NaMCM-36 assessed by the amount of pyridine adsorbed at room temperature were 0.38, 0.37 and 0.22 mmol/g, which agrees quite well with that from NH3-TPD results (0.37, 0.31 and 0.22 mmol/g, respectively). 3.3. Prins condensation activity 3.3.1. Activity over HMWW zeolites HMWW zeolites were tested for activity in Prins condensation reaction, and the results were summarized in Table 4. For HMCM-22, the conversion was quite low. This may be due to limited accessibility to the acid sites as the pore channels of MCM-22 (0.40 nm × 0.55 nm) are not large enough. For HITQ-2, a higher conversion was obtained. The catalyst is composed of platelets so that most of the acid sites are accessible to the β-pinene substrate [22]. For HMCM-36 with both mesopores and micropores, it is not difficult to understand that the conversion of β-pinene falls between that of HMCM-22 and HITQ-2. Unfortunately, the converted products were all mono-isomers of β-pinene rather than the intended product Nopol on HMCM-22 and HITQ-2. The selectivity for Nopol was also very low on MCM-36 (29%). As the C_C bond of β-pinene is terminal, β-pinene can easily form a carbenium ion. In the presence of Brønsted acid sites, the isomerization of β-pinene is facile. As reported, the first step of isomerization is the formation of a pinanyl cation. The carbenium ion then rearranges to form bicyclic and monocyclic isomers [22]. The formation of isomers is irreversible so that if isomerization is predominant, less β-pinene is available for Prins condensation. This is clearly seen in HITQ-2 where complete conversion was obtained but all the β-pinene was converted to other isomers and no Prins condensation of β-pinene
Table 3 Acidity of MWW zeolites determined by pyridine absorbed FT-IR.
Adsorbed NH3 (mmol/g)
Peak Temp. (°C)
I (120–350 °C)
II (350–600 °C)
Total
0.47 0.40 0.25 0.27 0.24 0.18
0.44 0.22 0.08 0.11 0.07 0.04
0.91 0.62 0.33 0.37 0.31 0.22
I 275 261 248 258 245 232
Catalysts
Brønsted
II 427 420 410 415 401 391
Amount of acid sites (μmol/g)
HMCM-22 HITQ-2 HMCM-36 NaMCM-22 NaITQ-2 NaMCM-36
B/L ratio desorption of 100 °C
Lewis
r.t
100 °C
200 °C
r.t
100 °C
200 °C
335 135 31 21 20 22
264 118 30 18 18 22
240 110 27 16 4 7
740 435 355 361 354 195
160 158 127 103 108 105
110 125 59 95 39 34
1.65 0.75 0.24 0.17 0.17 0.21
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Table 4 Activities of Prin reaction over the studied catalysts. Catalysts
HMCM-22 HITQ-2 HMCM-36 NaMCM-22 NaMCM-22 NaITQ-2 NaMCM-36 NaITQ-2 NaITQ-2 NaITQ-2 b c
Conv. of β-pinene (%)
Selectivity (%) Nopol
Camphene
Limonene
Othersc
80 80 80 80 80 80 80 70 85 90
24 a 100a 63a 16a 24b 60a 32a 46a 74a 94a
0 0 29 N99 95 87 85 85 69 10
43 29 27 0 1 7 8 8 15 33
55 51 39 trace 3 5 7 7 13 45
2 20 5 0 1 1 0 0 3 12
Yield of Nopol (%) 0 0 18 16 23 52 27 39 51 9.4
Conversion after reaction for 1 h. Conversion after reaction for 20 h. Including isomers of β-pinene such as terpinolenes, terpinenes and α-pinene.
with paraformaldehyde occurred. The lower Brønsted acidity of the external hydroxyl groups on HMCM-36 reduces the activity for isomerization so that some β-pinene could react with paraformaldehyde to form Nopol with 29% selectivity. 3.3.2. Activity over NaMWW zeolites To improve the yield of Nopol, the zeolites were ion-exchanged with sodium ions to remove the Brønsted acid sites, especially the strong ones, and thus suppress the activity for isomerization. Since the condensation or the isomerization reactions are both acidcatalyzed ones, the effects of the basic sites of NaMWW zeolites can be neglected here. An obvious increase of Nopol selectivity and yield was observed (Table 4). Compared to HMWW zeolites, β-pinene mainly reacted with paraformaldehyde to form Nopol. The selectivity to Nopol improved from just 29% in HMCM-36 to N80% for the NaMWW zeolites. Again, NaITQ-2 has the best conversion among the sodium-exchanged samples, which may be due to the high proportion of mesopores, since the micropores of MCM-22 are not large enough for the bulky reactants. Despite of low conversion, NaMCM-22 has the best selectivity to Nopol. The activity on NaMCM-22 was studied for a longer time. After reaction for 20 h, the conversion of β-pinene reaches 24% and the selectivity towards Nopol is 95%. It means that although the selectivity to Nopol is very ideal, the conversion of ß-pinene cannot reach a high standard due to the small pore size of MCM-22. 3.3.3. Effect of the temperature The conversion of β-pinene increased with higher reaction temperature (Fig. 4). At 90 °C, the conversion reached almost 100% after 4 h.
Conversion (%)
100 80 60 40
However, the selectivity was affected by the different temperatures. Below 80 °C, the selectivity towards Nopol was over 80% (Table 4). When the temperature was 85 °C, the selectivity was reduced to 60– 70%. With a further increase in the reaction temperature to 90 °C, about 90% of the β-pinene was converted to mono-isomers and the selectivity towards Nopol dramatically dropped. Hence, the optimal temperature for Nopol synthesis on micro- and mesoporous catalysts is below 90 °C [8,12]. In NaITQ-2 catalyst, the remaining weak Brønsted acid sites present after ion exchange may not be sufficient to form the pinanyl cation. With a higher reaction temperature of 90 °C, these weak Brønsted acid sites are activated resulting in an increase in the number of isomers formed. 3.3.4. Stability and reusability The reusability of the catalyst was studied (Table 5). After a batch run, the catalyst was recovered by filtration, washed with acetone for several times and dried at 80 °C overnight before reuse. After three runs, the yield for Nopol reduced from 57% to 48%. However, after recalcination at 500 °C for 3 h, the catalytic activity was recovered, indicating that the deactivation is largely due to coverage of the acid sites by the organic products. The Nopol yield of the recalcined sample was 55% which is comparable to that of the fresh catalyst (Table 5). 4. Conclusions MWW zeolites were studied as heterogeneous catalysts for the synthesis of Nopol via Prins condensation. The lamellar MWW zeolites contain sufficient acidity so that doping of metal such as Sn and Zn is not required. Additionally, MWW zeolites can be pillared or delaminated to
(d)
100
(c) (b) (a)
80
Selectivity (%)
a
React. Temp. (°C)
(b) (a) (c)
60 40
20
20
0
0
(d) 0
1
2
3
Reaction Time (h)
4
0
1
2
3
4
Reaction Time (h)
Fig. 4. Conversion and selectivity of ß-pinene on NaITQ-2 at different temperatures: (a) 70 °C, (b) 80 °C, (c) 85 °C and (d) 90 °C.
J. Wang et al. / Catalysis Communications 12 (2011) 1131–1135
financial support from the Fudan Graduate School for a short-term visit to National University of Singapore.
Table 5 Reusability of NaITQ-2 in the Nopol synthesis. Catalysts
Conversion of ß-pinenea (%)
Selectivity of Nopol (%)
Yield of Nopol (%)
Fresh 1st reuse 2nd reuse 3 rd reuse After regenerationb
71 64 61 59 72
81 79 82 82 77
57 51 50 48 55
a b
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Conversion was calculated after reaction for 4 h under 80 °C. Catalyst was washed with acetone and calcined at 500 °C for 3 h before reuse.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
expose more acid sites. However, the existence of Brønsted acid sites, especially the medium strong ones, will lead β-pinene to convert into monoisomers. Ion-exchanging MWW zeolites with sodium cations can remove most of the Brønsted acid sites while still preserving some Lewis acidity. In this case, the selectivity of Nopol can be enhanced to over 85%. An optimum Nopol yield of 57% was obtained over NaITQ-2 catalyst at 80 °C after 4 h. The NaITQ-2 is stable and reusable. Hence, it is shown that the acidity and porosity modulations over zeolites can be an efficient route to catalyze the selective synthesis of Nopol. Acknowledgments This work was supported by the Chinese Major State Basic Research Development Program (2006CB806103), the National Natural Science Foundation of China (20633030, 20773027 and 20773028) and the Science & Technology Commission of Shanghai Municipality (08DZ2270500). The author Jie Wang is grateful for the
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Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Acidity and porosity modulation of MWW type zeolites for Nopol production by Prins condensation Jie Wang a, Stephan Jaenicke b, Gaik Khuan Chuah b,⁎, Weiming Hua a, Yinghong Yue a,⁎⁎, Zi Gao a a b
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, 200433 Shanghai, PR China Department of Chemistry, National University of Singapore, 3 Science Drive 3, Kent Ridge, 117543 Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 17 February 2011 Received in revised form 21 March 2011 Accepted 22 March 2011 Available online 27 March 2011 Keywords: Prins condensation Nopol MWW zeolites Exposed acid sites Lewis acid sites
a b s t r a c t Prins condensation of β-pinene with paraformaldehyde was carried out over MCM-22, delaminated ITQ-2 and silica pillared MCM-36. The mesopore-containing MCM-36 and ITQ-2 catalysts exhibit higher conversion of βpinene due to more exposed acid sites. Lewis acid sites are responsible for Prins condensation while Brønsted acid sites favor the isomerization of pinene. The Brønsted acid sites can be removed mostly by ion-exchanging the zeolites with sodium cations. Thus, NaMWW zeolites had a higher selectivity towards Nopol. Of these, NaITQ-2 showed the highest activity and selectivity, and is a stable and reusable catalyst for production of Nopol. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Biomass carbohydrates, which are the most abundant renewable resources, are currently viewed as a feedstock for the green chemistry of the future. Among them, pinene is a worldwide available and inexpensive terpene, which is the major component of essential oil in pine trees [1]. Nopol, the condensation product of β-pinene and paraformaldehyde, is generally used in the agrochemical industry to produce pesticides and also in manufacturing household products such as soaps, detergents and polishes. However, the industrial methods for Nopol synthesis are high energy-consuming and form side products like monocyclic isomers along with Nopol [2]. In accordance with the concept of green chemistry, heterogeneous catalysts were widely explored in the past decades and also utilized in this reaction. Besides acid clays [3,4], mesoporous materials were studied as catalysts or catalyst supports for Nopol synthesis [5]. The most widely explored ones are mesoporous silicas, such as MCM-41 and SBA-15 [6]. Zinc, aluminum and tin were added to MCM-41 [4,6–10] and SBA-15 [11,12] to introduce Lewis acidic sites. Nevertheless, doping other metals into mesoporous silica is complicated and due to the amorphous nature,
⁎ Corresponding author. Tel.: + 65 65162839; fax: + 65 67791691. ⁎⁎Corresponding author. Tel.: + 86 21 65642409; fax: + 86 21 65641740. E-mail addresses: [email protected] (G.K. Chuah), [email protected] (Y. Yue). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.03.034
metal-doping mesoporous material may not be stable enough for industrial purpose. Zeolites, crystallized microporous aluminosilicas, are widely used in petrochemistry and fine chemical production due to their high hydrothermal stability and strong solid acidity. However, the pore size of conventional zeolites is too small so that the diffusion constraint of bulky molecules is severe. Moreover, for Nopol synthesis, the acidity of zeolites is too strong so that many mono-isomers of β-pinene are formed instead of Nopol [8]. The existence of Brønsted acid sites is also favorable to form monoisomers through pinene isomerization rather than Nopol through condensation. Therefore, the acid sites on zeolites should be modulated by ion-exchange with NH+ 4 [13] or alkali cations such as Na+[14,15]. MCM-22 zeolite has a unique pore structure and acid property [16– 18]. The half pockets on its external surface contribute most to the acidcatalyzed reactions involving bulky molecules. Furthermore, lamellar MWW zeolites can be swollen, exfoliated and pillared under suitable conditions. For instance, ITQ-2 [19] and MCM-36 [20] are two typical derivatives of the MCM-22 precursor. Both of these two materials have enhanced surface area and accessibility of the acidic sites. In this work, catalysts derived from MWW-type zeolites, MCM-22, delaminated ITQ-2 and silica pillared MCM-36 were synthesized and employed for Prins condensation of β-pinene with paraformaldehyde. Since the acid nature and acid strength are essential to the formation of Nopol, modulating the acidity of MWW-type zeolites by ion-exchanging with Na+ was
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J. Wang et al. / Catalysis Communications 12 (2011) 1131–1135
designed. The influence of temperature and the reusability of the catalyst were also studied.
X5
2. Experimental
(310) (220)
(101) (100)(102)
(d)
X5
2.1. Catalyst preparation 2.1.1. Synthesis of MCM-22, ITQ-2 and MCM-36 An MCM-22 precursor with a Si/Al = 20 gel ratio was synthesized in a similar manner to reported literature [21], except that a static and relatively long aging process was used [22]. MCM-22 was obtained directly from calcination of the MCM-22 precursor. ITQ-2 and MCM-36 were prepared according to methods reported in literature [19,20,22].
(c)
X5
(b) X5
(a) 2.1.2. Modulating the acidity on MWW zeolites The proton form of MWW zeolites was obtained by ion-exchanging the as-synthesized zeolites three times with 1 M NH4NO3 at 80 °C for 4 h. After that, the catalysts were calcined at 500 °C for 3 h. The samples were denoted as HMCM-22, HITQ-2 and HMCM-36, respectively. Ionexchange with sodium ions was carried out with 1 M NaCl under similar conditions. The catalysts were washed and dried overnight at 100 °C. The sodium-exchanged samples were denoted as NaMCM-22, NaITQ-2 and NaMCM-36.
2
4
5
10
15
20
35
40
effective carbon numbers (ECN). The products were confirmed by GCMS (Shimadzu-QP5000, DB5MS column).
3. Results and discussion
The powder XRD patterns were recorded on a Bruker D8 ENDEAVOR diffractometer (Cu Kα radiation, 40 kV, 40 mA). The bulk Si/Al ratio was determined using a Philips PW2404 XRF elemental analysis spectrometer. The N2 adsorption/desorption isotherms were measured with a Tristar 3000 porosimeter at liquid N2 temperature. Ammonia temperature programmed desorption (NH3-TPD) of the samples was carried out in a flow-type fixed-bed reactor at ambient pressure. The catalysts were pretreated in helium at 550 °C for 2 h. The NH3 adsorption temperature was 120 °C. Flushing with helium for 2 h was carried out prior to NH3 desorption. For NH3 desorption, the temperature was raised at a rate of 10 °C/min. The NH3 desorbed was detected by a thermal conductivity detector and collected in liquid N2 traps. The amounts trapped in the region of 100–350 °C and 350– 600 °C were quantified again after the desorption process. FT-IR spectra of pyridine absorption were recorded on a Bio-Rad Excalibur FT-IR spectrometer with a resolution of 4 cm− 1. The sample disk (10–20 mg) was pretreated in a glass cell at 300 °C for 2 h under a vacuum of 100 Pa and then cooled to room temperature. After the background spectrum was recorded, pyridine was introduced into the cell for 15 min. The cell was re-evacuated at room temperature for 1 h before measuring the IR spectrum. Further measurements were made after the sample had been evacuated at 100 °C and 200 °C for 1 h, respectively. Quantitative determination of Brønsted and Lewis acid sites was derived from the integrated areas of the IR bands at ca. 1540 and 1450 cm− 1, respectively. The integrated molar extinction coefficients were obtained from previous work of Emeis [23].
3.1. Textural properties of the catalysts XRD patterns of the MCM-22 precursor, calcined MCM-22, ITQ-2 and MCM-36 have similar peak positions for the strongest reflections of (100) and (310) at 2θ=7.2° and 25.9°, respectively [24] (Fig. 1). However, a broad peak occurs at 2θ in the region 7.5–11° and covers the strongest h0l reflections such as (101) and (102) for ITQ-2 and MCM-36. It indicates that ITQ-2 and MCM-36 possess wide a–b planes with large coherent domains, while the parameter along the c-axis is different. The N2 adsorption/desorption isotherms showed that the porosity of these three samples was different (Fig. 2). MCM-22 has a typical isotherm for zeolites (type I according to IUPAC), whereas ITQ-2 and MCM-36 have isotherms of type IV with H3 hysteresis loop, showing that mesopores were generated. The composition and textural properties of the catalysts are listed in Table 1. For MCM-22 and ITQ-2, the bulk Si/Al ratio was slightly lower than the initial gel. For MCM-36, the Si/Al ratio was 31, much
500
Volume adsorbed/desorbed (cm3/g)
The Prins condensation reaction for the synthesis of Nopol was carried out in a two-necked flask equipped with a condenser. Typically, 50 mg catalyst, 1 mmol β-pinene, 2 mmol paraformaldehyde and 5 ml toluene were added into the flask with magnetic stirring. The reaction was carried out between 70 and 90 °C. The catalysts were dried in 80 °C overnight before use. The products were analyzed by GC (Agilent GC6890 fitted with a HP-5 capillary column). Besides the condensation product Nopol, by-products of mono-isomers of β-pinene such as αpinene, camphene, limonene, terpinolene and terpinene were formed. The conversion and selectivity were calculated from the GC results using
30
Fig. 1. XRD patterns of (a) MCM-22, (b) MCM-22 precursor, (c) ITQ-2 and (d) MCM-36.
2.2. Characterization
2.3. Prins condensation reaction
25
2 Theta
(c)
450 400
(b)
350 300
(a)
250 200 150 100 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 2. Nitrogen absorption/desorption isotherms of (a) MCM-22, (b) MCM-36 and (c) ITQ-2.
J. Wang et al. / Catalysis Communications 12 (2011) 1131–1135
1133
(f)
Table 1 Composition and textural properties of the catalysts. Catalyst
Si/Ala
Stotalb (m2/g)
Sexternalc (m2/g)
Vtotald (cm3/g)
Vmicroc (cm3/g)
(e)
HMCM-22 HITQ-2 HMCM-36
19 19 31
524 676 558
127 625 210
0.38 0.69 0.59
0.21 0.03 0.18
(d)
a b c d
(c)
Determined by XRF. Surface area was calculated by BET method. Determined by t-plot method. Calculated by BJH method.
(b) (a)
higher than the MCM-22 precursor. The reason is that the pillaring material is pure silica so that the bulk silica content is increased.
3.2.1. NH3-TPD NH3-TPD was utilized to determine the total acidity on the catalysts (Table 2). On HITQ-2 and HMCM-36 samples, the amount of strong acid sites as well as the total acid sites decreased in comparison with HMCM-22. Furthermore, the acid strength decreased along with the swelling and pillaring process so that the peak temperatures were also decreased. After ion-exchange with NaCl, the acid amount of MWW zeolites was significantly lower than that of the protonated forms since the Brønsted acid sites was removed. Brønsted acidity was significantly decreased while Lewis acidity decreased by a lesser amount. 3.2.2. FT-IR spectra of pyridine adsorption The FT-IR of pyridine adsorption was used to study the nature of the acid sites. Fig. 3 shows the normalized intensity (by weight of sample) of the absorption bands after evacuation at 100 °C for 1 h. For HMCM-22 and HITQ-2 samples, absorption bands around 1540 cm− 1 for Brønsted acid sites and 1450 cm− 1 for Lewis acid sites were observed, showing that both Lewis and Brønsted acid sites are present. For HMCM-36 sample, the Brønsted/Lewis (B/L) ratio decreased due to a substantial decrease in the Brønsted acid sites (Table 3). HMCM-22 has the most acid sites and the highest B/L ratio. After swelling and exfoliation, the acidity of HITQ-2 was reduced along with a lower B/L ratio. As the swelling process occurred under basic conditions, dealumination could have occurred forming more Lewis acid sites on ITQ-2. For the pillared MCM-36, as the pillaring process involved mainly the external hydroxyl groups to form T―O―T moieties, its acidity was further reduced. After ion-exchange with NaCl, the Brønsted acid sites almost vanished but the Lewis acid sites were still present although their intensity was smaller. The decrease of Brønsted acid sites by ionexchange agrees with reported literature where a Na+ exchange rate of over 90% results in the disappearance of Brønsted acid sites [15].
Table 2 NH3-TPD data of the catalysts.
HMCM-22 HITQ-2 HMCM-36 NaMCM-22 NaITQ-2 NaMCM-36
1600
1500
1400
Wavenumber (cm-1) Fig. 3. Pyridine absorption spectra recorded after evacuation at 100 °C for (a) HMCM22, (b) HITQ-2, (c) HMCM-36, (d) NaMCM-22, (e) NaITQ-2 and (f) NaMCM-36.
3.2. Acidity measurement
Catalysts
1700
Furthermore, after ion-exchange, the acid density was sharply reduced for all the samples. The total acid sites remaining for NaMCM-22, NaITQ-2 and NaMCM-36 assessed by the amount of pyridine adsorbed at room temperature were 0.38, 0.37 and 0.22 mmol/g, which agrees quite well with that from NH3-TPD results (0.37, 0.31 and 0.22 mmol/g, respectively). 3.3. Prins condensation activity 3.3.1. Activity over HMWW zeolites HMWW zeolites were tested for activity in Prins condensation reaction, and the results were summarized in Table 4. For HMCM-22, the conversion was quite low. This may be due to limited accessibility to the acid sites as the pore channels of MCM-22 (0.40 nm × 0.55 nm) are not large enough. For HITQ-2, a higher conversion was obtained. The catalyst is composed of platelets so that most of the acid sites are accessible to the β-pinene substrate [22]. For HMCM-36 with both mesopores and micropores, it is not difficult to understand that the conversion of β-pinene falls between that of HMCM-22 and HITQ-2. Unfortunately, the converted products were all mono-isomers of β-pinene rather than the intended product Nopol on HMCM-22 and HITQ-2. The selectivity for Nopol was also very low on MCM-36 (29%). As the C_C bond of β-pinene is terminal, β-pinene can easily form a carbenium ion. In the presence of Brønsted acid sites, the isomerization of β-pinene is facile. As reported, the first step of isomerization is the formation of a pinanyl cation. The carbenium ion then rearranges to form bicyclic and monocyclic isomers [22]. The formation of isomers is irreversible so that if isomerization is predominant, less β-pinene is available for Prins condensation. This is clearly seen in HITQ-2 where complete conversion was obtained but all the β-pinene was converted to other isomers and no Prins condensation of β-pinene
Table 3 Acidity of MWW zeolites determined by pyridine absorbed FT-IR.
Adsorbed NH3 (mmol/g)
Peak Temp. (°C)
I (120–350 °C)
II (350–600 °C)
Total
0.47 0.40 0.25 0.27 0.24 0.18
0.44 0.22 0.08 0.11 0.07 0.04
0.91 0.62 0.33 0.37 0.31 0.22
I 275 261 248 258 245 232
Catalysts
Brønsted
II 427 420 410 415 401 391
Amount of acid sites (μmol/g)
HMCM-22 HITQ-2 HMCM-36 NaMCM-22 NaITQ-2 NaMCM-36
B/L ratio desorption of 100 °C
Lewis
r.t
100 °C
200 °C
r.t
100 °C
200 °C
335 135 31 21 20 22
264 118 30 18 18 22
240 110 27 16 4 7
740 435 355 361 354 195
160 158 127 103 108 105
110 125 59 95 39 34
1.65 0.75 0.24 0.17 0.17 0.21
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J. Wang et al. / Catalysis Communications 12 (2011) 1131–1135
Table 4 Activities of Prin reaction over the studied catalysts. Catalysts
HMCM-22 HITQ-2 HMCM-36 NaMCM-22 NaMCM-22 NaITQ-2 NaMCM-36 NaITQ-2 NaITQ-2 NaITQ-2 b c
Conv. of β-pinene (%)
Selectivity (%) Nopol
Camphene
Limonene
Othersc
80 80 80 80 80 80 80 70 85 90
24 a 100a 63a 16a 24b 60a 32a 46a 74a 94a
0 0 29 N99 95 87 85 85 69 10
43 29 27 0 1 7 8 8 15 33
55 51 39 trace 3 5 7 7 13 45
2 20 5 0 1 1 0 0 3 12
Yield of Nopol (%) 0 0 18 16 23 52 27 39 51 9.4
Conversion after reaction for 1 h. Conversion after reaction for 20 h. Including isomers of β-pinene such as terpinolenes, terpinenes and α-pinene.
with paraformaldehyde occurred. The lower Brønsted acidity of the external hydroxyl groups on HMCM-36 reduces the activity for isomerization so that some β-pinene could react with paraformaldehyde to form Nopol with 29% selectivity. 3.3.2. Activity over NaMWW zeolites To improve the yield of Nopol, the zeolites were ion-exchanged with sodium ions to remove the Brønsted acid sites, especially the strong ones, and thus suppress the activity for isomerization. Since the condensation or the isomerization reactions are both acidcatalyzed ones, the effects of the basic sites of NaMWW zeolites can be neglected here. An obvious increase of Nopol selectivity and yield was observed (Table 4). Compared to HMWW zeolites, β-pinene mainly reacted with paraformaldehyde to form Nopol. The selectivity to Nopol improved from just 29% in HMCM-36 to N80% for the NaMWW zeolites. Again, NaITQ-2 has the best conversion among the sodium-exchanged samples, which may be due to the high proportion of mesopores, since the micropores of MCM-22 are not large enough for the bulky reactants. Despite of low conversion, NaMCM-22 has the best selectivity to Nopol. The activity on NaMCM-22 was studied for a longer time. After reaction for 20 h, the conversion of β-pinene reaches 24% and the selectivity towards Nopol is 95%. It means that although the selectivity to Nopol is very ideal, the conversion of ß-pinene cannot reach a high standard due to the small pore size of MCM-22. 3.3.3. Effect of the temperature The conversion of β-pinene increased with higher reaction temperature (Fig. 4). At 90 °C, the conversion reached almost 100% after 4 h.
Conversion (%)
100 80 60 40
However, the selectivity was affected by the different temperatures. Below 80 °C, the selectivity towards Nopol was over 80% (Table 4). When the temperature was 85 °C, the selectivity was reduced to 60– 70%. With a further increase in the reaction temperature to 90 °C, about 90% of the β-pinene was converted to mono-isomers and the selectivity towards Nopol dramatically dropped. Hence, the optimal temperature for Nopol synthesis on micro- and mesoporous catalysts is below 90 °C [8,12]. In NaITQ-2 catalyst, the remaining weak Brønsted acid sites present after ion exchange may not be sufficient to form the pinanyl cation. With a higher reaction temperature of 90 °C, these weak Brønsted acid sites are activated resulting in an increase in the number of isomers formed. 3.3.4. Stability and reusability The reusability of the catalyst was studied (Table 5). After a batch run, the catalyst was recovered by filtration, washed with acetone for several times and dried at 80 °C overnight before reuse. After three runs, the yield for Nopol reduced from 57% to 48%. However, after recalcination at 500 °C for 3 h, the catalytic activity was recovered, indicating that the deactivation is largely due to coverage of the acid sites by the organic products. The Nopol yield of the recalcined sample was 55% which is comparable to that of the fresh catalyst (Table 5). 4. Conclusions MWW zeolites were studied as heterogeneous catalysts for the synthesis of Nopol via Prins condensation. The lamellar MWW zeolites contain sufficient acidity so that doping of metal such as Sn and Zn is not required. Additionally, MWW zeolites can be pillared or delaminated to
(d)
100
(c) (b) (a)
80
Selectivity (%)
a
React. Temp. (°C)
(b) (a) (c)
60 40
20
20
0
0
(d) 0
1
2
3
Reaction Time (h)
4
0
1
2
3
4
Reaction Time (h)
Fig. 4. Conversion and selectivity of ß-pinene on NaITQ-2 at different temperatures: (a) 70 °C, (b) 80 °C, (c) 85 °C and (d) 90 °C.
J. Wang et al. / Catalysis Communications 12 (2011) 1131–1135
financial support from the Fudan Graduate School for a short-term visit to National University of Singapore.
Table 5 Reusability of NaITQ-2 in the Nopol synthesis. Catalysts
Conversion of ß-pinenea (%)
Selectivity of Nopol (%)
Yield of Nopol (%)
Fresh 1st reuse 2nd reuse 3 rd reuse After regenerationb
71 64 61 59 72
81 79 82 82 77
57 51 50 48 55
a b
1135
Conversion was calculated after reaction for 4 h under 80 °C. Catalyst was washed with acetone and calcined at 500 °C for 3 h before reuse.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
expose more acid sites. However, the existence of Brønsted acid sites, especially the medium strong ones, will lead β-pinene to convert into monoisomers. Ion-exchanging MWW zeolites with sodium cations can remove most of the Brønsted acid sites while still preserving some Lewis acidity. In this case, the selectivity of Nopol can be enhanced to over 85%. An optimum Nopol yield of 57% was obtained over NaITQ-2 catalyst at 80 °C after 4 h. The NaITQ-2 is stable and reusable. Hence, it is shown that the acidity and porosity modulations over zeolites can be an efficient route to catalyze the selective synthesis of Nopol. Acknowledgments This work was supported by the Chinese Major State Basic Research Development Program (2006CB806103), the National Natural Science Foundation of China (20633030, 20773027 and 20773028) and the Science & Technology Commission of Shanghai Municipality (08DZ2270500). The author Jie Wang is grateful for the
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