MSU-S mesoporous materials: An efficient catalyst for isomerization of α-pinene

Bioresource Technology 101 (2010) 7224–7230

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MSU-S mesoporous materials: An efficient catalyst for isomerization of a-pinene Jie Wang, Weiming Hua, Yinghong Yue *, Zi Gao Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China

a r t i c l e

i n f o

Article history: Received 7 January 2010 Received in revised form 21 April 2010 Accepted 23 April 2010 Available online 18 May 2010 Keywords: a-Pinene Isomerization MSU-S(BEA) Accessible acid sites Zeolite-like subunits

a b s t r a c t MSU-S(BEA) and MSU-S(Y) mesoporous molecular sieves with different Si/Al ratios were prepared and characterized by XRD, XRF, N2 adsorption, 27Al MAS NMR, NH3-TPD and 2,6-di-tert-butyl-pyridine adsorption. Their catalytic behavior for the liquid phase isomerization of a-pinene has been investigated and compared with conventional zeolites and mesoporous molecular sieves. The activity correlates well with the amount of the accessible acid sites on the catalyst surface. MSU-S(BEA) with Si/Al ratio of 67 has the highest activity in comparison to others. Ninty-seven percent conversion of a-pinene and 91% yield for main products like camphene, limonene, tricylene and terpinolene can be obtained at 70 °C. The catalyst is stable and reusable, and the product yield is only reduced by 10% after four runs, which is probably caused by the slow dealumination in the framework wall during the reaction. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The 21st century is an era with a rapidly economic boost while with a sharp decline of petroleum and chemical resources. With a view to achieve sustainable development, alternative energies like biomass are especially attractive to researchers as well as to manufacturers. Biomass carbohydrates are the most abundant renewable resources available, and they are currently viewed as a feedstock for the green chemistry of the future (Lichtenthaler and Peters, 2004). Among them, a-pinene is a worldwide available and unexpensive terpene which is the major component of essential oil in pine trees. By isomerization of a-pinene, valuable products like camphene, limonene, tricylene, terpinene and terpinolene can be produced, which are important intermediates for use as fragrances, flavors, pharmaceuticals, solvents, and also chiral intermediates. The isomerization of a-pinene proceeds via two parallel pathways (Corma et al., 2007; Flores-Holguín et al., 2008), as shown in Scheme 1. The first way by ring expansion generates bi- and tricyclic products. The most profitable one via ring expansion is camphene, which is an intermediate compound for producing isoborneol, isobornyl acetate, and camphor. The second process leads to monocyclic products such as limonene, terpinolene, a- and c-terpinenes which are also valuable industrial chemicals. The industrial a-pinene isomerization process now is based on acidic TiO2 catalyst (Gscheidmeier et al., 1998), which proceeds in a close systems under 150–170 °C. However, the recovery rate on sulfuric acid covered on TiO2 is quite low while the high reaction * Corresponding author. Address: No. 220, Handan Road, Shanghai 200433, PR China. Tel.: +86 21 65642409; fax: +86 21 65641740. E-mail address: [email protected] (Y. Yue). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.04.075

temperature leads to huge energy consumption. Therefore, various heterogeneous catalysts are studied in order to find more stable catalysts or some milder reaction conditions, such as acid clays (Stefanis et al., 1995; Breen and Moronta, 2001; Volzone et al., 2001; Besün et al., 2002; Yadav et al., 2004; Volzone et al., 2005), silica supported heteropolyacid catalysts (Newman et al., 2005), sulfated zirconia (Flores-Moreno et al., 2001; Ecormier et al., 2005; Comelli et al., 2006), rare earth oxides (Yamamoto et al., 1999), modified zeolites (Severino et al., 1996; Lopez et al., 1998; Allahverdiev et al., 1999; Ozkan et al., 2003; Gündüz et al., 2005; Rachwalik et al., 2007; Mokrzycki et al., 2009) and some mesoporous materials (Yamamoto et al., 1998; Jarry et al., 2006; RománAguirre et al., 2007). Zeolites, the crystallized microporous aluminosilicates with good hydrothermal stability and strong solid acidity, are successfully commercialized in petro chemistry and bioresource catalysis. However, due to their relatively small pore size, the diffusion of bulky molecules through micropores is relatively slow, especially in liquid phase, which limits the application of zeolites involving bulky molecules, such as the isomerization of a-pinene. Therefore, modifications like dealumination (Rachwalik et al., 2007), desilication (Mokrzycki et al., 2009) were adapted via zeolites such as FER, MFI, MTW, MWW in order to enhance the conversion of a-pinene isomerization. On the other hand, the newly discovered mesoporous molecular sieves have attracted much interest because of their high surface area and uniform hexagonal arrays of cylindrical mesopores, which provide the potential for use as catalysts or catalyst supports for bulky molecules. However, owing to their amorphous nature, these materials are not as capable as zeolites with good framework stability and strong acidity. To enhance the hydrothermal stability

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Isomerization

Isomerization Camphene

α-Pinene

β-Pinene

Terpinolene Isomerization

Limonene Terpinene

Scheme 1. Routes for a-pinene isomerization.

and acidity of the mesoporous materials, a series of new mesoporous molecular sieves have been prepared recently by direct assembly of nanoclustered zeolitic precursors, which were named as MSU-S (Liu et al., 2000, 2001) or MAS (Liu et al., 2000, 2001; Zhang et al., 2001). These mesoporous molecular sieves have a certain degree of zeolite-like order in their framework wall and have shown enhanced activity and stability in the reactions like alkylation (Xu et al., 2006) and biomass pyrolysis (Triantafyllidis et al., 2007). In this work, MSU-S(BEA) and MSU-S(Y) with different Si/Al ratios were prepared and characterized by X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR), X-ray fluorescence (XRF), N2 adsorption, 27Al MAS NMR, NH3-temperature programmed desorption (NH3-TPD) and 2,6-di-tert-butylpyridine (DTBPy) adsorption. The catalytic performance of these catalysts towards a-pinene isomerization was investigated and compared with that of other catalysts reported before. The deactivation and reusability of the catalyst were also studied. 2. Methods 2.1. Catalysts preparation MSU-S(BEA) and MSU-S(Y) were prepared following the procedure (Liu et al., 2000, 2001) by direct assembly of nanoclustered zeolite BEA and zeolite FAU precursors, respectively. BEA seeds were prepared by the reaction of 20% TEAOH solution (6.7 mmol, Sinopharm Chemical Reagent), sodium aluminate (0.50 mmol, Sinopharm Chemical Reagent), and fumed silica (33.3 mmol, Sigma– Aldrich Chemicals) in water (1270 mmol) at 100 °C for 18 h. The similar stoichiometric ratio of sodium aluminate, water glass (25.86% SiO2, 7.34% Na2O, supplied by RIPP, Sinopec) and water was used to prepare a solution of FAU seeds at 100 °C for 18 h. After the initial aging period, CTAB (8.65 mmol, Shanghai Wenmin Biochemicals) was introduced as surfactant to assemble mesopores. The gel was introduced to autoclaves in 150 °C for 48 h. The as-made solid obtained was filtered, dried and finally calcined at 550 °C to remove the surfactant, then ion-exchanged with 0.1 mol/L NH4NO3 three times at 80 °C to remove exchangeable sodium ions, and calcined again at 450 °C. The final products are denoted as MSU-S(BEA)-X and MSU-S(Y)-X, where X represents the Si/ Al ratio of the reactant gel. Na-Beta zeolite with a Si/Al of 12.5 was supplied by Nankai University while Na-Y zeolite with a Si/Al of 2.6 was supplied by Wenzhou Catalyst Factory. Two zeolites were ion-exchanged to H-state with NH4NO3 solution and calcined at 450 °C before use. AlSBA-15 and AlMCM-41 were also synthesized according to the literature (Sun et al., 1997; Xu et al., 2005). 2.2. Characterization The small angle powder X-ray diffraction (XRD) patterns were recorded on a Bruker D4 ENDEAVOR diffractometer (Cu Ka radia-

tion, k = 1.542 Å, 40 kV, 40 mA) with a scan speed of 1 °/min. The d-spacing was calculated according to the Bragg Equation. The bulk Si/Al ratio was determined on a Philips PW2404 X-ray fluorescence (XRF) elemental analysis spectrometer. The IR-spectra of samples in the form of KBr pellets were recorded by using a Nicolet Avatar 360 FT-IR spectrometer. The N2 adsorption/desorption isotherms were measured on a Micromeritics ASAP2010 instrument at liquid N2 temperature. Specific surface areas of the samples were calculated from the adsorption isotherms by BET method, and pore size distributions from the desorption isotherms by BJH method. 27Al MAS NMR spectra were recorded using a Bruker MSL-300 spectrometer. A resonance frequency of 78.205 MHz, a recycle delay of 0.5 s, short 0.8 ls pulses, a spectral width of 15.625 kHz and a spin rate of 4 kHz were applied. NH3-TPD of the samples was carried out in a flow-type fixed-bed reactor at ambient pressure. The catalysts were pretreated at 500 °C for 2 h in He flow. The NH3 adsorption temperature was 120 °C, and the temperature was raised at a rate of 10 °C/min. The NH3 desorbed was collected in a liquid N2 trap and detected by gas chromatography. DTBPy adsorption in liquid phase was tested by adding 20 mg catalyst (40–60 mesh) into DTBPy/xylene solution. The mixture was stirred for 4 h under 80 °C. the remaining DTBPy were analyzed with a gas chromatograph equipped with a SE-30 capillary column (30 m  0.25 mm  0.3 lm) and a flame ionization detector while trace amount of tetradecane was utilized as internal standard. Hydrophobicity of MSU-S(BEA) samples was measured by calculating the amount of water adsorbed via thermogravimetric analysis (TG) on a Rigaku Thermoflex System. The catalysts (40–60 mesh) were hydrated in a desiccator over a saturated NaCl solution for 12 h prior to the measurements. 2.3. Activity measurement Isomerization of a-pinene was carried out in a three-necked flask equipped with a condenser. Typically, 0.2 g catalyst and 8 g a-pinene were added into the flask with magnetic stirring. The reaction arises in 70–100 °C for 4 h. The products were analyzed with a gas chromatograph (GC-122, Shanghai Analysis Instrument Co. Ltd.) equipped with a SE-30 capillary column (30 m  0.25 mm  0.3 lm) and a flame ionization detector. The capillary column was temperature programmed (initially retained at 80 °C for 11 min and onwards with an increase rate of 10 °C/min to 200 °C, then retained for another 7 min). The main products were camphene, limonene, tricyclene and terpinolene at retention time of 9.1, 13.2, 7.8, 15.4 min, respectively. The conversion and the yield are calculated in mol% by the equation: Conversion (%) = [1  (remaining a-pinene/total a-pinene)]  100%. Yield (%) = [Camphene (mol) + Limonene (mol) + Tricyclene (mol) + Terpinolene (mol)]/[total a-pinene (mol)]  100% The reaction data in the work were reproducible with a precision of less than 5%.

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3. Results and discussion 3.1. Catalysts preparations and characterizations MSU-S(BEA) with Si/Al ratios of 30, 50, 67, 100, 120, 150 as well as MSU-S(Y) with Si/Al ratios of 5, 10, 20, 50 were prepared, and their XRD patterns were measured. As two examples illustrated

(b)

(a) 0

2

4

6

8

10

2theta Fig. 1. XRD patterns for (a) MSU-S(BEA)-67 and (b) MSU-S(Y)-20.

(a)

600

3

Volume absorption, cm /g

700

(b)

500 400 3

Pore volume, cm /g

0.12

300 200

(b)

0.10 0.08

(a)

0.06 0.04 0.02 0.00

100

0

2

4

6

8

10

Pore diameter, nm

0.0

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 2. N2 adsorption (.)/desorption (N) isotherms and pore size distributions (inset) of (a) MSU-S(BEA)-67 and (b) MSU-S(Y)-20.

in Fig. 1, all the samples exhibit one intense diffraction peak which can be indexed to the (1 0 0) diffraction line characteristic of the hexagonal mesostructure. The calculated d-spacing was about 4.2 nm and 3.8 nm for MSU-S(BEA)-67 and MSU-S(Y)-20, respectively. The diffraction peak intensities for all the MSU-S catalysts are alike, suggesting that these samples have similar orderliness of the hexagonal mesostructure. However, the absence of crystallized zeolitic characteristic peaks in the region of 10–50° indicated that the FAU and BEA structure in MSU-S(Y) and MSU-S(BEA) catalysts were not in long-range order. Fig. 2 provides the N2 adsorption isotherms and the pore size distributions of two representative samples of MSU-S(BEA) and MSU-S(Y). All isotherms of MSU-S catalysts are similar, which were stepped like Type IV isotherms with two regions in their reversible parts, very similar to that of MCM-41 reported in the literature. The abrupt step in the P/P0 = 0.35–0.50 region implies the existence of mesopores. The narrow Gaussian pore size distributions shown in Fig. 2(inset) suggested that the samples had very regular mesoporous channels. The most probable pore diameter of MSU-S(BEA) were about 0.5 nm larger than MSU-S(Y), which parallel with the d-spacing difference obtained by XRD analysis. The composition and textural properties of MSU-S catalysts were given in Table 1. The bulky Si/Al ratios determined by XRF were slightly higher than the Si/Al ratios of initial gel, indicating that silicon was easier to enter into the mesoporous framework than aluminum during the synthesis. The BET surface areas and mesopore volumes of the samples were in the range of 623– 1183 m2/g and 0.87–1.36 cm3/g, respectively, which were close to the results reported in the literature. Fig. 3 shows the 27Al MAS NMR results for some of these Al-containing mesoporous catalysts. No extra-framework 6-coordinate aluminum at 0 ppm was observed, indicating that all aluminum was incorporated into the framework of these mesoporous catalysts. The chemical shift of framework 4-coodinate aluminum is 57 ppm in MSU-S(Y), differed from that in MSU-S(BEA) at 53 ppm, which implied that the subunits in MSU-S(Y) and MSU-S(BEA) are not the same. Meanwhile, the chemical shifts for MSU-S with FAU and BEA seeds were close to that for their corresponding zeolites, indicating these catalysts may contain small domains of zeolites with the BEA and FAU structure. The chemical shifts of tetrahedral AlO4 are related to the mean Si–O–Al bond angle (Liu and Pinnavaia, 2004). They increase with the decline of the bond angle. Generally, the zeolitic subunits containing five or six- membered ring units may lead to smaller Si–O–Al bond angles. That’s the reason why the chemical shifts for both MSU-S catalysts were larger than that of the amorphous mesoporous material, AlMCM41 (52 ppm). Additionally, the full-width half-height (fwhh) of MSU-S is slightly narrower than that of AlMCM-41, suggesting

Table 1 Composition and properties of the catalysts. Catalyst MSU-S(Y)-5 MSU-S(Y)-10 MSU-S(Y)-20 MSU-S(Y)-50 MSU-S(BEA)-30 MSU-S(BEA)-50 MSU-S(BEA)-67 MSU-S(BEA)-100 MSU-S(BEA)-120 MSU-S(BEA)-150 SiMCM-41 H-beta a b c

Si/Al a

6 11a 23a 56a 35a 59a 75a 108a 130a 168a 1 12.5b

Determined by XRF. Calculated from 29Si MAS NMR. Calculated from TG.

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

Water adsorbedc (wt.%)

623 698 908 880 875 962 999 932 1058 1183 988 543

0.87 1.06 1.09 1.36 0.90 0.96 1.13 1.13 1.21 1.33 1.40 0.23

2.6 3.0 2.8 2.8 3.1 3.1 3.4 3.7 3.5 3.4 2.7 –

– – – – 41 39 36 35 32 30 – –

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(a)

(b)

MSU-S(BEA)-150. Again, MSU-S(BEA)-67 has the highest activity. The above sequence shows that the activity does not only dominate by textural properties but also acid properties and hydrophobicity which we will discuss later in Section 3.3. The effect of reaction time and temperature on the isomerization of a-pinene over MSU-S(BEA)-67 was also studied and the results were shown in Table 3. An increased conversion and a reduced selectivity were observed with the increasing of reaction temperature, while both conversion and selectivity increased with the raising of reaction time. An optimum yield of 91.0% was obtained at the reaction temperature of 70 °C for 4 h. 3.3. Acidity and hydrophobicity

(c) 200

100

0

-100

Chemical Shift (ppm) Fig. 3.

27

Al MAS NMR for (a) MSU-S(BEA)-67; (b) MSU-S(Y)-10 and (c) AlMCM-41(20).

the orders of Al species in MSU-S samples were better than that of AlMCM-41 (Zhang et al., 2001). The existence of zeolitic domains in MSU-S catalysts was also proved by their FT-IR spectra (not shown here). A broad band at 550–600 cm1, which is characteristic of five- or six-membered ring subunits of zeolites (Liu et al., 2001; Zhang et al., 2001), was observed in both MSU-S catalysts, while for the amorphous mesoporous material AlMCM-41, the similar band did not exist. 3.2. Isomerization activity The activities of the prepared MSU-S samples for the isomerization of a-pinene were tested at 100 °C, and the results were listed in Table 2, together with that of conventional zeolites and mesoporous catalysts. Camphene, limonene, tricylene, terpinolene as well as other products like two kinds of terpinenes, ß-pinene and some unidentified polymerized products are formed during reaction. It can be seen that MSU-S(BEA) catalysts have the optimal activity. The conversion of a-pinene is the highest (98.4%) over MSU-S(BEA)-67 even after short period of reaction time (0.5 h). The isomerization activity of a-pinene under 70 °C over MSUS(BEA) catalysts with different Si/Al ratio was also shown in Table 2. The catalytic behavior of the catalysts changes with their Si/Al ratios. The activity of the catalysts is in the order of MSU-S(BEA)-67 > MSU-S(BEA)-100 > MSU-S(BEA)-50 > MSU-S(BEA)-30 > MSU-S(BEA)-120 >

Isomerization of a-pinene is a typical weak acid catalyzed reaction, which can take place on both weak acid sites and strong ones (Corma et al., 2007; Flores-Holguín et al., 2008). Hence, The total acidity of the MSU-S(BEA) samples was measured by NH3-TPD method and the results were listed in Table 4. Unsurprisingly, with the increase of Si/Al ratio, the total amount of acid sites as well as the amount of strong acid sites decreased, since aluminum was the main acid center for these catalysts. There were evidently two peaks on the TPD profiles of all the samples, corresponding to weak and strong acid sites. However, no obvious change in the peak temperature was observed with the increase of Si/Al ratio, showing that the distribution of acid strength on these samples was very close. Nevertheless, the order of the isomerization activity is not in parallel with the total amount of the acid sites obtained from NH3-TPD. This indicates that not all the acid sites, but those which can be accessible by the bulky reactant molecules at the reaction condition, were involved in the reaction. The kinetic size of apinene is about 144.8 Å3 (Gündüz et al., 2005) and the crucial diameter for a-pinene is about 6.7 Å (Yamamoto et al., 1998), which is close to the main pore diameters of zeolite BEA (7.6  6.4 Å2) and FAU (7.4  7.4 Å2). This may be the reason why H-beta with abundant acid sites has very low activity at 70 °C, since the pore channels are not large enough for a-pinene molecules to penetrate easily under that condition (70 °C, liquid phase). Only the acid sites externally or in the region of pore mouth are easily accessible to a-pinene molecules. To characterize the accessible acid sites of the catalyst for the bulky molecules, 2,6-di-tert-butyl-pyridine (DTBPy), which has a kinetic diameter of about 7.9 Å, has been employed as the base probe, since the previous study showed that the large DTBPy molecules only adsorbed on the Bronsted acid sites located on the

Table 2 The isomerization activity of a-pinene over various catalysts for 4 h. Sample

MSU-S(BEA)-67 MSU-S(Y)-20 H-beta(12.5) H-Y(2.6) AlMCM-41(20) AlSBA-15 (50) MSU-S(BEA)-30 MSU-S(BEA)-50 MSU-S(BEA)-67 MSU-S(BEA)-100 MSU-S(BEA)-120 MSU-S(BEA)-150 MSU-S(Y)-20 H-beta(12.5) a

Reaction time: 0.5 h.

Reaction temp.(°C)

100 100 100 100 100 100 70 70 70 70 70 70 70 70

Conv. (%)

98.4a 90.8 96.4 13.5 18.4 75.6 66.3 84.9 97.0 95.2 60.2 21.3 52.7 22.1

Selectivity (%)

Yield (%)

Camphene

Limonene

Tricyclene

Terpinolene

Others

38.9 43.1 45.1 60.3 40.1 43.3 48.3 48.4 48.0 45.4 44.7 42.9 49.7 50.0

26.4 33.6 26.2 12.3 29.5 33.7 36.3 33.4 31.4 33.8 33.5 39.8 34.8 28.9

7.7 1.7 2.7 0.0 1.1 1.6 1.2 1.4 2.7 1.5 1.2 0.4 1.1 0.0

6.7 9.7 10.5 3.7 8.4 9.0 8.1 9.1 11.8 10.1 11.4 7.5 7.1 7.0

20.3 11.8 15.4 23.7 20.9 9.4 6.1 7.7 6.1 9.2 9.2 9.4 7.3 15.1

78.4 80.1 81.5 10.3 14.6 66.2 62.2 78.4 91.0 86.4 56.8 19.3 48.8 18.8

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Table 3 The isomerization activity of a-pinene over MSU-S(BEA)-67 catalysts. Reaction time (h)

Reaction temp. (°C)

Conv. (%)

0.5 0.5 0.5 0.5 1 4

100 90 80 70 70 70

98.4 97.9 96.5 50.3 70.7 97.0

Selectivity (%)

Yield (%)

Camphene

Limonene

Tricyclene

Terpinolene

Others

38.9 37.2 42.2 44.0 46.2 48.0

26.4 22.5 26.5 37.3 31.9 31.4

7.7 6.7 6.6 1.1 1.7 2.7

6.7 9.8 6.5 8.7 9.9 11.8

20.3 23.8 18.2 8.9 10.3 6.1

78.4 74.7 78.9 45.8 63.4 91.0

Table 4 NH3-TPD and DTBPy adsorption data of the catalysts. Amount of adsorbed DTBPy (mmol/g)

I (120–350 °C)

II (350–600 °C)

Total

Total

0.17 0.13 0.11 0.10 0.08 0.07 0.48

0.14 0.11 0.10 0.08 0.06 0.06 0.82

0.31 0.24 0.21 0.18 0.14 0.13 1.30

0.28 0.32 0.37 0.36 0.27 0.20 0.08

external surface of zeolitic catalysts with 10-MR and unidirectional 12-MR pore channel systems (Corma et al., 1998). That is to say, the adsorption amount of DTBPy can simulate the accessibility of a-pinene on the catalysts’ surfaces. Table 4 shows the amount of adsorbed DTBPy over various MSU-S(BEA) and H-beta catalysts in the xylene solution, which was quite different from the amount of desorbed NH3 standing for total acid sites of the catalysts. In this case, MSU-S(BEA) catalysts had larger adsorption amounts than Hbeta. The result reveals that not all the acid sites of catalysts can be accessed by the bulky reactant molecules and the mesoporous MSU-S(BEA) catalysts had generally more accessible acid sites than H-beta. Additionally, the accessible acid sites on mesoporous MSUS(BEA) catalysts exhibits a volcano shaped curve versus the Si/Al ratio (illustrated in Fig. 4). This volcano shaped curve of the accessible acid sites correlates precisely with the change of the isomerization activities versus Si/Al ratio. The result can be explained from two perspectives. On one hand, the theoretical acid sites accessible to large molecule may decline with the decrease of aluminum content. On the other hand, the hydrophobicity of microporous and mesoporous aluminosilicates changes with the changing of Si/Al ratio (Corma, 2003). With the decrease of aluminum content, the MSU-S(BEA) catalysts became more hydrophobic, which facilitates the adsorption of organic DTBPy molecules on catalyst surface. The hydrophobicity of MSU-S(BEA) catalysts can be estimated by measuring the amount of water adsorbed via TG analysis. As listed in Table 1, with the decrease of Al content, the percentage of water adsorbed goes down, i.e., the hydrophobicity of the catalysts increases. From the above, MSU-S(BEA)-67 catalyst, because of the large pore channel and the hydrophobic surface, is the most accessible one for the bulky organic reactant molecule, thus has the highest isomerization activities. 3.4. Stability and regeneration of the catalyst The reusability of the MSU-S(BEA)-67 catalyst was also studied. After reaction, the catalysts were filtered, washed with ethanol several times, calcined in air at 450 °C to remove the adsorbed organic species, and then reused in the reaction. The regenerated catalysts for once, twice and three times are denoted as

0.4

100

0.2

50

0.0

Amount of absorbed DTBPy, mmol/g

MSU-S(BEA)-30 MSU-S(BEA)-50 MSU-S(BEA)-67 MSU-S(BEA)-100 MSU-S(BEA)-120 MSU-S(BEA)-150 H-beta(12.5)

Amount of adsorbed NH3 (mmol/g)

Yield, %

Catalysts

0 (a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 4. Relevance of the isomerization activity with the amount of adsorbed DTBPy: (a) MSU-S(BEA)-30; (b) MSU-S(BEA)-50; (c) MSU-S(BEA)-67; (d) MSU-S(BEA)-100; (e) MSU-S(BEA)-120; (f) MSU-S(BEA)-150 and (g) Hbeta.

MSU-S(BEA)-67-1, MSU-S(BEA)-67-2 and MSU-S(BEA)-67-3, respectively. The reaction data were listed in Table 5. The activity is quite stable after the slow drop in the first circle. The conversion of a-pinene remains at about 90% and the yield for aimed products is over 80% after three regenerations. The textural and acid properties of the regenerated catalysts were characterized by XRD, N2 adsorption, 27Al MAS NMR and DTBPy adsorption. The XRD patterns were similar before and after regeneration, indicating that the catalyst remained the hexagonal mesostructure after four runs. The 27Al MAS NMR result shows that about 20% of framework aluminum turned into non-framework aluminum (0 ppm) after the first regeneration, showing that dealumination occurs during the catalytic reaction. However, this dealumination process became much slower for the next two circles, and the octahedral aluminum had a percentage of 28 after the third regeneration eventually. The result is consistent with the N2 adsorption/desorption results illustrated in Table 5. The textural properties of the catalyst change more abruptly after the first run and then very slowly afterwards. The acid properties of the catalyst were also studied via DTBPy liquid phase adsorption. As seen from Table 6, the amount

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a b

Sample

Conv.a (%)

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

AlEx/Alb

Amount of adsorbed DTBPy (mmol/g)

MSU-S(BEA)-67 MSU-S(BEA)-67-1 MSU-S(BEA)-67-2 MSU-S(BEA)-67-3

97.0 92.0 90.2 90.1

999 915 893 853

1.13 1.04 1.01 1.04

3.4 3.4 3.4 3.3

0 0.20 0.27 0.28

0.37 0.35 0.34 0.34

Reaction temperature: 70 °C; reaction time: 4 h. Determined by 27Al MAS NMR.

Table 6 The summary of a-pinene isomerization activity. Catalyst

Temp. (°C)

Reaction time (h)

Cat./a-pinene (%)

Conv. (%)

Selectivity (%) Camphene

Limonene

MSU-S(BEA) MSU-S(Y) TiO2/H2SO4 FAU Beta/mesoBeta FER Desilicated ZSM-5, ZSM-12, MCM-22 Ga-SBA-15

70 70 150–160 150 100 90 90 80 150

4 4 1–2 2 3 3 3 1 2

2.5 2.5 0.7–1.5 2.8 2 5 – 6.8 –

97 53 >95 99 100 >95 70 57 100

48 50 80 50 25–27 45–50 – 37 39

31 35 – 20 – 40–44 – 32 –

Gscheidmeier et al. (1998) Severino et al. (1996) Gündüz et al. (2005) Rachwalik et al. (2007) Mokrzycki et al. (2009) Jarry et al. (2006) Román-Aguirre et al. (2007)

80 155 80 120 150 50

0.5 3 2 4 2 3

2.5 1 1.2 2 5 2.5

30 99 90 90 100 26

41 45 40 – 45–47 67

41 20 20–25 – 3 23

Yamamoto et al. (1998) Besün et al. (2002) Breen and Moronta (2001) Allahverdiev et al. (1999) Yadav et al. (2004) Yamamoto et al. (1999)

SO2 3 /MCM-41 FSM-16 Acid-treated clays Ion-exchanged clays Clinoptilolite Montmorillonite Ln(Yb)/SiO2

of accessible acid sites on the catalyst surface decrease after the first run and then slowly afterwards, which was in accordance with 27 Al MAS NMR results. Table 6 presents a summary of the catalytic activity of the various catalysts that have been investigated in the previous literatures. Highest yield was obtained over MSU-S(BEA)-67 in the present work under a mild reaction condition. 4. Conclusion Liquid phase a-pinene isomerization was carried out over various aluminosilicate catalysts. Among them, MSU-S(BEA)-67 has a benign activity compared to other catalysts reported before due to high accessible acid sites caused by its large pore channel and hydrophobic surface. The catalyst was rather stable, and the yield drops about 10% after three regeneration process, which was caused by slow dealumination of the zeolite-like subunits in the framework wall. The results given in this paper should be a convincing evidence of utilizing such kind of mesoporous materials with zeolite subunits into bulky molecular, especially biomass reactions. Acknowledgements 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 and Technology Commission of Shanghai Municipality (08DZ2270500). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2010.04.075.

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