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One-pot synthesis of mesostructured AlSBA-15-SO3H effective catalysts for the esterification of salicylic acid with dimethyl carbonate

Yan Zheng a,b, Junping Li a, Ning Zhao a, Wei Wei a, Yuhan Sun a,* a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

Received 4 November 2005; received in revised form 17 January 2006; accepted 17 January 2006 Available online 23 February 2006


Mesostructured AlSBA-15-SO3H materials with various Si/Al ratios were prepared by a one-pot method and characterized by XRD,

N2 adsorption, NH3-TPD and Si, Al MAS-NMR techniques. With the incorporation of Al into SBA-15-SO3H, the total acid sites substantially increased. Such materials showed high activity in the esterification of dimethyl carbonate (DMC) with salicylic acid

(SA), and the selectivity of methyl salicylate was correlated with the Bronsted acidity of the catalysts. 2006 Elsevier Inc. All rights reserved.

Keywords: Mesoporous molecular sieve; Esterification; Solid acid catalysts; Salicylic acid; Dimethyl carbonate

1. Introduction

The increasing demand for safe industrial processes requires the development and implementation of environmental friendly solid catalysts for value added acid catalyzed reactions. A variety of materials such as clays, zeolites, sulfated metal oxides and heteropolyacids were therefore developed as solid acid catalysts [1,2]. A problem for those catalysts is the hybrid acid sites and deconcentrated acidity, leading to low selectivity toward the targeted product. Thus, mesoporous materials functionalized by propyl-sulfonic groups [3,4], arenesulfonic groups [5–7], carboxylate groups [8] and peroxycarboxylic acid groups [9] were developed for a high accessibility of the ‘‘single site’’ and concentrated acid strength, which were promising catalysts for a variety of reaction such as the esterification of glycerol with fatty acids [10,1], etherification [12] and condensation reactions [13,14].

The esterification of salicylic acid with alcohols over acid catalysts is an important reaction in organic synthesis [15]. Using alcohol as alkylating agent, decarboxylation took place as a side reaction and water is a by-product as well, which lowered the selectivity and deactivated the catalyst. Thus, dimethyl carbonate (DMC), as an environmentally benign building block, was used as a safe substitute without the production of water [16–18]. In the present work, SBA-15 functionalized by propyl-sulfonic groups with ‘‘single site’’ was expected to be selective toward the targeted product for the esterification of salicylic acid with dimethyl carbonate. Moreover, the incorporation of Al into the material led to additional different Bronsted acidity and higher total acidity, and the activity of these new materials was compared to that of SBA-15-SO3H ‘‘single site’’.

2. Experimental 2.1. Synthesis of catalytic materials

The mesoporous materials were prepared from tetraethyl orthosilicate (TEOS) as the silica source under acidic

1387-1811/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.01.011

E-mail addresses: zhengyan@sxicc.ac.cn (Y. Zheng), yhsun@sxicc. ac.cn (Y. Sun).

w.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 92 (2006) 195–200 conditions using a triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123, molecular weight 5800, EO20-

PO70EO20, Aldrich) as a structure directing agent. The mesoporous silicas were modified using 3-mercaptopropyl- trimethoxysilane (MPTMS) as organosilica source and aluminum isopropoxide, Al(OiPr)3 as the Al source. In a typical synthesis, 4 g of P123 was added to 97.58 g of water.

After stirring for 2 h, a clear solution was obtained, then 1.97 g of 37 wt.% aqueous HCl was added to the solution with vigorous mechanical stirring. After complete dissolution, the solution was heated 40 C. 7.69 g of TEOS and the required amount of Al(OiPr)3 were added for prehydrolysis for 2 h, and then 0.81 g MPTMS and 8.37 g

H2O2 was added dropwise. The resulting mixture was stirred for 24 h at 40 C followed by aging at 100 C for another 24 h under static conditions. The molar composition of the mixture for 4 g P123 was 1TEOS:0.1MPTMS:-

(Si/Al = 7) and 0.00410 (Si/Al = 9). The template was removed from the as-synthesized material by washing with ethanol under reflux for 24 h. The samples were prepared by changing the molar Si/Al ratios and defined as

AlSBA-15-SO3H-10(Y), where Y denoted the molar Si/Al ratio of the samples. A series of SBA-15 containing differ- ent amounts of sulfonic acid were prepared according to a synthesis procedure of one-pot method described by Margolese et al. [3].

2.2. Characterization

X-ray diffraction powder data were acquired at room temperature on a Rigaku D Max I VC instrument with Ni filtered Cu Ka radiation (k = 1.5404 A ) at 40 kV and 20 mA, in the 2h range of 1–6 at a scan rate of 1 /min. The specific surface area, total pore volume and average pore diameter were measured by N2 adsorption–desorption method using a Micromeritics ASAP-2000 instrument

(Norcross, GA). The samples outgassed at 80 C and 10 4 Pa overnight and then the adsorption–desorption was conducted by passing nitrogen into the sample, which was kept under liquid nitrogen. Surface areas were calculated by the BET method, and the pore size distribution (PSD) was obtained by applying the BJH pore analysis to the desorption of the nitrogen adsorption–desorption isotherm. Elemental analysis for Si/Al ratios was performed with inductively coupled plasma (ICP) emission spectroscopy on an Atom Scan 16 instrument. The total acidities of samples were determined by temperature programmed desorption (TPD) of ammonia. A sample (ca. 0.1 g 40–60 mesh) was introduced into the stainless steel sample tube and pretreated in an argon flow for 2 h at 150 C. The sample was then cooled down to 50 C and several ammonia pulses were flushed through the sample tube. The saturation of the sample with ammonia was evidenced by the appearance of a constant peak area on the chart. After saturation, weakly adsorbed NH3 was eliminated by treatment with dry Ar at the same temperature, and the temperature subsequently rose to 600 C with a linear heating rate of 10 C/min under dry Ar. The amount of

NH3 evolved from the sample was determined using a BALZA Q-Mass spectrometer. The flow rate of Ar was kept at 50 ml/min. The Bronsted acidity in the samples was determined by the ion-exchange capacity, using aqueous solutions of sodium chloride (NaCl, 2 M) as exchange agents. In a typical experiment, approximately 0.05 g of samples was added to 25 ml of the salt solution and allowed to equilibrate. Thereafter, the resulting suspension was titrated by dropwise addition of 0.01 M NaOH (aq). From the volume of titrant solution, the acid exchange capacity was determined in units of mequiv of H+/g of the sample. 13C CP MAS NMR spectra were recorded on an Infinityplus-300 instrument at 75.4 MHz with a 5 s pulse delay using 7.5 m zirconia rotors at a spinning speed of 3.3 kHz. The 29Si and 27Al MAS NMR measurements were performed on a Bruker MSL 300 spectrometer. 29Si MAS NMR spectra were measured at 79.5 MHz with a 20 s pulse delays, using 7 m zirconia rotors spinning at 4.0 kHz. 27Al MAS NMR spectra obtained at 104.26 MHz with a 0.5 s pulse delays using 7 m zirconia rotors spinning at 10 kHz. The chemical shifts were reported in ppm relative to external tetramethylsilane (TMS) for 13C and 29Si, and

2.3. Catalytic test

The esterification of dimethyl carbonate (DMC) with salicylic acid (SA) to methyl salicylate (MS) was carried out over AlSBA-15-SO3H and SBA-15-SO3H in a stirred stainless steel autoclave (100 ml). A typical reaction mix- ture in the reactor contained 6.90 g of SA and 25.5 g of DMC. To this mixture 0.65 g of a freshly activated catalyst was added. The autoclave temperature was then slowly raised to 200 C and maintained at this temperature during 8 h. Samples were analyzed by gas chromatography using a capillary 30 m HP-5 column and an FID detector.

3. Results and discussion

3.1. Structure of AlSBA-15-SO3H

The elemental composition of AlSBA-15-SO3H samples synthesized with different Si/Al ratios in the gel are listed in

Table 1. In all cases, the Si/Al ratio of the samples was lower than the Si/Al ratio in the as-synthesized gel. This was closely related to the high solubility of the aluminum source in the acidic medium [19]. However, AlSBA-15-

SO3H samples with Si/Al ratios of 5–61 have been obtained. The powder XRD patterns of AlSBA-15-SO3H samples showed that the intensity of the (100), (110) and

(200) diffraction peaks gradually decreased with increasing Al content, indicating a decrease of long-distance order (see Fig. 1). Moreover, AlSBA-15-SO3H-10(5), where the

196 Y. Zheng et al. / Microporous and Mesoporous Materials 92 (2006) 195–200

number in the parentheses denotes the molar Si/Al ratio of the samples, did not show the hexagonal mesoporous structure, indicating a collapse of the pore structure.

Figs. 2 and 3 show the N2 adsorption/desorption isotherms and pore size distributions of the samples, and their textural properties are listed in Table 1. The N2 adsorption/ desorption isotherms of AlSBA-15-SO3H samples indicated type IV isotherms with a H1-type broad hysteresis

loop, which are typical of mesoporous solids [20]. All isotherms had a sharp condensation step at relative pressures in the range of 0.6–0.85 except for the sample AlSBA-15-

SO3H-10(5). AlSBA-15-SO3H-10(5) exhibited a slight condensation step, being indicative of a change of hexagonal structure, which is in agreement with the results of XRD. Furthermore, the capillary condensation step shifted to higher relative pressures with increasing aluminum content in the samples and the BJH pore size distribution broadened. This further suggested that the mesoporous structure decreased with the decrease in Si/Al ratio. 13C CP MAS NMR spectra (Fig. 4) showed no thiol resonance at 27.9 ppm and the resonances at 5.6, 17.2, and 1.3 ppm were attributed to propyl-sulfonic groups respec- tively for the C1 (adjacent to SO3H), C2, and C3 carbon [3,21,2]. The peaks at 16 ppm and 52 ppm are ascribed to unhydrolysed ethoxy species and methoxy species in the TEOS and 3-MPTMS moieties, respectively. Thus, it can be concluded that the thiol groups are not present in

Fig. 2. N2 adsorption/desorption isotherms of AlSBA-15-SO3H materials with different Si/Al ratios: (a) AlSBA-15-SO3H-10(5), (b) AlSBA-15- SO3H-10(9), (c) AlSBA-15-SO3H-10(20), (d) AlSBA-15-SO3H-10(61).

Fig. 3. Pore size distributions of AlSBA-15-SO3H materials with different Si/Al ratios: (a) AlSBA-15-SO3H-10(5), (b) AlSBA-15-SO3H-10(9), (c) AlSBA-15-SO3H-10(20), (d) AlSBA-15-SO3H-10(61).

Fig. 1. XRD powder patterns of AlSBA-15-SO3H materials with different Si/Al ratios: (a) AlSBA-15-SO3H-10(5), (b) AlSBA-15-SO3H-10(9), (c) AlSBA-15-SO3H-10(20), (d) AlSBA-15-SO3H-10(61).

Table 1 Textural parameters of AlSBA-15-SO3H samples with different Si/Al ratio

Sample Gel Si/Al Solid Si/Al ABET (m2/g) Pore volume (cm3/g) Pore diameter (nm) d100 (nm) Wall thicknessa (nm)

Y. Zheng et al. / Microporous and Mesoporous Materials 92 (2006) 195–200 197

the materials and were completely converted into propylsulfonic groups. The incorporation of organosiloxane into the materials could also be monitored by means of 29Si MAS NMR. Distinct resonances are observed for the

108 ppm is due to the presence of Si(OAl) (see Fig. 5). In aluminosilicates, there are mainly two kinds of Al species: tetrahedral Al and octahedral Al. Since only the tetrahedral Al species contribute to the ion-exchange ability and the Bronsted acidity of the materials, it is usually desired to obtain materials with more 4-coordinated Al species. 27Al MAS NMR spectroscopy is a widely used technique for discriminating between 4-coordinated and 6-coordinated Al. Fig. 6 show the 27Al MAS NMR spectra of AlSBA-15-SO3H samples prepared from different Si/Al ratio. Notably, the samples indicate the presence of both tetrahedral Al (53 ppm) and octahedral Al (0 ppm) except for AlSBA-15-SO3H-10(61), in which tetrahedral Al is the primary species. This result suggests that Al species can be introduced into the siliceous framework of SBA-15-

SO3H by a direct synthesis method at Si/Al ratio of 61. However, with increasing Al content, resonances at

53 ppm as well as at 0 ppm appeared, implying that the ratio of tetrahedral to octahedral Al in the samples was significantly affected by Si/Al ratio and the Al species in the as-synthesized gel was not completely incorporated into the silica framework.

Bronsted acid sites in the materials were measured by means of acid–base titration using sodium chloride as ion-exchange agent and the results are summarized in

Table 2. For the SBA-15-SO3H materials, the amount of surface Bronsted acid sites increased with the incorpora- tion of sulfonic acid groups, but upon incorporation of Al, the Bronsted acidity of the materials decreased with increasing amount of Al. The total acidity of the materials was measured by TPD of ammonia. In order to avoid the influence of H2O, 16m/z was used to monitor NH3 instead of 17m/z. The desorption temperature and areas of

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