行政院國家科學委員會專題研究計畫 成果報告
固態觸媒在特用化學品反應之應用(3/3)
計畫類別： 個別型計畫 計畫編號： NSC91-2113-M-002-046- 執行期間： 91 年 08 月 01 日至 92 年 10 月 31 日 執行單位： 國立臺灣大學化學系暨研究所 計畫主持人： 鄭淑芬 計畫參與人員： 汪學廣、陳志昌、劉銘璋 報告類型： 完整報告 報告附件： 出席國際會議研究心得報告及發表論文 處理方式： 本計畫可公開查詢中 華 民 國 93 年 2 月 11 日
行政院國家科學委員會補助專題研究執行進度報告
※※※※※※※※※※※※※※※※※※※※※※※※
※ 固態觸媒在特用化學品反應之應用(3/3) ※
※ Applications of solid catalysts in reactions of fine ※
※ chemicals (3/3) ※
※※※※※※※※※※※※※※※※※※※※※※※※
計畫類別：
√
個別型計畫 □整合型計畫
計畫編號：NSC91－2113－M－002－046－
執行期間：91 年 08 月 01 日至 92 年 10 月 31 日
計畫主持人：鄭淑芬
共同主持人：
執行單位：國立台灣大學化學系
中 華 民 國 93 年 2 月 01 日
行政院國家科學委員會專題研究計畫成果報告
固態觸媒在特用化學品反應之應用(3/3)
Applications of solid catalysts in reactions of fine
chemicals (3/3)
計畫編號：NSC91－2113－M－002－046－
執行期限：91 年 8 月 1 日至 92 年 10 月 31 日
主持人：鄭淑芬 執行機構及單位稱： 台灣大學化學系
Email: chem1031@ntu.edu.tw
計畫參與人員： 汪學廣、陳志昌、劉銘璋
一、中文摘要 利 用 共 沉 澱 法 製 備 含 磺 酸 基 的 SBA-15 中孔洞分子篩，比較洞壁表面的矽 氧連結到磺酸基的兩種碳氫練對磺酸基酸 性強度的影響，其一是丙烷鏈，另一是苯。 發現後者因可以非定域化磺酸基的負電 荷，可以得到較強酸度。使用所得的含苯 磺酸基中孔洞材料作觸媒，較以丙烷鏈磺 酸基的 SBA-15 可有效催化環己酮杇的 Beckmann 重排反應，生成醯胺。 關鍵詞：磺酸、中孔洞SBA-15 材料、酸催化、 Beckmann 重排反應 二、英文摘要SBA-15 mesoporous silica functionalised with propylene- and arene-sulfonic acid groups were prepared by co-condensation method, and the arene-sulfonic acid functionalised mesoporous silica was found to be more efficient than the one with propylene-sulfonic acid as the catalyst for the liquid phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam.
Keywords: sulfonic acid mesoporous SBA-15, acid catalyst, Beckmann rearrangement reaction
INTRODUCTION
The importance of the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, which is a valuable starting
material for the manufacture of nylon fibers, has been widely documented in the literature.1, 2 Current commercial processes for this reaction, however, require the use of oleum or sulfuric acid as a reaction medium.3 That caused problems in product separation, corrosion hazard of the reactor and the substantial amount of ammonium sulfate formed as by-product. The environmental regulations and process safety continue to drive the industry to develop solid acids to replace liquid acid processes. In the past decade, various solid catalysts have been applied to the Beckmann rearrangement of cyclohexanone oxime,4 especially the zeolite catalysts for vapor-phase reactions. However, the vapor-phase Beckmann rearrangement reaction over solid acid catalysts is usually carried out at temperatures higher than 250oC, and still rapid catalyst deactivation is encountered due to the coke formation and irreversible adsorption of the basic reaction products. In contrast, the solid/liquid phase catalytic system appears to be more promising, where the reaction can proceed at a moderate temperature and the catalyst deactivation is minimized due to the presence of solvent.
Acidic zeolites have been widely studied in the synthesis of fine chemicals due to their high surface area and confined domains.5 However, their uses are often restricted by the small pore diameters of less than 1nm. For reactions performed in the liquid phase, the reaction substrates and products need to diffuse rapidly toward and
away from the catalytic active sites. The discovery of mesoporous molecular sieves of M41S family attracted much attention of many research groups because these materials are likely to offer improved reaction activity in the conversion of larger substrate molecules in their well-defined channels with narrow pore size distribution.6 In 1998, another family of mesoporous silica, named SBA-15, of hexagonal-ordered pore structure was synthesized using amphiphilic triblock copolymers as pore-directing agents.7 The SBA-15 mesoporous silica has received great attention since then because of its larger pore diameter and better hydrothermal stability in comparison to MCM-41, its analog in the M41S family. However, in spite of having larger pore dimensions, acidity of mesoporous materials like Al-MCM-41 and Al-SBA-15, is much weaker than that of microporous zeolites.8 In order to overcome this drawback, ordered sulfonic-acid modified mesoporous silica has been prepared ether via post-grafting technique or through one-pot synthesis.9-14 The purpose of this study is to reveal the catalytic performance of the propylene- and arene-sulfonic acid- functionalised SBA-15 in liquid phase Beckmann rearrangement of cyclohexanone oxime to caprolactam.
EXPERIMENTAL
Catalyst Preparation
Propylene- and arene-sulfonic acid- functionalised SBA-15 materials were prepared by one-pot synthesis according to the procedures described in the literature.10 For the arene-sulfonic acid-functionalised SBA-15, 4 g of Pluronic 123 (Aldrich) was dissolved in 125 g of 1.9 M HCl solution at room temperature. After adding tetraethyl orthosilicate (TEOS, Aldrich), the resultant solution was equilibrated at 40oC for 45 min. before CSPTMS (2-(4-chlorosulfonylphenyl) ethyltrimethoxysilane, Gelest) and H2O2
were introduced into the solution. The molar composition of the gel versus 4 g of P123 was 0.0369 TEOS : 0.0041 CSPTMS : 0.0123 H2O2 : 0.24 HCl : 6.67 H2O, where
CSPTMS : TEOS = 1 : 9. The mixture was stirred at 40 oC for 20 h and then transferred
into a polypropylene bottle and reacted at 100oC for 24 h under static conditions. The solid product was recovered by filtration and dried at room temperature overnight. The template was removed from the as-synthesised material by refluxing in ethanol for 24 h. Finally, the material was filtered, washed several times with water and ethanol and dried at 50oC. The sample is abbreviated as SBA-arene-SO3H. Similarly,
propylene-sulfonic acid-functionalised SBA-15 (abbreviated as SBA-propyleneSO3H) was synthesized via
the co-condensation of MPTMS (mercaptopropyltrimethoxysilane) and TEOS (MPTMS : TEOS = 1 : 9).
RESULTS AND DISCUSSION
Characterization of Catalysts
As shown in Fig. 1, the small-angle X-ray powder diffraction (XRD) patterns of the three mesoporous samples, pure siliceous SBA-15 and those functionalised with propylene- and arene-sulfonic acids, all exhibit one very intense peak and two weak peaks indexed to (100), (110) and (200) reflections, indicating that the materials contained well-ordered hexagonal arrays of one-dimensional channel structure.9 Besides, the lattice expands as the sulfonic acid groups were introduced in the framework by co-precipitataion, in agreement with the results reported in the literature.10 Thermogravimetric analysis (TGA, Du Pont 951) in air and elemental analysis (EA, Heraeus) showed that the surfactants in the as-synthesized samples were almost removed by extraction. The N2 adsorption-desorption
isotherms showed characteristic of type IV of the IUPAC classification. The analysis of the isotherms of the sulfonic acid functionalised samples demonstrated no obvious change in BET surface area for SBA-propyleneSO3H
but slight decrease for SBA-areneSO3H, in
comparison to that of pure siliceous SBA-15 (Table 1). The mean pore size and pore volume were smaller for the sulfonic acid functionalised samples. Between the two functionalised samples, the values for SBA-propyleneSO3H were smaller than
cm3/g in comparison to 52 Å and 1.20 cm3/g. That is attributed to the relatively large size of the benzene ring in comparison to the propylene group. Nevertheless, the pore diameter still remained very narrow distributed with major fractions in 5.5- 7.0 nm. The acid capacity of the sulfonic acid modified samples was determined by acid-base titration.9, 15 The exchange values were both around 1.5 mmol of H+ per g of SiO2, and that was in consistence with the
results from TG analysis and sulfur elemental analysis.
Catalytic Studies
The Beckmann rearrangement of cyclohexanone oxime was carried out in chlorobenzene solvent at 132oC for 24 h under nitrogen. In order to remove adsorbed moisture in the catalyst, 0.2 g of the solid catalyst was pre-refluxed with toluene for 2 h, then in-situ dried in N2 flow at 150oC
overnight, then cooled to ca. 130oC before 2 mmol cyclohexone oxime solution in 10 mL chlorobenzene solvent was poured into the reactor. For comparison, several other acidic catalytic materials such as H-ZSM-5, H-mordernite, Al-MCM-41, and Al-SBA-15 were examined under similar conditions. The products were analyzed using a Chrompak CP 9000 gas chromatograph and identified with a HP 6890 GC-MS. It was found that other than the desired product, ε-caprolactam, cyclohexanone and small amount of high
molecular weight products were also observed. The reaction results were shown in Table 1. No conversion of cyclohexanone oxime was observed in the absence of catalyst. Pure siliceous SBA-15 only showed negligible activity, and caprolactam could hardly be detected in the products. H-ZSM-5 and H-mordernite showed very low conversion of cyclohexanone oxime and low selectivity to ε-caprolactam, though they possessed large amounts of acid sites and high acid strength in their small pores. The catalytic reaction might be performed only on the exterior surfaces due to the diffusion limitation of the liquid in the small pores. Al-MCM-41 and Al-SBA-15 showed 37 % and 20 % conversions of cyclohexanone oxime, however, the selectivities to ε-caprolactam were only 13 % and 19 %, respectively. In comparison, both sulfonic acid functionalised SBA-15 showed similar high conversions of substrate, but the selectivity to caprolactam on SBA-areneSO3H catalyst was much higher
than that on SBA-propyleneSO3H. This
phenomenon may be caused by two factors. On one hand, as an electron-withdrawing substituent, the phenyl ring adjacent to the sulfonic group can disperse the negative charge and stabilize the anion, and thus increase the acid strength.10 On the other hand, the phenyl group is large enough to make the active sites on the surface more hydrophobic than the propylene group, and thus facilitate the diffusion of the organic reactants and products in the liquid phase.13 Thermogravimetric analysis showed that the extracted SBA-areneSO3H contained ca. 5
wt% of water, which lost in the range of
30-100oC, while the extracted
SBA-propyleneSO3H had up to 15 wt% of
water. As shown in Table 1, the liquid phase Beckamnn rearrangement reaction of cyclohexanone oxime to ε-caprolactam strongly depends on the amounts of acid sites and the acid strength on the catalyst surface. The strong acid strength would enhance the selectivity of ε-caprolactam.
Leaching of sulfonic acid groups in the catalytic reaction was examined with SBA-areneSO H catalyst. Acid-base titration
0.5 1.0 1.5 2.0 c b a (200) (110) (100) Inte nsi ty (a . u ) 2 θ / o Fig. 1.
X-ray powder diffraction patterns of the extracted samples: (a) pure siliceous SBA-15, and those prepared with (b) 10 mol% propylenesulfonic acid, and (c) 10 mol% arenesulfonic acid
revealed that the acid amount was slightly reduced from 1.5 to 1.4 mmol of H+ /g SiO2
after the catalytic reaction at 132oC. However, elemental analysis of the used SBA-15 catalyst indicated slight increase in carbon and nitrogen contents after the reaction, in agreement with the result of thermogravimetric analysis, where a new weight loss (5~6%) was observed at ca. 330oC, assigned to the adsorbed organic species in the reaction. As a result, the leaching of sulfonic acid groups during the catalytic reaction is considered negligible and the slight decrease in acid amount is attributed to the increase in weight basis from adsorption of organic species during the reaction.14
Table 1.
Liquid phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam over various solid catalysts
Entry Catalyst SBET
( m2/g) Dp (Å) Conv.a (%) Select.b (%) 1 blank - - 0 - 2 SBA-15 786 52 < 5 - 3 H-ZSM-5c 402 5.5 9 44 4 H-Mordernited 500 6.5 12 18 5 Al-MCM-41e 036 26 37 13 6 Al-SBA-15f 877 56 20 19 7 SBA-15-arene-SO3Hg 652 38 43 82 8 SBA-15-propyl ene-SO3Hg 857 45 40 42
a Conversion of cyclohexanone oxime. b Selectivity of
ε-caprolactam. c Si/ Al = 115.
d Si/ Al = 10. e Si/ Al = 32. f Si/ Al = 60. gacid capacity = 1.5mmol H+ /g SiO
2
In conclusion, arenesulfonic acid-functionalised SBA-15 is a new and very effective acid catalyst for the liquid phase Beckmann rearrangement of cyclohexanone oxime. It shows high selectivity to ε-caprolactam and no leaching of acid sites during the reaction.
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