China Petroleum Processing and Petrochemical Technology Petrochemical Research
2012,Vol. 14, No. 4, pp 33-41
December 30, 2012
A “Green” Cyclohexanone Oxidation Route Catalyzed by Hollow Titanium Silicate Zeolite for Preparing ε-Caprolactone, 6-Hydroxyhexanoic Acid and Adipic Acid Xia Changjiu; Zhu Bin; Lin Min; Shu Xingtian (State Key Laboratory of Catalytic Material and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China) Abstract: Hollow titanium silicalite (HTS) molecular sieve has been synthesized, and information on its structure, physicochemical characterization, as well as surface property was investigated by a host of analytical methods, such as XRF, XRD, low-temperature N2 adsorption/desorption, TEM, FT-IR, UV-Vis, 29Si MAS NMR, and XPS techniques. The characterization results suggest that HTS zeolite has a special hollow crystal structure and its mesopore volume is larger than that of TS-1 zeolite. The titanium species in this zeolite are composed of the framework tetrahedral Ti (IV) ions and extra-framework octahedral Ti (IV) ions, which tend to disperse into its bulk phase. This zeolite material also has been applied to catalyze the cyclohexanone oxidation process, and the products are not completely consistent with those results obtained by using TS-1 zeolite, which might be caused by their difference in pore structure and pore volume, especially the mesopore volume. Cyclohexanone oxidation catalyzed by HTS zeolite is a representative consecutive reaction, the main target products of which are e-caprolactone, 6-hydroxyhexanoic acid and adipic acid. The effect of H2O2/cyclohexanone mole ratio on the cyclohexanone conversion, the total target product selectivity, the distribution of three target products selectivity and their variations along with reaction time is also researched and analyzed, which indicate that HTS zeolite shows a high performance for the Baeyer-Villiger reaction of cyclohexanone and catalytic oxidation of 6-hydroxyhexanoic acid under mild conditions, and the quantity of active surface titanium species as well as the pore structure and mesopore volume controlling the mass diffusion rate are the key factors determining the catalytic activity of HTS zeolite and product selectivity. Key words: catalytic oxidation; cyclohexanone; HTS zeolite; Baeyer-Villiger reaction; consecutive reaction
1 Introduction Titanium silicalite molecular sieve (TS-1), which has a MFI crystalline structure, with Ti4+ ions isomorphously substituting for Si 4+ ions in the silicalite-1 zeolite, is one of the most important and environmentally friendly catalytic materials used in the field of chemistry and petrochemical industries. It is well known that the active centers are the tetrahedral Ti4+ ions incorporated into the silica matrix of the MFI zeolite framework[2-5]. Thanks to its unique catalytic characteristics for the selective oxidation of organic molecules in the presence of aqueous hydrogen peroxide used as the oxidant, TS-1 zeolite is extensively applied in the industrial processes and academic researches, such as epoxidation of propylene, hydroxylation of aromatic compounds, ammoxidation of cyclo-
hexanone, and dehydrogenation of primary and secondary alcohols, respectively. It is a pleasure to see that some successful industrial and academic processes catalyzed by TS-1 zeolite have come to fruition, which can bring about remarkable environmental and social benefits[3-4]. One of the most prominent items is the “140 kt/a package technology project for production of caprolactam” owned by Sinopec’s Baling Petrochemical Co., Hunan province, China. Since the HTS zeolite used as catalyst for a singlestep “green process” to produce cyclohexanone oxime can replace the former four-step HPO (hydroxylamine phosphate oxidation) process, this company can reduce the equipment investment by 21.1% of the original value, Corresponding Author: Prof. Lin Min, Telephone: +86-1082368801; E-mail: [email protected]
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cut down 99% hazardous gas emissions, and simplify the production process. China Petrochemical Corporation’s Research Institute of Petroleum Processing (RIPP) has been carrying out the study on synthesis and modification of titanium silicalite since the 1990s, and nowadays has exploited the HTS zeolite with its own independent intellectual property rights[5-6]. Since the HTS zeolite has a unique hollow crystal structure and a hysteresis loop identified during lowtemperature N2 adsorption, it can improve the diffusion of reactants and products inside the pores of catalyst and perform better catalytic activity and stability than that of TS-1 zeolite. The HTS zeolite has been put into commercial production with the production capability reaching
thereby, we could infer the mechanism for the reaction
about 100 tons per year, and it has been fully proven as a very efficient catalyst for the selective oxidation of small organic molecules by dilute H2O2 solution.
transferred to a stainless steel autoclave and was heated
Today, ε-caprolactone is an important fine chemical, which is usually synthesized by the peroxyacid used as oxidant, and this reaction is called the Baeyer-Villiger oxidation[7-12]. The peroxyacid oxidation route is relatively mature, but it brings in a lot of disadvantages that limits its use as a best technical option, including the formation of large amount of organic wastes, low selectivity of the target product, difficulties in the separation and purification of products, high cost for manufacturing the peroxyacid, and potential danger in the process, which can obviously violate the principle of green chemistry. To avoid these drawbacks, various heterogeneous catalysts have been explored on this reaction[8-9]. The most significant catalyst for cyclohexanone oxidation is Sn-b zeolite[10-11], which can achieve a e-caprolactone selectivity of close to 100%. A. Bhaumik, et al. examined the Baeyer-Villiger oxidation of cyclohexanone using the TS-1 zeolite as catalyst, and they got good results, but the conventional TS-1 zeolite suffered from poor activity and stability, so there were no related reports about the outcome in the later years. In the present study, we introduced a new route for synthesis of ε-caprolactone from cyclohexanone with 30% hydrogen peroxide catalyzed by HTS zeolite. The HTS zeolite was prepared according to the US Patent No. 6475465 and characterized by some physical means to investigate the possible factors influencing the BaeyerVilliger oxidation process. Based on these data obtained
distilled water. The solid substance was dried at 110 ℃
involving the HTS catalyst and propose the optimal reaction conditions.
2 Experimental 2.1 Synthesis of the catalysts 22.5 g of TEOS (tetraethyl orthosilicate) was thoroughly mixed with 7.0 g of TPAOH solution and 59.8 g of distilled water. After the mixture was hydrolyzed at 60 ℃ under atmospheric pressure for 1.0 h, 1.1 g of tetrabutyl titanate and 5.0 g of anhydrous isopropanol were added slowly to the mixture followed by stirring at 75 ℃ for 3 h to obtain a clear and transparent colloid, which was then at 170 ℃ for 6 days. As the reaction terminated, the resultant crystallized solution was filtered and washed with for 1 h, followed by calcination at 550 ℃ in air for 4 h to obtain thereby the TS-1 molecular sieve. The TS-1 zeolite was mixed homogenously with tetraethylammonium hydroxide and water at a ratio of TS-1 zeolite (g):tetraethylammonium hydroxide (mol):water (mol)=100:0.6:60. Then, the mixture was transferred into a stainless steel autoclave, put inside a drying oven and heated at 175 ℃ for 3 days. After cooling and pressure relief of the system, the product was subjected successively to filtration, washing, drying and calcination at 550 ℃ in air for 3 h, and finally the HTS zeolite was obtained.
2.2 Characterization The X-ray fluorescence analysis (XRF) was performed in a Rigaku 3271E X-ray fluorescence spectrometer equipped with a semi-quantitative analysis system. X-ray powder patterns were obtained on a Bruker (Siemens) D 5005 diffractometer using nickel filtered CuKα radiation with scanning of 0.02°/step, at a wavelength of 0.15418 nm, and diffraction patterns were collected at room temperature in the scanning angle (2θ) range of 5°—35°. N 2 adsorption/desorption isotherms were collected at liquid nitrogen temperature using a Micromeritics ASAP 2010 apparatus. Before the measurement, approx. 50 mg of the sample was dehydrated under vacuum (at 10−3 torr)
Xia Changjiu, et al. A “Green” Cyclohexanone Oxidation Route Catalyzed by HTS Zeolite for Preparing ε-Caprolactone, 6-Hydroxyhexanoic Acid and Adipic Acid
at 300 ℃ overnight. The speciﬁc surface area data were determined from the linear part of the BET equation.
Table 1 Composition and structure of TS-1 and HTS zeolite samples
TEM pictures were taken on a G2 F20S-TWIN electron microscope with an accelerating voltage of 300 kV.
Fourier transform infrared (FT-IR) spectra were recorded
BET surface area, m2/g
Pore volume, mL/g
on a Nicolet 8210 infrared spectrometer with the wave-
numbers ranging from 400 cm-1 to 4 000 cm-1.
Si solid-state nuclear magnetic resonance (NMR) exper-
iments were performed with magic angle spinning (MAS) on an Inanity Plus-400 spectrometer. Samples were spun at 10 kHz in the 4-mm zirconium rotors. A classical crosspolarization sequence was used with 2 ms contact time and a recycle delay of 10 s. The diffuse reﬂectance ultraviolet-visible (UV-Vis) spectra were obtained on a Perkin-Elmer Lambda 20 UVVisible spectrometer, with the wavelength ranging from 200 nm to 800 nm.
The results of the elemental analysis of both TS-1 zeolite and HTS zeolite are listed in Table 1. It can be seen from Table 1 that both of these two zeolite samples are composed of SiO2 and TiO2, albeit with a small difference in their SiO2/TiO2 mole ratios. Figure 1 shows the X-ray diffraction patterns of both Ti-containing materials. We can find out that the main XRD diffraction peaks (2θ=23°—26°) agree well with the characteristics of MFI topology.
The XPS spectra were all recorded with a PHI Quantera SXM (Scanning X-ray Microprobe) in vacuum (6.7×10-8 Pa) at ambient temperature using unmonochromatized MgKα radiation operating at a constant power of 260W.
2.3 Catalytic reactions 0.1 mol of cyclohexanone, HTS zeolite and small magnetic stirring bars were put into a 100-ml three-necked round bottom flask. When the temperature reached 50 ℃ stably, 0.1x mol of hydrogen peroxide (x=0.33—3) was added to the reaction system while the temperature was set at the expected value, and thereafter the samples in the flask were picked up at desired time intervals. MnO2 was used to decompose the residual hydrogen peroxide and the samples were dissolved in acetone to form a homogeneous phase, which was analyzed by the Agilent 6890N gas chromatograph (GC) equipped with a 5% polar chromatographic column (Model HP-5).
3 Results and Discussion 3.1 Structural, physicochemical and surface properties of HTS The particular structure information, the physicochemical characterization, as well as the surface property of HTS zeolite samples were investigated by a variety of analytical techniques.
Figure 1 XRD patterns of TS-1 and HTS samples
Table 1 also summarizes the BET surface area and pore volume data of the TS-1 and HTS zeolite samples. The BET surface area of HTS zeolite was smaller than that of TS-1 zeolite, and the microporous volume of HTS zeolite was less than that of TS-1 zeolite, but the total porous volume of HTS zeolite was more than that of TS-1 zeolite, which means that the post-treatment has produced more mesopores. In order to illustrate this phenomenon, we have applied the low temperature N2 adsorption isotherms and the transmission electron microscopy (TEM) methods to analyze the HTS sample. Figure 2 exhibits the nitrogen adsorption-desorption isotherms of HTS zeolite sample, which indicated that the isotherm in the range from 0.4 to 1.0 was of type Ⅳ, which was caused by the adsorption-desorption of N2 in the mesopores of the zeolite particles. It is well known that the traditional ·
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TS-1 zeolite sample has no hysteresis loop in its N2 adsorption isotherm curve. So we could learn that HTS zeolite is of special hollow crystal structure, which may generate some mesopores during the pre-treatment procedure. This opinion could be further justified by combining the pore volume data with the micropore volume data listed in Table 1. The mesopore volume is equal to the pore volume data minus the micropore volume data, and the mesopore volume of TS-1 zeolite sample is 0.085 mL/g, while the mesopore volume of HTS zeolite sample is 0.160 mL/g. The same conclusion also could be obtained from the results of TEM analysis, Figure 3 (A) and (B) show the TEM images of TS-1 zeolite and HTS zeolite, respectively. By comparing with the data of TS-1 zeolite, the HTS zeolite showed an obviously hollow structure.
shown in Figure 4, both of TS-1 zeolite and HTS zeolite have adsorption peaks at 960 cm-1 in FT-IR spectra. Although the affiliation of this band is still in argument, it is generally accepted as the characteristic fingerprint band in terms of stretching vibration modes of [SiO4] groups perturbed by the presence of Ti (IV) ions in the form of [TiO4] and [O3TiOH], suggesting that Ti4+ ions are located in the framework of zeolite. The results show the state of Ti-containing species in HTS zeolite, and there is no serious impact on framework symmetry of HTS zeolite after post-treatment.
Figure 4 FT-IR spectra of TS-1 and HTS zeolite samples
Figure 2 N2-adsorption/desorption isotherms of HTS sample
The 29Si MAS NMR spectrum of HTS zeolite sample (in Figure 5) shows that only one main peak is identified at -114.1 ppm (Q4) and the peak at -102.1 ppm (Q3) is very weak. The peak at -102.1 ppm is attributed to a Si atom in the Si (OH)(OSi)3 environment, while the peak at -114.1 ppm occurs due to framework Si atoms in the Si(SiO4) circumstance. The results indicate that Si atoms are almost totally located in the framework of zeolite.
Figure 3 TEM images of: (A) TS-1 zeolite and (B) HTS
Generally speaking, the framework tetrahedral Ti (IV) ions of TS-1 zeolite are considered as the active sites for catalytic oxidation of organic compounds[2, 15]. In order to determine what is the active species in HTS zeolite sample, multiple analytical methods, such as FT-IR, 29Si MAS NMR, DR UV-Vis and XPS, have been used. [16-17] As ·
Figure 5 29Si MAS NMR spectra of HTS
X-ray photoemission spectroscopy (XPS) is a widely applied method for surface chemical analysis of zeolite[20-21]. We can learn from Figure 6 that the mole ratio of Ti/Si is
Xia Changjiu, et al. A “Green” Cyclohexanone Oxidation Route Catalyzed by HTS Zeolite for Preparing ε-Caprolactone, 6-Hydroxyhexanoic Acid and Adipic Acid Table 2 Oxidation of cyclohexanone over TS-1a) zeolite and HTSb) zeolite Test No.
TS-1 zeolite /H2O2
HTS zeolite /H2O2
Product selectivity, % CA
a) The results of TS-1 zeolite were cited from the work of A. Bhaumik, et al., as reported in Ref. . b) Reaction conditions: substrate/H2O2 ratio=1:1, catalyst dosage=20 wt% of TS-1 zeolite or 5 wt% of HTS zeolite, and the reaction temperature was 353K. c) “Three” means a system composed of solid catalyst + two immiscible phases (organic substrate + H2O2 solution). d) The cyclohexanone conversion and product selectivity were calculated for the case with a reaction time of 6 h.
0.25/24.74 that is equal to the value of 1/98.68, which is evidently lower than the ratio of Ti/Si determined by XRF analysis (with the value equating to 4.66/95.34), indicating that the titanium atoms incline to be dispersed into the bulk phase of HTS zeolite sample. The chemical state of titanium species are shown in Figure 4 (b), showing that the bands in 460.1 eV and 458.5eV are the characteristic peaks of the framework tetrahedral Ti (IV) ions and extra-framework octahedral Ti (IV) ions, respectively.
We could also obtain the percentages of different states of titanium species through fitting the peaks’ calculation data, the percentage of framework tetrahedral Ti (IV) ions is 49.07%, while the percentage of extra-framework octahedral Ti(IV) ions is 56.93%. It can be seen from Figure 7 that the HTS zeolite has two bands at 210 nm and 350 nm as identified by the DR UV-Vis spectra, which can be ascribed to the oxygen-tetrahedral Ti (IV) ions and the oxygen-octahedral Ti (IV) ions, respectively.
Figure 6 XPS spectra of HTS zeolite sample: (a) the total XPS spectra; ( b) the Ti2p XPS spectra
Figure 7 UV-vis spectra of HTS zeolite sample
3.2 Oxidation of cyclohexanone The different catalytic properties between HTS zeolite
and TS-1 zeolite are shown in Table 2. The results indicated that the main products of oxidation reaction catalyzed by TS-1 zeolite with 30% aqueous H 2O 2, included ecaprolactone, cyclohexane, hydroxyketone and diketones, while those of HTS zeolite included e-caprolactone, 6-hydroxyhexanoic acid, adipic acid and hydroxyketone, with the cyclohexanone conversion reaching 51.9%. By comparing the two results, we could find out that the mesoporous volume of HTS zeolite could enhance the transformation of cyclohexanone to e-caprolactone, which after being subjected to subsequent consecutive reactions could form 6-hydroxyhexanoic acid and adipic acid. Upon taking these factors into consideration, the ·
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e-caprolactone selectivity reached up to 89.12%, so the performance of HTS zeolite was much better than TS-1 zeolite for the Baeyer-Villiger oxidation of cyclohexanone under the same conditions, which could be attributed to the restraints on diffusion of reactants and products in the pores of zeolite. Herein the HTS zeolite material was used as catalyst to improve the cyclohexanone oxidation activity in a liquid-
liquid-solid three-phase system under mild reaction conditions. It is found that the main products of cyclohexanone oxidation in this system contained e-caprolactone (CA), 6-hydroxycaproic acid (HA) and adipic acid (AA) and hydroxyketones (HK) as identified by GC-MS. And the possible reaction route is shown in the following equation (Figure 8): It can be seen from Figure 8 that the cyclohexa-
Figure 8 Main reaction pathway for oxidation of cyclohexanone in the presence of HTS zeolite/H2O2 system
none oxidation reaction in the HTS zeolite/H2O2 system is a consecutively occurring reaction. The first step is a typical Baeyer-Villiger oxidation reaction, and it may start with the nucleophilic attack to the carbonyl group in ketones, followed by the subsequent migration of the carbonyl-substituent in the intermediate Criegee adduct with retention of its chemical configuration, which could give birth to e-caprolactone. In theory, the formation of one mole of lactone needs to consume one mole of H2O2. However, the e-caprolactone is very unstable in that system and it will be attracted by H2O molecules to form the corresponding 6-hydroxycaproic acid at the second step. Finally, the hydroxyl group of the oxyacid could be transformed into adipic acid. In the course of consecutive chemical reaction, the distribution of the target products depends on the energy barriers and effective collision. We have investigated the effect of the mole ratios of hydrogen peroxide/cyclohexanone and reaction temperature on controlling the cyclohexanone conversion, the target products selectivity, and the distribution of target products. Figure 9 shows the dependency between cyclohexanone conversion and reaction time at a mole ratio of H2O2/ cyclohexanone ranging from 3:1 to 1:3. It is evident that high H2O2/cyclohexanone ratio and long reaction time could lead to an increase in cyclohexanone conversion, it means that higher H2O2/cyclohexanone ratio could enhance the catalytic reaction rate. It is well known that in ·
Figure 9 Cyclohexanone conversion as a function of reaction time for different systems of reagent mole ratios The mole ratio H2O2/cyclohexanone is: ■—3:1; ●—2:1; ▲—1:1; ▼—1:2;
the catalytic oxidation system consisting of TS-1 zeolite/ H2O2, the activation of hydrogen peroxide process is the key step for initiating this reaction. The nucleophilic attack capability of low concentration H2O2 solution is too weak to oxidize the cyclic ketone into lactone . The activated H2O2 molecules catalyzed by the HTS zeolite can be involved in effective collision with cyclohexanone molecules to generate the corresponding e-caprolactone molecules, which are related with the cyclohexanone conversion during the sequential reaction. The experimental total target product selectivity curves (Figure 10) show that a high mole ratio of H2O2/cyclohexanone could improve the total target product selectivity of these reactions, which denotes that the H2O2/cyclohexa-
Xia Changjiu, et al. A “Green” Cyclohexanone Oxidation Route Catalyzed by HTS Zeolite for Preparing ε-Caprolactone, 6-Hydroxyhexanoic Acid and Adipic Acid
Figure 10 Comparison on various systems relating to total target product selectivity
product selectivity and reaction time
The mole ratio H2O2/cyclohexanone is: ■—3:1; ●—2:1; ▲—1:1; ▼—1:2;
Figure 11 Relationship between distribution of target
none ratio had a significant effect on the product selectivity. The total target product selectivity increased sharply with the increase in the H2O2/cyclohexanone mole ratio between the range from 2:1 to 1:3, which could explain that with an increasing mole ratio of H2O2/cyclohexanone, more H2O2 molecules could activate the active sites of titanium in the HTS zeolite and its nucleophilic attack capability was increased. Because the above-mentioned Baeyer-Villiger reaction of cyclohexanone is a typical nucleophilic addition reaction, it could accelerate the formation of target product and inhibit the formation of other byproducts by controlling the reaction conditions. It can be seen from Figure 4 that the target product selectivity obtained at a H2O2/cyclohexanone mole ratio of 3:1 was lower than the data obtained at the H2O2/cyclohexanone mole ratios between the range of 2:1 to 1:1, which might occur because so large an amount of water in this system could improve the aqueous solvent effect to reduce the concentration of oxidant and initiate other side reactions. So the optimized mole ratio of H2O2/cyclohexanone was specified at 2:1. As mentioned above, the main cyclohexanone oxidation process is a consecutive reaction which is one of the most complex reactions in organic chemistry. The distribution of main target product selectivity could be influenced by many factors, such as mole ratio, temperature, and reaction time. Figure 11 shows the effect of reaction time on the distribution of the target product selectivity at 80 ℃ and a H2O2/cyclohexanone mole ratio of 1:1. It can be seen from Figure 11 that the selectivity of e-caprolactone
(the mole ratio of Cyclohexanone and H2O2 is 1:1)
decreased with increase in the reaction time, while the selectivity of 6-hydroxyhexanoic acid increased sharply at the first 6 hours and then gradually leveled off; the adipic acid selectivity increased with an increasing reaction time. In this work during the first 8 hours, the respective target product selectivity value changed in the following order: 6-hydroxyhexanoic acid > adipic acid > e-caprolactone. It can be inferred that the reaction rate relating to formation of e-caprolactone from cyclohexanone was lower than the reaction rate of e-caprolactone hydrolysis, which could elucidate why at the beginning the e-caprolactone selectivity decreased and the 6-hydroxyhexanoic acid selectivity increased. However, the hydroxyl functional groups of 6-hydroxyhexanoic acid could be further oxidized by the HTS zeolite/H2O2 system to obtain the finial adipic acid product. When the concentrations of cyclohexanone and e-caprolactone were declining at a low level, the rate of 6-hydroxyhexanoic acid formation was less than its rate of transformation to adipic acid, which could explain the decreased selectivity of 6-hydroxyhexanoic acid and the increased adipic acid selectivity under the reaction conditions tested. It is interesting to note that the hollow titanium silicalite is an effective catalyst for both Baeyer-Villiger oxidation of cyclohexanone and oxyacid oxidation to synthesize dicarboxylic acid.
4 Conclusions Characterization results suggest that HTS zeolite has special hollow crystal structure, and its mesopore volume is larger than that of TS-1 zeolite. HTS zeolite has been ·
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employed to catalyze the Baeyer-Villiger oxidation of cyclohexanone. Both cyclohexanone conversion and desired product distribution obtained in the presence of HTS zeolite are different from those obtained on TS-1 zeolite as reported by A. Bhaumik, which might be ascribed to the difference in their mesopore volume. The cyclohexanone oxidation reaction catalyzed by HTS zeolite is a representative consecutive reaction, the main target products of which are e-caprolactone, 6-hydroxyhexanoic acid and adipic acid. We have studied the effect of H2O2/ cyclohexanone mole ratio on the cyclohexanone conversion, the total target product selectivity, the distribution of selectivity of three target products and their variations with reaction time. These results indicated that HTS zeolite exhibited high activity for oxidation of cyclohexanone and 6-hydroxyhexanic acid under mild conditions, So this catalytic material may play a significant role in the economic and social development in the future.
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Dalian Institute of Chemical Physics Realizes Green Synthesis of EG in Nano-reactor The team at the State Key Laboratory for Catalysis Fun-
catalytic hydration of EO the nano-reactor at 40 ℃ and
damentals headed by the academician Li Can and the
a H2O/EO molar ratio close to stoichiometric ratio (about
research fellow Yang Qihua at the CAS Dalian Institute of
2) could achieve a 98% conversion of EO and 98% se-
Chemical Physics has realized the manufacture of ethyl-
lectivity of MEG, with the ethylene glycol concentration
ene glycol (MEG) through catalytic hydration of ethylene
in the reaction system reaching over 75% to drastically
oxide (EO) in a nano-reactor featuring high efficiency,
reduce the energy consumption of MEG production.
green process and energy conservation.
In the meantime, this heterogeneous catalyst has avoided
Traditional MEG production process adopts the direct
the environmental pollution arising from application of
hydration of EO. In order to obtain high MEG selectivity,
traditional liquid or solid acid catalysts and can real-
the reaction system usually needs enormous excess water
ize separation and recycle of the catalyst to function as
(with the H2O/EO molar ratio equating to 20), leading to
a typical green catalytic process. Research has revealed
reduction of MEG concentration in the reaction liquid to
that the metal complex zeolite encapsulated in nano-cages
less than 10% coupled with a high energy consumption
is capable of free moving and can maintain its intrinsic
in the process of product distillation and purification. In
characteristics to display its higher intrinsic catalytic ac-
a bid to reduce the energy consumption the acid/caustic
tivity thanks to synergistic coupling effects of the dual
catalytic hydration process is generally applied. However,
active sites. The catalyst made by means of encapsulation
the traditional acid/caustic catalytic hydration process still
is a solid catalyst in macroscopic meaning. Therefore, the
needs a H2O/EO ratio that is too higher than the stoichio-
catalytic material in nano-reactor has both the advantages
metric ratio to attain a high MEG selectivity.
of heterogeneous and homogeneous catalysis. By virtue
This research team has discovered that Co(Salen) zeolite
of this strategy this research team has realized in the na-
encapsulated in silica-based nanocages has shown an ac-
no-reactor the dual-molecular catalytic oxidation coupled
celerated synergistic coupling effect in the chiral resolu-
with oxygen delivered from water molecules and dynamic
tion reaction, They further disclosed that in the course of
resolution reaction of epoxy compounds. ·