Mesoporous Titanium-Silicalite Zeolite Containing Organic Templates

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Mesoporous titanium-silicalite zeolite containing organic templates as bifunctional catalyst for the cycloaddition of CO2 and epoxides Dan Liu, Gang Li, Jiaxu Liu, Yue Wei, and Hongchen Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04759 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 16, 2018

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ACS Applied Materials & Interfaces

Mesoporous titanium-silicalite zeolite containing organic templates as bifunctional catalyst for the cycloaddition of CO2 and epoxides Dan Liu, Gang Li*, Jiaxu Liu, Yue Wei, Hongchen Guo State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China. Corresponding Author * E-mail: [email protected]

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Abstract: Mesoporous TS-1 (MTS-1) containing organic templates tetrapropylammonium hydroxide (TPAOH) and polydiallyldimethylammonium chloride (PDDA) is synthesized and treated with different concentrations of hydrochloric acid aqueous solution. The as-synthesized MTS-1 are characterized using XRD, FT-IR, SEM, TEM, UV-Vis,

13

C CP/MAS NMR, TGA,

CO2-TPD, N2 adsorption-desorption, XRF and EA. The results of 13C CP/MAS NMR show that the structures of organic templates are retained after acid washing treatment. Not required to be removed by calcination, the organic templates embedding in MTS-1 can take the role of basic sites and activate carbon dioxide (CO2), which is confirmed by FT-IR. Moreover, the amount of acid sites and basic sites in the samples before and after acid treatment is characterized by modified Hammett indicator method and CO2-TPD, respectively. The results show that both the acidity and the basicity in the material are improved after acid washing. Thus the sample after acid treatment contains two active sites, basic sites stemming from organic species and acid sites coming from framework Ti, which is beneficial to be used as bi-functional catalyst in cycloaddition reaction of CO2 and epoxides. It is highly active in the cycloaddition reaction, in which the conversion of epichlorohydrin (ECH) could achieve 97.8% and the selectivity of cyclic carbonate is 98.0% under 1.6 MPa at 393 K for 6 h when acetonitrile is used as the solvent. Moreover, the kinetic of the cycloaddition reaction is studied using ECH as substrate by varying the reaction parameters. More importantly, the organic-inorganic hybrid catalyst is reusable and stable against leaching of organic species in the cycloaddition reaction. Keywords: Mesoporous zeolites, Organic templates, Bifunctional catalyst, Cycloaddition reaction, Cyclic carbonate.


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1. Introduction Zeolites are a kind of microporous inorganic materials with regular intracrystalline cavities and channels of molecular dimensions. The microporous structure of zeolites provides large surface areas, unique size and shape selectivity and makes the zeolites be widely used as important heterogeneous catalysts in a number of industrial processes.1,

2

However, an

outstanding defect of such zeolite materials originating from their inherent micropores is the inhibitation to the diffusion of bulky reactants and products, which hinders their wide use in petrochemical processes and fine chemical. In the past few decades, many efforts have been devoted to synthesize hierarchical zeolites consisting of the typical micropores and additional mesopores by the strategy of developing new organic templates in order to overcome this disadvantage. 3-5 In general, synthetic systems of zeolites always require organic amines or quaternary ammonium salts as templates so as to gain ideal topological structure. However, in order to obtain the open zeolite channels and make sure the zeolite has a large surface area, almost without any exception, the occluded organic species are burned off. For some zeolites, the process of calcination may lead to the destruction of zeolite framework to some extent.6 More importantly, the calcination will result in air pollution. However, some of these organic amines or quaternary ammonium salts are very important homogeneous catalysts in the cycloaddition reaction of carbon dioxide (CO2). CO2 is not only one of the main greenhouse gas that leads to the global warming, but also a kind of cheap nontoxic and abundant C1 essential feedstock that can be used to synthesize fine chemicals.7 Therefore, some strategies for CO2 capture, separation/storage and its chemical transformation have been studied.8 Among them, catalytic CO2 conversion into value-added



industrial products has become an attractive craft due to its sustainability and eco-friendliness. One of the most efficient strategies is the synthesis of cyclic carbonates with epoxides, which is a green and atom-economic reaction. Otherwise, the generated cyclic carbonates can serve as valuable organic intermediates, aprotic polar solvents, fuel additives and monomers in polymerization reaction.9 Up to now, various heterogeneous catalytic systems have been developed for the fixation of CO2 with epoxides, including metal salts, zeolites, metal-organic frameworks, organo-catalysts, graphene oxide and modified graphitic carbon nitride.10-20 It is worth noting that these systems often used quaternary ammonium salts as co-catalyst for obtaining excellent catalytic performance. It is more environment-friendly and economical if the organic templates can be used as active sites in the cycloaddition reaction instead of being removed out of zeolites. However, in order to make full use of the organic templates-containing zeolite composite materials, they not only should satisfy the prerequisite that organic templates are rigid enough against decomposing under harsh reaction conditions and stable against leaching in the organic solution, but also could provide accessible zeolite channels that allow reactants to contact the active sites. For instance, the tetrapropylammonium hydroxide (TPAOH), a traditional structure-directing agent (SDA) for MFI zeolites, is situated in the intersections of 10-membered ring (MR) channels. Despite the TPA+ cations are rigid against leaching, they are occluded in the pore of zeolites, which make most of the active sites are generally hardly accessible to the reactants. Hence, new zeolite materials with particular structures basically different from traditional ones are highly needed. Little attention has been paid on utilizing the organic templates in the zeolites as active sites for the fixation of CO2 with epoxies. Srinivas group used the as-synthesized beta zeolite with tetraethylammonium cations (TEA+) in the cycloaddition reaction. The hybrid zeolite is proved


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to be a good base catalyst and can catalyze this reaction under mild conditions with the coexistence TEA+ cations.21 Recently, wu and co-workers synthesized a multilayered zeolite with SDA molecules occluded in micropores, which contains one quaternary ammonium of Geminitype SDA that were stably immobilized in the intersection of straight and sinusoidal 10-MR channels while the other one was exposed to the layer surface of the nano-sheets.22,

23

The

existence of SDA endows the multi-lamellar zeolites base sites, leading to a bi-functional catalyst that can effectively catalyze the cycloaddition reaction of CO2 with the post incorporated halogen anions as a nucleophile. The work is interesting, while multistep processes are needed for the synthesis of the lamellar structure zeolites with the specially designed surfactant. Herein, Mesoporous TS-1 zeolite (MTS-1) was prepared using easy available and low cost polydiallyldimethylammonium chloride (PDDA) as mesopore template. The organic templates endow as-synthesized mesoporous TS-1 zeolite basic sites, which cooperate synergistically with framework Ti species, leading to a bi-functional catalyst for cycloaddition reaction of CO2 with epoxides. The organic-inorganic composite is further treated with different concentrations of hydrochloric acid (HCl) aqueous solution and it presents excellent catalytic performance in the cycloaddition reaction. Meanwhile, the mesoporous catalyst is reusable and stable against leaching of organic species in the reaction. 2 Experimental 2.1 Catalyst preparation 2.1.1 Preparation of Mesoporous TS-1 Mesoporous TS-1 was prepared according to previously reported procedures with a slight change.24 The material was synthesized using TPAOH (25 wt. % aqueous solution, Shanghai Kairui) and PDDA (average Mw 1-2×105, 39-43 wt. % aqueous solution, Huangshan Haining) as



micropore and mesopore template, respectively. The molar composition of the resulting synthesis gel was 1SiO2: 0.02TiO2: 0.25TPAOH: 1IPA: 30H2O. In a typical synthesis, the mixture of TPAOH aqueous solution and deionized water were added drop-wise to certain amount of tetraethylorthosilicate (TEOS, Tianjin Kermel) and tetrabutylorthotitanate (TBOT, Tianjin Kermel) under vigorously stirring, respectively. The two solutions were hydrolyzed for 2-3 h at room temperature. After mixing the two solutions, the sol-gel was stirred for a period of time followed by removing alcohol at 343-353 K, and then PDDA (2-5g) was added in the mixture and further stirring for another 24 h. The synthesis gel was transferred into a Teflonlined stainless steel autoclave under autogenous pressure for 96 h at 443 K. Finally, the assynthesized solid was recovered by centrifugation, washed thoroughly with deionized water and dried at 373 K overnight. The as-synthesized zeolites were designed as MTS-1(1)-AS, MTS1(2)-AS, MTS-1(3)-AS, MTS-1(4)-AS, corresponding to 2/3, 3/3, 4/3, 5/3 of the PDDA/SiO2 mass ratio used in the initial sol-gel, respectively. To confirm the effect of the framework Ti species, silicalite-1 with mesoporous structure was hydrothermally synthesized under the similar condition without the addition of the TBOT, the sample was denoted as MS-1-AS (the PDDA/SiO2 mass ratio was 4/3). Otherwise, the conventional TS-1 (denoted as CTS-1-AS) with the same SiO2/TiO2 ratio was also synthesized following the similar procedure described above without the addition of the PDDA. Moreover, the organic template in CTS-1-AS was removed by calcination at 823K for 6 h and the sample was denoted as CTS-1. 2.1.2 Acid treatment of MTS-1(3)-AS The as-synthesized organic-inorganic zeolite (MTS-1(3)-AS) was treated with different concentrations of HCl aqueous solution (Tianjin No.3 chemical Reagent Factory). Typically, 1g


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of zeolite was stirred in 50 mL of 0.5-4 M HCl aqueous solution at 373 K for 3 hours. The process was repeated at the same condition twice. The acid washed and dried samples were designated as MTS-1(3)-AT-x, where x indicated the concentration of HCl aqueous solution (mol/L). 2.2 Catalyst characterization Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max 2400 diffractometer using Cu Kα radiation at 40 kV and 100 mA with a scan speed of 6o/min and a scan step size of 0.02o. Fourier transform infrared spectroscopy (FT-IR) spectra were collected on a Bruker EQUINOX55 spectrometer, using KBr pellet technique. Ultraviolet-visible spectra (UV-Vis) were recorded on a JASCOUV550 spectrometer. Scanning electron microscopy (SEM) images were collected on a FEI Quanta 450 to determine the morphology of the sample. Transmission electron microscopy (TEM) images were taken on FEI Company Tecnai G2 20 Stwin instrument with an acceleration voltage of 200 kV to confirm the existence of mesoporous. The sample was handled by first dispersing through supersonic in ethanol, then placing the drop on a carbon-coated copper grid. The N2 physical adsorption-desorption measurements were carried out at 77 K using a Micromeritics ASAP 2020 analyser after degassing of the sample under vacuum at 393 K. The specific surface area and total pore volume were calculated from adsorption data employing the Brunauer-Emmett-Teller (BET) method and the amount of nitrogen adsorbed at the relative pressure of 0.99, respectively. The pore-size distribution was estimated from Barrett-Joyner-Halenda (BJH) adsorption algorithm. Thermogravimetry analysis (TGA) was performed on a TA Instruments Q50 TGA by heating from 298 K to 1073 K under N2 at 10 K/min. Solid state

13

C cross polarization/magic angle spinning nuclear magnetic

resonance (13C CP/MAS NMR) spectra were obtained on a Bruker AVANCE III 500 NMR



spectrometer operating at a spinning frequency of 500 MHz. The MTS-1 samples adsorbed CO2 and were in-situ characterized using the Thermo Corp. Nicolet 6700 FT-IR spectrometer. Temperature-programmed desorption (TPD) profiles of CO2 were obtained using a self-made TPD devices and the desorbed CO2 was tested by a TCD detector. The acid strength distribution of the samples before and after acid treatment was studied by modified Hammett indicator method with neutral red (pKa= 6.8) and methyl red (pKa= 4.8) as indicator. Before the indicator was added, ultrasonic treatment had been used to make sure that probe molecule (n-butyl-amine, Tianjin Kermel) completely contacted with acid sites. The ratio of Si/Ti in zeolites was measured with Bruker SRS3400 X-ray fluorescence (XRF) spectrometer. The organic elemental analysis (EA) was measured on an Element Vario EL. 2.3 Cycloaddition of CO2 with epoxides In a typical reaction, 0.3 g catalyst, 2 mL epichlorohydrin (ECH), 22 mL acetonitrile and 1 mL methylbenzene as internal standard were put in an autoclave. After sealing the reactor, the air was purged three times with CO2. Then the cycloaddition reaction was implemented at a preset pressure and temperature. After six hours, the reactor was cooled for a period of time in an ice bath, then the unreacted CO2 was exhausted and the catalyst was separated through centrifugation, finally the products were analyzed by gas chromatography (Agilent 6890N equipped with HP-5 capillary column and a FID detector). In order to test the reusability, the catalyst after reaction was separated through centrifugation, washed with ethanol more than three times and dried at 393 K in an oven, then used directly for the next run on the same reaction condition. The transformation frequency (TOF) (molar amount of synthesized cyclic carbonate (CC) per molar basicity per hour) for each catalyst was calculated as equation (1):


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TOF

.

.% .% .

(1)

Where nCC was the molar amount (mmol) of CC, nECH was the molar amount (mmol) of ECH, t was reaction time (h) and ncatal. was the molar amount (mmol) of basic sites in the catalyst obtained by CO2-TPD analysis. 3. Results and discussion 3.1 Characterization of catalysts The XRD patterns of MTS-1-AS with different mass ratios of PDDA/SiO2 and a series of MTS-1(3)-AT with different concentrations of HCl aqueous solution are shown in Fig. 1. It can be clearly illustrated that all the materials exhibit characteristic reflections of MFI topology structure.25 In Fig. 1(B), the spectra of MTS-1(3)-AT have no obvious differences compared with MTS-1-AS, indicating the high stability of the as-synthesized samples. Moreover, all samples do not show diffraction peak at 2θ=25.3° except MTS-1(4)-AS suggesting that antatase TiO2 is not formed.26

Fig.1 XRD patterns of MTS-1-AS with different mass ratios of PDDA/SiO2 (A); MTS-1(3)-AT with different concentration of HCl and the reused catalyst (B)



Fig. 2 FT-IR spectra of MTS-1(3)-AS and MTS-1(3)-AT-2 before and after being reused The FT-IR spectra of MTS-1(3)-AS and MTS-1(3)-AT-2 are shown in Fig. 2. The absorption bands at 800 and 1100 cm-1 are attributed to the internal vibration of (Si, Ti) O4. The bands at 550 and 1230 cm-1 are assigned to the characteristic peaks of MFI structure, which are owned to the double 5-MR vibration of SiO4 and TiO4 tetrahedral. Moreover, the absorption peak at 970 cm-1 always related to the stretching vibration of Si-O-Ti bonds. It is hard to confirm quantitative correlations between the intensity of the 970 cm-1 peak and the framework Ti content of the catalyst, but its intensity is often directly proportional to the content of titanium in the framework.27-29 Just as shown in Fig. 2, MTS-1(3)-AT-2 exhibits a little stronger vibration at 970 cm-1 than MTS-1(3)-AS. The result indicates that the sample treating with acid has relative larger ratio of framework Ti species, which may be attributed to that some amorphous Ti species are eliminated by the acid washing treatment. In addition, MTS-1(3)-AS and MTS-1(3)-AT-2 both show the peaks at 1465 cm-1, 1635 cm-1 and 2940 cm-1, assigning to the C-H vibrations of TPA+ and PDDA, respectively.30, 31 This result confirms that the microporous and mesoporous organic templates are coexistence in the zeolites. Furthermore, a wide peak owning to the adsorption of water on the surface hydroxyl groups in the range from 3300 cm-1 to 3600 cm-1 is


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observed in spectra of MTS-1(3)-AS and MTS-1(3)-AT-2. The result also demonstrates that the high hydrophilicity of the two samples because of more silanol groups existing on the external surface without the calcination process.32

Fig. 3 UV-Vis spectra of MTS-1-AS with different mass ratios of PDDA/SiO2 (A); MTS-1(3)AT with different concentration of HCl aqueous solution (B) The UV-Vis spectra of MTS-1-AS with different mass ratios of PDDA/SiO2 and a series of MTS-1(3)-AT with different concentrations of HCl aqueous solution are represented in Fig. 3. All samples exhibit an absorption peak around 210 nm belonging to the charge transfer of O 2p electron to the empty 3d orbit of framework Ti atoms, which is the characteristic of isolated tetrahedral coordination Ti atoms. As shown in Fig. 3 (A), MTS-1-AS with different mass ratios of PDDA/SiO2 shows absorption peaks centered at 260-300 nm, which is attributed to pentacoordinated Ti atoms originated from the interaction of framework Ti atoms with water or hexacoordinated Ti atoms resulting from partially polymerization of non-framework Ti atoms by the formation of Ti-O-Ti bands.33 In particular, MTS-1(4)-AS has more obvious absorption bond at 260-300 nm suggesting the sample has more extra-framework Ti species than others, which



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might be caused by the excess PDDA. Besides, for the sample MTS-1(3)-AS, the weak signal near 330 nm represents the absence of antatase TiO2 in these samples, which is consistent with the result of XRD analysis. Fig. 3 (B) shows that the intensity of the bond at 285 nm for MTS1(3)-AT decreases slightly compared with that of MTS-1(3)-AS, which indicates some extraframework Ti species are removed by acid washing treatment, thus results in the increase of the relative content of framework Ti species. Moreover, the XRF test results show obvious difference of the ratio of Si/Ti between MTS-1(3)-AS (52.2) and MTS-1(3)-AT-2 (61.0), which is another evidence to prove that acid treatment can remove some extra-framework Ti species. Fig. 4 shows the N2 adsorption-desorption isotherms and pore size distributions of the samples before and after acid washing treatment. Due to the organic template molecules block the zeolite channels, leading to the two samples show a low N2 adsorption amount (Fig. 4. A). However, they still exhibit characteristic of type IV adsorption-desorption curves, which implies the existence of mesopore. The mesopore size distributions of the samples are calculated from the desorption branch of the isotherms based on the BJH model. The calculated results show that the two samples present mesopore size distributions in the range of 5-20 nm (Fig. 4. B). The molecular weight of PDDA is about 4×105 and its size is estimated at 5-40 nm according to the degree of polymerization, which is accord with the dimensions of the mesopores obtained from N2 analysis. Just as found in earlier research, the PDDA can be incorporated in the zeolite structure during crystallization and produce a new class of hierarchical zeolites that feature 3D continuous zeolitic frameworks with highly interconnected intracrystalline mesopores.34,

35

Moreover, MTS-1(3)-AT-2 shows a slightly higher values in Brunauer-Emmett-Teller surface areas (SBET=61.79 m2g-1) and total pore volume (Vtotal=0.13 cm3g-1) compared with MTS-1(3)AS (SBET=55.73 m2g-1, Vtotal=0.12 cm3g-1), which can be attributed to the impurities on the


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external surface or at the pore entrance of the zeolite are washed away by acid treatment. In a word, the acid treatment process does not change the textural properties of MTS-1(3)-AS obviously.

Fig. 4 N2 adsorption and desorption isotherms (A) and pore size distributions (B) of MTS-1(3)AS and MTS-1(3)-AT-2 13

C CP/MAS NMR characterization of MTS-1(3)-AS and MTS-1(3)-AT-2 are performed

(Fig. 5). The spectra clearly demonstrate that the two samples exhibit the peaks at 54, 71, 39 and 29 ppm associating with PDDA molecules and do not show the bands at 44, 47 and 64 ppm assigning to the product of the Hoffman degradation. These results indicate that PDDA is quite stable during crystallization and acid treatment process. So the PDDA are incorporated in the samples during crystallization to act as a “porogen” on the mesoscale and basically cannot be removed during acid treatment. In addition, the intensive peaks at 10.7, 11.7, 16.9 and 63.9 ppm in the spectra can be owned to TPA+ cation species, which indicate the existence of the microporous templates in the samples. In addition, the splitting at 10.7 and 11.7 ppm is due to the



differences in the Van der Waals interactions between the protons of the methyl group and the silicon species in the framework position.36

Fig. 5 13 C CP/MAS NMR spectra of MTS-1 (3)-AS and MTS-1(3)-AT-2 The TGA curves of the samples before and after acid washing treatment are shown in Fig. S1. They both show marked weight loss in the region of 473-923 K, corresponding to the decomposition of PDDA and TPAOH. The difference in weight loss between MTS-1(3)-AS (29.2 wt. %) and MTS-1(3)-AT-2 (26.0 wt. %) illustrates that the acid treatment process can effectively remove part of the organic species physisorbed on the external surface or loosely occluded in the mesopores channel, thus result in about 11.0 wt. % relative weight loss for the original organic species in as-synthesized samples. The remaining organic species are presumed to be immobilized stably in the zeolite.


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Fig. 6 SEM images of as-synthesized and the acid treatment samples: (A) MTS-1(3)-AS; (B) MTS-1(3)-AT-0.5; (C) MTS-1(3)-AT-2 and TEM image of MTS-1(3)-AT-2 (D) The SEM images of a series of MTS-1 are shown in Fig. 6 (A-C). It can be clearly illustrated that MTS-1(3)-AS shows the rod-like morphology and some crystals are aggregated to form bulk, which can be attributed to the strong interaction between electropositive PDDA and negatively-charged silicate species and the existence of PDDA enhances the intergrowth of crystal particles. The morphology of the sample hardly changes after acid treatment (Fig.6 (B, C)). Otherwise, the high magnification TEM image confirms the existence of mesopores in MTS-1(3)-AT-2 samples. The disordered mesopores can be easily observed in Fig. 6 (D), coinciding with the N2 adsorption-desorption result. This result indicates that generated mesopores are intracrystalline pore instead of the interparticle void. It is interesting that these mesopores are partially continuous and opened to the external surface of the material. This feature is favor of the interaction between the reactants and active sites and the diffusion of the products.



CO2 is usually considered to act as a Lewis acid, thus its adsorption followed by FT-IR analysis is an effective technique for characterization of basic sites of catalysts.37, 38 In this study, in order to check whether or not the synthesized material possesses basic sites available for chemical reaction, MTS-1(3)-AT-2 adsorbed CO2 and was in-situ characterized by FT-IR. As shown in Fig. 7, the peak near 2345 cm-1 is associated with free CO2. The new peak at 1057 cm-1 after the sample adsorbing CO2 can be assigned to the interaction of amine groups of organic templates with CO2 to form carbonate species.39,

40

The result shows that the material could

adsorb and interact with CO2 due to the existence of basic sites.

Fig. 7 FT-IR spectra of MTS-1(3)-AT-2 before (A) and after (B) interaction with CO2 CO2-TPD measurement is also used to study the basicity of a series of samples before and after acid treatment (Figure S2). Otherwise, considering the stability of organic template in MTS-1-AS, CO2-TPD is carried out less than 473 K. All samples exhibit obvious desorption peaks around 423 K, indicating that these materials contain weak basic sites. The CO2-TPD profiles imply the accessible basic sites to CO2 guest molecule rather than all basic sites in the samples. Undoubtedly, the existences of organic templates provide these materials with a lot of N-containing species which are accessible to CO2 and can be used as active sites. In addition,


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basing on the integrated peak areas, the accessible basicity of these samples increased in the order: MTS-1(1)-AS (247 µmol gcatal.−1) < MTS-1(2)-AS (283 µmol gcatal.−1) < MTS-1(3)-AS (308 µmol gcatal.−1) < MTS-1(3)-AT-2 (340 µmol gcatal.−1). The calculated results show that the accessibility of basic sites and the capacity of CO2 adsorption enhances with the increased amount of PDDA. More importantly, it also shows that acid treatment can indeed improve the amount of accessible basic sites. The conventional Hammett indicator method is widely used for studying the acid strength of solid material. Moreover, Benesi also introduces a kind of Hammett indicator method with noaqueous butyl-amine titration.41 This method is used to quantitatively analyze the acid strength of zeolites.42, 43 Herein the modified Hammett indicator method was used to investigate the acid strength of the samples and the results of the acid amounts with different strength are showed in Table 1. It can be clearly illustrated that MTS-1(3)-AT-2 possesses more accessible acid sites because the acid treatment can remove the extra-framework Ti species and impurities on the external surface and at pore entrance of the zeolite. Table 1 Acid strength distribution of MTS-1(3)-AS and MTS-1(3)-AT-2 Catalyst

Acid amount(mmol/g) H0≤+4.8

H0≤+6.8

MTS-1(3)-AS

0.025

0.040

MTS-1(3)-AT-2

0.055

0.10

3.2 Catalytic performance in cycloaddition of CO2 3.2.1 The role of framework Ti and organic template species in cycloaddition of CO2 The catalytic performance of various catalysts for the cycloaddition of epichlorohydrin (ECH) and CO2 is investigated and the results are summarized in Table 2. ECH scarcely reacts



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with CO2 in the absence of catalyst (entry 1, Table 2). Then various ECH conversions are obtained using different catalysts and the main reaction product is cyclic carbonate. Table 2 Cycloaddition of CO2 and ECH catalyzed by different catalysts Entry

Catalyst

Conversion/%

Selectivity/%

TOF(h-1)

1

no

trace

100.0

-

2

CTS-1

23.7

92.4

-

3

MS-1-AS

66.3

96.9

-

4

CTS-1-AS

40.5

97.5

-

5

MTS-1(3)-AS

79.7

97.7

33.2

6

MTS-1(3)-AT-2

97.8

98.0

40.0

Reaction conditions: temperature, 393 K; CO2 pressure, 1.6MPa; reaction time, 6 h; catalyst amount, 300 mg; substrate, ECH, 2 mL; solvent, acetonitrile, 22 mL. In order to clarify the role of framework Ti in the cycloaddition of CO2 with ECH, CTS-1 is used as the catalyst and a ECH conversion of 23.7% is obtained, which suggests that framework Ti species in TS-1 can appropriately catalyze the reaction (entry 2, Table 2). Otherwise, the lower conversion of ECH over MS-1-AS without Ti species compared with that of MTS-1(3)-AS also confirms the catalytically active role of the framework Ti species in the cycloaddition reaction (entry 3 and 5, Table 2). The role of microporous and mesoporous organic templates is also studied (entry 2, 4 and 5, Table 2). CTS-1-AS containing microporous template shows a higher conversion of ECH compared with CTS-1, which is owing to the existence of small amount of TPA+ cations locating at pore entrance and on external surface of the zeolite that can work as active sites to promote the reaction (entry 2 and 4, Table 2). However, most of the TPA+ cations are located at the intersection of the two 10-MR channels and inaccessible to guest molecules, thus limits their


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effective utilization. Otherwise, CTS-1-AS exhibits obviously lower conversion of ECH compared with MTS-1(3)-AS, which can be assigned to the existence of PDDA species in the latter material. PDDA can not only work as active sites but also produce mesopores to enhance the accessibility between active sites and reactants (entry 4 and 5, Table 2). Meanwhile, the greater increase on conversion of ECH over MTS-1(3)-AS also suggests that PDDA as basic sites plays the major role comparing with TPA+ cations for the cycloaddition reaction. It is worth noting that MTS-1(3)-AT-2 shows the best conversion of ECH among these catalysts and the higher TOF value than MTS-1(3)-AS (entry 5 and 6, Table 2). The reason is owned to some extra-framework Ti species and impurities on the external surface and at pore entrance of the zeolite are removed through acid treatment, thus the sample reveals more acid and basic sites. This can be confirmed by the result of Hammett indicator method and CO2-TPD, respectively. 3.2.2 Effect of the amount of PDDA (basic sites amount) in MTS-1-AS MTS-1-AS with different amount of PDDA is prepared and their catalytic performance in the cycloaddition of CO2 with ECH is shown in Fig. 8. All the catalysts present high selectivity of cyclic carbonate. The conversion of ECH enhances from 37.2% to 62.2% when mass of PDDA increases from 2 to 4 g (Fig. 8 (left)). The more PDDA in the MTS-1-AS sample, the more basic sites it possesses (Fig. 8 (right)). The enhanced basic sites in the catalyst are beneficial to the activation of CO2, leading to the increase of the ECH conversion. However, more extra-framework Ti species generate with excess PDDA being added in the synthesis of MTS-1(4)-AS (Fig. 3), which results in the activity of as-obtained catalyst decreases.



Fig. 8 Effect of the amount of PDDA in MTS-1-AS on its performance in the cycloaddition of CO2 (left) and the basic amount on the conversion of ECH (right) Reaction conditions: temperature, 373 K; CO2 pressure, 1.6MPa; reaction time, 6 h; catalyst amount, 300 mg; substrate, epichlorohydrin, 2 mL; solvent, acetonitrile, 22 mL. 3.2.3 Effect of HCl treatment to MTS-1(3)-AS MTS-1(3)-AS is treated with different concentrations of HCl solution and as-obtained samples are used as catalyst in the cycloaddition of CO2 with ECH (Figure S3). HCl solution can effectively remove the organic species physical adsorbed on the external surface or incompact occluded in the mesopores channel as well as the amorphous Ti species, thus the amount of accessible acid sites and basic sites both increases obviously (Table 1, Fig. S2). Hence, the conversion of ECH increases with the increased concentration of HCl solution to 2 M. Otherwise, further enhancing the concentration of HCl solution has less influence on the performance. On the basis of the plot of the conversion of ECH via the amount of basic sites in the catalysts (Fig. 8 (right)), the amount of basic sites seem to have the decisive effect on the activity


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in the cycloaddition of CO2. For MTS-1 in this paper, most basic sites originate from PDDA rather than TPA+ because most of the TPA+ cations are inaccessible to guest molecules. With the increase of the mass of PDDA in the synthesis, the amount of potential basic sites increases. The HCl treatment can improve the accessibility of these basic sites, thus the activity in the cycloaddition of CO2 is enhanced. 3.2.4 Effect of reaction parameters The reaction parameters (reaction temperature, CO2 pressure and reaction time) are optimized using MTS-1(3)-AT-2 as the catalyst. The effect of temperature on the cycloaddition reaction of CO2 and ECH is summarized (Fig. S4 (A)). The selectivity of cyclic carbonate is almost invariable, while the conversion of ECH increases from 35.5% at 353 K to 97.8% at 393 K, which is due to the high temperature making the activation of C=O bond in CO2 more easier. No further improvement in the conversion of ECH is derived when enhancing the temperature to 413 K，because the lower dissolution of CO2 at higher temperature would lead to an insufficient supply of CO2 molecules in the liquid phase.44 Similarly, the influence of CO2 pressure on the conversion of ECH is evaluated (Fig. S4 (B)). Variation of the CO2 pressure from 0.2 MPa to 1.6 MPa induces a continuous increase for the conversion of ECH from 54.3% to 82.2%, but further increasing the pressure has negligible influence on the conversion of ECH. The cycloaddition reaction mainly occurs in the liquid phase in the reaction system. The concentration of CO2 in the liquid phase and the interaction between the gas-liquid two phases both enhanced with increasing pressure, which leads to the increase of the conversion of ECH. However, the formation of CO2-epoxide composite is easier when excess acidic CO2 dissolves in the liquid phase thus hindered the conversion of ECH.45-47



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Finally, the effect of reaction time on the reaction results is also studied. The results illustrate that the conversion of ECH increases smoothly within 6 h (Fig. S4 (C)). However, further prolonging the reaction time to 8 h has no improvement for the conversion of ECH, which is due to the lower concentration of reactants in the late stage of the reaction would lead to the decrease of the reaction rate. In addition, the selectivity of cyclic carbonate remained about 98% throughout the course. Overall, 0.3 g catalyst, 393 K, 1.6MPa CO2 pressure and reaction time of 6 h are the optimal reaction conditions for the MTS-1(3)-AT-2 in ECH-CO2 cycloaddition reaction. 3.2.5 Reaction kinetic investigation The kinetic of this cycloaddition reaction is studied using ECH as substrate with the reaction condition at reaction time of 6 h, 0.3 g catalyst and 1.6MPa CO2 pressure. The common rate formula for cycloaddition reaction of CO2 is given as equation (2). Rate

[] !

k(ECH)( (CO) )* (catalyst)/

(2)

The k represents the rate constant. In this reaction system, the concentration of CO2 at 1.6MPa CO2 pressure can be considered as a constant because the amount of CO2 in this interval is excess. Moreover, the catalyst amount is 0.3 g that could be considered as a constant under the current kinetic investigation; hence equation (2) can be reducible to equation (3). Rate

[] !

k 01 (ECH)(

(3)

The kobs is represented the observed rate constant of this study. The cycloaddition reaction of CO2 is carried out at 393 K by varying the concentration of ECH from 1.02 M to 4.08 M while keeping the catalyst amount and CO2 pressure invariability. In order to confirm the effect of ECH concentration on the cycloaddition reaction rate, the rate is


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plotted versus ECH concentration (Fig. 9). A linear relationship between the reaction rate and the concentration of the ECH is found, thus in equation (3) a=1.

Fig. 9 Plots of reaction rate vs. ECH concentration 3.2.6 Cycloaddition of CO2 with different solvents and epoxies The influence of solvents on the reaction is studied under the same reaction conditions (entry 1-3, Table 3). Acetonitrile and dichloromethane give similar conversion of ECH and selectivity of cyclic carbonate. However, the conversion of ECH drastically reduces when chloroform is used as solvent, which can be owned to the competitive adsorption of chloroform on the active centers of the catalyst. According to this study, acetonitrile is the most appropriate solvent to transfer the reactants to the active sites of the catalyst to produce the corresponding cyclic carbonate. In addition, we also studied the reactive activity of MTS-1(3)-AT-2 using other epoxides as reactants to synthesize corresponding cyclic carbonates (entry 4-6, Table 3). The catalytic system is found to be efficient for propylene oxide (PO) and ECH. However, the styrene oxide (SO) and isobutylene oxide (IBO) show lower conversion among these epoxides, probably because of the low reactivity of the β-carbon atom or the high steric hindrance caused by the existence of



Page 24 of 35

branched chain.48, 49 Meanwhile, the selectivity of cyclic carbonate also declines, which can be attributed to the formation of by-products (such as diol). Table 3 Cycloaddition of CO2 with different solvents and epoxies Entry

Solvent

Epoxide

Conversion/%

Selectivity/%

TOF(h-1)

1

Acetonitrile

ECH

82.2

98.3

33.7

2

Dichloromethane

ECH

80.0

98.9

33.0

3

Chloroform

ECH

56.8

97.6

23.1

4

Acetonitrile

PO

80.1

97.8

32.8

5

Acetonitrile

IBO

35.3

84.4

12.4

6

Acetonitrile

SO

14.7

96.9

5.9

Reaction conditions: temperature, 373 K; CO2 pressure, 1.6 MPa; reaction time, 6 h; catalyst amount, MTS-1(3)-AT-2, 300 mg; substrate, 25.5 mmol. 3.2.7 Recycles of MTS-1-AT-2 catalyst Recyclability is an important and essential feature of the catalyst to be considered for industrial applications. The reusability of MTS-1(3)-AT-2 is examined under the optimal reaction conditions with acetonitrile as the solvent. The conversion of ECH is higher than 92% and the selectivity of the cycloaddition product maintains at 97% in the whole recycling test (Fig. S5). To investigate the structure stability, MTS-1(3)-AT-2 after four cycles is characterized using XRD, FT-IR, N2 adsorption-desorption and EA techniques. Reused catalyst gives similar XRD characteristic reflections of MFI structure as the fresh sample, suggesting that the mesoporous micro/mesoporous structure is very robust (Fig. 1(B)). No obvious change could be found in the FT-IR spectra of reused catalyst at 1465 cm-1, 1635 cm-1 and 2940 cm-1 (Fig. 2), suggesting that the organic composition of the reused catalyst is still reserved. The results of organic elemental analysis (CHN) show that there is no obvious decrease for the content of N element in reused


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catalyst, which confirm that the template agents are stable enough and cannot diffuse out the pore of zeolite. In addition, the reused sample shows similar values in Brunauer-Emmett-Teller surface area (SBET=68.21 m2g-1) and total pore volume (Vtotal=0.14 cm3g-1) compared with fresh one (SBET=61.79 m2g-1, Vtotal=0.13 cm3g-1), suggesting MTS-1-AT-2 has a good stability. The N2 adsorption-desorption isotherms of the reused sample are shown in Fig.S6. The high catalytic activity and recyclability of MTS-1(3)-AT-2 can be attributed to the highly crystalline zeolitic framework that provides sufficient framework Ti and large amount of organic species, and the generation of mesopores that penetrate zeolite framework also offers the fast diffusion channels for the reactants and product get into and depart from the active sites. 3.3 Reaction mechanism

Scheme 1 Plausible mechanism on the cycloaddition of CO2 with ECH over MTS-1(3)-AT-2 Generally speaking, there are three kinds of reaction mechanisms about the cycloaddition reaction of CO2 according to the different active sites (acid sites, base sites or both) in catalyst.5052

For example, C-TS-1 has Ti species only and shows low ECH conversion due to the absence

of base sites. This mechanism referring to the epoxide is activated by framework Ti species and CO2 inserts into the activated intermediate species. Otherwise, for MS-1-AS with basic sites but



no framework Ti species, a mechanism involves that CO2 is activated via the basic sites and inserts into the C-O bond of ECH by nucleophilic attack. In this study, a possible reaction mechanism is proposed for the bi-functional reaction system over the MTS-1(3)-AT-2. As shown in Scheme 1, in the first Step, framework Ti in the catalyst interacts with the oxygen atom of ECH leading to the activation of oxirane ring. Analogously (Step 2), the amine moieties of organic templates interact with the electrophilic carbon atom of the CO2 molecule to furnish carbonate species. In step 3, the carbonate species make the ECH ring opening to form carbamate salts by nucleophilic attack of the less hindered β carbon atom of ECH. In the final step, cyclic carbonate is obtained though intramolecular ring closure and the catalyst is regenerated at the same time. Here, the synergistic effects of the framework Ti in activating the oxirane ring and the generation of carbonate species though the interaction of the amine moieties of the organic templates with CO2 molecule cooperatively result in the formation of the corresponding cyclic carbonate.

4. Conclusion Mesoporous TS-1 is synthesized using TPAOH and PDDA as templates. Not required to be removed by calcination, the organic templates embedding in MTS-1 can take the role of basic sites and activate CO2. Thus the as-synthesized MTS-1 is an effective heterogeneous catalyst for the cycloaddition reaction of CO2 with epoxies. The organic quaternary ammonium salts in MTS-1 cooperatively work with the framework Ti species to catalyze the formation of the corresponding cyclic carbonates. The catalytic activity of MTS-1 can be further improved by treating using HCl solution. The modified Hammett indicator method and CO2-TPD show that both the acidity and the basicity in the material are improved after acid washing. Due to the synergetic effects of multi-functional active sites, MTS-1(3)-AT-2 exhibits a superior catalytic


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performance in the cycloaddition reaction of CO2 with ECH. The conversion of ECH is 97.8% and the selectivity of cyclic carbonate is 98.0% at 393 K under 1.6 MPa CO2 pressure for 6 h when acetonitrile is used as the solvent. Moreover, the organic-inorganic hybrid catalyst is reusable and stable in the cycloaddition reaction. In addition, the study also provides a clue to utilize the organic templates embedded in the materials as active sites in a chemical reaction instead of removing by calcination. Acknowledgment The authors acknowledge the financial support from the Program for New Century Excellent Talents in University (NCET-04-0270). Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Fig. S1 TGA curves of MTS-1(3)-AS and MTS-1(3)-AT-2; Fig. S2 CO2-TPD images of assynthesized and the acid treatment samples: (A) MTS-1(1)-AS; (B) MTS-1(2)-AS; (C) MTS1(3)-AS; (D) MTS-1(3)-AT-2; Fig. S3 Effect of HCl treatment to MTS-1(3)-AS on its performance in the cycloaddition of CO2; Fig. S4 Effect of reaction conditions on the cycloaddition of CO2 with ECH over MTS-1(3)-AT-2; Fig. S5 Reused test of MTS-1(3)-AT-2; Fig. S6 N2 adsorption and desorption isotherms of reused MTS-1(3)-AT-2. ORCID Gang Li: 0000-0003-0741-8023 References [1] Li, C. G.; Lu, Y.; Wu, H.; Wu, P.; He, M. Y.; A Hierarchically Core/Shell-Structured Titanosilicate with Multiple Mesopore Systems for Highly Efficient Epoxidation of Alkenes. Chem. Commun., 2015, 51, 14905-14908.



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Graphical abstract


Mesoporous Titanium-Silicalite Zeolite Containing Organic Templates

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