Synthesis, characterization and catalytic activity of hierarchical ZSM-5 templated by carboxymethyl cellulose

Powder Technology 320 (2017) 412–419

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Synthesis, characterization and catalytic activity of hierarchical ZSM-5 templated by carboxymethyl cellulose R. Sabarish, G. Unnikrishnan ⁎ Polymer Science and Technology Laboratory, Department of Chemistry, National Institute of Technology, Calicut, Kerala 673601, India

a r t i c l e

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Article history: Received 10 November 2016 Received in revised form 14 July 2017 Accepted 15 July 2017 Available online 19 July 2017 Keywords: Hierarchical ZSM-5 Carboxymethyl cellulose Template Catalytic activity Acetalization

a b s t r a c t Hierarchical ZSM-5 zeolite has been successfully synthesized using carboxymethyl cellulose (CMC) as a mesotemplate. The synthesized samples were characterized by using XRD, FT-IR, Al-NMR spectroscopy, SEM, TEM, TG, N2 adsorption–desorption and NH3-TPD. They exhibited characteristic MFI structure, which was confirmed by XRD and FT-IR analysis. 27Al MAS NMR spectra indicated the presence of tetrahedral coordinated Al in the hierarchical zeolite. SEM and TEM images of the sample showed significant mesoporosity. The N2 adsorption/desorption results supported the presence of mesopores. Catalytic activity of the prepared mesoporous ZSM-5 for the acetalization of cyclohexanone with methanol was examined. The effects of molar ratio, catalyst amount and time on cyclohexanone conversion were studied. It has been found that the catalytic activity of mesoporous ZSM-5 catalyst is superior to that of the conventional zeolite for the acetalization of cyclohexanone with methanol. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The presence of a wide range of porosity (micro, meso and macro pores) has made hierarchical zeolites better catalysts than conventional zeolites which possess only micropores [1–4]. Previous reports shows that the hierarchical zeolites offers high reactant diffusion rates, and hence they can effectively execute large molecular reactions such as acetalization, Friedal - Crafts reactions [5–9]. Synthesis of hierarchical zeolites has been successfully done using various strategies including post-treatment, templating method. Post-treatment method involves the dealumination and desilication processes, which then lead to a mesoporous structure in zeolites. In the templating method, various templating agents such as carbon black, surfactants, polymers and inorganic nanoparticles are introduced into the zeolite frame work during synthesis, with the subsequent removal of the templates to generate mesopores in the zeolite crystal [10–18]. Templating method is one of the commonly used methods to introduce mesoporosity in zeolite [19,20]. One of the drawbacks of the templating method is that most of the templates are expensive and not easily available. Thus significant attention is now being given to use less expensive and naturally available templates such as chitosan, starch, cellulose etc. [21]. Shi and co-workers developed hierarchical zeolite using ammonium-modified chitosan as a template [22]. They observed an enhanced catalytic activity for the hierarchical zeolite compared to conventional ZSM-5, for Claisen-Schmidt condensation and ⁎ Corresponding author. E-mail address: [email protected] (G. Unnikrishnan).

http://dx.doi.org/10.1016/j.powtec.2017.07.041 0032-5910/© 2017 Elsevier B.V. All rights reserved.

esterification reaction. A facial route to synthesise mesoporous hierarchical zeolites using starch derived bread as a template was reported by Xiao and coworkers [23]. Recently, Zhang et al. synthesized ZSM-5 zeolite with intracrystal mesopores using soluble starch as an in situ template [24]. Wang et al. successfully fabricated different types of zeolites (silicalite-1, ZSM-5, TS-1) using soluble starch and carboxy methyl cellulose as templates [21]. No detailed effort has yet been made on the carboxy methyl cellulose templated hierarchical zeolite generation and the examination of its catalytic activity on organic reactions. The present work focuses on the development of hierarchical ZSM-5 using carboxylmethyl cellulose as a mesotemplate. The catalyst efficiency of the generated hierarchical ZSM-5 has been confirmed by executing a large molecule reaction, i.e.; acetalization of cyclohexanone with methanol. Usually acetalization reactions are carried out in presence of a corrosive acid catalyst; typically H2SO4 or HCl [25,26]. The present strategy excludes the utilization of the corrosive acid and gives the product in good yield.

2. Materials & methods Tetrapropyl ammonium hydroxide [(CH3CH2CH2)4NOH; TPAOH], tetraethylorthosilicate (C8H20O4Si; TEOS) and aluminium isopropoxide (C9H21AlO3; AIP) were purchased from Sigma Aldrich Co. Ltd. (India). Cyclohexanone (C6H10O), methanol (CH3OH) and sodium carboxymethyl cellulose [C6H7O2(OH)x(OCH2COONa)y]n used were of reagent grade, procured from Merck, and were used without further purification. The molecular weight of CMC used was 700 kDa.

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2.1. Synthesis

2.3. Characterization

The hierarchical ZSM-5 zeolite was synthesized by a hydrothermal crystallisation route. In a typical synthesis, 0.03 g of aluminium isopropoxide (AIP) and 2.11 g of tetrapropylammonium hydroxide (TPAOH) were mixed to obtain a clear solution. 3.46 g of tetraethyl orthosilicate (TEOS) and 7 g of distilled water were added to it under constant stirring for 5–6 h. To the resulting solution (pH = 10), 0.2 g of CMC was added under constant stirring. It was then concentrated at 80 °C in a rotavapor for 20 min to get a transparent sticky solution. It was transferred into an autoclave, and kept at 80 °C for 24 h. The sample was later hydrothermally treated at 175 °C for 6 h. The obtained product was washed with deionized water, dried in air, and calcined at 550 °C for 5 h to remove the organic components. The hierarchical zeolite so obtained has been designated as ZSM-5 CMC. For comparison, another sample using the same procedure but without the mesogen i.e. CMC was also synthesized. The synthesis strategy is shown in Fig. 1.

X-ray diffraction (XRD) patterns were obtained with a Rigaku Miniflex 2200 diffractometer using CuKα radiation. Scanning electron microscopic images were obtained by using a Hitachi SU6600 Variable Pressure Field Emission Scanning Electron Microscope (SEM). FT-IR spectrum was recorded at room temperature using an FT-IR spectrometer Jasco 4700 in the range of 400–4000 cm−1. Thermogravimetric (TG) analysis of the uncalcined zeolite samples (TG) was done using a TGA Instrument Q50 at a heating rate of 10 °C/min in nitrogen. BET surface area and pore size distributions were measured by N2 adsorption–desorption using a Micromeritics Gemini V-2380 surface area analyzer. 27Al MAS NMR spectra were recorded on a Bruker Avance AV 300 spectrometer. Transmission electron microscopic images (TEM) were obtained with a JEOL JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. The temperature programmed desorption (TPD) patterns with ammonia on the samples were recorded on Micromeritics Chemisorb 2750. The Si/Al ratio of the samples was determined by electron dispersive spectroscopy (EDX) using JEOL JED-2300.

2.2. Catalytic reaction Acetalization of cyclohexanone with methanol was carried out in a 50 ml round bottomed flask equipped with a reflux condenser, magnetic stirrer and thermometer. In a typical reaction, 0.04 g catalyst, 4 mol methanol and 1 mol cyclohexanone were stirred vigorously at room temperature for 6 h. The product was collected after removing the catalyst using centrifugation and was analyzed by a Shimdazu gas chromatograph equipped with an FID detector and an Rtx@5 column. Nitrogen was used as the carrier gas. The conversion and product selectivity were calculated as follows: Cyclohexanone conversion ð%Þ ¼

Moles of cyclohexanone converted  100 Moles of cyclohexanone used

ð1Þ

Product selectivity ð%Þ ¼

Moles of diacetal formed  100 Moles of cyclohexanone converted

ð2Þ

3. Result and discussion Fig. 2 (a & b) shows the XRD patterns of ZSM-5 and ZSM-5 CMC samples respectively. Both the samples display the characteristic peaks of MFI structure in the range of 7–10° and 22–35°. Similarity of the XRD patterns of the hierarchical zeolite with the conventional zeolite clearly indicates the successful retention of crystallinity in the sample even in the presence of the template CMC [27].The MFI frame work for mesoporous zeolite was confirmed by FTIR analysis and the spectra are depicted in Fig. 3. The absorption bands at 455 cm− 1 (T\\O bend) 795 cm− 1 (external symmetric stretching), 1100 cm−1 (internal asymmetric stretching) and 1220 cm−1 (external asymmetric stretching) correspond to siliceous materials. The peak at 3640 cm−1 is attributed to the isolated silanol groups (Si\\O\\H) while the peak at 3453 cm− 1 correspond to the Al\\OH framework (Brønsted acid sites). Absorption at 546 cm−1 is due to the five-ring units in the structures of pentasil

0.03g of AIP + 2.11g of TPAOH

3.46g of TEOS + 7g of H 2O

Stirring for 5-6hrs

Clear solution CMC Rotavapor at 80oC

Aging for 24hrs

Hydrothermal treatment at 175oC for 6hrs

Drying

Calcination at 550oC Fig. 1. Flow chart for the synthesis of micro/meso ZSM-5 zeolite.

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Fig. 2. XRD patterns of: a) ZSM-5 b) ZSM-5 CMC.

Fig. 4. 27Al NMR spectrum of ZSM-5 CMC sample.

zeolites like ZSM-5 [28]. It is visible from the spectra that both conventional ZSM-5 and hierarchical ZSM-5 samples show absorption bands at 3640, 3453, 1642, 1225, 1150–1050, 796, 553 and 455 cm−1. The results agree well with the reported FT-IR value for MFI framework structure and hence confirm the characteristic MFI structure in the synthesized sample [29]. 27Al MAS NMR spectroscopy was employed to understand the coordination sites of Al in the sample. The 27Al MAS NMR spectrum of the hierarchical sample exhibits (Fig. 4) a large intense peak at δ = 54 ppm, which is attributed to the tetrahedrally coordinated aluminium in the framework. No visible peak was detected for the octahedrally coordinated site at δ = 0 ppm. Absence of octahedral coordination site further demonstrates the high crystallinity of the synthesized hierarchical sample [30]. Fig. 5a illustrates the adsorption-desorption isotherms of the ZSM-5 CMC. The isotherm corresponding to unmodified ZSM-5 (without CMC) is given at the inset. The N2 isotherm switches from type I (for unmodified) to type IV curve upon modifying zeolite with CMC. The modified sample initially shows high nitrogen uptake, and at a relative pressure

Fig. 3. FTIR spectra of: a) ZSM-5 b) ZSM-5 CMC.

Fig. 5. a) N2 adsorption isotherms of (I) ZSM-5 CMC (II) Inset ZSM-5. b) Pore size distribution of (I) ZSM-5 CMC (II) Inset ZSM-5.

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Fig. 6. SEM images of a) ZSM-5. (b, c & d) ZSM-5 CMC with three magnifications.

of P/P0 = 0.4–0.9, it shows a hysteresis loop; attributed to the capillary condensation of N2. This type of behaviour has been reported for samples containing both micro & mesopores only [30]. This confirms the development of micro/mesoporosity in the structure. The wide pore size distribution in Barrett-Joyner-Halenda (BJH) pore size distribution curve (Fig. 5b) strongly supports the formation of hierarchical zeolite structure with pronounced mesoporosity. The scanning electron micrographs of CMC ZSM-5 at different magnifications are shown in Fig. 6. It is evident from the images that the modified sample contains a range of pores covering micro and meso dimensions, confirming the modification after CMC treatment. HR TEM images of the samples (Fig. 7) have been found to be complementary to SEM images and further confirms the formation of mesopores in the sample. The TEM images of ZSM-5

CMC catalyst shows well-defined lattice fringes, demonstrating its high crystallinity (inset of Fig. 7). The electron dispersive spectroscopy (EDX) was performed to identify the Si/Al ratio of ZSM-5 CMC (Fig. 8).The EDX of the samples indicates that the sample possesses a Si/Al ratio of 93. A proposed mechanism for the formation of micro/ mesoporous ZSM-5 zeolite is shown in Scheme 1 [22,31]. The NH3-TPD was used to characterize the acidity of samples. Acidity patterns of ZSM-5 and ZSM-5 CMC are presented in Fig. 9. It evident that both samples display similar acidity patterns with two desorption peaks at b350 °C and N350 °C due to weak and strong acidic sites respectively. The minor acidity reduction for the modified catalyst, compared to the unmodified one, can be attributed to the formation of mesopores in the former. A similar observation has been reported earlier [31].

Fig. 7. TEM images of ZSM-5 CMC.

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Fig. 8. EDX spectrum of ZSM-5 CMC.

The results of the thermogravimetric analysis of ZSM-5 and ZSM-5 CMC are given in Fig. 10. The TG of ZSM-5 catalyst displayed an initial weight loss at 100 °C due to removal of physically adsorbed water molecules from the surface of the sample. The weight loss in the range of 400-500 °C could be due to the decomposition of structure directing agent TPAOH. The hierarchical zeolite i.e.; ZSM-5 CMC exhibits three distinct weight losses at 100, 200–250 and 350–500 °C which are attributed to the removal of physically adsorbed water molecules, degradation of the template-CMC and the decomposition of structure directing agent TPAOH respectively. (TG of CMC alone is presented in the supplementary data). (See Fig. 11.) 3.1. Catalytic performance of hierarchical zeolite for acetalization reaction The catalytic activity of synthesized hierarchical zeolite was tested for a large molecular reaction viz.; acetalization of cyclohexanone by methanol. Reaction conditions such as molar ratio of reactants, catalyst amount and time were optimised to obtain better yield for the reaction. The conversion of cyclohexanone to 1,1 dimethyl acetal a conventional zeolite catalyst happens through an acid catalysed route; with H+ being generated from zeolite (inset of Scheme 2). Owing to its acidic nature, zeolite initiates the reaction by protonating cyclohexanone. The protonated form then reacts with methanol to form hemiacetal, which on deprotonation forms an intermediate. The intermediate upon

Fig. 9. NH3-TPD patterns of a) ZSM-5 b) ZSM-5 CMC.

Fig. 10. TG curve of: a) ZSM-5. TG curve of: b) ZSM-5 CMC.

Fig. 11. TG/DTG curve of Carboxymethyl cellulose (CMC).

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TPAOH

417

H2O

Stirring

AIP

TEOS

Microporous

CMC

Calcination

Crystallisation

Mesoporous

Scheme 1. Formation of micro/mesoporous in ZSM-5.

dehydration and subsequent reaction with methanol forms the final product 1,1 dimethyl acetal. When the reaction is done with hierarchical zeolites a physical effect also supports the conversion because of the generation of mesopores. The mesopores (with bigger size compared to the micropores in an unmodified sample) offer better anchoring sites for the reactant. This facilitates more closer interaction of the reactant molecules with H+ ions and thus the catalytic efficiency is increased. A schematic representation of structure–activity relationship of conventional and hierarchical zeolites is depicted in Scheme 3. Hierarchical zeolite with mesopores could increase the diffusion of cyclohexanone and thus could enhance the yield of 1,1 dimethyl acetal.

3.1.1. Influence of molar ratio of the reactants Table 1 shows the effect of molar ratio of cyclohexanone to methanol on the conversion of cyclohexanone. Initially, with an increase in concentration of methanol from 1:1 to 1:4 the rate of reaction increases. However, at 1:5 M ratio the rate decreases, probably due to flooding of active sites with alcohol rather than ketone. Upon increasing the concentration of the ketone alone, (Table 1b) a decrease in the reaction rate has been observed. This can be attributed to the lesser accessibility to the reaction centres by the reactant (a crowding effect) due to its big size. The optimum concentration has been found to be 1:4 (cyclohexanone:methanol).

Scheme 2. Mechanism of acetal formation in presence of ZSM-5(H+ generation of zeolite shown in inset) [32].

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Scheme 3. Structure–activity relationship of conventional and hierarchical ZSM-5 zeolites.

Table 1 Effect of molar ratio on the acetalization of cyclohexanone. Table.1a Sl No

Molar ratio

Conversion of cyclohexanone (%)

Selectivity of 1,1 dimethylacetal

1 2 3 4 5

Cyclohexanone:methanol(1:1) Cyclohexanone:methanol (1:2) Cyclohexanone:methanol (1:3) Cyclohexanone:methanol (1:4) Cyclohexanone:methanol (1:5)

37 42 62 68 65

100 100 100 100 100

Table.1b Sl no.

Molar ratio

Conversion of cyclohexanone(%)

Selectivity of 1,1 dimethyl acetal

Reactant/catalyst ratio

1 2 3 4 5

Cyclohexanone:methanol(1:1) Cyclohexanone:methanol (2:1) Cyclohexanone:methanol (3:1) Cyclohexanone:methanol (4:1) Cyclohexanone:methanol (5:1)

37 35 32 29 25

100 100 100 100 100

24.5:1 49:1 73.5:1 98:1 122.5:1

3.1.2. Influence of catalyst loading The catalyst loading was optimised by varying the catalyst amount from 0.02 to 0.08 g. The effect of catalyst loading on cyclohexanone conversion is shown in Table 2. Upon increasing the amount of catalyst from 0.02 to 0.04 g the rate of conversion increases from 51% to 68%.

This is due to the availability of more surfaces for the acetalization reaction. With a further increase in the amount of catalyst, the rate is found to be almost constant. Therefore 0.04 g of the catalyst is proposed as the optimum amount to obtain the maximum conversion of cyclohexanone.

Table 3 Effect of time on acetalization.

Table 2 Effect of catalyst loading on the acetalization of cyclohexanone. Sl. no.

Catalyst amount(g)

Conversion of cyclohexanone (%)

Selectivity of 1,1 dimethylacetal

1 2 3 4

0.02 0.04 0.06 0.08

51 68 68 68

100 100 100 100

Sl. no.

Time(hrs)

Conversion of cyclohexanone (%)

Selectivity of 1,1 dimethylacetal

1 2 3 4 5

2 4 6 8 10

48 53 68 69 69

100 100 100 100 100

R. Sabarish, G. Unnikrishnan / Powder Technology 320 (2017) 412–419 Table 4 Overall effect of catalyst on acetalization. Sl. no.

Catalyst

Conversion of cyclohexanone (%)

Selectivity of 1,1 dimethylacetal

1 2 3

Without ZSM-5 CMC ZSM-5

18 39 68

100 100 100

Table 5 Reusability of hierarchical ZSM-5 catalyst for acetalization. Catalyst

Conversion of cyclohexanone (%)

Selectivity of 1,1 dimethyl acetal

Fresh 1st 2nd 3rd

68 67 66 64

100 100 100 100

3.1.3. Influence of reaction time The effect of reaction time on the acetalization of cyclohexanone has also been investigated and the results are shown in Table 3. It can be seen from the table that the conversion of cyclohexanone gradually increases to 48% in 2 h, to 53% in 4 h, to 68% in 6 h with time. However, after 6 h the rate remains practically constant. 3.1.4. Influence of type of catalyst A comparison of percentage conversion of cyclohexanone without catalyst, with unmodified zeolite and with modified zeolite is shown in Table 4. It is evident that the hierarchical zeolite executes a higher cyclohexanone conversion. The catalyst has been found to be re-usable, at least three times with washing with water and subsequent drying after every cycle of operation (Table 5). 4. Conclusion Hierarchical ZSM-5 zeolite was successfully synthesized using a biodegradable polymer, CMC, as a mesotemplate. The crystallinity and MFI structure of the samples were assessed by XRD and FT-IR. The mesoporosity of the modified sample was confirmed by N2 adsorption, SEM and TEM examination. The prepared catalyst was tested for acetalization reaction. The results suggest that the hierarchical ZSM-5 exhibits better catalytic performance than conventional ZSM-5 for the acetalization of cyclohexanone with methanol. The utilization of the modified catalyst for cyclohexanone conversion into 1,1 dimethyl acetal is a promising strategy in view of its environment – friendly performance. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgment We thank the Science & Engineering Research Board, New Delhi, India, for funding this research. [File No: SR/S1/OC-40/2011 dated 15/5/2012]. References [1] Y.S. Tao, H. Kanoh, L. Abrams, K. Kaneko, Mesopore-modified zeolites: preparation, characterization, and applications, Chem. Rev. 106 (2006) 896–910. [2] J. Cejka, S. Mintova, Perspectives of micro/mesoporous composites in catalysis, Catal. Rev. Sci. Eng. 49 (2007) 457–509.

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