Preparation of hydrophobic MFI zeolites containing hierarchical micro-mesopores using seeds functionalized with octyltriethoxysilane

Journal Pre-proof Preparation of hydrophobic MFI zeolites containing hierarchical micro-mesopores using seeds functionalized with octyltriethoxysilane Sara Novak, Thiago Faheina Chaves, Leando Martins, Celso Valentim Santilli

PII:

S0927-7757(19)31101-X

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124109

Reference:

COLSUA 124109

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

8 August 2019

Revised Date:

9 October 2019

Accepted Date:

12 October 2019

Please cite this article as: Novak S, Chaves TF, Martins L, Santilli CV, Preparation of hydrophobic MFI zeolites containing hierarchical micro-mesopores using seeds functionalized with octyltriethoxysilane, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124109

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Preparation of hydrophobic MFI zeolites containing hierarchical micro-mesopores using seeds functionalized with octyltriethoxysilane Sara Novaka, Thiago Faheina Chavesa, Leando Martinsa, Celso Valentim Santillia* a

State University of São Paulo (UNESP), Institute of Chemistry,

ro of

Rua Prof. Francisco Degni 55, Araraquara, SP, 14800-060, Brazil

Correspondence to: Celso Valentim Santilli (E-mail: [email protected])

ur

ABSTRACT

na

lP

re

-p

Graphical abstract

Jo

In this work, we report the synthesis of hydrophobic MFI zeolites presenting a mesopore size distribution and high specific external area (~288 m2 g-1), employing surface functionalization with octyltriethoxysilane (OTEOS). Previously, the MFI seeds were generated from a clear solution, followed by grafting with OTEOS and crystallization by hydrothermal treatment. The

modifications of the properties of MFI synthesized with different amounts of OTEOS (0-15 mol%) were confirmed by X-ray powder diffraction, thermogravimetric, and elemental analyses. Contact angle (θ ~140o) and

29

Si

NMR (T3 signals) measurements demonstrated that OTEOS was effectively grafted on the zeolite surface, resulting in powders with hydrophobic profiles. The grafting, which was mostly on the external surface, was crucial for avoiding the

ro of

aggregation of MFI crystals during the synthesis, since the OTEOS chains (C8) caused steric hindrance, while allowing fine-tuning of the intercrystalline mesopore volume and increasing the external surface area. This effect was less

-p

pronounced when grafting of the MFI zeolites was carried out using organosilanes

re

with longer carbon chain lengths (C12 and C16). This method enables the easy

lP

preparation of functionalized zeolitic materials, where the hierarchical pores and hydrophobic surface are essential characteristics for advanced technological

na

applications.

Keywords: Organosilane, functionalized zeolite, hydrophobic powder,

Jo

ur

hierarchical porosity.

1.

Introduction Zeolites are microporous crystalline aluminosilicates that can be employed

as adsorbents [1,2], ion-exchangers [1,3], biomaterials [4], and catalysts, among other uses [5-8]. Due to the sub-nanometer sizes of the micropores of these materials, there are diffusional restrictions on the transport of bulky molecules to active sites inside the solid, which has hindered exploitation of the potential of

ro of

zeolites [9]. In order to overcome this drawback, recent studies have focused on the introduction of mesopores (2-50 nm) or macropores (>50 nm) that interconnect the micropores of zeolites, resulting in a hierarchical porosity [10-

-p

13]. Hierarchical pores increase the access of reactants to active sites,

re

consequently improving the performance of zeolites in various applications [10,14,15].

lP

Among the strategies that can be used to introduce additional porosity,

na

organosilanes with hydrolysable methoxysilyl or ethoxysilyl groups attached at the end of the carbon chain have attracted attention, due to their unique properties

ur

[16]. The presence of the inorganic portion provides the organosilanes with chemical affinity that enables them to be grafted onto the surface of the zeolite,

Jo

while the carbon chains govern the formation of additional porosity during the synthesis [9,17,18]. Organosilane templates include silylated polymers, cationic surfactants, and neutral molecules [19]. The size of the templated pores depends on the polymer molecular weight, size, and nature of the chain components [18]. Additionally, the covalent bond formed with the zeolitic phase ensures strong

interaction, avoiding the formation of segregated phases containing micro- and mesopores in separated zones [17]. More recently, a material presenting hierarchical porosity associated with organosilane functionalization was demonstrated to have a useful application as an antireflective coating [20]. The high transparency of this coating based on zeolites grafted with 1H,1H,2H,2H‐ perfluorooctyl‐triethoxysilane (POTS) resulted from the combination of the

ro of

hierarchical porous structure and the hydrophobic profile presented by the functionalized material. On the other hand, Serrano et al. [21] demonstrated that some organosilanes presented low reactivity and had little effect in modifying the

-p

zeolite properties. Srivastava et al. [22] evaluated the use of different

re

alkyltriethoxysilanes (alkyl = methyl, propyl, or octyl) in the synthesis of MFI

lP

zeolites and suggested that organosilanes with long alkyl chains may be less effective for mesopore formation, due to their highly hydrophobic nature.

na

In this work, evaluation was made of the effect of the amount of octyltriethoxysilane (OTEOS) used in the synthesis on the porous structure and

ur

wettability of MFI zeolites. This neutral organosilane has been used previously

Jo

for functionalization of silica surfaces [23-26], but has been little explored for the modification of zeolites. Here, we demonstrate that despite of its highly hydrophobic nature, OTEOS (C8) can be grafted mainly on the MFI zeolite surface, allowing fine tuning of the specific volume of mesopores, in the range from 0.101 to 0.222 cm3 g-1. Evaluation was made of the chemical modifications on the MFI zeolite, considering the organosilane amount and carbon chain length

(C8, C12, or C16), using X-ray powder diffraction and solid-state NMR analyses, together with contact angle measurements (to determine the water wettability of the as-synthesized powders). Finally, investigation was made of the effect of the amount of OTEOS on the thermal decomposition profiles of the as-synthesized samples and the porous textures of the calcined zeolites.

2.

Materials and Methods

ro of

2.1. Synthesis

The MFI zeolites were synthesized based on the procedure described by

-p

Serrano et al. [27,28]. The chemicals used were as follows: tetraethyl orthosilicate (Si(OC2H5)4, TEOS, 98 wt%, Sigma-Aldrich), aluminum isopropoxide

re

(Al[OCH(CH3)2]3, AIP, StremChemicals), tetrapropylammonium hydroxide

and

deionized

water.

lP

((CH3CH2CH2)4N(OH), TPAOH, 1.0 mol L-1 aqueous solution, Sigma-Aldrich), The

organosilanization

agents

used

were

na

octyltriethoxysilane (CH3(CH2)7Si(OC2H5)3, OTEOS, 97.5%, Sigma-Aldrich), dodecyltriethoxysilane (CH3(CH2)11Si(OC2H5)3, C12, ≥95.0%, Sigma-Aldrich),

ur

and hexadecyltrimethoxysilane (CH3(CH2)15Si(OC2H5)3, C16, ≥85%, Sigma-

Jo

Aldrich).

The MFI zeolite precursor solution was prepared with the following molar

composition: 1Al2O3:60SiO2:11TPAOH:1500H2O. Firstly, water, AIP, and TPAOH were added (in this order) to a reaction vessel that was then closed and kept at 40 oC for 30 min under vigorous stirring. TEOS was then added and the

closed system was maintained under stirring at 40 oC for 24 h, for formation of the seeds (precrystallization step). In the grafting step OTEOS was added in a molar ratio of 5, 10, or 15%, relative to the molar silica content in the composition, and the system was maintained for 6 h under the same previous conditions (grafting step). The resulting solution was transferred to a Teflon-lined stainless steel autoclaves and maintained for 7 days at 150 oC under autogenous pressure

ro of

and stirred by tumbling at 60 rpm (crystallization step). After cycles of centrifugation and washing with deionized water until pH reached ~8, the solid product was dried at 60 oC for 48 h followed by calcination at 550 oC for 6 h in

-p

air atmosphere. As a reference, one sample was synthesized without the grafting

re

step. In order to demonstrate the comprehensive nature of this approach, syntheses

lP

were also performed using dodecyltriethoxysilane and hexadecyltrimethoxysilane as organosilanization agents, at fixed molar ratios of 5% (the results are shown in

na

Appendix B of the Supplementary Data).

2.2. Characterization

ur

The formation of the crystalline zeolite phase was followed by powder X-

Jo

ray diffraction (XRD), using a Siemens Diffrac Plus Commander D5000 diffractometer operating with Cu Kα radiation (λ = 1.5405 Å) monochromatized with a curved carbon single crystal. The diffractograms were recorded in steps of 0.02o, with an acquisition time of 4 s, in the 2θ range from 5o to 50o. Each diffractogram was normalized relative to the (501) peak at 2θ of ~23o. The

framework Si/Al ratio was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), using a PerkinElmer Optima 8000 spectrometer. Previously, 100 mg portions of the calcined samples were digested by treatment with an acidic aqueous solution prepared with 1 mL of deionized water, 1 mL of HF (99%, Sigma-Aldrich), and two drops (~0.5 mL) of H2SO4 (98%, Synth). Complementary information about the aluminum chemical 27

Al NMR measurements performed using a

ro of

environmental was obtained from

Bruker Avance III HD 400WB spectrometer operating at 14 kHz, with recycle delay of 2 s and aluminum chloride (AlCl3) as a reference.

and

13

C{1H}

CP-MAS

nuclear

magnetic

resonance

(NMR),

re

MAS

-p

The grafting of the zeolites was investigated using solid state 29Si{1H} CP-

lP

thermogravimetric analysis (TG-DTG), elemental analysis (CHN), and contact angle measurements. The solid-state NMR analyses were performed at room

na

temperature, using a Bruker Avance III HD 400WB spectrometer operating with a magnetic field of 9.4 T. The 29Si CP-MAS spectra were recorded with a 4.5 ms

ur

contact time for cross-polarization, a recycle delay of 5 s, and two-pulse phase-

Jo

modulated (TPPM) spin decoupling. The 13C CP-MAS spectra were recorded with 2.0 ms contact time and 5 s recycle delay. The 29Si and 13C NMR analyses both employed tetramethylsilane (TMS) as a reference. The TG-DTG measurements were performed using a TA SDT Q600 instrument, with the sample in an alumina crucible and heating to 1000 oC, at a rate of 10 oC min-1, under a flow of oxygen gas supplied at a rate of 100 mL min-1. Elemental analyses were carried out using

a PerkinElmer CHNS/O analyzer operating with oven and reduction coil temperatures of 1000 and 500 oC, respectively. The water droplet contact angle measurements of the as-synthesized compacted powders were performed using a Dataphysics OCA instrument. Each contact angle was obtained as the average of three measurements of a 7 µL droplet of deionized water. The powder morphology was observed by scanning electron microscopy,

ro of

using a JEOL JSM-7500F microscope operating at 2.0 kV. For this analysis, the calcined powder (~10 mg) was dispersed in 1 mL of water and ultrasonicated for 30 min. Volumes of 10 µL of supernatant were then deposited on substrates, dried,

-p

and sputter-coated with gold for 2 s. The particle size distribution was obtained

re

using ImageJ software [29]. Transmission electron microscopy images were

lP

acquired using the suspensions described previously, but deposited onto carboncoated grids, with analysis using a TECNAI F 20 HRTEM microscope operating

na

at 80 kV.

The textural properties of the calcined powders were assessed by N2

ur

physisorption. The adsorption-desorption isotherms were obtained at 77 K, with

Jo

relative pressures (p/p0) between 0.001 and 0.998, using a Micrometrics ASAP 2010 instrument. Before the measurements, the samples were maintained for 24h under vacuum of 10 µPa, at 200 oC, in order to remove water and other physisorbed gases. The specific external areas (Sext) and micropore volumes (Vmicro) were calculated by the t-plot method [30,31]. The total pore volume was calculated at p/p0 = 0.950 and the mesopore volume (Vmeso) as the difference

between the total volume (Vtotal) and Vmicro. The mesopore size distribution was determined from the desorption branch of isotherms, according to the BJH method [30,32].

3.

Results and Discussion

3.1. Structural characteristics of the as-synthesized zeolites The X-ray diffractograms of all the as-synthesized powders (Fig. 1) showed

ro of

the characteristic diffraction profile of the MFI zeolite. There were no significant differences in peak intensity or width between the bare and chemically modified

-p

zeolites, indicating that the crystalline domain sizes were similar. The same behavior was observed for these samples after calcination (Fig. A.1, Appendix A,

re

Supplementary Data), as well as for the powders synthesized with

lP

dodecyltriethoxysilane (C12) or hexadecyltrimethoxysilane (C16) (Figure B.1, Appendix B, Supplementary Data). As reported previously, organosilanes can act

na

as growth inhibitors, resulting in smaller crystals. On the other hand, some organosilanes have shown little [33] or no effect on size control, such as the

ur

octadecyltrimethoxysilane used by Serrano et al. [21] for chemical modification

Jo

of MFI zeolite. The present results indicated that the use of organosilanes with alkyl chain lengths from C8 to C16 led to similar sizes of MFI crystals. FIGURE 1

In the next step of the work, evaluation was made of the way that the organosilane was linked to the zeolite structure, considering the effects of the

chemical modification on some of the properties of the powders. The chemical bonds between the OTEOS and the zeolite were investigated by solid-state 29Si{1H} CP-MAS NMR (Fig. 2), in order to evidence the presence of organosiloxane species (Tm, where Tm = RSi(OSi)m(OH)3-m, with m = 1-3) and siloxane species (Qn, where Qn = Si(OSi)n(OH)4-n, with n = 1-4). All the samples presented resonance signals at around -111 ppm and -102 ppm, associated with

ro of

the Q4 (0Al) and Q3 species, respectively [34,35]. The zeolites synthesized with OTEOS presented a signal at around -68 ppm, corresponding to the T3 species [36]. This signal confirmed the grafting of OTEOS molecules onto the zeolite by

-p

means of the three ethoxysilyl groups of the organosilane (≡Si-C). The increase

re

of the amount of OTEOS favored increase of the T3 species and a concomitantly

lP

decreased Q3 (as shown by the Q4/Q3 ratios in Table 1), due to preferential grafting of the external surfaces of the zeolite crystals by OTEOS.

TABLE 1

na

FIGURE 2

ur

The presence of OTEOS in the zeolites was also noticed by 13C{1H} CP-

Jo

MAS spectra (Fig. 3). The unmodified C8-0 sample presented signals at 10, 11, 16, and 63 ppm, corresponding to carbon atoms from the propyl chains of the TPA+ cations. The signals at 10 and 11 ppm were both due to methyl groups, with the splitting being attributed to the two distinct chemical environments of propyl chains in MFI zeolite (the sinusoidal and straight channels) [37]. The zeolites synthesized with OTEOS also showed these signals, in addition to resonances at

14, 23, 30, 33, and 34 ppm, attributed to carbon atoms of the OTEOS chain [38]. The absence of signals for carbons from ethoxysilyl groups, at around 58 ppm, suggested that the OTEOS present in the powders was grafted in the zeolite structure [38]. Furthermore, the increased signal intensities for carbon atoms from OTEOS and the increased widths for the carbon from TPA+ indicated that the environments on the exterior surfaces of the MFI crystals became less ordered as

ro of

the amount of organosilane increased. This grafting on the external surfaces of the crystals induced significant textural changes in these calcined samples, as discussed in the next section.

-p

FIGURE 3

re

The CHN and TG analyses (Table 1) confirmed a correlation between the

lP

nominal amount of organosilane and the extent of grafting. Varying the amount of OTEOS from 5 to 15% resulted in total carbon contents of 2.5, 5.9, and 11.7

na

wt% for the C8-5, C8-10, and C8-15 samples, respectively, associated with a gradual increase of the total mass change in the TG analyses. These results were

ur

in agreement with the progressive increases of the T3 signals for the C8-10 and

Jo

C8-15 samples, as well as with the previous work of Serrano et al. [21], where the integrated intensities of the T3 signals (obtained using 29Si MAS NMR) were close to the expected theoretical value for phenylaminopropyltrimethoxysilane (PHAPTMS) added in the zeolite synthesis medium. Hence, the results obtained here supported the notion that easy tuning of the extent of zeolite grafting can be achieved by controlling the nominal amount of organosilane added in the

synthesis medium.

3.2. Tuning wettability by functionalization of the zeolite surface The OTEOS grafting led to considerable differences in the wettability of the powders. The water contact angles (Table 1) evidenced the favorable wettability of the reference sample (Fig. 4), while the grafted zeolites presented

ro of

unfavorable wettability. The contact angle increased with the nominal amount of OTEOS added in the synthesis medium, evidencing the continuous increase of the extent of organosilane grafting on the MFI crystals. The measured contact angles

-p

for the grafted zeolites (θ ~140o) were close to values reported for superhydrophobic materials (θ ~150o) [39]. Therefore, the OTEOS grafting

re

introduced nonpolar entities on the surface, functionalizing it and leading to

lP

highly hydrophobic powders. This finding was in agreement with the previous work by Zapata et al., [40], where contact angles of ~135o were obtained for H-

na

USY zeolites functionalized using organosilanes with different alkyl chain lengths, containing two (C2), six (C6), and eighteen carbons (C18). The authors

ur

also reported hydrophilic characteristics for lower degrees of functionalization,

Jo

independent of the chain length. In the present work, the contact angles not only confirmed the functionalization of the zeolites, but also suggest that OTEOS is a suitable emerging candidate for the preparation of highly hydrophobic MFI zeolites. The samples synthesized with dodecyltriethoxysilane (C12) and

hexadecyltrimethoxysilane (C16) (Fig. B.3, Appendix B, Supplementary Data) presented contact angles of 127o and 38o, respectively. The much lower contact angle for the sample obtained using C16, compared to the sample synthesized with C12, could be attributed to the smaller extent of surface functionalization, as evidenced by

29

Si{1H} CP-MAS NMR analysis (Fig. B.2, Appendix B,

Supplementary Data). Such a difference in functionalization efficiency can be

ro of

explained by the lower reactivity of organosilanes with longer alkyl chains, due to the greater hydrophobicity of the chains. Therefore, the extent of surface functionalization decreased in the order C8 > C12 > C16. This finding was

-p

consistent with previous studies reporting greater extents of functionalization

re

using organosilanes with short alkyl chains, such as C2 and C6 [40], or C4 [21],

lP

compared to use of a longer alkyl chain organosilane (C18). FIGURE 4

ur

zeolites

na

3.3. Thermal decomposition and porous textures of the calcined

The chemical modification of the MFI zeolites with OTEOS altered the two

Jo

main mass loss events observed in the TG and DTG profiles (Fig. 5). In the case of the unmodified C8-0 sample, the first event occurred in the range from 200 to 260 oC, attributed to the removal and oxidation of TPA+ cations present on the external surface of the zeolite [41]. The second region, at around 345-500 oC, could be explained by the removal of TPA+ from the micropores [21], together

with desorption of propylamine, which is one of the TPA+ oxidation products, at around 500 oC. The formation of propylamine is associated with the active sites of the zeolites and its degradation occurs according to Hoffman elimination reactions [42,43]. The chemically modified zeolites presented increased contributions of the first event, accompanied by decreases of the second event, again showing that

ro of

OTEOS was grafted mainly on the external surfaces of the MFI crystals. This was in agreement with the increasing trend of the Q4/Q3 ratio revealed by the NMR analyses, as well as the high contact angle values (Table 1). These results

-p

confirmed the high affinity of OTEOS for the zeolitic phase and its grafting

re

mainly on the external surfaces of the MFI crystals, which was consistent with the

lP

reported high affinity of OTEOS for silica surfaces [23,24]. Another interesting observation was the lower temperature of the second

na

thermal event for the grafted samples. According to an earlier study [44], conventional MFI zeolites present diffusional limitations that hinder the

ur

elimination of TPA+ thermal products from within the micropores, with their

Jo

removal only achieved at temperatures up to 700 oC. On the other hand, the Al content in zeolites also influences the TPA+ decomposition temperature [45]. Here, this contribution could be considered irrelevant, because no significant differences in the framework Si/Al ratio were observed, as can be seen in Table 2 and Fig. A.2 (Appendix A, Supplementary Data). Images of the calcined samples were acquired using SEM (Fig. 6) and TEM

(Fig. 7), in order to demonstrate the correlation between the thermal events associated with TPA+ elimination and decreased diffusional transport limitations promoted by reduced aggregation of the zeolite particles. The SEM and TEM images (Fig. 6 and 7) evidenced the effect of the OTEOS grafting on the morphology of the zeolite crystal aggregates. The non-functionalized sample presented crystal aggregates having regular sizes and well-defined disc-like

ro of

shapes, with average diameter and thickness of ~350 and ~175 nm, respectively, constituted by nanocrystals with 12-30 nm size. The addition of 5% OTEOS resulted in discs aggregates more dispersed, constituted by nanocrystals with 10-

-p

26 nm in smaller discs and with more poorly-defined geometry. Increase of the

re

amount of OTEOS to 10 and 15% led to progressive loss of the disc-like shapes,

lP

with the C8-15 sample presenting irregular aggregates with different sizes and shapes. In addition, the morphological changes in the zeolite discs were well

na

correlated with the lower temperature of the second event in the DTG analysis (Fig. 5(b)). These features provided clear evidence of facilitation of the removal

ur

of organic compounds from within the crystals, due to the shorter diffusion paths

Jo

resulting from their limited aggregation. Therefore, the chemical modification of MFI seeds with OTEOS promoted steric hindrance between the spheroidal particles during the hydrothermal treatment, resulting in improved mass transport from within the MFI channels when these samples were submitted to the thermal processes. FIGURE 6 and FIGURE 7

The effects of aggregate morphology variations on the porous textures of the zeolites were revealed by the adsorption-desorption isotherms (Fig. 8(a)). The unmodified and C8-5 samples presented type I isotherms, according to the IUPAC classification [46], characterized by an intense increase in the adsorbed amount at low relative pressures, followed by an extensive plateau corresponding to the occlusion of micropores by the layers of adsorbed N2 [46]. The increased N2

ro of

volume at high relative pressures could be attributed to capillary condensation of N2 in large voids between the aggregates, which in part depends on the spatial packing of aggregates [47]. However, the addition of 5% OTEOS led to an

-p

increase of the total amount adsorbed (considering the entire N2 isotherm) and the

re

appearance of subtle hysteresis loop for p/p0 > 0.4, characteristic of capillary

lP

condensation of N2 within the mesopores [46]. These modifications are associated with large mesopores (D >10 nm) observed on the BJH mesopores size

na

distribution (Fig. 8(b)), which is due to the enlargement of the intercrystalline spaces, as revealed in the TEM micrographs (Fig. 7). In contrast, this effect was

ur

much weaker in the isotherms for the zeolites synthesized with 5% of C12 and

Jo

C16. The similarity between these isotherms and the one for the unmodified zeolite was in agreement with the lower extent of grafting by the C12 and C16 organosilanes (Figs. B.5 and B.6, Appendix B, Supplementary Data). On the other hand, samples C8-10 and C8-15 both exhibited nearly horizontal hysteresis loops that were similar to the H2 type, associated with interconnected mesopores of different shapes and sizes [48], formed by aggregation of irregular particles. The

pore texture parameters (Table 2) evidenced continuous increases of both total pore volume and specific external area, according to the amount of OTEOS added in the synthesis. The C8-5 sample presented a specific external area around 30% greater than that of the non-functionalized zeolite. Nevertheless, the greatest difference was for the C8-10 and C8-15 samples, which exhibited approximately two and three-fold increases in the mesopore volume and external surface area,

ro of

respectively. According to the BJH distribution (Fig. 8(b)), the mesopore families induced by OTEOS grafting on the C8-10 and C8-15 samples had narrow size distributions centered at 4 nm. It is important to note that the effects of

-p

functionalization on the porous texture correlated well with the disaggregation of

re

the zeolites particles observed by SEM and TEM analyses, as well as with the less

lP

ordered environments of carbon atoms from TPA+, revealed by 13C NMR, and with the low temperature for TPA+ elimination, observed in the DTG analyses.

na

Additionally, the micropore volume (Vmicro =0.20 cm3 g-1) of C8-05, which is close to reported values [27, 49-53] undergoes a notable reduction on C8-10 and C8-15

ur

samples (Table 2). This feature suggests that the mesoporous structure in these

Jo

samples are likewise from a partial transformation of the zeolitic matrix into an amorphous Si/Al oxide due to OTEOS removal, similar to what occurs in desilication processes of zeolites but on a smaller extent [54]. Therefore, the OTEOS grafted mainly on the external surfaces restricted aggregation of the MFI crystals, which increased the external surface area and the mesoporosity, consequently resulting in improved diffusional transport.

FIGURE 8 TABLE 2 These findings were in agreement with previous work reporting the formation of intercrystalline mesopores and a significant increase in the specific external area, following grafting of phenylaminopropyl-trimethoxysilane (PHAPTMS) on MFI seeds [21,27]. On the other hand, the present results

ro of

contrasted with the observations of Srivastava et al. [22], who found little modification of zeolite functionalized with OTEOS, suggesting low interaction between these components. Therefore, the results obtained here showed not only

-p

that the nature of the organosilane can influence the interaction with the zeolite,

re

but also that the synthesis methodology needs to be considered. The findings

lP

demonstrated the viability of using octyltriethoxysilane for the functionalization

na

of MFI, which opens up new possibilities in the field of zeolite functionalization.

4. Conclusions

ur

Chemically modified MFI zeolites were successfully synthesized in the

Jo

presence of different amounts of OTEOS. This organosilane was mainly grafted on the MFI external surface, enabling control of the secondary aggregation of the crystals and leading to the formation of a tunable mesopore volume. The specific external areas of the calcined MFI zeolites were markedly increased when 10 and 15% OTEOS were employed. This set of results demonstrated the suitability of OTEOS for use as an organosilanization agent in the preparation of hydrophobic

zeolites with hierarchical porosity and improved diffusional transport. These zeolites may be useful in a wide range of applications requiring greater accessibility of active sites and/or a hydrophobic profile.

Conflicts of interest: None.

ro of

Acknowledgements

This study was financed in part by Coordenação de Aperfeiçoamento de Pessoal

-p

de Nível Superior - Brasil (CAPES, Finance Code 001), Conselho de Desenvolvimento Científico e Tecnológico (CNPq, grant number #304900/2011-

re

7), and the São Paulo State Research Foundation (FAPESP, grant numbers #2014

na

lP

50948-3, #465593/2014-3, and #2013/07296-2).

Jo

ur

Appendices A and B (Supplementary Data)

References

[7]

[8]

[9]

Jo

[10]

ro of

[6]

-p

[5]

re

[4]

lP

[3]

na

[2]

Y. Li, L. Li, J. Yu, Applications of Zeolites in Sustainable Chemistry, Chem. 3 (2017) 928-949. https://doi.org/10.1016/j.chempr.2017.10.009 G. V Brião, S.L. Jahn, E.L. Foletto, G.L. Dotto, Highly efficient and reusable mesoporous zeolite synthetized from a biopolymer for cationic dyes adsorption, Colloids Surf., A. 556 (2018) 43-50. https://doi.org/10.1016/j.colsurfa.2018.08.019 R.P. Townsend, E.N. Coker, Ion exchange in zeolites, in: H. Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen (Eds.), Stud. Surf. Sci. Catal., Elsevier Science B. V., London, 2001: pp. 467-524. L. Bacakova, M. Vandrovcova, I. Kopova, I. Jirka, Applications of zeolites in biotechnology and medicine a review, Biomater. Sci. 6 (2018) 974-989. https://doi.org/10.1039/c8bm00028j T.F. Degnan, Jr., Applications of zeolites in petroleum refing, Top. Catal. 13 (2000) 349-356. https://doi.org/10.1023/A:1009054905137 B. Yilmaz, U. Müller, Catalytic applications of zeolites in chemical industry, Top. Catal. 52 (2009) 888-895. https://doi.org/10.1007/s11244009-9226-0 N.V.C.B.L. Newalkar, F.C.C.Á. Lobs, Á. Mto, Á.Á.C. Alkylation, Use of zeolites in petroleum refining and petrochemical processes : recent advances, J Porous Mater. 18 (2011) 685-692. https://doi.org/10.1007/s10934-010-9427-8 R. Liu, S. Young, S. Dangwal, I. Shaik, E. Echeverria, D. Mcilroy, C. Aichele, S. Kim, Boron substituted MFI-type zeolite-coated mesh for oilwater separation, Colloids Surf., A. 550 (2018) 108-114. https://doi.org/10.1016/j.colsurfa.2018.04.038 M. Hartmann, W. Schwieger, S. Martin, Catalytic test reactions for the evaluation of hierarchical zeolites, Chem. Soc. Rev. 45 (2016) 3313-3330. https://doi.org/10.1039/C5CS00935A W. Schwieger, G.A. Machoke, T. Weissenberger, A. Inayat, T. Selvam, M. Klumpp, A. Inayat, Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity, Chem. Soc. Rev. 45 (2016) 3353-3376. https://doi.org/10.1039/C5CS00599J N. Masoumifard, R. Guillet-nicolas, F. Kleitz, Synthesis of Engineered Zeolitic Materials : From Classical Zeolites to Hierarchical Core - Shell Materials, Adv. Mater. 1704439 (2018) 1-40. https://doi.org/10.1002/adma.201704439 A. Feliczak-guzik, Hierarchical zeolites : Synthesis and catalytic properties, Microporous Mesoporous Mater. 259 (2018) 33-45. https://doi.rg/10.1016/j.micromeso.2017.09.030 X. Jia, W. Khan, Z. Wu, J. Choi, A. C. K. Yip, Modern synthesis strategies for hierarchical zeolites: Bottom-up versustop-down strategies, Adv.

ur

[1]

[11]

[12]

[13]

[20]

[21]

[22]

Jo

[23]

ro of

[19]

-p

[18]

re

[17]

lP

[16]

na

[15]

ur

[14]

Powder Technol. 30 (2019) 467-484. https://doi.org/10.1016/j.apt.2018.12.014 X. Dong, C.F. Zhou, M.B. Yue, C.Z. Zhang, W. Huang, J.H. Zhu, New application of hierarchical zeolite in life science: Fast trapping nitrosamines in artificial gastric juice by alkaline-tailored HZSM-5, Mater. Lett. 61 (2007) 3154-3158. https://doi.org/10.1016/j.matlet.2006.11.024 Y. Zhang, N. Ren, Y. Tang, Zeolite Nanocrystals. Hierarchical Assembly and Applications, in: V. Valtchev, S. Mintova, M. Tsapatsis (Eds.), Ordered Porous Solids, Elsevier Science B. V., Amsterdan, 2009: pp. 441-475. https://doi.org/10.1016/B978-0-444-53189-6.00017-2 A. Sachse, J. Garc, Surfactant-Templating of Zeolites : From Design to Application, Chem. Mater. 29 (2017) 3827-3853. https://doi.org/10.1021/acs.chemmater.7b00599 K. Möller, T. Bein, Mesoporosity a new dimension for zeolites., Chem. Soc. Rev. 42 (2013) 3689-707. https://doi.org/10.1039/c3cs35488a D.P. Serrano, J.M. Escola, P. Pizarro, Synthesis strategies in the search for hierarchical zeolites, Chem. Soc. Rev. 42 (2013) 4004-4035. https://doi.org/10.1039/c2cs35330j J. Pérez-Ramírez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen, Hierarchical zeolites: Enhanced Utilisation of Microporous Crystals in Catalysis by Advances in Materials Design., Chem. Soc. Rev. 37 (2008) 2530-2542. https://doi.org/10.1039/b809030k J. Zhang, J. Yu, Layer ‐ by ‐ Layer Approach to Superhydrophobic Zeolite Antireflective Coatings, Chin. J. Chem. 36 (2018) 51-54. https://doi.org/10.1002/cjoc.201700579 D.P. Serrano, J. Aguado, J.M. Escola, J.M. Rodriguez, A. Peral, Effect of the organic moiety nature on the synthesis of hierarchical ZSM-5 from silanized protozeolitic units, J. Mater. Chem. 18 (2008) 4210-4218. https://doi.org/10.1039/b805502e R. Srivastava, N. Iwasa, S. Fujita, M. Arai, Synthesis of Nanocrystalline MFI-Zeolites with Intracrystal Mesopores and Their Application in Fine Chemical Synthesis Involving Large Molecules, Chem. Eur. J. 14 (2008) 9507-9511. https://doi.org/10.1002/chem.200801113 A. Hasan, L.M. Pandey, Kinetic studies of attachment and re-orientation of octyltriethoxysilane for formation of self-assembled monolayer on a silica substrate, Mater. Sci. Eng. C. 68 (2016) 423-429. https://doi.org/10.1016/j.msec.2016.06.003 R.M. Blanco, P. Terreros, M. Fernández-Pérez, C. Otero, G. Díaz-González, Functionalization of mesoporous silica for lipase immobilization: Characterization of the support and the catalysts, J. Mol. Catal. B Enzym. 30 (2004) 83-93. https://doi.org/10.1016/j.molcatb.2004.03.012 F. Bernardoni, A.Y. Fadeev, Adsorption and wetting characterization of hydrophobic SBA-15 silicas, J. Colloid Interface Sci. 356 (2011) 690-698.

[24]

[25]

Jo

ur

na

lP

re

-p

ro of

https://doi.org/10.1016/j.jcis.2011.01.033 [26] X. Li, B. Shi, M. Li, Synthesis of highly ordered alkyl-functionalized mesoporous silica by co-condensation method and applications in surface coating with superhydrophilic / antifogging properties, J Porous Mater. 22 (2015) 201-210. https://doi.org/10.1007/s10934-014-9886-4 [27] D.P. Serrano, J. Aguado, G. Morales, J.M. Rodrı, A. Peral, M. Thommes, Molecular and Meso- and Macroscopic Properties of Hierarchical Nanocrystalline ZSM-5 Zeolite Prepared by Seed Silanization, Chem. Mater. 216 (2009) 641-654. https://doi.org/10.1016/j.jaap.2008.10.009 [28] D.P. Serrano, J. Aguado, J.M. Escola, A. Peral, G. Morales, E. Abella, Synthesis of hierarchical ZSM-5 by silanization and alkoxylation of protozeolitic units, Catal. Today. 168 (2011) 86-95. https://doi.org/10.1016/j.cattod.2010.12.040 [29] C. A. Schneider, W. S. Rasband, K. W. Eliceiri, NIH Image to ImageJ: 25 years of image analysis, Nat. Methods. 9 (2012) 671-675. https://doi.org/10.1038/nmeth.2089 [30] S. J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Porosity, second ed., Academic Press, London, 1982. [31] B. C. Lippens, J. H. de Boer, Studies on pore systems in catalysts: V. The t method, J. Catal. 5 (1965) 319-323. https://doi.org/10.1016/00219517(65)90307-6 [32] E.P. Barrett, L.G. Joyner, P.P. Halenda, The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms, J. Am. Chem. Soc. 73 (1951) 373-380. https://doi.org/10.1021/ja01145a126 [33] K. Qiao, F. Zhou, Z. Han, J. Fu, H. Ma, G. Wu, Synthesis and physicochemical characterization of hierarchical ZSM-5: Effect of organosilanes on the catalyst properties and performance in the catalytic fast pyrolysis of biomass, Microporous Mesoporous Mater. 274 (2019) 190-197. https://doi.org/10.1016/j.micromeso.2018.07.028 [34] P.C.M.M. Magusin, V.E. Zorin, A. Aerts, C.J.Y. Houssin, A.L. Yakovlev, C.E.A. Kirschhock, J.A. Martens, R.A. Van Santen, Templatealuminosilicate structures at the early stages of zeolite ZSM-5 formation. A combined preparative, solid-state NMR, and computational study, J. Phys. Chem. B. 109 (2005) 22767-22774. https://doi.org/10.1021/jp053217u [35] H.G. Karge, J. Weitkamp, General Aspects of Solid-State NMR Spectroscopy, in: Mol. Sieves Sci. Technol., Springer, Berlim, 2003: pp. 204-294. https://doi.org/10.1016/B978-0-444-86946-3.50005-X [36] C.W. Jones, K. Tsuji, M.E. Davis, Organic-functionalized molecular sieves (OFMSs): II . Synthesis , characterization and the transformation of OFMSs containing non-polar functional groups into solid acids, Microporous Mesoporous Mater. 33 (1999) 223-240. https://doi.org/10.1016/S13871811(99)00141-9

Jo

ur

na

lP

re

-p

ro of

[37] M. Kovalakova, B. H. Wouters, P. J. Grobet, 13C MAS NMR of organic templates in zeolites, Microporous Mesoporous Mater. 22 (1998) 193-201. https://doi.org/10.1016/S1387-1811(98)00097-3 [38] D. Banfi, L. Patiny, Resurrecting and Processing NMR Spectra On-line, Chimia. 62 (2008) 280-281. https://doi.org/10.2533/chimia.2008.280 [39] M. Ma, R.M. Hill, Superhydrophobic surfaces, Curr. Opin. Colloid Interface Sci. 11 (2006) 193-202. https://doi.org/10.1016/j.cocis.2006.06.002 [40] P.A. Zapata, Y. Huang, M.A. Gonzalez-borja, D.E. Resasco, Silylated hydrophobic zeolites with enhanced tolerance to hot liquid water, J. Catal. 308 (2013) 82-97. https://doi.org/10.1016/j.jcat.2013.05.024 [41] B.T. Holland, Transformation of mostly amorphous mesoscopic aluminosilicate colloids into high surface area mesoporous ZSM-5, Microporous Mesoporous Mater. 89 (2006) 291-299. https://doi.org/10.1016/j.micromeso.2005.10.039 [42] S. Bilger, M. Soulard, H. Kessler, J.L. Guth, Identification of the volatile products resulting from the thermal decomposition propylammonium cations occluded in MFI- type zeolites, Zeolites. 11 (1991) 784-791. https://doi.org/10.1016/S0144-2449(05)80056-9 [43] E. Bourgeat-lami, F. Di Renzo, F. Fajula, P.H. Mutin, M. Cedex, T. Des Courieres, Mechanism of the Thermal Decomposition of Tetraethylammonium in Zeolite beta, J. Phys. Chem. (1992) 3807-3811. https://doi.org/10.1021/j100188a044 [44] V.R. Choudhary, S.G. Pataskar, Thermal decomposition of TPA-ZSM-5 zeolites: effect of gas atmosphere and Si/Al ratio, Thermochim. Actu, 97. 97 (1986) 1-10. https://doi.org/10.1016/0040-6031(86)87001-0 [45] S. Bosnar, M. Vrankić, D. Bosnar, N. Ren, A. Šarić, Positron annihilation lifetime spectroscopy (PALS) study of the as prepared and calcined MFI zeolites, J. Phys. Chem. Solids. (2017). https://doi.org/10.1016/j.jpcs.2017.06.016 [46] K.S.W. Sing, D.H. Everett, R. a. W. Haul, L. Moscou, R. a. Pierotti, J. Rouquérol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with Special Reference to the Determination of Surface Area and Porosity, Pure Appl. Chem. 54 (1982) 2201-2218. https://doi.org/10.1351/pac198557040603 [47] J. Aguado, D.P. Serrano, J.M. Escola, Low temperature synthesis and properties of ZSM-5 aggregates formed by ultra-small nanocrystals, Microporous Mesoporous Mater. 75 (2004) 41-49. https://doi.org/10.1016/j.micromeso.2004.06.027 [48] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, San Diego, USA., 1999. [49] D. P. Serrano, J. M. Escola, R. Sanz, R. A. Garcia, A. Peral, I. Moreno and M. Linares. Hierarchical ZSM-5 zeolite with uniform mesopores and

[52]

[53]

Jo

ur

na

lP

re

[54]

ro of

[51]

-p

[50]

improved catalytic properties. New J. Chem. 40 (2016) 4206-4216. https://doi.org//10.1039/c5nj02856f A. Sayari, E. Crusson, S. Kaliaguine. External Surface Areas of H-ZSM-5 Zeolites. Langmuir. 7 (1991) 314-317. https://doi.org/10.1021/la00050a019 L. Meng, X. Zhu. W. Wannapakdee. R. Pestman. M. G. Goesten. L. Gao. A. J.F. van H. E. J.M. Hensen. A dual-templating synthesis strategy to hierarchical ZSM-5 zeolites as efficient catalysts for the methanol-tohydrocarbons reaction. J. Catal. 361 (2018) 135–142. https://doi.org/10.1016/j.jcat.2018.02.032 M. M. Antunes. S. Lima. A. Fernandes. A. L. Magalhães. P. Neves. C. M. Silva.] M. F. Ribeiro. D. Chadwick. K. Hellgardt. M. Pillinger. A. A. Valente. MFI acid catalysts with different crystal sizes and porosity for the conversion of furanic compounds in alcohol medium. ChemCatChem. 9 (2017) 2747-2759.https://doi.org/10.1002/cctc.201601236 W. Makowski. K. Mlekodaj. B. Gil. W. J. Roth. B. Marszałek. M. Kubu. P. Hude. A. Smiešková. M. Horňáček. Application of quasi-equilibrated thermodesorption of linear and di-branched paraffin molecules for detailed porosity characterization of the mono-layered zeolite MCM-56, in comparison with MCM-22 and ZSM-5. Dalton Trans. 43 (2014) 1057410583. https://doi.org/10.1039/c4dt00232f D. Verboekend, J. Perez-Ramírez. Design of hierarchical zeolite catalysts by desilication. Catal. Sci. Technol. 1 (2011) 879–890. https://doi.org/10.1039/c1cy00150g

Figure captions

na

lP

re

-p

ro of

Figure 1. XRD patterns of the as-synthesized zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C8-0), 5% (C8-5), 10% (C810), and 15% (C8-15).

Jo

ur

Figure 2. 29Si{1H} CP-MAS NMR analyses of the as-synthesized zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C80), 5% (C8-5), 10% (C8-10), and 15% (C8-15).

ro of -p re

Jo

ur

na

lP

Figure 3. 13C{1H} CP-MAS NMR analyses of the as-synthesized zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C80), 5% (C8-5), 10% (C8-10), and 15% (C8-15).

ro of -p re

Jo

ur

na

lP

Figure 4. Representative images of water drops on the surfaces of compacted powders of the as-synthesized zeolites: unmodified (C8-0) and chemically modified with 5% OTEOS (C8-5).

Figure 5. (a) TG and (b) DTG curves for the as-synthesized zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C8-0), 5% (C8-5), 10% (C8-10), and 15% (C8-15).

ro of -p re lP na ur

Jo

Figure 6. SEM-FEG micrographs of the calcined zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C8-0), 5% (C8-5), 10% (C8-10), and 15% (C8-15).

ro of -p

Jo

ur

na

lP

re

Figure 7. TEM micrographs of the calcined zeolites: unmodified (C8-0) and chemically modified with 5% OTEOS (C8-5).

ro of -p re lP

Jo

ur

na

Figure 8. (a) N2 adsorption-desorption isotherms and (b) pore size distributions for the calcined zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C8-0), 5% (C8-5), 10% (C8-10), and 15% (C8-15).

ro of

-p

re

lP

na

ur

Jo

Table captions Table 1. Mass losses, carbon contents, contact angles, and Q4/Q3 ratios for the assynthesized zeolites chemically modified with different nominal amounts of OTEOS. Sample

Carbon content (%wt) 7.4 9.9 13.3 19.1

Contact angle θ (o) 15.2 134.9 139.5 140.3

Q4/Q3 3.5 3.8 4.4 4.7

Jo

ur

na

lP

re

-p

ro of

C8-0 C8-5 C8-10 C8-15

Mass loss (%wt) 16.0 22.4 25.0 27.0

Table 2. Textural properties of the calcined zeolites chemically modified with different nominal amounts of OTEOS: Vtotal = total pore volume, Vmicro = micropore volume, Vmeso = mesopore volume, Sext = specific external area. Sample

Si/Al

C8-0 C8-5 C8-10 C8-15

54 54 57 57

Vtotal (cm3 g-1) 0.251 0.377 0.373 0.322

Vmicro (cm3 g-1) 0.150 0.203 0.130 0.102

Vmeso (cm3 g-1) 0.101 0.174 0.243 0.220

Sext (m2 g-1) 86 121 273 288

ro of

Figure captions - Appendix A (Supplementary Data) A.1 - XRD patterns of the calcined zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C8-0), 5% (C8-5), 10% (C8-10), and 15% (C8-15).

re

-p

A.2 - 27Al-MAS NMR analyses of the calcined zeolites chemically modified with different nominal amounts of OTEOS: unmodified (C8-0), 5% (C8-5), 10% (C810), and 15% (C8-15).

lP

Figure captions - Appendix B (Supplementary Data)

na

B.1 - XRD patterns of the as-synthesized zeolites, unmodified and prepared with different organosilanes: unmodified (C8-0), 5% dodecyltriethoxysilane (C12-5), and 5% hexadecyltrimethoxysilane (C16-5).

ur

B.1 - 27Si{1H} CP-MAS NMR analyses of the as-synthesized zeolites prepared with different organosilanes: 5% dodecyltriethoxysilane (C12-5) and 5% hexadecyltrimethoxysilane (C16-5).

Jo

B.2 - Representative images of water drops on compacted powders of the assynthesized zeolites prepared with different organosilanes: 5% dodecyltriethoxysilane (C12-5) and 5% hexadecyltrimethoxysilane (C16-5). B.3 - SEM-FEG micrographs of the calcined zeolites prepared with different organosilanes: 5% dodecyltriethoxysilane (C12-5) and 5% hexadecyltrimethoxysilane (C16-5). B.4 - N2 adsorption-desorption isotherms for the calcined samples prepared with different organosilanes: 5% dodecyltriethoxysilane (C12-5) and 5% hexadecyltrimethoxysilane (C16-5).

Jo

ur

na

lP

re

-p

ro of

B.5 - BJH mesopore size distributions of the calcined zeolites prepared with different organosilanes: unmodified (C8-0), 5% dodecyltriethoxysilane (C12-5), and 5% hexadecyltrimethoxysilane (C16-5).