Spectroscopic IR and NMR studies of hierarchical zeolites obtained by desilication of zeolite Y: Optimization of the desilication route

Microporous and Mesoporous Materials 281 (2019) 134–141

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Spectroscopic IR and NMR studies of hierarchical zeolites obtained by desilication of zeolite Y: Optimization of the desilication route

T

Mariusz Gackowskia,∗, Karolina Tarachb, Łukasz Kuterasińskia, Jerzy Podobińskia, Bogdan Sulikowskia, Jerzy Datkaa,b a b

Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, PL-30239, Krakow, Poland Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387, Krakow, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Hierarchical zeolites Desilication IR Spectroscopy

Optimization of the procedure of desilication of dealuminated zeolite Y in a NaOH/tetrabutylammonium hydroxide (NaOH/TBAOH) mixture was done in order to obtain zeolites of optimal crystallinity, porosity and acidity and therefore of promising catalytic properties in the isomerization of α-pinene. High-silica zeolite Y (Si/ Al = 31) was treated with NaOH/TBAOH mixture of various TBAOH content (0–100 mol %). Desilication was carried out at temperatures from 293 to 373 K. It was shown, that the optimal crystallinity, acidity, porosity and catalytic properties was obtained if zeolite was treated with the NaOH/TBAOH mixtures containing 10–70% of TBAOH. However, taking into account the cost of a desilication route (TBAOH is an expensive agent), the most economic variant of desilication using 10 mol % of TBAOH only was chosen. The experiments performed at different temperatures revealed that the optimal catalytic properties of a resultant material were obtained after desilication carried out at 353 K. The conversion of α-pinene on this sample was nearly doubled in comparison with other zeolites desilicated at lower or higher temperatures. IR experiments showed also that desilication performed at higher temperatures (above 318 K) produced samples with a new kind of OH groups at 3600 cm−1. Finally, the experiments of pyridine adsorption demonstrated that this maximum is composed of the two kinds of OH groups exhibiting various properties, namely the acidic groups vibrating at 3600 cm−1 and the non-acidic ones at 3620 cm−1.

1. Introduction

applications were presented in a monograph edited by J. Garcia-Martinez and Kunhao Li [12]. Discussion concerning the mechanism of desilication has been presented in Refs. [13–17]. Desilication of Y zeolites has been much less frequently studied. A standard zeolite Y with Si/Al ratio around 2.5 contains numerous AlO4− groupings protecting effectively the zeolite framework against the OH− attack. On the other side, a standard zeolite Y subjected first to dealumination by steaming followed by acid leaching to yield a highsilica material with Si/Al > 15 [18], is amenable to the reaction with OH− groups. Such zeolites were however unstable and could be destroyed not only in a diluted NaOH solution [19], but also in a very diluted ammonia solutions [20,21]. The addition of tetrapropylammonium ions (TPA+) to NaOH turned to be a crucial modification of the desilication route. TPA+ protects well the zeolite structure upon the mesopores formation [22–25]. Verboekend et al. [22] evidenced that the USY zeolite desilicated in a NaOH/TPAOH solution was characterized by a larger mesopore volume, better sorption capacity as well as better catalytic activity in

The most important advantages of zeolites as catalysts are related to the fact, that active sites are situated inside the micropores and the carbocation produced by the addition of protons to reactants is stabilized by the negative charge of the framework. Moreover, shape selectivity occurs in some zeolites. Another advantage of zeolites is the presence of very strong Brønsted acid sites. However, the disadvantage of zeolites might be a restricted diffusion of reactants in micropores. To improve the catalytic efficiency of zeolites, one can consider applying the use of hierarchical zeolites with developed pore system comprising mesopores. The desilication of zeolites in alkaline solutions turned out to be the most effective way of producing mesoporous zeolites. Even though a lot of desilication studies have been realized using various zeolites, the majority of desilication studies were focused on ZSM-5 type zeolite [1–11], from which samples of good porosity, acidity and catalytic activity could be prepared. Many data on the synthesis of mesoporous zeolites, their properties and catalytic

∗

Corresponding author. E-mail address: [email protected] (M. Gackowski).

https://doi.org/10.1016/j.micromeso.2019.03.004 Received 26 June 2018; Received in revised form 27 February 2019; Accepted 3 March 2019 Available online 06 March 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.

Microporous and Mesoporous Materials 281 (2019) 134–141

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2.2. Characterization methods

polyethylene pyrolysis, and alkylation of toluene with benzyl alcohol than the parent zeolite. The reason why tetraalkylammonium cations indeed stabilize a zeolite structure is not well elucidated. Li and Shanz [26] revealed strong bonding of tetraalkylammonium ions with silica entities protecting zeolitic Si against an alkali attack. Therefore, TBAOH itself [27] extracts only very small amounts of Si without a significant change of porosity. If tetraalkylammonium cations (known as a pore directing agent) are added to NaOH they act as an efficient pore-growth moderator during OH− assisted Si extraction [23]. It is possible that strong bonding of such cations with silica surfaces, shown by Li and Shanz [26] protects framework Si atoms against OH− attack. The properties of a high-silica zeolite Y with Si/Al = 31 desilicated further with NaOH, TBAOH, NaOH/TBAOH [27] mixture as well as with NH3 solutions of various concentrations were studied in our laboratory [21]. The treatment with NH3 solution led to amorphization of zeolite [20,21] and the loss of microporosity. Nevertheless, the mesopores of large volume and surface were simultaneously formed. The catalytic activity in α-pinene isomerization increased distinctly even though both concentration and the acid strength of protonic sites decreased. It can be therefore concluded that porosity of zeolites affects the catalytic properties in α-pinene isomerization more than the Brønsted acidity does. Similar effects were obtained by Nuttens et al. [28] who reported the increase of Brønsted and Lewis acid sites, and also increase of TOF in the isomerization of α-pinene. The objective of our study was to optimize the desilication route of zeolite Y in order to obtain zeolitic samples of the best acidity, porosity and hence of much better catalytic properties. The NaOH/TBAOH mixtures containing various amounts of TBAOH (0, 5, 10, 40, 70 and 100% of TBAOH) were used for desilication performed in the temperature range of 293–373 K. The concentrations of Brønsted and Lewis sites, their strength as well as heterogeneity of the SieOHeAl groups were followed by IR spectroscopy. The zeolites were characterized by chemical analysis, while the course of desilication was followed quantitatively by measuring the amount of extracted Si and Al. The porosity was studied by adsorption of nitrogen. The status of Al was investigated by 27Al MAS NMR spectroscopy. The catalytic properties of the zeolitic materials were finally evaluated in the liquid-phase isomerization of α-pinene, which is an appropriate test reaction for studying bulky molecules with an obvious practical importance [29]. This reaction produces limonene and camphene which are valuable feedstock for the chemical, cosmetics and food industries.

The X-ray powder diffraction (XRD) was recorded with a PANalytical X'Pert PRO MPD diffractometer with X'Celerator detector type at room temperature. The measurements were carried out continuously over a 2θ range from 5 to 50° with a 0.0167° and time per step 29.84 s. CuKα radiation (λ = 1.5418 Å) at 40 kV and 30 mA was used. The same amount of samples were placed in holders prior to data acquisition. Unit cell parameters were calculated using the formula: a0 = dhkl 2⋅(h2 + k 2 + l 2) , where dhkl − distance between layers, hkl – Miller indices. Crystallinity has been calculated as the sum of integrated area of reflexes at 15.7, 20.5, 27.2, 29.9, and 34.4° [31]. Si and Al contents in the zeolites as well as in the filtrate were determined by ICP OES spectroscopy on an Optima 2100DV (PerkinElmer) instrument. In order to determine the composition of zeolites, 70–80 mg of a zeolite sample was treated with the mixture of 0.3 ml HF and 3 ml of concentrated HCl in a Teflon vessel for 24 h. After the dissolution of zeolite, the liquid was diluted to 50 ml and Si and Al amounts were determined by ICP OES spectroscopy. The accuracy of measurement was ca. 5–10%. The sorption of nitrogen was followed at 77 K using an ASAP 2420 Micromeritics apparatus. Before experiment a sample was evacuated in situ in the Micromeritics cell at 670 K for 12 h. Surface area (SBET), micropore volume and surface (Vmicro, Smicro) were determined by applying the BET and t-plot methods, respectively. Pore size distribution and volume and surface area of mesopores (Vmeso) were obtained using the BJH model based on the adsorption branch of the isotherm. The accuracy of these measurements was ca. 10%. Prior to IR experiments self-supported zeolite wafers (diameter of 1 cm, m = 10–20 mg) were evacuated in situ in an IR cell at 720 K for 1 h. The spectra were recorded with a NICOLET 7600 spectrometer with the spectral resolution of 1 cm−1. The adsorption of carbon monoxide was performed at 170 K. The concentration of Brønsted and Lewis acid sites was determined quantitatively by IR spectroscopy of adsorbed pyridine. The acid strength of SieO1HeAl groups was followed by comparing the values of frequency shifts of hydroxyl groups interacting via hydrogen bonding with CO, according to the procedure described in Refs. [7,25]. The accuracy of determination of the concentration of acid sites was ca. 10%. The 27Al solid state Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS NMR) spectra were acquired on a Bruker Avance III 500 MHz WB spectrometer, operating at a magnetic field of 11.7 T. The 27 Al investigations were performed on fully hydrated samples. For this purpose, the samples were exposed to the vapour of a saturated Mg (NO3)2 solution at ambient temperature. 27Al MAS NMR spectra of the zeolitic materials were measured at 130.33 MHz using short 0.2 μs single-pulse excitations corresponding to the π/16 flipping angle and the repetition time of 0.5 s. Short pulses are necessary to ensure reliable, quantitative results for the 27Al spectra [32]. A Bruker high-speed MAS probe equipped with the 4 mm zirconia rotor and KEL-F cap was used to spin the sample at 12 kHz. Typically, 8192 transients were acquired for a spectrum. 27Al chemical shifts are quoted in parts per million from external 1 M aqueous Al(NO3)3 solution.

2. Experimental 2.1. Catalyst preparation The parent zeolite Y (PAR) with Si/Al = 31 was supplied by Zeolyst (CBV 760). Desilication was carried out with using NaOH/TBAOH mixture (total 0.2 M) of various proportions between NaOH and TBAOH (0, 5, 10, 40, 70 and 100 mol % of TBAOH) at various temperatures: 293, 318, 338, 353 and 370 K. The desilication time was 30 min and a mass ratio of a solution to zeolite was 30. After desilication, the suspension was cooled down in ice-bath, filtered and washed until neutral pH. Fourfold Na+/NH4+ ion-exchange with 0.5 M NH4NO3 was subsequently performed at 60 °C for 1 h. Afterwards, the samples were filtrated again, washed and dried at room temperature. Finally, the zeolites were calcined in air flow at 790 K for 10 h. Our earlier study [27] evidenced, that the repeatability of desilication procedure was good (the amounts of Si extracted in four desilication experiments were similar in all the cases: standard deviation was ca. 5%). It should be mentioned, that a good reproducibility of desilication procedure has been already reported for zeolites ZSM-5 [30].

2.3. Isomerization of α-pinene The catalytic tests of α-pinene isomerization were performed at 363 K in a glass flask equipped with a reflux condenser in batch conditions with intense stirring. In a typical experiment, 5.0 ml of α-pinene (98%, Aldrich) was heated to 363 K, and then 10 mg of a zeolite sample was transferred to the reactor. Prior to reaction, zeolites were evacuated at 493 K for 2 h. The reaction time was 120 min. Liquid aliquots withdrawn from the reaction mixture were analyzed by gas chromatography. More details on the protocol used are given elsewhere [33,34]. Our earlier study [27] revealed that the reproducibility of catalytic tests was good. The standard deviation of the conversion data 135

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Table 1 The amounts of Si and Al extracted, Si/Al values, crystallinity %, the unit cell parameter (a0), N2 physisorption derived properties, concentration of Brønsted and Lewis acid sites, shift of OH band after adsorption of CO of samples desilicated using different concentration of TBAOH and catalytic properties (conversion in αpinene isomerization after 5 min of reaction at 363 K). Sample

% extracted Si/Al

PAR NaOH 5% 10% 40% 70% 100%

31 10.8 15.1 17.3 18.1 21 29.5

Si

Al

Unit cell parameter

crystallinity

porosity

a0

%

Vmicro

%

%

Å

– 78 48 43 33 25 7

– 7.8 5.7 4.2 1.8 1.7 0.4

24.27 – 24.25 24.40 24.32 24.31 24.25

Vmeso

Smeso

Dmeso

B.a.c

catalysis L.a.c.

Δν

CO..OH −1

cm /g

cm /g

m /g

nm

μmol/g

μmol/g

cm

0.33 0.08 0.11 0.20 0.22 0.25 0.29

0.20 0.52 0.73 0.89 0.62 0.63 0.22

280 430 268 460 317 337 250

3.0 4.3 7.0 6.2 4.6 3.9 3.0

282 230 275 310 314 315 250

86 370 190 160 131 116 83

354 200 351 353 352 351 354

3

100 0 26 50 63 78 88

acidity

3

2

conversion mol% 1 1 11 14 14 12 2

are shown in Fig. S1. In all spectra, the signals at −107 ppm are the major ones and are assigned to Si(0Al) environments. The Si(0Al) signals are accompanied by much smaller at −102 ppm due to Si(1Al). In some spectra very weak and broad humps due to amorphous aluminosilicate can be discerned at ca. −110 ppm lines, in very good agreement with X-ray diffraction patterns, pointing to high crystallinity of the samples (Figs. 1 and 5). Because the Si(1Al) signals are relatively small, their integration were necessarily made with a larger error. Moreover, some contribution from the Si(1OH) environments occurring in the same region might disturb further the intensity of the true Si(1Al) line. All these uncertainties lead to underestimation of the real Si/Al ratios in the zeolite framework. Similar discrepancies were observed recently by Andreev and Livadaris [36], who were using the same commercial zeolite Y labelled CBV 760 as we studied here. Important information on the status of Al in the parent sample and the desilicated materials was obtained in NMR experiments. The 27Al MAS NMR spectra of zeolites desilicated using the NaOH/TBAOH mixtures are shown in Fig. 2A. The parent high-silica zeolite shows the Al signals at 61.4 and 0 ppm. The former was assigned to tetrahedrallycoordinated zeolitic aluminium and the latter to extraframework octahedral Al. The treatment of zeolite with NaOH, TBAOH and NaOH/ TBAOH caused vanishing of octahedral Al due to dissolution of these Al species. Significant amounts of Al were extracted (together with Si) from zeolite crystals during formation of mesopores. As only small amounts of Al were present in filtrate after desilication, it points to the fact that most of Al extracted from zeolite was reinserted into the framework. The 27Al MAS NMR spectra of desilicated zeolites show broadening of the tetrahedral aluminium signal, together with the small upfield shift of the peak, resulting probably from the formation of

in the series of four catalytic tests was better than ca. 7%.

3. Results and discussion 3.1. Various proportions between NaOH and TBAOH The data concerning the properties of zeolite Y desilicated in NaOH/ TBAOH solutions of various TBAOH contents are listed in Table 1. The amounts of both Si and Al extracted decreased with the TBAOH content. Pure NaOH extracted 78% of Si and 7.8% of Al, whereas pure TBAOH extracted much smaller amounts of Si and Al (7 and 0.4%, resp.). XRD experiments provided information on the crystallinity of samples and unit cell size. The results are presented in Figs. 1 and 5 as well as in Tables 1 and 2. According to the data given in Fig. 1 and Table 1 crystallinity of zeolites depends on the proportion between NaOH and TBAOH. The treatment with pure NaOH led to the complete amorphization of zeolite, the NaOH/TBAOH mixtures caused some decrease of crystallinity, nevertheless 26–78% of crystallinity was preserved depending on TBA+ content, in comparison with the parent zeolite. The crystallinity increased with the TBAOH content. As zeolites treated with contained TBA+ ions, the calcination of zeolites was an indispensable step in order to remove TBA+ from the pore system. According to the data presented in Fig. 1, the calcination did not affect the resultant samples crystallinity. The unit cell dimension (a0) has been calculated from XRD diagrams (Tables 1 and 2). While dealumination of zeolite (by steaming and acid treatment) led to the contraction of unit cell (a0 decreased from 24.69 [35] to 24.27 Å), further desilication caused only very small change of a unit cell size. 29 Si MAS NMR spectra of the parent zeolite and desilicated samples

Fig. 1. XRD diffractograms of zeolite samples before desilication (PAR) and after the treatment using different concentrations of TBAOH; left – non-calcined, right – after calcination. 136

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Table 2 The amounts of Si and Al extracted, Si/Al values, crystallinity %, the unit cell parameter (a0), N2 physisorption derived properties, concentration of Brønsted and Lewis acid sites, shift of OH band after adsorption of CO and catalytic properties conversion in α-pinene isomerization after 5 min of reaction at 363 K) of samples desilicated at different temperatures. Sample Si/Al

PAR 293 K 318 K 338 K 353 K 373 K

31 17.3 17.5 17.8 17.0 16.7

% extracted

Unit cell parameter

crystallinity

porosity

Si

a0

%

Vmicro

Al

%

%

Å

– 43 42 54 51 54

– 4.2 4.8 5.3 2.5 2.7

24.27 24.30 24.30 24.22 24.27 24.31

Vmeso

Smeso

catalysis

B.a.c

L.a.c.

Δν

CO..OH

conversion

cm /g

cm /g

m /g

nm

μmol/g

μmol/g

mol%

0.33 0.20 0.26 0.28 0.27 0.24

0.20 0.89 0.85 0.86 1.15 0.90

280 460 455 450 540 440

282 310 365 380 430 380

86 160 200 200 210 220

354 353 352 357 351 351

2 14 17 16 27 15

3

100 50 84 66 83 79

acidity

3

2

zeolitic material of relatively good mesoporosity and narrow mesopore diameter was formed. The addition of 5% of TBAOH improved slightly microporosity and produced mesopores of relatively large diameter and hence of a relatively small mesopore surface. Pure TBAOH yielded material of similar porosity as the parent zeolite did. The volume and surface of mesopores was optimal in the sample desilicated by the NaOH/TBAOH mixture containing 10% of TBAOH. The diameter of mesopores formed by NaOH/TBAOH decreased with the TBAOH content. This effect can be discussed by considering the role of pore-directing-agents (PDA) in the mesopore formation. Both TBA+ and Al (extracted from zeolite) may act as a PDA [15]. Verboekend and PerezRamirez reported that larger pores were formed with Al as PDA and more narrow ones in the presence of amine hydroxides serving as PDA [15]. Therefore, the increase of TBAOH content in the NaOH/TBAOH mixture produces zeolite with more narrow mesopores (Table 1). The spectra of the OH groups (Fig. 3) show the bands of SieOH, SieO1HeAl and SieO3HeAl groups at 3740, 3620 and 3550 cm−1 resp. The treatment with NaOH caused amorphization and loss of microporosity while destroying also the zeolitic SieOHeAl groups. On the contrary, the desilication with TBAOH and NaOH/TBAOH effectively preserved the zeolitic hydroxyls. The intensity of SieOHeAl bands practically does not depend on the proportion between NaOH and TBAOH. Only the zeolite treated with a mixture containing 5% of TBAOH shows somewhat lower SieOHeAl intensities – this might be related to lower crystallinity (see Fig. 1) and smaller micropore volume of the sample (Table 1). The concentration of both Brønsted and Lewis acid sites was determined by quantitative IR experiments of pyridine sorption, and the results are presented in Table 1. The NaOH treatment removed significant amounts of Si and led to amorphization of zeolite. Accordingly, the concentration and acid strength of the protonic sites decreased

another form of tetrahedrally-coordinated aluminium, namely the nonzeolitic one. We note that the signal of tetrahedral aluminium in the amorphous aluminosilicate was observed at 55.5 ppm. It can be concluded that octahedral aluminium, dissolved aluminium and Al extracted from the sample were reinserted to form tetrahedrally-coordinated non-zeolitic Al species. Reinsertion, sometimes called realumination, is a known phenomenon and was considered earlier for zeolites with different structures, for example ZSM-5 [37] or zeolite Y [38]. Recently, reinsertion of Al into a desilicated zeolite was demonstrated by us for zeolite ZSM-5 [8], as well as was postulated by Groen et al. [39]. Because the zeolite treated with TBAOH contains TBA+ ions located in the pore system, the calcination in air was necessary to remove TBA+ and empty the channels. In our study the zeolites treated with NaOH/ TBAOH were calcined in air at 790 K. Two desilicated zeolites were calcined: one of them was the zeolite treated with NaOH/TBAOH and the second one was the zeolite treated with NH4NO3 after desilication and next calcined. The corresponding 27Al MAS NMR are presented in Fig. 2 B. If the zeolite was treated with NH4NO3 the calcination produced octahedral Al (signal at ca. 0 ppm) and simultaneously the decrease of the tetrahedral non-zeolitic aluminium took place. This effect might be due to some dehydroxylation of SiOHeAl groups. The hydroxyls formed by non-zeolitic Al are indeed very sensitive for dehydroxylation [8]. At higher temperatures they condensate and form Lewis sites (Al signal at 0 ppm). No such effect was observed if desilicated zeolite non treated with NH4NO3. It indicates, that the dehydroxylation is possible only if protons were introduced by NH4+ decomposition. The information on porosity was obtained from the N2 sorption experiments (Table 1). NaOH led to the destruction of zeolite structure coupled with a significant reduction of the micropore volume. Non-

Fig. 2. 27Al MAS NMR spectra of non-calcined samples before (PAR) and after desilication using different concentrations of TBAOH (A). Impact of calcination and ion exchange with NH4NO3 on the sample desilicated using 10% TBAOH (B). 137

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Fig. 4. The conversion of α-pinene (mol %) at 363 K vs. reaction time for zeolites desilicated at 293 K and 353 K.

decreased upon desilication (the ΔνOH … CO shift decreased about 10–20 cm−1). This effect was explained and dwelled on in our previous paper [27] by considering the heterogeneity of the OH groups in those zeolites and a simultaneous presence of the more and less acidic hydroxyls. It was also found that desilication removed small amounts of Al atoms, and the Al atoms removed firstly were those responsible for the formation of the most acidic SieOHeAl groups. In the dealuminated zeolite Y studied in this work, all the SieOHeAl were homogeneous and the alkali treatment, which extracted also some small amounts of Al (see Table 1), changed the number but not the average acid strength of SieOHeAl only. Detailed information on the catalytic properties of desilicated zeolites was derived from the liquid phase tests of α-pinene isomerization. The reaction produces mainly camphene and limonene, which undergo further reactions to form γ-terpinene, α-terpinene, terpinolene and pcymene [29,33,34] according to the Scheme 1: This reaction was studied in details in our previous works on different zeolites and related solids [21,27,33,34]. The dependence of conversion at 363 K on reaction time is presented in Fig. 4, and the conversion after 5 min of reaction is given in Table 1. The Y non-desilicated zeolite revealed very low activity due to low accessibility of reactant to the active sites. Similarly, the zeolite treated with NaOH showed very low activity due to the destruction of the zeolite structure, loss of microporosity and important loss of protonic acidity. On the other hand, the desilication with NaOH/TBAOH increased significantly α-pinene conversion, what was related to the preservation of microporosity, formation of mesopores with large surface and volume, and maintaining high acidity. Comparing the results obtained with zeolite Y desilicated with NaOH/TBAOH mixtures of various proportions between both bases leads to the conclusion that, in general, α-pinene conversion practically does not depend on the proportions between bases. However, the zeolites treated with the mixture containing 10 and 40% TBAOH showed somewhat higher activity than that treated with solution containing 5 mol% of TBAOH only. By summing up one can state that the increase of TBAOH content in the NaOH/TBAOH mixture increases crystallinity and microporosity of desilicated samples in comparison with the zeolites treated with NaOH only, thus evidencing beneficial role of TBAOH in preserving the zeolite

Fig. 3. IR spectra of OH groups of samples before (PAR) and after desilication using different concentrations of TBAOH (calcined).

significantly and a lot of Lewis acid sites was formed. These Lewis sites were the product of dehydroxylation [8]. On the other hand, the treatment with TBAOH, which removed only a very small amount of Si, practically did not change the sample acidity. The desilication in NaOH/TBAOH mixtures containing 10–70% TBAOH, which extracted 43 - 25% of Si, increased a little the concentration of protonic sites from 282 to 310–315 μmol/g. It removes Si, increases the contribution of Al and therefore the amount of protonic sites. Similar effect, i.e. increase of the concentration of both Brønsted and Lewis sites in zeolites desilicated with NaOH/TPAOH was reported by Nuttens et al. [28], these authors detected however lower concentrations of sites than reported in our study. Rac et al. [40] studied desilication of zeolite Y by a NaOH/ TPAOH mixture, and reported a decrease of the concentration of protonic sites. It seems that different acidity of our zeolites and those studied by Rac et al. [40] may be due to some differences in the desilication route, and especially to the different PDA (TPAOH and TBAOH) used. The concentration of Lewis acid sites increased in comparison to the PAR sample. This increase was more pronounced if desilication was carried out using the NaOH/TBAOH mixtures more abundant in NaOH. As mentioned above, Lewis sites were formed by dehydroxylation. Very important information concerns acid strength of the SieOHeAl groups. As mentioned above, in the parent sample the acid strength of SieO1HeAl was very high (ΔνOH … CO = 354 cm−1) - only dealuminated mazzite showed the higher strength of SieOHeAl (ΔνOH −1 ). The treatment with NaOH decreased significantly … CO = 380 cm the acid strength of SieOHeAl. However, we note that the acid strength of SieOHeAl in the zeolites desilicated with the NaOH/TBAOH mixtures did not change significantly upon desilication (Table 1). This result seems to be of particular importance because in other zeolites: ZSM-5 [7], BEA [25] and MOR [41], the acid strength of SieOHeAl

Scheme 1. Isomerization of α-pinene. 138

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Fig. 5. XRD diagrams of zeolite samples before desilication (PAR) and after the desilication at different temperatures; left – non-calcinced, right – after calcination.

50 ppm. The latter was assigned to the pentahedrally-coordinated aluminium. IR results of pyridine adsorption (see infra) evidenced also the higher concentration of Lewis acid sites in the zeolite desilicated at 373 K rather than 293 K. The porosity of desilicated zeolites was studied by low temperature sorption of nitrogen. The results are collected in Table 2. As it is seen, the micropore volume is optimal in zeolites desilicated at 318–353 K, whereas mesopore volume is optimal (1.15 cm3/g) in the sample desilicated at 353 K. In other zeolites mesopore volume is somewhat smaller (0.85–0.90 cm3/g). Similarly, the mesopore surface is the largest in the sample desilicated at 353 K. The acidity of zeolites was followed by quantitative IR studies of pyridine and CO adsorption. The results are summarized in Table 2. The concentration of protonic sites increases with desilication temperature from 282 to 430 μmol/g if desilication temperature increases from room temperature to 353 K and subsequently decreases to 380 μmol/g upon further increase of desilication temperature. The acid strength of SieOHeAl groups practically does not depend on the desilication temperature (as the ΔνOH … CO shift is very similar for all zeolites desilicated in the temperature range of 273–373 K). The spectra of OH groups in zeolites desilicated at different temperatures are visualized in Fig. 7. The bands of SieOH, SieO1HeAl and SieO3HeAl practically do not change with the temperature of desilication; however, a new OH band appears at ca. 3600 cm−1 in all the samples desilicated above 318 K. The acidity of 3600 cm−1 hydroxyls was studied by pyridine adsorption using the zeolite desilicated at 373 K. By comparing the spectra recorded upon pyridine adsorption and the difference spectrum (the spectrum of a zeolite after adsorption minus the spectrum before adsorption) it becomes clear that the maximum around 3600 cm−1 describes the two kinds of hydroxyls: (i) non-acidic hydroxyls vibrating at 3620 cm−1, and (ii) acidic hydroxyls vibrating at 3600 cm−1, being Brønsted acid sites (Fig. 8). The presence non acidic hydroxyls vibrating at 3620 cm−1 was already reported by Kubelkova et al. [42]. The nature of 3600 cm−1 was already discussed: e.g. in Refs [43–47]. Makarova et al. [43] as well as Katada and Niwa [44–47] basing on experimental and DFT results evidenced that these hydroxyls interacted with extraframework Al characterized by Al NMR signal at 55 ppm [45]. The formation of strong protonic sites was accompanied in an increase of cracking activity [46]. The authors suggested that the increase of cracking activity was not correlated with formation of mesopores. The results of catalytic experiments in isomerization of α-pinene are presented in Fig. 4 and Table 2. The conversion after 5 min of reaction at 363 K is the highest for the sample desilicated at 353 K. This result agrees well with the data of N2 sorption and IR concerning acidity: this

structure. Both mesopore volume and surface were optimal for zeolite desilicated with the mixture containing 10% of TBAOH. The protonic acidity (concentration and acid strength of SieOHeAl) was optimal for zeolites treated with mixtures containing 10–70% TBAOH. As the consequence, the catalytic activities in α-pinene isomerization were optimal for samples treated with the mixtures containing 10 and 40% TBAOH. Taking into account that TBAOH is an expensive chemical, one can conclude, that the optimal content of TBAOH in the NaOH/TBAOH mixture used for desilication of zeolite Y (Si/Al = 31) should be 10%. All the results presented above were obtained for the samples desilicated at ambient temperature. The effect of desilication temperature (RT, 318, 338, 353 and 373 K) on the properties of desilicated materials will be presented in the next chapter. 3.2. Effect of desilication temperature The amounts of Si and Al extracted from zeolite Y during desilication with NaOH/TBAOH mixture containing 10% TBAOH at different temperatures are listed in Table 2. Generally, at higher temperatures somewhat larger amounts of Si and smaller amounts of Al are extracted. The XRD data demonstrating the effect of desilication temperature (RT to 373 K) on the crystallinity of zeolites are presented in Fig. 5 and Table 2. Desilication at all these temperatures essentially does not change crystallinity of samples. Only the treatment with NaOH/TBAOH at room temperature produces zeolite of somewhat lower crystallinity then desilication at higher temperatures. The data presented in Fig. 5 show that crystallinity did not change upon the calcination of desilicated materials. 27 Al MAS NMR spectra of zeolites desilicated at different temperatures are presented in Fig. 6 A. The increase of desilication temperature did not change visibly the status of Al, and the proportion between the amounts of tetrahedral zeolitic and non-zeolitic aluminium did not depend on temperature. A slight shift of the peak position is seen, from 61.4 ppm in the parent zeolite to 59.6 ppm in the sample desilicated at 373 K. It might be due to an increased contribution of non-zeolitic tetrahedral aluminium. The effect of calcination is presented in Fig. 6 B. If the desilicated zeolite, which was treated with NH4NO3 and subsequently calcined, the non-zeolitic tetrahedral aluminium was transformed into the extraframework octahedral one at −0.3 ppm due to dehydroxylation. By comparing the results obtained with the zeolite desilicated at room temperature and 373 K it becomes clear that the extent of dehydroxylation was bigger for zeolites desilicated at higher temperature. Such a behaviour was confirmed by a higher contribution of signals at −0.3 ppm and a visible increase of the baseline in the region from 10 to 139

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Fig. 6.

27

Al MAS NMR spectra of not calcined samples before (PAR) and after desilication at different temperatures (A). Impact of calcination and ion exchange (B).

Fig. 8. IR spectra of OH vibrations: a – before the adsorption of pyridine, b – after the adsorption of pyridine, c – difference between b and a. Fig. 7. IR spectra of OH groups of samples before (PAR) and after desilication at different temperatures (calcined). Top spectrum was recorded at 170 K, all other spectra were recorded at 400 K.

produce the zeolite of optimal crystallinity, porosity and acidity giving rise to the best catalytic performance, desilication was carried out by the NaOH/TBAOH solutions with different proportions between NaOH and TBAOH. The course of desilication was also studied as a function of temperature. It was demonstrated that the optimal and most economical composition of the desilicating agent should contain 10 mol% of TBAOH. The optimal temperature of desilication was found to be 353 K. Finally, it was additionally observed that desilication carried out above 313 K produced non-acidic hydroxyls vibrating at 3620 cm−1 and the acidic ones vibrating at 3600 cm−1.

zeolite exhibits the best mesoporosity coupled with the highest concentration of protonic sites responsible for isomerization. As it was mentioned above, Niwa et al. [46] suggested that the increase of cracking activity of steamed zeolites Y was due to increase of acidity and was independent from formation of mesopores. However, cracking is a reaction requiring strong acidity. α-Pinene isomerization proceeds under mild conditions in the liquid phase and does not demand very strong acidity, we suggest therefore that the main reasons of enhanced catalytic activity in this reaction were higher concentration of protonic sites and increase of mesoporosity. To summarize, the results concerning desilication of zeolite Y with the NaOH/TBAOH mixtures of variable compositions and desilication performed at different temperatures allow to conclude that optimal and the most economic variant comprises treatment of the dealuminated zeolite with Si/Al = 31 using a solution containing 10 mol % of TBAOH at 353 K. The resultant zeolitic sample is characterized by optimal mesoporosity, acidity and the highest conversion of α-pinene.

Acknowledgments This study was sponsored partly by the National Science Centre (Kraków, Poland) grant 2015/17/B/ST5/00023. B.S. gratefully acknowledges the Ministry of Science and Higher Education (Warsaw) for the solid-state NMR 500 MHz spectrometer investment Grant (project No. 75/E-68/S/2008-2).

Appendix A. Supplementary data 4. Conclusions Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.03.004.

In order to find optimal conditions of desilication of zeolite Y and to 140

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