IR and NMR studies of hierarchical material obtained by the treatment of zeolite Y by ammonia solution

Accepted Manuscript IR and NMR studies of hierarchical material obtained by the treatment of zeolite Y by ammonia solution

Mariusz Gackowski, Łukasz Kuterasiński, Jerzy Podobiński, Bogdan Sulikowski, Jerzy Datka PII: DOI: Reference:

S1386-1425(17)31015-6 https://doi.org/10.1016/j.saa.2017.12.042 SAA 15687

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

28 September 2017 8 December 2017 13 December 2017

Please cite this article as: Mariusz Gackowski, Łukasz Kuterasiński, Jerzy Podobiński, Bogdan Sulikowski, Jerzy Datka , IR and NMR studies of hierarchical material obtained by the treatment of zeolite Y by ammonia solution. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), https://doi.org/10.1016/j.saa.2017.12.042

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ACCEPTED MANUSCRIPT IR and NMR studies of hierarchical material obtained by the treatment of zeolite Y by ammonia solution Mariusz Gackowski1, Łukasz Kuterasiński1, Jerzy Podobiński1, Bogdan Sulikowski1, Jerzy Datka1,2

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Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland 2 Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland

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[email protected]

ABSTRACT

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Ammonia treatment of ultrastable zeolite Y has a great impact on its features. XRD showed a partial loss of crystallinity coupled with a loss of long-distance zeolite ordering. However, a typical short-range zeolite ordering, in the light of 29Si NMR studies, was largely preserved. 27Al MAS NMR spectra evidenced that most of Al was located in zeolitic tetrahedral positions, but some of them adopted a distorted configuration. Evolution of zeolites acidity was followed quantitatively by using IR. In particular, such studies revealed the presence of strongly acidic Si-OH-Al groups. IR studies suggest also heterogeneity of these OH groups. The heterogeneity of Si-OH-Al groups was a consequence of the less ordered structure of zeolites treated with ammonia solutions. It was also found that the treatment with ammonia solutions yields hierarchical material. The samples revealed promising catalytic properties in the liquid phase isomerization of α-pinene. Zeolites desilicated with ammonia may constitute an inexpensive route yielding viable hierarchical catalysts.

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Keywords: hierarchical zeolites, desilication, FT-IR, NMR, acidity

1. INTRODUCTION

Zeolites are very important catalysts in chemical industry [1–3]. The advantages of zeolites are following: high surface area, high hydrothermal and thermal stability, the presence of structural pores of molecular dimensions, and easiness of formation of strong Brønsted acid sites in their structure. The fact that active sites are situated inside micropores consequences in such advantages as stabilization of carbocations by negative charge of the zeolite framework and shape selectivity (in some zeolites). However, diffusional limitations are frequently observed when large molecules are processed. Moreover, restricted diffusivity can lead to fast deactivation of catalysts [4]. 1

ACCEPTED MANUSCRIPT Several methods were applied to improve the transport or reactants in zeolite crystals. One of the most efficient method used in the last decade is the desilication in alkali solutions, a treatment which extracts some amounts of Si and much smaller amounts of Al while forming secondary mesopores of relatively large volume [4–8]. Desilication is the most effective if the Si/Al ratio of a parent zeolite is around 30. If Si/Al is too low, the negative charge of AlO4- repulses OH- groups thus protecting zeolite framework against the OHattack. Conversely, if Si/Al is too high, there is no such protection, desilication is simply too extensive and leads to the dissolution of zeolite. ZSM-5 type zeolite with Si/Al around 30 was the most often desilicated aluminosilicate [9–16].

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Desilication of zeolites Y attained a great deal of attention in the last years. While pristine Y of Si/Al = 2.5 with the high concentration of AlO4- is resistant to OH- attack, zeolites which were first dealuminated by steaming followed by acid treatment were prone to reaction with bases. However, the action of NaOH and of such weak and diluted base as an ammonia solution, resulted in the amorphization of zeolite and loss of its microporosity [17– 20]. The remedy for this problem might be the addition of tetrapropylammonium (TPA) or tetrabutylammonium ions (TBA) to NaOH. The desilication with NaOH/TBAOH not only conserved the zeolite structure and its micropore system, but also produced remarkable amounts of secondary mesopores affecting significantly the catalytic activity [20–23].

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We undertook the studies of zeolites Y desilicated in NaOH, TBAOH (tetrabutylammonium hydroxide), and NaOH/TBAOH mixtures of various compositions. While NaOH caused destruction of zeolite, NaOH/TBAOH mixture caused only ca. 30% loss of crystallinity and micropore volume [20]. The mesopore volume and surface increased distinctly. Very important result concerned the acid strength of Si-OH-Al groups. Parent (nondesilicated) zeolite showed very high acid strength of bridging Si-OH-Al groups, this acids strength changed only a little upon desilication. As the consequence, the catalytic activity of zeolites in α-pinene isomerization increased significantly upon desilication with a NaOH/TBAOH mixture.

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The desilication with NaOH/TBAOH was found to be an efficient way to modify zeolites resulting in materials of good microporosity, high acidity and promising catalytic properties in the transformations of bulky molecules. However, tetrabutylammonium hydroxide, similarly as tetrapropylammonium hydroxide, is an expensive agent, so our aim was focused on developing a less expensive chemical suitable for desilication of zeolites. One of them, an aqueous solution of ammonia, was recently used for preparation of hierarchical zeolites by Aelst et al. [18]. These authors realized XRD and NMR studies on zeolite H-Y (Si/Al = 40) treated with 0.02 M ammonia solution. This procedure extracted only very small amounts of Si and Al, leading to amorphization of zeolites and formation of a dense, hydrated ammonium silicate phase. The progress of amorphization increased with the time of treatment. Our objective was to study a modification of an ultrastable zeolite Y (Si/Al = 31) with ammonia solutions of different concentrations, and in particular to follow the evolution of its acidity in order to prepare viable catalysts suitable for transformations of terpene class hydrocarbon (-pinene). Thus, high-silica ultrastable zeolite Y dealuminated was treated with 0.05 and 0.2 M ammonia solutions. The samples were characterized by XRD, NMR, detailed IR studies, N2 adsorption, as well as catalytic tests of α-pinene isomerization. As catalytic efficiency of zeolites is determined mostly by two factors, i.e. acidity and accessibility of acid 2

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sites to reactant molecules, a special attention was put on the IR studies of concentration, acid strength and heterogeneity of protonic sites (Si-OH-Al), and also on concentration and nature of Lewis sites. The concentration of sites was determined by quantitative IR experiments of pyridine adsorption. The acid strength and heterogeneity was studied by following hydrogen bonding of CO adsorbed at low temperature (ca. 170 K). The N2 adsorption experiments provided information on volume of micropores and mesopores, surface area and diameter of mesopores. The catalytic properties of zeolites were finally evaluated in α-pinene isomerization which is an appropriate test reaction for bulky molecules with an obvious practical importance. It has been previously used for evaluation of catalytic properties of hierarchical zeolites [8, 24, 25]. This reaction gives mainly limonene and camphene; both hydrocarbons constitute valuable feedstock for the chemical and food industries. 2. EXPERIMENTAL

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2.1 Catalyst preparation

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The parent zeolite Y of Si/Al = 31 was purchased from Zeolyst (CBV 760). The procedure of the treatment with NaOH has been described in our earlier work [20]. The zeolite was treated with 0.05 and 0.2 M ammonia solution at room temperature for 30 min with continuous stirring. After ammonia treatment the suspension was filtered and washed with distilled water until neutral pH and dried at room temperature. Prior to use in various experiments: XRD, N2 adsorption, catalytic tests, the samples were calcined in air flow at 723 K for 10 h.

2.2 Characterization methods

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The powder X-ray diffraction (XRD) measurements were carried out using a PANalytical Cubix X’Pert Pro diffractometer, with CuKα radiation, λ=1.5418 Å in the 2 angle range of 2-40°. Si and Al content in the parent and desilicated zeolites were determined by ICP OES spectroscopy on an Optima 2100DV (PerkinElmer) instrument.

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Adsorption of N2 at 77 K was studied on an ASAP 2420 Micromeritics apparatus, after activation in vacuum at 673 K for 12 h. Surface Area (SBET) and micropore volume (Vmicro) were determined by applying the BET and t-plot methods, respectively. Pore size distribution and volume of mesopores (Vmeso) were obtained by using the BJH model to the adsorption branch of the isotherm. For FTIR studies, the samples were pressed into the form of self-supporting discs (ca. 7 - 20 mg/cm2) and evacuated in a quartz IR cell at 720 K under vacuum for 1 h. Spectra were recorded with a NICOLET 6700 spectrometer equipped with a MCT detector. The spectral resolution was of 2 cm-1. The CO adsorption was performed at 170 K. The concentration of both Brønsted and Lewis acid sites was determined by quantitative IR studies of pyridine sorption. All acid sites were neutralized by pyridine at 450 K and physisorbed pyridine was removed by evacuation at the same temperature. The concentration of Brønsted and Lewis sites was calculated from the intensities of 1545 cm˗1 and 1450 cm˗1 bands of pyridinium ions (PyH+) and pyridine interacting with Lewis sites (PyL) and their extinction coefficients. The extinction coefficient of PyH+ 1545 cm˗1 band was determined as the slope of the linear 3

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correlation of the intensity of 1545 cm˗1 band versus the amount of pyridine sorbed in zeolite NaHY containing only protonic sites (a value of 0.075 cm2/μmol was obtained). The extinction coefficient of PyL band was determined in the experiments in which pyridine was sorbed in zeolite HY dehydroxylated at 1070 K containing practically only Lewis acid sites. The value of the extinction coefficient was calculated from the linear correlation of the 1450 cm˗1 band versus the amount of pyridine interacting with Lewis sites (the amount of pyridine sorbed minus the amount of pyridine reacting with protonic sites, the small amount of which remained upon pretreatment at 1070 K). A value of 0.130 cm2/μmol was obtained. The extinction coefficient of NH4+ band at 1450 cm˗1 was determined as the slope of the linear correlation of the intensity of this band versus the amount of ammonia sorbed in zeolite NaHY.

2.3 Catalytic tests

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2.3.1 Isomerization of α-pinene

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The solid-state 29Si and 27Al MAS NMR spectra were acquired at the basic resonance frequencies of 99.4 MHz and 130.3 MHz, respectively, on a Bruker Avance III 500 MHz WB spectrometer operating at a magnetic field of 11.7 T. The samples were spun in zirconia rotors. The 27Al and 29Si 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. 29Si MAS NMR spectra were acquired at a spinning rate of 8 kHz using high-power proton decoupling (SPINAL64), with 5.8 s (/3) pulses and the repetition time of 20 s. 27Al MAS NMR spectra were recorded at a spinning rate of 12 kHz with a short pulse length of 0.2 µs (π/12) and a recycle delay of 0.1 s. Short pulses are necessary to ensure reliable, quantitative results for the 27Al spectra [26]. The chemical shifts of 29Si and 27Al were externally referenced to TMS and 1 M aqueous Al(NO3)3, respectively. Deconvolution of the spectra was carried out using the Bruker TopSpin 3.1 software.

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Catalytic tests were performed at atmospheric pressure in a glass round-bottom flask equipped with a reflux condenser in a batch conditions with intense stirring (ca. 300 r.p.m.). In a typical experiment, 2.5 ml of α-pinene (98 %, Aldrich) was heated to 393 K, and then 0.12 mg of the sample activated in 493 K was transferred to the reactor. The reaction was carried out for 120 minutes. After selected times, aliquots of the reaction mixture were collected for a gas chromatography quantitative analysis (Hewlett-Packard HP 6890).

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3. RESULTS AND DISCUSSION The data on the amounts of Si and Al extracted in ammonia solutions (obtained from chemical analysis) are given in Table 1. The treatment with 0.05 and 0.2 M solution extracted only very small amounts of Si (1.8 and 3.5 %, resp.) and did not extract Al. This result agrees well with the earlier data of Aelst et al. [18] obtained with 0.02 M ammonia solution. The amounts of Si and Al extracted from the same zeolite FAU-31 in 0.2 M NaOH solution are presented as well for comparison purposes (Table 1). NaOH extracted much larger amounts of both Si (79%) and Al (7.8%) (cf. the results obtained in our earlier study – Ref. [20]). XRD (Fig. 1) results evidenced a significant loss of crystallinity after the treatment with 0.05 M ammonia solution, and almost complete amorphization after the treatment with 0.2 M solution. The decrease of XRD reflexes corresponds to the decrease of micropore 4

ACCEPTED MANUSCRIPT volume (vide infra - Table 1). A complete amorphization of zeolite was also observed after the treatment with NaOH [20]. As it is seen, calcination of zeolites treated with ammonia solution only slightly affected the crystallinity of samples (Fig. 1). 29

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Si MAS NMR spectra of the parent and modified samples are visualized in Fig. 2. In the starting sample two strong signals are accompanied by a third, much weaker and broadened. The strongest signal at -107 ppm corresponds to Q4 groupings in zeolite Y, that is to the Si(4Si) environment. The line at -102 ppm is assigned to zeolitic Q3 species, while the weak signal at -111 ppm is due to amorphous Q4 [27]. Upon desilication the intensity of the 111 ppm signal considerably increases, pointing to the continuous amorphization of the samples after treatment with the solutions of ammonia. This corresponds to the decrease of crystallinity, as evidenced by XRD diffractograms. On the contrary, the strongest signal Q4 at -107 ppm is significantly reduced after desilication, and the Q3 one is much less affected. Moreover, the zeolitic signals of Q4 and Q3 groupings are also broadened upon desilication, what is noticeably seen in the sample treated with 0.2 M NH3 (Fig. 2). Such behaviour suggests higher level of short-range disorder. Upon calcination, the intensity of the -107 ppm signal remains unaffected. We note that the decrease of crystallinity as evidenced by XRD is very high, up to 95 % for the 0.2 M NH3 calcined sample. Simultaneously, decrease of the intensity of the zeolitic Q4 signal at -107 ppm in comparison to the parent FAU-31 sample is much lower, retaining at least 20 % of its primary intensity (cf. Supporting info, Fig. S1). Hence, we can conclude that the long-range order is almost lost, while the short-range order is still preserved in these materials.

Nitrogen sorption results

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The results of N2 adsorption are presented in Table 1. The parent zeolite FAU-31 which originally was dealuminated by steaming and acid treatment contained micropores (the volume of which was 0.33 cm3/g) and mesopores (0.20 cm3/g). The treatment with ammonia solution decreased significantly the micropore volume (to 0.08 - 0.09 cm3/g) and produced mesopores of the relatively high volume (0.39 - 0.48 cm3/g). The surface of mesopores was also relatively high (430 - 460 m2/g). The average diameter of mesopores (3.4 - 3.8 nm) was somewhat higher than observed in the parent zeolite (3 nm).

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According to the data presented in Table 1 the degree of hierarchy of zeolites treated with ammonia solution was lower than of parent FAU-31. FAU-31 contained comparable amounts of micro- and mesopores, while zeolite treated with ammonia contained mostly mesopores and significantly smaller contribution of micropores. The data on porosity of the same parent FAU-31 treated with 0.2 M NaOH [20] are listed in Table 1. Generally, the porosity of zeolites treated with ammonia solutions and NaOH are similar. Both micropores and mesopores volumes as well as mesopore surface are comparable. The only difference is somewhat larger diameter of mesopores (3 and 3.4 3.8 nm for zeolites treated with NaOH and ammonia solution, resp.). However, the acid and catalytic properties (vide infra) of zeolites treated with NaOH and ammonia are different. Zeolite treated with ammonia shows also better acidity and catalytic efficiency, what will be discussed in details below.

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ACCEPTED MANUSCRIPT Acid properties The spectra of OH groups in the parent zeolite and samples treated with ammonia solution are presented in Fig. 3. The spectra show three bands: 3740 cm-1 of non-acidic SiOH, and two bands of acidic Si-O1H-Al and Si-O3H-Al at 3620 and 3550 cm-1 [28]. The treatment with 0.05 M ammonia solution decreases the intensities of both 3620 and 3550 cm -1 bands, what remains in good agreement with the XRD and nitrogen sorption data, evidencing the partial destruction of a zeolite framework. The more concentrated (0.2 M) ammonia solution leads to more advanced destruction of the zeolite framework and further decrease of the bands of both Si-O1H-Al and Si-O3H-Al groups.

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The zeolites which were treated with ammonia solution are in the ammonium form. The amount of NH4+ ions in zeolites treated with ammonia solution and studied before calcination was calculated from the absorbance of NH4+ band at 1450 cm-1. The samples were dehydrated in situ in IR cell at 370 K. The extinction coefficient of this band was equal to 0.130 cm/μmol. The amounts of NH4+ ions in zeolites treated with 0.05 and 0.2 M ammonia solutions obtained in this study were 520 and 511 μmol/g. These amounts are very close to the amounts of Al present in these zeolites of Si/Al = 31 and 30 (480 and 495 μmol/g) indicating, that all Al in zeolites treated with the ammonia solution is in tetrahedral AlO4positions. Such a conclusion agrees well with the NMR measurements. 27Al MAS NMR spectra revealed the Al signals at 59.6 and 56.5 ppm which can be attributed to tetrahedral Al in typically zeolite positions and "non-zeolitic" tetrahedral positions [18].

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The amounts of protonic (Brønsted) sites in zeolites treated by ammonia solutions and subsequently calcined at 720 K were determined by quantitative IR experiments of pyridine sorption at 443 K. These values are presented in Table 1. The treatment with ammonia solution decreased the concentration of protonic sites and produced Lewis sites. The concentration of protonic sites in zeolite treated with 0.05 and 0.2 M ammonia solution was respectively 182 and 100 μmol/g. The concentration of Lewis acid sites was 155 and 120 μmol/g. As protons in zeolites neutralize AlO4- tetrahedra the concentration of AlO4- is equal to concentration of H+. The fact, that concentration of AlO4- (and of H+) in calcined zeolites (182 and 100 μmol/g) is distinctly lower than of AlO4- (and of NH4-) in zeolite before calcination (520 and 511 μmol/g) is an evidence of the loss of AlO4- during calcination. This stems from dehydroxylation occuring during the calcination, so more amounts of Lewis acid sites are formed. Normally, the dehydroxylation of Si-OH-Al in zeolites occurs following the stoichiometry: 2 Si-OH-Al = Lewis acid site + H2O [29, 30], so the sum of concentration of Si-OH-Al plus twice concentration of Lewis sites (B + 2L) in dehydroxylated zeolite should be equal to the concentration of Si-OH-Al and of AlO4- in zeolite before dehydroxylation, and should not change further with the progress of dehydroxylation. According to the data presented in Table 1 for zeolite treated with 0.05 ammonia solution the sum B + 2L equal to 498 μmol/g is close to the concentration of NH4+ before calcination (520 μmol/g) and concentration of Al obtained from chemical analysis (480 μmol/g) evidencing, that Lewis acid sites (the concentration of which was determined with pyridine) were formed by dehydroxylation of protonic sites. The 27Al MAS NMR spectrum of the parent FAU-31 (Fig. 4) shows two signals: at 61.4 ppm of "zeolitic" tetrahedral Al and at 0 ppm of extraframework Al. The calcination of this zeolite did not change the status of Al, whereas the treatment with ammonia resulted in disappearance of octahedral Al at 0 ppm. This is not due to the dissolution of Al (as Al was 6

ACCEPTED MANUSCRIPT not found in a filtrate), most probably the octahedral Al was transformed into tetrahedral "non-zeolitic" species (signal at 56.5 ppm). 27

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Al MAS NMR spectrum of zeolite treated with 0.05 M solution (Fig. 4) shows two Al signals at 59.6 and 56.5 ppm, corresponding to "zeolitic" and "non-zeolitic" tetrahedral positions. The signal of octahedral Al is absent. Dehydroxylation during calcination is accompanied by decrease of both Al tetrahedral signals and formation of octahedral Al at 0 ppm. The decrease of amount of tetrahedral Al and formation of octahedral Al is consistent with the mechanism of dehydroxylation discussed long time ago [29, 30]. This mechanism assumes the condensation of two hydroxyls, the protons of which neutralize two AlO4tetrahedra with the formation of water molecule. Al from one such tetrahedron leaves framework position forming not well defined positively charged Al species which are neutralized by the second AlO4-. Therefore, one of the two AlO4- leaves framework forming Lewis site (octahedral Al) and the second one remains. As a result, the content of tetrahedral AlO4- is decreased by half. The data presented in Fig. 3 agree with this interpretation - the intensity of signals of tetrahedral Al is decreased by ca. half upon calcination.

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values are given in

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= 354 cm-1) of Si-

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Table 1. Parent zeolite FAU-31 shows very high acid strength (

OH...CO

O1H-Al. Most importantly, these hydroxyls were also found to be highly homogeneous [20]. The treatment with diluted ammonia solution (0.05 M) practically did not change the acid strength of Si-O1H-Al, OH...CO decreased insignificantly ( OH...CO decreased to 352 cm-1),

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but the more concentrated (0.2 M) ammonia solution led to more significant decrease of acidity ( OH...CO decreased to 344 cm-1).

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Interesting conclusion can be drawn by comparing the width of IR bands of Si-O1H-Al interacting by hydrogen bonding with CO. These bands (normalized to band height) are presented in Fig. 5 D . In the parent FAU-31 the band is narrow, thus due to homogeneity of the Si-O1H-Al groups (other arguments were presented in our earlier paper [20]). Homogeneity of the bridging Si-O1H-Al was explained by the same bridge geometry for all of these hydroxyls, and also by the fact that all of them have the same number of Al atoms near the bridge. As the 29Si MAS NMR spectrum of FAU-31 shows only two signals Si(0Al) and Si(1Al) all the bridging hydroxyls can be represented by the formula: (SiO)3Si-O1H-Al(OSi)3. The Si-O1H-Al bands in zeolites treated with ammonia solution are broader than in the parent sample, thus suggesting heterogeneity of the Si-O1H-Al groups. Another reason of heterogeneity of Si-O1H-Al may be the presence of hydroxyls exhibiting various bridge geometry. It is possible, that in zeolite in which partial destruction of the framework took place, the Si-O1H-Al bridges of various geometry are present. The width of the band of SiO1H-Al interacting with CO in zeolite treated with more concentrated ammonia solution (0.2 M) is larger than in that treated with diluted NH3, thus suggesting a more broad distribution of the acid strength of the bridging hydroxyls. It may be explained by higher extend of framework destruction (Fig. 1). To conclude, it is possible that the more broad distribution of the bridge angels is present in this material. 7

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Catalytic activity

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Main reaction scheme of the α-pinene isomerization is presented below (Scheme 1). The products of two parallel reactions are camphene and limonene. Under certain conditions, limonene can undergo further reactions yielding γ-terpinene, α-terpinene, terpinolene and pcymene.

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Scheme 1. Isomerization of α-pinene.

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The results of the catalytic tests inform how acidity and porosity of zeolites influence their activity and selectivity. The values of conversion: initial conversion - after 1 min of contact and conversion after 2 h are presented in Table 2. The dependence of conversion on the reaction time is given in Fig. 6.

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The parent FAU-31 shows a low initial α-pinene conversion (12 %). The treatment with NaOH, decreasing significantly samples acidity [20], decreased also the conversion (to 5 %). On the other hand, the treatment with diluted (0.05 M) ammonia solution increased the 1 min conversion to 21 %. This increase may be related to distinct increase of mesoporosity (both volume and surface increased) what in turn affected the catalytic activity more than decrease of the protonic sites concentration with simultaneous, insignificant decrease of their acid strength. On the contrary, the treatment with the more concentrated ammonia caused the decrease of conversion what might be explained by a distinct decrease of acidity. These results show clearly that the activity in α-pinene isomerization depends on both acidity and porosity. The zeolite treated with diluted ammonia (0.05 M) exhibits the optimal catalytic activity. The catalytic performance is therefore a compromise between the good porosity and the high acidity. The dependence of α-pinene conversion on reaction time is shown in Fig. 6. The parent FAU-31 loses the activity after 1 min of reaction, suggesting that the catalyst deactivation is most probably caused by deposition of oligomeric species formed by oligomerization of alkenes. Oligomeric deposits can block the zeolitic micropores in which active sites are situated. This blocking effect is less harmful for zeolites treated with NaOH and ammonia, because these samples contain mesopores facilitating the transport of reactants, and the conversion is increasing with the reaction time. For zeolite treated with diluted ammonia solution the conversion reaches 89 % after 2 h of reaction. This conversion value is significantly higher than 2 h conversion observed for parent FAU-31 (15 %) and a zeolite treated with NaOH (27 %). 8

ACCEPTED MANUSCRIPT From the above picture it is clear that the most important factor determining the increase in the activity in α-pinene isomerization is the increase of mesopore volume and surface, which have more important impact to the activity in the reaction of bulky molecule than some decrease of concentration and acid strength of Si-OH-Al groups. According to the scheme 1 and data in Tables 1 and 2 Lewis acid sites are not active (or show only very low activity) in α-pinene. This reaction proceeds via carbocation mechanism, moreover, the zeolite desilicated with NaOH which contains the highest amount of Lewis sites shows low activity in α-pinene isomerization.

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In general, the hierarchical material obtained by treatment with diluted ammonia results in relatively good catalytic performance in the transformation of relatively bulky αpinene molecule. This activity is not as high as observed for zeolite treated with NaOH/TBAOH [20] (which attained 100 % of conversion after 2 h of reaction). Preparation of hierarchical materials using non-expensive ammonia is more economic than with TBAOH.

4. CONCLUSIONS

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The data on the selectivity are presented in Table 2 after 1 min of reaction, but the selectivity practically did not change within period of time studied. Camphene and limonene are main products of α-pinene isomerization. Limonene undergoes further reactions (see scheme above). Some unidentified products (including oligomeric deposits) are produced, too. The contribution of camphene and limonene reaction paths depends mostly on the catalyst acidity. The parent zeolite FAU-31 of very high acidity produces mostly limonene and products of its further transformation. The decrease of acidity observed in zeolites treated with NaOH and NH3 affects the reaction selectivity. Yield of camphene is increased, while yield of limonene is decreased. The role of zeolite acidity in the selectivity of α-pinene isomerization to limonene and in further reactions of limonene was already discussed [31, 32]. Nevertheless, the most relevant finding of the presented data is the relatively high activity of zeolite treated with diluted ammonia.

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Our study shows that mild treatment of high-silica ultrastable zeolite Y (Si/Al = 31) using 0.05 & 0.2 M of ammonia solution (when compared to more severe desilication with 0.2 M NaOH [20]) has indeed a great impact on the structure and properties of desilicated samples. We note that effect is seen even if a highly diluted solution of ammonia (0.05 M) was used. The amount of silica extracted from zeolite crystals under these conditions is quite low, leading however to significant structural changes in the solids. Thus, high degree of amorphization was observed, as well as simultaneous changes of Al status and creation of high volume of mesopores. It was demonstrated that the samples treated with diluted ammonia solutions exhibited short-range order coupled with high Brønsted acidity. Moreover, the evolution of Brønsted acidity of the samples was followed quantitatively, taking into account the possibility of using such solids in acid-base catalysis. The latter result was confirmed further by the excellent catalytic performance of the resultant solids in the liquid phase transformations of -pinene. Such materials might therefore constitute promising catalysts for the liquid phase reactions, where the presence of an additional mesopore system is desirable. Finally, from the economic standpoint, the treatment of zeolites with inexpensive 9

ACCEPTED MANUSCRIPT solutions of ammonia seems to be much more convenient than a synthesis route of hierarchical zeolites based on the costly tetrabutylammonium hydroxide.

5. ACKNOWLEDGMENTS

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This study was sponsored by the National Science Center (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).

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ACCEPTED MANUSCRIPT zeolite catalyst during the methanol-to-hydrocarbon conversion process, J. Catal. 269 (2010) 219–228. doi:10.1016/j.jcat.2009.11.009. [11] K. Sadowska, K. Góra-Marek, J. Datka, Hierarchic zeolites studied by IR spectroscopy: Acid properties of zeolite ZSM-5 desilicated with NaOH and NaOH/tetrabutylamine hydroxide, Vib. Spectrosc. 63 (2012) 418–425. doi:10.1016/j.vibspec.2012.09.007.

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[13] K. Sadowska, K. Góra-Marek, M. Drozdek, P. Kuśtrowski, J. Datka, J. Martinez Triguero, et al., Desilication of highly siliceous zeolite ZSM-5 with NaOH and NaOH/tetrabutylamine hydroxide, Microporous Mesoporous Mater. 168 (2013) 195– 205. doi:10.1016/j.micromeso.2012.09.033.

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[14] K. Sadowska, K. Góra-Marek, J. Datka, Accessibility of acid sites in hierarchical zeolites: Quantitative IR studies of pivalonitrile adsorption, J. Phys. Chem. C. 117 (2013) 9237–9244. doi:10.1021/jp400400t.

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[18] J. Van Aelst, M. Haouas, E. Gobechiya, K. Houthoofd, A. Philippaerts, S.P. Sree, et al., Hierarchization of USY zeolite by NH4OH. A postsynthetic process investigated by NMR and XRD, J. Phys. Chem. C. 118 (2014) 22573–22582. doi:10.1021/jp5058594. [19] D. Verboekend, G. Vilé, J. Pérez-Ramírez, Hierarchical Y and USY zeolites designed by post-synthetic strategies, Adv. Funct. Mater. 22 (2012) 916–928. doi:10.1002/adfm.201102411. [20] M. Gackowski, K. Tarach, Ł. Kuterasiński, J. Podobiński, S. Jarczewski, P. Kuśtrowski, et al., Hierarchical zeolites Y obtained by desilication: porosity, acidity and catalytic properties, in press [21] D. Verboekend, J. Pérez-Ramírez, Desilication mechanism revisited: Highly mesoporous all-silica zeolites enabled through pore-directing agents, Chem. Eur. J. 17 (2011) 1137–1147. doi:10.1002/chem.201002589. [22] D. Verboekend, N. Nuttens, R. Locus, J. Van Aelst, P. Verolme, J.C. Groen, et al., 11

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 6

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ACCEPTED MANUSCRIPT Fig.1 XRD diffractograms of the parent FAU-31 zeolite and desilicated samples Fig.2 29Si MAS NMR spectra of the parent zeolite and desilicated samples before and after calcination Fig.3 IR spectra of the parent zeolite and desilicated samples in the range of OH vibrations

after calcination Fig.5

29Si

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Fig.4 27Al MAS NMR spectra of the parent zeolite and desilicated samples before and MAS NMR spectra of parent and desilicated zeolites

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before and after calcination

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Fig.5 The spectra of OH vibrations in: A - parent FAU-31, B - zeolite desilicated in 0.05 M NH3, and C - zeolite desilicated in 0.2M NH3, D – comparison of the width of normalized to intensity OH bands after the adsorption of CO for different samples

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before and after the desilication, a – before the adsorption of CO, b – after the

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Fig.6 The conversion of α-pinene (mol %) at 393 K vs. reaction time.

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ACCEPTED MANUSCRIPT Table 1. The amounts of Si and Al extracted, Si/Al values, N2 physisorption derived properties, concentration of Brønsted and Lewis acid sites. Sample Si/Al

Amount

Porosity

of species extracted

Volume (cm3/g)

Acidity

S D meso. meso. (m2/g) nm

Concentration of acid sites (μmoles/g)

OH..CO (cm-1)

NaOH

0.33

0.20

140

31

1.8

0

0.09

0.41

432

30

3.5

0

0.08

0.33

10.8

78

4.4

0.07

0.65

465

540

20

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86

354

182

155

352

3.8

100

120

344

3.0

230

370

200

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3.0

3.4

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Si

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ACCEPTED MANUSCRIPT Table 2. Catalytic activity in α-pinene isomerization (at 393 K): initial conversion (after 1 min of the reaction) and final conversion (after 120 min of the reaction) and selectivities to camphene, limonene and the products of further limonene transformations.

α-Pinene isomerisation Conversion at 393 K (mol %) after

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Selectivity (%)

2h

Camphene

Limonene + products 64

Others

FAU-31

12

15

20

NaOH

5

27

35

44

21

0.05 M NH3

21

89

24

55

18

0.2 M NH3

11

30

51

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1 min

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

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1. Ammonia treatment of ultrastable zeolite Y caused a partial loss of crystallinity and significant decrease of long-range ordering. 2. Desilicated samples partly preserved zeolitic short-range ordering and high Brønsted acidity. 3. Obtained samples exhibit high activity in liquid phase isomerisation of α-pinene. 4. Desilication of ultrastable zeolite Y using non-expensive ammonia solutions resulted in hierarchical material with promising catalytic properties.

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