- Email: [email protected]
D reactivity and acidity of Brønsted acid sites of MWW zeolites: Comparison with MFI zeolite
Applied Catalysis A, General 575 (2019) 180–186
Contents lists available at ScienceDirect
Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
H/D reactivity and acidity of Brønsted acid sites of MWW zeolites: Comparison with MFI zeolite Roman Buláneka, Martin Kubůb, Jan Vaculíka, Jiří Čejkac,
T
⁎
a
Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10 Pardubice, Czech Republic J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic c Department of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acidity Zeolites MWW MFI Isotopic exchange Adsorption enthalpy
Acid site strength in MWW and MFI zeolites was investigated by isotopic exchange reaction between deuterated hydroxyl groups of zeolites and ethane molecules. The course of the reaction was monitored by FT-IR spectroscopy through time changes of integral intensities of the Si(OD)Al Brønsted acid sites represented by absorption bands at 2644, 2655, and 2657 cm−1 for MFI, MWW and MCM-36 zeolite, respectively. The rate of the reaction was described by pseudo first order kinetics and acid site strength was compared using rate constants characteristic of individual zeolites at constant temperature. The exchange reaction is significantly faster over MFI zeolite compared with MWW zeolites, suggesting a higher acid strength of Brønsted sites in MFI zeolite than in MWW. In our previous study (Arean et al. Phys. Chem. Chem. Phys. 16 (2014) 10,129), we found notable discrepancy in the relation between OH group frequency shift induced by adsorption of weak base molecules like CO and N2 and the change of adsorption enthalpy of these probe molecules for MWW zeolites, which calls into question the use of frequency shifts as a measure of acidity. Our reported results based on isotopic exchange reaction rate measurement correlate with the change of the probe molecules adsorption enthalpy involved in hydrogen bonding. Therefore, we believe that enthalpy change or H/D exchange activity measurement is more reliable method for assessment of acid strength than OeH frequency shift probed by a weak base adsorbed at a low temperature.
1. Introduction
energy of base molecules derived from temperature programmed desorption (TPD) data, and measurement of the rates of probe acid catalysed reactions, were reported in the literature [7–18]. IR spectroscopy is by far the most commonly used technique for characterizing acid sites. The interaction of hydroxyl groups with the weak bases like CO or N2 leads to the formation of hydrogen bonded species that cause red-shift of the OeH vibration, which is very often used as an acidity scale (stronger acid will undergo a larger shift). This method was pioneered by Paukshtis and Yurchenko in 1980s [19] and by Makarova et al. in 1990s [20,21] and logarithmic relationship between frequency shift of OH group vibration upon adsorption of weak basis (ΔνOH) and zeolite acid strength was proposed. On the other hand, stronger acid sites will interact with weak base molecules by larger interaction energies. Therefore, the relationship between frequency shift of OH group vibration and enthalpy change (ΔHads) can be expected. Such correlation was found for CO and N2 adsorption on H-Y, HMFI and H-FER, but MWW zeolites do not follow this correlation [10,22]. MWW zeolites (specifically MCM-22 and MCM-56 zeolites)
Brønsted acid catalysed reactions, namely cracking, isomerizations, alkylations, hydrocracking, as well as methanol to olefin (MTO) or gasoline (MTG) conversions and alcohol dehydration or carbonylation are key processes in oil refining, petrochemistry, and fine chemical production [1–6]. Zeolites and related zeotypes have been used as acid catalysts for these processes since the 1960s due to their high catalytic activities, environmentally benign nature, and excellent thermal stability. Their catalytic activity is determined mainly by the strength and number of their Brønsted acid sites. Therefore, the knowledge of the factors controlling the acidic properties of zeolites and ability to assess (or compare) strength of acid sites in various zeolites is key challenge. To characterize acid sites strength, frequency of OeH bond vibration and OeH frequency shifts upon interaction of OH groups with a base molecules (ΔνOH), characteristic spectral bands of the base molecules adsorbed on acid sites, chemical shift of 1H signals in MAS NMR spectra, measurement of adsorption enthalpy or desorption activation
⁎
Corresponding author. E-mail address: [email protected] (J. Čejka).
https://doi.org/10.1016/j.apcata.2019.02.024 Received 27 November 2018; Received in revised form 16 January 2019; Accepted 9 February 2019 Available online 18 February 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
exhibited quite large frequency shift (316-320 cm−1), larger than MFI zeolite (303 cm−1) but the adsorption heats of CO are distinctly lower than for MFI (22.5 kJ/mol (MCM-22) and 20.0 kJ/mol (MCM-56) vs 29.4 kJ/mol for MFI). The same discrepancy was found also for dinitrogen. This peculiarity of MWW zeolites is worth examining in more detail, because understanding of MWW zeolites behaviour with respect to weak bases adsorption enthalpies and induced frequency shifts of OH vibrations would help to increase the understanding of zeolite acidity. Moreover, finding of proper method to determine the acid strength using molecules and experimental conditions similar to real acid-catalysed reactions would be useful and would help to optimize the catalytic performance of MCM-22 zeolite, which is industrially important zeolitic catalyst applied in Exxon-Mobil EBMax process of ethylbenzene production [23,24], Mobil-Raytheon process of cumene production [25] or 2,6-dimethylnaphtalene production (ExxonMobil-KobeSteel process) [26]. In addition MWW zeolites exhibit promising catalytic activities in many other reactions including toluene disproportionation [27] or alkylation [28], n-butane aromatization [29] or alpha pinene oxide isomerization [30,31] and many others. The aim of this study is to evaluate acid sites strength of MWW zeolite in bulk and pillared forms, compare it with MFI zeolite as a “benchmark”, and to assess the validity of the formerly proposed scaling factors for assessment of acid sites (ΔνOH and ΔHads). It should be stressed that using weak base molecules as a probe molecules for testing acid sites and assessment of their strength is sometimes questioned in literature. Major reservations relate to (i) application of experimental conditions significantly different from real conditions of catalytic reactions and (ii) using probe molecules with significantly different nature and size compared to real reactants. These reservations rise questions of accessibility of the acid sites by probe molecule and real reactant and not negligible contribution of secondary (lateral) interactions of molecules with zeolite framework depending on the molecule type and local geometry of acid site. These pitfalls are not easy to overcome. Recently, Kondo et al. [32,33] proposed the scaling zeolite Brønsted acid site strength by monitoring the rate of H/D isotopic exchange reaction between deuterated zeolite acid sites and ethane [32] or methane [33]. In order to avoid these reservations and to assess the acidity/reactivity of acid sites by an experimental approach independent of the above discussed approaches based on ΔνOH and ΔHads of the weak bases, we adopted and slightly modified the Kondo`s idea and applied it for the first time to the investigation of acidity of MWW zeolites represented here by 3D analogue (MCM-22) and layered form (MCM-36). In addition, this study can bring a light into the question of validity of frequency shift of OH groups upon interaction with probe molecule and adsorption enthalpy changes as a scaling factor for assessment of acid strength.
mixed with 77 ml of ion-exchanged C16TMA-OH (cetyltrimethylammonium hydroxide) and the slurry was stirred for overnight at ambient temperature. The product, denoted as MCM-22-sw, was separated by centrifugation, proper washed with water and dried at 60 °C. Then, pillaring was carried out with 5 g of MCM-22-sw in 150 ml of tetraethyl orthosilicate (1 g of zeolite per 30 ml of TEOS). The mixture was stirred and heated under reflux at 85 °C for 12 h. The solid was isolated by centrifugation and dried at 40 °C. Then, about 500 ml of water was added to 5.013 g of dried powder and stirred for 12 h (hydrolysis). The product was centrifuged again and dried at 60 °C. Final calcination was carried out at 540 °C for 6 h with temperature ramp of 2 °C/min. The calcined product MCM-36 was ion-exchanged into NH4+ form by treating four-times with 1.0 M NH4NO3 solution for 4 h at room temperature (100 mL of solution/1 g of zeolite ratio). NH4+-MFI with nominal Si/Al ratio 30 used in this study were supplied by Research Institute of Inorganic Chemistry, Ústí nad Labem, Czech Republic. 2.2. Zeolite characterization The structure and crystallinity of zeolites were determined by X-ray powder diffraction using Bruker AXS D8 Advance diffractometer equipped with a graphite monochromator and a position sensitive detector Våntec-1 using CuKα radiation in Bragg–Brentano geometry. Scanning electron microscopy (SEM) images were collected on JEOL JSM-7500 F microscope equipped with cold cathode-field emission at voltage of 1 kV in secondary electron imaging mode under high gentle beam conditions. Adsorption isotherms of nitrogen (at −196 °C) were measured on a Micromeritics ASAP 2020 static volumetric instrument. Prior to adsorption measurement, all samples were thoroughly degassed by slow heating (heating rate 0.5 °C/min) under turbomolecular pump vacuum up to 350 °C and degassed at target temperature for 12 h. The surface area (SBET) was evaluated by BET method using adsorption data in the p/p0 range of a relative pressure from 0.01 to pressure corresponding to maximum on Rouquerol plot [34]. The adsorbed amount at relative pressure p/p0 = 0.98 reflects the total adsorption capacity (Vtot). The tplot method [35] employing Harkins-Jura thickness equation [36] was applied to determine the volume of micropores (Vmic) and external surface (Sext). The concentration and the type of acid sites were determined by adsorption of pyridine as a probe molecule followed by FT-IR spectroscopy (Nicolet 6700 FTIR with DTGS detector) using the self-supported wafer technique. Prior to adsorption of probe molecules, self-supported zeolite wafers (with density of 8–12 mg/cm2) were activated in-situ by overnight evacuation at temperature 450 °C. Pyridine adsorption proceeded at 150 °C for 20 min at pyridine vapour pressure of 3 Torr, followed by 20 min evacuation at the same temperature. IR spectra were recorded with a 4 cm−1 optical resolution by accumulation of 128 scans for a single spectrum. The concentrations of Brønsted and Lewis acid sites were calculated from integral intensities of individual bands characteristic of pyridine on Brønsted acid sites at 1545 cm–1 and band of pyridine on Lewis acid site at 1454 cm–1 using molar absorption coefficients of ε(B) = 1.67 ± 0.1 cm/μmol and ε(L) = 2.22 ± 0.1 cm/ μmol, respectively [37].
2. Experimental 2.1. Preparation of materials 1.16 g of NaOH was dissolved in 251.2 g of water and mixed with 1.68 g of sodium aluminate (40–45% Na2O, 50–56% Al2O3). Then, 62 g of LUDOX (AS 30) was added. The mixture became thick and was stirred until homogeneous gel was formed. Then, 17.2 g of HMI (hexamethylenimine) was added and the final mixture was stirred for 2 h. The reaction mixture was charged into Teflon-lined steel autoclaves (4 x 90 ml). Crystallization proceeded at 150 °C under agitation and autogeneous pressure for 5 days. Solid product was collected by filtration, washed with distilled water and dried in the oven at 60 °C for 12 h. MCM-22 P was calcined at 540 °C for 6 h with temperature ramp of 2 °C. The calcined product, denoted as MCM-22, was ion-exchanged into NH4+ form by treating four-times with 1.0 M NH4NO3 solution for 4 h at room temperature (100 ml of solution/1 g of zeolite ratio). For synthesis MCM-36 material, 3.8 g of uncalcined MCM-22 P was
2.3. Isotopic exchange experiments To study Brønsted acid sites acidity/reactivity, isotopic exchange insitu experiments were carried out by using of AABspec #2000-A Multimode cell (AABspec Instrument Corp., Ireland) and FT-IR spectrometer Nicolet 6700 with MCT/A detector cooled by liquid nitrogen. The individual samples were pressed into thin self-supported wafers with density approximately 10 mg/cm2 and pre-treated by evacuation at 450 °C for 2 h (with slow heating rate of 2 °C/min). Then, investigated sample was cooled slowly down to 150 °C and exposed to the 20 mbar 181
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
volume of MCM-36 is caused by pillaring and incorporation of significant amount of amorphous silica in the form of pillars into resulting materials. Besides micropores, the MCM-36 exhibits also mesopores formed by additional void space in-between individual pillared lamellas. Analysis of mesopore size distribution of MCM-36 zeolite by NL DFT approach (using N2@77 K kernel for cylindrical pores and oxide surface) revealed pores with size between 2–3 nm. This finding is in a good agreement with interlayer distance calculated from XRD pattern of MCM-36 sample, taking into account the fact that we do not have a NL DFT model corresponding to the real nature and geometry of mesopores in the pillared zeolites (it is not exactly cylindrical pore). Hysteresis loops observable for all investigated materials in the range of p/p0 > 0.5 are related to nitrogen condensation in interparticle voids. Nature and concentration of acid sites were determined by wellestablished method based on adsorption of pyridine monitored by FT-IR spectroscopy [16,37,39]. Fig. 4 depicts IR spectra of all three zeolites in the range of (i) hydroxyl group vibration (Fig. 4A) and (ii) pyridine vibration (Fig. 4B). IR spectra of all samples in the region of OH groups vibration contain a band at 3747 cm−1 assigned to terminal silanol groups and a band related to Si(OH)Al Brønsted acid sites (BAS) centered at 3616 cm−1 in the spectrum of MFI sample and at 3622 cm−1 in the case of MWW zeolites [40,41]. No vibrational bands related to the OH groups on extra-framework or defective Al atoms were detected in all three samples. Adsorption of pyridine resulted in disappearance of BAS vibrational band, while silanols are almost intact. Simultaneously, typical three bands in the region from 1600 to 1400 cm−1 appeared. Band at 1545 cm−1 is related to the pyridinium ions coordinated to BAS (HPy+), band at 1454 cm−1 corresponds to the pyridine coordinatively bonded to Lewis acid sites (LPy) and band at 1490 cm−1 related to the superposition of the bands of pyridine on Brønsted and Lewis sites [12,37,42]. All three zeolites contain both Brønsted acid sites and Lewis acid sites, but in different amount. Quantitative analysis of IR spectra reported in Fig. 4B is summarized in Table 1. Ratio between BAS and LAS ranges from 1.19 for MCM-36 sample having largest population of Lewis sites to 3.84 for MFI zeolite with a quite low content of Lewis acid sites. Si/Al ratio determined for individual zeolites is equal to 34.6, 24.5 and 29.9 for MFI, MCM-22 and MCM-36 zeolite, respectively. Results of XRD, SEM and N2 adsorption isotherms measurements prove crystalline character of all three zeolitic materials with textural properties corresponding to the well-developed microporous of material and similar Si/Al ratio and content of BAS for all zeolites was find out by IR spectroscopy of adsorbed pyridine. To further investigate nature and strength of acid sites of MWW zeolites and to compare and assess the strength of Brønsted acid sites in MFI and MWW zeolites, D/H isotopic exchange experiments were performed on zeolites in D-form obtained by interaction with D2O at 150 °C prior to kinetic experiment. Experiments were carried out at temperatures in the range from 330 to 475 °C. Time-resolved IR spectra (for sake of clarity only some spectra were selected for each zeolite) measured during D/H isotopic exchange reaction over all three zeolites at 440–450 °C are shown in Fig. 5. IR spectra consist of three groups of vibrational bands assigned to: (i) O-D vibrations in the range of 28002500 cm−1, (ii) CeH vibrations of ethane in the range of 31002800 cm−1, and (iii) OeH vibrations in the range of 3750-3550 cm−1. It is noteworthy that position of OH group vibrational band is slightly red-shifted compared to the spectra recorded at room temperature (Fig. 4A) due to temperature effect of the OeH vibration (spectra were measured at 440–450 °C) [33,40]. Spectra of all zeolites recorded before ethane dosing exhibit bands in OD vibration region ascribed to the deuterated silanols (Si-OD) centered at 2752 cm−1 and deuterated Brønsted acid sites at 2644, 2655 and 2657 cm−1 for MFI (Fig. 5A), MCM-22 (Fig. 5B) and MCM-36 (Fig. 5C) zeolite, respectively. In addition, a residual SieOH band at 3730 cm−1 is also detectable for all three zeolites. On the other hand, Si(OH)Al Brønsted acid sites are completely exchanged into deuterated forms after reaction with D2O in all three cases. Upon contact with ethane, simultaneous increase in the
Fig. 1. XRD patterns of investigated zeolites. a - MFI, b – MCM-22 and c – MCM-36. Individual patterns are shifted along the y axis for sake of clarity.
of D2O for 30 min followed by additional 30 min of evacuation by turbomolecular pump. Upon interaction of zeolite with D2O, almost complete exchange (more than 95%, see Fig. 5) from protonic form to deuterated form of hydroxyls was observed. Deuterated sample was slowly heated to the target temperature (ranging from 330 to 475 °C) at which D/H isotopic exchange reaction was studied. After stabilization of required temperature (measured by thermocouple placed in wafer holder in the immediate vicinity of the sample), ethane (50 mbar) was introduced into IR cell and recording of time-resolved IR spectra measurement was started. IR spectra were collected continuously until conversion of OD groups into OH groups reached at least 60% with the optical resolution of 4 cm−1 by accumulation of 64 scans. The temperature of the sample was monitored during whole D/H isotopic exchange experiment and its variance during the IR spectra recording was less than 1 °C for all experiments. 3. Results and discussion Fig. 1 presents XRD patterns of the MFI, MCM-22 and MCM-36 zeolites. The diffraction pattern of MFI zeolite (Fig. 1a) contains exclusively only diffractions lines characteristic of the MFI structure. Sharp and well resolved diffraction lines, especially in the 20-25° 2θ range, prove the well-developed crystalline phase of MFI. In the case of MWW zeolites, the “fingerprint” region of the powder XRD is between 5 and 10° 2θ. The main diffraction lines in this region centered at 7.2, 8.1 and 10.2° 2θ are ascribed to the planes (100), (101), and (102). Diffractogram of MCM-22 zeolite (Fig. 1b) exhibits well separated lines of (101) and (102) planes demonstrating well-ordered periodic structure of MCM-22 zeolite. XRD pattern of MCM-36 sample contains additional low angle peak at 2.02° 2θ corresponding to d-spacing of 4.37 nm, which includes the c-parameter of MCM-22 unit cell (2.51 nm) and distance between individual layers forming the MCM-36 materials. The average interlayer distance of 1.86 nm was obtained from XRD pattern for MCM-36 sample. Moreover, broad and partially overlapped diffraction lines of (101) and (102) planes, typical for material with partially disordered (in ab plane) of MWW layers [38], are present in the XRD pattern of MCM-36 zeolite (Fig. 1c). SEM micrographs of all zeolites are displayed in Fig. 2. The MCM-22 and MCM-36 materials exhibit thin sheet like particles of micrometer size, which form cross-linked agglomerates. The MFI zeolite consists of globular particles having size from several hundred of nanometers to micrometer. Textural properties of investigated zeolites obtained from nitrogen adsorption isotherms (see Fig. 3) are summarized in Table 1. All investigated materials exhibit distinct microporous character with micropore volumes equal to 0.151, 0.178 and 0.113 cm3/g for MFI, MCM-22 and MCM-36, respectively. The decrease in the micropore 182
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
Fig. 2. SEM micrographs of the investigated zeolites. A – MCM-22, B – MCM-36 and C – MFI. Table 1 Basic characterization of the studied materials. Material
MFI MCM-22 MCM-36
SBET
Sext
Vmicro
a
b
c
(m2/ g)
(m2/ g)
(cm3/ g)
387 547 637
39 158 380
0.151 0.178 0.113
Vtot d (cm3/ g)
cL (Py)e (mmol/ g)
e
0.269 0.432 0.425
0.096 0.161 0.169
0.265 0.331 0.202
CB (Py)
Si/Al
e
k´ f (10−3 s-1)
(mmol/ g) 34.6 24.5 29.9
3.20 0.47 0.36
a The specific surface area determined by B.E.T. theory applied to the range of relative pressures following Rouquerol conditions [32]. b The specific area of the external surface of crystallites determined by the tplot method using Harkins-Jura equation. c Micropore volume determined by the t-plot method using Harkins-Jura equation. d The total pore volume determined from the amount adsorbed at p/ p0 = 0.98. e Calculated from IR spectra of adsorbed pyridine after desorption at 150 °C. f reaction rates of H/D isotopic exchange at 425 °C (values obtained by interpolation of measured data).
Fig. 3. N2 adsorption/desorption isotherms for MFI (a), MCM-22 (b) and MCM36 (c) zeolites. Empty points denote adsorption and full points denote desorption.
OH bands intensity at the expense of OD bands intensity was observed. Vibrational bands of protonated Brønsted acid sites at high temperature appeared at 3592, 3600 and 3603 cm−1 for MFI, MCM-22 and MCM-36 zeolites, respectively. After completing the experiment and removal of gas phase from the IR cell, no residual bands of some hydrocarbons or coke deposited on the surface of zeolites were detected. Also intensity of OD bands after repeated isotope exchange by D2O exhibited the same intensity as fresh sample before reaction with ethane. Therefore, conversion of ethane into higher hydrocarbons or carbon deposit remaining on the zeolite surface and blocking the Brønsted acid sites can be excluded. The IR spectra (Fig. 5) enable to assess that the rate of D/H exchange varies with type of zeolite. For more detailed inspection of isotopic exchange kinetics, the exchange reaction was studied at several temperatures for each sample and time changes of intensity of Brønsted
Si(OD)Al vibrational band were determined by deconvolution of the spectra. Generally, the D/H exchange reaction can be described by the following equation:
Z − OD + C2 H6 ⇄ Z − OH + C2 H5 D Due to more than thousand fold excess of ethane (when compared with the amount of OD group of the sample in the IR cell), probability of re-adsorption deuterated ethane and exchange of proton for deuterium is negligible and thus reverse reaction can be neglected. In addition, concentration of the ethane will be constant during the reaction due to big surplus, and kinetics can be described by pseudo-first order of reaction. If assumed that concentration of OD group on the surface is related to the intensity of the vibrational band (IOD), kinetics equation will be as follow 183
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
Fig. 4. IR spectra of OH group vibration (A) and adsorbed pyridine (B) on MFI (a), MCM-22 (b) and MCM-36 (c). Spectra denoted as I correspond to the surface of empty samples (after evacuation at 450 °C overnight), spectra denoted as II correspond to the surface with adsorbed pyridine after desorption for 20 min at 150 °C.
−
dIOD n = k pethane IOD = k ′ IOD dt
where k is rate constant, pethane is partial pressure of ethane, n is reaction order on the ethane pressure and t is time. Solving this equation led to the expression
I ⎞ = −k′ t ln ⎛⎜ OD 0 ⎟ ⎝ IOD ⎠ where I0OD is the initial integral intensity of acidic OD group vibrational band. Fig. 6 provides kinetics plots of natural logarithm of relative intensities of acidic OD band dependence on time at various temperatures for all three zeolites. All data fit the linear dependence so the assumptions mentioned above (i.e. multiple excess of ethane, constant ethane pressure, first order of reaction for OD) are reasonably fulfilled and they also agree with the reports of Kondo`s group [32]. From the slopes of the linear fits, the rate pseudo constant (k´) was derived. Values of k´ for all three zeolites in the range of temperature from 330 to 475 °C are shown in Fig. 6D. The k´ values exponentially increase with temperature as expected from the rate constant theory. It is evident that k´ of D/H isotopic exchange over MFI is significantly higher than over MWW zeolites. Kubota et al. [32,33] evaluated Brønsted acid sites strength of zeolites comparing activation energy of exchange reaction derived from temperature dependence of rate constants; the lower activation energy the stronger acid sites. The interpretation of activation energies is not straightforward due to necessity to take into account adsorbed state of the alkane and, therefore, heat of adsorption has to be considered. However, heat of adsorption is usually determined under completely different conditions (especially temperatures) than those applied for isotopic exchange reaction [43–45]. Heat of adsorption is not temperature independent as it was recently reported e.g. for interaction of CO with Brønsted sites in FER zeolite [46]. Therefore, direct rate constant comparison at constant temperature is proposed with assumption that faster isotopic exchange reaction will take place at the same temperature on stronger acid sites. Based on this assumption, it can be concluded that MFI zeolite exhibits stronger Brønsted acid sites than MWW zeolites. The strength of acid sites of MCM-22 and MCM-36 zeolite is almost the same. This finding is in line with report by Dumitriu et al. [47] concluding that pillaring process strongly influenced the total acidity but acid strength distribution remained similar like for MCM-22. Recent theoretical and experimental studied on ultrathin H-2dH zeolite layers claimed higher acid strength of sites on 2D materials, but the properties depends on thickness and flexibility of the layer and distance of the charge from the surface [48–50]. Relatively
Fig. 5. IR spectra of deuterated zeolites during D/H isotopic exchange by interaction with ethane (50 mbar). (A) MFI at 442 °C, (B) MCM-22 at 440 °C and (C) MCM-36 at 450 °C.
high thickness of the MWW layer including internal channel system is behind a small difference in strength of acid sites in MCM-22 and MCM36 zeolites. The order of the reaction rates (for illustration at 425 °C) MFI (3.2 10−3 s-1) > MCM-22 (4.7 10-4 s-1) ≈ MCM-36 (3.6 10-4 s-1) is in line with our results of carbon monoxide heat of adsorption measurement by VTIR spectroscopy and adsorption microcalorimetry [10,22] revealing significantly lower values of ΔHads for MCM-22 and MCM-56 compared with MFI and FER zeolites, as well as significantly lower adsorption heat of ammonia on MCM-22 zeolite with respect to MFI reported in the literature [51–53]. Thus, the frequency shift of OH group vibration upon interaction with bases as a scaling factor of the
184
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
Fig. 6. Natural logarithm of relative intensity of acidic OD group as a function of time at various temperatures for (A) MFI, (B) MCM-22 and (C) MCM-36 and temperature dependence of rate constants for all three zeolites (D).
reported statement that determination of the enthalpy change of probe molecules adsorption involved in hydrogen bonding is more reliable instrumental method for assessment of acid strength than OeH frequency shift probed by a weak base adsorbed at a low temperature.
acid sites strength, introduced by Paukshtis and Yurchenko [19], is not generally valid method and can lead to misinterpretation of the experimental data as in the case of MCM-22 zeolite [39]. A possible problem with evaluation of acid site strength by frequency shift of OeH vibration due to Fermi resonance effect was recently stressed by Hadjiivanov et al. [54–56] and based on theoretical modelling Rybicki and Sauer [49]. The results reported here bring further evidence that the Brønsted acidity ranking by OeH frequency shift probed by an adsorbed weak base can be misleading and that the correlation introduced by Paukshtis and Yurchenko [19] is in part the result of a coincidence of mutually compensating phenomena and serendipities.
Acknowledgement Financial support from the Czech Science Foundation for the project of the Centre of Excellence (P106/12/G015) is gratefully acknowledged.
4. Conclusions
References
The D/H isotopic exchange reaction of deuterated Brønsted acid sites with ethane carried out at various temperatures and monitored by FT-IR spectroscopy was used to assess the strength of acid site in MFI and MWW type zeolites. The following conclusions can be drawn from the experimental data:
[1] S. Al-Khattaf, S.A. Ali, A.M. Aitani, N. Zilkova, D. Kubicka, J. Cejka, Catal. Rev.-Sci. Eng. 56 (2014) 333–402. [2] G. Busca, Chem. Rev. 107 (2007) 5366–5410. [3] S. Singhal, S. Agarwal, S. Arora, N. Singhal, A. Kumar, Catal. Sci. Technol. 7 (2017) 5810–5819. [4] F. Su, Y.H. Guo, Green Chem. 16 (2014) 2934–2957. [5] N. Rahimi, R. Karimzadeh, Appl. Catal. A-Gen. 398 (2011) 1–17. [6] M.V. Opanasenko, W.J. Roth, J. Cejka, Catal. Sci. Technol. 6 (2016) 2467–2484. [7] E.G. Derouane, J.C. Vedrine, R.R. Pinto, P.M. Borges, L. Costa, M. Lemos, F. Lemos, F.R. Ribeiro, Catal. Rev.-Sci. Eng. 55 (2013) 454–515. [8] M. Boronat, A. Corma, Catal. Lett. 145 (2015) 162–172. [9] C. Lee, D.J. Parrillo, R.J. Gorte, W.E. Farneth, J. Am. Chem. Soc. 118 (1996) 3262–3268. [10] C.O. Arean, M.R. Delgado, P. Nachtigall, H.V. Thang, M. Rubes, R. Bulanek, P. Chlubna-Eliasova, Phys. Chem. Chem. Phys. 16 (2014) 10129–10141. [11] A.M. Zheng, S.B. Liu, F. Deng, Chem. Rev. 117 (2017) 12475–12531. [12] G. Busca, Microporous Mesoporous Mater. 254 (2017) 3–16. [13] G. Busca, Phys. Chem. Chem. Phys. 1 (1999) 723–736. [14] M. Hunger, Catal. Rev. 39 (1997) 345–393. [15] U. Fleischer, W. Kutzelnigg, A. Bleiber, J. Sauer, J. Am. Chem. Soc. 115 (1993) 7833–7838. [16] B. Gil, S.I. Zones, S.J. Hwang, M. Bejblova, J. Cejka, J. Phys. Chem. C 112 (2008) 2997–3007. [17] J. Sauer, J.R. Hill, Chem. Phys. Lett. 218 (1994) 333–337. [18] K.P. Schroder, J. Sauer, M. Leslie, C.R.A. Catlow, J.M. Thomas, Chem. Phys. Lett.
• Isotopic exchange reaction between deuterated hydroxyl groups and • • •
ethane at elevated temperatures can be described by pseudo first order kinetics. The rate constants of isotopic exchange are in the order MFI > MCM-22 ≈ MCM-36. The order of measured rate constants is consistent with previously reported heats of adsorption of CO or NH3. Thus, MFI zeolite contains significantly stronger Brønsted acid sites than MCM-22 and MCM-36. Strength of acid sites in MCM-22 and MCM-36 is comparable. Relatively high thickness of the MWW layer containing internal channel system is behind a small difference in strength of acid sites in MCM-22 and MCM-36 zeolites. Results of the isotopic exchange reaction support our previously 185
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
[37] C.A. Emeis, J. Catal. 141 (1993) 347–354. [38] M. Polozij, H.V. Thang, M. Rubes, P. Eliasova, J. Cejka, P. Nachtigall, Dalton Trans. 43 (2014) 10443–10450. [39] B. Gil, B. Marszalek, A. Micek-Ilnicka, Z. Olejniczak, Top. Catal. 53 (2010) 1340–1348. [40] K. Hadjiivanov, F.C. Jentoft (Ed.), Advances in Catalysis, Academic Press, 2014, pp. 99–318. [41] M. Bevilacqua, D. Meloni, F. Sini, R. Monaci, T. Montanari, G. Busca, J. Phys. Chem. C 112 (2008) 9023–9033. [42] M.C. Kung, H.H. Kung, Catal. Rev. 27 (1985) 425–460. [43] B.A. De Moor, M.F. Reyniers, O.C. Gobin, J.A. Lercher, G.B. Marin, J. Phys. Chem. C 115 (2011) 1204–1219. [44] G. Piccini, M. Alessio, J. Sauer, Y.C. Zhi, Y. Liu, R. Kolvenbach, A. Jentys, J.A. Lercher, J. Phys. Chem. C 119 (2015) 6128–6137. [45] J.A. Dunne, R. Mariwals, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Langmuir 12 (1996) 5888–5895. [46] M. Rubeš, M. Trachta, E. Koudelková, R. Bulánek, J. Klimeš, P. Nachtigall, O. Bludský, J. Phys. Chem. C 122 (2018) 26088–26095. [47] E. Dumitriu, D. Meloni, R. Monaci, V. Solinas, C. R. Chim. 8 (2005) 441–456. [48] J.A. Boscoboinik, X. Yu, E. Emmez, B. Yang, S. Shaikhutdinov, F.D. Fischer, J. Sauer, H.J. Freund, J. Phys. Chem. C 117 (2013) 13547–13556. [49] M. Rybicki, J. Sauer, Phys. Chem. Chem. Phys. 17 (2015) 27873–27882. [50] J. Hermann, M. Trachta, P. Nachtigall, O. Bludsky, Catal. Today 227 (2014) 2–8. [51] D.H. Zhou, Y. Bao, M.M. Yang, N. He, G. Yang, J. Mol. Catal. A-Chem. 244 (2006) 11–19. [52] Z. Zhu, Q. Chen, Z. Xie, W. Yang, C. Li, Microporous Mesoporous Mater. 88 (2006) 16–21. [53] C.-C. Tsai, C.-Y. Zhong, I. Wang, S.-B. Liu, W.-H. Chen, T.-C. Tsai, Appl. Catal. AGen. 267 (2004) 87–94. [54] K. Chakarova, K. Hadjiivanov, J. Phys. Chem. C 115 (2011) 4806–4817. [55] K. Chakarova, K. Hadjiivanov, Microporous Mesoporous Mater. 177 (2013) 59–65. [56] K. Chakarova, K. Hadjiivanov, Chem. Commun. (Camb.) 47 (2011) 1878–1880.
188 (1992) 320–325. [19] E.A. Paukshtis, E.N. Yurchenko, Russ. Chem. Rev. 52 (1983) 242. [20] M.V. Frash, M.A. Makarova, A.M. Rigby, J. Phys. Chem. B 101 (1997) 2116–2119. [21] M.A. Makarova, K.M. Alghefaili, J. Dwyer, J. Chem. Soc.-Faraday Trans. 90 (1994) 383–386. [22] M.R. Delgado, R. Bulanek, P. Chlubna, C.O. Arean, Catal. Today 227 (2014) 45–49. [23] J.S. Beck, A.B. Dandekar, T.F. Degnan, M. Guisnet, J.P. Gilson (Eds.), Zeolites for Cleaner Technologies, Imperial College Press, 2002, pp. 223–237. [24] B. Maerz, C.M. Smith, R.A. Meyers (Ed.), Handbook of Petrochemicals Production Processes, Mc Graw Hill, 2005, pp. 523–538. [25] A.B. Dandekar, N. Hryniszak, D.L. Stern, US patent 7868215 B2, 2011. [26] M. Motoyuki, K. Yamamoto, A.V. Sapre, J.P. Mc Williams, S.P. Donnelly, US patent 612 1501 A, 1997. [27] P. Wu, T. Komatsu, T. Yashima, Microporous Mesoporous Mater. 22 (1998) 343–356. [28] M.T. Portilla, F.J. Llopis, C. Martinez, S. Valencia, A. Corma, Appl. Catal. A-Gen. 393 (2011) 257–268. [29] N. Kumar, L.E. Lindfors, Appl. Catal. A-Gen. 147 (1996) 175–187. [30] M. Pitinova-Stekrova, P. Eliasova, T. Weissenberger, M. Shamzhy, Z. Musilova, J. Cejka, Catal. Sci. Technol. 8 (2018) 4690–4701. [31] M. Stekrova, M. Kubu, M. Shamzhy, Z. Musilova, J. Cejka, Catal. Sci. Technol. 8 (2018) 2488–2501. [32] T. Kubota, R. Osuga, T. Yokoi, J.N. Kondo, Top. Catal. 60 (2017) 1496–1505. [33] R. Osuga, T. Yokoi, K. Doitomi, H. Hirao, J.N. Kondo, J. Phys. Chem. C 121 (2017) 25411–25420. [34] J. Rouquerol, P. Llewellyn, F. Rouquerol, P.L. Llewellyn, F. Rodriquez Reinoso, J. Rouqerol, N. Seaton (Eds.), Characterization of Porous Solids Vii - Proceedings of the 7th International Symposium on the Characterization of Porous Solids, 2006, pp. 49–56. [35] B.C. Lippens, J.H. Deboer, J. Catal. 4 (1965) 319-&. [36] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, Pure Appl. Chem. 87 (2015) 1051–1069.
186
Contents lists available at ScienceDirect
Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
H/D reactivity and acidity of Brønsted acid sites of MWW zeolites: Comparison with MFI zeolite Roman Buláneka, Martin Kubůb, Jan Vaculíka, Jiří Čejkac,
T
⁎
a
Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10 Pardubice, Czech Republic J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic c Department of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acidity Zeolites MWW MFI Isotopic exchange Adsorption enthalpy
Acid site strength in MWW and MFI zeolites was investigated by isotopic exchange reaction between deuterated hydroxyl groups of zeolites and ethane molecules. The course of the reaction was monitored by FT-IR spectroscopy through time changes of integral intensities of the Si(OD)Al Brønsted acid sites represented by absorption bands at 2644, 2655, and 2657 cm−1 for MFI, MWW and MCM-36 zeolite, respectively. The rate of the reaction was described by pseudo first order kinetics and acid site strength was compared using rate constants characteristic of individual zeolites at constant temperature. The exchange reaction is significantly faster over MFI zeolite compared with MWW zeolites, suggesting a higher acid strength of Brønsted sites in MFI zeolite than in MWW. In our previous study (Arean et al. Phys. Chem. Chem. Phys. 16 (2014) 10,129), we found notable discrepancy in the relation between OH group frequency shift induced by adsorption of weak base molecules like CO and N2 and the change of adsorption enthalpy of these probe molecules for MWW zeolites, which calls into question the use of frequency shifts as a measure of acidity. Our reported results based on isotopic exchange reaction rate measurement correlate with the change of the probe molecules adsorption enthalpy involved in hydrogen bonding. Therefore, we believe that enthalpy change or H/D exchange activity measurement is more reliable method for assessment of acid strength than OeH frequency shift probed by a weak base adsorbed at a low temperature.
1. Introduction
energy of base molecules derived from temperature programmed desorption (TPD) data, and measurement of the rates of probe acid catalysed reactions, were reported in the literature [7–18]. IR spectroscopy is by far the most commonly used technique for characterizing acid sites. The interaction of hydroxyl groups with the weak bases like CO or N2 leads to the formation of hydrogen bonded species that cause red-shift of the OeH vibration, which is very often used as an acidity scale (stronger acid will undergo a larger shift). This method was pioneered by Paukshtis and Yurchenko in 1980s [19] and by Makarova et al. in 1990s [20,21] and logarithmic relationship between frequency shift of OH group vibration upon adsorption of weak basis (ΔνOH) and zeolite acid strength was proposed. On the other hand, stronger acid sites will interact with weak base molecules by larger interaction energies. Therefore, the relationship between frequency shift of OH group vibration and enthalpy change (ΔHads) can be expected. Such correlation was found for CO and N2 adsorption on H-Y, HMFI and H-FER, but MWW zeolites do not follow this correlation [10,22]. MWW zeolites (specifically MCM-22 and MCM-56 zeolites)
Brønsted acid catalysed reactions, namely cracking, isomerizations, alkylations, hydrocracking, as well as methanol to olefin (MTO) or gasoline (MTG) conversions and alcohol dehydration or carbonylation are key processes in oil refining, petrochemistry, and fine chemical production [1–6]. Zeolites and related zeotypes have been used as acid catalysts for these processes since the 1960s due to their high catalytic activities, environmentally benign nature, and excellent thermal stability. Their catalytic activity is determined mainly by the strength and number of their Brønsted acid sites. Therefore, the knowledge of the factors controlling the acidic properties of zeolites and ability to assess (or compare) strength of acid sites in various zeolites is key challenge. To characterize acid sites strength, frequency of OeH bond vibration and OeH frequency shifts upon interaction of OH groups with a base molecules (ΔνOH), characteristic spectral bands of the base molecules adsorbed on acid sites, chemical shift of 1H signals in MAS NMR spectra, measurement of adsorption enthalpy or desorption activation
⁎
Corresponding author. E-mail address: [email protected] (J. Čejka).
https://doi.org/10.1016/j.apcata.2019.02.024 Received 27 November 2018; Received in revised form 16 January 2019; Accepted 9 February 2019 Available online 18 February 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
exhibited quite large frequency shift (316-320 cm−1), larger than MFI zeolite (303 cm−1) but the adsorption heats of CO are distinctly lower than for MFI (22.5 kJ/mol (MCM-22) and 20.0 kJ/mol (MCM-56) vs 29.4 kJ/mol for MFI). The same discrepancy was found also for dinitrogen. This peculiarity of MWW zeolites is worth examining in more detail, because understanding of MWW zeolites behaviour with respect to weak bases adsorption enthalpies and induced frequency shifts of OH vibrations would help to increase the understanding of zeolite acidity. Moreover, finding of proper method to determine the acid strength using molecules and experimental conditions similar to real acid-catalysed reactions would be useful and would help to optimize the catalytic performance of MCM-22 zeolite, which is industrially important zeolitic catalyst applied in Exxon-Mobil EBMax process of ethylbenzene production [23,24], Mobil-Raytheon process of cumene production [25] or 2,6-dimethylnaphtalene production (ExxonMobil-KobeSteel process) [26]. In addition MWW zeolites exhibit promising catalytic activities in many other reactions including toluene disproportionation [27] or alkylation [28], n-butane aromatization [29] or alpha pinene oxide isomerization [30,31] and many others. The aim of this study is to evaluate acid sites strength of MWW zeolite in bulk and pillared forms, compare it with MFI zeolite as a “benchmark”, and to assess the validity of the formerly proposed scaling factors for assessment of acid sites (ΔνOH and ΔHads). It should be stressed that using weak base molecules as a probe molecules for testing acid sites and assessment of their strength is sometimes questioned in literature. Major reservations relate to (i) application of experimental conditions significantly different from real conditions of catalytic reactions and (ii) using probe molecules with significantly different nature and size compared to real reactants. These reservations rise questions of accessibility of the acid sites by probe molecule and real reactant and not negligible contribution of secondary (lateral) interactions of molecules with zeolite framework depending on the molecule type and local geometry of acid site. These pitfalls are not easy to overcome. Recently, Kondo et al. [32,33] proposed the scaling zeolite Brønsted acid site strength by monitoring the rate of H/D isotopic exchange reaction between deuterated zeolite acid sites and ethane [32] or methane [33]. In order to avoid these reservations and to assess the acidity/reactivity of acid sites by an experimental approach independent of the above discussed approaches based on ΔνOH and ΔHads of the weak bases, we adopted and slightly modified the Kondo`s idea and applied it for the first time to the investigation of acidity of MWW zeolites represented here by 3D analogue (MCM-22) and layered form (MCM-36). In addition, this study can bring a light into the question of validity of frequency shift of OH groups upon interaction with probe molecule and adsorption enthalpy changes as a scaling factor for assessment of acid strength.
mixed with 77 ml of ion-exchanged C16TMA-OH (cetyltrimethylammonium hydroxide) and the slurry was stirred for overnight at ambient temperature. The product, denoted as MCM-22-sw, was separated by centrifugation, proper washed with water and dried at 60 °C. Then, pillaring was carried out with 5 g of MCM-22-sw in 150 ml of tetraethyl orthosilicate (1 g of zeolite per 30 ml of TEOS). The mixture was stirred and heated under reflux at 85 °C for 12 h. The solid was isolated by centrifugation and dried at 40 °C. Then, about 500 ml of water was added to 5.013 g of dried powder and stirred for 12 h (hydrolysis). The product was centrifuged again and dried at 60 °C. Final calcination was carried out at 540 °C for 6 h with temperature ramp of 2 °C/min. The calcined product MCM-36 was ion-exchanged into NH4+ form by treating four-times with 1.0 M NH4NO3 solution for 4 h at room temperature (100 mL of solution/1 g of zeolite ratio). NH4+-MFI with nominal Si/Al ratio 30 used in this study were supplied by Research Institute of Inorganic Chemistry, Ústí nad Labem, Czech Republic. 2.2. Zeolite characterization The structure and crystallinity of zeolites were determined by X-ray powder diffraction using Bruker AXS D8 Advance diffractometer equipped with a graphite monochromator and a position sensitive detector Våntec-1 using CuKα radiation in Bragg–Brentano geometry. Scanning electron microscopy (SEM) images were collected on JEOL JSM-7500 F microscope equipped with cold cathode-field emission at voltage of 1 kV in secondary electron imaging mode under high gentle beam conditions. Adsorption isotherms of nitrogen (at −196 °C) were measured on a Micromeritics ASAP 2020 static volumetric instrument. Prior to adsorption measurement, all samples were thoroughly degassed by slow heating (heating rate 0.5 °C/min) under turbomolecular pump vacuum up to 350 °C and degassed at target temperature for 12 h. The surface area (SBET) was evaluated by BET method using adsorption data in the p/p0 range of a relative pressure from 0.01 to pressure corresponding to maximum on Rouquerol plot [34]. The adsorbed amount at relative pressure p/p0 = 0.98 reflects the total adsorption capacity (Vtot). The tplot method [35] employing Harkins-Jura thickness equation [36] was applied to determine the volume of micropores (Vmic) and external surface (Sext). The concentration and the type of acid sites were determined by adsorption of pyridine as a probe molecule followed by FT-IR spectroscopy (Nicolet 6700 FTIR with DTGS detector) using the self-supported wafer technique. Prior to adsorption of probe molecules, self-supported zeolite wafers (with density of 8–12 mg/cm2) were activated in-situ by overnight evacuation at temperature 450 °C. Pyridine adsorption proceeded at 150 °C for 20 min at pyridine vapour pressure of 3 Torr, followed by 20 min evacuation at the same temperature. IR spectra were recorded with a 4 cm−1 optical resolution by accumulation of 128 scans for a single spectrum. The concentrations of Brønsted and Lewis acid sites were calculated from integral intensities of individual bands characteristic of pyridine on Brønsted acid sites at 1545 cm–1 and band of pyridine on Lewis acid site at 1454 cm–1 using molar absorption coefficients of ε(B) = 1.67 ± 0.1 cm/μmol and ε(L) = 2.22 ± 0.1 cm/ μmol, respectively [37].
2. Experimental 2.1. Preparation of materials 1.16 g of NaOH was dissolved in 251.2 g of water and mixed with 1.68 g of sodium aluminate (40–45% Na2O, 50–56% Al2O3). Then, 62 g of LUDOX (AS 30) was added. The mixture became thick and was stirred until homogeneous gel was formed. Then, 17.2 g of HMI (hexamethylenimine) was added and the final mixture was stirred for 2 h. The reaction mixture was charged into Teflon-lined steel autoclaves (4 x 90 ml). Crystallization proceeded at 150 °C under agitation and autogeneous pressure for 5 days. Solid product was collected by filtration, washed with distilled water and dried in the oven at 60 °C for 12 h. MCM-22 P was calcined at 540 °C for 6 h with temperature ramp of 2 °C. The calcined product, denoted as MCM-22, was ion-exchanged into NH4+ form by treating four-times with 1.0 M NH4NO3 solution for 4 h at room temperature (100 ml of solution/1 g of zeolite ratio). For synthesis MCM-36 material, 3.8 g of uncalcined MCM-22 P was
2.3. Isotopic exchange experiments To study Brønsted acid sites acidity/reactivity, isotopic exchange insitu experiments were carried out by using of AABspec #2000-A Multimode cell (AABspec Instrument Corp., Ireland) and FT-IR spectrometer Nicolet 6700 with MCT/A detector cooled by liquid nitrogen. The individual samples were pressed into thin self-supported wafers with density approximately 10 mg/cm2 and pre-treated by evacuation at 450 °C for 2 h (with slow heating rate of 2 °C/min). Then, investigated sample was cooled slowly down to 150 °C and exposed to the 20 mbar 181
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
volume of MCM-36 is caused by pillaring and incorporation of significant amount of amorphous silica in the form of pillars into resulting materials. Besides micropores, the MCM-36 exhibits also mesopores formed by additional void space in-between individual pillared lamellas. Analysis of mesopore size distribution of MCM-36 zeolite by NL DFT approach (using N2@77 K kernel for cylindrical pores and oxide surface) revealed pores with size between 2–3 nm. This finding is in a good agreement with interlayer distance calculated from XRD pattern of MCM-36 sample, taking into account the fact that we do not have a NL DFT model corresponding to the real nature and geometry of mesopores in the pillared zeolites (it is not exactly cylindrical pore). Hysteresis loops observable for all investigated materials in the range of p/p0 > 0.5 are related to nitrogen condensation in interparticle voids. Nature and concentration of acid sites were determined by wellestablished method based on adsorption of pyridine monitored by FT-IR spectroscopy [16,37,39]. Fig. 4 depicts IR spectra of all three zeolites in the range of (i) hydroxyl group vibration (Fig. 4A) and (ii) pyridine vibration (Fig. 4B). IR spectra of all samples in the region of OH groups vibration contain a band at 3747 cm−1 assigned to terminal silanol groups and a band related to Si(OH)Al Brønsted acid sites (BAS) centered at 3616 cm−1 in the spectrum of MFI sample and at 3622 cm−1 in the case of MWW zeolites [40,41]. No vibrational bands related to the OH groups on extra-framework or defective Al atoms were detected in all three samples. Adsorption of pyridine resulted in disappearance of BAS vibrational band, while silanols are almost intact. Simultaneously, typical three bands in the region from 1600 to 1400 cm−1 appeared. Band at 1545 cm−1 is related to the pyridinium ions coordinated to BAS (HPy+), band at 1454 cm−1 corresponds to the pyridine coordinatively bonded to Lewis acid sites (LPy) and band at 1490 cm−1 related to the superposition of the bands of pyridine on Brønsted and Lewis sites [12,37,42]. All three zeolites contain both Brønsted acid sites and Lewis acid sites, but in different amount. Quantitative analysis of IR spectra reported in Fig. 4B is summarized in Table 1. Ratio between BAS and LAS ranges from 1.19 for MCM-36 sample having largest population of Lewis sites to 3.84 for MFI zeolite with a quite low content of Lewis acid sites. Si/Al ratio determined for individual zeolites is equal to 34.6, 24.5 and 29.9 for MFI, MCM-22 and MCM-36 zeolite, respectively. Results of XRD, SEM and N2 adsorption isotherms measurements prove crystalline character of all three zeolitic materials with textural properties corresponding to the well-developed microporous of material and similar Si/Al ratio and content of BAS for all zeolites was find out by IR spectroscopy of adsorbed pyridine. To further investigate nature and strength of acid sites of MWW zeolites and to compare and assess the strength of Brønsted acid sites in MFI and MWW zeolites, D/H isotopic exchange experiments were performed on zeolites in D-form obtained by interaction with D2O at 150 °C prior to kinetic experiment. Experiments were carried out at temperatures in the range from 330 to 475 °C. Time-resolved IR spectra (for sake of clarity only some spectra were selected for each zeolite) measured during D/H isotopic exchange reaction over all three zeolites at 440–450 °C are shown in Fig. 5. IR spectra consist of three groups of vibrational bands assigned to: (i) O-D vibrations in the range of 28002500 cm−1, (ii) CeH vibrations of ethane in the range of 31002800 cm−1, and (iii) OeH vibrations in the range of 3750-3550 cm−1. It is noteworthy that position of OH group vibrational band is slightly red-shifted compared to the spectra recorded at room temperature (Fig. 4A) due to temperature effect of the OeH vibration (spectra were measured at 440–450 °C) [33,40]. Spectra of all zeolites recorded before ethane dosing exhibit bands in OD vibration region ascribed to the deuterated silanols (Si-OD) centered at 2752 cm−1 and deuterated Brønsted acid sites at 2644, 2655 and 2657 cm−1 for MFI (Fig. 5A), MCM-22 (Fig. 5B) and MCM-36 (Fig. 5C) zeolite, respectively. In addition, a residual SieOH band at 3730 cm−1 is also detectable for all three zeolites. On the other hand, Si(OH)Al Brønsted acid sites are completely exchanged into deuterated forms after reaction with D2O in all three cases. Upon contact with ethane, simultaneous increase in the
Fig. 1. XRD patterns of investigated zeolites. a - MFI, b – MCM-22 and c – MCM-36. Individual patterns are shifted along the y axis for sake of clarity.
of D2O for 30 min followed by additional 30 min of evacuation by turbomolecular pump. Upon interaction of zeolite with D2O, almost complete exchange (more than 95%, see Fig. 5) from protonic form to deuterated form of hydroxyls was observed. Deuterated sample was slowly heated to the target temperature (ranging from 330 to 475 °C) at which D/H isotopic exchange reaction was studied. After stabilization of required temperature (measured by thermocouple placed in wafer holder in the immediate vicinity of the sample), ethane (50 mbar) was introduced into IR cell and recording of time-resolved IR spectra measurement was started. IR spectra were collected continuously until conversion of OD groups into OH groups reached at least 60% with the optical resolution of 4 cm−1 by accumulation of 64 scans. The temperature of the sample was monitored during whole D/H isotopic exchange experiment and its variance during the IR spectra recording was less than 1 °C for all experiments. 3. Results and discussion Fig. 1 presents XRD patterns of the MFI, MCM-22 and MCM-36 zeolites. The diffraction pattern of MFI zeolite (Fig. 1a) contains exclusively only diffractions lines characteristic of the MFI structure. Sharp and well resolved diffraction lines, especially in the 20-25° 2θ range, prove the well-developed crystalline phase of MFI. In the case of MWW zeolites, the “fingerprint” region of the powder XRD is between 5 and 10° 2θ. The main diffraction lines in this region centered at 7.2, 8.1 and 10.2° 2θ are ascribed to the planes (100), (101), and (102). Diffractogram of MCM-22 zeolite (Fig. 1b) exhibits well separated lines of (101) and (102) planes demonstrating well-ordered periodic structure of MCM-22 zeolite. XRD pattern of MCM-36 sample contains additional low angle peak at 2.02° 2θ corresponding to d-spacing of 4.37 nm, which includes the c-parameter of MCM-22 unit cell (2.51 nm) and distance between individual layers forming the MCM-36 materials. The average interlayer distance of 1.86 nm was obtained from XRD pattern for MCM-36 sample. Moreover, broad and partially overlapped diffraction lines of (101) and (102) planes, typical for material with partially disordered (in ab plane) of MWW layers [38], are present in the XRD pattern of MCM-36 zeolite (Fig. 1c). SEM micrographs of all zeolites are displayed in Fig. 2. The MCM-22 and MCM-36 materials exhibit thin sheet like particles of micrometer size, which form cross-linked agglomerates. The MFI zeolite consists of globular particles having size from several hundred of nanometers to micrometer. Textural properties of investigated zeolites obtained from nitrogen adsorption isotherms (see Fig. 3) are summarized in Table 1. All investigated materials exhibit distinct microporous character with micropore volumes equal to 0.151, 0.178 and 0.113 cm3/g for MFI, MCM-22 and MCM-36, respectively. The decrease in the micropore 182
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
Fig. 2. SEM micrographs of the investigated zeolites. A – MCM-22, B – MCM-36 and C – MFI. Table 1 Basic characterization of the studied materials. Material
MFI MCM-22 MCM-36
SBET
Sext
Vmicro
a
b
c
(m2/ g)
(m2/ g)
(cm3/ g)
387 547 637
39 158 380
0.151 0.178 0.113
Vtot d (cm3/ g)
cL (Py)e (mmol/ g)
e
0.269 0.432 0.425
0.096 0.161 0.169
0.265 0.331 0.202
CB (Py)
Si/Al
e
k´ f (10−3 s-1)
(mmol/ g) 34.6 24.5 29.9
3.20 0.47 0.36
a The specific surface area determined by B.E.T. theory applied to the range of relative pressures following Rouquerol conditions [32]. b The specific area of the external surface of crystallites determined by the tplot method using Harkins-Jura equation. c Micropore volume determined by the t-plot method using Harkins-Jura equation. d The total pore volume determined from the amount adsorbed at p/ p0 = 0.98. e Calculated from IR spectra of adsorbed pyridine after desorption at 150 °C. f reaction rates of H/D isotopic exchange at 425 °C (values obtained by interpolation of measured data).
Fig. 3. N2 adsorption/desorption isotherms for MFI (a), MCM-22 (b) and MCM36 (c) zeolites. Empty points denote adsorption and full points denote desorption.
OH bands intensity at the expense of OD bands intensity was observed. Vibrational bands of protonated Brønsted acid sites at high temperature appeared at 3592, 3600 and 3603 cm−1 for MFI, MCM-22 and MCM-36 zeolites, respectively. After completing the experiment and removal of gas phase from the IR cell, no residual bands of some hydrocarbons or coke deposited on the surface of zeolites were detected. Also intensity of OD bands after repeated isotope exchange by D2O exhibited the same intensity as fresh sample before reaction with ethane. Therefore, conversion of ethane into higher hydrocarbons or carbon deposit remaining on the zeolite surface and blocking the Brønsted acid sites can be excluded. The IR spectra (Fig. 5) enable to assess that the rate of D/H exchange varies with type of zeolite. For more detailed inspection of isotopic exchange kinetics, the exchange reaction was studied at several temperatures for each sample and time changes of intensity of Brønsted
Si(OD)Al vibrational band were determined by deconvolution of the spectra. Generally, the D/H exchange reaction can be described by the following equation:
Z − OD + C2 H6 ⇄ Z − OH + C2 H5 D Due to more than thousand fold excess of ethane (when compared with the amount of OD group of the sample in the IR cell), probability of re-adsorption deuterated ethane and exchange of proton for deuterium is negligible and thus reverse reaction can be neglected. In addition, concentration of the ethane will be constant during the reaction due to big surplus, and kinetics can be described by pseudo-first order of reaction. If assumed that concentration of OD group on the surface is related to the intensity of the vibrational band (IOD), kinetics equation will be as follow 183
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
Fig. 4. IR spectra of OH group vibration (A) and adsorbed pyridine (B) on MFI (a), MCM-22 (b) and MCM-36 (c). Spectra denoted as I correspond to the surface of empty samples (after evacuation at 450 °C overnight), spectra denoted as II correspond to the surface with adsorbed pyridine after desorption for 20 min at 150 °C.
−
dIOD n = k pethane IOD = k ′ IOD dt
where k is rate constant, pethane is partial pressure of ethane, n is reaction order on the ethane pressure and t is time. Solving this equation led to the expression
I ⎞ = −k′ t ln ⎛⎜ OD 0 ⎟ ⎝ IOD ⎠ where I0OD is the initial integral intensity of acidic OD group vibrational band. Fig. 6 provides kinetics plots of natural logarithm of relative intensities of acidic OD band dependence on time at various temperatures for all three zeolites. All data fit the linear dependence so the assumptions mentioned above (i.e. multiple excess of ethane, constant ethane pressure, first order of reaction for OD) are reasonably fulfilled and they also agree with the reports of Kondo`s group [32]. From the slopes of the linear fits, the rate pseudo constant (k´) was derived. Values of k´ for all three zeolites in the range of temperature from 330 to 475 °C are shown in Fig. 6D. The k´ values exponentially increase with temperature as expected from the rate constant theory. It is evident that k´ of D/H isotopic exchange over MFI is significantly higher than over MWW zeolites. Kubota et al. [32,33] evaluated Brønsted acid sites strength of zeolites comparing activation energy of exchange reaction derived from temperature dependence of rate constants; the lower activation energy the stronger acid sites. The interpretation of activation energies is not straightforward due to necessity to take into account adsorbed state of the alkane and, therefore, heat of adsorption has to be considered. However, heat of adsorption is usually determined under completely different conditions (especially temperatures) than those applied for isotopic exchange reaction [43–45]. Heat of adsorption is not temperature independent as it was recently reported e.g. for interaction of CO with Brønsted sites in FER zeolite [46]. Therefore, direct rate constant comparison at constant temperature is proposed with assumption that faster isotopic exchange reaction will take place at the same temperature on stronger acid sites. Based on this assumption, it can be concluded that MFI zeolite exhibits stronger Brønsted acid sites than MWW zeolites. The strength of acid sites of MCM-22 and MCM-36 zeolite is almost the same. This finding is in line with report by Dumitriu et al. [47] concluding that pillaring process strongly influenced the total acidity but acid strength distribution remained similar like for MCM-22. Recent theoretical and experimental studied on ultrathin H-2dH zeolite layers claimed higher acid strength of sites on 2D materials, but the properties depends on thickness and flexibility of the layer and distance of the charge from the surface [48–50]. Relatively
Fig. 5. IR spectra of deuterated zeolites during D/H isotopic exchange by interaction with ethane (50 mbar). (A) MFI at 442 °C, (B) MCM-22 at 440 °C and (C) MCM-36 at 450 °C.
high thickness of the MWW layer including internal channel system is behind a small difference in strength of acid sites in MCM-22 and MCM36 zeolites. The order of the reaction rates (for illustration at 425 °C) MFI (3.2 10−3 s-1) > MCM-22 (4.7 10-4 s-1) ≈ MCM-36 (3.6 10-4 s-1) is in line with our results of carbon monoxide heat of adsorption measurement by VTIR spectroscopy and adsorption microcalorimetry [10,22] revealing significantly lower values of ΔHads for MCM-22 and MCM-56 compared with MFI and FER zeolites, as well as significantly lower adsorption heat of ammonia on MCM-22 zeolite with respect to MFI reported in the literature [51–53]. Thus, the frequency shift of OH group vibration upon interaction with bases as a scaling factor of the
184
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
Fig. 6. Natural logarithm of relative intensity of acidic OD group as a function of time at various temperatures for (A) MFI, (B) MCM-22 and (C) MCM-36 and temperature dependence of rate constants for all three zeolites (D).
reported statement that determination of the enthalpy change of probe molecules adsorption involved in hydrogen bonding is more reliable instrumental method for assessment of acid strength than OeH frequency shift probed by a weak base adsorbed at a low temperature.
acid sites strength, introduced by Paukshtis and Yurchenko [19], is not generally valid method and can lead to misinterpretation of the experimental data as in the case of MCM-22 zeolite [39]. A possible problem with evaluation of acid site strength by frequency shift of OeH vibration due to Fermi resonance effect was recently stressed by Hadjiivanov et al. [54–56] and based on theoretical modelling Rybicki and Sauer [49]. The results reported here bring further evidence that the Brønsted acidity ranking by OeH frequency shift probed by an adsorbed weak base can be misleading and that the correlation introduced by Paukshtis and Yurchenko [19] is in part the result of a coincidence of mutually compensating phenomena and serendipities.
Acknowledgement Financial support from the Czech Science Foundation for the project of the Centre of Excellence (P106/12/G015) is gratefully acknowledged.
4. Conclusions
References
The D/H isotopic exchange reaction of deuterated Brønsted acid sites with ethane carried out at various temperatures and monitored by FT-IR spectroscopy was used to assess the strength of acid site in MFI and MWW type zeolites. The following conclusions can be drawn from the experimental data:
[1] S. Al-Khattaf, S.A. Ali, A.M. Aitani, N. Zilkova, D. Kubicka, J. Cejka, Catal. Rev.-Sci. Eng. 56 (2014) 333–402. [2] G. Busca, Chem. Rev. 107 (2007) 5366–5410. [3] S. Singhal, S. Agarwal, S. Arora, N. Singhal, A. Kumar, Catal. Sci. Technol. 7 (2017) 5810–5819. [4] F. Su, Y.H. Guo, Green Chem. 16 (2014) 2934–2957. [5] N. Rahimi, R. Karimzadeh, Appl. Catal. A-Gen. 398 (2011) 1–17. [6] M.V. Opanasenko, W.J. Roth, J. Cejka, Catal. Sci. Technol. 6 (2016) 2467–2484. [7] E.G. Derouane, J.C. Vedrine, R.R. Pinto, P.M. Borges, L. Costa, M. Lemos, F. Lemos, F.R. Ribeiro, Catal. Rev.-Sci. Eng. 55 (2013) 454–515. [8] M. Boronat, A. Corma, Catal. Lett. 145 (2015) 162–172. [9] C. Lee, D.J. Parrillo, R.J. Gorte, W.E. Farneth, J. Am. Chem. Soc. 118 (1996) 3262–3268. [10] C.O. Arean, M.R. Delgado, P. Nachtigall, H.V. Thang, M. Rubes, R. Bulanek, P. Chlubna-Eliasova, Phys. Chem. Chem. Phys. 16 (2014) 10129–10141. [11] A.M. Zheng, S.B. Liu, F. Deng, Chem. Rev. 117 (2017) 12475–12531. [12] G. Busca, Microporous Mesoporous Mater. 254 (2017) 3–16. [13] G. Busca, Phys. Chem. Chem. Phys. 1 (1999) 723–736. [14] M. Hunger, Catal. Rev. 39 (1997) 345–393. [15] U. Fleischer, W. Kutzelnigg, A. Bleiber, J. Sauer, J. Am. Chem. Soc. 115 (1993) 7833–7838. [16] B. Gil, S.I. Zones, S.J. Hwang, M. Bejblova, J. Cejka, J. Phys. Chem. C 112 (2008) 2997–3007. [17] J. Sauer, J.R. Hill, Chem. Phys. Lett. 218 (1994) 333–337. [18] K.P. Schroder, J. Sauer, M. Leslie, C.R.A. Catlow, J.M. Thomas, Chem. Phys. Lett.
• Isotopic exchange reaction between deuterated hydroxyl groups and • • •
ethane at elevated temperatures can be described by pseudo first order kinetics. The rate constants of isotopic exchange are in the order MFI > MCM-22 ≈ MCM-36. The order of measured rate constants is consistent with previously reported heats of adsorption of CO or NH3. Thus, MFI zeolite contains significantly stronger Brønsted acid sites than MCM-22 and MCM-36. Strength of acid sites in MCM-22 and MCM-36 is comparable. Relatively high thickness of the MWW layer containing internal channel system is behind a small difference in strength of acid sites in MCM-22 and MCM-36 zeolites. Results of the isotopic exchange reaction support our previously 185
Applied Catalysis A, General 575 (2019) 180–186
R. Bulánek, et al.
[37] C.A. Emeis, J. Catal. 141 (1993) 347–354. [38] M. Polozij, H.V. Thang, M. Rubes, P. Eliasova, J. Cejka, P. Nachtigall, Dalton Trans. 43 (2014) 10443–10450. [39] B. Gil, B. Marszalek, A. Micek-Ilnicka, Z. Olejniczak, Top. Catal. 53 (2010) 1340–1348. [40] K. Hadjiivanov, F.C. Jentoft (Ed.), Advances in Catalysis, Academic Press, 2014, pp. 99–318. [41] M. Bevilacqua, D. Meloni, F. Sini, R. Monaci, T. Montanari, G. Busca, J. Phys. Chem. C 112 (2008) 9023–9033. [42] M.C. Kung, H.H. Kung, Catal. Rev. 27 (1985) 425–460. [43] B.A. De Moor, M.F. Reyniers, O.C. Gobin, J.A. Lercher, G.B. Marin, J. Phys. Chem. C 115 (2011) 1204–1219. [44] G. Piccini, M. Alessio, J. Sauer, Y.C. Zhi, Y. Liu, R. Kolvenbach, A. Jentys, J.A. Lercher, J. Phys. Chem. C 119 (2015) 6128–6137. [45] J.A. Dunne, R. Mariwals, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Langmuir 12 (1996) 5888–5895. [46] M. Rubeš, M. Trachta, E. Koudelková, R. Bulánek, J. Klimeš, P. Nachtigall, O. Bludský, J. Phys. Chem. C 122 (2018) 26088–26095. [47] E. Dumitriu, D. Meloni, R. Monaci, V. Solinas, C. R. Chim. 8 (2005) 441–456. [48] J.A. Boscoboinik, X. Yu, E. Emmez, B. Yang, S. Shaikhutdinov, F.D. Fischer, J. Sauer, H.J. Freund, J. Phys. Chem. C 117 (2013) 13547–13556. [49] M. Rybicki, J. Sauer, Phys. Chem. Chem. Phys. 17 (2015) 27873–27882. [50] J. Hermann, M. Trachta, P. Nachtigall, O. Bludsky, Catal. Today 227 (2014) 2–8. [51] D.H. Zhou, Y. Bao, M.M. Yang, N. He, G. Yang, J. Mol. Catal. A-Chem. 244 (2006) 11–19. [52] Z. Zhu, Q. Chen, Z. Xie, W. Yang, C. Li, Microporous Mesoporous Mater. 88 (2006) 16–21. [53] C.-C. Tsai, C.-Y. Zhong, I. Wang, S.-B. Liu, W.-H. Chen, T.-C. Tsai, Appl. Catal. AGen. 267 (2004) 87–94. [54] K. Chakarova, K. Hadjiivanov, J. Phys. Chem. C 115 (2011) 4806–4817. [55] K. Chakarova, K. Hadjiivanov, Microporous Mesoporous Mater. 177 (2013) 59–65. [56] K. Chakarova, K. Hadjiivanov, Chem. Commun. (Camb.) 47 (2011) 1878–1880.
188 (1992) 320–325. [19] E.A. Paukshtis, E.N. Yurchenko, Russ. Chem. Rev. 52 (1983) 242. [20] M.V. Frash, M.A. Makarova, A.M. Rigby, J. Phys. Chem. B 101 (1997) 2116–2119. [21] M.A. Makarova, K.M. Alghefaili, J. Dwyer, J. Chem. Soc.-Faraday Trans. 90 (1994) 383–386. [22] M.R. Delgado, R. Bulanek, P. Chlubna, C.O. Arean, Catal. Today 227 (2014) 45–49. [23] J.S. Beck, A.B. Dandekar, T.F. Degnan, M. Guisnet, J.P. Gilson (Eds.), Zeolites for Cleaner Technologies, Imperial College Press, 2002, pp. 223–237. [24] B. Maerz, C.M. Smith, R.A. Meyers (Ed.), Handbook of Petrochemicals Production Processes, Mc Graw Hill, 2005, pp. 523–538. [25] A.B. Dandekar, N. Hryniszak, D.L. Stern, US patent 7868215 B2, 2011. [26] M. Motoyuki, K. Yamamoto, A.V. Sapre, J.P. Mc Williams, S.P. Donnelly, US patent 612 1501 A, 1997. [27] P. Wu, T. Komatsu, T. Yashima, Microporous Mesoporous Mater. 22 (1998) 343–356. [28] M.T. Portilla, F.J. Llopis, C. Martinez, S. Valencia, A. Corma, Appl. Catal. A-Gen. 393 (2011) 257–268. [29] N. Kumar, L.E. Lindfors, Appl. Catal. A-Gen. 147 (1996) 175–187. [30] M. Pitinova-Stekrova, P. Eliasova, T. Weissenberger, M. Shamzhy, Z. Musilova, J. Cejka, Catal. Sci. Technol. 8 (2018) 4690–4701. [31] M. Stekrova, M. Kubu, M. Shamzhy, Z. Musilova, J. Cejka, Catal. Sci. Technol. 8 (2018) 2488–2501. [32] T. Kubota, R. Osuga, T. Yokoi, J.N. Kondo, Top. Catal. 60 (2017) 1496–1505. [33] R. Osuga, T. Yokoi, K. Doitomi, H. Hirao, J.N. Kondo, J. Phys. Chem. C 121 (2017) 25411–25420. [34] J. Rouquerol, P. Llewellyn, F. Rouquerol, P.L. Llewellyn, F. Rodriquez Reinoso, J. Rouqerol, N. Seaton (Eds.), Characterization of Porous Solids Vii - Proceedings of the 7th International Symposium on the Characterization of Porous Solids, 2006, pp. 49–56. [35] B.C. Lippens, J.H. Deboer, J. Catal. 4 (1965) 319-&. [36] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, Pure Appl. Chem. 87 (2015) 1051–1069.
186