Deactivation behavior of alkali-metal zeolites in the dehydration of lactic acid to acrylic acid

Journal of Catalysis 329 (2015) 413–424

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Deactivation behavior of alkali-metal zeolites in the dehydration of lactic acid to acrylic acid G. Näfe a, M.-A. López-Martínez a, M. Dyballa a, M. Hunger a, Y. Traa a,⇑, Th. Hirth b,c, E. Klemm a a

University of Stuttgart, Institute of Chemical Technology, Faculty of Chemistry, Pfaffenwaldring 55, 70569 Stuttgart, Germany University of Stuttgart, Institute of Interfacial Process Engineering and Plasma Technology, Faculty of Energy Technology, Process Engineering and Biological Engineering, Nobelstraße 12, 70569 Stuttgart, Germany c Fraunhofer Institute for Interfacial Engineering and Biotechnology, Nobelstraße 12, 70569 Stuttgart, Germany b

a r t i c l e

i n f o

Article history: Received 4 February 2015 Revised 22 April 2015 Accepted 20 May 2015

Keywords: Lactic acid dehydration Acrylic acid Gas phase dehydration Deactivation behavior of zeolites Accumulation of polar compounds

a b s t r a c t A series of FAU, MFI, and MOR Na-zeolites with different nSi/nAl ratios and varying exchanged cations Li+, Na+, K+ and Cs+ are investigated as catalysts in the gas phase dehydration of lactic acid to acrylic acid. The conversions and selectivities as well as their time dependence prove to be greatly affected by the structure, the type of alkali-metal and the acidic/basic nature of the zeolite. Analyses of the catalysts after the reaction show a possible reason for the deactivation, viz. the blocking of the active sites due to the formation of deposits of clusters containing carboxyl groups. The deactivation rate is high if large adsorptive clusters can be formed and stabilized by intra- and inter-molecular hydrogen bonds. The conversion of lactic acid into acrylic acid without catalyst deactivation is possible if adsorptive accumulation is effectively excluded, e.g. when using Na-ZSM-5 zeolite. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction Biogenic platform chemicals can replace fossil based feedstocks for the production of commodity chemicals [1,2]. One of these biogenic platform chemicals is the 2-hydroxypropanoic acid, i.e. lactic acid (LA). LA can be produced from sugars or glycerol by means of biotechnological processes [3–5]. Due to its two functional groups, LA can thereafter be converted into a variety of compounds. Among the possible conversions, the dehydration of lactic acid to acrylic acid (AA) is of particular interest because large amounts of AA are demanded in industry as adhesive for lacquers and paints and as monomer for the production of superabsorbent polymers [6]. As it becomes discernible from Fig. 1, the main side products in the dehydration reaction are acetaldehyde (AcH), 2,3-pentanedione (PTD) and polymeric esters of LA (PLA) [7]. AcH is produced by decarbonylation (Fig. 1, path A) and decarboxylation (Fig. 1, path B) reactions. The decarbonylation is activated by the protonation of the carboxylic group of the LA molecule and is

⇑ Corresponding author. E-mail address: [email protected] (Y. Traa). http://dx.doi.org/10.1016/j.jcat.2015.05.017 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

thus catalyzed by Brønsted acid sites [8]. Similar to other carboxylic acids, decarboxylation is enabled by the nucleophilic activation of the carboxylic group. Besides redox-active metal sites such as Fe or Ni [9,10], only strong nucleophilic catalysts accelerate this reaction [11,12]. The mechanism of the dehydration of LA to AA is not yet understood in detail, although a postulation for the intermediate species was made by Gunter et al. [13] for the reaction over phosphate catalysts, which could be verified by other groups using infrared and Raman spectroscopy [14]. The phosphate anion acts both as electron withdrawing (El) and electron donating (Nu) group, allowing the liberation of water (Fig. 1, path C). For the formation of AA on alkali-metal modified zeolites, a plausible E2 elimination mechanism has been reported [15]. In addition, the formation of surface lactates on alkali ion exchanged zeolites as elemental step for the conversion of LA to AA has also been speculated [16]. 2,3-pentanedione is mainly produced over basic catalysts (Fig. 1, path D) and at elevated pressures [17]. The esterification to PLA (Fig. 1, path E) is catalyzed by Brønsted acid sites, as they occur in LA itself. Thus, the autocatalytic esterification can lead to the spontaneous (and undesired) oligomer formation provided that the LA concentration is high. For all important side reactions, the activation of the carboxylic group by protons is crucial as it can be seen from Eq. (1):

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devices, we have been able to successfully avoid the accumulation of such deposits in the catalyst bed. As a consequence, performance parameters in terms of selectivity and conversion could be achieved for the Na-Y zeolite catalyst, which significantly differ from the ones reported so far in the literature. In contrast to previous publications, the conversion of LA decreased remarkably with time on stream (TOS), whereas the selectivity to AA was reduced only slightly. Basically, this particular behavior can be explained by the assumption that catalytically active sites are blocked [31]. Since our optimized experimental setup is a crucial prerequisite for the unbiased study of the true deactivation behavior of a catalyst, it ensures a valuable contribution to elucidate the reason for the unique deactivation of alkali-exchanged zeolite catalysts. In the following, this will be demonstrated in the reaction of LA to AA with focusing on the catalytic performance of zeolites of the type FAU, MFI and MOR.

2. Experimental 2.1. Materials and catalyst preparation

Fig. 1. Products of lactic acid conversion. The participation of nucleophilic and electrophilic sites in the reactions is highlighted by the labels Nu and El.

ð1Þ

Since decarbonylation and decarboxylation are thermodynamically more favored than the dehydration reaction [18], the selective conversion of LA to AA in the gas phase requires the usage of a suitable catalyst by means of which the side reactions can be inhibited kinetically. In this way, bulk and supported nitrates [17,19], phosphates [13,19,20], alkali-metal salts [17,21], and hydroxyapatites [22] have been employed as catalysts. While selectivities to AA with values up to 60% were achieved on the latter [22], selectivities to AA up to 77% were found on a barium sulfate catalyst [23]. In general, there was no evidence of a severe deactivation of the catalyst. In addition, zeolites modified by alkali-metal salts have been used as catalysts [14–16,24–27]. In this way, high acrylic acid selectivity could be observed, reaching up to 68% in the case of K-modified Y zeolite [15] and up to 72% for a phosphate-loaded Na-Y zeolite [14,28]. However, the performance of such zeolites can significantly suffer from deactivation, by means of which the conversion rate and the selectivity are affected. The cause of the deactivation behavior is not yet fully understood. Coking of the catalyst and hydrothermal instability have been briefly discussed [14,23]. Besides alkali-metal zeolites, also zeolites that have strong Lewis and Brønsted acid sites, such as alkaline earth-metal zeolites and lanthanide zeolites have been used [25,26,29,30]. Recently, we have shown that the carbonaceous deposits formed by incomplete evaporation of LA may induce substantial misinterpretation of catalyst deactivation by pretending LA consumption [31]. By means of an optimized flow apparatus, in which evaporation and reaction are performed successively in separate

The commercially available catalysts Na-X zeolite with nSi/nAl = 1.1 and Na-Y zeolite with nSi/nAl = 2.4 were obtained from Strem Chemicals with LOT Nos. 14247S and 148960, respectively. NH4-Y zeolite with nSi/nAl = 5.2 was obtained from Condeka and Na-MOR zeolite with nSi/nAl = 5.2 was obtained from CU Chemie Uetikon AG. The Na-ZSM-5 zeolite with nSi/nAl = 18 was synthesized by hydrothermal treatment as described elsewhere [32]. Acetaldehyde, LiNO3, KI, NaNO3 and pyridine were obtained from Merck (all of them in their pure form), and CsCl (pure) and ammonium acetate (98%) were obtained from Sigma–Aldrich. Ammonium hexafluorosilicate (>99 wt.%) was purchased from Ventron. Aqueous L(+)-LA solution (food quality) with a concentration of 88 wt.% was obtained from PURAC. Tetradecane (olefin-free) was obtained from Fluka and was used as an internal standard for the gas chromatographic (GC) analysis. 2-butanone (HPLC-grade) was obtained from Alpha-Aesar and was used as solvent during the GC analysis. The as-received zeolites were stirred in a 1 M NaNO3 aqueous solution for at least 4 h at 353 K. Per 1 g of dry zeolite, 40 ml of NaNO3 solution was used. After the treatment, the zeolite was filtered and washed with demineralized water until the filtrate was free of nitrate. The sample was then dried at 353 K overnight. This procedure was repeated to ensure that all cation positions were occupied with sodium. Thereafter, the ion-exchanged Na-Y zeolite with nSi/nAl = 2.4 was further treated by stirring in a given volume of 1 M aqueous solution containing the corresponding amounts of Li-, K- or Cs-salt precursors for 4 h at 353 K. The ratio between the amount of alkali salt and the mass of the dry zeolite was 0.98 mmol g1 for the K, Li and Cs exchange. Finally, the zeolites were filtered and washed as described above. Dealumination (yielding the zeolite Y with nSi/nAl = 7.6) was carried out following a procedure described by Kühl [33]. Basically, 1.0 g (on a dry basis) of Na-Y zeolite with nSi/nAl = 2.4 was suspended in 10.4 g of a 10 mol l3 aqueous solution of ammonium acetate. The suspension was heated at 348 K. Afterward, 0.5 g of a 1 M (NH4)2SiF6 aqueous solution was added dropwise. The resulting suspension was heated at 363 K under stirring and kept for 3 h under these conditions. The zeolite was filtered and suspended again five times in 45 ml of hot water (368 K). Finally, the zeolite was calcined. The catalyst powders were stored in a desiccator over a saturated solution of calcium nitrate. The powders were pressed and sieved to yield particles of 200–315 lm in diameter.

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2.2. Characterization of the catalysts The water content in the fresh zeolites was determined using a Setaram Thermogravimetric Analyzer Setsys TG-16/18 by heating up the sample in a nitrogen flow from room temperature to 873 K with a heating rate of 20 K min1. Different from that, the water content of the catalysts, which had been used in the reaction of lactic acid, hereafter designated as used catalyst, was measured by heating up the samples in a flow of synthetic air from room temperature to 1223 K. The temperature-dependent mass loss manifests itself by two inverse sigmoid curves indicating the loss of water and carbonaceous deposits, respectively. An optical emission spectrometer (OES) of type Varian with inductively coupled plasma (ICP) (Vista-MPX CCD ICP-OES) was used for the chemical analysis of the catalysts. The samples were treated with diluted hydrofluoric acid and aqua regia at 453 K for 1 h in a microwave oven. The chemical composition of the deposits accumulated on the used catalysts was analyzed by means of a combustion analyzer Vario El by burning the samples at 1223 K in air and subsequently analyzing the flue gas with a thermal conductivity detector (TCD). To determine the morphology and the size of the crystals, scanning electron microscopy (SEM, Cambridge Cam Scan 44) was used at an excitation voltage of 5 kV. The samples were covered beforehand with an ultra-thin layer of gold using an Emitech sputter coating equipment K550. The zeolite structures were elucidated by powder X-ray diffraction (XRD). The diffraction patterns were obtained using a BrukerD8 Advance diffractometer with Cu Ka-radiation at k = 0.154 nm. The excitation voltage and the current intensity were 35 kV and 40 mA, respectively. The range of 2h values was 5–50° with a step size of 0.016 and a step time of 0.2 s. Solid-state MAS NMR spectra were recorded on a Bruker Avance III 400WB spectrometer. The 27Al MAS NMR spectroscopic studies were performed at the resonance frequency of 104.3 MHz and with 1200 accumulations. The spectra were recorded after single-pulse p/6 excitation and a repetition time of 0.5 s, using MAS NMR rotors with a diameter of 4 mm, and a spinning rate of 8 kHz. The samples under study had been previously stored over saturated Ca(NO3)2 aqueous solution at room temperature for at least 12 h. Furthermore, the used catalysts were analyzed by solid-state NMR spectroscopy of 13C nuclei in their natural isotopic abundance to identify the nature of the deposits. The spectra were recorded at the resonance frequency of 100.62 MHz via cross polarization (CP) experiments with a RAMP sequence [34], a contact time of 2 ms, a repetition time of 4 s, and ca. 15,000 accumulations (ca. 16 h per spectrum). MAS NMR rotors with a diameter of 4 mm and a spinning rate of 8 kHz were used. The specific pore volume (V), the specific micro pore volume (Vmicro) and the specific surface area (SBET) of the zeolites were measured by means of nitrogen physisorption. The adsorption and desorption isotherms were recorded with a Quantachrome Autosorb IIIb device at 77 K after degassing the sample at 623 K for 16 h in high vacuum. The mass of the zeolite was determined after the adsorption experiment. The relative pressure values were 1, 0.001–0.01 and 0.2–0.4 for the calculation of V, SBET (using the Brunauer–Emmet–Teller method) and Vmicro (using the t-plot method), respectively. The zeolite capacity for the irreversible adsorption of ammonia was specified by means of a Quantachrome Autosorb iQ apparatus. After degassing the samples as described above, the mass of the zeolite was determined. Thereafter, the sample was degassed for further 30 min and was then cooled down to 473 K. A first adsorption isotherm was obtained and the zeolite was afterward treated in vacuum for at least 120 min to remove any reversibly bound ammonia. An internal test routine was used to establish a minimal ammonia pressure in the cell by means of

a further evacuation, if required. After obtaining a second isotherm, the irreversibly bound ammonia was calculated by subtracting the two isotherms according to the method described by van Oers et al. [35]. FT-IR spectroscopic measurements with pyridine as probe molecule were carried out using a Bruker Vector 22 spectrometer and a high vacuum IR cell with CaF2 windows at 353 K. Self-supporting wafers were made from the zeolite powders that had been stored over saturated calcium nitrate solution. In a typical experiment, the catalyst was degassed at 623 K under vacuum for 16 h inside the upper part of the IR cell, which was heated by an external oven. A cell pressure of 2.2  10–6 Pa was reached after the degassing treatment. Once these conditions were achieved, the wafer was lowered in the part of the IR cell that contained the CaF2 windows and an FT-IR spectrum was recorded. The sample was then allowed to cool down to 423 K at which temperature it was held in contact with pyridine vapor with a partial pressure of 3.3 Pa for 30 min. After that, the cell was evacuated for 60 min to remove reversibly bound pyridine. At this point, the spectrum of the loaded catalyst was recorded. 2.3. Catalytic testing The flow apparatus used in the present work is described in detail elsewhere [31]. In accordance with the desired liquid hourly space velocity (LHSV), the flow reactor was charged with a known amount of catalyst particles or with a homogeneous mixture comprising catalyst particles and quartz glass beads (600–800 lm size, Vogelsberger GmbH, Germany). The reactor was heated up to 598 K in flowing nitrogen with a rate of 10 K min1 followed by a dwelling time of 30 min. In the meantime, the syringe pump was charged with the lactic acid feed. Prior to usage, the LA feed, derived from L(+)-LA 88 wt.% by dilution with the appropriate amount of demineralized water, was treated at 363 K under reflux for at least 7 days in order to hydrolyze any lactate esters. Due to this long treatment, the concentration of the lactic acid solution could not easily be kept at the same level for all of the experiments. Therefore, the lactic acid feed was used with a varying concentration ranging from 19 to 21 wt.%, as determined by titration with a NaOH standard solution (0.1 M, Sigma–Aldrich). The flow rate of 1 the aqueous LA solution was V_ 1 ¼ 2:5  102 ml min . The nitrogen flow was adjusted in such a way that the ratio between the mass flow rate of the liquid feed and the volume flow rate of the gas was 7.82104 g ml1. Due to the constant ratio, a persistently constant dilution of LA in the reactor was ensured for all runs, despite a slightly different LA concentration. Product sampling was commenced 45 min after the start of dosing and was undertaken over periods of 60 min. The samples were weighed to ascer_ out and were analyzed tain the mass that flows out of the reactor m using an HP/Agilent 6890 gas chromatograph equipped with a Phenomenex ZB-WaxPlus column (30 m length, 0.25 mm inner diameter, 0.25 lm phase thickness) and a flame ionization detector (FID). To minimize interaction between LA and the material of the GC inlet, an Agilent W-cup liner was employed. H2 was used as a carrier gas. _ in may As mentioned above, the mass flow into the reactor m change from one to the other experiment due to variation of the _ in , a separate experiment has LA concentration. For determining m to be carried out before or after the catalytic test, which is described elsewhere [31]. Mass balance was specified by the for_ out  100Þ=m _ in . The molar flow rate of lactic acid that goes mula ðm into the reactor n_ LA;0 was defined as:

n_ LA;0 ¼

_ in  wLA m 1

90:08 g mol

ð2Þ

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In Eq. (2), wLA is the mass content of the freshly hydrolyzed feed as determined by titration. The conversion XLA, yield Yj and selectivity Sj were calculated as follows:

X LA ¼

n_ LA;0  n_ LA;out  100 n_ LA;0

ð3Þ

Yj ¼

n_ j;out mLA   100 n_ LA;0 mj

ð4Þ

Sj ¼

Yj X LA

ð5Þ

In Eqs. 3–5, n_ j;out is the flow rate of the product sampled in the cooling trap as calculated from sampling time, sample weight and product concentration. The latter was determined by GC. m is the stoichiometric factor of lactic acid and of the product j, respectively. The values for conversion, yield and selectivity provided in Section 3.2 are integrals for a time period of 60 min. Owing to the metal-free flow apparatus applied in this work, the metal-catalyzed side reaction of LA is avoided [31]. Under these circumstances, LA can exclusively be converted on electrophilic (acidic) or nucleophilic (basic) sites. In a zeolite, aluminum is involved in the formation of either sites. For that reason and in order to compare the activity of zeolites of various chemical compositions and densities, we use a turnover frequency (TOF) that is related to the aluminum content of the zeolite. The TOF is calculated as follows:

TOF ¼

ðn_ LA;0  n_ LA;out Þ nAl

ð6Þ

where ðn_ LA;0  n_ LA;out Þ is the amount of converted LA per time unit, as determined by gas chromatography, and nAl is the amount of Al in the catalyst bed, as determined by means of OES and TGA. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Chemical composition and structure In Table 1, all properties of the zeolite catalysts studied in the present work are summarized. The catalyst names contain the following details: the subscripts at the alkali metals give the ion exchange degrees calculated from the nM/nAl ratios (where M = Li, Na, K or Cs), the hyphenated number at the end gives the nSi/nAl ratio. It becomes discernible from the third and fourth column that

the sum of the exchange degrees of the alkali-metal cations is higher than 85% for almost all of the catalysts. With exception of Na0.4-Y-5.2 and Na0.6-Y-7.6, it can thus be assumed that extra-framework species and Brønsted acid sites do not exist in the zeolites. In the aforementioned particular cases, the data of Table 1 display poor charge balancing, which clearly gives evidence of the presence of extra-framework aluminum species or Brønsted acid sites. This will be addressed in detail later. Although the Kand Cs-containing Y zeolites have been prepared by ion exchange from the Na0.9-Y-2.4 zeolite, i.e. by means of a method that leaves the zeolite lattice totally unaffected, the nSi/nAl values of the K- and Cs-containing Y zeolites slightly differ from that of the parent zeolite. Hydrolysis of Si–O and Al–O bonds in the cases of K0.2,Na0.8-Y-2.3 and Cs0.2,Na0.8-Y-2.6 may be the possible reason for such discrepancies. At any rate, the change of the nSi/nAl ratio due to ion exchange is not unknown in the literature [15,36]. The zeolites considered in this work are phase-pure, which results from comparing the XRD patterns (Supplementary Data) against simulated data of the structures of FAU [37], MOR [38] and MFI [39] zeolite types. However, for the K- and Cs-containing Y zeolite, an additional reflection at 2h = 12.6° emerges. The presence of the additional peak is accompanied by variation of the relative intensities of the reflections at 2h = 18.9° and 2h = 20.5°. These changes in the diffraction pattern may be caused by the substitution of sodium with the bigger alkali metals rather than by a structural decomposition of the zeolite lattice [40,41]. Octahedrally coordinated Al, that usually indicates the presence of extra-framework Al, does exclusively exist in the pore system of the Na0.4-Y-5.2 and Na0.6-Y-7.6 zeolites. This is revealed by means of 27Al MAS NMR spectroscopy (Supplementary Data). Therefore, the hydrolysis of lattice bonds in the case of the K0.2,Na0.8-Y-2.3 and Cs0.2,Na0.8-Y-2.6 zeolites can be excluded. For all of the zeolites, the crystal size, found by SEM, is in the range of 0.5–2 lm (Table 1 and Supplementary Data). With respect of the nitrogen physisorption results included in Table 1, the Na0.9-Y-2.4 zeolite shows the highest specific surface area of all of the Na-containing zeolites. However, when sodium cations are replaced by the smaller lithium during the ion exchange, the SBET slightly increases. Replacing Na+ by either K+ or Cs+ in the aforementioned zeolite results in ca. 20% decrease of SBET. While Na0.4-Y-5.2 and Na0.6-Y-7.6 zeolites show the lowest surface areas of the FAU series, both zeolites of type MOR and ZSM-5 exhibit the lowest specific surface areas of the whole series. The pore volume of the Na0.9-Y-2.4 zeolite increases around 8%

Table 1 Properties of the zeolite catalysts before and after (in parentheses) the reaction as well as properties of the deposits formed on the zeolites. nSi/ nAla

a b c d e *

nNa/ nAla

nM/ nAla

Crystal sizeb (lm)

dOc

Classification

Surface aread (SBET/m2 g1)

Pore volumed (V/mL g1)

Micropore volumed (Vmicro/mL g1)

w Ce / wt.%

nH/ nCe

0.33 0.44 0.51 0.48

0.26 0.27 0.21 0.13

(0.00) (0.04) (0.11) (0.02)

12.4 14.4 9.7 9.5

1.34 1.14 0.79 0.78

Na0.9-X-1.1 Na0.9-Y-2.4 Na0.4-Y-5.2 Na0.6-Y-7.6

1.14 2.42 5.15 7.59

0.87 0.93 0.37 0.61

n.a. n.a. n.a. n.a.

1–2 0.5–1.5 0.5–1.5 0.5–1.5

0.40 0.34 0.25 0.25

Nucleophilic Amphoteric Electrophilic* Electrophilic*

740 (10) 860 (140) 670 (350) 490 (120)

Li0.1,Na0.8-Y-2.4 K0.2,Na0.8-Y-2.3 Cs0.2,Na0.8-Y-2.6

2.41 2.32 2.63

0.81 0.78 0.83

0.09 0.16 0.19

0.5–1.5 0.5–1.5 0.5–1.5

0.28 0.35 0.36

Electrophilic Amphoteric Amphoteric

880 (35) 690 (40) 690 (30)

0.39 (0.06) 0.36 (0.07) 0.32 (0.07)

0.31 (0.01) 0.24 (0.01) 0.25 (0.00)

14.1 11.6 12.0

1.24 1.21 1.16

Na0.9-MOR-5.3 Na1.2-ZSM-5-18

5.25 18.14

0.88 1.18

n.a. n.a.

0.5–2.0 0.5–2.0

0.28 0.24

Electrophilic Electrophilic

420 (7) 300 (250)

0.51 (0.05) 0.22 (0.16)

0.21 (0.00) 0.10 (0.08)

4.2 2.6

1.29 0.78

Determined by OES/ICP. Determined by SEM. Calculated according to Sanderson [48]. Determined by N2 physisorption, reaction carried out with LHSV = 3 h1. Determined by elementary combustion analysis, reaction carried out with LHSV = 3 h1. Containing Brønsted acid sites.

(0.06) (0.14) (0.33) (0.32)

G. Näfe et al. / Journal of Catalysis 329 (2015) 413–424

when Li is incorporated in the structure, it remains the same when K is exchanged, and it decreases by about 11% when Cs is present in the zeolite. In general, the contribution of the micropore volume is in the range of 65–80% in the pore volume of the zeolites, with exception of Na0.9-MOR-5.3, Na1.2-ZSM-5-18, Na0.4-Y-5.2 and Na0.6-Y-7.6 for which the micropore volume accounts for about 30–45% of their pore volume. 3.1.2. Acid/base properties Fig. 2 depicts the IR spectra of pyridine adsorbed on the zeolites under consideration. Based on the well-known assignment of the vibrational mode v19b of adsorbed pyridine [42,43], the interaction of the probe molecule with cationic Lewis acid sites, i.e. alkali-metal cations, with strong Lewis acid sites, such as extra-framework Al, and with Brønsted acid sites, lead to IR bands at about 1440 cm1, 1450 cm1 and 1545 cm1, respectively. Therefore, the IR spectra suggest that all zeolites possess weak cationic Lewis acid sites. In addition, the Na0.4-Y-5.2 and Na0.6-Y-7.6 zeolites have strong acidic sites, i.e. both strong Lewis and Brønsted acid sites, which are responsible for the low exchange degree as observed by OES. The ratio between the amounts of strong Lewis acid sites and Brønsted acid sites cannot be determined quantitatively. In addition, if water is present (as during the dehydration reaction), the extra-framework Al sites are hydrolyzed and thus, Brønsted acid sites are formed [44]. Hence, Na0.4-Y-5.2 and Na0.6-Y-7.6 can be considered to be zeolites with strong Brønsted acidity. It becomes apparent from Fig. 2 that Na0.9-X-1.1, Na0.9-Y-2.4, Li0.1,Na0.8-Y-2.4 and K0.2,Na0.8-Y-2.3 are characterized by a broad IR band at about 1440 cm1 after contact with pyridine. This band seems to be caused by the overlapping of several narrower bands with different wavenumbers, which implies that the pyridine/catalyst interactions have different strengths [45]. Additionally, a shoulder of the IR band can be seen at 1437 cm-1. A band with such a low wavenumber is associated with physisorbed pyridine and with pyridine that is coordinated by hydrogen bonding [42,43]. A possible explanation for that occurrence of various types of pyridine molecules is the presence of pyridine clusters in the big cavities of the FAU zeolite. This is known from the adsorption of polar

1545 cm -1

1450 cm -1 1440 cm -1

Na1.2-ZSM-5-18 Na0.9-MOR-5.3 Absorbance / a. u.

Cs0.2,Na0.8-Y-2.6 K0.2,Na0.8-Y-2.3 Li0.1,Na0.8-Y-2.4 Na0.6-Y-7.6 Na0.4-Y-5.2 Na0.9-Y-2.4 Na0.9-X-1.1

1650

1600

1550

1500

1450

1400

Wavenumber / cm-1 Fig. 2. IR spectra of the zeolites with adsorbed pyridine (py) at 423 K. Shown are the subtracted spectra, as a result of which only the py bands are visible. The loaded sample was evacuated for 1 h before recording the spectra.

417

adsorptives such as ammonia, methanol and water onto nucleophilic and amphoteric zeolites [36,46,47]. Particularly in the case of big clusters, the polar adsorptives form numerous intra-molecular and inter-molecular hydrogen bonds as a result of which the adsorption enthalpy is significantly increased [46]. In this way, additional adsorption sites with higher strength are pretended. Protons are not detectable by OES. Therefore, it is helpful for the determination of the chemical composition of the zeolites that the existence of protons can be excluded for all zeolites, except for Na0.6-Y-7.6 and Na0.4-Y-5.2, by IR spectroscopy. Thus, the combination of both techniques provides useful information about the actual chemical composition from which the electronegativity of the zeolite lattice and the charge of the lattice oxygen dO can be calculated [48]. The resulting values of dO are listed in Table 1. It is generally assumed that if dO is in the range of 0.34 to 0.38, alkali-metal zeolites possess nucleophilic and electrophilic properties, i.e. they are amphoteric. Provided that the value is higher, nucleophilic properties become predominant, whereas for lower dO, the catalyst is electrophilic [49,50]. Following this classification, the zeolites under consideration are nucleophilic, amphoteric and electrophilic (Table 1). Note that only zeolites that do not have Brønsted acid sites provide possible nucleophilic sites [49]. That is why the zeolites Na0.6-Y-7.6 and Na0.4-Y-5.2 do not posses intrinsic basicity. Fig. 3 left illustrates the total uptake of ammonia adsorbed on the catalysts. The irreversibly bound ammonia in relation to the total uptake accounts for a strong ammonia/catalyst interaction (Fig. 3 right). As it becomes discernible from Fig. 3, Na0.9-X-1.1 and Li0.1,Na0.8-Y-2.4 show the highest ammonia uptake of the FAU zeolite series. Moreover, the comparison of the Li0.1,Na0.8-Y-2.4 zeolite to the Na-, K- and Cs-containing zeolites reveals that the uptake is lowered if Li+ is replaced by bigger alkali cations. This suggests that the space available in the pore systems, actually the space around the alkali-metal cations, is crucial for the adsorption capacity. For the same reason, the capacity of Na0.6-Y-7.6 and Na0.4-Y-5.2 is comparatively low since these zeolites have extra-framework Al and, thus, have less pore space available. For the zeolite Cs0.2,Na0.8-Y-2.3, oxygen charge and polarity are fairly high (see Table 1). Nevertheless, the amount of irreversibly bound ammonia on this catalyst is low, which indicates that the Cs-containing zeolite forms small clusters only with few hydrogen bonds. That means the substitution of Na+ by bigger cations gives rise to the reduction of the size of adsorptive clusters in the case of nucleophilic and amphoteric zeolites. Na0.9-X-1.1 is of nucleophilic nature and, thus, is expected to bind ammonia weakly. However, it is known [36] that the lattice oxygen charge of zeolite X, which correlates with a high zeolite polarity, leads to the formation of large adsorptive clusters with lots of hydrogen bonds. It is due to these bonds that the observable adsorptive/catalyst interaction appears to be rather strong. Hence, lattice oxygen charge plays also an important role in the strength of the ammonia adsorption. Na1.2-ZSM-5-18, the most electrophilic alkali-metal zeolite of the series, shows a very strong ammonia interaction. It is known that the adsorption of polar compounds, like ammonia [46] or CO2 [50], on alkali-metal zeolites is predominantly determined by the polarity of the zeolite and of the adsorptive and that the electrophilic/nucleophilic character of the latter does not significantly affect the adsorption process. That is why we believe that the capacity measured by ammonia adsorption also reflects the zeolite behavior toward the adsorption of LA or PLA. Following this idea, the zeolites with a high ammonia uptake can form big clusters made of LA or PLA and the zeolites with strong ammonia/catalyst interaction have the tendency to quickly form

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0.20

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-1

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a0 .9 -X a0 -1. 1 .9 N -Ya0 2 .4 .4 -Y Li Na0 -5 . 0. 1, .6-Y 2 N a K 0. 0.8 7.6 2 Cs ,Na Y-2 0. 0.8 .4 2, N -YN a0.8 2.3 a0 .9 -Y N -M -2.6 a1 .2 OR -Z -5 SM .3 -5 -1 8

0.00

N

N a0 .9 N -Xa0 1 .9 .1 N -Ya0 2 .4 .4 -Y Li Na0 -5 . 0. 1, .6-Y 2 N a K 0. 0.8 7.6 2 Cs ,Na Y-2 0. 0.8 .4 2, N -YN a0.8 2.3 a0 .9 -Y N -M -2.6 a1 .2 OR -Z -5 SM .3 -5 -1 8

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Irreversibily Bound Ammonia / Total Uptake

418

Fig. 3. Ammonia Adsorption. Left: Total uptake of ammonia on the zeolite catalysts at 473 K. Right: Irreversibly bound/reversibly bound ammonia on the zeolite catalysts at 473 K.

such clusters, especially when they have a polar lattice, i.e. negative lattice oxygen charge. 3.2. Catalytic testing 3.2.1. Na-FAU zeolites Fig. 4 (left) shows the time dependence of LA conversion over Na-FAU zeolites. For LHSV equal to 3 h1, Na0.9-X-1.1 is different from the Na-Y zeolites as it shows a conversion degree of 44%, while a conversion higher than 98% can be obtained over all of the other zeolites of the series at low TOS. On the Na0.4-Y-5.2 and Na0.6-Y-7.6 catalysts, full conversion is achieved over the whole experiment. When using a flow reactor, TOF can be determined only if the conversion is lower than 100%. Thus for Na0.4-Y-5.2 and Na0.6-Y-7.6, the LHSV has to be increased accordingly. On the other hand, for comparing activity and selectivities, the conversion should be at the same level. Thus for Na0.9-X-1.1, the LHSV has to be lowered. As it becomes discernible from Fig. 4 (middle), such an adjustment has the effect that the conversion at TOS = 105 min lies in the range 90% < XLA < 100% for all catalysts. In addition, the conversion now declines with respect of time on stream for each zeolite, although the extent of the decline varies correspondingly (Fig. 4, right). Since all of the Na-FAU catalysts have comparable crystal sizes, the catalytic activity is characterized by the value of their TOF. Therefore, the activity clearly increases in the order Na0.9-X-1.1 < Na0.9-Y-2.4 < Na0.6-Y-7.6 < Na0.4-Y-5.2 (Fig. 4 (right)). The same applies to the acidity expressed in terms of the oxygen charge, which proves a direct relationship between zeolite acid strength and catalytic activity in the dehydration reaction. Only Na0.4-Y-5.2 apparently deviates from the tendency. In the presence of water however, Na0.4-Y-5.2 has a higher concentration of extra-framework aluminum species than Na0.6-Y-7.6 (see Section 3.1.2). Thus, the former catalyst apparently has stronger [44] and more Brønsted acid sites under reaction conditions. Concerning the general character of the relationship between acidity and activity, it is worth mentioning that the present findings agree well with those found by Mok et al. [8] and Gunter et al. [51]. The generality is due to the fact that decarbonylation and polymerization (as the two thermodynamically favored and fast reactions) are catalyzed by electrophilic sites in zeolites with low oxygen charge d. The preference for the side reactions is even

more distinct as Brønsted acid sites are responsible (as it is the case for the zeolites Na0.4-Y-5.2 and Na0.6-Y-7.6). On the contrary, a high oxygen charge d induces a considerable number of lattice oxygen sites to act as a nucleophile [49]. By means of these sites, possible Brønsted acid sites in the zeolite as well as protons of neighboring carboxylic groups of LA and other acidic compounds, such as AA and PLA, are masked (Fig. 5). That is why a stable adsorptive species is formed that allows shifting the dynamic equilibrium shown in Fig. 5 toward the slower dehydration reaction. The effect, i.e. the shifting of the dynamic equilibrium, is smaller when the catalyst is less nucleophilic, i.e. has less lattice oxygen atoms that act as Nu. As a consequence of the role of Nu, the fast conversion of LA by decarbonylation and esterification gets inhibited with the result that activity decreases as the zeolite becomes more nucleophilic. As Fig. 4 (middle) shows, the conversion of LA over Na0.9-X-1.1 is remarkably reduced between 105 and 165 min, and becomes more stable at the end of the experiment. In contrast, the conversion of LA over Na0.9-Y-2.4 and Na0.6-Y-7.6 decreases slowly at the beginning of the reaction and abruptly at the end. For Na0.4-Y-5.2, conversion stays stable over the whole experiment. In the cases of Na0.9-X-1.1, Na0.9-Y-2.4 and Na0.6-Y-7.6, the deactivation rate, i.e. the decline of the conversion with increasing TOS, resulting from the time-dependent decrease of TOF (Fig. 4 (right)), amounts to about 60%, 60% and 28%, respectively, provided that the reaction lasts over 285 min. Considering the first 225 min of the reaction, TOF is lowered by 60%, 38% and 16%, respectively. In the same way, the specific surface area and the pore volume of the zeolites decrease under the influence of the reaction with LA (see Table 1). Hence, the loss of porosity is correlated with the deactivation behavior. It follows from X-ray diffraction and 27Al NMR spectroscopy results (Supplementary Data) that the reduction of the zeolite porosity is not accompanied by degradation of the zeolite lattice. Instead, a noticeable amount of carbonaceous species is found to be accumulated on the catalysts after the dehydration reaction (Table 1). In the light of these results, the deactivation of the catalyst must be understood as being caused by the blocking of the active sites of the zeolite due to the formation of deposits. In Fig. 6, the 13C CP/MAS NMR spectra of the used Na-FAU catalysts are shown. For all zeolites, coke-like deposits consisting of unsaturated sp2-hybridized carbon [52] are found. The signal at d13C  180 ppm, which indicates presence of carboxylic acids [52], is characteristic of Na0.9-X-1.1 and Na0.9-Y-2.4. For

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419

Fig. 4. Conversion of LA on zeolites Na-FAU at 598 K. Left: XLA with LHSV = 3 h1, middle: XLA with varying LHSV, right: TOF with varying LHSV (XLA: lactic acid conversion, TOF: turnover frequency as calculated by Eq. (6), LHSV: liquid hourly space velocity).

Fig. 5. Scheme of the conversion of LA on a catalyst which has electrophilic (El) and nucleophilic (Nu) sites in the presence of Brønsted acid sites (H+). A high stability of the surface species in which the carboxylic group is not activated allows for a high selectivity to acrylic acid.

Na0.9-X-1.1, an additional signal at d13C  70 ppm is discernible, which is related to the presence of alcohols [52]. In view of the nature of the compounds occurring in the reaction of LA, the fact that carboxylic acid and alcohol groups simultaneously occur on Na0.9-X-1.1 can only be caused by the existence of free LA. The high value of the ratio nH/nC of the carbonaceous deposits given in Table 1 suggests that the compounds formed on Na0.9-X-1.1 contain substantial amounts of heteroatoms, such as oxygen. Moreover, it follows from the results of 13C CP/MAS NMR experiments that overall nH/nC value must be due to deposits that are a

Fig. 6. 13C CP/MAS NMR spectra of the deposits formed on the Na-FAU zeolites upon the reaction of LA at 598 K and for LHSV = 3 h1 (assignment of specific signals: d13C  70 ppm corresponds to alcoholic C, d13C  125–145 ppm to unsaturated C and d13C  180 ppm to carboxylic C).

mixture of LA (nH/nC = 2.0), AA (nH/nC = 1.33), LA polymers (nH/nC = 1.33 to 1.66) and coke (nH/nC 6 1.0) [53].The nH/nC ratios for the remaining catalysts are lower implying that the major constituent of their deposits is coke. Accordingly, the color of the used

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catalysts gets darker as the ratio nH/nC decreases (Supplementary Data). Concerning the loss of activity as well as the nature of the deposits formed on the zeolites during the reaction, the catalyst deactivation must be caused by the accumulation of polar, acidic compounds rather than by the formation of coke-like deposits. Fig. 7 depicts the time dependence of the selectivities to AA and acetaldehyde. The selectivities to the byproducts propanoic acid and 2,3-pentanedione have also been determined as a function of time. However, for the sake of clarity, the latter data are not shown in Fig. 7, especially since the values are mostly below 2%. For TOS exceeding 165 min, the selectivity to acrylic acid slowly goes down on the Na0.9-X-1.1 and Na0.9-Y-2.4 zeolites. In the same period of time, the selectivity to acetaldehyde continuously increases. On the two Brønsted acidic Na-FAU zeolites Na0.6-Y-7.6 and Na0.4-Y-5.2, acetaldehyde is formed as major product, and the selectivity even rises with increasing TOS. On Na0.4-Y-5.2 and Na0.6-Y-7.6, the selectivity toward AA amounts to a maximum of 2% and 10%, respectively. As previously mentioned, the fast side reactions decarbonylation and decarboxylation are inhibited when the concentration of activated carboxylic groups of LA is kept low. This is achieved by the formation of stable, E2-type adsorptive intermediates (see Fig. 5). As a logical consequence, inhibition is not achieved if there are many free Brønsted acid sites and no nucleophilic sites in the catalyst as it is characteristic for the two Na-FAU zeolites with extra-framework Al. Therefore, the selectivity to acrylic acid

100

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SAA Na0.9-X-1.1

SAA Na0.6-Y-7.6

LHSV = 1.5h-1

LHSV = 6h-1

SAA Na0.9-Y-2.4

SAA Na0.4-Y-5.2

LHSV = 3.0h-1

LHSV = 12h-1

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SAcH Na0.9-X-1.1

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LHSV = 1.5h-1

LHSV = 12h-1

SAcH Na0.9-Y-2.4

SAcH Na0.6-Y-7.6

LHSV = 3.0h-1

LHSV = 6h-1

Fig. 7. Product formation upon the reaction of LA on zeolites Na-FAU at 598 K. Left: nucleophilic and amphoteric zeolites, right: electrophilic zeolites (SAA: selectivity to acrylic acid (filled symbols), SAcH: selectivity to acetaldehyde (open symbols)).

observed on Na0.6-Y-7.6 and Na0.4-Y-5.2 is low in comparison with that of amphoteric Na0.9-Y-2.4 and nucleophilic Na0.9-X-1.1. The time-dependent decline of the AA selectivity, as it becomes apparent from the behavior of non-acidic zeolites, points at a change of the zeolite properties with time. The reason of this is that carboxylic compounds are accumulated in the course of the reaction and consequently, this phenomenon affects the catalytic properties of the zeolites. The more free carboxylic groups are formed by accumulation on the catalyst, the more LA molecules react via the thermodynamically favored, Brønsted acid catalyzed reactions decarbonylation and esterification thus, decreasing the selectivity to AA. The decline of the catalytic activity, the loss of the porosity and the nature of the deposits give evidence of a specific deactivation mechanism: a high uptake capacity for polar adsorptives and a high lattice polarity lead to large adsorptive clusters stabilized by numerous hydrogen bonds [46]. Once a certain cluster size is reached, the deposits cannot leave the pores due to steric reasons. Consequently, the active sites of the zeolite become inaccessible with ongoing cluster formation. This causes deactivation. The higher the polarity of the zeolite, the faster is the formation of the clusters as well as the deactivation process. A weak adsorptive/zeolite interaction along with a low capacity for adsorbing polar compounds induce the cluster formation in the pores to become a slow process. Hence, such a moderate accumulation leads to a comparatively lower deactivation rate, which particularly takes places at the beginning of the dehydration reaction. Moreover, a catalyst with strong acid sites (Brønsted acidic and/or Lewis acidic) has a low deactivation tendency even though the adsorptive/zeolite interaction is strong (Na0.6-Y-7.6 and Na0.4-Y-5.2). This finding is due to the fact that adsorptive clusters that are too big to leave the pore system are preferentially decomposed to form coke. This is indicated by blackening. The tendency toward coke formation increases with increasing strength of the electrophilic sites of the catalyst, which is well known in the literature [54]. At the beginning of the reaction, it is mainly the electrophilic character of the zeolite lattice that influences the amount of acrylic acid. The higher the lattice oxygen charge of the zeolite (Table 1), the more nucleophilic sites are available for inhibiting the acid-catalyzed side reactions. Thus, the selectivity to acrylic acid is high. Very strong nucleophiles like in Na0.9-X-1.1 seem to be unfavorable. At higher TOS, the ability for the formation of carboxylic clusters becomes predominant. It determines the number of formed Brønsted acid sites and controls the acceleration of the decarbonylation and the polymerization. Unfortunately, a high polarity advances the cluster formation [36]. 3.2.2. Li-, K- and Cs-containing Na-FAU zeolites As discussed in Section 3.1.2, the type of alkali-metal influences the amount of ammonia adsorbed in the pore system of zeolite Y. Thus, ion exchange with different alkali-metal cations might influence the property of the zeolite toward LA accumulation and, as a consequence of the results discussed above, might have an impact on the catalytic performance. As it becomes discernible from Fig. 8 (left), the time-dependent conversion observable on the Li-, K- and Cs-containing Na-FAU zeolites is comparable with that found on Na0.9-Y-2.4 provided that the LHSV is equal to 3 h1. The conversion amounts to 100% at the beginning of the reaction which lowers as the reaction goes on, implying that all zeolites exhibit a similar catalytic activity and deactivate in the same manner as Na0.9-Y-2.4 does. Consequently, there is no remarkable effect of the nature of the Lewis acidic cation on the activity. The tendency for the deactivation, however, depends on the type of alkali metal used. Whereas TOF declines by about 47% during the experiment on the K- and Cs-containing

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Fig. 8. Reaction of LA with concentration of 20 wt.% on zeolites Na-FAU that contain Li, K and Cs carried out at 598 K and with LHSV = 3 h1 (XLA: lactic acid conversion (filled symbols), TOS: time on stream, TOF: turnover frequency as calculated by Eq. (6) (open symbols), SAA selectivity to acrylic acid (symbols filled in gray), SAcH selectivity to acetaldehyde (black symbols with gray edge)).

zeolites, it declines by 67% on Li0.1,Na0.8-Y-2.4 and 59% on Na0.9-Y-2.4. In general, the course of TOF gives evidence of the fact that K- and Cs-containing zeolites have a smaller tendency toward deactivation than the pure Na-zeolite and Li0.1,Na0.8-Y-2.4 (Fig. 8 (middle)). As already discussed in Section 3.2.1, the decomposition of the lattice of the Li-, K- and Cs-zeolites can be ruled out on the basis of the findings from 27Al MAS NMR spectroscopy and X-ray diffraction studies (Supplementary Data). As to the values in Table 1, the porosity of the modified catalysts is reduced during the reaction, which indicates blocking of the pores. The data for nH/nC ratio given in Table 1 imply that the deposits formed on the ion exchanged and on the pure Na-zeolites are of uniform nature. Despite the results for the ratio nH/nC, the results of 13C CP/MAS NMR spectroscopy (see Fig. 9) on Cs0.2,Na0.8-Y-2.6 reveal that a high percentage of carboxylic species and comparatively fewer COH species are deposited. The deposits formed on Li0.1,Na0.8-Y-2.4 lead to a relatively small carboxylic-C signal. In line with the 13C CP/MAS NMR results, the color of the used catalysts is the same in the case of the Li-zeolite but lighter for the K- and Cs-containing Na-FAU (Supplementary Data), if used Na0.9-Y-2.4 is taken as a reference. That means the amount of coke deposited on the Li- and Na-zeolite is higher than that on the other faujasites, which is in accordance with the electrophilic strength of the zeolites, i.e. with the oxygen charge of the lattice (see Table 1). The fact that the uptake capacity is reduced, as it is true for the K- and Cs-containing faujasites, means that the space available for bulky deposits is limited. The smaller size results in a smaller number of stabilizing hydrogen bonds. Thus, the driving force for the

formation of clusters is low [46], which is the reason why the active sites remain accessible for a longer time. In addition, small clusters can leave the pore system and those mainly consisting of LA can be converted on active sites as a result of which the pore blocking is circumvented. Consequently, the deactivation of the K- and Cs-containing zeolites is slowed down even though the pores are filled with deposits as it is suggested by the observed loss of porosity. The time dependence of the selectivity to acrylic acid and to acetaldehyde observed on the ion exchanged Na-Y zeolites is equivalent to that of the pure Na-Y. This becomes apparent from Fig. 8 (right). The highest selectivity to acrylic acid is achieved after TOS = 165 min, further on the selectivity decreases. The extent of the decline is lower for Cs0.2,Na0.8-Y-2.6. While the selectivity to acrylic acid decreases with time, the acetaldehyde formation increases. As already mentioned, it is mainly the electrophilic character of the zeolite lattice that influences the amount of acrylic acid at the beginning of the reaction (see Section 3.2.1 and Fig. 5). Very strong nucleophiles seem to be unfavorable in the Cs-zeolite, similar to the previously described situation of Na0.9-X-1.1. At higher TOS, the ability for the formation of carboxylic clusters becomes predominant. Both effects, i.e. enhancement of the formation of acrylic acid and reduction of selectivity with time, are responsible for the similar values of the selectivity to acrylic acid for the K- and Cs-containing zeolites after 225 min of reaction. Over the whole duration of the experiment, the best compromise between the strength of the nucleophilic sites and the accumulation rate of Brønsted acidic clusters can be achieved on Cs0.2,Na0.8-Y-2.6.

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TOS / min SAA Na1.2-ZSM-5-18 LHSV = 3 h-1 SAA Na1.2-ZSM-5-18 LHSV = 6 h-1

XLA Na1.2-ZSM-5-18

SAA Na0.9-MOR-5.3

LHSV = 3h-1 TOF Na1.2-ZSM-5-18

LHSV = 1.5 h-1

LHSV = 3h-1 XLA Na0.9-MOR-5.3 Fig. 9. 13C CP/MAS NMR spectra of the deposits formed on the Li-, K- and Cscontaining Na-FAU zeolites upon the reaction of LA at 598 K and LHSV = 3 h1 (assignment of specific signals: d13C  70 ppm corresponds to alcoholic C, d13C  125–145 ppm to unsaturated C and d13C  180 ppm to carboxylic C).

0 100 150 200 250 300

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SAcH Na1.2-ZSM-5-18 LHSV = 3 h-1 SAcH Na1.2-ZSM-5-18 LHSV = 6 h-1 SAcH Na0.9-MOR-5.3 LHSV 1,5 h-1

XLA Na1.2-ZSM-5-18 Under this circumstance, the acrylic acid yield is about 30% after 285 min of reaction. Ion exchange of zeolite Na-Y with other alkali-metal cations does not affect the activity in the LA conversion. Moreover, varying the electrophilic strength of the cations only has a significant impact on the selectivities at the beginning of the reaction. For higher TOS, it is the affinity of the zeolite toward accumulation of polar clusters that determines the deactivation and the decline of selectivity to acrylic acid. That is why for a long reaction time, the cluster formation can be slowed down, provided the formed clusters are comparatively small, i.e. in Cs-Y zeolites. 3.2.3. Na-MOR and Na-ZSM-5 zeolites Literature data on FT-IR spectroscopic studies demonstrate that alkali-metal zeolites of the type MOR and ZSM-5 do not form polar adsorptive clusters [36], seemingly because of their special structure without large cavities. On the other hand, the findings already discussed identify such adsorptives to be responsible for deactivation. Therefore, the Na0.9-MOR-5.3 and Na1.2-ZSM-5-18 zeolites may be considered as promising catalysts for the reaction of LA to AA without deactivation. The time-dependence of the conversion and the values of TOF as stated in Fig. 10 (left) suggest that the two zeolites of type MOR and ZSM-5 exhibit different catalytic activities. The conversion is 91 to 96% on Na1.2-ZSM-5 for the whole duration of the experiment and for a LHSV equal to 6 h1. This means there is no deactivation. The

LHSV = 6h-1 TOF Na1.2-ZSM-5-18 LHSV = 6h-1 Fig. 10. Reaction of LA on Na0.9-MOR-5.3 and Na1.2-ZSM-5-18 zeolites at 598 K. Left: XLA and TOF with respect of TOS. Right: selectivity with respect of TOS (XLA: lactic acid conversion (filled symbols), TOF: turnover frequency as calculated by Eq. (6) (oben symbols), SAA: selectivity to acrylic acid (symbols filled in gray), SAcH: selectivity to acetaldehyde (black symbols with gray edge), LHSV: liquid hourly space velocity).

catalyst is very active with the TOF being even higher than that of the strongly acidic faujasites (Fig. 4). On the MOR zeolite, however, the conversion is lower even at a LHSV of 1.5 h1. Moreover, a distinct deactivation is observable. As the data in Table 1 show, the Na0.9-MOR-5.3 catalyst possesses blocked micropores after the reaction. In contrast, the porosity of Na1.2-ZSM-5-18 is hardly affected in the reaction. Either used catalysts are loaded with small amounts of carbonaceous deposits. As the coke formation is less pronounced, the accessibility of the active sites gets higher with the result that the deactivation slows down. The 13C CP/MAS NMR spectra (Fig. 11) show signals of sp2-hybridized carbon for Na0.9-MOR-5.3. No signals are detectable for the ZSM-5 catalyst. Both zeolites are nearly white after the reaction (Supplementary Data). The analysis of the used catalysts illustrates that there is no accumulation on Na1.2-ZSM-5-18, which is the prerequisite for a

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Fig. 11. 13C CP/MAS NMR spectra of the deposits formed on the Na0.9-MOR-5.3 and Na1.2-ZSM-5-18 zeolites upon the reaction of LA at 598 K and LHSV = 3 h1 (assignment of specific signals: d13C  70 ppm corresponds to alcoholic C, d13C  125–145 ppm to unsaturated C and d13C  180 ppm to carboxylic C).

steady reaction. Apparently, the channels of the MOR zeolite are blocked by coke-like deposits, even though only a few carbonaceous residues can be found. It is known from the literature that MOR with its 1D pore system has unfavorable mass transport characteristics and that the channels of this catalyst can be blocked easily [53,55]. Therefore, it deactivates even though no substantial amount of molecules is deposited. On Na0.9-MOR-5.3, acetaldehyde is the main product (see Fig. 10 (right)). The selectivity to acrylic acid is only 26% and decreases with TOS. Apparently, once the channels are blocked, the active sites located at the outer surface of the zeolite largely control the reaction. At the outer surface of the catalyst crystals, the negative lattice charge is not balanced by the Lewis acidic sodium cations but by protons in silanol groups. These sites give rise to acceleration of the decarbonylation and, consequently, acetaldehyde is primarily formed. Na1.2-ZSM-5-18 does not only differentiate itself by a steady LA conversion, but also the product selectivity achievable on that catalyst is stable with respect of time. The selectivity to acrylic acid is between 54% and 57% and the selectivity to acetaldehyde is between 35% and 38%. In comparison with the results obtained on the faujasites, actually after 165 min of reaction, the percentage with respect to acetaldehyde is higher. The reasons are the low polarity of the framework of Na1.2-ZSM-5-18 and that there are no nucleophilic sites that might help preventing the side reactions of LA (see Fig. 5). Even though acetaldehyde is formed on the ZSM-5 catalyst, the yield of acrylic acid amounts to 53% after 285 min with the conversion still being 96%.

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acid and open-chain lactic acid polymers, proves to be the common reason for the deactivation of all catalysts taken into consideration. The blocking of the active sites causes the decline of the conversion whereas the acidic character of the deposits leads to a time-wise decline of the selectivity to acrylic acid by favoring the acid catalyzed side reactions. It is the structure and the acid-base properties of the zeolites that are responsible for the capability to form such clusters. The deactivation rate is high if big adsorptive clusters can be formed and can be stabilized by intra- and inter-molecular hydrogen bonds. This is true for zeolites with a high uptake capacity for polar compounds, e.g. zeolites with large pore space and a nucleophilic, polar lattice, exemplary Na-X zeolite. Unfortunately, from a mechanistic and thermodynamic point of view, especially the catalysts that have nucleophilic properties are capable of suppressing the side reactions decarbonylation and esterification. Zeolites with an electrophilic lattice, such as Brønsted acidic and/or extra-framework-Al-possessing catalysts, do not undergo cluster formation and thus, do not show deactivation. Yet again, their low nucleophilic character, i.e. low lattice polarity, does not allow for the side reactions decarbonylation and esterification to be inhibited. That is why on these catalysts only poor acrylic acid selectivity is obtained. In the case of nucleophilic or amphoteric zeolites, ion exchange of Na+ by K+ or Cs+ reduces the tendency toward cluster formation due to sterical reasons. Hence, only small clusters arise that can leave the pore system more easily. That is why K- and Cs-containing faujasites do deactivate slower than the corresponding Na-zeolites. In fact, these zeolites assure the highest selectivity to acrylic acid, actually about 62%, after 165 min time-on-stream, but thereafter they deactivate. Consequently, the values of the yield of acrylic acid after 165 min for the zeolites K0.2,Na0.8-Y-2.3 and Cs0.2,Na0.8-Y-2.6 are 59% and 58%, respectively, whereas after 285 min, the corresponding values are, 26% and 30%. Owing to the pore structure, adsorptive accumulation is effectively excluded in zeolite MFI. For instance, lactic acid reacts with stable conversion and stable selectivity on Na-ZSM-5 zeolite with nSi/nAl = 18. In this particular case, the yield of acrylic acid reaches up to 53% at a conversion of 96% after 285 min time-on-stream, which represents the highest values for the performance of all catalysts investigated in the present work. Regarding the optimization of the acrylic acid yield, a catalyst has to be found in which enough nucleophilic sites can be provided and in which at the same time the formation of acidic clusters is slow or not present at all. Following this concept, zeolite EMT can be a promising catalyst, as it allows establishing a similar lattice polarity like in zeolite FAU but with less pore space. Corresponding measurements as well as experiments with modified zeolite ZSM-5, i.e. ZSM-5 that provides a higher number of nucleophilic sites, will be the subjects of further work in our laboratory.

Acknowledgments G.N. acknowledges the support of Landesgraduiertenförderung and Carl-Zeiss-Stiftung. M.A.L.M. acknowledges Consejo Nacional de Ciencia y Tecnología (CONACyT-Mexico) for the scholarship granted (No. 207943). M.H. thanks Deutsche Forschungsgemeinschaft for financial support. We like to thank Utz Obenaus for conducting some of the NMR experiments.

4. Conclusions

Appendix A. Supplementary material

In the gas phase dehydration of lactic acid to acrylic acid, the accumulation of acidic hydrocarbons, such as lactic acid, acrylic

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2015.05.017.

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