Activity, selectivity and stability of Zn-exchanged NaY and ZSM5 zeolites for the synthesis of o-hydroxyacetophenone by phenol acylation

Microporous and Mesoporous Materials 143 (2011) 236–242

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Activity, selectivity and stability of Zn-exchanged NaY and ZSM5 zeolites for the synthesis of o-hydroxyacetophenone by phenol acylation C.L. Padró ⇑, E.A. Rey, L.F. González Peña, C.R. Apesteguía Catalysis Science and Engineering Research Group (GICIC), Instituto de Investigaciones en Catálisis y Petroquímica-INCAPE-(UNL-CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina

a r t i c l e

i n f o

Article history: Received 13 January 2011 Received in revised form 2 March 2011 Accepted 3 March 2011 Available online 8 March 2011 Keywords: Zn(Na)Y zeolites ZnZSM5 zeolites Phenol acylation o-Hydroxyacetophenone Acid catalysis

a b s t r a c t The gas-phase synthesis of o-hydroxyacetophenone (o-HAP) by phenol acylation with acetic acid was studied on Zn-exchanged NaY and ZSM5 zeolites. The density, nature and strength of surface acid sites were determined by temperature programmed desorption of NH3 coupled with infrared spectra of adsorbed pyridine. NaY zeolite contained only surface Lewis acid sites. The exchange of Na+ with Zn2+ increased both the density and strength of Lewis acid sites of parent NaY zeolite. The Lewis/Brønsted acid sites ratio on ZSM5 zeolite was about 1. The addition of Zn increased the density of Lewis acid sites but partially eliminated the protonic sites of parent ZSM5 zeolite. Thus, the Lewis/Brønsted acid sites ratio was 3.2 on a ZnZSM5 containing 1.19% Zn. The incorporation of Zn2+ to zeolite NaY increased the initial selectivity to o-HAP because o-HAP was preferentially formed through the attack of the acylium ion to the phenol molecule adsorbed in vertical orientation on Lewis acid sites. Nevertheless, the activity decay rate on ZnNaY samples increased with the Zn content. ZnZSM5 zeolites were more active but less selective to the o-HAP formation than ZnNaY zeolites. However, ZnZSM5 zeolites formed only small quantities of coke and exhibited a remarkable stability on stream. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Ortho- (o-HAP) and para-hydroxyacetophenone (p-HAP) isomers are intermediate compounds for the synthesis of fragrances and pharmaceuticals. For example, p-HAP is used in a new process for obtaining paracetamol [1], while o-HAP is involved in the synthesis of warfarin, a known anticoagulant drug [2], and of flavonones, via a Claisen–Schmidt condensation [3,4]. Both aromatic ketones are commercially obtained through the liquid-phase Fries rearrangement of the corresponding ester (phenyl acetate), using liquid catalysts such as metal halides (AlCl3) or mineral acids (HF or H2SO4) in super stoichiometric quantities. This synthesis technology has important environmental constraints because it is very corrosive and produces large amounts of toxic wastes. The possibility of obtaining o- and p-hydroxyacetophenone using solid catalysts that can be easily separated and recycled has been studied in the past few years. Strong solid acids, such as ionic resins, Nafion, and heteropolyacids, present moderate activity for the liquid-phase Fries rearrangement of phenyl acetate and form preferentially p-HAP, but also produce significant

⇑ Corresponding author. Tel.: +54 3424555279; fax: +54 3424533858. E-mail address: cpadro@fiq.unl.edu.ar (C.L. Padró). URL: http://www.fiq.unl.edu.ar/gicic (C.L. Padró). 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.03.005

amounts of phenol and are quickly deactivated because of coke formation [5]. The Fries rearrangement of phenyl acetate has been also investigated in gas phase, between 523 and 693 K, using acid zeolites [6–8]. Synthesis of o-HAP was favored under these reaction conditions, but formation of significant amounts of phenolic byproducts took place. In particular, Borzatta et al. [7] observed that commercial pentasil-type zeolite T-4480 (Süd-Chemie) forms o-HAP at high rates. In contrast, it was reported that high selectivities to p-HAP can be obtained by blocking the external surface of zeolite H3PO4/ZSM5 [8]. On all these studies, significant catalyst activity decay on stream was observed. Hydroxyacetophenone isomers can also be produced by acylation of phenol with acetic anhydride or acetic acid, whether in liquid or gas phase (Scheme 1). The gas-phase phenol acylation with acetic acid was studied on Al-MCM-41 and zeolite HZSM-5, at temperatures between 523 and 573 K. These solid acids produced preferentially o-HAP, but o-HAP yields were moderate because of the concomitant formation of phenyl acetate [9,10]. More active and stable solid catalysts have to be developed then in order to improve the o-HAP synthesis by phenol acylation. In particular, more knowledge on the exact requirements of strength, nature and density of surface acid sites is needed for efficiently promoting the C-acylation of phenol molecule in ortho position and for increasing the o-HAP yield. In a previous paper [11], we have studied the gas-phase acylation of phenol with acetic acid

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and 9.30%, Zn respectively, were obtained by ion exchange of zeolite NaY with 0.2 M and 0.5 M Zn(NO3)26H2O solutions at 353 K.

2.2. Catalysts characterization

Scheme 1. Reaction network for the acylation of phenol with acetic acid.

on different solid acids. We found that catalysts containing only Lewis acid sites, such as NaY zeolite, form selectively o-HAP but rapidly deactivate on stream. Samples containing strong Lewis and Brønsted acid sites, such as zeolites ZSM5 and HY produce oHAP at high rates because they catalyze the two main reaction pathways leading from phenol to o-HAP, i.e., the direct C-acylation of phenol and the O-acylation of phenol forming the PA intermediate, which is consecutively transformed via intermolecular phenol/ PA C-acylation (Scheme 1). We also found that zeolite ZSM5 was stable while zeolite HY quickly deactivated during the progress of the reaction. In this work we have extended these studies with the aim of improving the o-HAP synthesis from phenol acylation by regulating the zeolite surface acid properties. Specifically, we have exchanged zeolites NaY and ZSM5 with increasing amounts of Zn+2 cations in order to increase the density and strength of surface Lewis acid sites. Results show that on ZnNaY zeolites the initial selectivity to o-HAP increases with the Zn content reaching up to about 83% o-HAP. The addition of Zn to zeolite ZSM5 also favors the oHAP formation without affecting the unique stability that the parent zeolite exhibits for this reaction

BET surface areas (Sg) were measured by N2 physisorption at 77 K in a Quantochrome Corporation NOVA-1000 sorptometer. The crystallinity of the samples were determined by X-ray diffraction (XRD) using a Shimadzu XD-D1 diffractometer and Ni-filtered Cu Ka radiation. The chemical compositions were measured by atomic absorption spectroscopy. The density and strength of the acid sites were determined by temperature programmed desorption (TPD) of NH3 adsorbed at 373 K. Samples (100 mg) were treated in He (60 cm3/min) at 773 K for 2 h, cooled down to 373 K, and then exposed to a 1% NH3/He stream during 45 min. Weakly adsorbed NH3 was removed by flushing with He at 373 K for 90 min. Finally, the sample temperature was increased at 10 K/min in a He flow of 60 cm3/min and desorbed NH3 was analyzed by mass spectrometry (MS) in a Baltzers Omnistar unit. The nature and strength of the surface acid sites were determined by Infrared Spectroscopy (IR) in a Shimadzu FTIR Prestige21 spectrophotometer using pyridine as a probe molecule. Sample wafers were formed by pressing 20–40 mg of the catalyst at 5 ton/ cm2 and transferred to a sample holder made of quartz. An inverted T-shaped Pyrex cell containing the sample wafer was used. The two ends of the short arm of the T were fitted with CaF2 windows. Samples were initially outgassed in vacuum at 723 K during 2 h and then a background spectrum was recorded after being cooled down to room temperature. Spectra were recorded at room temperature, after admission of pyridine, adsorption at room temperature and sequential evacuation at 298, 423 and 573 K. The carbonaceous deposit formed on the catalysts during reaction was studied by temperature programmed oxidation (TPO). After 6 h of reaction, the catalyst samples were maintained at the reaction temperature in N2 flow 1 h before to perform the TPO experiment, in order to ensure that no molecules of reactants or products are retained on the catalyst surface. Samples (40 mg) were heated at 10 K/min in a 3% O2/N2 stream from room temperature to 1073 K. The evolved CO2 was converted into methane in a fixed bed reactor containing a methanation catalyst (Ni/kieselghur) at 673 K. Then, methane was analyzed using a flame ionization detector (gas chromatograph: SRI 8610C).

2. Experimental

2.3. Catalytic activity

2.1. Catalyst preparation

The gas-phase acylation of phenol (Merck, >99%) with acetic acid (Merck, 99.5%) was carried out in a fixed bed, continuous-flow reactor at 513 K and 101.3 kPa. Samples were sieved to retain particles with 0.35–0.42 mm diameter for catalytic measurements and pretreated in air at 773 K for 2 h before reaction. Phenol (P) and acetic acid (AA) were introduced (P/AA = 1) via a syringe pump and vaporized into flowing N2 to give a N2/(P + AA) ratio of 45. The effluent gas composition was analyzed on-line using an Agilent 6850 Gas Chromatograph equipped with a 30 m Innowax column (inner diameter: 0.32 mm, film thickness: 0.5 lm) and FID detector. Data were collected every 25 min for about 6 h. Main reaction products were phenyl acetate (PA), o-HAP and p-HAP; paraacetoxyacetophenone (p-AXAP) was detected in trace amounts. Phenol conversion (XP was calculated as: XP = RYi/(RYi + YP), where RYi is the molar fraction of products formed from phenol, and YP is the outlet molar fraction of phenol. The selectivity to product i (Si, mol of product i/mol of phenol reacted) was determined as: Si (%) = [Yi/RYi]100.

Zeolites Zn(0.82)ZSM5 and Zn(1.19)ZSM5 containing 0.82% and 1.19% Zn, respectively, were obtained from commercial ZSM5 zeolite (Zeocat Pentasil PZ-2/54, pore size: 5.5 Å, Si/Al = 20, 0.43 wt.% Na) by successive ion exchanges with aqueous solutions of Zn(NO3)26H2O (Riedel-de Haën, 98%) at 353 K. Zeolite Zn(0.82)ZSM5 was prepared by performing one exchange with a 0.05 M Zn(NO3)26H2O solution while zeolite Zn(1.19)ZSM5 was obtained by exchanging zeolite ZSM5 three times, with a 0.5 M Zn(NO3)2 solution. All the exchanged samples were filtered, washed with hot distilled water, dried at 373 k for 12 h, and finally calcined in air at 723 K for 3 h. Zeolite NaY was a commercial zeolite (UOP-Y 54, Si/Al = 2.4, pore size: 7.4 Å) of 700 m2/g. Zeolite HY was prepared by double ion exchange of zeolite NaY with a 1 M NH4Cl aqueous solution (Merck, 99.8%) at 353 K, and subsequent calcination in air at 723 K. Zeolites Zn(5.20)NaY and Zn(9.30)NaY containing 5.20%

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3. Results and discussion 3.1. Catalyst characterization The chemical composition, surface area and crystallinity of the samples are summarized in Table 1. The surface areas of parent ZSM5 and NaY zeolites were not significantly affected by ionic exchange procedures used for adding Zn2+ cations. Consistently, the XRD profiles (not shown here) revealed that the crystalline structures of parent zeolites did not change substantially following the ionic exchanges treatments; the crystallinity of Zn-exchanged zeolites was higher than 80% in all the cases. The Zn content on exchanged zeolites increased with both the concentration of the exchange solution and the number of exchanges. The largest concentrations of Zn achieved were 1.19 wt.% on ZSM5 and 9.30% on NaY corresponding to Zn/Al ratios of 0.23 and 0.29, respectively. The strength and density of surface acid sites were studied by NH3 TPD. The NH3 desorption profiles are shown in Fig. 1A (ZSM5-based samples) and Fig. 1B (zeolite Y-based samples). The TPD curves of ZSM5 and Zn-ZSM5 samples displayed a lowtemperature desorption peak with a maximum between 530 and 550 K arising from NH3 adsorbed on surface sites of weak and medium acidity, and a high-temperature band which accounted for the strong acid sites present on these materials. The maximum of this high-temperature band shifted to higher temperatures when the sample Zn content was increased. The surface densities of the acid sites were determined by deconvolution and integration of NH3 TPD profiles and are given in Table 2. It is observed that the concentration of medium and weak acid sites of zeolite ZSM5 remained practically unchanged while that of strong acid sites increased with the addition of Zn to the sample. The TPD profile of NaY presented a single low-temperature asymmetric peak at about 480 K (Fig. 1B) while zeolite HY showed Table 1 Chemical composition, surface area and crystallinity of the samples used in this work. Samples

ZSM5 Zn(0.82)ZSM5 Zn(1.19)ZSM5 NaY Zn(5.20)NaY Zn(9.30)NaY HY

Chemical composition Na (wt.%)

Zn (wt.%)

Si/Al

0.43 0.04 0.01 6.85 2.90 0.43 1.20

– 0.82 1.19 – 5.20 9.30 –

20 20 20 2.4 2.4 2.4 2.4

Surface area Sg (m2/g)

Crystallinity (%)

350 374 317 700 662 612 660

99.4 99.6 99.6 84.5 83.0 80.4 97.5

an additional broad desorption band at higher temperatures denoting the presence of stronger acid sites. The evolved NH3 from Zn(5.20)NaY sample gave rise to the low-temperature peak at 480 K and to a wide desorption band at higher temperatures. Zn(9.30)NaY presented an additional shoulder at about 550 K. These results showed that the exchange of zeolite NaY with Zn2+ cations not only increased the concentration of weak and medium acid sites but also generated strong acid sites. The FTIR spectra for ZSM5 and Zn-ZSM5 samples in the OH stretching region (3400–3800 cm1) after outgassing at 723 K are displayed in Fig. 2A. All the spectra showed a sharp band at 3745 cm1 corresponding to terminal silanol (SiOH) vibrations [12] and a band at 3600 cm1 that has been attributed to the stretching of hydroxyls in Si–OH–Al groups accounting for surface Brønsted acidity [13]. The silanol band at 3745 cm1 diminished with the Zn content probably because of the interaction of SiOH with ZnOH+ [14]. Similarly, Fig. 2A shows that the intensity of 3600 cm1 band decreased as the amount of Zn in the sample was increased, thereby indicating that protonic sites are involved in ZSM5 exchange with Zn+2 cations. Fig. 2B shows the FTIR spectra obtained on zeolites HY, NaY, Zn(5.20)NaY and Zn(9.30)NaY in the OH stretching region. NaY and Zn-exchanged NaY samples did not exhibit any absorption band, which revealed the absence of Brønsted acid sites on all these samples. In contrast, zeolite HY showed three OH stretching bands corresponding to terminal silanols (3740 cm1), Si–OH–Al groups located in accessible large cavities (3640 cm1), and Si–OH–Al groups located in sodalite cage and hexagonal prisms (3550 cm1) [15]. The nature of surface acid sites was studied by FTIR of adsorbed pyridine. On acid zeolites, the pyridinium ion (pyridine adsorbed on Brønsted acid sites) shows absorption bands at 1540, 1480– 1500 and 1640 cm1 [16,17]. Pyridine coordinatively bonded on Lewis acid sites gives rise to the characteristics bands at 1440– 1460, 1480–1500 and 1600 cm1. The 1600 cm1 band is the most sensitive band that shifts to higher frequency with the increase in the strength of the acid sites [18]. Fig. 3A shows the IR spectrum obtained on ZSM5 after pyridine adsorption at 298 K and the consecutive evacuation at 423 K. It is observed that the 1440– 1460 cm1 band was split in two overlapping peaks corresponding to pyridine adsorbed on Al (1450 cm1) and Na (1445 cm1) Lewis acid sites. The characteristic band of pyridinium ion at 1540 cm1 is also observed on the ZSM5 spectrum. The pyridine absorption bands in the 1600 cm1 region at around 1634, 1625 and 1600 cm1 arise from pyridine adsorbed on Brønsted, strong Lewis (Al cations) and weaker Lewis (Na cations) acid sites, respectively.

Fig. 1. TPD profiles of NH3 on: (A) zeolites ZSM5 and ZnZSM5 and (B) Y zeolites. Heating rate, 10 K/min.

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C.L. Padró et al. / Microporous and Mesoporous Materials 143 (2011) 236–242 Table 2 Sample acidity. Samples

TPD of NH3

IR of pyridine

Acid site density (lmol/g)

ZSM5 Zn(0.82)ZSM5 Zn(1.19)ZSM5 NaY Zn(5.20)NaY Zn(9.30)NaY HY a b c d

T = 423 K

T = 573 K

Weak and mediuma

Strongb

Total

B (area/g)c

L (area/g)c

L/(L + B)

B (area/g)d

L (area/g)d

L/(L + B)

204 201 195 280 1000 1470 567

271 394 382 – 492 651 863

475 595 577 280 1492 2121 1430

337 265 213 – 18 37 310

341 533 660 525 965 1200 465

0.50 0.67 0.76 1 0.98 0.97 0.60

240 191 155 – 6 9 209

232 379 455 181 557 880 177

0.49 0.66 0.75 1 0.99 0.99 0.46

Desorption temperature lower than 600 K. Desorption temperature higher than 600 K. By FTIR of pyridine adsorbed at 298 K and evacuated at 423 K (B: Brønsted sites; L: Lewis sites). By FTIR of pyridine adsorbed at 298 K and evacuated at 573 K (B: Brønsted sites; L: Lewis sites).

Fig. 2. FTIR spectra in the hydroxyls stretching region of: (A) zeolites ZSM5 and ZnZSM5 and (B) Y zeolites. Samples degassed at 723 K for 4 h.

Fig. 3. FTIR spectra of pyridine adsorbed at 298 K and evacuated at 423 K on: (A) zeolites ZSM5 and ZnZSM5, (B) Y zeolite samples.

Fig. 3A also shows the IR spectra obtained on Zn-ZSM5 samples. The exchange of Na+ by Zn2+ caused the disappearance of the 1445 cm1 band (pyridine adsorbed on Na cations) and the increase of the band at 1455 cm1 (pyridine adsorbed on Al and Zn cations); the positions of pyridine absorption bands on Zn2+ and Al3+ are similar, between 1455 and 1459 cm1 [19]. The replacement of Na+ by Zn2+ is also indicated by the disappearance of the 1600 cm1 band and the development of a new sharp band at 1615 cm1. The amounts of pyridine remaining on Brønsted (B) and Lewis (L) acid sites after evacuation at 423 and 573 K were obtained by

deconvolution and integration of pyridine absorption bands appearing at 1540 and 1440–1460 cm1, respectively. Results are given in Table 2. After evacuation at 423 K, the L/(L + B) ratio was about 0.5 on ZSM5. The exchange of Na+ with Zn2+ increased the amount of Lewis sites that was two times higher on Zn(1.19)ZSM5 than on ZSM5. In contrast, the concentration of Brønsted acid sites decreased monotonically with the Zn content which is in agreement with IR spectra of Zn-ZSM5 samples in the OH region (Fig. 2A) that suggested that the ZSM5 protonic sites are partially eliminated during the ionic exchange procedures. Thus, the Lewis/Brønsted sites ratio significantly increased with the Zn

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content reaching L/(L + B) ratios of 0.67 and 0.76 for Zn(0.82)ZSM5 and Zn(1.19)ZSM5, respectively. Similar results regarding the relative concentrations of Lewis and Brønsted acid sites on Zn-exchanged ZSM5 samples were observed after pyridine evacuation at 573 K (Table 2). The IR spectra obtained on Y zeolites are shown in Fig. 3B. Consistent with the IR characterization of hydroxyl groups in Fig. 2B, the pyridine absorption spectrum on zeolite NaY did not reveal the presence of surface Brønsted sites and presented only the bands arising from pyridine adsorbed on Na+ Lewis sites (1445 and 1600 cm1). In agreement with the results obtained by NH3 TPD (Fig. 1B and Table 2), data in Table 2 reveal that the addition of Zn generated stronger Lewis acid sites on the parent NaY zeolite. Indeed, after evacuation at 573 K zeolite Zn(9.30)NaY retained 75% of pyridine remaining after evacuation at 423 K; in contrast, zeolite NaY retained only 35% of pyridine. Data of Table 2 also show that the addition of Zn generated only very small amounts of Brønsted acid sites; the L/(L + B) ratio on ZnNaY samples was, in fact, higher than 0.97. The protonic form of Y zeolite (HY) showed the contribution of both Brønsted and Lewis acidity. The L/(L + B) ratio determined on this sample after pyridine evacuation at 423 K was 0.60 but decreased to 0.46 after evacuation at 573 K, thereby suggesting the predominance of strong Brønsted acidity. 3.2. Catalytic results In a previous paper [11], we identified the primary and secondary products for the acylation of phenol with AA and established the different reaction pathways on different acid catalysts. We proposed that o-HAP is obtained by two different pathways (Scheme 1): (i) directly by C-acylation of phenol in ortho position, (ii) indirectly from PA that is initially produced by O-acylation of

phenol. o-HAP is formed from PA either by Fries rearrangement or by acylation of phenol with PA. Fig. 4 shows the evolution of phenol conversion (XP), and selectivities (Si) as a function of time on Zn(0.82)ZSM5 and Zn(9.3)NaY samples and typically illustrates the time-on-stream behavior of the catalysts during the reaction. Zeolite Zn(0.82)ZSM5 formed only PA and o-HAP (Fig. 4A); phenol conversion and product selectivities remained practically constant during the reaction. Similar qualitative behavior was observed on ZSM5 and Zn(1.19)ZSM5. Zeolite Zn(9.30)NaY also formed only PA and o-HAP (Fig. 4B), but the selectivity to PA increased with time on stream at the expense of o-HAP in spite that XP did not change appreciably. A similar evolution of phenol conversion, PA and o-HAP selectivities during the progress of the reaction was observed on NaY, Zn(5.20)NaY and HY, but the latter formed in addition nonnegligible amounts of p-HAP. The catalytic properties of the samples were compared at initial conditions taking into account the observed deactivation on Y zeolites (Fig. 4B). Initial conversion and selectivities were obtained by extrapolating the corresponding curves to initial time on stream; results are given in Table 3. At the same contact time (W=F op ¼ 146 g h=mol) phenol conversion was two times higher on ZSM5 than on NaY, probably because ZSM5 contains a higher density of strong Brønsted and Lewis sites that catalyze more efficiently the conversion of phenol into both primary products, PA and o-HAP [20]. Table 3 compares the product selectivities obtained at 10% phenol conversion. The addition of small amounts of Zn to commercial ZSM5 increased the selectivity to o-HAP from 31.0% (ZSM5) to 38.8% (Zn(0.82)ZSM5). This result suggested that the formation of o-HAP via C-acylation of phenol (Scheme 1) is preferentially promoted on surface Lewis sites because data in Table 2 show that the presence of Zn in zeolite ZSM5 increased the density and strength of Lewis acid sites. Nevertheless, further addition of Zn

Fig. 4. Phenol conversion and product selectivities as a function of time on stream on: (A) Zn(0.82)ZSM5; (B) Zn(9.30)NaY. [513 K, 101.3 kPa total pressure, =146 g h/mol, P/AA = 1, N2/(P + AA) = 45].

Table 3 Catalytic and deactivation results for the gas-phase acylation of phenol with acetic acid. Samples

ZSM5 Zn(0.82)ZSM5 Zn(1.19)ZSM5 NaY Zn(5.20)NaY Zn(9.30)NaY HY *

Initial activity and selectivities (%)*

Deactivation*

Selectivity (XP = 10%, t = 0), (%)

X 0P

S0o-HAP

S0PA

S0p-HAP

S0o-HAP

S0PA

S0p-HAP

d0 (min1)  103

%C

18.0 20.8 21.3 9.1 11.6 11.8 10.4

52.7 62.8 62.2 71.8 77.7 82.7 69.5

47.3 37.2 37.8 28.2 22.3 17.3 22.7

– – – – – – 7.8

31.0 38.8 39.5 73.8 75.3 81.7 69.5

69.0 61.2 60.5 26.2 24.7 18.3 22.7

– – – – – – 7.8

0.70 0.90 0.89 16.0 18.0 22.0 14.5

3.7 4.3 4.0 10.1 10.8 13.7 9.0

T = 513 K, W=F op ¼ 146 g h=mol; P/AA = 1, N2/(P + AA) = 46.

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(Zn(1.19)ZSM5 sample) did not improve the formation of o-HAP in comparison with sample Zn(0.82)ZSM5. Commercial NaY zeolite was less active than ZSM5 but presented higher selectivity to o-HAP (Table 3). Besides, the incorporation of Zn2+ to zeolite NaY increased the o-HAP selectivitiy from 73.8% (NaY) to 75.3% (Zn(5.20)NaY) and 81.7% (Zn(9.30)NaY). In contrast, the o-HAP selectivity was 69.5% on zeolite HY, obtained by exchanging NaY with H+ cations. Zeolite HY also formed initially 7.8% of p-HAP.

3.3. Catalytic performance and surface acid properties Results in Table 3 show that the initial activity and selectivity of ion-exchanged ZSM5 and NaY zeolites for the acylation of phenol with acetic acid depend on the nature, density and strength of surface acid sites. The acetic acid reacts on both Brønsted and Lewis acid sites to form acylium ions (CH3CO+) that are the acylating agents, as depicted in Scheme 2 [20]. Generated electrophile CH3CO+ may then attack the phenol molecule either by electrophilic substitution of the ortho-hydrogen in the aromatic ring forming o-HAP or, alternatively, by O-acylation of the OH group producing PA. On the other hand, it has been reported that phenol can be adsorbed on Lewis acid centers in vertical orientation or on Brønsted sites in a parallel adsorption mode (Scheme 3) [21,22]. In the case of phenol adsorbed on vertical orientation, the attack of the acylium ion of benzene rings in ortho-position is promoted (o-HAP formation) because stabilization of the ortho-isomer intermediate is favored as compared to the intermediate formation in the para-position (p-HAP formation) [23]. When the phenol molecule interacts via the benzene rings with surface Brønsted acid sites adopting a parallel position to the surface, the attack of acylium ions to the oxygen of phenol forming PA would be predominant (Scheme 3). Zeolite NaY contains only weak and medium Lewis acid sites and formed initially 73.8% of o-HAP; the addition of Zn increased both the density and the strength of Lewis acid sites and, as a consequence, increased the o-HAP formation rate; the initial o-HAP selectivity on Zn(9.30)NaY was, in fact, 81.7% (Table 3). These results are consistent with the preferential formation of oHAP on Lewis acid sites, as depicted in Scheme 3. Nevertheless, Zn-exchanged NaY zeolites that contain very small amounts of

241

Brønsted acid sites (Table 2), formed initially also about 20% PA and the selectivity to PA increased with the progress of the reaction (Fig. 4B). This result suggests that formation of PA via the attack of adsorbed acylium ions to a gaseous phenol molecule cannot be discarded. Zeolite ZSM5 was more active than NaY because it contains a higher density of strong Brønsted and Lewis acid sites, which catalyze more efficiently the generation of acylating agent CH3CO+ from acetic acid. However, the initial o-HAP selectivity was lower on ZSM5 than on NaY (Table 3). The addition of Zn to zeolite ZSM5 increased the initial o-HAP selectivity, which confirms that o-HAP is preferentially formed on surface Lewis acid sites. 3.4. Catalyst deactivation and structure and surface acid properties Similar to the catalytic tests presented in Fig. 4, we observed on all the catalysts studied in this paper that phenol conversion remained practically unchanged during the 6-h catalytic experiments/runs. However, on Y zeolite-based catalysts, the product distribution changed with time on stream and formation of PA increased at the expenses of o-HAP (Fig. 4B). In contrast, on ZSM5-based catalysts the o-HAP selectivity remained practically unmodified during the run (Fig. 4A). In Fig. 5 we have plotted the evolution of the activity for the formation of o-HAP (ao-HAP) as a function of time on stream. The activity ao-HAP is defined as ao-HAP ¼ ro-HAP =r 0o-HAP , where ro-HAP and r0o-HAP are the rate of o-HAP formation at times t and zero, respectively. Fig. 5 shows the curves of activity versus time on stream obtained on HY, NaY, Zn(5.20)NaY and Zn(9.30)NaY. From the curves of Fig. 5 we calculated the initial deactivation rate (d0, min1) as the initial slope of the curves, i.e., d0 ¼ ðdao-HAP =dtÞt¼0 ; the obtained d0 values are listed in Table 3. Data in Fig. 5 and Table 3 show that the activity decay on Zn(1.19)ZSM5 was negligible; similar catalyst deactivation behavior was observed on ZSM5 and Zn(0.82)ZSM5 (deactivation curves not included in Fig. 5). The d0 values obtained on ZSM5 and ZnZSM5 samples were, in fact, close to zero (Table 3). In contrast, Y zeolite-based samples, rapidly deactivated on stream. In particular, the activity decay on Zn-exchanged NaY zeolites increased with the sample Zn content; the d0 value increased from 16 min1 on NaY to 22 min1 on Zn(9.3)NaY (Table 3). Carbonaceous residues formed on the samples during the catalytic tests were determined at the end of the runs by temperatureprogrammed oxidation (TPO). The carbon amounts on the samples were calculated by integration of the corresponding TPO profiles

Scheme 2. Formation of acylium ions on Brønsted and Lewis acid sites.

Scheme 3. Attack of the acylium ion to phenol adsorbed on Brønsted and Lewis acid sites.

Fig. 5. Activity for o-HAP formation (ao-HAP) as a function of time on stream [513 K, 101.3 kPa total pressure, =146 g h/mol, P/AA = 1, N2/(P + AA) = 45] ( ) Zn(1.19)ZSM5, (h) HY, (.) NaY, (s) Zn(5.20)NaY (j) Zn(9.30)NaY.

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(not shown here) and are presented in Table 3. On H(Na,Zn)Y zeolites, the %C increased from 9.0% on HY to 13.7% on Zn(9.3)NaY, following a trend similar to that noted for d0 values. This result suggested that the observed catalyst deactivation in Fig. 5 was mainly caused by the formation of carbonaceous residues. The %C values determined on ZnZSM5 samples were about 4%, i.e., significantly lower than on H(Na,Zn)Y zeolites. In a previous work [24], we studied the deactivation mechanism of o-HAP synthesis from phenol acylation with acetic acid on zeolites HY, HBeta and HZSM5. We found that o-acetoxyacetophenone (o-AXAP), is produced by reaction between o-HAP and acetic acid, is the key intermediate species responsible for the formation of carbonaceous deposits. Thus, catalysts forming higher amounts of o-HAP would promote the formation of o-AXAP and, consequently, of coke. This is exactly what we observed here in Fig. 5 for ZnNaY samples: the more active catalysts deactivated faster. The increase of the activity decay rate on ZnNaY samples containing higher Zn contents would reflects then an enhanced o-AXAP formation on Zn2+ Lewis acid sites. In contrast, the narrow pore size structure of zeolite ZSM5 would hinder the formation of bulky o-AXAP [24], thereby decreasing drastically the formation of coke on ZnZSM5 samples. Thus, the superior stability of ZnZSM5 zeolites is explained in basis of a shape-selectivity effect, irrespective of the surface acidity changes occurring when the Zn content on the sample is increased. 4. Conclusions The exchange of zeolites NaY and ZSM5 with Zn+2 cations changes the density and strength of surface acid sites without substantially modifying the sample surface area and crystallinity. Zeolite NaY contains only Lewis acid sites; the exchange of Na+ with Zn2+ increases both the density and strength of Lewis acid sites of parent NaY zeolite. Zeolite ZSM5 contains similar amounts of Lewis and Brønsted acid sites; the addition of Zn increases the amount of Lewis acid sites and eliminates protonic sites. Thus, the Lewis/Brønsted acid sites ratio on zeolites ZnZSM5 increases with the Zn content in the sample. Zeolite NaY produces preferentially o-hydroxyacetophenone (o-HAP) from the gas-phase acylation of phenol with acetic acid. The addition of Zn2+ to zeolite NaY increases further the o-HAP formation rate. Additionally, initial o-HAP selectivities of up to 82% are obtained on zeolites ZnNaY containing about 9.30% Zn. Zn-exchanged NaY zeolites efficiently catalyze the o-HAP formation because o-HAP is mainly formed through the attack of the acylating agent (acylium ions) to the phenol molecule adsorbed in vertical orientation on Lewis acid sites. Nevertheless, the activity decay rate on ZnNaY samples increases with the Zn content because surface Lewis acid sites also promote the formation

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