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Active sites in the alkylation of toluene with methanol: a study by selective acid–base poisoning
Applied Catalysis A: General 276 (2004) 207–215 www.elsevier.com/locate/apcata
Active sites in the alkylation of toluene with methanol: a study by selective acid–base poisoning A. Borgna*, J. Sepu´lveda, S.I. Magni, C.R. Apesteguı´a a
Catalysis Science and Engineering Research Group (GICIC), Instituto de Investigaciones en Cata´lisis y Petroquı´mica – INCAPE-(UNL-CONICET), Santiago del Estero 2654, (3000) Santa Fe, Argentina Received in revised form 6 August 2004; accepted 7 August 2004 Available online 22 September 2004
Abstract Selective acid–base poisoning of the alkylation of toluene with methanol was studied over alkali and alkaline-earth exchanged Y zeolites. Surface acid–base properties of the samples were determined by infrared spectroscopy using carbon dioxide and pyridine as probe molecules. Selective poisoning experiments were performed by co-feeding either acetic acid or 3,5-dimethyl pyridine. Kinetic deactivation parameters were determined by modeling experimental data with a deactivation model with residual activity (DMRA). MgY produced essentially xylenes and alkylated C9+ compounds. This zeolite was deactivated only by co-feeding 3,5-dimethyl pyridine, thereby indicating that exclusively acid sites are involved in the ring alkylation of toluene with methanol. In contrast, zeolite CsY formed mainly styrene and ethylbenzene, and was strongly deactivated by the addition of either acid or base compounds. Infrared characterization showed that zeolite CsY contains Lewis acid (Cs+)–base (O2) pairs capable of adsorbing bidentate carbonates but does not exhibit Bro¨nsted acidity. Thus, selective poisoning results and sample characterization showed that bimolecular side-chain alkylation reaction on CsY requires a specific surface acid–base pairs configuration not only for activating toluene and methanol, but also for promoting the rate-limiting step. # 2004 Elsevier B.V. All rights reserved. Keywords: Toluene alkylation; Basic zeolites; Selective poisoning; Styrene; Ethylbenzene
1. Introduction The alkylation of toluene with methanol is readily catalyzed on synthetic zeolites. Previous work has shown that the aromatic-ring alkylation of toluene with methanol takes place over acid zeolites [1], while the side-chain alkylation occurs preferentially over basic zeolites [2,3]. The side-chain alkylation of toluene with methanol for producing a mixture of styrene and ethylbenzene offers economical advantages compared with the conventional homogeneously catalyzed Friedel-Crafts process, which uses ethylene and benzene as reactants [4,5]. Zeolites are attractive materials for alkylation reactions because their acid–base properties can be easily modified by dealumination, isomorphous substitution or ionic-exchange. * Corresponding author. Tel.: +54 342 4555279; fax: +54 342 4531068. E-mail address: [email protected], (A.Borgna). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.08.007
The role of the acid–base properties of zeolite catalysts on the product distribution of aromatic alkylation reactions has been reviewed by Giordano et al. [6]. The ring-alkylation mechanism over acid catalysts would proceed via the formation of methoxonium ion, which requires Bro¨nsted acid sites [1,7]. Thus, the surface concentration of methoxonium ion, and therefore the catalytic activity for ring-alkylation, would depend on the density and strength of the Bro¨nsted acid sites. On the other hand, the side-chain alkylation of toluene with methanol over base catalysts has been known for several decades [8]. Although base catalysts such as MgO and CaO were successfully used for side-chain alkylation [9], recent studies indicate that the zeolite pores might be indispensable to efficiently catalyze the reaction [10]. Alkali-exchanged zeolites have demonstrated the highest activities and selectivities for side-chain alkylation of toluene with methanol [11]. Particularly, the side-chain
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alkylation of toluene with methanol requires basic sites to activate the carbon atom in the methyl group of toluene [12] and also to dehydrogenate methanol to formaldehyde, which is generally believed to be the actual alkylating agent to form styrene [10]. Formation of surface formates on alkaliexchanged zeolites after exposure to methanol has been observed [13–15]. On the other hand, strong base sites also catalyze methanol decomposition to form carbon monoxide, which is the major side reaction during toluene alkylation with methanol. Acid sites catalyze preferentially ring alkylation reactions, but surface acidity is also necessary to catalyze sidechain alkylations via combined acid–base pathways. Thus, it has been suggested that the side-chain alkylation of toluene with methanol requires a cooperative action of acid/base pairs for efficiently promoting the rate-limiting step in the reaction mechanism [12,16]. But other authors reported that the zeolite selectivity is essentially governed by the overall acid–base strength as measured by Sanderson electronegativity [6]. The exact requirements of acid site density and strength in the side-chain alkylation reaction mechanism is therefore still debated. In an attempt to obtain more insight into the active site requirements for the alkylation of toluene with methanol we present in this work a new approach for relating the solid surface properties with its catalytic performance based on selective acid–base poisoning. Specifically, we study the effect of the addition of acid and basic compounds on the deactivation kinetics for the alkylation of toluene with methanol over ion-exchanged Y zeolites employing a deactivation model with residual activity (DMRA). Results show that bimolecular side-chain alkylation would require not only a proper acid–base pair sites for activating both toluene and methanol, but also a specific acid–base pairs configuration to promote the adjacent adsorption of C1 species and bulkier toluenederived species.
2. Experimental 2.1. Catalyst preparation CsY, MgY, and HY zeolites were prepared by exchanging a commercial NaY zeolite (UOP Y-54, Si/Al: 2.5) with Cs acetate (Sigma, 99.8%), Mg(NO3)2, and ammonium acetate (Sigma, 99%), respectively. Ion exchanges were carried out at 353 K and 1 atm in four consecutive steps. Following each individual exchange step, the zeolites were filtered, washed with hot water, dried at 393 K, and finally calcined in flowing air at 723 K for 4 h. 2.2. Catalyst characterization The BET surface areas (Sg) were measured by N2 physisorption at 77 K in a Quantochrome Corporation NOVA-1000 sorptometer. The chemical composition of the
zeolites was determined by measuring elemental compositions by atomic absorption spectroscopy (AAS), using a Perkin-Elmer 3110 spectrometer. The structure of the zeolite samples was characterized by X-ray diffraction (XRD) methods using a Shimadzu XD-D1 diffractometer and Nifiltered Cu Ka radiation. The nature of surface acid sites was determined by Fourier transform infrared spectroscopy (FTIR) by using pyridine as probe molecule and a Shimadzu FTIR-8101M spectrophotometer. The spectral resolution was 4 cm1 and 50 scans were co-added. Sample wafers were formed by pressing 20–40 mg of the catalyst at 5 t/cm2 and transferred to a sample holder made of quartz. An inverted T-shaped Pyrex cell containing the sample pellet was used. The two ends of the short arm of the T were fitted with CaF2 windows. All the samples were initially outgassed at 723 K for 4 h and then a background spectrum was recorded after cooling the sample to room temperature. Data were obtained after admission of pyridine, adsorption at room temperature, and sequential evacuation at 298 and 423. Spectra were always recorded at room temperature. Difference spectra were obtained by subtracting the background spectrum recorded previously. The structure of CO2 chemisorbed on the samples was determined by Fourier transform infrared spectroscopy. Data were obtained using a Shimadzu FTIR-8101M spectrophotometer after CO2 adsorption at room temperature and sequential evacuation at 298 and 273 K. All the samples were pretreated for 2 h in vacuum at 773 K; spectra were taken at room temperature. 2.3. Catalytic testing The gas phase alkylation of toluene with methanol was carried out in a fixed-bed tubular reactor at 1 atm. Samples were sieved to retain particles with 0.35–0.42 mm diameter for catalytic measurements and treated in air at 723 K for 2 h before reaction in order to remove H2O, hydrocarbons, and CO2. A mixture of toluene (EM Science, 99.5%) and methanol (Merck, 99.8%) of 1:1 molar ratio was vaporized in a preheating section and delivered to the reactor. The reaction was carried out at 673 K, employing a space velocity (WHSV) of 2 h1. The reaction products were analyzed by on-line gas chromatography using a HewlettPackard 5890 chromatograph equipped with a Supelcowax 10TM column. Data were collected every 0.5 h during 8 h. Under our experimental conditions, the following main reaction products were detected: ethylbenzene, styrene, xylenes, higher alkylated compounds (C9+, tri and tetramethylbenzenes) and benzene. Toluene conversion (XT) was calculated as: XT (%) = [YT./ (SYj + YT)].100, where SYj is the molar fractions of the aromatic reaction products, including benzene, and YT is the outlet molar fraction of toluene. The selectivity to product j (Sj, mol of product j/mol of toluene reacted) was determined as: Sj (%) = [Yj/SYj]100. The SEt-Bz selectivity includes the sum of ethylbenzene and
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styrene, which are the side-chain alkylation products. In situ poisoning experiments were carried out by doping the toluene/methanol mixture with either acetic acid (Merck, 99.5%) or 3,5-dimethyl pyridine (Aldrich, >98%) in a concentration range between 0 and 15000 ppm.
3. Results and discussion 3.1. Catalyst characterization Table 1 summarizes the main characteristics of CsY, MgY, and HY ion-exchanged zeolites. Data for the NaY parent zeolite were also included as reference. The exchange degree [ED = (% Nainitial % Nafinal)/% Nainitial], as determined by chemical analysis, was higher than 65% for all the samples. The specific surface areas of exchanged zeolites were lower than that of the parent NaY zeolite. In agreement with other authors, we observe that the addition of alkaline metals by using the ion-exchange method modifies the physical properties of the zeolite [11,17]. Fig. 1 shows the X-ray diffraction patterns of NaY, CsY, and MgY zeolites. Significant changes in the relative diffraction line intensities are observed by comparing the XRD patterns of NaY with those of ion-exchanged zeolites, probably because the incorporation of alkaline metal cations bigger than Na+ changes the structure factors and X-ray absorption coefficients of the parent NaY zeolite [18,19]. In particular, a strong decrease of the diffraction peak intensities was observed on Cs-exchanged zeolites. Nevertheless, the structure of the parent zeolites seems to be preserved. The structure of chemisorbed CO2 species was determined by FTIR measurements of preadsorbed CO2. Fig. 2 presents the IR spectra obtained on the catalysts after CO2 adsorption and sequential evacuation at 298 and 373 K. The parent NaY zeolite exhibited infrared bands at around 1710, 1690, 1645 and 1370 cm1. Adsorption of CO2 on Csexchanged zeolites revealed similar IR bands: 1690, 1650 with shoulders at 1600, 1390 and 1330 cm1. According to the literature [20–22], bidentate carbonates form on Lewis acid–Bro¨ nsted base pairs (Mn+–O2, where Mn+ is the metal cation), and shows a symmetric O–C–O stretching at 1320–1340 cm1 and an asymmetric O–C–O stretching at 1610–1630 cm1. Unidentate carbonate formation requires
Fig. 1. XRD diffraction patterns of zeolites NaY, MgY, and CsY.
stronger basic sites and exhibits an asymmetric O–C–O stretching at 1510–1560 cm1 and a symmetric O–C–O stretching at 1360–1400 cm1. The IR spectra of Fig. 2 show therefore that NaY and CsY zeolites form essentially bidentate carbonate species. Nevertheless, the IR bands on CsY zeolite, were slightly shifted toward the positions of unidentate carbonate species, suggesting an increase of the surface oxygen basicity [23]. Carbonates species on NaY zeolite disappear after evacuation at 373 K; in contrast, a fraction of the bidentate carbonate species remains adsorbed on the CsY surface after outgassing at 373 K. This confirms that CsY zeolite contains stronger basic sites than NaY. Fig. 2 also shows that adsorbed CO2 on MgY gives rise to a strong infrared band around 1630 cm1 with a shoulder on high frequency side, and a weak broad band centered at about 1450 cm1. No IR bands were detected between 1300–1400 cm1. According to literature [20,21], bicarbonate species involves the formation of surface hydroxyl groups. Typically, bicarbonates show a C–OH bending mode at 1220 cm1 as well as a symmetric and asymmetric O–C– O stretching at 1480 and 1650 cm1, respectively [21,22]. Both symmetric and asymmetric O–C–O stretching modes are clearly observed in the IR spectra on MgY reported in Fig. 2. Then, these spectra are consistent with the formation
Table 1 Physicochemical properties, FTIR of pyridine, and catalytic results for the alkylation of toluene with methanol Zeolite
NaY CsY MgY HY a b
EDa (%)
– 68 81 93
Sg (m2/g)
700 430 540 660
XTb (%)
FTIR of pyridine L (area/g)
B (area/g)
L/B
525 155 660 465
30 5 285 310
17.5 31 2.3 1.5
Exchange degree: [(% Nainitial % Nafinal)/% Nainitial.]100. Values at t = 0, T = 673 K, P = 1 atm, toluene/methanol molar ratio = 1.
0.9 7.5 52.2 58.8
Selectivities (%)b SEt-Bz
SXy
Sþ C9
SBz
6.0 88.0 – –
88.5 12.0 54.8 58.5
5.5 – 43.9 18.8
– – 1.3 22.7
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Fig. 2. FTIR spectra of CO2 adsorbed at room temperature and evacuated at: (a) room temperature, (b) 373 K.
Fig. 3. FTIR spectra recorded in the hydroxyl-stretching region after sample evacuation at 773 K for 4 h.
of surface bicarbonate species. This observation is strengthened by the fact that the CO2 adsorption on acid HY zeolite gives rise to an IR spectrum qualitatively similar to that obtained on MgY. Bicarbonates on MgY remain adsorbed after outgassing at 373 K thereby suggesting the existence of strong Bro¨ nsted sites.
Fig. 3 shows the FTIR spectra in the hydroxyl-stretching region of CsY and MgY zeolites obtained after evacuation at 773 K for 4 h. Spectra obtained on NaY and HY zeolites were also included. No absorption bands were detected on CsY sample, thereby indicating that the hydroxyl group concentration on this zeolite is negligible. In contrast, MgY zeolite exhibited IR absorption bands at 3745, 3690 and 3620 cm1. The frequencies near 3745 and 3620 cm1 are close to those observed for HY zeolite and reflect the presence of Si–OH groups [24]. The band at 3690 cm1 is close to that observed on X zeolites for Al–OH groups [25]. The IR characterization reveals therefore the presence of structural hydroxyl groups (Bro¨ nsted acid sites) on MgY zeolites, and suggests that the exchanged cation is in the (MgOH)+ form [26]. The density and nature of surface acid sites were determined from the FTIR spectra of adsorbed pyridine. Fig. 4A shows the spectra obtained on zeolite HY after admission of pyridine, adsorption at room temperature, and sequential evacuation at 298, 423, 573, and 723 K. The peaks at 1450 y 1540 cm1 arise from pyridine adsorbed on Lewis and Bro¨ nsted acid sites on zeolites respectively [24,27,28]. These two peaks are present even after sample evacuation at 723 K. Fig. 4A also shows that the spectrum collected following evacuation at 298 K contains broad absorption bands characteristics of the presence of physisorbed pyridine. Pyridine molecules interacting via H-bonding with weakly acidic surface OH groups (bands at 1446 and 1597 cm1) were removed after evacuation at 423 K and the resulting FTIR spectrum showed well defined absorption peaks. We decided then to characterize the sample acidity by using spectra collected after evacuation at 423 K. Fig. 4B compares the spectra obtained on zeolites HY, NaY, MgY, and CsY after admission of pyridine, adsorption at room temperature, and evacuation at 423 K. The relative contributions of Lewis and Bro¨ nsted acid sites were obtained from experimental FTIR spectra by integration of pyridine peaks at 1450 y 1540 cm1, respectively. Results are given in Table 1. As expected, sample NaY shows essentially Lewis acidity (the areal peak relationship between Lewis and Bro¨ nsted sites, L/B, was 17.5) but after
Fig. 4. FTIR spectra of pyridine adsorbed on: (A) zeolite HY at 298 K and evacuated at increasing temperatures, (B) zeolites HY, MgY, NaY, and CsY at 298 K and evacuated at 423 K for 0.5 h. Dotted lines indicate the presence of Lewis (1450 cm1) and Bro¨ nsted (1540 cm1) sites.
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ion exchange with ammonium acetate the L/B value is 1.5 on the resulting sample HY because the Bro¨ nsted acid site density increases by one order of magnitude. The density of Lewis and Bro¨ nsted sites measured at 423 K on MgY was comparable to that determined on HY at the same temperature. But it was verified that pyridine adsorbed on Bro¨ nsted sites was more easily eliminated on MgY than on HY by evacuating the samples at temperatures higher than 423 K. This result showed that HY contains stronger Bro¨ nsted acid sites than MgY. Pyridine adsorption does not reveal any Bro¨ nsted acidity on zeolite CsY, which is consistent with results in Fig. 3, showing that the concentration of hydroxyl groups on CsY is negligible. 3.2. Catalytic testing with pure reactants The activity and selectivity for the reaction of toluene with methanol over modified Y zeolites were measured by using a poison-free toluene/methanol mixture as reactant. Toluene conversion diminished with time-on-stream on zeolites MgY and HY because of coke formation, and the initial activity was measured then by extrapolating the XT versus t curves to zero time. In contrast, the activity decay was negligible for CsY zeolite. Catalytic results are shown in Table 1. The HY zeolite, which is employed here as a reference sample, is active and selective for benzene-ring alkylation. Xylene isomers were the predominant products and the isomer distribution was close to the thermodynamic equilibrium distribution – para-xylene (p-Xy): 22.5%, metha-xylene (m-Xy): 50.0% and ortho-xylene (o-Xy): 27.5%. Significant amounts of benzene and higher alkylated compounds (C9+), such as tri and tetramethylbenzenes, were also observed. Similarly to HY sample, MgY promotes selectively the benzene-ring alkylation, producing essentially xylenes and C9+ compounds. The distribution of xylene isomers on MgY was also close to the thermodynamic equilibrium (Fig. 5). Previous work [1,7] stated that benzene-ring alkylation is mainly promoted by Bro¨ nsted
Fig. 5. Distribution of xylene isomers: T = 673 K, P = 1 atm, toluene: methanol = 1.
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acid sites, probably because formation of methoxonium ion is the first step in the reaction mechanism and requires surface hydroxyl groups. Our results are consistent with this assumption because the results of Figs. 3 and 4 shows that zeolites HY and MgY, which efficiently promote the benzene-ring alkylation, contain a high density of strong Bro¨ nsted acid sites. In contrast to MgY and HY samples, CsY shows moderate activity for toluene/methanol reaction but is highly selective for producing ethylbenzene and styrene, in agreement with previous reports showing that alkaliexchanged zeolites promote side-chain alkylation reactions [29]. Besides, regarding xylene isomer distribution, zeolite CsY favored the formation of o-xylene yielding more than 50% of o-Xy in the xylene mixture (Fig. 5). Results of Figs. 2 and 3 show that zeolite CsY does not exhibit Bro¨ nsted acidity but contains Lewis acid centers as well as surface Bro¨ nsted basic sites. This particular distribution of surface acid–base sites on CsY is different from the predominant strong Bro¨ nsted acidity determined on MgY and therefore accounts for the observed differences in catalytic activity and selectivity between both zeolites. Finally, zeolite NaY, which has neither surface basic nor Bro¨ nsted acid sites but contains a high density of Lewis acid sites, displays a very poor activity for converting toluene with methanol. To obtain further insight on the relationship between the surface acid–base properties of exchanged Y zeolites and the catalyst activity and selectivity for the alkylation of toluene with methanol, we performed additional catalytic tests by doping the reactant mixture with acid or basic poisoning molecules. Results are presented below. 3.3. Selective poisoning on MgY zeolite In situ poisoning experiments were carried out on MgY zeolite by doping the toluene/methanol mixture with either an acid (acetic acid) or a base (3,5-dimethyl pyridine). Fresh catalysts were initially tested for about 3 h using a pure toluene/methanol mixture before introducing the doped feed. Fig. 6 shows the activity and selectivity decay curves obtained as a function of time on MgY zeolite when doping
Fig. 6. Evolution of the alkylation activity and selectivity as a function of time. Effect of the addition of acetic acid. Zeolite MgY, T = 673 K, P = 1 atm, toluene:methanol = 1.
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obtained for different 3,5-DMP concentrations. The value of the deactivation function by coke, cc, was 0.7 104 min1. cp was related to poison concentration employing a pseudo-homogeneous kinetics, according to the Levenspiel deactivation model [33]: cp ¼ kd C ap
Fig. 7. Catalytic activity as a function of time and 3,5-DMP concentration, represented in a ln a vs. time plot. Zeolite MgY, T = 673 K, P = 1 atm, toluene:methanol = 1.
the feed with acetic acid. The activity is defined as a = rt/r0, where r0 and rt are the reaction rates at zero time and time t, respectively. Fig. 6 shows that the MgY activity diminishes with time on stream when using undoped reactants, probably because of the formation of carbonaceous deposits. The co-injection of 4700 ppm of acetic acid did not change the slope of the deactivation curve obtained with pure reactants, thereby revealing that the presence of acetic acid does not inhibit the toluene conversion reactions. Similarly, the zeolite selectivity was not affected by co-feeding acetic acid. In contrast, the addition of 3,5-dimethyl pyridine (3,5DMP) to the reactants caused the rapid deactivation of zeolite MgY (Fig. 7). The activity decline due to the presence of 3,5-dimethyl pyridine (3,5-DMP) cannot be determined directly from the experimental data because of the simultaneous catalyst deactivation caused by coke formation. To estimate the poison intrinsic effect, it can be assumed that both effects are additive, which implies that the overall deactivation rate is a simple sum of each individual deactivation rate (hypothesis of independence) [30]. Thus, according to mechanistic deactivation models for simultaneous deactivations [31,32], we can express the overall deactivation rate as:
da ¼ cc adc þ cp adp dt
(1)
(3)
where kd and a are the intrinsic deactivation constant and the poison order, respectively. kd and a were determined by linear regression of experimental data represented in a log cp versus log Cp plot. The values of these parameters are: a = 1.2 and kd = 6 108 min1 ppm1.2. In summary, selective poisoning experiments show that MgY zeolite is only deactivated by co-feeding a basic compound, thereby showing that the rate-limiting step of the reaction mechanism on MgY involves exclusively surface acid sites. This result is in line with previous work proposing that the ring alkylation of toluene with methanol is controlled by the density of Bro¨ nsted acid sites [1]. Promotion of the ring-alkylation mechanism on MgY essentially by surface Bro¨ nsted acid sites is also consistent with the fact that we determined a value of a = 1.2 for the deactivation order with respect to 3,5-DMP. Deactivation mechanistic models based on Langmuir-Hinshelwood kinetics predict, in fact, a poison order equal to 1 when the deactivation mechanism involves the poison adsorption on a single active site [30]. 3.4. Selective poisoning on CsY zeolite Fig. 8 shows the activity decay curves obtained on zeolite CsY upon co-injection of acetic acid or 3,5-DMP. In contrast with the results obtained on MgY, we observe that the activity of zeolite CsY for side-chain alkylation reaction declines by the addition of either an acid or a basic compound. Fig. 8A displays the a versus time plot corresponding to the catalytic data obtained on zeolite CsY during toluene alkylation with and without addition of acetic acid (AA). The catalyst activity remains nearly constant with time on stream when using undoped reactants,
where cc and cp are the deactivation functions related to coke and poison, respectively, and dc and dp are the kinetic orders with respect to coke and poison. As shown in Fig. 7, the deactivation curves are satisfactorily fitted using an exponential function, thereby indicating that the deactivation orders for both deactivating processes were close to 1. Therefore, Eq. (1) may be simplified to:
da ¼ ðcc þ cp Þa dt
(2)
By using the integrated form of Eq. (2), deactivation functions cc and cp can be obtained by linear regression. cc was determined from the deactivation curve obtained with undoped fed (cp = 0) and cp from the deactivation curves
Fig. 8. Time evolution of the CsY activity, poisoned by: (A) acetic acid, (B) 3,5-DMP. Points, experimental data; solid lines, model fits T= 673 K, P= 1 atm, toluene:methanol = 1.
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thereby suggesting that no deactivation by coke takes place on CsY. Hence, the poisoning effect can be obtained directly from the activity decay curves. Fig. 8A also shows that deactivation curves reach pseudo-steady state values different from zero for all acetic acid concentrations studied. In a previous work [34], we developed the deactivation model with residual activity (DMRA) for modeling deactivation processes reaching pseudo-steady states different from zero. This deactivation model with residual activity employs Langmuir-Hinshelwood kinetics and requires to ascertain both the main reaction and the deactivation mechanisms for determining the deactivation kinetic parameters. According to the literature [12,16], the following reaction mechanism accounts for the side-chain alkylation of toluene with methanol on basic zeolites: K1
C6 H5 CH3 þ l !ðC6 H5 CH3 Þl
where 0
cd ¼
kP pAA P ; 1 þ Ki pi
where m is the number of active sites involved in the controlling step of the main reaction. Then, from Eqs. (8) and (7) the deactivation rate can be written as: da ¼ ðcr þ cd Þa cr adm dt where
m1 ; m
dm ¼
ðC6 H5 CH3 Þl þ ðCH2 OÞl
cr ¼ mcr ¼ mkrp
k
K3
ðC6 H5 CH ¼ CH2 Þl ! C6 H5 CH ¼ CH2 þ l where l represents the surface active site, Ki the equilibrium adsorption constants and k the rate kinetic constant of the controlling step. The rate-limiting step is considered to be the surface reaction between formaldehyde and toluene adsorbed species. Assuming that the deactivation process takes place by reversible adsorption of acetic acid, the deactivation mechanism may be written as follows: kp ; krp
where kp and krp are the deactivation kinetic constants for the direct and reverse poisoning reaction. According to the proposed deactivation mechanism (reversible adsorption of acetic acid), the number of active sites involved in the controlling step of the deactivation reaction, h, is 1. Therefore, the deactivation rate is expressed as: dCAAl ¼ kp pAA Cl krp CAAl (4) dt and the balance of active sites at the catalytic surface as: X L ¼ Cl 1 þ Ki pi þ CAAl (5) where L is the concentration of active sites of fresh catalysts and C1 the concentration of vacant active site. L CAAl P Cl ¼ 1 þ Ki pi
(6)
Combining Eqs. (4) and (6), we obtain: 0 0 dCAAl ¼ cd ðL CAAl Þ cr CAAl dt
0
cd ¼ mcd ¼ m
(9)
kP pAA P ; 1 þ Ki pi
0
!ðC6 H5 CH ¼ CH2 Þl þ H2 O þ l
AA þ l $ AAl #
0
cr ¼ krp
From the definition of activity [35], the following relationship is derived: rt L CAAl m L CAAl (8) a¼ ¼ ) a1=m ¼ r0 pi ;T;xT L L
CH3 OH þ l !ðCH2 OÞl þ H2
K2
213
(7)
cd and cr are the deactivation functions for the direct and reverse poisoning reaction, respectively. Eq. (9) is similar to that developed for reversible coke deactivation processes [36,37]. In our case we assume that the reversible poisoning mechanism involves only one active site (h = 1), which would represent a surface Cs+–O2 pair site. Therefore, the deactivation order defined by Corella et al. [38,39], d = (m + h 1)/m, is equal to 1. The residual or steady-state activity, aSS, is the value of a for t ! 1, and can be obtained from Eq. (9) taking into account that for t ! 1 is da/dt = 0 [40]: m cr ðcr þ cd ÞaSS cr adSSm ¼ 0 ) aSS ¼ (10) cr þ cd According to the reaction mechanisms proposed above for both the main reaction and the deactivation reaction 1 h ¼ 1 , dm ¼ ; d ¼ 1 2 and Eq. (9) can be finally expressed as:
m ¼ 2;
pffiffiffi da ¼ ðcr þ cd Þa cr a dt
(11a)
or pffiffiffiffiffiffiffiffiffiffi da ¼ cG ða aSS a Þ dt where 2 cr aSS ¼ and cG ¼ cr þ cd cr þ cd
(11b)
(12)
Integrating Eq. (11b) for a differential reactor (cd and cr constants) we obtain: 2 pffiffiffiffiffiffi pffiffiffiffiffiffi 1 (13) a¼ aS þ ð1 aS Þ exp cG t 2
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Fig. 9. Influence of poison concentration on deactivation kinetic parameters determined on zeolite CsY from DMRA model.
Eq. (13) was employed to compare the DMRA predictions with the experimental data. Parameters cd and cr were determined by using nonlinear least-squares regression which minimizes the objective functions F = S (aexp acalc)2. Fig. 8A shows that the quality of fits was excellent when using DMRA to model the poisoning effect of acetic acid on the side-chain alkylation of toluene with methanol. Similar qualitative deactivation curves reaching residual activities different from zero were obtained on zeolite CsY when the feed was doped with 3,5-DMP (Fig. 8B). The activity decay curves of Fig. 8B were analyzed then by using the DMRA equations described above for acetic acid poisoning. It is observed in Fig. 8B that all experimental data, including high-time data, are satisfactorily correlated using DMRA predictions. The deactivation kinetic parameters determined by employing DMRA for modeling acetic acid and 3,5-DMP poisonings are summarized in Fig. 9. As predicted by this model, Fig. 9A and B clearly show a linear relationship between the deactivation function cd and the poison partial pressure for both acid and base poisoning. Furthermore, as also shown in Fig. 9A and B, reverse deactivation function cr seems to be independent of the poison concentration, which is in good agreement with the prediction of the DMRA deactivation model derived from mechanistic considerations. The reverse deactivation function, cr = 2krp, and the intrinsic deactivation kinetic constant for the direct reaction, ‘‘kp’’ = 2kp/(1 + SKiPi), were also obtained by multivariable non-linear regression. Regarding the value of direct deactivation constant ‘‘kp’’, it was obtained that ‘‘kp’’ was similar when CsY was poisoned with acetic acid (8.9 107 0.3 107 min1 ppm1) or with 3,5-DMP (6.7 107 0.4 107 min1 ppm1). In contrast, the reverse deactivation constant krp was significantly lower when reactants were doped with 3,5-DMP (0.0036 0.0005 min1) than with acetic acid (0.0094 0.0005 min1), thereby suggesting the stronger adsorption of the basic molecule on CsY. This later result is consistent with the lower values of aSS determined
for 3,5-DMP poisoning (Fig. 9C) and strongly suggests that differences in the deactivation behavior produced by either an acid or a base compound on CsY are related to the adsorption strength of the poison molecule. The weaker adsorption of acetic acid on CsY is also supported by the fact that FTIR spectra of adsorbed CO2 (Fig. 3) showed that surface basic sites on CsY are weak and display low basicity. In summary, selective poisoning experiments show that the side-chain alkylation of toluene with methanol on CsY zeolite is deactivated by the addition of either acid or base compounds, thereby suggesting that the surface actives sites promoting the rate-limiting step are Lewis acid (Cs+)–base (O2) pairs. Bimolecular side-chain alkylation reaction would require proper acid–base pair sites for activating both reactants, toluene and methanol, but also a specific acid– base pair configuration to promote the adjacent adsorption of C1 species and bulkier toluene-derived species. Consequently, the presence of either a basic or an acid compound poisons the combined acid–base pathway required for promoting the side-chain alkylation of toluene with methanol on zeolite CsY.
4. Conclusions Selective acid–base poisoning gives useful information for relating the surface properties of exchanged Y zeolites with their catalytic performance for the alkylation of toluene with methanol. MgY zeolite is only deactivated by cofeeding base compounds, thereby showing that exclusively acid sites are involved in the rate-limiting step of the reaction mechanism. In addition, the deactivation order with respect to the poisoning molecule is close to one, indicating that the deactivation mechanism involves the poison adsorption on a single active site, probably Bro¨ nsted acid sites. CsY zeolite is similarly deactivated by co-feeding either an acid or a base compound, demonstrating that the ratelimiting step of the reaction mechanism requires surface acid–base pairs. A deactivation model with residual activity
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derived by assuming that the rate-limiting step occurs on a single Cs+–O2 pair site satisfactorily interprets experimental data and allows the determination of both deactivation kinetic parameters and poison adsorption equilibrium constants. Results obtained from the deactivation model and infrared spectroscopy are consistent with the interpretation that Lewis acid/Bro¨ nsted base pairs with a specific configuration are required on CsY zeolite to form the intermediate complex between adsorbed formaldehyde and toluene which would be the key step of the reaction mechanism.
Acknowledgements The authors thank the Universidad Nacional del Litoral, Santa Fe, and CONICET (Argentina) for the financial support of this work. The authors also thank Prof. A. Monzo´ n for many helpful discussions on the development of kinetic deactivation models and H. Cabral for his technical assistance with infrared spectroscopy experiments.
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Active sites in the alkylation of toluene with methanol: a study by selective acid–base poisoning A. Borgna*, J. Sepu´lveda, S.I. Magni, C.R. Apesteguı´a a
Catalysis Science and Engineering Research Group (GICIC), Instituto de Investigaciones en Cata´lisis y Petroquı´mica – INCAPE-(UNL-CONICET), Santiago del Estero 2654, (3000) Santa Fe, Argentina Received in revised form 6 August 2004; accepted 7 August 2004 Available online 22 September 2004
Abstract Selective acid–base poisoning of the alkylation of toluene with methanol was studied over alkali and alkaline-earth exchanged Y zeolites. Surface acid–base properties of the samples were determined by infrared spectroscopy using carbon dioxide and pyridine as probe molecules. Selective poisoning experiments were performed by co-feeding either acetic acid or 3,5-dimethyl pyridine. Kinetic deactivation parameters were determined by modeling experimental data with a deactivation model with residual activity (DMRA). MgY produced essentially xylenes and alkylated C9+ compounds. This zeolite was deactivated only by co-feeding 3,5-dimethyl pyridine, thereby indicating that exclusively acid sites are involved in the ring alkylation of toluene with methanol. In contrast, zeolite CsY formed mainly styrene and ethylbenzene, and was strongly deactivated by the addition of either acid or base compounds. Infrared characterization showed that zeolite CsY contains Lewis acid (Cs+)–base (O2) pairs capable of adsorbing bidentate carbonates but does not exhibit Bro¨nsted acidity. Thus, selective poisoning results and sample characterization showed that bimolecular side-chain alkylation reaction on CsY requires a specific surface acid–base pairs configuration not only for activating toluene and methanol, but also for promoting the rate-limiting step. # 2004 Elsevier B.V. All rights reserved. Keywords: Toluene alkylation; Basic zeolites; Selective poisoning; Styrene; Ethylbenzene
1. Introduction The alkylation of toluene with methanol is readily catalyzed on synthetic zeolites. Previous work has shown that the aromatic-ring alkylation of toluene with methanol takes place over acid zeolites [1], while the side-chain alkylation occurs preferentially over basic zeolites [2,3]. The side-chain alkylation of toluene with methanol for producing a mixture of styrene and ethylbenzene offers economical advantages compared with the conventional homogeneously catalyzed Friedel-Crafts process, which uses ethylene and benzene as reactants [4,5]. Zeolites are attractive materials for alkylation reactions because their acid–base properties can be easily modified by dealumination, isomorphous substitution or ionic-exchange. * Corresponding author. Tel.: +54 342 4555279; fax: +54 342 4531068. E-mail address: [email protected], (A.Borgna). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.08.007
The role of the acid–base properties of zeolite catalysts on the product distribution of aromatic alkylation reactions has been reviewed by Giordano et al. [6]. The ring-alkylation mechanism over acid catalysts would proceed via the formation of methoxonium ion, which requires Bro¨nsted acid sites [1,7]. Thus, the surface concentration of methoxonium ion, and therefore the catalytic activity for ring-alkylation, would depend on the density and strength of the Bro¨nsted acid sites. On the other hand, the side-chain alkylation of toluene with methanol over base catalysts has been known for several decades [8]. Although base catalysts such as MgO and CaO were successfully used for side-chain alkylation [9], recent studies indicate that the zeolite pores might be indispensable to efficiently catalyze the reaction [10]. Alkali-exchanged zeolites have demonstrated the highest activities and selectivities for side-chain alkylation of toluene with methanol [11]. Particularly, the side-chain
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alkylation of toluene with methanol requires basic sites to activate the carbon atom in the methyl group of toluene [12] and also to dehydrogenate methanol to formaldehyde, which is generally believed to be the actual alkylating agent to form styrene [10]. Formation of surface formates on alkaliexchanged zeolites after exposure to methanol has been observed [13–15]. On the other hand, strong base sites also catalyze methanol decomposition to form carbon monoxide, which is the major side reaction during toluene alkylation with methanol. Acid sites catalyze preferentially ring alkylation reactions, but surface acidity is also necessary to catalyze sidechain alkylations via combined acid–base pathways. Thus, it has been suggested that the side-chain alkylation of toluene with methanol requires a cooperative action of acid/base pairs for efficiently promoting the rate-limiting step in the reaction mechanism [12,16]. But other authors reported that the zeolite selectivity is essentially governed by the overall acid–base strength as measured by Sanderson electronegativity [6]. The exact requirements of acid site density and strength in the side-chain alkylation reaction mechanism is therefore still debated. In an attempt to obtain more insight into the active site requirements for the alkylation of toluene with methanol we present in this work a new approach for relating the solid surface properties with its catalytic performance based on selective acid–base poisoning. Specifically, we study the effect of the addition of acid and basic compounds on the deactivation kinetics for the alkylation of toluene with methanol over ion-exchanged Y zeolites employing a deactivation model with residual activity (DMRA). Results show that bimolecular side-chain alkylation would require not only a proper acid–base pair sites for activating both toluene and methanol, but also a specific acid–base pairs configuration to promote the adjacent adsorption of C1 species and bulkier toluenederived species.
2. Experimental 2.1. Catalyst preparation CsY, MgY, and HY zeolites were prepared by exchanging a commercial NaY zeolite (UOP Y-54, Si/Al: 2.5) with Cs acetate (Sigma, 99.8%), Mg(NO3)2, and ammonium acetate (Sigma, 99%), respectively. Ion exchanges were carried out at 353 K and 1 atm in four consecutive steps. Following each individual exchange step, the zeolites were filtered, washed with hot water, dried at 393 K, and finally calcined in flowing air at 723 K for 4 h. 2.2. Catalyst characterization The BET surface areas (Sg) were measured by N2 physisorption at 77 K in a Quantochrome Corporation NOVA-1000 sorptometer. The chemical composition of the
zeolites was determined by measuring elemental compositions by atomic absorption spectroscopy (AAS), using a Perkin-Elmer 3110 spectrometer. The structure of the zeolite samples was characterized by X-ray diffraction (XRD) methods using a Shimadzu XD-D1 diffractometer and Nifiltered Cu Ka radiation. The nature of surface acid sites was determined by Fourier transform infrared spectroscopy (FTIR) by using pyridine as probe molecule and a Shimadzu FTIR-8101M spectrophotometer. The spectral resolution was 4 cm1 and 50 scans were co-added. Sample wafers were formed by pressing 20–40 mg of the catalyst at 5 t/cm2 and transferred to a sample holder made of quartz. An inverted T-shaped Pyrex cell containing the sample pellet was used. The two ends of the short arm of the T were fitted with CaF2 windows. All the samples were initially outgassed at 723 K for 4 h and then a background spectrum was recorded after cooling the sample to room temperature. Data were obtained after admission of pyridine, adsorption at room temperature, and sequential evacuation at 298 and 423. Spectra were always recorded at room temperature. Difference spectra were obtained by subtracting the background spectrum recorded previously. The structure of CO2 chemisorbed on the samples was determined by Fourier transform infrared spectroscopy. Data were obtained using a Shimadzu FTIR-8101M spectrophotometer after CO2 adsorption at room temperature and sequential evacuation at 298 and 273 K. All the samples were pretreated for 2 h in vacuum at 773 K; spectra were taken at room temperature. 2.3. Catalytic testing The gas phase alkylation of toluene with methanol was carried out in a fixed-bed tubular reactor at 1 atm. Samples were sieved to retain particles with 0.35–0.42 mm diameter for catalytic measurements and treated in air at 723 K for 2 h before reaction in order to remove H2O, hydrocarbons, and CO2. A mixture of toluene (EM Science, 99.5%) and methanol (Merck, 99.8%) of 1:1 molar ratio was vaporized in a preheating section and delivered to the reactor. The reaction was carried out at 673 K, employing a space velocity (WHSV) of 2 h1. The reaction products were analyzed by on-line gas chromatography using a HewlettPackard 5890 chromatograph equipped with a Supelcowax 10TM column. Data were collected every 0.5 h during 8 h. Under our experimental conditions, the following main reaction products were detected: ethylbenzene, styrene, xylenes, higher alkylated compounds (C9+, tri and tetramethylbenzenes) and benzene. Toluene conversion (XT) was calculated as: XT (%) = [YT./ (SYj + YT)].100, where SYj is the molar fractions of the aromatic reaction products, including benzene, and YT is the outlet molar fraction of toluene. The selectivity to product j (Sj, mol of product j/mol of toluene reacted) was determined as: Sj (%) = [Yj/SYj]100. The SEt-Bz selectivity includes the sum of ethylbenzene and
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styrene, which are the side-chain alkylation products. In situ poisoning experiments were carried out by doping the toluene/methanol mixture with either acetic acid (Merck, 99.5%) or 3,5-dimethyl pyridine (Aldrich, >98%) in a concentration range between 0 and 15000 ppm.
3. Results and discussion 3.1. Catalyst characterization Table 1 summarizes the main characteristics of CsY, MgY, and HY ion-exchanged zeolites. Data for the NaY parent zeolite were also included as reference. The exchange degree [ED = (% Nainitial % Nafinal)/% Nainitial], as determined by chemical analysis, was higher than 65% for all the samples. The specific surface areas of exchanged zeolites were lower than that of the parent NaY zeolite. In agreement with other authors, we observe that the addition of alkaline metals by using the ion-exchange method modifies the physical properties of the zeolite [11,17]. Fig. 1 shows the X-ray diffraction patterns of NaY, CsY, and MgY zeolites. Significant changes in the relative diffraction line intensities are observed by comparing the XRD patterns of NaY with those of ion-exchanged zeolites, probably because the incorporation of alkaline metal cations bigger than Na+ changes the structure factors and X-ray absorption coefficients of the parent NaY zeolite [18,19]. In particular, a strong decrease of the diffraction peak intensities was observed on Cs-exchanged zeolites. Nevertheless, the structure of the parent zeolites seems to be preserved. The structure of chemisorbed CO2 species was determined by FTIR measurements of preadsorbed CO2. Fig. 2 presents the IR spectra obtained on the catalysts after CO2 adsorption and sequential evacuation at 298 and 373 K. The parent NaY zeolite exhibited infrared bands at around 1710, 1690, 1645 and 1370 cm1. Adsorption of CO2 on Csexchanged zeolites revealed similar IR bands: 1690, 1650 with shoulders at 1600, 1390 and 1330 cm1. According to the literature [20–22], bidentate carbonates form on Lewis acid–Bro¨ nsted base pairs (Mn+–O2, where Mn+ is the metal cation), and shows a symmetric O–C–O stretching at 1320–1340 cm1 and an asymmetric O–C–O stretching at 1610–1630 cm1. Unidentate carbonate formation requires
Fig. 1. XRD diffraction patterns of zeolites NaY, MgY, and CsY.
stronger basic sites and exhibits an asymmetric O–C–O stretching at 1510–1560 cm1 and a symmetric O–C–O stretching at 1360–1400 cm1. The IR spectra of Fig. 2 show therefore that NaY and CsY zeolites form essentially bidentate carbonate species. Nevertheless, the IR bands on CsY zeolite, were slightly shifted toward the positions of unidentate carbonate species, suggesting an increase of the surface oxygen basicity [23]. Carbonates species on NaY zeolite disappear after evacuation at 373 K; in contrast, a fraction of the bidentate carbonate species remains adsorbed on the CsY surface after outgassing at 373 K. This confirms that CsY zeolite contains stronger basic sites than NaY. Fig. 2 also shows that adsorbed CO2 on MgY gives rise to a strong infrared band around 1630 cm1 with a shoulder on high frequency side, and a weak broad band centered at about 1450 cm1. No IR bands were detected between 1300–1400 cm1. According to literature [20,21], bicarbonate species involves the formation of surface hydroxyl groups. Typically, bicarbonates show a C–OH bending mode at 1220 cm1 as well as a symmetric and asymmetric O–C– O stretching at 1480 and 1650 cm1, respectively [21,22]. Both symmetric and asymmetric O–C–O stretching modes are clearly observed in the IR spectra on MgY reported in Fig. 2. Then, these spectra are consistent with the formation
Table 1 Physicochemical properties, FTIR of pyridine, and catalytic results for the alkylation of toluene with methanol Zeolite
NaY CsY MgY HY a b
EDa (%)
– 68 81 93
Sg (m2/g)
700 430 540 660
XTb (%)
FTIR of pyridine L (area/g)
B (area/g)
L/B
525 155 660 465
30 5 285 310
17.5 31 2.3 1.5
Exchange degree: [(% Nainitial % Nafinal)/% Nainitial.]100. Values at t = 0, T = 673 K, P = 1 atm, toluene/methanol molar ratio = 1.
0.9 7.5 52.2 58.8
Selectivities (%)b SEt-Bz
SXy
Sþ C9
SBz
6.0 88.0 – –
88.5 12.0 54.8 58.5
5.5 – 43.9 18.8
– – 1.3 22.7
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Fig. 2. FTIR spectra of CO2 adsorbed at room temperature and evacuated at: (a) room temperature, (b) 373 K.
Fig. 3. FTIR spectra recorded in the hydroxyl-stretching region after sample evacuation at 773 K for 4 h.
of surface bicarbonate species. This observation is strengthened by the fact that the CO2 adsorption on acid HY zeolite gives rise to an IR spectrum qualitatively similar to that obtained on MgY. Bicarbonates on MgY remain adsorbed after outgassing at 373 K thereby suggesting the existence of strong Bro¨ nsted sites.
Fig. 3 shows the FTIR spectra in the hydroxyl-stretching region of CsY and MgY zeolites obtained after evacuation at 773 K for 4 h. Spectra obtained on NaY and HY zeolites were also included. No absorption bands were detected on CsY sample, thereby indicating that the hydroxyl group concentration on this zeolite is negligible. In contrast, MgY zeolite exhibited IR absorption bands at 3745, 3690 and 3620 cm1. The frequencies near 3745 and 3620 cm1 are close to those observed for HY zeolite and reflect the presence of Si–OH groups [24]. The band at 3690 cm1 is close to that observed on X zeolites for Al–OH groups [25]. The IR characterization reveals therefore the presence of structural hydroxyl groups (Bro¨ nsted acid sites) on MgY zeolites, and suggests that the exchanged cation is in the (MgOH)+ form [26]. The density and nature of surface acid sites were determined from the FTIR spectra of adsorbed pyridine. Fig. 4A shows the spectra obtained on zeolite HY after admission of pyridine, adsorption at room temperature, and sequential evacuation at 298, 423, 573, and 723 K. The peaks at 1450 y 1540 cm1 arise from pyridine adsorbed on Lewis and Bro¨ nsted acid sites on zeolites respectively [24,27,28]. These two peaks are present even after sample evacuation at 723 K. Fig. 4A also shows that the spectrum collected following evacuation at 298 K contains broad absorption bands characteristics of the presence of physisorbed pyridine. Pyridine molecules interacting via H-bonding with weakly acidic surface OH groups (bands at 1446 and 1597 cm1) were removed after evacuation at 423 K and the resulting FTIR spectrum showed well defined absorption peaks. We decided then to characterize the sample acidity by using spectra collected after evacuation at 423 K. Fig. 4B compares the spectra obtained on zeolites HY, NaY, MgY, and CsY after admission of pyridine, adsorption at room temperature, and evacuation at 423 K. The relative contributions of Lewis and Bro¨ nsted acid sites were obtained from experimental FTIR spectra by integration of pyridine peaks at 1450 y 1540 cm1, respectively. Results are given in Table 1. As expected, sample NaY shows essentially Lewis acidity (the areal peak relationship between Lewis and Bro¨ nsted sites, L/B, was 17.5) but after
Fig. 4. FTIR spectra of pyridine adsorbed on: (A) zeolite HY at 298 K and evacuated at increasing temperatures, (B) zeolites HY, MgY, NaY, and CsY at 298 K and evacuated at 423 K for 0.5 h. Dotted lines indicate the presence of Lewis (1450 cm1) and Bro¨ nsted (1540 cm1) sites.
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ion exchange with ammonium acetate the L/B value is 1.5 on the resulting sample HY because the Bro¨ nsted acid site density increases by one order of magnitude. The density of Lewis and Bro¨ nsted sites measured at 423 K on MgY was comparable to that determined on HY at the same temperature. But it was verified that pyridine adsorbed on Bro¨ nsted sites was more easily eliminated on MgY than on HY by evacuating the samples at temperatures higher than 423 K. This result showed that HY contains stronger Bro¨ nsted acid sites than MgY. Pyridine adsorption does not reveal any Bro¨ nsted acidity on zeolite CsY, which is consistent with results in Fig. 3, showing that the concentration of hydroxyl groups on CsY is negligible. 3.2. Catalytic testing with pure reactants The activity and selectivity for the reaction of toluene with methanol over modified Y zeolites were measured by using a poison-free toluene/methanol mixture as reactant. Toluene conversion diminished with time-on-stream on zeolites MgY and HY because of coke formation, and the initial activity was measured then by extrapolating the XT versus t curves to zero time. In contrast, the activity decay was negligible for CsY zeolite. Catalytic results are shown in Table 1. The HY zeolite, which is employed here as a reference sample, is active and selective for benzene-ring alkylation. Xylene isomers were the predominant products and the isomer distribution was close to the thermodynamic equilibrium distribution – para-xylene (p-Xy): 22.5%, metha-xylene (m-Xy): 50.0% and ortho-xylene (o-Xy): 27.5%. Significant amounts of benzene and higher alkylated compounds (C9+), such as tri and tetramethylbenzenes, were also observed. Similarly to HY sample, MgY promotes selectively the benzene-ring alkylation, producing essentially xylenes and C9+ compounds. The distribution of xylene isomers on MgY was also close to the thermodynamic equilibrium (Fig. 5). Previous work [1,7] stated that benzene-ring alkylation is mainly promoted by Bro¨ nsted
Fig. 5. Distribution of xylene isomers: T = 673 K, P = 1 atm, toluene: methanol = 1.
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acid sites, probably because formation of methoxonium ion is the first step in the reaction mechanism and requires surface hydroxyl groups. Our results are consistent with this assumption because the results of Figs. 3 and 4 shows that zeolites HY and MgY, which efficiently promote the benzene-ring alkylation, contain a high density of strong Bro¨ nsted acid sites. In contrast to MgY and HY samples, CsY shows moderate activity for toluene/methanol reaction but is highly selective for producing ethylbenzene and styrene, in agreement with previous reports showing that alkaliexchanged zeolites promote side-chain alkylation reactions [29]. Besides, regarding xylene isomer distribution, zeolite CsY favored the formation of o-xylene yielding more than 50% of o-Xy in the xylene mixture (Fig. 5). Results of Figs. 2 and 3 show that zeolite CsY does not exhibit Bro¨ nsted acidity but contains Lewis acid centers as well as surface Bro¨ nsted basic sites. This particular distribution of surface acid–base sites on CsY is different from the predominant strong Bro¨ nsted acidity determined on MgY and therefore accounts for the observed differences in catalytic activity and selectivity between both zeolites. Finally, zeolite NaY, which has neither surface basic nor Bro¨ nsted acid sites but contains a high density of Lewis acid sites, displays a very poor activity for converting toluene with methanol. To obtain further insight on the relationship between the surface acid–base properties of exchanged Y zeolites and the catalyst activity and selectivity for the alkylation of toluene with methanol, we performed additional catalytic tests by doping the reactant mixture with acid or basic poisoning molecules. Results are presented below. 3.3. Selective poisoning on MgY zeolite In situ poisoning experiments were carried out on MgY zeolite by doping the toluene/methanol mixture with either an acid (acetic acid) or a base (3,5-dimethyl pyridine). Fresh catalysts were initially tested for about 3 h using a pure toluene/methanol mixture before introducing the doped feed. Fig. 6 shows the activity and selectivity decay curves obtained as a function of time on MgY zeolite when doping
Fig. 6. Evolution of the alkylation activity and selectivity as a function of time. Effect of the addition of acetic acid. Zeolite MgY, T = 673 K, P = 1 atm, toluene:methanol = 1.
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obtained for different 3,5-DMP concentrations. The value of the deactivation function by coke, cc, was 0.7 104 min1. cp was related to poison concentration employing a pseudo-homogeneous kinetics, according to the Levenspiel deactivation model [33]: cp ¼ kd C ap
Fig. 7. Catalytic activity as a function of time and 3,5-DMP concentration, represented in a ln a vs. time plot. Zeolite MgY, T = 673 K, P = 1 atm, toluene:methanol = 1.
the feed with acetic acid. The activity is defined as a = rt/r0, where r0 and rt are the reaction rates at zero time and time t, respectively. Fig. 6 shows that the MgY activity diminishes with time on stream when using undoped reactants, probably because of the formation of carbonaceous deposits. The co-injection of 4700 ppm of acetic acid did not change the slope of the deactivation curve obtained with pure reactants, thereby revealing that the presence of acetic acid does not inhibit the toluene conversion reactions. Similarly, the zeolite selectivity was not affected by co-feeding acetic acid. In contrast, the addition of 3,5-dimethyl pyridine (3,5DMP) to the reactants caused the rapid deactivation of zeolite MgY (Fig. 7). The activity decline due to the presence of 3,5-dimethyl pyridine (3,5-DMP) cannot be determined directly from the experimental data because of the simultaneous catalyst deactivation caused by coke formation. To estimate the poison intrinsic effect, it can be assumed that both effects are additive, which implies that the overall deactivation rate is a simple sum of each individual deactivation rate (hypothesis of independence) [30]. Thus, according to mechanistic deactivation models for simultaneous deactivations [31,32], we can express the overall deactivation rate as:
da ¼ cc adc þ cp adp dt
(1)
(3)
where kd and a are the intrinsic deactivation constant and the poison order, respectively. kd and a were determined by linear regression of experimental data represented in a log cp versus log Cp plot. The values of these parameters are: a = 1.2 and kd = 6 108 min1 ppm1.2. In summary, selective poisoning experiments show that MgY zeolite is only deactivated by co-feeding a basic compound, thereby showing that the rate-limiting step of the reaction mechanism on MgY involves exclusively surface acid sites. This result is in line with previous work proposing that the ring alkylation of toluene with methanol is controlled by the density of Bro¨ nsted acid sites [1]. Promotion of the ring-alkylation mechanism on MgY essentially by surface Bro¨ nsted acid sites is also consistent with the fact that we determined a value of a = 1.2 for the deactivation order with respect to 3,5-DMP. Deactivation mechanistic models based on Langmuir-Hinshelwood kinetics predict, in fact, a poison order equal to 1 when the deactivation mechanism involves the poison adsorption on a single active site [30]. 3.4. Selective poisoning on CsY zeolite Fig. 8 shows the activity decay curves obtained on zeolite CsY upon co-injection of acetic acid or 3,5-DMP. In contrast with the results obtained on MgY, we observe that the activity of zeolite CsY for side-chain alkylation reaction declines by the addition of either an acid or a basic compound. Fig. 8A displays the a versus time plot corresponding to the catalytic data obtained on zeolite CsY during toluene alkylation with and without addition of acetic acid (AA). The catalyst activity remains nearly constant with time on stream when using undoped reactants,
where cc and cp are the deactivation functions related to coke and poison, respectively, and dc and dp are the kinetic orders with respect to coke and poison. As shown in Fig. 7, the deactivation curves are satisfactorily fitted using an exponential function, thereby indicating that the deactivation orders for both deactivating processes were close to 1. Therefore, Eq. (1) may be simplified to:
da ¼ ðcc þ cp Þa dt
(2)
By using the integrated form of Eq. (2), deactivation functions cc and cp can be obtained by linear regression. cc was determined from the deactivation curve obtained with undoped fed (cp = 0) and cp from the deactivation curves
Fig. 8. Time evolution of the CsY activity, poisoned by: (A) acetic acid, (B) 3,5-DMP. Points, experimental data; solid lines, model fits T= 673 K, P= 1 atm, toluene:methanol = 1.
A. Borgna et al. / Applied Catalysis A: General 276 (2004) 207–215
thereby suggesting that no deactivation by coke takes place on CsY. Hence, the poisoning effect can be obtained directly from the activity decay curves. Fig. 8A also shows that deactivation curves reach pseudo-steady state values different from zero for all acetic acid concentrations studied. In a previous work [34], we developed the deactivation model with residual activity (DMRA) for modeling deactivation processes reaching pseudo-steady states different from zero. This deactivation model with residual activity employs Langmuir-Hinshelwood kinetics and requires to ascertain both the main reaction and the deactivation mechanisms for determining the deactivation kinetic parameters. According to the literature [12,16], the following reaction mechanism accounts for the side-chain alkylation of toluene with methanol on basic zeolites: K1
C6 H5 CH3 þ l !ðC6 H5 CH3 Þl
where 0
cd ¼
kP pAA P ; 1 þ Ki pi
where m is the number of active sites involved in the controlling step of the main reaction. Then, from Eqs. (8) and (7) the deactivation rate can be written as: da ¼ ðcr þ cd Þa cr adm dt where
m1 ; m
dm ¼
ðC6 H5 CH3 Þl þ ðCH2 OÞl
cr ¼ mcr ¼ mkrp
k
K3
ðC6 H5 CH ¼ CH2 Þl ! C6 H5 CH ¼ CH2 þ l where l represents the surface active site, Ki the equilibrium adsorption constants and k the rate kinetic constant of the controlling step. The rate-limiting step is considered to be the surface reaction between formaldehyde and toluene adsorbed species. Assuming that the deactivation process takes place by reversible adsorption of acetic acid, the deactivation mechanism may be written as follows: kp ; krp
where kp and krp are the deactivation kinetic constants for the direct and reverse poisoning reaction. According to the proposed deactivation mechanism (reversible adsorption of acetic acid), the number of active sites involved in the controlling step of the deactivation reaction, h, is 1. Therefore, the deactivation rate is expressed as: dCAAl ¼ kp pAA Cl krp CAAl (4) dt and the balance of active sites at the catalytic surface as: X L ¼ Cl 1 þ Ki pi þ CAAl (5) where L is the concentration of active sites of fresh catalysts and C1 the concentration of vacant active site. L CAAl P Cl ¼ 1 þ Ki pi
(6)
Combining Eqs. (4) and (6), we obtain: 0 0 dCAAl ¼ cd ðL CAAl Þ cr CAAl dt
0
cd ¼ mcd ¼ m
(9)
kP pAA P ; 1 þ Ki pi
0
!ðC6 H5 CH ¼ CH2 Þl þ H2 O þ l
AA þ l $ AAl #
0
cr ¼ krp
From the definition of activity [35], the following relationship is derived: rt L CAAl m L CAAl (8) a¼ ¼ ) a1=m ¼ r0 pi ;T;xT L L
CH3 OH þ l !ðCH2 OÞl þ H2
K2
213
(7)
cd and cr are the deactivation functions for the direct and reverse poisoning reaction, respectively. Eq. (9) is similar to that developed for reversible coke deactivation processes [36,37]. In our case we assume that the reversible poisoning mechanism involves only one active site (h = 1), which would represent a surface Cs+–O2 pair site. Therefore, the deactivation order defined by Corella et al. [38,39], d = (m + h 1)/m, is equal to 1. The residual or steady-state activity, aSS, is the value of a for t ! 1, and can be obtained from Eq. (9) taking into account that for t ! 1 is da/dt = 0 [40]: m cr ðcr þ cd ÞaSS cr adSSm ¼ 0 ) aSS ¼ (10) cr þ cd According to the reaction mechanisms proposed above for both the main reaction and the deactivation reaction 1 h ¼ 1 , dm ¼ ; d ¼ 1 2 and Eq. (9) can be finally expressed as:
m ¼ 2;
pffiffiffi da ¼ ðcr þ cd Þa cr a dt
(11a)
or pffiffiffiffiffiffiffiffiffiffi da ¼ cG ða aSS a Þ dt where 2 cr aSS ¼ and cG ¼ cr þ cd cr þ cd
(11b)
(12)
Integrating Eq. (11b) for a differential reactor (cd and cr constants) we obtain: 2 pffiffiffiffiffiffi pffiffiffiffiffiffi 1 (13) a¼ aS þ ð1 aS Þ exp cG t 2
214
A. Borgna et al. / Applied Catalysis A: General 276 (2004) 207–215
Fig. 9. Influence of poison concentration on deactivation kinetic parameters determined on zeolite CsY from DMRA model.
Eq. (13) was employed to compare the DMRA predictions with the experimental data. Parameters cd and cr were determined by using nonlinear least-squares regression which minimizes the objective functions F = S (aexp acalc)2. Fig. 8A shows that the quality of fits was excellent when using DMRA to model the poisoning effect of acetic acid on the side-chain alkylation of toluene with methanol. Similar qualitative deactivation curves reaching residual activities different from zero were obtained on zeolite CsY when the feed was doped with 3,5-DMP (Fig. 8B). The activity decay curves of Fig. 8B were analyzed then by using the DMRA equations described above for acetic acid poisoning. It is observed in Fig. 8B that all experimental data, including high-time data, are satisfactorily correlated using DMRA predictions. The deactivation kinetic parameters determined by employing DMRA for modeling acetic acid and 3,5-DMP poisonings are summarized in Fig. 9. As predicted by this model, Fig. 9A and B clearly show a linear relationship between the deactivation function cd and the poison partial pressure for both acid and base poisoning. Furthermore, as also shown in Fig. 9A and B, reverse deactivation function cr seems to be independent of the poison concentration, which is in good agreement with the prediction of the DMRA deactivation model derived from mechanistic considerations. The reverse deactivation function, cr = 2krp, and the intrinsic deactivation kinetic constant for the direct reaction, ‘‘kp’’ = 2kp/(1 + SKiPi), were also obtained by multivariable non-linear regression. Regarding the value of direct deactivation constant ‘‘kp’’, it was obtained that ‘‘kp’’ was similar when CsY was poisoned with acetic acid (8.9 107 0.3 107 min1 ppm1) or with 3,5-DMP (6.7 107 0.4 107 min1 ppm1). In contrast, the reverse deactivation constant krp was significantly lower when reactants were doped with 3,5-DMP (0.0036 0.0005 min1) than with acetic acid (0.0094 0.0005 min1), thereby suggesting the stronger adsorption of the basic molecule on CsY. This later result is consistent with the lower values of aSS determined
for 3,5-DMP poisoning (Fig. 9C) and strongly suggests that differences in the deactivation behavior produced by either an acid or a base compound on CsY are related to the adsorption strength of the poison molecule. The weaker adsorption of acetic acid on CsY is also supported by the fact that FTIR spectra of adsorbed CO2 (Fig. 3) showed that surface basic sites on CsY are weak and display low basicity. In summary, selective poisoning experiments show that the side-chain alkylation of toluene with methanol on CsY zeolite is deactivated by the addition of either acid or base compounds, thereby suggesting that the surface actives sites promoting the rate-limiting step are Lewis acid (Cs+)–base (O2) pairs. Bimolecular side-chain alkylation reaction would require proper acid–base pair sites for activating both reactants, toluene and methanol, but also a specific acid– base pair configuration to promote the adjacent adsorption of C1 species and bulkier toluene-derived species. Consequently, the presence of either a basic or an acid compound poisons the combined acid–base pathway required for promoting the side-chain alkylation of toluene with methanol on zeolite CsY.
4. Conclusions Selective acid–base poisoning gives useful information for relating the surface properties of exchanged Y zeolites with their catalytic performance for the alkylation of toluene with methanol. MgY zeolite is only deactivated by cofeeding base compounds, thereby showing that exclusively acid sites are involved in the rate-limiting step of the reaction mechanism. In addition, the deactivation order with respect to the poisoning molecule is close to one, indicating that the deactivation mechanism involves the poison adsorption on a single active site, probably Bro¨ nsted acid sites. CsY zeolite is similarly deactivated by co-feeding either an acid or a base compound, demonstrating that the ratelimiting step of the reaction mechanism requires surface acid–base pairs. A deactivation model with residual activity
A. Borgna et al. / Applied Catalysis A: General 276 (2004) 207–215
derived by assuming that the rate-limiting step occurs on a single Cs+–O2 pair site satisfactorily interprets experimental data and allows the determination of both deactivation kinetic parameters and poison adsorption equilibrium constants. Results obtained from the deactivation model and infrared spectroscopy are consistent with the interpretation that Lewis acid/Bro¨ nsted base pairs with a specific configuration are required on CsY zeolite to form the intermediate complex between adsorbed formaldehyde and toluene which would be the key step of the reaction mechanism.
Acknowledgements The authors thank the Universidad Nacional del Litoral, Santa Fe, and CONICET (Argentina) for the financial support of this work. The authors also thank Prof. A. Monzo´ n for many helpful discussions on the development of kinetic deactivation models and H. Cabral for his technical assistance with infrared spectroscopy experiments.
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