Continuous liquid phase acylation of toluene over HBEA zeolite: Solvent effects and origin of the deactivation

Journal of Molecular Catalysis A: Chemical 396 (2015) 231–238

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Continuous liquid phase acylation of toluene over HBEA zeolite: Solvent effects and origin of the deactivation Zhihua Chen, Wenqi Chen, Tianxia Tong, Aiwu Zeng ∗ State Key Laboratory of Chemical Engineering, Tianjin Chemical Synergy Innovation Center, Tianjin University, Institute of Chemical Engineering and Technology, Tianjin 300072, PR China

a r t i c l e

i n f o

Article history: Received 14 July 2014 Received in revised form 25 September 2014 Accepted 28 September 2014 Available online 7 October 2014 Keywords: Acylation Zeolite Toluene Solvent effect Deactivation

a b s t r a c t The continuous liquid phase Friedel–Crafts acylation of toluene (T) by acetic anhydride (AA) over HBEA zeolite was carried out in a fixed bed reactor, with acetic acid (AC) as a solvent. 4-Methylacetophenone (4MAP) was selectively formed in the initial reaction stage. However, a rapid catalyst deactivation occurred with a sharp decrease of the conversion of acetic anhydride, and this was mainly caused by 4-MAP and heavy compounds (‘coke’) existing in the zeolite pore, which poisoned the active sites of the catalyst. The use of excess toluene and moderate acetic acid enhanced catalyst activity and stability to some extent as it limited both the retention of 4-MAP and the formation of ‘coke’. Moreover, a considerable reduction of Broensted acid sites after deactivation revealed that the toluene acylation is primarily a Broensted acid catalyzed reaction. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Friedel–Crafts acylation is an important step for synthesizing aromatic ketones, which are used as prominent intermediates in the production of valuable industrial and fine chemicals such as pharmaceuticals, insecticides, plasticizers, dyes and perfumes [1–6]. This process conventionally has been carried out by using homogeneous catalysts Lewis acid (e.g., metal halides [7]) or Broensted acid (e.g., H2 SO4 , HCl, HF [3]), which leads to many problems concerning handling, safety, corrosion, waste disposal etc. [8]. To overcome these drawbacks, considerable efforts have been made to search for eco-friendly solid and reusable heterogeneous catalysts, particularly the zeolites such as ZSM-5 [9–11], H␤ [3,6,12,13], and HY [13–15]. Besides, it is a promising way to use less hazardous acylating agents, i.e., carboxylic acids and their anhydrides, instead of acyl halides. The acylation of active aromatic rings such as anisole [16–18] and 2-methoxynaphthalene [4,17], or higher active heteroaromatic compounds such as thiophene, pyrroles and furans [5,19,20], is carried out with very good yields and selectivities on zeolites. However, the results obtained from lower active aromatic rings like toluene and benzene are poorer. [21]

∗ Corresponding author. Tel.: +86 13920404701. E-mail address: [email protected] (A. Zeng). http://dx.doi.org/10.1016/j.molcata.2014.09.038 1381-1169/© 2014 Elsevier B.V. All rights reserved.

Many studies have been devoted to improving the activity and selectivity of acylation of toluene with acetic anhydride [17,21–26]. Derouane et al. [23] concluded that HBEA zeolite was definitely the preferred catalyst on acetylation of toluene with acetic anhydride due to its large and interconnected channels when compared with H-FAU(Y), H-MOR and H-MFI. Moreover, excellent selectivity (>98%) of 4-methylacetophenone has been obtained when HBEA zeolite was used as a catalyst [17,21,23,24]. However, zeolites easily suffered from fast deactivation because of the poisoning of the active sites by adsorption of the products and pore blockage due to “coke”-type large molecules, which strongly limited the use of zeolites in commercial processes [21]. Besides, we observed that almost all present studies on acylation of toluene by acetic anhydride over zeolites are carried out in batch reactors, which make the investigation of deactivation difficult and these researchers’ emphases were generally placed on the activity and selectivity but not the stability of the catalysts. In this work, continuous liquid phase acylation of toluene with acetic anhydride over HBEA zeolite (Si/Al = 27.6) was first carried out in a fixed bed reactor. Excess toluene and appropriate acetic acid were used. The aim of this paper is to understand the origin of deactivation of zeolites and the effects of solvent. Catalysts were recovered and extracted in a soxhlet with dichloromethane. The parent and deactivated HBEA zeolites were compared through the analysis of SEM, TGA, 13 C NMR MAS, 27 Al NMR MAS, and the FTIR spectroscopy of adsorbed pyridine. It shows that the organic

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Fig. 1. Acylation of toluene in fixed bed reactor apparatus diagram.

compounds adsorbed/retained in the zeolites (4-MAP and ‘coke’) are responsible for the deactivation of zeolites. Besides, a toluenerich reactant mixture and moderate acetic acid are preferable to limiting the deactivation to some extent. 2. Experimental 2.1. Materials Toluene (99.5%) and acetic anhydride (98.5%) were procured from Tianjin Jiangtian Chemical Technology Co., Ltd. Acetic acid (99.5%) and dichloromethane (99.5%) were supplied by Tianjin Guangfu Institute of Fine Chemistry. Tetraethyl ammonium hydroxide (TEAOH) was employed by Aladdin Reagent Co., Ltd. Aluminosilicate was purchased from Nankai University Catalyst Co., Ltd. All materials were used without further purification.

13 C NMR and 27 Al NMR spectra were recorded on a 300 MHZ Varian Infinity plus NMR spectrometer in order to investigate the kind of carbon deposit and the form of aluminum. The scanning frequency was 75.4 MHz and pulse delay was 5.0-s. The MAS speed (spin rate) was 8 kHz. Chemical shifts (ı) were reported in ppm on behalf of the kind of carbon deposit and the form of aluminum, respectively. The acidity of the zeolite samples before and after deactivation was determined by the pyridine adsorption–desorption method performed on a Nicolet 750 infrared spectrometer coupled to a conventional high vacuum system. The samples were calcined at 450 ◦ C for 120 min in an in situ IR gas cell under vacuum prior to pyridine adsorption. Then the temperature was cooled down to 90 ◦ C. Pyridine was adsorbed for 30 min and then the physisorption pyridine was evacuated at 200 ◦ C and 350 ◦ C for 20 min, respectively.

2.2. Catalyst preparation and characterization

2.3. Reaction procedure

The HBEA sample was made according to the method reported by Matsukata et al. [27]. Each 10 g powder of HBEA was mixed with 3 g Al2 O3 and 4 g nitric acid solution (10 wt%) into homogenate, extruded and cut into a short cylinder of 4 mm in length and 3 mm in diameter. The granular zeolite was dried at 120 ◦ C for 2 h, and then calcined at 450 ◦ C for 5 h in a muffle furnace. The bulk Si/Al molar ratios of the samples were determined by XRF. The BET surface area and total pore volume were determined using a Micromeritics ASAP2020 volumetric instrument at −196 ◦ C using nitrogen adsorption isotherms. The morphologies of the parent and deactivation zeolites were examined with a HIT-S4800 scanning electron microscopy (SEM). TG/DTG profiles were performed on a PerkinElmer Diamond TG/DTA instrument, with a heating rate of 15 ◦ C min−1 under a nitrogen flow rate of 20 ml/min.

Continuous experiments were carried out in a fixed reactor (Fig. 1) consisting of a stainless steel tube (8 mm in diameter, 28 cm in length) kept in an electrical heater mounted in a vertical manner. 5.6 (or 2.8) g of the HBEA zeolite (Si/Al = 27.6) was kept in the middle isothermal region of the reactor. The catalyst was activated in situ before the reaction by heating for 2 h at 200 ◦ C in a N2 flow (100 ml/min). Then the temperature was decreased to 120 ◦ C and the reaction pressure was controlled at 0.2 MPa in N2 flow (30 ml/min). Typically, the mixtures of toluene, acetic anhydride, and acetic acid with a molar ratio of 20:1:0.5 were introduced at the top of the reactor by a metering pump (flow rate 0.1 ml/min). The liquid products were taken at regular intervals and analyzed with a gas chromatography (GC, Agilent 7890A) equipped with a flame ionization detector (FID) and DB-FFAP capillary column (30 m in length and 0.25 mm in diameter).

Z. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 231–238

Since the toluene was overdosed compared with acetic anhydride (in molar), the conversion of substrate was calculated based on acetic anhydride (AA): (1)

The yield (mol%) and selectivity of 4-MAP were calculated as follows: Y4-MAP =

S4-MAP =

%mol (4-MAP at time = t) × 100 %mol (AA at time = 0)

(2)

GC peak area of 4-MAP × 100 sum of the GC peak of 2-MAP, 3-MAP and 4-MAP (3)

100

a Conversion (%) or Yield (mol%)

XAA

% (AA at time = 0) − % (AA at time = t) = × 100 % (AA at time = 0)

233

b

80 the rate of rise of 4-MAP yield the rate of rise of conversion AA

60

40

Conversion AA

20

4-MAP Yield 0

The organic compounds retained on the external zeolite surface and in the micropores were recovered at the end of the experiment. The used catalyst underwent a double-extraction methodology. The first extraction was made according to the description by Rohan et al. [28]. The deactivated catalyst was treated in a soxhlet for 8 h using dichloromethane as solvent. Then the dichloromethane was evaporated and the remaining organic material was analyzed by GC–MS. The zeolites therefore experienced a second extraction by dissolving in a solution of sodium hydroxide (10 mol/L), after that the organic species was extracted with dichloromethane and analyzed by GC–MS. Through such a methodology, most of the adsorbed compounds could be recovered.

0

1000

2000

3000

4000

5000

Time on Stream (min) Fig. 2. AA conversion (mol%) and 4-MAP yield (mol%) vs time during continuous acylation of toluene (T) with acetic anhydride (AA) over HBEA zeolite (a) and their rate of change (b). Experimental conditions: T/AA/AC molar ratio = 20:1:0.5; 120 ◦ C; LHSV = 0.426 h−1 .

gradually to 15 mol% and the rate of change is on the brink of 0 by 1500 min. In this paper, the catalyst is regarded as deactivation when the yield of 4-MAP is below 15 mol%.

3. Results and discussion

3.3. Effects of solvent

3.1. Characterization of zeolites

In the liquid phase synthesis of functional compounds, solvents are often used for some practical reasons such as solubilization of reactants and products, heat transfer with exothermic reactions and improvement of the rate, stability and selectivity of reactions [30]. Dimroth and Reichardt [30,31] regarded solvents as structured isotropic continuum composed of individual solvent molecules with their own solvent/solvent interactions, and took into account specific solute/solvent interactions such as hydrogen-bonding and EPD/EPA interactions. Thus, ET (30) and normalized values ETN , the empirical parameters of solvent polarity are proposed. For Friedel–Crafts acylation, Fromentin et al. [4] studied the influence of the polarity of solvents on the activity and selectivity of zeolite HBEA in the acetylation of 2-methoxynaphthalene with acetic anhydride. It is noted that high polar solvent, such as sulfolane (ETN = 0.410), can compete with the reactant molecules for diffusion inside the pores and for adsorption on the acid sites, reducing therefore the rates of the main reaction. High deacetylation rate is obtained with non-polar solvent such as 1-methylnaphthalene (ETN = 0.412) which cannot solvate the acylium ion intermediates. And high acetylation and isomerization rates are obtained in the presence of a solvent of intermediate polarity such as nitrobenzene (ETN = 0.324). The effect of solvent polarity is similar to the one found by Moreau et al. [32]: An increase in the solvent polarity lead to the increased competitive adsorption between solvent and reactants on the active sites. The research on solvents has always been a topic of interest, because many of the solvents generally used throughout both academia and industry are regarded as toxic, volatile, unsafe or high consumption in terms of environmental protection. This includes the development of environmentally benign neoteric solvents, constituting a series of novel solvents with desirable, less hazardous, new properties [30]. In fact, considering the development of a sustainable chemistry, the best solvent will be no solvent at all. Hence, in this work, toluene (ETN = 0.099) and acetic acid (ETN = 0.648), which were reactant or product as well, were considered as

The bulk Si/Al molar ratio of HBEA zeolite used in this work was 27.6 (determined by XRF), and the XRD crystallinity was 100% (XRD signal 2 = 7.8◦ and 22.5◦ ); The total surface area was 542 m2 /g (micropore surface area = 449 m2 /g; external surface area = 93 m2 /g) (determined by the BET method), the total pore volume was 0.209 cm3 /g (determined by the t-plot method). 3.2. Influence of time on the production of 4-MAP The acylation of toluene by acetic anhydride over 5.6 g zeolite HBEA was carried out in liquid phase at 120 ◦ C, 0.2 MPa in a fixed bed reactor, with acetic acid as a solvent. The mixed molar ratio of toluene, acetic anhydride and acetic acid was 20:1:0.5 (Table 1). LHSV for mixture (volume of mixture introduced per volume of catalyst per hour) was 0.426 h−1 . Whatever the time-on-stream (TOS), the selectivity to 4-MAP is very high (>96%). 3-MAP, 2-MAP, diacetylated toluene and traces of other heavy compounds appear as secondary products. It should also be remarked that some acetic anhydride undergoes hydrolysis to form acetic acid during the reaction, especially at short TOS. As seen from Fig. 2a, there is initially a huge difference between the yield of 4-MAP and the conversion of AA (maintains at 100%, approximately), due to the strong adsorption retention of 4-MAP [29] and acetic anhydride in the zeolite pores, as well as the hydrolysis of acetic anhydride. Meanwhile, the increase in the yield of 4-MAP can also illustrate the retention of 4-MAP. Fluctuation trend of the yield of 4-MAP is similar to the conversion of AA (Fig. 2b). And the yield of 4-MAP reaches a plateau (30 mol%) at 400–600 min on stream. This plateau is likely to be ascribed to the equilibrium between the rate of formation and adsorption of 4-MAP on the zeolites. After that, both the yield of 4-MAP and the conversion of AA rapidly decrease to a yield of about 26 mol%, after which the decreasing rates tend to be stable. The yield of 4-MAP drops

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25

32

T/AA=20

a

b

28

4-MAP yield (mol%)

4-MAP yield (mol%)

20

15 T/AA=10

10 T/AA=5

AC/AA=0.5

24

AC/AA=0

20

AC/AA=1.5 AC/AA=3

16 12

5 8

T/AA=1 100

0

200

300

400

500

600

700

800

Time on Stream (min)

100

200

300

400

500

600

Time on Stream (min) Fig. 3. 4-MAP yield (mol%) vs time as a function of the toluene to acetic anhydride (T/AA) during continuous acylation of toluene (T) with acetic anhydride (AA) over HBEA zeolite. Experimental conditions: 120 ◦ C; LHSV = 1.706 h−1 .

solvents in the acylation of toluene with acetic anhydride, without other outside solvents being introduced. Fig. 3 indicates that there is a significant improvement in the yield of 4-MAP when the T/AA molar ratio increased from 1 to 20. This is probably because the large excess of toluene helped to desorb the formed 4-MAP, and therefore reduced the inhibiting effect of ketone. So toluene can also be regarded as a solvent in this process. And toluene is generally much larger than the stoichiometric when acylated by acetic anhydride over zeolites [21–23,26] unless under the conditions of nitrobenzene as a solvent [17,24], or Fe-SBA as the catalyst [24]. Fe-SBA is a kind of mesoporous molecular sieves possessing larger pore diameter than the microporous ones, and this structure contributes to the diffusion of reactants and products. However, it is with less diffusion limitation that a lower selectivity of 4-MAP was got (85.6%) in comparison with the result of using HBEA (>98%). Nitrobenzene with intermediate polarity helps desorption of 4-MAP formed within the micropores and does not have remarkable competitive adsorption with reactants, and thus could favor the catalytic activity. However, it is in urgent need to find out a new solvent for the substitution of hazardous solvents such as nitrobenzene. In this work, acetic acid was considered as another solvent. The effect of acetic acid is illustrated in Fig. 4. Appropriate amount of acetic acid added in raw material could increase the yield of 4-MAP, while excess acetic acid would result in the

Fig. 4. 4-MAP yield (mol%) vs time as a function of the acetic anhydride to acetic acid (AC/AA) during continuous acylation of toluene (T) with acetic anhydride (AA) over HBEA zeolite (a) and the 4-MAP selectivity (b). Experimental conditions: T/AA molar ratio = 20:1; 120 ◦ C; LHSV = 1.706 h−1 .

opposite result (Fig. 4a). Different trend can be seen when it comes to the selectivity of 4-MAP, the selectivity increased with the increasing amount of acetic acid in raw material (Fig. 4b). This is because the process of acetic anhydride being dissociated into acetyl ions will be hindered in the presence of acetic acid. Thus the selectivity of 4-MAP will be improved. Though acetic acid might stabilize the acylium ion as well as the ionic intermediate formed by the attack of acylium ion with toluene and promote acylation reaction like nitrobenzene [7], superfluous acetic acid will compete with the reactant molecules for adsorption in the acid sites, which results in the decrease of the yield of 4-MAP (Fig. 4). 3.4. The origin of deactivation 3.4.1. Carbon deposits: composition and kinetic minimum cross-sectional diameter Zeolite BEA is generally described as a three-dimensional intersecting channel system, which consists of two mutually perpendicular straight channels, each with a cross-section of ˚ run in the x- and y-directions, 0.66 nm × 0.67 nm (or 6.6 A˚ × 6.7 A) and a sinusoidal channel of 0.56 nm × 0.56 nm runs parallel to the z-direction [33]. The average pore diameter of HBEA zeolite used in this work was 0.56 nm. Aluminum atoms in HBEA zeolite produce Broensted and Lewis acid sites. Broensted acid sites have external OH groups, which present both on the internal and external surface, while the Lewis acid sites are the exposed three-fold

Table 1 Acylation of toluene with acetic anhydride over zeolites in batch reactor. Year

Catalyst

T/AA molar ratio

Catalyst/AA weight ratio

Temperature (◦ C)

Time (h)

Solvent

1997

HY, HZSM-5

140:1

1:1

250

12

None

2000

BEA, Nano-BEA

20:1

0.5:1

150

4

None

2000 2004

HBEA HBEA, La-BEA

20:1 1:2

0.36:1 0.13:1

115 135

24 6

None Nitrobenzene

2005 2007 2013

HBEA FeSBA-1 Nano-BEA, Hierar-BEA

5:1 1:2 20:1

0.7:1 0.05:1 0.5:1

100 150 120

1 4 1.7

Ph-NO2 None None

HBEA

20:1

0.8:1

150

6

a b c

XAA : the conversion of acetic anhydride. YMAP : the yield of methylacetophenone. S4-MAP : the selectivity of 4-methylacetophenone.

None

XAA a

58% 83%

YMAP b 74 mol% 78 mol% 46 wt% (35 mol%) 80 wt% (61 mol%) 40 mol%

50% 66% 11% 77.2% 54.8% 76.8% 77.8% 36.3 mol%

S4-MAP c

Ref. [23]

>98% >98% ≥99.5% 100% 100% 98.7% 85.6%

96%

[22] [24] [25] [17] [26] [27] This work

Z. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 231–238

235

coordinated Al3+ ions, predominantly presenting on the internal surface [34–36]. The windows in zeolite crystal structures indicate molecular sieving capabilities and control access to the catalytically active internal sites [37]. The steric hindrance of remaining organic material on the external zeolite surface and in the micropores was evaluated from the calculated d (kinetic minimum cross-sectional diameters) of the organic molecules using ChemBio 3D Ultra 12.0. The concept of kinetic minimum cross-sectional diameter is derived from de (minimum cross-sectional diameters), which is amended by Lennard–Jones model [38] d =

de

(4)

21/6

It has been proposed on the assumption that the spherical symmetric molecules enter into the zeolite pore in the minimized state of barrier potential. It is expected to be very instructive for predicting shape selective adsorption of zeolites and their catalytic behaviors [39]. As shown in Table 2, the d of main components (components 1–4) are smaller than the pore opening of HBEA zeolite (0.66 × 0.67 nm, 0.56 × 0.56 nm), that is, the main components tend to not be controlled by the diffusion limitation of the zeolite. Thus it can account for their strong polarity adsorption on the acid sites rather than their steric blockage resulting in the retention in the zeolite pores. These are also the similar cases with components 6–8, 10 and 12. However, 4-methylphenyl anhydride (d = 0.84 nm) (component 5), butyl isobutyl phthalate (d = 0.80 nm) (component 9) and 3-phenyl-1-p-tolyl-propenone (d = 0.83 nm) (component 11) molecules are larger than the pore opening of the zeolite, which are probably trapped at the channel intersections. Fig. 5. The SEM images of the parent and deactivated samples.

3.4.2. Reasons for deactivation of zeolites In order to explore the reasons for deactivation of zeolites, the analytical methods of the SEM, TGA, 13 C MAS NMR, 27 Al MAS NMR, and the FT-IR spectroscopy of adsorbed pyridine were taken. The morphologies of the parent and deactivation HBEA zeolites were analyzed by SEM and the results are presented in Fig. 5. SEM image (Fig. 5a) shows the particles of the parent HBEA zeolite are in the range of 0.2–1 ␮m. They consist of loose aggregates of minute elementary crystals 30–100 nm in size and are evenly distributed. However, SEM image of the deactivation catalyst (Fig. 5b) shows there is a severe carbon deposition on the surface of the catalysts, which might block the acidic sites of the catalyst, leading to the decrease of the reaction rate. The amount of carbon deposit in the surface and channel of the zeolites has been determined by thermogravimetric (TG) and

differential thermal analysis (DTA), and the results are shown in Fig. 6a and b, respectively. Both the parent and deactivation samples have a peak of weight loss in the range of 50–200 ◦ C, 4 wt% and 8 wt%, respectively, which is mainly caused by surface moisture adsorbed from air and hydrated water. And for the deactivated zeolite, it also includes the adsorbed organic compounds of low molecular weight (fat hydrocarbon) (about 4 wt%) in the reaction process. There is no weight loss for the parent zeolite until in the range of 430–520 ◦ C (1 wt%), which is attributed to the template agent residue in the catalyst. However, the deactivated zeolite presents three exothermic bands at 200–250 ◦ C (2 wt%), 250–330 ◦ C (2 wt%) and 330–520 ◦ C (5 wt%). At the range of 200–330 ◦ C, the peak of deactivated zeolite represented the main product methylacetophenone (MAP) and some other by-products

A

100 98

-0.01

96

Deactivated zeolite

Parent zeolite

94

-0.02

92

-0.03

90

dm/dt

Weight percentage

Parent zeolite

B

0.00

88 86 84

Deactivated zeolite

82

-0.04 -0.05 -0.06 -0.07

80

-0.08

78 0

100

200

300

400

500

Temperature/

600

700

800

0

100

200

300

400

500

Temperature/

Fig. 6. The TG/DTG curves of the parent and deactivated samples.

600

700

800

236

Z. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 231–238

Table 2 Heavy reaction products (main components) and their kinetic minimum cross-sectional diameters. Components

Chemical structure

Molecular weight

Kinetic minimum cross-sectional diameter d (nm)

226

0.53

226

0.63

226

0.63

176

0.55

254

0.84

162

0.69

210

0.62

210

0.69

278

0.80

164

0.63

222

0.83

146

0.66

O

1. 4-Methylacetophenone

O

2. 3-Methylacetophenone

O

3. 2-Methylacetophenone

O

4. (4-Methylbenzoyl)acetone O O

O

O

5. 4-Methylphenyl anhydride

6. 1-(4-Methylphenyl)-1-butanone O

O

7. 4,4 -Dimethylbenzophenone

O

8. 3,3 -Dimethylbenzophenone

O O

9. Butyl isobutyl phthalate

O O O

10. Ethyl m-methylbenzoate

O

O

11. 3-Phenyl-1-p-tolyl-propenone

O

12. 4-Phenyl-but-3-en-2-one

Z. Chen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 231–238

237

Octahedral Al

Tetrahedral Al

Deactivated zeolite Deactivated zeolite

Parent zeolite

350

300

250

200

150

100

50

0

-50

-100 -150

Parent zeolite

ppm Fig. 7.

13

C MAS NMR of the parent and deactivated samples.

300

250

200

150

100

50

0

-50

-100 -150 -200 -250

ppm such as (4-methylbenzoyl) acetone, 4-methylphenyl anhydride and 4,4 -dimethylbenzophenone (Table 2), while at the range of 330–520 ◦ C the peak is mainly belong to the aromatic oligomers (4 wt%) which are difficult to be extracted with methylene chloride, except the template agent (1 wt%). The mass ratio of low molecular (4 wt%), MAP and its derivatives (4 wt%), and aromatic oligomers (4 wt%) was approximately 1:1:1. The distribution of different forms of carbon containing fat hydrocarbon (4 wt% + 2 wt%), aromatic carbon (4 wt% − 2 wt%) and aromatic oligomers (4 wt%) was roughly equal to 6:2:4. The 13 C MAS NMR spectra of the deactivated and parent zeolites are shown in Fig. 7. Derouane et al. [40] first studied the formation and stability of coke deposits by the method of 13 C MAS NMR during the conversion reaction of methanol to hydrocarbons. Carbon deposit mainly includes fat carbon, olefin carbon, aromatic carbon and poly aromatic carbon, etc. The peak signal of fat carbon is between 10 ppm and 40 ppm, the signal between 125 ppm and 145 ppm belong to aromatic carbon, while between 130 ppm and 200 ppm is the characteristic signal of poly aromatic. For the deactivated zeolites, the carbon deposit consisting of fat hydrocarbon, aromatic carbon and aromatic oligomers in the channel and surface is very serious, which presents two tall narrow bands at 10–40 and 170–190 ppm, as well as a low broad band at 125–145 ppm. It is difficult to obtain accurate coke content by 13 C MAS NMR spectra. However, the distribution of peak area of fat hydrocarbon, aromatic

Fig. 8.

27

Al MAS NMR of the parent and deactivated samples.

carbon and aromatic oligomers equates to roughly 4:1:2, which can be associated with the results (6:2:4) obtained by TG/DTA. The influence of carbon deposit on zeolites framework could be characterized by 27 Al MAS NMR, as shown in Fig. 8. The state of aluminum framework changes when the zeolites channel is filled with carbon deposit. There are two peaks corresponding to octahedral and tetrahedral Al sites [41] in the range of −10–40 and 40–80 ppm, respectively. The octahedral Al concentration tends to decrease on the deactivated zeolite compared to the parent sample, while the tetrahedral Al concentration keeps constant. This indicates that the change of octahedral Al should be due to the adsorption of carbon deposit in the zeolites. Besides, the tetrahedral Al represents Broensted acid sites, while the octahedral Al is on behalf of Lewis acid sites. Therefore, Fig. 8 may indicate that the Broensted acid is responsible for the Friedel–Crafts acylation reaction of toluene with acetic anhydride. The FT-IR spectra of the parent and deactivated HBEA zeolites adsorbed pyridine probe molecule are compared in Fig. 9. Band at 1545 and 1455 cm−1 can be attributed to pyridinium ions (PyH+ , Broensted acid site) and coordinative bound pyridine (PyL, Lewis acid site), respectively [18,42–45]. The band at 1489 cm−1 is associated with adsorption on both Broensted and Lewis acid sites [43].

Fig. 9. FTIR spectra of pyridine-adsorbed parent and deactivated HBEA zeolites: (a) desorption at 200 ◦ C and (b) desorption at 350 ◦ C.

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Table 3 Result of IR spectra of Py adsorbed on the parent and deactivated HBEA zeolites after desorption at different desorption temperatures. Sample

Parent zeolite Deactivated zeolite a

200 ◦ C

350 ◦ C

B acid site (mmol/g)

L acid site (mmol/g)

B/C

B acid site (mmol/g)

L acid site (mmol/g)

B/Ca

0.323 0.095

0.360 0.271

0.90 0.35

0.238 0.063

0.296 0.254

0.80 0.25

B/C: the ratio of B acid site to L acid site.

A reliable assessment of the concentration of surface acid sites can be calculated by the integrated peak area [44], and the results of Py-IR analysis of different samples are listed in Table 3. Calculated concentration of Broensted acid sites is decreased by 71% at 200 ◦ C, and 74% at 350 ◦ C for deactivated zeolite, in comparison with the parent one, while the concentration of Lewis acid sites does not reduce much. This can also demonstrate that toluene acylation is primarily a Broensted acid catalyzed reaction. 4. Conclusions The catalytic activity and stability of zeolite HBEA in a fixed bed reactor could be enhanced by addition of excess of toluene and appropriate acetic acid to some extent. However, the significant deactivation of the zeolite was still observed, which could be mainly due to the poisoning of the active sites by adsorption of the 4-MAP and the heavy compounds (‘coke’). The organic compounds retained on the external zeolite surface and in the micropores were recovered after soxhlet extraction and analyzed by GC–MS. The results of kinetic minimum cross-sectional diameters of the components show that heavy by-products such as 4-methylphenyl anhydride, butyl isobutyl phthalate and 3-phenyl1-p-tolyl-propenone are larger than the pore opening of the zeolite, which are probably trapped at the channel intersections. Moreover, it could be concluded confirmedly that the carbon deposit is mainly responsible for the catalyst deactivation through the comparison between the parent and deactivated zeolites through analysis of SEM, TG/DTA, 13 C MAS NMR and 27 Al MAS NMR. Acknowledgements This work is supplied by The National Key Technology R&D Program of China (2007BAB24B05) and Shanxi Qiaoyou Chemical Co. Ltd. References [1] [2] [3] [4]

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