Catalytic activity, deactivation and re-use of Al-MCM-41 for N-methylation of aniline

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

1299

Catalytic activity, deactivation and re-use of AI-MCM-41 for N-methylation of aniline J.M. Campelo, R.M. Leon, D. Luna, J.M. Marinas and A.A. Romero. Departamento de Quimica Orgfinica, Facultad de Ciencias, Universidad de C6rdoba, Campus Universitario de Rabanales, Edificio Marie Curie (C3), E-14014 C6rdoba, Spain.

A1-MCM mesoporous molecular sieves with Si/A1 ratio in the range 10-40 were characterized by XRD, N2 adsorption, MAS NMR, and DRIFT, and their acid properties were determined by pyridine (PY) adsorption. Aniline methylation was a pseudo-first-order process with respect to aniline concentration. N-methylation products were predominant with a selectivity of the 100 mol% at 573 K after two hours on stream. The aniline conversion and selectivity to NNDMA decreased whereas selectivity to NMA continuously increased with time on stream. Besides, the selectivity to NMA remained almost the same, irrespective of the aniline conversion, aluminum content, and acidity, this fact was strictly observed for A1-B-X catalysts. On the other hand, the re-used A1-MCM-41 catalysts would be described as amorphous aluminosilicates because of its structure deteriorate with reaction water. 1. INTRODUCTION Acid M41S materials have potential applications in the field of organic synthesis and production of fine chemical [1]. Alkylation of aniline is industrially important owing to the nomerous uses of various substituted anilines as raw materials for the synthesis of organic chemicals and chemical intermediates or additives in dyes, synthetic rubbers, explosives, herbicides and pharmaceuticals. The traditional route presents some disadvantages like high capital cost, reactor corrosion and formation of by-products that cannot be recycled. With the increasing awareness of environmental issues, various solid acid catalysts and alkylating reagents have been used for the reaction. Thus, vapor-phase aniline alkylation over environmentally safe solid acid catalysts is an answer to the conventional method of producing alkylanilines using mineral acids and Friedel-Crafts type catalysts. Recently, progress in the application of solid acid catalysts for aniline alkylation had been reviewed [2]. The main factors influencing activity and selectivity (N- and/or C-alkylation) are acid-base properties (number and strength) and shape-selectivity, in the solid acid catalyst, as well as reaction conditions (temperature, composition and feed rate). Moreover, the activity of acid catalysts for aniline alkylation might be suppressed by the adsorption of aniline, since it is a strong base. It therefore seems that the use of a strong acid catalyst is not suitable for this reaction. In the present work, we have carried out the vapor-phase N-methylation of aniline over A1-MCM-41 catalysts. We will focus on the effect of surface acidity upon activity and reaction selectivity, and on the deactivation process.

1300 2. EXPERIMENTAL SECTION 2.1. Catalysts A1-MCM-41 samples were prepared by two procedures. (i) Synthesis at room temperature (for catalytic comparison, gel A), following the procedure described by Griin and col. [3]. (ii) Synthesis gel A was filtered and the product was suspended in 100mL of filtration solution and the resultant suspensions were then hydrotermally treated, in a static teflon bottle at 383 K for 24 hours. The product thus obtained was filtered, dried at 298 K and calcined at 823 K in air for 24 hours. TEOS and A1C13.6H20 were used as Si and A1 sources, respectively, and cetyl-trimethyl-ammonium bromide as template. Samples are denoted A1-A-X (at room temperature) or A1-B-X (hydrothermally treated), where X = 10, 20, 30 and 40 are the Si/A1 ratios in the synthesis gel.

2.2. Characterization XRD patterns were carried out using a Siemens D-5000 diffractometer with CuK~ (~,=1.5418 A), a step size of 0.02 ~ and counting time per step of 1.2 s. Thermal analysis was performed by simultaneous TG-DTA measurement using the Setaram thermobalance Setsys 12. Samples were heated in the temperature range 293-1173 K at a heating rate of 10 K min 1. 27A1 (pulse: 1 ~s; recycle delay: 0.3 s) and 29Si (pulse: 6 ~ts; recycle delay: 600 s) MAS NMR spectra were recorded on a Bruker ACP-400 multinuclear spectrometer at 104.26 and 79.45 MHz, respectively. Nitrogen physisorption was measured with a Micromeritics instrument model ASAP 2000 at 77 K.

2.3. Surface Acidity 2.3.1. Pulse method The surface acidity (sum of Br6nsted and Lewis sites) was measured in a dynamic mode by means of the gas-phase adsorption of pyridine (PY) as probe molecule by using a pulse chromatographic technique [4]. Very small volumes of solutes were injected so as to approach conditions of gas-chromatographic linearity. The acidity measurements were repeated several times and good reproducibility of the results was obtained.

2.3.2. Temperature programmed desorption of pyridine Before adsorption experiments were started, the catalysts were pretreated in situ by passing nitrogen, at a flow rate of 50 mL min -1, and heating from 323 to 723 K at 10 K min-1; the temperature was maintained at 723 K for 10 min. After the activation treatment, the samples were cooled down to 373 K at which the adsorption experiment was carried out according to a chromatographic method described elsewhere [5]. Repeated adsorption/TPD experiments using the fresh sample did not show any change in the adsorption curve.

2.4. Catalytic Activity Measurements The reactions were conducted at 573 K in a vapor-phase continuous stainless-steel downflow fixed-bed reactor (6 mm ID) surrounded by an electric heater. An iron-constantan thermocouple was placed in the middle of the catalyst bed and the unit operated at atmospheric pressure. The substrate was delivered at a set flow rate using a liquid syringe pump (Harward Md. 44) and was vaporized prior to passing it through the catalyst bed in the presence of a flow of nitrogen carrier gas (3 L hl). The catalyst charges (W) were small,

1301 usually 0.03-0.06 g, retained by quartz wool at almost the center of the reactor. Standard catalyst pretreatment was carried out in situ at 573 K for 1 h under a stream of high purity nitrogen. In order to prevent any condensation of reactant and products all connections were heated at 490 K. Blank runs at 573 K showed that under the experimental conditions used in this work, the thermal effects could be neglected. The reaction products were on-line sampled every 15 min and analyzed by GC (FISONS Md. 8000) by using a stainless steel column (2 m x 3 mm) of 10% Carbowax 20 M/2% KOH on Chromosorb W-AW 80/100. Product characterization was performed by GC-MS (HP 5800 gas-chromatographic coupled with a VG AutoSpec high-resolution mass spectrometer) using products condensed in a cold trap. Reaction products were: N-methyl (NMA) and N,N-dimethylaniline (NNDMA). N,N-dimethyltoluidines (NNDMT, p - > o - ) were only present in very small amounts. Product such as diphenylamine was never found by highresolution mass spectrometry. Response factors of the reaction products were determined with respect to aniline from GC analysis using known compounds in calibration mixtures of specified compositions. The conversions reported here are on a methanol-free aniline basis and the selectivities are expressed as the ratio of moles produced (mol%). The process for reactivation of partially deactivated catalyst comprised the successive steps of: (a) A1-X catalysts (after 12 h on stream) were quickly heated from 573 to 823 K and then purged during 30 min under nitrogen flow (50 mL min-1); (b) the thermal reactivation was carried out under inert (N2, 50 mL minl), reductive (H2, 50 mL minl), or oxidative (02, 50 or 120 mL min ~) atmosphere during 1 h; and (c) then the catalysts were quickly cooled to reaction temperature under nitrogen flow. 2.5. DRIFT Measurements

DRIFT spectra were recorded on an FTIR instrument (Bomen MB-100) equipped with an "environmental chamber" (Spectra Tech, P/N0030-100) placed in a diffuse reflectance attachment (Spectra Tech, Collector). A resolution of 8cm -1 was used with 256 scans averaged to obtain a spectrum. Samples were equilibrated for at least 1 h at 473 K in flowing nitrogen (50 mL min -1) prior to collection of spectra. 3. RESULTS AND DISCUSSION The Si/A1 molar ratios of calcined A1-X samples (determined by EDX, not shown) were in close agreement with the composition of the gel mixtures. The quality of the XRD pattern and the pore wall thickness increased for A1-B-X with respect to A1-A-X samples. As can be expected, except for A1-B-10 sample, the BET surface area (A~ET) decreased (until ca. 30%) for hydrothermally treated samples. In all cases, the BJH plot for the physisorption of N2 on the aluminosilicate MCM-41 gave a remarkably narrow pore size distribution with a pore size of ca. 23 A. The sharp pore size distribution, with a ca. 6 and 3 ~, width at half-height for A1-A-X and A1-B-X samples, respectively, shows that the mesopores are exceptionally uniform (Table 1). The results of thermogravimetric (TG) and differential thermal analysis (DTA) of A1-X samples (not shown) were similar to that of Klinowski et al. [6] for M41S materials. The 295i MAS NMR spectra of aluminosilicate A1-X (not shown) were very broad showing that the silicon in A1-MCM-41 was therefore in a highly disordered environment. On the orther hand, the 27A1 MAS NMR spectra of A1-X samples (not shown) were similar, exhibiting an intense line at c.a. 54 ppm from 4-coordinate aluminum (Alt) and a low-intensity

1302 Table 1 Hexagonal unit cell parameter (ao = 2dl00/vt3), textural properties, wall thickness (ao-DBjH, e), surface acidity (vs PY/~tmol g-l, pulse method) and contributions to the total area for PY-TPD profiles of A1-MCM-41 catalysts a ao ABET DBJH E PY PY-TPD (Area %) Sample (~) (m 2 g-i) (~) (~) 573 K ~450 K ~600 K -700 K ,-850 K AI-A-40

39

1232

22(6)*

17

130

15

38

30

16

A1-A-30

39

1179

22(6)

17

131

19

36

31

14

A1-A-20

41

1250

23(6)

18

177

20

27

36

17

A1-A-10

43

1237

23(6)

20

251

24

29

30

17

A1-B-40

42

979

23(3)

19

130

34

32

24

10

A1-B-30

44

891

23(3)

21

141

28

36

24

12

A1-B-20

43

877

23(3)

20

187

31

37

24

8

A1-B-10

44

1173

22(7)

22

255

18

34

28

20

* DBjH is followed (in parentheses) by the width at half-height (in * ) of PSD curve. line at c.a. 0 ppm from 6-coordinated aluminum (Alo). The Alt/A1o ratios were c.a. 7 and 10, for all A1-A-X and A1-B-X samples, respectively. These results showed that the incorporation of the aluminum in the silicate network was improved for hydrothermally treated samples. The surface acidity of catalysts is given in Table 1 as the amount of pyridine adsorbed at saturation at 573 K temperature. The acidity measurements showed that the number and density of acid sites on MCM-41 catalysts was increased with the aluminum content and did not depend on the synthesis procedure. A representative pyridine-TPD curve of A1-MCM-41 samples is shown in Figure 1. Moreover, the TPD spectra were deconvolved assuming four independent types of parallel desorption processes, and thus, the theoretical desorption curves of the individual peak components summed to an overall contour, that was well-correlated to the experimental data. Thus, Figure 1 presents for sample A1-B-30 the experimental data, the individual components as the results of deconvolution, and the theoretical spectrum obtained by summing the individual peaks (standard deviations <3%). Analogous spectra were obtained for all A1-X samples. The PY-TPD data are fitted assuming that there are four Gaussian peaks. So, the low temperature pyridine peak at around 450 K is attributable to weak acid sites, pyridine peak at around 600 K is attributable to medium acid sites, whereas the third and fourth peaks (around 700 and 850, respectively) are assumed to be due to strong acid sites (Br6nsted and Lewis). The experimental data obtained from the TPD of pyridine for A1-X catalysts are given in Table 2. Acidity of A1-MCM-41 samples increased on increasing the A1 content (pulse method, see before), however this increase was not accompanied by any change in acid strength distribution. Thus, the PY-TPD profile was almost the same for these materials. Analogous acid properties were obtained for the A1-HMS materials in our laboratories [7]. In the absence of diffusion effects aniline conversion data (XAN) are fitted in a first-order rate equation: ln[1/(1-XAN)]=k(W/F)

(1)

1303

|

Ai-B-30 i

d p~

.i

o

I

I

I

I

I

I

373

473

573

673

773

873

973

Temperature / K Figure 1. TPD spectrum of pyridine desorbed from Ai-B-30 catalyst: Experimental data (solid line); calculated curves for desorption from four different types of sites and theoretical curve of overall desorption (dotted lines). where W is the catalyst weight and F the feed rate. Nevertheless, calculations are performed only in order to compare the reactivities of the different catalysts and are not aimed at finding the detailed rate equations. All values are reproducible to within about 8%. The initial reaction rate constants (k), XAN, and reaction selectivity to N-methylaniline (SNMA) are collected in Table 2. According to acidity data (Table 1) and the corresponding aniline methylation data (Table 2), we can conclude that on increasing the aluminum content of the catalysts, the acidity as well as the aniline conversion were increased. However, this relationship between surface acidity and catalytic activity was not strictly linear. Table 2 Aniline conversion (XAN, mol%), reaction rate constant (k, mol g'ls-1) and product selectivities (S, mol%) in aniline alkylation with methanol over AI-MCM-41 catalysts a 4 h on stream 8 h on stream 12 h on stream Catalyst XAN k x 10 6 SNMA XAN k x 10 6 SNMA XAN k x 10 6 SNMA AI-A-40

25.1

12.7

71.9

23.4

11.8

78.1

-

-

-

AI-A-30

27.9

14.7

72.8

23.8

12.2

76.1

23.5

12.0

76.8

AI-A-20

28.6

15.0

69.5

25.6

13.1

72.0

25.4

13.1

72.3

AI-A-10

41.3

23.4

64.9

37.7

20.8

67.0

35.5

19.3

69.9

AI-B-40

21.1

10.5

69.1

19.7

9.7

70.7

-

-

-

AI-B-30

28.2

14.6

69.3

25.5

13.0

71.0

23.7

11.9

72.5

AI-B-20

30.0

15.7

69.5

27.6

14.2

72.4

25.5

13.0

74.4

AI-B-10

35.0

18.9

70.4

30.5

16.0

72.7

28.5

14.7

74.2

a T=573 K; F=1.33 x 10 -6 mol s-l; WHSV= 14.8 h-l; 2 M aniline in methanol.

1304 Product selectivities were of the 100mo1% to the N-alkylation of aniline. NNDT (<2 mol%) were only presents at short time on stream (<2 h). The effect of time on stream is shown in Table 2. Here we can see that the aniline conversion and selectivity to NNDMA decreased whereas selectivity to NMA continuously increased. Besides, the selectivity to NMA remained almost the same, irrespective of the aniline conversion, aluminum content, and acidity, this fact was strictly observed for A1-B-X catalysts. These results are contrary to the expectation for aniline alkylation being a consecutive reaction and, also, as compared to previously studied A1PO4, A1POa-metal oxide, CrPO4-A1PO4 and A1-HMS catalysts [7-10]. Nevertheless, similar relation between Si/A1 ratio, total acidity and aniline conversion against selectivity to N,N-diethylaniline had been described for HZSM-5 samples, and it was suggested that, rather than the total acidity, probably acidity of particular strength and type were responsible for the conversion of aniline and selectivity of alkylanilines [2]. Another sets of experiment were carried out to establish the stability of the systems. TG, DTG and DTA thermal analysis curves showed that the decomposition process undergone by the organic on deactivated catalysts could be divided into three steps (Figure 2). (i) The first one, characterized in DTA by a broad endothermic zone, which did not generally extend far away than 413 K, could correspond to the removal of physically adsorbed water [10], arising from alkylation reaction. Weight losses at this stage were under 6%. (ii) Second step corresponds to the progressive removal of water and/or organic compound. TG curves showed a regular decrease in the 413-643 K region, constituting about 1,6% weight loss. (iii)In the third stage, a broad exothermal zone was displayed in DTA curves in the range 643-923 K in oxidative atmosphere and 643-1023 K for inert atmosphere, with weight losses being associated about 4%, and may be attributed to the removal of the carbonaceous compounds (coke and/or basic organic). These features were confirmed by DRIFT spectra of deactivated and reactivated catalysts (see below). On the other hand, XRD peak of the fresh A1-MCM-41 samples decreased in intensity after catalytic reaction, irrespective to the regeneration process (not shown). Thus, A1-MCM-41 structures are deteriorated as a consequence of the water produced in the alkylation of aniline with methanol, which is known well in the literature.

TG (mg, ~ )

DTG (mg min-1,...)

ATD (~V . . . .

0.0 -0.5

,,

.

/-

-3 1 A1-B-10, in air (50 mL min ) _ -6

/i ~ ~ f// ~ -1.0 - \. "~ ;.~" -0.09 ~

-9

-1.5 373

473

573

673

773

873

973

1073

Temperature / K Figure 2. TG, DTG and ATD obtained for the deactivated A1-B-10 sample.

-12 1173

)

1305

3739

A

B

"~ / N ~ 9

4000

3500

3000

1800

Wavenumber / cm -1

1700

1600

~

,,,

"~"

i

,4j

1500

1400

Wavenumber / cm -1

Figure 3. DRIFT spectra of A1-B-10 sample: a, a') after the reaction at 573 K for 12 h; b, b') reactivated at 823 K under oxygen flow (50 mL min-1); and c, c') reactivated at 873 K under oxygen flow (120 mL mina). Figure 3 shows the DRIFT spectra of the deactivated A1-B-10 catalyst as well as the reactivated one, obtained under thermal conditions and oxidative atmosphere. As M. Rozwadowski et al. [10], we found two main frequency regions in which the detected band occur (A:4000-2800 and B:1800-1350 cma). These bands could be attributed to paraffinic, olefinic and/or aromatic compounds, and coke (Figure 3Aa and 3Ba'). After the reactivation of the A1-B-10 catalyst at 823 K in flowing 02 (120 mL mina), it could be observed a broad band occurring at ca. 1625 cm -a (Figure 3.Bc') that is know as the coke band. Consequently, our reactivation procedure was not enough for the total removal of the coke, as could be expected from TG-DTA results. 60 50 ~" 40 o

g X

.

-

~

.

AI-A- 10

~

zx

30

20 10 ! 0

I

I

I

I

I

I

2

4

6

8

10

12

Time on stream (h) Figure 4. Time-on-stream dependence of aniline conversion at 573 K over A1-A-10 catalyst: (o) fresh catalyst, (0) reactivated at 823 K under nitrogen flow (50 ml minl), (V) reactivated at 823 K under oxygen flow (50 ml min-1), and (A) reactivated at 823 K under oxygen flow ( 120 ml min-1).

1306 Furthermore, the thermal reactivation at 823 K of deactivated A1-X catalysts (12 h on stream) strongly depends on the inert, reductive or oxidative atmosphere (N2, H2 or 02, respectively) and on its flow rate during the reactivation treatment. Thus, inert and reductive treatment did not improve the catalytic activities, whereas oxidative treatment restores up to ca. 80% of the AI-X catalysts activities in flowing 02 at 120 mL min 1 (Figure 4). However, additional thermal reactivation studies were not required because of A1-MCM-41 structure deteriorate with reaction water (see above) and, thus, the re-used catalysts would be described as amorphous aluminosilicates. 4. CONCLUSSION The number of active sites responsible for the mono-methylation of aniline did not increase with aluminum content and, besides, its decreased with time on stream on A1-MCM-41 catalysts. Thus, the deactivation of aluminosilicate MCM-41 catalysts can be explained in two ways: 1. MCM-41 structure deteriorates as a consequence of the reaction water in the alkylation of aniline with methanol. 2. The adsorption of aniline and/or its derivatives as well as coke formation on aluminosilicate MCM-41 is confirmed by DRIFT bands in 3000-2800cm -1 and 1700-1400 cm -1 regions. This fact can produce the pores occlusion and the lost of catalytic active sites. This research was subsidized by grants from Direcci6n General de Investigaci6n (Project BQU2001-2605), Ministerio de Ciencia y Tecnologia, and from the Consejeria de Educaci6n y Ciencia (Junta de Andalucia). References 1. G. 0ye, J. Sj6blom and M. St6cker, Adv. Colloid Interface Sci., 439 (2001) 89/90. 2. S. Narayanan and K. Deshpande, Appl. Catal. A, 199 (2000) 1. 3. M. Gr/in, K.K. Unger, A. Matsumoto and K. Tsutsumi, Microporous Mesoporous Mater., 27 (1999) 207. 4. J.M. Campelo, A. Garcia, D. Luna and J.M. Marinas, J. Mater. Sci., 25 (1990) 2513. 5. A.A. Romero, M.D. Alba and J. Klinowski, J. Phys. Chem. B, 102 (1998) 123. 6. J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, Thermochim. Acta, 265 (1995) 103. 7. J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and J.J. Toledano, Stud. Surf. Sci. Cat., 135 (2001) 281. 8. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, Stud. Surf. Sci. Cat., 108 (1997) 123. 9. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and M.R. Urbano, J. Catal., 172 (1997) 103. 10. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, Appl. Catal. A, 166 (1998) 39. 11. M. Rozwadowski, M. Lezanka, J. Wloch, K. Erdmann, R. Golembiewski and J. Kornatowski, Chem. Mater., 13 (2001) 1609.