tert-Butylation of diphenylamine over zeolite catalysts

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Applied Catalysis A: General 335 (2008) 74–81 www.elsevier.com/locate/apcata

tert-Butylation of diphenylamine over zeolite catalysts Part 1: Catalyst screening and optimization of reaction conditions Gabriel Kostrab a, Martin Lovicˇ a, Ivan Janotka b, Martin Bajus a, Dusˇan Mravec a,* a

Institute of Organic Chemistry, Catalysis and Petrochemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinske´ho 9, 812 37 Bratislava, Slovak Republic b Institute of Construction and Architecture, Slovak Academy of Sciences, Du´bravska´ cesta 9, 845 03 Bratislava, Slovak Republic Received 4 July 2007; received in revised form 12 November 2007; accepted 13 November 2007 Available online 19 November 2007

Abstract tert-Butylation of diphenylamine (DPA) with tert-butanol (TBA) in the liquid phase was studied over various zeolite catalysts H-MOR, H-BEA, H-Y and H-ZSM-5 (H-MOR CBV 21A and 90A, H-BEA CP 811E, 814E and 814Q, H-Y CBV 720 and H-ZSM-5 CBV 5020) to evaluate whether zeolite catalysts can be efficient as an alternative to the other catalysts in preparation of tert-butylated diphenylamines. The influence of catalyst and the reaction conditions on tert-butylation of diphenylamine with TBA as alkylating agent on catalytic activity and para-selectivity was studied. Catalyst screening revealed that H-BEA and H-Y are the most suitable zeolite catalysts in the alkylation of DPA with TBA. Among them, H-BEA CBV 814E showed the best results in the catalytic activity (91% conversion of DPA), selectivity (99% selectivity to 4TBDPA, 91% selectivity to 4,40 -DTBDPA) and dialkylation (0.62 4,40 -DTBDPA/4-TBDPA ratio). Subsequent optimization of reaction conditions in the tert-butylation of DPA with TBA over H-BEA CBV 814E showed that the most suitable reaction temperature is 180 8C. The study of influence of reactant’s molar ratio showed that TBA/DPA molar ratio 4:1 is the most suitable, but the excess of TBA is so high that it produces significant amount of isobutylene dimer (15%). However, TBA/DPA molar ratio 2:1 is comparable to 4:1, it is even better when considering the most important parameter—selectivity to 4,40 -DTBDPA and produces almost no isobutylene dimer (below 1%). The amount of catalyst charged into reaction system also significantly influences reaction parameters. Study shows that within wide range of catalyst charge (2.5% up to 30% as of DPA charge), the highest reasonable amount of catalyst with the best outcome can be obtained with 0.7 g (20% as of DPA charge) of catalyst charge in the reaction system. tert-Butylation of diphenylamine with tert-butanol over H-BEA zeolite catalyst appears to be an alternative to other catalysts in industrial preparation of tert-butylated diphenylamines. This paper presents the first original complex outlook on the tert-butylation of diphenylamine over zeolite catalysts. # 2007 Elsevier B.V. All rights reserved. Keywords: Alkylation; tert-Butylation; Diphenylamine; Zeolites; para-Selectivity

1. Introduction The acid catalyzed reactions as alkylation, acylation, isomerisation, etc. are important processes in hydrocarbon technology, petrochemistry as well as in fine chemicals syntheses [1,2]. Abbreviations: DPA, diphenylamine; TBA, tert-butanol; 4-TBDPA/4,40 , TBDPA-4-tert-butyldiphenylamine/4,40 -di-tert-butyldiphenylamine; XDPA, conversion of DPA; S4-TBDPA, selectivity = (4-TBDPA/STBDPA)  100; S4,40 -DTBDPA, selectivity = (4,40 -DTBDPA/SDTBDPA)  100. * Corresponding author. Tel.: +421 2 59 32 53 27; fax: +421 2 52 49 31 98. E-mail address: [email protected] (D. Mravec). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.11.019

Alkylated diphenylamines (dialkylated in fact) are important chemicals which are utilized as additives for stabilizing organic products that are subjected to oxidative, thermal, and/or lightinduced degradation. The additives can be added to numerous organic products widely used in engineering, for example, lubricants, hydraulic fluids, metal-working fluids, fuels, or polymers, to improve their performance properties. Alkylated diphenylamines (dialkylated) are industrially produced by alkylation of diphenylamine with diisobutylene (DIB), oligomers of isobutylene (IB) and propylene, 1-nonene, styrene, a-methylstyrene (AMS) and others using Friedel– Crafts alkylation catalysts to produce tert-butylated, octylated, nonylated, styrenated and cumylated diphenylamines which are

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the most important chemicals used in antioxidant formulations [3]. Typical Friedel–Crafts alkylation catalysts (e.g. AlCl3), F-C. alkylation catalysts combined with non-catalyst solids as support or in the presence of an ionic liquid can be efficiently used in production of various alkylated diphenylamines [4–6]. Also clay alkylation catalysts reported in the patent literature are claimed to be excelent in production of various alkylated diphenylamines [7–16]. Acid-treated and rare earth-modified clays have received considerable attention as catalysts for this industrially important reaction [17,18]. The F-series acidtreated clays provided by Engelhard are especially active catalysts for the alkylation of DPA with a-methylstyrene as the alkylating agent [18]. The desired alkylation products are mono and dicumyldiphenylamines (MCDPA and DCDPA). Particularly Engelhard F-24 was found to be a very effective catalyst for the alkylation of DPA with AMS and DIB. It was possible to obtain high selectivity of each of the alkylated products by a proper optimization of the reaction conditions. Authors also optimized regeneration of the used catalysts by partially regenerating of the deactivated catalysts with methanol under reflux and also by the use of ultrasonic irradiation. The conversion of DPA and AMS to the antioxidants MCDPA and DCDPA was studied by Liu et al. [19] over mesostructured aluminosilicate catalysts with different pore structures (hexagonal 2% Al-MCM-41, wormhole 2% Al-HMS and lamellar/vesicular 2% Al-MSU-G) and compared to commercial acid-treated clay catalyst Engelhard F-20 and supported H3PW12O40. All of the solid acid catalysts examined in their study were effective in converting DPA to MCDPA in an initial alkylation step using AMS. However, all three mesostructures are substantially more selective in comparison to the commercial F-20 clay and supported H3PW12O40 catalysts for the conversion of MCDPA to desired DCDPA antioxidant in a second alkylation step. The superior selectivity of the aluminosilicate mesostructures for the second alkylation process was attributable in part to an acid strength that allowed for protonation of the AMS and coadsorption of MCDPA, while at the same time minimizing the surface concentration of AMS for conversion to undesired dimers. Although zeolite catalysts have been successfully used for the production of fine organic chemicals [20–22] they have limited utilization for the transformations of large molecules in the liquid state due to diffusion limitations caused by the restricted pore size. The question remains whether these limitations can be considered as absolute or exceptions can be found. In our previous work, we have investigated catalytic activity and selectivity of large-pore commercial zeolites H-MOR, HBEA and H-Y, the influence of cerium modification of H-MOR and the influence of water in the liquid phase tert-butylation of toluene with tert-butanol and isobutylene [23–25]. Since our work is primarily focused on utilization of shape-selective zeolite catalysts in alkylation of aromatic compounds and to our best knowledge there are no papers reporting the use of zeolite catalysts in the production of alkylated/dialkylated diphenylamines, we were strongly challenged to investigate the

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possibility of utilization of zeolite catalysts in the alkylation of DPA despite of the facts mentioned in the previous paragraph. In the present work, we have investigated the possibility of utilization of zeolite catalysts in the alkylation of DPA with tertbutanol as alkylating agent in the liquid phase over different zeolite catalysts. The influence of catalytic activity and selectivity of various zeolite catalysts on alkylation of DPA with tert-butanol is evaluated, compared and discussed in this paper. 2. Experimental 2.1. Catalysts and chemicals Commercially avialable zeolite catalysts H-MOR (CBV 21A and CBV 90A) and H-BEA (CP 811E, CP 814E and CP 814Q), US H-Y (CBV 720) and H-ZSM-5 (CBV 5020) from Zeolyst Int. were used as alkylation catalysts. Characteristic properties of these zeolites are in Table 1. All zeolite catalysts were used as purchased. The samples of catalysts were activated by calcination in a stream of dry air at 500 8C during 6 h. All chemicals (diphenylamine, tert-butanol, and nheptane) were analytical grade purity purchased from Sigma–Aldrich GmbH, Germany. 2.2. Apparatus, procedure and analysis The alkylation of diphenylamine was carried out in a laboratory autoclave reactor (100 ml). In a typical run, diphenylamine (20 mmol), tert-butanol (40 mmol), 70 ml nheptane as a solvent and 0.7 g of freshly calcined zeolite catalyst kept at 200 8C after calcination were used in the case of alkylation reactions (charge of reactants and catalyst may vary in the experiments, see next chapters). The reactor was flashed thrice with nitrogen to replace air. Alkylation reactions were carried out at 180 8C and at the autogenous pressure. The samples of the reaction mixture were withdrawn periodically from the closed reactor and were analysed on CHROMPACK 9002 gas chromatograph equiped with CP Sil 5 CB column (25 m  0.53 mm) and FID detector. The temperature program was: 110 8C (5 min), from 110 to 275 8C with a slope of 3 8C/min. For quantitative analysis the method of the external standard was used. Table 1 Characteristics of zeolites Zeolite (Si/Al)

SBET (m2/g)

Vmicro,t (cm3/g)

Aciditya (mmol/g)

H-MOR (10) CBV 21A H-MOR (45) CBV 90A H-BEA (37.5) CP 811 E H-BEA (12.5) CP 814 E H-BEA (25) CP 814 Q H-ZSM-5 (25) CBV 5020 H-Y (15) CBV 720

485 512 670 707 613 445 784

0.197 0.193 0.270 0.196 0.225 0.139 0.286

1.38 0.38 0.55 1.03 0.68 0.68 0.56

a

Total acidity was determined by standard TPDA method.

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The products of the reaction were qualitatively identified on GC/MS QP5000 (Shimadzu) with EI and capillary column (HP-1, 50 m  0.2 mm  0.33 mm), carrier gas was helium (1 ml/min). Temperature program: from 110 8C with gradient 3 8C/min to 270 8C was used. 3. Results and discussion 3.1. Catalysts screening The influence of catalyst type on tert-butylation of diphenylamine (DPA) with tert-butanol (TBA) has been carried out over commercially available zeolite catalysts H-MOR (CBV 21A and CBV 90A), H-BEA (CP 811E, CP 814E and CP 814Q), US H-Y (CBV 720) and H-ZSM-5 (CBV 5020). Since reaction parameters could not be optimized prior to catalyst screening, we have decided to use the ones that we have optimized in our previous work for tert-butylation of mononuclear aromatics (toluene) [23–25]. The main reaction products have been identified as 4-tertbutyldiphenylamine and 4,40 -di-tert-butyldiphenylamine 1 (GCMS and H NMR) in all cases. Other reaction products were identified as 3-tert-butyldiphenylamine, monooctylated diphenylamine, isomers of di-tert-butylated diphenylamine and isomers of tert-butyl octyl diphenylamine. 2-tert-Butyldiphenylamine or other ortho-alkylated diphenylamines (mono or dialkylated) were not present in the reaction mixture (1H NMR). The main desired reaction products are 4-tert-butyldiphenylamine and 4,40 -di-tert-butyldiphenylamine ( para- or para, para’- isomers) which is logical because para- or para, para’- isomers are favoured by shapeselective interaction due to regular channel structure of zeolite catalysts. The formation of ortho- or ortho, ortho’- isomers is hindered by the position of –NH in Ph–NH–Ph and bulky tertbutyl group. As to theoreticaly possible N-alkylation of diphenylamine, there was not any presence of N-tert-butylated diphenylamine in the reaction mixture (1H NMR) probably due to sterical hindrance of –NH and given reaction conditions which, as we assume, prefer N-dealkylation. Fig. 1 shows the influence of catalyst type on tert-butylation of DPA with TBA. The highest conversion of DPA was obtained over all H-BEA catalysts (CP 811E, CP 814E and CP 814Q) and US H-Y (91.5, 93.6, 83.1 and 85.5%, respectively after 120 min). Both H-MOR catalysts (CBV 21A and CBV 90A) and H-ZSM-5 had significantly lower catalytic activity in the transformation of DPA into the alkylated diphenylamines (68.5, 45.8 and 61.2%, respectively after 480 min). The maximal conversion of DPA was obtained over H-BEA CP 814E (93.6%) after 120 min and further prolongation of reaction time showed the small decrease in the conversion of DPA. The same applies to other H-BEA catalysts and also US H-Y. This decrease of conversion suggests that with longer reaction time, the secondary reactions occur and the major one is the dealkylation of alkylated diphenylamines [23,24]. Fig. 2 shows the influence of catalyst type on the selectivity to 4-TBDPA in the tert-butylation of DPA with TBA. It can be clearly seen from Fig. 2 that all catalysts used in the screening

Fig. 1. The influence of catalyst on conversion of DPA (for reaction conditions see Section 2).

are highly selective to 4-TBDPA. Namely H-BEA CP 814 E and H-BEA CP 811E are superb, having 99% selectivity (after 480 min) to 4-TBDPA. The lowest selectivity to 4-TBDPA was obtained over H-ZSM-5 (92.9%). All catalysts maintained or have slightly increased their high selectivity to 4-TBDPA during whole period of reaction time except for US H-Y, which started on low selectivity at low conversion of DPA (80% selectivity to 4-TBDPA, under 50% conversion of DPA), but significantly increased selectivity to 4-TBDPA at high conversion of DPA (over 90% conversion of DPA, 98% selectivity to 4-TBDPA). Fig. 3 shows the influence of catalyst type on the selectivity to 4,40 -DTBDPA in the tert-butylation of DPA with TBA. It can be clearly seen from Fig. 3 that catalysts used in the screening are highly selective to 4,40 -DTBDPA. The highest selectivity to 4,40 -DTBDPA was obtained over all H-BEA catalysts (near 91%). Other zeolite catalysts showed lower selectivity to 4,40 DTBDPA. The poorest results were obtained for H-MOR CBV 90A and H-ZSM-5 (62.5 and 43.5% selectivity to 4,40 DTBDPA, respectively). Fig. 4 shows the influence of catalyst type on 4,40 -DTBDPA/ 4-TBDPA ratio in the tert-butylation of DPA with TBA which

Fig. 2. The influence of catalyst on selectivity to 4-TBDPA (for reaction conditions see Section 2).

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Table 2 The influence of external diffusion (after 240 min)

Fig. 3. The influence of catalyst on selectivity to 4,40 -DTBDPA (for reaction conditions see Section 2).

shows us how efficient the catalyst is to produce more desired dialkylated DPA (4,40 -DTBDPA). It can be clearly seen from Fig. 4 that catalysts used in the screening have different with reaction time increasing 4,40 -DTBDPA/4-TBDPA ratio. The highest 4,40 -DTBDPA/4-TBDPA ratio was achieved over H-Y, significantly lower was observed over all H-BEA catalysts and the poorest results were obtained over H-MOR and H-ZSM-5 catalysts. From the initial catalyst screening at given unoptimized reaction conditions it can be concluded that all H-BEA catalysts and H-Y are those with the highest potential in the alkylation of DPA with TBA over zeolite catalysts. Considering all above mentioned parameters (conversion of DPA, selectivity to 4TBDPA and 4,40 -DTBDPA and 4,40 -DTBDPA/4-TBDPA ratio) we have chosen H-BEA CP 814 E to be the only catalyst to continue with in our further studies.

Speed of stirring (min1)

800

1000

1200

XDFA (%) S4-TBDFA (%) S4,40 -DTBDFA (%) 4,40 -DTBDPA/4-TBDPA ratio

79.0 98.6 90.4 0.28

92.0 97.7 87.9 0.56

50.3 99.6 61.7 0.04

stirring has significant influence on conversion of DPA, selectivity to 4,40 -DTBDPA and on 4,40 -DTBDPA/4-TBDPA ratio. The selectivity to 4-TBDPA was only slightly influenced by the intensity of stirring. The speed of agitation at 1000 min1 allows for the highest conversion of DPA and the highest 4,40 -DTBDPA/4-TBDPA ratio, which are the most important factors in the alkylation of DPA. The other experiments were kept at stirring near 1000 min1. 3.3. The influence of reaction temperature

In order to exclude the influence of external diffusion on conversion of diphenylamine in the alkylation of DPA with TBA, we performed alkylation reaction over the most promising H-BEA CP 814E with different intensities of stirring in the range of 800–1200 min1. Table 2 shows the results obtained at the temperature of 180 8C and TBA/DPA molar ratio 2:1 after 240 min. As shown in Table 2, the intensity of

In the next, we have optimized the reaction temperature in the tert-butylation of DPA with TBA over H-BEA CP 814E. The reaction conditions were as follow: intensity of stirring near 1000 min1, charge of reactant as mentioned above in Section 2, TBA/DPA molar ratio 2:1, autogenous pressure and the reaction temperature was in the range from 140 to 200 8C. Fig. 5 shows the influence of reaction temperature on the conversion of DPA. It is obvious from Fig. 5 that with increasing temperature the conversion of DPA increases from 75% (140 8C) up to 92% (180 and 200 8C) and the highest conversion can be obtained after 240 min of reaction time. The influence of reaction temperature on the selectivity to 4TBDPA and 4,40 -DTBDPA is shown in Figs. 6 and 7. Increasing temperature from 140 to 200 8C increases selectivity to 4TBDPA monotonically from 95.5 up to 99.6%. The same applies to the influence of temperature on the selectivity to 4,40 DTBDPA but the 4,40 -DTBDPA selectivity reaches its maximal value (91%) at 180 8C and then it decreases with increasing reaction temperature. Fig. 8 also shows the influence of the reaction temperature on the 4,40 -DTBDPA/4-TBDPA ratio, which is also important

Fig. 4. The influence of catalyst on 4,40 -DTBDFA/4-TBDFA (for reaction conditions see Section 2, multiplied by 100).

Fig. 5. The influence of reaction temperature on conversion of DPA (for reaction conditions see Section 2).

3.2. The influence of external diffusion

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Fig. 6. The influence of reaction temperature on selectivity to 4-TBDFA (for reaction conditions see Section 2).

Fig. 9. The influence of TBA/DPA molar ratio on conversion of DPA (for reaction conditions see Section 2).

temperature of 180 8C might be the optimal one. The conversion of DPA, selectivity to 4-TBDPA and 4,40 -DTBDPA and 4,40 -DTBDPA/4-TBDPA ratio is strongly influenced by reaction temperature. The optimal reaction temperature for tertbutylation of DPA with TBA over H-BEA 814E was found to be 180 8C. In further experiments we have used this optimized reaction temperature. 3.4. The influence of TBA/DPA molar ratio

factor in the tert-butylation of DPA. As seen from Fig. 8 the influence of reaction temperature is rather significant, it increases from 0.25 (140 8C) to 0.62 (180 8C). Further increase of the reaction temperature to 200 8C has negative effect on the ratio, thus decreasing its value to 0.4. The investigation of the influence of reaction temperature clearly confirmed our assumption (seen Section 3.1, setting of reaction conditions prior to optimization) that reaction

The influence of reactant’s molar ratio on the alkylation of DPA with TBA was studied over previously optimized H-BEA CP 814E within wide range (TBA/DPA = 4:1, 3:1, 2:1, 1:1 and 1:2). Other reaction conditions were as previously mentioned in Section 2. The influence of TBA/DPA molar ratio on DPA conversion is shown in Fig. 9. As expected due to high excess of TBA, the maximum conversion of diphenylamine was obtained at TBA/DPA molar ratio = 4:1 and 3:1 (96.4 and 95.3%, respectively after 480 min), whereas the lowest conversion of DPA was reached at TBA/DPA molar ratio = 1:2 (56%). At the equimolar TBA/DPA ratio the conversion was 74%. When 4-TBDPA selectivity is taken into account (Fig. 10), the influence of TBA/DPA molar ratio was not significant at all because the selectivity to 4-TBDPA was suprisingly very high

Fig. 8. The influence of reaction temperature on 4,40 -DTBDFA/4-TBDFA ratio (for reaction conditions see Section 2, 4,40 -DTBDFA /4-TBDFA ratio multiplied by 100).

Fig. 10. The influence of TBA/DPA molar ratio on selectivity to 4-TBDPA (for reaction conditions see Section 2).

Fig. 7. The influence of reaction temperature on selectivity 4,40 -DTBDFA (for reaction conditions see Section 2).

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0

Fig. 11. The influence of TBA/DPA molar ratio on selectivity to 4,4 -DTBDPA and on 4,40 -DTBDFA/4-TBDFA (for reaction conditions see Section 2).

in all TBA/DPA molar ratio cases. The highest, near perfect, was achieved at TBA/DPA molar ratios 1:1 and 1:2 (both 99.5%), slightly lower was at TBA/DPA molar ratio 2:1 and 3:1 (both 99%) and at TBA/DPA molar ratio 4:1 the lowest selectivity to 4-TBDPA was achieved (98.5%). Fig. 11 shows the influence of TBA/DPA molar ratio on the selectivity to 4,40 -DTBDPA. The highest selectivity was achieved at TBA/DPA molar ratio 2:1 (91%). The higher excess of TBA—3:1 and 4:1 showed decrease in 4,40 -DTBDPA selectivity to 86%. At equimolar TBA/DPA molar ratio, 88% selectivity to 4,40 -DTBDPA was obtained. The selectivity to 4,40 -DTBDPA at 1:2 TBA/DPA molar ratio was comparable to the selectivities obtained with higher excess of TBA. At the higher excess of TBA (3:1 and 4:1), significant amount of isobutylene dimer was produced reaching almost 15% when counting dimer and alkylated aromatics (not included in presented results). Fig. 11 also shows the influence of TBA/DPA molar ratio on 4,40 -DTBDFA/4-TBDFA ratio in the tert-butylation of DPA with TBA. It can be clearly seen that with increasing TBA/DPA molar ratio from 1:2 to 4:1 the influence of molar ratio is significant and it increases from 0.16 (1:2) to 1.13 (4:1) after 480 min of reaction time. The study of influence of TBA/DPA molar ratio on tertbutylation of DPA with TBA over H-BEA CP 814E within wide range (TBA/DPA = 4:1, 3:1, 2:1, 1:1 and 1:2) revealed that TBA/DPA molar ratio significantly affects reaction parameters. Increasing TBA/DPA molar ratio increases the conversion of DPA up to 96%. It has no effect on the selectivity to 4-TBDPA since the selectivity is suprisingly high within whole studied TBA/DPA molar ratio range. The selectivity to 4,40 -DTBDFA is less affected by TBA/DPA molar ratio, the maximum selectivity is obtained at TBA/DPA molar ratio 2:1, other molar ratios showed lower selectivity to 4,40 -DTBDFA. 4,40 DTBDFA/4-TBDFA ratio was hugely affected by TBA/DPA molar ratio and increased within studied range from 0.16 to 1.13. It appears that TBA/DPA molar ratio 4:1 is the most suitable but the excess of TBA is so high that it produces significant

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Fig. 12. The influence of amount of catalyst on conversion of DPA (for reaction conditions see Section 2).

amount of isobutylene dimer (15%) which can be easily recycled but it complicates the reaction. On the other hand, TBA/DPA molar ratio 2:1 is comparable to 4:1, it is even better when considering selectivity to 4,40 -DTBDFA and produces almost no isobutylene dimer (below 1%). 3.5. The influence of amount of catalyst The influence of amount of catalyst on the alkylation of DPA with TBA was studied over previously optimized H-BEA CP 814E within wide range (0.0875, 0.175, 0.35, 0.5, 0.7 and 1.05 g or 2.5, 5, 10, 15, 20 and 30%, as the wt.% of DPA). Other reaction conditions were as previously mentioned in Section 2. The influence of amount of catalyst on DPA conversion is shown in Fig. 12. Fig. 12 shows the monotonously increasing influence of catalyst amount on the conversion of DPA. The conversion of DPA increases with increasing amount of catalyst from 65% to almost 92%. The lowest charges (0.0875 and 0.175 g) show the lowest but still reasonable catalytic activity, amounts of 0.35 and 0.5 g are comparable (85%) and the highest conversion of DPA was obtained with amounts of 0.7 and 1.05 g (91 and 92%, respectively).

Fig. 13. The influence of amount of catalyst on selectivity to 4-TBDPA (for reaction conditions see Section 2).

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Fig. 14. The influence of amount of catalyst on selectivity to 4,40 -DTBDPA (for reaction conditions see Section 2).

Fig. 13 shows the influence of catalyst amount on the selectivity to 4-TBDPA. As it can be seen from Fig. 13, the selectivity to 4-TBDPA is very high for all amounts of catalyst charged into the reaction system. It shows the same trend as the previously discussed influence on the conversion. The lowest selectivity is obtained for 0.0875 g (near 98%) and the highest for 1.05 g of catalyst in the reaction mixture, but the difference in the selectivities within the whole studied range is rather minimal. Fig. 14 shows the influence of amount of catalyst on the selectivity to 4,40 -DTBDPA. The effect of the increasing amount of catalyst charged into reaction system exhibits monotonous increase in the selectivity to 4,40 -DTBDPA within whole studied range. The lowest 4,40 -DTBDPA selectivity was obtained with 0.0875 g (77%) and the highest with 0.7 g and 1.05 g (89 and 91%, respectively) of catalyst charged. The other amounts of catalyst showed the selectivity to 4,40 -DTBDPA to be slightly higher than 80%. Fig. 15 shows the influence of catalyst amount on 4,40 DTBDPA/4-TBDPA ratio. As can be seen, the influence of catalyst amount charged into the reaction system is significant, since the 4,40 -DTBDPA/4-TBDPA ratio can be increased from 0.17 up to almost 0.9. The lowest ratio is obviously obtained

with the lowest charge of catalyst (0.0875 g) and as the amount of catalyst increases, the 4,40 -DTBDPA/4-TBDPA ratio increases and reaches the maximum with the highest charge of the catalyst (1.05 g). The study of the influence of catalyst amount charged into the reaction system revealed that with increasing amount of catalyst in the reaction mixture, the increasing effect on the conversion, selectivity to 4,40 -DTBDPA and 4,40 -DTBDPA/4-TBDPA ratio can be observed. The increasing amount of catalyst charged into the reaction system had no significant effect on the selectivity to 4-TBDPA mainly due to the fact that even with the lowest charge of catalyst the selectivity to monoalkylated DPA was very high and thus there was only little potential to improve this parameter. Considering all above mentioned, the most suitable amount of catalyst charged into the reaction system is 0.7 g, since all parameters reached their maximum at this amount. 4. Conclusion tert-Butylation of diphenylamine with tert-butanol in the liquid phase studied over various zeolite catalysts showed the possibility to utilize zeolite catalysts as an alternative to the other catalysts in preparation of tert-butylated diphenylamines. The study of the influence of catalyst and the reaction conditions on tert-butylation of diphenylamine with TBA as alkylating agent on catalytic activity and para-selectivity revealed that H-BEA zeolites are the most suitable zeolite catalysts in the alkylation of DPA with TBA. Among them, HBEA CBV 814E showed the best results in the catalytic activity (91% conversion of DPA), selectivity (99% selectivity to 4TBDPA, 91% selectivity to 4,40 -DTBDPA) and dialkylation (0.62 4,40 -DTBDPA/4-TBDPA ratio). Subsequent optimization of reaction conditions in the tert-butylation of DPA with TBA over H-BEA CBV 814E showed that the most suitable reaction temperature is 180 8C. The study of influence of reactant’s molar ratio showed that with TBA/DPA molar ratio 4:1 is the most suitable but the excess of TBA is so high that it produces significant amount of isobutylene dimer (15%) which can be easily recycled but it complicates the reaction. On the other hand, TBA/DPA molar ratio 2:1 is comparable to 4:1, it is even better when considering the most important parameter—selectivity to 4,40 -DTBDFA and produces almost no isobutylene dimer (below 1%). The amount of catalyst charged into reaction system also significantly influences reaction parameters. Study shows that within wide range of catalyst amount (2.5% up to 30% as of DPA charge), the highest reasonable amount of catalyst with the best outcome can be obtained with 0.7 g (20% as of DPA charge) of catalyst charged into the reaction system. tert-Butylation of diphenylamine with tert-butanol over HBEA zeolite catalyst appears to be an alternative to other catalysts in industrial preparation of tert-butylated diphenylamines. Acknowledgement

Fig. 15. The influence of amount of catalyst on 4,40 -DTBDFA/4-TBDFA (for reaction conditions see Section 2, multiplied by 100).

Authors thank for financial support from Slovak Grant Agency (Project SK-VEGA No. 1/3587/06).

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