Tert-butylation of Toluene with Tert-butyl Alcohol over Realuminated H-mordenite Zeolite*

Zhou Zhiwei (周志偉),Wu Wenliang (武文良), Wang Jun (王軍) and Zeng Chongyu (曾崇余)

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-butylation of Toluene with-butyl Alcohol over Realuminated H-mordenite Zeolite*

Zhou Zhiwei (周志偉),Wu Wenliang (武文良)**, Wang Jun (王軍) and Zeng Chongyu (曾崇余)

College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China

The realuminated H-mordenite catalysts (HM1-4) treated with different concentrations of NaOH and NaAlO2aqueous solutions were prepared, and characterized by inductively coupled plasma (ICP), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR) and temperature-programmed desorption of ammonia, They are of lower Si/Al ratio and higher acid amount while keeping a high relative crystallinity. Their catalytic performances were evaluated with the liquid-phase-butylation of toluene with-butyl alcohol in a 100 ml stainless steel batch reactor equipped with a stirrer. HM2 zeolite catalyst, obtained by treating HM in 0.1 mol·L－1NaOH followed by 0.05 mol·L－1NaAlO2aqueous solution, shows a higher catalytic activity because of its highest acid amount. For HM2 catalyst the influences of reaction conditions on catalytic performance were investigated. The conversion of toluene is 50.3% and the selectivity of--butyltoluene is 74.7% at a temperature of 180°C, 2 of molar ratio of-butyl alcohol to toluene, 4h of reaction timeand0.2 of(catalyst)/(toluene).

realuminated H-mordenite, toluene,-butyl alcohol,--butyltoluene

1 INTRODUCTION

In the liquid-phase-butylation of toluene, the conversion of toluene, the yield and selectivity of--butyltoluene increased with the decrease of the Si/Al ratio of Al-MCM-41 catalysts [8]. From this, one can suggest that zeolites with lower Si/Al ratio may perform higher catalytic activity in the-butylation of toluene. Realumination may be an effective way to lower the Si/Al ratio of zeolite. However, the effect of realumination of zeolites on their catalytic performances in the-butylation of toluene has not been widely reported so far.

In this work, the realuminated HM zeolites were prepared by the successive treatment with NaOH aqueous solution followed by the NaAlO2aqueous solution, and were used as catalysts in the-butylation of toluene. We find that the realuminated HM zeolites treated with appropriate concentrations of NaOH and NaAlO2solutions show substantially enhanced catalytic activities in the-butylation of toluene with-butyl alcohol.

2 EXPERIMENTAL

2.1 Catalyst preparation

A series of realuminated HM zeolite samples were prepared by impregnation method [18]. HM with Si/Al ratio of 11.3 was supplied by Beijing Petrochemical Corporation, China. 5 g of calcined HM zeolite was vigorously stirred in 50 ml of 0.05 mol·L－1aqueous NaOH solution at 50°C for 2 h. The material was recovered by filtration, washed with deionized water, dried at 110°C. The precursor was poured to 0.025 mol·L－1aqueous NaAlO2solution with stirring at 90°C for 6 h, and then recovered by filtration, washed with deionized water until neutral pH, and dried at 110°Covernight. The alkaline-treated samples were convertedinto H-form by three consecutive exchanges in 1 mol·L－1NH4NO3solution followed by air-calcination at 823 K for 5 h. The obtained sample was designated as HM1. HM2, HM3 and HM4 were treated by 0.10 mol·L－1, 0.15 mol·L－1, 0.20 mol·L－1aqueous NaOH solutions, followed by 0.05 mol·L－1, 0.075 mol·L－1, 0.10 mol·L－1aqueous NaAlO2solutions, respectively.

2.2 Catalyst characterization

Temperature-programmed desorption of ammonia (NH3-TPD) was carried out in a quartz U-tube reactor, and 300 mg sample was used for each measurement. The sample was pretreated in He stream at 550°C for 60 min and then cooled down to 100°C. After that, the NH3/He mixture was switched on for 30 min and then was swept by He stream for 40 min. The temperature was raised from 100°C to 600°C at a rate of 10°C·min－1. The consumption of NH3in the reactant stream was recorded by thermal conductivity detector (TCD).

Fourier transform infrared spectroscopy (FT-IR) spectra were registered by using a Thermo Nicolet NEXUS spectrometer in KBr pellets.

The Si/Al ratio of different realuminated HM zeolites was analyzed by inductively coupled plasma (ICP, Optima 2000 DV, PerkinElmer, USA) after the samples had been dissolved in HF solution .

2.3 Tert-butylation of toluene

The catalytic-butylation of toluene with-butyl alcohol was carried out in a 100 ml stainless steel autoclave equipped with a magnetically driven impeller. 0.2 g catalyst was added into a mixture of-butyl alcohol with toluene (various molar ratios) using-hexane as solvent. The reactor was flushed for multiple times with nitrogen to replace air.-butylation reactions were carried out at the initial pressure of 0.6 MPa and at a stirring speed 800 r·min－1. Each reaction was carried out at preset reaction temperatures with various reaction times. At the completion of the reaction, the reactor was cooled down to room temperature and the reaction mixture was analyzed on a SP-6890 gas chromatograph equipped with a SE-30 column (0.25 mm×50 m) and a flame ionization detector (FID).

3 RESULTS AND DISCUSSION

3.1 Catalyst characterization

The XRD patterns of the parent HM sample and the four realuminated samples (HM1-4) are shown in Fig. 1. It can be seen that the intensities of peaks assigned to mordenite zeolite decreased gradually with the increase of the content of the aqueous NaOH and NaAlO2solutions. The relative crystallinities of the realuminated samples were listed in Table 1, which were calculated by the summation of the five highest peaks in Fig. 1 assuming that the crystallinity of HM is 100%. Table 1 tells that HM1 and HM2 gave high relative crystallinities above 95%, whereas HM4 has a low value of 84% mostly because of the remarkable collapse of zeolite framework during the NaOH treatment with a high content (0.20 mol·L－1).

Figure 1 XRD patterns of the different realuminated HM zeolites

Table 1 The Si/Al ratio and relative crystallinities of the different realuminated HM catalysts

FT-IR spectroscopy of various catalysts is shown in Fig. 2. The bands for HM at 594 cm－1, 798 cm－1, 1088 cm－1and 1238 cm－1could be assigned to the vibration of double pentabasic rings, outside couple symmetry elastic vibration of the tetrahedron, antisymmetry elastic vibration of the inside tetrahedron and outside couple antisymmetry elastic vibration of the tetrahedron in the framework of mordenite, respectively [18]. The adsorption band for antisymmetry elastic vibration of the inside tetrahedron for HM4 shifted from 1088 to 1082 cm－1compared to HM sample. The result indicated that aluminum effectively inserted into the framework of mordenite zeolite during the treatment for HM in this work [19]. Table 1 also lists the Si/Al ratio measured by ICP, and shows the clearly decreased Si/Al ratio when the concentration of NaOH and NaAlO2aqueous solution increased for the realumination treatment of HM.

NH3-TPD profiles of different realuminated HM catalysts are shown in Fig. 3. Two ammonia desorption peaks, one at low temperature and another at high temperature, were observed for all the five samples. The total acid amount increased in the order: HM4＜HM＜HM3＜HM1＜HM2. The HM2 showed the highest acid amount. Although HM3 and HM4 possessed the lower Si/Al ratio, the decreased acid amount was observed for them, which is associated closely with their lower crystallinities.

Figure 2 FT-IR spectra of the different realuminated HM zeolites samples

Figure 3 NH3-TPD profiles of the different realuminated HM zeolites samples

3.2 Comparison of catalytic activities

Catalytic performances of different realuminated zeolites in the-butylation of toluene with-butyl alcohol are shown in Table 2. In all cases, the main products were identified as--butyltoluene and--butyltoluene.--butyltoluene was present in the reaction products only in trace amount. The formation of--butyltoluene is hindered by the ortho-position of methyl and voluminous-butyl group. The same steric effect allows only the formation of 3,5-di--butyltoluene (DTBT), where all alkyl groups are in the meta-position [8, 9]. 3,5-di--butyltoluene was also found in the reaction products, but only in trace amount. From Table 2 it can be seen that with the increase of concentration of NaOH and NaAlO2solution, the conversion of toluene increased for HM1 and HM2, and then dropped at higher concentration for HM3 and HM4. The most active catalyst was HM2, giving a toluene conversion of 34.5%, which may be owing to its highest acid amount (Fig. 3). The result is much higher than that in previous report [6](17.6%). The selectivity of-butyltoluene (TBT) remained near 100% over all samples. Rather than HM4, the selectivity of PTBT on other catalysts kept more or less constant, which was about 75.0%. The selectivity of PTBT was only 69.2% on the HM4 zeolite probably because of the partial framework. Because HM2 showed the highest catalytic activity, the following studies on effect of reaction conditions were focused on this catalyst.

Table 2 Catalytic performances of different realuminated HM zeolites in the tert-butylation of toluene with tert-butyl alcohol

①TBT: the molar fraction of TBT in products.

②PTBT: the molar fraction of PTBT in the TBT.

3.3 Effect of reaction conditions

3.3.1

Effect of reaction temperature on the-butylation of toluene over HM2 zeolite is shown in Table 3. Increasing the reaction temperature has a significant effect on both toluene conversion and PTBT selectivity. An increase of the temperature to 180°C led to a significant increase of toluene conversion from 1.7% to 34.5%. The conversion of toluene shows a plateau value (around 34.5%) at 180-200°C. With the further increase of the reaction temperature, the conversion of toluene decreased because the higher temperature caused the dealkylation of TBT with the formation of toluene and isobutene probably. Increasing the reaction temperature also led to a drop of the selectivity of PTBT, probably due to the isomerization of PTBT into MTBT, which is more stable at higher temperature [8]. When the reaction temperature increased up to 220°C, the selectivity of PTBT was only 53.2%.

3.3.2

Influence of the molar ratio of-butyl alcohol to toluene on the catalytic performance of HM2 catalyst is shown in Table 4. The maximum conversion of toluene at the molar ratio of-butyl alcohol to toluene 2 was 34.5%. An excess of-butyl alcohol with the ratio more than 2 has negative effect on toluene conversion. In that case HM2 deactivated, which may be due to oligomerization of the isobutene formed by intramolecular dehydration of-butyl alcohol, carbonization and then covered the active sites [6]. The selectivity of TBT and PTBT slightly rose with the increase of the molar ratio of-butyl alcohol to toluene from 1 to 4.

Table 3 Influence of the reaction temperature on the catalytic performance of HM2 zeolite

Table 4 Influence of the molar ratio of tert-butyl alcohol to toluene on the catalytic performance of HM2 zeolite

3.3.3

The liquid phase reaction of-butylation of toluene was carried out at various reaction times in the presence of HM2; the results are shown in Table 5. As seen from Table 5, the conversion of toluene increased as the reaction time was prolonged. It was 40.1% when the reaction time is up to 4 h. Because the-butylation of toluene is a continuous and reversible reaction [5], the conversion of toluene keeps constant with the reaction time being more than 4 h. One can draw that the-butylation of toluene reaches its steady state when the reaction time is up to 4 h on HM2 zeolite. The selectivity of PTBT decreased because of the isomerization of PTBT to MTBT when the reaction time was further prolonged.

Table 5 Influence of the reaction time on the catalytic performance of HM2 zeolite

Table 6 Influence of catalyst loading on the catalytic performance of HM2 zeolite

3.3.4

The influence of catalyst loading in reaction medium on the-butylation of toluene was investigated, and the results were shown in Table 6. From Table 6 it can be seen that catalyst loading affected the conversion of toluene and the selectivity of PTBT greatly. With the catalyst loading increasing, toluene conversion was largely promoted. When(catalyst)/(toluene) was 0.2, toluene conversion reached 50.3%. It kept constant as the catalyst loading increased further because the-butylation of toluene is a continuous and reversible reaction. The selectivity of PTBT decreased with the increasing of catalyst loading, which may be on account of the isomerization of PTBT to MTBT [7].

4 CONCLUSIONS

In this work, we reveal that the combined treatment of H-mordenite in the aqueous solution of NaOH followed by the aqueous solution of NaAlO2can result in the efficient catalyst for the-butylation of toluene with-butyl alcohol. By adjusting the concentration of treating solutions, the realuminated H-mordenite (HM2) is achieved with a high crystallinity, a lower Si/Al ratio and a substantially enhanced acid amount. At the optimal reaction conditions—reaction temperature 180°C, molar ratio of-butyl alcohol to toluene 2, reaction time 4 hand(catalyst)/(toluene) 0.2—a high conversion of toluene of 50.3% and a considerable selectivity of--butyltoluene of 74.7% is obtained.

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2008-05-20,

2008-11-09.

Major Basic Research Project of Natural Science Foundation of Jiangsu Province Colleges (07KJA53013).

** To whom correspondence should be addressed. E-mail: wwl@njut.edu.cn