Gas-Phase Methylation of Phenol over IronâChromium Catalyst

Article pubs.acs.org/IECR

Gas-Phase Methylation of Phenol over Iron−Chromium Catalyst Paweł Łysik, Agnieszka Górska,* and Stefan Szarlik Industrial Chemistry Research Institute, Department of Organic Technology and Separating Processes, Rydygiera 8 Street, 01-793 Warsaw, Poland ABSTRACT: Phenol methylation to 2,6-dimethylphenol was investigated in gas-phase conditions using an iron−chromium catalytic fixed bed. The experiments were carried out at 603 K, at the atmospheric pressure. The long-term stability of the catalyst and the effects of the reactor packing on the phenol conversion in one- and three-section reactor were studied. It was found that, in both types of the reactor, the phenol conversion exceeded 99%. 2,6-Dimethylphenol, 2-methylphenol, and 2,4,6trimethylphenol were identified in the outlet liquid, and methane, hydrogen, and carbon oxides were identified in the outlet gas. Selectivity to the main liquid product: 2,6-dimethylphenol of about 85% was obtained. Phenol conversion and selectivity to 2,6dimethylphenol decreased during the process, and therefore, the regeneration of the catalyst was necessary.

1. INTRODUCTION 2,6-Dimethylphenol is of high industrial importance, especially in the plastics industry.1 It is the monomer of several polymers and plastics used, for example, for the production of poly(2,6dimethyl)phenylene oxide resin.2,3 2,6-Dimethylphenol is produced by the alkylation of phenol.4 Phenol methylation can be performed either in the liquid phase (homogeneous or heterogeneous) or in the gaseous phase.5 It is known that gas phase processes carried out at the atmospheric pressure constitute a preferred technology due to their relatively low cost. Particularly, in such processes the use of a heterogeneous catalyst which, in addition to its high activity and stability, also demonstrates a high selectivity to receiving the expected product, is preferred. A wide spectrum of catalysts for the gasphase methylation of phenol have been reported in the literature, including a catalyst containing iron and chromium oxides.6−10 Such an iron−chromium catalyst is a TZC-3/1 catalyst from Grupa Azoty S.A. This catalyst is used industrially for a high temperature conversion of carbon oxide with water vapor in the production of hydrogen, syngas and ammonia. It consists of Fe2O3, Cr2O3, and CuO.11 In the previous paper, we studied the gas-phase alkylation of phenol with methanol on the industrial iron−chromium catalyst.12 The influence of the thermal effects in the catalyst bed, process temperature and the effect of catalyst loading were already studied. We found that the TZC-3/1 catalyst was active in the phenol methylation process. In this work, the effect of the activity of the TZC-3/1 catalyst during long-term experiments is investigated. This process was the initial stage of the testing in a pilot plant.

Figure 1. Effect of reaction time on the phenol conversion over iron− chromium catalyst at 603 K in the process carried out in a one-section reactor.

conversion in a long time process had a tendency to a slight decrease from 99.9% to 98.7% after 130 h of the process. High levels of phenol conversion were maintained for about first 10 h of the process and then slightly decreased. The catalyst regeneration cycles led to the restoration of phenol conversion to the level of 99%. The main products in the phenol methylation were 2,6dimethylphenol (2,6-xylenol), o-cresol (2-methylphenol), and 2,4,6-trimethylphenol. The change of the selectivity to various phenol derivatives for a long time experiment and the effect of regenerating the TZC-3/1 catalyst are shown in Figure 2.

2. RESULTS AND DISCUSSION 2.1. Stability of the Catalyst in Long Time. The catalysts deactivated during the alkylation of phenol with methanol because of coke formation. In Figure 1, the TZC-3/1 catalyst performance in the long time phenol methylation process is shown. After 40 and 80 h of the process, catalyst regeneration by the purging of the catalyst bed with air was performed. It was observed that the catalyst regeneration after 40 and after 80 h led to the restoration of the TZC-3/1 activity. The phenol © 2014 American Chemical Society

Received: Revised: Accepted: Published: 17558

July 20, 2014 October 14, 2014 October 16, 2014 October 16, 2014 dx.doi.org/10.1021/ie5028952 | Ind. Eng. Chem. Res. 2014, 53, 17558−17562

Industrial & Engineering Chemistry Research

Article

Figure 2. Effect of reaction time on the selectivity of the products for the phenol methylation over iron−chromium catalyst at 603 K in the process carried out in a one-section reactor.

Figure 3. Effect of reaction time on the selectivity of the products for the phenol methylation over iron−chromium catalyst at 603 K for process carried out in a three-section reactor.

the phenol methylation process in the three-section reactor was 2,6-dimethylphenol. Selectivity to 2,6-dimethylphenol in the first hours of the process was about 82%, then selectivity to 2,6dimethylphenol decreased down to about 76%. In this process, 2,4,6-trimethylphenol was also identified among products. Similarly to the one-section process, selectivity to o-cresol increased during the process run from 7% to 16.6%. Selectivity to 2,4,6-trimethylphenol (about 4.7%) was higher than in the one-section process. The regeneration of the catalyst (after 40 h) led to the restoration of high selectivity to 2,6dimethylphenol and lower selectivity to o-cresol. The change in the load of the reactor did not affect in a significant way the process of phenol methylation. The effect of the reaction time on the selectivity to 2,6-dimethylphenol for the process of phenol methylation in one- and three-section reactor is shown in Figure 4. Selectivity to 2,6-dimethylphenol in both processes is similar - it is lower for the three-section reactor.

Selectivity to 2,6-dimethylphenol reached the value of about 87% (after 48 h). Over time, selectivity to 2,6-dimethylphenol decreased down to 74%. In a long time process, an increase in selectivity to o-cresol from 7.9% up to 12.3%. was observed. At the same time, selectivity to 2,4,6-trimethylphenol remained at about 2.5%. The catalyst regeneration after 40 h led to the reclaim of high selectivity to 2,6-dimethylphenol and lower selectivity to o-cresol. Such an effect was maintained for a relatively short time, about 20 h. There was no significant effect on selectivity to 2,4,6-trimethylphenol of the catalyst regeneration. 2.2. Effects of Reactor Packing. The synthesis of 2,6dimethylphenol was carried out using also a three-section reactor. The long time process of the phenol methylation was carried out for 90 h, with one regeneration of the catalyst performed after 40 h (Figure 3). The conversion of phenol in the process carried out in the three-section reactor varied from 99.5% to 98.1%. In a one-section process, the main product of 17559

dx.doi.org/10.1021/ie5028952 | Ind. Eng. Chem. Res. 2014, 53, 17558−17562


Article

Figure 4. Effect of reaction time on the selectivity to 2,6-dimethylphenol for the process of phenol methylation carried out at 603 K over iron− chromium catalyst in one- and three-section reactors.

the system presented in Figure 5. The main reaction liquid products were 2,6-dimethylphenol, o-cresol, and 2,4,6-trimethylphenol. The main gaseous reaction products were methanol, methane, hydrogen, carbon dioxide, and carbon monoxide. 2.4. Catalyst Characterization. In Figure 6, XRD spectra of fresh and worn-out catalyst are shown. The catalyst composition changed after the process. During the process, the goethite FeO(OH) was reduced to Fe2O3; small signals from the FeO(OH) are not present on the X-ray diffraction pattern of the TZC-3/1 catalyst after phenol methylation. The chemical composition of the iron−chromium catalyst (Table 2), measured by the flame atomic absorption spectroscopy (FAAS) method, is practically the same before and after the process. This fact means that the process does not change the structure of the catalyst. Scanning electron microscopy (SEM) of the iron−chromium catalyst before the process (fresh catalyst) and after the process, shown in Figure 7 provides a direct visual comparison of particle morphology. SEM image shows that catalyst before and after the process is rather similar, but it can be seen that the long time process can result in a higher amount of larger crystallite sizes. However, the purging process of the catalyst bed with air which was performed at 503 K, caused the temperature increase up to around 670 K, thus the size of the crystallites aggregate which perhaps arose out of regeneration during the calcination process.

The reason the result of one-section was better than that of three-section supposedly was that the space of the reaction in the three-section reactor increased by approximately 8%. Consequently the residence time was extended. Increasing the residence time could result in a decrease in the conversion of phenol. 2.3. Material Balance for the Process of the Methylation of Phenol on the Iron−Chromium Catalyst. Simplified material balance calculations for liquid and gaseous phases are shown in Table 1. The calculations were made for Table 1. Material Balance Calculations for the Phenol Methylation over Iron−Chromium Catalyst at 603 K in the Process Carried out in One-Section Reactor stream no. component (g)

1

2

3

phenol methanol o-cresol 2,6-dimethylophenol 2,4,6-trimethylophenol water hydrogen methane carbon monoxide carbon dioxide others sum

104.0 176.0 0.0 0.0 0.0 20.0 0.0 0.0 0.0 0.0 0.0 300.0 300.0

1.0 0.3 12.2 114.8 8.0 16.7 0.0 0.0 0.0 0.0 0.0 153.0 300.9

0.0 23.7 0.0 0.0 0.0 0.6 9.4 4.8 4.8 88.6 16.0 147.9

3. CONCLUSIONS In the process of the production of 2,6-dimethylphenol on the iron−chromium catalyst in the one-section reactor, a decrease in the activity of the catalyst during the process was observed. A decrease in the activity of the TZC-3/1 catalyst caused a decrease in 2,6-dimethylphenol in the product and an increase in o-cresol. The decreasing activity resulted in a reduction in the degree of conversion of phenol. The same effects were also observed during the experiments in the three-section reactor. However, slightly better results were obtained for the process of the phenol methylation in the one-section reactor. The division

Figure 5. Simplified scheme of the experimental setup for material balance calculations: 1, feed; 2, liquid products; 3, gaseous products. 17560



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Figure 6. X-ray diffraction pattern of the TZC-3/1 catalyst before and after being used in the phenol methylation process: maghemite (Fe2O3), black triangle; goethite (FeO(OH)), black dot.

Table 2. Composition of the TZC-3/1 Catalyst before and after Being Used in the Phenol Methylation Process chemical composition

before process (wt %)

after process (wt %)

Fe2O3 Cr2O3 CuO

74.9 9.5 2.4

75.1 9.9 2.4

Figure 8. Simplified scheme of the experimental setup: 1, feed tank; 2, dosing pump; 3, heater (evaporizer); 4, reactor; 5, receptacle.

with a 30 m HP-PlotQ column (inner diameter = 0.32 mm, film thickness = 20 μm) and a thermal conductivity detector. Phenol conversion was calculated as

of the catalyst into sections did not give better results, but only complicated the introduction of such a deposit into the reactor.

phenol conversion (%) ⎡ concentration of phenol at the outlet ⎤ = 100 × ⎢1 − ⎥ concentration of phenol at the inlet ⎦ ⎣

4. EXPERIMENTAL SECTION 4.1. Gas-Phase Phenol Methylation. The gas-phase methylation of phenol was carried by the continuous method in the reaction system presented in Figure 8. Methanol, phenol, and water were warmed up to 553−573 K in an evaporizer (3 on Figure 8). Gases were fed using a dosing pump supplying from the top the reactor. The reactor was heated to 603 K. The gases from the reactor were cooled in a receptacle and then separated into the liquid and the gas phases (5 in Figure 8). The liquid samples were collected periodically for GC analysis. The liquid products of the phenol methylation were analyzed using a Hewlett-Packard 5890 chromatograph equipped with a 30 m HP-5 column (inner diameter = 0.32 mm, film thickness = 0.25 μm) and a flame ionization detector. Gaseous products were analyzed using Agilent 7890A chromatograph equipped

The selectivity to n component of the product (n = 2,6dimethylphenol, o-cresol, 2,4,6-trimethylphenol) was calculated as selectivity to n component of the product (%) concentration of n component produced = 100 × ∑ concentration of ni component produced

To study the stability of the catalyst in a long time, experiments were carried out continuously for about 130 h. Phenol, methanol and water (molar ratio 1:5:1) were introduced continuously into the reactor at a load of 0.05 g

Figure 7. SEM image of the TZC-3/1 catalyst before (A) and after (B) being used in the phenol methylation process. 17561



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of the phenol/g of the catalyst × h−1 under atmospheric pressure and at a temperature of 603 K. Additionally, after 40 and 80 h of the process of the purging of the catalyst bed with air at 503 K was performed in order to test if the catalyst could be regenerated to achieve the same activity as at the beginning of the process. The temperature increased during the firing of hydrocarbons to 670 K. The long-term catalytic behavior of TZC-3/1 catalyst for the alkylation of phenol with methanol was studied in a fixed-bed and down-flow steel reactor with an external diameter of 32 mm and 910 mm long (Figure 9a).

of the catalyst was carried out on a HZG 4 Seifert GmbH X-ray diffractometer using monochromatic CuKα radiation. The XRD data were collected from 10 to 80°(2θ) with a scanning rate of 2° (2θ)/min at ambient temperature. Moreover, Fe, Cr, and Cu content of the catalyst was determined using Flame Atomic Absorption Spectroscopy (FAAS) PerkinElmer, AAnalyst 800. Scanning Electron Microscopy (SEM) observations were performed on a JEOL JSM6490-LV electron microscope using SE detector.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the project No. UDAPOIG.01.03.01-14-058/09-00 “Complex Technology for Production of Engineering Polymers Based on Poly(phenylene oxide)” realized within The Operational Programme Innovative Economy, financial perspective between 2007-2013 is kindly acknowledged.

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REFERENCES

(1) Berkowicz, G.; Ż ukowski, W.; Baron, J. Effect of temperature on o-cresol methylation in a fluidized bed of commercial iron-chromium catalyst TZC-3/1. Polym. J. Chem. Technol. 2013, 15/3, 100. (2) Sad, M. E.; Padró, C. L.; Apesteguía, C. R. Study of the phenol methylation mechanism on zeolites HBEA, ZSM5 and HCMC22. J. Mol. Catal. A: Chemical 2010, 327, 63. (3) Cavani, F.; Maselli, L.; Passeri, S.; Lercher, J. A. Catalytic methylation of phenol on MgOSurface chemistry and mechanism. J. Catal. 2010, 269, 340. (4) Cresols and xylenols. Ullmann’s Encyclopedia of Industrial Chemistry, 5th completely revised ed.; Wiley-VCH: Weinheim, Germany, 1987; Vol. A8, p 47. (5) Crocella, V.; Cerrato, G.; Magnacca, G.; Morterra, C.; Cavani, F.; Maselli, L.; Passeri, S. Gas-phase phenol methylation over Mg/Me/O (Me = Al. Cr, Fe) catalysts: Mechanistic implications due to different acid-base and dehydrogenating properties. Dalton Trans. 2010, 39, 8527. (6) Biały, J.; Penczek, I.; Kopytowska, N.; Wrzyszcz, J.; Ć mielewska, J.; Grabowska, H.; Kaczmarczyk, W.; Mazur, K.; Mista, W. Sposób wytwarzania katalizatora procesu alkilowania fenolu lub o-krezolu. PL Patent 143487, 1984. (7) Baron, J.; Berkowicz, G.; Ż ukowski, W.; Kandefer, S.; Szarlik, S.; Zielecka, M.; Wielgosz, Z.; Jamanek, D. Synthesis of 2,6dimethylphenol in a catalytic fluidized bed of industrial iron-chromium catalyst. Przem. Chem. 2013, 95/2, 853. (8) Ito, M. Method for alkylation of phenols, and catalyst therefor. U.S. Patent 5059727, 1991. (9) Yonemitsu, E.; Togo, S.; Hashimoto, K.;. Ito, M.; Nishizawa, C.; Hara, N. Process for ortho-alkylation of phenol compounds. US Patent 3953529, 1976. (10) Ito, M. Catalyst for alkylation of phenols. US Patent 5128304, 1992. (11) Grupa Azoty. Iron-chromium catalyst. http://att.grupaazoty. com/en/oferta/chemikalia/2/3. (12) Łysik, P.; Górska, A.; Szarlik, S. Synthesis of 2,6-dimethylphenol by alkylation of phenol with methanol in gas-phase over iron catalyst. Przem. Chem. 2013, 92/5, 685.

Figure 9. Scheme of one-section reactor (a) and three-section reactor (b): i, inert balls A−C, catalyst beds.

The influence of the reactor packing was investigated using one- or three-section reactor (a and b on Figure 9) containing TZC-3/1 catalyst. The reactors in this process were filled with 400 mL of the catalyst with the particle size of about 6 × 6 mm. To hold this catalyst, the inert glass balls placed on the top and at the bottom of the reactor. The catalyst bed was held between these layers. In the first case, the catalyst consisted of one layer fixedbed (one-section reactor, Figure 9a). In the second case, the catalyst was divided into three layers fixed-bed sections (threesection reactor, Figure 9b). Among the beds were steel shavings. The inert balls formed upper and lower layers. A three layer reactor was used to induce the temperature profile. The reactors were heated by the outer electrical heater and the temperature was measured by the mobile thermocouple placed in the tube positioned concentrically inside the reactor. 4.2. Catalyst Characterization. Synthesis of 2,6-dimethylphenol was carried out using an industrial iron−chromium catalyst (TZC-3/1) produced by Grupa Azoty S.A, Poland. The main component of the TZC-3/1 catalyst in its unreduced form is iron(III) oxide. Chromium(III) oxide is a stabilizer and promoter of the catalyst. Copper(II) oxide enhances the catalyst selectivity and, to some extent, its activity. Catalyst physicochemical properties were analyzed by means of XRD, SEM, and AAS. The X-ray powder diffraction (PXRD) 17562


Gas-Phase Methylation of Phenol over IronâChromium Catalyst

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