Selective phenol methylation to 2,6-dimethylphenol in a fluidized bed of iron-chromium mixed oxide catalyst with o-cresol circulation
© Żukowski et al.; licensee Chemistry Central Ltd. 2014
Received: 25 April 2014
Accepted: 4 August 2014
Published: 16 September 2014
2,6-dimethylphenol (2,6-DMP) is a product of phenol methylation, especially important for the plastics industry. The process of phenol methylation in the gas phase is strongly exothermic. In order to ensure good temperature equalization in the catalyst bed, the process was carried out using a catalyst in the form of a fluidized bed - in particular, the commercial iron-chromium catalyst TZC-3/1.
Synthesis of 2,6-dimethylphenol from phenol and methanol in fluidized bed of iron-chromium catalyst was carried out and the fluidization of the catalyst was examined. Stable state of fluidized bed of iron-chromium catalyst was achieved. The measured velocities allowed to determine the minimum flow of reactants, ensuring introduction of the catalyst bed in the reactor into the state of fluidization. Due to a high content of o-cresol in products of 2,6-dimethylphenol synthesis, circulation in the technological node was proposed. A series of syntheses with variable amount of o-cresol in the feedstock allowed to determine the parameters of stationary states.
A stable work of technological node with o-cresol circulation is possible in the temperature range of350-380°C, and o-cresolin/phenolin molar ratio of more than 0.48. Synthesis of 2,6-DMP over the iron-chromium catalyst is characterized by more than 90% degree of phenol conversion. Moreover, the O-alkylation did not occur (which was confirmed by GC-MS analysis). By applying o-cresol circulation in the 2,6-DMP process, selectivity of more than 85% degree of 2,6-DMP was achieved. The participation levels of by-products: 2,4-DMP and 2,4,6-TMP were low. In the optimal conditions based on the highest yield of 2,6-DMP achieved in the technological node applying o-cresol circulation, there are 2%mol. of 2,4-DMP and 6%mol. of 2,4,6-TMP in the final mixture, whereas 2,4,6-TMP can be useful as a chain stopper and polymer’s molar mass regulator during the polymerization of 2,6-DMP.
A wide range of applications of products of phenol methylation makes that the process of their preparation the subject of numerous studies -. Strong bacteriostatic , bacteriocidal  and fungicidal properties  result in methyl-substituted derivatives of phenol being used as preservatives in the food industry , antimicrobial agents in the pharmaceutical industry , in decontamination and disinfection agents, such as Lysol, creolin  and also in the production pesticides .
2,6-dimethylphenol (2,6-DMP) is an important product of phenol methylation, especially for the plastics industry. A condensation of 2,6-DMP molecules takes place in the para position, because of locked ortho positions. The oxidative polymerization of this derivative of phenol leads to the formation of polyphenylene oxide (PPO) , possessing excellent mechanical, dielectric and chemical properties . Thus the PPO is applied in the automotive, electronics, electrical, building, and medical industries -. Apart from the production of PPO, 2,6-dimethylphenol is also used in the production of medicaments ,, pigments  and antioxidants .
There are known methods of preparation of 2,6-dimethylphenol, both in the liquid phase - and in the gaseous phase -. Carrying out the process in the liquid phase is not technologically preferred because of the long reaction time and the necessity of applying high pressure. An additional difficulty is the necessity to separate the catalyst from the products and unreacted substrates . These drawbacks cause the synthesis of 2,6-DMP to be mostly carried out in the gas phase. The reaction of phenol alkylation with methanol is carried out in the presence of various types of catalysts from the group of oxides, mixed oxides -, spinels -,,, and zeolites -.
The process of phenol methylation in the gas phase is strongly exothermic (ΔHor = -134,8 kJ/mol2,6DMP) and the adiabatic temperature rise equals 425°C for the process carried out under stoichiometric conditions at 330°C. In order to ensure good temperature equalization in the catalyst bed, it was proposed to carry out the process in a catalyst in the form of a fluidized bed. Intensive mixing in the fluidized bed catalyst allows maximum utilization of the catalyst surface and good temperature control, as well as ensures good heat and mass transfer at low pressure drop . It is known that iron oxide forms are part of catalysts for the phenol alkylation -. A catalyst TZC-3/1 is one of the industrial iron oxide catalysts. This catalyst is produced by Zakłady Azotowe in Tranów, Poland, and is intended for high-temperature conversion of carbon oxide with water vapor in the processes of obtaining hydrogen, syngas and ammonia . Preliminary research has indicated that the phenol methylation on this catalyst selectively leads to products of C-alkylation.
The purpose of this study is to investigate the influence of process parameters on the synthesis of 2,6-dimethylphenol, and to identify the main and simultaneous reactions, as well as to develop a method for on-line monitoring of the reaction extents and to define the parameters of a stable work of 2,6-DMP node with a o-cresol circulation.
Preparation of the catalyst TZC-3/1 fraction to obtain a stable fluidized-bed
Parameters of TZC-3/1 catalyst
Bed of TZC-3/1 catalyst, which was used during 2,6-DMP syntheses
Bulk density, kg/m3
1 017 ± 1
Pycnometric density, kg/m3
3 602 ± 4
Granular fraction, μm
75 - 150
72 - 90
7.0 - 11.5
0 - 0.095
1.5 - 4.0
Online monitoring of process of 2,6-DMP synthesis on TZC-3/1 catalyst
It can be predicted that the gas mixture leaving the reactor will contain: products of the methylation of aromatics, inorganic and organic components derived from the reaction of catalytic decomposition of methanol as well as unreacted substrates. Due to the ability of these compounds (except H2) to absorb electromagnetic radiation in the infrared range, it was possible to carry out quantitative analysis of the reaction mixture’s composition at intervals of several seconds using a FTIR spectrometer.
The Gasmet DX-4000 apparatus (with the firmware) was used for the analysis of the composition of the after-reaction gaseous mixture. It contains a Michelson interferometer and a gas cuvette with an optical path length of 5.0 m, preceded by particulate filter and heated to 180°C. This apparatus allows for obtaining the IR spectrum of the sample in the wave number range of 800-4000 cm-1. High temperature of 2,6-DMP synthesis and high product concentration in the exhaust gases resulted in the necessity of diluting post-reaction gases in two-steps with inert gases, by simultaneous cooling down to about 160°C, prior to introduction of the gaseous products into the analyzer. Such diluted mixture containing all the products of the reaction was passed through the demister, reheated to 180°C and directed to the gaseous cuvette of FTIR analyzer after filtration on ceramic and Teflon filters.
Evaluation of the synthesis by FTIR analysis
Selected ranges for FTIR analysis and the retention times of GC-MS analysis of components of the process
Range of FTIR analysis, cm-1
Retention time, min
see figure 5
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Determination of the minimum fluidization velocity
It was assumed that evaporated mixture of reagents: phenol, methanol and water, will affect the fluidization agent of the catalyst in the reactor. The reactant flow could not be too large, because its increase causes reduction of contact time between the catalyst and the reactants, but it had to be large enough to ensure stable state of fluidization of the catalyst. Moreover, the volume fraction of bubbles in the catalytic bed increase with rising gas velocity, making it a factor in reducing process efficiency. This results from the fact that the processes in the interiors of bubbles running without contact with the solid phase. This meant that the substrates should be fed to the reactor at a volume flow, providing that their velocity is slightly higher than the minimum fluidization velocity.
Minimum fluidization velocities of the catalyst TZC-3/1
Bed temperature of catalyst, °C
Minimum fluidization velocity, cm/s
Synthesis of 2,6-dimethylphenol from a mixture of phenol:methanol:water in molar ratio 1:5:1
With increased bed temperature, the molar stream of phenol in the post-reaction gases decreased (Figure 7) reaching the value about 6 mmol/h at 360°C, which corresponds to 90% consumption of this substrate. The molar stream of o-cresol - the first product of phenol methylation initially went up and went down because of subsequent alkylation after reaching 320°C. The p-cresol was not found in the product (neither by FTIR nor GC/MS analysis). The maximum yield of 2,6-DMP stream was achieved at 350°C. The streams of 2,4-DMP and 2,4,6-TMP by-products increased, but this increase was small (Figure 7b). At 350°C, the temperature of maximum 2,6-DMP yield, the total stream of 2,4-DMP and 2,4,6-TMP was approximately 24 times smaller than the stream of the target aromatic product.
Results of 2,6-DMP synthesis from mixture phenol:methanol:water in molar ratio 1:5:1
Temperature of bed
Conversion of phenol [%]
Selectivity of 2,6-DMP [%]
Selectivity of o-krezol [%]
Yield of 2,6-DMP [%]
Synthesis of 2,6-dimethylphenol from a mixture of phenol:methanol:water in molar ratio 1:8:1
Results of 2,6-DMP synthesis from mixture phenol:methanol:water in molar ratio 1:8:1
Temperatur of bed
Conversion of phenol [%]
Selectivity of 2,6-DMP [%]
Selectivity of o-krezol [%]
Yield of 2,6-DMP [%]
Comparison of the conversion and selectivity of 2,6-DMP, obtained at different excess methanol in the reaction mixture with phenol to methanol ratio of 1:5 and 1:8 (Tables 4 and 5) indicates that using a higher excess of methanol as methylating agent in the reaction of phenol with methanol does not increase the degree of conversion of phenol and does not improve the selectivity of 2,6-DMP. It was observed that, the use of a larger excess of methanol in the temperature range of 310-330°C leads to a slightly decreased 2,6-DMP yield and selectivity, and in the range above 330°C the conversion of methanol and selectivity of 2,6-DMP is practically the same at both of the methanol excesses applied. In both cases, the methanol is used in excess with respect to the stoichiometric requirements in the synthesis of 2,6-DMP (in the first case, the excess equals 2.5 and the in the second synthesis equals 4). Increase in excess of methanol significantly reduces the concentration of phenol in the feed, thereby leading to reduction in the rate of reaction, which results in lower conversion of phenol characteristic for the subsequent reactions and in a lower selectivity of dimethyl derivatives in favor of the intermediate product - o-cresol.
In the case of the process, in which, as a result of subsequent reactions in the mixture leaving the reactor, there is a large amount of an intermediate product, it may be more cost-effective to recycle it to the process. In a test arrangement without separating and recycling o-cresol, that kind of state can be created artificially by placing it in the mixture of substrates and selecting conditions for the synthesis process to ensure that the amount of this component before and after passing through the catalyst layer is the same.
The coefficient value of Φ = 1 means achievement of stationary state by the technological node. The value Φ < 1 means an increase of the o-cresol amount in the technological node, while Φ > 1 means decrease in the amount of o-cresol. When Φ ≠ 1, the value of selectivity and yield in the synthesis node do not have significant sense, because they are calculated at a specific time of unsteady state.
Synthesis of 2,6-DMP from a mixture of phenol:o-cresol:methanol:water in molar ratio 1:x:8:1
The green line highlighted in Figure 12c indicats the maximum yield of desired 2,6-DMP. This line crosses the black isoline Φ = 1 at the point of maximum yield of technological node in a stationary state.
In all the experiments, the catalyst loading was equal to 0,353 ± 0,013 gfeed/(gcatalyst.h). The volumetric flow rate, calculated at 310°C was equal 0,0199 ± 0,0020 m3/h. These fluctuations were caused by varying amounts of o-cresol in the feed at the constant liquid flow rate of starting material. The LHSV (liquid hourly space velocity) for all syntheses equals 0.339. During all the presented syntheses of 2,6-DMP, stable fluidization and constant value of pressure drop were observed.
The results of the experiments were presented in a graphic form of operational maps. The maps show dependence of efficiency, conversion, selectivity of selected reagents and the coefficient/factor Φ on bed temperature and o-cresol to phenol molar ratio in the mixture fed to the reactor. The steady-state conditions of the technological node of 2,6-DMP synthesis, stemming from the constant amount of o-cresol circulating in the system were indicated by isoline Φ = 1 in Figure 11. Stable work of technological node is possible in the temperature range 350-380°C, and o-cresolin/phenolin molar ratio of more than 0.48. If the system is initially in a non-stationary state of area A, where Φ > 1, the amount of o-cresol in the reactor will be decreased during the process until the system reaches equilibrium. Similarly, if the system is in area B (Φ < 1), then, as time goes by, the amount of o-cresol in the outlet will continuously increase until Φ = 1, which guarantees stability of the node.
Purified fraction of the iron-chromium catalyst with a grain size of 75-150 μm is a material, which undergoes stable fluidization. Experimentally determined minimum fluidization velocity allowed to select the necessary molar flow rate of substrates. Thanks to proper dilution system of the products stream, the laboratory installation enabled online reaction monitoring with an FTIR spectrometer. Wave numbers for the various process components selected in separate proceedings guarantee credible results of the FTIR analysis, confirmed by GC/MS analysis. The problem of high o-cresol content in the products of the 2,6-dimethylphenol synthesis can be solved by development of a technological node with o-cresol circulating in the system. Presented studies allow for determination of stationary conditions of 2,6-DMP synthesis node with o-cresol circulation. Optimal work of the technological node is possible in the temperature range of 350-380°C and with the o-cresolin/phenolin molar ratio of more than 0.48. Development of 2,6-DMP technology with o-cresol circulation, depending on temperature, should allow to obtain even 90% yield of 2,6-DMP with more than 90% phenol conversion in stationary conditions defined by plotted on both graphs isoline Φ = 1, which is evident by comparing Figures 12 and 13.
Work completed under the development project °Complex Technology for Production of Engineering Polymers Based on Poly(phenylene oxide).” No: WND-POIG.01.03.01-14-058/09. Project co-financed by the European Union through the European Regional Development Fund.
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