Fluorinated alumina for benzene propylation: reaction scheme and heterogeneity of the acid surface
Introduction
Previous works [1], [2] have studied fluorinated aluminas catalytic behaviour in benzene alkylation with propylene. They observed that while the fluorine content increases catalytic activity increases too, not only in the cumene but also in the dimers of propylene production. This dependence between the fluorine content and the activity could not be explained in terms of the evolution of the total acidity nor of the acid strength. These were necessary NH3-TPD, XRD, XPS, and FTIR-pyridine analysis to clear some aspects about the heterogeneity of the catalysts surfaces, distinguishing the presence of some compounds like AlF3·nH2O, Al(OH)xFy, and Al2O3.
It was [1], [2] discovered that only Brönsted acid sites were active for alkylation. Poisoning these sites with sodium ions decrease alkylation activity.
As an extension of previous publication, this work tries to elucidate the participation of each acid site in benzene propylation. To achieve this goal it is important to consider a more complex and detailed reaction scheme that could eventually be associated to the different active species in the fluorinated alumina.
At the same time that propylene alkylates benzene, it undergoes other reactions like oligomerization, disproportionate, polyalkylation, cracking or even the formation of cyclic hydrocarbons [3]. For example, propylene produces dimers over almost all acid catalysts, those dimers react with available propylene to produce heavier oligomers which can react with propylene again or with themselves [4].
Also aromatic compounds react in many ways. For example over acidic catalysts diisopropylbenzenes produce cumene via dealkylation generating propylene or in combination with benzene via transalkylation [5]. Thermodynamic information about the reactions and the species involved was also obtained from previous work [6], [7].
However, in most cases reaction pathways were proposed only regarding the feasibility and chemical formation of the compounds detected without verifying the model with numerical results from experiments [8]. To verify the reaction scheme, robust analytical methods must be employed in the identification and quantification of a considerable number of chemical species involved during the benzene alkylation. Such methods are GC–PIANO and GC–MS analysis [9]. On the other hand, statistical techniques like the Delplot method could be useful to determine complex reaction schemes but it requires a great number of data for each compound [10].
All these reactions must be considered in establishing a reaction scheme of benzene propylation over the desired acid catalysts. The reaction scheme is the first step to suggest a kinetic model that will lead to an activity measurement and its relation with the active surface of the catalysts [11].
Complementary analysis obtained from NH3-TPD and NH3-chemisorption can give additional information corroborating the existence of different active sites over the surface of the fluorinated aluminas [11], [12], [13], [14].
The isosteric heats of adsorption obtained from NH3-chemisorption can show the energetic differences among the acid sites [15], [16], [17]. Moreover, the desorption amount of ammonia from TPD analysis could be related to those energetic differences.
The aim of this paper is to establish a reaction scheme from experiments over alumina catalysts with different fluorine content. A simplified kinetic model will be obtained from the scheme as a way of measurement for the catalyst activity. After that we attempt to distinguish which kind of acidic sites are directly related with the activity shown by the catalyst.
Section snippets
Catalysts preparation
Three catalysts from γ-Al2O3 were synthesized, two of them were ground up to 200 mesh and the third was prepared in 1/8 in. pellets. They were calcined at 773 K for 18 h. Wetness impregnation was performed with two aqueous solutions, at 34 and 83 g/l of NH4F, in order to get up to 15 and 30 wt.%F. The γ-Al2O3 was immersed in the solution and stirred for a night. After impregnation, the solid was filtrated and washed with water using 100 ml water/10 g of solid. The aluminas were dried for 24 h at 393 K
Reaction scheme
For the three catalysts it was observed on the chromatograms distinguishable regions of different families of compounds: butenes, pentenes, six (6CNAH) and seven (7CNAH) non-aromatic hydrocarbons, cumene, dialkylated, and polyalkylated propylbenzenes, shown in Fig. 2, Fig. 3. In the case of 6CNAH and 7CNAH families olefin, paraffin, and cyclic compounds [11] were detected. Some results for each catalyst are shown in Table 2, where XB and XP are benzene and propylene conversions.
Even for
Discussion
Despite the different preparation of the catalysts from previous works, the trends in their properties considering the fluorine content were almost the same from those in references [1], [2] (quantity of acid sites, activity, XRD spectra, etc.). However, EDS and XRF had slight differences between their values, but they were both capable of showing the same trend in the fluorine content for the catalysts (Table 1). We also have to remember that XRF uses a calibration curve which depends on the
Conclusions
One reaction scheme allows us to compare different catalysts in terms of relative reactivity. This comparison between benzene alkylation reaction rate and propylene oligomerization reaction rate was a useful quantitative tool. In addition all the characterization techniques employed point out to the fact that the catalytic surface is heterogeneous, showing us four different types of acid sites.
Joining all of these information we tried to explained the catalyst activity in terms of its
Acknowledgements
This work was performed in the framework of Programme de Coopération Post-Graduée (PCP) France-Mexique with financial support from Consejo Nacional de Ciencia y Tecnologı́a (CONACyT), Ministère de Affaires Etrangères (France) and Petróleos Mexicanos. We are indebted to P. Altuzar for XRF analysis and C. Salcedo for XRD studies, both of the Unidad de Servicios de Apoyo a la Investigación (U.S.A.I.) in the U.N.A.M.
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