Simplified Kinetic Modeling of Propane Aromatization over Ga-ZSM-5

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Simplified Kinetic Modeling of Propane Aromatization over Ga-ZSM-5 Zeolites: comparison with experimental data Massimo Migliori, Alfredo Aloise, Enrico Catizzone, Alessio Caravella, and Girolamo Giordano Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02868 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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Industrial & Engineering Chemistry Research

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Simplified Kinetic Modeling of Propane Aromatization over Ga-ZSM-

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5 Zeolites: comparison with experimental data

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Massimo Migliori, Alfredo Aloise, Enrico Catizzone*, Alessio Caravella, Girolamo Giordano

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Department of Environmental and Chemical Engineering

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University of Calabria, I-87036 Rende (CS), Italy.

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*

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Phone:

+39 0984 496669

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Fax:

+39 0984 496655

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E-mail:

[email protected]

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ORCID ID: 0000-0002-3962-9493

To whom correspondence should be addressed

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Abstract

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This paper presents new results about experimental tests and kinetic modeling of propane

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aromatization on Ga-ZSM-5 zeolite. The presence of gallium as doping metal promotes the

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dehydrogenation reactions, limiting the formation of side-products (cracking fuel gas) and

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increasing the yield to aromatics. In this study, kinetic tests of propane aromatization at different

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values of reaction temperature (500, 525 and 550 °C) and contact time (0.07, 0.14 and 0.28 h) are

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performed in a multi-tubular reactor, aiming at investigating the complex reaction scheme of

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propane aromatization at a constant pressure (3 bar). Based on the obtained experimental data, a

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kinetic analysis is performed considering cracking products (methane and ethane/ethylene) and

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main aromatic compounds (benzene, toluene, xylene and ethyl benzene). This simplified approach is

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found to be a robust tool to predict products distribution when experimental data points are not

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enough to perform a reliable model parameters estimation. In the specific case, the adopted

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macroscopic kinetic model nicely predicts the catalyst behavior in terms of both propane

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conversion and products distribution. The kinetic parameters evaluation for the synthesized Ga-

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ZSM-5 sample also suggests a reaction temperature of 525 °C as an optimum value to favor the

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aromatization rather than the cracking of propane.

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Keywords: propane aromatization, gallium-doped ZSM-5, kinetic modeling

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1. Introduction

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In the last decades the transformation of light alkanes (i.e. LPG cut) into high-value aromatic

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compounds (mainly Benzene, Toluene, Xylenes - BTX) is receiving a growing attention because of

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its economic importance in terms of both exploitation of natural gas resources and valorization of

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light n-paraffin produced in petroleum refineries [1-4]. Among already available industrial

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solutions, UOP Cyclar® process operates from the middle 90s, even though the renewed availability ACS Paragon Plus Environment

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of light alkanes, mainly propane, as by-products in the transesterification of bio-oil extracted from

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oleaginous biomass sources (mainly microalgae) should be also considered, since their valorization

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is important to improve the eco-sustainability of this process [5, 6]. In particular, BTX represent

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important compounds in the organic chemical industry [7,8], being used (as an example) as a

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blending additive to enhance the octane number of gasoline. Propane aromatization reaction is an

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interesting way to produce these compounds, although a minimum of 60% of aromatics yield is

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requested in order to ensure an economic sustainability of the process [9]. For this reason, both

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academic and industrial research are focused on the optimization of the catalyst characteristics

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(typically molecular sieves) with the aim to improve BTX selectivity in this Gas-To-Liquid system.

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To this concern, MFI zeolite shows the best performances in terms of shape selectivity towards light

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aromatic compounds. Beside the acid function offered by HZSM-5 (MFI structure) catalyst, a redox

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function is necessary in order to drive the reaction towards aromatic compounds reducing the

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formation of light cracking-derived hydrocarbons (i.e. methane, ethane). A good choice as gold

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standard is represented by gallium [10-12].

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In fact, gallium-doped H-ZSM-5 zeolite is the most studied catalyst because it offers both high

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propane conversion and BTX selectivity compared with any other catalytic system investigated so

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far. For instance, zinc-doped HZSM-5 shows a fast deactivation during the regeneration cycle due

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to vaporization of Zn component [13], whereas the use of Ga-ZSM-5 improves catalytic

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performance towards propane conversion and p-xylene formation even after several regeneration

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cycles [14]. The high selectivity exhibited by Ga-modified-H-ZSM-5 is due to a combined effect of

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both the redox function performed from extra-framework gallium species and Brønsted acid sites

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offered from hetero-atom in the zeolite framework. Both acid and redox species are necessary to

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drive the reaction towards BTX formation [15, 16]. The dehydrogenation function of the catalyst is

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improved by increasing the gallium content [17], but a balance between acid and redox function is

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necessary to ensure high catalytic performances in terms of both propane conversion and BTX 3 ACS Paragon Plus Environment


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selectivity [18]. Gallium species can be introduced in MFI zeolite by several methods as:

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impregnation [19], ion exchange [20], mechanical mixing between Ga2O3 and H-ZSM-5 [17, 21], or

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directly during the synthesis of MFI structure [22, 23]. The latter seems to be the best method to

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obtain a catalyst with both high BTX selectivity and high resistance to deactivation [24, 25]. In this

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case, Gallium species are present both in framework and extra-framework position, allowing either

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acid or redox function. On the other hand, catalytic performances of Ga-MFI catalyst are very

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sensitive towards both synthesis and post-synthesis treatments. For instance, calcination

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temperature is an important parameter to tune because gallium atoms easily migrate to extra-

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framework position by thermal treatment affecting catalytic properties of the calcined material [26,

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27]. Furthermore, the acid sites type significantly impacts on the catalyst activity, as a linear

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correlation was found between the initial rate of propane aromatization and strong Lewis and

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Brønsted acid site concentration [28]. In fact, it is widely accepted that the acid sites strength is one

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relevant parameter for selectivity improvement, increasing the aromatics formation [29]. A highly

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active Ga/ZSM-5 was prepared by formic acid impregnation enhancing the dispersion of the Ga

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species and promoting the formation of highly dispersed GaO+ species. Also in this case, the super-

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catalytic behavior was attributed to the synergistic effect between the strong Lewis acid sites and

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the Brønsted acid sites [30]. Quite recently, it was emphasized that hierarchical bifunctional

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Ga/HZSM-5 catalyst are excellent solutions for propane aromatization. [31, 32]. Furthermore, the

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reaction operating conditions, in terms of contact time and reaction temperature, affect strongly

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selectivity towards BTX. Choudhary et al. [33] found that, increasing the contact time, the BTX

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selectivity increases simultaneously lowering the production of propylene, ethylene and C4

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hydrocarbons. On the other hand, increasing the reaction temperature, the selectivity towards

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aromatics decreases and a higher production of cracking compounds (methane, ethane) is observed

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[34, 35]. Also the catalyst porosity may be relevant in selectivity control, as mesoporous Ga-MFI

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showed an increased selectivity and the presence of induced intra-crystalline mesoporosity was

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found to be also a relevant parameter to shorten the contact time improving conversion and BTX 4 ACS Paragon Plus Environment

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selectivity with respect to the parental zeolites [36]. Another suggested way to increase the catalytic

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activity is the addition of Pd, which allows a better conversion under mild process conditions [36,

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37].

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Several mechanisms have been proposed to explain aromatic formation [38-41] and although

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several studies have been dedicated to this reaction, investigation about the mechanism of propane

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aromatization reaction is still an open challenge. Anyway, the first step of the reaction is the

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conversion of propane in ethylene and methane by protolytic C-C cracking or in propylene by a

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dehydrogenation reaction [42]. Starting from these intermediates, aromatic compounds are formed

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by combination, cyclisation and dehydrogenations reactions [43-45]. During these reactions

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methane and ethane are the most important by-products coming from the primary cracking of

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propane as well as from cracking of heavier olefins. BTX fraction usually consists of benzene,

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toluene, p-xylene and ethylbenzene, but the products distribution (selectivity) strongly depends on

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both catalyst characteristics and reaction conditions [34]. Also micro-kinetic modeling was recently

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used to study some mechanistic aspects of propane aromatization on Ga/HZSM-5 catalyst, finding

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that monohydric Ga sites (GaH2+) are predominant at low Ga/Al ratio whilst dihydric Ga sites

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(GaH2+) are present at high Ga/Al ratios, even though monohydric Ga sites still play a role in the

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reaction mechanism [46].

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On the other hand, due to the complexity of this reaction scheme, the kinetic analysis is an

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important tool to understand the reaction mechanism. The calculated kinetic parameters can also be

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helpful for reactor design and scale-up of this technology.

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In this contest, we report a kinetic analysis about propane aromatization reaction carried out over an

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home-made Ga-ZSM-5 zeolite. The sample was synthesized in order to obtain a catalyst with a high

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Ga/Al ratio (Si/Ga=25, Si/Al=200, Ga/Al=8) in order to improve the dehydrogenating function

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instead of the cracking activity [47]. The sample was characterized in terms of its chemical,

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physical and acidic properties using XRD, TG/DTA, atomic absorption spectroscopy, BET and 5 ACS Paragon Plus Environment


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NH3-TPD analysis. The catalytic tests were performed at different reaction temperatures and

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different contact times in order to estimate the kinetic parameters in a wide range of conversion

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values. Due the complexity of reaction mechanism, the analysis was restricted to cracking

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compounds (methane and ethane/ethylene) and aromatic compounds (benzene, toluene, C7+-rings).

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Despite the model does not consider the complete reaction mechanism, it is shown that it is possible

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to fairly predict the catalytic behavior of the material and assess the reaction conditions to favor the

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aromatics formation with respect to cracking.

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2. Experimental

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2.1 Synthesis of catalyst

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As the most effective synthesis method to obtain performing catalyst [24, 25], the synthesis was

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conducted by directly introducing the gallium in an alkaline-free gel synthesis by using tetrapropyl

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ammonium bromide as SDA (Structure Directing Agent) and methyl-ammine in order to obtain a

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basic medium allowing silica dissolution. The synthesis was performed using the following gel

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composition:

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1.08 CH3NH2 – 0.134 TPABr – 1 SiO2 – 0.0025 Al2O3 – 0.02 Ga2O3 – 40 H2O

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7.8 g of methylamine (anhydrous, ≥98%, Sigma-Aldrich), 8.3 g of tetra-propyl ammonium bromide

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(98%, Sigma-Aldrich) and 91 mg of aluminium hydroxide (Sigma-Aldrich) were dissolved in 111 g

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of distilled water. After complete dissolution, a solution composed by 56 g of distilled water and 2.4

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g of gallium nitrate (99%, Sigma-Aldrich) was mixed with the first solution. 14 g of precipitated

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silica gel (Merk) were then slowly added to the mixture and the resulting gel was stirred for 2 hours

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at room temperature. The resulting gel was thermally treated in a 250 ml PTFE-lined stainless steel

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autoclave kept at 175 °C for 7 days in a static oven. After crystallization the sample was recovered

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by filtration, washed several times with distilled water and dried at 80°C overnight. Acid form and

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elimination of SDA were obtained directly by calcination at 550 °C (8 hours, air flow: 20 mL/min).

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2.2 Chemical-physical characterization of the catalyst

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The crystallinity of the sample was verified by X-Ray powder diffraction (APD 2000 Pro) (region

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5° < 2θ < 50°, step 0.02°/ s); the morphology of the crystalline phase was observed with a scanning

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electron microscope (FEI model Inspect). The Si/Al ratio in the structure was measured by atomic

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absorption spectroscopy (GBC 932). The specific surface area and the micropores volume of the

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catalyst were obtained by performing a BET and t-plot analysis of porosimetry data (ASAP 2020

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Micromeritics) under nitrogen adsorption at 77 K, after a pre-treatment in vacuum at 200 °C for 12

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h. In order to analyse the thermal decomposition of the SDA molecules, thermoanalytical

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measurements were performed on the automatic TG/DTA instrument (SHIMADZU) operating with

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20 cc/min air flow (heating rate of 5°Cmin-1). Surface acidity was measured by NH3-TPD analysis

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(TPDRO1100, ThermoFisher) according with the following procedure. A dried sample (100 mg,

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pellet mesh 90-150 µm) was loaded in a linear quartz micro-reactor and pretreated at 300°C in

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helium flow for 1 h to remove any residual adsorbed water. The sample was cooled down to 150°C

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and saturated with 10% v/v NH3/He mixture with a flow rate of 20 STP mL·min-1 for 2 h. The

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ammonia physically adsorbed was removed by purging in helium at 150°C for 1 h until TCD

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baseline stabilization. The desorption measurement was carried out in the temperature range of 100-

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700°C (10 °C·min-1) using a helium flow rate of 20 STP mL·min-1 [48]. Peak analysis and

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deconvolution were performed by using a commercial software (PeakFit 4.12, Seasolve-USA).

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2.3 Catalytic tests

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The catalytic behaviour in propane aromatization reaction was evaluated by using a lab scale multi-

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reactor system (SPYDER, AmTech GmbH, Germany) equipped with a stainless steel reactor (I.D. 9

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mm, length 205 mm). During the test, the pressure was fixed at 3 bar and three values of

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temperature were investigated (namely 500, 525 and 550 °C). After sample loading into the reactor

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(catalyst pellets size: 300-500 µm), the temperature was increased in nitrogen flow from room

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temperature to the reaction temperature with a controlled thermal ramp (0.5 K/min) in order to 7 ACS Paragon Plus Environment


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prevent any catalyst damage. After stabilization of reaction conditions (pressure and temperature), a

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mixture of propane and nitrogen (15%v/v of hydrocarbon) was fed with a total flow rate at 60

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NmL/min. The Weight Hourly Space Velocity (WHSV) was varied between 0.07 and 0.28

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gpropane·gcat-1·h-1 by varying the weight of the catalyst loaded in the reactor. The outlet gas stream

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(methane, ethane, propane, benzene, toluene and xylenes) was quantitatively analyzed by an on-line

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GC (Agilent 7890A) equipped with a specific column (J&W 125-1032) and a FID detector and the

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first experimental data was recorded after 15 min of reaction in order to reach the steady state

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conditions. Any experimental test data was repeated over three independent samples and points

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reported throughout the paper are average with a coefficient of variation lower than 0.05.

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3. Kinetic modeling

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As already mentioned, the propane-to-aromatics conversion follows a complex mechanism

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consisting of several reaction steps from protolytic cracking, paraffins dehydrogenation to beta-

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scission, oligomerization, H-transfer and aromatization of alkenes, alkylation and dealkylation of

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aromatics [38]. Several simplified models have been proposed. For example, Bhan and co-workers

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[38], considered 11 reaction steps with 25 model parameters based on 240 experimental data of the

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considered species.

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Moreover, as discussed in the introduction section, the main challenge of the propane aromatization

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is to promote the aromatics formation (benzene, toluene and xylenes), inhibiting at the same time

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cracking reactions responsible for reactant losses due to the formation of methane and

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ethane/ethylene. A model with a reduced number of parameters was proposed by Ogunronbi et al.

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[32], lumping several species (e.g. cracking products such as methane and ethane/ethylene) and

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assuming BTX compounds as a unique class of species. This approach might miss some of the

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relevant objectives of the kinetic modeling (such as the BTX distribution prediction) but, despite its

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simplicity, model results are sufficiently precise. Therefore, it could be interesting to follow the

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same approach, exploiting some of the lumped species, in order to improve the model capability to 8 ACS Paragon Plus Environment

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fully predict the system behavior. For this purpose, in the present study propane conversion process

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is described via the simplified mechanism reported in Figure 1. This represents a relevant

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improvement of the above-mentioned approach [32] because (i) benzene, toluene and xylenes are

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considered as separate products as well as (ii) the cracking products (methane, ethane and ethylene),

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and (iii) because this allow the prediction of BTX distribution and catalyst selectivity expressed in

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terms of Aromatization/Cracking (A/C) ratio [35]. The following 1st order rate constants [h-1] are

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defined accordingly: kC rate constant of propane cracking to methane and ethane, kB rate constant of

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benzene formation, kT rate constant of toluene formation, kX rate constant of xylenes/ethyl benzene

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formation. In principle, it should be possible to define a kO rate constant formation of C4 species

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(mainly butane and butenes) but in this case, according to the carbon balance, a reaction mechanism

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has to be hypothesized. This could be interesting for sake of model generality but it requires a

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detailed kinetic model based on reaction mechanism involving cracking products and propane as

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reactant to produce C4 fraction. Therefore, to simplify the model, the C4 formation was not

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considered and for propane conversion, an overall rate constant kP was adopted, respecting the

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carbon balance. The reactor was considered as an isothermal and isobaric plug-flow reactor (PFR)

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since no significant change in either temperature or pressure were observed during the reaction.

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Furthermore, because of the high initial volumetric concentration of carrier (85% v/v), the system

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was also assumed at constant density.

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Therefore, according to the above described mechanism, for a continuous PFR in steady-state

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conditions, the differential material balance can be written using the molar concentration, being the

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reaction rate negative for disappearing species. In the case of propane, it holds:

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rP =

dCp dτ

= −k P C P

(1)

The rate of appearance of cracking products (methane + ethane/ethylene) is:



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dC C = kC C P dτ

(2)

rB =

dC B = k BC P dτ

(3)

rT =

dC T = kT C P dτ

(4)

The rate of toluene formation is:

222 223

rC =

The rate of appearance of benzene is:

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The rate of appearance of C7+-compounds (xylenes and ethyl benzene) is:

rX =

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dC X = kX C P dτ

(5)

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where ri [mol·cm-3·h-1] and C i [mol·cm-3] represent the reaction rate and molar concentration of

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each species in the system, respectively, whilst

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inverse of Weight Hourly Space Velocity (1/WHSV).

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The kinetic constant k i can be expressed as a function of the reaction temperature by an Arrhenius-

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type relation [49]:

τ

[h] is the contact time which is calculated as the

−

k i = ki 0 e

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Ei RT

(6)

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where Ei is the apparent reaction activation energy.

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The estimation of model parameters was performed with MATLAB R2012a by using fourth order

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Runge-Kutta method to solve the above differential equations coupled with Levenberg-Marquardt

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algorithm for non-linear regression analysis. A 95% confidential interval was adopted during the

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analysis.

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4. Results and discussion

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4.1 Chemical and physical properties of the catalyst

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The main chemical – physical characteristics of the catalyst used in this work are summarized in

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Table 1. XRD pattern of the synthesized catalyst is reported in Figure 2. From peaks position and

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intensity analysis, a highly pure and crystalline MFI structure was confirmed. Moreover, the

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scanning electron microscopy, reported in Figure 3, shows that the phase consists of >5 µm well-

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defined spherical crystals with inter-growths. N2 adsorption-desorption isotherm is reported in

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Figure 4, typical of microporous materials [50, 51].

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4.2 Catalytic test

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Figure 5 shows the effect both of contact time (expressed as 1/WHSV) and reaction temperature

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on propane conversion. The increasing of contact time positively affects the conversion of reactant

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even if this effect is more pronounced at higher temperature [52]. In particular, at 525 °C, by

250

increasing the contact time from 0.07 h-1 to 0.28 h-1 the propane conversion rises from ca. 0.09 to

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ca. 0.33 whilst at 500 °C, for the same increasing of contact time, the conversion increases from ca.

252

0.07 to ca. 0.14 only.

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The effect of contact time and reaction temperature on BTX overall selectivity is reported in Figure

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6. Selectivity towards BTX increases as the contact time increases, according to previous findings

255

of Choudhary and co-workers [52], rising from ca. 18-21% to ca. 31-33% when the contact time is

256

increased from 0.07 h-1 to 0.28 h-1. Furthermore, it appears that at 525 °C, investigated sample

257

usually shows the highest selectivity towards BTX. Figure 7 shows the selectivity values towards

258

benzene, toluene and xylenes observed at the highest contact time for different reaction

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temperature. It appears that toluene is the main product among the BTX compounds with a

260

selectivity around 20%, corresponding to 58% of the total BTX. Benzene selectivity is slightly 11 ACS Paragon Plus Environment


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below 10%, corresponding to 27% ca. of the overall BTX amount, whilst xylenes selectivity is

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always below 6% (15.0 – 19.3 % of total BTX). It is important to notice that the increase of reaction

263

temperature causes a reduction in xylenes selectivity, increasing the selectivity toward benzene and

264

toluene. In addition, it should be considered that contact time did not affect significantly the product

265

distribution and the same trend was observed also when varying the reaction temperature (See

266

Figures S1-S3 in the Supporting Information section). The observed reduction of the system to

267

form of xylene can be attributed to the cracking of a methyl group as confirmed by the increase of

268

light cracking-derived compound (such as methane) when increasing temperature and contact time

269

(Figure 8). For instance, methane selectivity rises from ca. 11% to ca. 14.4% when the reaction

270

temperature rises from 500 °C to 550 °C (at 1/WHSV=0.07 h-1). Figure 9 shows that the increasing

271

of reaction temperature also favour formation of ethane since ethane selectivity increases from ca.

272

25.8% to ca. 30.4% when the reaction temperature raises from 500 °C to 550 °C (at 1/WHSV=0.07

273

h-1). Unlike methane, ethane formation is favoured at low contact time. An analysis of the effect of

274

the operating conditions towards C4 – C5 loop selectivity (Figure 10) allows confirming that a low

275

gas hourly space velocity and intermediate reaction temperature (525°C is the optimal temperature

276

in the investigated conditions) are necessary to increase the selectivity towards the desired

277

compounds with an important decrease of by-products (e.g. methane, ethane and C4-C5

278

compounds). In addition, no compound heavier than BTX were detected by gas chromatographic

279

analysis in the reactor outstream, it was demonstrated the high shape selectivity towards light

280

aromatic molecules provided by MFI structure.

281

4.3 Comparisons between experimental data and model predictions

282

Rate coefficients of proposed model were estimated by the previously described fitting method and

283

summarized in Table 2. Figures 11-12 clearly show that the proposed model satisfactorily fits the

284

experimental data in terms of mole fraction of propane, cracking products (methane and ethane),

285

benzene, toluene and xylenes with a correlation factor always higher than 0.96. Figure 13 reports 12 ACS Paragon Plus Environment

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the comparison between predicted and measured mole fraction confirming the model agreement

287

with species distribution experimentally observed.

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All the rate coefficients (see Table 2) increase as the reaction temperature increases. The rate

289

coefficient of cracking reaction kC is always higher than aromatics rate coefficients, suggesting that

290

cracking reaction is an important side reaction of the process. The proposed model allows to predict

291

also aromatics distribution: toluene is the main aromatic compound with a predicted concentration

292

of ca. 56%wt/wt of the overall BTX amount while benzene represents ca. 27%wt/wt of the overall BTX

293

amount according to the previously discussed experimental results (see paragraph 4.2). Moreover,

294

proposed model predicts that xylene concentration decreases from ca. 18.5%wt/wt to ca. 15.5 %wt/wt

295

when the reaction temperature rises from 500 °C to 525 °C in totally agreement with experimental

296

observation reported in Figure 7. As previously mentioned, cracking reaction is an important side

297

reaction of the propane aromatization process. In this concern, it is possible, by using the model

298

predictions, to estimate the ratio between the aromatization-reaction rate and cracking-reaction one

299

(A/C) as follows:”.

300

A/C =

301

The values of A/C as a function of reaction temperature is reported in Figure 14. The trend clearly

302

suggests that a reaction temperature of 525 °C can be considered an optimum to favor aromatization

303

reaction over the cracking one. It is noteworthy that the same result was experimentally observed

304

(see paragraph 4.2), confirming that the simplified kinetic model proposed in this work is not only

305

able to predict species mole fraction but can be used as a useful tool to individuate estimate

306

parameters (e.g. BTX distribution, A/C ratio) that can be used for either reactor scale-up or

307

comparison between different catalysts.

rB + rT + rX k B + kT + k X = rC kC

308



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309

For each reaction loop, an apparent activation energy can be calculated according to Arrhenius-type

310

expression, whose results are reported in Table 3. It should be considered that the value of apparent

311

activation energy strongly depends on the catalyst properties: Ogunronbi and co-workers [32] found

312

that the apparent activation energy of cracking was 110 kJ/mol for microporous Ga-ZSM-5 and 130

313

kJ/mol for mesoporous Ga-ZSM-5, whilst higher values (about 213 kJ/mol) were found from Bhan

314

et co-workers [38] over a Ga-free HZSM-5. To this concern, the value of apparent activation energy

315

of cracking estimated in our modelling study drops within the range of data reported in the

316

literature. Also for the aromatization reactions, the calculated apparent activation energy is coherent

317

with a range of 140-220 kJ/mol, which can be extracted from literature data [32]. In particular, the

318

benzene formation shows an apparent activation energy higher than the toluene and xylene one, this

319

being in agreement with the theoretical prediction of Joshi and Thomson [45].

320

Conclusions

321

This paper mainly focuses on the BTX production via propane aromatization using a zeolite based

322

catalyst doped with Gallium. A Ga-H-MFI was synthesized and tested in the temperature range

323

500-550°C at different contact time, revealing how both parameters can significantly affect the

324

product distribution. It was found that the increase in contact time promotes the propane conversion

325

and the BTX selectivity was mainly affected by the reaction temperature. The most abundant BTX

326

product was toluene and the amount of Xylenes (the smaller BTX fraction) was found to decrease

327

when increasing temperature.

328

A kinetic model was also presented, based on reaction lumped reaction classes, that despite the

329

simple structure was able to discriminate between different component of the of BTX fraction and

330

to quantify the amount of cracking (main competitive reaction). Proposed model nicely fits

331

experimental data allowing to predict catalytic behavior of investigated catalyst as a function of

332

contact time and reaction temperature. In particular, referring to the analysis of competitive classes

333

of reaction (BTX formation and undesired propane cracking), the calculation of the related reaction 14 ACS Paragon Plus Environment

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334

rates allowed to assess a temperature of 525 °C as best process conditions to maximize the

335

Aromatization/Cracking ratio.

336 337

Acknowledgments

338

This work was supported by MIUR PRIN 2010-2011 2010H7PXLC Project on “Innovative

339

downstream processing of conversion of algal biomass for the production of jet fuel and green

340

diesel”.

341

342

Supporting Information

343

The supporting information section includes figures showing the effect of reaction time (WHSV) on

344

the BTX relative distribution at any investigated temperature.

345 346 347 348 349 350 351 352 353 354



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TABLES CAPTIONS Table 1 – Chemical and physical properties of investigated sample. The atomic ratio in molar units. Table 2 – Rate constants of investigated reaction classes at different reaction temperatures. Table 3 – Apparent activation energies of investigated reaction classes.

FIGURES CAPTIONS Figure 1 – Simplified scheme of propane aromatization reaction mechanism. Figure 2- XRD pattern of the investigated sample. Figure 3 – Scanning Electron Microscopy picture of the crystals of the investigated sample. Figure 4 – Nitrogen Adsorption – Desorption isotherms performed at 77 K on the investigated sample. Figure 5 – Propane conversion vs contact time at 500 °C (●), 525 °C (■) and 550 °C (▲) (dashed lines are only reader guideline). Figure 6 – BTX selectivity vs contact time at 500 °C (○), 525 °C (▲) and 550 °C (□). Figure 7 - Selectivity of Benzene (■), Toluene (▲) and Xylenes (●) vs reaction temperature at 1/WHSV=0.28 h. Figure 8 – Methane selectivity vs contact time at 500 °C (●), 525 °C (■) and 550 °C (▲). Figure 9 – Ethane selectivity vs contact time at 500 °C (●), 525 °C (■) and 550 °C (▲). Figure 10 – C4-C5 selectivity vs contact time at 500 °C (●), 525 °C (■) and 550 °C (▲).



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Page 24 of 42

Figure 11 – Comparison between model (lines) and experimental data (symbols) of propane conversion at 500 °C (■), 525 °C (▲) and 550 °C (●). Figure 12 – Comparison between model (lines) and experimental data (symbols) of products mole fractions at 500 °C (■), 525 °C (▲) and 550 °C (●). Figure 13 –Comparison between experimental mole fractions and model predictions. Figure 14 –A/C ratio predicted from the model vs reaction temperature.


Page 25 of 42

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Si/(Al+Ga)

18

Ga/Al

2.5

BET Surface Specific Area [m2/g]

344

Micropore volume [cm3/g]

0.110

NH3 – uptake [µmol/g]

630

ACS ParagonTable Plus1Environment


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Rate constant

Page 26 of 42

Reaction Temperature [°C]

[h-1]

500 °C

525 °C

550 °C

kP

0.614 ± 0.054

1.564 ± 0.046

2.101 ± 0.178

kC

0.409 ± 0.050

1.028 ± 0.039

1.572 ± 0.142

kB

0.025 ± 0.005

0.071 ± 0.004

0.097 ± 0.014

kT

0.045 ± 0.005

0.128 ± 0.004

0.168 ± 0.013

kX

0.013 ± 0.005

0.031 ± 0.005

0.040 ± 0.002

ACS ParagonTable Plus2Environment

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Reaction class

Activation energy [kJ/mol]

R2

Cracking

143.9 ± 27.1

0.966

Benzene formation

145.3 ± 41.4

0.925

Toluene formation

141.2 ± 44.0

0.911

Xylene formation

120.4 ± 34.7

0.923

27 ACS ParagonTable Plus3Environment


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Figure 1 ACS Paragon Plus Environment

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Figure3 ACS Paragon Plus Environment

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Page 42 of 42

GRAPHICAL ABSTRACT

42


Simplified Kinetic Modeling of Propane Aromatization over Ga-ZSM-5

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