Simplified Kinetic Modeling of Propane ... - ACS Publications

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

1

2

3

Simplified Kinetic Modeling of Propane Aromatization over Ga-ZSM-

4

5 Zeolites: comparison with experimental data

5 6 7

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

8 9 10

Department of Environmental and Chemical Engineering

11

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

12 13 14 15

*

16

Phone:

+39 0984 496669

17

Fax:

+39 0984 496655

18

E-mail:

[email protected]

19

ORCID ID: 0000-0002-3962-9493

To whom correspondence should be addressed

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

Abstract

21

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

22

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

23

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

24

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

25

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

26

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

27

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

28

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

29

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

30

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

31

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

32

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

33

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

34

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

35

aromatization rather than the cracking of propane.

36 37

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

38 39

1. Introduction

40

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

41

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

42

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

43

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

44

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

Page 2 of 42

Page 3 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45

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

46

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

47

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

48

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

49

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

50

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

51

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

52

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

53

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

54

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

55

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

56

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

57

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

58

standard is represented by gallium [10-12].

59

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

60

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

61

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

62

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

63

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

64

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

65

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

66

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

67

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

68

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

69

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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 42

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

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

78

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

79

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

80

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

81

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

82

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

83

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

84

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

85

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

86

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

87

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

88

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

89

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

90

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

91

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

92

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

93

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

94

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

95

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

Page 5 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


96

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

97

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

98

37].

99

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

100

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

101

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

102

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

103

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

104

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

105

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

106

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

107

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

108

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

109

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

110

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

111

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

112

reaction mechanism [46].

113

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

114

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

115

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

116

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

117

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

118

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

119

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

120

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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 42

121

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

122

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

123

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

124

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

125

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

126

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

127

aromatics formation with respect to cracking.

128

2. Experimental

129

2.1 Synthesis of catalyst

130

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

131

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

132

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

133

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

134

composition:

135

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

136

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

137

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

138

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

139

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

140

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

141

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

142

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

143

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

144

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

6 ACS Paragon Plus Environment

Page 7 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


145

2.2 Chemical-physical characterization of the catalyst

146

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

147

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

148

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

149

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

150

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

151

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

152

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

153

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

154

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

155

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

156

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

157

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

158

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

159

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

160

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

161

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

162

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

163

2.3 Catalytic tests

164

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

165

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

166

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

167

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

168

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

169

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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 42

170

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

171

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

172

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

173

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

174

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

175

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

176

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

177

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

178

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

179

3. Kinetic modeling

180

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

181

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

182

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

183

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

184

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

185

considered species.

186

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

187

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

188

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

189

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

190

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

191

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

192

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

193

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

194

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

Page 9 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


195

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

196

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

197

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

198

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

199

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

200

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

201

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

202

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

203

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

204

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

205

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

206

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

207

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

208

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

209

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

210

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

211

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

212

was also assumed at constant density.

213

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

214

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

215

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

216

217

rP =

dCp dτ

= −k P C P

(1)

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

218 219

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:

220 221

Page 10 of 42

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

rX =

224

dC X = kX C P dτ

(5)

225

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

226

each species in the system, respectively, whilst

227

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

228

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

229

type relation [49]:

τ

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

−

k i = ki 0 e

230

Ei RT

(6)

231

where Ei is the apparent reaction activation energy.

232

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

233

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

234

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

235

analysis.

236 10 ACS Paragon Plus Environment

Page 11 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


237 238

4. Results and discussion

239

4.1 Chemical and physical properties of the catalyst

240

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

241

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

242

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

243

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

244

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

245

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

246

4.2 Catalytic test

247

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

248

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

249

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

251

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.

253

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

254

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

259

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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 42

261

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

262

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

Page 13 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


286

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

287

with species distribution experimentally observed.

288

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 42

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

Page 15 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 42

355

REFERENCES

356

[1] Wood, D A.; Nwaoha, C.; Towler, B. F. Gas-to-liquid (GTL): A review of an industry offering

357

several routes for monetizing natural gas. J. Nat. Gas Sci. Eng. 2012, 9, 196 – 208.

358

[2] Seddon, D. Paraffin oligomerization to aromatics. Catal. Today 1990, 6, 351 – 372.

359

[3] Pyl, S.P.; Schietekat, C.M.; Reyniers, M.F.; Ramin, A.; Marin G.B.; Van Geem, K.M. Biomass

360

to olefins: Cracking of renewable naphtha. Chem. Eng. J. 2011, 176–177, 178–187.

361

[4] Ampelli, C; Perathoner, S.; Centi, G. CO2 utilization: an enabling element to move to a

362

resource- and energy-efficient chemical and fuel production. Philos. Trans. R. Soc. A. 2015,

363

373(2037).

364

[5] Peng, B.; Yao, Y.; Zhao, C.; Lercher, J. A. Towards quantitative conversion of microalgae oil to

365

Diesel-range alkanes with bifunctional catalysts. Angew. Chem.Vol. 2012, 124, 2114 – 2117.

366

[6] Gebreslassie, B. H.; Waymire, R.; You F. Sustainable design and synthesis of algae-based

367

biorefinery for simultaneous hydrocarbon fuel production and carbon sequestration. AIChE J.

368

2013, 59, 1599 – 1621.

369

[7] Al-Khattaf, S.; Ali, S.A.; Aitani, A.M.; Žilková, N.; Kubička, D.; Čejka, J. Recent Advances in

370

Reactions of Alkylbenzenes Over Novel Zeolites: The Effects of Zeolite Structure and

371

Morphology, Catal. Rev. 2014 56, 333-402.

372 373

[8] Perego C., Ingallina P., Combining alkylation and transalkylation for alkylaromatic production, Green Chem. 2004, 6, 274-279.

374

[9] Corbetta, M.; Manenti, F.; Pirola, C.; Tsodikov, M. V.; Chistyakov, A.V. Aromatization of

375

propane: techno-economic analysis by multiscale “kinetics-to-process” simulation. Comput.

376

Chem. Eng. 2014, 71, 457 – 466. 16 ACS Paragon Plus Environment

Page 17 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

377 378

379


[10]

Lukyanov, D. B; Gnep, N. S.; Guisnet, M. R. Kinetic modeling of propane aromatization

over HZSM-5 and GaHZSM-5. Ind. Eng. Chem. Res. 1995, 34, 515-523. [11]

Guisnet, M.; Gnep, N. S. Aromatization of propane over GaHMFI catalysts. Reaction

380

scheme, nature of the dehydrogenating species and mode of coke formation. Catal. Today 1996,

381

31, 275-292.

382 383

384 385

386

[12]

Bhan, A.; Delgass, W. N. Propane Aromatization over HZSM-5 and Ga/HZSM-5 catalysts.

Catal. Rev. 2008, 50, 19-151. [13]

Chester, A. W.; Chu, Y. F. Process for converting ethane to aromatics over gallium-

activated zeolite. US 4350835 A, 1982. [14]

Choudary, V. R.; Devadas, P. Regenerability of coked H-GaMFI propane aromatization

387

catalyst: influence of reaction-regeneration cycle on acidity, activity/selectivity and

388

deactivation. Appl. Catal., A 1998, 168, 187 – 200.

389

[15]

Choudhary, V. R.; Kinage, A. K.; Choudhary, T.V. Influence of binder on the acidity and

390

performance of H-Gallosilicate (MFI) zeolite in propane aromatization. Appl. Catal., A 1997,

391

162, 223-233.

392

[16]

Choudhary, V. R.; Mulla, S. A. R.; Banerjie S. Aromatization of n-heptane over H-AlMFI,

393

Ga/H-AlMFI, H-GaMFI and H-GaAlMFI zeolite catalysts: influence of zeolitic acidity and non-

394

framework gallium. Microporous and Mesoporous Mater. 2003, 57, 317-322.

395

[17]

Choudhary, V. R.; Mantri, K.; Sivadinarayana, C. Influence of zeolite factors affecting

396

zeolitic acidity on the propane aromatization activity and selectivity on Ga/H-ZSM-5.

397

Microporous and Mesoporous Mater. 2000, 37, 1 – 8.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

398

[18]

Page 18 of 42

Joshi, Y. V.; Thomson, K. T. The role of gallium hydride and Brønsted acidity in light

399

alkane dehydrogenation mechanisms using Ga-exchanged HZSM-5 catalysts: a DFT pathway

400

analysis. Catal. Today 2006, 105, 106 – 121.

401

[19]

Meraudeau, P.; Naccache, C. The role of Ga2O3 and proton acidity on the dehydrogenating

402

activity of Ga2O3-HZSM-5 catalysts: evidence of a bifunctional mechanism. J. Mol. Catal.

403

1990, 59, L31-L36.

404 405

406

[20]

Nakamura, I.; Fujimoto K. On the role of gallium for the aromatization of lower paraffins

with Ga-promoted ZSM-5 catalysts. Catal. Today 1996, 31, 335 – 344. [21]

Yakerson V. I.; Vasina T. V.; Lafar, L. I.; Sytnyk, V. P.; Dykh, G. L.; Mokhov, A. V.;

407

Bragin, O. V.; Minachev K. M. The properties of zinc and gallium containing pentasils – the

408

catalysts for the aromatization of lower alkanes. Catal. Lett. 1989, 3, 339 – 346.

409

[22]

Giannetto, G.; Leόn, G.; Papa, J.; Monque, R.; Galiasso, R.; Gabelica Z. A new way to

410

obtain acid or bifunctional catalysts. II. Straightforward calcination of as-synthesized [Ga,Al]-

411

ZSM-5 zeolites obtained from alkali-free media. Catal. Today 1996, 31, 317 – 326.

412 413

414

[23]

Nishi, K.; Komai, S.; Inagaki, K.; Satsuma, A.; Hattori, T. Structure and catalytic properties

of Ga-MFI in propane aromatization. Appl. Catal., A 2002, 223, 187 – 193. [24]

Choudhary, V. R.; Kinage, A. K.; Sivadinarayana, C.; Guisnet, M. Pulse reaction studies on

415

variations of initial activity/selectivity of O2 and H2 pretreated Ga-modified ZSM-5 type zeolite

416

catalysts in propane aromatization. J. Catal. 1996, 158, 23 – 33.

417

[25]

Bayanse, C. R.; van Hooff, J. H. C. Aromatization of propane over gallium-containing H-

418

ZSM-5 zeolites: influence of the preparation method on the product selectivity and the catalytic

419

stability. Appl. Catal., A 1991, 79, 127 – 140.


Page 19 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

420


[26]

Choudhary, V. R.; Sivadinarayana, C.; Kinage, A. K.; Devadas, P.; Guisnet, M. H-

421

Gallosilicate (MFI) propane aromatization catalyst: influence of calcination temperature on

422

acidity, activity and deactivation due to coking. Appl. Catal., A 1996, 136, 125 – 142.

423

[27]

Choudhary, T. V.; Kinage, A.; Banerjee, S.; Choudhary, V. R. Propane Conversion to

424

Aromatics on Highly Active H-GaAlMFI: Effect of Thermal Pretreatment. Energ. Fuels 2006,

425

20, 919-922.

426

[28]

de O. Rodrigues, V.; Eon, J.-G.; Faro, A. C. Correlations between Dispersion, Acidity,

427

Reducibility, and Propane Aromatization Activity of Gallium Species Supported on HZSM5

428

Zeolites. J. Phys. Chem. C 2010, 114, 4557–4567.

429

[29]

Satsuma, A.; Gon-no, A.; Nishi, K.; Komai, S.; Hattori, T. Contributions of three types of

430

Ga sites in propane aromatization over Ga2O3/Ga-MOR catalysts. Stud. Surf. Sci. Catal. 2006,

431

159, 257-260.

432

[30]

Xiao, H.; Zhang, J.; Wang, X.; Zhang, Q.; Xie, H.; Han, Y.; Tan, Y. A highly efficient

433

Ga/ZSM-5 catalyst prepared by formic acid impregnation and in situ treatment for propane

434

aromatization. Catal. Sci. Technol. 2015, 5, 4081–4090.

435

[31]

Wannapakdee, W.; Wattanakit, C.; Paluka, V.; Yutthalekha, T.; Limtrakul, J. One-pot

436

synthesis of novel hierarchical bifunctional Ga/HZSM-5 nanosheets for propane aromatization.

437

RSC Adv. 2016, 6, 2875–2881.

438

[32]

Ogunronbi, K.E.; Al-Yassir, N.; Al-Khattaf, S. New insights into hierarchical metal-

439

containing zeolites; synthesis and kinetic modelling of mesoporous gallium-containing ZSM-5

440

for propane aromatization. J. Mol. Catal. A 2015, 406, 1–18.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

441

[33]

Page 20 of 42

Choudhary, T. V.; Kinage, A.; Banerjee, S.; Choudhary, V. R. Influence of space velocity

442

on product selectivity and distribution of aromatics in propane aromatization over H-GaAlMFI

443

zeolite. J. Mol. Catal. A: Chem 2006, 246, 79 – 84.

444

[34]

Choudhary, T. V.; Kinage, A. K.; Banerjee, S.; Choudhary, V. R. Effect of temperature on

445

the product selectivity and aromatics distribution in aromatization of propane over H-GaAl-MFI

446

zeolite. Microporous and Mesoporous Mater. 2004, 70, 37 – 42.

447

[35]

Choudhary, V. R.; Devadas, P. Product selectivity and aromatics distribution in

448

aromatization of propane over H-GaMFI zeolite: influence of temperature. Microporous and

449

Mesoporous Mater. 1998, 23, 231 – 238.

450

[36]

Akhtar, M.N.; Al-Yassir, N.; Al-Khattaf, S.; Cejka, J. Aromatization of alkanes over Pt

451

promoted conventional and mesoporous gallosilicates of MEL zeolite. Catal. Today 2012, 179,

452

61-72.

453

[37]

Shen, X.C.; Lou, H.; Hu, K.; Zheng, X.M. Non-oxidative aromatization of C1 to C3

454

hydrocarbons over Pd-promoted Ga/HZSM-5 catalyst under mild conditions. Chinese Chem.

455

Let. 2007, 18, 479-482.

456 457

458 459 460 461

462 463

[38]

Bhan, A.; Hsu, S.; Blau, G.; Caruthers, J. M.; Venkatasubramanian, V.; Delgass, W. N.

Microkinetic modeling of propane aromatization over HZSM-5. J. Catal. 2005, 235, 35 – 51. [39]

Rodrigues, V. O.; Faro Júnior, A. C. On catalyst activation and reaction mechanisms in

propane aromatization on Ga/HZSM5 catalysts. Appl. Catal., A, 2012, 435 – 436, 68 – 77. [40]

Bayense, C. R.; van der Pol, A. J. H. P.; van Hoff, J. H. C. Aromatization of propane over

MFI-gallosilicates, Appl. Catal. 1991, 72, 87 – 98. [41]

Luzgin, M.V.; Gabrienko, A.A.; Rogov, V.A.; Toktarev, A.V.; Parmon, V.N.; Stepanov,

A.G. The “Alkyl” and “Carbenium” Pathways of Methane Activation on Ga-Modified Zeolite 20 ACS Paragon Plus Environment

Page 21 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


464

BEA: 13C Solid-State NMR and GC-MS Study of Methane Aromatization in the Presence of

465

Higher Alkane. J. Phys. Chem. C 2010, 114, 21555–21561.

466

[42]

Kazansky, V. B.; Frash, M. V.; Van Santen, R. A. A quantum – chemical study of hydride

467

transfer in catalytic transformations of paraffins on zeolites. Pathways through adsorbed

468

nonclassical carbonium ions. Catal. Lett. 1997, 48, 61 – 67.

469

[43]

Yu, S. Y.; Yu, G. J.; Li, W.; Iglesia, E. Kinetics and reaction pathway for propane

470

dehydrogenation and aromatization on Co/H-ZSM5 and H-ZSM5. J. Phys. Chem. B 2002, 106,

471

4714 – 4720.

472 473

474 475

476

[44]

Buchanan, J. S.; Santiesteban, J. G.; Haag, W. O.; Mechanistic consideration in acid-

catalyzed cracking of olefins. J. Catal. 1996, 158, 279 – 298. [45]

Joshi, Y. V.; Thomson, K. T. Embedded cluster (QM/MM) investigation of C6 diene

cyclization in HZSM-5. J. Catal. 2005, 230, 440 – 463. [46]

Krishnamurthy, G.; Bhan, A.; Delgass, W. N. Identity and chemical function of gallium

477

species inferred from microkinetic modeling studies of propane aromatization over Ga/HZSM-5

478

catalysts. J. Catal. 2010, 271, 370–385

479

[47]

Matsuoka, A.; Sakuma, S.; Onodera, M.; Kubota, H. Effect of Ga content and reaction

480

pressure upon the aromatization of propane over H-Ga-Al-bimetallosilicate catalysts. J. Porous

481

Mater. 2013, 20, 237 – 373.

482

[48]

Frusteri, F.; Migliori, M.; Cannilla, C.; Frusteri, L.; Catizzone, E.; Aoise, A.; Giordano, G.;

483

Bonura, G. Direct CO2-to-DME hydrogenation reaction: new evidences of a superior behavior

484

of FER-based hybrid systems to obtain high DME yield. J. CO2 Util. 2017, 18, 353– 361.

485 486

[49]

Migliori, M.; Aloise, A.; Catizzone E.; Giordano G. Kinetic analysis of methanol to

dimethyl ether reaction over H-MFI catalyst. Ind. Eng. Chem. Res. 2014, 53, 14885 – 14891. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

487 488

489

[50]

Page 22 of 42

Catizzone, E.; Aloise, A.; Migliori, M.; Giordano, G. Dimethyl ether synthesis via methanol

dehydration: effect of zeolite structure. Appl. Catal., A 2015, 502, 215 – 220. [51]

Catizzone, E.; Aloise, A.; Migliori, M.; Giordano, G. From 1-D to 3-D zeolite structure:

490

performance assessment in catalysis of vapour-phase methanol dehydration to DME, Microp.

491

Mesop. Mater. 2017, 243, 102 - 111.

492

[52]

Choudhary, V. R.; Devadas, P. Influence of space velocity on product selectivity and

493

distribution of aromatics and xylenes in propane aromatization over H-GaMFI zeolite. J. Catal.

494

1997, 172, 475 - 478.


Page 23 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 (▲).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Page 27 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Figure 1 ACS Paragon Plus Environment

Page 28 of 42

Page 29 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Figure3 ACS Paragon Plus Environment

Page 30 of 42

Page 31 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Page 32 of 42

Page 33 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Page 34 of 42

Page 35 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Page 36 of 42

Page 37 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Page 38 of 42

Page 39 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Page 40 of 42

Page 41 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 42 of 42

GRAPHICAL ABSTRACT

42


Simplified Kinetic Modeling of Propane ... - ACS Publications

Recommend Documents