Utilization of Dielectric Properties Assessment To Evaluate the

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Utilization of Dielectric Properties Assessment to Evaluate the Catalytic Activity and Rate of Deactivation of Heterogeneous Catalysts Tudor Ioan Sibianu, Georgios A. Dimitrakis, Juliano Katrib, Cristian Matei, Daniela Berger, Christopher John Dodds, Adrian V. Surdu, Ioan Calinescu, and Derek J Irvine Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04341 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 12, 2017

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Industrial & Engineering Chemistry Research

Utilization of Dielectric Properties Assessment to Evaluate the Catalytic Activity and Rate of Deactivation of Heterogeneous Catalysts Tudor I. Sibianu,a Georgios Dimitrakis,b Juliano Katrib,b Cristian Matei,a Daniela Berger,a Christopher Dodds,b Adrian V. Surdu,c Ioan Calinescua,* and Derek J. Irvineb,* a.

Department of Bioresources and Polymer Science, University “Politehnica” of Bucharest, Gh. Polizu Street, No. 1-7, Postal code 011061, Romania. b. Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, UK c. Department of Science and Engineering of Oxide Materials and Nanomaterials, University “Politehnica” of Bucharest, Gh. Polizu Street, No. 1-7, Postal code 011061, Romania

* Corresponding authors: I. Calinescu contact E-mail address [email protected] and Derek J. Irvine contact E-mail address [email protected] Keywords: Fischer-Tropsch, Dielectric properties, heterogeneous catalysis, carbon deposition

Abstract The use of dielectric property assessment to gauge the catalytic activity and rate of deactivation of heterogeneous catalysts is reported. Four supported catalysts containing a combination of Fe and Ni active sites and γ-Al2O3, ZSM-5, MCM-41, and SBA-15 supports were synthesized, characterized and utilized to catalyze a Fischer Tropsch process over a temperature range of 250° - 400°C that was specifically directed toward the production of lower olefins. Whilst the highest conversion was obtained from ZSM-5 and MCM-41 supports containing Fe and Ni as active sites at 350°C, all these catalysts were observed to be deactivated by the formation of carbon on their surface. The dielectric properties of the fresh, used catalysts and supports were evaluated and correlated with their catalytic activity and structural/textural properties. It was clearly shown that the dielectric property measurement could demonstrate both the presence and magnitude of

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carbon deposits on the catalyst via the differences in the values of fresh and used catalysts. Furthermore, the ability to differentiate between the levels of the carbon deposition observed was shown to be independent of the morphology exhibited by the carbon deposit demonstrating that this is a method that can be generally applied.

Introduction One very important current commercial application of heterogeneous catalysts is in the Fischer Tropsch (F-T) process, which synthesizes artificial fuels from syngas.1 This consists of the formation of higher chain length hydrocarbons from single carbon atom gaseous feedstock. However, in recent years there has been an increasing interest in the synthesis of lower olefins via F-T processes.2-5 This is because an efficient F-T method for manufacturing light olefins is predicted to be significantly less energy intensive than the current benchmark methods for produce these products (fluid catalytic cracking and pyrolysis processes). Therefore, such a process would reduce the chemical producing industries dependence upon crude oil as a feedstock.6 To date, metallic supported heterogeneous catalysts have been extensively used in the F-T synthesis. The first catalysts utilized amorphous solids, such as Al2O3, SiO2, and C, as supports. However, after the reduction treatments to deposit the metal active sites had been conducted, these systems were typically found to exhibit a broad, and sometimes multimodal, distribution of metallic crystal sizes. Consequently, the use of zeolites supports has been adopted in order to obtain the desired uniform dispersion of metal active sites throughout the support structure. However, whilst this was initially demonstrated to result in an improved active site structure, it was also shown that undesirable migration of Fe out of the pores occurred during use.7 This, in turn, has led to interest in materials such as mesoporous solids as F-T supports, because they


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exhibit narrower pore size distributions and larger pore diameters than the zeolite previously used.8 A common problem that occurs during F-T processes is catalyst deactivation due to surface carbon deposition, which decreases the conversion to product, service longevity and chemical selectivity that the process exhibits. This is especially true for a F-T process focused on the selective formation of light olefins, because the necessary operating conditions (high temperature and low H2:CO molar ratio) have been shown to promote both carbon deposition and formation of CO2 in excess.2 Therefore, from a process sustainability perspective, it would be highly desirable to be able to develop facile, non- intrusive methods to determine / monitor the level of fouling on such a catalyst. This would allow conditions to be systematically varied to optimize the process by both maintaining the efficiency of the reaction by maximizing the conversion toward the formation of desired final products, whilst simultaneously prolonging the catalysts operational lifetime by minimizing deactivation. It will also allow plant shutdowns for catalyst replacement to be systematically planned, so minimizing their impact of overall plant capacity. A number of recent literature studies have demonstrated the viability of dielectric spectroscopy to monitor chemical transformations in a non-invasive and non-destructive manner.9,10 This technique relies on the determination of changes in the complex permittivity of the reaction medium involved in the reaction as the transformation proceeds. The complex permittivity (ε*) determines both the extent and nature of how a material will interact with an incident electromagnetic (EM) field.11 It is expressed as a complex number with a real part (ε’), often termed dielectric constant, and an imaginary part (ε’’) termed dielectric loss (see Equation 1).

∗ = ′ − " (Equation 1)


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When the material is subjected to an external electric field, ε’ denotes the ability of a material to store energy in its structures, whereas ε’’ is a measure of the ability of the material to convert this stored energy into heat. Furthermore, ε* can be used to determine the level of penetration of EM energy into the material by calculating the penetration depth (D). D is defined as the distance into the bulk of a material where the initial electric field amplitude (Eo) of the incident EM wave, has decreased by 63%, and this can be calculated using Equation 2.

D=

λ ε′ (Equation 2) 2πε ′′

Where γ is the wavelength in meters of the EM wave (at the propagating frequency) and ε’, ε’’ have defined previously.

The complex permittivity of a material is highly depended on the interactions occurring between induced dipoles and electrical charges within the molecular structures of the materials that constitute the sample under study.12 This means that the presence of impurities that are highly dipolar or conductive in nature will have an influence on the materials permittivity. Therefore, since carbon is typically associated with high electrical conductivity, the authors proposed that its deposition on the surface of the catalyst would have a measurable effect on the total ε* of the sample. Furthermore, if this was the case, this influence of the deposited carbon on the observed value of ε* should therefore become more pronounced as the amount of the carbon deposited builds with time, leading to catalyst deactivation. As a result, it should be possible to determine the amount of fouling present on a catalyst by following the change in ε* of a catalyst bed in real time. Furthermore, if the penetration depth of the EM wave into the catalyst bed is sufficient to ensure that the entire volume of the bed is being monitored, then this deactivation assessment should be possible to conduct without the need to take a sample from the


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catalyst bulk, significantly simplifying and improving the safety of catalyst monitoring for such process as F-T which operate at high pressures and temperatures.13,14 The paper reports work to examine if the above hypothesis is valid by carrying out a dielectric property studies upon commonly used F-T catalysts and supports both in their “clean” form (i.e. pre-use), and after use when they have been contaminated with carbon depositions. The results of the study will be used to evaluate whether dielectric spectroscopy could be used to accurately assess and / or monitor the condition of the catalysts “in-situ” and therefore opening up new possibilities, for the improvement and optimization of F-T reactions.

Experimental methods Materials: All materials were used as received with no further purification. All supports were impregnated with metal sites following the methods described in prior publications.15,16 Catalyst preparation: Four types of F-T catalysts were synthesized by depositing Fe and Ni upon four supports with different molecular structures (γ-Al2O3, ZSM 5, MCM 41, SBA 15) using a concentrated aqueous solution containing iron nitrate and nickel nitrate with a molar ratio of Fe: Ni of 4:6.15,16,17,18 The volume of the concentrated aqueous impregnating solution required in each preparation was calculated based on the supports’ porosity (pore volume) to maximize active the phase content deposition achieved from a single impregnation step. In an example case, where γ-Al2O3 has a pore volume of 0.43 mL/g, 1 g of γ-Al2O3 was impregnated with 0.50 mL solution of Fe(NO3)3*9H2O and Ni(NO3)2*6H2O which corresponded to 0.72 g in total. The amount of nitrates used corresponded to a target 4 to 6 mole ratio based on the metal content. The impregnated supports were then dried and extruded into a cylindrical shape using hydroxylated alumina as binder in a 1: 1(wt) support-binder ratio and an extruder. After their extrusion, they were dried at 150 °C for 16 hours and then calcined at 550° for 6 hours. The


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predicted elemental compositions of the synthesized catalysts, based on the quoted molecular compositions of the raw materials used in this study, are shown in Table 1. Characterization of catalysts: Thermogravimetric Analysis (TGA): TGA was performed in air with a constant flow rate of 20 mL/min over the temperature range of 25 - 900 °C at a heating rate of 10 °C/min on a Netzch STA 449 F3 Jupiter TGA apparatus.

Table 1. Predicted elemental composition of the fresh catalysts. Theoretical Composition (%) Element γ-Al2O3 ZSM-5 MCM-41 SBA-15 O 41.35 44.9 42.97 43.1 Al 46.52 25.17 22.53 24.82 Si 0 19.3 18.62 17.86 Fe 4.82 4.16 6.31 5.65 Ni 7.31 6.31 9.57 8.57

Porosimetry: Porosities of both fresh and used catalysts were determined by nitrogen adsorption/desorption isotherms measured at 77 K on a Quantachrome Autosorb IQ2 porosimeter in the relative pressure, P/P0, in the range of 0.025 - 0.995. The specific surface area and average pore size values were determined by the Brunnauer-Emmett-Teller (BET) method and Barret-Joyner-Halenda (BJH) model, respectively. The samples were initially degassed under vacuum at 300 °C for 6h. Scanning Electron Microscopy (SEM): SEM analysis was performed on a Quanta Inspect F 50 electron microscope, at an acceleration voltage of 30 kV, coupled with an EDX spectrometer. SEM images were obtained on both the cross section and on the external surface of the extrudates. The EDX measurements were taken on the selected sites of the catalyst surface as well as on a larger cross section of the catalyst.


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Dielectric property measurements (DP): DP analysis was conducted in air, over a temperature range of 20 – 420°C with a step interval of 50 °C at both 912 and 2470 MHz using the cavity perturbation method described in previous literature reports.19-21 Cavity perturbation was adopted because it is known to be better suited for measurements of low microwave absorbing materials (i.e. those with low values of ε”, also termed as low loss materials) and powdered solid materials, such as these solid phase F-T catalysts used in the this study. Use of the cavity perturbation technique has for the determination of the dielectric properties of aluminosillicate materials via a contact free and non-invasive methodology has been reported in the literature.22 This, allowed the samples to be recovered, and avoided contact resistance and any interaction problems related to the electrodes.22 The system used was based upon a copper resonating cylindrical cavity with the following identified resonant TM0n0 modes; TM020 at 912 MHz and TM050 at 2470 MHz. No detailed calibration was required as the measurement is based on the differential measurement of an empty and loaded cavity relying on the monitoring of S21 scattering parameters. The measurement error for ε΄, typically associated with this technique, is less than 2%. Each measurement was repeated fifteen times at the different selected bulk densities of the powdered sample and the mean value of measurements, the associated standard deviation and the relative standard errors were determined. Determination of Catalytic Activity: The standard catalyst testing procedure was as follows. The test sample, 1 g fresh catalyst along with glass beads to occupy any free space in the catalyst bed, was loaded into the reactor (a quartz tube), which was then heated by an electric furnace to the target reaction temperature. The catalysts temperature was measured with a K-type thermocouple placed at the center of the catalyst bed and was digitally controlled via a feedback loop. The activation and regeneration of the catalyst was carried out in a N2:H2 mixture with a 1:1 molar


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ratio, at a flow rate of 40 mL/min. The catalytic tests were performed at atmospheric pressure and over a temperature range of 250 - 400°C, for each temperature a series of 3 experiments were taken each lasting 20 minutes. The syngas was obtained by mixing CO and H2 at a molar ratio 1: 1.23,24

Results and discussion The experimentally determined molecular properties of the catalysts synthesized in this study are presented in Table 2, which includes the specific surface area calculated using the BET - method, total pore volume and the average pore size determined by BJH model from the desorption branch of the isotherm.25,26 The addition of the binder was defined as the root cause of the reduced porosity exhibited by the catalyst samples when compared to the values of the standalone support.

Table 2. Textural properties of the fresh and used catalysts Pore diameter by Pore volume by Specific surface by Catalyst Typea BJH method, dBJH BJH method, Vp BET method, SBET 3 (nm) (cm /g) (m2/g) γ-Al2O3 unloaded support 5 0.43 239 γ-Al2O3 fresh catalyst 3.7 0.4 217 γ-Al2O3 used catalyst 3.7 0.34 200 ZSM-5 unloaded support 4.8 0.31 304 ZSM-5 fresh catalyst 3.5 0.25 214 ZSM-5 used catalyst 3.7 0.29 262 MCM-41 unloaded support 3.7 0.65 586 MCM-41 fresh catalyst 2.7 0.52 479 MCM-41 used catalyst 2.7 0.42 417 SBA-15 unloaded support 4.1 0.55 441 SBA-15 fresh catalyst 3.7 0.45 344 SBA-15 used catalyst 3.7 0.45 344 a Unloaded support is 1 : 1 (wt) ratio mixture of the corresponding support and γ-Al2O3 binder. Prior to use in the F-T activity test, the fresh catalysts were coextruded with hydroxylated alumina as binder. The later was added to enable the powdered catalysts to be shaped into a


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processable profile. Both the specific surface area and the total pore volume values for the γAl2O3 ZSM-5, SBA-15 and MCM-41 based catalysts were observed to be lower than the standalone supports (Table 2). These decreases were attributed to the partial blocking of catalyst pores by the binder. In certain cases, differences were also found between the molecular properties of the fresh and used catalysts, which also typically resulted in reduced pore volume/size in the used catalyst. This was attributed to the different levels of carbon deposition on the catalysts during use. This data was found to correlate well to the total level of carbon found on the used catalysts via TGA analysis. γ-Al2O3 and SBA-15 were found to exhibit lower levels of final carbon content/deposition (4.4 & 5.6%) than the ZSM-5 and MCM-41 catalysts (10.8 & 8.9%), which resulted in the latter exhibiting more pronounced reductions in pore volume and specific area (see Electronic Supplementary Information (ESI) Figure S1 for thermograms). The X-ray diffraction patterns showed the preservation of the crystalline structure of MCM-41 catalyst (Figure 1), SBA-15 (Figure 2) and ZSM-5 (see Electronic Supplementary Information (ESI) Figures S2 C and S2 D). Fresh catalyst Used Catalyst

80000 Intensity

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60000 40000 20000 0

0

2 4 6 8 10 Position (2°Theta) (Copper (Cu))

Figure 1. XRD pattern (small angle) for the MCM-41 fresh (left) and used (right) catalysts.


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50000

Fresh Catalyst Used Catalyst

40000 Intensity

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30000 20000 10000 0 0

2 4 6 8 10 12 Position (2°theta)(Copper(Cu)) Figure 2. XRD pattern (small angle) for the SBA-15 fresh (left) and used (right) catalysts.

The SEM investigation (Figure 3) showed similar surface morphology for three out of the four fresh catalysts samples, this similarity was attributed to the presence of the binder.

Figure 3. SEM micrographs of the surface of the fresh catalysts based on γ-Al2O3 (A), ZSM-5 (B), MCM-41 (C), SBA-15 (D).

Figure 4. SEM micrographs of the cross section for fresh catalysts based on γ-Al2O3 (A), ZSM-5 (B), MCM-41 (C), SBA-15 (D).


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However, the SBA-15-based sample was an exception as it exhibited elongated particle shape, and cuboid shape metallic particles can be observed on cross-section images (Figure 4 D). This shape differences were a possible cause of the reduced carbon build up and the associated negligible reduction in the porosity upon use. Differences between the fresh and used catalyst were also observed in the SEM images (see Figure 5).

Figure 5. SEM micrographs of the cross section for used catalysts based on γ-Al2O3 (A), ZSM-5 (B), MCM-41 (C), SBA-15 (D).

These took the form of the formation of fibber-like structures upon the used catalysts surface. These were concluded to be carbon deposits and they exhibited varied aspect ratios having a diameter in the range of 15 - 25 nm. In the case of ZSM-5 and MCM-41 based used catalysts it was observed that a higher amount of carbon had been deposited on their surfaces. This was confirmed by the combination of the TGA analysis the changes in porosity and surface area data detailed in Table 1 and Figure S1. XRD measurements performed on the cross-section of the fresh catalysts revealed the presence of Ni and Fe in a similar weight ratio to that of the impregnation solution used in their synthesis. However, an uneven distribution was found, with a smaller than expected amount of metals located on the surface. For the used catalysts, the presence of carbon was also detected in the XRD spectra (see ESI Figures S2D, S2F, S2A and S2H) and again the amount of carbon formed for ZSM-5 and MCM-41 based catalysts were found to be higher than the γ-Al2O3 and SBA-15.


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Evaluation of Catalytic Activity: The prepared catalysts were utilized in an F-T process a molar ratio of 1:1 CO:H2 to target the production of lower molecular weight olefins, following the method described in the experimental section. The experiments evaluated their catalytic activity for olefin synthesis over a reaction temperature range of 250 – 400 °C (see Figure 6 and Figure S3). It is known that utilizing such a feed ratio also favors the formation of carbon on the catalyst

60 40

ZSM 5 MCM 41 SBA 15

40

20

20

0

200 250 300 350 400 450 o Temperature ( C) 20 D 15 10 5 0

γAl2O3

60

200 250 300 350 400 450 o Temperature ( C)

Alkane Selectivity (%)

CO2 Selectivity (%)

80 B

100 A 80 60 40 20 0 200 250 300 350 400 450 o Temperature ( C) 80 C CH4 Selectivity (%)

CO Conversion (%)

surface and so lowering its overall catalytic activity.

Olefin Selectivity (%)

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0

200 250 300 350 400 450 o Temperature ( C) 20 E 15 10 5 0

200 250 300 350 400 450 o Temperature ( C)

Figure 6. Plots of CO feedstock conversion and selectivity’s toward the main products for the different catalyst types in the F-T process, where CO feedstock conversion = (A) and selectivity toward CO2 = (B), CH4 = (C), C2-C4 olefins = (D) and C2-C4 alkanes (E)


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Figure 6 contains the data from these experiments showing carbon monoxide feedstock conversion (Figure 6A) and selectivity toward CH4 (Figure 6B), CO2 (Figure 6C) and lower olefins (Figure 6D) for the different catalyst types. The use of higher temperatures was observed to promote conversion of CO2. The highest CO conversion (90 %) was obtained for ZSM-5based catalyst when run at a temperature of 400 °C. The product stream at these conditions mainly consisted of CO2 (~40% yield) and CH4 (~60 % yield). Production, of the target low chain olefins constituted only ~1% of the product stream at this temperature. Meanwhile, the MCM-41 and SBA-15-based catalysts exhibited the highest selectivity toward low olefin production (12.5% and 12.1%, respectively) at 350 °C, but CO conversion was only of the order of 15 – 12%). The total olefins yield (including methane) was also found to increase with temperature up to 350°C. However, at 400 °C the total olefin yields were noted to be slightly reduced. Closer inspection of the data defined that this was because there was almost 100% selectivity toward methane production over the olefins (Figures 6 C & 6B). Thus, 350 °C was concluded to the most favorable temperature to promote the formation of the target low molecular weight olefins (Figure 6D). Evaluation of dielectric properties and variation: The dielectric properties of the catalysts were then evaluated so that it could be determined if this measurement could give a more facile indication of the level of carbon deposition of different catalysts compare to the more involved techniques that are currently deployed. The data in Figure 7 showed that no major difference in the variation of ε’ values between the supports and fresh catalysts were noted within the temperature range of 20 - 420 °C for γ-Al2O3 and SBA 15. However, MCM 41 and ZSM 5 did exhibit a slight variation in the trend of ε’ between the support and fresh catalyst in the high temperature region.


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6

γAl2O3

12 ZSM-5

5

e'

e'

3 2 1

support fresh catalyst used catalyst

9

4

6 3

0

100 200 300 400 Temperature (°C)

0

8 MCM-41 e'

6 e'

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4 2 0

100 200 300 400 Temperature (°C)

100 200 300 400 Temperature (°C)

6 SBA-15 5 4 3 2 1 0 100 200 300 400 Temperature (°C)

Figure 7. Variation in Dielectric constant with temperature for the F-T catalysts measured at 2470 MHz (standard deviation ~ 0.1).

It would appear that coating the support with the metal catalyst has led to some alterations in the molecular response of MCM41 and ZSM 5 to the externally applied EM field. In the case of fresh MCM41, the ε’ values are suppressed for at high temperatures, whereas for ZSM 5 the values are increased. The origins of these effects will require further study. By comparison, all the used catalysts were found to exhibit both higher values of and less variation in ε’ with temperature relative to the support and fresh samples. Both effects were attributed the carbon deposition onto the catalyst during use. As carbon is a good microwave


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absorber, it was proposed that it now dominated the dielectric constant signature for the materials so increasing the value and significantly reducing/removing the temperature dependence.27 Similar trends in ε’’ values with temperature (Figure S4) were found, with the used catalysts values being distinctively higher than those of fresh catalysts and unloaded supports. However, these values and differences were smaller in magnitude that those of ε’. Thus, it was concluded that ε’ was the property that exhibited the greatest promise for determining catalyst fouling. The values of ε’ for all the used catalyst samples were noted to decrease across the 20-200 °C temperature region. This was proposed to be related to the presence of trace water entrained within the catalyst (Figure 8). As water is also a strong microwave absorber and so was expected to increase the interaction of the material with the EM field up to the point that it evaporates as the sample temperature was increased.28 This conclusion was supported by subsequent experiments with the SBA-15 supported catalyst, where a series of dielectric measurements were conducted on a sample that was dried in the oven at 150 °C and one that was kept in open air for 195 hours. From the data presented in Figure 8, an increase in the dielectric constant was observed that was equal to the dielectric constant drop between 20-200 °C for the analyzed samples which were kept in open air.

Support

Used Catalyst

Fresh Catalyst

4

4

4

2

2

2 0

100 Time (h)

200

910 Mhz 2470 Mhz

ε'

ε'

ε'

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0

100 Time (h)

200

0

100 200 Time (h)

Figure 8. Influence of the trace water upon the dielectric constant for the SBA-15 support, fresh and used catalyst.


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In practice, the 200-400 °C temperature region is of greatest interest for F-T processes. In this interval, the dielectric properties of the used (deactivated) catalysts were noted to be significantly different from those of the fresh catalysts or the original supports. Figure 7 illustrates clearly that the used catalysts exhibit higher values of ε’ compared to the support material alone and the fresh catalyst. Table 3 illustrates both the 912 and 2470 MHz data obtained by thermogravimetric analysis (TGA) on the used catalysts, where ∆ε’ represent the carbon contribution to the dielectric constant of the used catalyst which has been calculated as a difference between ε’ for the used catalyst and the fresh catalyst at optimal reaction temperature of 350 °C respectively (for ε’ data at 912 MHz see ESI Figure S5). Table 3. Physical & Material Properties of the Used Catalysts. Property Catalyst support γ-Al2O3 ZSM 5 MCM41 SBA 15 Carbon content (%)a 4.4 10.8 8.9 5.6 Carbon contribution to Dielectric 1.1 6.8 5.2 1.2 Constant (∆ε’) at 912 MHz Carbon contribution to Dielectric 0.97 5.8 4 0.93 Constant (∆ε’) at 2470 MHz CO Feed Conversion (%) 14.2 46.9 21.8 17.8 b Aspect ratio of carbon deposits Short Long Long Particles fibres fibres fibres a Carbon content is calculated from TGA (see ESI SI1); b Aspect ratio of carbon structures was estimated based on direct observation from SEM images (Figure 3 and 4).

Again, the 2470 MHz ε’’ data were also found to exhibit the same trend but the absolute values and difference were smaller (see ESI Figure S6) thus this data confirmed that ε’ was the preferable dielectric property to use to monitor for carbon deposition on heterogeneous catalysts. Thus, in a practical system, whether applied in either the laboratory or commercial scale environment, a single comparison of dielectric constant would be recommended to reduce overall data complexity.


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The amount of carbon deposits (determined from TGA measurements) was compared to the difference between the values of the ε’ of the used and fresh catalysts (∆ε’) measured at both 912 and 2470 MHz. Since carbon is expected to be more microwave absorbent that the support material, it would be expected to make a much higher contribution to the overall dielectric properties of the used catalyst.29 A clear correlation between the magnitude of carbon deposited and size of the ∆ε’ value observed for all the catalyst samples studied. However, this was not a linear relationship, rather the ∆ε’ difference noted to become more marked as the carbon deposit increased in magnitude. In the experience of the authors, whilst trends in dielectric data are discernible, repeatable and reliable, they are typically found to be non-linear with temperature and/or quantity. This non-linearity is typically due to a combination of changes that occur simultaneously as the concentration or temperature of the system changes. Influencing factors include the (a) physical, chemical, environmental characteristics of the materials, (b) the apparatus/reactor geometry/conformation and (c) the processing conditions adopted e.g. batch or flow. Thus the data in Table 3 both; (a) confirmed that the deposited carbon was dominating the dielectric response of the used catalysts and (b) defined that in any monitoring process a calibration curve would be needed to accurately gauge the absolute level of deposition by making allowance for the non-linear response. Additionally, as the trend in the magnitude of the data were the same with the different types of carbon morphology that the carbon deposits had been shown to exhibit on the catalysts (γAl2O3 = “short fibers”, ZSM-5 and MCM-41 = “long fibers” and SBA-15 = “particles” - see Figure 2, Table 3), it would appear that the physical morphology of carbon deposit does not influence the value ε’ recorded. This determined that a dielectric monitoring process would be of


17


broad applicability as it is independent of different types of carbon morphologies that may be deposited within a single process. The variation in penetration depth of the EM energy into the catalyst bed was also investigated, as this may influence the design / applicability of a dielectric monitoring process for an industrial asset. These data, which have been calculated using Equations ESI1 and ESI2, are contained in

γAl2O3

800 600 400 200 0

0

100 200 300 400 Temperature (°C)

1000 MCM-41 800 600 400 200 0

0

100 200 300 400 Temperature (°C)

Penetration depth (cm)

1000



Figures 9 and S7 for 2470 and 912 MHz respectively.


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support fresh catalyst used catalyst

1000 ZSM-5 800 600 400 200 0 0 100 200 300 400 Temperature (°C)

1000 SBA-15 800 600 400 200 0

0

100 200 300 400 Temperature (°C)

Figure 9. Variation in Penetration depth of Microwave energy with temperature for F-T catalysts at 2470 MHz.

For the used ZSM-5 and MCM-41 supported catalysts, which had the highest carbon deposit content of 10.8% and 8.9%, the largest decrease in the penetration depth were observed. This represents a reduction from 200 cm to 2 cm and 400 cm to 4 cm respectively at 350°C and 2470


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MHz. For the used catalyst supported on γ-Al2O3 and SBA-15, where the carbon content is much smaller (4.4% and 5.6%), the decrease in penetration depth was observed to be less 250 cm to 13 cm and 800 cm to 45 respectively at 2470 MHz. Such a reduction in penetration depth is to be expected as the level of absorber increases. The magnitude of this difference was attributed to the fact that carbon is a highly efficient EM energy absorber, so significantly reduced the level of energy that is able to penetrate through the catalytic bed. This confirmed that care should be exercised in the design of monitoring system for full scale F-T reactions to ensure that the correct frequency of EM energy is used so that the entire catalyst bed is being monitored. Comparison of the data from Figure 7 and Figure S4, Figure S5 and Figure S6 show that 912 MHz would be the preferred frequency of the two tested in this study. At a lower frequency, it was noted that both the penetration depth of the microwaves energy increased and the absolute size of the dielectric constant values increased. Thus, the associated differences were greater, making differentiation between fresh and carbon coated catalyst more sensitive. However, even at this lower frequency, the penetration depth is noted to have dropped less than 10 cm at the ~10 % level of carbon deposited on the used ZSM-5 and MCM-41 catalysts. Therefore, for large scale industrial application of dielectric catalyst monitoring even lower frequencies than 912 MHz would have to be investigated to ensure that the energy had sufficient penetration to monitor the whole volume of the bed uniformly. This correlates well with what could be expected for the catalyst systems based on the value of their dielectric properties and the fact that λ increases with low frequency.

Conclusions Four Fe-Ni supported F-T catalysts were prepared and co-extruded with a hydroxylated alumina in a 1:1 wt: wt support-binder ratio to form usable profiles. When tested in a Fischer-Tropsch


19


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process operated over a reaction temperature range of 250 – 400 °C with a feed gas molar ratio of 1:1 CO:H2 to favor the targeted lower olefin formation. It was found that conversion of the CO increased for all catalysts as the temperature was raised. Furthermore, the catalyst based on ZSM-5 support, exhibited the highest CO conversion by a significant margin. The optimum temperature to produce hydrocarbons was 350 °C, however even at this temperature the hydrocarbon was predominantly methane, with olefins (i.e. C2, C3 and C4 carbon length olefins). Thus, these catalysts were shown not highly selective toward the production of the target olefins. Furthermore, at this feed ratio the catalyst was also found to form carbonaceous deposits in the form of particles during use. Investigation of the dielectric properties of the supports, fresh and used catalysts showed that the greatest influenced upon their dielectric properties of the catalyst bed was the formation of carbon structures on the catalysts surface. Whilst the dielectric properties of the support and the fresh catalyst were essentially identical, those of the used catalysts presented significantly higher ε’ and ε’’ values. However, of these two properties the dielectric constant, ε’, was shown to be the more appropriate to use as the values and measurement differentials were greater. Furthermore, it was demonstrated that the magnitude of the ε’ value could be used to gauge, and differentiate down to the single percentage levels, the amount of carbon deposit present. Although this was noted not to be a linear relationship, rather the relative differences increasing as the carbon loading increased. Thus, when monitoring a real process, it is likely that a calibration curve will need to be produced to relate the dielectric property being monitored with the actual percentage carbon deposition that have been reached. The data also showed that the measurement was independent of the different morphology types of carbon deposits created on the surface, thus such a monitoring process should have a broad applicability. As the carbon deposit increased it was observed that the penetration depth is


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significantly reduced. It was shown that by moving to lower frequency both the level of differentiation exhibited in the dielectric constant and loss was increase and so was the level of penetration at higher carbon loading (~10%). Thus, even lower frequencies may be more suitable to a full-scale process to ensure that the entire catalyst bed is monitored. Thus, dielectric monitoring has been shown to present significant potential to enable accurate determination of the level of fouling that a catalyst is experiencing and this potentially could allow the process conditions to be optimized to maximize the sustainability of such a catalytic process and ensure that the timing of its replacement to be gauged accurately. Furthermore, it has the advantage that could be implemented as a non-destructive and non-invasive analytical technique.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ????.

This document contains seven figures presenting additional material analysis conducted upon the support and fresh / used catalysts that is complimentary to that presented in the manuscript, including thermogravimetric, X-ray diffraction, Dielectric Property and Penetration depth data, and background theory underpinning the cavity perturbation techniques used to collect the dielectric property data and the methods used to calculation of the penetration depth used.

Acknowledgements


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This study was supported by the Sectorial Operational Program for Human Resources Development project POSDRU/159/1.5/S/132397.

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12) Dimitrakis, G. A.; Garcia, I. J. V.; Lester, E.; Licence, P.; Kingman, S. W. Dielectric spectroscopy: a technique for the determination of water coordination within ionic liquids. Phys. Chem. Chem. Phys., 2008, 10, 2947-2951 13) Heng, P. W.; Loh, Z. H. Dielectric properties of pharmaceutical materials relevant to microwave processing: effects of field frequency, material density, and moisture content. J. of Pharm. Sci., 2010, 99, 941-957 14) Alam, M.; Lucero, A. Structure and dielectric characterization of a chemically vapor deposited diamond thick film, Mater. Sci. Eng., 1994, 27, 81-85 15) Fischer, N.; Steen, E.; Claeys, M. Structure sensitivity of the Fischer Tropsch activity and selectivity on alumina supported catalysts. J. Catal., 2013, 299, 67-80 16) Kang, S. K.; Bae, W. J.; Woo, J. K.; Prasad, S. P. S.; Jun, K. W. ZSM-5 supported iron catalysts for Fischer Tropsch production of light olefin; Fuel Process. Technol. 2010, 91, 399-403 17) Alvarez, A. M.; Bengoa, J. F.; Cagnoli, M. V.; Galleos, N. G.; Yeramihn, A. A.; Marchetti, S. G. Fischer Tropsch synthesis on iron catalysts supported on MCM-41 and MCM-41 modified with Cs. Stud. Surf. Sci. Catal., 2002, 142, 1339-1346 18) Cano, L. A.; Cagnoli, M. V. Effect of activation atmosphere on the activity of Fe catalysts supported on SBA-15 in the Fischer Tropsch Synthesis. J. Catal., 2011, 278, 310-320 19) Smith, A. D.; Lester, E.; Thurecht, K. J.; el Harfi, J.; Dimitrakis, G.; Kingman, S. W.; Robinson, J. P.; Irvine, D. J. Dielectric Properties of Free Radical Polymerizations: Molecular Symmetrical Initiators during Thermal decomposition. Ind. Eng. Chem. Res., 2010, 49, 1703-1710 20) Smith, A. D.; Lester, E.; Thurecht, K. J.; el Harfi J.; Dimitrakis, G.; Kingman, S. W.; Robinson, J. P.; Irvine, D. J. Temperature Dependence of the Dielectric Properties of 2,2’Azobis(2methyl-butyronitrile) (AMBN). Ind. Eng. Chem. Res., 2010, 49, 3011-3014 21) Navarrete, A.; Mato, R. B.; Dimitrakis, G.; Lester, E.; Robinson, J. R.; Cocero, M. J.; Kingman, S. W. Measurement and estimation of aromatic plant dielectric properties. Application to low moisture rosemary. Ind. Crops Prod., 2011, 33, 697-703 22) Katrib, J.; Folorunso, O.; Dodds, C.; Dimitrakis, G.; Kingman, S. W. Improving the design of industrial microwave processing systems through prediction of the dielectric properties of complex multi-layered materials. J. Mater. Sci., 2015, 50, 7591-7599 23) Li, T.; Wang, H.; Yanga, Y.; Xiang, H.; Li, Y. Study of an Iron Nickel bimetallic Fischer Tropsch synthesis catalyst. Fuel Process. Technol., 2013, 118, 1-8


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TOC Graphic Fresh Catalyst

12

4

Used Catalyst

9

ε'

ε' 2

6 3

0

200 400 Temperature ( oC) γAl2O3 ZSM 5 MCM 41 SBA 15

0

200 400 Temperature ( oC)

Carbon Content = 4.4 % Carbon Content = 10.8 % Carbon Content = 8.9 % Carbon Content = 5.6 %

Comparison of the variation of dielectric constant with temperature for fresh and used catalysts demonstrating that the deposition of carbon upon the catalyst during use has increased the value exhibited proportional to the amount of carbon deposited


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Utilization of Dielectric Properties Assessment To Evaluate the

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