The Effect of Zn on Offretite Zeolite Properties. Acidic

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The Effect of Zn on Offretite Zeolite Properties. Acidic Characterizations and NH3-TPD Desorption Models Yira Aponte, and Hugo I. de Lasa Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04474 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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Industrial & Engineering Chemistry Research

The Effect of Zn on Offretite Zeolite Properties. Acidic Characterizations and NH3-TPD Desorption Models Yira Aponte a, and Hugo de Lasa a*, a

Chemical Reactor Engineering Centre, Faculty of Engineering Science, University of Western Ontario, London, On., Canada N6A 5B9. *Corresponding author: Tel: 519-661-2144; Fax: 519-661-3498; E-mail: [email protected]

Abstract This article reports the effect of adding zinc to an offretite (OFF) zeolite. It is observed that zinc addition does not significantly change the specific surface area, the pore volume or the pore size distribution. It is noticed however, that zinc addition influences acid properties of the OFF significantly. For instance, when employing FTIR pyridine adsorption, it is observed that in the Zn-OFF, there are 2.5 times higher Lewis acid sites than Brönsted acid sites. Furthermore, when applying NH3-TPD, three acid sites were identified: a) abundant number of weak acid sites, b) fair number of moderate acid strength sites, and c) scarce number of strong acid sites. Furthermore, NH3 desorption kinetic parameters were also calculated. Two numerical methodologies, linear and nonlinear regression, were implemented. It was noticed that Ed augments with zinc as: Ed for OFF< Ed for Zn(2.0wt%)-OFF < Ed for Zn(3.5wt%)-OFF. Keywords: TPD; desorption kinetic; offretite desorption; Zn-offretite; NH3 TPD; desorption energies, desorption constants.

1. Introduction Zeolites of the offretite (OFF) family have been identified as having good prospects for a number of chemical processes such as: a) the transformation of toluene and the isomerization of mxylene 1,2, b) the catalytic selective reduction of NOx using Cu (II) ion exchange 3, c) methanol conversion using an OFF protonic form 4 d) n-hexane conversion to C3 hydrocarbons and oxylene conversion to p-xylene 5, and e) CO2 adsorption using K-OFF 6. Recently, the CREC research team at the Chemical Reaction Engineering Center, The University of Western Ontario proposed a new application for OFF zeolites 7,8 in FCC units. They can be used as an additive for sulfur reduction in gasoline (refer to Fig. 1). This selective adsorption takes advantage of the OFF topology consisting of 8 smaller member ring pores and OFF acid sites 7–10. Changes in the configuration of the cyclones allow one to collect a thiophenic rich stream from the outlet gases from the stripper. 1

ACS Paragon Plus Environment


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Fig. 1. Thiophene selective adsorption process using a sulfur removal additive in FCC units The effect of zinc in the framework (loaded by direct synthesis) of other zeolites, such as ZSM-5, ZSM-11 and Beta has already been reported 11–13. To our knowledge however, there is no published study that evaluates the Zn influence on OFF acidic properties. To accomplish this, NH3-TPD, a commonly used method for measuring the surface acidity of zeolites 14–17, is employed to study the following offretites: a) an OFF zeolite free of zinc, b) an OFF with a 2.0wt% Zn loading designated as Zn(2.0wt%)-OFF and c) an OFF with 3.5wt% Zn, loading designated as Zn(3.5wt%)-OFF zeolites. NH3-TPD is also employed to determine ammonia desorption kinetic parameters in these three OFF materials. NH3-TPD studies are complemented with FTIR. It is found, as advised by our research group 18–20 and as described later in this manuscript, that nonlinear numerical analysis is required, in order to assess desorption parameters. This rigorous method allows establishing realistic values for ammonia desorption parameters. This is the case for the three observed acid sites of the OFF. 2. Experimental Method 2.1. Syntheses of the OFF, the Zn (2.0wt%)-OFF and the Zn (3.5wt%)-OFF Zeolites The syntheses of the Zn-OFF additives are described in a previous work 9. In particular, the Zn(3.5wt%)-OFF and the Zn(2.0wt%)-OFF zeolites were synthesized by incorporating Zn during the zeolite preparation, using 0.04 and 0.02 ZnO/SiO2 molar ratios, respectively. The first step 2


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was to form a gel with: a) sodium silicate (26.7 wt% SiO2 and 10.8 wt% Na2O), b) aluminum hydroxide hydrate with a molar ratio of 0.082 (Al2O/SiO2), c) a tetramethylammonium chloride solution (TMA) at 5M with a molar ratio of 0.25-0.26 (TMA/SiO2), d) zinc nitrate hexahydrate (Zn(NO3)2, reagent grade), e) sodium hydroxide (NaOH, +99%), f) potassium hydroxide (KOH, +99%) and g) distilled water. All reactants were provided by Sigma-Aldrich. The gel suspension was stirred for 4 h. The resulting blend was heated in an autoclave at 160 °C during 80 h. Once the synthesis was completed, the resulting zeolite was treated as follows: a) filtered, b) washed with distilled water, c) dried at 120°C for at least 6 h, and d) calcined at 500°C during 7 h under air flow. This synthesis led to a sodium form of the zeolite. In order to activate it, the Zn-offretite zeolites were ion exchanged with an ammonium nitrate solution. Then, they were dried at 120ºC and calcined in air at 550 ºC during 7 h. The synthesized OFF were analyzed, after the various steps described above were carried out, using XRD for structure identification, and XRF and ICP-AES for chemical composition. On the basis of XRF and ICP, the following formulae were established 9: OFF: Na0.1K0.6Al3.6Si9.2O25.6 Zn(2.0wt%)-OFF: Na0.3K0.6Zn0.5Al3.4Si9.9O27.7 Zn(3.5wt%)-OFF: Na0.6K0.6Zn0.9Al3.3Si10.5O26.6 2.2. Physicochemical Characterization The physicochemical characterization of the OFF zeolites was developed using the following: a) ammonia temperature programmed desorption (NH3-TPD), b) CO2 temperature programmed desorption (CO2-TPD), c) N2 isotherms (BET, pore size distribution), d) Raman spectroscopy and e) Fourier Transform Infrared Spectroscopy (FTIR). N2 isotherms were used to evaluate specific surface area (BET and T-plot) and pore size distribution. The materials were degassed at 300 °C for 3 h. Nitrogen adsorption was developed using from covered the 10-6 to 1 relative partial pressures. The pore size distribution was obtained using the BJH method 21,22. NH3 -TPD was carried out using an AutoChem II 2920 Analyzer from Micromeritics. Ammonia was adsorbed for 1 h at 100 °C using a NH3/He gas mixture (19.04 molar ratio of He/ NH3). Samples were prepared by heating them under helium gas flow for 2 h at 500 °C. Then, the temperature was increased linearly using five different temperature ramps (β): 10, 15, 20, 25 and 30 °C/ min until 680 °C was reached. The experiments were performed 3-5 times using the OFF analyzed to ensure reproducibly. The following parameters remained unchanged during each repeat run 23,24: a) b) c)

The measured flow rate, Q, had a constant value of 50.25 ± 0.07 cm3 STP/min A NH3-He certified gas mixture had 4.99% ammonia and 95.01% helium The mass of the zeolites, mcat, was 0.12g ± 0.02g 3



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d) e)

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The particle size of the zeolites was 2-5µm 9 The system pressure was at 1 atm.

As well CO2 -TPD was implemented in the AutoChem II 2920 Analyzer from Micromeritics as follows: 1) Samples were pre-heated under helium gas flow for 1 h at 550 °C, 2) CO2 was adsorbed for 1 h at 50 °C using a CO2/He gas mixture (9.89 v% of CO2 in He), and 3) CO2 was desorbed using a 20 °C/ min temperature ramp until 680 °C was reached. Pyridine-FTIR measurements were established using a Bruker IFS55 FTIR Spectrometer at 4 cm-1 resolutions, with 100 scans per sample. In the first step, the OFF particles were heat treated with a N2 flow at 550 ºC for 2 h. In the second step, they were saturated with a continuous flow of N2 pyridine at 120 ºC for 1 h. Then, the materials were flushed under N2 flow at 120 ºC for 1 h. After this, the adsorbed pyridine was analyzed using a Diffuse Reflectance Fourier-Transform Infrared Spectrometer (DRIFTS) with OFF samples diluted with 50% KBr approximately. Raman spectra were obtained using a Renishaw Model 2000-Laser Raman Spectrometer. Excitation of the samples was carried out with a 633nm wave of Ar ion laser. 3. Results and Discussion 3.1. Structural Characterization Several structural characterizations of the OFF and Zn-OFF zeolites were developed by our research team9. These analyses included studies of X-Ray diffraction (XRD), Temperature Programmed of Oxidation (TPO, O2/He), Temperature Programmed of Reduction (TPR, H2/Ar), UV-Vis spectra and Scanning Electron Microscopy (SEM). A summary of the observations is stated as follows: • • •

•

XRD patterns for the OFF zeolites (with and without zinc) were in agreement with the reported in the literature 6,9,25–27. In addition, no new peaks for ZnO were observed. TPR and TPO analyses did not record any measurable hydrogen or oxygen consumption. UV-visible diffuse reflectance spectra for the various OFF zeolites showed peaks with shoulders at 230nm and 260nm. It was observed that the Zn included in the OFF did not display a 360nm band characteristic of ZnO. SEM showed that both the OFF and the Zn-OFF have crystallites of a 2-5µm in length.

This suggests that most of the zinc is an included in the OFF with very low zinc forming extra framework crystallites. Regarding BET surface areas, 430 m2/g, 460 m2/g and 438 m2/g were observed for the OFF, the Zn(2.0wt%)-OFF and the Zn(3.5wt%)-OFF, respectively. The total accessible micropore volumes of the OFFs is in the range of 0.19-0.20 cc/g. The zeolites display an Isotherm Type I without hysteresis in accordance with the IUPAC classification (highly microporous)28. 4


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Fig. 2 reports the pore size distribution calculated using BJH method. A cylindrical parallel model was considered in the calculations 8. One can notice a similar pore size distribution for all the OFF materials: 8 ring and 12 ring pore width dimensions of 4.3-4.4Å and 4.6-5.4 Å. This is in agreement with the observation of others7,29.

Fig. 2. Porosity Distribution calculated by BJH method for a) the OFF zeolite, b) the Zn(2.0wt%)-OFF zeolite and c) the Zn(3.5wt%)-OFF zeolite. Pore distribution is based on N2 adsorption at 77K. Furthermore, Fig. 3 provides Raman spectra for the three OFF zeolites of the present study. One can notice that all recorded bands in the OFF remain unmodified with zinc addition. One can also observe that the highest bands corresponding to the T-O-T (T=Al, Si or Zn) oxygen angles from bending vibrations 30–32. These are very close to the expected bands for the OFF (black line) at 317, 430 and 490 cm-1 33,34. These bands are assigned to the bending modes of 8-,6- and 4membered rings of the OFF zeolite.

5



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T-O-T bending motion

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Double Ring

T-O-T symmetric stretching vibration

T-O-T asymmetric Stretching vibration

Wavenumber, cm-1 Fig. 3. Raman Spectra for the zeolites Fig. 3 also shows that adding zinc in the OFF (e.g. 2.0wt% of zinc) increases the intensity of Raman strongest bands. In this respect, Yu et al. 31 reported that the stronger Raman signals, suggest higher structural order in zeolites. It can thus, be inferred that 2wt% of zinc improves somehow the structural OFF order. Regarding the Raman spectrum for 3.5wt% of zinc in the OFF, there is an increase of broad bands. One can speculate, that the broad bands may be caused by a small quantities of nanoparticle of Zn-O extra-framework 35,36. 3.2. Surface Acidity Characterization Fig. 4 reports NH3-TPD profiles for the OFF, the Zn(2.0wt%)-OFF and the Zn(3.5wt%)-OFF zeolites. The TPD profiles were obtained using temperature ramps with β=10, 15, 20, 25 and 30˚C/min. One can notice in Fig. 4a-c, the existence of three distinct temperature peaks. These three peaks appear consistently in each figure. For instance, Fig. 4a displays three peaks for the OFF, which are located at 430-532K (215-264°C), 641-730K (435-485°C) and at 857-927K (600-650°C) using different βs. These three peaks represent different types of acidity strengths: i.e. one weak, one of middle strength and one strong. One can notice that the first peak appears in the Low Temperature range (LT) while the other two peaks are in the High Temperature range (HT). In this respect, Ni et al. 37 reported a quick increase of Zn-ZSM5 direct synthesis during calcination. This led to higher Lewis acid sites and a shift of NH3-TPD peaks towards higher temperatures.

6


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Fig. 4. NH3-TPD Profiles at Various βs for one Run using: a) the OFF zeolite, b) the Zn(2.0wt%)-OFF zeolite and c) the Zn(3.5wt%)-OFF zeolite. The three NH3-TPD identified peaks were designated as Site I (weak acid site), Site II (moderate strength acid site) and Site III (strong acid site). On this basis, one can speculate that acid site characterizations as per the NH3-TPD, can provide insights into the adsorption properties of the Zn-OFF zeolites. Similar findings by others were obtained from the literature, with the OFF using NH3-TPD 38,39 and 1-Butene-TPD 40. In this respect, in these studies, it is claimed that both adsorption and catalytic offretite properties are influenced by the original form of the OFF and its modified forms such as H-OFF, K-OFF, Na-OFF, Cs-OFF and Cu-OFF 38,39,41,42. Regarding the NH3-TPD used to characterize the acid sites in the OFF zeolites, one can notice that Fig. 4a-c reports a progressive increase in the LT-Peak (Site I) as well as the MT and HTPeaks (Site II and III). This is the case when the OFF is modified with Zn. Thus, it can be concluded that the addition of zinc increases the strengths of the acid sites. On the other hand, Fig. 4b and 1c report a significant increase in the desorbed ammonia volume from both Site I and Site III. In this respect, one can also observe that Site III becomes more important in the case of the Zn (3.5wt%)-OFF. Furthermore, it can also be noticed that the acidity of Site III can be better quantified when using a temperature ramp above 15 ºC/min (β>15 ºC/min). Thus, it can be argued that these heating ramps leads to sharper NH3-TPD peaks with reduced overlapping. This is the result of the NH3-TPD which takes advantage of desorption 7



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activation energies in the order as follows: Ed,Site I ≅ Ed,Site II < Ed,Site III. The magnitude and relative order of these activation energies will be quantified later in Section 2.4. As shown in Fig. 4, more than one Zn-OFF site were recorded using NH3-TPD. Multiple types of adsorption sites are frequently accompanied with different desorption energies that should be assessed separately 23. In particular, three sites were identified in the OFF, using deconvolution with an Inverse Gamma distribution. Other distribution functions such as Gauss, Pearson IV and Beta Area were also considered. However, the inherent symmetry of these functions makes their adjustment difficult. As a result, the Inverse Gamma function was preferred given that it provided the lowest summation of the residuals. An example of the deconvolution performed is given in Fig. 5.

Fig. 5. Deconvolution of the OFF Sites (Sites I, II and III) for the Zn (3.5wt%)- OFF Zeolite with NH3-TPD using a β=25˚C/min ramp. Notes: a) Inverse Gamma Distribution individual areas are assessed with a R2=0.998. Summation of the 3 peaks areas are calculated with a 0.0623 residual. Table 1 reports the results of NH3-TPD deconvolution (refer to Fig. 5). One can notice that there is a significant rise in the total acidity in the Zn-OFF. In fact, the Zn-modified OFF shows an 80% acid density increase in Site I when compared to its parent OFF zeolite. Another important observation is also the increase of the acid Site II and III densities, when there is zinc in the OFF.

8


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Table 1. NH3-TPD after Deconvolution of the Peaks Using a β=30˚C/min Temperature Ramp (values are the average of three repetitions) OFF Zeolite

β=30˚C/min

Tp , K Acid amount, mmol/g STP

values 538 730 927 0.61 0.75 0.03 1.40

Site I Site II Site III Site I Site II Site III Total

St. Dev 7 37 12 0.03 0.01 0.01 0.04

Zn(2.0wt%)-OFF Zeolite values St. Dev 556 1 752 25 930 6 1.12 0.18 0.85 0.09 0.04 0.03 2.0 0.3

Zn(3.5wt%)-OFF Zeolite values St. Dev 578 2 742 26 940 18 1.01 0.17 0.98 0.15 0.12 0.03 2.1 0.4

Table 1 shows as well, the temperatures at which NH3-TPD peaks display maximum values (Tp). One can observe that the Tp for the Sites I and III increases gradually, with zinc content. On this basis, it can have concluded that the Zn in the OFF increases adsorption site density, as well as adsorption bonding strength. These results are further confirmed in Fig. 6, where the total desorbed ammonia for the 3 identified sites augments as a function of the Zn/Al ratio in the OFF. 2.5

Acid amount, mmol/g STP

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


Total

Site I

Site II

Site III

2.0 1.5

1.0

0.5

0.0 0

0.05

0.1

0.15

0.2

0.25

0.3

Zn/Al ratio, mol/mol

Fig. 6. Change in Acid Site Density in the OFF as a Function of the Zn/Al Ratio Fig. 7 reports the pyridine-FTIR spectra of all the zeolites used in this study. Fig. 7a displays the spectrum of zeolites between 1400-1800 cm-1. The Brönsted acid sites band was observed at 1545 cm-1. On the other hand, IR bands characteristic of pyridine interacting with Lewis acid sites were detected at 1440, 1451 and 1615 cm-1. Hydrogen bonded pyridine was also seen at 1631cm-1. In addition, a 1490 cm-1 band was recorded and assigned to the combined contribution of Brönsted and Lewis acid sites. Furthermore, there was an increase in the intensity of the peaks at 1615 cm-1 and 1451 cm-1 with Zn addition as well as in those of the double band at 1440-1451 cm-1. This showed that the addition of Zn led to the formation of new Lewis acid sites. These 9



observations are in agreement with the findings using other materials such as zinc silicates and Zn-β zeolite, where Zn addition modifies acidity 11,43. Furthermore, Beyer et al. 44 reported that the shoulder band at 1440-1451 cm-1 observed with the zinc addition is an indication of a modified crystallographic lattice. 0.25

a)

0.45

OFF Zeolite

1631

0.2

1615

Zn(2.0wt%)-OFF Zeolite

0.4

Zn(3.5wt%)-OFF Zeolite

0.35

3610

b) OFF Zeolite Zn(2.0wt%)-OFF Zeolite Zn(3.5wt%)-OFF Zeolite

0.3

1490

0.15

Absorbance units

Absorbance units

0.1

1451 1440

1545 0.05

0.25

3690 0.2 0.15

3745

0.1 0.05

0 1800

1750

1700

1650

1600

1550

1500

1450

1400

0 3790

3770

3750

Wavenumber, cm-1

0.49 0.44

3730 3710 3690 3670 Wavenumber, cm-1

3650

3630

3610

3590

c)

0.39 0.34

Absorbance units

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745

463

0.29

480

450

3770 3765 3760 3755 3750 3745 3740 3735 3730 3725 3720

0.24 0.19 0.14

OFF Zeolite Zn(2.0wt%)-OFF Zeolite Zn(3.5wt%)-OFF Zeolite

0.09 0.04 -0.01 1030

980

930

880

830 780 730 680 Wavenumber, cm-1

630

580

530

480

430

Fig. 7. Pyridine-FTIR Spectra of the OFF and Zn-OFF Zeolites Measured at: a) 1400-1800cm-1, b) 3590-3790 cm-1, and c) 430-1030 cm-1 Fig. 7b reports the 3590-3790 cm-1 FTIR spectra. This band allows determining the effects of zinc on the acid sites, particularly those on the Brönsted OH groups. On this basis, three significant bands were identified in the 3610, 3690 and 3745cm-1 OH stretching regions. Each band can be assigned to different OH vibrations. However, none of these bands showed a significant change, except for the 3745 cm−1 peak which provided information about non-acidic OH bands 43,45,46. This enhanced the non-acidic OH bands that can be assigned to the zinc located either on the external surface of the crystals or in the amorphous inclusions in the OFF structure 45. Furthermore, Fig. 7c showed a modest change in the 450-480 cm-1 FTIR bands, which may be attributed to the bending modes of T–O–T bridges 47. It can be speculated as a result, that zinc addition may modify these bridges, creating different strengths or acid site interactions 9. Brönsted and Lewis acid site concentrations were determined using the peaks at 1545 cm-1 and 1448 cm-1, respectively. Table 2 reports the FTIR area peaks and their relative Lewis/Brönsted 10


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acidities. One can observe that acidities are not proportional to the aluminum content. However, a raise of Lewis/Brönsted site ratios is observed with the increase of Al+Zn fractions. Table 2. Lewis and Brönsted Acid Site Concentrations Obtained from the Area of the FTIR Bands (au/cm). Zn(2.0wt%)-OFF Zn(3.5wt%)-OFF Zeolite Zeolite Lewis/Brönsted ratio 0.64 1.54 1.59 Brönsted 0.50 0.67 0.54 Lewis 0.32 1.03 0.86 Total 0.82 1.70 1.40 Al, mol/unit cell 3.60 3.40 3.30 Zn, mol/unit cell 0.00 0.50 0.90 Al+Zn, mol/unit cell 3.60 3.90 4.20 Note: Band ranges for Lewis acid sites:1430-1467cm-1 and for Brönsted acid sites: 1533-1560cm-1. OFF Zeolite

In summary, zinc addition leads to: a) A change in the acid site distribution as shown with TPD, b) An enhancement of Lewis acidity as observed with Pyridine-FTIR. These changes can be attributed to : a) the Zn incorporated into the OFF lattice that can cause a change in the strength of the bridging hydroxyl groups (Si-O-Zn) 48, b) the formation of new stronger Lewis acid sites and new Brönsted sites, c) Possible defects created by Zn 11, e) The ZnO clusters that are able to interact with the oxygen framework atoms and increase their charges, enhancing their acid-base characters 48. d) the zinc atoms placed in the neighborhood of Al atoms (e.g. Al-O-Zn) that may influence the charge distribution and basicity of the oxygen atoms 48. In this respect, Fig. 8 reports examples of possible acid site structures formed in the Zn-OFF.

Fig. 8. Possible Acid Sites Formed in the Zn-OFF Zeolites. One should also notice in this respect that, Meng et al.12 and Orazov et al. 11 reported alternative acid sites formed by framework zinc bonding with silanol groups (Zn-O-Si). Another important characterization of the OFF is to establish basicity changes given by electronegativity considerations. To accomplish this, one can consider a mean zeolite electronegativity parameter (Szeolite), as proposed by Sanderson 49. This Szeolite parameter, which increases with decreasing basicity 50, allows one to establish a zeolite basicity strength. Table 3 reports calculated values for the three OFFs of the present study. One can observe that the lowest 11



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electronegativity or the lowest Szeolite is displayed by the OFF with the highest Zn content. Thus, zinc in the framework may also lead to an increased density of negative basic sites and an increased basicity 50. Table 3. Mean Electronegativity of the Materials Name

Szeolite

OFF Zn(2.0wt%)-OFF Zn(3.5wt%)-OFF

Na0.1K0.6Al3.6Si9.2O25.6 Na0.3K0.6Zn0.5Al3.4Si9.9O27.7 Na0.6K0.6Zn0.9Al3.3Si10.5O26.6

3.997 3.956 3.906

Oxygen Charge -0.2554 -0.2641 -0.2746

Barthomeuf 50 described that the basic strength can be evaluated from CO2-TPD. It was reported that lowest desorption temperatures are evidences of higher basicity materials. The results of the CO2-TPD analysis for the OFF and Zn-OFF zeolites are reported in Table 4. Two peaks were identified in each of the OFFs, with minor differences in the maximum desorption temperatures. Table 4 also shows that the amount of CO2 desorbed increases with the zinc content in the OFF. It is believed that small ZnO cluster in Zn(3.5wt%)-OFF, as previously mentioned while discussing Raman analysis, can cause that increase. In this respect, Kim et al. 51 and Lasperas et al52 reported a direct connection between Cs loaded in X zeolite and CO2 desorbed amounts. It is expected however, that given the acid sites of the OFF Zn-OFF are the only ones involved on the methyl-thiophene adsorption, do not require additional basic site consideration. Table 4. CO2-TPD results for the zeolites at β=20˚C/min

T p, K CO2 amount, mmol/g STP

Site I Site II Site I Site II Total

OFF Zeolite 376 953 0.05 1.44 1.48

Zn(2.0wt%)-OFF Zeolite 375 953 0.32 1.24 1.56

Zn(3.5wt%)-OFF Zeolite 375 950 0.58 1.49 2.07

3.3. NH3-TPD Desorption Kinetics 3.3.1. Modeling Ammonia TPD Desorption To evaluate the desorption kinetic parameters, two cases were considered in this study: 12


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• •

Method 1: A first order kinetics without readsorption of ammonia, using linear regression: conventional method Method 2: A first order kinetics without readsorption of ammonia and employing nonlinear regression: non-conventional method 18

To develop the desorption kinetic model, some considerations were taken into account: a)

There is adsorption equilibrium between the adsorbed ammonia on the zeolite and the ammonia in the gas phase. As a result, the following equation can be considered to describe desorption-adsorption processes: ∗ ⇄ +

(1)

where (NH3)* or ϴ represents the adsorbed ammonia and ϴv stands for the vacant sites in the OFF. Therefore, the balance of acid sites can be represented as follows:

+ =

(2)

(3) where vd denotes the desorption volume and vm stands for the monolayer volume, both per unit of mass of the solid phase. b) The temperature in the OFF particle bed increases lineally with time, as follows: =

= +

(4)

where T denotes temperature at any time t, T0 stands for the initial temperature of ammonia desorption (i.e T0=100 ºC at t = 0 s), t represents the time and β indicates the heating rate coefficient. c)

There are no diffusional effects affecting the ammonia desorption. According to the literature 53,54, the following dimensionless criterion must be satisfied in order to be able to neglect the concentration gradients inside the OFF particles: ∙

∙ ∙∙

< 0.05

(5)

where Q is the carrier gas flow rate, R stands for the catalyst particle radius, N represents the number of particles used in the TPD analysis, and Dp denotes the particle diffusion coefficient. To evaluate these parameters, the following data were considered: c.1) The diffusion coefficient for ammonia in the OFF was approximated with the one reported by Sprynskyy 55 for mordenite. This Dp was reported to be in the 0.7-3.6×10−8 cm2/s range. This coefficient accounts for the diffusional effects in the mordenite zeolites with two types of channels: 6.7x7.0 Å and 2.9x5.7Å. Given the resemblance of the mordenite and the OFF pore network, this Dp value was considered as a good approximation. 13



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Page 14 of 26

c.2) The 2-5 µm catalyst particle radius was established using the average particle size as measured with SEM 9. c.3) Q and N parameters were calculated from the conditions utilized in the TPD analyses (Section 2.2). Using this approach, the dimensionless parameters as in Eq. (5) were calculated to be in the range of 6x10-11 - 3x10-10 for the three zeolites studied. These values were significantly lower than the recommended 0.05. d) The desorption kinetics was first rate. Thus, this rate can be expressed by:

−

& '

= (

(6)

where t is the time and kd stands for the rate constant of desorption. One can assume that adsorption kinetic parameters, kd and its changes with temperature can be expressed via the Arrhenius’ equation as follows, 1

)* = )*+ , ∙ -./ 0− 2* 3

(7)

where kdo stands for the intrinsic desorption constant in min-1, Ed is the desorption energy, and R denotes the universal gas constant. Given all the above, the procedure proposed by Cvetanovic et al., Konvalinka et al. and Niwi et al. 16,56–58, was implemented as follows: Combining Eq. (4), Eq. (6) and Eq. (7), one obtains the following: &

− 4 =

With kdo= kdo’/vm

567 8

The Effect of Zn on Offretite Zeolite Properties. Acidic

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