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Hydrothermal stabilization of rich Al-BEA zeolite by postsynthesis addition of Zr for steam catalytic cracking of n-dodecane Mohamed H.M. Ahmed, Oki Muraza, Ahmad Galadima, Anas Karrar Jamil, Emad N. Shafei, Zain H. Yamani, and Ki-Hyouk Choi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00087 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018
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Energy & Fuels 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 28 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
O | O –Si – O | O
O | O –Al – O | O
EnergyO & Fuels
Desilication
Dealumination
| H O– H H–O H | O
O Zr ion exchange | O –Zr – O | O
O | H Zr ion exchange O– H H–O H | ACS ParagonO Plus Environment
O | O –Zr – O | O
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Hydrothermal stabilization of rich Al-BEA zeolite by post- synthesis addition of Zr for steam catalytic cracking of n-dodecane Mohamed H.M. Ahmeda, Oki Murazaa ⃰, Ahmad Galadima a, Anas K. Jamil a, Emad N. Shafeib, Zain H. Yamania, Ki-Hyouk Choi b a
Center of Excellence in Nanotechnology and Chemical Engineering Department,
King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia b
Research and Development Center, Saudi Aramco, Dhahran 31311, Saudi Arabia ⃰Corresponding author, E-mail:
[email protected].
Abstract The steam cracking of n-dodecane was performed over Zr-modified BEA zeolite. The parent H-BEA zeolite was modified via Zr introduction following earlier treatment (desilication and dealumination). The procedure allowed achieving a Si/Zr molar ratio of 100. The changes in BEA crystallinity was observed by XRD. The EDX, FTIR, and NMR studies were used to confirm the quantity and the position of Zr particles in the framework. Changes in the catalyst acidity were also monitored using pyridine FTIR. The addition of zirconium to BEA framework prevents the dealumination due to present of steam as well as improved the catalyst life time from 30 min to 4h. Dodecane cracking results showed that the introduction of Zr to BEA framework after desilication is more efficient to protect framework Al in steam presence as compared to with dealumination post treatment.
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1. Introduction The upgrading of heavy hydrocarbons such as heavy oils, bottom distillation residues (BDR), atmospheric residues (AR) and vacuum residue (VR) is attracting much attention towards meeting the escalated demands of middle distillate products such as gasoline and diesel fuels [1-3]. Many catalytic and non-catalytic processes were explored to achieve more valuable and cleaner products from the heavy oil [3, 4]. The catalytic process is more promising in terms of products selectivity over the non-catalytic (thermal) process because slight adjustment in the catalyst pore size can shift the reaction towards the desired products [5]. Moreover, the catalytic method is cheaper compared with the non-catalytic processes which is usually associated with high energy inefficiency [6]. However, the capital cost of expensive metal catalysts such as NiMo [7] and CoMo [8] and other rare metals are the main disadvantage of the catalytic upgrading process. Therefore, this attracted the research on zeolite materials to compete with other catalysts since these aluminosilicates are cheaper as compared with common used metals catalysts. In this work, n-dodecane was used as a model compound of heavy naphtha to investigate the catalytic cracking performance of modified BEA zeolite under steam conditions. Among many zeolites frameworks, BEA zeolite was selected due to its large pore structure which is suitable for handling the diffusion challenges associated with any catalyst [9]. However, the as-synthesized BEA zeolite gave a wide range of products and it was not selective toward specific hydrocarbons when zeolite Beta was used as a catalyst for medium and long chain alkane cracking, the products selectivity of BEA zeolite was considerably enhanced by post-synthesis demetalation [10, 11] and by acidity modification [12]. On the other hand, the used steam in typical hydrocarbon cracking processes enhanced the catalytic stability by preventing its fast deactivation and retarding coke formation [13-15]. However, the presence of steam at high temperatures can cause a serious dealumination 2 ACS Paragon Plus Environment
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effect with negative consequences of catalyst deactivation especially when the amount of Al content is high (i.e when the Si/Al is low) [13, 14]. Therefore, the protection of framework Al under such conditions is highly required to secure longer catalyst life time. The synthesized BEA zeolite was modified by post synthesis treatments and isomorphic Zr incorporation to achieve better stability under steam conditions. The incorporation of Zr to the zeolite frameworks offer better steam resistance and preventing the unwanted extraction of Al species due to the hydrophobicity of zirconium [16, 17]. The introduction of active metals such as Zr, Ni, Cu, Zn and Fe to be internally connected to the zeolite framework by the post synthesis procedure represents easier route when compared with the in-situ routes as adopted in our earlier work involving Ni and Co species [18]. The in-situ route requires further investigation on the tailored synthesis conditions to achieve the required phase. However, by applying desilication or dealumination on the zeolite, the removed Si and Al ions will be substituted by H+. These ions can be easily replaced by Zr ions via simple wet ion exchange. In this work, Zr was explored to prevent the leaching of Al species from the zeolite framework due to the presence of steam, which was used here to prevent quick formation of coke.
2. Experimental 2.1 Synthesis of parent BEA zeolite BEA zeolite was synthesized by mixing 32 ml of distilled water with 17.4 g of colloidal silica (SNOWTEX, wt. 40% SiO2) over a magnetic stirrer in a rate of 800 rpm. A 30 g of tetraethylammounium hydroxide (TEAOH 40%, Sigma Aldrich) was added to the solution as a as an organic structure directing agent (OSDA). After the gel reached homogeneity a
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1.52 g of sodium aluminate was added and the gel was aged for 1 h. The gel composition was 1SiO2:0.083Al2O3:0.034Na2O:0.35TEAOH:11.6H2O. The dense gel was transferred to 100 ml autoclave and placed in a static oven at 150 oC for 72 h. The product was washed several times with distilled water and dried for 12 h. The Si/Al ratio of the synthesis gel was 6 and the sample was named as B-6. Other synthesis gels were prepared with different Si/Al ratios (12.5, 25, 50, and 100) by only changing either the amount of colloidal silica or sodium aluminate, the samples were named as B-12.5, B-25, B-50, and B-100 respectively. 2.2 Desilication treatment and zirconium substitution The sample B-12.5 was exposed to NaOH treatment to create some mesopores. The treatment was performed by adding of 1 g of calcined B-12.5 to 30 ml of NaOH solution (0.1 M). The treatment was performed on a hotplate at 65 oC for 15 min. The powder was then separated and washed several times with distilled water. A solution of 0.2 M of zirconium (IV) oxynitrate hydrate (ZrO(NO3)2, sigma Aldrich) was prepared and 1 g of the desilicated B-12.5 was added to 30 ml of this solution. The incorporation of zirconium was performed at 65 oC for 15 min. This Zr-desilicated sample was called as Zr-B-12.5-Dsi. 2.3 Dealumination treatment and zirconium substitution A 1 g of calcined B-12.5 was treated with 30 ml of 2 M concentration of nitric acid (HNO3). The treatment was carried out at 80 oC for 15 min. After that, the powder was washed, separated and dried. Later, a 1 g of dealuminated B-12.5 was treated with 30 ml of 0.5 M of ZrO(NO3)2 at 80 oC for 15 min. Again the product was washed and dried and then calcined at 650 oC for 12 h. The set of Zr-dealuminated sample was called as Zr-B-12.5Dal.
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2.4 Steam catalytic cracking Catalytic performance of BEA zeolite catalysts was evaluated in steam catalytic cracking of n-dodecane using a steam compatible packed bed reactor. All test reactions were performed at 350 oC and LHSV of 4 h-1. The experimental setup for the SCC process is presented in Fig. 1. It was conducted using a tubular flow system at atmospheric pressure. In addition, the n-dodecane and water used during the process were introduced using electric driven syringe pumps. The flow rates of n-dodecane and water were maintained at 3.6 cm3 h−1 and 0.4 cm3 h−1, respectively, to achieve dodecane to steam ratio of 9 (v/v). Nitrogen gas was employed as the carrier gas. The products were directly injected and analyzed using a GC-MS. Gas chromatography with a mass spectrometry detector (GCMSD) from Agilent was used in order to evaluate the hydrocarbon conversion. The GC-MS column used is an Agilent J&W HP-5ms with a length of 30 m, internal diameter 0.25 mm, and film thickness of 0.25µm, and the gas splitter ratio applied was 1:100 at the inlet temperature of 250°C. The initial temperature was set at 40°C for 1 min; then, the oven was ramped at 10°C/min in order to reach 280°C.
Please insert Figure 1 here. 3. Characterization Miniflex, a Rigaku diffractometer with Cu Kα radiation was used to record the XRD patterns of the powdered materials. The analysis was performed in the range of 5 to 50o of 2θ range with a scan step of 0.03o and a counting time of 4 s for each step. Field-emission scanning electron microscopy (FE-SEM) was used to study the morphology and chemical composition of the samples (LYRA 3 Dual Beam Tescan) equipped energy dispersive X-
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ray spectrometry (EDX, Oxford Instruments) operated at an acceleration voltage of 30 kV. N2 adsorption/desorption was measured using Micromeritics ASAP 2020 porosimeter. Prior to measurement, the samples were degassed at 623 K for 12 h to remove any possible adsorbed gases. Thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), experiments were carried out under Argon gas using a heating rate of 10 K/min up to 973 K. Pyridine adsorption followed by an infrared (IR) spectroscopy (Nicolet 6700 Spectrometer) in transmission mode. Spectra were recorded at 4 cm−1 spectral resolution, an undersampling ratio of 4, and a speed of 20 kHz. Samples of fresh catalysts were first pressed into thin wafers and then activated in situ in the IR cell under secondary vacuum (10−6 mbar) at 773 K. Afterwards, the sample was cooled down to 423 K and the pyridine introduced to the cell for 30 min.
4. Results and Discussion 4.1 The changes in BEA phase, crystallinity and morphology after the post treatment Based on the XRD results, the synthesis procedure adopted could only yield pure BEA zeolite with Si/Al ratios in the range of 6 to 25. Therefore, when higher Si/Al ratio gels were prepared (50 and 100), additional peaks were observed in the XRD patterns of the produced powder as shown in Figure 2. These new peaks that appeared at 2θ = 21.1 and 23.4o could be assigned to the formation of ZSM-12 (MTW) phase. Both the MTW and MFI zeolites have been reported to appear under typical synthesis conditions especially when the tetraethyl ammonium hydroxide is used as a template and high silicon content is represent in the solution [19, 20]. The relative crystallinity was systematically increased with the increase of Si/Al ratio from 6 to 25 as shown in Table 1. The highest crystallinity was observed with B-25 which used as a reference for other sample. The particle size of 6 ACS Paragon Plus Environment
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this sample was in the range of 700-900 nm. On the other hand, when the Si/Al ratio decreased the crystallinity and the particle size also decreased. The SEM images in Figure 4 are showing the size and the morphology of these samples. Please insert Figure 2 here. Since both treatments (desilication and isomorphic Zr substitution) have a serious destructive effects on zeolite crystals [10, 21], a severe decrease in Beta crystallinity was expected when a combination of both treatments were applied. The XRD patterns in Figure 3 showed a significant reduction in the intensity of the treated samples (Zr-B-12.5-Dsi and Zr-B-12.5-Dal) compared to the parent sample (B-12.5). Please insert Figure 3 here. The formation of mesoporosity upon the desilication and dealumination was observed by SEM images as presented in Figure 4. The SEM image of Zr-B-12.5-Dsi sample revealed both larger population and size of the mesopores compared to the Zr-B-12.5-Dal which is also reasonable since the vacant created upon Si extraction is larger than Al vacant. Please insert Figure 4 here.
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Table 1. Composition, crystallinity, and particle size of BEA samples.
Sample
Si/Al
Relative
Particles size
crystallinity (%)
(nm)
Si/Zr
B-6
6
-
46
200-400
B-12.5
12
-
67
400-600
Zr-B-12.5-Dal
20
106
-
-
Zr-B-12.5-Dsi
9
101
-
-
B-25
24
-
100
700-900
4.2 The incorporation of Zr to BEA structure and its effect on BEA textural properties The confirmation of Zr incorporation into BEA framework required correlation between the results of EDX, FTIR and NMR. The EDX results summarized in Table 1 show that the Zr substitution by different post-synthesis treatments gave almost the same Si/Zr ratio (101 and 106) over two different samples; namely Zr-B-12.5-Dsi and Zr-B-12.5-Dal, respectively. The FTIR spectra of the pre-heated parent B-12.5 and other treated samples (Zr-B-12.5-Dsi and Zr-B-12.5-Dal) in Figure 5 showed that the Zr treated samples have additional two peaks at 535 and 612 cm-1. These peaks were assigned to the tetragonal phase of Zr, which indicates that the Zr was connected to four oxygen atoms [22]. In particular, the peak at 612 cm-1 was a consequence of the stretching of Zr-O in the Zr-O-Si bond [23]. The intensity of the peaks in the desilicated sample Zr-B-12.5-Dsi is considerable higher than the intensity of Zr-B-12.5-Dal, which indicates higher quantity of Zr incorporation into BEA framework in the desilicated sample in the form of tetragonal 8 ACS Paragon Plus Environment
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coordination. However, since the EDX results showed similar Si/Zr ration in both samples, consequently this suggests that most of Zr in the dealuminated sample was placed on the BEA surface rather than the framework. Please insert Figure 5 here. 29
Si NMR was used to study the chemical shift on Si environment due to Zr incorporation
as shown in Figure 6. The parent B-12.5 and the treated samples (Zr-B-12.5-Dal and Zr-B12.5-Dsi) all showed two main peaks at -99.4 and -93.8 ppm. The peak located at -99.4 ppm can be assigned to Q1 Si(OSi)3(OAl), while the shoulder peak at -93.8 was assigned to Q2 Si(OSi)2(OAl)2 . The major difference observed in 29Si-NMR spectra between parent B12.5 and Zr-B-12.5-Dal, where the ratio of Q2/Q1 increased. This increase was affected by the considerable decrease in Q1 environment, which occurred because of Al removal via acid treatment. The observation is in a good agreement with FTIR result, which showed that the dealuminated sample had insignificant incorporation of Zr in BEA framework. On the other hand, the spectrum of Zr-B-12.5-Dsi showed a decrease in Q1 intensity, while the intensity of Q2 peak was significantly increased. This change can be attributed to substitution of the extracted Si ions by Zr, which probably caused a decrease in Si(OSi)3(OAl) environment, while a new environment of Si(OSi)2(OZr)2 was established. The integration of these three characterization results strongly confirms the successful incorporation of Zr in the framework of BEA zeolite especially in the desilicated sample Zr-B-12.5-Dsi. Please insert Figure 6 here. The BET areas of the catalysts derived from the nitrogen adsorption/desorption isotherms (Figure 7), presented in Table 2 showed an increase in the micropore surface area with the increase of the Si/Al ratio from 6 to 25. However, the mesopore surface area reached the 9 ACS Paragon Plus Environment
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maximum 151 m2/g for the B-12.5 sample as clearly shown by the hysteresis loop in Figure 6. The total surface area of the richest Al sample (B-6) was 189 m2/g. This low surface area indicates that the sample is not highly crystalline and partly amorphous which is in a good agreement with XRD results. The demetalation process was applied to allow the Zr to be connected inside the BEA structure and to keep the pores large after the introduction of large Zr atoms to the framework. The desilication and dealumination on BEA zeolite usually offer a significant increase in mesopores [24]. However, in this work, an insignificant increase in mesoporosity was observed while the microporosity reduced after the Zr substitution. This observation could be supported by the fact that the incorporated Zr atoms filled the additionally created pores. Please insert Figure 7 here. The pore size distribution results calculated by NLDFT method are presented in Figure 8 (B), which shows that the desilicated sample (Zr-B-12.5-Dsi) has the highest population of mesopore followed by the dealuminated sample (Zr-B-12.5-Dal) and the lowest population was associated to the parent B-12.5. Please insert Figure 8 here. Table 2. Textural properties of synthesized and treated BEA samples. Sample
Smicro (m2/g)
Smeso (m2/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
B-6
149
40
0.075
0.028
B-12.5
309
151
0.154
0.135
Zr-B-12.5-Dal
301
155
0.159
0.142
Zr-B-12.5-Dsi
276
164
0.138
0.148
B-25
430
74
0.214
0.054
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4.3 The role of Si/Al ratio in the n-dodecane cracking The concentration of Brønsted and Lewis acidity was calculated based of the pyridine absorbance peaks on the catalyst active sites as measured by FTIR and presented in Table 3. The results show that the richest Al sample (B-6) has the highest amount of Brønsted and Lewis acidity of 0.639 and 0.459 mmol/g, respectively. The amount of acidity decreased with the decrease of Al content in the following order B-6>B-12.5>B-25, which seems very reasonable since the presence of Al is the cause of having acidic active sites in aluminosilicate matrix. The conversion of dodecane over zeolite Beta catalysts showed a strong dependence on the Si/Al ratio of the BEA sample. It was observed that when the Si/Al ratio increased from 6 to 25, the conversion of dodecane increased from 17% to 45% together with considerable enhancement in the life time as presented in Table 4. The significant improvement in the catalytic activity and stability of the B-25 can be attributed also to the low amount of Al in the sample which results less amount of dealumination. Therefore, there was clear relation between the amount of Al present in the catalyst and the life-time of the catalyst. This strongly confirms that the used of steam in the cracking process causes serious framework Al extraction, which affect quick catalyst deactivation due to the deposition of extracted Al on the outer pores as well as increase the amount of Lewis acid sites [25]. This increased in Lewis acidity consequently promoted the production of more coke deposits [26]. The high Al content in sample B-6 and B-12.5 is responsible for the quick deactivation in these samples after less than an hour on stream. However, the lower Al content sample (B-25) was catalytically active for 6 h. Simply by comparing the color of the spent catalysts collected after the conversion of n-dodecane and the TGA results in Table 3, the color B-6
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and B-12.5 sample were light gray and contains 1.2 and 3.8% of coke, respectively. These amounts of coke are far smaller than the coke deposited on B-25 sample which was 15.6%. These results revealed that the main cause of deactivation in B-6 and B-12.5 was the leaching of Al framework due to presence of steam while B-25 sample was more stable. 4.4 The role of Zr and hierarchical BEA structure in the n-dodecane cracking As it was observed clearly in the previous section that the steam caused serious dealumination effect on BEA zeolite during the cracking of n-dodecane, therefore, zirconium ions were introduced as a hydrophobic agent to prevent the framework Al from leaching. In the dealuminated sample (Zr-B-12.5-Dal) where the Al content is less as compared to B-12.5 and Zr-B-12.5-Dsi, a slight improve in the catalyst stability was observed as demonstrated in Table 4. The existence of Zr in this sample allowed better deactivation resistance and the catalyst remained active for 1 h compared to the parent sample B-12.5. However, the initial activity didn’t significantly change as the conversion decreased from 25 in B-12.5 to 22% in Zr-B-12.5-Dal. The slight reduction in dodecane conversion can be explained by the reduction in Brønsted and Lewis acid sites as compared to B-12.5 (Figure 9). The Lewis sites were probably not seriously altered as compared to the Brønsted sites because the presence of Zr is usually gives a Lewis type of acidity in the same time causes a decrease in Brønsted sites [27, 28]. It seems that the incorporation of Zr after the dealumination did not fulfil the required purpose of protecting the framework Al from extracting in steam environment since the stability of Zr-B-12.5-Dal was not significantly enhanced as compared to B-12 as presented in Table 4. Please insert Figure 9 here. On the other hand, the desilicated sample Zr-B-12.5-Dsi showed a much better stability on stream for 4 h. The better stability of desilicated sample over the dealuminated one can be 12 ACS Paragon Plus Environment
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attributed to immerge of most of zirconium atoms to the internal structure of BEA zeolite as confirmed with FTIR results, which is clearly more effective in protecting framework Al. A clear evidence of the efficiency of Zr in protecting framework Al from steam leaching was confirmed by 27Al-NMR analysis of the fresh and spent samples as presented in Figure 10. The results are showing that the un-treated sample B-12.5 lost a considerable amount of framework Al which leached by steam during the reaction. This leached amount of framework Al was converted to non-framework Al species as confirmed by the peak appeared at 0 ppm. Meanwhile, the
27
Al-NMR results of Zr-B-12.5-Dsi are showing that
the framework Al was remained stable in the BEA structure. Please insert Figure 10 here. Table 3. The amount of acidity and coke of different BEA samples. Brønsted acid
Lewis acid
Sample
Amount of B/L ratio
mmol/g
mmol/g
coke %
B-6
0.639
0.459
5.7
1.2
B-12.5
0.589
0.422
1.3
3.8
Zr-B-12.5-Dal
0.202
0.380
0.53
5.6
Zr-B-12.5-Dsi
0.196
0.335
0.58
10.5
B-25
0.278
0.013
2.0
15.6
The TGA results in Figure 11 confirms that the desilicated sample has higher coke amount of 10.5% which can be the main reason for deactivation as compared with 3.8 and 5.6% for the parent sample (B-12.5) and the dealuminated one (Zr-B-12.5-Dal), respectively. This can be explained as the steam exposure created extra framework Al species for B-12.5 and Zr-B-12.5-Dal samples, which firstly affected the reactants diffusion as well as offered 13 ACS Paragon Plus Environment
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more Lewis acid site. The presence of higher quantity of Lewis acidity promotes the formation of coke in short time and consequently blocks the pores. Please insert Figure 11 here. Table 4. The conversion of n-dodecane over different BEA zeolites.
Sample
B-6 B-12.5 Zr-B-12.5-Dal Zr-B-12.5-Dsi B-25
Time on stream ½h
1h
2h
3h
4h
5h
17%
0%
25%
0%
22%
11%
0%
22%
15%
14%
14%
12%
0%
45%
40%
30%
23%
20%
21%
6h
15%
Conclusions The use of steam in the cracking n-dodecane over rich Al-BEA zeolite was seriously harmful to the structure of BEA due to the leaching of the framework Al. The presence of extra framework Al accelerated the deactivation of BEA zeolite by blocking the pores and increasing the Lewis acid site which enhanced the coke formation. The successful incorporation of zirconium to BEA structure offered prevention of framework Al from the possible extraction and prolonged the catalyst life-time. The main disadvantage observed in the current strategy by introduction of zirconium to BEA framework is the reduction of mesopore volume which was created through the desilication and dealumination. These mesopores are highly required to enhance the diffusion which will possibly reduce the coke 14 ACS Paragon Plus Environment
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formation since the products will diffuse out easily. The importance and the advantage of this work is represented by preventing the BEA zeolite from irreversible deactivation due to the extraction of framework Al. This leaching prevention by zirconium incorporation removes the limitation in the application of Al-rich zeolites in many applications where the steam presence is mainly required.
Acknowledgments The authors would like to thank the funding provided by Saudi Aramco for supporting this work through project contract number 6600011900 as part of the Oil Upgrading theme at King Fahd University of Petroleum and Minerals. The authors acknowledge the contribution from M. Qamaruddin on the analysis of textural properties.
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16 ACS Paragon Plus Environment
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A packed-bed reactor Flow meter N2
Experimental condition Carrier gas N2 : 29.3 [cm3.min-1] n-dodecane : 3.6 [cm3.h-1] H2 O : 0.4 [cm3.h-1] Catalyst volume : 1.5 [cm3]
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ACS Paragon Plus Environment Figure 1. A typical steam catalytic cracking experimental setup and conditions.
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Figure 5. FTIR spectra of BEA zeolite treated with different post-synthesis procedures.
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