Article pubs.acs.org/IECR
Study of the Relationship between Framework Cation Levels of Y Zeolites and Behavior during Calcination, Steaming, and n-Heptane Cracking Processes Bashir Y. Al-Zaidi, Richard J. Holmes, and Arthur A. Garforth* School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. ABSTRACT: The synthesis process and production of forms of the Y zeolite (i.e., NaY, NH4Y, HY, and USY) have been studied in depth, in order to investigate the thermal stability of the Y structure throughout calcination, steaming, and cracking processes. The results indicate that an increase in the cation content within the Y framework during calcination and/or steaming processes led to an increase in the catalyst stability and a reduction in the rate of any associated dehydroxylation reactions. It was also found that the presence of sodium ions hindered the extraction of Al atoms from the crystal lattice structure during the steaming treatment, which minimized partial and/or entire structural collapse at high temperatures. Finally, enhancements in the distribution of the acid sites, and an increase in the activity and selectivity of the produced USY catalysts during n-C7 cracking reactions, have also been observed.
■
INTRODUCTION Zeolites are generally defined as crystalline aluminosilicates,1 with the Y type having a three-dimensional structure similar to that of faujasite, where the unit cell is cubic and contains 192 total SiO4 and AlO4 tetrahedra,1,2 linked via oxygen bridges. Since silicon has a valence of 4 and aluminum has a valence of 3, the AlO4 tetrahedron carries a net negative charge,3,4 and a positive extraframework cation such as sodium is incorporated as a charge counterbalance that provides the zeolite with its ion exchange characteristics.3,5 The cations are localized at the center of the double six-membered rings found in the hexagonal prisms, sodalite cages, and supercages.6 Zeolites are mostly employed as acid catalysts, and the catalytic activities attributed to the generation of strong acidic sites on their surfaces, both Lewis and Brønsted acid sites, are easily created by the substitution of the surface cations with an exchange salt solution.1,7 A high temperature calcining process may then be used to generate the protonated forms of the zeolite;7 however, work has shown that the HY framework has a lower stability than the NaY form, which is prone to the formation of amorphous zones.8 Since the use of the zeolite-Y catalysts with stronger acid sites leads to a rapid deactivation resulting from coke formation,9 dealumination by steaming is often used to increase the lattice Si/Al ratio of the acidic catalyst, thereby changing the concentration of framework Al atoms.10,11 There have been many studies conducted into the optimization and enhancement of the catalytic properties of zeolite-Y catalysts in order to increase production yields. However, the majority of this work focuses on the reaction variables used during the modification processes for zeolite-Y structures. As such, this study focuses on the cation content within the Y zeolite, in order to establish a correlation between the dehydroxylation phenomenon and the collapse of the Y structure during calcination and/or steaming treatments. The effects of such modifications on the performance of zeolite-Y © 2012 American Chemical Society
catalysts in hydrocarbon cracking reactions have also been studied with a view to improving the cracking process and production yield.
■
EXPERIMENTAL SECTION Materials. The reagents used in the experimental work were anhydrous sodium aluminate (50.9 wt % Al2O3 + 31.2 wt % Na2O + 17.9 wt % H2O) and Ludox AS-40 colloidal silica (40 wt % suspension in water) from Sigma-Aldrich, sodium hydroxide (99 wt % NaOH) from Merck, ammonium nitrate as an ion exchange salt (>99.5 wt % NH4NO3) from Fluke, deionized (DI) water (1 ppm Na and 0.4 ppm Al), and liquid nheptane (≈99 wt % CH3(CH2)5CH3, with bp = 97−98 °C) from Sigma-Aldrich. Synthesis Process. The sodium form of zeolite-Y (NaY) was synthesized according to the chemical recipe for the FAU structure in the literature,12 with the feedstock and seed gel employed as a batch mixture to produce an overall gel composition with a molar formula of 4.62 Na2O:Al2O3:10 SiO2:180 H2O. Crystalline zeolite-Y product was obtained by dissolution of the reaction components in an aqueous base medium, following which the reaction mixture was heated for 24 h at 25 °C to both age and nucleate the seed gel. This was followed by crystallization for 18 h at 100 °C to stimulate overall gel crystal growth, with this time frame being found in preliminary work to be the most appropriate condition for hydrothermal synthesis. Finally, the warmed suspension was centrifuged and washed with DI water until the pH of the decanted solution dropped to 7, and the wet solid product containing the NaY zeolite was dehydrated overnight at 110 °C. Ion Exchange Process. Approximately 1 g of zeolite-Y was suspended in 100 mL of aqueous ammonium nitrate solution, Received: Revised: Accepted: Published: 6648
November 13, 2011 March 18, 2012 April 21, 2012 April 21, 2012 dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
Figure 1. SEM images of a zeolite-Y sample at 6000× and 24000× magnifications.
and the mixture was heated to 80 °C under reflux condition with constant stirring for 1 h. The exchanged Y zeolite was filtered off, washed with 1 L of DI water, centrifuged, and then dried at 110 °C overnight. A multistage ion exchange procedure using the above conditions was carried out to obtain the NH4 form of theY zeolite with 0.5, 1.5, and 3 wt % cation content. Calcination Processs. Approximately 10 mg of sample was calcined using a thermogravimetric (TG) instrument with an air-flow rate of 30 mL·min−1, in order to assess the thermal behavior of the synthesized Y zeolite. The temperature was ramped at 0.5 °C min−1 from room temperature to 120 °C and maintained at isothermal conditions for 16 h. The samples were then calcined at a constant rate of 1.25 °C min−1, from 120 °C to a maximum temperature (Tmax) of either 450, 500, or 550 °C, with the system maintained at isothermal conditions for 16 h. Steaming/Dealumination Process. NH4Y zeolite samples with 0.5 and 3 wt % Na+ contents were subjected to a steaming/dealumination process at either 400, 500, or 600 °C in a stainless steel reactor with an internal diameter of 10 mm and a length of 430 mm. A 2.5 g sample of the Y zeolite with a particle diameter range of 125−425 μm was packed into the reactor, and the bed was fixed with glass beads. The steaming process was conducted under two sets of nitrogen−steam mixture flow conditions: 150 mL·min−1 for 3 h or 75 mL·min−1 for 6 h. In all cases, the temperature was ramped to Tmax at a rate of 1.25 °C min−1 under dry air-flow conditions. The efficiency of the system was monitored using a material balance for water weight lost during the steaming experiments. Cracking Process. A 100 mg sample of USY zeolite in a granular form was sandwiched between two layers of quartz wool inside a cylindrical Pyrex microreactor of 4 mm i.d. and 400 mm length, and the reactor was inserted in the middle of a 25 mm i.d. and 300 mm long vertical Carbolite furnace. The sample was located at the predetermined stable temperature zone of the furnace and activated in situ prior to the cracking reaction using a dehydrated air flow of 35 mL·min−1 ramped from room temperature to 450 °C at a rate of 2 °C min−1. N2 gas at a flow rate of 33.3 mL·min−1 was used to transport the nC7 feed to the catalyst bed (W/F ≈ 44 g·h·mol−1), and the product gases from the outlet stream were sampled and
analyzed with an online gas chromatograph at 2, 17, 32, 62, 92, 122, and 300 min. Characterization Techniques. An X-ray diffractometer (Philips X'Pert Pro PW 3719) with copper radiation sources (Cu Kα1 and Cu Kα2) was used. The instrument settings at room temperature were as follows: scan speed = 0.040 759 2θ s−1, slit width = 1/8 and 1/4°, number of steps = 4368, and 2θ interval between 2 and 75°. The elemental composition of the zeolite samples was obtained using a Fisons Instruments, Horizon model ICP-AES, with an argon plasma, running at 8000 K. An acid digestion treatment using HF was carried out to destroy the zeolite structure and release the elements present within it. An FEI Quanta 200 scanning electron microscope (SEM) with large-field EDX detector attachment was employed to investigate zeolite morphology with a gold coating used to prevent charging effects. A Quantachrome Corp. Autosorb-6 surface area analyzer was used to measure the textural properties of the synthesized Y zeolites by nitrogen adsorption/desorption isotherms obtained at 77.4 K. 29 Si (79.49 MHz) and 27Al (104.26 MHz) solid state NMR spectra were obtained for the zeolite catalysts using a Bruker Avance III-400 spectrometer with adamantine as an external reference material. The zeolite sample in powder form was pressed inside a 4 mm ZrO2 rotor and a jet of driving air spun the rotor. Magic angle spinning at 54°74′ with a rotation frequency of 10 kHz and relaxation times of 40 s for 29Si and 5 s for 27Al was used. A Q5000 thermogravimetric analyzer with a sensitive microbalance was utilized to detect weight changes of less than 0.1 μg in the zeolite composition during calcination, with the TA universal analysis software used for data analysis. Finally, a Varian-3400 gas chromatograph with a He gas split ratio (SR) of 100:1 at the injector, running isothermally at 250 °C, and an open tubular PLOT Al2O3/KCl capillary column (50 m length and 0.32 mm i.d.) with a flame ionization detector (FID) operating at a H2/air ratio of 1:10 was employed to analyze the n-C7 cracking products. 6649
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
■
Article
RESULTS AND DISCUSSION Study of the NaY and NH4Y Zeolite Forms. During the synthesis of the NaY zeolite form from the raw materials, a
Table 1. Weight Loss during Calcination of NH4Y Zeolite sample
max calcination temp (°C)
weight loss (wt%)
NH4Y.001 NH4Y.002 NH4Y.003
450 500 550
29.27 30.89 31.26
Figure 4. Schematic diagram of the dehydroxylation phenomenon.
I4 + I3 + I2 + I1 + I0 ⎡ Si ⎤ = ⎢⎣ Al ⎥⎦ I4 + (0.75)I3 + (0.5)I2 + (0.25)I1 Framework
(1)
Additionally, TG analysis showed that the qualified sample had 24.8% water content, and ICP analysis indicated that it contained 8.2% sodium. Multipoint and Langmuir surface area measurements of the sample gave 665.6 and 971.1 m2 g−1, respectively, and the total pore volume was found to be 0.357 cm3 g−1. Powder X-ray diffraction patterns were used to determine the dimensions of the unit cell (ao), and the concentration of aluminum atoms (NAl) in the zeolite lattice was determined using eq 2:
Figure 2. Relationship between quantities of Na ion in exchanged solution (ppm) and zeolite-Y framework (wt %).
range of the crystallization times were optimized. Once the analysis of the formed samples was completed, the most appropriate sample was selected according to Figure 1. This meant that the qualified sample presented a more homogeneous composition than all other synthesized samples with Si/ AlBulk ≈ Si/AlFramework (NMR) = 2.23. The 29Si NMR spectrum (not shown) of the synthesized sample indicated five peaks with intensity (In) equating to the five possible environments of Si−(nAl) units in the Y zeolite framework. The 27Al NMR spectrum confirmed that all aluminum atoms were tetrahedrally coordinated at 60 ppm, with eq 1 used to determine the framework Si/Al(NMR) ratio.13
⎡a ⎤ NAl = 101.202⎢ o − 24.2115⎥ ⎣Φ ⎦
(2)
where Φ is the correction factor introduced by Jorik, with a value of 0.9966 for sodium Y type zeolites.1 Since the total number of AlO4 and SiO4 tetrahedral units is 192, the framework Si/Al(XRD) ratio was thus calculated from the following formula:14
Figure 3. TGA plots of NH4Y zeolite samples at different calcination temperatures. 6650
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
Figure 5. XRD patterns of steamed zeolite samples with 0.5 wt % Na content within the lattice.
⎡ Si ⎤ ⎣⎢ Al ⎦⎥
= Framework
192 −1 NAl
led to a small but significant increase in the sample weight loss, as seen in Table 1. The TG analysis illustrates two weight loss profiles for sample NH4Y.001, but three profiles for samples NH4Y.002 and NH4Y.003. It can be suggested that a high calcination temperature (above 450 °C) may lead to the removal of the original hydroxyl groups from a fragile zeolite structure containing 0.5 wt % Na via the dehydroxylation reaction (i.e., third phase of weight loss shown in TGA heating profiles). It has been reported16 that dehydroxylation reactions gradually lead to a collapse of the crystal lattice for a zeolite structure and amorphization, with 400 °C being sufficiently high for deammoniation of the NH4Y type, but not sufficiently high for the dehydroxylation of the resulting hydrogen form (HY). It was also claimed that the trigonally coordinated aluminum and silicon atoms are often attributable to the dehydroxylation and can be represented by Figure 4.17 Study of the USY Zeolite Form. In order to establish a relationship between the Na+ content inside the zeolite-Y lattice and the dehydroxylation phenomenon at a high temperature, the cation content within the lattice was altered during the steaming dealumination treatment. The properties of the 0.5 wt % Na−USY zeolite samples obtained using this treatment were characterized by XRD and NMR, with the results shown in Figures 5 and 6, while the characterization results of the 3 wt % Na−USY zeolite samples are shown in Figures 7 and 8. In addition, the number of extraframework aluminum (EFAl) species calculated as a
(3)
Accordingly, the ao value and the Si/Al(XRD) ratio for synthesized NaY zeolite samples were found to be 24.73631 Å and 2.11, respectively. Improvements in the kinetic properties of the hydrated NaY zeolite were achieved by means of a multistage ion exchange (IE) process, with the NH4Y form produced from the NaY form without affecting the environment of the Si−(nAl) building units within the zeolite lattice. The concentration of sodium ions in the precursor solution was found to increase from 1 ppm to 541, 667, and 762 ppm, as the percentage of Na+ content within the framework of exchanged zeolite decreased from 8.2 wt % to 3, 1.5, and 0.5 wt %, respectively (Figure 2). Study of the HY Zeolite Form. The anhydrous HY zeolite form was obtained by calcining the NH4Y form, during which the desorption of physisorbed and chemisorbed water together with the decomposition of the NH4+ cations into NH3 and H+ occurred, as shown in eq 4:15 Δ
NH4Y(s) → HY(s) + NH3(g)
(4)
Several calcination methods were optimized using TG analysis, with three zeolite-Y samples containing 0.5 wt % Na+ content providing the TGA results shown in Figure 3, indicating that increasing the calcination temperature with time 6651
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
Although the dealumination temperature remained constant in both steaming methods, the Si/Al ratio was slightly higher for the second method (75 mL·min−1 for 6 h) than for the first (150 mL·min−1 for 3 h), with this observation attributed to the change in weight hourly space velocity of steam (WHSV) through the dealumination experiments affecting the residence time of the steam molecules throughout the zeolite bed in the reactor. This means that deeper dealumination occurs with a lower flow rate and longer procedure as vapors have sufficient time to penetrate the porous structure and defuse in radial and longitudinal directions. Table 2 illustrates that there is an acceptable degree of correlation between the values of Si/Al(XRD) and the values of Si/Al(NMR) for each sample, with the framework Si/Al ratio increasing with increasing steaming temperature. In contrast, there is a clear difference between the Si/Al(XRD) value and the Si/Al(NMR) value for the steamed sample at 500 °C under 75 mL·min−1 for 6 h with 0.5 wt % Na+ content. The decrease in the peak height combined with an increase in the background signal can also be observed in the XRD pattern for this sample, as shown in Figure 5e, which indicates a partial structural collapse within the zeolite structure beginning at 500 °C. This results from enhanced dehydroxylation reactions inside the structure at high temperatures, combined with the use of longer steaming periods, and ultimately leads to an increase in the amorphous regions within the framework. Collapse of the entire structure was shown to occur in subsequent steamed samples when the steaming temperature was increased by just 50 °C to 550 °C, as shown in Figure 5c,f. Although the severity of steaming increased when the temperature was raised to 600 °C, the XRD patterns of the steamed samples with 3 wt % Na+, as shown in Figure 7c,f, confirm that the zeolite structure remained rigid, indicating that the increase in cation content inside the zeolite framework resulted in a USY catalyst with a stronger and more resilient structure that was less susceptible to decomposition by steaming. In general terms, the steaming rate (or substitution of aluminum by silicon molecules) was enhanced, while the dehydroxylation rate (i.e., the hydrolysis of the atomic bounds of Y zeolite) was hindered. Additionally, calcination of the NH4Y form with a very low percentage of Na+ content at high temperatures prior to steaming lead to the generation of the HY form with a weaker structure, which ultimately results in a rapid collapse under steaming. This conclusion is supported by data presented here and in the TGA curves (Figure 3), where the level of sodium content leads to enhanced stability during both calcination and steaming processes. It can also be shown that the Al−O bond length of 1.700 Å in HY is greater than those in NH4Y and NaY, with lengths of 1.636 and 1.620 Å, respectively,18 meaning that the Al−O bond is weakened within the crystal lattice structures of the HY type zeolites, and the electron density around the Al−O bonds is lower in HY than in the other two forms of Y zeolite. Figures 5 and 7 illustrate that the relative intensities of the XRD peaks decrease, while the background signal increases as a result of an increase in the steaming temperature, with the diffraction peaks shifting to the right-hand side of the patterns (higher 2θ values). This confirms that the unit cell of the zeolite had decreased under stabilization, with Table 2 also showing a reduction in the ao dimensions, or lower d spacings between the parallel planes in the atomic lattice upon dealumination, with these changes in the XRD patterns usually attributed to misplaced zeolite crystals found when mobile Al atoms move
Figure 6. Top: 29Si NMR spectra for USY zeolites with 0.5 wt % Na content steamed at different temperatures. Bottom: 27Al NMR spectra for USY zeolites with 0.5 wt % Na content steamed at different temperatures.
difference between the total numbers of aluminum atoms in the framework (FAl) before and after steaming, as shown by the Si/Al ratios, are given in Table 2. 6652
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
Figure 7. XRD patterns of steamed samples with 3 wt % Na content within the lattice.
substitution of Al−O bonds with shorter Si−O bonds2 since a tetrahedral Al−O bond (1.70 Å) in the HY zeolite form is larger than a Si−O bond (1.60 Å), with the Si−O bonds in the framework being more stable than Al−O bonds. Finally, NMR and XRD analyses for both steaming methods confirmed that increasing the sodium content by 3 wt % inside the Y zeolite framework during the steaming also inhibits the steaming process and, therefore, decreases both the Si/Al ratio and the percentage of EFAl species in the USY sample compared to the 0.5 wt % Na−USY sample (Table 2). Thus, it can be concluded that sodium ions hinder the extraction of aluminum atoms from the lattice matrix and thereby minimize the rapid contractions of the Y zeolite unit cell. Catalytic Performance of USY Zeolites. The steamed zeolite-Y samples were named as indicated in Table 3, and the conversions of n-heptane from a cracking reactions over these USY catalysts are plotted against the reaction time on stream (TOS), as illustrated in Figure 10. The effect of cation content within the framework of the USY catalyst on the percentage of n-C7 conversion was investigated using catalysts 005 and 006, which had lattice content of 3 wt % Na+, and the ion exchanged versions 007 and 008, which had a framework content of 0.5 wt % Na+. Experimental results indicated that a decrease in sodium content (i.e., an increase in the number of active acid sites) inside the structure of the USY catalyst increased the cracking conversion (Figure 10). In addition, the effects of temperature
from framework to extraframework positions allowing the silicate building units to obtain stable configurations. 29 Si NMR spectra (Figures 6, top, and 8, top) support the conclusions that silica-rich surface layers have been created at −105 and −101 ppm corresponding to Si(0Al) and Si(1Al) following increased steaming, such that the intensities of the peaks on the left-hand side of the 29Si NMR spectra show the Si(2Al) peak at −95 ppm decreasing due to an increase in the framework Si/Al ratio in comparison with the parent Na− NH4Y zeolite sample. Additionally, the 27Al spectrum of the parent Na−NH4Y sample consists of only one sharp resonance signal at 60 ppm corresponding to the tetrahedrally coordinated framework aluminum, with the spectra for the steamed samples in both Figures 6, bottom, and 8, bottom, showing two further peaks associated with nonframework aluminum species at 30 and 0 ppm, corresponding to five-pentahedral-coordinated and sixoctahedral-coordinated aluminum, with the concentration of guest EFAl species seen to increase as the severity of steaming increases (Table 2). A plot of the percentage EFAl species and the dimensions of the unit cell (ao) for the steamed samples against the Si/Al ratio provides a linear relationship, with a positive slope (Figure 9 a) due to an increase in the percentage of EFAl species related to the Si/Al(NMR) ratio, while the negative slope (Figure 9 b) is attributed to a decrease in the value of ao with increasing Si/ Al(XRD) ratio. These relationships can be attributed to the 6653
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
structures of catalysts 003 and 004 resulting from steaming treatment, as shown in Figure 5 b,e, caused this effect. Reducing the FAl concentration leads to an increase in the stability of the individual acid sites19 and in contrast decreases the overall acidity of the catalyst. Additionally, a 10% increase in EFAl species per unit cell is observed with catalysts 003 and 004 compared with catalysts 001 and 002 as shown in Tables 2 and 3; this increase resulted in blocked access to a large number of acid sites which was especially important during coke formation, where the association with EFAl species closed pore openings, forcing the cracking reactions to occur on the surface of the catalyst rather than within the pore structure. In addition to minimizing structural collapse especially at high temperatures, the distribution of the acid sites (both FAl and EFAl) has been adapted using the modified steaming method to provide greater enhancement in the behavior of the n-C7 cracking reaction, as shown with catalysts 007 and 008. While catalysts 003 and 007 have equivalent numbers of EFAl species, FAl atoms, and percentage cation content (Table 3), the performance of 007 is significantly better as clearly shown in the cracking data plotted in Figure 10. The tuned steaming conditions led to a reduction in the rate of associated dehydroxylation reactions, causing a uniform distribution of Al atoms within the Si−(nAl) building units and increasing the strength of each single acid site within the structure of catalyst 007. The selectivity of the USY catalyst was studied by monitoring the ratios of iC/C, C=/C, and (iC + C= + C)/nC7, as shown in Table 4, with the effects of cation incorporation being determined by comparing either the achieved cracking data obtained from catalyst 006 with that from catalyst 008, or the data from catalyst 005 with that from catalyst 007. The decrease in acidity of catalysts 005 and 006 leads to an increase in the ratio of C=/C and a decrease in the ratio of iC/ C in the final reaction products, due to a reduction in the number of active acid sites present to break down the double bond in the olefinic compounds (CC) and generate saturated bonds (i.e., light normal or branched paraffins). It was documented20 that the monomolecular mechanism dominated in n-heptane cracking, with the initial C3:C4 and C2:C5 ratios from a primary endothermic reaction being equal to 1, and therefore the overall paraffin-to-olefin (P/O) ratio in the final product is considered as unity. However, this was not observed in the experiments as a result of the presence of disproportionate reactions, with the rate of normal hydrocarbon formation seeming higher than the rate of double bond hydrocarbon formation over catalysts 007 and 008 than any other USY catalyst as shown in Table 4. On the other hand, the ratio of iC/C (i.e., favorable high octane number hydrocarbons in the products) also increased over those two catalysts, indicating that the hydrogenation of olefins as a secondary cracking reaction improves, when the strength and the number of available active acid sites within the framework of the USY zeolite catalysts are increased. Additionally, the organic ratios of (iC + C= + C)/nC7 were correlated against the previously plotted results in Figure 10, with the ratio of reacted nC7/unreacted nC7 increasing when both the activity of the USY catalyst and the nC7 cracking conversion increased, thus the highest ratio found on the most active catalyst 008 and the lowest ratio found with the least active catalyst 005.
Figure 8. Top: 29Si NMR spectra for USY zeolites with 3 wt % Na content steamed at different temperatures. Bottom: 27Al NMR spectra for USY zeolites with 3 wt % Na content steamed at different temperatures.
on the steaming process can be clearly seen in catalysts 001 and 003 and catalysts 002 and 004 with deeper dealumination at higher temperature leading to an increase in the framework Si/ Al ratio, such that the cracking conversion and catalyst activity are seen to decrease. It can be suggested that the decrease in the framework aluminum (FAl) per unit cell of the strong acid sites, as shown in Table 3, and the partial collapse in the 6654
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
Table 2. % EFAl Species and Si/Al Ratios of Steamed Zeolite-Y Samples sample
Si/Al(NMR)
% EFAl(NMR)
Si/Al(XRD)
ao(XRD) (Å)
3 wt % Na USY steamed sample under 150 mL·min−1 at 400 °C for 3 h 3 wt % Na USY steamed sample under 75 mL·min−1 at 400 °C for 6 h 0.5 wt % Na USY steamed sample under 150 mL·min−1 at 400 °C for 3 h 0.5 wt % Na USY steamed sample under 75 mL·min−1 at 400 °C for 6 h 3 wt % Na USY steamed sample under 150 mL·min−1 at 500 °C for 3 h 3 wt % Na USY steamed sample under 75 mL·min−1 at 500 °C for 6 h 0.5 wt % Na USY steamed sample under 150 mL·min−1 at 500 °C for 3 h 0.5 wt % Na USY steamed sample under 75 mL·min−1 at 500 °C for 6 h 3 wt % Na USY steamed sample under 150 mL·min−1 at 600 °C for 3 h 3 wt % Na USY steamed sample under 75 mL·min−1 at 600 °C for 6 h
4.471 4.720 4.832 5.026 5.630 5.918 6.121 6.370 6.153 6.832
≈40.9 ≈43.5 ≈44.6 ≈46.4 ≈51.2 ≈53.3 ≈54.6 ≈56.1 ≈54.8 ≈58.7
3.546 4.035 4.068 4.842 5.012 5.319 6.273 8.097 6.352 7.077
24.545 07 24.504 68 24.502 23 24.452 81 24.443 66 24.428 41 24.389 14 24.337 02 24.386 36 24.363 27
Table 3. Names and Al Content per Unit Cell of Selected USY Catalysts from the Steaming Process catal name 001 002 003 004 005 006 007 008
postsynthesis modification conditions
a
USY steamed at 400 °C, 150 mL·min−1, 3 h, with 0.5% Na content USY steamed at 400 °C, 75 mL·min−1, 6 h, with 0.5% Na content USY steamed at 500 °C, 150 mL·min−1, 3 h, with 0.5% Na content USY steamed at 500 °C, 75 mL·min−1, 6 h, with 0.5% Na content USY steamed at 600 °C, 150 mL·min−1, 3 h, with 3% Na content USY steamed at 600 °C, 75 mL·min−1, 6 h, with 3% Na content USY steamed at 600 °C, 150 mL·min−1, 3 h, with 0.5% Na content USY steamed at 600 °C, 75 mL·min−1, 6 h, with 0.5% Na content
FAl atoms per unit cellb
no. of EFAl speciesb
32.92
26.48
31.86
27.54
26.96
32.44
26.05
33.35
26.84
32.56
24.51
34.89
26.84
32.56
24.51
34.89
a
These conditions refer to the steaming experimental conditions, while the weight percentage of sodium refers to the concentration of cation within the structure of the catalyst during the steaming treatment. In addition, catalyst 007 was produced by means of subjecting catalyst 005 to an ion exchange stage after the steaming treatment without massive alteration in the structure, and the same procedure was applied to form catalyst 008 from catalyst 006. bThe summation of FAl atoms and EFAl species for each catalyst is equal to 59.4, which is the number of aluminum atoms in the unit cell of the parent Na−NH4Y zeolite before steaming. Figure 9. (a) Relationship between percentage EFAl species and Si/Al ratio obtained by NMR analysis for the steamed sample. (b) Relationship between size of unit cell in angstroms and Si/Al ratio obtained by XRD analysis for the steamed sample.
■
CONCLUSIONS The work presented here details the synthesis and production of the Na+, NH4+, H+, and US forms of the Y zeolite, comparing the thermal stability of the Y structure during calcination, steaming, and cracking processes in order to determine the optimum form of the Y zeolite and the effects of variations in sodium content on reaction stability. The optimum conditions required to produce the NaY zeolite precursor was found to require a gel aging time of 24 h at 25 °C, followed by 18 h of crystallization time at 100 °C. The use of three subsequent ion exchange processes resulted in the removal of ∼94% of the sodium content, giving an NH4Y zeolite with a structure that possesses properties similar to those of standard faujasite. Additionally, it was also shown that decreasing the concentration of Na+ inside the framework of
Figure 10. n-Heptane cracking reaction conversions at 450 °C over USY catalysts at different times on stream.
the Y zeolite caused no change in the aluminosilicate containing Si(nAl) units. 6655
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
Table 4. Selectivity of USY Catalysts during Deactivation Tests at 450 °C time on stream catalyst name
ratioa
2 min
17 min
32 min
62 min
92 min
122 min
300 min
001
iC/C C=/C (iC + C= + C)/nC7 conv mol % iC/C C=/C (iC + C= + C)/nC7 conv mol % iC/C C=/C (iC + C= + C)/nC7 conv mol % iC/C C=/C (iC + C= + C)/nC7 conv mol % iC/C C=/C (iC + C= + C)/nC7 conv mol % iC/C C=/C (iC + C= + C)/nC7 conv mol % iC/C C=/C (iC + C= + C)/nC7 conv mol % iC/C C=/C (iC + C= + C)/nC7 conv mol %
0.6207 1.4638 1.6981 62.0465 0.6444 1.3433 1.8048 63.4559 0.6833 1.8741 0.9586 48.0520 1.0389 4.5482 0.8687 45.5972 0.8355 7.3425 0.2163 16.8933 0.4174 8.1100 0.3408 24.5293 0.6593 0.5671 8.4949 88.5775 0.6111 0.6202 10.2081 90.1874
0.6108 1.4301 0.5491 34.5579 0.6306 1.4561 0.5347 33.9521 0.6239 1.7256 0.3022 22.3153 1.0829 3.0378 0.1059 8.6927 0.6499 6.2339 0.0822 6.7078 0.3687 2.9429 0.0694 5.6019 0.7556 0.7934 1.9704 65.4445 0.7268 0.7303 2.8433 73.0900
0.6043 1.4084 0.5289 33.7068 0.6306 1.4675 0.4674 30.9619 0.6668 1.9145 0.2246 17.4499 1.0155 3.3995 0.0849 6.9420 0.7336 5.3057 0.0545 4.2758 0.4367 3.4258 0.0555 4.3706 0.7948 0.8806 1.4683 58.5958 0.7571 0.8295 2.0219 66.0185
0.5944 1.4615 0.4513 30.2038 0.6075 1.5008 0.4348 29.4149 0.6423 1.9030 0.2051 16.1287 0.9308 3.1960 0.0777 6.3211 0.6341 3.4565 0.0315 2.1627 0.4045 3.1367 0.0485 3.7389 0.8006 1.0024 1.1814 53.2671 0.7813 0.9543 1.4777 58.7498
0.5814 1.4755 0.4337 29.3575 0.5999 1.5463 0.4039 27.8824 0.6264 1.9381 0.1942 15.3703 1.0412 3.0776 0.0791 6.4396 0.5339 5.2123 0.0419 3.1305 0.3852 3.0050 0.0476 3.6525 0.7974 1.1261 0.8519 45.1109 0.9854 0.8331 1.2327 54.3206
0.5753 1.4965 0.4036 27.9058 0.5870 1.5447 0.3926 27.3025 0.6151 1.9779 0.1646 13.2461 0.8038 2.6618 0.0762 6.1920 0.5182 4.9518 0.0374 2.7172 0.3662 2.7784 0.0481 3.6986 0.7913 1.2029 0.7661 42.4876 0.9507 0.6035 1.0558 50.4664
0.5932 1.7059 0.2737 20.6338 0.9819 1.7977 0.3743 26.3448 0.6316 2.1619 0.1401 11.3948 0.9434 3.0039 0.0723 5.8489 0.4314 4.7629 0.0359 2.5835 0.3904 3.4492 0.0536 4.1969 0.8370 1.3739 0.5414 34.2353 0.8766 0.8968 1.0943 51.3612
002
003
004
005
006
007
008
a
iC represents the isoparaffin compounds. C= denotes the olefin compounds. C indicates the paraffin compounds. nC7 refers to the unreacted heptane in the products.
used to reduce the framework Na+ content to 0.5 wt %. This resulted in enhanced activity and selectivity for the USY catalysts during n-C7 cracking reactions, since n-C7 cracking conversion is greatly increased when the concentration of framework Na+ is decreased after steaming.
Calcination of the NH4Y zeolite under air produced the protonic type (HY zeolite) with a diminished stability in the Y lattice with the majority of volatiles being expelled from the structure between 25 and 400 °C, while calcination above 450 °C was shown to cause a removal of framework OH groups via dehydroxylation reactions, as shown by the third weight loss step in the TGA heating profiles. However, the presence of greater concentrations of Na+ inside the lattice led to an increase in the stability of the zeolite structure during steaming processes, which in turn strengthened the USY framework and formed a more resilient structure. This was shown to result from sodium ions hindering the extraction of Al atoms from the Y aluminosilicate matrix and increased the probability of Si atoms filling the defect sites, thereby reducing the rate of dehydroxylation and minimizing both the rapid hydrolysis of Al−O bonds and the rapid contraction of the unit cell. This modified ultrastabilization process not only prevented the catalyst structure from partial and/or entire collapse at high steaming temperature, but also resulted in an improved distribution of either FAl atoms or EFAl species within the USY structure. Finally, catalysts 008 and 007 with 3 wt % Na+ content and optimum properties as defined from the experimental procedure were steamed, with post-steaming ion exchange
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We are indebted to the Ministry of Higher Education & Scientific Research in the Republic of Iraq for full financial support of this project. We would also like to acknowledge the assistance of the laboratory staff in the University of Manchester, U.K., where the majority of the experimental work was completed.
■
REFERENCES
(1) Bekkum, H. V.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introduction to Zeolite Science and Practice, 2nd ed.; Elsevier: Amsterdam, 2001.
6656
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
Industrial & Engineering Chemistry Research
Article
(2) Kaduk, J. A.; Faber, J. Crystal Structure of Zeolite Y as a Function of Ion Exchange. Rigaku J. 1995, 12 (2), 14−34. (3) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press, Inc.: London, New York, 1978. (4) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press, Inc.: London, New York, 1982. (5) Weitkamp, J. Zeolites and Catalysis. Solid State Ionics 2000, 131 (1−2), 175−188. (6) Hensen, E. J. M.; van-Veen, J. A. R. Encapsulation of Transition Metal Sulfides in Faujasite Zeolite for Hydroprocessing Applications. Catal. Today 2003, 86 (1−4), 87−109. (7) Decroocq, D. Catalytic Cracking of Heavy Petroleum Fractions; Editions Technip: Paris, 1984. (8) Hensen, E. J. M.; van-Veen, J. A. R. Encapsulation of Transition Metal Sulfides in Faujasite Zeolite for Hydroprocessing Applications. Catal. Today 2003, 86 (1−4), 87−109. (9) Occelli, M. L. Fluid Catalytic Cracking: Role in Modern Refining; American Chemical Society: Washington, DC, 1988. (10) Wang, Q. L.; Giannetto, G.; Torrealba, M.; Perot, G.; Kappenstein, C.; Guisnet, M. Dealumination of Zeolites II. Kinetic Study of the Dealumination by Hydrothermal Treatment of a NH4NaY Zeolite. J. Catal. 1991, 130 (2), 459−470. (11) Groen, J. C.; Moulijn, J. A.; Ramirez, J. P. Decoupling Mesoporosity Formation and Acidity Modification in ZSM-5 Zeolites by Sequential Desilication−Dealumination. Microporous Mesoporous Mater. 2005, 87 (2), 153−161. (12) Robson, H.; Lillerud, K. P. Verified Syntheses of Zeolitic Materials, 2nd ed.; Elsevier: Amsterdam, 2001. (13) Chiyoda, O.; Davis, M. E. Hydrothermal Conversion of Yzeolite using Alkaline-earth Cations. Microporous Mesoporous Mater. 1999, 32 (3), 257−264. (14) Gola, A.; Rebours, B.; Milazzo, E.; Lynch, J.; Benazzi, E.; Lacombe, S.; Delevoye, L.; Fernandez, C. Effect of Leaching Agent in the Dealumination of Stabilized Y Zeolites. Microporous Mesoporous Mater. 2000, 40 (1−3), 73−83. (15) Chandwadkar, A. J.; Kulkarni, S. B. Thermal Behaviour of Modified Faujasites. J. Therm. Anal. 1980, 19 (2), 313−320. (16) Beyer, H. K. Dealumination Techniques for Zeolites, Molecular Sieves Science & Technology; Springer-Verlag: Berlin/Heidelberg, 2002; Vol. 3. (17) Bhatia, S. Zeolite Catalysis: Principles and Applications; CRC Press: Boca Raton, FL, USA, 1990. (18) Koningsberger, D. C.; Miller, J. T. Local Structure Determination of Aluminium in Y Zeolite: Application of Low Energy X-Ray Absorption Fine Structure Spectroscopy. Catal. Lett. 1994, 29 (1−2), 77−90. (19) Weitkamp, J.; Puppe, L. Catalysis and Zeolites: Fundamentals and Applications; Springer: Berlin, 1999. (20) Cumming, K. A.; Wojciechowski, B. W. Hydrogen Transfer, Coke Formation, and Catalyst Decay and Their Role in the Chain Mechanism of Catalytic Cracking. Catal. Rev. 1996, 38 (1), 101−157.
6657
dx.doi.org/10.1021/ie2026184 | Ind. Eng. Chem. Res. 2012, 51, 6648−6657
