Hierarchical Zeolite Y with Full Crystallinity: Formation Mechanism and

Mar 15, 2017 - Hierarchical zeolite Y with high mesoporosity, integrate crystallinity, and hydrothermal stability is desired urgently for heavy oil co...
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Hierarchical Zeolite Y with Full Crystallinity: Formation Mechanism and Catalytic Cracking Performance Wenlin Li,*,†,‡ Jinyu Zheng,‡ Yibin Luo,‡ Chunyan Tu,‡ Yi Zhang,‡ and Zhijian Da*,‡ †

College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, People’s Republic of China ‡ State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, People’s Republic of China S Supporting Information *

ABSTRACT: Hierarchical zeolite Y with high mesoporosity, integrate crystallinity, and hydrothermal stability is desired urgently for heavy oil conversion. Here, an approach combining steam treatment, acid treatment, and alkaline treatment approachs is reported for fabricating hierarchical zeolite Y with high mesoporosity and extraordinarily high crystallinity. A robust zeolite precursor with good crystallinity was prepared first through an acid pretreatment step using fluorosilicic and hydrochloric acids, in which the zeolite crystal could be healed by the migration of Si into the skeleton. Moreover, it was deduced that the formation of hierarchical zeolite Y with extraordinarily high crystallinity was closely related to the unique desilication mechanism. The obtained hierarchical zeolite Y showed remarkable catalytic activities, high selectivity of liquid products, and low selectivity of bottom oil in the conversion of heavy oil. tion.20,21 However, the zeolites showed a relatively poor crystallinity and loss of microporosity.15,16 The pioneering work by Perez-Ramirez et al.16 reported that the hierarchical zeolite Y prepared in the above manner had a crystallinity of 31%. Further, the crystallinity of this hierarchical zeolite could be improved to as much as 45% with the increase of the reaction time to 72 h during the acid treatment process. Rive Technology17 designed a post-synthetic method to introduce mesoporosity into zeolite Y by a two-step process combining acid treatment and alkaline treatment with surfactant as the template. The interaction between surfactant CTAB and zeolite Y prevented the dissolution of crystals. The hierarchical zeolite Y prepared through the surfactant-based process showed a relatively high crystallinity of 50−72%, attributed to the protective role of the surfactant.7,22,23 As these examples show, it is still a major challenge to obtain hierarchical zeolite Y with both high mesoporosity and integrate crystallinity. It is known that the conversion of heavy oil into value-added products commonly takes place under severe hydrothermal conditions. Remarkably, the hydrothermal breakdown of zeolite with low crystallinity is really rapid under such conditions.4,9,24,25 Although the hierarchical structure of zeolite Y offers substantial improvement for the conversion, the effect of crystallinity, which is vital for efficient conversion under hydrothermal conditions, seems to be often overlooked. Hence, low crystallinity may be the real reason for some of the failures of industrialization. The purpose of this work is to prepare hierarchical zeolite Y with fully preserved crystallinity and provide further insight into the formation of mesopores. The close inner link between the

1. INTRODUCTION Zeolites are widely employed today as catalysts for renewable chemical and petrochemical industries as a result of their tunable acidities and confinement effects.1−5 Zeolite Y possessing three-dimensional pore channels with supercages of 1.2 nm in diameter is a leading catalyst in fluid catalytic cracking (FCC), hydrocracking, and alkylation processes. However, the sole microporous system of conventional zeolites leads to severe diffusion limitations when large reactant molecules are involved.6−10 Integrating hierarchical pores into zeolite structures has been proven to be an effective way to overcome the inherent diffusion limitations and improve the catalysis efficiency of zeolites. However, it is challengeable to introduce mesopores into FAU zeolite by direct synthetic approaches.11,12 As well-known, the conventional synthesis of zeolite Y does not need any organic templates, and therefore, the post-synthetic strategy would be the most promising route for industrial applications. The post-synthetic strategy, such as steaming, acid treatment, or alkaline treatment, has the advantage of low cost, and it is an easy way to prepare zeolites with good accessibility.13−15 In previous works, it has been demonstrated that hierarchical zeolite Y could not be created directly from industrial zeolite Y with alkaline treatment, because the substantial aluminum atoms could suppress the extraction of neighboring silicon species.9,16 In the case of zeolite Y, excessive aluminum atoms should be removed first to make the desilication process implementable.17−19 In a typical approach, a mild acid pretreatment was implemented to the ordinary zeolite Y and then alkaline treatment with cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), was implemented. Acid treatment could weaken the crystal structure of zeolite by removing Al atoms from the zeolite framework, while the mesoporous structure subsequently formed with Si dissolu© XXXX American Chemical Society

Received: December 22, 2016 Revised: March 15, 2017 Published: March 15, 2017 A

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spectrometer equipped with a magic angle spin probe. Fourier transform infrared (FTIR) and pyridine-adsorbed infrared (Py-IR) spectra were obtained with a Nicolet model 710. For pyridine adsorption, the sample was pretreated at 400 °C for 1 h. Then, sufficient pyridine was introduced at 170 °C. After that, the spectrum was recorded by evacuation at 350 °C. Temperature-programmed desorption of ammonia (NH3-TPD) was performed using an Autochem 2910 equipped with a thermal conductivity detector (TCD). The sample (ca. 200 mg, 20−40 mesh) was taken in a quartz U-shaped tube. It was flushed in He (30 mL/min) to 500 °C for 1 h. Then, the sample was cooled to 50 °C and adsorbed a 10% NH3−He mixture for 30 min. Subsequently, the sample was flushed with He until the baseline was stable. The desorption process was measured by increasing the temperature from 50 to 550 °C with a ramp rate of 10 °C/min. 2.3. Catalytic Cracking Performances. The catalytic performance of the zeolites was evaluated by the catalytic cracking of 1,3,5triisopropylbenzene (TIPB) and heavy oil in a fixed-bed flow reactor system. Actually, it is a batch system worked in a fluidized mode. In each reaction, a new catalyst was introduced at the start of each run. The properties of the heavy oil were listed in Table S1 of the Supporting Information. Catalytic cracking activity measurements of the prepared zeolites were carried out in a fixed-bed reactor unit manufactured by SINOPEC. Prior to each experiment, the zeolite was preactivated in situ at 500 °C for 2 h by nitrogen flow (0.1 mL s−1). After the activation of the zeolite, reacting feed was injected using a syringe pump via a vaporization line into the reactor. For the cracking of TIPB, 1.0 g of the zeolite was used and reaction conditions were 200 °C (no thermal cracking) and 960 s. In the case of heavy oil cracking, reaction conditions were as follows: the zeolite loading of 2 g, heavy oil amount of 0.83 g, reaction temperature of 500 °C, and reaction time of 960 s. After the reaction, the gaseous products were analyzed by a gas chromatograph (Agilent 7890). During the reaction step, the liquid products were collected in the bottom of the reactor, in which temperature was below −4 °C. The gaseous products were collected in a device by water displacement. The yields of gasoline [initial boiling point (IBP)−204 °C], light cycle oil (LCO, 205−350 °C), and heavy oil (above 350 °C) were analyzed on the basis of the weight of the liquid products and the results of simulated distillation (ASTM D2887). The coke content was determined by a CO and CO2 analyzer at the flue gas outlet. The conversion was defined as the sum of dry gas, liquid petroleum gas (LPG), gasoline, and coke. The TIPB conversion was calculated using

novel acid precursor and the formation of mesopores is illustrated. Moreover, benefiting from the synergistic effects of steam treatment, acid treatment, and alkaline treatment, an interesting hierarchical zeolite Y with full crystallinity is achieved. The synthetic process is rapid, economical, and easy to be commercialized. The design concept for hierarchical zeolite, which provides an efficient way to develop heterogeneous catalysts, has potential in conversion of heavy oil in industry.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Hierarchical Zeolite Y. Commercially available zeolite NaY with crystallinity of 91% and Si/Al atom ratio of 2.6 from Changling Catalyst Manufacturing Company of SINOPEC was used as the parent zeolite. The parent zeolite NaY was stabilized by 1-time ion exchange in 1.0 mol/L NH4Cl solution at 70 °C for 1 h, followed by calcination under steam flow at 600 °C for 2 h. The as-made zeolite was denoted as HY. The zeolite HY was further treated by ion exchange twice in 1.0 mol/L NH4Cl solution and calcination at 550 °C for 4 h. The resulting zeolite was denoted as HY-550. Hierarchical Y zeolite was prepared using an acid plus alkali strategy. In the acid treatment step (denoted as FSAH treatment), 4 g of aqueous NH4Cl solution and 10 g of zeolite HY were mixed at room temperature. Then, 0.06 M fluorosilicic acid (FSA) and 0.55 M hydrochloric acid were successively added to the above solution. The obtained solution was heated to 60 °C and kept stirring for 1 h. Then, the product was recovered by filtration, water washing, and drying. The as-made sample was denoted as zeolite FSAH-Y. Afterward, the alkaline treatment step was performed on zeolite FSAH-Y. Specifically, 5 g of zeolite FSAH-Y was dispersed in 0.6 and 1.0 M NaOH solution (50 mL). The as-made zeolites were recovered after filtration and drying. Then, the zeolites were treated by 5 consecutive ion exchanges in 1.0 mol/L NH4Cl solution (at 70 °C for 1 h). The alkaline-treated samples were denoted as zeolites AT-Y (L) and AT-Y (H), where L and H indicate the NaOH concentration of 0.6 and 1.0 mol/L, respectively. The samples after alkaline treatment could be converted to proton-type zeolites via ion exchange and calcination at 550 °C for 5 h. The resulting zeolites were denoted as AT-Y (L)-550 and AT-Y (H)-550, respectively. As for the catalytic cracking of heavy oil, zeolites HY, FSAH-Y, AT-Y (L), and AT-Y (H) were calcined in a 100% steam flow at 800 °C for 8 h and the as-made samples were named HY-800, FSAH-Y-800, AT-Y (L)-800, and AT-Y (H)-800, respectively. 2.2. Characterization. X-ray diffraction (XRD) analysis was performed on a Bruker diffractometer with Cu Kα radiation. The relative crystallinity (RC) was calculated by comparing the peak intensities of the zeolites to that of the parent NaY, which was considered to be 100% crystalline. The total intensities of the eight peaks assigned to the (331), (511), (440), (533), (642), (822), (555), and (664) reflections were used for comparison according to the ASTM D3906 method. The bulk Si/Al molar ratio of zeolite Y was obtained on a Philips MagicX X-ray fluorescence (XRF) spectrometer. A scanning electron microscopy (SEM) image of the sample was collected on field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan) with an operating voltage at 10 kV. Transmission electron microscopy (TEM) analysis was performed on TECNAI G2 F30 operating at 300 kV. Nitrogen adsorption−desorption analysis at a liquid nitrogen temperature (−196 °C) was carried out in a Micromeritics ASAP 2020M sorption analyzer. Prior to the measurement, the sample was outgassed at 300 °C. The specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method. The total pore volume was determined from the amount adsorbed at a relative pressure of around 0.99. The t-plot method was applied to obtain the microporous volume, external surface area, and mesoporous volume. The pore size distribution was calculated by the Barrett−Joyner−Halenda (BJH) method. 27 Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) measurements were conducted on a Bruker RX-400

conversion (%) = (TIPBfeed − TIPBproduct )/TIPBfeed × 100% where TIPBfeed and TIPBproduct are the amounts of TIPB in the feed and product, respectively.

3. RESULTS AND DISCUSSION Figure S1 of the Supporting Information is the XRD patterns for NaY, HY, FSAH-Y, AT-Y (L), and AT-Y (H) zeolites. The pattern of the parent zeolite NaY matches nicely with the characteristic Bragg reflections corresponding to the FAU structure in the wide angle region above 2θ = 5°. After steam treatment, there is a decrease in intensity, which is attributed partially to the non-crystal structure formed. It is expected that the steam treatment induces the formation of stable crystal zeolite Y. After FSAH treatment, the resultant samples still display the typical diffraction patterns of the FAU structure, suggesting that the treatment preserves the framework topology of zeolite and no new crystalline phases emerged. It has been reported that the removal of aluminum atoms from the framework of zeolite by acid could subsequently induce the formation of vacancies. Then, the resultant zeolite exhibits dramatic descending of the crystallinity.26 However, in this study, the crystallinity of FSAH-Y increases in comparison to B

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Energy & Fuels zeolite HY, indicating that the framework is much more robust through FSAH treatment. In FSAH treatment, extra-framework Al debris formed during the steam treatment could be removed and some extra-framework Si debris could be generated. Moreover, the condensation of the Si species formed in FSAH treatment could be able to fill up vacancies created in the framework by the extraction of aluminum atoms.27,28 It has been suggested that aluminum is removed in the form as a soluble fluoride complex, whereas the inserted species is monomolecular silicic acid, Si(OH)4. If a proper balance could be obtained between the rate of aluminum extraction and the rate of silicon insertion, the structural collapse could be avoided.29,30 Clearly, the FSAH treatment present here contributes to form a Si-rich structure and a perfect framework for the zeolite. As shown in Tables S2 and S3 of the Supporting Information, both the microporosity and Si/Al molar ratio of the zeolite are improved after the FSAH treatment. In comparison to zeolite FSAH-Y, zeolite ATY exhibits higher relative crystallinity (Figure S1 of the Supporting Information), indicating that the zeolite structure remains well and little amorphous phase emerges during the alkaline treatment process. The origin of the perfect crystallinity could be, originating from the proper precursors, prepared in the FSAH treatment. FSAH could act as a source for bonding Si atoms to the defect positions while generating some extraframework Si debris during the dealumination process.28,30,31 It is generally accepted that the Si/Al molar ratio of zeolites plays a crucial role in the formation of mesopores in zeolites and the zeolites with a high Si/Al ratio are easier for the elimination of silicon atoms in alkaline circumstances to form mesopores, which is due to the inhibition of dissolving silicon species in the alkaline circumstances by aluminum ions.2,32−34 As listed in Table S2 of the Supporting Information, both (Si/Al)surf and (Si/Al)bulk increase significantly. (Si/Al)frame of FSAH-Y was higher than that of NaY. Meanwhile, it is worth noting that (Si/Al)surf is much higher than (Si/Al)bulk of FSAHY and the ratio of (Si/Al)surf/(Si/Al)bulk is much higher than that of NaY. Obviously, the mixed acid pretreatment with hydrofluosilicic and hydrochloric acids was the most important reason for the formation of these above properties. This special role resulted in the crystallizing silicon-rich structure at the external surface of zeolite, which generated a composition gradient though the zeolite Y. Moreover, hydrofluosilicic acid treatment was employed as a directing agent to remove the aluminum ions and inserted silicon species simultaneously into the zeolite framework during the acid treatment process, thereby repairing the framework of zeolite timely and restoring crystallinity (Figure S1 of the Supporting Information). Notably, the (Si/Al)bulk ratio of sample AT-Y is markedly lower than that of FSAH-Y. This result indicated that predominant silicon atoms were removed from the interior of zeolite FSAH-Y. Moreover, the ratio of (Si/Al)surf/(Si/Al)bulk decreases from 1.7 (FSAH-Y) to 1.1 (AT-Y). It suggested that the mesopores were first generated closely to the external surface of the zeolite and then created inside of the crystals. Additionally, the volume of mesopores was found to rise distinctly with the increase of the alkali concentration, relating to the more powerful penetration gradient inside the zeolite. Nitrogen adsorption isotherms for different zeolites measured at 77 K are shown in Figure 1a. It can be seen that zeolite FSAH-Y exhibits higher micropore surface areas and micropore volumes compared to zeolite HY. This suggested that a integral crystalline structure was formed in zeolite FSAH-Y. The

Figure 1. (a) N2 adsorption and desorption isotherms and (b) BJH pore size distributions for zeolites NaY, HY, FSAH-Y, AT-Y (L), and AT-Y (H).

adsorption isotherms of zeolite AT-Y (L) and AT-Y (H) exhibit a sharp increase in the adsorbed amount in the range of 0.4 < P/P0 < 0, which demonstrates the generation of intracrystalline mesopores. Moreover, the low-pressure adsorption isotherms of zeolites AT-Y (L) and AT-Y (H) are higher than those of zeolites NaY and HY, suggesting the presence of supermicroporosity in the hierarchical zeolites. This also revealed that little damage to the internal pores of zeolite NaY crystals was caused by the sequent acid and alkaline treatment.26,35,36 However, it should be noted that the creation of mesopores through alkaline treatment involved the expense of destroying parts of the micropores in zeolite FSAH-Y, with the micropore volume decreasing from 0.333 mL/g for zeolite FSAH-Y to 0.297 mL/g for zeolite AT-Y (L) (Table S3 of the Supporting Information). In the present study, a breakthrough increase in the porosity of zeolite Y without any crystallinity loss is achieved, accompanying a huge amount of mesopores, which contribute to 61% of the total pore volume. Figure 2 shows the 29Si MAS NMR spectra for the samples before and after acid−alkaline treatment. A series of broad signals centered at ca. −107, −101, −96, and −91 ppm for zeolite NaY could be assigned to Si(0Al), Si(1Al), Si(2Al), and Si(3Al) structural groups, respectively. in comparison to zeolite NaY, the signal intensity of Si(3Al) and Si(2Al) for zeolite HY significantly decreases, indicating that aluminum atoms are eliminated from the framework of zeolite Y upon hydrothermal treatment. It can be seen that zeolite FSAH-Y exhibits a higher intensity of the Si(0Al) signal at the expense of the intensity of Si(2Al) and Si(3Al) in comparison to zeolite HY, indicating C

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Figure 2. 29Si MAS NMR spectra for zeolites (1) NaY, (2) HY, (3) FSAH-Y, (4) AT-Y (L), and (5) AT-Y (H).

Figure 3. 27Al MAS NMR spectra for zeolites (1) NaY, (2) HY, (3) FSAH-Y, (4) AT-Y (L), and (5) AT-Y (H).

that dealumination mainly occurs in the Al-rich zeolite Y framework and silicon inserts in the zeolite Y framework with the form of Si(0Al). In the case of zeolite FSAH-Y, a significant shoulder peak located from −115 to −110 ppm is detected, suggesting the occurrence of extra-framework silicon in zeolite.37 As presented in Figure 2, the signal assigned to Si(0Al) of zeolite FSAH-Y shows no distinct change before and after alkaline treatment with a lower concentration of NaOH, suggesting that no Si(0Al) silicon sites are extracted from the framework of zeolite FSAH-Y. However, amounts of mesopores has been generated in zeolite AT-Y (L), confirmed by BET analysis. These results demonstrated that the formation of mesopores in AT-Y (L) zeolite was not primarily induced by the desilication of Si(0Al) silicon sites in the framework of zeolite FSAH-Y but mainly resulted from the desilication from non-framework Si debris. Remarkably, extra-framework Si debris generated from FSAH treatment played an important role in the formation of mesopores. In comparison to zeolite FSAH-Y, the crystallinity of zeolite AT-Y (L) was significantly improved as a result of the removal of extra-framework silicon from the interior of zeolite FSAH-Y and little destruction of the zeolite framework during the alkaline treatment process. In addition, in comparison to parent zeolite NaY, it was surprisingly found that, zeolite AT-Y (L) showed a superior crystalline structure with higher crystallinity. However, in comparison to zeolite FSAH-Y, the signals assigned to Si(0Al) and Si(1Al) silicon sites of zeolite AT-Y (H) decrease, whereas the signals attributed to Si(2Al) and Si(3Al) silicon sites increase. It was suggested that Si(0Al) units were first extracted and, subsequently, Si(1Al) units of zeolite FSAH-Y were extracted by the higher alkaline treatment, leading to the decrease of crystallinity and framework Si/Al ratio. This result was consistent with XRD analysis. In 27Al MAS NMR spectra (Figure 3), it is worthwhile to note that zeolite NaY exhibits one resonance at 63 ppm, attributed to tetrahedrally coordinated aluminum. Zeolite HY exhibits two main peaks at 63 and 0 ppm, which are attributed to tetrahedrally coordinated aluminum and hexacoordinated extra-framework aluminum (hexacoordinated EFAL), respectively.26,38 In addition, a shoulder resonance around 50 ppm is observed in zeolite HY, which represents extra-framework aluminum with tetrahedral coordination. With FSAH treatment, the resonance at 0 ppm disappears completely and the resonance around 50 ppm decreases significantly. It indicated that the hexacoordinated EFAL species were completed removed and tetrahedrally coordinated FAL was partially

extracted by FSAH treatment. The results of 27Al MAS NMR spectra were consistent with the results reported in the literature, in which EFAL species would be removed preferentially, while FAL could be removed with the increased concentration of fluorosilicic acid.28,29,39 From the SEM images (Figure S2 of the Supporting Information), it can be seen that the hierarchical zeolite Y preserves complete crystal structures. The mesoporosity of the hierarchical zeolite Y is further investigated by TEM (Figure 4).

Figure 4. TEM images of zeolites (a and b) NaY and (c−g) AT-Y (L). D

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Energy & Fuels Mesopores with a pore size of 3−4 nm are found to distribute randomly throughout the whole zeolite AT-Y (L) crystals (panels c−f of Figure 4), which is consistent with the BJH results (Figure 1b). The complete crystalline structure of mesopores is demonstrated by the distinct lattice fringes, as presented in Figure 4g. Meanwhile, the newly formed mesopores, crossing the original microchannels, are functionalized as the communication channels among micropores. Thus, it can be concluded that the microporous structure was fully preserved after the desilication process, while the mesoporous secondary system was just formed significantly. Figure 5 is the infrared (IR) spectra in the region of OH group vibration for zeolites HY, FSAH-Y, AT-Y(L), and AT-

Figure 6. NH3-TPD spectra of zeolites (1) HY-550, (2) AT-Y (L)550, and (3) AT-Y (H)-550.

(330 °C), and its peak intensity decreases too. This result suggests that the decrease of the acid strength is owing to the acid treatment. It is well-established that the acid strength is related to the environment of aluminum sites in the zeolite framework.27,28,40 These observations are in good agreement with those reported in the literature, in which aluminum in Alrich regions associating with acid sites with weak acidity is removed and much more isolated and acidic sites could be formed.39,40 Table 1 presents the Py-IR acidity of zeolites HY-550, AT-Y (L)-550, and AT-Y (H)-550. It can be seen that zeolite AT-Y Table 1. Py-IR Acidity of Zeolites HY-550, AT-Y (L)-550, and AT-Y (H)-550 IR acidity at 200 °C (μmol g−1)

Figure 5. IR spectra in the region of OH group vibration of zeolites (1) HY, (2) FSAH-Y, (3) AT-Y (L), and (4) AT-Y (H).

Y(L). The small bands at 3660 and 3560 cm−1 are ascribed to the hydroxyl-containing cations bonding to extra-framework aluminum. The band at 3740 cm−1 is assigned to the so-called terminal silanols and Si−Al debris.16,37 For zeolite FSAH-Y, the bands at 3660 and 3560 cm−1 increase in comparison to that of zeolite HY and the bands at 3740 cm−1 increase significantly. Hence, it could be assumed that large amounts of hydroxyl nests and amorphous silica structures are generated by removal of Al species in the framework through fluorosilicic and hydrochloric acid treatments. There are almost no changes in the bridged hydroxyl group (at 3660 and 3560 cm−1) because little framework Si is removed in 0.6 M NaOH solution. The peak intensities of zeolite AT-Y(L) at 3560 and 3631 cm−1 are much stronger than those of zeolite AT-Y(H), which implies that the framework integrity of AT-Y(L) is better than that of AT-Y(H) because of little removal of framework Si. Figure 6 displays the NH3-TPD spectra of four different zeolite samples. The TPD profile of zeolite HY-550 shows two typical peaks, in which the peak at 170 °C is assigned to weak acidic sites and the peak at 368 °C is ascribed to strong acidic sites. Zeolite AT-Y (L)-550 exhibits a decreased peak intensity at 360 °C. Interestingly, with the increasing of the alkaline concentration, the peak of sample AT-Y (H)-550 ascribed to weak acidic sites shows little change, while the peak attributed to strong acidic sites distinctly moves to a lower temperature

IR acidity at 350 °C (μmol g−1)

sample

Brønsted

Lewis

Brønsted

Lewis

HY-550 AT-Y (L)-550 AT-Y (H)-550

637.4 562.9 463.4

216.1 97.8 126.9

555.3 463.1 384.9

157.4 88.8 109.3

(L)-550 possesses a higher concentration of both Brønsted acid sites and Lewis acid sites than zeolite HY-550. Consequently, the increase of the concentration of NaOH cannot enhance the concentration of Brønsted acid sites, but it benefits significantly from the increase of Lewis acid sites. In hierarchical zeolite Y, the majority of the Lewis acid sites origined from the reincorporation of Al atoms previously removed (together with Si) from zeolite during the formation of a mesopore. The realuminated atoms were distributed in the tetrahedrally coordinated sites of zeolite and may generate more acidic Lewis acid sites. According to the results obtained, a mechanism for the formation of mesopores was proposed, as shown in Figure 7. A perfect structure of the parent zeolite NaY crystal is presented in panel 1 of Figure 7. Arrows in panels 1 and 2 of Figure 7 denote the development trends of framework Al species to extra-framework Al species. When zeolite Y was subjected to steam treatment, a few Al species were selectively removed from the framework and the crystal structure of HY was made more stable, as shown in panel 1 of Figure 7. With the treatments of fluorosilicic and hydrochloric acids, amounts of Al species began to be extracted from the zeolite framework and then completely dissolved and extracted from the interior of zeolite in the form of acid solution (panel 2 of Figure 7). It was E

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Figure 7. Proposed mechanism for the formation of mesopores.

Table 2. Catalytic Performance of Zeolite HY-800 and Hierarchical Zeolite Y for Heavy Oil Cracking

a

zeolite

HY-800

FSAH-Y-800

AT-Y (L)-800

AT-Y (H)-800

H2 + C1 + C2 C3 + C 4 gasoline diesel bottom coke conversiona (mass %) gasoline + diesel (mass %) liquid yield (mass %) bottom/coke coke/conversion

1.52 9.97 40.54 19.95 17.70 10.32 62.35 60.49 70.46 1.72 0.17

1.37 10.41 40.64 18.97 17.25 11.36 63.78 59.61 70.02 1.52 0.18

1.54 11.46 46.22 17.64 13.64 9.49 68.72 63.86 75.32 1.44 0.14

1.42 11.84 49.05 16.50 9.77 11.41 73.72 65.55 77.39 0.86 0.15

Conversion = 100 wt % − diesel − bottom.

well-known that the removal of Al species from the zeolite framework and subsequent zeolite structure destruction could cause dramatic descending of the crystallinity.26,41,42 Nevertheless, in this study, fluorosilicic acid had the ability to insert Si species into the vacancies of the framework created by dealumination, as shown in panel 3 of Figure 7. Then, the zeolite structure was mended, and the crystallinity was preserved. In addition, some extra-framework Si species in the interior of the zeolite were generated in the dealumination process, which was demonstrated by the chemical composition

and NMR analyses. Further alkaline treatment could dissolve these extra-framework Si species, and mesopores were introduced into zeolite Y with preserved framework and improved crystallinity, as shown in panel 4 of Figure 7. If severe alkaline treatment was applied to the zeolite, it would induce partial collapse of the zeolite framework and decrease the crystallinity of the resulting hierarchical zeolite Y. Table S4 of the Supporting Information compares the catalytic cracking performance of TIPB over zeolite HY-550 and hierarchically structured Y zeolite. It can be seen that F

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zeolite HY-550 shows good catalytic performance with TIPB conversion of 64.34% at 200 °C and 68.96% at 300 °C. Zeolite AT-Y-550 is found to be more active than zeolite HY-550 for the cracking of TIPB. It is known that the cracking of TIPB mainly occurs on the external surface of zeolite. The cracking performance of zeolite HY-550 is enhanced with the alkaline treatment, probably attributed to the presence of superior crystallinity and high external surface areas coupling with strong acidity. After a very severe steaming treatment (at 800 °C and 100% steam for 8 h), the crystallinity of the hierarchical zeolite Y could be retained at 43%, the mesoporous volume increases from 0.240 to 0.396 mL/g, and the external surface area drops slightly from 178 to 147 m2/g (see Table S5 and Figure S3 of the Supporting Information), suggesting quite good hydrothermal stability of the hierarchical zeolite Y. The TPD curves given in Figure S4 of the Supporting Information show that there is one peak for all of the zeolites after steaming treatment. The only peak in the range from 140 to 180 °C corresponds to weak acid sites. It can be seen that the convenient zeolite and the hierarchical zeolite have almost the same acid properties after steaming treatment. For catalytic cracking, the formation of coke is often related to the strong acid sites.43 Therefore, proper acidities after steaming treatment in industry will have a better performance for catalytic cracking. This superior hydrothermal stability suggested the absence of defects, and the highly crystallinity is based on the subsequent treatment. The reaction results for zeolites HY-800 and FSAH-Y-800 and hierarchically structured zeolite AT-Y-800 in the catalytic cracking of heavy oil are shown in Table 2. In the case of zeolite AT-Y (H)-800, the heavy oil conversion and liquid yield reach up to 73.72 and 77.39%, respectively, while the bottom oil goes down to 9.77%. The higher heavy oil conversion for AT-Y-800 zeolite can be attributed to the mesoporosity and improved acid amount. The mesoporous structure produced by the alkaline treatment could greatly facilitate the accessibility of large molecules toward zeolite, decrease the residence time, and suppress secondary reactions, leading to the increase in the liquid yield.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03421. Properties of heavy oil (Table S1), XRD patterns of zeolites (Figure S1), relative crystallinity and chemical compositions of zeolites (Table S2), textural properties of zeolites (Table S3), SEM images of zeolites (Figure S2), conversion of 1,3,5-TIPB for zeolites (Table S4), textural properties of hierarchical zeolites (Table S5), N2 adsorption and desorption isotherms and BJH pore size distributions for hierarchical zeolite Y (Figure S3), and NH3-TPD curves of HY and two hierarchical zeolites (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86-0351-6018384. E-mail: liwenlin@tyut. edu.cn. *Telephone/Fax: +86-010-82368390.. E-mail: dazhijian.ripp@ sinopec.com. ORCID

Wenlin Li: 0000-0002-3761-3033 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the China Petroleum and Chemical Corporation (SINOPEC, ST 12088 and ST 15074). REFERENCES

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4. CONCLUSION The hierarchical zeolite Y has a volumetric ratio of 61% (Vmeso/ Vtotal) and retention of crystallinity of 100%. A robust zeolite precursor with good crystallinity is established through steam treatment and acid pretreatment steps using fluorosilicic and hydrochloric acids. The alkaline treatment eliminated amounts of Si debris, which was an essential process to introduce intracrystalline mesoporosity into zeolite Y. Consequently, the integrated steam−acid−alkaline treatment process facilitated the formation of mesopores and the acquisition of zeolite with a narrow mesoporous distribution and high crystallinity. Such an effective steam−acid−alkaline process also preserved the overall acidity of the resulting samples. The resulting hierarchical zeolite Y exhibited remarkably higher catalytic activity than conventional zeolite in the cracking reaction of TIPB. The excellent catalytic performance of hierarchical zeolite Y can be attributed to the combination of high crystallinity, large external surfaces, and appropriate acid properties. The synthetic scheme presented herein is facile, economical, and effective. This provides a new and easy path for the industrial application of hierarchical zeolite Y. G

DOI: 10.1021/acs.energyfuels.6b03421 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b03421 Energy Fuels XXXX, XXX, XXX−XXX