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Zeolite Y Mother Liquor Modified #-Al2O3 with Enhanced Brönsted Acidity as Active Matrix to Improve the Performance of FCC Catalyst Peng Bai, Mengjie Xie, Ubong Jerome Etim, Wei Xing, Pingping Wu, Yanan Zhang, Bowen Liu, Youhe WANG, Ke Qiao, and Zifeng Yan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04243 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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Industrial & Engineering Chemistry Research

Zeolite Y Mother Liquor Modified γ-Al2O3 with Enhanced Brönsted Acidity as Active Matrix to Improve the Performance of FCC Catalyst Peng Baia,*, Mengjie Xiea, Ubong J. Etim a, Wei Xingb, Pingping Wua, Yanan Zhanga, Bowen Liua, Youhe Wangb, Ke Qiaoa, Zifeng Yana,* a

State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis CPNC, College of Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China b

School of Science, China University of Petroleum (East China), Qingdao 266580, China

* Corresponding authors. Tel: +86-532-86981812. E-mail address:[email protected] (P. Bai), [email protected] (Z. Yan)

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Abstract: A new matrix material for the fluid catalytic cracking (FCC) catalyst was prepared by using the zeolite Y mother liquor to modify the surface acidity of γ-Al2O3. The modified γ-Al2O3 was characterized using a variety of techniques, and the relationship between surface acidity and catalytic performances in the catalytic cracking of vacuum gas oil (VGO) was correlated. Characterization results showed that Brönsted acid sites derived mainly from isolated silanol groups, which increased on modified γ-Al2O3, while Lewis acid sites reduced dramatically after modification. Correlation results indicated that increased Brönsted acid sites effectively improved the conversion of VGO. In addition, new medium strong acid sites engendered at the interfaces of γ-Al2O3/amorphous silica-alumina (ASA) or γ-Al2O3/zeolite Y played a critical role in determining the final product distribution, leading to higher yields of gasoline and liquefied petroleum gas (LPG) than those of the pure γ-Al2O3 derived catalyst.


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1. INTRODUCTION The fluid catalytic cracking (FCC) process, an important secondary processing technology for crude oils, has a practical significance for the improvement of petroleum processing depth and for the production of transportation fuels. It is generally accepted that the formation of carbocations initiates the reactions of β-scission, hydrogen transfer, isomerization, dealkylation and transalkylation occurring on solid acid catalysts, such as silica-alumina and molecular sieves.1 The type of carbocations is determined by the type of acid sites on the catalyst, tricoordinated carbenium ions are generally produced by protonation of olefins on Brönsted acid sites, while penta-coordinated carbonium ions usually form on Lewis acid sites by the dehydrogenation of alkane molecules.2 According to the reaction mechanism of FCC process, it is clear that the acidity of catalyst has a significant effect on heavy oil conversion and products selectivity. The extent, to which the reactants undergo cracking, depends on the density and strength of acid sites, the cracking activity increases linearly with the density of acid sites, but a higher acid density results in excessive hydrogen transfer reactions, producing more coke.3 Strong acidity enhances the cracking of hydrocarbon macromolecules into the gasoline range, whereas too large amount of strong acid sites lead to low gasoline yield and high coke formation.4-6 Therefore the catalyst matrix, as a pre-cracking site for hydrocarbon macromolecules, should have optimized acid type and strength to improve its performance for heavy oil cracking and light oil selectivity. Since the first commercial introduction into the refinery in 1962, zeolite Y has been the primary active component of FCC catalysts and extensively applied in the FCC process.7,

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Zeolite Y is constituted by building blocks of a faujasite structure and its catalytic active sites locate both in the internal cages and on the external surface, which makes it the major source of 3 ACS Paragon Plus Environment


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improvements in gasoline yield and octane value as well as in the production of other fuels with enhanced performance properties.8, 9 However, strong Brönsted acidity and small micropores of Zeolite Y tend to shift the product selectivity of valuable gasoline, light cycle oil (LCO) and liquefied petroleum gas (LPG) towards undesirable coke and dry gases by over-cracking.10 The catalytic performance of FCC catalysts not only depends on the properties of the active component, but also on its matrix. As early as 1940, amorphous silica-alumina (ASA) has been used as active matrices in FCC catalysts. Recently, advances in the use of ASA materials have been stimulated by the concept of matrix functionalization for the FCC process and by the requirement for active components with medium-strength acid sites for the hydrocracking process. As a result, renewed interests have been devoted to the research on surface acidity and pore structure of ASA.11, 12 ASA with more moderate Brönsted and Lewis acidities than zeolites is preferred to enhance the selectivity of gasoline fractions in catalytic cracking.13-17 In order to address the acidity and product distribution problems, various modification methods have been proposed, including incorporation of zeolites into ASA to create composites. Aghakhani et al.18 observed that covering Y-zeolite particles with a layer of ASA altered the FCC product distribution by pre-cracking heavy feed on the relatively weak acid sites of ASA to produce lowmolecular-weight intermediates, which were readily accessible to strong acid sites of Y-zeolite. Consequently, the ASA layer helped to reduce the coke formation and protect zeolite from destructive deactivation. In addition, Meng et al.19 reported that new acid sites formed between zeolite Y and ASA by wrapping NaY zeolite with small particles in ASA gel. The aged catalyst AHY/ASA showed good catalytic performance in cracking light diesel to gasoline fraction with reduced gas and coke formation. Kubicek et al.20 developed a new catalyst by wet embedding of zeolite Beta in ASA matrices. It was observed that new structural Brönsted acid sites were 4 ACS Paragon Plus Environment

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generated, which maintained a high catalytic activity for gas oil conversion and a high selectivity to gasoline and LPG after a high-temperature steam treatment. Given its good thermal/hydrothermal stability and high mechanical strength, γ-Al2O3 has been widely used as a matrix component of FCC catalyst. However, only Lewis acidity is observed on the surface of γ-Al2O3 after calcination. In a comparison to Brönsted acidity, Lewis acidity was found to have a lower activity in the conversion of heavy oil.21 Therefore, additional surface modification of γ-Al2O3 is desirable to tune its surface acidity to improve the catalytic performance of FCC catalyst. Intrigued by the advantages of Y/ASA composite materials as discussed above, it will be interesting to graft the surface of γ-Al2O3 matrix with the mother liquor of zeolite Y. The crystallinity of zeolite Y and the corresponding surface Brönsted acidity of the resulting modified γ-Al2O3 matrix can be rationally tuned by controlling the crystallization time of the mother liquor of zeolite Y. Therefore, in this work, we attempted to modify the surface acidity of γ-Al2O3 using the mother liquor of zeolite Y with the aim of introducing new structural Brönsted acidity to γ-Al2O3, possibly promoting the heavy oil conversion and increasing the gasoline yield. The modified samples of NaY/γ-Al2O3 and HY/γ-Al2O3 were thoroughly characterized by a variety of techniques to investigate the surface acidity and structural properties. The catalytic cracking performance of HY/γ-Al2O3 derived catalysts was evaluated in the cracking of vacuum gas oil (VGO) and compared with that of unmodified-γAl2O3 derived catalyst. The main influencing factors of VGO conversion and products distribution were discussed.



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2. EXPERIMENTAL SECTION 2.1. Materials. Sodium hydroxide (NaOH, ≥96.0 wt%, AR), sodium aluminate (NaAlO2, Al2O3 content is 41.0 wt%, AR), hydrochloric acid (HCl, 36.0~38.0 wt%, AR) and ammonium nitrate (NH4NO3, ≥99.0 wt%, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Silica sol (SiO2 content is 30.0 wt%) was purchased from Qingdao Haiyang Chemical Co., Ltd. Pseudoboehmite (Al2O3 content is 71.9 wt% on dry basis) was purchased from Henghui Chemical Co., Ltd. Kaolin clays, and zeolite USY were purchased from Huaxin powder Co., Ltd. All materials were used as received. Deionized water was used for all samples preparation. 2.2. Sample preparation. In a typical synthesis, a mixture solution consisting of NaOH, NaAlO2 and silica sol with initial molar composition of 4.8Na2O: Al2O3: 8SiO2: 120H2O was prepared, followed by stirring for 1 h at room temperature to form a homogeneous sol. The sol was then transferred into autoclaves for crystallization at 100 ˚C for different times (2, 4, 12 and 24 h) to obtain a white suspension, denoted as “Y-mother liquor”. The modification of γ-Al2O3 was conducted as follows. In a typical procedure, a certain amount of pseudoboehmite was dispersed in deionized water under stirring at 90 ˚C. Then a HCl solution (1.00 mol/L) was added to the above suspension until pH =3, followed by dropwise addition of a certain amount of Y-mother liquor. The resulting mixture was vigorously stirring for 6 h, and subsequently transferred into an autoclave for hydrothermal treatment at 100 ˚C for 24 h. The obtained material was filtrated, washed with deionized water and dried in an oven. The obtained powder was NaY-modified pseudoboehmite (denoted as J-n-ap). NaY-modified pseudoboehmite was finally calcined in air at 550 °C for 4 h at a heating rate of 2 °C/min to obtain the NaY-modified γ-Al2O3 (denoted as J-n-aa). In order to lower the sodium content, the 6 ACS Paragon Plus Environment

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NaY-modified pseudoboehmite was ion exchanged in a 1 mol/L NH4NO3 solution with a solidliquid ratio of 1 g: 30 mL. In this way, the NaY-modified pseudoboehmite sample was converted to NH4Y-modified pseudoboehmite (denoted as J-n-Np), which was calcined to HY-modified γAl2O3 (denoted as J-n-Ha). For easy comparison, the initial molar ratio of silicon atoms in Y-mother liquor to aluminum atoms in pseudoboehmite was fixed at 0.2 in the modification process. In the sample names, the number “n” stands for the crystallization time of “Y-mother liquor”. The “ap” stands for the NaY-modified pseudoboehmite and “aa” for the NaY-modified γ-Al2O3, “Np” for the NH4Ymodified pseudoboehmite, and “Ha” for the HY-modified γ-Al2O3. A USY zeolite (Si/Al =9.8, cell parameter =24.518 Å) was firstly treated with pure steam at 750 °C for 4 h (denoted as ST-USY). In order to avoid the modification of matrix acidity during catalyst preparation procedure, the catalysts containing 40 wt% of kaolin, 30 wt% of ST-USY, 20 wt% of HY-modified γ-Al2O3, and 10 wt% of industrial aluminum sol were prepared by physical mixing, drying at 120 °C for 24 h, and calcining at 550 °C in air for 4 h. For comparison purpose, the catalyst using commercial γ-Al2O3 as the matrix material was also prepared. Thus prepared catalysts are denoted as CatJ-4-Ha, CatJ-12-Ha and, Catγ-Al2O3, corresponding to the matrices of J-4-Ha, J-12-Ha and γ-Al2O3, respectively. All samples were crushed and sieved to 60-80 mesh before use. 2.3. Sample characterization. XRD patterns of synthesized samples were recorded on an X’pert PRO MPD diffractometer from PANalytical B.V., Netherlands, equipped with a nickel-filtered Cu Kα X-ray source radiation, which has a characteristic wavelength (λ) of 0.15418 nm, and operated at 40 kV and 40 mA. The scanning range was set from 5° to 75° at a speed of 10°/min. 7 ACS Paragon Plus Environment


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Nitrogen adsorption-desorption isotherms were measured at liquid nitrogen temperature (-196 °C) using a Micrometrics TRISTAR 3000 analyzer. Before analysis, samples were degassed at 300 °C under vacuum degree of 10-2 Torr for 4 h to remove the moisture and other adsorbed substances. Specific surface areas were calculated by the standard Brunauer-Emmett-Teller (BET) equation between relative pressure range of 0.05< P/P0 350 °C). Meanwhile, the elemental analyzer (Elementar Vario EL III) was used to measure the weight of carbonaceous materials deposited on the spent catalyst by analyzing the CO and CO2 quantities after combustion. 3. RESULTS AND DISCUSSION 3.1 Crystalline structure of samples. In order to investigate the influence of crystallization time on the crystallinity of NaY zeolites derived from the Y-mother liquor, the solids were retrieved from the synthesis mixture after different crystallization times and analyzed using XRD. XRD patterns of the samples are shown in Figure 1. Sample J-0 exhibits only one broad diffraction peak at about 2θ =23°, suggesting the formation of ASA rather than a crystalline zeolite.24 The characteristic peaks of NaY zeolites were observed in the XRD patterns of other samples (J-2, J4, J-12 and J-24) after crystallization, indicating the presence of the crystalline FAU-type zeolite framework.25 As shown in Table S1, the crystallinity of these NaY zeolite samples firstly increases and then has a slight decline with the extension of crystallization time. Typically, the crystallinity increases with longer crystallization time, however, zeolites are thermodynamically metastable products and will dissolve in alkaline aluminosilicate gel with sustained crystallization time.26 XRD patterns of NaY-modified pseudoboehmite and γ-Al2O3 are illustrated in Figure 2. As can be seen in Figure 2a, only pseudoboehmite phase is observed on samples J-0-ap and J-2-ap, while on samples J-4-ap, J-12-ap and J-24-ap, both pseudoboehmite and NaY diffraction peaks are observed. Similar intensities of NaY reflections are observed on samples J-4-ap and J-12-ap, while further increasing the crystallization time of Y-mother liquor, those of NaY zeolite reflections on sample J-24-ap decreased. Figure 2b shows that after calcination, samples J-0-aa 10 ACS Paragon Plus Environment

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and J-2-aa exhibit broad ASA peaks between 20° and 35°, and two crystalline peaks at around 2θ =46° and 2θ =67°, corresponding to the reflections of (4 0 0) and (4 4 0) crystal facets of the cubic γ-Al2O3. Moreover, the diffraction peaks observed in these two samples were broad and diffuse, corresponding to the poor intrinsic crystallinity of γ-Al2O3.27 On samples J-4-aa, J-12-aa and J-24-aa, evident NaY diffraction peaks are observed. More importantly, there are no marked shifts in the angular positions of these three samples compared to those of pure NaY zeolite, suggesting that cell parameter of the zeolite remained almost constant during the synthesis process.18 Comparing Figure 2b with Figure 1, a much lower crystallinity of NaY zeolite is observed for all the NaY-modified γ-Al2O3 samples than that of pure NaY samples, especially for sample J-2-aa, which has no discernible diffraction peaks assignable to NaY zeolite, revealing the crystalline structure of NaY in these samples were undermined when added to the acidic system. This is attributed to the non-stoichiometric decomposition of Y zeolite crystals in low pH solutions, and to the fact that the framework aluminum atoms of Y zeolite are easily dissolved in acidic solutions, resulting in the structural change from crystalline to amorphous.28 XRD patterns of NH4Y-modified pseudoboehmite and HY-modified γ-Al2O3 obtained from the ion exchange of NaY-modified pseudoboehmite are shown in Figure 3. It can be noticed that after the ion exchange, XRD patterns of the NH4Y-modified pseudoboehmite are similar to those in the Figure 2a, suggesting that the crystalline FAU framework of Y zeolite was retained during the ion exchange process. However, after calcination, on HY-modified γ-Al2O3 samples, the peak intensities of HY zeolite on samples J-4-Ha, J-12-Ha and J-24-Ha (shown in Figure 3b) decrease significantly, especially for sample J-24-Ha, the crystalline structure of Y zeolite was seriously deteriorated. This can be explained by the fact that the unit cell shrinkage and collapse



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of zeolite framework result in the reduction of crystallinity when subjected to ion exchange treatment and subsequent calcination.29 3.2. Textural properties of samples. Figure 4 and Figure 5 show the N2 adsorption-desorption isotherms and BJH pore size distribution (PSD) curves of NaY and HY modified γ-Al2O3. Textural properties of all samples are summarized in Table 2. It is apparent that all modified samples exhibit type IV isotherms (classification by the IUPAC), which are characteristic of mesoporous materials. The type H3 hysteresis loop, occurring at higher closure points (above P/P0 =0.9), indicates large pore size and broad pore size distribution with slit shaped pores, having non-uniform sizes formed by aggregation or agglomeration of ASA or γ-Al2O3 particles.30, 31 The formation of agglomerates could be due to the binding of surface hydroxyl groups (Si-OH) of neighboring ASA particles.32 The pore size distribution curves shown in Figures 4b and 5b verify that all modified samples exhibited the pore diameter of about 4-9 nm with a broad pore size distribution. The shape peak at ca. 4 nm is an artifact peak attributed to the tensile strength effect when using N2 as the adsorptive.33 In addition, the isotherm of sample J-0-aa with a relatively higher nitrogen adsorption capacity at P/P0 >0.8, when compared to the other modified samples, implies that there are more interparticle mesopores existing in this sample due to its ASA nature.34 Furthermore, less amount of N2 is adsorbed on samples J-2-aa and J-4-aa, indicating their low specific surface areas and pore volumes. During synthesis of sample J-2-aa, the low synthesis pH resulted in the destruction of its zeolite crystalline structure as confirmed by XRD (see Figure 2b), which in turn decreased the micropore surface area (Smicro) and micropore volume (Vmicro).18 In contrast, the higher Smicro and Vmicro of samples J-4-aa, J-12-aa and J-24-aa imply that there are more micropores in these samples due to the higher retention of zeolite Y crystallinity than samples J-0-aa and J-2-aa as 12 ACS Paragon Plus Environment

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confirmed by XRD results (Figure 2b). Samples J-12-aa and J-24-aa, although having surface areas of 319 m2/g and 321 m2/g, respectively, which are comparable to that of sample J-0-aa (341 m2/g), have relatively smaller pore volumes. This may be attributed to the partial decomposition of their zeolite crystalline structures in highly acidic solution, which results in the pore blockage by small ASA particles. In spite of the very similar shape of isotherms and BJH PSD curves of five HY-modified γAl2O3 samples as compared to those of all NaY-modified samples, ion exchange and subsequent calcination lead to a systematic change in the textural properties of the HY-modified samples. In addition to the slight changes in specific surface areas of samples J-2-Ha, J-4-Ha and J-12-Ha, the Smeso of J-0-Ha and J-24-Ha decrease significantly. This is probably due to the condensation of intermicellar units during synthesis, which leads to the increase of particle size and decrease of particle density after calcination.35 Meanwhile, changes in pore volumes of samples J-0-Ha and J-24-Ha are very different from each other. The former obviously decreases while the latter increases, which may be attributed to the structural change of J-24-Ha from crystalline to essentially amorphous. Among the five HY modified γ-Al2O3 samples, J-12-Ha exhibited the best pore structure with the largest Vmeso and Vtotal, which is favorable for alleviating diffusion resistance of heavy oil reactants when used as a matrix material for FCC catalyst, increasing the reactant concentration in the interior of the catalyst, thus enhancing the accessibility of the active sites to reactant molecules and improving the efficiency of the catalyst.19, 36 3.3. FT-IR spectroscopy analysis. The pyridine adsorption IR spectroscopy analysis is usually applied to characterize Brönsted and Lewis acid sites.37 Two different types of adsorption interactions between acid sites and pyridine are as follow: (i) protons transfer from Brönsted acid sites to pyridine to form pyridinium ions (PyH+) and (ii) electrons transfer from pyridine to 13 ACS Paragon Plus Environment


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Lewis acidic sites to coordinate with unsaturated Al3+, resulting in different IR spectroscopy response which can be used to distinguish Brönsted and Lewis acid sites.8 Pyridine adsorption IR spectroscopy measurements were recorded in the wavenumber range of 1700-1400 cm-1 and spectra are shown in Figure 6. As can be seen, peaks centered at 1546 and 1639 cm-1 are related to Brönsted acid sites.38 In contrast, bands centered at 1446, 1578, 1595 and 1614 cm-1 are attributed to Lewis acid sites, specifically, bands at 1446 and 1614 cm−1 correspond to the vibration mode of pyridine bonding with coordinatively unsaturated Al3+, band at 1578 cm−1 is related to α-pyridine species interacted with surface basic hydroxyl groups and band at 1595 cm−1 is attributed to the interaction between pyridine molecules adsorbed on the H-bond donor sites and surface OH groups.30 In addition, band centered at 1491 cm-1 is ascribed to synergistic effects of Brönsted and Lewis acid sites.39 The concentrations of Brönsted and Lewis acid sites of samples were evaluated by adopting the method reported elsewhere,40 and the data are presented in Table 3. As can be seen, the quantity of Brönsted acid sites is in the order: J-4-Ha >J-24-Ha ≈J-12-Ha >J-2-Ha >J-0-Ha. Similarly, the ratio of Brönsted to Lewis acid sites (B/L) follows the same order, which increases from 0.12 to 0.30 as the crystallization time of Y-mother liquor increases from 2 h to 4 h, and further decreases to 0.25 with the crystallization time extending to 12 h and above. It has been demonstrated that Brönsted acidity is attributed to the OH groups and there are three types of OH groups on the ASA surface: (i) a small amount of strong Brönsted acid sites ascribed to bridging OH groups; (ii) weaker ones with a higher quantity attributed to silanol groups coordinating with the coordinatively unsaturated Al3+; and (iii) nonacidic silanol groups with the largest amount.41 However, with regard to crystalline zeolites, bridging OH groups are the main sources of Brönsted acidity. FT-IR spectra of all HY-modified γ-Al2O3 samples after 14 ACS Paragon Plus Environment

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activation at 500 °C were recorded in the region of surface OH groups, and presented in Figure 7. The 3710, 3726 and 3734 cm-1 bands are characteristic peaks of surface hydroxyl groups on γAl2O3 corresponding to HO-µ2-AlV on the (110), HO-µ1-AlVI on the (111) and HO-µ1-AlV on the (110), respectively.42,

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The band at 3746 cm-1 due to the surface isolated silanol groups is

observed for all samples.44 With the increase in crystallization time of Y-mother liquor, the peak intensities of 3746 cm−1 OH groups experience an increase firstly and then gradually declines. The maximum peak intensity at 3746 cm−1 was observed on sample J-4-Ha, which is consistent with the trend of B/L ratio in Table 3. No peaks corresponding to bridging acidic OH groups was observed in the spectra, indicating that there are no bridging OH groups in the HY-modified γAl2O3 samples in this study. Thus, on samples J-4-Ha, J-12-Ha and J-24-Ha, the Brönsted acids were derived mainly from the isolated silanol groups rather than the bridging OH groups because of the low crystallinity of zeolite Y in these samples. 3.4. NH3-TPD measurements. Ammonia as a basic molecule with a kinetic diameter of 0.26 nm is small enough to adsorb on the acid sites located in very narrow pores of alumino-silicate materials.45, 46 Moreover, polar ammonia with the lone pair of electrons, has the ability to bond with many types of materials by dipolar interactions or hydrogen bond, and to selectively adsorb on acid sites of different strengths.47 Therefore, the NH3-TPD technique is commonly applied for a quantitative determination of the amount and strength of acid sites. NH3-TPD profiles obtained on γ-Al2O3, HY-modified γ-Al2O3 and zeolite HY are presented in Figure 8. Multiple peaks would be observed in the TPD profiles if a surface molecule has more than one binding state with significantly different adsorption enthalpies.48 It is clearly observed that, for all TPD curves of investigated samples, there is a broad asymmetric tail peak, which extends up to about 500 °C, and their intense desorption peaks are located at about 145 °C. 15 ACS Paragon Plus Environment


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Since the precracking of hydrocarbon macromolecules in the FCC process depends on both the strength and quantity of acid sites on matrix, Gaussian deconvolution method was used to deconvolute the NH3-TPD profiles in order to gain more detailed understanding of the surface acidity. NH3-TPD profiles were deconvoluted into four sub-profiles for all γ-Al2O3, HYmodified γ-Al2O3 and zeolite HY samples (the inset in Figure 8 takes J-12-Ha for example). Peaks I and II located at 130 ± 9 °C and 174.5 ± 16.5 °C, respectively, can be assigned to the desorption of NH3 physically adsorbed on weak acid sites, whereas peak III located at 243.5 ± 15.5 °C, is attribute to medium acid sites, and peak IV located at 342.5 ± 24.5 °C corresponds to strong acid sites.27 The quantities of different strength acid sites calculated from areas under the sub-profiles and the temperatures of desorption peaks with maxima (Tdi) are summarized in Table S2. Results show that the modification by Y-mother liquor obviously increases the total amount of acid sites of γ-Al2O3, except J-2-Ha. A less amount of acid sites on J-2-Ha than that of γ-Al2O3 can be explained by the significant decrease in the specific surface area, resulting in less exposed acid sites. However, even with higher specific surface areas, samples J-12-Ha and J-24-Ha have a similar amount of acidic sites to that of sample J-0-Ha, which may be due to a large amount of amorphous structures originating from the destruction of their zeolite crystal structures as confirmed by XRD results. No obvious trend is observed on the concentration of weak and strong acid sites after modification. Nevertheless, it should be noted that HY-modified γ-Al2O3 samples have more medium acid sites than γ-Al2O3, which may help to improve the pre-cracking performance of the matrix.21 The change in Tdi shows that peak positions corresponding to the medium and strong acid sites on all HY-modified γ-Al2O3 samples, shifted to higher temperatures, revealing that medium and strong acid sites are stronger on modified samples than 16 ACS Paragon Plus Environment

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those on γ-Al2O3. This also suggests that new strong acid sites may form at the interface of γAl2O3/ASA(zeolite) composites, which have more interconnected pores creating a higher acid site density and a stronger acid strength in comparison with the single crystal γ-Al2O3.18 3.5. NMR analysis. Figure 9 displays solid-state

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Al MAS NMR spectra of γ-Al2O3 and HY-

modified γ-Al2O3. For each spectrum, there are two major signals, an intense signal at 4.0-7.0 ppm and a broad one centered at 59.0-63.0 ppm, which are generally attributed to aluminum atoms in the octahedral (AlVI) and tetrahedral (AlIV) coordination centers, respectively. Moreover, another very weak signal at 34.0-40.0 ppm ascribed to the penta-coordinated aluminum atoms (AlV) is observed, which is commonly regarded as extra-framework species in dealuminated zeolites or interface species between alumina and mixed silica-alumina.30 After Gaussian deconvolution of all

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Al NMR spectra, the relative percentages of different

coordinated aluminum atoms were quantitatively calculated from the fitted peak areas and summarized in Table S3. It has been widely recognized that tetrahedral aluminum species contribute to Brönsted acidity, thus more tetrahedral aluminum species than the octahedral ones are desired.14,

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As can be seen in Table S3, the fraction of tetra-coordinated framework

aluminum species increases with increasing crystallization time of the modifier Y-mother liquor, which is well consistent with the change in the amount of Brönsted acid sites shown in Table 3. However, the fraction of octahedral-coordinated aluminum species generally decreases with the HY modified structures changing from amorphous to crystalline. In addition, the fraction of penta-coordinated aluminum species increases as the crystallization time of Y-mother liquor increases up to 4 h, while it decreases with a further increase in the crystallization time. It appears that penta-coordinated aluminum species may be related to strong acidity, due to the fact that the dehydration at a high temperature transforms aluminum atoms from pentahedral 17 ACS Paragon Plus Environment


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coordination to lower-coordination centers, which enhances the acid strength of the neighboring bridging hydroxyl groups.14 Therefore, sample J-4-Ha with the largest proportion of AlV has the largest amount of medium and strong acid sites. 29

Si NMR spectra with the chemical shift ranging from -70 to -120 ppm are shown in Figure

10, which are mainly corresponding to the chemical environments of silicon atoms in first and second coordination spheres. For all samples, the dominant resonances center around -81, -84, 86, -89, -95, -100, -105 and -109 ppm, corresponding to silicon atoms in different sites. Peaks at -81, -84, -95, -105 and -109 ppm arise from the tetra-coordinated silicon species surrounded by different numbers of Al-O- bonds, namely Si(OAl)4, Si(OSi)(OAl)3, Si(OSi)2(OAl)2, Si(OSi)3(OAl) and Si(OSi)4 or SiO2 groups，respectively.50-52 It was found that sample J-0-Ha has the most Si(OSi)4 or SiO2 groups, which is in line with its amorphous nature. In addition, peaks at -86 and -100 ppm are attributed to terminal silanol groups Si(OH)2(OSi)2 and Si(OH)(OSi)3, respectively.53 The area of these two peaks in samples J-12-Ha and J-24-Ha are larger than that of other three samples, implying that there are more silanol groups in these two samples. Meanwhile, the strong intensities of peaks at -89 ppm generally ascribed to the Si(OAl)3OH groups are observed in samples J-4-Ha, J-12-Ha and J-24-Ha with zeolite crystalline structures.54 The above analysis indicates that the amount of silanol groups is related to that of Brönsted acid sites, which is almost consistent with the FT-IR results. 3.6. Catalytic evaluation. The performance of catalysts using HY-modified γ-Al2O3 as matrices was evaluated in the catalytic cracking of VGO and compared with that derived from conventional γ-Al2O3 under identical conditions. Reaction results are summarized in Table 4.


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As can be seen from Table 1, the feedstock (VGO) with high-density contained a certain amount of aromatics and resins, having essentially some large molecules in these fractions that could not enter into the micropores of zeolite Y directly, but preferentially pre-cracked on the matrix surface during the FCC process. More importantly, these hydrocarbon macromolecules preferred to associate with other hydrocarbons as reactant molecules in the catalytic cracking process.21 Therefore, a considerable portion of reactant molecules were too large to penetrate into the Y-zeolite micropores directly. Based on the above analysis, this feedstock is considered to be a representative to investigate the pre-cracking performance of modified γ-Al2O3 matrix. Compared with γ-Al2O3 derived catalyst, catalysts using modified γ-Al2O3 as matrices exhibit a higher VGO conversion and a higher yield of gasoline. Especially, catalyst CatJ-4-Ha, using sample J-4-Ha with the highest concentration of surface Brönsted acid sites as matrix, effectively increases the conversion of VGO from 74.69% to 79.78%, and enhances the gasoline and LPG yields by 2.13% and 3.08% respectively. The obvious increase of VGO conversion and useful products selectivity (gasoline and LPG) on catalyst CatJ-4-Ha are mainly attributed to the high B/L value of matrix J-4-Ha as a result of increased Brönsted acid sites and a decrease in Lewis acid sites after modification. The introduction of certain amount of Brönsted acid sites and reduction of Lewis acid sites on the matrix surface enhance the conversion of heavy oil, which has been recognized before.55 These results also suggest that the matrix (J-4-Ha) had enough medium and strong acid sites (Table S2) to crack large hydrocarbon molecules. Moreover, the moderate Brönsted acid sites promoted the formation of carbenium ions and enhanced the hydrogen transfer reaction for olefin saturation, but not accelerating the deep cracking of low-carbon hydrocarbons in gasoline/LPG. Hence, the yield of gasoline/LPG was not reduced as a result of the increase in Brönsted acid sites concentration. In addition, the slight decrease of coke 19 ACS Paragon Plus Environment


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formation on modified matrix J-4-Ha is also probably attributed to its decreased amount of Lewis acid sites, given that Lewis acid sites usually induce high coke formation due to hydrocarbon reactions initiated by the dehydrogenation of hydrocarbons.55 However, a relatively lower heavy oil conversion (78.30%) and a lower LPG yield (8.67%) were observed on catalyst CatJ-12-Ha than those on catalyst CatJ-4-Ha. By correlating with the surface acidity, it is found that the VGO conversion and LPG yield are closely related to the Brönsted acidity of alumina matrix. As compared to matrix J-4-Ha, matrix J-12-Ha contains a lower amount of Brönsted acid sites and a higher amount of Lewis acid sites, which results in a lower VGO conversion and LPG yield due to the lower activity of Lewis acid sites in the FCC process as compared with Brönsted ones.21 In conclusion, the HY-modified γ-Al2O3 derived catalysts with a B/L acid ratio of 0.30 (Table 3) shows a good catalytic performance by promoting the conversion, and increasing gasoline and LPG yields in the catalytic cracking of VGO. By correlating structural properties with reaction results, no obvious effect of pore structure of HY-modified γ-Al2O3 matrix on the catalytic performance was observed as long as the specific surface area is greater than 200 m2/g and the pore volume is greater than 0.5 cm3/g. Therefore, we conclude that it is of minor significance for the modification of pore structure, while the surface acidity of alumina matrix plays a crucial role in the heavy oil conversion and final products distribution. The alumina matrix with rich Brönsted acid site of medium strength is desired for enhancing the heavy oil conversion and improving products distribution.


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4. CONCLUSIONS In this work, a catalyst exploiting a new functional matrix material of γ-Al2O3 modified by the zeolite Y mother liquor was prepared and compared with a conventional γ-Al2O3 derived catalyst in the catalytic cracking of VGO. Physicochemical properties of modified γ-Al2O3 were thoroughly investigated by various techniques, such as XRD, N2 sorption, FT-IR, NH3-TPD, and 27

Al/29Si MAS NMR. The characterization results showed that modified samples exhibited a

certain amount of Brönsted acid sites and a reduced amount of Lewis acidity sites with a B/L value up to 0.3. Additionally, it has been verified that there is existence of isolated silanol groups, which are responsible for the generation of Brönsted acid sites, and more medium strong acid sites on the modified γ-Al2O3. The activity of catalyst with modified γ-Al2O3 (CatJ-4-Ha) was higher than that of γ-Al2O3 derived catalyst, increasing the heavy oil conversion by 5.09%, yielding more gasoline and LPG by 2.13% and 3.08% respectively. Therefore, the method used in this study for modifying the γ-Al2O3 with zeolite Y mother liquor provides a new perspective for improving the heavy oil conversion and tailoring the FCC products distribution via precracking heavy oil molecules on the modified γ-Al2O3 matrix with Brönsted acid sites of medium strength. ACKNOWLEDGEMENTS This work was financially supported by the Joint Funds of the National Natural Science Foundation of China and China National Petroleum Corporation (U1362202), Natural Science Foundation of China (51601223, 21206195), the Fundamental Research Funds for the Central Universities (17CX05018, 17CX02056, 14CX02050A, 14CX02123A), Shandong Provincial



Page 22 of 44

Natural Science Foundation (ZR201702160196, ZR2012BM014), and the project sponsored by Scientific Research Foundation for Returned Overseas Chinese Scholars. SUPPORTING INFORMATION The supporting information is available free of charge on the ACS Publications website. Properties of Y zeolites synthesized at different crystallization times, Gaussian deconvolution results of NH3-TPD profiles of γ-Al2O3, HY-modified γ-Al2O3 and HY zeolite, Gaussian deconvolution results of solid-state 27Al MAS NMR spectra of γ-Al2O3 and modified γ-Al2O3. REFERENCES (1)

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List of Tables Table 1. Feedstock Properties of VGO properties

element

value

density (20 °C), kg/m3

903.2

carbon residue, wt%

Zeolite Y Mother Liquor Modified Î³-Al - ACS Publications

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