Catalyst Based on a Porous Composite Material Synthesized via an In Situ Technique

To overcome diffusion limitations and improve transport in microporous zeolite, the materials with a wide-pore structure have been developed. In this paper, composite microspheres with hierarchical porous structure were synthesized by an in situ technique using sepiolite, kaolin and pseudoboehmite as raw material. A novel fluid catalytic cracking (FCC) catalyst for maximizing light oil yield was prepared based on the composite materials. The catalyst was characterized by XRD, FT-IR, SEM, nitrogen adsorption-desorption techniques and tested in a bench FCC unit. The results indicated that the catalyst had more mesoand macropores and more acid sites than the reference catalyst, and thus can increase light oil yield by 1.31 %, while exhibiting better gasoline and coke selectivity.


Introduction
Microporous zeolite Y, which is the main active component in Fluid Catalytic Cracking (FCC) catalysts, plays an important role in the modern petrochemical industry.The narrow pores of microporous zeolite limit its application for the conversion of bulky molecules.The microporous structure typically results in relatively longer diffusion paths.[3] Zeolite Y can be prepared using a conventional gel synthesis method or in situ crystallization.5][6] Kaolin is usually used for the preparation of such catalyst.Since kaolin is, in essence, the pure mineral kaolinite, it is composed of aluminum and silicon oxides, i.e.Sepiolite presents a high specific surface and special structural characteristics based on the folding of the crystal structure when the zeolitic water has been removed by thermal treatment.[9] In this study, to overcome the challenges of using microporous zeolite, a new synthesis technology for the production of a hierarchical zeolite material with controllable hierarchical mesopore using in situ crystallization has been developed.The aim of the current work was to study the synthesis and properties of composite materials that have a wide-pore structure and were obtained using sepiolite, kaolin and pseudoboehmite, and to investigate the catalytic properties of the FCC catalyst prepared from the composite material.

Materials
Sepiolite was obtained from China Liuyang Sepiolite Mining Co., Ltd., kaolin was obtained from China Kaolin Co., Ltd., and pseudoboehmite was obtained from China Aluminum Co., Ltd.(The composition of compounds and mixtures is expressed in terms of the mass fraction.)Sodium silicate (containing 22.8 % SiO 2 , 6.9 % Na 2 O) and sodium metaaluminate (containing 20.8 % Na 2 O, 3.0 % Al 2 O 3 ) were purchased from the Yueyang Jucheng Chemical Co., Ltd.

Synthesis of composite materials
The typical synthesis of composite materials involved the following steps: Approximately 4 % of dispersant (sodium silicate) and 2 % of a pore-enlarging additive (ammonium bicarbonate) A Novel FCC Catalyst Based on a Porous Composite Material Synthesized via an In Situ Technique were blended with kaolin to form an aqueous slurry with a 35 % solid content, and precursor microspheres A (PMA) of particle size 0-150 μm, were produced by spray drying.The PMA were calcined at 960 °C for 2 h to produce type A microspheres (CMA).
Sepiolite and pseudoboehmite were blended in mass ratio 4 : 1 to form an aqueous slurry with a 35 % solid content.After the addition of a certain amount of a hydrochloric acid solution at room temperature, followed by stirring 2 h, washing, and filtering, the precursor microspheres B (PMB) of particle size 0-150 μm were produced by spray drying.The PMB were calcined at 750-800 °C for 2 h to produce type B microspheres (CMB).
A mixture of sodium silicate, sodium hydroxide, distilled water, zeolite initiator (i.e., the initiator was an amorphous gel that accelerated the formation of the NaY zeolite prepared with a composition of 16 Na CMA and CMB (CMA : CMB = 1 : 1) was mixed in a reactor and heated to 95-100 °C for 24-30 h to synthesize the composite material (SCM) containing Y zeolite as microspheres.The synthesis of SCM involves an inorganic reaction with a stoichiometry of Na 2 O : SiO 2 : Al 2 O 3 : H 2 O = 6.1 : 12 : 1 : 300.
After crystallization, the solid product was filtered and washed with distilled water followed by drying to afford SCM.

Catalyst preparation
The catalyst (CAT) was prepared from the as-synthesized SCM.First, SCM was exchanged one or more times with approximately 10 to 30 % of ammonium chloride to replace the sodium.Then, the microspheres were exchanged with 3 % of lanthanum chloride, followed by calcination at 500-600 °C with 100 % steam for 2 h to generate the active sites and appropriate pore structures necessary for catalytic cracking.

Characterization
The SiO 2 content was gravimetrically determined, and the Na 2 O and K 2 O contents were determined using flame photometry.In addition, Al 2 O 3 as well as Fe 2 O 3 and RE 2 O 3 were determined using complexometric and spectrophotometric methods, respectively.X-ray diffraction: Relative crystallinity, silica/alumina, crystalline unit cell size and phase of samples were recorded on a Rigaku Ultima IV diffractometer using Cu-Kα radiation (λ = 1.54056Å) at an operation voltage and current of 40 kV and 30 mA, respectively.The samples were scanned at 0.2° min −1 .The relative crystallinity of Y zeolite was estimated according to the equation relative crystallinity = peak height of product ⁄ peak height of reference, based on 2 θ range 22.0-24.5°.
IR spectroscopy (FT-IR): The IR spectra were recorded on an AVATAR 370 FT-IR spectrometer using KBr as compressed slices, in a range from 400 to 2000 cm −1 .
SEM: The morphology and size of the samples were determined using scanning electron microscopy (SEM) (JEOL JSM-6360) after coating with an Au evaporated film.
Nitrogen adsorption-desorption methods: The specific surface areas, pore volumes, and pore size distributions were measured on an ASAP 2020 sorptometer using adsorption and desorption isotherm plots at 77 K. Prior to the measurement, the samples were degassed at 623 K for 12 h.The surface areas were calculated using the Brunauer-Emmett-Teller (BET) method.The micropore volumes and external surfaces areas were calculated using the t-plot method.The pore parameters, mesopore surface and mesopore volume were calculated from the desorption branches of these isotherms using the BJH method.
Particle size distribution: A Malvern Micro-P particle size distribution analyser was used for determining the size distribution in the samples.
Attrition index: The attrition index of the catalyst was determined using attrition index analyser by air injection method.

Microactivity tests (MAT)
The activity of the catalyst was investigating using a microactivity test unit.The MAT conditions were as follows: reactor temperature of 460 °C, reaction time of 70 s, weight hourly space velocity (WHSV) of 15 h -1 and catalyst-to-oil mass ratio of 3.2.Prior to the MAT test, the fresh catalyst was steam-deactivated at 800 °C for 4 h or 17 h with 100 % steam.

Metal contamination
The catalysts were contaminated using the Mitchell method. 10A certain amount of NH 4 VO 3 and Ni(NO 3 ) 2 was dissolved in distilled water, and the obtained solution was mixed with the catalyst using the incipient wetness impregnation technique.The mixture was dried in an oven at 120 °C for 8 h and then calcined at 540 °C for 2 h.

Catalytic cracking tests
We chose a comparable commercial resid catalyst as the base reference FCC catalyst (RCAT).It was obtained from a domestic refinery that is based on REUSY zeolite with a surface area of 283.49m 2 g −1 and a pore volume of 0.22 cm 3 g −1 .The performance of the CAT and RCAT was tested in a small-scale fixed fluid bed reactor.The test conditions were as follows: reactor temperature of 520 °C, WHSV of 19 h -1 and catalyst-to-oil mass ratio of 6.In each run, 30 g of feedstock oil was used, and the feed injection time was 33 s.Prior to the test in fixed fluid bed reactor, the catalyst was steam-deactivated at 800 °C for 17 h with 100 % steam.The feedstock oil consisted of a mixture of 70 % vacuum gas oil (VGO) and 30 % vacuum tower bottom (VTB) and its properties are listed in Table 1.

Properties of SCM and CAT
In the in situ synthesis method, both the Al−O and Si−O species to form zeolite come from the kaolin mi-crospheres. 11In the current study, the Al−O and Si−O species were leached from CMA and CMB microspheres under caustic conditions and thus resulted in the formation of more meso-and macropores in calcined microspheres.The properties of SCM are listed in Table 2.The SCM microspheres containing zeolite Y in its sodium form had a relative crystallinity higher than 55 % with a silica/alumina amount ratio of 5.4.The composite microspheres exhibited a much larger external surface area, total pore volume, BJH pore volume, and average pore diameter.Properties of CAT and RCAT (Table 2) revealed the CAT possessed a larger surface area and pore volume.In comparison to RCAT, CAT had 150 m 2 g −1 more BET surface area and 130 m 2 g −1 more micropore surface area, increased by 52.8 % and 65.0 %, respectively.CAT had 0.17 cm 3 g −1 more total pore volume, 0.04 cm 3 g −1 more micropore volume and 0.12 cm 3 g −1 more BJH pore volume, increased by 77.3 %, 40.0 % and 85.7 %, respectively.CAT had 2.91 nm bigger average pore diameter, increased by 70.8 %.These results indicated that the CAT was richer in meso-and macropores.This phenomenon is also well illustrated in Fig. 4. The results had been confirmed that the residue of the caustic leached kaolin matrix can endow the catalyst with more meso-and macropores, and effectively improve the acidity and stability of the resulting FCC catalyst.CAT exhibited good attrition resistance, leading to less replenishment of fresh catalyst during the FCC process.In comparison to RCAT, MAT at 4 h, 17 h and contaminated condition of CAT increased by 7, 6, 14 %, respectively.The MAT results indicated that CAT had high activity and excellent hydrothermal stability, while levels of contaminant metals were present, the MAT results exhibited excellent vanadium and nickel passivation performance.

X-ray diffraction
The XRD patterns (Fig. 1) revealed that CMA was transformed from kaolin by heat treatment, under treatment temperature, which resulted in an amorphous phase as the major product.However, the characteristic peaks of the Si-Al spinel structure were also observed.The XRD patterns (Fig. 2) showed that CAT and RCAT exhibited the crystalline features of zeolite Y.In comparison to the XRD patterns of CAT, RCAT exhibited low intensity peaks due to CAT containing much of the zeolite phase, which is consistent with the XRD relative crystallinity analysis results.The acidities determined using FTIR analysis of CAT and RCAT are listed in Table 3.The acid strength distributions are quantitatively calculated from the pyridine adsorbed IR spectra at 200 and 400 °C (total acid amount and strong acid amount).Based on the results in Table 3, in comparison to RCAT, the strong Brönsted acid and total Brönsted acid amounts were higher for CAT.CAT enhanced the weak Lewis acidity.

Scanning electron microscopy
The SEM micrograph of SCM, Fig. 3(1), showed that the sizes of the Y zeolite particles were 0.4-0.8micrometers, which indicated that smaller crystals had agglomerated with the larger particles.The octahedral morphology of the Y zeolite was observed in all the images.Well-shape bipyramidal FAU crystals are not commonly observed in the reported synthesis from natural clay. 15ovarrubias reported that the characteristic bipyramidal-shaped crystals of the FAU zeolite can be obtained from kaolin. 16e SEM micrograph of CAT, Fig. 3(2), showed the typical FAU obtained by crystallization, the average particle size was about 0.6 µm in diameter.
(  As shown in Fig. 4(1), the isotherm for SCM exhibited the representative characteristics of type IV adsorption-desorption.The hysteresis loop that occurred in a relative pressure range 0.50-1.0 was due to the presence of mesopores and macropores.As shown in Fig. 4(2), the broad distribution of SCM indicated the existence of macropores.SCM possessed a wide pore structure with a trimodal distribution, the distribution was concentrated on approximately 4.0, 7.5, and 65 nm, respectively.
Based on the results in Fig. 4(3), the nitrogen adsorption and desorption branches of the isotherms for CAT exhibited a steeper decrease and a larger hysteresis loop than RCAT, indicating that meso-and macropores existed in CAT.Due to the unique synthesis route, the catalyst was endowed with more meso-and macropores.In addition, in CAT, the NaY zeolite grew in the pores of the calcined microspheres, and therefore, the mass-transfer resistance in the reaction process was effectively reduced. 17As shown in Fig. 4(4), CAT possessed a hierarchical porous distribution, the distribution was concentrated on approximately 3.8, 5.0, 9.0, 65 nm, respectively.The distribution of RCAT was observed at approximately 4.0 nm.The unique pore system of CAT should greatly enhance the accessibility of the catalytically active sites inside the microporous channels to larger reactant molecules leading to the accelerated diffusion of products and fewer secondary reactions.

Catalytic cracking tests
In comparison with RCAT, Table 4 shows that CAT had much lower heavy oil, 0.29 % lower LCO yield, 1.60 more gasoline yield, and 0.26 %, and 0.22 % lower dry gas and coke yield, respectively.The results indicated that CAT exhibited better heavy oil cracking capability and coke selectivity.The good product selectivity and higher liquid yields of CAT are obviously related to its wide pore structure and its active component.The mesopores and macropores in the catalyst can accelerate the diffusion of the product molecules, and therefore, the secondary reactions leading to dry gas and coke formation were reduced.The micropore channels effectively connect the macropore channels to the macropore channels in CAT, and may decrease unwanted secondary reactions that lead to the formation of dry gas and coke. 18By properly increasing the acidity, density, and strength in the pores of zeolite Y, the CAT effectively controlled the ratio of hydrogen transfer activity and crack ability in the catalytic reaction.
The properties of the cracked gasoline are summarized in Table 5.As shown in Table 5, in comparison to RCAT, CAT produced a reduction of 2.51 % in olefin, and the research gasoline and motor gasoline octane numbers increased by 0.7, 0.2 units.

Conclusions
In this study, new composite microspheres with hierarchical porous structure were synthesized by in situ technique using sepiolite, kaolin and pseudo-boehmite.The as-made composite microspheres containing zeolite Y in its sodium form had a relative crystallinity higher than 55 % with a silica/alumina molar ratio of 5.4.After the modification and steaming stabilization process, an FCC catalyst for maximizing light oil yield was prepared.The results of nitrogen adsorption-desorption isotherm measurements showed that the as-prepared catalyst had more meso-and macropores due to the unique synthesis route.The results of acidity characterization exhibited that the CAT had more acid sites than the commercial one.The catalytic cracking results showed that compared to the commercial catalyst, light oil yields of the CAT had increased by 1.31 %, the coke and dry gas yield had decreased by 0.26 % and 0.22 % points, respectively.

Table 2 -
Properties of SCM and CAT

Table 5 -
PONA analyses and gasoline octane number Financial support was provided by the National Natural Science Foundation of China (No. 21371055), Hunan provincial Natural Science Foundation of China (No. 11JJ2008), Hunan provincial Colleges and Universities Innovation Platform Open Fund Project (No. 15K049).