Synthesis Of SAPO-34 Molecular Sieves Research Articles

Effect of kaolin calcined temperature on the preparation and crystallization mechanism of SAPO-34 molecular sieve for methanol-to-olefins performance

SAPO-34 molecular sieves synthesized using calcined kaolin as partial raw material under three-stage of high-low-high temperature hydrothermal condition are explored systematically. Especially, the effect of kaolin calcination temperature on the crystallization mechanism of SAPO-34 and the methanol to olefin performance are investigated thoroughly. It is found that the sample (SP-500) synthesized by using kaolin calcined at 500 °C (MK-500) demonstrates clean cubic structure, large specific surface area and mild acidity, providing the initially activated Si and Al source for the synthesis of SAPO-34 molecular sieve due to the well scattered structure of MK-500. From the crystallization mechanism consideration, it can be proposed that the SAPO-34 molecular sieves will be controlled by the low degree of Si aggregation in metakaolin and quantity of Si (3Al,1Si) species. As a result, SP-500 displays excellent catalytic lifetime during methanol to olefin reaction for its special Si coordination environment, showing the highest stable light olefin yield of 80.6 % and a methanol conversion above 90 % for more than 720 min. In contrast, SP-800 sample with the most strong acid sites is only 390 min, even if showcasing the highest selectivity of light olefins (84.5%).

Rapid synthesis of SAPO-34 molecular sieve with cheap template for improving MTO performance

Low-silicon SAPO-34 was rapidly synthesized with the assistance of citric acid using inexpensive triethylamine as a template. The effects of citric acid on the crystal structure, morphology, and acidity were investigated using XRD, SEM, FT-IR as well as ICP characterization methods. The results show that citric acid promotes the growth of SAPO-34, shortens the crystallization time and improves the yield of solids. Furthermore, citric acid facilitates the synthesis of low-silicon SAPO-34, which exhibits improved MTO catalytic performance. Compared with conventional SAPO-34, low-silicon SAPO-34 generated higher selectivity of ethylene and propylene.

The effects of templates and seeds on the properties of nanosheet SAPO-34 molecular sieves and their catalytic performance in the MTO reaction

Nanoscale SAPO-34 molecular sieves were synthesized by adding different types of seed into hydrothermal synthesis systems with tetraethylammonium hydroxide (TEAOH) and triethylamine (TEA) & tetraethylammonium bromide (TEABr) as templates. The effects of different types of template and seed on the crystal structure, morphology, grain size and acidity of the molecular sieves were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 isothermal adsorption–desorption and ammonia temperature-programmed desorption (NH3-TPD). The methanol-to-olefins (MTO) reaction performance of the synthesized samples was investigated in a fixed-bed reactor. The results showed that crystalline supernatant and seed soaking solution could be used as liquid seeds to assist in the synthesis of SAPO-34 molecular sieves with a lamellar structure. The yield of SAPO-34 synthesized by seed increased from 38.64 to 59.68%, and the methanol conversion rate was significantly improved as compared with that of SAPO-34 synthesized without seed. The nano-thickness of SAPO-34 synthesized with TEA&TEABr instead of TEAOH as template decreased from 100–150 to 40–50 nm, and the lifetime increased from 360 to 400 min with the original yield kept constant.

Green synthesis of SAPO-34 molecular sieve using rice husk ash as a silica source: Evaluation of synthesis and catalytic performance parameters in methanol-to-olefin reaction

Reducing synthesis costs in the petrochemical industry leads to decreasing financial value of final products from raw materials. In this paper, SAPO-34 molecular sieve as a main catalyst of methanol-to-olefin process is newly synthesized by rice husk. The silica source of SAPO- 34 synthesis is achieved from this agricultural waste material by calcination and leaching methods with the purity of 97% based on XRF analysis. Between the series of samples synthesized, the sample with the highest crystallinity and textural properties is obtained by Si = 0.6 M concentration and hydrothermal synthesis at 200◦C for 18 h which has been proved by XRD, SEM, BET, EDX and NH3-TPD analyses. Also, this sample showed that has relatively higher selectivity toward light olefins compared to other samples. In the case of operating conditions, using methanol/water (20 wt%) instead of pure methanol as feed increased light olefins selectivity and reached the maximum value of 83.5% at 375 ◦C, however, highest light olefins mass (26 wt%) from the consumed feed acquired at 425 ◦C.

A sequential Box-Behnken Design (BBD) and Response Surface Methodology (RSM) to optimize SAPO-34 synthesis from kaolin waste

Kaolin waste from an industry in the Amazon region was selected as an economic source of Si and Al to optimize the synthesis of SAPO-34 molecular sieve. A sequential BBD, RSM and global desirability function (D) was used. The factors studied for optimization were the SiO2/Al2O3 molar ratio, the concentration of the structure-directing agent (SDA; morpholine) and the crystallization time. The responses evaluated in the synthesized SAPO-34 were the relative crystallinity (%), the enthalpy of formation of transition alumina (J.g−1) and the mass loss (%). The synthesis as a function of time and its products post-DSC and calcined at 1000 °C was performed to relate the enthalpy of formation of the transition alumina with the SAPO-34 formation. Prediction models and response surfaces were corroborated by the level values optimized by the desirability function. The ideal values based on the data used in this study included a crystallization time of 20 h, a xSiO2 value of 1.3 and a ySDA value of 1.6. The experimental validation based on the ideal values was within the established confidence level (±95%). The formation of the transition alumina at 985 °C influences the enthalpy of exothermic transformation identified at this temperature. Low values of this enthalpy are related to the effective formation of SAPO-34.

Synthesis of SAPO-34 Molecular Sieves via Novel Intermittent Hydrothermal Treatment and Its Effect on the Crystallization and Product Properties

Intermittent hydrothermal treatment was introduced into the synthesis of SAPO-34 molecular sieves to control the nucleation and the growth in the crystallization. The effect of the crystallization time, the order of long-time and short-time crystallization in two-stage crystallization, and frequency in multi-stage crystallization on synthesis, physicochemical properties and catalytic performance for conversion of methanol to light olefins (MTO) has been studied. The results show that pure SAPO-34 can be obtained with increasing crystallization time. The interruption of the initial crystallization is more beneficial for improving the Si distribution and the MTO catalytic performance of SAPO-34 molecular sieves. The sample obtained by repeatedly alternating heating and cooling during crystallization shows smaller particle size, higher acidity, longer lifetimes and higher yields of ethylene than that obtained by the conventional continuous crystallization at high temperature.

Organophosphorous surfactant-assistant synthesis of SAPO-34 molecular sieve with special morphology and improved MTO performance

SAPO-34s with special morphologies have been synthesized by using a self-designed organophosphorous surfactant and various microporous templates. The prepared SAPO-34s were evaluated by the MTO reaction, which showed improved catalytic performance.

Rapid synthesis of SAPO-34 nanocatalyst by dry gel conversion method templated with morphline: Investigating the effects of experimental parameters

SAPO-34 nanoparticles were successfully synthesized by a dry gel conversion method using morpholine as a structure-directing agent. In this study, the effects of synthesis parameters, such as crystallization time, gel drying temperature and water content added in the crystallization stage, on crystallinity, shape and particle size of SAPO-34 are investigated and the advantages of the dry-gel conversion (DGC) method are described for the synthesis of SAPO-34 molecular sieves. The products were characterized by XRD, SEM, TEM, EDX, BET and NH3-TPD. The results show that the favorable phase of crystallinity has been improved by increasing the crystallization time, gel drying temperature and water content. Most of the particles have particle sizes lower than 500nm and it is an excellence of employing the DGC method despite using morpholine as a structure-directing agent. The high nucleation density and slow crystal growth after nucleation for SAPO-34 crystals in the dry gel conversion method result in the formation of nanoparticles. Contrary to the hydrothermal method in which at least 24h of the synthesis time is required to obtain crystalline SAPO-34, dry-gel conversion synthesis of samples leads to formation of fully crystalline SAPO-34 crystals taking only 6h. This has been observed as an important influence of dry-gel conversion synthesis on the activity and selectivity of the samples in the MTO reaction.

Synthesis of SAPO-34 Molecular Sieves Templated with Diethylamine and Their Properties Compared with Other Templates

SAPO-34 molecular sieves were synthesized using diethylamine (DEA) as a template. Influencing factors, such as H3PO4 content, H2O content, Al source, and Si source, were investigated. Pure SAPO-34 could be obtained using the following synthesis gel conditions: 0.7 ≤ n(P2O5)/n(Al2O3) ≤ 1.2 and 25 ≤ n(H2O)/n(Al2O3) ≤ 100. The Al source has a great influence on the resulting sample composition. SAPO-34 could also be synthesized using triethylamine (TEA), morpholine (MOR), and a DEA-TEA mixture, respectively. The products were characterized using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, thermogravimetry, and 29Si MAS NMR. The level of silicon incorporation into the SAPO-34 framework decreased in the following order of SAPO-34 (DEA) > SAPO-34 (MOR) > SAPO-34 (TEA). The Si(4Al) contents in the SAPO-34 frameworks followed the order of SAPO-34 (DEA) ≈ SAPO-34 (MOR) > SAPO-34 (TEA). 以二乙胺 (DEA) 为模板剂, 合成了 SAPO-34 分子筛, 详细考察了磷酸用量、水用量以及硅源和铝源等因素的影响. 结果表明, 在 0.7 ≤ n(P2O5)/n(Al2O3) ≤ 1.2 及 25 ≤ n(H2O)/n(Al2O3) ≤ 100 范围内, 可以得到纯相 SAPO-34 分子筛, 铝源对所得样品的组成影响较大. 同时, 采用三乙胺 (TEA)、吗啉 (MOR) 以及二乙胺-三乙胺 (DEA-TEA) 混合模板剂合成了 SAPO-34, 通过 X 射线衍射、X 射线荧光分析、扫描电镜、热重和 29Si 固体核磁共振等手段对所得样品进行了表征. 结果显示, 不同模板剂促使硅进入 SAPO-34 骨架的能力顺序为 SAPO-34 (DEA) > SAPO-34 (MOR) > SAPO-34 (TEA); SAPO-34 骨架中所能容纳的最大 Si(4Al) 含量顺序为 SAPO-34 (DEA) ≈ SAPO-34 (MOR) > SAPO-34 (TEA).

A Facile Synthesis of SAPO-34 Molecular Sieves with Microwave Irradiation in Wide Reaction Conditions

Various reaction conditions uding temperature, time and type and concentration of templates have been changed in order to facilely synthesize, especially with microwave (MW) heating, SAPO-34 molecular sieves. SAPO-34 molecular sieve can be synthesized rapidly with microwave irradiation from a gel containing tetraethylammonium hydroxide (TEAOH) as a template. However, other several templating molecules lead to SAPO-5 molecular sieve under microwave irradiation even though SAPO-34 is obtained by conventional electric synthesis from the same reactant gels. Moreover, SAPO-34 can be obtained more easily by increasing the TEAOH or silica concentration or by increasing the reaction temperature. SAPO-34 can be obtained within 5 min in a selected condition (high temperature of 210 ) with microwave heating, which may lead to a continuous production of the important material. SAPO-34 synthesized by microwave irradiation is homogeneous and small in size and shows acidity and a stable performance in the dehydration of methanol and 2-butanol to olefins, suggesting potential applications in acid catalysis.

Crystallization and Si incorporation mechanisms of SAPO-34

In this study of the synthesis of SAPO-34 molecular sieves, XRD, SEM, XRF, IR and NMR techniques were applied to monitor the crystalloid, structure and composition changes of the samples in the whole crystallization process in order to get evidence for the crystallization as well as Si incorporation mechanism of SAPO-34. XRD results revealed that the crystallization contained two stages. In the first 2.5 h (the earlier stage), high up to ∼80% of relative crystallinity could be achieved and the crystal size of SAPO-34 was almost the same as that of any longer time, indicating a fast crystallization feature of the synthesis. In this stage, IR revealed that the formation of SAPO-34 framework structure was accompanied by the diminution of hydroxyls, suggesting that crystal nuclei of SAPO-34 may arise from the structure rearrangement of the initial gel and the condensation of the hydroxyls. NMR results reveal that the template and the ageing period are crucial for the later crystallization of SAPO-34. Preliminary structure units similar to the framework of SAPO-34 have already formed before the crystallization began (0 h and low temperature). Evidence from IR, NMR, and XRF shows that the formation of the SAPO-34 may be a type of gel conversion mechanism, the solution support and the appropriate solution circumstance are two important parameters of the crystallization of SAPO-34. Meanwhile, NMR measurements demonstrated that about 80% of total Si atoms directly take part in the formation of the crystal nuclei as well as in the growth of the crystal grains in the earlier stage (<2.5 h). Evidence tends to support that Si incorporation is by direct participation mechanism rather than by the Si substitution mechanism for P in this stage (<2.5 h). In the later stage (>2.5 h), the relative content of Si increased slightly with a little decrease of Al and P. The increase of Si(4Al) and the appearance of the Si(3Al), Si(2Al), Si(1Al) and Si(0Al) in this stage suggest that substitution of the Si atoms for the phosphorus and for the phosphorus and aluminum pair takes place in the crystallization. The relationship among structure, acidity and crystallization process is established, which suggests a possibility to improve the acidity and catalytic properties by choosing a optimum crystallization time, thus controlling the number and distribution of Si in the framework of SAPO-34.