Silicoaluminophosphate Zeolite Research Articles

Effect of zeolite topological structure in bifunctional catalyst on direct conversion of syngas to light olefins

Bifunctional catalyst composed of metal oxide and zeolite (OX-ZEO) is a promising strategy for the direct conversion of syngas to light olefins (STO), where the structure of zeolite plays a vital role in determining the selectivity of product. Herein, three kinds of silicoaluminophosphate zeolites with different topological structures, i.e., the ERI(SP17), AEI(SP18) and CHA(SP34), were hydrothermally synthesized, after the combination with Mn-Ga oxide, the prepared OX-ZEO was applied for STO reaction. The variation in the crystallization time for SP17 synthesis has a great impact on the generation of impurity phase of SAPO-5, where a crystallization time of 48–96 h is found to be beneficial in synthesizing SP17 zeolite with pure phase. SP17 zeolite with a crystallization time of 96 h, possesses the micropores and columnar morphology, where the small cage-defining 8-ring size of SP17 shows the olefins selectivity of 87.0% at a low CO conversion of 19.4%, significantly deviating towards the major fraction of ethylene (45.6%) than that of butene (8.2%). In a contrast, SP18 and SP34 zeolites with the same and large cage-defining 8-ring size, are richer in propylene and butene fractions than that of ethylene in overall similar olefins selectivity of 87.0% and 87.1% at CO conversion of 28.7% and 28.5%, respectively. Interestingly, it is further interpreted that the SP17 sample generated more carbon species during the reaction due to the small 8-ring size, while those amounts of carbon species were restricted in the hierarchical pore structure and plate-like morphology in SP18 and SP34 samples.

Plasma-Induced Desorption of Methane and Carbon Dioxide over Silico Alumino Phosphate Zeolites

Plasma-Induced Desorption of Methane and Carbon Dioxide over Silico Alumino Phosphate Zeolites

Synthesis and thermal stability of SOD-type SAPO-20 via interzeolite conversion of FAU-type SAPO-37 using tetramethylammonium hydroxide and piperidine

Interzeolite conversion (IZC) of zeolites has attracted significant attention; however, research on silicoaluminophosphate zeolite system is still in its early stages. Herein, an IZC of an FAU-type silicoaluminophosphate zeolite (SAPO-37) is performed using tetramethylammonium hydroxide (TMAOH) and piperidine as organic additives. For comparison, conventional hydrothermal synthesis using an amorphous precursor instead of SAPO-37 is also performed. Using TMAOH, SOD-type silicoaluminophosphate zeolites (SAPO-20) are obtained from both parent SAPO-37 and the amorphous precursor. However, in the case of piperidine, SAPO zeolites with different topologies are obtained from parent SAPO-37 and the amorphous precursor; that is, SOD-type SAPO-20 is obtained from parent SAPO-37, and CHA-type silicoaluminophosphate (SAPO-34) is obtained from the amorphous precursor. Piperidine molecules, as organic additives, are observed to be occluded in the cages of the obtained SAPO-20. The characteristics and thermal stability of the as-synthesized SAPO-20 are analyzed in detail.

Unraveling templated-regulated distribution of isolated SiO4 tetrahedra in silicoaluminophosphate zeolites with high-throughput computations.

Silicoaluminophosphate (SAPO) zeolites are well-known catalytic materials because of the mild acidity originating from the isolated SiO4 tetrahedra in their frameworks. Regulating the distribution of isolated SiO4 tetrahedra in SAPO zeolites is formidably challenging because SiO4 tetrahedra tend to agglomerate to form Si islands and the isolated SiO4 tetrahedra are difficult to determine using conventional characterization techniques. Here we synthesized Si-island-free SAPO-35 zeolites by using N-methylpiperidine as a new template, which exhibited excellent thermal stability compared to conventional SAPO-35 zeolites and a substantially improved methanol-to-olefins catalytic lifetime even comparable to that of commercial SAPO-34 zeolites. More strikingly, with the aid of high-throughput computations on 44 697 structure models combined with various state-of-the-art characterization techniques, for the first time, we reveal that the host–guest interactions between template molecules and SAPO frameworks determine the specific distributions of isolated SiO4 tetrahedra, which are responsible for the improvement in the chemical properties of zeolites. Our work provides an insight into the template-based regulation of isolated SiO4 tetrahedra in SAPO zeolites, which opens a new avenue in the discovery of promising zeolite catalysts with optimal SiO4 distribution.

Designed synthesis of single-crystalline silicoaluminophosphate zeolite via interzeolite transformation for n-dodecane hydroisomerization

Single-crystalline zeolites are highly desirable in this field owing to their high hydrothermal stability and high mass transfer efficiency. However, the preparation of single-crystalline silicoaluminophosphate SAPO-11 zeolite remains a significant challenge due to its complex synthesis process. Herein, we firstly report a facile and direct approach to synthesize single-crystalline SAPO-11 by interzeolite transformation via a kinetic-modulated process. The crystallization process of single-crystalline SAPO-11 was monitored by the time-dependent characterizations. A panel of characterizations by XRD, SEM, N 2 -adsorption and solid-state NMR provided evidence that the formation mechanism of single-crystalline SAPO-11 involved fast interzeolite transition process between two different topologies from AlPO 4 -8 (AET) to SAPO-11(AEL), thus it avoids the aggregation of small nanocrystals during the conventional polycrystalline SAPO-11 synthesis. In addition, the Brønsted acid sites of the prepared samples could be precisely regulated. The resulting single-crystalline catalyst exhibits superior isomer selectivity (92.3% vs. 70.6%) and yield (77.9% vs. 62.3%) in the n -dodecane hydroisomerization compared with the conventional polycrystalline sample due to the unique morphology and suitable acidity. This synthetic strategy provides a new orientation to design other single-crystalline zeolite catalysts. Single-crystalline SAPO-11 molecular sieve is synthesized through interzeolite transformation mechanism and has outstanding catalytic performance for the hydroisomerization of n -dodecane. • A new strategy to prepare a single-crystalline SAPO-11 by interzeolite transformation under kinetic-modulated process. • A clear recognition for the formation mechanism involved interzeolite transition process from AlPO 4 -8 to SAPO-11. • The acidity of single-crystalline SAPO-11 could be precisely regulated. • The Pt/S-SAPO-11 catalysts exhibit excellent catalytic performance for the hydroisomerization of n -dodecane.

Synthesis and Applications of SAPO‐34 Molecular Sieves

Silicoaluminophosphate zeolite (SAPO-34) has been attracting increasing attention due to its excellent form selection and controllability in the chemical industry, as well as being one of the best industrial catalysts for methanol-to-olefin (MTO) reaction conversion. However, as a microporous molecular sieve, SAPO-34 easily generates carbon deposition and rapidly becomes inactivated. Therefore, it is necessary to reduce the crystal size of the zeolite or to introduce secondary macropores into the zeolite crystal to form a hierarchical structure in order to improve the catalytic effect. In this review, the synthesis methods of conventional SAPO-34 molecular sieves, hierarchical SAPO-34 molecular sieves and nanosized SAPO-34 molecular sieves are introduced, and the properties of the synthesized SAPO-34 molecular sieves are described, including the phase, morphology, pore structure, acid source, and catalytic performance, in particular with respect to the synthesis of hierarchical SAPO-34 molecular sieves. We hope that the review can provide guidance to the preparation of the SAPO-34 catalysts, and stimulate the future development of high-performance hierarchical SAPO-34 catalysts to meet the growing demands of the material and chemical industries.

Induced Active Sites by Adsorbate in Zeotype Materials.

There has been a long debate on how and where active sites are created for molecular adsorption and catalysis in zeolites, which underpin many important industrial applications. It is well accepted that Lewis acidic sites (LASs) and basic sites (LBSs) as active sites in pristine zeolites are generally believed to be the extra-framework Al species and residue anion (OH-) species formed at fixed crystallographic positions after their synthesis. However, the dynamic interactions of adsorbates/reactants with pristine zeotype materials to "create" sites during real conditions remain largely unexplored. Herein, direct experimental observation of the establishment of induced active sites in silicoaluminophosphate (SAPO) by an adsorbate is for the first time made, which contradicts the traditional view of the fixed active sites in zeotype materials. Evidence shows that an induced frustrated Lewis pair (FLP, three-coordinated framework Al as LAS and SiO (H) as LBS) can be transiently favored for heterolytic molecular binding/reactions of competitive polar adsorbates due to their ineffective orbital overlap in the rigid framework. High-resolution magic-angle-spinning solid-state NMR, synchrotron X-ray diffraction, neutron powder diffraction, in situ diffuse reflectance infrared Fourier transform spectroscopy, and ab initio molecular dynamics demonstrate the transformation of a typical Brønsted acid site (Al(OH)Si) in SAPO zeolites to new induced FLP structure for hetereolytic binding upon adsorption of a strong polar adsorbate. Our unprecedented finding opens up a new avenue to understanding the dynamic establishment of active sites for adsorption or chemical reactions under molecular bombardment of zeolitic structures.

Resolving atomic SAPO-34/18 intergrowth architectures for methanol conversion by identifying light atoms and bonds

The micro-structures of catalyst materials basically affect their macro-architectures and catalytic performances. Atomically resolving the micro-structures of zeolite catalysts, which have been widely used in the methanol conversion, will bring us a deeper insight into their structure-property correlations. However, it is still challenging for the atomic imaging of silicoaluminophosphate zeolites by electron microscopy due to the limits of their electron beam sensitivity. Here, we achieve the real-space imaging of the atomic lattices in SAPO-34 and SAPO-18 zeolites, including the Al–O–P atoms and bonds, by the integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). The spatial distribution of SAPO-34 and SAPO-18 domains in SAPO-34/18 intergrowths can be clearly resolved. By changing the Si contents and templates in feed, we obtain two SAPO-34/18 catalysts, hierarchical and sandwich catalysts, with highly-mixed and separated SAPO-34 and SAPO-18 lattices respectively. The reduced diffusion distances of inside products greatly improve the catalytic performances of two catalysts in methanol conversion. Based on the observed distributions of lattices and elements in these catalysts, we can have a preliminary understanding on the correlation between the synthesis conditions and structures of SAPO-34/18 intergrowth catalysts to further modify their performances based on unique architectures.

Promoted crystallization of silicoaluminophosphate zeolites: an efficient way to accelerate crystallization rate and increase solid yield

SAPO-34 with 100% yield has been synthesized by an acid-assisted method, which is also applicable to the synthesis of other silicoaluminophosphate zeolites.

High Conversion of Syngas to Ethene and Propene on Bifunctional Catalysts via the Tailoring of SAPO Zeolite Structure

Direct conversion of syngas to light olefins using bifunctional catalysts containing both oxides and zeolites represents a promising alternative in comparison with Fischer-Tropsch synthesis. It is still a great challenge to achieve highly selective production of ethylene and propylene at a satisfying CO conversion for potentially industrial applications. Here, we report that a high selectivity to ethylene and propylene can be achieved by tailoring silicoaluminophosphate zeolite structure and acid site strength. By comparing bifunctional catalysts composed of ZnCr oxide and different zeolites with 8-membered ring openings, we achieve the total selectivity of 76.4% in ethylene and propylene at a CO conversion as high as 38.2% by catalysts containing SAPO-17 zeolite (ERI framework). The detailed hydrocarbon distribution is nearly identical to that in the methanol-to-olefins reaction. The high selectivity can be rationalized due to both smaller 8-membered ring openings and the weaker Brønsted acid strength of SAPO-17 zeolites.

The inorganic cation-tailored "trapdoor" effect of silicoaluminophosphate zeolite for highly selective CO2 separation.

Functional nanoporous materials are widely explored for CO2 separation, in particular, small-pore aluminosilicate zeolites having a “trapdoor” effect. Such an effect allows the specific adsorbate to push away the sited cations inside the window followed by exclusive admission to the zeolite pores, which is more advantageous for highly selective CO2 separation. Herein, we demonstrated that the protonated organic structure-directing agent in the small-pore silicoaluminophosphate (SAPO) RHO zeolite can be directly exchanged with Na+, K+, or Cs+ and that the Na+ form of SAPO-RHO exhibited unprecedented separation for CO2/CH4, superior to all of the nanoporous materials reported to date. Rietveld refinement revealed that Na+ is sited in the center of the single eight-membered ring (s8r), while K+ and Cs+ are sited in the center of the double 8-rings (d8rs). Theoretical calculations showed that the interaction between Na+ and the s8r in SAPO-RHO was stronger than that in aluminosilicate RHO, giving an enhanced “trapdoor” effect and record high selectivity for CO2 with the separation factor of 2196 for CO2/CH4 (0.02/0.98 bar). The separation factor of Na-SAPO-RHO for CO2/N2 was 196, which was the top level among zeolitic materials. This work opens a new avenue for gas separation by using diverse silicoaluminophosphate zeolites in terms of the cation-tailored “trapdoor” effect.

Polyacrylamide-assisted synthesis of hierarchical porous SAPO-34 zeolites with excellent MTO catalytic performance

Silicoaluminophosphate zeolite SAPO-34 with a CHA topological structure has been highlighted for its high methanol conversion and excellent selectivity for light olefins in the methanol-to-olefin (MTO) reaction. In this work, hierarchical porous SAPO-34 zeolite with excellent catalytic properties was first prepared by introducing water-soluble polyacrylamide (PAM) hydrogel into the synthesis system of conventional SAPO-34 by using a one-step hydrothermal synthesis method. The effects of PAM concentration and PAM type on the morphology and pore structure of hierarchical SAPO-34 catalysts and the formation mechanism of hierarchically structured zeolites were studied. Compared to conventional microporous SAPO-34, the hierarchically structured SAPO-34 exhibits excellent performance in the MTO reaction with an approximately three times longer catalytic lifetime (740 min) and remarkably improved selectivity for light olefins of up to 87.5%. A butterfly-shaped hierarchical porous SAPO-34 with a micro-meso-macroporous structure was directly synthesized in the presence of polyacrylamide hydrogel. The tri-level hierarchically structured SAPO-34 exhibits long catalytic lifetime and good thermal stability in the MTO reaction. • A hierarchical porous SAPO-34 with a micro-meso-macroporous structure was directly synthesized in the presence of PAM hydrogel. • The morphology and pore structure of SAPO-34 crystals could be controlled by adjusting the PAM amount and PAM type. • The hierarchically structured SAPO-34 exhibits long catalytic lifetime and good thermal stability in the MTO reaction.

Crystallization of SAPO Series Silicoaluminophosphate Zeolites and Effect of Their Composite Catalysts on CO Hydrogenation

Crystallization of SAPO Series Silicoaluminophosphate Zeolites and Effect of Their Composite Catalysts on CO Hydrogenation

Effect of zeolite topology on the hydrocarbon distribution over bifunctional ZnAlO/SAPO catalysts in syngas conversion

Recent studies demonstrate that bifunctional catalysts composed of metal oxide and zeolite can effectively control the hydrocarbon distribution in syngas conversion. In this effort, a set of eight silicoaluminophosphate (SAPO) zeolites with different topology structures are synthesized and integrated with a binary ZnAlO oxide for syngas conversion. The effects of micropore window size and cage size of SAPO zeolites, and reaction conditions on the hydrocarbon distribution are preliminarily investigated. It was found that the hydrocarbon distribution is primarily determined by the window size of SAPO zeolites, and a volcano trend was observed between the chain length of hydrocarbon products and the window sizes. The cage size of zeolites and reaction conditions only modifies the hydrocarbon distribution in a narrow range of chain length. The effect of zeolite topology on the hydrocarbon distribution in syngas conversion is overall in line with that in methanol conversion. These findings provide an efficient strategy to increase the selectivity of an individual hydrocarbon product in syngas conversion, such as ethylene, propylene, butenes, iso-butane and C5–C11iso-paraffins.

Lattice substitution and desulfurization kinetic analysis of Zn-based spinel sorbents loading onto porous silicoaluminophosphate zeolites

Green Zn-based spinel sorbents for hot coal gas desulfurization have been developed with the assistance of optimization procedures. The pilot study highlights an outstanding ordered mesoporous support (SBET = 323 m2 g−1, Da = 4.3 nm) of SAPO-34@as-prepared SBA-15 (SS) for loading active metal oxides. ZnCo2O4 spinel loaded onto SS (ZnCo2/SS) exhibits a prominent desulfurization performance compared to other sorbents whose partial Co is substituted by Mn or Fe in spinel B-site, owing to the slight effect of PO43− in SS. After systematic evaluation on role of sulfidation condition, 50 wt% ZnCo2/SS sorbent possesses the sulfur storage capacity of 138.08 mg g−1at550 °C and little loss of active species in 5 desulfurization-regeneration cycles. Results of high resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS) etc. demonstrate that 70.42% of initial sulfur capacity of ZnCo2/SS presented in the 2nd utilization is associated with zinc evaporation, existence of high stable sulfides and partial sintering. The improved deactivation kinetic model suitably describes that the H2S concentration distribution relates with the spatial position of fixed-bed reactor and desulfurization time.

Cu-SAPO-18 for NH3-SCR Reaction: The Effect of Different Aging Temperatures on Cu2+ Active Sites and Catalytic Performances

Cu-based silicoaluminophosphate zeolite with an AEI structure (Cu-SAPO-18) was directly synthesized through hydrothermal crystallization. The fresh and aged catalysts were applied for the selective...

Mesoporogen-Free Synthesis of Hierarchical SAPO-34 with Low Template Consumption and Excellent Methanol-to-Olefin Conversion.

Significant interest has emerged in the development of nanometer-sized and hierarchical silicoaluminophosphate zeolites (SAPO-34) because of their enhanced accessibility and improved catalytic activity for methanol-to-olefin (MTO) conversion. A series of nanometer-sized SAPO-34 catalysts with tunable hierarchical structures was synthesized in a Al2 O3 /H3 PO4 /SiO2 /triethylamine(TEA)/H2 O system by using a mesoporogen-free nanoseed-assisted method. The nanometer-sized hierarchical SH -3.0 catalyst (TEA/Al2 O3 =3.0) possessed the highest crystallinity, highest abundance of intracrystalline meso-/macropores, and the most suitable acidity among all obtained catalysts, showing the highest ethylene and propylene selectivity of 85.4 %. This is the highest reported selectivity for MTO reactions under similar conditions. Detailed analysis of the coke produced during the reaction revealed that the small-sized methyl-substituted benzene and bulky methyl-substituted pyrene were mainly located inside the crystals instead of on the surface of the crystals, which provided further insight into understanding the deactivation of the SAPO-34 catalyst during MTO reaction. Significantly, the simple and cost-effective synthetic process and superb catalytic performance of the nanometer-sized hierarchical SAPO-34 is promising for their practical large-scale application for MTO conversion.

Performance of Methanol-to-Olefins Catalytic Reactions by the Addition of PEG in the Synthesis of SAPO-34

SAPO-34, a silicoaluminophosphate zeolite, has been synthesized by the hydrothermal method with the addition of different molecular weights of polyethylene glycol (PEG), and has been characterized with XRD, SEM, N2 adsorption–desorption, FT-IR, and NH3 temperature-programmed desorption (NH3-TPD). We studied SAPO-34 as a catalyst in the methanol-to-olefins (MTO) reaction, in a fixed-bed reactor. The results show that the chain length of PEG has a great influence on the particle size and morphology of SAPO-34. PEG acts as inhibitor in the crystallization process. With the increase of the chain length of PEG used in the synthesis, from a relative molecular weight of 400–6000, the morphology of SAPO-34 changes gradually from cubic to nanoplate-like and then changes to cubic again. The particle size decreases markedly at first and then increases to some extent. The catalytic stability in the MTO reaction also increases first and then decreases, with all the catalysts having almost the same selectivity to olefins. When the sample is synthesized with PEG800, the particles become nanoplate-like with a thickness of 46 nm on average, and the catalytic stability is appreciably prolonged, which is attributed to the shorter diffusion paths of the reactants in the zeolite.

Krypton‐xenon separation properties of SAPO‐34 zeolite materials and membranes

Separation of the radioisotope 85Kr from 136Xe is an important target during used nuclear fuel recycling. We report a detailed study on the Kr and Xe adsorption, diffusion, and membrane permeation properties of the silicoaluminophosphate zeolite SAPO‐34. Adsorption and diffusion measurements on SAPO‐34 crystals indicate their potential for use in Kr‐Xe separation membranes, but also highlight competing effects of adsorption and diffusion selectivity. SAPO‐34 membranes are synthesized on α−alumina disk and tubular substrates via steam assisted conversion seeding and hydrothermal growth, and are characterized in detail. Membrane transport measurements reveal that SAPO‐34 membranes can separate Kr from Xe by molecular sieving, with Kr permeabilities around 50 Barrer and mixture selectivity of 25–30 for Kr at ambient or slight sub‐ambient conditions. The membrane transport characteristics are modeled by the Maxwell‐Stefan equations, whose predictions are in very good agreement with experiment and confirm the minimal competing effects of adsorption and diffusion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 761–769, 2017

In situ growth-etching approach to the preparation of hierarchically macroporous zeolites with high MTO catalytic activity and selectivity

Silicoaluminophosphate zeolite SAPO-34 with CHA topology is known as one of the best catalysts for methanol-to-olefin (MTO) conversion. In this work, we demonstrate a facile one-step hydrothermal synthesis of hierarchically macroporous SAPO-34 through the etching effect of hydrofluoric acid. The highly crystalline hierarchically macroporous SAPO-34 is prepared as central-holed rhombohedral crystals with particle size of ca. 5–10 μm that comprise intracrystalline parallel macrochannels of ca. 100 nm. The formation of macroporous structures via in situ growth-etching can be directly imaged by SEM. Strikingly, a particular crystal configuration consisting of eight pyramidal parts has been first observed during the zeolite crystal growth process, which eventually grows to form a perfect rhombohedral shape. The HF etching effect is further elucidated by the analysis of changes of pH values as well as of solid and liquid compositions following the evolution of crystallization. The texture properties, chemical environments of framework atoms, and acidity of the synthesized SAPO-34 are characterized by N2 adsorption/desorption, MAS NMR and NH3-TPD measurements. The hierarchically macroporous SAPO-34 shows larger micropore volume, slightly stronger acid strength, and lower external surface acidity than its conventional counterpart synthesized without using HF. Consequently, the hierarchically macroporous SAPO-34 exhibits excellent MTO catalytic performance, showing much higher selectivity to ethylene and propylene as well as longer lifetime than the conventional counterpart. In comparison with previously reported methods for the generation of hierarchical porosity, this one-step HF-assisted in situ growth-etching synthetic route is simple, straightforward and cost-effective, which offers a new approach to prepare hierarchically porous zeolites with improved catalytic activity.