Silicoaluminophosphate Research Articles

Selective production of olefins from methanol over a heteroatomic SAPO-34 zeolite.

The methanol-to-olefins (MTO) process has the potential to bridge future gaps in the supply of sustainable lower olefins. Promoting the selectivity of propylene and ethylene and revealing the catalytic role of active sites are challenging goals in MTO reactions. Here, we report a novel heteroatomic silicoaluminophosphate (SAPO) zeolite, SAPO-34-Ta, which incorporates active tantalum(V) sites within the framework to afford an optimal distribution of acidity. SAPO-34-Ta exhibits a remarkable total selectivity of 85.8% for propylene and ethylene with a high selectivity of 54.9% for propylene on full conversion of methanol at 400°C. In situ and operando synchrotron powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy and inelastic neutron scattering, coupled with theoretical calculations, reveal trimethyloxonium as the key reaction intermediate, promoting the formation of first carbon-carbon bonds in olefins. The tacit cooperation between tantalum(V) and Brønsted acid sites within SAPO-34 provides an efficient platform for selective production of lower olefins from methanol.

Microwave-assisted synthesis of solketal from glycerol and acetone in the presence of SAPO-34 and SAPO-5

Solketal is one of the most promising additives improving the properties of motor fuels. Investigations related to process intensification of synthesis of solketal from glycerol and acetone is gaining importance. The present work illustrates the use of the microwave mode for intensification of this process in the presence of silicoaluminophosphates (SAPO) as catalysts. The main focus of the study was placed on tuning the textural, acidic, and catalytic properties of SAPO-34 and SAPO-5 via variation of the synthesis parameters, such as the nature of Al and Si sources and the structure-directing agent. The catalytic properties of SAPO materials were investigated in the synthesis of solketal from glycerol and acetone under microwave assistance at the acetone/glycerol molar ratio of 2.4 in a methanol solution (glycerol/methanol: 1 g/1 mL) and 56 °C. It was demonstrated that the selectivity towards solketal was 91.1–98.6 % in the presence of the studied materials. The activities of mesoporous SAPO-34 and SAPO-5 prepared in the presence of triethylamine were higher as compared with microporous SAPO-34 prepared in the presence of tetraethylammonium hydroxide due to the high mesoporosity and a larger content of acid sites. The 90.4 % conversion of glycerol and 98.6 % selectivity towards solketal were observed in the presence of mesoporous 7.6%SAPO-34 for 90 min. The advantage of using microwave technology is shown. The solketal yield in the reaction under microwave irradiation was 3 times higher than that in the process performed under thermal heating conditions.

Preparation of silicoaluminophosphate-34 zeolite on different macroporous supports for permeability and selectivity of CO2/CH4

In this study, the zeolite silicoaluminophosphate-34 (SAPO-34) was synthesized using Al(OH)3 and SiO2 obtained from waste recovery as sources of aluminum and silicon. Tablets of compacted crystals were prepared and the crystal growth of SAPO-34 was promoted through several steps of hydrothermal treatment on non-conventional macroporous supports such as pumice stone and 500 mesh stainless steel. Homogeneous crystal growth was achieved on the surface of the supports after three treatments of 24 h. The CO2/CH4 separation selectivity was evaluated using the three aforementioned materials. A permeability of 5.4 × 10−7 mol/m2 s Pa and a CO2 selectivity of 47.55 were obtained with SAPO-34 supported on 500 mesh stainless steel. The pumice/SAPO-34 samples showed a permeability of 19.6 × 10−7 mol/m2 s and a selectivity of 52. Furthermore, this work demonstrated the potential of pumice stone supports for gas separation.

A Super-Hygroscopic Solar-Regenerated Alginate-Based Composite for Atmospheric Water Harvesting.

Global water scarcity is leading to increasingly tense competition across populations. In order to complement the largely fast-depleting fresh water sources and mitigate the challenges generated by brine discharge from desalination, atmospheric water harvesting (AWH) has emerged to support long-term water supply. This work presents a novel alginate-based hybrid material comprised of porous silico-aluminophosphate-34 (SAPO-34) as fast-transport channel medium as well as hydrophilicity and stability enhancer, and graphene-based sheets as light absorber for solar-enabled evaporation, both optimally incorporated in an alginate matrix, resulting in a composite sorbent capable of harvesting water from the atmosphere with a record intake of up to 6.85 gw gs -1. Natural sunlight is solely used to enable desorption achieving increase of the temperature of the developed network up to 60°C andresulting in release of the sorbed water, with impurities content well below the World Health Organization (WHO) upper limits. After 30 cycles of sorption and desorption, the composite hydrogel displayed unchanged water uptake and stability. This work provides an impactful perspective toward sustainable generation of water from humidity without external energy consumption supporting the emergence of alternative water production solutions.

The Synthesis of Amino-Acid-Anchored Two-Dimensional Silicoaluminophosphates and Congo Red Adsorption Application.

Two-dimensional (2D) materials with nanometer thickness have the advantage of a large specific surface area and excellent surface accessibility. They have great potential for adsorption, catalysis, and many other applications. 2D zeolitic silicoaluminophosphates (SAPOs) can be synthesized through a dual structure directing agent (SDA) strategy. But the materials have been primarily used in their organic-free form. In this work, we demonstrate that the same synthesis strategy is effective for developing amino-acid-anchored 2D SAPOs through a one-step synthesis. Because of the addition of amino acids, the surficial amino and carboxylic groups serve as active sites for adsorption and catalysis. Congo red adsorption is used to evaluate the potential of using the organic functional groups as active adsorption sites. The 2D SAPO materials have demonstrated excellent Congo red removal efficiency with close to complete removal for certain concentrations. The effects of the amino acid concentration and hydrothermal synthesis time on material morphology development will be discussed. Thorough characterization by SEM, TEM, FTIR, XRD, and nitrogen adsorption has been done to reveal the properties of the amino-acid-anchored 2D SAPOs.

Controlling product selectivity and catalyst lifetime by altering acid strength, cavity size of SAPO, and diffusion rate of methanol in the MTO reaction: DFT and MD calculations.

The initiation mechanisms of the MTO process over silicoaluminophosphate (SAPO) catalysts with zeolite-like structures using first-principles calculations have been investigated. The supramolecular system of silicoaluminophosphates consisting of inorganic cages with Brønsted acid sites and trapped organic compounds was used as a catalyst in the MTO reaction. To study the structure-property relationship in more detail, the effect of acidity and cage size of different types of SAPOs (SAPO-18, SAPO-34, and SAPO-17 with CHA, AEI, and ERI structures, respectively) in the aromatic cycle of hydrocarbon pool mechanism was investigated. The differences in reaction barriers can be explained by the cage size, pore topology, and environment of framework protons of materials. Product selectivity was controlled by using cavity-type zeolite, the steric constraint of the cavity for the formation of critical intermediates, and acidic strength. The results show that ethylene selectivity increases as the cavity size decreases, and the elliptical pore size of the structures decreases, thereby decreasing the acidity of the zeolite structure, leading to an increase in propylene selectivity. SAPO-18 exhibits the longest reaction lifetime and has the highest amount of carbonaceous material after reaction completion. SAPO-17 with small pore and cavity size is selective to ethylene, although it shows a rapid catalyst deactivation rate.

The Functionalization of PES/SAPO-34 Mixed Matrix Membrane with [emim][Tf2N] Ionic Liquid to Improve CO2/N2 Separation Properties

The use of ionic liquid [emim][Tf2N] as an additive in polyethersulphone (PES) and nano-sized silico-aluminophosphate-34 (SAPO-34) mixed matrix membrane was studied through the incorporation of different amounts of [emim][Tf2N] in the membrane composition, as presented in this work, varying from 10 to 40 wt%. Through gas permeation tests using CO2 and N2, the membrane composition containing 20 wt% [emim][Tf2N] led to the highest increase in CO2 permeability and CO2/N2 selectivity. The use of low concentrations of additive (10–20 wt%) promoted a state called antiplasticization; in this state, the permeability was even more regulated by the kinetic diameter of the species which, in this work, permitted achieving a higher CO2/N2 selectivity while increasing the CO2 permeability until an optimal condition. [emim][Tf2N] also promoted a better dispersion of SAPO-34 particles and an increase in the flexibility of the polymeric matrix when compared to a film with the same composition without [emim][Tf2N]. Moreover, the characterizations corroborated that the inclusion of [emim][Tf2N] increased the zeolite dispersion and improved the polymer/zeolite compatibility and membrane flexibility, characterized by a decrease in glass transition temperature, which helped in the fabrication process while presenting a similar thermal resistance and hydrophilicity as neat PES membrane, without affecting the membrane structure, as indicated by FTIR and a contact angle analysis.

Plasma Catalytic Conversion of Nitrogen and Hydrogen to Ammonia Over Silico Alumino Phosphate (SAPO) Zeolites

Plasma Catalytic Conversion of Nitrogen and Hydrogen to Ammonia Over Silico Alumino Phosphate (SAPO) Zeolites

Low-silica Cu-CHA Zeolite Enriched with Al Pairs Transcribed from Silicoaluminophosphate Seed: Synthesis and Ammonia Selective Catalytic Reduction Performance.

Cu-exchanged low-silica CHA zeolites (Si/Al ≤ 4) synthesized without organic templates are promising candidate catalysts for ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR), but their practical application is restricted due to the low hydrothermal stability. Here, inspired by the transcription from duplex DNA to RNA, we synthesized Al pairs enriched low-silica CHA zeolite (CHA-SPAEI, Si/Al = 3.7) by using silicoaluminophosphate (SAPO) featured by strict alternation of -Al-O-P(Si)-O-Al-O- tetrahedra as seed. The proportion of Al pairs in CHA-SPAEI is 78%, which is much higher than that in the conventional low-silica CHA (CHA-LS, 52%). After hydrothermal ageing at 800 °C for 6 h, Cu-exchanged CHA-SPAEI shows NO conversion above 90% within 225-500 °C under a gas hourly space velocity of 200,000 h-1, which is much better than that of Cu-exchanged CHA-LS. The spatial close proximity of Al pairs in CHA-SPAEI is confirmed by the 27Al double-quantum single-quantum two-dimensional NMR analyses. The strict -P(Si)-O-Al-O-P(Si)-O- sequence in the fragments from the dissolution of SAPO seed promotes the Al pairs with the -Al-O-Si-O-Al-O- sequence via a transcription process. The utilization of aluminophosphate-based zeolites as seeds opens up a new avenue for the regulation of the Al distribution in zeolites.

Low‐silica Cu‐CHA Zeolite Enriched with Al Pairs Transcribed from Silicoaluminophosphate Seed: Synthesis and Ammonia Selective Catalytic Reduction Performance

AbstractCu‐exchanged low‐silica CHA zeolites (Si/Al≤4) synthesized without organic templates are promising candidate catalysts for ammonia selective catalytic reduction of nitrogen oxides (NH3‐SCR), but their practical application is restricted due to the low hydrothermal stability. Here, inspired by the transcription from duplex DNA to RNA, we synthesized Al pairs enriched low‐silica CHA zeolite (CHA‐SPAEI, Si/Al=3.7) by using silicoaluminophosphate (SAPO) featured by strict alternation of −Al−O−P(Si)−O−Al−O− tetrahedra as seed. The proportion of Al pairs in CHA−SPAEI is 78 %, which is much higher than that in the conventional low‐silica CHA (CHA‐LS, 52 %). After hydrothermal ageing at 800 °C for 6 h, Cu‐exchanged CHA‐SPAEI shows NO conversion above 90 % within 225–500 °C under a gas hourly space velocity of 200,000 h−1, which is much better than that of Cu‐exchanged CHA‐LS. The spatial close proximity of Al pairs in CHA‐SPAEI is confirmed by the 27Al double‐quantum single‐quantum two‐dimensional NMR analyses. The strict −P(Si)−O−Al−O−P(Si)−O‐ sequence in the fragments from the dissolution of SAPO seed promotes the Al pairs with the −Al−O−Si−O−Al−O− sequence via a transcription process. The utilization of aluminophosphate‐based zeolites as seeds opens up a new avenue for the regulation of the Al distribution in zeolites.

Conquering the solubility barrier of di-n-octylamine as structure-directing agent in hierarchical silicoaluminophosphate synthesis by using phase-transfer synthesis

Silicoaluminophosphates (SAPOs) are a class of microporous solids employed as solid acid catalysts or adsorbents in petrochemical and coal chemical processes, which are often generated in hydrothermal synthesis using water-soluble organic structure-directing agents (OSDA), resulting in the formation of micron sized, rod-like crystals running along pore-channel direction for unidimensional SAPOs. The use of a low-cost, barely soluble di-n-octylamine (DOA) as OSDA in a water/toluene biphasic media to crystallize SAPO-31 is reported. The product is disclosed to be a hierarchically porous material built up of self-assembled nanocrystals (∼110 nm) with enhanced diffusion property and preserved acidity comparable to that of a standard sample derived using di-n-hexylamine as OSDA. The formation is mediated by the formation of a Pickering emulsion structure that favors crystal nucleation over growth, thus overcoming the solubility barrier of candidate OSDA. The obtained material exhibited higher isomer yield in catalytic hydroisomerization of n-dodecane compared to the control sample. Product distribution analysis discloses that mono-branched isomers are generated mainly inside the micropores, while di-branched isomers are produced via key-lock catalysis on pore-mouth sites. The increase in isomerization selectivity is attributed to the formation of more mono-branched isomers and their lowered cracking probability associated with enhanced diffusion property. This new synthetic protocol expands the toolkit of candidate OSDAs towards hydrophobic molecules and exemplifies porogen-free fabrication of hierarchical SAPOs. The uncovered origin of shape-selectivity offers a guidance towards manipulating hydroisomerization product slates.

CO2 captured by silicoaluminophosphate (SAPO) zeotypes

Excess carbon dioxide (CO2) in the atmosphere is causing great harm to the environment. Silicoaluminophosphate (SAPO) zeotypes have attracted great attention in CO2 capture. In this review, we comprehensively summarized and discussed the advances in the CO2 adsorption by SAPO zeotypes, the factors affecting the CO2 capture such as topologies, cation types, and amine modifications, and the interaction between the H2O, SOx, and NOx and the framework of SAPOs as well as their influence on the CO2 adsorption performance. At the end of the review, we raised the key challenges, current trends in the development of SAPO zeotypes, future research directions, and possible solutions to achieve the deployment of effective SAPO materials in CO2 capture.

Organic template-free synthesis of K-SAPO-34 zeolite for efficient CO2 separation

Silicoaluminophosphate (SAPO) zeolites as one of the most important members of zeolitic materials, providing equally high adsorption capacity of CO2 as aluminosilicate zeolites, can be used as promising candidate materials for selective adsorption and separation of CO2. In this work, desirable from both economic and environmental viewpoints, a green and cost-effective one step synthesis strategy of K-SAPO-34 zeolite was developed in a wholly inorganic system under hydrothermal conditions. The pure K-SAPO-34 crystals can be obtained in a wide range of initial gel compositions. The as-synthesized K-SAPO-34 has high proportion of framework Si (Si / (Si + Al + P) = 0.37) and framework charge density ((Al – P) / (Si + Al + P) = 0.36). Because the presence of silica islands in K-SAPO-34 framework leads to a decrease in the number of extra framework cations per unit cell, the K-SAPO-34 in this work has a high CO2 adsorption capacity (64.1 cm3/g) than the conventional K-SAPO-34 prepared by ion-exchange (1.9 cm3/g). Moreover, the K-SAPO-34 in this work has a superior separation performance for CO2/N2 and CO2/CH4.

Molecular Simulation of Biobutanol Recovery Using LTA and CHA Zeolite Nanosheets with an External Surface

LTA and CHA zeolites with silica and silico-alumino-phosphate (SAPO) composition have been recently proposed by Van der Perre et al. (ChemSusChem2017,10, 2968) to recover butanol from a mixture of butanol, ethanol, acetone, and water. Such a mixture is obtained from biomass fermentation, which is a sustainable alternative for butanol production. Here, we establish a computational methodology to simulate the three-step process, consisting of selective adsorption of butanol, with some ethanol and water, from the initial mixture, using a silica LTA nanosheet; desorption of the adsorbed mixture by increasing the temperature; and adsorption of the desorbed products in a CHA (silica or SAPO) nanosheet. A combination of Monte Carlo simulations, allowing to reproduce or predict single- or multiple-component gas adsorption isotherms, and molecular dynamics, giving the time evolution of the selective adsorption and desorption processes, have been used to model the experimental setup. A recently parameterized force field of general use for zeolites, AlPOs, and SAPOs has been tuned up in order to describe all interactions arising from this model, allowing to capture the thermodynamics of the system, the flexibility of the frameworks, and the effects of surface and bulk in the permeability of the nanosheets.

Alternative acid catalysts for the stable and selective direct conversion of CO2/CO mixtures into light olefins

Different acid catalysts (silicoaluminophosphates (SAPOs) -34, −18, and − 11, and HZSM-5 zeolite) were tested as components of In2O3-ZrO2/acid tandem catalysts in the direct synthesis of light olefins by hydrogenation of CO2, CO and their mixture. The conversion and olefins yield and selectivity evidence that the presence of the large amount of strongly acidic sites in SAPO-34 favors the extent of the reaction mechanism with methanol as intermediate, minimizing secondary methanation reactions. In addition, the shape selectivity of SAPO-34 boosts olefins selectivity (mainly of propylene), limiting the extent of the secondary reactions for the formation of other hydrocarbons. Using SAPOs as acid catalysts enhances olefins selectivity when co-feeding CO2 with CO. Despite all tandem catalysts undergo deactivation by coke deposition (mostly in the acid catalyst), a pseudo-steady state of stable remaining activity is acquired. From the study of the coke nature, soft and hard coke were discerned. For the complete regeneration of the SAPO-34 in the tandem catalyst, the stripping of the soft coke is not sufficient and the combustion at 500 °C of the hard coke (little developed) deposited on the micropores is required.

Synthesis of Hierarchical Silicoaluminophosphate (SAPO) Molecular Sieves by Post‐Synthetic Modification and Their Catalytic Application

AbstractSilicoaluminophosphates SAPO‐34 (CHA topology) and SAPO‐5 (AFI topology) molecular sieves are selectively prepared by hydrothermal method using templates tetraethylammonium hydroxide and triethylamine, respectively and suitably modified. Hierarchical SAPO‐34 & SAPO‐5 catalysts were post‐synthetically prepared by facile acid (HNO3 and HCl) and base (NaOH) etching. X‐ray diffraction (XRD), Thermogravimetric analysis (TG), FT‐IR and Scanning Electron Microscope (SEM) were employed to investigate structural changes of the molecular sieves. Hierarchical SAPO‐34 & SAPO‐5 were used for the ring opening reaction of propylene oxide with morpholine producing the corresponding β‐amino alcohols, and catalytic activity of the acid‐treated SAPO‐34 & SAPO‐5 was compared with that of alkali treated counterparts. The catalytic performance of the acid treated materials was found better than the base treated one. 91 % conversion was achieved using acid treated SAPO‐34, whereas 74 % conversion was achieved using alkali treated SAPO‐34.

Recent Progress of SAPO-34 Zeolite Membranes for CO2 Separation: A Review.

In the zeolite family, the silicoaluminophosphate (SAPO)-34 zeolite has a unique chemical structure, distinctive pore size, adsorption characteristics, as well as chemical and thermal stability, and recently, has attracted much research attention. Increasing global carbon dioxide (CO2) emissions pose a serious environmental threat to humans, animals, plants, and the entire environment. This mini-review summarizes the role of SAPO-34 zeolite membranes, including mixed matrix membranes (MMMs) and pure SAPO-34 membranes in CO2 separation. Specifically, this paper summarizes significant developments in SAPO-34 membranes for CO2 removal from air and natural gas. Consideration is given to a variety of successes in SAPO-34 membranes, and future ideas are described in detail to foresee how SAPO-34 could be employed to mitigate greenhouse gas emissions. We hope that this study will serve as a detailed guide to the use of SAPO-34 membranes in industrial CO2 separation.

A Review on SAPO-34 Zeolite Materials for CO2 Capture and Conversion.

Among several known zeolites, silicoaluminophosphate (SAPO)-34 zeolite exhibits a distinct chemical structure, unique pore size distribution, and chemical, thermal, and ion exchange capabilities, which have recently attracted considerable research attention. Global carbon dioxide (CO2 ) emissions are a serious environmental issue. Current atmospheric CO2 level exceeds 414 parts per million (ppm), which greatly influences humans, fauna, flora, and the ecosystem as a whole. Zeolites play a vital role in CO2 removal, recycling, and utilization. This review summarizes the properties of the SAPO-34 zeolite and its role in CO2 capture and separation from air and natural gas. In addition, due to their high thermal stability and catalytic nature, CO2 conversions into valuable products over single metal, bi-metallic, and tri-metallic catalysts and their oxides supported on SAPO-34 were also summarized. Considering these accomplishments, substantial problems related to SAPO-34 are discussed, and future recommendations are offered in detail to predict how SAPO-34 could be employed for greenhouse gas mitigation.

Post synthesis of hierarchical SAPO-34 via citric acid etching: Mechanism of selective desilication

Realizing the controllable adjustment on textural and acid properties of silicoaluminophosphates (SAPOs) is crucial for obtaining excellent-performance catalyst via acid-etching method. However, there are difficulties due to the lack of understanding of the mechanism of selective desilication for acid etching. In this paper, parent SAPO-34 were exposed to a series citric acid solutions aimed to investigate the influence of acid concentration on the physicochemical properties of the resulted SAPO-34 and elucidate the mechanism of selective desilication. The results showed that citric acid concentration plays a decisive role in efficiently manufacturing hierarchical pore structure and selectively extracting specific Si species. The diverse acid resistance of Si–O–Si and Si–O–Al bonds near or in different Si species (Si islands and isolated Si(4Al)) is responsible for the selective desilication of citric acid. The coupling effects of the differential distribution of Si species in parent SAPO-34 and the selective desilication influence the generation of the secondary pore and the strength and amount of acid sites of the resulted SAPO-34. Compared to the parent SAPO-34, the acid-treated SAPO-34 exhibited a longer lifetime and a higher selectivity for light olefins (ethylene and propylene) when used in methanol-to-olefins (MTO) reaction. • Hierarchical SAPO-34 was obtained via post-synthesis citric acid etching • The mechanism of selective desilication of citric acid was proposed • The diverse acid resistance of Si–O–Al bond resulted in selective desilication • The selective desilication influences the formation of secondary pore of SAPO-34

Investigation into the crystallization of molecular sieve DNL-6

Crystallization of DNL-6, a silicoaluminophosphate (SAPO) based molecular sieve with the RHO topology, was investigated under both the hydrothermal synthesis (HTS) and dry-gel conversion (DGC) conditions. Crystallization of DNL-6 under the HTS conditions is rather fast. But a combination of crystallization under the DGC conditions and reducing reaction temperature slow down the reactions, allowing for intermediates to be captured. Under the DGC conditions, DNL-6 crystallizes through a semi-crystalline layered phase. The nature of this intermediate is aluminophosphate (AlPO) rather than SAPO with most P atoms having a local environment of P(–O–Al)3(OH). The surfactant (cetyltrimethylammonium chloride) used for synthesis appears to be part of the layered intermediate. Si is directly incorporated in the DNL-6 framework via SM II mechanism when the semi-crystalline AlPO phase is transforming to DNL-6 with the assistance of a very small amount of water. Both the structure directing agent and the surfactant play a role in the formation of DNL-6, as they were found within the final synthesized products. SEM data show that hydrothermal synthesis produces a much more crystalline product. The facts that the semi-crystalline layered phase was also observed in the powder X-ray diffraction patterns of the solid samples obtained under the HTS conditions and that the evolution of the local structure around P and Al in the intermediate phases are similar imply that under the reaction conditions employed in the present study, the formation pathways of DNL-6 under the HTS and DGC conditions appear to have some similarities.