Silicoaluminophosphate Catalysts Research Articles

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.

Improvement of microporous silicoaluminophosphate properties by Fe and Ti insertion for photocatalytic hydrogen generation under visible light

Iron- and/or titanium-containing microporous silicoaluminophosphate catalysts have been synthesized, under hydrothermal conditions, using triethylamine structure directing, to obtain the aluminophosphate five phase (AFI). These solids were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), nitrogen adsorption measurements, UV–visible diffuse reflectance spectroscopy (UV–Vis-DRS) and X-ray photoelectron spectroscopy (XPS). The physico-chemical analyses have revealed that Fe- and/or Ti-containing materials have significant photocatalytic properties. Thus, they have been valorized in the photocatalytic hydrogen production from water. The reaction has been conducted, under visible radiations, and the process has been followed by photo-electrochemical characterization (PEC). Hydrogen is the only produced gas and the reaction takes place in the conduction band where water is reduced to hydrogen while the holes are entrapped using hole scavengers. The results show that our catalysts exhibit a remarkably high hydrogen evolution rate and remain stable during reuse up to at least five cycles.

Synthesis of hierarchical SAPO-34 catalysts modified with manganese nanoparticles for conversion of methanol to light olefins: a deactivation and regeneration study

The hierarchical type 34 silicoaluminophosphate (SAPO-34) catalysts containing manganese were synthesized in the presence of carbon nanotube as mesopore template and tested in the transformation of methanol to light olefin. Also, the regenerability of spent catalysts after methanol to olefins (MTO) reaction was studied. In order to investigate the effect of manganese on the framework and extra framework of SAPO-34 structure, manganese species were added to SAPO-34 by isomorphous substitution and impregnation methods. The physicochemical properties of modified hierarchical samples such as crystallinity, acidity, porosity and reducibility were strongly dependent on the metal addition method. The modified hierarchical samples showed superior performance in the terms of activity and longevity compared to unmodified hierarchical SAPO-34. Among the samples, hierarchical MnAPSO-34 catalyst prepared by isomorphous substitution exhibited the best performance in the MTO reaction. The characterization of the used hierarchical catalysts by TGA analysis revealed that the deposited coke on modified samples was lower compared to unmodified hierarchical SAPO-34. The regenerated modified hierarchical SAPO-34 samples showed more enhanced ethylene selectivity in the MTO reaction and longer lifetime due to the existence of more methylated naphthalenes and methylated benzenes in the residual coke after regeneration.

Hierarchical Silicoaluminophosphate Catalysts with Enhanced Hydroisomerization Selectivity by Directing the Orientated Assembly of Premanufactured Building Blocks

The ability to generate nanoscale zeolites and direct their assembly into hierarchical structures offers a promising way to maximize their diffusion-dependent catalytic performance. Herein, we report an orientated assembly strategy to construct hierarchical architectures of silicoaluminophosphates (SAPOs) by using prefabricated nanocrystallites as a precursor. Such a synthesis is enabled by interrupting the dry gel conversion process to prepare nanocrystallites, as crystal growth is shown to proceed predominantly by particle attachment. The orientation of assembly can be controlled to form either a three-dimensional, spongelike morphology or a two-dimensional “house-of-cards” structure, by modifying the additives. Structures with a high degree of control over crystal size, shape, architecture, pore network, and acidic properties are achieved. This versatile technique avoids the more tedious and expensive templating routes that have been proposed previously. The catalytic performance for the hydroisomeriza...

Effect of framework topology of SAPO catalysts on selectivity and deactivation profile in the methanol-to-olefins reaction

Catalytic performance in the methanol-to-olefins reaction was studied on four 8-ring silicoaluminophosphate (SAPO) molecular sieves with different framework topologies and cavity-windows of various dimensions: SAPO-35 (LEV), SAPO-56 (AFX), STA-7 (SAV), and SAPO-42 (LTA). The four catalysts were compared with the widely investigated SAPO-34 (CHA) and showed remarkable differences in lifetime and products selectivity. In particular, the STA-7 catalyst gave higher selectivity to C3–C4 olefins, while it had methanol conversion capacity similar to that of a SAPO-34 catalyst with similar crystal size and acidity. Organic species retained in the cavities of partially deactivated catalysts were analyzed at different times on stream. The changes in composition of trapped compounds were correlated with those of the effluent products to elucidate the reaction mechanism and the deactivation pathway. The differences in effluent product distribution and trapped coke species suggest that framework topology has a great influence not only on the stability of the materials but also on the products distribution.

Methanol to lower olefins conversion in a slurry reactor: Effects of acidity and crystal size of silicoaluminophosphate catalysts

Effects of the crystal size and acidity of silicoaluminophosphates on their catalytic behavior in the methanol to lower olefins conversion in a slurry reactor have been studied. It has been found that an increase in the crystal size of silicoaluminophosphate causes a reduction in its catalytic activity, which is due to an increase in diffusion limitations for both reagents and reaction products. An increase in the number of acid sites and their localization on the outer catalyst layer improve the time stability of silicoaluminophosphate performance and the selectivity of methanol conversion to lower olefins.

In-Situ Atomic Force Microscopy Study of the Dissolution of Nanoporous SAPO-34 and SAPO-18

In-situ atomic force microscopy has been used to investigate the dissolution behavior of industrially relevant silicoaluminophosphate catalysts SAPO-34 and SAPO-18. Spiral growth is prevalent on these materials and it is common for the spirals to be composites of multiple dislocation sources. The spirals dissolve via classical step retreat and the structure dissolves in a two-step process via unstable intermediates. The data support the proposition that the terminating surface of SAPO-34 is composed of double 6-rings. SAPO-34 and SAPO-18 both dissolve by removal of the same structural units with similar mechanisms.

Statistical Optimization for Production of Light Olefins in a Fluidized‐Bed Reactor

AbstractThe methanol‐to‐olefins reaction (MTO) was studied in a small‐scale fluidized‐bed reactor over synthesized silicoaluminophosphate (SAPO‐34) catalysts. Mesoporous nanocrystalline SAPO‐34 molecular sieves were synthesized hydrothermally by ultrasonic and microwave‐assisted aging processes in the presence of hexadecyltrimethylammonium bromide (CTAB) and tetradecyldimethyl(3‐trimethoxysilylpropyl)ammonium chloride (TPOAC) as surfactant agents. The Box‐Behnken experimental design method was applied to determine the optimum operating parameters of this process conducted in the fluidized‐bed reactor. The optimum conditions in terms of reaction temperature, ratio of inlet gas velocity to minimum fluidizing velocity, and MeOH weight fraction were evaluated.

Value addition to lignocellulosics and biomass-derived sugars: An insight into solid acid-based catalytic methods

For the synthesis of important platform chemicals such as sugars (xylose and arabinose) and furans (furfural and 5-hydroxymethylfurfural (HMF)) from carbohydrates (hemicellulose and fructose) solid acid catalysts are employed. Similarly, over solid acid catalysts, conversion of lignin into aromatic monomers is performed. It is observed that in the dehydration of fructose, because of higher hydrothermal stability, silicoaluminophosphate (SAPO) catalysts give better activity (78% HMF yield) compared with other solid acid catalysts (<63% HMF yield) at 175°C. Particularly, SAPO-44 catalyst can be reused at least 5 times with marginal decrease in the activity. Zeolite, HUSY (Si/Al = 15) is active in the conversion of isolated (pure) hemicellulose to produce 41% C5 sugars in water. The catalyst is also active in the selective conversion of hemicellulose from bagasse to yield 59% C5 sugars. It is possible to obtain high yields of furfural (54%) directly from bagasse if instead of water, water+toluene solvent system is used. Depolymerization of lignin using HUSY catalyst produced aromatic monomers with 60% yield at 250°C. A detailed catalyst characterization study is performed to understand the correlation between catalyst activity and morphology. To understand the effect of impurities present in the substrate over solid acid catalysts, metal-exchange study is carried out.

Spatial confinement effects of cage-type SAPO molecular sieves on product distribution and coke formation in methanol-to-olefin reaction

Three kinds of 8-membered ring silicoaluminophosphate (SAPO) molecular sieves with different cage structures, SAPO-34, SAPO-18 and SAPO-35, were employed in methanol-to-olefin (MTO) reaction. The main products over SAPO-34 and SAPO-18 were propene and butenes, whereas ethene and propene especially ethene were predominantly generated over SAPO-35. Coke species formation greatly depended on reaction temperature and varied systematically with cage size. The differences in production distribution and generated coke species in the MTO reaction suggest great spatial confinement effects imposed by cage structure of SAPO catalysts.

Efficient, Stable, and Reusable Silicoaluminophosphate for the One-Pot Production of Furfural from Hemicellulose

Development of stable, reusable, and water-tolerant solid acid catalysts in the conversion of polysaccharides to give value-added chemicals is vital because catalysts are prone to undergo morphological changes during the reactions. With the anticipation that silicoaluminophosphate (SAPO) catalysts will have higher hydrothermal stability, those were synthesized, characterized, and employed in a one-pot conversion of hemicellulose. SAPO-44 catalyst at 170 °C within 8 h could give 63% furfural yield with 88% mass balance and showed similar activity up to at least 8 catalytic cycles. The morphological studies revealed that SAPO catalysts having hydrophilic characteristics are stable under reaction conditions.

Influence of properties of SAPO's on the one-pot conversion of mono-, di- and poly-saccharides into 5-hydroxymethylfurfural

Synthesis of 5-hydroxymethylfurfural (5-HMF) from biomass derived mono- and poly-saccharides is gaining importance because of its usefulness in the preparation of important chemicals. In our work, we have synthesized several silicoaluminophosphate (SAPO) catalysts, and have shown that in the absence of any other pH modifying reagents, those are active in converting mono- and poly-saccharides into 5-HMF under biphasic reaction condition at 175 °C. Particularly, SAPO-44 catalyst showed the best activity in the conversion of fructose to yield 78% 5-HMF with 88% selectivity. On the contrary, all other catalysts showed lower yields (H-MOR: 63%, SAPO-5: 32%, 2DCT: 60%). Over SAPO-44, good yields for 5-HMF were observed when glucose (67%), maltose (57%), cellobiose (56%) and starch (68%) were used as substrates. Recycle study carried out with SAPO-44 catalyst in the fructose conversion reaction showed marginal decrease in the activity up to 3rd run and then afterwards constant activity was observed up to 5th run (1st: 78%, 2nd: 71%, 3rd: 66%, 4th: 65%, 5th: 65%). Catalyst characterizations revealed that SAPO catalysts have higher hydrophilic nature than H-MOR (Si/Al = 10) and hence it is postulated that this property may help in achieving better results. Further studies on the catalyst characterizations revealed that SAPO-44 undergoes modifications in its structure. However, ICP-OES data suggests that Al and/or P are not leached out in the solution indicating that change in local environment around elements is possible. The influence of acid amount, type of acid site etc. on the catalytic activity is discussed and found out that strong acid sites are required to boost the 5-HMF yields.

Methanol-to-Olefin Conversion on Silicoaluminophosphate Catalysts: Effect of Brønsted Acid Sites and Framework Structures

Silicoaluminophosphates (SAPO) with different framework structures and amounts of Bronsted acid sites, that is, SAPO-34, SAPO-41, SAPO-11, and SAPO-46, were synthesized and studied as catalysts for the methanol-to-olefin (MTO) conversion. Besides the well-known SAPO-34 catalyst, also SAPO-41 exhibits a good MTO performance with a methanol conversion of 100% and a selectivity to light olefins of about 70% at the reaction temperature of 723 K, which maintains up to a time-on-stream of 10 h. The Bronsted acid sites of silicoaluminophosphate catalysts employed in this study were characterized by means of 1H MAS NMR spectroscopy. These studies were performed with unloaded samples for studying the type and number of OH groups and after loading of ammonia for determining the amount of accessible Bronsted acid sites. The occluded organic species formed on the catalysts during the MTO reaction were analyzed by in situ UV/vis spectroscopy, thermogravimetry-differential thermal analysis (TG-DTA), and gas chromatogra...

FT-IR studies of the NO adsorption on rare earth (Eu/Gd/Tb) exchanged new generation silicoaluminophosphate catalysts of chabazite type

Novel rare earth exchanged silicoaluminophosphate catalysts (small pore, 0.43 nm) of chabazite type have been prepared. The rare earth elements europium (Eu), gadolinium (Gd) and terbium (Tb) were used for preparing the cation exchanged chabazite type SAPO-18 catalysts. Routine and sophisticated instrumental techniques were used for the material characterisation. The FT-IR technique was used to study the in situ interaction of NO on the rare earth exchanged SAPO-18 catalysts. The adsorption of NO on the Eu-, Gd-, Tb-exchanged SAPO-18 samples led to the formation of nitrous oxide, mono- and dinitrosyl complexes of Eu, Gd and Tb, nitrogen dioxide and nitrate species.

Comparing the Acidities of Microporous Aluminosilicate and Silico-Aluminophosphate Catalysts: A Combined Quantum Mechanics-Interatomic Potential Function Study

DFT-B3LYP calculations are performed on 4-ring models of aluminophosphates (AlPOs) and silico-aluminophosphates (SAPOs). The results are used to fit the parameters of ion pair shell model potential functions. The potentials obtained are tested in lattice energy minimizations for berlinite and the microporous materials AlPO-18, AlPO-40, AlPO-52, and VPI-5. Not only does the potential reproduce the observed structures (average error of the cell constants 1.3%), it also predicts vibrational frequencies over the whole frequency range equally well (maximum deviation 50 cm-1). The potential is used to predict the structures and properties of Bronsted acid sites in an aluminosilicate and a SAPO with the chabazite framework structure (HSSZ-13 and HSAPO-34). A new combined quantum mechanics-intermolecular potential function approach (QM-Pot) is used at the DFT level. Comparison is made with full periodic DFT calculations using plane wave basis sets. The deprotonation energies corrected for systematic errors of the methods used are 1 231-1 235 and 1 261-1 280 kJ/mol for HSSZ-13 and HSAPO-34, respectively. The same acid site has a lower acidity in a SAPO than in an aluminosilicate zeolite of the same structure.

Dehydrogenation of 2-picolyl alcohol to pyridine-2-car☐aldehyde over modified silico-aluminophosphate catalysts

The dehydrogenation of 2-picolyl alcohol was carried out over modified and unmodified amorphous/crystalline silico-aluminophosphate catalysts. The silico-aluminophosphates are mild and selective oxidation catalysts. The best catalytic performance is shown by Sb-K-SAPO (DPA) with the formation of 81.9% pyridine-2-car☐aldehyde. Pyridine and 2-picoline were also observed as byproducts.

Oxidation and ammoxidation of toluene and benzyl alcohol over silico-aluminophosphate and metal-silico-aluminophosphate catalysts

The silico-aluminophosphate (SAPO), vanadium-aluminophosphate (VAPO) and vanadium-silico-aluminophosphate (VSAPO) catalysts were synthesized by hydrothermal method. Depending on silicon and vanadium content, the as-synthesized molecular sieves were thermally stable or unstable up to 400–450°C. The catalysts were characterized by XRD, IR, TG and sorption methods. The oxidation and ammoxidation reactions of toluene and benzylalcohol were carried out over SAPO, VAPO and VSAPO catalysts. The catalysts were selective in the oxidation and ammoxidation of toluene. The yield of benzonitrile was 20–92 wt.-%. The vanadium and Sb 2O 3 modification enhances the yield of benzonitrile substantially. The benzoic acid was not observed in the products. In the ammoxidation of benzylalcohol, the yield of benzonitrile was 18%–74%. The selectivity of benzonitrile was comparatively lower. The reaction scheme is discussed. We report silico-aluminophosphate, VSAPO and TiSAPO, as-synthesized molecular sieves, as a new and novel class of oxidation and ammoxidation catalysts.

Ammoxidation of 3-picoline over vanadium-modified zeolites and crystalline/amorphous silicoaluminophosphate catalysts

The oxidation and ammoxidation of 3-picoline over vanadium-modified NaY, H-ZSM-5 (Si/Al = 30), crystalline SAPO (37), and crystalline/amorphous V-silicoaluminophosphate catalysts have been carried out. The yields of 3-cyanopyridine for V-ZSM-5, V-SAPO (37), and V-NaY were 62.6, 76.0, and 1 wt%, respectively. The aluminophosphate framework of SAPO enhances the yield of 3-cyanopyridine, and because of the shape selectivity the high boilers have been decreased in the product. We report vanadium-modified crystalline silicoaluminophosphate as a new class of oxidation and ammoxidation catalysts and compare them with amorphous materials.

Adsorption of methanol in zeolite, gallosilicate and SAPO catalysts

Abstract Methanol adsorption in zeolite, gallosilicate and silicoaluminophosphate (SAPO) catalysts has been investigated within the framework of ab initio molecular orbital calculations. Full optimization of all cluster models and their complexes has been carried out at the DZP/SCF level of theory. Physisorbed methanol and methoxonium cation complexed to the framework catalyst are found for SAPO catalysts, the latter complexes are observed only at high coverages, while only hydrogen-bonded physisorbed methanol complexes are obtained for gallosilicates and zeolitic catalysts. The conversion energy of the hydrogen-bonded physisorbed structure, H 3 SiOHAl(OH) 2 OPH 3 /[CH 3 OH] 2 to the methoxonium structure, H 3 SiOAl(OH) 2 OPH 3 ]/[CH 3 OH 2 + ][CH 3 OH], is about 6.69 kcal/mol. Comparison with hydrogen halides and related complexes of methanol shows that protonated SAPO/methanol is a very strong acid.

Formation of NH 4+ at the Brønsted site in SAPO catalysts

The catalytic properties of ammonia adsorption on silicoaluminophosphate (SAPO) clusters have been investigated within the framework of the ab initio self-consistent field method. Full optimization of strutures has been carried out at the DZ, DZP and TZ2P levels of theory. Two different types of ammonia adsorption on SAPO framework sites are proposed. In one of these the structures H 3SiOHA1(OH) 2OPH 3…NH 3 are stablilized on the bridging OH by a single site binding with an interaction energy of − 17.49 kcal/mol. The others is a type of the structure [H 3SiOA1(OH) 2OPH 3] [NH 4 + ], in which the ammonium cation forms two hydrogen bonds towards the unprotonated framework sites. Other possible structures like a “bifurcated” structure are less stable than the two H-bonded structures by about 0.48 and 0.1 kcal/mol at the DZP and TZ2P basis set levels respectively. This indicates the free rotation of the NH 4 + on the SAPO surface site at room temperature. The interaction energies for the structures [H 3SiOA1(OH) 2OPh 3] [NH 4 + are more stabe than for the structures H 3SiOhA1(OH) 2OPH 3…NH 3 by 0.5–1.36 kcal/mol depending on the basis sets. These calculated energy values are an inversion order from the zeolite/NH 3 complexes. Comparison of the SAPO complexes with hydrogen halides, silanol, and zeolite has demonstrated that the hydrogen-form SAPO is at least as acidic as zeolite.