SAPO-34 Pores Research Articles

Effects of ZrO2 crystalline phase on oxygen vacancy of GaZr oxides and their properties for CO2 hydrogenation to light olefins

A bifunctional catalyst, comprising GaZr oxide and SAPO-34 zeolite, manifests enhanced catalytic activity in CO2 hydrogenation to light olefins; nonetheless, the comprehensive analysis of the pivotal role played by the underlying structure of ZrO2 in Ga-Zr oxide has not been investigated. Herein, different crystalline structures of ZrO2 were prepared by the co-precipitation method and adopted as a support to deposit Ga to obtain ZrO2 with different ratios of monoclinic ZrO2 (m-ZrO2) to tetragonal ZrO2 (t-ZrO2) in GaZr oxides for CO2 hydrogenation to light olefins. Various characterizations demonstrated that the interface between Ga and the mixed phase of (m-t) ZrO2 produces more oxygen vacancies which favors the adsorption and activation of CO2, and the larger specific surface area and stronger H2 adsorption and dissociation capacity promote CO2 conversion. Interestingly, the GaZr oxide with high m-ZrO2 content exhibits superior catalytic activity than the GaZr oxide with high content of t-ZrO2. The highest light olefins yield (9.0%) and selectivity (77.9%) (CO free) with 27.9% CO2 conversion was achieved. In-situ DRIFT spectra further elaborated that the GaZr oxides with different ZrO2 crystalline phases follow the same reaction pathway to hydrogenate CO2 first to HCOO* and then to CH3O* on GaZr oxide surface. While compared with sole ZrO2, the introduction of Ga significantly promotes the hydrogenation of HCOO* to CH3O*, acting as a crucial reaction intermediate that subsequently diffuses into SAPO-34 pores to enhance the desired light olefins selectivity.

Sono-solvothermal decoration of pore size controlled SAPO-34 by nano-ceria for green fuel production via esterification reaction

In this research, the SAPO-34 nanocatalyst has been improved in order to produce biodiesel as a green fuel. According to large molecules of free fatty acids (FFAs) and difficulty in the diffusion of these molecules through small pores of SAPO-34, the active charcoal secondary template was used to create meso and macro pores. Also, in order to increase acidic strength and density of active sites, CeO2 particles were accommodated to the structure of mesopores SAPO-34 by sono-solvothermal treatment and using ultrasound irradiation powers of 0, 100, 200 and 300 W. The synthesized catalysts were evaluated by FESEM, FTIR, TPD-NH3, HRTEM, XRD, and BET-BJH analyses. Results of characterization and reactor tests indicate that increasing the ultrasound irradiation power from 0 to 300 W in the synthesis stage has a significant effect on improving nanocatalysts properties and their performance in producing biodiesel, such that the conversion percentage in esterification reaction increased from 77 to 94%. The BET analysis showed, firstly, that using active charcoal leads to an increase of mean pore diameter of SAPO-34 from 0.5 to 3.4 nm, and secondly, that by using ultrasound irradiation power of up to 300 W, the pore volume in meso and macro range increases by 2.5 times. Results of the reactor test for the Ce/MSAPO-34 catalyst revealed that increasing the temperature, reaction time, and mole ratio of methanol to oil leads to an increase in conversion of oleic acid to biodiesel. Studying the kinetics of esterification reaction showed that using ultrasound irradiation powers of 100, 200, and 300 W increased the reaction rate by 20%, 30%, and 40%, respectively.

Texture evolution of mesoporous SAPO-34 via a hard-templating sono-hydrothermal method for biodiesel production: Influence of carbon materials on nanocatalyst design

To solve the challenges of diffusion of large molecules of oil into the SAPO-34 pores, secondary carbon and polymeric templates are used in sonohydrothermal synthesis method. Structure of the catalysts was estimated by BET, XRD, TPD, EDX, FESEM, HRTEM, and FTIR analyses. The best pore distribution was obtained for the catalyst synthesized by charcoal active, in which the conversion was 83 % for the esterification reaction of oleic acid. To increase the activity of this catalyst, 5% CeO2 was accommodated on it by sono-solvothermal method which resulted in 94 % conversion. Kinetic studies showed that for all synthesized catalysts, the degree of esterification reaction was of the first order and CeO2 loaded sample had the highest rate constant, equal to 0.0013 min−1. By increasing the pore radius and acidic strength of the catalyst, the reaction rate constant increases due to the overcoming the penetration of large oil molecules and increased acidic activity.

Simultaneous Desulfurization and Denitrification on the SAPO-34@Al2O3 Core–Shell Structure Adsorbent

The SAPO-34@Al2O3 core–shell structure adsorbent was designed, prepared, and used to simultaneous desulfurization and denitrification. The SAPO-34@Al2O3 core–shell structure adsorbent showed better adsorption performance than SAPO-34, Al2O3, and SAPO-34 + Al2O3 for the core–shell structure. SAPO-34 was wrapped in irregularly shaped Al2O3, and the distribution of Al2O3 on the surface of SAPO-34 was uneven. The preparation of the core–shell structure presents an additional advantage of increasing the specific surface area. In comparison to MnOx and Fe2O3, SAPO-34 as the core material showed the best adsorption performance for the surface properties and pore structure of SAPO-34. SO2 and NO adsorption were mainly chemical adsorption, and Al2O3 played an important role in the adsorption process. NO and SO2 would be catalytically oxidized to NO2 and SO3 by Al2O3, and then most SO3 and a small amount of NO2 were adsorbed by shell material (Al2O3). The surface properties and pore structure of SAPO-34 made NO2 adsorb, while SO2 was blocked out of the core material. Al2O3 on the surface and pores of SAPO-34 could provide chemical adsorption sites, and the products of SO2 and NO adsorption were sulfate and nitrate.

Effect of toluene adsorption on permeation through SAPO-34 membranes

When 0.65 mol% toluene was added to SAPO-34 membrane feeds (172-kPa feed pressure), the CO2 and N2 permeances decreased by approximately 50%, even though toluene is too large to diffuse into the SAPO-34 pores. Single-component adsorption isotherms for CO2 and toluene were used to fit a dual-site Langmuir model to estimate external surface coverages. The lower CO2 permeances qualitatively correlated with the reduced CO2 surface coverages. Changes in permeances for 10-kPa feed pressures were also measured so that surface coverages could be estimated more accurately using ideal adsorbed solution theory. The estimated decreases in CO2 coverages on the membrane surface due to competitively-adsorbed toluene reduced the trans-membrane CO2 driving force and accounted for the observed decreases in permeance.

Role of Manganese Oxide in Syngas Conversion to Light Olefins

The key of syngas (a mixture of CO and H2) chemistry lies in controlled dissociative activation of CO and C–C coupling. We demonstrate here that a bifunctional catalyst of partially reducible manganese oxide in combination with SAPO-34 catalyzes the selective formation of light olefins, which validates the generality of the OX-ZEO (oxide-zeolite) concept for syngas conversion. Results from in situ ambient-pressure X-ray photoelectron spectroscopy, infrared spectroscopy, and temperature-programmed surface reactions reveal the critical role of oxygen vacancies on the oxide surface, where CO dissociates and is converted into surface carbonate and carbon species. They are converted to CO2 and CHx in the presence of H2. The limited C–C coupling and hydrogenation activities of MnO enable the reaction selectivity to be controlled by the confined pores of SAPO-34. Thus, a selectivity of light olefins up to 80% is achieved, far beyond the limitation of Anderson–Shultz–Flory distribution. These findings open up pos...

Increasing H2/N2 separation selectivity in CHA zeolite membranes by adding a third gas

Adding propane or n-butane in low concentrations to a H2/N2 feed increases the H2/N2 separation selectivity in SAPO-34 membranes by decreasing the N2 permeance more than the H2 permeance. More than two days were required to reach steady state because propane and n-butane diffuse slowly into the zeolite pores, but they adsorb strongly and thus decrease H2 and N2 loadings. Although the loading of the more-weakly adsorbed H2 is expected to decrease more, the N2 permeance decreased more than the H2 permeance apparently because the diffusivity of the larger N2 molecule decreased more. Propane more than doubled the H2/N2 selectivity at 298K, but its effect on selectivity decreased as the temperature increased because the propane loading in the SAPO-34 pores decreased. Ethane also decreased H2 and N2 permeances, but it did not change H2/N2 selectivity. As with propane, ethane decreased H2 loadings more than N2 loadings, and decreased N2 diffusivity more than H2 diffusivity. However, the smaller ethane had less effect on the N2 diffusivity than did propane, so the net effect was that the selectivity did not change. The effects of alkane adsorption in SAPO-34 membranes were reversed when the alkanes were removed from the feed.

Measuring Mixture Adsorption by Temperature-Programmed Desorption

Temperature-programmed desorption (TPD) was used to measure single-component and mixture adsorption of CO2 and C3H8 in zeolite 13X and SAPO-34 crystals. The TPD was carried out with the adsorbing gases in the feed stream so that adsorption could be determined for weakly adsorbing molecules. The single-component loadings measured by TPD and by volumetric adsorption were within 5% of each other. Carbon dioxide preferentially adsorbs on zeolite 13X from CO2/C3H8 mixtures, whereas C3H8 preferentially adsorbs on SAPO-34. Adsorption equilibrium was quickly reached on zeolite 13X. However, more than 12 h were required to obtain equilibrium for CO2/C3H8 mixtures in SAPO-34 because C3H8 diffuses much slower than CO2 in SAPO-34 pores, but adsorbs more strongly and thus displaces half of the CO2 that initially adsorbs. This behavior helps explain the slow approach to equilibrium observed for CO2/CH4 separations with SAPO-34 membranes when C3H8 was added to the feed.

Separating molecules by size in SAPO-34 membranes

SAPO-34 membranes that have CO2/CH4 separation selectivities of ~100 at 295K and pressures up to 4.6MPa also show high molecular sieving separations of mixtures containing i-butane (0.50-nm kinetic diameter) or CF4 (0.47nm). The separation selectivities were 15,000–20,000 for CO2/i-butane from 295 to 420K, and 12,000–20,000 for CO2/CF4 for feed pressures up to 0.5MPa at 295K. These may be the highest gas-phase separation selectivities reported for zeolite membranes, and they are likely lower limits because of concentration polarization, leakage around O-rings, and i-butane diffusion through the silicone O-rings. The low defect concentration was also confirmed by spatial defect distributions measured with i-butane. Thus, less than 1% of the CH4 permeation during CO2/CH4 separations is through defects larger than 0.47nm. Isobutane, even though it is too large to adsorb in SAPO-34 pores (0.38-nm diameter), appeared to inhibit CO2 permeation by adsorbing on the external surface. These SAPO-34 membranes also separated H2/i-butane with lower limits on selectivities of 6000 at 295K and 10,000–14,000 at 423K, and they were also selective for He/CH4 separation (selectivity=20) and H2/CH4 separation (selectivity=35–45). In these separations, both He and H2 appear to speed up CH4 permeation, as expected from molecular simulations reported previously. Also, CH4 appears to slow diffusion of CO2, He, and H2.

Mechanisms of the Deactivation of SAPO-34 Materials with Different Crystal Sizes Applied as MTO Catalysts

SAPO-34 materials with comparable Brønsted acid site density but different crystal sizes were applied as methanol-to-olefin (MTO) catalysts to elucidate the effect of the crystal size on their deactivation behaviors. 13C HPDEC MAS NMR, FTIR, and UV/vis spectroscopy were employed to monitor the formation and nature of organic deposits, and the densities of accessible Brønsted acid sites and active hydrocarbon-pool species were studied as a function of time-on-stream (TOS) by 1H MAS NMR spectroscopy. The above-mentioned spectroscopic methods gave a very complex picture of the deactivation mechanism consisting of a number of different steps. The most important of these steps is the formation of alkyl aromatics with large alkyl chains improving at first the olefin selectivity, but hindering the reactant diffusion after longer TOS. The hindered reactant diffusion leads to a surplus of retarded olefinic reaction products in the SAPO-34 pores accompanied by their oligomerization and the formation of polycyclic aromatics. Finally, these polycyclic aromatics are responsible for a total blocking of the SAPO-34 pores, making all catalytically active sites inside the pores nonaccessible for further reactants.

Preparation and characterization of SAPO-34 – Matrimid® 5218 mixed matrix membranes for CO2/CH4 separation

Different SAPO-34 zeolite loaded Matrimid® 5218 mixed matrix membranes (MMMs) were prepared by solution casting method and characterized using XRD and SEM analysis. Findings showed that semi crystalline neat polymer becomes more crystalline after thermal treatment at higher temperatures close to Matrimid® 5218 glass transition temperature. Furthermore, incorporation of crystalline filler particles of SAPO-34 zeolite resulted in more and more crystallinity of the MMMs. SEM images also exhibited acceptable contacts between the filler particles and the polymer chains. Permeation measurement showed that CO2 permeabilities and CO2/CH4 selectivities of the MMM with 20wt% loading of SAPO-34 zeolite particles up to 6.9 (Barrer) and 67, respectively. This can be attributed to size discrimination of SAPO-34 pores that falls between CO2 and CH4 kinetic diameters.

Coke deposition in the SAPO-34 membranes for examining the effects of zeolitic and non-zeolitic pathways on the permeation and separation properties in gas and vapor permeations

SAPO-34 membranes were hydrothermally synthesized using 75 nm-sized SAPO-34 crystals as seed crystals. The SAPO-34 membrane showed a high selectivity to water for water/2-propanol (H2O/IPA) vapor permeation (separation factor>3200). Selective coke deposition within the pores of SAPO-34 via methanol-to-olefins reaction was applied to examine the effects of zeolitic and non-zeolitic pathways on the separation properties using various SAPO-34 membranes with different qualities. The results imply that H2O molecules are selectively adsorbed and the permeation of IPA is blocked even through the non-zeolitic pores (less than 1 nm) of SAPO-34 membranes. The size and hydrophilicity of non-zeolitic pores are important factors for inhibiting the IPA permeation.

Spatially resolved gas permeation through SAPO-34 membranes

Abstract A technique to measure spatially resolved permeances through tubular membranes with a moveable mass spectrometer probe at the membrane surface is described. This technique was used to determine spatial permeance distributions for SAPO-34 zeolite membranes, alumina membrane supports, and a Vycor glass membrane. The distribution of local permeances provides more detailed information about membrane properties than global permeances. The CO 2 and CH 4 permeances of SAPO-34 membranes that had high permeances and selectivities at 4.6 MPa were not spatially uniform across the membrane area. Membranes prepared using Al(OH) 3 as the aluminum precursor had large regions that were nearly impermeable, whereas membranes prepared with Al( i -C 3 H 7 O) 3 were more uniform, and thus had higher overall permeances. Scanning electron micrographs indicated that these differences in permeance were not due to differences in membrane thickness, and spatially uniform permeance distributions for alumina supports suggest that they were not due to non-uniformities in the supports. Defect locations in the SAPO-34 layers were detected using i -butane permeation, because i -butane is too large to enter the SAPO-34 pores, and the defect concentrations were low. These spatially resolved measurements indicate the potential to increase both CO 2 /CH 4 selectivity and CO 2 permeance, and help understand the factors that affect membranes properties.

Adsorption and Separation of C1−C8 Alcohols on SAPO-34

Adsorption and separation of 1-alcohols were studied on the SAPO-34 molecular sieve, which is the catalyst of choice for the methanol-to-olefins (MTO) process. Vapor phase adsorption isotherms of methanol and ethanol were measured at 343 K using the gravimetric technique. Liquid phase isotherms of pure 1-alcohols and mixtures were obtained by batch adsorption measurements at room temperature. These experiments highlighted the occurrence of a window effect, giving rise to a strongly chain length-dependent adsorption and diffusion. Lower alcohols such as methanol and ethanol were able to fill the entire pore volume. Alcohols larger than 1-butanol only adsorb in very small amounts after 3 h, as their adsorption in the SAPO-34 pores is sterically hindered, which gives rise to diffusional limitations. Binary batch experiments showed preferential adsorption of short chain alcohols (i.e., ethanol) from longer chain molecules. Breakthrough separation experiments with several alcohol mixtures were performed at 298−473 K and varying flow rates from 0.1 to 4.0 mL/min. Separation of ethanol from hexanol could be achieved at room temperature. SAPO-34 is also able to selectively remove ethanol from 1-propanol at room temperature. An increase in temperature to 348 K, however, improves the separation as a result of the improved ethanol diffusivity. The observed effect can be used to separate short chain molecules from longer ones.

H2 Separation Using Defect-Free, Inorganic Composite Membranes

Defect-free, microporous Al(2)O(3)/SAPO-34 zeolite composite membranes were prepared by coating hydrothermally grown zeolite membranes with microporous alumina using molecular layer deposition. These inorganic composite membranes are highly efficient for H(2) separation: their highest H(2)/N(2) mixture selectivity was 1040, in contrast with selectivities of 8 for SAPO-34 membranes. The composite membranes were selective for H(2) for temperatures up to at least 473 K and feed pressures up to at least 1.5 MPa; at 473 K and 1.5 MPa, the H(2)/N(2) separation selectivity was 750. The H(2)/CO(2) separation selectivity was lower than the H(2)/N(2) selectivity and decreased slightly with increasing pressure; the selectivity was 20 at 473 K and 1.5 MPa. The high H(2) selectivity resulted either because most of the pores in the Al(2)O(3) layer were slightly smaller than 0.36 nm (the kinetic diameter of N(2)) or because the Al(2)O(3) layer slightly narrowed the SAPO-34 pore entrance. These composite membranes may represent a new class of inorganic membranes for gas separation.

Blocking defects in SAPO-34 membranes with cyclodextrin

The CO 2/CH 4 separation selectivity of SAPO-34 membranes increased by 20–150% after soaking them in 1–2% aqueous β-cyclodextrin (CD) solution and drying them at 473 K. The percentage increase was higher at higher pressures, and the selectivities increased more for membranes with more defects. The CD decreased the CO 2 fluxes at high pressures by about 10–20%, whereas CH 4 fluxes decreased by 30–60%. The flux of CF 4, which is too large to enter the SAPO-34 pores and thus only permeates through defects, decreased 80% for one membrane (5.7 MPa feed pressure) after CD deposition; this confirmed that defects were effectively blocked. Even before CD exposure, the CF 4 permeance at 2.8 MPa was only 0.25% of the CO 2 mixture permeance for that membrane. Permporometry also showed that CD blocked defects; the maximum defect size decreased from greater than 30 nm to less than 20 nm. About 91% of the overall defect area in the original membrane was from defects smaller than 8 nm, whereas 99% was for defects smaller than 2 nm after CD deposition. The membrane separation performance did not change during 3 days of continuous measurements, nor did it change after long term storage at 473 K in air.

Key role of the pore volume of zeolite for selective production of propylene from olefins

A plausible reaction mechanism for propylene (C(3)H(6)) production from ethylene (C(2)H(4)) was investigated, based on the amounts of effluent hydrocarbons and hydrocarbons produced in the pores of SAPO-34. Propylene was produced via an oligomerization-cracking mechanism. On the basis of this mechanism, the conversions of C(2)H(4), pentenes, and hexenes were examined. The catalytic performance was compared, in order to investigate the role of the pore volume of zeolites with 8-, 10-, and 12-membered rings in the selective production of C(3)H(6). The selectivity for C(3)H(6) was crucially dependent upon the pore volume of the zeolite. Highly selective production of C(3)H(6) from olefins (C(2)H(4), pentenes, and hexenes) can be accomplished by employing a new concept: adjusting the pore volume of a zeolite to accommodate the volume of an olefin and/or its carbenium cations, as opposed to a conventional molecular sieve approach. For example, an unimolecular cracking of pentenes into C(3)H(6) and C(2)H(4) involving primary cations can be controlled by the pore volume of a zeolite.

Carbon dioxide selective mixed-matrix membranes formulation and characterization using rubbery substituted polyphosphazene

Substituted polyphosphazene (PPZ) used in the formulation of mixed-matrix membranes conforms to make good interface contact with the molecular sieve particles due to its rubbery nature. The methoxy ethoxy ethanol (MEE) substituted polyphosphazene is CO 2 selective with CO 2/N 2 and CO 2/CH 4 ideal separation factors of 43 and 15, respectively, at 22 °C. This study shows the improvement of the CO 2 separation factor over light gases by formulating mixed-matrix membranes using silicoaluminophosphate (SAPO-34) and modified 2D layered aluminophosphate (ALPO). The synthesis and characterization of these molecular sieve materials is described. The CO 2/CH 4 and CO 2/N 2 ideal separation factors increase to 17 and 54 for the membrane formed by PPZ and SAPO-34. A more significant increase in separation factors is observed with decreasing membrane temperature. The pore blocking effect by CO 2 in the PPZ–SAPO mixed-matrix membranes was not observed even though this behavior was reported in literature for SAPO-34 molecular sieve membranes. This is potentially due to lower adsorption of CO 2 in the pores of SAPO-34 when dispersed in the continuous polymer phase as compared to SAPO-34 particles. The layered aluminophosphate zeolite was exfoliated to create particles with high aspect ratio that were dispersed in the PPZ polymer to form composite membranes. The CO 2/H 2 separation factor observed in these composite membranes was 12.0 as compared to a value of 8.5 for PPZ membrane at 22 °C. The structure of these membranes is expected to be CO 2 selective flakes with high aspect ratio dispersed in the polymer which retard the transport of less interactive gases resulting in the improvement of separation factors. A comparison of gas transport data between the two types of mixed-matrix membranes also provides information regarding the structure of these membranes.

Hydrogen purification using a SAPO-34 membrane

A SAPO-34 membrane separated CO 2/H 2 and H 2/CH 4 mixtures at feed pressures up to 1.7 MPa. Strong CO 2 adsorption inhibited H 2 adsorption and decreased H 2 permeances significantly, especially at low temperatures, so that CO 2 preferentially permeated and CO 2/H 2 selectivities were higher at low temperatures. At 253 K, CO 2/H 2 separation selectivities were greater than 100 with CO 2 permeances of 3 × 10 −8 mol m −2 s −1 Pa −1. The CO 2/H 2 separation exceeded the upper bounds (selectivity–permeability plot) for polymer membranes. The SAPO-34 membrane separated H 2 from CH 4 because CH 4 is close to the SAPO-34 pore size and has a lower diffusivity than H 2. The H 2/CH 4 separation selectivity had a small maximum with temperature, and decreased slightly with feed pressure and CH 4 feed concentration.

Modification of Zeolite Membranes for H2Separation by Catalytic Cracking of Methyldiethoxysilane

Boron-substituted ZSM-5 and SAPO-34 membranes were silylated by the catalytic cracking of methyldiethoxysilane (MDES) to increase their selectivity for H2 separation from light gases. The MDES reacted in the B-ZSM-5 pores and reduced their effective pore diameter, so that silylation significantly increased their H2 selectivity. The H2/CO2 separation selectivity at 473 K increased from 1.4 to 37, whereas the H2/CH4 separation selectivity increased from 1.6 to 33. However, silylation decreased the H2 permeances more than 1 order of magnitude in the B-ZSM-5 membranes. The H2 permeance and H2/CO2 and H2/CH4 separation selectivities increased with temperature. At 673 K, the H2 permeance was 1.0 × 10-7 mol·m-2·s-1·Pa-1 and the H2/CO2 separation selectivity was 47. Methyldiethoxysilane does not fit into SAPO-34 pores, but silylation apparently decreased the pore size of the nonzeolite pores in the SAPO-34 membranes. After silylation, the H2 permeances and H2/CO2 and H2/N2 separation selectivities were almost unc...