Extra-framework Aluminum Research Articles

Understanding HZSM-5's shape-selectivity improvement through tiny autonomous dealumination

Abstract The decisive factor that determines HZSM-5's shape-selective capability is obscure due to the interference from zeolite acidity, pore size and crystal size. p-xylene selectivity was found to increase from 45% to 82% in toluene alkylation reaction with the same HZSM-5 catalyst but storage period difference. However, no instrument detectable zeolite acid sites strength, type and quantity changes were observed. NMR revealed slight progressive increase of extra-framework aluminum by dealumination, and nitrogen adsorption-desorption isotherm verified the tiny decrease of micropore volume. Pulse chromatography tests confirmed p-xylene diffusion resistance increase in HZSM-5's channels. The finding made it possible to clarify the interactions between shape-selectivity improvement, p-xylene diffusion resistance increase and extra-framework aluminum while excluding all the other interfering factors, indicating the diffusion resistance control of molecules in HZSM-5's channels is the key to improve its shape-selectivity.

Development of hierarchical and phosphorous-modified HZSM-5 zeolites by sequential alkaline/acid treatments and their catalytic performances for methanol-to-olefins

An efficient and simple method was proposed to synthesize hierarchical and phosphorous-modified HZSM-5 zeolites by sequential alkaline and acid treatment. In the first NaOH alkaline treatment, intracrystalline mesoporosity was introduced into the zeolites by the crystal defect formation and growth of crystal defects due to desilication. The excessive defect growth caused amorphization of the zeolites, dramatically decreasing their surface area and micropore volume. The aluminum-related debris resulting from the desilication deposited on the extra-framework positions and had a dramatic influence on the crystallinity and microporosity of the zeolites. In the subsequent H3PO4 treatment for the alkaline-treated HZSM-5, the deposited extra-framework aluminum species were selectively removed, thus increasing the micropore volume and surface area of the zeolites. Along with the dealumination, the phosphorous species were introduced into the framework to occupy the silicon sites, leading to a crystalline structure repair. The post-treatments could modify the structural and acidic properties of the HZSM-5 zeolite. Catalytic tests showed that the hierarchical and phosphorous-modified HZSM-5 zeolite exhibited higher methanol conversion and propylene selectivity in methanol-to-olefins reactions over a longer period compared with the parent HZSM-5.

Catalytic oligomerization of isobutyl alcohol to jet fuels over dealuminated zeolite Beta

Various dealuminum methods of zeolite Beta to generate the jet fuels by converting isobutyl alcohol has been investigated in detail. And related factors of the zeolite catalyst such as acid site, pore structure, and pore size which can influence reactive activity, were also studied utilizing XRD, SEM, XRF, Nitrogen adsorption-desorption, NH3-TPD, Py-FTIR, and 27Al solid-state NMR. Removal of extra-framework aluminum or part of framework aluminum by hydrochloric acid dealumination, is able to improve the diffusion property of the products in the channel of zeolite Beta via increasing the ratio of Lewis/Brønsted. Consequently, isobutyl alcohol can be quantitatively oligomerized over dealuminated zeolite Beta with the selectivity of C8−16 exceeding 50 % at a conversion of 98 %. Moreover, the deactivated catalyst can be easily regenerated by calcining it in flowing air. High conversion, high jet fuels selectivity, and facile regeneration make it an attractive potential catalyst for isobutyl alcohol oligomerization reaction.

Rapid and green synthesis of SAPO-11 for deoxygenation of stearic acid to produce bio-diesel fractions

A grinding synthesis approach is employed to prepare water containing gel. SAPO-11G was successfully prepared by a dry gel conversion method using water containing gel assisted by microwave radiation (120–180 °C, 2 h). Compared to hydrothermal system, approach employed in the present study show green characters such as conservation of energy and time, low cost of organic filler, and reducing wastewater discharge. Deoxygenation of stearic acid shows that Ni/SAPO-11 catalysts have a high selectivity to paraffin and a low yield to cracking products. Ni/SAPO-11G has a comparable activity (conversion> 95%) with Ni/SAPO-11H molecular sieves. Coke deposition in SAPO-11G is lower than that in SAPO-11H due to a lower concentration of extra-framework aluminum species (i.e., Lewis acid sites). These results propose a rapid way to prepare zeolites in an environmental friendly route and are important to produce bio-diesel fractions via a deoxygenation reaction. • A grinding synthesis method is employed to prepare water containing precursor. • SAPO-11 is prepared using dry gel conversion heated by microwave. • Ni/SAPO-11 catalysts exhibit a high selectivity to C15-18 paraffin in deoxygenation of stearic acid.

Effect of sulfosalicylic acid treatment on the properties of Beta zeolite and performance of NiW/Beta-based catalysts in hexadecane hydrocracking

Effect of sulfosalicylic acid treatment on the properties of Beta zeolite (BEA and BEA-SA) and performance of NiW/Beta-Al2O3 catalysts in hexadecane hydrocracking have been studied. The acid treatment had almost no effect on porous structure of Beta zeolite while it resulted in significant changes in acidity. The decrease in concentration of Brønsted (35 to 25 μmol/g) and Lewis acid sites (119 to 54 μmol/g) was observed that could be related to the removal of extra framework aluminum. NiW/BEA-Al2O3 catalyst having zeolite with higher acidity demonstrated higher activity in hexadecane hydrocracking. The hexadecane conversion was 54.5–95.6% and 32.4–93.9% for NiW/BEA-Al2O3 and NiW/BEA-SA-Al2O3, respectively. However, NiW/BEA-SA-Al2O3 demonstrated higher selectivity to target products (C5-C15 hydrocarbons and iso-C16) than NiW/BEA-Al2O3 (63.8 and 14.4 % vs. 62.4 and 4.5 %, respectively) at similar hexadecane conversion of ∼95 %. This can be attributed mainly to the effect of sulfosalicylic acid treatment on Beta zeolite acidity.

A Periodic DFT Study of the Synergistic Mechanisms between Extraframework Aluminum Species and Bro̷nsted Acid Sites in HY Zeolites

A series of model probes with different sizes and polarity, such as ammonia, pyridine, and thiophene, have been used to gain insight into the influence mechanisms of extraframework aluminum (EFAl) ...

Synthesis of all-silica hollow zeolites by selective demetallation

Exploiting the protective role of extra-framework aluminum species on the crystal rim, all-silica hollow zeolites are efficiently produced by demetallation.

Understanding structure-function relationships in HZSM-5 zeolite catalysts for photocatalytic oxidation of isopropyl alcohol

The HZSM-5 molecular sieves are a kind of special photocatalyst with photocatalytic activity, while their relationship between structure and function has been a matter of debate. Here, we report that for HZSM-5, the distinguished transformation of coordination environment derived from the removal of tetrahedral framework aluminium by thermal treatment will result in a considerable change in photocatalytic behaviours. The established NMR analysis combined with the theoretical calculations demonstrates the formation of extra-framework aluminium (EFAL) upon calcinations. This leads to a changed reaction path of photocatalytic oxidation of isopropyl alcohol in gas-phase over calcined HZSM-5 catalysts. DFT calculation results show the formed of EFAL species can increase the interaction energy between reactant molecules and zeolites which affords an outstanding photocatalytic reactivity and a controllable photocatalytic selectivity in the oxidation of isopropyl alcohol. This clearly indicates that the framework aluminium (FAL) species are responsible for acetone formation, while the EFAL species are apt to promote the deep mineralization of isopropyl alcohol to inorganic CO2. This study provides new understanding in the issue of photocatalysis for zeolite molecular sieves.

Selective catalytic reduction of NOx with methanol on H-ZSM-5: The effect of extra-framework aluminum

Abstract A series samples with different types of acid sites: Bronsted acid sites, Lewis acide sites, and extra-framework aluminum (EFAL) species were investigated in selective catalytic reduction of NOx with methanol (methanol-SCR). Comparing with Bronsted acid sites (BAS) and Lewis acid sites (LAS) present in zeolites, EFAL species exhibit much higher catalytic activity. EFAL species were confirmed in calcined HZSM-5 by FT-IR with pyridine and in-situ DRIFT. Reaction intermediates were investigated by in-situ DRIFT measurements, and the reaction mechanism was proposed. Formamide and methyl nitrite species was the key intermediates of methanol-SCR reaction and formamide further react with nitrous compounds to produce nitrogen and nitride oxide. EFAL species promoted the adsorption of methyl nitrite and nitromethane species and the formation of formamide.

Extra-Framework Aluminum Species of Zeolite that Surrogate the Growth of Metal Organic Framework from Zeolite Matrix.

Constructing a robust hybrid material with a porous inorganic and a porous organic framework is highly intriguing owing to its diverse functionality and porosity. However, the line of synthesis is not straightforward, since their nucleation and crystal growth processes are incompatible. Here, a simple method for the fabrication of hybrid zeolite/metal-organic framework of different framework structures is developed wherein the less-useful extra-framework aluminum species present in the zeolite surrogate the growth of metal organic framework (MOF) from the zeolite matrix in the presence of organic linkers of the corresponding MOF. An NMR study confirms that all the octahedral Al species are converted to Al-MOF. TGA analysis shows that 32 % Al of H-Beta is converted to Al-MOF. Furthermore, NH3 TPD analysis shows that most of the weak acid sites disappear but strong acid sites are preserved suggesting the utilization of weakly bound Al species of H-Beta in the growth of Al-MOF. The synthesis strategy is successfully demonstrated using H-Beta, H-ZSM-5, and H-Y zeolites for the growth of MIL-53 and MIL-96 MOFs from the zeolite matrix. This synthesis strategy enables application-based engineering of the framework structures, functionality, and porosity of the materials.

Pyridine assisted desilication of mordenite

Pyridine assisted alkaline desilication has been elaborated as new post-synthetic modification strategy of mordenite. The addition of pyridine during the desilication treatment revealed to impact materials properties through a dual action on textural and chemical properties; substantially different in comparison to the classical alkaline treatment or CTAB assisted desilication. As far as texture is concerned the use of pyridine permits to greatly modify the desilication scheme, hence yielding aggregates of smaller zeolite particles with substantially higher amount of external silanols. The presence of pyridine in the as-synthesized mordenites allows to in situ generate extra-framework aluminium (EFAL) species during final calcination step. Both of these features, smaller particle size and the presence of EFAL species improve catalytic properties considerably. Whilst the achievement of smaller particles reduces diffusion path length, the presence of EFAL species allow for exalting acidity and hence catalytic activity in n-hexane cracking and propene transformation.

Effect of Er(NO3)3 on thermal activation of kaolinite and alkaline reaction behavior of metakaolin

This study investigated the products of kaolinite thermal activation and the process of the alkaline reaction, using XRD, FTIR, 27Al MAS NMR, SEM, TEM and XPS methods; moreover, the effect of Er(NO3)3 addition on this process was analyzed. The results demonstrated that the addition of Er(NO3)3 enhanced the destruction of the microstructure in the thermal activation process of kaolinite, increased the disorder degree, and promoted the transformation of the thermal activation product from AlVI to AlV. With the progress of the alkali reaction and increase in the alkali concentration, the zeolite A phase was gradually transformed into a sodalite phase by dealumination. In this process, extra-framework aluminium derived from dealumination was enriched on the surface of the alkali reaction products. The addition of Er(NO3)3 accelerated the formation of zeolite and the transformation of the zeolite phase into the sodalite phase.

Ultra-deep adsorptive removal of thiophenic sulfur compounds from FCC gasoline over the specific active sites of CeHY zeolite

Abstract Adsorption desulfurization performance of NaY, HY and CeHY zeolites is evaluated in a miniature fixed-bed flow by model gasoline containing with thiophene, tetrahydrothiophene, 2-methylthiophene, benzothiophene or mixed sulfur compounds. The structural properties of adsorbents are characterized by XRD, N2-adsorption and XPS techniques. Adsorption desulfurization mechanisms of these sulfur compounds over the specific active sites of adsorbents as a major focus of this work, have been systematically investigated by using in situ FT-IR spectroscopy with single and double probing molecules. Desulfurization experimental results show that the CeHY adsorbent exhibits superior adsorption sulfur capacity at breakthrough point of zero sulfur for ultra-deep removal of each thiophenic sulfur compound, especially in the capture of aromatic 2-methylthiophene (about ca. 28.6 mgS/gadsorbent). The results of in situ FT-IR with single probing molecule demonstrate an important finding that high oligomerization ability of thiophene or 2-methylthiophene on the CeHY can promote the breakthrough adsorption sulfur capacity, mainly resulting from the synergy between Bronsted acid sites and Ce(III) hydroxylated species active sites located in the supercages of CeHY. Meanwhile, the result of in situ FT-IR with double probing molecules further reveals the essence of oligomerization reactions of thiophene and 2-methylthiophene molecules on those specific active sites. By contrast, the oligomerization reaction of benzothiophene molecules on the active sites of CeHY cannot occur due to the restriction of cavity size of supercages, but they can be adsorbed on the Bronsted acid sites via protonation, and on Ce(III) hydroxylated species and extra-framework aluminum hydroxyls species via direct “S-M” bonding interaction. As to the tetrahydrothiophene, adsorption mechanism is similar to that of benzothiophene, except in the absence of protonation. The paper can provide a new design idea of specific adsorption active sites in excellent desulfurization adsorbents for elevating higher quality of FCC gasoline in the future.

Highly stable seed-derived ferrierite for carbonylation of dimethyl ether to methyl acetate: Effects of seed content to catalytic stability

Abstract The seed-derived ferrierite zeolite (FER) synthesized at different contents of FER seed component without using any organic structure-directing agent (OSDA) showed a higher catalytic activity and stability for a gas-phase carbonylation of dimethyl ether (DME) to methyl acetate (MA) at an optimal amount of the previously synthesized FER-seed with 15wt%. The optimal amount of FER-seed material played an important role to optimize the numbers of active Bronsted acid sites in 8-membered ring (8-MR) channels with less formation of coke precursors by decreasing defect sites through their recrystallization during the preparation step. The defected Lewis acidic extra-framework aluminum (EFAL) sites on the seed-derived FER surfaces were responsible for preferential coke depositions and for a retarded CO insertion by decreasing the rate of the gas-phase DME carbonylation reaction.

Promotion of protolytic pentane conversion on H-MFI zeolite by proximity of extra-framework aluminum oxide and Brønsted acid sites

The activity of strong Brønsted acid sites in H-MFI zeolites for pentane cracking and dehydrogenation is enhanced by the presence of extra-framework aluminum in spatial proximity. The steric proximity of these two sites is deduced from the perturbation of OH groups on extra-framework aluminum oxide by pyridine or pentane adsorbed on Brønsted acid sites manifested in the IR spectra. The turnover frequencies of overall cracking and dehydrogenation on such sites are about 50 and 80 times higher than on isolated strong Brønsted acid sites. While pentane does not adsorb stronger, the site pair stabilizes cracking and dehydrogenation transition states mainly via increasing the activation entropy. This is interpreted as a later transition state for the cracking and dehydrogenation. The results suggest that controlled steaming can be used to enhance catalytic activity of zeolite Brønsted acid sites.

Thin-felt hollow-B-ZSM-5/SS-fiber catalyst for methanol-to-propylene: Toward remarkable stability improvement from mesoporosity-dependent diffusion enhancement

A hollow-B-ZSM-5/SS-fiber catalyst is developed through alkali leaching of the full-silica core of the silicalite-1@B-ZSM-5 in situ structured onto a thin-felt stainless-steel fiber (20 μm SS-fiber), which is synthesized by a seed-assisted dry-gel vapor-phase transport method. As-obtained catalyst shows marked improvement in the mesoporosity-dependent diffusion (by o-xylene diffusion measurement). Boron incorporation is essential for preventing the framework dealumination during alkali leaching treatment due to the increased framework negative charges. The hollow-B-ZSM-5/SS-fiber catalyst shows remarkable stability improvement in the methanol-to-propylene (MTP) reaction because of the enhanced diffusion of the nano-hollow-structure and the boron-incorporation stabilized framework aluminum. A boron-free hollow-ZSM-5/SS-fiber is also obtainable with the textural properties comparable to the hollow-B-ZSM-5/SS-fiber but shows undesired degeneration of the tetra-coordinated aluminum. The preferential generation of coke in the micropores of the hollow-ZSM-5/SS-fiber causes a rapid deactivation even compared to the parent silicalite-1@ZSM-5/SS-fiber, due to the existence of extra-framework aluminum.

Mechanistic insights into structural and surface variations in Y-type zeolites upon interaction with binders

Zeolite Y is one of the most important solid acid catalysts that find immense applications in catalytic processes such as fluid catalytic cracking (FCC), acylation and cumene cracking. The technical grades of FCC catalyst require specifications that are met by dispersing the zeolite in amorphous binders. On contact with binders, zeolite properties in composite catalysts alter. This article reports an insight into structural and surface modifications of Y zeolites caused by binders’ incorporation. Experiments employing various characterization techniques, including solid state MAS-NMR and FTIR reveal alumina incorporation adjusts zeolite framework in comparison with silica binder. Incorporation of alumina binder into zeolite Y polarizes the framework by extra-framework alumina species (such as Al3+, AlO+ and Al(OH)2+) coming from the binder. The polarization, apparent with USY and REY, is considered as the operating mechanism for distortion of zeolite framework, involving the attraction of aluminum species from the binder to the framework of zeolite and its subsequent framework structure reorientation. Compensating framework Na+ cations restrain the polarization of NaY framework due to its deficiency of protons, revealing Brönsted acid sites as active centers for the zeolite framework polarization. The resulting alumina bound catalysts are amorphized by layers of alumina materials that implicate the Brönsted acidity as a function of its weight in the catalyst. Catalytic reaction reveals that polarization of the zeolite framework by extra-framework alumina species decreases the transformation of paraffin to carbenium ion, increasing the hydrogen transfer reaction pathway.

On How Copper Mordenite Properties Govern the Framework Stability and Activity in the Methane-to-Methanol Conversion

Herein we investigated the activity of copper mordenites in the methane-to-methanol conversion and the material de- and realumination. From four parent materials, a library of copper mordenites was synthesized by liquid- and solid-state ion-exchange techniques. Two key properties govern the activity of these materials in the methane conversion: the parent counterion and the copper ion-exchange procedure. H-form parents result in more active materials. The optimum stoichiometry between silicon, aluminum, and copper leads to a methanol productivity of up to 169 μmol/g. This equals a stoichiometry of up to 0.47 methanol molecules formed per copper atom. The methanol productivity is constant over up to three cycles. The stability of the mordenite framework was monitored by SEM, EDX, 27Al, and 29Si MAS NMR spectroscopy. No detectable copper nanoparticles formed. However, a dealumination of the mordenite framework and the formation of extra-framework aluminum (EFAl) species in quantities of up to 12% were obser...

Additional Lewis acid sites of protonated fibrous silica@BEA zeolite (HSi@BEA) improving the generation of protonic acid sites in the isomerization of C6 alkane and cycloalkanes

This paper describes a study of the nature of metal-free protonated fibrous silica BEA (HSi@BEA) and its ability to generate protonic acid sites originating from molecular hydrogen and from C6 alkanes and cycloalkanes. HSi@BEA was successfully synthesized by a microemulsion system and subsequently applied in n-hexane, cyclohexane and methylcyclopentane isomerization. The HSi@BEA consisted of microspheres with a bicontinuous concentric lamellar structure morphology. Its core was mainly composed of an aluminosilicate framework of BEA, while the lamellar structure consisted of silica. The particle size of HSi@BEA was in the range of 400 nm to 700 nm with pore diameters between 3.6 nm to 5.6 nm and 15 nm to 25 nm. 27 Al MAS NMR results showed that the additional silica lamellar structure of the HSi@BEA increased the extra-framework aluminum (AlOH) which acts as Lewis acidic sites in the catalyst. FTIR results revealed that these additional Lewis acid sites in the metal-free catalyst generated a high amount of protonic acid sites by playing a role as electron acceptors after the dissociation of H2 and C6 alkane and cycloalkanes. Significantly, the participation of more protonic acid sites enhanced the catalytic performance of the C6 alkanes and cycloalkanes, n-hexane, cyclohexane and methylcyclopentane, at a low reaction temperature of 423 K. The presence of a silica lamellar structure resulted in a two to threefold higher isomer yield of n-hexane, cyclohexane and methylcyclopentane compared to bare HBEA catalyst which mainly produces cracking products. In addition, a kinetics study showed that the activation energy could be lowered by two to three times at elevated temperatures in the range of 423 K to 623 K.

Influence of the Binder Type on the Properties of Nanocrystalline Zeolite Beta-Based Catalysts for Benzene Alkylation with Propylene

The effect of different binders (aluminum hydroxide (sample K-1), silica gel (sample K-2), pseudoboehmite (sample K-3), a kaolin–silica sol mixture (sample K-4), and a kaolin–aluminum hydroxide mixture (sample K-5)) on the set of the textural, acidic, and catalytic properties of catalysts based on nanocrystalline zeolite Beta has been studied. It has been shown that the introduction of aluminum hydroxide, silica gel, and a kaolin–aluminum hydroxide mixture as binders does not lead to the blocking of the pore structure of the zeolite, while the introduction of pseudoboehmite and a kaolin–silica sol mixture into the catalyst composition leads to a decrease in the micropore volume. Using the NH3 TPD method, it has been found that the use of aluminum hydroxide, pseudoboehmite, and a kaolin–aluminum hydroxide mixture as a binder leads to an increase in the number of acid sites of the catalyst compared with the respective parameter of the original BEA-25 zeolite. The observed changes are attributed to the migration of aluminum from the binder to the zeolite structure to form new acid sites, as evidenced by 27Al MAS NMR data. It has been assumed that an increase in acidity for the K-4 sample is associated with the interaction of silica with the extra-framework aluminum of the Beta zeolite. With respect to activity in the benzene alkylation with propylene, the catalysts can be arranged in the following order: K-1 > K-3 > K-2 > K-5 ≈ BEA > K-4, which correlates with the number of acid sites in the samples. The best process parameters have been achieved in the presence of the K-1 sample exhibiting a stable on-stream behavior for 10 h and providing a selectivity for alkylation products (cumene + DIPB) of 99.7% and a cumene selectivity of 89.7 wt % at 100% propylene conversion.