Nano-sized ZSM-5 Research Articles

Para-selective methylation of toluene with methanol over nano-sized ZSM-5 catalysts: Synergistic effects of surface modifications with SiO2, P2O5 and MgO

The nano-sized ZSM-5 catalysts were modified by surface coating with SiO2, P2O5, MgO and their combinations; the catalytic properties were investigated in the shape-selective methylation of toluene with methanol. The catalysts were characterized by XRD, XRF, N2 adsorption–desorption, temperature programmed desorption of ammonia (NH3-TPD), Fourier-transform infrared spectra of adsorbed pyridine/2,6-di-tert-butylpyridine, and adsorption of n-hexane/cyclohexane. The passivation of Lewis acid sites occurs prior to that of the Brönsted acid sites over ZSM-5 modified by SiO2, while P2O5 or MgO preferentially neutralizes the Brönsted acid sites of ZSM-5. The deposition of MgO is more efficient in passivating the acid sites and narrowing the pore openings, compared to SiO2 or P2O5 modification with the same oxide content (<9wt%). The single modification could not completely passivate the external surface acid sites and simultaneously narrow the pore openings to a proper extent; so the selectivity to para-xylene does not exceed 90% even at the highest oxide loading. The multiple modification by SiO2, P2O5 and MgO, with a suitable sequence can efficiently eliminate external surface acid sites, and simultaneously narrow the pore openings, which led to a higher para-selectivity (∼98%). The combined modification with SiO2, P2O5 and MgO in a proper sequence can lead to a synergistic effect for tailoring the acid property and pore mouth of the catalyst, thus enhancing the para-selectivity to ∼98% and improving catalytic stability, as demonstrated by flow test for 1000h on stream.

Atmospheric pressure synthesis of nanosized ZSM-5 with enhanced catalytic performance for methanol to aromatics reaction

We develop an atmospheric pressure synthetic route for nanosized ZSM-5 with 100% yield, by decoupling its nucleation and growth process. The product exhibited excellent performance in catalytically transferring methanol into aromatics.

Synthesis of SAPO-34/ZSM-5 Composite and Its Catalytic Performance in the Conversion of Methanol to Hydrocarbonsc

SAPO-34/ZSM-5 composite was synthesized with nano-sized ZSM-5 zeolite as seeds and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen sorption, Fourier transform infrared (FTIR) and temperature-programmed desorption of ammonia (NH3-TPD); its catalytic performance in the conversion of methanol to hydrocarbons (MTH) was investigated. The results indicated that the as-synthesized SAPO-34/ZSM-5 composite takes the form of sphere-like particles with a diameter ranging from 2.3 to 4.8 µm, through the accumulation of tiny ZSM-5 and SAPO-34 crystals; a large quantity of mesopores was formed in the interspaces between the ZSM-5 and SAPO-34 crystals. Owing to its proper acidity and porosity, the SAPO-34/ZSM-5 composite exhibits higher stability in MTH and favors the formation of aromatics and alkanes, compared with the parent SAPO-34 and ZSM-5 zeolites and their mechanical mixture.

A fast route for synthesizing nano-sized ZSM-5 aggregates

Nano-sized ZSM-5 aggregates have been rapidly synthesized from leached metakaolin by solid-like state conversion. Compared with the conventional hydrothermal route, the nano-sized ZSM-5 aggregates obtained by this method showed more excellent catalytic performance, C2–C4 olefin selectivity and longer lifetime to endure coke deposition.

Highly acidic mesoporous aluminosilicates assembled from zeolitic subunits generated by controllable desilication of ZSM-5 in Na2SiO3 solution

Mesoporous aluminosilicates (ZM) were hydrothermally synthesized by assembling zeolitic subunits, which were obtained by the controllable desilication of nano-sized ZSM-5 zeolite particles in Na2SiO3 aqueous solution, with cetyltrimethylammonium bromide (CTAB) as the template. The characterization results indicated that ZM possessed a mesoporous structure similar to MCM-41 and ZSM-5 subunits were present in the pore walls. Compared with conventional Al-MCM-41, ZM had less isolated silanol groups, larger pore wall thickness, and higher percentage of tetrahedrally coordinated Al species. The acidity site density, the proportion of strong acidity sites, and the B/L ratio of HZM, which was obtained by ion-exchange of ZM followed by calcination, were significantly higher than those of HAl-MCM-41, but slightly lower than HZSM-5 zeolite. In cumene cracking, HZM showed higher activity than HAl-MCM-41, and much better stability than HZSM-5. In the bulky molecules-involved Friedel–Crafts alkylation of benzene with 1-dodecene and in the liquid phase benzylation of benzene with benzyl alcohol, HZM exhibited higher catalytic performance than HAl-MCM-41 and HZSM-5.

Enhancing shape selectivity without loss of activity – novel mesostructured ZSM5 catalysts for methylation of toluene to p-xylene

An increase in p-xylene selectivity was observed without losing the catalytic activity over novel mesoporous nano-sized ZSM5 crystals covered with an external SiO2 overlayer created by deposition of tetraethyl orthosilicate.

Synthesis and characterization of partially crystalline nanosized ZSM-5 zeolites

Partially crystalline nanosize ZSM-5 with high surface area (678m2/g) was synthesized successfully without using organic template by a two-step temperature process. Aluminum nitrate was used as aluminum source for the first time to synthesize ZSM-5 materials. The presence of MFI structure of the materials was analyzed by XRD, FTIR, Raman spectroscopy and TEM techniques. Addition of commercial HZSM-5 as a seeding agent to the reactants resulted an increase in crystallinity of the ZSM-5 sample and subsequent decrease in specific surface area. The partially crystalline samples exhibited low microporosity and remarkably high meso/macropore volume with pore diameters around 30nm.

Ethylation of coking benzene with ethanol over nano-sized ZSM-5 zeolites: Effects of rare earth oxides on catalyst stability

Abstract The ethylation of coking benzene with ethanol over nano-sized ZSM-5 zeolites modified with Y 2 O 3 , La 2 O 3 and CeO 2 was investigated in a fixed-bed reactor. Nano-sized ZSM-5 zeolite modified with Y 2 O 3 was more stable than the other two samples during the reaction. The results from NH 3 -TPD, Py-IR, and TG-DTA show that the strong Bronsted acid sites were eliminated, consequently suppressing coke deposits during the reaction. Combined with the results from 1 H NMR and 129 Xe NMR spectra, it is concluded that the hydrated Yttrium ions with small size reached deep location in zeolite channel.

Selective synthesis for light olefins from acetone over ZSM-5 zeolites with nano- and macro-crystal sizes

Production of light olefins such as ethylene, propylene and isobutylene from acetone was examined over ZSM-5 zeolites. These light olefins are produced from acetone over the acid sites of the zeolite via a series of consecutive reactions where olefins such as ethylene and propylene are obtained by cracking of isobutylene produced from aldol condensation products of acetone. Macro- and nano-sized ZSM-5 zeolites were prepared by conventional hydrothermal and emulsion methods, respectively, and the ZSM-5 zeolites with nearly the same acidity and BET surface area were obtained regardless of the crystal sizes. From SEM observations, the crystal sizes of the zeolites were approximately 2000 nm and 30–40 nm. These zeolites with different crystal sizes were applied to light olefins synthesis from acetone, and the effect of crystal size on catalytic activity and stability was investigated. As compared with the macro-seized zeolite, the nano-sized zeolite exhibited a high activity over a long lifetime. However, because the nano-sized zeolite possesses a large external surface area, undesirable reactions to form aromatics from the produced light olefins occurred on the acid sites located near the external surface. To inhibit aromatics formation, selective deactivation of the acid sites located near the outer surface of the zeolite was achieved via the catalytic cracking of silane (CCS) method using diphenyl silane (DP-silane). The CCS method was effective in deactivating the acid sites located near the external surface of the ZSM-5 zeolite. Moreover, the nano-size zeolite after the CCS treatment using DP-silane exhibited high olefins and low aromatics yields under high acetone conversion conditions.

Effect of acidity and diffusibility on coke deactivation over nano-sized HZSM-5 zeolites

Four series of nano-sized HZSM-5 zeolite catalysts were prepared with different steaming and HNO3 leaching processes. These samples were studied by XRD, NH3-TPD, FT-IR, n-hexane and cyclohexane adsorption, and fixed-bed aromatization of C4 liquefied petroleum gas (C4 LPG). Results show that, with the steaming temperature increasing, the acidity of catalyst decreased gradually, which was attributed to the removal of Al species from the framework of the zeolite. When steaming is combined with acid leaching, we found that acid leaching-steam catalysts exhibit stronger acidity than steam-acid leaching catalysts. Besides, acid leaching also played an important role in enhancing the diffusibility of catalysts. Catalysts which underwent acid leaching twice exhibit the best microporous diffusibility. Through correlating the acidity and diffusibility of the catalysts with their aromatization performance, it is found that the product distribution of aromatization is mainly determined by catalyst acidity rather than diffusibility. On the other hand, the average coking speed of the catalysts is not only influenced by the acidity of the catalyst, but also by its microporous diffusibility. The improvement of microporous diffusibility remarkably favors suppressing the coking deactivation of nano-sized ZSM-5 zeolite in C4 LPG aromatization.

ChemInform Abstract: Confined Space Synthesis. A Novel Route to Nanosized Zeolites.

Confined space synthesis is a novel method in zeolite synthesis. It involves crystallization of the zeolite inside the pore system of an inert mesoporous matrix. In this way it is possible to prepare nanosized zeolites with a controlled size distribution by proper choice of the inert matrix. Here, confined space synthesis was adopted to prepare nanosized ZSM-5, zeolite Beta, zeolite X, and zeolite A with tailored crystal size distributions using mesoporous carbon blacks as inert matrices. All zeolites were characterized by X-ray powder diffraction, transmission electron microscopy, and nitrogen adsorption/desorption prior to and after removal of the carbon matrix. ZSM-5 with Si/Al ratios of 50, 100, and infinity (silicalite-1) were synthesized with controlled average crystal sizes in the range 20-75 nm. Nanosized zeolite Beta (7-30 nm), zeolite X (22-60 nm), and zeolite A (25-37 nm) were prepared similarly. Removal of the carbon matrix by controlled combustion allows a convenient method for isolation of the pure and highly crystalline zeolites. Therefore, confined space synthesis appears to be an attractive method for preparation of zeolites with a controlled size distribution.

Ion conductivity of nano-scaled Al-rich ZSM-5 synthesized in the pores of carbon black

A novel way to obtain nano-sized Al-rich ZSM-5 crystallites by constrained growth in the pores of a carbon black, followed by removal of the carbon matrix by oxidation is reported. Tailoring the size of the formed zeolite particles by selecting a suitable templating carbon material with requested porosity and the easy separation of the synthesized product by simple filtration instead of centrifugation, are the two main advantages of this method. The carbon black is infiltrated with the synthesis solutions in two steps: First an ethanolic solution containing the Al-source is used, followed by a second infiltration with a tetraethylorthosilicate (TEOS) solution after evaporation of the alcohol. During a subsequent hydrothermal treatment (48 h, 180 °C) the zeolite nanocrystals grow inside the pores. Because of the separate addition of the aluminium and silicon sources, this procedure enables to vary the silica to alumina ratio in a wide range. For reaching high alumina contents, decreasing of the silicon concentration in the synthesis mixture is more suitable than increasing the amount of infiltrated aluminium. About 50 nm ZSM-5 particles with Si/Al ratios down to 10 could be obtained with acceptable crystallinity. According to impedance spectroscopy measurements the nano-ZSM-5 shows higher ion conductivity than μm-sized ZSM-5 and small ZSM-5 particles obtained by ball-milling since the latter procedure damages the crystal zeolite structure. This finding demonstrates that small particle sizes as well as a good crystallinity are important to reach high ion conductivities. This statement is in all likelihood transferable to other zeolites.

Catalytic properties of nano-sized ZSM-5 aggregates

Nano-ZSM-5 (NZ) with well-defined porosity was synthesized by organic silica method. The material exhibited more than a twofold increase in pore volume when compared to the normal ZSM-5 (HZ). The increase in pore volume is observed in the entire range of the pores with diameter from 5 Å to 1500 Å. The order of increase in volume is macropores (>500 Å) > micropores (up to 20 Å) > mesopores (20–500 Å). Although macropore and mesopore formation is responsible for the major increase in the pore volume, significant increase in zeolitic pore volume was also observed in NZ. Both the ZSM-5 materials were tested for esterification of cyclohexanol with acetic acid under autogeneous pressure conditions where NZ showed better activity (69% conversion).

Synthesis of ZSM-5 zeolite membranes without organic templates using a varying-concentration technique

Template-free nanosized ZSM-5 seeds with an average size of 15 nm were prepared from a synthesis solution with the composition 12Na 2O:100SiO 2: 2Al 2O 3:2500H 2O. By the use of these seeds, thin ZSM-5 zeolite membranes were prepared on the outer surface of a porous α-alumina tube with a pore size of 2 μm in a gel system by varying-concentration synthesis with organic-free template. The first composition synthesis sol-gel was the same as seeds of molar composition and the second one was 12Na 2O: 100SiO 2: 2Al 2O 3: 5000H 2O at 180 °C for 10 h, respectively. XRD shows that the film consists of well-crystallized ZSM-5 zeolite. SEM investigation indicats that the zeolite films on the supports are defect free and the film thickness is approximately 8 μm. The permeances for H 2, N 2, CH 4 and CO 2 are 8.94 × 10 −7, 3.27 × 10 −7, 3.9 × 10 −7, 3.14 × 10 −7 and 0.874 × 10 −7 mol·m 2·s −1·Pa −1, respectively. The ideal selectivity of membrane at room temperature for H 2/CO 2, H 2/N 2, H 2/CH 4 are 2.84, 2.73 and 2.29, respectively.

Effect of template type and template/silica mole ratio on the crystallinity of synthesized nanosized ZSM-5

The crystallinity of nanosized ZSM-5 zeolite from precursors mixtures containing different template types, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide, and different template/silica mole ratios, 0.215, 0.322, 0.43 and 0.537, has been studied. The produced samples were investigated using XRD, SEM, FT-IR and BET surface area measurements. Physicochemical investigations showed that the obtained products by different type of templates and different template/silica mole ratios were ZSM-5 phase. The as-synthesized ZSM-5 sample prepared by using tetrapropylammonium hydroxide template and 0.215 template/silica mole ratio at 230 °C for 45 h had the highest crystallinity. It was found that the average crystallite size increased in the following order; tetramethyl < tetrabutyl < tetraethyl < tetrapropyl and surface area increased in the following order: TMAOH < TEAOH < TBAOH < TPAOH. By increasing template/silica mole ratios from 0.215 to 0.537 the surface area decreased and average crystallite size increased.

A novel carbon/ZSM-5 nanocomposite membrane with high performance for oxygen/nitrogen separation

A novel carbon/ZSM-5 nanocomposite membrane is successfully prepared by incorparating nano-sized ZSM-5 into polymeric precursor(polyimide), which shows excellent permselectivities for separation of oxygen/nitrogen gas pairs.

Low temperature synthesis and properties of ZSM-5 aggregates formed by ultra-small nanocrystals

ZSM-5 formed by aggregates of nanocrystals with ultra-small crystal size (10–20 nm) has been prepared by hydrothermal crystallization of clear synthesis mixtures at low temperatures (70–90 °C) under atmospheric pressure. Compared with the conventional autoclave treatment, this method leads to a significant reduction in the crystal size, although with lower zeolite yields, which has been assigned to the mild conditions used (low temperature and atmospheric pressure), as they may prevent the crystal growth by aggregation/densification of primary units in the nanometer range. Characterization of the samples obtained at long synthesis times by both X-ray and electron diffraction show they are highly crystalline materials, indicating that the ultra-small crystal size is compatible with the detection of crystallinity. After calcination, most of the Al atoms remain tetrahedrally coordinated, which is a proof of their stability. The nanosized ZSM-5 exhibits a very high external surface area (typically around 200 m 2/g), as a consequence of its small crystallites, making this material a potentially interesting catalyst for the conversion of bulky molecules. Likewise, a mathematical model has been developed which correlates the variation of the textural properties of the ZSM-5 zeolite with its crystal size.

Synthesis and characterization of nano-sized zeolites

Synthesis of zeolite ZSM-5 and silicalite-1 by hydrothermal crystallization of clear supersaturated homogeneous synthesis mixture has been achieved. The influence of the templates, synthesis temperature, synthesis time and SiO2/Al2O3 ration on the properties of the nano-sized zeolite has been investigated. The properties of the samples were characterized by X-ray powder diffraction, N2 adsorption/desorption, scanning electron microscopy, and transmission electron microscopy. The results show that the temperature at 170℃ was favorable to synthesize the nano-size zeolites. The use of tetrapropylammonium hydroxide as the template in the hydrothermal synthesis conditions employed was favorable to form nanozeolite than tetramethylammonium hydroxide. The synthesis time also plan an important role. 1 day was required to obtain a fully crystallized product. The longer crystallization time would produce bigger particles. A decrease in Al2O3 content would decrease the crystallite size. The nanosize ZSM-5 and silicalite with crystallite size of 150 nm have been successfully synthesized.

ZSM-5 zeolite membranes prepared from a clear template-free solution

Template-free nanosized ZSM-5 seeds were prepared from commercially available ZSM-5 powder. By use of these seeds, thin and hydrophilic ZSM-5 zeolite membranes were prepared on the outer surface of a porous α-alumina tube in a clear solution free from organic template. The membranes showed high thermal stability to withstand pretreatment at 400 °C. At 270 °C, the ideal selectivities for H 2/ i-butane and N 2/SF 6 were 61 and 20, respectively. The membrane was effective to separate butane isomers at high temperatures. The maximum of the n/ i-butane ideal selectivity was 36 at 300 °C. The separation factors for a 50/50 n/ i-butane mixture were as high as 13 in the temperature range from 300 to 400 °C.

Confined space synthesis. A novel route to nanosized zeolites.

Confined space synthesis is a novel method in zeolite synthesis. It involves crystallization of the zeolite inside the pore system of an inert mesoporous matrix. In this way it is possible to prepare nanosized zeolites with a controlled size distribution by proper choice of the inert matrix. Here, confined space synthesis was adopted to prepare nanosized ZSM-5, zeolite Beta, zeolite X, and zeolite A with tailored crystal size distributions using mesoporous carbon blacks as inert matrices. All zeolites were characterized by X-ray powder diffraction, transmission electron microscopy, and nitrogen adsorption/desorption prior to and after removal of the carbon matrix. ZSM-5 with Si/Al ratios of 50, 100, and infinity (silicalite-1) were synthesized with controlled average crystal sizes in the range 20-75 nm. Nanosized zeolite Beta (7-30 nm), zeolite X (22-60 nm), and zeolite A (25-37 nm) were prepared similarly. Removal of the carbon matrix by controlled combustion allows a convenient method for isolation of the pure and highly crystalline zeolites. Therefore, confined space synthesis appears to be an attractive method for preparation of zeolites with a controlled size distribution.