H-type Zeolites Research Articles

Preparation of ZSM-5 zeolites by sodium-free method in rotating packed bed

In industry, H-type zeolites are usually synthesized under hydrothermal conditions in the sodium-containing system, in which great deal of ammonia nitrogen wastewater will be produced during the transformation of zeolites from Na-type to H-type. In this work, we prepared ZSM-5 zeolites in the sodium-free system using organic amines in a rotating packed bed (RPB), which can greatly reduce the amount of wastewater. It was found that the ZSM-5 zeolites synthesized by isopropanolamine (MIPA) had the highest acid amount with a value of 2.331 mmol/g compared to that by other three amines, and exhibits the highest conversion of benzaldehyde with a value of 77.63% in the acetal reaction of benzaldehyde and ethylene glycol. More importantly, RPB premixing can improve the transfer of Al into the framework of ZSM-5 zeolites relative to conventional stirring technique. This work provided a new route to prepare ZSM-5 zeolites with high acid amount in sodium-free system.

Morphology-Controlled Synthesis of H-type MFI Zeolites with Unique Stacked Structures through a One-Pot Solvent-Free Strategy.

H-type aluminosilicate zeolites are extensively used as solid-acid catalysts and support materials in industrial catalysis. However, the conventional synthesis methods involving hydrothermal syntheses and ion-exchange processes suffer from severe water pollution and toxic gas emissions. Herein, H-type MFI zeolite catalysts with a unique stacked structure were directly synthesized in the presence of NH4 F and with the help of zeolite confinement through a solvent-free route without further ion-exchange procedures. A range of ex situ and in situ characterization procedures were used to provide evidence of the simultaneous use of pre-made ZSM-5 and NH4 F as a confined Al source and mineralizer, respectively. The confined zeolite framework of ZSM-5 prevented the formation of AlFx species between NH4 F and Al atoms, ensuring that the prepared samples had desirable acidic properties. Moreover, the resulting morphology could be controlled by using different silica substrates. The obtained H-type MFI zeolites showed excellent catalytic performance in methanol-to-gasoline reactions owing to their unique structure and directly exposed acidic sites. The developed one-pot strategy provides an alternative method for the facile synthesis of H-type zeolites with defined morphology.

Rational Synthesis of Chabazite (CHA) Zeolites with Controlled Si/Al Ratio and Their CO2 /CH4 /N2 Adsorptive Separation Performances.

Separation of CO2 from CH4 and N2 is of great significance from the perspectives of energy production and environment protection. In this work, we report the rational synthesis of chabazite (CHA) zeolites with controlled Si/Al ratio by using N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as an organic structure-directing agent, wherein the dependence of TMAdaOH consumption on the initial Si/Al ratio was investigated systematically. More TMAdaOH is required to direct the crystallization of CHA with higher Si/Al ratio. Once the product Si/Al ratio is larger than 24, the amount of TMAdaOH consumption remains nearly constant. CHA zeolites with different Si/Al ratios and charge-compensating cations were then applied for the separation of CO2 /CH4 /N2 mixtures. The equilibrium selectivities predicted by ideal adsorbed solution theory (IAST) and ideal selectivities calculated from the ratio of Henry's constants for both CO2 /CH4 and CO2 /N2 decrease with the zeolite Si/Al ratio increasing, whereas the percentage regenerability of CO2 presents the opposite trend. Therefore, there is a trade-off between adsorption selectivity and regenerability for the adsorbents. There is a weaker interaction between CO2 molecules and the H-type zeolites than that on the Na-type ones, thus a higher regenerability can be achieved. This study indicates that it is possible to design CHA zeolites with different physicochemical properties to meet various adsorptive separation requirements.

Hydrolysis of cellulose to produce glucose with solid acid catalysts in 1-butyl-3-methyl-imidazolium chloride ([bmIm][Cl]) with sequential water addition

Selective glucose production by cellulose hydrolysis with initial or sequential water addition in the presence of solid acid catalysts in 1-butyl-3-methyl-imidazolium chloride ([bmIm][Cl]) under microwave irradiation was investigated at 120 °C and 1.2 MPa of N2 atmosphere. For revealing the effect of water on cellulose hydrolysis in [bmIm][Cl], the phase behavior during the reaction was observed. Colored compounds formed at lower amounts of water, while some amounts of cellulose precipitated at higher amounts of water. Glucose yield was highest (65 mol%) at 25 wt% water in [bmIm][Cl] for the case of initial water addition. To improve the glucose yield, sequential water addition in which water was added in steps as the reaction proceeded was examined. Glucose yields in the presence of Amberlyst-15 in [bmIm][Cl] were 68.5 and 75.0 mol% by two-step and three-step water addition, respectively. Employing three-step water addition, Dowex 50wx8-100 (ion-exchange resin), hydrothermally synthesized carbon catalyst and three types of nano-zeolites were assessed. The carbon catalyst-modified fuming sulfuric acid and H-type zeolites (mordenite and faujasite) gave comparable glucose yields as Amberlyst-15. Sequential water addition is an effective technique for improving reaction efficiency in cellulose/solid acid catalyst/ionic liquid reaction systems.

Zeolite-promoted hydrolysis of cellulose in ionic liquid, insight into the mutual behavior of zeolite, cellulose and ionic liquid

In this paper, a variety of H-type zeolites were demonstrated as efficient catalysts for hydrolysis of cellulose in ionic liquid (IL) to afford high yield of glucose under mild condition without a pretreatment process. By supplying water gradually as the reaction progresses, HY with the acid amount of 11.1mol% promoted the highest glucose yield of 50.0% at 130°C, and the yield of the three main products (glucose, cellobiose and 5-hydroxymethylfurfural) was up to 97.2%. It was found that the pore size, the acid amount, the water amount and the timing of water addition are critical factors for high hydrolysis efficiency. XRD characterization suggested that the framework structure of zeolite was particularly stable in 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), while the cell parameter were enlarged due to the dilatation effect of [BMIm]Cl. Elemental analysis of the IL-treated HY demonstrated that IL entered the channel of HY. As such, part of the intra-channel space of HY was occupied, making the average pore diameter, pore volume and the BET surface area of IL-treated HY all decreased, which were demonstrated by physisorption characterization. FT-IR through pyridine adsorption showed that H+ cations generated in-situ from the Brönsted acid sites of zeolite were the key active species for the good hydrolysis performance, while the Lewis acid sites did not exhibit high activity. By simple calcination, the recovered HY showed excellent reusability. Finally, the proposed reaction pathway and recycle of the catalyst were presented.

Undesorbed Dichloromethane in Zeolites Studied by Solid-State NMR

Abstract We have studied the state of adsorbed dichloromethane molecules not removed by evacuation in several H-type zeolites, ZSM5, mordenite, β, and Y, by means of solid-state NMR. The amount of undesorbed dichloromethane was determined by quantitative analysis of 1H MAS NMR spectra. The strength and the amount of Brønsted acid sites could not explain the amount of the undesorbed dichloromethane. The channel size is the most probable to determine the amount. The 1H and 13C MAS NMR spectra as well as the 2H static NMR spectra demonstrated that undesorbed dichloromethane is rather mobile.

Effect of acidity and pore diameter of zeolites on detection of base molecules by zeolite thick film sensor

In order to investigate molecular recognition by zeolite thick film gas sensors, the effect of zeolite acidity and pore diameter of zeolites on the detection of base molecules (water, acetonitrile, ammonia, benzonitrile, pyridine, aniline, and triethylamine) was investigated. The impedance spectra of the thick film sensors of H-type zeolites (MFI, MOR and BEA) showed a semicircle attributable to an RC parallel equivalent circuit from 4 Hz to 1 MHz, and resistance R decreased in the presence of 300 ppm of base molecules at 673 K. For smaller molecules having molecular diameter of less than 0.4 nm (water, acetonitrile, and ammonia), a good correlation was observed between the response magnitude of R and the proton affinity of base molecules. It was indicated that acid–base interaction is the major factor when molecular diameter is smaller enough than zeolite pore diameter. As for larger molecules (benzonitrile, pyridine, aniline, and triethylamine) of which the molecular diameter is nearly the pore diameter of zeolites, the response magnitudes were lower than those of smaller molecules. Quantification of adsorbed species by in situ FT/IR revealed that the response magnitude for larger molecules significantly depends on the mobility of migrating species controlled by relative size of molecules to zeolite pore diameter.

Effects of Transition-Metal Composition of Protonated, Layered Nonstoichiometric Oxides H1−xNb1−xMo1+xO6 on Heterogeneous Acid Catalysis

Protonated, layered transition-metal oxides H1−xNb1−xMo1+xO6 with various Nb and Mo ratios were examined as solid acid catalysts. The effects of transition-metal composition on layered crystal structure, solid acid proprieties, and catalytic activities were investigated. X-ray diffraction (XRD) patterns revealed that a layered crystal structure was maintained in the range of x = −0.15 to 0.15. Beyond this range, the oxides did not form a layered structure, which resulted in negligible acid catalytic activity. The acid strength and acid amount determined by ammonia temperature-programmed desorption measurements indicated that increasing Mo concentration enhanced the acid strength. On the other hand, an increase in Nb concentration increased the acid amount. These layered oxides possess intercalation availability for benzyl alcohol, determined by XRD and thermogravimetric analysis, where the amount of intercalated benzyl alcohol tended to correspond to the acid amount of the layered metal oxides. Layered Nb−Mo oxides exhibited remarkable catalytic activity in Friedel−Crafts alkylation of toluene with benzyl alcohol, exceeding that of niobic acid, ion-exchange resins, and H-type zeolites, and this was attributed to the intercalation behavior. The alkylation activity over layered Nb−Mo oxides increased with increasing Nb concentration, which was related to the acid amount rather than the acid strength. For hydrolysis of cellobiose and esterification of lactic acid, on the other hand, the acid catalytic activity increased with increasing Mo concentration, H0.9Nb0.9Mo1.1O6 > HNbMoO6 > H1.1Nb1.1Mo0.9O6, indicating that the acid strength was more important in these reactions.

Conversion of α-methylstyrene over pentasil zeolites with various silica ratios

It has been shown that α-methylstyrene in the presence of H-HSZ (SiO2/Al2O3 = 40–120) and Fe-HSZ (SiO2/Al2O3 = 60) pentasils in a nitrogen atmosphere (80–120°C, 5–20 wt % catalyst) is converted into linear (4-methyl-2,4-diphenylpent-1-and-2-enes) and cyclic (1,1,3-trimethyl-3-phenylindan) dimers. The H-type high-silica (SiO2/Al2O3 = 120) and ferrosilicate zeolites in air facilitate the conversion of α-methylstyrene into acetophenone and α-phenylpropionaldehyde.

Greener synthesis route for Jet Propellant-10: the utilization of zeolites to replace AlCl3

This paper summarizes our recent developments in the utilization of H-type zeolites to replace AlCl3 in exo-tetrahydrodicyclopentadiene production. H-type Y zeolites with large pore size (0.74 nm) had sufficient activities for isomerization of endo-tetrahydrodicyclopentadiene (endo-THDCPD) to form exo-tetrahydrodicyclopentadiene (exo-THDCPD). As for acidity strength, the moderate strength of zeolite acidity was favorable. The incorporation of fluorine generated more moderate acids, which improved the catalytic performance of zeolites to exo yield = 92.58%, exo selectivity = 96.47%, and suppressed the adamantane (ADM) yield = 2.32%. When NH4F/SSY = 6.57%, calcined at 500 °C, this showed an equivalent performance as AlCl3.

Liquid-phase oxidation of benzene to phenol by copper catalysts in aqueous solvent with a high acetic acid concentration

The liquid-phase catalytic oxidation of benzene was carried out under mild reaction conditions over Cu-impregnated zeolite catalysts (Cu/NaX, Cu/HX, Cu/NaY, Cu/HY, Cu/NaZSM-5, Cu/HZSM-5, Cu/Namordenite, Cu/Hmordenite, and Cu/Hbeta) using both molecular oxygen and ascorbic acid as the oxidant and the reducing reagents, respectively. Phenol was exclusively produced as the oxidation product and no other product was detected. Among the catalysts tested in this study, the Cu/HY catalyst had the highest activity for phenol formation. H-type zeolites were more effective supports for phenol formation than the corresponding Na-type ones. The yield of phenol was found to increase with the simultaneous increase in the amounts of both the supported Cu and the added ascorbic acid. The Cu species supported on HY began to elute as Cu2+ species in the solvent from the HY support with the initiation of the oxidation of benzene. The dissolved Cu2+ species were reduced to form precipitated Cu1+ species during the benzene oxidation. The homogeneous Cu species dissolved from the Cu/HY catalyst were thus suggested as mainly contributing to the phenol formation. The precipitated Cu1+ species that had no activity for the benzene oxidation were found to be regenerated in their oxidation activity toward phenol formation by calcination of the Cu1+ species in flowing air, although parts of the Cu species were found to be converted to aggregated CuO during the calcination.Key words: supported copper catalysts, zeolite, oxidation, benzene, phenol, oxygen, ascorbic acid.

Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO and the Formation of Stable Pd Clusters in Zeolites

Spontaneous dispersion and clustering processes of Pd were measured by means of the energy-dispersive EXAFS method. The spontaneous dispersion of bulky metal Pd into highly dispersed PdO was directly observed on the H-type zeolite in the atmosphere of O2. In contrast to H-type zeolites, simple oxidation of the agglomerated Pd was observed on Na-ZSM-5. The structural change of Pd was followed in the atmosphere of hydrogen. The clustering processes of metal Pd depended on the kind of zeolite, and these were categorized into three groups. The first group, i.e., Na-ZSM-5 and H-beta, showed monotonic agglomeration of metal Pd by increasing the reduction temperature. The second group consisted of H-ZSM-5 and H-mordenite where the formation of Pd6 clusters was found. On these zeolites, the generation of Pd6 clusters was reversibly observed upon the repetition of reduction and oxidation treatments. The third group consisted of H-Y and USY zeolites where the formation of Pd13 clusters was observed. From these findings, it was concluded that the crystal structure and acid sites of zeolites had profound influences on the dynamic behavior and the genesis of Pd clusters with various structures.

Pt catalysts supported on H-type zeolites for the catalytic combustion of chlorobenzene

The deep oxidation of chlorobenzene was investigated over Pt catalysts supported on H-type zeolites (H-ZSM5 and H-beta). Pt/zeolite catalysts showed a higher activity compared to Pt/γ-Al2O3 samples which were tested for comparison. Within each class of zeolite, the activity of Pt/zeolite catalysts was found to be higher on the samples with lower SiO2/Al2O3 ratio. Amounts of polychlorinated benzenes (PhClx) were produced in the order Pt/H-ZSM5<Pt/H-beta<Pt/γ-Al2O3 and were found to be roughly independent of the SiO2/Al2O3 ratio of the zeolitic support. The trend in the PhClx formation observed on Pt/zeolite samples, both in terms of total amount and relative distribution, was explained on the basis of a product shape selectivity effect induced by the zeolite, a lower size of zeolite channels hindering the further chlorination of PhCl to PhClx.

Formation of the V Center on H-Type Zeolites with Thermal Treatment

The effect of thermal treatment on H−MCM-22 and H−ZSM-5 zeolites was investigated using the electron spin resonance technique. A six-line signal (denoted as A, g = 2.048, A = 22.15 G) was detected on H−MCM-22 after He purging at high temperatures, whose intensities increased with the treating temperature. The same signal was also found on H−ZSM-5 zeolites with different crystal sizes. The paramagnetic center was identified as a V center, namely, a hole of an electron trapped on an oxygen atom bonding to a nearby aluminum atom. These signals appeared only on a dealuminated sample or a sample concomitantly with dealumination. The formation of the hole might involve an electron transferring from the lattice oxygen to a nonframework aluminum species, and the hyperfine splitting is caused by the interaction between the electron hole locating on the p orbit of oxygen and the framework aluminum bonding with the oxygen. The signal disappeared after the sample was exposed to air or oxygen at room temperature. Howe...

Measurement of Heat of Argon Adsorption for the Evaluation of Relative Acid Strength of Some Sulfated Metal Oxides and H-type Zeolites

The relative acid strength of sulfated SnO2, sulfated ZrO2, silica−alumina, H−Y, H−ZSM-5, and H-mordenite was estimated by the heat of Ar adsorption, calculated from a temperature dependence of amo...

Carbonylation of tert-butyl alcohol over H-zeolites

The Koch-type carbonylation of tert-butyl alcohol was studied over H-type zeolites. It was found that the catalytic carbonylation of a large amount of tert-butyl alcohol relative to the acidic sites of the H-zeolites in organic solvents requires an elevated temperature and CO pressure, although previous solid state NMR studies have revealed that the transformation of tert-butyl alcohol of an amount comparable to the acidic sites into 2,2-dimethylpropanoic acid proceeds just upon the CO co-adsorption in the H-zeolites at room temperature and atmospheric pressure. The catalytic performance of different H-zeolites and the influence of CO pressure, H2O addition and solvent effects on the carbonylation of tert-butyl alcohol have been investigated. H-ZSM-5 gives the highest selectivity for 2,2-dimethylpropanoic acid due to its adequate pore dimensions. The present work indicates the possible industrial application of solid acids as carbonylation catalysts instead of liquid acids for the Koch reaction to produce tert-carboxylic acids.

Comparative study of the oxidative decomposition of trichloroethylene over H-type zeolites under dry and humid conditions

H-type zeolites (H-Y, H-ZSM-5, and H-MOR) have been investigated for the catalytic oxidative decomposition of trichloroethylene under dry and humid (1000, 7500 and 15,000 ppmv of water) conditions, between 200 and 550°C. The activity order was found to be: H-MOR>H-ZSM-5>H-Y. Temperature-programmed desorption of ammonia and diffuse reflectance FT-IR measurements of adsorbed pyridine revealed that strong Brønsted acidity plays an important role in controlling the oxidative activity of the H-type zeolites. The main oxidation products were CO, CO 2, HCl and Cl 2. Small amounts of tetrachloroethylene were also detected. The addition of water to the feed stream did not alter the activity order of the zeolites observed in dry conditions. Better conversions were anticipated over H-MOR and H-ZSM-5 at low temperatures and excess of water concluding that both oxygen and water accounted for the TCE destruction. In contrast, H-Y showed a low activity in humid air due to its strong hydrophilic character. It was found that water promoted the formation of CO 2 and limited the generation of undesired chlorinated by-products (Cl 2 and C 2Cl 4). XRD analysis indicated that H-ZSM-5 was the most resistant zeolite to dealumination by chlorination.

Performance of zeolites and product selectivity in the gas-phase oxidation of 1,2-dichloroethane

The deep oxidation of 1,2-dichloroethane (DCE) over H-type zeolites (H-Y, H-ZSM-5 and H-MOR) was evaluated. Experiments were performed on conditions of lean chlorocarbon concentration (around 1000 ppmv) under dry and humid conditions, between 200 and 550°C in a conventional fixed-bed reactor. The high density of Brønsted acid sites, proved by temperature-programmed desorption (TPD) of ammonia and diffuse reflectance FT-IR of adsorbed pyridine measurements, make H-ZSM-5 zeolite an effective catalyst for DCE decomposition. Vinyl chloride was identified as an intermediate in 250–400°C range. When vinyl chloride was destroyed at higher temperatures, all the zeolites showed a great selectivity (>90%) to HCl. CO was promoted in quantity reflecting the difficulty of its oxidation over these zeolite catalysts. The activity of the zeolites was reduced in the presence of water vapour (15,000 ppmv). It was noticed that the addition of water to the feed stream did not alter the order of activity observed in the dry experiments. Moreover, the presence of water in the DCE decomposition changed significantly the reaction product distribution. Vinyl chloride formation was found to be significantly lowered over the three zeolites, and selectivity to CO 2 formation was largely enhanced. The X-ray powder diffraction (XRD) analysis of the deactivated samples indicated partial destruction of the zeolite crystal structure during reaction.

Evaluation of H-type zeolites in the destructive oxidation of chlorinated volatile organic compounds

The deep oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over H-type zeolites (H-Y and H-ZSM-5) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppmv) in dry air, between 200 and 550°C in a conventional fixed-bed reactor. H-ZSM-5 zeolite resulted more active than H-Y zeolite in the decomposition of both chlorinated volatile organic compounds. It was established that Brønsted acidity plays an important role in controlling the catalytic behaviour of the H-type zeolites. DCE was completely decomposed at 400°C, whereas TCE required higher temperatures (550°C). Vinyl chloride was identified as an intermediate in the DCE oxidation in the range of 250–400°C. When vinyl chloride is destroyed at higher temperatures, both zeolite catalysts show a high selectivity (>90%) towards HCl formation. Trace amounts of tetrachloroethylene were detected in the TCE oxidation, which peaked at 500°C. CO was promoted in quantity in the destruction of both DCE and TCE reflecting the difficulty of carbon monoxide oxidation over H-type zeolites.

Deactivation of zeolites in n-hexane cracking

The deactivation process of the H-type zeolites (H-mordenite, H-Y zeolite, and H-ZSM-5) in n-hexane cracking was studied using a pulse method in which the emphasis was put on the clarification of the initial stage of the deactivation of these H-type zeolites. The adsorption behaviors of n-hexane and ammonia on fresh and used zeolites were compared in order to elucidate the relationship between the micropore structure of the zeolites and the deactivation by the carbonaceous deposits (coke). The main deactivation of the H-mordenite (HM) zeolite was found to be caused by pore blockage of the deposited coke. The H-Y zeolite (HY) was suggested to be deactivated by the proton coverage of the coke. No deactivation of the H-ZSM-5 (HZ) was observed under the present pulse conditions. These deactivation behaviors of the H-type zeolites were found to be consistent with the results obtained by a gravimetric flow method. The initial activity, defined as the conversion at the first pulse, could be correlated with the strong acidity, irrespective of the micropore structure of the zeolites used. The correlation of both the initial aromatic and coke yields with the strong acidity was discussed in connection with the micropore structure of the H-type zeolites.