External Acid Sites Research Articles

Understanding the roles of different acid sites in beta zeolites with different particle sizes catalyzed liquid-phase transalkylation of diethylbenzene with benzene

Moderate and strong Brønsted acid sites were active sites and external acid sites had more contribution to the reaction.

Shape-selective alkylation of benzene with ethylene over a core–shell ZSM-5@MCM-41 composite material

A series of ZSM-5@MCM-41 core–shell composite materials prepared via a multi-cycle-sol–gel coating strategy is investigated as the catalyst for benzene alkylation with ethylene, in which both ethylbenzene and para-diethylbenzene ( p- DEB) are aimed as the target products. With multi-cycle-sol–gel coating, the external acid sites on the samples are gradually passivated by the inert MCM-41 shell. As a result, the shape selectivity to p- DEB is greatly enhanced. Nevertheless, the coating of mesoporous MCM-41 shell on ZSM-5 accelerates deactivation of the catalyst only due to the dilution effect of ZSM-5 content in the catalyst at the same space velocity, which is a reason that core–shell ZSM-5@MCM-41 will potentially be a practical catalyst in shape selective alkylation of benzene. In order to enhance the yield of p- DEB on ZSM-5@MCM-41, the reaction conditions at the fixed bed reactor including temperature, the molar rate of benzene to ethylene and GHSV, are also optimized.

Spontaneous Pillaring of Pentasil Zeolites.

Conventional methods to prepare hierarchical zeolites depend upon the use of organic structure-directing agents and often require multiple synthesis steps with limited product yield and Brønsted acid concentration. Here, it is shown that the use of MEL- or MFI-type zeolites as crystalline seeds induces the spontaneous formation of self-pillared pentasil zeolites, thus avoiding the use of any organic or branching template for the crystallization of these hierarchical structures. The mechanism of formation is evaluated by time-resolved electron microscopy to provide evidence for the heterogeneous nucleation and growth of sequentially branched nanosheets from amorphous precursors. The resulting hierarchical zeolites have large external surface area and high percentages of external acid sites, which markedly improves their catalytic performance in the Friedel-Crafts alkylation and methanol to hydrocarbons reactions. These findings highlight a facile, commercially viable synthesis method to reduce mass-transport limitations and improve the performance of zeolite catalysts.

From Colloidal Dispersions of Zeolite Monolayers to Effective Solid Catalysts in Transformations of Bulky Organic Molecules: Role of Freeze-Drying and Dialysis.

We investigated the properties and catalytic activity of zeolites with MWW topology obtained by unprecedented liquid exfoliation of the MCM-56 zeolite into solutions of monolayers and isolation/reassembly of the dispersed layers by various methods, with optional purification by dialysis or ammonium exchange. The layers were recovered by flocculation with alcohol or ammonium nitrate and freeze-drying. Flocculation alone, even with ammonium nitrate, did not ensure removal of residual sodium cations resulting in catalysts with low activity. Dialysis of the solutions with dispersed monolayers proved to be efficient in removing sodium cations and preserving microporosity. The monolayers were also isolated as solids by freeze-drying. The highest BET area and pore volume obtained with the freeze-dried sample confirmed lyophilization efficiency in preserving layer structure. The applied test reaction, Friedel–Crafts alkylation of mesitylene, showed high benzyl alcohol conversion due to increased concentration of accessible acid centers caused by the presence of secondary mesoporosity. The applied treatments did not change the acid strength of the external acid sites, which are the most important ones for converting bulky organic molecules. Zeolite acidity was not degraded in the course of exfoliation into monolayers, showing the potential of such colloid dispersions for the formation of active catalysts.

Investigation of crystal size effect on the NOx storage performance of Pd/SSZ-13 passive NOx adsorbers

The crystal size effect on the NOx adsorption and desorption over Pd/SSZ-13 passive NOx adsorbers (PNA) was investigated. SSZ-13 zeolites with various crystal size (0.4, 0.8, and 2.3 μm) presented highly textural uniformity. 0.4 μm Pd/SSZ-13 showed the highest NOx storage capacity and robust hydrothermal stability. According to the combined XRD, HAADF-STEM, H2-TPR, and CO-TPR results, ionic Pdn+ ions were dispersed better on Pd/SSZ-13 with a smaller crystal size. The nature of Pdn+ ion distribution was investigated by bulky and external surface acidic characterization. It was found that more external surface acid sites were formed on as-prepared smaller-sized SSZ-13 zeolites, which contributed to easier Pdn+ ion diffusion. The kinetic study showed that the NOx adsorption on Pd/SSZ-13 obeyed the pseudo-second-order model and the NOx adsorption activation energy for Pd/SSZ-13 was 25∼38 kJ/mol, independent of the crystal size. This work might guide the design of state-of-the-art zeolite supported PNAs.

Structure-Catalytic Properties Relationship in Friedel Crafts Alkylation Reaction for MCM-36-Type Zeolites Obtained by Isopropanol-Assisted Pillaring

MWW type zeolites are characterized by the presence of zeolitic layers of 2.5 nm thickness, containing 10-member ring sinusoidal channels inside and supercavities with 12-member ring openings located on their surfaces. Expansion and pillaring of layered zeolites increase the access to active sites and can enable or facilitate catalytic activity towards larger reactant molecules. This goal is explored in this work reporting the pillaring of layered zeolite MCM-56 with MWW topology by tetraethylorthosilicate (TEOS) treatment with the assistance of isopropanol, aimed at obtaining hierarchical micro-mesoporous systems. MCM-56 (Si/Al = 12) was synthesized with hexamethyleneimine as a structure-directing and aniline as a structure-promoting agent. Hierarchical porous systems were obtained using two different pillaring methods: (1) with TEOS only and (2) with TEOS mixed with isopropanol. The MWW framework was preserved during swelling/pillaring in both methods. Pillared zeolites obtained via alcohol-assisted pillaring possessed unique intermediate micro-mesopores with the size of about 2 nm. IR study revealed a decrease in the concentration of accessible acid centers upon pillaring. However, the fraction of acid sites on the external surface, accessible for adsorption of large molecules, increased by up to 90%. Catalytic activity was evaluated in the Friedel-Crafts alkylation of mesitylene with benzyl alcohol. Pillaring resulted in reduction of the acid site concentrations, but the materials retained high catalytic activity. Pillaring in the presence of alcohol produced increased turnover frequency values based on the concentrations of the external acid sites.

Alkylation of resorcinol with tertiary butanol over zeolite catalysts: Shape selectivity vs acidity

The catalytic performance of various zeolites such as H-ZSM-5, H-Y, H-beta, H-Mordenite in resorcinol alkylation with tertiary butanol demonstrated that pore characteristics have major influence on product selectivity, whereas acid strength and number of acid sites influenced resorcinol conversion. The passivation of external surface of H-beta zeolite by silylation and amine poisoning produced shape selectively O-alkylated resorcinol methyl tert butyl ether (RMTBE) and 4-tert butyl resorcinol (4-TBR). We propose that 4-TBR formation goes over external acid sites through RMTBE isomerization, whereas formation of 4-TBR takes place inside the pores through direct C-alkylation mechanism.

Hierarchical Mg/ZSM-5 catalysts for methanol-to-propylene reaction via one-step acid treatment

One-step acid treatment for template- and solvent-free synthesized NaZSM-5 was developed to obtain hierarchical HZSM-5. Hierarchical Mg/ZSM-5 was obtained by the incipient impregnation method, which further increased the propylene yield of the methanol-to-propylene (MTP) reaction. The physicochemical properties of the samples were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), N2 adsorption–desorption, pyridine adsorption-IR (Py-IR) and NH3 temperature-programmed desorption (NH3-TPD). The results indicated that one-step acid treatment (250 °C, 12 h) could increase the SiO2/Al2O3 molar ratio from 29.43 to 53.33 and maintain the well-distributed state of acid sites. The blocked pore structure was opened to introduce additional mesoporosity. Additionally, NaZSM-5 was synchronously converted to HZSM-5, thereby avoiding ion-exchange procedures. Synchronization of dealumination, desodiation and introduction of the mesoporous structure was achieved in one step, providing a good matrix for further impregnation with Mg species. The obtained hierarchical Mg/ZSM-5 (0.3 M–MgO) showed low diffusion hindrance and reduction in external surface acid sites (especially Bronsted acid sites), affording high propylene selectivity (53.34%) and good resistance to carbon deposition (~ 0.77%) in the MTP reaction.

A highly selective FER-based catalyst to produce n-butenes from isobutanol

The conversion of isobutanol to light olefins over zeolites was investigated by IR operando spectroscopy. FER zeolites showed a surprisingly high selectivity for the direct conversion of iso-butanol to linear butenes, hence catalyzing dehydration and skeletal isomerization in one step. As a fundamental understanding of the separate reactions is required to design superior catalysts, an in-situ and operando FT-IR mechanistic study of the reaction was carried out. The spectroscopic data, the derived species participating and their respective role are analyzed using chemometric (PCA and MCR-ALS) tools. We highlighted the role played by the external acid sites and the requirement of a specific distance between these sites to ensure the n-butene selectivity and point that the internal Brønsted acidity has an adverse effect on the catalyst stability. Moreover, there is no correlation between the carbon species formed on the surface and the exceptional selectivity towards n-butenes, allowing to exclude a “carbon pool” mechanism.

Role of the Morphology of Sulfonic Resin Catalysts in the Etherification of Ethanol with iso-Butylene: A Review

The influence of the morphology of the sulfonic resins on their catalytic properties for the etherification of ethanol with isobutylene has been analyzed. It is shown that the influence of the morphology is complex, and can be manifested through various factors: the accessibility and energetics of acid sites, the adsorption–desorption dynamics of reagents, and the mass transfer conditions. It has been established that the effect of morphology on the acidity of sulfonic resin is manifested in the difference of the energetics of external and internal acid sites. Regardless of morphology, the stoichiometry of ethanol sorption on sulfonic resins corresponds to 1 molecule per 1 acid site. The volcano-shape temperature dependence of the activity of sulfonic resins for ethanol etherification is due to the thermodynamics of the reaction (high-temperature branch) and the mass transfer characteristics of the catalyst (low-temperature branch). Due to various conditions of the mass transfer, the morphology determines the optimal process temperature and, consequently, the activity of the catalyst.

Local environment and catalytic property of External Lewis acid sites in hierarchical lamellar titanium Silicalite-1 zeolites

Abstract Hierarchical two-dimensional (2D) lamellar zeolites represent a young emerging field and a novel expression of the zeolite frameworks. Accompanied with synthesis and catalytic performance investigation, assessment on the local environment and catalytic behavior of acid sites in 2D zeolites are needed. Here, we described the local environment of Lewis acid sites and evaluated catalytic activities of external acid sites (i.e. sites on external surface and in mesopores) in 2D lamellar titanium silicalite-1 (TS-1) zeolites. For comparison purpose, four TS-1 zeolites, including 3D TS-1, multilamellar 2D TS-1 without pillarization and with pillarization by silica and silica/titania, respectively, were prepared. The local environment of Ti-sites, including coordination status of Ti-atom, speciation of hydroxyl (-OH) group that hints degree of confinement, acid site strength and accessibility, was collectively assessed by spectroscopic and organic base titration measurements. Kinetic studies of cyclooctene (C8H14) epoxidation with hydrogen peroxide (H2O2) discerned catalytic activity of external Ti-sites that decreased with increasing mesoporosity due to decrease in site confinement.

Carbonylation of dimethyl ether on ferrierite zeolite: Effects of crystallinity to coke distribution and deactivation

Abstract Different distributions and locations of coke precursors on the highly crystalline seed-derived ferrierite (FER) altered the catalytic stability during carbonylation of dimethyl ether (DME) in a gas-phase. The larger amounts of heavy cokes on external Bronsted acidic sites of the FER were responsible for a fast deactivation, and their distributions on the active internal Bronsted acid sites such as 8-membered ring (8-MR) channels were largely affected by their crystallinity. Although the surface cokes on the FERs were inevitably formed, the FER having an optimal Bronsted acid site concentration in its 8-MR channels which were formed by recrystallization of Lewis acidic extra-framework Al species during a seed-assisted OSDA-free synthesis step, revealed slower deactivation rate at the proper seed content in the range of 7–15 wt%. The slow deactivation phenomena on the seed-derived crystalline FERs, especially on the optimal FER(S15) and FER(S7) having the proper amount of crystallization sites were originated by minimizing the coke depositions on the external surfaces.

Controllable synthesis of ultra-tiny nano-ZSM-5 catalyst based on the control of crystal growth for methanol to hydrocarbon reaction

A series of aggregated nano-ZSM-5 with controlled crystal size (<100 nm) were prepared via low temperature hydrothermal synthesis and utilized for the study of particle size effect of ZSM-5 on methanol to hydrocarbon (MTH) reaction for the first time. The textural and acidic properties of ZSM-5 at different crystal growth process were characterized and analyzed by N2-sorption, XRD, NH3-TPD, TEM, SEM, TG, Py-IR and XPS. Two stages of crystal growth were found during the synthesis which could be used to guide the controlled synthesis of ultra-tiny nano-ZSM-5. According to catalytic performance, it was found that compared to external acid sites, internal acid sites play a crucial role on the synthesis of gasoline products. A positive relationship was found between the acidity and liquid hydrocarbon yield. In nanometer scale (<100 nm) the crystal size and acidity of ZSM-5 still strongly influenced the catalytic performance and a balance existed between the crystal size control and acidity regulation for the catalytic stability. This work could attract more attention on the structure fine control of nanoscale ZSM-5 catalyst.

Probing the Brønsted Acidity of the External Surface of Faujasite-Type Zeolites.

We outline two methodologies to selectively characterize the Brønsted acidity of the external surface of FAU-type zeolites by IR and NMR spectroscopy of adsorbed basic probe molecules. The challenge and goal are to develop reliable and quantitative IR and NMR methodologies to investigate the accessibility of acidic sites in the large pore FAU-type zeolite Y and its mesoporous derivatives often referred to as ultra-stable Y (USY). The accessibility of their Brønsted acid sites to probe molecules (n-alkylamines, n-alkylpyridines, n-alkylphosphine- and phenylphosphine-oxides) of different molecular sizes is quantitatively monitored either by IR or 31 P NMR spectroscopy. It is now possible, for the first time to quantitatively discriminate between the Brønsted acidity located in the microporosity and on the external surface of large pore zeolites. For instance, the number of external acid sites on a Y (LZY-64) zeolite represents 2 % of its total acid sites while that of a USY (CBV760) represents 4 % while the latter has a much lower framework Si/Al ratio.

Hydrophobic-hydrophilic balance of ZSM-5 zeolites on the two-phase ketalization of glycerol with acetone

Herein, ZSM-5 zeolites of several Si/Al mole ratios, and varying hydrophobicity were used as catalysts in the ketalization of glycerol. The water formed in the reaction adsorbs on acidic sites, hampering the catalytic conversion of glycerol. The conversion drops even more by adding water in the reaction mixture, intensifying the adsorption in hydrophilic zeolites. However, the samples with the higher Si/Al ratio and abundant hydrophobic ≡Si-O-Si≡ linkages had the uppermost turnover frequency. The reaction proceeds at the interface of the glycerol and acetone phases. Since the catalyst preferentially stays in the glycerol phase due to gravity action, the full catalytic action of the catalyst is hindered. Contact angle measurements with the glycerol/zeolite systems confirmed that the hydrophobic zeolite had the lowest affinity with glycerol, thus improving the interfacial solubilization of glycerol/acetone in the vicinity of active sites turning it much more efficient. The catalytic action was further enhanced by partially ion-exchanging external acid sites with minor quantities of bulky n-hexylammonium cations, turning the zeolite more hydrophobic and a strong candidate for other acid-catalyzed interfacial liquid-phase reactions.

Development of mesoporous ZSM-5 zeolite with microporosity preservation through induced desilication

Alkali treatment is a widely used strategy to create mesopores for microporous zeolites to decrease the diffusion limitation in micropores. However, many of the micropore losses existed due to the uncontrollable destruction of framework. In this paper, a series of mesoporous ZSM-5 with well-maintained micropores was synthesized through induced desilication strategy. To induce the diffusion of alkali solution for the homogenizing desilication and mesopore formation, abundant and uniform small mesopores were firstly built for ZSM-5 by protective desilication in tetrapropylammonium hydroxide solution. The pore structure and acidity of the obtained ZSM-5 were characterized by XRD, TEM, N2 physisorption, NH3-TPD and Py-IR. Through induced desilication, the external surface area of obtained ZSM-5 increased from 42 m2 g−1 of parent ZSM-5 to 148 m2 g−1 and only 17% of micropore surface area was sacrificed, which was far lower than 37% of the loss in traditional NaOH treatment. Meanwhile, the strong acid sites were well preserved and the external acid sites were reduced. Compared with the sample treated by NaOH solution, the obtained catalyst exhibited higher catalytic stability and aromatics, especially PX selectivity. Good regenerated performance with long catalytic lifetime and high liquid hydrocarbon yield could also be found for the obtained catalyst. The strategy proposed in this paper is simple and ingenious in tailoring the porosity since it only relies on the desilication method, which is different from the usual coupling of dealumination and desilication.

Ex-situ catalytic upgrading of vapors from microwave-assisted pyrolysis of bamboo with chemical liquid deposition modified HZSM-5 to enhance aromatics production

The ex-situ catalytic upgrading of vapors from the microwave-assisted pyrolysis of bamboo sawdust for aromatics-rich bio-oil production was investigated. It was found that the sequential two-stage reactor, with microwave-assisted fixed bed and independent vertical reforming furnace, could separately optimize both the pyrolysis and catalytic upgrading processes. Besides, for purpose of decreasing coke generation and enhancing aromatics yield during the catalytic fast pyrolysis (CFP) of bamboo, chemical liquid deposition (CLD) was used to modify HZSM-5, which played a significant role in reducing external acid sites and narrowing the pore-opening size of catalyst. Experimental results illustrated that the yield of pyrolyzed oil initially increased, and then decreased, with increasing pyrolysis temperature, achieving its maximum at 550 ℃. In addition, the effect of different CLD modifying conditions (modifier type, deposited amount, and deposition time) on aromatics proportion of the obtained bio-oil was further explored. The consequences presented the TiO2-modified HZSM-5 (deposited amount, 4%; deposition time, 4 h) provided the maximal yield of BTX (benzene, toluene and xylene, 23.58%) compared to the original HZSM-5 (BTX: 16.96%), and the coke generation clearly declined. Simultaneously, the catalytic temperature and catalyst to biomass mass ratio (C/B) in ex-situ upgrading process were also investigated, and it was found that the concentration of BTX was maximized at catalytic temperature of 500 ℃, and C/B = 1.

Comparative study on pyrolysis of bamboo in microwave pyrolysis-reforming reaction by binary compound impregnation and chemical liquid deposition modified HZSM-5

The deactivation of catalyst is a significant reason for its limited application during the catalytic fast pyrolysis (CFP) process. To reduce the coke formation, binary compound impregnation (BCI) and chemical liquid deposition (CLD) were used to modify HZSM-5 catalysts. At the same time, the self-designed microwave reactor separated the pyrolysis of bamboo and catalytic upgrading of primary vapor, which made the catalytic effect more thorough. Experimental results indicated that CLD used TiO2 deposition to cover external acid sites, while BCI by phosphorus-nickel could cover and partly destroy superficial acid sites through two different ways. Within the scope of the loaded amount studied, the yield of aromatic hydrocarbons in the oil phase increased at first and then decreased, while the coke formation reduced continuously. BTX (benzene, toluene and xylene), the most valuable product in bio-oil, drastically increased by 39.1% and 22.6% respectively over the CLD and BCI modified catalysts. Considering the catalytic performance as well as cost, CLD over HZSM-5 has more advantages in the CFP process to upgrade bio-oil.

Low-cost synthesis of nanoaggregate SAPO-34 and its application in the catalytic alcoholysis of furfuryl alcohol

Silicoaluminophosphate-34 (SAPO-34) molecular sieves have important applications in the petrochemical industry as a result of their shape selectivity and suitable acidity. In this work, nanoaggregate SAPO-34 with a large external surface area was obtained by dissolving pseudoboehmite and tetraethylorthosilicate in an aqueous solution of tetraethylammonium hydroxide and subsequently adding phosphoric acid. After hydrolysis in an alkaline solution, the aluminum and silicon precursors exist as Al(OH)4− and SiO2(OH)−, respectively; this is beneficial for rapid nucleation and the formation of nanoaggregates in the following crystallization process. Additionally, to study the effect of the external surface area and pore size on the catalytic performance of different SAPO-34 structures, the alcoholysis of furfuryl alcohol to ethyl levulinate (EL) was chosen as a model reaction. In a comparison with the traditional cube-like SAPO-34, nanoaggregate SAPO-34 generated a higher yield of 74.1% of EL, whereas that with cube-like SAPO-34 was only 19.9%. Moreover, the stability was remarkably enhanced for nanoaggregate SAPO-34. The greater external surface area and larger number of external surface acid sites are helpful in improving the catalytic performance and avoiding coke deposition.

Improved performance of mesoporous HZSM-5 designed with selective deactivation of external surface acidity in the isomerization of styrene oxide

Abstract A series of mesoporous HZSM-5 designed by phosphorous modification with or without hexadecyltrimethylammonium bromide (CTAB) pretreatment were prepared and tested in the isomerization of styrene oxide. CTAB pretreatment changed the external surface properties of mesoporous HZSM-5 with positive charged, which could selectively adsorb the phosphate anions and deactivate the external Bronsted acid sites caused by a framework dealumination during calcinations. The phosphate species on the external surface could not migrate into the micropores. There were only a few phosphate species formed in the micropores and most of the internal Bronsted acid sites were preserved. External surface deactivation whilst retaining the hierarchical pore structure of mesoporous HZSM-5 was achieved by phosphorous modification with CTAB pretreatment, thus providing a potential industrial application catalyst which remarkably improves the catalytic stability (lifetime 59.8 h) while preserving the high activity (>99%) and selectivity (>96%).