H-mordenite Research Articles

Selective enrichment of Brønsted acid site in 8-membered ring channels of MOR zeolite to enhance the catalytic reactivity of dimethyl ether carbonylation

Directional adjusting of the distribution of Brønsted acid site (BAS) in zeolite to control its catalytic performance is highly attractive but remains challenging. This study aims to tailor the distribution of BAS in 8-membered ring (B8-MR) of H-Mordenite (HMOR) by adding aniline or p-phenylenediamine as additives to the sol‐gel. The results reveal that the number of B8-MR is significantly increased but with a slight change in the number of BAS in 12-membered ring channels. When using dimethyl ether (DME) carbonylation as the probe reaction, the conversion of DME to methyl acetate over the synthesized HMOR zeolite is increased by nearly 2 times. Our finding not only provides a facile strategy to regulate the acid distribution in different channels of the HMOR zeolite but also offers a promising candidate for the catalytic DME carbonylation, which is a critical intermediate step for ethanol synthesis from syngas.

Zinc enhances catalytic performance of pyridine-adsorbed HMOR: Dimethyl ether carbonylation

Dimethyl ether (DME) carbonylation to methyl acetate (MA) on H-mordenite (HMOR) zeolite is of critical importance for ethanol synthesis from syngas in industry, but it is generally accepted that carbonaceous deposits at Brønsted acid sites (BASs) in the twelve-membered ring (12-MR) lead to rapid deactivation of HMOR. Pyridine (Py) significantly prolong the catalytic lifetime of HMOR by selectively enter into the 12-MR to occupy the BASs. Moreover, loading Zn species into HMOR dramatically improve the MA formation. However, the role, active type and location of Zn species in HMOR are unclear. Herein, we comparatively dissected the catalytic mechanisms of DME carbonlyation in HMOR, pyridine-adsorbed HMOR (Py-HMOR), and three types of Zn-modified Py-HMOR systems by density functional theory (DFT) calculations, and confirmed Zn2+ ion doped Py-HMOR (Zn2+-Py-HMOR) system with the Zn2+ located between T3O31/T3O31′ and T3O33 sites in the 8-MR shows the optimal catalytic activity for DME carbonylation to MA. During the whole cycle of DME carbonylation, only the reaction step of acetyl intermediate with CH3OH is able to restore the active T3O31/T3O31′. Our Bader charge and charge density differences analyses revealed that the catalytic activity improvement of Zn2+-Py-HMOR system for DME decomposition and MA formation is stem from the enhancement of electrostatic interaction, more framework O sites in 8-MR involving in electron transfer, and matched spatial confinement. This work will provide indispensable inspiration for further modification of zeolite catalysts with 8-MR for DME carbonylation reaction.

Acidity and Local Confinement Effect in Mordenite Probed by Solid-State NMR Spectroscopy.

Herein, utilizing acetonitrile as the probe molecule, the acidity and host-guest interactions of H-mordenite (H-MOR) zeolites are investigated comprehensively by solid-state NMR spectroscopy and theoretical calculation. The locations and local configurations of Brønsted acid sites (BASs) in H-MOR are revealed by multinuclear and multidimensional NMR experiments with adsorption/coadsorption of acetonitrile (CD3CN) and trimethylphosphine (TMP). Moreover, the confinement effect of dual pores in MOR has been characterized via the quantitative determination of host-guest interactions between CH3CN and BASs. The 1H-15N dipolar measurement results and DFT calculations demonstrate that there are two kinds of acetonitrile molecules adsorbed in 12-membered ring (12MR) main channels with distinct mobility, where acetonitrile undergoes either partially restricted or highly flexible motion in the time scale of nanoseconds to microseconds. These two types of acetonitrile can exchange with temperature rising. In contrast, the mobility of acetonitrile in 8-membered ring (8MR) channels is very restricted due to the confinement of the framework.

Identifying the Active Silver Species in Carbonylation of Dimethyl Ether over Ag−HMOR

AbstractAs one of the key steps in ethanol production from syngas, dimethyl ether (DME) carbonylation to methyl acetate (MA) catalyzed by zeolite has drawn much attention. H‐mordenite (HMOR) modified with metal has been continuously developed for improving catalytic performance. Here, we investigated the role of Ag species through three series of Ag−HMOR catalysts prepared through altering Ag loading, varying reduction temperature and additional ion exchange. TEM, XPS, CO FTIR and UV‐Vis spectroscopy were employed to identify the nature and the amount of Ag species in xAg−HM with different Ag loading. Taken together with catalytic performance evaluation, 5Ag−HM with the moderate size of Ag0 species (Agnδ+ and Agm clusters) was proved to have greatest promotion effect on DME carbonylation. This was also evidenced by further improved MA formation rate over catalysts via elevating reduction temperature or via additional NH4Cl ion exchange, which contained more Ag0 and fewer Ag+ species respectively. These provide insight into metal‐modified HMOR, inspiring design and fabrication of zeolite catalysts with multi‐active sites.

Monomolecular Dehydration of Ethanol into Ethylene over H-MOR Studied by Density Functional Theory.

The framework effect of H-mordenite (H-MOR) zeolite on monomolecular dehydration of ethanol to ethylene has been simulated based on density functional theory. It is indicated that both the reaction mechanism and the activation energy barriers are significantly affected by the pore-confinement effect. In the 12-membered ring (12-MR), the energy barriers of the stepwise mechanism and the concerted mechanism are 35.0 and 42.4 kcal mol–1, respectively, suggesting that ethylene can be competitively formed through both pathways. While in the 8-membered ring (8-MR), the barrier of the concerted mechanism is 43.4 kcal mol–1, which is much lower than that of the stepwise mechanism with the ethoxy intermediate formation barrier of 53.7 kcal mol–1. Furthermore, the water molecule acts as the intermediate to stabilize the transition states (TSs) for both stepwise and concerted mechanisms and helps to transport protons during the reaction.

Mechanistic insights into controlled depolymerization of Chitosan using H-Mordenite

Kinetics of chitosan depolymerization were studied in dilute acetic acid solution, in presence of H-Mordenite (H-MOR). Rate constants for chitosan depolymerization were determined by measurement of molecular weight, using Gel permeation Chromatography (GPC). Depolymerization rate of chitosan was altered in presence of an acidic, porous material like H-MOR. Maximum concentration of H-MOR studied during process led to minimal increase in energy of activation, from 20.54 kJ/moL to 23.25 kJ/moL. Infra-red spectroscopy, adsorption studies and rheological assessment indicated adsorption /grafting of chitosan onto porous H-MOR surface as the possible mechanism for facilitation of the depolymerization process. Under extreme conditions investigated during process optimization, H-MOR resulted in a three-fold reduction in 5-Hydroxy Methyl Furfural (5-HMF) formation and over ten times decrease in glucosamine content, as compared to reactions conducted without H-MOR. Therefore, presence of H-MOR is imperative to cleave chitosan in controlled manner and obtain products of desired molecular weight, with fewer impurities.

Stable and facile ethanol synthesis from syngas in one reactor by tandem combination CuZnAl-HZSM-5, modified-H-Mordenite with CuZnAl catalyst

A direct ethanol synthesis route from syngas in one reactor is achieved through tandem combination of CuZnAl-HZSM-5, H-Mordenite (HMOR) with CuZnAl catalyst. The HMOR zeolite plays a key role determining the whole catalytic performance. It is found that, among the studied HMOR, HOMR-20 (Si/Al = 20) exhibits the best catalytic performance. After an induction period (around 7 h), the CO conversion and ethanol selectivity reaches 6.4% and 50.1%. Modifying HOMR-20 with Zn can improve the catalytic performance. The initial CO conversion and ethanol selectivity is increased to 7.4% and 69.5%. However, Zn-HMOR-20 still suffers from deactivation. Modifying Zn-HMOR-20 with pyridine can further remarkably enhance the stability. After a long running time of 200 h, no deactivation is observed and CO conversion and ethanol selectivity are maintained at the values higher than 7.0 and 52.0%. Modifying with pyridine can effective inhibit carbon deposition over Zn-HMOR-20, which is mainly responsible for the excellent stability.

Origin of High Selectivity of Dimethyl Ether Carbonylation in the 8-Membered Ring Channel of Mordenite Zeolite

The mechanisms of dimethyl ether (DME) carbonylation in the 8-membered ring (MR) and the 12-MR channels of H-mordenite (H-MOR) zeolite were separately investigated by quantum-chemical methods to unravel the origin of high selectivity of the methyl acetate product in the 8-MR channel. It is shown that not only the formation of surface methoxy species (SMS) but also its following carbonylation reaction are more energetically feasible in the 8-MR channel. Based on the kinetic analysis of the carbonylation reaction in the two channels, the effective rate constants were determined to reveal the reactivity difference. It was found that at 423 K, the effective rate constant in the 8-MR channel was ca. 13 magnitudes higher than that in the 12-MR channel, revealing the unique selectivity for acetyl intermediate formation in the 8-MR channel. The calculated host–guest interaction energies (Eint) provided evidence that the DME carbonylation reaction exclusively occurring in the 8-MR channel can be ascribed to the strong pore confinement. Among the different components of the Eint, the Pauli repulsive interaction (EPauli) is responsible for the SMS formation route, while the electrostatic (Ees) and orbital terms (Eoi) dictate the high selectivity of the carbonylation step in the 8-MR channel. These results suggested that the experimental observation of acetyl intermediate species solely in the 8-MR channel could be mainly ascribed to the difference of the Pauli repulsion, electrostatic interaction, and orbital interaction in the two channels. The theoretical results provide new insights into the DME carbonylation reaction mechanism over H-MOR zeolite.

Spacial hindrance induced recovery of over-poisoned active acid sites in pyridine-modified H-mordenite for dimethyl ether carbonylation

Zeolite catalysts, such as H-mordenite (H-MOR), are readily deactivated by coke deposition in carbonylation reactions. Pyridine modification of H-MOR can improve its stability but can lead to an undesirable loss in catalytic activity. Herein, we report the intrinsic impact of the pyridine adsorption behavior on H-MOR and the spacial hindrance of the zeolite frameworks on dimethyl ether (DME) carbonylation at a molecular level. We discovered that acid sites at O2 positions, located on common walls of eight-membered ring (8-MR) side pockets and 12-MR channels, were active in DME carbonylation, but were unfortunately poisoned during pyridine modification. Density functional theory calculations revealed that the pyridine-poisoned acid sites at the O2 positions could be easily regenerated due to the spacial hindrance of the zeolite frameworks. Accordingly, they can be facilely regenerated by proper thermal treatment, which induces 60% promotion in the catalytic activity along with a high stability. Our findings demonstrate the determining role of O2 positions in H-MOR for DME carbonylation and provide a new avenue for the rational design of other efficient zeolite-relevant catalytic systems.

Promoting dimethyl ether carbonylation over hot-water pretreated H-mordenite

Herein, we report the effect of the hot-water pretreatment on the catalytic activity of the H-mordenite (H-MOR) zeolite for DME carbonylation. The structure and acidity of the H-MOR catalysts pretreated by hot water were systematically characterized by XRD, nitrogen adsorption, SEM, H2O-TPD, NH3-TPD, 1H NMR and NH3-DRIFTS technologies. With the increase of the duration of the hot-water pretreatment, the catalytic activity shows a volcano-type variation. The H-MOR catalyst pretreated by hot water for 10 min at 573 K displays the highest catalytic activity among all of the catalysts. The appropriate duration of the hot-water pretreatment can generate new medium strong and strong Brønsted acid sites, which changes the microenvironment in the pores of H-MOR. Additionally, the newly generated Brønsted acid sites will accelerate the first step of DME carbonylation, that is, DME reacts with the Brønsted acid sites of the zeolite forming surface methoxy groups and methanol, improving the catalytic activity of the catalysts. However, too long duration of the hot-water pretreatment will remain water molecules in H-MOR, which inhibits DME carbonylation.

Rationalizing Acid Zeolite Performance on the Nanoscale by Correlative Fluorescence and Electron Microscopy.

The performance of zeolites as solid acid catalysts is strongly influenced by the accessibility of active sites. However, synthetic zeolites typically grow as complex aggregates of small nanocrystallites rather than perfect single crystals. The structural complexity must therefore play a decisive role in zeolite catalyst applicability. Traditional tools for the characterization of heterogeneous catalysts are unable to directly relate nanometer-scale structural properties to the corresponding catalytic performance. In this work, an innovative correlative super-resolution fluorescence and scanning electron microscope is applied, and the appropriate analysis procedures are developed to investigate the effect of small-port H-mordenite (H-MOR) morphology on the catalytic performance, along with the effects of extensive acid leaching. These correlative measurements revealed catalytic activity at the interface between intergrown H-MOR crystallites that was assumed inaccessible, without compromising the shape selective properties. Furthermore, it was found that extensive acid leaching led to an etching of the originally accessible microporous structure, rather than the formation of an extended mesoporous structure. The associated transition of small-port to large-port H-MOR therefore did not render the full catalyst particle functional for catalysis. The applied characterization technique allows a straightforward investigation of the zeolite structure–activity relationship beyond the single-particle level. We conclude that such information will ultimately lead to an accurate understanding of the relationship between the bulk scale catalyst behavior and the nanoscale structural features, enabling a rationalization of catalyst design.

Modifying the acidity of H-MOR and its catalytic carbonylation of dimethyl ether

Among the reactions catalyzed by zeolites there are some that exhibit high selectivity due to the spatial confinement effect of the zeolite framework. Tailoring the acidity, particularly the distribution and location of the Brönsted acid sites in the zeolite is effective for making it a better catalyst for these reactions. We prepared a series of H-mordenite (H-MOR) samples by varying the composition of the sol-gel, using different structure directing agents and post-treatment. NH3-TPD and IR characterization of adsorbed pyridine were employed to determine the amount of Brönsted acid sites in the 8-membered ring and 12-membered ring channels. It was shown that controlled synthesis was a promising approach to improve the concentration of Brönsted acid sites in MOR, even with a low Al content. Using an appropriate composition of Si and Al in the sol-gel favored a higher proportion of Brönsted acid sites in the 8-membered ring channels. HMI as a structure-direct agent gave an obvious enrichment of Brönsted acid sites in the 8-membered ring. Carbonylation of dimethyl ether was used as a probe reaction to examine the modification of the acid properties, especially the Brönsted acid sites in the 8-membered ring channels. There was a linear relationship between methyl acetate formation and the number of Brönsted acid sites in the 8-membered ring channels, demonstrating the successful modification of acid properties. Our results provide information for the rational design and modification of zeolites with spatial constraints.

Direct observation of DME carbonylation in the different channels of H-MOR zeolite by continuous-flow solid-state NMR spectroscopy.

The dynamic evolution of acetyl intermediates in the two different channels of H-mordenite (H-MOR) zeolite during dimethyl ether (DME) carbonylation is tracked by using in situ solid-state NMR spectroscopy under continuous-flow conditions. Thus, the reaction path via methyl acetate produced over active sites in 8 member ring (MR) channels, followed by diffusion into 12 MR channels, is proposed.

Ethanol direct synthesis from dimethyl ether and syngas on the combination of noble metal impregnated zeolite with Cu/ZnO catalyst

In this report, ethanol (EtOH) was selectively synthesized from dimethyl ether (DME) and syngas (CO+H2) with a dual-catalyst bed in a single reactor. H-Mordenite (H-MOR) zeolite was used to convert DME into methyl acetate (MA) through carbonylation reaction in the first stage reaction, and then EtOH was produced by MA hydrogenation over the Cu/ZnO catalyst at the second stage. In addition, the metals such as Pt, Pd or Ru were used to impregnate H-MOR catalyst by impregnation method to improve its catalytic activity. The combination of Pt/H-MOR catalyst with Pt loading amount of 0.4wt.% and Cu/ZnO catalyst showed excellent catalytic performance compared with the conventional combination of pure H-MOR catalyst and Cu/ZnO catalyst.

Coking on micrometer- and nanometer-sized mordenite during dimethyl ether carbonylation to methyl acetate

As compared to conventional micrometer-sized mordenite particles, nanometer-sized mordenite gave a much higher activity and better stability in dimethyl ether carbonylation to methyl acetate by suppressing the deposition of hard coke. A structural analysis revealed that intra-crystalline diffusion limitation of the reactants and products to and from the active sites inside the channels was greatly decreased by reducing the zeolite size to the nanometer size. Nanometer-sized H-mordenite (HMOR) showed a much higher reaction rate and stability than micrometer-sized HMOR, resulting from the improvement of molecule transportation efficiency by reducing the crystalline size.

Insight into Dimethyl Ether Carbonylation Reaction over Mordenite Zeolite from in-Situ Solid-State NMR Spectroscopy

Carbonylation of dimethyl ether (DME) with CO over H-mordenite (H-MOR) zeolites from 423 to 573 K was studied by in-situ C-13 solid-state NMR spectroscopy. The reaction was monitored separately in the 8-membered ring (MR) and in the 12-MR channels of the zeolite under identical conditions. The experimental results indicated that both 8-MR and 12-MR channels were capable of producing methyl acetate product but with remarkably different selectivities. At low-reaction temperature, surface acetyl species (CH3CO-) as stabilized acylium cation was solely identified in the 8-MR channels, which was confirmed by measurement of its characteristic chemical shift anisotropy (CSA) parameters. Additionally, the intermediate role of the acetyl species was evidenced by the fact that it could react with DME to form methyl acetate. This demonstrated the theoretically proposed specificity of the 8-MR channels for generation and stabilization of the acetyl intermediate which was considered as the rate-limiting step of carbonylation reaction. While in the 12-MR channels, the acetyl intermediate was not found during the reaction, and formation of hydrocarbons was favored. The absence of acetyl intermediate might account for the lower reactivity of the 12-MR channels to generate methyl acetate product.

Solid-State MAS NMR Studies of Brønsted Acid Sites in Zeolite H-Mordenite

(17)O-(1)H double resonance NMR spectroscopy was used to study the local structure of zeolite H-Mordenite. Different contact times were used in cross-polarization magic angle spinning (CPMAS) NMR, CP rotational-echo double resonance (CP-REDOR) NMR, and heteronuclear correlation (HETCOR) NMR spectroscopy to distinguish between Brønsted acid sites with different O-H distances. The accessibility of the various Brønsted acid sites was quantified by adsorbing the basic probe molecule trimethylphosphine in known amounts. On the basis of these experiments, locations of different Brønsted acid sites in H-Mordenite (H-MOR) were proposed. The use of (17)O chemical shift correlations to help assign sites is discussed.

Highly regioselective di-tert-amylation of naphthalene over reusable H-mordenite zeolite

Highly regioselective di-tert-amylation of naphthalene using different alcohols can be achieved over a H-mordenite (HM) zeolite. For example, the tert-amylation of naphthalene using tert-amyl alcohol in cyclohexane over HM (Si/Al = 10) zeolite has been optimised to give a 70% yield of 2,6-dialkylnaphthalenes, of which 2,6-di-tert-amylnaphthalene was produced in 46% yield along with 2-tert-amyl-6-tert-butylnaphthalene (23%) and 2,6-di-tert-butylnaphthalene (1%). This has been achieved by varying the reaction time, temperature, pressure and amounts of tert-amyl alcohol and zeolite. No 2,7-dialkylnaphthalenes were seen under the conditions tried. The zeolites can be easily regenerated by heating and then reused.

Conversion of Methanol Using Modified H-MOR Zeolite Catalysts

The conversion of methanol was carried out over various mordenite zeolite catalysts to evaluate their catalytic performance. A post-preparation treatment of the H-MOR catalyst by halogenation with NH 4 Cl or NH 4 F and by hydrohalogenation with HCl or HF was carried out and its effect on methanol conversion reactions at 100–300 °C in a continuous flow reactor was investigated. The as-synthesized H-mordenite (H-MOR) is generally more active during dimethyl ether (DME) production than the NH 4 -MOR and Na-MOR. Fluorinated treatment with HF or NH 4 F significantly improved the catalytic activity during methanol conversion and the formation of DME in comparison to chlorinated treatments with HCl or NH 4 Cl. This is principally attributed to the higher Si/Al ratio and an increase in the number of acid sites and their strength. Halogenation treatment with the acids of F − or Cl − gave the highest conversion activity for DME production compared to halogenation treatments with the salts of the same halogens. Zeolite dealumination by the acids was more profound than that by the halogen ion salts, which resulted in a decrease in the crystallinity and crystallite sizes of the zeolite. Dimethyl ether (DME) production from methanol was optimized because of its industrial importance as a future diesel fuel. An increase in the acidity and a decrease in the crystallite size of H-MOR by treatment with NH 4 Cl, NH 4 F, HCl, or HF led to an increase in DME production. However, hydrohalogenation was found to be more effective than halogenation.

Stability Enhancement of H-Mordenite in Dimethyl Ether Carbonylation to Methyl Acetate by Pre-adsorption of Pyridine

The carbonylation of dimethyl ether to methyl acetate over H-mordenite (HMOR) and pyridine-modified HMOR was compared. The catalytic stability of HMOR was improved significantly by pyridine pre-adsorption, and a yield of methyl acetate ∼30% was still obtained after 48 h on stream at 473 K. In situ infrared spectroscopy and ammonia temperature-programmed desorption revealed that pyridine preferentially occupied the acidic sites in 12-membered ring pores but not the acidic sites in 8-membered ring pores. 129Xe NMR studies suggested that the channels of HMOR were blocked by coke in the reaction but those in the pyridine-modified HMOR were not. The acidic sites in the 12-membered ring pores were responsible for the deactivation of HMOR, and the reaction can be directed to occur mainly on the acidic sties in the 8-membered ring pores by the selective adsorption of pyridine in the 12-membered ring pores. 对比研究了二甲醚在氢型丝光沸石和吡啶修饰的氢型丝光沸石上的二甲醚羰基化反应. 结果表明, 吡啶预吸附大大提高了二甲醚羰基化反应的稳定性, 并且在 473 K 反应 48 h 能够保持约 30% 的乙酸甲酯收率. 原位红外光谱和 NH3 程序升温脱附研究发现,吡啶吸附在 12 元环内, 而 8 元环内的酸性位基本不受干扰. 129Xe 核磁共振研究表明, 未经修饰的丝光沸石反应后孔道严重堵塞, 而吡啶修饰的分子筛反应后孔道基本不变. 因此, 12 元环内酸性位是积炭失活位, 吡啶吸附抑制了 12 元环内积炭的生成, 二甲醚羰基化反应能够在 8 元环内活性位上顺利进行.