Location Of Acid Sites Research Articles

Solvent structure and dynamics over Brønsted acid MWW zeolite nanosheets

In the liquid phase of heterogeneous catalysis, solvent plays an important role and governs the kinetics and thermodynamics of a reaction. Although it is often difficult to quantify the role of the solvent, it becomes particularly challenging when a zeolite is used as the catalyst. This difficulty arises from the complex nature of the liquid/zeolite interface and the different solvation environments around catalytically active sites. Here, we use ab initio molecular dynamics simulations to probe the local solvation structure and dynamics of methanol and water over MWW zeolite nanosheets with varying Brønsted acidity. We find that the zeolite framework and the number and location of the acid sites in the zeolite influence the structure and dynamics of the solvent. In particular, methanol is more likely to be in the vicinity of the aluminum (Al3+) at the T4 site than at T1 due to easy accessibility. The methanol oxygen binds strongly to the Al at the T4 site, weakening the Al–O for the bridging acid site, which results in the formation of the silanol group, significantly reducing the acidity of the site. The behavior of methanol is in direct contrast to that of water, where protons can easily propagate from the zeolite to the solvent molecules regardless of the acid site location. Our work provides molecular-level insights into how solvent interacts with zeolite surfaces, leading to an improved understanding of the catalytic site in the MWW zeolite nanosheet.

Regulating the distribution of acid sites in ZSM-11 zeolite with different halogen anions to enhance its catalytic performance in the conversion of methanol to olefins

Purposive regulation of the spatial location and distribution of acid sites in a zeolite catalyst, though still rather challenging, has proved to be an effective measure to enhance its performance in the conversion of methanol to olefins (MTO). Herein, a series of ZSM-11 zeolites were synthesized using different sodium halides (with tetrabutylammonium hydroxide (TBAOH) as the structure-directing agent); the effect of halogen anions (Cl-, Br- and I-) in the synthesis gel on the Al siting and thereon the spatial location and distribution of acid sites in the ZSM-11 zeolites as well as their relation to the catalytic performance in MTO was elaborately investigated. The results indicate that for the synthesis of high Si/Al ratio ZSM-11 zeolites (Si/Al > 60), the Cl- anions present in the synthesis gel can induce more Al atoms siting on the external surface of ZSM-11 crystals, thereby leading to a short catalytic lifetime in MTO due to the rapid coke deposition. In contrast, larger Br- or I- anions in the synthesis gel, owing to their stronger covalent interaction with the TBA+ cations, may moderate the formation of crystal silicate nuclei and lead to a uniform distribution of Al species in the inner straight channels, which can effectively suppress the surface coke deposition and inhibit the aromatic-based cycle for MTO. As a result, the high Si/Al ratio ZSM-11 zeolites obtained with Br- or I- in the synthesis gel exhibit much better catalytic stability in MTO and higher selectivity to propene and butene, in comparison with the ZSM-11 counterpart synthesized using Cl-. These results help to clarify the relation between the catalytic performance of ZSM-11 in MTO and the location of acid sites and thus bring forward a facile strategy to enhance the catalytic performance of zeolites through purposively regulating the spatial distribution of acid sites.

Clarification of acid site location in MSE-type zeolites by spectroscopic approaches combined with catalytic activity: comparison between UZM-35 and MCM-68.

MSE-type zeolites synthesized by different organic structure-directing agents (OSDAs), UZM-35 and MCM-68, were prepared. The location of Brønsted acid sites derived from the framework Al atoms and acidic properties were investigated based on 27Al MQMAS NMR and in situ IR techniques combined with the evaluation of the catalytic activity. We have successfully found a significant difference in the location of Brønsted acid sites in the MSE-type framework; 61 and 33% of acid sites were located at the 12-ring channel for MCM-68 and UZM-35, respectively. The differences in the location of the acid sites yielded their unique catalytic activities for the hydrocarbon cracking reactions, indicating that a well-chosen type of OSDAs for the synthesis is one of the possibilities for controlling the distribution of the framework Al atoms in the MSE-type framework.

Catalytic roles of the acid sites in different pore channels of H-ZSM-5 zeolite for methanol-to-olefins conversion

H-ZSM-5 zeolite is a typical catalyst for methanol-to-olefins (MTO) conversion. Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework, the catalytic roles of the acid sites in different pore channels of the H-ZSM-5 zeolite are not well understood. In this study, the MTO reaction network, involving the aromatic cycle, alkene cycle, and aromatization process, and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel, sinusoidal channel, and intersection cavity of H-ZSM-5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations. The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels. In contrast, the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels, whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities. Consequently, the catalytic performance of H-ZSM-5 zeolite for MTO conversion, including activity and product selectivity, can be regulated properly through the purposive alteration of the acid site distribution, viz., the location of Al in the zeolite framework. This study helps to elucidate the relation between the catalytic performance of different acid sites in the H-ZSM-5 zeolite framework for MTO conversion, which should greatly benefit the design of efficient catalyst for methanol conversion.

Proton Mobility, Intrinsic Acid Strength, and Acid Site Location in Zeolites Revealed by Varying Temperature Infrared Spectroscopy and Density Functional Theory Studies.

The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable characterization of zeolites' intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials ( Eact. < 40 kJ mol-1) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si-(OD)-Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O-(H/D) bond strength. This observation allowed the extrapolation and estimation of precise v(O-D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of the position of a proton in ZSM-5 zeolites under catalytically relevant conditions ( T > 423 K), which has so far never been achieved by any other technique.

Selective Introduction of Acid Sites in Different Confined Positions in ZSM-5 and Its Catalytic Implications

Controlling the location of acid sites in zeolites can have a great effect on catalysis. In this work we face the objective of directing the location of Al into the 10R channels of ZSM-5 by taking advantage of the structural preference of B to occupy certain positions at the channel intersections, as suggested by theoretical calculations. The synthesis of B-Al-ZSM-5 zeolites with variable Si/Al and Si/B ratios, followed by B removal in a postsynthesis treatment, produces ZSM-5 samples enriched in Al occupying positions at 10R channels. The location of the acid sites is determined on the basis of the product distribution of 1-hexene cracking as a test reaction. The higher selectivity to propene and lower C4=/C3= ratio in the samples synthesized with B and subsequently deboronated can be related to a larger concentration of acid sites in 10R channels, where monomolecular cracking occurs. Finally, several ZSM-5 samples have been tested in the methanol to propene reaction, and those synthesized through the B-...

Accessibility and Location of Acid Sites in Zeolites as Probed by Fourier Transform Infrared Spectroscopy and Magic Angle Spinning Nuclear Magnetic Resonance

The understanding of location and accessibility of zeolite acid sites is a key issue in heterogeneous catalysis. This paper provides a brief overview of Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) characterisation of acidity in zeolites based on the application of test molecules with a diverse range of basicity and kinetic diameters. Many zeolites, including ZSM-5 and BEA, have been characterised by monitoring the interaction between the zeolite acid sites and test molecules, such as 1,3,5-triisopropylbenzene, pyridine and alkylpyridines, to probe the location, accessibility and strength of the Brønsted acid sites. 1,3,5-triisopropylbenzene can be used to distinguish Brønsted acid sites located on the external and internal surface in most medium and large pore channel zeolites. Brønsted acid sites on the external surface of medium pore zeolites can also be quantified using 2,6-di-tert-butyl-pyridine and 2,4,6-trimethylpyridine. It is concluded that using a combination of probe molecules, including co-adsorption experiments, affords differentiation between acid sites located in channels and cavities of different sizes and on the external and internal surfaces of various zeolitic structures.

Butylene catalytic cracking to propylene over a hierarchical HZSM-5 zeolite: Location of acid sites controlling the reaction pathway

A hierarchical ZSM-5 zeolite was prepared by steam-assisted crystallization (SAC) method and then the acidity of zeolites was modified by using organic silane compounds (catalytic cracking of silane, or the CCS method). The obtained zeolite was finally applied in the process of butylene catalytic cracking to produce propylene. High yield of propylene, 40 wt%, was achieved and it was much higher than that achieved on other prepared zeolites. This rise in propylene yield is mainly ascribed to the specific location of framework aluminum of zeolite in crystal sphere. Compared with the ZSM-5 zeolite synthesized by hydrothermal method, the ZSM-5 zeolite prepared from SAC method shows more acid sites in straight and sinusoidal channels rather than the intersection of channels, which is beneficial to reinforcing the reaction pathway of producing propylene and simultaneously suppressing the main side reactions in butylene catalytic cracking. And since the external acid sites of zeolite were covered by using the CCS method, the main side reactions were further inhibited. These results also help to elucidate the catalytic behavior of acid sites in different locations of HZSM-5 in butylene catalytic cracking and better understand the processing of SAC method.

Understanding Brønsted-Acid Catalyzed Monomolecular Reactions of Alkanes in Zeolite Pores by Combining Insights from Experiment and Theory.

Acidic zeolites are effective catalysts for the cracking of large hydrocarbon molecules into lower molecular weight products required for transportation fuels. However, the ways in which the zeolite structure affects the catalytic activity at Brønsted protons are not fully understood. One way to characterize the influence of the zeolite structure on the catalysis is to study alkane cracking and dehydrogenation at very low conversion, conditions for which the kinetics are well defined. To understand the effects of zeolite structure on the measured rate coefficient (kapp ), it is necessary to identify the equilibrium constant for adsorption into the reactant state (Kads-H+ ) and the intrinsic rate coefficient of the reaction (kint ) at reaction temperatures, since kapp is proportional to the product of Kads-H+ and kint . We show that Kads-H+ cannot be calculated from experimental adsorption data collected near ambient temperature, but can, however, be estimated accurately from configurational-bias Monte Carlo (CBMC) simulations. Using monomolecular cracking and dehydrogenation of C3 -C6 alkanes as an example, we review recent efforts aimed at elucidating the influence of the acid site location and the zeolite framework structure on the observed values of kapp and its components, Kads-H+ and kint .

Determination of Acid Site Location in Dealuminated MCM-68 by 27Al MQMAS NMR and FT-IR Spectroscopy with Probe Molecules

A series of MCM-68 zeolites with different Si/Al ratios were prepared by treatment with nitric acid and compared with beta zeolites. Speciation of aluminum and location of acid sites changed depending on the Si/Al ratio. The location of Bronsted acid sites in MCM-68 samples was able to be classified by FT-IR measurements with pyridine and 2,6-di-tert-butylpyridine as probe molecules, and the number of Bronsted acid sites was quantified according to the locations. For high-aluminum MCM-68, Bronsted acid sites were broadly distributed in both the 12-ring channel and 10-ring windows as well as inside the supercage. The Bronsted acid sites in the 12-ring channel were easily removed by the acid treatment, and consequently, high-silica MCM-68 had Bronsted acid sites predominantly in the 10-ring windows and inside the supercage. 27Al MQMAS NMR spectra of high-silica MCM-68 showed two specific peaks assignable to T6 and T7 sites, which did not face the 12-ring channel, forming the Bronsted acid sites highly toler...

Mechanistic insights into the catalytic role of various acid sites on ZSM-5 zeolite in the carbonylation of methanol and dimethyl ether

The catalytic performance of ZSM-5 in carbonylation is strongly related to the location of acid sites in the zeolite framework.

External or internal surface of H-ZSM-5 zeolite, which is more effective for the Beckmann rearrangement reaction?

The influence of Brønsted acid site location on the Beckmann rearrangement reaction over H-ZSM-5 zeolite has been explored.

Inelastic Neutron Scattering Study on the Location of Brønsted Acid Sites in High Silica LTA Zeolite

The presence of Bronsted acid sites (SiOHAl) is crucial in many of the applications of zeolites in petroleum refining and related industries. This acidity relies on the number and location of acid sites, but the latter is difficult to know accurately in spite of a large number of experimental techniques available. Zeolites usually contain many types of acid sites depending on the crystallographic different O-sites and their chemical environment. In the present work we show how inelastic neutron scattering (INS) can be employed for an accurate determination of the location of Bronsted acid sites in zeolites. This is achieved using a sample of LTA zeolite with few defects of connectivity, few (O1H, O2H, O3H) crystallographically different oxygens, and high Si/Al ratio. The narrow and intense peaks corresponding to SiOH bending modes ca. 1100 cm–1 have been shown, using periodic DFT calculations, to strongly depend on the type of oxygen to which the proton is bonded. A comparison between calculations and INS...

Modelling metal centres, acid sites and reaction mechanisms in microporous catalysts.

We discuss the role of QM/MM (embedded cluster) computational techniques in catalytic science, in particular their application to microporous catalysis. We describe the methodologies employed and illustrate their utility by briefly summarising work on metal centres in zeolites. We then report a detailed investigation into the behaviour of methanol at acidic sites in zeolites H-ZSM-5 and H-Y in the context of the methanol-to-hydrocarbons/olefins process. Studying key initial steps of the reaction (the adsorption and subsequent methoxylation), we probe the effect of framework topology and Brønsted acid site location on the energetics of these initial processes. We find that although methoxylation is endothermic with respect to the adsorbed system (by 17-56 kJ mol(-1) depending on the location), there are intriguing correlations between the adsorption/reaction energies and the geometries of the adsorbed species, of particular significance being the coordination of methyl hydrogens. These observations emphasise the importance of adsorbate coordination with the framework in zeolite catalysed conversions, and how this may vary with framework topology and site location, particularly suited to investigation by QM/MM techniques.

Control of the Al Distribution in the Framework of ZSM-5 Zeolite and Its Evaluation by Solid-State NMR Technique and Catalytic Properties

The effects of the organic structure-directing agents (OSDAs) and Na cations for the synthesis of ZSM-5 on the location of Al atom in the framework as well as the acidic and catalytic properties were investigated. To achieve these purposes, ZSM-5 zeolites were synthesized by using four kinds of OSDAs including tetrapropylammonium hydroxide cations, dipropylamine, cyclohexylamine, and hexamethylenimine with or without Na cations. In situ FT-IR spectroscopy using CO as probe molecule was applied to the evaluation of the acid property of the ZSM-5 zeolites. The location of Al atoms was examined by high resolution 27Al MAS and MQMAS NMR techniques. The constraint index (CI) has also been used to estimate the distribution of acid sites in the micropores. The location of acid sites was investigated based on the difference in the transition-state shape-selectivity through the cracking of n-hexane and 3-methylpentane. Furthermore, the cracking of various types of paraffins and the conversion of aromatic compounds were conducted to clarify the acid site distributions.

Catalytic cracking of n-alkane naphtha: The impact of olefin addition and active sites differentiation

An extended dual kinetic model allows to fit the n-heptane cracking results working in a wide range of reaction conditions. The duality of the model is provided by the contribution of monomolecular and bimolecular cracking mechanisms. It takes into account the role played by the olefins formed on the global cracking or added within the feed. Furthermore by means of this model and the kinetic parameters obtained when cracking n-heptane on ZSM-5, it has been observed that, while some characterization techniques show a homogeneous zeolite surface from the point of view of the active sites, rigorous kinetic experiments point to the possibility that the reactant sees a heterogeneous surface with, at least, two groups of cracking active sites. Those differentiated active sites give different cracking rates and different activation energies for the process and, in the case of ZSM-5, could be assimilated to sites pointing to the 10R channels and sites pointing into the crossing of the 10R channels, mainly due to differences in acid site location and confinement effects.

Comparative Study of Acidic Properties of UZM-5 Nanosheets by Stepwise Adsorption Assisted by a Selective External Poisoning Approach

UZM-5 zeolite was hydrothermally synthesized using a TMA+/TEA+ mixed-template as a structure-directing agent (SDA). The resulting sample displays a sheet-like morphology and a uniform particle size of 100–300 nm. The thickness of the particles is ca. 15 nm, corresponding to 5 unit cells. UZM-5 has an 8-membered ring (8-MR) micropore system with a pore diameter of 3.8 A, a secondary mesopore system with a narrow pore size distribution centered at 3.5 nm, specific surface area of 576 m2/g and significantly large external surface area of 122 m2/g. The nature and the location of acid sites were determined from the temperature-programmed desorption (TPD) of basic probes with various sizes and an FT-IR method, combined with topological analysis. It was found that the density of the acid sites on the external surface of particles is higher than that in the micropores. Regarding the acid sites on the external surface of particles, the highly exposed double 4-ring (D4R) units represent dominantly strong Bronsted-t...

Cooperative effects of secondary mesoporosity and acid site location in Pt/SAPO-11 on n-dodecane hydroisomerization selectivity

Two different methods, organosilane templating and carbon templating, were used to generate secondary mesoporosity within SAPO-11. The former method produced mesoporous SAPO-11 samples with a large amount of external acidity, while the latter produced mesoporous samples containing predominantly internal acid sites. It was shown that, depending on the location of Brønsted acid sites, Pt supported on mesoporous SAPO-11 samples can exhibit widely ranging maximum isomerization yields from 13% to 84%, while solely microporous SAPO-11 showed 44% isomerization yield. The results indicate that the presence of secondary mesoporosity can be either remarkably beneficial or detrimental to hydroisomerization selectivity depending on the spatial location of the acid sites. The present results clearly showed that hydroisomerization selectivity of the catalysts can be significantly enhanced by facilitating the hydrocarbon diffusion via the formation of secondary mesopores while suppressing the formation of external acid sites that can non-selectively catalyze consecutive cracking reactions.

Periodic quantum chemical studies on anhydrous and hydrated acid clinoptilolite.

Periodic quantum chemistry methods as implemented in the crystal09 code were considered to study acid clinoptilolite (HEU framework type), both anhydrous and hydrated. The most probable location of acid sites and water molecules together with other structural details has been the object of particular attention. Calculations were performed at hybrid and pristine DFT levels of theory with a VDZP quality basis set in order to compare performances. It arises that PBE0 provides the best agreement with experimental data as concerns structural features and the most stable Al distribution in the framework. The role of the water molecule distribution in the stability of the systems, the most probable structure that they induce in the material, and their eventual influence on further chemical modification processes, such as dealumination, are discussed in detail. Results show that, apart from the usually considered interactions of water molecules with the zeolite framework, that is, a H-bond with Brönsted acid sites and coordination with framework Al as Lewis ones, it is necessary to consider cooperation of other weaker effects so as to fully understand the hydration effect in this kind of materials.

Evidences for the relationship between surface structure and reactivity of goethite nanoparticles based on advanced molecular-probe methods

Surface properties of two goethites have been studied in order to compare the amount of acid surface sites and their distribution over the various surface domains. For this purpose, ammonia, pyridine and nitrogen were used as basic molecular probes. Calorimetry measurements of ammonia adsorption provided the image of the average surface acidity being moderate. This conclusion was supported by the moderate resistance of the adsorbed pyridine molecules to degassing conditions. Adsorption and desorption of pyridine prior to gaseous nitrogen adsorption resulting in masking/unmasking of acid surface sites on the goethite surface allowed confirmation of the acid character of the specific adsorption sites characterized by the high-energy adsorption of electron-donating molecular nitrogen. The amount of acid sites probed by nitrogen and ammonia were of the same order of magnitude but systematically higher for ammonia. The subsequent analysis of the argon and nitrogen derivatives of first-layer adsorption isotherm led to determine the distribution of {101} and {121} crystallographic faces and discuss the location of acid sites on these surface domains.