Acid Sites In Zeolites Research Articles

Construction of Bifunctional Co/H-ZSM-5 Catalysts for the Hydrodeoxygenation of Stearic Acid to Diesel-Range Alkanes.

Bifunctional Co/H-ZSM-5 zeolites were prepared by a surface organometallic chemistry grafting route, namely, by the stoichiometric reaction between cobaltocene and the Brønsted acid sites in zeolites. The catalyst was applied to a model reaction of the catalytic hydrodeoxygenation of stearic acid (SA). The cobalt species existed in the form of isolated Co2+ ions at the exchange positions after grafting, transformed to CoO species on the surface of the zeolite, stabilized inside the zeolite channels upon calcination in air, and finally reduced by hydrogen to homogeneous clusters of metallic cobalt species approximately 1.5 nm in size. During this process, the Brønsted acid sites of the H-ZSM-5 zeolites were preserved with a slightly reduced acid strength. The as-prepared bifunctional catalyst exhibited an approximately 16 times higher activity for the hydrodeoxygenation of SA (2.11 gSA gcat-1 h-1 ) than the reference catalyst (0.13 gSA gcat-1 h-1 ) prepared by solid-state ion exchange and a high C18 /C17 ratio of approximately 24. The remarkable hydrodeoxygenation performance of the bifunctional Co/H-ZSM-5 was owed to the effective synergy between the uniformed metallic cobalt clusters and the Brønsted acid sites in H-ZSM-5. The simplified reaction network and kinetics of the SA hydrodeoxygenation catalyzed by the as-prepared bifunctional Co/H-ZSM-5 zeolites were also investigated.

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.

Hydrodeoxygenation of phenolic compounds to cycloalkanes over supported nickel phosphides

SiO2, HZSM-5 and Al2O3 were used to support nickel phosphides to prepare hydrodeoxygenation (HDO) catalysts. The nickel loading was kept at 20 wt% while the Ni/P molar ratio was varied among 3, 2, and 1 in the preparation by incipient wetness impregnation. XRD characterization revealed that Ni3P, Ni12P5, and Ni2P as the major crystal phases were obtained at Ni/P ratio of 3, 2, and 1, respectively, on SiO2 and HZSM-5. When Al2O3 was used as the support, nickel metal rather than nickel phosphides was generated. Among SiO2-supported nickel phosphides, Ni3P exhibited highest hydrogenation activity and catalytic performance in phenol HDO. Ni3P/HZSM-5 showed the high catalytic performance in HDO of phenol as well as catechol and o-cresol, with Ni3P as the hydrogenation site and the acid sites in HZSM-5 zeolite as the dehydration site. The strong acidity in HZSM-5 also facilitated the isomerization of cycloalkanes at elevated temperatures.

Robust nickel cluster@Mes-HZSM-5 composite nanostructure with enhanced catalytic activity in the DTG reaction

Isoparaffin-rich gasoline plays an essential role in the global energy consumption. In this regard, there have been extensive studies to prepare catalysts for efficient synthesis of isoparaffin-rich gasoline. In this paper, we report a protocol to incorporate Ni nanoclusters (NiNC) into mesoporous Mes-HZSM-5 zeolite for the synthesis of NiNC@Mes-HZ catalysts. The catalysts are prepared via impregnation of Ni6(PET)12 clusters into Mes-HZSM-5 zeolite, followed by annealing at 550 °C in air. The NiNC@Mes-HZ catalysts are characterized by TEM, XRD, H2-TPR, NH3-TPD, ICP-MS, as well as N2-physical adsorption method. The Ni clusters (average diameter: ca. 1.2–2.7 nm) are found to locate at mesoporous area of the zeolite and interact with Brønsted acid sites of the HZSM-5 zeolite, evidenced by the pyridine adsorption Fourier-transform infrared spectrum analysis. The NiNC@Mes-HZ catalysts (Ni cluster loading = 0.11 wt%) exhibit 100% conversion of dimethyl ether (DME) with 66.4C% selectivity towards C5-11. Results show 53.5C% of C5-11 product contains isoparaffin. This is considerably higher than that for Mes-HZSM-5 zeolite (27.5C% isoparaffin selectivity) and xNiNP@Mes-HZ (73.4% DME conversion) catalysts in the DTG process at 350 °C. The higher selectivity of the 0.11NiNC@Mes-HZ catalysts towards isoparaffin production is deemed to associate with the ease of DTG reaction to take place at the interface of the Ni nanoclusters and acidic sites of the zeolite. Further, it is shown that the nickel nanoclusters largely improve the durability of Mes-HZSM-5 zeolite (over 205 h), which is mainly due to the inhibition of carbon deposition production during the DTG process, evidenced by TPO-MS analysis of the used NiNC@Mes-HZ. The Ni nanoclusters in the zeolite show good hydrogenation capacity and cracking performance, enhancing the olefin methylation pathway on the acidic sites of Mes-HZSM-5 and attenuating the aromatic methylation cycle during the DTG reaction.

Specific zone within 8-membered ring channel as catalytic center for carbonylation of dimethyl ether and methanol over FER zeolite

The DFT investigation of methanol and dimethyl ether (DME) carbonylation at different sites of FER zeolite is carried out in an attempt to explore whether some specific acid sites are present to selectively catalyze the desired reaction. It is shown that the surface methoxy groups (SMG), generated by the reaction of methanol or DME with the Brønsted acid sites of zeolites, preferentially forms in the 8-membered ring (8MR) and 10-membered ring (10MR) channels. Further study of the CO attack on the SMG shows that, the carbonylation reaction occurs selectively at the 6MR zone of 8MR channel of FER zeolites. This result not only provides a theoretical perspective for the experimentally observed higher carbonylation reactivity of SMG located inside the 8MR channels, but also makes the location of the reaction more accurate.

Exploring the acid catalyzed isomerization of phenanthrene under confinement in mordenite

The isomerization reaction of 1,2-dihydrophenanthrene catalyzed by a mordenite zeolite is explored theoretically in the present study. It was found that the electrostatic adducts which resemble ion-pair and π complexes, detected for zeolite catalysts, could enable the reaction to proceed through an energy favorable channel and explain its selectivity. The reaction pathway found for the interaction of 1,2-dihydrophenanthrene with the acid site of a mordenite zeolite proceeds firstly through a hydrogen abstraction, followed by a concerted cyclization pathway with C-C bond β-scission assisted by the reagent interaction with the zeolite catalyst. Then, two sequential migrations take place over the five-ring cycled product leading to unstable carbocation intermediate species which could be hydrogenated by an H2 molecule. The Al site within the MOR zeolite framework, does not activate the H-H bond. The C-C bond scission-cyclization process is detected as the energetic limiting step that could be fostered due to the confinement effect induced by the zeolite porous structure.

Enhanced chemical activity and wettability at adjacent Brønsted acid sites in HZSM-5

The adjustable acid sites in zeolites and its well-defined pore structure allows for a fine-tuning of the catalytic performance. The activity and selectivity of several reactions have been shown to be dependent on the location and distribution of the acid sites in the zeolite. However, the underlying mechanisms responsible for this dependence remain to be explored. Here, we report density functional theory calculations, through which we investigate the impact of proximity of Brønsted acid sites in the HZSM-5. We find that Brønsted sites with close spatial proximity can significantly strengthen the adsorption of water, which is used as a molecular probe for the local activity. We find that a water molecule can form H-bonds with two adjacent sites with increased adsorption energy. This increase is attributed to enhanced polarization of the water molecule through pronounced interfacial charge transfer. In some case, we observe that the proton becomes detached from the zeolite framework and forms a hydronium ion. We further show that hybrid functional calculations are essential for accurately describing the structural stability when two sites are located in close proximity of each other. Both, the enhanced polarization and proton delocalization may affect activity and selectivity for zeolite-catalyzed reactions.

Effect of Nature of Heteroelement (Ba, Ga, Al) on Adsorption and Acid Characteristics of Hierarchical Porous Zeolites of MOR, BEA, and MTW Structural Types

The strength of the acid sites in isostructural hierarchical porous zeolites increases in the order BSiO < GaSiO < AlSiO, and the concentration in the structural types increases in the order MTW < BEA < MOR. Zeolite materials with BEA and MOR topology, which consist of nanocrystals with ultrathin-layer morphology, are characterized by a marked decrease of the adsorption potential compared with isostructural materials with nanorods or nanosphere morphology due to a 1.5-fold decrease in the isosteric heats of adsorption of triethylamine.

Methanol Desorption from Cu-ZSM-5 Studied by In Situ Infrared Spectroscopy and First-Principles Calculations

The dynamic interaction of methanol and its derivatives with Cu-exchanged ZSM-5 during methanol temperature-programmed desorption from 30 to 450 °C has been investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy and first-principles calculations. The results emphasize that defects in the framework structure of the zeolite and Bronsted acid sites constitute ion-exchange sites for Cu ions. The Cu sites introduced in ZSM-5 actively interact with methanol adsorbed at moderate temperature, i.e., below 250 °C, and take roles in further oxidation of the adsorbed species to formate and CO. Moreover, spectra recorded at higher temperatures, i.e., above 300 °C, after adsorption of methanol show strong interaction between methoxy groups and the zeolite framework, suggesting that, under mild conditions, proton extraction for methanol production during direct partial oxidation of methane to methanol over Cu-ZSM-5 is necessary.

Brønsted/Lewis Acid Synergy in Methanol-to-Aromatics Conversion on Ga-Modified ZSM-5 Zeolites, As Studied by Solid-State NMR Spectroscopy

1H–71Ga internuclear spatial proximity/interaction between Bronsted acid site (BAS) and cationic Ga species (Lewis acid sites) in Ga-modified ZSM-5 zeolites, which leads to a synergic effect in the methanol-to-aromatics (MTA) conversion, was identified with solid-state NMR spectroscopy. The internuclear distance between BAS and Ga species was measured, which is similar to that of a neighboring BAS pair located in the six-membered rings of ZSM-5. The Bronsted acidity of the Ga-modified zeolite was considerably enhanced due to the synergic effect, and the synergic active sites were quantified by 1H–71Ga double-resonance solid-state NMR, which shows a correlation with the aromatics selectivity in the MTA reaction.

Catalytic Pyrolysis of Chilean Oak: Influence of Brønsted Acid Sites of Chilean Natural Zeolite

This paper proposes the Chilean natural zeolite as catalyst on bio-oil upgrade processes. The aim of this study was to analyze chemical composition of bio-oil samples obtained from catalytic pyrolysis of Chilean native oak in order to increase bio-oil stability during storage. In order to identify chemical compounds before and after storage, biomass pyrolysis was carried out in a fixed bed reactor at 623 K and bio-oil samples were characterized by gas chromatography/mass spectrophotometry (GC/MS). A bio-oil fractionation method was successfully applied here. Results indicate that bio-oil viscosity decreases due to active sites on the zeolite framework. Active acids sites were associated with an increment of alcohols, aldehydes, and hydrocarbon content during storage. Higher composition on aldehydes and alcohols after storage could be attributed to the occurrence of carbonyl reduction reactions that promotes them. These reactions are influenced by zeolite surface characteristics and could be achieved via the direct contribution of Brønsted acid sites to Chilean natural zeolite.

Hydroisomerization of n-hexadecane using Pt/alumina-Beta zeolite catalysts for producing renewable diesel with low pour point

The performance of bifunctional Pt/Alumina-Beta zeolite catalysts for the hydroisomerization (HDI) of n-C16 was studied by varying the zeolite content in the support. A relationship between overall activity and support acidity was observed, indicating that the skeletal isomerization over zeolite acid sites is the rate-determining step. Isomerization and cracking products distributions are independent of the zeolite content in the support for the entire range of n-C16 conversion. The catalysts of the present study are more selective than those previously reported in the literature with Beta zeolite, which is probably associated to the use of a high Si/Al zeolite. Finally, it was observed that the pour point of the hydroisomerized products decreases with the overall conversion and correlates with the primary and secondary carbon contents of the products.

Effect of Enhanced Accessibility of Acid Sites in Micromesoporous Mordenite Zeolites on Hydroisomerization of n-Hexane

This paper describes a study of the nature and the accessibility of the acid sites in micromesoporous mordenite zeolites obtained by desilication and dealumination and analysis of their activity and selectivity in the hydroisomerization of n-hexane. Alkaline–acid, acid–alkaline–acid, and fluorination–alkaline–acid postsynthesis treatments were employed for the preparation of micromesoporous mordenites. The FTIR spectra of adsorbed d3-acetonitrille, 27Al MAS NMR, HR-TEM, and N2 adsorption were used for quantitative analysis of the Bronsted and Lewis sites, the coordination of Al atoms, and the textural properties. The alkaline treatment causes desilication, preferably occurring along the crystal defects and resulting in the formation of a secondary mesoporous structure characterized by 5–20 nm cavities and the formation of extraframework (AlEx) species and terminal Si–OH groups. The AlEx species formed by hydrolysis of perturbed or dislodged framework Al easily restrict part of the pseudomonodimensional ch...

A first-principles evaluation of the stability, accessibility, and strength of Brønsted acid sites in zeolites

Catalytic reactions of zeolites occur mainly at their acid sites; however, there is a lack of comprehensive method to characterize the acid sites by multiple aspects due to their complexity associated with both the host structures and the origins of formation. Here we propose a comprehensive computational approach, from both thermodynamic and kinetic points of view, to evaluate jointly the energetic stability, space accessibility, and acid strength (SAS) critical to the catalytic performance of acid sites in zeolites. In this work, we evaluated the SAS systematically for various potential Brønsted acid sites in zeolites, using density functional methods and surface area calculations. The total energy was used to compare the relative stabilities of various acid sites of the same type. The deprotonation energy (DPE) was calculated to represent the acid strength. Moreover, we proposed a concept of local accessible surface area (LASA) to measure the accessibility of a localized surface region of molecular sites of complex shape. The SAS were evaluated comprehensively for various Brønsted acid sites including bridging hydroxyl (BH), nest hydroxyl (NH), and terminal silanol (TS) in the internal pores and on the external surfaces of MFI-type zeolite. We found the most stable acid sites are the Al2/H2, Al9/H2, and Al1/H1 sites, respectively, for intra-crystalline BH, and BHs on the (010) and (100) external surfaces. The most stable NH sites are located at the T4, T7, and T5 defective sites, respectively, in the internal pores, and on the (010) and (100) external surfaces. The accessibility of an acid site depends on the OH orientation at the studied acid sites. Most of the NH sites are difficult to access due to the connected hydrogen bonds. The acid strength of the studied Brønsted acid sites decreases as BH>NH>TS. Both the pore geometry and hydrogen bonds can enhance the acid strength. The first-principles computation of the SAS provides a comprehensive approach to characterize acid sites that facilitates evaluation of catalytic performance for the design of zeolite catalysts.

Motions of Trimethylphosphine Oxide in Carbon Nanotubes as Revealed by Solid‐state NMR

Trimethylphosphine (TMP) has been adsorbed in carbon nanotubes and oxidized to trimethylphosphine oxide (TMPO). 31P spin–lattice relaxation T1 measurements at temperatures from 203 to 333 K have been employed to investigate the motions of TMPO. The 31P T1 is found to be monoexponential and isotropic. The activation energy is measured as 4.4 ± 0.3 kcal•mol−1, which is comparable to that of TMP adsorbed on Lewis acid sites in zeolites.

Crucial Role of Weak Acid Sites for Catalytic Performance of Zeolites in Ethyl tert-butyl Ether Synthesis

Catalytic activity of different zeolites: H,NH4-form of mordenite-containing rock (H-CMK) and H-Beta with a Si/Al ratio of 15–407 (H-ВЕА) in ethyl tert-butyl ether (ETBE) synthesis in a packed-bed flow reactor at 80–180°C and 1 MPa has been studied. Acid characteristics of zeolites were determined by stepwise (Quasi-Equilibrium) ammonia thermodesorption. Three types of acid sites of different strength has been found, which is marked as weak (ENH3 = 60–75 kJ/mol), medium (ENH3 = 86–123 kJ/mol), and strong (ENH3 = 112–145 kJ/mol). The correlation between ETBE productivity and the concentration of weak acid sites has been found. Thereby, it was established that weak acid sites of zeolites are the active sites in ETBE synthesis.

Reaction and deactivation of propylene over SAPO-34 at low temperature

We report the low temperature conversion of propylene with SAPO-34 zeolite as the catalyst, as a branched route in process of methanol to propylene (MTP). The maximum conversion of propylene occurred at 300°C, corresponding to the activation of propylene over low temperature acid sites of zeolite. With rate constant calculation and coke analysis at low temperature, we confirmed that adamantane is easily produced from propylene and it does not contribute to the increased conversion of reactants, but results in the cage blockage of zeolite. By detailed coke analysis and product distribution analysis, we proposed cyclic olefins, not aromatics, played the important role in hydrocarbon pool. In this regard, low temperature shows negative effect on MTP process considering the quick conversion of propylene to other olefins and the formation of adamantane, ignore of the increased ratio of propylene to ethylene ratio. The result provided new insight on the reaction mechanism of MTO/MTP system.

Influence of Zirconium Modified USY on Coupled Hydrogenation and Ring Opening of Tetralin Over NiW/USY + Al2O3

There is increasing interest in modification USY in hydrotreating catalysts to attain high activity and targeted product selectivity. Zirconium modification on USY (Zr-USY) was designed in this study. Zirconium highly dispersed, and greatly enriched on the surface of USY zeolites. Zirconium modification slightly induced framework dealumination. Introduction of zirconium increased total acid sites of USY zeolites, probably ascribed to Zr4+ polarizing H2O. The corresponding catalysts NiW/Al2O3 + Zr-USY-1 (40.6 wt%) and NiW/Al2O3 + Zr-USY-2 (41.4 wt%) showed higher selective ring opening products yield compared to NiW/Al2O3 + USY (31.7 wt%). The enhanced ring opening products yield may be due to sufficient cooperation between WS2 and acidity. The results provide further insight into USY modification and its role on hydrotreating catalysts.

In situ catalytic fast pyrolysis of crude and torrefied Eucalyptus globulus using carbon aerogel-supported catalysts

Nickel and iron supported on thermostable cellulose-derived carbon aerogels (CAG), were used for the catalytic fast pyrolysis (CFP) of crude and torrefied Eucalyptus globulus. Pyrolysis vapors produced from torrefied biomass under non-catalytic conditions were reduced by 70% in carboxylic acids with respect to vapors originating from crude biomass, but also were richer in ketones and aromatics. In the case of CFP of both crude and torrefied E.globulus on CAG-supported iron and nickel (Fe/CAG and Ni/CAG), the metallic clusters were activated for ketonization, while Ni0 provided additional hydrogenation sites, increasing the production of monoaromatics (c.a.18% selectivity), leading to a high acids-to-ketone (ξA-K = 1.3) and heavy-to-light (ξH-L = 1.0) ratios. Accordingly, Ni/CAG and Fe/CAG were more effective than HZSM-5 –a traditional cracking catalysts– for upgrading CFP vapors to light compounds. The combination of zeolite acid sites with the oxophilic Ni0 and Fe0 allows the deoxygenation of vapors while the ratio of polyaromatics to light aromatics (ξPAHs) was reduced by nearly 50%.

Cooperativity of adjacent Brønsted acid sites in MFI zeolite channel leads to enhanced polarization and cracking of alkanes

The spatial proximity between Brønsted acid sites (BAS) in HZSM-5 zeolites is demonstrated by 1H double quantum (DQ) MAS NMR measurements. This proximity results in more pronounced polarization of adsorbed acetone and alkanes in zeolites, evident from 13C MAS NMR spectra. The adjacent BAS with synergistic interactions on alkane reactants (propane, n-butane, and n-pentane) cooperatively catalyze alkane cracking at higher turnover rates than on isolated BAS. Apparent activation energies are similar on HZSM-5 catalysts with different concentrations of isolated and adjacent BAS, while apparent activation entropies become less negative at higher BAS concentrations. Kinetic experiments in conjunction with adsorption measurement and DFT calculations prove that cracking rates at these Al-site pairs are mainly due to more positive intrinsic activation entropies, suggesting that the protonation transition state occurs later along the reaction coordinate on adjacent BAS. Adjacent Brønsted acid sites favor cracking over dehydrogenation and favor central cracking over terminal cracking.