Internal Acid Sites Research Articles

Hollow Zeolite Nanoreactor with Double Shells for Methanol Aromatization: Explicit Recognition on Catalytic Function of Inverse Elemental Zone and Shell-Cavity.

Core@shell catalyst composited of dual aluminosilicate zeolite can effectively regulate the distribution of acid sites to control hydrocarbon conversion process for the stable formation of target product. However, the diffusion restriction reduces the accessibility of inner active sites and affects synergy between core and shell. Herein, hollow ZSM-5 zeolite nanoreactor with inverse aluminum distribution and double shells are prepared and employed for methanol aromatization. It is demonstrated that the intershell cavity alleviated the steric hindrance from zeolites channel and provided more paths and pore entrance for guest molecule. Correspondingly, olefin intermediates generated from methanol over the external shell are easier to adsorb at internal acid sites for further reactions. Importantly, the diffusion of generated aromatic macromolecules to the external surface is also promoted, which slows down the formation of internal coke, and ensures the use of internal acid sites for aromatization. The aromatics selectivity of the nanoreactor remained at 8% after 154h, while that of solid core@shell catalyst decreased to 2% after 75h. This finding promises broader insight to improve internal active site utilization of core@shell catalyst at the diffusion level and can be great aid in the flexible design of multifunctional nanoreactors to enhance the relay efficiency.

Tandem catalytic pyrolysis of mixed plastic packaging wastes to produce BTEX over dual catalysts

The ubiquitous challenges caused by the accumulation of mismanaged plastic solid wastes have seriously endangered ecological health. For enhancing the sustainable and environment-friendly conversion of mixed plastic packaging wastes into high-value benzene, toluene, ethylbenzene, and xylene (BTEX) while effectively reducing the coke deposition on catalysts, tandem catalytic pyrolysis of mixed packaging plastics over dual catalysts (MCM-41 and HZSM-5) was explored via a combination of in-situ catalysis and ex-situ catalysis in a two-stage fixed bed reactor. The selectivity of total and monocyclic aromatic hydrocarbons was nearly 100% and 85% with MCM-41 (500 °C) followed by HZSM-5 (550 °C), respectively. The BTEX production (245.33 mg/g) was considerably improved as compared to ex-situ catalysis of HZSM-5 (185.30 mg/g) and MCM-41/HZSM-5 (209.14 mg/g). Moreover, the total coke amount of tandem catalysis decreased to 14.25 mg/g than that of ex-situ catalysis of HZSM-5 (30.72 mg/g). The mechanisms analysis revealed that the pre-reforming effect of MCM-41 suppressed polycyclic aromatic hydrocarbons (PAHs) formation and coke deposition on the following HZSM-5 and meanwhile improved the utilization of internal acid sites for obtaining high-yield BTEX.

Elucidating the Nature of the External Acid Sites of ZSM‐5 Zeolites Using NMR Probe Molecules

The identification of acid and nonacid species at the external surface of zeolites remains a major challenge, in contrast to the extensively-studied internal acid sites. Here, it is shown that the synthesis of zeolite ZSM-5 samples with distinct particle sizes, combined with solid-state NMR and computational studies of trimethylphosphine oxide (TMPO) adsorption, provides insight into the chemical species on the external surface of the zeolite crystals. 1 H-31 P HETCOR NMR spectra of TMPO-loaded zeolites exhibit a broad correlation peak at δP ∼35-55 ppm and δH ∼5-12 ppm assigned to external SiOH species. Pore-mouth Brønsted acid sites exhibit 31 P and 1 H NMR resonances and adsorption energies close to those reported for internal acid sites interacting with TMPO. The presence of an external tricoordinate Al-Lewis site interacting strongly with TMPO is suggested, resulting in 31 P resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Brønsted sites.

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.

Diffusion-dependent upgrading of hydrocarbons synthesized by Co/zeolite bifunctional Fischer–Tropsch catalysts

The design of efficient zeolite-supported Co catalysts is essential for the further development of the one-step production of liquid fuels via the low-temperature Fischer–Tropsch synthesis (FTS). Herein, Co/zeolite catalysts were prepared from zeolites SSZ-13, ZSM-22, ZSM-11, ZSM-5, Mordenite, and Y. Their textural and acidic properties, Co dispersion, and internal diffusion behaviors were comprehensively analyzed, along with their FTS product mixtures. Counterintuitively, the degree of isomerization and hydrocarbon decomposition did not correlate with the acidic properties of the zeolite support. Instead, catalysts with high characteristic diffusivity delivered product mixtures with high degree of isomerization and decomposition, suggesting that the access of paraffinic hydrocarbons to the internal acid sites of the zeolite determines the upgrading performance of the bifunctional Co/zeolite catalyst in the low-temperature FTS reaction. Hence, the present findings support upcoming efforts on the design of new and effective FTS catalysts.

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.

Quantification of rhenium oxide dispersion on zeolite: Effect of zeolite acidity and mesoporosity

The anchoring nature and spatial distribution of rhenium oxide (ReOx) supported on zeolites (ReOx/zeolite) were determined as a function of the support acidity and mesoporosity. The ReOx/zeolite samples were characterized collectively using Argon isotherm, XRD, Raman, XAS, DRIFTS, and organic chemical titrations. The results show that the ReOx species formed the isolated distorted tetrahedral ReO4− structure. On the zeolite support, ReOx species anchored predominantly on Brønsted acid sites (Si-OH+-Al) enclosed in micropores (internal acid sites), by replacing the protons in Si-OH+-Al sites at a H+/Re ratio of ∼1.1 in zeolites with high acidity. The decrease in zeolite acidity led to the anchoring of ReOx species onto both Si-OH+-Al and silanol group (Si-OH). The increase in zeolite mesoporosity migrated the ReOx species onto both internal acid sites and those on the external surface and mesopores (external acid sites), as well as Si-OH groups. The implication of distribution of ReOx in zeolite was explored by direct non-oxidative methane conversion reaction.

ZSM-5 decrystallization and dealumination in hot liquid water.

Zeolites have recently attracted attention for upgrading renewable resources in the presence of liquid water phases; however, the stability of zeolites in the presence of liquid-phase water is not completely understood. Accordingly, the stability of the ZSM-5 framework and its acid sites was studied in the presence of water at temperatures ranging from 250 to 450 °C and at pressures sufficient to maintain a liquid or liquid-like state (25 MPa). Treated samples were analyzed for framework degradation and Al content and coordination using a variety of complementary techniques, including X-ray diffraction, electron microscopy, N2 sorption, 27Al and 29Si NMR spectroscopy, and several different types of infrared spectroscopy. These analyses indicate that the ZSM-5 framework retains >80% crystallinity at all conditions, and that 300-400 °C are the most aggressive. Decrystallization appears to initiate primarily at crystal surfaces and share many characteristics in common with alkali promoted desilication. Liquid water treatment promotes ZSM-5 dealumination, following a mechanism analogous to that observed under steaming conditions: initiation by Al-O hydrolysis, Al migration to the surface, and finally deposition as extra framework Al or possibly complete dissolution under some conditions. As with the framework, dealumination is most aggressive at 300-400 °C. Several models were evaluated to capture the non-Arrhenius effect of temperature on decrystallization and dealumination, the most successful of which included temperature dependent values of the water auto-ionization constant. These results can help interpretation of previous studies on ZSM-5 catalysis in hot liquid water and suggest future approaches to extend catalyst lifetime.

Role of Biopolymers in the Deactivation of ZSM-5 during Catalytic Fast Pyrolysis of Biomass

Rapid coking and catalyst deactivation are significant problems during catalytic fast pyrolysis of biomass. Cellulose and lignin were found to coke via different mechanisms, resulting in two distinct types of catalyst deactivation. Lignin pyrolysis vapors cause coke formation mainly by external surface coking without limiting access to the active acid sites in the microchannels. Lignin deactivates the surface cracking capability of ZSM-5, resulting in unreacted primary vapors passing through while maintaining aromatization reactions. Cellulose pyrolysis vapors generate coke mainly as an extension of the aromatization reactions on the acid sites, which leads to occlusion of the internal acid sites. This deactivates the upgrading reactions, resulting in decreased aromatics formation, generation of oxygenated intermediates and increased alkylation of 1-ring aromatics and reduced multi-ring aromatics selectivity. The results indicate that the decrease in aromatics formation observed during catalytic pyrolysis...

Zeolite–supported nickel phyllosilicate catalyst for C[sbnd]O hydrogenolysis of cyclic ethers and polyols

In this study, a series of zeolites, including small pore (SAPO–34), medium pore (Ferrierite, ZSM–5, and ZSM–11), and large pore (MOR and BETA) zeolites were explored as supports for nickel catalysts. The catalysts were prepared by the deposition–precipitation (D–P) method to obtain ca. 10 wt% Ni loading. The structures of prepared catalysts were elucidated using a wide range of characterization techniques such as BET, XRD, TEM, H2–TPR, NH3–TPD, FTIR, and XPS. It was found that 1:1 nickel phyllosilicate structures is formed over D–P nickel catalysts, while 2:1 nickel phyllosilicate was identified only over Ni/BETA catalyst. Furthermore, Ni/ZSM–5 catalyst was also prepared by impregnation method as a reference to investigate the effect of preparation method. The impact of the topological structures of zeolite on the catalytic activity of reactants (i.e., cyclic ethers derived from furanic compounds), reaction intermediates (i.e., polyols) was subsequently investigated. The ring–opening of THFA can proceed over Ni/Ni phyllosilicate species on the external surface of the catalyst, producing pentane polyols. Then, the pentane polyol intermediates can further undergo ring–closure pathway, predominantly over the internal acid sites within zeolite structures (except for Ni/SAPO–34). It was found that medium–pore zeolites (ZSM–5 and ZSM–11) with mild acidity exhibit the highest reaction rate with a THP yield up to ca. 64%. Similarly, the highest catalytic activity during THPM conversion was achieved over ZSM–5– and ZSM–11– supported nickel catalysts, with up to 51% selectivity towards 16HDO. Comparing the zeolite maximum pore sizes and the kinetic diameters of the reactants and products with the catalytic performance, it is clear that topological structures of ZSM–5 and ZSM–11 have pore confinement effect on the reaction mechanism and catalytic activity in the hydrogenolysis of cyclic ethers and polyols.

Effect of Acid Site Localization in Sulfonic Resin Amberlyst 15 on its Catalytic Properties in Ethyl tert-butyl Ether Synthesis

The role of the localization of acid sites in macroporous sulfonic resin Amberlyst 15 in the synthesis of ethyl tert-butyl ether was investigated. By partial blockage of the acid sites by Na+ cations it was established that internal acid sites are primarily active in the reaction while a side reaction with the formation of tert-butanol takes place at the external acid sites.

One-pot synthesis of nano-crystalline MCM-22

Nano-crystalline MCM-22 zeolite was synthesized in a one-pot procedure by the use of an organosilane (dimethyl-octadecyl-(3-trimethoxysilylpropyl)-ammonium chloride, TPOAC) in the zeolite synthesis gel. This crystal growth inhibition procedure introduced mesopores in the MCM-22 crystallites. The lower mechanical stability of the nano-crystalline MCM-22 zeolite compared with bulk MCM-22 can be countered to some extent by pillaring. The increased external surface of the microporous zeolite domains resulted in increased accessibility of the Brønsted acid sites, as followed from the better performance in liquid-phase benzene alkylation with propylene as compared with bulk MCM-22. The increased accessibility of the internal acid sites in Mo-loaded hierarchical MCM-22 was also evident from the improved benzene selectivity during methane aromatization. Silylation of hierarchical Mo/MCM-22 was detrimental for the catalytic performance in MDA. The nano-crystalline MCM-22 has physico-chemical and catalytic properties intermediate between those of MCM-22 and ITQ-2 with the benefit over ITQ-2 that it can be synthesized in a single step.

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.

Acidity characterization of rare-earth-exchanged Y zeolite using 31P MAS NMR

Detailed qualitative and quantitative information on the effects of rare-earth (RE) cations on the types (Brönsted and Lewis), strengths, and distributions of acid sites on Y zeolite was studied by solid-state 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, using adsorbed trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO) as probe molecules. A total of seven 31P resonance peaks, with 31P NMR/TMPO chemical shifts at δ = 78, 70, 65, 62, 58, 55, and 53, corresponding to sites with different acid strengths, were identified. The peaks at δ = 78 and 70 arose from external and internal acid sites, the peaks at δ = 65, 62, 58, and 53 were from internal Brönsted acid sites, and the peak at δ = 55 was from internal Lewis acid sites. With increasing RE content, the number of medium strength Brönsted acid sites (δ = 62 and 58) increased significantly, whereas those of strong Brönsted acid sites (δ = 65) and weak Lewis acid sites (δ = 55) decreased. These experimental results were explained in terms of the influence of framework Al, extra-framework Al, and RE cations on the Y zeolite acidity.

Role of Delamination in Zeolite-Catalyzed Aromatic Alkylation: UCB-3 versus 3-D Al-SSZ-70

Delaminated zeolite UCB-3 exhibits 2.4-fold greater catalytic activity relative to its three-dimensional (3D) zeolite counterpart, Al-SSZ-70, and 2.0-fold greater activity (per catalyst mass) when compared with industrial catalyst MCM-22, for the alkylation of toluene with propylene at 523 K. The former increase is nearly equal to the observed relative increase in external surface area and acid sites upon delamination. However, at 423 K for the same reaction, UCB-3 exhibits a 3.5-fold greater catalytic activity relative to 3D Al-SSZ-70. The higher relative rate enhancement for the delaminated material at lower temperature can be elucidated on the basis of increased contributions from internal acid sites. Evidence of possible contributions from such acid sites is obtained by performing catalysis after silanation treatment, which demonstrates that although virtually all catalysis in MCM-22 occurs on the external surface, catalysis also occurs on internal sites for 3D Al-SSZ-70. The additional observed enhancement at low temperatures can therefore be rationalized by greater access to internal active sites as a result of sheet breakage during delamination. Such breakage leads to shorter characteristic internal diffusion paths and was visualized using TEM comparisons of UCB-3 and 3D Al-SSZ-70.

Designing MFI-based catalysts with improved catalyst life for [formula omitted] oligomerization to high-quality liquid fuels

Light olefin oligomerization is an important alternative for the production of clean liquid automotive fuels and can be performed in the presence of solid acid catalysts. Among these, medium-pore zeolites, and especially ZSM-5, have been widely described in the open literature. In this work, the relative importance of intracrystalline diffusion path lengths for the product molecules (depending on the zeolite crystal size and the presence of mesopores in the crystallites) and Brønsted acid site density are discussed for two different olefins, propene and 1-pentene. Thus, ZSM-5 samples with (a) the same crystallite size and different acid site density, (b) the same density of acid sites and different crystallite size, (c) postsynthesis generation of mesopores by different desilication severities, and (d) samples with similar crystal size, mesoporosity, and acid site density, but with a different ratio of external to internal acid sites have been prepared and studied for oligomerization of propene and 1-pentene. The results obtained suggest that the properties required for a best-performing catalyst (maximum conversion and lowest deactivation rate) are different for these two alkenes. Whereas Brønsted acid site density is determinant for propene oligomerization when intracrystalline diffusion path lengths are below a certain critical value, the presence of a large number of Brønsted acid sites is not sufficient in the case of 1-pentene, and additional mesoporosity becomes crucial. Thus, mesoporous ZSM-5 samples prepared by postsynthesis desilication treatments present a greater improvement in initial conversion and catalyst life for 1-pentene oligomerization than for conversion of propene.

Spatial distribution, strength, and dealumination behavior of acid sites in nanocrystalline MFI zeolites and their catalytic consequences

The distribution, acid strength, and steam stability of internal and external acid sites were systematically investigated for nanocrystalline MFI zeolites with different crystallite size (2–300nm), which were obtained using gemini-surfactants having different alkyl chain lengths. FT-IR spectroscopy using pyridine and 2,6-di-tert-butylpyridine as a base probe revealed that the concentration of external acid sites increased with a decreasing crystallite size. The external sites were weaker than the internal sites in acid strength, but could still catalyze n-octane cracking reactions that require the strongest acid strength among various acid-catalyzed reactions. Most notably, the external acid sites of zeolites exhibited remarkably higher steam stability at 873K than the internal acid sites. Such an observation was supported by the catalytic results in methanol to dimethylether (DME) and gasoline conversion, and the Claisen–Schmidt condensation involving bulky molecules.

The isopropylation of diphenyl ether over H-mordenite catalysts

The isopropylation of diphenyl ether (PE) was examined over dealuminated H-mordenite (MOR) under 0.8 MPa of propene at 175–350 °C. 4,4′-Diisopropyldiphenyl ether (4,4′-DIPPE) was selectively formed at low and moderate reaction temperatures (175–250 °C). The selectivities for 4,4′-DIPPE decreased with increasing reaction temperature, and resulted in the formation of thermodynamically more stable 3,4′- and 3,3′-DIPPE at high temperatures (300–350 °C). The selectivities for DIPPE isomers in encapsulated products had similar features to the selectivities of bulk products. The shape-selective isopropylation of PE consecutively occurs at low and moderate reaction temperatures: 4-isopropyldiphenyl ether (4-IPPE) forms preferentially from PE, and then rapidly convert to 4,4′-DIPPE. However, the isomerization of 4,4′-DIPPE occurred on internal acidic sites in the channels as well as on external acidic sites at high temperatures. MOR channels were large enough for the isomerization of 4,4′-DIPPE at high temperatures because of the fluctuations of zeolite structure and organic molecules. These features of the isopropylation of PE were quite different from the isopropylation of biphenyl (BP), where no isomerization of 4,4′-diisopropylbiphenyl occurred inside the channels. These differences were due to the conformation of the transition states between PE and BP in MOR channels. Flexible conformation of 4,4′-DIPPE also enabled the isomerization inside the channels.

Low temperature conversion of linear C 4 olefins with acid ZSM-5 zeolites of homogeneous composition

Conversion of linear butenes over ZSM-5 zeolites leads to a large variety of products, from olefins to aromatics. At low temperature, these products are mainly olefins from C 3 = to C 13 =, with a predominance of dimeric products. The analysis of the product stream obtained by the reaction of n-butenes over several materials with MFI topology, but with different aluminum content provides evidence for an oligomerization–cracking–realkylation mechanism. The reactions occur predominantly in the catalyst bulk or at the external surface, depending on the amount of coke developed inside the catalyst. This behavior is determined by the concentration of framework Al and associated silanol groups, inducing coking. For Al-rich MFI samples, the amount of hard coke is sufficient to block the access of reactants to internal acid sites. As a result, extra-crystalline catalysis dominates, while the appearance of the individual products is consistent with that predicted by classical carbocation theory. For siliceous samples, hard coke formation is less pronounced, and evidence for shape selective product formation is present. Expected relations among structural, morphological and acidic properties of all MFI materials have been confirmed by XRD, SEM and BET and by 27Al MAS NMR, NH 3-TPD and FTIR.

Catalytic Behavior of Cobalt (II) Phthalocyanine Immobilized on Bentonite Clay in Bulk Polymerization of Methyl Methacrylate

The catalytic behavior of cobalt (II) phthalocyanine (CoPc) immobilized on bentonite clay, with different complex loadings ranging between 0.2 and 2.2 wt%, in the presence of n-butyl amine solvent, was investigated in bulk polymerization of methyl methacrylate without using an activator or cocatalyst. The interaction of CoPc molecules with a bentonite surface, encouraged by the amine, involved the inclined stacking model, i.e., interaction between N-atoms of the macro-ring system and OH's of the support. Two different mechanistic pathway regions could be suggested, depending on complex loadings. The first was in the range, 0.2−1.0 wt% CoPc, behaving in ionic fashion as the bare bentonite. Isolated oriented molecules probably activated bentonite through the exposure of new internal acid sites during intercalation. The second, in the range of 1.4−2.2 wt% CoPc, proceeded via combined mechanisms, ionic functioned by bentonite support and free radical functioned by complex packed oriented stacks, staggered in clay galleries to cover a fraction of internal active acid sites. The number of polymer chains formed per one CoPc center (reaction turnovers) was >2–∼10 with diluted catalyst samples. With higher loadings, one chain was formed per active site. Such behaviors, without a detectable effect on polymer characteristics or enchainment sequence, were studied in light of intercalation, orientation and accessibility models.