Acid Sites Of HZSM-5 Research Articles

Efficient solvent- and hydrogen-free upcycling of high-density polyethylene into separable cyclic hydrocarbons.

Plastic pollution is a planetary threat that has been exacerbated by the COVID-19 pandemic due to the surge in medical waste, personal protective equipment and takeaway packaging. A socially sustainable and economically viable method for plastic recycling should not use consumable materials such as co-reactants or solvents. Here we report that Ru nanoparticles on zeolitic HZSM-5 catalyse the solvent- and hydrogen-free upcycling of high-density polyethylene into a separable distribution of linear (C1 to C6) and cyclic (C7 to C15) hydrocarbons. The valuable monocyclic hydrocarbons accounted for 60.3 mol% of the total yield. Based on mechanistic studies, the dehydrogenation of polymer chains to form C=C bonds occurs on both Ru sites and acid sites in HZSM-5, whereas carbenium ions are generated on the acid sites via the protonation of the C=C bonds. Accordingly, optimizing the Ru and acid sites promoted the cyclization process, which requires the simultaneous existence of a C=C bond and a carbenium ion on a molecular chain at an appropriate distance, providing high activity and cyclic hydrocarbon selectivity.

Direct conversion of CO2 into aromatics over multifunctional heterogeneous catalysts

ABSTRACT The conversion of CO2 into fuels and value-added chemicals over heterogeneous catalysts may help realize a carbon-neutral or carbon-negative society. Recently, the direct conversion of CO2 to aromatics has been extensively explored. However, the requirement of multifunctional active sites, complexity of tandem cascade reactions, and difficulty in understanding reaction mechanisms present significant challenges to the design of effective heterogeneous catalysts. In this review, catalytic systems used for the direct production of aromatics from CO2 via the methanol-mediated and modified Fischer–Tropsch synthesis pathways are critically discussed. The initial section discusses the role of acid sites in HZSM-5 for the formation of aromatics. This is followed by a thorough review of recent progress on aromatic syntheses in terms of their catalytic performance, Si/Al ratios of zeolites, proximity between metal oxide and zeolite, and reaction conditions. The reaction mechanisms and the stability of existing catalysts are also discussed. Finally, the current challenges and future prospects of the catalytic conversion of CO2 to aromatics are highlighted.

Selective conversion of syngas to aromatics over metal oxide/HZSM-5 catalyst by matching the activity between CO hydrogenation and aromatization

Syngas conversion to aromatics has been studied over metal oxide/HZSM-5 bifunctional catalyst. In this reaction, CO is activated on metal oxide and hydrogenated to C1/C2 intermediate. The formation of aromatics involves C–C coupling, cyclization and dehydrogenation on acid sites of HZSM-5. The matching between CO hydrogenation and aromatization activity is key to the overall catalytic performance. High aromatic selectivity of up to 82.5% may be achieved over various metal oxide/HZSM-5 catalyst among which Cr2O3/HZSM-5 is the best in activity and aromatic selectivity. The CO activation and hydrogenation over Cr2O3 are facilitated by doping of Zn. The effect of Zn doping is revealed by systematic study on phase, morphology, reducibility and oxygen defect concentration. To enhance CO conversion and to prevent excessive hydrogenation, the strict control of Zn/Cr ratio and the phase of Zn species is required. The activity of HZSM-5 may be altered by changing Si/Al ratio and zeolite/oxide ratio and a moderate acid strength and density is necessary to balance the competition between aromatization and side reactions. Furthermore, the stability of the optimal catalyst 1Zn-3Cr/HZSM-5(18) is demonstrated with no significant deactivation after 100 h on stream.

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.

Increasing light aromatic products during upgrading of lignite pyrolysis vapor over Co-modified HZSM-5

To upgrade pyrolysis vapors derived from the catalytic fast pyrolysis (CFP) of Baiyinhua lignite (BYHL) in a drop tube reactor, various Co-loading (1, 3, 6 and 10 wt%) zeolites were prepared via wet impregnation. The effect of Co-loading and pyrolysis temperature ranged from 400 to 700 °C on product yields and tar properties were investigated in this work. The results showed that the product yield and the composition of pyrolysis tar were significantly affected by Co-modified HZSM-5 and parent HZSM-5 compared with no catalyst. Incorporation of metal-Co in HZSM-5 had an additional effect on the performance of the parent HZSM-5. The total content of benzene (B), toluene (T), ethylbenzene (E), xylene (X) and naphthalene (N) significantly increased and it reached 24.2 mg/g at Co-loading of 3 wt% (3Co-H-5) at 600 °C. 3Co-H-5 exhibited also a promising performance in lowering the oxygen content from 49.8% without a catalyst to 14.6% by GC–MS analysis. The deoxygenation mechanisms over metal-Co active centers and acid-catalyzed reactions, such as isomerization and cracking, over acid sites of HZSM-5 were speculated in this work. In addition, the temperature of 600 °C is not only appropriate for fast pyrolysis of lignite to obtain a higher pyrolysis tar, but also for catalytic pyrolysis to obtain BTEXN.

Nickel Phosphides Supported on HZSM-5 for Catalytic Hydrodeoxygenation of Eugenol: Effect of Phosphorus Content

Nickel phosphides supported on HZSM-5 zeolite with different initial P/Ni molar ratios were prepared for catalytic hydrodeoxygenation (HDO) of eugenol to produce hydrocarbons. When P/Ni<0.5, the increase of P content gave smaller and more uniform particles which was helpful for hydrogenation of aromatic ring, while the total acid sites of the catalyst decreased due to the coverage of more acid sites of HZSM-5 by adding P which was unfavourable for the removal of oxygen. At P/Ni=0.5, the active phase supported on HZSM-5 was mainly Ni3P with the smallest average particle size and minimum amount of acid sites which could account for the best hydrogenation activity and the lowest selectivity of hydrocarbons, respectively. Further increasing the P/Ni ratio up to 1 and 2, the active phase became Ni2P and a lot of nonuniform Ni2P nanorod crystals appeared on and out of the support HZSM-5 which caused a sharp drop in the hydrogenation activity of the catalyst. For a combination of hydrogenation and deoxygenation considerations, The P/Ni=0.25 is the best ratio for nickel phosphides supported on HZSM-5 zeolite to HDO of lignin-related phenols.

Pt/HZSM-5 catalyst synthesized by atomic layer deposition for aqueous-phase hydrogenation of levulinic acid to valeric acid

A Pt/HZSM-5 catalyst was prepared by atomic layer deposition (ALD) for aqueous-phase hydrogenation of levulinic acid (LA) to valeric acid (VA). 5Pt/HZSM-5 produced with 5 cycles of Pt ALD was identified as a highly active and stable bifunctional catalyst, and a high yield of VA (91.4%) was achieved in aqueous solution. A close interaction between Pt and acid sites of HZSM-5 is favor for the selective generation of VA. The microporous structure and the acid sites of HZSM-5 were not changed after Pt ALD, and some Pt nanoparticles were located in the micropore channel of HZSM-5. This reveals that the Pt ALD has the advantage to protect the structure of zeolite. The average particle size of Pt nanoparticles, electric state of surface Pt, and surface acid sites are nearly not changed with the increase of Pt ALD cycle number. However, the ratio of Pt in the pore channel to that out of the pore decreases with the increase of ALD cycle numbers, resulting in a decrease of TOF of VA yield. For comparison, Pt nanoparticles supported on HZSM-5 were also produced by impregnation. But the pore structure of HZSM-5 was damaged, and more micropore were formed by impregnation method for Pt loading. Moreover, it exhibited very low catalytic activity, selectivity of VA, and stability.

Mechanism of reaction of CeO2–CaO–Pd/HZSM5 catalyst in the Syngas process in the presence of sulfur-containing impurities

CeO2–CaO–Pd/HZSM5 catalyst, prepared using HZSM5 supports, with CeO2–CaO as the additive components, HZSM5 as the acid component and Pd as the hydrogenation component, have been prepared by the impregnation method, and the catalytic and sulfur-tolerant performance in the Syngas-To-Dimethyl ether (STD) reaction with thiophene and H2S as probe sulfur poisons were examined. The prepared catalyst exhibited higher catalytic performance and better sulfur resistance than a conventional catalyst based on Cu/ZnO/Al2O3 and HZSM5. The sulfur content, Pd sulfidity and acid concentration were determined in order to ascertain the adsorption sites of sulfides on the surface of the catalyst, and the products of the STD reaction from sulfur-containing syngas in an autoclave were analysed by GC-MS at different reaction times in order to determine the sulfur-tolerance mechanism of the catalyst. The study confirms that almost no H2S is absorbed on the surface of CeO2–CaO–Pd/HZSM5 catalyst, while thiophene is absorbed and activated on the acid sites of HZSM5, and hydrogen is adsorbed dissociatively on Pd active sites, followed by active hydrogen spillover to the acid sites to promote hydrogenation and thus convert the adsorbed thiophene to hydrocarbons and hydrogen sulfide.

The Effect of Hydrothermal Treatment on the Aromatization Performance of HZSM-5 Modified by Zn, P

Zn and P modified HZSM-5 zeolite catalysts were prepared by impregnation method using HZSM-5 zeolite prior to hydrothermal treatment. The surface area, the acid center style, and the acid strong distributing on hydrothermal treatment HZSM-5 zeolite were characterized by XRD, BET, NH3-TPD, and FT-IR techniques. Meanwhile, the influence of hydrothermal treatment temperature on the aromatization performance of the catalyst was investigated using FCC gasoline (50–100°C fraction) as feedstock. The result showed that, along with the temperature of hydrothermal treatment heightening, the surface area reduced and pore volume increased a little, however, the pore size clearly increased; at the same time the total acid amount and the number of strong acid sites of HZSM-5 were reduced significantly. With the hydrothermal treatment temperature increasing, the catalyst initial activity gradually decreased and the stability presented the trend of first increase then decrease. The HZSM-5, which was treated by hydrothermal at 550°C modified by zinc and phosphor, exhibited preferable aromatization performance. After reacting 16 h, the conversion of olefins, the contents of aromatic, and the yield of liquid products were 90.26, 54.06, and 10.02%, respectively.

Tetramethylbenzenium radical cations as major active intermediates of methanol-to-olefin conversions over phosphorous-modified HZSM-5 zeolites

The reaction intermediates of methanol-to-olefin (MTO) conversion over phosphorous-modified HZSM-5 (P-MFI) catalysts were investigated by electron spin resonance (ESR) and gas chromatography–mass spectroscopy (GC–MS). Phosphorous modification partially neutralized the strong acid sites in HZSM-5. The phosphorous compounds suppress the migration of polymethylbenzenium radical cations, lowering spin–spin interactions and thus preserved their hyperfine splitting. The ESR spectra of the organic species formed in P-MFI zeolites during MTO conversion were similar to those of the cation species generated from 1,2,4,5-tetramethylbenzene in mordenite. GC–MS of organic extracts revealed the predominant formation of polymethylbenzenes with 4–6 methyl groups. The correlation between conversion and the number of spins of tetramethylbenzenium radical cations exhibited that these radical cations might be major active intermediates of MTO conversion over P-MFI catalysts.

Comparison of kinetics, activity and stability of Ni/HZSM-5 and Ni/Al2O3-HZSM-5 for phenol hydrodeoxygenation

We have investigated the detailed kinetics of phenol hydrodeoxygenation in liquid aqueous medium over Ni supported on HZSM-5 or HZSM-5 with 19.3 wt.% γ-Al2O3 binder. The individual reaction steps on two Ni catalysts followed the rate order r1 (phenol hydrogenation)<r2 (cyclohexanone hydrogenation)<r3 (cyclohexanol dehydration)≪r4 (cyclohexene hydrogenation), so that phenol hydrogenation was the rate determining step. As Ni/Al2O3-HZSM-5 showed up to five times higher catalytic activity for phenol hydrogenation than Ni/HZSM-5, it also delivered higher rates for overall phenol hydrodeoxygenation, verified by both the kinetic study monitored by GC and in situ IR spectroscopy to trace the product concentrations. Under the present conditions, Ni leaching was almost negligible from Ni/Al2O3-HZSM-5 after 90h. The HZSM-5 support was stable, but the Al2O3-HZSM-5 support lost 7wt.%. The catalytic activity gradually decreased when catalyst was recovered and reused, mainly due to Ni particle sintering tracked by transmission electron microscopy. The change of acid sites on the fresh and used catalysts monitored by IR of adsorbed pyridine demonstrated that Brønsted acid sites of HZSM-5 could be irreversibly transformed to Lewis acid sites during calcination.

Impact of Forming and Modification with Phosphoric Acid on the Acid Sites of HZSM-5

The individual and combined effect of phosphoric acid modification, the addition of an alumina-based binder, and steaming on the acid−base and catalytic properties of HZSM-5 have been investigated. Whereas all three processes reduce the concentration of Brønsted acid sites, the underlying mechanisms and the extent of the effects vary. Upon addition of an alumina-based binder or during steaming of the parent zeolite dealumination is the sole process occurring. Impregnation of HZSM-5 with phosphoric acid leads to hydrolysis of framework aluminum and to the condensation of Brønsted acid sites with POH groups. This latter process can be reversed in the presence of an aqueous phase. In the forming process, this leads to the reversible formation of phosphate anions and phosphoric acid and the redistribution of phosphate species between the zeolite and the alumina binder. This mobility is mitigated by introducing phosphate species to the (alumina) binder indicating that in aqueous phase equilibrium between phosphate ions and phosphoric acid the zeolite and the (alumina) binder exists. Whatever the first step of modification of the parent material is (doping with H3PO4 or addition of either alumina or phosphated alumina binder), steaming at elevated temperatures essentially reduces and equalizes the concentration of Brønsted acid sites. For all samples derived from one common parent zeolite, the rate of protolytic cracking of propane depends linearly on the concentration of strong Brønsted acid sites; that is, all modified materials have the same turnover frequency. This turnover frequency is notably higher than that of the parent material.

Catalytic conversion of CH 3Br to aromatics over PbO-modified HZSM-5

The catalytic conversion of CH 3Br to aromatics was investigated over PbO-modified HZSM-5. The effects of PbO loading, SiO 2/Al 2O 3 molar ratio of HZSM-5, and calcination conditions of catalyst on catalytic performance were studied. It was found that the most suitable PbO loading and SiO 2/Al 2O 3 ratio are 5 wt% and 70, respectively. The 5 wt% PbO/HZSM-5 calcined at 450 °C for 8 h performs the best. Also, PbO is superior to the other metal oxides selected to modify HZSM-5, and HZSM-5 is better than SiO 2, Al 2O 3 or ZrO 2 as support. The characterization results indicate that (i) the PbO species is highly dispersed on the HZSM-5 support, (ii) the role of PbO is to promote hydrocarbon aromatization, and (iii) the acid sites of HZSM-5 are indispensable for CH 3Br conversion into aromatics. We found that the deactivation of catalyst is due to pore blocking by carbonaceous deposits. The overall results demonstrate that the method is efficient for the conversion of CH 4 to aromatics.

Study on Mo/HZSM-5 catalysts modified by bulky aminoalkyl-substituted silyl compounds for the selective methane-to-benzene (MTB) reaction

The methane dehydroaromatization reaction was carried out over SiO 2-modified Mo/HZSM-5 catalysts at 1023 K under 0.3 MPa with 2700 ml g −1 h −1 of 6% H 2 added methane. The molecular tuning effect of silanation treatment for HZSM-5 crystals was studied using various silyl compounds, including 3-aminopropyl-triethoxysilane (APTES), 3-aminopropyl-trimethoxysilane (APTMS), N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (TESPDHI), triphenylsilylamine (TPSA), N-propyl-triethoxysilane (PTES), and tetraethoxysilane (TEOS). It was found that the optimum addition (0.5 wt% as SiO 2) of basic group-substituted bulky silyl compounds such as APTES, APTMS and TESPDHI resulted in the fine tuning of the external microporous apertures of HZSM-5 having ca. 5.0 Å diameter similar to that of MCM-22, and improve the benzene selectivity of above 90% (benzene + toluene) in the methane dehydroaromatization reaction on 6 wt% Mo/HZSM-5 catalysts due to the marked suppression of naphthalene and coke formation. In contrast, the other bulky silyl compounds are not effective for the surface modification of Mo/HZSM-5 in the MTB reaction. The IR of pyridine adsorption studies revealed that the surface Brønsted acid sites of HZSM-5 was selectively transformed to Lewis acid sites by the treatment of the selected silyl agents such as APTES, APTMS, and TESPDHI, resulting in highly stable and selective catalytic performance of Mo/HZSM-5 in the MTB reaction.

Effects of Rhodium Addition to Mo/HZSM-5 Catalyst for Methane Dehydroaromatization

Abstract Rhodium added 6 wt % Mo/HZSM-5 catalyst (Rh/Mo = 0.2) exhibited highly stable and active performances in the methane dehydroaromatization reaction at 1023 K under 0.3 MPa with 2700 mL g−1 h−1 of the methane feed with 6% hydrogen addition due to the effective suppression of the coke deposition on the Brønsted acid sites of HZSM-5.

DFT Study of Structural and Electronic Properties of PdO/HZSM-5

A density functional theory (DFT) study has been conducted to investigate the structural and electronic properties of PdO/HZSM-5 and thereby the relationship between PdO and the acid sites of HZSM-5. Different cluster models of PdO/HZSM-5 based on experimental and theoretical works are presented. The study shows that the local structure of PdO supported on HZSM-5 is very similar to the four-coordinated square planar structure of bulk PdO. This is consistent with the reported EXAFS analysis in the literature. The average distances between the Pd and the four coordinated oxygen atoms obtained by DFT calculations with different cluster models are close to the reported experimental results. The analysis of the Mulliken population and the electron density difference of the cluster models shows that there exists a charge transfer from the four coordinated oxygen atoms to the Pd. The components of the frontier molecular orbital of these cluster models come mainly from the Pd and the four coordinated oxygen atoms...

Acid Sites in HZSM-5 Upon Copper Exchange by FTIR and DSC Using Ammonia

The acidity of the parent HZSM-5 zeolite and Cu-HZSM-5 catalyst has been examined by FTIR and DSC using ammonia desorption. Sites of different strength were found in HZSM-5, desorbing ammonia at a relative temperature difference of about 100 °C. Upon copper exchange a fraction of Bronsted sites were transformed to Lewis sites, but the acid strength of the remaining Bronsted sites was increased. Aside from Lewis sites originating from copper exchange, there might be some additional sites formed from precipitated copper. This could explain the quantity of adsorbed NH3 on Cu-ZSM-5 which is higher than theoretically expected. While changing their nature, acid sites of higher strength keep their location, which is manifested by some diffusion effect towards ammonia.

Alkylation of benzene with ethane over platinum-loaded zeolite catalyst

Alkylation of benzene with ethane was carried out using various zeolite catalysts at temperature ranges of 400–550 °C. Loading of platinum onto zeolite greatly enhanced the yield of ethylbenzene. Among the zeolites tested, H-ZSM5 and H-MCM22 showed catalytic activities. By contrast, mordenite did not yield ethylbenzene. Moderate acid strength distribution is the key factor of zeolite catalysts. Optimum catalyst and conditions for this reaction are as follows. The platinum-loaded H-ZSM5 catalyst containing 6.8 wt% Pt, at a reaction temperature of 500 °C, afforded ethylbenzene and styrene formation rates of 14.2 and 0.8 mmol h−1 g-cat−1, respectively (benzene-based yields 7.3 and 0.4%). In the alkylation of benzene with ethane over platinum-loaded H-ZSM5, ethene was initially formed from ethane over the metallic platinum. Then the alkylation proceeded over the acid sites of H-ZSM5.

Partial oxidation of methane to formaldehyde over Mo/HZSM-5 catalysts

Two Mo/HZSM-5 catalyst series have been tested in the partial oxidation of methane to formaldehyde at atmospheric pressure. Conventional aqueous impregnation with ammonium heptamolybdate was used for preparing catalysts with Mo loading ranging from 0.3 to 3.3 wt.%. Two different molybdenum species were detected in these catalysts: tetrahedral monomeric Mo(VI) species and octahedral coordinated polymolybdate species. For molybdenum loading higher than 1.3 wt.%, polymolybdate species are predominant causing a decrease in the dispersion and a partial blocking up of the HZSM-5 zeolite channels. A second catalyst series were prepared from HZSM-5 zeolite by aqueous impregnation and subsequent washing with an aqueous ammonia solution, yielding Mo/HZSM-5 catalysts with Mo contents ranging from 0.08 to 0.45 wt.%. A well-dispersed molybdenum oxide phase mainly constituted of tetrahedral monomeric species joined to the zeolite surface was characterized. Both catalyst series show different activity in methane conversion. Polymolybdate species of impregnated Mo/HZSM-5 catalysts are more reactive in methane activation than acid sites of HZSM-5 but lead to low HCOH selectivity values. In contrast, treated catalysts show lower methane conversion but higher HCOH selectivity. These results demonstrate the specificity of the MoO sites of monomeric species for HCOH production.

FTIR and 19F MAS NMR investigation of the interactions between p-fluoroacetophenone and MFI structure type zeolites

Interactions between PFA molecules and MFI structure type zeolites were studied by FTIR and 19F MAS NMR spectroscopies. In the purely siliceous ZSM-5 sample, PFA molecules were physisorbed. In the acid samples, the interactions implicated the carbonyl group of the PFA molecules and the Brönsted acid sites of HZSM-5. This interaction could be a hydrogen bond or a protonation of the carbonyl group. The complementarity of the IR and 19F NMR spectroscopies was shown: IR specified the type of interaction whereas NMR allowed quantification of the strength of the interaction and the amount of active acid sites of the zeolite.