Acidic OH Groups Research Articles

Highly Efficient and Selective Synthesis of Renewable Isoprene Over Mo-Fe-O and MgO/Mg(OH)2 Composite Catalysts.

Synthesis of renewable isoprene continues to attract significant interest, yet catalysts still need improvement for industrial applications. We report herein an efficient and novel Prins tandem approach for highly selective production of isoprene from bio-based methanol and isobutene over Mo-Fe-O and MgO/Mg(OH)2 acid-base bifunctional catalysts. Hydration decreases the number of strong basic sites O2- ions and increases OH groups over MgO, thus improving the isoprene selectivity. Basic OH on Mg(OH)2 promotes the Prins condensation of formaldehyde and isobutene, while acidic OH promotes dehydration of isoprenol to isoprene. Suitable contents and proportions of basic and acidic OH groups over Mg(OH)2 increase isoprene selectivity and methanol conversion to 90 and 92 % on the Mo-Fe-O+Mg(OH)2 composite catalysts, respectively. It exhibits good recycling stability, with isoprene selectivity remaining 90 % after five successive regenerations. DFT calculations reveal OH groups formed after hydration reduce the energy barrier of the H transfer step, thereby promoting the overall reaction.

A New Type of Acidic OH-Groups in the LTL Zeolite

Acidic properties of ion-exchanged LTL zeolites have been studied using FTIR spectroscopy, complemented by X-ray powder diffraction, SEM-EDX, XRF and N2 physisorption. Infrared spectra of the ion-exchanged zeolites show the presence of two intense bands of the bridging OH-groups: a narrow band at ~3640 cm−1 that is attributed to Si(OH)Al groups freely vibrating in 12 MR and a broad, intense band at ~3250 cm−1 that is assigned to bridging OH groups forming hydrogen bond with neighbouring oxygen atoms, e.g., in six-membered rings. The former can be selectively removed by caesium or rubidium cations with up to 3 Cs+ or Rb+ per unit cell readily ion-exchanged into the LTL zeolite, replacing an equivalent number of acidic OH-groups or K+ cations within the structure. The cation migration of the larger cation, evaluated by the Rietveld refinement method, occurs mostly via the main 12 MR channels. By contrast, less than 1 Li+ or Na+ cation per unit cell can be introduced under similar conditions. Accordingly, the concentration of Si(OH)Al groups in back-exchanged NH4-K-LTL with smaller cations (Li+, Na+) does not differ considerably from the concentration of Brønsted acid sites in the original NH4-K-LTL. Lower concentrations of acid sites have been detected in the samples back-exchanged with Cs+, Rb+ and K+. In addition, the acidic properties of NH4-LTL samples have been compared with a structurally related NH4-MAZ zeolite.

Carboxylate-Catalyzed C-Silylation of Terminal Alkynes.

A carboxylate-catalyzed, metal-free C-silylation protocol for terminal alkynes is reported using a quaternary ammonium pivalate as the catalyst and commercially available N,O-bis(silyl)acetamides as silylating agents. The reaction proceeds under mild conditions, tolerates a range of functionalities, and enables concomitant O- or N-silylation of acidic OH or NH groups. A Hammett ρ value of +1.4 ± 0.1 obtained for para-substituted 2-arylalkynes is consistent with the proposed catalytic cycle involving a turnover-determining deprotonation step.

INFLUENCE OF MODIFICATION ON PROPERTIES OF ZSM-5 ZEOLITE IN THE REACTION OF TOLUENE ALKYLATION WITH ISOPROPANOL

Bimetallic catalysts based on ZSM-5 zeolite modified with ytterbium and nickel have been prepared. The influence of the concentration of ytterbium (1.7–5.0 wt.%) and nickel (1.0–1.5 wt.%) on the acidic, textural and catalytic properties of the zeolite in the reaction of alkylation of toluene with isopropanol in the temperature range of 300–3500C was studied. It has been shown that the unmodified zeolite HZSM-5 exhibits low selectivity (54.6–58.2%) for isopropyltoluenes (3-IPT, 4-IPT, and 2-IPT) than the modified catalysts. In addition, the content of the valuable 4-IPT isomer does not exceed 44.2%. Modification of HZSM-5 with ytterbium in an amount of 5.0 wt.% leads to an increase in the selectivity for isopropyltoluenes up to 68.7%, and the selectivity for 4-IPT up to 68.8%. Additional modification of the 3%-Yb-HZSM-5 catalyst with nickel in the amount of 1.0 wt.% prevents the formation of 2-IPT and promotes an increase in the yield of 4-IPT. The modified catalysts were characterized by XPA, BET, BJH, and ammonia TPD. Modification, calcination, and successive treatment of the catalyst with air and hydrogen leads to a dispersed distribution of Yb2O3 and metallic nickel oxide nanoparticles in the outer surface and in the pores of the zeolite, which causes a decrease in the specific surface area, total pore volume, and an increase in the fraction of mesopore volume. As a result of the interaction of modifiers with acidic OH groups of the zeolite, the concentration of strong Brønsted acid sites decreases and new, stronger Lewis acid sites form. All this has a decisive effect on the selectivity of 4-IPT formation and the stability of the bimetallic catalyst. The maximum selectivity for 4-IPT on a 3% Yb 1.0% Ni/HZSM-5 bimetallic catalyst is achieved at 3300C and is 71.5% at a toluene conversion of 24.6%.

Comparative study of the adsorption performance of NH2-functionalized metal organic frameworks with activated carbon composites for the treatment of phenolic wastewaters

To better understand the role of the —NH2 group in adsorption process of phenolic wastewaters, NH2-functionalized MIL-53(Al) composites with activated carbon (NH2-M(Al)@(B)AC) were prepared. The results showed that the NH2 group could increase the mesopore volume for composites, which promotes mass transfer and full utilization of active sites, because hierarchical mesopore structure makes the adsorbent easier to enter the internal adsorption sites. Furthermore, the introduction of the NH2 group can improve the adsorption capacity, decrease the activation energy, and enhance the interaction between the adsorbent and p-nitrophenol, demonstrating that the NH2 group plays a crucial role in the adsorption of p-nitrophenol. The density functional theory calculation results show that the H-bond interaction between the —NH2 group in the adsorbent and the NO2 in the p-nitrophenol (adsorption energy of –35.5 kJ·mol−1), and base-acid interaction between the primary —NH2 group in the adsorbent and the acidic OH group in the p-nitrophenol (adsorption energy of –27.3 kJ·mol−1) are predominant mechanisms for adsorption in terms of the NH2-functionalized adsorbent. Both NH2-functionalized M(Al)@AC and M(Al)@BAC composites exhibited higher p-nitrophenol adsorption capacity than corresponding nonfunctionalized composites. Among the composites, the NH2-M(Al)@BAC had the highest p-nitrophenol adsorption capacity of 474 mg·g−1.

Promoting active site renewal in heterogeneous olefin metathesis catalysts.

As an atom-efficient strategy for the large-scale interconversion of olefins, heterogeneously catalysed olefin metathesis sees commercial applications in the petrochemical, polymer and speciality chemical industries1. Notably, the thermoneutral and highly selective cross-metathesis of ethylene and 2-butenes1 offers an appealing route for the on-purpose production of propylene to address the C3 shortfall caused by using shale gas as a feedstock in steam crackers2,3. However, key mechanistic details have remained ambiguous for decades, hindering process development and adversely affecting economic viability4 relative to other propylene production technologies2,5. Here, from rigorous kinetic measurements and spectroscopic studies of propylene metathesis over model and industrial WOx/SiO2 catalysts, we identify a hitherto unknown dynamic site renewal and decay cycle, mediated by proton transfers involving proximal Brønsted acidic OH groups, which operates concurrently with the classical Chauvin cycle. We show how this cycle can be manipulated using small quantities of promoter olefins to drastically increase steady-state propylene metathesis rates by up to 30-fold at 250 °C with negligible promoter consumption. The increase in activity and considerable reduction of operating temperature requirements were also observed on MoOx/SiO2 catalysts, showing that this strategy is possibly applicable to other reactions and can address major roadblocks associated with industrial metathesis processes.

Mechanisms of antimony release from lacustrine sediments with increasing temperature

Antimony (Sb) is more mobile in lacustrine sediments with seasonal warming. However, the mechanisms of Sb mobility in sediments are still unclear, especially considering the interactions among Sb, iron (Fe), manganese (Mn), and dissolved organic matter (DOM). In this study, high-resolution dialysis (HR-Peeper) and multi-spectral techniques simultaneously investigated changes in Sb, Fe, Mn, and DOM in two different ecological types (algal and grass) sediments with increasing temperature. We found that the dissolved Sb rapidly increased with the increase in temperature. The oxidation of Sb(III) to Sb(V) by Fe/Mn oxides in oxygen (O2) rich overlying water and surface sediment layers was one of the reasons for Sb concentration enhancement in pore water. Further, using excitation-emission matrix and parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectroscopy, fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy (2D-COS) revealed that complexation with DOM was the other reasons for Sb concentration increasing in sediments. This was demonstrated by the similar distribution pattern and significant correlation between Sb and tryptophan-like components. Titration experiments further revealed that Sb was more stably bound to tryptophan-like components in the aromatic C–H (660 cm−1), alcoholic C–O (1115 cm−1), alkene CC (1615 cm−1), and carboxylic acid OH (3390 cm−1) groups. The tryptophan-like components from the algae region had a higher binding force than that from the macrophytes region. Our study effectively promotes an understanding of Sb mobilization in lacustrine sediments.

Microhydration of the metastable N-protomer of 4-aminobenzoic acid by condensation at 80K: H/D exchange without conversion to the more stable O-protomer.

4-aminobenzoic acid (4ABA) is a model scaffold for studying solvent-mediated proton transfer. Although protonation at the carboxylic group (O-protomer) is energetically favored in the gas phase, the N-protomer, where the proton remains on the amino group, can be kinetically trapped by electrospray ionization of 4ABA in an aprotic solvent such as acetonitrile. Here, we report the formation of the hydrated deuterium isotopologues of the N-protomers, RND3 +·(H2O)n=1-3, (R = C6H4COOD), which are generated by condensing water molecules onto the bare N-protomers in a liquid nitrogen cooled, radiofrequency octopole ion trap at 80K. The product clusters are then transferred to a 20K cryogenic ion trap where they are tagged with weakly bound D2 molecules. The structures of these clusters are determined by analysis of their vibrational patterns, obtained by resonant IR photodissociation. The resulting patterns confirm that the metastable N-protomer configuration remains intact even when warmed by the sequential condensation of water molecules. The attachment of H2O molecules onto the RND3 + head group also affords the opportunity to explore the possibility of H/D exchange between the acid scaffold and the proximal water network. The spectroscopic results establish that although the RND3 +·(H2O)n=1,2 clusters are formed without H/D exchange, the n = 3 cluster exhibits about 10% H/D exchange as evidenced by the appearance of the telltale HOD bands. The site of exchange on the acid is determined to be the acidic OH group by the emergence of the OH stretching fundamental in the -COOH motif.

Cu(II) complexes of flavonoids in solution: Impact of the Cu(II) ion on the antioxidant and DNA-intercalating properties

• Complexes of Cu(II) with flavonoids are studied in two solvents: DMSO and methanol. • Formation of complexes is monitored by UV-Vis spectroscopy and DFT calculations. • Kaempferol, luteolin and fisetin complexes show enhanced radical scavenging effect. • Molecular docking study indicates the interaction with DNA via intercalation. • Complexes may represent perspective therapeutics for neurodegenerative disorders. Flavonoids are naturally occurring polyphenolic compounds showing antioxidant and metal chelating properties. Chelation of the metal ion, e.g. , Cu(II), may significantly affect their activity in biological systems. Due to DNA-intercalating ability, metal complexes of flavonoids can act as therapeutics in the treatment of various human diseases, such as cancer or neurodegenerative diseases. In this work, antioxidant and DNA-intercalating properties of Cu(II)-flavonoid complexes are investigated using spectroscopic techniques and various computational approaches. The formation of stable Cu(II)-flavonoid complexes of kaempferol, luteolin, fisetin, and apigenin in two metal:ligand ratios (1:1 and 1:2) is confirmed by UV-Vis spectroscopy. DFT calculations indicate that Cu(II) ion adopts square planar coordination in Cu(II)-flavonoid (1:2) complexes. Radical scavenging activity of flavonoids in the presence/absence of Cu(II) ion is monitored using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay. Except for apigenin, studied complexes show enhanced radical scavenging activity. Calculation results also imply that the presence of Cu(II) ion enhances acidity of OH groups in flavonoids. The molecular docking studies demonstrate that the formation of Cu(II)-flavonoid complexes (1:1 and/or 1:2) improves the DNA-intercalating ability of the parent flavonoids.

Electronic and mechanical anharmonicities in the vibrational spectra of the H-bonded, cryogenically cooled X- · HOCl (X=Cl, Br, I) complexes: Characterization of the strong anionic H-bond to an acidic OH group.

We report vibrational spectra of the H2-tagged, cryogenically cooled X- · HOCl (X = Cl, Br, and I) ion-molecule complexes and analyze the resulting band patterns with electronic structure calculations and an anharmonic theoretical treatment of nuclear motions on extended potential energy surfaces. The complexes are formed by "ligand exchange" reactions of X- · (H2O)n clusters with HOCl molecules at low pressure (∼10-2mbar) in a radio frequency ion guide. The spectra generally feature many bands in addition to the fundamentals expected at the double harmonic level. These "extra bands" appear in patterns that are similar to those displayed by the X- · HOD analogs, where they are assigned to excitations of nominally IR forbidden overtones and combination bands. The interactions driving these features include mechanical and electronic anharmonicities. Particularly intense bands are observed for the v = 0 → 2 transitions of the out-of-plane bending soft modes of the HOCl molecule relative to the ions. These involve displacements that act to break the strong H-bond to the ion, which give rise to large quadratic dependences of the electric dipoles (electronic anharmonicities) that drive the transition moments for the overtone bands. On the other hand, overtone bands arising from the intramolecular OH bending modes of HOCl are traced to mechanical anharmonic coupling with the v = 1 level of the OH stretch (Fermi resonances). These interactions are similar in strength to those reported earlier for the X- · HOD complexes.

Preparation and Characterization of the Halogen-Bonding Motif in the Isolated Cl-·IOH Complex with Cryogenic Ion Vibrational Spectroscopy.

In the presence of a halide ion, hypohalous acids can adopt two binding motifs upon formation of the ion-molecule complexes [XHOY]- (X, Y = Cl, Br, I): a hydrogen (HB) bond to the acid OH group and a halogen (XB) bond between the anion and the acid halogen. Here we isolate the X-bonded Cl-·IOH ion-molecule complex by collisions of I-·(H2O)n clusters with HOCl vapor and measure its vibrational spectrum by IR photodissociation of the H2-tagged complex. Anharmonic analysis of its vibrational band pattern reveals that formation of the XB complex results in dramatic lowering of the HOI bending fundamental frequency and elongation of the O-I bond (by 168 cm-1 and 0.13 Å, respectively, relative to isolated HOI). The frequency of the O-I stretch (estimated 436 cm-1) is also encoded in the spectrum by the weak v = 0 → 2 overtone transition at 872 cm-1.

Vibrational Signatures of HNO3 Acidity When Complexed with Microhydrated Alkali Metal Ions, M+·(HNO3)(H2O)n=5 (M = Li, K, Na, Rb, Cs), at 20 K.

The speciation of strong acids like HNO3 under conditions of restricted hydration is an important factor in the rates of chemical reactions at the air-water interface. Here, we explore the trade-offs at play when HNO3 is attached to alkali ions (Li+-Cs+) with four water molecules in their primary hydration shells. This is achieved by analyzing the vibrational spectra of the M+·(HNO3)(H2O)5 clusters cooled to about 20 K in a cryogenic photofragmentation mass spectrometer. The local acidity of the acidic OH group is estimated by the extent of the red shift in its stretching frequency when attached to a single water molecule. The persistence of this local structural motif (HNO3-H2O) in all of these alkali metal clusters enables us to determine the competition between the effect of the direct complexation of the acid with the cation, which acts to enhance acidity, and the role of the water network in the first hydration shell around the ions, which acts to counter (screen) the intrinsic effect of the ion. Analysis of the vibrational features associated with the acid molecule, as well as those of the water network, reveals how cooperative interactions in the microhydration regime conspire to effectively offset the intrinsic enhancement of HNO3 acidity afforded by attachment to the smaller cations.

Effect of alumina surface chemistry on chromia dispersion and dehydrogenation activity of CrOx/η-Al2O3 catalysts with high Cr content

In this work, effect of alumina surface chemistry on physicochemical properties and isobutane dehydrogenation activity of CrOx/η-Al2O3 catalysts with ca. 13 wt. % Cr was investigated. For this purpose, three catalysts were synthesized from η-Al2O3 samples which have identical textural properties but different density of Lewis acid sites (LAS) and OH groups as revealed by IR spectroscopy of adsorbed CO and pyridine. Systematic investigation of CrOx/η-Al2O3 catalysts with a number of techniques (chemical analysis, N2 adsorption, X-ray diffraction, Raman spectroscopy and transmission electron microscopy) showed that (a) Cr6+ and Cr3+ quantities as well as surface areas are the same; (b) quantity of low-active large α-Cr2O3 (> ca. 5 nm) and very small CrOx species (< ca. 1 nm) diminishes while (c) content of partially crystallized Cr2O3 particles from ca. 1 to about 5 nm size increases with the LAS density on parent η-Al2O3 and its dehydroxylation. Correspondingly, isobutane dehydrogenation activity at low times on stream (up to 40 min at 570 °C) increases. This activity difference was observed in the course of 50 dehydrogenation-regeneration cycles. Therefore, alumina surface chemistry strongly affects the Cr2O3 dispersion and dehydrogenation performance of high-loaded chromia/alumina catalysts. On the basis of the results, a model of surface structure for CrOx/Al2O3 catalysts was proposed including surface migration of chromium being dependent upon LAS and OH groups of Al2O3.

Acidic property of YNU-5 zeolite influenced by its unique micropore system

YNU-5 zeolite has the YFI -type framework with a 12-12-8-ring system (2-dimensional 12-oxygen membered ring pores 3-dimensionally connected by twin 8-ring channels), and isolated 8-ring channels separated from the system by a thin (mono-atomic silicate) wall. In this study, the acidic property of YNU-5 zeolite was analyzed mainly by means of the ammonia IRMS-TPD (infrared/mass spectroscopy temperature-programmed desorption) method. In addition, the accessibility of acid sites was evaluated through adsorption of pyridine. The number of Brønsted acid sites approximately agreed with the number of Al atoms, and the amount of Lewis acid sites was negligible, indicating that most acid sites have the nature of bridging Si(OH)Al group in the YFI framework. Most of the Brønsted acidic OH groups on the YNU-5 zeolites (including the dealuminated samples) reacted with pyridine vapor at 343 K to form pyridinium cations, indicating that the Brønsted acid sites are highly accessible through the 12-12-8-ring system. Enthalpy of ammonia desorption from the Brønsted acid site, which can be an index of acid strength or reactivity of the acid site with a basic reactant, was in the following order: FAU < MOR (12-ring) ≈ *BEA < MFI < MWW ≈ YFI < MOR (8-ring), indicating the markedly large ammonia desorption enthalpy on the YNU-5 zeolite compared to the other 12-ring zeolites. The DFT calculations suggest that the Brønsted acidic protons with especially high ammonia desorption enthalpy are located in the isolated 8-ring but accessible from the 12-12-8-ring system side. In addition, dealumination at a high temperature under the reflux conditions with nitric acid resulted in the preferential removal of Brønsted acid sites with low ammonia desorption enthalpy, probably in the 12-12-8-ring system. The presence of reactive Brønsted acid sites in the isolated 8-ring, accessibility to them from the 12-12-8-ring system, and preferential removal of the Brønsted acid sites from the 12-12-8-ring system are reasonably consistent with the previously reported catalytic properties for dimethyl ether-to-olefin reaction. • Enthalpy of ammonia desorption (Δ H ) from the Brønsted acid site on the YNU-5 zeolite was high. • The Brønsted acidic protons with high Δ H were located in the isolated 8-ring but accessible from the 12-12-8-ring system. • Dealumination in the reflux conditions of nitric acid preferential proceeded in the 12-12-8-ring system.

Effect of rare earths on acidity of high-silica ultrastable REY zeolites and catalytic performance of NiMo/REY+Al2O3 catalysts in vacuum gas oil hydrocracking

A series of low-sodium ultrastable Y zeolites (SiO 2 /Al 2 O 3 ≈ 25) partially exchanged with different amounts of rare earth ions (RE 2 O 3 = 0.55–3.32 wt%) was prepared and characterized by XRD, IR spectroscopy, elemental analysis, N 2 adsorption, as well as 29 Si and 27 Al NMR. The effect of RE content on the number and relative strength of acid sites of REY zeolites was studied by IR spectroscopy of absorbed CO and pyridine. The increase in the RE content in the zeolites leads to a decrease in the concentration of strong Brønsted acid sites (BAS) in faujasite cages (bridged Si–O(H)–Al groups), especially the concentration of stronger BAS associated with Si–O(H)–Al groups polarized by extra-framework aluminum species. At the same time, the concentration of less acidic OH groups connected with extra-framework ions increases possibly due to the formation of additional RE–O(H)–Al groups. The total number of BAS in all the samples remains close. An increase in the RE 2 O 3 content from 0.55 to 3.32 wt% decreases almost by half the ratio of strong BAS in faujasite cages to less acidic extra-framework BAS. The catalytic performance of NiMo/(30 wt% REY + 70 wt% Al 2 O 3 ) catalysts was evaluated in hydrocracking of vacuum gas oil. The enhanced hydrocracking activity and selectivity to middle distillates are mainly achieved due to the optimal acidity of the zeolites. • Synthesis of USY zeolites (Si/Al≈12.5) exchanged with different amount of RE ions. • Strong BAS/less acidic extraframework BAS ratio decreases with RE content increase. • Stronger BAS (Si–O(H)–Al groups polarized by EFAL species) are exchanged first. • Additional BAS are associated with the formation of RE–O(H)–Al groups in supercages. • NiMo/REY + Al 2 O 3 with optimal acidity exhibits improved hydrocracking performance.

Effect of two-component amorphous silica-alumina (ASA) with different Si/Al molar ratios on hydrocracking reactions for increasing naphtha over NiW/USY-ASA

The combination of ASA-0.4 and ASA-2 produces new acidic OH groups by increasing the contact points between silicon and aluminum. These OH groups improve the density of acid sites and increases the area of the active adsorption area of the support.

XRD, NMR, FT-IR and DFT structural characterization of a novel organic-inorganic hybrid perovskite-type hexabromotellurate material

The synthesis of a new hexabromotellurate of triazole ring-containing amino acid is described. XRD, NMR and DFT analysis of the structure of protonated [(5-amino-4-phenyl-4H-1,2,4-triazol-3-yl)sulfanyl]acetic acid testifies the protonation of the first nitrogen atom of the triazole ring. Hirshfeld surface analysis identifies numerous intermolecular interactions in the newly synthesized organic-inorganic hybrid hexabromotellurate. The DFT based comprehensive analysis of the structures of possible conformers also testifies the higher thermodynamic stability of the 1-protonated triazole cation, whereas the starting [(5-amino-4-phenyl-4H-1,2,4-triazol-3-yl)sulfanyl]acetic acid exists in solution in neutral form with the intermolecular hydrogen bond between the carboxylic acid OH group and the second nitrogen of the triazole ring. The experimentally observed NMR upfield shift of the signal of the exocyclic amino group was studied via GIAO computations and was explained through the analysis of the HOMOs of the neutral and protonated forms of the triazole compound.

Elucidation of the versatile Brønsted acidity of nanosized ZSM-5 materials

Abstract Studying a series of nanosized and microsized H-ZSM-5 zeolites with nSi/nAl ratios of 31–88 revealed systematic differences in the crystallinity, morphology, and Bronsted acidity of these materials. While the aluminium atoms in all samples under study are tetrahedrally coordinated framework species, only 32–34% of these atoms in the nanosized materials contribute to the formation of Bronsted acidic bridging OH groups (Si(OH)Al), which are characteristic for common microsized H-ZSM-5 zeolites. The remaining framework aluminium species cause formation of silanol groups with weakly enhanced Bronsted acidity (Si(OH)⋯Al) comparable with those in amorphous aluminosilicates. These properties of nanosized ZSM-5 materials are explained by the assumption of hybrid particles consisting of internal crystalline ZSM-5 domains containing Si(OH)Al groups covered by X-ray-amorphous surface layers with Si(OH)⋯Al groups. Direct evidence for these surface layers is found with transmission electron microscopy, but the other characterisation methods are also in line with this explanation.

Impact of proximity between NiMoS and zeolitic HY sites on cyclohexene hydroconversion: An infrared operando study of sulfide catalysts

This work evaluates the effects of sulfide phase location on zeolite-containing catalysts in cyclohexene hydroconversion using the infrared operando technique. Parallels between activity, product yield, and surface species changes with time on stream and under pyridine poisoning were assessed on pure HY zeolite, sulfided NiMo/HY, NiMo/Al2O3, and NiMo/Al2O3 + HY. Isomerization activity could be related to zeolitic acidic OH groups and hydrogenation activity to sulfide phase sites. From the changes due to pyridine pulse injection, intrinsic activities could be calculated for isomerization (1-methyl-1-cyclopentene) and hydrogenation (cyclohexane) routes. The turnover frequency value for isomerization is greater than that for hydrogenation by one order of magnitude. Hence, the poisoning effect of one N-molecule on isomerization will be 10 times higher than that on the hydrogenation route. Whatever the location of the sulfide phase, the coke is mostly formed on the zeolite. As a consequence, on NiMo/HY, the close proximity between acidic and sulfide sites results in lower formation of hydrogenated products, due to blockage of some sulfide sites by coke. In contrast, the hydrogenation activity of NiMo/Al2O3 + HY is less affected by coke, NiMoS being mostly located on alumina. However, the results show that the close proximity between acid and sulfide sites allows a higher degree of hydrogenation of the isomerization product (1-methyl-1-cyclopentene). Thus, the higher proximity has a beneficial effect on methylcyclopentane formation.

Effects of Brønsted acid site proximity in chabazite zeolites on OH infrared spectra and protolytic propane cracking kinetics

The arrangement of framework Al heteroatoms in zeolite lattices influences the energetics of proton siting and thus the distribution of protons among crystallographically-distinct oxygen atoms and void environments, as shown here for H-CHA zeolites of fixed bulk Al content (Si/Al = 15) but varying amounts of proximal Al sites (Al-O-(Si-O)x-Al, x = 1,2) in a six-membered ring (6-MR). Stretching vibrations of Brønsted acidic OH groups give rise to composite infrared (IR) features whose shapes and integrated intensities are invariant with temperature (448–748 K) on H-CHA zeolites containing predominantly 6-MR isolated Al sites. In sharp contrast, IR peak shapes change and integrated OH areas decrease reversibly with increasing temperature on H-CHA zeolites containing 6-MR paired Al sites (quantified by Co2+ titration). Periodic density functional theory (DFT) calculations show that the four distinct OH configurations at an isolated Al site exhibit distinct vibrational frequencies and molar absorptivities, but are isoenergetic (within 10 kJ mol−1) rendering equilibrium proton populations and OH IR spectra insensitive to temperature. In contrast, paired proton configurations differ in energy by up to ~40 kJ mol−1, with higher energy configurations having lower oscillator strengths; as a result, increasing temperatures shift proton populations toward higher energy configurations that result in a decrease in composite OH IR peak areas. First-order rate constants of protolytic propane cracking (748 K, per H+) are ~12× higher on paired than isolated protons, despite similar apparent activation energies, reflecting apparent activation entropies that are less negative at paired proton sites. Propane cracking rates (748 K, per g) decrease linearly with Na+ content upon partial titration of H+ sites in CHA zeolites containing 6-MR isolated sites, but decrease more strongly at low Na+ contents on CHA zeolites containing 6-MR paired sites, consistent with experimental OH IR spectra and DFT calculations evincing the preferential exchange of Na+ at 6-MR paired Al sites. These conclusions about proton proximity, founded on the interrogation of well-defined and characterized CHA zeolite samples by experiment and theory, provide new insights regarding its consequences for the temperature sensitivity of OH IR spectra and protolytic alkane cracking rates in Brønsted acidic zeolites. The finding that proximal proton sites accelerate high-temperature alkane activation turnovers via non-polar transition states represents a mechanistically distinct extension of our prior reports, wherein proximal protons accelerate low-temperature alkanol dehydration turnovers via polar transition states stabilized by hydrogen bonding interactions, heralding the catalytic diversity among zeolites containing different framework Al arrangements.