Synthesis and Characterization of Spinel‐Type Gallia‐Alumina Solid Solutions

By precipitation with ammonia of ethanolic solutions containing the appropriate proportions of gallium and aluminium nitrate, following by calcination of the resulting gels at 773 K, mixed Ga₂O₃/Al₂O₃ oxides having Ga:Al ratios of 9:1, 4:1, 1:1, 1:4 and 1:9 were obtained. Powder X‐ray diffraction showed that these mixed metal oxides form a series of solid solutions having the spinel‐type structure; also shown by γ‐Al₂O₃ and γ‐Ga₂O₃. The specific surface area (determined by nitrogen adsorption at 77 K) was found to range from 160 m² g^?1 for the mixed oxide having Ga:Al = 9:1 up to 370 m² g^?1 for that having Ga:Al = 1:9. High resolution MAS NMR showed that Ga³⁺ and Al³⁺ ions occur at both tetrahedral and octahedral sites in the spinel‐type structure of the mixed metal oxides, although there is a preferential occupation of tetrahedral sites by Ga³⁺ ions. A proportion of penta‐coordinated Al³⁺ ions was also found. IR spectra of carbon monoxide adsorbed at 77 K showed that the mixed metal oxides have a considerable Lewis acidity, related mainly to tetrahedrally coordinated metal ions exposed at crystal surfaces. The characteristic infrared absorption band of coordinated (adsorbed) CO appears in the range 2205–2190 cm^?1, and its peak wavenumber is nearly independent of Ga:Al ratio in the mixed gallia‐alumina oxides.     

Vergleichende Untersuchungen zur Struktur von 4‐Dimethylaminopyridin‐Addukten

Comparative Structural Studies on 4‐Dimethylaminopyridine‐Adducts Lewis acid‐base adducts of the type dmap—MMe₃ (M = Al 1 , Ga 2 , In 3 , Tl 4 ) as well as dmap—AlCl₃ ( 6 ) and dmap—Al(t‐Bu)₃ ( 7 ) were synthesized by reaction of MR₃ with 4‐dimethylamino‐pyridine (dmap) whereas dmap—AlH₃ ( 5 ) was obtained from AlH₃·Et₂O. 1 — 7 were characterized by means of NMR (¹H, ¹³C{¹H}) and mass spectrometry and elemental analysis. In addition, their solid state structures were determined by single crystal X‐ray diffraction studies. A comparison of the structural parameters reveales the influence of both electronic (Lewis acidity of the group 13 atom) and steric interactions on the structure and stability of as prepared Lewis acid‐base adducts.     

Exploring the structural landscape of 2‐(thiophen‐2‐yl)‐1,3‐benzothiazole: high‐Z′ packing polymorphism and cocrystallization with calix[4]tube

We report here for the first time a cocrystal of the so‐called neutral calix[4]tube, which is two tail‐to‐tail‐arranged and partially deprotonated tetrakis(carboxymethoxy)calix[4]arenes, including three sodium ions, with 2‐(thiophen‐2‐yl)‐1,3‐benzothiazole, namely trisodium bis(carboxymethoxy)bis(carboxylatomethoxy)calix[4]arene tris(carboxymethoxy)(carboxylatomethoxy)calix[4]arene–2‐(thiophen‐2‐yl)‐1,3‐benzothiazole–dimethyl sulfoxide–water (1/1/2/2), 3Na⁺·C₃₆H₃₀O₁₂^2?·C₃₆H₃₁O₁₂^?·C₁₁H₇NS₂·2C₂H₆OS·2H₂O, which provides a new approach into the host–guest chemistry of inclusion complexes. Three packing polymorphs of the same benzothiazole with high Z′ (one with Z′ = 8 and two with Z′ = 4) were also discovered in the course of our desired cocrystallization. The inspection of these polymorphs and a previously known polymorph with Z′ = 2 revealed that Z′ increases as the strength of intermolecular contacts decreases. Also, these results expand the frontier of invoking calixarenes as a host for nonsolvent small molecules, besides providing knowledge on the rare formation of high‐Z′ packing polymorphs of simple molecules, such as the target benzothiazole.     

Zero‐, one‐ and two‐dimensional hydrogen‐bonded structures in the 1:1 proton‐transfer compounds of 4,5‐dichlorophthalic acid with the monocyclic heteroaromatic Lewis bases 2‐aminopyrimidine,nicotinamide and isonicotinamide

The structures of the anhydrous 1:1 proton‐transfer compounds of 4,5‐dichlorophthalic acid (DCPA) with the monocyclic heteroaromatic Lewis bases 2‐aminopyrimidine, 3‐(aminocarbonyl)pyridine (nicotinamide) and 4‐(aminocarbonyl)pyridine (isonicotinamide), namely 2‐aminopyrimidinium 2‐carboxy‐4,5‐dichlorobenzoate, C₄H₆N₃⁺·C₈H₃Cl₂O₄⁻, (I), 3‐(aminocarbonyl)pyridinium 2‐carboxy‐4,5‐dichlorobenzoate, C₆H₇N₂O⁺·C₈H₃Cl₂O₄⁻, (II), and the unusual salt adduct 4‐(aminocarbonyl)pyridinium 2‐carboxy‐4,5‐dichlorobenzoate–methyl 2‐carboxy‐4,5‐dichlorobenzoate (1/1), C₆H₇N₂O⁺·C₈H₃Cl₂O₄⁻·C₉H₆Cl₂O₄, (III), have been determined at 130 K. Compound (I) forms discrete centrosymmetric hydrogen‐bonded cyclic bis(cation–anion) units having both R₂²(8) and R₁²(4) N—H...O interactions. In (II), the primary N—H...O‐linked cation–anion units are extended into a two‐dimensional sheet structure via amide–carboxyl and amide–carbonyl N—H...O interactions. The structure of (III) reveals the presence of an unusual and unexpected self‐synthesized methyl monoester of the acid as an adduct molecule, giving one‐dimensional hydrogen‐bonded chains. In all three structures, the hydrogen phthalate anions are essentially planar with short intramolecular carboxyl–carboxylate O—H...O hydrogen bonds [O...O = 2.393 (8)–2.410 (2) Å]. This work provides examples of low‐dimensional 1:1 hydrogen‐bonded DCPA structure types, and includes the first example of a discrete cyclic `heterotetramer.' This low dimensionality in the structures of the 1:1 aromatic Lewis base salts of the parent acid is generally associated with the planar DCPA anion species.     

Synthesis,Coordination Behavior,and Reduction Chemistry of Cymantrenyl‐1,3‐bis(2,3,4,5‐tetraphenyl)borole

We describe the synthesis of base‐free bisborole [Cym^?(BC₄Ph₄)₂]—Cym^?=(OC)₃Mn(η⁵‐C₅H₃)—and its transformation into two fully characterized Lewis acid–base adducts with pyridine bases of the type 4‐R? NC₅H₄ (R=tBu, NMe₂). The results of electrochemical, as well as NMR and UV/Vis spectroscopic studies on [Cym^?(BC₄Ph₄)₂] and the related monoborole derivative [Cym(BC₄Ph₄)]—Cym=(OC)₃Mn(η⁵‐C₅H₄)—provided conclusive evidence for 1) the enhanced Lewis acidity of the two boron centers that result from conjugation of two borole fragments, and 2) the fact that Mn? B bonding interactions between the Lewis acidic borole moieties and the Mn center are considerably less pronounced for bisborole [Cym^?(BC₄Ph₄)₂]. In addition, the reduction chemistry of [Cym^?(BC₄Ph₄)₂] has been studied in detail, both electrochemically and chemically. Accordingly, chemical reduction of [Cym^?(BC₄Ph₄)₂] with magnesium anthracene afforded the corresponding tetraanion, which features a rare Mg? OC bonding mode in the solid state.     

Noble‐Metal‐Free Single‐Atom Catalysts CuAl4O7–9− for CO Oxidation by O2

The single copper atom doped clusters CuAl₄O_7–9^? can catalyze CO oxidation by O₂. The CuAl₄O_7–9^? clusters are the first group of experimentally identified noble‐metal free single atom catalysts for such a prototypical reaction. The reactions were characterized by mass spectrometry and density functional theory calculations. The CuAl₄O₉CO^? is much more reactive than CuAl₄O₉^? in the reaction with CO to generate CO₂. One adsorbed CO is crucial to stabilize Cu of CuAl₄O₉^? around +I oxidation state and promote the oxidation of another CO. The widely emphasized correlation between the catalytic reactivity of CO oxidation and Cu oxidation state can be understood at the strictly molecular level. The remarkable difference between Cu catalysis and noble‐metal catalysis was discussed.     

Enhanced catalytic performance of Pd‐Ga bimetallic catalysts for 2‐ethylanthraquinone hydrogenation

A series of Pd and Pd‐Ga bimetallic catalysts were prepared by a co‐impregnation method for 2‐ethylanthraquinone (EAQ) hydrogenation to produce hydrogen peroxide. Compared with 0.6Pd catalyst, the hydrogenation efficiency of 0.6Pd1.2Ga catalyst (11.9 g L^?1) increases by 32.2%, and the stability of 0.6Pd1.2Ga catalyst is also higher than that of 0.6Pd catalyst. The structures of the samples were determined by N₂ adsorption–desorption, ICP, XRD, CO chemisorption, TEM, H₂‐TPR, in situ CO‐DRIFTS and XPS. The results suggest that incorporation of Ga species improves Pd dispersion and generates a strong interaction between Ga₂O₃ and Pd interface or between Pd and support. DFT calculation results indicate that the strong adsorption of carbonyl group on Ga₂O₃/Pd interface facilitates the activation of EAQ and promotes the hydrogenation efficiency.     

A Comparative Study of CO Oxidation on Nitrogen‐ and Phosphorus‐Doped Graphene

The geometry, electronic structure, and catalytic properties of nitrogen‐ and phosphorus‐doped graphene (N‐/P‐graphene) are investigated by density functional theory calculations. The reaction between adsorbed O₂ and CO molecules on N‐ and P‐graphene is comparably studied via Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms. The results indicate that a two‐step process can occur, namely, CO+O₂→CO₂+O_ads and CO+O_ads→CO₂. The calculated energy barriers of the first step are 15.8 and 12.4 kcal mol^?1 for N‐ and P‐graphene, respectively. The second step of the oxidation reaction on N‐graphene proceeds with an energy barrier of about 4 kcal mol^?1. It is noteworthy that this reaction step was not observed on P‐graphene because of the strong binding of O_ads species on the P atoms. Thus, it can be concluded that low‐cost N‐graphene can be used as a promising green catalyst for low‐temperature CO oxidation.     

Aluminum‐ and Gallium‐Containing Open‐Dawson Polyoxometalates

Al‐ and Ga‐containing open‐Dawson polyoxometalates (POMs), K₁₀[{Al₄(μ‐OH)₆}{α,α‐Si₂W₁₈O₆₆}] · 28.5H₂O ( Al₄ ‐ open ) and K₁₀[{Ga₄(μ‐OH)₆}(α,α‐Si₂W₁₈O₆₆)] · 25H₂O ( Ga₄ ‐ open ) were synthesized by the reaction of trilacunary Keggin POM, [A‐α‐SiW₉O₃₄]^10–, with Al(NO₃)₃ · 9H₂O or Ga(NO₃)₃ · nH₂O, and unequivocally characterized by single‐crystal X‐ray analysis, ²⁹Si and ¹⁸³W NMR, and FT‐IR spectroscopy as well as elemental analysis and TG/DTA. Single‐crystal X‐ray analysis revealed that the {M₄(μ‐OH)₆}⁶⁺ (M = Al, Ga) clusters were included in an open pocket of the open‐Dawson polyanion, [α,α‐Si₂W₁₈O₆₆]^16–, which was constituted by the fusion of two trilacunary Keggin POMs via two W–O–W bonds. These two open‐Dawson structural POMs showed clear difference of the bite angles depending on the size of ionic radii. In cases of both compounds, the solution ²⁹Si and ¹⁸³W NMR spectra in D₂O showed only one signal and five signals, respectively. These spectra were consistent with the molecular structures of Al₄ ‐ and Ga₄ ‐ open , suggesting that these polyoxoanions were obtained as single species and maintained their molecular structures in solution.     

Co‐Adsorption of O2 and CO on Au2−: Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2O2(CO)n]− (n=2–6)

The co‐adsorption of O₂ and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au₂O₂(CO)_n]^? (n=2–6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au₂O₂(CO)_n]^? (n=2–6) complexes were characterized to have a core structure involving one CO and one O₂ molecule co‐adsorbed on Au₂^? with the other CO molecules physically tagged around. The CO stretching frequency of the [Au₂O₂(CO)]^? core ion is observed around =2032–2042 cm^?1, which is about 200 cm^?1 higher than that in [Au₂(CO)₂]^?. This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O₂ and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co‐adsorbed O₂ and CO on Au₂^? may have relevance to the elementary reaction steps on real gold catalysts.     

Synthesis and Structure of Dicyclohexylammonium Tri‐tert‐pentoxysilanethiolate and 5‐Aminopentylammonium Tri‐tert‐pentoxysilanethiolate

Tri–tert–pentoxysilanethiol, (EtMe₂CO)₃SiSH, reacts with dicyclohexylamine and 1,5–diaminopentane yielding ion‐quadruples connected by hydrogen bonds: [{(C₆H₁₁)₂NH₂}⁺{(EtMe₂CO)₃SiS}^?]₂ ( 1 ) and [{H₂N–C₅H₁₀–NH₃}⁺{(EtMe₂CO)₃SiS}^?]₂ ( 2 ). The compounds were characterized by X–ray diffraction, elemental analysis, IR and NMR. Both compounds form dimers, composed of two thiolate anions and two ammonium cations. The dimers have a central core of eight–membered rings built due to the formation of four charge–assisted N⁺–H···^?S–hydrogen bonds. Additional N⁺–H···N hydrogen bonds are found in the case of 2 , linking the dimers into infinite two–dimensional sheets.     

Hydrogen bonding in two benzene‐1,2‐diaminium pyridine‐2‐carboxylate salts and a cocrystal of benzene‐1,2‐diamine and benzoic acid

The isostructural salts benzene‐1,2‐diaminium bis(pyridine‐2‐carboxylate), 0.5C₆H₁₀N₂²⁺·C₆H₄NO₂^?, (1), and 4,5‐dimethylbenzene‐1,2‐diaminium bis(pyridine‐2‐carboxylate), 0.5C₈H₁₄N₂²⁺·C₆H₄NO₂^?, (2), and the 1:2 benzene‐1,2‐diamine–benzoic acid cocrystal, 0.5C₆H₈N₂·C₇H₆O₂, (3), are reported. All of the compounds exhibit extensive N—H…O hydrogen bonding that results in interconnected rings. O—H…N hydrogen bonding is observed in (3). Additional π–π and C—H…π interactions are found in each compound. Hirshfeld and fingerprint plot analyses reveal the primary intermolecular interactions and density functional theory was used to calculate their strengths. Salt formation by (1) and (2), and cocrystallization by (3) are rationalized by examining pK_a differences. The R₂²(9) hydrogen‐bonding motif is common to each of these structures.     

2,6‐Bis[bis(2‐pyridiniomethyl)aminomethyl]‐4‐tert‐butylphenol dichloride diperchlorate hydrate

The title compound, C₃₆H₄₄N₆O⁴⁺·2Cl^?·2ClO₄^?·0.132H₂O, is shown to be protonated at all the pyridine N atoms; the two chloride ions are hydrogen bonded to three pyridine N atoms and to the phenolic O atom of the same cation [Cl?N = 3.045 (2)–3.131 (2) Å and Cl?O = 2.938 (2) Å], and the remaining pyridine N atom is hydrogen bonded to the phenolic O atom [N?O = 2.861 (2) Å]. The mean value of the C—N—C angle of the protonated pyridine rings is 123.4 (1)°, which is significantly larger than that found for unprotonated pyridine rings.     

Room‐Temperature Activation of H2 by a Surface Frustrated Lewis Pair

Surface frustrated Lewis pairs (SFLPs) have been implicated in the gas‐phase heterogeneous (photo)catalytic hydrogenation of CO₂ to CO and CH₃OH by In₂O_3?x(OH)_y. A key step in the reaction pathway is envisioned to be the heterolysis of H₂ on a proximal Lewis acid–Lewis base pair, the SFLP, the chemistry of which is described as In???In‐OH + H₂ → In‐OH₂⁺???In‐H^?. The product of the heterolysis, thought to be a protonated hydroxide Lewis base In‐OH₂⁺ and a hydride coordinated Lewis acid In‐H^?, can react with CO₂ to form either CO or CH₃OH. While the experimental and theoretical evidence is compelling for heterolysis of H₂ on the SFLP, all conclusions derive from indirect proof, and direct observation remains lacking. Unexpectedly, we have discovered rhombohedral In₂O_3?x(OH)_y can enable dissociation of H₂ at room temperature, which allows its direct observation by several analytical techniques. The collected analytical results lean towards the heterolysis rather than the homolysis reaction pathway.     

Synthesis,characterization, and optimum reaction conditions of oligo‐2‐[(pyridine‐2‐yl‐methylene) amino] phenol

The reaction conditions of the oxidative polycondensation of 2‐[(pyridine‐2‐yl‐methylene) amino] phenol (2‐PMAP) with air O₂, H₂O₂, and NaOCl were studied in an aqueous alkaline medium between 60 and 90 °C. Oligo‐2‐[(pyridine‐2‐yl‐methylene) amino] phenol (O‐2‐PMAP) was characterized with ¹H NMR, Fourier transform infrared, ultraviolet–visible, size exclusion chromatography (SEC), and elemental analysis techniques. Moreover, solubility testing of the oligomer was performed in polar and nonpolar organic solvents. With the NaOCl, H₂O₂, and air O₂ oxidants, the conversions of 2‐PMAP into O‐2‐PMAP were 98, 87, and 62%, respectively, in an aqueous alkaline medium. According to SEC, the number‐average molecular weight, weight‐average molecular weight, and polydispersity index of O‐2‐PMAP were 2262 g mol^?1, 2809 g mol^?1, and 1.24 with NaOCl, 3045 g mol^?1, 3861 g mol^?1, and 1.27 with air O₂, and 1427 g mol^?1, 1648 g mol^?1, and 1.16 with air H₂O₂, respectively. Also, thermogravimetric analysis showed that O‐2‐PMAP was stable against thermooxidative decomposition. The weight loss of O‐2‐PMAP was 96.68% at 900 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2717–2724, 2004     

Semiconductor/Covalent‐Organic‐Framework Z‐Scheme Heterojunctions for Artificial Photosynthesis

A strategy to covalently connect crystalline covalent organic frameworks (COFs) with semiconductors to create stable organic–inorganic Z‐scheme heterojunctions for artificial photosynthesis is presented. A series of COF–semiconductor Z‐scheme photocatalysts combining water‐oxidation semiconductors (TiO₂, Bi₂WO₆, and α‐Fe₂O₃) with CO₂ reduction COFs (COF‐316/318) was synthesized and exhibited high photocatalytic CO₂‐to‐CO conversion efficiencies (up to 69.67 μmol g^?1 h^?1), with H₂O as the electron donor in the gas–solid CO₂ reduction, without additional photosensitizers and sacrificial agents. This is the first report of covalently bonded COF/inorganic‐semiconductor systems utilizing the Z‐scheme applied for artificial photosynthesis. Experiments and calculations confirmed efficient semiconductor‐to‐COF electron transfer by covalent coupling, resulting in electron accumulation in the cyano/pyridine moieties of the COF for CO₂ reduction and holes in the semiconductor for H₂O oxidation, thus mimicking natural photosynthesis.     

Two succinic acid derivatives of 2‐ethyl‐6‐methylpyridin‐3‐ol

Crystals of bis(2‐ethyl‐3‐hydroxy‐6‐methylpyridinium) succinate–succinic acid (1/1), C₈H₁₂NO⁺·0.5C₄H₄O₄²⁻·0.5C₄H₆O₄, (I), and 2‐ethyl‐3‐hydroxy‐6‐methylpyridinium hydrogen succinate, C₈H₁₂NO⁺·C₄H₅O₄⁻, (II), were obtained by reaction of 2‐ethyl‐6‐methylpyridin‐3‐ol with succinic acid. The succinate anion and succinic acid molecule in (I) are located about centres of inversion. Intermolecular O—H...O, N—H...O and C—H...O hydrogen bonds are responsible for the formation of a three‐dimensional network in the crystal structure of (I) and a two‐dimensional network in the crystal structure of (II). Both structures are additionally stabilized by π–π interactions between symmetry‐related pyridine rings, forming a rod‐like cationic arrangement for (I) and cationic dimers for (II).     

Synthesis and Characterization of the Lewis Acid‐Base Complexes SbCl5 · LB (LB = ICN,BrCN, ClCN, 1/2(CN)2, NH2CN,Pyridine) – a Combined Theoretical and Experimental Investigation. The Crystal Structures of SbCl5 · NCCl and SbCl5 · NCCN · SbCl5

The Lewis acid‐base complexes SbCl₅ · LB (LB = ICN, BrCN, ClCN, ¹/₂(CN)₂, NH₂CN, pyridine) were prepared. The products formed were characterized by Raman and NMR spectroscopy. Density functional theory (B3LYP) was applied to calculate structural and vibrational data. Vibrational assignments of the normal modes for these Lewis acid‐base adducts was made on the basis of their Raman spectra in comparison with computational results. The stability of the complexes was investigated by calculating the bond dissociation enthalpy. SbCl₅ · NCCl and SbCl₅ · NCCN · SbCl₅ were characterized by X‐ray structural analysis. NBO analyses were performed on the crystallographic data.     

Three‐dimensional hydrogen‐bonded structures in the 1:1 proton‐transfer compounds of l‐tartaric acid with the associative‐group monosubstituted pyridines 3‐aminopyridine, 3‐carboxypyridine (nicotinic acid) and 2‐carboxypyridine (picolinic acid)

The 1:1 proton‐transfer compounds of l ‐tartaric acid with 3‐aminopyridine [3‐aminopyridinium hydrogen (2R,3R)‐tartrate dihydrate, C₅H₇N₂⁺·C₄H₅O₆⁻·2H₂O, (I)], pyridine‐3‐carboxylic acid (nicotinic acid) [anhydrous 3‐carboxypyridinium hydrogen (2R,3R)‐tartrate, C₆H₆NO₂⁺·C₄H₅O₆⁻, (II)] and pyridine‐2‐carboxylic acid [2‐carboxypyridinium hydrogen (2R,3R)‐tartrate monohydrate, C₆H₆NO₂⁺·C₄H₅O₆⁻·H₂O, (III)] have been determined. In (I) and (II), there is a direct pyridinium–carboxyl N⁺—H...O hydrogen‐bonding interaction, four‐centred in (II), giving conjoint cyclic R₁²(5) associations. In contrast, the N—H...O association in (III) is with a water O‐atom acceptor, which provides links to separate tartrate anions through O_hydroxy acceptors. All three compounds have the head‐to‐tail C(7) hydrogen‐bonded chain substructures commonly associated with 1:1 proton‐transfer hydrogen tartrate salts. These chains are extended into two‐dimensional sheets which, in hydrates (I) and (III) additionally involve the solvent water molecules. Three‐dimensional hydrogen‐bonded structures are generated via crosslinking through the associative functional groups of the substituted pyridinium cations. In the sheet struture of (I), both water molecules act as donors and acceptors in interactions with separate carboxyl and hydroxy O‐atom acceptors of the primary tartrate chains, closing conjoint cyclic R₄⁴(8), R₃⁴(11) and R₃³(12) associations. Also, in (II) and (III) there are strong cation carboxyl–carboxyl O—H...O hydrogen bonds [O...O = 2.5387 (17) Å in (II) and 2.441 (3) Å in (III)], which in (II) form part of a cyclic R₂²(6) inter‐sheet association. This series of heteroaromatic Lewis base–hydrogen l ‐tartrate salts provides further examples of molecular assembly facilitated by the presence of the classical two‐dimensional hydrogen‐bonded hydrogen tartrate or hydrogen tartrate–water sheet substructures which are expanded into three‐dimensional frameworks via peripheral cation bifunctional substituent‐group crosslinking interactions.     

Highly Dispersed SiOx/Al2O3 Catalysts Illuminate the Reactivity of Isolated Silanol Sites

The reaction of γ‐alumina with tetraethylorthosilicate (TEOS) vapor at low temperatures selectively yields monomeric SiO_x species on the alumina surface. These isolated (‐AlO)₃Si(OH) sites are characterized by PXRD, XPS, DRIFTS of adsorbed NH₃, CO, and pyridine, and ²⁹Si and ²⁷Al DNP‐enhanced solid‐state NMR spectroscopy. The formation of isolated sites suggests that TEOS reacts preferentially at strong Lewis acid sites on the γ‐Al₂O₃ surface, functionalizing the surface with “mild” Brønsted acid sites. For liquid‐phase catalytic cyclohexanol dehydration, these SiO_x sites exhibit up to 3.5‐fold higher specific activity than the parent alumina with identical selectivity.