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Two-stage flow synthesis of isobutyl-modified MAO using 3D printing.

We present the first 3D-printed continuous flow platform for the green synthesis of methylaluminoxane (MAO), the most powerful olefin polymerization co-catalyst with metallocenes. This platform consists of an on demand designed and 3D-printed liquid–liquid mixer, reactors, and a separator to enable precise control of the highly exothermic hydrolytic reaction of trimethylaluminum (TMA). Moreover, during TMA hydrolysis, solid by-products are formed and methane gas is released, both of which impede reagents blending and MAO formation, thereby making it a challenging implementation with a continuous flow strategy. By optimizing the platform configuration and reaction conditions, we obtain an MAO product with a high yield and superior co-catalytic activity, as measured by two different groups of polymerization tests. This low-volume process, which avoids the accumulation of any explosive reagents, offers long-term stability and inherent safety and facilitates scale up with multiple reactors. In addition, such an efficient, scalable, and safe flow platform preserving the mixing and heat transfer advantages delivers insights into handling other highly exothermic reactions involving the formation of solids and gases.

The co-reaction method provides an efficient strategy for methane conversion under mild conditions, which is of urgent importance for direct valorization of natural gas to liquid hydrocarbons. However, a vague mechanism and the neglect of methanol conversion make methane conversion difficult to achieve. In this work, the methane conversion pathway and catalysis mechanism are investigated by temperature programmed surface reaction-mass spectrometry (TPSR-MS) and reaction pathway regulation, and methane conversion can be achieved at the mild conditions of 300 °C and 1 atm by co-reaction with methanol using the Zn/ZSM-5 catalyst. It is observed that methane participates in an auto-catalysis process in which the resultant/intermediates, formed from the “aromatics-based cycle” of the methanol-to-aromatic (MTA) reaction, perform a critical function in place of methanol in the methane conversion. In an opposite way, C4–C6 alkene and/or alcohol additives suppress the cycle even at low addition amounts. Subsequent kinetic experiments demonstrate that the activation barrier for methane conversion can be significantly reduced by the addition of a small amount of aromatics. Furthermore, an effective increase in methane conversion (from 7.3% to 14.1%) and aromatic concentration (from 57.5% to 74.3% in C%) can be achieved by co-feeding 3 mol% p-xylene. This work provides a new strategy to promote methane conversion under mild conditions and to understand the role of reaction intermediates in methane conversion for catalyst design.

Novel NiPt/graphitic carbon nitride nanosheets (g-C3N4 NSs) have been successfully constructed through a simple and facile wet chemical reduction approach. Interestingly, benefiting from the unique structure of g-C3N4 NSs and their strong interaction with NiPt, Ni0.6Pt0.4/g-C3N4 NSs demonstrate outstanding catalytic activity toward hydrogen generation from hydrous hydrazine, giving 100% H2 selectivity with a high turnover frequency (TOF) value of 2194 h−1 at 323 K.

Ketonization of carboxylic acids provides an attractive way for upgrading biomass feedstocks into biofuels. Here, a series of Mg–Al hydrotalcite (HT) catalysts doped with different Ce contents were prepared and used for the ketonization of propionic acid. Mg3Al0.9Ce0.1 displayed the highest ketonization activity (90.6% conversion of propionic acid) under mild reaction conditions (350 °C). By applying SEM, TEM, H2-TPR, CO2/NH3-TPD, XPS, etc., the mechanism for the improvement of catalytic activity and stability was obtained. When the doping content of Ce is low, such as in Mg3Al0.9Ce0.1, cerium oxide is uniformly dispersed on the HT support. At the same time, the strong interaction between the CeO2 species and Mg–Al hydrotalcite led to the improvement of redox properties and modified acid–base sites, which is beneficial for the ketonization reaction. However, the structure of the HT support is destroyed by the agglomerated cerium oxide when a more doped Ce is employed, and the specific surface area and the surface Ce3+ ratio are remarkably lowered, thereby causing a significant decrease in the ketonization activity. By using the temperature programmed surface reaction (TPSR) technique, a deeper insight into the mechanism of reaction pathways over different catalysts is gained. The significant improvement in the reaction stability of the ketonization reaction over Mg3Al0.9Ce0.1 as compared to unmodified catalysts could be ascribed to the strengthening of the M–O bond of the catalyst after the introduction of Ce.

To improve the aromatics yield of methanol to aromatics conversion (MTA) over zeolite, which has become a potential route for producing aromatics, modified ZSM-5 catalysts with equimolar metals denoted as EM-X/ZSM-5 (X = Zn, Cu, Ag, and Ni) were investigated under CO2 and N2 flow for MTA in a fixed-bed reactor. The physicochemical properties were characterized by atomic absorption spectroscopy (AAS), N2 adsorption–desorption isotherms, X-ray diffraction (XRD), and NH3 temperature-programmed desorption (NH3-TPD). Comparison with the results obtained in pure N2 flow showed that catalysts doped with Zn, Ni, and Ag could promote aromatization activity and BTX yield in the presence of CO2. Among these, EM-Zn/ZSM-5 showed an aromatics yield of 59.05%, with an increase of 8.1%, whereas EM-Cu/ZSM-5 was found to reduce the aromatization activity in the presence of CO2. Moreover, the interaction mechanism of the active sites of the catalysts with CO2 for the MTA reaction was explored on the basis of the absorbability of the catalysts for CO2, which was studied by CO2 temperature-programmed desorption (CO2-TPD); the activation ability for CO2 to combine with hydrogen was investigated by the catalytic reaction of CO2 + H2, and the verification experiments for the coupling behavior of ZSM-5 doped with different contents of Zn in the presence of CO2 were carried out.

A series of Ce1−xTixO2−δ mixed oxide catalysts were synthesized using a homogeneous precipitation method and characterized, and then these catalysts were applied to convert acetic acid to acetone using a gas-phase ketonization reaction. Ti-doped Ce1−xTixO2−δ catalysts (x = 0.1–0.5) exhibited much better ketonization performance than their parent catalysts of CeO2 and TiO2, and such Ti-doping-induced catalysis improvement is attributed to the formation of a Ce–O–Ti structure depending on the Ti content. Among the different Ce1−xTixO2−δ catalysts, the Ce0.7Ti0.3O2−δ catalyst calcinated at 500 °C showed the best catalysis activity and high stability. A combination of techniques (i.e. TEM, FTIR, H2-TPR, NH3/CO2-TPD and XPS) further revealed that the formation of the Ce–O–Ti structure modified the surface acid–base properties and thus enhanced the redox properties. Moreover, the introduction of Ti into CeO2 also increased the number of oxygen vacancies on the catalysts’ surface that favored the ketonization of acid molecules. This work provides valuable insights into the design of highly efficient CeO2-based catalysts for acid removal in the upgrading process of bio-oil.

In order to overcome problems of Au–Cu bimetallic catalysts for acetylene hydrochlorination reaction such as instability, Au–Cu–SH/AC catalysts were prepared through the introduction of thiol and tested to examine their activity and stability. It was found that performances of Au–Cu–SH/AC catalysts were quite excellent, with significantly higher catalytic activity and better stability than performances of Au/AC and Au–Cu/AC catalysts. The contents of Cu and thiol additives were also optimized and the optimum molar ratio of Au/Cu/SH was 1 : 1 : 10. Catalyst samples were characterized by scanning electron microscopy (SEM), nitrogen adsorption/desorption (BET), X-ray diffraction (XRD), transmission electronic microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). It was demonstrated that the Au–Cu–SH/AC catalysts were Au0-based catalysts, due to thiol reducing Au3+ to Au0 species during the preparation process. Au0 species exhibited better catalytic activity than Au3+ species for acetylene hydrochlorination, according to the comparison with the composition of active species in different samples through XPS. Furthermore, the sulfhydryl of thiol could bond to the surface of gold nanoparticles (Au NPs). It helped in mitigating the oxidation of Au0 by HCl, protecting Au NPs from structure damage, stabilizing Au NPs in a nearly constant particle size and keeping a more active structure in the reaction environment. Thus, improved dispersity of active species and protection of the active structure of the Au NPs resulted in the better catalytic activity and stability of Au–Cu–SH/AC.

In order to reduce the simultaneous production of insoluble polymers during the bis(imino)pyridine (BIP) iron-catalyzed ethylene oligomerization, a series of phenolic compounds were introduced as modifiers. It was found that the polymer share in the total products would be largely reduced with the increasing dosage of the phenols and the enlargement of para-substituent size from methyl to tert-butyl. Further 1H NMR studies showed that the phenols could provide methylaluminoxane (MAO) profound structural modifications, giving rise to larger MAO aggregates and decoration of phenoxy groups on its surface. This would thus facilitate the active ion pair separation, leading the phenols to become effective polymer-retarding modifiers. Starting from the reaction between 4-tert-butylphenol, AlMe3 and water, a novel phenoxy-aluminoxane could be prepared. Its combination with AlMe3 enabled the catalyst activation, and gave us a further verification about the important role of phenoxy groups on the MAO surface. Furthermore, the introduction of electron-withdrawing groups would improve the reactivity of the –OH group, promoting the interaction between the phenols and MAO. A series of para-halogen substituted phenols were thus developed. With the relatively large size of the bromo group and the highest reactivity of the –OH group, 4-bromophenol was proved to be the most efficient polymer-retarding modifier among the studied phenols in this work. An almost polymer-free ethylene oligomerization could be achieved by this strategy without altering the mono ortho-methyl substituted BIP ligand.

A new impregnation method, involving a mixture of solvents and a vacuum drying process, was used to prepare a gold catalyst (MIV-1Au/C1) for acetylene hydrochlorination. It was found that MIV-1Au/C1 was twice as active as the catalyst prepared through the traditional method (PI-1Au/C1). The new method is green, mild and simple. Moreover, it appears to be controllable by tuning solvent and temperature. Excellent Au dispersion in MIV-1Au/C1 was revealed by transmission electron microscopy (TEM). In addition, X-ray photoelectron spectroscopy (XPS) profiles proved that Au0 was the only active species of MIV-1Au/C1 at the initial/highest point of testing. Further XPS results showed that Au0 could be oxidized to Au3+ during the reaction along with the deactivation of MIV-1Au/C1. Thus, Au0 appeared to be preferred for the catalytic route. Our findings demonstrate that this new method has potential as a catalyst for acetylene hydrochlorination. Moreover, this study highlighted the importance of Au0 in this field.

Silica was deposited onto activated carbon through TEOS hydrolysis and this composite was used as a support for the Au catalyst in acetylene hydrochlorination. Silica content and the catalyst synthesis process were both optimized. It was found that 1Au/5SiO2/AC showed improved stability, while being as active as 1Au/AC. Thermogravimetric analysis (TGA) quantitatively revealed the synchronism of TEOS hydrolysis and Au deposition, which was believed to be the linchpin in 1Au/5SiO2/AC synthesis. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) demonstrated that amorphous silica dispersed on the carbon surface uniformly as spherical particles. Silica deposition brought down the surface area of the catalyst while leading to better distribution of gold nanoparticles. Higher gold distribution degree guaranteed the catalytic activity of 1Au/5SiO2/AC despite surface area loss. A higher level of resistance to acetylene, excellent surface property stability and less carbonaceous deposition were determined as the origin of the improved stability of 1Au/5SiO2/AC.