Prospects for the combination of mechanochemistry and flow applied to catalytic transformations

The urge of developing modern alternatives regarding industrial production has led to the creation of novel techniques that help overcome critical disadvantages from traditional batch mechanochemistry. One promising strategy includes the merging of flow processes with mechanochemistry. Two technologies are herein highlighted: twin-screw extrusion (TSE) and Impact (induction) in Continuous flow HEated Mechanochemistry (I-CHEM) allowing not only the industrial production of polymers but also active pharmaceutical ingredients, synthesis of (nano)materials, and the obtention of high-added value products from biomass and wastes valorization.     

Solubility study and intensification of extraction of phenolic and anthocyanin compounds from Oryza sativa L. ‘Violet Nori’

Oryza sativa L. ‘Violet Nori’ is an Italian cultivar of spontaneous growing aromatic purple rice which is particularly rich in polyphenolic compounds, especially anthocyanins, conferring it an excellent antioxidant capacity. The present study aimed at increasing the extraction yields of its antioxidant compounds with green strategies and it is divided into two steps. The first step concerned a solubility study of the targeted polyphenols in different ethanol:water mixtures by means of a theoretical prediction method, using the simulation program COSMO-RS, and the subsequently confirmation of the computational results by practical experiments. Once the best extraction mixture was identified, the second step of the work was performed, with the purpose of intensifying the extraction yield. Therefore, various innovative green extraction techniques, including ultrasound, using both the probe system and the ultrasonic bath, bead milling, microwave and accelerated solvent extractions were tested and compared to conventional maceration.Results, expressed in terms of total phenolic and total monomeric anthocyanin contents, showed that the best extracting solvent for ‘Violet Nori’ rice was the mixture ethanol:water (60:40 v/v), being COSMO-RS computational predictions in good correlation with the experimental results. Moreover, the most efficient techniques to extract the antioxidant compounds resulted to be both ultrasound-assisted extraction probe and bead milling, that in only 5 min got the same extractive efficiency obtained after 3 h of conventional maceration.     

Conventional vs. Microwave- or Mechanically-Assisted Synthesis of Dihomooxacalix[4]arene Phthalimides: NMR,X-ray and Photophysical Analysis §

Direct O-alkylation of p-tert-butyldihomooxacalix[4]arene (1) with N-(bromopropyl)- or N-(bromoethyl)phthalimides and K₂CO₃ in acetonitrile was conducted under conventional heating (reflux) and using microwave irradiation and ball milling methodologies. The reactions afforded mono- and mainly distal di-substituted derivatives in the cone conformation, in a total of eight compounds. They were isolated by column chromatography, and their conformations and the substitution patterns were established by NMR spectroscopy (¹H, ¹³C, COSY and NOESY experiments). The X-ray structures of four dihomooxacalix[4]arene phthalimide derivatives (2a, 3a, 3b and 5a) are reported, as well as their photophysical properties. The microwave (MW)-assisted alkylations drastically reduced the reaction times (from days to less than 45 min) and produced higher yields of both 1,3-di-substituted phthalimides (3a and 6a) with higher selectivity. Ball milling did not reveal to be a good method for this kind of reaction.     

Comprehensive performance of a ball-milled La0.5Pr0.5Fe11.4Si1.6B0.2Hy/Al magnetocaloric composite

Due to the hydrogen embrittlement effect, La(Fe,Si)₁₃-based hydrides can only exist in powder form, which limits their practical application. In this work, ductile and thermally conductive Al metal was homogeneously mixed with La_0.5Pr_0.5Fe_11.4Si_1.6B_0.2 using the ball milling method. Then hydrogenation and compactness shaping of the magnetocaloric composites were performed in one step via a sintering process under high hydrogen pressure. As the Al content reached 9 wt.%, the La_0.5Pr_0.5Fe_11.4Si_1.6B_0.2H_y/Al composite showed the mechanical behavior of a ductile material with a yield strength of ~44 MPa and an ultimate strength of 269 MPa accompanied by a pronounced improvement in thermal conductivity. Due to the ease of formation of Fe-Al-Si phases and the several micron and submicron sizes of the composite particles caused by ball milling process, the magnetic entropy change of the composites was substantially reduced to ~1.2 J/kg· K-1.5 J/kg· K at 0 T-1.5 T.     

Mechanochemical Approach for Air-Tolerant and Extremely Fast Lithium-Based Birch Reductions in Minutes

Birch reduction has been widely used in organic synthesis for over half a century as a powerful method to dearomatize arenes into 1,4-cyclohexadiene derivatives. However, the conventional Birch reduction reaction using liquid ammonia requires laborious procedures to ensure inert conditions and low temperatures. Although several ammonia-free modifications have been reported, the development of an operationally simple, efficient, and scalable protocol remains a challenge. Herein, we report an ammonia-free lithium-based Birch reduction in air without special operating conditions using a ball-milling technique. This method is characterized by its operational simplicity and an extremely short reaction time (within 1 min), probably owing to the in situ mechanical activation of lithium metal, broad substrate scope, and no requirement for dry bulk solvents. The potential of our flash Birch reaction is also demonstrated by the efficient reduction of bioactive target molecules and gram-scale synthesis.     

Mechanically Activated Solid-State Radical Polymerization and Cross-Linking via Piezocatalysis

Piezocatalysis offers a means to transduce mechanical energy into chemical potential, harnessing physical force to drive redox reactions. Working in the solid state, we show here that piezoelectric BaTiO₃ nanoparticles can transduce mechanical load into a flux of reactive radical species capable of initiating solid state free radical polymerization. Activation of a BaTiO₃ powder by ball milling, striking with a hammer, or repeated compressive loading generates highly reactive hydroxyl radicals (⋅OH), which readily initiate radical chain growth and crosslinking of solid acrylamide, acrylate, methacrylate and styrenic monomers. Control experiments indicate a critical role for chemisorbed water on the BaTiO₃ nanoparticle surface, which is oxidized to ⋅OH via mechanoredox catalysis. The force-induced production of radicals by compressing dry piezoelectric materials represents a promising new route to harness mechanical energy for solid state radical synthesis.     

Direct In Situ Investigation of Milling Reactions Using Combined X‐ray Diffraction and Raman Spectroscopy

The combination of two analytical methods including time‐resolved in situ X‐ray diffraction (XRD) and Raman spectroscopy provides a new opportunity for a detailed analysis of the key mechanisms of milling reactions. To prove the general applicability of our setup, we investigated the mechanochemical synthesis of four archetypical model compounds, ranging from 3D frameworks through layered structures to organic molecular compounds. The reaction mechanism for each model compound could be elucidated. The results clearly show the unique advantage of the combination of XRD and Raman spectroscopy because of the different information content and dynamic range of both individual methods. The specific combination allows to study milling processes comprehensively on the level of the molecular and crystalline structures and thus obtaining reliable data for mechanistic studies.     

Improved Dermal and Transdermal Delivery of Curcumin with SmartFilms and Nanocrystals

Poor aqueous solubility of active compounds is a major issue in today’s drug delivery. In this study the smartFilm-technology was exploited to improve the dermal penetration efficacy of a poorly soluble active compound (curcumin). Results were compared to the dermal penetration efficacy of curcumin from curcumin bulk suspensions and nanocrystals, respectively. The smartFilms enabled an effective dermal and transdermal penetration of curcumin, whereas curcumin bulk- and nanosuspensions were less efficient when the curcumin content was similar to the curcumin content in the smartFilms. Interestingly, it was found that increasing numbers of curcumin particles within the suspensions increased the passive dermal penetration of curcumin. The effect is caused by an aqueous meniscus that is created between particle and skin if the dispersion medium evaporates. The connecting liquid meniscus causes a local swelling of the stratum corneum and maintains a high local concentration gradient between drug particles and skin. Thus, leading to a high local passive dermal penetration of curcumin. The findings suggest a new dermal penetration mechanism for active compounds from nano-particulate drug delivery systems, which can be the base for the development of topical drug products with improved penetration efficacy in the future.     

Ag_3PO_4/MoS_2纳米片复合催化剂制备及可见光催化性能

采用机械球磨法成功制备Ag_3PO_4/MoS_2纳米片复合催化剂。运用X射线衍射仪(XRD)、透射电子显微镜(TEM)、扫描电子显微镜(SEM)、紫外可见漫反射光谱(UV-Vis)和荧光发射光谱(PL)对复合催化剂的结构和形貌进行了表征。结果表明,Ag_3PO_4纳米粒子均匀地附着在MoS_2纳米片层结构上,两者形成紧密结合。以亚甲基蓝为模拟污染物,研究复合催化剂在可见光照射下的光催化特性;通过循环实验考察复合催化剂的稳定性。结果显示,含有1%的MoS_2纳米片与Ag_3PO_4形成的复合催化剂在30 min内对亚甲基蓝的降解率为95%,其降解动力学常数是纯相Ag_3PO_4的2倍。经过5次循环实验后复合催化剂对于亚甲基蓝的降解率为84%,而纯Ag_3PO_4对于亚甲基蓝的降解率仅为35%。Ag_3PO_4/MoS_2纳米片复合催化剂具有优良的光催化活性和高稳定性,主要归因于二硫化钼纳米片与磷酸银形成异质结,磷酸银激发的电子和二硫化钼纳米片产生的空穴直接复合,从而促使光生电子从磷酸银晶体表面快速分离,减轻了磷酸银的光电子腐蚀,同时也提高了复合物的光催化活性。     

机械球磨制备三苯胺基PAF-106s及C2烃吸附性质

以三苯胺为单体, 无水三氯化铁为催化剂, 二甲醇缩甲醛为外交联剂, 通过机械球磨不同比例的三苯胺、 三氯化铁和二甲醇缩甲醛, 合成了PAF-106s(PAF-106a~PAF-106c, PAF: porous aromatic framework). 红外光谱、 元素分析、 X射线光电子能谱和固体核磁共振波谱等表征结果证明发生了聚合反应. 氮气吸附结果表明, 三苯胺、 三氯化铁和二甲醇缩甲醛的比例影响PAF-106s的多孔性能. 三氯化铁和三苯胺摩尔比从3∶1增加到12∶1时, PAF-106c的BET比表面积从PAF-106a的135 m²/g增加到280 m²/g. 引入二甲醇缩甲醛后, PAF- 106d~PAF-106g的BET比表面积随三氯化铁和二甲醇缩甲醛摩尔比的增加而逐渐降低. 在273和298 K下, 测试了PAF-106c的C2烃吸附性能, 并采用理想吸附溶液理论计算了C₂H₂/C₂H₄和C₂H₆/C₂H₄分离比.