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Abstract: The liquid phase oligomerization of 1-hexene at 200°C and 5MPa using n-octane as solvent towards hydrocarbon mixtures useful as fuels (gasoline and diesel) was tested over several acid catalysts: micrometer (μ-) and nanocrystalline (n-) HZSM-5 zeolites, mesoporous hydrothermal Al-MCM-41, and sol–gel Al-MTS and Al-SBA-15 catalysts. The conversion was always above 75% except for μ-HZSM-5 (just 8.4%) due to its low external surface area (5m2 g−1) and the fast deactivation in the reaction conditions used in this work. The total selectivity towards oligomers was around 95% and the highest share of C9–C12 dimers (47%), C13–C18 trimers (33%) and heavy C19–C30 compounds (33%) were yielded over Al-SBA-15, n-HZSM-5 and sol–gel Al-MTS, respectively. The remarkable oligomerization performance of n-HZSM-5 was ascribed to its high external surface area (102m2 g−1) and for the mesoporous catalysts, to their large BET surface area. In particular, Al-MTS showed the best behaviour due to its higher BET surface area and slightly weaker acidity. All the catalysts exhibited steady-state performance with time on stream (TOS) without drastic changes in activity up to 180min. Simulated distillation analyses proved that the lighter fuel (gasoline+diesel) was obtained over Al-MTS (final distillation temperature=463°C; C26–C32 =8.4%) while the heaviest was obtained over n-HZSM-5 zeolite (final distillation temperature=524°C; C28–C40 =11.7%), probably related to its stronger acidity and microporous nature. The similar nature of hydrocarbons compounds retained over the catalysts after reaction proved by FTIR spectroscopy together with the thermogravimetric analyses results, showed the stronger adsorption of the reaction products promoted by the microporous nature of zeolites. [Copyright &y& Elsevier]

Abstract: Self-metathesis of 1-hexene was carried out at 40°C over Re2O7 supported on two micelle templated aluminas (sol–gel mesoporous alumina (SGAL) and MSU), prepared with cationic and neutral surfactant, respectively, and a reference Re2O7/γ-Al2O3 catalyst with the same rhenium oxide content (∼9wt%). Both mesostructured catalysts (Re2O7/MSU and Re2O7/SGAL) showed similar activity and far higher than that of the reference Re2O7/γ-Al2O3 catalyst, with a selectivity towards the self-metathesis products above 99%. Among the tested solvents, dodecane yielded the highest conversion likely because of its low polarity and better product removal. Similar TOF values were obtained for Re2O7/SGAL catalysts with mesopore diameter within the 5.0–12.0nm range. In relation with the rhenium content, an optimum in TOF values was detected for rhenium oxide loadings within 6–10wt%. The increase in activation temperature within 550–700°C led towards a greater rise in activity over Re2O7/SGAL catalysts than over Re2O7/γ-Al2O3. DRIFT chemisorbed pyridine spectra disclosed only Lewis acid sites over the activated catalysts and in higher amounts over both mesostructured catalysts. 31P MAS NMR spectra of chemisorbed triethylphosphine oxide (TEPO) indicated the presence of three kind of acid sites (δ ∼58, 68 and 80ppm), with a higher content of strong Lewis acid sites (80ppm) over both mesostructured catalysts. The remarkable performance of both ordered mesoporous catalysts was ascribed to the presence of these strong Lewis acid sites, likely stemming from the interaction of linked rhenium species with the weak Lewis acid sites of the support. [Copyright &y& Elsevier]

Abstract: Catalytic cracking of both LDPE and HDPE has been studied using three different samples of nanocrystalline HZSM-5 zeolite as catalysts. The zeolite samples were synthesized under different conditions in order to obtain materials with different crystal sizes in the nanometer range (10–60nm) and high external surface areas (78–242m2/g). The plastic cracking reactions were carried out at 340°C for 2h using a plastic/catalyst mass ratio of 100. Despite these really mild conditions, high activities were observed over the three catalysts. The plastic conversion and the turnover frequency increased strongly with the amount of external surface area of the catalysts, showing the importance of the non-sterically hindered external acid sites for the conversion of the bulky plastic molecules. The main products obtained in the cracking reactions over the three catalysts are light hydrocarbons (C3–C5 fraction) with a considerable amount of olefins, which indicates the possibility of addressing the zeolite selectivity towards hydrocarbons useful as raw chemicals. These products are formed via an end-chain cracking pathway of the polymer molecules over the zeolite acid sites. The occurrence of this mechanism has been confirmed by DSC analysis of the LDPE and HDPE residues remaining within the reactor after the catalytic tests, showing they still consist of plastic molecules with melting temperatures similar to those of the raw polymers. [Copyright &y& Elsevier]

Abstract: Six acid solids of varying acid and textural characteristics has been investigated for their catalytic activity in the degradation of plastic polymers using thermogravimetric and differential thermal analysis. The polymers included pure low-density polyethylene, pure high-density polyethylene and two samples of recycled polyethylene of urban and agricultural origins. The catalysts were three zeolitic materials (standard ZSM-5, nanocrystalline n-ZSM-5 and Beta) and three mesostructured solids (sol–gel Al-MCM-41 (sg), hydrothermal Al-MCM-41 (hy) and Al-SBA-15). The catalytic activity of each acid solid has been related to its capacity to shift the degradation reaction to lower temperatures. Despite showing strong acid properties, standard ZSM-5 zeolite exhibited a very low catalytic activity on most plastics, which was attributed to diffusional impediments that affected the access of the bulky polymer molecules to its internal active sites. These impediments were partly overcome in the case of Beta zeolite and even more markedly with nanocrystalline n-ZSM-5, which exhibited the strongest catalytic activity due to a combination of strong acid properties and large external surface area. Owing to their non-crystalline nature, mesostructured solids showed weaker acid properties than their zeolitic counterparts. However, this disadvantage was partly compensated by the presence of larger pores that reduced diffusional hindrances. Thus, Al-MCM-41 (hy) exhibited one of the highest catalytic activities, largely surpassing the performance of crystalline solids with stronger acid properties. The catalytic activity of all the acid solids tested in this work was significantly reduced when they were used on waste plastics. This deactivating effect was more notable in mesostructured solids. [Copyright &y& Elsevier]

Abstract: This paper investigates the effect of various acid catalysts in the conversion of tyre rubber into hydrocarbon products. A used tyre rubber and three pure elastomers (cis-1,4-polyisoprene, polystyrene-co-butadiene and cis-polybutadiene) were characterised for their chemical composition and thermal behaviour in order to determine the main constituent of the waste material. Five acid solids of comparable aluminium contents but varying textural and/or acid characteristics were used as catalysts, including three zeolites (standard ZSM-5, nanocrystalline n-ZSM-5 and beta) and two mesostructured materials (Al-MCM-41 and Al-SBA-15). The presence of catalysts did not affect the degradation temperature of waste tyre rubber, although significant differences were observed in the nature of the resulting hydrocarbons. All zeolitic materials showed a marked selectivity towards the formation of aromatic species, particularly toluene (up to 19.82%), m/p-xylene (up to 16.91%) and benzene (up to 10.28%). This behaviour was slightly more marked in zeolites with stronger acid properties and reduced steric impediments, like nanocrystalline n-ZSM-5. Mesostructured catalysts (Al-MCM-41 and Al-SBA-15) not only exhibited strong aromatisation properties but also a marked benzene alkylation capacity, which resulted in the formation of a wider range of alkylaromatic products. This behaviour was associated with their weaker Lewis type acid character and also to their larger pore sizes, which posed no impediment to the formation of larger molecular size products. [Copyright &y& Elsevier]