High temperature treatments of porous activated carbon

The use of biomass waste for the preparation of activated carbon is of great industrial interest for reducing costs and increasing the sustainability, especially in the field of energy storage. A high temperature treatment is required to obtain a more ordered carbon material, thus increasing its conductivity. However, this high temperature treatment entails as a disadvantage a significant reduction in porosity. Therefore, a method to prepare activated carbons with a high porosity development as well as high conductivity could be of great interest for many applications. The aim of this work is to analyze the possible influence of phosphorus compounds on the physical-chemical properties of different carbon materials thermally treated at relatively high temperatures (1600 ºC). With this goal, it has been prepared activated carbons from different precursors (olive stone, lignin and hemp) and different conformations (powder, fibers and monoliths) by physical and chemical activation, with CO2 and H3PO4, respectively. Once the different activated carbon materials were prepared, they were thermally treated at 1600 ºC under inert atmosphere. The different samples were characterized by N2 and CO2 adsorption at 77 and 273 K, respectively, XPS, XRD and Raman techniques. The oxidation resistance was also evaluated in a thermogravimetric balance. High temperature treatments of activated carbon without the presence of P surface groups produced an important contraction of the porosity (from 900 to 150 m2 g-1). However, temperature treatments of phosphorus-activated carbon allowed for preparing carbon materials with a relatively high structural order and a well-developed porosity (c.a. 1100 m2 g-1), with a significant contribution of mesoporosity. These results suggest that these P-surface groups are responsible for the low contraction observed for the porous structure, avoiding, in a large extent, its collapse.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. MINECO (CTQ2015-68654-R). MINECO (PTA2015-11464-I)

Modification of the morphology, porosity and surface chemistry of lignin-based electrospun carbon materials

Lignin is a biopolymer that can be found as the main component of plants. It is obtained as a coproduct in the papermaking and biofuel industries. Owing to its high carbon and aromatic content, high availability and reduced cost, it is an excellent precursor for the preparation of highly valued carbon materials. Electrospinning is a suitable top-down technique for the preparation of polymeric fibers using high voltage electrical fields and polymer solutions of proper viscosity and conductivity. Organosolv lignins, which are extracted from lignocellulosic biomass using organic solvents, are soluble in ethanol, obtaining a solution that matches the requirement of the electrospinning process. In this way, it is possible to produce lignin-based porous carbon fibers using a coaxial electrospinning device [1]. This contribution summarizes our findings about the preparation of carbon materials with different morphologies and composition by processing lignin using electrohydrodynamic forces. Lignin spheres, beaded fibers, straight fibers, beaded tubes and straight tubes are obtained by using coaxial and triaxial spinnerets that allows the electrospinning of two or three different solutions at once [1], Fig. 1. Thermal stabilization in air is needed in order to avoid melting of lignin fibers during carbonization. Stabilization times of 48-96 hours are usually required in this step, decreasing the sustainability of the production process. Phosphoric acid can be added in small amounts in the lignin solution, shortening the time for achieve a successful thermostabilization of the fiber [2]. The carbonized materials show narrow microporosity and large surface area values (SBET from 600 to 1000 m2g-1) and additional pore size and volume can be developed by controlled gasification.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. This work was supported by the Spanish Ministry of Economy and Competitiveness and FEDER (CTQ-2015-68654-R)

Alcohol decomposition on basic/acid lignin-derived submicron diameter carbon fibers

The use of lignin, the second most abundant polymer in nature, along with a simple and versatile technique, electrospinning, represents an advantageous and promising approach for the preparation of carbon fibers. In previous studies, we have demonstrated that the incorporation of H3PO4 to the initial lignin solution allows for shortening the carbon fibers preparation process and that the resulting carbon fibers present P-surface groups that are of great interest for heterogeneous catalysis. Different carbon fibers catalysts have been prepared by electropinning of Alcell lignin in the absence or presence of H3PO4 as chemical activating agent. Carbonization at different temperatures between 500 and 1600 ºC allows for preparing carbon fibers with a high variety of porosity and chemical surface properties. Diverse oxygen surface groups are presented on the carbon catalysts surface. The isopropanol decomposition has been used as a catalytic test to study the acid or basic character of the prepared carbon fibers. Carbon fibers without phosphorus surface groups generate acetone as the main product of the isopropanol decomposition reaction, from 400 to 600 ºC, suggesting the basic character of these catalysts. On the contrary, phosphorus-containing carbon fibers show high acid character, producing selectivity to propylene of 100 % at temperatures between 250 and 350 ºC. The most acid carbon fiber catalyst produced a high selectivity to ethylene and dimethyl ether for the decomposition of ethanol and methanol, respectively. The conversion enhancement that the presence of oxygen in the gas phase produced for all these reactions was also studied.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. MINECO (CTQ2015-68654-R). MINECO (PTA2015-11464-I)

A perspective on the preparation of value-added carbon materials from lignin

The thermochemical conversion of lignin into different added-value carbon materials constitutes an alternative approach for valorization of this co-product that can be integrated in pulping and biorefinery processes. Such approach is based on the relatively high carbon content and the abundance of aromatic rings in the structure of raw and technical lignins. In this way, our research group have been preparing carbon molecular sieves, activated carbons, electrospun nanofibers, nanostructured and highly ordered carbons from different types of lignins during the last three decades.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Kinetic study of methanol dehydration over Zro2 supported-activated carbons

The growing concerns about climate change and energy consumption have been the driving force in seek of alternative fuels such as DME, mainly produced via methanol dehydration over a solid acid catalyst. The use of activated carbons for this aim has been little studied up to date. Only a few studies can be found in the literature, reporting all of them materials with a low thermal stability of the acid surface groups, which results into a fast deactivation of the catalyst. In this work, the preparation of activated carbons via chemical activation with phosphoric acid, their modification with different ZrO2 loads, and their application as methanol dehydration catalysts have been studied. The catalytic results showed that the best methanol conversion and selectivity towards DME were achieved with the activated carbon prepared with an impregnation mass ratio value (H3PO4 /precursor) of 2 and an activation temperature of 800 ºC, loaded with a 7 % (wt) of ZrO2 . This catalyst exhibits high steady state methanol conversion values even at temperatures as high as 400 ºC (XCH3OH= 80%, 0.1 g·s/μmol, PCH3OH= 0.08 atm in helium), keeping a selectivity to DME higher than 96%. The effect of oxygen in the reaction atmosphere was also analysed. In this sense, an increase of 15 % in the DME yield was obtained when using air instead of helium as reaction atmosphere (350 ºC, 0.1 g·s/μmol, PCH3OH= 0.04 atm). A kinetic study has been carried out on this catalyst in which two mechanisms (Eley Rideal and Langmuir Hinshelwood) for methanol dehydration have been analysed. The models proposed also consider the presence of oxygen in the reaction media.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. MINECO (CTQ2015-68654-R). MECD (FPU13/02413)

Highly porous and conductive functional carbon fibers from electrospun phosphorus-containing lignin fibers

Functional carbon fibers were prepared by carbonization of thermostabilized electrospun lignin-fibers at 500–900 °C followed by a high temperature thermal treatment at 1200–1600 °C. The effect of the preparation temperature on their surface chemistry, structural order, textural properties and electrochemical behavior has been stablished. Maximum porosity development is obtained at 900 °C. The addition of phosphoric acid in the electrospinning lignin solution shortens the stabilization time of the fibers, increases the carbonization yield, generates phosphorus functional groups (P content as high as 3% wt.) and increases the BET surface area of carbon fibers from 840 to 1143 m2 g−1. Interestingly, when phosphorus-containing carbon fibers are treated even at very high temperature, 1600 °C, most of the porosity is preserved (ABET = 822 m2 g−1). XPS depth profile reveals the presence of reduced phosphorus in the core of carbonized fibers. XRD and TEM analysis make evident that the presence of phosphorus induces curvature of the graphitic layers, which seems to hinder the stacking of the graphene layers, explaining the preservation of microporosity after the thermal treatment at high temperature. However, Raman and XRD analyses point out that the presence of phosphorus does not affect the lateral growing of the crystallites. Thus, phosphorus preserves the porosity and allows the development of the electrical conductivity after the thermal treatments. Gravimetric capacitances of 79 F g−1 and capacitance retention of 63% at 68 A g−1 have been determined for phosphorus-containing carbon fibers prepared at 1200 °CWe gratefully thank MICINN (RTI2018-097555-B-100) and Junta de Andalucía (UMA18-FEDERJA-110 and P18-RT-4592) for financial support. Funding for open access charge: Universidad de M ́alaga / CBUA

Kinetic study of steam gasification of lignocelullosic biomass char obtained from pyrolysis

Hydrogen plays a key role in the energy transition towards a more sustainable model based on renewable energies, as it has the capacity to solve two major problems: i) reducing dependence on fossil fuels and ii) reducing greenhouse gas emissions. Currently, approximately 85 % of the world hydrogen production comes from fossil fuel technologies without CO2 capture. In this sense, a promising alternative for the substitution of these fuels may be biomass gasification with water vapor, due to its high availability and low cost, where the gas produced can be used directly as fuel or raw material to produce high value-added chemicals. The aim of this work is to perform the steam gasification of the solid residue obtained from biomass pyrolysis to optimize the hydrogen production. Three types of lignocellulosic biomass (almond shell, olive stone and hemp) were evaluated as raw material for steam gasification. Biomass samples were obtained by conventional pyrolysis in a fixed-bed reactor at a temperature of 800 °C, a heating rate of 10 °C/min for 1 h, obtaining a char that was later gasified in a downdraft fixed-bed reactor, using steam as the reacting agent (30 % vol.), in a temperature range between 800 °C and 900 °C. A kinetic model that takes into account the gas product distribution and the weight loss during the gasification reaction was developed using representative gas-solid models such as the volumetric model (VM), the grain model (GM) and the random pore model (RPM).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Catalytic fast pyrolysis of olive stone for bio-oil deoxygenation.

In this work, conventional, fast and catalytic fast pyrolysis of olive stone residues were studied with a commercial catalyst with mild acidity, γ-Al2O3, an activated carbon from olive stones (AC) prepared by chemical activation with phosphoric acid and the same activated carbon loaded with Zr as active phase (ACZr). The results showed that fast pyrolysis has a higher content of liquid fraction (64 %wt) than that obtained in conventional pyrolysis (52 %wt), due to cracking reactions occur at a lesser extent. In the catalytic fast pyrolysis, the acid character of the catalysts favors the cracking and deoxygenation reactions that reduce the production of the liquid fraction, mainly producing an increase in the phenols species and a decrease in the acid species. On the other hand,a clear increment in the content of CO can be observed (conventional and catalytic fast pyrolysis, respectively), due to decarbonylation reactions, which are favored in the presence of catalysts.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Coordination problems in engineering degrees

This work focuses on the analysis of the satisfaction level of the learning for the students of the degree of Chemical Engineering (CE), Engineering of Industrial Technologies (EIT), Engineering of the Energy (EE) and Engineering in Industrial Organization (EIO), Computer Science (CS) and Telecommunications Engineering (TE). A methodology has been proposed for monitoring and evaluating the correct coordination of execution of how to implement the possible changes in the syllabus and guidelines of the above degrees. This methodology consists of a revision of the syllabus of all the subjects of first course; the establishment of indicators to easily identify the subject requiring improvements, data gathering from students by surveys and interviews and analysis of the results. From the data obtained with the participation of more than 700 students, some changes have been proposed to the coordinators of the degrees, involving temporary schedule of contents and guideline modifications. In addition, the indicators are applicable to other degrees, improving the system of coordination at different levels.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

A Kinetic Model Considering Catalyst Deactivation for Methanol-to-Dimethyl Ether on a Biomass-Derived Zr/P-Carbon Catalyst

A Zr-loaded P-containing biomass-derived activated carbon (ACPZr) has been tested for methanol dehydration between 450 and 550 °C. At earlier stages, methanol conversion was complete, and the reaction product was mainly dimethyl ether (DME), although coke, methane, hydrogen and CO were also observed to a lesser extent. The catalyst was slowly deactivated with time-on-stream (TOS), but maintained a high selectivity to DME (>80%), with a higher yield to this product than 20% for more than 24 h at 500 °C. A kinetic model was developed for methanol dehydration reaction, which included the effect of the inhibition of water and the deactivation of the catalyst by coke. The study of stoichiometric rates pointed out that coke could be produced through a formaldehyde intermediate, which might, alternatively, decompose into CO and H2. On the other hand, the presence of 10% water in the feed did not affect the rate of coke formation, but produced a reduction of 50% in the DME yield, suggesting a reversible competitive adsorption of water. A Langmuir–Hinshelwood reaction mechanism was used to develop a kinetic model that considered the deactivation of the catalyst. Activation energy values of 65 and 51 kJ/mol were obtained for DME and methane production in the temperature range from 450 °C to 550 °C. On the other hand, coke formation as a function of time on stream (TOS) was also modelled and used as the input for the deactivation function of the model, which allowed for the successful prediction of the DME, CH4 and CO yields in the whole evaluated TOS interval.This research was supported by the Spanish Ministry of Science, Innovation and Universities and Junta de Andalucia through RTI2018-097555-B-I00 and UMA18-FEDERJA-110 projects, respectively. J.T.-L. also acknowledges the assistance of the Spanish Ministry of Economy, Industry and Competitiveness for the award of a predoctoral contract to become a Ph.D. (BES-2016-079237). Partial funding for open access charge: Universidad de Málag