A novel amine-based adsorbent for CO₂ capture from air was developed, which uses biogenic raw materials and an environmentally benign synthesis route without organic solvents. The adsorbent was synthesized through freeze-drying an aqueous suspension of nanofibrillated cellulose (NFC) and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS). At a CO₂ concentration of 506 ppm in air and a relative humidity of 40% at 25 °C, 1.39 mmol CO₂/g was absorbed after 12 h. Stability was examined for over 20 consecutive 2-h-adsorption/1-h-desorption cycles, yielding a cyclic capacity of 0.695 mmol CO₂/g.

A temperature-vacuum swing (TVS) process, capable of extracting pure CO 2 from dry and humid atmospheric air, is experimentally analyzed. Adsorption/desorption cycles utilizing a packed bed of a sorbent material made of diamine-functionalized commercial silica gel are performed under equilibrium and non-equilibrium (short-cycle) conditions. Thereby, the CO 2 capture capacity of the material is determined over a wide range of operational parameters, namely 10-150 mbar abs desorption pressure, 74-90 C desorption temperature, and 0-80% relative humidity during adsorption. Up to 158 ml of CO 2 (6.8 ml per gram sorbent) with a purity of up to 97.6% is recovered per cycle. Adsorption isotherms of the sorbent material are experimentally determined by thermogravimetry and fitted to isotherm models, which are successfully applied to predict desorption capacities achieved in the TVS process. Under dry conditions, desorption pressures above 100 mbar abs lead to strongly decreasing CO 2 capture capacities below 0.03 mmol g À1 . Under humid conditions with 40% relative humidity during adsorption, the desorption pressure can be raised to 150 mbar abs with capture capacities remaining above 0.2 mmol g À1 . Stable performance of the sorbent material in the TVS process is demonstrated over 40 consecutive adsorption/desorption cycles.

A fundamental analysis of single-component and binary CO2 and H2O adsorption of amine-functionalized nanofibrillated cellulose is carried out in the temperature range of 283-353 K and at CO2 partial pressures in the range of 0.02-105 kPa, where the ultralow partial pressure range is relevant for the direct capture of CO2 from atmospheric air. Single-component CO2 and H2O adsorption experimental data are fitted to the Toth and Guggenheim-Anderson-de Boer models, respectively. Corresponding heats of adsorption, derived from explicit solutions of the van't Hoff equation, are -50 kJ/mol CO2 and -48.8 kJ/mol H2O. Binary CO2/H2O adsorption measurements for humid air reveal that the presence of H2O at 2.55 kPa enhances CO2 adsorption, while the presence of CO2 at 0.045 kPa does not influence H2O adsorption. The energy demand of the temperature-vacuum-swing adsorption/desorption cycle for delivering pure CO2 from air increases significantly with H2O adsorption and indicates the need to reduce the hygroscopicity of the adsorbent.

In recent years Direct Air Capture (DAC) has established itself as a promising approach to atmospheric Carbon Dioxide Removal (CDR) also referred to as Negative Emissions. However, due to the amounts likely needed to be removed CDR technologies like DAC will only become climate relevant if they rapidly reach gigaton scale, around the middle of this century. Here we give a brief insight into DAC and in particular, the modular low temperature DAC technology developed by Climeworks of Switzerland. We discuss potential co benefits, in particular in relation to the Sustainable Development Goals (SDGs) of the United Nations and conclude by suggesting some policy approaches on how a climate relevant scale could be achieved in time.

A temperature-vacuum swing (TVS) cyclic process is applied to an amine-functionalized nanofibrilated cellulose sorbent to concurrently extract CO 2 and water vapor from ambient air. The promoting effect of the relative humidity on the CO 2 capture capacity and on the amount of coadsorbed water is quantified. The measured specific CO 2 capacities range from 0.32 to 0.65 mmol/g, and the corresponding specific H 2 O capacities range from 0.87 to 4.76 mmol/g for adsorption temperatures varying between 10 and 30 °C and relative humidities varying between 20 and 80%. Desorption of CO 2 is achieved at 95 °C and 50 mbar abs without dilution by a purge gas, yielding a purity exceeding 94.4%. Sorbent stability and a closed mass balance for both H 2 O and CO 2 are demonstrated for ten consecutive adsorption−desorption cycles. The specific energy requirements of the TVS process based on the measured H 2 O and CO 2 capacities are estimated to be 12.5 kJ/mol CO2 of mechanical (pumping) work and between 493 and 640 kJ/mol CO2 of heat at below 100 °C, depending on the air relative humidity. For a targeted CO 2 capacity of 2 mmol/g, the heat requirement would be reduced to between 272 and 530 kJ/mol CO2 , depending strongly on the amount of coadsorbed water.