Application of thermogravimetric analysis to the evaluation of aminated solid sorbents for CO<Subscript>2</Subscript> capture

In this work a series of solid sorbents were synthesized by immobilizing liquid amines on the surface of a mesoporous alumina. The samples were chemically characterized and BET surface areas calculated from the N₂ adsorption isotherms at 77 K. The CO₂ capture performance of the sorbents and their thermal stability was studied by thermogravimetric methods. The effect of amine loading on the CO₂ capture performance of the prepared sorbents was also evaluated. Analysis of TG-DTG curves showed that thermal stabilization of the amines is significantly improved by immobilizing them on an inorganic support. Temperature-programmed CO₂ adsorption tests from 298 K up to 373 K at atmospheric pressure, proved to be a useful technique for assessing the capacity of sorbents for CO₂ capture. Alumina impregnated with diethylenetriamine presented the highest CO₂ adsorption capacities throughout the tested temperature range.     

Effect of various aminosilanes functionalized inside nanoporous silica on CO<Subscript>2</Subscript> adsorption performance

Three different aminosilanes ((3-aminopropyl)trimethoxysilane (1NS), N-[3-(trimethoxysilyl) propyl]ethylenediamine (2NS), N¹-(3-trimethoxysilylpropyl)diethylenetriamine (3NS)) were grafted covalently inside nanoporous silica (NPS-1) with a large surface area to prepare CO₂ adsorbents. The prepared CO₂ sorbents were evaluated for their CO₂ sorption capacity, kinetic behavior, temperature programmed desorption (TPD) and textural properties. Grafting efficiency of 1NS was better due to the smaller molecular size compared to 2NS and 3NS, which are difficult to react with the hydroxyl group of the silica surface due to steric hindrance. The highest adsorption capacity of 7.0 wt% was observed for the 2NS/NPS-1 adsorbent, followed by 5.2 wt% for 1NS/NPS-1, then 5.0 wt% for 3NS/NPS-1. The adsorption capacity of 2NS/NPS-1 was highest at 30 °C, and it gradually decreased as the adsorption temperature increased. TPD analysis showed that the reaction of primary amine of 2NS with CO₂ inside the nanoporous silica could form less thermally stable carbamic acid and carbamate compared to 1NS and 3NS.     

Facile Synthesis of Polyethylenimine and Nano-TiO<Subscript>2</Subscript> Particles Functionalized PolyHIPE Beads for CO<Subscript>2</Subscript> Capture

Polyethylenimine (PEI) and titanium dioxide nanoparticles (nano-TiO₂) functionalized poly- HIPE beads were synthesized by suspension polymerization of styrene/divinylbenzene high internal phase emulsion (HIPE) containing PEI and nano-TiO₂ particles in inner phase. The products are uniform and spherical beads with average diameter of 1 mm. Characterization results showed good thermal stability and desired mechanical strength. CO₂ adsorption tests were performed with CO₂/H₂O/N₂ (1 : 1 : 8) gas mixture. Nano-TiO₂ particles distinctly improved the CO₂ adsorption performance of the polyHIPE beads, resulting in enhanced CO₂ adsorption capacity and fast adsorption/desorption kinetics. Besides, the functionalized polyHIPE beads exhibited remarkable cycle stability.     

Synthesis,Characterization and Application of Amine-Functionalized Hierarchically Micro-Mesoporous Silicon Composites for CO2 Capture in Flue Gas

An efficient CO₂ adsorbent with a hierarchically micro-mesoporous structure and a large number of amine groups was fabricated by a two-step synthesis technique. Its structural properties, surface groups, thermal stability and CO₂ adsorption performance were fully investigated. The analysis results show that the prepared CO₂ adsorbent has a specific hierarchically micro-mesoporous structure and highly uniformly dispersed amine groups that are favorable for the adsorption of CO₂. At the same time, the CO₂ adsorption capacity of the prepared adsorbent can reach a maximum of 3.32 mmol-CO₂/g-adsorbent in the actual flue gas temperature range of 303–343 K. In addition, the kinetic analysis results indicate that both the adsorption process and the desorption process have rapid adsorption/desorption rates. Finally, the fitting of the CO₂ adsorption/desorption experimental data by Avrami’s fractional kinetic model shows that the CO₂ adsorption rate is mainly controlled by the intra-particle diffusion rate, and the temperature has little effect on the adsorption rate.     

Dry Potassium-Based Sorbents for CO<Subscript>2</Subscript> Capture

Dry potassium-based sorbents were prepared by impregnation with potassium carbonate on supports such as activated carbon (AC), TiO₂, Al₂O₃, MgO, CaO, SiO₂ and various zeolites. The CO₂ capture capacity and regeneration property of various sorbents were measured in the presence of H₂O in a fixed bed reactor, during multiple cycles at various temperature conditions (CO₂ absorption at 50–100 °C and regeneration at 130–400 °C). The KAlI30, KCaI30, and KMgI30 sorbents formed new structures such as KAl(CO₃)₂(OH)₂, K₂Ca(CO₃)₂, K₂Mg(CO₃)₂, and K₂Mg(CO₃)₂·4(H₂O), which did not completely convert to the original K₂CO₃ phase at temperatures below 200 °C, during the CO₂ absorption process in the presence of 9 vol.% H₂O. In the case of KACI30, KTiI30, and KZrI30, only a KHCO₃ crystal structure was formed during CO₂ absorption. The formation of active species, K₂CO₃·1.5H₂O, by the pretreatment with water vapor and the formation of the KHCO₃ crystal structure after CO₂ absorption are important factors for absorption and regeneration, respectively, even at low temperatures (130–150 °C). In particular, the KTiI30 sorbent showed excellent characteristics with respect to CO₂ absorption and regeneration in that it satisfies the requirements of a large amount of CO₂ absorption (87 mg CO₂/g sorbent) without the pretreatment with water vapor, unlike KACI30, and a fast and complete regeneration at a low temperature condition (1 atm, 150 °C). In addition, the higher total CO₂ capture capacity of KMgI30 (178.6 mg CO₂/g sorbent) than that of the theoretical value (95 mg CO₂/g sorbent) was explained through the contribution of the absorption ability of MgO support. In this review, we introduce the CO₂ capture capacities and regeneration properties of several potassium-based sorbents, the changes in the physical properties of the sorbents before/after CO₂ absorption, and the role of water vapor and its effects on CO₂ absorption.     

Triamine-grafted pore-expanded mesoporous silica for CO<Subscript>2</Subscript> capture: Effect of moisture and adsorbent regeneration strategies

Adsorption of carbon dioxide (CO₂) was investigated on triamine-grafted, pore-expanded MCM-41 mesoporous silica (TRI-PE-MCM-41). Measurements of adsorption capacity using mass spectrometry showed an enhanced CO₂ adsorption capacity in humid streams compared to dry CO₂. This was corroborated with breakthrough experiments, which also showed that TRI-PE-MCM-41 offered a practically infinite selectivity towards CO₂ over nitrogen. Cyclic measurements of pure CO₂ and CO₂:N₂ = 10:90 mixture using different regeneration modes showed that amine-grafted PE-MCM-41 is particularly suitable for CO₂ removal using temperature swing adsorption (TSA) at adsorption temperatures higher than ambient, while temperature-vacuum swing adsorption (TVSA) may be attractive at ambient temperature.     

FT-IR study on CO<Subscript>2</Subscript> adsorbed species of CO<Subscript>2</Subscript> sorbents

The stability of amine-functionalized silica sorbents prepared through the incipient wetness technique with primary, secondary, and tertiary amino organosilanes was investigated. The prepared sorbents were exposed to different gaseous streams including CO₂/N₂, dry CO₂/air with varying concentration, and humid CO₂/air mixtures to demonstrate the effect of the gas conditions on the CO₂ adsorption capacity and the stability of the different amine structures. The primary and secondary amine-functionalized adsorbents exhibited CO₂ sorption capacity, while tertiary amine adsorbent hardly adsorbed any CO₂. The secondary amine adsorbent showed better stability than the primary amine sorbent in all the gas conditions, especially dry conditions. Deactivation species were evaluated using FT-IR spectra, and the presence of urea was confirmed to be the main deactivation product of the primary amine adsorbent under dry condition. Furthermore, it was found that the CO₂ concentration can affect the CO₂ sorption capacity as well as the extent of degradation of sorbents.     

Effect of PEG with different <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> as template direction reagent on preparation of porous TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> with assistance of supercritical CO<Subscript>2</Subscript>

Titania–silica composite have been prepared using polyethylene glycol (PEG) with different molecular weights (M _w), PEG20000, PEG10000, and PEG2000, as template in supercritical carbon dioxide (SC CO₂). The composite precursors were dissolved in SC CO₂ and impregnated into PEG templates using SC CO₂ as swelling agent and carrier. After removing the template by calcination at suitable temperature, the titania–silica composite were obtained. The composite were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen sorption–desorption experiment. Photocatalytic activity of the samples has been investigated by photodegradation of methyl orange. Results indicate that there are many Si–O–Ti linkages in the TiO₂/SiO₂ composite; the PEG template has a significant influence on the structure of TiO₂/SiO₂. In addition, the TiO₂/SiO₂ prepared with PEG10000 exhibited high photocatalytic efficiency. So this work supplies a clue to control and obtain the TiO₂/SiO₂ composite with different photocatalytic reactivity with the aid of suitable PEG template in supercritical CO₂.     

Preparation and characterization of H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript>/ZrO<Subscript>2</Subscript> catalyst for carbonation of methanol into dimethyl carbonate

A H₃PW₁₂O₄₀/ZrO₂ catalyst for effective dimethyl carbonate (DMC) formation via methanol carbonation was prepared using the sol–gel method. X-ray photoelectron spectra showed that reactive and dominant (63%) W(VI) species, in WO₃ or H₂WO₄, enhanced the catalytic performances of the supported ZrO₂. The mesoporous structure of H₃PW₁₂O₄₀/ZrO₂ was identified by nitrogen adsorption–desorption isotherms. In particular, partial sintering of catalyst particles in the duration of methanol carbonation caused a decrease in the Brunauer–Emmett–Teller surface area of the catalyst from 39 to 19 m²/g. The strong acidity of H₃PW₁₂O₄₀/ZrO₂ was confirmed by the desorption peak observed at 415 °C in NH₃ temperature-programmed desorption curve. At various reaction temperatures (T?=?110, 170, and 220 °C) and CO₂/N₂ volumetric flow rate ratios (CO₂/N₂?=?1/4, 1/7, and 1/9), the calculated catalytic performances showed that the optimal methanol conversion, DMC selectivity, and DMC yield were 4.45, 89.93, and 4.00%, respectively, when T?=?170 °C and CO₂/N₂?=?1/7. Furthermore, linear regression of the pseudo-first-order model and Arrhenius equation deduced the optimal rate constant (4.24?×?10^?3 min^?1) and activation energy (E_a?=?15.54 kJ/mol) at 170 °C with CO₂/N₂?=?1/7 which were favorable for DMC formation.     

Adsorption behaviors of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> on zeolites JSR and Na<Subscript>n</Subscript>JSR using the GCMC simulations

The adsorption behaviors of CO₂ and CH₄ on new siliceous zeolites JSR and Na_nJSR (n = 2, 8, 16) were simulated using the Grand Canonical Monte Carlo method. The adsorption isotherms of CO₂ became higher with an increase in the Na⁺ number at a low pressure range (<150 kPa), whereas the isotherms showed a crossover with increasing pressure and the adsorption amount became smaller at a high pressure range (>850 kPa). With an increase in Na⁺ number, the pore volume decreased as the pore space was occupied by increasing Na⁺ ions. Additionally, two energy peaks on the interaction energy curves implied that CO₂ was adsorbed on two active sites. On the other hand, the adsorption amount of CH₄ decreased with an increase in the Na⁺ number and only one energy peak was observed. Adsorption isotherms were well fitted with the Langmuir and Freundlich equations up to 1000 kPa and the adsorption affinity of CO₂ on Na₁₆JSR zeolite was highest. The adsorption capacities of CO₂ in the studied zeolites were up to 38 times higher than those of CH₄. Diffusion constants of CO₂ and CH₄ decreased with an increase in the adsorbed amount and Na⁺ number. Considering the adsorbed amount, adsorption selectivity and affinity, zeolites JSR with a low Na⁺ number (JSR and Na₂JSR) is a good candidate for a pressure swing adsorption in the separation of CO₂/CH₄ mixture whereas JSR zeolites with high Na⁺ ratios (Na₁₆JSR and Na₈JSR) may be a better selection for a vacuum swing adsorption.     

Effect of metal substitution for cobalt on the oxygen adsorption properties of YBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript>

YBaCo₄O₇ compound is capable to intake and release a large amount of oxygen in the temperature range of 200–400°C. In the present study, the effect of Zn, Ga and Fe substitution for Co on the oxygen adsorption/desorption properties of YBaCo₄O₇ were investigated by thermogravimetry (TG) method. Due to fixed oxidation state of Zn2+ ions, the substitution of Zn²⁺ for Co²⁺ suppresses the oxygen adsorption of YBaCo_4−xZn_xO₇. The substitution of Ga³⁺ for Co³⁺ also decreases the oxygen absorption capacity of YBaCo_4−xGa_xO₇. This can be explained by the strong affinity of Ga³⁺ ions towards the GaO₄ tetrahedron. Compared with Zn- and Ga-substituted samples, the drop of oxygen adsorption capacity is smallest for Fe-substituted samples because of the similar changeability of oxidation states of Co and Fe ions.     

Equilibrium isotherms and isosteric heat for CO<Subscript>2</Subscript> adsorption on nanoporous carbons from polymers

Four nanoporous carbons obtained from different polymers: polypyrrole, polyvinylidene fluoride, sulfonated styrene–divinylbenzene resin, and phenol–formaldehyde resin, were investigated as potential adsorbents for carbon dioxide. CO₂ adsorption isotherms measured at eight temperatures between 0 and 60 °C were used to study adsorption properties of these polymer-derived carbons, especially CO₂ uptakes at ambient pressure and different temperatures, working capacity, and isosteric heat of adsorption. The specific surface areas and the volumes of micropores and ultramicropores estimated for these materials by using the density functional theory-based software for pore size analysis ranged from 840 to 1990 m² g^?1, from 0.22 to 1.47 cm³ g^?1, and from 0.18 to 0.64 cm³ g^?1, respectively. The observed differences in the nanoporosity of these carbons had a pronounced effect on the CO₂ adsorption properties. The highest CO₂ uptakes, 6.92 mmol g^?1 (0 °C, 1 atm) and 1.89 mmol g^?1 (60 °C, 1 atm), were obtained for the polypyrrole-derived activated carbon prepared through a single carbonization-KOH activation step. The working capacity for this adsorbent was estimated to be 3.70 mmol g^?1. Depending on the adsorbent, the CO₂ isosteric heats of adsorption varied from 32.9 to 16.3 kJ mol^?1 in 0–2.5 mmol g^?1 range. Overall, the carbons studied showed well-developed microporosity and exceptional CO₂ adsorption, which make them viable candidates for CO₂ capture, and for other adsorption and environmental-related applications.     

Removal of Cu(II) from CuSO4 Aqueous Solution by Mg‐Al Hydrotalcite‐like Compounds

Hydrotalcite‐like compound (HTlc) with a Mg/Al molar ratio of 2:1 was synthesized by using a coprecipitation method and the sorption removal of Cu(II) by the Mg‐Al HTlc sample from CuSO₄ solution was investigated. It was found that the Mg‐Al HTlc showed a good sorption ability for Cu(II) from CuSO₄ solution, indicating that the use of hydrotalcite‐like compounds as promising inorganic sorbents for the removal of heavy metal ions from water is possible. The sorption kinetics and the sorption isotherm of Cu(II) on the HTlc obeyed the pseudo‐second order kinetic model and Langmuir equation, respectively. The percent removal of Cu(II) by the HTlc was strongly dependent on the initial pH of bulk solution. It increased sharply with the increase of initial pH value in the range of 5–7, and was relatively small in the initial pH range of 4–5, while it reached about 100% after initial pH was higher than 7. The presence of AlCl₃ might obviously lower the equilibrium sorption amount (q_e) of Cu(II) on the HTlc. However, the presences of NaCl and MgCl₂ might increase the q_e. The presences of ligands (citric acid and EDTA) in the studied concentration range might obviously decrease the q_e of Cu(II) on the HTlc. The removal mechanism of Cu(II) cations by HTlc in the presence of SO₄^2? anions may be attributed to the surface‐induced precipitation of Cu(II) hydroxides and the surface complex adsorption by the linking effect of SO₄^2? between the HTlc and Cu(II) cations, and the removal ability arising from the surface‐induced precipitation is much higher than that from the linking effect of SO₄^2?.     

Molecular Mechanisms of the Effect of Water on CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> Mixture Adsorption in Slitlike Carbon Pores

The grand canonical ensemble Monte Carlo method has been used to study adsorption of carbon dioxide, methane, and their mixtures with different compositions in slitlike carbon pores at a temperature of 318 K and pressures below 60 atm. The data obtained have been used to show the effect of fixed amounts of pre-adsorbed water (19, 37, and 70 vol %) on the adsorption capacity and selectivity of carbon micro- and mesopores. The presence of water reduces the adsorption capacity throughout the studied pressure range upon adsorption of gaseous mixtures containing less than 50% CO₂, as well as in narrow micropores (with widths of 8?12 Å). Upon adsorption of mixtures with CO₂ contents higher than 50%, the adsorption capacity of pores with low water contents appears, in some region of the isotherm, to be higher than that in dry pores. In the case of wide pores (16 and 20 Å), this region is located at low and moderate pressures, while for mesopores it is located at high pressures. The analysis of the calculated data has shown that the molecular mechanism of the influence of preadsorbed water on the adsorption capacity is based on the competition between the volume accessible for adsorption (decreases the capacity) and the strength of the interaction between carbon dioxide molecules and water molecules (increases the capacity). Therewith, the larger the surface area of the water–gas contact, the stronger the H₂O–CO₂ interactions.     

Preparation of piperazine-grafted amine-functionalized UiO-66 metal organic framework and its application for CO<Subscript>2</Subscript> over CH<Subscript>4</Subscript> separation

UiO-66 amine functionalized was synthesized by solvothermal method. Post-synthetic modification of UiO-66-NH₂ with piperazine, a known promoter to enhance the chemisorption rate of CO₂ uptake, was carried out and analyzed to understand its crystalline structure, morphology and porous structure. Results show that piperazine is an effective agent for enhancing the capacity of absorption of CO₂. This porous product exhibits an improved CO₂ uptake at pressures up to 3000 kPa via physisorption and chemisorption mechanisms. The CH₄ adsorption and desorption isotherms on UiO-66, UiO-66-NH₂ and pip-UiO-66-NH₂ at temperature of 298.15 K and pressures ranging from 0 to 5000 kPa were carried out. IAS theory for a mixture of 0.05 bar CO₂, 0.85 bar CH₄ and 0.1 bar other gas revealed a selectivity factor of 19.09 for CO₂/CH₄ from pip-UiO-66-NH₂. Results show that these materials are effective adsorbents for CO₂ and CH₄ uptakes.     

Adsorption equilibria of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> in cation-exchanged zeolites 13X

Ion-exchange with different cations (Na⁺, NH₄ ⁺, Li⁺, Ba²⁺ and Fe³⁺) was performed in binderless 13X zeolite pellets. Original and cation-exchanged samples were characterized by thermogravimetric analysis coupled with mass spectrometry (inert atmosphere), X-ray powder diffraction and N₂ adsorption/desorption isotherms at 77 K. Despite the presence of other cations than Na (as revealed in TG-MS), crystalline structure and textural properties were not significantly altered upon ion-exchange. Single component equilibrium adsorption isotherms of carbon dioxide (CO₂) and methane (CH₄) were measured for all samples up to 10 bar at 298 and 348 K using a magnetic suspension balance. All of these isotherms are type Ia and maximum adsorption capacities decrease in the order Li > Na > NH₄–Ba > Fe for CO₂ and NH₄–Na > Li > Ba for CH₄. In addition to that, equilibrium adsorption data were measured for CO₂/CH₄ mixtures for representative compositions of biogas (50 % each gas, in vol.) and natural gas (30 %/70 %, in vol.) in order to assess CO₂ selectivity in such scenarios. The application of the Extended Sips Model for samples BaX and NaX led to an overall better agreement with experimental data of binary gas adsorption as compared to the Extended Langmuir Model. Fresh sample LiX show promise to be a better adsorption than NaX for pressure swing separation (CO₂/CH₄), due to its higher working capacity, selectivity and lower adsorption enthalpy. Nevertheless, cation stability for both this samples and NH₄X should be further investigated.     

Preparation and characterization of waste ion-exchange resin-based activated carbon for CO<Subscript>2</Subscript> capture

Waste ion-exchange resin was utilized as precursor to produce activated carbon by KOH chemical activation, on which the effects of different activation temperatures, activation times and impregnation ratios were studied in this paper. The CO₂ adsorption of the produced activated carbon was tested by TGA at 30 °C and environment pressure. Furthermore, the effects of preparation parameters on CO₂ adsorption were investigated. Experimental results show that the produced activated carbons are microporous carbons, which are suitable for CO₂ adsorption. The CO₂ adsorption capacity increases firstly and then decreases with the increase of activation temperature, activation time and impregnation rate. The maximum adsorption capacity is 81.24 mg/g under the condition of 30 °C and pure CO₂. The results also suggest that waste ion-exchange resin-based activated carbons possess great potential as adsorbents for post-combustion CO₂ capture.     

CO<Subscript>2</Subscript> adsorption and dehydration behavior of LiNaX,KNaX, CaNaX and CeNaX zeolites

In this study, NaX synthetic zeolite was modified by following the conventional cation exchange method at 70°C. 82, 81, 79 and 48% of sodium were exchanged with Li⁺, K⁺, Ca2⁺ and Ce3⁺, respectively. Thermal analysis data obtained by TG/DSC was used to evaluate the dehydration behavior of the zeolites. The strongest interaction with water and the highest dehydration enthalpy (ΔH) value were found for Li-exchanged form and compared with the other forms. The temperature required for complete dehydration increased with decreasing cation size (cation size: K⁺>Ce³⁺>Ca2⁺>Na⁺>Li⁺). CO₂ adsorption at 5 and 25°C was also studied and the virial model equation was used to analyze the experimental data to calculate the Henry’s law constant, K _o and isosteric heat of adsorption at zero loading Q _st. K _o values decreased with increasing temperature and the highest Qst was obtained for K rich zeolite. It was observed that both dehydration and CO₂ adsorption properties are related to cation introduced into zeolite structure.     

High-performance CO<Subscript>2</Subscript> capture on amine-functionalized hierarchically porous silica nanoparticles prepared by a simple template-free method

The adsorption of CO₂ on polyethyleneimine (PEI)-functionalized hierarchically porous silica nanoparticles (PSNs), prepared by using rice husk as a silica source via a simple template-free method, was reported in this study. Compared with traditional alkaline fusion and surfactant-templating methods for preparing waste-derived porous silica materials as CO₂ adsorbents, this method holds specific important advantages in being an inexpensive, and energy-saving process with faster production rate. The results revealed that the (NH₄)₂SiF₆ salt formed during the synthetic process served as an effective porogen, which can be readily removed by washing with water. Additionally, the total pore volumes of PSNs materials were strongly correlated to the amount of (NH₄)₂SiF₆. When evaluated as a support of PEI for CO₂ adsorption, 55PEI/PSNs(12/14) could reach 159 mg/g at 75 °C under 15 % CO₂, which was remarkably superior to those using waste silicate precursors reported in the previous literature. It was demonstrated that both PEI loading, and total pore volume of the PEI/silica composite sorbents, played key roles on CO₂ adsorption. Besides, 55PEI/PSNs(12/14) also showed high stability during 20 cycles of adsorption–desorption operation, implying its high potential in post-combustion CO₂ capture.     

Effects of CO<Subscript>2</Subscript> pressure on isotropic transition of amphiphilic side-chain type liquid crystalline di-block copolymers

The effects of high pressure carbon dioxide (CO₂) on the isotropic transition of three different amphiphilic di-block copolymers, PEO_m-b-PMA(Az)_n, namely PEO₁₁₄-b-PMA(Az)₄₀, PEO₂₇₂-b-PMA(Az)₄₆ and PEO₄₅₄-b-PMA(Az)₄₇, and on PMA(Az)₃₀ homopolymer have been investigated by scanning transitiometry. Under CO₂ pressure, the isotropic transition temperature decreased with the increase of pressure up to around 30 MPa due to CO₂ sorption and increased above 40 MPa. Transition entropy of the isotropic transition indicated that the depression of the isotropic transition temperature was caused by the adsorption of CO₂ into the azobenzene moieties and that the increase above 40 MPa was caused by the desorption of CO₂ into the azobenzene moieties. Comparison between PEO_m-b-PMA(Az)_n copolymers and PMA(Az) homopolymer clarified PEO domain acted CO₂ pathway to approach the equilibrium state rapidly.