PROCESS FOR CATALYTIC CO2 DESORPTION AND CATALYST FOR SAID PROCESS

Abstract

The present invention relates to a process for CO2 desorption in an overall CO2 absorption/desorption procedure. The present invention in particular relates to a process and a system for the catalytic CO2 desorption steps, in the presence of at least one Zr-based catalyst. The present invention also relates to a suitable Zr-based catalyst for the CO2 desorption process, and to methods for preparing such a catalyst.

Description

FIELD OF THE INVENTION

The present invention relates to a process for CO₂ desorption in an overall CO₂ absorption/desorption procedure. The present invention in particular relates to a process and a system for the catalytic CO₂ desorption steps, in the presence of at least one Zr-based catalyst. The present invention also relates to a suitable Zr-based catalyst for the CO₂ desorption process, and to methods for preparing such a catalyst.

BACKGROUND

The rapidly increasing carbon dioxide (CO₂) concentration in the atmosphere has caused great climate changes. From the latest report of the Intergovernmental Panel on Climate Change (IPCC), limiting warming to close to 1.5° C. or even 2° C. will be beyond reach, unless there are immediate, rapid, and large-scale reductions in CO₂ emissions. However, the risks and cost of non-fossil energy alternatives such as nuclear, biomass, solar energy, etc., cannot meet current energy demands. Additionally, any rapid changes to non-fossil energy sources, even if possible, would result in large disruptions to the existing energy supply infrastructure with substantial consequences to the global economy. The fossils fuel energy is believed to remain the dominant energy source from the Energy Information Administration. Under that premise, CO₂ capture and storage (CCS) and CO₂ capture and utilization (CCU) become promising routes to limit the savagely increasing CO₂ emission. However, for both routes, CO₂ capture is always the first step. In order to meet mid-to-long-term CO₂ reduction targets, it is necessary to develop a cost-effective CO₂ capture technology.

For the CO₂ from power plants, one of the dominant CO₂ emission sources, Post-Combustion Capture (PCC), using amine solvent as a CO₂ absorber, is the most mature and widely used technology. Moreover, it is believed to be the most promising technology to provide an energy-efficient and timely solution for decreasing CO₂ emissions from fossil power plants. A typical amine-based CO₂ capture and regeneration process involves the entire absorption-desorption cycle of CO₂ using amine solvents, as shown in FIG. 1. The CO₂-containing flue gas stream is introduced into a packed bed absorber column where it flows counter currently as it contacts lean amine, allowing for efficient absorption. After absorption, the rich amine solvent flows through a rich-lean heat exchanger before being introduced into a stripper column for thermal regeneration. In the stripper column, heated steam is supplied to strip out the captured CO₂ and increase the temperature. This free (lean) amine solvent then returns to the absorber column for a new cycle of absorption.

Typical amine solvents for CO₂ capture include primary amines like monoethanolamine (MEA), secondary amines like diethanolamine (DEA), and tertiary amines like methyldiethanolamine (MDEA). The mechanism of CO₂ absorption in different amine solvents is shown as follows:

CO₂+2R₁R₂NH↔2R₁R₂NH₂⁺+R₁R₂NCOO⁻  Primary/secondary amine:

CO₂+R₁R₂R₃N+H₂O↔R₁R₂R₃NH⁺+HCO₃⁻  Tertiary amine:

MEA is the most widely used amine solvent because of the tremendous CO₂ absorption rate, high capacity, high mass-transfer performance, and low-price properties. The absorption of CO₂ in MEA solvent follows a Zwitterion mechanism. One CO₂ is absorbed and forms one carbamate and one protonated MEA. As shown in FIG. 2, the desorption route of CO₂ from MEA mainly comprises two steps: (1) proton transfer, which includes proton transferring from protonated amine to water and proton transferring from H₃O⁺ to carbamate; (2) carbamate breakdown and release of CO₂. The proton transfer steps are unfavorable at low temperatures, forcing the industry to run under a high desorption temperature for an acceptable solvent regeneration efficiency. However, the high regeneration temperature (100˜140° C.) consumes more than 60% of the overall energy cost for PCC, and also causes degradation of amine solvent and corrosion of equipment.

Extensive studies have been carried out to reduce the energy cost during the solvent regeneration step of the primary amine. New amine solvents and bi-phasic amine solvents can significantly reduce the energy cost during desorption while maintaining a relatively high CO₂ absorption rate. However, they are still economically inefficient due to the high price of the solvent and complex reactor.

From a catalytic perspective, using a proper catalyst can decrease the activation energy of CO₂ desorption and boost the reaction rate, thereby decreasing the stripping time and increasing the solvent regeneration efficiency. Moreover, some catalysts allow CO₂ desorption at a temperature lower than 100° C., which could significantly reduce the energy consumption and solvent loss.

WO 2011120138 discloses a method, which applies solid catalysts into the CO₂ absorption and desorption process. It was reported that by using an acid catalyst like H-ZSM-5 in the stripper, CO₂ desorption from 5 M MEA solvent could be detected at a temperature lower than 50° C. The reason for low-temperature desorption is considered to be the direct donation of the proton from the Bronsted acid (B acid) site of H-ZSM-5 to carbonates. The catalyst performance between the B acid site and the Lewis acid (L acid) site with H-ZSM-5 and y-Al₂O₃ was also compared. B acid catalysts like H-ZSM-5 showed better performance on CO₂ desorption than Lewis acid catalysts like γ-Al₂O₃ during the CO₂ rich stage (temperature ramping stage) but showed poorer performance during CO₂ lean stage (isothermal stage).

There are many following reports on using solid acid catalysts, like HY, H-Beta, CeSO₄/ZrO₂, CMK-3-SiO₂, SO₄₂/ZrO₂/SBA-15 for CO₂ desorption from amine solvent. US2016030880A1 discloses a series of metal oxides which increase the CO₂ desorption amount from 3 M MEA. It also used acidic material (metal oxide, zeolites, proton-exchange resins) to help amine regeneration. The metal oxides contained metals from rows 3-5 in the Periodic Table. V₂O₅, MoO₃, WO₃, Cr₂O₃, MgO, γ-Al₂O₃, VO_X/Al₂O₃, MoO_x/Al₂O₃, WO_x/Al₂O₃, H-zeolite-Y, proton-exchange resin (Amberlyst 15 (H)) were tested. Among other materials, MoO₃ showed the best catalytic performance, which caused CO₂ desorption to start at 40° C. and increased CO₂ desorption amount by 27%. However, almost 96% of MoO₃ dissolved during the reaction. The dissolving amount decreased to 68% after using γ-Al₂O₃ as support. However, the CO₂ desorption amount also decreased. It seems that the improvement of the desorption is highly related to the dissolution of the metal oxide. This increased the difficulties on catalyst separation after the solvent regeneration process, which greatly limited the application for those types of catalysts.

US20160250591A1 reports TiO(OH)₂ as an efficient catalyst for CO₂ desorption from amine solvent. TiO(OH)₂ can drastically improve the CO₂ desorption rate of a CO₂-saturated 20% MEA solution by 100% during the temperature ramping stage at a low temperature of 88° C., which was much higher than the previous catalysts investigated.

US2010/209323A1 discloses DeNOx catalysts for the reduction of NOx compounds and porous catalyst support materials are provided. The inventive catalysts comprise an active metal catalyst component and mixed TiO₂/ZrO₂ porous support particles that comprise a) a crystalline phase comprising titanium dioxide and/or a titanium/zirconium mixed oxide, b) an amorphous phase comprising zirconium, and c) a small amount of one or more metal oxide(s) or metalloid oxide(s) deposited on the amorphous outer layer. The inventive catalysts exhibit superior activity and ammonia selectivity.

According to the reported results, using solid catalyst has already shown some improvement on low-temperature CO₂ desorption from amine solvent but still cannot meet the application requirement due to the limitation of poor catalytic performance or catalyst dissolution problems. An undissolved solid catalyst with better and stable catalytic performance is strongly needed to decrease the energy consumption of the amine regeneration process for PPC.

Therefore, there is a need for improved catalysts and improved processes for CO₂ desorption.

SUMMARY OF THE INVENTION

It has now been found that the above objectives can be attained either individually or in any combination by using the specific and well-defined processes and catalysts as disclosed herein.

The Applicants have developed a Zr-based catalyst, preferably comprising ZrO(OH)₂, and a method for preparing said catalyst. This catalyst shows advantageous properties when applied in a CO₂ desorption process. For example, the catalytic activity of catalysts according to the invention, and embodiments thereof, is far greater than any previously reported catalyst. The catalytic activity increases with increase in catalytic loading. The catalytic action is confirmed as the catalytic activity is maintained over cycles. The present invention, and embodiments thereof, also allows for a reduction in size of the desorption column leading to lower CAPEX. The present invention, and embodiments thereof, also allows for a reduction in temperature of CO₂ desorption, which in turn leads to lower energy requirements (OPEX). The present invention, and embodiments thereof, also allows to reduce the cost of regeneration of solvents in the absorption process.

More specifically, the Applicants have found that a fine designed undissolved solid Zr-based catalyst according to the invention, or embodiments thereof, allows to increase the CO₂ desorption rate under low temperature in the solvent regeneration process for the amine-based CO₂ Post-Combustion Process. The catalyst material preferably comprises a specific O:Zr atomic ratio. The catalyst material preferably comprises ZrO(OH)₂, and/or is synthesized under a preferred pH with a preferred basic precipitating agent.

The Applicants have demonstrated that a Zr-based catalyst according to the invention can boost the CO₂ desorption rate at a low temperature. The synthesis method influences the ratio of acid and basic hydroxyl groups and the surface charge property, which is demonstrated herein to cause an additional improvement in catalytic performance.

The catalysts prepared according to the invention, or embodiments thereof, can be used in MEA solvent and other amines in a CO₂ capture plant. The boosting of CO₂ desorption rate with the catalysts makes it possible for the solvent regeneration process to run under a lower temperature which can significantly decrease the energy cost of PPC and prevent the amine solution from degradation and evaporation. The catalyst is stable and insolubilized in amine solvent, which makes it easy to be separated and regenerated. Furthermore, the catalyst can be deposited on a structured support (inert packing) to replace traditional packing, or can be shaped in solid structure to replace traditional packing.

In a first aspect, the present invention provides a method for preparing a Zr-based catalyst for CO₂ desorption, the catalyst preferably comprising ZrO(OH)₂. The method preferably comprises the steps of:

• • mixing a solution comprising a basic precipitating agent into a solution comprising a Zr source, thereby preparing a synthesis solution; and,
  • precipitating the Zr-based catalyst, preferably comprising ZrO(OH)₂, from the synthesis solution, thereby obtaining a precipitated Zr-based catalyst.

The method is preferably characterized in that the pH of the synthesis solution is at least 3 and at most 8, preferably at least 4 and at most 7, for example about 5.

In some preferred embodiments, the Zr-based catalyst comprises Zr, 0, and H. Preferably, the O:Zr atomic ratio is at least 2.1, preferably at least 2.2, for example at least 2.3, for example at least 2.4, for example at least 2.5, for example at least 2.6, for example at least 2.7, for example at least 2.8, for example at least 2.9. In some embodiments, the O:Zr atomic ratio is at least 3.0, for example at least 3.5, for example at least 4.0.

In some preferred embodiments, the Zr-based catalyst comprises ZrO(OH)₂. Preferably, the precipitated Zr-based catalyst comprises at least 60% by weight ZrO(OH)₂ compared to the total weight of the precipitated Zr-based catalyst, preferably at least 70%, preferably at least 80%, preferably at least 90%, for example at least 95%, for example at least 98%, for example at least 99%.

In some preferred embodiments, the basic precipitating agent is a hydroxide, preferably selected from the group comprising: NaOH, NH₄OH (NH₃*H₂O), and KOH. Preferably, the basic precipitating agent is NaOH.

In some preferred embodiments, the Zr source is ZrO(NO₃)₂.

In some preferred embodiments, the method further comprises the step of:

• • calcining the precipitated Zr-based catalyst at a calcination temperature Tc.

Preferably, the calcination temperature Tc is at most 400° C., preferably at most 350° C., preferably at most 300° C.

In a second aspect, the present invention provides a Zr-based catalyst for CO₂ desorption, prepared using the method according to the first aspect, and embodiments thereof. Preferably, the O:Zr atomic ratio is at least 2.1. Preferably, the Zr-based catalyst comprises ZrO(OH)₂. (Preferred) embodiments of the first aspect are also (preferred) embodiments of the second aspect, and vice versa.

In some preferred embodiments, the ratio of acidic OH to basic OH of the Zr-based catalyst is at least 0.1 and at most 1.3, preferably at least 0.2 to at most 1.2, preferably at least 0.5 to at most 1.2, preferably at least 0.8 to at most 1.1, preferably about 1.0.

In some preferred embodiments, the Zr-based catalyst has a surface charge Zeta potential of at least −25 mV, preferably at least −15 mV, preferably at least −5 mV, for example at least 0 mV, for example at least 5 mV.

In some preferred embodiments, the Zr-based catalyst comprises other metals, preferably selected from the group comprising: Hf, Ce, or Zn. In some preferred embodiments, the purity of Zr compared to the other metals in the Zr-based catalyst is at least 80%, preferably at least 85%, preferably at least 90%, preferably at least 95%.

In a third aspect, the present invention provides a process for CO₂ desorption from an amine solvent. The process preferably comprises the steps of:

• • providing an CO₂-containing amine solution comprising CO₂ absorbed in an amine solvent;
  • supplying a Zr-based catalyst according to the second aspect, and embodiments thereof, to the CO₂-containing amine solution;
  • heating the amine solution comprising the Zr-based catalyst to a desorption temperature Td; and,
  • desorbing CO₂ from the amine solution comprising the Zr-based catalyst during a residence time.

(Preferred) embodiments of the second aspect are also (preferred) embodiments of the third aspect, and vice versa.

In some preferred embodiments, the Zr-based catalyst acts as a non-solubilized heterogeneous catalyst.

In some preferred embodiments, the amine solvent comprises a primary amine, secondary amine, or tertiary amine. Preferably, the amine solvent comprises monoethanolamine (MEA) or 2-amino-2-methyl-1-propanol (AMP), preferably monoethanolamine (MEA).

In some preferred embodiments, the residence time is at least 4 h and at most 168 h, preferably at least 8 h and at most 72 h, preferably at least 16 h and at most 48 h, preferably at least 20 h and at most 36 h, for example about 24 h.

In a fourth aspect, the present invention provides a process for CO₂ absorption and desorption. Preferably, the process comprises the steps of:

• • absorbing CO₂ in an amine solvent, thereby obtaining a CO₂-containing amine solution; and,
  • desorbing CO₂ from the CO₂-containing amine solution using the process according to the third aspect, or embodiments thereof, thereby regenerating the amine solvent.

(Preferred) embodiments of the third aspect are also (preferred) embodiments of the fourth aspect, and vice versa.

In a fifth aspect, the present invention provides the use of a Zr-based catalyst, preferably a catalyst according to the second aspect, or embodiments thereof, in a process according to the third or fourth aspect, or embodiments thereof.

(Preferred) embodiments of the second, third, or fourth aspect are also (preferred) embodiments of the fifth aspect, and vice versa.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a carbon capture process schematic.

FIG. 2 shows the reaction route for CO₂ desorption from a primary and secondary amine solvent.

FIG. 3 illustrates a schematic lab-scale CO₂ desorption apparatus.

FIG. 4 illustrates (A) CO₂ loading concentration versus reaction time and (B) a kinetic study during the isothermal stage in 5 M CO₂-saturated MEA with different catalysts.

FIG. 5 illustrates (A) CO₂ loading concentration versus reaction time, (B) a kinetic study during the isothermal stage in 5 M CO₂-saturated MEA with different loading amounts of ZrO(OH)₂, and (C) the relationship between catalytic reaction rate constant (kCAT) and catalyst loading amount.

FIG. 6 illustrates (A) a scheme for solvent recycled experiment, (B) CO₂ loading concentration versus reaction time, and (C) a kinetic study of kCAT during the isothermal stage in recycled MEA solvent without solid catalysts.

FIG. 7 illustrates the CO₂ desorption in 5 M CO₂-saturated MEA solution for a 168 h reaction with and without ZrO(OH)₂.

FIG. 8 illustrates (A) a scheme of two catalyst regeneration methods: wash recycled and directly recycled, (B) CO₂ loading concentration versus reaction time in 5 M CO₂-saturated MEA with ZrO(OH)₂ regenerated by different methods.

FIG. 9 illustrates (A) CO₂ loading concentration versus reaction time in 5 M CO₂-saturated MEA with ZrO(OH)₂ synthesized under different pH, and (B) the relationship between kCAT and the synthesis pH of ZrO(OH)₂.

FIG. 10 illustrates (A) CO₂ loading concentration versus reaction time and (B) a kinetic study during the isothermal stage in 5 M CO₂-saturated MEA with ZrO(OH)₂ synthesized by different methods.

FIG. 11 illustrates a thermogravimetric analysis (TGA) result of the ZrO(OH)₂ catalyst under N₂ atmosphere.

FIG. 12 illustrates the relationship between ZrO(OH)₂ synthesis pH and density of basicity —OH, acidity —OH and total —OH.

FIG. 13 illustrates the relationship between kCAT and the Acidity —OH/Basicity —OH ratio.

FIG. 14 illustrates (A) CO₂ loading concentration versus reaction time, and (B) a kinetic study during the isothermal stage in 5 M CO2-saturated MEA with ZrO(OH)₂ calcined under different temperatures.

FIG. 15 illustrates the XRD results of commercial ZrO₂ and different-temperature-calcined ZrO(OH)₂.

FIG. 16 illustrates (A) CO₂ loading concentration versus reaction time and (B) a kinetic study during the isothermal stage in 5 M CO₂-saturated MEA with different MZrO(OH)₂ catalysts.

FIG. 17 illustrates the relationship between kCAT and Zeta potential of different ZrO(OH)₂ based catalysts at pH 9.3.

FIG. 18 illustrates (A) CO₂ loading concentration versus reaction time and (B) a kinetic study during the isothermal stage in 2.5 M CO₂-saturated AMP with and without different catalyst.

FIG. 19 illustrates a scheme for a continual CO₂ desorption reactor.

FIG. 20 illustrates a zoomed in section of the scheme for a continual CO₂ desorption reactor illustrated in FIG. 19.

DETAILED DESCRIPTION OF THE INVENTION

When describing the invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

In the following passages, various aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

The terms “comprising”, “comprises” and “comprised of” as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements, or method steps. It will be appreciated that the terms “comprising”, “comprises” and “comprised of” as used herein comprise the terms “consisting of”, “consists” and “consists of”.

As used in the specification and the appended claims, the singular forms “a”, “an,” and “the” include plural referents unless the context clearly dictates otherwise. By way of example, “a step” means one step or more than one step.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1, 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

The term “about” as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of +/−10% or less, preferably +/−5% or less, more preferably +/−1% or less, of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier “about” refers is itself also specifically, and preferably, disclosed.

The terms “wt %”, “vol %”, or “mol %” refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, which includes the component. When describing the present invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

Preferred features, embodiments, and uses of this invention are set herein below. Each embodiment of the invention so defined may be combined with any other embodiment unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. Hereto, the present invention is in particular captured by any one or any combination of one or more of the below embodiments, with any other aspect and/or embodiment.

In a first aspect, the present invention provides a method for preparing a Zr-based catalyst for CO₂ desorption, the catalyst preferably comprising ZrO(OH)₂. The method preferably comprises the steps of:

• • mixing a solution comprising a basic precipitating agent into a solution comprising a Zr source, thereby preparing a synthesis solution; and,
  • precipitating the Zr-based catalyst, preferably comprising ZrO(OH)₂, from the synthesis solution, thereby obtaining a precipitated Zr-based catalyst.

The method is preferably characterized in that the pH of the synthesis solution is at least 3 and at most 8, preferably at least 4 and at most 7, for example about 5. In some embodiments, the pH of the synthesis solution is at least 3, preferably at least 4, for example at least 5. In some embodiments, the pH of the synthesis solution is at most 8, preferably at most 7, for example at most 6.

A catalyst prepared this way was found to provide a significant improvement on CO₂ desorption properties without significant dissolution, as illustrated in the example section. More specifically, the present synthesis method influences the ratio of acid and basic hydroxyl groups and the surface charge property, which is demonstrated herein to cause an additional improvement in catalytic performance.

In some preferred embodiments, the Zr-based catalyst comprises Zr, O, and H. Preferably, the O:Zr atomic ratio is at least 2.1, preferably at least 2.2, for example at least 2.3, for example at least 2.4, for example at least 2.5, for example at least 2.6, for example at least 2.7, for example at least 2.8, for example at least 2.9, for example about 3.0. In some embodiments, the O:Zr atomic ratio is at least 3.0, for example at least 3.5, for example at least 4.0. In some embodiments, the O:Zr atomic ratio is at most 5.0, for example at most 4.9, for example at most 4.8, for example at most 4.7, for example at most 4.6, for example at most 4.5, for example at most 4.4, for example at most 4.3, for example at most 4.2, for example at most 4.1, for example about 4.0. In some embodiments, the O:Zr atomic ratio is at least 2.1 and at most 5.0, for example at least 2.2 and at most 4.9, for example at least 2.3 and at most 4.8, for example at least 2.4 and at most 4.7, for example at least 2.5 and at most 4.6, for example at least 2.6 and at most 4.5, for example at least 2.7 and at most 4.4, for example at least 2.8 and at most 4.3, for example at least 2.9 and at most 4.2, for example at least 2.9 and at most 4.1, for example about 3.5.

A catalyst with this atomic ratio was found to provide a significant improvement on CO₂ desorption properties.

The O:Zr atomic ratio is herein defined as the bulk ratio and can be measured by the weight loss of the catalyst between 170° C. to 800° C. assuming the weight loss is from the loss of hydroxyl groups forming H₂O, and the rest of the solids are ZrO₂. The weight loss can be measured by Thermalgravimetric analysis (TGA) tested under N₂ flow and a temperature rate of between 5 to 20° C./min, preferably 10° C./min, assuming all the weight decrement after a temperature of 180° C. forms water and the final solid species above 600-800° C. is considered ZrO₂.

In some preferred embodiments, the Zr-based catalyst comprises ZrO(OH)₂. Preferably, the precipitated Zr-based catalyst comprises at least 60% by weight ZrO(OH)₂ compared to the total weight of the precipitated Zr-based catalyst, preferably at least 70%, preferably at least 80%, preferably at least 90%, for example at least 95%, for example at least 98%, for example at least 99%.

A catalyst with this atomic structure was found to provide a significant improvement on CO₂ desorption properties, as illustrated in the example section.

In some embodiments, the solution comprising a precipitating agent comprises at least 1.00 M and at most 1.50 M, for example about 1.25 M precipitating agent.

In some preferred embodiments, the basic precipitating agent is a hydroxide, preferably selected from the group comprising: NaOH, NH₄OH (NH₃*H₂O), and KOH. Preferably, the basic precipitating agent is NaOH.

In some embodiments, the solution comprising a Zr source comprises at least 2.00 M and at most 4.00 M, for example about 3.00 M Zr source.

In some preferred embodiments, the Zr source is ZrO(NO₃)₂. ZrO(NO₃)₂ is a common Zr source, and it may be easier to form ZrO(OH)₂ compared to Zr(NO₃)₄.

In some embodiments, the synthesis solution is prepared by dripping the solution comprising a precipitating agent into the solution comprising a Zr source under stirring. This allows the final pH to be easier controlled. Furthermore, dripping the solution comprising a Zr source into the solution comprising a precipitating agent might irreversibly form a sediment closer to ZrO₂ in a basic environment.

In some embodiments, a co-precipitation method is used.

In some preferred embodiments, the method further comprises the step of:

• • calcining the precipitated Zr-based catalyst at a calcination temperature Tc.

Preferably, the calcination temperature Tc is at most 400° C., preferably at most 350° C., preferably at most 300° C.

It was found that a calcination step at too high temperatures removed the OH groups and reduced the catalyst to ZrO₂, thereby severely reducing the catalytic activity, as illustrated in the example section.

In a second aspect, the present invention provides a Zr-based catalyst for CO₂ desorption, prepared using the method according to the first aspect, and embodiments thereof. Preferably, the O:Zr atomic ratio is at least 2.1 or other values as described herein. Preferably, the Zr-based catalyst comprises ZrO(OH)₂, preferably at percentages as described herein. (Preferred) embodiments of the first aspect are also (preferred) embodiments of the second aspect, and vice versa.

Such a catalyst was found to provide a significant improvement on CO₂ desorption properties without significant dissolution, as illustrated in the example section.

In some preferred embodiments, the ratio of acidic OH to basic OH in the Zr-based catalyst is at least 0.1 and at most 1.3, preferably at least 0.2 to at most 1.2, preferably at least 0.5 to at most 1.2, preferably at least 0.8 to at most 1.1, preferably about 1.0.

Controlling the ratio of acidic OH to basic OH was found to provide a significant improvement on CO₂ desorption properties, as illustrated in the example section.

The ratio of acidic OH to basic OH is preferably measured by Thermogravimetric Analysis (TGA), for example as conducted on a TGA Q500 from TA Instruments. The TGA may be performed under a 10 mL/min N₂ atmosphere, with a temperature ramping from 50° C. to 150° C. for 30 min to make sure all free water is released, and then to 800° C. The temperature ramping rate may be 10° C./min.

In some preferred embodiments, the Zr-based catalyst remains in an amorphous phase upon heating/calcining in ambient/inert until 200-300° C. The inventors have surprisingly found that the best-performing catalysts do not show the tetragonal crystalline t-ZrO₂ phase upon heating/calcining in ambient/inert until 200-300° C., but remain in an amorphous phase, whereas inferior catalysts do give clear formation of the t-ZrO₂ phase (as measured using classic state of the art powder X-ray diffraction analysis, for example as illustrated in the example section) upon the same thermal treatment. In some preferred embodiments, the Zr-based catalyst comprises at most 20.0 wt % of the tetragonal crystalline t-ZrO₂ phase, preferably at most 10.0 wt %, preferably at most 5.0 wt %, preferably at most 2.0 wt %, preferably at most 1.0 wt %, preferably essentially no tetragonal crystalline t-ZrO₂ phase, after heating/calcining in ambient/inert until 200-300° C.

In some preferred embodiments, the Zr-based catalyst has a surface charge Zeta potential of at least −25 mV, preferably at least −15 mV, preferably at least −5 mV, for example at least 0 mV, for example at least 5 mV.

Catalysts with such a high surface charge were found to provide improved CO₂ desorption properties, as illustrated in the example section.

The Zeta potential is preferably measured on a NanoPlus HD with an Auto-Titrator from Particulate Systems. Typically, 40 mg well-grinded sample powder may be mixed with 40 g ultrapure water, followed by a 20 min ultrasonic treatment. The finely dispersed sample solution may be used for the testing. The Zeta potential may be tested under a pH of 9.3±0.2. 0.1 M HCl and 0.1 M NaOH may be used for pH adjustment.

In some preferred embodiments, the Zr-based catalyst comprises other metals, preferably selected from the group comprising: Hf, Ce, or Zn. Other metals than Zr might be used for reasons of cost and/or stability. In some preferred embodiments, the purity of Zr compared to the other metals in the Zr-based catalyst is at least 80%, preferably at least 85%, preferably at least 90%, preferably at least 95%.

Such catalysts were found to also provide suitable catalytic activity.

In a third aspect, the present invention provides a process for CO₂ desorption from an amine solvent. The process preferably comprises the steps of:

• • providing an CO₂-containing amine solution comprising CO₂ absorbed in an amine solvent;
  • supplying a Zr-based catalyst according to the second aspect, and embodiments thereof, to the CO₂-containing amine solution;
  • heating the amine solution comprising the Zr-based catalyst to a desorption temperature Td; and,
  • desorbing CO₂ from the amine solution comprising the Zr-based catalyst during a residence time.

(Preferred) embodiments of the second aspect are also (preferred) embodiments of the third aspect, and vice versa.

The present invention allows to reduce the cost of regeneration of solvents in the absorption/desorption process.

In some preferred embodiments, the Zr-based catalyst acts as a non-solubilized heterogeneous catalyst. The catalyst is typically stable and insolubilized in amine solvent, which makes it easy to be separated and regenerated.

In some preferred embodiments, the amine solvent comprises a primary amine, secondary amine, or tertiary amine. Examples of suitable amines include but are not limited to: monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA), piperazine (PZ), ammonia, amines, alkanolamines, derivatives and/or combinations thereof. Preferably, the amine solvent comprises monoethanolamine (MEA) or 2-amino-2-methyl-1-propanol (AMP), preferably monoethanolamine (MEA).

In some embodiments, the amine solvent comprises a combination of two or more amines, for example AMP with PZ as a co-solvent. A combination of amines can increase absorption rate and cyclic capacity to reduce energy of regeneration (heat duty), application of high amine solution concentration and minimized degradation.

An AMP/PZ (3 M/1.5 M) ratio is considered to be the preferred ratio. The proper ratio of AMP/PZ can increase the absorption rate and the cyclic capacity while avoiding the precipitation of PZ.

In some embodiments, the amine solvent is present in the amine solution at a molar concentration of at least a 1 M to at most a 10 M, preferably of at least a 2 M to at most an 8 M, preferably of at least a 4 M to at most a 6 M, for example at about a 5 M concentration. These concentrations were found to provide optimal results, as illustrated in the example section.

In some embodiments, the Zr-based catalyst is provided in a concentration of at least 1.0 g to at most 8.0 g, preferably of at least 2.0 g to at most 6.0 g, preferably of at least 2.5 g to at most 4.0 g, for example about 3.0 g Zr-based catalyst per 100 mL amine solution.

These amounts were found to provide optimal results, as illustrated in the example section.

In some embodiments, the desorption temperature Td is at most 500 K, preferably at most 450 K, preferably at most 400K, preferably at most 380 K, preferably about 361 K.

The present invention has the significant advantage that the desorption temperature can be kept low. The boosting of CO₂ desorption rate with the catalysts makes it possible for the solvent regeneration process to run under a lower temperature which can significantly decrease the energy cost of PPC and prevent the amine solution from degradation and evaporation.

In some preferred embodiments, the residence time is at least 4 h and at most 168 h, preferably at least 8 h and at most 72 h, preferably at least 16 h and at most 48 h, preferably at least 20 h and at most 36 h, for example about 24 h.

These residence times were found to provide optimal results, as illustrated in the example section.

In a fourth aspect, the present invention provides a process for CO₂ absorption and desorption. Preferably, the process comprises the steps of:

• • absorbing CO₂ in an amine solvent, thereby obtaining a CO₂-containing amine solution; and,
  • desorbing CO₂ from the CO₂-containing amine solution using the process according to the third aspect, or embodiments thereof, thereby regenerating the amine solvent.

(Preferred) embodiments of the third aspect are also (preferred) embodiments of the fourth aspect, and vice versa.

The present invention allows to reduce the cost of regeneration of solvents in the absorption/desorption process.

In some embodiments, the process is a batch process. In some more preferred embodiments, the process is a continuous process.

In some embodiments, the process further comprises the step of:

• • recycling the Zr-based catalyst into the process.

The catalyst of the present invention has the advantage that it can be easily recycled, particularly since the catalyst typically acts as a non-solubilized heterogeneous catalyst.

In some embodiments, the step of recycling the Zr-based catalyst comprises the step of:

• • directly recycling the Zr-based catalyst and amine solvent into the process, by using the Zr-based catalyst and amine solvent in a re-absorption step without separation.

In some embodiments, the step of recycling the Zr-based catalyst comprises the step of:

• • removing the Zr-based catalyst from the amine solvent, preferably through filtration;
  • washing the Zr-based catalyst with water;
  • optionally, centrifuging the Zr-based catalyst;
  • drying the Zr-based catalyst; and,
  • adding the dried catalyst to an amine solvent.

In a fifth aspect, the present invention provides the use of a Zr-based catalyst, preferably a catalyst according to the second aspect, or embodiments thereof, in a process according to the third or fourth aspect, or embodiments thereof. (Preferred) embodiments of the second, third, or fourth aspects are also (preferred) embodiments of the fifth aspect, and vice versa.

Use of a Zr-based catalyst as described herein allows for a reduction in size of the desorption column leading to lower CAPEX. Use of a Zr-based catalyst as described herein also allows for a reduction in temperature of CO₂ desorption, which in turn leads to lower energy requirements (OPEX), and/or less solvent degradation. For example, desorption at lower temperatures allows for less degradation of the amine(s).

Examples

The following examples serve to merely illustrate the invention and should not be construed as limiting its scope in any way. While the invention has been shown in only some of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the scope of the invention.

Experimental Methods

An experimental apparatus for solvent regeneration (CO₂ desorption) is shown in FIG. 3. It consists of a 250 mL round-bottom flask equipped with a thermometer, a heater controlled by a temperature controller, a condenser, a magnetic field stirrer. For a typical CO₂ desorption experiment run, 150 mL 5 M MEA solution with initial CO₂ loading (0.52 mol/mol) and 3 g catalyst is mixed in the reactor under a 500 mL/min N₂ flow from the top of the condenser. Then the solution was heated to 361.1 K in 50±5 min and kept constant for 180 min with a stirring rate of 500 rpm. The time when the solution reached 361.1 K is noted as 0 min. For a long time (168 h) CO₂ desorption experiment, 6 g ZrO(OH)₂ is used in 150 mL 5 M MEA. The CO₂ concentration of the solution is analyzed by Chittick equipment with the average absolute relative deviation less than 5%. Typically, 500 μL CO₂ loaded amine solution was added to the flask by a pipette, then 1 mL of 1.5 M H₂SO₄ was injected into the solution under a violent stirring. The volume of released CO₂ is noted to further calculate the CO₂ concentration of the sample. Every sample was tested three times, and the average value was adopted.

The catalysts are evaluated in two aspects: CO₂ deposition amount during the temperature ramping stage and the catalytic reaction rate constant (k_CAT).

The k_CAT is calculated as follows:

$k_{\mathrm{CAT}}=k_{\mathrm{overall}}-k_{\mathrm{blank}}$

k_overall is the overall reaction rate constant calculated through the CO₂ concentration of 0 min, 30 min, 60 min, 90 min, 120 min and 180 min after the temperature gets stable (isothermal stage). k_blank is calculated from the changes of the CO₂ concentration in MEA solvent without any catalysts during the first 3 h desorption in the isothermal stage. Without specific description, the value of k_blank is considered to be −1.3*10⁻⁴ min⁻¹.

X-ray powder diffraction (XRD) was conducted on a high-throughput STOE STADI P Combi diffractometer in the transmission mode with focusing Ge(111) monochromatic X-ray inlet beams (λ=1.5406 Å, Cu Kα source).

Thermogravimetric analysis (TGA) was conducted on TGA Q500 from TA Instruments. Typically, the TGA was performed under a 10 mL/min N₂ atmosphere, with a temperature ramping from 50° C. to 150° C. for 30 min to make sure all free water is released, and then to 800° C. The temperature ramping rate was 10° C./min.

Zeta potential was tested on NanoPlus HD with an Auto-Titrator from Particulate Systems. Typically, 40 mg well-grinded sample powder was mixed with 40 g ultrapure water, followed by a 20 min ultrasonic treatment. The finely dispersed sample solution was used for the testing. The Zeta potential was tested under a pH of 9.3±0.2. 0.1 M HCl and 0.1 M NaOH were used for pH adjustment.

The O:Zr ratio for different pH synthesized Zr-based catalyst samples prepared according to the invention were found to be: 4.27, 4.50, 4.85, 3.39, 2.85, and 2.49.

Example 1: CO₂ Desorption Performance with Different Catalysts

The performance of different catalysts is shown in FIG. 4 and Table 1. The O:Zr ratio for these catalysts was between 3:1 and 5:1. Compared with Blank, all three of H-ZSM-5, TiO(OH)₂ and ZrO(OH)₂ show improvement on CO₂ desorption amount during the temperature ramping stage, while ZrO(OH)₂ shows 150% improvement on CO₂ desorption amount compared to H-ZSM-5 and TiO(OH)₂. After the temperature stabilizes, there is no noticeable improvement on CO₂ desorption with H-ZSM-5, but TiO(OH)₂ and ZrO(OH)₂ still show a significant improvement in CO₂ desorption rate. A kinetic study on the isothermal stage found that the k_CAT value for H-ZSM-5 is almost 0, while ZrO(OH)₂ shows the highest k_CAT value of −3.1*10⁻⁴ min⁻¹, which is 2.4 times the value of TiO(OH)₂. MoO₃ shows a very high increment of CO₂ desorption amount at the temperature ramping stage but a lower CO₂ desorption rate than ZrO(OH)₂ at the isothermal stage. The k_CAT for MoO₃ is only −1.8*10⁻⁴ min⁻¹, which means that the catalytic effect of MoO₃ is only about 58% of ZrO(OH)₂. To be noted, MoO₃ is completely dissolved after the temperature reaches 88° C., but there is no significant dissolution for ZrO(OH)₂. Considering the k_overall for blank MEA solvent is −1.3*10⁻⁴ min⁻¹, using ZrO(OH)₂ provides a 238% improvement on the reaction rate constant of the CO₂ desorption from MEA at 88° C.

TABLE 1
CO2 desorption Performance with Different Catalysts Summary
		CO2 desorption	kCAT
	Catalyst	amount (mmol)	(*10−4 min−1)
	Blank	41.0	0
	H-ZSM-5	43.6	−0.06
	TiO(OH)2	48.9	−1.3 ± 0.2
	MoO3	56.0	−1.8
	ZrO(OH)2	48.9	−3.1 ± 0.2

Reaction condition: 3 g of catalyst, 150 mL CO₂-saturated 5 M MEA, N₂ flow of 0.5 L/min from the top of the condenser, 88° C. The CO₂ desorption amount is calculated at the end of the temperature ramping stage.

Example 2: CO₂ Desorption Performance for Different Catalyst Amounts

CO₂ desorption performance with different ZrO(OH)₂ amounts is shown in FIG. 5 and Table 2. The CO₂ desorption amount during the temperature ramping stage increased with the amount of the catalyst. The k_CAT in the isothermal stage and the catalyst amount show a linear relationship when the catalyst amount is below 4 g. However, when the catalyst amount increased to 6 g, the k_CAT is no longer increased linearly but relatively lower due to the mass transfer limitation.

TABLE 2
CO2 desorption Performance with
Different ZrO(OH)2 Loading Amount
ZrO(OH)2	CO2 desorption	kCAT
Amount (g)	amount (mmol)	(*10−4 min−1)
0	41.0	0
1.5	47.6	−1.8 ± 0.2
2	48.9	−2.2 ± 0.3
3	48.9	−3.1 ± 0.2
4	50.0	−3.6 ± 0.4
6	52.8	−4.7 ± 0.4

Reaction condition: 150 mL CO₂-saturated 5 M MEA, N₂ flow of 0.5 L/min from the top of the condenser, 88° C. The CO₂ desorption amount is calculated at the end of the temperature ramping stage.

To confirm that the active sites of ZrO(OH)₂ do come from the solid phase instead of the solubilized species, the solid phase ZrO(OH)₂ was removed from the MEA solution by centrifuge after a typical CO₂ desorption reaction, and the remaining MEA solution was collected, re-absorbed with CO₂, and went for another desorption test without any catalyst, noted as “solvent recycle”. The experimental scheme and results are shown in FIG. 6. The removal of the catalyst ceases the catalytic effect as the desorption performance shows no noticeable difference with the blank. The desorption rate constant decreased by more than 90% than that of ZrO(OH)₂.

In a long-time desorption experiment (FIG. 7), ZrO(OH)₂ improved almost 70% CO₂ desorption amount compared with blank at 24 h. After that, due to the thermodynamic limitation, the CO₂ desorption rate with ZrO(OH)₂ was suppressed, and the CO₂ concentration of the solution approached the blank and finally reached a similar level after 168 h. This also indicates that the CO₂ desorption with ZrO(OH)₂ is not a stoichiometric reaction but a catalytic reaction.

A catalyst regeneration test was also carried out in two ways: water washed regeneration and directly recycled. The scheme for the two methods is shown in FIG. 8A. For washed recycled, the spent ZrO(OH)₂ is washed with water, centrifuged, and dried under 100° C. overnight, then used in a new CO₂ desorption test. For directly recycled, the reacted mixture (lean MEA solution and ZrO(OH)₂) directly went for a re-absorption followed by the desorption process. As shown in FIG. 8B, both directly recycled and water washed recycled ZrO(OH)₂ show similar CO₂ desorption performance as the fresh ZrO(OH)₂. The result indicate that ZrO(OH)₂ is not a one-time catalyst and can be easily recycled.

Example 3: Effect of Synthesis pH on Catalyst Synthesis

A typical ZrO(OH)₂ is synthesized through a precipitation method. NaOH solution was dripped into a ZrO(NO₃)₂ solution until pH reached 5 under a violent stirring. After ageing for 3 h, the sediment was centrifuged and washed three times, then dried at 100° C. for 48 h. The outstanding catalytic performance of ZrO(OH)₂ on CO₂ desorption from the amine solvent is due to the carefully controlled synthesis process and the finely designed surface property.

By changing the amount of the basic precipitating agents, the synthesis pH of ZrO(OH)₂ can be easily controlled. As shown in FIG. 9 and Table 3, ZrO(OH)₂ synthesized under different pH showed vastly different catalytic performance.

TABLE 3
CO2 desorption Performance With
ZrO(OH)2 Synthesized Under Different pH
ZrO(OH)2	CO2 desorption	kCAT
synthesis pH	amount (mmol)	(*10−4 min−1)
Blank	41.0	0
3	52.8	−0.7
4	46.2	−2.4 ± 0.2
5	48.9	−3.1 ± 0.2
6	47.6	−2.2 ± 0.3
7	46.2	−1.8
8	41.0	−1.0
12	38.3	0

Reaction condition: 3 g of catalyst, 150 mL CO₂-saturated 5 M MEA, N₂ flow of 0.5 L/min from the top of the condenser, 88° C. The CO₂ desorption amount is calculated at the end of the temperature ramping stage.

The relationship between k_CAT and the synthesis pH shows a volcano curve, where k_CAT reaches the maximum with 3.1*10⁻⁴ min⁻¹ at a synthesis pH of 5. When the synthesis pH went either higher or lower, an apparent drop in k_CAT appears. To be noted, ZrO(OH)₂ synthesized under lower pH like pH 3 shows a high CO₂ desorption amount during the temperature ramping stage with very low catalytic effect (k_CAT=−0.7*10⁻⁴ min⁻¹), which indicated that the improvement might be due to the remaining acid during the synthesis process.

Example 4: Effect of Synthesis Method on Catalyst Synthesis

This example compares the catalytic performance of ZrO(OH)₂ synthesized by a method according to an embodiment of the invention, and ZrO(OH)₂ synthesized from a reported synthesis method for metal hydroxyl oxide.

Following the reported method of synthesis of high-performance TiO(OH)₂, ZrO(OH)₂ (iso) catalysts were synthesized. Zirconium isopropoxide was added into deionized water with a molar ratio of H₂O:Zr being 1400:1, followed by stirring for 4 h. The precipitate powder was filtered, rinsed three times with deionized water and ethanol, then dried at 100° C. for ˜10h.

The ZrO(OH)₂ can also be synthesized through a hydrothermal method. ZrO(NO₃)₂ was mixed in a water-ethanol solvent, and kept at 80° C. and circulated for 2 h. The precipitate powder was then filtered, rinsed three times with deionized water, and dried at 100° C. for ˜10h. The dried powder is noted as ZrO(OH)₂ (hydro).

TABLE 4
CO2 desorption Performance With
ZrO(OH)2 Synthesized With Different Methods
		CO2 desorption	kCAT
	Catalysts	amount (mmol)	(*10−4 min−1)
	Blank	41.0	0
	TiO(OH)2	48.9	−1.3 ± 0.2
	ZrO(OH)2	48.9	−3.1 ± 0.2
	ZrO(OH)2 (iso)	47.6	−1.6
	ZrO(OH)2 (hydro)	51.5	0.9

Reaction condition: 3 g of catalyst, 150 mL CO₂-saturated 5 M MEA, N₂ flow of 0.5 L/min from the top of the condenser, 88° C. The CO₂ desorption amount is calculated at the end of the temperature ramping stage.

The catalytic performance of ZrO(OH)₂ synthesized with different methods for CO₂ desorption is shown in Table 4 and FIG. 10. Although ZrO(OH)₂ (hydro) shows the highest CO₂ desorption amount during the temperature ramping stage, it shows a k_CAT of 0.9*10⁻⁴ min⁻¹, which has an even negative effect on the CO₂ desorption rate during the isothermal stage. ZrO(OH)₂ (iso) shows a similar improvement on CO₂ desorption amount as ZrO(OH)₂ does. During the temperature ramping stage, the k_CAT value for ZrO(OH)₂(iso) is −1.6*10⁻⁴ min⁻¹, which is only 51% of ZrO(OH)₂ but still higher than that of TiO(OH) (1.3*10⁻⁴ min⁻¹). The result further proves the advantage of the current synthesis methods and shows that the ZrO(OH)₂ itself is more suitable for CO₂ desorption than TiO(OH)₂.

Example 5: Effect of Hydroxyl Groups

The hydroxyl groups for ZrO(OH)₂ are believed to show either acidity or basicity depending on the binding type. Mono-binding —OH can be considered as weak bases. It has lower binding energy with Zr and can be removed at a lower temperature (below 400° C.). While for bridged-binding —OH groups, they have a stronger binding with Zr and can be considered weak B acid. The removal temperature for bridged-binding —OH is relative higher (400-600° C.).

Since the hydroxyl group is releasing as H₂O with the raising of the temperature, the hydroxyl groups amount can be quantified by TGA. A typical TGA result for ZrO(OH)₂ is shown in FIG. 11. The water released between 200° C. −400° C. is considered to be from weak bonding —OH (basic —OH), and the water released between 400° C. to 600° C. is considered to come from strong bonding —OH (acidic —OH).

The hydroxyl group density for ZrO(OH)₂ synthesized under different pH values is shown in FIG. 12. It can be seen that the synthesis pH has a significant influence on the —OH group. When the synthesis pH increased from 3 to 5, the total density of the hydroxyl group only slightly decreased from 25.7 mmol/g to 23.3 mmol/g, with a slight increase of basic —OH (from 10.8 to 11.6 mmol/g) and a decrease of acidic —OH (from 14.9 to 12.1 mmol/g). When the synthesis pH further increased from 5 to 7, both total —OH density and acidic —OH decreased significantly, but basic —OH remained almost not changed. By further increasing the pH to 12, the drop on both types of —OH density occurred.

A volcano curve relationship between k_CAT and the basic/acid —OH ratio is shown in FIG. 13. The k_CAT reaches the top when the basic and acidic —OH ratio is close to 1.0. The results indicate that there is a synergetic effect between acidic and basic —OH; thus controlling the basic and acidic —OH ratio of the catalysts may be one of the critical factors for the high-performance ZrO(OH)₂.

To further prove the importance of the hydroxyl group of the catalyst, ZrO(OH)₂ is calcined from 200° C. to 600° C. to partly remove the —OH group from ZrO(OH)₂. From FIG. 14 and Table 5, it can be seen that the CO₂ desorption amount at the temperature ramping step decreased with the increasing calcination temperature. However, ZrO(OH)₂ calcined at 200° C. and 300° C. only shows a slight decrease in catalytic performance. By further increasing the calcining temperature to 400° C., the k_CAT further drops to −1.3*10⁻⁴ min⁻¹. The XRD results (FIG. 15) show that below 400° C. calcination, ZrO(OH)₂ remained amorphous and after a 400° C.-calcination, the ZrO(OH)₂ crystalized and formed t-ZrO₂ with a crystal size of 30 nm. However, compared to commercial ZrO₂, ZrO(OH)₂ (400° C.) shows a twofold k_CAT. The better performance of the ZrO(OH)₂ compared with ZrO₂ may be due to the higher density of the hydroxyl groups, which makes it a better catalyst than ZrO₂. On the other hand, by calcining ZrO(OH)₂ under a temperature of 300° C., a more stable catalyst can be achieved with only a slight drop in catalytic performance. The O:Zr ratio for ZrO(OH)₂ (400° C.) was 2.17:1, while for the other catalysts it was higher than 3:1.

TABLE 5
CO2 desorption Performance With
ZrO(OH)2 Calcined under different temperature
	Catalysts	CO2 desorption	kCAT
	(calcination temperature)	amount (mmol)	(*10−4 min−1)
	Blank	41.0	0
	ZrO(OH)2	48.9	−3.1 ± 0.2
	ZrO(OH)2 (200° C.)	48.9	−2.3
	ZrO(OH)2 (300° C.)	47.6	−2.0
	ZrO(OH)2 (400° C.)	43.6	−1.4
	ZrO2 (commercial)	42.3	−0.6

Reaction condition: 3 g of catalyst, 150 mL CO₂-saturated 5 M MEA, N₂ flow of 0.5 L/min from the top of the condenser, 88° C. The CO₂ desorption amount is calculated at the end of the temperature ramping stage.

Example 6: Catalytic Performance of Mixed Metal Hydroxyl Oxide

Mixed Metal hydroxyl oxides are investigated based on the ZrO(OH)₂. Typically, 10% of the metal nitrate is mixed with Zirconium precursor, followed by the same synthesis method with ZrO(OH)₂. The catalyst is noted as 10% MZrO(OH)₂, (M=Hf, Ce, Zn).

As shown in FIG. 16 and Table 6, 10% ZnZrO(OH)₂ showed 25% improvement on CO₂ desorption amount compared with the blank, which had a 10% improvement to ZrO(OH)₂. However, the k_CAT of 10% ZnZrO(OH)₂ is only −1.6*10⁻⁴ min⁻¹, which is lower than 10% HfZrO(OH)₂ (−1.9*10⁻⁴ min⁻¹), and 10% CeZrO(OH)₂ (−2.3*10⁻⁴ min⁻¹). ZrO(OH)₂ still shows the highest k_CAT of −3.1*10⁻⁴ min⁻¹. To be noted, even for the lowest k_CAT of 10% MZrO(OH)₂, which is 10% ZnZrO(OH)₂, it still shows a 25% improvement to the k_CAT of TiO(OH)₂.

TABLE 6
CO2 desorption Performance With Different MZrO(OH)2
		CO2 desorption	kCAT
	Catalysts	amount (mmol)	(*10−4 min−1)
	Blank	41.0	0
	ZrO(OH)2	48.9	−3.1 ± 0.2
	10% HfZrO(OH)2	48.9	−1.9
	10% CeZrO(OH)2	47.6	−2.3
	10% ZnZrO(OH)2	51.5	−1.6
	TiO(OH)2	48.9	−1.3 ± 0.2

Reaction condition: 3 g of catalyst, 150 mL CO₂-saturated 5 M MEA, N₂ flow of 0.5 L/min from the top of the condenser, 88° C. The CO₂ desorption amount is calculated at the end of the temperature ramping stage.

Example 7: Surface Charge of the ZrO(OH)₂ Based Catalysts

The surface charge of the ZrO(OH)₂ based catalysts are tested through a Zeta potential analyzer under pH 9.3, which is the typical pH of the CO₂-MEA solvent during the desorption. The value of Zeta potential reflects the charge property of the particles' surface. A positive Zeta potential indicates that the particle's surface is positively charged, and the absolute value for the Zeta potential reflects the density of the charges. FIG. 17 shows a clear relationship between the k_CAT and the Zeta potential of the ZrO(OH)₂. The higher the Zeta potential is, the higher the catalytic effect is achieved. The higher Zeta potential results in a higher concentration of carbamate species adsorbed on the surface of the catalyst, which is favorable for the carbamate breakdown step and results in showing a higher catalytic effect.

Example 7: CO₂ Desorption Performance with ZrO(OH)₂ in 2.5 M CO₂—Saturated AMP Solution

2-amino-2-methyl-1-propanol (AMP) has been proposed as a commercially attractive new CO₂ absorbent because of its advantages in absorption capacity, absorption rate, degradation resistance, and regeneration energy.

TABLE 7
CO2 desorption Performance With and Without Different Catalysts
		CO2 desorption	kCAT
	Catalyst	amount (mmol)	(*10−4 min−1)
	Blank	110	0
	TiO(OH)2	115	−1.3
	ZrO(OH)2	116	−4.6

Reaction condition: 3 g of catalyst, 150 mL CO₂-saturated 2.5 M AMP, N₂ flow of 0.5 L/min from the top of the condenser, 88° C. The CO₂ desorption amount is calculated at the end of the temperature ramping stage.

To show the CO₂ desorption performance with ZrO(OH)₂ catalyst in other amine solvents, a CO₂ desorption experiment was conducted in a CO₂-saturated 2.5 M AMP solution. The CO₂ desorption performance is shown in FIG. 18 and Table 7. AMP already showed a very high desorption rate without any catalysts. The k_blank of AMP is −4.8*10⁻⁴ min⁻¹ which is about three times compared with that of MEA. However, ZrO(OH)₂ still shows improvement in both the temperature ramping stage and the isothermal stage. And compared with TiO(OH)₂ (−1.3*10⁻⁴ min⁻¹), the k_CAT of ZrO(OH)₂ showed a 250% improvement. The results proved that ZrO(OH)₂ does not only work well in MEA solvent, but can also be applied in other amine solvents for boosting the CO₂ desorption rate under low temperature.

Example 8: Industrial Desorption Process

To simulate the continual desorption process in industry, the catalyst is also tested in a self-built continual reactor (see FIG. 19, and a zoomed in section in FIG. 20). The CO₂-rich amine solution was fed over the catalysts bed from the bottom at a flow rate of 3.4 mL min⁻¹ under 85° C. CO₂ released from the amine solution was carried out to the analysis system by N₂ flow blowing from the top of the liquid level. The CO₂ desorption flowrate is calculated by the intensity of m/z=44 signal from mass spectroscopy (MS). The results were double checked by titrating CO₂-lean amine solution.

The calculation methods are shown as followed:

The CO₂ desorption rate calculated through MS:

${\mathrm{CO}}_2\left(\%\right)=A*I$ $F_{\mathrm{CO}2}=\frac{F_{N2}\times {\mathrm{CO}}_2\left(\%\right)}{1-{\mathrm{CO}}_2\left(\%\right)}$

• • I: CO₂ signal intensity from MS
  • A: Calibration factors of MS signal (%)
  • CO₂ (%): CO₂ concentration (%)
  • F_N2: N₂ flow rate (mL/min)
  • F_CO2: released CO₂ flow rate (mL/min)

The CO₂ desorption rate is also checked through titrating liquid samples after reaction, which is calculated as below:

$F_{\mathrm{CO}2}=\left(\mathrm{ct}-c0\right)\times F_{\mathrm{MEA}}$

• • ct: CO₂ concentration in the reacted solution (mL_CO2/mL_CO2)
  • c0: CO₂ concentration in the fresh solution (mL_CO2/mL_CO2)
  • F_MEA: Flow rate of MEA solution

Table 8 shows the results of CO₂ desorption rate in continual reactor with different catalysts. The test without catalyst (called blank) contained glass beads of comparable size as that of the catalysts, which showed a relative low CO₂ desorption rate by 9.7 mL/min. By using TiO(OH)₂, the CO₂ desorption rate increased to 21.0 mL/min. The CO₂ desorption rate with ZrO(OH)₂ is 35.50 mL/min, which showed 266% improvement compared with that of the blank and 69% compared with TiO(OH)₂.

TABLE 8
CO2 desorption rate and cycling capacity with different catalysts
          CO2 desorption rate
Catalysts (mL/min)
Blank     9.7
TiO(OH)2  21.0
ZrO(OH)2  35.50

Reaction condition: N₂: 100 mL/min, CO₂ saturated MEA solution flow: 3.4 mL/min, 85° C., 3 cm catalyst.

FIGS. 19 and 20 use following reference numbers: 101—Liquid sampling; 111—Rich amine tank; 112—Lean amine tank; 121—Pump; 122—Coriolis circular heat exchanger; 123—Heated transfer line; 124—Drain valve; 125—Heating laboratory bath; 126—Cold finger trap; 127—Process cooling water; 128—Variable vacuum pump; 129—Flow indicator; 130—Catalyst; 140—Mass spectroscopy.

Claims

1-15. (canceled)

16. A method for preparing a Zr-based catalyst for CO2 desorption, the method comprising the steps of: mixing a solution comprising a basic precipitating agent into a solution comprising a Zr source, thereby preparing a synthesis solution; and,precipitating the Zr-based catalyst from the synthesis solution, thereby obtaining a precipitated Zr-based catalyst;

17. The method according to claim 16, wherein the Zr-based catalyst comprises ZrO(OH)2, and wherein the ratio of acidic OH to basic OH of the Zr-based catalyst is at least 0.1 and at most 1.3.

18. The method according to claim 16, wherein the precipitated Zr-based catalyst comprises at least 95% by weight ZrO(OH)2 compared to the total weight of the precipitated Zr-based catalyst.

19. The method according to claim 16, wherein the basic precipitating agent is a hydroxide; and/or wherein the Zr source is ZrO(NO3)2.

20. The method according to claim 16, further comprising the additional step of: calcining the precipitated Zr-based catalyst at a calcination temperature Tc;

21. A Zr-based catalyst for CO2 desorption, prepared using the method according to claim 16, wherein the O:Zr atomic ratio is at least 2.1; preferably wherein the Zr-based catalyst comprises ZrO(OH)2; and wherein the Zr-based catalyst comprises at least 90% by weight ZrO(OH)2 compared to the total weight of the Zr-based catalyst.

22. The Zr-based catalyst according to claim 21, wherein the ratio of acidic OH to basic OH of the Zr-based catalyst is at least 0.1 and at most 1.3.

23. The Zr-based catalyst according to claim 21, wherein the Zr-based catalyst has a surface charge Zeta potential of at least −25 mV, preferably at least −5 mV.

24. The Zr-based catalyst according to claim 21, wherein the Zr-based catalyst comprises other metals selected from the group comprising: Hf, Ce, or Zn; preferably wherein the purity of Zr compared to the other metals in the Zr-based catalyst is at least 90%.

25. A process for CO2 desorption from an amine solvent, the process comprising the steps of: providing an CO2-containing amine solution comprising CO2 absorbed in an amine solvent;supplying a Zr-based catalyst according to claim 21 to the CO2-containing amine solution;heating the amine solution comprising the Zr-based catalyst to a desorption temperature Td; and,desorbing CO2 from the amine solution comprising the Zr-based catalyst during a residence time.

26. The process according to claim 25, wherein the Zr-based catalyst acts as a non-solubilized heterogeneous catalyst.

27. The process according to claim 25, wherein the amine solvent comprises monoethanolamine (MEA) or 2-amino-2-methyl-1-propanol (AMP), preferably monoethanolamine (MEA).

28. The process according to claim 25, wherein the residence time is at least 4 h and at most 168 h, preferably at least 20 h and at most 36 h.

29. A process for CO2 absorption and desorption, comprising the steps of: absorbing CO2 in an amine solvent, thereby obtaining a CO2-containing amine solution;and,desorbing CO2 from the CO2-containing amine solution using the process of claim 25, thereby regenerating the amine solvent.

30. Use of a Zr-based catalyst in the process according to claim 25.