Synthesis of Propiolic and Butynedioic Acids via Carboxylation of CaC 2 by CO 2 under Mild Conditions

: Carbon dioxide (CO 2 ) is a greenhouse gas, and its resource use is vital for carbon reduction and neutrality. Herein, the nucleophilic addition reaction of calcium carbide (CaC 2 ) to CO 2 was studied for the first time to synthesize propiolic and butynedioic acids by using CuI or AgNO 3 as catalyst, Na 2 CO 3 as additive, and triphenylphosphine as ligand in the presence/absence of a hydrogen donor. The effects of the experimental conditions and intensification approach on the reaction were investigated. The reactivity of CaC 2 is closely associated with its synergistic activation by the catalysts, solvent, and external intensification, such as the ultrasound and mechanical force. Ultrasound helps to promote the reaction by enhancing the interfacial mass transfer of CaC 2 particulates. Mechanochemistry can effectively promote the reaction, yielding 29.8% of butynedioic acid and 74.8% of propiolic acid after 2 h ball milling at 150 rpm, arising from the effective micronization and interfacial renewal of calcium carbide. The present study sheds a light on the high-value uses of CO 2 and CaC 2 and is of reference significance for the nucleophilic reaction of CaC 2 with other carbonyl compounds.


Introduction
CO 2 is a greenhouse gas, and its high-value utilization is vital for the global "carbon peak and neutrality".CO 2 is stable with little reactivity except for its acid-base reaction.Usually, it only reacts with high-energy reagents, e.g., hydrogen [1,2], ethylene oxide, Grignard reagents, alkyl metals, and metal hydrides under appropriate catalysis.Therefore, its resource utilization is challenging.At present, CO 2 is mainly used for the synthesis of methanol [3,4] and alkanes [5] through catalytic hydrogenation, or carbonates and polycarbonates [6,7] via a reaction with ethylene oxide.CO 2 is a carboxylation reagent with atom economy for terminal alkynes to prepare alkynyl carboxylic acids [8,9].These reactions, however, are of low efficiency and require specified catalysts, such as Ag [10], Au [11], Cu [12], Ni [13], Mg [14], Mo [15], or rare earth metals [16].Recently, a terminal alkyne carboxylation with CO 2 has been reported [17] under alkaline conditions.
CaC 2 is an important coal chemical with an annual production of over 30 million tons in China.It is mainly used for the production of acetylene and its derivatives [18], such as vinyl chloride [19,20], vinyl acetate [21,22], tetrachloroethylene, trichloroethylene, 1,4-butanediol, acetylene black, etc.The industrial application of acetylene has been being replaced by petroleum chemicals with the rapid development of the oil-refinery industry in the past decades.The calcium-carbide industry is confronted with a survival crisis due to the lack of high-value downstream products.Thus, it is necessary to explore the new reactions of calcium carbide to promote the development of calcium carbide chemistry [23].In fact, calcium carbide is the cheapest industrial carbanion resource, and its alkyne anion can be used as a nucleophilic reagent to react with various chemicals with carbonium characteristics, such as carbonyl and halogenated compounds [24,25].Even if the Grignard reagents or alkyl metals can be partly replaced by calcium carbide, a technology innovation and revival of the calcium-carbide industry may be expected.Under the context of the high-value utilization of CaC 2 and the resource uses of CO 2 for carbon reduction and neutrality, we speculated the potential nucleophilic reactivity of CaC 2 with CO 2 to form the calcium salts of butynedioic or propionic acids.Further, the reaction intensification by ball mill and ultrasound was studied.This study provides a novel synthesis of propiolic and butynedioic acids at mild conditions, which is of significance for the resource utilization of CO 2 and the development of calcium-carbide chemistry and industry.

Results and Discussion
2.1.Preparation of Butynedioic Acid 2.1.1.Thermal Reaction Yield versus Time Thermal reaction was conducted in an oxygen bomb reactor at 333 K and CaC 2 :CO 2 ≈ 1:6 (mole ratio) under magnetic stirring.The reaction is assumed as a nucleophilic addition of the carbanion (C 2 2− ) to CO 2 , forming calcium acetylenedicarboxylate, which is converted to butynedioic acid via acidification by hydrochloric acid via Equations ( 1) and (2).As shown in Figure 1a, the reaction is very slow, yielding only 0.65% of butynedioic acid after 36 h reaction.Additionally, the yield increases gradually with time, reaching 6.35% after 84 h reaction.The slow reaction may be ascribed to the insolubility of CaC 2 in acetonitrile and its restricted interfacial reaction with CO 2 .Moreover, the resultant calcium butynedioate is also insoluble and remains on the CaC 2 surface, which hindered the interfacial mass transfer and reaction of CO 2 .As an ionic crystal with high lattice energy, CaC 2 is insoluble in all organic solvents at ambient conditions.Thus, mass transfer is the controlling step here, and appropriate process intensification is vital via, e.g., ultrasound irradiation or ball mil.
CaC 2 +2CO 2 = Ca(OOC-C ≡ C-COO) ( 1) by adding Na2CO3.Apparently, both Na2CO3 and PPh3 can enhance the reactivity of CaC2 to varying degrees.In the presence of AgNO3-PPh3-Na2CO3, the product yield increased steadily with time from 1.0% (6 h) to 4.7% (8 h) and 13.3% (12 h), which far surpass the yield of thermal reaction after 84 h (6.4%).Overall, the ultrasonic intensification effect is limited here.This is because the ultrasonic cavitation and micro-jet are unable to break the crystal structure of CaC2 but, instead, promote the interfacial mass transfer.Adversely, the ultrasonic degassing effect is prone to decrease gas dissolution and reactivity.

Mechanochemical Intensification of the Reaction
Mechanical milling helps to reduce the particle size, increase the surface area, promote the interfacial renewal and mass transfer, and facilitate the reaction of calcium carbide [28].The reaction result under different ball mill conditions is presented in Table 2 and Figure 1b.The yield is only 0.5% after 6 h reaction at 50 rpm (Table 2), indicating a weak mechanochemical effect at a low rotation rate.The yield increases rapidly to 28.0% with the increased rotation rate from 50 to 150 rpm, which demonstrates the significant role of the rotation rate or the mechanical energy intensity on the reaction.This may be ascribed to the greatly enhanced shear stress and colliding pressure on the tiny CaC2 granules and the surface renewal with the increased rotation rate [29][30][31].However, the product yield decreased slightly from 28.0% to 25.1% as the rotation rate further increased to 200 rpm; meanwhile, the yield at 6 h (25.1%) is even lower than that at 2 h (27.0%).This may be attributed to the side reactions after a longer reaction time and higher mechanical energy intensity, i.e., the excessive nucleophilic substitution of CaC2 to CO2 or the carboxylic products (calcium acetylenedicarboxylate or calcium propiolicate), forming alkynyl carbon materials [32].

Ultrasonic Intensification of the Reaction
Ultrasonic intensification arises from the cavitation and micro-jet of ultrasound, which promotes the interfacial mass transfer and reactions [26,27].Thus, the reaction of calcium carbide and carbon dioxide was conducted in an oxygen-bomb reactor placed in an ultrasonic cleaner (200 W, 40 kHz) for 6-12 h at room temperature.In addition, the effects of the AgNO 3 catalyst, PPh 3 complexing agent, and Na 2 CO 3 additive on the reaction were investigated.Herein, Na 2 CO 3 was used to activate CaC 2 via their possible metathesis, forming CaCO 3 and sodium acetylide (Na 2 C 2 ) at least partly, along with the nanorization of CaC 2 with the increased interface and accessibility of the carbanions.AgNO 3 is prone to react with CaC 2 to form silver acetylide (Ag 2 C 2 ) due to their favorite Lewis soft acid-soft base interaction.Additionally, Ag 2 C 2 tends to complex with PPh 3 , which helps its dissolution and homogeneous reaction with the dissolved CO 2 .
As seen from Table 1, 1.6% of yield is achieved after 12 h reaction in the presence of AgNO 3 , which is much higher than that after 36 h stirring reaction without AgNO 3 (0.65%).Adding PPh 3 can further enhance the yield to 2.1%, which is superior to the yield of 1.9% by adding Na 2 CO 3 .Apparently, both Na 2 CO 3 and PPh 3 can enhance the reactivity of CaC 2 to varying degrees.In the presence of AgNO 3 -PPh 3 -Na 2 CO 3 , the product yield increased steadily with time from 1.0% (6 h) to 4.7% (8 h) and 13.3% (12 h), which far surpass the yield of thermal reaction after 84 h (6.4%).Overall, the ultrasonic intensification effect is limited here.This is because the ultrasonic cavitation and micro-jet are unable to break the crystal structure of CaC 2 but, instead, promote the interfacial mass transfer.Adversely, the ultrasonic degassing effect is prone to decrease gas dissolution and reactivity.

Mechanochemical Intensification of the Reaction
Mechanical milling helps to reduce the particle size, increase the surface area, promote the interfacial renewal and mass transfer, and facilitate the reaction of calcium carbide [28].The reaction result under different ball mill conditions is presented in Table 2 and Figure 1b.The yield is only 0.5% after 6 h reaction at 50 rpm (Table 2), indicating a weak mechanochemical effect at a low rotation rate.The yield increases rapidly to 28.0% with the increased rotation rate from 50 to 150 rpm, which demonstrates the significant role of the rotation rate or the mechanical energy intensity on the reaction.This may be ascribed to the greatly enhanced shear stress and colliding pressure on the tiny CaC 2 granules and the surface renewal with the increased rotation rate [29][30][31].However, the product yield decreased slightly from 28.0% to 25.1% as the rotation rate further increased to 200 rpm; meanwhile, the yield at 6 h (25.1%) is even lower than that at 2 h (27.0%).This may be attributed to the side reactions after a longer reaction time and higher mechanical energy intensity, i.e., the excessive nucleophilic substitution of CaC 2 to CO 2 or the carboxylic products (calcium acetylenedicarboxylate or calcium propiolicate), forming alkynyl carbon materials [32].Both Na 2 CO 3 and triphenylphosphine could promote the reaction, as shown from the results in arrays 8-10.Herein, Na 2 CO 3 helps to convert CaC 2 to amorphous Na 2 C 2 and CaCO 3 at least partly through their metathesis due to the high electrostatic affinity between Ca 2+ and CO 3 2− , and triphenylphosphine, as a ligand, is prone to complex with silver acetylide, facilitating its dissolution and reactivity in acetonitrile.
The 13 CNMR and 1 HNMR spectra of the product is presented in Figure 2a and Figure S1, respectively.In the 13 CNMR spectrum, the peaks at 156.74 ppm and 75.66 ppm are attributed to carboxyl and alkynyl carbons, respectively, and the peaks in the range of 120-140 ppm are attributed to the aromatic carbon of triphenylphosphine as an impurity of the product.As shown in Figure S1, there is a sharp proton peak at 4.71 ppm for water, while the carboxyl proton is invisible.This may be ascribed to the fast proton exchange between carboxyl group and water; thus, the carboxyl proton is usually hard to detect in 1 HNMR.The weak signals within 7.46-8.09ppm belong to the proton of the benzene ring of PPh 3 , being consistent with the 13 CNMR spectrum.The NMR analysis confirmed the sole product of butynedioic acid.This suggests the possibility of preparing butynedioic acid via the carboxylation of CaC 2 by CO 2 , but the reaction conditions need to be optimized further in terms of the solvent, catalyst, additives, and appropriate activation modes of calcium carbide.
Both Na2CO3 and triphenylphosphine could promote the reaction, as shown from the results in arrays 8-10.Herein, Na2CO3 helps to convert CaC2 to amorphous Na2C2 and CaCO3 at least partly through their metathesis due to the high electrostatic affinity between Ca 2+ and CO3 2-, and triphenylphosphine, as a ligand, is prone to complex with silver acetylide, facilitating its dissolution and reactivity in acetonitrile.
The 13 CNMR and 1 HNMR spectra of the product is presented in Figure 2a and Figure S1, respectively.In the 13 CNMR spectrum, the peaks at 156.74 ppm and 75.66 ppm are attributed to carboxyl and alkynyl carbons, respectively, and the peaks in the range of 120-140 ppm are attributed to the aromatic carbon of triphenylphosphine as an impurity of the product.As shown in Figure S1, there is a sharp proton peak at 4.71 ppm for water, while the carboxyl proton is invisible.This may be ascribed to the fast proton exchange between carboxyl group and water; thus, the carboxyl proton is usually hard to detect in 1 HNMR.The weak signals within 7.46-8.09ppm belong to the proton of the benzene ring of PPh3, being consistent with the 13 CNMR spectrum.The NMR analysis confirmed the sole product of butynedioic acid.This suggests the possibility of preparing butynedioic acid via the carboxylation of CaC2 by CO2, but the reaction conditions need to be optimized further in terms of the solvent, catalyst, additives, and appropriate activation modes of calcium carbide.Figure 13 CNMR spectra of the product butynedioic acid (a) and propiolic acid (b) using phenolic resin as hydrogen donor.

Preparation of Propiolicic Acid
By adding the appropriate hydrogen donor HA to the above reaction system, the calcium acetylide (ACaC≡CH) intermediate may be formed via protonation of CaC 2 .
Additionally, calcium propiolicate is synthesized via the nucleophilic addition of the carbanion HC≡C − to CO 2 and then converted to propiolicic acid via acidification by hydrochloric acid, as shown below.

The Reaction Intensification by Ultrasound and Mechanochemistry
The reaction result under thermal, ultrasonic, and ball mill conditions is compared in Table 3.Firstly, 40.2% of propiolicic acid yield is achieved after 24 h thermochemical reaction at 333 K in an oxygen bomb reactor filled with CaC 2 , phenolic resin, as the hydrogen donor, acetonitrile, catalysts (CuI, Na 2 CO 3 ), and CO 2 gas at 1 MPa.The reaction is highly selective to propiolicic acid, as justified by the liquid chromatographic analysis.When the same reaction was conducted in an ultrasonic cleaner (200 W, 40 kHz), 31.5% of yield is achieved after 6 h reaction at room temperature and other fixed conditions.Clearly, ultrasound can promote the reaction.For the mechanochemical reaction in planetary ball mill at 298 K, 74.2% of yield was achieved after 6 h reaction under the above proportioned materials.Obviously, ball milling is more efficient than thermal and ultrasonic chemical reactions, with a significantly shortened reaction time and increased product yield.Reaction conditions: (1) CH 3 CN 10 mL, 1 g CaC 2 , CO 2 : CaC 2 = 8 (mole ratio), 0.37 g phenolic resin, 0.03 g CuI, 0.37 g Na 2 CO 3 , 300 mL oxygen bomb reactor; (2) the same reactor as (1) is placed in an ultrasonic cleaner; (3) CH 3 CN 15 mL, 1 g CaC 2 , CO 2 : CaC 2 > 6 (mole ratio), 0.03 g CuI, 0.37 g phenolic resin, 0.37 g Na 2 CO 3 , 375 g S.S balls, 250 mL jar, planetary ball mill.

The Reaction Yield at Different Mechanochemical Conditions
The effect of milling rate on the yield of propiolicic acid was studied.The reaction is found to be negligible after 2 h ball mill at 50 rpm.This is because the rotation rate is too low to lead to an efficient miniaturization of solid reagents and catalysts and effective collision with CO 2 in acetonitrile solvent.As shown in Table 4, the yield of propiolicic acid increased notably with the rising rotation rate in the range of 50~150 rpm, and 74.8% of yield is obtained after 2 h milling at 150 rpm.Further increasing the milling rate to 200 rpm even decreases the yield slightly to 72.9%.This may be ascribed to the partial carbonation [32] of CaC 2 by CO 2 and propiolicate at high-mechanical-energy intensity.Thus, milling rate has a significant effect on the reaction, and a higher rate helps to destroy the lattice structure of CaC 2 , increase the colliding pressure, frequency, and reaction rate.It is noteworthy that the yield is almost unchanged after 2 h milling.This is because the initial reaction rate is quite high due to the high exposure of acetylide sites for the nucleophilic reaction, and the reaction decreases greatly due to the depletion and coverage of the reactive sites by the solid calcium salt product.

Influence of Hydrogen Donor and Solvents on the Reaction
The phenolic resin used is hard to reclaim from the solid mixture.In order to make the process economical and environmentally friendly, a reusable hydrogen donor needs to be explored.The reaction effects of some hydrogen donors were evaluated at 150 rpm ball mill conditions, including t-butanol, phenol, imidazole, NaHCO 3 , NH 4 HCO 3 , acetylene, acetone, and CH 3 CN.The results are shown in Table 5.As seen in Table 5, the yield of propiolic acid is low (10~26%) when imidazole, acetylene, acetone, and acetonitrile are used as the hydrogen donor due to their higher pKa (14~25), low acidity, or proton-donating ability.The product yield is also not high (15~24%) when NaHCO 3 and NH 4 HCO 3 are used even though they have comparable acidity (pKa = 9.4~10.4)with phenol.This may be ascribed to their sparse solubility in the solvent.In contrast, the product yield is high (40-60%) when phenol or t-butanol is used, and t-butanol can be used as an alternative of phenolic resins due to its cheapness, ready availability, and recyclability.The 13 C and 1 H NMR spectra of the product with deuterium water solvent are shown in Figure 2b and Figure S2, respectively.
As seen in Figure 2b, three obvious peaks appeared, i.e., 74.77 ppm for the terminal carbon of acetylene, 77.01 ppm for the alkynyl carbon connected with the carboxyl group, and 157.64 ppm for the carboxylic acid carbon.In Figure S2, two distinct proton peaks are observed at 4.65 ppm for water and 3.27 ppm for terminal acetylene, and the proton of carboxylic acid is too active to be detectable.
To study the solvent effect, the yield of propiolicic acid in three aprotic polar solvents is compared.As shown in Table 5, the reaction yield in chloroform and BMIMCl is lower than that in acetonitrile at other similar conditions.For example, when t-butanol is used as the hydrogen donor, the yield of propiolicic acid in CH 3 CN, BMIMCl, and chloroform is 63.1%, 39.5%, and 1.05%, respectively, which demonstrates the superiority of the acetonitrile here.Further, CH 3 CN is a widely used solvent of nucleophilic reactions due to its strong polarity, non-protonic, low viscosity, ease availability, cost effectiveness, and rational volatility for recycled uses.Considering the solvent effect on the reactivity of calcium carbide [33], this reaction in other solvents is to be explored in the future.

Materials and Equipment
Analytical-grade chemicals were purchased from varying companies, including CaCO 3 , Na 2 CO 3 , CuI, AgNO 3 , CO 2 , triphenylphosphine, phenolic resin, acetylene, and 1-butyl-3methyl imidazolium chloride (BMIMCl), etc., and used as received.The detailed information of these chemicals is provided in Table S1 of the Supplementary Materials.
The equipment used mainly include a planetary ball mill, oxygen bomb reactor, ultrasound cleaner, rotary evaporator, high-speed centrifuge, high-performance liquid chromatograph(HPLC), GC-MS, NMR spectrometer, etc., and their detailed information is provided in Table S2 of the Supplementary Materials.

Experimental Methods
For the thermochemical reaction, a specified amount of CaC 2 powder (400 mesh), Na 2 CO 3 , acetonitrile, triphenylphosphine, and AgNO 3 /CuI was added to the oxygen bomb reactor (hydrogen donor was added for the preparation of propiolic acid) and then charged with 0.6 MPa CO 2 and stirred magnetically for a period of time at a set temperature.For the ultrasonic reaction, the above oxygen bomb reactor was placed in an ultrasound cleaner (22.5 L, 200 W, 40 kHz, size 500 × 300 × 150 mm) at room temperature, and its temperature was controlled by the circulating tap water.For the mechanochemical reaction, the above chemicals were added to the milling jar, sealed and charged with CO 2 to 0.15 MPa.The jar was fixed on a planetary ball mill, run for a period of time at a set rotation rate, and recharged CO 2 every 2 h to remain the gas pressure.After the reaction, acetonitrile was removed by rotary evaporator at a reduced pressure, and the solid was acidified with dilute hydrochloric acid to pH = 3, and then vacuum dried to get a solid mixture.The as-obtained solid was dissolved in acetonitrile and filtrated, and the filtrate was analyzed by HPLC.
The liquid samples were diluted with acetonitrile and analyzed by HPLC under the following conditions: C18 column, UV detector at 220 nm, and methanol aqueous (MeOH: H 2 O = 6:4, v/v) as mobile phase.The column was first purged for 30 min with 1.0 mL/min of methanol, followed by 1.0 mL/min of methanol aqueous solution until the baseline was stable.The injection volume was 2-3 times of the quantitative loop (20 µL).The mobile phase was ultrasonically degassed for 30 min before use, and the test samples were filtered by 0.45 µm membrane.The liquid sample was evaporated, and the residue solid was dissolved in D 2 O reagent for 1 HNMR and 13 CNMR tests.
The yield of propiolic and butynedioic acids is obtained based on the usage of CaC 2 and their corresponding amount formed, i.e., the mole ratio of the acid product formed and the CaC 2 added, as CO 2 is excessive in the reaction system.The content of propiolic acid and butynedioic acid in acetonitrile solution is determined by HPLC along with the pre-established standard curves (Figures S3 and S4).The detailed analytical procedure and the standard curves are provided in the Supplementary Materials.

Figure 2 .
Figure2.13 CNMR spectra of the product butynedioic acid (a) and propiolic acid (b) using phenolic resin as hydrogen donor.

Table 1 .
Effect of ultrasonic reaction conditions on the yield of butynedioic acid.

Table 1 .
Effect of ultrasonic reaction conditions on the yield of butynedioic acid.

Table 2 .
Yield under different ball mill conditions for the reaction of CaC2 and CO2.

Table 3 .
Reaction results at different conditions and intensification approaches.

Table 4 .
Mechanical reaction of CaC 2 and CO 2 under different conditions.

Table 5 .
Yield of propiolic acid under different conditions and hydrogen donors.