**2. Results and Discussion**

Referring to our previous work [56], monodentate N-heterocyclic compounds are potentially a new family of stabilizing agents that could be a starting point for design new catalytically active nanoparticles with higher catalytic e fficiency. In this work, the catalytic activity of PdNPs is compared with e ffectiveness of catalytic system based on Pd(II) complexes in the carbonylation of aniline (AN) to N,N--diphenylurea (DPU) by CO2/CO/O2 mixture. Based on our previous results, we proposed the mechanism of AN carbonylation by CO/O2 in the presence PdCl2(XnPy)2 complexes (where: Py = pyridine, X = -Cl or -CH3, *n* = 0–2), with Pd(II) reduced to Pd(0) in situ in the catalytic cycle, see Scheme 1. Partial precipitation of inactive Pdblack reported by Ragaini [57] is one of the proofs of the Pd<sup>0</sup> presence in the system, further supported by our isolation of palladium black precipitated during the reaction of PdCl2(PhNH2)2 complex with carbon monoxide [15]. The next step is reoxidation of Pd(0) to Pd(II) and in this cycle both molecular oxygen and iodine are supposed to be potential oxidants responsible for recycling Pd(II) from Pd(0), step 1a–1b. Although oxidation of Pd(0) by oxygen is possible, it is very slow [58]. Alternatively, Pd(0) in 1a may be oxidized during the oxidative addition of I2 to Pd(0), according to the equation: Pd(0) + I2 → PdI2. Then, HI (instead of water) is released and this HI is oxidized by molecular oxygen: 4HI + O2 → 2I2 + 2H2O [1]. The intermediate 1b is able to coordinate aniline with subsequent insertion of CO to NH-Pd bond in 1c, creating a new carbon-nitrogen bond, intermediate 1d reductive elimination gives N,N--diphenylurea, generating Pd(0) species [15]. According to the proposed mechanism presented in Scheme 1, Pd(0) stabilized by pyridine ligands plays a crucial role as an acceptor of oxidizing agen<sup>t</sup> (O2 or I2). In order to verify the hypothesis on the participation of Pd(0) in the catalytic cycle, we planned experiments with Pd(0) nanoparticles (PdNPs), and the results (conversion, selectivity, and yield of carbonylation) were compared with the same parameters for process catalyzed by Pd(II) complexes. Prior to that, we searched for the optimal conditions to make both processes, catalyzed by PdNPs and Pd(II) complexes, comparable.

**Scheme 1.** Proposed path of the carbonylation of aniline by CO/O2 catalyzed by PdCl2(XnPy)2 complexes.

### *2.1. Optimization of Reaction Conditions of AN Carbonylation with CO2*/*CO*/*O2 Mixture*

Conversion of aniline, selectivity towards DPU, and TOF for DPU are presented in Table 1, indicating a strong correlation between the rate of reaction and the amount of CO2 used. Relatively high conversion and selectivity are observed when no CO2 is loaded into the system (entry 5, Table 1). However, even a moderate addition of CO2 is associated with the increasing rate of reaction, and its optimal amount in the gaseous mixture is ca. 50% (entry 4). High yield of DPU (96%) was also observed by Gabriele et al. for carbonylation of aniline performed in the presence of CO2 and a different Pd-based catalytic system. Authors applied a very simple catalyst (K2PdI4), without any co-catalyst and additives; however, a long reaction time (24–72 h) was required [44]. Figure 1 shows that conversion of aniline and selectivity towards DPU decrease drastically when more than 50% of CO2 is introduced. Eventually, no formation of DPU is observed at 100% fraction of CO2 (entry 1, Table 1). Investigation of the impact of CO/CO2 ratio was complemented by additional experiment in which carbonylation of aniline under conditions from entry 4 in Table 1 was conducted in the presence of Ar/CO/O2 instead of CO2/CO/O2 mixture, and no difference in DPU yield between the two cases was observed. Obtained results sugges<sup>t</sup> that replacing CO with certain amount of CO2 allows for achieving higher yields of DPU, although, as indicated by the experiment with argon, CO2 itself does not seem to act as a carbonylating agent. We propose two possible explanations for the beneficial effect of CO2. First, it is likely that the observed optimal CO/CO2 ratio is a result of the balance between enhancement of mass transfer by CO2 (as commonly observed in GXLs) and minimal amount of CO necessary for efficient carbonylation of aniline. In the presence of 88% of CO2 content, the amount of CO (12%) is lower than required according to stoichiometry, and therefore, unsurprisingly, is associated with lower conversion and TOF values. On the other hand, the presence of CO2 might prevent (inhibit) possible side reactions, e.g., oxidation of CO to CO2 (which to some extent may occur in the presence of palladium catalyst). Unless the necessary amount of CO is provided in the system, no DPU is produced and such results confirm that CO2 is not a source of carbonyl group in this reaction. However, we performed further processes in the presence of CO2 as an additional gas due to its economic benefits (CO2 is a natural waste in CO production).


**Table 1.** Parameters obtained for the carbonylation a of aniline with CO/CO2/O2 mixture, catalyzed by Pd(II) complexes: conversion (CAN), selectivity (SDPU) and turnover frequency (TOFDPU) of the catalyst, depending on the contents of CO2 in CO2/CO, iodine, and iron.

a Reaction conditions unless stated otherwise: PdCl2(2,4Cl2Py)2 = 0.056 mmoL, AN = 54 mmoL, ethanol = 20 mL, 3.8 MPa CO + CO2, 0.6 MPa O2, (100 ◦C, 60 min, Py = pyridine, AN = aniline, DPU = N,N-diphenylurea. b For CO/O2: (0.6 MPa O2 and 3.8 MPa CO) the molar ratio CO:O2 = 6:1 ca. For CO/O2/CO2 (0.6 MPa O2, 1.7 MPa CO, 2.1 MPa CO2) the molar ratio CO:O2 = 3:1 ca. c Selectivity toward DPU expressed as (mmoL DPU) × (mmoL converted AN)−<sup>1</sup> [%]. d TOFDPU (turnover frequency for AN) = [mmoL of AN reacted selectively to DPU] × [mmoL of Pd(II) complex used]−<sup>1</sup> × h−1, e PdCl2Py2 used instead of PdCl2(2,4Cl2Py)2. f 0.6 mL of 2,4-Cl2Py added.

**Figure 1.** Effect of percentage of CO2 in CO2/CO mixture ( **A**) and amount of iodine (**B**) on the conversion (Conv.) and yield and selectivity (Select.) of the catalyst. For the reaction conditions see footnotes in Table 1.

The substituent e ffect in the pyridine ring was investigated for the optimized content of CO2 in CO2/CO mixture (i.e., 56%). On the basis of obtained results, there is no consistent trend in the e ffect of derivatives of pyridine in PdCl2 complexes on the rate of reaction (Figure S1 in Supplementary Material). The slightly higher yield of DPU (comparing to other Pd-based complexes) is noticed for PdCl2(2,4-Cl2Py)2 complex, and most further studies in this area are performed in the presence of this complex.

The e ffect of iodine on the rate of carbonylation was also investigated. As shown in Table 1, when no iodine is used, the desired reaction does not proceed, regardless of the amount of iron present in the system (entries 6–7, Table 1). Increasing amount of iodine results in higher conversion, selectivity and TOF values, with the maximum at 0.04–0.12 mmoL of I2 (entry 9 and 10, Table 1). This observation, in agreemen<sup>t</sup> with previous reports [1,59–62], indicates that iodine might play various crucial roles such as: (i) recovery of the catalytic system, perhaps by oxidation of iron powder to iron(II), (ii) generation of palladium complexes [Pd(CO)3I]<sup>−</sup>, considered to be the catalytically active species, (iii) reoxidation of Pd(0) to Pd(II). However, if a large excess of iodine is used, the yield of DPU decreases, and the TOF value decreases (entry 11, Table 1). This effect may be attributed to undesirable side reactions, such as formation of insoluble anilinium iodide, which limits the amount of free aniline in the system [60,63]. Moreover, excess of iodide ions may ge<sup>t</sup> coordinated to Pd(II), effectively competing with other reagents, which is commonly referred to as catalyst poisoning [10,60,64].

The effect of iron on the rate of reaction is minor (entries 12–16, Table 1). Although a small addition of iron to the mixture seems to increase the conversion of aniline and selectivity towards DPU (see entries 12 and 13, Table 1), it is not a significant change. As more iron is introduced to the system, conversion of aniline decreases slightly (entry 14, Table 1). These observations indicate that a certain amount of iron is beneficial and it slightly increases TOF values during DPU formation. In agreemen<sup>t</sup> with our report [15], Fe(0) is oxidized by I2 to Fe(II) and the possible role of Fe(II) is to react with Pdblack in order to return Pdblack to the catalytic cycle as shown in Scheme 1. In the literature, we can find reports for [1] and against [65] the suggested role of iron. Lower conversion of aniline, when excess of iron is used, can be attributed to the reaction between iron and oxygen, which in turn decreases the amount of necessary oxidizing agen<sup>t</sup> (O2). Both reactions of metallic iron, with iodine and with O2, occur easily [66]. Our previous research indicates that even traces of iron (from stainless steel reactor and stirring element) are kinetically significant.

The reaction rate seems to be strongly dependent on the temperature settings selected (see Table 2): at 80 ◦C the reaction proceeds only to some extent (entry 1). Optimal value for the synthesis of DPU is 100–120 ◦C (entry 2 and 3) with even higher temperature (140 ◦C) leading to formation of EPC (entry 4, Table 2, selectivity and TOF values for EPC are placed in parentheses). These results sugges<sup>t</sup> that, after achieving the activation parameters suitable for the formation of DPU, further increase of temperature does not enhance the catalyst activity and may even have a slightly negative impact on the conversion of aniline, possibly due to occurrence of side reactions such as formation of N-ethylaniline, 2-methylquinoline, polyaniline, and EPC (entry 4, Table 2). Further studies were conducted at 100 ◦C because one of our aims was to operate at desirable energy-saving conditions, i.e., at the lowest temperature allowing formation of satisfactory amount of DPU. The temperature of 100 ◦C was also chosen for other practical reasons—it was the most appropriate temperature for comparative tests (high conversion and selectivity of Pd(II) complex were observed at this temperature during our previous studies reported in [15]). Although NPs might be more active at 120 ◦C than at 100 ◦C, the goal of this work was not to achieve the highest possible activity of catalyst but to study and compare activity of various types of pre-catalysts at the same temperature.


**Table 2.** Parameters obtained for the carbonylation of aniline by CO/CO2/O2 catalyzed by PdCl2(2,4-Cl2Py)2: conversion (CAN), selectivity (SDPU) and turnover frequency (TOFDPU), depending on the temperature a.

a Reaction conditions unless stated otherwise: PdCl2(2,4Cl2Py)2 = 0.056 mmoL, I2 = 0.04 mmoL, AN = 54 mmoL, ethanol = 20 mL, 3.8 MPa CO + CO2, 0.6 MPa O2, 100 ◦C, 60 min, Py = pyridine, AN = aniline, DPU = N,N-diphenylurea. b Selectivity toward DPU expressed as (mmoL DPU) × (mmoL converted AN)−<sup>1</sup> [%]. c TOFDPU (turnover frequency for AN) = [mmoL of AN reacted selectively to DPU] × [mmoL of PdCl2(2,4ClPy)2]−<sup>1</sup> × h−1. d Selectivity toward EPC (ethyl N-phenylcarbamate), e TOFEPC = (mmoL of EPC formed)(mmoL of PdCl2(2,4-Cl2Py) used)−1h−1.
