2.1. Characterization of the Samples
The morphology of the pristine rCBs (i.e., rCB_1 obtained through periodic pyrolysis and rCB_2 being a product of so-called Molten technology) is displayed in
Figure 1. For the sake of comparison, SEM images of a commercial carbon black (CB) are also shown (
Figure S1 in Supplementary Material).
In
Figure S1a,b, it can be observed that a commercial sample displayed characteristic spherical primary particles, typical of carbon blacks, fused together to form aggregates and agglomerates of irregular shapes and considerable sizes. Additionally, some free spaces and channels were visible in the material structure, giving the sample morphology a resemblance to a coral reef.
At first glance, the morphological features of rCB_1 and rCB_2 differed significantly from those of the commercial CB.
Figure 1a, representing rCB_1, and
Figure 1b, representing rCB_2, show that the obtained tire-derived samples consisted of compact particles of several micrometers and oval or irregular shapes.
The larger particle sizes were observed in the case of rCB_2 produced through continuous pyrolysis. This sample also presented a more homogeneous and “dense” particle surface. On the other hand, in the case of rCB_1 obtained in a batch process, the particle structure resembled that of a cauliflower, with a surface roughness morphology.
Higher-magnification SEM images presented in
Figure 1c (representing rCB_1) and
Figure 1d (representing rCB_2) exhibited that the produced recovered carbon blacks showed some structural similarity to commercial CB, as tiny spherical primary particles were visible in both rCB_1 and rCB_2 samples. Interestingly, some brighter spots, attributed to the presence of inorganic compounds [
50], were also observed in the images. The primary particles formed a three-dimensional network of aggregates, with some inner spaces between the formed structures in the case of rCB_1 produced in batch pyrolysis (
Figure 1c). On the other hand, the continuous Molten technology was observed to result in the formation of more aggregated particles and solid surfaces (
Figure 1d).
To verify the influence of the acidic treatment on the rCB morphology, scanning electron microscopy was employed to investigate rCB_1-SA and rCB_2-SA. The obtained SEM images of the modified carbons are presented in
Figure 1d,f. As observed, the acidic treatment affected the morphology of the samples. For rCB_1 SA, the particle surface remained rough but appeared more compact compared to rCB_1. Furthermore, the micrograph of the modified rCB_1 carbon showed a lower number of bright spots (assigned to the presence of inorganic matter [
50]) compared with the image of the raw material. This can suggest partial removal of inorganic matter from rCB_1 due to the sample’s acidic treatment. For rCB_2-SA, the changes in sample morphology primarily involved reduced particle sizes compared to the unmodified carbon.
The morphology of the pristine rCB samples was confirmed by the TEM micrographs obtained for these materials, presented in
Figure 2. As can be observed, the produced rCBs were predominantly composed of nearly nano-sized spherical particles, each with a diameter of less than 100 nm, quite similar to those in commercial carbon blacks [
51]. However, it is important to note that there was a broad and varied particle size distribution within the obtained rCB samples.
The results of the elemental analysis of the rCB and CB samples are presented in
Table 1. As can be observed, both post-pyrolytic solids showed quite comparable compositions; however, they were different from that of commercial carbon black. The contents of carbon in rCBs were in the range of 75–80%, whereas a commercial CB sample contained 99.6% of C. Noticeably, rCBs also indicated quite a significant amount of S in their structures (as opposed to CB), i.e., 2.4–2.8%, which was due to the presence of sulfur in the tire feed material, coming from a vulcanization process. These S values were quite similar to those reported by other authors for the tired-derived rCBs [
52,
53]. Importantly, an essential parameter differing the obtained rCBs from the commercial carbon blacks was the quantity of ash; namely, rCB_1 and rCB_2 showed a considerable amount of ash (about 21–27%), whereas the commercial CB sample did not contain mineral matter. The presence of minerals and metals at the surface of the rCBs was also confirmed by the results of the microscopic studies using an EDX detector (
Figure S2 in Supplementary Material). The obtained data revealed that the surface of rCBs contained several inorganic elements, notably Zn and S, in substantial quantities. Furthermore, some amounts of Ca, Na, Fe, and other elements were also found. It should be noted, however, that some differences were observed in the surface composition of rCB samples prepared according to different pyrolysis procedures. Importantly, as shown in
Table 1, the total acidities measured for the commercial CB sample and rCB solids were close to 0 mmol H
+/g, indicating that these materials did not contain acidic groups on their surfaces (both S- and O-containing).
To induce surface acidity, the obtained rCBs as well as the commercial CB sample were functionalized with concentrated sulfuric acid. The results of elemental analysis, ash content determination, and total acidity measurements achieved for the initial and functionalized materials are depicted in
Table 1. As can be seen, the sulfonation procedure resulted in obtaining samples showing higher sulfur contents compared to the pristine commercial and recovered CBs, i.e., in the range of 1.0–3.8%. Importantly, a direct effect of the applied functionalization on the samples’ surface chemistry was quite a significant increase in their total acidities (A
tot; up to 0.76 mmol H
+/g), which was most likely due to the introduction of surface sulfonic groups to rCBs and CB. It cannot be excluded, however, that acidic oxygen moieties were also formed during the functionalization of samples, as reported earlier for the sulfonation of various carbon-type materials [
55,
56,
57]. In general, the obtained rCBs were more susceptible to modification with H
2SO
4, for which the A
tot values after the sulfonation were up to 0.76 mmol H
+/g compared to 0.30 mmol H
+/g for CB_SA. It is also worth noting that the use of concentrated sulfuric acid for sulfonation led to a significant decrease in the samples’ ash contents. Similar observations were also made by Cardona-Uribe et al. [
58].
The crystallographic structure of the produced rCBs was examined using an X-ray diffraction technique. The XRD pattern of the pristine samples in
Figure 3 revealed a broad signal at 2θ around 24° and a weak reflex at 2θ of about 43° (both indexed as ‘C’), typical for amorphous carbon materials [
59]. Furthermore, the obtained diffractograms confirmed the presence of several impurities in the samples, which is also in line with the results of the ash determinations (see
Table 1) and EDX analysis (see
Figure S2 in Supplementary Material). For both pristine rCBs, diffractions at ca. 2θ = 28, 36, 48, and 56° were probably related to the presence of ZnS (indexed as ‘ZnS’) and ZnO (indexed as ‘ZnO’) [
60,
61]. On the other hand, the reflection at ca. 2θ = 29° observed in the pristine rCB_1 and rCB_2 samples indicated the presence of CaCO
3. Additionally, the diffractogram of rCB_1 showed a strong reflection at 2θ equal to 26.6°, most likely associated with the presence of SiO
2 (indexed as ‘SiO
2’) [
59,
62,
63,
64]. All the compounds identified are added or formed in situ during tire manufacture.
The diffractograms of the sulfonated rCBs differed from those of the pristine materials. Following the acid treatment with sulfuric acid, the peaks associated with the crystalline phases vanished (the exception was the rCB_1 sulfonated sample where the reflection at 2θ of ~26° was still present), confirming that the H
2SO
4 acid treatment can effectively eliminate the minerals and other impurities originally present in the samples. This finding is also in accordance with the results of the decreased ash contents noted for the functionalized rCB samples (see
Table 1) and the SEM findings (
Figure 1).
Figure 4 presents the results of the thermogravimetric (TG) analysis performed for commercial and recovered carbon blacks, both pristine and after the acidic functionalization. As observed in
Figure 4a, commercial CB exhibited minimal weight changes throughout the TG measurement. In contrast, the initial rCB samples displayed quite noticeable weight decreases which started to be significant at about 400 °C. Finally, the rCB samples lost approximately 7% of their initial weight. According to the DTG results in
Figure 4a, the most remarkable weight losses were observed at about 580 °C and 680 °C, i.e., at temperatures higher than the temperatures of the sample preparation (see also
Section 3.1). These signals were probably due to the decomposition of some less thermally stable inorganic additives as well as to the further carbonization of a rubber-derived carbonaceous solid.
The TG and DTG plots obtained for the functionalized samples are shown in
Figure 4b.
As presented, the DTG patterns of the sulfonated CB and rCB samples showed peaks at the temperature <100 °C, attributed to the release of water, and broad signals with the maxima at about 220, 250, and 380 °C, which can be ascribed to the presence of sulfonic and oxygen functionalities [
57,
65]. The appearance of the latter indicates the effective introduction of functional groups onto the surface of investigated samples. The smallest changes were observed for the commercial CB, suggesting the lowest degree of functionalization of CB-SA amongst the prepared samples, which is also in accordance with the results of the S and O contents in this material presented in
Table 1. Interestingly, the signals originally present in the DTG patterns of the pristine rCBs in a temperature range of 400–700 °C were flatter in the DTG plots of the acid-treated materials. This can indicate a partial removal of mineral matter and impurities from rCBs and is also in line with the conclusions drawn from the ash determination, XRD, and SEM analyses.
To study the composition and the state of chemical species in the obtained rCBs, an X-ray photoelectron spectroscopy (XPS) analysis was performed.
The XPS survey spectra of a selected rCB sample (pristine rCB_1) are depicted in
Figure 5. Two clearly resolved C 1s and O 1s photoelectron peaks were observed. Furthermore, small photoelectron peaks originating from the presence of other atoms such as S, Si, and Zn were also reported. The presence of these elements on the surface of rCBs comes from an original recipe of tires and is also in accordance with the previous results (see the discussion on the ash, XRD, and SEM analyses).
The deconvoluted high-resolution XPS S 2p spectra of a selected sample before and after functionalization are shown in
Figure 6. As observed, different sulfur species were present on the surface of the post-pyrolytic rCB_1 solid, namely the peaks at lower BEs (162–165 eV region) can be ascribed to the reduced forms of S, such as inorganic sulfides (BE~162 eV) and aliphatic or aromatic sulfur (BE~164–165 eV) [
66,
67]. The sample also contained some amounts of S in an oxidized state, probably in the form of sulfates, as indicated by the presence of a peak at a BE~169.5 eV [
66]. Importantly, an intense signal at ca. 168 eV appeared in the S 2p spectrum of the functionalized rCB_1, suggesting the effective incorporation of -SO
3H groups on the surface of this sample. This also agrees well with the increased A
tot reported for rCB_1-SA compared to that of rCB_1 (see
Table 1). It is also clearly visible that some sulfur forms (the reduced ones; BE~162 eV) were leached from the prepared rCB_1 after its acidic treatment, which is consistent with the XRD data. The relative concentration of different sulfur species in the rCB_1 and rCB_1-SA is presented in
Figure 6. The incorporation of -SO
3H species onto the sample surface was also confirmed for rCB_2 (see
Figure S3 in Supplementary Material).
The high-resolution XPS C 1s spectra of the obtained rCB (raw and modified ones) are displayed in
Figure S4 in the Supplementary Material. As presented, the virgin samples showed profiles typical for carbon materials, with a main signal at 284.6 eV assigned to sp
2 and sp
3 carbon [
56]. Additionally, the deconvoluted spectra presented peaks at a BE of about 286.0 ± 0.2 eV, 287.4 ± 0.2 e, 289.5 ± 0.2 eV, and 290.5 ± 0.2 eV, typically ascribed to C-O, C=O, O-C=O, and pi-pi* transitions, respectively [
68]. It seems, however, that in our case, the peaks in the 286.0–289.5 eV region corresponded at least partially to C-S, C=S, and CO
32− moieties [
69,
70,
71], as the virgin rCBs contained quite a lot of sulfur and inorganic matter in their structures (see also
Table 1). Nevertheless, due to this complex composition of rCBs, the unequivocal assignment of the peaks is not straightforward. Further, as observed in
Figure S4a,c), there were some differences in the spectra of rCB_1 and rCB_2, suggesting that the method of pyrolysis affected the sample surface composition, e.g., batch pyrolysis yielded a material with a higher contribution of functionalities assigned to the peaks at 286.0, 287.4, and 289.5 eV compared to rCB_2. Additionally, in both cases, the sulfonation of rCBs with concentrated sulfuric acid altered the rCB surface characteristics, removing some of the C-X moieties.
The results of the textural analysis of the pristine and modified samples are presented in
Table 2.
The relatively high BET surface area of 68 m
2/g obtained for the commercial CB material was comparable to that of rCB_1 and rCB_2, which showed
SBET values of 66 m
2/g and 63 m
2/g, respectively. These BET surface values obtained for the produced rCBs fall within the range of those previously reported for tire-derived solids [
72,
73] and resulted only from the presence of pores of higher sizes (as
Vmicro in all cases was not detected). Furthermore, based on the analysis of the adsorption/desorption isotherms depicted in
Figure S5 in the Supplementary Material, it can be inferred that rCB samples exhibited type IV behavior, characteristic of mesoporous adsorbents [
74].
After the modification with sulfuric acid, the BET surface areas of the rCBs samples slightly increased, indicating a partial purification of the sample surface from unbound particles, referred to as ash [
58]. This finding is also in line with the previous results (please refer to
Table 1 and
Figure 1,
Figure 3 and
Figure 4 for specific details).
2.2. Catalytic Results
Figure 7 presents the catalytic results obtained in glycerol acetylation using a selected functionalized sample (rCB_1-SA) versus time (expressed as the conversion of glycerol and selectivity to individual acetins). To be sure that the observed catalytic effect is due to the sample modification and not the presence of mineral matter (see also the ash content for rCB_1-SA in
Table 1), the pristine material (i.e., rCB_1) was also tested.
As can be seen in
Figure 7a, the glycerol conversion obtained after 1 h of the reaction over unmodified rCB-1 was about 40%. This parameter increased with time; however, at each measuring point, the achieved results were comparable to those obtained for the blank (please see also
Figure 8 and the relevant discussion). This simply means that the pristine rCB-1 (and the contaminants present in the sample) did not show any catalytic effect in the tested process. This finding agrees well with the negligible rCB_1 acidic properties (see
Table 1) and the lack of acidic sulfonic groups in this material (see
Figure 6a). rCB_1-SA showed an improved catalytic performance in the reaction compared with its unmodified counterpart. The observed enhancement in the sample activity was most likely related to the functionalization process and the introduction of acidic sites on the rCB-1 surface, as suggested by comparing A
tot values of rCB_1 and rCB_1-SA in
Table 1 and XPS results in
Figure 6. The same behavior was also reported for the rCB_2 pair of samples, i.e., raw (which was practically inactive in the reaction) and sulfuric-acid-treated (which showed an improved catalytic performance compared to the initial carbon).
Figure 7b–d depicts the distribution of different products obtained in glycerol acetylation over the pristine and modified rCB_1 versus time. As can be seen, mainly MA was initially formed when the untreated rCB_1 was applied. The use of rCB_1-SA as a catalyst resulted in considerable changes in the distribution of acetins, promoting the formation of mainly DA and TA products, i.e., the compounds of special interest. Furthermore, the selectivity to higher substituted esters generally increased with time at the expanse of MA, suggesting a subsequent transformation of MA to DA and TA. This observation is in accordance with the suggested mechanism of glycerol acetylation over carbon-type catalysts [
75]. Finally, after 24 h, the combined selectivity to diacetins and triacetin obtained using rCB_1-SA was over 80%.
Figure 8 compares the catalytic results obtained in the reactions over the functionalized tire-derived carbons (i.e., rCB_1-SA and rCB_2-SA), modified commercial carbon black (i.e., CB-SA), and in the blank test (reaction without a catalyst).
As observed, the reaction of glycerol with acetic acid is an autocatalytic process. This is due to the presence of a hydrogen atom in a CH
3COOH compound. Thus, the blank test showed about 45% conversion of glycerol after 1 h, which increased with time to about 90% after 24 h. About a 38% yield of MA was obtained in the first 6 h of the reaction, while the yields of DA and TA obtained within this time were only about 40% and 7%, respectively. As can be seen, the functionalized recovered CBs worked efficiently in the process, giving significantly higher conversions of glycerol and yields of higher substituted acetins, i.e., DA and TA, compared with the reaction without a catalyst, in a short time. Thus,
YDA and
YTA obtained using modified rCB_1 and rCB_2 were about 53% and 18% after 6 h of the reaction. The extension of the reaction time resulted in only slight changes in the
YDA, whereas
YTA increased to about 25–32% after 24 h. Importantly, the catalytic performance of the produced catalysts was comparable to that exhibited by the modified commercial CB sample. This agreement aligns well with the similar sulfur contents introduced to the commercial CB (1.0%) and rCBs (increase by approximately 1.0% and 1.3%) after their modifications (see
Table 1).
To assess the catalytic stability of the obtained catalysts, a selected sample (rCB_1-SA) was tested in four subsequent reaction runs (time of reaction of 4 h), and the obtained results are presented in
Figure 9. As can be observed in the graph, the conversion of glycerol practically did not alter when the catalyst was recycled, and in the fourth run, the
XGly measured after 4 h was still about 95%. Generally, the yields of the most valuable products, i.e., DA and TA, did not change considerably in the successive cycles, although some drop in the TA yield was observed after the first reaction run. Further changes in
YTA were not so significant, and the yield of triacetin obtained in the fourth reaction over rCB_1-SA was still higher than that produced in the blank. The slight drop in the catalyst activity was probably related to the partial leaching of the active sites of the reaction. This was suggested by the slight decreases in the S content and sample total acidity of rCB-1_SA after the fourth reaction cycle compared with the fresh rCB_1-SA (please compare the results in
Table 1 and
Table 3). Interestingly, the ash contents in the fresh and reused rCB_1-SA were comparable, suggesting that the mineral matter was not dissolved in the reaction medium. Therefore, an expensive purification step of the produced rCBs before their use in glycerol acetylation can be omitted.
Table 4 presents a comparison of the catalytic results obtained in glycerol esterification over tire-derived rCBs and other carbon-type catalysts. As can be observed, the glycerol conversions achieved using typical carbons described in the literature were very high and comparable (above 90%) in all cases. However, the samples exhibited varying activities towards the formation of DA and TA, and the conditions for obtaining satisfactory results differed. For example, de la Calle et al. [
76] found that sulfonated hydrothermal carbon produced from glucose (SHTC) could give about a 60% selectivity to diacetins (DA) and a 30% selectivity to triacetin (TA) within 4 h. However, a large excess of acetic acid (Gly:AA molar ratio of 1:9) was necessary to achieve those results. Other types of carbons, such as C_glycerol obtained by partial carbonization with concentrated sulfuric acid or thermally reduced graphene modified with diazonium salt (TRGO_BDS), worked effectively at a more economically favorable Gly:AA molar ratio (1:6), giving selectivity to TA of about 20% within 2 h. On the other hand, the carbon produced from the Karanja seed shell at 400 °C (KJ-400) yielded only about 40% and 4% selectivities to DA and TA, respectively, within 4 h. In view of the above, the catalytic performance of the functionalized rCBs prepared in this research was very promising as the samples gave a significant combined selectivity to DA and TA (~75%) within a short time at an economically acceptable Gly:AA molar ratio. This clearly shows that problematic end-of-life tires can be successfully valorized into efficient carbon-type catalysts with promising activities in acetin production, contributing to the recycling of troublesome rubber wastes.