Integration of Nano-Sized HZSM-5 with ZnZrO_x as a Bifunctional Catalyst to Boost Benzene Alkylation with Carbon Dioxide and Hydrogen

Abstract

Benzene alkylation with CO2/H2 is a doubly beneficial option to upgrade benzene toward valuable products, in which benzene can be converted into valuable toluene and xylene, while CO2 can be utilized as an alkylation reagent to mitigate the greenhouse effect. It is quite challenging to ascertain the critical rule of zeolite regulation for the synergistic effect in oxide-zeolite composite (OXZEO) catalysts, a series of bifunctional catalysts were investigated in this work, consisting of ZnZrO x solid solution and nano-sized HZSM-5 with uniform size distribution and distinct acid properties. Key parameters affecting the catalyst performance were analyzed and optimized in this tandem reaction, namely, the mass ratio of two components, the SiO2/Al2O3 ratio of HZSM-5, and the proximity between ZnZrO x and HZSM-5. Due to the increased active site accessibility originating from nano-sized HZSM-5, the combined selectivity of toluene and xylene reaches 94.6% at 31.3% benzene conversion, respectively. Moreover, low ethylbenzene selectivity (<1%) has been achieved. It is worth noting that the long-term stability test indicates a faster deactivation in the nano-sized HZSM-5 system as compared with the micro-sized counterpart. Based on EDS mappings, it is found that nano-sized HZSM-5 experiences a more severe metal migration issue, resulting in lower catalyst stability. The reaction mechanism with formate-methoxy intermediates and the synergistic effect of CO2 hydrogenation-alkylation were further confirmed by in situ diffuse reflectance infrared Fourier transformations spectroscopy and gas chromatography–mass spectrometry. By correlating zeolite size and acidity with catalytic performances, these findings could facilitate the further rational design of OXZEO catalysts toward CO2 catalytic conversions.

Boosting Benzene Alkylation Conversion with CO2/H2 via a Triple Composite Catalyst

“…51 The 54,55 The weak ν(CH 3 ) and ν(CO) peaks of H 3 CO* species were observed at around 2820 and 1047 cm −1 . 38,56 These results suggest that the catalyst adopts an HCOO*-H 3 CO*-mediated mechanism for CO 2 hydrogenation to methanol. On the contrary, with the introduction of Al 2 O 3 , the relative amount of H 3 CO* species in ZZO/0.3Al 2 O 3 was significantly lower than that in pure ZZO, and a similar pattern was found for the HCOO* species.…”

“…Consequently, the catalytic activity of the ZSM-5 catalyst is weakened. Yin et al studied the interaction between zinc species and ZSM-5 of different crystal sizes and found that ZSM-5 with a millimeter scale exhibited superior catalytic performance compared to nanoscale ZSM-5 due to the weak interaction between millimeter-scale ZSM-5 and oxides . For comparison, the spatial distance between ZZO and the acidic sites of the zeolite was greater when ZZO and Z5 were mixed as granules.…”

“…Yin et al studied the interaction between zinc species and ZSM-5 of different crystal sizes and found that ZSM-5 with a millimeter scale exhibited superior catalytic performance compared to nanoscale ZSM-5 due to the weak interaction between millimeter-scale ZSM-5 and oxides. 38 For comparison, the spatial distance between ZZO and the acidic sites of the zeolite was greater when ZZO and Z5 were mixed as granules. The conversion of benzene was increased steadily to 8.5% due to the prolonged diffusion time of the intermediate methanol from the ZZO surface to the acidic sites of the zeolite.…”

“…These light hydrocarbons can alkylate with benzene to form byproducts such as ethylbenzene, which inhibits the alkylation of benzene to xylene. Yin et al optimized the concentration of Brønsted acid sites and Lewis acid sites through adjustment of the Si/Al ratio of the zeolite, and both the self-reaction of methanol and the formation of ethylbenzene could be suppressed . Thus, development of a catalytic system utilizing DME as an efficient alkylation reagent during the alkylation reaction of benzene with CO 2 /H 2 still needs to strengthen the efficient alkylation of benzene.…”

Upgrading Trimethylbenzene to Durene by CO2-Mediated Methylation over Cu-Boosted ZnZrOx Integrated with HZSM-5

“…Actually, the direct contribution of CO 2 to TetraMB in the presence of 1,2,4-TriMB might be much lower than 214.2 mg g oxide −1 h −1 since the energy barrier of *OCH 3 /methanol self-coupling is higher than that for its involvement in the methylation of aromatics, 23 and active intermediates would be rapidly consumed by methylation. 30 CO 2 hydrogenation to methanol is thermodynamically favored at a high pressure and a low temperature (Figure S3), 5,6 typically requiring an optimal temperature range of 180−320 °C depending on the catalyst, and alkylation of aromatics demands a higher temperature for effective conversion. Additionally, the reaction complexity increases when the aromatic substrate is a multimethylbenzene like TriMB instead of benzene or toluene.…”