Recycling Spent Cr Adsorbents as Catalyst for Eliminating Methylmercaptan

Abstract

Waste adsorbents generated from treating Cr(VI)-containing wastewater are hazardous materials and generally landfilled or treated by acid or base desorption, with concomitant high cost and toxic effects. The present work shows that these Cr adsorbents can be reused as highly efficient catalysts for treating sulfur-containing VOCs (CHSH), not only avoiding the economic and environmental impact from the conventional approaches, but also achieving the efficient treatment of sulfur-containing waste gas. Importantly, these reused Cr adsorbents exhibit enhanced activity and stability compared with the catalysts reported elsewhere, indicating a new avenue of green chemistry. The highly toxic adsorbed Cr(VI) species are reduced to a CrO crystalline phase by calcination and finally immobilized as a CrS solid phase while converting and eliminating CHSH. Still, the presence of Cr(VI) species on the reused Cr adsorbent provides enough reactive sites for reaction, but high concentration of Cr(VI) species causes serious accumulation of coke deposit on the catalyst, leading to fast deactivation of the catalyst.

Investigation of the Isolated Cr(VI) Species in Cr/MCM‐41 Catalysts and its Effect on Catalytic Activity for Dehydrogenation of Propane

“…Thus, it could be concluded that the poor dispersion of Cr species over the 15Cr/MCM‐41 catalyst gives rise to the formation of isolated Cr(VI) states. In fact, it's worth mentioning that another method for the synthesis of Cr/MCM‐41 catalyst, called as Chromium Column Adsorption process and reported in our previous work, shows a better dispersion of Cr species over the Cr/MCM‐41 sample, as displayed in Figure S6 in ESI (according to the XRF data, the Cr content from this method is close to 15 %wt, results are not shown). Surprisingly, no characterization of the isolated state of Cr(VI) species are seen from the H 2 ‐TPR profile (as shown in Figure S7B in ESI), and meanwhile, its activity towards PDH reaction is higher than that over the 15Cr/MCM‐41 sample, as exhibited in Figure S7A in ESI).…”

“…The mesoporous MCM‐41 was prepared in our laboratory as previously reported . The Cr modified MCM‐41 catalysts with different weight of Cr loadings (5, 10, and 15 wt%) were synthesized by the wet impregnation method, due to its superiority of low cost and operational easy.…”

Development of a Strategy To Reuse Spent Cr Adsorbents as Efficient Catalysts: From the Perspective of Practical Application

“…One attractive option for dealing with these undesired Cr adsorbents is to recycle them as highly efficient catalysts. 10 In fact, the outstanding characteristic of the present Cr adsorbent, regarded as organic−inorganic hybrid silicon material, is its calcined product, which can eventually become MCM-41, and is known to be a catalyst support for many heterogeneous catalysts. As a matter of fact, waste Cr adsorbents were recycled by calcination and reused as Cr/MCM-41 catalyst to eliminate methyl mercaptan (CH 3 SH), a typical compound of sulfur-containing VOCs, 11,12 as shown in our recent study, 10 which indicates a new avenue of green chemistry and has important implications for environmental protection.…”

“…10 In fact, the outstanding characteristic of the present Cr adsorbent, regarded as organic−inorganic hybrid silicon material, is its calcined product, which can eventually become MCM-41, and is known to be a catalyst support for many heterogeneous catalysts. As a matter of fact, waste Cr adsorbents were recycled by calcination and reused as Cr/MCM-41 catalyst to eliminate methyl mercaptan (CH 3 SH), a typical compound of sulfur-containing VOCs, 11,12 as shown in our recent study, 10 which indicates a new avenue of green chemistry and has important implications for environmental protection. However, it was proposed to use the chemical reagent K 2 Cr 2 O 7 to prepare Cr(VI)-containing solutions, and the influence of any additional impurity ions was not considered, so the results might not be applicable for real spent Cr adsorbents, because many other chemical species in a given wastewater would be coadsorbed or/and compete for adsorption with Cr on the adsorbent during wastewater treatment.…”

“…Thus, it would be very important to study the effect of these other additional impurity ions on the physicochemical properties and catalytic reactivity of the reused Cr adsorbent. In fact, investigation of the effects of other chemical species such as additional impurity anions and cations in the wastewaters on the catalytic performance of the reused Cr adsorbents has already been carried out in our previous work, 10 and it was shown that these impurity ions caused a difference in the physicochemical properties of the reused Cr adsorbent, thus leading to the difference in catalytic reactivity for CH 3 SH abatment. However, detailed analysis and explanations of these results are not given.…”