Influence of dealumination and treatments on the chromium speciation in zeolite CrBEA

Two samples of chromium-containing BEA zeolite, CrAlBEA and CrSiBEA prepared by different methods, are investigated by XRD, TPR and FTIR of CO adsorbed at 100 K and NO at ambient temperature. CrAlBEA is prepared by conventional ion exchange, and CrSiBEA, by a two-step post-synthesis method (dealumination of TEABEA zeolite by treatment with nitric acid followed by incorporation of chromium into resulting SiBEA by impregnation with Cr(NO3)3 aqueous solution). In both calcined and activated samples, the major part of chromium is in the form of Cr6+. A small amount of Cr3+ is also detected in CrAlBEA by adsorption of CO (a Cr3+–CO band at 2189 cm−1) and NO (a Cr3+–NO band at 1875 cm−1).The TPR pattern of CrSiBEA exhibits a main peak at 726 K, while two main peaks, at 520 and 760 K are observed with CrAlBEA. All these peaks are due to reduction of Cr6+ species. In line with the TPR results, the speciation of chromium in CrSiBEA does not change upon treatment of the sample at 573 K in H2. In contrast, the same treatment leads to the reduction of CrAlBEA with creation of Cr3+ as shown by the appearance, after CO adsorption, of Cr3+–CO bands at 2208–2202 cm−1. When CrAlBEA is treated with H2 at 773 K, some Cr2+ appear which can form tricarbonyls (as evidenced by IR bands at 2214, 2206 and 2179 cm−1). The treatment of CrSiBEA with H2 at 773 K leads to the formation of similar Cr2+ sites. However, with this sample, some Cr3+ is also present, as shown by a Cr3+–CO band at 2198 cm−1 appearing after CO adsorption. In the presence of NO, Cr2+ ions are slowly oxidized to Cr3+. These results demonstrate the important effect of dealumination of BEA zeolite and the treatment of Cr-containing BEA on the chromium speciation.