The structure of alkyl substituents on tertiary amines can impact catalytic performance for the isomerization of glucose to fructose. When using the same concentration, homogeneous amines with similar values of pK(a) result in different glucose conversions. The observed differences can be attributed to how the steric bulk of substituents on the amine impact the accessibility. Amines with less bulky substituents achieve higher glucose conversion than sterically hindered amines, indicating that the amine is involved in a catalytically relevant step. Analogues of these homogeneous tertiary amines with different alkyl substituents are immobilized onto mesoporous silica materials. Contrary to the trend with homogeneous amines, the steric bulk of isopropyl substituents for tertiary amine functionalized silica materials leads to higher glucose conversion than less bulky substituents. The differences are associated with interactions with the silica surface. Catalytic testing with homogeneous amines in presence of silica demonstrate that the amines with bulky substituents are impacted less by the silica than less sterically hindered amines. Overall, these results demonstrate that amines are a tunable catalyst for the isomerization of glucose to fructose. (C) 2019 Elsevier Inc. All rights reserved.