Wide mesoporous alumina composites are produced by an “in situ reaction” route comprising agglomeration of an alumina powder that is capable of rehydration together with a second reactive powder such as carbonate. In one method of production, the powders are fed to a rotating forming device that is continuously sprayed with liquid under conditions to form particulates. The discharging beads are then subjected to curing and thermal activation to produce the final catalyst or adsorbent. The alumina participates in a pore altering process involving the carbonate component upon formation of hydroxycarbonate intermediates such as Dawsonite. Large fraction of the pore volume of the final product consists of wide mesopores in the 15-50 nanometers range. The alumina composites exhibit a characteristic trimodal pore structure that includes also small micro-meso pores and macropores larger than 200 nanometers.