. . . "It is desirable that (a212) has a small unsaturation degree (0.1 meq/g or smaller, preferably 0.05 meq/g or smaller, particularly preferably 0.02 meq/g or smaller), and it is desirable that (a212) has a content of a primary hydroxyl group of at least 30%, preferably at least 50%, particularly preferably at least 70%." .